What Is Pressure Sensitive Adhesive?
The full name of pressure-sensitive adhesive is pressure-sensitive adhesive, also known as pressure-sensitive adhesive, referred to as pressure-sensitive adhesive. Pressure-sensitive adhesive products include pressure-sensitive adhesive tape, pressure-sensitive adhesive label paper, and pressure-sensitive film. Their full name is pressure-sensitive adhesive tape, pressure-sensitive adhesive label paper, pressure-sensitive adhesive sheet, commonly known as tape, self-adhesive label paper, and pressure-sensitive film. This component has been adjusted to achieve better product performance.
There are many types of pressure-sensitive adhesives, which can be classified from different angles.
(1) Classification of the main components of pressure-sensitive adhesives
Elastomer-type pressure-sensitive adhesive
The elastomer used in this type of pressure-sensitive adhesive was the earliest natural rubber, and gradually expanded to various synthetic rubber and thermoplastic elastomers. According to the elastomer used, this type of pressure-sensitive adhesive can be further divided into natural rubber pressure-sensitive adhesive, synthetic rubber pressure-sensitive adhesive, and thermoplastic elastomer pressure-sensitive adhesive.
a. Natural rubber pressure sensitive adhesive
This is the earliest type of rubber-type pressure-sensitive adhesive developed and still has a large output. They are based on natural rubber elastomers and are complex mixtures of additives such as tackifier resins, softeners, antioxidants, pigments, fillers and cross-linking (vulcanizing) additives. Natural rubber has high cohesive strength and elasticity It can also be miscible with many tackifying resins to obtain a high degree of viscosity and good wettability to the adhered material, so natural rubber is an ideal type of pressure-sensitive adhesive main material. Its main disadvantage is the existence of unsaturated double bonds in the molecule, which has poor aging resistance to light and oxygen. However, after cross-linking and the use of antioxidants, it can improve its weather resistance and heat resistance. Natural rubber pressure-sensitive adhesives can be used to make almost all types of pressure-sensitive adhesive products.
b. Synthetic and recycled rubber pressure sensitive adhesives
Mainly made of styrene-butadiene rubber, polyisoprene rubber, polyisobutylene and butyl rubber, and synthetic rubbers such as neoprene and nitrile rubber, with pressure-sensitive resins, softeners, antioxidants and other additives. Adhesives have their own characteristics. But none of them is as important as natural rubber pressure-sensitive adhesives. Recycled rubber, especially pressure-sensitive adhesives made from recycled natural rubber, also have good performance and are relatively inexpensive, so they are also valued.
c. Thermoplastic elastomer pressure sensitive adhesive
Thermoplastic elastomers represented by styrene-butadiene-styrene block copolymers (SBS) and styrene-isoprene-styrene block copolymers (SIS) are the mainstays in the manufacture of hot-melt pressure-sensitive adhesives. raw material. Because hot-melt pressure-sensitive adhesives do not use solvents, they do not cause environmental pollution and high production efficiency. In today's society where the call for energy saving and elimination of environmental pollution is increasing, the importance of such pressure-sensitive adhesives is increasing.
Resin-type pressure-sensitive adhesives The resins used in this type of pressure-sensitive adhesives are polyacrylate, polyurethane, polyvinyl chloride, polyvinyl ether, etc. Among them, polyacrylate is currently the most used, and its output has exceeded natural rubber pressure-sensitive adhesive. [2]
There are four main types of pressure-sensitive adhesives used in industry: solvent-based pressure-sensitive adhesives, emulsion-type pressure-sensitive adhesives, hot-melt pressure-sensitive adhesives, and radiation-curable pressure-sensitive adhesives. According to their polymers, pressure-sensitive adhesives are divided into rubber-based pressure-sensitive adhesives, polyacrylate-based pressure-sensitive adhesives, polyvinyl ether resins, polyurethane resins, polyisobutylene and other emulsion-type pressure-sensitive adhesives. A major feature of China's pressure-sensitive adhesive industry, emulsion pressure-sensitive adhesives, especially acrylate emulsion pressure-sensitive adhesives, have special importance in China. [3]
In recent years, with the expansion of the demand for packaging, decoration, office supplies and various labels, the demand for pressure-sensitive adhesives has also increased. Acrylate pressure-sensitive adhesive emulsions have become the development direction of pressure-sensitive adhesives because they are non-toxic and pollution-free, and have excellent aging resistance, weather resistance, pressure sensitivity, and adhesiveness, and have been widely used. In view of the characteristics of pressure-sensitive adhesives, such as easy to peel, easy to peel, not torn, and not to dry up for a long time, the main monomers and second monomers of acrylic esters were selected, and a variety of types were introduced. Functional group monomers, through repeated experiments and reasonable adjustment of the process formula, screen and synthesize a multi-component copolyacrylate pressure-sensitive adhesive emulsion with good comprehensive properties, which can be applied to double-sided tapes and trademarks that require transfer coating processes Production of paper, etc. In addition, acrylic pressure-sensitive tapes, double-sided tapes, protective tapes, masking tapes, label tapes, and medical tapes have been greatly improved not only in yield, but also in bonding and coating performance, and are suitable for various uses. Tape products are constantly emerging. Nevertheless, the acrylic pressure-sensitive adhesive and tape industry are still general-purpose, with a single variety, and cannot meet the needs of the market in terms of adhesive properties, water resistance, heat resistance and humidity resistance. Special tapes still need to be imported. Such as high-strength double-sided tape, high temperature resistant masking tape, flame retardant tape, magic tape, solar film and marking tape. Through the use of various modification methods, the development of polyacrylate emulsion pressure-sensitive adhesives is changing rapidly. With the synthesis of various new acrylate monomers and the introduction of components with special functions into the polyacrylate emulsion pressure-sensitive adhesive system at the same time, products with different properties will be obtained, which will surely broaden its application field and make it more functional and diverse. Development direction.
