Abstract
We evaluated the feasibility of producing biodegradable mulch fabrics from bast fibers using a low-cost nonwoven fabric production process. Commercially available low-cost hemp and linseed flax fibers were carded, lightly needle-punched and then consolidated using a hydroentanglement process to produce fabrics around 200 g/m2 and 0.5 mm thickness. The hydroentanglement process liberated micro and nanofibers that formed a continuous fibrous network entangling and linking the parent fibers to produce fabrics with good tensile properties. Preliminary field trials of the fabrics demonstrate that they can be used to suppress weeds when colored with a commercially available carbon black-based mulch colorant. When in contact with soil the fabric biodegraded and lost strength in a matter of a few months.
Weeds compete with crops or pasture for space, nutrients, water and light and depending on the degree of competition can reduce crop yields by 10 to 25%. 1 Weeds can be controlled by a range of methods with different levels of effectiveness. Herbicides are highly effective, particularly for broad-acre farming, but their inappropriate use can lead to herbicide resistance and environmental impact. Chemical-free weed management is highly desirable particularly for high value, organically grown fresh produce. One option is to use mulch to control weeds. Mulch can also lead to improved crop yield and earliness, and to improved water efficiency.2,3 The growth of weeds relies on photosynthesis and an effective mulch fabric for weed control should prevent light transmission. Plants deprived of sunlight are unable to photosynthesize CO2 and water into plant tissue and oxygen. This is due to the inability of chlorophyll, the plant pigment responsible for converting light energy to chemical energy in the plant, to absorb radiant energy. The deficiency causes yellowing of the plant known as chlorosis, and then death (or necrosis). The absorption maximum for chlorophyll is around the wavelength from 680 nm to 700 nm.
Plastic mulch produced from polyethylene is often used. A recent review of polyethylene mulch, including developments in biodegradable and photodegradable plastic mulch has been published. 3 Polyethylene mulch can be black or clear and produced as a film, or a fabric of woven film strips or synthetic fibers. Woven or nonwoven plastic fabrics allow water and nutrient penetration from above. The main disadvantage of this type of mulch is the disposal of the plastic after use. Removal is time consuming and any fragments remaining at the end of a growing season are burnt or sent to landfill. Fragments may also be buried, contributing to plastic pollution. Recycling is limited as the mulch is heavily contaminated with dirt and debris.
Mulch derived from natural materials, either of animal (manure, wool) or plant (e.g. straw, wood chips) origin may also be used. While these materials biodegrade, contributing to organic matter in the soil, they are often bulky, retard soil warming in the spring and harbor pests. Natural mulch may not always be available in adequate amounts (seasonal), requires additional labor to spread, does not always provide effective weed control and may even carry weed seeds. 3 Shredded paper has also been used as a mulch and paper (sheet) coated with wax, vegetable oil or polyethylene to reduce the rate of degradation during use has also been trialled. 4 Paper sheeting can be placed and allows seedlings to be transplanted mechanically. 4
Natural fiber mulch mats are also available. Examples include thick jute nonwoven felts for terrestrial weed control, and thin woven jute fabrics for short-term soil stabilization and aquatic weed control. 5 The thick jute weed mats are made from textile fibers by the needle-punching method. Their main disadvantages are bulkiness and low mechanical strength, causing difficulties for machine-laying. The relatively loose structure of the needle-punched felts also makes them an ineffective light barrier. Woven fabrics are considerably more expensive due to the need to spin yarns and then weave the fabrics.
The straw of many crops, such as linseed, flax, hemp, jute and kenaf, contain considerable quantity of (bast) fibers that can be extracted for the production of biodegradable mulch fabrics. The traditional fiber extraction method 6 includes dew or water retting followed by manually stripping or mechanical scutching to remove fibers from the stem. High quality flax fibers are obtained by hackling and these are made into fine count yarns through a wet spinning route. A key feature of the traditional fiber extraction method is to keep the straw aligned from the field to the processing line. However the traditional long fiber extraction and linen spinning processes are considered too expensive for the production of mulch fabric.
