Hai Vu Pham
email: hai-vu.pham@agrosupdijon.fr

Tuong Lan Dinh

The Vietnam's food control system: Achievements and remaining issues

Keywords:

Since the beginning of the twenty-first century, food safety has become a societal issue that has received considerable attention in Vietnam. Cases of food intoxication are copiously reported in the press, while scientific reports show that the risk is imminent and is particularly underestimated due to lack of statistics and appropriate knowledge regarding the food chain. This crisis forced the Vietnam government to adopt structural solutions and construct its national food control system. In 2010, the Parliament approved the Food Safety Law, thanks to which the country started applying modern principles of administration. It implemented regular food safety controls, allocated and relayed the responsibility to decentralized public agencies, and required food information. This article discusses Vietnam's food control system after the promulgation of the Food Safety Law. We show that in addition to the inherent problems such as credibility of risk analysis, the main challenges arise from the daily implementation of control activities. Rule enforcement is weak because of the informal economy in agriculture and food supply sectors. Improvements in rule enforcement are urgently required in this fast changing economy in the middle run.

Introduction

In 2016, Vietnam was shocked by a massive fish death scandal due to an environmental disaster along its 200 km coastline caused by erroneous industrial discharge (BBC, 2016). Fishing and fish consumption was banned at that time, which was later resumed without any proper risk analysis. This, along with many other scares and scandals, has made food safety one of the top concerns of Vietnamese citizens in recent years (USAID, 2015). Currently, a significant part of the population believes that what they consume daily is not safe (Figuié, Bricas, & Truyen, 2004); (Ha, Shakur, & Pham Do, 2019) and questions the credibility of food control activities.

Vietnam has shifted from a planned economy to the market system from 1986. At the beginning, the shift did not induce large changes in the food sector, which was under the control of national cooperatives and distribution state-run companies. However, from 1995, urbanization and demographic growth stretched this on-privatization system to its limits. Area of cultivated land is reduced while agricultural production is intensified [START_REF] Lam | Emerging health risks from agricultural intensification in Southeast Asia: A systematic review[END_REF]. Studies show that the volume of pesticides used for agriculture has increased from 100 tons per year in the 1950s, to 35,000 tons in 2002 and to about 105,000 tons in 2012 in Vietnam [START_REF] Hoi | Pesticide use in Vietnamese vegetable production: A 10-year study[END_REF]. Production intensification but also industrial development (by ripple effects) make food safety problems more frequent, while food chains have become more complex. Moving quickly from the status of "having enough food to eat" to that of "needing safe food", Vietnam is confronted with serious problems of food control at present [START_REF] Nguyen-Viet | Research and training partnership to assist policy and capacity building in improving food safety in Vietnam[END_REF].

Many developing countries experiencing rapid changes in their food systems have also encountered similar situations [START_REF] Reardon | Rapid transformation of food systems in developing regions: Highlighting the role of agricultural research & innovations[END_REF]. Reports show that low-and middle-income countries in South Asia, South-East Asia, and Sub-Saharan Africa account for 41% of the global population, but are afflicted with 53% of all food-borne illness and 75% of related deaths [START_REF] Jaffee | The safe food imperative: Accelerating progress in low-and middle-income countries[END_REF]. Building the capacity to better assess and control the food system is definitively among the most important objectives of these countries for reaching the Sustainable Development Goals (United Nations, 2015).

The domestic market is not the only driver of change in Vietnam. Along with the adoption of the market mechanism, Vietnam has signed many trade agreements that aim to export agricultural and food products. The export to foreign markets such as the USA, European Union (EU), and Japan, is crucial for economic growth of the country, that also forces its farmers and food producers to comply with international standards. In particular, the Association of Southeast Asian Nations (ASEAN) Economics Community (AEC) agreement has been signed in 2015 (see Table 1 for the acronyms). This document envisions a https://doi.org/10.1016/j.foodcont.2019.106862 Received 4 June 2019; Received in revised form 30 August 2019; Accepted 31 August 2019 common market between ASEAN countries, similar to the European Economic Community (EEC) in Europe. Trades of food and agricultural products within the AEC are expected to increase sharply. Implementation of enhanced and transparent activities related to food control is the first condition to meet international requirements and render this economic integration effective.

This study describes the National Food Control System (NFCS) of Vietnam and critically reviews its main evolution from 2010. It is crucial to understand how the system is organized and how it functions from a criticized standpoint to assess its real capacity. The year 2010 is an important milestone and has been chosen as the starting point of the study. This year marks the approval of the Vietnam Food Safety Law (VFSL), which opened the door to application of modern principles of food administration. Since then, important changes have been recorded, particularly in big cities such as Hanoi or Ho Chi Minh City. Vietnam's achievements in food control are now visible as mentioned later. However, the country is still facing challenges that cannot be solved using the current methods of control, that we will also discuss.

The article consists of three main sections. First, the aim, scope and methodology of the review will be defined. Next, the main characteristics of the Vietnam's system and its evolution will be described and analyzed, respectively in terms of legislation, food control management bodies, food inspection and risk analysis, and policy priorities. Finally, the article discusses the weaknesses and challenges of the system related to the problem of rules enforcement, and generally to that of managing informal agents in a market economy.

Aims and scope of the review, and the methodology used

Aims of the review

The paper was motivated by the need to thoroughly assess certain aspects of the NFCS of Vietnam and to provide a comprehensive overview of how it functions. The Vietnamese government has set up a food safety policy, following the experiences learned from developed countries, especially the EU and the USA. However, development of a food control system does not involve only the drafting of regulatory documents or establishment of institutions responsible for quality control. A critical review is therefore needed.

In addition, as this is not an isolated case among developing countries encountering problems related to food control, this review may also be useful in other situations. Although each country is specific in its legislative setting and administrative organization, there are generic factors that may explain why and how a control system may or may not function. They are for example, the level of resources (technical and human) mobilized, clarity and transparency of control activities, or economic conditions in the food sector. Then, by analyzing these elements, helpful lessons could be learned and applied when the same problems are confronted.

Scope of the review

The concept of NFCS has been defined in Food and Agriculture Organization (FAO) "Guidelines for strengthening national food control system" as "the integration of a mandatory regulatory approach with preventive and educational strategies that protect the whole food chain" (FAO, 2006) which should include effective enforcement of mandatory requirements, training and education, community outreach programs, and promotion of voluntary compliance. Basically, it is an integration of regulatory regimens for providing safe and wholesome food to all concerned [START_REF] Shukla | Evaluating elements of national food control system: Indian context[END_REF]; but the NFCS is not only related to the state's legislation: it also refers to other forms of mutual relationship between public authorities and the food chain's stakeholders (education and voluntary standards). However, the NFCS should be distinguished from a food quality system. Food control deals with issues of safety, which are compulsory and often non-negotiable, while food quality is more flexible and is related to customer satisfaction [START_REF] Mainguy | La qualite dans I'agroalimentaire[END_REF]. Food quality can be assumed by private companies, while food control can only be implemented by, or under control of public agencies.

According to FAO (2006), a NFCS is formally composed of five blocs: (1) food law and regulation, (2) food control management, including a set of operational agencies or administrations in charge of implementation, (3) inspection service, (4) laboratory service, and (5) information, education, communication, and training (IECT). In practice, countries may pay attention to some of these blocs in a selective manner. For example, when studying the fish supply chain in Uganda [START_REF] Bagumire | National food safety control systems in Sub-Saharan Africa: Does Uganda's aquaculture control system meet international requirements[END_REF], proposed to tackle four components of their NFCS: legislation, identified public authorities, laboratory system, and food inspection. [START_REF] Pham | La securite des aliments au Vietnam -etat des lieux pluridisciplinaire sur les enjeux au niveau de la production, de la distribution, des politiques publiques et des risques microbiologiques[END_REF] showed that the EU food control system functions with three main components: (1) food administration (Directorate General Health and Food Safety -DG SANTE, and agencies of the state members' ministries), which promulgates law and regulations and are in charge of food inspection, (2) risk assessment, which is independently performed by the European Food Safety Authority (EFSA) and the food safety agencies of the state members, and (3) the food operators' responsibility regarding public regulation and inspection (for example, Hazard Analysis and Critical Control Point (HACCP) and traceability).

Risk analysis is an important principle of food control activities although it is not always mentioned explicitly as an operational bloc. The definitions and working principles for risk analysis have been developed for use by the Codex Alimentarius Commission. Since 2000, risk-based approach has been mobilized by most developed countries [START_REF] Giorgi | The pitfalls of european risk governance: A question of design or application? Some empirical results from the food safety sector. Innovation: The european[END_REF]. For example, the EU clearly mentions three steps in risk analysis: risk assessment, risk management, and risk communication [START_REF] Alemanno | Foundation of EU food law and policy-ten years of the european food safety authority[END_REF]. However, in developing countries, the scope of risk analysis is often reduced owing to the lack of capacity and understanding for conducting risk analysis in appropriate manner. In the case of Vietnam, risk analysis becomes an emblematic point that merits mention.

To discuss the case of Vietnam in detail, it is decided to introduce certain adjustments. Instead of following the initial 5-bloc configuration suggested by the FAO guideline, the review focuses on four blocs:

-Law and regulation -Food control management bodies -Food inspection vs. risk analysis -Policy, education, and training Blocs 1 and 2 are not often communicated by the government, which usually leads to misunderstanding among the stakeholders. Bloc 3 maintains an ambiguity between food inspection and risk analysis, which will be discussed separately in a section. Regarding bloc 4, although these elements are at the core of Vietnam's food safety policy, they constitute objectives rather than actions of an operational program.

Finally, the review looks at the conditions of implementation of the NFCS. A good design is not sufficient for smooth functioning of a system. Equally critical are the manner in which they are implemented by the local authorities and the food chain's stakeholders who are supposed to comply with the rules, but do not actually cooperate for various reasons.

