Acrylic emulsion adhesive

Acrylic polymers have unique weather resistance and aging resistance, and have excellent antioxidant and UV aging resistance. They have high bonding strength and good water resistance, so they are widely used. Emulsion adhesives can take into account the advantages of water-soluble and solvent-based adhesives, and have strong controllability in preparation, which is an important development direction of adhesives.

Acrylic emulsion adhesives have the following excellent properties:

1. Using water as the dispersion medium, no organic solvents are used, no toxic or flammable hazards, and are environmentally friendly products;

2. There are many types of acrylic monomers, and the ester, carboxyl, hydroxyl and other functional groups contained have strong polarity. It is easy to emulsion copolymerize with other monomers such as vinyl acetate, styrene, vinyl chloride, etc. to make emulsion adhesives with various properties;

3. Acrylic polymers have excellent color retention, light resistance and weather resistance, are not easy to oxidize, and are not sensitive to ultraviolet degradation;

4. Acrylic polymers have high bonding strength and shear strength.

There are two ways to classify acrylic emulsion adhesives. According to the use of the product, the emulsion can be divided into: emulsion for interior walls, emulsion for exterior walls, elastic emulsion, waterproof emulsion, sealing emulsion, etc. According to the composition of the product, it can be divided into: pure acrylic emulsion, silicone acrylic emulsion, styrene acrylic emulsion, vinegar acrylic emulsion, etc.

However, acrylic emulsion adhesives also have certain disadvantages: poor water resistance, long drying time, and easy to mildew.

2. Introduction to the study of modified acrylic emulsion adhesives

Polyacrylate is a kind of polymer with multiple properties and wide applications. Its emulsion is generally based on methyl acrylate, ethyl acrylate or butyl acrylate as the main monomer, and is copolymerized with methacrylate monomers, styrene, acrylonitrile, etc. to form an emulsion. Improving the structure or polymerization method of the polymer can make the modified acrylic adhesive have better performance.

1. Silicone modification

Silicone resin has excellent high and low temperature resistance and water resistance. The use of silicone to modify polyacrylate emulsion adhesives has become a hot topic in recent years. Organic functional alkoxysilanes are widely used in adhesives, sealants and coatings as adhesion promoters and crosslinkers. Some experts have studied the application of an organic silane Wz-A specifically for water-based systems in water-emulsified polyacrylic acid sealants. This water-based silane can significantly improve the mechanical properties and bonding properties of the sealant without changing the stability of the product. The addition amount of Wz-A is more suitable at 0.8%-1.6%. Experts used hydroxy silicone oil and silane coupling agents A-151 and KH-570 to chemically modify acrylates through emulsion polymerization. The hydroxy silicone oil and acrylates were connected with the carbon-carbon double bond and siloxane structure in the silane coupling agent. The results showed that the acrylate emulsion adhesive modified by KH-570 had significant improvements in various properties. Vinyl-modified silicone emulsions were prepared by ring-opening polymerization of octamethylcyclotetrasiloxane and a silane coupling agent with a vinyl end group. When the reaction temperature is 80℃, the catalyst is dodecylbenzenesulfonic acid, and the silane coupling agent is A-151, the monomer conversion rate of vinyl-modified silicone emulsion in the obtained core-shell emulsion is high and the emulsion stability is good. This emulsion is used as a seed emulsion for the modification of polyacrylate emulsion to obtain a coating printing adhesive with good softness and color fastness.

