Poly(methyl methacrylate) Automotive Lacquer Technology - Industrial


Jun 1, 1979 - Poly(methyl methacrylate) Automotive Lacquer Technology ... Industrial & Engineering Chemistry Product Research and Development...
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979 San Antonio, Texas, Feb 21-23, 1977. (7) Anderson, G. P., "Analysis and Testing of Adhesive Bonds", Academic Press, New York, 1977. (8) Hotten. B. W.,US. Patent 3 491 047 (1970). (9) Holliday, L., Ed., "Composite Materials", Chapter 9,Elsevier, New York, 1966. (10) Craus, J., Ishae, I., Sides, A,, Proceedings of the Annual Meeting of the Association of Asphalt Paving Technologists, Florida, Feb 13-15, 1978. (11) Plancher, H., Green, E. L.. Petersen, J. C., Proceedings of the Annual Meeting of the Association of Asphalt Paving Technologists, New Orleans, Feb 16-18. 1976. (12) Grosmangin, J., Verschave, A., Marvillet. J., French Patent 2 201 336 ( 1 974). (13) Plummer, M. A., Schroeder, D. E., Zimmerman, C. C., Canadian Patent

(15)Makowski, H. S.,Lundberg, R. D., Singal, G., US. Patent 3870841 (1975). (16) Cantor. N. H.. US. Patent 3642728 (19721. (17) Lundberg, R. D., Makowski. H. S.,Wesierman, L., US. Patent B 487467 (1976);see also Lundberg, R. D., Polym. Prep., Am. Chem. SOC.,Div. Polym. Chem., 19 [I], 455 (1978). (18) British Patent Specification No. 493 905. (19) Alexander, J. A., M.S. Thesis, Massachusetts Institute of Technolo y, 1968;B. F. Goodich Chemical Division, "New Product News [Ehstomersg", Vol. IV, No. 1, Feb 1969. (20) Eisenberg, A., King, M., "Ion Containing Polymers", Vol. 1 and 2,Academic Press, New York, 1977. (21)Holliday, L.. "Ionic Polymers", Wiley, New York. 1975.

Received for reuieu; May 15, 1978 Accepted March 1, 1979

1011759 (1977). (14) Boca, P. L., Pacor, P., Canadian Patent 1010390 (1977).

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POLYMER COATINGS SECTION Poly(methyl methacrylate) Automotive Lacquer Technology Werner S. Zlmmt E.

I. Du Pont

de Nemours & Company, Inc., Marshall R&D Laboratory, Philadelphia, Pennsylvania 19146

Attempts to develop durable automotive lacquer finishes from poly(methy1 methacrylate) uncovered some difficulties that had to be overcome in order for these new systems to become commercially acceptable. This report describes some of these problems, their physical causes, and the methodology used to find acceptable solutions. Most of the difficulties involved polymer physics, especially rheology, and an understanding of the principle involved often explained why the empirical solutions were effective.

The development of new coatings is a slow and frustrating process. False starts, unexpected difficulties, and constantly changing objectives are the daily experience. Often, the effects that are observed seem not to correlate with any quantitative measurements that are being used to guide the researcher. Often, theoretical considerations seem useless, so that only experience and luck appear relevant. This work, although describing the research on a product that is now commercial, has not been published in any detail and to the extent that it will be described here. The esters of acrylic acid and methacrylic acid have been known since the early part of the century. They always had the potential to make attractive polymers and plastics, but for many years the difficulty of preparing the monomers restricted their use. In the 1930's the problem of large-scale production was solved and they became available for many end uses. One of the early uses proposed was as a substitute for varnish. Before World War 11, the Rohm & Haas Company was selling solutions of acrylic polymers for minor coating uses. One such use was "stop off' lacquer, which was used as a barrier for metal plating. Another use was in luminous coatings for use on gun sights and also on airplane instrument dials (I). During World War 11, poly(methy1 methacrylate) was used as an airplane glazing material. As such, it proved to have superb durability, being resistant to various weather and atmospheric exposures. Its availability and excellent outdoor durability made it a natural candidate 0019-7890/79/1218-0091$01.00/0

for use in lacquers and coatings. Of all the acrylic polymers and copolymers, methyl methacrylate homopolymer is the most difficult to use for lacquers. Its solutions are the most viscous at any given molecular weight and concentration. The early literature from Rohm & Haas already mentions that only dilute solutions are sprayable. Nevertheless, an attempt was made to see if a solution lacquer with good outdoor durability could be developed from poly(methy1 methacrylate). The incentives for such a development were twofold: (1) existing nitrocellulose automotive lacquers were generally poor in gloss retention and needed extensive polishing to keep them shiny; (2) the requirements to obtain the maximum durability from nitrocellulose lacquers limited their color range and the styling that could be obtained. It was hoped that the introduction of a more durable vehicle, such as poly(methy1 methacrylate), would allow substantial increase in the color range available for automotive styling and at the same time improve the outdoor durability of the finish. During this development program a number of difficulties were encountered that had to be overcome before lacquers based on poly(methy1 methacrylate) could be commercialized. 1. Spraying

The first such problem was the difficulty of finding a polymer that could be sprayed at a reasonable concentration. For a coating to be commercially successful, it 0 1979

American Chemical Society

92

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18. No. 2, 1979 ?OlYmH

ltmlmml

Polymer P-1 None , Polymer P.1 Milled' 45 min. Polymer P - l Milkd'60 min. Polymer P.1 Milled' 30 min. Polymer P.1 Healed to 250°C Solution None Solution Milled. 10 mi".

