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- Practical Guide -
Page 37 of 153
structural elements and functional groups as the toxicologically investigated
monomers and, therefore, the toxicological profile is assumed to be the same.
3.1.2 Monomers and other starting substances for thermoset polymers.
Thermoset polymers (often called "resins"): Polymers that will undergo a chemical
reaction through the application of heat and pressure, catalysts, ultraviolet light, etc.,
leading to an infusible state. Typical of the plastics in the thermosetting family are the
aminos (melamine, benzoguanamine and urea), most polyesters, alkyds, epoxies, acrylics,
polyurethanes and phenolics. (Plastics Engineering Handbook of The Society of the
Plastics Industry, Inc., edited by Michael L. Berins, 1991).
Thermoset systems: These are systems based on thermoset polymers which after
polymerisation (crosslinking, drying, curing, hardening) solidify to a three dimensional
crosslinked matrix that cannot be remelted without destroying its original characteristics.
Typical examples are organic coatings, composites, etc.
Figure 1 shows the manufacturing process of a crosslinked thermoset food contact
material e.g. a cured organic coating
+ D (curing agent)
(A+B) -----------------------> (AB)
n
------------------------------>-------(ABD)

PHASE I
PHASE II
PHASE III
Figure 1. Phases of manufacturing of a crosslinked thermoset food contact material.
(A, B) = starting substances for the manufacturing of thermoset polymers;
(AB)
n
= thermoset polymer often called "resin", incorporating a variable number of base units (AB);
(ABD) = final product of the polymerisation process e.g. a crosslindek/cured coating.
The manufacturing of the crosslinked thermoset food contact material can be described as
follows:
Phase I describes the manufacturing process of the thermoset polymers (AB)
n
from the
starting substances A and B. Depending on the number of repeating units in the backbone,
the thermoset polymers (AB)
n
may be oligomeric or polymeric. Phase I also includes the
manufacturing of oligomeric or polymeric curing or crosslinking agents (D) which are
also considered thermoset materials.
The thermoset polymers (AB)
n
as such are not used as food contact materials, they are
merely used as reactive intermediates or prepolymers that require further
polymerisation/curing in a second stage (Phase II) to form the final crosslinked thermoset
food contact material e.g. a cured organic coating (Phase III)
Phase II describes the manufacturing (curing) process of the final crosslinked thermoset
food contact material by reacting the thermoset polymers (AB)
n
with curing/crosslinking
agents (D) or by other means of polymerisation (heat, pressure, UV light, catalyst activated
cure). As mentioned before, the curing/crosslinking agent is also a thermoset material that
can be monomeric, oligomeric or polymeric in nature.