Crystallization and fiber/matrix interaction during the molding of PEEK/carbon composites

Crystallization and Fiber/ Matrix Interaction During the Molding of PEEK/ Carbon Composites

J. DENAULT and T. W-KHANH

Industrial Materials Institute National Research Council Canada

Boucherville, Quebec, Canada, J4B 6Y4 and

Department of Mechanical Engineering Ecole Polytechnique de Montrkal

Montreal, Quebec, Canada, H3C 3A7

The objective of this work was to investigate the effects of molding conditions (molding temperature, residence time at melt temperature, and cooling rate) on the crystallization behavior and the fiber/matrix interaction in PEEK/carbon composites made from both prepreg and commingled forms. In order to investigate the crystallization behavior of the PEEK matrix, the molding process was simulated by differential scanning calorimetric analysis, DSC. The results show that the prepreg and commingled systems do not have the same matrix morphology; prepreg tape was found to be at its maximum of crystallinity, whereas the commingled system was found to be only partially crystalline. The results show that processing must be camed out at a temperature sufficiently high to destroy the previous thermal history of the PEEK matrix; this is an essential requirement to produce efficient fiber/matrix adhesion in the commingled fabric system. Optical microscopic observations also suggest that matrix morphology near the fibers is dependent on the melting conditions: a well-defined transcrystalline structure at the interface is observed only when the melt temperature is sufficiently high. However, the high temperature of molding can easily result in degradation of the PEEK matrix such as chain scission and crosslinking reactions. Thermal degradation of the matrix during processing is found to affect the crystallization behavior of the composites, the fiber/matrix adhesion, and the matrix properties. This effect is more important in the case of a commingled system containing sized carbon fibers because the sizing agent decomposes in the molding temperature range of PEEK/carbon composites. This produces a decrease of the matrix crystallinity and an elimination of the nucleating ability of the carbon fibers. A transition between cohesive and adhesive fracture is observed when the cooling rate increases from 30"C/min to 71"C/min for the composite made from the commingled fabric. This critical cooling rate is found to closely correspond to a change in the mechanism of crystallization of the PEEK matrix.

INTRODUCTION ODed on the carbon fiber surface. Similarly, in the ecent studies have shown that the physical and Rm echanical properties of PEEK resin and its com-

posites can be greatly affected by the processing cycle (1-9). In fact, investigations carried out on a wide range of semicrystalline polymers (10-13, 14) have shown that the number of surviving nuclei in the melt is dependent on the residence time and the temperature at which the polymer was held before cooling. Lee and Porter (14) have found that increas- ing the residence time of the samples in the molten state produces a decrease in the number of spherulites present in the bulk matrix, and a well-de fined transcrystalline region is subsequently devel-

cise of the PPS/carbon fiber system (15), it has been found that the appearance of the transcrystalline structure is observed only at temperatures at which all of the previous crystalline entities of the matrix have been destroyed. Moreover, transverse tensile tests done on unidirectional PEEK/carbon composite containing a transcrystalline phase show higher strength values than that of the matrix, suggesting a strong interfacial bond between the carbon fiber and the PEEK matrix (14).

While PEEK is recognized for its resistance at high temperature, it has also been found that the PEEK resin degrades in the molding temperature range.

POLYMER COMPOSITES, OCTOBER 1992, Vol. 13, No. 5 361

J . Denault and T. Vu-Khanh

Under oxidizing conditions, in air for example, chain scission occurs and crosslinks are formed (16, 17). causing reduction of crystallizability and discol- oration (1 8).

Consequently, in order to optimize the processing cycle of the PEEK/carbon composite, upper and lower limits of molding temperature and molding time must be defined. In the present work, the APC-2 prepreg and two commingled systems, NCS-1025 and NCS- 1057, have been analyzed. In order to control the final morphology of the PEEK matrix in the molded composites, the compression molding process was simulated by differential scanning calorimetry. The effects of the thermal processing conditions (molding temperature, residence time, and cooling rate) on the crystallization behavior of the PEEK matrix and on the fiber/matrix interaction and matrix degradation are demonstrated. Since fiber/matrix adhesion has been found to be dependent on thermal treatment, the optimum molding time and temperature ranges will also be determined as a function of fiber/matrix interaction.