Yw IW Spray SDlidf MWMn

413,000 127,000

110,000 60.000 64,000 120,000 120,000

4.5%

3.1

23% 25% 32%

1.2

-

17.9% 30.8%

1.2 1.1

3.7

-

'lill.d.1 Ill'C

Nitrocellulose Lacquer

PMMA Lacquer

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Figure 1. Cohwehhing of poly(methy1 methacrylate) solution.

Molecular Weight lltlo']

Figure 2. Relationship of molecular weght with physical properties and web-free solids.

must satisfy two major requirements: (1)it must meet certain standards of performance and appearance and (2) there must he a practical way of applying it. Neither requirement can he independent of the other. It matters not how great the performance of the coating if it cannot he applied, nor how easy it is to apply if it does not perform well. Two types of poly(methy1 methacrylate) were in commercial manufacture hy Du Pont at the time this work was undertaken. One grade (Polymer P-1) had a viscosity average molecular weight of about 500000, and the other grade (HG-40) ahout 150000. Initial studies with these two polymers indicated that they were not suitable for spray application. The solutions were viscous, but even when the viscosity was reduced by dilution a phenomenon known as webbing was observed (Figure 1). Instead of breaking up into discreet droplets in the air stream from the spray gun, the polymer solution came out as thin fibers that looked like spider wehs. When a polymer solution webs the drops do not coalesce to form a continous, coherent film after arriving at the target. Maximum web-free solids were determined hy reducing the solids content until the solution could be sprayed without webbing. For Polymer P-1this was just under 4%, while the HG-40 solution was sprayahle at about 10%. What disturbed us was that the Brookfield viscosity of a 20% HG-40 solution was similar to that of nitrocellulose solutions at 20% solids. However, while the latter was sprayahle at 20%, the methacrylate was not. Obviously, solution viscosity, as measured by a Brookfield viscosimeter, was not an indicator of sprayability. Nevertheless, some relationship between molecular weight and web free solutions seemed indicated. Polymers of 30000 to 60000 molecular weights were synthesized. These could he sprayed at practical solids hut their physical properties were poorer than those of the plastic grade polymers. This approach did not seem promising. Good mechanical properties are a direct function of molecular weight; high web-free solids are an inverse function of molecular weight (Figure 2). The question that needed answering was-is it possible to prepare polymers

Figure 3. Effect of various treatments on the molecular weight and sprayability of PMMA solutions.

that have both good physical properties and reasonable application properties? Initally attempts were made to formulate the polymers at hand, since these already were in reasonably large-scale production. Attempts to find additives or to use different solvents proved fruitless. The first breakthrough actually was a matter of serendipity. Many pigment dispersions are prepared by grinding the pigment on a two-roll ruhher mill with the desired polymer. When this was done with HG-40 it was found that these lacquers could now be sprayed at nearly 20% solids, rather than the 10% at which they had been sprayahle originally. The polymers were isolated and examined and it was found that the molecular weight had decreased from 150000 to 120000. What seemed more significant was that the molecular weight distribution had been narrowed suhstantially. A study was conducted of various techniques of ohtaining polymer with a narrower molecular weight distribution. We considered mechanical degradation, fractional precipitation, fractional extraction, ultrasonic degradation, or thermal degradation. Two-roll milling of polymers was an established process in the rubber industry, equipment was available, and costs could be calculated. Precipitation or fractionation of polymers did not seem to he a feasible process, but some high spot studies indicated that a narrower distribution could be obtained. Ultrasonic degradation, while affecting some reduction of the 50000 mol wt polymer was ineffective for polymers of molecular weight of 150000. Thermal degradation did not result in any narrowing of the molecular weight distribution (Figure 3). All these techniques, however, had one difficulty in common. They all would cause a substantial increase in the cost of the polymer, compared to direct polymerization. Since economics are important, a study was undertaken of the solution polymerization of methyl methacrylate. The variables studied included solvents, monomer concentrations, temperature, type and quantity of initiator, and/or chain transfer agents. Solution polymers having molecular weights ranging from 60 to 140000 were prepared and examined. A reasonahle correlation between molecular weight and web-free solids was found. That the polymers were not identical with milled polymers was shown by the solution viscosity. The milled polymer of a starting molecular weight of 145000 had a much higher viscosity at the web-free spray point then the solution polymers. Nevertheless, the properties OF fully formulated lacquers made from some solution polymers and milled polymers were almost indistinguishable. The question still remained as to what affected the webbing of high molecular weight polymer solutions. Obviously, it was not simply the solution viscosity. Polymers as well as their solutions are viscoelastic materials. Generally, in coatings we consider only the viscous component of this complex property. The flow behavior of most low molecular weight substances can he described adequately by viscosity measurements alone.

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979 SOLIDS/MSCOS'JY RELATIONSUP FDR PMMA

726 000 -0,le 000 - A

X - WEB FREE SOLIDS

,

NORMAL STRESS DIFESNCE A J 2000 SEC FOR PMMA OF MW 726000-0. AND l69,OGO-A. X - WEB FREE SOL IDS

,

.

