Reprinted from the RP/C th Annual Conference Preprint The Society of the Plastics Industry, Inc.

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1 Reprinted from the RP/C 98 0th Annual Conference Preprint The Society of the Plastics Industry, Inc. 0th Annual Corrference, Reinforced Plastics/ Composites Institute, The Society of the Plastics Industry, Inc. Jan. 28-Feb., 98 RESIN TEARING: A POLYESTER GELCOAT SURFACE DEFECT by DAVID L.ELLSWORTH * ABSTRACT Polyester gelcoat has been a useful and popular coating for RP fabrication for over twenty years. The gelcoat has not only been important as a protective coating for the fiberglass laminate in a large variety of construction applications, but in many cases has provided a cosmetically desirable finish that enhances the attractiveness and market value of the RP part. In recent years as the manufacture of RP has become more sophisticated and production schedules have required increased production rates, equipment suppliers have improved spraying equipment to meet these new demands. This new equipment, such as high pressure airless spray systems, and the faster production rates using the more conventional pressure pot systems have led to fabricators encountering surface defects, caused by resin separation, on the finished gelcoat parts. These defects are variously known as resin tearing, streaking, or snow- fencing. Our observation of this surface defect on finished parts in RP fabrication shops and our ability to duplicate the phenomenon in a controlled laboratory environment led us to ask certain questions about this gelcoat problem.. What is resin tearing? 2. What does it look like? 3. Why does it occur?. How can the problem be corrected? This study was undertaken to try to answer these questions. The data obtained from microscopic and X-ray studies have indicated clearly what resin tearing is and what it looks like. The actual spraying of a variety of different gelcoat formulations under different conditions in our laboratory in conjunction with actual field observations in various RP fabrication shops have produced a reasonable theory as to why the problem occurs. In order to determine how the problem might be corrected a large number of factors were considered.. The conditions under which the gelcoat was sprayed including equipment used, type of mold, application techniques, and the use of thinning solvents. 2. The effect of various fillers and pigments. 3. Different gelcoat resins and pigment grinding vehicles were examined for their effect.. The use of different thixotropes and thickening agents and the rheology of the system as a whole including the role of various solvents was studied.. The study of various wetting agents and air release agents was undertaken to determine what effect they might have on the elimination of the defect. *Fiber Resin Supply Co. & King Fiber Glass Corp. 366 W. Nickerson St. Seattle, WA 989 Our conclusions from our studies and recommendations for fabricators and gelcoat manufacturers for improved gelcoated parts are presented in this paper. INTRODUCTION Resin tearing is a gelcoat surface defect that appears on a molded fiberglass part as a streaking discoloration, usually linear in nature and often wrongly identified as a pigment separation. On the surface of most gelcoats such as a gray or blue it looks like black streaks. In the off-whites or browns it can appear as a dark brownish streak. The defect is observed most frequently in gelcoats that are: (l) light brown, (2) offwhite, and (3) light gray or blue. Darker colors seem to effectively hide the defect if it is there. An example of resin tearing is shown in Figures -3. The problem seems to occur most often in applications that have sharp corners or a tight radius where the applicator has difficulty achieving the desired gelcoat thickness and is required to spray the material at a difficult angle. The defect may occur in flat areas, but this is usually rare and the flat area is often adjacent to a difficult curve or radius. Spas, hot tubs, jacuzzi bath tubs, recreation vehicles and tops are some examples of RP products that often produce this problem. The separation occurs most often with high pressure, airless gelcoat systems which subject the fluid to very high shear forces as the material is sprayed, or in cases where more conventional pressure pots are used in pot pressures of -0 psi or more. Thick gelcoat application which produces obvious sagging or slumping of the coating or the blasting of the coating by holdingthe gun too close to the mold or at an angle which is not perpendicular to the mold also seems to aggravate the problem. Gelcoat manufacturers have been concerned about this problem for some time and have discussed it and made numerous suggestions as to how to eliminate it (l-3). A manufacturer of hydrophobic wetting agents and air release agents for polyester gelcoat systems has also made mention of the defect (). The problem has continued to plague RP manufacturers, however, and as a supplier of liquid polyester gelcoats to the RP industry our customers and field representatives have brought the problem to our attention and we undertook a study to examine it. This paper describes a three-year study of resin tearing in RP products. We have attempted to identify the nature of the defect, the causative factors, and what steps might be taken to minimize it. EXPERIMENTAL In order to determine the actual nature of the streaking discoloration, samples