Encyclopedia of Synthetic Raw Materials for Polyvinyl Acetate Emulsions.
Publish time：2020-10-23 Authors：admin Browse：
The main synthetic raw materials of polyvinyl acetate emulsion adhesive include monomer, dispersing medium, initiator, emulsifier, protective colloid, plasticizer, regulator, filler, defoamer, freeze-thaw stabilizer and so on.
Vinyl acetate is the main monomer of emulsion polymerization, also known as vinyl acetate, referred to as VAc, structural formula, relative molecular weight 86.1, colorless flammable liquid, volatile, harmful to the central nervous system, and will stimulate the mucous membrane and cause tears. VAc slightly soluble in water (20 ℃ in water solubility of 2.5g), easy to hydrolyze, hydrolysis products acetic acid will interfere with the polymerization reaction. During storage, VAc is prone to polymerization, so diphenylamine, copper acetate, hydroquinone and other polymerization inhibitors need to be added. In the vinyl acetate industrial products, such as containing 5 ~ 15mg/kg of hydroquinone polymerization inhibitor, it can be directly polymerized without prior removal.
The dispersion medium is mainly water, non-polluting, low cost and easy to control temperature. In the use of water, it is necessary to consider the influence of iron ions, chloride ions and sulfate ions and other impurities in the water.
The initiating agent is usually per-sulfate, such as potassium persulfate and ammonium persulfate, and sometimes hydrogen peroxide is used. The amount of the initiator is usually 0. 1% to 1% of the monomer mass. The initiating efficiencies of potassium persulfate and ammonium persulfate are very close, but since the solubility of potassium persulfate in water at room temperature is 2%, and the solubility of ammonium persulfate in water can be more than 20%, ammonium persulfate is more frequently used in industrial production.
When selecting an emulsifier, consider its critical micelle concentration (CMC), HLB value, and ion type. Commonly used are octylphenol ethoxylates ( OP - 10), sodium alkyl sulfate, sodium alkyl benzene sulfonate (0. 01% to 5%), sodium oleate, etc..
The main role of the protective colloid is to form a protective layer of hydration on the surface of latex particles with a spatial potential barrier effect to prevent coalescence of latex particles. The commonly used protective colloids are polyvinyl alcohol, animal glue, gelatin, PVA, methyl cellulose, carboxymethyl cellulose, gum arabic, sodium polyacrylate, etc. PVAc emulsions often use PVA as a protective colloid, the amount of 1% to 4%. The use of protective colloids and emulsifiers can control the size of the latex particle size and its distribution, and improve the reaction stability and storage stability.
The role of the buffer is to adjust the pH value of the dispersion medium. When the pH value is high, the initiator decomposes faster, forming more active centers and polymerization reaction is faster. However, with the decomposition of the pertechnetate initiator, hydrogen ions will be gradually released to lower the pH of the system. But if the pH value is too low, not only will the initiating speed is slowed down significantly, but also reduce the stability of the system. Commonly used are carbonate, phosphate, acetate, etc., the amount of monomer mass 0. 3% ~ 5.0%.
The role of plasticizers is to give PVAc emulsion adhesive at lower temperatures and good film-forming adhesive force. Commonly used plasticizers are alkyl phthalate esters and aromatic phosphate esters, such as dibutyl phthalate (DBP), dioctyl phthalate (DOP), trimethyl phosphate (TCP), and so on. When the amount of plasticizer is 10% to 25% of the monomer mass, it can significantly improve the low temperature film-forming properties and adhesion of PVAc.
The regulating agent, also known as chain transfer agent, acts to control the molecular weight of the polymer chain. Commonly used are carbon tetrachloride, mercaptans, polysulfides and so on.
Adding fillers in the system can reduce costs, increase solid content and viscosity, reduce permeability, and improve filling properties. Filler is divided into two kinds of organic and inorganic. The amount of organic filler is generally 5% to 10%, inorganic filler can be as high as 50%.
PVAe emulsion is not easy to mildew, but in the addition of starch or cellulose, it is necessary to add 0.2% to 0.3% of the preservatives, such as formaldehyde, phenol, quaternary ammonium salt, etc. Sometimes it is also necessary to add 0.2% to 0.3% of the cellulose. Sometimes it is necessary to add 0.2% to 0.3% of the silicone oil or advanced alcohols as defoamers. In addition, the addition of 2% of the emulsion of 2% to 10% of the mass of methanol, ethylene glycol, glycerol, etc. as freeze-thaw stabilizer, can prevent its freezing at low temperatures or stability after ablation is destroyed.
