evaluation of degradable spun-melt 100% polylactic acid ...7) l. wadsworth.pdf · evaluation of...

Journal of Engineered Fibers and Fabrics 50 http://www.jeffjournal.org Volume 8, Issue 4 – 2013

Evaluation of Degradable Spun-Melt 100% Polylactic Acid Nonwoven Mulch Materials in a Greenhouse Environment

Larry C. Wadsworth, PhD1, Douglas G. Hayes, PhD2, Annette L.Wszelaki, Ph.D.3, Tommy L. Washington1,

Jeffrey Martin3, Jaehoon Lee, PhD2, Robert Raley2, C. Tyler Pannell2, Sathiskumar Dharmalingam2, Carol Miles, PhD4, Arnold M. Saxton, PhD5, Debra Ann Inglis, PhD6

1Depart. of Materials Science and Engineering, University of Tennessee, Knoxville, TN UNITED STATES

2Depart.of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN UNITED STATES

3Depart. of Plant Sciences, University of Tennessee, Knoxville, TN UNITED STATES

4Depart. of Horticulture & Landscape, Washington State University, Mount Vernon, WA UNITED STATES

5Depart. of Animal Science, University of Tennessee, Knoxville, TN UNITED STATES

6Depart. of Plant Pathology, Washington State University NWREC, Mount Vernon, WA UNITED STATES

Correspondence to: Larry Wadsworth email: [email protected]

ABSTRACT The tendency of most commercially available plastic agricultural mulches to undergo only partial fragmentation with time leads to their long-term persistence in soil, resulting in potentially detrimental environmental hazards. Nonwovens composed of biobased polymers such as poly(lactic acid) (PLA) with micron-sized fibers may be potentially valuable for agricultural mulches due to their high mechanical strength and potential ability to undergo complete mineralization. To assess the performance of 100% PLA spunbond (SB) and meltblown (MB) mulches, and commercially available cellulosic mulch, a greenhouse bench study was conducted where the mulches were buried in soil augmented with either lime or compost for 10 and 29 wk to accelerate biodegradation and mineralization. At 10 and 29 wk, MB and SB mulches, respectively, lost considerable mechanical strength for all soil treatments while showing only minimal signs of loss in molecular weight. INTRODUCTION Plastic agricultural mulches fulfill many important roles in production of specialty crops, particularly in weed control; they can also increase soil temperature and extend the growing season in the early spring and late fall, and enhance water utilization efficiency

leading to increased crop yields and quality [1, 2]. Black polyethylene (PE) mulch has been the most commonly used mulch material in specialty crop production since the early 1960’s. However, PE mulches must be retrieved at the end of the crop growing cycle, thereby requiring costly labor. In addition, the PE mulches are not commonly recycled at the end of the growing season and are sent to a landfill, resulting in low sustainability. Also, mulch fragments that remain in the field after the growing season persist indefinitely, leading to potentially harmful environmental consequences. There is a critical need to develop new mulch material that can be tilled into the soil at the end of the production season that will undergo full microbial assimilation, thereby not negatively impacting the soil ecosystem. A healthy microflora/fauna and no release or accumulation of foreign or toxic substances are critical soil ecosystem requirements [3, 4]. Although several commercially available mulches advertised as “biodegradable” have achieved moderate success in undergoing partial microbial utilization in soil and / or industrial composting conditions, no product has fully achieved all of these criteria (reviewed by us in [5]). This greenhouse study provides supporting data regarding the (i) potential biodegradability of three mulch products, and (ii) effects of two soil


