6 bonding mechanisms of adhesives

6 bonding mechanisms of adhesives


News 23 Mar 2021

1.What is adhesive?

A substance that can bind the adherends together through adhesion and cohesion. It can be natural or synthetic, organic or inorganic. The structure can be described as follows,

1.Substrate material

Main body material, main body of adhesion

2.Hardener

The main ingredients, processing additives that solidify substances

3.Thinner

The additives reduces the viscosity of the adhesive itself, wetting the filler

4.Coupling agent

a substance that improves the ability of the adherend to bond with the adhesive.

5.Additives

Substances that improve and enhance the overall performance of the adhesive.

6.Filler

A non-sticky solid substance added to improve the performance of the adhesive or reduce the cost.

2.Adhesive types

Adhesive is 1 of the 6 major polymer materials. According to incomplete statistics, there are currently more than 6000 adhesive product. Because of its wide variety, classifying methods are also different, which could be mainly classified by purpose, application method, application object, and morphology.

Adhesive classifications:

1.By purpose

Metal, plastic, fabric and paper, etc.

2.By application method

Thermoset type, hot melt type, room temperature curing type and pressure sensitive type etc.

3.By application object

Structure type, unstructured type and special adhesive etc.

4.By morphology

Water-soluble type, water emulsion type and solvent type etc.
pvc electrical tape adhesive

3.The theoretical basis of adhesive bonding

1.The mechanism of adhesive involves 3 main concepts

1.Substrate: Bonded object.

2.Adhesion: Refers to all the forces or mechanisms that make the adhesive and the substrate bond together, which exist at the interface between the matrix and the adhesive.

3.Cohesion: Refers to all the forces or mechanisms that make glue itself together.

2.Good bonding effect must meet 2 conditions

1.Enough close contact surface. (Surface clean, wet, spread)

2.Intermolecular attraction or forming adhesive bonds. (curing)

4.6 bonding mechanism of adhesives

1. Bonding (curing) mechanism

In order to meet the above 2nd condition, the adhesive force across the interface is formed after curing. The nature and magnitude of this force are extremely important for the adhesive effect.

Simply put, bonding is accomplished under the combined actions of chemical bonding and physical bonding (mechanical interlocking, diffusion, electrostatic adsorption, weak boundary layer), and they coexist and influence each other.

3.6 bonding mechanism of adhesives

1.Mechanism interlocking theory

The point of view of theory: The adhesive penetrates into the voids on the surface of the adherend, and is embedded in these voids after curing to be tightly combined. The binding force is generated form friction.

The theoretical background was proposed by Mcbain J.W. and Hopkins of the former Soviet Union in 1952, and once played an active role, but with the establishment and development of other adhesive theories, it was almost ignored. The existence of this theory can be proved by microscopic instrument observation.

Applied on smooth metal surface, the strength of direct bonding is not high. Generally, surface treatment is required to make a large number of micro-cavities before bonding, and then the bonding strength can be improved dramatically. The defect is the inability to explain the bonding of non-porous materials.

2.Diffusion theory

When two compatible polymers are in close contact, the phenomenon of mutual diffusion due to the Brownian motion of molecules. This diffusion effect is an interaction across the adhesive and the interface of the adherend.

The main supporter of this mechanism is Voyutski. He believes that macro-molecular chains or chain fragments can fully move and dissolve each other. In fact, the main concept related to this theory is the dynamics of polymer chains in the interface zone, which was proposed by De Gennes more than 20 years ago.

It is applied in compatible adhesives. The defect is that it cannot explain the adhesion of polymer materials and metal, glass or other hard bodies, because it is difficult for polymers to diffuse into such materials.

3.Electrostatic theory

When the adhesive and the adherend system are the same electron acceptor-donor combination, electrons will be transferred from the donor (such as metal) to the acceptor (such as polymer), forming a double Electric layer, which creates electrostatic attraction (adhesion).

Proposed by Deryaguin and his colleagues in 1948. They believe that substrates and adhesives with different electronic layer structures will undergo electron transfer to maintain Fermi balance.

The theory successfully explained the experimental facts about adhesion work and peeling speed.

The defect is that it cannot explain the bonding phenomenon of adhesives and conductive adhesives with carbon black as filler, and cannot explain the effect of temperature, humidity and other factors on the peeling force.

4.Absorption theory

The theory believes that the bonding process is divided into two stages: 1. Adhesive molecules diffuse to the surface of the adherend through Brownian motion, which makes the polar gene clusters and molecular segments of the two closer to each other; 2. Inter-facial molecules generate mutual attraction, the distance between molecules is further shortened to the maximum stable state.

This theory is proposed by Debryan et al. in the late 1940s. Through the study of the epoxy resin adhesive bonding metal aluminum, they found that there is a similar rule between the bonding strength and the hydroxyl group concentration in the epoxy resin.

It is used for spontaneous infiltration adhesives to fill uneven surfaces. The defect is that it cannot explain 1. The adhesive force between the adhesive and the adherend is greater than the strength of the cohesion itself, 2. The relationship between the adhesive strength and the separation speed between molecules, 3. The influence of water on the glue.

5.Weak boundary layer theory

The weak boundary layer theory believes that the generation of the weak boundary layer (WBL) is due to the unevenness of the interface layer between the adhesive and the adherend, and the unevenness of the interface layer is due to 1. The formation of air bubbles in the interface, 2. .Insoluble impurities in the cured adhesive.

It was proposed by Bikerman. The creation of this theory has attracted people's attention and formed the concept of "thick interface layer" or "interface phase" widely used in bonding science. No matter what the nature of the adhesive and the substrate are, the interface phase will be formed.

The impact of this theory is that the stress relaxation and crack development of WBL are different from ordinary interfaces, which greatly affects the overall performance of materials and products.

6.Chemical bond formation theory

The theory believes that in addition to the physical bond between the adhesive and the adherend molecules, there are sometimes chemical bonds that are produced 1. ionic bonds, 2. covalent bonds, and 3. metal bonds.

It was proposed by Gent and Ahagon who had contribute greatly to the theory appearance. This theory is applied in adhesion of vulcanized rubber to copper-plated metal, the effect of coupling agent on adhesion, and the adhesion of isocyanate to metal and rubber. The defect of the theory  is a sticky phenomenon that cannot be explained without a chemical reaction.

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