Common knowledge of coatings: characteristics and development of water-based non stick coatings

Core tip: non stick coating is a kind of special coating which is not easy to be adhered or removed by other sticky substances This kind of coating is widely used in household appliances, cooking kitchenware, automobile, machinery, chemical industry and other industries because of its low surface energy, small friction coefficient and easy sliding According to the different medium and state of coating, non stick coating can be divided into water-based, solvent based and powder based types The water-based non stick coating is the most popular and the largest production because of its less organic solvent, good safety and less environmental pollution In recent years, the world demand for non stick coating is 2 Paint agent, top 10 Chinese wall paint brands Recommended introduction: interior wall paint and exterior wall paint can achieve beautiful, clean and pleasant effects, but the effects in different environments are different, so the color of the paint will be different There are ten brands of Chinese paint, ten brands of global paint, ten brands of Chinese wall paint, and ten brands of Chinese paint, Famous trademark of Chinese paint and coating In private rooms, most of us choose according to our own personality, personal hobbies and living habits The younger generation will choose the interior wall paint with rich colors and bright colors, while the older ones will have some working experience Paint Home News: non stick coating is a kind of coating surface is not easy to be adhered by other sticky substances or easy to be removed after adhesion This kind of coating is widely used in household appliances, cooking kitchenware, automobile, machinery, chemical industry and other industries because of its low surface energy, small friction coefficient and easy sliding According to the different medium and state of coating, non stick coating can be divided into water-based, solvent based and powder based types The water-based non stick coating is the most popular and the largest production because of its less organic solvent, good safety and less environmental pollution In recent years, the world demand for non stick coating is increasing at the rate of 20% - 25% per year The research and development of non stick coating in China is still in the initial stage of development, and there are few products, far from meeting the domestic demand Based on this, this paper focuses on the basic idea of preparing water-based non stick coating with fluororesin and binder as the main raw materials, and analyzes the film-forming mechanism and main influencing factors appropriately 1 The basic film-forming substances of water-based non stick coating and film-forming principle 1 The basic film-forming substances of fluororesin non stick coating are fluororesin and binder The selection of fluororesin plays an important role in the non stick performance of non stick coating There are many kinds of fluororesins Among them, the friction coefficient of polytetrafluoroethylene (PTFE) is the smallest, so PTFE is the preferred fluororesin for non stick coating In order to obtain better coating performance or to facilitate construction, generally on the basis of PTFE, resin such as polytetrafluoroethylene perfluoroalkyl vinyl ether copolymer (PFA) or polytetrafluoroethylene hexafluoropropylene copolymer (FEP) is added, so the fluorine resin used for non stick coating is generally a mixture mainly of PTFE The molecular structure of PTFE is - CF2 - CF2 - Because the carbon chain in the molecule is surrounded by fluorine atoms with strong electronegativity, forming a stable C-F bond In order to break the bond, it needs more than 500J of heat energy, so the heat resistance of PTFE is very good At the same time, because it can mask the C atom on the main chain, so that its surface is inert, almost all viscous substances can not adhere to this surface, and can not be bonded by ordinary methods In the PTFE molecule, fluorine atoms repel each other greatly, and the C-F molecular chain is arranged in a spiral shape in space, so the molecular chain is relatively rigid and the melting viscosity is very high Although the molecular chain of PTFE is rigid, it is easy to slip between molecules, which endows PTFE with beneficial self lubrication and low friction coefficient The structure of PTFE is regular, the crystallinity is high, it is almost insoluble in any solvent under normal temperature, and the melting temperature is high, the melting viscosity is large, the coating fluidity is not good, which directly affects the non viscosity of the non stick coating Based on the above reasons, the fluororesin used for non stick coating is 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, which makes the porosity of the coating lower, especially suitable for the humid and hot environment of baking food factory PFA has the chemical resistance and thermal stability close to PTFE, but the toughness of PFA is higher than that of PTFE, which can improve the wear resistance of the coating The addition of PFA can prevent the formation of large crystal area on the coating surface Generally, adding more than 40wt% PFA has obvious effect However, the miscibility of PTFE and FEP is limited, and crystalline zone will be formed in the separated part of each polymer, sometimes film defects will occur Large porosity and film defects are not allowed in a coating used to prevent water and / or air permeability of the substrate To a large extent, these defects are related to the composition of fluoropolymer and other components in the paint formulation The method and conditions of using the coating are also important in controlling the structure of the coating (e.g cooling rate) The addition of copolymers such as FEP and PFA, due to their low crystallinity, contributes to the formation of a coating with a higher degree of amorphous than that with pure PTFE Therefore, the mixing of PTFE with FEP or PFA in non stick coating can improve the impermeability of the coating and prevent the formation of bubbles or corrosion spots and other defects Since fluoropolymers are almost insoluble in any solvent, they can be made into water dispersions It is required that the particle size of the micro powder should reach the micron level, and because the surface energy of fluoropolymer is very low, special surfactant should be used to wet its surface to form a stable aqueous PTFE dispersion The water-based PTFE dispersion can not be directly coated on the surface of the substrate after surface treatment This is because the surface energy of PTFE is very low, and the adhesion of the coating formed after high temperature melting and cooling is poor, so the adhesive with good