Marc Baron

Mamy-Daniel Rakotorinina

Mohamed Ihab

El Assil

Yohann Guillaneuf

Didier Gigmes

Didier Siri

Anouk Siri

Jean-Jacques Flat

Sébastien Quinebeche

Philippe Cassagnau

Anouk Gaudel-Siri

Emmanuel Beyou
email: beyou@univ-lyon1.fr

Melt radical grafting of diethylmaleate and maleic anhydride onto oligoamide-11 (OA11) and polyamide-11 (PA11) in presence of acyloxyimide derivatives: Toward the compatibilization of PA11/EVOH blends

Keywords: Melt Radical grafting, polyamide-11, H-abstractors

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

Introduction

The melt radical addition of monomers such as maleic anhydride, diethylmaleate and vinyltrialkoxysilanes to saturated polymer chains yields functional derivatives that are valued for tye layer in co-extrusion, blend compatibilization and filler reinforcement applications [START_REF] Ciardelli | Functionalization of polyolefins in the melt[END_REF][START_REF] Passaglia | [END_REF][3][4][5][6][7]. In this way, the reactive extrusion technique has proved to be an effective way to introduce a variety of functional groups in macromolecular chains without the use of any solvent [START_REF] Ciardelli | Functionalization of polyolefins in the melt[END_REF]8].

In the latter case, peroxides are usually use to abstract hydrogen atoms onto the polymer backbone at high temperature (T>160°C to reach convenient viscosity of the polymer under shearing) leading to the formation of radical sites on the polymer backbone that may initiate the graft copolymerization of a monomer [9,10]. However, the use of peroxides does not permit to control the final macromolecular structure because it leads to side reactions such as Β-scission (polypropylene) and coupling reactions or cross-linking among macroradicals (polyethylene) [11][12][13][14][15][16]. In contrary, when using diethylmaleate (DEM) as monomer, crosslinking and coupling reactions are totally prevented in presence of PE as polymer matrix because hydrogen transfer reactions between DEM-based macroradicals and PE chains [17,18]. Thus, nitroxyl ethers (NORs) based radical generators were developed and their activity is based on their thermolysis which leads to the formation of either alkoxy and aminyl radicals or alkyl and nitroxyl radicals, as discussed by Pfaender [19,20]. For exemple, the nitroxyl ether Irgatec 76 (CIBA) was used for the free radical modification of polyolefins by maleic anhydride with a very high efficiency at processing temperatures above 250 °C [19].

Recently, a new family of radical initiators based on the N-acetoxyphtalimide (NAPI) was designed and these compounds are supposed to produce acyl and cyclic amidinyl radicals upon heating at high temperature (T > 190 °C) and the latter species are claimed to be a good alternative to peroxides molecules [21][22][23][24]. In particular, we have shown that NAPI was a better hydrogen abstractor than di-cumyl-peroxide in presence of polyethylene at 200°C and permiting us to keep under control the undesired polyethylene crosslinking reactions [24].

In case of polyamide-based materials, the melt surface modification of polyamides is not usual to prepare grafted copolymers which may be useful as compatibilizing agents for polymer blends and may improve physical and chemical properties such as weather and heat resistance [25]. Indeed, most of the work is based on the chemical modification of polyamides with different initiator systems and using solvent such as ethanol and dioxane [25][26][27][28].

Nevertheless, a few years ago Passaglia et al [29,30] described the in situ-free radical grafting of polyamides in the melt by using functional agents in the presence of peroxides.

However, it was shown that the active free radicals produced by the thermal decomposition of peroxides are responsible for branching [29].

Herein, for the first time we have studied the melt functionalization oligoamide-11 and polyamide-11 with dethylmaleate in presence of new grafting agents producing very efficient cyclic amidinyl radicals toward H-abstraction. Indeed, it is expected that this new grafting agents will produce new functionalized polyamides with a high functionalization degree that can be used as compatibilizing agent in PA11/EVOH blends. We have studied grafting reactions in regards to hydrogen abstractor chemical structures, their concentration, the reaction temperature and the monomer concentration and using an autoclave for oligoamide-11 and a microextruder for polyamide-11. The modified oligomers and polymers were analysed by nuclear magnetic resonance (NMR) spectroscopy. Finally, the effect of the incorporation of DEM-grafted-PA11 in PA11/EVOH blends at different concentrations was evaluated by the use of scanning electronic microscopy.

Experimental

Materials

The oligoamide-11 (3-4 units) and the Rilsan® polyamide 11 were provided by Arkema.

Diethylmaleate (99%, Aldrich) and Dicumyl peroxide (DCP, Aldrich) were used without any further purification. All other solvents were used as received.

The phtalimide-based H-abstractors (NAPI-, NANPI-, NASI-derivatives) were easily prepared according to procedures described in the literature [31][32][33].

Methods

Grafting of diethylmaleate (DEM) onto oligoamide-11 (OA11) and polyamide-11 (PA11 in an autoclave and a mini-extruder in presence of acyloxyimide derivatives

For the chemical modification of OA11, the reaction is carried out in a 450 mL autoclave (Parr) equipped with temperature and pressure devices. In a typical process, 50 g of OA11, DEM (4.6 mL, 11.5mol%), NAPI-or NANPI-or NASI-derivatives (0.74-0.95g, 1.46 mol%) were added simultaneously in the autoclave at temperatures of 190°C for 60 min and 230°C for 7.5 min to ensure fully decomposition of NAPI-, NANPI-and NASI-derivatives. The unreacted products (DEM and decomposition products) were extracted from the modified materials with boiling acetone.

For the chemical modification of PA11, 5 g of PA11, DEM (0.5 mL, 11.5mol%), NAP-or NANP-or NASI-derivatives (0.074-0.095g, 1.46 mol%) were added in a preheated mini-extruder (Thermo Scientific HAAKE™ MiniCTW micro-conical twin screw compounder, 7 ml) at a screw speed of 60 rpm and a temperature of 110°C. This type of extruder allowed us to control the processing residence time from the recirculation of the molten polymer in the chamber. After 20 min., the temperature was increased to 190°C (or 230°C) for 60 min or 7.5 min, respectively. The unreacted products (DEM and decomposition products) were extracted from the modified materials with boiling acetone.

