Metallic Fiber and Yarn:
Metallic fiber is manufactured fiber composed of metal, plastic-coated metal, metal-coated plastic, or a core completely covered by metal. These are light weight and relatively inexpensive yarns. Metallic fibers are an attractive class of fibers for composite applications considering their exceptional mechanical properties; however, the specific strength or strength to weight ratio (strength divided by density) of metallic fibers is inferior to high performance carbon, polymer or ceramic fibers due to their higher density. They also have disadvantages like poor corrosion resistance and inferior bonding strength with matrix. But, the surface of metallic fibers can be coated with ceramics to address these problems. Till today, they are the popular choice for applications like infrastructure building, electromagnetic interference shielding (EMI shielding), etc.
Gold and silver have been used since ancient yarns for fabric decoration. More recently, aluminium yarns and aluminized plastic nylon yarns have replaced gold. The metal generally used is aluminium, and the plastic used is cellulose acetate butyrate or mylar. They are made through laminating process. Coated metallic filaments help to minimize tarnishing. It is available only in filament form.
Any ductile metal, such as gold, copper or silver, can be drawn into fine filaments that can be rounded or flat. Expensive metals, such as gold and silver, may have inner cores made out of cheaper base metals, e.g. copper or polyamide. These filaments and threads are used mainly for decoration in embroidery, fringing, braids or are woven into fabrics such as brocades or tapestries. Some metallic threads tarnish, so they are replaced with threads that look similar but are made of a cheap metal e.g. tinfoil, which are placed between two thin layers of polyester film called Mylar and laminated; for silver color the film is clear and for gold the film is pigmented but they can be produced in any color. These threads are often known as Lurex. Metal threads are usually made with a core of cotton yarn wrapped within a ribbon of metal. Gold thread and silver wire are used for expensive fabrics. Imitation gold and silver wires are also prevalent, but they get tarnished easily.
Modern metallic yarns are produced from colored aluminium ribbons sandwiched between two layers of transparent plastic material. Aluminium, the basic metal used is softer, lighter in weight and cheaper than the more precious metals and is not apt to tarnish or cause discoloration.
Types of Metallic Fiber:
A number of metals are available in various fibrous forms. However, a number of practical factors limit the number of fiber types for composite applications. The most important influencing factors are availability, abundance, processing difficulties and cost. Cost of metal fibers is determined by a number of factors such as fiber type, abundance, production procedure, diameter, form and so on.
300 and 400 series stainless steel, nichrome, inconel, hastelloy x, carpenter 20cb3, nickel, 80/20 nickel chromium, titanium and tantalum are widely used to produce metal fibers.
Stainless steel fibers are the most potential metal fibers in terms of availability, application and material properties.
Aluminum is the major material used in light weight vehicle structures such as aircrafts. Aluminum alloys are used in most of the aerospace components. They are of low density, and possess high electrical and thermal conductivities and good resistance to chemical corrosion. Aluminum is also recyclable and properties do not degrade with the recycling process. Aluminum fiber composites showed improved mechanical, thermal and electrical properties than glass fiber reinforced composites.
Among the other fibers used, copper has been used in a number of applications as it is tough, ductile, malleable and has exceptionally high electrical and thermal conductivities.
Properties / Characteristics of Metallic Fiber:
Though fibers can be amorphous (glass), polycrystalline (carbon, boron, alumina, etc.) or single crystals (silicon carbide, alumina, beryllium and other whiskers), metallic fibers are mostly polycrystalline. In general, strength and stiffness properties of a fiber are significantly higher compared to the bulk material due to less crystal defects and higher orientation of crystallites along fiber length direction. The orientation of crystallites along the fiber direction also helps considerably in improving the strength properties. A whisker, being a single crystal, is not prone to crystal defects unlike polycrystalline fibers and provides very high strength and stiffness.
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Due to high electrical conductivity, metal fibers impart significant electrical conductivity at very low fiber concentrations.
Metal fiber dispersed composites appear to be light grey in color, hence molded articles cannot be colored with all colors. Mostly orange, beige and some other colors are possible.
Metal fibers have minimal adverse effect on mechanical properties of composites as compared to other fillers which are abrasive when used at high loading levels. Metallic fibers are several times finer than a human hair.
Tensile and thermal properties of metal fibers are very good; however, due to their high density the specific strength and modulus are lower as compared to high performance fibers like Kevlar and carbon fiber.
Properties of metallic fiber are point out below:
- Can be expensive (e.g. silver and gold)
- Lustrous and will tarnish
- Bright and appealing
- Often Heavy (e.g. silver and gold)
- High-Maintenance (aftercare can be problematic)
- The yarns are flexibility and some extensibility
- Good strength and hence can be used as warp or weft yarn
- All metallic yarns are moth proof
- Good chemical and biological resistance
- Laminated metallic threads:
- Resist bleach
- Resistant to vat dyeing
- Resistant to wet finishes on fabrics
- Drape well
- Machine washable
Typical properties of metal fibers include high density (steel: 8 g/cm3), high tensile strength (steel: 200–1600 MPa), high elongation at break, high E-modulus (steel: 210,000 MPa), high heat and electrical conductivity, resistance to corrosion and many chemicals, and flame resistance.
Manufacturing of Metallic Fiber:
The earliest production of metal fibers dates back to around 3000 BC when metal wires were used to decorate textiles. During the industrial revolution many metal-working businesses were established, and wire drawing developed from a craft into an industry. A thin endless metal object with a diameter below 100 μm is called ‘filament’; above that it is considered to be a ‘wire’. Apart from pure metals (such as copper), often alloys of different metals are processed (widespread for aluminium- and steel-based fibers).
