Production Process and Application Prospect of UHMWPE
Due to its numerous excellent properties, UHMWPE has shown great advantages in the high-performance fiber market, including mooring ropes in offshore oil fields and high-performance lightweight composite materials. They play a crucial role in modern warfare, aviation, aerospace, maritime defense equipment, and other fields.
Production Process and Application Prospect of UHMWPE
The main production processes of ultra-high molecular weight polyethylene fibers (UHMWPE) are as follows: Preparation of raw materials - twin screw extruder - spinning box - spinneret plate - extraction - drying - heating and stretching - winding and forming.
Preparation of raw materials
Currently, the preparation methods of raw materials at home and abroad vary, with different solvents used and varying solid content. Therefore, there is no fixed unified mode, and the production equipment is also very different, while the conventional melt spinning is solvent-free. But no matter which method is adopted, the desired effect can ultimately be achieved. Due to the continuous production process, the ratio of raw materials should not fluctuate and should always be uniform and consistent. Although increasing the solid content is one of the important means to increase production, the stretching ratio also increases accordingly, and the overall speed needs to be correspondingly accelerated, which increases the difficulty of operation. The production of yarn is significantly increased, making it difficult to grasp. But if the percentage of solid content can be controlled within an appropriate concentration, it is still possible, so it should be done according to one's own situation and according to one's own abilities. Increasing the speed of the metering pump is also an effective way to increase production.
Mixing materials
The screw extruder plays a role in conveying, stirring, heating, and pressurizing materials. Firstly, the slurry before entering the "screw" should be de foamed and should not contain water vapor. The material should be thoroughly mixed and stirred during transportation. The heating temperature of each zone should be set according to the position of the kneading block on the screw, and a certain conveying pressure should be ensured. The setting of screw kneading blocks is highly theoretical, and different combinations will have different effects on the mixing of materials.
Spinning
The main function of a spinning box is to provide insulation; Temperature control; Evenly distribute materials to each spinning component.
Spinneret
The material is extruded into filaments by a metering pump, which is achieved through a spinneret plate. The pore size and planing surface shape of the spinneret are important technical parameters, which play a crucial role in the forming and tensile properties of fibers. Match the temperature between the spinning box and the spinneret, and set the temperature parameters by observing the melting state of the sprayed silk. But to achieve precise control, it is necessary to have some specific technical conditions and practical experience.
Extraction
The main method is to extract and replace a large amount of solvent from the filament, in order to obtain "pure" high-strength polyethylene fibers. The selection of extractants varies from manufacturer to manufacturer, and the production process also varies. So far, it is difficult to find an ideal extractant that is economical, practical, safe, environmentally friendly, has good extraction effects, is non-toxic, and odorless. Internationally, it is also a long-term problem that cannot be solved.
In the process from spinning to extraction, the filament is randomly and continuously stretched. From the appearance, it changes from coarse to fine, from semi transparent to semi milky white, and the stretchability of the filament gradually improves, giving it some strength. If viewed from the inside of the silk, there is no significant change in the molecular structure of the raw material, the large molecules are not oriented and arranged, still in an disordered state, and the molecules are wrapped and isolated by a large amount of solvents, unable to form molecular chains. If the molecular chains cannot form, the silk cannot have true strength. At this point, the interior of the fibers actually resembles a circular network, with polyethylene molecular particles in its network. As the fibers continue to stretch and refine, solvents continuously precipitate, and the shape of the network changes from round to long, from comb to dense. The density between material molecules gradually increases, and the arrangement of macromolecules gradually transitions from a disordered state to a partially ordered state.
Desiccation
The main purpose of the drying process is to remove the extractant adhered to the silk strip and dry it for use in stretching. This process may seem simple to control, but it is actually quite difficult. A slight mishandling of the process temperature and tension can lead to a large amount of merging and stiff threads, resulting in the inability of semi-finished fiber bundles to continue processing. The key lies in the control of drying temperature and drying length. This process should not be underestimated as it directly affects the quality of the product after stretching.
Heating and stretching
The drafting process of ultra-high molecular weight polyethylene fibers is basically the same as that of conventional polyester staple fibers in terms of form, but the precision required for control is greatly different. This fiber must adopt a multi-stage stretching method to achieve high strength and high modulus characteristics. During each stage of under stretching, there are significant changes in the intermolecular structure. With stretching, the macromolecules move from disordered to ordered, oriented arrangement, and the crystallinity gradually increases. Only when the orientation of the macromolecules in the fiber along the fiber axis increases, will the number of macromolecular chains generated be greater, the cohesion will be greater, and the strength of the fiber will naturally be higher. The crystallinity of fibers increases, and the initial modulus naturally increases. Under the action of external force resistance, the smaller the elongation of fibers, the smaller the deformation.
