From boat hulls and boat decks to fiberglass body panels and components in the automotive industry to bathroom and kitchen fixtures, gelcoats play an important role in providing a high-quality surface finish and protective layer in products made from thermoset composites.

UPE gelcoats are one of the most common types of gelcoats and are made with unsaturated polyester (UPE) resin. They are formulated from several components, including the resin/polymer, reactive monomer, pigments, fillers, thixotropic agents, promoters, inhibitors and specialty additives, which can include dispersants. Once cured, they form a hard, durable and chemically cross-linked surface.

Aesthetically, UPE gelcoats offer a smooth, glossy surface. Pigments can be added to offer a variety of colors to enhance the appearance of the finished product. From a protection standpoint, gelcoats form a high molecular weight network polymer matrix through gelation and curing. This acts as a barrier against environmental factors like UV radiation, water and chemicals, preventing damage to the underlying composite structure. UPE gelcoats exhibit good durability, fade resistance, chalking and corrosion, especially those made with isophthalic acid/neopentyl glycol (ISO/NPG) resins.

Adding Color to Gelcoats

Gelcoat pigments are used to add color, and are often in paste or liquid form. They are dispersed within the gelcoat resin to create a desired color or to match existing colors for repairs. Gelcoat pigments are added before the catalyst to ensure even distribution and proper curing.

The specific pigments used determine the color of the gel coat and fabricated part, and can also significantly affect performance characteristics of the coating. The pigment type and concentration affect the hiding capacity (opacity) of the gelcoat. Bright yellows, oranges, reds and dark blues will have slightly less hiding capacity than pastels. The high pigment loads necessary to achieve better hide with these bright colors can cause spray and cure challenges if not properly dispersed.

Careful pigment selection can also slow chalking that occurs with outdoor exposure and provide an abrasion resistant coating that can be readily cleaned, waxed and buffed to a high gloss. The most commonly used pigment in gelcoats is titanium dioxide. It is the primary pigment in white gelcoats and is also used in combination with other pigments in non-white colors.

Applying Gelcoats

Gelcoats are applied in a liquid state to the mold before composite material is laid down. The gelcoat is sprayed or brushed into a polished mold surface as the first layer and then allowed to partially cure to a tacky surface before the next layers are added. The next composite layers are reinforcing fiber and thermoset polymer (resin), which could also include additional additives (e.g., fillers, air release agents, dispersants). Gelcoats are designed to be compatible with the resins used in the composite to enable strong chemical bonds between the gelcoat and the subsequent layers (structural part) of the composite. The entire part cures, locking the gelcoat to the composite substrate as a tough, integrated outer skin.

The application of gelcoat is an involved operation with its success dependent on the material, equipment, environment and gelcoat operator. Most gelcoats are designed for spray application, with a well-sprayed part having good cosmetics and optimal performance. Spray-up is good for low- to medium-volume production and is a faster process than hand lay-up. It can be used on smaller parts, but it is also very well suited to larger parts like swimming pools or boat hulls.

The Importance of Dispersants

When pigments are used in gelcoats, dispersants are a necessary addition to ensure optimal performance. Dispersants are used to wet, disperse and stabilize pigments to ensure uniform, long-lasting dispersion. They are essential for achieving consistent color, surface quality and application properties in gelcoat systems, especially when blending colors together.

Pigment dispersants have an anchoring group that provides strong adsorption onto the pigment surface. Polymeric chains, which are attached to the anchor group, provide the stabilization. Without dispersants, pigments and fillers can clump together, creating inconsistent color and poor surface finish. They can also settle out over time, reducing shelf-life and requiring frequent mixing, and cause viscosity spikes, making the gelcoat harder to apply consistently and lead to sagging, especially in spray applications.

A dispersant has an important job to disperse the pigment and improve the gelcoat performance—but it also has another important job. That job is to not interfere with the resins/catalysts in the ambient curing system.

Solplus™ D610 for Dispersion into UPE

One dispersant that performs well in gelcoats is Lubrizol SOLPLUS™ D610. It is a 100% active liquid polymeric dispersant designed for the wetting, dispersion and fluidization of pigments for dispersion into UPE gelcoats. This novel anchored dispersing agent provides a number of benefits, including increased pigment loading, improved flow and self-levelling plus resistance to settling. Other benefits include strong rheology characteristics with reduced viscosity, better resistance to color separation, and an improved rate of pigment wetting for faster dispersion times and higher color development.

Solplus™ D610 improves the workability of UPE Gelcoats with minimal impact on fumed silica thixotropic anti-sag agents.

SOLPLUS D610: Improved Gelcoat Workability

Improved gelcoat chart

Solplus™ D610 – UPE Colorants

Colorant chart

Lubrizol has decades of experience in pigment dispersion. For gelcoat formulators and gelcoat users, Lubrizol offers high-performance dispersants as well as a range of additional additives for use in UPE gelcoats and thermoset composites, including air release agents, styrene suppressants and rheology control additives. Lubrizol also continues to develop new additives, including ones to support the trend toward more sustainable styrene-free systems.

Contact us to learn more about how we can help deliver performance benefits to UPE gelcoats and thermoset composites.

Pigment Dispersants in UPE Gelcoats Elevate Color and Performance in Thermoset Composites