Plastic lumber is the ultimate in downcycling, Bill McDonough's term for materials coming back in their next life as a lower valued product than before. Some plastics are harder to recycle than others; toner cartridges are full of toxic toner powder. Most go into landfill or are incinerated; Waycam says
"Land fill releases harmful toxins and chemicals into the land whilst also taking 1000’s of year to degrade, whilst incineration pollutes the environment by putting the toxins in to the Air as well as increasing the carbon emissions, as the cartridges are burnt."
The stuff isn't too pretty, but they "have a process that transforms cartridges into Plastic Wood call TRI Wood which we can then turn into fence posts and garden furniture with 100% recycling and no landfill or incineration."