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Polymer Degradation and Stability 97 (2012) 496e503

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

A new generation of wood polymer composite with improved thermal stability

Fabien Sliwa a,b,*, Nour-eddine El Bounia a, Gérard Marin b, Fatima Charrier c, Frédéric Malet d

aCanbio, Hélioparc, 2 Avenue du Pr. Angot, 64053 Pau Cedex 09, Franceb IPREM-EPCP, UMR CNRS 5254, Université de Pau et des Pays de l’Adour, Hélioparc, 2 Av. du Président Angot, 64053 Pau Cedex, Francec IPREM-EPCP, UMR CNRS 5254, Université de Pau et des Pays de l’Adour, IUT des Pays de l’Adour, 40004 Mont de Marsan Cedex, FrancedARKEMA CERDATO, PSM 27470 Serquigny, France

a r t i c l e i n f o

Article history:Received 20 September 2011Received in revised form16 January 2012Accepted 18 January 2012Available online 26 January 2012

Keywords:DegradationThermogravimetric analysis (TGA)Wood compositePebaxThermal properties

* Corresponding author. Canbio, Hélioparc, 2 AvenCedex 09, France.

E-mail address: fabien.sliwa@gmail.com (F. Sliwa)

0141-3910/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2012.01.023

a b s t r a c t

We have studied the thermal stability of a new family of wood polymer composites (WPC) which usea thermoplastic elastomer matrix (pebax� copolymers). These copolymers are poly(ether-b-amide)thermoplastic elastomers which show a significant elongation at break and a melting point below200 �C which helps prevent degradation of wood fibres upon processing. Moreover these polymerspresent a hydrophilic character able to interact with wood fibres. Another important feature is that thesepolymers are synthesized from renewable sources. We have been using two types of pebax� matricesand two species of wood flour as fillers. Composites were made by using a laboratory-size twin-screwextruder to obtain homogeneous composite pellets prior to injection moulding into tensile test samples.

The thermal stability of the matrix, wood fibres and composites was investigated using thermogra-vimetric analysis under air and nitrogen atmosphere. In our study, we have shown a spectacularimprovement of thermal stability of the composites under air atmosphere, as opposed to measurementperformed under nitrogen. The presence of wood in pebax� hinders the thermo-oxidation in air by theformation of char residue in the earlier stage of degradation. We have also determined an optimal rangeof wood content in which we observe the protective synergism.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Interest in wood plastic composites (WPC) has grown rapidly inrecent years mainly in outdoor applications and constructionindustries. The two most common WPC are polyolefin (poly-ethylene PE and polypropylene PP) matrices and wood flour/fibres(WF). But use of poly(vinyl chloride) (PVC) is expected to grow dueto its superior performance and excellent weatherability comparedto polyolefins [1,2].

These composites have beenwidely studied. Properties of WPCsdepend on the characteristics of matrix and fillers, the fractionalcomposition of wood fillers, chemical interaction between woodfibres and polymer, humidity absorption and processing conditions[2e22].

One of the main issues is the compatibility between fibres andthe polymer. In particular, the hydrophobic character of polyolefinswhich contrasts with the hydrophilic character of wood fibres.Many studies have focused on the compatibility of natural fibres

ue du Pr. Angot, 64053 Pau

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and the matrix. The main solutions found to improve compatibilityare the use of coupling agents, pretreatment of wood fibre and/orthe polymer through surface coating treatment or graft copoly-merisationwhich improvemechanical properties, water absorptionand dispersion [14,23e29].

Another limitation of using wood fibres in WPC applications istheir low thermal resistance, thermal degradation beginningaround 200 �C. Therefore most polymers used for WPC have rela-tively lowmelting temperatures. This degradation is determined bythe wood species and composition [30e35].

It has been shown in previous investigations that the polymermust possess functional groups able to interact with the functionalgroups of wood. Furthermore the polymer must have highmechanical properties and a melting point lower than 200 �C inorder to process the WPC at lower temperatures.

