Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives

The last few decades have witnessed significant advances in the development of polymeric-based foam materials. These materials find several practical applications in our daily lives due to their characteristic properties such as low density, thermal insulation, and porosity, which are important in packaging, in building construction, and in biomedical applications, respectively. The first foams with practical applications used polymeric materials of petrochemical origin. However, due to growing environmental concerns, considerable efforts have been made to replace some of these materials with biodegradable polymers. Foam processing has evolved greatly in recent years due to improvements in existing techniques, such as the use of supercritical fluids in extrusion foaming and foam injection moulding, as well as the advent or adaptation of existing techniques to produce foams, as in the case of the combination between additive manufacturing and foam technology. The use of supercritical CO2 is especially advantageous in the production of porous structures for biomedical applications, as CO2 is chemically inert and non-toxic; in addition, it allows for an easy tailoring of the pore structure through processing conditions. Biodegradable polymeric materials, despite their enormous advantages over petroleum-based materials, present some difficulties regarding their potential use in foaming, such as poor melt strength, slow crystallization rate, poor processability, low service temperature, low toughness, and high brittleness, which limits their field of application. Several strategies were developed to improve the melt strength, including the change in monomer composition and the use of chemical modifiers and chain extenders to extend the chain length or create a branched molecular structure, to increase the molecular weight and the viscosity of the polymer. The use of additives or fillers is also commonly used, as fillers can improve crystallization kinetics by acting as crystal-nucleating agents. Alternatively, biodegradable polymers can be blended with other biodegradable polymers to combine certain properties and to counteract certain limitations. This work therefore aims to provide the latest advances regarding the foaming of biodegradable polymers. It covers the main foaming techniques and their advances and reviews the uses of biodegradable polymers in foaming, focusing on the chemical changes of polymers that improve their foaming ability. Finally, the challenges as well as the main opportunities presented reinforce the market potential of the biodegradable polymer foam materials.

Carbon Capture Utilization for Biopolymer Foam Manufacture: Thermal, Mechanical and Acoustic Performance of PCL/PHBV CO2 Foams

Biopolymer foams manufactured using CO₂ enables a novel intersection for economic, environmental, and ecological impact but limited CO₂ solubility remains a challenge. PHBV has low solubility in CO₂ while PCL has high CO₂ solubility. In this paper, PCL is used to blend into PBHV. Both unfoamed and foamed blends are examined. Foaming the binary blends at two depressurization stages with subcritical CO₂ as the blowing agent, produced open-cell and closed-cell foams with varying cellular architecture at different PHBV concentrations. Differential Scanning Calorimetry results showed that PHBV had some solubility in PCL and foams developed a PCL rich, PHBV rich and mixed phase. Scanning Electron Microscopy and pcynometry established cell size and density which reflected benefits of PCL presence. Acoustic performance showed limited benefits from foaming but mechanical performance of foams showed a significant impact from PHBV presence in PCL. Thermal performance reflected that foams were affected by the blend thermal conductivity, but the impact was significantly higher in the foams than in the unfoamed blends. The results provide a pathway to multifunctional performance in foams of high performance biopolymers such as PBHV through harnessing the CO₂ miscibility of PCL.

Multi-Fold Enhancement in Compressive Properties of Polystyrene Foam Using Pre-delaminated Stearate Functionalized Layer Double Hydroxides

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.

Effect of Crosslinking Agent and Branching Agent on Morphological and Physical Properties of Poly(Butylene succinate) Foams

To enhance a viscosity of poly(butylene succinate) (PBS), a crosslinking agent (i.e. dicumyl peroxide, DCP) or branching agent (i.e. Desmodur®N3300, N3300) was incorporated with various amount ranging from 0 to 4 phr via an internal mixer. The thermal transition, rheological properties (e.g. storage modulus, loss modulus, and complex viscosity) and gel content of the compound were determined via differential scanning calorimeter, rheometer, and gel content measurement, respectively. The results revealed that less degree of crystallinity, higher viscosity and more crosslinked structure were detected as increasing amount of DCP and N3300. Subsequently, a compression moulding technique was used to prepare PBS foam with a chemical blowing agent, i.e. azodicarbonamide (ADC). Scanning Electron Microscopy was acquired to examine cell size, cell size distribution, cell structure, and cell density. Besides, physical properties, e.g. foam density and physical appearance, were investigated. The results indicate that the degree of crystallinity, viscosity, and crosslinked structure affect the morphological and physical properties of PBS foams, i.e. greater viscosity and crosslinked structure causing obstacle for cell growth, and less degree of crystallinity allowing easier cell nucleation and cell growth.

Interconnected porous poly(ɛ-caprolactone) tissue engineering scaffolds fabricated by microcellular injection molding

In tissue engineering applications, a scaffold containing an interconnected porous structure is often highly desirable since these interconnected pores allow nutrients and signaling molecules to reach all of the cultured cells. In this study, microcellular injection molding, a mass production method for foamed plastic components, was combined with chemical foaming and particulate leaching methods to fabricate an interconnected porous structure using poly(ɛ-caprolactone) (PCL). Sodium bicarbonate (SB) was employed as the chemical foaming agent while carbon dioxide (CO2) was used as the physical foaming (blowing) agent. The results showed that interconnected porous structures of PCL, which depend on the composition of the materials used, could be successfully produced. Sodium bicarbonate not only generated CO2 to supplement the supercritical fluid microcellular injection molding, but also served as the nuclei for heterogeneous cell nucleation. Sodium bicarbonate and its byproduct, sodium carbonate, were also the porogens in the particulate leaching process, which further enhanced the porosity and interconnectivity. The morphologies and mechanical properties of the samples with different material compositions and porosities were discussed. The results of cell viability assays of 3T3 fibroblasts suggested that the resulting interconnected porous PCL scaffolds exhibited good biocompatibility. Cell spreading was affected by the porosity of the scaffold because of the physical restriction effect on the cell migration. Highly improved interconnectivity of the scaffold provided more space for the cells to spread.

An experimental study on foaming of linear low-density polyethylene/high-density polyethylene blends

In this research work, foaming behavior of selected polyethylene blends was studied in a solid-state batch process, using CO2 as the blowing agent. Special emphasis was paid towards finding a relationship between foamability and thermal and rheological properties of blends. Pure high-density polyethylene, linear low-density polyethylene, and their blends with two weight fraction levels of high-density polyethylene (10 and 25%wt.) were examined. The dry blended batches were mixed using an internal mixer in a molten state, and then the disk-shaped specimens, 1.8 mm in thickness, were produced for foaming purposes. The foaming step was conducted over a wide range of temperatures (120–170℃), and the overall expansion and cellular morphology were evaluated via density measurements and captured SEM micrographs, respectively. Three-dimensional structural images were also captured using a high resolution X-ray micro CT for different foamed samples and were compared. Rheological and DSC tests for the virgin and blends were also performed to seek for a possible correlation with the formability. Based on the results, blended polyethylene foams exhibited remarkable expansion and highly enhanced cell structure compared to pure polymers. Bulk density, as low as 0.33 g/cm3, was obtained for blends, while for the virgin high-density polyethylene  and linear low-density polyethylene, bulk density lower than 0.5 g/cm3 was not attainable. The lowest density was observed at a foaming temperature of 10–20℃ above the melting (peak) temperature obtained via DSC test. Rheological characteristics, including storage modulus and cross-over frequency value, were also found to be the indicators for the materials foaming behavior. Moreover, blends with 25% wt. of high-density polyethylene exhibited the highest expansion values over a wider range of temperature compared with 90% linear low-density polyethylene/10% high-density polyethylene.