Factors influencing the selection of wood adhesives
Introductory reading: The basic function of wood adhesive is to coat the surfaces of the glued objects to make them connected. When making adhesive parts, we should first consider selecting suitable wood adhesives from different types of wood adhesives. The following are eleven factors that affect the selection of wood adhesives.
1 Bonding assembly The whole processing process of bonding assembly has more or less influence on the selection of wood adhesive. The surface produced by the processing before bonding is directly related to the bonding effect. If it cannot be changed, it is necessary to choose the wood adhesive matching this surface. After the bonding is completed, the processing methods (cutting, punching, heating, freezing, dipping, etc.) of the whole bonded parts should also be considered when selecting wood adhesives. If the whole bonding part is a unit on the production line, in order to synchronize with other units, the operating performance of the wood adhesive used must adapt to the speed of the production line. The form (liquid, paste, film or solid) of the same wood adhesive should also match the processing method (manual or mechanical) of the adhesive.
- The physical and chemical properties of the glued wood and the required surface preparation before bonding are another important factor to be considered when selecting wood adhesives. At this time, the selection goal is to expand the bonding degree on the joint, so that the bonding of the joint under test is cohesive failure rather than joint failure on the wood adhesive/adherend interface. Only in this way can the highest strength of the adhesive be achieved.
3 Low strength wood, such as fabric, felt or some similar wood, may be more fragile than wood adhesive, so joint damage occurs in the wood content. For this type of application, the selected wood adhesive allows the bonding part to be used under any physical conditions that its wood can withstand without the risk of bonding damage. At this time, the use of high-strength wood adhesive will make the bonding wood super standard or high priced.
4 The thickness of the glued wood The thickness and strength of the glued body are equally important, especially where the elastic coefficient of the wood adhesive is related to the joint design. Flexible wood, such as rubber, thin metal sheet, plastic film, etc., are easy to bend in use. Do not use hard and broken wood adhesives to bond them, because such wood adhesives may break, resulting in reduced bonding strength. The difference in the flexibility or thermal expansion of the adherend can cause internal stress in the adhesive layer. Such stress may lead to premature failure of the joint before it bears any applied load. Such connections are particularly dangerous when used at sub zero temperatures. To a certain extent, the stress can be reduced to the minimum by joint design, but there is still stress in the cured adhesive layer. The minimum stress exists objectively between the same wood. By adjusting the composition of wood adhesives and selecting wood adhesives, it can be expected that a wood adhesive has the same rheology, thermal expansion and chemical stability as the glued wood. The expansion of one of the joint wood in the chemical medium (gas, liquid) will lead to the stress close to the interface between the wood adhesive and the adherend.
Another factor that cannot be ignored is the form of wood adhesive. Solvent based wood adhesives wrinkle the edge of the joint when bonding thermoplastic film or its composite film, and their effect on hard thermoplastic wood often reduces the requirements for wood surface treatment. The special shape of the glued wood usually contributes to the application of special forms of wood adhesives. The edge joint of aluminum honeycomb structure to flat metal sheet is best bonded with liquid primer and thermosetting adhesive film (glass fiber cloth as lining). It is convenient to make copper fins and aluminum tubes into heat exchangers with paste glue. For porous wood, it is more appropriate to use wood adhesive with special viscosity or paste.
