Contact adhesive is a special kind of pressure-sensitive adhesive, and its biggest feature is that it produces bonding effect after "contact", that is, the same adhesive is coated on the surfaces of two adherends, and bonding occurs through the mutual contact of the two coated surfaces. After the contact adhesive is coated, the adhered surface needs to be dried to form a transparent and non-stick polymer film, which is the main difference between contact adhesive and ordinary pressure sensitive adhesive.
1. waterborne polyurethane transfer contact adhesive
Waterborne polyurethane is mainly used to prepare transfer contact adhesive. Transfer glue is slightly different from ordinary glue. The adhesive coated on the two bonding surfaces can be the same polymer or the same polymer, but the bonding strength with the substrate is different. When two bonding surfaces contact, the two adhesives will stick together; Once the two adhesive parts are peeled off, one adhesive coating is usually separated from the substrate and transferred to the surface of the other adhesive coating, so this adhesive is usually disposable.
Krampe et al. first coated 15% aqueous polyamide dispersion on paper, and immediately dried it at 120℃ to form an isolation layer on the substrate, and then coated 35% aqueous polyurethane dispersion on the surface of the isolation layer to form a transferable polyurethane coating. 5min In addition, waterborne polyurethane coating (solid content 35%) was coated on the corona treated HDPE film and dried at 80℃ for 5min. Polyurethane of the above two coating materials can be bonded at room temperature.
Waterborne polyurethane transfer contact adhesive can be used to make all kinds of sealing tapes, and the sealing can be used once. By selecting different release agents and coating processes, transfer coatings and adhesive layers can also be coated on other polymer film substrates to form transfer contact adhesive products.
2. Natural latex contact adhesive
Natural latex itself can be used as a contact adhesive. For example, KL series "cold sealant" produced by NationalStarch is actually contact glue. This kind of adhesive is coated on the base material or basement membrane, and can be rolled up and stacked without adhesion after drying, but under certain pressure, the adhesive coating will have a firm adhesion effect.
The biggest feature of natural latex is its fast adhesion, which is suitable for making "quick-breaking" glue. This is mainly because under the action of pressure and rapid shear, the protective colloid on the surface of latex particles is easily destroyed, exposing the rubber molecular chain of natural latex and forming a dense and good polyisoprene film, thus showing good bonding performance.
When using natural latex as contact glue, it is generally necessary to modify it, which is mainly divided into chemical modification and * * * mixed modification. Thailand is the first country to chemically modify and industrialize natural latex. At present, the latex added to its products accounts for most of the market share of chemically modified products. Great progress has also been made in the research of chemical modification of natural latex for contact rubber in China, but there are no industrialized products so far.
Mixing with other water-based products is the most commonly used way of natural latex as contact glue, such as mixing with acrylic emulsion polymer. At present, there are many commercial emulsion products on the market that can be used to modify natural latex, such as Hycar series emulsion of BFGoodrich, which is made of butyl acrylate, styrene, acrylamide, butadiene, itaconic acid and so on. There are also contact adhesive products obtained by polymerizing butadiene and styrene and then modifying natural latex.
3. Contact adhesive between neoprene latex and neoprene latex
In the field of adhesives, the functions and properties of neoprene latex are similar to those of natural latex. In addition to using natural latex in contact adhesives, neoprene latex is also widely used in this field. 1975, Brath used neoprene latex as base material, p-tert-butyl phenolic resin prepared by alkali catalyst as tackifier resin, and then added a little zinc oxide to prepare contact adhesive with high peeling strength. In addition, natural tackifying resin can also be used to modify chloroprene rubber emulsion and cross-link with metal oxides.
The water-dispersible contact adhesive was obtained from the modified chloroprene latex with carboxyl group and a resin emulsion with emulsifying ability. The contact viscosity was improved after low pressure, low temperature, high humidity and drying, and the cohesion and heat resistance were improved.
