Resins, formaldehyde-based

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

Another possible solution to an odor problem is to substitute a less noxious or more acceptable odor within a process. An example of this type of control is the substitution of a different resin in place of a formaldehyde-based resin in a molding or forming process. 

However, the most important furan resins are those produced with 2-furfuryl alcohol, for example, the 2-furfuryl alcohol-formaldehyde-based resins, which are normally synthesized by a condensation reaction catalyzed by acidic sites and promoted by heat [224] or the poly(furfuryl alcohol) thermosetting resin that is usually synthesized by the cationic condensation of its monomer 2-furfuryl alcohol, which polymerizes exothermically in the presence of a catalyst such as acid and iodine in methylene chloride, producing black, amorphous, and branched and/or cross-linked structures [225],... 

The first demonstration of the industrial importance of heme peroxidases in grafting applications has been the development of hybrid resins from renewable sources to replace phenol-formaldehyde based resins. Phenolic resins are widely used in surface coatings, adhesives, laminates, molding, friction materials, abrasives, flame retardants, carbon membranes, glass fiber laminates, fiberboards, and protein-based wood adhesives, [5]. Table 7.1 and Fig. 7.2 summarize some of the... 

Table III shows the properties of particleboards prepared with Phase I whey permeate-based resin. Table IV shows the properties of rice-hull-reinforced building boards using Phase II resin. Low-quality boards are prepared with rice hulls, but their qualities may be improved by using ground hulls or adding sawdust to the formulation. Although whey-based resins have been found to be excellent adhesives for binding solid lignocellulosic materials, these resins tend to require higher cure temperatures and longer cure times as compared to formaldehyde-based resins.

Many consumer products containing formaldehyde-based resins release formaldehyde, leading to consumer annoyance and health-related complaints (1-8). This release has led to various symptoms, the most common of which are irritation of the eyes and of the upper respiratory tract (2z5). Formaldehyde also produced nasal carcinomas in mice and rats after exposure to 14.1 and 5.6... 

Formaldehyde as a pollutant in the indoor air is usually connected with the use of formaldehyde based resins in e.g. building materials and in furniture. This article presents measurements of the formaldehyde emission from various products containing urea-formaldehyde (UF) or phenol-formaldehyde (PF) resins. The emission from all test objects have been measured in a ventilated test chamber at the standardized testing atmosphere 23 C, 50 % RH according to the International Organization for Standardization (ISO). The emission from woodbased panels and other materials have been measured at a loading factor of 1.0 m /m and at an air change rate of 1.0 h . ... 

In the future the Swedish formaldehyde rules may include other UF-bonded products as MDF-boards and the requirements also may be formulated as emission rates. On the behalf of the National Board of Physical Planning and Building, the Swedish National Testing Institute has performed a study on the emission from products bonded with formaldehyde based resin. The measurements have been performed in a ventilated test chamber at standardized climate in agreement within the work of the European Organization for Standardization, CEN. 16 West European countries are represented in CEN. 

The chemical participation of lignin macromonomers in polymerization or copolymerization reactions has been focussed mostly on the reactivity of both types of OH groups, and hence in the synthesis of polyesters, polyurethanes and polyethers, although some research has also dealt with their intervention through the unsubstituted aromatic sites in different formaldehyde-based resins in partial replacement of phenol [58, 59]. 

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). 

It is interesting to remark that other reactive materials which will readily undergo transesterification analogous to that of propylene carbonate with methanol are trialkyl borates, tetraalkyl titanates, and trialkyl phosphates in an alkaline environment. Also gas injection of methyl borate (and carbon dioxide) has been found to enhance the results of wood composites bonded with formaldehyde-based resins [29], just as the addition of propylene carbonate and glycerol triacetate have been shown to do in wood composites bonded with phenolic resins. 

Although PVA adhesives for nonstructural applications and formaldehyde-based resins for structural applications have price advantages over epoxies and offer excellent... 

