applied sciences

Article Physical and Degradable Properties of Mulching Films Prepared from Natural Fibers and Biodegradable Polymers

Zhijian Tan *, Yongjian Yi, Hongying Wang, Wanlai Zhou, Yuanru Yang and Chaoyun Wang *

Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, Changsha 410205, China; [email protected] (Y.Y.); [email protected] (H.W.); aruofl[email protected] (W.Z.); [email protected] (Y.Y.) * Correspondence: [email protected] (Z.T.); [email protected] (C.W.); Tel.: +86-731-8899-8517 (Z.T.); +86-731-8899-8501 (C.W.); Fax: +86-731-8899-8501 (C.W.)

Academic Editor: Samuel B. Adeloju Received: 2 March 2016; Accepted: 6 May 2016; Published: 12 May 2016

Abstract: The use of film in has the serious drawback of producing vast quantities of waste. In this work, films were prepared from natural fibers and biodegradable polymers as potential substitutes for the conventional non- film used as mulching material in agricultural production. The physical properties (e.g., mechanical properties, heat preservation, permeability, and photopermeability) and degradation characteristics (evaluated by micro-organic culture testing and soil burial testing) of the films were studied in both laboratory and field tests. The experimental results indicated that these fiber/polymer films exhibited favorable physical properties that were sufficient for use in mulching film applications. Moreover, the degradation degree of the three tested films decreased in the following order: fiber/starch (ST) film > fiber/poly(vinyl alcohol) (PVA) film > fiber/polyacrylate (PA) film. The fiber/starch and fiber/PVA films were made from completely biodegradable materials and demonstrated the potential to substitute non-biodegradable films.

Keywords: mulching film; biodegradation; natural fiber;

1. Introduction The practice of using mulching film to improve the growth and yield of annual and perennial crops has long been recognized [1]. Mulching film preserves heat and moisture, reduces pressure from and pathogens, and conserves water and fertilizer [2–4]. In these ways, mulching film contributes to sustainable agricultural production. Plastic films (e.g., (PE), poly(), , copolymers of ethylene with vinyl acetate) are the most widely used materials because of their excellent mechanical properties and low cost. However, the major problem associated with the use of non-biodegradable plastic films is that they can pollute soil when they are buried in landfills; in addition, the removal and disposal of these plastic residues from the field either before or after harvest is costly and time-consuming [1,5]. For these reasons, farmers usually incorporate used films into the soil; occasionally, they burn the used plastic films, causing harmful pollution [6]. In order to increase the sustainability of agricultural practices and to overcome the disposal problems associated with conventional plastic films, the development and use of mulching films based on biodegradable materials is highly desirable. Until now, various films capable of being degraded by microorganisms in the soil have been developed, including oxo-degradable plastic film [7], paper film [8], and biodegradable film [9–11]. Oxo-degradable plastic is low-cost and easy to install, but the parts of the film that are buried in the soil do not break down and must be exposed to light and air for degradation [12]. Paper film has been

Appl. Sci. 2016, 6, 147; doi:10.3390/app6050147 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 147 2 of 11 considered as an alternative to plastic film; however, paper film suffers from very rapid degradation and is easily broken down after exposure to soil, rain, and wind [13]. Furthermore, in order to cause no undesirable effects to the performance of agricultural soil, no harmful substances resulting from the degradation of biodegradable materials used as mulching film should accumulate in the soil [14]. The fibrous materials derived from renewable crops or from their by-products or agro-industrial wastes are good candidates for use in blends and composites [15]. Natural fibers have good mechanical properties and are biodegradable, which makes them highly suitable for use in films in combination with biodegradable polymers. Some natural fibers, such as ramie, flax, hemp, and cotton, are abundant in China and are mainly used for textiles. However, the waste fiber has not been suitably recycled after industrial production. Some biodegradable polymers such as starch, poly(vinyl alcohol) (PVA), poly(butylene -co-terephthalate) (PBAT), poly(butylene succinate-co-adipate) (PBSA), and poly(lactic acid) (PLA) have been used to prepare biodegradable agricultural films [13,14,16–20]. Some films based on natural fibers and polymers were also prepared, such as an orange fiber residue/PVA film [21] and an Osage orange wood/PLA film [10]. These films were prepared using the mixture of grinded fibers and a polymer aqueous solution. The aim of this study was to prepare films based on natural fibers and biodegradable polymers as potential substitutes for plastic films. In this work, three mulching films were severally developed from natural fibers (a blend of waste ramie and cotton fiber) and polymers (PA, ST, and PVA) using the method of air laying in nonwoven fabric. The biodegradability of the films was studied in detail.

