Wood bonding technology plays an important role in the wood industry. The bonding properties of wood bonding products are largely affected by the properties of the wood itself. The factors affecting the bonding properties of wood are both inherent in the wood and in the processing of the wood. The former includes wood density, tree species, extracts, pH, etc., and the latter includes wood moisture content, surface roughness, and the like.
1 Wood density
The strength of wood is usually proportional to its density, because the amount of wood cell wall material contained in a unit volume is the material basis that determines the strength of the wood, and the strength of the wood increases as the density of the wood increases. For ideal bonding systems, bond failure should occur in the xylem.
The bonding test of many hardwoods proves that when the wood density is less than 0.8 g/cm3 when the wood density is less than 0.8 g/cm3, the cohesive strength of the urea-formaldehyde resin adhesive itself and the interface strength of the urea-formaldehyde resin adhesive and wood More than the strength of the wood itself, the bond failure often occurs in the weakest link in the bonding system. Therefore, the bonding strength is closely related to the strength of the wood. Generally speaking, the bonding strength increases with the increase of the wood density. In the case where the wood density is greater than 0.8 g/cm3, since the cohesive strength of the urea-formaldehyde resin adhesive itself and the interface strength of the urea-formaldehyde resin adhesive and wood are less than the strength of the wood itself, the bond failure does not occur in the wood portion but occurs in the case The glue layer or interface, therefore, in this case, the bond strength is almost independent of the wood density. When resorcinol formaldehyde resin adhesive is used to bond all needles and hardwood materials, since the resorcinol formaldehyde resin adhesive itself has a large cohesive force, the bonding strength of wood glued products bonded with resorcinol formaldehyde resin adhesive is used. Increased as the density of bonded wood increases.
Although a high-density wood-glued product has a high bonding strength, the stress due to a change in its moisture content is also large. In the case where the adhesive is determined, the bonding durability of the wood having a high density tends to be worse than that of the wood having a low density. For example, a eucalyptus laminated material made of an aqueous polymer isocyanate adhesive has a relatively large compressive shear strength, but when the humidity of the use environment changes greatly, cracking of the rubber layer is more likely to occur.
In addition, in order to achieve the desired bonding effect when bonding dense wood, it is required that the adhesive used should have a high cohesive force so that the strength of the wood itself can be fully utilized. For wood with a particularly high density, such as ebony, iron wood beans, etc., the adhesive is not easily absorbed, so that the adhesive drying speed is very slow and the bonding strength is low. Therefore, for wood with a particularly high density, in order to allow the adhesive to penetrate into the cells, it is required to be double-sided sanded, double-coated, and wait for a sufficient time for the adhesive to penetrate before applying pressure. Sometimes, polyurethane adhesives are required because the adhesion of such adhesives does not depend on their penetration, primarily chemical bonding.
2. Wood fiber direction and cutting plane direction
Wood is an anisotropic material in which the direction of the wood fibers within the bonding system can be perpendicular to each other, parallel to one another or at an angle to each other.
When the two pieces of wood are bonded in the same fiber direction, the bonding strength is the largest. Such products, such as laminated timber and veneer lumber (LVL), etc.; common wood products in which the fiber directions are perpendicularly bonded to each other, there are plywood and three layers. Parquet and so on. As the fiber angle between the two bonded woods increases, the bonding strength of the bonding material gradually decreases. When the fiber directions are perpendicularly bonded to each other, the bonding strength is the lowest, and the bonding strength is only 1/4 to 1/3 of that when the fibers are parallel to each other when the fibers are bonded to each other in parallel. The direction in which the bonding faces are cut also has an effect on the bonding strength.
Martin et al. studied the effect of wood fiber direction on the penetration of liquid adhesive onto the surface of wood. It shows that when the adhesive is applied to the end face of the wood, the adhesive will penetrate into the cell cavity of these cells due to the openness of the wood conduit and the trachea on the end cut surface. It is easy to make the end face lack of glue, which causes the wood glue strength to decrease. Therefore, under the same bonding conditions, the bonding strength of the end face and the end face is lower than the bonding strength of the diameter surface and the diameter surface, the chord surface and the chord surface.
In actual work, if end-face bonding is necessary, in order to prevent the lack of glue, it is necessary to increase the viscosity and solid content of the adhesive; apply two coats of glue, or apply glue on both sides of the substrate, or increase the thickness of the glue layer; use moderate pressure And prevent the glue from being squeezed out. For example, when the wood is longitudinally lengthened by the docking method, the above measures can be taken to avoid the lack of glue at the ends.
