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SPE Library content related to rheology
Theoretical And Experimental Investigation Of Bubble Growth In High-Pressure Foam Injection Molding
We researched a novel simulation strategy that predicts bubble growth phenomenon tailored to high-pressure foam injection molding (HP-FIM) processes. This was done via systematic HP-FIM experiments using a visualization technique. The mathematical model that we developed was based on the well-known “cell model”. To improve the model’s robustness and accuracy, we used the Simha-Somcynsky equation of state for the PS/CO2 mixture, which in turn offers an accurate prediction of the initial bubble radius. Moreover, to capture the fluid flow and mass transport behavior during bubble growth, the transport and rheological properties (that is, its diffusion coefficient, surface tension, viscosity, and relaxation time) that were adopted in this work were functions of the temperature, the pressure, and the gas concentration. In this work, instead of solving the cavity temperature and pressure separately, the temperature and pressure profile inside the cavity were respectively simulated using MoldFlow and experimentally obtained. By inputting the initial gas concentration and the transient pressure and temperature profiles, the proposed model could accurately predict the bubble growth profile under different HP-FIM conditions. The proposed model was validated using experimental data obtained from a series of visualized HP-FIM trials. In both cases, qualitative and good quantitative agreements were achieved between the simulated and the measured bubble growth data.
Strain Hardening Of Linear Polymer Enhanced By Heat Shrinking Fibers
The strain hardening behavior of polymers has important roles in processing such as foaming, film formation, and fiber spinning. The most common method to enhance strain hardening is to introduce a long-chain branching structure on the backbone of a linear polymer, but this method is costly and challenging to tailor the behavior. We hypothesized that in situ shrinking fibers can increase the strain hardening of linear polymers, and the degree can be efficiently controlled. In this study, we show that heat-activated shrinking fibers compounded in linear polypropylene enhance strain hardening and foamability. Moreover, changing processing conditions, such as temperature, can amplify the degree of enhancement. Rheological measurements and physical foaming tests are shown to support our hypothesis.
A System For Visualizing And Measuring Stress Of Plastic Flows Under Shear Conditions
Shear stress on polymers has been shown to have a strong effect on morphological and thus mechanical properties of the final structure. In this study, an in-situ visualization system was developed to i) visualize crystal nucleation and growth with high spatial and temporal resolutions and ii) have capability to measure the local shear stress and viscosity of a saturated polymer in isolated, simple shear. The system allows for easy control of experimental parameters: applied shear strain, shear strain rate, temperature, heating/cooling rate, pressure, polymer, and saturation gas. An early verification of the shear stress measuring capability was conducted of the This visualization/measuring system provides a reliable way of determining both rheological and optical properties of plastics simulated under dynamic conditions like that of industrial plastic processes.
A New Method To Determine TF And Clash Berg Stiffness (ASTM D1043), Using A Rotational Rheometer
Thermo-rheological testing is important for the vinyl industry, as it indicates the temperature range over which a given vinyl formulation can be used in a specific application. A test that has been used for many years is described in D1043, the Clash-Berg stiffness test. The test typically consists of determining at what temperature a material will have a shear modulus of 310.3 MPa (45,000 psi) after 5 seconds of stress applied in torsion. The instrumentation that is used for this test is antiquated and has become difficult to procure. Modern rotational rheometers are well-suited for this test and can be considered as replacements for the older equipment. In this presentation, we will show test results from Clash-Berg tests on TA Instruments DHR rotational rheometer and will demonstrate the excellent correlation between results from the rotational rheometer and the torsion tester.
In-Situ PP/PET Nano-Fibrillated Composites: The Effect Of Viscosity Ratio On Fibrillation And Foaming Behavior
It is widely accepted that the manufacturing of high expansion PP foams with fine cell morphology is a challenging task due to the low melt strength and the weak rheological behavior of the linear polypropylene. In this study we present a novel method to manufacture high cell density, large expansion microcellular foam through nano-fibrilation PP/PET composites. Various studies have been conducted to improve the processability of linear PP foams. Until now, the most successful industrial approach is using the branching PP as it expressed the strain hardening response and the increased melt strength behavior. However, the commercial price of branching PP resins are still doubled or even tripled comparing with linear PP resins, which dramatically limits the branching PP’s applications. Inducing chemical cross-linking is proven to be another effective way to improve the melt strength of PP. However, the cross-linked structure causes difficulty in recycling PP resins. Furthermore, the cross-linking reaction is not evenly initiated throughout the matrix rendering non-uniform cell structure in the final foam product. Implementing inorganic/organic filler is another alternative route for enhancing the foamability. PP reinforced with those fillers has higher viscosity and better elasticity at melting state. Nonetheless, the well-recognized challenging issue is to achieve well distribution and dispersion of nano-size fibers inside the polymer matrix. Because of the large surface to volume ratio, the nano-fibers tend to agglomerate. The well-established methods usually requires complex experimental conditions and normally involves dealing with chemical hazards. By implementing nano-fibrillation technology, all above mentioned draw-backs were overcome. The nano-fibrillation technology is used to manufacture polymer-polymer fibril composite in this study. The nano-fibrillation technology can generate high aspect ratio nano-fibrils uniformly dispersed inside the polymer matrix. The processing can be briefly summarized as: (i) blending immiscible polymer matrix (A) and polymer reinforcement (B) to make polymer (B) dispersed in spherical shape (the melting temperature of polymer B should be at least 30oC higher than polymer A); (ii) applying large deformation on the polymer extrudate by either hot stretching or cold stretching; (iii) carefully choosing a temperature between the melting temperature of polymer A and polymer B to melt the composite without damaging the fibril morphology of polymer B. In this study, three kinds of PPs with different viscosity are reinforced with PET nano-fibrils via melt spinning. The study shows that the high viscosity PP is preferred to generate low diameter nano-fibrils (~200 nm) in a wide concentration range; while the diameter of fibrils in low viscosity PP decreased with raising PET concentration. The oscillatory shear behavior is studied by comparing the storage modulus (G’) and phase angle (tanδ) of the non-fibrillated and fibrillated samples. Differential scanning calorimetry and birefringence optical microscope were employed to study the crystallization kinetics of PP/PET fibril composites. The rheological properties and crystallization kinetics were significantly improved with the presence of PET fibrils. Crucially, benefit from the strengthened rheological behavior and crystallization kinetics, the batch foaming of PP/PET nano-fibril composite is able to product a high cell density polymer foams.