2. Materials and Methods

2.1. Materials and Reagents Three mulching films (fiber/PVA, fiber/PA, and fiber/ST, originally white) were prepared in our cooperative enterprise (YuanJiang Jingzhu Science and Technology Co. Ltd, Yiyang, China). The fiber/PA, fiber/ST, and fiber/PVA films all had an approximate thickness of 0.35 mm and weight of 40 g/m2, which contained approximately 16% polymer (polyacrylate, starch, or poly(vinyl alcohol), respectively) and 84% waste fibers in the textile industry (ramie and cotton fibers with a mass ratio of 4:1). The properties of ramie fiber are as follows: approximately 2–5 cm in length, 30 µm in diameter, a density of 1.49 g/cm3, and 6.5% moisture content. The properties of cotton fiber are as follows: approximately 13 mm in length, 20 µm in diameter, a density of 1.58 g/cm3, and 7.2% moisture content. All the polymers were of technical grade. Polyacrylate (30% emulsion) was mixed copolymers of methyl acrylate, ethyl acrylate, and butyl acrylate, with an average molecular weight of 1,2000 and purchased from Zhuzhou Chemical Factory, Zhuzhou, China; corn starch was purchased from Suzhou Shuanghuan Chemical Technology Co., Ltd., Suzhou, China, with an average molecular weight of 15,000 and purity of ě 98%; PVA (model 100-40H) was purchased from Shanxi Sanwei Group Co., Ltd., Linfen, China, with an average molecular weight of 14,000 and alcoholysis degree ě 99%. In addition, a commonly used PE film (white and black) was purchased from the market with a thickness of 0.04 mm.

2.2. Physical Properties of Mulching Films

2.2.1. Mechanical Properties The tensile strength and percentage elongation of each film were measured using a Tininus Olsen testing machine (Model H5KL, Tinius Olsen Testing Machine Company, Horsham, PA, USA) according to ASTM D4964-64 (Test method for tension and elongation of elastic fabrics). The test was carried out before the trial in the field, and each sample was determined in triplicate.

2.2.2. Heat Preservation Heat preservation was investigated by performing a field test that involved the cultivation of Chinese cabbage in the garden of Wangcheng District, Changsha, China (latitude: 28˝181 N, longitude: Appl. Sci. 2016, 6, 147 3 of 11

112˝511 E, altitude: 50 m). The soil temperature was recorded by a soil temperature recorder (model TZS-6W-G, Zhejiang Top Instrument Co., Ltd., Hangzhou, China). The heat preservation behavior of the three fiber/polymer films was similar; therefore, only the results from the fiber/PVA film have been reported. The films were laid on the field, and the temperature at the position of 0 cm, 10 cm, and 20 cm under the soil surface was recorded with no film as a control using the reported method by Moreno [22]. The test was carried out over a period of 39 days from November 8, 2014 to December 17, 2014.

2.2.3. Water Permeability The films’ water permeability was assessed using 6-cm-diameter Fisher Payne permeability cups that contained 5 mL of water and were sealed by the films. No film was set as control [21]. The experiment was performed in an air-conditioned room (25 ˝C temperature and 60% relative humidity (RH)) and variations in the water weight over time were recorded.

2.2.4. Photopermeability The percentage of light transmittance was measured at different wavelengths (450, 650, and 850 nm) using an ultraviolet-visible (UV-Vis) spectrophotometer (UV-2100, Unico, San Diego, CA, USA), with the sample placed in the light path using the reported method by Kijchavengkul [23].

2.2.5. Thermal Stability Thermal gravimetric analysis (TGA) was performed to analyze the thermal stability of the film samples using a TGA/DSC 1 SF/1382 analyzer (METTLER TOLEDO, Shanghai, China). A sample of 10 mg was placed in an aluminum pan and heated from 25 to 600 ˝C at a heating rate of 10 ˝C/min under a nitrogen atmosphere.

2.3. Biodegradability Tests

2.3.1. Micro-Organic Culture Test Microbial measurements were based on the dilution method of plate counting. Suspensions of soil collected from the garden of Wangcheng District, Changsha, China, which had been covered with the fiber/PVA and PE films for the cultivation of Chinese cabbage, were prepared. The properties for the soil were as follows: Thesoil pH was 6.05, the soil carbon was 24.52 g/kg, total nitrogen was 1.28 g/kg, total phosphorus was 0.85 g/kg, and total potassium was 17.37 g/kg. The test was carried out over a period of 49 days from November 1, 2014 to December 19, 2014. The fiber/PVA film was used for the micro-organic culture test by laying the film at the bottom of the culture dish. Beef-protein agar medium was used to culture bacteria at 37 ˝C for 48 h, Martin substratum was used to culture fungi at 28 ˝C for 72 h, and Gause’s No. 1 synthetic medium was used to culture Actinomycetes at 28 ˝C for 72 h. The measurement of microorganism was performed in triplicate.

