Composites part 1 - e

Question 1

How are polyester resins formed? How is it cross linked?

Answer 1

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.

Question 2

How can the cross linking of polyester be speeded up?

Answer 2

An accelerator may be added to speed up the reaction

Question 3

What is the shrinkage of polyesters on curing typically?

Answer 3

Between 4 and 7% by volume

Question 4

How are vinyl-esters similar to polyesters?

Answer 4

In that they cure by radical initiated polymerization.

Question 5

How are vinylesters formed?

Answer 5

From the reaction of an epoxy resin with acrylic or methacrylic acid and properties can be varied using different epoxy resins.

Question 6

What are the advantages of vinyl-esters?

Answer 6

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.

Question 7

How are epoxies formed?

Answer 7

By condensation of epichlorhydrin and polyhydroxy compounds.

Question 8

How are epoxies normally supplied?

Answer 8

As a single constituent, the resin, with a second constituent, the hardener or cross-linking agent, that has to be added.

Question 9

How are a range of properties possible with epoxies?

Answer 9

By combining different resins and hardeners

Question 10

What are the advantages of epoxies?

Answer 10

• higher strength and adhesion to fibres than polyesters.

Question 11

What is the shrinkage of epoxies on curing?

Answer 11

between 0.25 and 2% by volume.

Question 12

How are phenolics formed?

Answer 12

By condensation of phenol and aldehyde. The condensation reaction is usually promoted by heat but can also be initiated using a strong acid catalyst.

Question 13

What are the advantages of phenolics?

Answer 13

Good fire resistance with low smoke and toxic fume emission characteristics

Question 14

What are the disadvantages of phenolics?

Answer 14

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%

Question 15

What is gel coat?

Answer 15

A dense, void-free layer of resin on the exterior of moulding to improve the surface finish.

Question 16

How is gel coating applied?

Answer 16

It is applied to the mould and then partly cured prior to applying the resin.

Question 17

What are the advantages of gel coat?

Answer 17

Tough, resilient films, most can be pigmented

Question 18

What is pre-preg?

Answer 18

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.

Question 19

What are the parameters that can typically be specified for pre-pregs?

Answer 19

• fiber type and grade
• surface treated?
• resin type
• resin content
• cured ply thickness

Question 20

What are the disadvantages of pre-pregs?

Answer 20

• limited shelf life due to partially cured state

- has to be stored at low temps in a freezer

Question 21

How are pre-pregs processed?

Answer 21

• hand lay-up, vacuum bad and autoclave moulding