Polypropylene Foam Research Articles

Experimental investigation on impact and after-impact compression (AIC) behavior of foam core composite panels

This study investigates the impact and after-impact compression behavior of sandwich structures with carbon fiber composite face-sheets consisting of various polymeric core structures. In particular, the performance of polypropylene foam core with closed cell structure, which is new to engineering applications, against other polymeric core structures was investigated. Sandwich panels were subjected to three different impact energies and after-impact compression tests were conducted on each sample to reveal their damage tolerances and load-carrying capabilities. In-situ acoustic measurements were made to fully understand the damage and evolution mechanisms in the after-impact compression tests. Honeycomb polypropylene (PP) foam, polyvinyl chloride (PVC), and extruded polystyrene (XPS) foam were used as core materials. As a result of the tests, it was obtained which damage types occurred at which force levels. PP core panels exhibited different impact behavior compared to conventional sandwich core structures and were successful in preventing damage to the face sheets.

Properties of chemically foamed polypropylene materials for application to automobile interior door panels

Abstract In recent years, alternative approaches have been implemented in the automotive sector to reduce raw material costs and protect the environment. An increase in weight causes both fuel consumption and CO2 emissions to rise. This study aims to reduce exhaust emissions due to weight reduction by using foamed polypropylene in the door panel production of a subcompact crossover SUV car and saving energy by shortening the injection cycle time. The newly produced 2 % ITP 822 chemical foaming agent added door panel was compared with the current door panel performances. As a result of foam morphology structure, impact, and hardness tests, it was decided that ITP 822 is a suitable chemical foaming agent. In addition, a weight reduction of 5.2 % was achieved. Moreover, the injection cycle time has been reduced by approximately 12 %, reducing the total cycle time from 35 s to 31 s.

Preparation of Open-Cell Long-Chain Branched Polypropylene Foams for Oil Absorption.

This study aimed to prepare open-cell foams using a blend of long-chain branched polypropylene and polyolefin elastomer (LCBPP/POE) for the production of reusable oil absorbents. The supercritical CO2 foaming process was conducted using a two-step batch rapid depressurization method. This unique two-step foaming approach significantly expanded the temperature and pressure windows, resulting in more uniform cells with smaller sizes, ultimately leading to higher expansion ratios and an increased open cell content. The foaming process was optimized by adjusting parameters, such as the LCBPP/POE ratio, foaming temperature, and foaming pressure, reaching a maximum open cell content of 97.6% and a maximum expansion ratio of 48. The influence of polypropylene (PP) crystallization was investigated with the aid of scanning electron microscopy and differential scanning calorimetry. Furthermore, the hydrophobic and lipophilic characteristics of the LCBPP/POE open-cell foam were determined via contact angle measurements and oil/water separation tests. Oil absorption tests revealed that the blended LCBPP/POE foam has a higher oil absorption capacity than that of the pure LCBPP foam. The cyclic oil absorption tests demonstrated the outstanding ductility and recoverability of the LCBPP/POE open-cell foam in comparison to those of the pure LCBPP foam. Over 10 cycles, the LCBPP/POE foam maintained a substantial adsorption capacity, retaining 99.3% of its initial oil absorption capacity. With its notable features, including a high open cell content, excellent hydrophobic and lipophilic characteristics, superior oil absorption capacity, impressive cyclic oil absorption performance, and robust reusability, LCBPP/POE open-cell foams exhibit significant promise as potential oil adsorbents for use in oil spill cleanup applications.

A unified approach for evaluating mechanical compression tests for polymer bead foams

AbstractIn the realm of material characterization, the mechanical properties of polymer foams play a pivotal role in shaping their applicability across diverse industries. In this pursuit, we present a novel approach to standardize and automate the assessment of key mechanical parameters for Expanded Polypropylene (EPP) foams using a self‐developed Python script, made freely available for the scientific community. We precisely determine the compression modulus, plateau onset, and onset of densification strain for EPP foams across various densities. The script's effectiveness is demonstrated through comparisons with manual evaluations and established standards, highlighting its superiority, consistency, and suitability for a wide range of materials and conditions. Moreover, the script enables the analysis of energy absorption, shedding light on the intricate relationship between density and energy dissipation. Finally, our approach was extended to other foams to provide insight on their mechanical properties. The automated methodology ensures accuracy, reproducibility, and efficiency, thereby advancing the understanding of foam materials and facilitating informed design decisions. This research contributes to laying a foundation for the standardized assessment of foam mechanical properties, which could potentially facilitate their effective use in other applications.

