Smoothing the Surface

Gel coats and veils provide the finishing touch.

Composites manufacturers today use surface finishes like gel coats and veils to improve their parts’ appearance and durability. But when their predecessors in the 1950s developed gel coats, they weren’t concerned about appearance. They were searching for a way to minimize the damage that fiberglass products were causing to their tools. “Companies took the same type of resins that they used to manufacture glass fiber laminates, thickened them up, added filler, painted them on the mold and then made the parts,” says Rick Pauer, market manager at Polynt. “People started liking the look that the gel coat provided, so the next level was to give color or surface enhancements to the FRP parts.” Boat builders saw the potential of gel coat and began asking manufacturers to incorporate other properties into it like ultraviolet (UV) and blister protection and water resistance. The marine industry is still the largest customer for gel coats, but manufacturers of transportation products (RVs, trucks and buses) and sanitary products (tub and shower surrounds) are also big purchasers. For all of these markets, “the performance of a gel coat is very much about aesthetics; there is the element of surface protection, but a significant amount of it is appearance,” says Harry Certain, business manager, Interplastic Corp. “People are saying ‘I want my RV to look great 15 years from now; I want my boat to look brand new after a number of years.’” Gel coats provide one big advantage over thinner coatings like paint. “Having a uniform color for 20 mils allows easy repairs to be made in a gel-coated surface,” says Pauer. “The agriculture, transportation and marine markets greatly appreciate the reparability of gel-coated composites where scratches and graffiti can be easily buffed back to their original surfaces. In the architectural market, the thicker gel coat is often sand blasted to provide a textured surface that looks much like concrete or terra-cotta stone.” A Balancing Act Developing cost-effective, quality gel coats is not an easy task. Scott Crump, director of research and development – gel coats/colorants at Interplastic, says that a typical gel coat formula contains between 15 and 25 different components. A change to one ingredient to improve a particular property can adversely affect other properties. “The challenge is trying to balance those ingredients and manage all the expectations of the fabricator and the end user, because there are always tradeoffs,” says Crump. “The formulation concepts leading to a coating with really good spray characteristics are not necessarily the best in terms of compliance with VOC/HAP requirements. In fact, they go in opposite directions.” The complexity of the task may be one reason why the number of gel coat manufacturers has shrunk from 20 at one time to four or five today. “You look at the technology, at the economies of scale, you look at what it takes to be consistent and you can see it has gone from mom-and-pop operations to a more industrial design product in a plant,” says Crump. “When I first started, the company I was working for had literally started in a garage. They would not be able to do that today for a variety of reasons.” Gel coat manufacturers have to design their products for both the immediate customer – the fabricator – as well as the end user. For fabricators, a gel coat has to produce consistent results. “It’s like paint, but it has to be able to be sprayed and it can’t run or drip,” Pauer says. That’s challenging with a material that is usually applied at 15 to 35 mils in thickness... . In addition, gel coats must perform well over a range of temperatures, since some fabricators may not have good environmental controls in their shop.” Porosity is also an issue, Pauer adds. “We generally spray or brush on this material, but you can’t have any type of air voids because you could affect its continuous film characteristics. If there’s a hole in that film, if there’s porosity or a fish eye, that allows water to ingress and it looks like a defect on the surface. It also affects corrosion resistance, water resistance, the UV resistance – all of those things that you are using the gel coat for come into play.” Gel coat formulations also vary by application method. Crump says the vast majority of composite fabricators in the U.S. today spray gel coats into an open mold as the first layer of a composite part. “The gel coat forms a strong chemical bond to the resin,” he says. “The individual layers become monolithic, because they are essentially bonded together. That’s one of the reasons that we use gel coats instead of just painting the outside of the surface with some other type of coating. It’s really durable because it’s not a physical bond to the surface, it’s a chemical bond.” Composite product manufacturers apply the remainder of gel coats to existing structures as a post-finish layer. These surface coats add a thick, durable layer to a product’s surface. Required drying time is another factor in gel coat formulation. Several of Interplastic’s customers produce composite parts (bath/shower units, transportation panels, etc.) using high throughput, continuous processes such as conveyor lines. “The gel coat has to be ready to laminate in less than 15 minutes. If it’s not, the production rate of the entire