Even in an eco-conscious city like San Francisco, more than 9,000 tons of recyclables are dumped in landfills every year largely due to one culprit: flimsy plastics. Low-grade plastics, such as shopping bags, padded online shipping envelopes and food packaging, are piling up in recycling centers. Part of the reason is that overseas markets such as China and the Philippines — which used to buy America’s trash by the shipload — are turning most plastics away. California lawmakers say the state must act to stop plastics from crowding landfills and polluting the ocean. They’ve proposed sweeping legislation to require manufacturers to reduce the reliance on single-use plastics.