A scalable versatile methodology to construct micro/nano open-cell polypropylene foam with high oil adsorption capacity and speed

A scalable versatile methodology to construct micro/nano open-cell polypropylene foam with high oil adsorption capacity and speed

Lateral compression behavior of expanded polypropylene foam–filled carbon and glass fiber composite tubes

Abstract The crashworthiness and deformation behavior of circular composite tubes that were internally supported with expanded polypropylene (EPP) foams were investigated under lateral compression tests. Carbon woven (CFRP) and glass woven fiber/epoxy (GFRP) composites and EPP foam with densities of 30, 60, and 75 kg·m−3 were used. According to results, empty CFRP and GFRP tubes absorbed almost the same amount of energy; however, the GFRP tube had a higher specific energy absorption value due to its lower weight compared to the CFRP tube. EPP foam filling has a more significant effect on the crashworthiness of CFRP tubes compared to GFRP tubes. The best results in CFRP tubes, in terms of specific energy absorption, were obtained as 2.67 J g−1 at 75 kg·m−3 EPP foam-filled sample; however, 60 kg·m−3 EPP foam–filled sample exhibited the best configuration in terms of force efficiency. For the GFRP tubes, the best configuration was obtained at 60 kg·m−3 EPP foam–filled sample for all of the crashworthiness parameters. It is seen that the crushable length of composite tubes was shortened with the increase of EPP foam density. Lastly, the deformation behaviors of composite tubes showed that the CFRP tubes were more brittle than the GFRP tubes.

Synergistic effect of thermoplastic polyester elastomer and polytetrafluoroethylene in situ fibrillation on the resilience and compression strength of long‐chain branched polypropylene foam

AbstractExcellent resilience and compression resistance are the key characteristics of cushioning and water–oil separation materials. Although polypropylene (PP) offers excellent performance and affordability of PP, it exhibits poor resilience. To address this shortcoming, polymer blends with long‐chain branched PP (LCBPP), thermoplastic polyester elastomer (TPEE), and polytetrafluoroethylene (PTFE) were prepared via melt blending, and their mechanical properties were tested. The LCBPP/TPEE/PTFE composites were subjected to physical foaming using supercritical CO2 (scCO2) as the blowing agent. The synergistic toughening mechanism of the elastomer/nanofiber and the effect of the structure and crystallization of the dispersed phase on the foam resilience and foam compression strength were investigated. The results revealed that fibrous PTFE and spherical TPEE formed a composite dispersed phase in LCBPP. The impact and tensile strengths of the unfoamed LCBPP were found to have been simultaneously optimized owing to the synergistic effect of the elastomeric toughening of TPEE and the reinforcement of the PTFE fiber–induced fibrous crystal structure. Compared with pure LCBPP, the impact strength of the LCBPP/TPEE/PTFE composite increased by 216%, whereas its tensile strength decreased by only 6%. In particular, the synergistic effect of TPEE/PTFE improved the resilience and compression strength of the LCBPP foam. Compared with the pure LCBPP foam, the permanent strain of the LCBPP/TPEE/PTFE foam was reduced by 13.8% after five cycles of compression and its compression strength increased by 27.3%. The experimental results of our study could be used to prepare LCBPP foams with exceptional resilience, high compression strength, and good application prospects.

