Cohesion or cohesive strength is another important property of pressure sensitive adhesives in addition to adhesion and tack. Any material will be deformed or even destroyed when subjected to external forces. The so-called cohesion of the material refers to the ability of the material itself to resist the external force. The effect of the external force on the adhesive joint is nothing more than three loading methods: positive pulling, shearing and peeling. When a good pressure-sensitive adhesive product is subjected to a peeling external force (especially a rapid peeling external force), the adhesive interface is generally broken, and when subjected to a positive pulling or shearing force, the adhesive layer mainly undergoes cohesive failure. Therefore, the tensile strength or shear strength of a pressure-sensitive adhesive can be used to characterize its cohesive strength, but in most pressure-sensitive adhesive applications, it is slow or permanent. Shear force effect. For example, when using a double-sided pressure-sensitive adhesive tape to fix an object on a wallboard, a box adhesive tape is subject to the long-term effect of the gravity of the object, and so on. At this time, the slippage of the pressure-sensitive adhesive tape is due to the creep failure of the pressure-sensitive adhesive under the action of a permanent shearing force. Since this type of damage also occurs primarily in the adhesive layer, people more often use pressure sensitive adhesives to resist the ability to sustain creep damage caused by external shear forces, ie, shear creep retention (also known as tack retention). To characterize the size of their cohesion. In addition, it has also been proposed to characterize the cohesion of a pressure-sensitive adhesive using positive tensile creep retention and T- peel creep retention. However, it is not as good as the shear creep retention is closer to the actual situation, and can better reflect the cohesive properties of pressure-sensitive adhesives, and the test method of pull-creep retention is cumbersome and therefore not often used by people.
Â Â Â Â Â Many countries have developed standards for testing adherence, such as ASTM D-3653 and PSTC-7 in the United States, JISZ2037 and JISZ1528 in Japan. The specific method is to stick the adhesive tape to a standard adherent (usually stainless steel) test piece with a certain area (length l0Ã— width b ), and hang a certain quality ( W ) at the end of the vertically hanging adhesive tape. The weight is so that the direction of force and the adhesive surface are completely parallel (at 180Â° ), and kept at a certain temperature, the adhesive tape is slipped until the falling time t0 or the adhesive tape is moved down for a certain period of time. The distance L , as a measure of the shear creep retention (holding force) of the tape, is related to the properties of the adherend, the size of the adhesive joint, the weight of the weight, and the test temperature. When recording tack, these test conditions must be indicated.
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