Composites part 1 - d

Question 1

What are preforms?

Answer 1

Dry fiber mats, usually CSM or CFRM, that have been preformed into a desired shape prior to resin impregnation. The mats are a special preformable grade coated with a thin thermoplastic binder which may be in emulsion (or film) form or in powder form.

Question 2

How is the preforming operation normally carried out?

Answer 2

In a heated press which melts the thermoplastic binder present on this grade of mat and forms the desired shape. On removal of heat and pressure the mat retains its formed shape. The handleable preform is removed from the press and taken to another mould for the resin impregnation operation. The preforms have sufficient integrity to be stacked in quantity prior to subsequent moulding.

Question 3

What is the role of the matrix in FRP?

Answer 3

To maintain the structural integrity by bonding the reinforcement together. The matrix also serves to provide environmental protection for the reinforcement.

Question 4

What does the choice of matrix depend on?

Answer 4

the mechanical, thermal and chemical properties required as well as the processing methods available and permitted cost

Question 5

What are the two polymer matrix classifications?

Answer 5

1. thermoplastic

2. thermoset

Question 6

What is the difference between thermoplastics and thermosets?

Answer 6

Thermoplastics remain permanently fusible so that they will soften and eventually melt when heat is applied, whereas cured thermoset polymers do not soften, and will only char and break down at high temps.

Question 7

Why do thermoplastics melt and thermosets don’t?

Answer 7

Thermoplastics have relatively weak forces of attraction between the chains, which are overcome when heat is applied.
Thermosets have strong chemical bonds cross-linking the molecules so it is effectively one large molecule with no crystalline structure.

Question 8

What are the difference in properties between thermoplastics and thermosets?

Answer 8

Compared with thermoplastics, thermosets are generally harder, more rigid and more brittle and their mechanical properties are not heat sensitive. They are also less soluble in organic solvents.

Question 9

What is the most common choice for the matrix?

Answer 9

thermoset

Question 10

What are common types of thermosets chosen?

Answer 10

polyesters, epoxies and phenolics

Question 11

What is the selection criteria for resin?

Answer 11

1. Stress and strain limits
2. Heat distortion temperature
3. shrinkage
4. adhesion to fibers
5. fire resistance
6. cost of materials
7. ease of processing
8. shelf life and storage
9. safe handling and processing