Fernando Machado De Mello

Essaid Bilal
email: bilal@emse.fr

Gustavo Neves
email: gustavonevescm@gmail.com

Maria Eduarda Loureiro
email: me.loureiro@yahoo.com.br

Dos Reis

Thiago Peixoto De Araujo
email: thiagopeixotoaraujo@gmail.com

Resumo

Compared Background and Reference Values in Sources of Cadmiumenriched Soils from Brazil

Keywords: Soil, Cadmium, Contamination, Legislation, Reference Values

The complexity of the assessment process of variations in contamination levels of soils is in part due to distinct sources, natural or by some anthropic activities. Compare these different substances sources, backgrounds and guiding values is very important to introduce directions and contributions for regulations or environmental protection legislation in general, for heavy metals contents, among other chemical components, and are pointing to more restrictive use of phosphate fertilisers, rich in those pollutant elements, in many countries around the world. Recognize and characterize contaminated soils is a serious concern with issues that whole society have to deal with today and one of the largest and most neglected problems in Brazil. This works intends to obtain an insight in the natural sources and loads of heavy metals, especially cadmium to soils in Brazil, mainly with the application of huge volume of mineral fertilisers, directly linked with their dangerous concentrations. This is a threat to public health in many regions, and it is one of the priorities to protect it from all sources of anomalous concentration in the ground. Decreasing soil quality by means of contamination and pollution by heavy metals is now recognized also as a serious risk for environmental quality and to the health of people.

INTRODUCTION

Soils enriched in Heavy Metals Elements (HME) became a major worldwide environmental issue, not only by the well know toxicology risk linked with consumption of contaminated tobacco and food, but by the long-term effects related to its slow and continuous concentration in soils and water. This concentration is driven through their strong geochemical and physical affinity for some special soils and geologic terrains, specially a long residence time in surface.

The cadmium contamination is largely due to agricultural practices, mining and metallurgy. The current paper has the purpose to analyse and compare how this major issue is receiving attention in Brazil and in some developed countries, analyze the guiding values and behaviour of heavy metals in the soil, with emphasis in Cd from the fertilisers and mining industry; to compare protocols of sampling, methods of detection and regulations around the world; and to compare the minimum concentrations to establish levels of intervention and assess the risk. The distribution of most elements in soil shows a pattern related to geology and/or mineralisation. Past climates and the prevailing particular tropical conditions in Brazil caused strong argillic and ferrallitic weathering of some rocks, with a change in mineralogy, and a modification of vertical and lateral distribution patterns of most major elements, but the main source of the anomalous enrichment in arable soils remains the HME generated through time by agricultural activities on the environment, particularly in Latosols over volcano-sedimentary basins of Brazil. The current legal brazilian norm, stablished by the National Environmental Council [START_REF]Resolução nº 420, de 28 de dez. de 2009. Dispõe sobre critérios e valores de qualidade do solo quanto à presença de substânciasquímicas e estabelece diretrizes para o gerenciamento ambiental de áreas contaminadas por essas substâncias em decorrência de atividades antrópicas[END_REF] through resolution 420/2009proposed different values of reference for inorganic substances.

Quality control and risk assessment associated to the soil contamination requires the knowledge of the total content in those elements and the influence of source rock contents in HME in the various constituent compartments of the ground. In the present work, HME are considered as the sense of [START_REF] Alloway | Cadmium In Heavy Metals in Soils[END_REF], the metals/metalloids which behaves geochemically as Siderophiles (Co, Ni, Au, Mo, Pb, As), Calchophiles (Cu, Ag, Au, Zn, Cd, Hg, Pb, As, Sb, Se, Tl, Mo) and the Lithophiles (V, Cr, Mn, U, Ti).

In phosphate fertilisers some HME, like Cd contents, are expressed in mg Kg-1 of P or P 2 O 5 . 1 kg of P is equivalent to 2.29 kg of P 2 O 5 . Comparing different types of units is sometimes difficult and may require conversions between P and P 2 O 5 or back calculating mg Cd Kg-1 P 2 O 5 based on an assumed fertiliser application rate. Phosphorus Pentoxide (P 2 O 5 ) has 44% P. To obtain the value expressed in Cd Kg-1 P one must divide it by 2.29 to express in the Cd Kg-1 P 2 O 5 form.

Soil contamination by HME became a problem in large agricultural areas, especially in extensive plantations, with the continuous use of phosphate based fertilisers. The option to protect this natural resource is by using the legislation to establish, in a clear manner the rules, regulations and norms. This is the base to support and increase the controls and monitoring of those elements. In the last years, biomedical research has shown a strong correlation caused by cadmium in diseases of humans. A recent line of research has concluded that cadmium is a major human toxicant; there is a need to limit exposure from as many sources as possible. The World Health Organization -WHO (2010) presented a list containing ten substances that threatens the public health and cadmium was the fifth one.

STANDARTS CADMIUM CONTENTS IN ROCKS, SOILS AND FERTILISERS

Cadmium has low crustal abundance, in the region of 0.1 mg kg-1 [START_REF] Alloway | Cadmium In Heavy Metals in Soils[END_REF] and it is found rarely in its elemental form, in greenockite (CdS), octavite (CdSe) and monteponite (CdO) (Kabata-Pendias and Mukherjee, 2007). Cadmium is often found in association with zinc-bearing and zinc-bearing lead ores, which are the main source of cadmium production [START_REF] Greenwood | Zinc, cadmium and mercury[END_REF]. The high cadmium contents are also found in black shales and in phosphate rocks, varying considerably from one source to another, in general with two extremes, igneous rocks having the lowest contents compared to sedimentary [START_REF] Mclennan | Earth's continental crust[END_REF][START_REF] Mclennan | Geochemistry of sediments[END_REF].

The phosphate rocks which are mined in Brazil, Russia, South Africa and Finland are igneous rocks and have very low cadmium contents (sometimes below 10 mg kg-1 P 2 O 5 ). In the other hand, those found in North and West Africa and in the Middle East are sedimentary rocks formed in environments rich in organic matter, and generally have much higher cadmium levels. In Tunisia, Togo, Senegal, they reaching frequently values of 60 mg kg-1 P 2 O 5 while in Morocco deposits, the most important supplier of Brazil and European Union (EU), deposits leads cadmium contents in fertilisers above 60 mg kg-1 P 2 O 5 . Most natural soils contain less than 1 mg kg-1 of cadmium resulted from the weathering of parent materials [START_REF] Alloway | Cadmium In Heavy Metals in Soils[END_REF]. Generally lower levels are found in acid igneous rocks (mean granite 0.09 mg kg-1) than basic (mean basalt 0.13 mg kg-1). In seds, sandstone and limestone shows lower levels, higher contents are found in black shales (0.3 to 219 mg kg-1), organic-rich seds or marine manganese nodules and phosphorite (Fergusson,1990). Elevated Cd values are generally linked with Pb and Zn of sulphide mineralisation from Sedex or VHMS mineralisation. The average Cd contents in soil surface is estimated to be 0.53 mg kg-1, with all higher values reflecting anthropogenic influences [START_REF] Kabata-Pendias | Trace Elements in Soils and Plants, Third Edition[END_REF].

