Polyurethane Raw Materials and Production
Another form of plastic coating attached to a fabric base for synthetic leather is called polyurethane. Polyurethane is composed of isocyanates, polyester polyols, and additives. The isocyanates are methylene diphenyl diisocyanate (MDI) and toluene diisocyanates (TDI) (Lee, 2002). In 2000, 4.4 million tons of MDI and TDI were produced (Lee, 2002). Polyols react with isocyanates and produce polyurethane polymers powered by hydroxyl groups (Lee, 2002). In 2000, there were 850,000 tons of polyester polyols produced with a growing demand of 4-5% per year (Lee, 2002). Polyol blends include additives like catalysts, fire retardents, blowing agents, coloring agents, and fillers.
MDI isocyanate is derived from benzene (Lee, 2002). Concentrated nitric and sulphuric acids and benzene are blended to yield nitrobenzene (Lee, 2002). Nitrobenzene is hydrogenated to aniline by dissolving iron in hydrochloric acid with nitrobenzene (Lee, 2002). The aniline is purified, then reacted with formaldehyde to generate a polyamine mixture called methylene dianiline (MDA) (Lee, 2002). Excess aniline is removed and recycled. The amine groups in MDA must be phosgenated to convert to isocyanates (Lee, 2002). A phosgene molecule reacts with the amine group and emits hydrogen chloride gas that must be boiled off and used as a raw material for other processes (Lee, 2002). Isocyanates are the worlds largest suppliers of hydrogen chloride gas (Lee, 2002). The excess phosgene and solvent are removed and recycled (Lee, 2002). The crude diisocyanate stream and residue is divided into pure MDI and a mixed isomer stream (Lee, 2002).
Toluene is converted into the isocyanate TDI and is processed much like MDI. Toluene is nitrated into dinitrotoluene which produces an isomer mix (Lee, 2002). The isomers are hydrogenated to crude toluene diamine (TDA) (Lee, 2002). TDA is purified by distillation to remove mixed isomers that are disproportional, meaning they have the wrong level of hydrolysable chlorine and acids that would influence rates of chemical reactions (Lee, 2002). Purified TDA is phosgenated similarly to the process in MDI, and excess phosgene is recycled. The isocyanate mixture is distilled to generate a liquid TDI product and a residue (Lee, 2002).
Polyester polyol raw materials include dibasic acids like adipic acid and AGS mixed acids, glycols like ethylene glycol, propylene glycol, 1,4-butane diol, and 1,6-hexane diol, and branching agents like glycerol and pentaerythritol (Lee, 2002). The closer together the ratio of glycols to adipic acid in a polyol results in an extended polymerisation that is desireable for flexible foams (Lee, 2002). I am assuming flexible or elastomer foams are used in synthetic leather due to its more flexible qualities as a plastic. Triols such as glycerol or trimethylolpropane increase functionality of a polyester polyol, which leads to branching of the polyester backbone (Lee, 2002). In the production of polyester polyols, the raw materials are first heated under pressure. Water is distilled off, which wastes some acid groups and slows the rate of polymerisation (Lee, 2002). Azeotropic distillation can be applied to decrease this loss by using a vacumm and or adding nitrogen to improve the polymerisation reaction (Lee, 2002). Pigment additives can be organic or inorganic. Inorganic pigments include titanium dioxide, chromium oxide, carbon black, and iron oxide (Lee, 2002). Fillers are added to they polyol blend to reduce cost, increase stiffness, and increase temperature stability. Glass fibre is most commonly used, although carbon fibre is becoming more popular as its price is dropping (Lee, 2002). Stabilisers are added to prevent microbial attack by enzymatic hydrolysis (Lee, 2002). They are usually composed of metal organics relating to antimony, copper, or arsenic (Lee, 2002). A blend of stabilisers are used for UV resistance. Benzophenone, benzotriazoles, and amines all work as UV absorbers (Lee, 2002). Fire retardants like solid melamine, graphite or aluminium trihydrate and other low viscosity liquid compounds are added to polyols (Lee, 2002). It is typical for fire retardants to contain bromine, chlorine, or phosphorous (Lee, 2002).