Rheology of wood plastics melt. Part 1. Capillary rheometry of HDPE filled with maple

Shear and extensional flow properties of the melts of high‐density polyethylene (HDPE)‐maple composites were studied with capillary rheometry to understand the effects of the wood content, particle size, and maleated polyethylene (MAPE). The viscosity data were compared with the values for neat matrix resin for reference. The effects of commercial wood particle size grades were examined at 60% by weight of wood loading. It was found that both shear and extensional viscosities increase with wood content but the filler content dependence is not as significant as for suspensions of inorganic fillers at similar filler loadings. Commercial wood particle size grades were found to result in less change in viscosity than wood content. The Mooney analyses conducted on the lower branch of the capillary shear flow data revealed a significant contribution of wall slip and confirmed the presence of a yield stress at higher filler contents. The internal lubrication role of MAPE was also illustrated in detail through the changes in both shear and extensional flow. POLYM. ENG. SCI., 45:549–559, 2005. © 2005 Society of Plastics Engineers     

Effects on the melt rheology of systems of Nd‐Fe‐B particles suspended in a poly(phenylene sulfide) and liquid crystalline polymer blend

The melt rheology of blends of a liquid crystalline polymer (LCP) and poly(phenylene sulfide) (PPS) and their composites with ferromagnetic Nd‐Fe‐B particles (MQP) was studied. We investigated the effects of LCP concentration, Nd‐Fe‐B particle volume fraction and size, distribution, and shear rate on the rheological properties of these composites. Enthalpy of fusion changes that were observed resulted from the addition of the LCP and Nd‐Fe‐B particles to the polymer blends/composites. The shear rate and frequency dependencies of the materials revealed a viscosity reduction at low (1–3 wt%) and moderate (10–15 wt%) LCP concentrations, and strong effects on the shear‐thinning characteristics of the melt. The suspensions of polydispersed Nd‐Fe‐B particle configurations in PPS that were of lower size ratios gave better processability, which is contradictory to previously reported behavior of suspensions containing spherical particles. Specifically, the compositions with unimodal and a bimodal distribution of Nd‐Fe‐B particles gave the lowest viscosities. The experimental data were correlated with semi‐empirical viscosity model equations of Maron‐Pierce, Krieger‐Dougherty, Eilers, and Thomas and were found to be consistent with the data. The maximum packing fraction, ϕ_m, of the MQP particles was estimated to be within the range of 0.78 ϕ ≤_m ≤ 1.0 through graphical and parametric evaluation methods.     

Rheological behavior and microstructure of bimodal suspensions of core‐shell structured swollen particles

The rheological behavior and microstructure of bimodal suspensions of core‐shell structured swollen particles have been examined with changing volume ratio of two different sized particles. As the volume fraction of large particles increases, the viscosity, degree of shear‐thinning, and the critical shear stress σ_c decreases, while the interparticle distance ξ of the microstructure increases. The suspensions exhibit single mode rheological behavior and have a single diffraction peak in the SAXS profiles. These results suggest that the bimodal suspensions of the core‐shell structured swollen particles behave likely to unimodal suspensions of hard spheres with alloy like single mode microstructure composed of hypothetical intermediate size particle. The relationship between σ_c and ξ can be represented as σ_c = 3kT/4πξ³, which corresponds to the dynamics of the Brownian hard sphere model with ξ being the particle diameter. These findings indicate that the shear‐thinning of the suspensions can be attributed to dynamical competition between the thermal motion and the hydrodynamic motion under shear flow and that the mechanism can be applied to bimodal suspensions of the swollen particles as well as unimodal suspensions of hard spheres. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 102: 2212–2217, 2006     

Effects of polymer molecular weight and filler particle size on flow behavior of wood polymer composites

The influence of polymer matrix molecular weight and filler particle size on rheological properties and extrudate distortions of metallocene polyethylene (mPE)/wood flour (WF) composites has been investigated by rotational and capillary rheometers. It was found that at low shear rates smaller filler particles provide higher shear viscosity than the larger sized filler. At high shear rates and WF loadings above 30 wt%, the effect of particle size on the melt flow properties becomes negligible. The relative increase of the storage modulus with decreasing particle size is more pronounced in the case of low molecular weight polymer matrix than that in higher molecular weight polyethylene based composites. The wood filled polyethylenes exhibit extrudate surface defects, which are complex function of the shear rate, polymer matrix molecular weight, and filler particle size. Increasing the shear rate results in pressure oscillations and spurt‐flow. It was also observed that the evolution of the extrudate surface tearing is strongly dependent on the pressure during a single pressure oscillation cycle in the spurt flow regime. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Investigation of Crystal Morphology and Shock Sensitivity of Cyclotrimethylenetrinitramine Suspension by Rheology