EXPERIMENTAL.

Materials

The commingled systems, consisting of hybrid yams of intimate blends of continuous thermoplastic ( 150G) PEEK filaments with AS-4 graphite filaments, were obtained from BASF Structural Materials, Inc. The nonwoven fabric NCS-1057 is made from sized AS-4 carbon fibers commingled with (150G) PEEK filaments, and the nonwoven fabric NCS- 1025 con- sists of unsized AS-4 fibers Commingled with (150G) PEEK filaments. The conventional APC-2 prepreg tapes, consisting of "Victrex" PEEK reinforced with unidirectional AS-4 graphite fibers, were obtained from ICI.

Thermal Analysis

In order to investigate the crystallization behavior of the PEEK matrix in the composites, the processing cycle was simulated by differential scanning calorimetry, DSC. The tests were done on a Perkin- Elmer DSC-7. Samples of around 10 mg were used for the analysis.

The decomposition temperature range of the sized graphite fibers was determined by thermogravimetry using a thermal gravimetric analyzer Setaram Model €370. The analysis was made under nitrogen atmosphere on a sample of around 40 mg at a heating rate of 10"C/min .

Processing

Square plaques 150 mm X 150 mni X 3 mm thick were compression molded in a matched mold and pressed in a programmable Wabash press, permitting reproducible molding cycles. The plaques were molded between 365°C and 475°C; the temperature of the melt was measured with a thermocouple inserted in the laminates. Heating was done without pressure,

and when the molding temperature was reached, high pressure was applied. The residence time at 400°C was varied from 1 to 120 min. The plaques molded at 400°C were also cooled at different rates by transfer- ring the mold to a second press set at different temperatures or by modifying the cooling system of the first press. The cooling rate reported was calculated from the time required to cool the melted sample from 400°C to 100°C. assuming that the PEEK crystallization under 100°C is negligible (19).

Microscopy

Pure PEEK resin containing a few carbon fibers was melted between two microscope cover glasses on a hot plate a t 365°C and 400°C and held at the melting temperature for 5 min. After this treatment, the melted samples were cooled slowly to around 260°C and then quenched to room temperature for observation of the matrix morphology. The crystalline structures of the PEEK matrix were observed using a polarized light transmission microscope Leitz/Wetz- lar, Dialux 20.

The short beam shear fracture surfaces of the composites were examined by means of a Jeol TSM-220 scanning electron microscope (SEMI.

RESULTS AND DISCUSSION

The mechanical performance of thernioplastic rom- posites is closely related to the matruc morphology, which is dependent on the thermal history. In order to control the final morphology of the PEEK matrix in the molded composites, it is essential to understand the effect of thermal history on its crystallization behavior. For this purpose the comprcssion molding process was simulated by DSC. The thernial cycle of the DSC experiments is presented in Fig. I . Under nitrogen atmosphere, samples of APC-2 prepreg, NCS- 1025 fabric, NCS- 1057 fabric. and pure (150G) PEEK resin removed from the commingled fabric were heated at 5"C/min from room temperature to a temperature of treatment in the molten state, T,,,, ranging from 365°C to 475°C. After a given residence time, t,, ranging from 1 to 120 min a t Tmq the samples were cooled at various rates ranging from 0.5"C/min to 16O"C/min. During the heating and cooling periods. the endothermal peak of the melting and the exothermal peak of crystallization were respectively recorded.

Thermal cycle

Fig. 1. 'Riemrrl cycle ILsedjofor the DSC experiments

362 POLYMER COMPOSITES, OCTOBER 7992, Vol. 73, No. 5

Cnjstallization and Fiber/ Matrix Interaction

0 5 308 c.

First, the melting behavior of the PEEK matrix in the prepreg and the fabric materials was analyzed by DSC. Typical thermograms of these materials are shown in Fig. 2. These melting curves reveal that the morphology of the PEEK matrix in the APC-2 prepreg is different from that in the NCS- 1025 and the NCS- 1057 fabrics. The presence of a n exothermal peak in the heating stage of the fabrics indicates that crystallization occurs and the PEEK filaments are probably only partially crystalline. In the case of APC-2 prepreg the absence of this exotherm peak suggests that the PEEK matrix is at its maximum crystallinity.

