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Figure 6. This large meniscus coating apparatus is 38.1 em wide.
solid block of TFE Teflon. The meniscus contact area which is 17 mm wide has been artificially blackened to outline the exit ends of the capillaries. Figure 5 shows a second Teflon coating head with the meniscus contact area 25 mm long and blackened as before. It is fitted atop a glass jar solution reservoir and is surmounted by a 3-rpm motor driving a chain pump to maintain a constant solution level in the coater head. This was the most important component in a tape coating system designed to operate a t elevated temperature in a
1980, 19. 316-319
laboratory vacuum oven without any manual manipulations or adjustments. Figure 6 shows a capillary fed meniscus coating head on a 38.1 cm (15 in.) wide film casting machine built in the laboratory. The head is rotated about 90" up from its normal operating position to permit this photograph. It is made of normal 2-in. IPS pipe with a flat surface milled on one side completely through the pipe wall. A bar of Teflon containing the capillaries is fastened to that opening by a series of spring-loaded clamps. An inflow of solution to the coating head reservoir takes place at the center of concentric channels a t either end of the pipe. The excess solution overflows into the 3/rin. IPS end pipes and then returns to storage. The capillaries were 0.76 mm (30 mil) in diameter and equispaced at 5.9/cm (15/in.). Conclusion A new coating technique has been demonstrated in which the coating thickness is substantially independent of the liquid meniscus thickness. Flat and uniform coatings are therefore possible without high precision coating machinery. A totally enclosed coating solution reservoir also contributes to high-quality coatings by eliminating solvent losses. Reproducible coatings can be made with small amounts of coating solution. Received for review April 14, 1980 Accepted June 10,1980 Presented at 179th National Meeting of the American Chemical Society, Houston,TX, Mar 25,1980,Division of Organic Coatings and Plastics Chemistry.
Polydimethylsiloxane-Poly(alky1ene oxide) Block Copolymers as Flow-Out Additives for Epoxy Resin Powder Coatings Michael P. Hill Dow Corning Ltd., Bany, United Kingdom
Michael J. Owen* Dow Corning Corporation, Midland, Michigan 48640
Block cop0 ymers of polydimethylsiloxaneand poly(ethyleneoxide) are effective in promoting the flow-out during men'ng of epoxy resin powder coatings. The effect of copolymer compos:tion was investigated Lsing a liquid epoxy resin on an aluminum suostrate. Two properties were studied as a function of time, the surface tension of the resin and the contact angle with the substrate. A direct correlaton is obtained between mese two properties implying that the lowering of the surface tension is the mechanism whereby the flow-out improvement is effected. The 50-60% pa ysiloxane content copolymers have the desired oalance of surfaceactivity and soluoility that gives the best performance. Homopolymeric polydimethylsiloxaneis incompatiole and inhibits flow-out. The inclusion of titania f ller markedly compiicates the behavior of the addt've and indicates specific interaction between copolymer and filler.
Introduction These studies were prompted by the need for additives to improve the flow-out on melting of powder coatings. The principal powder coatings used are epoxy resins. Preliminary studies with one such material, Shell DX55, revealed that polydimethylsiloxane (PDMS)-poly(ethy1ene
oxide) (PEO) block copolymers were effective in this application. A small piece of Shell DX55 containing the additive was placed in an oven a t 195 " C for 10 min and the contact angle (0) so produced was measured a t room temperature. Significant contact angle lowerings were produced by certain of the PDMS-PEO block copolymers.
