Acrylic polyols

Acrylic polyols represent a special group of amorphous polyols, of molecular weight (MW) of 8000-13000 daltons, obtained by radical copolymerisation of acrylic monomers (ternary or quaternary copolymers), such as acrylic or methacrylic acids and esters. The source of hydroxyl groups in these acrylic polyols is the utilisation in the radical copolymerisation reaction of hydroxyalkyl acrylates or hydroxyalkyl methacrylates [1,2] as comonomers. The acrylic polyols are used in high performance polyurethane (PU) coatings. 

The general radical copolymerisation reaction for synthesis of acrylic polyols is shown in reaction 10.1. It is obligatory that one of the comonomers is a hydroxyalkyl acrylate or hydroxyalkyl methacrylate (mainly hydroxyethylacrylate and hydroxyethylmethacrylate) in order to introduce hydroxyl groups (as lateral groups, not as terminal groups) available for the reaction with -NCO groups of diisocyanates (reaction 10.1). 

Generally, the radical copolymerisation reactions of acrylic comonomers are performed in an adequate solvent, by drop wise addition of monomer - initiator (peroxides) mixture. 

The resulting performances of acrylic polyol based PU coatings depend profoundly on the chemical nature of the monomers used. Thus, methylmethacrylate (MMA) confers exterior durability, excellent light stability, hardness and water resistance. Styrene confers hardness, water stability but, unfortunately, poor light stability. Butyl and 2-ethylhexyl acrylates and methacrylates confer flexibility and acrylic and methacrylic acids confer adhesion to metals and solvent/grease resistance. 

Generally, the acrylic polyols, which are amorphous solids at room temperature, are used as solutions (40-100% solids), in various solvents, but sometimes, special structures, which are liquid at room temperature, are used without solvents (for example the structures very rich in butyl esters). The most used solvents for acrylic polyols are xylene, naphtha, butyl acetate, 1-methoxypropyl acetate, and butyl glycol. The viscosity of acrylic polyols varies from 1000-9000 mPa-s at 25 °C. The equivalent weight varies from 400-700 (OH number varies between 80-140 mg KOH/g). Due to the presence of acrylic acid units, the acidity of acrylic polyols is relatively high (2-5 mg KOH/g). Acrylic polyols have an excellent appearance, are very light in colour (they are practically colourless polyols), the Gardner colour frequently being 1. 

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The acrylic polyol component is then mixed with the water contaminated isocyanate component and allowed to cure via the standard polyol isocyanate reaction ... 

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of polymer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7], Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. 

Pieper R, Ekin A, Webster DC, Casse F, Callow JA, Callow M, E. (2007) A combinatorial approach to study the effect of acrylic polyol composition on the pProperties of crosslinked sUoxane-polyurethane fouling-release coatings. J Coatings Techn Res 4 453 61... 

Kugel AJ, Jarabek LE, Daniels JW et al. (2009) Combinatorial materials research applied to the development of new surface coatings XII novel, environmentally friendly antimicrobial coatings derived from biocide-functional acrylic polyols and isocyanates. J Coat Technol Res 6 107-121... 

Secondly, the reaction was inhibited by both strong and weak acids. Strong acids, such as HBF4, completely stopped the reaction. Weaker acids, snch as acetic acid, had a much less pronounced and concentration-dependent effect. It has been snggested that the concept of the ionic mechanism mnst be viewed with some degree of caution, since the reaction proceeded faster in non-polar solvents, snch as cyclohexane, compared with a dipolar aprotic solvent, snch as dimethylformamide, whereas one wonld expect that the polarity of the solvent wonld significantly stabilize the ionic catalyst intermediates. However, Urban et al. have demonstrated that an ionic mechanism is likely operative in the reaction of hexamethylene diisocyanate with an acrylic polyol, nsing DBTDL as catalyst. 

The most important comonomers used as raw materials in acrylic polyols syntheses are shown in Figure 10.1. 

Figure 10.1 Comonomers used as raw materials for acrylic polyol syntheses...

Two representative acrylic polyols, quaternary copolymers of various acrylic monomers, are shown in Figures 10.2 and 10.3. 

Figure 10.2 A representative structure of an acrylic polyol for hard PU coatings (with a...

The MW of the acrylic polyol (Figure 10.2) is around 13000 daltons and the glass transition temperature (T ) is around 50 °C. The high content of methylmethacrylate confers hardness to the resulting PU coating. 

The molecular weight of the acrylic polyol (Figure 10.3), used in elastic PU coatings, is around 11600 daltons. The Tg of the acrylic polyol (Figure 10.3) is around - 20 °C, proving its high flexibility at room temperature. 

It is very interesting that acrylic polyols can be used as precursors to synthesise hybrid structures, such as acrylic - polyester polyols, by the polymerisation of some cyclic monomers such as e-caprolactone, initiated by hydroxyl groups of acrylic polyols (reaction 10.2). 

It is interesting that the systems containing acrylic polyols are readily dispersed in water after the neutralisation of -COOH groups with ammonia or with dimethylethanolamine [2]. The hydroxyl content is around 2-4% and the nonvolatile content around 40-45%, somtimes a cosolvent such as butyl glycol or naphtha (0-10%) is used. The viscosity of this kind of dispersion is low, around 200-1500 mPa-s at 25 °C. 

Desmophen . (Bayer Miles] Polyester, polyether, or acrylic polyols. 

Urethane resins are synthesized from isocyanates and chemical compounds with hydroxyl or urethane groups (—N—C—O—) including water, polyesters, epoxies, and acrylics [8,9]. The chemical structure of urethane coating is shown in Fig. 13.4. Polyester and epoxy have better barrier resistance to moisture and chemical attack than the acrylic polyol. Ahphatic isocyanate-based coatings are resistant to UV hght and have excellent gloss and color retention. 

Chem. Descrip. Pigment disp. in acrylic polyol Uses Colorant for acrylic urethanes Features Offers exc. chem. resist. 

Chem. Descrip. Mutlifunctional aromatic urethane acrylate contg. an acrylated polyol diluent... 

Chem. Descrip. Hexatunctional aliphatic urethane acrylate containing acrylated polyol diluent Uses Urethane-acrylic in coatings and inks Features High cure speed good scratdVabrasion resist. 

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