Mason Delacroix
Polyvinyl alcohol (PVA) is a colorless and odorless polymer commonly supplied in bead form. The viscosity of PVA is critical to its spinnability, spinning stability, optimal spinning conditions, and fiber quality control. The apparent viscosity of PVA can be determined using various methods, including nonlinear regression with models such as Ferrys, Robertson-Stiff, Williamson, Sisko, and Ellis de Haven. Another approach involves using an Ostwald viscometer to measure the efflux time of PVA solutions of varying concentrations. Additionally, a self-made hydrothermal kettle with a built-in online viscometer can be employed to study the changes in apparent viscosity under different temperatures, concentrations, and polymerization degrees. Understanding the viscosity behavior of PVA is essential for optimizing its applications and enhancing the mechanical properties of PVA fibers.
| Characteristics | Values |
|---|---|
| Formula | [CH2CH(OH)]n |
| State | Colorless (white) and odorless |
| Stock solution | 1% PVA solution prepared by dissolving 1 g of solid powder PVA in 50 mL distilled water by boiling and diluting the solution at room temperature to 100 mL |
| Viscosity calculation | Comparing its viscosity with a standard solvent like water (1 centipoise) |
| Rheological behaviour | Non-Newtonian fluid independent of time |
| Temperature range | 20-35 °C |
| Concentration range | 4-10% |
| Viscometer | Ostwald viscometer |
| Viscometer usage | Solution is sucked through Arm 2 and clamped to a burette stand, then allowed to flow freely and the time is recorded |
| Viscosity change | Dissolution temperature affects the transparency of the solution |
| Variables affecting ηap | Top, CPVA and PD |
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What You'll Learn
Using an Ostwald viscometer
To calculate the apparent viscosity of polyvinyl alcohol (PVA) using an Ostwald viscometer, follow these steps:
Firstly, it is important to understand the basics of viscosity. Viscosity is the measure of a fluid's internal resistance to flow, or how well a fluid pours. It is a way of quantifying the density or thickness of a fluid. For example, honey has a higher viscosity than water because it flows slower and resists movement. The SI unit of viscosity is the newton-second per square meter (N·s/m2), which can also be expressed as pascal-second (Pa·s) or kilogram per meter per second (kg·m−1·s−1).
Now, onto the procedure for using an Ostwald viscometer. The viscometer consists of a U-shaped glass tube with two arms and two bulbs, labelled Bulb A and Bulb B. Below Bulb B, there is a fine capillary glass tube, and above and below this bulb, there are two markings: an upper mark C and a lower mark D.
Firstly, clean the viscometer. Add distilled water to Bulb A through Arm 1, then suck the water up into Bulb B through Arm 2, either by mouth or using a rubber bulb. Force the water downwards by blowing through Arm 2, so that it moves down through the fine capillary tube. Repeat this process several times with fresh distilled water to ensure the capillary tube is clean.
Next, clamp the clean viscometer to a burette stand. Introduce the PVA solution through Arm 1 into Bulb A. Suck the solution up into Bulb B through Arm 2, again either by mouth or with a rubber bulb. Allow the solution to flow down freely through Arm 2 and record the time it takes to flow from mark C to mark D using a stopwatch. This is known as the efflux time and is directly proportional to the apparent viscosity.
By comparing the efflux time of the PVA solution to the efflux time of a liquid of known viscosity, you can determine the viscosity of the PVA solution. It is important to ensure accurate results by carefully preparing the sample and calibrating the viscometer, as well as controlling the temperature, as viscosity is temperature-sensitive.
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Statistical treatment of shear stress and shear speed data
The statistical treatment of shear stress and shear speed data is an important aspect of understanding the rheological behaviour of polyvinyl alcohol (PVA) solutions. Rheology is the study of the deformation and flow of matter, and it plays a crucial role in characterising the viscosity of PVA solutions.
Shear stress and shear rate are fundamental concepts in rheology. Shear stress refers to the force exerted on a fluid parallel to its cross-sectional area, while shear rate measures the change in velocity of adjacent fluid layers. The ratio of shear stress to shear rate defines viscosity, which quantifies the fluid's resistance to flow.
To analyse the rheological behaviour of PVA solutions, various non-Newtonian fluid models are employed, including the Ferrys, Robertson-Stiff, Williamson, Sisko, and Ellis de Haven models. These models are evaluated using statistical techniques, such as nonlinear regression analysis, to determine their applicability to experimental data. The correlation index statistics and determination coefficients are calculated to assess the goodness of fit for each model.
