Final Report: High Strength Degradable Plastics From Starch and Poly(lactic acid)

EPA Grant Number: R829479C012
Subproject: this is subproject number 012 , established and managed by the Center Director under grant R829479
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: The Consortium for Plant Biotechnology Research, Inc., Environmental Research and Technology Transfer Program
Center Director: Schumacher, Dorin
Title: High Strength Degradable Plastics From Starch and Poly(lactic acid)
Investigators: Sun, Susan Xiuzhi , Seib, Paul
Institution: Kansas State University
EPA Project Officer: Lasat, Mitch
Project Period: July 1, 2002 through June 30, 2004
RFA: The Consortium for Plant Biotechnology Research, Inc., Environmental Research and Technology Transfer Program (2001) RFA Text |  Recipients Lists
Research Category: Targeted Research , Hazardous Waste/Remediation

Objective:

The objective of this research project was to develop affordable, durable, and degradable bioplastics from starch and poly(lactic acid) (PLA). Specific research objectives were to:

  1. identify plasticizers that can enhance the flowability (processibility) of starch and PLA blends while retaining strength and improving flexibility of the blends;
  2. study aging behaviors of the starch/PLA/methylenediphenyl diisocyanate (MDI) blends as affected by moisture, temperature, and storage time;
  3. and determine degradability of the starch/PLA/MDI blends.

Summary/Accomplishments (Outputs/Outcomes):

In PLA/starch blends, PLA serves as the matrix, and starch as the biodegradable filler. The thermal transition behaviors of the starch/PLA blends are similar to that of extruded pure PLA. PLA is incompatible with starch. The dried starch remains in its granular shape in the PLA matrix. As starch content increases, the mobility of the PLA molecule is restricted by starch, and the crystal growth rate could be reduced. The storage modulus of the blends increases as starch content increases for both corn and wheat starch systems, especially in the temperature range of glass transition and crystallization. As the starch content increases, the PLA matrix becomes discontinuous, and the effective cross-section area of the continuous phase reduces, resulting in reduced strength and elongation. Model analysis indicates that some adhesion forces probably exist between the starch and PLA. The adhesion force between the two phases is caused by polar interaction between the two components, and hydrogen-bonding force exists between the carbonyl group in PLA and the hydroxyl group in starch.

High-strength bioplastics from starch and PLA have been obtained by using coupling reagents. MDI is the most effective coupling reagent to improve the interfacial and tensile strength between starch and PLA. The amounts of MDI as well as the PLA/starch ratios were optimized. A low level of MDI (0.25-0.5 wt percent) has a significant positive effect on the mechanical properties of a 55/45 (w/w) PLA/starch blend. A 0.5 weight percent MDI gave the (55/45) PLA/starch, a 66.7 MPa tensile strength, much higher than that without MDI (36 MPa), and higher even than raw PLA (62.1 MPa), even though the flexibility was retained as in raw PLA.

Few individual starch granules are observed from SEM observation, and those that are distinguishable appeared to be coated with matrix PLA. Moreover, fracturing of the blend occurred through the starch granules rather than at their interface. The mechanical properties of blends are not significantly affected by MDI concentration above 0.5 percent. With MDI, a covalent linkage likely formed at the PLA/starch interface so that the interfacial adhesion was enhanced and consequently improved tensile strength. In the blend, PLA’s molecular mobility was restricted by rigid starch granules, which had a high modulus, and the covalent MDI bonding at the interface; therefore, the PLA molecules were less free to stretch resulting in a high Young’s modulus. The blend with 0.5 percent MDI has the highest storage modulus whereas the raw PLA has the lowest. The storage modulus of the blend PLA/starch with MDI at high temperature is high enough for the blend to be rigid and almost the same as that of the blend without MDI at room temperature. As the blend coupled with MDI, tensile strength of the blends at all starch levels was similar to that of pure PLA. The blend with 45 percent starch has the highest tensile strength whereas the one with 20 percent starch has the lowest strength.

