A Comprehensive Understanding Of The Basics Of Adhesives
First, the definition of adhesive
1. Definition of adhesive:
A natural or synthetic, organic or inorganic substance capable of joining two or more parts or materials together by adhesion of an interface and cohesion of a substance, collectively referred to as an adhesive (adhesive) Also known as adhesives, it is customarily called glue.
2, bonding (bonding):
Refers to the state in which the two surfaces are combined by chemical force, physical force, or both.
The adhesive must first adhere to the surface of the adherend due to the adhesion between the two phases, which is derived from the secondary or primary valence force.
That is, each particle in a single substance is bound by a main valence bond force (including an ionic bond, a covalent bond, a coordinate bond, a metal bond, etc.), and a secondary bond force (including van der Waals force, hydrogen bond). The cohesive force of the adhesive is related to factors such as intermolecular force, relative molecular mass, degree of crosslinking, crystallization and molecular entanglement.
Second, the requirements of the adhesive
1: No matter what state, it should be liquid when applied;
2: The surface of the adherend can be completely spread and fully wetted;
3: It must be able to transform the liquid into a solid or gel state in some way to form a tough and stable glue layer;
4: There should be a certain strength after curing. Reliable connection, transfer of stress and resistance to damage;
5: It can withstand temperatures above 0 °C and withstand a certain period of time.
Third, the classification of adhesives
1: Classified by appearance
1.1: Solvent type
1.2: Emulsion type (aqueous)
1.3: cream or paste
1.4: Solid type
1.5: film type
2: According to the curing method
Water-based evaporation type, solvent evaporation type, hot melt type, chemical reaction type, pressure sensitive type.
Fourth, the basic composition of the adhesive
1, the basic composition of the adhesive:
The components of the adhesive include a binder, a curing agent, a solvent, a plasticizer, a filler, a coupling agent, a crosslinking agent, a promoter, a toughening agent, a tackifier, a thickener, a diluent, an antioxidant, and a polymerization inhibitor. Agent, flame retardant, initiator, photosensitizer, antifoaming agent, preservative, stabilizer, complexing agent, emulsifier.
2, the type of base material:
The base material is the main component of the adhesive, the main agent or the host polymer, and acts as a binder. Depending on the chemical composition, the source of the binder is an inorganic compound and an organic compound. The organic compound is a polymer base of a polymer synthesis type.
2.1 Commonly used organic compounds
Thermosetting synthetic resins: phenolic resins, epoxy resins, amino resins, polyurethane resins, unsaturated polyesters, and acrylic resins.
Thermoplastic synthetic resin: rubber base material, natural polymer material, water-based latex base material
Rubber base adhesive
2.2 The principle of selection of adhesive base
Depending on the object of the adhesive application, the application and the performance of the application. At the same time, the molecular structure of the molecular binder is closely related to the bonding properties.
1). Polarity of polymer materials
2). Crystallization properties of polymer materials
3). Molecular weight of polymer materials
3, curing agent (hardener)
3.1. Curing agent (hardener)
The ground state raw material is converted into a linear high polymer or a network polymer by a chemical reaction, polymerization, polycondensation or cross-linking reaction, and is a substance having mechanical strength and stability of the bonded joint.
3.2. Selection of curing agent:
1): It is preferably a non-toxic, colorless, odorless substance of liquid;
2): the curing agent reacts with the cured product to be stable, and the heat release is small to reduce the cohesive force of the adhesive layer;
3): heat resistance, using a curing agent having a large number of reactive groups in the molecule;
4): toughness, use a curing agent with a long molecular chain;
4, adhesive solvent
4.1. Adhesive solvent
Refers to a liquid that can reduce some solid or liquid intermolecular forces and disperse the dissolved matter into a molecular or ionic homogeneous system. The commonly used solvent is a low viscosity liquid substance. There are mainly aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, esters, alcohols, ketones, acids, ethers, terpenes, amides and waters. However, most organic solvents have certain toxicity, flammability, explosiveness, pollution to the environment, safety and hidden dangers, and harm to the human body.