2.3.2. Soil Burial Test Soil burial biodegradation tests were performed in a flowerpot with a capacity of approximately 1000 mL. Soil samples were obtained from the test garden in Wangcheng District, Changsha, China. All of the film samples (50 mm ˆ 50 mm) were buried in the pot under 10 cm of soil with 65% moisture. The temperature was 10–25 ˝C, and the air humidity was 25%–45%. The test was carried out over a period of 45 days in the laboratory from November 4, 2013 to December 18, 2013. At different time points, samples were collected from the pot, gently cleaned by rinsing with water and brushing, dried to a constant weight at 105 ˝C, and weighed. The percentage weight loss of the samples was calculated according to the following equation:

Weight loss “ rpM0 ´ M1q{M0s ˆ 100% , (1) Appl. Sci. 2016, 6, 147 4 of 11

where M0 is the initial mass of the films before the test, and M1 is the residual mass of the films after the test. Fourier transform-infrared spectroscopy (FT-IR) was used to analyze the samples before and after degradation by the standard KBr pellet method. The IR spectra were obtained using a Thermo Nicolet 380 Fourier transform infrared spectrometer (Thermo Nicolet Corporation, Madison, WI, USA) in the range of 4000–600 cm´1. The films were tested before and after degradation using the transmission method. Scanning electron microscopy (SEM) was performed to observe surface changes on the films before andAppl. after Sci. 2016 degradation., 6, 147 The film samples were coated with gold, and the surface4 wasof 11 observed using a HitachiFourier S-4800 transform scanning‐infrared electron spectroscopy microscope (FT‐IR) (Hitachi, was used Tokyo, to analyze Japan) the samples with a before stated and resolution of 1.4 nm at 1after kV degradation acceleration by the voltage. standard KBr pellet method. The IR spectra were obtained using a Thermo Nicolet 380 Fourier transform infrared spectrometer (Thermo Nicolet Corporation, Madison, WI, 3. ResultsUSA) and in Discussion the range of 4000–600 cm−1. The films were tested before and after degradation using the transmission method. Scanning electron microscopy (SEM) was performed to observe surface 3.1. Mechanicalchanges Properties on the films before and after degradation. The film samples were coated with gold, and the surface was observed using a Hitachi S‐4800 scanning electron microscope (Hitachi, Tokyo, Japan) The resultswith a stated from resolution the tensile of 1.4 strength nm at 1 kV test acceleration and the percentage voltage. elongation of the four tested films are shown in Table1; these results indicated that the tensile strength and elongation of the fiber/polymer 3. Results and Discussion films were poorer than that of commonly used PE film. The fiber/polymer films of this tensile strength were sufficiently3.1. Mechanical strong Properties for use as a mulching film in our field test. The results from the tensile strength test and the percentage elongation of the four tested films Tableare 1. Tensileshown strengthin Table at1; breakthese ofresults the different indicated mulching that the tensile films. ST:strength Starch; and PVA: elongation Poly(vinyl of the alcohol); PA: Polyacrylate;fiber/polymer PE: films Polyethylene were poorer thana. that of commonly used PE film. The fiber/polymer films of this tensile strength were sufficiently strong for use as a mulching film in our field test. Machine Percentage Cross-Machine Percentage Type of Film Table 1. Tensile(average, strength at MPa) break of the differentElongation mulching (%) films. ST: Starch;(Average, PVA: MPa)Poly(vinyl alcohol);Elongation (%) PA: Polyacrylate; PE: Polyethylene a. Fiber/ST film 2.92b 24.77b 3.52a 9.06b Fiber/PVA film 3.58a,bMachine 23.71bPercentage Cross‐ 2.62bMachine Percentage 10.92b Type of Film Fiber/PA film 3.69a(average, MPa) 10.92bElongation (%) (Average, 3.74a MPa) Elongation 18.31b(%) PE filmFiber/ST film 12.96c2.92b 132.3a24.77b 3.52a 12.96c 9.06b 132.3a aFiber/PVAThe different film lowercases indicate3.58a,b significant difference23.71b at P < 0.05 level2.62b in the same column.10.92b Fiber/PA film 3.69a 10.92b 3.74a 18.31b PE film 12.96c 132.3a 12.96c 132.3a 3.2. Heat Preservation a The different lowercases indicate significant difference at P < 0.05 level in the same column. The fiber/PVA and PE films were used to culture Chinese cabbage; the control used no film. 3.2. Heat Preservation The results in Figure1 show that the ability of the films to preserve temperature followed the The fiber/PVA and PE films were used to culture Chinese cabbage; the control used no film. The following order: PE film > fiber/PVA film > no film. The ability of the fiber/polymer film to results in Figure 1 show that the ability of the films to preserve temperature followed the following preserve heatorder: was PE film poorer > fiber/PVA than film that > no of film. the The PE ability film; of however, the fiber/polymer our field film testto preserve demonstrated heat was that the fiber/polymerpoorer film than couldthat of the be PE beneficial film; however, under our certain field test conditions, demonstrated such that the as fiber/polymer during the film summer could season or in high-temperaturebe beneficial under regions. certain conditions, such as during the summer season or in high‐temperature regions.

