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Additives: modified starch-based biodegradable plastics.

ADDITIVES

The use of a renewable natural polymer in the manufacture of plastic products has long been sought, and the search intensified following the oil crisis of the early 1970s. Although modified cellulosics have been used, particularly in packaging films and fibers, only recently has the use of starch been widely commercialized.

Using starch as a cost-effective additive was developed in the 1970s, but it was also realized at that time that standard starch was unsuitable. This led to the discovery of the benefits of modifying the starch/polymer interface by making the normally hydrophilic starch surface hydrophobic, and the need to reduce the moisture content of starch so that it could be processed in polymer melts above 160[deg.]C.

Further development resulted in an additive system that renders common polymers--such as polyethylene, polypropylene, polystyrene, and polyurethane--biodegradable.

One of the major obstacles to the development of the technology was the lack of interest in degradable polymer systems, with the exception of some work in the area of photodegradable mulch films. This situation has changed radically in the last twelve months. Numerous jurisdictions, from city to federal, in the United States have passed or proposed legislation that will ban or restrict the use of nondegradable plastic products, ranging from shopping bags to fast-food containers to disposable diapers. This legislative threat has generated interest in using biodergradable plastic systems based on starch.

Degradable Plastics

Degradable plastics are those polymers containing materials that enhance the already existing, but rather slow, photodegradation and biodegradation processes.

Simply removing the stabilizers already incorporated into a polymer is usually not sufficient to render it biodegradable because stabilizers are, in most cases, included to facilitate processing.

The photodegradation process of common polymers is well understood, and extensive development of UV stabilizers has been carried out over the years. Biodegradation of polymers, however, is less well understood.

To improve understanding, studies of the effects of biological attack on low-molecular-weight fragments of polyethylene have been conducted. Similarly, studies measuring sup.14.CO.sub.2 evolution from sup.14.C-labeled polyethylene have shown that over long periods of time, oxidative degradation of the polymer occurs and produces low-molecular-weight carbonyl-containing groups that can be metabolized by microorganisms, thus producing sup.14.CO.sub.2 as a product of biological activity.

Biodegradation

Biodegradation is the breakdown of materials by the action of living organisms. For plastics the most important organisms are bacteria, fungi, and actinomycetes, although larger organisms can play a part.

The breakdown caused by microorganisms can be of three different types:

1. A biophysical effect, in which cell growth can cause mechanical damage.

2. A biochemical effect, in which substances from the microorganisms can act on the polymer.

3. Direct enzymatic action, in which enzymes from the microorganisms attack components of the plastic product, leading to splitting or oxidative breakdown.

There are polymer systems such as caprolactam and polyhydroxybutyrate/valerate that are subject to enzymatic attack directly. Because of cost and processing requirements these materials are, at least for the present, more suitable to speciality applications, such as those in the medical field.

More suitable for commodity plastics are those based on additive systems most commonly employing starch in either gelatinized or granular form. In these systems, although the starch is degraded and the plastic item can break down, accelerator systems must be incorporated to facilitate breakdown of the polymer chain to the point that it can become metabolized.

Biodegradation can be evaluated by measuring changes in physical properties of the product, by studying chemical changes in the film, or by assessing biological activity.

The Ecostar System

Plastics made with the Ecostar system incorporate a starch modified to make the normally hydrophilic surface of the starch hydrophobic. This starch is then dried to less than 1% moisture (compared with the 10% to 12% moisture of normal starch). Several different starches can be used. Although corn-starch is the most readily available and widely used, rice starch can be used for products requiring fine particle size.

Ecostar, as treated above, is a free-flowing white powder available in granular sizes of 15 microns for cornstarch, 5 microns for rice starch, and 80 microns for potato starch. The density of cornstarch is 1.28 g/cc. It is stable to 230[deg.]C, and its moisture content is less than 1%. A fatty acid or autoxidant (or autooxidant) is added to the system to facilitate breakdown of the polymer.

Biodegradation

Mechanism

In the Ecostar system, degradation proceeds by two interactive mechanisms. Starch is present in the polymer as granules, as shown in Fig. 1. These granules are attacked by microorganisms, such as fungi and bacteria, until they are completely removed, as illustrated in Fig. 2. This weakens the polymer matrix as well as greatly increases the surface area of the plastic.

The second mechanism is a result of the formation of peroxides by the autoxidant when it comes into contact with metal salts present in soil, fresh water, or sea water. These peroxides begin to break the polymer chain. This second mechanism is tremendously enhanced by the increase in surface area provided by the first mechanism.

Breakdown of the polymer chain not only weakens the material, but reduces the chain length, and thus molecular weight, to a level that the polymer can then be metabolized by microorganisms. As this process is purely biological, the products of degradation are those of normal biological activity, namely carbon dioxide and water.

