#Adhesion basics
Types of adhesives
There are many types of adhesives for different purposes. Various adhesives are used to meet customer’s production processes or applications.
Adhesives are categorized in different ways as follows :
— CLASSIFICATION BY MAIN COMPONENTS
MEC process
In this category, there is cement and other materials that endure a high temperature of 1000℃ or higher, however this category is minor now.
- Silicone-based (water glass and
silicate) - Calcium-based (cement and plaster)
Organic adhesives
Most adhesives are organic.
- Thermoplastic resins
- Heat-curing resins
- Rubber/elastomer
— CLASSIFICATION BY FORMS
Moisture Scattering type
They are water-based adhesives that are made by emulsifying the resin and dispersing it in water.
- Epoxy resin-based
- Polyvinyl acetate-based
- Nitrile rubber-based
Solvent type
They are adhesives that are made by dissolving a resin into a solvent, like volatile-solvent type adhesives.
- Phenol resin
- Polyvinyl acetate
- Chloroprene rubber-based
Non-solvent type
They are adhesives that cure by the chemical reaction including the silicone-based and the epoxy-based.
- Epoxy resin-based
- Acrylic resin-based
- Silicone rubber-based
Solid type
They are solid type adhesives in the form of lumps, powders, and films.
- Epoxy resin-based
- Styrene-butadiene rubber-based
— CLASSIFICATION BY CURING METHODS
Volatile solvent type
They cure by vaporizing organic solvents.
Example : Rubber pastes and adhesives for plastic models.
- Polyvinyl acetate
- Nitrile rubber-based
Moisture curing type
It cures when reacting with moisture in the air. Example : Instant adhesives and silicone based liquid gaskets.
- Cyanoacrylate-based
- Silicone rubber-based
Heat curing type
The hardener in the resin is activated and cured by heating. Example : One-part epoxy resins
- Epoxy resin-based
- Acrylic resin-based
Curing-agent-mixed type
Curing occurs, when the base agent is mixed with a hardener.
Example : Two-part epoxy resins
- Epoxy resin-based
- Silicone rubber-based
- Acrylic resin-based
Anaerobic curing type
It cures when air is repelled at the metal part. It is used for screws, joints, and pipes.
- Acrylic resin-based
UV-curing type
It cures in a short time by irradiating ultraviolet rays. It is used as a bonding sealant and adhesive for glass and transparent plastics.
- Acrylic resin-based
- Epoxy-resin-based
Heat melting type
It is coated at a molten state and cures and bonds as it gets cold. It is used for wrapping and bookbinding.
- Styrene-butadiene rubber-based
Pressure-sensitive type
It bonds by applying pressure at room temperature. It is used for adhesive tapes and seals.
- Acrylic resin-based
Two-time humidifying type
This adhesive can glue stamps and other items after wetting the dry adhesive surface with water, to recover its adhesion.
- Chloroprene rubber-based
- Starch type
Adhesive theory
So, what is exactly adhesion ?
Let’s explain briefly the concept of adhesion. Let us think of factors that decide the strength of adhesion. Strength of adhesion will be one of the key points in your design requirements.
— FACTORS THAT DETERMINE THE STRENGTH OF ADHESION
Wetting and contact angle
It is necessary that the adhesive soaks on the adherend. Adhesive strength differs depending on the soaking of the adhesive and the contact angle of two adherends.
How the bond works
There are various theories on bonding mechanisms. Adhesive strength differs depending on each bonding process.
Chemical bond theory
The adhesive causes a chemical reaction between the adhesive and the adherend. As this chemical reaction joins moleculars together, the adhesion becomes strong.
Absorption theory
A hydrogen bond is formed between the adhesive and the adherend, creating a force that attracts them to each other. The binding energy of hydrogen bonds is weaker than that of chemical bonds.
Anchoring effect theory
The adhesive penetrates into pits on the surface of the adherend and bonds to the adherend. The adhesive force depends on how strong the anchoring effect is.
Electrostatic theory
The bonding force is generated by the effect of static electricity. The bonding force is rather weak.
Mutual diffusion theory
Bonding takes place between two adherends, the surfaces of which are dissolved by the adhesive. Due to both adherends having dissolved surfaces, the bonding force is strong.
How the adherend receives a peeling force
There are various ways in which force may be applied to the adherend to result in peeling. The adhesive selection and application method are derived through consideration of the following ways in which force is applied.
Pulling force upward and downward (tension)
Shifting force (shearing)
Peeling force when pulling from the edge (cleavage)
Rolling up force (peeling)
How to choose an adhesive
When choosing an adhesive, you need to consider the following.
From the viewpoint of the magnitude of external force and load applied to the joint :
- Required adhesive strength
- Presence or absence of chemicals
- Structural design of joints
- Durability
- Temperature and humidity during production
From the viewpoint the material and surface condition of the substrate :
- Contamination of oil or dirt
- Material that is hard to bond/oxide coating
- Surface roughness and smoothness
From the viewpoint of operability and automation :
- Allowable curing time and aging time
- Ease of handling
- Ease of application
- Ease of storage
From the viewpoint of health and safety :
- Impact on the external environment
- Laws and regulations (Fire Service Act, Industrial Safety and Health Act, etc.)
Health and safety measures and first aid for workers