Introduction

W. G. SIMPSON

'Natural' and Other Finishes

The finish of an uncoloured plastic film or moulding is said in the industry to be 'natural'; such finishes can be serviceable and perfectly adequate in some uses (as in pulley wheels or other unseen parts in mechanical assemblies).

However, coloured or uncoloured materials may be finished in a great variety of ways. For certain purposes (even for 'natural'), quenching or annealing immediately after processing are necessary in order to obtain the molecular arrangements and therefore the mechanical or other properties required. In other words, the finishing is an important factor in deciding the suitability of the object for its purpose, and whether or not it will meet a specification.

The world would be more uniform, less colourful and stimulating, if it were not for the human habit of decorating and finishing objects of all kinds — both useful and ceremonial — often almost regardless of the types of substance from which they were made (like gilded or lacquered timber). Brooches and other items of jewellery, buttons, fasteners, seals, and more mundane things have been made for centuries by working bone or horn, and from malleable compositions like clays or filled resins, which were set hard just by drying under ambient conditions or were fired. A striking example of benefit from the habit was the skills developed in the nineteenth century in finishing a common material like cast iron (for a balustrade in the Royal Pavilion at Brighton, Sussex, for instance, this heavy dull metal was transformed by clever casting and painting into an unusually tough and long-lasting bamboo). Besides being converted by such means into a variety of 'timbers' iron was made to represent different types of stonework and decorative plaster (it was the 'versatile' building material of its day).

When 'natural', many plastics are transparent or translucent — which means, in effect, that they can be produced as transparent goods as well as in translucent or opaque colours. With materials that essentially are transparent, the range of colours possible is very wide. Other plastics are opaque, or dark in original colour — so that the opportunities for coloration by blending and mixing are more restricted.

With some plastics, the 'natural' appearance of the surface (especially in the case of opaque materials) is matt and not especially attractive; on the other hand, a polymer such as polystyrene in 'natural' form has a glossy surface which is described sometimes as 'glass-like'. Paradoxically enough, it may be the object of the technologist by finishing a material in a special way to make it more attractive and arresting in appearance than otherwise it would be — or, on the other hand, to make a glossy and rather brittle material less so. Success in finishing may mean the consistent production of a 'rough' finish as well as a smooth one.

It always is difficult to generalize, but apart from the desire to change and to improve the appearance of a substrate, perhaps to disguise it, there can be a variety of technical reasons for selecting methods of finishing (at times with objectives which may seem confusing or even contradictory) — such as to change the chemical, electrical, or physical properties, to match the appearance of some adjacent components, or merely to convey information by colour-coding or printing. Some possible approaches to and applications for finishing technology are summarized in the form of a list below. (It is not proposed in this volume to attempt to encompass the use of plastics and synthetic resins in finishing other substances — in forms such as paints, coatings, or coloured films applied (say) to protect timber boards, and sheets in ferrous or non-ferrous metal.)

(i) To provide a skin on the surface of a product and so to impart altered or improved properties (as examples, to make the surface conductive electrically, sensitive to light, more resistant to weathering — including degradation by ultra-violet light, and harder or stronger in a mechanical sense)

(ii) to provide a distinctive surface, either generally, or in a particular part (such as to abrade other substances, or to resist wear; to break or to tear easily in pre-arranged circumstances)

(iii) to assist in fabrication by marking out and cutting lines or shapes for sealing or welding later

(iv) to apply to the surface of the plastic a layer of another material — perhaps another plastic or something of different nature entirely (like organic fibres or silicates) — so that the resulting composite combines the properties of added materials with those of the original (typically, in packaging a film of another plastic might be applied in order to enhance resistance by the base material to permeation by gases, or to improve adhesion)

(v) to make for some specific purpose a new composite material (like a laminate in different colours-for engraving or otherwise machining for fancy effects, a flexible film to make signs that will reflect light, or a rigid but light 'sandwich' for structural purposes in aircraft and other applications where weight is an important consideration)

(vi) so that a surface will be more suitable for further processing (in preparation for electroplating, lamination, metallizing, printing, and so forth)

(vii) to mimic, deceive, or mislead, by giving the plastic the appearance of other materials (like brass, ivory, marble, leather, or wood)

(viii) to make an object similar in appearance (and therefore a match to or a contrast with) adjacent objects in other materials

(ix) to extend a material (and thus to reduce its cost) — perhaps physically by stretching it in some way, or (through the introduction of 'blowing agents' or gaseous expansion) by separating the particles or portions of the feedstock by means of closed or open voids

(x) to make decorative items such as prints for furnishings in domestic premises, hotels, and other public buildings, accessories for clothing, and so forth

(xi) for advertising and to convey information by printing logotypes, trade marks, instructions for use, details of composition, computer codes, etc.

(xii) as the...