Six major adhesion mechanisms of adhesives and physicochemical factors affecting bonding strength
2018-07-09
Adhesion is the result of the interaction between different materials after contact at their interface. Therefore, the role of the interfacial layer is a fundamental issue in the study of adhesive science. Factors such as the interfacial tension between the adherend and the adhesive, surface free energy, functional group properties, and interfacial reactions all affect adhesion. Adhesion is a highly complex technology with many influencing factors, and existing adhesion theories all explain their principles from a certain aspect, so there is no comprehensive and unique theory to date. Several aspects of adhesion mechanisms are introduced below. The adhesion mechanisms of these adhesives are also very useful for engineers in solving the problem of adhesion of coatings on different substrates.
I. Adsorption Theory
The theory that considers the adsorption of a solid to an adhesive as the main reason for adhesion is called the adsorption theory of adhesion.
The theory suggests that the main source of adhesive force is the intermolecular force of the adhesive system, namely van der Waals force and hydrogen bonding. The adhesive force between the adhesive and the adherend surface has some similar properties to the adsorption force. The interaction process between the adhesive molecules and the adherend surface molecules involves two processes:
The first stage is the diffusion of liquid adhesive molecules towards the adherend surface by Brownian motion, bringing the polar groups or segments of the two interfaces closer together. In this process, increasing the temperature, applying contact pressure, and reducing the viscosity of the adhesive all help to enhance Brownian motion.
The second stage is the generation of adsorption force. When the distance between the adhesive and adherend molecules reaches 5-10Å, mutual attraction occurs between the interfacial molecules, further shortening the intermolecular distance to the state of maximum stability.
According to calculations, due to the van der Waals force, when two ideal planes are 10Å apart, the attractive force between them can reach 10-1000 MPa; when the distance is 3-4Å, it can reach 100-1000 MPa. This value is far greater than the strength achievable by the best structural adhesives today. Therefore, some believe that as long as two objects are in good contact, that is, the adhesive fully wets the bonding interface and reaches an ideal state, the action of dispersion forces alone is sufficient to produce high bonding strength. However, the actual bonding strength differs greatly from the theoretical calculation, because the mechanical strength of a solid is a mechanical property, not a molecular property, and its magnitude depends on the local properties of each part of the material, and is not equal to the sum of the intermolecular forces. The calculated value assumes that two ideal planes are in close contact, and ensures that the interaction between each pair of molecules on the interface layer is destroyed simultaneously, which means that it is impossible to ensure that the interaction between each pair of molecules occurs simultaneously.
If the adhesive is too polar, it can sometimes seriously hinder the wetting process and reduce the adhesive strength. Intermolecular forces are a factor in providing adhesive strength, but not the only factor. In some special cases, other factors can also play a dominant role.
II. Chemical Bonding Theory
The chemical bonding theory suggests that in addition to intermolecular forces, chemical bonds are sometimes formed between adhesive and adherend molecules. For example, studies on the bonding interface between vulcanized rubber and copper-plated metal, the role of coupling agents in bonding, and isocyanates on the bonding interface between metal and rubber have all shown the formation of chemical bonds. The strength of chemical bonds is much higher than that of van der Waals forces; the formation of chemical bonds can not only improve the adhesion strength but also overcome the drawbacks of desorption that causes the destruction of the adhesive joint. However, the formation of chemical bonds is not universal; certain conditions must be met to form chemical bonds, so it is impossible to make all contact points between the adhesive and the adherend form chemical bonds. Moreover, the number of chemical bonds per unit adhesion interface is much smaller than the number of intermolecular interactions, so the adhesion strength from intermolecular forces cannot be ignored.
III. Weak Boundary Layer Theory
When the liquid adhesive cannot wet the surface of the adherend well, air bubbles remain in the gaps, forming a weak zone. For example, when impurities are soluble in the molten adhesive but not in the cured adhesive, another phase will be formed in the cured adhesive layer, creating a weak boundary layer (WBL) between the adherend and the adhesive as a whole. In addition to process factors, the formation of WBL can also be caused by the inhomogeneity of the interfacial structure produced during the polymer network formation or melt interaction molding process, and in the thermodynamic phenomena such as surface adsorption of the adhesive. Inhomogeneous interfacial layers will result in WBL. The stress relaxation and crack development of this WBL will be different, thus greatly affecting the overall performance of the material and product.