amendments (lime, compost) commonly used in specialty crop field production on mulch degradability. Two experimental degradable mulch candidates were prepared by using fibers of PLA [5, 6] and nonwoven fabric production techniques [7]. The first, a spunbond (SB) PLA spun-melt nonwoven is a strong, thermally point-bonded web of continuous filaments with fiber diameters of 12 to 15 µm. The second, a meltblown (MB) PLA nonwoven, is weaker than the SB mulch, but possesses smaller fiber diameters of 4 to 8 µm which can potentially lead to enhanced biodegradation. PLA was chosen as the mulch material due to its relatively high abundance and low cost and high mechanical strength compared to other biobased polymers [5,6]. PLA mulch candidates were included in the study to help determine if the fibrous structures with their corresponding higher surface areas and porosity allow for accelerated rates of degradation, and if MB PLA degrades more readily than SB PLA. The third mulch is a commercially available cellulosic product. Although it is generally accepted that PLA undergoes nearly complete microbial assimilation under composting conditions at a relative humidity of 98% and temperature above 60 °C [8], one report suggests that biodegradation can also occur under near-ambient conditions [9]. Hakkarainen et al. incubated 1.8 mil thick PLA film samples at 30 °C in a mixed culture of microorganisms extracted from compost. After 5 wk of incubation, the microorganism-treated film had degraded to a fine powder. In contrast, under control treatment, the PLA film remained intact; indicating that application of a broad range of readily available microorganisms from compost can accelerate mineralization. Thus in this study, soil from a USDA certified organic farm and soil amendments of composted dairy cow manure (rich source of microorganisms) and lime (the latter to promote hydrolysis of PLA) were selected. This greenhouse study was designed as a companion study for a larger study on the production of tomatoes in both high tunnels and open fields where five mulch treatments are being evaluated for impacts on tomato yield and soil environment during the growing season and after soil incorporation for up to two years [10]. Burial times (10 and 29 wk) of mulch specimens in this greenhouse study coincide with time to first flower and final harvest in the companion field trial.

EXPERIMENTAL Description of Mulches A brown cellulosic product (WeedGuard Plus, Sunshine Paper Co., LLC, Aurora, CO, USA) with a specification weight of 107 g/m2, was included in this study as a biodegradable control [11]. The SB PLA was produced at a specified weight of 90 g/m2 and width of 1.1 m by the Saxon Textile Research Institute (STFI), Chemnitz, Germany from Ingeo™ PLA 6202D (melt index {M.I.} of 15-30), provided by NatureWorks LLC, Blair, NE. The MB PLA was produced at a specified weight of 80 g/m2 and width of 0.32 m by Biax-Fiberfilm Corporation, Greenville, WI, USA from a blend of 80% Ingeo 6201D PLA (M.I. of 15-30) and 20% Ingeo PLA 3251D (M.I of 70-85) with both PLA grades provided by NatureWorks. NatureWorks was selected because it is the world’s largest producer of PLA for plastics, textiles and nonwovens. PLA Grades 6201D and 6202D are typically utilized in the production of SB fabric and 20% PLA 3251D with its higher M.I. and corresponding lower molecular weight was added to reduce the melt viscosity of the blend as required in the MB process. Experimental Design The experimental design was completely randomized and consisted of three mulches, three soil treatments, two burial times and three replicates within each mulch, soil and time combination. The soil (Dewey Silt Loam) in all three soil treatments was obtained from a USDA certified organic farm and was placed in plastic trays measuring 52 cm L x 25 cm W x 6 cm D. The mulch specimens were cut to 61 cm in the longitudinal direction and 38 cm in the traverse direction and the edges extending beyond the perimeter of the trays were not tested. The soil treatments consisted of a control (non-amended soil), lime-amended soil (4.5 t ha-1 or 27.2 g per tray; lime obtained from Oldcastle Industrial Minerals, Thomasville, PA, USA), and compost-amended soil (45 t ha-1 or 585 g/tray; employing Black Kow composted dairy cow manure from Black Gold Composting Co., Oxford, FL, USA). The mulch samples were buried beneath 2 cm of each soil treatment. To each tray, 1.0 L of water was added per 48 hour period. The study was initiated on March 19, 2010 (initial time); the 10 wk samples were retrieved May 28, 2010; and the 29 wk samples on October 7, 2010. The ambient light in the greenhouse was not supplemented and averaged 11:13 hrs L:D (light: darkness) in Spring and Fall and 12:12 hrs L:D during the summer. During this time, the greenhouse RH was 80 ± 10% and the temperature was 29 ± 9.4 °C.


