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Rheology
SPE Library content related to rheology
HIGH STRAIN RATE RHEOMETRY OF POLYMER MELTS RELEVANT TO MICROMOLDING AND THIN-WALLED INJECTION MOLDING
The rheology of polymer melts has been measured at strain rates up to 107 s-1, relevant to micromolding and thin walled molding processes, using an instrumented high speed injection moulding machine. Deviations from shear thinning behavior were observed for commercial grades of polyethylene, polypropylene, polystyrene and PMMA, and shear thickening behavior occurred for some of the polymers examined. Off line parallel plate rheometry and twin bore capillary rheometry were used to provide rheological data at low and medium shear strain rates respectively. 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 found, with polymers containing larger side groups reaching the rate independent plateau at lower strain rates than those with simpler structures. These results have implications for micromolding of particular polymer architectures.
THERMAL STABILIZATION OF BIODEGRADABLE POLY-HYDROXYBUTYRATE (PHB) IN MELT EXTRUSION PART 1: POLY-OLIGOMERIC-SILSESQUIOXANE
Poly-Oligomeric-Silsesquioxane POSS nano modifier was examined as a thermal stabilizer for PHB.Melt compounding of Poly-Hydroxy-Butyrate PHB copolymers with different POSS moieties was performed.Reactive and non-reactive POSS nano modifiers were used. The effect of modification on PHB thermal stability was evaluated by changes in rheology and molecular weight. POSS modifiers with unique core-shell structures were found to significantly reduce the loss in molecular weight during melt mixing possibly by decreasing viscous-heating effects.
DESIGN AND THEORETICAL ANALYSIS OF MINI-PLUNGER SPRAYER USED IN SW2502 FUSED DEPOSITION MODELING RAPID PROTOTYPING SYSTEM
The fused deposition modeling rapid prototyping system is based on the manufacturing principle of layer by layer. The sprayer is the key component. Based on the polymer rheological properties, the structure design and theoretical analysis of the mini-plunger sprayer of SW2502 fused deposition modeling system are studied. The plasticizing capacity of the mini-plunger sprayer is analyzed in theoretically. It is very important for the fused deposition modeling process to maintain a stable plasticizing capacity. The driving force of ABS feed is related to several parameters such as structure form of sprayer and material property, etc. The barrel and nozzle design should be fit for mobility, purity, and mechanical property of ABS feed. The modified ABS feed adopts self-pressure-adding driving method to decrease the pressure losses of barrel and nozzle. The plasticizing capacity of the sprayer is more than 20g/h.
MICROCELLULAR EXTRUSION FOAMING FOR LINEAR AND LONG-CHAIN-BRANCHED POLYLACTIDE
In this work, microcellular extrusion foaming for both linear and long-chain-branched (LCB) polylactide (PLA) was processed on a single-screw extrusion system with CO2 as a blowing agent. The rheological experiments were conducted on an advanced rheometric expansion system (ARES) rheometer to compare the rheological properties among linear and LCB-PLAs without blowing agent. The characterization for foamed samples shows that in comparison to the linear PLA, the LCB-PLA foams have larger volume expansion ratios, decreased cell-opening, smaller average cell sizes and higher cell densities, due to the higher viscosity, higher melt strength and higher crystallinity derived from long-chain branching.
MELT RHEOLOGY AND X-RAY ANALYSIS OF GRADIENT COPOLYMERS: SEQUENCE DESIGN FOR PROCESSIBILITY
Gradient copolymers have great versatility in terms ofsequence distribution of monomers along the polymerbackbone for control over their level of nanophaseheterogeneity and flow properties. Using a gradientcomposition rather than a block-type distribution, it ispossible to design longer chains which undergonanophase segregation at lower temperatures yet becomemore homogeneous and melt processible at accessibletemperatures. These behaviors are investigated for a rangeof block and gradient architectures using melt rheologyand small-angle x-ray scattering.
