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Plasticizers

Plasticizers are added to thermoplasts or elastomers to make them more flexible, improve processability, or allow them to foamed. A distinction is also made between primary and secondary plasticizers.

Generally, plasticizers are low-molar-mass liquids, and only seldom are they low-or high-molar-mass solids. Elastomers are mostly plasticized with mineral oils: typical rubber tires, for example, contain about 40% mineral oil.Phthalic esters dominate plasticizers for thermoplasts, and here, di (2-ethyl hexyl) phthalate (“dioctyl phthalate “, DOP) is the most used.Polymeric plasticizers are only used in a relatively small number of cases. They are mostly polyesters or polyethers. High-molar-mass polyesters are used for polymer blends, but low-molar-mass polyesters are used as actual plasticizers. Since the latter are produced by polycondensation, they have a broad molar mass distrubution, and thus monomer and oligomer components. High monomer factions mean low polymer fraction, but quite high oligomer fractions. In such cases, they are called oligomeric plasticizers.

A distinction is also made between primary and secondary plasticizers. Primary plasticizers interact directly with the polymer chains, where sencondary plasticizers are actually only diluents for the primary plasticizers. For this reason, secondary plasticizers are also called extenders. This, depending on the polymers, a given plasticizer can act as either a primary or a secondary plasticizer. For example, heavy oils are extenders for PVC, but primary plasticizers for elastomers.

Eighty to eighty-five percent of all plasticizers are used to produce plasticized PVC. The phthalates are preferentially used to plasticize certain polyurethanes, polyester resin, and phenolic resin. Phosphaste esters are good plasticizers for melamine resins, unstaturated polyesters, phenolic resins, polyamides, and cellulose acetate. A total of about 500 different plasticizers are commercially available on the market.

Plasticizers incerease the chain segment mobility by different molecular effects. Polar plasticizers produce the gauche effect with polar polymer chains, that is, they increase the gauche conformation fraction at the expense of the trans conformations, and so reduce the mean rotational energy barrier. Acting as more or less good solvents, plasticizers dissolve helix structure and crystalline regions. In addtion, chain segments become more separated on account of the dilution effect. On the other hand, solvation does not increase chain mobility since a solvent sheath acts like a substituent and consequently increase the rotational energy barrier.

Because of the increased chain segment mobility, the glass transition temperatures, moduli of elasticity, tensile strengths, and hardness are decreased, whereas the extension at break is increased. The change in these parameters can thus be used as a macroscopic meaure of the effectivity of the plasticization. Of these parameters, only the glass transition temperature depends solely on the polymer chain mobililty, all other parameters contain contributions from other effects. Thus, measurements on plasticization effectively using glass transition temperatures, moduli of elasticity, tensile strengths, elongations at break, and hardness can not yield identical results.

To increase segment mobility, the plasticizer must be able to form a thermodynamically stable mixture with the polymer, that is, it must be compatible with the polymer, but solvents which are too good stifffen the chain by solvation. Thus, plasticizers must be solvents which are as poor as possible.