Gel coats are a crucial component in many composite products. The Gel coat is generally a polyester resin mixed with pigment. Gel coats are the exterior protection of composite products such as fiberglass boats, wind turbine blades, bathtubs, and more.
The application of a Gelcoat is almost as much an art form, as it is a science. A quality Gelcoat operator is hard to come by, and experience is priceless. Additionally, there are countless variables that play a role in the performance and quality of a gel coat. These include:
Chemistry
Not all Gelcoats are equal, chemistry wise. Gel coats by nature are thick, partly due to the resin and the large particle size of the pigments used. To thin gel coats, styrene is added. In fact, most gel coats always had upwards of 45% styrene.
Different gel coats from different manufacturers will perform differently. Each has different chemical formulas, and each might have a different percentage of styrene. It is because of this, that extra attention needs to be given prior to working with an unfamiliar gel coat.
Shelf Life
A gel coat will generally have a short shelf life, between 90-120 days usually. This shelf life is reduced dramatically if the gel coat is stored in temperatures about 26oC degrees, a common rule of thumb is that the shelf life is reduced in half for every 5 degrees above 26oC.
As a gel coat ages, it can thicken, not cure properly, or tiny specks begin to appear called “seeding.” Either way, this can affect the performance of the gel coat. Having a fresh gel coat is essential to making high-quality and consistent composite products.
Temperature
Gel coats are made to be used at a temperature above 15°C. Below 15°C, several factors are affected. The lower the temperature:
- The higher the viscosity
- The lower the thixotropy
- The longer the gel time
- Catalyst viscosity increases (can influence catalyst injector readings)
All of these affect flow rates and atomization and make sag possible. Cold weather can also result in a less cosmetically pleasing product. Slower gel and cure times of gel coats caused by cold conditions may lead to post-cure. This can be visible as print-through or distortion.
Pieces of advice for using gel coats in cold conditions:
Increased viscosity of the gel coat and viscosity makes equipment calibration more important.
Drums can require two to three days to warm, even inside a shop. Three to four days may be required for extremely cold temperatures. Cold floors can lengthen this warming time. Test material temperature before using.
Place insulation, such as wood, underneath drums on cement floors. It prevents heat from being drained by the concrete.
Keep a close eye on inventory and place orders in advance.
Application:
As mentioned earlier, proper application of a gel coat is crucial to the quality. For example, the person spraying the Gelcoat needs to be in proper distance from the mold and move the gun at just the right speed to apply around 6 mils of gel coat.
This is not an easy task and when gel coating a large part, it can become confusing knowing how many passes of gel coat have been applied. Ideally, the gel coat applicator will apply 18 mils total gel coat, in 3 passes. Applying a single color of gel coat can be difficult, but imagine trying to apply multiple colors of gel coat to the same part, this can become quite the art form.
In the following, we discuss some of the features, advantages and disadvantages of different types of resins used in the production of gelcoats:
Polyesters
| Advantages | Disadvantages |
|---|---|
| Ease of use | Medium mechanical resistance |
| The lowest price among other types of resins | High shrinkage during curing process |
| – |
Limited working time after adding primer |
| – | High styrene diffusion while molding |
Vinyl esters
| Advantages | Disadvantages |
|---|---|
| Ease of use | Needs additional curing to achieve very good properties, |
|
Very high chemical and environmental resistance |
High shrinkage during curing process |
|
Higher mechanical resistance than polyesters (more like epoxy resins) |
Low UV resistance
(compared to ISO polyester resin) |
|
Medium price |
High styrene diffusion |
Epoxies
| Advantages | Disadvantages |
|---|---|
| High mechanical resistance | vulnerable to mixing and stirring |
|
Water resistance |
Extreme damage in expose to UV |
|
Long working time |
Tendency to turn yellow in expose to UV and heat |
|
High thermal resistance (up to 140°C in humid conditions and up to 220°C in dry conditions). |
Requires safety precautions while relocating and utilizing |
|
Low shrinkage |
Much more expensive than vinyl esters and polyesters |
Conclusion:
Gel coat is a crucial component of aesthetic composite products. It provides protection and it is the outward expression of the composite material. It is important that the gel coat is applied properly, and all the variables which can affect the gel coat are reduced. By doing so, a beautiful and long-lasting product can be produced.
- General purpose gel coat
- UV resistant gel coat
- Marine industry gel coat
- Fire retardant gel coat
- Tooling gel coat
- High performance gel coat
- Antibacterial gel coat