Figure 4. Rehological behavior of a high molecular weight and a low molecular weight polymer: a, solids-viscosity; b, solids-normal stress difference. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 10, 2015 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/i360070a003

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Table I. Relation of Viscosity and Webbing in Polymer Solutions PMMA PMMA (MW (fir, 726 000) 1 6 9 000) web free web free solids 7% solids 19.5% viscosity at web free point normal shear difference at web free point ( 2 0 0 0 - ' )

0.45 P

2.2 P

520

500

However, as molecular weight increases, the elastic properties become significant. Anyone who has ever had resins gel in the reactor has seen the solution climb up the stirrer, just before or after gelation. This is probably one of the most common examples of the elastic component of polymer systems. The effects of molecular weight and of concentrations of poly(methy1 methacrylate) were studied using a Weissenberg Rheogoniometer, Model R-17. This instrument is capable of measuring the elasticity as well as the viscosity of polymer solutions. In steady-state shear, both the shear stress and the first normal stress difference are measured. The former is a function of viscosity, whereas the latter is a function of the elasticity of the system. The normal stress difference (NSD) of solutions of PMMA of two different molecular weights (726000 and 169000) were compared. If the NSD's at different concentrations are extrapolated to 2000 s-l (a shear rate representative of the shear during spray application) and then plotted vs. concentration, one finds that webbing occurs at approximately the same value of the NSD. No similar correlation between webbing and viscosity was observed (Figure 4a, b and Table I). It appears that webbing correlates better with elasticity than with viscosity. The velocity field for homogeneous simple shear in (Figure 5) rectangular Cartesian coordinates may be expressed by the function (2) u1 = a(&(t)xz; up = 0; u3 = 0 in which the coordinates xl, x 2 , x 3 are measured from a suitably chosen reference point. Subscripts 1, 2, 3 denote the direction of flow, the direction normal to the shearing planes, and the third or neutral direction; & ( t )represents the shear rate. The normal shear stress difference is a function of the shear rate and the molecular weight. These normal shear stresses are due to the entanglements of long chains in relatively concentrated polymer

VI

Y(t) x,

vz.

0

v3-

0

c ,

SHEAR R A l S

Figure 5. Shear velocity field in Cartesian coordinates.

solutions. The idea of entanglement has been used for a long time. Berry and Fox described the relationship between melt viscosity and molecular weight by two equations (3) vo = K M M C M ,

vo = KM3.4; M > M , where M , represents the molecular weight at which chain entanglement occurs. For PMMA this is reported to be about 30000 (4). For polymers below that value one would expect no webbing, but neither would one expect to obtain satisfactory mechanical properties. As polymer solutions are diluted, the molecular weight required for entanglements increases. This can be shown by elasticity vs. molecular weight curves for solutions a t different concentrations. In fact, such curves might conceivably be consolidated into a single superimposable curve, somewhat similar to the WLF modulus-temperature-cycle time superposition principle. The early experimental data led to the conclusion that two-roll milling caused a small change in the viscosity average molecular weight, as well as a narrowing of molecular weight distribution and a substantial increase in the web-free spray solids. The theoretical treatment of the relationships between entanglements and molecular weight has been inadequate in dealing with polydispersity, but all observations indicate that these relationships are a function of a higher molecular weight average than the weight average molecular weight. Since the viscosity average molecular weight is a lower average, the lack of sensitivity is not surprising. It is believed that entanglement influences only the large-scale motions of a polymer chain. Relaxation times associated with the higher normal modes corresponding to cooperative motions over distances smaller than the entanglement spacing are assumed to be unaffected by entanglement. Motions which exceed the entanglement spacings and which require cooperation will be strongly affected by entanglement. Such motions will be severely slowed and their relaxation times will be shifted to larger values. Since relaxation time is a reciprocal function of shear rate the effect of entanglement will be observed much more acutely during spraying (a high shear rate treatment) than during Brookfield viscosity measurement. In summary then, linear polymer molecules become entangled with each other when their molecular weight exceeds a critical value. This is true of solutions as well as of bulk polymers. These entanglement points act like weak cross-links, causing the elastic properties. The number of entanglements in a unit volume of polymer solution is a function of both the molecular weight and the

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Slight Failure

Severe Failure

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Figure 6. Typical recoat craze patterns for PMMA lacquer film.

concentration of polymer in the solution. If the web-free solids represent a constant concentration of entanglement it becomes reasonable to expect that as the concentration of very high molecular weight molecules is increased the concentration of the total polymer in solution must he decreased in order to avoid wehhing. Low molecular weight liquids when expelled through a narrow orifice at high shear rates exit in a stream that rapidly breaks up into small droplets under the influence of surface tension. In the presence of long chain molecules that have many points of entanglement, this stream is prevented from breaking up into droplets since break-up requires long distance cooperation, and thus webs are formed. Only when the concentration of long chains is sufficiently low so that the concentration of entanglement points is small can the stream break up into fine droplets. This explains why wehhing is not a function of the low shear viscosity of the system, hut is a function of the elasticity of the system. If the higher molecular weight fractions of a polymer with a broad distribution are removed, higher web free solids result even though the solution viscosity or the molecular viscosity average weight may not have been reduced significantly. Conversely, the addition of small quantities of very high molecular weight polymer multiplies the concentration of entanglements many times and completely ruins the spray properties of an otherwise sprayahle polymer solution.