IV. Diffusion Theory
On the premise that two polymers are compatible, when they are in close contact with each other, mutual diffusion occurs due to Brownian motion of molecules or chain segment swing. This diffusion process occurs across the interface of the adhesive and the adherend. The result of diffusion is the disappearance of the interface and the formation of a transition zone. The adhesion system cannot explain the adhesion of polymer materials to metals, glass, or other hard materials using diffusion theory, because polymers are difficult to diffuse into these materials.
V. Electrostatic Theory
When the adhesive and adherend system is an electron acceptor-donor combination, electrons will transfer from the donor (such as metal) to the acceptor (such as polymer), forming a double electric layer on both sides of the interface region, thus generating electrostatic attraction.
When the adhesive layer is quickly peeled off from the metal surface in a dry environment, the light and sound phenomena of discharge can be observed with instruments or the naked eye, confirming the existence of electrostatic action. However, electrostatic action only exists in adhesive systems that can form a double electric layer, so it is not universal. In addition, some scholars have pointed out that the charge density in the double electric layer must reach 1021 electrons/cm2 for the electrostatic attraction to have a significant effect on the adhesive strength. However, the maximum value of the double electric layer migration charge density is only 1019 electrons/cm2 (some believe it is only 1010-1011 electrons/cm2). Therefore, although electrostatic force does exist in some special adhesive systems, it is definitely not the dominant factor.
VI. Mechanical Force Theory
From a physicochemical point of view, mechanical action is not a factor in generating adhesive force, but a method of increasing the adhesion effect. The adhesive penetrates into the gaps or unevenness of the adherend surface, and after curing, an interlocking force is generated in the interface region. These situations are similar to the joining of nails and wood or the rooting of roots in soil. The nature of mechanical connection force is friction. When bonding porous materials, paper, fabrics, etc., the mechanical connection force is very important, but for some solid and smooth surfaces, this effect is not significant.
Physical factors affecting the adhesive strength of adhesives
1. Surface Roughness
When the adhesive wets the surface of the adherend well (contact angle θ < 90°), surface roughening helps to improve the wetting of the adhesive liquid on the surface, increase the density of contact points between the adhesive and the adherend, and thus improve the bonding strength. Conversely, when the adhesive wets the adherend poorly (θ > 90°), surface roughening is not conducive to improving the bonding strength.
2. Surface Treatment
Surface treatment before bonding is the key to successful bonding, and its purpose is to obtain a strong and durable joint. Due to the existence of oxide layers (such as rust), chromium plating layers, phosphating layers, release agents, etc., which form a "weak boundary layer" on the adherend, the surface treatment of the adherend will affect the bonding strength. For example, the polyethylene surface can be treated with hot chromic acid oxidation to improve the bonding strength. Treatment at 70-80℃ for 1-5 minutes will result in a good bondable surface. This method is suitable for polyethylene plates, thick-walled pipes, etc. However, when treating polyethylene films with chromic acid, it can only be carried out at room temperature. If it is carried out at the above temperature, the surface treatment of the film uses plasma or micro-flame treatment.
When treating the surface of natural rubber, styrene-butadiene rubber, nitrile rubber, and chloroprene rubber with concentrated sulfuric acid, it is hoped that the rubber surface will be slightly oxidized, so the sulfuric acid should be thoroughly washed away shortly after the acid is applied. Excessive oxidation will leave more fragile structures on the rubber surface, which is not conducive to bonding.
When locally bonding the surface of vulcanized rubber, the release agent is removed from the surface treatment, and a large amount of solvent washing should not be used to prevent the release agent from diffusing to the treated surface and hindering bonding.
For the surface treatment of aluminum and aluminum alloys, it is hoped that an aluminum oxide crystal will be formed on the aluminum surface, but the naturally oxidized aluminum surface is a very irregular and rather loose aluminum oxide layer, which is not conducive to bonding. Therefore, the natural aluminum oxide layer needs to be removed. However, excessive oxidation will leave a weak layer in the bonded joint.
