A Review of Sulfur Vulcanization of Rubber-Juniper Publishers
JUNIPER
PUBLISHERS- Academic Journal of Polymer Science
Opinion
Charles Goodyear discovered in 1839 that heating raw
rubber with sulfur modified the rubber to retain its shape. Over the
years, there has been an increasing trend to use more chemical curatives
in rubber compounds. Excessive use of the chemical curatives is harmful
to health, safety and the environment and their use is restricted by
the new European chemicals policy, Registration, Evaluation,
Authorisation and Restriction of Chemicals (REACH) and various
legislations for environment and safety [1,2]. Sulfenamide accelerators
and zinc oxide (ZnO) are used extensively to cure a wide range of
industrial rubber articles. However, these chemicals cause eye, skin and
respiratory irritation in human and are harmful to aquatic life.
Zinc oxide was treated with
N-tert-butyl-2-benzothiazole sulfenamide (TBBS) accelerator in ethyl
acetate solvent to provide a convenient single material component to use
as an additive. The quantity of TBBS required to provide monomolecular
coverage of the zinc oxide was determined to be 35mg/g based on the
approximate surface areas of the TBBS molecule (6x10-19m2) and the ZnO
(50m2) used. Gradually, the amount of TBBS to coat the zinc oxide was
increased from 100mg/g to 350mg/g to find a material with optimum
properties. The material with 35 mg/g TBBS led to a very slow cure, but
material with 350mg/g gave a good cure comparable to much higher
loadings of TBBS. A large batch of the surface modified zinc oxide was
then prepared with this ratio from 202.0g of ZnO and 70.7g of TBBS which
was mixed in 100ml of ethyl acetate in a 500ml beaker. The suspension
stirred magnetically for 15min at room temperature (21.5 °C) to ensure
uniform coating. The mixture was filtered under suction using an
electric diaphragm vacuum
pump (capable of achieving 50mmHg). The white solid was left to dry
overnight and then further dried in a vacuum oven at 50
°C. Evaporation of the filtrate on a rotary evaporator indicated the
mass lost was 0.110g, indicating the bulk of the TBBS was absorbed onto
the ZnO. The surface modified additive will be referred to as the
“powder”. In another experiment, the loading of the powder with 350 mg/g
of TBBS was increased progressively to determine its effect on the cure
properties of the rubber. The raw rubber was mixed with the powder and
sulfur to produce compounds. The cure properties of the rubber compounds
were measured at 160±2°C according to the British Standard ISO 3417.
All the indications are that a much lower consumption of ZnO and TBBS in
sulfur vulcanization, by almost 77wt%, yields significantly shorter
cure cycle and a more efficient cure. For example, the optimum cure time
of the powder-based system was 32% shorter than that of some commercial
sulfur cure systems and its rate of cure higher by 41%. Other benefits
include improvement in health, safety and the environment as well as
major cost reduction. It seems that treating the surface of ZnO with
TBBS to produce a single additive component is a more efficient method
of using these chemicals in rubber vulcanization than the methods which
are currently in use. This has the added advantage of eliminating
secondary accelerators and stearic acid which is used as secondary
activator with ZnO, and minimizing ZnO in the cure system. The sulfur
cure systems currently in use in industrial rubber articles are
inefficient, too costly, damaging to human health and the environment
and no longer viable. A cure system based on a single TBBS/ZnO composite
powder and sulfur is a more efficient way to reduce chemical curatives
and to make green compounds for industrial applications.
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