Sustainable Materials for Horticultural Application

David Grewell 1*, Gowrishanker Srinivasan1, Dept.of Agricultural and Biosystems Engineering, James Schrader 2, William Graves2 Department of Horticulture, Michael Kessler 3 Dept .of Material science and Engineering

Iowa State University. Ames, IA, USA

Abstract

Bioplastic materials were compounded utilizing soy, poly-lactic acid (PLA), and poly-hydroxyalkanoate (PHA) biopolymers along with ethanol industry co-products and biomass additives to manufacture horticultural plant containers. Various formulations and processing conditions were studied to improve mechanical properties of the plastics. These materials were developed and compounded at Iowa State University and subsequently injection molded into 4.5-inch greenhouse pots at R&D/Leverage, Lee's Summit, Missouri. The bioplastic pots were evaluated for their performance by studying plant growth of vegetable and ornament crops grown in them under greenhouse and field conditions. The pots were also characterized for degradation and water retention. Commercial polypropylene pots, 4.5” green color, were used as the control treatment for the study. Comprehensive growth studies along with degradation results identified numerous bioplastic types that performed as well as or better than commercial polypropylene plant containers. Among the different material types, SP.A-PLA, a blend of soy and PLA resins, was observed to produce the best results in terms of plant growth compared to polypropylene plastic pots during plant production. This is attributed to the slow release of fertilizing compounds during the degradation of soy protein. Certain bioplastic pot types were observed to retain soil moisture content over a longer time period than pots made from other environmentally friendly materials, such as paper or peat moss. Such properties are considered beneficial during the plant production cycle when using horticultural pots because they require less watering.

Introduction Bioplastics for mass production applications are yet to achieve wide-spread market acceptance in the plastics industry, despite the commercial introduction of viable bioplastic resins. The adoption of bioplastics by the plastic industry has largely been influenced by limited properties such as water sensitivity or brittleness and lack of processing know-how for this class of materials. Other factors influencing broader introduction of this material class include a weak-recovering economy together with price fluctuations of agricultural raw materials caused by recent drought conditions in the United States. However, a

proposed solution is to develop materials or compounds that are both cost-effective and provide an additional advantage of being multi-functional for specific applications. Such an approach would help promote their commercial acceptance. Developing multifunctional materials involves combining the positive characteristics of two or more components to formulate the ideal material for a certain application/field such as the horticultural industry. Studies and estimates indicate that under ideal conditions the fruit produce industry in the state of California alone disposes up to 107,000 tons of agricultural plastics waste annually [1]. In addition, it is estimated that more than 2 billion plant-pot containers are generated as waste by the container-crops horticultural industry nationwide [2]. The use of biodegradable-multifunctional plastic materials, such as soy-protein plastics for horticultural pots may significantly reduce waste. Because agricultural products like these have a relatively short service life, these applications are ideal for plastics that decompose within a similar time frame. In more detail, products such as pots, temporary ground covers, mulch film, and temporary field markers are products that only need to last a year or less and are ideal target applications for protein-based plastics. In addition, protein-based plastics have an inherent fertilizing effect, making them beneficial for horticultural applications such as plant pots [3, 4]. They are formulated from plant proteins, which are polymerized amino acids. Degradation of these materials releases nitrogen and other important plant nutrients. Thus, these plastics are multi-functional, serving as structural materials and as fertilizers. However, soy-protein plastics are hydrophilic, and so far, higher water stability has been accomplished only at the detriment of mechanical strength [5]. A promising solution is to use soy-protein plastics compounded with polylactic acid (PLA) for the production of agricultural plastic products. This approach improves the mechanical properties and water stability of the resulting soy protein-PLA based plastics. For these applications, PLA is a good polymer for compounding with soy, because the products are intended to decompose in the field, and PLA, being compostable, causes no adverse environmental impacts related to disposal. Under natural conditions, decomposition of PLA requires relatively long time periods (1-2 years or more). Decomposition of PLA can be accelerated by applying commercial composting conditions where humidity and

temperature are relatively high. However, the addition of plant protein or natural biomass fillers along with oligomers such as polyethylene glycol (PEG) accelerates the decomposition and overall degradability of the PLA matrix in soil because degradation of the proteins / fillers increases the exposed surface area, which in turn increases the possible reaction sites for hydrolysis of the PLA and opens the material to microbial attack. We investigated the use of compounded soy, PLA, and PHA plastics along with agricultural co-products for horticultural applications with service lives shorter than 1 year. The bioplastic pots were designed to serve from plant germination to the time at which the entire pot was transplanted in the ground with the plant root system. The engineered slow rate of degradation of the plastic would provide a fertilizer or soil-conditioning effect for the plant during the growing season. Compared to other commercially available bio-based pots, such as paper-fiber or peat-moss pots, the proposed formulations have the advantage that they are relatively water proof and do not require extra watering in order to assure plant growth. In addition, many of the existing degradable materials, other than the proposed material, are mechanically weak and can be torn easily during handling. The proposed design is cost-effective and provides multi-functional advantages: 1) Water retention 2) Fertilizer effect 3) Structural stability 4) Degradability In order to optimize the bioplastics, various formulations (e.g., with varying soy protein and flour content) were produced and compared to conventional plastic containers. Performance was characterized in terms of material strength, shelf life, and biodegradation, in addition to plant health, growth, and fruit production.

