ADHESIVES – Bonding
Post 243 ⇒ by Gautam Shah →
Adhesive joining is one of the materials joining technology, others are:
Mechanical joining techniques include Nails, Screws, Seam forming, wedge or pin fixing and interlocking. Fusion joining entails melting of, one or both surfaces through heat, solvent or chemical action, used in welding of steels and plastics, metal forging, PVC solvent joining. Gravity-positioning use the earth’s gravity for stabilization of carpets, floor-spreads, cobble stones, etc. which stay-put when placed parallel to the gravity.
Advantages of adhesives are many. Adhesives can join substances that are materially and dimensionally different and form-wise very difficult. Adhesive joints are thin, overlapping and smooth. Adhesives allow uniform stress distribution, unlike screws and nuts which create localized stress points. Adhesive Joints may be designed as required, to be elastic or rigid. Relatively low process temperature involved in adhesive bonding does not affect the crystallographic structure of the metal. Adhesives can create very extensive, multi layered laminar compositions without physically cutting or puncturing the materials.
Limitations of adhesives are few, but important. Adhesives require elaborate surface treatments, specific application conditions, curing procedures and considerable expense of time for setting. Inspection of the joint is difficult. Joint design becomes very critical compared with other mechanical and thermal processes. The adhesive itself may corrode the materials it is joining, or induce stresses during curing.
BONDING MECHANISM OF ADHESIVES
An adherend is a material or its surface attached to another by means of adhesion. To prevent failure of an adhesive it is necessary to prepare the adherend (surface or substrate). Surface preparations include cleaning, roughening or smoothening, and in some cases its chemical modification or coating.
An adhesive generally should have strength not greater than the strength of the adherend. It should be as rigid or flexible as the adherend. Adhesive should have a high bonding strength than a high structural strength and thermal expansion properties similar to the surfaces being joined. Adhesives must not contain solvents or volatile that can affect the adherend. Adhesives must not adversely affect the plasticizer or any constituents of the adherend.
The adhesive bonding primarily facilitated by mechanical factors, in which the surface roughness or absorption properties of the adherends provide a key for the adhesive to grip. Adhesion for smooth materials such as metals, plastics and ceramics depends on the molecules of the adhesive and the surface molecules of the adherents. Rough surfaces have greater contact area. Mechanical adhesion can be increased by special surface treatments.
Polyethylene surfaces need to be modified by flame treatment for adhesive bonding. Aluminium requires anodizing with phosphoric acid. Carbon steel parts are coated with a zinc or iron phosphate conversion coating as an alternative to sand blasting or hand cleaning to roughen up the surface.
In order to form a bond adhesives harden via processes such as evaporation of the solvent or water, reaction induced with radiation such as heat or UV, polymerization, chemical reaction or phase change.
In formation of an adhesive bond, a transitional zone arises in the interface between adherend and adhesive. In this zone, called the inter-phase, the chemical and physical properties of the adhesive may be considerably different from those in the non-contact portions. The inter-phase composition controls the durability and strength of an adhesive joint and is primarily responsible for the transference of stress from one adherend to another. The inter-phase region is frequently the site of environmental attack, leading to joint failure.
The mechanical behaviour of the bonded structure is also influenced by the joint design, and by the way in which the applied loads are transferred from one adherend to the other. The quality of an adhesive is usually judged against the adherend (i.e., the components being joined -metal alloys, plastics, composites, etc.) and the surface (that is the nature of the surface and its treatment).
The strength of the bond depends on two factors: Adhesion and Cohesion.
Adhesion is the ability of the bonding material -the adhesive to stick -adhere to the materials being joined -the adherends. The development of an adhesive bond occurs through more than one of these processes. First, a mechanical interlocking, occurs when the adhesive flows into pores and micro projections on the surface. Second, Interdiffusion results when liquid adhesive dissolves and diffuses into the adherend materials. The third mechanism occurs through, adsorption and surface reaction, when adhesive molecules adsorb onto a solid surface and chemically react with it. Finally, electrostatic attraction, forces develop at an interface between materials.
Cohesion is the ability of the adhesive and/or the adherend to resist the applied forces within itself. Cohesion could occur even without an adhesive material if there is molecular attraction between surfaces that are in very intimate contact, as happens between two panes of glass. Materials with good surface wetting qualities, like water, also can help in cohesion. Cohesive failure is a constitutional failure of the adhesive, when two items separate out with adhesive remaining on both the substrate surfaces.
There are three ways in which an adhesive bonded joint can fail:
- A very strong adhesive will not allow a joint to open out, so there is a rupture elsewhere in the material.
- Too weak an adhesive fails and separates into two distinct layers.
- An adhesive may fail to adhere to one face.
PREVENTION OF ADHESIVE FAILURES
- careful design of the joint,
- correct selection of the adhesive,
- careful preparation of the joint surfaces,
- controlled application
- environment at the joint (cleanliness, temperature and humidity).
With correctly prepared surfaces, the adhesion at the interface is usually greater than the strength of the adhesive itself, and failures occur within the adhesive film. Failure of the adhesive film is usually caused by the propagation of cracks accelerated by the presence of discontinuities and flaws. Therefore, thin layered adhesives provide the strongest joints. Usually the adhesive selected should have similar strength characteristics to be adherends being bonded together. An exception would be where boding is only temporary pending another joining processes to be used. Most adhesives show optimum strength characteristics when in tension or compression closely followed by, shear. Often the high strength, thermosetting adhesives form brittle bonds that are adversely affected by vibration and impact loading, causing the bond to crack or shatter. Under such conditions a slightly weaker but more resilient adhesives may perform more satisfactorily. Adhesives may show a satisfactory strength characteristic under test conditions, but will tend to creep under sustained loads in service.
Most woodwork adhesives are stronger than the wood, so it is the convenience of viscosity, hardening time, cost, durability, etc. that play important role in the selection. A gap of 0.076 to 0.15 mm in wood joints suffices for a thin adhesive layer to give optimum results.
SELECTION OF AN ADHESIVE
This depends on several factors such as:
- Economics, batch size, and quantity,
- Impermeable joints to seal liquids and gases,
- Thermal or electrical insulation requirements,
- Vibration and fatigue resistance,
- Corrosion capacity.