Analysis of the basic principles of coating adhesion

Adhesion theory and
When two objects are placed together to achieve close interface molecular contact, so that a new interface layer is generated, adhesion is generated. Adhesion is a complex phenomenon involving the physical effects and chemical reactions of "interfaces". Since each observable surface is usually associated with several layers of physical or chemically adsorbed molecules, the true number of interfaces is not known precisely, and the question is where the two surfaces are demarcation and where attachment

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when the coating is built on the substrate and adheres to it during drying and curing. The size of these forces depends on the properties of the surface and the bonding material (resin, polymer, substation). Broadly speaking, these forces can be divided into two categories: the main price force and the secondary price force. The chemical bond is the main price force, which has a much higher adhesion than the sub-price force, and the sub-price force is based on a much weaker physical force represented by the hydrogen bond. These forces are more common on substrates with polar groups, such as pyrides, and less common on non-polar surfaces such as polyethylene. Key strength and key energy strength/type/energy (kca/molar)/example:
co-price key primary price force 15 to 170 Most organic matter
hydrogen bond secondary price force <12
dispersion force sub-price force <10 The exact <10 molecular
-even effort sub-price force <5 polar organic matter
induced force sub-price force <0.5 non-polar organic matter
coating attachment is not fully understood. However, the force that connects the two objects together may result in mechanical connections, electrostic attraction, or chemical bonding due to the diffusion of the substrate and coating through the coating. Depending on the physical and chemical properties of the substrate surface and the coating used, attachment may take one or more of the above-mentioned processes. Some of the proposed theories are discussed below.
1. Mechanical Connection Theory
This coating mechanism is suitable for coating penetration when the coating is constructed on a substrate containing holes, holes, fissures or holes. In this case, the role of paint is very much like the nail when wood is flattened, playing a mechanical anchoring role. When the substrate has grooves and fills with cured paint, it is more difficult to remove the coating due to mechanical action, similar to putting two knotted pieces of wood together. Instrument analysis and mapping of various surfaces (form maps) show that coatings do penetrate into grooves or cracks in complex "tunnel" shapes, providing mechanical attachment when cured hardened. The attachment of various coatings to old or weathered coatings, as well as to blast substrates, is part of this process. Zinc phosphate or iron has a large contact area with the coating, which can improve adhesion and corrosion resistance.
the roughness of the surface affects the interface area of the coating and substrate. Because the force required to remove the coating is related to the geometric area, the force attached to the substrate by the coating is related to the actual interface contact area. As the surface area increases, the difficulty of removing the coating increases, which is usually achieved by mechanically grinding rough surfaces. The surface area is increased by sand blasting, resulting in an increase in adhesion. Obviously due to many other factors, attachment does not increase in the same proportion, but a significant increase is usually visible.
Only when the coating completely penetrates to the irregular surface, improve surface roughness is beneficial, if not fully penetrate, then the contact between the coating and the surface will be smaller than the corresponding geometric area, and there is a gap between the coating and substrate, the air bubbles that reside in the void will lead to the accumulation of moisture, resulting in the loss of adhesion.
Often scrubed with cured coatings, inter-layer adhesion (especially in automotive coatings) can be improved, especially in primer/varnish systems, requiring smooth varnishes, lightness and low surface performance, so the attachment of the second layer of varnish is difficult. This problem becomes more serious when the coating cures or bakes for longer than the original temperature, in which case a light polishing of the surface indicates that adhesion can be significantly improved. Although surface roughing can improve adhesion, care must be taken to avoid deep and sharp shapes, due to the sharp summit generated by roughness leading to transmission (see substrate), in some cases do not expect such a deep and sharp bulging will form an unearraed coating, resulting in stress concentration points, reduced adhesion, and thus reduced durability.
As long as the coating film is slightly fluid, the coating film shrinks, the thickness is uneven and the three-dimensional size changes will rarely produce non-releaseable stress, but with the increase in viscosity and coating rigidity and the gradual formation of adhesion to the substrate will generate a large number of stresses, and remain in the dry paint film. Obviously, when the construction parameters (wet film and dry film thickness) are fixed, the coating thickness of the raised part is smaller than the depression, resulting in different physical properties. This unearral coating has a high internal stress and, when put into use, is further affected by erosion or aging of the paint solvent, which occasionally exceeds the stress tolerance of the coating film, resulting in cracks, peeling or other reductions in coating integrity.
the adhesion of electroplating metals to polyethylene and ABS plastics is proven to be derived from mechanical connections. The metal plating process involves first treating the plastic surface, generating a large number of mechanical depressions that facilitate mechanical connections, then inculuating it with a tin chloride solution, depositing Pd in a Pd2-plus solution, depositing nickel without electricity, and then electroplating the required metals, such as chromium. The adhesion of electroplating metal to plastic is strong only when the connection depression is generated after plastic treatment. Different pre-treated metals not only alter the chemical composition of the surface, but also produce surface connection points, and mechanical connections play a considerable role, if not the most critical, to such surfaces.
surface form of unprocessed and phosphate-treated cold-rolled steel plates, a large number of interstate iron phosphate microchips can be found on the surface after phosphation, and the space between the chips provides a large number of physical connection points.
