characterization of thermoplastic poly(ethyleneterephthalate)-glass fibre composites, 1....

Die Angewandte Makromolekulare Chemie 212 (1993) 19-33 (Nr. 3713)

' Laboratoire d'Etudes des Mattriaux Plastiques et des Biomattriaux, Universitt Claude Bernard, Lyon I, 43, Boulevard du 11 Novembre 1918,

69622 Villeurbanne CCdex, France CEMES, CNRS, Toulouse, France

Characterization of thermoplastic poly(ethy1ene terephtha1ate)-glass fibre composites, 1

Crystallization study

Catherine Gauthier', Jacques Chauchard', Bernard Chabert', Mostafa Saktoun2, Alain Boudet2

(Received 5 October 1992, revised 10 May 1993)

SUMMARY This paper presents the influence of glass fibres on crystallization kinetics and on

matrix morphology for poly(ethy1ene terephthalate) (PET)/glass fibre composites. The following parameters are also considered: fusion-crystallization conditions, thermal stability and the addition of nucleating agents in the matrix (talc or sodium benzoate). It clearly appears that the influence of those additives on the crystallization of PET is predominant compared to the effect of stiffening fibres. Moreover, the application of shear stresses at the PET/glass fibre interface promotes the growth of a different crystalline superstructure.

ZUSAMMENFASSUNG: Der EinfluD von Glasfasern auf die Kristallisationskinetik und die Matrixmorpholo-

gie von Composites aus Poly(ethylenterephtha1at) und Glasfasern wurde untersucht. Die Schmelz- und Kristallisationsbedingungen, die thermische Stabilitat und die Addi- tion von Kristallisationskeimen (Talkum oder Natriumbenzoat) wurden ebenfalls berucksichtigt. Der EinfluD dieser Additive auf die Kristallisation uberwiegt den der Verstarkungsfasern. Daruber hinaus fordern Scherspannungen an der PET/Glasfa- ser-Grenzflache das Wachstum einer anderen kristallinen Uberstruktur.

Introduction

Over the years, attention has been paid to thermoplastic composites for various reasons: high production rate, long storage period, reformability, recyclability, etc' . Processing of semi-crystalline polymeric matrix composites

CI 1993 Hiithig & Wepf Verlag, Basel CCC OOO3-3146/93/$05.00 19

C. Gauthier, J. Chauchard, B. Chabert, M. Saktoun, A. Boudet

is usually performed by heating the parts a few degrees above the matrix melting point. Then the molten matrix compostite can be formed using the appropriate process (thermoforming, pultrusion, compression moulding). Finally, the product is cooled down to room temperature.

The effects of the imposed processing conditions on crystallization kinetics and the morphology have been extensively reported for high-performance semi-crystalline matrices such as PEEK2$ or PPS4. Both previous melt history and cooling rate are important parameters to be ~ o n t r o l l e d ~ . ~ .

Moreover, in thermop1astWfibre composites, the behaviour of the polymer during crystallization and the final morphology can be modified to a great extent due to the role of nucleating agents played by the fibrous fillers7 and also arising from the presence of shear stresses*.

PET is a semi-crystalline polymer which exhibits excellent solvent resistance, low water uptake and a quite satisfactory thermal resistance. These properties make PET very attractive as the matrix for glass fibre composites in the automotive industry. The crystallization of PET has been previously studied as a function of various parameters such as the molecular structure of the polymer9 and different physical factors including temperature, cooling conditions, moisture, pressure and additivesl0, l .

However, the effect of glass fibre on the development of crystallinity and on the final morphology of PET has not been studied so far. During this investigation, we will study the anisothermal crystallization of neat PET and corresponding glass fibre-reinforced matrices formed under different processing conditions, and will in turn consider three different aspects: the influence of the temperature in the molten state, the kinetical and finally the morphological aspect.

Experimental

PET resin was provided by RhBne Poulenc in a powder form with referencial name 92T, which represents a R.P. viscosity index in melt state. The number-average molecular weight was M, = 20000, with a polydispersity index M,/M, = 2.

Because of its slow crystallization rate, the addition of nucleating agents to PET has been widely practiced in plastic fabrication for shortening of cycle times in processes. Two different nucleating agents were chosen in our study, talc and sodium benzoate. The added quantities were less than 1 wt.-%.

The glass fibres were supplied by VERTROTEX (glass E) in the form of rovings (320 Tex). The diameter of a single filament was about 14 pm. The glass fibres were coated

20

Characterization of thermoplastic PET-glass fibre composites, I

with a sizing agent (VERTROTEX ref. EC 590) including a polyester as sticking agent and an organosilane as coupling agent.

