Synthesis and Formulation Example of Polyvinyl Acetate Emulsion Adhesive (I)

Disperse in water as a stable emulsion in the form of polymer particles (with a particle size of 0.1-10 μm), known as polymer emulsions, referred to as emulsions. Polyvinyl acetate emulsion adhesive, also known as white plastic, white plastic. It is based on vinyl acetate monomer, water as a dispersion medium, obtained by emulsion polymerization. Polyvinyl acetate emulsion adhesive because it is a water-based adhesive, non-polluting, non-combustion, with high adhesive strength, faster curing speed, easy to use, non-toxic, low cost, resistant to acid, alkali, oil, storage The characteristics such as longer period, strong adhesion to porous materials such as wood, paper, cotton, leather, ceramics, etc., and high initial bonding strength, are widely used in wood processing, furniture manufacturing, building decoration, paper Products processing, book binding, fabric processing, cigarette joints, automotive interior decoration, pencil production, handicraft manufacturing, leather processing, ceramic tile bonding, label fixing, paper industry, latex paint manufacturing and many other fields. The amount of polyvinyl acetate emulsion in China is roughly 53% (mass) of adhesive, 11% (quality) of printing, 12% (quality) of cigarette, 18% (quality) of coating and construction, and 1% (quality) of fabric processing. , other 5% (quality).

Polyvinyl acetate emulsion adhesives also have some shortcomings, such as poor water resistance, especially into boiling water; easily hygroscopic, easy to open the plastic in the wet environment, not durable; poor heat resistance, the adhesive layer after curing has thermoplasticity; low softening point (40-80 °C) As the temperature rises, the strength decreases sharply, and creep phenomena are also prone to occur. They cannot be used in applications where the temperature is high, the resistance to freezing and thawing is poor, and the freezing below -5°C results in demulsification and storage in winter. Special attention should be paid to insulation during transportation.

The polyvinyl acetate emulsion can be used as an adhesive directly or it can be modified by adding other additives such as plasticizers, antioxidants, defoamers, preservatives, antifreeze agents, fillers, etc. to the polyvinyl acetate emulsion. Into the required adhesive.

1 Synthesis of polyvinyl acetate emulsion adhesive

1.1 The main raw materials (1) The monomer monomer is vinyl acetate, also known as vinyl acetate (VAC), and the structural formula is CH3C0OCH=CH2.

Emulsion polymerization is the same as free radical type polyaddition reaction. It requires high purity of the monomer. When the monomer is stored, it is often added with a polymerization inhibitor to prevent self-polymerization. The polymerization inhibitor should be removed before polymerization, or the amount of polymerization inhibitor Add some initiator method to remove the inhibitor.

(2) Emulsifier reactions Two nonionic emulsifiers are generally used, namely nonylphenol polyoxyethylene ether (OP-10) and polyvinyl alcohol (PVA). In addition to the emulsifier, polyvinyl alcohol also functions as a protective rubber and a thickening agent, so its amount is large. A small amount of anionic emulsifier may also be added in an appropriate amount. The type and amount of the emulsifier have a great influence on the stability of the emulsion, the distribution of the particle size, and the properties of the rubber.

(3) Initiator persulfates such as persulfate hinge (APS), potassium persulfate can be used as initiators. Emulsion polymerization is generally used water-soluble initiator, the amount of 0.5% (mass) of the amount of monomer. The addition of persulfate will reduce the pH of the system, resulting in a slower reaction rate, coarser emulsion particles, and even gel demulsification. Therefore, it is necessary to add sodium bicarbonate to adjust the pH during use.

(4) Other additives Plasticizer dibutyl phthalate (DBP)
Preservatives Sodium nitrite, Sodium benzoate Defoamer octanol Disperse and kill methanol Antifreeze ethylene glycol

(5) Deionized water is used for water because tap water contains metal ions and it has a polymerization inhibitory effect on the polymerization reaction. If tap water is used as a raw material, it is necessary to add an ethylenediaminetetraacetic acid (TPA).

1.2 Polyvinyl Acetate Emulsion Synthesis Mechanism The initiation of emulsion polymerization takes place within the micelles. The polymerization is mainly carried out in the initiated micelles and the resulting polymer particles. Monomer droplets act as monomers. The role of storage. Since the number of micelles is very large, almost one free radical may be contained in one micelle, so the chain termination speed is significantly reduced, resulting in waste of the emulsion polymerization speed, and the resulting polymer has a high molecular weight. Vinyl acetate emulsion polymerization is a type of radical polymerization reaction. It follows the general rules of free radical polymerization and requires three major stages of chain initiation, chain growth and chain termination. The mechanism of the polymerization reaction is described below.

