A phenomenological constitutive model for the viscoelastic deformation of elastomers

This study proposes a one-dimensional constitutive model for elastomeric materials based on recent observations regarding the separation of elastic and viscous contributions in uniaxial cyclic tensile experiments on EPDM rubber. The focus is on capturing the changes in constitutive behaviour and energy dissipation associated with the Mullins effect. In the model, this is achieved through the evolution of both permanent set and hyperelastic parameters of an Edwards-Vilgis function to account for the Mullins effect, and with a viscosity associated with the effective stretch rate of the network to describe the non-linear flow stress. The simulations are able to reproduce the observed constitutive response and its change with increasing levels of pre-deformation. The model is less able to accurately reproduce the virgin loading response, which is achieved via extrapolation to zero pre-strain. However, for practical purposes, where scragging of elastomeric products is the norm, the model is able to predict the cyclic response and the dissipated energy, and their change with different scragging levels in good agreement with experimental data

Towards Exceptional Icephobicity with Chionophile-inspired Durable Biomimetic Coatings

Liquid-infused polymeric surfaces have demonstrated promising icephobicity. However, the capability to maintain the icephobic performance after material damage has been a challenge, both in terms of conserving a smoother surface and the replenishment of infused liquid. Cetacean skin possesses a microscopically smooth texture in the form of cells lubricated with lipid proteins and consists of structural fibres that ensure durability. Concerning the structure of cetacean skin, glycerol infused fibre-reinforced polyurethane coatings (GIFRP) were proposed. Instead of hosting the lipid proteins, the coatings were infused with glycerol, a known cryoprotectant to induce the supercooling of water, a strategy inspired by wood frogs and red flat dark beetles to prevent freezing. The inclusion of glycerol delayed water droplet freezing duration by 659%, while negligible frost accumulated on the fabricated coatings during anti-icing tests. The reinforcement of fibres was effective and the surface damage was reduced by a factor of 4, compared to the pure polyurethane coatings under erosion impact. The incorporation of fibres has proven to be beneficial for infused-liquid replenishment and the slow-releasing capabilities of GIFRP coatings. Minimised surface deterioration and the continued presence of glycerol on GIFRP coatings demonstrated a small increase in ice adhesion from 0.22 kPa to 0.77 kPa after the erosion tests, one of the lowest values reported in the literature after substantial surface damage. The concept inspired by cetacean skin and the cryoprotective features of chionophiles was instrumental in keeping the ice adhesion under 1 kPa after erosion impact

Challenges to the industrial melt-processing of conductive plastics

In this work, we investigate the relationship between the timescales available for polymer mobility during processing and post-processing and the electrical resistivity of melt-processed thermoplastics filled with carbon nanoparticles. Post-process annealing below the glass transition temperature was one avenue explored to uplift electrical conductivity. Detailed analysis of available literature on thermoplastics filled with either graphite nanoplatelets or carbon nanotubes, and of relevant processing data suggests that the required timescale for shaping process or post-processing to obtain conductive material needs to be sufficiently longer than that of the base polymer characteristic relaxation time τd. Four factors have been identified that promote the formation of a conductive filler network in thermoplastics: filler loading content, polymer molar mass, processing temperature and processing timescales

Exploiting Generative Design for 3D Printing of Bacterial Biofilm Resistant Composite Devices

open access articleAs the understanding of disease grows, so does the opportunity for personalization of therapies targeted to the needs of the individual. To bring about a step change in the personalization of medical devices it is shown that multi-material inkjet-based 3D printing can meet this demand by combining functional materials, voxelated manufacturing, and algorithmic design. In this paper composite structures designed with both controlled deformation and reduced biofilm formation are manufactured using two formulations that are deposited selectively and separately. The bacterial biofilm coverage of the resulting composites is reduced by up to 75% compared to commonly used silicone rubbers, without the need for incorporating bioactives. Meanwhile, the composites can be tuned to meet user defined mechanical performance with ±10% deviation. Device manufacture is coupled to finite element modelling and a genetic algorithm that takes the user-specified mechanical deformation and computes the distribution of materials needed to meet this under given load constraints through a generative design process. Manufactured products are assessed against the mechanical and bacterial cell-instructive specifications and illustrate how multifunctional personalization can be achieved using generative design driven multi-material inkjet based 3D printing