Although uneven distributed by different regions of Brazil, the dominant soil type is the Latosol, with 56,30% in total area of the country, followed by the Argisol (20,68%) and Neosol (9,38%) [START_REF] Coelho | Recurso natural solo[END_REF]. That distribution is mostly due to the geologic ground and by extreme climatic differences in the country, inducing complexes and unique pedogenic processes. Proposed HME baseline values of natural concentrations in brazilian soils by Amaral Sobrinho (1993) suggests that different values of HME could be found in the same class and level of soil or between different classes as a function of the variation of soil characteristics. Estimation of natural contents of HME in some soil types of Brazil was performed by [START_REF] Fadigas | Proposição de valores de referência para a concentração natural de de metais pesados em de solos brasileiros[END_REF], dividing the soils in seven groups (see TABLE 1), the first one (1) distinguished by high contents in Mn, Fe and clay is composed by Red Distrophyc Latosol, Brun Latosol and Red Argisol mainly formed in terrains of basaltic compositions, some of them over the huge Paraná Sedimentary Basin. This is the group that naturally concentrates the highest values of HME. The second group (2) includes those with high levels of silt, Mn and high CEC (Cation Exchange Capacity), including Cherno sols, Luvisols, eutrophic soils, and some samples of Yellow Latosol, Red-Yellow Latosol and Red Argisol. The sixth and seventh groups (7 and 8) shows the lowest levels in HME and share the same composition but are differentiated by clay and Fe contents, includes Yellow Dystrophic Latosols and Argisols, and in minor quantity by soils derived by Tertiary and Quaternary sediments. Third, fourth and fifth (3, 4 e 5) groups share intermediate characteristics of those cited above and are mainly constituted by a great variety of of Latosols and Argisols, and, with minor importance by Plinthosol, Cambisol, Nitosol with dystrophic character. Average values showed in TABLE 1 are near the quality reference values [START_REF] Casarini | Proposta de valores de referência de qualidade e intervenção para solos e águas subterrâneas no estado de São Paulo[END_REF] for the State of Sao Paulo, Brazil, in which the concentrations, in mg kg-1 are: Cd (0.5), Co (12.5), Cu (35.1), Cr (40.2), Ni (13.2), Pb (17), Zn (59.9). The adsorption/desorption of Cd and Zn presents a great sensibility to pH, compared with Cu and Pb. This chemical behaviour could contribute to explain the accumulation Zn and Cd in soil surficial layers in locals with higher pH (Alloway, 1990), i.e. in topsoils, the upper, outermost layer of soil, usually the top 5 cm to 20 cm depth layer. It has the highest concentration of SOM (Soil Organic Matter) and microorganisms and is where most of the biological soil activity occurs, where plants nourishment by roots concentrates.

Phosphate fertilisers contain between 5 and 100 mg kg-1 Cd and up to 300 mg kg-1 Cd may be present in sewage sludge. Cd is a trace element in fertilisers, which have been applied extensively to arable and pasture land around the world. In United Kingdom (UK) the ECB (2007) reported that current fertilisers contain around 79 mg kg-1 Cd of P. Based on the use of fertilisers in the 1980s and early 1990s, [START_REF] Alloway | Cadmium In Heavy Metals in Soils[END_REF] estimated that around 4.3 g of cadmium per hectare per year has been added to agricultural soils in the UK. Across the European Union, 231 tonnes of cadmium are added to agricultural soils each year from fertiliser use (EC, 2007). The average cadmium levels in fertiliser was estimated in 2000, showing that levels in cultivated soil have a trend, lower in the countries of northern Europe (about 2.5 mg Cd Kg-1 P in fertiliser and 0.21 mg Cd Kg-1 in soil) and much higher in other parts of Europe (about 138 mg Cd Kg-1 P in fertiliser and 0.5 mg kg-1 soil). It was concluded that this average cadmium content of European fertilisers, 138 mg Cd Kg-1 P (or 60 mg Cd Kg-1 P 2 O 5 ), would lead to a radical increase in the concentrations of cadmium in soil and crops, and in cadmium leaching.

Only 10% of the applied P as a fertiliser is taken by the plants, differing in this aspect by the higher taken of N and from K. This difference is also attributed by the higher P fixation in tropical soils, with high Fe-Al oxides [START_REF] Raij | Fósforo no solo e interação com outros elementos[END_REF]. The Brazilian consumption of P 2 O 5 was in 2002 about 2.777.000 t [START_REF] Lopes | Reservas de fosfatados e produção de fertilizantes fosfatados no Brasil e no mundo[END_REF], 43% of monoammonium phosphate (MAP), 30% in the form of simple superphosphate (SSP), 15% triple superphosphate (TSP) and 12% of other sources of the market. [START_REF] Langenbach | Teor de cádmio nos fertilizantes fosfatados brasileiros[END_REF] compared the Cd concentration in eleven brazilian phosphates and observed that they Cd contents are lesser than 2.0 mg kg-1 Cd. The brazilians phosphate rock from Catalão-GO presents 4 mg kg-1 Cd, 19 mg kg-1 Cr e 58 mg kg-1 Pb, and the fine apatitic concentrate from Araxá-MG presents 7 mg kg-1 Cd, 44 mg kg-1 Cr e 127 mg kg-1 Pb (Gabe and Rodella,1999, in [START_REF] Alcarde | Qualidade e legislação de fertilizantes e corretivos[END_REF]. Amaral [START_REF] Amaral Sobrinho | Metais pesados em alguns fertilizantes e corretivos[END_REF] presented typical ranges of HME in phosphate fertilisers as 0.1-170 mg kg-1 Cd, 7-225 mg kg-1 Pb, 7-38 mg kg-1 Ni, 1-300 mg kg-1 Cu and 50-1450 mg kg-1 Zn.

HEALTH PROBLEMS CAUSED BY CADMIUM

The ground contamination by heavy metals elements is a major environmental problem for two main reasons. Beside the well know ecotoxicology risk in food, this contamination can have very long-term effects because its continuous concentration and strong chemical and physical affinity for the solid matrix of distinctive geodiversity terrains and a long residence time in soils (Echevarria and More, 2006).

The kidney is the most sensitive organ to cadmium excess and the toxicity is driven by the diet, but the cadmium in food is only the second factor, smoking is the dangerous contribution to cadmium human body burden. Cadmium is also retained in liver and, once absorbed, is not easily excreted, its biological half life ranging between 10 to 30 years. Cd food intakes are in decreasing order of importance: cereals and cereal products; vegetables; meat products, offal (inner organs), fish and seafood. The highest inputs of phosphate fertilisers are in vegetables and wheat. Cd causes the augmentation of glucose excretion and amino acids, litiasis on kidney and urine calcium, decalcification of bones promoting fractures, enfisema of lungs and fibrosis peribronchial and perivascular [START_REF] Johri | Heavy metal poisoning: the effects of cadmium on the kidney[END_REF] The EU community promote long lived studies concerning cadmium exposure and have recently implemented fertiliser limits envisaging food safety standards that are more stringent than USA standards. EU concern about cadmium in food, since 1980, was triggered by a rising body burden in some segments of the population that approached the level of onset of adverse health effects. While the overall average cadmium intake from food alone does not pose a risk to most people, certain populations are considered at higher risk. As examples, vegetarians could consume more of the metal due to their diet, women may increase their absorption due to low levels of Fe, and tobacco consumers are exposed to an additional source of Cd.

ANALYTICAL METHODS COMPARED

Comparison of three extraction procedures was performed by Campos et al. (2005) to assess HME in mineral fertilizers (Embrapa, 1999; USEPA 3051A and USEPA 3050B) for Cd, Cr, Cu, Ni, Pb, and Zn from Brazil and the main fertilizers commercialized there. The methods described do not fully solubilize the solid fraction of phosphates and are more indicated than total extractions for soil pollution studies, due to representation of maximum potential bioavailability of a certain pollutant (FIGURE 1).

