Shrinkage modeling of polyester shrink film

A model was developed to quantify the thermal shrinkage behavior of polyester shrink film under isothermal and nonisothermal conditions. This model employs the straightforward first order reaction kinetics to an otherwise microscopically complex relaxation process. It offers three unique parameters for quality control and material specification of shrink film. In the isothermal condition, the induction time, the shrink constant, and the ultimate shrinkage are the three parameters used by the model to quantify the shrinkage of shrink film under an elevated but constant temperature. In the nonisothermal condition, the shrink film is heated using a constant heating rate. Three parameters, which are the onset shrink temperature, the shrink constant, and the ultimate shrinkage, characterize the shrinkage of shrink film. The model fits the experimental data very well. The thermal shrinkage of several polyester homopolymers, copolymers, and polyester blends was investigated and compared to that of poly(vinly chloride) (PVC) shrink film. Two polyester blends containing glycol modified poly(ethylene terephthalate) (PETG) copolyester behave similar to PVC in shrinkage behavior. The effect of stretch temperature, stretch ratio, and stretch rate on the shrinkage characteristics of oriented PETG film was also studied. The stretch temperature has the highest influence on the shrinkage behavior of oriented film.     

A nonlinear mechanical model for solid-filled rubbers

A nonlinear mechanical model has been proposed for application to nonlinear viscoelastic elastomers. The model consists of four nonlinear elements similar to the Burgers model. Based on the theory of reaction rate proposed by Eyring and the statistic concept of entropy-spring developed in rubber elasticity, four relevant nonlinear elements are used to construct the model. The deformation, or the time rate of deformation of each element is described by a nonlinear stretch parameter A. According to the model and the parameter A, two groups of equations are derived to describe the creep or relaxation behavior of solid-filled and pure rubbers, respectively. In order to examine the applicability of the model as well as the nonlinear stretch parameter, a series of experiments for solid-filled polybutadiene rubbers has been carried out, which include constant rate of tensile tests, creep and relaxation tests with and without aging effects. It is found that the nonlinear stretch parameter satisfies the Eyring equation and that the model agrees the experiments extremely well.     

Experimental study and numerical simulation of the injection stretch/blow molding process

The injection stretch/blow molding process of PET bottles is a complex process, in which the performance of the bottles depends on various processing parameters. Experimental work has been conducted on a properly instrumented stretch/blow molding machine in order to characterize these processing parameters. The objective being a better understanding of the pressure evolution, preform free inflation has been processed and compared with a simple thermodynamic model. In addition, a numerical model for the thermomechanical simulation of the stretch/blow molding process has been developed. At each time step, mechanical and temperature balance equations are solved separately on the current deformed configuration. Then, the geometry is updated. The dynamic equilibrium and the Oldroyd B constitutive equations are solved separately using an iterative procedure based on a fixed-point method. The heat transfer equation is discretized using the Galerkin method and approximated by a Crank-Nicholson's scheme over the time increment. Successful free blowing simulations as well as stretch/blow molding simulations have been performed and compared with experiments.     

Distributive mixing in conveying elements of a ZSK-53 co-rotating twin screw extruder

A fluid dynamics analysis package (FIDAP), using the finite element method, was implemented to simulate the 3-D isothermal flow patterns in the conveying element of a ZSK-53 co-rotating twin screw extruder. The fluid was described by a power-law model. The dynamics of distributive mixing was studied numerically by tracking the motion of particles in the mixer. The extent of distributive mixing was characterized in terms of length and area stretch as well as strain distributions. The length stretch reflects the overall capability of the mixing device to spread minor component particles away from their neighbors originally present in the same cluster. The area stretch reflects the evolution of intermaterial area when mixing two fluids with a passive interface. We observed an oscillatory behavior for the average intermaterial area stretch, which was explained in terms of a stretching and folding mechanism. Folding occurs during material takeover from one screw to the other. Operating at higher rotational speeds enhances distributive mixing efficiency.     

