Melt fracture,melt viscosities,and die swell of polypropylene resin in capillary flow

The melt fracture, shear viscosity, extensional viscosity, and die swell of a polypropylene resin were studied using a capillary rheometer and dies with a 0.05‐cm diameter and length/radius ratios of 10, 40, and 60. A temperature of 190°C and shear rates between 1 and 5000 s^?1 were used. A modified Bagley plot was used with consideration of pressure effects on both the melt viscosity and end effect. The shear viscosity was calculated from the true wall shear stress. When the true wall shear stress increased, the end effect increased and showed critical stresses at around 0.1 and 0.17 MPa. The extensional viscosity was calculated from the end effect and it showed a decreasing trend when the strain rate increased. Both the shear and extensional viscosities correlated well with another polypropylene reported previously. The die swell was higher for shorter dies and increased when shear stress increased. When the shear rates increased, the extrudate changed from smooth to gross melt fracture with regular patterns (spurt) and then turned into an irregular shape. In the regular stage the wavelength of the extrudates increased when the shear rate increased. The frequency of melt fracture was almost independent of the shear rate, but it decreased slightly when the die length increased. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1587–1594, 2003     

Observation of melt fracture of polypropylene resins in capillary flow

Melt fracture, shear viscosity, extensional viscosity, and die swell of two polypropylene resins were studied using a capillary rheometer. A modified Bagley plot with consideration of pressure effects on melt viscosity and end effect was used. From the true wall shear stress the shear viscosity was calculated. Extensional viscosity was calculated from the end effect. Both shear and extensional viscosities of different molecular weights and temperatures correlated well under the time-temperature Williams-Landel-Ferry (WLF) superposition. Die swell increased when shear stress increased, and was higher for shorter dies at a given shear rate. When shear rates increased the extrudate staged from smooth to gross melt fracture with regular patterns (spurt), and then turned into irregular shapes. In the regular stage the wavelength of extrudates was measured, and corresponding frequency was calculated. The frequency increased when molecular weight decreased and when melt temperature increased. The shift factor based on shear viscosity also brought frequency data of different molecular weights and temperatures into master curves. The frequency decreased slightly when die lengths increased from L/R=10 to 60. A small maximum was observed when shear rates increased.     

Shear viscosity,extensional viscosity,and die swell of polypropylene in capillary flow with pressure dependency

The shear and extensional viscosities of a polypropylene resin were studied using a capillary rheometer and capillary dies of 1‐mm diameter and length of 10, 20, and 30 mm. Melt temperatures at 190, 205, and 220°C and shear rates between 100 and 5000 s^?1 were used. At the highest shear rate a visible melt fracture was observed. An equation relating the pressure drop and die length was derived with consideration of pressure effects on melt viscosities and the end effect. After the correction for pressure effects the true wall shear stress and end effect at zero pressure were calculated. The end effect showed a critical stress of melt fracture around 10⁵ Pa, and increased rapidly when shear stress increased above the critical stress. From shear stress the shear viscosity was calculated, and a power law behavior was observed. Extensional viscosity was calculated from the end effect and showed a decreasing trend when strain rate increased. After time–temperature superposition shift shear viscosity data correlated well, but an upward trend was observed in extensional viscosity when melt fracture occurred. Die swell ratio at different temperatures can be plotted as a function of wall shear stress and was higher for shorter dies. © 2002 Wiley Perioodicals, Inc. J Appl Polym Sci 84: 1269–1276, 2002; DOI 10.1002/app.10466     

聚丙烯熔体拉伸流变行为的研究

采用双料筒毛细管流变仪，对两种不同摩尔质量的等规聚丙烯的剪切及拉伸流变行为进行了研究，并采用Cogswell方法计算了熔体的拉伸强度。结果表明：聚丙烯为假塑性流体，随着剪切速率的增加，熔体的表观剪切黏度下降，呈现出假塑性流体典型的“剪切变稀”行为。在相同的温度和剪切速率下，平均摩尔质量较小、分布较宽的聚丙烯的表观剪切黏度及拉伸黏度均较小，但熔体的拉伸强度较大，意味着其具有较好的流动性能，并具有较高的可拉伸性，可以达到较大的拉伸比。     

Flame retardation of polypropylene: Effect of organoantimony compounds on the melt flow behavior

The melt flow behaviour of polypropylene filled with organoantimony compounds, triphenylstibinedibromide and its derivatives with tribromo-, trichloro-, and pentachloro-phenols, as flame retardants has been studied in the temperature range of 180 to 220°C, and at shear rates of 29.5 to 588.8's^?1, using a capillary rheometer. A decrease in the melt viscosity at all shear rates and temperatures was noticed on addition of these flame retardants, The melt viscosity further decreased upon increasing the concentration (from 5 to 20 phr) of the flame retardants. The die swell was measured in order to determine the melt elasticity of the filled systems. The minimum melt viscosity of filled propylene was observed approximately where the maximum die swell (melt elasticity) occurred.     

聚对苯二甲酸丙二酯熔体拉伸流变性能研究

使用毛细管流变仪研究了PTT熔体在单轴拉伸流场中的流变性能。结果表明,PTT熔体拉伸流动属于拉伸变稀型,熔体的拉伸粘度随拉伸应力增加而降低,随相对分子质量提高,熔体的拉伸变稀现象更加明显。熔体的相对分子质量越高,拉伸速率对熔体的拉伸粘度影响越大。随拉伸速率提高,熔体的拉伸应力增大,拉伸粘度减小,易产生拉伸共振现象。因此,PTT熔体纺丝时应该采取较低的喷头拉伸倍数,即适当提高熔体挤出时的纺丝速度,或降低卷绕速度,以此降低熔体拉伸比和防止拉伸共振现象,改善纺丝成形的稳定性。     

高黏度聚酯流变行为的研究

采用毛细管流变仪研究了高黏度聚酯(PET)的表观黏度及黏流活化能随温度(280～300℃)及剪切速率(20～104s-1)的变化。结果表明:高黏度PET熔体随着剪切速率的增加出现切力变稀现象,随着熔体温度升高,剪切速率对熔体的表观黏度的影响降低;高黏度PET的黏流活化能随着剪切速率的提高而降低;在温度为300℃,剪切速率为3 000 s-1时,高黏度PET熔体具有较好的流动性。     

Interrelationship of strength and flow characteristics of polystyrene

This study investigated the interrelationship between strength and flow characteristics of general-purpose polystyrene (GPPS) used in injection molding applications. The ease of flow was chosen as a measure of processability and was evaluated using the melt flow rate and capillary rheometer techniques. Of the different strength tests that were examined, flexural and notched tensile strength tests were most effective in differentiating between commercial grades of high and low molecular weight GPPS. While characterizing strength of injection molded specimens, the degree of molecular orientation was taken into consideration. For unplasticized resins, increasing the weight average molecular weight by about 100,000 enhanced the flexural strength by 10%, but also increased the viscosity at low shear rates (10 to 100 s^?1). The increase in molecular weight had virtually no effect on viscosity at the highest shear rates (up to 10,000 s^?1). Plasticized resins displayed a 6% loss in flexural strength as well as a significant reduction in viscosity (throughout the shear rate range) as compared with the unplasticized resins. As expected, the improvement in strength achieved by increasing molecular weight leads to a simultaneous increase in the viscosity, i.e., a deterioration of processability. In addition, our study indicates that for samples without preferential molecular orientation, narrowing the molecular weight distribution significantly improves the balance of strength and melt flow rate properties.     

