Peeling rate

Most types of PSAs have found some application in the label industry. Block copolymer-based adhesives are perhaps the most popular because of their high adhesion to a variety of surfaces, their low cost, their good performance over a range of temperatures and peel rates, and their ease of processing. For applications where high temperature performance is required, block copolymers have been formulated with high T end block associating resins or polymers. 

Fig. 25. Evolution of the tack of polychloroprene-aromatic hydrocarbon resin blends as a function of the resin content. Tack was obtained as the immediate T-peel strength of joints produced with 0.6 mm thick styrene-butadiene rubber strips placed in contact without application of pressure. Peeling rate = 10 cm/min.

All of the above peel data were collected using a 0.5 mm/min peel rate. A brief study was conducted on the effect of the peel rate on the measured peel strength in the PMDA-ODA/APS/Si02 system without any F contamination. These data are summarized in Table 5. 

It is evident from Table 5 that an order of magnitude change in the peel rate when peeling PMDA-ODA polyimide does not make a significant difference in the measured peel strength. Actually, the difference between consecutive table entries may be considered to be within the experimental error. 

PM DA-ODA/Si02 peel strength as a function of the peel rate... 

The peel strength of PMDA-ODA on SiOz is insensitive to an order of magnitude change in the peel rate. 

The reducing ends are the starting points for the well-known peeling reaction. The conditions for the peeling to proceed require alkaline media and a free 2-OH group. In case of a substituted hydroxyl at C2, as it occurs for instance in hemicelluloses, the peeling rate is drastically reduced. 

The metallic layers were examined either by conventional or cross-section TEM in a Jeol 200 Cx microscope. For the cross section preparation a sandwich of two laminates is made, glued face to face with an epoxy, cut in small pieces, mechanically polished, and then ion milled to a final TEM observation thickness. The plane section TEM sample are prepared by dissolving the PET in trifluoroacetic acid for 5 to 10 mn. The area observed, on plane section TEM, for the grain size calculation is close to 0.2 urn. For the adhesion measurements, test pieces consist of aluminum support (1 mm thick) double sided tape (Permacel P-94) PET (12pm) / evaporated aluminum/ ethylene acrylic acid (EAA) copolymer film. These laminates are prepared for the peel test by compression under 1.3 105 N.m2 at 120°C for 10 seconds. The peel test is performed by peeling the EAA copolymer sheet from the laminate in an INSTRON tensile tester at 180° peel angle and 5 cm min peel rate. 

Initially, pectinases were evaluated for peeling and segmenting activity at equivalent protein levels, using the Lowry procedure as modified by Potty (5) for protein determination. In later work, pectinases were tested at a number of concentrations to determine the amount required to yield a specific peel rating using regression analysis. 

Pectinase Peel Rating Section Rating Fluid Loss %... 

At concentrations providing equivalent peeling ratings, pectinases should exhibit similar activity levels of the responsible enzyme(s). However, when the concentrations necessary for a 4 peel rating were determined for each pectinases by regression analysis, and the individual enzyme activities calculated for those concentrations, no such similarity was evident (Table II.)... 

Table II. Enzyme Activities of Pectinase Infusions Giving a Peel Rating of 4 ...

Figure I. Temperature dependence of peel force for Polymer I (O) peel rate at 0.1 in./min, ( ) peel rate at 12 in./min (A) adhesive failure, (B) Estar base broke, (C) cohesive failure...

The gradation, as a function of rate of peeling, has been shown (12) to be directly related to the local temperature rise. When the peeling rate is sufficiently slow, the deformation process is one of isothermal, macroscopic creep and if the adhesive layer is thick enough, separation at a low level of peeling force may occur, entirely in the adhesive phase. 

The adhesive strength of post-crossHnked samples was evaluated by a 180° peel test which measured the force of separation at a constant peel rate (5 mmmin ) at room temperature. The results reported in Fig. 24.3 show the effect of contact time (ranging from 1 h up to one month) in the second step of joint formation at room temperature. Up to approximately 300 h of contact, no effect of time is seen. For longer times, the peel strength increases slightly. Although the scatter is important, this weak trend can be related to crosslinks formed by irradiation in the molecular interphase. This is confirmed by the fact that no separation is observed for these joints immersed in a good solvent such as cyclohexane, whereas spontaneous delamination is observed for shorter contact times. 

Fig. 24.3 Peel strength versus contact time of contact at room temperature for EPDM joints made of partially cross-linked networks which are post-crosslinked by an electron beam (peel rate = 5 mm min" ).

Joint Crosslinking conditions Peel rate [mm mln ] Peel strength [k] m- ]... 

In the nonsymmetric, nonhomogeneous joints (SBR/IR and IR/SBR), failure occurred in the SBR sheet but the peel strength compared at the same peel rate varied significantly. The existence of a gradient of properties due to the migration of sulfur or sulfur intermediates was then proposed [9]. 

Fig. 24.7 Peel strength C versus peel rate R influence of the contact time at 40°C after pre-crosslinking of the sheets separately and before post-crosslinking.

The locus of failure depends on parameters such as the state of crosslinking of PUa before contact and the characteristics of the HTPB used (molecular weight distribution, functionality, etc.) but also on testing conditions (peel rate, air or liquid). It can be related to the properties of the material close to the interface. 

Effectively, when viscoelastic losses are negligible (i.e., when performing experiments at very low peel rate or high temperature), 4>-> 1 and G must tend toward IV. However, the resulting threshold value Gq is generally 100 to 1000 times higher than the thermodynamic work of adhesion, IV. 

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