Materials and Methods

A total of 14 injection-moldable plant-pot formulations were studied. They included 9 bio-composite materials along with 5 as-supplied biopolyester resins and one commercially available standard polypropylene pot type. The formulations evaluated during this work included PLA, PHA, and a blend of soy flour /soy protein isolate. In addition, fillers such as DDGs (Dry Distillers Grains with solubles), a co-product from corn-to-ethanol production, and corn stover were also studied. The soy-protein formulations are detailed in Table 1 and the rest of the plastics were commercially-available resins compounded with various bio-fillers. The following sections discuss the processing of the soy-based formulations and bioplastic composites in detail.

Soy Plastics and Blends Raw materials: Soy protein isolate (SPI, ~90% protein) was obtained from Solae Company, St. Louis, MO. Defatted soy flour (SF) was purchased from Archer Daniels Midland (IL). Injection-molding grade polylactic acid (PLA) 3001D was procured from NatureWorks LLC. Plasticizer and salts, i.e. glycerol (Gly), sodium sulfite, sorbic acid (potassium salt) and polyethylene glycol (PEG 8000), were obtained from Fisher Scientific (Pittsburgh, PA). Chemicals phthalic anhydride (PA), and adipic acid (AA) were procured from Sigma Aldrich and Acros Organics. Preparation and Processing: The resin formulations were compounded with two fractions, a solid and a liquid fraction. The solid fraction consisted of 1 Kg of SPI:SF (50:50) for all formulations. The liquid fraction consisted of a mixture of water, glycerol, and both sodium sulfite and sorbic acid (K-salt) for both SP and SP.A formulations. For both SP and SP.A formulations, the measurements were based on their final percentage (%) concentration of each component in the molded plastic and these concentrations are as detailed in Table 2. The mass of anhydride chemistry added was compensated by reducing the glycerol in order to maintain a consistent total mass of the liquid fraction for all formulations. Once prepared, both the solid and liquid fractions were mixed together in a high speed mixer (Henschel Mixers American, Inc., Houston, TX) to obtain a powdery soy resin mixture. This mixture was further extruded on a twin-screw compounder (Liestriz Micro 27, L/D ratio 30, American Liestriz Corp., Somerville, NJ) to obtain an extrudate, which was pelletized with a pelletizer (Scheer Bay Inc. WI) to produce plastic pellets. The temperature profile during extrusion followed a gradation of 95 to 110°C between the hoppers to the die for all formulations except for phthalic anhydride formulations which ranged between 95 and 120°C. The plastic pellets were air dried to a moisture level of 10-15%. Both SP-PLA and SP.A-PLA formulations were processed by compounding SP or SP.A pellets with PLA at a 50:50 concentration. Prior to compounding, PLA melt temperature was modified/lowered by compounding the PLA with 20% PEG 8000 by weight. This melt-temperature modified PLA was utilized for SP-PLA and SP.A-PLA to avoid thermal degradation of soy resin during compounding. Biopolyester composites Raw Materials: Injection-molding grade polylactic acid (PLA) 3001D was procured from Nature works LLC. Molding grade polyhydroxy alkanoates (PHA) resin grades, P1003, P1004, P1008 and P4010 were secured as