In the manufacture of the composite prepregs, glass fibres were impregnated with PET using the powder impregnation method developed by IrchaI2. The glass fibre content was approximatively 40% by volume.

Non-isothermal crystallizations were performed on a PERKIN ELMER differential scanning calorimeter (DSC 4) operated under a constant nitrogen flux. From 8 to 12 mg of the PET powder was weighed in open aluminium DSC pans. The temperature scale of DSC was calibrated with indium.

For each experiment, crystallization kinetics was evaluated from the temperature of the maximum in the crystallization exotherm (Tc); the degree of crystallinity was estimated from the crystallization enthalpy (A Hc).

The morphology was examined using a transmission electron microscope (TEM) with an accelerating voltage of 2 MV. PET samples were prepared between two microscope slides using a hot stage with a temperature regulator (Mettler FP5). Occasionally, during isothermal crystallization, the glass fibre was pulled across the matrix with a controlled speed to apply a shear stress. Before observation, samples were embedded in epoxy resin and cut with an ultramicrotome to a thickness of approxi- mately 0.1 pm.

Dark field micrographs obtained from diffracted electron beams were used to examine spherulites which became visible by diffraction contrast.

Results and discussion

Influence of melt conditions

I n order to control the thermal history and polymer stability, the samples were heated at 80 K min-' to a chosen Ti and held in the molten state at this temperature for a chosen time ti. A scan was then recorded from Ti to ambient temperature at a rate of - 10 K min-'. The Ti values varied from 270 to 350"C, while ti was 5 min. After the cooling scan, subsequent meltings were also recorded during heating at 10 K min- ' . Typical DSC traces obtained for the PET matrix are shown in Fig. 1 . One unique melting endotherm near 250 "C was observed.

It has been shown in the literature that the magnitude and the shape of the melting endotherm depend strongly on previous processing h i ~ t o r y ' ~ - ' ~ . In particular, after previous isothermal crystallizations, two melting processes can take place. The smaller endotherm at lower temperature is associated with the continuous melting and recrystallization of the crystallites which were previously formed at lower temperatures. Its position varies anywhere from a

21


5 0 100 150 200 250 300 TIOC)

Fig. 1. Typical DSC thermograms of PET: (A) cooling scan at 10 K min-'; (B) subsequent heating at 10 K min-' (1 cal = 4.1868 J).

few degrees above the temperature of isothermal crystallization to a shoulder of the melting temperature (Tf). This phenomenon has not been observed during our experimentations at 10 K min-'. Nevertheless, as the crystallinity of the sample may change during the heating scan, the degree of crystallinity should not be evaluated from the melting enthalphy AHf but from the crystallization enthalpy AH,.

Fig. 2 shows the variations of the crystallization temperature T, as a function of Ti for the neat PET (without nucleating agent) and for the PET-glass prepreg. It can be seen that there are three regions: Region I: At low treatment temperature, T, decreases with Ti. In this region

the number of crystalline entities (which remain in the melt) is reduced when Ti is increased, resulting in a decrease of the crystallization temperature.

Region 11: The crystallization temperature, T, does not change with Ti. This could be explained by the fact that all the crystalline entities have been destroyed. Hence, the number of crystalline nuclei is constant: the homogeneous nuclei of the polymer are molten, but stable heterogeneous nuclei (minerals particles, catalytic residues, etc.) may subsist.

To assess this proposition, the equilibrium melting temperature of PET, T,, has been estimated. PET samples have been isothermally crystallized at various temperatures between 220 and 245°C in DSC sample pans after

22

Characterization of thermoplastic PET-glass fibre composites, 1

Region I Region II Region Ill

2LO 280 320 360 Ti(OC)

Fig. 2. Crystallization temperature T, versus treatment temperature in the molten state Ti: ( o ) PET, ( 0 ) glass-PET composite.

premelting at 290 "C for 5 min. The observed melting temperatures (T,) were obtained from the DSC melting thermograms by reheating the samples at a rate of 10 K min-'.

Fig. 3 shows the changes in observed melting temperatures (T,) as a function of crystallization temperature (T,). The melting temperatures closely follow a straight line. The extrapolation of this straight line intersects the T, = T, line at TFo = 280°C. We can notice that this temperature nearly corresponds to the beginning of region 11 and is in good agreement with literature resultsl6, *'. Region 111: At high Ti, T, slightly increases. Thermogravimetric experiments

using a DuPont Instruments TGA 2000 indicate that significant relative weight loss of samples appears when Ti is higher than 310°C. Due to bond ruptures and reduction in molecular weight, the molecular rearrangements become easier, resulting in an early crystallization and an increase in T,. In conclusion, in this temperature region, various chemical phenomena can take place (degradation, bond ruptures, reticulation, hydrolysis, etc.) and lead to irreversible changes. Further study should include detailed work on molecular weight evolution. Nevertheless, it is already possible to eliminate this region for further processing conditions.