Methodology

The methodology used consist of reviewing documents, coupled with information collected from different interviews with public officers and experts. In the first step, analysis reports from experts having worked on the issue were studied. Limited in number, these reports come from World Bank (2017), World Bank ( 2016 In the second step, all texts of law and regulations that are mentioned in the above reports have been identified. For the sake of coherence, only texts that have been implemented after 2010 were chosen. As the volume of juridical documents is still large, focus are made on most important ones, which are obtained thanks to the interviews with experts and public authority officers.

Finally, critical reviews were made by comparing result of desk studies with information collected from field surveys, which are carried out in parallel. Owing to the area of research of the authors, these surveys were limited to the meat and vegetable sectors. The authors have met with stakeholders and public officers in these fields and discussed the implementation of food control activities with them.

It's worth noting that a systematic review of the literature has also been performed. However, owing to the dearth of scientific writings in both English and the native language, the results will not be presented here.1 Whereas, the article did not aim at establishing the causality between policy changes and the trend of food-borne diseases because of lack of data. Access to data is one of the biggest challenges when dealing with food safety in Vietnam. The lack of transparency in the food chain statistics and risk communication has been mentioned by other researchers (World Bank, 2017). But data is still an important issue, and is discussed below (Box 1).

The food control system of Vietnam: major points

Food law and related regulations

The Vietnam food safety law 2010 (VFSL) and its most related regulatory documents

The VFSL2 or precisely the Food Safety Law No. 55/2010/QH12 is the most important text in the legislative blocs. Adopted in 2010, it is a remarkable reform of Vietnam's regulatory framework in terms of food safety (USDA, 2013). It sets forth innovative approaches and principles aligned with international approaches (World Bank, 2017). In addition, it affirms that organizations and individuals participating in the food chain are obliged to ensure food safety. Although trivial, these principles are essential for ensuring that stakeholders are first persons who shall respect safety requirements.

The law defines mandatory technical regulations that should be respected by all. Procedures of food testing and inspection, and food safety risk analysis have been formally rendered. Traceability has been evoked and should be applied in case of identification of unsafe food or at the request of competent authorities. In terms of control, an ex-ante control step ("the certificate of compliance with food safety conditions") is required for food operators, with derogation for BRC (British Retail Consortium standard) or IFS (International Food Standard respectively) holders.

The VSFL delegates the state's responsibilities for food control and food safety management to the government. Specifically, three ministries share this important charge: the Ministry of Health (MOH), the Ministry of Agriculture and Rural Development (MARD), and the Ministry of Industry and Trade (MOIT). Three inter-ministerial regulations frame this cooperation, the most important one being the interministerial circular number 13/2014/TTLT-BYT-BNNPTNT-BCT.

Some other issues were completed by subordinate regulation enacted by the government and concerned ministries. In particular, the decree 38/2012/ND-CP (today superseded by decree 15/2018/ND-CP) specified the procedures of certification and declaration of conformity with technical regulations, food safety controls for Genetically Modified Organisms (GMOs), state control of imported and exported foodstuffs, food labeling, and advertising.

Related laws and regulations

Vietnam's legislation framework on food safety is also concerned with other laws, which are connected to the VFSL in different manners. Among them, the most important ones are: a. Law on Quality of Products and Commodities b. Law on Standards and Technical Regulations c. Law on Inspection Explanation is given in the Table 2. In addition, there are other laws and regulations regarding food control issues, such as the Law on Administrative Sanction, Law on Aquaculture and Fisheries, Ordinance of Veterinary Control, Ordinance of Plant Protection and Disease Control, Ordinance of Plant Varieties Protection, and Ordinance of Animal Species Protection. Compared to the three precedent laws, these texts are more sectorial, and not always associated with the control of food safety.

Food control management body

According to FAO (2006) food control management is the continuous process of planning, organizing, monitoring, coordinating, and communicating, in an integrated way, a broad range of risk-based decisions and actions to ensure the safety and quality of domesticallyproduced, imported, and exported food for national consumers and export markets, as appropriate. In Vietnam, this task is shared between three ministries: the MOH, MARD, and MOIT; the main and final responsibility lies with the MOH. Besides, it is important to mention that food control is also managed by local authorities. There are 63 people's committees at the provincial/city level, 678 people's committees at the district level, and sometimes several in the ward level, as in some of Vietnam's big cities or provinces.

Government level

Management competences are distributed among the three ministries based on two criteria: (1) the stage in the food chain and (2) the category of food. The MARD is responsible for controlling the production of agricultural products for human consumption. It also follows the first processing operation of fresh products (cleaning, slaughtering, and cutting) and their trading at the wholesale stage. The MOIT is responsible for processed foods, and especially the retail sale of all food products (including fresh food) in circulation at traditional markets, supermarkets, and food shops. Finally, the MOH is responsible for the food additives and chemical agents used in food processing. It is also in charge of the last stage of the food chain, which includes control at restaurants, collective kitchens, and canteens (see Fig. 1 below). This is a clear improvement, as prior to 2010, the MOH had to control all the food products that were circulated in the market.

The coordination between the three ministries is realized via a national directive committee on food safety. The committee acts officially as a bureau of policy, under the lead of the vice prime-minister in charge of food safety. The role of the central committee is to provide policy guidance to assist the Government in decision-making. However, inter-sectorial coordination is rather weak. According to experts, no efficient exchange of information or resource has actually occurred in this instance.

Inside each ministry, responsibility allocation follows a vertical topdown configuration.

• The MARD scatters food control responsibility through its different departments. The main task belongs to the National Agro-Forestry-Fisheries Quality Assurance Department (NAFIQAD) whose missions are to manage the quality and safety of foodstuffs, to test and certify the safety of imported food, and to perform necessary tests in accredited laboratories. NAFIQAD is strongly supported by two other departments: the PPD (Plant Protection Department) and the DAH (Department of Animal Health or Department of Veterinary Sciences). In addition, its activities are related to more or less those of the three others departments under the MARD: Plant production, Animal husbandry, and Processing and Trade promotion (recently created in 2017).

• At the MOH, the responsibility rests with the Vietnam Food Box 1 Dispersed data on Vietnam's food safety issues.

In 2011, a project of law based on a report from the Ministry of Health (MOH) provided the first data from 2004 to 2009 (MOH, 2011) regarding food safety issues. The document provides several statistics confirming problems with food safety. For example, from among the 60 breeding farms randomly selected for the study, 86% of the animal feed samples contained residual antibiotics higher than that approved by legal standards; 50% of the urine samples contained the prohibited growth hormone. These findings converged to an independent study by the International Livestock Research Institute (ILRI) [START_REF] Hoang | Securite alimentaire de viande porcine -de la perception des consommateurs vietnamiens a la politique[END_REF], according to which 60% of the meat samples in Vietnam's traditional markets do not follow the microbiological standards (90% in Hanoi) and 10% contain antibiotic residuals exceeding maximum residue limits MRL.

Pham & Dao (2016) collated several dispersed food hazard-related studies regarding vegetables. One of them shows that 539 of 1050 vegetables (51%) sampled from Hanoi's traditional markets was contaminated with heavy metals and dangerous pesticides at levels surpassing the MRL. This study was performed by the international non-governmental organization (NGO) VECO (now Rikolto) in collaboration with Vietnamese research centers in 2010. Another 2010 study from the University of Health of Hanoi showed that 72% of the 660 analyzed samples of periurban vegetables contained Escherichia coli and Salmonella.

Publications focusing on the use of pesticides and antibiotics in agriculture [START_REF] Hoi | Pesticide use in Vietnamese vegetable production: A 10-year study[END_REF][START_REF] Nguyen-Viet | Rapid integrated assessment of food safety and nutrition related to pork consumption of regular consumers and mothers with young children in Vietnam[END_REF], safety issues in food sub-sectors (T. T. T. [START_REF] Tran | Food safety and the political economy of food governance: The case of shrimp farming in Nam Dinh Province, Vietnam[END_REF], [START_REF] Tran | Governance of global value chains in response to food safety and certification standards: The case of shrimp from Vietnam[END_REF], [START_REF] Luu | Seafood safety compliance with hygiene regulations within Vietnamese domestic distribution chains[END_REF], [START_REF] Dang-Xuan | Quantitative risk assessment of human salmonellosis in the smallholder pig value chains in urban of Vietnam[END_REF]; and the impact of new practices of production and distribution [START_REF] Moustier | The role of farmer organizations in supplying supermarkets with quality food in Vietnam[END_REF]; (Wertheim-Heck & Spaargaren, 2016); ( [START_REF] Van Hoi | Market governance for safe food in developing countries: The case of low-pesticide vegetables in Vietnam[END_REF] are available in the Web of Science from 2010 till date, the number of which is increasing slightly every year. Despite the originality and the quality of these studies, the publications are dispersed and their scopes are limited, considering the societal aspects of the issue. The most probable reason is the difficulty in accessing data, while the cost of collecting data itself is high.

The 2011 official report of the MOH mentioned 1058 food poisoning outbreaks during the period 2004-2009. The number of cases was 1007 between 2011 and 2016, with 30.395 victims and 164 deaths. In 2017, the World Bank reported considerably different statistics on foodborne diseases (VFA)-from more than 7000 outbreaks each year in 2006-2008 to 5500 in 2012-2013, and from 57 to 28 deaths in these two periods. Despite the differences in the results, both reports emphasize the involvement of microbiological agents. According to the Ministry of Health, these incidences were caused by pathogens (41%), followed by biological toxins (28%) and chemicals (4%), with 34% cases in the northern mountainous area alone.