of gelcoat that clearly demonstrated the defect were examined by microscopic analysis. Microtome cuts of the gelcoat were examined under an electron microscope and then scanning electron micrographs were taken of the defects. A sample of one of the separated areas was subjected to energy dispersive X-ray analysis (EDAX) to determine primarily the presence and distribution of the elements silicon (from the fumed silica) and Titanium (from the white TiO 2 pigment), which would normally be present in a typical polyester gelcoat. The separation itself and the surrounding area were both studied and the elemental distribution plotted. A large number of different gelcoat formulations were made for testing purposes by mixing them in the standard fashion using a Cowles Dissolver. The formulations were varied by using different combinations of pigments, wetting agents, thixotropes, thixotropic agents, fillers and resins. Some formulations were altered by changing viscosity with different solvents or different amounts of a particular solvent. The sample gelcoats were sprayed on a jacuzzi-type tub mold (Figure ), that we felt was fairly typical of the RP industry. Session 2-D, Page

2 This particular mold contains a variety of curves, radii, flat areas, and areas that are difficult to spray. The spraying equipment used was a standard Binks 8 spray gun with a 66PE tip and a one- or two-quart pressure pot. The line pressure was kept constant at 70 psi. The pot pressure varied from psi to 0 psi depending upon the conditions of the test. The gel coat was usually sprayed at 8 ±2 mils thickness. In some cases in order to test the formulation s resistance to separating, it was sprayed 30 0 roils thick and allowed to sag severely, or it was sprayed on the mold at less than a 90 angle and the spray gun was held about six inches from the mold to induce considerable stress and shearing effects in the gelcoat. The catalyst level was l-2% MEKP depending on the promotion level of the gelcoat. The gelcoat was removed from the mold by using compressed air as soon as it was cured sufficiently to do so. A surface agent of wax in styrene was often added to the gelcoat before spraying to keep the exposed side of the coating as free from air inhibition, and as tack free as possible for ease of handling. RESULTS AND DISCUSSION Resin tearing when first observed with the unaided eye appears to be a type of pigment separation, with linear streaks. The streaking when observed under magnification is seen to actually be a separation of the solids leaving resinous voids in the material that absorb and scatter light in a different manner than the surface around them (). This may be illustrated clearly in Figures 3. The higher magnification shows the presence of a clear resin-filled hole in what is otherwise a solid uniform surface. This is shown more clearly by the microtome cuts of the electron micrographs in Figures -8. The separations appear as the clear or light areas and do not contain pigments. The clear zones are only filled with resin and they seem to be distributed throughout the gelcoat. Air bubbles can t be identified. The scanning electron micrographs are shown in Figures 9-3. In these photographs the color of the separations are reversed as the dark areas are now the resinous voids. The burst surface is caused by the microtome cutting knife used to obtain the thin layers. The cutting process causes a break up of the pigment particles producing the burst effect. This evidence demonstrates conclusively that there is resin in the separated area but there appears to be a marked absence of filler or active pigment. In an attempt to determine what might be present or absent in a resinous void an energy dispersive X-ray analysis (EDAX) was performed on the separation shown in Figure 2. The EDAX analysis is shown in Figure. In the separation the concentration of the titanium is very low compared with the distribution outside the separation. On the other hand the concentration of the silicon seems to be very close to the same value whether it is measured inside or outside of the resinuous void. Interestingly enough the filler that was used in this gelcoat formulation was alumina trihydrate rather than a standard talc. Thus, the results for the elemental silicon must be due entirely to the presence of dispersed fumed silica. Furthermore, a peak for aluminum appears in the analysis and it correlates with the titanium results as the elemental concentration is much higher outside the void than inside it. These results clearly show that the inert filler and the active pigment both separate from the base resin leaving a void containing finely dispersed fumed silica. Thus, the thixotrope itself seems relatively immune to the tearing phenomenon. In an effort to better understand what actually causes the separation to occur we sprayed some gelcoats using different pressures on the fluid. Table I shows the results when each gelcoat was sprayed at a low pressure and then a significantly higher pressure. The appearance rating is simply an arbitrary Session 2-D, Page 2 rating of the amount and severity of resin tearing in the cured gelcoat and maybe interpreted as follows: Excellent No observable separations Very Good Small areas of minor separations accept part 3 Good Some areas of separations borderline part 2 Fair Several areas of separation reject part Poor Severe separations in several areas O Very Poor Severe separations in all areas. In each case the appearance of the cured coating was improved significantly by lowering the pressure. This information correlates with previous literature which has indicated the incidence of resin tearing maybe shear related (l). Our experience in spraying test gelcoats on our mold and observing operators in the field have convinced us that resin tearing is a shear related mechanical problem. It may occur any of three ways.. High fluid pressure in airless systems or high air pressure on the pressure pot, forcing the fluid out the tip under severe shearing stress. 