Adhesives Knowledge: Basic Introduction
PART One
What is an adhesive?
Introduction to Adhesives
An adhesive is a material used to bond surfaces of objects through an adhesive bonding process.
An adhesive is a substance capable of bonding two parts together to form a single, integrated whole.
History of Adhesives
History of adhesives
Why use adhesive?
Why do you use adhesives –
▪ Uniform stress distribution (avoid stress concentration)
▪ No damage to the substrate (eliminates voids, prevents thermal damage, and preserves the protective coating)
▪ Weight reduction (by using thinner metal substrates)
▪ Reduced labor requirements (no drilling or weld seam grinding required)
▪ Simultaneous bonding and sealing
▪ Reduce galvanic corrosion
▪ Adhesion of dissimilar materials
▪ Enhance aesthetics with unique design options
▪ Soundproofing and vibration reduction
▪ Possesses a certain degree of flexibility, particularly compared to mechanical connection methods
adhesive
Around us
Applications in the adhesives industry
Composition of the adhesive
Definition of Adhesive Technology
Adhesion technology is a method of firmly bonding like or unlike materials together by leveraging the adhesive force generated by adhesives on solid surfaces.
Primary forms of bonding
Structural bonding
The bonding phenomenon that securely fixes structural units together.
Non-structural bonding
This primarily refers to surface coating, sealing, and functional bonding.
Adhesive curing method
PART TWO
Common Parameters for Adhesives
– Common expressions –
dynamic viscosity>>
Viscosity refers to the internal friction of a liquid and, to a certain extent, characterizes its flow properties. Viscosity is expressed as dynamic viscosity, with units of po (ps) or cP (cps); other commonly used units include Pa·s and mPa·s.
Extrusion ratio>>
The unit for characterizing viscosity is the number of grams of adhesive extruded per unit time through a pipe of a given diameter under a specified pressure, expressed in g/min.
Surface drying time>>
This refers to placing a small PE film (approximately 2 cm × 5 cm) over the surface of an adhesive such that, upon removal, no residual adhesive remains on the surface. The product itself retains its adhesive properties, but no marks are left on the PE film.
hardness>>
It denotes the material’s resistance to external compression—that is, the force required to indent the material with a rigid object—and its magnitude reflects the inherent hardness or softness of the material.
Shore hardness commonly employs the grades Shore D, A, and 00, with the hardness decreasing in this order; the Shore 00 grade is typically used for gel-based materials that are extremely soft.
Loading modes in bonded joints. In tension, the load pulls the bonded surfaces apart perpendicular to the bond plane. In shear, the load acts parallel to the bond plane and tends to slide the bonded parts relative to one another. In peel, the load lifts a flexible bonded part progressively from an edge. The loading direction and joint geometry must be considered when comparing strength results.
bonding strength>>
The force per unit bonding area exerted on a bonded specimen when it fails under the loading of the adhesive layer.
Locating the failure. Interfacial (adhesive) failure separates the adhesive from a bonded surface. Cohesive failure leaves the fracture within the adhesive layer, with adhesive remaining on both bonded surfaces. Failure within the adherend is substrate failure. Examine both separated surfaces to distinguish the failure location from the direction of the applied load.
cohesive failure>>
Bond failure occurs within the adhesive or within the bonded materials.
Interface damage>>
Failure phenomena occurring at the interface between an adhesive and the adherend
dielectric strength>>
A measure of the electric strength of a material when it acts as an insulator. It is defined as the maximum voltage per unit thickness that a specimen can withstand before breakdown, expressed in kV/mm.
coefficient of heat conductivity>>
Characterizes the thermal conductivity of a material. It is defined as the amount of heat transferred per second from one plane to another, within a material, across two parallel surfaces separated by one meter and each having an area of 1 m², perpendicular to the direction of heat flow, when the temperature difference between the two surfaces is 1 K; the unit is W/(m·K).
Comparing thermal materials. Air is a poor heat conductor compared with the metals shown in the material comparison: iron, aluminium, copper and silver. Water is a liquid heat-transfer medium and must be considered separately from solid metal parts. For an adhesive joint, select using the cured adhesive’s own datasheet, bond-line thickness and stated test conditions; bulk-material comparisons do not establish a product’s thermal performance.
catalyst damage>>
The catalyst loses its catalytic activity due to reactions with other materials.