- The force on the adhesive: The wood adhesive changes from a flexible and wet wood to a tough and hard solid, and its cohesive strength property changes greatly, with a range of several thousand N/cm2. The increase of cohesive strength is the fundamental reason why the adhesive layer can withstand different stresses, but the formation of cohesive strength is also an important reason for the stress in the adhesive body. In some cases, only wood adhesives may be required for temporary bonding. For example, when positioning and locking the parts, make the parts fixed. It is necessary to consider the bonding stress in the application of wood adhesives with special strength requirements. In particular, the nature and size of the stress, and the application conditions of the adhesive. After the selection of the adhesive, the performance of the wood adhesive on the joint also depends on many factors, the most important being the joint design, the state of the connected surface, the bonding technology used, the thickness of the adhesive layer, and the strength, thickness or shape of the bonded parts. The type and size of the stress provided by the wood adhesive largely depend on the joint design. The adhesive may be subjected to shear, tension or compression, or splitting or peeling forces, as well as any combination of these stresses. Most wood adhesives show good compressive strength; Some wood adhesives may have low peel strength but high shear strength, or vice versa. It is often possible to achieve the required joint strength, even with low strength wood adhesives. Of course, where large area joints cannot be designed, it is inevitable to use high-strength wood adhesives. It is of special significance to select suitable wood adhesive to meet the required strength for the thickness of joint adhesive film. The wood adhesive with high modulus can obtain higher tensile strength and shear strength when the thickness of adhesive film is small. In order to obtain the best strength of thermosetting resin, the thickness of adhesive film is usually 0.03-0.12 mm, and the strength below 0.03 mm is usually reduced. The strength is related to the smoothness of the bonded surface, and the joint is dangerous due to lack of glue. On the other hand, when using elastic wood adhesive, increasing the thickness of adhesive film will produce higher peel strength. Especially when the adhesive film is more than 0.13mm thick, the best strength is usually achieved. In order to reduce joint stress, wood adhesives that are harder than the glued wood after curing are not used. The conditions under which the adhesive is subjected to external stress must be specified. Not all wood adhesives have exactly the same effect on loads that may be supported by bonded joints, such as intermittent loads or vibration loads. Some wood adhesives form a hard and fragile adhesive layer, which is easy to crack under vibration load; While other wood adhesives can withstand intermittent load, they cannot support continuous load. For many wood adhesives, increasing the loading rate will significantly improve the bonding strength (such as impact or shear strength), which is also a factor worth considering.
7 Compatibility between the adherend and the wood adhesive When the adherend is incompatible with the wood adhesive, the bonding of the adhesive will be damaged. This is true even when one of the components of the wood adhesive is incompatible with the adherend. For example, metal parts are corroded by acidic (or alkaline) wood adhesives; The plasticizer in the flexible plastic migrates to the wood adhesive, resulting in the destruction of the interface bonding; The effect of solvent or volatile matter in wood adhesive on plastic film. Whenever possible, when providing samples of wood adhesives, provide detailed descriptions of their properties, which is undoubtedly beneficial to manufacturers of wood adhesives or practitioners of bonding processes. For electronic components and printed circuit boards, it is generally required that wood adhesives do not corrode copper and other component wood under the conditions of use or storage. When bonding explosives or similar initiating explosive wood, other chemical reactions may damage the bonding, or even have adverse effects on explosives (i.e. sensitization or passivation).
8 Requirements of bonding process The bonding conditions of wood adhesives are also important criteria for selecting the correct wood adhesives. In factory or assembly line production, the determined assembly environment may limit the selection of wood adhesives for bonding products. At this time, the operability of wood adhesives considered may often abandon the potential interests of users. Typical factors involved in the bonding process include: the form of wood adhesive, the preparation and application method of wood adhesive, the storage period of wood adhesive, the application period of wood adhesive, the necessary means or equipment for bonding, the variability of the bonding process, the allowable time between gluing and bonding (overlapping), the drying time and temperature of the adhesive layer, the curing temperature and use temperature of the adhesive layer Change rate of bonding strength at different temperatures, special requirements and preventive measures, such as odor, flammability and toxicity. The selection method of wood adhesive applied to the workpiece depends not only on the physical properties of the wood adhesive, but also on the size and shape of the parts, the number of coated components and the size of the parts. The form of wood adhesive varies from thin liquid to paste and solid. However, different means of use should be adopted for different forms. For example: spray, brush or roll coating for thin liquid; For paste wood adhesive, use glue applicator or scraper type glue applicator. For the adherend, the adhesion of wood adhesive is usually important. It plays a decisive role in the tacking or adhesion of the parts that have been coated with glue and will be assembled. During assembly, the adhesion time range determines the time interval between application and assembly of adhesive parts. Therefore, the adhesive properties will determine the necessary use conditions of wood adhesives (i.e. the form, mass transfer rate, mixing time and application method of wood adhesives). In contrast to thermoplastic wood adhesives, thermosetting wood adhesives usually have less adhesion. Adhesion varies greatly, which depends on the structure and aggregation morphology of wood adhesives. Latex wood adhesive becomes sticky only when its liquid dispersion medium (carrier) is removed (volatilized); Solvent based rubber becomes sticky even when it contains a significant amount of solvent. Both adhesives show good adhesion. For some adhesive parts, the curing temperature of wood adhesive affects the selection of wood adhesive. Many thermosetting wood adhesives need to be heated and pressurized to form adhesive parts, and it is impossible to achieve these conditions in the processing process, usually using cold curing wood adhesives. The selection of wood adhesive can also be determined by the geometry and arrangement of the components of the adhesive. Generally, the loose fitting components need the interstitial wood adhesive; On the contrary, low viscosity wood adhesive shall be used between closely matched parts.
