Gel Coat Quality Has To Be Skin Deep

Most of the time, the gel coat on a cast polymer product is a clear coating that is designed to showcase the color and effects of the cast polymer product while protecting the cast polymer surface. That’s a lot to ask for a layer that is typically only 20 mils thick.

Because the gel coat is so thin and expected to do so much, close attention to chemical details and skillful application techniques are essential. Because so little material is involved, gel coat appearance and performance is significantly affected by variations.

Let’s look at ten key gel coat issues and guidelines for optimizing your process for higher quality and profitability. The guidelines are not presented in any particular order of importance.

  1. Control gel coat cure by controlling thickness.
    The rate of thermoset polymer cure is affected by the mass of thermoset polymer component. At a higher mass, a thermoset generates more exothermic heat which speeds the cure rate. The typical clear gel coat thickness is approximately 20 mils. Variations should be kept to within ±2 mils. All other parameters being equal, a consistent thickness means a consistent cure rate time after time. And that means a more predictable part time after time.
  2. Control gel coat color by controlling thickness.
    In spite of their name, clear gel coats have some degree of color in them. As one would expect, the greater the gel coat thickness, the more color from the gel coat is introduced to the final part. This is especially true at lower temperatures which have an even greater influence on the appearance of “hidden” gel coat color. Some operations apply more gel coat to a vanity’s bowl area to increase the moisture resistance of that area. In these cases, expect the extra gel coat mass to produce a different color in the bowl when compared to the appearance of the vanity’s top.
  3. Promote the promoter
    The fact that clear gel coats require clear ingredients limits the type of promoters that can included in the gel coat formulation. Clear promoters are generally less “forgiving” because most process enhancement additives make it difficult, if not impossible, to achieve a good degree of clarity.

    Consequently, the general practice is to use the exothermic heat of the matrix to assist gel coat cure. Here is where paying attention to the interrelationship of system components is especially important. Any changes to the matrix resin will change its cure profile, thus changing the cure profile of the gel coat.
  4. Master the science of catalysts.
    Catalyst addition is the most important gel coat-related chemistry you perform. For each part or family of parts, establish the optimum catalyst addition level - typically between 1% and 3%. Any change in catalyst type or conditions will inevitably create a change in gel coat cure, working time, viscosity and/or color. When catalyst levels are too high, physical and mechanical properties tend to be below target levels. Higher catalyst levels usually increase the yellowness of the gel coat. When catalyst levels are too low, cure is not complete, color is poor, and poor adhesion between the gel coat and matrix leads to delamination.
  5. Keep equipment and tooling in prime condition.
    Most of these guidelines address the gel coat chemistry and formulation. Related factors to consider are the equipment used to apply the gel coat and tooling to which the gel coat is applied. The catalyst can be manually mixed into the gel coat base in the spray system pressure pot. In some shops, the catalyst is atomized and externally mixed with the gel coat during spray application. All equipment should be kept clean and calibrated. The gel coat surface will reflect the surface of your molds. Tooling should be free of physical defects which can cause localized differences in cure. The tooling surface should be free of contaminants, including dirt, solvents, water and mold release agent or wax.
  6. Control all your temperatures.
    Temperature control is essential to process control. The optimum temperature range for gel coat cure is 120°F and 170°F. If the temperature is too high, the gel coat will experience yellowing which change the intended color and effect of the matrix. If the temperature is too low, cure will be incomplete. Manufacturers can account for seasonal temperature swings by changing catalyst levels. When economically feasible, another strategy is to maintain a constant shop temperature. This results in higher utilility heat costs to heat the shop environment during colder weather and air conditioning the shop during warmer weather. Because the gel coat is the first material applied, it is significantly affected by mold surface temperature. Molds in the morning are cooler and may act as a heat sink, causing parts made earlier in the day to appear different. Part quality is also be affected by gel coat storage temperature. Another sound practice is to allow the molds and gel coat to achieve a common ambient temperature.
  7. Adapt to the nuances of low VOC formulations.
    Switching to a low VOC gel coat reduces shop and environmental emissions but calls for some adjustments in the manufacturing process. With less styrene a typical unsaturated polyester becomes more viscous. As a result, the gel coat operator should use a spray tip orifice that is one size larger than one typically used. Using similar spray pressures, the lower-VOC formulation should be applied with a tighter fan pattern that reduces overspray. The spray droplets will be larger with a low-VOC gel coat, so the wet-applied gel coat will not look the same as a gel coat that is atomized during application. Optimum wet gel coat thickness is still in the 18 to 22 mil range. However, the cured low-VOC gel coat will demonstrate less shrinkage because there is less styrene to evaporate out.
  8. Keep good records.
    Good recordkeeping is the essential first step in any quality program. Save the manufacturing and quality control information provided by the gel coat supplier. Using properly calibrated equipment, periodically measure and record:
    • the rheology of the gel coat,
    • gel coat cure parameters (gel time and gel-to-peak exotherm time), and
    • gel coat color.
    This statistical information will help you establish which parameters are best for gel coat quality, consistency and performance. Using these parameters as a control, you can try evolutionary changes and measure their ability to bring about improvements.
  9. Make changes infrequently and methodically.
    The key to product consistency is establishing consistency in your formulation and shop conditions. Through incremental changes, you will eventually arrive at optimum formulation and process conditions that best balance your manufacturing, performance and cost needs. Having an established process does not rule out change. However, changes should be evolutionary; and a detailed outcome of any change should be recorded.
  10. Work with your supplier’s technical service organization.
    Tell your gel coat supplier what you plan to make. Be specific about the type and amount of catalyst you use. Explain the nuances of your shop conditions. Then you and your gel coat supplier can work together to optimize your formulation and process for higher quality, consistency and profits.

    Written by Dan Oakley, Gel Goat Product Leader for AOC.

    The World of AOC

    Headquartered in Collierville, Tennessee, AOC is a leading global supplier of resins, gel coats, colorants, additives and B-sides for composites and cast polymers. Backed by the industry’s best technical support, AOC products are manufactured in facilities strategically located in United States, Canada, Mexico, the United Kingdom and Thailand. AOC-owned facilities are certified as meeting ISO 9001:2000 standards for quality control.

    For more information on AOC’s complete package of marine resins, gel coats and technical support, phone Steve Martin at (901) 854-2846, e-mail smartin@aoc-resins.com, or visit www.aoc-resins.com on the Internet.