Effect of Molding Temperature

Figure 3 shows the effect of melting temperature, T,,, on the temperature a t the onset of crystallization, Tci, of the three PEEK/carbon systems. In these experiments the residence time in the melt was fixed at 5 min and the cooling rate was kept constant at 5”C/min. For purposes of comparison, the result on the pure (150G) PEEK resin is also presented. It is possible that the PEEK matrix in APC-2 may differ from the (1 50G) PEEK resin in terms of molecular wcight and additive content. However, according to the information provided by ICI, the ( 150G) PEEK is the closest commercial grade to the PEEK matrix in APC-2.

As has been observed in the case of PPS/carbon prepreg (1 5) , three distinct regions can be seen from the curves of Fig. 3. The first region, which corresponds to the decrease of TCi with T,,, at low melting temperature, can be associated with the decrease of the number of remaining crystalline entities in the melt when T,,, increases. The second region, a plateau region, corresponds to the melting condition in which all of the pre-existent crystalline entities are destroyed. At high melting temperatures, there is a third region where Tci decreases as T, increases. suggesting that modification of the PEEK matrix oc-

-

NCS-1025

N T 50 70 90 110 I S 0 150170 190 210 230 250 270 290 310 330 310 370 391

TEMPERATURE (“C)

Fig. 2. ‘Ajpical thermograms oj- melting OJ fhe PEEK matrix presents in the APC-2 prepreg, and NCS-1025 and NCS- 105 7 .fabrics .

0 - 326 I

322

320

NCS-1057 + NCS-1025 0 APC-2

310 1 \ \ \I

E 0 I- Melting temperature (“C)

Fig. 3. Variation of temperature of the onset oJ crystallizu tion. T,,. as a ftcnction of T,,, for the PEEK resin, APC 2 prepreg, and NCS- 1025 and NCS- 1057 fabrics.

curs. It has been found that the PEEK matrix evolves a t high temperature: chain oxidation and a crosslinking reaction were the proposed mechanisms for its thermal degradation ( 16- 18).

Taking into account the experimental error, which is approximately 0.5”C in the determination of T,,, the following observations can be made from the curves in Fig. 3. In the plateau region, it can be observed that the crystallization temperatures of the APC-2 and the NCS- 1025 are slightly higher than that of the pure PEEK resin, whereas the NCS- 1057 fabric shows lower Tci than that of the neat resin. Good nucleating ability of the AS-4 carbon fibers ( 14, 18) can explain the higher crystallization temperatures observed in the APC-2 and NCS- 1025. Figure 4 shows the cross- polar optical niicrographs of crystallized PEEK resin in the presence of sized (NCS-1057) and unsizcd (NCS- 1025) AS-4 carbon fibers. As expected, these pictures show that unsized AS-4 carbon fibers have a strong nucleating ability in the PEEK matrix. A transcrystalline structure can be easily observed around these fibers. I t should be noted that the columnar crystalline structure was observed only when the melt temperature was sufficiently high to destroy all of the preexisting crystalline entities. At lower temperatures it was not possible to distinguish any specific structure around the fiber surface.

In the case of sized carbon fiber no particular crystalline region was observed at the fiber surface, regardless of the melting temperature (Fig. 4 ) . In fact, thermogravimetric analysis of the sized carbon fibers of the NCS-1057 revealed that degradation of the sizing agent occurs between 300°C and 400°C (Fig . 5). exactly in the melting temperature range of the PEEK matrix. Consequently, the perturbation of the crystallization behavior of the PEEK matrix in the


J. Denault a n d T. Vu-Khanh

(b)

Fig. 4. Cross-polar optical micrographs of PEEK crystallized in presence of (d sized and (b) unsized carbonfibers.

101.0

100.5

m5 t Temperature, 'C

Fig. 5. 'lhermvgravimetric analysis of sized carbon fibers present in the NCS-1057 fabric.

presence of sized carbon fibers can be created by the decomposition of the sizing agent.

In the third region of Fig. 3 it can be observed that the presence of sizing on carbon fibers also induces

premature matrix degradation. The drop of Tci occurs at around 420°C for the NCS-1057, in comparison with 460°C for the APC-2, the NCS-1025, and the neat resin.

