A Thermoanalytical Study on Solid-state Cure of Poly( p-Phenylene Sulfide)

MIN PARK, KWANG HEE LEE, and CHUL RIM CHOE

High Performance Polymer Laboratory Korea Institute of Science & Technobgy

Cheongryang. Seoul, Korea

and

WON HO JO

Department of Tzu3Y.e Engineering Seoul National University

Seoul 151 -742, Korea

Differential scanning calorimetry (DSC) was used to investigate the solid-state cure process of poly(pheny1ene SulfideXPPS) resin. Virgin PPS resin in an open sample pan was cured in DSC cell. Either air or oxygen was used as a curing atmosphere. Cure temperatures were in the range of 200 and 250 “C, which are below the melting point of PPS resin. Cure temperature as well as atmospheric condition influenced the cure behavior of PPS in the solid state. Both the rate and the amount of cure increased with increasing cure temperature. On the other hand, the time to reach the maximum cure rate was independent of cure tempera- ture. Changing the atmosphere from air to oxygen increased both the cure rate and the amount of cure. The size effect of PPS particles on the cure reaction was also discussed.

INTRODUCTION

oly(pheny1ene sulfideXPPS) is a high performance P thermoplastic recognized for its unique combina- tion of properties, including good mechanical proper- ties, thermal stability, and chemical resistance ( 1-3). Among its various properties, the curability has been considered as the most interesting and significant characteristic. This cure property of PPS allows pre duction of many products with varying molecular weight from a single starting material (4). The curabil- ity of PPS means that this polymer undergoes chemi- cal reactions when exposed in an oxidative environ- ment at elevated temperatures. The melt flow of this polymer is reduced by these chemical reactions (5). The term “cure” has been generally used in indus- tries to describe these chemical reactions, although not identical to those of thermoset resins. Although the cure process (especially solid-state cure) has been widely used for increasing the molecular weight of virgin PPS, it has been recognized as a serious “bot- tleneck in the overall production of PPS resin owing to the requirement of relatively long process time (over 6 h). Much effort for reducing the cure process time, therefore, has been expended ever since the commercialization of this polymer (6-12). The cure reactions of PPS can be divided into two categories:

melt cure and solid-state (or pre-melt) cure. The pro- cedures and usage of cure processes are described elsewhere in detail (13). The elucidation of the cure mechanism by a chemical method is difficult because of the inherent excellent chemical resistance of PPS resin. Hawldns (14) studied the cure mechanism us- ing a model compound and FT-IR He susested that the cure reaction of PPS consists of chain scission, extension, and cross-linkings. Joshi and Radhakrish- nan ( 15) also studied the melt cure of low molecular weight PPS using an IR technique. They reported that the crosslinking produces 1,3,5-tri-substitution or 1,3,4,5-tetra substitution in the units of the PPS chain. In particular, they suggested the formation of five-membered rings during the melt cure process. Recently, Ma et aL, (16) also studied the cure mecha- nism of PPS using various analytical techniques, such as FT-IR, solid-state CI3-NMR, and rheometric mea- surements. They concluded that the two most impor- tant reactions in the melt cure of PPS were crosslink- ing and chain extension and that the distribution of molecular weight became broader and shifted toward hgher molecular weight with increasing cure time. Love11 and Still ( 17) reported that the cure tempera- ture was an important factor in the cure of PPS based on their solubility experiment in l-chloronaph- thalene. They found that an appreciable amount of

POLYMER ENGfNEERiNG AND SCIENCE, JANUARY 1994, Vol. 34, No. 2 81

Min Park, Kwang Hee Lee, Chul Rim Choe. and Won Ho Jo

cure reaction could be obtained even under nitrogen environment merely by increasing the cure tempera- ture.

The change of internal structure of PPS resulting from the cure reactions inevitably causes the change in crystallization behavior, which is closely related to the morphology developed during processing. The ef- fect of cure on morphology and properties of PPS has been investigated by Cheng and Ho (18). Recently, investigators (19, 20) also pointed out the increasing crystallization rate with increasing the number of cycles of the melting-crystallization process. Al- though progress has been made in the study of the crystallization of PPS (2 1-27), the systematic studies of the cure process are few, except for some patents. For this reason, we have decided to undertake sys- tematic studies on the cure behavior of PPS and its effect on the structure and properties of PPS prod- ucts. As a first step of our research, we have pre- sented the phenomenological observations on the cure behavior of PPS using the isothermal DSC (dif- ferential scanning calorimetry) technique.