2. Repair Crazing One of the major advantages of lacquers for automotive finishes is the ability to carry out spot repairs, which is inherent in the use of permanently soluble vehicles. When lacquers hased on PMMA were first tested for repair, it was found that they crazed badly at low temperatures. Scientists who have been involved in plastics have heen using the word crazing for a specific phenomenon (5). In the case of lacquer coatings hased on poly(methy1 methacrylate) the phenomenon described by this term may he superficially similar hut can hardly he considered identical. When a film of plasticized poly(methy1 methacrylate) on a metal panel is exposed to solvent vapors, to liquid solvents, or recoated with another layer of lacquer, the original coating has a tendency to crack, forming patterns, which under mild conditions look like little crows' feet, and under severe conditions look like mud cracks running in random patterns (Figure 6). This effect depends on the composition of the lacquer, on the substrate, on the solvent used, and on the temperature of the lacquer at the time it is exposed to the solvent. Free films of the same composition and under the same conditions do not craze.

For any given polymer/plasticizer composition, crazing occurs within a specific temperature range. Below the lower limit of that temperature range the polymer does not craze, prohahly because the solvent evaporates before it can penetrate into the lacquer; above the upper limit crazing also does not occur. The temperature range in which crazing does occur can he manipulated by the extent of plasticization of the polymer. The more plasticizer is used, the lower the crazing range. It is believed that solvent crazing of lacquer films is caused by the stresses that develop in the film during the drying process. These stresses are due to the fact that the coefficient of thermal expansion of the metal substrate is approximatelyljl0 of that of the polymeric coatings. These stresses form independently of the history of the system; they occur when the panel is baked to remove solvent and then cooled, or when the solvent is allowed to evaporate at ambient temperature. Generally automotive coatings are heated to a temperature of 90 to 160 'C. If the glass transition temperature of the system is around 50-55 "C, the coating becomes rigid when it is cooled below that temperature. It can then no longer relax. Since the metal shrinks less than the coating, the latter, if it adheres, can no longer contract as demanded by its own coefficient of expansion. The result is that the coating is under tension in two dimensions. The greater the difference between the ambient temperature and the glass transition temperature, the greater is the tension to which the film is subjected. Because the film is below its glass transition temperature, these stresses remain indefinitely. The stresses caused by the resistance to shrinkage can he substantial. If a coating is applied and dried on one side of an aluminum foil that is restrained during the drying period, the foil will roll into a coil after the restraints are removed. If a coating is applied to one side of a foil spring the spring will he bent, and one can measure the forces exerted on the spring if the proper constants are known. If the film is not baked hut simply allowed to lose solvent at ambient temperature, it will pass through the glass transition temperature a t the point where the quantity of solvent that is left is such that solvent plus polymer plus plasticizer form a system which has a glass transition temperature a t the ambient conditions. It is known that this is approximate, since the film is not homogeneous hut contains more solvent near the bottom than near the top. However, solvent continues to evaporate slowly, raising T,and reducing volume. Since the film is now rigid and attached to a nonshrinking substrate it cannot shrink laterally and stresses develop. It has heen shown that this theory is reasonable by coating and haking a metal panel and a nylon panel with the same topcoat. When the two were exposed to solvent the metal panel crazed and the nylon panel did not. Nylon has about the same coefficient of thermal expansion as poly(methy1 methacrylate). No stresses are developed when these panels are cooled after haking the lacquer. The craze-free temperature of plasticized PMMA is about 20 " C below its glass transition temperature. If solvent is placed on a coating its rate of penetration is a function of temperature, specifically of the difference hetween the glass transition temperature and the ambient temperature. If the latter is near Tg:the solvent will dissolve the film rapidly and no crazing IS observed. If the temperature is in the crazing range, dissolution is slow and the film will craze. Two factors are prohahly operating simultaneously: (1)the rate of diffusion and (2) the extent

Ind. Eng. Chem. Prod. Res. Dev., VOI. 18, No. 2, 1979 95

I

I 0

(lime) H min.

2

3 4 Virne) H Mi".

5

oc I%I.l"l.ll0. 0, I,

Ill"nl,m Ira"",

Figure I . Effect of plasticizer content on the rate of solvent absorption of films without Bentone 34.

