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Polyurethane Plastic

What is polyurethane and how is it produced?

March 12, 2019

Polyurethane coating protects the Metropol Parasol in Plaza de la Encarnacion
The striking Metropol Parasol uses a polyurethane coating to protect the bonded timber from corrosion.

Polyurethane is a polymer joined by urethane links. These links are formed by reacting a di- or poly-isocyanate with a polyol. Polyurethane is unique in that it is not manufactured like many other plastics. Most polymers, such as polyethylene, are produced in the form of a powder and then molded into a desired form. Contrastingly, polyurethanes are typically formed directly into the final product.

What is polyurethane plastic used for?

Polyurethane is the base component for various commercial applications and building materials. It can also be found in apparel, sporting goods, and traffic management devices.

Surfboards with polyurethane components
Polyurethane plastic is often used as a base component in surfboards and marine equipment.

Plastic parts

Polyurethane plastic can be rigid or flexible. Rigid polyurethane is used in structural parts for industrial and commercial applications. It is also used for surfboard and boat components, as well as electronic instrument bezels. Flexible polyurethane is a common material for creating grips in sporting equipment, such as golf clubs and tennis rackets.

Polyurethane flexible bollards in a row
Flexible bollards are durable and cost-effective—designed for non-abrasive, low-impact traffic management.

Flexible bollards

Polyurethane flexible bollards offer traffic management solutions in parking lots and city streets. Low-impact bollards are durable and cost-effective, used as bike lane delineators or to mark the end of a parking stall. For areas with changing traffic needs, a removable flexible bollard allows for varied configurations.

Woman running on track wearing spandex clothing
Spandex, comprised of at least 85% polyurethane, is prominently used in activewear.

Fabric

Polyurethane can be converted into fibers using a dry spinning technique. These fibers are widely used in fabric for apparel from sportswear to hosiery. Chemically, the fabric, known as Spandex, Lycra, or elastane, is comprised of at least 85% polyurethane.

Polyurethane wheels on shopping carts
Polyurethane's high load-bearing characteristic makes for durable elastomeric wheels.

Wheels

Polyurethane has a high load-bearing capacity and is the base material for elastomeric wheels and tires. These can be found on shopping carts, elevators, roller coasters, and roller skates.

Bowling alley with polyurethane coated floor
Polyurethane-coated floors are protected against abrasion and impact.

Floorings and infrastructure

Polyurethane coatings are ideal for surfaces that require abrasion and corrosion resistance, such as hardwood floors and outdoor infrastructure. Through regular maintenance, these surfaces can be protected from daily wear and tear.

Polyurethane dashboard of a car
Semi-flexible polyurethane foams have firmer properties, ideal for car dashboards and door liners.

Foams and cushions

Flexible foams are soft with high absorption characteristics. Semi-flexible polyurethane foams are firmer and used for car dashboards and door liners. High-resilience foam products are suitable for foam seating, seals, and insulation panels.

Polyurethane sealant used on windows
Polyurethane foam sprays fill up crevices around windows for better insulation.

Adhesives and sealants

Polyurethane is used in high-performance adhesives and surface sealants. These sealants fill gaps while allowing for expansion and contraction in building materials. The elastomeric properties of the sealant allow for 25–50% movement. Foam sprays are also developed from polyurethane and fill up crevices for better building insulation.

Polyurethane properties

The polymer densities of polyurethane can be controlled, resulting in a range of properties. Polyurethane benefits include high elasticity and moldability, making it an ideal base material for many plastic products, as seen above.

POLYURETHANE PROPERTIES
Advantages

High abrasion and solvent resistance

High load bearing capacity and elasticity

High insulation

Good molding quality

Disadvantages

Low thermal capability

Toxic properties (Isocyanates)

Low tolerance to sunlight

Flammable

Polyurethane structure and production

Polyurethane is produced by the exothermic reaction between two molecules, isocyanates and polyols, that create a urethane linkage. It is important to first understand isocyanates and polyols as polyurethane is a result of these two compounds.

1) Isocyanates

Isocyanate has the formula R–N=C=O. It is an organic compound that is reactive, and often contains polymeric materials. When an isocyanate has two isocyanate groups, it is referred to as a di-isocyanate (TDI). TDI is used to react with polyols in the production of polyurethane.

2) Polyols

Polyols can be either polyether or polyester polyols. Polyether polyols are made when epoxides react with an active hydrogen containing compounds. Polycondensation occurs between the multifunctional carboxylic acids and polyhydroxy compounds. Polyols with a higher molecular weight result in flexible polyurethanes. Lower molecular weight polyols produce rigid polyurethanes.

