How are polyester resins formed? How is it cross linked?
by a condensation reaction between glycol and an unsaturated dibasic acid. The unsaturated resin is dissolved in a monomer solvent, usually styrene, and cross linked by the addition of a catalyst and heat.
How can the cross linking of polyester be speeded up?
An accelerator may be added to speed up the reaction
What is the shrinkage of polyesters on curing typically?
Between 4 and 7% by volume
How are vinyl-esters similar to polyesters?
In that they cure by radical initiated polymerization.
How are vinylesters formed?
From the reaction of an epoxy resin with acrylic or methacrylic acid and properties can be varied using different epoxy resins.
What are the advantages of vinyl-esters?
They are generally tougher, have improved chemical resistance and are capable of higher operating temps than polyesters. They fall between polyesters and epoxies in terms of performance and cost.
How are epoxies formed?
By condensation of epichlorhydrin and polyhydroxy compounds.
How are epoxies normally supplied?
As a single constituent, the resin, with a second constituent, the hardener or cross-linking agent, that has to be added.
How are a range of properties possible with epoxies?
By combining different resins and hardeners
What are the advantages of epoxies?
- higher strength and adhesion to fibres than polyesters.
What is the shrinkage of epoxies on curing?
between 0.25 and 2% by volume.
How are phenolics formed?
By condensation of phenol and aldehyde. The condensation reaction is usually promoted by heat but can also be initiated using a strong acid catalyst.
What are the advantages of phenolics?
Good fire resistance with low smoke and toxic fume emission characteristics
What are the disadvantages of phenolics?
During cure they produce highly volatile contents, predominantly water and are unstable at room temperature. The pot life of the resin is therefore low and a high proportion of the mass of the hardened resin is made up of water. The resin cannot be pigmented, the color being unstable. The shrinkage upon curing is between 8 and 10%
What is gel coat?
A dense, void-free layer of resin on the exterior of moulding to improve the surface finish.
How is gel coating applied?
It is applied to the mould and then partly cured prior to applying the resin.
What are the advantages of gel coat?
Tough, resilient films, most can be pigmented
What is pre-preg?
A thin sheet of partially cured, or b-staged, resin containing reinforced fibers. The fibers can be woven or unidirectional. The pre-preg sheet usually comes with a plastic backing sheet or release film which is removed prior to laminating. Normally supplied as rolls.
What are the parameters that can typically be specified for pre-pregs?
- fiber type and grade
- surface treated?
- resin type
- resin content
- cured ply thickness
What are the disadvantages of pre-pregs?
- limited shelf life due to partially cured state
- has to be stored at low temps in a freezer
How are pre-pregs processed?
- hand lay-up, vacuum bad and autoclave moulding