Short fiber reinforced thermoplastics. I. Rheological properties of glass fiber reinforced Noryl

An experimental study on the flow behavior of glass fiber reinforced Noryl (a commercial poly(phenyleneoxide)/polystyrene blend) using a capillary rheometer is described. The effect of fiber concentration on shear viscosity and die swell was studied at various temperatures. Breakage of glass fibers during flow through the rheometer is discussed; it was found that the average fiber length (about 230 μm) was not significiantly altered, except at the highest shear rate (575 s⁻¹) studied. The incorporation of short fibers into thermoplastic polymer melts increases their viscosity without changing the basic rheological character-shear rate dependency. No discernible viscosity changes were measured by incorporating 10 weight percent fibers, and upon further increase of fiber concentration from 20 to 30 weight percent no appreciable increase in viscosity was noted. It is shown that short glass fibers cause a large reduction in extrudate swell. The presence of voids and fiber orientation contribute to the decrease of the die swell, an effect greater than expected from volumetric considerations alone.     

PMMA熔体黏度影响因素实验研究

针对不同类型的聚甲基丙烯酸甲酯(PMMA)材料熔体的黏度开展实验研究。结果表明,3种不同的PMMA材料因相对分子质量大小的不同而使其流变特性存在一定的差异,在相同的温度下和压力下,相对分子质量越大,剪切黏度越大。随着挤出压力的增加,剪切速率则增加,剪切黏度减小,呈现剪切变稀的特征。同时剪切黏度随温度的变化也较为显著,随着温度的增加,剪切黏度变小,当温度达到280 ℃,3种材料的剪切黏度逐渐变得相等。     

High shear strain rate rheometry of polymer melts

The rheology of a range of polymer melts has been measured at strain rates above those attained during conventional rheometry using an instrumented injection molding machine. Deviations from shear thinning behavior were observed at high rates, and previously unreported shear thickening behavior occurred for some of the polymers examined. Measured pressure and volumetric throughputs were used to calculate shear and extensional viscosity at wall shear strain rates up to 10⁷ s^?1. Parallel plate rheometry and twin bore capillary rheometry were used to provide comparative rheological data at low and medium shear strain rates, respectively. Commercial grades of polyethylene, polypropylene, polystyrene, and PMMA were studied. Measured shear viscosity was found to follow Newtonian behavior at low rates and shear thinning power law behavior at intermediate strain rates. At shear strain rates approaching or above 10⁶ s^?1, shear viscosity reached a rate‐independent plateau, and in some cases shear thickened with further increase in strain rate. A relationship between the measured high strain rate rheological behavior and molecular structure was noted, with polymers containing larger side groups reaching the rate‐independent plateau at lower strain rates than those with simpler structures. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Melt rheology of segmented polyamides: Effect of block molecular weight

Segmented polyamides, also known as polyether‐ester‐amides, are composed of polyether and polyamide structural units. The rheological behavior of segmented polyamides with respect to the variations in the molecular weight of hard and soft blocks has been studied using a Monsanto Processability Tester. These systems exhibit pseudoplastic flow behavior. The shear viscosity of the segmented polyamides decreases with a decrease in hard block molecular weight up to 1500. However, at low shear rates, the shear viscosity shows marginal change with an increase in soft segment molecular weight. The equilibrium die swell increases with an increase in shear rate, but decreases with increasing temperature. The stress relaxation study of the segmented polyamides reveals that the stress developed during extrusion relaxes exponentially for all the systems. The equilibrium die swell at a fixed temperature and shear rate, the time required to relax a fixed amount of stress and the stress developed after a certain time interval decrease with a decrease in hard block molecular weight up to 1500, but increase with an increase in soft segment molecular weight. The activation energy of the melt flow process increases with the rate of shear in most of the cases. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1739–1747, 1999     

Melt spinning flow behaviour of high-density polyethylene blended with low-density polyethylene

ABSTRACT

The melt spinning flow behaviour of a high-density polyethylene (HDPE) blended with a low-density polyethylene (LDPE) was studied using a melt spinning technique in temperature ranging from 160 to 200°C and die extrusion velocity varying from 9 to 36?mm?s^?1. The results showed that the melt apparent extension viscosity of the blends was higher than those of the LDPE and HDPE; the melt apparent extension viscosity decreased with increasing temperature; while the melt apparent extension viscosity increased with increasing extension strain rate when the extension strain rate was lower than 0.2?s^?1, and then decreased; the melt apparent extension viscosity reached up to a maximum value when extension strain rate was about 0.2?s^?1; the relationship between the melt apparent extension viscosity and the LDPE weight fraction did not follow the mixing rule.     

