Cellular Polymer Films Research Articles

Optimization of the cellular morphology of biaxially stretched thin polyethylene foams produced by extrusion film blowing

This work presents the production of cellular polymer films using extrusion blowing to impose biaxial stretching on the cellular structure while processing. The materials selected are linear low-density polyethylene (LLDPE) and low density polyethylene (LDPE) as the matrix, azodicarbonamide as the chemical blowing agent, and talc as the nucleating agent. The processing parameters, namely, the temperature profile, screw speed, feed rate, take-up ratio, blow-up ratio, and the matrix composition were all optimized to produce a homogeneous cellular structure with defined morphologies. The optimized films had a thickness below 300 µm, a relative density around 0.6, a cell density above 2 × 106cells/cm3, and biaxially stretched cells with aspect ratios above 4 longitudinally and 3.8 transversally.

Piezoelectric cellular polymer films: Fabrication, properties and applications

Piezoelectricity can be defined as the ability of certain materials to provide mechanical–electrical energy conversion. In addition to traditional ferroelectric polymers (such as polyvinylidene fluoride, PVDF) and ceramics (such as lead zirconate titanate, PZT), charged polymer film foams have also shown important piezoelectric activity. In fact, when cellular polymers are exposed to high electrical fields, positive and negative charges are created on the opposite faces of each cell surface. As a result, charged cellular polymers can exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. The piezoelectric effect of these materials is known to be affected by several parameters such as the cellular structure (cell density, shape and size), relative density and elastic stiffness. However, a careful morphology control is mandatory to optimize the piezoelectric response.<br />Ferroelectrets have recently received a great deal of academic and industrial interest due to their wide range of technological applications associated with high piezoelectric constants, low cost, flexibility and low weight. In this paper, an overview of different piezoelectric cellular polymers is presented with recent developments and applications.

Cellular Polymer Ferroelectret: A Review on Their Development and Their Piezoelectric Properties

ABSTRACTBecause of their ability to show ferroelectret behavior when exposed to an external electric field, cellular polymers have been recently considered for ferroelectret applications. These cellular polymer films can be produced by stretching or foaming, but depending on the application and conditions, different polymers, such as polypropylene (PP), poly(ethylene terephthalate), poly(ethylene naphthalate), poly(tetrafluoro ethylene), cross‐linked PP, and some cyclo‐olefines, have been considered. Nevertheless, cellular PP was the most investigated material because of its outstanding properties such as high piezoelectric d33 coefficient, flexibility, good fatigue resistance, good charge trapping properties, and low cost. In this review, recent advances related to the materials used for ferroelectret applications and their processing are discussed. The effect of different parameters such as pressure, electrical breakdown strength of the gas phase, presence of fillers, and service temperature on the d33 coefficient is presented and discussed.

The effective electromechanical properties of cellular piezoelectret film: finite element modeling

Cellular space-charge polymer film, also called cellular piezoelectret, has very large piezoelectric effect due to their unique microvoid structure. In this article, the cellular piezoelectret film is considered to be a periodic composite material with closed-cell microvoids aligned periodically. Three dimensional finite element modeling is carried out to obtain the effective elastic modulus and piezoelectric coefficients. Sensitivity analysis was presented by modeling the effective electromechanical properties with different individual variable, including material constants and void shape parameters. By assuming a relation between void shape and void volume fraction, the finite element model can simulate quite well the inflation experiments of the voided charged Polypropylene film published in literature. Finally, the finite element model is used to explore the voided charge polymer film with non-uniform distribution of the trapped charges on the internal surface of voids. It was found that the resultant overall piezoelectric coefficients will be more significant if charges are closely gathered in the central area, and sparse in the around area of the internal surface of voids.

Effective electromechanical properties of cellular piezoelectret: A review

Due to the large quasi-piezoelectric d 33 coefficient in the film thickness direction, cellular piezoelectret has emerged as a new kind of compliant electromechanical transducer materials. The macroscopic piezoelectric effect of cellular piezoelectret is closely related to the void microstructures as well as the material constants of host polymer. Complex void microstructures are usually encountered in the optimum design of cellular piezoelectret polymer film with advanced piezoelectric properties. Analysis of the effective electromechanical properties is generally needed. This article presents an overview of the recent progress on theoretical models and numerical simulation for the effective electromechanical properties of cellular piezoelectret. Emphasis is placed on our own works of cellular piezoelectret published in past several years.

Theoretical modeling and experiments on the piezoelectric coefficient in cellular polymer films

AbstractThis article presents an analytical model allowing the determination of the, d33, piezoelectric coefficient in cellular polymer films. The cellular film is identified to an equivalent effective layered structure made of alternating gas and solid layers that are electrically charged at their interfaces. The model expresses the d33 piezoelectric coefficient as a function of porosity, film thickness, free‐stress surface charge density, and the film modulus, which depends on frequency. The temperature is indirectly taken into account as it is involved in the parameters appearing in the final equation of the model. The obtained results showed that the piezoelectric activity of charged cellular polymer films decreases with the film thickness and increases with the film expansion and the surface charge density. The variation with the percentage of porosity shows a nonlinear trend, with an increase in the d33 piezoelectric coefficient with the void fraction, and then a decrease when such a percentage exceeds a thickness‐dependent critical value. Such quantitative description of the piezoelectric activity can help the design of cellular films with enhanced piezoelectric performances. The model predictions were also compared with some experimental results obtained on cellular piezoelectric films made of polypropylene filled with 10% of calcium carbonate microparticles. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Multiple Hydrogen-Bond Array Reinforced Cellular Polymer Films from Colloidal Crystalline Assemblies of Soft Latex Particles.

