The preparation points of water-based thermal reflection coatings are discussed

0 Foreword
In the design of the building energy-saving external wall insulation system, the application of more white thermal reflection insulation coating, with hollow microbeads and high refractive index TiO2 as the main functional pigment fillers, the thermal reflection ratio of sunlight can reach more than 80%. With the increasingly urgent need for building energy saving, a single white thermal reflection coating can not meet the requirements of multi-color exterior paint. Choose infrared reflective pigments, through the coloring principle of color preparation of dark color coatings, energy-saving coatings to become the mainstream of the development of
China
. Usually titanium dioxide and yellow pigments have a good thermal reflection effect, but if you need to prepare black, blue, gray, red paint, ordinary pigments are difficult to have a good thermal reflection effect. Once added titanium dioxide can greatly improve thermal reflectivity, but will significantly dilute the color concentration. BASF, Clariant and other companies have introduced thermal reflection color organic pigments, Hensmeier, Panhua and other companies have introduced special titanium dioxide thermal reflection, in the full spectrum of visible light band has a relatively low reflectivity, therefore reduced the brightness value and cover force, highlighting a specific color, but in the near-infrared band still has a high thermal reflectivity. These two types of pigments are used together, so that color coatings have a certain color, but also to achieve a certain thermal reflection effect.
thermal reflection coating as a functional coating, the use of thermal reflection pigments to help achieve energy saving and consumption reduction, but does not necessarily mean that the best results can be achieved. Resins, additives and other fillers in coating formulations can still have a significant impact. This paper gives a brief discussion on the effect of the main components of coating formulation on thermal reflective coatings, hoping to arouse the concern of the industry and discuss them in depth.
1 Trends in environmentally friendly functional coatings
At present, thermal reflection coatings for wall or metal plates are developing towards water-based.
1.1 Main features of water-based coatings
With the improvement of human awareness of environmental protection, it has become common to prepare polymer liquid coatings with water instead of organic solvents. In water-based coatings, the solvent is a polar solvent and the film-forming aid is a non-polar (or weakly polar solvent). High polypolarity is low, in the water needs to be emulsified dispersion or neutralization of salt, how to improve the water resistance of the coating film after dehydration at the same time try to control low VOC is an important subject, especially the single-part temperature self-drying coating.
1.2 To improve the water-based thermal reflection coating film resistance of the main link

(1) pigment volume concentration (PVC). Appropriately increase the PVC to reduce the volume of the resin and increase the volume of inorgers, and prepare high-solid, low-stick inorder slurry with appropriate wetting/dispersants.
(2) volume solid content. Minimize the volume of solvents and water, and select suitable combined thickening and fluidizing systems.
(3) supplemented by an appropriate deblister and substrate moisturizer.
(4) resin and inorganic: the couple between organic/inorganic/substrate.
between the resin (5) resin and the couple between the resin and the surfactant of the inorger.2 Water-based thermal reflection coating
2.1 definition of thermal reflection
thermal reflection refers to the solar radiation near infrared (see Figure 1) has a high reflection and low absorption capacity, can be the building roof, the outer wall surface of the vast majority of the solar radiation reflected out, reduce conduction and reflow are not as effective as the reflected heat radiation source.
2.2 varieties of thermally reflective pigments
thermally reflective pigments can be divided into organic and inorganic pigments, as can general-purpose pigments, which have different chemical and chemical properties. Table 1 shows the performance comparison of organic and inorganic pigments.
2.3 Reference formula for water-based thermal reflective coatings (see Table 2)
Table 2 Reference formulations for thermal reflection coatings, if the total solar reflectivity (TSR) needs to be increased, special titanium dioxide powder needs to be considered in order to increase the amount of titanium dioxide powder, supplemented by thermal reflection color pigments or slurry.
2.4 Resin varieties used in water-based thermal reflection coatings
water-based acrylic resins include acrylic emulsions, acrylic water dispersions (water dilutes acrylic) and acrylic aqueous solutions. The emulsion is mainly synthesized by oily ole monosomy in water caused by water-based free-based triggers, while resin water dispersions are synthesized by different processes such as free-based solution polymerization or progressive solution polymerization. Water-based acrylic resin is mostly anion type, co-polymer resin monomer selected the appropriate amount of unsaturated carponic acid such as acrylic, methyl acrylic, shunbutylate, methylenebutyl acid, etc. , so that the side chain with a carbide, and then use organic amine or ammonia water and salt to obtain water solubility. In addition, the resin side chain can also be selected by the appropriate monosome to introduce - OH, - CONH2 or - O - and other hydro-based groups and increase the water solubility of the resin. Neutralized acrylic resin can be dissolved in water, but its water solubility is not very strong, often form a milky turbid liquid or a high viscosity solution, so in the water-soluble resin must be added a certain proportion of hydro-assisted solvents to increase the water solubility of the resin, which involves the formula of the two parents balance and polyelectric stability. Resin not only has good adhesion, water resistance, but also requires stable aging resistance, yellow denaturation and light transmission, because thermal reflection coatings are very high optical requirements.