2. Epoxy modification

Acrylate is grafted and copolymerized with epoxy resin, and the epoxy groups are cross-linked under the action of a curing agent, so that the system can eventually form an interpenetrating network structure with an epoxy resin cross-linking network as the skeleton and acrylic resin running through it. This modified polyacrylate emulsion has the characteristics of high strength and good adhesion of epoxy resin and excellent weather resistance and flexibility of acrylate. Chemical experts use core-shell polymerization technology. When the mass fraction of epoxy resin is 30% and the glass transition temperature of the copolymer is 243.15K, a curable emulsion pressure-sensitive adhesive can be obtained. After curing, the bonding strength to aluminum alloy can reach 10MPa. Adjusting the proportion of epoxy resin and the glass transition temperature of the polymer can change the performance of the pressure-sensitive adhesive. Epoxy resin is added to the polymerization system, and epoxy resin modified vinegar-acrylic emulsion adhesive is prepared by emulsion polymerization. The bonding strength, film-forming properties, coating hardness and stain resistance of the adhesive have been improved to varying degrees. The product can produce good results when used as an adhesive.

3. Blending modification

Crosslinked and copolymerized modified acrylate adhesives sometimes have disadvantages such as poor weather resistance, low operating temperature, and poor stability. Experts use high-heat-resistant benzoxazine and acrylate copolymer emulsion to blend to obtain modified polyacrylate adhesives with excellent performance. Under the action of heating and catalyst, benzoxazine undergoes ring-opening polymerization to generate a nitrogen-containing phenolic resin-like network structure, so that the cured adhesive forms an interpenetrating network structure. The modified adhesive has the characteristics of high temperature resistance, dimensional stability, flame retardancy, and environmental friendliness. The use of benzoxazine improves the heat resistance of the material substrate. The substrate made of the modified adhesive can be placed in a 350°C tin bath without stratification or bubbling within 10 seconds.

3. Application of acrylic emulsion adhesive

1. Wood product adhesive

Using vinyl acetate, butyl acrylate, N-hydroxymethyl acrylamide and acrylic acid as monomers, a quaternary emulsion adhesive for the wood industry was prepared by semi-continuous emulsion polymerization, which improved the bonding strength, water resistance and minimum film-forming temperature of the emulsion adhesive. Compared with the unmodified homopolymer emulsion, the copolymer emulsion modified with functional monomers has greatly improved the bonding strength and water resistance of the emulsion adhesive.

2. Packaging industry

Acrylic emulsion adhesives have the advantages of easy use, low cost, strong adhesion, and good weather resistance, and are widely used in the packaging field. The acrylic emulsion system is copolymerized and modified with hydrogenated rosin to obtain a water-based adhesive for plastics. When the amount of hydrogenated rosin is 5%, the copolymerized modified acrylic emulsion has good adhesion to PE, PET, and PP, and the adhesion is increased from 110min to 294min, and the peel strength is increased from 2.8N/cm to about 7.2N/cm.

3. Fabric printing

Pigment printing is to attach non-affinity and non-reactive pigments to the fiber surface with the help of adhesives to form the desired pattern. The feel, color brightness, and fastness of the printed fabrics depend to a large extent on the quality of the adhesive. At present, the adhesives used for paint printing are mainly acrylates, which have strong adhesion to fabrics and good film-forming properties. The formed film is transparent, flexible and elastic. Therefore, the preparation of high-performance polyacrylate adhesives is an important goal to improve the quality of paint printing.

4. Pressure-sensitive adhesive

Polyacrylate emulsion type pressure-sensitive adhesive is the most widely used type of pressure-sensitive adhesive. Since the early 1980s, the product categories include packaging tape, electrical insulation tape, surface protection tape, double-sided tape, medical tape trademark paper and pressure-sensitive label paper, etc., and have been widely used in packaging, printing medical, home appliance industry and manufacturing industry. By introducing monomers containing polar functional groups, the structure of colloidal particles can be controlled, so that the surface properties and cohesion of pressure-sensitive adhesives can be improved at the same time, which is conducive to achieving high shear strength, high peel strength and high viscosity.