1 One of the main factors affecting the mechanical properties of emulsion pressure-sensitive adhesives is the glass transition temperature Tg of the copolymer in the adhesive.
The glass transition temperature of the pressure-sensitive adhesive should generally be kept in the range of -20 ° C to -60 ° C. Of course, the pressure-sensitive adhesive formula system with different requirements has different optimal Tg values. The glass transition temperature can be adjusted by selecting a soft monomer with a very low glass transition temperature and a hard monomer with a higher glass transition temperature to copolymerize in a certain proportion, so that it can have a good initial viscosity and maintain a certain cohesion. Hold sticky [4].
Monomers and glass transition temperatures of commonly used acrylic pressure-sensitive adhesives | | |
Monomer category | Monomer name | Glass transition temperature / |
Viscous (soft) monomer | Ethyl acrylate | -twenty two |
Butyl acrylate | -55 | |
Isooctyl acrylate | -70 | |
Cohesive (hard) monomer | Vinyl acetate | twenty two |
Acrylonitrile | 97 | |
Acrylamide | 165 | |
Styrene | 80 | |
Methyl methacrylate | 105 | |
Methyl acrylate | 8 | |
Modified (functional) monomer | Methacrylate | 228 |
acrylic acid | 106 | |
Hydroxyethyl methacrylate | 86 | |
Hydroxypropyl methacrylate | 76 | |
Diaminoethyl methacrylate | 13 | |
2Acrylic copolymer emulsion monomers are usually divided into 3 categories:
(1) Soft monomers that function as adhesion, mainly acrylates with 4-8 carbon atoms, such as butyl acrylate, isooctyl acrylate, etc .;
(2) Cohesive hard monomers, such monomers and soft monomers are copolymerized to maintain a balance of adhesion and cohesion, such as methyl methacrylate, vinyl acetate, styrene, vinyl chloride, etc.
(3) Modified functional monomers can impart reactive properties to the adhesive through its own reactive functional groups to improve the adhesive's adhesive properties and water resistance, weather resistance, solvent resistance and other properties, such as acrylic acid, methyl Acrylic acid, methacrylamide, glycidyl acrylate, etc. [5].
3 When preparing an ester emulsion pressure-sensitive adhesive, the appropriate soft and hard monomers and the amount must be selected so that the ratio of the soft and hard monomers is appropriate to achieve the coordination and balance of the pressure-sensitive adhesive's adhesive properties and cohesive strength However, for pressure-sensitive adhesives, the choice of functional monomer is also very important. It can not only increase the cohesion of the pressure-sensitive adhesive, improve the adhesion to the substrate, but also provide active groups for further crosslinking. In addition, the amount of emulsifier has a great effect on the stability of the emulsion. When the amount of emulsifier is small, the stability of the emulsion in the polymerization is poor, and it is prone to demulsification. With the increase in the amount of emulsification, the emulsion gradually stabilizes. However, if the amount of emulsifier is too high, the water resistance of the pressure-sensitive adhesive will be reduced, and there will be too much foam during sizing, which will affect the construction performance. In actual application, an anti-freezing agent, an antifoaming agent, an antifungal agent, a colorant, etc. may be added to a complete emulsion pressure-sensitive adhesive formula.
According to different methods and fields of use, emulsion pressure-sensitive adhesives have different performance testing requirements. But the basic properties can be roughly divided into two categories: emulsion properties and pressure-sensitive adhesive mechanical properties. The emulsion performance refers to some basic properties of the emulsion itself, such as: solid content, pH value, dilution stability, mechanical stability, viscosity, pH stability, etc. The mechanical properties are evaluated from the use of adhesives, including: initial tack, holding tack, 180 ° peel strength, etc. A good pressure-sensitive adhesive should well balance these three major mechanical properties. Also includes construction performance, coloring performance, and more.
