5 Must-Have Features in a melt extracted stainless steel fiber

May. 13, 2024

Melt-Spun Fibers for Textile Applications - PMC

Textiles have a very long history, but they are far from becoming outdated. They gain new importance in technical applications, and man-made fibers are at the center of this ongoing innovation. The development of high-tech textiles relies on enhancements of fiber raw materials and processing techniques. Today, melt spinning of polymers is the most commonly used method for manufacturing commercial fibers, due to the simplicity of the production line, high spinning velocities, low production cost and environmental friendliness. Topics covered in this review are established and novel polymers, additives and processes used in melt spinning. In addition, fundamental questions regarding fiber morphologies, structure-property relationships, as well as flow and draw instabilities are addressed. Multicomponent melt-spinning, where several functionalities can be combined in one fiber, is also discussed. Finally, textile applications and melt-spun fiber specialties are presented, which emphasize how ongoing research efforts keep the high value of fibers and textiles alive.

Check now

In the last 80 years melt-spun fibers became by far the most important fibers for apparel, but even more so for technical textiles, where they spawned a myriad of novel applications. The aim of this review is to provide information about the current state of research and development regarding melt-spun fibers. The field has shown steady and continuous progress, and it is high time to summarize the technology from a contemporary point of view. This review provides insights in thermoplastic polymers as well as extrusion and bicomponent technology, with a strong focus on the main markets for melt-spun fibers.

Today, chemical fibers are spun by drawing a melt or solution of a polymer or an inorganic material from a spinneret into a medium (quenching or solvent removal by air/gas, water or coagulation bath) where it solidifies. Drawing can either be applied by godets (rollers) and winders, by a high-velocity air stream, or by an electrostatic or centrifugal force. lists fiber spinning methods used to produce filaments, staple fibers and nonwovens.

Man-made fibers have a long history. Robert Hooke first brought up the idea to create silk-like fibers in 1665, followed by René-Antoine Ferchault de Réaumur, who actually produced the first artificial filaments from different kinds of varnish in 1734 [ 1 ]. In 1883 Joseph Swan injected dissolved nitro-cellulose into a coagulation bath and thus obtained filaments for light bulbs [ 2 ]. In 1938 DuPont de Nemours (Wilmington, DE, USA) launched the production of Nylon ® (PA 6.6), the first commercial melt-spun fiber, invented by Wallace Carothers [ 3 ]. In the same year Paul Schlack developed Perlon ® (PA 6), a fiber declared vital to the war by Nazi Germany [ 4 ]. The first polyester fiber, Terylene ® (PET), was created in 1941 by Imperial Chemical Industries (ICI) [ 5 ]. The commercial production of polyolefin fibers started in 1957, based on the Ziegler-Natta catalyst recognized by a Nobel Prize in 1963 [ 6 ].

2. Raw Materials for Melt-Spinning

2.1. Polymers and Their Spinnability

Most commonly used materials for melt-spinning are polyamides, polyesters and (linear) polyolefins [7]. lists a selection of polymers used for fiber melt-spinning, together with some relevant properties. Basic requirement for melt-spinning is that the polymer becomes fusible below its degradation temperature. Note that the maximum allowed extrusion temperature can fall well below the decomposition temperature Td quoted in , while some polymers have their melt-processing window near Td. In addition, such thermoplastic polymers should ideally have the following properties to ease processability and to yield sufficient fiber properties [8,9,10]:

Table 2

PolymerDensity [g/cm3]Tg [°C]Tm [°C]Td [°C]TPResChRARUVFRPA 61.1450225387+++++++++PA 6.61.1450260407+++++++++PET1.3975260402+++++++PBT1.3350220373++++++++PLA1.2560165321+++-++PP0.91−15170399++-+++--LDPE0.92−125110440+-++---HDPE0.95−125130436++-+++--PVDF1.78−40170431-++++-++++PEEK1.32145335569++++++++++++PPS1.3485285494++++++++-++PEI1.27215-515++++++++++++PMMA1.18110-334-+-+++-PC1.20150-471-+--+++Open in a separate window

The polydispersity of commercially available polymers ranges from two to 12 or more; as a rule of thumb, the polydispersity should not exceed three for stable melt-spinning [11]. Temperature, moisture, air humidity, residence time and shear forces significantly promote molecular weight degradation during extrusion and spinning. Local shear heating may increase the spinning temperature by as much as 10–15 °C [12].