23


I [ I I I I

I 200 LJ

220 230 210 250 260 270 280 T d O C )

Fig. 3. Plot of the observed melting temperatures versus crystallization temperature, where the equilibrium melting temperature of PET is estimated by extrapolation. The lower solid line represents T, = T,.

Prior thermal history influences not only the non-isothermal crystallization of bulk PET but also that of the glass-PET composite. The presence of glass fibres has little effect on the shape of the curve: the three different regions are still present. Moreover, crystallization temperature is always higher for the glass-PET composite. For different other thermoplastic composite s y ~ t e m s ~ ~ ~ ~ , similar results have been explained by the nucleating role of stiffening fibres; this interpretation will be discussed later.

Fig. 4 shows the variation of T, as a function of Ti for the two nucleated matrices and the corresponding composites.

First, we can notice that the crystallization temperatures of both matrices nearly follow the same curve. The crystallization temperatures of nucleated PET are always higher than that of the pure resin, that is to say the crystallization phenomenon occurs earlier in the presence of talc or sodium benzoate. For example, T, in region I1 for the talc-nucleated matrix is 215 "C, whereas that of pure PET is only 192°C.

The curves for both materials (nucleated matrix and corresponding prepreg) now overlap. Hence, in this case, the influence of the presence of glass fibres is negligible in comparison with that of the additives.

24

zoo

190 - 240 280 320 360

Ti(OC)

-

Fig. 4. Crystallization temperature T, versus treatment temperature in the molten state Ti: (0) sodium benzoate-nucleated PET, ( 0 ) talc-nucleated PET, (W) corresponding nucleated PET-glass composites.

Finally it should be noticed that there are only slight differences between region I and regin I1 for nucleated products. The elimination of the homogeneous, crystalline nuclei has no effect on the nucleated polymers where heterogeneous nuclei are predominant.

Kinetic aspects

After completely melting the crystals (5 min at 290 "C), samples of PET and of glass-PET prepregs were cooled to room temperature at different cooling rates between 2 and 40 K min-'.

In order to study the crystallization kinetics, the variation of the transformed fraction as a function of temperature must be studied. The transformed fraction X = XT/X, was calculated from the exotherm area (XT represents the crystallinity attained at a temperature T and X, the final crystallinity).

Fig. 5 shows the variation of X versus T for the three different matrices (with and without nucleating agents) at a cooling rate of 10 K min-'.

The influence of nucleating particles mainly appeared at the beginning of the phenomenon which corresponds to the nucleation step. In fact, the nuclea-

25


220 195 170 1L5 120 T ( O C )

Fig. 5 . Variation of transformed fraction X versus T for the three PET matrices (V = 10 K * min-I); ( 0 ) PET + sodium benzoate, (A) PET + talc, (W) pure PET.

ting agents promote both earlier crystallization and faster crystallization rate. Nevertheless, Fig. 5 confirms that there are only slight differences, concerning the crystallization kinetics, between the two nucleating agents.

The influence of glass fibres on the crystallization kinetics is presented by Fig. 6 and 7. These figures show the variation of X versus T at two different cooling rates for the matrix and the corresponding composite, respectively for the pure PET (Fig. 6 ) and the sodium benzoate-nucleated matrix (Fig. 7).

For both systems, when the cooling rate increases, the crystallization temperature decreases significantly because the system moves away from equilibrium conditions. But in the nucleated systems, the crystallization kinetics are less sensitive to the change of cooling rate.

In Fig. 6, it can be observed that the crystalllization kinetics of PET in pultruded ribbons is significantly modified; we notice a higher crystallization temperature for PET-glass composite since the beginning of the crystallization (nucleation step). Moreover, at high cooling rate, the crystallization process of the neat polymer is slowed down. This results in a shorter transformation half- time for the composite samples than for the neat PET. This effect could probably be attributed to the limitation of homogeneous sporadic nucleation of the polymer at high cooling rate. Hence, in this case, the crystal growth rate seems to become diffusion-controlled rather than nucleation-controlled. So,

26


1.0 C

0.8 2 ?

0.6 - u -0

0.1 5 0.2 5

- ul

c 0

220 200 180 160 110 TI'C)

Fig. 6 . Variation of transformed fraction X versus T at two different cooling rates for the pure PET matrix (M) and for the composite ( 0 ) ; A: 5 K min-'; B: 40 K min-I.

