Page 352 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.32 CHAPTER 5

easier bonding-debonding equilibrium, which favors molecular mobility and flexibility.
(3) Lower-molecular-weight plasticizers have more mobility and favor greater flexibility.

5.4.3 Effects on Other Properties
5.4.3.1 Processability. Plasticizers are often used to increase melt flow during process-
ing, particularly in rubber, cellulosic plastics, and coatings. In vinyl plastisol technology,
powdered resin is slurried in nonsolvent plasticizer at room temperature for easy process-
ing in low-cost equipment and then heated to dissolve and cooled to gel into the final prod-
uct.

5.4.3.2 Mechanical Properties. Addition of plasticizer lowers hardness and strength
and increases elongation and creep. It increases adhesion and friction.
5.4.3.3 Acoustic Properties. Addition of plasticizer produces damping of vibration (au-
ditory frequencies) and thus reduces the resulting noise.
5.4.3.4 Thermal Properties. Addition of plasticizer slides the modulus-versus-tempera-
ture curve horizontally toward lower and lower temperatures, in proportion to the amount
of plasticizer added. This lowers the stiffening temperature of flexible plastics. It also low-
ers the softening temperature of rigid plastics, which limits maximum use temperature but
improves melt processability.
When epoxidized fatty esters are used to synergize thermal stabilization of PVC
(Sec. 5.1.3.2), these are liquids which therefore also act as secondary plasticizers.
Flammability is also a function of the plasticizer used. When phthalates are added to
PVC, they reduce the total chlorine content, which reduces flame retardance. On the other
hand, when organic phosphate esters are used as plasticizers, they actually increase flame-
retardance (Sec. 5.7). This makes them particularly important in cellulosic plastics.

5.4.3.5 Electrical Properties. Addition of plasticizer increases mobility of ionic impuri-
ties, which lowers electrical resistance (increases conductivity). Plasticizer increases the
mobility of polar groups in the polymer, which increases dielectric constant and shifts the
loss peak to higher frequencies.
5.4.3.6 Chemical Properties. Plasticizer increases free volume and molecular mobility,
which increases permeability of small molecules of liquids and gases. Toxicity of some
plasticizers to small laboratory animals has led to considerable environmental and political
concern, but they have never caused any trouble to human beings; on the contrary, the in-
ventor of plasticized PVC lived healthily to the age of 101.

5.4.4 Permanence
Plasticizer failure can shorten the useful life of the plastic product. Failure can be either
loss of plasticizer (it is “fugitive”) or degradation of the plasticizer (“aging”).
5.4.4.1 Fugitivity. Plasticizer may escape for a number of reasons. Volatility is gradual
evaporation at higher temperatures, such as fogging of auto windshields in hot weather,
and shrinkage and embrittlement of electrical wire and cable insulation. Extraction of
plasticizer by gasoline, motor oil, solvents, or even soapy water causes stiffening of jack-
ets and gloves in dry cleaning and of shower curtains and baby pants in soapy water. Exu-

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