Biodegradable mulch film

Abstract

This invention provides a biodegradable mulch film that is difficult to biodegrade while in use as a covering material, but easily biodegrades after use. [Solution] The biodegradable mulch film comprises a group of colored layers 2 containing resin components and colorants, and an outer layer 5 and an inner layer 6 arranged to sandwich the group of colored layers 2. The group of colored layers 2 includes a white layer 3 containing a white pigment as a colorant and a black layer 4 containing a black pigment as a colorant. The outer layer 5 and inner layer 6 contain aromatic aliphatic polyester.

Description

Technical Field

[0001] The present invention relates to a biodegradable multi-film.

Background Art

[0002] An agricultural multi-film is known as a covering material for covering fields for growing crops. The agricultural multi-film is used for the purpose of promoting stable growth of crops by ensuring heat retention of the soil, suppressing weeds, suppressing evaporation of moisture, and the like.

[0003] By the way, as an agricultural multi-film, a film having biodegradability (hereinafter referred to as "biodegradable multi-film") may be used for the purpose of omitting recovery and disposal after use and reducing the burden on users.

[0004] In relation to the above, Patent Document 1 discloses an invention of a biodegradable resin film including a reflective layer and a coloring layer. The reflective layer is composed of a resin composition containing a biodegradable aliphatic aromatic polyester resin and titanium oxide, and the coloring layer is composed of a resin composition containing a biodegradable aliphatic aromatic polyester resin and carbon black. According to the invention of Patent Document 1, the biodegradable resin film is biodegradable by, for example, plowing it into the soil after use.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] From the perspective of rapid biodegradation after use, a high biodegradability of biodegradable mulch film is desirable. However, if the biodegradability is high, decomposition may progress even while the film is being used as a covering material. In particular, if pigments are added as coloring agents, the film is more likely to disintegrate. If decomposition progresses while the film is being used as a covering material, its original purpose of promoting crop growth may be hindered.

[0007] Therefore, the object of the present invention is to provide a biodegradable mulch film that is difficult to biodegrade while in use as a covering material, and easily biodegradable after use. [Means for solving the problem]

[0008] A biodegradable mulch film according to one aspect of the present invention comprises a group of colored layers containing a resin component and a colorant, and an outer layer and an inner layer arranged to sandwich the group of colored layers. The group of colored layers includes a white layer containing a white pigment as a colorant and a black layer containing a black pigment as a colorant. The outer layer and the inner layer contain aromatic aliphatic polyester. [Effects of the Invention]

[0009] According to the present invention, a biodegradable mulch film is provided that is difficult to biodegrade while in use as a covering material, and easily biodegradable after use. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic cross-sectional view illustrating the biodegradable mulch film according to this embodiment. [Figure 2] Figure 2 is a graph showing the evaluation results of the biodegradation performance of Example 1 and Comparative Example 1. [Figure 3] Figure 3 is a graph showing the evaluation results of the biodegradation performance of Reference Examples 1 to 3. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described below with reference to the drawings.

[0012] Figure 1 is a schematic cross-sectional view illustrating the biodegradable mulch film 1 according to this embodiment. As shown in Figure 1, the biodegradable mulch film 1 comprises a colored layer group 2 containing resin components and colorants, and an outer layer 5 and an inner layer 6 arranged to sandwich the colored layer group 2. The biodegradable mulch film 1 according to this embodiment is used in the agricultural field as a covering material for covering, for example, ridges where crops are grown. When using the biodegradable mulch film 1, the side exposed to sunlight is the outer layer 5, and the side facing the soil is the inner layer 6.

[0013] The colored layer group 2 has a white layer 3 containing a white pigment as a coloring agent and a black layer 4 containing a black pigment as a coloring agent. In other words, the biodegradable mulch film 1 is a so-called black and white mulch film.

[0014] Here, the outer layer 5 and the inner layer 6 contain aromatic aliphatic polyester.

