Abstract
Developing an environmentally benign styrene foam is a critical environmental need. Supercritical CO2 use in foams has proven to be a valuable path. Adding fillers to increase bubble nucleation has been pursued concurrently. A prominent filler used is high surface area fillers, such as smectic clays. However, all studies to date show a limit of 152% in compressive moduli and 260% in the compressive stress. The values, even with such gains, limit structural application. A seminal work in 1987 by Suh and Cotton proved that carbonyl linkages in calcium carbonates and CO2 interact and impact nucleation efficiency and performance in supercritical CO2 foams. In this paper, a high surface area clay (layer double hydroxides) which begins in an exfoliated state, then functionalized with a long chain alkyl carboxylate (stearic acid) is synthesized. The result is a remarkable multi-fold improvement to the compressive properties in comparison to polystyrene (PS); a 268% and 512% increase in compressive modulus and strength, respectively. Using a pre-delaminated approach, the higher surface area was achieved in the clays. The presence of the stearate improved the interactions between the clay galleries and PS through hydrophobic-hydrophobic interactions. The glass transition temperature of the nanocomposites was observed to shift to higher values after foaming. The results point to a new path to increase performance using a pre-delaminated clay with functional groups for environmentally benign foams.
Highlights
Polystyrene (PS) is one of the major commodity plastics widely used and commercially produced [1], with an annual production of 19.6 million tons, as of 2015, and has continued to grow ever since [2]
The approach of stearate functionalization to improve nucleation in supercritical CO2 foamed polymer composites espoused by Cotton and Suh in 1987 proved effective in conjunction with clay filled polymers
A synthetically prepared layered double hydroxide was functionalized with stearate and introduced into polystyrene in weight fractions of 1, 3 and 5% w/w
Summary
Polystyrene (PS) is one of the major commodity plastics widely used and commercially produced [1], with an annual production of 19.6 million tons, as of 2015, and has continued to grow ever since [2]. It is used for applications in various sectors, such as building and piping, automotive, electronics and electrical equipment, furniture and food packaging, just to mention a few [2,3]. Physical foaming techniques have evolved over the years, and the especially the expansion of polystyrene using foaming agents, such as water and supercritical carbon dioxide (scCO2 ) has led Polymers 2020, 12, 8; doi:10.3390/polym12010008 www.mdpi.com/journal/polymers
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