Pierre Brosselard

Thierry Bouchet

Dominique Planson

Sigo Scharnholz

Gontran Pâques

Mihai Lazar

Christophe Raynaud

Jean-Pierre Chante

Emil Spahn

A 3.5 kV thyristor in 4H-SiC with a JTE periphery

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

A 3.5 kV thyristor in 4H-SiC with a JTE periphery

Pierre Brosselard a, b , Thierry Bouchet c , Dominique Planson b , Sigo Scharnholz a , Gontran Pâques a , Mihai Lazar b , Christophe Raynaud b , Jean-Pierre Chante b , and Emil Spahn a a) French-German Research Institute of Saint Louis (ISL), B.P. 34, F-68301 Saint Louis cédex, France b) Centre de Génie Electrique de Lyon (CEGELY) INSA-Lyon, UMR 5005 CNRS, Bât. Léonard de Vinci, 20 av. Albert Einstein, F-69621 Villeurbanne, France c) IBS Z.I. Peynier-Rousset Rue Gaston Imbert Prolongée 13790 Peynier France corresponding author: pierre.brosselard@insa-lyon.fr Today, at ISL, there are pulsed power applications under investigation which handle electric energies up to 10 MJ using switching elements as sparc gaps or high power semiconductors based on silicon [1]. Since silicon based technology is reaching its physical limits concerning blocking and power handling capability, GTO thyristors based on SiC are under investigated for compact future pulsed power systems we are investigating [2]. Reaching breakdown voltage of 19 kV [3], the potential of SiC based devices appears very interesting.

According to preparatory device simulations using the finite element code MEDICI TM , the developed GTO-thyristors should be able to block voltages up to 6 kV. The models and parameters used by the simulator are described by Brosselard and al. in [4]. Simulations with different fixed charge densities at the oxide/SiC interface have been realised with a 1.5 µm oxide thickness. The results are shown in figure 2. A negative charge induces a positive charge in SiC which limits the extension of the space charge so that the breakover voltage decreases.

The n-type 4H-SiC wafer material was purchased from Cree Research TM , including a P + PNP + (from the wafer up to the top) epitaxial layer structure (figure 1). The process sequence has been divided in five photolithographic levels. The realization has begun with two different dry etchings, the first allowing to take the gate contact, the second one necessary for the mesa protection. A nitrogen implantation followed by 1700°C/30mn annealing were needed to realise the JTE. After an oxide deposition to passivate the thyristor structure, a nickel metallisation has been realized.

The static characterisations used to evaluate the breakover voltage were performed using a 12.5 kV high voltage source. The maximum breakover voltage equals to 3.5 kV, as illustrated in figure 2. The previous simulations show that the fixed charge density is negative and included between 2 and 3x10 12 cm -2 . Other samples must be produced and characterised to extract the charge density from C(V) measurements.

All devices on the sample have been electrically characterised with a 1100 V Keithley K2410. Figure 3 shows the location of the breakover voltage on the wafer surface. The dark ones hold a voltage up to 1kV. This figure clearly shows that the repartition of the better ones, is not uniform. We can see that most of them are placed in the upper left side of the sample, whereas nearly no thyristor, able to hold a higher voltage, are placed in the left border region. That illustrates the non uniform distribution of wafer defects []. We can also see through figure 4 that the device area has a large impact on the breakover voltage. Indeed only the smallest devices (0,2 mm²) were able to hold voltages above 3000 V.

We conclude, that the results are encouraging for the use of the thyristors in power applications. The turn-on characterisations are in progress and will be described is the final paper.

figure 1 :

 1 figure 1 : Thyristor description with JTE protection


Schoefs Franck
email: schoefs@physique.univ-nantes.fr

Rguig Mustapha

RISK BASED INSPECTION OF JACKETS SUBMITED TO THROUGH-WALL CRACKS

The actual challenge for the requalification of existing offshore structures through a rational process of reassessment leads to state the importance of Risk Based Inspection methodology. This paper points out the inspection results modelling and their contribution to decision aid tools. The study of the impact of through cracks on structural integrity of jacket platforms is still a challenge. The detection of large cracks is first addressed. In order to minimize inspections and maintenance costs, all the available data from inspection results, such as probability of detection and probability of false alarm, must be addressed, as well as the probability of crack presence. This can be achieved by the use of the decision theory. These capabilities of Non Destructive Testing give a first input for the risk study. A cost function is suggested to introduce this modelling into a risk analysis and is devoted to help rank the NDT tools. The case of large through-wall cracks is specifically addressed.

INTRODUCTION

Fixed offshore platforms such as jackets offer harsh environmental conditions for in situ inspections and are submitted to extreme events. Most of them currently reach or will reach soon their initial design lifetime and need structural integrity assessment from an economical point of view (Moan 2000). Regarding the fatigue effects on jackets platforms, cracks at the weld connection in tubular nodes are propagating. These are surface cracks, which reach the wall thickness with time: this is the through cracks. Then the tube detaches from the node; this is not satisfactory from design criteria point of view where the thickness defines the limit state of crack depth. Through crack is then considered as the critical size. It means that beyond this crack depth, repair is advocated whatever the consequence on the structural behaviour. To avoid such damages, the structure is managed using Inspection, Maintenance and Repair plans ([1]- [START_REF] Faber | Aspects of inspection planningquality[END_REF]). In order to optimize costs induced by these plans, research have been carried out: optimization on inspection planning ( [START_REF] Goyet | Optimal inspection and repair planning: case studies using IMREL software[END_REF], [START_REF] Faber | Risk based inspection -the framework[END_REF]), Risk-Based Maintenance/Inspections ( [START_REF] Goyet | Integration Approach for RBI of offshore installations[END_REF], [START_REF] Tanaka | Cost based optimal relation between inspection time and assessment time for random fatigue crack growth[END_REF]- [START_REF] Onoufriou | Reliability based inspection planning of offshore structures[END_REF]) and Reliability Centered Maintenance ( [START_REF] Reicks | USCGC Healy (WAGB 20) -A case study for implementing Reliability Centered Maintenance[END_REF]). They provide suitable models of inspections results in order to perform mechanical and fatigue computing as well as reliability updating. The definition of probability updating is devoted to this aim. Several challenges which may conflict have to be worked out : minimize inspections, failure and maintenance costs, minimize uncertainty on inspection results, obtain the most complete information on structural integrity. This can be achieved by the mean of mechanical criteria for the ranking of critical nodes and decision aid-tool for the risk based ranking of inspection capability. Moreover, to analyze evolution with time, one must provide probabilistic models of crack growth and methods for inspections updating. This questioning is not treated in this paper. First, a risk based analysis of inspection performance for offshore structures is proposed. Detection of cracks is of great importance and a miss, or a spurious indication can lead to maintenance costs overrun. In order to minimize inspections and maintain costs, all the available data from inspections results such as probability of detection, probability of false alarm must be used, as well as the probability of crack presence. This can be achieved by the use of the decision theory. In particular the transfer of detection probabilistic information obtained from tests in laboratory to decision aidtool after in-situ inspection is presented. In a second part, several approaches of NDT tool ranking are compared. First, decisions based on PoD information are presented. Then R.O.C curve is introduced as graphical support which contains all the probabilistic information of inspection result. The conditioning on events "detection of crack" and "presence of crack" is inversed in view to provide decision aidtools from in-situ inspection results. It leads to introduce a new probability: the probability of crack presence. Theses concepts are finally illustrated with numerical examples.