a donation from Metabolix (Lowell, MA). DDGs and corn stover were obtained from local sources. Preparation and Processing: All composites of the respective resins were processed by compounding DDGs or stover at 10% concentration (by weight) utilizing a twin-screw compounder (Liestriz Micro 27, L/D ratio 30, American Liestriz Corp., Somerville, NJ) to obtain an extrudate, which was pelletized with a pelletizer (Scheer Bay Inc. WI) to produce plastic pellets for injection molding Injection molding Tensile test samples In order to characterize the strength of the various materials, pellets from each formulation were injection molded into ASTM tensile samples using a 22s Boy machine (20 ton clamping force). The injection molding was completed at recommended processing temperatures for virgin as-supplied resins and at temperatures below degradation temperatures of the biocomponents for the composites and blends. Samples were tested for their tensile strength after storage at room conditions for a minimum of 48 hours, following ASTM D638-08 procedures. Tensile strengths reported are the averages of seven samples. The mechanical properties of conventional plastic pots, commonly polypropylene, were not tested. Pots- 4.5 inch standard Prototype 4.5-inch pots (11.5 cm OD at top) were molded at R&D Leverage, Lee’s Summit, MO, using a sprue-less mold design. The mold included a three-piece module where the cavity side was free from the base plate as seen in Figure 1. This design was selected to avoid machine down time caused by recurrent sprue failure for high-percentage soy-protein plastics, a known issue with these plastics. The sprue failure for the soy-plastic resin was because of the low mechanical properties of the soy plastic formulation caused by the inclusion of soy flour. Soy flour was utilized to reduce the overall cost of the resulting resin. Greenhouse and Garden Evaluations Horticulture evaluations were performed by growing five container-crop species (marigold, petunia, salvia, pepper, and tomato) in bioplastic and control containers under standard greenhouse conditions for five weeks, then transplanting the container/plant units into a test garden near Ames, Iowa and growing them for eight more weeks. In the garden evaluation, plants were grown after the containers were removed, crushed, and placed in the soil near the plant roots. The results were compared with those for plants grown in standard petroleum-plastic (PP) pots and planted in standard fashion, where the containers are removed and discarded.

A total of 225 plant/container units were evaluated during this phase of the study (15 container types x 5 plant species x 3 replications) as seen in Figure 2. It is important to note that plant pots made from high-percentage soy polymers (SP and SP.A) were not evaluated in the garden trial because they failed structurally during the greenhouse portion of the study. In the greenhouse trial, container/plant units were rated for container performance (0 to 5 rating, 0 = worst, 5 = best, based on grower’s expectations of durability and functionality) and were measured for plant health (0 to 5 rating) and plant size (height x width x width of the plant shoot). Garden units were measured for plant health, plant size, and fruit production on peppers and tomatoes (fresh weight). In addition, container materials were evaluated for biodegradation by measuring weight loss after 20 weeks in soil. Results for plant size, fruit production, and biodegradation were normalized in order to express them on a scale of 0 to 5, so they could be plotted with plant health and container ratings to show cumulative results for container performance across the measured parameters.

Results and discussion Mechanical Properties Figure 3 shows base material strength for the materials studied. It is seen that PLA exhibited the highest strength (60 MPa), which was slightly reduced when nano-clays were added (54 MPa). In addition, it was seen that PHA was also relatively strong (10-15 MPa). The protein-based plastics (SP and SP.A) were relatively weak (<10 MPa); however, when compounded with PLA the strength increased to approximately 10 MPa. In addition, it was seen that when PLA and PHA were compounded with DDGs and corn stover, the strength ranged between 12 and 18 MPa. Figure 4 (a) shows a photograph of a few of the plant/container units early in the greenhouse production trial. Figure 4 (b) shows a photograph of plant/container units approximately 1 month after planting. Pots made from different material formulations were randomly placed throughout the greenhouse to reduce any effects of location or environmental conditions, such as slight variations in temperature. Figure 4 (c) shows a photograph of a pot that was produced from SP formulation, which had higher water stability compared to the SP.A formulation [5]; however, it is seen that the level of water stability achieved by this formulation was not sufficient to keep the pot stable during the numerous watering cycles. Because of these results, the greenhouse units for high-percentage soy (SP and SP.A) were not transplanted during the subsequent garden trial.

Greenhouse and Garden Trials Ten of the fourteen bioplastics and biocomposites that were evaluated performed as well or better than the standard petroleum-based container in both the greenhouse and garden trials (Figures 5 and 6). Two of the basic formulations, high-percentage soy plastics SP and SP.A, were not included in the results because pots made of these materials failed structurally before the end of the fifth week of greenhouse culture. In the greenhouse trial, container ratings based on grower’s expectations of durability and functionality were highest for containers made of materials containing Mirel PHA and for the polypropylene control (Figure 5). Plant health was similar across all container-material treatments, but there were differences in plant size. After 5 weeks of greenhouse culture, plants grown in containers made of a 50/50 blend of SP.A + PLA and a 50/50 blend of SP + PLA were the largest, followed by plants grown in containers that consisted of all or mostly PLA (Figure 5). Based on the overall material scores during the greenhouse trial, all of the container materials evaluated, with the exception of high-percentage SP, high-percentage SP.A, and Mirel PHA P4010, appear to be viable replacements for petroleum-based plastic in plant containers, and two of the materials, 50/50 blended SP.A + PLA and Mirel PHA P1008, appear to be slightly superior than petroleum-based (polypropylene) plastic during greenhouse production. In the garden trial, plant health was similar across all container material types when the bioplastic containers were removed, broken to pieces, and installed near the plant roots; but there were differences in plant size and fruit production after 8 weeks of garden culture (Figure 6). Plants grown with pieces of blended SP.A + PLA plastic near their roots grew larger and produced more fruit than plants in the other treatments (Figure 6). Plants grown with Mirel-PHA P4010 plastic near their roots were smaller than plants in the other treatments, and plants grown with Mirel-PHA P1004 near their roots produced less fruit than the control. After 20 weeks in soil, degradation was highest for Mirel-PHA P4010, SP + PLA, and SP.A + PLA materials. Including DDGs or corn stover to form composite materials had no effect on performance during greenhouse evaluations, but improved the performance of PHA- and PLA-based materials with respect to both plant growth and material degradation during the garden trial. While nearly all of the bioplastic materials tested performed as well or better than the petroleum-plastic control in the garden trial, containers made of 50/50 SP.A + PLA were ranked among the best for each of the four measured parameters and had the greatest overall material score (Figure 6). These strong results with blended SP.A + PLA bioplastic containers are consistent with research evaluating the

