Cascade (Dithio)carbonate Ring Opening Reactions for Self-Blowing Polyhydroxythiourethane Foams.

Polyurethane (PU) foams are very common materials that have found many applications over the years. Their use is constantly improving due to their unique physical properties and easy blowing which does not require the addition of a blowing agent. Greener routes have been explored in the recent years to replace isocyanates. One of the most promising routes is leading to polyhydroxyurethanes (PHU). However, with PHUs, external blowing agent are usually required to obtain a foam. Thus, the work focuses on PHU foam synthesis using in situ reaction to produce NIPU foam. Hence, the aminolysis of thiocyclic carbonate triggers Pearson reaction between released thiols and cyclic carbonates which serves as a chemical blowing agent.

Phenolic Foam Preparation Using Hydrofluoroolefin Blowing Agents and the Toughening Effect of Polyethylene Glycol.

In this work, a new class of fourth-generation, zero ozone depletion potential, hydrofluoroolefin-based blowing agents were used to prepare phenolic foam. While hydrofluoroolefin blowing agents have been used previously to prepare polyurethane foams, few studies have been reported on their use in phenolic foams. We introduce an effective method for foam preparation using two low-boiling blowing agents, cis-1,1,1,4,4,4-hexafluoro-2-butene and trans-1,1,1,4,4,4-hexafluoro-2-butene, and their combinations with hexane. Traditionally, phenolic foams have been prepared using chlorofluorocarbons and hydrochlorofluorocarbons, which can have harmful effects on the environment due to their high ozone depletion potential or global warming potential. Conductor-like screening model for real solvents (COSMO-RS) modeling studies were performed to understand the effects of different blowing agent combinations on their boiling points. A series of phenolic foams were prepared by varying the concentration of the hydrofluoroolefin and the hydrofluoroolefin-hexane blowing agent combinations. The concentrations of the surfactant, Agnique CSO 30, and the toughening agent, polyethylene glycol, were also varied to yield a formulation with the optimal properties. The foams formulated with the hydrofluoroolefin-hexane mixture displayed a higher compressive strength and a lower thermal conductivity than those prepared with either hydrofluoroolefin or hexane alone. The cell microstructure of all the foams was examined using scanning electron microscopy. By introducing flexible chains into the resin matrix, PEG facilitates proper distribution of hydrofluoroolefin-hexane blowing agents and other reagents and thereby increases the mechanical strength of the foam.

Liquid foaming of TPU with Methylal.

This work investigates the peculiarities of using a liquid blowing agent, namely dimethoxymethane (Methylal) to foam a thermoplastic polyurethane (TPU) in the laboratory practice of batch foaming equipment. We preliminarily measured thermodynamic properties of the polymer/gas system relevant to foaming, namely the vapor-liquid pressures at the TPU foaming temperatures. Three different paths were then explored for foaming. First, we used Methylal under its liquid-vapor equilibrium condition, in which both liquid and vapor are present. Secondly, we used Methylal in the liquid state to experiment with liquid foaming strategies. We have observed specific aspects, details, and issues related to the use of liquid blowing agents and devised strategies to deal with them. Finally, we used Methylal as a co-blowing agent together with CO2. In all cases, we examined the impact of pressure, pressure drop rate, and temperature on foam density and morphology. Overall, liquid foaming has proven to be a viable technique and Methylal an effective blowing agent, especially in cooperation with other gaseous blowing agents, where it significantly improves the expansion ratio of the final product.

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.

Cryogenic Insulation-Towards Environmentally Friendly Polyurethane Foams.

