What are Composites?

Below you will find articles and case studies on different GRP and FRP operation carried out at Composite Mouldings. Ideal research material if you are interested in the composites industry. If you require any further information or advice please do not hesitate to contact our technical team who will be happy to help.

Epoxies and polyesters are both families of thermosetting resins. The fundamental difference between these two resins is the type of reactive chemical group that is involved in the crosslinking reaction. For epoxies, the reactive group is a small 3-membered circle of two carbon atoms and an oxygen atom – called the epoxy circle.

For polyesters, the reactive group is a double bond between two carbon atoms. We can appreciate the differences in crosslinking reactions and conditions without getting into it on a scientific level. Epoxies do not use organic peroxides. Curing epoxy is accomplished by adding a curing agent (hardener), but the conditions for curing can vary widely depending on the natures of resin and the curing agent, as well as the requirements of the manufacturing operation and the properties of the final product

Polyesters are crosslinked by adding a small amount of organic peroxide (catalyst) to a solvent system of the resin. Then, either with applying heat or at room temperature the resin will cure. When compared with the polyester crosslinking process and the properties of polyester products, the epoxy system and product properties are far more versatile. This greater flexibility is both good and bad. Good because the manufacturer can select curing conditions and properties that are just right for the product and manufacturing application. Bad because the system can become so complex that the level of skill needs to be very high in order to choose the proper components of the resin mixture, the type of curing agent, and the environment conditions.

The production of epoxy resins requires the optimization of cost, performance, and processing aspects. Typically ingredients are selected for their chemical and physical attributes and then various formulations are tried in small test mixes until the desired properties are achieved.

But it is not just what is put together that matters, how the ingredients are assembled and used affects the consistency of properties, the processing characteristics, safety in scale up, and full-scale production practice.

To summarise polyester resins are tried and tested to be suitable for mass production but the flexibility and superior technical qualities of epoxy does attract the more demanding projects. At the end of the day it comes down to cost and how extensively a product needs to be engineered.

We hope you have found this article helpful in determining which material best for your production requirements.

Epoxy From Wikipedia, the free encyclopedia

Epoxy or polyepoxide is a thermosetting polymer formed from reaction of an epoxide “resin” with polyamine “hardener”. Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives.


The first commercial attempts to prepare resins from epichlorohydrin were made in 1927 in the United States. Credit for the first synthesis of bisphenol-A-based epoxy resins is shared by Dr. Pierre Castan of Switzerland and Dr. S.O. Greenlee of the United States in 1936. Dr. Castan’s work was licensed by Ciba, Ltd. of Switzerland, which went on to become one of the three major epoxy resin producers worldwide.

Ciba’s epoxy business was spun off and later sold in the late 1990s and is now the advanced materials business unit of Huntsman Corporation of the United States. Dr. Greenlee’s work was for the firm of Devoe-Reynolds of the United States. Devoe-Reynolds, which was active in the early days of the epoxy resin industry, was sold to Shell Chemical (now Hexion, formerly Resolution Polymers and others).


The applications for epoxy-based materials are extensive and include coatings, adhesives and composite materials such as those using carbon fiber and fiberglass reinforcements (although polyester, vinyl ester, and other thermosetting resins are also used for glass-reinforced plastic). The chemistry of epoxies and the range of commercially available variations allows cure polymers to be produced with a very broad range of properties.

In general, epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and very good electrical insulating properties. Many properties of epoxies can be modified (for example silver-filled epoxies with good electrical conductivity are available, although epoxies are typically electrically insulating). Variations offering high thermal insulation, or thermal conductivity combined with high electrical resistance for electronics applications, are available.

Did you know?

Vinegar is an effective and safe solvent to clean up tools, brushes, skin, and most surfaces contaminated with epoxy resin or hardener. Acetone can also be used, but it is very volatile and flammable, unlike vinegar. Vinegar is safer for cleaning epoxy resin from human skin than acetone: both liquids will dissolve the resin, but the resin/acetone solution can easily pass through the skin into the bloodstream, unlike vinegar.

White vinegar can even clean up epoxy resin that is beginning to cure/harden. One should always avoid getting epoxy on skin. Citrus-based waterless hand cleaners will help to remove fresh resin from the skin. Vinegar and rubbing alcohol can also be effective in removing fresh uncured epoxy from skin. One should then follow with washing with soap and water. DME (Dimethoxyethane) is also a good solvent for epoxy resin and hardener that gives off very little vapor.

from Wikipedia

Polyester resins are unsaturated resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Among other uses, it is the basic component of sheet moulding compound and bulk moulding compound.

Unsaturated polyesters are condensation polymers formed by the reaction of polyols (also known as polyhydric alcohols, organic compounds with multiple alcohol or hydroxy functional groups) and polycarboxylic that contain double bonds. Typical polyols used are glycols such as ethylene glycol. The usual polycarboxylic acids used are phthalic acid and maleic acid. Water, which is a by-product of this esterification reaction, is removed from the reaction mass as soon as it is formed to drive the reaction to completion.

Unsaturated polyesters differ from saturated polyesters such as polyethylene terephthalate which constitutes the polyester films and fibers of commerce in that acids or glycols having double bond unsaturation are included in the formula to provide reactive olefinic unsaturation in the unsaturated polyester alkyd. Polyester resins are thermosetting; “thermosetting” means the plastic softens when initially heated, but sets permanently rigid once it has cooled (as opposed to “thermoplastics”, which re-soften with heat).

Polyester resin is often purchased in liquid form for the production of glass-reinforced plastic. In this case, a catalyst (typically methyl ethyl ketone peroxide (MEKP) (also known as butanone peroxide) is used to initiate the polymerization reaction; benzoyl peroxide is a somewhat less hazardous alternative suitable for some purposes. The process of curing polyester resins using a catalyst is an exothermic process. The use of excessive catalyst can cause an excess of heat during the cure process and thus damage the resin by charring or even cause ignition.

Uses of Polyesters

Polyester resin is used for casting, auto body repair, wood filling, and as an adhesive. It has good wear and adhesive properties, and can be used to repair and bond together many different types of materials. Polyester resin has good longevity, fair UV resistance, and good resistance to water. It is important to recognize that all polyester resin products are not created equal; their chemical makeup is complex and can have a wide range of properties.

As a filler in auto repair, for example, this material is formulated for superior adhesion to paints and metals, but cures very hard to resist surface trauma; it is therefore only marginally sandable. As a filler for millwork, however, polyester must be softer than the wood substrate so it can be sanded without leaving fills standing proud above surfaces. Polyester resins adhesion to some materials can be excellent such as in fiberglass composites but others such as timber the adhesion is much weaker.