The USEPA 3050B method, from the Environmental Protection Agency, EUA (Usepa, 1998a), takes 1 g of the material, while USEPA 3051A (Usepa, 1998b) less material, 0.5 g to 1.0 g. The Embrapa Method (Embrapa, 1999) uses approximately 0.5 g of the material added to 20 mL of HCl 2 mol L-1. The quantification of the trace elements content was performed (Campos et al., 2005) by air acetylene flame atomic absorption spectroscopy. Among the studied phosphates, the thermophosphate presented significantly greater concentrations of Cd, Cr, Cu, Ni, and Zn whereas Cu, Ni, Pb, and Zn were found in greater contents in the natural phosphate 2. The reactive phosphate 3 presented the greatest quantity of Cd (145±13 mg kg-1) and the natural phosphate 2, the highest quantity of Pb (234±9 mg kg-1).

The tested methods can be applied in studies concerning heavy metals in samples of brazilian natural phosphate fertilisers phosphates (FN1 e FN2) showed an average of 8.7 mg kg-1 of Cd content, non-brazilian ones presented higher values, in average, 77 mg kg-1 Cd (Campos et al., 2005). Phosphate rocks presented contents between 5±0,6 e 145±13 mg kg-1 (Figure 1). Those values are in the range cited by Kabata-Pendias & Pendias ( 2001) phosphate fertilisers, 0,1-170 mg kg-1 of Cd. Amaral [START_REF] Amaral Sobrinho | Metais pesados em alguns fertilizantes e corretivos[END_REF] encountered lower contents for apatite rocks (2-7 mg kg-1 Cd). As for Cu content, there was no difference between the extraction methods (TABLE 3). For Ni and Zn contents, there was no statistical difference between USEPA 3051A and USEPA 3050B, but they were superior than the Embrapa method (1999). The USEPA 3050B extracted more Cd than other methods, while for Cr contents, Embrapa method extracted the most.

GUIDING VALUES IN BRAZIL AND EUROPE

The Brazilian Environmental Council (CONAMA), through Resolution n.420/ 2009, established that each state in the country must determine quality reference guiding values for heavy metal concentrations based on a set of soil samples that represent the local geomorphology, lithology and pedogenic processes. This was decided because the international values or those from other regions might result in erroneous interpretation regarding areas suspected of contamination. The brazilian resolution establishes three types of guiding values: Quality Reference Values (QRVs), which should be determined by each state, Prevention Values (PVs) and Investigation Values (IVs), that are established and is valid for the whole country. In São Paulo state the limits were determined by CETESB mainly in topsoil and are, in mg kg-1 (As -15), (Cd -1.3), (total Cr -75), (Cu -60), (Hg -0.5), (Ni -30), (Pb -72) and (Zn -1,900) to class it at risk.

Cadmium in fertilizer phosphate rock, animal manures and land applied municipal sewage sludge is increasingly regulated in different parts of the world. Initially, cadmium was regulated primarily because of concerns about the metal leaching from fertilized soil into ground and surface waters. However, as knowledge increased about cadmium as a major human toxicant in our food, the emphasis on allowable cadmium in fertilizer has shifted to the amount of uptake seen in agricultural commodities grown on soils that include added cadmium in fertilizer. The limits on cadmium in fertilizer have taken several forms, most commonly: (i) a limit on the amount of cadmium allowed per unit of phosphorous (mg Cd Kg-1 P) and (ii) a limit on the amount of cadmium allowed per unit of phosphorous oxide (mg Cd Kg-1 P 2 O 5 ). This limit is used because American Plant Food Control officials adopted P 2 O 5 as the standard for guaranteed analysis of phosphorous content based on an ancient measurement procedure. The method was replaced, but the form designation is in use today in fertiliser packs. This measure may be ambiguous because the actual phosphorous applied to soil is about 44 % of the P 2 O 5 weight percentage designated on pack.

In The Netherlands the Soil Quality Regulation ( 2006) and the Soil Remediation Circular (2009) focuses on the elaboration of the remediation criterion used to determine whether urgent remediation is necessary. To be compared worldwide, all values presented in TABLE 2 are taken only from soils with 10% of organic matter and 25% of Clay. This restriction is due to strong correlation between HME, clay and SOM contents. Higher the SOM content, higher levels of HME. Sandy soils are poor in HME than argillic ones and the SOM. The highest allowed levels in EU community are 3 mg Cd kg-1, 150 mg Zn kg-1;140 mg Cu kg-1 and 50 mg Pb kg-1 [START_REF] Chaudri | Enumeration of indigenous Rhizobium leguminosarum biovar trifolii in soil previously treated with metal-contaminated sewage sludge[END_REF]. In Poland, those limits are, for arable soil 3 mg Cd kg-1, 300 mg Zn kg-1 and 200 mg Pb kg-1 [START_REF] Chlopecka | Forms of Cadmium, Lead, and Zinc in contaminated Soils from Southwest Poland[END_REF]. According to EU recommendations, soil treated with sewage sludge should not contain more than 3 mg kg-1 Cd (86/278/EC/12-6-1986). Soil Guideline Values (SGVs) for cadmium in the Netherlands, are presented according to land use in TABLE 2. The SGVs apply only to cadmium and its inorganic compounds. For residential and allotment land uses, SGVs are based on estimates representative of lifetime exposure. Although young children are generally more likely to have higher exposures to soil contaminants, the renal toxicity of cadmium are based on considerations of the kidney burden accumulated over 50 years or so (Environment Agency, 2009). It is therefore reasonable to consider exposure not only in childhood but averaged over a longer time period.

PROPOSED REDUCTION OF CADMIUM CONTENTS OF PHOSPHATE FERTILISERS IN EU

European Commission recently proposed a regulation (European Union Regulation -EC 2016) envisaging stringent limits of cadmium in phosphate fertilisers. Besides the proposals of no action and general actions for market incentives, they are synthetically: (i) an initial limit of 60 mg Cd Kg-1 P 2 O 5 will apply as soon as the regulation comes into force;. (ii) more stringent limit of 40 mg Cd Kg-1 P 2 O 5 will phase in three years later; (iii) the lowest limit of 20 mg Cd Kg-1 P 2 O 5 will come into force nine years after the regulation initiation and (iv) a new Regulation setting a Community limit value for cadmium content in phosphate fertilisers at 60 mg cadmium/kg P 2 O 5 decreasing over time to 40 and eventually 20 mg cadmium/kg P 2 O 5 if decadmiation becomes available on industrial scale.

A new Regulation converges to a common proposal: establish an EU limit value of 60 mg cadmium/kg P 2 O 5 as a starting point. This limit would take effect after an appropriate transition period of e.g. 2 to 3 years. Flexibility should be given to allow Member States to set limit values at either 40 or 20 mg cadmium/kg P 2 O 5 in the light of specific conditions in their territories. Fertilisers would be labelled to provide an indication which limit value for cadmium they comply with.

DISCUSSION AND CONCLUSIONS

The metallic elements tracks (Cd, Cr, Cu, Hg, Ni, Pb, Zn) are rapidly increasing in soils mainly due to the indiscriminate use of fertilisers and, recently, urban sewage sludge. As some of them are potentially toxic and present no agronomic interest, their presences generate a major concern. The agricultural use of the residual mud allows the recycling of precious components such as the organic matter and many nourishing elements of the plant [START_REF] Logan | Physical characteristics of alkaline stabilized sewage sludge (Nvitro soil) and their effects on soil properties[END_REF]. Residual muds can replace or reduce the use of this imported and expensive fertilisers. Economic and environmental issues are in the center of this debate, and this paper intends to expose the main advances in the knowledge.

Muds are products susceptible to supply in the cultures of the nourishing elements useful fortheir growth and for their development. Furthermore, certain sewage sludge (composted or limed) can play the role of amendment, by allowing maintaining or to improve the structure of the ground, its biological activity or still control its acidity. Nevertheless, from an agronomic point of view, and provided that the farmer respects the agronomic advice, the economy on the purchases of fertilisers can be considerable.

