Polyols are reactive stocks that contain at least 2 functional groups and through these groups, they react with isocyanate (NCO) groups to form their polyurethane structure. Polyols can be classified as several different groups in their areas of usage. Polyols are separated into two groups such as hydroxyl and amino-terminated groups. The features of polyurethane products are mostly affected by polyols in their substance. Most of the chemical, physical and mechanical features of polyurethane products are directly connected with the chemical structures of polyols that are used in their structures. The characterization of the polyols that are used for preparation of polyurethanes is performed in several ways. The characterization of the polyols is performed based on their numbers, functionality, molecular weight and range, equivalence weight, water cornet, reactivity, viscosity, intensity, colour and acid value. Considering their amount used in polyurethane systems the most common ones are polyether polyols and polyester polyols. Apart from these, amino-terminated polyether polyols and a little amount of polycarbonate polyols are also utilized. All of these chemical structures of polyol structures are stated below.
Usage Areas of Polyols
Polyols are one of the two main components in polyurethane systems. Polyurethane structures are formed by the reaction of at least two hydroxyl functional groups on polyol structures with isocyanate groups. Polyurethanes are used for several industrial areas, particularly sectors including footwear, construction, furniture, automotive, heating-refrigeration. The most important reaction partners of isocyanates are polyols. Polyol compounds are characterized based on their hydroxyl numbers (OH, mgKOH/g). The hydroxyl numbers decrease depending on the increase in the molecular weights of the polyols. Polios are primarily separated as two groups: polyethers and polyesters. 80-90% of the polyols used today are polyether polyols. The general chemical representation of the polyurethane reaction is given below.
Polyester polyols are produced with the condensation reaction of dicarboxylic acids and glycols.
Polyester polyols are produced by polyesterification of glycols and diacids or their derivatives. Polyester polyols are divided into three groups based on diacids that used for their preparation: aliphatic polyesters, aromatic polyesters and caprolactone-based polyesters. As dicarboxylic acid in aliphatic polyesters; adipic acid, succinic acid, phthalic acid and maleic anhydride or anhydrides of acids are preferred. In aliphaticpolyester polyols; ethylene glucose, diethylene glucose, propylene glucose, 1.4 butanediol and 1.6 hexanediol are used. In aromatic polyester polyols; acids containing cyclic groups such as terephthalic acid, isophthalic acid are used. Molecular weight of aromatic polyesters is lower than molecular weight of aliphatic polyesters.
Different monomer combinations and weight of molecules of polyester polyol determine the properties of the obtained polyurethane material. The highly branched polyester polyol enables the formation of rigid polyurethane foam with high temperature and chemical resistance, while the less branched polyester polyol provides flexible polyurethane foam with high elasticity. In similar way, rigid polyurethane foam is obtained with low molecular weight polyester polyols, whereas flexible polyurethane foam is obtained with high molecular weight polyester polyols.
Polyester polyols and polyurethanes provide superior characteristic properties. They provide better crystalline structure within urethane structures, superior secondary strengths between polyester chains and therefore heat and fire resistance compared to polyether polyols. In addition, polyester polyol-based polyurethanes provide better solvent resistance compared to polyether polyol-based polyurethanes. The disadvantage of polyester polyols is that they are prone to hydrolysis under high humidity and temperature conditions.
Polyether polyols are polymeric structures in chain or network structure polymerized in such a way that ether bonds are established between monomers. Polyether polyols are hydroxyl functional oligomers that are commonly used in the production of polyurethanes. They are produced by polymerization of alkylene oxide. They are generated by polymerization of cyclic etheric structures. They are polymeric structures produced by ethoxylation/propoxylation of cyclic ethers in the presence of catalysts. Polyether polyols are obtained by the addition reaction of propylene oxide and ethylene oxide.