Classes of Plasticizers

Plasticizers can be classified according to their chemical structure as shown in Table 2.1. Plasticizers may be also classified into primary and secondary types (14). Primary plasticizers are used solely as plasticizer, i.e., as the basic component of the plasticizer formulation. Secondary plasticizers are blended with primary plasticizers in order to improve some of the properties. 

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Polyolefins are manufactured and used in much greater quantity than any other class of plastics. The principal polyolefins are polyethylenes of various densities (LDPE, LLDPE, HDPE) and polypropylene (PP) (see Olefin polymers). 

In all of the examples given so far in this chapter the product of polymerisation has been a long chain molecule, a linear polymer. With such materials it should be possible for the molecules to slide past each other under shear forces above a certain temperature such that the molecules have enough energy to overcome the intermolecular attractions. In other words above a certain temperature the material is capable of flow, i.e. it is essentially plastic, whereas below this temperature it is to all intents and purposes a solid. Such materials are referred to as thermoplastics and today these may be considered to be the most important class of plastics material commercially available. 

Subsequent chapters deal with individual classes of plastics. In each case a review is given of the preparation, structure and properties of the material. In order to prevent the book from becoming too large I have omitted detailed discussion of processing techniques. Instead, with each major class of material an indication is given of the main processing characteristics. The applications of the various materials are considered in the light of the merits and the demerits of the material. 

Plasticizers can be classified according to their chemical nature. The most important classes of plasticizers used in rubber adhesives are phthalates, polymeric plasticizers, and esters. The group phthalate plasticizers constitutes the biggest and most widely used plasticizers. The linear alkyl phthalates impart improved low-temperature performance and have reduced volatility. Most of the polymeric plasticizers are saturated polyesters obtained by reaction of a diol with a dicarboxylic acid. The most common diols are propanediol, 1,3- and 1,4-butanediol, and 1,6-hexanediol. Adipic, phthalic and sebacic acids are common carboxylic acids used in the manufacture of polymeric plasticizers. Some poly-hydroxybutyrates are used in rubber adhesive formulations. Both the molecular weight and the chemical nature determine the performance of the polymeric plasticizers. Increasing the molecular weight reduces the volatility of the plasticizer but reduces the plasticizing efficiency and low-temperature properties. Typical esters used as plasticizers are n-butyl acetate and cellulose acetobutyrate. 

As reviewed within each one of the major classes of plastics (PE, PVC, PC, etc.) there are usually a very wide variety of specific formulations, each of which has slightly different properties and/or processing capabilities at various costs. Prices, too, will tend to vary depending upon the supplier, the current state of the market, and the volume of plastic that the processor is prepared to purchase. 

With regard to the common European market, the European Economic Community (EEC) has undertaken numerous activities concerned with materials and material information systems. In one demonstration program for material databases eleven such databases from various countries in the EEC are being cooperatively developed with joint standards for terminology, data presentation, database access, and the user interface of search commands, aids, and menus. For the materials class of plastics, Polymat was selected to participate in this cooperative work. Interesting developments occur from which the users of central material databases in the entire EEC area can benefit. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Class of Plastic Modulus Yield Stress Ultimate Tensile Strength Elongation at Break Examples... 

Figure 16.5 Classes of plastics by shape of stress-strain curve. (Source WittcofF and Reuben, Industrial Organic Chemicals in Perspective. Part Two Technology, Formulation, and Use, John Wiley Sons, 1980. Reprinted by permission of John Wiley Sons, Inc.)...

With this increased emphasis on testing foods, activity in searching for alternative simulants has diminished. This is to be welcomed. The use of food simulants in compliance testing for the general case (see Chapter 15) is still important. But the quest for alternatives that mimic simulants that in turn are intended to mimic foodstuffs represents a cul-de-sac. A simulant has to resemble a food in its interaction with the food contact material. It is clearly the case that different simulants are needed for different foods and for different materials, paper versus ceramics versus plastics for example. And maybe even different simulants could be needed for different classes of plastics and different types of substances. But if simulants have to be fine-tuned and tailored for individual applications then they lose much of their utility. 

The third class of plastic that is defined in the USP is PP. These containers can be used for oral solids or liquids, making their application among the most versatile of pharmaceutical packaging materials. Specifications are given once again in both the /5 / F3] Code of Federal Regulations PP... 

With the rapid growth of the plastics industry in the last ten to fifteen years, a problem in the comparability of figures in each class of plastic material has developed. For example, with a plastic material made of a phenolic resin and rosin, the problem is whether to classify this combination as phenolic or as rosin. It is important for each specific class of material that these mixed commodities be considered consistently in one class or in another. In the published figures this particular combination is handled in a number of different ways. How they should be handled is an arbitrary matter the most important thing is to handle them consistently. 

The data thus far have shown that S-PS can be plasticized effectively with respect to backbone and ionic domain plasticizers. By appropriate choice of the plasticizer type either the PS backbone or the ionic domains can be plasticized preferentially. By appropriate control of the metal sulfonate content and the polarity of the plasticizer used, flexible S-PS compositions possessing useful tensile properties are feasible. While this approach has substantial merit, it is apparent that simply increasing the level of a phthalate plasticizer to improve melt flow results in a substantial decrease in useful tensile properties. It would be desirable to use a given level of backbone plasticizer and adjust the melt flow of the entire composition by independently plasticizing the ionic domains. One approach to achieve this objective has been described in the plasticization of ionic groups in metal-sulfonated ethylene propylene terpolymers (9). In those systems, the incorporation of metal carboxylates as plasticizers can improve both flow behavior and tensile properties. It is of interest to determine if this class of plasticizers can be combined with the phthalate plasticizers used for the S-PS backbone to provide an improved balance of flow behavior and tensile properties for S-PS s. 

Different PET chemolysis methods have been developed aimed at the production of TPA, DMT or BHET, all of them being possible monomers for the reconstruction of fresh polyesters. The exact monomer formed by PET depolymerization depends on the type of chemical agent used to break down the polymeric chains. In certain processes, the final product of PET chemolysis is a mixture of polyols, useful in the formulation of other polymers such as unsaturated polyesters, polyurethanes and polyisocyanurates. This is an interesting case of chemical recycling because the breakdown of one polymer leads to the raw materials for the preparation of a quite different class of plastics. 

Polyolefin - Polyolefins are a large class of carbon-chain elastomeric and thermoplastic polymers usually prepared by addition (co)polymerization of olefins or alkenes such as ethylene. The most important representatives of this class are polyethylene and polypropylene. There are branched and linear polyolefins and some contain polar pendant groups or are halogenated. Unmodified polyolefins are characterized by relatively low thermal stability and a nonporous, nonpolar surface with poor adhesive properties. Processed by extrusion, injection molding, blow molding, and rotational molding. Other thermoplastic processes are used less frequently. This class of plastics is used more and has more applications than any other. Also called olefinic resin, olefinic plastic. 

The next most important demand for methyl alcohol is as a raw material in the synthesis of many important organic compounds, including formaldehyde acetic acid chloro-methanes, compounds in which the hydroxyl group and/or one or more hydrogen has been replaced by fluorine, chlorine, bromine, and/or iodine methyl methacrylate, a compound from which acrylic plastics are made methylamines, the source of another important class of plastics, dimethyl terephthalate, the monomer for yet another class of plastics and other products. 

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