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Structure, Properties and Applications of Polymeric Foams

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Advanced Composites".

Deadline for manuscript submissions: closed (31 December 2020) | Viewed by 36613

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Dr. Aleksander Hejna

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Department of Polymer Technology, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland
Interests: polymer composites; reactive processing; lignocellulose fillers; filler modification; recycling; waste management; polyurethane foams
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Special Issue Information

Dear Colleagues,

The journal Materials is preparing a Special Issue entitled “Structure, Properties and Applications of Polymeric Foams”.

Foaming of polymeric materials enables weight reduction, very important from the economic point of view, but most of all, it should be considered as an excellent means to provide new properties to various polymeric materials. Permanent developments in foaming technologies allow manufacturing of foams with micro- and even nano-sized pores, expanding the already very wide range of their applications. Except for the most conventional applications, which include damping materials, thermal and acoustic insulation, packaging materials or absorbents, applications in catalysis, fuel cells, tissue engineering, and electromagnetic shielding, often associated with nanocellular structures, have become more and more popular.

Moreover, having in mind the ongoing trends and law regulations, it is important to remember the environmental impact of polymeric foams. Recent technological developments often involve biodegradability of foams, application of environmentally friendly raw materials, and innovative recycling methods.

Because of the richness of potential innovations and future developments, the Editors are pleased to launch this Special Issue and invite researchers to contribute their original research papers and reviews on the structure, performance, and applications of polymeric foams.

Topics include:

Innovations in physical and chemical foaming methods;
Thermoplastic and thermosetting polymeric foams and composites;
Polymeric foams based on renewable raw materials;
Structure–property relationships of polymeric foams;
Insulation properties of polymeric foams;
Novel applications of polymeric foams;
Reduction of foams’ environmental impact;
Biodegradable and sustainable polymeric foams;
Recycling of polymeric foams.

Dr. Aleksander Hejna
Guest Editor

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Keywords

Polymeric foams
Cellular polymers
Polymer composites
Structure–property relationships
Cellular structure
Foaming technologies
Foaming agents
Insulation properties
Recycling
Biodegradable polymers

Published Papers (11 papers)

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Editorial

Jump to: Research, Review

2 pages, 168 KiB

Open AccessEditorial

Special Issue: Structure, Properties and Applications of Polymeric Foams

by Aleksander Hejna

Materials 2021, 14(6), 1474; https://doi.org/10.3390/ma14061474 - 17 Mar 2021

Viewed by 1178

Abstract

The Special Issue “Structure, Properties and Applications of Polymeric Foams” aimed to gather the numerous reports associated with the different aspects of polymeric foams [...] Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

Research

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14 pages, 2011 KiB

Open AccessArticle

Large Deformation Finite Element Analyses for 3D X-ray CT Scanned Microscopic Structures of Polyurethane Foams

by Makoto Iizuka, Ryohei Goto, Petros Siegkas, Benjamin Simpson and Neil Mansfield

Materials 2021, 14(4), 949; https://doi.org/10.3390/ma14040949 - 17 Feb 2021

Cited by 5 | Viewed by 2594

Abstract

Polyurethane foams have unique properties that make them suitable for a wide range of applications, including cushioning and seat pads. The foam mechanical properties largely depend on both the parent material and foam cell microstructure. Uniaxial loading experiments, X-ray tomography and finite element analysis can be used to investigate the relationship between the macroscopic mechanical properties and microscopic foam structure. Polyurethane foam specimens were scanned using X-ray computed tomography. The scanned geometries were converted to three-dimensional (3D) CAD models using open source, and commercially available CAD software tools. The models were meshed and used to simulate the compression tests using the implicit finite element method. The calculated uniaxial compression tests were in good agreement with experimental results for strains up to 30%. The presented method would be effective in investigating the effect of polymer foam geometrical features in macroscopic mechanical properties, and guide manufacturing methods for specific applications. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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17 pages, 6511 KiB

