What are the principles and applications of wetting agents?
I. The concept of wetting agent
The basic feature of the molecular structure of the wetting agent is that one end of the molecule has a hydrophilic group (chain segment), the other end has a hydrophobic group (chain segment) of the chemical substance. In general terms, this means that the molecules are hydrophilic and hydrophobic, respectively.
When the resin is a water-based resin, I understand the wetting mechanism (mainly for water-based resins) as follows:
Mechanism: When the water-based resin is coated on the surface of the substrate, a part of the wetting agent is at the bottom of the coating, which is in contact with the surface to be wetted, the lipophilic chain segments adsorbed on the solid surface, and the hydrophilic groups reach outward into the water. The contact between water and the substrate is turned into a contact between water and the hydrophilic groups of the wetting agent, forming a sandwich structure with the wetting agent as the middle layer. Make the water phase easier to spread, so as to achieve the purpose of wetting. Another part of the wetting agent, exists on the surface of the liquid, its hydrophilic group extends to the liquid water, hydrophobic groups exposed to the air, the formation of a single molecule layer, reduces the surface tension of the coating, prompting the coating better wetting of the substrate, so as to achieve the purpose of wetting.
Second, what is the liquid wetting performance
Wetting performance is a measure of the affinity of liquid substances to solid substances. The main forms of performance are: a, the solid surface of the wetting; b, in the solid surface of the spread; c, in the solid surface of the penetration.
Simply put, good wetting properties of the liquid is easy to spread on the solid surface, easy to penetrate the solid surface of the gap.
Third, the influence of the liquid wetting performance of the intrinsic factor
Wetting performance is a relative form of expression, that is to say, and the characteristics of the liquid and solid itself, the most important of which is the relative size of the liquid and solid surface tension. The smaller the surface tension of the liquid, the larger the surface tension of the solid, the better the wetting performance of the liquid on the solid, the liquid will be able to spread well on the solid surface.
Fourth, the measurement of liquid wetting ability
The size of the liquid wetting ability can be used to spread the liquid on the solid surface, the formation of the contact angle θ to measure. The smaller the contact angle θ, the better the wetting performance of the liquid on the solid, θ is equal to zero, the best wetting performance. Where θ = 90 ° is an important parameter, because θ < 90 °, the liquid can spontaneously spread in the solid surface wetting; and θ > 90 °, the liquid can no longer be solid surface spontaneous spreading wetting.
The contact angle can be calculated by the following formula: cosθ= (γs-γsl)/γl
where:
γs is the surface tension of the solid.
γl is the surface tension of the liquid.
γsl is the interfacial tension between the liquid and solid surfaces. γsl is very small relative to γs and γl, and can sometimes be ignored in calculations.
In addition to the contact angle θ to measure the wetting performance, the spreading coefficient can also be used to indicate the size of the wetting capacity. Its physical significance for a certain volume of liquid can be in the solid surface wetting area, expressed in cm2/g. The better the wetting performance of the liquid, the larger the wetting area. Spreading coefficient expressed in S, the formula is: S = γs – γsl – γl; when S is greater than zero, the liquid can be spontaneously in the solid surface wetting.
V. Factors affecting the wetting ability
1, the chemical structure and composition of the liquid and the solid being wetted. Mainly affect the size of the surface tension and affect the wetting ability.
2, the degree of roughness of the solid surface. For example, θ <90 °, the surface roughness will increase the contact angle will reduce the wetting performance; θ> 90 °, the surface roughness increases the contact angle becomes larger and difficult to wet.
3, the degree of contamination of solid surfaces. Solid surface pollution is generally not conducive to wetting. So the substrate should be decontaminated before coating.
4, surfactant. Adding surfactants to the liquid can effectively reduce the surface tension and facilitate wetting.
5、Temperature has a direct effect on the surface tension of the material, which should be considered in the practical work.
Sixth, the application of theory
It can be concluded from the above basic theory that whether the coating can produce wetting effect on the substrate depends on the surface tension of the coating. When the surface tension of the coating is equal to or less than the surface tension of the solid substrate, it will spread well on the solid surface.
In practice, there is also a measure of the choice of wetting agent, we should choose a wetting agent that can effectively reduce the surface tension of the coating to improve the selectivity of the material.
VII. Surface Tension Table of Common Substances
| material | surface tension[mN/m{dyn/cm}] |
| Water | 72.2 |
| Glycol | 48.4 |
| o-Xylene | 30 |
| Ethylene glycol monoethyl ether acetate | 28.7 |
| n-Butyl acetate | 25.2 |
| Rosin | 24 |
| n-Butanol | 24.6 |
| Methyl isobutyl ketone | 23.6 |
| Methyl ethyl ketone | 24.6 |
| Melamine resin (HMMM type) | 58 |
| Epoxy resin (Epikote 828) | 45 |
| Methyl polymethyl acrylate | 41 |
| 65% Soybean Oil Fatty Acid Alkyd Resin | 37 |
| Oil-free alkyd resin | 47 |
| Modaflow leveling agent | 32 |
| Tinplate (uncoated/coated) | 35~45 |
| Phosphate Treated Steel | 40~45 |
| Aluminum | 37~45 |
| Alkyd resin primer | 70 |
| Glass | 70 |
| Polymer | Yc(达因/cm) |
| Urea-formaldehyde resin | 61 |
| Cellulose | 45 |
| Polyacrylonitrile | 44 |
| Polyethylene Oxide | 43 |
| Polyethylene Terephthalate | 43 |
| Nylon 66 | 42.5 |
| Nylon 6 | 42 |
| Polysulfone | 41 |
| Polymethylmethacrylate | 40 |
| Polyvinylidene Chloride | 40 |
| Polyvinyl Chloride | 39 |
| Polyvinyl Alcohol Acetal | 38 |
| Chlorosulfonated Polyethylene | 37 |
| Polyvinyl Acetate | 37 |
| Polyvinyl Alcohol | 37 |
| Polystyrene | 32.8 |
| Nylon 1010 | 32 |
| Polybutadiene(cis) | 32 |
| Polyethylene | 31 |
| Polyurethane | 29 |
| Polyvinyl chloride | 28 |
| Polyvinyl Butyral | 28 |
| Butyl Rubber | 27 |
| Polyvinylidene Chloride | 25 |
| Polydimethylsiloxane | 24 |
| Polytrifluoroethylene | 22 |
| Silicone Rubber | 22 |
| Polytetrafluoroethylene | 18.5 |
| Perfluoropropylene | 16.2 |
| Perfluorooctyl methacrylate | 10.6 |