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The degradation of plastic litter in rivers: implications for beaches - page 4 / 10

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Williams, A.T. & Simmons, S.L.

At Merthyr Mawr beach, South Wales, December 16, 1994, survey records revealed:

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    sanitary items: 75 per km;

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    metal cans: 210 per km;

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    plastic bottles: 550 per km (340 plastic, 210 poly-

ethylene).

On December 6, 1995, 2856 containers were counted along the same 1 km stretch of strand line. This beach is a litter sink par excellence. The NUC (Anon. 1995) study also indicated that about half of Britain’s coast- line is polluted, with an average of 20 sanitary items, 17 metal cans and 22 plastic bottles occurring per km of coast.

The unusually high quantities of plastic found within the Bristol Channel are a threat to fishermen’s liveli- hoods (Anon. 1992b, c; Williams et al. 1993b). Circula- tion patterns and slow flushing times within the Bristol Channel are likely to result in long residence times before this material is released into the open sea. The Channel may therefore act as an eventual sink for plastic sheeting and other mobile materials. If inputs of such material continue, a build-up within the Bristol Channel may occur.

Test material

The test material used was a brand of panty-liner marketed as a daily use towel. Panty-liners generally consist of a (non woven polypropylene/polyethylene/ rayon) cover enclosing cotton pulp which is backed by a polyethylene shield. A strip along the shield is coated with pressure-sensitive adhesive and covered with a sili- cone-coated release tape. The release tape is removed upon initial application and generally the remaining prod- uct is flushed intact after use. The towel is then trans- ported by the sewerage system to a sewage treatment works, where appropriate treatment is carried out. Final effluent may be discharged to inland waters or sea and the sludge to the land, sea or atmosphere. Unfortunately, during the sewage treatment process there is a tendency for the plastic backing strip to become dissociated from the cotton pulp. In this form the strips constitute a major screening problem. “Certain types of plastics and cotton bud sticks appear to align themselves so that they give least resistance to flow and as a result a higher proportion get past the screens than would be expected” (Huntingdon 1990, p. 3). Even under optimum conditions, sanitary towel backing strips often find their way into water- courses. This problem is exacerbated during periods of heavy rainfall as flows during storm conditions have to be discharged after only coarse screening (often 12 - 25 mm bar screens) unless suitable storage facilities exist. Concurrently, under the same conditions, Storm Water

Overflows also operate, commonly discharging com- pletely unscreened effluent into receiving waters. Once present in the watercourse, it is generally only the plastic backing strip of a sanitary towel which persists and ends on a beach. As such, tensile testing of just the backing strips was felt to be appropriate.

Methodology

Control samples were required in order to develop appropriate methods for test piece preparation and opti- mum test parameters. To obtain the test pieces, release tapes were first removed from each of the panty-liners. The panty-liners were then submerged in water for 1-2 days until separation of the backing strip from the cotton pulp could be achieved without damaging or stretching the test material.

Guidelines exist regarding standard tensile testing procedures for plastic sheeting (ASTM D882-83, Anon. 1983a, b; BSI 2782, Anon. 1986). A 19-mm strip was cut from the centre of the specimen; this width conforms with the standard testing procedure. A steel rule was used as a template and using a scalpel, clean parallel edges were cut along the length of the backing strip (150 mm). Following BSI 2782 (1986) recommendations, tensile testing was carried out at a variety of speeds, initial gauge lengths (distance between grips), grip types and loads. Due to the unique nature of the test piece, a trade off was required between these factors in order to produce the most consistent results. After several trials and discussions with materials experts (Wild pers. comm.) the test parameters were finally set. Under these constant conditions, twenty control specimens were tested with which samples could be compared after exposure.

Test procedure

A JJM30K tensile testing instrument was used for the degradation analyses. The instrument’s standard gnurled grips were unsuitable for the test material as they caused tear failure at the grip interface. This was overcome by lining the grips with rubber. The digital displays were programmed to the chosen settings (maxi- mum load 0.1 KN, maximum extension 500 mm, grip separation speed 25 mm/min) and the plotter linked to produce a graphical printout. The grips were set to the initial gauge length (90 mm) and specimens marked to ensure the test was carried out using the centre of the test piece. Once the specimen was properly aligned and both the tensile instrument and plotter zeroed, the test was initiated. The grips pulled apart at a constant rate of 25 mm/min whilst the digital displays monitored the

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