Moisture can strongly influence processability and cause degradation of polymers in extrusion, thus drying of polymers is very important before extrusion [8]. This is especially true for polyesters like PET, PBT or PLA, which can suffer considerable loss in molecular weight by hydrolytic degradation (hydrolysis) of the melt in presence of water [13]. For water removal, either a batch procedure in fluidized bed or vacuum (tumble) dryers can be applied, or continuous drying in the feed hopper with desiccated air or nitrogen at normal pressure. However, over-drying polyamide by more than two orders of magnitude below the equilibrium moisture content can negatively influence extrusion processing, because moisture impacts the chemical equilibrium of the polycondensate and acts as a plasticizer for polyamide [14]. Non-hygroscopic polymers like polyolefins usually need not to be dried before processing. However, even hydrophobic fluoropolymers like PVDF should be dried to remove surface moisture, which otherwise could dissolve hydrogen fluoride (HF) monomers to form the highly corrosive and toxic hydrofluoric acid.

Polymers for man-made fibers can not only contain residual water, but also dissolved and dispersed gases, as well as volatile liquids and solids (e.g., unreacted monomers, reaction by-products) that boil at processing temperatures [15]. During extrusion of the polymer, these substances are kept in the melt by hydrostatic pressure. As their solubility decreases with the pressure drop at the die exit, gas bubbles and/or a pitted surface can evolve in the melt strand, which impair fiber quality or hinder spinnability [16]. Volatiles evaporating from the spinneret must be removed by an exhaust, both to protect operators and to avoid agglomeration at the die exit.

By far most of the man-made fibers are spun from semi-crystalline polymers. The crystalline structure stabilizes the highly orientated molecular chains, which otherwise tend to recoil above Tg, resulting in pronounced fiber shrinkage [17]. In consequence, mainly amorphous polymers with high Tg, like PEI and PC, are used for fiber melt-spinning ( ).

2.2. Polyamides

Globally, PA 6 and PA 6.6 are by far the most used polyamides that are also significant for large-scale production of melt-spun fibers [29]. PA 6.6 is produced by the condensation reaction of hexamethylenediamine and adipic acid, while PA 6 is synthesized by ring-opening polymerization of ε-caprolactam [30]. Both fiber types exhibit similar properties, i.e., outstanding wear and abrasion resistance, high tenacity and toughness, excellent fatigue behavior and good resilience ( ); slight dissimilarities mainly stem from differences in molecular weight distribution and draw-induced molecular orientation [18]. For industrial applications, fibers are drawn with DR 4–5 to achieve high mechanical performance, while DR 2-2.5 is applied for apparel applications to achieve high uniformity in dye diffusion [18]. Other well-tried fiber-forming aliphatic polyamides are PA 11 (Tm~185 °C), PA 12 (Tm~180 °C), PA 6.12 (Tm~210 °C), PA 6.10 (Tm~215 °C), PA 4 (Tm~260 °C), PA 4.6 (Tm~295 °C). In the nomenclature PA x.y, x and y represent the respective number of carbon atoms in the diamine and diacid monomer, respectively [31].

PA 5.6, which can be synthesized by direct polycondensation of 1,5-pentamethylenediamine obtained from L-lysine and adipic acid, is a biobased alternative to PA 6 and PA 6.6, with a high potential in fiber applications [32]. With Tm~250 °C, PA 5.6 shows thermal properties and a heat resistance comparable to the commercially available PA 6 [33]. The melt-spinning of PA 5.6 in the form of segmented pie bicomponent fibers has been reported in combination with PET [34].

Some partially aromatic polyamides like polyphthalamides (e.g., PA 6T, Tm~325 °C, and PA 9T, Tm~300 °C) or PA MXD6 (Tm~237 °C, produced from m-xylenediamine and adipic acid) can also be melt-spun; respective filaments are commercially available but scarce [35,36]. Aromatic polyamides (aramids), on the other hand, cannot be melt-processed, since their melting temperature exceeds the degradation temperature; aramid fibers are produced by solution spinning.

