2 Natural and synthetic fibres for composite nonwovens S.MUKHOPADHYAY,Indian Institute of Technology Delhi,India D01:10.1533/9780857097750.20 Abstract:This chapter reviews the use of natural and synthetic fibres in composite nonwovens.It begins by reviewing the use of natural fibres including the most widely used:cotton,jute,kenaf and flax.Most nonwoven fibres are still,however,synthetic.The rest of the chapter discusses the use of polypropylene,polyester,polyethylene and nylon.The chapter concludes by reviewing the use of bicomponent fibres to combine the properties of different fibre types in maximising the functionality of nonwovens. Key words:natural fibres,synthetic,bicomponent. 2.1 Introduction This chapter reviews the use of natural and synthetic fibres in composite nonwovens.It begins by reviewing the use of natural fibres including the most widely used:cotton,jute,kenaf and flax.Most nonwoven fibres are still,however, synthetic.The rest of the chapter discusses the use of polypropylene,polyester, polyethylene and nylon.The chapter concludes by reviewing the use of bicomponent fibres to combine the properties of different fibre types in maximising the functionality of nonwovens. 2.2 Natural and biodegradable fibres for nonwovens The environmental impact of disposable products such as baby diapers,adult incontinence and feminine hygiene products has meant that there is growing interest in the use of natural or biodegradable fibres.Natural fibres for nonwovens include cotton,jute,kenaf and flax as well as smaller quantities of hemp,coir, sisal,milkweed,wood and some animal fibres.Synthetic biodegradable fibres that have also been used for nonwoven applications include: regenerated cellulosic fibres such as cellulose acetate,rayon and lyocell: synthetic fibres fibres such as polylactic acid (PLA),poly(caprolactone) (PCL),poly(hydroxybutyrate)(PHB),poly(hydroxybutyrate-co-valerate) (PHBV),polytetramethylene adipate-co-terephthalate(PTAT)and poly(vinyl acetate)(PVA). Among natural fibres,cotton is most widely used for nonwoven applications? and it has been estimated that cotton accounts for 8%of the global market in 20 2014 Woodhead Publishing Limited
20 © 2014 Woodhead Publishing Limited 2 Natural and synthetic fi bres for composite nonwovens S. MUKHOPADHYAY, Indian Institute of Technology Delhi, India DOI: 10.1533/9780857097750.20 Abstract: This chapter reviews the use of natural and synthetic fi bres in composite nonwovens. It begins by reviewing the use of natural fi bres including the most widely used: cotton, jute, kenaf and fl ax. Most nonwoven fi bres are still, however, synthetic. The rest of the chapter discusses the use of polypropylene, polyester, polyethylene and nylon. The chapter concludes by reviewing the use of bicomponent fi bres to combine the properties of different fi bre types in maximising the functionality of nonwovens. Key words: natural fi bres, synthetic, bicomponent. 2.1Introduction This chapter reviews the use of natural and synthetic fi bres in composite nonwovens. It begins by reviewing the use of natural fi bres including the most widely used: cotton, jute, kenaf and fl ax. Most nonwoven fi bres are still, however, synthetic. The rest of the chapter discusses the use of polypropylene, polyester, polyethylene and nylon. The chapter concludes by reviewing the use of bicomponent fi bres to combine the properties of different fi bre types in maximising the functionality of nonwovens. 2.2 Natural and biodegradable fi bres for nonwovens The environmental impact of disposable products such as baby diapers, adult incontinence and feminine hygiene products has meant that there is growing interest in the use of natural or biodegradable fi bres. Natural fi bres for nonwovens include cotton, jute, kenaf and fl ax as well as smaller quantities of hemp, coir, sisal, milkweed, wood and some animal fi bres. Synthetic biodegradable fi bres that have also been used for nonwoven applications include: 1 • regenerated cellulosic fi bres such as cellulose acetate, rayon and lyocell; • synthetic fi bres fi bres such as polylactic acid (PLA), poly(caprolactone) (PCL), poly(hydroxybutyrate) (PHB), poly(hydroxybutyrate- co-valerate) (PHBV), polytetramethylene adipate- co-terephthalate (PTAT) and poly(vinyl acetate) (PVA). Among natural fi bres, cotton is most widely used for nonwoven applications 2 and it has been estimated that cotton accounts for 8% of the global market in
