Definitions of adhesives
Adhesives—also known as bonding agents or colloquially referred to as “glue” —are natural or synthetic, organic or inorganic substances designed to bond two or more workpieces or materials together through mechanisms such as interfacial adhesion and material cohesion.
① Adhesion: The state in which two surfaces are bonded together by chemical forces, physical forces, or a combination of both.
② The adhesive must first adhere to the surface of the substrate, as this results in interfacial adhesion force between the two phases, which originates from secondary valence bond forces or primary valence bond forces.
Cohesion refers to the state in which the particles within a single substance are held together by primary bonding forces (including ionic bonds, covalent bonds, coordinate bonds, metallic bonds, etc.) and secondary bonding forces (including van der Waals forces and hydrogen bonds). The cohesion of an adhesive is influenced by factors such as intermolecular forces, relative molecular mass, degree of crosslinking, crystallization, and molecular entanglement.
Requirements for Adhesives
① Regardless of its state, the material shall remain in a liquid form during application.
② Fully spread over and thoroughly wet the surface of the substrate.
③ The liquid must be capable of being converted, by some means, into a solid or gel state to form a tough and stable gel layer.
④ After curing, the material shall possess adequate strength to ensure reliable bonding, effective stress transfer, and resistance to failure.
⑤ Can withstand temperatures above 0 °C and withstand prolonged exposure testing.
Classification of Adhesives
Classification by appearance: solvent-based and emulsion-based (water-based); paste or putty-based; solid-type; film-type.
Based on the curing mechanism, they can be classified as: water-borne evaporation type, solvent-volatile type, hot-melt type, chemical reaction type, and pressure-sensitive type.
Basic Composition of Adhesives
Basic composition of adhesives: The components of an adhesive include the binder, curing agent, solvent, plasticizer, filler, coupling agent, crosslinking agent, accelerator, toughening agent, tackifying agent, thickening agent, diluent, anti-aging agent, polymerization inhibitor, flame retardant, initiator, photoinitiator, defoaming agent, preservative, stabilizer, complexing agent, and emulsifier.
Types of base material: The base material serves as the primary component, main agent, or core polymer of an adhesive, providing its adhesive functionality. Depending on their chemical composition, base materials can be derived from inorganic or organic compounds. Organic compounds consist of synthetic polymers.
① Commonly used organic compounds: thermosetting synthetic resins—phenolic resin, epoxy resin, amino resin, polyurethane resin, unsaturated polyester, and acrylic resin.
② Thermoplastic synthetic resins: rubber-based resins, natural polymer materials, waterborne latex-based resins, and other base materials
③ Rubber-based adhesives
Principles for selecting adhesive base materials: Selection is based on the target application, the operating environment, and the required performance characteristics of the adhesive. Also, there is a profound correlation between the molecular structure of the base material and its adhesive properties.
① This is related to the polarity of the polymer materials.
② Crystallization properties of polymer materials
③ Molecular weight of polymer materials
Curing agent (hardening agent): A substance that, through chemical reactions—such as polymerization, polycondensation, or cross-linking—converts the base raw materials into linear polymers or network polymers, thereby endowing adhesive joints with mechanical strength and stability.
Selection of curing agent:
① Ideally, it should be a liquid that is non-toxic, colorless, and odorless.
② The reaction between the curing agent and the substrate should proceed steadily with minimal heat release, thereby reducing the cohesive force of the adhesive layer.
③ Heat resistance: select a curing agent whose molecule contains a greater number of reactive groups.
④ Toughness: select a curing agent with longer molecular chains.
Adhesive solvents refer to liquids capable of reducing the intermolecular forces between certain solid or liquid molecules, thereby enabling the solute to disperse into a homogeneous system of molecules or ions; commonly used solvents are low-viscosity liquid substances. The primary types include aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, esters, alcohols, ketones, acids, ethers, terpenes and olefins, amides, and water. However, most organic solvents possess certain characteristics, such as toxicity, flammability, explosiveness, environmental pollution potential, safety hazards, and adverse effects on human health.
Role of the solvent:
① Reaction medium for polymerization: ensures a mild reaction environment and facilitates temperature control.
② Dissolving the base material: preparation of solvent-based adhesives
③ Diluent: Dilutes the adhesive to achieve a specific viscosity, facilitating wetting and simplifying application.
④ Adjust the volatilization rate of the adhesive
⑤ Prevent gelation to enhance storage stability
⑥ Surface cleaning agent prior to bonding
⑦ Direct bonding; applicable to certain plastics
⑧ Reduce costs and enhance profitability
Selection of adhesive solvents:
① Polarity: Select a solvent whose polarity is identical to or similar to that of the base material (based on the principle of “like dissolves like”).
② Solubility parameter (related to polarity): The solubility parameter of the solvent should be close to that of the polymer matrix.
③ Volatility of the solvent: Select a solvent with appropriate volatility, or a mixture of solvents with varying volatility rates. If the solvent evaporates too rapidly, a film may readily form on the surface of the adhesive slurry, while the solvent beneath the film may not evaporate adequately; if the solvent evaporates too slowly, the surface drying time will increase, thereby compromising work efficiency.
