Question

From material's perspective, what are some of the typical challenges for the processing of thermosetting polymers, and what are the measures to alleviate these issues?

Answer 1

There are some of typical challenges for processing of thermosetting polymers,

Thermosetting resins are processed in two or more stages. For instance, consider those used for fabricating fiber-reinforced laminates. First, a laminating varnish is developed from curing agent, resin monomer, other functional additives and a solvent. The varnish is then applied to a reinforcing fabric made of high modulus fibers or glassfiber. The solvent is then removed and the resin reacts partially to form a prepreg.

At the B-stage, the prepreg is partly soluble and is a low-melting thermoplastic solid consisting of unreacted monomer, adduct and higher molecular weight oligomers. Adduct is a re-action product in which the ratio of resin to curing agent is 1:1.

Certain resins like epoxies need to be secured from moisture and refrigerated to forestall “advancement”, that is, continuation of the reaction. For product fabrication, a multilayer sandwich of prepreg is created, placed in an autoclave or a press, and heated to “cure”.

Several things need to be achieved at this stage – the fibers must be thoroughly wetted with resin, the laminate needs to be consolidated to the required thickness, excess resin must be removed, trapped air, and moisture and solvent need to be vented out to avoid porosity, and the cross-linking reaction is advanced to the preferred level.

With a rise in temperature, prepreg melts and the viscosity drops precipitously, after which it levels and then climbs again. The viscosity behavior is due to the intervention of a competing situation – the rising temperature triggers additional reactions among monomers, oligomers and curing agent improving the viscosity of the prepreg. The multi-functional reactants cause crosslinks to rapidly form between the polymer chains in all directions. The system gels and flow stops suddenly. The matrix is solid at this point and the shape and thickness of the composite are constant. Additional heating is needed, however, for the matrix to form the balance of ist potential crosslinks and to develop, for the most part, ist ultimate level of stiffness. To accomplish this, an additional out-of-mold post-cure step is generally used.

Single-Stage Processing

Thermosets are also processed in a single stage method known as reaction injection molding (RIM). RIM has gained popularity for a range of recreational and automotive application. RIM is similar to thermoplastic injection molding. The main distinction is that RIM uses polymerization in the mold instead of cooling to enable formation of a solid polymer.

Rheology of Thermosets

Polymerization is also used by other reaction molding processes for solidifying the molded piece. In thermoset injection molding, reactants are subjected to heat of 200°C for reaction activation. In RIM, the reactants are combined at a considerably lower temperature as impingement mixing activates the reaction, not the external heat.

In the RIM process, complicated plastic components are rapidly produced directly from low viscosity reactants. Two or more liquids are impingement mixed at low pressures and temperatures when they are introduced into the mold. Phase separation of segmented block copolymers forms a solid polymer, and parts can now be ejected from the mold within a minute.

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