Particulate Filler Use by Polymer Type Unsaturated Polyester Resins (UPR)

Unsaturated polyester resins are used as viscous liquids made by dissolving low molecular weight polymers in vinyl monomers, mainly styrene. This allows easy molding or shaping of the resin into a desired form before curing to the final rigid shape. Crosslinking is achieved by free radical addition polymerization of the styrene monomer.

Typical applications for UPR are in products which are reinforced with fiberglass, such as shower stalls, boat hulls, vehicle panels, and autobody parts and trim. These also usually use mineral fillers in conjunction with the glass fibers. Mineral-filled

UPRs without glass fiber are used in products such as synthetic marble and onyx countertops and in autobody putty.

The main particulate filler used in UPR is natural calcium carbonate. Significant amounts of ATH are also used where halogen-free fire retardancy is required and for some solid surface applications. Talc is also used in products such as automotive body fillers. There is reported to be a significant use of calcium sulfate dihydrate (gypsum) as a filler in the USA, but it is not used to any extent in Europe.

Two of the most important forms of UPR which use mineral fillers are bulk and sheet molding compounds (BMC and SMC). The BMC compounds are the easiest to fabricate, but SMCs have superior properties.

Bulk molding compound (BMC) is a blend of mineral fillers (usually calcium carbonate and/or ATH), glass fiber reinforcement, catalysts, stabilizers, and pigments which are made into the form of a viscous, “puttylike” molding compound. The glass reinforcement is present at between 10% and 30% w/w, and the glass length is typically less than 12.5 mm. Bulk molding compounds have excellent flow characteristics and can be formulated for good mechanical properties, flame and tracking resistance, electrical insulation, corrosion, and stain resistance. As such they are well-suited to a wide variety of applications.

Sheet molding compound (SMC) is similar to BMC in outline, but contains longer glass fibers and higher glass levels and is made for compression molding purposes and larger parts and higher mechanical strength is needed. Glass reinforcement can be up to 60% w/w and glass length up to 25 mm.

SMC is produced in using a continuous in-line process. A paste of all the components except the glass is made first. This is then spread evenly onto a polymer film (usually nylon or polyethylene). Chopped glass fibers are then randomly deposited onto the paste. Another polymer film is introduced to make a sandwich structure which is rolled and allowed to mature. The top and bottom polymer films act to prevent autoadhesion of the roll. Sheet molding compounds can be molded into complex shapes which, after curing, have excellent mechanical properties and surface appearance, together with good electrical insulation, and are ideal for automotive Class A body panels, high-strength electrical parts, and many other applications.

While the glass fibers are the main contributor to their mechanical properties, particulate fillers also have an effect. Table 5 taken from trade literature shows the effect of an ATH filler on the flexural strength and modulus of a typical SMC

Table 5 The effect of an ATH filler on flexural properties of a typical SMC formulation (rounded values)

formulation and how this varies with filler loading. Note that while the flexural strength starts to decrease at high loadings, it is still above the unfilled, even at 300 phr addition level.