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Being highly crystalline and absolutely linear (60-70%
crystallinity) Biomer polyesters liquify when heated and freeze when
cooled. Crystallization speed is fast between 80°C and 100°C.
Below 60°C or above 130°C the speed of crystallization is
rather slow. The material then remains amorphous and sticky for
hours.
Don't use barrier screws because of
the temperature profile!
The sharp transition fluid/solid can be used to achieve very fast processing speeds. To obtain this it is best to melt the material right behind the filling zone and to lower its temperature towards the die (see temperature profile on the reverse side). The material then has a viscosity similar to PP of a MFI of about 30-40.
As most materials left over in the machine after the last run have high viscosity at 130°C, they will not be displaced by the low viscous PHB. Such materials can be replaced by adding a colored batch of a low melting polymer such as PCL (polycaprolactone) or a high MFI PP.
As all polyesters PHB based resins contain bound water (not only surface bound one!). Therefore it is necessary to dry the pellets. Best results are obtained in dry air dryers: >2 hours at 80°C. Please note that the pellets regain the original humidity within 30 minutes if they are removed from the dryer.
Crystallization speed depends on many (local) factors. We recommend to find the set points by following these steps:
Start conditions:
Cooling time 20 seconds (keep fixed till the end of the optimization).
Temperature profile of 185°C (hopper) to 165°C (die, see temperature profiles below).On large machines start the temperature profile only in zone 2 to not to expose the resins to excess thermal heath.
Set mold temperature to 45°C.
Optimization:
Cool barrel in 5°C steps at the tip (and zones in front of tip accordingly, but keep zone 1 at 185°C) till the form no longer is filled. Increase the temperature by 5°C.
Vary the mold temperature by 5°C up or down so that the melt temperature is cooled to about about 90°C.
Reduce cooling time till the article sticks to die. Increase time in 1 – 2 second steps.
- Barrier screws are unsuitable for PHB since pellets are transported into the zones of lower temperatures where they don’t melt.
-
The material depolymerizes at high
temperatures, therefore
do not exceed 185°C (be aware of the
friction energy induced by backpressure)
flush after a standstill
and restart.
- Degradation temperature: 195°C (inflammable gases).
- Molten PHB sticks to the skin.
- Clean barrel and screw with PP.