Open AccessFeature PaperArticle

Impact of Different Epoxidation Approaches of Tall Oil Fatty Acids on Rigid Polyurethane Foam Thermal Insulation

by Arnis Abolins, Ralfs Pomilovskis, Edgars Vanags, Inese Mierina, Slawomir Michalowski, Anda Fridrihsone and Mikelis Kirpluks

Materials 2021, 14(4), 894; https://doi.org/10.3390/ma14040894 - 13 Feb 2021

Cited by 18 | Viewed by 2847

Abstract

A second-generation bio-based feedstock—tall oil fatty acids—was epoxidised via two pathways. Oxirane rings were introduced into the fatty acid carbon backbone using a heterogeneous epoxidation catalyst-ion exchange resin Amberlite IR-120 H or enzyme catalyst Candida antarctica lipase B under the trade name Novozym^® 435. High functionality bio-polyols were synthesised from the obtained epoxidated tall oil fatty acids by oxirane ring-opening and subsequent esterification reactions with different polyfunctional alcohols: trimethylolpropane and triethanolamine. The synthesised epoxidised tall oil fatty acids (ETOFA) were studied by proton nuclear magnetic resonance. The chemical structure of obtained polyols was studied by Fourier-transform infrared spectroscopy and size exclusion chromatography. Average molecular weight and polydispersity of polyols were determined from size exclusion chromatography data. The obtained polyols were used to develop rigid polyurethane (PU) foam thermal insulation material with an approximate density of 40 kg/m³. Thermal conductivity, apparent density and compression strength of the rigid PU foams were determined. The rigid PU foams obtained from polyols synthesised using Novozym^® 435 catalyst had superior properties in comparison to rigid PU foams obtained from polyols synthesised using Amberlite IR-120 H. The developed rigid PU foams had an excellent thermal conductivity of 21.2–25.9 mW/(m·K). Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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18 pages, 9224 KiB

Open AccessArticle

The Impact of Ground Tire Rubber Oxidation with H₂O₂ and KMnO₄ on the Structure and Performance of Flexible Polyurethane/Ground Tire Rubber Composite Foams

by Aleksander Hejna, Adam Olszewski, Łukasz Zedler, Paulina Kosmela and Krzysztof Formela

Materials 2021, 14(3), 499; https://doi.org/10.3390/ma14030499 - 21 Jan 2021

Cited by 22 | Viewed by 2590

Abstract

The use of waste tires is a very critical issue, considering their environmental and economic implications. One of the simplest and the least harmful methods is conversion of tires into ground tire rubber (GTR), which can be introduced into different polymer matrices as a filler. However, these applications often require proper modifications to provide compatibility with the polymer matrix. In this study, we examined the impact of GTR oxidation with hydrogen peroxide and potassium permanganate on the processing and properties of flexible polyurethane/GTR composite foams. Applied treatments caused oxidation and introduction of hydroxyl groups onto the surface of rubber particles, expressed by the broad range of their hydroxyl numbers. It resulted in noticeable differences in the processing of the polyurethane system and affected the structure of flexible composite foams. Treatment with H₂O₂ resulted in a 31% rise of apparent density, while the catalytic activity of potassium ions enhanced foaming of system decreased density by 25% and increased the open cell content. Better mechanical performance was noted for H₂O₂ modifications (even by 100% higher normalized compressive strength), because of the voids in cell walls and incompletely developed structure during polymerization, accelerated by KMnO₄ treatment. This paper shows that modification of ground tire rubber is a very promising approach, and when properly performed may be applied to engineer the structure and performance of polyurethane composite foams. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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15 pages, 26149 KiB

Open AccessEditor’s ChoiceArticle

Static Mechanical Properties of Expanded Polypropylene Crushable Foam

by Przemysław Rumianek, Tomasz Dobosz, Radosław Nowak, Piotr Dziewit and Andrzej Aromiński

