Polyurethane foam

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An assortment of polyurethane foam products for cushioning and insulation Polyurethane Products.png
An assortment of polyurethane foam products for cushioning and insulation

Polyurethane foam is a solid polymeric foam based on polyurethane chemistry. As a specialist synthetic material with highly diverse applications, polyurethane foams are primarily used for thermal insulation and as a cushioning material in mattresses, upholstered furniture or as seating in vehicles. Its low density and thermal conductivity combined with its mechanical properties make them excellent thermal and sound insulators, as well as structural and comfort materials.

Contents

Polyurethane foams are thermosetting polymers. They cannot be melted and reshaped after initially formed, because the chemical bonds between the molecules in the material are very strong and are not broken down by heating. Once cured and cooled, the material maintains its shape and properties. [1]

Classification of polyurethane foams

Polyurethane foams are the most widely used representatives of thermoset foams. Depending on their cellular structure, they can be classified as open or closed-cell foams. Looking at mechanical properties, there are two main types of polyurethane foam; flexible (soft) and rigid (hard) foams. [2] Generally speaking, flexible polyurethane foams have an open-cell structure where the pores are interconnected, smaller in size and irregularly shaped; contrary to rigid polyurethane foams that have a closed-cell structure, where the pores are not interconnected. [3] The market share between these two types is largely equal. [4]

There are various processing technologies in the production of polyurethane foams. Depending on the properties of the end application, the two most often used at large scale production are moulding and slabstock (block) foaming. [5] Next to these, other prominent types include cavity-filling foam (e.g. car fillings used for acoustic insulation); and spray foam (e.g. roof thermal insulation). These are known as semi-flexible foams behind appropriate overlays. [6]

Flexible polyurethane foam

The flexible polyurethane foam (FPUF) is produced from the reaction of polyols and isocyanates, a process pioneered in 1937. [7]  Depending on the application the foam will be used for, a series of additives are necessary to produce high-quality PU foam products. FPUF is a versatile material that can be tailored to exhibit different properties. It allows for superior compression, load-bearing and resilience that provides a cushioning effect. Because of this property, lightweightness, and efficient production process, it is often used in furniture, bedding, automotive seating, athletic equipment, packaging, footwear, and carpets. [7]

Flexible polyurethane foams with a high volume of open pores have been greatly regarded as an effective noise absorption material and are widely used as acoustic insulation in various sectors, from construction to transportation. [8] It is also a very resilient material that does not deteriorate over time and its lifetime is typically linked to the lifetime of the application it is used in. [9]

Types of Flexible Polyurethane Foams based on Manufacturing Technology

Flexible polyurethane foams can be manufactured through a continuous (slabstock) production or moulding process. In the continuous process, the mixed ingredients are poured on the conveyor belt. The chemical reaction occurs instantly, causing the foam to rise within seconds and then solidify. In theory, foam blocks of several kilometres in length could be produced this way. In reality, the foam blocks are typically cut at a length of between 15 and 120m, cured and stored for further processing. [10]

Contrary to slabstock foam, moulded foam production is a discontinuous process. Moulded foam articles are made one at a time by injecting the foam mixture into moulds. When the foam rises and expands, it occupies the whole space in the mould. It solidifies almost instantly and the produced part can then be removed from the mould, either mechanically or manually. [11] This is the biggest advantage of moulded PU foams –they can be moulded into specific desired shapes, eliminating the need for cutting and reducing waste fractions. They can be produced with multiple zones of hardness and with reinforcements for further easier assembly. [12] This is why moulded foam technology is widely used in the production of seat cushions used in the transport industries.

Based on the production process, other types of flexible polyurethane foams may include rebonded (or recycled), reticulated and auxetic PU foams.