Such factors are the reason why in this research we selectedpebax� copolymers as the composite matrix. These copolymers arePolyether-b-amide thermoplastic elastomers with an importantelongation at break and have a melting point below 200 �C thatprevent the degradation of wood fibres. Moreover this polymer hasa hydrophilic character able to interact with wood fibres. Pebax�

copolymers consist of rigid polyamide segments, acting as physicalcrosslinks, and flexible polyether segments [36e39].

F. Sliwa et al. / Polymer Degradation and Stability 97 (2012) 496e503 497

Another important feature for selecting this polymer is that itcomes from renewable sources. Our final objective is to designcomposites mainly issued from natural resources.

The characteristics of the polymer, in particular its elastomericproperties and hydrophilic character, defines it as a technicalpolymer. Compared withmostWPCs developed from polyolefins orPVC matrices, one can therefore imagine specific novel and tech-nical applications for this new type of WPC.

In this study we examined the interactions betweenwood fibresand pebax� copolymers without any specific compatibilising agent.The thermal behaviour was investigated by thermogravimetricanalysis and differential scanning calorimetry. The thermal stabilityof the matrix, wood fibres and composites was investigated bythermogravimetric analysis under air and nitrogen atmosphere inorder to characterize the potential fire performance of composites.

2. Experimental

2.1. Materials

The basic materials used in this study were two types of woodflour and two grades of pebax�.

The first wood flour (WF1) was supplied by Rettenmaier (Sprucewood reference C120) [23]. The WF1 characteristics given by thesupplier are listed in Table 1.

The second wood flour (WF2) was Maritime Pine woodproduced by grinding then sieving in the Sylvadour Laboratory inMont de Marsan (France). Its general characteristics are also givenin Table 1.

Pebax� 25R53 SP01 and pebax� 55R53 SP01 copolymers weresupplied by Arkema (CERDATO, Serquigny, France) in powder form.In this article, copolymers will be respectively quoted as pebax�25and pebax�55. The two polymers were selected in order to haveone “soft” grade and one “hard” grade, with a melting point below200 �C. Pebax�25 and pebax�55 have tensile moduli of respectively15 MPa and 145 MPa and melting temperatures of 136 �C and167 �C.

2.2. Processing

2.2.1. CompoundingWFs were dried at 105 �C for 6 h and pebax� at 60 �C under

vacuum for 5 h. Pebax� and WFs were then dry-blended prior tocompounding. Five levels of filler content (10, 20, 30, 40 and 50 wt%) were used in the preparation of samples. Compounding wascarried out by using a laboratory-size twin-screw extruder (ThermoScientific, Eurolab 16) in order to produce homogeneous compositepellets. The barrel temperature ranged from 90 �C to 170 �C.

2.2.2. MouldingThe compound pellets were dried for 16 h at 80 �C prior to

injection moulding of the tensile test samples (ISO 527-2) by usinga 65-ton DK injection moulder. The mould temperature was 15 �Cand the barrel profile temperature of the injection machine rangedfrom 160 �C to 200 �C depending on composites wood fibre loading.

Table 1Wood fibre characteristics.

Characteristics Lignocel C120 Maritim pine wood

Wood specie Spruce Maritim pineParticle range (mm) 70e150 60e450Cellulose (%) w50 w42Lignin (%) w25 w28Hemicellulose (%) w25 w27

2.3. Testing and analysis

2.3.1. Thermogravimetric analysis (TGA)Thermogravimetric analysis was performed with a Perkin Elmer

(TGA 4000) analyzer. Matrix, fibres and composites were analyzedto determine their temperatures and times of degradation.

Dynamic analysis were performed under nitrogen and air ata heating rate of 10 �C/min, from 30 �C to 600 �C.

Isothermal tests were performed under air at temperaturesof 220 �C for 720 min and 300 �C for 240 min, after heating at10 �C/min from 30 �C up to the test temperature.