Grafting of diethylmaleate (DEM) and maleic anhydride (MA) onto polyamide-11 (PA11) in an extruder in presence of acyloxyimide derivatives

The grafting reaction of DEM and MA onto PA11 in the molten state was performed in an intermeshing co-rotating twin-screw extruder (LEISTRITZ LSM 30-34) at a rotor speed of 250 rpm and a processing temperature of 290°C. The concentration of DEM and MA was 3wt% while the H-abstractor one was 1.60wt%. Then, the corresponding material was dried under vacuum at 120°C for two hours to eliminate the residual monomer.

Polymer blends

The preparation of the polymer blends (Table 5) was performed at 230°C for 5 min in a preheated mini-extruder (Thermo Scientific HAAKE™ MiniCTW micro-conical twin screw compounder, 7 ml) at a rotor speed of 60 rpm.

Characterizations

H NMR spectra for polymeric samples were recorded on a Bruker ALS 300 spectrometer, operating at magnetic field of 9.4 T. The experiments were performed at room temperature using 5 mg of polymer diluted in1 mL of in D 2 SO 4 . A spectral width of 6410.3 Hz was used with a relaxation delay of 1.5 s and 512 scans per point.

Scanning electron microscopy (SEM) images were taken by a commercial FEI Quanta 250 FEG. The polymer blends were fractured in liquid nitrogen and deposited on carbon tape. The observation was made at 15 kV. In a typical experiment, each sample was cut in 1 mm thick band, cooled in liquid nitrogen and fractured inside a cryotransfer GATAN Alto 2500 at -150 °C using a scalpel. The observation was done in cryo mode at -150 °C with an accelerating voltage of 2 kV.

Results and discussion

The original biosource PA (PA11, Rilsan) are becoming increasingly popular in many application areas such as in automotive, electronics and sports. Since PA11 displays good processability, high impact strength at low temperature and low moisture absorption, it has been developed for various applications including fuel lines in automotive, pipe for oil and gas applications, air break hoses for truck and medical appliances. However, to design new applications, it may be desirable to develop novel PA11-based alloys and blends. In this context, scanning the factors influencing the selectivity toward hydrogen abstraction on nylon 11 and other side reactions is of particular interest. Hence, amidinyl-based molecules socalled acyloxyimide have been selected to generate radicals in presence of a commercially available OA11 and PA11 from Arkema (Scheme 1). Indeed, the thermal homolytic decomposition temperatures of acyloxyimide affording potential hydrogen abstractors are in the range of temperatures close to the ones expected during reactive extrusion of polyamides, typically few minutes at 190-250°C. As shown in scheme 1, the DEM grafting position benefiting from the electron attractive -C=O group is preferred [29]. Passaglia et al. [30] studied the grafting of DEM onto PA-6 in presence of dicumyl peroxide as H-abstractor and they obtained the higher DEM grafting degree (4.3 mol%) when using 0.73 mol% of DCP and 11.5 mol% of DEM at 230°C.

Grafting of DEM onto OA11: optimization of the experimental conditions 8

Herein, considering that one mole of dicumyl peroxide generates two moles of radicals at full efficiency, OA11 was first modified in presence of 1.46 mol% of NAPI (two fold order of DCP, Fig. 1) as H-abstractor and 11.5 mol% of DEM at 230°C. After reaction, the resulting polymeric sample was extracted with boiling acetone to remove the unreacted DEM and analysed by 1 H NMR (Fig. 1). [30]. Considering the integral value of the latter peak, we have calculated the degree of functionalization (FD), as the number of DES groups per hundred OA units and we obtained a value of 8, corresponding to a grafting yield of 70%. A similar functionalization degree was obtained by using DCP at temperatures of 190°C and 230°C. Then, we have checked the optimal experimental conditions for the grafting of DEM onto OA11 by varying the reaction temperature, and both the DEM and the NAPI concentrations. The results are summarized in Table 1. Regarding entries 1 to 4, we can conclude that increasing the temperature from 230°C to 250°C leads to a decrease of the DES functionalization degree (about 7%) while the presence of by-products is detected by 1 H NMR. On the other hand, the use of a lower NAPI amount (0.73%) dramatically reduces both the DES functionalization and the grafting yield whatever the temperature is (230°C or 250°C). For clarity, all the results reported in Table 1 have been plotted on graphs (Fig. 2) allowing us to determine the optimal grafting conditions. Indeed, Fig. 2a clearly shows that the use of low amounts of NAPI (between 0.37mol% and 0.73 mol%) do not permit to obtain a DES functionalization degree higher than 5% at 230°C.

Moreover, a plateau is obtained from a NAPI concentration of 1.46 mol% with a fuctionalization degree of 8% (Fig. 3a). In addition, it can be observed from Fig. 2b that the DEM concentration may tailor the DES FD at 230°C with DES FD varying from 5.9% to 11.8% by increasing the DEM concentration from 6.2mol% to 23.0 mol% while the higher grafting yield (95%) is obtained for the lowest DEM amount (6. The results are summarized in Table 2 and it is observed a similar DES functionalization degree for all de H-abstractors at 230°C (7-9%) close to the value obtained with DCP (8%)

while the use of a lower reaction temperature (190°C) leads to a huge decrease of the DES FD for most of the H-abstractors. The melting points of NAPI-derivatives and NASI-derivatives are about 130°C and 190°C, respectively, while NANPI-derivatives do not exhibit any melting point before their degradations at temperatures above 250°C which may explain their similar reactivity at a temperature of 230°C. Moroever, the results obtained at 190°C suggest that each H-abstractor may tailor the DES FD at 190°C that is related to their different rate of decomposition in the corresponding reaction media.

From Table 2, it was concluded that the most efficient H-abstractors at both reaction temperatures 190°C and 230°C are NAPI (1), tBu-NASI (5) and Phe-NASI (6) which allow a DES FD similar to the one obtained with DCP.

Grafting of DEM onto PA11

Then, the grafting of DEM onto the PA11 was studied in the molten state using an intermeshing co-rotating twin-screw mini-extruder at a processing temperature of 190 °C for 1h to avoid too much DEM evaporation. It should be mentioned that the volatility of NAPIand NASI-derivatives powders can be neglected during the grafting reaction because their melting points are about 130°C and 190°C, respectively, suggesting very high corresponding boiling points. The results for some NAPI-and NASI-based H-abstractors are gathered in Table 3. Surprisingly, it is observed that the NAPI derivatives are much more efficient than the NASI ones allowing a higher DES FD (approximately by a factor 4). It may be attributed to the lower solubility and/or a lower diffusion coefficient of NASI derivatives in the molten PA11.

