Polyurethane: What It Is, History, and How It’s Made

Polyurethane is a super-versatile polymer with a wide range of applications, appreciated for its durability, flexibility, and toughness. The manufacturing process is based on the cross-linking reaction of polyols (polyester or polyether) with diisocyanates (toluene diisocyanate or methylene diphenyl diisocyanate). Polyurethane's relatively easy synthesis and huge morphological versatility have led to its widespread use across industries ranging from construction and automotive to furniture and footwear. It is one of the most important and ubiquitous polymers in modern manufacturing, construction, and decorative/protective roles.

This article will discuss what polyurethane is, including its history, and how it’s made.

What Is Polyurethane?

Polyurethane is a typical thermoset polymer compound characterized by durability, flexibility, toughness, and resistance to abrasion, acids/alkalis, solvents, and weathering. It is formed through a covalent polymerization reaction between a polyol group and an isocyanate group. This reaction creates a varied range of crystalline structures and properties across the family of polyurethane materials. Most commonly it is encountered as rigid and flexible foams, hard and soft elastomers, adhesives, coatings, and sealants. Polyurethane is exploited across most industries and sectors of activity, from construction and large-scale manufacture to furniture and consumer electronics, with all forms finding applications across most sectors. Its versatility allows for customized tuning of properties such as: hardness, elasticity, and thermal resilience. This makes it applicable to a huge diversity of products and roles.

When Was Polyurethane Invented?

Polyurethane was derived in the late 1930s by Otto Bayer and a research group at IG Farben in Leverkusen, Germany. Like many at the time, this research team focused on research into new materials for textiles. However, polyurethane’s diversity of potential applications rapidly expanded as its properties were further explored.

By the 1940s, polyurethane foam was being used in aircraft seating. Its usage continued to grow rapidly in the subsequent decades across various industries due to its versatility and beneficial properties.

How Is Polyurethane Made?

The manufacture of polyurethane constituents involves the production of the raw materials for polyurethane, polyols, and diisocyanates. These are synthesized through chemical processes whereby epoxides are reacted with components that have an excess of hydrogen atoms. This is also commonly achieved by an alcohol reaction with an organic compound containing multiple hydroxyl groups.

Diisocyanates are synthesized from organic compounds containing two isocyanate groups. On a mass-production scale, various methods are used including the phosgenation of petroleum-sourced diamines. Once the raw materials are prepared, they are stirred together in an appropriate ratio to form the liquid polyurethane precursor. This mixture is then subjected to a polymerization reaction, often catalyzed by additional compounds or initiators and often exploiting foaming agents, to form the final polyurethane product.

The manufacturing process will vary depending on the specific details of the intended resultant polyurethane being produced, such as: flexible foam, rigid foam, thermoset elastomers, or thermoplastic polyurethane (TPU). Each type requires different formulations and processing conditions to achieve the desired properties.

What Are the Properties of Polyurethane?

Table 1 below highlights the physical properties of polyurethane:

Table 2 below highlights the chemical properties of polyurethane:

What Are the Uses of Polyurethane?

Polyurethanes are intrinsic and preferred constituents in various industries because of their versatility and highly tunable properties. Some of their uses include:

1. Foam insulation is used in: panelized building insulation foam, spray foam insulation, and rigid foam boards.
2. Sealants and adhesives for construction, automotive, and industrial applications, possessing strong bonding capabilities.
3. Coatings and paints, varnishes, etc. for surfaces requiring durability and resistance to abrasion, chemicals, and weathering.
4. Flexible and rigid foams for mattress and furniture cushions, automotive seats, packaging, and buoyancy aids, benefiting from light weight and cushioning properties.
5. Elastomers in wheels, rollers, gaskets, seals, and bushings in various industrial applications due to excellent resilience and abrasion resistance.
6. Footwear, in the production of shoe soles and insoles due to their cushioning, shock absorption, and durability.
7. Automotive parts in car interiors, bumpers, and exterior panels because of light weight, impact resistance, and durability.
8. Marine applications in boat hulls, buoys, and marine coatings due to their resistance to water, chlorides, and UV radiation.
9. Textiles and apparel in synthetic leather, coated fabrics, and waterproof garments requiring durability and water resistance.
10. Medical devices such as: prostheses, catheters, and medical tubing due to great biocompatibility and flexibility.

What Manufacturing Methods Are Best To Use With Polyurethane?

Polyurethanes can be integrated into products using many methods, with a range of processing approaches better suited to particular forms and applications. These manufacturing methods include:

1. Reaction Injection Molding (RIM): RIM involves mixing precursor components under high pressure and injecting them into a mold in which they react to reflect the cavity shape faithfully. RIM is suitable for producing large, complex parts with tight tolerances like: automotive body panels, bumpers, and structural components.
2. Casting: Casting involves pouring premixed liquid polyurethane resin into a mold and allowing it to cure to form the desired shape. This is suitable for producing prototypes, small batches, and parts with detailed features, such as: decorative elements, figurines, and complex prototype components.
3. Compression Molding: Compression molding involves placing preheated polyurethane material into a mold cavity and compressing it under high pressure and temperature to induce rapid cure. Compression molding is suitable for producing parts with moderate complexity and is commonly used for manufacturing: gaskets, seals, and relatively simply shaped components.
4. Extrusion: Extrusion involves forcing pre-mixed polyurethane material through a die to create continuous shapes with a consistent cross section. Heat in the extrusion tool accelerates cure, so the material is partially cured and shape-resilient at the exit. Extrusion is commonly used for producing continuous profiles for: tubing, hoses, and seals with uniform dimensions and long lengths.
5. Spray Coating and Foaming: This involves spraying polyurethane coatings or foams onto surfaces to provide protection, insulation, or decorative finishes. This method is widely used in construction, automotive, and furniture manufacturing for surface protection and bonded, conforming insulation.
6. Injection Molding: Injection molding involves pumping molten thermoplastic (TPU) OR premixed thermoset polyurethane material into a mold cavity under high pressure and allowing it to cool and solidify (thermoplastic) or rapidly cure (thermoset). Injection molding is eminently suitable for producing high-volume, precision parts with complex geometries. 