Crystal morphology and shock sensitivity of a series of cyclotrimethylenetrinitramine (RDX) particles suspended from ethylene glycol were investigated. Flow rheology was employed to measure the rheological properties of the suspensions at constant temperature; it was observed that the stress‐shear rate and viscosity behavior of the suspensions were controlled by the particle morphology. The viscosity of the RDX suspensions changed with the roundness/smoothness of RDX crystals at all applied shear rates. The suspensions containing crystals with smoother morphology showed reduced viscosity. When the viscosity data was compared to the shock sensitivity results from the RS‐RDX Round Robin study, a good correlation was obtained. This study has validated the use of flow rheology to indicate the morphology and shock sensitivity of crystalline particles.     

Rheology and curing kinetics of fumed silica/cyanate ester nanocomposites

A low‐viscosity bisphenol E cyanate ester (BECy) monomer was combined with fumed silica with average primary particle diameters of 12 and 40 nm to form high‐temperature adhesives with processability at ambient temperatures. Rheological evaluation revealed that for silica loadings below 15 vol%, suspensions of both particle sizes exhibited shear thinning and thixotropic behavior. Samples with high silica loadings (>15 vol%) of 40‐nm silica also showed intense shear thickening at shear rates above 10 s^?1. Thixotropy was most pronounced for the 12‐nm silica, but the formation of a gel was slow, indicating that the polar nature of the BECy monomer was responsible for disrupting hydrogen bonds between silica particles. Rheokinetic evaluation of catalyzed samples showed that increasing silica content reduced gel time and increased gel viscosity, and this effect was most pronounced for the 12‐nm silica. Differential scanning calorimetry confirmed that the silica's hydroxyl groups have a minor catalytic effect on the polymerization kinetics, such that the activation energies of the catalyzed suspensions were decreased with increased nanoparticle loading and decreased particle size. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Mechanical behavior of highly filled natural CaCO3 composites: Effect of particle size distribution and interface interactions

The mechanical behavior of poly(di‐methyl siloxane) (PDMS) composites containing high volume fractions of natural CaCO₃ particles of various particle size distributions was studied under tensile and oscillatory bending stresses, emphasizing the unique behavior of high filler loaded compositions. Composites containing the maximal possible solid loading of raw CaCO₃ were investigated for the effect of fatty acids surface treatment. The elastic modulus increased with increasing filler loading, following Chantler's model for dental composites when correlated with the absolute filler volume fraction. Good fit to “traditional” models, e.g., Frankle‐Acrivos and Halpin‐Tsai, was obtained by correlating the modulus values with the volume fraction relative to the maximal possible filler loading. A master curve of different particle size distributions and filler levels composites was obtained by using the relative volume fraction values, illustrating the effect of particle packing characteristics on small deformation mechanical behavior. A minor increase in T_g was found in parallel to the appearance of a T_m relaxation peak at approximately −40°C. A peak temperature shift at T_m and a pronounced increase in this peak with increasing filler fraction was found as well. The changes in the melting transition are attributed to the constraints of the filler particles acting on the crosslinked melting polymer. Surface treatment with fatty acids significantly degraded the tensile properties. Interestingly, an increase of 4 vol% filler was enabled owing to the surface treatment, while restoring reasonable tensile properties. No significant effect was observed for excess of fatty acids resulting from physically adsorbed acids. Tan δ curves reveal low PDMS‐CaCO₃ particles interactions, and mobility of the PDMS chains in the increased filler fraction as in the treated 64 vol% composite, both higher than those in the raw composite. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

The rheology of filled polymers

The flow properties of polymer melts containing fillers of various shapes and sizes have been examined. If there is no failure of either the filler or polymer in the solid state, then the modulus enhancement for randomly distributed filler is equal to the melt viscosity enhancement under medium shear stress conditions (10⁴ Nm^?2) in simple shear flow or in oscillatory shear flow. Submicron-size fillers, in particular, can form weak structures in the melt that greatly increase the low shear rate viscosity without changing the modulus of the solid proportionately. The highly pseudo-plastic nature of polymer melts at shear stresses of 10⁶ Nm^?2 means that, even without orientation of filler particles toward the flow direction, the viscosity enhancement is less than at lower shear stresses.     