There is a range of production processes available:
- Wire drawing (coarse fibers)
- Bundle drawing (medium to fine fibers)
- Cutting (staple fibers)
- Taylor (very fine fibers)
- Melt drag (coarse fibers)
Below Figure-2 depicts the principle design of a wire-drawing machine. It is one of the oldest production processes. The wire is pulled through subsequent drawing dies, which cause it to become thinner and longer. An annealing process stabilizes the inner structure of the drawn fibers.
In the Taylor process, a metal rod is molten inside a glass tube with a slightly higher melting point. The electric current in the coil heats up both materials, the wire melts, and the glass tube softens. Then the glass tube is drawn, causing the metal to become very thin. After cooling, the fine metal fibers are taken up whereas the glass tube is crushed, using ultrasound. fiber diameters can be lower than 50 μm, but production speed is low and the process can only be used for specific metals that do not fuse with the glass tube (Figure 3).
Metal fibers include fiber produced from pure metals, alloys, and metalloids using various mechanical or thermal methods. Furthermore, synthetic fiber materials and metal fibers can be specifically metalized to improve properties. The production of the metals, the manufacture of alloys, and metallization will not be included in the following.
The mechanical wire or metal fiber production is based on the conventional wire drawing and nozzle drawing process, and the subsequent bundle wire drawing method. In wire drawing, the wire is subjected to multiple drawing stages reducing the diameter (depending on the material) from 8 to 2 mm. Steel wire is then annealed at 600–900oC before being quenched. To achieve small filament diameters, conventionally drawn thin wires are embedded in a ductile and chemically more instable matrix (e.g. copper). This composite is subjected to a drawing process, tapering the total diameter. After chemically removing the matrix, metal multifilaments with very small diameters of 4–25 μm are left. By breaking these filaments into staple fibers of 50–150 mm in length and subsequent spinning, metal spun fiber yarns are produced. Other variations of mechanical metal fiber production are based on cutting, e.g. by so-called chasing or by vibrating cutter heads, resulting in fiber dimensions of 10–250 μm. However, these mechanical methods can only process certain, e.g. some iron- and copper-based materials.
Thermal methods (rapid solidification processes) are based on producing the metal fibers and wires by direction extrusion from the hotmelt phase, with a subsequent quenching process. The Taylor method (which is described before) achieves fiber diameters of 50 μm, melt spinning in rotating fluids creates fiber diameters of 50–500 μm. Especially brittle fibers like aluminum, copper, and zinc, or fibers made from aluminum and copper alloys, are produced by melt extraction.
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Depending on the production method, fiber cross-sections and surfaces differ. Fibers produced by cutting methods have a rough surface and a lower fineness related maximum tensile force, due to the notch effect.
The combination of certain metals exhibits the shape memory effect (SME) in their alloys. Metals with this property are referred to as shape memory alloys (SMA). These include nickel/titanium alloys, alloys made from copper, zinc, aluminum, or tin, as well as alloys made from copper, aluminum, and nickel. Regarding the memory effect, a distinction is made between thermal and mechanical shape memory. Applications of these materials in the composite field are promising.
Uses of Metallic Fiber:
Metallic fibers are mainly used for woven and knitted fabrics. Typical applications of metal fibers are filters (as in polymer melt spinning), antistatic applications (e.g., filters, protective clothing), reinforcement structures (tire cord), and also sensors (smart textiles) and architecture (metal fabric façade).
When suitable adhesives and films are used, they are not affected by salt water, chlorinated water in swimming pools or climatic conditions. If possible anything made with metallic fibers should be dry-cleaned. Ironing can be problematic because the heat from the iron, especially at high temperatures, can melt the fibers. They are used mainly for decorative purposes. Because of its brightness and appeal in apparels, the metal yarns are mainly used for decorative purposes in women’s dresses, blouses and skirts.
Most of these fibers are used to produce non-shielding items such as filter media, abradable seals; aircraft sound insulation, air bag filters and anti–static textiles; however, they are extensively used as composites for EMI shielding and structural applications.
Luxury goods made of real metals, ecclesiastical robes, evening wear, handbags, upholstery and cushions. Lurex fabrics are used in a wide range of garments, such as tops, scarves, jumpers and dresses. The use of metallic yarns is affected tremendously by the dictates of current fashion. There are very few items in which metallic threads have not appeared.
Advantages and Disadvantages of Metal Fiber:
As reinforcement, metal fibers have many advantages. They are easily produced using several fabrication processes and are more ductile, apart from being not too sensitive to surface damage and possess high strengths and temperature resistance.
However, their weight and the tendency to react each other through alloying mechanisms are major disadvantages. Ceramic fiber improve vastly in performance when a fine metal outline is incorporated with refractory ceramics by improving their thermal shock and impact resistance properties. Metal wires can also be used to reinforce polymers or plastics. Such combinations ensure high strength, light weight and good fatigue resistance. Continuous metal fibers can be easily handled. Better flexural properties are observed in some metal fiber reinforced plastic composites which also offer improved strength and weight than glass fibers. However, their poor tolerance to high temperature and the resultant steep variations in thermal expansion coefficient limit their application. Oxide fibers have high temperature tolerance, but they lack in ductility. Hence, metal fibers are often coated with ceramics (oxides and carbides) to overcome their limitations as reinforcements.
Other advantages of metal fibers are the similarity of their shrinkage to unfilled resins, excellent abrasion and corrosion resistance, minimal alteration of base resin properties (long life), and cost effectiveness.
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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.