3. Results and discussion

3.1. Non isothermal thermogravimetric analysis

3.1.1. Thermal stability under nitrogenThermal behaviour of the matrix, wood fibre (WF1) and

composites was investigated by thermogravimetric analysis undernitrogen at a heating rate of 10 �C/min, from 30 �C to 600 �C. Fig. 1shows the weight loss as a function of temperature. Sampledegradation occurs in a single stage for the matrix as well as for thewood fibre. We can notice that the degradation curves of thecomposites lie between the curves corresponding to the purematrix and fibres. Composite decomposition temperatures dependon wood flour content and decrease with the amount of wood.

We have reported in Fig. 2 the first derivative of weight loss asa function of temperature, which characterizes the decompositionrate of a sample. One observes that degradation of compositesoccurs in two stages, corresponding respectively to the degradationof the matrix and fibres. The intensity of peaks depends on woodcontent. We can also observe two degradation peaks for pebax�25,the first one at 435 �C and a smaller peak at 460 �C. The first peakcorresponds to the degradation of polyether segments and thesecond peak to the degradation of polyamide. The thermal degra-dation of pebax�25 under nitrogen atmosphere is due to randomchain scission of several types of bonds. It occurs by a depolymer-isation process which takes place through a chain scissionmechanism mainly initiated in the thermally unstable polyetherblocks [39].

One can observe on the wood fibre degradation curve a singlepeak at 385 �C with a shoulder at 345 �C. Thermal degradationbegins at 200 �C; lignin degrades first at a rate slower than other

Fig. 1. TGA curves under nitrogen of pebax�25, WF1 and composites at various fillercontent.

Fig. 2. DTG curves under nitrogen of pebax�25, WF1 and composites at various fillercontent.

Fig. 4. Effect of wood content on T0,1 and T0,5 upon degradation in air atmosphere.

F. Sliwa et al. / Polymer Degradation and Stability 97 (2012) 496e503498

components, but within an extended temperature range (up toalmost 600 �C). The main degradation peak at 385 �C correspondsto cellulose degradation, while the shoulder at the lower temper-ature can be attributed mainly to hemicellulose degradation[32e35,40e42].

3.1.2. Thermal stability under airIn order to characterize the potential fire performance of

composites, thermal behaviour was investigated under air bythermogravimetric analysis at a heating rate of 10 �C/min, from30 �C to 600 �C.

Fig. 3 shows the weight loss as a function of temperature.Degradation occurs in multiple stages. We observe an unexpectedphenomenon: composites present a degradation temperaturehigher than that of their individual components, whatever thewood fibre ratio. This phenomenon appears only under air atmo-sphere as we do not observe the same feature under nitrogenatmosphere. Our results demonstrate an anti-degradation synergybetween the polymer and wood. The wood flour and the pebax�25protect themselves mutually: as composite degradation is delayedcompared with the degradation kinetics of the individual compo-nents. We have derived from TGA curves specific values in order tocharacterize the mass loss delay. We have reported in Fig. 4 thevalues of T0,1 and T0,5 which correspond to the temperatures at

Fig. 3. TGA curves under air of pebax�25, WF1 and composites at various fillercontent.

which 10 wt% and 50 wt% degradation occurred (mass losses). T0,1increases with increasing wood content up to a maximum value at30% wood, then decreases as wood content increases. The T0,1values of composites are much larger than the values correspond-ing to the purematrix and wood.We observe a 50 �C increase of T0,1at 30% wood, compared with the pure components. There isa significant increase (20 �C) even at 5%wood content. The values ofT0,5 increase from 50 �C to 70 �C as wood content increases from 5%to 50%, compared with the pure components.

Pebax�25 degradation occurs in three stages. The initial step,beyond 250 �C, matches the oxidation of the less thermally stablesegment (polyether). The second stage lies between 400 �C and500 �C and corresponds to simultaneous thermal and oxidativedegradation in the residual polyether and the polyamide segments.The third phase (above 500 �C) corresponds to the oxidativedegradation of the char formed in the earlier degradation stages[39,43e45].

The first stage of wood degradation under air atmospherefollows a process similar to that observed under nitrogen, but thepresence of oxygen causes additional degradation processes.Whereas degradation is enhanced at low temperatures, a seconddegradation stage occurs between 400 �C and 500 �C. This seconddegradation process is due to the oxidation of char residue gener-ated in the early stage and therefore, unlike with experimentsunder nitrogen atmosphere, no residue is left at the end[32,40e42,46].