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1

of stress. As the temperature drops below T, the stress caused by the restraints exercised by the metal on film shrinkage will increase. As solvents diffuse into the stressed film and weaken it, the stresses are relieved by fracture of the film (crazing). One obvious solution to the crazing problem is the addition of more plasticizer to the vehicle to decrease the glass transition temperature. However, unlike poly(viny1 chloride), PMMA rapidly becomes soft and tacky when it is heated ahove T,. Since the craze-free range would require a Tgof about 35 "C and the surface of cars can attain 60 "C or higher, such a low T, would be impractical. The most effective method we found of avoiding solvent crazing is the addition of 448% Bentone 34, a modified montmorillonite clay, also called Bentonite clay, in which some of the inorganic interstitial cations have been replaced by dimethyldioctadecylammonium ions. When added to poly(methy1 methacrylate) solutions, Bentone 34 imparts a substantial measure of craze resistance to the film, increases the viscosity, and reduces gloss. In order to obtain a high gloss, coatings made from these formulations have to be buffed. However, since commercial films in use a t the time acrylic lacquers were developed also needed buffing, this represented an acceptable compromise. An effort was made to determine how Bentone reduces film crazing tendency. A number of observations made have already been noted. It was observed that a soft film which absorbs solvent very rapidly will not craze, that films on substrates that have the same coefficient of thermal expansion as poly(methy1 methacrylate) do not craze, and that free films 1-2 mils thick will not craze when exposed to solvent vapor. It was also observed that solvents which do not swell or dissolve PMMA generally will not craze the film, or that if the temperature is very low, crazing does not seem to occur. A study of how Bentone affects crazing in lacquer coatings shows the following. When lacquer films have sufficient plasticizer so that they could be exposed to solvent vapor without crazing, they absorb solvent vapors very rapidly; when the plasticizer level is sufficiently low so that crazing will OCCUI, solvent vapor absorption is slow initially, and only increases at ahout the same time as the f i i starts crazing (Figure 7). In the presence of Bentone 34 the solvent vapor absorption of the film which would otherwise craze is rapid and similar to that of a film which has sufficient plasticizer to be craze-free (Figure 8). A study of the distribution of monomeric plasticizer in acrylic films as a function of the distance from the surface showed that at above room temperature the surface is rich in plasticizer, but plasticizers appear to recede from the surface when the films are cooled. It was shown that Bentone accumulates at the surface. The open structure

6 *mwn,

Figure 8. Effect of plasticizer content on the rate of solvent absorption of clear acrylic films Containing Bentone 34.

Figure 9. Benard cells formed by solvent evaporationfrom reference I.

of the clay prevents a surface skin from forming when the film is baked or air dried. The absence of a skin appears to promote rapid penetration of solvent into the film. Other studies of the behavior of lacquer films during solvent evaporation show that extensivevortex action takes place and that BBnard cells are formed (6). Such cells are formed during evaporation of any solution that contains polymers which have higher surface tension than the solvent. Anand showed the structure of these cells (Figure 9) (7). Weak spots are formed at the houndaries between cells. The ideal shapes of these cells are hexagons. If solvent penetrates between the edges of the cells, where the filmis weak, and rupture occurs, some of the ruptures should look like crow's feet, as is actually observed when only minor crazing occurs. Bentone 34, among other properties, has the effect of increasing the low shear viscosity of the film very rapidly, so that vortexing is strongly suppressed. As a result, it would he expected that films containing Bentone, even at temperatures at which solvent absorption is not very rapid, would still be stronger and more resistant to crazing than similar films not containing Bentone. Thus it appears that the role of Bentone 34 in improving the craze resistance of acrylic lacquers is twofold: it reduces vortex action and thus reduces the formation of BBnard cells and the accompanying weak spots between the cells, and it increases the rate of solvent absorption, which leads to uniform, rapid softening of the film, thus allowing relief of buildup stresses without breaking the film. 3. Cracking As mentioned earlier, coatings for automobiles must have good exterior durability, which includes retention of both gloss and film integrity. When lacquers based on PMMA plasticized with dibutyl phthalate (DBP) were exposed in Florida they failed sporadically by cracking. This failure often occurred under the flap that covers about

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-4

Figure 10. Loss of plasticizer on heating of PMMA lacquer films: S-160 = butyl benzyl phthalate: DBP = dibutyl phthalate.

r PLASTICIZER LOSS ON HEA'IINP P M A FILMS. [FILM THICUYW:

~-

H--.

ami-)

-

one fourth of the panel on the exposure rack. This failure appeared to be independent of exposure to light. Further investigation suggested that cracking was influenced by the primer, the plasticizer, the film thickness, and the time and temperature of the film bake. Since such a severe failure of the film could make PMMA based lacquers unacceptable for commercial use, an understanding of the processes involved as well as a solution to the problem was required. The observation that nature and quantity of the plasticizer had a significant effect on the cracking of the film suggested that loss or migration of the plasticizer might be involved. Thus, butyl benzyl phthalate cracked much less than dibutyl phthalate. A study of plasticizer evaporation showed that DBP evaporated more rapidly than butyl benzyl phthalate (BBP) both alone and from the film (Figures 10 and 11). Loss of plasticizer will embrittle the paint film and increase the stresses, since it cannot shrink. Extraction studies with silica gel or mineral spirits showed that DBP was also extracted much more rapidly than BBP. These studies implicated the permanence of the plasticizer as a significant variable in the cracking process. Furthermore, the observation that lower initial plasticizer also reduces cracking is predictable. Only a part of the plasticizer is lost due to evaporation, depending on the temperature or conditions of exposure. Once the limiting concentration is reached no further losses occur. If the initial plasticizer content of the film does not exceed the

-

TIME D N b

1

D

Figure 12. Plasticizer migration from topcoat to undercoat as a function of temperature and presence or absence of solvent.