There is no significant change in elongation for starch blends compared to pure PLA until the starch level reached 30 percent. Between 30-60 percent, elongation leveled off remaining near 5.0 percent. Young’s modulus for blends increases as starch content increases up to 45 percent. Beyond 45 percent starch content, Young’s modulus remains the same (1.75-1.81 GPa). Forty-five percent starch and 0.5 weight percent MDI was an optimum combination, which gives a calculated mole ratio of PLA to MDI of about 1/40. These data suggest that each starch level may have an optimum MDI content. At 10 or 15 percent moisture starch, the strength properties decline because water would compete with PLA and starch in reactions with MDI. At 20 percent moisture starch, besides the reaction of water with MDI, starch granules become swollen and the mechanical strength and elongation is greatly reduced. Moreover, poly(vinyl alcohol), as a readily available biodegradable material, also has the ability to enhance compatibility and improve mechanical properties of PLA/starch blends.

Maleic anhydride (MA) is highly reactive with PLA free radicals induced by an initiator 2,5-bis(tert-butylperoxy)-2,5 dimethylhexane (L101). The anhydride group also reacted with hydroxyl groups from starch to form ester linkages. Thus, MA was a good compatibilizer for the PLA and starch blends. Mechanical properties of PLA/starch blends were significantly improved. A PLA/starch (55/45) blend with 1.0 percent MA and 10 weight percent L101 (MA basis) had tensile strength of 52.4 MPa and 4.1 percent elongation, which was close to neat PLA. There is no significant difference in mechanical properties prepared by one-step and two-step extrusion. More important, MA is less toxic compared to MDI.

The mechanical and physical performance of various plasticizers of PLA/starch blends were examined. Three groups of plasticizers were used: water, low-molecule weight nontoxic citrate esters, and polymeric plasticizers. The mechanical, thermal, and water absorption properties of the plasticizers were examined. Water is a ready-to-use plasticizer, completely miscible with starch, and destructurizes starch in its blending with PLA to achieve a fine dispersion and, consequently, obtains desirable product properties. The tensile strength of water-plasticized blends was reduced but there was no significant increase in elongation. The most apparent disadvantage for water as plasticizer was the quick thermal loss during processing, as determined by dynamic mechanical analyzer and isothermal treatment at a certain high temperature. Citrate esters as plasticizers significantly reduced glass-transition temperatures of blends, and improved the elongation at break and worked as effective plasticizers. The elongation was dramatically improved compared to 2.7 percent elongation of PLA/starch at the same ratio, however, this was at the expense of tensile strength. PLA/starch ratios influenced the plasticization. Fifteen percent citrate gave the PLA/starch (55/45 weight) blend 130 percent elongation, and a 10 MPa tensile strength; 94 percent elongation was achieved at 60/40 (PLA/starch), 83 percent at 65/35 (PLA/starch), and 81 percent at pure PLA. Both water and citrate esters as plasticizers for PLA/starch blends gave the blends unstable mechanical and physical properties in long-term runs. The presence of plasticizers (water or citrate esters), however, reduced the efficiency of coupling reagents. Certain citrate esters had thermal loss during processing and isothermal treatment. The weight loss is directly proportional to plasticizer concentration in the blend and temperatures. Therefore, reducing processing temperature and processing time are necessary to avoid loss of plasticizer.

Low-molecular-weight plasticizers have another drawback—that of leaching in liquid mediums. Poly(ethylene glycol), poly(proplylene glycol) are polyethers with low molecular weight and are used widely as plasticizers because they have better plasticization on starch than sorbitol. Glycerol also significantly reduces the tensile strength of the blends although the plasticized starch of the blend was above 15 percent in continuous phase. Tensile strength of blends containing sorbitol increases slightly as sorbitol content increases. The morphology of the blend containing sorbitol is more solid than those of the blends containing other plasticizers. Polymeric plasticizer also significantly reduced the glass transition temperature of the blends, and improved elongation at break. Ten percent plasticizer gave the blend 20 MPa tensile strength and 24 percent elongation, which was much better than citrate esters at this concentration. An advantage to blending such plasticizer with a PLA/starch blend was that no plasticizer experienced thermal losses during processing, and isothermal treatment gave the blends a stable mechanical property.