20 18 Fiber/PVA film 16 PE film 14 No film 12 10 20 cm

) 8 6 ℃ 20 18 16 14 12 10 cm 10 8

20 Temperature ( 18 16

14 12 0 cm 10 8

Dec.3-Dec.7 Dec.8-Dec.12 Nov.28-Dec.2 Nov.8-Nov.12 Dec.13-Dec.17 Nov.13-Nov.17 Nov.18-Nov.22 Nov.23-Nov.27 Figure 1. Temperatures measured under soil (at depths of 0, 10, and 20 cm) covered by different Figure 1. mulchingTemperatures films (fiber/PVA measured and PE) under and no soil film. (at depths of 0, 10, and 20 cm) covered by different mulching films (fiber/PVA and PE) and no film. Appl. Sci. 2016, 6, 147 5 of 11

Appl. Sci. 2016, 6, 147 5 of 11 3.3. Water Permeability Appl.3.3. Water Sci. 2016 Permeability, 6, 147 5 of 11 The water permeability of the three fiber/polymer films was measured under constant conditions of 253.3.˝C Water andThe 60% waterPermeability RH. permeability The results of shown the three in Figure fiber/polymer2 reveal thatfilms the was water measured permeability under constant of different conditions of 25 °C and 60% RH. The results shown in Figure 2 reveal that the water permeability of treatmentsThe decreased water permeability in the following of the order:three nofiber/polymer film > fiber/ST films >was fiber/PVA measured > under fiber/PA. constant It can be different treatments decreased in the following order: no film > fiber/ST > fiber/PVA > fiber/PA. It can inferredconditions that this of 25 arrangement °C and 60% RH. is in The line results with shown the hydroscopicity in Figure 2 reveal of that the the films water and, permeability indirectly, of is in be inferred that this arrangement is in line with the hydroscopicity of the films and, indirectly, is in accordancedifferent with treatments the hydrophilic decreased ability in the following of polymers. order: Both no film fiber/polymer > fiber/ST > fiber/PVA films allowed > fiber/PA. water It tocan easily accordance with the hydrophilic ability of polymers. Both fiber/polymer films allowed water to easily be inferred that this arrangement is in line with the hydroscopicity of the films and, indirectly, is in permeatepermeate into into the soilthe soil and and maintain maintain the the moisture moisture contentcontent of of the the soil, soil, which which had had a better a better water water holding holding accordance with the hydrophilic ability of polymers. Both fiber/polymer films allowed water to easily capacitycapacity than than PE film.PE film. permeate into the soil and maintain the moisture content of the soil, which had a better water holding capacity than PE film. 100 Fiber/PA film Fiber/ST film 100 80 Fiber/PAFiber/PVA film film Fiber/STNo film film 80 Fiber/PVA film 60 No film

60 40

40

Water permeated (%) Water 20

Water permeated (%) Water 20 0 012345 0 Time (days) 012345 Figure 2. Water permeability Timeof the ( dathreeys) fiber/polymer films. Figure 2. Water permeability of the three fiber/polymer films. 3.4. Photopermeability Figure 2. Water permeability of the three fiber/polymer films. 3.4. Photopermeability 3.4. PhotopermeabilityThe determination of photopermeability can provide the data of the extent to which the white Thefiber/polymer determination films can of photopermeabilityprohibit the light. Figure can provide3 shows that the datathe light of the transmittance extent to which of the thethree white The determination of photopermeability can provide the data of the extent to which the white fiber/polymerwhite fiber/polymer films can films prohibit is below the 25%, light. much Figure lower3 shows than that that of thethe lightPE film; transmittance the black PE film of the had three fiber/polymer films can prohibit the light. Figure 3 shows that the light transmittance of the three alight transmittance below 3%. Therefore, the fiber/polymer films were able to prohibit the growth whitewhite fiber/polymer fiber/polymer films films is is below below 25%, 25%, much much lower than than that that of of the the PE PE film; film; the theblack black PE film PE filmhad had of weeds by reducing the transmitted light to some extent. Some natural dyes, such as carbon black, alightalight transmittance transmittance below below 3%. 3%. Therefore, Therefore, the the fiber/polymerfiber/polymer films films were were able able to prohibit to prohibit the growth the growth tannin, and humic substances, can be blended with the films to produce different colors. of weedsof weeds by reducing by reducing the the transmitted transmitted light light to to somesome extent. Some Some natural natural dyes, dyes, such such as carbon as carbon black, black, tannin,tannin, and and humic humic substances, substances, can can be be blended blended with with the films films to to produce produce different different colors. colors. 100 450 nm 90 100 650 nm 80 450850 nm 90 650 nm 70 80 850 nm 60 70 50 60 40 50

30 40 Light transmittance/ % transmittance/ Light 20 30 Light transmittance/ % transmittance/ Light 10 20 0 10

A 0 /ST PE r/PA r/PV e er e A Fib Fib Fib Black PE /ST PE r/PA r/PV e er e Fib Fib Figure 3. PhotopermeabilityFib of the four tested films.Black PE

FigureFigure 3. 3.Photopermeability Photopermeability of the the four four tested tested films. films. Appl. Sci. Sci. 20162016,, 66,, 147 147 66 of of 11 11 Appl. Sci. 2016, 6, 147 6 of 11 3.5. Thermal Stability 3.5. Thermal Stability The TGA curves in Figure 4 show that weight loss began to occur for the three fiber/polymer The TGA curves in Figure 4 show that weight loss began to occur for the three fiber/polymer filmsThe at approximately TGA curves in 300 Figure °C, indicating4 show that that weight their lossthermal began stability to occur was for poorer the three than fiber/polymer that of the PE films at approximately 300 °C, indicating that their thermal stability was poorer than that of the PE film;films however, at approximately they were 300 sufficiently˝C, indicating stable that for their use as thermal a mulching stability film was at temperatures poorer than that occurring of the PEin film; however, they were sufficiently stable for use as a mulching film at temperatures occurring in thefilm; natural however, environment. they were sufficiently stable for use as a mulching film at temperatures occurring in the naturalthe natural environment. environment.