PROBABILISTIC MODELING OF CRACK DETECTION

The detection of cracks in steel offshore jacket structures is a great challenge, first, by the cost induced. Through crack detection is cheaper using F.M.D. (Flooded Member Detection) techniques, than classic ones such as M.P.I, A.C.F.M or Ultrasonic. However the kind of information given is quite different: F.M.D is only able to show whether or not a through crack has been detected whereas other techniques can detect smaller cracks in order to follow or repair them. These cracks do not have the same impact from a structural integrity point of view. The harsh environment and bad conditions of underwater inspections lead to lower detection performances than in laboratory tests. From the detection point of view, it means that during an inspection campaign, crack detection does not imply crack presence. This is known as the probability of false alarm PFA or false indication, whereas the case of detection of an existing crack is referred to the probability of detection PoD. If tests are performed in laboratory under ideal conditions, these grandeur are closely linked to those obtained with the signal theory. Then the definition of PoD can be expressed in the following form : PoD(a) = P(a > a d ) [START_REF] Goyet | Integration Approach for RBI of offshore installations[END_REF] Where a d is the detectable crack size, under which it is assumed that no detection is done and a the crack size. More the crack size is great, more the PoD is close to 1. Figure 1 illustrates the PoD (lab) and the PFA (lab) caculated for a low signal/noise ratio of about 2 and a specific detection threshold at 8. Noise and signal-noise are, for the illustration, supposed to be normally distributed. Negative crack size are due to the noise. The usual definition of PoD is the probability to detect a crack in a crack class range (see below). To obtain a global sight of the NDT tool performance, the PoD (lab) can be plotted as a function of the PFA (lab) . This is the Receiver Operating Characteristic (abbreviated R.O.C) curve which is the intrinsic tool performance for each crack size (see figure 2). Two R.O.C curves are plotted one for a high signal/noise ratio (about 5 for NDT 2) and the second for a lowest signal/noise ratio (about 2 for NDT 1). It is to notice that uncertainties on PoD and PFA curve assessment have to be introduced. In fact PoD theoretical curve should be continuous, monotonically increasing as it is a probability distribution function. However, the experimental PoD curve is discrete and not necessarily monotonically increasing. Each point is representative of a crack class range and the probability of detection in that class is the number of actual detected cracks divided by the total number of existing cracks in that class. As a consequence, it is not necessarily an increasing function. In particular, such a curve is representative of complex tubular joint inspections, with inclined braces on the chord and no distinction of shape or geometrical characteristics. As some area of such nodes are less easy to reach for the diver or the R.O.V, the inspection performance decreases: the typology and accessibility of the joint have a great influence on PoD. This shows that the first model (equation ( 1)) is not satisfactory for inspection data use without evaluation of the uncertainty on PoD. Several authors ( [START_REF] Barnouin | Underwater inspection reliability trials foroffshore structures[END_REF]) propose a reliable PoD curve or a lower bound estimate of population PoD at 95 % confidence ( [START_REF] Rudlin | Reliability of inspection for fatigue cracks in offshore structures[END_REF]). Let us note X the random variable that takes the value X = 1 in case of crack presence, X = 0 otherwise. To inspect is to make a decision on the state of the inspected area. Thus a detection is modeled by the random decision function d(.) on the state X of the inspected area: in case of crack detection d(X) = 1 and otherwise d(X) = 0. Finally, the probability of detection and the probability of false alarm can be modelled as follows:

PoD(X) = P(d(X) = 1 | X = 1) (2) PFA(X) = P(d(X) = 1 | X = 0) (3)
Considering now in situ inspections, the complex environment and harsh conditions of the divers make generally not optimal the condition of use of N.D.T tools. This leads to lower performances than expected. There is no way actually to provide a function able to deduce the new R.O.C. curve from the previous obtained in laboratory. The only way is to assess directly these characteristics in realistic in-situ conditions. This was made within the ICON project [START_REF] Barnouin | Underwater inspection reliability trials foroffshore structures[END_REF]. Moreover the aim of this project was to give an unified overview of several tool performances in the same realistic conditions. The decision scheme from inspection results is quite different as this one. In fact the conditioning of events is inversed. The problem is to take a decision on the structural state from an inspection result and not to decide conditionally to a real structural state as it is presented in equations ( 2) and (3) (see figure 3).

The events E i related to crack presence/absence conditionally to crack detection/non-detection are:

-E 1 : no presence of crack conditionally to no crack detection, P(E 1 ) = P(X = 0 | d(X) = 0) P(E 1 ) + P(E 3 ) = 1 ; P(E 2 ) + P(E 4 ) = 1 [START_REF] Tanaka | Cost based optimal relation between inspection time and assessment time for random fatigue crack growth[END_REF] These probabilities are then expressed ( [START_REF] Schoefs | Risk-Based Inspection on in-service offshore platforms : a probabilistic approach of the crack detection and global integrity criteria[END_REF], [START_REF] Rouhan | Structural integrity evaluation of existing offshore platforms based on inspection data[END_REF]) as function of POD (eq. 2), PFA (eq. 3) and PCP, the Probability of Crack Presence : where 1 is an unknown function and 2 is described by the non-linear equations ( 9)- [START_REF] Reicks | USCGC Healy (WAGB 20) -A case study for implementing Reliability Centered Maintenance[END_REF]. It leads to conclude that the usual introduction of PoD as single parameter in the decision scheme is equivalent to consider that PoD = P(d(X) = 1). It implies that two conditions are respected {PCP = 1 ; PFA = 0}.

)) X ( PCP 1 )).( X ( PFA 1 ( ) X ( PCP )). X ( PoD 1 ( )) X ( PCP 1 )).( X ( PFA 1 ( P(E1) - - + - - - = (9) )) X ( PCP 1 ).( X ( PFA ) X ( PCP ). X ( PoD )) X ( PCP 1 ).( X ( PFA P(E2) - + - = (10) )) X ( PCP 1 )).( X ( PFA 1 ( ) X ( PCP )). X ( PoD 1 ( ) X ( PCP )). X ( PoD 1 ( P(E3) - - + - - = (11) )) X ( PCP 1 ).( X ( PFA ) X ( PCP ). X ( PoD ) X ( PCP ). X ( PoD P(E4) - + = (12)
Parametric studies can then be performed when modifying the range of PoD, PFA and PCP [START_REF] Rouhan | Probabilistic modelling of inspections results for offshore structures[END_REF]. In particular, the effect of PFA can be addressed. Thus the transfer of information during inspection can be modelled with successive steps from inspector in laboratory condition to decision-maker. An illustration is given on figure 4.

Inspection results

Area to be inspected good detection bad detection

Laboratory conditions

PoD (lab) P FA (lab) .

In situ conditions

Diver + NDT Performance PoD ; PFA

Structural History PCP

Inspection planning

Decision-maker P(E 1 ) ; P(E 2 ) P(E 3 ) ; P(E 4 )

NDT TOOL RANKING BASED ON COST FUNCTION

Risk based Inspection is devoted to provide to the decisionmaker powerful information to decide which NDT to use, in which part of the structure and when. A complete R.B.I should introduce stochastic model for taking into account the loss of safety with time [START_REF] Madsen | Stochastic modeling of fatigue crack growth and inspection[END_REF]. As only detection are considered here, the problem is to select N.D.T tool relatively to his performance for a specific application. Costs of inspection and of maintenance policy have to be introduced as well as the so called cost of failure. With respect to the modelling presented on figure 4 several level of decision aid-tools based on NDT tool performance are available :

-PoD based decision aid-tool :

The ranking of techniques is available for a specific range of crack size. A global technique capability upon the total range of crack size is deduced from the area under the PoD curve. A ponderation can be introduced when multipliing each PoD with the PCP of the crack range. The risk is expressed by a summation of the expectation for each cost. When introducing a systematic repair strategy (repair if detected) the costs to introduce is the cost of inspection C i , of repair C r and of failure C f . . The probability of failure Pf is deduced from the consequence of probability of no-crack detection (1 -PoD). Moreover, the confidence level on the technique performance can be introduced with the reliability curve. Several conclusions can be deduced in particular about the plot of new P.o.D curves based on sub-populations (typology and depth) or about the modeling of a d as a random variable.

-PoD-PFA based decision aid-tool :

Here the couple (PoD,PFA) provides the complete probabilistic information on the tool performance. In fact PoD and PFA are not complementary. The decision is based on the position of the "performance points" on R.O.C curve. An over-cost of repair should be introduced to take into account the displacement of the diver and material at the crack location in case of false alarm. The major difficulty resides in P.F.A assessment based on suitable data.