3) Polyurethane

The type of isocyanate and polyol used in polyurethane production influences its properties. Polyols are responsible for controlling the flexibility and elasticity of the polymer. Longer chains and low crosslinking create a stretchy polymer, while shorter chains result in a harder polymer. There are many choices available for isocyanates and polyols, and a variety of processing conditions are used for production.

Additives are also involved in the polyurethane manufacturing process. These chemicals control polyurethane-forming reactions and properties.

ADDITIVES USED IN POLYURETHANE PRODUCTION

Catalysts

Speeds up chemical reaction

Cross-linking agents

Mechanical reinforcement

Flame retardants

Reduces flammability

Plasticizers

Reduces hardness

Fillers

Improves material properties and reduces costs

Blowing agents surfactants

Controls cell structure of foam polyurethane

Smoke suppressants

Reduces the level of smoke when burnt

Pigments

Adds color for aesthetics

Classifications of polyurethane

Polyurethane can be classified into two groups based on their reactions to heat:

Thermoplastic

Thermoplastic polyurethane is versatile in that it can be re-heated and remolded numerous times. When heated, it turns to liquid and can be molded into the desired shape without any changes to the plastic’s properties. Thermoplastic polyurethane is best suited for end-products that require heat tolerances of over 250°F.

Thermoset

Thermoset polyurethane cannot be formed more than once. When thermoset polyurethane hardens, it experiences a chemical reaction, making the change permanent. Thermoset polyurethane is the more durable option for weight-bearing and abrasion resistant products.

Types of polyurethane

Polyurethane can be categorized into four main types: elastomers, coatings, flexible foams, and cross-linked foams. For each of the types, the most suitable class of polyurethane should be chosen—either thermoplastic or thermoset. Depending on the specific design and application, different requirements for heat, weight, and abrasion must be considered.

Polyurethane flexible bollards
The elastomeric properties of flexible bollards allow them to bend under vehicles without losing their form.

Elastomers

Polyurethane elastomers have very high elasticity. Traffic safety devices, such as flexible bollards, benefit from these elastic properties. Designed to bend under vehicles, flexible bollards can quickly return to their original form after being flattened.

Coatings

Polyurethane coatings provide resistance from harsh solvents such as industrial cleaners. They also offer some protection against impact. Polyurethane coatings are ideal for surfaces that need abrasion resistance. Water-based solutions are also available.

Flexible foams

Flexible foams have high impact strength and cushioning. Semi-flexible polyurethane foams are also available as firmer alternatives.

Cross-linked foams

Cross-linked foams offer a more rigid and thicker structure compared to flexible foams.

Polyurethane plastic maintenance

Polyurethane is easy to take care for and low maintenance. It is resistant to solvents, oils, and corrosion—and highly durable against weathering and abrasion. Regular maintenance is as simple as wiping down the surface with a damp cloth. For instances with more surface damage, detergents and cleaning agents may be used. However, it is best to save the harsh cleansers for occasional use as they can affect the gloss level of some polyurethane finishes.

Polyurethane plastic recycling

Polyurethane can be recycled in two ways: mechanical or chemical recycling. Mechanical recycling reuses the material in its polymer form, while chemical recycling takes the material back to its chemical components.

1) Mechanical recycling

Rebond

Polyurethane waste is chopped into small pieces, sprayed with a prepolymer, and bound to the desired grade of foam. A popular method for recycled carpet underlays.

Regrind

Polyurethane industrial trim is grinded into a fine powder. The powder can then be combined with virgin materials to produce new polyurethane foam.

Adhesive pressing

Polyurethane parts are granulated and blended, then put under heat and pressure to form boards or moldings. The resulting particleboards can be used in sound proofing products and furniture applications.

2) Chemical recycling

Glycolysis

Industrial and post-consumer polyurethanes are mixed with diols at high heat. New polyols are created through a chemical reaction, which can then be used to make recycled polyurethane.

Hydrolysis

A reaction between used polyurethanes and water results in polyols that can be used as fuel. The intermediate chemicals can be used as raw materials to create recycled polyurethane.

Pyrolysis

Gas and oil are created by breaking down polyurethanes in an environment that is free of oxygen.

Hydrogenation

Gas and oil are created from used polyurethanes through applying heat, pressure, and oxygen.

The future of polyurethane

Pile of old polyurethane foam
Old polyurethane foam removed during excavation work can be recycled through mechanical or chemical means.

Polyurethane is an impressive resource with its combination of workable characteristics. Polyurethane products also boast variety—ranging from plastics, foams, coatings, to sealants—that benefit a multitude of industries.

Polyurethane products have a cyclical life span as they continue to hold value even after reaching the end of their product life cycle. In 2010, American automobile manufacturer, Chrysler, announced that it would use 180,000 pounds of recycled polyurethane foams for their seats and headrests. As the world goes green, the polyurethane industry follows suit with sustainable practices.

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