Biodegradable nonwovens & usage

Nonwoven fabrics can be used in a wide variety of applications, which may be limited life, single-use fabrics as disposable materials or durable fabrics for automotive and civil engineering applications, explain Rajanna L Gotipamul, SG Kulkarni and Pranshil Gourkar, in the 2nd and concluding part of the article. The 1st part appeared under Spotlight section of March 2017 issue.

The fibre components were prepared by separately opening and then hand mixing the two fibre types for homogeneity. The blend of fibre was then carded to form a web using a modified Hollingsworth card with the conventional flats installed at the licker-end of the machine. The resulting carded webs had the basis weights of about 40 g m?2. After carding, acetone solvent or water dip-nip treatment was applied to some of the carded webs. Then the treated or untreated webs were fed for thermally point-bonding using a Ramisch Kleinewefers 60 cm-wide calender. The embossed roll had a diamond pattern, covering approximately 16.6 per cent of the surface area, i.e. the bonded area was around 16.6 per cent.

Cotton/cellulose acetate biodegradable nonwovens

The first studied biodegradable cotton-based nonwoven fabrics were produced by cotton and ordinary cellulose acetate (OCA) fibre. Bonding temperatures used for thermal calendering were 150 °C, 170 °C and 190 °C based on the ordinary cellulose acetate?s high softening temperature (Ts : 180?205 °C). The tensile strengths of the nonwoven fabric made with cotton/cellulose acetate nonwoven blend is quite low and is not suitable for consumer application when it is processed under the temperatures associated with cellulose acetate?s softening temperature. Solvent treatment has been introduced in order to modify the softening temperature of cellulose acetate fibre and to lower the calendering temperature, while maintaining enhanced tensile properties. Acetone, a good solvent for cellulose fibre, was chosen for the solvent pre-treatment; 20 per cent acetone solvent pre-treatment was applied to cotton/cellulose acetate nonwovens to decrease the softening temperature and further lower the calendering temperature.

The results showed that these solvent treatments could decrease the softening temperature of cellulose acetate fibre and produce comparatively high tensile strengths. Because acetone is a flammable solvent there is a preference not to use it in commercial processes, so two alternative methods were further applied for cotton/cellulose acetate nonwovens. Water dip-nip treatment was used instead of acetone solvent pretreatment to make the process more environmentally friendly. It was observed that water could be used as an external plasticiser instead of 20 per cent acetone solvent without compromising web strength and the process is environmentally friendly.

A plasticised cellulose acetate fibre was developed in which an internal plasticiser was added during fibre manufacture to lower the softening temperature of ordinary cellulose acetate and further lower the bonding temperature during the thermal calendering process. The peak load was improved by using PCA instead of OCA, especially at higher bonding temperatures. Further comparison of external plasticiser (water) and internal plasticiser shows that there is no significant difference between them. Thus, an internal plasticiser (PCA) can be used in place of the external plasticiser (water) without compromising web strength, and the process is more economical. Based on the above analysis, it seems that the optimal processing conditions are either for cotton/OCA with water dip-nip treatment or cotton/PCA without treatment bonded at 190 °C for both the blend ratios. The optimal strength of the lightweight biodegradable nonwoven was around 0.8 kg, which is sufficient for many applications.

Cotton/Eastar biodegradable nonwovens

The desired calendering temperature of PCA bonded nonwovens was still too high to achieve good tensile properties. So, a newly introduced biodegradable copolyester unicomponent (Eastar) fibre, which has a relatively low softening temperature (~80 °C), was further selected as a binder fibre instead of cellulose acetate fibre. It has been reported that this binder fibre can be totally degraded into CO2, H2O and biomass.23 Because of the low softening temperature of the binder fibre (Ts : ~80 °C), the bonding temperatures used are 90 °C, 100 °C, 110 °C, and 120 °C. The tensile strengths of the cotton/Eastar fabrics are higher than those of cotton/ OCA nonwovens but much lower than those of cotton/PCA nonwovens.

Unicomponent Eastar Bio® GP copolyester fibres are soft and somewhat difficult to crimp due to the high elasticity of the fibre. For the carding process, relatively stiffer fibres are preferred. One disadvantage of using Eastar as a binder fibre is that it is hard to get the binder fibres well distributed, which may cause the low tensile properties of the final calendered nonwoven fabrics. Thus, a bicomponent fibre with Eastar Bio® GP copolyester as a sheath on a stiffer PP core was produced by Eastman Chemical Company, Kingsport, NC, US to offer more stiffness than a 100 per cent unicomponent Eastar Bio® GP copolyester fibre and to further improve the tensile properties of the nonwoven fabrics. This bicomponent binder fibre has higher tenacity, higher crimps and lower peak extension compared to that of the Eastar monocomponent binder fibre as listed in Table.

Wetlaid disposable nonwovens with flax fibre

The use of bleached elementary flax fibre in modern disposable nonwoven products was recently studied by van Roekel et al. Due to the long elementary fibre length and high cellulose content of flax bast fibres, they are an excellent substitution for synthetic fibres in disposable nonwovens. Wetlaid nonwoven sheets were produced and spunlaced on a pilot unit, however, further improvements are reported to be needed for the process. Usually, wetlaid disposable nonwovens are manufactured on Fourdrinier type paper machines, stock preparation and the headbox are modified to long fibres, and surfactants are applied to help disperse the long fibres in the primary water cycle. The machine for wet-laying flax nonwovens needs to be fast rewetting, have easy dispersion in the existing stock preparation system, and homogeneous formation. Various blends of 18 mm cut flax and PET fibre, supplemented with fluff pulp fillers, were produced; no finishing was applied for the flax fibre for the process. A 1.5 m wide, 80 g m?2 web at about 100 m min?1 was formed. It was observed that the strength properties of the web disappear completely with the increase of flax content. When extrapolated to 40 per cent flax content, strength can be fully attributed to the fluff pulp, and the strength of the web is not improved by adding more flax. Since the individual flax fibre has sufficient strength, the absence of tensile strength in the web was believed to be from the poor formation and bonding properties of the web. Therefore, further improvement of the wet-laid process is needed either by using shorter flax fibre or applying finish to flax fibre to improve its dispersion.

Nonwovens from animal fibres

Wool has been one of the most widely used animal fibres. The first nonwovens were produced from wool fibres as felts by mechanically interlocking the woollen fibres, taking advantage of their natural surface scales. Wool has excellent thermal properties and is one of the best insulating fibres. Because it is more expensive than many of the synthetic fibres used in nonwovens, it has not been one of the popular fibres. More recently, there has been an increasing effort to incorporate wool fibres in special nonwoven applications. Using nonwoven processes, it is possible to produce low-cost lightweight woollen fabrics with high stretch. Recent work24 has shown that nonwoven fabrics from wool can be produced with properties that are not possible to achieve by knitting and weaving. Some of the nonwoven products that are produced from merino wool include three-dimensional coating fabrics, stretch fabrics, windproof fabrics and footwear accessory fabrics.

Thermal blankets produced from wool fibres have excellent insulation and comfort properties. They are waterproof and pack into a small volume, making them suitable for lightweight blankets used in search and rescue operations. The combination of properties such as wicking ability, moisture and sound absorption, resiliency and thermal insulation makes wool and wool-blend nonwovens suitable for many automotive uses. Thus, there is increasing effort to take advantage of wool?s properties in many emerging applications. One such example is blending 20?35 per cent wool with rayon to produce affordable WoolFelt® nonwovens by National Nonwovens.

Silk, considered the queen of fibres, is an expensive fibre with many rich properties and is a natural protein fibre that is known to be biodegradable. Because of the cost, this is not a fibre targeted for nonwovens. However, there have been efforts to produce silk nonwovens for niche applications; one advantage is that waste and poor quality silk can be used to produce many of the nonwoven products, thereby helping control the cost.

Recently, spunlaced silk nonwovens with very low basis weight of 25 g m?2 have been developed using the Jetlace 2000 water jet machine from Rieter Perfojet.These lightweight nonwovens are targeted for sanitary materials and medical applications such as gauze and wound dressings, cosmetics and skincare products, where the property demand might be stringent. Also, by using the hydroentangling process, using any other chemical additive is avoided. These fabrics have softness, elasticity, moisture absorption, heat preservation, breathability and are not harmful to the body in any way. Some of these nonwovens can also be used in high value garments as liners for overcoats, jackets, suits or fashion fabrics. There is likely to be continuing research and development in this area as the market realises the potential for such fabrics and with the simultaneous reduction of costs by using waste silk.

