Gel-spinning process

In 4.3 we have already seen that polymers, in the rubber or fluid condition, crystallize much more rapidly when their chains are oriented. Therefore a stretched rubber, if stereospecific in its molecular structure, is able to crystallize at a temperature considerably above its equilibrium thermodynamic melting point. Also a thermoplast such as polyethylene, when in the molten state or in solution, can crystallize spontaneously when the chains are being orientated in elongational flow. The latter case is utilized when polyethylene is spun from a diluted solution (gel spinning process), resulting in fibres of super-high strength and stiffness ( Dyneema fibres). 

De Gennes scaling concepts for polymer solutions and melts and the concept of reptation movement of polymer chains in melts Pennings discovery of chain extension and shish-kebab formation in stirred solutions of very high MW polyethylene this eventually led to the ultra-high modulus gel-spinning process of polyethylene... 

Figure 4.11 Flow diagram of the gel-spinning process of making ultra high molecular weight polyethylene fiber.

Recently elastic polyolefin fibres have also been introduced (generic name lastol), they are cross-linked and stable at temperatures up to 220 °C and above.- High-tenacity polyethylene fibres, with ultra high molecular weight (UHMW-PE, Dyneema) have been available for some time they are produced by a gel-spinning process at high dilution. 

PAN by use of gel-spinning or other techniques to create a completely extended chain. Their estimate, based on a comparison of the rod-like PAN conformation with helical chain conformations of other polymers, is that the maximum tensile modulus of atactic PAN would be about 55 GPa. They conclude that PAN with ultrahigh modulus cannot be made by gel spinning, or by any other means, due to the intrinsic chain properties. It is proposed that the strong intramolecular nitrile repulsions that cause the PAN to adopt a rod-like, semiextended conformation do not allow the chain to unravel completely from its semiex-tended conformation. Attempts to develop gel-spinning processes in acrylic fibers are discussed in Section 12.5.4. 

H. Yasuda et al.. Gel spinning processes, in Advanced Fiber Spinning Processes, P.T. Nakajima, Ed., Woodhead Publishing Limited, Cambridge, p. 174 (1994). 

Highly oriented flexible-chain polymers, in particular ultrahigh-molecular-weight polyethylene (UHMWPE) fibers prepared by gel spinning process, as suggested by Smith and Lemstra [322], exhibit the extremely high mechanical strength and modulus of elasticity rather close to the theoretical estimates. At the same time, these fibers reveal decreased creep resistance which substantially restricts their application potential. 

Note the maximum value for polyethylene (PE) at In A = 4.6, implying a draw ratio of 100. The polar polymers, polyester and polyamide(s), which are not superdraw-able and not candidates for a gel spinning process, are at the other extreme. There is an interesting group of polymers with intermediate polarity and drawability, however polypropylene (PP, DR = 47.5), poly(vinyl alcohol) (PVA, DR = 30), polyacrylonitrile (PAN, DR = 28), poly-L-lactic add (PLLA, DR = 20). 

Figure 9.19 Gel-spinning process schematic (reprinted from ref. 186 with permission of Wiley).

The general gel spinning process can be divided into the following four steps ... 

Molecule type molecule type determines the use of the product. The type of molecule chosen for the gel spinning process is usually polymers of chain thermo-plasticity, and commonly used gel spinning molecules include UHMWPE, PVA and PAN. 

Many factors affect the gel spinning process. The gel fibre can obtain the high strength and high modulus only by properly adjusting these factors. 

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