Table 2

Important laws related to food control activities in Vietnam.

a. Law on Quality of Products and Commodities

This law governs the control and inspection of food as merchandise in the market. It focuses on market regulation, the most vulnerable stage in the Vietnamese food chain. Under this law, the decree No. 43/2017/ND-CP on product labeling requires information on food products. However, the decree covers only packed and processed food, and excludes fresh food and processed-but-unpacked food from its field of application.

b. Law on Standards and Technical Regulations

Legal framework for standards, technical regulations, and compliance assessment in general and for the food sector in particular. Technical regulations in Vietnam are not fully complied with the Codex Alimentarius; In 2016, [START_REF] Pham | An toàn thực phẩm nông sản. phẩm, hệ thống sản xuất phân phối và chính sách nhà nướcHanoi: NXB Nong Nghiep[END_REF] reported that only about 65% of food safety technical regulations are equivalent to the Codex norms.

c. Law on Inspection

This law has been modified to consider specialized inspection related to food safety control. It determines the conditions of inspecting food activities and food operators.

Administration (VFA). The VFA is in charge of elaborating the safety standards and providing advice to assist the MOH in implementing state control and state legal application regarding food safety. It plays an important role in establishing the legal framework of food control. The MOH also created the National Institute of Food Control (NIFC), which is not involved in safety management but performs food inspection and risk assessment.

• The role of MOIT in safety control is rather modest. Their main mission is to control the legal circulation of foodstuff in markets.

According to the report of MOH ( 2011), most of the non-compliance is related to the conditions of marketing of food. The Vietnam Directorate of Market Surveillance (VDMS) under the MOIT has then the important charge of controlling food label and food information.

It controls all traditional markets, supermarkets, and food shops where foodstuffs are sold as merchandise. 3 Besides, the Directorate for Standards, Metrology and Quality (STAMEQ) under the MOIT contributes to the elaboration of legal standards for products and processing schemes.

NAFIQAD (MARD) and VFA (MOH) are the two most important agencies in charge of food safety control management in Vietnam. They have antenna at almost 63 provinces. The PPD and DAH (both under MARD) also play important roles as upstream controls are primordial in Vietnam; many safety issues are detected at this stage.

Decentralized food control management bodies

Provincial and district agencies have progressively accepted new responsibilities of Vietnam's NFCS. They are decentralized services of the state, but mobilized to work together horizontally inside an intersectorial framework. Their coordination is ensured by the People's committee. 4 The directive 13/CT-Ttg on 13 May 2016 reinforced the responsibility of local authorities by stating that the president of the people's committee should be responsible for safety issues in his/her territory of competence.

Concretely, there is an inter-sectorial directive committee, and different control groups operating in each province/city. A (provincial/ city) inter-sectorial directive committee is similar to the central committee, albeit limited to the administrative boundary, and is led by the president of the province. Its members are essentially from the three departments under the three ministries, DARD, DOIT, and DOH, and other different public and civil institutions. The latter structures continue to receive vertical guidance from their ministries. But, their dayto-day operations are decided horizontally by the people's committee of the city or province they belong to.

Control groups are units who implement field controls and do not participate in the management (Fig. 2). Next, they are organized at different levels, such as provincial, district, and commune levels. The multiplication of controls at many levels reduces risk and clearly increases the degree of compliance of stakeholders. This contributes to the success of Vietnam's NFCS (Government, 2017). The disadvantage is that overlapping controls disturb the food operator and lead to unoptimized use of resources.

The most advanced experience of decentralization was a pilot program implemented in 2017 in three cities: Ho Chi Minh, Danang, and Bac Ninh. The program created the city's Food Safety Management Authorities (FSMA) in each city. The FSMA is a public establishment, which has higher degree of freedom than the directive committee. It can apply new food management methods, for example, traceability of food using technology (QR code reader for consumers with smartphones) or the promotion of organic food items. The fledgling program is, however, too immature to be evaluated.

In overall, the decentralized solution is limited by the available workforce. With 10,805 rural and urban communes, Vietnam is clearly lacking qualified agents for ensuring controls at every administrative level. Rural communes do not have a control group, while many urban communes (wards) work with incomplete teams. For example, the control group may contain one agent from the MARD, one agent from the MOIT, but no one from the MOH. Lack of training capacity is also another challenge that retards the recruitment of food control agents.

Food inspection -risk analysis

Food inspection

According to the FAO (2019), inspection activities have focused historically on sampling and testing of end products to determine their compliance with regulations. Today, it focuses more on ensuring the compliance of systems for food safety management, implying implementation of the general principles for food hygiene and HACCPbased approaches. The aim of the new "preventive" approach is to reduce food safety risks. Food inspection in Vietnam still follows the historic configuration. It insists on verifying whether the food complies 3 According to the report of MOH (2011), most of the non-compliance is related to the conditions of marketing of food products. 4 Name of local governments in Vietnam.

with legal safety requirements. Two tools of inspection have been developed in Vietnam: the certificate of compliance with food safety conditions and the safety test.

• Valid during a period of 2 year, the Certificate of compliance with food safety conditions is addressed to individual and legal persons, who operate in the food chain. 5 The certificate does not certify the safety of foodstuffs, but only the capacity of food operators for complying with the legal requirements. In particular, it certifies that food operators have minimal knowledge regarding the physical conditions at work to ensure food safety. Inspection is performed to verify whether the stakeholders respect all the conditions. For producers, the scope of inspection includes all inputs such as soil and water conditions, pesticides, and animal feeds.

• The safety test is used to verify if food stuff follows the technical regulations. The principle is that food is sampled and then dispatched to laboratories for microbiological and chemical analysis.

Sampling can be performed directly by public authorities such as NAFIQAD and VFA for hazardous controls. NAFIQAD has also developed rapid tests that can be used directly at the selling stalls at open wet markets. The VFA inspects imported food and performs sampling tests at restaurants, collective kitchens, and school canteens.

Test analysis is performed by accredited laboratories accredited by the government. In 2017, 37 certification bodies, including 13 from MOH and 24 from MARD, and 101 laboratories, both private and public, have been accredited. Among them, there are 19 laboratories from MOH, 6 from MOIT, and 76 from MARD, which are involved in the inspection of fertilizers, animal feed, and veterinary medicament (Government, 2017).

In 2009, the MOH created the National Institute for Food Control (NIFC), the most important structure in charge of food inspection in Vietnam. Its objectives include food hygiene testing for quality products and additives, providing processing aids for domestic food production and imports, and risk assessment of food. The NIFC also acts as a national referee in the field of food inspection for safety and quality control. It offers professional guidelines and assesses the capacity of testing laboratories nationwide.

Food inspection vs. risk analysis

Food inspection, despite the fact that it received many attention from the Vietnam's government, should not be confused with risk analysis. In particular, it should be distinguished with risk assessment, the first component of the risk-based approach. According to the WHO (2019), risk assessment is the scientific evaluation of known or potential adverse health effects resulting from human exposure to food-borne hazards. The principle of independent risk assessment is fundamental [START_REF] Jouve | Principles of food safety legislation[END_REF]. Estimate of risk should be based on sound scientific studies to establish good management strategies.

In Vietnam, risk assessment is mentioned in official documents; however, there is an ambiguity between the definitions of this operation and food inspection. The article 50 of the VFSL defines that risk assessment consists of verification and inspection of risk to public health with respect to the presence of physical, chemical, and microbiological factors. The law states that the ministries can legally and separately perform risk assessment (the responsibility of VFA and NAFIQAD). 6 In reality, assessments are outsourced to accredited laboratories in a contractual relationship, followed by discussion on the results only among concerned public authorities. Solution of management is given in a second phase, without any explanation. Then, risk communication is considered to be bad (World Bank, 2017) and the credibility of the entire process of risk analysis is at stake. Thus, there is not really assessment of risks to health, and most of the inspection activities are legal hazard tests for verifying the level of compliance.

A risk assessment taskforce was initiated in 2007 by universities, research institutes, and technical experts from MOH and MARD to initiate a risk-based approach in Vietnam, which has recently been evaluated [START_REF] Nguyen-Viet | Research and training partnership to assist policy and capacity building in improving food safety in Vietnam[END_REF]. The results show that the process requires time to become accessible, mostly because its impact is not directly identifiable in terms of decrease in the frequency of food-borne 5 Smallholders are, however, exempted from obtaining this certificate. 6 Only the MARD has issued guidance on risk analysis (circular number 02/ 2013/TT-BNNPTNT in 2013) and the MOH is working on a draft of the circular.

diseases or protection of the population. Such result is intuitively easy to understand: spending on risk assessment is important, but "returns" on the investment are not directly observable, that posits a challenge for low income countries who want to see policy's results expressly.

Food safety strategy, education, and training

Food safety policy is important as it paves the way for the evolution of food control measures. As Vietnam's NFCS is at an early stage of development, it can decide the evolution of the system. Hence, the following points are important.

The strategic vision to 2030

The most important food safety policy document is the decision approving the national food safety strategy in the period 2011-2202, vision to 2030. Released in 2011, the document focuses on five main objectives:

• Improve the understanding on food safety of different categories of stakeholders in the food chain (food producers, food processor, consumers, and state officers.)

• Building the capacity for the state's food safety administration (ap- plication of ISO standard, constructing a rapid alert system, implementation of risk analysis for all cities with > 2 million inhabitants)

• Improve the use of standard and certification of food safety (appli- cation of HACCP, GMP, GHP, ISO 9001, ISO 22000, and VietGAP)

• Improve food safety conditions at commercial units (restaurant, collective kitchen, market)

• Reduce cases of food intoxication. Capacity to control along the whole food chain in 2030.

To summarize, the official vision is to reinforce the state's control, improve the information and communication regarding food safety, and provide knowledge to increase citizen's understanding. This policy was implemented by the government and its national inter-sectorial directive committee. This vision, materially translated in a national program of action, shall be respected by all local governments. Specifically, the program defines main actions as following:

• Develop volunteer safety standard (VietGAP, VietGAHP) and other private standards.

• Modernize traditional markets where food is sold under poor hy- gienic conditions.

• Use modern technologies in food production and food supply.