100 100 80 80

60 Fiber/PVA film 60 Fiber/STFiber/PVA film film 40 Fiber/PAFiber/ST film 40 PEFiber/PA film film Weight /% Weight PE film Weight /% Weight 20 20 0 0 100 200 300 400 500 600 100 200 300 400 500 600 Temperature/ ℃ Temperature/ ℃ Figure 4. Thermal gravimetric analysis (TGA) curves for the four tested films. Figure 4. Thermal gravimetric analysis (TGA) curves for the four tested films.films. 3.6. Biodegradability 3.6. Biodegradability 3.6.1. Micro‐Organic Culture Test 3.6.1. Micro Micro-Organic‐Organic Culture Test The three fiber/polymer films behaved similarly in the micro‐organic culture test through our The three fiber/polymerfiber/polymer films films behaved behaved similarly similarly in in the the micro micro-organic‐organic culture test through our primary experiments; therefore, only the results from the fiber/PVA film had been reported. The soil primary experiments; therefore, therefore, only only the the results results from from the the fiber/PVA fiber/PVA film film had had been been reported. reported. The Thesoil microbial quantities are presented in Figure 5 and show that the amount of microbes increased over soilmicrobial microbial quantities quantities are presented are presented in Figure in Figure 5 and5 andshow show that thatthe amount the amount of microbes of microbes increased increased over time. The bacteria and actinomycetes were richest using fiber/polymer films, and the fungi was overtime. time. The Thebacteria bacteria and andactinomycetes actinomycetes were were richest richest using using fiber/polymer fiber/polymer films, films, and and the the fungi fungi was richest using the PE film. The covering of films can produce more microbial, which can be caused by richest using the the PE PE film. film. The The covering covering of of films films can can produce produce more more microbial, microbial, which which can can be becaused caused by the degradation of the film or heat generated beneath the films; thus, the increase in bacteria and bythe the degradation degradation of ofthe the film film or or heat heat generated generated beneath beneath the the films; films; thus, thus, the the increase increase in in bacteria and actinomycetes might be beneficial for the degradation of this fiber/polymer film. actinomycetes might be beneficialbeneficial for the degradation ofof thisthis fiber/polymerfiber/polymer film. film.

FigureFigure 5. 5. TheThe amount amount of of microorganisms microorganisms in in soil soil covered covered by by different different films films between between November November 1, 2014 2014 Figure 5. The amount of microorganisms in soil covered by different films between November 1, 2014 andand December 19, 2014. and December 19, 2014. 3.6.2. Soil Soil Burial Burial Test Test 3.6.2. Soil Burial Test TheThe weight lossloss ofof thethe samples samples in in the the soil soil burial burial test test is shownis shown in Tablein Table2; the 2; resultsthe results indicated indicated that The weight loss of the samples in the soil burial test is shown in Table 2; the results indicated thatthe degradationthe degradation degree degree of the of the three three fiber/polymer fiber/polymer films films decreased decreased in in the the following following order: order: fiber/ST fiber/ST that the degradation degree of the three fiber/polymer films decreased in the following order: fiber/ST filmfilm > fiber/PVA fiber/PVA film film > > fiber/PA fiber/PA film. film. film > fiber/PVA film > fiber/PA film. Appl. Sci. 2016, 6, 147 7 of 11

Table 2. The weight loss of fiber/polymer films after different degradation time by soil burial test.