A model for the compressible, viscoelastic behavior of human amnion addressing tissue variability through a single parameter

A viscoelastic, compressible model is proposed to rationalize the recently reported response of human amnion in multiaxial relaxation and creep experiments. The theory includes two viscoelastic contributions responsible for the short- and long-term time- dependent response of the material. These two contributions can be related to physical processes: water flow through the tissue and dissipative characteristics of the collagen fibers, respectively. An accurate agreement of the model with the mean tension and kinematic response of amnion in uniaxial relaxation tests was achieved. By variation of a single linear factor that accounts for the variability among tissue samples, the model provides very sound predictions not only of the uniaxial relaxation but also of the uniaxial creep and strip-biaxial relaxation behavior of individual samples. This suggests that a wide range of viscoelastic behaviors due to patient-specific variations in tissue composition

Reversibility of the Mullins effect for extending the life of rubber components

This paper investigates the recovery of the Mullins effect by heat-treatment of predeformed (300% tensile strain) carbon-black filled ethylene-propylene-diene rubber (EPDM) in an air-circulating oven. The tensile response was measured, and the secant modulus at 100% strain, energy to deform to 300% strain, ultimate tensile strength, strain to failure and permanent deformation were compared to undeformed specimens subjected to the same heat-treatments, 3–24 h at temperatures of 60–80°C. Recovery of properties is dependent on time and temperature of treatment, increasing with longer times and higher temperatures. Recovery of secant modulus follows time-temperature superposition (TTS) with an activation enthalpy of 111 kJ mol−1; TTS is shown to be applicable to the other measures, probably because of additional cure in the rubber resulting from the heat-treatment. Treatments under vacuum led to similar effects as in an air-circulating oven

Large Deformations in Oriented Polymer Glasses: Experimental Study and a New Glass-Melt Constitutive Model in Wiley InterScience (www.interscience.wiley.com)

ABSTRACT: An experimental study was made of the effects of prior molecular orientation on large tensile deformations of polystyrene in the glassy state. A new hybrid glass-melt constitutive model is proposed for describing and understanding the results, achieved by parallel coupling of the ROLIEPOLY molecularly-based melt model with a model previously proposed for polymer glasses. Monodisperse and polydisperse grades of polystyrene are considered. Comparisons between experimental results and simulations illustrate that the model captures characteristic features of both the melt and glassy states. Polystyrene was stretched in the melt state and quenched to below T g , and then tensile tested parallel to the orientation direction near the glass transition. The degree of strain-hardening was observed to increase with increasing prior stretch of molecules within their entanglement tubes, as predicted by the constitutive model. This was explored for varying temperature of stretching, degree of stretching, and dwell time before quenching. The model in its current form, however, lacks awareness of processes of subentanglement chain orientation. Therefore, it underpredicts the orientation-direction strain hardening and yield stress increase, when stretching occurs at the lowest temperatures and shortest times, where it is dominated by subentanglement orientation. INTRODUCTION As demonstrated by a wide range of polymer products, molecular orientation is one of the fundamental parameters that determine the mechanical response of a melt processed, thermoplastic polymer. 1 The degree of orientation present within a particular product is a complex function of its rheology and the process parameters employed. The rheological behavior itself is intrinsically linked to the molecular weight and its distribution, and the chain architecture, as well as the presence of any additives. Consequently, as industrial polymer processing engineers have long known, mechanical properties of practical importance such as Young's modulus, yield stress, and fracture toughness in any given direction are highly sensitive to the grade of polymer employed and the flow history encountered during processing. 2-9 Therefore, it is of great practical interest to understand better the development of frozen-in molecular orientation during melt processing, and the relationships between this orientation and resulting solid-state properties. In particular, there is an engineering need to achieve a predictive capability, for use in optimizing polymer products