0 1980 American Chemical Society 0196-4321/80/1219-0316$01.00/0
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Moreover, the greater the contact angle lowering the better the performance in actual coatings application by Vanguard Powder Coatings Ltd. (U.K.). Polydimethylsiloxane homopolymers, which were insoluble in the epoxy resin, did not lower the contact angle-they increased it-nor did they perform well in the actual coatings. This behavior indicated that a balance between surface activity and compatibility was needed to produce the desirable low contact angles. The assumption was that the additives were able to lower the surface tension of the melt and thereby lower the contact angle. In order to better understand these observations we took a series of PDMS-PEO block copolymers of different composition and studied their effect on the surface tension and contact angle of an epoxy resin that was liquid at room temperature. Within the time frame of these experiments all contact angles were greater than zero although in several cases it appeared that this ultimate value was being approached asymptotically. The Young equation can be rigorously applied only to the finite wetting case at equilibrium. However, by applying it to the present dynamic situation, and also assuming that no component of the molten drop is adsorbed on the substrate to produce a surface pressure term, it follows that cos 0 =
asv - OLS CLV
where aSv is the surface tension of the substrate, aLv is the surface tension of the epoxy resin, and aLs is the experimentally unmeasurable interfacial tension between the resin and the substrate. A lowering of aLv will increase cos 0, i.e., reduce 8. A direct correlation between a L V and cos 6 over a wide range of additive composition would suggest that this is the principal mode by which the additive affects the flow-out behavior and that there is no complicating effect 011 the interfacial tension. It is very unlikely that the optimum composition for maximum surface activity will coincide with the optimum composition for maximum interfacial activity. An example of this difference is behavior of such silicone-containing block copolymers at different interfaces has already been given for the air/water and silicone/ water interfaces (Kanellopoulos and Owen, 1971). Experimental Method Materials. The epoxy resin used was Shell Epikote 828, a low molecular weight viscous liquid at room temperature. I t was used as received. Four substrates were tested initially, aluminum, mild steel, stainless steel, and glass. The first three showed parallel behavior but the glass gave very different results, some of the additives markedly increasing the contact angle. This behavior is ascribed to specific additive-glass adsorption interaction.. The most consistent results were obtained on aluminum and this was chosen as the standard substrate. Toluene w,as used to obtain a grease-free surface. The composition of the PDMS-PEO block copolymers is given in Table I. They are all based on a linear polydimethylsiloxane chain with varying numbers of the same poly(ethy1ene oxide) chain attached to the PDMS backbone via non-hydrolyzable Si-C linkages produced by the familiar hydrosilylation reaction between =SiH and the allyl end group of the poly(ethy1ene oxide) using chloroplatinic acid as the catalyst. All the PEO chains have terminal hydroxyl groups except copolymer PDMSPE0/5, which is capped with acetoxy groups. The term block copolymer is used generally and covers the case
Table I. Composition of the Copolymers
copolymer no. PDMS-PEO/1 PDMS-PEO/2 PDMS-PE0/3 PDMS-PEO/4 PDMS-PEOJ5 PDMS-PEO/6 PDMS-PE0/7
approx. no. of Siatoms in PDMS block 7 7 12 14 18 46 73
%
polysiloxane" 13 30 33 50 53 59 60
approx. no. of PEO blocks 5 2 3 2 2b 4 6
a Includes all mono-, di-, and trimethylsiloxy units in Terminal hydroxyls capped with acetoxy copolymer. POUP.
where the E S i H groups are on the ends of the PDMS chain and also where they are along the chain. In every case these additives were used at 0.5% w/w concentration. Techniques. Surface tension was measured using the pendant drop technique. Details of this have already been published (Kendrick, et al., 1967). Two techniques were used for contact angle determination; the commercially available Rame' Hart instrument and one built according to the design of Fort and Patterson (1963). No systematic differences were noticed between the two techniques nor was there any difference in the spread of values of the contact angle obtained on a given sample between the instruments (usually *3O). All measurements were made at room temperature 21 l "C. Results and Discussion Both the surface tension/time and contact angle/time plots approached equilibrium values asymptotically with most of the change occurring in the first 25 min although values up to at least 125 min were collected in all cases. The results are best summarized by taking the values at a chosen time and plotting as a function of copolymer composition. Ten minutes was chosen as a reasonable practical time although almost any time gives a similar picture. Figure 1 is the summary plot for surface tension and Figure 2 the corresponding plot for contact angle. The close correlation between these two plots, cos 6 increasing as aLv decreases, strongly supports the original assumption that the lowering of aLv is the mechanism whereby the flow-out improvement is effected. It is likely that the 50-60% polysiloxane region is a maximum in efficacy because 100% polysiloxane (PDMS homopolymer) results in a much diminished cos 0, less than no additive at all. No examples of copolymers in the 60-100% polysiloxane range were synthesized because it was evident they would not be compatible with the epoxy resin. However, a related series of copolymers containing a random poly(ethy1ene oxide)/poly(propylene oxide) (PEO/PPO) copolymer segment showed a distinct minimum in the surface tension-composition curve. This system was not pursued as the actual lowerings of surface tension achieved were significantly less than the PDMSPEO system. In line with this observation, the contact angles were higher and flow-out behavior in practice was inferior to the PDMS-PEO system. These PDMSPEO/PPO materials were of larger molecular weight than the PDMS-PEO series. Their slower diffusion through the resin to the surface would account for their poorer surface activity. We have reported previously (Hill et al., 1974) that very strong adsorption of polydimethylsiloxane-polystyrene (PDMS-PS) block Copolymers occurs on aluminum via the PDMS portion. This does not appear to happen in the
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Figure 2. Dependence of contact angle at 10 min on copolymer composition.
