Non-stick coating is a special coating whose coating surface is not easily adhered by other viscous substances or easily removed after being adhered. This kind of coating is widely used in household appliances, cooking utensils, automobiles, machinery, chemical industry and other industries because of its anti-adhesion characteristics such as extremely low surface energy, low friction coefficient and easy sliding. According to the medium and state of the coating, non-stick coatings can be divided into water-based, solvent-based and powder-based types. Water-based non-stick coatings are most popular with users and have the largest output because they contain less organic solvents, are safe, and have little environmental pollution. In recent years, the world demand for non-stick coatings has increased at a rate of 20% to 25% per year. The research and development of non-stick coatings in my country is still in its infancy, and there are few products, which are far from being able to meet domestic demand. Based on this, this article focuses on exploring the basic idea of preparing water-based non-stick coatings with fluororesin and binder as the main raw materials, and appropriately analyzes the film formation mechanism and main influencing factors.
1 Basic film-forming substances and film-forming principles of water-based non-stick coatings
1.1 Fluorine resin
The basic film-forming materials of non-stick coatings are fluororesin and binder. The choice of fluororesin plays a vital role in the non-stick performance of non-stick coatings. There are many types of fluororesin. Among many fluororesins, polytetrafluoroethylene (PTFE) has the smallest friction coefficient, so PTFE is the first choice for non-stick coatings. In order to obtain better coating performance or ease of construction, it is generally added on the basis of PTFE, such as tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) or tetrafluoroethylene-hexafluoropropylene copolymer (FEP) The fluororesin used in non-stick coatings is generally a mixture of PTFE.
The molecular structure of PTFE is -CF2—CF2—. Since the carbon chain in the molecule is surrounded by highly electronegative fluorine atoms, a stable C—F bond is formed. To break this bond, more than 500 J of heat energy is required, so the heat resistance of PTFE is extremely excellent. At the same time, because it can shield the C atoms on the main chain, it makes the surface inert. Almost all viscous substances cannot adhere to this surface, nor can it be adhered by ordinary methods. In the PTFE molecule, the mutual repulsion force between the fluorine atoms is large, and the spatial arrangement of the C-F molecular chain is spiral, so that the molecular chain is relatively rigid and the melt viscosity is high. Although the molecular chain of PTFE is stiff, it is easy to slip between molecules. This feature gives PTFE beneficial self-lubricity and low friction coefficient.
PTFE has a regular structure, high crystallinity, almost insoluble in any solvent under normal temperature conditions; and high melting temperature, high melt viscosity, and poor fluidity of the coating, which directly affects the non-stickiness of the non-stick coating. For the above reasons, the fluororesin used in non-stick coatings is generally a mixture of PTFE and other fluororesins, such as FEP, PFA, etc. Compared with PTFE, FEP has the advantages of better melt fluidity and lower crystallinity, so that the formed coating has a lower porosity, which is especially suitable for the hot and humid environment of baked food factories. PFA has the chemical resistance and thermal stability very close to PTFE, but the toughness of PFA is higher than that of PTFE, which can improve the wear resistance of the coating. Adding PFA can prevent the formation of large crystal areas on the surface of the coating. Usually adding more than 40wt% of PFA has obvious effects. However, the miscibility of PTFE and FEP is limited, and crystalline regions are formed in the part where each polymer is separated, and coating defects sometimes occur. Large pores and film defects are not allowed in a coating used to prevent water and/or air permeability of the substrate. These defects are largely related to the composition of the fluoropolymer, and may also be related to other ingredients in the coating formulation. The method and conditions of using paint are also very important in controlling the structure of the coating film (such as cooling rate). The addition of copolymers like FEP and PFA, due to their lower crystallinity, helps to form a coating with a lower permeability and a higher degree of amorphous than pure PTFE. Therefore, mixing PTFE with FEP or PFA in non-stick coatings can improve the permeability of the coating film and prevent the occurrence of defects such as bubbles or corrosion spots.
Since the fluoropolymer is almost insoluble in any solvent, it can be made into an aqueous dispersion. The particle size of the micropowder is required to reach the micron level, and because the surface energy of the fluoropolymer is extremely low, it is required to wet its surface with a special surfactant to form a stable aqueous PTFE dispersion. Water-based PTFE dispersions cannot be directly coated on the surface of the substrate after surface treatment. This is due to the extremely low surface energy of PTFE, and the poor adhesion of the coating formed after high temperature melting and cooling, so it needs a good adhesion to the substrate. It can be said that the non-stick coating is composed of fluororesin and adhesive. Polymer alloy.
1.2 Adhesive components for non-stick coatings
The main function of the binding component in the non-stick coating is to make the fluororesin adhere to the substrate, and to modify the fluororesin to strengthen the various properties of the coating, so that the special functions of the fluororesin can be fully utilized. Fluororesin has excellent anti-sticking properties, self-lubricating properties, heat resistance, etc., but due to its special molecular structure, fluororesin cannot be used as a single film-forming product for coatings. It is necessary to add bonding ingredients to the non-dry coating to enhance the adhesion, hardness, durability and other properties of the fluororesin non-stick coating. In solvent-based non-sticky materials, polyethersulfone resin, silicone resin, and epoxy resin are usually added. If the temperature is not high, acrylic resin can be used. When higher temperature is required for water-based non-stick coatings, polyphenylene sulfide resin and polyamideimide (PAI) are used more often.