Start Time/Initial End Time/Weight Type of Film Weight Loss (%) Time, Months Weight (g) (g) Appl. Sci. 2016, 6, 147 Nov. 4/0.2578 Nov. 18/0.2065 19.9% 0.57 month of 11 Fiber/PVA Nov. 4/0.2893 Dec. 4/0.1896 34.5% 1 month Table 2. The weightNov. 4/0.2697loss of fiber/polymer Dec. films 18/0.056 after different degradation 79.2% time by soil burial test. 1.5 month Start Time/Initial End Weight Loss Fiber/PAType of Film Nov. 4/0.3448 Dec. 4/0.2895 16.0%Time, Months 1 month Weight (g) Time/Weight (g) (%) Fiber/ST Nov. 4/0.3527 Dec. 4/0.1634 53.7% 1 month Nov. 4/0.2578 Nov. 18/0.2065 19.9% 0.5 month PE filmFiber/PVA Nov.Nov. 4/0.1050 4/0.2893 Dec.Dec. 4/0.1030 4/0.1896 34.5% 0% 1 month 1 month Nov. 4/0.2697 Dec. 18/0.056 79.2% 1.5 month Fiber/PA Nov. 4/0.3448 Dec. 4/0.2895 16.0% 1 month In the soil burial test, the three films were degraded for four weeks. The samples were Fiber/ST Nov. 4/0.3527 Dec. 4/0.1634 53.7% 1 month characterizedPE by film FT-IR beforeNov. and 4/0.1050 after degradationDec. 4/0.1030 to identify the0% changes in1 spectra month intensities in the films’ functional groups [23]. The results are shown in Figure6. Before degradation, the fiber/PVA filmIn the showed soil burial characteristic test, the three absorption films were bands degraded at 3331 for cmfour´ 1weeks.(O–H The stretching), samples were 2902 cm´1 characterized by FT‐IR´ before1 and after degradation to identify ´the1 changes in spectra intensities in (C–H stretching), 1628 cm (C–O stretching), and 1316 cm (–CH2 bending). In addition, the absorptionthe films’ bands functional at 1159 groups cm´ [23].1 (anti-symmetric The results are shown stretching in Figure of 6. Before the C–O–C degradation, bridge) the fiber/PVA and 1028 cm´1 film showed characteristic absorption bands at 3331 cm−1 (O–H stretching), 2902 cm−1 (C–H (skeletal vibrations involving C–O stretching) were characteristic of a saccharide structure. The IR stretching), 1628 cm−1 (C–O stretching), and 1316 cm−1 (–CH2 bending). In addition, the absorption spectra forbands the at fiber/ST 1159 cm−1 film(anti‐ andsymmetric the fiber/PA stretching film of the were C–O–C similar bridge) to and that 1028 of the cm− fiber/PVA1 (skeletal vibrations film, although ´ the fiber/PAinvolving film C–O had stretching) a characteristic were characteristic absorption of a band saccharide at 1727 structure. cm 1 (C=OThe IR stretching).spectra for the Figure fiber/ST6 shows major differencesfilm and the offiber/PA absorption film were intensity similar to in that spectra of the fiber/PVA for both film, the fiber/PVAalthough the andfiber/PA fiber/starch film had films before anda characteristic after degradation, absorption whereas band at 1727 the cm fiber/PA−1 (C=O stretching). film spectra Figure show 6 shows little variancemajor differences before of and after absorption intensity in spectra for both the fiber/PVA and fiber/starch films before and after degradation. The IR spectra indicates that the contents of the samples changed with the absorption degradation, whereas the fiber/PA film spectra show little variance before and after degradation. The intensity,IR also spectra changing indicates some that characteristicthe contents of peaks.the samples These changed findings with also the indicate absorption that intensity, the fiber/PVA also and fiber/starchchanging films some degraded characteristic rapidly peaks. during These this findings period, also while indicate the that fiber/PA the fiber/PVA film degraded and fiber/starch more slowly by contrast.films degraded rapidly during this period, while the fiber/PA film degraded more slowly by contrast.

Fiber/PA film

Fiber/PVA film Absorbance

Fiber/ST film

4000 3500 3000 2500 2000 1500 1000

-1 Wavenumber (cm )

Figure 6.FigureFourier 6. Fourier transform-infrared transform‐infrared spectroscopy spectroscopy (FT-IR)(FT‐IR) spectra spectra for for the the three three fiber/polymer fiber/polymer films films before (solid line) and after (dashed line) degradation. before (solid line) and after (dashed line) degradation. SEM micrographs of films taken before and after the soil burial test are presented in Figure 7. SEMThe micrographs samples had of many films ripples taken and before holes and after after 1 themonth soil of burial degradation, test are indicating presented the in partial Figure 7. The samplesbiodegradation had many ripples of the and polymer holes and after fiber; 1 month however, of degradation,degradation of indicatingthe fiber requires the partial more time. biodegradation The of the polymerSEM micrographs and fiber; showed however, that degradation the degradation of the degree fiber of requires the fiber/PVA more time.and fiber/ST The SEM films micrographs was greater than that of the fiber/PA film. showed that the degradation degree of the fiber/PVA and fiber/ST films was greater than that of the fiber/PA film. Appl. Sci. 2016, 6, 147 8 of 11 Appl. Sci. 2016, 6, 147 8 of 11

Figure 7. Scanning electron microscopy (SEM) images of film samples before and after degradation. Figure 7. Scanning electron microscopy (SEM) images of film samples before and after degradation. (A‐1) original fiber/starch film; (A‐2) fiber/starch film after 4 weeks of degradation. (B‐1) original (A-1) original fiber/starch film; (A-2) fiber/starch film after 4 weeks of degradation. (B-1) original fiber/PVA film; (B‐2) fiber/PVA film after 4 weeks of degradation. (C‐1) original fiber/polyacrylate fiber/PVA film; (B-2) fiber/PVA film after 4 weeks of degradation. (C-1) original fiber/polyacrylate film; (C‐2) fiber/polyacrylate film after 4 weeks of degradation. film; (C-2) fiber/polyacrylate film after 4 weeks of degradation. Both the field and laboratory tests indicate the potential usefulness of the fiber/starch film; however, because of its rapid degradation in soil (approximately 1–2 months in different seasons), it can only be used as a mulching film for crops with short growth periods. The fiber/PVA and fiber/PA Appl. Sci. 2016, 6, 147 9 of 11