The polymeric plasticizer dioctyl maleate (DOM) at low concentrations (< 8%) acted as a compatibilizer for similar chemical structures with MA to improve interfacial adhesion between PLA and starch. At higher concentrations (> 8%), it acted as a plasticizer to improve flexibility of the blend. Fifteen percent of DOM gave the PLA/starch (55/45) blend 36.0 percent elongation and 16.2 MPa tensile strength. This plasticizer does not leach or have thermal loss at liquid medium but does absorb moisture. Both citrate esters and polymeric plasticizers improved blend elongation by a percolation manner. Above the percolation limitation, elongation was enhanced significantly.

Starch as a nucleation agent of PLA crystallization kinetics also was investigated by isothermal crystallization at various temperatures and dynamic crystallization. Starch effectively increases the crystallization rate of PLA, even at concentration as low as 1 percent. Starch significantly promoted the cold-crystallization ability, but the crystallization temperature was not affected when the starch content was less than 10 percent. The crystallization rate increased slightly as starch content in the blend increased. The Avrami exponent (n) values located between 2 and 3 indicted that spherulitic development arises from a thermal heterogeneous nucleation. Addition of starch into PLA did not affect significantly tensile strength, but slightly improved elongation when added at a relatively low concentration (< 10%). The starch-induced crystallinity improved the tensile strength of PLA/starch blends, but reduced the flexibility of the blends.

The water absorption of the blends increases when starch content increases. Water absorption of the starch/PLA blends significantly increased on the first day and then leveled off after three days. Water absorption increased slowly at starch concentrations of less than 60 percent, but increased rapidly with starch concentration over 70 percent. Water absorption is caused mainly by starch. At low-starch content, PLA formed a very good continuous phase that covered the starch. As starch content increased to 60 percent or more, the PLA phase was discontinuous, and starch granules were not covered completely by the PLA matrix, resulting in large water uptake. Water absorption of the blend in boiling water was much higher than in water absorption at room temperature.

Physical aging at room temperature with various moisture levels, temperatures, and storage times were conducted. Aging reduced the mechanical strength of PLA/starch blends, and made a significant thermal relaxation around the glass transition area. The aging can be removed by heating the blends to melting temperature and then cooling down. Service life of PLA/starch blends were predicted based on the physical aging and thermal degradation investigation.

Starch granule size affects the mechanical properties of PLA/starch blends. Reducing starch granule size and creating more opportunities for coupling reagents to react with starch, improved the mechanical properties of the blends.

Biodegradable foam from PLA and cornstarch were extruded successfully. Water acts as a good blowing agent. Experiments were conducted on extrusion conditions, including extrusion temperature setting die diameters and screw speed, and material compositions such as PLA/starch ratios, water concentration, and nucleation agent content affecting the foam formation and the final density and properties. The water resistance of the foams improved significantly with the presence of PLA. With 20 percent PLA, starch loss of the foam, after soaking in water at room temperature for 1 week, was about 60 percent; the foam with 40 percent PLA had starch loss of about 20 percent. The foam bulk densities were less than 0.2 g/cm 3 and suitable for cushioning and packaging application. Foams made from starch solubilized after soaking in water for a short time.

Conclusions:

  • Biodegradable starch significantly reduces the cost of raw materials when blended with PLA.
  • MDI is the most effective coupling reagent for improving interfacial adhesion between PLA and starch and resulted in high-tensile strength materials, higher than neat PLA at certain concentrations.
  • MA is an effective compatibilizer with less toxicity than MDI. Though mechanical properties of compatibilized PLA/starch by MA are lower than those with MDI, it is still affordable, especially for food contact items.
  • Water is a good plasticizer for starch to make a better dispersion in PLA matrix. Tensile strength and elongation, however, were reduced.
  • Various biocompatible plasticizers were identified. Citric esters are good plasticizers, which are compatible with PLA and significantly improve the flexibility of PLA/starch materials but at the expense of tensile strength.
  • Both water and citric esters are easily lost during processing, long time storage, and contact with liquid medium.
  • High molecular weight plasticizers balance mechanical performance at a certain concentration and avoid plasticizer loss at high temperature processing and in liquid medium.
  • PLA improves the compression strength and water resistance of starch based foams. The foam’s structure is combined with opened and closed cellular morphology.


