Polyurethane Elastomers can be many things. They can be strong, stiff, flexible, tough, resilient, durable, long-lasting…and they can also be both viscous and elastic. The technical name for a polymer which possesses both properties, viscosity and elasticity, is an elastic polymer, which is commonly shortened to elastomer.
Polyurethane Elastomers are known for their extreme resilience and have therefore begun to see use in the manufacture of a number of vehicle and machinery parts. In particular, polyurethane elastomers are often used to produce fenders, fascia, trims, interior and exterior vertical panels, doors, chassis fairings, and window surrounds. They are also frequently seen as a component material in tires and wheels. A manufactured part which contains elastomers is described as elastomeric in nature.
The following guide expands on the definition, properties and benefits of elastomers and their myriad uses in the manufacture of injection molded parts for a number of industries.
The Definition of Polyurethane Elastomer
As discussed above, a polymer becomes an elastomer when it possesses elastic characteristics, much like rubber. Elasticity measures the resistance of a substance to distorting forces. It specifically focuses on the substance’s ability to return to its original shape after these forces have been applied. Many materials are considered to be at least somewhat elastic; common examples include spandex, Lycra, nylon and rubber bands.
Because of their elastic properties, most polyurethane elastomers feature long chains of polymers held together by fairly weak inter-molecular forces. The molecular structure of an polyurethane elastomer is frequently compared to “spaghetti” due to the length and intermixture of the chains. These long chains respond well to stress and can easily return to their original form without tearing or breaking.
Thermoset (Solid) and Thermoplastic Elastomers
Like other polymers, elastomers can be divided into thermoplastic and thermoset varieties. Thermoplastic elastomers can be repeatedly melted down and re-molded into a variety of shapes, while thermoset elastomers are cured once into a single shape which they then retain permanently. The molecules within thermosets are irreversibly bonded via cross-links – this means that the thermoset, once solid, will not melt even when exposed to extremely high temperatures.
Thermoset elastomers are typically cured via a process known as vulcanization. Vulcanization solidifies the elastomer via the addition of sulfur, and has been proven to increase the strength and resilience of the resulting material. Curing solid elastomers via vulcanization typically does not involve the use of blowing agents such as water, air, carbon dioxide or nitrogen. Another thermosetting method to create elastomers is mixing isocyanate and polyol together to create polyurethanes with elastic properties. By modifying the makeup of these two chemicals, the properties of the polyurethane elastomer can be tailored to meet the demands of the specific application it is intended for.
A small subset of elastomers exist which are thermoset in nature but cannot be successfully cured via the vulcanization process. These include silicone, ethylene propylene, and most fluoroelastomers, and are collectively referred to as saturated elastomers.
Common Examples of Elastomers
Elastomers occur in the natural world in the form of natural rubber, alternately called latex. Natural rubber is extracted as a thick white liquid from the rubber tree (Hevea brasiliensis) which is native to the Amazon rainforest and is today widely cultivated throughout South America and Asia. This liquid natural rubber can be cured and used in the manufacture of a number of goods, from erasers and rubber bands to mattresses. Natural rubber is typically thermoplastic in nature. Elastomers as a category of materials are sometimes referred to as rubbers, although not all elastomers are made from or contain natural rubber.
Most thermoset elastomers are manmade. Commonly used thermoset elastomers include neoprene, nitrile, and butyl. Elastomers are frequently terpolymers, polymers which are produced via the combination of exactly three monomeric components. Other polymers, such as polyurethane, are not always elastomeric in nature but include some varieties which meet the qualifications to be considered an elastomer. At Romeo RIM, we use a variety of elastomeric thermoset polyurethanes, primarily our Bayflex and Spectrum Solid series, the properties and benefits of which are discussed later in this guide.
The Polyurethane Elastomer Molding Process
Like other thermoset polymers, elastomeric polyurethane is molded into parts via the reaction injection molding (RIM) process. Two liquid components, isocyanate and polyol, are stored in large tanks and transported via high-pressure pumps. The components are mixed inside an impinging mixer, where the chemical reaction of the two, within a mold, cross-link to form an elastomeric solid material.
As with all of our reaction injection molding parts, elastomeric parts utilize lightweight closed molds, usually constructed from aluminum. Heat and low pressure are applied during a short curing process of mere minutes. The elastomeric part emerges from the mold in a fully cured solid and ready for use. Romeo RIM also offers the unique opportunity of in-mold painting (IMP), in which the paint is applied directly to the mold surface, then it chemically bonds and transfers to the part within the molding cycle, eliminating the need for difficult and time-consuming post-mold painting.
Elastomeric polyurethane is also an excellent candidate for reinforced reaction injection molding (RRIM), in which glass, mineral, or carbon fibers are added to the liquid polymer to reinforce the completed parts. RRIM parts are stronger and tougher without needing to sacrifice the resilient elastic qualities for which elastomers are famed.
Benefits of Polyurethane Elastomers
Because of their elastic nature, elastomers produce the most resilient and damage-resistant molded parts. They are extremely resistant to a number of potentially deforming factors including heat, oils, chemicals, abrasion, impacts, extreme weather conditions, and natural wear and tear.
Because of their excellent tensile strength and tear resistance, elastomers are an excellent choice for parts which will frequently experience high speeds, such as the tires and body panels of automobiles and machines. Elastomeric parts will remain strong, functional and even aesthetically pleasing for years and years of sustained use.
In addition, elastomers are highly moldable, stable, and not tacky, allowing for the production of large or complex parts. Elastomeric parts can be highly detailed and can be made to the most particular of customer specifications.
Polyurethane elastomers are also environmentally friendly and do not present health or safety risks to those who come in contact with them. Elastomeric parts are halogen free, nontoxic, and do not emit smoke, gases or volatile organic compounds (VOCs) which can both harm the atmosphere and cause respiratory damage to people who inhale them.
Romeo RIM is proud to offer four distinct series of high-quality elastomeric polyurethanes, each with a number of unique advantages. The Bayflex 110 series boasts one of the shortest cure times of any of our products, at an amazing 30 seconds. Meanwhile, the Bayflex XGT series is strong, sturdy and perfect for the manufacture of large parts.
Contact Romeo RIM today to learn more about manufacturing parts with our excellent line of elastomeric polyurethanes.