Friday, May 1, 2020

ADHESIVE

Adhesives :

An adhesive is a material (liquid/semi liquid/solid form) that adheres/bonds at least two surfaces together in a strong and stable manner. 

Bonding in Adhesives :

Mechanical Chemical Van der Wall forces (the attraction between two molecules, each of which has a region of slight positive and negative charge) Moisture aided diffusion (Diffusive bonding occurs when species from one surface penetrate into an adjacent surface while still being bound to the phase of their surface of origin)

There are two types of sources :
1) Natural – starch, animal glue, albumen, etc. 
2) Synthetic - elastomers, thermoplastics, emulsions, etc.

Categories of adhesives :

Non-reactive - 
1. Pressure sensitive adhesives – Pressure Sensitive Adhesive (PSA) adhesives that form a bond with the substrate on application of light pressure. PSA can be of water based, solvent based and hot melt. Ex. acrylate based polymers 
2. Hot application adhesives - Hot Melt Adhesives also known as hot glue is commonly supplied in solid cylindrical sticks of various diameters, designed to be melted in an electric hot glue gun.

Reactive -
Basically the adhesives in this class are thermoplastic in nature which means they are heated to a sufficient temperature where they will flow and wet the substrates and then set and develop the bulk strength on cooling. 
1. UV Light curing adhesives- UV cure adhesives contain photoinitiator molecules which are activated and / or decompose when they absorb the energy emitted by the ultraviolet light; producing free radicals which initiate and accelerate the curing process of the adhesive achieving solidify state in order of seconds. Ex. UV Epoxy, UV Acrylic, UV Silicone, UV Cyanoacrylate, UV Anaerobic. 
2. Heat curing adhesives- When heat is applied the components react and crosslink. Ex. thermoset epoxies, urethanes, and polyimides. 
3. Moisture curing/One Part adhesives- Moisture curing adhesives cure when they react with moisture present on the substrate surface or in the air. This type of adhesive includes cyanoacrylates and urethanes.

Where are Adhesives used in Construction?

To bond ceiling, wall and floor tiles, timber and timber products, concrete, asphalt and fabrics, metals etc.

Characteristics of a good Adhesive

1. They must behave as a liquid, at some time during bond formation, to flow over and wet (make intimate contact with) the adherents. 
2. They form surface attachment through adhesion (the development of intermolecular forces). 
3. They must harden to carry sometimes continuous, sometimes variable load throughout their lives. 
4. They transfer and distribute load among the components in an assembly. 
5. They must fill gaps, cavities, and spaces. 
6. They must work with other components of the assembly to provide a durable product. 
7. They should accommodate temperature differences.

What are the advantages of using Adhesives?

a) Dissimilar materials can be joined 
b) Bond is continuous 
c) Large areas can be bonded in less time 
d) Bonding is more accurate 
e) Adhesives seal and join in one process Increased production speed 
f) Better finishing at no/low costs 
g) Choice of fast or slow curing 
h) Easily combined with other joining methods

Advantages of an adhesive bond :
 
1. Provides large stress-bearing area. 
2. Provides excellent fatigue strength. 
3. Damps vibration and absorbs shock. 
4. Minimizes or prevents galvanic corrosion between dissimilar metals. 
5. Joins all shapes and thicknesses. 
6. Provides smooth contours. 
7. Seals joints. 
8. Joins any combination of similar or dissimilar materials. 
9. Often less expensive and faster than mechanical fastening. 
10. Heat, if required, is too low to affect adhering materials.

Disadvantages of an adhesive bond

1. Surfaces must be carefully cleaned. 
2. Long cure times may be needed. 
3. Limitation on upper continuous operating temperature. 
4. Heat and pressure may be required. 
5. Jigs and fixtures may be needed. 
6. Rigid process control usually necessary. 
7. Inspection of finished joint difficult. 
8. Useful life depends on environment. 
9. Environmental, health, and safety considerations are necessary. 
10. Special training sometimes required. 

Sealants typically have lower strength and higher elongation than adhesives do. Many Adhesive technologies can be formulated into sealants. 
 

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