Then, the program of action envisions a modern food system, with a large place for private stakeholders, who can comply with the standard and manage the implementation of food control (Vietnam Parliament, 2017). These stakeholders are expected to invest in technological solutions to generate value-added products that meet the standard. For example, Vietnam is attempting to replace traditional markets by supermarkets and trade centers, which will be controlled by the operators [START_REF] Wertheim-Heck | Food safety and urban food markets in Vietnam: The need for flexible and customized retail modernization policies[END_REF]. This will empower private stakeholders (enterprises, supermarkets), who can establish selfcontrol. At the same time, farmers and smallholders will obviously face increased challenges in this fast-changing economy.

Generally, the policy may conduct to a shift of control power from public authorities to private food companies. Such change presents a potential threat because the government of Vietnam is going to establish many food standards in collaboration with stakeholders. If a company engaging in this process obtain in addition the possibility of implementing self-control, it could generate unfair conditions of competition without necessarily improving food safety. It's worth reminding that food control has the characteristic of a public good, meaning that it requires public intervention because it is objectively not a profitable activity.

Traceability

Inspired by the EU, where traceability is obligatory in the feed and food chain, the Vietnam government is looking forward to implementing the concept of traceability, in the perspective of putting control along the food chain. However, no clear regulatory framework has been defined. The VFSL itself is unclear on traceability as it requires only traceability of unsafe food. It states that "traceability for unsafe food should be implemented in case of request by competent authorities or when the food operator detects by himself a safety problem". In these cases, food operators shall provide numbers of the affected batches and inform its business representatives about them. In 2011, the MARD issued the circular number 74/2011/TT-BNNPTNT requiring one-step-back-onestep-forward traceability. As the only regulation on traceability, the circular is to be applied by all producers and traders of food under the control of the MARD. But only hand-written tracking documents are required, which can be easily falsified.

In reality, traceability has already been implemented by modern food processing companies, not for the sake of compliance but for their own management requirements. The situation regarding smallholders is different, and unique methods of traceability in the food sector are not available. The MARD identified important disconnects between their management of farms, slaughterhouses, and market vendors inside their own system (IPSARD, 2016). For instance, for animal-origin products tracking, the ear notch, which is the basis of the animal identification system, has only been experimented in North Vietnam in 2016. That clearly shows that the MARD's circular was not operational.

Traceability is a challenging issue for Vietnam, as many agents participating in the food sector do not possess any business licence. This implies that even if a national program on traceability is implemented, the government has to develop a new independent system of agent identification as it cannot use the system of business identification (see informal economy later).

Promotion of standards

The government is proactive in elaborating and disseminating standards both via standardization and regulatory processes. The most remarkable example is where 13 regulatory documents have been issued to regulate the production and commercialization of safe vegetables RAT ("rau an toan") [START_REF] Dinh | From 'rau an toan' to a proliferation of standards: Where are we at in Vietnam with addressing food safety through standards?[END_REF]. The national strategy on food safety expects that by 2020, 60% of tea and vegetable areas in Vietnam will be produced under standards of safe vegetable and tea.

State actors play central roles in promoting the standard. However, achievement of the standard-use policy is weaker than expected. The reason is that standards are considered signs of quality, which is not accessible to the low-and middle-income classes. [START_REF] Pham | Referentiels de production de legumes au Vietnam[END_REF] showed that the national surface of VietGAP in 2016 was less than 6% of the total vegetable production, mostly because of difficulties in identifying stable consumers with purchasing power. The objective of achieving 60% of production by 2020 as defined by the national food safety strategy appears to be a distant dream. Furthermore, evidence showing that the use of GAP might improve food safety is minimal [START_REF] Nguyen-Viet | Food safety in Vietnam: Where we are at and what we can learn from international experiences[END_REF]. Reports show that 72% consumers never purchased certified poultry and nearly 40% respondents regularly buying chicken with governmental certification do not view it as a credible certification.

Moreover, the management of food quality and food safety is a matter of permanent confusion in Vietnam. For example, both NAFIQAD and VFA have vocation to work on food quality, even though their main activities are control of food safety. Then, the distinction between mandatory legal standards and voluntary standard (of quality) is unclear for many food operators, not to say also for the consumer.

Education and to the informal food sector

Education and training is the last important part of the food safety strategy of Vietnam. They help to consolidate the responsibility of food operators. Trainings are organized by different public institutions such as NAFIQAD and VFA. The beneficiaries are farmers and food businessmen. In Vietnam, a successful completion of training is the first condition to be reached to obtain the certificate of compliance with food safety conditions. The training is compulsory for all food operators, except farmers and smallholders, for whom it is optional. People who receive the brief training are asked to appear in a short control test such as a traffic code examination, where they should select good answer options. The government expects to generate large number of effective trainees every year. It expects that 100% of the food safety staff and 82% each of the food operators and consumers will be trained by 2020. However, the quality of training has not been thoroughly evaluated.

Remaining issues and challenges

With a fledgling NFCS, the achievements of Vietnam from 2010 are encouraging. Today, public officers are more familiar with the Codex Alimentarius and the harmonization with the Codex is on going. Control of production inputs (plant protection substances and animal feeds), as well as veterinary controls at different stages of the food chain, have been regularized. Vietnam is now looking toward implementation of food labeling and traceability. The results confirm that the country is on its way toward consolidation of NFCS. However, important issues and challenges persist, which require detailed discussion.

Challenge of the informal sector

The informal sector or informal economy is the part of the economy which is not regulated by the state. The adverse effect of informal economy is related to the protection of workers (ILO, 2018), lack of tax payment and smuggling. Such problems are endemic in many developing countries. The informal sector is also a challenge for food control activities, because regulations and controls first target the legal stakeholders, i.e., operators with a legal status.

Vietnam started legalizing the activities of private agents in the market economy from 1986 when it departed from the planned economy. However, a large part of Vietnam's agriculture and food economy involves informal stakeholders, who have worked before for national cooperatives. Today, farmers and many food distributors are still unknown to the administrator, because their activities have not been legally registered. The World Bank (2017) report states that in 2010, the country had 8.9 million agriculture-based households, but only 2536 agricultural enterprises (meaning with a legal status). The Vietnam government also inventories a number of 500,000 food processing establishments in 2017, of which 85% have the status of smallholder business. The latter is a special status in Vietnam reserved for small enterprises of less than 10 units of labor, which are exempted from declaring detailed activities. Only a lump sum tax is due after a declared general turnover. As a consequence, fresh food such as rice, meat, and vegetable, was sold in markets without any form of control, including fiscal controls.

From 2010, the VFSL is urging the government to strictly control food operators. The MARD was handed the important portfolio of food control, and have to deal with the large number of farm households, food semi-processors and food collectors, of which the activities are not registered. Concerning the MOIT, besides the control of formal sector such as industrial processing companies, supermarkets, and special food shops, they have the charge of controlling the wet markets which are the most important channel to access food in Vietnam. In 2017, the landscape was still dominated by traditional wet markets: 8600 official ones versus 700 supermarkets in all the country [START_REF] Nguyen-Viet | Research and training partnership to assist policy and capacity building in improving food safety in Vietnam[END_REF]. According to public officers, an important part of vendors operating in these wet markets are not registered. Moreover, many of them operate outside the official perimeter defined by the authorities. Then, the control of their activities is very difficult since these agents are unknown by the administration.

Vietnam has tried to solve the problem by decentralization of food control activities. Local agents can produce good results because they have better understanding of the field, and can design appropriate solutions to adapt the compliance framework to the local context. However, in the long run, combating informal economy via local solution may be expensive. Furthermore, the food chain today does not contain only localized stakeholders, but also collectors and wholesalers to whom local control is clearly not enough. These latter are extremely mobile and can obtain supplies from remote uncontrolled production areas. Without regulating their practices via a centralized management, it is difficult to have an efficient NFCS.

Rule enforcement in a fast changing economy

Generally, rule enforcement is a challenge for all control activities. The rapid economic development adds a new layer of difficulty. Developing countries experience mass concentration in cities where people can earn more. At the same time, the demand for food is high, while access to safe and good-quality food becomes difficult [START_REF] Reardon | Rapid transformation of food systems in developing regions: Highlighting the role of agricultural research & innovations[END_REF]. In this situation, food operators tend to overlook the rules for profit. Thailand, Cambodia, India, and China are facing the increasing problem of food safety parallel to economic growth [START_REF] Jaffee | The safe food imperative: Accelerating progress in low-and middle-income countries[END_REF]. For example, in 2008, China was affected by the crisis of melamine-tainted baby formula, which claimed six lives and caused over 290,000 nationwide cases of renal disease [START_REF] Zhou | The regulatory regime of food safety in China, a systemic not accidental failure[END_REF]. This highlights that Vietnam is not an isolated case of faulty food control.

Experiences in different countries show that regulation is necessary, but it is the daily implementation of rules that matters the most [START_REF] Ni | Law enforcement is key to China's food safety[END_REF]. Correct implementation of rules is as important, if not more, than the rules themselves. Training and information are helpful, but the core responsibility of a NFCS is that of control. To achieve this, clear rules should be established and should be backed by clear sanctions. At the same time, the institutional framework should be enhanced to take into account informal sector, without that it is difficult to establish correctly the target of control. Of course, this approach should not reduce the importance of cooperative solutions between the government and food chain's stakeholders, including informal ones who are numerous in developing countries. However, enforcement is the backbone of an effective NFCS, and appears to be the best way to bolster the system's efficacy.

SWOT analysis

To summarize, a SWOT analysis is provided hereunder, which will enable synthetic reading of the paper (See Table 3).

Conclusion

The article performs in-depth analysis on successes and caveats of the NFCS of Vietnam, that aims to provide information for policy makers and stakeholders of the food chain who are working in or with Vietnam. The system has started to operate after the promulgation of the Food Safety Law in 2010. It has made good progress, thanks to application of modern principles and reinforced allocation of responsibility to local authorities. One of the successes is the use of local governments for strictly controlling food chain stakeholders, including those in the informal sector. The use of standards, traceability, education, and modernization of the food system is underway. These achievements are due to the willingness of the government to fight against unsafe food products in the domestic market and to develop the agricultural product and food export sector. But at early stage, the system is also facing serious structural problems. First is the lack of transparency in risk assessment, which was not considered during implementation of the policies. In addition, the absence of the status of stakeholders is worrying, because it directly affects the effectiveness of control in a market system. The informal sector is a matter of concern, considering their share in the Vietnamese economy. Finally, recent changes in the policy are in favor of private companies who have capacity of investment to comply with the rules, but the solution goes with the risk of loosing public control in the hand of the companies. Such threats require progressive but determined solutions from the government in the future.