Physical Testing and Analysis of Mulches Mulch specimens were analyzed at three times in this study: initial as-received, 10 wk after burial, and 29 wk after burial. Buried mulch specimens were carefully removed from the soil, laid over nylon organza fabric supported on a wire mesh, carefully washed with a very low-pressure water hose to remove loose soil without apparent damage to the mulch specimens, air dried, and allowed to equilibrate over night to standard textile testing conditions before being tested for physical properties. For each physical test, except air permeability for which it was not necessary to cut subsamples from the replicate samples, 5-10 subsamples measuring 2.5 cm X 15 cm in the machine direction (MD) were cut. However, MB PLA specimens underwent significant deterioration according to visual observation; therefore, the subsample dimensions were modified to 2.5 cm X 7.6 cm in MD. For each subsample, the following parameters were measured according to standard protocol: weight to the nearest mg [12], thickness [13], and tensile properties [14]. The tensile tests were determined using a United Testing System, Model E-Vi-60CX, United Calibration Corporation, Huntington Beach, CA. The gauge length in the tensile tester was 7.6 cm for the 2.5 cm X 15 cm subsamples, and was 2.5 cm for the 2.5 cm X 7.6 cm subsamples. Air permeability was determined only on mulch subsamples that had no visible holes in a circular area with a diameter of at least 7.6 cm. For this assessment, subsamples were secured by the circular clamp around the TextTest air permeability orifice. Subsamples were moved over the air permeability orifice and air permeability was determined at six different locations [15]. To perform scanning electron microscopy (SEM), the subsample was mounted on a 1.2 cm diameter aluminum disk using double side adhesive carbon tape. Then the subsample was sputter-coated with a thin layer of gold (less than 5 nm) in a vacuum chamber using argon gas and a small electric current of approximately 3 mA. Digital photomicrographs were made of the SB PLA and MB PLA mulch samples initially at soil burial and after 29 wk by viewing at 100X, 500X and 1000X magnifications with a LEO 1525 field emission scanning electron microscope (Zeiss, Germany). With the initial mulch subsamples, the average fiber diameter was determined from photomicrographs at 500X using Image J software created by While Rasband, downloaded from the National Institute of Health website (rsb.info.nih/gov/ij). For each image, a line was drawn on the digital image parallel to the edge of a randomly selected fiber and a perpendicular line was constructed from one edge of the fiber to the

other to measure fiber diameter. A total of 33 fiber diameter measurements were recorded per image; one image was taken for each of the three subsamples, resulting in a total of 99 measurements per sample. Gel permeation chromatography (GPC) was used to determine the number-averaged molecular weight (Mn) and polydispersity index (PDI) of the mulch specimens based on polystyrene standards. Two replicates were analyzed per mulch specimen. Samples (approximately 20 mg) were dissolved in 5 mL of chloroform and stirred for one hour. Chloroform-dissolved samples were then centrifuged at 10,000 rpm (radius of 5.5 cm) for 1 min, and filtered (nylon, 0.45μm). The solution (200 L) was injected into a 300 x 7.5 mm ID PLgel Mixed C column from Varian, Inc. (Walnut Grove, CA), possessing packing of nominal 5 m. Chloroform (0.8 mL min-1) was used as the mobile phase, with the HPLC system containing an evaporative light scattering detector. Determination of Soil Chemical Properties Soil chemical properties, including pH, cation exchange capacity (CEC), total nitrogen (N) and total carbon (C), were measured. A soil sample (100-150 g) was randomly removed from each replicate of each treatment and was combined to provide a bulk soil sample of each treatment. Soil tests were performed at the Soils and Plant Analysis Laboratory of University of Tennessee, Nashville, TN. Soil pH was measured using the 1:1 water method [16] and total carbon was measured using the Dry Combustion method [17]. Total nitrogen was measured using Dumas method [18] and CEC was calculated from the summation of cations from the Mehlich 1 soil extraction method [19]. Statistical Analysis Analyses of the physical testing, GPC data and soil chemistry data were performed using SAS 9.2 (2011) software (SAS Institute Inc., Cary, NC USA). Each data point reported in the tables is the mean compared using Fisher’s Least significant difference (LSD) in SAS 9.2 with p<0.05. Since the three mulches are inherently different in their physicochemal nature, their physical properties were directly compared statistically only at 0 wk. The physical properties of each the SB PLA and MB PLA mulches were analyzed statistically for burial duration and soil treatment as separate datasets, with the initial or “time zero” measurements included in the datasets. Since values for thickness and MD breaking load of the mulches at 0 wk were not normally distributed, power and log transformation


were performed, respectively. Therefore, for these two measurements, back transformed means with respective standard errors were reported. RESULTS AND DISCUSSION SEM photomicrographs of the cellulosic, SB PLA and MB PLA initially and after 29 wk of burial in the soil, are shown in Figure 1. The cellulosic fibers, initially, were flat and ribbon-like in shape (Figure 1A) while the original SB PLA and MB PLA fibers were essentially round (Figure 1 B-C). As shown in Table I, the average width (i.e., fiber diameter) of the flat cellulosic fibers, was 20.8 µm, while the SB and MB PLA fibers were 14.8 µm and 6.3 µm,

respectively, prior to their soil burial. The latter two measurements are consistent with typical fiber sizes reported for SB and MB textiles [6]. No visible breaks in the filaments were observed with the SB and MB mulch specimens prior to treatment, or for the SB PLA exposed to any of the soil treatments after burial for 29 wk. However, MB PLA buried for 29 wk in all soil treatments had visible fiber breaks, indicating deterioration (Figure1 D-F). There were no visible fragments of the cellulosic mulches after 10 wk for all soil treatments, suggesting they underwent at least partial microbial assimilation, consistent with the disappearance of the cellulosic mulch in the open field studies conducted by us [10].

FIGURE 1. SEM photomicrographs of initial cellulosic (A), SB PLA (B), and MB PLA (C) specimens at 1000X, and of MB PLA at 500X after 29 wk of burial in control (D), lime-amended (E), and compost-amended (F) soil treatments.

As shown in Table I, values for each given property of the mulches prior to soil burial were inherently different (p<0.0001). The cellulosic mulch had the highest initial weight of 110.3 g/m2, but the lowest thickness of 0.255 mm. The initial air permeability of the SB PLA was almost three times greater than that of MB PLA, with the lower air permeability of the latter being attributed to the greater cover (lower porosity) afforded by the finer fibers. As would be

expected with the denser cellulosic mulch, initial air permeability was extremely low relative to the initial air permeability of SB and MB mulches. The MD breaking load and elongation of the SB was significantly higher than the MB, as expected [6], with the cellulosic mulch possessing the highest breaking load but the lowest breaking elongation of the three mulches. Cellulosic mulch had the greatest fiber diameter while MB PLA had the lowest.


TABLE I. Comparison of initial (0 wk) physical properties of mulches.

1 Means compared using Fisher’s Least significant difference (LSD) in SAS 9.2 (2011) with p values reflecting statistical significance; mean values in the same column with different letters are statistically different. 2 Standard error of the mean. 3The data were not normally distributed; hence, the power transformation was performed and the back transformed means with respectivestandard errors reported. 4 The data were not normally distributed; hence, the log transformation was performed, and the back-transformed means values with respective standard errors reported.

As revealed in Table II, the soil treatment did not statistically affect any of the measurements for SB PLA with the exception of weight (p<0.0001). Weight increased 45% over the duration of the experiment (p<0.0001). There were no differences due to treatment at 10 wk whereas compost resulted in a larger increase in weight at 29 wk (p<0.0001). Air permeability decreased 32% over time (p<0.0001) and at 10 wk was greatest for the lime treatment and at 29 wk was least for the compost treatment (p=0.0009). Thickness increased significantly with time for all soil treatments. These differences in mulch weight, air permeability, and thickness reflect the strong adsorption of soil particles onto the mulches. Moreover, the particulates could not be removed without further damaging the sample. These findings demonstrate that weight, thickness, and air permeability cannot be used to assess the occurrence of microbial degradation during

the 29 wk period of soil burial employed in this study. However, generally, such a time frame typically corresponds to an initial period during which the mulch material is “opened up,” to allow accessibility for microorganisms [5]. This event is often catalyzed by abiotic events, such as hydrolysis [5]. In addition, the strong adsorption by soil particulates may play an important role in the abiotic processes referred to above. Importantly, the MD breaking load decreased between 10 and 29 wk for all soil treatments, resulting in an approximately 75% reduction in strength from the initial value of 8.99 N to 2.2 N at 29 wk. Likewise, the MD elongation decreased significantly between 10 and 29 wk, although this reduction was only 16%. Therefore, SB PLA underwent significant weakening during the 29 wk soil burial period. The authors believe this deterioration reflects the “opening up” of the fibers, as described above.


TABLE II. Physical properties of SB PLA at 0 wk (initial) and after burial in three different soil treatments for 10 and 29 wk.

1Means compared using Fisher’s Least significant difference (LSD) in SAS 9.2 (2011) with p values reflecting statistical significance; mean values in the same column with different letters are statistically different whereas mean values with the same letter or no letter are statistically equivalent. 2Standard error of the mean. 3Soil treatments (main effect) were not statistically significant, except for weight that was shown in the interaction; hence, only p values were displayed.

In Table III, it can also be seen that MB PLA mulch underwent significant weakening during only 10 wk of soil burial suggesting it has a more rapid deterioration than the SB PLA mulch. The average MD breaking load of the MB PLA at 10 wk for all three soil treatments was 1.39 N, compared to 6.87 N for the initial MB PLA, a loss of approximately 80%. The MD breaking elongation also decreased with soil burial time (p<0.0001), from an initial value of 5.98% to 3.79% at 10 wk, a 37% reduction. As with SB PLA, the soil treatments did not significantly affect physical properties for MB PLA; also the weight and thickness of MB PLA increased appreciably with duration, presumably due to retained soil particulates. However, air permeability of the MB PLA increased significantly from 0 wk to 10 wk, which may refer to extensive “opening up” of the material (consistent with the observation of partial fragmentation at the macroscopic level) that is only partially offset by blockage via adsorbed soil particulates. As described in Table IV, the Mn and PDI of both nonwoven PLA mulches did not change due to soil treatment nor with burial duration, and there were no interactions between any variables. These results suggest the absence of hydrolysis or mineralization to

a significant extent. The authors believe biodegradation would occur during a longer duration, as the mulches undergo further weakening, resulting in increased exposure of individual fibers to microorganisms. The molecular weight of MB PLA mulch is significantly lower than that of SB PLA, reflecting the inclusion of the high melt index, hence low molecular weight, Ingeo PLA 3251D feedstock at a 20% level in the MB PLA. Soil chemical properties are presented in Table V. There was a significant increase in soil pH during the 29-wk study period for all mulch materials and soil treatments compared to the control non-amended soil. Soil pH levels were increased in all treatments above the threshold for healthy production of most vegetable crops (pH 7.0), indicating a potential need to adjust soil pH downwards if mulches are degraded in situ. CEC values did not differ significantly due to mulch type or duration in the study but did differ due to soil treatment in that they were higher for lime and compost treatments compared to non-amended soil. However, these values raise no concerns for vegetable crop production, and are similar to findings reported in other studies where composted manure was added to soil [20-22].


Total N and Total C in the compost-amended soil were higher than for other soil treatments, due to the inherent N and C content of the compost. Total N and Total C across all soil and mulch treatments did not

decrease significantly over the course of this study. These results suggest that N and C are not being added to the soil due to mulch degradation.

TABLE III. Physical properties of MB PLA at 0 wk (initial) and after soil burial in different treatments for 10 wk.

1 Means compared using Fisher’s Least significant difference (LSD) in SAS 9.2 (2011) with p values reflecting statistical significance; mean

values in the same column with different letters are statistically different whereas mean values with the same letter or no letter are statistically equivalent.

2 Standard error of the mean. 3 NT=Not tested due to large holes in the mulches 4Soil treatments (main effect) were not statistically significant; hence, only p values were displayed.

TABLE IV. Molecular weight based on polystyrene standards (Mn) and polydispersity index (PDI) of SB and MB PLA mulches at 0 wk and after burial in different soil treatments for 10 and 29 wk.

1 Means compared using Fisher’s Least significant difference (LSD) in SAS 9.2 (2011) with p values reflecting statistical significance; mean values in the same column with different letters are statistically different whereas mean values with the same letter or no letter are statistically equivalent. 2 Standard error of the mean. 3 Soil treatments (main effect) were not statistically significant; hence, only p values were displayed.


TABLE V. Soil chemical properties of the control (non-amended) and amended soils at 0 wk, 10 wk and 29 wk with cellulosic, SB and MB PLA mulches.

1 Means compared using Fisher’s Least significant difference (LSD) in SAS (2011) with p values reflecting statistical significance. Mean values in the same column with different letters are statistically different whereas mean values with the same letter are statistically equivalent. 2 Standard error of the mean. 3Mean values encompass both with and without mulches.

CONCLUSIONS The results from this greenhouse study suggest that PLA mulches, particularly the MB nonwoven, underwent deterioration when buried in soil for a 29 wk period. Moreover, the SB PLA and MB PLA mulch experienced a decrease in breaking load in the machine direction of 75% and 80%, respectively, for durations of 29 wk and 10 wk, respectively. This reflects achievement of the first stage of biodegradation, the “opening up” of the mulch material to allow for accessibility for microorganisms, typically catalyzed by abiotic events such as hydrolysis [5]. The type of soil treatment did not affect the extent of mechanical strength loss. SEM photomicrographs revealed that fiber breakage occurred in the MB PLA mulch after 29 wk for all soil treatments; however, fiber breakage was not observed for SB PLA. By 10 wk the cellulosic control had disappeared in all soil treatments, and there were no visible fragments available for testing.

These findings indicate that PLA nonwovens may be potentially valuable for agricultural mulches, particularly mulches that are compostable and useful in sustainable perennial and ornamental crop production systems. However, the formulations that were evaluated in this study are prototypes not yet likely suitable for use as mulches in annual cropping systems that would be expected to undergo full microbial assimilation within a 1-2 year period after being plowed into the soil. Additional greenhouse studies involving materials with design modifications tested under a broader set of environmental conditions are ongoing. ACKNOWLEDGEMENTS This research was funded in part through a grant from the NIFA Specialty Crops Research Initiative, USDA SCRI-SREP Grant Award No. 2009-02484.


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AUTHORS’ ADDRESSES Larry C. Wadsworth, PhD. Tommy L. Washington Department of Materials Science and Engineering The University of Tennessee 414 Ferris Hall 1508 Middle Drive Knoxville,TN 37996-2100 UNITED STATES Douglas G. Hayes, PhD Jaehoon Lee, PhD Robert Raley C. Tyler Pannell Sathiskumar Dharmalingam

Department of Biosystems Engineering and Soil Science The University of Tennessee 2506 E.J. Chapman Drive Knoxville, TN 37996 UNITED STATES Annette L.Wszelaki, Ph.D. Jeffrey Martin Department of Plant Sciences The University of Tennessee 2431 Joe Johnson Drive Knoxville, TN 37996 UNITED STATES Carol Miles, PhD Department of Horticulture & Landscape Architecture, Washington State University 16650 State Route 536 Mount Vernon, WA 98273 UNITED STATES Arnold M. Saxton, PhD Department of Animal Science The University of Tennessee 2640 Morgan Circle Drive Knoxville, TN 37996 UNITED STATES Debra Ann Inglis, PhD Department of Plant Pathology Washington State University NWREC State Route 536 Mount Vernon, WA 98273 UNITED STATES

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