Recyclers should be worried about hydro-biodegradable or "compostable" plastics, because they will certainly compromise an oil-based recycling stream. Oxo-biodegradable plastics will not.
The Oxo-biodegradable Plastics Association supports the recycling industry, but recycled plastics are not normally degradable and will, like ordinary plastics, accumulate for decades if they get into in the open environment. However, recycled plastic and ordinary plastic can now be made oxo-biodegradable by the inclusion of a pro-degradant formulation at the extrusion stage.
Oxo-biodegradable plastics are now a fact of life. They provide a significant environmental benefit and they are increasingly popular. In 2008 one company alone sold enough formulation to make more than 5 billion plastic products.
This briefing is intended to assist recyclers and the public to understand the relationship between biodegradability and recyclability of plastics.
1(a) OXO-BIODEGRADABLE PLASTICS
Oxo-biodegradable plastics have been in commercial use since the 1970s, and are based on commodity polyolefins, particularly polyethylene and polypropylene. Their performance during manufacture and use is indistinguishable from that of regular polyolefins, and their biodegradation is caused by formulations that promote transition metal ion oxidation in the presence of oxygen.
The length of the useful life of an oxo-biodegradable plastic product is determined by antioxidants (processing stabilisers and UV stabilisers) contained within the formulation, which can be modified so that the plastic product degrades according to whatever timescale is required.
Obviously if any plastic is going to be recycled it will have to be collected and recycled before it has become embrittled. Oxobiodegradable products will normally have a useful life before embrittlement of at least 18 months, and if they have not been collected and recycled by then, they probably never will be.
A. New oxo-biodegradable products made with recyclate
If a new product is to be made with recycled polymer which contains or might contain a pro-degradant formulation and the new product is intended to be degradable, the process is obviously straightforward, as a pro-degradant effect is actually desired. This applies particularly to recycling of oxo-biodegradable offcuts in plastic factories, or where used oxo-biodegradable "back-of-shop" plastics (e.g. shrink-wrap pallet-wrap, bread-wrapping etc) are sent back for recycling into more oxo-biodegradable products.
B. Short-life products
If the new product to be made from recyclate which contains or might contain a pro-degradant formulation, is intended for short-life products such as refuse-sacks, bin-liners, shopping bags, bread wrappers etc. the effect of any pro-degradant formulation is unlikely to manifest itself during the intended service-life, and biodegradability for such items is in any event desirable. It is desirable because a proportion of these items will always find their way into the land or sea environment, where they would otherwise subsist for decades after they had been discarded.
C. Long-life products
Since polymers lose stabilisers every time they are reprocessed, it is good practice to add new stabilisers each time, whether the feedstock contains oxo-biodegradable plastic or not. If suitably formulated, the stabilisers will also neutralise any pro-oxidant which may still be effective.
C(1) Building Films
If the new product to be made is a plastic film intended for long-term durability - such as a building film for damp-proofing or waterproofing - the specification in some countries for some of these films requires the use of a virgin polyolefin compound1 and recyclate is not therefore relevant. For all other building films the specification will usually require the use of stabilisers where necessary2. There will of course be no pro-degradant formulation in recyclate chosen from in-house scrap, or from other feedstock whose origin is known.
In the case of lower-grade building films, where no guarantee is given, these are often made from recyclate whose origin is not known, and the manufacturer should always add stabilisers as above, whether the feedstock contains a pro-degradant formulation or not.
(a) ISO Standard 8779 "Plastics piping systems - Polyethylene (PE) pipes for irrigation" provides at para. 4.2 that only clean reprocessable material generated from a manufacturer's own production may be used if it is derived from the same resin as used for the relevant production. As the origin of the material will be known, it will not therefore be used for this purpose if it could contain any pro-degradant formulation.
(b) SABS piping is manufactured to a specification which permits the use of recyclate only from "in-house scrap." Small bore piping class 6 and 10 is usually LDPE and, larger sizes, HDPE.
"In-house scrap" is scrap which has been generated during manufacture of the SABS grade pipe which can be chipped up and added back.
There is therefore no difficulty with the manufacture of such piping, as the origin of the recyclate is known and it will not therefore be used for this purpose if it contains any pro-degradant formulation.(c) "SABS Equivalent" piping is manufactured from 100% recycled material according to the SABS specification but is not marked. Usually HDPE with from 5-20% LDPE blended for flexibility. For a quality product where a guarantee is demanded, clean industrial scrap is used where product history (material source and material grade) is known. This will not therefore contain a pro-degradant formulation.(d) Agricultural and Domestic piping is manufactured in South Africa from 100% LDPE scrap. Normally the same scrap is used as in (c) above, but it should only be used in low-tech situations if the origin of the recyclate is unknown. Stabilisers should always be added if there is any doubt about the origin of the recyclate, and there is a case for an industry specification for this category of piping, which would include a requirement to add stabilisers.
"Low tech situation" refers to small bore piping Class 3 and 6 used for piping water to cattle or game troughs or on domestic irrigation systems, essentially at low pressures.
1(b) HYDRO-BIODEGRADABLE PLASTICS
The second class of biodegradable plastics is the hydro-biodegradables (or "compostables"), which are generally based on intermediates of biological origin derived from crops. Crop-based plastics were developed some 20 years after their oxo-biodegradable counterparts, and there are two sub-classes of different origins.
The earliest was poly (3-hydroxy butyrate). PHB is produced biologically from sucrose. This is an expensive product with a relatively low thermal decomposition temperature, which was partially overcome by varying the alkanoate structure (PHA). The second sub-group of hydro-biodegradable polyesters are the synthetic aliphatic polyesters, which are in some cases based on biological intermediates (e.g. polylactic acid - PLA).
Both sub-groups are physically incompatible with main stream packaging wastes (PE, PP, ABS and PET) and aliphatic polyesters cannot be readily reprocessed with commercial polyesters, due to their thermal incompatibility
Plasticised-starch is a different type of bio-based plastic used in packaging. This material has acceptable initial properties but is sensitive to hydrolysis during use, and cannot normally be re-processed for use in the same application. Like other bio-based plastics, they are not compatible with mainstream plastics used in packaging and cannot be co-recycled into useful secondary products.
Hydro-biodegradable plastics, unlike oxo-biodegradable plastics, cannot therefore be recycled with the most abundant components of plastic waste. They therefore have to be segregated from the waste stream and treated separately, with considerable increase in cost. Furthermore it is difficult for the manufacturers of recyclate to physically distinguish between hydro-biodegradable and normal plastic.
Hydro-biodegradable plastics (often referred to as bioplastics) have been called into question by recyclers4 and Recoup's project manager has warned that starch-based plastics could "have a negative impact on plastics recycling as a whole5. .... the fear is that bioplastics will increasingly find their way into the plastics recycling stream - impacting on quality and un-doing the work done on raising public awareness of plastics recycling."
Recyclers should therefore be concerned to see that hydro-biodegradable plastics are not encouraged.