Melamine crosslinking

Common crosslinkers for hydroxyl-terminated PUD s are water-dispersible isocyanates and melamines. Unlike other crosslinkers, melamine crosslinkers usually have good stability in water. Two major drawbacks are associated with melamine crosslinking. In the first place, the bond must be heated. It will not... 

Table V). The ED behaviour was much improved by chemical incorporation of the monoepoxide, CARDURA ElO (no. 5j Table V). Unfortunately, in spite of the many hydroxyl groups present the cure of these binders with external crosslinkers (melamine or phenolic resins) was marginal. This undoubtedly is due to the lack of catalysing acidic functions and to the retarding influence of the amino groups.

Stable nitroxides, independent of the way of their formation, are traditionally considered as the prerequisite and the principal species of the free-radical scavenging activity of HAS [15,125]. More pathways and oxidants certainly participate in NO formation (Eq.15). The specificity of NO" generation via oxidation with ROOH may serve for a rapid evaluation of the weatherability of HAS doped crosslinked melamine and urethane acrylic copolymers [152,159]. 

Highly dispersed crosslinked melamine-formaldehyde structures have been prepared by Reimer [37]. In a typical open pore melamine-formaldehyde preparation, a melamine resin that has a low memelamine fonnaldehyde molar ratio (1 1.6) and that contains 15% sucrose as an additive was converted into an open pore melamine—formaldehyde oligomer by acidification with 7% phosphoric acid (based on resin solid) at 50-60°C. 

The reaction of melamine and formaldehyde in nearly neutral solution produces methylol melamine or its low molecular weight polycondensates. Applying the melamine solution, then removing water by drying at elevated temperature, and finally reacting the condensate with heat produces hardened and crosslinked melamine-formaldehyde oligomers that are nonmelting and insoluble. 

Renner, A., Highly dispersed, crosslinked melamine-formaldehyde polymers, Mafe-romol. Chem., 120, 68, 1968. 

After a de-nib, spray surfacer is applied to build up the film thickness before top-coating. The surfacer contains a high level of pigment and extender (at least 35% by volume) and frequently a saturated polyester resin with a melamine —or urea —formaldehyde crosslinker. The coating is applied at thicknesses up to 35 ftm and stoved for 20 min at 150-165°C. 

Sanding is carried out at this stage and, after clean-up, the final colour or top-coat is applied. There is some variation in the resin chemistry used. Alkyds crosslinked with melamine-formaldehyde are widely used for non-metallic pigmentation. Metallics are usually based on acrylics for better durability. The acrylic may be thermoset with melamine-formaldehyde or a thermoplastic lacquer (plasticised copolymer of methyl methacrylate). A thickness of about 50ftm is applied and stoved for 20 min at 130°C (lacquers receive a bake-sand-bake process for a smoother appearance). 

The melamine resins have many more reactable groups and so less are needed for crosslinking (25% of total compared with 50% of total with UF resins), and have greater heat resistance than the urea resins because of the pseudo-aromatic nature of the six membered ring. 

Amino resins are those polymers prepared by reaction of either urea or melamine with formaldehyde. In both cases the product that results from the reaction has a well crosslinked network structure, and hence is a thermoset polymer. The structures of the two parent amino compounds are shown in Figure 1.1. 

When crosslinked, amino resins are very resistant to most organic solvents, though they tend to be attacked by both acids and alkalis. Urea-formaldehyde polymers are more susceptible to attack than those prepared from melamine and formaldehyde. 

Thermosets differ molecularly from thermoplastics in that their individual chains are anchored to one another through crosslinks. The resulting network creates cohesive materials that demonstrate better thermal stability, rigidity, and dimensional stability than thermoplastics. Some examples of traditional thermosets are melamine-formaldehyde resins, which are used to treat fabrics to make them wrinkle-free, and Bakelite (a phenol-formaldehyde resin), a historically important polymer used in many applications, such as costume jewelry, electrical switches, and radio casings. 

Kinetic Model. All of the coatings used in this study are hydroxy functional acrylic copolymers crosslinked with melamine formaldehyde crosslinkers. The chemistry of crosslinking with melamine formaldehyde crosslinkers has been discussed in detail elsewhere (5.11). The type and rate of the reactions depend primarily on... 

The question whether the intramolecularly crosslinked microparticles of non-aqueous polymer dispersions are really microgels is also justified, considering non-aqueous dispersions prepared from acrylic copolymers and melamine/formaldehyde crosslinker with particle sizes of about 300 nm. [45, 343]. In any case, these crosslinked polymeric microparticles are useful constituents of high-solids coatings, imparting a yield stress to those solutions which probably involves attractive forces between the microparticles. 

Crosslinked polymer particles with a rather complex structure, which have also been designated by the name microgels and recommended as components of metal effect paints, consist of carboxyl-terminated oligoesters of 12-hydroxy stearic acid which were reacted with glycidyl methacrylate, subsequently copolymerized with MMA and hydroxymethyl methacrylate and then crosslinked by hydroxy melamine [346]. 

The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxymethyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-sty-rene) to afford a negative-tone resist material which does not swell upon development in aqueous base. 

The polymer used in this study was a m,p-cresol novolak ( 1 =14000, Mn =1300). Melamine resins were obtained from American Cyanamid Co. Polymer/crosslinker formulations were prepared using 23 wt % novolak in cellosolve acetate and 15 parts per hundred crosslinker based on novolak solids. p-Toluenesulfonic acid was... 

The crosslinkers examined in this study were aminoplast resins 1-4 selected from melamine-formaldehyde, urea-formaldehyde, benzoguanamine-formaldehyde, and glycoluril-formaldehyde resins, all of which undergo the crosslinking sequence shown in Scheme 1. The response of these crosslinkers to acid catalysis in thin films is compared on a relative basis to the well studied methylated melamine, 1 19-11). 

Acid Concentration Required for Crosslinking Resist Films. The strong acid catalyzed reaction of methylated melamine 1 with novolak in a film was studied first in order to estimate the amount of acid generated in experimental AHR resists upon exposure. Incremental amounts of a 1% solution of p-toluenesulfonic acid (pTSA) were added to a resist solution which contained no RSAG. The solutions were coated onto wafers and subjected to a typical bake cycle. The dissolution rate... 

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