Recycling thermosets: The use of high-pressure high-temperature sintering (HPHTS) and degraded material as means of producing new products

• Main Author: Williams, Drew E
• Format: Dissertation
• Language: English
• Subjects: chemistry, polymer; engineering, materials science
• ISBN: 0496133136; 9780496133130

Thermosetting materials have long been considered impossible to reuse since they do not melt or dissolve. Few technologies have been developed to recycle waste thermosets compared to those available for the recycling of metals, glasses, and thermoplastics (meltable polymers). As rubbers are a sub-category of thermosetting materials, they also suffer from these limitations to recycling. Currently, scrap rubber tires and waste polyurethanes represent two of the largest recycling dilemmas facing our society. The work herein offers two potential solutions to this problem of recycling thermosets.
The first technique, High-Pressure High-Temperature Sintering (HPHTS), allows parts to be produced from 100% recycled material (current techniques typically use less than 10% recycled content). Several thermosetting systems were investigated in efforts to understand why certain thermosets are more recyclable via HPHTS than others. The goal of this work was to understand the mechanism of HPHTS and design and/or synthesize thermosets that are more easily recycled when they reach the waste stream. During this study, it was realized that Chemical Stress Relaxation (CSR) techniques offered excellent insight into the HPHTS process. As such, a section of the thesis is focused on Chemical Stress Relaxation and its correlation with HPHTS. The last sections devoted to HPHTS involve the utilization of additives in the HPHTS process as a means of increasing mechanical properties and engineering the backbone of thermosets in efforts to enhance recyclability. These chapters target the end uses of these materials; their purpose being to increase properties so that the materials can be utilized for real world products.
The second technique, degradation/devulcanization of thermosets (specifically rubber), is carried out at high temperatures (>280°C) under a variety of conditions (in a melt press while under pressure, in a Parr-reactor, etc.) The resulting viscous liquid-like material has numerous uses that include revulcanization, asphalt modification, and oil replacement in the compounding and molding of virgin rubber. Many of these uses not only increased the amount of rubber recycled, but also offered potential improvements over prior art. Materials incorporating over 35% recycled content were produced and maintained all of the original mechanical properties of the control (oil compounded) specimen.