Gel Spinning Process

Gel Spinning Process:
Gel spinning is defined as a spinning method for high strength fibers through a gel-like state as intermediate substance. The industrially prosperous spinning method called “gel spinning” was invented by Dutch researchers, Smith and Lemstra, in DSM in the early 1980s. Gel spinning technology is one of the major production methods for high-strength and high-mechanical-property fibers. In gel spinning, the degree of polymerization is the most critical property; therefore, for polyethylene the ultra-high molecular weight above 600,000 g/mol is used. Gel spinning, also known as semi melt spinning, is a method that prepares high-strength and high-elastic modules fiber in the gel state. After the extrusion of the polymer solution or plasticised gel from the spinnerets, it is cooled in solvent or water before being stretched into gel fiber by ultra-high extension. Gel spinning can promote the polymer of fiber’s molecular weight, degree of orientation, crystallinity and fiber density effectively, and that contributes to increasing the fiber’s strength and modules. Gel spinning causes relatively ideal structures for ordinary flexible-chain polymers (e.g. PE, PAN and PVA) to produce high-strength and high-modulus fiber. In the near future, gel spinning will be able to produce many more special fibers with excellent properties.

Gel spinning process is as a method of achieving high strength fibers through an intermediate gel-like state. In gel spinning, the most important feature of the polymer is the degree of polymerization. Polymers with an average molecular weight above 600 000 g/mol are effectively processed by this technique. Although gel spinning is a relatively long-standing process, in its current adaptation, polymer of an extremely high molecular weight is dissolved in a solvent at low concentrations of about 1–2 percentage of polymer by weight, making a highly viscous solution. The solution is then dry- or wet-spun to a fiber that retains most of the solvent and is actually a gel of polymer and solvent. The gel spun fiber is further drawn with the removal of the solvent to produce a very highly oriented and ordered structure with high strength and modulus.

Recent development of a type of solution spinning called gel spinning has resulted in commercialization of several high performance polyethylene fibers most notably SpectraR and DyneemaR. These fibers have tenacities approaching that of KevlarR (3 GPa) but with lower modulus and much lower melting temperature. The process uses an ultra-high molar linear polyethylene (UHMWPE) that is dissolved in solvent. The ratio of solvent to polymer is generally in the 1–5% range. This dope is metered from the mixing unit through a multi-hole spinneret and into an extrusion tank usually filled with water. As the dope is cooled and solvent is extracted, gel-like filaments are formed in a type of network arrangement. These as-spun filaments, still containing large amounts of solvents, are then transferred to an oven and very slowly drawn up to 50 × or more. The resulting fibers are then wound on packages. The equipment and machinery used for manufacturing these products has to date been built in-house by the fiber producers and is a closely guarded secret.

The essences of the gel-spinning method are summarized as follows: (1) the reduction of number density of entanglements by diluting the solution with a solvent, (2) the larger molecular weight between entanglements using an ultrahigh molecular weight polymer, (3) the fix of less entangled state in the solution via the crystallization after the throughput from a spinneret, and (4) the high draw ratio at an elevated temperature.

Gel spinning fiber is applied in high strength and high tensile fabrics, including bulletproof jacket, belts, gloves and sportswear. The most popular gel spinning fiber, UHMWPE, can be used to manufacture safety protective equipment, high-tension ropes, fishing nets and sports leisure goods.

Factors Affecting Gel Spinning:
Many factors affect gel spinning processing. The product’s spinning property, stability and quality are subject to factors such as,

Molecule type,
Molecular weight,
Solvent type,
Solvent concentration,
Laddering rate / ladder speed,
Spinning temperature,
Extractant type,
Stretching temperature,
Stretching rate, and
Stretching magnification etc.

Steps in Gel Spinning Process:
The general gel spinning process can be divided into the following four steps:

1. Dissolution: The dissolution of polymer in solvent. The molecular chains of the solid polymers may be entangled, and the entanglements will affect the molecular chain stretching. The dissolution of polymers in solvent can disentangle these entanglements to aid the gel fiber stretching.

2. Spinning and formation: Extruding the solvent from the spinnerets by gear pump, and cooling it rapidly by air or water. The rapid cooling process can lead to the formation of crystals and retain the disentangled state of polymers to aid the formation of high-strength and high-elasticmodulus fiber.

Fig: Schematic diagram of gel spinning process

3. Remove solvent: The most commonly used methods to remove solvent in gel spinning include the natural drying method and the use of extractant, which mainly aims to remove the solvent residuals in gel fiber. The extractant can displace the gel fiber solvent based on diffusion and penetration principles. In gel spinning, the selection of solvent and extractant directly affect the stretching stability, and is key to the gel spinning method.

4. Stretching: The high magnification stretching of the gel fibre. High magnification stretching can change the folded polymer molecular chains to straight chains, and improve polymer crystallinity and orientation. This is a unique feature of gel spinning, and is the key to enhancing gel fiber performance.

Types of Fibers Produced Using Gel Spinning:
Gel spinning is usually applied in the processing of high molecular weight or ultra-high molecular weight polymers. The entanglement of long chain polymers is fully stretched by appropriate solvent and temperature. After spinning, high magnification stretching can further stretch the polymer molecular chain to get the high strength of the fiber. Gel spinning is most widely applied in production of UHMWPE, Polyvinylalochol (PVA) polymer and polyacrylonitrile (PAN) fiber.

References:

Advances in Filament Yarn Spinning of Textiles and Polymers Edited by Dong Zhang
Advanced Fiber Spinning Technology by T. Nakajima
High-Performance and Specialty Fibers: Concepts, Technology and Modern Applications of Man-Made Fibers for the Future by The Society of Fiber Science and Technology, Japan
Handbook of Fiber Chemistry, Third Edition Edited by Menachem Lewin

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Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.