CORRELATION OF FILM CASTING BEHAVIOR WITH NON-LINEAR
RHEOLOGY FOR A SERIES OF HDPE’S WITH DIFFERENT DEGREES OF
SPARSE LONG CHAIN BRANCHING
The degree of film-width reduction or necking during
film-casting is analyzed for several metallocene-catalyzed
high density polyethylenes HDPE with varying degrees
of sparse long-chain branching LCB. It is found that the
addition of sparse LCB reduces the degree of necking to a
greater degree than broadening the MWD. Analysis of the
uniaxial extensional and dynamic shear rheology with the
pom-pom constitutive model reveals that a distribution of
branches along shorter relaxation time modes is important
in reducing necking at higher drawdown ratios.
CORRELATION OF FILM CASTING BEHAVIOR WITH NON-LINEAR RHEOLOGY FOR A SERIES OF HDPE'S WITH DIFFERENT DEGREES OF SPARSE LONG CHAIN BRANCHING
The degree of film-width reduction or necking during film-casting is analyzed for several metallocene-catalyzed high density polyethylenes, HDPE, with varying degrees of sparse long-chain branching, LCB. It is found that the addition of sparse LCB reduces the degree of necking to a greater degree than broadening the MWD. Analysis of the uniaxial extensional and dynamic shear rheology with the pom-pom constitutive model reveals that a distribution of branches along shorter relaxation time modes is important in reducing necking at higher drawdown ratios.
TRACKING PHASE SEPARATION KINETICS OF BLOCK COPOLYMER SOLUTIONS USING DYNAMIC MECHANICAL MEASUREMENTS
Rheology is used to track the phase separation
kinetics of block copolymer solutions during processing
and then modeled using the Avrami equation. Previous
studies dealt explicitly with disorder-order transitions
following shallow temperature quenches. To make this
work applicable to the processing of solution-cast films,
styrenic block copolymers in solvents of varying
selectivity are cast as thin, dilute samples and dried
isothermally so that various constant-concentration
kinetics can be tracked. Rheology proves to be useful in
tracking both rapid and slow kinetics of diblock and
triblock styrenic copolymers in toluene. It is found that
the diblock copolymer undergoes faster ordering than a
triblock copolymer of comparable molecular weight. A
competition between thermodynamic driving forces and
kinetic suppression of the phase separation exists as
concentration changes. AFM images are provided to
confirm the kinetic data and to understand the meaning
of the Avrami exponent values determined for the
different systems studied.
THE RELATIONSHIP BETWEEN TUBULAR LOW-DENSITY POLYETHYLENE (LDPE) BLOWN-FILM OPTICS AND MOLECULAR STRUCTURE
LDPE product development is accelerated by understanding the relationships between process, structure, rheology, fabrication, and end-use properties. This paper describes the data analysis of a large set of tubular LDPEs.Focus is put on film optics in relation to structure, including fabrication condition effects. It is found that haze, gloss, and clarity are largely independent in the good-optics range. Good optics is found to be related to narrow molecular weight distribution, whereas melt index and density play a secondary role. It is advised to use topology-generating models to further quantify structural features in relation to optics.
3D FORMING OF GENUINE WOOD VENEER AND IN-MOLD LAMINATION WITH WOOD-PLASTIC-COMPOSITES (WPC) IN ONE INJECTION MOLDING CYCLE
This paper covers the development of an innovative composite material together with a specialized process for 3D formed parts made of Wood-Plastic-Composites (WPC) with genuine wooden surface. Special preprocessed veneers are 3D formed and in-mold laminated with polypropylene-based WPC simultaneously. Questions relating to adhesive strength and warpage of the two components and the injection molding of WPC are analysed and discussed. Another key issue is the scientific derivation of the rheological behaviour of WPC, the optimization of the injection molding process for WPC and the analysis of the warpage between the two materials veneer and WPC
ENHANCING PROPERTIES OF PP-IMPACT COPOLYMERS BY CHEMICAL MODIFICATION
Polypropylene impact copolymers are widely used in
automotive applications. They are required to comply
with many criteria. Customers demand high-performance
materials which also exhibit good aesthetical properties.
The challenge is to balance properties as high impact
strength, good flow ability and absence of surface defects,
like tiger stripes.