EXPERIMENTAL

A PPS resin used in this study was Ryton V- 1 in the form of off-white neat powder from the Phillips Petroleum Company. Molecular weight estimated from intrinsic viscosity measurement using Mark-Houwink equation parameters reported by Stacy (28) was about 18,000, which was in good agreement with other reported values (29). The detailed procedure for deter- mining molecular weight from viscometry is de- scribed elsewhere (30). Commercial PPS pellets hav- ing a cylindrical shape were also supplied by the Phillips Petroleum Company. This material is merely known as a base polymer with no reinforcing agents. The samples larger than the neat PPS particles were prepared from the melt-extrudate of the neat PPS. A Brabender twin screw extruder was used for produc- ing melt-extrudate. The melt-extrudate of neat PPS was ground into powder and then sieved using a series of meshes. The sue of the particles was esti- mated using a video-microscope system.

DSC offers a convenient method for gathering pre- cise and reproducible data on various thermal events occurring in polymers. All the calorimetric measure- ments were performed on a Rigaku DSC8230B mod- ule equipped with a TAS-100 data station. A pure indium reference was used as a standard material for DSC calibration. For curing neat PPS resin, the isothermal DSC mode was used. After thermal equili- bration between the sample and equipment had been attained at a predefined cure temperature, the nitro- gen atmosphere was changed to either air or oxygen environment. Cure temperatures employed were 200, 2 10,220,230,240, and 250 "C, all of which are below the melting temperature of PPS (285 "C). The flow rate of atmospheric gases during a DSC run was held at 50 ml/min. The partial integration of the DSC peaks to obtain degree of cure data with cure time

was done on a personal computer using commercial software.

RESULTS AND DISCUSSION

Figure 1 shows typical isothermal DSC thermo- grams of PPS samples heat-treated at 250 "C under different atmospheres. Contrary to the conspicuous exothermic peak under an air environment, there is no peak when the sample is heat-treated in a nitro- gen atmosphere even for a prolonged time. Since the only difference between the two DSC measurements is the atmosphere employed, we think the oxygen in air might be responsible for the exothermic peak on the DSC thermogram. When considering that the rate of heat liberated from the sample would be directly proportional to the rate of cure reaction, the ordinate in the DSC trace may correspond to the cure rate of PPS. In an air atmosphere, the cure rate rapidly reaches its maximum value, then slows down as the intrusions of oxygen molecules into the particle be- come difficult with increasing crosslinking density near the surface. This leads to a very long tail in the exothermic cure peak.

F'lgure 2 shows the normalized DSC thermograms of the samples cured in air at various temperatures. It is noted that the higher the cure temperature, the greater the heat of cure. I t is known that the perme- ability is a product of solubility and diffusivity, and that the sorption of incompressible light gases includ- ing oxygen and nitrogen is almost independent of the temperature in the range of our consideration (3 1). Accordingly, the permeability of oxygen molecules into

2 0 4 0 6 0

Time (mm)

Fig. 1 . Typical DSC thennogram of PPS cured in air and in a nitrogen atmosphere (cure temperature = 250 "C).

1 0

I I I I I I I I

0 20 40 6 0 80 1 0 0 120 1 4 0 1 Time (mi")

0

Fig. 2. Normalized DSC thermograms of PPS cured at dLzer- ent temperatures in air.

82 POLYMER ENGINEERING AND SCIENCE, JANUARY 1994, Vol. 34, No. 2

Solid-state Cure of PPS

PPS resin would depend on temperature mainly through the diffusivity term. It should be pointed out that there exists a critical crosslink density near the surface above which diffusion of sorbed oxygen molecules becomes difficult. Since the sorption rate of oxygen at the surface is relatively independent of temperature, the heat of cure would be proportional to the amount of oxygen diffused into PPS particles before the critical crosslink density has been reached. Therefore, the increase of overall heat of cure is at- tributed to the increased permeability through diffu- sivity with increasing cure temperature. In addition, as the cure temperatures employed were near the melting temperature of PPS (285 "0, the PPS mol- ecules would experience repeated partial melting and recrystallization at these temperatures. It is probable that the solid-state cure occurs dominantly in amor- phous regions. The increase of amorphous regions resulting from the partial melting would provide more opportunities for the solid-state cure reaction. The increases in the diffusivity of sorbed oxygen molecules and in the amorphous region of PPS resin with in- creasing cure temperature are largely responsible for the increase of heat of cure. On the other hand, it is noticeable that the time to reach the maximum cure rate is independent of cure temperature, which is different from the cure behaviour of other usual ther- moset resins. The time to reach critical crosslink density may depend largely on the oxygen concentra- tion near the gas-solid surface. Because the sorption rate is almost independent of temperature, the time to reach the maximum cure rate, which is related in some way to the time of critical crosslink density, would be almost independent of temperature.