4.2. The role of the solvent:
1) The medium of polymerization: the reaction is moderate, the temperature is easy to control,
2) Dissolving base: preparing solvent-based adhesive
3) Thinner: Dilute the adhesive to a certain viscosity, which is good for wetting and easy to coat.
4) Adjust the volatilization speed of the adhesive
5) Prevent gels and increase storage stability
6) Surface cleaning agent before bonding
7) Direct bonding, some plastics
8) Reduce costs and improve efficiency
4.3, the choice of adhesive solvent
1). Solvent selection
A. Polarity: Select the solvent with the same or similar polarity as the base material (similar principle of compatibility).
B. Solubility parameter (related to polarity): The solubility parameter of the solvent is similar to the solubility parameter of the polymer matrix.
C. Solvent volatility: Select a solvent with a proper volatility or a solvent that is mixed slowly.
The solvent evaporates too quickly: the surface of the glue is easy to form a film, and the solvent under the film is not volatile; the volatilization is an endothermic process, and the temperature of the surface of the film is too fast to condense water vapor, which affects the bonding quality.
The solvent evaporates too slowly: the surface drying time is slow, which affects the work efficiency.
D, the choice of solvent should take into account the price, toxicity and source.
5.1. The role of softener:
Softeners are not a component of SBS costs. Their role is to adjust hardness, fluidity, and cost. Oil-filled SBS makes blending easier. The most commonly used is naphthenic oil, which is dark, toxic, and unusable. Paraffin oil does not improve physical properties and can cause squirting. The domestic Karamay KN series environmentally friendly naphthenic oil (such as KN4010, etc.) is of superior quality.
5.2. Type of softener:
According to the hydrocarbon composition, it is divided into three types: paraffin group, cycloalkyl group and aromatic group.
5.3. Characteristics of softener:
1) Paraffin-based rubber oil: good oxidation resistance and light stability, but relatively poor emulsifying, compatibility and low temperature;
2) Naphthenic rubber oil: It has the advantages of both paraffin-based and aromatic groups. It has moderate emulsifying properties, compatibility, no pollution, no toxicity, and wide application range.
3) Aromatic base rubber oil: good compatibility, large filling amount and low price, and the obtained rubber product has high strength. However, the color is deep, the toxicity is high, and the environment is polluted.
6.1. Definition of plasticizer
It is a substance which lowers the glass transition temperature and melting temperature of a polymer compound, improves the brittleness of the adhesive layer, and improves the melt fluidity.
6.2. The role of plasticizers:
1) Dispersing the active groups of the polymer compound, weakening the intermolecular forces, thereby reducing the interaction between the molecules.
2), increase the thermal, elongation and cold resistance of the polymer compound, and reduce its cohesive strength, elastic modulus and heat resistance.
6.3, the choice of plasticizer:
The polarity affects the compatibility of the plasticizer with the host material, and the principle works like a solvent.
The physico-mechanical properties of the glue are affected by the loss of exudation, migration and volatilization. High boiling or high molecular weight plasticizers are selected.
3), molecular weight and status:
The higher the molecular weight, the better the bond strength.
6.4. Commonly used plasticizers:
Phthalates, phosphates, adipates and sebacates.
7.1. Definition of packing
A solid material that does not chemically react with the substrate in the adhesive, but can change its properties and reduce cost.
7.2. Commonly used fillers are classified into organic fillers and inorganic fillers:
Organic filler: improve the brittleness, high hygroscopicity and low heat resistance of the resin
Inorganic filler: The relative density of the glue is increased, the brittleness is increased, but the heat resistance, the properties of the medium, the shrinkage rate, and the like are all improved.
7.3. Selection of packing:
1): no activity reacts with other components of the adhesive;
2): Good dispersibility and good wettability of the base material;
3): no moisture, harmful substances, grease, no hygroscopicity;
4): Uniform particles, non-toxic;
5): Wide source, convenient processing and low price;
6): the density of the filler and the density of the binder cannot differ too much;
7): The dosage is appropriate.
7.4. Commonly used fillers: silica, calcium carbonate, magnesium carbonate, aluminum powder, phenolic resin, etc.
8, coupling agent
8.1. Coupling agent
A compound that simultaneously produces a certain binding force with polar substances and non-polar substances, and is characterized by substances having both polar and non-polar parts in the molecule. Commonly used coupling agents are organic chromium coupling agents, silicone coupling agents, and titanate coupling agents.