Figure 6 shows the evolution of the heat of crystallization of these materials as a function of the melting temperature. It can be seen that as opposed to the variation of T,, (Fig. 31, the heat of crystallization is not affected by T, in the low temperature range: in all cases, it remains relatively constant with increas- ing T,. up to the region of matrix degradation. This result indicates that the melting temperature affects only the number and the size of the crystalline entities, as observed by several authors (10-13, 21) but does not alter the ultimate level of crystallinity of the composites. From Fig. 6, it can also be observed that the presence of unsized carbon fibers does not affect the level of crystallinity of the PEEK matrix; however, the presence of sized carbon fibers produces a continuous decrease in the heat of crystallization with melting temperature. From these results it appears that the degradation of the sizing agent also produces a significant decrease in the crystallizability of the PEEK matrix in the composite.

Figure 7 shows the nonisothermal crystallization behavior of the different systems melted at 400°C. Assuming that the crystallization enthalpy is directly related to the crystallized polymer, the transformed fraction, X(t ) /X( .o) of the PEEK matrix during the crystallization process can be obtained by the inte- gration of the exotherm peak (22). The transformed fraction of PEEK is plotted as a function of time, t , which is defined by the relation t = (To - T)/+. In this relation, To is the starting temperature of the crystallization exotherm as defined in (221, T is the temperature corresponding to the time t , and + is the cooling rate.

At 400°C a temperature corresponding to the situ-

20 '

10 '

x PEEK resin

+ NCS-1025

NCS-1057

0 APC-2

360 380 4 0 0 4 2 0 4 4 0 4 6 0 4 8 0

Melting temperature ("C)

Fig. 6. Variation of the heat of crystallization of PEEK resin, APC-2 prepreg, and NCS-1025 and NCS-1057 fabrics. as a function of T, .


C ystall ization and Fiber/ Matrix Interaction

0.8 -

a6 - a4 -

0.2 - .

0 AFJC-2 NCS-1057

0.0 0 2 4 6 8 1 0

Time (min) Fig. 7. Evolution of the transformedfraction of PEEK during the cooling (SC/ mid of the PEEK resin, the APC-2 prepreg, and the NCS-1025 and the NCS-l057fabrics, which were melted at 400°C.

ation at which all of the previous crystalline entities are destroyed, it can be observed that the presence of the unsized carbon fibers does not seem to affect the overall rate of crystallization of the PEEK matrix. This result is rather surprising, since the nucleating ability of the unsized carbon fibers was easily observed by optical microscopy (Fig. 4). Hence, the nucleating ability of the unsized carbon fibers favors the initia- tion of the crystallization process, as shown by the increase of Tci (Fig. 3), but does not affect the development of these crystalline entities. However, at this temperature the presence of sized carbon fibers produces a decrease of the crystallization rate. In fact, to reach a transformed fraction of 0.9, it takes 3.1 min for the PEEK resin, the APC-2 prepreg, and the NCS- 1025 fabric, whereas for the NCS-1057 fabric it takes 4.2 min. The retardation in the crystallization p r e cess is probably also a result of the degradation of the sizing agent. As discussed above, the sizing agent on the carbon fibers in the NCS- 1057 fabric decomposes between 300°C and 400"C, resulting in a perturbation in the crystallization behavior of the PEEK matrix. Figures 8 and 9 show the evolution of the transformed fraction of the PEEK matrix during the cooling of the NCS-1025 and the NCS-1057 fabrics, respectively, melted at various temperatures. From these curves it can be observed that from the melting temperature around which matrix degradation could be detected, a significant decrease of the rate of transformation of the PEEK matrix occurs.

Figure I 0 shows micrographs of short beam shear- ing fracture surfaces of the three composites molded at 365°C. 400°C, and at temperatures corresponding to matrix degradation: 475°C for the APC-2 and the NCS- 1025, and 460°C for the NCS- 1057 composite. I t can be seen that while the fiber/matrix interaction in the case of NCS-1025 is strongly dependent on the molding temperature, the NCS- 1057 and the APC-2

1 .o

0.8

0.6

0.4

0.2

0.0 0 5 1 0 1 5

Time (min) Rg. 8. Evolution of the transformed fraction of PEEK during the cooling (SC/min) of the NCS-1025 fabric, melted at various temperatures.

