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17 pages, 5759 KiB

Open AccessArticle

The Effects of a Crosslinking Agent on the Microrheological Properties and Cellular Structure of Silicone Rubber Foam Prepared via a Green Process

by Hongyu He, Lulu Li, Hong Liu, Bin Luo, Zhipeng Li and Wenhuai Tian

Materials 2024, 17(3), 707; https://doi.org/10.3390/ma17030707 - 01 Feb 2024

Viewed by 559

Abstract

Chemical foaming technology is widely used in the preparation of silicone rubber foam and is attributable to its one-step molding capability and eco-friendly production processes. The microrheological properties of silicone rubber play a pivotal role during the foaming process. In this study, Rheolaser [...] Read more.

Chemical foaming technology is widely used in the preparation of silicone rubber foam and is attributable to its one-step molding capability and eco-friendly production processes. The microrheological properties of silicone rubber play a pivotal role during the foaming process. In this study, Rheolaser Lab (Formulaction, Toulouse, France) was used to conduct in situ examinations for the influence of a crosslinking agent on the microrheological properties of silicone rubber foam for the first time. This study monitors the entire reaction process of silicone rubber foam from liquid to solid, as well as the matching of crosslinking and foaming reactions. Various parameters, including solid–liquid balance, elasticity index, and macroscopic viscosity index, are measured to analyze the microrheological properties of silicone rubber foam. The results show that the silicone rubber foam exhibits good microrheological properties, thereby demonstrating excellent performance at a crosslinking agent content of 2%. Through adjusting the experimental conditions, a sustainable and efficient approach was proposed for better cellular structure control in the industrial preparation of silicone rubber foam. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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13 pages, 8894 KiB

Open AccessEditor’s ChoiceArticle

Biocomposite Foams with Multimodal Cellular Structures Based on Cork Granulates and Microwave Processed Egg White Proteins

by Giorgio Luciano, Adriano Vignali, Maurizio Vignolo, Roberto Utzeri, Fabio Bertini and Salvatore Iannace

Materials 2023, 16(8), 3063; https://doi.org/10.3390/ma16083063 - 13 Apr 2023

Cited by 2 | Viewed by 1296

Abstract

In an effort to reduce greenhouse gas emission, reduce the consumption of natural resources, and increase the sustainability of biocomposite foams, the present study focuses on the recycling of cork processing waste for the production of lightweight, non-structural, fireproof thermal and acoustic insulating [...] Read more.

In an effort to reduce greenhouse gas emission, reduce the consumption of natural resources, and increase the sustainability of biocomposite foams, the present study focuses on the recycling of cork processing waste for the production of lightweight, non-structural, fireproof thermal and acoustic insulating panels. Egg white proteins (EWP) were used as a matrix model to introduce an open cell structure via a simple and energy-efficient microwave foaming process. Samples with different compositions (ratio of EWP and cork) and additives (eggshells and inorganic intumescent fillers) were prepared with the aim of correlating composition, cellular structures, flame resistance, and mechanical properties. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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23 pages, 18763 KiB

Open AccessArticle

Novel Expandable Epoxy Beads and Epoxy Particle Foam

by Du Ngoc Uy Lan, Christian Brütting, Christian Bethke, Johannes Meuchelböck, Tobias Standau, Volker Altstädt and Holger Ruckdäschel

Materials 2022, 15(12), 4205; https://doi.org/10.3390/ma15124205 - 14 Jun 2022

Cited by 6 | Viewed by 2811

Abstract

Expanded polymeric beads offer the advantage of being able to produce parts with complex geometries through a consolidation process. However, established polymeric beads are made of thermoplastics, deform and melt beyond their temperature services. In this manuscript, a new technique is proposed to [...] Read more.