Rheology and flow behavior of suspensions of nanosized plate‐like particles in polyester resins at the startup of shear flows; Experimental and modeling

Applications of organoclays to polymer resins are potentially of great technical interest. This research targets preparation and characterization of nanocomposites of organoclays in an unsaturated polyester resin (UP). The nanocomposites with two distinct dispersion levels were compounded in a complex multisteps mixing process at three concentrations and subsequently their microstructures were characterized by XRD. The solution‐state rheological properties of layered silicate/UP were measured in the start‐up of steady state in the simple shear mode and analyzed based on our observations of internal structure of nanoparticles dispersion. Strong shear thinning behavior caused by increasing particle alignment induced by the shear field was observed for the suspensions. Effects of nanoparticles loadings and shearing on transient viscosity of the UP resin were examined. The experimental results show that increasing shear time in nanosuspension preparation steps and hence improvements of nanoplate dispersion in the polyester resin give rise to viscosity of the suspensions. A theoretical analysis of the motion of rigid ellipsoidal particles in a Newtonian fluid based on the thermodynamic approach and Jeffery's analysis of the motion of ellipsoid particles with an interaction term was developed to determine the orientation state of disc‐shape layers and then, the expression for the extra stress tensor was calculated to predict transient viscosity and normal stress differences. It is found that the thin oblate particles have significant orientation preference in the shearing direction. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Effect of volume fraction and particle size on wall slip in flow of polymeric suspensions

For especially highly concentrated suspensions, slip at the wall is the controlling phenomenon of their rheological behavior. Upon correction for slip at the wall, concentrated suspensions were observed to have non‐Newtonian behavior. In this study, to determine the true rheological behavior of model concentrated suspensions, “multiple gap separation method” was applied using a parallel‐disk rheometer. The model suspensions studied were polymethyl methacrylate particles having average particle sizes, in the range of 37–231 μm, in hydroxyl terminated polybutadiene. The effects of particle size and solid particle volume fraction on the wall slip and the true viscosity of model concentrated suspensions were investigated. It is observed that, as the volume fraction of particles increased, the wall slip velocity and the viscosity corrected for slip effects also increased. In addition, for model suspensions in which the solid volume fraction was ≥81% of the maximum packing fraction, non‐Newtonian behavior was observed upon wall slip correction. On the other hand, as the particle size increased, the wall slip velocity was observed to increase and the true viscosity was observed to decrease. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 439–448, 2005     

Development of freeze flow apparatus to study filled polymer melts

A novel method to study the distribution of filler particles and polymer orientation of a polymer melt within a capillary die has been developed. Material within the die is quench‐cooled and then removed to provide information about the flow regime at the instant it was frozen. The equipment has been used to examine calcium carbonate‐filled high density polyethylene under high shear. The samples were examined using Energy Dispersive X‐ray Spectrometry (EDS) as well as being studied using X‐ray Diffraction (XRD). The distribution of filler particles across the radius of the capillary has been studied at high and low wall shear rates using EDS. A constant particle distribution across the radius of the die was observed for both flow regimes. The arrangement of crystalline structures within the specimens was examined by XRD. An increase in crystalline order was noticed with increasing wall shear rate. POLYM. ENG. SCI., 47:1937–1942, 2007. © 2007 Society of Plastics Engineers     

Dynamic and capillary shear rheology of natural fiber‐reinforced composites

An extended dynamic and capillary rheological study of molten flax and sisal polypropylene (PP) composites was performed. Fiber concentration varied from 20 to 50 wt% and shear rate from 0.1 rad s^?1 to 10,000 s^#142;?1. Maleic anhydride‐grafted‐PP was used as compatibilizer; it strongly reduces PP and composite viscosity. Composites are yield‐stress shear‐thinning fluids with solid‐like behavior being more pronounced at high fiber content. Composites do not obey Cox–Merz rule, which was explained by different macrostructures of the molten composites in parallel plates and capillary die geometries: random fiber orientation versus strong alignment in the flow direction, respectively. Theories describing the viscosity of suspensions of solid particles were applied to the composites studied and rheological parameters and maximal packing fiber volume fraction were calculated. POLYM. ENG. SCI., 53:2582–2593, 2013. ©2013 Society of Plastics Engineers.     

