Thermoplastic Elastomer Extrusion

Thermoplastic elastomers (TPEs) behave like an elastomeric rubber at room temperature while being processed as a thermoplastic polymer. Compared to thermoset rubbers,   TPES are mechanical flexible, light, easy processing, and recycled, which make them a ideal choice for a sustainable circular economy.

Block co-polymers such as SEBS styrene-diene triblock copolymers are the most important category of thermoplastic elastomers.They can be blended with large quantities of additives without a drastic effect on properties. 
In almost all applications, SEBS or other similar styrenic-based polymers is less than 50%. For extremely soft products they are formulated with only SEBS, oil, filler and additives. For harder formulation, polypropylene is added.

Process setup for SEBS-based TPS:
Basic formula: SEBS+pp+oil+filler+additives
The elastomer/polymer/oil are fed into barrel 1,the filler is most time introduced downstream,but can also fed into main feed barrel.

While both TPO and TPV formulations contain many of the same ingredients, TPV describes TPO with a crosslinked rubber component .The most significant difference between the two product types is that TPO is a physical mechanical blend of the mechanical blends of polyolefin and rubber,while TPV is a physical mechanical blend of ingredients overlaid on top of an in-situ elastomer cross-linking reaction process.

So,each of the two  requires a different compounding extruder set up.TPO are  process in a machine configuration similar to that shown  for TPE-S.The obvious difference is the length of the twin-screw compounder.TPO system is shorter with a L/D of approximately 32 vs 60 for the TPV(either greater of less depending on sepefice formulation). The reason for the difference on L/D is the TPV system requires more residence time for crosslinking and disperion of EPDM.

Process setup for EPDM/PP-based TPV:

Basic formula: EPDM+PP+oil+filler+crosslink agent
The PP EPDM and small amout of fillers are fed into first barrel for melt mixing.
The crosslink agent is introduced downstream 

While most other TPE base elastomers are compounded with additional ingredients to provide hardness, softness, stiffness etc, TPU products are customized  during polymerization.A typical chemical reaction starts with a blend of lower viscosity liquids and then through reaction produces a higher viscosity final product.Successful polymerization of TPU places several strict requirements on a twin screw compounding extruder as shown below:

Precise feeding of liquid raw materials
Required residence time of between 1 and 3 min
Narrow residence time distribution to avoid reduced reactions
Good homogenization of the reactants
Removal of  heat of reaction energy through barrel cooling

The first is usually addressed with a full automatic TPU casting machine . Narrow residence time, good homogenization of reactants and removal of heat can be accomplished with screw design and efficient barrel tempering. The more challenging requirement is residence time. A residence time of 1 to 3 minutes can have a significant negative impact on production rates. A typical compounded TPO has a residence time of 10 to 20 seconds. The longer TPV process section approaches a minute residence time. Therefore, a high internal, lower shear design twin-screw is optimal for this polymerization process. 

Typical Process setup for TPU polymerization