Expanded polymeric beads offer the advantage of being able to produce parts with complex geometries through a consolidation process. However, established polymeric beads are made of thermoplastics, deform and melt beyond their temperature services. In this manuscript, a new technique is proposed to fabricate expandable epoxy beads (EEBs), then expand and fuse them to produce epoxy particle foams (EPFs). This technique is called solid-state carbamate foaming technique. For production of EEBs, a mixture of epoxy, carbamate and hardener is prepared and poured into a 10 mL syringe. The mixture is manually extruded into 60 °C water to obtain a cylindric shape. The extrudate is then further cured to obtain an epoxy oligomer behaving rheological tan delta 3 and 2 at 60 °C. The extrudate is cut into pellets to obtain EEBs. The EEBs are then loaded into an aluminum mold and placed in an oven at 160 °C to expand, fuse to obtain EPFs of 212 kg/m³ and 258 kg/m³. The obtained EPFs provide a T_g of 150–154 °C. The fusion boundaries in EPFs are well formed. Thus, the produced EPFs exhibit a compressive modulus of 50–70 MPa, with a torsion storage modulus at 30 °C of 34–56 MPa. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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10 pages, 4909 KiB

Open AccessArticle

Liquid Silicone Rubber Foamed with Thermoplastic Expandable Microspheres

by Svenja Marl, Ralf-Urs Giesen and Hans-Peter Heim

Materials 2022, 15(11), 3779; https://doi.org/10.3390/ma15113779 - 25 May 2022

Cited by 8 | Viewed by 2184

Abstract

To reduce the material costs as well as the density of Liquid Silicone Rubber (LSR), LSR foams can be produced in an injection molding process. Expandable thermoplastic microspheres can be used as blowing agents. This publication deals with the analysis of the cell [...] Read more.

To reduce the material costs as well as the density of Liquid Silicone Rubber (LSR), LSR foams can be produced in an injection molding process. Expandable thermoplastic microspheres can be used as blowing agents. This publication deals with the analysis of the cell structure of these LSR foams. For this purpose, cylindrical disks are injection molded and examined for their cell structure as a function of different proportions of microspheres using a scanning electron spectroscope. In addition, the density of the samples is determined. It was found that a very homogeneous cell structure is produced in this process, that heat transport has a significant influence on the expansion of the microspheres, and that the formation of a filler network limits the expansion at higher proportions of blowing agent. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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15 pages, 3823 KiB

Open AccessArticle

Effect of Cell Morphology on Flexural Behavior of Injection-Molded Microcellular Polycarbonate

by Kübra Güzel, Jan-Christoph Zarges and Hans-Peter Heim

Materials 2022, 15(10), 3634; https://doi.org/10.3390/ma15103634 - 19 May 2022

Cited by 3 | Viewed by 1311

Abstract

The quantitative study of the structure and properties relationship in cellular materials is mostly limited to cell diameter, cell density, skin layer thickness, and cell size distribution. In addition, the investigation of the morphology is generally carried out in two dimensions. Therefore, the [...] Read more.

The quantitative study of the structure and properties relationship in cellular materials is mostly limited to cell diameter, cell density, skin layer thickness, and cell size distribution. In addition, the investigation of the morphology is generally carried out in two dimensions. Therefore, the interrelation between morphological properties and mechanical characteristics of the foam structure has remained in an uncertain state. In this study, during the physical foaming process, a foam morphology is locally created by using a mold equipped with a core-back insert. The variation in morphology is obtained by modifying the mold temperature, injection flow rate, and blowing agent content in the polymer melt. X-ray microtomography (μCT) is used to acquire the 3D visualization of the cells structure. The Cell Distribution Index (CDI) is calculated to represent the polydispersity in cell size distribution. The relationship between the wide range of morphological qualities and relevant flexural properties is made explicit via a statistical model. According to the results, the morphology, particularly cell shape, characterizes the mechanism of the linear elastic deformation of the closed-cell foams. IR-thermography reveals the bending failure of cellular structures in the tensile region despite the differences in cell diameter. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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11 pages, 5418 KiB

Open AccessArticle

Scale-Up and Testing of Polyurethane Bio-Foams as Potential Cryogenic Insulation Materials

by Maria Kurańska, Ugis Cabulis, Aleksander Prociak, Krzysztof Polaczek, Katarzyna Uram and Mikelis Kirpluks

Materials 2022, 15(10), 3469; https://doi.org/10.3390/ma15103469 - 12 May 2022

Cited by 3 | Viewed by 1541

Abstract

This article compares the properties of closed-cell PUR bio-foams produced on a laboratory scale and on an industrial scale. In the formulation used, the polyol premix contained 40 wt.% of a bio-polyol based on rapeseed oil. Selected useful properties of the foams obtained [...] Read more.

