Rigid Polymer Foams Research Articles

Wood-based 3D printing: potential and limitation to 3D print building elements with cellulose & lignin

Under urgent sustainability targets, the building industry craves for renewable and recyclable biomaterials as cellulose is a fiber; Lignin is a plant-derived low-cost polymer with remarkable properties, yet its valorization is in its infancy. Recent studies have shown potentials to combine cellulose and lignin into a renewable bio-based material for the built environment, with the use of additive manufacturing to allow geometric customization and local control of material. However, previous studies also highlighted crucial issues to be solved. One main challenge is the lack of knowledge on combinations of lignin and cellulose with different binders to achieve a paste suitable for 3D printing, leading to a material applicable in the built environment. To contribute overcoming the challenge, this research aimed to explore various combinations of cellulose, lignin, and binders and to study the extrudability of the resulting paste using a clay extruder installed on a robotic arm. Several combinations were explored, evaluated, and compared. The four recipes with the highest scores were used to produce samples for tensile and three-point bending tests, water absorption and retention tests, and microscope analysis. The overall outcome has shown similarities between the mechanical properties of the mixture developed using methylcellulose as the binding agent and rigid polymer foams, such as the ones commonly used as insulation panels. Moreover, the material mix with the highest score in the preliminary assessment was further applied to fabricate samples with varied geometries to assess its potential and limitations combined with the fabrication process. Finally, two demonstrators were produced to explore the printing process for different geometric configurations: conceptual window frame and structural node were designed, and 3D printed as proof of concept.

A Review of Rigid Polymeric Cellular Foams and Their Greener Tannin-Based Alternatives.

This review focuses on the description of the main processes and materials used for the formulation of rigid polymer foams. Polyurethanes and their derivatives, as well as phenolic systems, are described, and their main components, foaming routes, end of life, and recycling are considered. Due to environmental concerns and the need to find bio-based alternatives for these products, special attention is given to a recent class of polymeric foams: tannin-based foams. In addition to their formulation and foaming procedures, their main structural, thermal, mechanical, and fire resistance properties are described in detail, with emphasis on their advanced applications and recycling routes. These systems have been shown to possess very interesting properties that allow them to be considered as potential substitutes for non-renewable rigid polymeric cellular foams.

A Model of Moist Polymer Foam and a Scheme for the Calculation of Its Thermal Conductivity

This paper proposes a method for determining an effective value of the thermal conductivity for moist, highly porous rigid polymer foams. The model of moist foam based on an ordered structure with interpenetrating components was developed in accordance with the moisture distribution in the pore space. With small moisture content, isolated water inclusions are formed, and the pore space is considered as a binary system (vapor-gas mixture and water) with isolated inclusions. With an increase in moisture content, isolated water inclusions merge, forming a continuous layer, and pore space is considered as a binary system of interpenetrating components. The thermal conductivity of the vapor-gas mixture is represented as the sum of the thermal conductivity of the dry gas and the thermal conductivity of the vapor caused by the diffusion transfer of vapor in the pore space, taking into account the coefficient of vapor diffusion resistance. Using the proposed scheme of calculation, a computational experiment was performed to establish the influence of the vapor diffusion, moisture content, and average temperature of the foam on its thermal conductivity.

Statistical Characteristics of Polymer Grouting Material Microstructure

The two‐component foaming polyurethane is a kind of grouting material that has recently been widely used in engineering structural repair. Its physical and mechanical properties are closely related to its microstructure. Therefore, the qualitative and quantitative analysis of polymer microstructure has always been a research hotspot. The statistical characteristics of polymer grouting material microstructure are investigated by six groups of specimens with different densities, and the density varies from 0.1 to 0.6 g/cm3. The microstructure morphology of polymer was observed and described by scanning electron microscopy (SEM), and the microstructure feature parameters were extracted and calculated by image processing technology. The quantitative analysis of section cell roundness distribution, cell diameter distribution, and polymer porosity shows that low‐density polymer materials have anisotropy. While the density exceeds 0.3 g/cm3, the cell structure tends to be spherical. The section cell diameters obey a normal distribution, and when the density increases, the cell diameter decreases. The porosity of the polymer has a linear negative correlation with the density. The polymer matrix has a density of 1.21 g/cm3. The microstructural information obtained in this study will help establish a cell‐based model to explain the mechanical response of rigid polymer foams.

Improved compression properties of polypropylene extrusion foams by supramolecular additives

Owing to the high lightweight design potential polymer foams become increasingly important. For rigid polymer foams, requiring high dimensional stability under load, a high compression modulus is a key feature. Here, we demonstrate how supramolecular additives can be utilized to control the foam morphology and to significantly improve the compression behavior of extruded foams made of linear isotactic polypropylene. Three different 1,3,5-benzenetrisamides were selected as supramolecular additives. These additives are soluble in the polymer melt and form a supramolecular nanofiber network upon cooling, acting as nucleating sites for both, foam cells and polymer crystals. It is shown that the in situ formed nanofiber network is very effective in reducing the cell size of extruded foams. Depending on the molecular structure and the concentration of the supramolecular additives, the compression modulus of polypropylene-polymer foams can be significantly increased compared to a reference foam with talc. Unexpectedly, an improvement of 100% with a concentration of only 0.02 wt% of a supramolecular additive compared to the neat polypropylene foam featuring similar densities is achieved. This improvement cannot be correlated with the foam morphology and is most likely attributed to the presence of the supramolecular nanofiber network.