Characterizing friction for fiber reinforced composites manufacturing: Method development and effect of process parameters

In automated layup manufacturing processes of fiber-reinforced polymer composites, the quality of the manufactured part is strongly dependent on frictional behavior. Improper control of frictional forces can lead to defect formation. Frictional sliding rheometry tests provide an innovative methodology to accurately characterize the tool-ply friction of unidirectional (UD) prepreg employing unique annular plate geometries. The effect of processing parameters (temperature, velocity, and normal force) on the frictional response of a carbon fiber prepreg was studied. Moreover, utilizing custom designed plate geometries coupled with optically transparent fixtures allowed for in-situ quantification of the prepreg-rigid surface contact area along with simultaneous characterization of the process parameter-dependent frictional mechanisms. Our findings highlight the reduction in frictional forces with increasing temperature, attributed to the increased resin flowability, while increases in sliding rates resulted in a pronounced increase in the frictional forces. The effect of applied load on the frictional characteristics was more complicated due to contributions from both the adhesive and normal forces. Finally, the results were interpreted in light of the contact area measurements performed at different temperatures, normal force, and sliding rate

The acoustic properties of latex foam made from deproteinized natural rubber latex and epoxidized natural rubber latex

Acoustic foam materials are used to control noise pollution in a wide range of applications, including buildings and the transportation industry. This study explores the feasibility of replacing conventional acoustic foam made of synthetic material with natural rubber (NR) latex foam. The acoustic properties of two types of specialty NR latex foams, deproteinized natural rubber (DPNR) latex foam and epoxidized natural rubber (ENR) latex foam, were investigated using an impedance tube according to ISO 10534-2. It was found that the sound transmission loss (STL) performance is directly proportional to the thickness and density of the latex foam samples. A comparison between ENR, DPNR, and LATZ latex foams showed that the ENR foam exhibits the highest STL curve at all thickness and density levels. The sound absorption coefficient (SAC) performance was assessed at low and high frequencies. At low frequencies, the SAC was found to be affected by the density and thickness levels of the materials, whereas at higher frequencies, it was the morphological characteristics that influenced the SAC. The noise reduction coefficient (NRC) was used to evaluate the overall noise absorption performance of the foams. It was found that high-density DPNR latex foam exhibits a higher NRC value than high-density ENR latex foam. Overall, both ENR and DPNR latex foams meet the specifications for acoustic applications in buildings and the transportation industry, making them strong contenders for the next generation of environmentally friendly acoustic foams

Development of Latex Foam Pillows from Deproteinized Natural Rubber Latex

Currently there is significant demand for pillows with improved pressure-relief features made from natural materials, alternatives to petrochemical-based foams. In order to meet the requirements, this study's approach is to develop latex foam pillows from deproteinized natural rubber (DPNR) latex with a unique dual-density cervical-shaped structure. In this work, DPNR latex foam pillows were produced at three different density levels which are high-density, medium-density and low-density. Extractable protein content of latex foam made from DPNR was confirmed lower than latex foam made from low ammonia NR latex (LATZ) and of commercial NR latex foams, making DPNR pillows more hypoallergenic than others. The physical properties of the produced DPNR latex foams were examined in accordance with Malaysian Standard MS679, and were found to comply with all requirements stipulated in standard specifications. A novel dual-density cervical-shaped DPNR latex foam pillow prototype was produced where the pillow has lower density at the upper part and higher density at the lower part. Pressure-mapping was used to visualize the pressure distribution patterns and to measure the average peak pressure when a mannequin head was placed on top of the pillow. The study observed that decreasing the density increases the softness of the DPNR latex foam. Softer latex foams led to larger surface contact area, and hence a reduced average peak pressure value. This cervical-shaped structure further increased the surface contact area between the pillow and mannequin head, and thus reduced further the average peak pressure value