Like human skin, plastics are susceptible to degradation due to ultraviolet (UV) light from the sun. Solar UV light that reaches our planet's surface has a wavelength between 280 and 400 nanometers, so it's not visible to the human eye, which sees light in wavelengths from roughly 390 to 750 nanometers.
Outdoors, UV light is present in amounts great enough to blitz polymer molecules. With enough exposure, UV light can cause a chemical reaction in the plastic, which results in scission, or severing, of those big polymer molecules.
Old-school plastics aren't particularly sensitive to sun exposure and as such, they could last for a very long time even in direct sunlight. However, with some clever molecular manipulation, or by integrating additives, engineers can make plastics that photodegrade much more quickly.
Certain chemical additives can make plastics more light-sensitive. Common additives (also called promoters, photosensitizers or accelerants) include ketone carbonyl, carbon monoxide carbonyl and different types of metal blends.
The carbonyls are types of organic compounds that are interlaced with plastic molecules. Other additives include metal salts, such as iron, cobalt, nickel which help to initiate a two-stage degradation process.
In the first stage, the additives absorb UV light and cause weak links in the polymers, so those big synthetic molecules get weaker and weaker. The second stage of the process happens as environmental factors like wind and waves bash and abuse the product, which leads to its eventual crumbling.
Blending plastic resins and additives takes trial and error, as engineers balance strength and non-toxicity, as well as degradation rates.