Polymers have undergone different names over the decades, sometimes creating confusion

Like all materials of great diffusion, both historical and geographical, polymers bring with them different linguistic approaches, which have, over time, increasingly moved away from a correct classification or attribution of precise technical meaning.

Then there are generalizations of the terms or confusion about them, which do not indicate a specific polymer but a family of products, apparently all the same, but different from other types of non-plastic materials.

If you have heard of people who experienced the launch and industrialization of plastic materials in the 60s of the last century, through the marketing of commonly used household products for example, you will have heard the words bakelite or moplen mentioned, which they were nothing more than a way of indicating an article made with the new raw material, plastic, of apparently inferior quality to traditional rigid materials such as aluminium, copper, brass, cast iron or wood.

An article made in Bakelite was light, beautiful to look at, waterproof and, above all, cheap, suitable for that large segment of the population who was filling their homes with items for daily life but who were very careful about spending.

If we go more into a technical approach to the problem, the classification of polymeric materials is made difficult by the imprecision of certain denominations, which have established themselves in the technological field, and which have been introduced into common use before there were exact ideas on the structure and properties of polymers.

Polymers which can be distinguished in relation to the conditions of their practical applications, in elastomers and plastomers, whose designations have a mechanical basis :

The first polymers have a tendency (at ordinary temperatures) to very high elastic deformations, with low average elastic moduli.

The second polymers however, always at room temperature, have rather modest elastic deformations, with relatively high modules and, in general, an interval of plastic slips until failure.

Elastomers, with appropriate technologies, among which vulcanization is of fundamental importance, are transformed into elastic rubber products ("vulcanized").

Vulcanization introduces a limited number of cross-links into the elastomer which, while they do not produce major obstacles to the stretching and shrinking mechanism of the polymer chains (under the action of an external load), blocks viscous flows.

In some cases, vulcanization is not an essential operation for obtaining elastic products (non-vulcanizable elastomers).

Plastomers, often also called resins, can in turn be divided into two categories:

The first, more widespread, is that of materials which can be formed into manufactured articles by the action of pressure and temperature (resins molding), with reversibility of formability with respect to temperature (thermoplastic resins or thermoplastics).

The second with irreversibility due to the intervention of chemical processes that modify the structure (thermosetting resins).

It is precisely the characteristic of formability that gave rise to the denomination "plastic materials". Typical technological operations for the transformation of formable elastomers into plastic material products are moulding, extrusion, pressing, etc.

Thermosetting resins are, therefore, polymers which during forming, normally performed by hot pressing, are transformed from essentially polymeric products linear cross-linked polymers.

The cross-linking is produced by chemical reaction at high temperature between the base resin and a "cure" agent, or by reaction, favored by high temperatures, between still free functional groups present in the polymeric chains of the base resin.

Polymers with a crosslinked structure can also be obtained by cold chemical reaction between a base resin, generally liquid, and a curing agent called “hardener”: this category of products is called “reticulated casting resins” and unsaturated polyester resins belong, for example.

Less frequently, in resins, which are therefore more valuable, the ability to form, from the molten state of solution, filaments or very thin sheets (film ) which, with suitable solid-state stretching operations, undergo a considerable mechanical reinforcement.

It is from these fibrogenic or film-forming polymers that, with suitable technologies, the most valuable synthetic fibers are obtained (or different monofilaments such as bristles and horsehair) or even most of the transparent or translucent films, widely used in the packaging sector or other applications.

In the classification scheme shown below, the double dashed horizontal arrow indicates the technological possibility of transforming a plastomeric material into an elastomeric one (for example the case of the plasticization of rigid resins) and vice versa (for example the case of the transformation of natural rubber into ebonite by forced vulcanization). Transformation in the first sense is of great economic importance, since it allows the use of resins which are in themselves limitedly useful in widely requested manufactured articles.

Machine translation. We apologize for any inaccuracies. Original article in Italian

Source: Angelo Montebruni