[0015] According to the above configuration, a biodegradable mulch film 1 is provided that is difficult to biodegrade while in use as a covering material, and easily biodegradable after use. That is, although the aromatic aliphatic polyester contained in the outer layer 5 and inner layer 6 is biodegradable, its biodegradation rate is relatively slow. Because the aromatic aliphatic polyester, which is difficult to decompose, is contained in the outer layer 5 and inner layer 6, even if the colored layer group 2 has a biodegradable structure, initial decomposition can be suppressed. In other words, the progress of biodegradation can be suppressed while in use as a covering material. Furthermore, after the decomposition of the outer layer 5 and inner layer 6 has progressed to a certain extent, the colored layer group 2 will be exposed. Therefore, if a highly biodegradable structure is adopted for the colored layer group 2, the biodegradation rate after use can be increased. As a result, a biodegradable mulch film 1 is realized that is difficult to decompose during use and easily decomposes after use.

[0016] The above is an overview of this embodiment. Next, the detailed configurations of the colored layer group 2, the outer layer 5, and the inner layer 6 will be described. <Colored layer group> As previously described, the colored layer group 2 has a white layer 3 and a black layer 4. The white layer 3 is located on the outside, and the black layer 4 is located on the inside. In the colored layer group 2, the coloring agent is dispersed in the resin. That is, each layer included in the colored layer group 2 contains resin, and the coloring agent is dispersed in that resin.

[0017] The resin content in colored layer group 2 is, for example, 50% by mass or more, preferably 70% by mass or more. The resin content may differ between the white layer 3 and the black layer 4. In this case, the resin content in colored layer group 2 refers to the total resin content in the entire colored layer group 2. For example, even if the resin content in the white layer 3 is less than 50% by mass, the total resin content in the entire colored layer group 2 may be 50% by mass or more depending on the resin content in the black layer 4. This also applies to the content of other components described below. (Aliphatic polyester) A suitable resin to be included in the colored layer group 2 is aliphatic polyester. Aliphatic polyester has high biodegradability. Therefore, by using aliphatic polyester, decomposition after use can be accelerated.

[0018] In this specification, "aliphatic polyester" means polyester that does not have repeating units containing aromatics. Therefore, "aromatic aliphatic polyester" is not included in "aliphatic polyester" as defined herein.

[0019] Examples of aliphatic polyesters include at least one selected from the group consisting of polylactic acid (PLA), polyhydroxyalkanoic acid (PHA), polybutylene succinate adipate (PBSA), polycaprolactone (PCL), polyglycolic acid (PGA), polyethylene adipate, polybutylene adipate, polyhydroxyadipate, polycyclohexylene dimethyl adipate, and compounds containing repeating units represented by the following formula (A) (hereinafter referred to as "aliphatic polyester A").

[0020] [Chemical formula]

[0021] (In the above formula (A), m1 and m2 are integers from 1 to 4.) Preferably, the aliphatic polyester contains the above-mentioned aliphatic polyester A and / or polyhydroxyalkanoic acid. Examples of the compound corresponding to the aliphatic polyester A include polybutylene succinate (PBS) and polyethylene succinate (PES), etc., and PBS is more preferable from the viewpoint of degradability. As PBS, commercially available products derived from plants (biomass-derived) are available. Using plant-derived PBS is also preferable from the viewpoint of environmental load. Examples of the polyhydroxyalkanoic acid include polyhydroxybutyric acid (PHB) and 3-hydroxybutyrate-co-3-hydroxyhexanoate (PHBH), etc.

[0022] The content of the aliphatic polyester contained in the colored layer group 2 is preferably 30% by mass or more. The content of the aliphatic polyester in the colored layer group 2 is preferably 40% by mass or more, and more preferably 40 to 80% by mass. (Aromatic aliphatic polyester) The colored layer group 2 may contain a resin other than the aliphatic polyester. Preferable examples of the other resin include aromatic aliphatic polyesters. In this specification, "aromatic aliphatic polyester" refers to a polyester having both a repeating unit having an aromatic group and a repeating unit not having an aromatic group. By blending an aromatic aliphatic polyester, the degradation rate during use can be controlled.

[0023] The aromatic aliphatic polyester is not particularly limited. Examples of the aromatic aliphatic polyester include a compound having two types of repeating units represented by the following formula (B) (hereinafter referred to as "aromatic aliphatic polyester B").

