Study on Acrylate Pressure Sensitive Adhesives

Acrylic pressure-sensitive adhesives can be prepared by solvent polymerization, emulsion or polymerization, suspension polymerization and the like, and can be classified into five categories according to their use forms: solvent, emulsion type, hot melt type, water soluble type, and radiation-curable type, among which solvents Types and emulsions have developed more mature.

(1) Composition of acrylic pressure sensitive adhesive

Acrylate pressure-sensitive adhesives mainly consist of solutions or emulsion copolymers obtained by solution, emulsion or suspension polymerization of various acrylic monomers. Some still need to add tackifier resin, cross-linking agent, softener, pigment filler and other additives.

One, monomer

The monomers for preparing acrylate pressure sensitive adhesives can be roughly divided into three categories: soft monomers, hard monomers, and functional monomers.

The soft monomer is the main monomer for the preparation of the pressure-sensitive adhesive, and its function is to produce a polymer with a lower vitrification temperature (Tg) and a tackiness. Ethylene acrylate (EA), n-butyl acrylate (BA), and 2-ethylhexyl acrylate (2-EHA) homopolymers (average relative molecular weight 103-105) with Tg below -200C. It has pressure-sensitive adhesive properties at room temperature. These low glass transition temperatures are generally not high in cohesive strength and therefore cannot generally be used alone as a pressure sensitive adhesive.

The hard monomer, also known as the second monomer, is a (meth)acrylate or other olefinic monomer that produces a higher Tg homopolymer and is copolymerizable with the soft monomer. Commonly used are methyl acrylate (MA), methyl methacrylate (MMA), ethyl methacrylate (EMA), n-butyl methacrylate (BMA), acrylonitrile (AN), vinyl acetate (VAc) Styrene (St) and so on. The hard monomer and the soft monomer can only be copolymerized with a proper mixing ratio to obtain a better pressure-sensitive adhesive with higher cohesive strength and service temperature.

Functional monomers, also known as functional monomers, are vinyl monomers with various functional groups that can be copolymerized with soft monomers. The pressure-sensitive adhesive can produce a certain degree of cross-linking, so that the cohesive strength, heat resistance and aging resistance are greatly improved. The commonly used functional monomers are (meth)acrylic acid, β-hydroxyethyl (meth)acrylate, β-hydroxypropyl (meth)acrylate, glycidyl (meth)acrylate, ethylene glycol methacrylate. (Meth)acrylamide, N-methylolacrylamide, maleic anhydride, itaconic acid, diethylbenzene, and the like.

Second, the glass transition temperature of the copolymer

Acrylate copolymers used as pressure-sensitive adhesives are generally prepared by free-radical copolymerization of the above three types of monomers under the action of a free radical initiator. The commonly used initiators for solution polymerization are benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN); emulsion polymerization uses water-soluble ammonium persulfate (APS) or potassium persulfate (KPS) as the initiator. When the amount of the three types of monomers used in the copolymerization is taken into account, the viscosity and the cohesive force balance are taken into consideration. The glass transition temperature of the copolymer reflects the performance of the pressure-sensitive adhesive to some extent. Therefore, one often uses the glass transition temperature value to predict a copolymerization. Whether the material is suitable for use as a pressure sensitive adhesive can also guide how to improve the mechanical properties of the copolymer. Only when the glass transition temperature of the copolymer is lower than -200C, pressure-sensitive adhesiveness will be produced at room temperature; if a pressure-sensitive adhesive undergoes peeling test under the standard conditions of room temperature, internal destruction of the adhesive layer occurs, then try to improve the glass. The temperature can improve its pressure-sensitive adhesive properties.

When preparing an acrylate copolymer suitable for pressure-sensitive adhesives, first, when using the FOX formula to design the proportions of various comonomers, the glass transition temperature Tg of the copolymers is controlled within a certain range, and Tg can be as follows: The FOX formula is calculated:

1/Tg = W1/Tg 1 +W2/Tg 2+...Wn/Tgn

In the formula, W1, W2...Wn are the mass percentage of each monomer; Tg1, Tg2... Tgn are the glass transition temperature (K) of each monomer homopolymer, respectively.

Third, the relative molecular mass and its distribution on the performance of pressure sensitive adhesive

The relative molecular weight of the copolymers has a great influence on their mechanical properties and adhesive properties. Pressure-sensitive adhesives made of low molecular weight copolymers, although the initial tack may sometimes be good, but the peel strength and holding power are generally Not high, a good pressure-sensitive adhesive has a rapid increase in its holding power as the relative molecular mass increases, and the initial tack and peel strength drop to a stable level, and the peeling occurs stable interface failure. Therefore, when preparing acrylate pressure-sensitive adhesives, the relative molecular mass of the copolymer must be controlled within a certain range. The composition and structure of the copolymer are different, and the optimal range of relative molecular mass is also different.

