Polyol

In organic chemistry, a polyol is any compound that carries multiple hydroxyl groups (–OH). The exact meaning of the term shifts a bit depending on whether you're talking about food science or polymer chemistry. When a polyol has exactly two, three, or four hydroxyl groups, it's called a diol, triol, or tetrol, respectively.

Polyols can be sorted by their chemical makeup. Common types include polyether, polyester, polycarbonate, and acrylic polyols. Polyether polyols break down further into polyethylene oxide (also called polyethylene glycol, or PEG), polypropylene glycol (PPG), and polytetrahydrofuran (PTMEG). These have two, three, and four carbon atoms per oxygen atom in their repeating units. Polycaprolactone polyols are also sold commercially. There is a growing trend toward using biobased, renewable polyols.

Polyether polyols are used in many ways. For instance, polyurethane foam relies heavily on them. Polyester polyols can make rigid foam and come in aromatic, aliphatic, and mixed aromatic-aliphatic versions—the mixed kind often uses recycled raw materials like polyethylene terephthalate (PET). Acrylic polyols are typically reserved for high-performance jobs that need good ultraviolet light stability and low volatile organic compound (VOC) coatings. They also go into direct-to-metal coatings. Because they're used where UV resistance matters—like automotive coatings—the isocyanate component is usually UV-resistant too, often based on isophorone diisocyanate. Caprolactone-based polyols create polyurethanes that resist hydrolysis better. Polycarbonate polyols are pricier than other types, so they're used in demanding applications; they've been made into isophorone diisocyanate prepolymers for glass coatings and can appear in reactive hotmelt adhesives. All polyols can be turned into polyurethane prepolymers, which then find their way into coatings, adhesives, sealants, and elastomers.

Low molecular weight polyols are common in polymer chemistry, where they act as crosslinking agents and chain extenders. Alkyd resins, for example, use polyols in their production and are used in paints and casting molds. They are the main binder in most commercial oil-based coatings, with about 200,000 tons made each year. These resins link reactive monomers through ester formation. The polyols used to make commercial alkyd resins are glycerol, trimethylolpropane, and pentaerythritol. In polyurethane prepolymer production, a low molecular weight diol like 1,4-butanediol can serve as a chain extender to boost molecular weight, though it also increases viscosity because it introduces more hydrogen bonding.

Sugar alcohols are a class of low molecular weight polyols usually made by hydrogenating sugars. Their formula is (CHOH)nH2, where n ranges from 4 to 6. They are added to foods because they have fewer calories than sugars, though they are generally less sweet and are often paired with high-intensity sweeteners. They also appear in chewing gum because mouth bacteria don't break them down into acids, so they don't cause tooth decay. Common sugar alcohols include maltitol, sorbitol, xylitol, erythritol, and isomalt. Some sugar alcohols—like myoinositol, pinitol, and mannitol—help cells maintain water balance and respond to drought or cold stress.

The term "polyol" also covers various molecular backbone chemistries. Polyols can react with diisocyanates or polyisocyanates to make polyurethanes. MDI is widely used in polyurethane foam production. Polyurethanes go into flexible foam for mattresses and seating, rigid foam insulation for refrigerators and freezers, elastomeric shoe soles, fibers like Spandex, and coatings, sealants, and adhesives. The word "polyol" is also applied to other molecules with hydroxyl groups. For example, polyvinyl alcohol has the formula (CH2CHOH)n, with n hydroxyl groups that can number in the thousands. Cellulose is a polymer with many hydroxyl groups, but it is not called a polyol.

There are polyols made from renewable sources like plant-based materials, including neem oil, castor oil, and cottonseed oil. Vegetable oils and biomass are also potential renewable raw materials. Seed oil can even be used to produce polyester polyols.

Because the term "polyol" simply comes from chemical naming and only means several hydroxyl groups are present, no single set of properties applies to all polyols. However, they are usually viscous at room temperature due to hydrogen bonding.