VULCANIZING REAGENTS AND ACCELERATORS
A variety of materials are included in natural rubber latex formulations
used in glove manufacturing. The residues from these materials have
become a major concern because of their ability to sensitize users and
elicit Type IV allergic reactions.
The major classes of materials that are used in formulations and are
involved in the sensitization process are the vulcanizing reagents,
accelerators and, to a lesser extent, the antioxidants.
For natural rubber to be transformed into a useful material it must be
vulcanized or cross-linked. Vulcanization is the process by which rubber
is heated in the presence of sulphur or sulphur donors to form a film
which has good physical strength but still remains soft and flexible.
The universal vulcanizing agent for rubber is sulphur but more efficient
materials are sulphur donors such as thiurams. Formulations containing
thiurams allowed manufacturers to produce gloves at higher outputs thus
reducing the overall cost of the gloves. One of the major thiurams used
was tetramethyl thiuram disulphide (TMTD).
TMTD is well known to cause allergic reactions. However, there are a
wide variety of thiurams available to the manufacturer. In the book titled
"Rubber Chemicals" by J. Van Alphen over 12 different thiurams are
listed many of which are suitable for glove manufacture.
Due to differences m chemical structure, not all thiurams have the same
propensity to elicit allergic responses.
The thiurams breakdown during vulcanization and liberate sulphur and
consequently if formulations have been optimized there will be no trace
of the original thiuram in the cured glove but its breakdown products,
such as carbamates and secondary amines will be present.
As manufacturers searched for improvements in processes and the
physical characteristic of rubber products, Efficient Vulcanizing (EV)
systems were developed.
These EV systems for glove manufacturing usually incorporate
dithiocarbamates as accelerators. As their name implies, the dithiocarbamates
boost the cure system and accelerate the vulcanization process.
The most commonly used accelerators are the zinc salts of the
dithiocarbamates, such as ZDEC, ZDMC and ZDBC. Again there are
over 34 different varieties available to manufacture.
The genetic structure for the dithiocarbamates which strangely resembles
thiurams, is shown in the diagram above:
REDUCTION OF RESIDUAL CHEMICALS
Several options are available to the manufacturer to reduce or eliminate
the presence of residual vulcanizing reagents and accelerators.
Reformulation - New formulas could be developed which do not
contain the traditional vulcanizing reagents or accelerators.
One approach still under investigation is the use of radiation-cured
latex. In this process, latex is compounded with organic materials
which become highly reactive when exposed to radiation. The
activated organic compounds readily cross-link with the rubber
latex particles which, after drying, form cured films without the
use of sulphur.
The main drawback to this approach is that the resultant rubber films
are not up to the standard of sulphur-cured films and they still contain
residues of the radiation sensitive organic materials.
Optimization of Cure Systems - As stated previously, the
vulcanizing reagents and accelerators are meant to breakdown during
the curing of the glove.
By optimizing the cure system, the amount of accelerators and
vulcanizing reagents added to the latex can be minimized.
Consequently, all the reagents should be consumed during
vulcanization and only minimal residues will remain.
Obviously it will be necessary to choose reagents whose breakdown
products are low-risk with respect to allergenicity.
Effective Leaching or Washing - From the natural rubber protein
allergy research, it has been shown that if effective leaching and
washing is employed the risk of natural rubber protein allergy can
be significantly reduced.
This is also directly applicable to chemical allergens. If gloves are
extensively washed or leached after the vulcanization process the
residual chemical level can be dramatically reduced.
CONCLUSION
In the future, it is likely that manufacturers may change latex formulas to
utilize single accelators, use less sensitizing chemicals and/or increase
post-process washing of latex products to remove residual chemicals.
However, it is essential that these changes not affect the barrier
properties of gloves.