Indeed, as discussed before, the melting points of NAPI-based derivatives are about 130°C while the NASI-based derivatives ones are about 190°C. This behavior will be confirmed in a future work.

These operating conditions were finally partly used to make a pilot scale test in a twin-screw extruder at a temperature of 290°C and a flow rate of 1 kg/h (required experimental conditions for Arkema, CERDATO) with maleic anhydride (MA) and DEM as monomers.

From the 1 H NMR analyzes (figures S2 and S3), we calculated a MA FD of 5.3mol% and a DES FD of 2.3 mol%. The latter low DES FD in comparison with the one obtained at 190°C in a mini-extruder (i.e. 5 mol%) may be attributed to high DEM evaporation during the grafting reaction at 290°C and we were not able to compare this value with the one obtained 14 in a presence of a peroxide such as luperox 101 (2,5-Bis(tert-butylperoxy)-2,5dimethylhexane) or DCP because of their too low halt time life at this temperature (less than 1 s). Even if the FDs are lower than the lab experiments, acyloxyimide derivatives allowed to prepared unprecedented functionalized PA11 using industrial conditions.

Effect of the incorporation of MA-g-PA11 and DEM-g-PA11 into PA11:EVOH blends

The grafted copolymers synthesized at a pilot scale (MA-g-PA11 and DEM-g-PA11) grafted DEM) were finally used as compatibilizing agents at different weight content in PA11/EVOH (ethylene-vinyl alcohol copolymer) blends (Table 4). For the use of MA-g-PA11, it is also expected that it reacts with EVOH producing in situ a grafted EVOH-based copolymer. In addition, in order to probe the impact of the presence of the grafted copolymers on the rheological behavior and morphology of the studied polymer blends, we first carried out complex viscosity measurements on the homopolymers and the MA-g-PA11 compatibilizing agent (Fig. 3). At low frequency (0.1 rad / s), it can be observed that the complex viscosity of EVOH is 5500

Pa.s while PA11 displays a complex viscosity of 365 Pa.s. Moreover, it can be observed that the grafting of MA onto the PA11 increases its viscosity by a factor 5. Nevertheless, the rheological behavior of the various studied blends (Table 4) (with or without the grafted copolymers) are nearly the same so that it was not possible to demonstrate any significant influence of the presence of the grafted copolymer (Figure 4). For clarity, only the rheological behaviours and the SEM images of PA11/EVOH (65/35) and PA11/EVOH/MA-g-PA11

(65/30/5) (sample 1, Table 4) blends are presented. The SEM images show that the dispersed phase diameter does not decrease significantly by adding the MA-g-PA11 copolymer with the presence of particles dimension about 1-2 µm (Figure 5b). It can also be observed the presence of particle footprints suggesting poor adhesion of the EVOH particles with the PA11 matrix. This behaviour could be attributed to the very high PA11/EVOH viscosity ratio (about 40 at 1 rad/s, Figure 4) that may not favour a compatibilizing effect in presence of MA-g-PA11 and some hydrolysis of the in situ formed EVOH-based grafted copolymer in presence of hydrated PA11 despite its drying under vacuum.

Conclusions

Herein, it was shown that the grafting of monomers such as diethylmaleate and maleic anhydride onto oligoamide-11 and polyamide-11 is efficient when using acyloxyimide derivatives as H-abstractors. Under optimal experimental conditions, the DES FD was high as 9 mol% onto OA11 with tBu-NAPI as H-abstractor at 230°C in an autoclave reactor. The latter radical grafting reaction was then transposed to the grafting of DEM on PA11 at 190°C in a mini-extruder and we obtained a lower DES FD (6 mol%). Moroever, the grafting reaction at an industrial scale did not permit to reach such high DES FD (by a factor 2, below). This difference may be mainly attributed to extrusion rates and setpoint temperatures much higher than those used in the mini-extruder model study. Finally, the use of MA-g-PA11 and DEM-g-PA11 as compatibilazing agents for PA11/EVOH blends was not successful enough with both the presence of EVOH particles diameters ranging from 1µm to 2µm and EVOH particle footprints suggesting poor adhesion between the two phases. Future work will consist in optimizing the flow rate, the temperature and the amount of H-abstractors in order to obtain higher FD at a pilot scale and consequently a higher compatibilizing effect.

Scheme 1 .

 1 Scheme 1. Mechanism of DEM grafting onto OA11 and PA11 in presence of acyloxyimide compounds.

Fig. 1 .

 1 Fig. 1. 1 H NMR of DEM-g-OA11 in D 2 SO 4 , as solvent.

Fig. 1

 1 Fig. 1 confirms the grafting of diethylsuccinate (DES) groups onto the oligoamide backbone

Fig. 2 .

 2 Fig. 2. Effect of a) NAPI concentration at 230°C, b)DEM concentration at 230°C and c)reaction temperature onto the FD of DES onto OA11.

Scheme 2 .

 2 Scheme 2. Acyloxyimide H-abstraction agents based on N-acetoxy-phtalimide structure.

Fig. 3 .

 3 Fig. 3. Comparison of the complex viscosity behaviours of a)EVOH, b)MA-g-PA11 and c)PA11 as a function of the angular frequency.

Fig. 4 .

 4 Fig. 4. Rheological behavior and SEM image of a) PA11/EVOH (65/35) and b) PA11/EVOH/MA-g-PA11 (65/30/5) blends.

Fig. S1. 1 H

 1 Fig. S1. 1 H NMR spectra of neat OA11 in D 2 SO 4 , as solvent.

Fig. S2. 1 H

 1 Fig. S2. 1 H NMR spectra of MA-g-PA11 in D 2 SO 4 , as solvent.

Table 1 .

 1 DES functional degrees and their corresponding grafting yields as a function of NAPI concentration,

	DEM concentration and temperature.
	Entry	NAPI (mol%)	DEM (mol%)	T (°C)	FD (%) a	Grafting Yield (%)
	1	1.46	11.5	230	8.4	70
	2	1.46	11.5	250	7.4	65
	3	0.73	11.5	250	4.9	45
	4	0.73	11.5	230	3.8	35
	5	0.37	11.5	230	3.8	35
	6	2.92	11.5	230	8.4	70
	7	1.10	11.5	230	7.0	65
	8	1.46	23.0	230	11.8	50
	9	1.46	17.5	230	10.2	60
	10	1.46	6.2	230	5.9	95

a Degree of functionalization calculated form 1 H RMN spectra in D 2

SO 4

Table 2 .