高等级HDPE管材树脂毛细管流变性能研究

采用单管毛细管流变仪,在不同温度下对高等级高密度聚乙烯(HDPE)管材树脂的熔体流动行为进行了研究,考察了剪切应力(τ<,w>)、剪切速率(γ)、挤出胀大及温度之间的关系.结果表明,高等级HDPE管材树脂熔体的剪切流动基本上服从幂律定律;熔体的表观黏度(η<,a>)对温度的依赖性大致上符合Arrhenius方程;η<,a>随τ<,w>和γ的增加而非线性减小;挤出胀大比随温度的升高而下降,随γ的增加呈非线性增大.     

Effect of degree of polymerization on gelation and flow processability of poly(vinyl chloride)

The effect of degree of polymerization (DP) on the gelation and flow processability of poly(vinyl chloride) (PVC) was studied. Sheets with adjusted degree of gelation were prepared by rolling rigid pipe formulation suspension PVC compounds with DPs of 800, 1050 and 1300 by changing the milling temperature. Their degrees of gelation were measured with DSC and their capillary flow properties were measured with a capillary rheometer at 150, 170 and 190°C and the effect of DP on the relation between gelation and flow processabilities was studied. Because of the higher shearing heat during milling, the sample with the higher DP had a higher history temperature and thus tended to show a higher degree of gelation. The viscosity increased as the gelation increased. The dependency of viscosity on DP was higher at higher milling and extrusion temperatures and thus at a higher degree of gelation and a lower shear rate. This was assumed to be attributed to the more prominent uniform molecular flow as against the particle flow. The die swell increased with increasing the milling and extrusion temperatures and hence with increasing the gelation. A sample with a lower DP tended to show a larger die swell and this tendency was even more pronounced at the higher extrusion temperature. The melt fracture easily occurred when a sample with advanced gelation was extruded at low temperature. Whereas at low milling temperatures a sample with the lower DP showed a lower critical shear rate at onset of melt fracture, and thus easily generating melt fracture, at high milling temperatures it showed a higher critical shear rate and hence scarcely generated melt fracture. These experimental results were explained by the fact and concept that a sample with a lower DP shows a higher increase in the gelation during extrusion and/or the slighter feature of particle flow as against the uniform molecular flow at the same gelation level. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1915–1938, 2004     

Experimental studies on radial extrudate swell and velocity profiles of flowing PS melt in an electro‐magnetized die of an extrusion rheometer

This article investigates the radial extrudate swell and velocity profiles of polystyrene melt in a capillary die of a constant shear‐rate extrusion rheometer, using a parallel coextrusion technique. An electro‐magnetized capillary die was used to monitor the changes in the radial extrudate swell profiles of the melt, which is relatively novel in polymer processing. The magnetic flux density applied to the capillary die was varied in a parallel direction to the melt flow, and all tests were performed under the critical condition at which sharkskin and melt fracture did not occur in the normal die. The experimental results suggest that the overall extrudate swell for all shear rates increased with increasing magnetic flux density to a maximum value and then decreased at higher densities. The maximum swelling peak of the melt appeared to shift to higher magnetic flux density, and the value of the maximum swell decreased with increasing wall shear rate and die temperature. The effect of magnetic torque on the extrudate swell ratio of PS melt was more pronounced when extruding the melt at low shear rates and low die temperatures. For radial extrudate swell and velocity profiles, the radial swell ratio for a given shear rate decreased with increasing r/R position. There were two regions where the changes in the extrudate swell ratio across the die diameter were obvious with changing magnetic torque and shear rate, one around the duct center and the other around r/R of 0.65–0.85. The changes in the extrudate swell profiles across the die diameter were associated with, and can be explained using, the melt velocity profiles generated during the flow. In summary, the changes in the overall extrudate swell ratio of PS melt in a capillary die were influenced more by the swelling of the melt around the center of the die. Polym. Eng. Sci. 44:2298–2307, 2004. © 2004 Society of Plastics Engineers.     

