Experimental and theoretical fracture toughness investigation of PUR foams under mixed mode I+III loading

The polyurethanes (PU) are a very versatile family of materials mainly obtained by combinations of polyols and polyisocyanates. Based on their annual worldwide production of around 20 million tons and a global market of $50 billion (2016), PUs rank 6th among all polymers. Through their value chain, PUs involve different players: (i) the chemists producing most of PUs raw materials, (ii) the PUs producers from the raw materials, (iii) the compounders/assemblers who formulate PUs into their final products, and finally (iv) the end-users. Due to the multiplicity of their structures, PUs can be used in various forms and applications. Cellular materials are the largest part of this market (more than 60 %) with segments including the furniture, automotive, bedding, insulation, building or construction markets. Two main types of foam can be fabricated: (i) flexible with open cells, stress and tensile properties, e.g., furniture or bedding, and (ii) rigid with closed cells, low thermal conductivity, low density and high dimensional stability mainly for thermal insulation, e.g., building industries. The formulation step significantly influences the microstructure or morphology of these cellular materials and impacts the final foam properties. Even if some partially biobased compounds (polyols) can be used, commercial PU cellular materials are till now mainly based on fossil resources. However, future materials will combine high performance with low environmental impact in order to fulfill societal expectations. In this way, new biobased compounds combining different fields such as biotech, chemistry, science and materials engineering are more and more used in complex formulations for renewable foams, leading to specific renewable macromolecular architectures. This review aims to highlight the main biobased components (polyols, polyisocyanates and additives) used in formulations for PU foams, in relation to the corresponding fabrications, morphologies and properties. The main renewable sources come from (mono and poly)sugars, oleo-chemistry, polyphenols (lignins, tannins …), or different compounds from white biotech processes from agro-wastes … The impact of these different components on material performances is discussed more particularly for rigid polyurethane foams. The structure-property relationships are analyzed with a scope on cellular morphology, mechanical, thermal properties, fire resistance, and insulation behavior. Finally, an analysis focus on future perspectives on biobased PU foams is conducted.

Structure-properties relationships of cellular materials from biobased polyurethane foams

Failure analysis and modeling of foam sandwich laminates under impact loading

A literature review and own investigations have been a base for presenting various methods of the synthesis of bio-polyols from renewable raw materials as well as their potential applications in the formulations of flexible and rigid polyurethane foams (Table 1). A characteristics of selected bio-polyols (Table 2) and their effect on foaming process of different polyurethane compositions are shown (Fig. 1, Table 3). Examples of the influence of selected bio-polyols on cell structures and physical and mechanical properties of both flexible (Fig. 2) and rigid (Table 4) foams are also presented.

Porous polyurethane plastics synthetized using bio-polyols from renewable raw materials

A series of flame-retardant rigid polyurethane foams (RPUFs) containing nonreactive phosphonate (5-ethyl-2-methyl-1,3,2-dioxaphosphorinan-5-yl) methyl dimethyl phosphonate P-oxide (EMD) and expandable graphite (EG) were prepared by water blown. The flame-retardant properties and mechanism of EMD/EG on RPUFs were systematically investigated. The EMD/EG system effectively increased the limiting oxygen index (LOI) value and decreased the values of total heat release (THR), av-effective heat of combustion (EHC), pk-heat release rate (HRR), total smoke release (TSR) of RPUFs. The impact values of LOI, THR, and av-EHC resulted by EMD/EG system are nearly equal to the sum of the impact values by EMD and EG individually in RPUFs, which implies the addition flame-retardant effect from EMD and EG. EMD alone exerted excellent gas-phase flame-retardant effect by releasing PO fragments with quenching effect. The firm residue produced by EMD combined well with the loose and worm-like expanded graphite from EG further to form compact and expanded char layer, which brought excellent barrier effect and filtration effect to matrix. That's why pk-HRR and TSR values of RPUF reduced. Depending on the simultaneous actions of EMD/EG system in gas phase and condensed phase during combustion, the flame-retardant effects from nonreactive phosphonate and EG on RPUFs were added together. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45960.