2.5 Two-parent balance of water-based thermal reflection coatings
2.5.1 Contradiction between water-based thermal reflection coating preparation and storage and construction stability and resistance after dehydration into film
However, all surfactants remain in the coating film after the film is formed and damage the resistance of the coating film.
in the process of dehydration and solvent, viscosity will increase, PVC will form different degrees of condensation of hydrophobic substances. As long as there is unevenness, it may affect the thermal reflection effect. How to control dehydration speed and balance between parents is the key to achieve the original formula design goals. The coating is generally pro-water, film-forming is better, but the coating film is less water resistant;
2.5.2 The importance of the balance of two parents of water-based thermal reflection coatings
The emulsion system should ensure the balance of the two parents, storage/construction should ensure that the water is oil-packed, after construction, moisture evaporation, penetration, rapid milk breaking, film formation, then may form oil-packed water, resulting in rapid surface drying and early anti-water coating. PVC in hydrophobic formulations should not be too low, which improves film hardness and resistance to contamination. Improving the breathability of the coating film reduces the risk of foaming, the overall hydrophobicity of the coating film, and improves the permeability.
2.6 Variety and principle of action of wetting dispersants in hydrothermal thermal reflection coatings
2.6.1 Hydrophobic modified anion dispersants
alkaline inorgetic pigments and fillers usually use anion dispersants to prepare the slurry. It is mainly the principle of collory chemistry, in the water phase of these polyelectrute ionized, so that the surface of inorganic particles saturate adsorption anion dispersant, the formation of static rejection, supplemented by a small amount of cellulose, to make the slurry stable.
commonly used anion dispersants in water-based coatings are mainly divided into polycarbonates, or acrylic, Malay acid and styrene polypolymers. In terms of adhesive properties, the dispersion effect of polycarbonate is better, but the suspension stability and water resistance are poor. The effect of multi-co-dispersant static rejection is poor, but the anti-sinking effect of spatial resistance and the hydrophobic contracting with HEUR rheative agent are better.
there are many suppliers of these dispersants, such as BASF, Dow, Nopko, Jiangsu Yantai and so on. The use of high-speed dispersion machines and dispersants to disperse inormeric materials and prepare suspended stable slurry, acrylic ionum and thyme co-polymer have this function. However, acrylic oxide is more suitable for high PVC coatings and with HEC or ASE, emulsion do not use much. The thermal reflection coating of medium and low PVC can use ternary copolymers and be paired with hydrophobic modified PU-connected fluid thickener.
2.6.2 non-ion moisturizing dispersant
in the coating formulation wetting agent can adjust the different water content and PVC of the coating two parent balance. Control drying speed and freeze-thaw stability with polar solvents.
low PVC coating needs more film-forming additives, need appropriate wetting agent to balance the time it starts to swell emulsion granules, choose the appropriate emulsion varieties and dosage to prepare low PVC coating. In medium-high PVC coatings, more attention is paid to the role of wetting agents. The basic paint formula needs to be colored, paying attention to the variety and dosage of wetting agents and thickeners. The dispersion of pigments in the base paint is affected not only by the dispersants it absorbs, but also by the surface state of other particles in the formula, and by the rebalancing of oil and water after dehydration during film-forming. The non-ion moisturizer is mainly fatty alcohol polyoxyethylene ether. Fatty alcohol surfactants, whether synthetic or natural alcohols are mainly straight-chain products. However, as a wetting agent, branch chain products have many special properties.
many foreign companies have developed many of these products, which meet the requirements of low-temperature, low-foam industrialization process. Figure 2 is the structure of this type of wetting agent.
usually fat alcohol polyoxyethylene ether coagulation point is above 10 degrees C, which brings a lot of inconvenience to use, in addition to more bubbles is also a practical problem. The low coagulation point wetting agent is a branch chain alkyl polyol comb structure ethyl ether, no APEO, no solvent, has good water solubility, at low temperature has a good flow, can effectively improve the dispersion of pigments, improve the color display of water-based coatings, can help improve the film gloss, low foaming.