% POLYSILOXANE
Figure 1. Dependence of surface tension at 10 min on copolymer composition.
Table 11. Effect of Titania Fillers on Contact Angle cos 0 (10 min)
RG2 titania Epikote 828 alone + 0.5% PDMS-PE0/4 +0.5% PDMS-PEO/5 + 0.5% PDMS-PEO/7
0.62 0.18 0.72 0.48
BTP “sulfate” titania 0.57 0.88 0.79
present case. We presume that the epoxy resin can compete successfully for the aluminum adsorption sites and thereby prevent adsorption of the copolymer, whereas polystyrene cannot. These PDMS-PS materials have some compatibility with the epoxy resin and were evaluated in this flow-out application. They provided some improvement but were not as effective as the best of the PDMSPEO copolymers. One thing to be expected with relatively low molecular weight materials is a significant end-group effect. Only PDMS-PE0/5 has a different end group in the present series. I t did not deviate in behavior in the simple system so far considered. However, a profound effect was observed with titania-filled materials. This is of interest as in practice many titania-filled systems are used. Table I1 shows the effect on cos 0 at 10 min for some of the additives in titania-filled epoxy resin using mild steel as the substrate. Two titanias were used, RG2 from Laporte (U.K.) and a British Titan Product (BTP) material manufactured by the sulfate process. This latter material is typical of those grades used in powder coatings. In each
case 75 parts of titania was used to 100 parts of epoxy resin as recommended by Vanguard Powder Coatings Ltd.
(U.K.). For RG2, PDMS-PE0/5 with its acetoxy capped PEO chains gives the expected good increase in cos 8 but the two additives with termiiial hydroxyl groups on the PEO chains inhibit spreading markedly. They are worse than no additive at all. For the BTP sulfate process material the pattern is totally different. Here both additives promote spreading with the uncapped additive giving the better performance. Some very specific filler-additive interactions must be occurring. With RG2, adsorption of the terminal hydroxyls on the titania setting up a three-dimensional network is a strong possibility. This would increase the resistance to flow of the resin. Maybe with the sulfate process titania the adsorption does not lead to bridging but to a silicone-treated filler that interacts less with the resin than the case without additive thereby decreasing the resistance to flow. This is an area where much more work will be needed. Conclusion In the simple epoxy resin system there is a direct correlation between surface tension lowering and the change in contact angle with the substrate. The presence of titania filler complicates the situation dramatically. PDMS-PEO block copolymers are the best system yet identified for this application with a 50430% polysiloxane content giving the desired balance between surface activity and solubility. Many of the benefits of adding this type of copolymer to plastics and resin systems, such as good release behavior, improved surface lubrication, etc., can also be achieved by
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the incorporation of high molecular weight homopolymeric silicone fluid. This is not the case in this spreading application. PDMS homopolymer is detrimental, probably because of adsorption on the substrate producing a low surface energy material over which the resin cannot spread. Literature Cited Fort, T., Jr.; Patterson, H. T. J . Colloid Sci. 1963, 78, 217. Hill, M. P. L.; Millard, P. L.; Owen, M. J. "Polymer Science and Technology", Lee, L-H, Ed.; Plenum Prees: New York, 1974, Vol. 5B, p 469. Kaneiiopoulos, A. G.; Owen, W.J. J. Colloid Interface Sci. 1971, 35, 120.