 2 DES functional degrees at 230°C and 190°C as a function of the H-abstractor chemical structure and using a DEM concentration of 11.5 mol%.

	H-abstractor (1.46 mol%)	*FD (%) at 230°C for 7.5 min	*FD (%) at 190°C for 1h
	NAPI (1)	8.4	7.0
	tBu-NAPI (2)	8.1	4.2
	Phe-NAPI (3)	8.9	5.3
	NASI (4)	8.0	5.1
	tBu-NASI (5)	9.0	8.1
	Phe-NASI (6)	8.1	7.9
	NANPI (7)	7.0	2.1
	tBu-NANPI (8)	8.1	2.2
	Phe-NANPI (9)	7.2	3.0
	DCP	8.0	7.9

*Degree of functionalization calculated form 1 H RMN spectra in D 2 SO 4

Table 3 .

 3 DES functional degrees at 190°C for the grafting of DEM onto PA11 as a function of the H-abstractor

	chemical structure.
	H-abstractor (1.46 mol%)	FD(%) a at 190°C
	NAPI (1)	5.0
	tBu-NAPI (2)	6.1
	NASI (4)	1.5
	tBu-NASI (5)	1.5
	Ph-NASI (6)	1.5

a Degree of functionalization calculated form 1 H RMN spectra in D 2

SO 4

Table 4 .

 4 Compositions of the PA11/EVOH-based blends.

	Entry	PA11 (wt%)	EVOH (wt%)	MA-g-PA11 (wt%)	DEM-g-PA11 (wt%)
	1	65	30	5	-
	2	65	30	_	5
	3	65	25	10	-
	4	65	25	-	10
	5	80	15	5	-
	6	80	15	-	5

Acknowledgement

The financial supports from the French National Agency for Research (ANR-14-CE07-0015) and the competitiveness cluster Plastipolis are acknowledged.

SUPPORTING INFORMATION

Melt radical grafting of diethylmaleate and maleic anhydride *beyou@univ-lyon1.fr *beyou@univ-lyon1.fr


I M Zendjebil

Outdoor Augmented Reality: State of the Art and Issues

Keywords: Outdoor augmented reality, 3D localization, generation of 3D models, 3D visualization, 3D interaction, AR technology

The goal of an outdoor augmented reality system is to allow the human operator to move freely without restraint in its environment, to view and interact in real time with geo-referenced data via mobile wireless devices. This requires proposing new techniques for 3D localization, visualization and 3D interaction, adapted to working conditions in outdoor environment (brightness variation, features of displays used, etc.).

This paper surveys recent advances in outdoor augmented reality. It resumes a large retrospective of the work carried out in this field, especially on methodological aspects (localization methods, generation of 3D models, visualization and interaction approaches), technological aspects (sensors, visualization devices and architecture software) and industrial aspects.

Introduction

Augmented Reality (AR), which enriches the real world by adding virtual entities, offers great potential and opens doors for new applications. In recent years, many augmented reality systems have emerged. However, the vast majority of these systems are intended for small indoor environments easy to control that limit the problems often encountered in AR (brightness variation, occlusion, 3D registration, etc.).

The advances made in computer vision and mobile computing allow to develop more complex AR applications, intended for outdoor environment, where working conditions are hard to control (sunlight, unrestricted mobility, etc.), and where different types of sensors can be used. Outdoor augmented reality presents several problems that are still the subject of intense research, including the 3D localization, visualization and interaction on mobile devices, the use of geo-referenced data, etc.

This paper presents a recent state of the art on outdoor augmented reality organized in six sections: In the first section, we will present the challenges and issues of mobile AR. The second section will expose outdoor AR problems. Then, we will present and describe the various techniques developed for AR mobile. The fourth section is a technological survey. An overview of the major projects in this field will be described in the fifth section.

Outdoor AR challenges

The development of an outdoor AR system presents real challenges whether in technology, in methodology or in industry. Indeed, the development of an AR mobile platform will:

-Explore the synergy between augmented reality and mobile computing; -Develop an architecture (hardware and software) suited for outdoor environments; -Explore and develop new 3D localization methods which take into account the real conditions of outdoor environments (variations in brightness, occlusion, etc.); -Give the operator access to complex technical data during its interventions; -Study augmented reality architecture that allows to view geo-referenced data; -Develop new interaction methods and visualization approaches on mobile devices. Industrial issues of outdoor AR applications are very important, especially in cultural heritage, environment and tourism. Indeed, the technological advances allow to deploy and to democratize AR in industrial domains.

Outdoor AR issues

The scientific issues relating to outdoor AR mainly consist in developing new techniques on the fields of: localization, 3D visualization and interaction on handheld devices.

The mobility of the operator in an unrestricted and unprepared environment makes difficult localization process. Usually, we combine an absolute location with a relative localization to improve a continuous operator's localization. The absolute location is to estimate position using for example a GPS. The relative location uses sensors' motion like accelerometer or gyroscope to estimate the relative displacement of the operator referred to a reference position. 3D models of the environment are typically used to initialize localization procedure. The problem of accurate localization in natural environment and 3D registration (unknown surrounding, multiple sources of information, risk of occultation) is still an opened field of research.

In addition, outdoor augmented reality applications need a 3D virtual representation of the environment that reproduces enough realistically the real environment. Indeed, the 3D models are very useful for the pose estimation from a 2D/3D matching. The construction of simple and rich 3D models is a major challenge for AR.

The information available to the operator in the ground is often huge. Display these data, in real time; on handheld devices which have a limited processing power and low storage space is another problem that arises in mobile augmented reality.

Finally, the AR applications must allow handling and interacting with 3D data displayed to the operator. This task is accomplished through an interface. In conventional terminals such as PC, tools for interaction are based on WIMP concepts (windows, icons, menus, etc.). Outdoor augmented reality applications introduce new challenge due to the new mobile devices used (HMD, PDA, cellular phone, etc.). This requires proposing new Human/Machine Interaction approaches.