[0024] [ka]

[0025] (In formula (B) above, n1 and n2 represent the molar amounts of each repeating unit. The molar ratio (n1:n2) is, for example, 0.3:1.0 to 10:1.0. Preferably, the molar ratio (n1:n2) is 1.0:1.0 to 1.3:1.0. Also, m3, m4, and m5 are integers from 1 to 4. Furthermore, R is a substituent, representing a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms.) Preferably, as aromatic aliphatic polyester B, examples include polybutylene adipate terephthalate (PBAT) and polybutylene succinate terephthalate (PBST). Among aromatic aliphatic polyesters, PBST is more resistant to degradation. Therefore, by using PBST, the initial degradation can be further delayed. Plant-derived PBST is commercially available. By using plant-derived PBST, the environmental impact can be reduced.

[0026] In the colored layer group 2, the content of aromatic aliphatic polyester is preferably less than the content of aliphatic polyester. In other words, it is preferable that the amount of aliphatic polyester is greater than the amount of aromatic aliphatic polyester. This can further promote biodegradation after use. (white layer) Next, we will explain the white layer 3. As previously mentioned, the white layer 3 contains a white pigment. The white pigment consists of reflective particles.

[0027] While there are no particular limitations on the white pigment, titanium dioxide is preferred because it has extremely high reflectivity and can efficiently promote the biodegradation of the white layer 3.

[0028] The content of the white pigment in the white layer 3 is, for example, 15% to 40% by mass, preferably 20% to 30% by mass.

[0029] If the white pigment content is 15% by mass or more, the white layer 3 can achieve sufficient reflectivity. As a result, when covering ridges or other structures, the white layer 3 reflects light incident from the outside, suppressing excessive rise in soil temperature. Furthermore, it accelerates the breakdown of the resin during biodegradation. This further accelerates the biodegradation of the white layer 3.

[0030] The thickness of the white layer 3 is, for example, 1 to 20 μm, preferably 5 to 15 μm, from the viewpoint of ensuring sufficient reflectivity and biodegradability of the biodegradable mulch film 1.

[0031] In this embodiment, the colored layer group 2 may have one white layer 3 or two white layers 3. (Black layer) Next, we will explain the black layer 4. As previously mentioned, the black layer 4 contains black pigment. The black pigment is a light-shielding particle.

[0032] Examples of black pigments include carbon black, titanium black, acetylene black, lamp black, graphite, aniline black, cyanine black, and perylene black. In particular, it is preferable to include carbon black because it has extremely high light-shielding properties and can efficiently promote the biodegradation of the black layer 4.

[0033] The content of the black pigment in the black layer 4 is, for example, 1 to 15% by mass, preferably 3 to 10% by mass.

[0034] If the black pigment content is 1% by mass or more, the black layer 4 can achieve sufficient light-blocking properties. As a result, when covering ridges or other structures, the black layer 4 blocks light entering from the outside, suppressing weed growth. In addition, carbon black, like titanium dioxide, promotes the breakdown of the resin during biodegradation. This further accelerates the biodegradation of the black layer 4.

[0035] The thickness of the black layer 4 is, for example, 1 to 10 μm, preferably 1 to 5 μm, from the viewpoint of ensuring sufficient light-shielding and biodegradability of the biodegradable mulch film 1. (Aliphatic polyester content in the black and white layers) Preferably, the aliphatic polyester content (mass%) in the black layer 4 is greater than that in the white layer 3. The amount of white pigment required to obtain the desired reflectivity tends to be greater than the amount of black pigment required to obtain the desired light-shielding properties. In other words, when optimizing reflectivity and light-shielding properties, the amount of white pigment in the white layer 3 tends to be greater than the amount of black pigment in the black layer 4. That is, from the viewpoint of pigment content, the black layer 4 is less prone to biodegradation than the white layer 3. Therefore, by increasing the aliphatic polyester content (mass%) in the black layer 4 compared to that in the white layer 3, it becomes easier to equalize the biodegradability of the white layer 3 and the black layer 4. As a result, biodegradability after use can be further improved. (Total thickness of colored layer group 2) The thickness of the colored layer group 2, which includes the white layer 3 and the black layer 4, is, for example, 5 to 20 μm, preferably 10 to 15 μm. (Other additives) The colored layer group 2 may also contain other additives as needed. Examples of other additives include decomposition accelerators, lubricants, weathering agents, slip agents, heat insulating materials, antiblocking agents, plasticizers, film-forming aids, thickeners, pigment dispersants, and heat stabilizers.

