Comparing Thermoplastic Elastomers and Thermoset Rubber

Thermoplastic elastomers (TPE) are an important class of materials and are used in a wide variety of applications. In general, TPEs encompass multiple different types of polymeric material structures, and are noted for being soft and flexible like thermoset rubber, while being melt-processable and -reprocessable like thermoplastics.

Essentially, TPEs are block copolymers or a physical mix of polymers which exhibit simultaneous thermoplastic and elastomeric properties. As a family, TPEs encompass a special group of polymeric materials that undergo a high level of elastic deformation without crosslinking. They exhibit characteristics of both thermoplastics and thermoset rubber simultaneously.

Structurally, TPEs are divided into two distinct categories: blends and block copolymers. Regardless of whether the TPE is a blend or a block copolymer, the polymer system has crystalline and amorphous domains. For blends this is achieved by a mechanical mixture of semi-crystalline and amorphous polymers. Alternately, block copolymers are composed of discrete blocks of crystalline and amorphous domains within the same polymer chain.

Regardless of whether the TPE material is a copolymer or a blend, the hard block will have a melting point, or less commonly a glass transition temperature, well above room temperature. Accordingly, the soft block will have a glass transition temperature, or less frequently a melting point, well below room temperature.

Specific properties can be obtained and tailored by selective combination of the structure and ratios of individual hard phases and soft phases. While both the hard and soft phases contribute to the overall physical and mechanical properties of a TPE, some key properties may be more closely associated with one domain or the other.

In contrast to thermoplastic elastomers, thermoset rubbers are single-phase materials, without the dual hard and soft phases. Rubber materials are natural or synthetic polymer macromolecules, and can be polymerized as homopolymers or random copolymers/terpolymers.

The structure of rubber is amorphous, exclusive of crystalline domains. Because of this, rubber materials undergo a glass transition, but no melting point. By definition, thermoset rubber materials have a glass transition temperature below room temperature. This is in contrast with thermoset plastic materials, which have a glass transition temperature above ambient conditions.

In addition to the intermolecular bonds holding the polymer chains together, similar to that of TPEs, rubber materials have non-reversible covalent chemical bonds binding the individual chains together. These chemical bonds, or crosslinks, are formed during molding through a process known as vulcanization, alternatively known as crosslinking or curing.

The crosslinking process within thermoset rubber is a chemical reaction that takes place at a relatively high temperature during the molding process. The most common crosslinking agents are sulfur, sulfur-containing chemicals, and peroxides.

Both thermoplastic elastomers and thermoset rubber materials get their principal properties from the base polymer. However, both types of materials contain formulation additives that modify and enhance the final properties of the compounds. These additives commonly include reinforcing fillers, non-reinforcing fillers, plasticizers, stabilizers and anti-degradants, process aids, and many types of specific performance enhancers. Thermoset rubber compounds also contain curatives, as well as cure activators and accelerators to enhance the crosslinking process.

Compared with other thermoplastic materials, TPEs offer advantages in properties including:

• softness: low hardness;

• flexibility: low modulus;

• impact strength;

• tear and abrasion resistance;

• fatigue resistance;

• desirable compression set;

• broad use-temperature range;

• chemical resistance;

• low specific gravity.

However, thermoset rubber compounds offer distinct performance advantages over TPEs due to their crosslinked structure. Absolute properties will be highly dependent on the exact compounds being compared.

The advantages of TPEs are predominately in the area of processing, while thermoset rubber typically is superior in regards to performance properties, especially those related to temperature, chemical contact, and sustained stress. In all fairness, the advantages indicated in ease of compounding and ease of molding are based upon one’s viewpoint. For those familiar with working with thermoset rubber compounds, a difference of opinion may be held.

Both types of materials, thermoplastic elastomers and thermoset rubbers, are diverse classes of polymeric materials offering a wide range of properties. A comparison shows that the inherent properties are dependent on the different structures comprising the two sets of materials, as well as formulation additives.

The best material for a particular application will depend on many parameters, including the design of the component and the service conditions. Designers of articles and assemblies should be well familiar with both thermoplastic elastomers and thermoset rubbers in order to choose the most appropriate material to ensure the best chance for product success.