There has been extensive research and development in recent times on mass decortication technology to produce low-cost, natural fibers for industrial use, for example in biocomposites. 7 Straw alignment is no longer required, and instead, randomized straw is collected into larger bales to allow high volume production to be carried out conveniently. Modern decorticating systems include a bale opener, a series of breaking machines and a number of cleaning machines. These machines mechanically break down the adhesive bonds between the woody core of the plant stem and the fiber bundles to be extracted. Broken core materials (called shives, or hurds) are removed by mechanical loosening and shaking. Several product lines may be produced by a decorticator and longer fibers can be further processed into nonwoven mats and coarse count yarns using textile machinery. After decortication the fiber is pressed into bales and transported to a textile mill for further processing.
At the mill, fibers are disentangled to produce a thin web of individual fibers in a carding machine. The web is then folded into a batt of suitable thickness using a cross-lapping system. The cross-laid batt is very bulky and has little mechanical strength. It needs to be consolidated to a manageable density, usually by a needle-punching machine. In needle punching, barbed needles are forced through the thickness of the fiber batt to drive a portion of the fibers from one side of the batt to the other. Thousands of barbed needles per meter of working width are employed on a needle board which penetrates the fabric through a reciprocating action. The needle-punching action interlocks the fibers by friction. 8 The needle-punched batt or fabric has some degree of mechanical integrity and has been used as a weed mat in the past. The needle-punched fabric as a weed mat is however not ideal, as it is thick for its weight (low density) and not strong enough to sustain handling, severe weather conditions (for example strong winds) or attack by small animals, such as birds.
In a recent study, 9 we found that flax fibers in woven fabric could be fibrillated on the surface by the high-pressure water jets used in the hydroentanglement process (alternatively known as spunlace). The micro- and nano-sized fibrils from neighboring fibers are interconnected by entanglement and bonding, and hence significantly strengthen the woven fabric structure. A 10-fold improvement in abrasion resistance was demonstrated. The hydroentanglement process is highly productive and can produce fabrics of widths up to 4 meters at speeds as high as 100 meters per minute.
In this paper, we evaluate the feasibility of producing biodegradable mulch fabrics from bast fibers using a low-cost hydroentanglement nonwoven process. Preliminary field trials were also undertaken to evaluate the effectiveness of the resultant fabric at suppressing weeds compared to polyethylene (PE) woven mulch fabrics. The initial results are promising, demonstrating the concept and providing impetus for further research in large-scale field trials to establish the environmental and production benefits of these fabrics and the extent to which they address the deficiencies of natural mulches used in the past. An economic analysis of fabric production using current process was also undertaken.
Materials and methods
Fibers
Two types of commercially available decorticated bast fibers were used in the production of biodegradable mulch fabrics: (a) flax extracted from linseed straw in Canada, commercially available as “Durafibre” and (b) hemp fiber extracted by “green decortication” (i.e. without retting), commercially available as “DA Therm” from industrial hemp in The Netherlands. Both types of fiber contain less than 5% shives, the woody part of the plants. The retted flax fiber appeared longer in length, finer in diameter and darker in color than the un-retted hemp fiber.
Fabric production – nonwoven process
The nonwoven line used to produce the weed mat fabrics consists of a number of machines for opening and blending (Trutzschler™ FBF1000 consisting of three hoppers), carding (Spinnbau™ 201), cross-lapping (Autefa™), needling (Dilo™ CBF20) and winding up of the resulting nonwoven mat. In the production of the mulch fabric samples, the cross-lapped bast fiber webs were given a very light needle-punching action (called pre-needling). This was done so that they had sufficient integrity for transferring to the next process, hydroentanglement. In the CSIRO pilot plant, the hydroentanglement machine was not directly linked to the web-forming machine line as it would be for commercial production. The pre-needling can be omitted in commercial production.
A Rieter™ Perfojet pilot hydroentanglement machine was used to fibrillate the bast fibers in the pre-needled nonwoven fabric. This laboratory scale machine is 450 mm wide and equipped with three lines of injectors. The pre-needled web was first pre-wetted at a low water pressure (20 bar) and then treated at high water pressure (200 bar) on one side of the web. The hydroentangled fabric was then dewatered by a pair of pressure rollers and dried by passing through a Gyson™ through-air oven. A schematic of the processing steps used to prepare a number of different fabrics is shown in Figure 1.