A commonly used silane coupling agent in adhesives.
8.2. The role of the coupling agent:
1). Increase the intermolecular force of the main resin itself and improve the cohesive strength of the adhesive.
2). Increase the bonding between the resin and the adherend, and play a certain bridging role.
Different coupling agents have different reactive groups, and different reactive groups have different effects on bonding properties.
9, other additives
Under certain conditions, the free radical-generating material can be decomposed. Generally, an initiator is added to the compound containing an unsaturated bond. Commonly used initiators are dibenzoyl peroxide, cyclohexanone peroxide, cumene peroxide, and even. Nitrogen diisobutyl cyanide
A substance capable of lowering the decomposition temperature of the initiator or accelerating the reaction of the curing agent with the resin rubber.
A substance which can delay the aging of a polymer compound, and is suitable for aging and deterioration of an adhesive used under high temperature and exposure, and an anti-aging agent is added at the time of compounding
A low viscosity adhesive that increases the viscosity of the adhesive
Inhibitors and stabilizers:
A substance that prevents or retards the polymer containing unsaturated bonds and self-crosslinks during storage (hydroquinone)
Third, change the design principle of adhesive properties
Improved design of bond strength
1: Select a resin (epoxy resin, polyurethane) with high adhesion and cohesion;
2: adding a toughening agent, reducing brittleness, increasing the toughness of the rubber layer, and reducing internal stress;
3: A thermosetting resin is used in combination with a thermoplastic resin or a rubber. The thermosetting resin provides a strong and heat-resistant element, and the thermoplastic resin provides a tacky and tough element. The combined results will result in higher bond strength;
4: Introducing a polar group or adding a resin with good compatibility and high polarity can improve the bonding strength of the rubber type adhesive;
5: a suitable crosslinking agent;
6: Add appropriate amount of suitable filler, reduce shrinkage, whisker or nano Tennessee effect is excellent;
7: adding a suitable coupling agent;
8: Add diluent to reduce viscosity and increase wettability.
Improved design of heat resistance
1: Use high temperature resistant resin or rubber (phenolic resin, silicone, fluororubber, heterocyclic polymer);
2: increase the density of the ring;
3: increase the crosslinking agent;
4: Appropriately increase the crystallinity;
5: use a curing agent with good high temperature resistance;
6: adding a heat resistant filler;
7: reduce high temperature oxidative decomposition, adding antioxidants;
8: A silane coupling agent is added.
Improved design of cold resistance
1: Select a cold resistant polymer, such as polyurethane;
2: adding a plasticizer or a toughening agent;
3: reduce the degree of crosslinking;
4: lowering the crystallinity;
5: Reduce the amount of filler.
Improved design of solvent resistance
1: using a solvent-resistant resin or rubber;
2: increase the degree of crosslinking;
3: Appropriately increase the amount of filler;
4: Use less or no plasticizer.
Improved design of acid and alkali resistance
1: increase the degree of crosslinking;
2: use inert filler;
3: increase the amount of filler;
4: Ester plasticizers are not resistant to acids.
Water resistance modified design
1: The polymer containing less -CN, -NH2, -OH, -COO-, etc. in the molecule is low in water absorption and is not easily hydrolyzed;
2: increase the amount of adhesive base;
3: use a water resistant curing agent;
4: increase the crosslink density;
5: The filler used is a type with less water absorption;
6: A coupling agent is added to improve the interface properties.
Improved design for aging resistance
1: Select a base material with good water resistance, weather resistance and aging resistance;
2: increase the degree of crosslinking;
3: adding an active filler;
4: Some anti-aging agents or antioxidants may be added to some adhesives;
5: adding an appropriate amount of an organosilane coupling agent;
6: Use a high temperature curing agent.
Improved design of flame retardancy
1: Select flame retardant resin and rubber flavor base;
2: a flame retardant plasticizer;
3: adding a flame retardant such as antimony trioxide or zinc borate;
4: A flame retardant curing agent is used.
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