Advantages in extrusion: thermoset vs. thermoplastic.

In the complex world of the rubber industry, processors can choose from a wide variety of polymers, and deciding which material is most appropriate for a specific application can be overwhelming. To simplify the decision, elastomeric materials can be classified into one of two broad families--thermoset rubber and thermoplastic rubber. Each family has distinct processing and application advantages, as well as some disadvantages, or at least potential shortcomings.

When deciding which family of materials to use, keep the following factors in mind:

* End use application;

* required physical properties;

* raw material costs; and

* processing and packaging costs.

You will also want to consider the relative availability of various elastomers; your existing processing, testing and manufacturing equipment: raw material storage requirements; and material mixing and processing experience.

In an industry more than 160 years old, numerous products have been developed since Charles Goodyear first discovered vulcanization. Considering and evaluating the issues described above will help you determine which family of materials--the thermosets or the thermoplastics--will best suit your needs.

Advantages of thermoset rubber

On a molecular scale, a thermoset rubber compound is best described as chemically connected "spaghetti strands." Fillers and additives such as carbon black or plasticizers impart unique properties to the compound, such as UV protection, a smooth surface or flame retardancy. Manufacturing a thermoset rubber part usually involves three basic steps:

* Mixing the raw rubber, various fillers, oils and cure chemicals in an internal mixer or on a mill to form the compound;

* shaping the compound into the finished part via molding or extrusion; and

* curing or vulcanizing the finished part and allowing it to cool.

Processors have used a wide variety of both natural and synthetic thermoset rubber, including EPDM, SBR, NBR and polychloroprene in industrial, automotive and commercial applications for ninny years. Thermoset rubber can be used in both injection molding and extrusion processes. Thermoset rubber offers several distinct advantages over its elastomeric counterparts. Because thermoset rubber is more widely used and accepted in the industry, processors tend to have more experience with these materials. Thermoset rubber offers an increased range of physical properties such as hardness, heat resistance and compression set. As a result, these materials can be used in more applications. In addition, no pretreatment o, drying is needed prior to processing.

Advantages of thermoplastic rubber

On the other hand is thermoplastic rubber, which behaves like a rubber material but can be processed on conventional plastic processing equipment.

Thermoplastic rubber is generally a blend between a conventional cured or uncured thermoset rubber such as EPDM and a hard thermoplastic such as polypropylene or polystyrene. Thermoplastic rubber can be classified into three basic groups:

* Styrenic block copolymers (SBS, SEBS, SIS);

* thermoplastic/thermoset rubber blends (TPV, TPO, PVC/nitrile)--the thermoset rubber phase may be fully cured, partially cured or simply mechanically blended; and

* multi-block copolymers, including mechanical blends (polyurethane/rubber blends, nylon/rubber blends, polyester/rubber blends).

Thermoplastic rubber offers unique advantages compared with its thermoset counterparts. Thermoplastic rubber does not need to be compounded or mixed prior to use. In addition, only part or profile cooling is required instead of vulcanization. Because of the continuous manufacturing process, the consistency of the material is improved.

Thermoplastic rubber has a lower specific gravity than most thermoset rubber, yielding more parts per pound. Thermoplastic rubber can be recycled, yet still retain its original properties. By comparison, recycled thermoset rubber can only be used as ground filler in mechanical blends.

As with thermoset rubber, thermoplastic rubber can be processed into finished parts via extrusion and injection molding. Thermoplastic rubber has an additional advantage because it can be blow-molded and thermoformed.

Both families of rubber offer unique features, properties and advantages. Each has its respective applications and places in the marketplace. Choosing the most appropriate product for a particular application requires careful consideration of the performance needs for the end-use, required physical properties, available processing equipment and, last but certainly not least, cost.

Real-world application: Profile extrusion
Processing and equipment differences

Thermoset rubber                       Thermoplastic rubber
Wide variety of applications           More limited applications
                                        but has a lower specific
                                        gravity

No pretreatment or drying              Predrying required to pre-
 before processing                      vent porosity or blisters

Extrude with general                   Extrude with high shear
 purpose screw                          screw

After extrusion, vulcanize             After extrusion, cool part
 and cool part before                   before packaging
 packaging

Limited recycling potential            Recycled material retains
                                        original properties

Scott Hargest is an extrusion specialist with PolyOne's Elastomers and Performance Additives Group.