Pre-Pressing Of Plywood Panels In A Cold Press Using Urea Formaldehyde Resin

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Pre–pressing has become the standard practice now-a-days in modern plywood production. A number of advantages can be derived from pre-pressing if consolidation of pre-pressed panels is fair or good. It is common practice to pre-press plywood panels in a cold press to activate the cold tack of the resin before transferring the veneer stack into a multi-daylight hot press. This procedure and the cold tack of the adhesive are necessary for the veneers to stick together after the adhesive mix was applied. Sufficient cold tack is essential for the step of feeding the hot-press, particularly in plywood panel production, where the glued veneer layers have to stick together to fit into the individual openings of the press. To ensure the intimate contact of the single veneers within a panel lay-up, the cold tack of the resin has to fulfill certain requirements. The process involves a temporary bond formation between the glue coated and non coated veneers before hot pressing .The veneer assembly is compressed near to its final thickness during the short pre –pressing cycle. Bond formation during pre-pressing is the result of gelling of the adhesive by loss of water however, final curing of the resin does not occur at this stage.

Different types of adhesive are used in the plywood production depending on the desired material properties. Phenol formaldehyde (PF) resins belong to the group of phenol-plastic binders and are appreciated for their high water and weathering resistance. Urea formaldehyde (UF) resins belong to the group of amino plastic binders most important to wood industry. Unmodified UF resin is applied when the final product is used for interior purposes with limited water resistance. UF resins are also described as being easy to handle, low priced and widely available. Moreover, they ensure a high strength of the bond under dry conditions. Another benefit of UF is the possibility of cold tack, which is the focus of the present study. The ability of urea formaldehyde (UF) resins to develop cold tack is needed in plywood production during prepressing in a cold press to ensure that the veneers stick together, can be transported, and fit into a multi-daylight hot press.

Tack of a resin is defined as the property to form a bond immediately when in contact with another surface and is usually referred to as cold tack. The cold tack of UF is assumed to be influenced by various adhesive related factors such as resin synthesis procedure, adhesive age, addition of additives, or the open assembly time as well as by adherent related factors such as wood species, moisture content, surface age, drying mechanism, PH of the glue line etc. The cold tack of a UF resin depends on its molar ratio F/U. A higher cold tack can therefore be achieved by using specific resin cooking procedures or by the addition of additives to the resin like POLYMATE-888 as a resin booster during the cooking of the resin to make the resin high cold tack and high viscosity. They state that the drying behavior of the resin applied to the surface of a panel is a key factor for the development of cold tack with UF resins. This drying behavior is time dependent, shows a maximum after a certain time, and leads to a reduction in cold tack when this maximum has been exceeded. Although cold tack behavior was identified to be of importance, information on systematic investigation of cold tack behavior for plywood production is scarce. However, it was aimed for a better understanding of the cold tack of UF for plywood production. The drying behavior of the resin is a key factor for cold tack, which is supposed to be influenced by various factors such as lay-up time, resin amount, resin age, moisture content, veneer temperature, and prepress time. A better understanding of the change in cold tack with these factors will contribute to a better control of this step in the plywood production.

Higher tensile shear strength was again observed for high temperature, high resin amount, and a low lay-up time. No obvious change in tensile shear strength with a change in resin age or lay-up time was apparent. Additionally, interactions between the factors (1) lay-up time and moisture content, (2) resin amount and moisture content, (3) lay-up time, resin amount, and resin age, as well as (4) lay-up time, resin amount, and veneer temperature were marked as significant.

The moisture content has the highest impact on the tensile shear strength, followed by the veneer temperature. Increasing moisture content negatively affects the tensile shear strength, whereas increasing veneer temperature shows a positive effect. The coded factors indicate increasing tensile shear strength for low moisture content or high temperature. They attributed the higher cold tack to the increase in the solid content by the faster water removal at higher temperatures. A slightly negative effect was calculated for the lay-up time, which can be explained by the importance of drying of the resin for the cold tack development; the drying out process,

however, may not exceed a certain time, since the cold tack decreases again. Even though the resin amount as a main factor showed no significance for the model, the interaction with the moisture content has a minor negative effect, which leads to the presumption that a high moisture content and high resin amount decrease the tensile shear strength.

Based on the results of this study it was proven that the lay-up time, veneer moisture content, veneer temperature, and the pre-press time, together with the interaction of (1) lay-up time and moisture content as well as (2) resin amount and moisture content, had a significant impact on the tensile shear strength and thus, the cold tack of the investigated UF resin mix.

The basic advantages of doing pre –pressing of plywood before hot pressing to minimize the formation of defective plywood,repairing and repairing cost, reduction of steam pocket, increase the production efficiency ,veneer saving etc. To obtain good prepress tack the adhesive quality and spread must be correlate to veneer temperature, veneer moisture, assembling time, procedure typically needed for production and equipment capabilities etc.

AUTHOR: S. C. Sahoo, Scientist, Indian Plywood Industries Research &Training Institute, (IPIRTI)

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