Biodegradable plastics from wheat starch and polylactic acid (PLA)

Principal Investigators: X. Susan Sun, Biomaterial Processing Engineering, KSU Dept. Of Grain Science and Industry and Paul Seib, Starch Chemistry and Technology, KSU Dept. of Grain Science and Industry

Introduction and Background Advanced technology in petrochemical polymers has brought many benefits to mankind. However, it becomes more evident that the ecosystem is considerably disturbed and damaged as a result of the non-degradable materials for disposable items. The environmental impact of persistent plastic wastes is growing more global concern, and alternative disposal methods are limited. Incineration may generate toxic air pollution, and satisfactory landfill sites are limited. Also, the petroleum resources are finite and are becoming limited. It becomes important to find durable plastic substitutes, especially in short-term packaging and disposable applications. Recently, the continuously growing concern of the public for the problem has stimulated research interest in biodegradable polymers as alternatives to conventional nondegradable polymers such as polyethylene and polystyrene etc.

Starch may offer a substitute for petroleum based plastics. Starch is a renewable degradable carbohydrate biopolymer that can be purified from various sources by environmentally sound processes. Starch, by itself, has severe limitation due to its water solubility. Articles made from starch will swell and deform upon exposure to moisture. To improve some of the properties, starch is often blended with hydrophobic polymers during the past decades by a number of researchers with petroleum polymers to increase biodegradability, and reduce the usage of petroleum polymer.

Fully biodegradable synthetic polymers have been commercially available since 1990, such as poly (lactic acids) (PLA), polycaprolactone (PCL), and polyhydroxybutyrate~valerate (PHBV). Among these biopolymers, PLA was extensively studied in medical implants, suture, and drug delivery systems since 80s due to its biodegradability. PLA has been attractive for disposable and biodegradable plastic substitutes due to its better mechanical properties. However, PLA is still more expensive than conventional plastics. Also, the degradation rate is still slow as compared to the waste accumulation rate.

The goal of this proposed research are 1) to investigate the compatibility of wheat starch and PLA compared to the blends of corn starch and PLA, 2) to develop wheat starch based formulation and processing technology for biodegradable plastics. The specific objective of this proposed research is to enhance flexibility of wheat starch and PLA blends.

Results from Previous Funding

The major accomplishment from previous research is that one chemical has been identified for co-polymer or diblock formation in reactive blending of wheat starch and PLA, that would link starch and PLA resulting in high strength. For example, with about 0.5% of this chemical, the tensile strength of a blends (starch:PLA = 50:50 wet base) was about 6lMPa, which was very close to 64MPa for pure PLA, and was significantly increased as compared to the blends (50:50) without the chemical (about 30 Mpa). Water absorption of the blend was less than 10% after ten days of water soaking test. In addition, the modulus of the blends above its glass transition temperature (about 60⁰C) was significantly improved, which is the drawback of pure PLA.

Tensile strength and elongation are two major mechanical properties for a plastic to have market potential. The elongation for pure PLA is about 6%, and the elongation of the starch/PLA blends was about 4.5%, which is brittle for many packaging or fast food utensil materials.

It is the purpose of this proposal to request $19,500 for FY 2001 starting July 1, 2000 to enhance the flexibility of the starch and PLA blends, which will have great potential for commercialization.

Rational and Significance

Kansas is number one in the production of wheat. About 70% of the components of wheat is starch. This proposed research will enhance the utilization of wheat starch in industrial materials to increase rural economy. In addition, environmental protection has become one of the highest priorities of both national and international government agencies. The public also supports this priority. This research plan will provide technology for producing low cost, durable, and degradable environmental wheat starch based plastics.

As mentioned before, large solid-plastic wastes are produced from packaging, public or medical services and travel locations, grocery bags, trash bags, fast food serving and eating utensils, and even flower/plant pots. The newly developed plastics from this research could provide possible solutions to the problem. Also, the plastics from this proposed research could be used as biomedical and pesticide slow-releasing carrying matrix. The outcome of this research will enhance the utilization of wheat as an industrial material. The results obtained from this research will also be useful to both academic and industrial scientists.