Mobile augmented reality techniques

Pose Estimation methods

In AR applications, it is important to know the position and orientation of the camera (also called camera pose) because it ensures the spatial and temporal coherence of the augmented scene. Indeed, camera pose allows to create a virtual camera with the same characteristics which is used to rend virtual entities overlaid the real images. This allows us to properly align real and virtual worlds. There are two main classes of pose estimation methods: marker-based methods (or fiducials) and markerless methods.

Marker-based methods use artificial markers placed in the real scene to estimate pose estimation. These markers are easy to detect and can contain codes that help to distinguish between them. Their positions in the real world are known a priori. Pose estimation contains the following steps: markers detection, 2D/3D matching, and finally estimate the camera pose. Two types of markers are commonly used: circular ones and square ones. The circular markers are quite invariant to perspective distortions, and their gravity center provides a stable 2D position. We mention the Concentric Contrasting Circle (CCC) markers [START_REF] Hoff | Computer visionbased registration techniques for augmented reality[END_REF] (black circle on a white background or vice versa), and colored concentric rings [START_REF] State | Superior augmented reality registration by integrating landmark tracking and magnetic tracking[END_REF] [START_REF] Cho | A multi-ring color fiducial system and an intensity-invariant detection method for scalable fiducial tracking augmented reality[END_REF]. The square markers were introduced by [START_REF] Koller | Real-time vision-based camera tracking for augmented reality applications[END_REF]. With the success encountered by ARToolKit library [START_REF] Kato | Marker tracking and hmd calibration for a video-based augmented reality conferencing system[END_REF], their use has become widespread. The markers used in this library have black borders on a white background. The internal "pattern" contains a code to identify the marker. The main drawback of marker-based methods is theirs need of artificial markers placed in the real scene which is not always possible.

Markerless methods exploit the natural features existing in the real scene such as the corners [START_REF] Lepetit | Fully automated and stable registration for augmented reality applications[END_REF], edge and line segments [START_REF] Drummond | Real-time tracking of complex structures for visual servoing[END_REF][55] [START_REF] Comport | A real-time tracker for markerless augmented reality[END_REF], etc. The 2D data extracted from the images of the scene are matched with 3D data extracted from a 3D model of the scene. The use of models can improve the robustness and performance of the pose estimation methods. For example, in [START_REF] Lepetit | Fully automated and stable registration for augmented reality applications[END_REF], Lepetit & al. propose a pose estimation method based on points. The method uses a partial 3D model of the scene. Their algorithm contains two phases: an automatic initialization phase during which the points of interest are detected in the reference images of the object to track, and a tracking phase. The pose is estimated using a 2D/3D matching and RANSAC algorithm. Edge-based methods [START_REF] Drummond | Real-time tracking of complex structures for visual servoing[END_REF] consist in extracting the edges of a 3D model and project them in image according to the previous pose estimation. The pose is obtained by minimizing the difference between the predicted position of contours and their current position in the image. In addition, Comport & al. [START_REF] Comport | A real-time tracker for markerless augmented reality[END_REF] present a reliable method formulated as a virtual visual servoing approach. The model-based method proposed uses local edge tracking. The control law of closed loop minimizes the error between the current position and the desired position of the camera. Comport uses an Mestimator for handling outliers. This method is robust to partial occlusions and changes of illumination. The proposed approach converges fast enough for small displacements.

Simon & al. [START_REF] Simon | Markerless tracking using planar structures in the scene[END_REF] have developed a method to track planes and recover the pose of the camera. They use the RANSAC algorithm to estimate robustly the best homography that relates the points belonging to the same plan in successive frames. The method is distinguished by its robustness to occultation, its speed. This method is easy to implement and very accurate. However, it presents a risk of drift due to the errors accumulation. We can also find texture-based tracking approach to estimate camera. It does not require the extraction of features such as contours or points of interest, but generally use all information of the pattern [START_REF] Lucas | An iterative image registration technique with an application to stereo vision[END_REF][17][24] [START_REF] Stricker | Real-time and markerless vision-based tracking for outdoor augmented reality applications[END_REF]. Jurie and Dhome [START_REF] Jurie | Real time 3d template matching[END_REF] proposed a template-based method. The idea is to estimate the differences between the current image and the image warped using predicted pose. These differences allow to update pose parameters. In ARCHEOGUIDE project, Stricker & al. [START_REF] Stricker | Real-time and markerless vision-based tracking for outdoor augmented reality applications[END_REF] described an approach based on a formal correlation matching between current image and references images of the real scene. The algorithm selects reference image with the highest score which allows to deduce 2D transformation between this image and the current image. From this transformation, we can extract camera parameters. Edge-based methods have the advantage of being robust to changes in illumination, but they are unsuited to textured scenes. However, the textures-based methods are robust to occlusions but remain highly sensitive to illumination changes. So, a new category of methods has been emerging called "hybrid methods". These methods combine different visual features, such as edges and texture [45][36]. For example, the approach developed by Vacchetti & al. [START_REF] Vacchetti | Combining edge and texture information for real-time accurate 3d camera tracking[END_REF] combines contours and interest points. The camera pose is obtained simultaneously by minimizing point's re-projection error and distance between the edges and projected of 3D edges on image.

4.2

Generation of 3D models Most of existing tools need a human hand to intervene in the reconstruction process. This makes them inadequate for large-scale reconstruction. Among these softwares, we can cite the photogrammetric software developed by GTA Geoinformatik Company, and SketchUp from Google Company. Concerning automatic approach, Lafarge [START_REF] Lafarge | Modèle paramétrique pour la reconstruction automatique en 3d de zones urbaines denses à partir d'images satellitaires haute résolution[END_REF] (laboratory MATIS) proposed methods using aerial data (photo, NSM, DTM, etc.). These approaches provide good results but are not yet in industrial phase. Also, we find other cloud points based methods provided with 3D scanners to construct 3D meshes. Some techniques are based on building prints. The system CityEngine proposed by Müller [START_REF] Müller | [END_REF] uses a grammar for a parameterizable reconstruction of urban environments (see figure 2.b). It also allows to adjust the level of details depending on the selected quality. However, it is necessary to define the grammar for the city we would like to reconstruct. Müeller [START_REF] Müller | Image-based procedural modeling of facades[END_REF] improves his CityEngine by adding a module for analysis facade's photos to automatically generate the adapted grammar for each building.

