Aerospace Materials: Plastics - Thermosets

Vocabulary and key concepts from the lecture on Thermosets, including chemical processes, property definitions, and specific polymer systems used in aerospace applications.

Thermosets

Cross-linked polymers with a 3-D-network structure and covalent bonds linking the chains, characterized as comparatively stiff, brittle, and non-meltable.

Stoichiometric ratio

The specific mixing ratio of resin and curing agent (hardener) required because the hardener is a reaction partner that builds a network with the polymer.

One-component resin systems

Resin systems that contain a pre-dosed amount of curing agent already in the resin, which is activated by heating.

Accelerator

A component added to a resin system to increase the reaction speed.

Inhibitor

A component added to a resin system to decrease the reaction speed.

Gelation (Gel point)

The critical point during curing when branched structures extend throughout the whole sample, the resin no longer flows well, and the network becomes insoluble.

Potting time

The time window in which a resin can be processed, defined by the intersection of the viscosity curve with a specified boundary viscosity.

Exothermic reaction

A chemical reaction between resin and hardener that releases heat, causing a temperature increase in the reaction mass.

Cold-Curing

A curing process typically triggered at room temperature by the decay of an accelerator over time.

Step cure

Heat curing carried out stepwise at different temperatures to better control temperature-induced shrinkage and stress development.

Post-curing

A process of heating cold-hardened parts at higher temperatures, usually in the range of the maximum achievable glass transition temperature, to allow complete cure.

TTT Diagram (Time-Temperature-Transition)

A diagram used to understand the curing process during isothermal curing by showing different state ranges of a thermoset relative to time and temperature.

Degree of Cure (DoC)

A measure of the percentage of reactive groups in the resin and hardener that have reacted, where higher degrees result in higher stiffness, strength, and heat resistance.

Glass transition temperature ($T_g$)

A property used to characterize the degree of cure; it increases as curing progresses and reaches its maximum level at complete curing.

Epoxy resin (EP)

High-quality thermosets formed by addition polymerization from chain-shaped uncrosslinked resins, often used in aerospace for airframe structures.

Phenolic resin (PF)

Thermosets with favorable fire properties and high stiffness, often used in aircraft interiors and as a binder for brake pads.

Unsaturated polyester resins (UP)

Resins with high to very high stiffness and hardness that are resistant to fuels and diluted acids but unresistant to permanent hot water contact.

Polyimides (PI)

Aromatic polymers that exhibit the highest heat resistance (up to $650\,^{\circ}\text{C}$ for short periods) and are used for MLI blankets on satellites.

Multi Layer Insulation (MLI)

Blankets typically consisting of 15 to 40 layers of films and fabrics, often using aluminized Kapton film to reflect thermal radiation away from spacecraft.

Yield strength of Carbon/epoxy composite

The stress level at which the material begins to deform plastically, specified as $760\,\text{MPa}$ for a [0/+-45/90] structure with $60\,\%$ fiber volume content.

Semi-finished product

Materials like Prepregs, BMC (bulk moulding compound), and SMC (sheet moulding compound) used as precursors for part production.

Creep

The tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses.