Melt Relaxation Time Research Articles

Research on improved gas‐assisted extrusion of plastic sheets based on time–temperature equivalence principle

AbstractBased on the time–temperature equivalence principle, the relationship between the melt relaxation time (MRT) and melt heating temperature (MHT) in an improved gas‐assisted extrusion (GAE) of a polypropylene (PP) sheet was investigated by numerical simulations. In the simulations, the MRT was varied and the inherent properties of the MHT were controlled using experimental methods. With an increase in MRT, the velocity in all directions, pressure drop, and shear rate of the melt increased; however, increasing the MRT does not always yield a positive effect on the GAE process of the melt. For instance, the melt velocity increased vertically at the die exit with MRT, resulting in an outward extrudate swell. In addition, the extruded product was intact when the melt was heated to 220°C, which verified the principle of time–temperature equivalence.

Steady-state modeling of extrusion cast film process, neck-in phenomenon, and related experimental research: A review

This review provides the current state of knowledge of steady-state modeling of the extrusion cast film process used to produce flat polymer films, as well as related experimental research with a particular focus on the flow instability neck-in. All kinematic models used (i.e., 1-, 1.5-, 2-, and 3-dimensional models) together with the utilized constitutive equations, boundary conditions, simplified assumptions, and numerical methods are carefully summarized. The effect of draw ratio, Deborah number (i.e., melt relaxation time related to experimental time), film cooling, second to first normal stress difference ratio at the die exit, uniaxial extensional strain hardening, and planar-to-uniaxial extensional viscosity ratio on the neck-in is discussed.

Meltblown fibers: Influence of viscosity and elasticity on diameter distribution

Both melt viscosity ( η o) and elasticity (correlated here with the longest melt relaxation time λ 1) were found to control the diameter distribution of meltblown fibers. Fibers were formed by melt blowing binary polystyrene (PS) blends containing widely differing component molecular weights using a custom-built laboratory apparatus. Varying the concentration and molecular weight of a high molecular weight PS provided independent control over η o and λ 1. These rheological parameters influence the average diameter ( d av) and the distribution of diameters (coefficient of variation, CV) of meltblown fibers in different ways. Increasing η o leads to an increase in d av but has little impact on CV. On the other hand, increasing λ 1 beyond a threshold value reduces CV while simultaneously increasing d av. A one-dimensional slender-jet theoretical model with both upper convected Maxwell and Phan–Thien and Tanner constitutive equations was developed to investigate the influence of viscoelasticity and processing parameters on the properties of meltblown fibers. This model predicts a strong dependence of fiber diameter on the air shear stress and variations in fiber diameter with viscoelasticity that are in qualitative agreement with the experimental results. We believe these results suggest that carefully controlling the viscoelastic profile of polymers used in melt blowing is a viable approach for producing nanofibers with narrow fiber diameter distributions using current commercial equipment.

Fiber Spinning, Structure, and Properties of Poly(ethylene terephthalate-co-4,4‘-bibenzoate) Copolyesters

Thermal, rheological, and fiber spinning behavior of poly(ethylene terephthalate-co-4,4'-bibenzoate) random copolymers containing 5-65 mol % bibenzoate (BB) units have been studied. Copolymer randomness was determined by 13 C solution NMR. Copolymers with >40 mol % BB units spin like thermotropic liquid crystals and have mechanical properties approaching those of liquid crystals; however, no evidence of liquid crystallinity was observed in these fibers based on thermal, optical, and rheological studies. Thus, we believe these polymers are frustrated liquid crystalline polymers. The α-relaxation attributed to the glass transition temperature is completely suppressed in the fully drawn and heat-treated PETBB55 fibers. Structure development during fiber spinning as well as fiber mechanical properties have been studied. Fibers containing more than 40 mol % bibenzoate show the development of a new crystal structure and exhibit much higher tensile modulus than the copolymers containing a lower bibenzoate percentage. Copolymers containing more than 40 mol % bibenzoate also exhibit high melt relaxation times compared to the lower bibenzoate copolymers.