What Are the Advantages of Polyurethane?

Polyurethane offers many flexible and selectable/tunable advantages that make it a versatile material. Here are some of those advantages:

1. Polyurethane can be tuned to deliver varied requirements: flexibility, hardness, resilience, toughness, and chemical resistance.
2. These materials exhibit one of the highest durability and resistance to abrasion of all elastomers. Resistance to tearing and weathering facilitates long-lasting performance in demanding environments.
3. They can be tailored to achieve a broad hardness spectrum, from gels (25 Shore OO) to bowling balls (75 Shore D).
4. Hard polyurethanes can withstand high loads and pressures without deforming. This makes them suitable for load-bearing components in various industries, such as: automotive, construction, and mining. In softer forms, they can withstand huge distortion under load, without permanent deformation.
5. They exhibit excellent resistance to oils, solvents, greases, and many aggressive chemicals, making them suitable for harsh environments.
6. Polyurethanes are relatively low density compared to metals and many polymers. This contributes to weight savings in automotive components and aerospace structures. Their solid densities are further reduced by foaming while retaining great rigidity.
7. They can be processed using various methods, facilitating options for the efficient production of complex shapes and parts.
8. Polyurethanes offer cost-effective solutions for diverse applications compared to alternative materials.

What Are the Disadvantages of Polyurethane?

While polyurethanes offer significant advantages, some disadvantages must be considered, such as:

1. Polyurethane materials can degrade when exposed to prolonged sunlight or UV. This leads to discoloration, reduced mechanical properties, and curtailed life span in outdoor applications. The use of antioxidants and stabilizers can counteract this to a large degree.
2. They may soften or become brittle at elevated temperatures, limiting their utility in some applications.
3. Some formulations demonstrate limited resistance to hydrolysis. This results in material degradation when exposed to water, particularly in humid or warm/wet environments.
4. A limited number of solvents and fuels may react with polyurethane materials, leading to degradation or damage to the material.
5. Typical polyurethane formulations contain isocyanates and other components that pose health and environmental risks during manufacturing, use, and disposal.
6. Polyurethane materials often have a higher initial cost compared to some materials, such as PVC or light metals.
7. Recycling polyurethane materials is extremely challenging due to their complex chemical composition. Hence, most recycling streams do not include them in their processing. 
8. Some polyurethane products, particularly foams and coatings, release volatile organic compounds (VOCs) and other toxic chemicals during production or use.

Is Polyurethane Safe To Use?

Yes, polyurethane is considered safe to use when handled and processed according to established safety guidelines.

Those working with polyurethanes in the precursor or curing state must systematically wear appropriate personal protective equipment (PPE), to prevent skin contact and inhalation of fumes or dust. Ventilation must be provided in areas in which polyurethanes are used or processed to minimize personnel exposure to airborne contaminants. Polyurethane precursor components — isocyanates and polyols — must be stored in appropriate sealed containers in well-ventilated areas away from heat and direct sunlight. Unused or waste materials must be disposed of according to local regulations and best practices.

Manufacturers of polyurethane products provide Material Safety Data Sheets (MSDS) containing critical safety information. It's essential to review and understand these before use. Workers handling polyurethane materials should receive appropriate training on safe handling practices, emergency procedures, and the potential hazards associated with these materials. 

What Are the Effects of Polyurethane on the Environment?

Polyurethane can have varied and significant environmental impacts in its production, use, and disposal. The production of polyurethane involves the use of petroleum-based chemicals and energy-intensive processes, contributing to resource depletion and CO2 emissions. Polyurethane production typically releases VOCs, including irritant/toxic isocyanates and other hazardous chemicals, during manufacturing. Polyurethane products can contribute to solid waste generation at the end of their life cycle. They persist in the environment for long periods without biodegrading. They also release various toxins when incinerated at normal disposal temperatures.

Polyurethane vs. Epoxy

Polyurethanes and epoxies are divergent but versatile polymers used in a wide spectrum of applications with distinct characteristics and properties. However, there are few real overlaps in application, other than in coatings.

Polyurethanes are polymers composed of organic blocks tied together by urethane links, formed through the reaction of isocyanates and polyols. Epoxy resins are thermosetting polymers formed through the reaction of epoxide groups (resin) with curing agents (hardener or catalyst).

Polyurethane exhibits flexibility, impact resistance, abrasion resistance, and good chemical resilience. Epoxy offers high strength, excellent adhesion, chemical resistance, and dimensional stability.

Common applications of polyurethane include: coatings, adhesives, sealants, elastomers, foams, and flexible parts like gaskets and seals. Epoxy is widely used in coatings, adhesives, laminates, composites, electronic encapsulation, and structural applications like: flooring, marine coatings, and aerospace components.

Polyurethanes are flexible and resilient, making them suitable for applications requiring impact resistance and toughness. Epoxy offers high strength and great rigidity, making it ideal for applications in which structural integrity and dimensional stability are crucial.

Summary

This article presented polyurethane, explained it, and discussed its history and how its made. To learn more about polyurethane, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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