Ingestion and biodegradation of disposable surgical masks by yellow mealworms Tenebrio molitor larvae: Differences in mask layers and effects on the larval gut microbiome

During the COVID-19 pandemic, the usage and production of face masks considerably increased, resulting in large quantities of mask waste accumulating in the natural environment. To investigate whether masks of polypropylene (PP) material could be ingested and degraded by insect worms like PP foam plastic, yellow mealworms were provided with different layers of disposable surgical masks as sole diets for 30 d. Although mask layers, especially the middle layer of melt-blown filter, could be ingested by yellow mealworms, sole mask layer diets had adverse effects on the larval survival and growth. Analyses of Fourier transform infrared spectroscopy, differential scanning calorimeter and thermogravimetric, and gel permeation chromatography demonstrated the changes of functional groups, thermostability and molecular weights in frass compared to original masks, indicating the partial oxidation and degradation of masks. And the depolymerization of the middle layer of masks by yellow mealworms was different from that of other layers. The larval gut bacterial and fungal microbiomes were assessed by Illumina MiSeq, indicating that both of them shifted upon sole layer mask diets. Changes in relative abundances of dominant bacterial and fungal genera demonstrated the strong association between gut microbiome and mask degradation. For instance, unclassified Enterobacteriaceae was closely associated with outer layers degradation. Lactococcus and unclassified Ascomycota were responsible for middle layers degradation, while Lactococcus and Morganella for inner layers degradation. In conclusion, disposable surgical masks of PP material could be ingested and biodegraded by yellow mealworms. The diversities of gut bacterial and fungal microbiomes were associated with the differences in rigid crystalline structures of the layer masks.

Enhancing the sound and thermal insulation properties of polypropylene foam by preparing high melt strength polypropylene.

High-performance polypropylene (PP) foam is a vital polymer product in industrial areas. However, the poor melt strength of ordinary PP homopolymer limits its foaming molding. In this work, high melt strength polypropylene (HMSPP) was prepared by using styrene (St) and tripropylene glycol diacrylate (TPGDA) as comonomers, and then PP foams were prepared by mold foaming method. The results showed that adding St in the grafting process of TPGDA will obviously improve the melt strength of the PP matrix, and its melt strength (28184Pa.s) is 7.4 times higher than that of pure PP. HMSPP foam has more regular and uniform cells and higher cell density, which significantly improves the sound and thermal insulation properties of PP foam. Compared with pure PP foam, the average sound transmission loss (52.9dB) of HMSPP foam with a low foaming ratio increased by 64%, and the thermal conductivity (0.0867W/mK) decreased by 46%. Therefore, the obtained HMSPP foam can be used in sound insulation or thermal insulation area. This work provides an available route for the high-performance utilization of PP foam. This article is protected by copyright. All rights reserved.

Conversion of Polypropylene (PP) Foams into Auxetic Metamaterials

In this work, a simple and environmentally friendly process combining low pressure (vacuum) and mechanical compression is proposed to convert recycled polypropylene (PP) foams (28 kg/m3) into low density foams (90–131 kg/m3) having negative tensile and compressive Poisson’s ratios (NPR). The main objective of the work was to determine the effect of processing conditions (vacuum time, temperature and mechanical pressure). Based on the optimized conditions, the tensile Poisson’s ratio of the resulting auxetic foams reached −1.50, while the minimum compressive Poisson’s ratio was −0.32 for the same sample. The foam structure was characterized via morphological analysis (SEM) to determine any changes related to the treatment applied. Finally, the tensile and compressive properties (Young’s modulus, strain energy, energy dissipation and damping capacity) are also presented and discussed. It was observed that the mechanical properties of the resulting auxetic foams were improved compared to the original PP foam (PP-O) for all tensile properties in terms of modulus (19.9 to 59.8 kPa), strength (0.298 to 1.43 kPa) elongation at break (28 to 77%), energy dissipation (14.4 to 56.3 mJ/cm3) and damping capacity (12 to 19%). Nevertheless, improvements were also observed under compression in terms of the energy dissipation (1.6 to 3.6 mJ/cm3) and the damping capacity (13 to 19%). These auxetic foams can find applications in sport and military protective equipment, as well as any energy mitigation system.