Test preparation
Due to the pressure-sensitive adhesive synthesized in this test, pressure-sensitive adhesives with different properties can be synthesized under different raw material ratios. It should be soft monomers, hard monomers, and modified functional monomers. Their composition is interdependent and restricted. To balance the pressure-sensitive adhesive's adhesion, cohesion, adhesive properties, and water resistance, weather resistance, and solvent resistance, the ratio of the various raw materials must be determined. After consulting related literatures, the ratio of methyl methacrylate to butyl acrylate should be 1:70; the ratio of hydroxyethyl acrylate to butyl acrylate should be 1:60 [6]; 0.5% of the volume [7]; the amount of emulsifier reaches a CMC value of 1.5% of the monomer [7]; 2.2% of the amount of the protective glue positioning monomer [7]. It was determined that the oil-water ratio was 54:46. The performance of the product thus obtained was relatively good.
Test principle
This experiment uses emulsion polymerization to polymerize. This method can obtain glue with fine particles and good quality. Thermal decomposition initiators are a class of materials that can decompose active free radicals when heated. One initiator molecule usually decomposes two free radicals through homogeneous splitting. The process of thermal decomposition in solution is as follows:
An oxidation-reduction system is composed of two or more components. These components generate initiating free radicals through an oxidation-reduction reaction. For example, a persulfate-bisulfite-initiated system can generate free radicals through the following reactions:
The following reactions can also occur in multivariate polymerization:
nCH2CHCOO (CH2) 3CH3 [CH2CHCOO (CH2) 3CH3] n
Composition of pressure-sensitive adhesive
Adding polymers, tackifiers, vulcanizing agents, etc. to the prepared glue solution can obtain pressure-sensitive adhesive products. The polymer used as the pressure-sensitive adhesive can be natural rubber, polyisobutyl rubber, butyl rubber, or various resins, acrylic resin, silicone resin, fluororesin, and the like. Tackifying resins for pressure-sensitive adhesives include rosin esters, phenolic resins, and petroleum resins. The following table: The composition of the pressure-sensitive adhesive [9]:
Component | polymer | Tackifier | Plasticizer | filler | Viscosity modifier | Antioxidant | Vulcanizing agent | Solvent |
Dosage% | 30-50 | 20-40 | 0-10 | 0-10 | 0-10 | 0-2 | 0-2 | Right amount |
effect | Give the cohesive layer sufficient cohesive strength | Increase adhesive force | Increase adhesive fast adhesion | Increase the cohesive strength of the adhesive layer and reduce costs | Adjust adhesive layer viscosity | Increase service life | Improve the cohesive strength and heat resistance of the adhesive layer | Easy to apply |
Common raw materials | Various rubber, random polypropylene, cis-vinyl copolymer, polyvinyl ether, fluororesin, etc. | Rosin, petroleum resin | Phthalates, quinates | ZnO, TiO2, MnO2, Clay | Castor oil, soybean oil, liquid paraffin, motor oil, etc. | Antioxidant A, Antioxidant D | Sulfur, peroxide | Gasoline, toluene, ethyl acetate, acetone |
Experimental reagent
(Soft monomer) 57 g of butyl acrylate (Hard monomer) 0.63 g of methyl methacrylate
(Hard monomer) 0.74g of hydroxyethyl methacrylate
(Functional monomer) 1.2 g of methacrylic acid (buffer) 0.25 g of sodium bicarbonate
(Initiator) 0.29g of ammonium persulfate (protective gel) 1.3g of polyvinyl alcohol
(Emulsifier) K12 (sodium dodecylsulfonate) 0.5g OP-10 (octylphenol polyoxyethylene (10) ether) 0.4g
3% ammonia water 0.7g distilled water 45g Note: Except ammonia water is AR grade, the rest is CP grade [5]
Under the premise that the soft and hard monomers and the process conditions remain unchanged during the experiment, as the content of acrylic acid and methacrylic acid increases, the peel strength decreases, the stickiness increases, and the viscosity also increases. Better than the thickening effect of methacrylic acid. This is because acrylic acid is copolymerized on the particle surface, and most of it is adsorbed on the particle surface, while methacrylic acid is more lipophilic and easily penetrates into the polymer main chain. The high molecular weight part of the polymer determines the adhesive strength of the pressure-sensitive adhesive, while the low molecular weight part contributes more to the initial adhesion and peel strength. The introduction of polar groups (OH, COOH) into the molecule causes hydrogen bonding between the molecules, and a slight network cross-linked structure, which makes the molecular weight distribution of the copolymer relatively wide, and the copolymer with a broad molecular weight distribution Adhesives are easier to achieve a better uniformity among the three major properties of pressure-sensitive adhesives, and the above experiments also prove this.