1 .o

0.8

0.6

0.4

0.2

0.0 0 5 1 0 1 5 0

Time (min)

Rg. 9. Evolution of the transformedfraction of PEEK during the cooling (SC/min) of the NCS-1057 fabric, melted at various temperatures.

composite always show good interfacial adhesion. This could be explained by the fact that although the APC-2 and the NCS-1025 materials consist of almost the same constituents [Victrex PEEK and AS-4 carbon fibers for the APC-2 prepreg, and (150G) PEEK and AS-4 carbon fibers for the NCS- 1025 fabric], they are not in the same physical state. In the case of the APC-2 system, strong fiber/matrix interaction al- ready exists in the prepreg form, whereas in the NCS-1025 the PEEK filaments are separate from the carbon fibers fabric before molding. Consequently, the fiber/matrix interaction in the NCS- 1025 fabric has to be created during the molding process. It is worth noting that this adhesion is not necessarily due to the transcrystalline structure, since the etched surfaces of the molded laminates with good fiber/ma-


J . Denault and T. Vu-Khanh

APC - 2

NCS-1025

NC S -1057

b) FWJ. 10. Micrographs of fracture surfaces of the composites molded (a) at 3 6 5 C . (bl a t 400"C, and (c) at temperature corresponding to matrix degradation: 475°C for the APC-2 and the NCS-1025, and 460"Cfor the NCS-1057 composile.

trix adhesion and well-developed spherulites did not show any transcrystallinity at the fiber/matrix interface. The same observation has been made by Pea- cock, et al. (23-25). In fact, from most of the morphe logical studies reported on PEEK/carbon composites, it appears that the transcrystalline structure is a negligible element of the global crystallinity of the PEEK matrix. Lustiger (26) pointed out that the columnar crystalline structure along the carbon fibers is observed only in a specific region of a very slowly cooled specimen.

In the case of NCS-1057 fabric, the matrix retention is probably related to a completely different mechanism. Although the sizing agent on the AS-4 carbon fibers is found to decompose in the temperature range of 300°C to 400°C, it still promotes interaction between the fibers and the matrix, and good matrix retention on the fibers can be seen on the fracture surface regardless of the molding temperature. The fracture surfaces of the composites molded at relatively high temperatures (475°C for APC-2 and NCS- 1025, and 460°C for NCS- 1057) also show that the matrix ruptures in a brittle mode, while at lower temperatures the mode of rupture is rather ductile with some microscopic stretching of the matrix.

Effect of Residence Time

To simulate the effect of residence time on the crystallization behavior of the PEEK matrix, the DSC experiments were conducted as follows: The samples were heated at 5"C/min from room temperature to a treatment temperature of 400°C. After a given residence time ranging from 1 to 120 min, the samples were cooled to room temperature at 5"C/min. While molding temperature was found to strongly affect the temperature at the onset of crystallization, qi ( R g . 3), the effect of residence time was found to be less important. Figure 1 1 shows the effect of residence time on the temperature at the onset of crystallization, T,,, of the APC-2, the NCS-1025, and the NCS- 1057 composites. The results show that T,, remains relatively constant as a function of residence time until the time at which matrix degradation occurs for the APC-2 and the NCS- 1025, suggesting therefore that at this treatment temperature the crystallization process is not altered by the residence time. However, in the case of NCS-1057, T,, continuously decreases with the residence time. The result suggests that matrix degradation occurs in this composite, probably induced by the degradation of the sizing agent at this temperature.

366 POLYMER COMPOSITES, OCTOBER 1992, Vo/. 13, No. 5

Crystallization and Fiber/ Matrix Interaction

oa Y

C - 0 320 Q I1 c

N

2 2 300 !! 0 ' 1 0 1 0 0 1000 P E Residence time at 400°C (mln) r-"

Fig. 1 1 . Variation of (he temperature of the onset of crystd lizaiion, Tc,, ofthe PEEK matrix in t h e m - 2 prepreg, and the NCS-1025 and the NCS-1057 fabrics, as a function of the residence time at 400°C.