3. Penetration
Bonded joints are often penetrated by other low-molecular-weight substances under the action of the ambient atmosphere. For example, water molecules penetrate into the adhesive layer in a humid environment or underwater; solvent molecules penetrate into the polymer in the polymer adhesive layer in an organic solvent. The penetration of low-molecular-weight substances first causes the adhesive layer to deform, and then enters the interface between the adhesive layer and the adherend. This reduces the strength of the adhesive layer and leads to the destruction of the bond.
Penetration not only starts from the edge of the adhesive layer, but for porous adherends, low-molecular-weight substances can also penetrate into the adherend from the pores, capillaries, or cracks of the adherend, and then invade the interface, causing defects or even destruction of the joint. Penetration not only leads to a decrease in the physical properties of the joint, but also causes chemical changes at the interface due to the penetration of low-molecular-weight substances, generating a rust zone that is not conducive to bonding, causing the bonding to completely fail.
4. Migration
Adherends containing plasticizers, such as PVC materials, are prone to migration from the polymer surface or interface because these small molecules have poor compatibility with the polymer macromolecules. If the migrated small molecules accumulate at the interface, they will hinder the bonding between the adhesive and the adherend, causing bonding failure.
5. Pressure
When bonding, applying pressure to the bonding surface makes it easier for the adhesive to fill the pits on the surface of the adherend, and even flow into deep holes and capillaries, reducing bonding defects. For adhesives with low viscosity, excessive flow will occur when pressure is applied, resulting in a lack of adhesive. Therefore, pressure should be applied when the viscosity is higher, which also promotes the escape of gas on the surface of the adherend, reducing pores in the bonding area.
For thicker or solid adhesives, applying pressure during bonding is essential. In this case, it is often necessary to appropriately increase the temperature to reduce the viscosity of the adhesive or liquefy the adhesive. For example, the manufacture of insulating laminates and the molding of aircraft rotors are carried out under heating and pressure.
In order to obtain higher bonding strength, different pressures should be considered for different adhesives. Generally, high pressure is applied to solid or high-viscosity adhesives, while low pressure is applied to low-viscosity adhesives.
6. Adhesive Layer Thickness
Thicker adhesive layers are prone to bubbles, defects, and premature fracture, so the adhesive layer should be as thin as possible to obtain higher bonding strength. In addition, the thermal expansion of the thick adhesive layer after heating causes greater thermal stress in the interface area, which is more likely to cause joint failure.
The stress acting on the actual joint is complex, including shear stress, peel stress, and alternating stress.
(1) Shear stress: Due to the action of eccentric tension, stress concentration occurs at the end of the bond, and in addition to shear force, there are tensile forces consistent with the interface direction and tearing forces perpendicular to the interface direction. At this time, under the action of shear stress, the thicker the adherend, the greater the strength of the joint.
(2) Peel stress: When the adherend is a soft material, peel stress will occur. At this time, tensile stress and shear stress act on the interface, and the force is concentrated on the bonding interface between the adhesive and the adherend, so the joint is easily damaged. Because the peel stress is very destructive, the design should avoid using joint methods that will produce peel stress.
(3) Alternating stress: The adhesive on the joint gradually fatigues due to alternating stress and is destroyed under conditions far below the static stress value. Tough and elastic adhesives (such as some rubbery adhesives) have good fatigue resistance.
7. Internal Stress
(1) Shrinkage stress: When the adhesive cures, shrinkage occurs due to volatilization, cooling, and chemical reactions, causing shrinkage stress. When the shrinkage force exceeds the adhesion force, the apparent bonding strength will decrease significantly. In addition, the stress distribution is uneven at the bonding ends or around the voids of the adhesive, also causing stress concentration, increasing the possibility of crack formation. Crystalline adhesives have a larger volume shrinkage due to crystallization during curing, also causing internal stress in the joint. If a certain amount of rubbery substance that can crystallize or change the crystal size is added, the internal stress can be reduced. Adding tougheners to thermosetting resin adhesives is a good example. For example, in phenol-formaldehyde-acetal resin, when the acetal content is below 40%, the joint undergoes simple interfacial failure; while above 40%, it is cohesive failure, and the bonding strength is significantly enhanced.