-P(Ei) based decision aid-tool :

The probabilities ( 4)-( 7) describe surfaces varying in level with PoD and PFA for specific values of PCP. For a tool which is used in several inspection context (value of a d ), the projection of its R.O.C curve on this surface give a sight of sensitivity to the conditions of inspection. These curves are plotted for the non-complementary probability P(E2) and P(E3) on figures 6 and 7 for two values of PCP (named γ). These values will be commented below in the paper. [START_REF] Rouhan | Probabilistic modelling of inspections results for offshore structures[END_REF]. For illustration, the repair strategy is based on the following policy:

Let us consider now an utility function based on cost considerations. The expectation of a cost function E(C) is introduced to this aim

-no crack detection leads to no action, -crack detection leads to repair. Considering this particular policy, expressions of E(C) become:

-In case of no crack detection:

) P(E C C E(C) 3 1 1 + = (15)
-In case of crack detection :

) P(E C ) P(E C E(C) 2 2 4 4 + = (16)
In this case, the cost overrun is defined by :

) P(E C E(C) 2 2 = (17)
Where C 1 is the cost of inspection, C 4 is the cost including repair in case of crack presence, C 2 is the cost of inspection and repair and i C is the cost overrun resulting from bad decisions due to bad inspection results, here the financial penalty due to a non-detected crack (cost of failure). The different relative costs of failure, inspection and repair considered here for illustration are presented in table 1. The knowledge of these data allows a comparison of the two techniques with performances expressed with the couple (PoD, PFA). Four performances are considered : case (a) and (b) for NDT 1 and case (c) and (d) for NDT 2 (see figure 2 andtable 2). The distribution of large cracks is assumed to be exponential with parameter λ = 10 cm and the class range δ = 5 cm [START_REF] Moan | In-Service Observations of Cracks In North Sea Jackets. A Study on Initial Crack Depth and POD values[END_REF]. Among the two classes [0;5] and [40;45] in centimeter, the values of γ are : a ∈ [0;5] ⇒ γ = 0,4 ; a ∈ [40;45] ⇒ γ = 0,01 These two classes are representative respectively of small and large through cracks. R.O.C are supposed to be constant whatever γ. The expected costs overrun in case of detection (eq. 17) and the expected cost in case of non-detection are reported in table 2. The best technique is the one that minimize costs both in case of detection and in case of non-detection. Second, consider now each inspection. The more the probability of crack presence is high, the less the PFA plays an important role. That is to say that the cost corresponding to non-detection event is higher in case of large cracks and moreover in case of bad situation of inspection (high PFA).

Third, for low probability of crack presence, the cost overrun in case of crack detection becomes higher and play a dominant role. This effect is more significant with high PFA values. Thus the percentage of the cost over-run in case of detection varies from 3 to 77 % in case (b). More analyses can be deduced from a complete parametric study [START_REF] Rouhan | Probabilistic modelling of inspections results for offshore structures[END_REF].

It can be difficult to determine precisely the performance point in terms of couple (PoD;PFA). Sensitivity studies can be used to analyse the effect of this uncertainty on the risk. Another way is to consider the mean of probabilities P(Ei) among the points of the ROC curve. Their mathematical expressions are given in eq. 17. [START_REF] Rouhan | Probabilistic modelling of inspections results for offshore structures[END_REF] where l ROC denotes the curvilinear abscissa on the projection of ROC curve on the surface P(Ei), L ROC the length of the ROC curve in the (PoD;PFA) plane and lOC i) E ( P the value of P(Ei) at this abscissa. The corresponding area are drown on figure 8 for P(E 2 ) and NDT 1 and 2. Obviously, these probabilities on the ROC curve are complementary and : Figure 9 presents the same area for P(E 3 ) where the discrepancy between the areas obtained for the two NDT are not so large. The mean expected cost on the ROC curve has the convenient form (deduced from eq. 16 and 17):

∫ = ROC ROC lOC ROC ROC l d i) E ( P L 1 m(P(Ei))
-In case of no crack detection: It allows a global overview of the NDT performance. It leads to conclude that these tools are equivalent in case of large cracks and that the cost in case of non detection is globally higher for small cracks. NDT 1 appears to lead to higher costover-run for this range of cracks. This analysis must be completed by introducing a difference between the costs of the both inspections. If NDT 1 keeps the same value of 0.002 and NDT 2 which has "a better ROC" a cost of 0.004, then the last columns of table 3 is modified and presented in table 4.

The analysis of cots for small cracks detection is particularly interesting under this hypothesis. In fact, tools 1 and 2 lead to the same cost over-run and are similar in terms of risk analysis. This conclusion is of interest beacause of the price of FMD (Flooded Member Detection) which is low with PFA which are low too. More studies should investigate the assessment of PFA.

CONCLUSION

The use of NDT when inspecting existing structures leads to state the importance of a probabilistic modeling. In this paper the detection is pointed out with specific definition of the PoD and PFA. An cost function based on a mean performance of NDT tool is proposed and shown to be very suitable for ranking purposes whether of the inspection performance tools. This decision aid-tool can be introduced as an alternative to other classical aid-tools especially in the cases where the uncertainty on ROC points position is significant. It leads to conclude that better PoD cuves don't lead to better NDT tool in terms of risk analysis. PFA acts as important parameter.

Figure 1 .

 1 Figure 1. THERORETICAL DEFINITIONS OF POD AND PFA.

  (lab) PFA (lab) noise pdf signal+noise pdf crack size [mm]

Figure 2 .

 2 Figure 2. EXAMPLE OF R.O.C. CURVES FOR TWO N.D.T TOOLS AND ONE CRACK SIZE OF 10 MM MEAN.

7 )

 7 no presence of crack conditionally to crack detection, P(E 2 ) = P(X = 0 | d(X) = 1) (5) -E 3 : presence of crack conditionally to no crack detection, P(E 3 ) = P(X = 1 | d(X) = 0) (6) -E 4 : presence of crack conditionally to crack detection, P(E 4 ) = P(X = 1 | d(X) = 1) (Some events are complementary and :

Figure 3 .

 3 Figure 3. CASES OF CRACK PRESENCE/ABSENCE CONDITIONALLY TO DETECTION/NON DETECTION.

Figure 4 .

 4 Figure 4. TRANSFER OF DETECTION DATA IN THE DECISION CHAIN.

Figure 5 .

 5 Figure 5. EFFECT OF P.o.D AS UNIQUE DECISION AID-TOOL.

Figure 6 .Figure 7 .

 67 Figure 6. EVOLUTION OF P(E 2 ) IN THE (PoD ; PFA) plane FOR PCP VALUES OF 0.1 AND 0.5.

Here, C 1 =

 1 C insp . ; 1 C = C insp. + C failure ; C 2 = C insp. )nd 0.002 25 0.002 10

Figure 8 .

 8 Figure 8. INTEGRATED AREA OF P(E2) FOR NDT 1 (upon) and 2 (below).

Figure 9 .

 9 Figure 9. INTEGRATED AREA OF P(E3) FOR NDT 1 (upon) and 2 (below).

Table 2 .

 2 EXPECTED COST DEPENDING ON PoD, PFA, γ First, consider the classes of crack size (columns of the table). The third inspection case where the PFA is lower and the PoD greater offers the best compromise in terms of cost whatever γ. It should be emphasis that having the best PoD do not let to the best NDT tool: PFA affects global performances. Hence, case (d) is not the best choice (high PoD but High PFA too).

Table 3

 3 presents results obtained for L ROC , length of ROC curve in the plane (PFA;PoD), m(PE3), m(PE2),

	d E(C) and

Table 3 .

 3 EXPECTED MEAN COST OVER ROC

Table 4 .

 4 EXPECTED MEAN COST OVER ROC

ACKNOWLEDGMENTS

The work reported here had been carried out in collaboration with IFREMER. Its support is gratefully acknowledged.


Artificial Finger for Textile Fabrics Brigitte Camillieri, Marie-Ange Bueno Laboratoire de Physique et Mécanique Textiles, Ecole Nationale Supérieure d'Ingénieurs Sud Alsace, University Haute Alsace marie-ange.bueno@uha.fr

STATEMENT OF PURPOSE

The goal is to design an artificial finger to replace the human finger during friction test on textile surfaces. In this study, four different kinds of textile fabrics are chosen because they represent a wide range of textile fabrics for clothing industry. Then alternative friction tests are carried out by humans on these fabrics. The evolution of the coefficient of friction during a displacement cycle are compared for the four fabrics. This helped to select the fabric used to develop the artificial finger. Then the artificial finger is designed by mimicry by studying the influence of the main morphological and mechanical criteria of the finger (shape, texture, Young's modulus) on the friction signal. Finally, the developed artificial finger is validated by comparing the results obtained for the four kinds of fabrics with the artificial and human fingers.

