In general, plywood is made of adhesive, inside wood core veneer and top and bottom outside face veneer made of three parts. According to the current situation of domestic production, We mainly analyzed separately from the close relationship between the permeability and the glue, from the above three parts .
First, the adhesive is generally urea formaldehyde used for pre pressing. Due to the vast majority of the local small and medium production, it is inevitable that a lot of existence, such as low solid content, low viscosity, low bonding strength and high formaldehyde. Especially environmental protection glue, in order to reduce the formaldehyde release, it will cause serious problems, such as stability is too poor, initial viscosity is too small, pre pressing time is too long and the bonding strength is too low. Because UF prepressing glue itself easy penetration, coupled with low solid content, low viscosity, low initial viscosity and prepressing time is long, the more easy penetration, especially the green plastic easy penetration, local plywood factory in any way, to increase the amount of flour, and the corresponding increase of amount of glue, to resist too much glue penetration, to ensure that the prepressing and bonding strength. As for the hot pressing of urea formaldehyde glue, generally used without pre pressure plate plate, mainly because of no prepressing and only bonding strength, the penetration time is very short, little penetration, ammonium chloride can be under a certain amount, after hot pressing rapid film, and obvious surface glue (province province the glue thin, gelatinize), so less urea formaldehyde adhesive than the pre not much difference between, worthy of our consideration. In fact, in contrast, it is not difficult to launch, urea formaldehyde glue prepressing prepressing , is particularly important to have no backing plate prepressing influence, only more easy prepressing , more easy to film, the penetration time is shorter, less penetration, can be more close to the UF hot glue, the the easy realization, rubber surface.
Secondly, the core veneer is generally poplar. As poplar wood is soft wide hole material, is one of the fast penetration of tree species, especially in the growing number of artificial poplar fast growing, because the structure is more loose, and its penetration faster. Although better than hardwood, because it is hard wide ring porous wood, the structure is loose, permeability is the fastest, but inferior to birch, because the hardwood diffuse porous wood, fine structure, penetration is slow. From the analysis, we can explain why in the actual production, achieve the same pre cold pressing and bonding strength ,poplar used less wheat flour and glue than birch In particular, more fat containing wood, such as softwood and hardwood such as a small part, although the penetration of the slowest, but also less wheat flour and glue and the adhesive bonding capacity difference is too different.
Finally, the face veneer is generally thin wood veneer. Because the face veneer is very thin, so easy glue penetration caused by adhesive penetration “the most common defect”, especially bingtangor face veneer easily penetrate the face veneer and easily penetrate the wet face veneer. At the same time, the veneer thickness and substrate moisture content is low, speed up the glue inward penetration, so it is prone to lack of glue and internal surface of high water content, resulting in the bond strength of common low, “face bubble”, “face crack/open splits” and the defects of higher water content. In order to avoid the occurrence of the above defects, by prolonging the storage time, the evaporation of water and to increase the initial viscosity and adhesive less permeability less permeability, have a certain effect, but limited, because time is too long, around the plywood and face veneer is dry, and internal glue and a wet face, and cause the face veneer stress is not even more easily caused by defects which two platform crack face “. The local plywood factory in any way, to greatly increase the amount of flour, and a moderate increase of amount of glue.
Based on the above reasons, the local ordinary glue for veneer core lay-up, general flour usage was 25% – 30%, for veneer, generally 45% – 50%, while the environmental protection glue used in veneer core lay-up, generally 35% – 40%, for veneer, generally 55% – 60%. In addition, affected by climate, summer is generally get lower, winter take the upper limit, and in winter than in summer is much larger amount of glue, the reason is: because the summer has accelerated the evaporation of water, rapidly to form film, greatly shorten the preloading time, a corresponding reduction in the glue because of excessive penetration, while summer prepressing and bonding strength, not winter than province affected by rubber. Thus, as the most important flour filler, plays an important role, mainly for holding water, glue, adhesive, low permeability, promoting evaporation, strengthening and toughening etc.. But the water holding, glue, thickening, less infiltration and evaporation promoting effect is poor, the film performance is worse, preloading time, using a large amount of flour; toughening is very good, but increased first and then decreased, the reason is more than 25% of the flour, flour paste is followed by hot pressing the use of large quantities of flour, visible, although the flexibility, but the bond strength of plywood is also low, this should not exceed 30% , especially the environmental glue lowest, because of its bonding strength is low, coupled with the use of large amount of flour. Although some of the local plywood factory, in view of the above reasons, has begun to use high gluten flour, but the efficiency is not significant. In summary, if the flour using the amount of more than 30%, while the poor quality of the gelatin itself, no glue or plate, are required to be improved is, relying solely on the flour, not only of poor quality, and the cost is high!
In view of the existing urea formaldehyde glue and flour pre pressing deficiencies, some advanced technology including local plywood factory, has started the glue mixing , make sure the glue can be modified by adding other additives, such as other fillers (soybean powder, wood powder and peanut shell powder etc.), tackifier , reinforcing agent (melamine, phenol), foaming agent (blood meal, nekal, for the purpose to promote less glue infiltration evaporation, glue coat fast pre pressing, save wheat flour and glue, curing agent (do), ammonium chloride, formic acid, commonly used in environmental protection glue, can effectively shorten the curing time, but for the pre pressure had little effect, and the excessive amount of the adhesive activity period is too short and poor toughness etc.), formaldehyde scavenger and mildew preventive . It is worth mentioning that some of the advanced technology of plywood factory, existing in the veneer and glue is good, the most commonly used is mixed with part of modified polyvinyl acetate emulsion modified white latex, it mainly because of the penetration, good initial tack, make good glue coat-forming and prepressing quickly, thus can effectively avoid the “glue penetration, low bonding strength, face veneer drum