Rheological Behavior of Vinyl Ester Resin

Rheological and thermal behaviour of vinyl ester resin

The melt viscosities of three low density polyethylenes of widely varying melt indices were studied as a function of temperature over a broad range of shear rates and shear stresses. Apparent viscosities at constant shear stress could be fitted adequately to a simple Arrhenius equation over the entire temperature range studied. However, pronounced curvature of the log ηa–1/T curves was observed for apparent viscosities at constant shear rate. The apparent activation energies for viscous flow at constant shear stress were found to decrease slowly with increasing shear stress and also to decrease with decreasing molecular weight. A temperature‐shear rate superposition was demonstrated to hold, and the shift factor dependence on temperature was determined.

The viscosities of a number of monodisperse polystyrene melts have been measured using a capillary rheometer. The materials covered a molecular weight range of 43,000–460,000. Shear rates of 1.54–1540 sec.−1 and temperatures of 350–450°F. were studied. The effect of molecular weight distribution of polydisperse polystyrene was also measured. It was found that while low shear viscosity was dependent on Mw, higher shear melt viscosities depended on averages between Mw and Mn until at 1000–2000 sec.−1, Mn controlled viscosity. Agreement with the 3.4‐power dependence of zero shear viscosity was good. Similar exponential relationships were found, with higher rates of shear, corresponding to smaller values of the exponent. Constant values of the exponent were found at constant shear stress but not at constant shear rate. Agreement with the constancy of the activation energy for viscous flow for various molecular weights and distributions at constant shear stress was good. However at constant shear rate, ΔE decreased as the molecular weight average increased and as the distribution broadened. Viscosity versus shear rate master curves were constructed by using the Buehe‐Harding procedure. All monodisperse polystyrenes showed excellent fit with the master curve. Other molecular weight distributions did not. Master curves also were constructed for measurements of dynamic viscosity versus frequency for monodisperse polystyrene. These curves when compared to steady state viscosities failed to confirm the correspondence of ηa to either |η*| or to η′.

In this work, rheological and mechanical properties of acrylonitrile–butadiene–styrene/corn starch composites (ABS/starch) were studied. The composites were prepared using a laboratory-scale, single-screw extruder. Rheological properties were determined using the single-screw extruder, apparent shear rate (γ a ), apparent shear stress (τ a ), apparent viscosity (η a ), non-Newtonian index (n), and flow activation energy at a constant shear rate (E γ) and constant shear stress (E τ). Mechanical properties in terms of tensile tests were performed using Testometric M350-10KN, stress at break, strain at break, and Young's modulus were determined. Rheological results showed that the composites are pseudo plastic in behavior, and the apparent viscosity of the composites increases with increasing starch content above the additive rule, which indicates a partial compatibility in the composite. It was also found that the flow activation energy of the composite increases with increasing starch content. The mechanical results showed that the strain at break of the composite decreases sharply by the presence of starch, whereas the Young's modulus increases with increasing starch content.

The rate of breakage of duplex DNA molecules by laminar flow through a capillary has been studied. For λb2b5c DNA (molecular wt., M = 25 × 106) the point at which breakage occurs is normally distributed around the center of the molecule with a standard deviation of 12.5% of the molecular length. At constant shear stress or shear rate, the breakage rate is independent of ionic strength. Thus, shear induced local denaturation is not a rate limiting, preliminary step in breakage. In experiments at constant temperature with varying solvent viscosity (controlled by added sucrose) the breakage rate is a function of shear rate, not of shear stress. The rate of opening of hydrogenbonded circles into linear molecules by hydrodynamic shear is also shown to be a function of shear rate and not of shear stress. The breakage rate at constant shear rate is not greatly dependent on temperature. The shear rate required to achieve breakage is inversely proportional to M1,2. The breakage rate constant, k varies as a very high power of the shear rate; at 25°C, d In k/d In Gm ∼ 15; at 10°C, d In k/d In Gm ∼ 26, where Gm is the maximum shear rate at the capillary wall. The unexpected result that breakage rate is mainly dependent on shear rate, not shear stress, supports a model in which the DNA molecule is distorted with a driving force which depends on the hydrodynamic shear stress, ηG, but the rate limiting step is segment diffusion into a highly extended configuration. The characteristic time to achieve this configuration is proportional to solvent viscosity, η, hence the breakage rate is dependent on ηG/η or G, the shear rate.