adhesion to the substrate is needed It can be said that the non adhesive coating is a polymer alloy composed of fluororesin and adhesive 1.2 the main function of the bonding component in the non stick coating is to make the fluorine resin adhere to the substrate, and to modify the fluorine resin, strengthen the various properties of the coating, and make the special functions of the fluorine resin fully play Fluororesin has excellent anti viscosity, self lubrication, heat resistance, etc., but due to its special molecular structure, fluororesin can not be used as a single film-forming material for coatings In order to enhance the adhesion, hardness, durability and other properties of the fluororesin non stick coating, it is necessary to add bonding components into the non dry coating Polyethersulfone resin, organosilicon resin and epoxy resin are usually added in solvent-based non stick materials If the temperature is not high, acrylic resin can be used Polyphenylene sulfide (PPS) resin and polyamide imide (PAI) are used more when the temperature of water-based non stick coating is higher Pai is a kind of aromatic amorphous polymer with amide and imide groups in the main chain Its glass transition temperature Tg is high The friction coefficient of Pai is low, and it also has good wear resistance at high temperature Pai and PTFE have good compatibility, and their combination has a wide range, which is conducive to the development of different components of non stick coating to meet various needs In addition to the above features, Pai, as the bonding component of non stick coating, has the following features: the preparation of water-based Xu material by making Pai into an aqueous solution and forming a single phase with water, which is conducive to the preparation of water-based Xu material; although its proportion in Xu material is small, it can firmly attach PTFE to the tomb material; the film has good fluidity, thixotropy, high smoothness of the coating; good dispersion of pigment, easy coloring Because Pai has many advantages in water-based non stick coatings, many foreign companies use Pai as the bonding component The non stick coating containing Pai can be made into single-layer and double-layer coating Xu material, and the general coating equipment can meet the needs of paint making 2 Self stratification and the influence of additives on it The polymer alloy coating composed of fluororesin and binder should contain as much fluororesin as possible in the surface layer and as much as possible in the bottom layer, and there is a concentration gradient from the surface layer to the bottom layer, that is to say, there is so-called self stratification The purpose of this method is to make the bottom layer and substrate have good adhesion, the surface layer has good non adhesion, and there is no peeling phenomenon in the middle of the coating It is one of the key technologies for the preparation and construction of water-based non stick coatings that how to prepare stable water dispersion of fluororesin and binder, how to make them compatible with each other, and how to layer in the process of coating formation There are many factors that affect the self stratification of coating, such as the composition of coating, the compatibility of resin, the density and size of coating particles, the modification of particle surface by additives, the plasticizing process of coating and the preparation process of coating The function of additives is to improve the surface properties of each component of the paint, and to promote the abrasiveness, stability and construction of the paint In addition to the wetting and dispersing stability of each component of the coating, how to improve and adjust the compatibility between the resins and promote the self stratification of the coating should also be considered to obtain good adhesion and non adhesion In terms of wetting and dispersing surface, the main consideration is the effect of raw material particle surface and dispersing medium water As the coating composition includes non-polar fluororesin and bonding resin to some pigments and fillers with high polarity, the surface polarity of each component is far from the same, and the requirements for wetting agent, dispersion and stabilizer are also very different, which must be considered comprehensively and mutually In terms of improving the compatibility and self stratification of resin, the main consideration is the force between resin and resin The bonding between the resins is firm, which is not easy to separate from each other, the fluororesin cannot be moved to the surface, the self stratification of the coating cannot be realized, and the coating is not sticky; the compatibility of the resins is not good, and the resins are very easy to separate from each other, the abrasion dispersion and construction performance of the coating are poor, and there is obvious phase separation between the resins, and the coating shows two parts of obvious bonding resin and fluororesin, with the performance of each single part The coating has low strength and poor comprehensive performance The function of auxiliary agent is to make the compatibility between resins and the self stratification of coating harmonious and unified, so as to achieve the best comprehensive performance of coating 3 Coating non stick performance test the non stick of non stick coating is mainly judged by measuring the contact angle of droplets (water or oil) on the coating The lower the solid surface tension is, the worse the ability of another liquid to expand on its surface, that is, the worse the wettability is, the better the non viscosity is In general, the expansion capacity of liquid on solid surface is measured by expansion coefficient s In the formula of S = γ s-γ sl-γ L: γ s - surface tension of solid; γ SL - surface tension of solid-liquid interface; the smaller the value of γ L - surface tension of liquid is, the weaker the ability of liquid to expand on solid surface is, and the better the invisibility of solid surface is γ s - γ SL = γ LCOS θ s = γ L (COS θ - 1) If a substance is selected as the standard liquid, then γ L is certain The contact angle of the standard liquid on different solid surfaces is measured The spreading coefficient S1S2 Λ Sn can be obtained The larger the θ is, the smaller the S is, and the liquid tends to reduce the area of contact with the solid, the better the invisibility of the solid surface In other words, the contact angle θ can be used to quantitatively evaluate the non stick property of the coating The influence of the process includes three steps: surface pretreatment, spraying and sintering Generally, the sintering temperature is 360-380 ℃, and the sintering time is 15-30min Higher sintering temperature