[0036] In particular, the biodegradability of the colored layer group 2 after use can be further enhanced by adding the aforementioned decomposition accelerator. Examples of decomposition accelerators include inorganic substances other than white pigments. Examples of such inorganic substances include calcium carbonate. <Outer layer and inner layer> As previously described, the outer layer 5 and inner layer 6, which form the surface layer of the biodegradable mulch film 1, contain aromatic aliphatic polyester. The aromatic aliphatic polyester used here is the same as that described for the colored layer group 2.

[0037] Particularly preferable, the outer layer 5 and inner layer 6 may contain PBST (polybutylene succinate terephthalate) from the viewpoint of sufficiently suppressing the progression of biodegradation during use as a covering material.

[0038] The aromatic aliphatic polyester content in the outer layer 5 and the inner layer 6 is, for example, 60% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more, respectively.

[0039] The thickness of the outer layer 5 and the inner layer 6 is, for example, 0.5 to 10 μm, preferably 1 to 5 μm, from the viewpoint of sufficiently suppressing the progress of biodegradation while they are used as covering materials. The outer layer 5 and inner layer 6 may contain other additives as needed. Examples of other additives include lubricants, weathering agents, slip agents, heat insulating materials, antiblocking agents, plasticizers, film-forming aids, thickeners, and heat stabilizers and hydrolysis inhibitors. However, it is preferable that inorganic pigments are not included in order to avoid unnecessarily accelerating decomposition during use. Furthermore, the content of components other than aromatic aliphatic polyesters is, for example, 20% by mass or less, preferably 15% by mass or less, and more preferably 10% by mass or less. (Thickness of biodegradable mulch film) The overall thickness of the biodegradable mulch film 1 is, for example, 10 to 30 μm, preferably 15 to 20 μm. (Reflectivity) The biodegradable multifilm 1 according to this embodiment has high reflectivity. For example, the reflectivity of the biodegradable multifilm 1 at a wavelength of 550 nm is 65% or more. In a preferred embodiment, the reflectivity is 67% or more. (Light blocking property) The biodegradable multi-film 1 according to this embodiment has high light-shielding properties. For example, the light-shielding rate of the biodegradable multi-film 1 at a wavelength of 550 nm is 95% or more. In a preferred embodiment, the light-shielding rate is 97% or more. (Biomass content) As previously described, plant-derived resins can be used as the resin contained in the biodegradable mulch film 1. For example, the biodegradable mulch film 1 has a biomass content of 25% by weight or more. Preferably, the biomass content is 30% by weight or more. Such a biomass content can be achieved, for example, by using plant-derived PBS as the aliphatic polyester in the colored layer group 2 and plant-derived PBST as the aromatic aliphatic polyester contained in the outer layer 5 and inner layer 6. (Manufacturing method) The method for producing the biodegradable multifilm 1 is not particularly limited. For example, the biodegradable multifilm 1 having a multilayer structure can be produced by co-extrusion. For example, the inflation method can be used as the extrusion molding method. [Examples]

[0040] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples unless it exceeds the essence of the invention. (1) Evaluation of the biodegradability of biodegradable mulch films (Example 1) A composition for the colored layer, an outer layer composition, and an inner layer composition were prepared. The prepared compositions were molded using a co-extrusion extrusion method (inflation method) to obtain the biodegradable mulch film according to Example 1. Table 1 shows the composition and thickness of each layer. In Table 1, Bio-PBST refers to plant-derived PBST, and Bio-PBS refers to plant-derived PBS. Furthermore, both Bio-PBST and PBAT used in this embodiment had the structure shown by formula (B) described above, and the molar ratio of the constituent units (n1:n2) was in the range of 1.0:1.0 to 1.3:1.0.

[0041] Furthermore, the biomass content of the biodegradable mulch film in Example 1 was 34.1% by weight. Furthermore, for Example 1, the light reflectance and light shielding rate at a wavelength of 550 nm were measured using an ultraviolet-visible-near-infrared spectrophotometer (JASCO, V770). The light reflectance and light shielding rate were calculated based on the following formulas, by measuring the incident light intensity, reflected light intensity, and transmitted light intensity when light was irradiated from the outer layer side.

