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Rheology of Pre-Cure Mix

Thermoset resins, unlike most other polymer types, start off as a liquid monomer of oligomer, which flows into a mold for curing. Addition of fillers (and glass fibers) inevitably increases the viscosity of the system and affects its flow properties. To some extent the increased viscosity can be beneficial to filling and molding operations and can be useful in preventing filler settling during storage of the premix. Indeed specialized thixotropic agents are sometimes used to prevent such settling. However, if the viscosity increase goes too far, then it can cause problems. As well as inhibiting mold filling, high viscosity can prevent proper degassing, which is needed to remove air entrapped during filler addition and molding operations. These viscosity effects are most noticeable at high filler addition levels such as employed for maximum flame retardancy when using ATH.

Two tools are at the formulator’s disposal to counteract deleterious viscosity effects. The first is the use of dispersant additives, of which several specialist ranges exist. When choosing these additives, consideration must be given to the stability of the viscosity over time, as this can often change. The second tool is to use fillers with tailored particle size distributions that allow for maximum packing. These distributions can be made up by the formulator (see Ferrigno 1978) or supplied by the filler producer. Oil absorption of a filler is a good guide for this, with a low value giving least effect on viscosity.

An example of the use of a proprietary dispersing agent on the viscosity of an ATH at 275 phr in an unsaturated polyester resin system is given in Fig. 4. The effect of particle size distribution on viscosity of a similar system is given in Table 4.

Particle shape also has an important bearing on resin viscosity and this is particularly relevant to ATH, which can be produced by two different methods, grinding or precipitation. Grinding produces platy particles, while precipitation gives blocky ones with lower oil absorption and less effect on viscosity.

Use of a dispersing agent to reduce the viscosity of a highly filled UPR (275 phr ATH)

Fig. 4 Use of a dispersing agent to reduce the viscosity of a highly filled UPR (275 phr ATH)

Table 4 The effect of optimizing particle size distribution on the viscosity of an unsaturated polyester system containing 150 phr of an ATH filler (all fillers with 12 pm average particle size)

ATH type

Linseed oil absorption (g/100 g)

Viscosity (cP)

Standard milled grades



Broad particle size distribution


10 - 12,000

Highly optimized broad distribution


Below 10,000


Most thermoset resins, being amorphous, are quite transparent. Fillers, such as calcium carbonate and ATH have higher refractive indices than the polymers and so significantly reduce transparency (an effect which varies with particle size). This is not usually an issue in many applications. Where it is, then fillers of lower refractive index, and nearer to the polymer matrix, such as glass and crystalline silicas, are more suitable.

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