I

1.0

C 0.8 .g

2 0.6 -

U

U a,

0.L ; c ul C

+ 0.2 2

0 220 200 180 160

T("C)

Fig. 7. Variation of transformed fraction X versus T at two different cooling rates (5 or 40 K min-') for PET (W) and composite ( 0 ) nucleated with sodium benzoate; A: 5 K m i x 1 , B: 40 K min-I.

the role of the fibres in the PET-glass pultruded ribbons becomes more predominant when the homogeneous crystallization of the matrix is un- favoured.

On comparing the sodium benzoate-nucleated PET/glass fibre composite kinetics data with those of the sodium benzoate-nucleated polymer, one

27


observes that the rates of crystallization for both products are not significantly different. This is shown in Fig. 7.

Finally, the results concerning the crystallization kinetics have to be considered in relation to the processing history of the samples. For example, when PET is mixed with glass fibres in the DSC pan, the modification of crystallization kinetics is no more observed. So, we suggest that the glass fibres do not play a strong role as nucleating agent. To explain the change of crystallization kinetics of PET in pultruded ribbons, one should probably consider shear stresses during the process and/or the repartition of the polymer in the fibre network. This assumption will be checked in the morphology study of the samples.

Finally, a main difference between pure and nucleated PET is also demon- strated by Fig. 8 which shows the dependence of crystallization enthalpy AH, on the cooling rate. One should remind that we are principally interested in crystallinity developed during the process. So, the crystalline contents of the samples could not be calculated from the area of melting endotherms and we decided to use crystallization enthalpy AH, as crystallinity indicator. We can observe that, when cooling rate increases, the crystallization enthalpy (and consequently the degree of crystallinity) decreases, but the decrease of the enthalpy values is considerably reduced in the case of nucleated PET.

15 r I

0 10 20 30 LO V lK/min)

Variation of crystalline enthalpy (AH,) versus cooling rate V (1 cal = 4.1868 J); pure PET (U), matrix nucleated with (0) sodium benzoate and ( o ) talc, respectively.

Fig. 8.

28


Morphology in PET-glass composites

Finally, we consider the morphology of the samples. Generally, the presence of extraneous surfaces strongly influences the morphology of thermoplastic polymers. The nucleation of a polymer on substrates can be complex, depen- ding on factors such as temperature gradient, shear field, etc. This part of the study is only concerned with the pure PET matrix.

It is generally accepted that PET forms spherulites in the bulk when it is crystallized from the melt. Fig. 9 shows PET spherulites obtained after an non-isothermal crystallization at low cooling rate (0.1 K min-I). There is a size distribution with an average size near 1 or 2 vm.

Fig. 9. Spherulites in PET obtained after an anisothermal crystallization at low cooling rate (0.1 K min-').

In order to study the PET/glass interface, we used both isothermal and anisothermal modes. Similar results are observed for both modes although isothermal crystallization (in this case at 240 " C ) promotes bigger entities which are easier to examine.

In Fig. 10, the low nucleating capability of the glass surface is shown. In fact, most of the spherulites develop from the bulk polymer. Occasionally, the

29

C . Gauthier, J. Chauchard, B. Chabert, M. Saktoun, A. Boudet

Fig. 10. PET/glass interface obtained after an isothermal crystallization at 240 "C without shearing; spherulite develops from the bulk polymer.

crystalline entities grow from the glass surface and take the shape of half a spherulite (Fig. 11). The glass fibres appear to be weakly nucleating agents for the PET crystallization.

We found previously that PET crystallizes at higher temperatures (lower supercooling) in the case of pultruded ribbons, the processing history of the ribbons should be taken into account to explain these kinetic results. So, in order to complete the morphology study, we considered the influence of shear

Fig. 1 1 . PET/glass interface obtained after an isothermal crystallization at 240 "C without shearing; spherulite develops from the glass surface.

30


stresses which are present to the great extent when composites are processed as pultruded ribbons.

We observed that, when the glass fibre is pulled with a controlled speed during crystallization, the number of homogeneous crystallization nuclei grows considerably. This multiplication of the entities prevents a three- dimensional spherulitic development of the lamellae. The latter develop in different directions and sometimes in one direction only, that being normal to the filler surface (Fig. 12). This point has already been observed in a poly- propylene-glass fibre systems and has been explained by the fact that the application of a shear stress at the fibre/matrix interface allows the orientation of the macromolecular chains in the vicinity of the fibres and the growth of a sub-critical nucleus.