2.3. Polyesters

PET is the predominant polyester used for fiber production, not only because of its good end-use properties and economy of production but in particular because of the ease of physical and chemical modification, suppressing negative and enhancing positive properties of PET [37]. Due to its relatively high glass transition temperature (Tg~75 °C), as-spun PET forms a stable, supercooled melt with molecular orientation in fiber direction, which develops oriented crystallites only when fully drawn [19]. Their excellent properties ( ) are responsible for polyester fibers and filaments finding use in all fields of fiber application [19]. To obtain higher molecular weight PET for improved performance, solid phase polymerization is applied below Tm, where pre-crystallized chips are heated in a stream of hot inert gas or agitated in a vacuum drier to remove small traces of volatiles [37].

Other commercially viable polyesters suitable for fiber production are PBT and PTT, which exceed PET in crystallization rate, resilience, elasticity and dyeability (they can be dyed at 100 °C, while PET requires 130 °C) [22,37]. 1,3-propanediol, the crucial substrate to polymerize PTT (Tm~230 °C), can either be derived petrochemically, or by enzymatic fermentation of renewable resources [38]. PEN, the last of this melt-spinnable semi-aromatic polyester family, has higher melt temperature (Tm~270 °C), tensile modulus, chemical and UV-resistance than PET and as such is beneficial for industrial fibers [22,39]. Respective high performance melt-spun fibers are commercially available but scarce [40].

2.4. Polyolefins

The most prominent polyolefins used for melt-spinning are PP, LDPE and HDPE, consisting essentially of saturated aliphatic hydrocarbon macromolecules [41]. Technologies to convert polyolefins into fibers and fabrics include monofilament and multifilament spinning, staple fiber, spunbond, melt blown, and slit film [20]. Polyolefin-based spunbond and melt blown fabrics are the material of choice for disposable hygiene and medical applications like diapers, incontinence pants, sanitary napkins, surgical gowns and masks [42]. Polyolefin filaments, being polymeric hydrocarbons, possess luster and a waxy handle, which can be reduced by non-circular fiber cross-sections like triangular or cross-shaped [41]. The main properties and characteristics of polyolefin fibers are summarized in . Worth mentioning is their lightness (density below 1 g/cm3), poor dyeability and adhesion, as well as low resilience and high tendency to creep [20]. UHMWPE yields fibers with extraordinary tensile properties, but its very high molar mass hinders melt-spinning; the polymer needs to be gelled in a solvent before being extruded through a spinneret (gel-spinning) [43]. Toyobo (Manufacturer, Tokyo, Japan) introduced a melt-spun high-strength polyethylene fiber under the brand name Tsunooga® [44].

well supply professional and honest service.

2.5. Chemically Inert Polymers

Filaments from chemically resistant polymers like fluoropolymers, polyetherketones, polysulfides and polyetherimide find applications in e.g., hot medium filters or protective textiles [45]. The best-known fluoropolymer is PTFE (Tm~330 °C), but its high molecular weight hinders flowability to an extent that it cannot be melt-spun [46]. Respective fibers are produced by paste extrusion, where PTFE powder is mixed with a lubricant and transformed into film to be calendered, slit, sintered and stretched [45]. A melt-processable PTFE material (Tm~315 °C), comprising small amounts of perfluoropropylvinylether, was launched in 2006 under the brand name Moldflon® (ElringKlinger Kunststofftechnik, Bietigheim-Bissingen, Germany) [47]. PVDF (Tm~170 °C), PVF (Tm~200 °C), and co-polymers of tetrafluoroethylene with e.g., hexafluoropropylene, have lower melting points and can be melt-spun to filaments with good tensile properties and high chemical resistance [24,27].