Effect of specimen size, compressibility and inertia on the response of rigid polymer foams subjected to high velocity direct impact loading

The influences of specimen length-to-diameter ratio, material compressibility, and inertia on direct impact response of high density closed-cell polymeric foam are investigated. High speed photography and stereovision digital image correlation are conducted to measure the full-field deformation response of the material subjected to direct impact. Inertia stress developed in the specimen is calculated from the acceleration distribution obtained from full-field measurements. Total axial stress magnitude along the axis of the specimen is then reconstructed from inertia and boundary-measured stresses. It is clearly shown that there is an appreciable degree of spatial variability in strains, strain rates and stresses developed in the impacted foam specimens, whereas the degree of such axial variability is more significant at higher length-to-diameter ratios. The study is further extended to take advantage of such spatial variability to identify the rate sensitivity of the examined material over a wide range of strain rates from 1000 s−1 to 5000 s−1. The approach proposed here is shown to facilitate the identification of viscoplastic constitutive response of low impedance materials using a minimum number of experiments.

Modelling of the strain rate dependent deformation behaviour of rigid polyurethane foams

Rigid polymer foams exhibit excellent mechanical properties for the use in impact and crash components of lightweight structures, such as core layer of sandwich-structures. The design of these lightweight structures strongly depends on secured knowledge of the stress-strain response at various strain rates. For the purpose of predicting the strain rate dependent behaviour under compression loading, an analytical model based on a Burgers model was developed and validated with comprehensive experimental tests of a rigid closed-cell polyurethane foam with different densities (230 kg/m3, 420 kg/m3 and 610 kg/m3) using a servo-hydraulic high speed testing machine at defined strain rates, ranging from 0.0004 s−1 to 8 s−1.

Estimation of the elastic constants of highly porous cellular plastics reinforced with fibres embedded in foam struts

In order to enhance the mechanical properties of polymer foams, fillers of different materials and sizes are being applied. Fibrous fillers shorter than the characteristic foam cell dimensions have the potential of efficient reinforcement, due to their high aspect ratio, without detrimentally interfering with foam structure. A theoretical model is developed for evaluation of the effect of filler on foam stiffness. The elastic response of rigid polymer foams filled with short fibres, of length commensurable with that of foam struts, is modelled by using the orientational averaging technique. Explicit expressions for components of the stiffness tensor of composite foams are derived in terms of elasticity characteristics of the polymer and reinforcing fibres, fibre content, foam density, and parameters of the orientational distribution of foam struts. The model is applied to describing the foam stiffness as a function of fibre loading of polyurethane foams filled with milled carbon fibres.

Preparation and Properties of Rigid Polyurethane-Imide Foams

A series of rigid polyurethane foams are synthesized via the reaction of isocyanate terminated polyimide prepolymers with polyether polyol. Deionized water and n-pentane are used for blowing agents. The prepolymers and polymers are characterized by conventional methods, and physical, mechanical and thermal properties are studied. The results show that in comparison to pure polyurethane foams, these rigid polymer foams exhibit improved thermal stability as well as good compressive property. SEM of the compressed body of rigid polyurethane-imide foams show that the destructive forms are open-type tear of the film and the breakdown of the cell body wall.

Modeling the Ultraviolet Photodegradation of Rigid Polyurethane Foams

Before the Montreal Protocol of 1987 and the subsequent phasing-out of chlorofluorocarbons (CFCs) in industrial applications, rigid polymer foams were made using these compounds as secondary blowing agents. The CFCs remain trapped in the gaseous part of the cellular foam structure, and once discarded these foams constitute a significant reservoir for the environmental release of ozone-depleting chemicals. Environmental degradation of the foam accelerates this process. Of particular interest in this work is the degradative effect of ultraviolet (UV) light on rigid polyurethane foams. Foams were subjected to accelerated weathering conditions and then viewed with a scanning electron microscope. The thin cell membranes near the foam surface degrade when exposed to UV light, leaving only a network of polymer struts that offers negligible resistance to the escape of CFCs or any other gases contained within. This effect has been reproduced qualitatively through simulated weathering of a computer-generated foam s...

Fundamentals of Membrane Transport

Mass transport through thermally activated molecular dispersal (diffusion) in multicomponent mixtures comprising membrane systems can exhibit complexities which require different levels of physical and theoretical descriptions of which we give examples. At the simplest level mean field level component flows are derived from gradients of chemical potential. We will focus primarily in our talk on the mean field description in homogeneous or inhomogeneous polymer systems. Penetrant diffusion in homogeneous “rubbery” polymer films or membranes can be relatively successfully described by a simple “free volume” theory. Inhomogeneities due to e.g. polymer crystallinity are a further complication and are an essential aspect in diffusion in very porous systems such as rigid polymer foams. Further complications arise in mechanically relaxing polymer films with memory which are not in mechanical equilibrium. Such effects as well as penetrant stress driven diffusion and dual mode sorption can arise in polymer penetrant systems below their glass transition temperature.