It is known that peroxide modification whilst
increasing the flow ability of polypropylene impact
copolymers deteriorates the basic mechanical and
aesthetical properties.
Work was performed in which a PP-impact
copolymer was subjected to peroxide aided chain-scission
under simultaneous presence of the co-agent 1,4-
butanedioldimethacrylate (1,4-BDDMA). Results show
that samples made with 1,4- BDDMA exhibit superior
cold impact resistance and tiger stripe performance
compared to the materials made with only peroxide. In
addition, morphology, molecular weight distribution, and
rheological behaviour of the continuous and dispersed
phases of the modified PP impact copolymer were
studied.
ENHANCING PROPERTIES OF PP-IMPACT COPOLYMERS BY CHEMICAL MODIFICATION
Polypropylene impact copolymers are widely used in automotive applications. They are required to comply with many criteria. Customers demand high-performance materials which also exhibit good aesthetical properties. The challenge is to balance properties as high impact strength, good flow ability and absence of surface defects, like tiger stripes. It is known that peroxide modification whilst increasing the flow ability of polypropylene impact copolymers deteriorates the basic mechanical and aesthetical properties. Work was performed in which a PP-impact copolymer was subjected to peroxide aided chain-scission under simultaneous presence of the co-agent 1,4-butanedioldimethacrylate (1,4-BDDMA). Results show that samples made with 1,4- BDDMA exhibit superior cold impact resistance and tiger stripe performance compared to the materials made with only peroxide. In addition, morphology, molecular weight distribution, and rheological behaviour of the continuous and dispersed phases of the modified PP impact copolymer were studied.
COMPLEX THERMAL HOT-RUNNER BALANCING – A METHOD TO OPTIMIZE FILLING PATTERN AND PRODUCT QUALITY
Depending on the material and process parameters
geometrically balanced runners can show rheological
imbalances. In hot runner systems these imbalances are
handled by controlling the nozzle tempering. However this
balancing method leads to high polymer temperatures and
causes a product quality that differs from part to part.
This paper presents and discusses the preferential
polymer flow paths inside hot-runner systems and their
dependency on the plastic system as simulated and
analyzed by SIGMASOFT. It will be shown that a
complex thermal hot-runner tempering layout tweaked by
simulation reduces the imbalances and improves the
product quality at the same time.
COMPLEX THERMAL HOT-RUNNER BALANCING – A METHOD TO OPTIMIZE FILLING PATTERN AND PRODUCT QUALITY
Depending on the material and process parameters geometrically balanced runners can show rheological imbalances. In hot runner systems these imbalances are handled by controlling the nozzle tempering. However this balancing method leads to high polymer temperatures and causes a product quality that differs from part to part. This paper presents and discusses the preferential polymer flow paths inside hot-runner systems and their dependency on the plastic system, as simulated and analyzed by SIGMASOFT. It will be shown that a complex thermal hot-runner tempering layout tweaked by simulation reduces the imbalances and improves the product quality at the same time.
THE MECHANICAL PROPERTIES OF MEDIUM MOLECULAR WEIGHT HIGH DENSITY POLYETHYLENE FILMS: MAKING THE LINK BETWEEN ORIENTATION AND RHEOLOGY
This paper describes a study of the mechanical properties of films produced from five different medium molecular weight homopolymer high density polyethylene resins. The machine direction (MD) tear strength and dart impact strength of these films track together. This is due to the tendency of these films to fail during dart impact testing by tearing in the machine direction. The ratio of MD and transverse direction (TD) tear properties correlates with low frequency linear viscoelastic measurements. There is a straightforward relationship between melt index measurements and low frequency rheology for the resins in our study, which relates film properties directly to readily available resin properties and film processing conditions.