Cryogenics is the science and technology of very low temperatures, typically below 120 K. The most common applications are liquified natural gas carriers, ground-based tanks, and propellant tanks for space launchers. A crucial aspect of cryogenic technology is effective insulation to minimise boil-off from storage tanks and prevent frost build-up. Rigid closed-cell foams are prominent in various applications, including cryogenic insulation, due to their balance between thermal and mechanical properties. Polyurethane (PU) foam is widely used for internal insulation in cryogenic tanks, providing durability under thermal shocks and operational loads. External insulation, used in liquified natural gas carriers and ground-based tanks, generally demands less compressive strength and can utilise lower-density foams. The evolution of cryogenic insulation materials has seen the incorporation of environmentally friendly blowing agents and bio-based polyols to enhance sustainability. Fourth-generation physical blowing agents, such as HFO-1233zd(E) and HFO-1336mzz(Z), offer low global warming potential and improved thermal conductivity. Additionally, bio-based polyols from renewable resources like different natural oils and recycled polyethylene terephthalate (PET) are being integrated into rigid PU foams, showing promising properties for cryogenic applications. Research continues to optimise these materials for better mechanical performance and environmental impact.

High performance plant-derived thermoplastic polyester elastomer foams achieved by manipulating charging order of mixed blowing agents.

Plant-derived thermoplastic polyester elastomer (TPEE) is an environment friendly polymer known for its exceptional tear strength and mechanical properties, whose monomers are generated from crops. To prepare high-performance TPEE foams is still challenging due to the intrinsic shrinkage behavior. Herein, two microcellular foaming routes with different charging orders of mixed blowing agents, namely "CO2 firstly charging process (CO2-F-process)" and "N2 firstly charging process (N2-F-process)", were developed to elucidate the effects of mixed blowing agents on foaming behavior. Compared with the case in N2-F-process, more carbon dioxide and less nitrogen were adsorbed in CO2-F-process. Thus, TPEE foams prepared by N2-F-process show less shrinkage and higher creep recovery ratio than those prepared by CO2-F-process. Thanks to better structural stability and smaller shrinkage, TPEE foams prepared by N2-F-process exhibited enhanced strength and resilience. For the foams with similar density, compression strength can be increased by 52 %, and energy loss coefficient can be reduced to 50 %, by using N2-F-process. Thus, not only biomass TPEE foams with enhanced mechanical performance shows promising prospects in those areas that needs lightweight, insulation and high resilience, but also novel microcellular foaming technique with mixed blowing agents opens a new way for developing high-performance polymeric foams.

Influence of Reactive Amine-Based Catalysts on Cryogenic Properties of Rigid Polyurethane Foams for Space and On-Ground Applications.

Rigid polyurethane (PUR) foams have outstanding properties, and some of them are successfully used even today as cryogenic insulation. The fourth-generation blowing agent Solstice® LBA and commercial polyols were used for the production of a low-density cryogenic PUR foam composition. A lab-scale pouring method for PUR foam preparation and up-scaling of the processes using an industrial spraying machine are described in this article. For the determination of the foam properties at cryogenic temperature, original methods, devices, and appliances were used. The properties at room and cryogenic temperatures of the developed PUR foams using a low-toxicity, bismuth-based, and low-emission amine catalyst were compared with a reference foam with a conventional tin-based additive amine catalyst. It was found that the values of important cryogenic characteristics such as adhesion strength after cryoshock and the safety coefficient of the PUR foams formed with new reactive-type amine-based catalysts and with the blowing agent Solstice® LBA were higher than those of the foam with conventional catalysts.

Enhanced Low-Density Silicone Foams Blown by Water-Hydroxyl Blends.

Water, alcohols, diols, and glycerol are low-cost blowing agents that can be used to create the desired silicone foam structures. Although their combined use can be beneficial, it remains unclear how it affects the physical properties of the resulting materials. We conducted a comparative study of these hydroxyl-bearing blowing agents in fumed silica- and mica-filled polymer composite systems for simultaneous blowing and crosslinking to obtain a low-density, uniform porosity and superior mechanical properties. The foams were optimized for a uniform open-pore structure with densities ranging from 75 to 150 kg‧m-3. Varying the diol chain length (Cn) from one to seven carbons can alter the foam density and structure, thereby enhancing the foam tensile strength while maintaining a low density. Replacing 10 mol% of water with 1,4-butanediol decreased the density by 26%, while increasing the specific strength by 5%. By combining glycerol and water blowing, the resulting foams exhibited a 30% lower apparent density than their water-blown analogs. The results further showed that Cn > 4 alkane chain diols had an odd-even effect on the apparent density and cell wall thickness. All foamable compositions had viscosities of approximately 7000 cSt and curing times below 2 min, allowing for quick dispensing and sufficient time for the foam to cure in semi-industrial volumes.