process is reduced,” Crump says. Boat builders, on the other hand, can allow gel coats to cure for much longer intervals. Designs for End Customers For owners of boats, RVs, tub surrounds and other end users, the priorities are durable gel coats that keep the desired high-gloss appearance for as long as possible. They don’t want to see blotchy colors or a chalky finish. “The level of expectation has gone up; we’re making larger, more expensive, more high-end boats, and the customer wants them to retain that aesthetic as long as possible,” says Ryan Wilkins, North American marine gel coat product manager, Ashland. Longevity is not the only end user requirement, however. “There’s adjustment and adaptation depending on where the product is going to be used,” Certain says. “For example, a transportation product doesn’t see as much water exposure as a marine product, so we can compromise a little on the water resistance but work very hard for UV resistance for that market.” A cultured marble sink top with a gel coat finish won’t have problems with UV exposure, but will need to withstand thermal expansion and contraction caused by hot and cold water running over it. A wind blade on an energy-producing turbine may move at 300 mph, so its gel coat needs to have excellent abrasion and fatigue resistance as well as outdoor durability, including resistance to heat, light and moisture. End customers have challenged gel coat manufacturers with their demands for a greater variety of colors with more depth and intensity. Black and other dark colors are popular with manufacturers of personal watercraft, for example, but developing the gel coats that maintain those colors over several years is difficult. Style trends also impact gel coat formulations. Years ago, metal flake finishes were popular in the bass boat market; now, consumers want boats with a look that emulates today’s automotive finishes, which have very tiny flecks that reflect light. When designing for the transportation industry, gel coat manufacturers have a different challenge. Truck rental and RV companies that use FRP wood panels for their vehicles’ sidewalls want the same color and finished look for the composite portions of their trucks as for the metal sections. Some manufacturers design gel coats for smaller-scale products. Diversified Structural Composites, for example, produces the shafts for electric trolling motors for fishing boats. “It’s a fiberglass-reinforced pultrusion that comes straight out of our die with a gloss, Class A finish,” says Rob Klawonn, company president. “We produce a very uniform, consistent surface finish for a low cost. We also add some UV inhibitors to the resin mix that gives it a long life as a composite outside; they can last 10 to 20 years in a rugged environment while maintaining a good surface finish without powdering or chalking.” A combination of processing techniques, resin chemistry and tool design enables the company to achieve this result, he says. Putting on the Veil Surface veils provide an extra layer of protection for composite parts. They are thin, lightweight materials weighing from 17 to 68 grams per square meter. “With surface veils, we’re basically putting a fiber into a fabric manufacturing process to impart surface smoothness or to block the underlying reinforcements from blooming to the surface when exposed to UV or corrosive chemicals,” says Brandon Ratcliffe, market manager at Precision Fabrics Group. “The use of a veil creates a resin-rich surface layer that enhances the corrosion properties of the FRP composites. A lot of veils are used in FRP corrosion-resistant pipes and corrosion-resistant tanks that hold aggressive corrosive agents to improve durability and longevity.” The fiber choice and the means by which the veil is made into a nonwoven greatly affect processing and end use performance. Veils can also provide UV protection and fire resistance. Veils fall into three major categories: glass, carbon and synthetic. Glass veils are common in everyday use because they process very easily and wet out well in compatible resin systems. They’re found in flooring and in corrosion-resistant products where the chemicals don’t attack the glass. Composite manufacturers use carbon veils in niche applications, such as high-temperature caustic services or where the product needs electrical static dissipation. “FRP is highly insulative, which is great for electrical applications but can result in static charge build up in some pipe, tank or ducting applications. Conductive veil is often specified when there is a fire or explosive hazard, because it allows you to ground the equipment and prevent static build up and fires,” Ratcliffe says. Both glass and carbon veils must be compatible with the resin they’re going to be used in. That’s not the case with synthetic veils. “They are resin agnostic, so you can use them in any resin system,” Ratcliffe says. Synthetic veils provide better corrosion resistance with severe pH chemicals such as hydrofluoric acid, sodium hypochlorite and many others, he adds. They also work very well in any composite that is subjected to UV radiation. Printed veil fabrics can provide graphic characteristics, such as a wood grain or camouflage look. Colored veils enable manufacturers to produce variations of their products without having to completely change over a line. As with gel coats, a