The degradation of plastic litter in rivers: implications for beaches -
Fig. 2. Panty-liner degradation trial samples.
increase in load and extension. When specimen failure occurred, digital outputs for maximum load and exten- sion were recorded, along with the extension at yield point (“first point on the force-extension curve at which an extension occurs without an increase in force”; BSI 2782, Anon. 1986, p. 2). This procedure was repeated for the ten specimens in the sample and for each subse- quent sample of ten test pieces.
Panty-liner degradation trials were initiated on a weekly/monthly sampling programme, using ten test pieces to be measured per unit time, a number felt to be suitable based on BSI 2782 (Anon. 1986) recommenda- tions of a minimum five test pieces per sample. Having separated the backing strips from the remainder of the towel, these were secured to hardboard exposure plates in sets of ten, using carefully placed drawing pins. River bank attachment height for exposure plates was gov- erned by the level of indigenous litter stranding, i.e. the previous flood level. Three exposure plates were se- cured to the bank, three were buried and the remainder attached to nearby tree branches. As all strips were positioned at a natural stranding level they were ex- posed to equivalent environmental conditions as those stranded naturally (Fig. 2).
Results and Discussion
Beneficial properties of plastic products such as durability and strength have led to their widespread use in society. Plastics are expected to retain these proper- ties throughout their service lifetime in order to fulfil their required function. Specific mechanical properties may be measured to predict material durability, aiding determination of potential applications, and may also act as a reference with which to monitor the breakdown of plastics. Tensile testing of materials to record load/ extension measurements, allow mechanical properties such as tensile strength and elongation to be calculated. Tensile strength is “the maximum tensile stress which the test piece is capable of supporting” (BSI 2782, Anon. 1986, p. 3) and may be calculated using the equation:
Where: = maximum tensile strength (MPa); F = maximum force (N); A = Initial mean cross-sectional area (mm).
Elongation is “the elongation produced in the gauge length of the test piece at break” (BSI 2782, Anon. 1986, p. 2) and is expressed as a percentage of the original gauge length. Elongation at break may be calculated using the equation: