Effect of curing on the internal structure of polyphenylene sulphide coatings

Thin Solid Films, 142 (1986) 2 13-226 PREPARATION

AND

213

CHARACTERIZATION

EFFECT OF CURING ON THE INTERNAL POLYPHENYLENE SULPHIDE COATINGS

STRUCTURE

OF

S. G. JOSH1 AND S. RADHAKRISHNAN

Polymer Science and Engineering Group, Chemical Engineering Division. National Chemical Laboratory. Pune 41 I 008 (India) (Received

August

13, 1985; revised December

3, 1985; accepted

January

24, 1986)

Internal structural changes in polyphenylene sulphide coatings have been investigated as a function of curing temperature by means of scanning electron microscopy, X-ray diffraction and IR spectroscopy. The adhesion, recrystallization behaviour and chemical resistance of the films were found to be dependent on the curing temperature. These properties have been correlated with the physical and chemical changes taking place during the curing process.

1.

INTRODUCTION

Polymeric coatings used for high temperature and corrosion-resistant applications have in the past been mostly based on fluorinated polyolefins’. However, in recent years coatings of polyphenylene sulphide (PPS) have found increasing application since they are more economical, easy to formulate and apply, nonpolluting during both the coating process and the bake cycle, and, in some instances, even better than polytetrafluoroethylene (PTFE) in hardness, adhesion etc.2-4 PPS has also drawn considerable attention from physicists and electrical engineers because it can be made conducting by doping with charge transfer agents such as arsenic pentafluoride’-‘. PPS coating can be applied to various kinds of substrates by many different techniques such as water-dispersed slurry spraying, fluidized-bed coating, electrostatic powder coating and flocking *. In all these techniques, the first application step has to be followed by a bake cycle at high temperature (above 35OC) in order to cure the polymer and to form a hard tough coalesced coating which adheres well to the substrate. Although some reports are available from the major manufacturer of this polymer regarding the application techniques and durability of the films etc. ‘*‘O, detailed investigations of the structural changes taking place at various stages of the curing cycle have not been reported previously. These studies are important for understanding the relationship of various parameters, such as polymer melt index, curing time and temperature schedule, with the properties of the final product.

0040-6090/86/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

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2. EXPERIMENTAL

S. G. JOSHI, S. RADHAKRISHNAN

DETAILS

A sample of PPS having low melt index (Ryton grade Vi) obtained from Phillips Petroleum Co., U.S.A., and laboratory-made virgin polymer having high melt index, were used in making the coating formulations. The synthesis and characterization of the virgin polymer have been described elsewhere”. The polymer was mixed with pigment, propylene glycol, wetting agent and water, and ball milled for 16 h to form a slurry. The pigment used was TiO, (Anatase) or carbon black (SAF channel black). The composition contained 100 parts by weight PPS, 30 parts TiO,, 60 parts propylene glycol, 3.5 parts wetting agent (Triton X) and 100 parts water. In the case of unpigmented coating, the formulation contained 100 parts PPS, 20 parts propylene glycol, 2.5 parts wetting agent and 130 parts water. The slurry was spray coated onto clean glass substrates, microscope slides (Microaids), aluminium sheets or steel plates, using a conventional paint-spraying unit with an air pressure of about 25 lbf in-‘. The coated substrates were directly passed into a ventilated oven preheated to the curing temperature, which ranged from 350 to 450 “C, and left there for 30 min. The samples were removed after the prescribed time into the ambient conditions, thus completing the curing cycle. The coating thickness was measured by means of a Fischer Multiscope 650, and ranged from 25 to 30 urn depending on the substrate-to-spraying unit distance. In order to investigate the internal structure of the coatings, the substrates together with the coatings were fractured vertically after scribing a deep line on the back side. A thin film (less than 200 A) of Au-Pd alloy was then vacuum deposited onto the fracture surface to increase the contrast, and the surface was examined by scanning electron microscopy (SEM) using a Stereoscan Cambridge model 150. The crystalline content of the coatings was estimated from a wide-angle X-ray diffraction (WAXS) technique in a manner similar to that reported earlier”. The changes in chemical and also supermolecular structure were monitored by means of attenuated total reflectance (ATR) IR spectroscopy for which flexible aluminium substrates coated and cured simultaneously with the others mentioned above were used. In some cases, transmission IR spectra of free-standing films were also recorded. A Perkin-Elmer 283 spectrophotometer along with an ATR attachment (KRS-5 prism) was used for this purpose. In order to study the resistance of the coatings to chemical environment, the films were dipped in 50% solutions of sulphuric and chromic acids as well as 10% solutions of NaOH and NaCl for short duration (10 min) and then washed and dried. The changes in IR spectra or surface electrical conductivity were then monitored. A Keithley 614 digital electrometer was used for measuring the electrical resistivity. 3. RESULTS AND DISCUSSION The coatings were light tan to dark brown or black in appearance, depending on the curing temperature used and the type of pigment added. The adhesion of both pigmented and unpigmented coatings was highest (as tested qualitatively by a Scotch-tape type of method) on aluminium substrate, while for the glass substrates it depended on the curing temperature. For curing temperatures of 350-425 “C it was

