Open Molding & Lamination Technical Support

What Are Laminating Resins?

Laminating resins are polyester or vinyl ester resins specifically formulated for ambient-temperature open mold processes such as hand lay-up and spray-up. These resins are designed to wet fiberglass reinforcement efficiently while maintaining proper working time and resistance to sag.

Laminating resins may be supplied as neat resin systems or combined with inorganic fillers depending on application requirements, cost considerations, and regulatory constraints.

How Laminating Resins Are Formulated

The performance of laminating resins is controlled through careful formulation. Resin manufacturers balance chemistry, processing characteristics, emissions regulations, and cost targets to create materials suitable for production environments.

Polymer Selection

The polymer backbone determines the structural properties of laminating resin systems. Most laminating resins are based on unsaturated polyester or vinyl ester chemistry.

• Polyester Resins: Widely used for general laminating applications. Orthophthalic, isophthalic, and DCPD-based systems offer different balances of performance, emissions, and cost.
• Vinyl Ester Resins: Provide improved strength, corrosion resistance, and water resistance. These resins are often selected for marine structures and higher-performance laminates.

Hybrid or blended resin systems may also be used to balance performance and cost requirements.

Reactive Monomers

Reactive monomers serve two essential roles in laminating resins. They participate in the curing reaction and reduce viscosity to workable levels for fiberglass wet-out.

• Control viscosity and reinforcement wet-out
• Influence cure speed and working time
• Affect peak exotherm temperature
• Impact final mechanical properties

Environmental regulations may restrict certain monomer types and concentrations.

Thixotropy & Sag Control

Laminating resins exhibit thixotropic behavior, meaning viscosity decreases under shear during application but increases when the material is at rest.

This property allows resin to flow easily during spraying or wet-out while minimizing drainage and sagging on vertical surfaces.

Thixotropic performance is commonly measured using a Thixotropic Index (TI), which compares viscosity under low and high shear conditions.

Thixotropy is achieved through additives such as fumed silica, organoclays, and specialized rheology modifiers.

Cure Control: Promoters & Inhibitors

Promoters and inhibitors regulate the curing behavior of polyester and vinyl ester laminating resins.

• Promoters (Accelerators) activate peroxide catalysts for ambient-temperature cure
• Co-promoters assist with cure speed and hardness development
• Inhibitors provide storage stability and control working time

Maintaining the correct balance is critical. Excessive exotherm can damage molds or distort laminates, while insufficient cure can lead to poor mechanical performance.

Specialty Additives

Additional additives may be incorporated into laminating resin systems to improve processing behavior or enhance finished part performance.

• Air release and wetting agents
• Catalyst color indicators
• Odor-reduction additives
• UV stabilizers
• Flame-retardant systems

Laminating Resin Specifications & Quality Control

Resin manufacturers perform quality control testing to verify consistent performance. These values are typically reported through a Certificate of Analysis (COA).

Actual laminate cure behavior may differ from mass cup gel tests due to laminate thickness, reinforcement content, and heat dissipation.

Need Help Selecting a Laminating Resin?

Selecting the correct laminating resin prevents common problems such as poor fiberglass wet-out, sagging laminates, slow cure, or excessive heat buildup.