Journal Articles on this Report : 11 Displayed | Download in RIS Format

Other subproject views: All 19 publications 12 publications in selected types All 11 journal articles
Other center views: All 211 publications 48 publications in selected types All 44 journal articles
Type Citation Sub Project Document Sources
Journal Article Ke TY, Sun XZ. Melting behavior and crystallization kinetics of starch and poly(lactic acid) composites. Journal of Applied Polymer Science 2003;89(5):1203-1210. R829479 (2006)
R829479C012 (2003)
R829479C012 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Journal Article Ke TY, Sun XZS. Thermal and mechanical properties of poly(lactic acid)/starch/methylenediphenyl diisocyanate blending with triethyl citrate. Journal of Applied Polymer Science 2003;88(13):2947-2955. R829479 (2006)
    R829479C012 (2003)
    R829479C012 (Final)
  • Abstract: Wiley InterScience
    Exit
  • Journal Article Ke TY, Sun SXZ, Seib P. Blending of poly(lactic acid) and starches containing varying amylose content. Journal of Applied Polymer Science 2003;89(13):3639-3646. R829479 (2006)
    R829479C012 (2003)
    R829479C012 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Journal Article Ke TY, Sun XZS. Starch, poly(lactic acid), and poly(vinyl alcohol) blends. Journal of Polymers and the Environment 2003;11(1):7-14. R829479 (2006)
    R829479C012 (Final)
  • Abstract: Springer-Abstract
    Exit
  • Journal Article Wang H, Sun XZ, Seib P. Mechanical properties of poly(lactic acid) and wheat starch blends with methylenediphenyl diisocyanate. Journal of Applied Polymer Science 2002;84(6):1257-1262. R829479 (2006)
    R829479C012 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Journal Article Wang H, Sun XZ, Seib P. Effects of starch moisture on properties of wheat starch/poly(lactic acid) blend containing methylenediphenyl diisocyanate. Journal of Polymers and the Environment 2002;10(4):133-138. R829479 (2006)
    R829479C012 (Final)
  • Abstract: Springer-Abstract
    Exit
  • Journal Article Wang H, Sun XS, Seib P. Properties of poly(lactic acid) blends with various starches as affected by physical aging. Journal of Applied Polymer Science 2003;90(13):3683-3689. R829479 (2006)
    R829479C012 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Journal Article Zhang J-F, Sun XZ. Mechanical properties of poly(lactic acid)/starch composites compatibilized by maleic anhydride. Biomacromolecules 2004;5(4):1446-1451. R829479 (2006)
    R829479C012 (Final)
  • Abstract from PubMed
  • Abstract: ACS-Abstract
    Exit
  • Journal Article Zhang J-F, Sun XZ. Mechanical properties and crystallization behavior of poly(lactic acid) blended with dendritic hyperbranched polymer. Polymer International 2004;53(6):716-722. R829479 (2006)
    R829479C012 (2003)
    R829479C012 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Journal Article Zhang J-F, Sun XZ. Mechanical and thermal properties of poly(lactic acid)/starch blends with dioctyl maleate. Journal of Applied Polymer Science 2004;94(4):1697-1704. R829479 (2006)
    R829479C012 (Final)
  • Abstract: Wiley-Abstract
    Exit
  • Journal Article Zhang J-F, Sun XZ. Physical characterization of coupled poly(lactic acid)/starch/maleic anhydride blends plasticized by acetyl triethyl citrate. Macromolecular Bioscience 2004;4(11):1053-1060. R829479 (2006)
    R829479C012 (Final)
  • Abstract from PubMed
  • Abstract: Wiley-Abstract
    Exit
  • Supplemental Keywords:

    bioplastics, biopolymers, starch, poly(lactic acid), plasticization, mechanical property, biodegradation, water absorption, thermal property, plasticizer, compatibilizer, compatibilization, sustainable industry, sustainable business, waste, agricultural engineering, bioremediation, environmental engineering, geochemistry, new technology, innovative technology, bioaccumulation, biodegradation, bioenergy, bioengineering, biotechnology, phytoremediation, plant biotechnology, sustainable industry/business, environmental chemistry,, Scientific Discipline, TREATMENT/CONTROL, Sustainable Industry/Business, Genetics, Geochemistry, Technology, New/Innovative technologies, Environmental Engineering, Agricultural Engineering, polylactic acid, bioengineering, biodegradable plastics, plant genes, biotechnology, plant biotechnology, novel plastics

    Relevant Websites:

    http://www.oznet.ksu.edu/dp_grsi/sun Exit
    http://www.cpbr.org Exit

    Progress and Final Reports:

    Original Abstract
  • 2003 Progress Report

  • Main Center Abstract and Reports:

    R829479    The Consortium for Plant Biotechnology Research, Inc., Environmental Research and Technology Transfer Program

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R829479C001 Plant Genes and Agrobacterium T-DNA Integration
    R829479C002 Designing Promoters for Precision Targeting of Gene Expression
    R829479C003 aka R829479C011 Biological Effects of Epoxy Fatty Acids
    R829479C004 Negative Sense Viral Vectors for Improved Expression of Foreign Genes in Insects and Plants
    R829479C005 Development of Novel Plastics From Agricultural Oils
    R829479C006 Conversion of Paper Sludge to Ethanol
    R829479C007 Enhanced Production of Biodegradable Plastics in Plants
    R829479C008 Engineering Design of Stable Immobilized Enzymes for the Hydrolysis and Transesterification of Triglycerides
    R829479C009 Discovery and Evaluation of SNP Variation in Resistance-Gene Analogs and Other Candidate Genes in Cotton
    R829479C010 Woody Biomass Crops for Bioremediating Hydrocarbons and Metals
    R829479C011 Biological Effects of Epoxy Fatty Acids
    R829479C012 High Strength Degradable Plastics From Starch and Poly(lactic acid)
    R829479C013 Development of Herbicide-Tolerant Energy and Biomass Crops
    R829479C014 Identification of Receptors of Bacillus Thuringiensis Toxins in Midguts of the European Corn Borer
    R829479C015 Coordinated Expression of Multiple Anti-Pest Proteins
    R829479C016 A Novel Fermentation Process for Butyric Acid and Butanol Production from Plant Biomass
    R829479C017 Molecular Improvement of an Environmentally Friendly Turfgrass
    R829479C018 Woody Biomass Crops for Bioremediating Hydrocarbons and Metals. II.
    R829479C019 Transgenic Plants for Bioremediation of Atrazine and Related Herbicides
    R829479C020 Root Exudate Biostimulation for Polyaromatic Hydrocarbon Phytoremediation
    R829479C021 Phytoremediation of Heavy Metal Contamination by Metallohistins, a New Class of Plant Metal-Binding Proteins
    R829479C022 Development of Herbicide-Tolerant Energy and Biomass Crops
    R829479C023 A Novel Fermentation Process for Butyric Acid and Butanol Production from Plant Biomass
    R829479C024 Development of Vectors for the Stoichiometric Accumulation of Multiple Proteins in Transgenic Crops
    R829479C025 Chemical Induction of Disease Resistance in Trees
    R829479C026 Development of Herbicide-Tolerant Hardwoods
    R829479C027 Environmentally Superior Soybean Genome Development
    R829479C028 Development of Efficient Methods for the Genetic Transformation of Willow and Cottonwood for Increased Remediation of Pollutants
    R829479C029 Development of Tightly Regulated Ecdysone Receptor-Based Gene Switches for Use in Agriculture
    R829479C030 Engineered Plant Virus Proteins for Biotechnology