Nonwoven processing flow chart.
Initially a solvent based (pressurized can), black pigment colorant was used to color some of the test fabrics. This was replaced by a less expensive, environmentally friendly black colorant for mulch from Colorbiotics, 10 which contains carbon black as the pigment and is UV stable. The colorant was diluted 1:10 with water and pressure sprayed on the fabric to an uptake of 1:2 liquor:fabric mass ratio.
As a comparison to the natural fiber trial fabrics, a commercial black PE split film woven fabric (70 grams per square meter (gsm)) purchased from a local garden supplies shop was used.
Tensile strength
Tensile strength in both the length direction (machine direction (MD)) and the width direction (cross direction (CD)) was tested using standard size fabric strips (50 mm wide, gauge length of 200 mm) at an extension rate of 100 mm/min according to Australian Standard AS2001.2.3.1 – 01. 11 Bursting strength was tested using a ball bursting attachment and tested according to Australian Standard AS2001.2.19 – 1988. 12 Test measurements were undertaken on the as-produced fabrics and after exposure for the first two months of the preliminary field trials, which commenced in early spring.
Light transmittance
Diffuse transmittance spectra of fabrics were obtained using a Cary 300 Bio UV-visible spectrophotometer (Varian, Melbourne) fitted with a 70 mm diameter integrating sphere (DRA-CA-301, Labsphere, New Hampshire, USA) machined from Spectralon©, a pure form of polytetrafluorethylene (PTFE). Spectra were obtained by mounting the specimens (25 x 25 mm) in the optical path of the beam entering the sphere, and using a 0° wedge and a Spectralon© reference standard over the reflectance port.
Water transport through fabrics
The ability of the fabrics to minimize water runoff and allow water and nutrient absorption from above, for example from rain or above-ground irrigation was determined through the impact of (tap) water droplets on the surface of fabric inclined at 20°.
The rate at which the fabrics were able to absorb moisture was determined using the commonly used vertical wicking test. Strips of the fabric samples (50 mm wide) were held in an upright position with their lower ends dipped in a (tap) water bath. The height of the wicking water front on the fabric strip was recorded at different times. The vertical fabric strip wicking test method has been reviewed in great detail by Patnaik et al. 13
Preliminary field trials
Preliminary field trials were conducted using the fabrics produced to determine their efficacy at inhibiting weed growth through chlorosis and to determine their durability.
A trial area in a large grassed (pasture) field at CSIRO Belmont site in Geelong, Victoria, Australia, was chosen with no natural obstructions from sunlight and rain and with a variety of broadleaf weeds and grasses. The species identified in the trial area include Trifolium subterranean (subterranean clover), Plantago lanceolata (ribwort or ribgrass), Phalaris minor (lesser or annual canary grass), Medicago polymorpha (burr medic), Oxalis corniculata (creeping oxalis), Chenopodium album (fat hen), Rumex crispus (curled dock) and Agropyron repens (couch grass). 14
The chlorosis field trial commenced on 14 September 2011 (two weeks into the spring of Southern Hemisphere) and was completed towards the end of autumn (17 April 2012). In this preliminary trial the mulch fabrics were placed adjacent to each other on an area of the field and pinned to the ground using a U-shaped wire peg approximately every 0.5 m around the perimeter. The initial length of the weeds and grasses meant that the fabric was not placed directly on the ground but raised somewhat, and this allowed some air circulation under the fabric. Photographs from different stages of the trails were used to record any change in weed density, height and color. The soil moisture and temperature under each fabric were also monitored during the coverage period using a Delta-T HH2 digital moisture meter and a Fluke 80TK thermocouple that was attached to a multimeter.