Figure 12 shows micrographs of short beam shear- ing fracture surfaces of the NCS-1025 laminates molded at 400°C for various residence times. It has been demonstrated (6, 8, 9) that in short beam shear samples, fracture occurs mainly at the fiber/matrix interface, and the short beam shear strength reflects the fiber/matrix adhesion. From these micrographs it can be observed that the creation of a fiber/matrix interaction in the commingled fabric made with unsized carbon fiber is dependent on the residence time at this molding temperature. Fiber/matrix adhesion can be observed only after 5 min of residence time, and in addition, the amount of matrix retention o h served on the carbon fiber surface seems to increase as the residence time increases, suggesting stronger fiber/matrix interaction. Moreover, the critical residence time corresponding to the appearance of the fiber/matrix adhesion in the NCS- 1025 composite was found to increase to 10 min when the molding temperature is reduced to 380°C (9). These results suggest that fiber/matrix interaction in PEEK/ carbon composite is a complex kinetic process that depends on both time and temperature. Inan investi- gation on polycarbonate/carbon fiber composites, Brady and Porter (27) also pointed out that adsorp tion is likely the primary mechanism for the fiber/matrix interaction. In the case of PEEK, adsorp tion is probably not the only primary mechanism, but a chemical interaction between the matrix and the highly reactive AS-4 carbon fibers can also play a major role since an increase in the adhesion is o h served at very long residence times when the matrix is degraded to a certain extent (Fig. 1 1 ) . I t is worth noting that the cooling rate in these laminates was

[C) Fg. 12. Micrographs offracture surfaces of the NCS-1025 laminates as afunction of residence time at 400°C; d 1 min, b) 5 min, c) 120 min.

kept constant at about 20"C/min. Consequently, the observed enhancement in fiber/matrix adhesion is not due to the crystalline structure of the matrix around the fiber surface.

Effect of Cooling Rate

Cooling rate is another major parameter in the molding cycle; it can have a strong effect on the matrix morphology in the composite and consequently on the performance of the final part. To simu-


J. Denault and T. Vu-Khanh

late the effect of the cooling rate on the crystallization behavior of the PEEK matrix in the composites, the DSC experiments were conducted as follows: Under nitrogen atmosphere the samples were heated at 5"C/min from room temperature to a melting temperature of 400°C. After 5 min of residence at 400°C. the samples were cooled at various cooling rates ranging from 0.5"C/min to 160°C/min. In order to characterize the final morphology of the materials, the samples were reheated again at 20"C/min to a temperature of 400°C. During the cooling and the reheating periods, the exothermal peak of crystallization and the endothermal peak of melting were respectively recorded.

The effects of cooling rate on the temperature at the maximum of the peak of crystallization, Tcl,. of the APC-2 and NCS-1025 composites are presented in Rg. 13. For purposes of comparison, the results of the same analysis done on the neat (150G) PEEK resin are also presented. Flynn and Wall (28) and Ozawa (29, 30) have shown that the relationship

1.052 Ea l n 0 =

R T C P

between the cooling rate, 0, and the temperature at the maximum of the crystallization peak can be used to characterize a kinetic phenomenon such as polymer crystallization, assuming that the crystalline conversion at the maximum of the peak is constant. In this equation, R is the gas constant. In Fig. 14 is reported the relative degree of crystallinity, X,,/X,, at the maximum of the crystallization peak. This value is determined at the inflection point of the integrated heat flow curves. As discussed by Ozawa (29, 30). the constant value of X t p / & over the entire cooling rate range permits the application of Eq 1 in the interpretation of the nonisothermal crystallization data obtained for the pure PEEK resin and the

A

r

C .- E

Y

E \

c Q

W C

0 u

.- v

0.0

slope 3

0 APC-2

I I 0 . 0 0 1 9 5

-1.0 I U.UO165 0 .00175 0 . 0 0 1 8 5

Fig. 13. Effect of cooling rate on the temperature at the maxC mum of the peak of crystallization for the APC-2 and NCS- 1025 composites and pure PEEK resin.