The calculated Cd increase with the application of 200 kg ha-1 of the FR3 incorporated in 0.1m of soil depth by but and would reach 0.094 mg kg-1, by the way, it would 111 applications to attain the intervention level of 10 mg kg-1 de Cd of CETESB (2001). In the other hand, they noticed that in five years, Cd contents could double, from one average soil of 0.5 mg kg-1 Cd (Campos et al., 2005).

The possibility of overcome the maximum permissible concentration, not only for Cd, but also for other elements evaluated, increases with larger quantities of phosphates fertilizers associated with sewage sludge and other fertilizers containing trace elements. [START_REF] Ramalho | Acúmulo de metais pesados em solos cultivados com cana-de-açúcar pelo uso contínuo de adubação fosfatada e água de irrigação[END_REF] availed the use of polluted water and phosphate fertilisers combined, and observed that soils that received 25 years of phosphate fertilisers, showed a noticeable increase of Cd (0.66 mg kg-1) when compared with control area (0.5 mg kg-1), without however elevate it to critical levels. The conditions that determine the adsorption capacity, such as pH, organic matter content, content of clay and oxides of Fe and Al, affect the availability and mobility of heavy metals present in phosphates [START_REF] Resumo | Heavy metals and rare earth elements in phosphate fertilizer components using instrumental neutron activation analysis[END_REF]. It is also important to consider local conditions that could lead to losses by erosion of soil particles enriched with heavy metals, transported to other areas or water bodies.

Current limits of cadmium on fertilisers in Brazil are insufficient to meet health and environmental protection goals. The states that have previously established base levels need to review recent research on the health effects of cadmium, the increment of cadmium in soils, and the contribution of fertiliser to cadmium loading in surface waters. These states should lead the way toward health protection adopting more stringent limits that certainly will reduce exposures and prevent adverse health effects.

Figure 1 -

 1 Figure 1 -Average contents of Cd in brazilian natural phosphates FN1 an FN2, brazilian triple superphosphate and reactive non-brazilian phosphates FR1, FR2, FR3 and FR4, determined by methods USEPA 3051A (in black bars), USEPA 3050B (white bars) and EMBRAPA (dashed bars). Higher traces are the standard deviation of average values. Modified from Campos et al., 2005.

Table 1 -

 1 Normal considered values of Cd, Co, Cr, Cu, Ni, Pb and Zn in natural soils, proposed as a Reference Value (RV) Concentration considered normal for the soils belonging to each group and corresponding to the value of the upper quartile (75%) of the frequency distribution of the sample data in each group.

	ELEMENT

1

. Modified from

Fadigas et al., 2006. 2

Mean upper quartile between groups

Table 2 -

 2 Background values, Intervention Values and Maximum Values in soil in function of its destination (Soil Remediation Circular 2009 e Soil Quality Regulation 2006). Modified from[START_REF] Ribeiro | Contaminação do solo por metais pesados[END_REF]

	Elements	Background values) a	Sediment a (mg/kg)	Max Values Maximum Maximum	Intervention Value) a	Intervention Value) b
		(mg/kg)		values for	values for	(mg/kg)	(mg/kg)
				residential	industrial soil
				soil quality class) a	quality class) a
				(mg/kg)	(mg/kg)
	As	20	29	27	76	76	76
	Cd	0.6	0.8	1.2	4.3	13	13
	Cr total	55	100	62	180	180	-
	Cu	40	36	54	190	190	190
	Hg Total	0.15	0.6	0.83	4.8	36	-
	Pb	-	-	-	-	-	4
	Ni	50	85	210	530	530	530
	Zn	140	140	200	720	720	720
	Legend: a) Soil Quality Regulation 2006; b) Soil Remediation 2009.


Sébastien Roland

Mahdi Moghaddam

Sylvie Tencé-Girault

Bruno Fayolle

Evolution of mechanical properties of aged poly(ether ketone ketone) explained by a microstructural approach

Keywords:

In this work, the thermo-oxidative behavior of PEKK films (~50 μm thick) in solid state were investigated. Since the mechanical properties of PEKK samples, especially the strain at break, are clearly affected after long ageing time, the modifications of the PEKK structure have been monitored at the molecular and the macromolecular scales using different techniques such as gel permeation chromatography, X-Ray scattering, differential scanning calorimetry, and dynamic mechanical analysis. If the embrittlement of the PEKK specimens seems to appear due to a predominant branching/cross-linking phenomenon occurring at the macromolecular scale, crystalline morphology is also slightly modified due to a chain scission mechanism occurring simultaneously. increase of publications number is probably related to the fact that Arkema has commercialized in 2016 the Kepstan® 60 0 0, 70 0 0, and 80 0 0 having a T/I ratio of 60/40, 70/30, and 80/20, respectively.

 .

The use of polymers is often limited by a chemical ageing, such as hydrolysis or oxidation, which leads to embrittlement of these polymers. It is associated with chain scission or branching/crosslinking processes occurring during ageing. In order to characterize, to predict, or to limit this embrittlement, relationships between the chemical structure modified by ageing and the mechanical behavior, especially at failure, are required. We can note that this requirement meets the more general topic in the polymers field which is to understand the effect of the chemical structure of polymers on their fracture properties, except that, in the case of thermal ageing, the chemical structure is modified over time, which can be exceedingly long. This challenging question re-

Introduction

The increasing market share of composites compared to metals traditionally used in aeronautical structures has led to significant weight savings. While thermosets remain largely the majority of composite matrices compared to thermoplastics, the introduction of the latter is now being considered to increase the temperature performance of composites and facilitate their welding and recycling. Amongst the high-performance thermoplastic arylether ketones (PAEK) family, poly(ether ketone ketone) (PEKK) copolymers consisting of para and meta isomers appear to be ideal candidates as a matrix for composites since their properties are similar to poly(ether ether ketone) PEEK and can show a wider processing window [START_REF] Gardner | Structure, crystallization and morphology of poly (aryl ether ketone ketone)[END_REF]. Indeed, the chemical structure of PEKK can be tuned by varying the initial ratio of the constitutive repetitive units (terephthalic or isophthalic acid units, so-called T/I ratio), which modifies the crystallization kinetics and the final crystallinity ratio.

The earlier publications concerning PEKKs are associated with the first synthesis more than 30 years ago [START_REF] Chang | PEKK as a new thermoplastic matrix for high-performance composites[END_REF]. The first papers about PEKK properties are about crystallization aspects [START_REF] Gardner | Structure, crystallization and morphology of poly (aryl ether ketone ketone)[END_REF][START_REF] Cheng | Polymorphism and crystal structure identification in poly(aryl ether ketone ketone)s[END_REF][START_REF] Ho | Crystal morphology and phase identification in poly(aryl ether ketone)s and their copolymers. 3. Polymorphism in a polymer containing alternated terephthalic acid and isophthalic acid isomers[END_REF][START_REF] Hsiao | Isothermal crystallization kinetics of poly(ether ketone ketone) and its carbon-fibre-reinforced composites[END_REF] where PEKKs containing terephthaloyl (T) and isophthaloyl (I) moieties have been studied in depth: the crystal structures, the polymorphism, and the crystallization kinetics have been investigated as a function of T/I ratios. During the first decade of the 20th century, only few studies are devoted to the PEKK copolymers synthesis [START_REF] Cai | Synthesis and properties of novel poly(aryl ether ketone)s containing both diphenyl moiety and amide linkages in the main chains[END_REF] or composite applications [START_REF] Dan-Jumbo | Life Prediction of Carbon Fiber/PEKK Thermoplastic Composite Material for Structures Design[END_REF]. Since 2016, several publications on PEKK are focused on crystallization kinetics modelling [START_REF] Choupin | A more reliable DSC-based methodology to study crystallization kinetics: application to poly(ether ketone ketone) (PEKK) copolymers[END_REF], on mechanical behavior in melt state [START_REF] Coulson | Dynamic rheological behavior of poly(ether ketone ketone) from solid state to melt state[END_REF] or in solid state [START_REF] Quiroga Cortés | Morphology and dynamical mechanical properties of poly ether ketone ketone (PEKK) with meta phenyl links[END_REF]. This

∼ mains an open question, which can be tackled by using chemical ageing to modify the chemical structure of the macromolecules and to investigate the consequences of these modifications on the mechanical properties.