Chitin is a safe natural substance found in the shells of crabs, shrimp and lobster, and in the wings of butterflies and ladybirds, etc. Chitin is one of the three most abundant polysaccharides in nature, with glucose and starch. It ranks second to cellulose as the most abundant organic compound on earth. Chitin and its derivatives, chitosan, chitin oligosaccharide and chitosan oligosaccharide, have many useful properties that make them suitable for a wide variety of health-related applications. Also, chitin products are known to be anti-bacterial, anti-fungal, antiviral, non-toxic and non-allergic. Nonwoven webs can be formed from chitin fibres for use in medical applications, such as chitin artificial skin, a newly developed patented product. The chitin nonwoven is produced by a special wetlaid process and has the properties of three-dimensional structures: soft handle, absorbency, breathability, non-chemical additive, compact texture, softness and smoothness. Thus it is the ideal dressing for extensive burns, scalds and other traumas.

Its main features are: inhibition of bacterial growth avoiding cross-infection and control of the loss of the exudates; good biocompatibility; excellent bioactivity; stimulation of new skin cell growth; accelerated wound healing; no adverse reaction of abnormal immunity, repelling or irritation. As well as artificial skin, other chitin-based nonwoven products include wound protective bandages, wound dressings and skin beauty packs. Feather products have been used in bedding and some outerwear for cold climates. Nonwoven battings made from chicken feather fibres have been evaluated as possible insulating materials. When compared with goose and synthetic fibres, chicken feather batts show better insulating properties than those of synthetic fibres and close to that of downs. The chicken feather battings also have good resiliency, which is important for insulation battings. One disadvantage is that the properties of chicken feather, in both size and tenacity, vary depending on how they are separated from the quill.

Technologies for biodegradable nonwovens

Spunbond PTAT nonwovens: The Eastar Bio® GP copolyester (PTAT) can be melt spun into spunbond and meltblown fabrics. It has been reported that uniform spunbond fabrics have been produced on Ason spunbond equipment using slotted air technology and Reifenhauser Reicofil equipment at conventional spinning speeds. Fabrics with finer fibres, higher throughputs, higher spinning speeds (> 4500 m min?1) and basis weight ranges from 14 to 130 g m?2 have been successfully obtained. The resultant spunbonded fabrics are semi-crystalline with good drapeability, soft hand, and elastic properties. The fabrics can be gamma radiation sterilised, radio frequency bonded and ultrasonically sealed, which make the fabrics suitable for medical applications, such as hospital surgical packs, wipes, bondages, face masks, etc. The fabrics can also be used for agricultural and other absorbent disposable products such as diapers, seed mats, ground cover, etc.

Spunbond PLA nonwovens polylactic acid: PLA first received considerable attention because of its biodegradability and biocompatibility; in recent years, researchers have been paying more attention to biodegradable nonwoven products. PLA was spunbonded and meltblown at the University of Tennessee in 1993.30 The following year, Kanebo, a Japanese company, introduced Lactron® (poly-L-Lactide) fibre and spunlaid nonwovens. Biesheim-based Fibreweb (France) developed nonwoven webs and laminates made of 100 per cent PLA in 1997 and introduced a range of meltblown and spunlaid PLA fabrics under the brand name of Deposa?.

The PLA polymers are processed using conventional spunbond or meltblown techniques. The plies of the nonwovens can either be hot calendered, needle punched, hydroentangled, or chemically bonded. They are intended for disposable hygiene, agriculture, and medical applications such as diapers, sanitary napkins, protective clothing, surgical masks and drapes. An example of the Biodegradable materials for nonwovens 57 three-ply nonwoven for medical application consists of the first spunbond web with a basis weight of 10?20 g m?2 and the linear density of the fibres between 1.5 and 2.5 dtex, the meltblown web with basis weight of 5?15 g m?2 and the linear density of the fibres between 0.1 and 0.3 dtex, and the second spunbond with a basis weight of 10?20 g m?2 and the linear density of the fibres between 1.5 and 3.0 dtex. The total weight is from 25?55 g m?2. The calendering temperature for the laminate is between 65 and 120 °C depending on the type of raw material and the calendering speed. The laminate has a bonded surface area of 8?15 per cent. The strength of the composite is between 40 and 100 N and the elongation at break is from 30?60 per cent.