The contact adhesive with good contact viscosity, high adhesive strength and good creep resistance at high temperature can be obtained by mixing chlorine partial emulsions with different glass transition temperatures (Tg) prepared by polymerization of vinylidene chloride and acrylate. Padget used vinylidene chloride and acrylate as monomers to prepare two kinds of emulsions with low Tg at -50 ~ 0℃ and high Tg at 0 ~ 30℃. Using the contact viscosity of low Tg polymer and the creep resistance provided by high Tg, the contact viscosity and retention viscosity of adhesive can be improved simultaneously by mixing different Tg polymers.
4. Vinyl ester polymer contact adhesive
The glass transition temperature of polyvinyl acetate is 27℃, and it has good adhesion to various substrates. Generally, it can be polymerized with internal plasticizing monomer (such as ethylene), and the adhesion can be greatly improved. Ethylene-vinyl acetate emulsion (EVA) with proper ethylene content can also be used as contact adhesive. Compared with polyacrylate and rubber contact adhesive, EVA contact adhesive often shows the shortcomings of poor contact viscosity (initial viscosity) and adhesive strength, which can be improved by increasing the amount of plasticizer.
Tam et al. used EVA emulsion with ethylene content of 23% ~ 27% and solid content of 40% ~ 70%, in which 3% of other monomers were added besides ethylene and vinyl acetate, and finally a contact adhesive with good contact viscosity and high bonding strength was prepared, especially a nonpolar surface.
EVA emulsion with N- hydroxymethylacrylamide (NMA) participating in * * * poly is a special polymer emulsion (EVA/NMA emulsion). The Tg of the emulsion can be controlled between -30 ~ 30℃, and the Tg of polymer emulsion commonly used as contact glue is-16 ~ 5℃. After adding NMA, the adhesive strength of the system was significantly improved, and the wetting effect of the emulsion on the substrate was also improved.
By using vinyl ester and acrylate poly, selecting other suitable monomers and adjusting the proportion of monomers, the obtained emulsion adhesive can be directly applied to the contact bonding of materials such as floors and decorative laminates without adding other additives. In addition, vinyl acetate and neoprene latex can be directly mixed into tackifying resin to obtain water-dispersible contact adhesive with good initial adhesion and excellent heat and water resistance.
5. Acrylate polymer contact adhesive
Like ordinary pressure-sensitive adhesives, contact adhesives can also be prepared from acrylate polymers. There are many kinds of acrylic monomers, the Tg of different acrylic polymers is quite different, and the polymerization characteristics of acrylic monomers and various vinyl unsaturated monomers are also different, which makes the physical properties of this series of products very designable. The difference between ordinary pressure-sensitive adhesive and contact adhesive is mainly manifested in the Tg design of polymer, and the Tg of contact adhesive needs to be higher to meet the requirement that the film is not sticky at room temperature. Therefore, by selecting the best ratio of soft and hard monomers and controlling the Tg of the polymer, the water-emulsion contact adhesive with excellent performance can be prepared by using acrylic monomers.
Cold sealants such as envelope sealants and food bag sealants are also typical contact adhesives. In the past, natural latex was often used as the main adhesive, but at present, the products made of styrene-acrylic emulsion have better performance. By comparing this type of contact adhesive with natural latex sealant, Duct found that the sealing strength, adhesion, mechanical strength, stability, oxidation stability, service life and other performance parameters of the synthesized sealant were better.
Sanderson et al. prepared acrylate polymer emulsion contact adhesive with wide molecular weight distribution through a certain polymerization process. They used a variety of acrylate monomers to prepare emulsion polymers with high molecular weight (5×102 ~/kloc-0 /×105), and then added chain transfer agents at the later stage of polymerization to prepare low molecular weight (/kloc-0 /×105 ~ 2×/kloc-0). The contact glue consists of 5% ~ 70% low molecular weight polymer and 30% ~ 95% high molecular weight polymer. Polymerization monomers mainly include soft monomers such as butyl acrylate and 2- ethylhexyl acrylate, unsaturated carboxyl monomers such as methyl methacrylate, styrene, methacrylic acid and itaconic acid, crosslinking monomers such as butanediol diacrylate and N- hydroxymethylacrylamide and functional monomers.
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