Catalyst

Substance introduced into a material to initiate the reaction. Without it, the reaction cannot occur (e.g., peroxides in polyester, vinyl ester or polyurethane resins).

The catalyst concentration has a direct influence on the reaction rate.

Synonyms: initiator, activator.

Chimiorheology

Also referred to as rheokinetics.

Crosslinking

Formation of one or more three-dimensional networks through chemical or physical processes. Chemical bonds (or crosslinks) are created between macromolecular chains.

This process is irreversible. The material becomes insoluble and infusible.

Exothermicity

Heat release resulting from the chemical reaction of a mixture of components. It reaches its maximum at the exothermic peak.

Geltime

Time required for a reactive system to gel and become so viscous that it can no longer be processed or handled. It corresponds to the formation of the first insoluble fraction.

If the gel state is disturbed, the final product will have properties that are not fully established.

Synonyms: gelation, coagulation, gelatinization, setting.

Pot life

Time required for the viscosity of a given amount of a resin/hardener mixture to double. In some cases, when the initial viscosity is very low (e.g., 500 mPa·s), the pot life can be defined as the time required for the viscosity to increase by a factor of 4, 5, or another predefined value.

Rheokinetics

Evolution of the rheological behavior of a system during a chemical reaction.

Also sometimes referred to as chemorheology.

Shrinkage

Dimensional change of the resin during the transition to the solid state. The increase in density results in a volume reduction that can reach 8% in some cases. It may lead to crack formation when processing thick sections.

Shrinkage can have two origins:

* chemical origin (reduction of free volume);
* thermal origin (thermal contraction).

Synonyms: retraction, contraction.

Step B

State obtained when the crosslinking reaction is stopped by rapidly cooling a formulation with an appropriate stoichiometry for complete crosslinking.

The product is maintained in this intermediate state, with a relatively short shelf life (compared with a prepolymer). The reaction can be reactivated without adding a catalyst by increasing the temperature.

Vitrification

Transformation of a material into a glassy state or transition to the glassy state.

This occurs either by cooling a material below its glass transition temperature (e.g., glass manufacturing) or during a curing reaction when the instantaneous glass transition temperature of the material reaches the actual temperature of the product.

Working time

Time available for the operator to process the material before it reaches a viscosity level that is too high for proper handling.

Therefore, this is a specific parameter that must be determined and defined according to the processing conditions and the manufacturing process used: infusion, contact molding, low-pressure injection, resin transfer molding, etc.