A polyol is an organic compound containing multiple hydroxyl groups. The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, three and four hydroxyl groups are diols,[1][2] triols,[3] and tetrols,[4][5] respectively.

Classification

Polyols may be classified according to their chemistry.[6] Some of these chemistries are polyether, polyester,[7][8][9][10] polycarbonate[11][12] and also acrylic polyols.[13][14] Polyether polyols may be further subdivided and classified as polyethylene oxide or polyethylene glycol (PEG), polypropylene glycol (PPG) and Polytetrahydrofuran or PTMEG. These have 2, 3 and 4 carbons respectively per oxygen atom in the repeat unit. Polycaprolactone polyols are also commercially available.[15] There is also an increasing trend to use biobased (and hence renewable) polyols.[16][17][18][19]

Uses

Polyether polyols have numerous uses.[20][21] As an example, polyurethane foam is a big user of polyether polyols.[22]

Polyester polyols can be used to produce rigid foam[23][24] They are available in both aromatic and aliphatic versions.[25][26] In addition they are also available in mixed aliphatic-aromatic versions often made from recycled raw materials, typically Polyethylene terephthalate (PET).[27][28]

Acrylic polyols are generally used in higher performance applications where stability to Ultraviolet light is required[29] and also lower VOC coatings.[30][31][32] Other uses include DTM -Direct to metal coatings.[33] As they are used where good UV resistance is required, such as automotive coatings, the isocyanate component also tends to be UV resistant and hence isocyanate oligomers or prepolymers based on Isophorone diisocyanate are generally used.[34]

Caprolactone based polyols produce polyurethanes with enhanced hydrolysis resistance.[35][36] They have other uses too.[37][38][39][40]

Polycarbonate polyols are more expensive than other polyols and are thus used in more demanding applications.[41][42] They have been used to make an Isophorone diisocyanate based prepolymer which is then used in glass coatings.[43][44] Even though they are produced in smaller volumes than other polyols, the market size is valued at 400 million dollars by 2031.[45] They maybe used in reactive hotmelt adhesives.[46]

All polyols maybe used to produce polyurethane prepolymers.[47][48][49][50][51] These then find use in Coatings, Adhesives, Sealants and Elastomers.

Low molecular weight polyols

Structure of an idealized alkyd resin derived from the polyol glycerol (red, a low molecular weight polyol) and phthalic anhydride.
Structure of an idealized alkyd resin derived from the polyol glycerol (red, a low molecular weight polyol) and phthalic anhydride.

Low molecular weight polyols are widely used in polymer chemistry where they function as crosslinking agents and chain extenders. Alkyd resins for example, use polyols in their synthesis and are used in paints and in molds for casting. They are the dominant resin or "binder" in most commercial "oil-based" coatings. Approximately 200,000 tons of alkyd resins are produced each year. They are based on linking reactive monomers through ester formation. Polyols used in the production of commercial alkyd resins are glycerol, trimethylolpropane, and pentaerythritol.[52] In polyurethane prepolymer production, a low molecular weight polyol-diol such as 1,4-Butanediol maybe used as a chain extender to further increase molecular weight though it does increase viscosity because more hydrogen bonding is introduced.[53]

Low molecular weight polyols
Pentaerythritol.svg

Pentaerythritol
Xylitol Structural Formula V.1.svg

Xylitol

Sugar alcohols

Sugar alcohols, a class of low molecular weight polyols, are commonly obtained by hydrogenation of sugars.[54] They have the formula (CHOH)nH2, where n = 4–6.[55]

Sugar alcohols are added to foods because of their lower caloric content than sugars; however, they are also, in general, less sweet, and are often combined with high-intensity sweeteners. They are also added to chewing gum because they are not broken down by bacteria in the mouth or metabolized to acids, and thus do not contribute to tooth decay. Maltitol, sorbitol, xylitol, erythritol, and isomalt are common sugar alcohols.

Polymeric polyols

Polymeric polyols
Polyether Polyol Structural Formula V3.svg

Polyether polyol

(The oxygen atoms of the ether linkages

are shown in blue.)

Polyester Polyol Structural Formula V.3.svg

Polyester polyol

(The oxygen and carbon atoms

of the ester groups are shown in blue.)

The term polyol is used for various chemistries of the molecular backbone. Polyols may be reacted with diisocyanates or polyisocyanates to produce polyurethanes. MDI finds considerable use in PU Foam production.[56] Polyurethanes are used to make flexible foam for mattresses and seating, rigid foam insulation for refrigerators and freezers, elastomeric shoe soles, fibers (e.g. Spandex), coatings, sealants and adhesives.[57]

The term "polyol" is also attributed to other molecules containing hydroxyl groups. For instance, polyvinyl alcohol is (CH2CHOH)n with n hydroxyl groups where n can be in the thousands. Cellulose is a polymer with many hydroxyl groups, but it is not referred to as a polyol.

Polyols from recycled or renewable sources

Apart from geopolitical issues to reduce reliance on petroleum and hydrocarbon sources, there is a growing trend to have polyols based on either recycled or renewable sources. The ideal is plant-based materials. Peer reviewed research is in progress.[58] Castor oil is a naturally occurring plant based material that has multiple hydroxyl functional groups and thus a polyol. Vegetable oils and biomass may also be used to produce a renewable polyol.[59]

Properties

Since the generic term polyol is only derived from chemical nomenclature and just indicates the presence of several hydroxyl groups, no common properties can be assigned to all polyols. However, polyols are usually highly viscous (when polymeric) to solid (when low-molecular weight) at room temperature due to hydrogen bonding.

See also

References

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