Particulate Fillers in Thermoset Plastics g

Roger Rothon

Contents

Definitions....................................................................................... 112

Introduction...................................................................................... 113

Processing of Thermoset Composites........................................................... 113

Effect of Particulate Fillers on Properties....................................................... 114

Cost Saving.................................................................................. 115

Exotherm Control and Shrinkage Reduction................................................ 115

Stiffness (Modulus).......................................................................... 115

Thermal Expansion.......................................................................... 117

Thermal Conductivity ....................................................................... 117

Flame Retardancy ........................................................................... 118

Aesthetics.................................................................................... 119

Abrasion Resistance......................................................................... 119

Toughness.................................................................................... 119

Rheology of Pre-Cure Mix.................................................................. 120

Transparency................................................................................. 121

Particulate Filler Use by Polymer Type......................................................... 121

Unsaturated Polyester Resins (UPR)........................................................ 121

Polymethylmethacrylate (PMMA).......................................................... 123

Phenolic Resins.............................................................................. 123

Epoxy Resins................................................................................ 123

Polyurethanes................................................................................ 123

Recycling and Sustainability.................................................................... 123

Cross-References................................................................................ 124

References....................................................................................... 124

R. Rothon (*)

Rothon Consultants and Manchester Metropolitan University, Guilden Sutton, Chester, UK e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

© Springer International Publishing Switzerland 2017 111

R. Rothon (ed.), Fillers for Polymer Applications, Polymers and Polymeric Composites:

A Reference Series, DOI 10.1007/978-3-319-28117-9_77

Abstract

Although thermoset polymers are significant users of particulate fillers, the scientific literature is much scarcer than for other polymer types. This is because they are usually used in combination with glass fibers, which then dominate the properties of the composite.

The processing of thermosets is different to that for the other two polymer types, with the filler being added to a relatively low viscosity, often liquid, phase; where the high shear that helps dispersion in other polymer types is missing. On the other hand, there is less damage to the filler particles, which means that it is easier to use fillers such as mica, wollastonite, and glass fibers. It is also easier to incorporate hard fillers, such as crystalline silicas, and temperature-sensitive ones like cellulosics, than it is with other polymers. Thermoset polymers are also able to tolerate larger particle size fillers. Because they are polymerized during molding, thermosets can also exhibit higher mold shrinkage and controlling this is an important role of particulate fillers. Fillers are also important in reducing polymerization exotherms, which can otherwise cause problems.

The main general purpose filler used in thermosets is calcium carbonate in various forms. This is mainly employed for cost reduction, shrinkage, and exotherm control. Large quantities of aluminum hydroxides are also used for low smoke and fume flame-retardant and aesthetic purposes. Epoxy printed circuit boards use fillers such as alumina to impart high thermal conductivity while retaining low electrical conductivity. Thermosets make more use of hard fillers such as crystalline silicas than other polymer composites. These are used to improve abrasion resistance in flooring and solid surface applications.

Most thermosets are polar, and this means that they can wet and interact well with many types of fillers, especially minerals like carbonates. This reduces the need for surface-modifying species, but dispersants and coupling agents may still be utilized, especially with siliceous fillers. Coupling agents are also often used to help property retention under adverse environmental conditions rather than to improve initial properties.

Keywords

Thermosets • Unsaturated polyesters • Polyurethanes, epoxy resins • Calcium carbonate • Aluminum hydroxide

Definitions

Thermoset plastics are polymers which are produced “in-situ” from relatively low molecular weight precursors, usually in a mold cavity. The final shape is thus set by chemical reaction (curing), as opposed to cooling of a melt, which occurs with thermoplastics. Most elastomers are also thermoset, but start from high molecular weight polymers and have much less cross-linking than thermosets.

Particulate fillers are powdered substances, with low aspect ratio particles usually less than 100 pm in size, which are added to polymers to reduce cost, to improve processing, and/or to modify one or more properties. They can be inorganic or organic in nature and may be of natural origin or synthetic.

Solid surfaces are an important, glass fiber-free, application for fillers in thermoplastics. The term refers to a solid, non-porous surfacing material mainly used in kitchens and homes for counter tops, sinks, and bathroom fittings. They are usually made from highly filled polyester or acrylic resins and, by judicious choice of fillers, can have a very attractive appearance, resembling polished minerals, but much more easily shaped and cut. They also need to be formulated to resist heat, stains, and impact and to either be very scratch resistant or able to be easily resurfaced by polishing. The main fillers used in this application are ATH and crystalline silicas.

Introduction

In common with other polymers, thermosets are significant users of particulate fillers, often at high addition levels. However, they do differ in the processing methods, reasons for use, and the filler types most often met. As with many particulate filler topics, the bulk of the literature is in trade publications and conference proceedings, rather than in peer-reviewed journals.

The principal thermoset polymers using fillers are unsaturated polyester resins (UPR), polymethyl methacrylates (PMMA), phenolics, epoxies, and polyurethanes. The main fillers used and the reason for their choice are summarized in Table 1. More detail is given in sections “Processing of Thermoset Composites” and “Effect of Particulate Fillers on Properties.”

 
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