Materials 2021, 14(2), 249; https://doi.org/10.3390/ma14020249 - 06 Jan 2021

Cited by 14 | Viewed by 4906

Abstract

Closed-cell expanded polypropylene (EPP) foam is commonly used in car bumpers for the purpose of absorbing energy impacts. Characterization of the foam’s mechanical properties at varying strain rates is essential for selecting the proper material used as a protective structure in dynamic loading application. The aim of the study was to investigate the influence of loading strain rate, material density, and microstructure on compressive strength and energy absorption capacity for closed-cell polymeric foams. We performed quasi-static compressive strength tests with strain rates in the range of 0.2 to 25 mm/s, using a hydraulically controlled material testing system (MTS) for different foam densities in the range 20 g/dm³ to 220 g/dm³. The above tests were carried out as numerical simulation using ABAQUS software. The verification of the properties was carried out on the basis of experimental tests and simulations performed using the finite element method. The method of modelling the structure of the tested sample has an impact on the stress values. Experimental tests were performed for various loads and at various initial temperatures of the tested sample. We found that increasing both the strain rate of loading and foam density raised the compressive strength and energy absorption capacity. Increasing the ambient and tested sample temperature caused a decrease in compressive strength and energy absorption capacity. For the same foam density, differences in foam microstructures were causing differences in strength and energy absorption capacity when testing at the same loading strain rate. To sum up, tuning the microstructure of foams could be used to acquire desired global materials properties. Precise material description extends the possibility of using EPP foams in various applications. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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26 pages, 7784 KiB

Open AccessArticle

Change in Conductive–Radiative Heat Transfer Mechanism Forced by Graphite Microfiller in Expanded Polystyrene Thermal Insulation—Experimental and Simulated Investigations

by Aurelia Blazejczyk, Cezariusz Jastrzebski and Michał Wierzbicki

Materials 2020, 13(11), 2626; https://doi.org/10.3390/ma13112626 - 09 Jun 2020

Cited by 8 | Viewed by 3358

Abstract

This article introduces an innovative approach to the investigation of the conductive–radiative heat transfer mechanism in expanded polystyrene (EPS) thermal insulation at negligible convection. Closed-cell EPS foam (bulk density 14–17 kg·m⁻³) in the form of panels (of thickness 0.02–0.18 m) was tested with 1–15 µm graphite microparticles (GMP) at two different industrial concentrations (up to 4.3% of the EPS mass). A heat flow meter (HFM) was found to be precise enough to observe all thermal effects under study: the dependence of the total thermal conductivity on thickness, density, and GMP content, as well as the thermal resistance relative gain. An alternative explanation of the total thermal conductivity “thickness effect” is proposed. The conductive–radiative components of the total thermal conductivity were separated, by comparing measured (with and without Al-foil) and simulated (i.e., calculated based on data reported in the literature) results. This helps to elucidate why a small addition of GMP (below 4.3%) forces such an evident drop in total thermal conductivity, down to 0.03 W·m⁻¹·K⁻¹. As proposed, a physical cause is related to the change in mechanism of the heat transfer by conduction and radiation. The main accomplishment is discovering that the change forced by GMP in the polymer matrix thermal conduction may dominate the radiation change. Hence, the matrix conduction component change is considered to be the major cause of the observed drop in total thermal conductivity of EPS insulation. At the microscopic level of the molecules or chains (e.g., in polymers), significant differences observed in the intensity of Raman spectra and in the glass transition temperature increase on differential scanning calorimetry(DSC) thermograms, when comparing EPS foam with and without GMP, complementarily support the above statement. An additional practical achievement is finding the maximum thickness at which one may reduce the “grey” EPS insulating layer, with respect to “dotted” EPS at a required level of thermal resistance. In the case of the thickest (0.30 m) panels for a passive building, above 18% of thickness reduction is found to be possible. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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14 pages, 2771 KiB

