Abstract
While existing foam studies have identified processing parameters, such as high-pressure drop rate, and engineering measures, such as high melt strength, as key factors for improving foamability, there is a conspicuous absence of studies that directly relate foamability to material properties obtained from fundamental characterization. To bridge this gap, this work presents batch foaming studies on one linear and two long-chain branched polypropylene (PP) resins to investigate how foamability is affected by partial melting (Method 1) and complete melting followed by undercooling (Method 2). At temperatures above the melting point, similar expansion was obtained using both foaming procedures within each resin, while the PP with the highest strain hardening ratio (13) exhibited the highest expansion ratio (45 ± 3). At low temperatures, the foamability of all resins was dramatically improved using Method 2 compared to Method 1, due to access to lower foaming temperatures (<150 °C) near the crystallization onset. Furthermore, Method 2 resulted in a more uniform cellular structure over a wider temperature range (120–170 °C compared to 155–175 °C). Overall, strong extensional hardening and low onset of crystallization were shown to give rise to foamability at high and low temperatures, respectively, suggesting that both characteristics can be appropriately used to tune the foamability of PP in industrial foaming applications.
Highlights
Polymer foaming is a well-established technology and has involved numerous research efforts that aim to achieve the advantageous properties of cellular plastics, such as their high strength-to-weight ratio, enhanced impact strength, thermal insulation, sound absorption, and lightweight qualities [1,2,3]
The effect of strain hardening is highlighted in the positive correlation between the strain hardening ratio (SHR) values and the volume expansion ratios at high temperatures, such as 160 and 170 ◦C
Compared to the other two resins, Resin A was able to tolerate the largest amount of extensional stress induced by cell growth owing to its high SHR value, which is reflected in its high volume expansion under both methods
Summary
Polymer foaming is a well-established technology and has involved numerous research efforts that aim to achieve the advantageous properties of cellular plastics, such as their high strength-to-weight ratio, enhanced impact strength, thermal insulation, sound absorption, and lightweight qualities [1,2,3]. Chen et al used a different approach to increase the apparent solubility of scCO2 in PP by introducing a pressure swing saturation that led to the dissolution of gas in a shorter amount of time compared to that in a conventional batch foaming process [28] As a result, this strategy widened the foaming temperature range and increased the expansion ratio, which may be due to Polymers 2022, 14, 44 the repeated depressurization steps inducing the nucleation of cells that effectively served as nucleation sites for subsequent bubble nucleation. The introduction of this new temperature protocol allows batch foaming to be brought one step closer to industrial foaming processes, thereby establishing greater common grounds between both approaches for better chances of batch foaming being employed as a method for screening the foamability of newly developed resins prior to their application in continuous processes
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