Applications of biodegradable nonwovens

There are many companies both big and small involved in various aspects of nonwovens, from producing polymers, fibres, additives, to making machinery, producing nonwoven roll goods and converting the nonwovens into final products. Biodegradable nonwovens can be used for almost all areas of nonwoven applications. In sanitary and medical industries, a hair cap made of a poly(L-lactic acid)-based thermoplastic resin nonwoven fabric showed good hair-capturing property. Natural coconut fibres (coir) have been applied for biodegradable erosion control mats in the geotextile industry. In the automotive industry, most of the European automotive producers already use car interiors made of natural fibres.

In Germany, flax, sisal and jute were used for car interiors.. A variety of mouldable, cellulosic-based nonwoven composites for automotive applications with excellent thermal insulation properties, which were fabricated from kenaf, jute, flax and waste cotton using recycled polyester and substandard polypropylene. In the filtration industry, refuse bags and drain filters have been made by using fine denier biodegradable polylactic acid nonwovens for the application of sink drains; and biodegradable pleated filter material and filter units for air purification and liquid filtration.

Flushable nonwovens Liquid waste system disposal is quite attractive compared to solid waste disposal where the infrastructure may not be well developed. In many instances, landfill and solid waste disposal techniques have significant environmental problems. Considering this, the wastewater systems is more convenient, hygienic and environmentally sound; there is already a massive infrastructure in place as wastes from houses go to industrial biodegraders in the form of sewage farms, or local biodegraders or septic tanks. In such situations, many disposable products can be flushed through the system rather than thrown away as solid waste. Flushable nonwoven diaper liners and wipes have been on the market for a while. For such products, flushability is desirable and technically it is possible to develop such products. However, lack of convenience and cost issues have driven the market towards non-degradable plastics in diapers as well as feminine hygiene products.

In designing and developing flushable nonwovens, one of the challenging requirements is that the products have to be strong enough to be stored and/or used when wet, but at the same time, should be weak enough to breakdown in the sewage system. Flushability itself is not well defined and there is no accepted standard method to evaluate and certify flushability of such products. The current efforts involve comparing fabrics by agitating them in a standard volume of water for a standard time and observe the fragmentation degree or determine the time for achieving full dispersion. For a nonwoven material to be claimed flushable, the fabric must break up immediately in a toilet bowl and be small enough to be transported from the toilet bowl to the sewage system in a single flush. It should not lead to blocking of pipework and there should not be any accumulation in subsequent flushes.

There have been consumer studies that have shown that many flushable wipes in the market lead to clogging of pipes. In addition to the fact that they have to break down, they should not contain any chemicals that might affect the functioning of the sewage farm or the quality of the treated water. This means that all the materials used have to be biodegradable. In these situations, the enhancement of wet strength will retard flushability.

For wet wipes, other alternatives suggested are to use a system where the wipes remain dry till they are ready to use, then a wet additive is incorporated just as it is being dispensed. Another suggestion is to possibly modify toilets and flushing systems to handle new materials, where the breakdown is accelerated either by additional chemical or mechanical actions. When one looks at all the available materials and processing technologies, air laid and wetlaid systems are more suitable. The problem with spunbond types is the difficulty in breaking down continuous fibres; using short fibres to form webs and binding them with fibres that are biodegradable or water soluble will be the best approach. the introduction of a new biodegradable polymer Nodax®(a polyhydroxyalkanoate), which may be used for flushable nonwoven products, and other biodegradable nonwoven materials as well.

References

• http://www.innovationintextiles.com
• http://www.technica.net/
• http://www.textileworld.com/
• Biodegradable materials for nonwovens G. B H a T, the University of Tennessee, USA; and D. V. P a R I K H, Southern Regional Research Center, USDA, USA,

Rajanna L Gotipamul and SG Kulkarni are from the DKTE Society?s Textile Engineering Institute, Rajwada, Ichalkaranji-416115, Kolhapur, Maharashtra Pranshil Gourkar is from Global Nonwovens