A Bird's Eye View of Elastomers
Did you know that Christopher Columbus was probably the first European to see natural rubber? He found the natives of the Amazon Valley playing with bouncing balls during his second voyage in the 1490’s. It wasn’t until 1770 that John Priestly coined the term “rubber”.
An “elastomer” can be defined as a polymer with the property of elasticity. It’s a polymer that deforms under stress and returns to its original shape when the stress is removed. Elastomers are amorphous polymers with tremendous segmental motion. Their molecular form is compared often to a spaghetti and meatball like structure. The meatballs signify a cross-link between polymer chains. The polymer chains are made up of many monomer subunits which can consist of carbon, hydrogen, and oxygen atoms.
Elastomers are not just natural latex but includes other polymers such as SBR (Styrene-butadiene rubber), an all-purpose rubber composed from a copolymer of styrene and butadiene. SBR has a similar property make-up as natural rubber and is the highest volume elastomer available. SBR is also tough and resistant to heat and flex cracking. CR (Neoprene) is another example and it is an excellent all-purpose elastomer that is flame resistant with moderate resistance to oil and gasoline. Neoprene can be classified into two groups sulfur-modified and mercaptan-modified. Sulfur-modified neoprene has high tear strength and resilience while mercaptan-modified neoprene has a high resistance to heat and compression.
While natural rubber is still the most commonly used elastomer, the reality is that raw latex, from the Hevea brasiliensis tree, is of little use in its occurring form.
In 1844 Charles Goodyear patented the process of vulcanization and named it after Vulcan, the Roman god of fire. Vulcanization is a chemical process for converting natural rubber or related polymers into more durable materials with the addition of sulfur or other curatives and accelerators. They modify the polymer by forming cross-links between individual polymer chains. Once vulcanized, depending on temperature, the rubber changes its properties. A number of changes can happen after vulcanization including the following:
- Reduction of tackiness
- Increase in tensile strength
- Decrease in solubility in solvents
- Decrease in cold flow and plasticity
- Increase in elasticity
- Decrease in temperature sensitivity
All of these changes in properties happen in elastomers through the modern use of vulcanization.
Here are the different types of polymers:
- Commodity Thermoplastics
- Engineering Thermoplastics
- High-temperature resistant Thermoplastics
- Thermoplastic Elastomers
Thermoset or thermosetting polymer is a pre-polymer material that cures irreversibly. The cure may be induced by heat generally above 200 degrees Celsius through a chemical reaction, or sustainable irradiation. Natural rubber has many positive advantages, such as, high resilience and tensile strength. It also has excellent resistance to tear abrasion and rebound elasticity. But it also has poor resistance to heat and sunlight. Natural rubber has little resistance to oil, gasoline, and hydrocarbon solvents. Synthetic polyisoprene also has high resilience and resistance to tear and abrasion. It has great rebound elasticity and good flexibility. Because the chemical make-up of synthetic polyisoprene and natural rubber is so close they have very closely related positives and negatives.
Thermoplastic elastomer (TPE) materials combine the functional performance and properties of thermoset rubbers with the process ability of thermoplastics. TPEs permit fabrication of “rubber-like” articles with the speed, efficiency, and economy of plastic manufacturing. Thermoplastic elastomers are generally low modulus, flexible materials that can be stretched to more than twice their original length and will be returned to approximate original length when the tension is released. Unlike thermosets, thermoplastics can be re-melted and re-molded.
Commodity thermoplastics are a common name for the least expensive thermoplastics that make up the majority of the total plastic production, such as polypropylene and certain grades of polyethylene. Polyethylene is inert, extremely resistant to water, food, and other water based liquids. Polyethylene is cheap and easy to mold and fabricate. It can also be colored, as well as, transparent, translucent or opaque. It can also be textured and metal coated. Polyethylene and polypropylene are used to manufacture products like, vacuum-formed food packaging, disposable drinking cups, window frames, and wire insulation.
Engineering thermoplastics are a subset of plastics that are used in applications generally requiring higher performance in areas of heat resistance, chemical resistance, impact, fire retardant, and mechanical strength. Engineering Thermoplastics are often compared to commodity thermoplastics, but are much more expensive and manufactured at a lower rate. These thermoplastics are used to make car bumpers, dashboards, and motorcycle helmets.
High-temperature thermoplastics retain their physical properties at higher temperatures and exhibit thermal stability even in the longer run. These thermoplastics have higher heat deflection temperatures and continuous use temperature. High-temperature thermoplastics can be used for a diverse set of industries like electrical, medical devices, automotive, telecommunications, aerospace, and many other specialized applications.