fertilizer effect of soy plastic and soy + PLA blended plastic containers for horticultural use. Schrader et al. found that plant-available nitrogen, the most essential component of commercial fertilizer [6, 7], is released from containers made of soy plastic and blended soy + PLA plastic during greenhouse production [4]. Their results indicate that containers made of high-percentage soy similar to those evaluated in our trials release nitrogen at a rate (total N = 621 mg·L-1 in leachate at 3 weeks of culture) that is excessive for container crops during greenhouse production, but that containers made of soy polymer blended 50/50 with PLA release nitrogen at a rate (total N = 168 mg·L-1 in leachate at 3 weeks of culture) that is beneficial for plant growth and health [4]. Their results also show that plant-available nitrogen released from blended soy + PLA containers improves plant health, growth, and quality during greenhouse production and increases plant growth, quality, and fruit production in the landscape when the containers are allowed to degrade in the soil near plant roots [4].

Conclusions Bioplastics and biocomposites show strong potential for use as sustainable replacements for petroleum plastics in agricultural applications, and are especially promising for single-use applications where the plastics have a short service life. Our results demonstrate that bioplastic products can be manufactured on existing plastics-processing equipment, they can perform as well or better than petroleum-based plastics, and they can degrade to indiscernible organic matter in soil within a reasonable length of time. The addition of low-cost natural fillers such as DDGs or corn stover can improve the function and degradation of bioplastics, and at the same time, lower the cost of the bioplastic product. Blending protein polymers such as soy with carbohydrate-based polymers can improve material performance for applications such as plant pots and can provide an intrinsic fertilizer effect for growing plants [4].

References 1. S. Hurley. Postconsumer Agricultural Plastic Report,

California integrated waste management board (2008).

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<http://www.agcensus.usda.gov/Publications/2007/Online_Highlights/Census_of_Horticulture_Specialties>.

3 M. Helgeson, W.R. Graves, D. Grewell, G.

Srinivasan. J. Envir. Hort. 27(2)123-127, June (2009).

4 J.A. Schrader, G. Srinivasan, D. Grewell, K.G. McCabe, W.R. Graves.. HortScience (2013) (under consideration).

5 G. Srinivasan, S. Carolan, D. Grewell. 68th ANTEC,

SPE Proceedings, (2010). 6. K. Mengel and E.A. Kirkby. Principles of plant

nutrition. 5th ed. . (2001). 7. The Fertilizer Institute. Fertilizer fundamentals.

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Table 2 Soy Plastic formulations Component Formulation

SF(g) SPI(g) Base Solution PA(g) AA(g)

Water(ml) SS(g) KS(g) Glycerol(g)

SP 500 500 650 10 5 225 75 - SP.A 500 500 650 10 5 150 75 75 Figure 1 Schematics of a three piece injection mold module with a false bottom. The seperation of the female cavity and base plate funtions as the ejection mechanism as the molded pot stay with the female side that could be easily ejected manually

Figure 2 Various pot types prepared for planting

[Movement of Male and Female cavity per molding cycle]

Drain hole studs

Injection Gate

Base plate fixed on machine

Female cavity with false bottom

Figure 3. Tensile strength for the materials compounded and studied.

(a) (b) (c)

Figure 4. Photograph of selected pots during greenhouse production (a,b) and (c) high-percentage soy container during the greenhouse trial.

a b

Figure 5. Plant size, plant health, and grower rating of plant-container units cultured in a greenhouse for five weeks. Each of the three parameters is represented on a scale of 0 to 5, with a maximum container-material score of 15, the sum of all three parameters.

Figure 6. Plant health, size, and fruit production of plant-container units after 8 weeks in a garden, and degradation of container materials after 20 weeks in soil. Each of the four parameters is represented on a scale of 0 to 5, with a maximum container-material score of 20, the sum of all four parameters.