Types of resin systems

Composites are a combination of fiber reinforcement and a resin matrix. The resin system holds everything together, and transfers mechanical loads through the fibers to the rest of the structure. In addition to binding the composite structure together, it protects from impact, abrasion, corrosion, other environmental factors and rough handling. Resin systems come in a variety of chemical families, each designed and designated to serve industries providing certain advantages like economic, structural performance, resistance to various factors, legislation compliance, etc. Only the most common resins of the thermoset family and the ones mostly used in composite construction are described below. Those are Polyester (orthophthalic and isophthalic), vinyl ester, epoxy, and phenolic. Fibermax carries only epoxy resin systems.

 

Unsaturated polyester resins are the simplest, most economical resin systems that are easiest to prepare and show good performance. Millions of tons of this material is used annually around the world. They are manufactured by the condensation polymerization of various diols (alcohols) and dibasic acids (e.g. maleic anhydride or fumaric acid) to give esters, a very viscous liquid that is then dissolved in styrene, a reactive monomer. Styrene lowers the viscosity to a level suitable for impregnation or lamination. Generally, polyesters exhibit somewhat limited thermal stability, chemical resistance, and processability characteristics. Applications include transportation markets (large body parts for automobiles, trucks, trailers, buses), marine (small and large boat hulls and other marine equipment), building (panels, bathtub and shower shells), appliances etc.

 

Is also refered to as ortho or General Purpose Polyester (GP) was the original polyester developed. It has the lowest cost and is still very widely used in FRP industry. It is commonly used in applications where high mechanical properties, corrosion resistance, and thermal stability are not required. Although the upper temperature limit is only 50oC, it performs satisfactory in water and sea water. It is normally not recommended for use in contact with chemicals.

 

Often referred to as Iso, it is improved polyester. It has a slightly higher cost, improved strength, thermal stability (55oC) and mild resistance to corrosion conditions. Improved resistance to water permeation has prompted its use as a gel barrier coat in marine applications. Improved chemical resistance has led them to extensive use in underground petroleum tanks (in gas stations) with satisfactory service life. They are also used in salts and mild acids.

 

Even further improved polyester, it is bisphenol chlorinated, or a combination of polyester and epoxy. Its curing, handling and processing characteristics are those of polyester, and it exhibits higher test results in corrosion temperature resistance and strength and has higher cost. Modifications of the molecule have produced even higher properties.

 

Phenolic resin is a reaction of phenol and formaldehyde. It can be cured via heat and pressure, without the use of catalysts or curing agents. It is one of the oldest thermosetting resins available and sells at a very reasonable cost. Cured phenolic resins are fire resistant without the use of mineral fillers or fire retardant additives. Phenolic composites have excellent high-temperature properties and if properly formulated and cured, they can form carbon to carbon composites with outstanding temperature resistance. Phenolics are also unique in their chemical resistance. Disadvantages of these resins include high curing temperatures and pressures, longer curing times than polyesters, and limited color range. The use of phenolic resins in composites is growing, primarily due to regulative legislation on flame spread, smoke generation, and smoke toxicity. It is used extensively in automobiles, appliances, electronics, and as an industrial adhesive both in higher and lower temperature applications.

 

Epoxy resins are a broad family of materials. The most common ones are prepared from the reaction of bis-phenol A and epichlorohydrin and contain a reactive functional group in their molecular structure. Epoxy resin systems show extremely high three dimensional crosslink density which results to the best mechanical performance characteristics of all the resins. The most demanding strength/weight applications use epoxy almost exclusively. It has excellent strength and hardness, very good chemical heat and electrical resistance. Disadvantages include higher cost, processing difficulty (quantities of resin and hardener need to be measured precisely. Also, often heat curing is required.) Epoxy systems are used in applications like aerospace, defense, marine, sports equipment, adhesives, sealants, coatings, architectural, flooring and many others.

 

Gel coats are prepared from a base resin and additives. The base resin can be polyester, vinyl ester, phenolic or epoxy. Additives are thixotropic agents, fillers, pigments and other. The gel coat, as the name implies, has a gel texture. This makes the gel coat capable to "stay" on vertical surfaces of molds without draping. It is placed first in the mold, so it becomes the outer surface of the construction. It is used to replace paint, for cosmetic purposes, and to protect from the environment.