Pressure-sensitive tape, peeling

Low viscosity cellulose acetate is used in lacquers and protective coatings for paper, metal, glass, and other substrates and as an adhesive for cellulose photographic film because of its quick bonding rate and excellent bond peel strength (135) (see Coatings). Heat-sensitive adhesives for textiles have also been prepared from cellulose acetate (136). Extmded cellulose acetate film makes an excellent base for transparent pressure-sensitive tape (137) (see Adhesives). 

The release of elastic energy stored in the fibrils themselves, when they rupture, must be mentioned in developing a model of the separation process for adhering systems. Retraction of fibrils after rupture can be observed visually, in the peeling of a pressure-sensitive tape (11,23). See Figure 2. D611 (2b) reports for PMMA,... 

Figure 2. Fibrillar structure in peeling a pressure-sensitive tape. Note the retraction of fibrils that have separated from the solid. (Reproduced with permission from Ref. 21. Copyright 1969 John Wiley Sons, Inc.)...

As mentioned above, in the peeling separation of a pressure-sensitive tape, a temperature rise of 1 to 10°C is observed, with drive rates that are very modest compared to the rate of advance of a freely propagating crack in a brittle polymer. This temperature rise lowers the relaxation time, even though the polymer may be 10 to 50 above its Tg already. With a "hot melt adhesive" polymer, the Tg may be in the range of about 40 to 80 C. Such an adhesive may exhibit extremely "tough" mechanical behavior, no doubt because it resembles the pressure-sensitive tape case, described above, more than the brittle polymer case. 

We will also apply the concepts discussed above, to systems in which the polymer is not brittle, e.g., the peeling of pressure-sensitive tapes. For such polymer-solid systems, when separation... 

Peel Adhesion of Pressure-Sensitive Tape at 180°, D3330, American Society for Testing and Materials, Philadelphia. 

Chem. Descrip. Urethane diacrylate oligomer Uses Flexibilizer, toughener for coatings, inks, UV/EB adhesives (pressure-sensitive tapes) applied to flexible substrates Features Imparts high flexibility and elasticity, adhesive peel sir., toughness high elong. 120%... 

Nonstructural adhesives are those having bond strength that is less than 1000 psi but sufficient for applications such as pressure sensitive tapes, labels, laminates, and so on. Nonstructural adhesives are usually employed where production speed, convenience, and high peel strength are required. They generally have sufficient permanence for the applications mentioned. 

Pressure-sensitive adhesives are specifically formulated for good flexibility, tack, and peel strength. Most pressure-sensitive adhesives are applied to plastic, paper, foil, or fibrous material as suitable backings. Pressure-sensitive tapes are made with adhesive on one side or on both sides (double-faced tape). Double-faced foam tapes are available to suit applications where substrates have surface irregularities, contours, and significant gaps in the bond line. 

It is well established that, in the peeling of pressure-sensitive tapes off solids, fflaments of adhesive exist, extending from the adhesive layer on the tape backing, down to the solid. i" ) See Figure 4. (An exception to the formation of filaments may be observed with certain tapes that are designed for cohesive separation, and have very thick layers of adhesives. During... 

FIGURE 4. Disbondment zone of a pressure-sensitive tape Kaelble s Schematic. . . peel profile and experimental boundary distribution of normal stresses. (From Reference 14.)... 

The polymeric adhesives for modem, pressure-sensitive tapes are usually tailored such that, on peeling off a solid (or at least, off the solid for which the tape was designed), no residue remains on the solid. On the other hand, the polymer may be of a design such that the adhesive splits the fibers may mpture, and a residue of polymer may remain on the solid. 

The peeling force, for a pressure-sensitive tape, is expended (dissipated) in the elongation of the filaments.(1A local temperature rise is observed in peeling(i 3) it has been attributed to the plastic and elastic deformation that occurs when these filaments are drawn.(D A large, transient temperature rise has been observed in the fracture of PMMA and of polystyrene. See the review of the thermal effects on pp. 136-139 in Doll.d )... 

For computation by Eq. (30), we need values of at rates of extension that are relevant to peeling of pressure-sensitive tapes. Kaelble(i4) has reported that the zone in which the fiber elongation takes place is about 1 mm wide. See Figure 4. At a peel rate of 1 mm/sec (a low rate) this zone will travel past a point in space in 1 second. This means that the time to break, t, is about 1 second for this peeling rate, and = 3 sec i. 

The rate of true strain, , for a filament in the separation zone of a pressure-sensitive tape that is being peeled off a solid, will decrease as the fiber elongation progresses. So the value of... 

We have mentioned, earlier, the fact that a local, transient temperature rise has been observed in the peeling of pressure-sensitive tapes.0 3) peak temperature rise of 12.5 °C has been measured, in peeling at 1(X) cm/min (Figure 12). The temperature rise ATj appears to be associated with dissipative processes occurring in the filaments that are drawn, in the disbond-ment zone, when a pressure-sensitive tape is peeled off a solid. In fracturing a brittle polymer, the work (per unit area) done is converted into heat, and most of this heat is released in the craze filaments.2) A transient temperature rise, above 500 °C, has been reported with PMMA, and above 400 °C for polystyrene O) (see below). 

Finally, we may look at the theory that we have developed from the viewpoint of catastrophe theory. Filament rupture is intrinsically a catastrophe, and so is interfacial separation. The peeling of a pressure-sensitive tape and the propagation of a fracture front are macroscopically continuous processes, but the change in behavior of individual filaments or fibers in the separation front is discontinuous either they detach from the solid or they rupture. Thus, the microscopic course of events is subject to a bifurcation, i.e., a choice between two catastrophes. 

The role of the deformation energy can best be demonstrated by peeling a pressure-sensitive tape from a rigid substrate. During separation typically the adhesive becomes filamented and the legs are stretched to such degree that they break or separate from the surface. The former is a cohesive and the latter an adhesive failure. The energy required to achieve this is the peel force. 

Typically, pressure-sensitive tapes are applied in region B. The effective pulling rate of the standard peel test at 180° is 2.5x10 m/s, which is in the middle of region B (indicated by an arrow in Fig. 12). At very low temperature, region C may be reached. For application under freezer conditions, special tapes with lower Tg are available. 

Adhesive properties of pressure-sensitive tapes are characterized primarily by three parameters peel adhesion, shear adhesion and tack. 

Peel adhesion is the force required to remove a pressure sensitive tape or label from a test surface at a specified angle and rate of peel. This gives a measure of the ultimate strength obtained by the adhesive and the dwell time before peeling must be specified. In this test, the adhesive tape or label is applied to the test surface with a specified force. 

Rubber-based solvent cements are adhesives made hy combining one or more rubbers or elastomers in a solvent. These solutions are further modified with additives to improve the tack or stickiness and the degree of peel strength, flexibility, and the viscosity, or body. Rubber-based adhesive is used in a wide variety of applications, such as contact adhesive from plastic laminates used for counter tops, cabinets, desks, and tables. Adhesive is used on pressure-sensitive tapes as floor tile adhesive and carpeting adhesive. Self-sealing envelopes and shipping containers use rubber cements. Solvent-based rubber adhesives have been the mainstay of the shoe and leather industry. 

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