APPROACH

The four investigated fabrics are: o A plain woven fabric with a low surface pile. o A knitted polar fleece with a pile over the two sides of the fabric. The bristles have variable lengths and their orientation is random on the fabric surface. o A knitted velvet. The bristles are parallel and of equal lengths (about 2 mm). o A knitted artificial fur which bristles are parallel and of equal lengths (about 5 mm).

For pile fabrics as velvet or fur, the pile has a preferred direction for laying down as with the fur of an animal. This preferred direction is called further along pile and the other against pile main direction.

Two reciprocating linear tribometers have been used to measure the friction: a tribometer for fabrics equipped with a slider (artificial finger) and a tactile tribometer where the slider is a human finger. In both cases, the fabric sample is fixed on an oscillating table and the normal and tangential forces are measured.

A preliminary study showed that the touch exploration of textiles is carried out under a pressure of about 3 kPa with a sliding nominal speed of 20 mm/s (Breugnot). For both tribometers, the chosen sliding distance is 50 mm. For human finger, the choices are: using the index of the dominant hand with a finger tilt angle of 25 relative to the horizontal (Bueno et al.). Several cycles are performed but only the second one is exploited.

In addition, each artificial finger has an apparent contact area of 1.5 cm 2 , which corresponds to the surface of the finger in contact when the pressure is 3 kPa (Breugnot). Consequently, the nominal normal force is 0.5 N.

RESULTS AND DISCUSSION

First study

The first study with a panel of seventeen people allowed to see each fabric presents a specific evolution of the coefficient of friction (COF). From Figure 1, because of the specific and complex COF evolution, the velvet has been chosen to develop the artificial finger.

Slider design

To design the slider used as an artificial finger for textile fabrics, the chosen approach is to study in a tribological point of view the influence of different characteristics of a finger: its rounded shape, the texture of the fingertips, i.e. the dermatoglyphics and its hardness. In that purpose, different shapes of slider have been investigated (Fig. 2) and two materials. The selected slider is made of silicone and presents a grooved sole and rounded leading edges and covered with a specific fabric. Figure 3 shows the evolution of the COF for this slider.

Validation of the artificial finger

For all the fabrics investigated, the curves obtained with a human finger or with the artificial finger have very similar appearance except in the transitory slopes. Therefore, qualitatively the artificial finger has the same behaviour as a real finger. Quantitatively the artificial finger gives a value of COF that is included in the range of COF obtained by our panel of subjects. The values obtained with the artificial finger are consistently lower than those obtained by the human fingers; an explanation of this difference will be given.

DISCUSSION

There are some differences between the curves obtained by the two experiments and especially the transitory slopes. This can be explained by a different displacement of real and artificial fingers relative to the fabric in both directions.

CONCLUSION

This study shows the influence of each morphological and mechanical human finger features on the friction behavior with textile fabrics: rounded shape, finger texture and hardness. The rounded shape of the finger is not really relevant in this case but the finger texture is fundamental. In fact, the results obtained with smooth and grooved sliders allow us to bring some conclusions:

 With a smooth sole, the friction cycle depends on the length of the slider as the pile at the ends of the sliding region is not stressed in both directions.  With a grooved sole, the pile is forced in both directions because the bristles interlock into the grooves.

The grooves, that is to say the dermatoglyphics, thus play an important role in the friction behavior of the finger against pile surfaces.

The influence of finger hardness has been evaluated with the help of rigid (made of aluminum) and soft (made of silicone) sliders. With the soft slider made in silicone, the adhesion is too important with textile fabrics. Therefore it has to be covered by a chosen material. A specific fabric has been identified.

The results obtained with the grooved rigid slider and the grooved soft slider covered with a suitable fabric are closed together in qualitative and quantitative points of view. However, the artificial finger made of silicone and covered with a structured fabric gives with velvet, a friction cycle closest to that obtained with a real finger, in particular as regards the transient due to changes of fabric direction. In addition, the coefficients of friction obtained with a real finger and with this slider are of the same order of magnitude.

The artificial finger developed gives good results qualitatively and quantitatively for all the different fabrics tested representing a wide range of textile fabrics.

Figure 1 :

 1 Figure 1: Typical evolution of the COF between a human finger and the four fabrics investigated a) plain woven fabric b) polar, c) velvet fabric and d) artificial fur. For velvet and artificial fur, the COF is positive along pile and negative against pile main direction.

Figure 2 :

 2 Figure 2: Different shapes of the slider used.

Figure 3 :

 3 Figure 3: Evolution of the COF on velvet obtained with the artificial finger selected.

  Breugnot, Cécile. "Contribution À La Caractérisation Mécanique Du Toucher Des Surfaces Textiles À Partir De Critères Neurosensoriels." PhD Thesis. Université de Haute Alsace, 2005. Bueno, Marie-Ange, et al. "Costam: Tool Design for a

Table I

 I

		gathers COF values obtained between the four
	investigated fabrics and the artificial finger or a human
	finger. For fabrics with oriented pile (velvet and fur fabrics),
	the COF along and against pile main direction are
	considered separately.
	Fabric	Artificial finger	Human fingers
		COF	COF	Panel size
	plain	0.34	0.43 0.12	40
	polar	0.59	0.70 ± 0.21	40
		COF along	COF against	COF along	COF against
	velvet 0.45	0.44	0.58 ± 0.23 0.61 ± 0.16 17
	fur	0.50	0.64	0.60 ± 0.13 0.85 ± 0.24 17

Table I :

 I Values of COF for the four fabrics obtained with the artificial finger and the human fingers.


Adel Elamri

Khmais Zdiri

Omar Harzallah
email: omar.harzallah@uha.fr.

Abdelaziz Lallam

Chapter PROGRESS IN POLYETHYLENE TEREPHTHALATE RECYCLING

Keywords: PET, recycling, blends, thermal and rheological

published or not. The documents may come

INTRODUCTION

The production of plastic materials in its various declinations comes mainly from petroleum. However, we know that the depletion of fossil resources is inevitable. The economy of the main material, in this case oil, becomes a goal in itself. This objective can be achieved both through the use of renewable materials, or recycling of materials, if operations are technically and economically feasible.

In this work, the treatment is focused on the technical aspect of thermomechanical recycling of PET waste from mineral water bottles. The thermomechanical recycling presents the advantage of being fast and cheap. In contrast, thermomechanical recycling leads to loss of important mechanical properties by thermal degradation mechanism. It is possible to overcome the loss of these properties, either by recycling under vacuum, which requires heavy and expensive equipment, or by the addition of virgin material to the primary material for recycling. In this chapter, it is the second procedure which is described.

VIRGIN PET

Virgin PET has come to be considered as one of the most important engineering polymers in the past two decades. It is regarded as an excellent material for many applications and is widely used for making liquid containers (bottles). It has excellent thermomechanical and chemical properties [START_REF] Zhang | The consumption and recycling collection system of PET bottles: A case study of Beijing, China[END_REF]. Many companies produce virgin PET giving it different trade names [START_REF] Westerhoff | Antimony leaching from polyethylene terephthalate (PET) plastic used for bottled drinking water[END_REF]. Some of the common trade names of commercially available PET are summarized in Table 1. Commercial PET has a wide range of intrinsic viscosity [η] that varies from 0.45 to 1.2 dL.g -1 with a polydispersity index generally equal to 2. The PET repeating unit is shown in Figure 1.

Table 1. Trade names of common PET and their manufacturer

PET Synthesis

PET production process involves two different starting reactions. The first starting reaction is an esterification reaction (Figure 2a) where terephthalic acid (TPA) reacts with ethylene glycol (EG) at a temperature between 240°C and 260°C and a pressure between 300 and 500 kPa. The second reaction is a trans-esterification reaction (Figure 2b) where dimethyl terephthalate (DMT) reacts with EG at 180 -210°C and 100 kPa [START_REF] El Toufaili | PhD Thesis: Catalytic and mechanistic studies of polyethylene terephthalate synthesis[END_REF]. Trans-esterification is the much preferred process due to easier purification. The output of both these processes is bis(hydroxyethyl) terephthalate (BHET). The pre-polymerisation step follows, in which BHET is polymerised to a degree of polymerization (DP) of up to 30 (Figure 2c). Prepolymerisation reactions conditions are 250-280°C and 2-3 kPa. The third stage is the polycondensation process where the DP is further increased to 100. The polycondensation process conditions are 280-290°C and 50-100 Pa.

