Page 31 - Plastics News October 2020

some crosslinked materials, an additional problem that
can arise is blistering of the part. This is caused by
volatile byproducts that are naturally produced during
condensation polymerization reactions. In the case of
post-baking phenolic, the compound that is given off is
ammonia. If the ammonia cannot diffuse rapidly enough
through the wall of the part, it will produce a distortion
in the part.
The time required for post-baking will depend upon the
objective. Unlike the process of annealing semi-
crystalline thermoplastics, one of the important
consequences of post-baking a crosslinked material is an
increase in the Tg. This increase is dependent upon both
time and temperature, and the relationship is non-
linear. So, it is important to understand the material, the
state it is in when it comes out of the mold, and the
performance needed for the application. Another key mold temperature and the Tg of the polymer in the part.
It should be obvious that with a higher mold
difference between annealing in the crystals in a semi-
temperature, there will be less work to do in post-baking
crystalline polymer and increasing the crosslink density to achieve the desired level of performance.
of thermoset polymers is that in semi-crystalline
thermoplastics the annealing temperature must exceed In our next column we will review annealing practices in
the Tg of the polymer. This is not necessarily the case in thermoplastic polyurethanes, where some remarkable
thermosets. A phenolic resin with an as-molded Tg of 175 benefits can be achieved in a relatively short period of
C can be post-baked at 160 C and the Tg will increase. time.

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Figure 1 shows the relationship between time and the with your details in the Directory on a
rise in Tg in a phenolic material, from work done by Ted
Morrison at Plenco. This shows that an increase in Tg of complimentary basis.
approximately 30° C can be attained in about 18 hr of
post-baking. But an additional increase of the same Kindly send your updated contact
magnitude will require 146 hr following the model
established in the graph. A higher post-bake details along with your company logo
temperature can be used, but this risks problems with on publication@aipma.net for printing
blistering and warpage.
in the Directory.
The alternative, as is the case in all of the materials we
have discussed to this point, is to develop more structure
in the part during the molding process by using a higher
mold temperature. Figure 2 shows another result from
Morrison's study that makes the connection between

October 2020 31 Plastics News