concentration that remains under a given set of conditions, no losses occur and hence no additional stresses are introduced by exposure. Some simple measurements have been made of the order of magnitude of the stresses that can be generated by the loss of plasticizer. If a film of PMMA and BBP is immersed in mineral spirits while kept at fixed dimension, the initial rate of stress development is about 50 psi/h. After removal and drying the stresses reached 180 1b/ine2. If the film was allowed to shrink while plasticizer was extracted and then returned to its original dimensions, the stress required exceeded 1000 lb/in.2. When BBP was substituted for DBP only about half the stresses developed, presumably because BBP is less fugitive. The difference in stress development between extraction in the restrained mode and the relaxed mode is due to the creep or stress relaxation that can occur continously when the film is under stress, thus reducing the final stress values observed. The change in plasticizers helped but did not eliminate cracking. The effect of the undercoat was also found to be significant. It was found that the plasticizer, in addition to evaporating during baking or exposure, can also migrate into the undercoat. This results in the embrittlement of the topcoat and the softening of the undercoat, a situation that is known to lead to failure when such systems are stressed, Studies with '*C-labeled plasticizers have shown that migration of plasticizers occurs during application of the topcoat; in fact, in the absence of solvents very little migration occurs. If a dry film of undercoat and a film of topcoat containing labeled plasticizer are pressed together and heated to 120 O F , a very slow migration of plasticizer occurs, which ceases after the first few days. If this system is now exposed to solvent vapor (such as toluene) a very rapid migration occurs which reaches equilibrium in a few hours (Figure 12). Different plasticizers reach different equilibrium conditions. Cross-linked undercoats soften less than uncross-linked undercoats. Thick topcoats soften a given undercoat more than thin topcoats, probably because more plasticizer can migrate into the undercoat. The effect of moisture on plasticizer migration, while slower than the effect of solvent, may be more damaging in the long run. When our first exposures came back from the test fence, it was observed that cracking was especially severe under the flap. This was shown to be due to the retention of moisture. In laboratory tests, cycling the coating between warm moist conditions and cold dry conditions gave film failures that closely resembled those

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979

Table 11. Factors Affecting Lacauer Cracking undercoat

topcoat

solubility of plasticizer thickness hardness cure plasticizer content thickness volatility extractability

I

I

directly directly inversely inversely directly directly directly directly

97 1

1

CROSS-LINKED

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Y

on exposed panels. Results of laboratory cycling correlated well with exposure in Florida, so that rapid screening tests could be carried out. The nature of the plasticizer also had a significant effect; the less soluble it was in the primer, the lower the cracking tendency. From such tests it could be shown that cracking failure was directly proportional to topcoat film thickness, plasticizer content, and primer underbake (Table 11). The final answer to the cracking problem had to be coordinated with the requirements of all properties. While low concentrations of certain plasticizers reduce the tendency to crack, they increase the tendency to solvent craze during repair. Some primers, when overbaked, improve crack resistance; however, overbaking reduces topcoat adhesion. Thus the final system must have a balance that minimizes various faults and yields an acceptable overall balance of properties. 4. Development of Nonbuffed High Gloss Coating Technology The three previous problems all were uncovered during testing as observations that required explanation by application of theory. Sometimes it took several years before all data needed to test a theory became available. After acrylic lacquers went into commercial use, experience on the production line suggested that buffing in order to obtain gloss was undesirable. It is true that nitrocellulose also needed buffing; however, the mechanical properties of poly(methy1 methacrylate) are such that it was more difficult to buff and therefore it became an objective to find a method of obtaining craze resistance that would avoid the need to use Bentone. The first attempt to develop nonbuffing coatings involved the replacement of Bentone with cellulose acetate butyrate and raising the bake temperature. Originally acrylic lacquers had been baked a t 110 "C. Later it was found that gloss increased as bake temperature was increased. Substitution of cellulose acetate butyrate for Bentone and the capacity to bake at 125 "C allowed the introduction of the bake-sand-bake process on the assembly line. In this process the coating is applied and baked just enough to make it hard. The coating is lightly sanded and rebaked at a higher temperature to reflow the sand scratches and produce a very smooth, glossy finish. However, as the bake temperatures were increased, the plasticizer tended to evaporate. The properties of an amorphous polymer at any given temperature are a function of the molecular weight and the relationship between the temperature at which it is being tested and the glass transition temperature (Figure 13) (8). At temperatures below Tgpolymers are generally brittle and hard. At temperatures above Tgthermoplastic materials are either rubbery or soft and easily deformed; thermoset coatings can be soft but they are less readily deformed because of the cross-linking. In the region of the glass transition temperature thermoplastic materials are generally hard without being brittle. This would be the most desirable range for their utilization. Unfortunately, the glass transition temperature range is very narrow,

0

0

6

J

5

4

50

100

50

2 50

200

T E M P E R A T U R E C'.

Figure 13. Plots of E, (10) vs. temperature for crystalline isotactic polystyrene for samples A and C and for highly cross-linked atactic polystyrene (ref 8).

