Acrylic Polyol Research Articles

Quantitative study of water transport during the hydrolysis of polymer coatings exposed to water vapor

Thermoset acrylic–melamine resins are widely used for automobile exterior coatings. These materials are formulated by reacting an acrylic polyol with an alkylated melamine. Because the reactions are reversible, acrylic–melamine coatings tend to hydrolyze during exposures in moist environments. During hydrolysis, water in the coating film is consumed. To keep the moisture content in the film in equilibrium, water must be transported from regions of high water concentration to regions of low water concentration. An approach based on Fourier transform infrared (FTIR) spectroscopy analysis of the coating degradation fitted to a transport model is presented to estimate the diffusion coefficients and velocities of water transport during the hydrolysis of an acrylic–melamine coating exposed to different relative humidities (RHs). Theoretical prediction agreed well with the experimental FTIR data of coating hydrolytic degradation. Generally, both the diffusion coefficient and velocity of water transport in the coating increased with increasing RH. Since water transport resulting from the hydrolysis reactions is a very slow and complex process, the approach presented here provides a viable means for obtaining valuable data for quantitative analyses of coating hydrolytic degradation at different RHs.

X-Ray Photoelectron Spectroscopy and Atomic Force Microscopy of High-Solid Polyurethane Resins

High-solid polyester polyol resins and high-solid acrylic polyol resins were synthesized, cured by isophorone diisocyanate trimer (IPDI) to obtain polyurethane films, and then characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). XPS analysis showed that the top layers of these polymer films are dominated by the soft segments of the resins; the IPDI segments tend to remain inside the bulk. The polyester resins cured by IPDI at room temperature show more soft segments than these resins cured by IPDI at 120°C. Thechange in mole ratios of hydroxyl to carboxylic and/or anhydride groups during the synthesis of the resins does not give rise to distinct variation in surface composition in our experimental range. AFM topographic images demonstrate that high-solid resins cured by IPDI exhibit numerous grains or aggregates on the surfaces, while low-solid resins cured by IPDI produce surfaces with only a few aggregates. Curing temperature, such as room temperature or 120°C, does not cause distinct differences in topographic images. Higher hydroxyl content of the resins results in smaller grains or aggregates on the surfaces. High-solid acrylic polyol resins cured by IPDI give rise to separated aggregates surfaces, which is different from high-solid polyester polyol resins cured by IPDI. High-solid Polyester polyol resin Acrylic polyol resin XPS AFM

Kinetics and mechanisms of catalyzed and noncatalyzed reactions of OH and NCO in acrylic polyol–1,6‐hexamethylene diisocyanate (HDI) polyurethanes. VI

AbstractFourier transform infrared spectroscopy was used to study the kinetics of noncatalyzed and catalyzed polyurethanes. These studies show that for noncatalyzed acrylic polyol–hexamethylene diisocyanate (HDI) trimer reactions, the reactions between OH and NCO of HDI exhibit second‐order kinetics, with first‐order kinetics with respect to NCO and OH. On the other hand, when dibutyltin dilaurate (DBTDL) is used as a catalyst in acrylic polyol–HDI trimer reactions, the reaction rate is first order with respect to NCO and 0.5 order in OH and DBTDL concentrations. A mechanism for the catalyzed acrylic polyol–HDI trimer crosslinking reactions is proposed and it appears that an equilibrium involving associations between OH and DBTDL exists, resulting in the formation of an active anion, which interacts with NCO to generate polyurethanes. To further verify this mechanism, the influence of acidity on the reaction rate constant was investigated. When the acidity of the system is increased, retardation of urethane formation occurs. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2322–2329, 2002

New low-VOC acrylic polyol dispersions for two-component polyurethane coatings

Two-pack water-borne polyurethane coatings are a relatively new technology which promises excellent performance properties combined with a low content of volatile organic compounds (VOCs). In this paper the effects of a polymer structure of acrylic polyol dispersions on film-formation and final coating properties (pot life, hardness, gloss) are investigated. Two kinds of water-borne acrylic polyols have been investigated. Both water-borne polyols show good results in applicability, hardness and chemical stability when elevated temperatures can be used. The polyol based on a single-step synthesis route is superior in pot life, hardness and chemical stability. The low paint VOC of 130g/l indicates that this polyol already fits demands of future VOC regulations.

