What Is Polyester Polyol?

Contact Kerton Chemicals Right Now !

Broadly speaking, polyester polyols include conventional polyester polyols, polycaprolactone polyols, and polycarbonate diols, all of which contain ester groups (COO) or carbonate groups (OCOO); however, in practice, the term “polyester polyol” typically refers to polyester polyols produced through polycondensation reactions involving dicarboxylic acids and diols.

Conventional Polyester Polyols

Polyester polyol is one of the main raw materials for polyester polyurethane, which can be divided into aliphatic polyols and aromatic polyols based on whether they contain benzene rings. Among them, aliphatic polyols are mainly composed of adipic acid-based polyester diols.

Adipic Acid-based Polyester Diols

Common aliphatic polyester polyols are primarily polyester diols, which are generally produced by polycondensation of adipic acid (with a small number of products using sebacic acid) with one or more diols, such as ethylene glycol, propylene glycol, 1,4-butanediol, or diethylene glycol (i.e., diethylene glycol).

Physical and Chemical Properties

Polyester diols are milky white, waxy solids or colorless to pale yellow viscous liquids at room temperature. Solid polyester has a melting point of 25–50°C and becomes a viscous liquid after heating. It is slightly soluble in water. The acid value of polyester is generally less than 1.0 mg KOH/g.
Polyester polyols are essentially non-toxic; however, if they accidentally enter the eyes or splash onto the skin, they should be immediately rinsed with tap water. Prolonged skin contact may cause mild irritation; it is recommended to wear safety goggles and gloves during handling.
The chemical industry standard HG/T 2707—1995, “Specifications for Polyester Polyols,” specifies the physicochemical performance indicators for several types of adipic acid-based polyester polyols and adopts the polyester nomenclature recommended by the industry standard. Currently, the color and acid value of polyester polyols produced by most manufacturers have met or exceeded these specifications.

Preparation Methods

The synthesis of polyester is divided into two main stages. The first stage is to add dicarboxylic acids and diols (and trace catalysts) into the reactor and carry out esterification and condensation reactions at 140-220°C. The top temperature of the distillation tower (column) is controlled at 100-102°C, and most of the by-product water generated is evaporated at atmospheric pressure. After that, the acid value is generally reduced to 20-30 mg KOH/g at 200-230°C for 1-2 hours. The second stage involves vacuuming and gradually increasing the vacuum degree to remove trace amounts of water and excess diol compounds under reduced pressure, allowing the reaction to proceed towards the formation of low-acid-value polyester polyols, which can be referred to as the "vacuum melting method." It is also possible to continuously introduce inert gases such as nitrogen to carry water, which is called the "carrier gas melting method." It is also possible to add azeotropic solvents such as toluene to the reaction system and slowly remove the generated water using a water separator during toluene reflux. This method is called "azeotropic distillation."

Characteristics and Applications

Polyester polyurethane has a large number of polar groups, such as ester and urethane groups, with strong cohesive strength and adhesion and high strength, wear resistance, and other properties. Therefore, aliphatic polyester diols are commonly used in the production of cast polyurethane elastomers, thermoplastic polyurethane elastomers, microporous polyurethane shoe soles, PU leather resins, polyurethane adhesives, polyurethane inks and pastes, fabric coatings, etc.
Polyester diols produced from adipic acid and 1,4-butanediol, 1,6-hexanediol, or ethylene glycol are waxy solids; the resulting polyurethane elastomers exhibit high crystallinity and strong initial tack. Polyesters such as PMA and PPA, produced from diols with side chains, are liquid at room temperature and flexible; they are used in inks, soft leather, and other applications. PMA exhibits good hydrolytic stability.

Aromatic Polyester Polyols

Aromatic polyester polyols are polyester polyols containing benzene rings, generally referring to polyester polyols synthesized from aromatic dicarboxylic acids (or anhydrides, esters) and diols (or polyols) as raw materials. The raw materials for polyester are generally phthalic anhydride (PA), terephthalic acid (PTA), isophthalic acid (IPA), etc. The commonly used diol raw materials are diethylene glycol (DEG), and other diols can also be used. Adding a small amount of polyol can give polyester polyol molecules a branched structure.

The aromatic polyester polyols used in the polyurethane industry are currently mainly phthalic anhydride polyester polyols. In addition, terephthalate polyester polyols are produced through ester exchange reactions between polyester waste, PTA residue, and other raw materials such as diethylene glycol, as well as medium to high molecular weight aromatic copolyester diols obtained by condensation of dicarboxylic acids such as terephthalic acid, isophthalic acid, adipic acid, and sebacic acid with diols such as ethylene glycol, diethylene glycol, and neopentyl glycol.

Physical and Chemical Properties

Aromatic polyester polyols are pale yellow to reddish-brown, viscous, transparent liquids. They are stable, have a slight aromatic odor, are non-toxic and non-corrosive, are insoluble in water, and are highly compatible with most organic compounds. They are non-flammable and non-explosive. Aromatic polyols are predominantly polyester diols, with a functional group number generally ranging from 2 to 3. The viscosity of aromatic polyester diols is higher than that of aliphatic polyester diols of the same molecular weight.

Preparation Methods

The manufacturing method of aromatic polyester polyols is similar to that of aliphatic polyester polyols, except that phthalic anhydride is used as the main dicarboxylic acid component, and less water is produced during the condensation process. Most aromatic polyester polyols have high hydroxyl values, and there is a relatively high excess of alcohol raw materials compared to acids during synthesis.
Aromatic polyester polyols can be prepared by an ester exchange reaction between polyester waste, PTA residue, and diethylene glycol. Due to its low cost, it is quite competitive.

Characteristics and Applications

Polyurethane made from aromatic polyester has excellent hydrolysis resistance, heat resistance, and adhesion.

Phthalic anhydride polyester polyol and aromatic polyester polyol prepared from waste polyester/waste PTA are generally used to manufacture rigid polyurethane foam. The flame retardancy of rigid foam based on high hydroxyl value aromatic polyester polyol is better than that based on polyether polyol. In the polyurethane foam plastic industry, aromatic polyester polyols are often used to replace some or all polyether polyols in polyurethane foam plastic and polyisocyanate rigid foam plastic formulations. Adding some aromatic polyester polyols to the formula of refrigerator combinations in winter can also improve the toughness and adhesion of foam. Phthalic anhydride polyester polyol is particularly suitable for polyisocyanurate (PIR) foam. The foam plastic contains a large number of benzene rings, which not only improves the heat resistance of the foam but also improves the flame retardancy of the products.
One or both of the dicarboxylic acids, such as terephthalic acid, isophthalic acid, adipic acid, decanedioic acid, and other dicarboxylic acids; and one or two of the diols, such as ethylene glycol, diethylene glycol, neopentyl glycol, and methylpropanediol. These medium- to high-molecular-weight aromatic copolyester diols are generally used to prepare two-component solvent-based adhesives, which are commonly used in composite films for flexible food packaging and aluminum-plastic laminates.