Open AccessArticle

A Design of Experiment Approach for Surface Roughness Comparisons of Foam Injection-Moulding Methods

by Gethin Llewelyn, Andrew Rees, Christian Griffiths and Martin Jacobi

Materials 2020, 13(10), 2358; https://doi.org/10.3390/ma13102358 - 20 May 2020

Cited by 14 | Viewed by 2607

Abstract

The pursuit of polymer parts produced through foam injection moulding (FIM) that have a comparable surface roughness to conventionally processed components are of major relevance to expand the application of FIM. Within this study, 22% talc-filled copolymer polypropylene (PP) parts were produced through FIM using both a physical and chemical blowing agent. A design of experiments (DoE) was performed whereby the processing parameters of mould temperatures, injection speeds, back-pressure, melt temperature and holding time were varied to determine their effect on surface roughness, Young’s modulus and tensile strength. The results showed that mechanical performance can be improved when processing with higher mould temperatures and longer holding times. Also, it was observed that when utilising chemical foaming agents (CBA) at low-pressure, surface roughness comparable to that obtained from conventionally processed components can be achieved. This research demonstrates the potential of FIM to expand to applications whereby weight saving can be achieved without introducing surface defects, which has previously been witnessed within FIM. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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16 pages, 5042 KiB

Open AccessArticle

Evaluation of the Zero Shear Viscosity, the D-Content and Processing Conditions as Foam Relevant Parameters for Autoclave Foaming of Standard Polylactide (PLA)

by Tobias Standau, Huan Long, Svenja Murillo Castellón, Christian Brütting, Christian Bonten and Volker Altstädt

Materials 2020, 13(6), 1371; https://doi.org/10.3390/ma13061371 - 18 Mar 2020

Cited by 21 | Viewed by 3333

Abstract

In this comprehensive study, the influence of (i) material specific properties (e.g., molecular weight, zero shear viscosity, D-content) and (ii) process parameters (e.g., saturation temperature, -time, -pressure, and pressure drop rate) on the expansion behavior during the autoclave foaming process were investigated on linear Polylactide (PLA) grades, to identify and evaluate the foam relevant parameters. Its poor rheological behavior is often stated as a drawback of PLA, that limits its foamability. Therefore, nine PLA grades with different melt strength and zero shear viscosity were systematically chosen to identify whether these are the main factors governing the foam expansion and whether there is a critical value for these rheological parameters to be exceeded, to achieve low density foams with fine cells. With pressure drop induced batch foaming experiments, it could be shown that all of the investigated PLA grades could be foamed without the often used chemical modifications, although with different degrees of expansion. Interestingly, PLAs foaming behavior is rather complex and can be influenced by many other factors due to its special nature. A low molecular weight combined with a high ability to crystallize only lead to intermediate density reduction. In contrast, a higher molecular weight (i.e., increased zero shear viscosity) leads to significant increased expandability independent from the D-content. However, the D-content plays a crucial role in terms of foaming temperature and crystallization. Furthermore, the applied process parameters govern foam expansion, cell size and crystallization. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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22 pages, 18939 KiB

Open AccessArticle

Study on the Structure-Property Dependences of Rigid PUR-PIR Foams Obtained from Marine Biomass-Based Biopolyol

by Paulina Kosmela, Aleksander Hejna, Jan Suchorzewski, Łukasz Piszczyk and Józef Tadeusz Haponiuk