2. Improper spraying techniques or difficult angles where the fluid hits the mold at an oblique angle or is blasted on, shearing on the mold as it is sprayed. 3. Putting gelcoat on too thick in the wet film causing slumping or sagging of the material due to gravitational forces and shearing the gelcoat as a result. We have determined that proper spraying techniques, correct pressures, and proper tip selection can aid materially in minimizing the problem. There are circumstances, however, that produce resin tearing even when the fabricator has done all that is possible for them to do in the way of good techniques and proper equipment. It was because of this fact that we examined possible alterations in the gelcoat formulation to help overcome the more difficult cases. One of the first approaches was to try eliminating basic ingredients one by one to determine if the problem might be directly related to one of them. The standard polyester gelcoat is made up of a few basic ingredients: (a) the gelcoat resin, (b) talc or some other inert pigment filler, (c) a thixotrope, commonly fumed silica, (d) active colored pigments, (e) thixotropic agents to react with the thixotrope to produce sag resistance and proper rheology, (f) styrene monomer and occasionally solvents such as acetone used for thinning, (g) a promoter such as cobalt octoate, and (h) various additives for improving spraying, flow, appearance, air release, etc. One of the first areas of concern is that of active pigments and their role in the separation phenomenon. An effort was made to use a pigment vehicle that was the best available and as many of the pigments as possible were ground in it. The goal was to create a pigment system as compatible with the resins as possible. The role of the inert pigments or fillers was studied as well to determine the extent of their effect in causing the separations. Table II shows some of the results that were obtained. The unpigmented gelcoats gave some of the best results regardless of the filler, but it was possible to produce a small separation in two of them. This result is quite significant because it means that the active pigments cannot be the sole causative factor for the separations. On the other hand the filler cannot be the sole factor either as the sample that is pigmented and has no filler had severe tearing. Interestingly the removal of all filler, thixotropes and additives, using only resin, pigment and promoter results in no separations at all. The results shown in Table III are an attempt to discover if the use of a different thixotrope has any material effect on the occurrence of tearing. They seem to indicate that hydrophobic

3 fumed silica may be better than hydrophilic fumed silica or precipitated silica. The precipitated silica gave mixed results but it may be said it is not a panacea for the curing of the problem. Hydrophilic fumed silica is the most common thixotrope used in polyester gelcoats and can also be found to give mixed results. The hydrophobic fumed silica which in this case has a special silicone treatment has consistently given good results, regardless of the filler or pigment combination. In Table IV we see the effect of altering the rheology of the gelcoat by simply thinning it with solvent. The results were quite disappointing. If the rheology is to be changed to make the gelcoat easier to spray it cannot be done simply by thinning with a solvent or the risk of resin tearing will be increased. Some other points of interest that came to light during the study were: () the talc and silica seem to give consistently better results in resisting separations than the alumina trihydrate, (2) the gelcoat resin can make a difference as the ortho NPG resin consistently produced gelcoats that under similar conditions were superior to the standard iso and iso NPG resins insofar as incidence of resin tearing was concerned, and (3) the proper use of air release agents and wetting agents seem to positively affect resistance to separation in all the gelcoat formulations that were studied. CONCLUSIONS Since resin tearing is a mechanical problem, the first step in its solution should be to maximize the techniques and equipment of the fabricator. Fluid or air pressures should be at the minimum level which allows the fabricator to spray his mold at the rate which permits him to meet his production schedule. The selection of the correct tip for his gun can be helpful in allowing him to lower his pressure and still spray the mold adequately. The gelcoater should use a fan pattern that allows the spraying of the mold from 8 to 36 away and the thickness of the material should be kept at 8 ± 2 roils. The material should be sprayed in two to three passes, allowing a few seconds between each pass. The gelcoater should avoid cold temperature conditions and the thinning of the gelcoat. The catalyst should be kept at the