Does plastic bag degrade in marine environment

You must have seen this prank on the internet when the 5p plastic bag charge kicked in.  In fact, according to The Guardian, over 60% of the respondent of their survey claimed that they are strongly or slightly agree with the policy.  Why would the government implement such regulation and what are the environmental impact on the plastic carrier bags?

Production of plastics has increased from 0.5m tonnes in 1950 to 260m tonnes in 2007 (Plastic Europe, 2008).  Amongst the total quantity of plastic produced, about 37% of it are made as disposable item for packaging.  While only 14% of the plastic bag are being recycled (EPA office of Solid Waste, 2013), most of the plastic bag accumulates in landfill and wildlife habitat.  The problem is significant in marine environment.  About 60-80% of marine litter is plastic (Derraik,
2002) and for example, it's estimated that more than 300,000 plastic particles km-2 circulated in the North Pacific Gyre (Moore et al, 2001).

Does plastic bag degrade in sea?  As we all know, plastic shows great resistance to aging and minimal biodegradation.  When plastic are exposed to UVB radiation in sunlight, together with the oxidative and hydrolytic properties of seawater, polymers will oxidate.  The only problem is that it takes a further 100 years more for the mineralisation of polyethylene to complete, by then, the plastic would become bio-available.

Standard method indicating degradation are to test the reduction in tensile strength and loss of surface area.  O'Brine (T. O'brine, 2010) has performed a test on the degradation rate of different form of carrier bags, including bio-degrable bag (made by corn-starch, vegetable oils and compostable polyesters) and standard polyethylene bag.  Unsurprisingly, from figure 2, there were only 2% surface loss of the standard polyethylene after 40 weeks.  While for compostable polyester, no samples are remained at 24 weeks.  On the other hand, tensile strength of all plastic type decreased over time but at a different rate (Figure 1).

Figure 1

Figure 2

O'Brine provides some explanation of why some plastic simply won't degrade over time.  One of the reason is when plastic is immersed in the seawater, the UV light it received reduce by 90%.  This is significant as UV light can initiate the oxidative process which leads to deterioration of plastics.  The reduction in temperature in the marine environment would also reduce degradation rate.  According to Ho et al. (1999), degradation rate of plastics is positively correlated to temperature and relative humidity.

However, degradable plastic bag are nothing magical.  Different formulation of degradable material have different reactions.  Some may merely disintegrate plastic into small pieces that would enter the food chain more easily.  When degradable plastics are broken into small particles, they are no more degradable than conventional plastics.

For sure that the 5p carriers bag charge would alleviate the problem.  Wales and Northern Ireland has recorded 78% and 81% reduction in carriers bag usage after the implementation of the policy.  So why not bringing your own bag for your next shopping in the supermarkets.