Natural and synthetic fibres 21 nonwoven fibre products.'The advantages of cotton include biodegradability, superior wet strength and a quick drying surface which is particularly useful for wipes.Bleached cotton fibres are used because of their superior appearance and high levels of absorbency as well as being soft to the touch and breathable. Uses of cotton nonwovens include disposable products such as swabs,wipes, filters,wadding,personal care products such as babies'diapers and feminine hygiene products as well as semi-durable products such as bedding,pillow fillers and household furnishings.An area of recent growth is spunlaced cotton fibres for cosmetic wipes.Typical production techniques involve web formation using one of three techniques: ·dry laid process 。wet laid process polymer laid process(involving spun laid and melt blown web formation). Bonding is achieved through needle punching,hydroentangling,stitchbonding and chemical or thermal bonding techniques.Cotton has been combined into bicomponent fibres for nonwoven applications using fibres such as lyocell and cellulose acetate.Biocomponent fibre technology is discussed later in this chapter. Useful properties of jute fibres for nonwoven applications include high strength,modulus and dimensional stability as well as good moisture absorption and breathability.3 Jute fibres are typically pretreated to improve properties such as crimp to aid further processing.Manufacturing techniques for jute include stitch bonding,hot calandering,needle punching,thermal bonding and hydroentanglement.Needle punching is particularly suitable for developing nonwovens with properties such as good acoustic insulation.Applications of jute nonwovens include floor coverings (where they provide thermal and acoustic insulation),filters,composites and geotextiles.A particularly important application of jute nonwovens is automotive interiors (e.g.door,floor and boot lining materials)due to the combination of light weight,durability,low flammability and other mechanical properties such as drapability,tensile strength and impact performance. Kenaf fibres are typically pretreated to reduce stiffness,while they may be blended with cotton,polyester and polypropylene.'Nonwoven mats are typically formed by the air laid process followed by needle punching and thermal bonding. Kenaf nonwoven mats are used in composites and geotextile applications as well as automotive interiors.*The high cellulose content and length of flax fibres also makes them a good substitute for synthetic fibres in disposable nonwovens. Fibres are prepared by pulping and bleaching.They can then be processed using wet laying followed by hydroentangling or spunlacing.The wet laid process can result in poor fibre formation in the web which can be corrected through good control of fibre length and the use of appropriate fibre finishes.Dry laid jute nonwovens have been shown to have more consistent properties.Applications of jute nonwovens include filters.10
Natural and synthetic fi bres 21 nonwoven fi bre products. 3 The advantages of cotton include biodegradability, superior wet strength and a quick drying surface which is particularly useful for wipes. Bleached cotton fi bres are used because of their superior appearance and high levels of absorbency as well as being soft to the touch and breathable. 4 Uses of cotton nonwovens include disposable products such as swabs, wipes, fi lters, wadding, personal care products such as babies’ diapers and feminine hygiene products as well as semi- durable products such as bedding, pillow fi llers and household furnishings. An area of recent growth is spunlaced cotton fi bres for cosmetic wipes. Typical production techniques involve web formation using one of three techniques: • dry laid process • wet laid process • polymer laid process (involving spun laid and melt blown web formation). Bonding is achieved through needle punching, hydroentangling, stitchbonding and chemical or thermal bonding techniques. Cotton has been combined into bicomponent fi bres for nonwoven applications using fi bres such as lyocell and cellulose acetate. Biocomponent fi bre technology is discussed later in this chapter. Useful properties of jute fi bres for nonwoven applications include high strength, modulus and dimensional stability as well as good moisture absorption and breathability. 5 Jute fi bres are typically pretreated to improve properties such as crimp to aid further processing. Manufacturing techniques for jute include stitch bonding, hot calandering, needle punching, thermal bonding and hydroentanglement. Needle punching is particularly suitable for developing nonwovens with properties such as good acoustic insulation. Applications of jute nonwovens include fl oor coverings (where they provide thermal and acoustic insulation), fi lters, composites and geotextiles. A particularly important application of jute nonwovens is automotive interiors (e.g. door, fl oor and boot lining materials) due to the combination of light weight, durability, low fl ammability and other mechanical properties such as drapability, tensile strength and impact performance. Kenaf fi bres are typically pretreated to reduce stiffness, 6 while they may be blended with cotton, polyester and polypropylene. 7 Nonwoven mats are typically formed by the air laid process followed by needle punching and thermal bonding. Kenaf nonwoven mats are used in composites and geotextile applications as well as automotive interiors. 8 The high cellulose content and length of fl ax fi bres also makes them a good substitute for synthetic fi bres in disposable nonwovens. 9 Fibres are prepared by pulping and bleaching. They can then be processed using wet laying followed by hydroentangling or spunlacing. The wet laid process can result in poor fi bre formation in the web which can be corrected through good control of fi bre length and the use of appropriate fi bre fi nishes. Dry laid jute nonwovens have been shown to have more consistent properties. Applications of jute nonwovens include fi lters. 10
22 Composite Nonwoven Materials 2.3 Polypropylene and polyester fibres Polypropylene fibres account for 60%of all the fibres used for nonwoven production.They come under the general category of polyolefins.Filaments are produced from the polymer by melt-spinning.Generally,the polypropylene used in filaments is of the iostactic variety while 65%of polypropylene fibre nonwovens are used for hygiene applications.Its oleophilic nature makes the nonwoven fabric efficient in absorbing and retaining oil from oil-water mixtures.Polypropylene fibres have the advantages of being lightweight,dry (due to low moisture absorption)with good strength,good abrasion resistance,high modulus and good resistance to deterioration from chemicals and microorganisms as well as having a soft feel.Wettability can be improved by various treatments such as plasma treatment.Polypropylene has a major limitation in terms of its thermal properties -its low glass transition temperature and melting point restricts some potential applications. Polyester is the second most widely used fibre next to polypropylene in nonwoven production.Polyester is used in quilted fabrics,bedspreads and other home furnishings.The term polyester generally refers to polyethylene terephthalate or PET.There are also other polyester fibres available such as polybutylene terephthalate (PBT)and polytrimethylene terephthalate (PTT) However PET is still the most widely used polyester fibre.Polyester is mainly produced by melt spinning.Fibres are relatively strong and have between 60% and 85%crystallinity.The high crystallinity of polyester and the presence of the aromatic ring in the fibre structure makes fibres low in water absorption and resistance to acids and alkalis.The diameter of polyester fibres most widely used to produce nonwovens and composite nonwovens varies from 0.38 to 50 um and a cut length of 0.9 to 150mm. The advantages of polyester fibres include high strength,high modulus, high toughness,good abrasion resistance,good resilience,very low moisture absorbency,high melting temperature and heat distortion temperature,resistance to hazardous chemicals,oxygen barrier,inertness and biocompatibility.They can be processed using any of the main methods of nonwoven manufacture.The wettability of polyester fibres can be improved by plasma treatment. In a study on geotextiles by Abdelmalek et al.the in-plane and cross-plane water-retention characteristics of polyester geotextiles was studied.The materials used in the investigation were continuous filaments nonwoven needle punched polyester geotextiles varying in their apparent opening size.It was found that cross-plane water-retention data demonstrated their hydrophobic nature,with both specimens being essentially non-conductive to water beyond suction heads of 0.2-0.3kPa.Both specimens exhibited significant hysteresis in their water- retention function,such that at a given suction a geotextile contained more water when drying than when wetting
