A thermoplastic adhesive that is polymerized from vinyl acetate monomer under the action of an initiator. According to its different polymerization methods, it can be divided into two types: solution type and emulsion type. The emulsion type is generally used for wood gluing.
The 1914 German patent (281687) reported that vinyl acetate can be polymerized into colorless and transparent blocks under photoinitiation. During the period from 1915 to 1925, free radical vinyl monomers were extensively studied. Polymer, Planson first prepared polyvinyl acetate emulsion using emulsion polymerization in 1929, and achieved industrial production around 1937. After World War II, it was widely used in coatings, textiles, adhesives and other industries. China began researching it in the mid-1950s, and mass-produced it in Tianjin, Shanghai, Beijing and other places in the mid-1960s, and began to replace animal glue in the furniture manufacturing industry.
Polymerization methods
Generally, two methods are used: solution polymerization and emulsion polymerization. Solution polymerization usually uses benzoyl peroxide or azobisisobutyronitrile as initiator to obtain polymers with lower molecular weight. Emulsion polymerization uses the action of emulsifiers (anionic or nonionic surfactants) to disperse monomers in the medium for polymerization, and then initiates polymerization with water-soluble initiators to obtain emulsion polymers. The polymerization reaction of polyvinyl acetate is divided into three consecutive stages: chain initiation, chain propagation, and chain termination.
Chain initiating stage
When the free radical or ion of the initiator itself is connected to the vinyl acetate monomer, an initiating effect occurs, resulting in the rearrangement of the double bond electrons and the The active sites are transferred to vinyl acetate monomer.
Agents are usually free radicals produced by peroxides such as benzoyl peroxide, hydrogen peroxide, persulfate, etc.
Chain growth (polymerization) stage
The initiated monomer free radicals further react with the monomer molecules through the same transfer mechanism to form chain free radicals, causing the molecular chain to continue to grow. .
Chain termination stage
When the active point disappears, chain growth can be terminated. There are two reaction mechanisms for chain termination: double-radical bonding and double-radical disproportionation. The proportion of the two chain termination modes changes with different polymerization conditions.
Properties and uses
Polyvinyl acetate latex, commonly known as "white glue", is a commonly used wood adhesive. It is produced from vinyl acetate monomer, polyvinyl alcohol as emulsifier, ammonium persulfate (or potassium persulfate) as initiator, water as medium, and emulsion polymerization at 70 to 90°C. Its appearance is a milky white viscous liquid, slightly acidic (pH=4.0~5.5), and resistant to dilute acids and dilute alkali. When encountering strong acid and alkali, it will cause hydrolysis to generate polyvinyl alcohol, with a solid content of 40 to 50, a viscosity of 1500 to 4000 millipaseconds (20°C), and a particle size of 1 to 2 microns. Polyvinyl acetate latex is water-based and therefore easy to handle and wash. Non-toxic, no risk of fire and explosion, no irritation to human breathing and skin.
Polyvinyl acetate latex is a self-drying adhesive that can be evaporated by moisture at room temperature (20°C), and the particles connect themselves into macromolecules to form a translucent adhesive film without the need for heating and By adding a curing agent, it can be cured in a shorter time. The initial bonding strength and dry bonding strength are relatively high. The storage life of latex can be as long as 1 to 2 years.
Polyvinyl acetate latex has poor water resistance, heat resistance and solvent resistance. When the air humidity is 65, the water absorption is 1.3 of the glue weight. When the humidity is 95, the water absorption is 3.5 of the glue weight. The softening point is 45 to 90°C. The cured adhesive layer is prone to creep, which reduces the bonding strength. These shortcomings can be improved by using heavy metal salts such as aluminum chloride and zinc chloride as cross-linking agents by mixing with thermosetting resins or by polymerizing vinyl acetate monomer with other monomers such as acrylate monomers. and heat resistance (table below).
The appropriate storage temperature for polyvinyl acetate latex is 10 to 40°C. If the temperature is too low, it will easily freeze and affect its use. If the latex is frozen, it must be completely thawed in an environment of 30 to 40°C before use.
Polyvinyl acetate latex is suitable for gluing porous materials.
Can glue wood to wood, wood to paper, wood to plastic veneer, etc. It can be glued at room temperature or heated. The pressure during gluing is generally 0.5 MPa. Room temperature gluing usually takes 2 to 4 hours in summer and more than 6 hours in winter. Hot pressing gluing is usually not used. If hot pressing is needed, hot pressing can be done at a temperature of 70 to 80°C and a hot pressing time of 1 to 1.5 minutes per millimeter. .
Solution-type polyvinyl acetate adhesive has good bonding strength to non-polar surfaces and can be used to bond leather, cellophane, mica sheets, ceramics and plastic products.