Jamming rheology of model cementitious suspensions composed of comb‐polymer stabilized magnesium oxide particles

The colloidal microstructure of concentrated suspensions containing anionic comb‐polymer‐stabilized magnesium oxide (MgO) particles in water was analyzed by shear rheometry for indications of changes in particle microstructure based on particle size and comb‐polymer usage. As the suspensions were sheared at different rates, jamming in the sheared MgO suspensions was observed as shear stress overshoots. The shear‐induced evolution of the suspension's microstructure was strongly related to the perceived interactions between neighboring MgO particles in the suspension. In the jammed state, interactions are believed to be enhanced by the formation of entanglements between opposing comb‐polymer side‐chains. Steric repulsion between side‐chains was lessened for large particles on account of their diameters, which further enabled side‐chain entanglement during close particle contact under shear. Suspensions with relatively wide particle size distributions (0.5–400 μm) were theorized to form hydrocluster aggregates, while suspensions with narrower particle size distributions (0.5–40 μm) most likely resulted in networked microstructures under the influence of the chain entanglements from the adsorbed comb‐polymer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40429.     

Relative viscosity models and their application to capillary flow data of highly filled hard‐metal carbide powder compounds

The rheological behavior of highly filled polymer systems used in powder injection molding (PIM) technology strongly influences the final properties of the products. In this study, the capillary flow data of multi‐component polymer binders—based on polyethylene, paraffin, ethylene‐based copolymers, and polyethylene glycol—compounded with three various hard‐metal carbide powders were employed. The rheology of such highly filled (up to 50 vol%) multiphase systems is necessarily a complex phenomenon characterized by strain dependent, non‐Newtonian properties complicated by flow instabilities and yield. Over 15 mathematical models proposed for highly filled systems were tested, some of them calculating the maximum filler loading. Due to the complex structure of the filler (irregular shape, particle size distribution) and a multi‐component character of the binder, the applicability of these models varied with the powder‐binder systems studied. However, the particular values of maximum loadings are in good accordance with the predictions based on powder characteristics. Simple modification of Frankel‐Acrivos model to the systems containing unimodal hard‐metal carbide powders with particles of an irregular shape and broad particle size distribution gave precise agreement between experimental data and model prediction. POLYM. COMPOS., 26:29–36, 2005. © 2004 Society of Plastics Engineers.     

Poly(phenylene sulfide) magnetic composites. II. Crystallization,thermal, and viscoelastic properties

Poly(phenylene sulfide)/ferrosoferric oxide composites (PPS/Fe₃O₄) with various loading levels were prepared by melt compounding. The crystallization, thermal, and viscoelastic properties were characterized respectively by the DSC, DMA, TGA and parallel plate rheometer. The results reveal that the well‐dispersed Fe₃O₄ particle restricts the segmental motion of polymer chain, leading to a remarkable increase of the glass transition temperature and thermal stability of the PPS due to the strong physical association and interactions between particles and matrix. The presence of the Fe₃O₄ particles, however, can not facilitate crystallization of PPS, because the particles only play the role of inert filler and have no heterogeneous nucleating effect. The crystallization kinetics, as a result, decreases with increasing particles loadings due to the increasing restriction of the chain mobility and high viscosity. In addition, the rheological percolated network of the Fe₃O₄ particles is very sensitive to the steady shear deformation, and also to the temperature. The network structure is easily broken by the steady shear flow due to the sharply reduced particle–particle interactions. However, the percolation threshold reduces with increase of temperature, because those interactions are temperature‐dependent. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

First Studies on the Rheological Behavior of Suspensions in Ionic Liquids

The suspension rheology of hematite in the ionic liquid Ecoeng^TM.212 was studied in detail and compared to the pure ionic liquid. This is the first report on the rheological behavior of suspensions in ionic liquids, and it is postulated that colloidal stability and rheology must be considered to understand these results, and to overcome limitations on the production of nanosized particles in industrial applications. Concentrated suspensions of particles in the nanometer range show non‐Newtonian flow behavior including shear thinning and shear thickening. These phenomena are mainly caused by particle‐particle interactions in the suspension, and control of these interactions is critical. The influences of temperature and solid concentration on flow behavior were shown for the pure liquid and the suspensions. It is seen that the ionic liquid follows the Arrhenius equation for non‐associating electrolytes. It is possible to shift all hematite suspension curves to a master curve according to the model of Gleißle and Baloch. Furthermore, the flow behavior of the suspensions can be modeled with the well‐known Herschel‐Bulkley plot. A 10 wt % suspension of Fe₂O₃ follows Newtonian behavior over the entire range, similar to the pure ionic liquid. It is believed that the ionic liquid has an influence on the stability of the particles, leading to a decrease of attractive particle‐particle forces.     