Different surfactant substances on the physical adsorption of pigments are very different, by the external conditions of interference factors are also many, if the use of anchoring base segment co-concentration will avoid the surface active substances in the dewatering process may appear desorption, thereby improving the resistance of the coating film. Non-ion moisturizers can be adsorbed to organic or oil-dependent pigment surfaces in the water phase, but the adsorption rate on hydro-hydro-alkaline pigment surfaces is zero. Therefore, in the hydro-hydro inorder filling dispersion process, wetting agent only plays a role in reducing the water surface pressure wetting. For organic or oil-free pigments can be used as dispersants, but the molecular weight is too low, or no anchoring action of the wetting agent can easily spread into the water.
on the surface of the organic pigment, the lower HLB dispersant has a higher adsorption, but the water dispersion of such surfactants is poor. The adsorption of LYC additives is lower than that of single group additives, and the adsorption of dispersants of ion structure is lower than that of non-ion structure.
2.6.3 A segment copolymer dispersant
liquid coating that has anchoring function on the pigment surface releases moisture and solvent aid when it is converted to a solid coating film. With the decrease of water, the proportion of hydrophobic substances in the formula increases, and the phenomenon of milk-breaking transition occurs. At this point the particles in the formula begin to reare, and the Benard vortex, formed by the increased viscosity and dehydration process, causes similar particles to congregate with each other. It is also possible to desorpt material that was previously physically adsorbed on the surface of particles. This causes particles in the original homogenous water-based coating to begin to show uneven clustering, which affects the resistance and decorativeness of the coating film. Anchored to the pigment particle surface with a parent dispersant, it does not disengage from the particle surface during dehydration and can continue to be compatible with the resin.
In recent years, polymer dispersants have appeared on the market, in the 1980s, DuPont used group transfer polymerization to synthesize methyl acrylates hydro-oil-based segment polymers, which have a good effect on the dispersion of organic pigments in water. BASF synthesizes EFKA 4585 using controlled free-based polymerization technology. BYK introduced disperser DISPERBYK 2010 and 2015 based on controlled polymerization technology.
Unlike general anion dispersants, the molecular chain of polymer dispersants needs to contain anchoring group parts and stabilizing parts, and anchoring group parts and stabilizing parts need to be properly organized. The use of reversible-de-active free-based polymerization process can be used to synthesize segment-type polymer dispersants (structure shown in Figure 3), which can provide spatial bit resistance and electrostatic dual stability for dispersed organic pigment particles in the water-based system, which has positive significance for thermal reflection coatings.
thermal reflection of special titanium dioxide is suitable for high TRS requirements of the coating, it needs a special dispersant, the requirements of both titanium dioxide binding force and resin compatible, which not only helps the slurry water dispersion can also improve the volume of the coating solid content. The role of anchoring groups in polymer dispersants is to provide a strong enough binding force with titanium dioxide or other inorgeable pigment fillers to make it difficult to fall off the surface of titanium dioxide particles. Phosphate, phosphate, carboxy acid, nitric acid and titanium dioxide can form a co-priced bond, distribution bond, and titanium dioxide binding force. The polymer structure of this type of acrylic segment is shown in Figure 4.
these parent-type insert polymer dispersants need to be neutralized into salt and used to improve the compatible of titanium dioxide and resin after anchoring the surface of titanium dioxide. The dispersant is an acrylic polymer, which is heat-resistant, weather-resistant, yellow-resistant and stable.
2.7 and pigments, resins and substrates can react with the coupled agent
thermal reflection coating in addition to consider organic or inorganic pigment preparation into slurry and coating, but also consider the commonly used building materials have good adhesion. The coupled agent can be used to achieve the organic/inorganic/substrate between the coupled, by connecting the polysilioxane molecule to the side chain or active function groups, such as alkyd acid, ethyl oxide, propylene-based groups, can improve compatible with most systems and obtain good performance. In addition, due to the access to active errands, polysilica molecules can react with resins during film-forming processes, with the aim of avoiding migration.
2.8 Effects of different connective cooperation in the water phase
After the preparation of hydrophobic modified high-solid content thermal reflection slurry, in order to ensure that the coating has good suspension and fluidity, it is necessary to establish the coating particles and water molecules between the connectivity cooperation.
types of co-operation include: (1) hydrogen bonding cooperation; (2) ion-to-ion co-operation; (3) collosus co-operation; and (4) hydrophobic co-operation. After the polymer concentration is higher than a certain critical concentration, the large molecular chain is gathered by hydrophobic connection, forming a supermodulue structure based on intermodal connectivity -dynamic physical cross-linking network, increasing the volume of fluid mechanics and greatly increasing the viscosity of the solution.
polyurethane, polyether-type compound thickener is a low-to-medium molecular weight non-ion polymer, its hydrophobic end and emulsion particles or inormerables by the secondary force (van der Huali and hydrogen bonds) to form a connective network structure. This co-operation is the cohesion between non-bonded groups and molecules. This is different from gathering