9 Service conditions For each type of assembly, when selecting wood adhesive for it, the strength that the adhesive must maintain under service conditions during the expected whole service life must be taken into account. Therefore, in order to specify a suitable wood adhesive, the designer must understand all the conditions that need to be met. In general, the strength and durability requirements are important, and the relevant factors have been mentioned in the stress condition of the adhesive. The response of different types of wood adhesives to different stresses and application rates varies widely. Thermoplastic wood adhesives are not suitable for structural applications because they tend to break when supporting low loads and soften when heated. Thermoplastic wood adhesive can not withstand the vibration stress for a long time, although it shows greater strength than thermosetting in the short-term continuous test. Thermoplastic rubber wood adhesive usually has high peel strength, but its tensile strength and shear strength are relatively low. On the contrary, thermosetting resins are often used as basic components of structural wood adhesives. The structural wood adhesive becomes a relatively hard adhesive layer at room temperature, and retains most of its strength. Therefore, generally speaking, it is desirable to apply thermosetting structural wood adhesives when high strength and high damage resistance stability are required. The wood adhesive with thermosetting resin or rubber/resin as the main component is also suitable for supporting vibration load, but its strength is relatively poor for supporting peeling stress or splitting stress. Under impact load, color resilient wood adhesive will prove to be more satisfactory than fragile thermosetting wood adhesive. Another important factor worth considering is the effective temperature range required by wood adhesives. Generally, the service temperature of adhesive parts refers to the adhesive layer temperature, not the ambient temperature. The temperature of the adhesive is often lower than the ambient temperature, especially when the adhesive is exposed to the heating environment intermittently or for a short time. Under high temperature, all adhesives will lose strength to varying degrees, and some 540 wood adhesives will soften or decompose into useless things. Many kinds of thermosetting and thermoplastic wood adhesives can be used below 70 ° C; However, at 120 ° C, there are only a few thermoplastic wood adhesives. The service life of wood adhesives usually requires a service life for wood adhesives, that is, the service life of adhesive parts. In fact, it requires that the bonding validity or life of wood adhesives should be longer than that of adhesive parts. For example, the wood adhesive used for bonding the grindstone to the metal seat must maintain satisfactory bonding until the grindstone is exhausted. Of course, in some cases, short service life is another requirement for wood adhesives. During assembly, sometimes it is necessary to temporarily stick the parts with wood adhesive, so that they can be installed by other means; Or, during the processing, the workpiece is “clamped” on the workbench with wood adhesive, such as the grinding of glass, stone, magnetic head or metal. The bonding of porcelain powder before sintering and the bonding of cast sand and wood are another example of temporary bonding. There are special requirements for the bonding of casting sand, that is, after the casting is shaped, the bonding will become invalid, and the sand mold will disintegrate and no longer attach to the casting. Waxes, lacquers, shellac, etc. are often used as temporary wood adhesives.
10 The storage requirements of adhesive parts and the service conditions of adhesive parts have been generally paid attention to, but the storage conditions of adhesive parts can not be ignored. The correct way is to pay attention to the extreme temperature and collision (impact) load that the adhesive may be subjected to during transportation and storage when selecting wood adhesives. For example, the jet engine works in a hot state and may be stored in an environment below – 40 ° C or transported under such conditions. If the storage and transportation conditions are ignored when selecting wood adhesives, the adhesive parts will be damaged before use.
The cost considers the bonding process as a whole, and it is more important to select the proper properties of wood adhesives than the cost of wood adhesives. In addition to the price of wood adhesives, the production efficiency of the selected wood adhesives and other factors should also be taken into account. Only the lowest cost wood adhesives, regardless of the following related factors, may not achieve good economic results: (1) the total efficiency of bonding (related to the bonding area and the number of components); (2) Ease of equipment required for application or processing (equipment includes mold, fixture, presser, heating furnace, applicator, etc.); (3) The time required for the process (including assembly time, preparation time and curing time of bonded components); (4) Labor cost for assembly and inspection of bonded components; (5) The quantity of waste material compared with other connection methods. Generally, it is preferable to use fast positioning wood adhesive with simple bonding process, as long as complex assembly fixtures or molds are not used, even if the price of wood adhesive is high.