Sustainability of Flexible Polyurethane Foams

Since the invention of polyurethane chemistry there have been constant innovations in the industry, driven by the need to decrease the toxicity of chemical substances used in production processes. Some examples include reducing Volatile Organic Compounds emissions or using blowing agents with a lower global warming potential (GWP) as well as ozone-depleting potential (ODP). [13]

In the last decades, the main focus of the FPUF industry has been improving the environmental impact of its products and processes. A cradle-to-gate analysis of flexible (TDI slabstock) PU foam shows that (by far) the largest effect on the life cycle of the PU foams is due to raw materials extraction and production. Depending on the parameters, these account for about 90% of the total Greenhouse Gas (GHG) emissions. [14]

Traditionally nearly all raw materials used for flexible PU foam production have been of fossil origin. Today, it is possible to make flexible PU foams from alternative, non-fossil sources, thus significantly improving its environmental footprint. [15] These include bio-polyols, recycled polyols and CO2-based polyols.

As a thermosetting polymer PU foam cannot just be melted at the end of its useful life to make new products. For PU foam-containing products, there are various recycling technologies available and in broad use today:

  1. Physical (or mechanical) recycling. Physical recycling changes the physical properties of the material to a form more suitable for further processing. [16] With this recycling process, the chemical composition of the PU material is not changed. [6] The most common method is called rebonding, in which the flexible PU foam (production cut-offs and end-of-life foam) is transformed into so-called trim (foam flocks), which in turn can become rebonded foam used in products such as: e.g. carpet underlay, gym mats, acoustic insulation, as well as mattresses and furniture cushioning. Other types of mechanical recycling of PU foam include regrinding (powdering), compression moulding and adhesive pressing of powdered PU waste. [6] For example, regrinding includes shredding PU material into a fine powder and mixing it as filler with a polyol component to make new PU foams [17]
  2. Chemical recycling (or depolymerisation). Chemical recycling methods are focused on recovering monomers, which can be used to synthesise new polymers. [16] The chemical composition of the waste PU foam is changed by breaking down and reforming the targeted bonds, to recover the original raw materials. [6] Flexible polyurethane foam is broken down into its specific constituent chemical raw materials, which can be used again to make fresh foam. Technology has been in use at an industrial scale (in Europe) since 2013 for post-industrial flexible PU foam. [18] Differentiated by the base material used to dissolve PU foam, depolymerisation technologies include hydrolysis, aminolysis, alcoholysis and glycolysis. [6]
  3. Feedstock (or thermo-chemical) recovery. Feedstock recycling includes thermal processing of (often) mixed waste materials (of which PU can be one constituent), disintegrating them at a molecular level and recovering synthesis (syngas) and fuel gas - products which can be further used as new raw materials for the petrochemical industry. [19] Mass balance accounting is needed to account for recycled materials. [20]   For applications that are too difficult to dismantle or too contaminated to recycle, thermo-chemical recycling is the best option. This technology in particular allows the production of new “virgin-equivalent” raw materials, which are specifically appropriate for the production of applications that need to comply with stringent requirements, e.g. in the automotive industry. [21]

Globally today the most often used waste management methods are landfilling and energy recovery. These should only be used when recycling methods are not available or cost-effective. Energy recovery processes include combustion, incineration and thermal degradation of PU. [16]

Rigid polyurethane foams

Rigid polyurethane foam has many desirable properties which has enabled increased use in various applications, some of which are quite demanding. [22] [23] These properties include low thermal conduction making it useful as an insulator. It also has low density compared to metals and other materials and also good dimensional stability. [24] A metal will expand on heating whereas rigid PU foam does not. They have excellent strength to weight ratios. [25] Like many applications, there has been a trend to make rigid PU foam from renewable raw materials in place of the usual polyols. [26] [27] [28]

They are used in vehicles, planes and buildings in structural applications. [29] They have also been used in fire-retardant applications. [30]

Space shuttles

Polyurethane foam has been widely used to insulate fuel tanks on Space Shuttles. However, it requires a perfect application, as any air pocket, dirt or an uncovered tiny spot can knock it off due to extreme conditions of liftoff. [31] Those conditions include violent vibrations, air friction and abrupt changes in temperature and pressure. For a perfect application of the foam there have been two obstacles: limitations related to wearing protective suits and masks by workers and inability to test for cracks before launch, such testing is done only by naked eye. [31] The loss of foam caused the Space Shuttle Columbia disaster. According to the Columbia accident report, NASA officials found foam loss in over 80% of the 79 missions for which they have pictures. [31]