The melt-spinnable PEEK (Tm~335 °C) is the foremost member of the aromatic thermoplastic polyetherketones [24]. The advantage of PEEK fibers is their ability to operate in extreme conditions (high temperature, chemical impact and abrasion) over long lifetimes [45]. PPS (Tm~285 °C) is inherently flame-resistant, has outstanding high-temperature stability and oil and solvent resistance (no known solvent below 200 °C) [15,26]. PEI (Tg~215 °C) is an amorphous polymer and can be melt-spun into fibers which are resistant against specific chemicals, have a lower strength and melting point than PEEK or PPS and are more extensible [24].

LCPs, characterized by a highly ordered fluid state, are resistant to virtually all chemicals [48]. The principal monomer in all commercial thermotropic LCPs is hydroxybenzoic acid [49]. By subjecting a LCP to shear and extension forces via melt-spinning, the molecular chains become highly oriented without post-drawing [50]. Subsequent heat-treatment (annealing) for solid phase polymerization under reduced pressure results in filaments with superior tensile properties [51,52]. The first commercially available, melt-spun LCP fiber was introduced in 1990 under the brand name Vectran®, now manufactured by Kuraray (Osaka, Japan) [53].

2.6. Thermoplastic Elastomers

Elastomeric fibers, produced from polymers with thermoreversible physical cross-links, are characterized by high elastic recovery (up to 99%) and high extensibility (up to 500%) [54,55]. Their elasticity performance mainly stems from the combination of soft and hard segments of the polymer structure [56]. Most conventional elastomeric fibers are produced by dry spinning of PU, but melt-spun products are available; although they often lack in yarn uniformity and recovery, they are of interest due to ecological and economic advantages [57]. In filaments, melt-spun from TPU, their outstanding elasticity originates from the chemical composition of polyols (soft segments) and isocyanates (hard segments) [58]. PEE, a multiblock copolymer comprising segments of semicrystalline polyester (hard, mostly PBT) and noncrystalline polyether (soft), is a low-cost melt-spinnable polymer with adequate characteristics as thermoplastic elastomer [54,59]. TPOs are melt-spun to produce elastic filaments that are chemically resistant [56,60]. Elastic recovery, heat-resistance and long-term stability of thermoplastic elastomers can be achieved by subsequent covalent crosslinking [59,61].

2.7. Amorphous Polymers

The morphology of solid-state polymers typically fluctuates continuously between ideal crystalline and fully amorphous states, where “amorphous” is widely used in polymer science to mean non-crystalline [62,63]. Polymers with irregular molecular structures cannot crystallize under any condition, so the only important morphological feature that can be changed through processing is molecular orientation [64]. X-ray diffraction studies have confirmed the presence of oriented non-crystalline domains in filaments, melt-spun from fully amorphous polymers, indicating that the degree of orientation is directly proportional to the fiber draw ratio [65,66].

Fully amorphous polymers tend to be transparent, in contrast to semicrystalline polymers, which typically are translucent or opaque due to a usually heterogeneous crystalline structure that leads to refractive index inhomogeneities and thus light scattering at the interfaces between crystalline and amorphous regions [67,68]. Most prominent examples of polymers used for melt-spinning transparent fibers are PMMA (Tg~110 °C), PC (Tg~150 °C) and PS (Tg~95 °C) [26,69]. The application temperature of respective fibers is below Tg, since their oriented macromolecules tend to recoil above Tg, resulting in strong shrinkage of the fibers.

2.8. Biopolymers

The term “biopolymer” generally refers to biobased (produced from biogenic substances which are considered renewable resources), but is every so often used for biodegradable (degradable by biological means), biocompatible (no adverse effect on humans or animals) or bioresorbable polymers (dissolved or absorbed in the body). The main biopolymers considered for fiber melt-spinning are PLA, PCL, PGA, PBAT, PEF and PHAs.

PLA is produced from lactic acid, whose raw material is naturally occurring starch, which is usually extracted from corn [23]. Fiber grade PLA, mostly consisting of L lactic acid (LLA) containing less or equal to 8% D lactic acid (DLA), is commercially one of the most promising bio-based, biodegradable and biocompatible polymers (PLA with a D-isomer level exceeding 8% does not crystallize) [21,23,70]. The biodegradability of PLA in the natural environment is lower than that of other biopolymers, since it is less susceptible to microbial attack [71]. Its main drawback regarding melt-processing is the low thermal stability in the presence of moisture (hydrolysis) [72]. However, mechanical performance and thermal resistance can be enhanced by adjusted LLA/DLA mixing and adequate spinning parameters to obtain stereo-complex crystals with strong interaction between LLA and DLA sequences [28,73,74,75].