Addition flame-retardant effect of nonreactive phosphonate and expandable graphite in rigid polyurethane foams

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

In the present study, it could be shown that by integration of a piezoceramic transducer in a fiber-reinforced door side panel, a flat loudspeaker can be realized. Taking into account the given restrictions, the integration position has been identified, where the geometry decouples the vibrating membrane from the supporting surface. With the help of an acoustic finite-element simulation, the main design variables of the integration position were found and the relevant effects for sound radiation were made visible. The manufacturing of the test specimen with piezoceramic transducers was performed using vacuum-assisted resin infusion and the long fiber injection procedure. The effect on the real sound radiation behavior of the door side panel with a material-immanent loudspeaker was experimentally determined using laser scanning vibrometry and sound pressure measurements. The presented work shows, for the first time, the high potential of acoustic functionalization of lightweight structures during the manufacturing process for the realization of lightweight and space-saving loudspeakers in a production-ready process.

Structure-Integrated Loudspeaker Using Fiber-Reinforced Plastics and Piezoelectric Transducers—Design, Manufacturing and Validation

Since different X-ray tomographic techniques offer advantageous characteristics, they frequently complete or even replace standard methods based on acoustic emission, thermography and ultrasonic. Especially the detection and evaluation of voids and cellular structures can be performed with a resolution of up to 1 μm and 3D visualization. Within this paper, a novel in situ CT device is used to perform compression tests on closed-cell polymeric foams with a defined foam density. Next to the determination of the cell structure with consideration of the pore size distribution and their morphology, the elastic compression behaviour is investigated. Following the analysis of the foam structure, the implementation of an implicit finite element model is developed based on computer tomographic scans. By the introduction of a phase field function to implicitly describe the foam structure, technical difficulties associated with meshing the complex structure can be avoided and the equations to describe the deformation behaviour are effectively solved on high performance computers. Beyond that, the results gained by the in situ CT device help to adapt and complete the currently available elastic deformation models and therefore offer the opportunity to predict the material behaviour for various load scenarios of polymeric foams.

Deformation Analysis of Polymer Foams under Compression Load using in situ computed Tomography and Finite Element Simulation Methods

Over the last decade, carbon fibre-reinforced composites (CFRP) are increasingly used as lightweight material for various industrial applications. Due to the anisotropic material structure and its corresponding properties, novel design concepts and processing technologies were developed to further harness the material inherent lightweight potential. However, the material degradation in long-term use and failure behaviour is still considered a challenging issue for material scientists and engineers in particular. Therefore, concepts for structural health monitoring and their suitable implementation is still a major research topic. Among others, one solution uses the conductivity of carbon fibre yarns and their suitability to act as in-situ strain sensors. In the present work, the measurement principle bases on the usage of the piezo-resistive effect, meaning that every mechanical strain of the roving filaments causes a correlative change of the measurable electrical resistance. Since, these sensory elements need shielding from their surrounding carbon filaments of the composite structure, suitable fibre-based dielectric jackets have been developed with a wide range of suitable materials and textile processing technologies. In this contribution, the influence of the integrated carbon fibre sensors on the resulting mechanical performance of the composite structure is evaluated using an analysis of variances approach. Beyond that, the local composite morphology is analysed to evaluate the composite microstructure.

Influence of Carbon Roving Strain Sensory Elements on the Mechanical Properties of Carbon Fibre-Reinforced Composites

In contrast to conventional manufacturing methods for sandwich structures, in this paper a foamable polyurethane system is used to impregnate textile layers and to create the sandwich foam core in one single process step. Given this simultaneous process, a high adaptability of the resulting mechanical properties can be achieved by varying the textile reinforcement of the sandwich top layers, the foam core density and the sandwich thickness, respectively. Within this paper, a design of experiment approach is implemented by using statistical analysis of variance (ANOVA) to investigate the influence of selected geometrical and material parameters on the resulting bending properties of these novel sandwich structures. A partial factorial design helps to identify the sensitivity of individual parameters and their interaction, as well as the correlation between sandwich morphology and bending performance. Based on the gained results, statistical models will help to identify parameters with statistical significance. Therefore, optimized sandwich structures according to stress-related requirements and geometrical constraints can be designed.

Oliver Weißenborn

Papers

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

Structure-Integrated Loudspeaker Using Fiber-Reinforced Plastics and Piezoelectric Transducers—Design, Manufacturing and Validation

Deformation Analysis of Polymer Foams under Compression Load using in situ computed Tomography and Finite Element Simulation Methods

Influence of Carbon Roving Strain Sensory Elements on the Mechanical Properties of Carbon Fibre-Reinforced Composites

Modelling the Bending Behaviour of Novel Fibre-Reinforced Sandwich Structures with Polyurethane Foam Core