Let us recall some approaches to relate polymer structure and its ductility. By considering only amorphous polymers as a simple case compared to semi-crystalline polymers, two different approaches are often proposed in the literature to explain ductility below T g : the first one is to consider that ductility is associated with local mobility below T g as a sub-glass transition often called T β : the more intense the latter transition, the more ductile the polymer [START_REF] Monnerie | Deformation, yield and fracture of amorphous polymers: relation to the secondary transitions[END_REF]. Interestingly, this approach has been applied to linear polymers such as PMMA and its copolymers but also to networks such as epoxy thermosets. According to this approach, embrittlement of epoxies by oxidation is often associated with the sub-glass transition disappearance [START_REF] Ernault | Origin of epoxies embrittlement during oxidative ageing[END_REF].

By considering only linear amorphous polymers, another approach is to consider that ductility is governed by the entanglement density ( ν e ): the greater the entanglement density, the more ductile the polymer [START_REF] Kramer | Microscopic and molecular fundamentals of crazing[END_REF]. For these linear polymers, complete embrittlement is reached when the molar mass is below five times the molar mass between entanglements (M e ∝ 1/ ν e ) and is thus associated with entanglements network disappearance. The critical molar mass for embrittlement is often called M' C (M' C = 5M e ) [START_REF] Fayolle | A critical molar mass separating the ductile and brittle regimes as revealed by thermal oxidation in polypropylene[END_REF]. For a given polymer, embrittlement is then directly linked to a chain scission mechanism due to an ageing process. We have proposed to extend the latter approach to semi-crystalline polymers when they are submitted to a chain scission process. It appears that if a critical molar mass has been put in evidence when the ductile-brittle transition occurs, the physical meaning of this critical molar mass remains unclear [START_REF] Fayolle | Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state[END_REF]. Indeed, in addition to a decrease of the molar mass associated with chain scission phenomena, an increase of the crystallinity ratio which also promotes embrittlement is also witnessed. This latter process is often called the chemi-crystallization process [START_REF] Fayolle | Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state[END_REF]. The contributions of the two mechanisms, both promoting embrittlement, is intricate to unravel.

In the case of the chemical ageing of PEKK, a clear understanding of the relationships between these chain scission and/or crosslinking mechanisms and the loss of ductility remains a challenge to be faced to establish the relevant modifications limiting the so-called polymer end of life for a given long-term exposure. We propose here to investigate the embrittlement of a PEKK 70/30 occurring during exposure at 280° C. This temperature of exposure has been chosen to balance between a temperature high enough to initiate detectable oxidative ageing, knowing the PEKK is highly stable from the oxidation point of view, and a temperature below the melting peak onset. As a result, we report here for the first time a comprehensive study of the PEKK embrittlement in an oxidative atmosphere by characterizing the polymer at microscopic scales. Chemical modifications and kinetic aspects will be investigated in a second article.

Experimental

The as-received material used for this study is a poly(ether ketone ketone) (subsequently named PEKK 70/30) film of 50 μm in an amorphous state provided by Arkema. Ter ep hthaloyl (T) and Isophthaloyl (I) are the two moieties in the monomers used for the synthesis of PEKK 70/30. For this copolymer, the T/I ratio is 70/30. Pieces of PEKK films with dimensions of 5 ×10 cm² were cut and put on Teflon sheets in perforated aluminum boxes in a ventilated oven at 280° C. The boxes were taken from the oven periodically with exposure times from 24 h to 1200 h and cooled in ambient atmosphere while the cover of the box was closed. The 24 h sample (cold crystallized) was considered as the unaged reference sample. All films with different exposure times were ana-lyzed similarly with different methods from macroscopic to molecular scales.

At the macroscopic scale, the mechanical behavior of exposed films was analyzed by tensile tests using a tensile machine (Instron 4301). Dumbbell samples with dimensions of 2 ×10 mm ² in the gauge length were cut out of the exposed films by using a sharpened punch, having the main axis aligned with the extrusion direction of the films. Tests were done at room temperature with the strain rate of 1 mm/min on 4-6 samples for each exposure time.

At the macromolecular scale, all samples with different exposure times were analyzed by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), Small and Wide-Angle X-Ray Scattering (SAXS-WAXS), and Gel Permeation Chromatography (GPC).

DSC analyses were carried out on a TA Instruments Q10 0 0 calorimeter with ca. 8 mg of each sample sealed in a hermetic aluminum pan. The thermal analysis is divided into three stages consisted of a first heating, a cooling, and a second heating with similar heating and cooling rates of 10 °C/min under nitrogen flow (50 mL/min). In the first heating, the sample was equilibrated at 20 °C and heated up to 400 °C. The sample was then maintained for 5 minutes at 400 °C. In the cooling stage, the sample was cooled down to 20 °C and, then, was maintained for 1 minute to equilibrate. In the second heating, the sample was subsequently heated up to 400 °C.

DMA analyses were done using a TA Instruments Q800 dynamic mechanical analyzer on samples with dimensions of 5 ×10 mm ² that were cut out of the main exposed pieces using a press machine and a mobile punch. The tension mode with an oscillation strain of 0.1% was used at 1 Hz. Each sample was equilibrated at -130 °C for 2 minutes, then it has been heated up to 300 °C with a heating rate of 3 °C/min.

Simultaneous Small and Wide Angle X-rays Scattering experiments were performed on the Xenocs Nano-inXider SW system in transmission mode using Cu K α radiation ( λ = 1.54 Å) from an X-ray microsource (GeniX3D) operating at 50 kV-0.6 mA (30 W). Scattering patterns were collected using the combination of two Pilatus3 (Dectris) detectors operating simultaneously in SAXS and WAXS positions allowing a continuous q range between 0.01 Å -1 and 4.2 Å -1 (2 θ range between 0.15 °and 62 °). From SAXS spectra, we calculated the electron density function using the XSACT software [START_REF]XSACT: X-ray scattering analysis and calculation tool[END_REF] and deduced the crystalline lamellae thickness L c and periodicity L P . We decomposed WAXS spectra in crystalline and amorphous components using Fityk software [START_REF] Wojdyr | Fityk : a general-purpose peak fitting program[END_REF] and deduced the crystalline phase and the weight crystalline ratio. The detailed procedure used for the SAXS-WAXS quantitative analysis was published in previous publications [ 20 , 21 ].

GPC measurements were performed by Arkema on a Waters Alliance 2695 device with a Waters 2414 RID detector.

Results

First, we propose to put in evidence mechanical behavior changes during exposure at 280 °C in air. Second, we investigate all modifications that can be responsible for these mechanical changes.

Mechanical property changes

Fig. 1 shows the evolution of the elongation at break for PEKK 70/30 as a function of ageing time. Inset shows typical strain-stress curves of selected tensile test experiments at three ageing duration, i.e. 24 h, 300 h, and 1000 h. The ductility of PEKK is unaltered at very short times. However, as exposure duration reaches a critical value of 300 h, the elongation at break decreases. At 400 h of exposure, the material shows low ductility since failure occurs when the necking process is initiated. We define an arbitrary criterion at 30% (green dotted line), below which PEKK loses its ductility. For very long exposure times, strain at break tends to an asymptotic value close to the yield strain value i.e. ~10%. We can note that properties at small strain such as Young's modulus is not significantly affected by ageing (results are not shown here).