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moderate but above 425 “C it was good. In contrast, the coatings on aluminium plates or foils adhered so well that even after repeated bending hardly any peeling was noticed. In order to investigate the internal structure of the films, and also bonding at the coating-substrate interface, the samples were sectioned vertically and observed by SEM. In the case of poor adhesion, large numbers of voids were visible, and clear separation between the coating and substrate was noticed at the level of a few micrometres. Figures l(a), l(b) and l(c) show the SEM micrographs of TiO,pigmented coatings with poor adhesion, those with good/excellent adhesion at low

(4

lb)

04 Fig. 1. SEM micrographs adhesion. Magnifications:

ofTiO,-pigmented PPS coatings: (a), (b) 5000x ;(c) 104x.

(a) with poor adhesion;(b),(c)

with excellent

216

S. G. JOSHI, S. RADHAKRISHNAN

magnification (5000 x ) and the same at high magnification (104x) respectively. It can be seen in Figs. l(b) and l(c) that a typical internal structure developed, comprising a network of the polymer with TiO, interspersed uniformly. This is especially evident at high magnification. The fracture surface is also similar to that observed for a ductile material rather than a brittle one, suggesting the flexible amorphous and/or cross-linked nature of the coatings. The crystallinity of the polymeric phase in these coatings was determined from the WAXS data in the same manner as reported earlier12, by taking into account only the major reflections for PPS (corresponding to the 110 and 200 planes) and as such was found to be less than 5% for all the coatings as made. In a separate experiment the bulk polymer was cured at various temperatures ranging from 30 to 300°C and the crystallinity estimated from WAXS data. Figure 2 shows the

Fig. 2. Crystallinity as a function of curing temperature: temperatures I2 showing a maximum at 205 “C.

---,

theoretical

crystallization

rate for various

EFFECT OF CURING ON STRUCTURE OF PPs

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crystallinity as a function of curing temperature (T,), and it can be seen that, for samples cured above 300 “C, the crystallinity is almost zero in agreement with the findings for the coatings. However, when these coatings were annealed for 3-4 h at 200-205 “C, the temperature at which maximum crystallization takes place, about 15% crystallinity was noted in the coatings cured at lower temperature (350-370 “C). The coatings cured at high T, values did not show any tendency to crystallize (in the PPS phase) even after prolonged annealing. The recrystallization behaviour is important for the retention of properties in long-term aging and/or high temperature use. In the cases where crystallization occurs, formation ofcracks, voids etc. may take place owing to shrinkage of the material at the crystalline sites. This suggests that for high temperature application such as coated cookware it may be preferable to cure the PPS coatings at higher temperature (above 370 “C). The IR absorption spectra for these coatings were recorded by the ATR technique, in which the coatings together with the substrate can be mounted in the spectrophotometer. However, in order to avoid extraneous effects due to improper contact, and also to avoid damage to the reflecting prism, samples having flexible substrates (a foil) were preferred for these studies. Figure 3 shows the ATR spectra in the region 4000-800 cm-’ for the pigmented samples cured at 350 “C (curve II) and 450 “C (curve III) together with that of pure polymer in the form of a compressed disc (curve I). Only the portions where pronounced changes occur are indicated in the figure. The spectra for the pigmented coatings could not be recorded below 800 cm- ’ because of strong absorption from TiO, rendering the sample opaque in that region. It is interesting to note in Fig. 3 that there are some new absorption bands in the ATR spectra of the coatings which are not seen in the transmission spectra of the pure polymer. The noteworthy changes are the occurrence of a broad weak band at 3600 cm-‘, and weak but sharp peaks at 3100,296O and 2850 cm-‘. The shoulder at 1530cm-’ and absorption at 970 and 960 cm-’ show different intensity values in the coatings from those in the bulk polymer. Further, there were changes in the intensities, together with sharpness of various absorption bands, in the region 1900-900 cm-’ with increasing curing temperature (cf: curves II and III). These became especially clear when the spectra were recorded in transmission mode for portions chipped off the substrate or by dissolving the substrate in alkaline medium. Figure 4 shows the transmission spectra of unpigmented polymer coatings cured at 350 “C (curve I) and 450 “C (curve II). In this case the changes in the relative intensities of the various absorption bands at 1380,1290,960,940,630,570,490,425 and 410 cm-’ are quite evident. Additionally, new bands are seen for samples cured at high temperature at 1410 (shoulder), 1375,1140,860,640 and 660 cm-‘. The IR absorption of PPS together with the changes occurring in it as a result of processing such as compression moulding etc. have been well investigated and documented by us earlier I2 . It was shown there that the bands in the region 1000-600 cm- ’ are sensitive to supermolecular structure such as crystallinity of the sample. Now, the broad band at 3600cm-’ and the low intensity sharp peaks at 3100,291O and 2850 cm- ’ are clearly due to chemical groups not belonging to the intrinsic polymer, since these were not present in its spectrum. However, considering the probable moieties which can give rise to absorption in this region, namely the hydroxyl group for the broad absorption and (-CH-) asymmetric-symmetric