[0042] Light reflectance (%) = reflected light intensity / incident light intensity x 100 Light shading rate (%) = (1 - transmitted light intensity / incident light intensity) × 100 As a result, the light reflectance of Example 1 was 69.2%, and the light shielding rate was 99.1%. These values ​​were comparable to those of typical black and white multi-films.

[0043] [Table 1]

[0044] (Comparative Example 1) As Comparative Example 1, a biodegradable mulch film (20 μm thick) related to commercially available product A was prepared. Commercial product A had a white layer and a black layer, but lacked an outer layer and an inner layer. The resin components contained in the white and black layers were a mixture of PBAT and PLA, and the resin component content relative to the total mass of the film was 74% by mass. In addition, inorganic substances, including titanium dioxide as a white pigment, were contained at a content of 26% by mass relative to the total mass of the film. (Evaluation method) The biodegradability of Example 1 and Comparative Example 1 was evaluated by the following method.

[0045] The film was cut into 50mm x 50mm pieces, and their initial weight was measured. The measured film pieces were placed in a mesh net. Next, a culture medium containing high-temperature resistant bacteria, adjusted to a moisture content of approximately 25%, was spread in the container. The film pieces, still in the net, were buried in the culture medium, and the container was stored at 60°C and 90% RH. During storage, the culture medium dried out, so moisture was periodically added using a spray bottle to compensate for the weight loss. After two weeks of storage, the film pieces were removed from the culture medium, and their weight was measured again. At this time, as much of the culture medium adhering to the film pieces as possible was removed by brushing and washing. The decomposition rate was calculated from the weight loss before and after burial using the following formula.

[0046] Decomposition rate (%) = 100 - (Weight after storage / Initial weight) × 100 After weighing, the film pieces were placed in a net and buried in the growing medium, and stored again under the same conditions. After another two weeks, the weight of the film pieces was measured again, and the decomposition rate was calculated using the formula described above. Based on the calculated decomposition rate, the biodegradability of the sample film and the comparison film was evaluated. (Evaluation results) Figure 2 shows the evaluation results. Specifically, Figure 2 shows the relationship between elapsed time (days) and the decomposition rate as a graph for Example 1 and Comparative Example 1.

[0047] As shown in Figure 2, Example 1 showed a lower decomposition rate compared to Comparative Example 1. However, compared to Comparative Example 1, Example 1's decomposition rate was 10% (=13-3%) lower after 14 days, while after 28 days it was 5% (=38-33%) lower. From this, it can be understood that Example 1, which has aromatic aliphatic polyester in both the outer and inner layers, shows a lower initial decomposition rate (slope) compared to Comparative Example 1, which lacks both outer and inner layers, and gradually exhibits a decomposition rate comparable to or higher than that of commercially available products. In other words, Example 1 is initially difficult to biodegrade, but becomes easier to biodegrade over time. (2) Examination of the relationship between polyester type and biodegradability (Reference example 1) As a reference example 1, a film (30 μm thick) was prepared from a resin composition containing 90% by mass of Bio-PBS (aliphatic polyester) and 10% by mass of PBAT (aromatic aliphatic polyester). (Reference example 2) As a reference example 2, a film (30 μm thick) composed solely of PBAT (aromatic aliphatic polyester) was prepared. (Reference example 3) As reference example 3, a film (30 μm thick) composed solely of Bio-PBST (aromatic aliphatic polyester) was prepared. (evaluation) For Reference Examples 1-3, the materials were buried in a field, and the decomposition rate over time was measured. The results are shown in Figure 3. As shown in Figure 3, Reference Examples 2 and 3, which consist of aromatic aliphatic polyesters, showed more suppressed initial degradation than Reference Example 1, which consists almost entirely of aliphatic polyesters. This confirms the effect of aromatic aliphatic polyesters in suppressing initial degradation. Furthermore, Reference Example 3, which consists of PBST, showed even greater suppression of initial degradation than Reference Example 2, which consists of PBAT. From this, it can be understood that using PBST can further suppress initial degradation compared to using PBAT. [Note] The main embodiments of the present invention and their effects are summarized below as an appendix. (Note 1) A biodegradable mulch film comprising a group of colored layers 2 containing resin components and colorants, and an outer layer 5 and an inner layer 6 arranged to sandwich the group of colored layers 2, wherein the group of colored layers 2 includes a white layer 3 containing a white pigment as a colorant and a black layer 4 containing a black pigment as a colorant, and the outer layer 5 and inner layer 6 contain aromatic aliphatic polyester.