Archivideo proposes FastBuilder for the large scale reconstruction of urban environment (Figure 2.a). This system is based on heuristics to fill in the gaps. The reconstruction uses a library of facades adapted to the city (~500 facades) and classification buildings depending on their style to fit the model. The reconstruction rules can be changed throughout the city, street or building. This system allows rapid reconstruction and can be tailored to specific needs.

3D visualization

The techniques proposed in computer graphics require adaptations depending on features of mobile devices (limited processing and storage space). We can adopt a remote rendering approach that follows a client/server scheme. In client part, we can recover user interaction and display images calculated by a distant server.

More recently, different implementations of these approaches have been proposed (see [START_REF] Pouderoux | Création semi-automatique de modèles numériques de terrain -Visualisation et interaction du terminaux mobiles communicants[END_REF]) adapted to mobile devices such as personal digital assistants (PDA). These techniques give good results but require excellent connectivity between server and client, which is often difficult to guarantee in a real mobility context.

The adaptive rendering techniques include algorithms that can regulate in real time the amount data to be displayed according to available client resources. The strong point of these technologies is it can operate on a class of heterogeneous devices like handhelds. For example, Magellan [START_REF] Marvie | Visualisation Interactive d'Environnements Virtuels Complexes à travers des Réseaux et sur des Machines à Performances Variables[END_REF] is a visualization platform developed at IRISA which aims to ease the development of new solutions for 3D interactive visualization on heterogeneous devices. It provides a set of system classes encapsulating callback system (sockets, threads, etc.) in order to ensure portability. Magellan system is then multi-platform (Windows/ Linux/SunOS). Iparla team extends Magellan to mobile devices using mobile Windows Operating System (for Pocket PC and Smartphone). The new version, called Elkano, includes: a module of distributed rendering, modeling language X3D, the ability for the server to build scene graphs, etc.

3D interaction

The interactions between a user and an application are through interfaces. In desktop computers case, these interfaces are mainly based on the model WIMP (Windows, Icons, Menus, Pointing device.).

The constraints imposed by mobile devices are different. Then, the Human/Machine Interaction has to change. For example, in case of input text, T9 and Triple Tap have been developed to address the absence of keyboard. For 3D interaction, we need to create similar interface.

Interacting with 3D computer-graphics is a complex process. Usually, there are two major tasks: manipulation and navigation in 3D scenes:

1. Manipulation: any object's movement in space can be represented by 6 independent geometric transformations, which are translations along the 3-axes (x, y, z) and rotations around the same axis. Thus, if we wish to allow the user to move an object freely, it is necessary to allow it to operate on 6 Degrees of Freedom (DOF).

2. Navigation: it can be divided into two components: travel component which allows the movement of the actual view to another and way finding component also called cognitive component which corresponds to processes set to identify, apprehend 3D environment, or find a path to achieve a specific task (eg exploration, research, predefined movement).

There are several devices for 3D interaction on handheld devices, namely touch screens, buttons, cameras, and the positions and orientations of mobile devices. For example, for the touch screens, the interactions are based on gesture, drawing or pointing. The use of menus is difficult because of the small screen size (that would make it hide a large part of the view). In addition, several interfaces are based on gestures such as Scriboli [START_REF] Hinckley | Guimbretiere: Design and analysis of delimiters for selection-action pen gesture phrases in scriboli[END_REF] (Figure 3). In a browser, it is possible to designate the point of interest [START_REF] Mackinlay | Rapid controlled movement through a virtual 3d workspace[END_REF]. This technique is widely used in 3D applications under the name of "Go to". The displacement velocity will be depending on the distance between the observer point and the arrival point. Other techniques, such as using a "virtual Sphere" [START_REF] Chen | A study in interactive 3-d rotation using 2-d control devices[END_REF] or Ware set techniques, Eye in Hand, Scene in Hand, Flying Vehicle [START_REF] Ware | Exploration and virtual camera control in virtual three dimensional environments[END_REF], which are now standard tools, can be used.

Figure 3: Scriboli approach [19]

A lot of research is conducted on the use of a single button to move. Among the techniques, we found HoverCam [START_REF] Khan | Hovercam: interactive 3d navigation for proximal object inspection[END_REF] or UniCam [START_REF] Zeleznik | Unicam -2d gestural camera controls for 3d environments[END_REF]. These techniques are based on the recognition of gestures, and prediction on the user intents. In the case of object manipulation, the techniques developed for Pc are used, such as "Jack" [START_REF] Bier | Skitters and jacks: Interactive 3d positioning tools[END_REF] or "Skitter" [START_REF] Nielson | Direct manipulation techniques for 3d objects using 2d locator devices[END_REF], which are controllers. However, it should be noted that the control of these interfaces is more difficult with a stylus as a mouse. The advent of touch screens with several points on mobile devices makes possible to consider the use of techniques such as method proposed by Hancock & al. [START_REF] Hancock | Shallow depth 3d interaction: Design and evaluation of one-, two-, and three-touch techniques[END_REF] or approach developed by Zeleznik & al. [START_REF] Zeleznik | Two pointer input for 3d interaction[END_REF]. These techniques allow manipulating 3D objects using multiple points, to define the center and angle of rotation, or by "grabbing" object to handle. In the case of 3D interaction, the use of the keyboard is a problem due to the binary signal. For example, in the navigation case, it is difficult to determine the movement velocity when pressing a button. But several strategies have been developed. For example, the selection of a point by using keys allows to move a cursor, but this shift can be long.

To improve the selection phase, some studies have been conducted. To select a particular item on the screen, Hachet & al. propose Jump and Refine technique [START_REF] Hachet | jump and refine" for rapid pointing on mobile phones[END_REF] (figure 4), which divides the task into two phases: coarse selection in a grid then accurate selection inside this zone. ZoneZoom [START_REF] Robbins | Zonezoom: map navigation for smartphones with recursive view segmentation[END_REF] proposed by Robbins & al. has the same concept. The method consists in expanding area interest. Embedded cameras in handheld devices are also used in augmented reality applications such as NaviCam developed by Rekimoto [START_REF] Rekimoto | The world through the computer: Computer augmented interaction with real world environments[END_REF]. These approaches are based on the detection of markers previously placed in real scene. Tangimap [START_REF] Hachet | A camerabased interface for interaction with mobile handheld computers[END_REF] (figure 5) is a bimanual interface which has been developed to interact with 3D scenes. By moving a target in front of the camera, the user can control simultaneously 3 DOF. Finally, a lot of input interfaces are based on the movements of mobile terminal taken in hand. The ScrollPad [START_REF] Fallmana | Scrollpad: Tangible scrolling with mobile devices[END_REF] is a PDA equipped with an optical mouse and allows documents visualization by moving it on a table. In the Peephole interface [START_REF] Yee | Peephole displays: pen interaction on spatially aware handheld computers[END_REF], the user moves the PDA (connected to a mouse by a system of cables and pulleys) in space.