Procedures/Methodology

Plasticizers usually play an important role in polymer compounding. Many factors involved in plasticizer selections including molecular structures, polarities, required product qualities and properties, and costs. In this research, various possible plasticizers at different amounts and different delivering methods will be explored. Types of plasticizers, optimum amount of plasticizer, and delivery methods, and better plasticizer combinations for wheat starch will be identified through this proposed research project.

An intensive mixer and laboratory scale twin-screw extruder will be used. The mixture will be compression molded using the Hot Press (Carver) for early stage quality evaluation. Mechanical properties, such as tensile strength and elongation, will be measured, and simple water absorption curve will be determined to roughly evaluate the formulation potential.

Other Related Works

Blending, grafting and blocking starch with synthetic polymers started in the 1 960s (Brockwy, 1965, Bagley et al., 1977) and recently advanced by several researchers. Jane and Spence (1995) investigated the possibility of developing thermoplastics from pure corn starch and various proteins. The mixture could be molded and extruded into different shaped articles. The strength of the protein-starch thermoplastics were fairly good. However, the water resistance was very low.

Synthetic hydrophobic biodegradable polymers have been available since 1990. However, these synthetic polymers are usually more expensive than petroleum polymers and also has a slow degradability. Blending starch with these degradable synthetic polymers has recently become the focus of researchers. Mayer et al. (1994) advanced the research results obtained by Otey et al. (1982) on blending of starch with polyvinyl alcohol and ethylene vinyl alcohol for degradable films. The films had low melting temperature in the range of 40 to 130⁰C. Kotnis et al. (1995) prepared biodegradable plastic substitutes by blending up to 25% of starch with degradable polyhydroxybutyrate-valerate (PHBV). The cost is still high due to the high percentage of PHBV. Also the mechanical strength properties of the blends were greatly reduced. Krishna and Naryan (1996) prepared a new degradable polymer by blending up to 45% starch with degradable polycaprolactone (PCL). This new material is not strong enough because the melting temperature of PCL is only 60⁰C. Also, PCL gets soft when temperature is above 40⁰C. These drawbacks greatly limit the applications of the starch-PCL blends. PLA is a new synthetic biopolymers (Gruber et al., 1 996a, and I 996b) and limited information bas been available.

KANSAS WHEAT COMMISSION QUARTERLY REPORT OF PROGRESS

Principal Investigators:
X. Susan Sun - Grain Science and Industry
Paul Seib, Grain Science and Industry

Title: "Biodegradable Plastics From Wheat Starch and Polylactic Acid"

Department Heads: Dr. Brendon Donnelly

Progress report: Third Quarter FY 01 (Jan. 1, - Mar. 31, 2001)

Accomplishments:

Preparation of manuscripts:

Strengthening blends of poly(lactic acid) and starch with methylenediphenyl diisocyanate. J. of Applied Polymer Science (accepted)

Coupling of poly(lactic acid) and wheat starch with methylenediphenyl diisocyanate. J. of Applied Polymer Science (submitted)

Effects of Starch Moisture on Properties of Wheat Starch/Poly(lactic acid) Blend Containing Methylenediphenyl Diisocyanate. J. of Polymers and the environment (Submitted).

High strength degradable plastics from reactive blending of starch and poly(lactic acid), has been approved (issue number is pending)

Effects of Plasticizers:

We are continuing to work on the effects of plasticizers on starch/PLA/MDI blends. Three groups of plasticizers were selected: 1) acetyl triethyl citrate (AC) and triethyl citrate (TC); 2) poly(ethyl glycol)(PEG) and poly(propylene glycol) (PPG) of low molecular weight and containing an ether group that might interact with PLA; and 3) glycerol and sorbitol, which are miscible with starch. Experiments on AC, TC, PEG and PPG were finished. We are doing data analysis, and detailed results will be reported next report.

source: http://www.kswheat.com/research/2001KSUResearch/Projects/biodegradable.htm