* 1.0

.- C 0.9

E 0.8

- .- - - cn ?' 0.7 0

0.6 0

0.5 al ? 0.4 m 0) '0 0.3

m 0.1

0) > .- c. - al a 0.0

0 1 0 0 2 0 0

Cooling rate ("C/rnin)

FUJ. 14. Variation of the relative degree of crystallinity at the maximum of the nonisothermal crystallization peak as afunction of cooling rate.

PEEK/carbon composites. In a study made on the nonisothermal crystallization behavior of the (450G) PEEK resin and APC-2 composites, Cebe (31) has found that the relative degree of crystallinity corresponding to the maximum of the crystallization peak increases with the decreasing cooling rate for the neat resin, while that of APC-2 remains constant. Hence, as opposed to what is observed by this au- thor, the results in Figs. 1 3 and 14 suggest that crystallization of the neat PEEK follows the same kinetic as that of PEEK/carbon composite. This could be due to a different grade of the PEEK resin in this work.

The plots in Fig. 13 show two changes in slope, which occur at cooling rates of approximately 5"C/min and 40"C/min. These transitions suggest two changes in the mechanism of crystallization for the PEEK matrix, and this phenomenon does not seem to be affected by the presence of carbon fiber. While the first transition at 5"C/min corresponds exactly to that reported by Tung and Dynes (321, the second transition has never been observed. Although no sufficient experimental results are presently available to interpret this transition, the high cooling rate involved above 40"C/min could correspond to an incomplete level of crystallization. In fact, as proposed by Cebe and Chung (33), the crystallization of a thermoplastic matrix such as PEEK can be represented by a three-step model. The first step is characterized by the born spherulites growing radially into a sea of remaining amorphous material. As the spherulites grow, they eventually impinge upon their neighbors, resulting in a decrease in the rate of transformation of the amorphous phase into the crystalline phase: this constitutes the second step of the proposed model. Finally, the third stage of crystallization is characterized by many small crystals, which grow between existing lamellae. This phenomenon (called secondary crystallization) has been experimentally



B B APC-2 APC-2

+ 30

0 5 0 1 0 0 1 5 0 2 0 0 2

Cooling rate ("Clmin)

Fig. 15. Percentage of Crystallinity in the APC-2 and NCS- 1025 composites as afunction of cooling rate.

verified by Bassett's work (34), in which the secondary in filling lamellae have been revealed by TEM on etched surfaces. While this model for PEEK crystallization has been developed for isothermal crystallization, it could be easily extended to the case of nonisothermal crystallization. In fact, at a high cooling rate, little time is allowed for spherulite develop ment, resulting in incomplete crystallization, probably without complete impingement. Moreover, at a slow cooling rate, small crystallites have enough time to develop into the principal lamellae of the impinged spherulites, leading to secondary crystallization.

Based on this model, the transitions in Fig. 13 at a cooling rate of 40"C/min are probably due to the fact that spherulite development is stopped before impingement, leading to material of low crystallinity. Moreover, cooling rates lower than 5"C/min lead to secondary crystallization of the PEEK matrix, and highly crystalline material. This interpretation is in accordance with the strong increase of the crystallinity of the samples cooled at low rates (Fig. 15). The secondary crystallization can be observed on the DSC thermogram of the melting behavior of the crystalline phase of the composite cooled at low rates, as shown in Fig. 16. For cooling rates lower than 5"C/min, the melting peak shows a shoulder before the primary peak occurs. This shoulder is probably related to the melting of secondary crystalline regions between the main crystallites (29, 32, 34-36).

Between 5"C/min and 40"C/min, the observed slope could be related to the second stage of crystallization model, corresponding to the beginning of the impingement process and the secondary crystallization. In fact, secondary crystallization can probably start before the completion of impingement. The crystals probably nucleate and grow before complete impingement, but they are thinner and smaller in extent as a consequence of their having grown in a

more constrained volume than the dominant lamellae. The calculated activation energy associated with the three slopes of curves of Fig. 13 are reported in Table 1. The activation energies obtained from slopes 2 and 3 correspond to those reported by Tung and Dynes (321, who have calculated 1 15 kcal/mol for the slow quench rates and 75 kcal/mol for the fast rates.

