Polyols may be classified according to their chemistry.[5] Some of these chemistries are polyether, polyester,[6] polycarbonate[7][8] and also acrylic polyols.[9][10] 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.[11] There is also an increasing trend to use biobased (and hence renewable) polyols.[12][13][14][15]
Uses
Polyether polyols have numerous uses.[16][17] As an example,
polyurethane foam is a big user of polyether polyols.[18]
Polyester polyols can be used to produce rigid foam.[19][20] They are available in both
aromatic and
aliphatic versions.[21][22] They are also available in mixed aliphatic-aromatic versions often made from recycled raw materials, typically
polyethylene terephthalate (PET).[23]
Acrylic polyols are generally used in higher performance applications where stability to
ultraviolet light is required[24] and also lower
VOC coatings.[25][26] Other uses include direct to metal coatings.[27] 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.[28]
Caprolactone-based polyols produce polyurethanes with enhanced hydrolysis resistance.[29][30]
Polycarbonate polyols are more expensive than other polyols and are thus used in more demanding applications.[31][32] They have been used to make an
isophorone diisocyanate based prepolymer which is then used in glass coatings.[33] They may be used in reactive hotmelt
adhesives.[34]
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.[40] In polyurethane prepolymer production, a low molecular weight polyol-
diol such as
1,4-butanediol may be used as a chain extender to further increase molecular weight though it does increase
viscosity because more
hydrogen bonding is introduced.[38]
Sugar alcohols, a class of low molecular weight polyols, are commonly obtained by
hydrogenation of sugars.[41]: 363 They have the formula (CHOH)nH2, where n = 4–6.[42]
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.
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.[43] 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.[44]
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
There are polyols based on renewable sources such as
plant-based materials including
castor oil and
cottonseed oil.[45][46][47] Vegetable oils and biomass are also potential renewable polyol raw materials.[48] Seed oil can even be used to produce polyester polyols.[49]
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 viscous at room temperature due to hydrogen bonding.
^Howarth, GA (2003). "Polyurethanes, polyurethane dispersions and polyureas: Past, present and future". Surface Coatings International Part B: Coatings Transactions. 86 (2): 111–118.
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^WO2011129940A1, Montgomery, Steven; Brown, Shawn & Sonnenschein, Mark et al., "Polycarbonate polyols and polyurethanes made therefrom", issued 2011-10-20
^Abraham, T.W.; Höfer, R. (2012). "10.03 - Lipid-Based Polymer Building Blocks and Polymers". In Matyjaszewski, K; Möller, M (eds.). Polymer Science: A Comprehensive Reference. Elsevier. pp. 15–58.
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^McAdams, Carina; Farmer, Steven (September 2003). "Stabilization of Rigid Systems Containing Aromatic Polyester Polyol and Water". Journal of Cellular Plastics. 39 (September 2003): 369–386.
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^Boustead, I. (2005).
"Polyurethane rigid foam"(PDF). Eco-Profiles of the European Plastics Industry. Brussels: PlasticsEurope. Archived from
the original(PDF) on 2013-09-25.