Additionally, the viscosity factor (VF) is introduced as a complementary statistical indicator. It expresses the relationship between apparent viscosity and differential viscosity and helps to discern the stability of the model across different shear rates. The VF takes into account the coefficient of variation and the linearity between these two viscosity measures.
By examining the statistical behaviour of shear stress and shear rate data through these models and indicators, researchers can gain insights into the complex rheological behaviour of PVA solutions. This statistical treatment aids in understanding the flow characteristics, stability, and viscosity variations of PVA solutions across different concentrations and temperature ranges.
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The Ellis model
Overall, the Ellis model is a valuable tool for characterizing the rheological behaviour of aqueous solutions of polyvinyl alcohol, particularly in the specified concentration and temperature ranges. Its stability, reliability, and ability to adjust experimental data make it a preferred choice for studying the complex flow behaviour of PVA solutions.
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The effect of temperature
The viscosity of polyvinyl alcohol (PVA) solutions is dependent on temperature. The effect of temperature on the viscosity of PVA solutions has been studied over a range of temperatures, typically between 20°C and 80°C. At higher temperatures, the viscosity of PVA solutions decreases, while at lower temperatures, the viscosity increases. This relationship between temperature and viscosity is consistent with the behaviour of non-Newtonian fluids, which are used to model the rheological behaviour of PVA solutions.
The Ellis de Haven model has been found to be the most stable and reliable model for representing the rheology of PVA solutions over a range of temperatures from 20°C to 35°C and concentrations from 4% to 10%. This model takes into account the non-linear relationship between apparent viscosity and differential viscosity, which is influenced by temperature. The model also considers the correlation between the quotient of apparent viscosity and differential viscosity and the stability of the solution.
The Ferrys, Robertson-Stiff, Williamson, and Sisko models have also been used to study the effect of temperature on PVA solutions. However, these models have presented some irregularities in the consistency of the viscosity factor (VF). The VF is a statistical indicator that expresses the relationship between apparent and dynamic viscosity and is used to evaluate the stability of the solution.
In addition to the effect of temperature on the viscosity of PVA solutions, the molecular weight and concentration of PVA also play a role. In the low molecular weight region, the molecular weight dependence of viscosity varies with temperature and concentration. The activation energy for viscous flow is similar to that of cellulose derivatives, and the viscosity can be expressed in terms of a single function of concentration and molecular weight within a certain temperature range.
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The effect of concentration
The viscosity of polyvinyl alcohol (PVA) solutions is highly dependent on concentration. The concentration of PVA in water can vary from 1% to as high as 25% by weight. Typically, however, the concentration of PVA in aqueous solutions studied falls between 4% and 10%.
At a constant temperature, the viscosity of PVA solutions increases with concentration. This relationship is not linear, and the viscosity increases rapidly at higher concentrations. For example, the viscosity of a 15 wt% PVA solution is significantly higher than that of a 10 wt% solution.
The addition of xanthan gum (XG) to PVA solutions can also affect viscosity. XG is a non-Newtonian pseudoplastic fluid, meaning its viscosity decreases with increasing shear rate. The viscosity of PVA/XG mixtures increases with XG concentration, although the overall viscosity of the mixture is lower than that of pure PVA solutions due to XG's pseudoplastic properties.
The concentration of PVA also affects the choice of rheological model used to describe the solution's behaviour. The Ellis de Haven model is generally considered the most stable and reliable for describing the rheology of PVA solutions across all concentrations and temperatures. However, other models, such as Ferrys, Robertson-Stiff, Williamson, and Sisko, may be more appropriate in specific cases. These models are independent of time and are used to determine the relationship between apparent and dynamic viscosity.
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Frequently asked questions
Viscosity is the resistance to flow of one layer of liquid over another layer. It represents the concept of "thickness" in liquids. For example, oil has a higher viscosity than water.
The SI unit of viscosity is the newton-second per square meter (N·s/m2), also frequently expressed in the equivalent forms pascal-second (Pa·s) and kilogram per meter per second (kg·m−1·s−1).
The apparent viscosity of polyvinyl alcohol can be calculated using an Ostwald viscometer, which consists of a U-shaped glass tube with two arms and two bulbs. The time it takes for the polyvinyl alcohol solution to flow from a marked point on the upper bulb to a marked point on the lower bulb is recorded and used to calculate viscosity.
The viscosity of polyvinyl alcohol solutions depends on the concentration of the solution and the temperature. As the concentration of the polymer solution increases, the viscosity also increases. Increasing the dissolution temperature decreases the viscosity due to reduced physical interaction and increased thermal energy.
