K-Value of PVC
The K-value of PVC (Polyvinyl Chloride) refers to the polymer's inherent viscosity, which is a measure of its molecular weight. It's a crucial parameter in determining the processing and performance characteristics of PVC.
The K-value is typically determined by measuring the relative viscosity of a solution of PVC in a specific solvent under standardized conditions. The higher the K-value, the higher the molecular weight of the PVC polymer, indicating better mechanical properties and heat resistance. Pre-configured instruments availble for ISO 1628 part 2 and ASTM D1243.
The K-value can vary depending on the specific grade and formulation of PVC, but common K-values for PVC range from around 55 to 70.
RPV configurations and accessories shown below are suitable for testing K-Value of PVC.
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K-Value of PVC - ISO 1628-5, ASTM D1243
Measuring the K-value of PVC involves determining the relative viscosity of a solution of PVC in a specific solvent under standardized conditions.
The first step is to dissolve a known mass of PVC sample in a specified volume of a suitable solvent. Common solvents used for this purpose include cyclohexanone or tetrahydrofuran (THF). The concentration of the PVC solution is typically in the range of 0.2% to 0.5% by weight.
Use a suitable glass viscometer, such as an Ubbelohde or Cannon-Fenske viscometer, to measure the relative viscosity of the PVC solution. The viscometer should be calibrated prior to use. Temperature control is achieved by using a constant temperature viscometer bath.
Charge the viscometer with the PVC solution and measure the flow time of the solution through the capillary at a specific temperature. The temperature is usually maintained at 25°C (77°F) for standardization.
Calculation: Calculate the relative viscosity of the PVC solution. Then use the relative viscosity to calculate the K-value using the appropriate equation or table provided by standards organizations such as ASTM International or ISO.
The equation or table relates the relative viscosity to the K-value for PVC.
Perform measurements on multiple samples and take the average to ensure accuracy and reliability of the results.
It's essential to follow standardized procedures and adhere to safety guidelines when performing these measurements, as working with solvents and PVC solutions may involve handling hazardous materials.
Which solvent do I need?
Cyclohexanone is used for dissolving the PVC. This is also used for cleaning the sample path. The drying solvent is acetone.
What is the temperature of measurement?
25 Degrees C is the standard temperature for measuring the K-value of PVC. This is achieved in a constant temperature viscometer bath.
What are the installation requirements?
A fume hood located in an air-conditioned laboratory.
How long does it take to measure K-Value of PVC
One complete test cycle on the RPV polymer viscometer will take approximately 30 minutes. This includes cleaning and drying of the sample path.
Can I measure K-Value without using a solvent?
The short answer is no. A K-Value determination requires measuring the polymer solution in a calibrated viscometer using a constant temperature bath.