Up to this stage, PET is suitable for applications that do not require high molecular weight (Mw) or intrinsic viscosity [η] such as fibers and sheets. A solid state polymerization (SSP) step might be required when a high Mw PET is produced. SSP is used to increase the DP to 150, and also increasing Mw. SSP operating conditions are 200-240°C at 100 kPa and 5-25 h [START_REF] Ravindranath | Polyethylene terephthalate-chemistry, thermodynamics and transport properties[END_REF]. Bottle grade PET that has an [η] of 0.7-0.85 dL.g -1 is normally produced by SSP at 210°C for around 15-20 h [START_REF] Al-Ghatta | Production of foam grade PET[END_REF][START_REF] Al-Ghatta | New technology for solid-state polymerization of polymers: polyethylene terephthalate solid-state polyaddition[END_REF]. Some virgin PET manufacturers have tended in recent years to produce PET co-polymer; such as isophthalic acid modified PET, rather than homopolymer PET. Bottles are then made from co-polymer PET because of its lower crystallinity, improved ductility, better process ability and better clarity [START_REF] Scheirs | Polymer recycling: Science, Technology and Applications[END_REF]. The PET chain is considered to be highly stiff above the glass transition temperature (Tg) unlike many other polymers. The low flexibility of the PET chain is a result of the nature of the short ethylene group and the presence of the p-phenylene group. This chain inflexibility significantly affects PET structure-related property. The standard physical and chemical properties of commercial PET are shown in Table 2.

PET Applications

PET is used broadly in products such as bottles, electrical and electronic instruments, in different sectors of automotive industry, housewares, lighting products, power tools and material handling equipment. PET films and fibers are the oldest applications of PET. Films are produced by biaxial orientation through heat and drawing. PET films are used in photographic applications, x-ray sheets and in food packaging. PET films are also reported to be used in electrical and dielectrics applications due to the severe restriction of the electric dipole orientation at room temperature that is well below the transition temperature [START_REF] Brydson | Plastics materials[END_REF]. PET fibers are another important application of PET and are produced by forcing molten PET through small holes in a die. Fiber strength is achieved by applying tension to align the chains through uniaxial stretching. Bottle production requires the use of PET with high molecular weight. This is explained in part by the manufacturing process (bi-stretching, blow -extrusion), and by the need to obtain sufficient barrier properties for this application, often related to beverage packaging. PET chains with high molecular weight will allow a 'mesh' at most ends of the chains after bi-stretching which will limit the diffusion of gaseous molecules from the outside to the content and vice versa [START_REF] Goshel | Thermally stimulated structural changes in highly oriented glassy PET[END_REF].

Virgin PET is produced at different specifications because different applications require various properties. Examples of PET's [η] with respect to the required application are shown in Table 3.

Thermal and Crystallization Behaviors

Commercial PET has a melting temperature (Tm) between 255 and 265°C and for more crystalline PET it is situated between 255 and 270°C [START_REF] Ravindranath | Polyethylene terephthalate-chemistry, thermodynamics and transport properties[END_REF]. The Tg of virgin PET varies between 67 and 140 C. The thermal transitions and crystallization of virgin PET with a focus on reversing crystallization and melting have been analyzed by several researchers [START_REF] Kong | Multiple melting behavior of poly(ethylene terephthalate)[END_REF][START_REF] Wunderlich | Reversible crystallization and the rigid-amorphous phase in semicrystalline macromolecules[END_REF]. An interesting phenomenon was reported in which the virgin PET experiences multiple endothermic transitions during thermal analysis [START_REF] Avila-Orta | On the nature of multiple melting in poly(ethylene terephthalate) (PET) and its copolymers with cyclohexylene dimethylene terephthalate (PET/CT)[END_REF]. It was reported that this phenomenon is attributable to morphological and structural re-organization. As the temperature increases, better crystal structures are achieved because of the re-organization of the less perfect crystals.

Virgin PET is well known for having a very slow crystallization rate. The highest crystallization rate takes place between 170°C and 190°C. Cooling PET rapidly from the melt to a temperature below Tg can produce an amorphous, transparent PET. Semi-crystalline PET can be obtained by heating the solid amorphous PET to a temperature above Tg where 30% crystallinity can be achieved [START_REF] Charrier | Polymeric materials and processing. Plastics, elastomers and composites[END_REF].

The rate of crystallization of virgin PET depends greatly on temperature and reaches its maximum at a temperature of 150-180°C. The rate of crystallization also depends on other factors such as Mw, the presence of nucleating agents, the degree of chain orientation, the nature of the polymerization catalyst used in the original production of PET and the thermal history.

RECYCLED PET

Due to its increasing consumption and non-biodegradability, PET waste disposal has created serious environmental and economic concerns. Thus, management of PET waste has become an important social issue. In view of the increasing environmental awareness in the society, recycling remains the most viable option for the treatment of waste PET. On the other hand, as the price of virgin PET remains stable, new and cheaper technologies for recycling PET give an added value to the PET recycling industry by providing industry with relatively cheaper PET.

Many researchers reported that in order to achieve successful PET recycling, PET flakes should meet certain minimum requirements [START_REF] Pawlak | Characterization of scrap poly(ethylene terephthalate)[END_REF][START_REF] Giannotta | Processing effects on poly(ethylene terephthalate) from bottle scraps[END_REF]. Examples of the minimum requirements for the post consumer PET (POSTC-PET ) flakes are summarized in Table 4. The major factor affecting the suitability of POSTC-PET flake for recycling is the level and nature of contaminants present in the flakes.

Table 4. Minimum requirements for POSTC-PET flakes to be reprocessed

Recycled PET Contamination

Contamination of POSTC-PET is the major cause of deterioration of its physical and chemical properties during re-processing. Minimizing the amount of these contaminants leads to better quality of recycled PET. POSTC-PET is contaminated with many substances such as:

Acid Generating Contaminants

The most harmful acids to the POSTC-PET recycling process are acetic acid, which is produced by poly(vinyl acetate) closures degradation, rosin acid and abietic acid that are produced by adhesives and hydrochloric acid that is produced by PVC. The acids act as catalysts for the chain scission reactions during POSTC-PET melt processing [START_REF] Fann | Processing Characteristics of recycled PET as melt spun fibers[END_REF]. Paci and La Mantia [START_REF] Paci | Influence of small amounts of polyvinylchloride on the recycling of polyethylene terephthalate[END_REF] investigated the influence of small amount of PVC during the melt processing of POSTC-PET. They reported that the presence of PVC (as little as 100 ppm) would increase POSTC-PET chain scission due to the catalytic effect of hydrogen chloride evolving during the degradation of PVC. The presence of PVC also results in discoloration of POSTC-PET during processing.

Water

Water reduces Mw during POSTC-PET recycling through a hydrolysis reaction. Moisture contamination should be below 0.02% to avoid the Mw reduction. Most water content comes from the flake washing process but can be reduced substantially by proper drying.

Coloring Contaminants

Fragments of colored bottles and printed ink labels cause undesirable colors during processing. Enhancement of sorting and washing processes in bottle recycling may reduce coloring contaminants.

Acetaldehyde

Acetaldehyde is present in virgin PET and POSTC-PET. It is a by-product of PET degradation reactions. The migration of acetaldehyde into food products from PET containers was a major concern in the early stages of developing the POSTC-PET recycling process. The high volatility of acetaldehyde means that it can be minimized by processing under vacuum or by drying. Stabilizers such as (4-aminobenzoic acid, diphenylamine and 4,5dihydroxybenzoic acid) are added to PET in order to minimize the amount of the generated acetaldehyde [START_REF] Villain | Thermal degradation of polyethylene terephthalate: study of polymer stabilization[END_REF].

Other Contaminants

The public use PET bottles for storing other substances such as detergents, fuel, pesticides, etc... The remains of these substances could be a health hazard if traces of these substances remain after POSTC-PET recycling [START_REF] Demertzis | Studies on the development of a quick inertness test procedure for multi-use PET containers-sorption behavior of bottle wall strips[END_REF]. The increase of people's awareness of the danger of such practices to public health has minimized the amount of these contaminants significantly.