A possible explanation of the improved composites thermalstability is the formation of a char skin-layer at the earlier stage ofdegradationwhich hinders the thermo-oxidation process, acting asbarrier. This barrier effect can explain the mass loss delay observedfor composites, which is due to a slowdown of diffusion of volatilethermo-oxidative products and oxygen to the polymer. Anothercomplementary explanation of the improved thermal stability ofcomposites is linked to a particle effect, incorporating particles intoa polymer causes an apparent increase in stability. Indeed, theweight loss is a measurement of products diffusion from a viscousmelt, the parameters of this diffusion are likely to be dependent onpath tortuosity introduced by impermeable particles of wood[43,47,48].

Furthermore the lignin, component of wood, may play a role oncomposites thermal stability. Indeed, different studies with poly-propylene and rubber matrixes have shown that lignin can act as anantioxidant. They concluded that lignin serves the role of thermalstabilizer. Hence, the lignin acts on the synergism effect helping toimprove thermal stability [49e52].

Fig. 5. Effect of wood content on W400 �C and W500 �C upon degradation in airatmosphere.

Fig. 7. TGA curves under air of pebax�55, WF1 and composites at various filler content.

F. Sliwa et al. / Polymer Degradation and Stability 97 (2012) 496e503 499

To better understand the degradation phenomena, we haveextracted from TGA curves the W400 �C and W500 �C values corre-sponding to the percentage of residue at 400 �C and 500 �C,respectively. One can observe in Fig. 5 that the values of W400 �Cincrease with increasing wood content up to 10%, then the corre-sponding values remain stable up to 40%, before decreasing at 50%.At 5% wood content, there is 10% more residue compared with thepure polymer and 20% more compared with wood. For compositesranging from 10% to 40% of wood content, there is 30% moreresidue compared with the matrix and about 40% compared withwood. The W500 �C values increase with increasing wood contentup to 50% and are still higher than the pure matrix and woodvalues.

Given that at 400 �C and 500 �C the residue ratio of compositedegraded samples (which are char at this temperature) is above thepure component values, one can conclude that there is more characting as protective barrier to oxidation. The residue ratio ofcomposites is above the pure component values starting from thebeginning of degradation. Hence one may conclude that charformation occurs in the earlier stages of degradation, slowing downthe degradation of composites. The amount of residue at 500 �Cincreases with wood content for composites whereas it is negligible

Fig. 6. TGA curves under air of pebax�25, WF2 and composites at various fillercontent.

for pure WF. One may notice that the composite mass loss delay isquite similar whatever the WF content; this strengthens thehypothesis that we only have a skin-layer acting as a protectivebarrier.

As we observe the same phenomenon whatever the woodcontent, we can assume that the composites degradation behaviourmay be explained by the development of interactions betweenwood and polymer. We performed further experiments to under-stand these phenomena.

Firstly, we performed thermogravimetric analysis experimentsunder air with the same matrix, changing the nature of woodfibres (maritime pine wood (WF2)). One can see in Fig. 6 thatcomposites with WF2 have an higher thermal degradationtemperature, as observed previously with WF1. Interactionsbetween pebax� and wood fibres shift degradation to highertemperatures, acting as a protective barrier for each other. Thisconclusion is relevant to the fact that the degradation curves ofWF1 and WF2 are similar.

For a better understanding of these results, we have performedanother series of experiments by changing the matrix. Firstly, inorder to analyze the effect of polyamide or polyether segments, wehavemade a series of composites using a second pebax� copolymermatrix with a higher polyamide content (pebax�55).

Fig. 8. DTG curves under air of pebax�55, WF1 and composites at various fillercontent.

Fig. 9. TGA curves under air of PA11, WF1 and composites at various filler content.

Fig. 10. TGA curves under air of PTMG, WF1 and composites at various filler content.

