High Performance Polyethylene Fibers – An Overview
Muhammad Imteaz Anjum
School of Textile and Design
University of Management and Technology Lahore, Pakistan
Gel-spun polyethylene fibers are ultra-strong, high-modulus ﬁbers that are based on the simple and ﬂexible polyethylene molecule. They are called high-performance polyethylene (HPPE) ﬁbers, high-modulus polyethylene (HMPE) ﬁbers or sometimes extended chain polyethylene (ECPE) ﬁbers. The gel-spinning process uses physical processes to make available the high potential mechanical properties of the molecule. This has been quite successful but there is still ample room for improvement.
Basic Structure of Polyethylene Fibers
The polyethylene is a long chain aliphatic hydrocarbon and it is thermoplastic. The Tg is approximately -120ºC. Tm depends upon the structure which ranges from 108-132ºC. It has high molecular weight alkane and has a good resistance to chemical attack. Because it is a crystalline material and does not interact with any liquids, there is no solvent at room temperature.
Manufacturing of Polyethylene Fibers
Gel-spun high-performance polyethylene fibers are produced from polyethylene with a very high molecular weight (UHMW-PE). This material is chemically identical to normal high-density polyethylene (HDPE), but the molecular weight is higher than the commonly used PE grades. It is in the range that is used in abrasion-resistant engineering plastics. Different from all other high-performance ﬁbers, the molecules in high performance polyethylene ﬁbers are not ‘preformed’ to form high tenacity and modulus ﬁbres. In aramids and comparable ﬁbres, the molecules tend to form rod-like structures and these need only be oriented in one direction to form a strong ﬁbre. Polyethylene has much longer and ﬂexible molecules and only by physical treatments can the molecules be forced to assume the straight (extended) conformation and orientation in the direction of the ﬁbre. All the physical and chemical properties of polyethylene remain in the ﬁbers. The differences result from the high chain extension (stretching), the high orientation and the high crystallinity. The gel-spun ﬁbres have properties that are superior to those made by solid-state processes.
High performance polyethylene ﬁbers are commercially produced under the trade names Dyneemaby DSM High Performance Fibers in the Netherlands and by the Toyobo/DSM joint venture in Japan, and Spectra by Honeywell (formerly Allied Signal or Allied Fibers) in the USA. The basic theory about what a super-strong polyethylene ﬁbers should look like was already available in the 1930s from the ideas of Carothers, but it took almost half a century to produce HPPE ﬁbres. The basic theory of how to produce a super-strong ﬁbre from a polymer such as polyethylene is easy to understand. In normal polyethylene the molecules are not orientated and are easily torn apart. To make strong ﬁbres, the molecular chains must be stretched, oriented and crystallised in the direction of the ﬁbre. Furthermore, the molecular chains must be long to have sufﬁcient interaction and for this reason polyethylene with an ultra-high molecular weight (UHMW-PE) is used as the starting material. Usually extension and orientation are realised by drawing. The problem is that spinning these ﬁbres from the melt is almost impossible due to the extremely high melt viscosity. Furthermore, the drawing of a melt processed UHMW-PE is only possible to a very limited extent owing to the very high degree of entanglement of the molecular chains. In the gel spinning process these two problems are solved: the molecules are dissolved in a solvent and spun through a spinneret. In the solution the molecules become disentangled and remain in that state after the solution is spun and cooled to give ﬁlaments. Because of its low degree of entanglement, the gel spun material can be drawn to a very high extent. The main steps in the process are the continuous extrusion of a solution of ultra high-molecular weight polyethylene (UHMW-PE) Spinning of the solution, gelation and crystallization of the UHMW-PE. This can be done either by cooling and extraction or by evaporation of the solvent. Super drawing and removal of the remaining solvent gives the ﬁbre its ﬁnal properties but the other steps are essential in the production of a ﬁbre with good characteristics. In the gel-spinning process, not only do all the starting parameters have an inﬂuence on the ﬁnal properties of the ﬁbre, the different process steps also inﬂuence all the following stages in the production of the ﬁbre. So, starting from the same principles, Dyneema and Spectra may use very different equipment to produce comparable ﬁbres.