Rheological characterization of fibril-reinforced fiber-based polymer blend
Introducing a fibrillar morphology into a polymer blend enhances the adsorption and mechanical strength of the resultant composite.
SPE Applied Rheology Division April 2018 Newsletter
Read the latest issue of the SPE Applied Rheology Division newsletter.
Poly(hydroxamic acid) hydrogels for the removal of dyes from aqueous solutions
The crosslinker used in the preparation of a poly(hydroxamic acid) hydrogel has a greater effect on its dye-sorbent properties (i.e., the removal efficiency) than does the type of dye.
Rheology of polyacrylamide/layered double hydroxide nanocomposites
The viscoelastic properties and stability of hybrid gels, synthesized via in situ polymerization, are improved upon addition of magnesium-aluminum layered double hydroxide.
Transient modeling of viscosity
The viscoelastic behavior of polymer melt is a significant factor during rheological characterization and should be taken into account during analysis and viscosity modeling.
High-efficiency hot embossing for polypropylene
A new plate-to-plate isothermal hot embossing method with a low cycle time enables highly uniform microstructures to be introduced in semicrystalline polymers for use in practical devices.
Real-time viscosity monitoring during graft copolymerization of gelatinized starch
A combined stirrer-torque meter provides a novel and effective way to obtain continuous, in-line measurements of reaction mixtures.
Polypropylene/cotton stalk biocomposites with enhanced characteristics
The influence of cotton stalk concentration, chemical treatment, and presence of a compatibilizer on the mechanical, rheological, and morphological properties of samples is investigated.
Novel indicator of weld line strength in extruded parts
A combined numerical and experimental approach provides an improved understanding of the correlation between flow conditions and the strength of weld lines.
Enhanced cellulose nanofibril/polypropylene composites for 3D printing
Novel composites containing natural spray-dried nanofibers have improved rheological properties over conventional filled polypropylene composites and may be suitable for fused filament fabrication.
Effects of Processing Parameters on Colour Variation and Pigment Dispersion during the Compounding in Polycarbonate Grades
The objective of this work is to study the variations of how independent processing parameters such as temperature, speed, and feed rate affect the dependent responses for consistent output colour (L*, a*, b*, dE*). In this study, the compounded material was processed on an intermeshing twin-screw extruder (TSE) and injection molded to evaluate their effect on the colour stability, rheology and dispersion of the polycarbonate resins. Focus was extended to the interaction of the speed, which correlates to the dispersion and colour changes.
Influence of Rheology on Part Dimensions and Production in Injection Molding
The present work was conducted to assess the influence of polymer viscosity variation from batch to batch on the part dimensions and production interruptions. The results show however that parameters such as mold temperature, barrel temperature profile and holding pressure have much more influence on these two production quality indicators than the polymer viscosity.
Creep and Recovery of Polylactic Acid and its Clay Nano-Composite
This work focuses on exploring the long-term rheological behavior of polylactic acid and its nano-composites containing 3% clay. Creep and recovery experiments were performed at 160°C for the neat PLA and its composite. Zero-shear rate viscosity was determined and used to determine the terminal relaxation time. Also, the continuous retardation spectra were calculated from the creep data and found to be consistent with those from the oscillatory data.
Synergistic Effects of Antioxidants (AO) on Properties of Homo Polypropylene
Antioxidants (AO) are used to protect the polymer from deterioration either during extrusion or after production. Probably two types of Antioxidants are used to protect polymer, primary antioxidant (phenol type) and secondary antioxidant (phosphate type). The performance ratio of both the antioxidants is depend on its process technology, processing conditions and its application. To understand the effect of each antioxidants on HomoPP, different samples at different AO dosage were extruded. Relevant product properties such as mechanical, physical and rheology were measured. The test result obtained elucidates that the polymer recipe is a balance of additives to meet the requisite end product properties under the employed processing conditions.
Effect of Screw Speed on Polyethylene-Calcium Carbonate Composites Produced Using Twin and Quad Screw Extruders
The effect of ultra high screw speed on mixing was investigated using polyethylene microcomposites with 1 wt% calcium carbonate compounded on novel twin and quad screw extruders. The screws had similar designs and the screw speeds were 300 to 2000 rpm. Extruder type influenced the effects screw speed had on extruder residence time, melt temperature, drive torque, and head pressure. Parallel plate rheology indicated significant chain scission of the polymers and better filler dispersion at higher screw speeds of 900 and 1500 rpm, especially with the quad screw extruder. In the quad screw extruder, the lower melt temperatures and greater shear allowed better mixing at higher screw speeds than the twin screw extruder. The level of mixing in the quad screw extruder also depended on resin viscosity.
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