Styrofoam Recycling: Relaxation-Densification of EPS by solar heat
Authors: Dennis Robert , Dennis Andromeda , Dennis William 
Published: October 2023

The problem of Styrofoam waste is multifaceted. Not only do we need to properly handle the current and future stream of newly manufactured Styrofoam objects, we also have many decades of inert Styrofoam occupying a large fraction of both our active and full/decommissioned landfills, and floating in our oceans and waterways. Solutions are therefore required for recycling current waste streams as well as mitigating huge volumes of legacy Styrofoam waste, distributed globally wherever human populations, landfills, waterways, terminal lakes, reservoirs, or ocean gyres exist. Simply banning Styrofoam, aside from packing peanuts, has been of limited effect because Styrofoam is simply too useful and inexpensive. It is essentially a super-material that is hard to replace and difficult to do without. Every environmentalist who owns a modern television unpacked it from a Styrofoam shell inside a large cardboard box.
Polystyrene, the fundamental constituent of Styrofoam, is a common plastic that recycles well. The problem with recycling Styrofoam is that as a foam it is about 98% air by volume, so it must be densified before transport for recycling: the air has to be removed from the foam to return it to its original form as a solid plastic again. The shipment of un-densified Styrofoam for recycling is not economically viable, it would take about 50 trucks to transport only one truck load of fully densified plastic. This equates to un-densified Styrofoam costing more money for paying the costs of factors such as fuel for the vehicle, more drivers, etc. This means, waste Styrofoam must be densified at or near the point of its end-of-use.
The focus of our experiments was to see how well zero-carbon, widely distributable methods of heat exposure could densify different types of expanded polystyrene packing peanuts. We show that this can readily be accomplished using the range of temperatures achievable by a typical solar oven (at or above 300°F) — well below the melting temperature of polystyrene (~ 500°F).

Rather than using solar heat for this experiment, the proof of concept was demonstrated in a temperature-controlled laboratory oven to maintain well-controlled and stable temperatures to allow the generation of accurate temperature-time-densification curves.
At the highest temperatures (~ 300°F) and the longest exposure time (10 minutes), the densification factor ranged from 50 to 53. This is very close to the theoretical limit. Lower temperatures result in partial densification with higher variability. Larger pieces would need longer heat exposure times.
To achieve full densification, we suggest using a temperature of at least 300°F (149°C) for 10 or more minutes, for pieces of Styrofoam about the size of packing peanuts. This is easily achievable in solar ovens
This process can be readily done in distributed locations at or near the location of end-of-use for Styrofoam products wherever solar ovens can achieve temperatures of about 300°F, thereby making the recycling and upcycling of Styrofoam economically viable for most inhabited regions of the globe, and especially in equatorial areas where plastic waste is especially troublesome.