Declaration of interest

I declare having no conflict interest with any stakholders who may concern in this scientific paper.

I understand that if I, my family members and close relatives and personal friends have any direct or indirect interest with any of them, I shall make a declaration to the Editorial Board of the Food control journal.

Table 3 SWOT analysis of the Vietnam NFCS.

Strengths

-Firm and complete regulatory framework for building good policy.

-Strong local institutions in big cities/province capable of implementing control at the smallest administrative level. -Establishing testing laboratories throughout the country -Societal awareness for facilitating regulation implementation -Food control of export products (rice, sea-food, high-value fruits) is correctly done by big companies.

Weaknesses -Lack of qualified agents for food control, mostly outside urban areas.

-Not completely harmonized with Codex Alimentarius -No control along the whole supply chain. Essentially downstream control when the food comes to the markets. -Risk analysis is theoretical. Risk assessment has not been implemented.

-Large number of informal operators participating in the food system.

Opportunities

-Young system with potential to learn and adapt.

-Clear policy focusing on food labeling, certification, and traceability.

-Possibility to use shared knowledge and experience at low cost. Example: Adapting to the Codex and international standards instead of investing on risk estimate.

Threats

-Ineffective "farm to fork approach", as control efforts are only made in big cities for urban consumers, while production is realized in rural areas. -Lack of transparency in risk analysis, especially in risk assessment, resulting in bad policy in terms of health protection. -Shift of control power from public authorities to private food operators who are favored by the current policy.

  ), Vietnam Government (2017), Ministry of Health (MOH) (2011), Institute of Policy and Strategy for Agriculture and Rural Development (IPSARD) (2016), and USDA (2013).

Fig. 1 .

 1 Fig. 1. Distribution of food management competences in Vietnam, cited from (Pham and Dao, 2016).

Fig.

  Fig. Organization of food control management in Vietnam.

Table 1

 1 List of acronyms.

	Acronyms	Full name
	ASEAN	Association of Southeast Asian Nations
	AEC	ASEAN Economics Committee
	BRC	Bristish Retailer Consortium
	DAH	Department of Animal Health
	EFSA	European Food Safety Authority
	FAO	Food and Agriculture Organization
	FSMA	Food Safety Management Authorities
	HACCP	Hazard Analysis and Critical Control Point
	IECT	Information, education, communication, and training
	ILO	International Labor Organization
	IPSARD	Institute of Policy and Strategy for Agriculture and Rural
		Development
	IFS	International Food Standard
	MARD	Ministry of Agriculture and Rural Development
	MOH	Ministry of Health
	MOIT	Ministry of Industry and Trade
	NAFIQAD	National Agro-Forestry-Fisheries Quality Assurance Department
	NIFC	National Institute of Food Control
	NFCS	National Food Control System
	PPD	Plant Protection Department
	STAMEQ	Vietnam Directorate for Standards, Metrology and Quality
	USDA	United State Department of Agriculture
	VDMS	Vietnam Directorate of Market Surveillance
	VFA	Vietnam Food Administration
	VFSL	Vietnam Food Safety Law
	VietGAP	Vietnam Good Agricultural Practices

A search on two scientific databases, Web of Sciences and Science Direct, shows that the scope of these writings is limited. Two keywords "food safety" AND "Vietnam" returned 141 results in the Web of Science, and 835 in Science Direct, including conference papers. Regarding abstracts, the inventory provided only 10 and 11 relevant publications in the two databases, respectively. The papers focused on safety controls in different sub-sectors of the food system, for example Fisheries and Aquaculture, Meat, and Fruits & Vegetables. They also discussed the attitude and behavior of the food chain's stakeholders toward public regulation and standard promotion. Other keywords such as "food control" and "food protection policy" also returned similar results, but with a more modest scope. The latter are mostly included in the results from the two terms, "food safety" AND "Vietnam" as mentioned above.

2 Promulgated on 28 June 2010, this law replaces the Ordinance on Food Safety of 2003, which constituted the precedent regulatory framework.


Yann Carpier

Benoît Vieille
email: benoit.vieille@insa-rouen.fr

Nicolas Delpouve

Eric Dargent

Isothermal and anisothermal decomposition of Carbon fibres polyphenylene sulfide composites for fire behavior analysis

Keywords: fire, thermal decomposition, thermoplastic, isothermal, anisothermal

scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In aeronautics fire is one of the most critical conditions in service for materials. When plane parts consist of Polymer Matrix Composite (PMC) materials, the criticality is even higher because most PMCs usually cannot bear significant mechanical loading at temperature higher than 200-250°C. High-performance thermoplastic-based (TP) composites (e.g. PEEK, PPS, PEI…) are promising materials for structural applications in aeronautics as they are resistant to impact. They have a good damage tolerance as well as they have a very good retention of their mechanical properties under fire conditions. Among the most common PMCs used in load-bearing aircraft structures Carbon/epoxy (thermosetting) composites are flammable and readily decompose when exposed to heat and fire. In the literature most of the studies dealing with the fire behavior of composite materials focus on thermosetting-based composites [START_REF] Mouritz | Fire properties of polymer composite materials. N° 143 Solid mechanics and its applications[END_REF] but very few on thermoplastic-based composites [START_REF] Browne | A model for the structural integrity of composite laminates in fire[END_REF] [START_REF] Gibson | High temperature and fire behaviour of continuous glass fibre/polypropylene laminates[END_REF]. To better understand the changes in the thermal and mechanical behaviors of polymer-based composites under fire exposure the first step usually is to investigate the influence of thermal heat fluxes, hence temperature, resulting from a fire. Such heat fluxes lead to significant temperature differences within composite structures. From thermal analyses it is therefore possible to study the decomposition mechanisms as well as their kinetics under different testing conditions (heating rate, isothermal and anisothermal, inert or oxidizing atmosphere). The decomposition mechanisms are closely associated with the chemical nature of the polymer [START_REF] Vieille | Correlation between post fire behavior and microstructure degradation of aeronautical polymer composites[END_REF].

Figure 1 presents the characteristic behaviour of a thermoplastic material subjected to a heat flux on one side. The heat conduction within the material causes physical and chemical transformations. Through the thickness there are different transformations such as thermal degradation and thermal decomposition in a cross section called the "mesophase". When the material undergoes a thermal decomposition there are changes in the chemical composition and the structure of the material depending on the atmosphere (inert or oxidizing) in which the decomposition occurs (see Fig. 1). Pyrolysis of polymer matrix produces a liquid called tar and a non-condensable gas. Char is produced by repyrolysing the tar [START_REF] Bai | High temperature performance of polymer composites[END_REF] and results from the thermal decomposition of organic resins and fibres. It is a highly porous material that can consist of crystalline (i.e. graphitic) and/or amorphous regions [START_REF] Vieille | Post fire behavior of carbon fibers Polyphenylene Sulfide-and epoxy-based laminates for aeronautical applications: A comparative study[END_REF]. Under oxidizing atmosphere decomposition gases can ignite and cause the material to ignite. All these chemical changes are accompanied by sudden changes in physico-chemical and mechanical properties [START_REF] Guillaume | Modélisation de la décomposition thermique des matériaux en cas d'incendie[END_REF].

In the mesophase gas bubbles form within a phase whose viscosity decreases with temperature. On the surface the bubbles burst and release combustible gases that may fuel the flame. If portions of material remain non-gasified, then they form viscous tars that accumulate on the surface. From a thermal point of view this thermal decomposition has many consequences. Indeed the chemical mechanisms of decomposition (pyrolysis, oxidation, inflammation) are accompanied by heat exchanges, which influence the energy balance: endothermic reaction for the first, exothermic for the other two. The resulting change in properties is also accompanied by a significant perturbation of heat exchanges within the material. The effective conductivity decreases through the thickness due to the formation of porosity but also on the exposed surface due to the change in material emissivity.

