The production of redispersible polymer powder is mainly divided into two steps, namely, emulsion polymerization and drying, as shown in the figure.
(l) Emulsion Polymerization
Monomers adopted for emulsion polymerization determine the types of redispersible polymer powder. Polymer monomers used for the production of redispersible polymer powder are mainly olefins and unsaturated monomers, including a variety of vinyl esters and acrylic esters. The redispersible polymer powder is mainly used in building binders and adhesives, while the vinyl acetate polymer has such advantages as low price, high bond strength, non-toxic, harmless, safe and convenient for production and use. Therefore, it has the largest amount used in the polymer emulsion of building binders and adhesives.
Generally speaking, there is no particular limit on the polymerization method of preparing the emulsion used for the redispersible polymer powder. Various emulsion polymerization methods using water as the dispersion medium may be used. However, continuous or semi-continuous emulsion polymerization methods are the most preferably used. The seed emulsion polymerization method can also be used. Generally protective colloids and anionic or nonionic emulsifiers are used, or without emulsifiers. The solid content of polymer emulsion obtained from the preparation of redispersible polymer powder is usually between 40% and 60%, which can be properly adjusted depending on the performance of dryer, performance requirements of product and other additives required to be added before drying. For the ethylene-vinyl acetate copolymer emulsion, it should be diluted to 40% or less.
In order to improve the redispersibility of redispersible polymer powder and prevent caking during drying and storage, protective colloids or surfactants (emulsifiers) should be added before drying, making the redispersible polymer powder with strong hydrophilicity and alkali sensitivity. The most commonly used protective colloid is partially hydrolyzed polyvinyl alcohol (PVA). PVA contains a lot of hydroxyl groups, with quite poor water resistance. And the vinyl acetate polymer itself is poor in the water resistance, especially hot water resistance, due to its polar ester and carboxyl groups. In the redispersible polymer powder containing PVA and carboxyl groups, polyvalent metal salts can be added to improve its water resistance, especially hot water resistance, since PVA and carboxyl groups can react with metal salts to become insoluble in water. In the emulsion containing PVA, aldehydes can also be added for PVA acetalization and then to reduce its water absorption. In addition to PVA, some other protective colloids with good water resistance can also be selected to ensure the water resistance of product, such as polyacrylic acids and modified polyacrylic acids.
Before emulsion drying, some other adjuvants such as defoamers, thickeners and hydrophobic agents can be dried together with emulsion dispersions.
(II) Emulsion Drying
The most commonly used drying method of preparing the redispersible polymer powder is spray drying. Reduced pressure drying and freeze drying methods can also be used.
Drying is a difficulty in the preparation of redispersible polymer powder. Not all emulsions can be converted into dispersible polymer powder because these thermoplastic polymer emulsions, film-forming or even tacky at room temperature, must be converted to free-flowing powders at high temperatures. The diameter of emulsion particle in the emulsion dispersion is about several μm. In the spray drying process, emulsion particles will condense, so the particle size of redispersible polymer powder is usually 10-500μm. It can be seen from a scanning electron microscope (SEM) that a hollow structure will be formed by condensation of emulsion particles. After redispersible polymer powders redisperse, the diameter of emulsion powder is generally 0. 1-5μm. The particle size distribution of redispersed liquid emulsion particles is one of the main quality indexes of redispersible polymer powder during dispersion. It determines the bonding capacity of redispersible polymer powder and various effects as an additive, so appropriate dispersion and drying methods should be chosen. Try to use the dispersion liquids with as similar particle size distribution to original emulsions as possible, in order to ensure the redispersed liquid with near-identical properties to original emulsions.
Most of the redispersible polymer powder uses the parallel spray drying process, that is, the motion direction of powder is the same with hot air. Some also uses the counter-current spray drying process, with air or nitrogen generally used as the drying medium. During spray drying, emulsion particles are prone to condensation, discoloration and other problems, so it is necessary to strictly control the emulsion additives, dispersion and solid content, as well as the spray form, spray pressure, droplet size, inlet and outlet hot air temperature, air speed and other process factors. In general, dual nozzle or multi nozzle has superior effects heat utilization to single nozzle. Usually the nozzle pressure is about 4 x 105Pa, the inlet temperature of hot air between 100 and 250℃, the outlet temperature about 80℃. Adding such inert mineral anti-caking agents as kaolin, diatomaceous earth and talcum powder can prevent caking. But if added before drying, then anti-caking agents may be encapsulated by polymers into microcapsules and lose power. Most are sprayed with emulsion respectively and independently at the top of dryer, but it is also easy to lose and crust on the dryer and pipeline by air currents. The better method for adding additives is a two-part method. One part is sprayed with compressed air at the top of dryer, and the other part enters with the cold air at the bottom. In order to prevent caking, the remaining part can be saponified during the emulsion polymerization process when polymerization reaches 80% to 90%. Or melamine-formaldehyde condensates may be added to the emulsion. Also, a certain kind of emulsifier emulsion can be utilized.
In the production process of redispersible polymer powders, polymer powders are polymer “solid” particles transformed from monomer emulsified droplets. Strictly speaking, these particles are not solid indeed, since the polymers considered here are thermoplastic, which will become solids only below a certain critical temperature, referred to as the glass transition temperature (Tg). Only above this temperature will the thermoplastic body lose all of its crystalline properties. However, since polymers are intertwined like nets, this kind of material is in fact still in a quasi-solid state.
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