By 2009 researchers created a superior polyimide foam to insulate the reusable cryogenic propellant tanks of Space Shuttles. [32]

Related Research Articles

<span class="mw-page-title-main">Polyurethane</span> Polymer composed of a chain of organic units joined by carbamate (urethane) links

Polyurethane refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term does not refer to the single type of polymer but a group of polymers. Unlike polyethylene and polystyrene polyurethanes can be produced from a wide range of starting materials resulting various polymers within the same group. This chemical variety produces polyurethanes with different chemical structures leading to many different applications. These include rigid and flexible foams, and coatings, adhesives, electrical potting compounds, and fibers such as spandex and polyurethane laminate (PUL). Foams are the largest application accounting for 67% of all polyurethane produced in 2016.

<span class="mw-page-title-main">Polystyrene</span> Polymer resin widely used in packaging

Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a poor barrier to air and water vapor and has a relatively low melting point. Polystyrene is one of the most widely used plastics, with the scale of its production being several million tonnes per year. Polystyrene is naturally transparent, but can be colored with colorants. Uses include protective packaging, containers, lids, bottles, trays, tumblers, disposable cutlery, in the making of models, and as an alternative material for phonograph records.

<span class="mw-page-title-main">Epoxy</span> Type of material

Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also collectively called epoxy. The IUPAC name for an epoxide group is an oxirane.

<span class="mw-page-title-main">Thermosetting polymer</span> Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

<span class="mw-page-title-main">PET bottle recycling</span> Recycling of bottles made of polyethylene terephthalate

Although PET is used in several applications, as of 2022 only bottles are collected at a substantial scale. The main motivations have been either cost reduction or recycle content of retail goods. An increasing amount is recycled back into bottles, the rest goes into fibres, film, thermoformed packaging and strapping. After sorting, cleaning and grinding, 'bottle flake' is obtained, which is then processed by either:

<span class="mw-page-title-main">Memory foam</span> Component primarily utilized for making cushions or mattresses

Memory foam consists mainly of polyurethane with additional chemicals that increase its viscosity and density. It is often referred to as "viscoelastic" polyurethane foam, or low-resilience polyurethane foam (LRPu). The foam bubbles or ‘cells’ are open, effectively creating a matrix through which air can move. Higher-density memory foam softens in reaction to body heat, allowing it to mold to a warm body in a few minutes. Newer foams may recover their original shape more quickly.

In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups. The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, three and four hydroxyl groups are diols, triols, and tetrols, respectively.

A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More specialized polyolefins include polyisobutylene and polymethylpentene. They are all colorless or white oils or solids. Many copolymers are known, such as polybutene, which derives from a mixture of different butene isomers. The name of each polyolefin indicates the olefin from which it is prepared; for example, polyethylene is derived from ethylene, and polymethylpentene is derived from 4-methyl-1-pentene. Polyolefins are not olefins themselves because the double bond of each olefin monomer is opened in order to form the polymer. Monomers having more than one double bond such as butadiene and isoprene yield polymers that contain double bonds (polybutadiene and polyisoprene) and are usually not considered polyolefins. Polyolefins are the foundations of many chemical industries.

<span class="mw-page-title-main">Foam rubber</span> Rubber manufactured with a foaming agent

Foam rubber is rubber that has been made with a foaming agent so that its structure is an air-filled matrix. Commercial foam rubber is generally made of synthetic rubber, natural latex, or polyurethane. Latex foam rubber, used in mattresses, is well known for its endurance. Polyurethane is a thermosetting polymer that comes from combination of methyl di-isocyanate and polyethylene and some chemical additives.