In conclusion, the morphology study has shown the weakly nucleating agent effect of the glass fibre for PET crystallization. Nevertheless, the shear flow during processing allows a strong orientation of the macromolecular chains. This strong orientation of polymer chains is not completely eliminated because of the high percentage of unidirectional glass fibre. Finally, pultrusion process can significantly modify the nucleation step of crystallization and consequently the final morphology at the PET/glass interface.

Fig. 12. PETIglass interface obtained after an isothermal crystallization at 240 "C with shearing.

31


Conclusions

In this work, we were interested in the crystallization of neat PET and corresponding glass fibre reinforced matrices in relation with processing conditions.

First, the influence of the treatment temperature Ti in the molten state on the anisothermal crystallization was studied. Prior thermal history influences the non-isothermal crystallization of both bulk PET and glass-PET composite. We observed that T, versus Ti plot presents three different regions. For lower treatment temperatures (region I), the number of homogeneous nuclei (and consequently T,) varies with the melting conditions. However, under more severe melting conditions (region 111), the microstructure of PET might considerably change (chain ruptures, degradation, etc.). Hence, optimal processing conditions could be chosen in region I or I1 to prevent polymer degradation.

In the case of pure PET (without nucleating agent), the crystallization kinetics is considerably modified when the polymer is present in pultruded form: crystallization temperature is always higher for the glass-PET composite. For different other thermoplastic composite systems, similar results have been explained by the nucleating role of stiffening fibres. But, in this case, the morphology study indicates that the glass fibres only play a weakly nucleating role for PET crystallization. Nevertheless, this aspect could be explained by considering the role of shear stresses, which are present to a great extent when ribbons are pultruded. Those shear stresses are probably responsible for a strong orientation of the macromolecular chains which can significantly modify the nucleation step of the crystallization process and consequently the final morphology at the PET/glass interface.

Finally, for nucleated polymers, melting parameters become less influenced by the presence of glass fibres because the number of homogeneous nuclei is always negligible in comparison with heterogeneous ones. Those particles also limit the decrease of the crystalline content with cooling rate. Moreover, the influence of glass fibres is completely negligible when nucleating agents are added. There are only slight differences between talc and sodium benzoate concerning the crystallization kinetics, but the morphological aspect should be considered in future investigations.

The authors thank the French society Renault for support for this research, and B. Saur and C . Jouret for their assistance for performing the TEM studies.

32


F. N. Cogswell, Int. Polym. Process 1 (1987) 157, C.A. 107 (1987) 177382e * Y. Deslandes, M. Day, N. F. Sabir, T. Suprunchuk, Polym. Compos. 10 (1989) 360

B. S. Hsiao, T. Y. Chang, B. B. Sauer, Polymer 32 (1991) 2799 J. M. Kenny, A. Maffezzoli, Polym. Eng. Sci. 31 (1991) 607 A. Jonas, R. Legras, Polymer 32 (1991) 2691 P. T. Curtis, P. Davies, I. K. Partridge, J. P. Sainty, Proc. International Conference on Composite Materials 6 (ICCM 6) and European Conference on Composite Materials (ECCM 2), London, July 1987, Elsevier Applied Science, London 1987, p. 4401 B. Chabert, J. Chauchard, Ann. Chim. (Paris) 16 (1991) 173 E. Devaux, B. Chabert, Polym. Commun. 32 (1991) 464 F. Van Antwerpen, D. W. Van Krevelen, J. Polym. Sci., Part A-2 10 (1972) 2423

l o S. A. Jabarin, J. Appl. Polym. Sci., 34 (1987) 97 P. J. Philips, H. T. Tseng, Macromolecules 22 (1989) 1649

j 2 E. Morel, G. Mavel, Pat. 7401975 Fr. A 2 258254 (1974) and Pat. 8303938, Fr. A 2 542338 (1983)

l 3 G. E. Sweet, J. P. Bell, J. Polym. Sci., Part A-2 10 (1972) 1273 l 4 J. Chauchard, G. Lachenal, Thermochim. Acta 62 (1983) 53 l 5 R. Elenga, R. Seguela, F. Rietsch, Polymer 32 (1991) 1975

B. Wunderlich, “Macromolecular Physics”, Academic Press, London 1980, Vol. 2, p. 389 P. J. Phillips, H. T. Tseng, Macromolecules 22 (1989) 1649 C. Verdeau, These, Ecole Nationale SupCrieure des Mines de Paris, Paris 1988

’

l 8

33

characterization of thermoplastic poly(ethyleneterephthalate)-glass fibre composites, 1....

Documents