A strategy for extending the processing temperature for polypropylene in foam extrusion and its theoretical validation

The quite narrow PP foaming temperature window is the main challenge for continuous extrusion foaming of polypropylene (PP) using supercritical CO2. In this study, high impact polystyrene (HIPS) is added to PP to widen the polypropylene foaming temperature range by reducing the temperature sensitivity of the melt strength of the blend. The behavior of crystalline, dynamic rheology, and extensional rheological of PP/HIPS blends are analyzed. The results show that the crystallinity and crystallization temperature of PP/HIPS blends decreased significantly, the temperature dependence of the blends’ extensional viscosity and melt strength became weaker, and the activation energy of extensional viscosity and melt strength activation energy decreased, while the elastic modulus and viscosity and the system relaxation time increased. It means that the addition of HIPS reduces the crystallinity of PP, improves the cell morphology and to a certain extent overcomes the problem of a sharp decrease in PP melt strength with increasing temperature. The addition of HIPS extends the foaming temperature range of the PP material from 4 K to a maximum of 12 K. Furthermore, we simply estimated the temperature window for extruded foams using the Arrhenius equation. The estimated values have the same trend as the experimental results and are analyzed.

Guest Editorial: Polymer electrets and ferroelectrets

Guest Editorial: Polymer electrets and ferroelectrets

A compound‐structured piezoelectret system and its applications in wearable health monitoring

AbstractPiezoelectrets, also called ferroelectrets, can exhibit promising piezoelectric properties and have plenty of applications in wearable health monitoring. Usually, the cellular structure of piezoelectrets is of outstanding importance for their sensing properties, and structure improvement and optimisation would be a possible way to realise high‐performance piezoelectrets. The authors proposed a compound‐structured piezoelectret system, where a layer of polypropylene foam was sandwiched between two layers of solid polytetrafluoroethylene, resulting in a combination of a foam‐structured and a layer‐structured piezoelectrets. The compound systems are thin and flexible, they can exhibit stable electrical outputs, they have relatively broader linear working range under pressure, and promising mechanical sustainability for multiple testing. The results reveal that the compound system can be considered as a simple addition of both components, and each component contributes linearly and independently to the whole system. The application potential of this proposed compound system has been demonstrated by sleep monitoring together with carotid and radial pulse recordings, where many useful physiological information including breath, heartbeat, and pulse details can be extracted from the signals acquired by the compound system. A type of flexible sensor system that is very competitive for future portable and wearable applications may be provided.

Ferroelectret Polypropylene Foam-Based Piezoelectric Energy Harvester for Different Seismic Mass Conditions

Energy harvesting technologies and material science has made it possible to tap into the abundant amount of surrounding vibrational energy to efficiently convert it into useable energy providing power to portable electronics and IoT devices. Recent investigations show that the piezoelectric effect is created in cellular polymers called ferroelectrets. These cellular-compliant polymers with polarized pores have a piezoelectric response to generate electrical energy when subjected to mechanical strain or surrounding vibration. It is found that there is a significant difference between ferroelectret polarized cellular polypropylene foam and traditional piezoelectric polymers such as polyvinylidene fluoride (PVDF). The former has approximately ten times higher piezoelectric coefficient than the latter. This means that with an acceleration of 9.81 m/s2 force on this material, ferroelectrets generate up to 39 (µW/g/mm3) power output. Designing a polypropylene-based piezoelectric energy harvester based on the d33 mode of vibration can be challenging due to several factors, as it requires balancing multiple factors such as mechanical stability, piezoelectric response, circuit topology, electrode size, spacing, placement relative to the piezoelectric material, and so on. This paper proposes the preliminary experimental investigation of ferroelectret cellular polypropylene foam in harvesting performance. Suggestions of different approaches for the structural design of energy harvesters are provided. The vibration-dependent response and generated output are examined concerning pulse or sinusoidal input excitation. The voltage generated for both excitations is compared and suggestions are provided regarding the suitable kind of excitation for the chosen ferroelectret material. Finally, conclusions and prospects for ferroelectret materials used in energy-harvesting applications are given.

Crushing Responses of Expanded Polypropylene Foam.

