"Improved End-of-Life of Plastic Mulches"; illustration of large strawberry with tractor tilling mulch into a strawberry field.

What is a Soil-Biodegradable Plastic Mulch Composed Of?

Summary

Soil-biodegradable mulches (BDMs) are increasingly used in agriculture to replace conventional plastic mulch and reduce waste and disposal challenges. BDMs are designed to be tilled into the soil after use at the end of growing season. Growers and agricultural professionals have questions regarding the formulation of BDMs. This is an introduction to what BDMs are made from.

Project funding acknowledgement

Strawberry plants growing in BDM plastic mulch. — Figure 1. Strawberry grown with soil biodegradable plastic mulch. Photo by L.W. DeVetter.

Soil-biodegradable plastic mulches (BDMs) are an alternative mulch technology designed to biodegrade in soil upon tillage through the activity of native soil microorganisms. They were created in the early 1990s as an alternative to non-degradable plastic polymers, primarily polyethylene (PE), that reduce plastic waste generation in agriculture. Functionally, BDMs should be 100% biodegradable and provide the same horticultural benefits as non-degradable mulches including weed suppression, soil temperature moderation, soil moisture retention, and overall increased yields and crop quality. BDMs should biodegrade in field soil or a composting environment and form no environmentally harmful residues.

But what is a BDM composed of and what provides the various properties in this alternative mulch? This factsheet and the List of primary ingredients that may be found in a soil-biodegradable plastic mulch provides a general list of ingredients that may be found in commercial BDMs and lists their various functions. In general, 75-95% by mass of BDM films are composed of polymeric feedstocks with the remainder being additives/minor components. Feedstocks are the primary raw material used to make a BDM and are biobased, derived from fossil fuels, or a blend of the two. Commercially available BDMs are a blend of both biobased and fossil fuel feedstocks with biobased content ranging from 10-50%. Biobased content does not correlate with biodegradability. One example of BDM created for an experiment contained 78.7% polymers, 13.3% binder (calcium carbonate), 4.4% plasticizer and processing aid, 2.0% carbon black, and 1.6% slip agent (Hayes et al., 2017). Individual mulch manufacturer formulations will vary, however, as they are proprietary and are unique to the company.

Note that a glossary of terms associated with soil-biodegradable mulches for specialty crops can also be found online.

List of primary ingredients that may be found in a soil-biodegradable plastic mulch.

Polymeric Feedstocks

Biobased – Starch

Extracted from natural materials and includes thermoplastic starch (TPS) and cellulose. TPS is made from high-amylose starch derived from a number of sources, such as potato, corn, sugar beets, etc. The high-amylose starch is processed at high temperatures with water and alcohols to make TPS. TPS is one of the most common biobased feedstock due to its cheap cost.

Biobased – Polylactic acid (PLA)^{See Notes 1}

Lactic acid units are chemically linked together to form long chains known as polylactic acid (PLA). Lactic acid is made industrially using genetically modified bacteria that ferment carbohydrates into lactic acid. PLA is used in other applications, such as 3-D printing to make models and prototypes, including objects used in the medical field. PLA is a thermoplastic polyester.

Biobased – Polyhydroxyalkanoates (PHA)

A biodegradable polyester and fatty acid biopolymers produced from genetically modified microorganisms (e.g., bacteria and yeasts). Includes poly(hydroxybutyrate) (PHB) and poly(hydroxyvalerate) (PHV).

Fossil fuel – Polybutyleneadipate-co-terephthalate (PBAT) ^{See Notes 2, 3}

A biodegradable co-polymer or co-polyester made from organic materials including adipic acid, 1,4-butanediol, and terephthalic acid (DMT). Description of organic materials provided below. Properties are similar to PE.

Fossil fuel – Poly-caprolactone (PCL)

Biodegradable polyester.

Fossil fuel – Poly-butylene succinate (PBS) and poly-butylene succinate Adipate (PBSA) ^{See Notes 2, 3}

Biodegradable polyester with properties similar to non-degradable polyethylene.

Notes

Although the monomer, lactic acid, is biobased, PLA is a “synthetic” because the polymerization is performed chemically using a catalyst.
Feedstocks prepared by some manufacturers are partially biobased; for instance, the monomers 1,4-butanediol and succinic acid.
Low molecular weight oligomers and monomers (e.g., 1,4-butanediol and adipic acid) may be present at trace levels (< 1%).

Additives

Plasticizers

Added to improve flexibility and processing properties of brittle polymers. Glycerol, sorbitol, and triethyl citrate are examples of plasticizers that may be used to make BDMs. Phthalate-based plasticizers were once used, but no longer due to human-health concerns.

Lubricants

Modifies material viscosity and improves processing properties (e.g., vegetable oils such as canola or castor oil) (Mitrovich et al. 2012).

Fillers

Includes wood, silica, clay, or other polymers. Added to improve mulch properties and reduce costs. Also added to improve processing and in-field performance.

Nucleating or clarifying agents

Increases crystallization of polymeric ingredients, which in turn can increase physical mulch properties like stiffness and tensile strength (e.g., thermopalstic starch) (Kang et al., 2008).

Antioxidants

Prevent loss of strength, flexibility, thermal stability, and/or col-or to mulch film from oxidation during mulch manufacturing and after in-field application (e.g., flavenoids) (Arrigo and Dintcheva, 2017).

UV Stabilizers

Prevents polymer break down by UV-light. Example is carbon black, which is a pigment and also a UV-stabilizer.

Antibacterial additives

Added to prevent bacterial growth on mulch (e.g., zinc pyrithione, silver nanoparticles).

Colorant

Synthetic organic material used to provide color to a material. Can also function as a stabilizer.

Pigment

Inorganic material used to provide color to a material. Example is carbon black, which is commonly and may or may not be naturally derived. Titanium dioxide (TiO₂) may be used for white mulch, but has human and environmental health concerns and may be replaced by talc or calcium carbonate (CaCO₃). Pigments can also function as a stabilizer.

References

Arrigo, R. and N.D. Dintcheva. 2017. Natural anti-oxidants for bio-polymeric materials. Archives in Chemical Research. 1:2.
Hayes, D.G., L.C. Wadsworth, H.Y. Sintim, M. Flury, M. English, S. Schaeffer, and A. Saxton. 2017. Effect of diverse weathering conditions on the physicochemical properties of biodegradable plastic mulches. Polymer Testing 62:454-467.
Kang, K.S., S.I. Lee, T.J. Lee, R. Narayan, and B.Y. Shin. 2008. Effect of biobased and biodegradable nucleating agent on the isothermal crystallization of poly (lactic acid). Korean Journal of Chemical Engineering 25:599-608.
Mitrovich, M.J., B. Lanz, and M. Molz. 2012. Biodegradable plastic based lubricant compositions. Canadian Patent CA2827702A1.

This material is based upon work that is supported by Western Sustainable Agriculture Research and Education, under award number WPDP19-05. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and should not be construed to represent any official USDA or U.S. Government determination or policy

U.S. Department of Agriculture, National Institute of Food and Agriculture

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