If this embrittlement process is usual for polymers under oxidative atmosphere, we propose here to investigate by a multi-scale approach the physico-chemical reasons for such behavior. As seen in the introduction, this kind of embrittlement process can be associated with several causes such as molecular mobility modifications, especially sub-glass transitions, molar mass changes, or crystallinity increase. In the next parts, we propose to investigate experimentally each cause.

Molecular mobility

Dynamic mechanical analysis (DMA) remains the best way to investigate molecular mobility changes during ageing. This classical tool enables the characterization of local/sub-glass mobility and cooperative/glass transition. Fig. 2 represents the DMA results obtained for a selected set of PEKK samples exposed at 280 °C.

It shows the evolution of the storage modulus and the tan( δ) as a function of temperature (from -120 °C to 260 °C, from glassy to rubbery states). Tan( δ) curves include mechanical manifestation of the glass transitions indicated by strong peaks at temperatures higher than 150 °C and sub-glass transitions manifested by numerous weaker peaks at temperature mainly below 0 °C.

The intensity of sub-glass transitions would be greater in shear mode but it requires thicker samples [START_REF] Coulson | Dynamic rheological behavior of poly(ether ketone ketone) from solid state to melt state[END_REF] . However, DMA in elongation mode was preferred since samples are films in order not to have DLO oxidation. This mode was used to characterize oxidized epoxy films in a recent publication [START_REF] Ernault | Origin of epoxies embrittlement during oxidative ageing[END_REF] and a clear modification of the sub-glass transition was observed. In the case of PEKK films in elongation mode, the sub-glass transition is weak, especially in dry conditions [START_REF] Coulson | Dynamic rheological behavior of poly(ether ketone ketone) from solid state to melt state[END_REF] , compare to that of the epoxy systems in the same experimental conditions. As a result, it is difficult to conclude about the impact of the degradation on the sub-glass transition. On the other hand, by increasing the exposure time, the maximum of the alpha peak indicating the mechanical manifestation of the glass transition (T α ) shifts to higher values. As the exposure time goes by, the intensity of the glass transition peak decreases while the basal width of the peak increases. This basal width increase could be attributed to a wider distribution of relaxation times, which could be interpreted as an increase of dynamical heterogeneities around the alpha transition [START_REF] Masurel | Role of dynamical heterogeneities on the viscoelastic spectrum of polymers: a stochastic continuum mechanics model[END_REF] . The increase of T α as a result of increasing the exposure time suggests that crosslinking occurs predominantly but a chain scission process cannot be excluded. The same conclusion has been drawn for similar exposure in the case of PEEK where T g changes has been witnessed [START_REF] Courvoisier | Multi-scale and multi-technique analysis of the thermal degradation of poly(ether ether ketone)[END_REF] .

Storage modulus curves as the other group of curves in Fig. 2 , are composed of glassy state, which ends with the modulus dropping at the rubbery state. If the modulus measured below T α is almost indifferent to the exposure time, one can witness that T α increases with exposure time whereas rubbery modulus increases. More precisely, Fig. 3 shows the evolution of the storage modulus at room temperature (T < T α ) and at 250 °C (T > T α ) extracted from modulus curves in Fig. 2 . The magnitude of the modulus in the glassy state remains almost constant while it increases in the rubbery state as the exposure time elapses. This increase in rubbery modulus value measured above T α can be associated with an increase of the crystallinity or to a crosslinking process. This point will be investigated later.

Macromolecular modifications

Fig. 4 a shows the evolution of the molar mass M w as well as the percentage of insoluble part obtained by GPC as a function of exposure time at 280 °C. Until 125 h of exposure, the samples are totally soluble in the GPC solvent and the molar mass (red) increases. Thereafter, the molar mass of the soluble part (blue) drops while the insoluble part (black) increases. The increase of the molar mass in the first stage, followed by insoluble fraction formation, undoubtedly confirms a predominant branching/crosslinking process. Given that M w increases sharply from the very beginning, the number of chain scissions (s) can be considered as minor compared to branching/crosslinks (x).

In order to assess the crosslinking mechanism during the exposure, we propose to apply Saito theory [START_REF] Saito | Theory of gel-dose curves for polymers undergoing simultaneous crosslinking and scission[END_REF] valid up to the gel point (100 h) where crosslinks are easily related to weight average molar masses as follows:

2 x = 1 M w - 1 M w 0 ( 1
)
After the gel point, Charlesby-Pinner theory [START_REF] Charlesby | Analysis of the solubility behaviour of irradiated polyethylene and other polymers[END_REF] can be used with the insoluble fraction data knowing the initial molar mass M w0 and by considering that chain scissions are negligible:

sol + sol = 1 x M w 0 (2)
In Fig. 4 b, we report the cross-linking number during exposure before gel point thanks to Saito theory and after the gel point according to Charlesby-Pinner theory. It appears clearly that the crosslinking process starts from the beginning of the exposure and crosslinking is the major degradation phenomenon, as it was also observed for PEEK [ 23 , 26 ]. It is noteworthy that crosslink number calculated by both ways is consistent before and after the gel point. In Fig. 5 a, the melting peak for the sample with an exposure time of 24 h is wide and shows two shoulders on both sides of the minimum, which indicates either different crystalline phases or lamellae thicknesses. As ageing is increased to 100 h, a stronger melting peak appears with shoulders above this melting temperature. As the exposure time increases, the shoulders are not visible anymore due to the shift of the melting peak to higher temperatures, as seen for the 200 h sample, for which only a sharp peak is observed. For exposure times of 300 h to 10 0 0 h this peak sharpens and as time goes by, the melting temperature increases.

Crystallization aspects

The first heating was followed by a cooling stage (at 10 °C/min). In Fig. 5 b, crystallization peaks emerges during cooling at temperatures higher than 200 °C. For exposures below 200 h, chains have a considerable mobility and crystallize in a great amount so that crystallization appears with an exothermic peak during cooling. For exposure above 300 h, crystallization seems to be limited: whereas the crystallization temperature drops, the crystallization exotherm is flattened. For exposure of 10 0 0 h, chains do not crystallize significantly, as the crystallization kinetics becomes too slow compared to the applied cooling ramp.

Fig. 6 shows the evolution of the heat of melting as a function of exposure time, measured during the first and second heating. In the first heating, the heat of melting increases for the first hundred hours and then stabilizes as the exposure time increases. In the second heating section following the cooling stage at 10 °C/min, the heat of melting decreases almost linearly from the very beginning of exposure until 800 h to become almost negligible. In other words, the increase of the heat of melting during the first heating reflects an annealing process that can be associated with a chemicrystallization mechanism. In this case, chains initially trapped in the amorphous region by the entanglement network, can be released and then incorporated in the crystalline phase. Knowing this mechanism requires a chain scission mechanism, we can conclude that even if the crosslinking mechanism is predominant, a chain scission mechanism should occur simultaneously. The fact that the crosslinking mechanism is predominant is confirmed by the decrease of the enthalpy of melting measured during the second heating, since the resulting crosslinks no longer allow the crystallization process to proceed. This latter phenomenon is often observed in semi-crystalline polymers in which crosslinking is the main degradation mode as was observed for PEEK under irradiation [START_REF] Sasuga | Ion irradiation effects on thermal and mechanical properties of poly(ether-ether-ketone) (PEEK)[END_REF] .