This paper aimed to experimentally clarify the crushing mechanism and performance of expanded polypropylene foam (EPP) and analyze the influence of density and thickness on its mechanical behavior and energy absorption properties under static crushing loadings. Hence, a series of compression tests were carried out on EPP foams with different densities and thicknesses. For foam with a density of 60 kg/m3, the mean crushing strength, energy absorption (Ea), energy absorption efficiency (Ef), specific energy absorption (SEA), and energy absorption per unit volume (w) increased by 245.3%, 187.2%, 42.3%, 54.3%, and 242.8%, respectively, compared to foam with a density of 20 kg/m3. Meanwhile, compared to foam with a thickness of 30 mm, the mean crushing strength, energy absorption (Ea), energy absorption efficiency (Ef), SEA, and energy absorption per unit volume (w) for foam with a thickness of 75 mm increased by 53.3%, 25.2%, -10.8%, -4.7%, and -10.6%, respectively. The results show that foam density has a significantly greater influence on static compressive performance than foam thickness. The microstructures of the EPP foam before and after static compression were compared by observing with a scanning electron microscope (SEM), and the failure mechanism was analyzed. Results showed that the load and energy as well as the deformation and instability processes of its cells were transferred layer by layer. The influence of density on the degree of destruction of the internal cells was obvious. Due to its larger mass and larger internal damping, thicker foams were less damaged, and less deformation was produced. Additionally, the EPP foam exhibited a considerable ability to recover after compression.

Low-profile broadband microwave absorber based on magnetic coating and artificial electromagnetic structures

In order to realize electromagnetic stealth of large equipment, we propose a low-cost, mass-producible composite microwave absorber in this paper. The developed novel EM absorber comprises a pattern-impregnated honeycomb structure with the absorbent of carbon black (CB), a resistive frequency selective surface (FSS) embedded in polypropylene (PP) foam, and a mixture coating covered on a metal plane whose absorbent is flaky carbonyl iron (FCI). CB particles and FCI particles provide electrical and magnetic losses at the microscopic scale. Moreover, the subwavelength-scale artificial electromagnetic structure achieves broadband matching to free space by adjusting macroscopic geometric parameters. The consistency between measurements and simulations proves that the composite absorber achieves over 90% absorption ratio with a thickness of 8.3 mm from 2.06 to 18 GHz (corresponding to a fractional bandwidth of 158.9%). Additionally, this composite absorber possesses good mechanical property with a maximum uniaxial tensile stress of 67.43 MPa.

In Situ Dispersion of Lignin in Polypropylene via Supercritical CO2 Extrusion Foaming: Effects of Lignin on Cell Nucleation and Foam Compression Properties.

Supercritical CO2 (scCO2) extrusion foamed high-melt-strength (HMS) polypropylene (PP) often suffers from low cell density, large cell sizes, and poor cell structure uniformity due to the poor nucleation rates of CO2 in the PP. To remedy this, various inorganic fillers have been used as heterogeneous nucleation agents. Although their efficient nucleation effects have been demonstrated, the preparation of these fillers causes some adverse effects on the environment/human health or involves relatively expensive processes or non-eco-friendly chemicals. In this work, biomass-based lignin is studied as a sustainable, lightweight, and cost-effective nucleating agent. It is found that scCO2 could assist in situ dispersion of lignin in the PP in the foaming process, leading to significantly increased cell density, smaller cells, and improved cell uniformity. The Expansion Ratio is also simultaneously improved due to reduced diffusive gas loss. The PP/lignin foams with low lignin loadings exhibit higher compression moduli and plateau strengths than the PP foams with the same densities owing to the improved cell uniformity and probably also the reinforcing effect of the small lignin particles in cell walls. Moreover, the energy absorption capability of the PP/lignin foam with 1 wt% lignin could match the PP foam with similar compression plateau strengths; even the density of the former is 28% lower than the latter. Therefore, this work provides a promising approach to a cleaner and more sustainable production of HMS PP foams.