Results and discussion
Fabric production
Optical and scanning electron microscope (SEM) images of the raw hemp and flax fibers, after needle punching and after the hydroentanglement process are shown in Figure 2(b). The fabric thickness was greatly reduced due to the densification brought about by the high-pressure water jets. SEM images of the cross-section of the treated bast fiber fabrics show that the high-pressure water jets created micro- and nanofibers from the much larger diameter parent fibers and these fibers form a continuous fibrous network entangling and linking the parent fibers together. The fiber surface before treatment was smooth and clean. Fibrils of different bundle sizes appeared on the fiber surface after hydroentanglement, as shown in Figure 2(c) and (d). Many of these surface fibrils were found to be in the range 10 nm to 50 nm in diameter. Figure 2 also shows that the surface fibrils produced from flax fiber and hemp fiber had no noticeable differences. As it will become clear later, the fabric properties are dominated by the surface fibrils. The fabrics made from the two types of fibers were used interchangeably in this study.
Images of the product at various stages during the manufacturing process: (a) optical images of the bast fibers; (b) optical images of initial needle-punched nonwoven mats (bottom) and resulting fabrics (top) after the hydroentanglement process; (c) and (d) scanning electron microscopic images of the fabrics after hydroentanglement. Note that images on the right and left hand sides are flax fiber based product and hemp fiber based product respectively.
Mechanical strength
Adequate mechanical strength is required for mat handling, laying and minimizing damage due to the scratching of birds and small animals.
Fabric tensile strength and bursting strength were measured and normalized according to the fabric mass per unit area (area density in gsm) of the test specimens. The needle-punched nonwoven flax fabric had a normalized strength of 0.013 N/gsm in both directions (50 mm fabric test width). Fabric strength increased more than 50 fold after hydroentanglement. We attribute this dramatic improvement in fabric mechanical strength to the formation of the micro- and nanofibrils by the high-pressure water jets and entanglement and surface bonding of these to the parent fibers (Figure 2). In the needle-punched fabric, the individual fibers are held together only by friction and a small amount of fiber entanglement. In the hydroentangled fabric, the confinement of fibers within the fabric has been increased enormously by the newly formed network of surface fibrils. It is also possible that hydrogen bonding between the nano-sized cellulose fibers and adhesive gummy materials (such as lignin, hemicellulose and pectin) released from the parent fiber due to surface fibrillation helps the liberated fibrils to adhere more strongly to each other and to their parent fibers.
The normalized tensile strength of the hydroentangled flax fabrics increased marginally with fabric area density (Figure 3). The spread in fabric area density reflects the variability in the production process for the fabric both in the width and length direction. This variability is typically reduced significantly during long production runs on full scale commercial equipment that has been optimized for the fiber and end product. The polypropylene fabric had an absolute tensile strength of 450 N, not greatly higher than the absolute (mass dependent) strength of the flax fabrics, but, due to the higher strength and lighter weight of the polypropylene fabric, had a normalized tensile strength of 6 N/gsm. The flax fabric bursting strength results were much less consistent than the tensile results also reflecting the unevenness in the fabric density.
Normalized flax fabric mechanical strength as a function of fabric area density: (a1), (b1) and (c1) after production and (a2), (b2) and (c2) after field trial; (a1) and (a2) tensile strength in the MD; (b1) and (b2) tensile strength in the cross machine direction; (c1) and (c2) bursting strength.
Water transport through fabric
While the polypropylene mat is of a woven construction, the material is sufficiently hydrophobic with small pore size between the interlacing split film threads to restrict water penetration. When placed on the fabric, drops of tap water bead and roll off the surface without absorption (Figure 4). Fabrics constructed from the flax fibers absorb the water drops immediately where placed.
Water droplets placed on the inclined fabric are shed by PE (black) but absorbed by the two flax fabrics.
Strips (50 mm wide) of PE and flax fabric samples were held in an upright position with their lower ends dipped in a water bath. The height of the water front on the flax fabric strip was 70 mm after 25 minutes and 180 mm after four hours (Figure 5). Conversely the PE split film woven fabric showed no sign of wicking (height = 0).
Wicking height test. Left: after 25 minutes; right: after three hours (scale bars = 180 mm). Two flax fabrics of nominal 150 gsm and PE fabric (black).
Both tests demonstrate the hydrophobic nature of the PE split film woven fabric compared to the hydrophilic properties of the flax fabric and the potential for natural fiber fabrics to minimize water runoff particularly on high sloping soils.
Preliminary field trials
Photographic images of typical pasture grasses and weeds present in the field trial are shown in Figure 6.
