Adhesive tack range

In the empirical world of tack experts, many professional terms are employed, like green strength (as initial tack is sometimes called), which relate to the resistance to separation before the adhesive has had a chance to vulcanize or crosslink. This characteristic may also be called quick tack or aggressive tack . It may be one of the most important properties in determining the suitability of an adhesive, such as that placed on a pressure-sensitive tape, for a certain application. Associated with tack is dry tack , which is a property of certain adhesives to stick to one another even though they seem to be dry to the touch. Autohesive tack (or autohesion) is the dry tack between materials having similar chemical compositions. Tack range is the time that an adhesive will remain in a tacky condition. 

Tack range (tack stage)— The period of time in which an adhesive will remain in the tacky-dry condition after application to an adherend under specified conditions of temperature and humidity. 

Tackifier n. A substance (e.g., rosin ester) that is added to synthetic resins or elastomeric adhesives to improve the initial tack and extend the tack range of the deposited adhesive film. 

Other important characteristics related to tack and commonly measured are dry tack and tack range. Dry tack is the property of certain adhesives to stick to one another even though they seem dry to the touch. Tack range is the time that an adhesive will remain in a tacky condition after apphcation to the adherend. 

Most versatile rubber adhesive. Superior resistance to oil and hydrocarbon solvents. Inferior in tack range but most dry tack-free, an advantage in precoated assemblies. Shear strength of 150-2,000 Ib/in. higher than neoprene, if cured... 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Plasticizers reduce hardness, enhance tack and reduce cost in rubber base adhesive formulations. A plasticizer must be easily miscible and highly compatible with other ingredients in the formulations and with the surfaces to which the adhesive is applied. The compatibility and miscibility of plasticizers can be estimated from the solubility parameter values. Most of plasticizers have solubility parameters ranging between 8.5 and 10.5 hildebrands. However, the high miscibility and compatibility also lead to easier diffusion of the plasticizer to the surface, decreasing the adhesion properties. Therefore, plasticizers should be carefully selected and generally combinations of two or more of them are used. 

Plasticizers soften the film and increase the adhesion and the setting speed. The most common are phthalates, adipates and benzoates. The amount added can be in a broad range of 10-50%. They affect the swelling and softening of the PVAc emulsion particles, ensure film formation at room temperature, and the tack of the still wet adhesive. They also provide improved moisture resistance of the bond. Disadvantages are the lower resistance of the bond line against heat, possible migration of the plasticizers and enhanced cold flow. 

The main property that distinguishes a pressure-sensitive adhesive from other types of adhesives is that it exhibits a permanent and controlled tack. This tackiness is what causes the adhesive to adhere instantly when it is pressed against a substrate. After it has adhered, the PSA should exhibit tack, peel and shear properties that are reproducible within narrow limits. This requires that the adhesive layer be only slightly cross-linked.113 PSAs are based on polymers with low Tg, typically in the range-74 to +13°C.114... 

Semiautomatic Labeling. In this operation the machine selects, glues, and applies the label, but the item to which it is applied is placed into position by hand. Labels may be picked up by vacuum or the adhesive. The machine must be set up correctly, labels must be produced to certain critical limits, and the adhesive must be specially selected. A higher tack is necessary than that used for hand labeling. Speeds range from 25 to 60 per min (i.e., 3,600 per hr maximum). 

Figure V demonstrates the effect of adhesive dry coating weight on 180° peel strength and rolling ball tack with OH/NCO ratios of 1.7 and 1.3 for Hycar 2103 and Hycar 2106, respectively. Static shear tests run both at room temperature and 70°C show no creep for both systems throughout the thickness range tested.

The range is the time over which an adhesive, once applied to a substrate, retains a useful degree of tack. It is a function of the adhesive formulation. For an adhesive to form a useful bond between two substrates, the range of the adhesive must be at least as long as the open time. 

Other important uses are adhesives and coatings. A wide variety of resins, plasticizers, fillers, and other ingredients commonly used in adhesives and coatings can be used with styrene-diene-styrene triblock copolymers. With these ingredients properties such as tack, stiflfiiess, softening temperatures, and cohesive strength can be varied over a wide range. With aliphatic resin additives the block copolymers are used for permanently tacky pressure-sensitive adhesives, and in conjunction with aromatic resins they are used for contact adhesives. The copolymers can be compounded into these adhesives by solution or hot-melt techniques. 

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