Michael Stephens
Symphony Plastics, UK

Abstract. Currently 25% of global polyethylene agricultural consumption is used in Europe, although only 5% of the world population resides there.  In excess of 450,000 hectares are covered by plastic mulch in Europe.  Italy alone covers the same area as utilised in the whole USA.  Why Europe is so advanced in biodegradable materials?  The spectrum of degradable products in use in Europe includes garbage bags, compost sacks, check out carrier bags and packaging films. Consumption is gauged to be in the order of 35,000 tons this year.

Starch-based technologies, aliphatic polyesters, oxo-degradable polymers are the technologies available.  The first two materials are hydro-degradable and rely on microbial active environments to degrade. The oxo-degradable materials are inherently degradable, will degrade through photo, thermic and molecular scission in a time controllable manner.

The European Directive on Packaging Waste permeates all packaging and agricultural activities. The work of the CEN Standard working groups and the developing definitions of degradability and bio-degradability. CEN Technical Committees 249 and 261, Working Groups WG9 and WG2. Current definitions and new revisions/standards enroute will be discussed.

Plastic is the inherently degradable material.  The innovative approach to plastic mulch disposal will be discussed, including such factors as how it works and what it is not.

For many years there has been an approach to creating biodegradable polymers by the addition of organic starch materials into a petrochemical product. Unfortunately there are a number of flaws to this system. The two materials are entirely incompatible. The properties of what was once a highly efficient and minimalist material are highly compromised.

Strength can not be maintained for the same cross section thickness. Thus the material is heavier, with all the associated disadvantages that brings. It relies entirely on microbial activity to bio-degrade. No microbes –no degradation.

The familiar alternative is photo degradation. Invariably activated by a transition metal this is a familiar material in agricultural applications. A significant drawback is the unpredictable nature of photo degradation. The exposure to ultra violet radiation is variable and non existent when material is buried. Thus there is insufficient ultraviolet radiation; leading to insufficient degradation.

The more reliable development is the Symphony technology that manages a balance of thermic and photo degradable properties.  To understand the advantages of this material it is important to appreciate how polyethylene is affected by the Symphony additive.

Polyethylene is a member of the polyolefin family. It is a simple hydrocarbon made up from a molecular structure based upon hydrogen and oxygen. Polymers such as these work because they have a huge molecular structure that provides lightweight strength, and imperviousness. To provide an idea of how large these structures are it helps to remember that carbon dioxide has a molecular weight of 44 and water one of 18—polyethylene’s weight is between 250,000 and 300,000!

The transition metal in the Symphony additive causes an irreparable scission between the hydrogen elements and their carbon bonds. The long molecular chains are reduced in size and more and more carbon is exposed – to be oxidised or, later on consumed by micro organisms.

When the molecular weight is reduced to around 100,000 Daltons the plastic embrittles and flakes. Below 40,000 the molecular structure becomes bio-degradable. The structure becomes water wettable and microbes and fungi can attach to it to convert carbon to cell wall structure.

The degradation process is initiated at the extrusion process which takes place at temperatures of around 700 degrees Fahrenheit. It is important to make a distinction between the initiation of the degradation process and its manifestation. Although the process commences in the extruder it is delayed in actually damaging the plastic by a special balance of inhibitors. They are geared to the end application and its anticipated life expectancy. For example a garbage bag can be created that will degrade in 2 years, a produce bag in 9 months  and a banana tree bag in 19 weeks.

Heat, ultra-violet radiation and stress (through tearing or shredding) can accelerate the degradation process. It can be accelerated, but not stopped.  Thus when an application is identified and its life expectancy, properties and environmental profile measured a specific additive can be produced.  Thorough eco-toxicity testing of the system has been carried out on two continents. The material successfully passes the familiar tests incorporating daphnia, cress seeds, earthworms etc.  Interestingly the films produced by Symphony are also safe for direct food contact.

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