Wednesday, April 2, 2008

Chemicals Composition

The addition of limited amount of urea to resin to the resin solution just before use, either directly or in a hardener is a well-known and often recommended way of reducing the smell of formaldehyde. But it is not easy to determine quantitatively what type of product this contributes, whether resin or insoluble methylene urea compound; it depends on the molar ratio, the pH on addition, the rate at which the pH falls during curing, and the temperature.

It will be noticed that the term “water tolerant” is used. This expression, for reasons that are given later, is often more accurate and therefore is preferable to “water-solubility.” Here, the term is used to apply to the behaviour of the uncured resin, and should not be confused with “water-resistance,” a term that is applied to the cured adhesive.

Formaldehyde is used in the form of an aqueous solution (formalin), and the reaction with urea normally proceeds from liquid aqueous reactants to a solid end-product, but the manufacturer stops the reaction at an intermediate stage while the product is still liquid; the user initiates the final stage of the reaction to convert the liquid to the solid end-product. The final stage-the curing reaction is a continuation of the condensation reaction started by the manufacturer. Unlike some phenolic resins, a urea resin normally requires the addition of a hardening agent to initiate the final stage. As received by the user, therefore, a U.F. glue is generally, but not always, a two package adhesive.

The simplest reaction products of formaldehyde and urea are methylol compounds. Monomethylol and dimethylol urea are the primary products of the reaction, and their realative proportions are determined by the molecular ratio of formaldehyde to urea. The methylol urea are not resinous materials but crystalline compounds of simple molecular structure formed by the addition of a molecule of formaldehyde to an amine group; they are not themselves adhesives. The methylol ureas are produced under neutral or weakly alkaline conditions (heat may not be applied), and in the preparation of a resin they are not separated from the reaction mixture. After an appropriate period of time, the reaction mixture is made weekly acid to promote the condensation reaction leading to formation of the resin. At this stage the three parameters, pH, temperature and time, have to be considered together in determining the desired end-point. An alternative method entails determining the temperature at which a clear resin becomes cloudy on cooling; or the two methods can be combined.

After arresting the reaction by neutralisation, the solution can be evaporated to increase the resin concentration and viscosity, or it can be spray-dried to produce a powder that is re-constituted with water by the user of the glue.

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