Waterborne polymer films made from soft polymer latex dispersions generally suffer from deterioration of chemical resistance and physical barrier properties under high humidity conditions and upon solvent exposure. Here we demonstrate the fabrication of robust polyhedral cellular polymer films from poly(methyl methacrylate-co-butyl acrylate) latexes, which were made by emulsion polymerization using a 2-ureido-4-pyrimidinone (UPy) functional methacrylate comonomer. Multiple hydrogen bond (MHB) arrays provided by UPy groups arrest the film formation process thereby creating a cellular reinforcement. The cellular polymer films exhibit impressive physical and mechanical properties. Upon solvent exposure, the films show colloidal crystalline-type Bragg diffraction features and do not suffer excessive and deteriorative uptake of water and, more remarkably, can absorb high amounts of organic solvents, thereby turning into an organogel with preservation of shape, up to a 14-fold volumetric swelling ratio of the polymer films in case of chloroform.

Piezoelectret microphones.

Piezoelectret microphones, a recently described new class of electret microphones, are based on the strong piezoelectric effect of internally charged, cellular polymer films. Such piezoelectret films have a large number of internal voids within the polymer. The upper and lower walls of these voids are oppositely charged. Films of this kind, when metallized on both sides, show high-piezoelectric activity and can be directly used as microphones. Recent work on these transducers has concentrated on stacking of piezoelectret films to improve sensitivity, on directional microphones, and on investigations of new film materials with improved charge stability at elevated temperatures. Stacked transducers have sensitivities proportional to the number of piezoelectret layers. With six layers, values of approximately 15 mV/Pa can be achieved. Directivities include omnidirectional, bidirectional, and cardioid patterns as well as dimensional characteristics. The new materials are based on fluoropolymers. These are, as opposed to the originally used polypropylene, not available in suitable cellular form. However, useful films can be made by fusing thin fluorocarbon films such that an array of small air voids is created between the layers. Transducers consisting of charged films of this kind withstand temperatures of 90 °C without significant loss of sensitivity.

Airborne phased array for airborne applications based on cellular polymer

Object recognition, advanced distance measurements and other inspection scenarios have an increasing demand in versatile airborne ultrasonic phased arrays for acoustic scanning without moving parts. Based on cellular polymer film with high piezoelectric effect we have realized an array structure with a pitch of 0.5 mm and element length of 10 mm. The working frequency of the material was measured to 250 kHz. From pointspread simulation with the small ratio of pitch/wavelength of 0.35 we could expect good beam steering and focusing characteristics. A first test array was realized and characterized. There was good agreement between measurement results and simulations. Additionally a low frequency electronic beamformer system was developed for generating the first B-image of an airborne phased array. Measurements showed that cellular polymer is a well suitable material for airborne applications. It can easily be structured to the desired shape. It allows especially the realization of phased arrays for applications like surface or profile measurement, access control, attendance check, robot guidance etc. New airborne array types like linear, phased, curved, or circular arrays are now possible. Also single element transducers with varying apertures (rectangular, oval), shaped apertures [focusing (line- or point-focusing), defocusing] or combinations of both are possible.

Optimized Preparation of Elastically Soft, Highly Piezoelectric, Cellular Ferroelectrets from Nonvoided Poly(ethylene Terephthalate) Films

AbstractElectrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro‐acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided poly(ethylene terephthalate) (PETP) films by means of foaming with CO2, biaxial mechanical stretching, controlled void inflation, and bipolar electric charging. The nonvoided PETP films were foamed with supercritical CO2 at a suitably high pressure and subsequently annealed for stabilization. The cellular foam structure was further optimized by means of well‐controlled biaxial stretching in a commercial stretcher and sometimes subsequent inflation in a pressure chamber. Bipolar electric charging of the internal voids was achieved through the application of high electric fields in an SF6 atmosphere. The new optimized PETP ferroelectrets exhibit quite large piezoelectric coefficients up to almost 500 pC N–1, for which unusually low elastic stiffnesses of only around 0.3 MPa are essential. The PETP‐foam ferroelectrets possess unclamped thickness‐extension resonance frequencies between approximately 120 and 250 kHz, and are thus highly suitable for several established as well as novel ultrasonic‐transducer applications.

High-sensitivity piezoelectric microphones based on stacked cellular polymer films (L)

Improvements of the sensitivity of piezoelectric microphones based on charged cellular polymer films are reported. The improvements are achieved by (1) an increase of the piezoelectric d33-coefficient of the cellular polypropylene films by pressure expansion and (2) stacking of the films. Microphones consisting of a single film of such material have sensitivities of about 2 mV/Pa at 1 kHz, independent of size, while for a microphone with five stacked films a sensitivity of 10.5 mV/Pa was measured. The equivalent noise level is about 37 dB(A) for the single-film transducer and 26 dB(A) for the stacked version. Advantages of these new piezoelectric transducers include their simple design, low cost, and small weight, as well as a large range of shapes and sizes possible.

Understanding the role of the gas in the voids during corona chargingof cellular electret films - a way to enhance their piezoelectricity

The influence of the corona-charging process on the piezoelectrictransducer coefficient d33 of a cellular electret film has been investigated.An increased corona voltage can be considered as a way to enhance the chargedensity and thus also the resulting piezoelectric effect. Highercorona-charging voltages are possible with increased ambient pressure or insuitable dielectric gases. The effect of the gas inside the voids has alsobeen studied. Enhanced transducer coefficients were obtained by coronacharging in N2 or N2O gas atmospheres at 100-450 or 100-140 kPapressures, respectively. The highest transducer coefficients of about790 pCN-1 were obtained when N2 gas was filled into the voids of a cellularpolymer film by means of consecutive vacuum and high-pressure treatments at295 or 313 K.