In addition to the above mechanisms, it has been shown that macrodegradation of thin films can take place through attack by certain insects, such as wood lice, as shown in Fig. 3.

As the degradation mechanism is biological in nature, the rate of degradation depends greatly upon factors such as moisture, presence and type of microorganisms, temperature, pH, presence of metal salts, relative proportions of polymer and active ingredients, polymer type, and the surface area and thickness of the article. An example of the loss of strength of samples of starch-filled plastics exposed to composting conditions is shown in Fig. 4.

Processing and

Properties

Suitable masterbatch formulations containing both hydrophobic starch and autoxidant have been produced for conventional and linear low-density polyethylene as well as high-density polyethylene, polypropylene, and polystyrene.

Processing precautions are limited to avoiding moisture pickup and temperatures in excess of 230[deg.]C. Although dielip buildup has been reported in a few cases, this problem has been relieved by the addition of a processing aid such as a fluoroelastomer. Physical properties of LLDPE film made with the additive are shown in Table 1. The loss of strength shown by starch-filled films is being addressed by studies examining the starch-polymer interface and the effect of various starch surface treatments on the strength of that bond.

Apart from the biodegradability imparted to the plastic item, inclusion of Ecostar has shown other benefits, such as antiblock and low gloss finish in films. Increased dimensional stability in injection molding and increased stiffness in blowmolding have been demonstrated as well.

Recent Improvements

Enhanced biodegradability and photodegradability. For many applications, definitive and rapid degradation times are not required. For example, products such as shopping bags, garbage bags, and films used for hygienic products have demanding specifications during use; however, the rate of degradation is non-specific. For other applications such as plastic compost bags, mulch films, and medical implants the degradability rate is often more rapid and specific. In these cases the environment can be detailed moderately, but not exactly.

Attempts at using Ecostar for compost showed dramatic reductions in elongation, as shown in Fig. 4. These were, however, insufficient to meet the demands of the composting site. In conjunction with Pavag in Switzerland, we have developed an accelerated degradation system known as Ecostar Plus.

As mentioned previously, the Ecostar system contains an autoxidant to promote attack on the polymer chain. In Ecostar Plus, accelerators are added to hasten the process. This has resulted not only in enhanced biodegradability, as shown in Fig. 5, but also enhanced photodegradability, as shown in Fig. 6. Figure 7 shows fragmentation of a bag exposed in a laboratory simulation for 30 days at 60[deg.]C with no exposure to sunlight. This closely simulates the temperature conditions present in well-aerated compost.

Bags made with this material have been tested for the past year in several municipal composting programs in Switzerland. Agricultural mulch film trials are commencing.

It is important that for this system as well as for Ecostar applications, attention be paid not only to the additive, but to the basic components of the polymer being processed, such as stabilizer and antioxidant levels.

Breathable rapidly biodegradable films. Porvair Ltd. in the UK has developed Ecolan, a breathable, rapidly biodegrading film. It is being developed for hygiene products requiring a fast degradation system.

The manufacturing technology is very different from normal blown film, and the resulting material can have a level of over 50% Ecostar. The polymer is polyurethane, and the resulting structure is microporous with holes (or, more correctly, interconnecting pores) of controlled size.

The performance characteristics of an early sample of the film are summarized in Table 2. Tensile strength is not of paramount importance since the resulting products derive much of their strength from the nonwoven part to which the Ecolan is bonded. As would be anticipated, a material containing 50% Ecostar displays rapid biodegradability. Preliminary results of the resulting tensile strength after exposure to two environments are shown in Table 3.

Biodegradability testing. Efforts to find suitable accelerated methods for testing the biodegradability of products such as those described above are under way.

Oven testing has been found useful for predicting the performance of formulations designed for elevated temperature environments such as compost.

ASTM methods G-21 and G-22 have often been used to determine the susceptibility of materials to fungal and bacterial attack. We have also used a method developed at the Wool Industries Research Association, in which luciferase, the enzyme used by fireflies, is added to the fungal suspension, and the amount of light emitted represents the biological activity. Comparisons can easily be made between products containing no biodegradable additive and products with starch along with a standard such as wool.

Summary

The market for biodegradable and photodegradable products is growing not just in technical applications such as mulch film and composting bags, but also in the area of commodity products, as consumer awareness and legislation drive the demand.

These products are currently available at a cost increase of 5% to 15% over conventional nondegradable materials.
COPYRIGHT 1989 Society of Plastics Engineers, Inc.
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Author:Maddever, Wayne J.; Chapman, Graham M.
Publication:Plastics Engineering
Date:Jul 1, 1989
Words:1746
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