Substances bearing plurality of hydroxyl groups are termed as spolyols. They may also contain ester, ether, amide, acrylic, metal, metalloid and other functionalities, along with hydroxyl groups. Polyester polyols (PEP) consist of ester and hydroxylic groups in one backbone. They are generally prepared by the condensation reaction between glycols, i.e.,
ethylene glycol, 1,4-butane diol, 1,6-hexane diol and a dicarboxylic acid/anhydride (aliphatic Polyurethane: An Introduction 7 or aromatic). The properties of PU also depend upon the degree of cross-linking as well as molecular weight of the starting PEP. While highly branched PEP result in rigid PU with good heat and chemical resistance, less branched PEP give PU with good flexibility (at low temperature) and low chemical resistance. Similarly, low molecular weight polyols produce rigid PU while high molecular weight long chain polyols yield flexible PU. An excellent example of naturally occurring PEP is Castor oil. Other vegetable oils (VO) by chemical transformations also result in PEP. PEP are susceptible to hydrolysis due to the presence of
ester groups, and this also leads to the deterioration of their mechanical properties. This problem can be overcome by the addition of little amount of carbodiimides. Polyether polyols(PETP) are less expensive than PEP. They are produced by addition reaction of ethylene or propylene oxide with alcohol or amine starters or initiators in presence of an acid or base catalyst. PU developed from PETP show high moisture permeability and low Tg, which limits their extensive use in coatings and paints. Another example of polyols is acrylated polyol (ACP) made by free radical polymerization of hydroxyl ethyl acrylate/methacrylate with other acrylics. ACP produce PU with improved thermal stability and also impart typical characteristics of acrylics to resultant PU. These PU find applications as coating materials. Polyols are further modified with metal salts (e.g., metal acetates, carboxylates, chlorides) forming metal containing polyols or hybrid polyols (MHP). PU obtained from MHP show good thermal stability, gloss and anti-microbial behavior. Literature reports several examples of VO based PEP, PETP, ACP, MHP used as PU coating materials. Another example is VO derived fatty amide diols and polyols (described in detail in chapter 20 Seed oil based polyurethanes: an insight), which have served as excellent
starting materials for the development of PU. These PU have shown good thermal stability and hydrolytic resistance due to the presence of amide group in the diol or polyol backbone.

Declaration:The article is quoted © 2012 Sharmin and Zafar, licensee InTech . Only for communication and learning, do not do other commercial purposes, does not represent the company’s views and opinions, if you need to reprint, please contact the original author, if there is infringement, please contact us immediately to do delete processing.