Лако-красочные материалы — производство

It is very important in the commercial production of UF resins to be able to control the size of the molecules by the condensation reaction, since their properties change continuously as they grow larger. The most perceptible change is the increase in viscosity. Low-viscosity syrups are formed first. These change into high-viscosity syrups, which are clear to turbid. Molecular weight may vary from a few hundred to a few thousand, with a wide range of molecular size. These molecules are built up by water splitting off at random between reactive groups of neighboring molecules, thereby increasing their size. Once their solubi­lity, viscosity, pH, concentration, and so on, have been determined, they constitute the resins available commercially. The most important factors influencing the final properties of aminoplastic resins in industrial manufacture are the purity of the reagents, the molar proportions of the materials used, the preparation process used, and the pH variation and control.

The most common method of preparation for commercial UF resin adhesives is the addition of a second amount of urea during the preparation reaction. This consists of reacting urea and formaldehyde in more than equivalent proportions. Generally, an initial urea/formaldehyde molar ratio of 1:2.0 to 1:2.2 is used. Methylolation can in this case be carried out in a much shorter time, by using temperatures of up to 90 to 95°C. The mixture is then maintained under reflux. When the exotherm subsides (usually after 10 to 30min), the methylol compounds have formed, and the reaction is completed under reflux by adding a trace of an acid to decrease the pH to the UF polymer-building stage (pH 5.0 to 5.3). As soon as the right viscosity is reached, the pH is increased to stop polymers building and the resin solution is cooled to about 25 to 30°C. More urea (called second urea) is added to consume the excess of formaldehyde, until the molar ratio of urea to formaldehyde is in the range 1:1.1 to 1:1.7. After this addition of urea, the resin is left to react at 25 to 30°C for as long as 24 h. The excess water is eliminated by vacuum distilla­tion until a resin solids concentration of 64 to 65% is reached, and the pH adjusted to achieve suitable shelf life or storage life.

The final addition of urea can be done in one operation, or the urea may be added at suitable intervals in smaller lots. Second or further ureas can be added at a temperature slightly higher than ambient or can be added at higher temperatures, 60 to 90°C, according to the type of final resin wanted [13-16]. Increasing second or further urea additions tends to improve bond quality, especially at low formaldehyde/urea molar ratios [13-16]. Higher-molar-ratio resins tend to exhibit an overall better initial bond quality [14], but present an exponentially increased formaldehyde emission pro­blem [16], most often disqualifying them from many, or most, modern uses. Some UF resins used for joinery are also produced without a final or second urea addition. The pH used during the condensation reaction (not the methylolation) is generally in the range 4.8 to 5.3.

Control of the average molecular size of the finished resin is essential for correct flow in plywood and particleboard applications while in the hot press prior to curing. Too low a level of condensation (i. e., low-molecular-weight resins) may give too much flow; the resin ‘‘runs away’’ from the wood or sinks into it rapidly under pressure, leaving ‘‘starved’’ glue lines. This can be corrected by lowering the pH by adding an acid or acid-producing substance, usually a curing agent, hardening catalyst, or simply, hardener. If a resin of too high a condensation stage (i. e., high-molecular-weight resins) is on hand, its flow under normal pressure and temperature may be too low to produce good results. This can usually be corrected by adding flow agents to it, provided that at least some flow is left in the resin. It is generally an advantage to produce resins with ample flow in the factory. Their storage life is longer and finishing can be done at any time, at short notice, to specification, particularly by adjusting the flow and speed of cure.

Resins that have lost part of their flow during manufacture or storage must be corrected by the addition of a flow agent. The simplest means is often the addition of water sprayed on the compound and mixed in well. If a resin is still capable of flowing, this procedure produces a resin with properties that are still acceptable. In cases where moist­ure content control is critical, it may be necessary to allow a little more time for ‘‘heating’’ to let the added moisture escape. However, if the flow is very low, and large quantities of water must be used to bring the flow back to normal, this method is not recommended. The large amount of water would cause longer ‘‘breathing’’ times to be necessary due to excessive volatile components, and excessive shrinkage may take place, causing too much stress on the glue lines. It must be kept in mind that excessive water addition causes UF resin precipitation. The best way to correct flow in these cases is to mix the resin with large amounts of an equal resin of the same quality that has a higher flow. Any proportion may be used to bring the flow back to normal. If increased flow is desired, 0.5 to 2.0% of spray- dried UF or melamine-formaldehyde resin can also be added to function as a flow agent. Methylol compounds, such as dimethylolurea, also increase flow, but they increase the water released during reaction more than do spray-dried resins. Lubricating agents such as calcium stearate are also able to give a fair degree of flow increase.

Many substances have been suggested as curing agents. These include the following acid products: (1) boric acid, (2) phosphoric acid, (3) acid sulfates, (4) hydrochlorides, (5) ammonium salts of phosphoric or polyphosphoric acid, (6) sodium or barium ethyl sulfate, (7) acid salts of hexamethylenetetramine, (8) phthalic anhydride, (9) phthalic acid,

(10) acid resins such as poly(basic acid)-poly(hydric alcohol), (11) oxalic acid or its ammo­nium salts, and many others. However, the most widely used curing agents in the wood products industry are still ammonium chloride or ammonium sulfate. Their effect can be altered by retarding the reaction of the resin. This is done by the simultaneous addition of small amounts of ammonia solution (which is eliminated during hot curing) to lengthen the pot life of the glue mix. Latent catalysts that produce acid only on heating may also be used, such as dimethyloxalate and other easily hydrolizable esters, or halogenated sub­stances such as 0.1 to 0.2% of bromohydrocinnamic acid and others (Fig. 2).

The driving force in the use of these salts as hardeners is their capacity to release acid, which decreases the pH of the resin and thereby accelerates curing. The speed of the reaction between the ammonium salt and formaldehyde (or ammonia and formaldehyde when this is present) also determines, together with the amount of heat supplied, the rate of acid release and therefore the rate of curing:

4NH4Cl + 6HCHO! 4HCl + (CH2)6N4 + 6H2O

Hexamethylene

tetramine

Ammonium chloride is a better hardener than hydrochloric acid, as the latter produces weaker joints. The effect of a fixed amount of ammonium chloride on the pH change and on the rate of resin curing as a function of time and temperature is shown in Fig. 2.

Often, particularly in cold-setting UF resins for joinery, hardeners consisting of mixtures of a salt such as ammonium chloride or ammonium sulfate with an acid such as phosphoric acid, citric acid, or others are used to regulate pot life and rate of curing. Both pot life and rate of curing of the resin can then be regulated (1) by varying the concentration of the hardener in the resin, (2) by changing the relative proportions of acid and salt, and (3) by changing the type of acid and/or salt composing the hardener. Acting on these three principles, setting times of between a few minutes and several hours can easily be obtained.