Rubber
Compounds Used in the Survey and Survey Results
Why
ExxonMobil Chemical Elastomers Resist Chemical Attack
Compounding
for Maximum Chemical Resistance
ExxonMobil Chemical elastomers
offer good resistance to:
• Water
• Inorganic salt solutions
• Synthetic hydraulic fluids
• Ethylene glycol base antifreeze formulations
• Most inorganic acids
Although the "average" compounds
reported in our survey are attacked
by some of the concentrated acids, compounds specially designed for
acid resistance can be recommended for severe acid environments.
When immersed in organic
compounds, ExxonMobil Chemical
elastomers show good resistance to attack by acids, amines and
oxygenated compounds (alcohols, aldehydes, esters, ethers and
ketones). Resistance to soaps is excellent whether in concentrated
form (as purchased) or in 1% solutions which approximate the
concentration of these materials commonly used in household
applications.
ExxonMobil Chemical elastomers
display good resistance to attack by
animal or vegetable oils. However they are like other petroleum derived
synthetic polymers: their resistance to the attack of hydrocarbons, oils
and other petroleum-based chemicals is low.
Rubber
Compounds Used in the Survey and Survey Results
Chemical immersion results
are reported in terms of volume change,
tensile strength retention, elongation retention, change in Shore A
hardness and surface condition of the sample.
Why
ExxonMobil Chemical Elastomers Resist Chemical Attack
No single factor explains the ability of our elastomers to resist attack by
many chemicals.
Rather, the explanation for their chemical resistance behavior should
consider several of their basic characteristics that in combination enable
their compounds to withstand reaction and absorption, the two primary
means of chemical attack.
Of utmost importance are chemical structure, polarity, solubility, cohesive
energy density and reactivity.
Polymers with very low unsaturation are susceptible to attack by chemical
reaction at only a few points along the molecular chain. ExxonMobil
Chemical elastomers differ significantly from other common elastomers
in unsaturation content. Vistalon EPM polymers are completely saturated.
Exxon butyl, chlorobutyl and Vistalon EPDM polymers contain only the
minimal unsaturation necessary to permit effective vulcanization by
conventional sulfur-based cure systems.
Compounding for Maximum Chemical Resistance
Selecting the compounding ingredients best suited for specific service
conditions provides compounds with maximum resistance to different
chemical environments. The base polymer, type and amount of fillers,
additives, cure system and state of cure are all factors that have an
effect on the chemical resistance of vulcanizates.
For best results, the compounding ingredients should be chosen for
maximum resistance to the particular chemicals to which the compound
will be exposed.
Polymer Grade
The choice of polymer grade depends on the nature of the chemical
attack that the compound must resist.
Strong acids, certain inorganic salt solutions such as permanganates
and dichromates, and other oxidizing agents attack rubber compounds
by reacting at the double bond sites. ExxonMobil Chemical elastomer
grades with the lowest unsaturation levels offer the best resistance to
materials that attack by chemical reaction. In many cases, completely
saturated materials and low molecular
weight polyethylene can be added to reduce the level of unsaturation.
Oxygenated solvents, vegetable oils and most inorganic salt solutions
do not appreciably attack a polymer through its unsaturation. Instead,
they do their damage primarily through the surface phenomenon of
absorption. To resist this mode of attack, grades which develop a
high cross-link density at optimum cure should be selected.
High cross-link density is an index of a tight elastic network (also
signified by a high modulus) that is resistant to chemical absorption
and associated swelling. ExxonMobil Chemical elastomer grades with
the greatest unsaturation are recommended.
Fillers
The choice of fillers also depends to some extent on the specific
chemical in mind. Reactive fillers like calcium carbonate are not
recommended for acid resistance since the acid would react with the
CaCO3 and thereby degrade the compound. Fillers with high surface
area should not be used with fluids that attack the polymer by absorption
because such fillers tend to imbibe the chemical. Hygroscopic fillers should
be avoided when resistance to water, salt solutions or weak acids and
bases is a requisite. Although the latitude to adjust filler content may be
limited by the physical property requirements of a compound, it is usually
best to keep the filler concentration relatively low for good chemical
resistance.
Plasticizers
As a general rule, plasticizer
content should be relatively low for best
chemical resistance. Many chemicals can extract the plasticizer from a
compound and subsequently be absorbed into the compound. Extractable
plasticizers should not be used unless abolutely essential for processing
the compound.
Vulcanization Systems
For resistance to most chemicals, a sulfur cure is recommended, usually at
levels of 1.5 to 2.0 phr of sulfur. However, specialized acceleration systems
such as low sulfur-high accelerator, sulfur donor, quinoid, peroxide (Vistalon
grades only) and resin cures offer excellent chemical resistance along with
improvements in other properties such as heat stability and compression set.
State of Cure
For maximum resistance to any chemical, compounds should be vulcanized
for the optimum cure time. Either over-curing or undercuring will adversely
affect chemical resistance.
