Adhesive bonding

Meaning:

Joint design in adhesive bonding
Fig 1 Various joint design in adhesive bonding

Adhesive bonding is a joining process in which a filler material is used to hold two (or more) closely spaced parts together by surface attachment.
The filler material that binds the parts together is the adhesive.
It is a nonmetallic substance usually a polymer.
The parts being joined are called adherends. Adhesives of greatest interest in engineering are structural adhesives, which are capable of forming strong, permanent joints between strong, rigid adherends.
A large number of commercially available adhesives are cured by various mechanisms and suited to the bonding of various materials.
Curing refers to the process by which the adhesive’s physical properties are changed from a liquid to a solid, usually by chemical reaction, to accomplish the surface attachment of the parts.
The chemical reaction may involve polymerization, condensation, or vulcanization.
Curing is often motivated by heat and/or a catalyst, and pressure is sometimes applied between the two parts to activate the bonding process.
If heat is required, the curing temperatures are relatively low, and so the materials being joined are usually unaffected—an advantage for adhesive bonding.
The curing or hardening of the adhesive takes time, called curing time or setting time.
In some cases this time is significant—generally a disadvantage in manufacturing.
Joint strength in adhesive bonding is determined by the strength of the adhesive itself and the strength of attachment between adhesive and each of the adherends.
One of the criteria often used to define a satisfactory adhesive joint is that if a failure should occur due to excessive stresses, it occurs in one of the adherends rather than at an interface or within the adhesive itself. The strength of the attachment results from several mechanisms, all depending on the particular adhesive and adherends:
(1) chemical bonding, in which the adhesive unites with the adherends and forms a primary chemical bond upon hardening;
(2) physical interactions, in which secondary bonding forces result between the atoms of the opposing surfaces; and
(3) mechanical interlocking, in which the surface roughness of the adherend causes the hardened adhesive to become entangled or trapped in its microscopic surface asperities.
For these adhesion mechanisms to operate with best results, the following conditions must prevail:
(1) surfaces of the adherend must be clean—free of dirt, oil, and oxide films that would interfere with achieving intimate contact between adhesive and adherend; special preparation of the surfaces is often required.
(2) the adhesive in its initial liquid form must achieve thorough wetting of the adherend surface; and
(3) it is usually helpful for the surfaces to be other than perfectly smooth—a slightly roughened surface increases the effective contact area and promotes mechanical interlocking. In addition, the joint must be designed to exploit the particular strengths of adhesive bonding and avoid its limitations.

Adhesive types:

A large number of commercial adhesives are available.
They can be classified into three categories: (1) natural, (2) inorganic, and (3) synthetic.
Natural adhesives are derived from natural sources (e.g., plants and animals), including gums, starch, dextrin, soy flour, and collagen.
This category of adhesiveis generally limited to low-stress applications, such as cardboard cartons, furniture, and bookbinding; or where large surface areas are involved (e.g., plywood).
Inorganic adhesives are based principally on sodium silicate and magnesium oxychloride. Although relatively low in cost, they are also low in strength—a serious limitation in a structural adhesive.
Synthetic adhesives constitute the most important category in manufacturing.
They include a variety of thermoplastic and thermosetting polymers.
They are cured by various mechanisms, such as
(1) mixing a catalyst or reactive ingredient with the polymer immediately prior to applying,
(2) heating to initiate the chemical reaction, (3) radiation curing, such as ultraviolet
light, and
(4) curing by evaporation of water from the liquid or paste adhesive.
In addition, some synthetic adhesives are applied as films or as pressure-sensitive coatings on the surface of one of the adherends.

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Adhesive application technology:

Industrial applications of adhesive bonding are widespread and growing.
Major users are automotive, aircraft, building products, and packaging industries; other industries include footwear, furniture, bookbinding, electrical, and shipbuilding.

Surface Preparation:

Characteristics behavior of (a) brittle and (b) tough adhesives in a peeling test
Fig. 2 Characteristics behavior of (a) brittle and (b) tough adhesives in a peeling test

In order for adhesive bonding to succeed, part surfaces must be extremely clean.
The strength of the bond depends on the degree of adhesion between adhesive and adherend, and this depends on the cleanliness of the surface.
In most cases, additional processing steps are required for cleaning and surface preparation, the methods varying with different adherend materials.
For metals, solvent wiping is often used for
cleaning, and abrading the surface by sand blasting or other process usually improves adhesion.
For nonmetallic parts, solvent cleaning is generally used, and the surfaces are sometimes mechanically abraded or chemically etched to increase roughness.
It is desirable to accomplish the adhesive bonding process as soon as possible after these treatments, since surface oxidation and dirt accumulation increase with time.

Application Methods:

The actual application of the adhesive to one or both part surfaces is accomplished in a number of ways.
The following list, though incomplete, provides a sampling of the techniques used in industry:
Brushing, performed manually, uses a stiff-bristled brush. Coatings are often uneven.
Flowing, using manually operated pressure-fed flow guns, has more consistent control than brushing.
Manual rollers, similar to paint rollers, are used to apply adhesive from a flat container.
Silk screening involves brushing the adhesive through the open areas of the screen onto the part surface, so that only selected areas are coated.
Spraying uses an air-driven (or airless) spray gun for fast application over large or difficult-to-reach areas.
Automatic applicators include various automatic dispensers and nozzles for use on medium- and high-speed production applications.
Roll coating is a mechanized technique in which a rotating roller is partially submersed in a pan of liquid adhesive and picks up a coating of the adhesive, which is then transferred to the work surface.
Variations of the method are used for coating adhesive onto wood, wood composite, cardboard, and similar materials with large surface areas.

Advantages and Limitations:

Advantages of adhesive bonding are
(1) the process is applicable to a wide variety of materials.
(2) parts of different sizes and cross sections can be joined—fragile parts can be joined by adhesive bonding.
(3) bonding occurs over the entire surface area of the joint, rather than in discrete spots or along seams as in fusion welding, thereby distributing stresses over the entire area.
(4) some adhesives are flexible after bonding and are thus tolerant of cyclical loading and differences in thermal expansion of adherends.
(5) low temperature curing avoids damage to parts being joinedjoined.
(6) sealing as well as bonding can be achieved; and
(7) joint design is often simplified (e.g., two flat surfaces can be joined without providing special part features such as screw holes).

Principal limitations of this technology include:

(1) joints are generally not as strong as other joining methods.
(2) adhesive must be compatible with materials being joined.
(3) service temperatures are limited.
(4) cleanliness and surface preparation prior to application of adhesive are important.
(5) curing times can impose a limit on production rates; and
(6) inspection of the bonded joint is difficult.

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