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Laminating Resins Used In Composites

Understand The Basics of General Purpose Laminating Resins


Craftsman coating surf board at his works
ULTRA.F/Taxi Japan/Getty Images

What are laminating resins?

Traditional "laminating resins" are generally either polyester or vinyl ester resins. They are made for use in open mold, spray-up, or hand lamination at room temperatures. Out of all the composite resins, laminating resins are the most versatile, are fit to make products ranging from surfboards to wind turbine blades and they can be specifically designed to use as both a filled and unfilled resin.

Laminating Resin Formulation

Laminating resins are formulated from components including:

  • Polymer Reactive monomer
  • Thixotropic agents
  • Promoters
  • Inhibitors
  • Specialty additives

Which components are used and their amounts, are dependent upon the end use requirements for performance and cost. Another determining factor to consider are emission regulations.

Processing requirements include:

  • Sprayability
  • Wetout
  • Sag
  • Working time
  • Trim time
  • Cure
  • Peak exotherm temperature

Finished part performance requirements may include:

  • Part appearance
  • Physical properties
  • Water resistance
  • Weathering resistance
  • Bond strength
  • Flame retardancy


Polymer type and grade determine the end characteristics. This is in addition to the processing and application properties. Polymer grade is first decided when choosing a laminating resin. Differences in cost between the grades as well as the mechanical strength and processing features should all be considered.

In the past, orthophthalic and isophtalic-based resin were the most popular grades among laminating resins. Both types offer mechanical strength and secondary bonding. When comparing the two, isophthalic-based resins give better chemical, heat, and moisture resistance as well as higher tensile properties. Both isophthalic and orthophthalic resins have been replaced by Dicyclopentadiene (DCPD) resins for certain applications.

Over the last 15 to 20 years, DCPD grade resins have become increasingly popular in open molding because of three main reasons.

  • Consumers' demands for superior smoothness on finished parts. (DCPD resins offer a smoother surface and less print-through and distortion.)
  • The heightened rules regulating styrene emissions. (DCPD polymers can be made as usable laminating resins at considerably lower styrene levels. DCPDs can be made with styrene contents less than 30% while those of isophthalic-based resins are 45 to 50%.)
  • Cost. (DCPD-based resins are more affordable while, at the same time, offering lower void content, accordance with environmental regulations, and less fiber-print.)

All of the polyester polymers offer cross-linking. This occurs when an unsaturated acid component is present, usually maleic anhydride. This leaves a site for the cross-linking reaction.


Two roles of the monomer in laminating resin:

    1. To change from a liquid to a rigid solid, a monomer must be chemically reactive, thus cross linking with the polymer.
    2. A monomer is used to reduce the viscosity of the polymer to a level that is easy to work with during the laminating application. Unsaturated polyesters polymers in laminating resin are very viscous at room temperatures, in essence, laminating resins are simply just large mixtures of polymers held in a monomer.

      Monomers commonly used in laminating resins:

      • Styrene
      • Vinyl toluene
      • Methyl methacrylate (MMA)
      • Alpha methyl styrene

      Amounts and combinations effect:

      • Glass fiber wetout
      • Exotherm temperature reached during curing
      • Cure rate
      • Physical properties of the laminate (strength, hardness, clarity, dielectric properties, etc.)

      Thixotropic Agents

      If a laminating resin is thixotropic, it means that it has a viscosity that is dependent on shear rate. It is important that laminating resins are formulated in this way because a low viscosity is needed during high shear processes such as:

      • Pumping
      • Spraying
      • Wetout

      Once these processes are finished, the resin needs to return to its highly viscous state to prevent sag and draining. Thixotropic agents work by forming a hydrogen bond network with the polymer. When exposed to high shear processes, the hydrogen bond network breaks down and viscosity is decreased. Once the high shear process is done, the bonds are recreated thus reinstating the highly viscous state. The more quickly this network can be recreated, the less there is a chance for sag and drainage. In filled systems, it also aids in holding fillers in suspension.

      Thixotropic Index (TI) is the means for measuring the thixotropy of a resin. It is the ratio between the low shear viscosity and the high shear viscosity of the resin. The high shear viscosity of an unfilled laminating resin is typically between 400 and 700 cps. TIs are usually between 2 and 4. Lower TIs are usually seen in filled laminating resins.

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