Recycling Methods

PET recycling has been one of the most successful and widespread among polymer recycling. PET recycling methods can be categorized into three groups namely primary, secondary and tertiary recycling. There is also a so-called 'zero-order' recycling technique, which involves the direct reuse of a PET waste material [START_REF] Hopewell | Plastics recycling: challenges and Opportunities[END_REF].

Zero-order Recycling

This technique consists in washing of PET bottles in hot water with detergents following the similar procedure of that practiced for glass bottles. The collected bottles are first subjected to hot washing with detergents and then to a control by sensors that detect and eject bottles containing volatile or liquid pollutants. The main disadvantage of this method is that it does not allow to effectively eliminate any contaminants absorbed by the PET [START_REF] Romão | Poly (ethylene terephthalate), PET: A review on the synthesis processes, degradation mechanisms and its recycling[END_REF].

Primary Recycling

Primary recycling, also known as mechanical recycling, was applied for the first time in the 1970s. It involves separating the polymer from its contaminants and reprocessing it to granules via mechanical means. Mechanical recycling steps include sorting and separation of wastes, removal of contaminants, reduction of size by crushing and grinding, extrusion by heat, and reforming. The more complex and contaminated the waste is, the more difficult it is to recycle mechanically. Among the main drawbacks of primary recycling are the heterogeneity of the solid waste, and the degradation of the product properties each time it is recycled. Mechanical recycling of PET includes 4 phases (Figure 3).

Secondary Recycling

Secondary recycling, more commonly known as chemical recycling, involves the transformation of the PET polymer chain. Usually by means of solvolytic chain cleavage, this process can either be a total depolymerization back to its monomers or a partial depolymerization to its oligomers and other industrial chemicals. Since PET is a polyester with functional ester groups, it can be cleaved by some reagents such as water, alcohols, acids, glycols, and amines. Also, PET is formed through a reversible polycondensation reaction, so it can be transformed back to its monomer or oligomer units by pushing the reaction to the opposite direction through the addition of a condensation product. These low molecular products can then be purified and reused as raw materials to produce high quality chemical products [START_REF] Carta | Chemical recycling of poly(ethylene terephthalte) (pet) by hydrolysis and glycolyis[END_REF]. Among the recycling methods, chemical recycling is the most established and the only one acceptable according to the principles of 'sustainable development', defined as development that meets the needs of the present generation without compromising the ability of future generations to meet their needs, because it leads to the formation of the raw materials (monomers) from which the polymer is originally made [START_REF] Karayannidis | Chemical Recycling of Poly(ethylene terephthalate)[END_REF].

As shown in Figure 4, there are three main methods in PET chemical recycling depending on the added hydroxyl bearing molecule: glycol for gylcolysis, methanol for methanolysis, and water for hydrolysis. Other methods include aminolysis and ammonolysis. Over the past five decades, since the start of PET chemical recycling research numerous researches have been done in order to fully understand the chemical pathways of the depolymerization methods, and improve desired products yield from these methods.

Tertiary Recycling

Tertiary recycling represents the recovery of energy content from the plastic waste by incineration. When the collection, sorting and separation of plastics waste are difficult or economically not viable, or the waste is toxic and hazardous to handle, the best waste management option is incineration to recover the chemical energy stored in plastics waste in the form of thermal energy. However, it is thought to be ecologically unacceptable due to potential health risks from the airborne toxic substances.

Increasing PET Intrinsic Viscosity

During the melt processing of recycled PET at high temperatures, R-PET undergoes a series of thermal and hydrolytic degradation reactions in the presence of water and contaminants, such as adhesives, poly(vinyl chloride) (PVC), etc… This leads to the decrease in intrinsic viscosity (η) by formation of low molecular mass PET with a significant increase of carboxyl and hydroxyl end groups. Oligomers and volatile compounds are produced during PET processing above its melting temperature, which contribute to the depression of Mw [START_REF] Freire | Thermal stability of polyethylene terephthalate (PET): oligomer distribution and formation of volatiles[END_REF]. Scheirs [START_REF] Scheirs | Polymer recycling: Science, Technology and Applications[END_REF] reported that cyclic and linear oligomeric compounds are formed during the reprocessing of PET as a result of thermal degradation reactions. He also showed that the oligomers content increased from 0.9 wt.% in virgin PET to 1.8-3.0 wt.% in R-PET. These products also serve as a catalyst to increase the rate of degradation reactions. Furthermore, as some of the degradation products have carboxyl ends, this increases the overall PET carboxyl content which leads to a decrease in PET thermal stability.

Overcoming the reduction of R-PET's Mw or [η] has been the objective of many investigations [START_REF] Torres | Chemical modification of virgin and recycled PET by adding of chain extenders during processing[END_REF][START_REF] Itim | Effect of multiple extrusions and influence of PP contamination on the thermal characteristics of bottle grade recycled PET[END_REF]. Different methods and processes have been reported over the past two decades to restore or maintain R-PET properties during processing, which are outlined below.

Reprocessing Under Vacuum

The most recent PET recycling units are equipped with vacuum systems. All volatile substances present or produced while processing, including water vapor, are removed constantly during processing. Volatile components further promote the degradation reactions and the removal of the volatile compounds during processing has recently been regarded as essential. Currently, intensive drying to remove moisture, as well as vacuum degassing processing, is introduced in plastic recycling systems to minimize the effect of degradation reactions, resulting in higher R-PET intrinsic viscosity in comparison with normally extruded POSTC-PET.

Stabilizers

PET processing stabilizers mainly have the function of reducing the effect of PVC contamination and are also associated with thermal stabilization. The majority of the PET process stabilizers are metal based such as butyl tin mercaptide, antimony mercaptide and lead phthalate. Adding organic phosphate to PET during re-processing helps to stabilize it by decomposing hydroperoxide groups to non-radical products. Scheirs [START_REF] Scheirs | Polymer recycling: Science, Technology and Applications[END_REF] reported that bis(2,4,di-tertbutylphenyl) pentaerythritol diphosphite can prevent Mw loss, inhibit yellowing reactions and reduce the production of acetaldehyde. The major disadvantage of using stabilizers is the significant added cost to the PET recycling process.

Solid State Polymerization

The SSP is a process in which POSTC-PET is heated to above its Tg but below the Tm. Condensation reactions occur at a temperature range of between 200 and 240°C, which is lower than the melt processing temperature (280°C). Degradation reactions have little effect at the low temperature of processing leading to an increase in R-PET Mw. The SSP byproducts are removed continuously by vacuum or by applying an inert gas stream [START_REF] Gao | Modeling of solid state polycondensation of poly(ethylene terephthalate)[END_REF].

Karayannidis et al. [START_REF] Karayannidis | Solid-state polycondensation of PET recycled from postconsumer soft-drink bottles[END_REF] investigated the use of SSP to increase the POSTC-PET's Mw. They investigated the effect of reaction temperature and time on the resulted PET [η]. They reported that modified PET is produced with improved Mn after solid stating at 230°C for 8 h. However, SSP is considered too slow and expensive to be applied on an industrial scale.

Chain Extension

PET or POSTC-PET chain extension is a process where a di-or poly-functional low Mw material is reacted with PET carboxyl and/or hydroxyl end groups to rejoin the broken chains that result from PET chain scissions during melt processing.

The mechanism of the PET chain extension reaction was introduced by Inata and Matsumura [START_REF] Inata | Chain extenders for polyesters IV. Properties of the polyesters chain extended by 2,2'-bis(2-oxazoline)[END_REF]. They suggested that for a 2,2'-bis(2-oxazoline) chain extender there are three types of reactions with PET:  A blocking reaction where a molecule of chain extender reacts with one chain of PET.  A coupling reaction where a molecule of chain extender joins two PET chains.  No reaction where the chain extender molecule fail to react. Chain extenders can be classified according to the PET functional end group that they react with. The chain extenders that react with carboxyl end groups are effective, in addition to the chain lengthening, by reducing the overall carboxyl content leading to high hydrolytic and thermal stability which in turn maintains Mw during melt processing [START_REF] Haralabakopoulos | Chain extension of poly(ethylene terephthalate) by reactive blending using diepoxides[END_REF]. The other type of chain extenders is hydroxyl-reactive chain extenders. They are more effective with low Mw PET produced by melt polycondensation because the hydroxyl content predominates over the carboxyl end groups. The hydroxyl-reactive chain extenders will increase [η] rapidly when reacting with PET that is produced by melt polycondensation PET [START_REF] Awaja | Recycled poly(ethylene terephthalate) chain extention by a reactive extrusion process[END_REF]. An example of hydroxyl reactive chain extenders are 2,2'-bis(3,1-benzoxanin-4one). Many researchers reported disadvantages associated with operating with some chain extenders. Cardi et al. [START_REF] Cardi | Chain extension of recycled poly(ethylene terephthalate) with 2,2'-Bis(2-oxazoline)[END_REF] reported that undesired side reactions could take place when using bis(2-oxazoline) (BO) to chain extend PET due to the sensitivity of the oxazolinic ring to acidic compounds. Structure of bis(2-oxazoline) is shown on Figure 5. Torres et al. [START_REF] Torres | Chemical modification of virgin and recycled PET by adding of chain extenders during processing[END_REF] reported that diisocyanates are more effective chain extenders than BO and diepoxides; however, they reported product discoloration when using diisocyanates.