2. Chemical Bond Theory
may form co-priced bonds between interfaces and are more likely to occur in thermoso-solid coatings, which are the most strongly connected and have the best durability, but require a reactive chemical substrate firmly integrated on the substrate and coating. Because the interface layer is very thin, the chemical bonds on the interface are difficult to detect. However, as discussed below, interface bonding does occur, which greatly increases the bonding strength. Some surfaces, such as coated surfaces, wood, compounds and some plastics, have a variety of chemical function groups that form chemical bonds with coating materials under suitable conditions.
organic pyrane is widely used in glass fiber primers to enhance the adhesion of glass in resin and fiber-reinforced plastics, or as a primer or an integrated mixture to promote resin adhesion to ore, metals and plastics. Essentially, the application produces a niobium alcohol-based, which can be used with a niobium-based on the glass surface, or may form a strong ether bond with other metal oxides. These chemical bondings can occur between metal hydroxides and tantalum-containing coatings on the surfaces of glass, ceramics and some metal substrates.
An example of the fact that reactive groups such as hydroxyl and carboxyl-based coatings tend to adhere more firmly to substrates containing similar clumps is the excellent adhesion of melamine-cured acrylic finishes to melamine-cured polyester primers, and one possible explanation is that the remaining base of the cured primer reacts with the finished melamine curing agent, effectively pulling the primer and the finisher together. When the coating is over-baked (too long to bake and/or the curing temperature is too high), the adhesion of the paint is significantly reduced, sometimes even without adhesion. The contribution of residual hydroxyl to adhesion can be confirmed by the IR spectrum: standard baked primers are rich in hydroxyl, while over-baked primers have very little, if any.
when the substrate contains reactive hydroxyl, it also reacts with thermosterged polyurethane coatings under appropriate conditions.
chemical bonding is also fully suitable for interpreting the excellent adhesion of epoxy coatings to cellulose substrates. Obviously, as confirmed by the infrared spectrum, the epoxy base of epoxy resin and the hydroxyl of cellulose on the interface react, resulting in the disappearance of the telescopic vibration peak 3350cm-1 and C-O telescopic vibration peak 1100cm-1, while the epoxy 915cm-1 peak of epoxy resin and the symmetrical telescopic vibration peak of oxygen bridge disappear 1160cm-1.
some polymers attach weakly to the crosslinked polymer surface, resulting in interface defects. There are reports that adding a small amount of certain nitrogen-containing groups can greatly improve adhesion. For example, amino polymers have a strong adhesion to crosslinked alcoholic acid resins, because the ammonia-ester exchange reaction occurs between the two phases of the interface, forming aamide bond.
can easily be found in model compounds that use butamine as an amino polymer. When amines are added to a toluene solution of unsterified alcoholic acid resins, they react easily at room temperature to form didybendiamide, which crystallizes and crystallizes. FTIR spectroscopy to detect the mixture of amino resins and uncarified alcoholic acid resins found that the mixture after baking the amine-based absorption peak decreased, while the emergence of aamide absorption peak, indicating that the interface did occur ammonia-ester exchange reaction.
3. Static theory
It can be imagined that the electrostic force in the form of a charged double-charged layer is formed on the interface of the coating surface, the coating and surface are accompanied by residual charge, scattered in the system, the interaction of these charges can improve some adhesion. Static power is mainly the dispersion force and the interaction force derived from the permanent even pole. The attraction between molecules containing permanent even polar matter is caused by the interaction between the positive and negative regions of one molecule and the negative electrode region of the other.
The degree to which a coating wets a solid surface induces attraction between dippoles by contact angle, known as London force or dispersion force, which is a type of Van der Wort force and also contributes to adhesion, providing most of the attraction between the coating and substrate for certain substrate/coating systems. It should be noted that these interactions are only short-range interactions, inversely inversely related to the six or seven squares of the distance between coatings/substrates. Close contact between the coating and substrate is necessary because the effect of these forces decreases significantly when the distance exceeds 0.5 nanometers (5 E).
4. Diffusion Theory
When the two phases of coatings and substrates (polymers) reach molecular contact through wetting, depending on the nature of the material and curing conditions, certain fragments of the polymer may spread to varying degrees to the other side of the interface. This phenomenon needs to be completed in two steps, that is, after wetting the chain segments through the interface to diffuse each other to form a staggered mesh structure.
because long chain properties are different and diffusion coefficients are low, non-similar polymers are generally incompatible, so it is not possible for complete large molecules to diffuse through the interface. However, theoretical and experimental data show that local chain diffusion is easy to occur, and between polymers to form a diffusion interface layer of 10 to 1000 E. The diffusion of coatings is also indirectly confirmed by the influence of contact time, curing temperature and molecular structure (molecular weight, molecular chain flexibility, side chain group, polarity, double bonding and physical compatibility). The direct evidence includes the determination of diffusion coefficient, the observation of interface structure by electroscope, radiant thermoluminescence technology and optical microscope. Obviously, this diffusion occurs most easily on polymer substrates such as engineering plastics because the free volume between molecules is larger and the distance between molecules is much greater than that between metals.
attachment formation mechanism
when two materials that are not similar reach "close" contact, the two free surfaces in the air disappear, forming a new interface. The nature of the interface interaction determines the strength of the bond between the coating and the substrate, the degree of this interaction is basically determined by the wetting of one phase by the other phase, when using liquid coating, the fluidity of the liquid phase is also very helpful, so wetting can be regarded as close contact between the coating and substrate. In order to maintain the adhesion of the coating and substrate, in addition to ensuring the initial wetting, it is important to keep the bonding condition unchanged after the film is completely moisturized and cured.