Baseline Stiffness Modulates the Non-Linear Response to Stretch of the Extracellular Matrix in Pulmonary Fibrosis

Pulmonary fibrosis (PF) is a progressive disease that disrupts the mechanical homeostasis of the lung extracellular matrix (ECM). These effects are particularly relevant in the lung context, given the dynamic nature of cyclic stretch that the ECM is continuously subjected to during breathing. This work uses an in vivo model of pulmonary fibrosis to characterize the macro- and micromechanical properties of lung ECM subjected to stretch. To that aim, we have compared the micromechanical properties of fibrotic ECM in baseline and under stretch conditions, using a novel combination of Atomic Force Microscopy (AFM) and a stretchable membrane-based chip. At the macroscale, fibrotic ECM displayed strain-hardening, with a stiffness one order of magnitude higher than its healthy counterpart. Conversely, at the microscale, we found a switch in the stretch-induced mechanical behaviour of the lung ECM from strain-hardening at physiological ECM stiffnesses to strain-softening at fibrotic ECM stiffnesses. Similarly, we observed solidification of healthy ECM versus fluidization of fibrotic ECM in response to stretch. Our results suggest that the mechanical behaviour of fibrotic ECM under stretch involves a potential built-in mechanotransduction mechanism that may slow down the progression of PF by steering resident fibroblasts away from a pro-fibrotic profile.     

Limitations of Reptation Theory for Modeling the Stress-Dependent Rheological Behavior of Polyethylene Terephthalate Above the Glass-Transition Temperature

The biaxial stretch blow molding is an established process for manufacturing plastic containers, in which preforms are stretched both in circumferential and axial directions while being blown into a mold. In the development phase of these products, computer-aided analysis tools are extensively used to increase the material and process efficiency. The accuracy of these tools depends on the underlying material models and parameters. The aim of this article is to investigate the suitability of reptation theory for the prediction of the strain-dependent rheological behavior of polyethylene terephthalate (PET) in the stretch blow molding process. Reptation theory has already been successfully applied to a number of polymer melts in the past decades. However, the practical applicability of reptation theory for predicting the strain-dependent rheological behavior of highly viscous polymers slightly above the glass-transition temperature, as is the case with stretch blow molding, has not yet been fully investigated. In the first step, the constitutive material model equation of reptation theory is implemented and the necessary model parameters are determined using various measurement methods. However, the measurements could not be conducted with the same accuracy as in the case of polymer melts, because the measurement methods used showed instabilities in the glass-transition temperature range, which led to high measurement uncertainties. Consequently, the application of the material model does not match quantitatively to biaxial stretch tests. Qualitatively, on the other hand, the material model successfully reproduces the stress–strain behavior of PET films at low strains. In case of temperature dependence, the model results are neither qualitatively nor quantitatively satisfactory. The temperature dependency of the material model has been further investigated in the second step. It was shown that the derivative of the Doi–Edwards memory function with respect to the temperature has an inflection point if the stretching duration is equal to the disengagement time. For very small disengagement times compared to the stretching duration, the results of the model match the experimental observations. For high disengagement times induced by the large viscosities near the glass-transition temperature and for low stretching times induced by high strain rates; however, the Doi–Edwards memory function cannot predict the experimental observations correctly. The investigations show that reptation model qualitatively predicts the strain behavior of biaxial stretched PET films at low strains correctly. However, different measurement approaches for a more accurate and reproducible determination of the material properties and a modification of the model are required in order to adapt the model to highly viscous melts above the glass-transition temperature. The results have shown that the process conditions of the two-stage stretch blow molding, such as high strain rates and low processing temperatures, exceed the validity limits of reptation theory. POLYM. ENG. SCI., 60:765–772, 2020. © 2020 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.     