22 Composite Nonwoven Materials 2.3 Polypropylene and polyester fi bres Polypropylene fi bres account for 60% of all the fi bres used for nonwoven production. They come under the general category of polyolefi ns. Filaments are produced from the polymer by melt- spinning. Generally, the polypropylene used in fi laments is of the iostactic variety while 65% of polypropylene fi bre nonwovens are used for hygiene applications. Its oleophilic nature makes the nonwoven fabric effi cient in absorbing and retaining oil from oil–water mixtures. Polypropylene fi bres have the advantages of being lightweight, dry (due to low moisture absorption) with good strength, good abrasion resistance, high modulus and good resistance to deterioration from chemicals and microorganisms as well as having a soft feel. Wettability can be improved by various treatments such as plasma treatment. Polypropylene has a major limitation in terms of its thermal properties – its low glass transition temperature and melting point restricts some potential applications. Polyester is the second most widely used fi bre next to polypropylene in nonwoven production. Polyester is used in quilted fabrics, bedspreads and other home furnishings. The term polyester generally refers to polyethylene terephthalate or PET. There are also other polyester fi bres available such as polybutylene terephthalate (PBT) and polytrimethylene terephthalate (PTT). However PET is still the most widely used polyester fi bre. Polyester is mainly produced by melt spinning. Fibres are relatively strong and have between 60% and 85% crystallinity. The high crystallinity of polyester and the presence of the aromatic ring in the fi bre structure makes fi bres low in water absorption and resistance to acids and alkalis. The diameter of polyester fi bres most widely used to produce nonwovens and composite nonwovens varies from 0.38 to 50 μ m and a cut length of 0.9 to 150 mm. The advantages of polyester fi bres include high strength, high modulus, high toughness, good abrasion resistance, good resilience, very low moisture absorbency, high melting temperature and heat distortion temperature, resistance to hazardous chemicals, oxygen barrier, inertness and biocompatibility. They can be processed using any of the main methods of nonwoven manufacture. The wettability of polyester fi bres can be improved by plasma treatment. 11 In a study on geotextiles by Abdelmalek et al.12 the in- plane and cross- plane water- retention characteristics of polyester geotextiles was studied. The materials used in the investigation were continuous fi laments nonwoven needle punched polyester geotextiles varying in their apparent opening size. It was found that cross- plane water- retention data demonstrated their hydrophobic nature, with both specimens being essentially non- conductive to water beyond suction heads of 0.2–0.3 kPa. Both specimens exhibited signifi cant hysteresis in their waterretention function, such that at a given suction a geotextile contained more water when drying than when wetting
Natural and synthetic fibres 23 2.4 Polyethylene and nylon fibres Polyethylene(PE)is the second type of polyolefin which is also used in nonwoven applications.Depending on the kind of structure,they are classified as HDPE (high density polyethylene),LDPE (low density polyethylene)or LLDPE(linear low density polyethylene).There has been limited research on polyethylene nonwovens,but the pre-irradiation induced emulsion graft polymerisation method has been used to introduce acrylonitrile onto PE nonwoven fabrics.3 In another study,Ma et al.4 studied impregnation of nonwoven HDPE fibrous samples with a nonionic surfactant (N,N-dimethyldodecylamine N-oxide)using supercritical carbon dioxide as the solvent.The results showed that although the supercritical fluid(SCF)was absorbed into the polymer,no structural changes or loss of mechanical strength was observed.The wetting properties of this highly hydrophobic material were significantly improved. Nylon is a polyamide.The most common types are nylon 6 and nylon 66. Nylon 6 is polymerised from caprolactam whereas nylon 66 is produced from hexamethylene diamine with adipic acid as the starting material.Both polymers are meltspun to form continuous filaments.The most commonly used nylon fibres have a diameter of 15 to 25 um.Nylon fibres have low heat resistance and show poor conduction of heat.Their advantages in practical terms include high durability,relatively high glass transition and melting temperatures,high tensile and tear strengths,good elastic recovery and low static electric charge generation. Disadvantages include poor resistance to exposure to light and poor wet strength. The physical properties of nylon 6 nonwoven mats produced from solutions with formic acid have been studied.'5 Nonwoven electrospun mats from various solutions with different concentrations were examined regarding their morphol- ogy,pore size,surface area and gas transport properties.Each nonwoven material with average fibre diameters from 90 to 500nm was prepared under controlled electrospinning parameters.From the results,the authors observed that fibre diameter was strongly affected by the polymer concentration(polymer viscosity). In addition,the results showed that the pore size and gas transport property of electrospun nylon 6 nonwoven mats were affected by fibre diameter. Saraf et al.created a superhydrophobic/superoleophobic surface by preparing a metastable Cassie-Baxter(CB)surface.To create a CB surface it was essential to have low surface energy and properly constructed surface morphology.The researchers explored three different techniques to achieve superhydrophobicity and superoleophobicity using hydroentangled nylon nonwoven fabric:pulsed plasma polymerisation of 1H,1H,2H,2H-perfluorodecyl acrylate (PFAC8). microwave-assisted condensation of IH,1H,2H,2H-perfluorodecyltrimethoxysi- lane (FS),and FS condensation through wet processing.Nonwoven fabric materials prepared using these three techniques were superhydrophobic and superoleophobic as shown by their very high contact angles for both water(contact angles of 168-174)and dodecane(contact angles of 153-160)
Natural and synthetic fi bres 23 2.4 Polyethylene and nylon fi bres Polyethylene (PE) is the second type of polyolefi n which is also used in nonwoven applications. Depending on the kind of structure, they are classifi ed as HDPE (high density polyethylene), LDPE (low density polyethylene) or LLDPE (linear low density polyethylene). There has been limited research on polyethylene nonwovens, but the pre- irradiation induced emulsion graft polymerisation method has been used to introduce acrylonitrile onto PE nonwoven fabrics. 13 In another study, Ma et al.14 studied impregnation of nonwoven HDPE fi brous samples with a nonionic surfactant (N,N-dimethyldodecylamine N-oxide) using supercritical carbon dioxide as the solvent. The results showed that although the supercritical fl uid (SCF) was absorbed into the polymer, no structural changes or loss of mechanical strength was observed. The wetting properties of this highly hydrophobic material were signifi cantly improved. Nylon is a polyamide. The most common types are nylon 6 and nylon 66. Nylon 6 is polymerised from caprolactam whereas nylon 66 is produced from hexamethylene diamine with adipic acid as the starting material. Both polymers are meltspun to form continuous fi laments. The most commonly used nylon fi bres have a diameter of 15 to 25 μ m. Nylon fi bres have low heat resistance and show poor conduction of heat. Their advantages in practical terms include high durability, relatively high glass transition and melting temperatures, high tensile and tear strengths, good elastic recovery and low static electric charge generation. Disadvantages include poor resistance to exposure to light and poor wet strength. The physical properties of nylon 6 nonwoven mats produced from solutions with formic acid have been studied. 15 Nonwoven electrospun mats from various solutions with different concentrations were examined regarding their morpho logy, pore size, surface area and gas transport properties. Each nonwoven material with average fi bre diameters from 90 to 500 nm was prepared under controlled electrospinning parameters. From the results, the authors observed that fi bre diameter was strongly affected by the polymer concentration (polymer viscosity). In addition, the results showed that the pore size and gas transport property of electrospun nylon 6 nonwoven mats were affected by fi bre diameter. Saraf et al.16 created a superhydrophobic/superoleophobic surface by preparing a metastable Cassie–Baxter (CB) surface. To create a CB surface it was essential to have low surface energy and properly constructed surface morphology. The researchers explored three different techniques to achieve superhydrophobicity and superoleophobicity using hydroentangled nylon nonwoven fabric: pulsed plasma polymerisation of 1H,1H,2H,2H-perfl uorodecyl acrylate (PFAC8), microwave- assisted condensation of 1H,1H,2H,2H-perfl uorodecyltrimethoxysilane (FS), and FS condensation through wet processing. Nonwoven fabric materials prepared using these three techniques were superhydrophobic and superoleophobic as shown by their very high contact angles for both water (contact angles of 168–174°) and dodecane (contact angles of 153–160°)
24 Composite Nonwoven Materials 2.5 Bicomponent fibres The objective of spinning bicomponent fibres is to overcome the limitations of conventional single component spinning.The principal purpose of blending polymers to produce bicomponent fibres is to achieve improved processing and functional properties for a specific end use. Bicomponent fibres can be mainly classified into three types: Side by side (S/S)-where each polymer is divided along the length into two or more distinct regions of the cross-section. .Core sheath type (C/S)-as the name suggests,in this type of fibre one of the components is completely surrounded by another component.The core sheath type fibres can be self bonding (e.g.two components with varying