I~

-100

-80

-60

-40

0 20 T E M P E R A T U R E ,.C

-20

a0

SO

00

100

120

Figure 14. Effect of butadiene content on the modulus-temperature relationship of butadiene-styrene copolymers (ref 9).

much narrower than the normal temperature range encountered by exterior coatings. Automotive coatings can be expected to be exposed to temperatures from below -20 "C to above 65 "C. The glass transition temperature of poly(methy1 methacrylate) is generally reported to be between 105 and 115 "C. Obviously, the polymer would be brittle at all temperatures below 100 "C. To make it useful, it has to be plasticized, either by adding an external plasticizer or by copolymerization with an internal one. Both methods only shift Tg,but do not broaden it (Figure 14) (9). In order to obtain the high-temperature properties that are needed to allow the lacquer to be used in a warm climate, the glass transition temperature of the f i i should be about 55-65 "C. Such a film would have a craze-free temperature of about 34-45 "C, which means that any repairs carried out below that temperature would cause the film to craze. Repairs on automobiles are carried out at temperatures as low as 15 "C. Therefore, to obtain craze resistance a method to reduce the inherent stresses in the film at those temperatures had to be developed. I t is well known that thermoplastic block or graft copolymers can have a broad temperature range of utility if one of the components has a high glass transition temperature and the other component has a low glass transition temperature. The temperature-modulus curve of such a polymer has a very high modulus at temperatures below the glass transition temperature of the soft block, an intermediate modulus in the range between the two glass transitions, and a rapid drop in modulus when the

98

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, NO. 2. 1979

I

Table I11

1

Q

e

0.74 0.43

0.40 0.53

POLYBLENOS

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MMA BA

TEMPERATURE.

1.813 0.542

O2V

.C

Figure 15. Modulus-temperature behavior of polymer blends (ref 10) of polystyrene and a 30 IO butadlen-tyene copolymer. Numbers on the curves refer to weight percent of polystyrene in the blend.

temperature exceeds the ?,' of the hard block. That is exactly the behavior that we felt was required to obtain craze resistance lacquers. Block and graft copolymers differ from random copolymers in their structure and in the manner which the polymer chains behave in bulk. Random copolymers usually form a single phase; graft and block copolymers generally form two phases, each of which is rich in only one of the chemical species. It is well known that styrene butadiene copolymers of various compositions can he blended to some extent (Figure 15) (10). It seemed desirable to determine if systems containing two phases could also he obtained by blending acrylic polymers. While mutual solubility or Compatibility is a common phenomenon among low molecular weight organic species, it is uncommon among polymers. Thus, for instance, poly(methy1 methacrylate) is not compatible with poly(butyl acrylate) or poly(ethy1acrylate). The explanation that is generally given is based upon the thermodynamics of solutions. The entropy of mixing two species depends on the number of random associations that are availahle. In low molecular weight materials, where the volume of the individual molecules is small, the entropy value is substantial; as a result even when the heat of mixing is positive (heat is absorbed), the free energy can be negative and solutions will be stable. In high molecular weight substances, because of the large volume of the individual molecules and the fact that the total number of positions they can assume is limited, the entropy of mixing is very small. As a result, unless the enthalpy of mixing is either negative or 0 the free energy of mixing is either 0 or uositive and the mixture is unstable relative to separation into two phases. Polv(methv1methacrvlate) can form clear solutions and clear iilms ;hen mixed with copolymers of methyl methacrylate and other acrylic monomers. Not a l l blends that give clear solutions give clear films. By choosing comonomers that reduce the glass transition temperatures, especially butyl acrylate or 2-ethylhexyl acrylate, and controlling the compositions, nearly compatible blends can be obtained, one component of which has a glass transition temperature that is 25-35 "C lower than that of the other. The nature of these copolymers and the effect of synthesis method on compatibility and film properties have been studied.

r,

rz

I

-50

I

I

I

0 50 Temperature. 'C

I

I

LOO

Figure 16. Logarithmic decrement vs. temberature for a two-phase film; 404020 PMMA/(MMA/BA)/plasticizer (baked 90 min at 163

"C).

Figure 17. Electron micrograph of a polymer blend film, 404020 PMMA/(MMA/BA)/plastieizel, heated 30 min at 300 OF.

Generally speaking, copolymers of methyl methacrylate with butyl acrylate have a wide compositional distribution. Methyl methacrylate and butyl acrylate have the copolymerization constants (11)listed in Table 111. In hatch reactors the initial copolymer contains only 6(t70% of the butyl acrylate that has been charged. Above 95% conversion the instantaneous molecules formed are very rich in butyl acrylate. A polymer solution prepared from 77/23 MMA/BA mixture can be hlended with a solution of poly(methy1 methacrylate) of similar molecular weight to give clear solutions and, if properly handled, clear films. When the glass transition temperature of this film is measured with a torsion pendulum one can obtain two distinctive glass transition temperatures or damping peaks (Figure 16). Electron micrographs of such films show two phases (Figure 17). One interesting aspect observed is that both glass transition temperatures occur at a lower temperature than those of the individual polymers systems. The glass transition temperature of the poly(methy1methacrylate)

Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979 99 EFFECT OF BLENDING OF P M M A WITH P I M M A / B A ) O N OBSERVED Tg

HCMOPCLYWER

CO PO LY NE R

75

IO8

19

BLEND

Tg

HIGH

LOW

95

60

Figure 18. Glass t r a n s i t i o n temperature of polymer b l e n d films.