During the stretching process of fibers, the stretching ratio should be as high as possible to allow for sudden stretching changes in the fibers, which can better promote the ordered orientation and high crystallization of macromolecules. The internal crystallization of fibers occurs simultaneously with the formation of high orientation, resulting in a crystalline transformation. Due to the high molecular weight and strong resistance to external forces of this type of fiber, hot stretching technology can only be used in production. So, it is necessary to have a higher stretching temperature in order to achieve high magnification stretching. At each stage of stretching, the temperature varies and depends on the state of the filament in the previous process. There is no fixed number, but it must be within the temperature range that the fiber itself can withstand. In production, the temperature generally does not exceed 155 degrees Celsius. Otherwise, there will be the production of hard and stiff threads.
Winding forming
The requirements for silk roll forming: the silk tube should have no collapsed edges or burrs, and the tow should be of fixed length and weight. The so-called fixed length and weight are not simply referring to the requirements for the length and weight of the tow. Its connotation is very deep, and it is very difficult to accurately grasp it. It is a requirement that all production processes must be normal and stable, and the fiber size must always be uniform and consistent in order to be guaranteed. If anyone can truly achieve the technical level of fixed length and weight, they will reach the peak in the field of high-strength fibers.
Application prospects of UHMWPE
Due to its numerous excellent properties, UHMWPE has shown great advantages in the high-performance fiber market, including mooring ropes in offshore oil fields and high-performance lightweight composite materials. They play a crucial role in modern warfare, aviation, aerospace, maritime defense equipment, and other fields.
In terms of national defense
Due to its excellent impact resistance and high energy absorption, this fiber can be used in military applications to make protective clothing, helmets, and bulletproof materials, such as armor plates for helicopters, tanks, and ships, radar protective casings, missile covers, bulletproof vests, stab proof vests, shields, etc. Among them, the application of bulletproof vests is the most eye-catching. It has the advantages of softness and better bulletproof effect than aramid, and has now become the main fiber occupying the US bulletproof vest market. In addition, the specific impact load value U/p of ultra-high molecular weight polyethylene fiber composite materials is 10 times that of steel, and more than twice that of glass fiber and aramid. Bulletproof and riot helmets made of resin composite materials reinforced with this fiber have become substitutes for steel helmets and aramid reinforced composite helmets abroad.
Aviation
In aerospace engineering, due to its lightweight, high strength, and good impact resistance, this fiber composite material is suitable for various aircraft wing tip structures, spacecraft structures, and buoy aircraft. This fiber can also be used as a deceleration parachute for space shuttle landings and as a rope for hanging heavy objects on airplanes, replacing traditional steel and synthetic fiber ropes. Its development speed is exceptionally fast.
Civil aspect
(1) The application of ropes and cables: Ropes, cables, sails, and fishing gear made of this fiber are suitable for marine engineering and were the initial use of this fiber. Commonly used for negative force ropes, heavy-duty ropes, salvage ropes, towing ropes, sailboat ropes, and fishing lines. The rope made of this fiber has a fracture length of 8 times that of steel rope and 2 times that of aramid under its own weight. This rope is used as a fixed anchor rope for super oil tankers, ocean operation platforms, lighthouses, etc. It solves the problems of corrosion, hydrolysis, and UV degradation caused by steel cables and nylon and polyester cables in the past, which lead to a decrease in cable strength and breakage, and require frequent replacement.
(2) Sports equipment and supplies: Safety helmets, skis, sail boards, fishing rods, rackets and bicycles, gliders, ultra lightweight aircraft components, etc. have been made on sports equipment, and their performance is better than traditional materials.
(3) Used as a biomaterial: This fiber-reinforced composite material is used in dental support materials, medical implants, and plastic sutures. It has good biocompatibility and durability, high stability, and will not cause allergies. It has been clinically applied. It is also used in medical gloves and other medical measures.
(4) In industry, this fiber and its composite materials can be used as pressure vessels, conveyor belts, filtering materials, automotive buffer plates, etc; In terms of architecture, it can be used as a wall, partition structure, etc. Using it as a reinforced cement composite material can improve the toughness of cement and enhance its impact resistance.