Morphological study of HDPE blown films by SAXS, SEM and TEM: a relationship between the melt elasticity parameter and lamellae orientation

Evidence is presented showing how the long relaxation times in the melt affect the subsequent morphology of blown films made from medium molecular weight (MMW) HDPE homopolymers. The blown film morphology was studied using the small-angle X-ray scattering (SAXS) technique, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SAXS data were used to obtain the lamellae orientation functions using the Herman's orientation function equation commonly used for wide-angle X-ray diffraction (WAXD). The blown film results showed an increase in orientation function with increase in ER, a normalized melt elasticity parameter that reflects an increase in the longest melt relaxation times. In our study, such an increase in the longest relaxation times results from an increase in long-chain branching (LCB). As expected, compression molded samples show random orientation by SAXS. Considerable differences were observed in the lamella organization between the blown films of low and high ER resins. TEM micrographs show the presence of fibril nuclei several microns long. Lamellae stack perpendicular to the machine direction (MD) in films made from higher ER resins. In contrast, stacks of lamellae arranged randomly characterize the micrographs of blown films made from resins of low ER value. A model is proposed to explain the relationship between orientation function and melt relaxation times.

Influence of molecular weight distribution on the melt extrusion of high density polyethylene (HDPE): Effects of melt relaxation behavior on morphology and orientation in HDPE extruded tubular films

The influence of molecular weight distribution and extrusion processing variables on the morphological features and orientation of high density polyethylene (HDPE) uniaxially extruded tubular films was investigated. In order to gain a better understanding of the orientation–crystallization behavior occurring during extrusion processing, the melt flow properties of the two HDPE resins with identical M̄n (14 600) values but different molecular weight distributions (M̄w/M̄n=10.3, 15.1), utilized in our previous study, were characterized by dynamic rheological experiments over the temperature range from 150 °C to 230 °C within the angular frequency range from 0.1 to 100 rad/s. The experimental data were shifted to produce master flow curves. The flow activation energy calculated from the shifting process was found to be 25.9 kJ/mol for resin 1 and 29.1 kJ/mol for resin 2. The characteristic relaxation time at 190 °C obtained by use of a Carreau–Yasuda analysis for resin 2 having the broader molecular weight distribution was found to be 6.5 times greater than that of resin 1. This observation further supports our previous conjecture that the prominence of the fibril nuclei in resin 2 is due to its longer melt relaxation time behavior. The extrusion processing variables of melt temperature at the die exit, quench height (which is the distance from the exit of the die to the cooling ring), flow rate of the air through the cooling ring, film line speed, and die gap were varied to control the melt relaxation time of HDPE resins and the processing time frame for cooling. The results show that a longer melt relaxation time and a shorter cooling processing time can enhance the formation of fibril nuclei. The importance of melt relaxation behavior in influencing the final morphological structure in HDPE extruded films and their associated properties is clearly made evident in this paper.

On the significance of pressure relaxations in capillary or slit flow

It is occasionally proposed by Leblanc (1976) and GOttfert (1994) that information about the viscoelasticity of molten polymers can be obtained by recording the pressure as a function of time when the piston driving a capillary or slit rheometer is suddenly stopped. In the cited reference by G6ttfert (1994), a single exponential is fitted to the pressure decay curve in order to estimate a measure of the melt's relaxation time, while in the other work cited by Leblanc (1976) a "relaxation spectrum" is inferred from the pressure data. The purpose of this note is to demonstrate that the pressure decay depends primarily on the viscosity and compressibility of the melt and that the contribution of melt elasticity is too small to be measured using a standard capillary rheometer. Hatzikiriakos and Dealy (1992, 1994) have modelled the pressure transients in a capillary rheometer, and it will be convenient to build on their analysis. For purposes of the present demonstration, I consider the case of a fluid with constant viscosity, as this makes it possible to obtain an analytical solution. As an initial condition, we consider that the piston speed, Vp, the reservoir pressure, P, and the flow rate, Q, are at steady state. At t = 0, the piston is suddenly stopped, and we wish to calculate P ( t ) and see how it is related to the properties of the fluid. We make the following simplifying assumptions:

Viscosities and Normal Stress Coefficients of High Impact Polystyrene Fluids

The viscosities and primary normal stress coefficients are presented for well characterized, rubber‐modified polystyrene as a function of shear rate, temperature, rubber phase volume fraction, and molecular weight. From these data, melt relaxation times and shear compliances are calculated and related to the polystyrene matrix molecular weight and rubber phase volume fraction. The matrix and rubber‐modified melt compliance data are compared to values predicted from molecular and continuum theories.

Viscosities and normal stress coefficients of high impact polystyrene‐mineral oil mixtures

AbstractMeasured melt viscosities and primary normal stress coefficients are presented for a well‐characterized, rubbermodified polystyrene as a function of shear rate and mineral oil diluent concentration. The logarithmic blending relationship accurately predicts the observed data. Melt relaxation times and shear compliance data are also calculated. The compliance numbers are consistent with molecular models based on a simple dumbbell (appropriately modified for rubber and mineral oil addition).