PCL (Tm~60 °C) is a petroleum-based biodegradable and biocompatible aliphatic polyester with good mechanical properties, consisting of a sequence of methylene units with in-between ester groups [76]. Its slow biodegradation rate in the human body make PCL suitable for implantable long-term drug delivery systems [77]. PCL is highly miscible and combines well with other polymers, and thus has been investigated as a polymer blend component or copolymer for various applications [78,79,80,81]. PCL in the form of filaments is also of interest for technical textiles, but its low melting point limits the application and thus only a few studies discuss the conventional melt-spinning of PCL homo-component fibers [82,83,84,85].

PGA (Tm~225 °C), a highly biocompatible and biodegradable petroleum derived aliphatic polyester of simple molecular structure, can be melt-spun in fibers with good mechanical properties [77,86]. Since the product placement of the first man-made absorbable suture named Dexon® in 1972, PGA and its copolymers dominate the biodegradable suture market [87,88]. However, the narrow processing window makes it difficult to spin high-strength fibers under ordinary industrial conditions, while laboratory trials with modifications in the spinning line led to fibers of high tensile strength and toughness [89,90].

PBAT (Tm~120 °C) is an aliphatic-aromatic copolyester, which degrades within a few weeks with the aid of naturally occurring enzymes [91]. Considering both tensile and biodegradation properties, a random copolymer with 44 mol% of polybutylene terephthalate was found to be ideal [92]. PBAT shows good melt-spinnability, and the copolymer’s hard (aromatic) and soft (aliphatic) monomer segments yield elastic fibers with low modulus and high resilience [93]. The copolyester is also considered a good candidate for the enhancement of PLA [91].

PEF (Tm~210 °C) is produced by polycondensation of ethylene glycol and furan dicarboxylic acid (as such chemically analogous to PET), which can be derived from plant-based resources [94]. Despite its slow crystallization and low Tm, PEF has recently gained attention as a potential bio-based replacement for PET [95]. In analogy to PET, solid phase polymerization is the key to attain high molecular weight and thus suitability for engineering applications [95]. Own unpublished data show that PEF can be melt-spun into fibers with properties similar to PET fibers.

PHAs are produced by bacteria for intracellular carbon and energy storage [96,97,98,99,100]. PHAs, either in the form of homopolymers or copolymers of various hydroxyalkanoic acids, are thermoplastic, biodegradable, biocompatible and nontoxic [101,102,103]. The bacteria-synthesized, perfectly linear and isotactic polymer chains promise superior properties [104,105,106]. However, their rapid thermal degradation at temperatures just above the melting temperature, variations in quality and molecular weight, and lack of purity commonly resulting from the biotechnical production process, hamper melt-spinning of virgin PHAs [107]. Melt-spinning trials have been reported for P3HB (Tm~180 °C) [107,108,109,110,111,112], PHBV (Tm~170 °C) [113,114,115] and PHBH (Tm~145 °C) [116,117,118,119]. In 2007, Tepha (Lexington, KY, USA) launched P4HB (Tm~60 °C) mono- and multifilaments for medical applications [120,121].

melt extracted ss stainless steel fiber

Protections for this product

Delivery via

Expect your order to be delivered before scheduled dates or receive a 10% delay compensation

Secure payments

Every payment you make on Alibaba.com is secured with strict SSL encryption and PCI DSS data protection protocols

Refund policy

Claim a refund if your order doesn't ship, is missing, or arrives with product issues

Want more information on melt extracted stainless steel fiber? Feel free to contact us.

Comments

0/2000

All Comments (0)

Previous: How Does Pearl Necklaces Work?

Next: Can stainless steel be melted down?

Guest Posts

If you are interested in sending in a Guest Blogger Submission,welcome to write for us!