This article compares the properties of closed-cell PUR bio-foams produced on a laboratory scale and on an industrial scale. In the formulation used, the polyol premix contained 40 wt.% of a bio-polyol based on rapeseed oil. Selected useful properties of the foams obtained on the two scales and the use of one-step and spraying methods were compared. In the case of the spraying method, the experimental system was compared to a commercial one. Given the possibility of applying the bio-foams in insulation systems for cryogenic and liquefied natural gas (LNG) applications, a compressive strength analysis of the foams was carried out at room temperature as well as at −196 °C. It was found that the foams modified with the bio-polyol were characterized by a higher compressive strength at low temperatures than commercial foams based on a petrochemical polyol. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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18 pages, 7926 KiB

Open AccessArticle

Role of Air Bubble Inclusion on Polyurethane Reaction Kinetics

by Cosimo Brondi, Mercedes Santiago-Calvo, Ernesto Di Maio and Miguel Ángel Rodríguez-Perez

Materials 2022, 15(9), 3135; https://doi.org/10.3390/ma15093135 - 26 Apr 2022

Cited by 5 | Viewed by 2705

Abstract

In this study, we investigated the influence of mixing conditions on the foaming process of water blown polyurethane (PU) foams obtained at different mixing speeds (50, 500, 1000 and 2000 rpm). In particular, the morphological evolution during the foaming process, in terms of [...] Read more.

In this study, we investigated the influence of mixing conditions on the foaming process of water blown polyurethane (PU) foams obtained at different mixing speeds (50, 500, 1000 and 2000 rpm). In particular, the morphological evolution during the foaming process, in terms of the bubble size and bubble density, was studied via optical observations, while the effects on the reaction kinetics were monitored using in situ FTIR spectroscopy. At the slow mixing speed (50 rpm), no air bubbles were included and the early foaming process was characterized by the formation of new bubbles (CO₂ nucleation), provided by the blowing reaction. Later on, it was observed that the coalescence affected the overall foaming process, caused by the gelling reaction, which was inhibited by the indigent mixing conditions and could not withstand the bubbles expansion. As a result, a PU foam with a coarse cellular structure and an average bubble size of 173 µm was obtained. In this case, the bubbles degeneration rate, dN/dt, was −3095 bubble·cm⁻³·s⁻¹. On the contrary, at 500 rpm, air bubbles were included into the PU reaction system (aeration) and no formation of new bubbles was observed during the foaming process. After this, the air bubbles underwent growth caused by diffusion of the CO₂ provided by the blowing reaction. As the gelling reaction was not strongly depleted as in the case at 50 rpm, the coalescence less affected the bubble growth (dN/dt = −2654 bubble·cm⁻³·s⁻¹), leading to a PU foam with an average bubble size of 94 µm. For the foams obtained at 1000 and 2000 rpm, the bubble degeneration was first affected by coalescence and then by Ostwald ripening, and a finer cellular structure was observed (with average bubble sizes of 62 µm and 63 µm for 1000 rpm and 2000 rpm, respectively). During the first foaming stage, the coalescence was less predominant in the bubble growth (with dN/dt values of −1838 bubble·cm⁻³·s⁻¹ and −1601 bubble·cm⁻³·s⁻¹, respectively) compared to 50 rpm and 500 rpm. This occurrence was ascribed to the more balanced process between the bubble expansion and the PU polymerization caused by the more suitable mixing conditions. During the late foaming stage, the Ostwald ripening was only responsible for the further bubble degeneration (with dN/dt values of −89 bubble·cm⁻³·s⁻¹ and −69 bubble·cm⁻³·s⁻¹, respectively). Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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13 pages, 5794 KiB

Open AccessEditor’s ChoiceArticle

Acoustic Characteristics of Microcellular Foamed Ceramic Urethane

by Jin Hong and Sung Woon Cha

Materials 2022, 15(6), 2007; https://doi.org/10.3390/ma15062007 - 08 Mar 2022

Cited by 2 | Viewed by 1943

Abstract

Noise pollution critically degrades the quality of human life, and its effects are becoming more severe due to rapid population growth and the development of industry and transportation. Acoustic wave aggregation in the 30–8000 Hz band can have a negative impact on human [...] Read more.

