Composites

Mechanism of Action

Unsaturated polyester resins (UPR) refer to a large variety of products all containing reactive double bonds in the polymeric chain. The presence of double bonds provides the product with the ability to crosslink in the presence of a monomer, also acting as a diluent, via a free radical mechanism.

Example of chemical structure

The reactivity, the viscosity and the final properties of UPR can be adjusted by:

• Chemical composition (aliphatic, aromatic, number of double bonds)
• Resin molecular weight
• Nature and concentration of the crosslinking monomer (containing at least one double bond)

The general crosslinking mechanism is shown here:

The crosslinking monomer

In most cases the crosslinking monomer is Styrene introduced at concentrations between 25 and 45 wt%, but other monomers can also be used.

Curing catalyst

To initiate the curing reaction a source of radicals is needed. In most cases, organic peroxides are used.

• The first step is the decomposition of the peroxide.
• The second step is chain propagation reaction via styryl radical formation capable to react with a double bond

For cold curing, initiators such as amines or metallic salts are also introduced.

Inhibitors

In order to prevent premature curing and to extend pot-life, inhibitors are also introduced into the system. Inhibitors are free radical scavengers like hydroquinone derivatives, terbutylcatechol, cresol derivatives.

Inhibitors
Examples of inhibitors used in UPR

Hydroquinone	Excellent for stabilization during storage No color issues
Tertiobutylhydroquinone	Can be used in a wide range of temperatures Very efficient with systems initiated by Dibenzoylperoxide
Ditertiobutylhydroquinone	Provide very good stability to the resin until 60°C Used when short gelling time is needed (cold and hot process)
Toluhydroquinone	Can be used in a wide range of temperature Very efficient with systems initiated by Dibenzoylperoxide Provide excellent thermal resistance
Benzoquinone	Excellent performances Provides yellow color to the system
Tertiobutylcatechol	Active until 40°C Rapidly absorbed by the resin Excellent adjustment of pot-life

Monomers
Monomers used for UPR crosslinking

Methylmethacrylate	Mainly used for translucent parts Always used in combination with styrene (50:50) Excellent weathering
Alphamethylstyrene	Used for some special applications where a good control of the reactivity and good control of shrinkage is needed. Unpleasant odor
Divinylbenzene	High reactivity Provide high chemical resistance Lower impact resistance
Diallyl phthalate	Low volatility Used in molded parts (mainly by BMC) with good electrical performances
Vinyl acetate	Used in special applications like stratification of PS foam. Low hydrolysis resistance