Under the condition that the fibers are bonded to each other in parallel, the bonding surface has a diameter section and a chord section. In some cases, there is also a difference in the bonding strength of the cross-section and the chord-faced bonded article. When the cohesive force of the adhesive and the interface strength between the adhesive and the wood are greater than the strength of the wood itself, wood damage is likely to occur. If there is a difference in the strength between the string cutting surface and the diameter cutting surface of the wood, the bonding strength between the string cutting surface and the string cutting surface bonding product and the diameter cutting surface and the diameter cutting surface bonding product are also different.
For example, larch wood has a radial shear strength that is greater than its chordwise shear strength. When the larch board is bonded with an aqueous polymer isocyanate adhesive, the normal compression shear strength of the larch diameter cutting board is higher than that of the string cutting board. The wood-break rate of the larch string and the string-cutting board is relatively high, basically the wood is destroyed, and most of the wood damage occurs at the junction or wheel boundary where the material is rapidly changing. Therefore, the API is used. When the adhesive produces larch glulam, it should be bonded by a cutting board as much as possible.
3 wood moisture content
The moisture content of wood has a great influence on the bonding performance. When the moisture content of the wood is high, the adhesive applied on the bonding surface is diluted, the viscosity thereof is lowered, and excessively penetrates into the wood structure, causing lack of glue, resulting in a decrease in the bonding strength. Wood is dried at high temperatures for a long time, and its moisture content is too low, which also weakens the wetting of the adhesive and reduces the bonding strength. Generally, the bonding strength is high when the wood moisture content is 5% to 16%.
In addition, the wood has the characteristics of shrinkage and wetness. If the moisture content of the wood is too low, the product will be damp and swell; if the moisture content is too high, it will desorb and shrink. In general, the adhesive itself is not resistant to the very high stresses caused by wood expansion and drying, which is a major cause of cracking and deformation of wood products. This situation occurs when furniture is produced and used. The production of furniture is often inconsistent with the moisture content of the wood used in the place where it is used. This requires that the moisture content of the wood used to produce furniture be as close as possible to the humidity of the environment in which it is used. For example, furniture purchased in Guangzhou is prone to chapped when it is shipped to Xinjiang. This requires the furniture manufacturer to use wood with a moisture content close to that of the customer who purchased the furniture. Considering the dry hygroscopic hysteresis of wood, the final moisture content of wood drying and the equilibrium moisture content of wood in the area of ​​use are generally about 2%.
4 Wood shrinkage expansion rate
Due to the natural shrinkage of the adhesive when cured and the difference in the properties of the adhesive and wood, internal stresses are present in the bonded joint, and internal stress concentration will reduce the bond strength. In order to reduce the stress generated by the alternating heat or high temperature solidification cooling, the adhesive should be as close as possible to the coefficient of thermal expansion of the wood.
There are two main ways to reduce internal stress: one is to add fillers, and the other is to use adhesives with good elasticity.
When the wood is bonded to other materials, if the shrinkage ratio between the materials is too different, measures should be taken to reduce the dimensional change of the wood or to bond with an elastic adhesive. For example, when metal-wood is bonded, the hydroxyl component in the wood is wetted or desorbed to cause the wood to shrink and swell, which is the main reason for the poor dimensional stability of the wood.
By blocking the hydroxyl groups of the wood and forming chemical crosslinks between the hydroxyl groups, the dimensional stability of the wood can be improved and the strength of the modified wood can be improved. Therefore, the wood is treated with a phenolic resin in advance, and the size is fixed and then bonded to the metal to reduce the stress caused by the difference in shrinkage ratio between the two. For example, when a single board or a plywood is used to form a partition board with a foam board, the veneer or the plywood is first fixed in size, and then the composite effect with the foam board is better.
5. Surface processing method
The bonding plane of the wood must be smooth and straight. If it is not flat (curved or warped), the two planes cannot be in close contact, which will affect the strength and quality of the bonding. Therefore, the wood is machined prior to the bonding operation, which may cause mechanical damage to the wood cells, and the degree of damage varies with the type of machining and the degree of processing. During the planing process, the surface wood structure is less damaged, and the inner cavity of the wood cells is in an open state, and the adhesive is easily formed into an effective glue nail after being immersed. In order to reduce the damage to the wood tissue, the planer must be kept sharp, otherwise the damage of the wood tissue will be more serious.
A. P. The Singh study found that the surface of P.radiate wood planed with a sharp planer has a normal shape in addition to the tubular cells adjacent to the glue layer. The glue layer between the wood surfaces is thin and has a normal width (see Figure 3). In contrast, the surface of radiata pine wood planed with a relatively blunt planer is severely damaged by axial tracheids and rays. Due to the bluntness of the planer, the cells are severely crushed. In particular, the crushing of the cells next to the glue layer is likely to occlude the cell cavity and prevent the adhesive from penetrating into the cells, resulting in a thick layer and uneven thickness.