With long-chain, ﬂexible polymers the high orientation required can be obtained by drawing up to a very high draw ratio (50–100 times). Melt processed UHMW-PE can be drawn up to ﬁve times only, as the interaction between the molecular chains is too high because of the molecular entanglements. In solution, the molecules disentangle but there remain a number of cross-overs determined by the concentration and the length of the molecules. The ﬂexible molecules assume a roughly spherical shape with a diameter proportional to the cubic root of the molecular weight. For the UHMW-PE chains the diameter of such a ball is about 1% of the total chain length. As soon as strain is applied when the solution is pressed through the spinneret, the molecules are forced into more elongated form For maximum ﬁbre strength, the polyethylene molecules should be as long as possible. From an economic point of view the concentration of the solution should be as high as possible. However, these two factors together result in a solution that has a viscosity that is far too high to spin. Careful optimisation of these parameters is an essential part of the process.
Gelation and Crystallization
The solvent used in the polyethylene gel-spinning process should be a good solvent at high temperatures (>100°C) but at lower temperatures (<80°C) the polymer should easily crystallize from the solution. After the spinneret, the solution is cooled in the quench, the solvent is removed and a gel ﬁbre is formed. This can be done by evaporation or by extraction of the solvent.
The ﬁnal properties of the ﬁbre in the gel-spinning process are achieved in the super drawing stage. All the preceding steps are needed to make this possible. The strength and modulus are directly related to the draw ratio. The maximum attainable draw ratio appears to be related to the molecular weight and the concentration. The attainable draw ratio increases with decreasing concentration, but for each molecular weight there is a minimum concentration below which drawing is not possible, due to insufﬁcient molecular overlap.
Properties of Polyethylene Fiber
1. Tensile properties: The primary properties of the Dyneemaand Spectra ﬁbres are high strength and high modulus in combination with the low density. HPPE ﬁbres have a density slightly less than one, so the ﬁbre ﬂoats on water. Whereas the strength and modulus are already very high. The tenacity is 10 to 15 times that of good quality steel and the modulus is second only to that of special carbon. Elongation at break is relatively low, as for other high-performance ﬁbres, but owing to the high tenacity, the energy to break is high.
2. Energy absorption: Dyneema and Spectra ﬁbres can absorb extremely high amounts of energy. This property is utilized in products for ballistic protection. But it makes the ﬁbre equally suited for products such as cut-resistant gloves and motor helmets. The ﬁbres can also be used to improve the impact strength of carbon or glass ﬁbre-based composites. In these applications, not only the high tenacity is used but also the high energy absorption.
3. Fatigue Fatigue: Is very important in, for example, rope applications. HPPE ﬁbres are the ﬁrst high-performance ﬁbers that not only have a high tenacity but that also have tension and bending fatigue properties comparable with the commonly used polyamide and polyester grades in ropes.
4. Abrasion resistance: Abrasion resistance is very important in ropes, also in gloves. In many of applications it is at least one of the factors that determines wear and tear and so the service life. The high molecular weight polyethylene used for HPPE ﬁbres is also a well-known engineering plastic.
5. Effects of water: Polyethylene fibers are not hygroscopic and does not absorb water. The ﬁbres have a very low porosity, therefore water absorption in the ﬁbre is negligible.
6. Chemical resistance: HPPE ﬁbres are produced from polyethylene and do not contain any aromatic rings or any amide, hydroxylic or other chemical groups that are susceptible to attack by aggressive agents.The result is that polyethylene and especially highly crystalline, high molecular weight polyethylene is very resistant against chemicals.
Applications of Polyethylene Fibers:
- Medical implants
- Cable and marine ropes
- Sail cloth
- Composites like Pressure vessel boat hulls, sports equipment, impact shields
- Fish netting
- Concrete reinforcement
- Protective clothing
- Can be used in radar protective cover because of its low dielectric constant
- Can be used as a lining material of a pond which collects evaporation of water and containment from industrial plants.
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