melting points with the lower melting point polymer on the sheath),surface tailored fibres (e.g.sheath containing expensive additives)or filled fibres (e.g.a core of recycled material covered by a sheath which has desired properties). Matrix fibril (MF)-where many fine fibrils of one polymer are dispersed randomly in size and location but with axial alignment in a matrix of another component. There are various variants of bicomponent fibres with varying phases (in terms of quantity and location)and tailored interfaces.Bicomponent fibre spinning is a type of blend spinning.Depending on the choice of constituents,they can be either compatible or non-compatible.The first category results in a homogenous single phase solution and their spinning resembles the spinning of single component homopolymers.Incompatibility between the polymers results in phase separation.The control of morphology becomes more challenging. The different cross-sections of biocomponent fibres are shown in Fig.2.11s including: citrus,wedge,or segmented pie; hollow or non-hollow; regularly round cross-section; irregularly non-round cross-section,including flat ribbon,multilobal,triangle, paralleled strip,etc. Every type of bicomponent fibre has specialised end uses.Side-by-side fibres and eccentric core/sheath fibres demonstrate self-crimping properties.C/S fibres with polypropylene sheath around a nylon core may potentially produce a fibre with the wear resistance ofa nylon fibre and display the stain resistance ofpolypropylene fibre.Trilobal biocomponent fibres are used for speciality filtration applications. In the meltblown industry,sheath core fibres are used as a thermobonding fibre.A strong bond must be made between the two materials to prevent fibre splitting. A fibre made of chemically different species will require specialised treatment to enhance the strength of the interface.The splitting of fibres is done by thermal, mechanical or chemical means
24 Composite Nonwoven Materials 2.5Bicomponent fi bres The objective of spinning bicomponent fi bres is to overcome the limitations of conventional single component spinning. The principal purpose of blending polymers to produce bicomponent fi bres is to achieve improved processing and functional properties for a specifi c end use. Bicomponent fi bres can be mainly classifi ed into three types: • Side by side (S/S) – where each polymer is divided along the length into two or more distinct regions of the cross- section. • Core sheath type (C/S) – as the name suggests, in this type of fi bre one of the components is completely surrounded by another component. The core sheath type fi bres can be self bonding (e.g. two components with varying melting points with the lower melting point polymer on the sheath), surface tailored fi bres (e.g. sheath containing expensive additives) or fi lled fi bres (e.g. a core of recycled material covered by a sheath which has desired properties). • Matrix fi bril (M/F) – where many fi ne fi brils of one polymer are dispersed randomly in size and location but with axial alignment in a matrix of another component. There are various variants of bicomponent fi bres with varying phases (in terms of quantity and location) and tailored interfaces. Bicomponent fi bre spinning is a type of blend spinning. 17 Depending on the choice of constituents, they can be either compatible or non- compatible. The fi rst category results in a homogenous single phase solution and their spinning resembles the spinning of single component homopolymers. Incompatibility between the polymers results in phase separation. The control of morphology becomes more challenging. The different cross- sections of biocomponent fi bres are shown in Fig. 2.1 18 including: • citrus, wedge, or segmented pie; • hollow or non- hollow; • regularly round cross- section; • irregularly non- round cross- section, including fl at ribbon, multilobal, triangle, paralleled strip, etc. Every type of bicomponent fi bre has specialised end uses. Side- by-side fi bres and eccentric core/sheath fi bres demonstrate self- crimping properties. C/S fi bres with polypropylene sheath around a nylon core may potentially produce a fi bre with the wear resistance of a nylon fi bre and display the stain resistance of polypropylene fi bre. Trilobal biocomponent fi bres are used for speciality fi ltration applications. In the meltblown industry, sheath core fi bres are used as a thermobonding fi bre. A strong bond must be made between the two materials to prevent fi bre splitting. 19 A fi bre made of chemically different species will require specialised treatment to enhance the strength of the interface. The splitting of fi bres is done by thermal, mechanical or chemical means