Thermal decomposition in PMCs

First of all polymers have diverse chemical compositions and structures, which involve various thermal decomposition mechanisms [START_REF] Drysdale | An introduction to fire dynamics[END_REF]: (i) depolymerization, (ii) random chain scission, (iii) chain stripping, (iv) cyclisation. Although the thermal decomposition of a polymer can theoretically be observed according to only one of these mechanisms, it is generally the combination of several of them as the temperature increases and results in the formation of many decomposition products. It can be considered complete when any solid residue is fully vaporized or gasified. Among the commonly used polymer materials for aeronautics applications epoxy resin is known for its poor fire resistance [START_REF] Mouritz | Fire properties of polymer composite materials. N° 143 Solid mechanics and its applications[END_REF], phenolic resins are also widely used as thermal protection whereas fire-improved epoxy resins are sometimes considered as ablative protection for internal thermal insulation [START_REF] Grange | Determination of thermophysical properties for carbon-reinforced polymer-based composites up to 1000°C[END_REF]. Compared to common thermosetting resins (phenolic and epoxy) used for high temperature conditions polyether-ether-ketone (PEEK) and PolyPhenylene Sulfide (PPS) thermoplastic resins known for their high-temperature stability proved to have higher thermal performance, particularly with a higher decomposition temperature [START_REF] Vieille | Post fire behavior of carbon fibers Polyphenylene Sulfide-and epoxy-based laminates for aeronautical applications: A comparative study[END_REF]. In TP polymers, melting is a general stage of char formation and their decomposition results in the production of char [START_REF] Grand | Fire retardancy of polymeric materials[END_REF]. In addition to temperature, the thermal degradation of a material is highly dependent on the atmosphere in which it occurs. The presence of dioxygen greatly modifies the reaction kinetics, which generally accelerate although the opposite effect is sometimes observed [START_REF] Ma | Studies on Thermogravimetric Properties of Polyphenylene Sulfide and Polyetherether Ketone Resins and Composites[END_REF]. but it can also induce the formation of new reaction products that would not be observed under an inert atmosphere [START_REF] Price | Polymer degradation and the matching of FR chemistry to degradation[END_REF]. Thermogravimetric analyses (TGA) conducted on PPS resin show that decomposition under inert atmosphere begins 20°C later than in air [START_REF] Ma | Studies on Thermogravimetric Properties of Polyphenylene Sulfide and Polyetherether Ketone Resins and Composites[END_REF] but results in a slightly greater mass loss. When its formation is completed the char begins to decompose at a slow rate (12% in 300°C), as shown by Yao et al. who observed slow rate decomposition in PEEK-based composites [START_REF] Yao | The thermal decomposition kinetics of poly(ether-etherketone) (PEEK) and its carbon fiber composite[END_REF]. Indeed pyrolysis (i.e. the first stage of decomposition) does lead to a greater mass loss under inert atmosphere, but this is only because it is completed. Conversely, due to oxidation, decomposition under air results in the total gasification of char at 750°C. Second of all the presence of fibres in an organic matrix greatly influences thermal decomposition, since fibres have a higher thermal resistance than polymers. For example glass fibres begin to soften between 700 and 1000°C [START_REF] Berthereau | Fibres de verre de renforcement[END_REF]. Carbon fibres have a particularly high thermal resistance with a sublimation temperature above 3000°C in an inert atmosphere. However they are very sensitive to oxidation with complete decomposition when the temperature rises to 950°C under air [START_REF] Feih | Tensile properties of carbon fibres and carbon fibre-polymer composites in fire[END_REF]. Under isothermal conditions the oxidation of carbon fibres starts at 500°C and results in their total decomposition in 3 hours at 650°C. A thermal degradation study was carried out by Tadini et al. [START_REF] Tadini | Thermal degradation analysis of innovative PEKK-based carbon composites for high-temperature aeronautical components[END_REF] on carbon fibres reinforced composites (PEKK-and phenolic-based). To evaluate the PEKK behavior and to understand the impact of composite nature in terms of structural strength under fire TGA tests were performed under different non-isothermal heating conditions (between 30 and 1000°C). Under inert atmosphere PEKK-based composites are characterized by one single global reaction between 500-700°C. Phenolic-based composites are characterized by two reactions and earlier pyrolysis (around 200°C). The better performance of PEKK composites is attributed to the ether and ketone bonds between the three aromatic groups of the monomer.

Thermal decomposition of PPS

The anisothermal thermal decomposition of PPS has been specifically addressed by different authors [START_REF] Ehlers | Thermal degradation of polymers with phenylene units in the chain. I. Polyphenylenes and poly(phenylene oxides)[END_REF][START_REF] Christopher | Thermal degradation of poly(phenylene sulfide) and perfluoropoly(phenylene sulfide)[END_REF][START_REF] Montaudo | Primary thermal degradation processes occurring in poly(phenylenesulfide) investigated by direct pyrolysis-mass spectrometry[END_REF][START_REF] Budgell | Thermal degradation of poly(phenylene sulfide) as monitored by pyrolysis-GC/MS[END_REF][START_REF] Perng | Thermal decomposition characteristics of poly(phenylene sulfide) by stepwise Py-GC/MS and TG/MS techniques[END_REF][START_REF] Peters | The thermal degradation of poly(phenylene sulphide) -Part 1[END_REF][START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF][START_REF] Hawkins | Chemistry of the Cure of Poly(p-phenylene sulfide)[END_REF][START_REF] Li | High-resolution thermogravimetry of polyphenylene sulfide film under four atmospheres[END_REF]. Depending on the heating rate, it starts between 450 and 500°C and initially leads to a mass loss of between 45 and 65% depending on the considered atmosphere. The chemical mechanisms of this decomposition were studied using different characterization techniques: Pyrolysis (Py) -Mass Spectrometry (MS) [START_REF] Montaudo | Primary thermal degradation processes occurring in poly(phenylenesulfide) investigated by direct pyrolysis-mass spectrometry[END_REF], Gas Chromatography (GC) [START_REF] Budgell | Thermal degradation of poly(phenylene sulfide) as monitored by pyrolysis-GC/MS[END_REF], Py-GC-MS and TGA-MS coupling [START_REF] Perng | Thermal decomposition characteristics of poly(phenylene sulfide) by stepwise Py-GC/MS and TG/MS techniques[END_REF], etc. Not all studies agree on the chronology of chemical mechanisms probably because of the different formulations of the PPS resin. Nevertheless the decomposition seems to be a competition between depolymerization and random chain scission resulting in the formation of benzenethiol and hydrogen sulfide H2S, carbon disulfide CS2 and benzene respectively [START_REF] Perng | Thermal decomposition characteristics of poly(phenylene sulfide) by stepwise Py-GC/MS and TG/MS techniques[END_REF][START_REF] Peters | The thermal degradation of poly(phenylene sulphide) -Part 1[END_REF]. Cyclisation reactions between different decomposition products then lead to the formation of char [START_REF] Montaudo | Primary thermal degradation processes occurring in poly(phenylenesulfide) investigated by direct pyrolysis-mass spectrometry[END_REF]. The latter, like what is observed during the decomposition of the PEEK, then undergoes a slow thermal decomposition, which results in a relatively small mass loss. This observation is only true for decompositions in an inert atmosphere. Under air, char undergoes oxidation leading to its complete volatilization before 800°C.

As shown by Patel et al. in PEEK-based composites, the first stage of pyrolysis is relatively unaffected by the atmosphere [START_REF] Patel | Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies[END_REF]. When it comes to thermal decomposition, T5% refers to the temperature at the onset of decomposition often defined as the temperature corresponding to a mass loss of 5%. Nevertheless in PPSbased composites, mechanisms of cross-linking under air, occurring at temperatures lower than T5%, can increase the thermal resistance of PPS and delay the onset of its decomposition [START_REF] Hawkins | Chemistry of the Cure of Poly(p-phenylene sulfide)[END_REF]. This phenomenon has been highlighted by Ma et al. [START_REF] Ma | Studies on Thermogravimetric Properties of Polyphenylene Sulfide and Polyetherether Ketone Resins and Composites[END_REF]. The study of isothermal decomposition between 410 and 470°C reveals that from 440°C, the mass loss after 2 hours is higher under N2 than under air (see Fig. 2b). This result explains why mass loss begins later in TGA (anisothermal conditions) under air (see Fig. 2a). Note that the inflection in the residual mass under air, which is a characteristic of a new decomposition mechanism (here oxidation), is observed for residual mass of PPS around 75%, far before the end of pyrolysis under inert atmosphere, suggesting an overlap of the two mechanisms.

Objectives of the study

The study of fire resistance aims to understand and characterize how and for how long the composite material can bear a mechanical loading when one face is subjected to heat fluxes. It results in temperature differences transversely on the sample exposed surface as well as through the materials thickness. The knowledge of the temperature distribution on both exposed and back surfaces is required to determine the temperature range within the materials with respect to its characteristic temperatures (glass transition, melting, pyrolysis onset). Before considering the combined influence of heat and mechanical loadings, it is also necessary to investigate the influence of temperature on the thermal decomposition of C/PPS composites and its constitutive materials depending on the testing conditions. Thus TGA tests were carried out for different heating rates, in both isothermal and anisothermal conditions, under inert or oxidizing atmospheres.

Material and methods

Materials and specimens

The studied composite materials are carbon fabric reinforced laminates consisting of a semi-crystalline high-performance PPS supplied by the Ticona company. The woven-ply prepreg, supplied by SOFICAR, consists of 5-harness satin weave carbon fiber fabrics (T300 3K 5HS) whose weight fraction is 58%. The prepreg plates are hot pressed according to a quasi-isotropic [(0,90)/(±45)/(0,90)/(±45)/(0,90)/(±45)/(0,90)] stacking sequence (see Fig. 3). The glass transition temperature of the material is Tg=98°C and its melting temperature Tm=280°C, as measured by DSC [START_REF] Blond | Correlation between physical properties, microstructure and thermo-mechanical behavior of PPS-based composites processed by stamping[END_REF]. The degree of crystallinity of PPS matrix is close to 30% and the porosity ratio is very small (less than 2%).

Experimental set-up

Radiative heat flux exposure and temperature measurements

To study fire resistance a cone calorimeter is usually used. A rectangular sample (100*150mm² and a 2.2mm thickness) is placed under a fume hood and subjected to different heat fluxes (30-40-50kW/m²) resulting from the radiation of a conical heating resistor. The temperature distribution within the material must be measured in order to understand thermally-induced phenomena such as the thermal decomposition and degradation of mechanical properties [START_REF] Carpier | Mechanical behavior of carbon fibers polyphenylene sulfide composites exposed to radiant heat flux and constant compressive force[END_REF]. Temperature measurements are conducted on the exposed and the back surfaces of the specimens. In the present study, two methods are used to measure the temperature: by infrared thermography (back surface) or by means of thermocouples (exposed surface).

The first method is the most accurate because it provides the temperature mapping over the entire surface.

The infrared thermography measurement was carried out using a Thermacam camera PM 575 manufactured by FLIR Systems. To estimate the temperature, the camera requires the knowledge of the material emissivity. The value was set at 0.90 according to the value frequently used in the literature for composite materials with carbon fibres [START_REF] Boulet | On the influence of the sample absorptivity when studying the thermal degradation of materials[END_REF]. However infrared thermography measurement is difficult to implement on the exposed surface because it requires the measurement of the temperature by observing the specimen through the cone calorimeter. Such measurement presents many sources of errors: hot atmosphere, pyrolysis gas, reflection of a part of the radiation on the surface of the sample. As a result type K thermocouples are used to measure the temperature distribution on the exposed surface.