A low-density polyethylene (LDPE) sample and an isotactic polypropylene (PP) sample were chemically cross-linked in molten state with dicumyl peroxide using liquid 1,2-polybutadiene (L-PB) as an auxiliary for PP. The effects of the degree of cross linking on their capillary flow properties have been studied. The apparent viscosity ηa of LDPE system at a constant shear rate rapidly increases until a gel content of 1.1 wt%, which is due to the increase in molecular weight, and after that log ηa linearly increases with the gel fraction, which is due to an action of gel particles as filler. The dependence of ηa on the gel fraction is more notable at lower shear rate. The value of ηa at a constant shear rate of PP system with an L-PB content of 5 phr drops until a gel content of about 15 wt%, and after that shows a maximum at a gel fraction of about 25 wt%, and gradually decreases with the gel content. For the system of an L-PB content of 20 phr, ηa gradually increases with the gel fraction. The end correction coefficient ν shows a maximum in a range of gel fractions from 15 to 40wt%. The flow activation energy at a constant shear rate, ΔHγ, of LDPE system decreases with the gel fraction and the flow activation energy at a constant shear stress, ΔHτ, shows a maximum at a gel fraction of about 70wt%.

A low-density polyethylene (LDPE) sample and an isotactic polypropylene (PP) sample were chemically cross linked in molten state with dicumyl peroxide using liquid 1,2-polybutadiene (L-PB) as an auxiliary for PP. The effects of the degree of cross linking on their capillary flow properties have been studied. The apparent viscosity ηa of LDPE system at a constant shear rate rapidly increases until a gel content of 1.1 wt. %, which is due to the increase in molecular weight, and after that log ηa linearly increases with the gel fraction, which is due to an action of gel particles as filler. The dependence of ηa on the gel fraction is more notable at lower shear rate. The value of ηa at a constant shear rate of PP system with an L-PB content of 5 phr drops until a gel content of about 15 wt. %, and after that shows a maximum at a gel fraction of about 25 wt. %, and gradually decreases with the gel content. For the system of an L-PB content of 20 phr, ηa gradually increases with the gel fraction. The end-correction coefficient ν shows a maximum in a range of gel fractions from 15 to 40 wt. %. The flow activation energy at a constant shear rate, ΔHγ̇, of LDPE system decreases with the gel fraction and the flow activation energy at a constant shear stress, ΔHτ, shows a maximum at a gel fraction of about 70 wt. %.

: Styrene and methacrylated fatty acid (MFA) monomers were used as reactive diluents in the vinyl ester resins. The viscosities of these resins were measured as a function of reactive diluent content and type, temperature, and vinyl ester molecular weight to determine the operating window for composite manufacture. The viscosity decreased exponentially with reactive diluent content. Styrene and MFA monomers affected the viscosity in the same manner, but not to the same extent. The viscosities of resins using both diluents were accurately predicted using a logarithmic rule of mixtures from the two-component viscosity functions. The viscosity increased exponentially and predictably as a function of the vinyl ester number average molecular weight. Increasing the temperature decreased the viscosity in an Arrhenius manner. The activation energy for viscous flow decreased linearly as the diluent content increased, but was unaffected by vinyl ester molecular weight, fatty acid chain length, and unsaturation level. Overall, the resin viscosity was modeled as simple functions of the resin temperature, vinyl ester molecular weight, styrene content, MFA content, MFA chain length, and MFA unsaturation level, which are all known quantities for a formulated resin. Therefore, the operating window for various liquid molding operations was predicted for standard DGEBA-based vinyl ester resins and low VOC/HAP resins.

Properties modification by blending polymers has been an area of immense interest. In this work, rheological and mechanical properties of poly(lactic acid)/polystyrene (PLA/PS) blends were investigated. PLA/PS blends in different ratios were prepared using a laboratory scale single screw extruder to obtain (3 mm) granules. Rheological properties were studied using a capillary rheometer and the Bagley’s correction was performed. True shear rate (γr), true shear stress (τr), and true viscosity (ηr) were determined, the relationship between true viscosity and (1/T) was studied for PLA70 blend and the flow activation energy at a constant shear stress (Eτ) and a constant shear rate (Eγ) was determined. The mechanical property measurements were performed at room temperature. Stress at break and strain at break were determined. The results showed that PLA/PS blend exhibited a typical shear-thinning behavior over the range of the studied shear rates, and the viscosity of the blend decreased with increasing PLA content. Also it was found that no equal-viscosity temperature exists between PLA and PS. The mechanical results showed immiscibility between PLA and PS in the blend.

The variation with shear of the concentration dependence of the reduced viscosity was studied in the system polystyrene‐toluene at 20, 40, and 60°C. The slope constant k′ was determined at constant shear stress k′T and at constant shear rate k′D; k′T was found to increase, k′D to decrease, with increasing shear. Alternatively, the concentration dependence was expressed in terms of Peterlin's effective viscosity. For this system the effective viscosity at constant shear stress was independent of shear but the effective viscosity at constant shear rate decreased with shear. The decrease in k′D, and in effective viscosity at constant shear rate with shear are attributed to molecular entanglement and an explanation is proposed for the observed differences in behaviour at constant shear stress and constant shear rate. In appendices a new formula for the calculation of viscosity ratios from the relative flow times is derived, and a procedure is outlined to compute intrinsic viscosities and limiting slope constants that will be free from absorption effects.

