Polyester Polyol Raw Materials

The main raw materials for polyester polyols are polyols and dicarboxylic acids. Polyols include diols and triols. Ethylene glycol (EG), diethylene glycol (DEG), 1,2-propanediol (PG), 1,4-butanediol (BDO), neopentyl glycol (NPG), and 2-methyl-1,3-propanediol (MPD) are the most commonly used diols in polyester polyol synthesis. Other diols such as 1,6-hexanediol (HDO) are also used in the synthesis of polyester diols. Trimethylolpropane (TMP) and glycerol can also be used in small amounts in the synthesis of polyester polyols to regulate the degree of branching, resulting in a polyester with a hydroxyl functionality greater than 2. Typical physical properties of commonly used diol raw materials in polyester synthesis are shown in Table 5-8.

In principle, all aliphatic diols containing primary or secondary hydroxyl groups can be used in polyester synthesis. In addition to the commonly used diols mentioned above, there are also 1,3-butanediol, 1,3-propanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, dipropylene glycol, 1,4-dihydroxymethylcyclohexane, 1,4-cyclohexanediol, hydroxypivalic acid hydroxypivalyl ester, 2-butyl-2-ethyl-1,3-propanediol, 2-ethyl-1,3-hexanediol, dodecanediol, dodecylcyclohexanediol, tricyclic 12-carbon primary hydroxyl diol, etc. The physicochemical properties of most commercial diols will be described in detail in the “Aliphatic Diols” section.

Among these diols, diols with an even number of carbon atoms (such as 1,4-butanediol and 1,6-hexanediol) produce polyester diols with higher crystallinity when reacted with adipic acid, and are often used in the production of polyurethane adhesives requiring high initial bond strength and high-strength elastomers. Polyester diols prepared from diols with side groups, such as neopentyl glycol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, and 2,2,4-trimethyl-1,3-pentanediol, exhibit good flexibility and hydrolysis resistance.

Perstorp’s NS20 polyol, produced by the Swedish company Perstorp, is a diol synthesized from neopentyl glycol and propylene oxide in a 1:2 molar ratio. It has a molecular weight of 220, a hydroxyl value of 480–530 mg KOH/g, and a typical viscosity of 170 mPa·s, and is used as a raw material for the synthesis of polyester polyols.

Due to the high cost of many diols, they are not widely used in the production of polyester diols. However, with the increasing industrialization of certain diols, decreasing raw material prices, and the growing demand for special functionalized polyester diols, some special diols will likely be used in bulk for the synthesis of polyester polyols. For example, Kuraray Co., Ltd. in Japan uses its own 3-methyl-1,5-pentanediol as a raw material to produce a series of polyester diols and polyester triols.

The most commonly used dicarboxylic acid in polyester synthesis is adipic acid. Sebacic acid is also used in small quantities to synthesize special polyester diols requiring hydrolysis resistance, but its use is limited due to its higher price compared to adipic acid. Terephthalic acid, phthalic anhydride, and isophthalic acid are also commonly used raw materials for the synthesis of aromatic polyester polyols. Special polyester polyols can also use small amounts of trimellitic anhydride as a raw material to create a certain degree of branching.

Other dicarboxylic acids (anhydrides, esters) that can be used in polyester synthesis include succinic acid, glutaric acid, azelaic acid, dodecanedioic acid, 1,4-cyclohexanedicarboxylic acid, dimer acid, and mixed dicarboxylic acids. Table 5-9 shows the melting points, molecular weights, and other parameters of some dicarboxylic acids (anhydrides/esters). Typical physical properties of dicarboxylic acids and polycarboxylic acids (anhydrides) are detailed in the “Dicarboxylic Acids” section.