Aminoplasts hardening

Cross-linking with aminoplasts and phenoplasts constitutes an important class of hardeners for high molecular-weight epoxy resins that require elevated temperature cures (see Amino resins). 

Reactivity and hardening reactions of aminoplastic adhesive resins... 

MUPF/PMUF-resins harden in the main under similar acid conditions to UFs and MUFs. Because MUF-resins harden in the acid pH range, but phenolic resins have a minimum reactivity under these conditions, there is the real danger that the phenolic portion of the resin might not really be incorporated into the aminoplastic... 

Upon hardening of the composition the plastometer will record the process of relaxation of the stresses with the working bar being idle. Stresses arising in the material with the progress of time fall off to an equilibrium value Tq. Figure 5 shows the hardening process of the aminoplasts at temperatures 140 and 1 and 160 °C under a continuous deformation and the relaxation of stresses which attains the maximum value Tq. 

The hardening rate of an aminoplast is determined by the amount of time necessary for a complete hardening of the specimen in the form of a conical test core. The specimen is pressed under conditions specifically indicated in the standards on the aminoplast ... 

In Ref. [33] there is adiscussion of the method for calculating optimal temperature regimes and seasoning time in the processing of aminoplastics by taken account of the heating and hardening rates of the materials for different temperatures of preliminary heating (60 and 80 °C). 

For the sake of simplification Xjeas. regarded as a sum of the consequtive processes of heating (Xheat.) hardening (Xhard.) of the aminoplastic. The time of heating is determined from the equation of heat transfer i.e. Eq. (2). 

To determine the hardening time of aminoplastics, use is made of the Kanavets plastometer. An equation is deduced for determining the seasoning time in the mould ... 

At present the progress in the pressure casting method as applied to aminoplasts involves the automation of control for the casting process, as well as the elimination of the hardening of the composition melt in the distribution gates. 

The physico-mechanical properties of aminoplasts in the articles are determined by the degree of hardening and macrostructural defects. In the cooling of the articles down to room temperature reactive groups in the polymer are still retained, but their interaction is made difficult due to the loss of mobility caused by the molecules of the reticular polymer because of the latter s vitrification. Simultaneously a nonequilibrium supramolecular structure is recorded. Heat treatment of the articles does not alter the supramolecular structure, the latter remaining invariable. Heat treatment at a temperature below the vitrification temperature may only cause either a certain additional hardening of the binder or increase the... 

Urea-formaldehyde resins and similar aminoplast precondensates form the greatest proportion of all the resins used as additives. Mono-methylated and dimethylated ureas are used, as are the analogous condensation products of formaldehyde with melamine. The monomeric compounds penetrate into the intermicellar space in the cellulose in aqueous solution, and there harden with heat to form insoluble resins (cf. also Section 28.2). Since the formation of mono- and dimethylated urea is reversible, CH2O occurs in equilibrium. Formaldehyde can form methylene cross-link bridges between the individual chains. In addition, longer cross-linking... 

For a general introduction, see Wood adhesives - basics. Aminoplastic resins are polycondensation products of the reaction of aldehydes with compounds carrying amine or amide groups. Formaldehyde is by far the primary aldehyde used. The name amino-plastic is traditional, but is a misnomer these are thermosetting, irreversibly hardening resins. Two main classes of these resins exist urea-formaldehyde (UF) resins and melamine-formaldehyde (MF) resins. Today, for wood adhesives, pure MF resins are not used anymore because of cost, and only melamine-urea-formaldehyde (MUF) adhesives are used. The main use of these resins is in composite products such as particle board, plywood, medium density fibreboard (MDF), oriented strand board (OSB), glu-lam/flngerjointing and furniture. 

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