PMDA has previously been reported as an effcient chain extender or branching agent. It is thermally stable, produces no side products on reaction with PET, is tetra functional, commercially available and economical [START_REF] Xanthos | Identification of rheological and structural characteristics of foamable poly(ethylene terephthalate) by reactive extrusion[END_REF].

Reactive Extrusion Process

Reactive extrusion processes facilitate polymer modifications greatly in the sense of providing a modified polymer with better specifications for different applications. Reactive extrusion is basically achieved by using an extruder as a reactor. For the POSTC-PET modification process, reactive extrusion has been attractive because of its advantages of combining a reactor and an extruder in one system. Many researchers used reactive extrusion systems to modify POSTC-PET [START_REF] Incornato | Structure and rheology of recycled PET modified by reactive extrusion[END_REF] in order to achieve better chemical and rheological properties. When applying POSTC-PET chain extensions in a reactive extrusion system, process parameters such as the twin-screw extruder performance and stability are closely monitored to ensure successful operation. Awaja et al. [START_REF] Awaja | The effect of chain extension on the thermal behavior and crystallinity of reactive extruded recycled PET[END_REF] investigated a reactive extrusion system with added chain extenders. They obtained lower carboxylic content using different types of chain extenders. For the POSTC-PET reactive extrusion processes, single or twinscrew extruders can be used. Single screw extruders have the advantage over twin-screw extruders lower capital cost which is the reason why they are used widely in the extrusion industry. Twin-screw extruders are well known for their high mixing ability and are used mainly in compounding and reactive polymer processes.

As described in this section, several efforts have been made by researchers to overcome the problem of intrinsic viscosity loss of POSTC-PET when extruded. Nevertheless, the proposed techniques have two main disadvantages: the high cost of required chemical products and the complication of the process. Indeed, the application of these techniques on the industrial scale is both relatively difficult and costly.

ENHANCEMENT OF RECYCLED PET PROPERTIES

Using recycled polymer blends with virgin polymer of the same nature is a common solution for upgrading post-consumer materials. Scarfato and La Mantia [START_REF] Scarfato | Characterization of monopolymer blend of virgin and recycled polyamide 6[END_REF], have studied blends of recycled PA6 and virgin PA. They found that the blends prepared in specific conditions show rheological and mechanical properties close to those of the virgin polymer. Kukaleva et al. [START_REF] Kukaleva | The Effect of Blending on the Viscosity Reduction of Recycled Milk Bottle Grade HDPE[END_REF], investigated blends of recycled and virgin HDPE polymers. They stated that mechanical properties (modulus and elongation) of the blends have predictable linear behavior. Earlier Wenguang [START_REF] Wenguang | Processing and mechanical properties of recycled PVC and of homopolymer blends with virgin PVC[END_REF], prepared and characterised homopolymer blends of recycled/virgin PVC. He found that the properties of the blends are between those of the pure compounds.

In many cases it is proposed to use blends of recycled PET and virgin PET to maintain the properties of the recycled polymer sufficiently high during the transformation processes.

Raw Polymers

In this context, the authors of this chapter conducted experiments based on three types of polymers: two PET bottle waste polymers and a virgin one to obtain melt spun yarns. Chips of crushed and cleaned bottles, purchased from an industrial plastic waste collector, were used as the source for recovered PET materials. Recycled PET (PET-A) comes from blue post-consumer bottles and recycled PET (PET-B) arises from heterogeneous deposits of various colored bottles (white, green, etc…). A fiber grade PET (PET-C) was used as the virgin PET resin. The characteristics of these three starting polymers are recapitulated in Table 5:

Melt Processing

The moisture content of PET should not exceed 0.005%. So, recycled chips and virgin pellets were dried under vacuum during 12H at 90°C in order to eliminate the residual moisture. Then, blends of virgin/recycled PET (PETV/R) were obtained using a Haake-Rheocard twin screw extruder under the following conditions: die temperature 270°C, extrusion speed 40 rpm and residence time of 5mn. The extrudate is cooled in water then passed through a pelletizing unit. We considered the following compositions of blends: 90/10, 70/30, 50/50 and 25/75 (PETV/ PETR w/w).

Methods of Analysis

Solution viscosity measurements were carried out on a Scött Gerate viscosimeter equipped with Ubbelohde capillary in a solution of phenol and 1,1,2,2-tetrachloroethane (60/40 w/w %) at 25°C. the intrinsic viscosity, [η], of polymers was extrapolated using the Huggins equation. The average molecular weights, Mw and Mn, were then computed from the values of [η] according to the following relations, Berkowitz [START_REF] Berkowitz | Viscosity-molecular weight relationships for poly(ethylene terephthalate) in hexafluoroisopropanol-pentafluorophenol using SEC-LALLS[END_REF] and Kokkalas [START_REF] Kokkalas | Effect of the Sb2O3 catalyst on the solid-state postpolycondensation of poly(ethylene terephthalate)[END_REF]:

Eq. 1 Eq. 2 A TA Instruments calorimeter was used to obtain thermograms of virgin, recycled and blended PET polymers. The swept temperature lies between 30 and 280°C under nitrogen atmosphere and an empty capsule as reference. The calorimeter is attached to a thermal analysis data station that calculates glass transition temperature Tg, crystallisation temperature Tc, melting temperature Tm, enthalpy of crystallisation Hc, enthalpy of melting Hm of samples. The percent of crystallinity,  c , for all PET samples was calculated from equation 3 whenever a crystallisation exotherm was present during heating: Eq. 3 where

, is the heat of fusion of a 100% crystalline PET [START_REF] Starkweather | The heat of fusion of poly(ethylene terephthalate)[END_REF].

Dynamic and steady shear rheological measurements were carried out on a dynamic rheometer MCR500 from Physica. This operates at both controlled stress and strain modes. The rheological properties of molten materials were determined using parallel plate geometry: 25 mm diameter and a constant gap of 1 mm. Steady shear viscosity measurements were carried out in a shear rate range varying from 0,1 to 300 s -1 . For very higher shear rates the data obtained were inconsistent due to the fact that molten polymer ejected outside of measurement gap because of the important centrifuge force that develops at high rotation speed of superior plate. For dynamic tests, the strain () values are chosen in order to perform the frequency sweeps in the linear viscoelastic range (LVR) i.e., the limiting strain under which the rheological parameters (η*, G', G"…) remained constant. For this purpose, preliminary strain sweeps were made, at constant frequency (1 Hz) and temperature (270°C), and the LVR was determined for each polymer. The frequency range investigated was 0,1-100 Hz with a constant shear strain of 10% (within the LVR of polymers).

RECYCLED/VIRGIN PET BLENDS' BEHAVIOR

Thermal Analysis of PET-V/R Blends

DSC analysis of PET-V/R blend pellets was carried out following this experimental protocol:

 a heating step from 20 to 280°C at a rate of 10°C.mn -1  a thermal treatment at 280°C for 3 minutes  a crystallization step from 280°C down to 20°C at 10°C.mn -1 .

