In 1937, Otto Bayer followed the first synthesis of polyurethane. It has since become an essential element in a range of products and sectors, including in medicine as medical device housing, in transport integrated in vehicle parts, in athletic equipment, in construction as an insulated thermal and acoustic element, and in impermeable coverings and on road surfaces to name a few.
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Polyurethane is obtained through a reaction of two components: one that contains two less functional alcohol groups- functional group -OH, called polyol, and one that contains at least two -NCO functional groups, called polyisocyanate. Several additives, along with the characteristics of the polyol and the polyisocyanate used, will determine the final properties of the product.
Types of Polyurethanes
Polyurethane can be thermostable or thermoplastic.
Thermostable polyurethanes can be classified as:
- Flexible and rigid polyurethane foams
- RIM whole skin and polyurethane foams
- Bio-based polyurethanes
- Generalized paints, varnishes, and coatings (pavements, waterproofing membranes)
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Flexible Polyurethane Foams
Flexible polyurethane foams have limited resistance to load application. Their internal structure is open cell and permeable in the air, and they possess elastic qualities in the presence of deformation. They are commonly used in mattresses, upholstery, and the automotive industry.
Rigid Polyurethane Foams
Rigid polyurethane foams have some of the lowest thermal conductivity of other insulated materials. They also have excellent soundproofing properties.
This type of polyurethane has a number of applications, such as insulation for refrigerators and freezers, industrial plants (e.g., tanks, pipes, shipbuilding, and the construction sector), and other thermal insulation and soundproofing projects in construction and automotive.
PUR insulation can be injected inside wall cavities to create an energy-efficient barrier. The foam can migrate to small spaces and create an air-tight seal. According to the Insulation Manufacturer Association, PUR offers “the best thermal performance of all practical full cavity insulants.”
It is also possible to continuously spray PUR foam onto any surface. It is generally less expensive than other materials, making it well-suited for renovation projects.
RIM Integral Skin and Polyurethane Foams
A material called “integral skin” (DIN 7726) can be created by carefully balancing all the reactions that occur during the formation of polyurethane foams. This is done by adjusting mold temperature, the quantity of foaming agent, and the packing of cells, enabling the formation of molded pieces with a cellular nucleus structure covered by an exterior skin made from the same material.
The pieces carried out from the integral polyurethane foam provide a high degree of reactivity due to the introduction of new transformation processes, which enable the dosing and mixing of a wide variety of primary substances in a short time. The growth of these processing technologies has resulted in what is known as the Reaction Injection Molding (RIM) process.
RIM integral skin and polyurethane foams provide many benefits in the design and construction of molds because of their variety of mechanical properties and the increase in surface finish variety and density distribution.
The automotive industry is unique in its relevance because it uses pieces of integral skin to lower part weight, which satisfies the mechanical properties needed to fundamentally reduce vehicle weight, reduce CO2 emissions, and maintain security guidelines. These types of products can also be used in sports equipment, furniture, electronic devices, or medical housings.
Bio-Based Polyurethanes
There has been a significant increase in demand for petroleum monomers to synthesize polymers, even with the decline and price fluctuation of fossil resources.
Developing bio-based and renewable products has become more significant in the polymer sector, particularly in developing bio-based polyurethane. The reaction between polyol polyester or polyether and isocyanate creates polyurethane.
In bio-based polyurethane, one or both components can be substituted.
Polyol with functional hydroxyl groups can be found in biomass resources of functionalized vegetable oils, lignin derivatives, cellulose derivatives, and agro-industrial waste.
Isocyanate can be derived from biomass through fatty acid or lignin derivatives, as well as bio-based amino acids. The resulting polyurethanes will have a determined percentage of bio-based contents, positively impacting their carbon footprint.
Bio-based polyurethanes can be used in many applications, such as rigid and flexible foams, elastomers, coatings, and a polymeric matrix for composites. This depends on the biomass resources used in the polymer synthesis. Bio-based polyurethanes are used in several industrial sectors and the construction sector.
Non-Isocyanate Polyurethanes
According to the new horizons, all polyurethanes can create non-isocyanate polyurethanes (NIPUs). However, isocyanate should be avoided due to its toxicological issues (REACH nº1907/2006). This removes hazardous chemical compounds from the synthesis and results in polymeric materials with unique and tunable characteristics.
Non-isocyanate polyurethane (NIPU), or polyhydroxy urethanes (PHUs), are generally derived from a reaction between cyclic carbonate and polyfunctional amines. Recently, a number of strategies have been developed that seek to use renewable resources for the synthesis of NIPUs, meeting the prerequisites of green chemistry and paving the way for applications in the PU industry. There is potential for their use in coatings, adhesives, foams, sealants, and elastomers.
Hybrid non-isocyanate polyurethane (HNIPU) is another option, as it addresses the issue of using harmful substances. It can be developed using a solvent-free and catalyst-free synthesis in an eco-friendly manner to arrive at the proposed materials without stress on product purification and waste treatment. This material is still being researched but has potential applications in coatings, interior and exterior paints, flooring, and packaging. Hybrid Coating Technologies has developed Hybrid Non-Isocyanate Polyurethane (HNIPU) through patented technology.
This information has been sourced, reviewed and adapted from materials provided by AIMPLAS.
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