Yearbook 2013/2014 - ehedg

Aspects of compounding rubber materials for contact with food and pharmaceuticals 123
Figure 1 assumes two different compound recipes of an
EPDM 70 Sh A material, both of which aim for FDA Aqueous
Food compliance. The “good” compound is of a very good
quality, while the other “cheap” compound is made from
low-cost materials. Several factors can be used to compare
the two recipes in order to determine the differences, and
ultimately, judge the material performance parameters.
For example. in looking at the EPDM polymer, one can see
that this could either be a very pure material with no residues
from the catalysts and no residual monomers (Good) or low
molecular weight oligomers (Cheap). The polymerisation is
very well controlled, giving a uniform molecular architecture
and molecular weight distribution. Also, the batch-to-batch
variation is kept at a minimum. Or it could be the opposite,
which clearly would reduce the cost. Both compounds can
be formulated to meet the same standards, ie. FDA or
BfRFrom an end user point of view, this relates to durability,
compression set, taste and smell, uniformity of the product
and extraction of residues to the product.
The next functional group is carbon black, which acts as a
reinforcing agent. Basically, this is soot, which is produced
by combusting a hydrocarbon source in a controlled
atmosphere. The type and amount is regulated to some
extent. For the good compound as shown in Figure 1,
a carbon black is used for which the hydrocarbon source
is clean and well defined. For the cheap compound, the
hydrocarbon source has a higher content of sulphur and
consists of many different molecules, preventing a uniform
end product. The end user will see a difference in taste and
smell and extraction of residues.
When aiming for a cheap compound, it is common practice
to “dilute” the compound by using chalk. This will increase
the hardness of the material and so it is necessary to add
more plasticiser in order to reach the same hardness. The
usage of chalk will increase swelling in aqueous solutions,
and the chemical resistance will suffer.
Plasticiser is added to these compounds to ensure
homogeneity and to adjust the hardness. For EPDM mineral
oil is used. This can be either a medical grade oil, which
is also used as edible oil and in healthcare products, or a
technical grade oil, which will have a higher content of
naphthenics and aromatics. Again, the user will notice the
difference in the taste and smell, as well as extractables.
Finally, a curing system must be decided upon. This is what
makes the final product elastic. While a thermoplast, which
is uncured, will deform permanently upon load, rubber will
regain the original shape due to the cross-linking of the
polymer chains. For EPDM, two curing systems are normally
used. Peroxide curing gives excellent thermal stability,
compression set, taste and smell and chemical resistance,
but the manufacturing process is more expensive. As
an alternative, a sulphur system may be used. The
manufacturing cost goes down, but so does the performance
as described for the peroxide system.
The example illustrates the complexity of choosing
materials and suppliers. As the figure illustrates, end users
of food-contact rubber materials in food or pharmaceutical
manufacturing lines should specify functional requirements
rather than material, ask for documentation, and choose
a rubber supplier who can successfully translate specific
needs into rubber solutions.