Details An odourless fume, used as a hardener for formaldehyde-based resins. 

The presence of numerous hydroxyl groups able to react with formaldehyde makes starch-derived products suitable chemicals for formaldehyde-based resins. Research on this subject started many years ago and showed that in a number of applications it is possible to partially replace or extend urea formaldehyde, phenol formaldehyde and melamine formaldehyde resins without significantly affecting the finished product s performance. In many applications, adhesive systems based on formaldehyde resins incorporate a polysaccharide component. More than 4.5 Mio mto of formaldehyde-based resins have been produced in Western Europe alone. The use of carbohydrates allows lower consumption of oil-based resins and, consequently, reduced release of formaldehyde in the environment. 

The use of hydroxymethylated resorcinol (HMR) as an adhesion promoter to enhance solid wood bonding is a recent development that holds considerable promise in improving the durability of the bonds between solid wood and adhesives. HMR consists of a mix of methylolated resorcinol monomers and low molecular weight oligomers [1]. Its usefulness as an adhesion promoter has been shown for a number of different wood species and different adhesives formaldehyde-based resins, epoxies and polyurethanes [2-7]. 

The best adhesive formulation for phloroglucinolic tannins, such as pine tannin extracts is, instead, a comparatively new and is also capable of giving excellent results when using resorcinol tannins such as a wattle tannin extract [68-71]. The adhesive gluemix consists only of a mix of an unmodified tannin extract 50 per cent solution to which paraformaldehyde and polymeric nonemulsifiable 4,4 -diphenylmethane diisocyanate (commercial pMDI) are added [68-71]. The proportion of tannin extract solids to pMDI can be as high as 70/30 w/w, but can be much lower in pMDI content. This adhesive is based on the peculiar mechanism by which the pMDI in water, is hardly deactivated to polyureas because it reacts faster with the hydroxymethyl groups of a formaldehyde-based resin, be it a tannin or another resin [69,71]. 

Isocyanates by themselves cannot be used to bond plywood because of the characteristic balance of rheology and rate of hardening, which are characteristic of PMDl. However, recently, they have been used as minority additives (10-30%), copolymerized during panel hot-pressing with formaldehyde-based resins such as PF and even UF to yield very strong bondlines. These combinations have been used industrially to yield excellent bonding both in plywood and in particleboard. ... 

Out of the approximately 6.5 million ton of formaldehyde-based resins (in dry form) used as adhesives in the wood-based panels industry, more than 900,000 ton are based on melamine (133). The resins are produced mainly by the chemical industry itself, as well as partly by the wood-based panels industry on its so-called megasites. In 1999 approximately 28%, or 196,000 ton, of the melamine that was produced worldwide was converted into wood adhesives (133). 

Dessipri, E., et al., Use of FT-NIR Spectroscopy for On-Line Monitoring of Formaldehyde-Based Resin Synthesis. Europ. Polym. J., 2003.39 1533-1540. 

Varcum resins. Phenol-formaldehyde base resins used in plastics and var-nishes,... 

Formaldehyde-based resins were the first network polymers prepared by step polymerization to be successfully commercialized. They are prepared in two stages. The first involves the formation of a prepolymer of low molar mass which may either be liquid or solid. In the second stage the prepolymer is forced to flow under pressure to fill a heated mould in which further reaction takes place to yield a highly crosslinked, rigid polymer in the shape of the mould. Since formaldehyde is difunctional, the coreactants must have a functionality, /, eater than two and those most commonly employed are phenol (/= 3), urea (/= 4) and melamine (/= 6)... 

The theories of gelation presented here can only be applied when it is clear that the assumption of equal reactivity of functional groups is satisfactory. In terms of the polymerizations described earlier, the theories generally are applicable to the formation of polyester and polyurethane networks, but not to the formation of formaldehyde-based resins and epoxy resins. Failure of the principle of equal reactivity for the latter systems results from modification of the reactivity of a particular functional group by reaction of another functional group in the same molecule of monomer. 

---