Current and Future Status of Biodegradable Plastics
The Tomato Magazine
Feburary 2005
By John Warner
Agriculture and Agri-Food Canada
Greenhouse and Processing Crops Research Center
Harrow, Ontario, Canada N0R 1G0
Plastic (polyethylene) mulches have been used in commercial
vegetable production since the early 1960s. Benefits include soil temperature
modification, earlier crop production, higher yields, enhanced produce
quality, improved soil water retention by reduced evaporation, reduced
fertilizer leaching, reduced soil erosion, better management of certain
insect pests and fewer weed problems.
Plastic mulches are relatively inexpensive to purchase, easy to use and
readily available in a variety of colors and forms. Furthermore, crop
management systems are well established for plasticulture. It is estimated
that over 4,000 acres of vegetables re grown on plastic mulches in Ontario,
Canada.
Disposal Problems
Plastic mulches, however, have a negative impact on the environment. They
are produced using non-renewable resources, are not biodegradable, are
often disposed of after a single season and disposal of used mulch is
becoming more difficult.
Disposal may involve incineration, burying in a landfill site or recycling.
Open burning of plastic is not allowed in most jurisdictions. Incineration
to recover the fuel value is an alternative; however, incineration facilities
that will accept waste plastic are not readily available. Land filling
plastic mulches is expensive; the plastics persist for many years after
disposal and restrictions are placed on the disposal of agricultural plastics
in landfills. Some landfills refuse to accept used plastic mulches. Recycling
of plastic mulches is difficult because of plant material, soil, moisture
and possible chemical residues associated with them after their use in
the field.
Some growers are stockpiling used plastic mulch because no other suitable
alternatives exist. The pick up and disposal cost in U.S. dollars of plastic
mulch has been estimated at over $100 per acre.
An alternative to polyethylene mulch is biodegradable mulches. Organic
mulches such as hay, straw, sawdust, etc., provide some of the benefits
of plastic mulch but may be more expensive, harder to handle and do not
provide the soil warming effects encountered with plastic. Paper mulches
have also been used, however, paper disintegrates at the shoulder where
the mulch is buried and paper pieces tend to blow around.
Recently, biodegradable polymers have been developed for agricultural
uses. Biodegradable films are often thinner than traditional polyethylene,
but otherwise are quite similar. Biodegradable films are available in
clear, black and a variety of colors. They may be made from renewable
resources such as starch, cellulose or degradable polymers.
Film Testing
There are two types of biodegradable polymer films presently being investigated.
One is a polyethylene which has a proprietary additive which allows oxidative
degradation of the polyethylene film. The breakdown is a two-step process.
The oxidative degradation is triggered by sunlight, heat and mechanical
stress. The film becomes brittle, and the molecular fragments are then
further broken down by microbial action to CO2, water and natural substances.
The second step is the microbial action. Depending on how the film is
formulated will affect the length of time required for it to break down
(from several months to several years). These are not the same as the
photodegradable mulches that were previously available that left plastic
residues in the fields.
The other type of film under test is starch-based polyester. Starch is
added to the formulation to break up the long polymer (carbon) chains
to shorter lengths which are then subject to further breakdown through
microbial activity to CO2 and water. Polyester is also biodegradable.
There are also polyethylene films with starch added to enhance breakdown,
but the problem is that molecular fragments of polyethylene may remain
in the soil.
There are compostable standards in the U.S. (ASTM) and Europe (EN) which
state that in order for a product to be compostable the following criteria
must be met:
1. Disintegration-the ability to fragment into non-distinguishable pieces
and safely support bioassimilation and microbial growth
2. Biodegradation-conversion of carbon to CO2 to the level of 60 percent
over a period of 180 days (ASTM) standard) and 90 percent in 180 days
(EN standard)
3. Safety-that there is no evidence of eco-toxicity in the finished compost
and soils can support plant growth, and
4. Toxicity-that heavy metals concentrations are less than 50 percent
recommended values.
Few products presently meet the compostable standards. However, products
may be biodegradable, but in a slower time frame, or may not be adequately
tested to meet the U.S. or European standards. Companies may also be promoting
products which are not truly biodegradable. In Ontario, there is limited
use of biodegradable mulches. Biodegradable mulches are just starting
to become available and the cost tends to be higher than for polyethylene.
Testing needs to be done to determine the length of time that the products
will last in the field, how consistent they perform from year to year,
to determine their soil warming effects and effects on crop maturity,
yield and quality.
Testing of sweet corn on clear biodegradable mulches and peppers on black
biodegradable mulches at Agriculture and Agri-Food Canada, Harrow and
at Ridgetown College, University of Guelph, Ridgetown, Ontario, has shown
that performance of the biodegradable mulches can be similar to polyethylene.
Biodegradable films generally are thinner and therefore, extra care must
be exercised when laying the films to prevent tears and punctures. Weed
control must be good under the mulch as weed will grow through the biodegradable
films rather than being smothered as often occurs under the stronger polyethylene
films.
Editor's Note: This research was presented Dec.
8, 2004, during the Plasticulture Session of the 2004 Great Lakes Fruit,
Vegetable and Farm Market Expo in Grand Rapids, Mich.
© 2005 Columbia Publishing
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