Augmented Reality technologies

Sensors

Several sensors are used for localization in 3D AR mobile applications, including:

Global Positioning System (GPS)

Developed by the U.S. Department of Defense, the system is used to determine a global position. It is based on a satellite positioning (see figure 6). GPS uses geodesic coordinate system WGS84.

Equipped with a GPS receiver, position, time and velocity of the operator are estimated from the different signals provided by satellites. Indeed, each satellite has an atomic clock and internal ephemeris used to calculate the predicted coordinates. The accuracy of these systems is about 10 meters for public GPS and for 1 meter for professional GPS.

Figure 6: Global Positioning System

In addition, DGPS (Differential GPS) can reach centimetric accuracy. This system uses signals emitted from both satellites and earth stations whose positions are known. These stations compute separately the error measurement for each satellite and transmit data correction. DGPS receivers merge these data with their own measures to provide a global position of the target.

Inertial Sensor

Two types of inertial sensors are commonly used: accelerometers and gyroscopes. The accelerometers are used to measure linear acceleration of target object. The position is estimated by a double integration of obtained data. However, the integration process tends to accumulate errors which produce a drift between (drift) estimated position and current position. The gyroscopes measure the angular velocity, and provide orientation by a single integration. These two types are generally combined.

Hybrid sensors

Each sensor provides errors measures in position and/or orientation. Hence the idea is to combine two or more heterogeneous sensors for handling errors. Of course, the use of multiple sensor technologies increases the systems complexity, but the fusion of the various available data improves the accuracy of the estimate of the position and orientation.

Among combinations found in literature, we can cite: magnetic sensor combined with a camera in [START_REF] Auer | The integration of optical and magnetic tracking for multi-user augmented reality[END_REF][51], inertial sensor attached to a camera [START_REF] Ribo | Hybrid tracking for outdoor augmented reality applications[END_REF], or a GPS with an inertial and a camera [START_REF] Piekarski | Hybrid indoor and outdoor tracking for mobile 3d mixed reality[END_REF].

Display devices

In augmented reality systems, we have many displays devices that allow viewing virtual data, thus enhancing the perception of real world. These devices can be subdivided, technologically, into two classes:

Monitor based AR-displays

Monitor-based systems provide to the operator the opportunity to see virtual objects overlaid on real world without being equipped for many special glasses. In laboratories, these types of device permit to test quickly the experimental system. The operator is equipped with a camera. The acquired image is enhanced with the virtual object and then returned on the screen. There is a large number of this type of device. The most basic is obviously screen PC or Hand-held devices (for example, the magnifying glass approach). In addition, we find a new generation of Hand-held devices (figure 7) which are also used as display interfaces such as the Tablet PC, PDA (Personal Digital Assistants) and cellular phone. Among these devices, the tablet PC is the most powerful component; it has been used in several outdoor augmented reality projects such as MARS, ARCHEOGUIDE, and GEIST.

See-through AR-displays

Also called HMD (Head Mounted Display) (figure 9), this class of displays represented the most used devices in AR community. There are two different technologies: optical see-through and video see-through. Video see-through are equipped with two cameras through which the real environment is enhanced with virtual entities. In optical see-through, the display of the augmented scene is achieved using a system of mirrors that allows to overlay computer-generated graphics on the sight of the real scene.

Outdoor augmented reality projects

In recent years, many outdoor AR projects have in military domain, civil engineering, cultural heritage, etc. In this section, we will present some of these projects.

6.1

Applications for military Many military conflicts are deployed into urban space. These new battlefields are complex because of restricted visibility, unfamiliar environment, lack of communications between the various troops and the difficulty of distinguishing between friends and foes. Developed in Naval Research Laboratory, (Washington, 2000), the project BARS "Battlefield Augmented Reality System" [START_REF] Julier | Bars: Battlefield augmented reality system[END_REF] aims to propose a military navigation and localization system which allows soldiers to see relevant information such as inside building plans or position of snipers for example. Equipped with GPS coupled to inertial sensor (for localization) and see-through HMD (figure 9), the soldier can view data and interact with them using a mouse or voice commands.

6.2

Applications for Navigation Started in 1996 by the Computer Graphics and User Interfaces Lab (Columbia University), MARS project [START_REF] Hollerer | Exploring mars: Developing indoor and outdoor user interfaces to a mobile augmented reality system[END_REF] (figure 10), acronym of "Mobile Augmented Reality System", was one of the first outdoor AR projects. It aims to explore the synergy between augmented reality and mobile computing. The platform provided information on the environment where user is moving. This created a touring Machine. The system is composed of a laptop and a DGPS receiver to obtain a more accurate user's position. Feiner & al. associated a differential corrector to GPS receiver. The orientation of point of view is determined using an inertial magnetometer. The user is also equipped with a tablet PC and a stylus to interact with the browser engine.

Other works were inspired by the MARS project such as Tinmith platform [START_REF] Piekarski | Tinmith-evo5architecture for supporting mobile augmented reality environments[END_REF] (Wearable Computer Lab, University of Australia, 1998) (figure 11) and the collaborative mobile AR platform developed by Studierstube project [START_REF] Reitmayr | Collaborative augmented reality for outdoor navigation and information browsing[END_REF] (Graz University of Technology, University of Vienna, 2004). Started in 2000, ARCHEOGUIDE project (Augmented Reality-based Cultural HEritage Onsite GUIDE) [START_REF] Gleue | Design and implementation of a mobile device for outdoor augmented reality in the archeoguide project[END_REF] aims to develop new interactive methods for accessing to cultural heritage information. This system includes a laptop, a HMD, headphones, a microphone and a camera. The system filter the information designed through user's position and orientation. Moreover, the proposed interface allows choosing between several themes and media. ARCHEOGUIDE provides an opportunity to visualize the 3D reconstructed damaged site (see figure 12).