Evaluation of alkane sulfonic acid catalysts and derivatives for curing activity in a melamine/acrylic polyol coating formulation using fourier transform infrared spectroscopy

FTIR measurements may be able to replace more laborious physical property determinations and thus serve as an efficient screening methodology in catalysis driven processes. This possibility was considered in evaluating alkane sulfonic acids catalyst performance to promote the cure of a melamine resin with acrylic polyols. The example chosen here is an evaluation of alkane sulfonic acid derivatives to measure catalytic efficiency of the cure reaction between hexamethoxymethylmelamine (HMMM) and an acrylicpolyol. The IR measurements provided information on the extent of reaction, via monitoring the disappearance of the OH stretching mode at 3472 cm−1 associated with the acrylic polyol. This method may also be used to provide detailed information on reaction kinetics and insight into the cure mechanism. As the reaction proceeds, the coating’s properties change and IR measurements can serve as a probe for these changes.

Synthesis of acrylic resins for high-solids coatings by solution and separation polymerization

Conventional solution polymerization under monomer-starved conditions was compared with separation polymerization, also known as monomer-starved, as a method for making acrylic resins with low polydispersity (D=Mw/Mn). Separation polymerization employs aliphatic or cycloaliphatic solvents that are good solvents for the monomers but poor solvents for the resin; thus, the resin separates during polymerization. Various process conditions, initiators, chaintransfer agents, and solvents were studied, focusing mainly on a monomer line-up of methyl methacrylate, styrene, ethyl acrylate, and 2-hydroxy ethyl methacrylate in a 15/15/40/30 weight ratio. Two initiators, t-amyl peroxy 2-ethyl hexanoate and t-butyl peroxy 2-ethyl hexanoate gave about equal, excellent results. 2-Mercapto ethanol was selected as a chain transfer agent. With these ingredients, the separation polymerization method is capable of producing oligomeric acrylic polyol resins with polydispersities (D) of about 1.7 to 1.8 when Mn is in the range 1350 to 1600. These resins have substantially lower solution viscosities than a commercial benchmark resin, which has Mn=1230 and D=2.03. In preliminary tests of 2K polyurethane coatings, the film properties obtained with acrylics made by separation polymerization were, on balance, superior to those obtained with a commercial benchmark resin.

Effect of particle size distribution on the performance of two-component water-reducible acrylic polyurethane coatings using tertiary polyisocyanate crosslinkers

A two-component waterborne polyurethane system with a 250 g/l VOC has been formulated to meet the performance requirements of the automotive refinish market. This paper discusses an experimental study to develop a two-component water-reducible polyurethane coating using a tertiary aliphatic polyisocyanate crosslinker with dry times, appearance, and reactivity equivalent to a two-component solventborne system. The effects of acrylic polyol monomer composition, glass transition temperature (Tg), and hydroxyl concentration on drytime and ambient cure film performance have been determined. The effects of particle size distribution of the formulated coating on performance were examined. It was found that smaller particle size provides overall improved film properties, i.e., faster dry time, increased hardness, and faster development of solvent resistance. The relationships of both acrylic polyol composition and formulation variables on the particle size distribution are discussed.

Optimization of acrylic polyols for low VOC two-component water reducible polyurethane coatings using tertiary isocyanate crosslinkers

Polyol development is critical in order to develop a two-component waterborne polyurethane coating that can attain a 250 g/l VOC while maintaining performance requirements. This paper discusses a statistical study to develop a 250 g/l, two-component water reducible polyurethane coating using a tertiary isocyanate crosslinker with dry times and reactivity equivalent to a two-component solventborne system. The effects of acrylic molecular weight, glass transition temperature ( T g), and hydroxyl concentration on viscosity and dry time have been analyzed and statistical models identified that describe the relationships between these experimental factors and responses. Contour plots show tradeoffs between obtaining a low viscosity resin and maintaining fast dry time of the applied formulation. The data from the statistical study suggests that it is difficult to achieve the overall performance desired for an automotive refinish system with the simple four-component acrylic system investigated. The addition of either a longer chain hydroxyl functional monomer or styrene in the acrylic polyol improved the overall performance of the system. Faster cure was also demonstrated with an alternate catalyst.

Application of graft copolymers using macromonomer method to two-component polyurethane coatings

Graft copolymers as acrylic polyols in two-component polyurethane coatings were prepared by the free-radical copolymerization of methyl methacrylate, n-butyl methacrylate, n-butyl acrylate, acrylic acid, 2-hydroxyethyl methacrylate and poly(methyl methacrylate)-macromonomers. The poly(methyl methacrylate) macromonomer was prepared by the polymerization of methyl methacrylate in the presence of thiopropionic acid as a transfer agent followed by the reaction with glycidyl methacrylate. These polymers, whose numbers of poly(methyl methacrylate) macromonomer branches and the molecular weights of the poly(methyl methacrylate) macromonomer branches were controlled, offer an advantage over conventional resins with respect to the application/appearance of coatings as well as the final film properties. Some of these advantages were higher solids and a better control of the coating rheology, an increase in the cross-linking reactivity of the polyols with polyisocyanate and improvement in film toughness. The change in the morphological structure of the films under tensile stress was of particular interest.