It is well known that PEKK copolymers exhibit a polymorphism depending on their chemical composition, namely the T/I ratio, and their crystallization kinetics and pathway [START_REF] Cheng | Polymorphism and crystal structure identification in poly(aryl ether ketone ketone)s[END_REF] . WAXS and SAXS measurements were performed in order to check possible crystalline modifications, nature, and amount, during the exposure at 280 °C. Fig. 7 shows diffraction spectra for samples aged at different exposure times. After 24 h at 280 °C, the film sample, which was initially amorphous, underwent a cold crystallization and can be considered as fully crystallized [START_REF] Choupin | A more reliable DSC-based methodology to study crystallization kinetics: application to poly(ether ketone ketone) (PEKK) copolymers[END_REF] .

As the initial film is amorphous, the samples have been cold crystallized during the thermal treatment at 280 °C. It is known that cold crystallization of PEKK, with T/I around 70/30, leads to coexistence of form I and form II while melt crystallization yields to mostly form I [START_REF] Gardner | Polymorphism in poly(aryl ether ketone)s[END_REF] . In Fig. 7 a, both form I and form II are indeed observed. Each Bragg peak is indexed with the (hkl) planes of the crystal lattices of form I and form II, respectively [ 4 , 21 ]. The ageing does not seem to impact the polymorphism distribution as the diffractograms remain unchanged even until 10 0 0 h.

The decomposition of the WAXS spectra in crystalline and amorphous components leads to the calculation of the weight crystallinity ratios [START_REF] Tencé-Girault | Simultaneous SAXS-WAXS Experiments on Semi-Crystalline polymers: example of PA11 and its brill transition[END_REF] . Fig. 8 shows the evolution of this weight crystalline ratio as a function of exposure time. The crystallinity ratio increases significantly from 0.22 to 0.28 after 1200 h of exposure time.

We recorded simultaneously WAXS and SAXS signals. Scattering at small angle (SAXS), reported in Fig. 7 b, characterizes the crystalline lamellae organization. From these SAXS curves, we can calculate the correlation function of the electronic density and deduce the crystalline lamellar thickness (L c ) and periodicity (L P ) [START_REF] Tencé-Girault | Simultaneous SAXS-WAXS Experiments on Semi-Crystalline polymers: example of PA11 and its brill transition[END_REF] . These data are reported in Fig. 8 . For exposure time below 200 h, we observe a slight increase of the lamellar thickness, from 31 Å to 33 Å, while the periodicity remains constant at 126 Å. For higher exposure time, L P decreases first to 118 Å at 700 h then remains constant beyond, while L c decreases slowly and continuously until 1200 h to reach 30 Å. If all the crystalline and the amorphous phases are localized into the lamellae stacks, then the L C /L P ratio represents a volume crystal ratio. This data, reported in Fig. 8 , evolves only very slightly: a first increase below 200 h is followed by a plateau between 200 h and 600 h before a small decrease.

Discussion

In this part, we focus on structure-mechanical property relationships, which could explain the properties changes observed during degradation. As we have seen, both phases of the polymer are impacted by the degradation: scission/crosslinking in the amorphous phase and crystalline lamellae modifications, which effects are still to be accounted for.

Amorphous and crystalline phase changes

From DSC and WAXS, we measure a significant increase of the weight crystallinity ratio from 22% to 28% for exposure times between 24 h and 1200 h. On the other hand, the volume crys-tallinity does not evolve significantly. This difference can be attributed to the amorphous density modification as a result of branching/crosslinking [START_REF] Chen | Radiation-induced crosslinking: II. Effect on the crystalline and amorphous densities of polyethylene[END_REF][START_REF] Kacarevic-Popovi Ć | Effect of gamma irradiation on density and thermal expansion changes of uniaxially oriented linear lowdensity polyethylene[END_REF][START_REF] Mukherjee | Effect of gamma radiation on low density polyethylene[END_REF] . Indeed, the evolution of the long period L P ( Fig. 8 ), well correlated with the branching number ( Fig. 4 b), mainly reflects an evolution of the amorphous phase thickness. For low exposure times, there is no significant branching, L P remains constant. For exposure times between 200 h and 700 h, the number of branching and crosslinks increases drastically while L P decreases from 126 Å to 118 Å. In the meantime, we also observe that the lamellar thickness L c remains almost unchanged with exposure time. The slight increase of L c until 200 h could be interpreted as a small annealing process, which might be related to the chain scission mechanism. At the beginning of ageing, even though the chain scission processes are minor, they are effective enough to produce chemi-crystallization while crosslinking/branching acts are not sufficient enough to prevent this phenomenon. Consequently, L c slightly increases from the beginning of ageing. As ageing proceeds (after vertical dashed line at ca. 200 h), crosslinking/branching acts become more effective, which prevents the chemi-crystallisation from pursuing. This would explain how L c reaches a plateau after 200 h.

Modulus changes

We have seen in Fig. 3 that if the modulus at 23 °C remains constant, the modulus measured at 250 °C, i.e. above T g , increases from 120 to 270 MPa during the degradation process. The first possible origin of this increase can be ascribed to the crosslinking mechanism since the crosslink number ( ν) after 10 0 0 h of exposure, reaches a value close to 0.03 mol.kg -1 ( Fig. 4 b). However, if we assess that the rubbery modulus of the amorphous phase increases only up to 0.4 MPa thanks to the rubbery theory E = 3 ×ν×RT ρ with volumic mass ρ at 250 °C (623 K) of 1.17 g.cm -3 [START_REF] Li | Prediction of PEKK properties related to crystallization by molecular dynamics simulations with a united-atom model[END_REF] , it is obvious that the crosslinking process can not contribute to the modulus increase. The second possible origin of the modulus increase is the crystallinity change. We have seen in Fig. 8 that the crystallinity is significantly modified. In order to ascertain a possible correlation between both quantities, the storage modulus measured at 250 °C (T g0 DMA + 80 °C) is plotted as a function of crystallinity measured by WAXS in Fig. 9 . A clear trend is evidenced and confirms qualitatively the dependence of the modulus on the crystallinity. To examine more quantitatively the physical meaning of this dependence, we can compare these data with the data shown in Choupin et al. [START_REF] Choupin | Influence of thermal history on the mechanical properties of poly(ether ketone ketone) copolymers[END_REF] where PEKK (60/40) tensile modulus measured higher than T g (180 °C) is related to crystallinity. In this study, the crystallinity in terms of melting enthalpy was modified by varying the crystallization conditions. Even though it is complicated to compare directly these data, since the experimental conditions (DMA vs. static tensile testing) are not similar, it appears that the modulus dependence relates to what was observed by Choupin et al. and we can conclude that the increase of the modulus above T g is clearly related to crystallinity.

A mixed criterion for the ductility loss during ageing

In Fig. 1 , the green dotted line represents an arbitrary criterion that delimits the high and the low ductility regions. As this ductility loss is clearly evidenced during the exposure at 280 °C in air, we propose here to discuss all modifications that can be responsible for this change of mechanical property. According to the literature seen in the Introduction part, different modifications associated with chemical ageing have been proposed to understand the loss of ductility of polymers: i Local (molecular sub-glass) mobility decrease ii Crosslinking leading to limited chain extensibility iii Chain scission leading to a reduction of the molar mass below a critical value M' C related to the molar mass between entanglements iv Crystallinity increase or crystalline morphology change in the case of semi-crystalline polymers Our investigations in the case of PEKK 70/30 under air show that the first mechanism (i) appears to be irrelevant since the subglass transition is not impacted by the ageing process. However, considering that this mechanism only concerns amorphous polymers, the presence of a crystalline phase in our case could also hide this mechanism.