Catalysts for addition-curable silicones may lose activity due to reactions with certain materials (e.g., compounds containing sulfur, phosphorus, or nitrogen), resulting in the failure of the adhesive to cure.
Temperature shock>>
Also known as the high-and low-temperature cyclic test, this procedure involves subjecting adhesive specimens to specified cycles of high and low temperatures to evaluate changes in their performance characteristics.
age>>
The phenomenon whereby the performance of adhesives degrades over time in operating environments, such as yellowing, embrittlement, deformation, and easy delamination.
Adhesives Knowledge: Read Material Properties with the Test Conditions
Useful adhesives knowledge should explain how a reported property relates to the actual joint. Viscosity describes resistance to flow under the measurement conditions, but it does not establish whether a bead will remain in place on a vertical surface. A material can flow readily while dispensing and resist movement after the shear is removed. Temperature and the way a material is sheared can therefore matter when comparing processing behavior. Ask how the property was measured before using it to select a pump or coating method. Hardness is another property that needs context. A cured material’s resistance to indentation does not by itself establish adhesion to a substrate, resistance to tearing or suitability for a moving joint. A soft material may buffer movement while needing separate support; a hard material may hold position while transferring more stress into adjacent parts. Strength values also describe a test configuration. The adherends, surface preparation, adhesive layer and loading method influence the result. What does a high strength value tell an engineer if the production joint fails by a different loading mode? It’s more useful to compare data that relate to the intended substrate and geometry than to rank unrelated figures. The JOME industrial adhesive catalogue can help identify candidate material families, while a technical data sheet should be read with its test conditions. Thermal conductivity and electrical insulation similarly answer separate questions. Heat-transfer behavior in a complete assembly depends on contact and geometry as well as the bulk material, and insulating behavior must be assessed with the electrical design. A favorable property in isolation cannot describe every interaction in a bonded part. Use the material description to identify relevant trials rather than treating it as a substitute for them. Record which properties are needed for dispensing, which are needed for assembly handling and which are needed in service. That distinction turns a list of adhesive terminology into a selection process that can be checked against the real application.
Distinguish the curing route from the evidence of a finished bond
The practical value of adhesives knowledge is knowing what to observe during bonding. Start with the curing route and the access it needs. A carrier-based adhesive needs a suitable drying path, a moisture-curing material needs the required moisture access, and a light-curing material needs an appropriate exposure route. A chemically mixed system depends on the selected formulation’s preparation instructions. These mechanisms should not be reduced to the same instruction to wait until the surface feels dry. The visible surface can become stable before the interior or hidden interface reaches its intended condition. Likewise, handling strength and complete cure describe different stages of assembly. Record when the joint closes, how it is held and what happens before it reaches its final processing condition. This helps explain a failure that appears only after a part is moved or exposed to another manufacturing step. Surface preparation is part of the bond rather than a separate housekeeping detail. The adhesive meets the actual surface layer, which may be a coating, contamination or a weakened region of the substrate. After a suitable mechanical test, inspect where the joint separated. Material left mostly on one adherend suggests an interface to investigate. Separation within the adhesive points toward a different path, while substrate tearing or coating detachment identifies another weak region. These observations are clues, not a complete diagnosis on their own. Compare them with cure history, surface condition and the loading method before drawing a conclusion. Then condition representative joints for the relevant environment and repeat the evaluation with the same method. Changes after moisture, temperature or chemical exposure may involve the adhesive, either interface or the substrate itself. Keep those possibilities open when interpreting the result. An application enquiry through JOME adhesive technical enquiries is more informative when it includes the substrate finish, joint drawing, process sequence and observed failure location. The selection can then address a concrete bonding problem, rather than relying on the assumption that a particular chemistry is universally strong, flexible or resistant.
Selection of the right grade from the full JOME product catalogue usually starts with drawings and service conditions. The companion piece on The Most Comprehensive Classification of Adhesives in sits alongside this guide, the the JOME adhesive manufacturer profile lists every family, and the contact JOME for project review handles enquiries.