How to pick proper model vulcanization systems
By Scott Solis, Brendan Rodgers,
Bharat B. Sharma, Nitin Tambe
and Walter H. Waddell
ExxonMobil Chemical Co.
(Second of two parts)
Vulcanization of butyl rubber
The major applications utilizing butyl
rubber are tire innertubes, curing bladders, pharmaceutical closures and engineered products such as mounts, sheeting, automotive flaps and damping pads.
These applications require good compound aging resistance, oxidative resistance and heat stability. Depending on
the application of the final product, one
of the following three cure systems have
been used11, 12.
● Quinone dioxime
● Resin cure
● Thiuram (i.e. TMTD) accelerated
Quinone cure systems
The crosslinking of butyl rubber with
para-quinone dioxime (QDO) or para-quinone dioxime dibenzoate (DBQDO)
proceeds through an oxidation step that
forms the active crosslinking agent para-dinitrosobenzene. The use of metal oxides
(such as PbO2, Pb3O4, MnO2) or MBTS
as an oxidizing agent, increases the vulcanization rate allowing use in room temperature cures such as in cements3.
Fig. 9 is a schematic illustrating the
The vulcanization of isobutylene-co-isoprene rubber (IIR), brominated
isobutylene-co-isoprene rubber (BIIR), chlorinated isobutylene-co-isoprene
rubber (CIIR) and brominated isobutylene-co-para-methylstyrene elastomer (BIMSM) differs from that of general-purpose rubbers.
Butyl rubber has about 2 percent unsaturation in the backbone. Halobutyl
rubber (BIIR and CIIR) incorporates the butyl backbone with either bromine or
chlorine, which significantly increases the chemical reactivity of the isoprenyl
units located in the butyl backbone.
In BIMSM the bromine atom is bonded to the para-methylstyrene group, affording the completely saturated polymer backbone a site of chemical reactivity.
These properties are excellent vapor impermeation, resistance to heat degradation and improved chemical resistance as compared to general-purpose rubbers. However, this low amount of reactivity requires special consideration to
vulcanize these isobutylene-based polymers.
The type of vulcanization system selected is a function of the composite
structure in which it is used and the cured product performance requirements.
Therefore, vulcanization systems vary and may include an accelerator package along with resins, zinc oxide, zinc oxide and sulfur, and quinoid systems.
This review will discuss the types and selection of appropriate vulcanization
systems for isobutylene-based elastomers.
Equation 2. Formation of zinc salts of stearic acid.
function of the reactivity of the phenol-methylol groups in the reactive phenol
Curing resins such as a heat reactive
octylphenol formaldehyde resin, which
contains methylol groups, can be used3.
Table IV contains a model formulation
illustrating the resin cure system. For a
faster cure, addition of a halogen donor
such as polychloroprene or stannous
chloride is required13. In this instance,
polychloroprene has not been considered as part of the total compound polymer content.
A more reactive resin cure system not
requiring an activator is obtained when a
portion of the methylol groups are replaced by bromine. An example of this
resin is brominated octylphenol formaldehyde resin3.
Fig. 10 is a schematic illustrating the
reaction of a curing resin (heat reactive
octylphenol formaldehyde) in the vulcanization of butyl rubber. Following
the elimination of water in the reaction
sequence, the exomethylene group and
carbonyl oxygen react with an isoprenyl
unit in butyl rubber to form a chroman
ring. Chromonone ring structures are
very stable and are frequently found in
See Systems, page 15
Equation 3. Increase in concentrations of Zn(OH)Br, Zn(Br)(St) and ZnBr2.
Edited by Harold Herzlich
reaction of a dioxime in the vulcanization
of butyl rubber. The dioxime vulcanization system can be used for dry rubber
applications. One example is electrical
insulation systems, which contain butyl
rubber for improved ozone resistance.
Resin cure systems
Phenol formaldehyde curing resins
are classified as resols, i.e. three-dimensional resin systems forming crosslink
networks which can serve as reinforcing
resins versus novolac resins, which are
linear. Resin curing of butyl rubber is a
Equation 4. Hydrogen bromide reacts with zinc oxide to form Lewis acids.
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