Alcohol-Based Chain Extenders and Crosslinkers

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Common diol chain extenders and curing agents mainly include 1,4-butanediol, ethylene glycol, diethylene glycol, 1,6-hexanediol, and HQEE.

Common polyol crosslinkers include glycerol, trimethylolpropane, pentaerythritol, and low molecular weight polyether polyols.

Dimethylolpropionic acid, dimethylolbutyric acid, and some self-synthesized carboxyl-containing diols are a class of chain extenders containing hydrophilic groups, used in the preparation of water-based polyurethanes.

Most diols and polyols with specific structures will be introduced in the “Polyhydroxy Compounds” section. Some special polyhydroxy crosslinkers are introduced below.

Chemtura’s Vibracure A250 is a diol mixture with a hydroxyl equivalent weight of 45.0, mainly used for curing PPDI prepolymers. It is easier to mix with prepolymers than BDO. The resulting elastomer has high tensile strength and elongation, high tear strength, resilience, and dynamic mechanical properties. Vibracure A931 is a diol mixture with a melting point of approximately 93°C and a hydroxyl equivalent weight of 87.6. The cured elastomer has a hardness range of Shore A 55-60, with high tensile strength, elongation, tear strength, and low compression set.

Small molecule diols are conventional chain extenders, but some companies also recommend higher molecular weight diol chain extender (curing agent) products.

For example, Chemtura’s Vibracure A120 is a liquid polyether diol (Mw=1000), used as a flexible chain extender for low-hardness polyurethane elastomers such as rollers. It can be used alone or in combination with curing agents such as BDO, HQEE, or MOCA. Vibracure A122 is a white waxy solid polyether diol (PTMEG-2000), which can be used for TDI or MDI-based prepolymers, with a curing temperature of 55-115°C. Vibracure A125 is a polyester diol with a melting point of 49°C (Mw=1000), which can be mixed with BDO in various proportions and used as a curing agent for MDI-polyester prepolymers; when mixed with MOCA, it can be used as a curing agent for TDI prepolymers to obtain softer elastomers.

Addolink 3530 from Rhein Chemie Rheinau GmbH (Germany) is a polyester diol with a hydroxyl value of approximately 245 mg KOH/g, a viscosity (25°C) of approximately 300 mPa·s, and a pour point of approximately -4°C, used as a curing agent for polyurethane elastomers.

In addition, small amounts of low molecular weight polyethers are used as chain extenders and crosslinkers for foams or elastomers, as briefly introduced in Chapter 2, “Polyether Polyols,” and other sections on product applications. Many polyether manufacturers supply low molecular weight polyether polyol crosslinkers with molecular weights of several hundred, used in polyurethane foam systems and “CASE” elastomeric polyurethane systems. Application areas include rigid foam, semi-rigid foam, high-resilience foam, flexible foam, coatings, adhesives, and elastomers.

For example, Perstorp Polyols (a subsidiary of Perstorp, Sweden) produces a series of crosslinkers that can be used in polyurethane foams and elastomers, and some products are also used in polyurethane coatings. These are low molecular weight adducts of one trimethylolalkane or pentaerythritol molecule with 3 to 8 ethylene oxide or propylene oxide molecules. Among them, the polyol crosslinker Polyol 3610 is ethoxylated trimethylolpropane, with the structural formula CH3CH2C(CH2OCH2CH2OH)3, i.e., an adduct of one trimethylolpropane molecule with three ethylene oxide molecules. The corresponding product names from the former Upjohn and Dow companies were Isonol 93 and Voranol 234-630, respectively (both are now discontinued). Products with very low water content (e.g., 0.03% water content) can be used as curing agents for polyurethane prepolymers in non-foamed polyurethanes. HP-Link 600, produced by Shanghai Hongpu Chemical Technology Co., Ltd., is a trimethylolpropane-based liquid polyol crosslinker. It is a colorless, transparent, viscous liquid with a viscosity (25℃) of 600–800 mPa·s and a hydroxyl value of (600±20) mg KOH/g. It has been widely used in polyurethane high-resilience foams, microcellular foams, elastomers, and high-density rigid foams, improving the resilience of foam products and creating a dense surface. HP-Link 105C is an aliphatic amine-initiated polyol, a brown viscous liquid with a viscosity (25℃) of 500–700 mPa·s and a hydroxyl value of (500±20) mg KOH/g. It can replace conventional diethanolamine and triethanolamine crosslinkers, improving the resilience of high-MDI system high-resilience foams while accelerating product curing and demolding.

BASF’s Pluracol PEP series are secondary hydroxyl polyether tetraols obtained from the reaction of pentaerythritol and propylene oxide. Pluracol 858 is a small-molecule polyether triol. Bayer’s Arcol Polyol 1004A/1004, Arcol Polyol 1030, Arcol Polyol 1070, Multranol 4011 (Desmophen 4001T), and Multranol 4012 (equivalent to their Desmophen 4012R) are also polyoxypropylene polyols. Arcol Polyol 1004A is a low-acid-value polyether diol used as a chain extender and curing agent for elastomers.

In addition, Solvay’s low-molecular-weight polycaprolactones, such as CAPA2054, 2043, 3031, 3022, 3050, 3091, and 4101, can also be used as chain extenders and crosslinkers for polyurethane elastomers, coatings, and adhesives. Their physical properties can be found in the “Polycaprolactone Polyols” section.