Crosslink physical

It should be noted that AM is only a measure of an apparent crosslink density of compounds. It is beyond the scope of the present work to investigate in detail the effective crosslinking (physical and chemical). However, for a qualitative assessment it can be concluded that the apparent crosslink density decreases or is influenced by the E-beam irradiation of PTFE powder. PTFE500kGy-EPDM composites show much lower AM and hence lower apparent crosslink densities. It can be inferred that the state of cure and crosslinking efficiency are strongly dependent on irradiation dose. Table 3 shows the optimum curing time (f90, time required to reach 90% of the AM) as a function of PTFE loading and irradiation dose for different PTFE-filled EPDM composites. 

One characteristic is that the polymer becomes insoluble. In addition to chemical crosslinks, physical crosslinks may exist, for example in (semi)-crystalline polymers where the piy stallites act as crosslinks or in block copolymers where the domains of the dispersed phases act as physical crosslinks. 

Covalently crosslinked Physically crosslinked Interpenetrating networks... 

Chemical crosslinking Physical crosslinking regardless of the amount of medium (water) By forming a crosslinking structure, it will not dissolve... 

From a topological point of view, the IPN s are closely related to polymer blends, block and graft copolymers, AB-crosslinked copolymers " and ionomeric blends. Some interesting hybrids exist between the IPN s and other polymer materials. The thermoplastic IPN s contain physical crosslinks rather than chemical (covalent) crosslinks. Physical crosslinks can be formed from block copolymers, ionomers or semi-crystalline polymers. When the temperature is raised above the softening point of the respective components, the material flows like a thermoplastic. At service temperatures, it behaves like an IPN, with thermoset behavior. Table 1 summarizes some of the terminology used to describe IPN structure and morphology. 

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

Earlier, successful attempts at physically crosslinking acrylic PSAs have been disclosed in the work of Husman et al. [98], Mancinelli and colleagues [99,100] and others. Instead of making ABA type structures, these authors studied and developed the use of high macromers in acrylic copolymers. Macromers are... 

Vulcanization changes the physical properties of rubbers. It increases viscosity, hardness, modulus, tensile strength, abrasion resistance, and decreases elongation at break, compression set and solubility in solvents. All those changes, except tensile strength, are proportional to the degree of cross-linking (number of crosslinks) in the rubber network. On the other hand, rubbers differ in their ease of vulcanization. Since cross-links form next to carbon-carbon double bonds. 

Polymer High MW (>10,000) Tg usually < RT Physically crosslinks on cooling Strong Strength Hot tack Viscous loss —> peel force... 

The two-component waterborne urethanes are similar in nature to the one-component waterborne urethanes. In fact, many one-component PUD s may benefit from the addition of a crosslinker. The two-component urethanes may have higher levels of carboxylic acid salt stabilizer built into the backbone than is actually needed to stabilize the urethane in water. As a result, if these two-component urethane dispersions were to be used as one-component adhesives by themselves (without crosslinker), they would show very poor moisture resistance. When these two-component urethane dispersions are used in conjunction with the crosslinkers listed in Fig. 8, the crosslinkers will react with the carboxylic pendant groups built into the urethane, as previously shown in the one-component waterborne urethane section. This accomplishes two tasks at the same time (1) when the crosslinker reacts with the carboxylic acid salt, it eliminates much of the hydrophilicity associated with urethane dispersion, and (2) it crosslinks the dispersion, which imparts solvent and moisture resistance to the urethane adhesive (see phase V in Fig. 5). As a result of crosslinking, the physical properties may be modified. For example, the results may be an increase in tensile properties and a decrease in elongation. Depending upon the level of crosslinking, the dispersion may lose the ability to be repositionable. (Many of the one-component PUD s may... 

Once the crosslinker is added, it is important to apply the adhesive and dry off the water. Most of the commonly used crosslinkers will react with water over a period of time and lose effectiveness. In some two-component PUD s, the system may increase in viscosity and even gel, giving the user some idea of when the useful life of the crosslinker is approaching its end. In other instances, no viscosity increases or other visible indications signal that the crosslinker has reached the end of its useful life. The improvements in physical properties, solvent resistance, and water resistance normally provided by a crosslinked PUD adhesive would not be fully realized, in this case. 

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