Assigned: January 28, 2002
Due: February 4, 2002

1. For a number of epoxy composites the heat distortion temperature (or glass transition temperature) of a composite structure is closely correlated with the final cure temperature for the composite. Explain why this trend occurs.

It is difficult for reactions to occur in the glassy state. Therefore the highest curing temperature is likely to be the epoxy's glass transition temperature for rigid epoxies. More flexible epoxies have glass transitions dominated by the flexible segments present. In these cases the glass transition temperature never rises up to the highest cure temperature.

2. An epoxy, DGEBPA with a number average molecular weight of 1,000 g/g-mole, is cured with an amine (MDA - see lecture slides). Assume the cure temperature is low enough that any hydroxyls (-OH) formed do not react. However, any hydrogen in an amine group can be abstracted. Assume a mix ratio of 100 g DGEBPA and 50 g MDA.

1. What is the functionality of each reactant?

DGEBPA: 2 epoxy groups react once so f = 2

MDA: Each amine can loose a H so f = 4

2. What is the equivalent molecular weight for each reactive group?

MW(epoxy) = 1,000/2 = 500 g/g-mole epoxy groups

MW (MDA) = 198 CH₂ (C₆H₄)₂ (NH₂)₂

MW (-H) = 198/4 = 49.5

3. Predict the extent of reaction at gelation.

r = n(-H)/n(epoxy)=(50/49.5)/(100/500) = 5.05

(1/p)² = r(f-1) = 5.05(4-1) = 15.15 or p (gel) = 0.26

4. What is the molecular weight per crosslink when the extent of reaction = 1?

3. The chemical composition and molecular weight of the polyester prepolymer in a polyester resin system can vary substantially. For example, the number of carbon-carbon double bonds might range from zero to 10. These double bonds participate in the addition polymerization with styrene, which is commonly used as the reactive diluent. Styrene has one double bond which reacts to form two covalent links.

Assuming the resin contains 40 wt. % styrene as the reactive diluent, determine the following for a set of 11 polyesters with zero to 10 double bonds, if each of the prepolymers has the same initial molecular weight of 1,500.

Basis: 100 g polyester + styrene

N (PE) = 0.6 (100)/1500 = 0.04 moles

MW (styrene) = 104

N (styrene) = 0.4 (100)/104 = 0.385 moles

F (styrene) =2

F (PE) = 2 (# of double bonds)

1. whether the material cross links.

b) the percent of reaction that must occur before gelation.

Note, f = the functionality of the polyester, r = (0.385*2)/(0.04*f) , (1/pgel)² = r(f-1)

4. An investigation of the flexural strength of unidirectional SiC reinforced glass composites revealed an increase in strength as temperature increased up to 1000°C. Not until the test temperature reached 1100°C did the strength drop below the strength obtained at room temperature. Explain why this trend was observed. Would you expect a similar trend in metallic or polymeric matrix composites?

Glass is brittle and flaw growth happens readily. As the temperature approaches the glass transition temperature the ductility of the glass starts to increase, blunting crack growth. Hence the composite shows an increase in strength around 1000 C and then the strength drops as the glass transition is reached.

Brittle polymers below their Tg and brittle metals could show similar trends.

5. Bismaleimide (BMI) resins are made by first creating the imide groups in low molecular weight prepolymers and then combining these prepolymers by carrying out an addition polymerization by opening double bonds as shown in Figure 2-8 in your handouts. In contrast, the direct route to polyimides entails condensing polyamic acid as shown in Figure 2-5 in your handouts. Why do manufacturing engineers prefer the BMI method of cure?

The direct route leads to the formation of water as a byproduct, which can be difficult to remove. The cure of BMI doesn't lead to any byproducts, leading to happy manufacturing engineers.

PS Solvent removal is a concern for both approaches. Both may be processed as powders or are chemically modified to be liquids at room temperature - a major compromise.