④ The selection of solvents should take into account cost, toxicity, and availability.
The role of the softener: It is an indispensable component of SBS; its functions are to regulate hardness and fluidity, reduce production costs, and facilitate the mixing and processing of the compound. Naphthenic oil is the most commonly used type; however, aromatic oils are highly colored, toxic, and therefore unsuitable for use, while paraffin oil fails to improve material properties and may even cause splattering. Domestic Karamay KN-series environmentally friendly naphthenic oils (e.g., KN4010) offer superior quality.
Types of softening agents: classified according to their hydrocarbon composition into paraffin-based, cycloalkyl-based, and aromatic-based categories.
Characteristics of the softener:
① Paraffin-based rubber oil: exhibits excellent antioxidant and light-stability properties, but demonstrates relatively poorer emulsification, compatibility, and low-temperature performance.
② Cycloalkyl rubber oil: Combines the advantages of paraffin-based and aromatic-based rubbers, exhibits moderate emulsifying and compatibility properties, is non-polluting and non-toxic, and has a wide range of applicable applications.
③ Aromatic rubber oil: It exhibits excellent compatibility, allows for high filler loading, is cost-effective, and yields rubber products with high strength. However, it tends to produce dark-colored products, possesses high toxicity, and poses significant environmental pollution risks.
Definition of plasticizer: A substance that lowers the glass transition temperature and melting temperature of polymer compounds, reduces the brittleness of adhesive layers, and enhances the melt flow properties.
Functions of plasticizers:
① Disperse the active functional groups of polymer compounds, thereby weakening intermolecular forces and reducing intermolecular interactions.
② Enhance the thermal stability, elongation, and cold resistance of polymer compounds, while reducing their cohesive strength, elastic modulus, and heat resistance.
Selection of plasticizers:
① Polarity: The degree of polarity influences the compatibility between the plasticizer and the base material; the underlying principle is analogous to that of solvent interactions.
② Durability: The physical and mechanical properties of the rubber may be adversely affected by loss due to exudation, migration, and volatilization. Therefore, plasticizers with high boiling points or high molecular weights should be selected.
③ Molecular weight and state: the higher the molecular weight, the greater the bonding strength.
Commonly used plasticizers include phthalates, phosphates, adipates, and sebacates, among others.
Definition of filler: A solid material added to an adhesive that does not undergo a chemical reaction with the matrix, but can modify its properties and reduce costs.
Commonly used fillers are categorized into organic fillers and inorganic fillers: organic fillers—reduce the brittleness of the resin, exhibit high hygroscopicity, and demonstrate poor heat resistance; inorganic fillers—increase the relative density of the resin and enhance its brittleness, while also improving heat resistance, medium compatibility, and shrinkage rate.
Selection of packing material:
① It is inactive and does not react with other components of the adhesive.
② Good dispersibility and excellent wettability toward the base material
③ Free from moisture, harmful substances, and oils/fats; non-hygroscopic.
④ Particles are uniform and non-toxic.
⑤ Wide availability, easy processing, and low cost
⑥ The density of the filler shall not differ significantly from that of the base material.
⑦ Appropriate dosage
Commonly used fillers: silica, calcium carbonate, magnesium carbonate, aluminum powder, phenolic resin, etc.
Coupling agents: compounds capable of establishing a certain degree of bonding affinity with both polar and non-polar substances; they are characterized by possessing both polar and non-polar regions within their molecular structure. Commonly used coupling agents include organochromium coupling agents, organosilicon coupling agents, and titanate coupling agents. In adhesives, silane coupling agents are frequently employed.
Role of the coupling agent:
① Enhance the intermolecular forces within the matrix resin itself to increase the cohesive strength of the adhesive.
② It enhances the bonding between the resin and the substrate, thereby serving as a bridge. Different coupling agents possess distinct reactive groups, and these varying reactive groups exert differing effects on the bonding performance.
Initiator: A substance that, under specific conditions, decomposes to generate free radicals; initiators are typically added to compounds containing unsaturated bonds. Commonly used initiators include benzoyl peroxide, cyclohexanone peroxide, isopropyl peroxide, and azobisisobutyronitrile.
Promoter (catalyst): A substance capable of lowering the decomposition temperature of the initiator or accelerating the reaction between the curing agent and the rubber resin.
Anti-aging agents: Substances capable of delaying the aging of polymer compounds. For adhesives used under high temperatures or prolonged exposure to sunlight—where they are prone to aging and degradation—anti-aging agents are added during the adhesive formulation process.
Thickening agent: A substance added to low-viscosity adhesives to increase their viscosity.
Antioxidants and stabilizers: substances (e.g., hydroquinone) that prevent or delay the spontaneous cross-linking of polymers or monomers containing unsaturated bonds during storage.
Selection of the right grade from the full JOME product catalogue usually starts with drawings and service conditions. The companion piece on OCA Optical Adhesive You’ve never compared the sits alongside this guide, the the JOME adhesive manufacturer profile lists every family, and the contact JOME for project review handles enquiries.