Лако-красочные материалы — производство

3.1 Introduction

There are now so many thermoset plastics available that it can be difficult to differentiate between the various types. Thermoset resins are used for the manufacture of a wide range of parts from canoes to wind turbine blades and from polymer concrete to epoxy adhesives and so there is only a brief discussion of thermoset plastics included in this guide as it is essentially aimed at the bonding of small parts.

Thermosetting plastics (thermosets) are polymer materials that irreversibly cure, to a chemically crosslinked stronger form [1]. Once formed and cooled they cannot be reprocessed and will decompose before they can melt. Thermoset materials are generally stronger than thermoplastic materials due to their rigid three-dimensional network of bonds and are also better suited to high-temperature applications up to the decomposition temperature of the material, see Figure 3.1.

Figure 3.1 Thermoset plastics

Therefore, a thermoset material cannot be melted and re-shaped after it is cured and so cannot be recycled, except as filler material [2].

Some examples of thermosets are:

• Vulcanised rubber,

• Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plasticware),

• Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard),

• Melamine resin (used on worktop surfaces),

• Epoxy resin (used as an adhesive and in fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic), and

• Polyimide (used in printed circuit boards and in body parts of modern airplanes). Some methods of moulding thermosets are:

• Reactive injection moulding (used for objects like milk bottle crates),

• Extrusion moulding (used for making pipes, threads of fabric and insulation for electrical cables),

• Compression moulding (used to shape most thermosetting plastics), and

• Spin casting (used for producing fishing lures and jigs, gaming miniatures, figurines and emblems as well as production and replacement parts).

Thermoset plastics differ from thermoplastics in that they are crosslinked when they are polymerised and so do not melt prior to thermal degradation.

Some of the advantages of thermosets include:

• Generally improved temperature resistance over thermoplastics,

• Lower creep properties and stress relaxation than thermoplastics,

• Thermosets are often harder and more scratch resistant than thermoplastics, and

• Improved solvent and UV resistance over thermoplastics.

The main disadvantages are:

• Thermosets can not be recycled,

• Low strain at break,

• Slower production rates compared with hot melt thermoplastic processes, and

• The resins and catalysts are often more hazardous to health.