Effects of stretch induced softening to the free recovery behavior of shape memory polymer composites

During a cyclic tension test, many elastomeric materials exhibit an appreciable softening in their mechanical properties resulting from the previous stretch, known as the Mullins effect. This paper explores the influence of the stretch induced softening effect to the free recovery behavior of an acrylate shape memory polymer (SMP) composite by incorporating carbon black (CB) as filler materials. The observed softening effect in this SMP composite is considered to be a consequence of stretch induced alternation of filler–polymer interactions inside the composite. Further experiments find that a larger prior stretch gives a larger increase in material softening, which in turn decreases the shape recovery speed. To capture the experimental observations, a multi-branch one dimensional (1D) model is applied, where the modulus in the equilibrium branch is modeled to decrease with stretching deformation following a damage-like softening function. It is found that the loss in modulus due to softening consequently reduces the driving force for recovery and thus results in a slow recovery. Parametric studies further demonstrate that the discounted shape recovery speed will finally reach a saturated level when gradually increasing the programmed strain level in a shape memory cycle.     

A constitutive model for large multiaxial deformations of solid polypropylene at high temperature

Experiments on a blow‐molding grade of polypropylene have been performed at 135°C using a biaxial testing machine. Both simultaneous and sequential equibiaxial tests were performed at strain rates relevant to solid phase processing regimes. A constitutive model has been developed that includes a single Eyring process and two Edwards‐Vilgis networks. The effectiveness of this model for predicting the observed stress‐strain behavior is explored. Predictions of simultaneous stretching and the first stretch in sequential experiments are excellent. The second stretch in sequential experiments is less well predicted, but the model's performance is useful overall. The model is incorporated into a commercial finite element code and its practicality is demonstrated. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

MIXING WITH DIFFUSION AND FAST CHEMICAL REACTION IN SIMPLE SHEAR FLOW: A COMPARISON OF STRETCH MODEL TO TWO-DIMENSIONAL DIFFUSION WITH PERIODIC BOUNDARY CONDITIONS

The lamellar stretch model for describing mixing with simultaneous diffusion and fast chemical reaction is evaluated for simple shear flow between two parallel plates by comparing the predicted concentration profiles to those computed from the governing partial differential equation. A direct algorithm is devised to solve the sparse matrix originating from the discretization of the PDE. Under conditions where initial striation thickness equals the characteristic dimension of the flow geometry, the lamellar stretch model turns out to be quite accurate for k ≥ 10⁶ or k ≤ 1, where k is the product of the Reynolds number and the Schmidt number. In the intermediate range of k values, where mechanical and diffusional mixing are of comparable importance, the model is less accurate     

Micromechanics of the growth of a craze fibril in glassy polymers

The primary objective of this work is to model the growth and eventual failure of a craze fibril in a glassy polymer, starting from a primitive fibril. Experimental investigations have shown that properties like the entanglement density of a polymer play a pivotal role in determining whether macroscopic failure of a polymer occurs through crazing or shear yielding. Failure is seen to be related to the formation of a soft ‘active zone’ at the craze-bulk interface, through disentanglement. The present work aims at explaining some of the experimental findings about fibril growth and failure in glassy polymers on the basis of a continuum model of a craze with a constitutive model that accounts for yield, network hardening and disentanglement. The results show that this approach is capable of providing explanations for experimentally observed facts such as the propensity to crazing in polymers with low entanglement density and the linearity between the stretch in a fibril and the maximum stretch of a molecular strand in the fibril.     