(1) Chain Initiation of Polymerization A monomer, water, an emulsifier, an initiator, and the like are added to a reactor and an emulsion is formed by stirring. In the emulsion polymerization of vinyl acetate, persulfate is usually used as an initiator, such as potassium persulfate or ammonium persulfate, and the molecules of the initiator in the reaction system are thermally decomposed to generate free radicals upon heating. Usually the reaction system water is a continuous phase, which dissolves a small number of monomer molecules, surface active molecules present in the single-molecule state, as well as micelles in the state of aggregation, micelles and monomer droplets dissolved in monomer molecules. The initiator radicals are absorbed by the micelles and enter the micelles. When the radicals diffuse into the monomer-solubilized micelles, the sulfate ion-based white radicals combine with the vinyl acetate monomers to form monomer radicals. .

(2) Chain growth of the polymerization reaction The monomer radicals are then combined with monomers to initiate polymerization, forming chain radicals, and the consumed monomers are continuously replenished by monomer droplets passing through the aqueous phase and entering the micelles to be replenished. The initiation of the polymerization of the monomer molecules initiates the continuous growth of the polymer chain to obtain a high molecular weight polymer. The micelles are expanded by the resulting polymer to form monomer-swollen polymer-active particles. It continues to react until the second radical diffuses into the microparticle causing the chain to terminate. In this way, polymer latex particles having a monomolecular emulsifier layer adsorbed on the surface are formed. With the continued decomposition of the initiator, the initiation reaction in the micelles continues to occur, and the number of active microparticles increases rapidly. At this time, the chain termination rate is still small, so the total polymerization rate tends to increase. When the monomer conversion rate reaches 10%-20%, the emulsifier molecules in the reaction system are mostly adsorbed on the surface of the polymer particles in the form of a monolayer, and the concentration of the emulsifier in the aqueous phase is reduced to the critical micelle. At a concentration, no new micelles are formed and no new polymer particles are formed. The monomer continues to be replenished by the droplets of monomer entering the active particles, and the polymer particles are continuously expanding. Polymerization proceeds at a constant rate, and at the same time, the surface tension of the aqueous phase increases significantly. When the monomer conversion reaches 60%-70%, the monomer droplets gradually become smaller and then disappear.

(3) Chain termination of the polymerization reaction At this time, the monomer droplets are no longer present, and the remaining monomers present in the polymer fine particles are adsorbed or swelled by the polymer.

1.3 Polyvinyl acetate emulsion synthesis process

(1) Reaction Temperature The reaction temperature is 75-85°C. The reaction temperature affects the polymerization rate and the average molecular weight of the emulsion. Improve the reaction temperature, speed up the production of free radicals. The monomer activity increases, the chain growth rate constant increases, and the polymerization rate increases. As the reaction temperature increases. The decomposition rate constant of the initiator becomes larger. When the initiator concentration is constant, the rate of radical generation is large, resulting in an increase in the chain termination rate in the latex and a decrease in the average molecular weight of the emulsion.

Increased reaction temperature also increases the number of latex particles. The average diameter decreases; when the reaction temperature increases, collision occurs between the latex particles, the coalescence rate increases, and the hydration layer on the surface of the latex particles becomes thinner, which leads to a decrease in emulsion stability. If the reaction temperature is equal to or higher than the cloud point of the emulsifier, the emulsifier loses its stabilizing effect and causes demulsification.

(2) Reaction time is 8-9 hours. The reaction time is also an important factor affecting the quality of the polyvinyl acetate emulsion. The reaction time is mainly reflected in the length of time of vinyl acetate dropping. The longer the time of vinyl acetate dropping, the smaller the latex particles, the more uniform the particle size, the greater the emulsion strength, the better the stability, the transparency of the film, and the water resistance improve.

(3) Feeding method even with the same emulsion polymerization formula. Due to different operating methods, the resulting emulsions will have differences in particle size distribution, relative molecular mass, and other aspects. There are three types of feeding methods.

1 One-time feeding method All the components were simultaneously added to the reactor for polymerization. Because the vinyl monomer in the polymerization, the thermal effect is larger, and the emulsion polymerization reaction rate is faster, so for industrial-scale devices, this feeding method to the temperature control has brought greater difficulties. Therefore, this method is only used when water and oil are relatively large. Sometimes in order to control the rate of heat release to maintain a certain polymerization temperature, Liu will be triggered to add in batches. The one-time feeding method is used less in actual production.

2 The monomer dropping method is to add the monomers slowly and continuously to the aqueous solution of the emulsifier. The aqueous solution of the initiator is often added dropwise at the same time, and the polymerization temperature is controlled at the dropping speed. The method is easy to operate and the polymerization reaction is easy to control, so it is widely used.

(3) Addition of emulsified droplets: The materials are pre-mixed to form an emulsion, and then gradually added dropwise to the reaction system for polymerization. The polymerization temperature is relatively easy to control, but this method requires pre-emulsification, but generally under the condition of emulsion polymerization formula, the emulsion stability of the monomer is not good, and the layering is easy. Therefore, pre-emulsification equipment must be provided, which increases the investment in equipment. And power consumption, it is less used.