Macromolecular modifications, i.e. crosslinking and/or chain scissions, have been investigated by different ways: T g , molar mass, and gel fraction increase, which clearly indicates that crosslinking is largely predominant over chain scission. The crystallization behavior confirms this process since new crosslinks avoid chain folding and thus crystals formation. However, could this crosslinking process limit the extensibility of chains into the amorphous phase, which could reduce the strain at break? This scenario is often discussed for elastomers having different crosslinking density [START_REF] Bueche | Molecular theory for the tensile strength of gum elastomers[END_REF] .

For instance, the proportional relation λ fail ∝ ν -1/2 has been proposed, where λ fail is the elongation at break [START_REF] Smith | Ultimate tensile properties of elastomers. III. Dependence of the failure envelope on crosslink density[END_REF] . In our case, the fact that λ fail is inversely proportional to ν is consistent to this latter scenario.

The second scenario is to consider that embrittlement is mainly governed by the crystallinity ratio, as for the modulus. As it has been proposed in a review devoted to semi-crystalline polymers [START_REF] Fayolle | Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state[END_REF] , embrittlement due to chain scission process, the ductilebrittle transition results in an increase of the crystallinity ratio corresponding to a decrease of the amorphous thickness: below a critical value, a cavitation process initiated in the amorphous zone would be promoted during the deformation over plasticity leading a macroscopic brittle failure. In our case, we also observe a decrease of the amorphous thickness. However, this decrease could be assessed to a densification of the amorphous phase rather than an increase of the crystalline thickness.

As a result, during the degradation, the predominant crosslinking is accompanied with a chemi-crystallization process. To compare the two scenarios (branching/crosslinking vs. crystallinity ratio), Fig. 10 was plotted. As can be seen in this figure, the change in crystallinity is so little that it cannot be accounted for the loss of ductility. Thus, we consider that the main reason to explain this loss should be the branching/crosslinking acts that must be even more effective at a temperature below T g , since the molecular mobility is limited. In order to test this criterion, different ways of chemical degradation could be performed such as different temperatures of exposure or irradiation conditions.

Conclusions

In this study, the thermo-oxidative degradation at elevated temperatures (280 °C) below the melting temperature of PEKK 70/30 has been evidenced. A prominent branching/crosslinking process has been identified at the molecular level, as both the molar mass and the glass temperature transition increase with exposure time. A chain scission process cannot be totally excluded as chemi-crystallization is suspected through the evolution of the degree of crystallization. The DSC and the WAXS analyses indicate an increase of the crystallization that can be caused most probably through a minor chain scission process. This increase in crys-tallinity can be correlated to the increase of the modulus above the glass transition temperature. Overall, the observed loss of ductility of aged PEKK 70/30 is in all likelihood induced by a branching/crosslinking process occurring in the amorphous phase impeding the molecular mobility during the plasticization at large deformation.

Fig. 1 .

 1 Fig. 1. Strain at break of PEKK 70/30 as a function of exposure time at 280 °C. The black dotted line is a guide to the eye. The green dotted line represents strain at break of 30%. Inset shows typical stress-strain curves of selected aged PEKK samples exposed at 280 °C for 24 h, 300 h, and 10 0 0 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 2 .

 2 Fig. 2. Evolution of the storage modulus (left axis) and tan( δ) (right axis) as a function of temperature for PEKK 70/30 exposed at 280 °C during 24 h, 100 h, 200 h, 575 h, and 10 0 0 h.

Fig. 3 .

 3 Fig. 3. Evolution of the storage modulus E' as a function of exposure time at 23 °C (upper half) and 250 °C (lower half).

Fig. 4 .

 4 Fig. 4. (a) Evolution of the molecular weight M w (red and blue) and the insoluble part (black) as a function of exposure time. The dotted lines are guides to the eye. (b) Branching number calculated using equations 1 (in red) and 2 (in black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 5

 5 Fig. 5 shows DSC thermograms in first heating on the left (a) and cooling on the right (b). Sigmoid-shape signals around 150 °C to 200 °C in both groups of thermograms are the manifestation of glass transition. Since the mechanical manifestation of the glass transition has been more clearly discussed in the DMA part, we will focus here on the melting and crystallization behavior.In Fig.5a, the melting peak for the sample with an exposure time of 24 h is wide and shows two shoulders on both sides of the minimum, which indicates either different crystalline phases or lamellae thicknesses. As ageing is increased to 100 h, a stronger melting peak appears with shoulders above this melting temperature. As the exposure time increases, the shoulders are not visible anymore due to the shift of the melting peak to higher temperatures, as seen for the 200 h sample, for which only a sharp peak is observed. For exposure times of 300 h to 10 0 0 h this peak sharpens and as time goes by, the melting temperature increases.The first heating was followed by a cooling stage (at 10 °C/min). In Fig.5b, crystallization peaks emerges during cooling at temperatures higher than 200 °C. For exposures below 200 h, chains have a considerable mobility and crystallize in a great amount so that crystallization appears with an exothermic peak during cooling. For exposure above 300 h, crystallization seems to be limited: whereas the crystallization temperature drops, the crystallization exotherm is flattened. For exposure of 10 0 0 h, chains do not crystallize significantly, as the crystallization kinetics becomes too slow compared to the applied cooling ramp.Fig.6shows the evolution of the heat of melting as a function of exposure time, measured during the first and second heating. In the first heating, the heat of melting increases for the first hundred hours and then stabilizes as the exposure time increases. In the second heating section following the cooling stage at 10 °C/min, the heat of melting decreases almost linearly from the very beginning of exposure until 800 h to become almost negligible. In other words, the increase of the heat of melting during the first heating reflects an annealing process that can be associated with a chemicrystallization mechanism. In this case, chains initially trapped in the amorphous region by the entanglement network, can be released and then incorporated in the crystalline phase. Knowing this mechanism requires a chain scission mechanism, we can conclude

Fig. 5 .

 5 Fig. 5. First heating (a) and cooling (b) DSC thermograms of PEKK 70/30 exposed at 280 °C for times ranging from 24 to 10 0 0 h.

Fig. 6 .

 6 Fig. 6. Evolution of the heat of melting measured during the first heating ( ) and the second heating ( •) as a function of exposure time.

Fig. 7 .

 7 Fig. 7. WAXS (a) and SAXS (b) spectra for PEKK 70/30 exposed at 280 °C for 24 h, 200 h, and 1000 h.

Fig. 8 .

 8 Fig. 8. Weight crystallinity ratio from WAXS ( █ ) and volume crystallinity ratio (L C /L P ) from SAXS ( ) as a function of exposure time at 280 °C (left), L C ( •) and L P ( ) from SAXS.

Fig. 9 .

 9 Fig. 9. Storage modulus measured at 250 °C as a function of the crystallinity ratio measured by WAXS.

Fig. 10 .

 10 Fig. 10. Crystallinity ratio -branching map for the loss of ductility.

Acknowledgments

The authors acknowledge the financial support of the French National Research Agency (ANR IMPEKKABLE project). The authors wish to thank the LEM laboratory (Arkema, Serquigny), especially Sylvie Lebreton and François Bargain for facilitating SAXS-WAXS experiments and Christine Thieulin and Stéphane Bizet for GPC measurements. Stéphane Bizet and Benoît Brulé are warmly thanked for fruitful discussion, reading, and clarifying this article. We would also like to thank Rakhi Sood for some experimental parts in this work. This work was performed within the framework of the Industrial Chair Arkema (Arkema/CNRS-ENSAM-Cnam).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Sébastien Roland: Writing -review & editing, Supervision, Conceptualization, Visualization. Mahdi Moghaddam: Writing -original draft, Investigation. Sylvie Tencé-Girault: Writing -review & editing, Supervision, Conceptualization, Investigation. Bruno Fayolle: Writing -review & editing, Supervision, Conceptualization, Project administration, Funding acquisition.