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. 2025 May 6;17(9):1270.
doi: 10.3390/polym17091270.

An Analysis of Foams Produced from Recycled Polyolefins and Low-Cost Foaming Agents: Benchmarking Using Pore Size, Distribution, Shear Effects, and Thermal Properties

Krishnamurthy Prasad  1 , Fareed Tamaddoni Jahromi  1 , Shammi Sultana Nisha  1 , John Stehle  2 , Emad Gad  1 , Mostafa Nikzad  1
Affiliations

An Analysis of Foams Produced from Recycled Polyolefins and Low-Cost Foaming Agents: Benchmarking Using Pore Size, Distribution, Shear Effects, and Thermal Properties

Krishnamurthy Prasad et al. Polymers (Basel). .

Abstract

Foamed specimens were fabricated from virgin and recycled polyethylenes (linear low-density polyethylene, LLDPE, and low-density polyethylene, LDPE) using low-cost citric acid and sodium bicarbonate foaming agents. The foaming agents chosen showed decomposition behaviour either without phase change (sodium bicarbonate, NaB) or liquefaction followed by decomposition (citric acid, CA). The manufactured polyethylene foams were then benchmarked against a polyurethane foam. Two types of mixing were used prior to foaming, viz., solid-state pulverisation or high-shear internal mixing, and the effect of mixing on properties critical for foam viability were analysed. These properties included density, pore size, shape and distribution, crystallinity, and porosity. It was found that the virgin LLDPE and recycled LDPE showed similar trends in terms of narrow pore size distribution and reduced crystallinity, while the recycled LLDPE tended towards more pore coalescence. This difference in behaviour was attributed to the more mixed phase nature of the recycled LLDPE as opposed to the majorly single-phase virgin LLDPE and recycled LDPE. Lowered densities obtained for the NaB foaming compared to CA can be speculated to be because of the ionic and simple NaB decomposition as opposed to the complex radical pathway for the CA decomposition.

Keywords: density; foaming; pore size distribution; recycled polyolefins; shear.

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Conflict of interest statement

Author John Stehle was employed by the company Robovoid Pty Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 10
Figure 10
DTA graphs of the foaming agents NaB and CA showing the peak decomposition temperatures.
Figure 11
Figure 11
DSC curves of pulverised foam materials: (a) melting curve of LDPE foams; (b) recrystallisation curves of LDPE foams; (c) melting curve of LLDPE foams; (d) recrystallisation curves of LDPE foams; (e) melting curve of V_LLDPE foams; (f) recrystallisation curves of V_LLDPE foams.
Figure 12
Figure 12
DSC curves of batch mixed foam materials: (a) melting curve of LDPE foams; (b) recrystallisation curves of LDPE foams; (c) melting curve of LLDPE foams; (d) recrystallisation curves of LDPE foams; (e) melting curve of V_LLDPE foams; (f) recrystallisation curves of V_LLDPE foams.
Figure 13
Figure 13
Degree of crystallinity values of all fabricated foams: (a) via pulverisation; (b) via batch mixing.
Figure 1
Figure 1
Scheme of degradation reactions and products thereof of citric acid.
Figure 2
Figure 2
Foaming of recycled LDPE with citric acid. (a) Dry mixed polymer and foaming agent powder before being placed in the oven. (b) Foam product after being removed from the oven and the mould.
Figure 3
Figure 3
(a) Dough obtained after internal mixing of V_LLDPE with NaB. (b) Foamed polymer obtained from dough.
Figure 4
Figure 4
(a) Comparison of V_LLDPE CA pulverised foam with commercial PU foam. Optical microscopy images of (b) V_LLDPE CA pulverised foam; (c) commercial PU foam; (d) V_LLDPE CA batch mixed foam.
Figure 5
Figure 5
Comparison of the pore size distribution of pulverised and batch-mixed foams of V_LLDPE produced with different chemical foaming agents: (a) NaB; (b) CA.
Figure 6
Figure 6
Span of pore sizes observed in V_LLDPE foams made from CA and NaB under pulverisation and batch mixing conditions benchmarked with PU mattress foam.
Figure 7
Figure 7
Batch mixed CA foam: (a) LDPE; (b) LLDPE, pulverised NaB foam; (c) LDPE; (d) LLDPE.
Figure 8
Figure 8
Pore size distributions from the microstructure of recycled polyolefin foams: (a) LDPE with CA foams; (b) LLDPE with CA foams; (c) LDPE with NaB foams; (d) LLDPE with NaB foams.
Figure 9
Figure 9
Span of pore sizes observed in recycled LLDPE, and recycled LDPE foams made from CA and NaB under pulverisation and batch mixing conditions benchmarked with PU mattress foam.
Figure 14
Figure 14
Densities and VP of manufactured foams vs. raw materials with PU mattress benchmark: (a,b) via pulverisation; (c,d) via batch mixing.
Figure 15
Figure 15
(a) The required strength for compressing the foams to 50% of the original thickness for different compositions. (b) Compression modulus of the tested foam samples.
Figure 16
Figure 16
Compression set under deflection of 50% for LDPE foams with different compositions.
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