The thermograms recorded during heating run of PET-C/A blends are reported on Figure 6:  an exothermic peak known as "cold crystallisation" [44], corresponding to the crystallisation of amorphous phase. This peak is typical for semicrystalline polymers such as PET or PEEK The phenomenon of cold crystallisation is explainable. As the frozen chains are heated, a critical mobility for crystallisation is reached and the material organises with an exothermic process. From the Table 6, we observe that the values of the cold crystallisation temperature (Tcc, min ) of our blends increase with increasing recycled PET content.  an endothermic peak associated with the fusion of the whole crystalline structure. We can see on Figure 5 that the values of melting temperatures (Tm, max), that correspond to the maximum of the melting peak, of the PET-V/R blends decrease with the PETR concentration and are between those of the starting materials i.e., PETR and PETV. This absence of melting depression or synergy was explained by Fann et al. [START_REF] Fann | Kinetics and thermal crystallinity of recycled PET. I. Dynamic cooling crystallization studies on blends recycled with engineering PET[END_REF] by the fact that PET/PET blends are crystalline-crystalline isomorphous polymer blends of the same material (PET). DSC curves obtained from the cooling run of PET-V/R samples are plotted on Figure 7. Contrary to other samples, PETC exhibits a rather broad temperature range of crystallisation starting at a lower temperature than other materials. However, recycled PET polymers (PETA and PETB) show higher crystallisation temperatures Tc ( + 20°C) with a sharper and narrower temperature range of crystallisation than PETV. This is explainable by the lower molecular weight (Mw) of PETR polymers and the physical contaminations that they may contain that will act as nucleation agents for crystallisation kinetics. On the other hand, PET-V/R blends show various degrees of improvement of the crystallisation process. Their crystallisation is narrower and sharper than parent polymers. We can also deduce from Table V that minimum crystallisation temperature (Tc, min) of the blends increases with the increase of PETR composition.

Rheological Behavior of PET-V/R Blends

Steady state rheological properties of virgin/recycled PET blends were determined at shear rates ranging between 0,1-300 s -1 at constant temperature 260°C. The flow curves obtained for different blends are reported on Figure 8. We remark that the apparent viscosity increases with the increase of PETR percentage for the two types of blends PET-C/A and PET-C/B. Also the viscosities of all the blends lie between those of pure PET materials.

Furthermore, it can be noted that the viscosity of the blends PET-C/B is higher than that of the blends PET-C/A. This is probably due to the fact that the PET-A is more likely to degrade than the PET-B. Indeed, during the preparation of the blends the polymers undergo shearing, thermal and mechanical stress (temperature and pressure).

The zero shear viscosity of the PET-C/A blends is plotted against the PETV weight fraction (Figure 9). The experimental curves show a negative deviation (ND) from the logarithmic additivity rule frequently used to model polymer/polymer blends viscosity [START_REF] Utracki | Melt rheology of polymer blends[END_REF].

Dynamic rheological properties (*, G' = ''/ and G'' = '/) for the PETV/R blends and extruded pure polymers (PETR and PETV) were measured with constant strain of 10% at 260°C. As expected the dynamic viscosity (*) decreases with frequency (Figure 10). Also, as for steady state shear viscosity, the complex melt viscosity increases with virgin PET concentration. In order to analyse the dynamic rheological data, the Cole-Cole plot (G' vs. G'') was used. This criterion was developed by Han and Kim [START_REF] Chang | Rheological technique for determining the orderdisorder transition of block copolymers[END_REF]. In this method, if a blend is miscible the same slope is observed between the blend compositions and pure components; otherwise it is considered to be immiscible or a phase separated blend. On Figure 11, the plots showed almost the same slope for all the PET-V/R blends which is a good criterion proving the miscibility of recycled and virgin PET polymers. The independence of composition and the absence of phase separation would mean that PETV and PETR are miscible down to the macromolecular level. The PET-V/R blends would thus form an isotropic medium that has no separate domains occupied by the respective constituent components.

CONCLUSION

In addition to the environmental incentive, recycling of PET as an industry derives its driving force from the increasing value and applications of virgin and modified PET. As described in the first sections, many scientific findings have been made in the field of PET recylcing. Particular focus in this study was given to the methods used to enhance properties of recycled PET.

The study of thermal behavior of virgin/recycled PET blends points out the their good microstructural morphology. The absence of melting depression or synergy was explained by the fact that PETR/PETV blends are crystalline-crystalline isomorphous polymer blends of the same material. This statement was in accordance with rheological measurements. The steady and dynamic tests showed the absence of phase separation that would mean that PETV and PETR are miscible down to the macromolecular level. The PET-V/R blends can be considered as forming an isotropic medium that have no separate fields occupied by the respective constituent components.

Thus, the study reveals that the mechanical blending of recycled PET chips and virgin PET pellets improves the melt processing of recycled PET.

Figure 1 .

 1 Figure 1. PET repeating unit.

Figure 2 .

 2 Figure 2. PET synthesis reactions (a) esterification of TPA with EG (b) trans-esterificationof DMT with EG and (c) polymerization.

Figure 3 .

 3 Figure 3. Phases of mechanical recycling of PET bottles.

Figure 4 .

 4 Figure 4. Different solvolysis methods for PET chemical recycling.

Figure 5 .

 5 Figure 5. Chemical structure of BO chain extender.

Figure 6 .

 6 Figure 6. Thermal behavior during crystallization run of PET-C/A blends.The same behavior was also observed for PET-C/B blends. It can be deduced from PETV/R thermograms that:

Figure 7 .

 7 Figure 7. Thermal behavior during crystallization run of PET-C/B blends.

Figure 8 .

 8 Figure 8. Viscosity η vs shear rate for PET-C/B blends (at 260°C).

Figure 9 .Figure 10 .

 910 Figure 9. Negative deviation of η 0 vs. composition for PETC/A blends.

Figure 11 .

 11 Figure 11. Cole-Cole analysis of viscoelastic modulus of PET-C/A blends.

Table 2 . Physical and chemical properties of PET [8]

 2

	Property	Test method	Value (Unit)
	Molecular weight (of repeating unit)	-	192 (g.mol -1 )
	Weight-average M w	GPC	30,000 -80,000 (g.mol -1 )
	Density	-	1.41 (g.cm -3 )
	Glass transition temperature	DSC	69-115 (°C)
	Melting Temperature	DSC	265 (°C)
	Breaking strength	Tensile	50 (MPa)
	Tensile strength (Young's modulus)	-	1700 (MPa)
	Yield strain	Tensile	4 (%)
	Water absorption (after 24h)	-	0.5 (%)

Table 3 . Intrinsic viscosity of PET depending on its application [11] PET [η] (dL.g -1 )

 3

	Fibers	Textiles	0.40 -0.70
		Techniques	0.72 -0.98
		bi-oriented	0.60 -0.70
		Thermoforming	0.70 -1.00
	Bottles	for water	0.70 -0.78
		for soft drinks	0.78 -0.85

Table 5 . Intrinsic Viscosity, molecular weight and MFI of the three PET polymers

 5

		[ ] 	M w	M n	MWD	MFI
		(dL.g -1 )	(g.mol -1 )	(g.mol -1 )	(M w /M n )	(g.10')
	PET-A	0,67	35500	17700	2,03	26
	PET-B	0,63	32300	16100	2,01	30
	PET-C	0,74	42100	20200	2,1	20

Table 6 . Thermal characteristics of the PET-V/R blends

 6

		Tg	T m, max	Hm	T cc, min	Hcc		T c, min	Hc
		(°C)	(°C)	(J.g -1 )	(°C)	(J.g -1 )	(%)	(°C)	(J.g -1 )
	PET-C	83	255	55.8	-	0	41	199	36
	(100%)
	PET-C/A	83	257	39.5	121	26.1	10	211	44.6
	(90/10)
	PET-C/B	83	257,5	38,7	119	23	11,5	212	44
	(90/10)
	PET-C/A	84	255	38.1	124	27.1	8	213	42.9
	(70/30)
	PET-C/B	84	254.5	35.7	122	21.2	10.6	213.5	45.1
	(70/30)
	PET-C/A	84	253.5	40.6	125	25.5	11	213	44.3
	(50/50)
	PET-C/B	84	253	38.2	124	22.1	12	214.5	43.9
	(50/50)
	PET-C/A	84	252	37.5	129	23.8	10	212	43.6
	(25/75)
	PET-C/B	84	255	44.5	127	20.7	17.5	217.5	44.1
	(25/75)
	PET-A	85	251	45.5	131	21	18	217	38.6
	(100%)
	PET-B	85	254	44.3	129	22	16	218	37.8
	(100%)