Melamine formaldehyde system

The melamine-formaldehyde system is somewhat more complicated because of the basicity of the melamine. Okano and Ogata found the hydroxymethylation to be second order, but the rate increased with increasing pH in the range of pH 3.0-10.6. 

The urea - formaldehyde (UF) and melamine-formaldehyde systems represent similar hazards. Free formaldehyde, which can be present in trace amounts, may be liberated to the air when resins are processed or even slowly afterwards, which can irritate the mucous membranes. Formaldehyde is a metabolite occurring normally in the human body and is converted to formic acid by enzymic oxidation. Formaldehyde in the cured resin is believed to be due to the unreacted free formaldehyde left (there are also claims that it may be due to demethylolation reaction and/or cleavage of methylene-ether bridges as well). A model specification to set out the health hazards is presented for polymer mortar surfacings (out of epoxy, polyester and PU thermosets), intended for their use as indoor floor tappings [39]. 

Of the technologies used in the industry, the gelatin capsule had the largest market share in 1986. Over the last 20 years, the trend has been toward melamine-formaldehyde systems for the same application for reasons of cost, ease of manufacture, and quality. 

All major manufacturers of microcapsules with scented oils as the core material now use different variations of melamine-formaldehyde systems. 

But there is no doubt that the pure MF system has some unbeatable advantages on account of its great functionality and reactivity. It s not for nothing that the pure cross-linked melamine-formaldehyde system is the hardest commercially available plastic of all. ... 

Figure 2.15. Comparison of the heat capacity change as a function of reaction conversion for an epoxy(/ = 2)-amine(/ = 4), epoxy-anhydride, unsaturated polyester and melamine-formaldehyde system.

Scheme 11.3. Gelation mechanism of the melamine-formaldehyde system.

Another focus was put on the melamine-formaldehyde system. Through a microemulsion templating sol-gel route [237], thermal conductivities of the samples slightly below 0.025 Wm K have been obtained [238]. Most recently, polyimide aerogels have been developed by NASA s Glenn Research... 

Melamine—formaldehyde resins may be used in paper which contacts aqueous and fatty foods according to 21 CFR 121.181.30. However, because a lower PEL has been estabUshed by OSHA, some mills are looking for alternatives. Approaches toward achieving lower formaldehyde levels in the resins have been reported (66,67) the efficacy of these systems needs to be estabUshed. Although alternative resins are available, significant changes in the papermaking operation would be required in order for them to be used effectively. 

It has also been found that there can be interactions between hydrolytic degradation and photochemical degradation. Especially in the case of melamine-formaldehyde cross-linked systems, photochemical effects on hydrolysis have been observed. 

The term aminoplastics has been coined to cover a range of resinous polymers produced by interaction of amines or amides with aldehydes. Of the various polymers of this type that have been produced there are two of current commercial importance in the field of plastics, the urea-formaldehyde and the melamine-formaldehyde resins. There has in the past also been some commercial interest in aniline-formaldehyde resins and in systems containing thiourea but today these are of little or no importance. Melamine-phenol-formaldehyde resins have also been introduced for use in moulding powders, and benzoguanamine-based resins are used for surface coating applications. 

There is also growing interest in multi-phase systems in which hard phase materials are dispersed in softer polyether diols. Such hard phase materials include polyureas, rigid polyurethanes and urea melamine formaldehyde condensates. Some of these materials yield high-resilience foams with load deflection characteristics claimed to be more satisfactory for cushioning as well as in some cases improving heat resistance and flame retardancy. 

All wet strength agents are bi- or multi-functional molecules with the capability to cross-link with each other or with cellulose. The choice of chemistry depends to a large extent on pH. In acid systems, the main wet strength agents are urea-formaldehyde (U/F) and melamine-formaldehyde (M/F) resins, whereas in neutral and alkaline systems polyamine-polyamide-epichlorohydrin resins are more effective. However, these are not the only systems in use, and a summary of these and other available methods is provided in Figure 7.22. 

Conditions to be met in oven drying enamels depend also on the composition of the binder. Paint systems containing melamine-formaldehyde or urea-formaldehyde resins, for instance, harden by polycondensation with other resins, such as epoxy resins, short-oil alkyd or acrylic resins at elevated temperatures. Baking is carried out at temperatures between 100 and almost 200°C and may last from a few minutes to more than an horn. A general trend towards energy conservation has shifted public attention towards binders which require low baking temperatures. 

PolyHIPE materials have also been prepared by polycondensation in high internal phase emulsions [153]. Thus, a resorcinol-formaldehyde (RF) porous copolymer was synthesised from an o/w HIPE of cyclohexane in an aqueous solution of resorcinol, formaldehyde and surfactant. Addition of an acid catalyst to the emulsion, followed by heating, resulted in copolymerisation. Other systems prepared included urea-formaldehyde, phenol-formaldehyde, melamine-formaldehyde and a polysiloxane-based elastomeric species. 

Urea—formaldehyde and melamine—formaldehyde reagents are resin formers, which not only cross-link cotton but also copolymerize with themselves. These have been used both as simple cross-linkers or prepolymer systems. If too much of the polymerization is concentrated on the fiber surface, the fabric may be sufficiendy stiffer that it takes on a boardy character. As such, the finisher must control the action of agent to give the desired crisp hand but prevent the development of boardiness. Melamines have been recommended for applications when complete shrink resistance is required. However, both finishes were rejected for the white-shirt market because of loss of strength when hypochlorite bleach is used on account of vulnerable NH groups and the ensuing discoloration (37). 

Polyelectrolyte multilayer microspheres, prepared by alternating adsorption of dextran sulfate and protamine on melamine formaldehyde cores followed by the partial decomposition of the core, were used to immobilise the peroxidase and glucose oxidase. Retention of enzymic activity of the peroxidase/glucose oxidase system incorporated into the microspheres was demonstrated. These bienzyme system immobilised in the microspheres can be applied for kinetic glucose assays [ 156]. 

Experimental verification of Eq. (12) was carried out for a number of quite different systems curing both in a solution and bulk such as epoxy, epoxy silicone and silicone oligomers, melamine-formaldehyde and carbamide resins, derivatives of furan resins [11,28,32,63, 81]. Typical results plotted according to Eq. (12) are given in Fig. 18. After phase segregation (t > tp), the experimental dependences fl(t) are completely described by formula (12). The viscosity variation of the dispersion medium is a function of molecular mass andis calculated on the basis of Eq. (12). 

In the following discussion, only the most widely used adhesive types are described. These are the urea-formaldehyde (UF) resins, melamine-formaldehyde (MF) resins, phenol-formaldehyde (PF) resins, diisocyanates, polyisocyanates, polymers and copolymers of vinyl acetate, and polyamides. These are all predominantly thermosetting resin systems. 

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