6.3

Applications for architecture and construction UM-AR-GPSROVER [3] (Department of Civil and Environmental Engineering, University of Michigan) allows to view virtual models of construction in an urban environment (figure 13). The system is composed of a GPS receiver for position and an inertial sensor for orientation, a HMD, a camera and a laptop. ARQuake [START_REF] Piekarski | Arquake: the outdoor augmented reality gaming system[END_REF] (see figure 14) represents one of the first game based on outdoor augmented reality paradigms. It is an adaptation of the popular game Quake. The aim of the project is to construct a game that run in a real environment and where player can move freely. The virtual objects are aligned with respect to player's head position and orientation. The pose is estimated by the fusion of inertial sensor data and visual tracking method using ARToolKit Library. This platform uses Tinmith architecture. Other interactive outdoor AR games have emerged, such as Game-City [START_REF] Cheok | Game-city: a ubiquitous large area multi-interface mixed reality game space for wearable computers[END_REF], Human Pacman [START_REF] Cheok | Human pacman: a mobile, widearea entertainment system based on physical, social, and ubiquitous computing[END_REF] which is an adaptation of popular game Pacman.

Visualizing geo-referenced data

Using Tinmith architecture, ARVino platform allows to visualize viticulture GIS data. The system is composed of a laptop, a video, a GPS receiver for position and a magnetic sensor for orientation. It is placed on a rotating table mounted on a tripod.

Started in April 2006, the project Vidente [START_REF] Mendez | Interactive context driven visualisation tools for augmented reality[END_REF] (collaboration between Institute of Computer Graphics and Vision (ICG) and GRINTEC GmbH) aims to develop a mobile augmented reality solution to visualize subsurface characteristics. Users will be equipped with a mini-Tablet PC coupled with camera and various heterogeneous sensors. From user's localization, it is possible to visualize in real time 3D representation of network of cables and pipes hidden underground (see figure 15). The data were provided via a GIS (Geographic Information System). The project's aim is to provide an additional tool to existing mobile GIS solutions, and contribute to an optimized processing for data access.

Conclusion

Outdoors augmented reality presents several challenges on technological aspect (sensors, mobile terminals, etc) methodological aspect (3D localization techniques, generation of 3D environment, 3D visualization and interaction), as well as industrial aspect. The development of such systems requires solving several AR mobile scientific hard points: 3D localization in outdoor environment, visualization and 3D interaction via mobiles devices.

The survey presented in this paper has been conducted in RAXENV project. RAXENV (Outdoor Augmented Reality applied to environment sciences) aims to demonstrate the feasibility of outdoor augmented reality system dedicated to environment science and technology for the environment, both in terms of technology but also adoption by users. RAXENV is an exploratory project supported by ANR (the French National Research Agency), whose main goals are:

-To develop a 3D localization system using several sensors for virtual/real registration. This system should be robust to illumination conditions, occlusions, etc; -To propose an acquisition and representation system of geo-referenced data; -To develop methods for representing subsurface objects (pipes, cavities, etc) in an augmented environment; -To propose new 3D interaction methods; RAXENV differs from the conventional augmented reality approaches often used in the industry. It is dedicated to unfamiliar outdoor environments and based on Geographic Information Systems (GIS). Also, it explores ways to interact with data in mobility situation where the use of devices such as keyboard/mouse is impossible or inappropriate.

Figure 1 :

 1 Example of data used for 3D reconstruction (a) Vector data (b) NSMAugmented reality systems need a virtual representation of the real environment. Indeed, this 3D model will be used for localization process and occlusions handling. Therefore, the generation of this environment is based on real data. These data are provided by different sources such as:-Aerial photography: images acquired from aircraft, helicopters, satellites, etc; -Numerical Surface Model (NSM): a digital representation of the topography of the environment (see figure1.b) (forms and details visible on the ground); -Vector data: include building prints (see figure1.a) (the surface or up to a height of the building) and elements ridge (geometry of the roof of buildings); -Oblique photos; -Clouds points acquired by a scan; -Terrain's photos and video;

Figure 2 :

 2 Examples of 3D models of urban environment: (a) FastBuilder (b) CityEngineHoeim[START_REF] Hoiem | Automatic photo pop-up[END_REF] uses image-based method for the reconstruction of a simple model. The method consists in segmenting the image in different classes (such as sky, ground, vertical, etc.) which can divide images in planes (like Children books). Idem, Cornelis[START_REF] Cornelis | 3d city modeling using cognitive loops[END_REF] detects objects outside the perspectives plans. Then, it becomes possible to add a 3D representation in the model and expand the area of validity of the model (wider area is covered).

Figure 4 :

 4 Figure 4: Jump and Refine [16]

Figure 5 :

 5 Figure 5: Tangimap [15]

Figure 7 :

 7 Figure 7: Handheld display devices: (a) Tables Pc (b) PDA (c) cellular phone

Figure 8 :

 8 Figure 8: Examples of Head Mounted Display 5.3 Software ArchitectureIn augmented reality, the developed software architectures aim at implementing an abstraction layer between the different technologies and algorithms (tracking, data fusion, etc.).-DART: The Designer's Augmented Reality Toolkit[START_REF] Macintyre | Dart: A toolkit for rapid design exploration of augmented reality experiences[END_REF] (Columbia University, 2003) is a tool for rapid development of AR applications. It enables designers to create informal content for experiments. Indeed, it offers a rapid prototyping to a wide public. -COTERIE: Object-oriented Testbed for Exploratory Research in Interactive Environments [32] (Computer Graphics and User

Figure 9 :

 9 Figure 9: BARS platform [23] tested by a soldier

Figure 10 :

 10 Figure 10: MARS platform [22]

Figure 11 :

 11 Figure 11: Tinmith platform [43]

Figure 12 :

 12 Figure 12: 3D Greek temple viewed via ARCHEOGUIDE [14]

Figure 13 :Figure 14 :

 1314 Figure 13: Example of augmented construction view [3] 6.4 Entrainment and Game applications

Figure 15 :

 15 Figure 15: Vidente project [37]

Figure 16 :

 16 Figure 16: Raxenv in urban scenario: Conceptuel view.