Water-free synthesis of polyurethane foams using highly reactive diisocyanates derived from 5-hydroxymethylfurfural.

This paper reports on the synthesis of a new highly reactive diisocyanate monomer based on hydroxymethylfurfural. It further describes its catalyst-free conversion to linear-chain thermoplastic polyurethanes as well as to cross-linked polyurethane foams. In addition, a novel strategy for the synthesis of polyurethane foams without the necessity of using water is developed. Nitrogen is utilized herein as blowing agent which is formed during Curtius rearrangement of a new furan based carboxylic azide into its corresponding diisocyanate.

Development of a Rheology Die and Flow Characterization of Gas-Containing Polymer Melts.

We present a novel measurement die for characterizing the flow behavior of gas-containing polymer melts. The die is mounted directly on the injection-molding cylinder in place of the mold cavity and thus enables near-process measurement of viscosity (i.e., under the conditions that would be present were a mold attached). This integration also resolves the issue of keeping gas-containing polymer melts under pressure during measurement to prevent desorption. After thermal characterization of the die, various correction approaches were compared against each other to identify the most suitable one for our case. We conducted measurements using polypropylene in combination with two different chemical blowing agents. Increasing the blowing-agent content to up to 6% revealed an interestingly low influence of gases on melt viscosity, which was confirmed by elongational viscosity measurements. For verification, we compared our results to corresponding measurements taken on a high-pressure capillary rheometer and found that they were in excellent agreement. Our die cannot only be used for rheological characterization. Combined with ultrasound sensors, it provides an innovative way of measuring the volumetric flow rate. This development represents an important step in improving the sustainability of gas-containing polymer processing.

Porous PLAs with Controllable Density by FDM 3D Printing and Chemical Foaming Agent.

This paper shows how fused decomposition modeling (FDM), as a three-dimensional (3D) printing technology, can engineer lightweight porous foams with controllable density. The tactic is based on the 3D printing of Poly Lactic Acid filaments with a chemical blowing agent, as well as experiments to explore how FDM parameters can control material density. Foam porosity is investigated in terms of fabrication parameters such as printing temperature and flow rate, which affect the size of bubbles produced during the layer-by-layer fabrication process. It is experimentally shown that printing temperature and flow rate have significant effects on the bubbles' size, micro-scale material connections, stiffness and strength. An analytical equation is introduced to accurately simulate the experimental results on flow rate, density, and mechanical properties in terms of printing temperature. Due to the absence of a similar concept, mathematical model and results in the specialized literature, this paper is likely to advance the state-of-the-art lightweight foams with controllable porosity and density fabricated by FDM 3D printing technology.

Improved Cell Morphology and Surface Roughness in High-Temperature Foam Injection Molding Using a Long-Chain Branched Polypropylene.

Long-chain branched polypropylene (LCB PP) has been used extensively to improve cell morphologies in foaming applications. However, most research focuses on low melt flow rate (MFR) resins, whereas foam production methods such as mold-opening foam injection molding (MO-FIM) require high-MFR resins to improve processability. A systematic study was conducted comparing a conventional linear PP, a broad molecular weight distribution (BMWD) linear PP, and a newly developed BMWD LCB PP for use in MO-FIM. The effects of foaming temperature and molecular architecture on cell morphology, surface roughness, and mechanical properties were studied by utilizing two chemical blowing agents (CBAs) with different activation temperatures and varying packing times. At the highest foaming temperatures, BMWD LCB PP foams exhibited 887% higher cell density, 46% smaller cell sizes, and more uniform cell structures than BWMD linear PP. Linear PP was found to have a surface roughness 23% higher on average than other resins. The BMWD LCB PP was found to have increased flexural modulus (44%) at the cost of decreased toughness (-88%) compared to linear PP. The branched architecture and high molecular weight of the BMWD LCB PP contributed to improved foam morphologies and surface quality in high-temperature MO-FIM conditions.