Extent of pasture growth and chlorosis under the various fabrics. A, B, C and D established 14 September 2011, imaged 18 November 2011. E and F established 18 October 2011, imaged 18 November 2011. G and H established 20 January 2012, imaged 17 April 2012. Coverage was for 65 days for A, B, C and D and 31 days for E and F during spring and 88 days for G and H during summer. Lower images are of pasture broad leaf weeds and grasses growing in the field trial area at the commencement of the trial.
Three natural colored fabrics constructed from flax fiber at 150, 250 and 350 gsm and the commercially available PE fabric were laid out in the trial area in mid-September 2011. After one month the fabrics were rolled back and an assessment of weed/grass suppression made. Grasses and weeds under the PE mat showed clear signs of yellowing (chlorosis) with some having died. On the other hand, while most broad leaf weeds had withered under the flax fabrics the grasses appeared to show little evidence of reduced growth (Figure 6).
The trial areas were re-covered with the fabrics and two additional trial plots were established four days later (18 October 2011) adjacent to the original plots discussed previously. These plots were covered with 150 gsm and 350 gsm flax fabrics that were sprayed with the solvent based black pigment colorant after laying.
All fabrics were left in place for one month (until 18 November 2011) and then removed for examination (Figure 6). The soil moisture and temperature under each fabric were monitored at the beginning and end of the coverage period but there was no significant difference between the various fabrics. Under the PE control fabric, all grasses became yellow while the broad leaf weeds had died. Most broad leaf weeds had died under all three natural colored flax fabrics but the grasses had survived, although there was evidence of slight chlorosis of the grasses under the heavier 350 gsm fabric. All broad leaf weeds had died and there was severe chlorosis of the grasses under the black colored flax fabrics, particularly the 350 gsm fabric. The black colorant significantly increased the extent of chlorosis at the end of the 31 days coverage period compared to the uncolored, natural fabric from 65 days coverage.
The alternative, less expensive and more environmentally friendly colorant (Colorbiotics) was applied to a nominal 200 gsm hemp fabric. This fabric was then laid in mid-January 2012 (summer) and left in place for three months. The trial results are also shown in Figure 6 (G and H) and indicate that this hemp fabric blackened with the carbon-based colorant was as effective as the PE control fabric at killing the pasture weeds and grasses.
The results of these chlorosis trials can be explained by the light transmittance of the mulch fabrics. Before hydroentanglement, the needle-punched web (250 gsm flax fabric) allows up to 10% transmission of incident light at 680 nm and 20% at 850 nm, while after hydroentanglement the increased fiber density of the fabric reduces the transmitted light to below 1% at 680 nm (Figure 7).
Diffuse transmittance spectra of nonwoven flax fabric (250 gsm) before and after hydroentanglement.
The diffuse transmittance of flax fabrics of three different area densities was compared with the commercial PE woven fabric (Figure 8). Approximately 0.5% light of all wavelengths is transmitted through the PE woven fabric, presumably through the voids between the crossing split film threads in the woven structure. The light transmittance of the natural colored flax fabrics is dependent on the fabric density and on the wavelength. Ultraviolet wavelengths shorter than 400 nm are completely blocked, while longer wavelength visible light can pass, red wavelengths (λ > 600 nm) having the highest transmittance. Transmittance is higher for the 150 gsm fabric compared to the 350 gsm fabric. Spraying a hemp fabric with the black mulch colorant containing carbon black reduced the transmittance to essentially zero and below that of the commercial black PE fabric.
Comparison of light transmittance through woven polypropylene (PE) and nonwoven flax and hemp fabrics: (a) before 2-month field trial and (b) after a 2-month field trial.
Three fabrics, a black PE control, a 200 gsm natural color hemp fabric and the same fabric colored with the Colorbiotics black mulch colorant were then placed on turned loose soil in two different trial plots, each approximately 5 m × 3 m with the edges of the fabric sealed under soil and the temperature monitored. The main difference between the two patches was that the soil in patch 2 appeared to be finer and drier than patch 1. Moist soil can be heated by covering with plastic sheets to trap solar radiation over several weeks 15 leading to soil pasteurization or solarization to control soil pathogens and weeds. 16
Soil temperature (℃) recorded under the fabrics
Degradation of mulch fabrics during use
The normalized (to area density) tensile and bursting strength of flax fabrics after the two months of field trial exposure commencing in early spring are shown in Figure 3. The tensile strength was reduced by around 30% compared to the as-produced fabrics. The bursting strength results also showed a reduction.
These flax fabric samples were also tested for light transmittance after the two months of exposure, as shown in Figure 8(b). The light transmittance of the 250 gsm and 350 gsm fabrics did not show significant changes compared to before exposure whereas transmittance for the 150 gsm flax fabric increased substantially across the entire spectrum, more than 10% at 680 nm.
The transmittance of the PE mat was reduced to zero after the field trial, presumably due to the closure of the voids between the film strips due to weathering.
The hemp fabric treated with carbon black-based pigment also showed zero light transmittance after the field trial, demonstrating that the hemp fabric can match the commercial PE material over a two month timescale. The natural colored flax and hemp fabrics tended to photo-bleach on exposure to the elements during the trial whereas the hemp fabric colored with the Colorbiotics mulch colorant appeared to darken.
Layers of hemp fabric were left lying on the ground in contact with the soil over four months until early July 2012. The trial region (Geelong, Australia) experienced higher than usual rainfall during the trial period. Worms and insects had colonized the fabrics. The fabric degraded over time becoming weak enough to be readily cut using a garden spade, demonstrating that the fabric biodegrades during use, particularly when in intimate contact with soil. This is particularly important to enable agricultural machinery to break up the fabric when used on a large scale. Alternatively the fabric could be gathered and composted.
Economic considerations
The hemp fibers used were sourced from Europe and purchased from an Australian distributor at around 1.25 AUD/kg (1 AUD ≈ 1 USD) in a 500 kg order. For bulk quantities we estimate a cost of 1 AUD/kg input fiber, and with a conversion efficiency of 90%, the fiber cost is 1.10 AUD/kg in fabric form. For low-cost commodity fiber, the fiber cost typically represents 50% of the selling price of the hydroentangled nonwoven product. Hence the natural colored fabric price will be around 2.20 AUD/kg. The price of the carbon black colorant used was 4 AUD/liter. Based on a 10:1 dilution ratio at 50% pickup on the fabric, coloration will add an additional 0.2 AUD/kg of fabric or a total price of 2.40 AUD/kg of colored fabric. For our calculations we assume a fabric mass of 150 gsm. This fabric mass is at the lighter end of the fabrics we produced but at full scale commercial production on optimized equipment a lightweight, consistent product should be possible. On this basis the fabric manufacture price is estimated to be less than 0.4 AUD/m2. The mass of the bast fiber fabric is around twice that of the polyethylene fabric, but the bast fiber cost is around half that of the polyethylene so overall manufacturing costs should be somewhat similar. The retail price and wholesale price for 1800 mm wide black PE weed mat fabric are presently 1.44 AUD/m and 1.19 AUD/m, respectively. 17
Estimated cost for fabric manufacture
Machines required for fabric production post decortication include the following: Chinese produced –weighing bale opener, blending hopper, stabilizing feeding hopper, pressurized hopper feeder, airlaying machine (or alternatively, card and cross-lapper), batt drafter, hydro entanglement machine, dryer and winding machine. These core production machines are complemented by a water treatment plant with filtration systems and a high-pressure pump of up to 400 bar.
Conclusion
Fabrics suitable as mulch for weed suppression can be produced from bast fibers using a low-cost nonwoven fabric process that includes fiber extraction, carding and hydroentanglement. The hydroentanglement process generates micro- and nano-fibers from the fibers to produce a fabric with good tensile properties. Preliminary field trials of the bast fiber fabrics indicated that when colored black, the fabrics are as effective at suppressing weeds as black polyethylene woven mulch fabrics. The fabric biodegrades when it is in contact with moist soil, and hence there will be no additional cost of removal and disposal compared to polyethylene.
Footnotes
Funding
We would like to thank Andrew Jones for manufacturing the fabrics, Badar Zaidi for mechanical testing of the fabrics and the Rural Industrial Research and Development Corporation, Australia for funding the research through its National Weeds Programme.