A GPC-Mx Approach Of Improving Rheology-Mwd Prediction For Polypropylene
A new GPC polydispersity parameter, MxR, was introduced in this study. The MxR value, based on the GPC-Mx concept originally proposed by Yau [1], was found to be more suitable to account for the effect of different parts of the polymer molecular weight distribution (MWD) on rheology measurements at different shear rates than do the traditional polydispersity index based on the ratio of Mz, Mw and Mn values. By using this Mx-approach on a set of polypropylene homopolymers, we show that a much improved correlation indeed exists and can be achieved between GPC measurement with the rheology polydispersity indices of PDI and ModSep. Also presented in the paper is the improved correlation between melt flow rate (MFR) and molecular weight (MW) by using this Mx approach.
TRANSIENT RHEOLOGY OF A POLYPROPYLENE MELT REINFORCED WITH LONG GLASS FIBERS
The purpose of this research is to
understand fiber orientation of long glass fibers
(> 1mm) in polymer melts and the associated
rheology in well-defined simple shear flow.
Specifically, we are interested in associating the
rheological behavior of glass fiber reinforced
polypropylene with the transient evolution of
fiber orientation in simple shear in an effort to
ultimately model fiber orientation in complex
flow. A sliding plate rheometer was designed
to measure stress growth in the startup and
cessation of steady shear flow. Results were
confirmed by independent measurements on
another sliding plate rheometer13. A fiber
orientation model that accounts for the
flexibility of long fibers, as opposed to rigid rod
models commonly used for short fibers, was
investigated and results are compared with
experimentally measured values of orientation.
The accuracy of this model, when used with the
stress tensor predictions of Lipscomb, is
evaluated by comparing against experimental
stress growth data. Samples were prepared with
random initial orientation and were sheared at
different rates. Results show that fiber
flexibility has the effect of retarding transient
fiber orientation evolution. Additionally, it is
shown that the stress growth measurements
provide results that are not fully explained by
the chosen models.
THE USE OF RHEOLOGICAL AND THERMAL FRACTIONATION METHODS FOR THE ASSESSMENT OF NUCLEATING AGENT EFFICIENCY IN POLYPROPYLENE
The increasing diversity of commercial polypropylene products in recent years, related to the increase in the production volumes of PP, has led to a high number of studies dealing with structure-specific nucleating agents (NAƒ??s). Experiments on sorbitol-based NAƒ??s suggest that the nucleation function of this group of nucleating agents is a result of hydrogen bonding and self-assembly. So far, nucleation studies have been conducted in an indiscriminate manner with little reference to the exact effect of PP structure on nucleating behaviour of NAƒ??s. However, given the variety of commercial PP structures produced today, the question on the existence of synergy between polypropylene structure and nucleating efficiency of NAƒ??s remains open. The SIST (Stepwise Isothermal Segregation Technique) has been used in this study as a fast and efficient method to characterize a group of random PPƒ??s having different structures, giving the lamellar thickness distribution. In addition, temperature sweep measurements were performed to study the effect of flow on the interaction between NA and PP. In this way, small discrepancies between different materials can be pointed out to assess the nucleating efficiency of the nucleating agent.
PROCESSING AND CHARACTERIZATION OF BLENDS OF POLY LACTIC ACID (PLA) AND SAPINDUS TRIFOLIATUS.
We investigated thermal mechanical rheological properties of a binary blend of poly lactic acid (PLA) and Sapindus trifoliatus as additive in it. The blends with different percentages of Sapindus Trifoliatus were extruded and their molded properties were examined. DSC thermograms show improvement in the crystallization. Rheological properties changed drastically with increasing percentage of Sapindus Trifoliatus. Tensile strength and modulus increased by addition of Sapindus Trifoliatus. Immiscibility was measured with thermal data. It is also observed that thermal stability of blends was lower than the pure PLA.
EFFECT OF HEAT AND SHEAR ON THE GELATINIZATION OF THERMOPLASTIC STARCH WITH VARIOUS PLASTICIZERS
Different compositions of starch/water/plasticizer slurries are prepared in a batch mixer followed by an equilibration time. The slurries were then subjected to different thermal and shear treatments. A rheological technique has been developed to track the influence of shear on the gelatinization process and the results are also supported by DSC and polarized light microscopy for static systems. Shear has a dramatic effect on gelatinization and the dependence of the onset and conclusion temperatures with slurry composition vary widely depending on the type of plasticizer used.
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