During the "90th anniversary of the founding of the military" parade, the production workshop for bulletproof helmets was opened to the public for the first time in a live television broadcast. When it comes to bulletproof materials, it is necessary to mention ultra-high molecular weight polyethylene fibers and aramid fibers.
The main production processes of ultra-high molecular weight polyethylene fibers are as follows:
Preparation of raw materials twin-screw extruder spinning box spinneret extraction drying heating drafting winding 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 proportion of raw materials should not fluctuate and should always be uniform and consistent. Although the increase in solid content is one of the important means to increase production, the stretching ratio also increases, and the overall speed needs to be correspondingly accelerated, increasing the difficulty of operation. The production of wool 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 one of the effective means 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" needs to be deaerated and must not contain water vapor. The material needs to be thoroughly mixed and stirred during transportation. The heating temperature of each zone should be set based on the position of the kneading block on the screw, and a certain delivery pressure should be ensured. The setting of screw kneading blocks is highly theoretical, and different combinations have different effects on the mixing of materials.
Spinning
The main function of the spinning box is to keep warm; Temperature control; Distribute materials evenly to each spinning component.
Spinneret
The material is extruded into silk strips by the metering pump through the spinneret. The aperture size and planed surface shape of the board are its important technical parameters, which play a vital role in the fiber forming and tensile properties. The temperature at the spinning box and the spinneret is matched, and the temperature parameters are set by observing the melting state of the ejected yarn. 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 to obtain "pure" high-strength polyethylene fibers. The selection of extractants varies from manufacturer to manufacturer, and the production process is also different. So far, it is difficult to find an ideal extractant that is economical, practical, safe, environmentally friendly, has good extraction effects, and 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 filaments are randomly and continuously stretched, from coarse to fine in appearance, from semi transparent to semi milky white, and the stretchability of the filaments gradually improves, giving them a bit of "strength". If viewed from the inside of the silk, the molecular structure of the raw material does not change significantly, the macromolecules are not aligned in a directional manner, but still in a disordered state, and the molecules are wrapped and isolated by a large number of solvents, which cannot form molecular chains. If the molecular chains are not shaped, the silk cannot have real strength. At this point, the interior of the fiber actually resembles a circular network, with polyethylene molecular particles in its network. As the fiber continues to stretch and refine, solvents continue to precipitate, and the shape of the network also changes from round to long, from comb to dense. The density between material molecules gradually increases, and the arrangement of large molecules gradually transitions from a disordered state to a partially ordered state.
Dry
The main purpose of the drying process is to remove the extractant adhered to the silk strip and dry it for drafting purposes. This process may seem simple to control, but it is actually quite difficult. If the process temperature and tension are not properly controlled, a large number of merging and stiffening phenomena will occur, resulting in the inability to continue processing the semi-finished tow. The key lies in the control of drying temperature and drying length. This process cannot be underestimated as it directly affects the quality of the product after stretching.
Heating draft
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 drawing method in order to achieve the characteristics of high strength and high modulus. During each stage of under stretching, there are significant changes in the intermolecular structure. With stretching, the macromolecules shift 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 be generated, and the cohesion will be greater, resulting in a higher strength of the fiber. The crystallinity of fibers increases, and the initial modulus naturally increases. Under the action of external force resistance, the elongation and deformation of fibers decrease.
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 degree of orientation. 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. Therefore, it is necessary to have a higher stretching temperature in order to achieve high drawing power. Each stage of stretching has a different temperature, which 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 fixed in 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 in order to be guaranteed. If anyone can truly achieve the technical level of fixed length and weight, they will reach the pinnacle in the field of high-strength fibers.
Application Prospects of Ultra High Molecular Weight Polyethylene Fibers
Due to its numerous excellent properties, ultra-high molecular weight polyethylene fibers have shown great advantages in the high-performance fiber market, including mooring ropes for 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.
National Defense
Due to its good impact resistance and large specific energy absorption, the fiber can be made into protective clothing, helmets and bulletproof materials in the military, such as the armor plates of helicopters, tanks and ships, the protective covers of radars, missile covers, bulletproof vest, stab resistant clothing, shields, etc., of which the application of bulletproof vest is the most eye-catching. It has the advantage of gentleness, and its bulletproof effect is better than aramid. It has become the main fiber occupying the American bulletproof vest market. In addition, the specific impact load value U/p of ultra-high molecular weight polyethylene fiber composite material is 10 times that of steel, and more than twice that of glass fiber and aramid fiber. 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 landing and a rope for hanging heavy objects on airplanes, replacing traditional steel and synthetic fiber ropes, and its development speed is exceptionally fast.
Civil aspect
(1) Application of ropes and ropes: ropes, ropes, 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 breaking length of the rope made of this fiber under its own weight is 8 times that of steel rope and 2 times that of aramid. This rope is used as a fixed anchor rope for super oil tankers, offshore operating platforms, lighthouses, etc. It solves the problems of corrosion, hydrolysis, UV degradation, etc. that have been encountered in the past when using steel cables and nylon and polyester cables, resulting in reduced cable strength and fracture, and requiring frequent replacement.
(2) Sports equipment and supplies: Safety helmets, snowboards, sail boards, fishing rods, rackets and bicycles, gliders, ultra light aircraft components 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 grafts, and plastic sutures. It has good biocompatibility and durability, high stability, and does 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.