Thermogravimetric Analysis (TGA)

To carry out these experiments a thermo-gravimetric analyzer TA Instruments Discovery is used. Thermal decomposition is characterized under three atmospheres: nitrogen, air and dioxygen. The gas flow rate is set at 20 mL.min -1 .The extent of decomposition is characterized by a degree of progress in the reaction defined by the following expression [START_REF] Flynn | The historical development of applied nonisothermal kinetics[END_REF]:

= (1)
With the initial mass of the sample, is the mass at a time t and is the mass after decomposition.

When the analysis is carried out in an inert atmosphere, corresponds to the degree of pyrolysis and its value is 1 when all the pyrolysis is completed. Under oxidizing atmosphere, it corresponds to the degree of decomposition and its value is 1 when the entire solid residue is completely gasified.

The study of the PPS thermal decomposition has been conducted using anisothermal heating from room temperature to 700°C. These analyses are extremely useful for determining the primary steps of the decomposition: T5% refers to pyrolysis onset, which is the temperature at which 5% of the polymer mass has decomposed and Tmax refers to the maximum peak of decomposition rate. Such analyses can also be considered for studying the thermal decomposition kinetics. However they are not suitable for clearly identifying the chemical mechanisms involved. Two types of analyses were conducted on samples whose initial mass is about 8mg:

• Anisothermal conditions at a heating rate of 10°C/min: linear evolution of temperature as a function of time. These experiences are supposed to be representative of the increase in material temperature during the transient phase of its thermal response.

• Isothermal conditions at a heating rate of 100°C/min: rapid temperature increase to a value of which is maintained for a period of 1h30 and 5h, depending on the test temperature. These experiments aim to characterize decomposition when the temperature of the material has reached its permanent state.

Modulated TGA

Derived from the modulated DSC analyses proposed by Flynn [START_REF] Flynn | The historical development of applied nonisothermal kinetics[END_REF] and Reading [START_REF] Reading | Modulated Temperature Differential Scanning Calorimetry: Theoretical and Practical Applications in Polymer Characterization[END_REF], the MT-TGA method consists in subjecting a sample to a sinusoidal temperature ramp. It depends on the evolution of the degree of conversion between the peak (maximum heating rate) and the trough (minimum velocity) of a cycle. The activation energy Ea can be calculated [START_REF] Blaine | Obtaining Kinetic Parameters by Modulated Thermogravimetry[END_REF] and related to the main decomposition mechanisms from a single experiment [START_REF] Cheng | A modulated-TGA approach to the kinetics of lignocellulosic biomass pyrolysis/combustion[END_REF][START_REF] Gracia-Fernández | Study of the degradation of a thermoset system using TGA and modulated TGA[END_REF]:

= ( ) (2)
Where T is the average value of oscillatory temperature; A is the amplitude of the temperature oscillation; and is the ratio for adjacent peaks and valleys, of the periodic rate of reaction. The modulation parameters were: amplitude = 5K, period = 200s and heating rate = 5K/min.

Results and discussion

Radiative heat flux exposure

On the exposed and the back surfaces the changes in the surface temperature depend on the applied heat flux. The thermal response starts with a transient phase during which the temperature increases rapidly. On both surfaces temperature eventually becomes constant when the balance is reached between heat sources (radiation from the heat source) and losses (radiation of the material, convection and thermal decomposition). The lower the heat flux is, the longer it takes to reach this steady state (see Fig. 5a).

Finally, by comparing the temperature profiles on both exposed and back surfaces of C/PPS laminates subjected to 50kW/m² heat flux, the through-the-thickness temperature difference can be estimated (see Fig. 4). In the steady state a significant difference (about 320°C) is observed through-the-thickness but also on the back surface (see Fig. 5b). The onset of decomposition temperature is reached on the exposed surface but not on the back surface. It is therefore reasonable to assume that the temperature profile through-the-thickness is nonlinear and that the laminates thickness increases as the material decomposes.

The knowledge of the temperature difference is of the utmost importance to determine: (i) the gradient in both thermal and mechanical properties, (ii) gradient in the chemical composition, (iii) thermally-induced damages.

To better understand the influence of a heat flux on the temperature difference on the outer surfaces and within the laminates it is necessary to investigate the influence of temperature on the thermal decomposition of C/PPS composites and its constitutive materials depending on the testing conditions (see Fig. 1). TGA provides information on the temperatures at which the matrix is pyrolyzed and possibly at which the char and then the fibres oxidize in the composite. This will be useful in the future to establish a temperature profile according to thickness. Thus, TGA experiments were carried out in both isothermal and anisothermal conditions, under inert or oxidizing atmospheres (see Fig. 1) to identify the characteristic temperatures on the outer surfaces as well as within the materials. 10

TGA under anisothermal conditions

Inert atmosphere

Under nitrogen the pyrolysis of the PPS starts at 503°C and ends around 640°C resulting in a mass loss of about 55% (Figure 6a). Over this temperature range the evolution of the decomposition rate or Mass Loss Rate (often abbreviated MLR) has a virtually symmetrical peak with a maximum close to 550°C (Figure 6b). The presence of a single peak may indicate that pyrolysis occurs according to a meta-mechanism, a combination of random chain scission and depolymerization failures (see Section 1.1). The product of this decomposition, representing 45% of the initial mass, is char which is resistant at high temperatures with a mass loss of only 11% at 900°C (5% of the initial polymer mass -see Fig. 6a).

In C/PPS, consisting of 43% by weight of PPS, pyrolysis only results in a mass loss of 26% with a maximum decomposition rate at 550°C. Indeed the thermal decomposition of carbon fibres under inert atmosphere does not occur at this temperature level [START_REF] Feih | Tensile properties of carbon fibres and carbon fibre-polymer composites in fire[END_REF]. It is thus possible to calculate the mass loss of the matrix in the composite material, as well as its decomposition rate, assuming that only it decomposed while the fibres remained intact. It is interesting to note that despite the high thermal resistance of carbon fibers the decomposition of the matrix starts 20°C earlier in the composite material (T5%= 483°C at 10°C/min) as fibre/matrix interfaces are preferential areas for initiating a degradation mechanism. It could also be caused by the higher thermal conductivity of carbon fibres compared to the PPS matrix, bringing heat in the sample at a faster rate. Decomposition is also slightly more extensive with a polymer residual mass after pyrolysis of 60% compared to only 55% in the plain PPS. The presence of fibres also results in a decomposition rate of the matrix slower than for the resin alone (Figure 7a). The residue, consisting of char and fibres, is also very different from the decomposition of the plain PPS. Fibres limiting the swelling associated with the formation of pyrolysis gas, the residue of C/PPS is a highly porous fibrous network but only slightly thicker than the composite at room temperature (see Fig. 8a) while the product of the decomposition of the PPS (see Fig. 8b) is in the form of a bubble resulting from the "swelling" of the material under high internal pressure. Although fibres do not decompose they have a significant influence on matrix decomposition.

Another illustration of this phenomenon concerns the evolution of the activation energy of decomposition mechanisms as a function of the pyrolysis degree [START_REF] Cheng | A modulated-TGA approach to the kinetics of lignocellulosic biomass pyrolysis/combustion[END_REF][START_REF] Gracia-Fernández | Study of the degradation of a thermoset system using TGA and modulated TGA[END_REF], studied here by modulated TGA (Figure 7b).

Comparison of curves = (!) shows that the activation energy required to initiate decomposition (< 0.1) is much lower in the case of C/PPS, which could explain why it takes place at a lower temperature.

During most of the decomposition, is higher for PPS alone resulting in an ever higher decomposition rate (Figure 7a). Finally, we can observe that the last peak of activation energy, associated with char formation, starts later in C/PPS (! =0.84 compared to 0.76 for matrix). This may explain the lower residual polymer mass in the case of PPS only. The curves representing the activation energy is divided into three parts supposedly associated with the mechanisms of PPS pyrolysis: depolymerization, random chain scission and ultimately cyclisation through char formation.

Oxidizing atmosphere (Dioxygen and air)

As indicated in section 1.1 the decomposition of carbon fibre-reinforced composites under oxidizing atmosphere is faster than the one of other fibers-reinforced composites such as glass fibres for example, because carbon fibres are sensitive to oxidation phenomena just like polymeric matrices. For practical reasons decomposition under air has been little studied. The oxidation phenomena are here mainly characterized with TGA analyses under dioxygen.

Under dioxygen, the thermal decomposition of PPS begins at 502°C, approximately the same temperature as under an inert atmosphere (see Table 1). The curve representing the Mass Loss Rate of plain PPS resin as a function of temperature MLR = f(T) is characterized by 3 peaks (see Figure 9). The first peak corresponds to a mass loss of about 15%. Thus it may be associated with the first pyrolysis mechanism, depolymerization (Figure 10b). Following this reasoning, the second peak, which extends to ∆# = $%%, may be associated with the second pyrolysis mechanism. Even before the pyrolysis ends a new mechanism starts and may be associated with the simultaneous oxidation of PPS matrix and the pyrolysis residue as it extends until the entire initial material is carbonated (Figure 10a). The maximum oxidation rate is virtually 3 times higher than the pyrolysis rate.

In C/PPS thermal decomposition begins at 515°C, i.e. 10°C later than under N2 (see Table 1). Similar observations have already been made by Ma et al. in the study of the decomposition of C/PPS under air and can be explained by the behaviour thermoplastic-thermosetting PPS [START_REF] Ma | Studies on Thermogravimetric Properties of Polyphenylene Sulfide and Polyetherether Ketone Resins and Composites[END_REF]. Indeed when heated in an oxidizing atmosphere above its melting temperature PPS can crosslink like a thermosetting polymer thus having a higher thermal stability. If the delay in the onset of decomposition is only observed in C/PPS composite, this suggests that these phenomena are promoted by the presence of fibres. As for the PPS matrix alone pyrolysis and oxidation of char overlap in the C/PPS. Nevertheless the fact that the second peak of MLR ends with a mass loss of 43% equivalent to the matrix mass in the composite, it tends to indicate that oxidation of char and fibres occurs separately and not simultaneously (Figure 10a). Analysis of the evolution of the activation energy Ea indicates a rather good correlation with decomposition rate except when the material is strongly degraded as the MLR decreases with Ea. It is also possible to infer that the oxidation mechanisms are broken down into several sub-mechanisms represented by different peaks on the Ea vs polymer mass loss (Figure 10b). Finally, pyrolysis under oxidizing atmospheres requires higher Ea than under nitrogen, which might be consistent with the cross-linking hypothesis (Figure 11).

