Materials
Thermoplastic Elastomers (TPE)
Product Description
Also referred to as Thermoplastic Rubber (TPR), TPEs (thermoplastic elastomers) have the properties and performance of rubber, but are processed like plastic. They are ideal for applications such as window and door seals as well as commercial glazing seals. TPEs provide:
- Excellent weatherability
- Outstanding ozone resistance
- Benefit of being co-extrudable
- UL and NSF approval with certain TPE grades
Thermoplastic Elastomers (TPEs) are a class of polymers that provide the functional performance of thermoset rubbers, but are processed like plastic. TPEs are flexible materials that can be repeatedly stretched to at least twice their original length at room temperature and return to the approximate length of the original shape upon stress release.
Also referred to as Thermoplastic Rubber (TPR), these elastomeric materials can be easily reprocessed and remolded, offering a level of design and fabrication freedom that is not commonly available to normal thermoset rubber compounds.
Thermoplastic Elastomers provide:
- Slip resistance
- Excellent weatherability
- Shock absorption
- Outstanding ozone resistance
- Flexibility
- Soft texture
- Benefit of being co-extruded
- UL and NSF approval with certain TPE grades
Benefits of choosing TPE over thermoset rubber can include:
- Simplified processing — no mixing or vulcanization involved
- Lower part costs through lower density and thinner wall sections
- TPEs are colorable
- Recyclable scraps and parts
Typical TPE products include:
- Rubber Gaskets
- Stoppers
- Rubber Seals
- Housings
- Rollers
- Valves
- Wheels
- Casters
- Bumpers
- Pads
- Strain Reliefs
- Switch Contact Points
- Cosmetic Cases
- Fuel Line Covers
- Bellows
- Cable Jacketing
- Housings
- Wrist Straps
- Shoe Soles/Heels
- Airbag Doors
- Cushions
- Keycaps
- Grips
- Pushbuttons
- Handles
- Knobs
What is TPE Material?
All TPE materials are composed of both crystalline and amorphous domains. These compositions can be either a blend or an alloy of crystalline and amorphous polymers. They can also be block co-polymers containing blocks of crystalline and amorphous domains along the same polymer chain. In this dual domain structure of thermoplastic elastomers, the crystalline domains are typically referred to as the "hard phase" domains, providing the TPEs with their thermoplastic character and acting as the heat-fugitive cross-links.
Contrarily, the amorphous or "soft phase" domains give the TPEs their elastomeric characteristics. Combined, both phases contribute to the overall mechanical and physical properties of a thermoplastic elastomeric compound. However, some key properties can be associated to one phase or the other and can thereby act as a guide in the selection or design of a custom TPE compound.
"Soft Phase" — Elastomeric Properties include:
- Hardness range
- Compression and tension set
- Flexibility
- Lower service temperature limits
"Hard Phase" — plastic properties include:
- Tensile strength
- Tear strength
- Modulus
- Heat resistance
- Processing temperatures
- Continuous use temperature
- Chemical and fluid resistance
- Adhesion to inks and over-molding substrates
Important Considerations in Selecting a TPE Rubber Compound
Chemical or fluid exposure
- In making a good material selection it is important to know what, if any, fluids or chemicals the TPE material may be exposed to in its application
- Various TPE families will exhibit a wide range of chemical and fluid resistances, giving each its own strengths and weaknesses when exposed to acids, bases, greases, organic solvents and other substances
Service Temperature
- The continuous use temperature is defined as the highest temperature at which TPE material will retain its elastomeric characteristics while continuing to function in its application
- Remember — the very thing that allows TPEs to be processed as thermoplastics will also restrict the use of these materials in very high temperature applications
Hardness vs. other property requirements — Don't be fooled
Hardness is defined as a materials' resistance to indenture and is probably the most common property identified with TPE. However, for certain applications, hardness may be the least relevant property that should be considered. Durometer is the international standard for measuring the hardness of rubber, sponge rubber, plastic, and other nonmetallic materials. Many rubbers, soft plastics and TPEs are rated on the Shore A scale. Thermoplastic resins hardness is rated on the Shore D scale.
Other important engineering properties include*:
- Tensile strength
- Elongation at break
- Tensile set
- Tensile modulus
- Flexural modulus
- Compression set
*while it is true that there is a general correlation between these properties and hardness within a given TPE family, this correlation may not hold across different families of TPE rubber.
Example- It is possible for two different TPE materials to have the same hardness while having a completely different flexural modulus.
Tear Strength
- Measured in pounds per linear inch (pli) or Newtons per millimeter (N/mm), tear strength is a measure of a material's ability to resist tearing
- Tear strength in plastics is measured as the force required to tear a specific test specimen divided by the specimen thickness
- This test may also be used to illustrate the anisotropy of a material
Peel Strength
- A measure of how well a TPE material has bonded to a rigid substrate
- Various testing methods are employed by suppliers to determine a specific TPE materials proper peel strength. These tests methods are derived from the accepted thermoset rubber and adhesive test methods
Compression Properties
Important in sealing and gasket applications, two measurements of a TPE materials performance under compressive loads include:
- Compression set — the ability to return to its original shape after being subjective to a compressive load or strain.
- Reported as a percent of compression that is not recovered
- Compressive stress relaxation — the ability of a TPE material to continuously push back against a compressive load over a period of time.
- Reported as either a percent or an absolute level of initial force that is no longer present
Both of the above compression properties result from "cold flow" or plastic deformation of the TPE and are affected by time, temperature and fluid or chemical exposure.