<span class="mw-page-title-main">Polyisocyanurate</span> Type of plastic typically used for thermal insulation

Polyisocyanurate, also referred to as PIR, polyol, or ISO, is a thermoset plastic typically produced as a foam and used as rigid thermal insulation. The starting materials are similar to those used in polyurethane (PUR) except that the proportion of methylene diphenyl diisocyanate (MDI) is higher and a polyester-derived polyol is used in the reaction instead of a polyether polyol. The resulting chemical structure is significantly different, with the isocyanate groups on the MDI trimerising to form isocyanurate groups which the polyols link together, giving a complex polymeric structure.

A blowing agent is a substance which is capable of producing a cellular structure via a foaming process in a variety of materials that undergo hardening or phase transition, such as polymers, plastics, and metals. They are typically applied when the blown material is in a liquid stage. The cellular structure in a matrix reduces density, increasing thermal and acoustic insulation, while increasing relative stiffness of the original polymer.

<span class="mw-page-title-main">Building insulation material</span> Insulation material

Building insulation materials are building materials that form the thermal envelope of a building. As thermal insulation they reduce the heating energy requirement or the cooling load of buildings ; they can also provide soundproofing.

Reaction injection molding (RIM) is similar to injection molding except thermosetting polymers are used, which requires a curing reaction to occur within the mold.

Polybutylene (polybutene-1, poly(1-butene), PB-1) is a polyolefin or saturated polymer with the chemical formula (CH2CH(Et))n. Not be confused with polybutene, PB-1 is mainly used in piping.

<span class="mw-page-title-main">Spray foam</span> Building material

Spray foam is a chemical product created by a chemical reaction of two component parts, commonly referred to as side A and side B. Side A contains very reactive chemicals known as isocyanate. Side B contains a polyol, which reacts with isocyanates to make polyurethane, and a mixture of other chemicals, including catalysts, flame retardant, blowing agents and surfactants. These react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place. This expansion makes it useful as a specialty packing material which forms to the shape of the product being packaged and produces a high thermal insulating value with virtually no air infiltration.

Natural oil polyols, also known as NOPs or biopolyols, are polyols derived from vegetable oils by several different techniques. The primary use for these materials is in the production of polyurethanes. Most NOPs qualify as biobased products, as defined by the United States Secretary of Agriculture in the Farm Security and Rural Investment Act of 2002.

<span class="mw-page-title-main">Insulated pipe</span>

Insulated pipes are widely used for district heating and hot water supply. They consist of a steel pipe called "service pipe", a thermal insulation layer and an outer casing. The insulation bonds the service pipe and the casing together. The main purpose of such pipes is to maintain the temperature of the fluid inside the service pipes. Insulated pipes are commonly used for transport of hot water from district heating plants to district heating networks and for distribution of hot water inside district heating networks.

Biofoams are biological or biologically derived foams, making up lightweight and porous cellular solids. A relatively new term, its use in academia began in the 1980s in relation to the scum that formed on activated sludge plants.

Covalent adaptable networks (CANs) are a type of polymer material that closely resemble thermosetting polymers (thermosets). However, they are distinguished from thermosets by the incorporation of dynamic covalent chemistry into the polymer network. When a stimulus (for example heat, light, pH, ...) is applied to the material, these dynamic bonds become active and can be broken or exchanged with other pending functional groups, allowing the polymer network to change its topology. This introduces reshaping, (re)processing and recycling into thermoset-like materials.

References

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  2. ↑ Skleničková, Kateřina; Abbrent, Sabina; Halecký, Martin; Kočí, Vladimír; Beneš, Hynek (2022-01-17). "Biodegradability and ecotoxicity of polyurethane foams: A review". Critical Reviews in Environmental Science and Technology. 52 (2): 157–202. doi:10.1080/10643389.2020.1818496. ISSN   1064-3389.
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  6. 1 2 3 4 5 Ullmann's encyclopedia of industrial chemistry. Vol. 19: A (5th rev. ed.). Weinheim: VCH Verl.-Ges. 1991. ISBN   978-0-89573-169-2.
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  21. ↑ EURO-MOULDERS (2024). "The Mass Balance Approach – An Ideal Way to Reduce the Carbon Footprint of Polyurethanes in the Automotive Sector".
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