14 Polymer Blend No PIIIIICIZW

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18

Polymer BIend/Plailismn 10114

6 4 2

CYMA Pl.stkkr

crud

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Time, Minutes

Figure 19. R a t e o f solvent absorption o f lacquer f i l m s a t various plasticizer contents (acetone vapor absorption a t 75 O F ) .

used is about 108 "C, and that of the copolymer about 75 "C. The two glass transition temperatures observed experimentally were 95 and 60 "C. (Figure 18). This may seem odd, but is quite logical. Apparently some of the MMA rich fraction of the copolymer is soluble in the PMMA phase, thus introducing enough dissolved BA into that phase to reduce its Tr In turn, the removal of MMA from the copolymer phase increases the BA content of that phase, thus reducing the Tgof the copolymer phase. Methyl methacrylate/ butyl acrylate 80:20 copolymer made by a process which gives a uniform polymer composition is incompatible with the poly(methy1 methacrylate) and does not give clear films. This suggests that it may be necessary to have some polymer molecules that are soluble in both phases, in order to control the size of the dispersed phase in such a way that clear films can be obtained. Coatings made from blends of polymers with plasticizers were prepared and tested for film properties. We found that these had substantially lower craze-free temperatures than ones from PMMA with a much larger quantity of plasticizer. The solvent vapor absorption of polymer blends was compared with plasticized poly(methy1 methacrylate). Figure 19 shows the rate of absorption of solvent vs. time for the following systems: poly(methy1 methacrylate) /plasticizer in a crazing system, poly(methy1 methacrylate)/plasticizer in a system which does not craze at room temperature and two polyblends. The noncrazing homopolymer system absorbs solvent very rapidly, while the crazing system absorbs it very slowly until there is a sudden break when the rate of absorption increases. At that point in time craze marks could be seen on the panel surface. The absorption of the polyphase system is intermediate between these two extremes but essentially is close to that of the noncrazing homopolymer system. A study of the effect of thermal history on film clarity showed that a clear two-phase film was obtainable only when the film viscosity was increased rapidly during and after solvent removal. If the film became too hot and its viscosity dropped, the dispersed phase would coalesce into

larger aggregates and the film became hazy. If evaporation was allowed to proceed at room temperature without restraints, clear films were formed. If, however, room temperature evaporation was restricted by placing a cover only 4 mm above the film so that the area directly over the film remained rich in solvent vapor, cloudy films were formed. It is believed that slow evaporation allowed the dispersed phase to separate into larger particles because the viscosity did not increase fast enough to freeze the film a t the optimum stage. Once the principles of the effect of copolymer composition and compatibility had been worked out it became necessary to find the most practical combination and to develop commercial lacquer systems. Because of the effect of baking on clarity, the composition of the copolymer had to be carefully controlled. Plasticizers that would not interfere with the attainment of the,desirable structure also had to be developed. It was found that low molecular weight polyesters make excellent plasticizers and do not volatilize as do monomeric phthalates. Low molecular weight polyesters had previously been tested as plasticizers for PMMA hompolymer during crazing studies. While attractive compositions could be formulated, these caused cracking when they were applied under or between compositions that were plasticized with monomeric plasticizers. The unexpected observation was that the use of such polyester plasticizers in the two-phase blends did not cause cracking. While no definitive experiments were run, it was speculated that the monomeric plasticizers, when they diffused, would accumulate preferably in the soft, dispersed phase, thus not appreciably softening the two-phase film. Since this is much harder than the plasticized homopolymer, the tendency toward differential movement and thus cracking would be greatly reduced. This is speculation and not supported by evidence except the hardness data and the cracking data. In conclusion, the benefits obtainable when one can explain observations on the level of physical and chemical phenomena should be stressed. One does not always have the time or opportunity to do this in industry. This puts the burden more heavily on the academic community. Paint systems are immensely complex mixtures. Technologists are just starting to be able to model some of the interactions of solvents and binders and pigments and it may be a long time before the total system is understood. However, to the extent that fundamentals can be used, they should be. Literature Cited Resinous Products Company Bulletin: Acryloid-Acrysol (1944). Graessley, W. W., Adv. Po/ym. Sci., 16, 20 (1974). Berry, G. C., Fox, T. G , Adv. Polym. Sci., 5 , 261-357 (1968). Graessiey, W. W., Adv. Polym. Sci., 16, 55 (1974). (5) "Fracture Processes in Polymer Solids", Bernard Rosen, Ed., "Brittle Behavior of Polymeric Solds", J. [email protected],p 223, Interscience,New York, N.Y., 1964. (6) Hansen, D. M., Pierce, P. E.. Ind. f n g . Chem. Prod. Res. Dev., 12, 67 (1973). (7) Anand, J., Balwinski, R. Z.,J . Colloid Interface Sci., 31, 196 (1969). (8) Tobolsky, A. V., "Properties and Structure of Polymers", p 75, Wiley, New York, N.Y., 1960. (9) Tobolsky, A. V., "Properties and Structure of Polymers", p 79, Wiley, New York, N.Y., 1960. (10) Tobolsky, A. V., "Properties and Structure of Polymers", p 82, Wiley, New York. N.Y., 1960. (11) Rohm & Haas Company, private communication. (1) (2) (3) (4)

Received for review S e p t e m b e r 14, 1977 Accepted M a r c h 5 , 1979