recommended level. These steps will minimize the three types of shearing mentioned in the previous section. Careful attention to catalyst levels, temperature and the avoidance of solvent addition will improve the rheology of the gelcoat for easier handling and will allow the gel time to occur fast enough to make the resin separations less likely to occur. The gelcoat should be thoroughly mixed each time before it is sprayed, ensuring that the solids will be properly suspended in the resin and less likely to separate. The gelcoat manufacturer can help the fabricator by supplying a coating that has the viscosity and thixotropy that will allow it to be sprayed at lower pressures without the addition of solvents and that will gel properly at the correct catalyst level. The coating manufacturer also needs to consider the formulation of a gelcoat that has the best balance of pigments, fillers, resins and additives. We suggest the use of silicas or talcs as fillers at a level that will improve the flow and air release of the gelcoat. Consideration of a hydrophobic silica in place of the traditional hydrophilic type may be necessary to help in stabilizing the more polar fillers in the nonpolar polyester resin. Pigments should be ground in one vehicle that is specifically compatible with. the gelcoat resin and they should be wet out properly during the grinding process. The more harmonious the blend between the pigments and the gelcoat resin the less likely separations will occur. The proper use of hydrophobic wetting agents and air release agents can do a great deal toward balancing the various components of the gelcoat and especially improving the wet-out of the fillers and pigments by the resin. The better these major components are bound to each other the less likely resin tearing will appear. A final area that might be seriously examined would be the use of thixotropic agents such as low molecular weight glycols that produce resistance to sag in combination with fumed silicas. It may be necessary to run tests to select an agent that is less polar and produces a more stable system with the resin, fillers and pigments. These more polar agents can sometimes aggravate the separations. By careful formulating and working with the fabricators to improve techniques in their shops, we believe gelcoat manufacturers can do a great deal to minimize and perhaps eliminate resin tearing as a polyester gelcoat problem. ACKNOWLEDGEMENTS To Fiber Resin Supply Co. and King Fiber Glass Corp. for the permission to publish this work; to the several manufacturing companies in the polyester coatings field whose personnel assisted this work; with a special thanks to Degussa Corp. for their microscopic and X-ray analysis of the resin tearing samples. To Kenny Wright of Fiber Resin Supply Co., the technician who helped do the testing; to the personnel of King Fiber Glass Corp. and its affiliates, many of whom helped in a material way to further this work; and to Ken Stewart, Vice President and General Manager of Fiber Resin Supply who authorized and encouraged this work in every way. REFERENCES. Stahlke, N. P., and Lester, M., Modern Plastics, October Polyester Application Manual, Cook Paint and Varnish Company, th Edition, p. 29, Gel-Kote Application Manual, Glidden Coatings and Resins Division of SCM Corp., p. 3.. Additives For Plastics, Byk-Mallinckrodt Air Release Agents In Polyester Composites, Technical Bulletin 0, p., 980. BIOGRAPHY David L. Ellsworth was born in Ballston Spa, New York, in 9. He received his Bachelor of Science degree in Chemistry from Utah State University in 967. He later obtained a Ph.D. Degree from the University of Washington in 976 for a dissertation on the kinetic and thermodynamic properties of the electrostatic catalysis of organic reactions in non-polar media. He served as a lecturer of Organic Chemistry in the Department of Chemistry, University of Washington, until 978 when he joined Fiber Resin Supply Co., and affiliate of King Fiber Glass Corp. He has served as the chemist in charge of quality control, product development and technical service in the coatings laboratory at Fiber Resin Supply since 978. He resides with his wife and two daughters in Seattle, Washington. Session 2-D, Page 3

4 TABLE I Relationship of Spraying Pressure to Incidence of Resin Tearing Gelcoat Color Resin Filler Pot Pressure (p.s.i.) Appearance Rating Light Gray Light Gray 2 Light Blue Light Blue 20 2 TABLE II Relationship of Pigment and Filler to Incidence of Resin Tearing Gelcoat Color Unpigmented Resin Iso NFG Filler Pot Pressure (p.s.i. ) O Appearance Rating 0 Unpigmented Unpigmented Unpigmented Light Reddish Brown Iso Iso Talc 0 Talc Talc Talc * None 0 Iso NFG None 0 Session 2-D, Page * This sample contains resin, pigment and promoter only.

5 TABLE III Comparison of Various Thixotropes for Incidence of Resin Tearing Gelcoat Color Resin Thixotrope Filler Pot Pressure (p.s.i. ) Appearance Rating Hydrophilic Fumed Hydrophilic Fumed 3 Hydrophilic Fumed 30 3 Hydrophobic Fumed 30 Hydrophobic Fumed 3 0 Precipitated Precipitated Iso NFG Precipitated Precipitated 2. TABLE IV The Effect of the Addition of Solvent on Incidence of Resin Tearing Gelcoat Color Resin Filler Solvent Pot Added Pressure Appearance (% by weight) (p.s.i.) Rating None 0 % Acetone 20 % Acetone 30 0 % Acetone 30 0 Talc 20% Acetone 0 % MEK 20 2 % MMA 20 % Methylene 20 Chloride Session 2-D, Page

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