Kendrick, T. C.; Kingston. 6.M.; Lloyd, N. C.; Owen, M. J. J . ColbH Interface Sci. 1967, 24, 135.
Received for review April 10, 1980 Accepted May 22, 1980 This paper was originally presented at a symposium on Mechanisms of Film Formation from Powders, Melts, and Solutions at the 179th National Meeting of the American Chemical Society, Houston, Texas, March 23-28,1980, Division of Organic Coatings and Plastics Chemistry.
IV. Symposium on New Concepts in Coatings and Plastics Chemistry R. H. Lalk, Chairman 178th and 179th National Meetings of the American Chemical Society, Washington, D.C., September 1979, and Houston, Texas, March 1980
Diethanolamine as a Hardener for Epoxy Resins C. V. Lundberg Bell Laboratories, Murray Hi//, New Jersey 07974
The chemical changes occurring during the reaction between diethanolamine and epoxy resin are followed by infrared analysis; and physical, mechanical, and electrical properties are related to these chemical changes. Properties critically examined are glass transition temperature ( Tg),hardness, linear shrinkage, dissipation factor, and insulation resistance. Complete cure does not occur at 40 O C within 100 days, and yet, after 3 days at 40 OC, the system is sufficiently cured so that on postcuring at 100 OC no additional shrinkage occurs. A T of 60 OC is developed in 3 days at 40 O C and slowly rises to 75 "C in 100 days. Curing at 100 "C results in a h l cure and a T, in the vicinity 'of 100 "C. The dissipation factor at lo3 Hz increases as time of cure increases and doubles in value at full cure. Hardness and insulation resistance increase in value as the epoxy converts from liquid to solid, but they quickly reach constant values and become insensitive to additional cure.
Introduction Diethanolamine has been used as a hardener for epoxy resins for 15 or more years. Within the past 2 years we have had occasion to examine some of the physical, mechanical, and electrical properties of diethanolamine-cured epoxy compounds and have uncovered some interesting information. In addition we have attempted to follow the chemical changes occurring during cure at several temperatures by means of infrared analysis and have attempted to correlate these changes with the mechanical and electrical property changes. Epoxy resin cured with diethanolamine, whose structural formula and properties are shown in Figure 1,converts to a solid a t room temperature in approximately 1 day. It was observed that tranaformers impregnated and encapsulated with this compound and cured at room temperature developed corona extinction voltages an order of magnitude higher than when cured at 100 "C. It was quite natural to attribute the improved corona extinction voltage to the low shrinkage associated with the low-temperature cure. Low shrinkage decreases the likelihood of the formation of internal voids and cracks--the most likely source of low corona extinction voltages aside from trapped air.
The state of cure of the epoxy was determined by measuring its electrical resistance. When this measurement reached a plateau value, it was thought that a full epoxy cure had been obtained. By this method a full cure is obtained in 8 days at 24 "C, 3 days a t 40 O C and 18 h at 75 " C . In the present study we have used the glass transition temperature ( T ) as an indicator of chemical change or degree of cure. tures similar to the 40 "C cure above yield T , values in the 55-60 "C range. On continued exposure at 100 " C for 3 days, the Tgincreases to 97 " C , and, on further heating at 125 "C, it advances to 103 "C. If the transformers referred to above had been tested after temperature exposures sufficiently long to increase the T,of the epoxy to the 97-103 " C range, other property changes would most likely have taken place along with the chemical change indicated by the increase in the Tg, and the corona extinction voltage may have been decreased. The word "may" is used because changes associated with the increase in T, could result in stress buildup in the epoxy without formation of internal cracks. This transformer was small in size-ca. 2 in. in diameter by ll/*-in. high-and small castings resist cracking more effectively than large castings.
0196-432 1180112 19-0319$01.OO/O 0 1980 American Chemical Society