正交试验法在PAN原丝纺丝工艺中的应用

利用正交设计方法确定了PAN湿法纺丝过程中影响原丝性能最主要的因素。结果表明 :以计量泵转速、凝固浴温度、负拉伸以及蒸汽拉伸倍数作为考察对象 ,影响原丝性能的最显著因素是蒸汽拉伸倍数 ,其次是负拉伸。以负拉伸、第二拉伸、第三拉伸以及蒸汽拉伸的倍数作为考察对象 ,影响原丝性能最显著的是蒸汽拉伸倍数 ,负拉伸几乎没有影响。利用正交实验结果 ,适当调节蒸汽拉伸倍数 ,制得强度为 6.5～ 7.0cN/dtex ,线密度为 1.1～ 1.3dtex的原丝     

Numerical simulation of gas-assisted polymer-melt electrospinning: Parametric study of a multinozzle system for mass production

In this study, we developed a multiphysics model for simulation of a gas-assisted melt-electrospinning (GAME) process, focusing on jet formation and propagation behavior. By numerically calculating the stresses acting on the jet during a single-nozzle GAME process, the shear viscous stress was identified as the main factor with respect to jet stretch; thus, the relationship between shear viscous stress and jet thickness was investigated. The jet stretch ratio increased sharply when shear viscous stress reached the level at which jet sharpening occurred, leading to stable jet formation. We defined this stress as the critical shear viscous stress to determine stable spinnability. By imposing an electric field distribution calculated for a multi-nozzle array (number of nozzles, tip-to-tip distance, and applied voltage) on the boundary condition of the single-nozzle GAME simulation model, multinozzle GAME was simulated; this enabled proposal of a spinnability diagram for stable spinning.     

Evolution of polyacrylonitrile precursor fibers and the effect of stretch profile in wet spinning

Wet spinning of polyacrylonitrile-based polymers is a common technique to manufacture carbon fiber precursors. Understanding the role of stretch profile on structural evolution will support efforts to reduce cost and improve process robustness. Fiber stretch generally occurs in three sequential stages: jet stretch, wet stretch (first draw), and hot draw (second draw). In this study, total fiber stretch was kept constant, but distributed differently across the stretch stages yielding three different fiber variants. Samples were collected and analyzed after each stretch stage in order to correlate process parameters to structural information. For all variants, orientation of the ordered phase increases gradually for each stage of stretch while activation energy for the structural relaxation decreases. Alternatively, crystallite size increases substantially during hot draw, which is shown to have the most pronounced effect on cyclization behavior. Given the process conditions, the variant with the lowest jet stretch and highest hot draw demonstrates the highest tenacity and modulus along with the greatest orientation, crystallite size, and highest peak exotherm temperature.     

Application of surface-renewal-stretch model for interface mass transfer

A new surface-renewal-stretch (SRS) model was developed to correlate experimental data for the time-average overall mass transfer coefficient, K_L,av, in liquid-liquid and gas-liquid mass transfer systems. The model is based on the equation of continuity, which includes both turbulent and convective mass transfer at the liquid-liquid and gas-liquid interfaces. The model incorporates Dankwerts surface-renewal model with the penetration theory for surface stretch proposed by Angelo et al. [Angelo, J.B., Lightfoot, E.N., Howard, D.W., 1996. Generalization of the penetration theory for surface stretch: application to forming and oscillation drops. A.I.Ch.E. Journal 12 (4) 751-760]. We used our new SRS mass transfer model to correlate successfully the existing interface mass transfer experimental data from published literature. As a result, the experimental mass transfer coefficient data was predicted with a high degree of accuracy.     

PET注拉吹制品壁厚分布的研究

通过注射拉伸吹塑实验分析了二次吹胀压力、拉伸杆速度以及吹胀延迟时间对瓶子轴向壁厚分布的影响。实验结果表明：吹胀压力的改变对瓶子轴向壁厚分布影响不大,拉伸杆速度与吹胀延迟时间的变化对瓶子轴向壁厚分布有显著影响。吹胀延迟时间的改变会形成不同的型坯轮廓发展模式,在注拉吹可视化实验结果的基础上,结合应力和温度场两方面的因素解释了型坯轮廓发展模式的成因。     