Adjustable Twin‐Slit Rheometer for Shear Viscosity Measurement of Extruded Complex Starchy Melts

An online rheometer with an innovative system of height‐adjustable and independently temperature‐controlled slits was designed to measure the shear viscosity of extruded wheat bran fiber‐containing starchy materials. The range of melt pressures and temperatures, obtained with a die, could be covered by the rheometer. A close ingredient thermomechanical history in the extruder was achieved both with the die and the rheometer, while covering an apparent shear rate from 5 to 30 s^–1. Although minor technical problems remained, first rheological data were obtained and showed a pseudoplastic flow behavior for all recipes. The flow curves were fitted by a power law model. Wheat bran fiber addition influenced both the K‐ and n‐values, leading to more shear‐thinning melt behavior and an increase in true shear viscosity. Only a limited effect on these values was found in these preliminary experiments when further increasing the fiber content.     

Effects of extrusion conditions on rheological behavior of acrylonitrile–butadiene–styrene terpolymer melt

The influence of temperatures and flow rates on the rheological behavior during extrusion of acrylonitrile–butadiene–styrene (ABS) terpolymer melt was investigated by using a Rosand capillary rheometer. It was found that the wall shear stress (τ_w) increased nonlinearly with increasing apparent shear rates and the slope of the curves changed suddenly at a shear rate of about 10³ s^?1, whereas the melt‐shear viscosity decreased quickly at a τ_w of about 200 kPa. When the temperature was fixed, the entry‐pressure drop and extensional stress increased nonlinearly with increasing τ_w, whereas it decreased with a rise of temperature at a constant level of τ_w. The relationship between the melt‐shear viscosity and temperature was consistent with an Arrhenius expression. The results showed that the effects of extrusion operation conditions on the rheological behavior of the ABS resin melt were significant and were attributable to the change of morphology of the rubber phase over a wide range of shear rates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 606–611, 2002     

Rheological and chemorheological studies of cyclic aryl ether ketone and aryl ether thioether ketone oligomers containing the 1,2-dibenzoylbenzene moiety

The melt stability, shear rate, and temperature dependence of steady-state shear viscosity of molten cyclic aryl ether ketone and thioether ketone oligomers containing the 1,2-dibenzoylbenzene moiety have been investigated. The isothermal chemorheology of the ring-opening polymerization of cyclic oligomers 4 and 9 in the presence of a nucleophilic initiator was also conducted. The cyclic aryl ether ketone oligomers are thermally stable in the melt, and their melt viscosity is several orders of magnitude lower than their high molecular weight linear counterparts. At a given temperature, the steady-state shear viscosity of the molten cyclics initially undergoes shear thinning as the shear rate increases, and once the shear rate is above 10 s⁻¹, the molten cyclic oligomers behave like Newtonian fluids. For the amorphous cyclic oligomers studied, the steady-state shear viscosity at 100 s⁻¹ at a given temperature only depends on their glass transition temperature. The cyclic aryl thioether ketone oligomers are thermally unstable in the melt and undergo ring-opening polymerization in the absence of an initiator to form high molecular weight linear polymers with a concomitant rapid increase in viscosity. The rate of change in viscosity increases with temperature and is promoted by the addition of a catalytic amount of elemental sulfur or a disulfide such as 2,2-dithiobis(benzothiazole). It is hypothesized that the ring-opening polymerization is initiated by the in situ generated thiyl radical(s) and proceeds via a free radical route. © 1996 John Wiley & Sons, Inc.