F. Sliwa et al. / Polymer Degradation and Stability 97 (2012) 496e503500

As pointed out for pebax�25, one may observe in Fig. 7 thatcomposites exhibit a degradation temperature above the degra-dation temperatures of the matrix and WF.

Degradation of composites begins at lower temperaturescompared with pebax�55, at a slower rate however. The onset ofdegradation occurs at temperatures which are lower for woodfibres than for pebax�55, which explains why the onset ofcomposite degradation occurs between wood fibres and pebax�55.One observes however the same general features as for pebax�25composites at higher temperatures: the residue ratios at 400 �C -after the first degradation stage - and at 500 �C - before the thirddegradation step - are higher than for the pure components. Wecan explain the pebax�55 degradation process according to theprevious comments about pebax�25 degradation. The maindifference is the percentage of weight loss at each stage, due to thedifference in the PTMG/polyamide ratio.

We have reported in Fig. 8 the weight loss derivative as a func-tion of temperature for the pebax�55 series. These curves, whichcharacterize the decomposition rate, confirm that matrix and fibredegradation starts at temperatures which are lower than forcomposites. The first degradation peak in Fig. 8 at 340 �C corre-sponds to the first stage of wood degradation and the second peakat 380 �C corresponds to the polyether segments degradation.These peaks are not observed at the same temperature in the

Fig. 11. Isothermal TGA curves under air (T ¼ 2

composite degradation curves. Themaximumpeak temperature forcomposites is delayed and corresponds to the second degradationpeak of matrix and wood. There is also a second peak at 550 �Ccorresponding to the degradation of char, which confirms theformation of char already degraded for wood.

Pebax� is a segmented block copolymer based on PTMG and PAblocks. In order to understand the interaction between wood andeach copolymer block, we have decided to test the thermaldegradation of composites made with the homopolymer matrixcorresponding to each block and wood fibre. The first experimentswere performed with polyamide 11 (the molecular weight Mn is600 g/mol).

We do not observe in Fig. 9 the thermal degradation synergismbetweenwood and polyamide as previously observed with pebax�.Composites degradation is intermediate between the matrix andWF in temperatures ranging from 300 �C to 500 �C, the pure matrixhaving the best thermal stability. We may point out however thatabove 500 �C the composites residue ratio is higher than for thepure matrix and wood. This result is particularly interesting as itconfirms that char residue above 500 �C observed on pebax� TGAcurves arises from interactions between wood and polyamideblocks of pebax� [43,45,53].

The literature shows that there is no residue beyond 500 �C forPA under nitrogen [39,45,53]. The strong protective effect appearsonly under air atmosphere as char residue beyond 500 �C appearsonly under air. This is an important experimental fact to understandcomposites degradation under air.

20 �C) for pebax�25, WF1 and composites.

Fig. 12. Weight loss as a function of wood content at 220 �C and 600 min.

F. Sliwa et al. / Polymer Degradation and Stability 97 (2012) 496e503 501

The second analysis was achieved with a pure PTMG homo-polymer having a molecular weight Mn of 2000 g/mol.

One can observe Fig. 10 a behaviour which differs from whatwas observed previously. The onset of composites degradationbeyond 300 �C is intermediate between the matrix and WF.Contrary to polyamide, the PTMG matrix has the lowest degra-dation temperature whereas WF exhibits a better thermalresistance. One can hence conclude that the presence of WFincreases the degradation temperature of PTMG. One can observefor composites a stabilization effect of the PTMG phase byWF due to the fact that WF presents a higher degradationtemperature.

We can assume that during the onset of pebax� compositesdegradation (which corresponds to the decomposition of PTMGblock), degradation is moved towards high-temperatures, given thefact that PTMG degradation occurs before wood degradation.

3.2. Isothermal thermogravimetric analysis

The aim of isothermal thermogravimetric analysis is to charac-terize (i) the composites degradation behaviour under air atconstant temperature and (ii) the interactions betweenwood fibresand matrix as previously demonstrated by dynamic TGA analysis.

A first series of tests with pebax�25 and WF1 were performedunder air at a temperature of 220 �C for 720 min, after heating at10 �C/min from 30 �C.