manufacturer’s decision on whether or not to use a veil often comes down to cost. “A lot of it is solving the problem for the right price,” Ratcliffe says. “Imparting improved UV protection is a complex problem to solve and often requires a solution that is more than just a basic veil fabric. You have to put a coating on the veil that incorporates different UV absorbers or antioxidants to improve UV protection. The same is true for improved flame and smoke mitigation.” Customers are looking for better solutions that they can integrate into their manufacturing processes in one step, Ratcliffe adds. When deciding between veils, gel coats, in-mold coatings and post-paint processes, manufacturers typically consider total cost of the materials, processing cost and manufacturing scrap rates to determine the best way to proceed for their specific solution. Measured Advances Progress in surface materials comes incrementally – an improvement in a gel coat’s resin can enhance durability, a new type of fabric can offer better corrosion resistance. But those changes make a real difference over time. One of Ashland’s earliest marina gel coats had an expected lifetime of 1,000 hours; its latest generation of products can reach 4,000 to 4,500 hours. While gel coat manufacturers are reluctant to give specifics about the new products their companies are working on, they will discuss the industry’s direction in general terms. [caption id="attachment_3919" align="alignleft" width="250"] Gel coats designed for wind turbine blades provide added protection against abrasion, fatigue, heat, light and moisture. Photo credit: Polynt[/caption] “The main drivers of change related to the performance of composite surface finishes include the performance in the field (for example, weathering, water resistance, etc.), economics and environmental/safety issues," Crump says. One environmental driver has been reduction of the amount of styrene used to make gel coats; manufacturers had to redesign their formulas to get the same properties they had previously offered in higher styrene-content formulas. Regulators are likely to continue the push toward lower styrene content and lower VOC materials in general. Wilkins believes that ease of application and repair will continue to be a major focus at Ashland and other gel coat providers. “There is a lot of work being done around making gel coat products more forgiving when applied in changing environmental conditions,” he says. Interplastic has implemented some new quality control rheological tools and techniques that measure how a gel coat is going to spray, level and sag. “Controlling the application characteristics of the coating from batch-to-batch is one way to reduce variation for our customers,” says Crump. “They can spray the same way every time, and it flows into the mold the same way every time.” Customers and manufacturers are always looking for gel coats with new and different colors to help meet their design needs. Often times these gel coats are required for production on short notice. The use of gel coat quick tint systems, developed to meet this need, has grown over the last several years. Ashland’s Instint^™ system, for example, operates similar to a paint mixer by rapidly tinting and mixing clear and white base gel coats to provide customers with just-in-time service. While gel coat manufacturers work hard to meet the increased expectations of their largest customers, they are also looking for new opportunities. One potential market is construction. Diversified Structural Composite is already producing a garage door component with a class A satin finish. Certain thinks the construction market could grow. “Gel coat is a pretty versatile material, and with some changes to its mechanical properties it might open up some opportunities,” he says. “Most of the gel coats out there today are designed to be very hard, high-gloss surfaces. But imagine that you had an in-mold coating that was more elastomeric. You could use it for other types of applications where you are not really looking for gloss or cosmetics, but for wearability.” Gel coats could also play a role in 3-D printing technology. Polynt is working with a partner, TruDesign of Knoxville, Tenn., at the Oak Ridge National Laboratory, where they are producing molds for composite parts using a Big Area Additive Manufacturing (BAAM) printer from Cincinnati Inc. “With BAAM, they are putting down a very heavy bead of material,” says Pauer. “That leaves a very textured, corduroy surface on the mold.” Researchers have been using a mill to grind off that rough surface, but Pauer thinks they could use specialized coatings and putties that bond well to the current thermoplastic printing materials. “You could under-print the part by ⅛ or ¼-inch thickness, then add a layer that bonds to the printed substrate with a material that is much easier to finish than carbon fiber-filled ABS. Then you can gel coat the mold to provide a durable and repairable mold surface,” says Pauer. “Some initial test results show that the concept works well for prototype and some limited production parts in both traditional open and closed molds, as well as in 350 degrees Fahrenheit autoclave molds.” The gel coat layer would quickly provide the smooth, hard surface that the composite part makers require. That is, after all, what gel coats and other surface finishes do best.