PAI is a product made by introducing amide bonds into polyimide molecules. It is an aromatic amorphous polymer with both amide and imide groups on the main chain, and its glass transition temperature Tg is relatively high. PAI has a low coefficient of friction and has good wear resistance at high temperatures. PAI and PTFE have good compatibility, and the range of compounding between the two is wide, which is conducive to the development of non-stick coatings with different compositions to meet various needs. In addition to the above characteristics, PAI as a bonding component of non-stick coatings has the following characteristics: PAI is made into an aqueous solution and forms a single phase with water, which is conducive to the preparation of water-based materials; although the proportion of PAI in the materials is Less, but it can firmly adhere PTFE to the tomb material; the coating film has good fluidity and thixotropy, and the coating smoothness is high; it has good pigment dispersion and easy coloring. Because PAI has many advantages when used in water-based non-stick coatings, many foreign companies use PAI as a bonding component in water-based non-stick coatings. Non-stick coatings containing PAI can be made into single-layer and double-layer coating materials, and general-purpose coating equipment can meet the needs of paint making.
2 Self-stratification phenomenon and the influence of additives on it
Ideal non-stick coating of polymer alloy composed of fluororesin and binder. The surface layer should contain as much fluororesin as possible, and the bottom layer should contain as much adhesion punishment as possible, and there is a concentration gradient from the surface layer to the bottom layer. The so-called self-stratification phenomenon. The purpose of this is to make the bottom layer and the substrate have good adhesion, the surface layer has good non-stickiness, and there is no peeling phenomenon in the middle of the coating. How to prepare stable fluororesin water dispersion and binder aqueous solution or water dispersion, how to make them compatible with each other, and the delamination phenomenon occurs during the coating formation process, is one of the key technologies for the preparation and construction of water-based non-stick coatings. One. There are many factors that affect the self-delamination of the coating, such as the composition ratio of the coating, the compatibility of the resin, the density and size of the coating particles, the modification effect of additives on the particle surface, the plasticization process of the coating and the preparation of the coating Process etc.
The role of additives is to improve the surface properties of each component of the coating, and to promote the grinding, stability and construction properties of the coating. In addition to the main consideration of the wetting, dispersion and stabilization of the coating components, it is also necessary to consider how to improve and adjust the compatibility between the resins and promote the self-delamination of the coating to obtain good adhesion and non-stickiness. In terms of wetting and dispersing, the main consideration is the effect of the surface of the raw material particles and the dispersion medium water. Since the coating components include non-polar fluororesins and bonding resins to certain pigments and fillers with high polarity, the surface polarity of each component is There is a big difference, and the requirements for wetting agents, dispersing agents, and stabilizers are also very different. They must be considered comprehensively and take into account each other. In terms of improving resin compatibility and self-delamination, the main consideration is the force between the resin and the resin. The resins are firmly bonded and cannot be separated from each other. The fluororesin cannot migrate to the surface, the self-delamination of the coating cannot be achieved, and the coating is not sticky; the resin compatibility is not good, and the resins are easily separated from each other. The grinding and dispersion of the coating The construction performance is poor, there is obvious phase separation between the resins, and the coating is manifested as two parts of obvious bonding resin and fluororesin. The performance is their individuality, no complementarity, low coating strength, and poor overall performance. The role of additives is to harmonize the compatibility between the resins and the self-layering of the coating, so as to achieve the best overall performance of the coating.
3 Coating non-stick performance test
The non-stick properties of non-stick coatings are mainly judged by measuring the contact angle of droplets (water or oil) on the coating. The lower the solid surface tension, the worse the ability of another liquid to spread on its surface, that is, the worse the wettability, the better the non-stickiness. In general, the spreading ability of a liquid on a solid surface is measured by the spreading coefficient S.
where: γs——surface tension of solid; γSL——surface tension of solid-liquid interface; γL——surface tension of liquid
The smaller the S value, the weaker the ability of the liquid to spread on the solid surface, and the better the non-stickiness of the solid surface.
If a substance is selected as the standard liquid, then γL is certain. By measuring the contact angle θ1θ2Λθn of the standard liquid on different solid surfaces. The spreading coefficient S1S2ΛSn can be obtained.
The larger the θ, the smaller the S, the liquid tends to reduce the area in contact with the solid, and the better the non-stickiness of the solid surface. In other words, the contact angle θ can be used to quantitatively evaluate the non-stick performance of the coating.
4 Process influence
Water-based non-stick coating spray molding includes three steps of substrate surface pretreatment, spraying and sintering. The sintering temperature is generally 360～380℃, and the sintering time is 15～30min. The higher the sintering temperature, the stronger the adhesion of the coating, but if the sintering temperature is too high, the coating will fade, scar and bubbling, or even scorch. In addition, if the sintering temperature is too high, the fluororesin will decompose to produce highly toxic fluoride. When the sintering temperature is too low, the coating cannot be fully melted and leveled, and pinholes are easily formed in the coating, which fails to meet the technical requirements of the design. When the coating film is still in a molten state, the coating film quenched in cold water has low crystallinity and good coating density. For PTFE/PFA mixtures, the quenched coating film quickly loses its crystalline structure. About 40% of the copolymer content will have a significant effect on the coating film cooled at room temperature, but if the coating film is quenched quickly, only 20% PFA (even without any PFA) is very effective.
As a kind of environmentally friendly special paint with strong practicability and high technical content, water-based non-stick coating has attracted more and more attention from paint research and development workers. At present, a variety of water-based anti-sticking and wear-resistant coatings have been developed abroad, especially Teflon from DuPont of the United States, which is still in the development stage in China. Due to the complex technology of water-based non-stick coatings, how to disperse fluorine resin powder, how to select binding resins, and how to make the resins compatible with each other are still important topics in the development of water-based non-stick coatings.