The relative molecular mass distribution also has an effect on the performance of the pressure sensitive adhesive. In general, the high relative molecular mass part determines the sticking force of the pressure sensitive adhesive, and the low relative molecular mass component influences the initial adhesive force and the peel strength. Under the same circumstances, the copolymer adhesive with wider relative molecular mass distribution can more easily balance the three major properties of the pressure-sensitive adhesive. Actually, in a cross-linked acrylate pressure-sensitive adhesive with a relatively high molecular weight, a small amount of a non-cross-linked acrylate copolymer having a relatively low molecular weight is mixed in, and the distribution of the relative molecular weight of usage right is broadened, and can be maintained relatively. In the case of good adhesion, the initial tack and peel strength are greatly improved.

(2) Crosslinking of acrylate copolymers

To make acrylic ester pressure-sensitive adhesives have good tackiness, the copolymer must have a relatively high weight average molecular weight. However, such adhesives, especially solution pressure-sensitive adhesives, tend to be too viscous and difficult to achieve. Coating. If the copolymer, which has a relatively low molecular weight and thus low viscosity, is suitably cross-linked during or after coating, the contradiction between performance and workability can be solved. At the same time, cross-linking can improve the high-temperature resistance, solvent resistance and aging resistance of pressure-sensitive adhesives, so cross-linking of acrylate copolymers has very important practical value.

Specific cross-linking methods can be summarized as the following five

(1) Cross-linking by the condensation reaction of an active methylol group, and an acrylate copolymer obtained by copolymerization with N-methylol acrylamide, when heated alone or with an acid catalyst, may be due to active N-methylol groups Condensation self-crosslinking, known as self-crosslinking, cross-linking can also be cross-linked, known as the diplomatic union. The hydroxyl group- or carboxyl-containing acrylate copolymer and the cross-linking agent containing an active methylol group, when heated or heated with an acid catalysis, undergo condensation reaction between the methylol group and the hydroxyl group or the carboxyl group due to the methylol group or etherification. Cross-linked.

(2) The acrylate copolymer containing a hydroxyl group, a carboxyl group, and a secondary amine group is cross-linked by an isocyanate reaction and can be cross-linked with a polyisocyanate and its addition. The cross-linked pressure-sensitive adhesive obtained by this method has excellent overall performance.

(3) Cross-linking acrylate copolymers containing amine groups, hydroxy groups, and/or carboxyl groups without epoxy groups can be cross-linked with a cyclic or polycyclic oxime compound, while epoxy-containing acrylate copolymers can be used with polyamines or Compounds such as polybasic anhydride crosslink,

(4) Alkoxy metal compounds such as tetrabutoxytitanium, tetraisopropoxytitanium, and triisopropoxyaluminum are cross-linked by the reaction of metal alkoxides, and can react with acrylates at room temperature or lower temperatures The carboxyl or hydroxyl groups of the copolymers are rapidly cross-linked by esterification or alkoxy exchange reactions.

(5) Ionic cross-linking metal salts, such as zinc, lead, sodium, cobalt and other polyvalent or monovalent metal acetic acid, octanoic acid, naphthenic acid, or lauric acid and other organic acid salts and carboxyl-containing acrylate copolymers Cross-linking.

(3) Effect of tackifier resin and other additives

In general, it is not necessary to add a tackifier resin and other additives to the acrylate pressure-sensitive adhesive. However, various additives may be added in order to improve the adhesive properties of the hard-to-stick material or to reduce costs, coloring, and other purposes. The addition of a tackifying resin or plasticizer improves the initial tack and the 1800 peel strength. Especially at low temperature (00C) peel strength. When adding a tackifying resin to an emulsion type acrylate pressure sensitive adhesive, the tackifying resin must first be made into an emulsion, and the rosin ester tackifying resin emulsion can greatly improve the adhesive properties of the acrylate emulsion pressure sensitive adhesive to the polyolefin plastic. For example, if about 50% of MBG-64 emulsion is added, the peel strength of 1800 will increase sharply; when the amount of addition reaches 70%, the peel strength will reach a maximum value.

Inorganic fillers add solution or emulsion acrylate pressure-sensitive adhesive can reduce costs.

Adding flame retardants such as phosphates and yttrium oxide can make acrylate pressure sensitive adhesive flame retardant.

(4) Solvent Acrylic Pressure-Sensitive Adhesives

Solvent-based acrylate pressure-sensitive adhesives are viscous liquids obtained by radical polymerization of acrylate monomers in an organic solvent, with or without other additives. Solvent-based acrylic pressure-sensitive adhesives have the advantages of lower average molecular weight, good wettability, greater initial tack, faster drying speed, and better water resistance. Although there are problems in environmental protection, resources, energy, and safety, it still occupies a considerable proportion and cannot be completely replaced by other types of pressure-sensitive adhesives. Solvent-based acrylate pressure-sensitive adhesives are further classified into three types: non-cross-linked, cross-linked, and non-water-dispersed.

Non-crosslinked acrylate pressure sensitive adhesive

Several kinds of acrylic esters were selected according to the appropriate ratio, suitable solvents and initiators were selected, and free radical copolymerization was performed in organic solvents using suitable process conditions. The resulting copolymers had glass transition temperatures of -250C to -650C. Relative molecular weight 30 to 1 million, solid content 30 to 50%, viscosity 1000~2000mPa.s, with less good pressure-sensitive adhesive properties and coating performance, this solution is a non-cross-linked acrylic pressure-sensitive adhesive .