Page 30 - Plastics News October 2020

Annealing Tips for Cross linked Polymers: Part 5

As with semi-crystalline thermoplastics, annealing can molding resin.
be used in thermosetting polymers to obtain a level of In this form the material has a low melting or softening
cross linking that may not be possible within the molding temperature and an even lower Tg. However, when this
cycle material is subjected to elevated temperatures, which
Mike Sepe are usually provided by a heated mold, a chemical
Just as annealing is used in semi-crystalline reaction continues the polymerization process,
thermoplastics to perfect the crystalline structure of the increasing the molecular weight of the polymer by
polymer, the same process can be used to obtain a level forming crosslinks between the already formed chains,
of crosslinking in thermosetting polymers that may not as well as extending those chains. This is a very
oversimplified description of polymerization in a
be achievable within the context of the molding cycle.
The property changes that are associated with an thermosetting material.
increased level of crosslinking are in many ways very But of primary concern for this discussion is the fact that
similar to those related to an increased degree of in the process of forming the part we are also creating
crystallinity. the finished material. The properties of the part will
But crystallization and crosslinking, while they respond depend to a significant degree upon the degree of
to the same processing and post-processing influences, crosslinking that is established and this, in turn, is
are fundamentally different processes. Thermoplastics determined by the mold temperature and time that the
have been built to a useful chain length before they part is in the mold. Ideally, the part that emerges from
the mold is composed of a material with a high Tg that is
arrive at the processing plant, and crystallization occurs
spontaneously from the melt as temperature declines. related to the degree of crosslinking.
At some point in the cooling process, we observe a sharp But just as molders may not achieve all of the desired
transition in the material structure that is a function of crystallinity in a semi-crystalline thermoplastic, they
the chemistry of the material and the ambient or applied may also not attain all of the desired crosslinking in a
pressure. thermosetting polymer within the allotted cycle time. In
Once that critical point is attained, the crystallization those cases, annealing is performed to enhance the
process continues, provided that the material remains degree of crosslinking. In the parlance of the industry,
above the glass-transition temperature. That this is often referred to as post-baking. The idea behind
post-baking is to drive the degree of crosslinking to a
temperature (Tg) is essentially a constant for any given
polymer, as long as the molecular weight is high enough higher level without extending the molding cycle time or
to be associated with useful mechanical properties, so resorting to higher mold temperatures. It is particularly
the annealing conditions needed to promote additional useful in polymers such as phenolics and polyimides that
crystallization are predictable. crosslink through a process known as a condensation
mechanism. These types of materials have the ability to
Crosslinked materials arrive at the processing plant as a
undergo additional crosslinking to a significant degree
work in process. The chemistry of the material has been under the influence of the elevated temperature
established through a chemical reaction that has been associated with post-baking.
stopped before polymerization can truly begin, a state
The benefits of post-baking to achieve a higher degree of
often referred to as “prepolymer.” This material is
capable of undergoing further reactions to create the crosslinking in thermosetting polymers are similar to
fully developed polymer. These reactions are promoted those obtained through annealing semi-crystalline
by elevated temperatures and rely on the presence of thermoplastics. Mechanical strength and modulus
reactive groups that are part of the prepolymer, and a increase, and with those changes come improvements in
catalyst. creep and fatigue resistance. Dimensional stability at
elevated temperatures will also be enhanced, while
Phenolic, the first truly synthetic polymer, is a well- ductility will decline. And just as there can be problems
known member of this material family. This material with dimensional changes during annealing of semi-
starts from a reaction of phenol with formaldehyde. As
crystalline thermoplastics, the same issues can occur
the initial stages of the reaction take place, the product with post-baking.
increases in viscosity and may at some point become a
sticky, viscous material that can be useful as an In the case of semi-crystalline thermoplastics, we
adhesive. If the process is continued, the material can touched on the fact that if too little crystallinity was
become a solid with a relatively low melting point. It can obtained during the molding process, the attempt to
make up the difference with annealing can result in
then be pulverized and combined with catalyst and the
appropriate fillers, at which point it has become a unmanageable problems with shrinkage and warpage. In

October 2020 30 Plastics News