(2) Thermal stress: At high temperatures, when the molten resin cools and cures, volume shrinkage occurs, and internal stress is generated at the interface due to the constraints of bonding. When there is a possibility of slippage between molecular chains, the internal stress disappears.
The main factors affecting thermal stress include the coefficient of thermal expansion, the temperature difference between room temperature and Tg, and the elastic difference.
In order to alleviate the thermal stress caused by the difference in the coefficient of thermal expansion, the coefficient of thermal expansion of the adhesive should be close to that of the adherend. Adding fillers is a good method, and powders or fibers of such materials can be added.
Chemical factors affecting the bonding strength of adhesives
The chemical factors affecting the bonding strength mainly refer to the polarity of the molecules, molecular weight, molecular shape (number and size of side groups), molecular weight distribution, crystallinity of the molecules, stability of the molecules to the environment (transition temperature and degradation), and the properties of other components in the adhesive and adherend, such as pH value, etc.
1. Polarity
The polarity of the adhesive and adherend molecules affects the bonding strength, but it does not mean that an increase in the polarity of these molecules will necessarily increase the bonding strength. From the perspective of polarity, in order to improve the bonding strength, it is better to change the polarity of the surface of the interface region than to change the polarity of all the molecules of the adhesive and adherend. For example, after polyethylene, polypropylene, and polytetrafluoroethylene are subjected to plasma surface treatment, many polar groups such as hydroxyl, carbonyl, or carboxyl groups are generated on the surface, thus significantly improving the bondability.
2. Molecular weight
The molecular weight (or degree of polymerization) of the polymer directly affects the intermolecular forces of the polymer molecules, and the magnitude of the intermolecular forces determines the melting point and boiling point of the substance, and determines the glass transition temperature Tg and melting point Tm of the polymer. Whether the polymer is used as an adhesive or an adherend, its molecular weight affects the bonding strength.
The relationship between molecular weight and bonding strength is limited to the case of unbranched linear polymers, including two types.
The first type is cohesive failure of the adhesive in the entire molecular weight range. In this case, the bonding strength increases with the increase of molecular weight, but remains unchanged when the molecular weight reaches a certain value.
The second type is different due to different molecular weights. In this case, cohesive failure occurs in the low molecular weight range, and the bonding strength increases with the increase of molecular weight; when the molecular weight reaches a certain value, the cohesive force of the adhesive is equal to the adhesion force, resulting in mixed failure; when the molecular weight further increases, the cohesive force exceeds the adhesion force, the wettability is poor, and interfacial failure occurs.
3. Side chains
Side groups on long-chain molecules are important factors determining the properties of polymers. Considering intermolecular forces, the effect of polymer branching is that when the branch is small, increasing the branch length reduces the intermolecular force. When the branch reaches a certain length, it begins to crystallize, and increasing the branch length increases the intermolecular force. This should be the reason for decreasing or increasing the bonding strength.
4. PH value
For some adhesives, their pH value has a close relationship with the service life of the adhesive, affecting the bonding strength and bonding life. Generally, strong acids and strong alkalis, especially when acids and alkalis have a great impact on the bonding materials, are often harmful to bonding, especially porous wood, paper, and other fiber materials are more easily affected.
Since the curing process of thermosetting phenol-formaldehyde resins and urea-formaldehyde resins is greatly affected by the pH value, a higher acidity is often required. For example, p-toluenesulfonic acid or phosphoric acid is added to phenol-formaldehyde resin during curing, and ammonium chloride or hydrochloric acid is added to urea-formaldehyde resin. Therefore, in situations where high acidity is not desired but bonding is required, the use of neutral m-phenol formaldehyde resin is suitable.
Pre-treating the wood surface with alkali can generally obtain a strong joint. However, attention must also be paid to the pH value of the adhesive layer, which has a greater impact on the adhesive layer than on the surface to be bonded.