Figure 3 shows the degree of cure with time for various temperatures in air. From these observa- tions, we confirmed that the cure temperature has a great influence on the rate of cure. ( 17).

To exarnine the effect of oxygen concentration on the cure process, the PPS samples were cured in a pure oxygen atmosphere. Figures 4 and 5 show the normalized DSC thermograms and the degree of cure

1 .o

0.8

m 2 0.6 ; m 'y

0.4

0.2

0.0 I I I I 5 0 1 0 0 1 5 0 2 0 0

Time (min)

Fig. 3. Lkgree of cure with time for various temperatures in air.

with time for various temperatures, respectively. Though the shape of the exothermic peaks obtained both in air and in oxygen are similar, the time to reach the maximum cure rate is different: about 2 min in oxygen and about 3 min in air.

The variation in heat of cure with cure temperature under different atmospheres is shown in m. 6. The trend of increased heat of cure with increasing cure temperature is obvious in both air and oxygen atme spheres. The difference is that the heat of cure in oxygen is greater than that in air at all temperatures.

9 " C a 100

20 4 0 6 0 80 Time (min)

0. 4. Normalized DSC thermograms of PPS cured at varC ous temperatures in an oxygen atmosphere.

0 . o v I I 1 I 0 2 0 4 0 6 0 8 0

Time (min)

0. 5. Degree of cure with time for various temperatures in an awygen atmosphere.

,/*' .,/ 0-

200 220 2 4 0

Temperalure ("C)

Fig. 6. Heat of cure with temperatures for dt'erent atmo spheres.

POLYMER Effi I#EERfff i AND SCIENCE, JANUARY 1994, Vol. 34, No. 2 83

Min Park, Kwang Hee Lee, Chul Rim Choe, and Won Ho Jo

The difference in the amount of oxygen penetrated into PPS particles before the critical crosslink density has been reached would be responsible for the sub- stantial difference in heats of cure between different environments.

Figure 7 shows the half-cure time, denoting the time to achieve half of cure reaction, with cure tem- .--------- ~. . ~ . . .~

peratures, for different atmospheric conditions. The cure rate in oxygen is faster than that in air for all

centration is preferable for reducing the cure time. To see the effect of the size of PPS particles on the

cure reactions, samples of different particle size were prepared and cured under the same conditions. It was estimated that virgin PPSRyton V-1) has the largest surface area, and the commercial PPS pellet (GR02, from the Phillips Petroleum Co.) in the form of short cylindrical shapes has the smallest. Other sam- ples with intermediate particle sizes are designated as fine EPPS and coarse EPPS. These were prepared from the melt-extrudate of neat PPS. Diameters of various samples were determined using a video-mi- croscope system. Since the shapes of the ground and sieved particles were somewhat irregular, the average value was used as the equivalent particle size. In Table 1, the average diameter and heat of cure of these particles are presented. In the case of PPS pellets, the heat of cure is very small compared to those of finer size. Figure 8 shows the DSC thermc- gram of these samples. It was found that the cure behavior is affected systematically by the size of the sample. As the particle size increases, the exothermic curve becomes broader and the peak tends to shift to a longer time.

Figure 9 shows the effect of particle size on the cure rates. Half-cure time increases with increasing par- ticle size, implying that reducing the particle size

20 4 0 6 0 0 0 100

temperatures, indicating that increasing oxygen con- Time (mm)

Fig. 8. The size effect of PPS particles on DSC thermograms (cure temperature = 250 "C).

Table 1. Sample Size and Heat of Cure (AHcure).

Sample Average Diameter (mm) AHcu,e(cal/g)

Neat PPS 10 14.1 Fine EPPS 140 13.2 Coarse EPPS 350 13.0 PPS pellet 2000 1.9

200 2 2 0 2 4 0 260

Temperature ("C)

iQg. 7. Half-cure time with temperatures for different atme spheres.

Time (mm)

Flg. 9. The sue effect of PPS particles on degree ofcure with time (cure temperature = 250 "C).

causes a speed-up of the cure reaction. However, the heat of cure, indicative of the amount of cure achieved during the entire cure process, is nearly constant unless there is significant difference in surface area between the samples.