In this work, the flow behavior of polycarbonate / acrylonitrile-butadiene-styrene (PC/ABS) was investigated. The present blends with different ratios of PC and ABS were prepared by means of internal bath mixer. From the results of capillary rheometer, apparent shear rate, apparent shear stress, apparent viscosity, power law index, and flow activation energy at a constant shear rate and shear stress were determined. The results showed that the blends are pseudo plastic in behavior, and PC behaves in a relatively Newtonian manner but ABS exhibited significant shear thinning. In addition, the flow activation energy of the blends decreases with increasing shear rate while it increases with increasing shear stress.

Melt rheology and extrudate swell properties of talc filled polyethylene compounds

The rheological properties of a sodium tallow-coconut oil soap (15% water) have been determined using a high pressure capillary extrusion viscometer over shear rates of 14.7 to 2560 sec−1 and temperatures of 70–103°C. Capillary flow measurements were also made on sodium stearate (25% water) at 90°C. The data indicated shear thinning characteristics and were fitted to an equation of the form: $$\log \tau _R = \log A + n\log \dot \gamma _R $$ over the above shear rate range. The flow indices (n) of 0.337–0.437 were comparable to those obtained from polyethylene data in the literature. A zero shear activation energy of 57.2 Kcal/mole was calculated for the tallow-coconut soap. The activation energy at constant shear stress was greater than that at constant shear rate and decreased with increasing shear stress and shear rate. The soap flow unit was estimated to contain about 2 · 104 molecules.

At high shear rates a steady state of shear flow with constant shear rate, constant shear stress, and constant recoverable shear strain is observed in the short-time sandwich rheometer after some few shear units already. The melt exhibits rather high elastic shear deformations and the recovery occurs at much higher speed than it is observed in the newtonian range. The ratio of first normal stress difference and twice the shear stress, being equal to the recoverable strain in the second-order fluid limit, significantly underestimates the true elastic shear strains at high shear rates. The observed shear rate dependence of shear stress and first normal stress difference as well as of the (constrained) elastic shear strain is correctly described on the basis of a discrete relaxation time spectrum. In simple shear a stick-slip transition at the metal walls is found. Necessary for the onset of slip is a critical value of shear stress and a certain amount of elastic shear deformation or orientation of the melt.

Cardiovascular blood-recirculating devices provide life saving cardiopulmonary support to critically ill populations. Unfortunately, common device-related thrombotic complications continue to result in significant mortality, neurological morbidity, and limb loss. Past studies have shown that increased wall shear stress from blood-recirculating devices activates circulating monocytes and platelets. Less well studied is the enhancement of thrombosis by platelet-derived (PMPs) and monocyte-derived microparticles (MoMPs). These are small (0.1-1 micron) cell-derived membrane vesicles released from activated cells. Studies using a biological stimulant demonstrate that MoMPs initiate clotting whereas PMPs enhance clotting. Few if any studies have defined the contributions of device-produced microparticles (MPs) to changes in thrombotic potential. Our hypothesis is the increased dynamic or constant shear stress generated by a blood-recirculating device promotes release of prothrombotic MPs. Using an Anton Paar MCR Rheometer we exposed whole human blood in vitro to both dynamic (70 dynes/cm 2 at 0.05hz) and constant (70 and 35 dynes/cm 2 ) shear stress conditions for two minutes. After shear, MPs were collected with dual centrifugation and then quantified using a BD FACS Canto II flow cytometer with forward scatter photomultiplier tube (resolution=200nm). Then we pelleted and washed MPs to measure changes in thrombin generation with a Calibrated Automated Thrombogram (CAT). Dynamic shear stress generated 3841±1171 PMPs/μL and 3522±1090 MoMPs/μL, (MEAN±SE, n=6). Constant shear stress at 70 dynes/cm 2 and 35 dynes/cm 2 generated PMPs/μL of 4937±1655 and 1880±389 and MoMPs/μL of 6935±2029 (n=3) and 2262±1041 respectively (MEAN±SE, n=3 for each). All shear conditions generated an increase in MP concentration over static conditions of 644±120 PMPs/μL and 538 MoMPs/μL (p<0.01). Dynamic and constant sheared MPs increased the estimated thrombin potential and peak thrombin generation by at least 30% compared to static (p<0.05). This study is the first to demonstrate that device relevant constant and dynamic shear stresses can generate PMPs and MoMPs that promote thrombosis.