Fine planing of the wood surface helps to ensure the uniformity of the adhesive layer on the surface of the bonding material.
Planing of the surface of the material exposes the S2 layer of cell wall with a very strong polarity, which is very advantageous for bonding. Although the sanding process will make the surface of the wood very smooth, the inner cavity of the wood tissue is damaged, and some of the cell surface is very smooth, and the cavity is also easily blocked by the sanding powder. Therefore, the bonding strength is lower than that of the planing process. .
6 surface properties of the material
Since the wood is a porous material, when the two wood surfaces are bonded, only a small portion of the apparent area is actually contacted, and this area varies depending on factors such as the structure of the wood, the surface roughness, and the applied pressure. That is to say, even if the applied pressure is not large at the time of bonding, the bonding material may be subjected to a relatively high pressure or even crushed in an area where a small portion is in contact.
Generally speaking, the surface of a certain roughness, the concavities and convexities, the fine holes and the grooves facilitate the penetration of the adhesive and can produce a mechanical bond. However, if the roughness is too large and the concave holes and grooves on the surface are too deep, the residual air or the adsorbed moisture may hinder the penetration of the adhesive and may be unfavorable for bonding.
7 surface passivation of wood
Keeping the surface of the wood clean and preventing it is the basic condition for its good adhesion. In addition to air pollution (such as dust particles in the air), the surface of the wood is prone to a "self-contamination" over time, which changes the surface properties of the wood and affects the bonding properties of the wood.
Within a few hours after the cutting of the wood, the change in the surface of the "weak interface layer" that the oleophilic, low molecular weight material moves to its surface to form a low surface energy is often referred to as "surface passivation", blunt The rate and extent of the change will vary depending on the wood species and storage temperature.
The oleophilic substances in wood are mainly composed of resins, fatty acids and esters thereof, paraffin waxes, and terpene compounds. Acidic and neutral adhesives do not readily penetrate this oleophilic layer, but alkaline binders can saponify fatty acids to some extent, causing the oleophilic layer to disappear, allowing the adhesive to penetrate well into the wood. R. M. Russbaum found that the surface wettability of the European spruce (Picea abies) wood decreased significantly after 3 days of cutting. Therefore, he pointed out that the longest storage time for European spruce materials used for finishing and bonding to avoid surface passivation is 3 days, and should be used within 2 to 3 days after processing to avoid natural passivation of the surface and affect its bonding strength. .
In the production of wood glued products, it should be emphasized that the wood should be bonded within 24 hours after planing or sanding. For example, in the production of laminated timber, the wood should be bonded and pressed as soon as possible after the planing or milling of the finger, from the planing or milling of the finger to the bonding pressure of no more than 24h.
In order to eliminate the influence of “self-contamination†on the surface of the wood and the pollution of the surrounding environment on the surface wetting of the wood, the surface of the wood is effectively pretreated before bonding. Wood surface treatment can be mechanical and chemical: cleaning with solvent, cleaning the bonding surface of the wood, making the bonding surface as large as possible; removing mechanical surface (planing or sanding) to remove surface contaminants while improving surface roughness To increase the surface area of ​​the bond.
8 wood extracts
Wood extracts have an effect on the wetting, penetration and solidification of the adhesive. In general, it is difficult to sufficiently wet the wood with a large amount of ingredients, and the bonding strength is poor.
Wood that is difficult to glue, such as dipterocarp, boiled in hot water at 100 ° C for 24 h, will give water-soluble gum ingredients of 5% to 8% by weight of wood; and wood with good bonding properties Boiled under the same conditions, the extracting ingredients only accounted for 0.1% to 0.2%. Surfactants can also be added to hot water or alkaline water for cooking, but this requires a lot of cost, so there are certain difficulties in practical applications.
For resin-containing wood such as larch and masson pine, it is first dried at a high temperature to allow the resin to exude from the wood, followed by washing the surface with a solvent, removing the resin, and then shaving the surface. Immediately after the surface treatment of the wood is completed, the resin is prevented from overflowing again. Phenolic adhesives are recommended for bonding.
9 Wood pH
The pH of the wood affects the curing time of the adhesive. For example, poplar, especially Daqingyang, has an alkaline pH value, which can prolong the curing time of the urea-formaldehyde resin adhesive. Because urea-formaldehyde resins are cured under acidic conditions, the alkalinity of poplars reduces the acidity of the adhesive. Adding more curing agent during the adjustment of the glue and adjusting the pH to 4.0~5.0 is the easiest way to solve the effect of poplar alkali on the curing time of urea-formaldehyde resin. In actual production, NH4C1 or a mixed curing agent may be used according to actual conditions, such as adding an acid to the ammonium salt.
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