When did plastics become regarded as bad for the environment? I would like to have some valid references.
The idea that plastics are bad is usually based on two observations:
Firstly, there are giant garbage patches in oceans full of large plastic waste. These consist of large plastic items. The observation of these garbage patches is fairly old. According to Wikipedia, at least in 1945 the Great Pacific garbage patch was present.
Secondly, more recently, an observation has been made that every location on this planet if full of microplastics. The water we drink has microplastics. The food we eat has microplastics. The term "microplastics" was created in 2004 by professor Richard Thompson. Presumably the first observations of microplastics were made earlier.
Note neither of these two indicate that every use of plastics is bad. If plastic waste is properly handled, it doesn't create any environmental risks.
In areas with proper waste handling, the harm mainly comes from people carelessly laying waste around and trash cans that allow birds to open them, distributing plastic waste to the environment, which the wind carries to oceans. Also plastic clothing if washed in a washing machine creates huge amounts of microplastics in the wastewater. Currently water treatment facilities don't remove the microplastics, and washing machines don't have microplastics filters.
Biodegradable plastics could be a solution, although biodegradation means plastics won't be as durable as non-biodegradable plastics. It may be the case microbes are evolving the ability to eat plastic, like they evolved ability to eat lignin and cellulose (stopping the accumulation of coal reserves from dead forests). If this is the case, every plastic may become biodegradable in the future. It affects the longevity of plastic parts negatively, but also affects the environmental risks created by plastic pollution positively.
I think concerns about plastic slowly increased over time. The earliest mention I am aware of is 1966 when a survey was held that reveiled plastic fragments in dead albatrosses. This scientfic article mentions that:
Some of the earliest accounts of plastic debris in the marine environment are of fragments and pellets ingested by seabirds in the 1960s (e.g. Kenyon & Kridler 1969; Harper & Fowler 1987)
plastic fragments found in body cavity of dead laysan albatrosses during 1966 survey (Kenyon & Kridler 1969)
The 1960s was also the time when the image of plastics changed from wonderful new material that opened up possibilities to something cheap and flimsy. However, it wasn't until the 1970s that the negative effects of plastic became more widely known.
Plastic’s reputation fell further in the 1970s and 1980s as anxiety about waste increased (source)