5. Construction adhesive

Polyacrylate emulsion adhesive is one of the commonly used adhesives in the construction industry, with the advantages of easy use, non-toxic and harmless, and low price. By selecting polyacrylic acid emulsion and changing the poly-powder ratio, appropriately increasing the poly-powder ratio and adding effective plasticizers, the elongation at break of the sealant and the low-temperature flexibility of the sealant products can be improved. The copolymer emulsion of acrylic acid/butyl acrylate/vinyl acetate/hydroxymethyl acrylamide prepared by Chen Yuanwu et al. can be used as the bonding base material of building elastic putty, waterproof coating and waterproof coating. The ethylene-vinyl acetate copolymer acrylate adhesive can be used in cement-based repair mortar.

IV. Formula composition

1. Monomer

The monomers for synthesizing acrylic ester emulsion copolymer adhesives are generally acrylic acid and its C1-C8 alkyl acrylates. As the alkyl chain length increases, the homopolymer gradually softens, the glass transition temperature decreases, and the texture becomes soft. After octyl acrylate, due to the increase of alkyl carbon atoms, the side chain tends to crystallize and the polymer becomes brittle.

In acrylic ester emulsion adhesives, the composition of the copolymer monomer is divided into three parts. The first part is a soft monomer with a low glass transition temperature, which gives the adhesive bonding properties, such as methyl acrylate, ethyl acrylate, butyl acrylate, isooctyl acrylate, etc.; the second part is a hard monomer with a high glass transition temperature, which gives the adhesive cohesion, such as methyl methacrylate, styrene, acrylonitrile, vinyl acetate, vinylidene chloride, etc.; the third part is a functional group monomer, which introduces monomers with functional groups to give the adhesive reaction properties, such as hydrophilicity, heat resistance, water resistance, and crosslinking.

2. Initiator

The initiators of this system are mostly water-soluble persulfates, and ammonium persulfate, potassium persulfate, and sodium persulfate are commonly used. If the amount of initiator is too little, it is not easy to initiate polymerization; if the amount of initiator is too much, the polymerization is unbalanced. The more suitable initiator amount is 0.2% to 0.8% of the total monomer amount. Among them, the use of 0.2% to 0.4% of the initiator can make the prepared polyacrylate emulsion appear blue, the particle size of the emulsion particles is small, and the stability of the emulsion is good.

3. Emulsifiers

Emulsifiers include nonionic, cationic and anionic systems. At present, my country mostly uses a composite system of anionic emulsifiers and nonionic emulsifiers. Commonly used anionic emulsifiers are sodium alkyl sulfate, sodium alkylbenzene sulfonate, sodium dialkyl-2-sulfosuccinate, sodium alkyl allyloxy polyoxyethylene phosphate, sodium salt of polyoxyethylene alkylphenol ether maleic anhydride adduct; nonionic emulsifiers include polyoxyethylene alkyl ether, polyoxyethylene phenol ether, etc.

The type and concentration of the emulsifier will directly affect the initiation rate and chain growth rate. When choosing a suitable emulsifier, the lipophilic and hydrophilic balance value (HLB value) should be considered first, and then the affinity between the monomer and the emulsifier should be considered. Generally, the more similar the molecular structure, the better the affinity.

When the HLB value of the mixed emulsion is low, the chain growth rate during emulsion polymerization is fast, the particle size is large, the emulsion polymerization conversion rate is low, and it is easy to coagulate or even break the emulsion; when the HLB value of the mixed floatation agent is high, the chain growth rate during emulsion polymerization is slow, the particle size is small, and the emulsion polymerization conversion rate is also low.

4. Crosslinking agent

When emulsion-type acrylate is polymerized, adding a crosslinking agent can improve its adhesion performance. There are external crosslinking, self-crosslinking (ionic crosslinking) and multi-crosslinking processes in the polymerization. Among them, self-crosslinking refers to the direct crosslinking reaction between macromolecular chains; external crosslinking is often carried out by adding urea-formaldehyde resin or melamine resin to carboxyl latex. According to the crosslinking temperature, it can be divided into high-temperature crosslinking and room-temperature crosslinking.

The main room temperature crosslinking agents are:

(1) Acid diacyl trap system proposed by BASF;

(2) Zinc o-methoxybenzoate system with reversible process proposed by Rohm-Hass;

(3) Metal ion crosslinking, such as Zn(Ac)2, Al(Ac)3, etc.

In addition, monomers with certain polar groups can be selected as reactive modifiers to make the copolymer slightly crosslinked, so that the chemical bonds forming the molecular network replace the simple intermolecular forces, and to a certain extent improve the rigidity of the copolymer.

In the copolymerization reaction of acrylic ester emulsion, two or more active groups are introduced to achieve the purpose of medium and low temperature self-crosslinking: the synthesized self-crosslinking emulsion glue can be used for bonding polyimide to copper foil. Using epoxy-containing and amide-containing crosslinking agents, the curing temperature is 180-190℃, and the substrate prepared has high peel strength and good weather resistance, which can be comparable to external crosslinking acrylic ester emulsion glue.

5. Other additives

Commonly used tackifying resins include rosin, rosin-modified phenolic resin, terpene, petroleum resin, etc.

Thickening of acrylic emulsion can be achieved by adding ammonia or sodium hydroxide solution. A homemade 30% (mass fraction) polyacrylic acid emulsion thickener is used, which is a high molecular weight polyacrylic acid emulsion with low viscosity and high concentration of carboxyl groups, which is easy to mix evenly with acrylic copolymer emulsion. The dosage of the thickener is 1% to 5% (mass fraction). After adding the thickener, the pH value needs to be adjusted to 9 with ammonia. After standing for a period of time, the viscosity of the water phase will suddenly increase and the emulsion will become viscous.

Sometimes, in order to reduce costs, improve bonding strength and water resistance, and reduce the shrinkage rate of emulsion adhesives, fillers, reinforcing agents or other polymer emulsion systems are added.

5. Polymerization process of acrylic emulsion adhesive

There are many processes to choose from for producing polymer emulsion and emulsion polymer, such as intermittent process, semi-continuous process, process with additional emulsifier, continuous emulsification process, seed polymerization process. The polymerization process can adopt ordinary polymerization method and seed polymerization method. Different polymerization processes produce different distributions of monomers in the emulsion particles. Hard monomers are arranged in the shell layer of the emulsion particles, which can make the strength of polyacrylate emulsion adhesive greater; conversely, soft monomers are arranged in the shell layer, which can make it have strong adhesion.

Vinyl acetate emulsion is used as the modified substance, and acrylic acid, butyl acrylate, and methyl methacrylate are used as monomers to synthesize modified polyacrylate hydrosol. Compared with the ordinary polymerization method, the seed polymerization method can improve the stability of the emulsion and the permeability of the hydrosol. The light transmittance of the hydrosol increases and the bonding strength improves.

The polymerization process has a great influence on the morphology of latex particles, the most important of which is the feeding method. Through the emulsion copolymerization of butyl acrylate-vinyl acetate-acrylic acid, the impact of the specific operating process on the viscosity and stability of the product in semi-continuous emulsion polymerization was studied. It is believed that during the semi-continuous operation process:

1. Increasing the ratio of emulsifier in seeds and drops can initially increase the viscosity of the emulsion rapidly and improve its stability. However, when the ratio increases to a certain level, the increase in emulsion viscosity is no longer obvious;

2. As the amount of seeds increases, the viscosity of the emulsion gradually increases. When the amount of seeds accounts for 25% to 30% of the monomer, the viscosity is the largest. Subsequently, as the amount of seeds increases, the viscosity decreases and the reaction approaches intermittent polymerization;

3. The addition of water-soluble functional group monomer acrylic acid (AA) can stabilize the system, but the adding method has a greater impact on performance. In the later stage of the reaction, acrylic acid is added dropwise to adjust the pH value to alkaline. The carboxyl groups in the polymer dissociate, producing The charge forms a repulsive force, which significantly improves the mechanical stability of the emulsion.