Raman Studies of Hydrogen Bonding in Polyamides

We report Raman and pre-resonant Raman spectra of the polyamides N,N- tetramethylene bisbutamide, N,N-hexamethylene bishexamide, and 7,14-diazacyclotetradecane-1,6-dione, as well as of a low molecular weight prepolymer of nylon 6,6 aimed at characterizing the nature of hydrogen bonding in these compounds. The relative intensities of the C=O stretch and C-N stretch bands, the frequency width of the C=O band, the frequency of the N-H stretch band, and the frequency width of the N-H band indicate that these compounds do not show the Raman signatures of hydrogen bonding that we have established in our previous studies of many model amides. In this regard, these polyamides behave like the lactams 2-azacyclononanone and 2-azacyclotridecanone, for which all the Raman spectral indicators of hydrogen bonding are similarly negative. These results indicate that whatever the nature of the intermolecular interactions in these polyamides, they do not result in the changes in vibrational and electronic structure that we believe are associated with hydrogen bonding.     

An analysis of stretch forming of thermoplastic composites

Thermoplastic composite sheet materials have found an increasing number of applications through the use of matched‐die compression molding. The combination of high strength, low weight and moderate manufacturing costs makes the material an attractive alternative to sheet metal stampings in a number of applications. However, a significant range of conventional sheet forming techniques may also prove suitable for these materials, provided the effects of strain rate and temperature can be properly understood and exploited, leading to a further reduction of manufacturing costs and an increasing number of potential applications. In this work, limiting strains in biaxial stretch forming were explored using the hemispherical stretch forming test. The materials tested contain 20, 35, and 40 percent by weight of randomly oriented glass fiber in a polypropylene matrix. The forming tests were performed at temperatures ranging from 75°C to 150°C and at punch speeds of 0.01cm/sec, 0.1cm/sec, and 1cm/sec. A rotationally symmetric, anisotropic material model with rate sensitivity was developed and incorporated into an axisymmetric finite element model of the stretch‐forming process. The model parameters were temperature‐dependent, though the temperature distribution in the formed part was assumed to be uniform. Strain distributions in the formed parts are compared to finite element method results, and the results are good up to the point when localized necking begins to dominate the strain distribution. These forming limit strains are compared with predictions based on Marciniak's imperfection theory, with good results.     

Axisymmetric Adhesive Contact under Equibiaxial Stretching

This article experimentally examines axisymmetric adhesive contact under equi-biaxial stretch of the substrate. It is motivated by recent theoretical models which predict that the contact radius decreases sharply beyond a critical strain, with an instability that can result in spontaneous detachment or gross slipping across the contact area. The model system in the present experiments consists of convex glass lenses resting on poly(dimethyl siloxane) (PDMS) sheets, which are subjected to equibiaxial stretch in a specially designed experimental setup. It is shown that the evolution of the contact area is well described by the theoretical model after accounting for the mode-mixity dependence of work of adhesion at the contact edge. More significantly, the conditions of instability observed in the experiments were well predicted by the model. The findings are expected to be significant in predicting soft material contact behavior, such as that in biological adhesion.     

塑料拉伸吹塑中型坯再加热阶段的研究进展

介绍了拉伸吹塑的相关理论。从型坯的取向和结构、红外辐射在拉伸吹塑再加热阶段的应用、型坯的温度分布对再加热拉伸吹塑制品性能的影响等方面对塑料拉伸吹塑中型坯再加热阶段进行了综述;并对今后的研究提出了展望。     

Contour-variable model of constitutive equations for polymer melts

Based on a modified expression of the rate of the convective constraint release, we present a new contour-variable model of constitutive equations in which the non-uniform segmental stretch and the non-Gaussian chain statistical treatment of the single chain are considered to describe the polymer chain dynamics and the rheological behavior of an entangled system composed of linear polymer chains. The constitutive equations are solved numerically in the cases of steady shear and transient start-up of steady shear. The results indicate that the orientation and stretch, as well as the tube survival probability, have strong dependence on the chain contour variable, especially in the high-shear-rate region. However, the inclusion of the non-uniform features in the constitutive models has little modification comparing with the uniform models in determining the rheological properties both qualitatively and quantitatively.