Page 294 - Handbook of Plastics Technologies

ELASTOMERS

4.86 CHAPTER 4

cessing aids but will be considered separately. Unlike petroleum oils, the term plasticizer
will be generally applied to synthetic ingredients, which are frequently added to lower the
T of the composition (i.e., to impart low-temperature flexibility).
g
Petroleum Oils. Petroleum oils are generally mixtures of paraffinic, naphthenic, and
aromatic hydrocarnbons. The relative amounts of these components determine the com-
patibility of a particular oil with a particular rubber. The paraffinic oils are more compati-
ble with EPDM and IIR. The more aromatic oils are more compatible with the more polar
rubbers (e.g. CR, NBR, and CSM). Most petroleum extender oils are compatible with NR,
IR, BR, and SBR. The effects of adding extender oil are to lower viscosities of uncured
compounds and allow the use of greater amounts of filler, and with respect to the vulcani-
zates, to reduce hardness, reduce modulus, and somewhat reduce tensile strength.
The viscosity and volatility of the oil are important. Generally, low-viscosity oils give
vulcanizates of lower glass transition temperatures. The lower-molecular-weight paraffinic
oils generally have lower viscosities, but they are also more volatile and thus somewhat fu-
gitive, especially at elevated temperatures.
As well as acting as plasticizers, the extender oils are considered to be process aids be-
cause of the reduced viscosities of the rubber compounds wherein they are used. This al-
lows easier processing, especially with rubber stocks that are highly loaded with filler.
Process Aids. Fatty acids, their metal salts (soaps), fatty acid esters, fatty alcohols,
and other substances are used to improve processing characteristics of rubber compounds.
Many such additives are available. They can have strong influences on processability.
They act as lubricants for flow during extrusion, molding, and so forth, allowing easy slip-
page between the rubber stock and the metal surfaces. They can also improve the disper-
sion of fillers, and so forth. In addition to aiding in flow during molding and extrusion, the
presence of lubricating process aids reduces the temperature of mixing in internal mixers.
Fatty acids are used in small amounts, with zinc oxide, as vulcanization activators. In
addition to their activating effect in the vulcanization process, the acid and its in-situ-
formed zinc soap do act as lubricants as well as activators.
In addition to the fatty acids and their metal soaps, fatty acid esters and fatty alcohols
are used, because they give outstanding processing improvements but without other types
of action—for example, cure and activation or breakdown enhancement during the masti-
cation of NR or IR. Pentaerythritol tetrastearate is a example of an ester-type process aid
with a broad range of applications. It does not bloom or exert unwanted effects.
Tackifiers. Pine tar, coumarone-indene resins, zylol-formaldehyde, and other resins
are used to increase the tack of rubber compounds. Tack, here, means stickiness of the un-
cured rubber stock to itself, rather than to other things, such as metal surfaces. Tack has
also been called autoadhesion. It is extremely important for building up structures such as
tires. Natural rubber inherently has good natural tack, but most synthetic rubbers do not.
Synthetic Plasticizers. The most important types of synthetic plasticizers are esters.
Phthalate esters are used to improve elasticity and low-temperature flexibility, especially
in NBR and CR vulcanizates. Common examples are dibutyl phthalate (DBP), di(2-ethyl-
hexyl) phthalate (DOP), diisooctyl phthalate (DIOP), and diisononyl phthalate (DINP).
They are generally used at levels of 5 to 30 phr.
Adipate and sebacate esters are used, in particular when low-temperature flexibility is
especially desired. Examples are di-2-ethylhexyl adipate (DOA) and di-2-ethylhexyl seba-
cate (DOS). Azelaic acid esters are also used. Trimellitates [e.g., triisooctyl trimellitate
(TIOTM)] are plasticizers with extremely low volatility. Phosphate esters are use to give
softness when flame retardance is also required.
Other ester plasticizers include polyesters of adipic and sebacic acids and 1,2-propyle-
neglycol. These are used where nonvolatile and nonmigrating plasticizers are needed.
Other types of esters are also used, such as citrates, ricinoleates, and octyl-iso-butyrate.

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