Aminoplast polymers

Other polycondensation reactions which lead to finely dispersed polymers in liquid polyethers are the polycondensation reactions of urea and melamine with aqueous formaldehyde [92-95]. The reaction medium is usually polyether polyols, PO homopolymers or PO-EO copolymers (random or block copolymers), with MW of 3000-5000 daltons. During the polycondensation reaction, the aminoplast polymer precipitates, being insoluble in polyether and water (water from formaldehyde solution and reaction water), is eliminated by vacuum distillation. A variant of this reaction is to develop the polycondensation in water, and water containing the aminoplast polymer (as a viscous solution) is added to a polyether polyol, under vacuum, and at high temperature (100-130 °C), water being continuously eliminated from the reaction medium. The aminoplast insoluble polymer precipitates in the form of fine particles. 

Aminoplast dispersions, in spite of the accessibility and low cost of raw materials, are not produced industrially, due to the risk of toxic formaldehyde elimination, especially when the resulting aminoplast polymer polyol is used for slabstock foams and, of course, for moulded flexible foams used for seating or interior automotive parts. 

They account for approximately 80% of the production of aminoplastic polymers, their annual production being estimated to 1.2 million tons. The precursor consists of mono-, di-, and trimethylolurea. 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

The tonnage of plasticisers consumed each year exceeds the annual tonnage consumption of most plastics materials. Only PVC, the polyolefins, the styrene polymers, the aminoplastics and, possibly, the phenolics are used in large quantity. 

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. 

Another POLIDCASYR extension ensures that structural features of the backbone can be distinguished from those occurring in side chains. The system furthermore augments structure codes for polymers using a controlled vocabulary of keywords, such as epoxy resin , aminoplast or phenoplast . 

In far too many instances trade-name polymer nomenclature conveys very little meaning regarding the structure of a polymer. Many condensation polymers, in fact, seem not to have names. Thus the polymer obtained by the step polymerization of formaldehyde and phenol is variously referred to a phenol-formaldehyde polymer, phenol-formaldehyde resin, phenolic, phenolic resin, and phenoplast. Polymers of formaldehyde or other aldehydes with urea or melamine are generally referred to as amino resins or aminoplasts without any more specific names. It is often extremely difficult to determine which aldehyde and which amino monomers have been used to synthesize a particular polymer being referred to as an amino resin. More specific nomenclature, if it can be called that, is afforded by indicating the two reactants as in names such as urea-formaldehyde resin or melamine-formaldehyde resin. 

In an alternate process organic pesticide was microencapsulated in aminoplast shellwalls starting with the urea-formaldehyde or melamine-formaldehyde prepolymers dissolved in the aqueous phase [16]. Growing polymer chains wrap around the pesticide emulsion... 

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... 

The first truly synthetic resin was developed by Baekeland in 1911 (phenol-formaldehyde). This was soon followed by a petroleum-derived product called coumarone-indene, which did indeed have the properties of a resin. The first synthetic elastomer was polychloroprene (1931) originated by Nieuwland and later called neoprene. Since then many new types of synthetic polymers have been synthesized, perhaps the most sophisticated of which are nylon and its congeners (polyamides, by Carothers), and the inorganic silicone group (Kipping). Other important types are alkyds, acrylics, aminoplasts, polyvinyl halides, polyester, epoxies, and polyolefins. 

The acrylic resin used was Rohm and Haas Co.s QR-496 ( 4), so the exact nature of the polymer is not known. It is hydroxy functional, thus requiring an aminoplast for proper cure commercially modified hexa-methylol melamine resin was used for this purpose. During the first part of the investigation the acrylic resin was used without the melamine resin, but for final evaluation it was incorporated to give a more realistic commercial coating system. 

As a coating composition containing a polymer and an amino resin is heated under acidic conditions, reaction occurs between the active hydrogen groups and the condensable groups of the aminoplast as follows ... 

Formaldehyde is employed in the production of aminoplasts and phenoplasts, which are two different but related classes of thermoset polymers. Aminoplasts are products of the condensation reaction between either urea (urea-formaldehyde or UF resins) or melamine (melamine-formaldeliyde or MF resins) with formaldehyde. Phenoplasts or phenolic (phenol-formaldehyde or PF) resins are prepared from the condensation products of phenol or resorcinol and formaldehyde. 

The principal feature that distinguishes thermosets and conventional elastomers from thermoplastics is the presence of a cross-linked network structure. As we have seen from the above discussion, in the case of elastomers the network structure may be formed by a limited number of covalent bonds (cross-linked rubbers) or may be due to physical links resulting in a domain structure (thermoplastic elastomers). For elastomers, the presence of these cross-links prevents gross mobility of molecules, but local molecular mobility is still possible. Thermosets, on the other hand, have a network structure formed exclusively by covalent bonds. Thermosets have a high density of cross-links and are consequently infusible, insoluble, thermally stable, and dimensionally stable under load. The major commercial thermosets include epoxies, polyesters, and polymers based on formaldehyde. Formaldehyde-based resins, which are the most widely used thermosets, consist essentially of two classes of thermosets. These are the condensation products of formaldehyde with phenol (or resorcinol) (phenoplasts or phenolic resins) or with urea or melamine (aminoplastics or amino resins). 

Aminoresins or aminoplastics cover a range of resinous polymers produced by reaction of amines or amides with aldehydes [14,46,47]. Two such polymers of commercial importance in the field of plastics are the urea-formaldehyde and melamine-formaldehyde resins. Formaldehyde reacts with the amino groups to form aminomethylol derivatives which undergo further condensation to form resinous products. In contras to phenolic resins, products derived from urea and melamine are colorless. 

In the encapsulation process depicted in Figure 5.82(c), polymerization is initiated in the water phase of oil-in-water emulsion. As the molecular size of the polymer increases, it deposits at the water-oil interface where it continues to grow forming a cross-linked polymer capsule wall. In a typical example, methylol urea or methylolmelamine is added to the aqueous phase along with an ionic polymer. The pH is adjusted to 3.5—4.5 and the mixture allowed to react for 1-3 h at 50-60°C. The ionic polymer in the aqueous phase assists deposition of the aminoplastic at the water-oil interface. 

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