Plastics are high molecular weight (macromolecular or polymeric) organic substances that have usually been synthesized from low molecular weight compounds. They may also have been obtained by chemical modification of high molecular weight natural materials (especially cellulose). The raw materials are most often petroleum, natural gas, and coal. They can be reacted with air, water, or sodium chloride to prepare reactive monomers. The most important industrial synthetic processes for the preparation of plastics from monomers may be classified according to the mechanism of the formation reaction of the polymer, such as polymerization and condensation reactions. Since several chemically identical or similar plastics can be prepared in several different ways and from different raw materials, this classification has little meaning for the analysis of unknown plastics samples. On the other hand, in addition to chemical investigations, the appearance of a plastic as well as its behavior on heating yields useful information for its identification.
Each plastic analysis begins with screening tests. In addition to the observation of several characteristics, such as solubility, density, softening, and melting behavior, an important role is played by heating in a combustion tube (pyrolysis test) and in an open flame (flame test). If these preliminary tests do not yield a positive identification, examine the materials for the presence of heteroatoms such as nitrogen, halogens (especially chlorine and fluorine), and sulfur. Then begin a systematic analysis by testing the solubility, and proceed to simple specific tests. In addition, try to identify the possible presence of organic or inorganic fillers or other additives such as plasticizers or stabilizers. Unfortunately, the simple approaches discussed here seldom give reliable information about the type and amounts of such additives.
Among the many solvents for plastics, the most widely used are toluene, tetrahydrofuran, dimethylformamide, diethyl ether, acetone, and formic acid. In certain cases, chloroethylene, ethyl acetate, ethanol, and water are also useful. It should be noted that the flammability and toxicity of many solvents requires special care in handling. Benzene should be avoided as much as possible. Tables 3.1 and 3.2 show a compilation of the behaviors of the most important plastics in various solvents. For the systematic analysis of plastics, the distinction between soluble and insoluble polymers provides the first separation into two groups. One can then apply chemical methods to investigate these two groups further.
The previously described simple screening tests are not always sufficient to identify an unknown plastic with certainty. In some cases the use of chemical reactions for identification purposes cannot be avoided. First, one tests for heteroatoms, those elements which are present in the plastic in addition to carbon and hydrogen, such as nitrogen, sulfur, chlorine, fluorine, silicon, and sometimes phosphorus. Unfortunately, there is no simple direct method for the certain identification of oxygen, so it is not possible to test for oxygen in a qualitative manner. The following reactions presuppose a certain amount of experimental skill and the necessary care.
On the basis of the screening tests described in previous chapters and with the use of certain specific reactions, the most important plastics can be identified through simple separation procedures. Test first for heteroatoms (Chapter 4), and then for solubility in different solvents (Section 3.1). If necessary, test for other characteristic physical properties or carry out chemical reactions.
The reactions described in this section are useful as screening tests for certain groups of plastics, but also for testing for specific cleavage products, such as phenols or formaldehyde, from some plastics.
Earlier chapters focused primarily on an analysis of today’s dominant plastic materials, which for the most part, date back to the 20th century. Before that era, not only natural resins, such as beeswax, amber, copal resins, gutta-percha, shellac, bitumen, horn, tortoise shell, and materials derived from cellulose derivatives (cellulose nitrate and acetate), from blood protein (mostly as Bois Durci), or from casein are used to find a variety of applications. Our ancestors used these natural materials to produce, among other things, pieces of jewelry, decorative items, or articles of daily use such as tins, picture frames, or desktop utensils where these materials often en replaced metals, wood, paper, or ceramics.
Previous chapters have described simple methods of identification, and generally these are sufficient in order to assign an unknown sample to a certain group of plastics. Of course, with these simple methods, especially in dealing with plastics of complicated composition, one can only obtain some qualitative information. In order to obtain more detailed information it is necessary to use more advanced methods of analysis (Table 8.1), which can only be carried out by specially trained personnel.