The decomposition of PPS begins at 508°C at 20°C/min under air, which is the only experimental condition tested under this atmosphere. For the same heating rate, decomposition begins at 523°C and 530°C respectively under N2 and O2 (see Table 2). Pyrolysis results in a mass loss of 39%, which is lower than that observed under inert atmosphere (Figure 12a), which is consistent with the observations made in the case of a PEEK resin by Patel et al [23][34]. As under dioxygen, oxidation begins before the pyrolysis ends. Nevertheless the appearance of the characteristic peak of pyrolysis on the MLR curve MLR = f(T)

indicates that pyrolysis is almost completed when oxidation begins under air (Figure 12b) while it did not even reach its maximum rate when this transition occurs under O2. In C/PPS, decomposition begins at 517°C compared to 525 and 526°C under N2 and O2 (see Table 2).Thus the mechanisms of crosslinking and improvement of thermal resistance observed under O2 do not appear to occur under air. During isothermal tests under air, the latter have not been observed in C/PPS by Ma et al. However the decomposition temperatures cannot be readily discussed from one atmosphere to another as the operating conditions differ in term of heating rate and testing machine. Oxidation appears to start faster under O2 simply because there are more reactive species and the atmosphere is more oxidizing.

TGA under isothermal conditions

Inert atmosphere

The isothermal decomposition of C/PPS under N2 was studied for a temperature range between 465 and 545°C. Lower temperatures than in an oxidizing atmosphere were chosen. Indeed the inert atmosphere is representative of what the specimen undergoes at core whose temperature is lower than that of the exposed side subjected to an oxidizing atmosphere (see Fig. 1b). In addition the temperature profiles measured in section 3.1 are considered here to justify the choice of isothermal temperature conditions. As expected the mass losses are at least equal to or greater than those observed during anisothermal decomposition (Figure 13) for all test temperatures except one (530°C). At 465°C for example, while only 2% of the mass of polymer in C/PPS are gasified under anisothermal conditions, mass loss reaches 12% (28% of the matrix) after 30 minutes of exposure and 29% (69% of the matrix) at the end of the test. The higher the test temperature, the higher the mass loss. Indeed whereas for high temperatures (530 and 545°C), the mass loss seems to reach a plateau, which is not the case for lower temperatures (at least over the temperature range studied). However the isothermal conditions are not really isothermal for the highest temperatures at which the decomposition has already clearly started at lower T5% decomposition temperatures (whose accurate estimate is difficult), which calls into question the possibility of direct comparison.

Oxidizing atmosphere (Dioxygen and air)

Under oxidizing atmosphere isothermal decomposition was studied for four temperatures: 520, 550, 580 and 610°C. The second and fourth were chosen because these are the maximum temperatures measured on the front panel for specimens of C/PPS exposed to flux densities of 40 and 50 kW/m2 (see Section 3.1).

One of the difficulties of the analysis is that the material can be quite significantly decomposed during the temperature rise up to the test temperature. Nevertheless this temperature ramp is partially representative of what the material undergoes during fire exposure and thus contributes to the understanding of fire behaviour. Fire exposure combines both radiative and convective fluxes leading to heat fluxes higher than 100kW/m², whereas the fluxes resulting from cone calorimeters are usually lower than 80kW/m². The oxidizing atmosphere in the vicinity of the surface exposed to a cone calorimeter allows the combustion of pyrolysis gases. On the contrary, there is very little dioxygen in the vicinity of the sample surface exposed to a flame. Under real fire conditions, it is therefore expected that the thermal decomposition of C/PPS laminates would differ from the one observed under a radiative heat flux.

As under inert atmosphere, the same decomposition mechanisms as under anisothermal conditions may be at stake. However the times required for decomposition are shorter than under N2 (Figure 14). The distribution of decomposition rate peaks and therefore the decomposition mechanisms change with temperature. For example, under the air, the lower the temperature, the more pyrolysis is characterized by a low mass loss (Figure 15). The air crosslinking mechanisms highlighted by Ma et al. could partly explain this phenomenon [START_REF] Ma | Studies on Thermogravimetric Properties of Polyphenylene Sulfide and Polyetherether Ketone Resins and Composites[END_REF]. However this is no longer true when pyrolysis starts. For lower temperatures, as the PPS matrix decomposition is slow, a partial crosslinking of the matrix occurring at the same time could result in less extensive pyrolysis and therefore explain an earlier oxidation. Decomposition rates differ under the two atmospheres. Under air, they are approximately 4 times lower than those observed under O2, which can be related to the percentage of dioxygen in the air. The fastest mechanisms are also different under both atmospheres. Under air, pyrolysis occurs more rapidly while under dioxygen it is the oxidation of char that occurs faster.

Conclusion

The investigations on the relationship between local temperature resulting from the local balance energy exchange, the local atmosphere and the mass losses are essential in understanding the influence of fire on polymers and polymer matrix composites. The main objective of this work was to better understand the thermal decomposition mechanisms in both PPS plain resin and PPS thermoplastic-based composites subjected to radiative heat fluxes representative of fire exposure.

The first step consisted in investigating the influence of thermal heat fluxes (hence temperature) resulting from fire of medium intensity. Such heat fluxes (typically 50kW/m²) lead to significant thermal gradients within composite structures, leading to (i) a gradient in both thermal and mechanical properties, (ii) a gradient of chemical composition and (iii) thermally induced damages.

By comparing the temperature profiles on both exposed and back surfaces of C/PPS laminates subjected to a 50kW/m² heat flux, the through-the-thickness temperature gradient has been shown to be significant (about 320°C in the steady state). For such temperature level, the onset of decomposition temperature is reached on the exposed surface but not on the back surface, leading to a significant gradient of the chemical composition through the thickness.

In order to predict the evolution of properties for other heat fluxes, decomposition mechanisms as well as their kinetics and their activation energy under different heating conditions (isothermal and anisothermal), or testing atmosphere (inert and oxidizing), have been studied by means of TGA and MT-TGA tests. If the influence of the atmosphere has shown similar behaviors to what is conventionally observed on other materials and corroborated by other studies on the PPS [START_REF] Perng | Thermal decomposition characteristics of poly(phenylene sulfide) by stepwise Py-GC/MS and TG/MS techniques[END_REF][31], questions were raised about the influence of the heating rate in anisothermal conditions. Even if the heating rates tested in this study (10 and 20°C/min) are low compared to those observed in a real fire (several hundred degrees per minute), differences were observed in the increase of Td,onset when the heating rate increases, whether the testing atmosphere is oxidizing or not.

Considering the influence of heating conditions in isothermal conditions, an increase in the testing temperature leads to increasing decomposition rates, as expected. Oxidation starts before the end of pyrolysis under anisothermal conditions, whereas pyrolysis is complete before oxidation under isothermal conditions. As far the influence of fibers on the thermal decomposition mechanisms in PPS-based composites is concerned, the cross-linking may be promoted by the presence of fibers resulting in delaying the onset of pyrolysis under oxidizing atmosphere. Such cross-linking is not significant enough to observe such a delay under air. In agreement with conclusions drawn in the literature on PPS and PPS-based composites, MT-TGA tests suggest the existence of three different mechanisms during pyrolysis under inert atmosphere: depolymerization, random chain scission and ultimately cyclisation (char formation).

List of figure

Fig. 1 -Fig. 2 -Fig. 3 -Fig. 4 -Fig. 5 -Fig. 6 -Fig. 7 -Fig. 8 -Fig. 9 -Fig. 11 -Fig. 12 - 21 Fig. 13 -Fig. 14 -Fig. 15 -

 123456789111221131415 Fig. 1 -Physico-chemical processes and phase distribution during the thermal decomposition of: (a) thermoplastic matrix (adapted from [7]) -(b) TP-based composites Fig. 2 -(a) Anisothermal thermal decomposition of PPS and C/PPS (b) Isothermal thermal decomposition of PPS (according to [11]) Fig. 3 -Through-the-thickness microscopic observation of the mesotructure in quasi-isotropic C/PPS laminates Fig. 4 -Temperature changes in C/PPS samples subjected to a radiant heat flux (50kW/m²) Fig. 5 -Temperature measurements on the back surface by means of infrared thermography: (a) Influence of heat flux (at specimen center) -(b) Temperature distribution at 50kW/m² (after t=180s) Fig. 6 -(a) TGA curves of PPS and C/PPS under N2 (b) Evolution of the decomposition rate as a function of temperature Fig. 7 -Comparison of the decomposition of pure PPS or PPS associated with carbon fibres: (a) Decomposition rate as a function of pyrolysis degree ( -(b) Activation energy of the pyrolysis reaction as a function of the degree of pyrolysis Fig. 8 -Microscopic observations of TGA specimens after thermal decomposition under N2: (a) C/PPS -(b) PPS Fig. 9 -Evolution of residual mass and decomposition rate (MLR) of PPS and C/PPS under O2 as a function of temperature Fig. 10 -(a) Evolution of the decomposition rate under O2 as a function of the mass loss (b) Evolution of activation energy under O2 as a function of polymer mass loss Fig. 11 -Evolution of C/PPS activation energy under O2 and N2 as a function of mass loss Fig. 12 -Decomposition of plain PPS and C/PPS composites at a heating rate 20°C/min: (a) Residual mass and decomposition rate (MLR) under air in as a function of temperature (b) Decomposition rate as a function of degree of decomposition