5. Crosslinking
The cohesive strength of the polymer increases with the increase of the crosslinking density, but when the crosslinking density is too high, the polymer becomes hard and brittle, thus reducing the impact strength of the polymer. The strength of the crosslinked polymer is closely related to the number of crosslinking points and the length of the crosslinking molecules. As the number of crosslinking points increases, the crosslinking distance shortens, and the length of the crosslinking molecules shortens, the crosslinked polymer becomes harder and more brittle.
6. Solvents and plasticizers
The bonding strength of solvent-type adhesives is naturally affected by the amount of residual solvent in the adhesive layer. When the amount of solvent is large, although the wettability is good, the cohesive force of the adhesive is reduced, and the cohesive strength is reduced. When the affinity between the adhesive polymers is large, the bonding strength increases with the volatilization of the solvent. When there is no affinity between the two, the adhesion of the adhesive is larger when some solvent remains, and the strength decreases with the volatilization of the solvent. For example, polyvinyl acetate cannot bond polyethylene, but it can bond after adding a small amount of solvent. Obviously, the solvent plays a role in increasing the affinity between the two.
Plasticizers and solvents have similar functions; sometimes, even if adhesion is not achieved, adding an appropriate plasticizer can achieve adhesion. Plasticizers also volatilize over time, seeping to the surface, and the adhesive strength decreases as the plasticizer decreases. Conversely, sometimes the plasticizer in the adherend will migrate into the adhesive layer, softening the adhesive and losing its cohesive adhesive strength. Or the plasticizer accumulates at the interface, causing the adhesive interface to separate.
7. Fillers
Incorporating fillers into adhesives has the following effects:
(1) Increase the cohesive strength of the adhesive;
(2) Adjust viscosity or processability (e.g., thixotropy);
(3) Improve heat resistance;
(4) Adjust the coefficient of thermal expansion or shrinkage;
(5) Increase the gap's fillability;
(6) Provide conductivity;
(7) Reduce cost;
(8) Improve other properties.
8. Crystallinity
The polymerization state of high-crystallinity polymers is regular. If the melting point is not high, heating the crystalline polymer will cause the ordered molecular arrangement within the crystalline region to become disordered, and the molecules will begin to transition to a molten state. Therefore, high-crystallinity polymers are suitable for hot-melt adhesives.
9. Decomposition
During use, the decomposition of the adhesive is an important factor that reduces the adhesive strength. The causes of adhesive decomposition include water, heat, irradiation, acids, alkalis, and other chemicals. The reaction and decomposition of polymers with water is called hydrolysis. Heating often leads to polymer crosslinking. The hydrolysis resistance of polymers varies depending on the chemical bonds in their molecules. Most water-soluble polymers are easily hydrolyzed. The hydrolysis of water-insoluble polymers is very slow, while the water absorption capacity of polymers plays an important role in hydrolysis. Polymer hydrolysis is also significantly affected by crystallinity and chain conformation. Since trace amounts of acids or alkalis can accelerate the hydrolysis of certain polymers, polyester condensation resins are easily hydrolyzed when in contact with acids or alkalis. The moisture resistance of epoxy resins varies significantly depending on the type of curing agent and the operating environment. Epoxy resins cured with polyamide are destroyed due to the hydrolysis of amide bonds; epoxy resins cured with polybasic anhydrides disintegrate due to the breakage of ester bonds; polyurethanes are also often destroyed due to ester bond hydrolysis. Polymers with ether bonds and carbon-carbon bond structures, such as phenolic resins, styrene-butadiene, and nitrile rubber, are not easily hydrolyzed and have good water resistance.
Excessive heating of polymers will cause the following changes:
(1) Decomposition of polymer molecules;
(2) Continued crosslinking;
(3) Escape of volatile and migratory components; These processes will result in a decrease in the cohesive strength of the adhesive or a decrease in the interfacial force.
Polymers undergo degradation and crosslinking at high temperatures. Degradation breaks the polymer chains, reducing the molecular weight and the strength of the polymer. Crosslinking forms new chemical bonds between molecules, increasing the molecular weight and the strength of the polymer. Continuous crosslinking of the polymer at the adhesive joint will make the polymer brittle and worsen the joint strength.
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