06. Compatibility of the adhesive and the adhesive

       When the bonded part is incompatible with the adhesive, the bonding of the bonded part will be destroyed. This is true even when one of the components of the adhesive is incompatible with the bonded part. Examples include: corrosion of metal parts by acidic (or alkaline) adhesives; migration of plasticizers from flexible plastics into the adhesive, leading to interfacial bond breakdown; and the action of solvents or volatiles in the adhesive on plastic films.

       Whenever possible, samples of the adhesive are provided along with a detailed description of its properties, which is undoubtedly beneficial to either the manufacturer of the adhesive or the implementer of the bonding process. For electronic components and printed circuit boards, it is usually required that the adhesive not corrode copper and other component materials under conditions of use or storage. In the case of bonding of explosives or similar pyrotechnic materials, other chemical reactions occur that may disrupt the bond or even adversely affect the explosives (i.e., sensitization or passivation).

07. Requirements for the bonding process

       The conditions under which the adhesive is bonded are equally important criteria for selecting the correct adhesive. In factory or assembly line production, the defined assembly environment may limit the choice of adhesive for the bonded product. The operational properties of the adhesive under consideration may often discard the potential interest of the user.

       Typical factors implicated in the bonding process include: the form of the adhesive, the method of preparation and application of the adhesive, the storage period of the adhesive, the period of application of the adhesive, the means or equipment necessary to apply the adhesive, the variability of the bonding process, the time permitted between gluing and bonding (laminating), the time and temperature for drying of the layer of adhesive, the temperature at which the layer of adhesive is to be cured and the temperature at which the adhesive is to be applied, and the rate of change of the strength of the adhesive bond at different temperatures, Special requirements and precautions, such as odor, flammability and toxicity, etc.

       The adhesive properties of the adhesive are usually important for the object to be bonded. For parts that are to be assembled after the adhesive has been applied, the tackiness or bonding properties play a decisive role. During assembly, the time frame for tacking determines the time interval between coating and assembly of the bonded part. The nature of the tack will therefore determine the conditions under which the adhesive must be used (i.e., form of adhesive, mass transfer rate, mixing time, and method of application).
       In contrast to thermoplastic adhesives, thermoset adhesives typically have less adhesion. Adhesion is highly variable, depending on the structure and aggregation pattern of the adhesive molecules. Latex adhesives become tacky only when their liquid dispersion medium (carrier) is removed (volatilized); solvent-based rubbers become tacky even when they contain appreciable amounts of solvent. Both types of adhesives show very good adhesion. For some bonded parts, the curing temperature of the adhesive affects the choice of adhesive. Many thermosetting adhesives have to be heated and pressurized to form a bonded part, and since these conditions are not possible during processing, cold-curing adhesives are often used.

       The choice of adhesive can also be determined by the geometry of the bonded parts and their arrangement, usually loose-fitting parts need gap adhesives; conversely, tight-fitting parts need to use low-viscosity adhesives between.

08. Conditions of use

       For each type of assembly, the selection of an adhesive for it must take into account the strength that must be maintained under the conditions of use of the bonded parts throughout the desired period of use. Therefore, in order to specify a suitable adhesive, the designer must be aware of all the conditions that need to be met.

       Typically, the important ones are the strength and durability requirements, and the relevant factors for these are covered in the stressing of the bonding body. There is a wide range of differences in the response of different types of adhesives to different stresses and rates of application. Thermoplastic adhesives are unsuitable for structural applications because they tend to break down when supporting lower loads and soften when subjected to heat. Thermoplastic adhesives are not able to withstand prolonged vibratory stresses, although they show greater strength than thermosets in short duration tests.
       Thermoplastic rubber-type adhesives usually have high peel strength, but their tensile and shear strengths are relatively low. In contrast, thermoset resins are often used as the basic component of structural adhesives. Structural adhesives become a relatively hard adhesive layer at room temperature and retain most of their strength. Therefore, it is generally desirable to apply thermoset structural adhesives where high strength and high damage stability are required. Adhesives with thermosetting resin or rubber/resin as the main component are also suitable for supporting vibration loads, but their strength is relatively poor for supporting peeling or splitting stresses. Under impact loading, color resilient adhesives will prove more satisfactory than the use of friable thermoset adhesives.

       Another important factor to consider is the range of effective temperatures required by the adhesive. Usually the temperature at which a bonded part is used is the temperature of the adhesive layer, not the ambient temperature. The temperature of the bonded part is often lower than the ambient temperature, especially if the bonded part is intermittently or briefly exposed to a heated environment. At high temperatures all glues lose strength to varying degrees and some 540 adhesives soften or decompose to nothing. Many classes of thermoset and thermoplastic adhesives can be used below 70°C; however, at 120°C, only a few thermoplastic adhesives have a service life.

       Often adhesives are required to have a service life, i.e., the service life of the bonded part, and in practice the adhesive is required to have a longer bonding period or life than the bonded part. For example, the adhesive used to bond a whetstone to a metal seat must maintain a satisfactory bond until the whetstone is exhausted.
       Of course, in some cases, a short service life is another requirement for the adhesive. In assembly, it is sometimes necessary to use adhesives to temporarily adhere the parts in order to use other means to make it installed; or in the process of using adhesives to “clamp” the workpiece on the table, such as on the glass, stone, magnetic head or metal, etc., when the case of grinding. Bonding of ceramic powders before sintering and bonding of casting sand materials are two other examples of temporary bonding. The bonding of casting sand also has special requirements, i.e. after the casting has been shaped, the bonding fails and the sand disintegrates and is no longer attached to the casting. Waxes, lacquers, shellacs, etc. are often used as temporary adhesives.

09. Requirements for the storage of bonded parts

       While the conditions of use of the bonded parts are usually taken into account, the storage conditions of the bonded parts should not be overlooked. The correct approach is to select the adhesive with an eye to the fact that in transportation and storage the bonded parts may be subjected to abnormal extremes of temperature and collision (shock) loads. For example, jet engines are operated in a hot state, and may be stored in an environment below -40 ° C or transported under such conditions. If storage and transportation conditions are ignored when selecting an adhesive, the bonded parts can be damaged before use.

10. Cost

       Considering the bonding process as a whole, the appropriate nature of the selected adhesive is more important than the cost of the adhesive. In addition to the price of the adhesive, but also take into account the productivity of using the selected adhesive and other factors, only the lowest cost adhesive, regardless of the following relevant factors, may not receive good economic results:

(1) The total efficiency of bonding (related to the bonding area and the number of components);

(2) Ease of application or equipment required for the process (equipment includes molds, fixtures, pressurizing tools, heating ovens, applicators, etc.);

(3) The time required for the process (including assembly time, preparation time for the adhered elements, curing time, etc.);

(4) The labor cost of assembly and inspection of the adhered components;

(5) The amount of waste material compared to other joining methods.