Model filled polymers: The effect of particle size on the rheology of filled poly(methyl methacrylate) composites

The effect of size of crosslinked monodisperse spherical polymer particles on the steady shear and dynamic rheology of filled poly(methyl methacrylate) (PMMA) composites was studied for PMMA and polystyrene (PS) particles in the range from 0.1 to 1.3 micron particle size. For PMMA matrices filled with crosslinked PS particles, reduction in filler size increases non‐Newtonian behavior. Particle size effects on the rheology of filled PMMA were much less pronounced for PMMA filler. The rate of growth of steady shear viscosity with aging time was much larger for PMMA filled with PS particles than with PMMA particles. The apparent yield stress of filled PMMA composites was estimated from Casson plots. The yield stress was negligible for PMMA filler but increased with decreasing particle size for PS filler. We suggest that PS particles are rejected by the PMMA matrix and form clusters, causing large enhancements in viscosity and moduli. Polym. Eng. Sci. 44:452–462, 2004. © 2004 Society of Plastics Engineers.     

DNP/HMX熔铸炸药的流变性能

为了解3,4-二硝基吡唑(DNP)/HMX悬浮液在不同影响因素下的流变行为,采用Brookfield R/S Plus流变仪对其流变性能进行测试,分析了HMX含量、粒度、颗粒级配、体系温度以及不同添加剂对悬浮液流变性能的影响。结果表明,DNP单质为牛顿流体,表观黏度约为16.4mPa·s,比TNT高82%,比DNAN高140%;同一剪切速率下,DNP/HMX悬浮液表观黏度随固含量的增加而增加,当HMX质量分数为30%时,悬浮液近似牛顿流体;HMX质量分数高于30%时,表观黏度随剪切速率的增加呈指数型下降的趋势愈发明显;悬浮液表观黏度随颗粒粒径的增大和温度的增加而降低,当温度从95℃升到105℃时,黏流活化能(E)从29211J/mol增至38458J/mol;固含量为60%时,平均粒径(d50)分别为16.6μm和575.6μm的HMX颗粒的最佳质量比为1∶5,此时悬浮液表观黏度最小。N-甲基-4-硝基苯胺(MNA)降低了悬浮液的表观黏度,乙酸丁酸纤维素(CAB)和微晶蜡-80(MV80)增加了悬浮液的表观黏度。     

Synthesis of high solid content waterborne polyurethanes with controllable bimodal particle size distribution

A series of waterborne polyurethane dispersions were synthesized by one‐pot reaction and step‐wise reaction, respectively. The effects of synthetic methods and DMPA content on the particle size distribution (PSD), solid contents and viscosity were studied by laser particle size analyzer, Brookfield viscometer and TEM analysis. High solid content and low viscosity waterborne polyurethanes (WPUs) with controllable bimodal PSD were prepared by one‐pot reaction using 2,2‐dimethylol propionic acid (DMPA) as the only self‐emulsifier. Meanwhile, 40% solid content WPUs with unimodal PSD were obtained by step‐wise reaction at the same formula. With the increment of DMPA content, the ratio of large particles to small particles decreased and two peaks of the particle size finally became one peak by one‐pot reaction while the PSD remained unimodal by step‐wise reaction. The reason leading to the difference of PSD between one‐pot reaction and step‐wise reaction was analyzed and the relationships among PSD, viscosity and solid content were discussed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40420.     

Dynamic mechanical properties of particulate-filled composites

The relative shear moduli of composites containing glass spheres in a rubbery matrix obey the Mooney equation, analogous to the relative viscosity of similar suspensions in Newtonian liquids. However, when the matrix is a rigid epoxy, the relative shear moduli are less than what the Mooney equation predicts but greater than what the Kerner equation predicts. Relative moduli are less for rigid matrices than for rubbery matrices because (1) the modulus of the filler is not extremely greater compared to that of the rigid matrix; (2) Poisson's ratio is less than 0.5 for a rigid matrix; (3) thermal stresses in the matrix surrounding the particles reduce the apparent modulus of the polymer matrix because of the nonlinear stress—strain behavior of the matrix. This latter effect gives rise to a temperature dependence of the relative modulus below the glass transition temperature of the polymer matrix. Formation of strong aggregates increases the shear modulus the same as viscosity is increased by aggregation. Torsion or flexure tests on specimens made by casting or by molding give incorrect low values of moduli because of a surface layer containing an excess of matrix material; this gives rise to a fictitious increase in apparent modulus as particle size decreases. The mechanical damping can be markedly changed by surface treatment of the filler particles without noticeable changes in the modulus. The Kerner equation, which is a lower bound to the shear modulus, is modified and brought into closer aggrement with the experimental data by taking into account the maximum packing fraction of the filler particles.