Significantly Improved Foamability and Mechanical Properties of Polypropylene through a Core–Shell Structure via Low Gas Pressure Foam Injection Molding

Lightweight plays an important role in energy conservation and emission reduction. Foam injection molding (FIM) is a promising technology for the mass preparation of structural parts. But it is still a challenge to prepare foam with good cellular structure via low gas pressure foam injection molding (LGP-FIM). Polypropylene (PP) as a kind of universal plastic exhibits good mechanical properties, but its mechanical properties are notably deteriorated after foaming. In this study, ethylene-propylene-diene monomer (EPDM) rubber and high-density polyethylene (HDPE) were added to PP to improve its melt strength and increase the gas diffusion rate. As a result, the cell size of the ternary blend attained was ∼2 μm, and the cell density was increased to 1010 cells/cm3. Meanwhile, the tensile toughness of ternary blend foam increased by 668% compared to PP foam. The notched impact strength of ternary blend foam increased by 178% compared to PP foam. Considering the excellent performance of PP foam and the low cost of LGP-FIM, this work presents a method for fabricating high mechanical performance foams that could be easily industrialized.

In Situ Nanofibrillar Polypropylene-Based Composite Microcellular Foams with Enhanced Mechanical and Flame-Retardant Performances

With the increasing demand for plastic components, the development of lightweight, high strength and functionalized polypropylene (PP) from a cost-effective and environmentally friendly process is critical for resource conservation. In situ fibrillation (INF) and supercritical CO2 (scCO2) foaming technology were combined in this work to fabricate PP foams. Polyethylene terephthalate (PET) and poly(diaryloxyphosphazene)(PDPP) particles were applied to fabricate in situ fibrillated PP/PET/PDPP composite foams with enhanced mechanical properties and favorable flame-retardant performance. The existence of PET nanofibrils with a diameter of 270 nm were uniformly dispersed in PP matrix and served multiple roles by tuning melt viscoelasticity for improving microcellular foaming behavior, enhancing crystallization of PP matrix and contributing to improving the uniformity of PDPP's dispersion in INF composite. Compared to pure PP foam, PP/PET(F)/PDPP foam exhibited refined cellular structures, thus the cell size of PP/PET(F)/PDPP foam was decreased from 69 to 23 μm, and the cell density increased from 5.4 × 106 to 1.8 × 108 cells/cm3. Furthermore, PP/PET(F)/PDPP foam showed remarkable mechanical properties, including a 975% increase in compressive stress, which was attributed to the physical entangled PET nanofibrils and refined cellular structure. Moreover, the presence of PET nanofibrils also improved the intrinsic flame-retardant nature of PDPP. The synergistical effect of the PET nanofibrillar network and low loading of PDPP additives inhibited the combustion process. These gathered advantages of PP/PET(F)/PDPP foam make it promising for lightweight, strong, and fire-retardant polymeric foams.

The Cellular Structure and Toughness of Hydrogenated Styrene-Butadiene Block Copolymer Reinforced Polypropylene Foams

Polypropylene nanocomposites containing varying amounts of Styrene-ethylene-butadiene-styrene block copolymer (SEBS) were prepared through the supercritical nitrogen microcellular injection-molding process. Maleic anhydride (MAH)-grafted polypropylene (PP-g-MAH) copolymers were used as compatibilizers. The influence of SEBS content on the cell structure and toughness of the SEBS/PP composites was investigated. Upon the addition of SEBS, the differential scanning calorimeter tests revealed that the grain size of the composites decreased, and their toughness increased. The results of the rheological behavior tests showed that the melt viscosity of the composite increased, playing a role in enhancing the cell structure. With the addition of 20 wt% SEBS, the cell diameter decreased from 157 to 66.7 μm, leading to an improvement in the mechanical properties. Compared to pure PP material, the impact toughness of the composites rose by 410% with 20 wt% of SEBS. Microstructure images of the impact section displayed evident plastic deformation, effectively absorbing energy and improving the material's toughness. Furthermore, the composites exhibited a significant increase in toughness in the tensile test, with the foamed material's elongation at break being 960% higher than that of pure PP foamed material when the SEBS content was 20%.