Materials 2020, 13(5), 1257; https://doi.org/10.3390/ma13051257 - 10 Mar 2020

Cited by 25 | Viewed by 3158

Abstract

The paper describes the preparation and characterization of rigid polyurethane-polyisocyanurate (PUR-PIR) foams obtained with biopolyol synthesized in the process of liquefaction of biomass from the Baltic Sea. The obtained foams differed in the content of biopolyol in polyol mixture (0–30 wt%) and the isocyanate index (I_ISO = 200, 250, and 300). The prepared foams were characterized in terms of processing parameters (processing times, synthesis temperature), physical (sol fraction content, apparent density) and chemical structure (Fourier transform infrared spectroscopy), microstructure (computer microtomography), as well as mechanical (compressive strength, dynamic mechanical analysis), and thermal properties (thermogravimetric analysis, thermal conductivity coefficient). The influence of biopolyol and I_ISO content on the above properties was determined. The addition of up to 30 wt% of biopolyol increased the reactivity of the polyol mixture, and the obtained foams showed enhanced mechanical, thermal, and insulating properties compared to foams prepared solely with petrochemical polyol. The addition of up to 30 wt% of biopolyol did not significantly affect the chemical structure and average cell size. With the increase in I_ISO, a slight decrease in processing times and mechanical properties was observed. As expected, foams with higher I_ISO exhibited a higher relative concentration of polyisocyanurate groups in their chemical structure, which was confirmed using principal component analysis (PCA). Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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15 pages, 4142 KiB

Open AccessArticle

Enhancement of the Antibacterial Activity of Natural Rubber Latex Foam by Blending It with Chitin

by Nanxi Zhang and Hui Cao

Materials 2020, 13(5), 1039; https://doi.org/10.3390/ma13051039 - 26 Feb 2020

Cited by 21 | Viewed by 3357

Abstract

To enhance the antibacterial activity of natural rubber latex foam (NRLF), chitin was added during the foaming process in amounts of 1–5 phr (per hundred rubber) to prepare an environmentally friendly antibacterial NRLF composite. In this research, NRLF was synthesized by the Dunlop method. The swelling, density, hardness, tensile strength, elongation at break, compressive strength and antibacterial activity of the NRLFs were characterized. FTIR and microscopy were used to evaluate the chemical composition and microstructure of the NRLFs. The mechanical properties and antibacterial activity of the NRLF composites were tested and compared with those of pure NRLF. The antibacterial activity was observed by the inhibition zone against E. coli. NRLF composite samples were embedded in a medium before solidification. The experimental results of the inhibition zone indicated that with increasing chitin content, the antibacterial activity of the NRLF composites increased. When the chitin content reached 5 phr, the NRLF composite formed a large and clear inhibition zone in the culture dish. Moreover, the NRLF–5 phr chitin composite improved the antibacterial activity to 281.3% of that of pure NRLF against E. coli. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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Review

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16 pages, 3180 KiB

Open AccessReview

Recent Progress in Processing Functionally Graded Polymer Foams

by Supitta Suethao, Darshil U. Shah and Wirasak Smitthipong

Materials 2020, 13(18), 4060; https://doi.org/10.3390/ma13184060 - 13 Sep 2020

Cited by 36 | Viewed by 4978

Abstract

Polymer foams are an important class of engineering material that are finding diverse applications, including as structural parts in automotive industry, insulation in construction, core materials for sandwich composites, and cushioning in mattresses. The vast majority of these manufactured foams are homogeneous with respect to porosity and structural properties. In contrast, while cellular materials are also ubiquitous in nature, nature mostly fabricates heterogeneous foams, e.g., cellulosic plant stems like bamboo, or a human femur bone. Foams with such engineered porosity distribution (graded density structure) have useful property gradients and are referred to as functionally graded foams. Functionally graded polymer foams are one of the key emerging innovations in polymer foam technology. They allow enhancement in properties such as energy absorption, more efficient use of material, and better design for specific applications, such as helmets and tissue restorative scaffolds. Here, following an overview of key processing parameters for polymer foams, we explore recent developments in processing functionally graded polymer foams and their emerging structures and properties. Processes can be as simple as utilizing different surface materials from which the foam forms, to as complex as using microfluidics. We also highlight principal challenges that need addressing in future research, the key one being development of viable generic processes that allow (complete) control and tailoring of porosity distribution on an application-by-application basis. Full article

(This article belongs to the Special Issue Structure, Properties and Applications of Polymeric Foams)

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