CONCLUSION

In this study, we have demonstrated that differen- tial scanning calorimetry can be a useful analytical technique to monitor the solid-state cure of PPS resin. For the solid-state cure of PPS, the atmosphere em- ployed was important to the cure reaction. Increasing cure temperature results in increases in both cure rate and amount of cure, presumably through in- creased permeability of oxygen molecules into PPS particles. Increasing the oxygen concentration of the cure atmosphere has a similar effect on cure rate and the amount of cure achieved during the entire cure process. Particle size also affects the solid-state cure behavior of PPS; finer particles lead to a faster cure rate.

In conclusion, to reduce the solid-state cure prc- cess time of PPS, a higher cure temperature and an atmosphere of higher oxygen concentration are preferable. Using finer particles is another way to increase the cure rate.

REFERENCES 1. H. W. Hill, Jr.. and D. G. Brady, Polgrn Eng. Sci., 16,832 (1976).

a4 POLYMER ENGlNEERiNG AND SCIENCE, JANUARY 7994, Yo/. 34, No. 2

Solid-state Cure of PPS

2. L. C. Lopez and G. L. Wilkes, JMsReu. Macromol Chem

3. C. C. Martin, J. E. OConnor, and A. Y. Lou, SAMPE 9.. 17. A. B. Port and R H. Still. Polym Deg. Stabil, 2, 1 (1980).

Phy.. C29(1). 83 (1989). 18.H. Cheng and G. Ho. Angew. MakromoL, 127, 103

12 (1984). 19. L. Caramaro, B. Chabert, J . Chauchard, and T. Vu-

20. N. A. Mehl and L. Rebenfeld, submitted to Polym Eng.

21. A. J. Lovinger, D. D. Davis, and F. J. Padden, Jr., Polk

22. J. P. Jog and V. M. Nadkami, J. AppL Polym sci. 30,

23. A. J . Lovinger, F. J. Padden, Jr., and D. D. Davis, Poly

24. L. C. Lopez and G. L. Wilkes, Polymer, 30, 147 (1989). 25. L. C. Lopez and G. L. Wilkes, Polymer, 30. 882 (1989). 26. S. S. Song, J . L. White, and M. Cakmak, Polyrn Eng. Sci,

27. G. P. Desio and L. Rebenfeld, J. AppL Polym Sct, 39,

28. C. J . Stacy, J. AppL Polym Sct, 32, 3959 (1986). 29. L. C. Lopez and G. L. Wilkes, Polymer, 29, 106 (1988). 30. C. R Choe and M. Park, in "A Study on Carbon Fiber/

Thermoplastic Composite," Rep. No. UCN745( 1)-4263-6, KIST, Korea, 1990.

31. C. E. Rogers, in Polymer Permeability, Ch. 2, "Permea- tion of Gases and Vapors in Polymers," J . Comyn, ed., Elsevier, New York (1985).

( 1984).

4. D. G. Brady, J. AppL Polym Sci. 36, 231 (1981). 5. D. G. Brady, J. AppL Polym Sci, 20, 2541 (1976). 6. J. T. Edmond, Jr.. and H. W. Hill. Jr., U S . Pat. 3,524.835

to Phillips Petroleum Co. (August 1970). 7. J. S. Scoggin, US. Pat. 3,793,236 to Phillips Petroleum

Co. (February 1974). 8. S. E. Jesus, Eur. Pat. 64,300 to Phillips Petroleum Co.

(May 1982). 9. R L. Dupree, US. Pat. 4,383,080 to Phillips Petroleum

Co. (May 1983). 10. W. H. Beever and D. G. Brady, U S . Pat. 4,405.767 to

Phillips Petroleum Co. (September 1983). 11. K C. Chales, Eur. Pat. 309.916 to Phillips Petroleum Co.

(September 1988). 12. J. F. Geibel, Eur. Pat. 331,964 to Phillips Petroleum Co.

(February 1989). 825 (1990). 13. H. W. Hill, Jr., and D. G. Brady. in Encyclopedia of

Chemical Technology, Vol. 18. p. 793, Wiley, New York, ( 1982).

14. R T. Hawkins, Macromolecules, 9, 189 (1976). 15. S. G. Joshi and S. Radhakrishnan, Thln Solid Films,

16. C. M. Ma, L. Hsiue, W. Wu, and W. Liu, J. AppL Polym

Khanh, Polym Eng. Sci. 31, 1279(1991).

sci

mer. 26, 1595 (1985).

997 ( 1985).

mer, 29, 229 (1988).

30, 944 (1990).

142, 213 (1986).

Sci, 39. 1399 (1990).

POLYMER ENGINEEfflNG AND SCIENCE, JANUARY 7994, Vd. 34, No. 2 85