Figures 17 and 18 show, respectively, the fracture surfaces of the short beam samples of the APC-2 and NCS- 1025 laminates molded at 400°C for a residence time of 5 min and cooled at various rates. While fiber/matrix interaction is always present in the APC-2 laminates, matrix residue on the carbon fiber after fracture decreases significantly as the cooling rate increases. For the NCS-1025 composites, a transition between cohesive and adhesive fracture is o b served between cooling rates of 30"C/min and 7 l"C/min. These microscopic observations suggest that a critical cooling rate between 30"C/min and 71°C/min probably results in a change in the fiber/matrix interaction for the PEEK/carbon composite made from both the commingled and prepreg systems.

This critical value corresponds closely to that o b served above (see Fig. 13) for the variation of Tcp as a function of the cooling rate, suggesting a dependence between the strength of the fiber/matrix interface and the crystalline morphology of the PEEK matrix around the fiber surface. However, as previously mentioned (8, 91, interfacial crystallization is not the sole parameter responsible for the creation of a fiber/matrix interaction in PEEK/carbon composite.

1 O ' k W b W kt0 * W & W 24000 28pW b M &oPa At0

TenpeP.turel'C1

Flg. 16. DSC thennogram of the melting behavior of the NCS- 1025 composite previously cooled at I T / min.

Table 1. Activation Energies of the Crystallization Process

E, (Kcallmol)

Slope 1 Slope 2 Slope 3

(150G) PEEK 19 53 107 NCS-1025 18 65 131

APC-2 19 58 100


J. Denault and T. Vu-Khanh

td Fg. 17. Micrographs of fracture surfaces of the APC-2 laminates molded at 400°C for a residence time of 5 inin and cooled at various rates: (d T C / rnin, (b) 20"C/ rnin. (c) 63°C / rnin.

In fact, it has been found that crystallinity is efficient only when a fiber/matrix interaction has been created in the composite. Since a fiber/matrix interaction is present in the prepreg form, annealing of the rapidly cooled (200"C/min) laminate at 300°C for 4 h leads to an increase of the short beam shear strength from 73 MPa to 86 MPa, reaching approximately the value of the slowly cooled laminate. However, the same annealing experiment performed on the rapidly

cooled NCS- 1025 laminate leads to equivalent im- provement only if the residence time at a given molding temperature is sufficient to permit the creation of a fiber/matrix interaction. This interaction appears to be a complex kinetic process, which depends on both time and temperature.

CONCLUSION

In this work the effects of processing on the crystallization behavior and on the microstructure of PEEK/carbon composites have been discussed. It is shown that the prepreg and commingled composites do not have the same matrix morphology; prepreg tape is found to be at its maximum of crystallinity, whereas PEEK filaments in the commingled systems are only partially crystalline before molding. To destroy all the existing crystalline entities in the PEEK matrix, molding must be carried out at sufficiently high temperatures. When the molding temperature is increased, the crystallization temperatures of the APC-2, NCS-1025 and NCS-1057 composites decrease, attain a plateau region, and decrease again with the melt temperature. The plateau region probably corresponds to the molding temperature range in which all the crystalline entities are eliminated. At lower temperatures, these entities remain in the melt and play a nucleating role. At higher molding temperatures, matrix degradation occurs, resulting in a decrease in the crystallization temperature, the heat of crystallization, and the overall rate of crystallization. The use of a sizing agent does not result in good fiber/matrix adhesion because the degradation of the sizing agent occurs at the melting temperature of the PEEK matrix, resulting in premature matrix degradation.

In the APC-2 prepreg, good fiber/matrix bonding is always present. However, in the commingled fabric, this interaction must be created during the molding process. Fiber/matrix adhesion in the commingled system depends on molding temperature and residence time at melt temperature, as well as cooling rate.

A transition between cohesive and adhesive fracture is observed when the cooling rate increases from 30"C/min to 71°C/min for the NCS-1025 composite. This critical cooling rate is found to correspond to a change in the mechanism of crystallization of the PEEK matrix. An Arrhenius representation of the crystallization behavior of the pure PEEK resin and the PEEK present in the composites shows two changes in the mechanism of PEEK crystallization. While no sufficient experimental results are presently available to properly interpret this phenomenon, at high cooling rates incomplete crystallization results in a lower fiber/matrix adhesion in the composites.

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Crystallization and fiber/matrix interaction during the molding of PEEK/carbon composites