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.