Resol PF resins

However, there is practically no research work pubUshed focussing on the preparation of kraft lignin-PF hybrid resins for the impregnation of papers intended for decorative laminates. Here, typically resol PF resins of low viscosity and rapid curing characteristics are applied. Kraft papers are impregnated with PF and stacks of such papers are cured in a press at elevated temperature to form cured panel products that are preferably used as exterior claddings and in kitchen and bathroom surfaces where high moisture resistance is required. 

Resole PF resins are produced by the reaction of phenol with excess formaldehyde (P F molar ratio 1 1.8 to 1 2.2) in the presence of an alkali catalyst (Fig. 3). Because resole resins contain reactive methylol groups, they are self-curing resins that can condense with active sites on the phenol rings to form a cross-linked network in the presence of heat even without additional hardener. Resole resins have a very branched structure (Fig. 3) and are by far the more important of the two types of PF resins for wood composites. 

The lower cost of the urea-modified PF resins is a combination of PF solids extension by lower cost urea and improved adhesion and distribution capabilities. The improvements in storage stability stem from the thinning and dilution effects as well as from the formaldehyde scavenging. Liquid PF resoles with high free formaldehyde contents tend to be less stable in storage. 

In the manufacture of pure resorcinol resins, the reaction can be violently exothermic unless controlled by the addition of alcohols. Because the alcohols perform other useful functions in the glue mix, they are left in the liquid adhesive. PRF adhesives are generally prepared firstly by reaction of phenol with formaldehyde to form a PF resol polymer, that has been proved to be in the greatest percentage, and often completely, linear [95], In the reaction step that follows the resorcinol chemical is added in excess to the PF-resol to react it with the PF-resin -CH2OH groups to form PRF polymers in which the resorcinol groups can be resorcinol chemical or any type of resorcinol-formaldehyde polymer. 

Phenol-formaldehyde (PF) resins were synthesized to manufacture non-flammable insulating foam. When alkali catalyst, for example, barium hydroxide (Ba(OH)2), was present, lesol resins are produced[l]. In the analj s of molecular species of resol, capillary GC-MS had been used to separate hemiformal-type compoimds(acetylated hydroxybenzylhaniformals)... 

All 10 PF resins were produced with Ba(OH)2 catalyst for 300 min with varying F/P molar ratio, OH/P wt %, and reaction temperatureffable 1). The resins were stored frozen at -18H until analysis. Molecular species in resol were analyzed by GO after trimethylsilylation of sample with N,0-bis(trimethylsilyl)trifluoracetamide in pyridine[2]. A glass column (3 m x 2 mm I.D.) packed with 3% OV-1 on 100-120 mesh Chromosorh W HP was applied. Injection... 

Fig. 15.2.5. 15.1 MHz C CP/MAS NMR spectra of (a) resole type resin (b) cured resin after treatment with 1.0 N sodium hydroxide under N2(g) for 65°C for 3 days, and (c) cured PF resin after treatment with 36.8% formal under N2(g) at 25°C for 1 day.

In the A stage, simple PF resins are readily soluble in alcohol, esters, ketones, phenols, and some ethers, and insoluble in hydrocarbons and oils. As a class, resols tend to be more soluble in alcohols and water, and novolaks tend to be more soluble in hydrocarbons. In the early stages of condensation, resols are often soluble in water, owing to the presence of methylolphenols, especially polyalcohol. This is more pronounced with resols that are derived from phenol. Cresilic resols are less soluble, and xylenolic resols are almost insoluble in water. The solubility of A-stage resins in dilute aqueous sodium hydroxide or in mixtures of water and alcohols follows the same trend. 

Phenolic resins are also extensively used in the binding of foundry molds. Both resol and novolak resins are used for this application. The sand is coated with the phenolic resin at a rate of 3 to 4%. The PF resin can be used both as an organic solvent solution and in powder form. Coating of the substrate can be done both at ambient or at higher temperature. In higher-temperature coatings novolaks are the preferred resins and in this application, waterborne resins (75% resin) can also be used. Hexamethylenetetramine as well as wax are added. Hexamine is often added separately from the resin to avoid precuring. 

Choi and Chung [16] were the first to prepare phenolic resin/layered sihcate nanocomposites with intercalated or exfoliated nanostructures by melt interaction using linear novolac and examined their mechanical properties and thermal stability. Lee and Giannelis [10] reported a melt interaction method for phenolic resin/clay nanocomposites, too. Although PF resin is a widely used polymer, there are not many research reports on PF resin/montmorillonite nanocomposites, and most of the research investigations have concentrated on linear novolac resins. Up to now, only limited research studies on resole-type phenolic resin/layered silicate nanocomposites have been published [17-19] and there is still no report on the influence of nano-montmorillonite on phenolic resin as wood adhesive. Normally H-montmorillonite (HMMT) has been used as an acid catalyst for the preparation of novolac/layered silicate nanocomposites. Resole resins can be prepared by condensation reaction catalyzed by alkaline NaMMT, just as what HMMT has done for novolac resins. 

Whereas celluloid was the first plastic material obtained by chemical modification of cellulose, the phenol-formaldehyde (PF) resin was the first commercially successful synthetic plastic. This phenolic plastic was discovered by L.Fi. Baekeland in Belgium in 1907, and Bakelite was produced industrially in 1910. Baekeland used the term resole to describe PF resins made with an alkaline catalyst, and those made with an acidic catalyst were called novolac. The ability of formaldehyde to transform some products in resinous materials was observed by Butlerov (1859) and Bayer (1872) [3]. 

Phenol-Formaldehyde (PF) Adhesives. PF resins are produced by the reaction of a phenol and formaldehyde. Depending upon the phenol-to-formaldehyde (P F) molar ratios and the type of catalyst, two types of PF resins can be produced resole and novolac PF resins (Figs. 3 and 4). 

Conversely, novolac PF resins are produced by the reaction of excess phenol with formaldehyde (P F molar ratio 1 0.8 to 1 1) in the presence of an acid catalyst. Unlike resole resins, novolac PF resins have a more linear structure (Fig. 4) and do not self-cure because they lack the residual reactive methylol groups of resoles. Therefore, an external curing agent such as hexamethylenetetraamine must be added to novolac resins to yield a cross-linked structure. Novalac PF resins cured under acidic conditions are not recommended for wood composites for long-term structural applications. 

We distinguish the following reaction states during the crosslinking of PF resins A-state Linear initial state resole or novolac liquid or solid meltable and soluble... 

Insulation. Resole resins are sprayed on hot fibers with the matted fibers then heated until cured. The resins are generally phenol based and prepared at high F/P mole ratios, e.g., 2.5-3.5, and cooked at low temperatures. These conditions produce primarily uncondensed polymethylolated phenols. The catalysts are commonly alkaline earth hydroxides. The resin is applied in 10-15% aqueous solutions.The PF resin imparts good dimensional quality to the glass wool mats and provides its important nonfianunability characteristics. 

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