Fig. 13. TGA curves under air, isothermal at 30

One can observe in Fig. 11 that there is a protective synergybetween the matrix andWF. The weight loss as a function of time islower for composites than for the pure matrix and WF. Compositesdo present a higher thermal stability compared with the purecomponents.

We can correlate the isothermal analysis data with the previousnon isothermal tests. For isothermal experiments at 220 �C, thecomposites residue ratio is higher than for the matrix and WF: oneconfirms that there is a protective effect starting from the onset ofdegradation. As the residue ratio is larger for composites, one canconclude that there is formation of a degradation char starting fromthe beginning of degradation, which can explain this protectiveeffect. The fact that the composites residue ratio is higher - startingfrom the onset of degradation - explains why the mass loss isdelayed during non isothermal analysis.

We notice also that the weight loss is related to wood content.We have plotted in Fig. 12 the weight loss at 600 min as a functionof wood fibre content. We observe that the minimum weight lossfor composites lies at about 20% WF content. One can yet observea significant protective effect between wood fibres and pebax�

even at low WF content (5%).The residue ratio depends on wood content but there is no

significant difference between 10% and 40%. We can correlate thisobservation with non isothermal analysis where we have shownthat themass loss delay is quite similar whatever thewood content.Therefore one can conclude that the "barrier effect" is due to a skin-layer formed during degradation which depends of the polymer/wood ratio. This protective effect is however mostly efficient forcomposites with wood content between 10% and 40% WF content,with a maximum at 20%.

A second series of experiments were performed with pebax�25and WF1 at a higher temperature under air (300 �C for 240 min)after heating at 10 �C/min from 30 �C. One can observe on Fig.13 thesame features as for isothermal analysis at 220 �C, the weight lossbeing higher at 300 �C and the degradation of composites beingclearly improved compared with the copolymer matrix and WFdata. The difference between the degradation kinetics of compos-ites and their pure components is enhanced at higher temperature,the composites degradation residue ratio being larger whichconfirms that the barrier effect is due to the degradation charproduct. The protective effect depends on the degradation of woodin presence of polymer; hence it depends on wood content, thelargest effects being observed at low (5%) and high (50%) WFcontent. The residue ratio curves are quite close between 10% and

0 �C of pebax�25, WF1 and composites.

F. Sliwa et al. / Polymer Degradation and Stability 97 (2012) 496e503502

40% with a maximum however at 20%. Hence there is an optimalrange of WF content where we observe the best protective syner-gism between wood and pebax�.

4. Conclusions

A new WPC made of a bio-sourced thermoplastic elastomermatrix and wood fibre as filler is presented in this study. We haveshown a major improvement of thermal stability of compositesunder air, as opposed to measurement performed under nitrogen.The degradation of pebax�25 composites is improved comparedwith the pure matrix and wood fibres under air, while the oppositeis observed under nitrogen. The presence of wood in pebax�25hinders the thermo-oxidation by the formation of char residue inthe earlier stage of degradation, in air atmosphere. The antioxidanteffects of lignin are also likely to play a role in the improved thermalstability.

We also observe an improved thermal stability under airatmosphere for pebax�55 composites beyond 400 �C.

Thermogravimetric analysis of PA under air atmosphere showsformation of char residue beyond 500 �C which is not observedunder nitrogen. Thermal degradation of PA composites under airatmosphere lead to a higher residue ratio for composites beyond500 �C, compared with pure PA. This result demonstrates inter-actions between wood fibre and polyamide during the degrada-tion. Therefore one can conclude that polyamide protects the woodfibre.

Isothermal thermogravimetric analysis at 220 �C and 300 �Cunder air atmosphere confirms the improved thermal stability, asthe level of char residue of pebax�25 composites is clearly higherthan for the pure components all through the isothermal analysisexperiment.

The characterization of thermal stability of this family of WPcomposites presented in this paper is the first part of a compre-hensive study of these composites. In particular, the mechanicaland structural properties of these materials will be presented infollowing papers.

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