218

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S. RADHAKRISHNAN

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EFFECT OF CURlNG ON STRUCTURE OF PPS

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stretching for the latter peaks, together with the fact that propylene glycol was used in the coating process, it may be concluded that small amounts of propylene glycol or its degradation products remain in the sample even after the curing cycle. Further, since ATR spectra show these bands more clearly than others, and this technique scans surface layers of a few micrometres (at 45’ incident angle with the KRS-5 prism)13 these impurities are likely to be present mostly near the surface of the coating. The band assignment for the other absorption peaks has already been documented, and corresponds to various modes arising from the phenyl and sulphur linkages. From the intensity pattern of absorption bands in the region 1900-1700 cm- ‘, and the occurrence of new bands at 860,640 and 610 cm-’ which are related to the overtone bands of the phenyl group or -CHout-of-plane deformation14, it may be concluded that 1,3,5-tri-substituted or even 1,3,4,5-tetra-substituted phenyl groups may be present in the cured samples, in addition to the normal 1,4-disubstituted ones which are expected for PPS. Such groups can be introduced by the cross-linking process of PPS which occurs during the high temperature curing step. Typically the thermal curing may proceed asl’

leading to the formation of cross-linked structures such as

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or

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which is based on the fact that oxygen also plays an important role in the curing of PPS in air16. Although the exact chemical structure is difficult to resolve, it can be said with certainty from the IR data that cured coatings contain groups with fivemember rings (absorption in the range 640-625 cm-‘) and the above-mentioned moieties. The intensities of the absorption bands arising from these groups progressively increased with increase of curing temperature. This is in agreement with the fact that, for the same curing time, the cross-linked density increases (the curing rate is higher) with increase in temperature. When the cured samples (350 “C) were subjected to thermal annealing at 200-205 “C for 3 h, the absorption bands sensitive to crystallinity6*12*‘7, namely 1350, 1290, 1170, 960, 810, 630, 550 and 400 cm- l, showed corresponding changes in intensities from which crystallinity was deduced”, by comparison with amorphous samples, to be about 20%. In contrast, no such changes were noted for samples cured at 450°C even after prolonged annealing. Since these samples contain highly cross-linked polymer chains, the possibility of their crystallization is quite low.

EFFECT OF CURING

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In order to determine the chemical stability of these coatings, the films were peeled off the substrate (glass) and dipped in solutions of various chemicals for about lOmin, washed, dried, and their IR spectra recorded. Some major changes were noted in the IR spectra for films dipped in sulphuric and chromic acids, which are known to attack the polymeric coatings. No such changes were seen in the case of other chemicals such as NaOH, NaCl and HCI. Figure 5 shows the IR spectra in the regions of interest for films cured at 350 “C and 450 “C and treated in sulphuric acid (50%). It may be seen that the absorption peaks at 710cm-’ (A), 740cm-’ (B), 960 cm-’ (C) and 1170 cm-’ (D) are especially sensitive to the acid treatment. The relative intensities of these bands normalized with respect to the intensity of the unaffected well-defined absorption band (1000 cm- ‘) as a function of curing temperature with and without acid treatment, are shown in Fig. 6. The normalization procedure was followed in order to eliminate the effect of thickness variationig. It is seen that, without the acid treatment, the intensities of the bands at B and D decrease, while those at A and C increase, with increase of curing temperature. After the acid treatment, however, some interesting features are seen in the intensity variation. While, for low and high T,, the absorption bands at A and B show a large decrease in intensity, at intermediate curing temperatures (400-425 “C) the intensities of these bands are close to the original values. There is a similar trend in the

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1400 WAVE

1000

NUMBERS

60

(cm-‘)

Fig. 5. IR spectra of PPS films after sulphuric T, values of 350 “C and 450 “C respectively.

acid treatment

(SO%, 10 min, 23 “C). Curves I and II are for

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CURING

350

400

TEMPERATURE

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450

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Fig. 6. The relative intensities of various absorption bands indicated in Fig. 5 as a function of curing temperature. Before acid treatment: 0, A; x , B; fl, C; 0, D. After acid treatment: .,A; *, B; &C; m. D. Theinset shows therelativechange(AI)in theintensityofeach bandafteracid treatment at variouscuring temperatures.