Cure of secondary carbamate groups by melamine-formaldehyde resins

Melamine-formaldehyde (MF) resins have been used as crosslinkers for hydroxyl-functional coreactants in thermoset coatings for about 60 years. Crosslink densities of films prepared from oligomeric urethane polyols suggested that the methoxymethyl groups of MF resins could react with urethane groups (i.e., secondary carbamate groups) as well as reacting with hydroxyl groups. Co-reactants that contain secondary carbamate groups and no hydroxyl groups have been prepared with several types of backbone structures. Cure of such co-reactants by MF resins has been studied using a gradient oven with determination of impact resistance, solvent resistance, and hardness. Several formulations from these cure profile sets have been selected for crosslink density determinations by dynamic mechanical analysis (DMA). Crosslink densities of cured films are consistent with complete conversion of secondary carbamate groups at temperatures only slightly higher than those used for cure of hydroxyl groups. The -OH groups on certain acrylic polyols were converted to secondary carbamate groups. The original acrylic and the converted acrylic were both cured with MF resins. Acid resistance was much better for films prepared from the acrylic that contained secondary carbamate groups.

Crosslinking with malonate blocked isocyanates and with melamine resins

Automotive clearcoats with improved acid etch resistance are being formulated using a combination of a dialkyl malonate blocked polyisocyanate, a melamine crosslinker, and an acrylic polyol,1,2 These coatings contain lower levels of melamine crosslinker compared to conventional acrylic/HMMM systems and show excellent acid etch resistance. We explored the reaction mechanism of this complex crosslinking system and found explanations for the good chemical resistance properties.

Star oligomers for low VOC polyurethane coatings

Hydroxyl functional star oligomers prepared by ring-opening polycondensation have a narrow molecular weight and composition distribution. Such oligomers give a solids/viscosity advantage over linear structures at the same molecular weight and can be used in low VOC two component polyurethane coatings. The solids/viscosity advantage is a combination of the highly branched structure and the narrow molecular weight distribution. Acrylic polyols formulated at the same molecular weight/polarity/average functionality and hydroxyl value give poorer drying properties. This difference can be explained by the fact that the free radical polymerisation technique leads to polymer compositions having a broad molecular weight and functionality distribution and high fractions of polymer with a low functionality. The potlife of acrylic polyols based formulations is adversely affected by the higher fractions of high molecular weight material and with a high average functionality. In low VOC polyurethane formulations star oligomers need to be formulated within a narrow range of compositions (TG, polarity), functionality and molecular weight to get an optimum balance of drying properties and potlife.

Waterborne two-pack isocyanate-free systems for industrial coatings

Two-pack paint systems are used in various segments of the industrial coatings market. Stringent legislation concerning the emissions of organic solvents has led to an increasing demand for green alternatives, for the current solventborne products. Water, in this respect, is one of the most interesting technologies for ambient curing systems. A novel waterborne isocyanate-free two-pack system for high gloss topcoats is herewith described. The system is based on two waterborne acrylics with different functional groups. Properties of the system were compared with the performance of waterborne two-pack polyurethanes based on polyisocyanates and waterborne acrylic polyols. Solventborne versions of both two-pack systems were also used as references.

Fabrication and characterization of polyester and acrylic polyurethane optical waveguides

Asystematic study of the fabrication and characterization of planar optical waveguides based on polyester and acrylic polyurethane coatings has been made. Mode effective refractive-index measurements have been made on a large number of these waveguides fabricated on glass prepared in different conditions. The results show that the mixture of polyester and acrylic polyol in the initial solution can be used to control the refractive index from 1.563 to 1.595. The propagation losses are estimated to be 10-20 and 6-12 dB/cm for polyester and acrylic-based waveguides, respectively.

Mechanism and kinetics of the reaction of acrylic polyols with hexa (methoxymethyl) melamine

AbstractNew procedures have been developed to study the mechanism and kinetics of the crosslinking of acrylic polyols with hexa (methoxymethyl) melamine. Both the rate of gel formation and alcohol release are found to be affected by the concentration and the nature of the catalyst and to be proportional to the hydrogen ion concentration. The rate of gel formation can be modeled by using the Miller and Macosko procedure with the input consisting of the polymer composition and the rate of release of reaction products. The correlation between calculated and experimentally determined quantity of gel formed is good when the reactions are slow, but becomes poor when gel formation is rapid, suggesting that the formation of large rings is favored by rapid reaction. The rate of gel formation shows much less variation among different polymer systems than does the rate of release of reaction products from the crosslinking reaction.