Extension hardening

The programme on branched polymers is driven both by long-standing puzzles in the extension-hardening (yet shear thinning) of commercial branched polymers and the radically different rheology seen in model mono-disperse branched melts such as stars. Recent extensions of the tube model... 

A very recent move to apply the structures emerging from molecular rheology to non-linear models of polydisperse complex-architecture melts has met with considerable success. The simple insight that the stress is a composite, not a structural, variable, with orientational and scalar components of different relaxation times, vastly improves the ability to model LDPE melts quantitatively. It also explains how such melts may be shear thinning yet extension-hardening. 

Figure 6-27. Unpublished data of Brindley (1996) (reported in Brady et al. (1996a)) showing that although the oxidation-resistance of the orthorhombic-based alloy Ti-22Al-20Nb-2Ta-lMo at.% is superior to that of Ti-48Al-2Cr-2Nb at.% at 800°C in air, it still suffers from extensive hardening during elevated-temperature exposure in air. (a) Cyclic oxidation data (b) Knoop microhardness data (25 g/15 s).

Cp is tire number of elasticity active chains per volume unit. The comparison between experimental data and tire prediction by (C2.1.20) shows a reasonable agreement up to large defonnation (figure C2.1.16). For large values of X, strain hardening arises because of tire limited extensibility of tire chains or because of shear-induced crystallization. 

At one time urea-formaldehyde was used extensively in the manufacture of plywood but the product is today less important than heretofore. For this purpose a resin (typically U-F molar ratio 1 1.8)-hardener mixture is coated on to wood veneers which are plied together and pressed at 95-110°C under pressure at 200-800 Ibf/in (1.38-5.52 MPa). U-F resin-bonded plywood is suitable for indoor application but is generally unsuitable for outdoor work where phenol-formaldehyde, resorcinol-fonnaldehyde or melamine modified resins are more suitable. 

The low molecular weight polysulphides have found somewhat greater use. Of general structure HS—R—SH and with molecular weights of approximately 1000 they will react with the epoxy group to cause chain extension but not cross-linking. The normal hardeners must therefore be employed in the usual amounts (Figure 26.20). 

PRF adhesives in which a liquid phenol-resorcinol-formaldehyde adhesive and a powder or liquid hardener are used are currently the most commonly used industrially. Pure resorcinol-formaldehyde (RF) adhesives were used extensively... 

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Part A of the adhesive is again a standard phenol-resorcinol-formaldehyde (PRF) cold-setting adhesive, with powder hardener added at its standard pH. Part B can be either the same PRF adhesive with no hardener and the pH adjusted to 12, or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho, or pine bark extract, with no hardener [118,135-137], The results obtained with these two systems are good and the resin not only has all the advantages desired but also the use of vegetable tannins and the halving of the resorcinol content makes the system considerably cheaper [118,135-137]. 

Nitrocellulose alone will not give a continuous coating. It must, therefore, be blended with other components comprising a plasticiser and a hardening resin. An extensive range of such products is available, the ultimate choice depending on the properties required. 

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