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EFFECT

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intensities of absorption at C and D, with the difference that there is no maximum of intensity with curing temperature after acid treatment as noted for cases A and B. These various findings may be analysed simply by plotting the ratio of intensity difference (AZ) noted in the bands with and without chemical treatment to that of the reference peak intensity (1000 cm -I). The inset to Fig. 6 shows such a plot for the absorption bands A, B, C and D for various curing temperatures. It is seen that the intensity difference in all cases, and hence the extent of chemical attack, is a minimum when the curing temperature is in the region of 400 “C, while it is large on either side of this temperature. Among the various possibilities for the action of strong oxidizing agents on polymers, it is well known that amorphous regions are more prone to chemical structures are attack than the crystalline regions *‘*‘i. However, cross-linked expected to be more stable than the linear structures. Also the extent to which a chemical will react with the film material depends on how far it penetrates by diffusion into the intermolecular regions or amorphous regions, or permeates in the cracks, microvoids etc. in the film structure. Considering the fact that all the films cured in the temperature region 310-450 “C were amorphous (without annealing) and that cross-linking was greater in the case of high T,, it seems likely that the chemical resistance of the PPS coating (as revealed by changes in IR spectra) is controlled mainly by the diffusion and/or permeation of the acid molecules into the structural defects in the film, and that there is an optimum temperature of curing at which this process is minimized. In order to determine the porosity, density of microvoids etc., at least qualitatively, the coatings were dipped into a solution of ionic conductors (NaCl or NaOH) which were found to be chemically non-reactive to PPS. The films thus treated were then dried and the surface conductivity was measured. Figure 7 shows the surface resistance (2 mm gap) of unpigmented coatings dipped in NaOH or NaCI, corresponding to curves A and B respectively. The primed curves A’ and B’ are those for the pigmented coatings treated in the respective solutions. It can be seen that for the unpigmented coatings the surface resistance after chemical treatment is low for a T, value of 350 “C but increases with increase in T,, and in all cases the NaCl-treated films have higher resistance than those treated with NaOH. It may be mentioned here that the ionic conductivity of the former is much less than that of the latter2’, which could be the reason for this difference. The pigmented coatings treated by these solutions showed much lower resistance (higher conductivity), being about an order of magnitude less than that found in the unpigmented case. Further, it is interesting to note that the surface resistance is low at T, values of 350 and 450 “C, while it is high at intermediate temperatures (about 400 “C). Now, the bulk resistivity of PPS (undoped)23 is more than lOi R cm and it can be reduced (made highly conducting) by doping with charge transfer agents such as arsenic trifluoride or pentafluoride 5*6.24. The chemicals used in the present experiment are known to be non-reactive chemically or otherwise with PPSz5. The increase in conductivity can thus be mainly attributed to the diffusion of ionic species into the coating, especially in the microvoids, cracks, surface pits etc. which are normally expected to be present in many coatings. It thus appears from the resistivity behaviour that, with increase in curing temperature, such diffusional sites

SURFACE

RESISTANCE

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EFFECT OF CURING

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are reduced in number for unpigmented coatings, while there is an optimum curing temperature for the pigmented coatings, at which these sites are minimized. These findings are very much in agreement with the above-mentioned hypothesis regarding the chemical resistance of these coatings. 4.

SUMMARY AND CONCLUSIONS

Pigmented and unpigmented PPS coatings have been investigated for internal structure after curing at various temperatures. It was found that the coatings showing good adhesion to substrates such as glass and aluminium plates exhibit a network-like structure of the polymer with the pigment particles interspersed therein. The crystallinity of as-made layers was negligible but, on annealing at 200 “C for 3 h, about 15% crystallinity was induced only in the samples made at low curing temperatures. The IR studies revealed that certain absorption bands in the region 1500-500 cm-‘, which are sensitive to the type of substitution in the phenyl ring, change in intensity and/or sharpness with increase in curing temperature, suggesting that the cross-linking process produces 1,3,5-tri-substitution or 1,3,4,5-type tetrasubstitution in these units of the polymer chain. Crystallization also produces changes in the IR spectra. The chemical resistance of the coatings, as seen from the intensities of certain absorption bands after chemical treatment with oxidizing acids, was found to be dependent on the curing temperature T,, being maximum for a T, value of about 400°C under the present preparative conditions. The surface conductivity of samples dipped in ionic salt solutions gave an indication of the density of microvoids or microporosity of the samples, which was also found to be dependent on the curing temperature. For the pigmented coatings the number of diffusional sites was a minimum at an optimum curing temperature of 400 “C. It can be seen from the above studies that the internal structure, and hence the properties such as adhesion, chemical resistance and protective capacity, of the PPS coatings is very much governed by the curing temperature, and, in order to produce the highest quality films, this needs to be optimized. REFERENCES 1 2 3 4 5 6 7 8 9

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