Both the field and laboratory tests indicate the potential usefulness of the fiber/starch film; however, because of its rapid degradation in soil (approximately 1–2 months in different seasons), it can only be used as a mulching film for crops with short growth periods. The fiber/PVA and fiber/PA films can be completely degraded in 2–3 months and 3–4 months, respectively, during different seasons according to the findings of our field test. PVA and starch were reported as biodegradable polymers; however, it reported that polyacrylate was not a biodegradable polymer [24–27]. Therefore, the fiber/PA film will not be suitable for large-scale applications. On the whole, the fiber/polymer films used in this study have a much shorter degradation time, which can only be used for crops with a short growth periodicity. Therefore, much work should be done by us to prolong the degradation time of these fiber/polymer films in our further studies.

4. Conclusions The use of biodegradable films in agriculture can promote sustainability, reduce , and increase the use of renewable raw materials obtained from agro-industrial waste. The wastes of some fibers such as ramie and cotton can be recycled, and PVA and starch are biodegradable polymers that can be used to prepare mulching film. The tested fiber/polymer films showed good physical properties. The water permeability and heat preservation tests demonstrated that the films had good water permeability and heat preservation capability. The micro-organic culture and soil burial tests demonstrated the good biodegradability of the films. The soil burial test and the FT-IR and SEM analyses indicated that the degradation degree of the three films decreased in the following order: fiber/ST film > fiber/PVA film > fiber/PA film. In the view of the biodegradability, fiber/ST and fiber/PVA films are more suitable than non-degradable films for use as mulching film for crops, but much work should be done to prolong the degradation time of these fiber/polymer films.

Acknowledgments: This work was financially supported by the earmarked fund for the China Agriculture Research System (No. CARS-19), and the Agricultural Science and Technology Innovation Program (No. ASTIP-IBFC07). Author Contributions: Zhijian Tan and Chaoyun Wang conceived and designed the experiments; Yongjian Yi, Hongying Wang, and Wanlai Zhou performed the experiments; Zhijian Tan and Yuanru Yang analyzed the data; Zhijian Tan wrote and revised the paper. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: PVA poly(vinyl alcohol) PA polyacrylate ST starch PE polyethylene PBAT poly(butylene adipate-co-terephthalate PBSA poly(butylene succinate-co-adipate PLA poly(lactic acid) RH relative humidity TGA thermal gravimetric analysis FT-IR Fourier transform-infrared spectroscopy SEM scanning electron microscopy

References

1. Shogren, R.L.; Rousseau, R.J. Field testing of paper/polymerized vegetable oil for enhancing growth of eastern cottonwood trees for pulp. Forest Ecol. Manag. 2005, 208, 115–122. [CrossRef] Appl. Sci. 2016, 6, 147 10 of 11

