Hydrolysis of acrylic melamine

Structure are the hydrolysis of acrylic-melamine crosslinks and the subsequent foimiatlon of melamine-melamine crosslinks. The rate of hydrolysis during exposure depends on both the humidity and the ultraviolet light Intensity as well as the coating composition. The enhancement by ultraviolet light has been attributed to a singlet-triplet transition by the melamine molecule. 

It is the purpose of this paper to further characterize the changes in crosslink structure that occur on weathering. In particular, a detailed study of the dependence of the rate of methoxy disappearance on humidity and light intensity is presented which verifies the photoenhancement of hydrolysis. The effect of changes in the composition of the acrylic/melamine coating and the effect of common photostabilizers on the rate of photohydrolysis has been determined. Finally, possible mechanisms for photoenhanced hydrolysis are discussed. 

The ether linkage formed during the crosslinking reaction of amino formaldehyde resins with hydroxy functional polymers is sensitive to acid hydrolysis. [2.197] Exposure to acid rain is sufficient to damage high solids acrylic/melamine coatings. The use of either active CH [2.198] or NH functional polymers [2.199] creates linkages... 

The major component of the paint is an alkyd-melamine or acrylic-melamine resin. When hydrolysis is completed, the dimethylether bonds (—CHj—O—CH2—) giving the crosslinks, are broken. This can be easily detected by infrared spectroscopy. 

The pressurized hydrolysis technology is available for all TP bumpers of Toyota cars, which are painted with acrylic-melamine resin or alkyd-melamine resin. However, the recovered bumpers contain some repaired ones which are repainted with urethane paint film. The pressurized hydrolysis technology is not available for the urethane paint film because this film is not hydrolyzable with water vapor in such conditions. To maintain the high performance of the recycled material, the repaired bumper must be sorted from the recovered TP bumpers. Considering the difference of the chemical reaction of the paint films with dye agents, a paint film dyeing method was develop>ed. A nonrepaired bumper which is painted with acrylic- or alkyd-melamine resin becomes red and fluorescent color when a mixture of dye agent of Acid Red 52 and a solvent of lactic acid are applied. On the other hand, a repaired bumper... 

FTIR techniques in combination with or as complement to other measurement techniques have been used in a wide range of photochemistry studies on polymers. These include bisphenol-A polycarbonate [173], polycarbonate coatings on mirrors [174], PMMA [175], poly( -butyl acrylate) [176] and polypropylene [177]. DSC and FTIR studies have been used in conjunction to investigate the nature of y-radiation-induced degradation and its effect on the 19°C and 30°C phase transitions in PTFE [178]. IR studies of the hydrolysis of melamine-formaldehyde crosslinked acrylic copolymer films have shown that copolymer-melamine formaldehyde crosslinks are broken and that crosslinks between melamine molecules are formed [179]. The thermal and photo-degradation mechanisms in an IR study of cured epoxy resins were found to be related to the autoxidative degradation processes for aliphatic hydrocarbons [180]. 

While epoxy coatings based on DGEBA and other aromatic epoxies are limited to undercoats and under-the-hood applications because of their poor UV resistance, GMA-based coatings have been developed for improved acid-etch performance automotive top coats. They compete with traditional acrylic polyol-melamine topcoats that are highly susceptible to acid rain-induced hydrolysis, and offer better mar resistance and less worker exposures than isocyanate-based topcoats (242,243). BMW has coverted to a GMA-acrylic powder clear coat developed by PPG. 

A wide variety of other specialty monomers are also used to provide specialized performance properties for coating applications. For example, amine functional monomers can be used to improve adhesion to aged alkyd substrates. Specialized monomers can also be used to improve exterior durability, for example VEOVA (vinyl ester of vesatic acid) monomers can improve the hydrolysis resistance of vinyl acetate polymers, and n-butyl methacrylate can be used to enhance the durabiHty of BA-MMA acrylics. Polymer hydrophobicity can be fine tuned by varying the levels of hydrophobic and hydrophilic monomers in the composition and styrene or ethyl hexyl acrylate are used to increase film hydrophobicity and reduce water permeability in BA-MMA systems. Specialty monomers are also used to provide specific chemical functionality to polymer compositions. For example, hydroxyethyl methacrylate can be used to provide hydroxyl functionality to acrylic resins, allowing these polymers to be used in cross-linkable thermoset coatings which cure via melamine chemistry. While specialty monomers are used at relatively low levels in polymer compositions, they frequently provide the performance features needed for the successful application of emulsion polymers in many coating areas. 

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