Industrial plastics: rigid and flexible foams

Rigid polyurethane foams in sandwich forms have wide applications as building components. They are also the best insulants known today and so have wide application in refrigeration and in buildings, where they are applied in fitted slab form or are foamed into cavities at the building site. They can also be applied by spraying about six millimetres thickness with each pass of the spray gun. The ability to spray a foaming mixture through a single nozzle is a great advantage in application.

A very important use of rigid foam is for furniture parts to reproduce wood structures; these can be injection molded. Polyurethane foam can be screwed and nailed with a retention about equal to white pine lumber. A major advance in the manufacture of sandwich structures is a new method of injection molding, in which a large machine is used to produce moldings up to 1.2 metres square. Moldings of great strength and any desired surface are obtained.

Flexible foams

Flexible foams, usually polyurethane, are made in slab form up to 2.4 metres in width and as much as 1.5 metres high; these are then cut to required shapes or sizes or are molded. The molded foams may be hot molded.

This involves filling heated aluminum castings and gives a product having high resistance to compression, as for automobile seats; or they may be cold molded, a process used particularly for semi-flexible foams with high load-bearing properties. Used almost exclusively by the automobile industry for crash pads, armrests, and dashboard covers, the process involves machine mixing the ingredients and pouring them into aluminum molds lined with vinyl or acrylo-nitrile-butadiene-styrene skins, which become the cover material for the part.

Polystyrene foams are made in a wide range of densities, from expandable beads, either by extrusion through slot-shaped openings to 40 times the original volume to form boards directly or by foaming in steam chests to form large billets. Using small beads in stainless steel molds, cups can be molded with thin sections.

Thin sheet for packaging can also be made by the tube extrusion technique. Though packaging is a major use for forms made in closed molds, the largest use is for building panels; they can be plastered directly.

Acrylonitrile-butadiene-styrene can be expanded from pellets and is particularly suitable for wood-grain effects and for the production of heavy sections. Expanded vinyls can be made from plastisols for flooring or textile linings by calendering with a blowing agent and laminating to a fabric base, and by injec­tion molding for insulation and such articles as shoe soles. An improved material is now obtained from cross-linked polyvinyl chloride and competes with polyester in glass reinforced plastic.

Unit 14

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Chemistry is the science of substances — of their structure, their properties and their reactions that change them into other substances. This is a very large field of study because the number of different kinds of substances is very great, and each kind has its own characteristic qualities. Chemistry occupies a central position among the basic sciences. It is closely connected with physics and biology. It plays an important part in the development of geology and physiology, and enters every industry as well as medicine and agriculture.

The early chemists studied two types of matter: inorganic and organic. But the materials which are provided by nature cannot satisfy modem science and technology now and man is turning more and more to the help of chemistry.

In present-day organic chemistry new compounds are being synthesized daily. With its plastics, synthetic fibres and other artificial materials organic chemistry is the chemistry of modern times. Inorganic chemistry is quite a different thing, it was studied long ago and is as old as the Old World.

Chemical science is successfully solving many complex problems and its rapid development will be raising the living standard of people.

YOU SHOULD KNOW...

...that there exist some rules which you must observe when you are working in a chemical laboratory. Let me name some of them.

1. Every vessel which you use for the experiment must be absolutely clean. Otherwise it can influence the reaction and therefore the results of the experiment may not be precise.

2. One should be very careful when working with glass things and mixing different liquids.

3. One should use the precise amount of the chemical substance or water which is recommended for the experiment.

4. When you are making experiments put down everything that you watch during your work. You must put down the names, the amounts and weights of substances which you use, the rise in temperature, the boiling and melting points, the change in colour, etc. You should also put down the final result of each experiment.

In other words, cleanliness, carefulness and watchfulness are the three main points which must be observed by every student.