Homework #2
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.

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?
2. What is the equivalent molecular weight for each reactive group?
3. Predict the extent of reaction at gelation.
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.

a) whether the material cross links.

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

Put your results into a table.

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?

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?