2. Liu, M.; Huang, Z.; Yang, Y.J. Analysis of biodegradability of three biodegradable mulching films. J. Polym. Environ. 2010, 18, 148–154. [CrossRef] 3. Liu, E.; He, W.; Yan, C. ‘White revolution’ to ‘white pollution’—Agricultural plastic film in china. Environ. Res. Lett. 2014, 9, 091001. [CrossRef] 4. Li, C.; Moore-Kucera, J.; Miles, C.; Leonas, K.; Lee, J.; Corbin, A.; Inglis, D. Degradation of potentially biodegradable plastic mulch films at three diverse us locations. Agroecol. Sust. Food 2014, 38, 861–889. [CrossRef] 5. Tocchetto, R.; Benson, R.; Dever, M. Outdoor weathering evaluation of carbon-black-filled, biodegradable copolyester as substitute for traditionally used, carbon-black-filled, nonbiodegradable, high-density polyethylene mulch films. J. Polym. Environ. 2001, 9, 57–62. [CrossRef] 6. Briassoulis, D.; Babou, E.; Hiskakis, M.; Kyrikou, I. Analysis of long-term degradation behaviour of polyethylene mulching films with pro-oxidants under real cultivation and soil burial conditions. Environ. Sci. Pollut. R. 2015, 22, 2584–2598. [CrossRef][PubMed] 7. Corti, A.; Sudhakar, M.; Chiellini, E. Assessment of the whole environmental degradation of oxo-biodegradable linear low density polyethylene (LLDPE) films designed for mulching applications. J. Polym. Environ. 2012, 20, 1007–1018. [CrossRef] 8. Anderson, D.F.; Garisto, M.A.; Bourrut, J.C.; Schonbeck, M.W.; Jaye, R.; Wurzberger, A.; DeGregorio, R. Evaluation of a paper mulch made from recycled materials as an alternative to plastic film mulch for vegetables. J. Sustain. Agric. 1995, 7, 39–61. [CrossRef] 9. Russo, R.; Malinconico, M.; Petti, L.; Romano, G. Physical behavior of biodegradable alginate-poly(vinyl alcohol) blend films. J. Polym. Sci. Pol. Phys. 2005, 43, 1205–1213. [CrossRef] 10. Finkenstadt, V.L.; Tisserat, B. Poly(lactic acid) and Osage Orange wood fiber composites for agricultural mulch films. Ind. Crop Prod. 2010, 31, 316–320. [CrossRef] 11. Scarascia-Mugnozza, G.; Schettini, E.; Vox, G.; Malinconico, M.; Immirzi, B.; Pagliara, S. Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment. Polym. Degrad. Stab. 2006, 91, 2801–2808. [CrossRef] 12. Cirujeda, A.; Aibar, J.; Anzalone, A.; Martin-Closas, L.; Meco, R.; Moreno, M.M.; Pardo, A.; Pelacho, A.M.; Rojo, F.; Royo-Esnal, A.; et al. Biodegradable mulch instead of polyethylene for control of processing tomato production. Agron. Sust. Dev. 2012, 32, 889–897. [CrossRef] 13. Shogren, R.L. Preparation and characterization of a biodegradable mulch: Paper coated with polymerized vegetable oils. J. Appl. Polym. Sci. 1999, 73, 2159–2167. [CrossRef] 14. Kapanen, A.; Schettini, E.; Vox, G.; Itävaara, M. Performance and environmental impact of biodegradable films in agriculture: A field study on protected cultivation. J. Polym. Environ. 2008, 16, 109–122. [CrossRef] 15. Chiellini, E.; Cinelli, P.; Chiellini, F.; Imam, S.H. Environmentally degradable bio-based polymeric blends and composites. Macromol. Biosci. 2004, 4, 218–231. [CrossRef][PubMed] 16. Su, J.F.; Huang, Z.; Liu, K.; Fu, L.L.; Liu, H.R. Mechanical properties, biodegradation and water vapor permeability of blend films of soy protein isolate and poly(vinyl alcohol) compatibilized by glycerol. Polym. Bull. 2007, 58, 913–921. [CrossRef] 17. Xiong, H.; Tang, S.; Tang, H.; Zou, P. The structure and properties of a starch-based biodegradable film. Carbohyd. Polym. 2008, 71, 263–268. [CrossRef] 18. Bilck, A.P.; Grossmann, M.V.; Yamashita, F. Biodegradable mulch films for strawberry production. Polym. Test 2010, 29, 471–476. [CrossRef] 19. Kijchavengkul, T.; Auras, R.; Rubino, M.; Selke, S.; Ngouajio, M.; Fernandez, R.T. Biodegradation and hydrolysis rate of aliphatic aromatic . Polym. Degrad. Stabil. 2010, 95, 2641–2647. [CrossRef] 20. Yamamoto-Tamura, K.; Hiradate, S.; Watanabe, T.; Koitabashi, M.; Sameshima-Yamashita, Y.; Yarimizu, T.; Kitamoto, H. Contribution of soil esterase to biodegradation of aliphatic polyester agricultural mulch film in cultivated soils. AMB Express 2015.[CrossRef][PubMed] 21. Imam, S.; Cinelli, P.; Gordon, S.; Chiellini, E. Characterization of biodegradable composite films prepared from blends of poly(vinyl alcohol), cornstarch, and lignocellulosic fiber. J. Polym. Environ. 2005, 13, 47–55. [CrossRef] 22. Moreno, M.; Moreno, A. Effect of different biodegradable and polyethylene mulches on soil properties and production in a tomato crop. Sci. Hortic. 2008, 116, 256–263. [CrossRef] Appl. Sci. 2016, 6, 147 11 of 11

23. Kijchavengkul, T.; Auras, R.; Rubino, M.; Ngouajio, M.; Fernandez, R.T. Assessment of aliphatic-aromatic copolyester biodegradable mulch films. Part I: Field study. Chemosphere 2008, 71, 942–953. [CrossRef] [PubMed] 24. Chandra, R.; Rustgi, R. Biodegradable polymers. Prog. Polym. Sci. 1998, 23, 1273–1335. [CrossRef] 25. Leja, K.; Lewandowicz, G. Polymer biodegradation and biodegradable polymers—A review. Pol. J. Environ. Studies 2010, 19, 255–266. 26. Amass, W.; Amass, A.; Tighe, B. A review of biodegradable polymers: Uses, current developments in the synthesis and characterization of biodegradable , blends of biodegradable polymers and recent advances in biodegradation studies. Polym. Int. 1998, 47, 89–144. [CrossRef] 27. Siracusa, V.; Rocculi, P.; Romani, S.; Rosa, M.D. Biodegradable polymers for food packaging: A review. Trends Food. Sci. Technol. 2008, 19, 634–643. [CrossRef]

© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).