Noise pollution critically degrades the quality of human life, and its effects are becoming more severe due to rapid population growth and the development of industry and transportation. Acoustic wave aggregation in the 30–8000 Hz band can have a negative impact on human health, especially following continuous exposure to low-frequency noise. This study investigates the acoustic performance of microcellular foams made of a mixture of brittle and soft materials and their potential use as absorption materials. It is common to use porous materials to improve acoustic properties. Specimens prepared by mixing ceramic and urethane were made into microcellular foamed ceramic urethane by a batch process using carbon dioxide. The specimens were expected to exhibit characteristics of porous sound-absorbing materials. After measuring the acoustic characteristics using an impedance tube, a significant sound-absorption coefficient at a specific frequency was noted, a characteristic of a resonance-type sound-absorbing material. However, the sound-absorption properties were generally worse than those before foaming. Differences based on the size, shape, and structure of the pores were also noted. It will be necessary to check the effects of cellular morphological differences on the absorption properties by controlling the variables of the microcellular foaming process in a future study. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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Review

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24 pages, 25313 KiB

Open AccessReview

Surface Flame-Retardant Systems of Rigid Polyurethane Foams: An Overview

by Yuping Jiang, Hongyu Yang, Xiang Lin, Simeng Xiang, Xiaming Feng and Chaojun Wan

Materials 2023, 16(7), 2728; https://doi.org/10.3390/ma16072728 - 29 Mar 2023

Cited by 4 | Viewed by 2155

Abstract

Rigid polyurethane foam (RPUF) is one of the best thermal insulation materials available, but its flammability makes it a potential fire hazard. Due to its porous nature, the large specific surface area is the key factor for easy ignition and rapid fires spread [...] Read more.

Rigid polyurethane foam (RPUF) is one of the best thermal insulation materials available, but its flammability makes it a potential fire hazard. Due to its porous nature, the large specific surface area is the key factor for easy ignition and rapid fires spread when exposed to heat sources. The burning process of RPUF mainly takes place on the surface. Therefore, if a flame-retardant coating can be formed on the surface of RPUF, it can effectively reduce or stop the flame propagation on the surface of RPUF, further improving the fire safety. Compared with the bulk flame retardant of RPUF, the flame-retardant coating on its surface has a higher efficiency in improving fire safety. This paper aims to review the preparations, properties, and working mechanisms of RPUF surface flame-retardant systems. Flame-retardant coatings are divided into non-intumescent flame-retardant coatings (NIFRCs) and intumescent flame-retardant coatings (IFRCs), depending on whether the flame-retardant coating expands when heated. After discussion, the development trends for surface flame-retardant systems are considered to be high-performance, biological, biomimetic, multifunctional flame-retardant coatings. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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24 pages, 4351 KiB

Open AccessEditor’s ChoiceReview

Developing Insulating Polymeric Foams: Strategies and Research Needs from a Circular Economy Perspective

by Lucia Doyle, Ingo Weidlich and Ernesto Di Maio

Materials 2022, 15(18), 6212; https://doi.org/10.3390/ma15186212 - 07 Sep 2022

Cited by 12 | Viewed by 2866

Abstract

Insulating polymeric foams have an important role to play in increasing energy efficiency and therefore contributing to combating climate change. Their development in recent years has been driven towards the reduction of thermal conductivity and achievement of the required mechanical properties as main [...] Read more.

Insulating polymeric foams have an important role to play in increasing energy efficiency and therefore contributing to combating climate change. Their development in recent years has been driven towards the reduction of thermal conductivity and achievement of the required mechanical properties as main targets towards sustainability. This perception of sustainability has overseen the choice of raw materials, which are often toxic, or has placed research efforts on optimizing one constituent while the other necessary reactants remain hazardous. The transition to the circular economy requires a holistic understanding of sustainability and a shift in design methodology and the resulting research focus. This paper identifies research needs and possible strategies for polymeric foam development compatible with Circular Product Design and Green Engineering, based on an extensive literature review. Identified research needs include material characterization of a broader spectrum of polymer melt–gas solutions, ageing behavior, tailoring of the polymer chains, detailed understanding and modeling of the effects of shear on cell nucleation, and the upscaling of processing tools allowing for high and defined pressure drop rates. Full article

(This article belongs to the Special Issue Polymer Foams: Materials, Processing and Properties)

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