[0048] According to the above configuration, the white layer 3 and black layer 4 included in the colored layer group 2 each contain coloring agents, and are therefore easily biodegraded, for example, when tilled into the soil. On the other hand, the outer layer 5 and inner layer 6 contain aromatic aliphatic polyester, which is difficult to biodegrade, and are therefore difficult to biodegrade while used as a covering material. (Note 2) A biodegradable mulch film as described in Appendix 1, wherein the colored layer group 2 contains an aliphatic polyester.

[0049] According to the above configuration, the colored layer group 2 contains easily biodegradable aliphatic polyester, and therefore, after being used as a covering material, it can be easily biodegraded by, for example, tilling it into the soil. (Note 3) A biodegradable mulch film as described in Appendix 1 or 2, wherein the aliphatic polyester content in the colored layer group 2 is 30% by mass or more.

[0050] The above configuration can suppress the efficient progress of biodegradation. (Note 4) A biodegradable mulch film as described in any one of the appendices 1 to 3, wherein the aliphatic polyester content (mass%) in the black layer 4 is greater than the aliphatic polyester content (mass%) in the white layer 3.

[0051] The above configuration makes it easier to equalize the biodegradability of the white layer 3 and the black layer 4. As a result, the biodegradability after use can be further enhanced. (Note 5) A biodegradable mulch film as described in any one of the appendices 1 to 4, wherein the colored layer group 2 further contains inorganic substances other than white pigments.

[0052] The above configuration can accelerate decomposition after use. (Note 6) A biodegradable mulch film as described in any one of the appendices 1 to 5, wherein the white pigment contains titanium dioxide.

[0053] According to the above configuration, the biodegradable mulch film 1 can achieve high reflectivity. Furthermore, the biodegradation process can be efficiently accelerated. (Note 7) A biodegradable mulch film as described in any one of the appendices 1 to 6, wherein the black pigment contains carbon black.

[0054] According to the above configuration, the biodegradable mulch film 1 can achieve high light-blocking properties. Furthermore, the biodegradation process can be efficiently accelerated. (Note 8) A biodegradable mulch film as described in any one of the appendices 1 to 7, wherein the biomass content is 25% by weight or more. [Explanation of Symbols]

[0055] 1: Biodegradable mulch film, 2: Colored layers, 3: White layer, 4: Black layer, 5: Outer layer, 6: Inner layer

Claims

1. A group of colored layers containing resin components and colorants, An outer layer and an inner layer are arranged so as to sandwich the aforementioned group of colored layers, Equipped with, The aforementioned colored layer group is The aforementioned coloring agent includes a white layer containing a white pigment, The aforementioned coloring agent comprises a black layer containing a black pigment, The outer layer and inner layer contain aromatic aliphatic polyester. Biodegradable mulch film.

2. A biodegradable mulch film according to claim 1, The aforementioned colored layer group includes an aliphatic polyester, Biodegradable mulch film.

3. A biodegradable mulch film according to claim 2, The aliphatic polyester content in the aforementioned colored layer group is 30% by mass or more. Biodegradable mulch film.

4. A biodegradable mulch film according to claim 2 or 3, The aliphatic polyester content (mass%) in the black layer is greater than the aliphatic polyester content (mass%) in the white layer. Biodegradable mulch film.

5. A biodegradable mulch film according to claim 1 or 2, The aforementioned colored layer group further contains inorganic substances other than the white pigment. Biodegradable mulch film.

6. A biodegradable mulch film according to claim 1 or 2, The aforementioned white pigment contains titanium dioxide. Biodegradable mulch film.

7. A biodegradable mulch film according to claim 1 or 2, The aforementioned black pigment contains carbon black. Biodegradable mulch film.

8. A biodegradable mulch film according to claim 1 or 2, The biomass content is 25% by weight or more. Biodegradable mulch film.

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