The natural rubber, synthetic rubber, and acrylic polymer used as the main components in pressure-sensitive adhesives are all thermoplastic polymers. When improving the performance and operating temperature of pressure-sensitive adhesives, the creep resistance of these polymers will become a problem.
To improve these shortcomings, the following methods can be used to address them:
1. Using high molecular weight raw materials
2. Add fillers to prevent slippage of rubber molecular segments.
3. Like thermoplastic elastomers such as SIS and SBS, styrene copolymerization is used to improve the creep performance of rubber.
4. Various cross-linking reactions that utilize rubber sulfurization or modify thermoplastic resins.
Among the above methods, crosslinking is the most effective way to improve the creep resistance of pressure-sensitive adhesives. Crosslinking can effectively improve the creep resistance of pressure-sensitive adhesives, as well as enhance their solvent resistance, moisture resistance, and chemical resistance. Many cross-linking methods have been applied in the pressure-sensitive adhesive industry. The types of elastomers are different, and the crosslinking agents and crosslinking methods are also different. Most crosslinking methods for rubber elastomers involve adding a vulcanizing agent to the elastomer and subjecting it to heating or other treatment to crosslink the elastomer molecules, forming a three-dimensional network structure. The crosslinking of acrylic ester polymers is achieved through the reaction between the functional groups and crosslinking agents contained in the copolymer. The degree of crosslinking is determined by the required properties of the pressure-sensitive adhesive, especially the initial viscosity.
1、 Sulfur and other sulfur crosslinking agents
Sulfur and other sulfur crosslinking agents are collectively referred to as sulfur crosslinking agents. Sulfur crosslinking can be used in pressure-sensitive adhesives based on natural rubber, styrene butadiene rubber, and other diene rubbers. This method is the preferred crosslinking method before using phenolic resin for crosslinking. The main disadvantage of sulfur crosslinking is the instability of carbon sulfur bonds. Sulfur is corrosive in electrical adhesive tapes, and sulfur-containing adhesives are polluting. The crosslinking reaction between sulfur or sulfur donor compounds and rubber is quite slow when heated. Adding sulfur accelerators to the formula can shorten the crosslinking time, lower the vulcanization temperature, and reduce the amount of sulfur used.
The typical sulfurization accelerator for natural rubber is zinc dibutyltin thiocarbamate. If minimal pollution is required, a combination of tetramethyl thiuram disulfide, zinc oxide, and zinc dibutyltin thiocarbamate can be used.
2、 Phenolic resin
Thermosetting alkylphenol formaldehyde resin can crosslink with double bonds in rubber. Its characteristic is good heat resistance and it belongs to non polluting crosslinking agents. Phenolic resin has a longer crosslinking time and slower speed when used alone. To reduce crosslinking time, accelerators or catalysts such as zinc oxide, zinc resinate, p-toluenesulfonic acid, etc. can be added.
Compared with ordinary phenolic resin, bromine based phenolic resin is the most effective crosslinking agent. In addition to phenolic resin, the condensation products of urea formaldehyde and alcohol, as well as the reactants of bisphenol and formaldehyde, can be used for crosslinking, with the latter being particularly effective for butyl rubber.
3、 Melamine formaldehyde resin and other resins
Melamine formaldehyde resin, abbreviated as amino resin, is a reactant of melamine and formaldehyde. Acrylic or rubber elastomers with hydroxyl or carboxyl groups can be crosslinked using melamine formaldehyde resin with butyl or methyl etherification. Reactive resins such as urea and formaldehyde reactants, epoxy resins, etc. can sometimes be used as crosslinking agents in pressure-sensitive adhesives.
4、 Organic peroxides
Non polluting pressure-sensitive adhesives can be obtained by crosslinking elastomers such as butadiene rubber, styrene butadiene rubber, and natural rubber with organic peroxides.
Organic peroxides are heated and decomposed to generate free radicals, which take away the hydrogen atom on the allyl group in the elastomer or add to the double bond, turning the elastomer into a new free radical. In this way, according to the usual principle of free radical reaction, the product can achieve cross-linking. The crosslinking efficiency of organic peroxides is relatively high, and the crosslinking rate depends on the decomposition rate of peroxides, which increases with temperature. Representative compounds of this type are benzoyl peroxide and diisopropylbenzene peroxide.
5、 Polyisocyanates
Polyisocyanates are used in the crosslinking of rubber in the form of monomers or prepolymers, and can also be used as crosslinking agents for acrylic pressure-sensitive adhesives. Polyisocyanates are effective crosslinking agents for solvent based adhesives. But as cross-linking progresses, the viscosity of the pressure-sensitive adhesive solution will rapidly increase. In this case, prepolymers can be used, which are reactants of polyisocyanates and hydroxylated oligomers.
The characteristic of polyisocyanate crosslinking is that it can be crosslinked at room temperature, so it is generally added before coating. Due to the absorption of moisture from solvents and air by isocyanates, the adhesive solution becomes turbid. Therefore, it is necessary to strictly control the moisture content. Polyisocyanates are also one of the commonly used crosslinking agents in acrylic pressure-sensitive adhesives.
6、 Metal oxides
Alkali metal oxides such as zinc oxide, lead oxide, magnesium oxide, etc. mixed with high molecular weight polymers containing carboxyl groups can easily cause crosslinking and the formation of three-dimensional network structures under temperature or heating. In the periodic table, when mixed with high molecular weight polymers containing carboxyl groups, Group IIA metal oxides represented by calcium, strontium, and barium will not form a network structure even when heated in completely dry conditions. However, in the presence of trace amounts of moisture such as humidity in the air, they will solidify and form a three-dimensional network structure
7、 Other crosslinking agents
Water soluble polyamide epichlorohydrin resin, zirconium compounds, multifunctional aziridine derivatives, etc. are effective crosslinking agents of carboxyl containing lotion pressure sensitive adhesive. Adding a small amount of diamine or polyamine to ordinary pressure-sensitive adhesive can improve its creep resistance. If amines and peroxides are used together, the effect will be better. Derivatives of maleimide and quinone derivatives (benzoquinone oxime) can be used as crosslinking agents for diene rubber. Aldehydes can be used to crosslink natural rubber, styrene butadiene rubber, butyl rubber, and other elastomers in acidic environments. The most effective aldehyde crosslinking agents are formaldehyde formaldehyde, polyformaldehyde, and formaldehyde formaldehyde.
