What Are the Main Types of Polyol?

22 7 月, 2024
2:13 上午

What Are Polyols?

Polyols are versatile compounds with multiple hydroxyl groups that play a crucial role in various industries, particularly in the production of polyurethanes. These compounds are essential components in the manufacturing of foams, coatings, adhesives, sealants, and elastomers. Understanding the different types of polyols is vital for selecting the right material for specific applications and achieving desired product properties.

Different Types of Polyols

The two main categories of polyols are polyether polyols and polyester polyols. Each type has unique characteristics and subcategories, making them suitable for different applications. Generally speaking, polyester-type PUs have better heat resistance but poorer water resistance, whereas polyether-type PUs are the opposite.

1. Polyether polyols

Polyether polyols (referred to as polyethers) are produced through a polyaddition reaction involving a starter (a compound containing active hydrogen groups) and epoxides such as ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO) in the presence of a catalyst. Glycerol (propane-1,3-diol) is commonly used as the starter, and a combination of PO and EO is typically employed as the epoxides. Various types of polyether polyols can be produced by varying parameters such as the method of addition (mixed or separate), ratio, and sequence of addition of PO and EO.

They are chemicals used extensively for manufacturing general-purpose polyurethane foam plastics, adhesives, elastomers, and more. Specialized polyether polyols are also utilized as defoamers, surfactants, and other applications. At room temperature, polyether polyols are typically viscous liquids ranging from colorless to brown, soluble in aromatic hydrocarbons, halogenated hydrocarbons, alcohols, and ketones, and possess hygroscopic properties.

Common types of polyether polyols are detailed as follows:

1.1 Polyoxypropylene Glycol

Polyoxypropylene glycol, also known as polypropylene glycol (PPG), is prepared in lined glass or stainless steel reactors. A mixture of starter (1,2-propanediol or dipropylene glycol) and catalyst (potassium hydroxide) is added to the catalyst vessel. Heating is conducted to 80-100°C, solvent from the catalyst is removed under vacuum to facilitate alcohol formation.

The catalyst is then transferred to the reaction vessel and heated to 90-120°C. At this temperature, propylene oxide is added to maintain vessel pressure at 0.07-0.35 MPa. Propylene oxide undergoes continuous polymerization until reaching a specified molecular weight. After removing residual propylene oxide, the polyether mixture is neutralized in a neutralization vessel, filtered, refined, and stabilized to obtain the final product.

1.2 Polytetrahydrofuran Glycol

Polytetrahydrofuran glycol (PTHF), also known as polytetramethylene ether glycol (PTMEG, PTMG, PTMO), is synthesized by ring-opening polymerization of tetrahydrofuran in the presence of a cationic catalyst.

Production process involves adding tetrahydrofuran to a reactor, lowering the temperature below -5°C, and adding fuming sulfuric acid catalyst under vigorous stirring. Water is added in a measured quantity while maintaining low reaction temperatures. The temperature is then raised to 70-90°C to vaporize unreacted tetrahydrofuran monomers. After settling, neutralization, filtration, and vacuum drying, polytetrahydrofuran glycol is obtained.

Polyether polyols like PTHF are priced higher and typically used in the production of high-performance polyurethane materials due to their excellent properties such as low-temperature resistance, water resistance, oil resistance, abrasion resistance, and resistance to mold.

1.3 Tetrahydrofuran-Oxirane Copolymer Polyol

Tetrahydrofuran-oxirane copolymer polyol is produced by ring-opening copolymerization of tetrahydrofuran and propylene oxide catalyzed by Lewis acids. It undergoes processes such as neutralization, water washing, dehydration, and filtration.

This copolymer polyol is less expensive than pure polytetrahydrofuran glycol (PTMG) but offers similar performance characteristics, making it particularly suitable for manufacturing cold-resistant polyurethane materials (able to withstand temperatures as low as -200°C).

2. Polyester polyols

Polyester polyols are typically synthesized through the condensation reaction of dicarboxylic acids or their derivatives with glycols. Polyester polyols offer several advantageous properties, including excellent solvent resistance, superior adhesion to various substrates, and enhanced mechanical properties compared to their polyether counterparts.

Generally speaking, synthetic polyesters used in the production of polyurethane are classified according to the types of carboxylic acids. They typically include the following categories:

2.1 Aliphatic Polyester Polyols

Depending on the saturation of their molecular chains, aliphatic polyester polyols are categorized into saturated polyester polyols and unsaturated polyester polyols. Additionally, caprolactone polyester polyols also belong to the category of aliphatic polyesters.

2.1.1 Saturated Aliphatic Polyester Polyols

Saturated aliphatic polyester polyols are characterized by a molecular structure with hydroxyl end groups and no unsaturated segments. They consist of low-functionality linear oligomers. Due to their numerous ester and urethane polar groups within the molecular chain, they exhibit high strength, abrasion resistance, and adhesion.

These excellent properties make saturated aliphatic polyester polyols widely used in the production of polyurethane inks, thermoplastic polyurethane elastomers, microcellular polyurethane soles, polyurethane adhesives, castable polyurethane elastomers, PU synthetic leather resins, fabric coatings, and more.

