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People Basics

People Basics Polyurethanes H H H O C N C N C O + HO C C HO H H H By: O H O H H Angela Austin C N C N C O C C O Editor in Chief, PU Magazine International Dr. Heinz Gupta Verlag Ratingen, Germany H Fig. 1: Aromatic isocyanate reacts with polyol to form the polyurethane molecule H H H H n In 2012, the polyurethane industry celebrated 75 years since the first form of this versatile material was made in a laboratory in Leverkusen, Germany. Initially, research led by Otto Bayer in 1937, created a sticky resin that could be spun into a fibre. It was to compete against nylon fibres being produced in the US. Research was delayed during WW II, but upon resumption in 1945 chemists started to experiment with different raw materials and stoichiometric mixes to create the many forms of polyurethane we know today in our homes, cars, clothing and footwear. Polyurethane can be foamed to produce a rigid foam, typically used as a thermal insulation material in buildings and refrigerators or a flexible foam found in mattresses, upholstered furniture, car seats and textiles. Polyurethane (PU) can also be produced in forms suitable for use in adhesives, sealants, coatings, elastomers, fibres and even as the resin in light weight composites for boats, truck floors, tractor hoods and most recently in the chassis of the BMW i3 and i1 electric models. PU is produced by an exothermic reaction between a molecule with two or more –OH groups (polyols) and an isocyanate with two or more - NCO groups (Fig. 1). The polyaddition reaction can take place at -20°C to 100°C depending upon the PU formulation, but typically ambient temperature is sufficient. The reaction speed can be controlled through the use of catalysts, often tin or amine types. In addition, the final form of polyurethane can be changed by varying the ratio of polyol: isocyanate, plus the use of additives such as flame retardants, surfactants and blowing agents (typically hydrofluorocarbons, hydrocarbons or water). Long, flexible segments, contributed by the polyol, create a soft, elastic material. High amounts of crosslinking due to the –NCO group create tough or rigid polymers. The crosslinking creates a polymer that consists of a threedimensional cellular structure where the cell size and the contents of the cell effects the properties of the polyurethane. Polyurethanes is typically a thermoset resin meaning that it does not melt when heated, however, thermoplastic polyurethanes (TPU) can also be produced. The unique property of polyurethane is the range and versatility of the end forms that can be produced. Basic raw materials Isocyanates are very reactive materials. This makes them useful in making polymers but also requires special care in handling and use. The aromatic isocyanates, diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) are more reactive than aliphatic isocyanates, such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). Most of the isocyanates are difunctional, meaning that they have exactly two isocyanate groups per molecule. An important exception to this is polymeric diphenylmethane diisocyanate, which is a mixture of molecules with two-, three-, and fouror more isocyanate groups. Polyols are polymers in their own right and have on average two or more hydroxyl groups per molecule. Polyether polyols (Fig. 2) are mostly made by co-polymerising ethylene oxide and propylene oxide with a suitable polyol precursor. Polyester polyols (Fig. 3) are made from reacting a dicarboxylic acid and a glycol. Polyols used to make polyurethanes are not “pure” compounds because they are often mixtures of molecules that contain different numbers of hydroxyl groups. The length of the polyol chain and the functionality contribute to the properties of the final polymer. Polyols used to make rigid 50 bioplastics MAGAZINE [02/14] Vol. 9

Basics R 1 R R 1 1 H 2 H 2 H 2 HO C C O C C O C C OH H H H n Fig. 2: Polyether polyol O O HO R O C R C O R O H n Fig. 3: Polyester polyol Fig. 4: Indian flexible foam producers use colours to identify densities (Courtesy Prime Comfort Products Pvt Ltd.) those used to make flexible polyurethanes have molecular weights up to ten thousand or more. Typical forms of polyurethane PU comes in many forms, here are the main ones which are found in our everyday lives; Flexible foams (moulded and blocks) These soft foams are typically made from combining a long chain polyether polyol with TDI (toluene diisocyanate) in a ratio of 2:1. (MDI, methylene diisocyanate can also be used but is more typically used in moulded flexible foams). The foam is made like a giant cake on a moving conveyor. A mixture of chemicals is dispensed onto a layer of kraft paper. As the mixture moves along the conveyor it expands due to the exothermic reaction and the formation of gas bubbles which form the cell structure of the foam. Such machines produce large blocks of foam at the end of a line which can easily be e.g. 20 metres long. The block can be cut and transported to a nearby warehouse to cool, before being cut further or sold to mattress and furniture manufacturers. The foam can be produced in a variety of densities and with specific properties (Fig. 4). Other applications include the use of flexible foam as a growth media for green roofs and walls, in hydroponics (Fig. 5) and when cut into an egg box style, the foam is an excellent acoustic insulator (Fig. 6). Flexible foams can also be made with viscoelastic properties. These foams, also called memory foams, rememberr and support your body shape when used in mattresses, medical chairs and body supports (Fig. 7). Moulded flexible foams are produced using a mould and are typically used in automotive seating and commercial furniture because of their higher density and greater support. These foam are made by injecting the raw material mix into an open or sealed mould. Heat and or pressure is applied and the reaction occurs inside the mould in a matter of a few minutes. When the foam is cured (set), the mould is opened to reveal a piece of seat foam. Rigid PU foams Rigid foams are made from a mixture of MDI and polyol in a 2:1 ratio. Blowing agents are also used to promote the expansion of the foam and to fill the cells formed during the reaction. The cells used to be filled with CFCs, which acted as excellent thermal insulation. For environmental reasons, these gases have now been replaced with water, methyl formate, hydrofluorocarbons, hydrocarbons and most recently with hydrofluoroolefins. These foams are produced for use as thermal insulation, but they can be used for packaging, structural strength and noise absorption. These foams are found in all refrigerators and freezers, they can be applied to surfaces such as walls and roofs using a spray gun, and they can be foamed between metal sheets, wooden panels and other materials including foil and glass fibre sheet to make panels for the construction industry. PU Coatings Polyurethane coatings are among the most versatile types of coatings. Polyurethane top coats are used to provide corrosion and UV resistance. In addition, they can bioplastics MAGAZINE [02/14] Vol. 9 51

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