Among saturated aliphatic polyester polyols, adipic acid-based polyester polyols have the widest application range and are typical representatives of polyester polyols. Additionally, there are other saturated acid-based polyester polyols such as succinic acid, glutaric acid, and decanedioic acid-based polyols.

2.1.2 Unsaturated Aliphatic Polyester Polyols

Unsaturated aliphatic polyester polyols are a type of polyester containing double bonds in their molecular structure, synthesized through esterification and condensation reactions between unsaturated polyacids and polyols. At high temperatures, these double bonds undergo cross-linking reactions. However, the biodegradability of the resulting unsaturated aliphatic polyester decreases with increasing cross-linking. They find significant medical applications, such as serving as scaffolds, drug delivery media, or for repairing cartilage tissues.

2.1.3 Caprolactone-based Polyester Polyols

Polycaprolactone (PCL) polyols are linear aliphatic polyesters synthesized by ring-opening polymerization of ε-caprolactone monomer (ε-CL) with diols, triols, or polyolamines in the presence of catalysts like titanium tetraisopropoxide or tin octoate. It is a semi-crystalline polymer with a narrow molecular weight distribution that matches well with its initiators.

It exhibits good thermal stability and the polyurethanes produced from it possess high tensile strength, low-temperature flexibility, excellent elasticity, water resistance, abrasion resistance, tear resistance, high-temperature adhesion, resistance to hydrocarbon solvents, and chemical durability.

PCL is a commonly used biomaterial approved by the US FDA, often employed as implants and drug release materials in medical applications. Due to its flexibility, processability, and extended degradation period, PCL is particularly favored for postoperative anti-adhesion membranes and tissue engineering scaffolds, maintaining structural integrity over extended periods without degradation.

2.2 Aromatic Polyester Polyols

Aromatic polyester polyols contain rigid benzene rings in their molecular chains and are typically synthesized from phthalic anhydride with diols or triols. The inclusion of benzene ring rigid groups in the molecule enhances product strength, heat resistance, and flame retardancy.

Aromatic polyester polyols are commonly used in the production of rigid polyurethane foam plastics and non-foam polyurethanes such as polyurethane coatings, adhesives, and elastomers. High hydroxyl value polyester polyols in rigid foam formulations exhibit better flame retardancy compared to polyether polyol-based foam plastics.

In the polyurethane foam industry, aromatic polyester polyols are increasingly replacing some or all of the polyether polyols. Currently, aromatic polyester polyols are widely applied domestically and internationally in the production of sandwich panel insulation boards and on-site spray coatings in the construction industry, owing to their excellent performance characteristics.

2.3 Mixed Acid Polyester Polyols

Mixed acid polyester polyols are synthesized from a combination of various fatty acids, acid anhydrides, or phthalic anhydrides as starting materials for polyurethane production. The purpose of using mixed acids is to compensate for the performance limitations of polyurethanes synthesized from single raw materials. There are several types of mixed acid polyester polyols, such as alcohol-acid polyester polyols, acrylic acid polyester polyols, mixed diacid polyester polyols, and aromatic-aliphatic copolyester polyols.

2.3.1 Alcohol-Acid Polyester Polyols

These are oil-modified polyesters (e.g., soybean oil) primarily used in polyurethane coatings, adhesives, and other applications.

2.3.2 Acrylic Acid Polyester Polyols

Typically used in photosensitive polyurethane coatings. Advantages of acrylic emulsion wood coatings include high solid content, high hardness, low cost, fast drying speed, and good weather resistance.

2.3.3 Mixed Diacid Polyester Polyols

Synthesized from diacids and diols, often using C4-C6 diacids as by-products from adipic acid industrial production. Widely used plasticizers are a type of chemical product directly synthesized using mixed diacid.

2.3.4 Aromatic-Aliphatic Copolyester Polyols

These copolyesters contain both aromatic and aliphatic polyester structural units in their molecular chains. Aliphatic polyester segments exhibit good biodegradability but poor thermal and processing properties, while aromatic polyester segments offer excellent heat resistance and processing properties but degrade less readily. By incorporating aliphatic polymers into aromatic polyester, copolyester materials can be obtained that combine the advantages of both polymers, resulting in good mechanical properties, high thermal stability, biocompatibility, and biodegradability.

Conclusion

Polyols, essential in polyurethane and polymer production, divide into polyether and polyester categories, each with distinct properties and applications. Polyether polyols like PPG and PTMEG, including their copolymers, are valued for versatility and cost-efficiency. Polyester polyols, encompassing aliphatic, aromatic, and mixed acid varieties, offer superior mechanical strength and solvent resistance.

Selecting between polyether and polyester polyols depends on specific needs such as hydrolytic stability, mechanical properties, chemical resistance, and cost, critical for developing tailored, high-performance products. Ongoing advancements in the polymer industry are expected to introduce new polyol types and enhancements, expanding customization options and optimizing performance across diverse applications.

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