Evaluating Thermal Oxidation and Rheological Drift in Recycled Polyolefins
Tracking melt flow changes and oxidation time prevents part failure from degraded recycled polyolefin resins.
Pressure Filter Value
Meaning
Quantitative measurements of the pressure increase recorded across a fine mesh screen during the extrusion of a polymer melt reveal the concentration of solid contaminants. Pressure filter value is a critical quality metric used in the plastics industry, particularly for recycled materials and masterbatches. This test involves pumping a specific amount of molten resin through a standardized filter pack and monitoring the pressure before the filter.
As the contaminants in the melt clog the screen, the pressure rises. The final value is a direct measure of the cleanliness of the material. A low value indicates a clean resin that is suitable for demanding applications like thin films or fibers.
A high value indicates a contaminated material that may cause defects or processing issues.
Melt Cleanliness
Identifying the presence of even small amounts of undissolved material is essential for ensuring the quality of the final product. In the measurement of pressure filter value, the solid contaminants can include degraded polymer, unmelted particles, metal fragments or organic waste. These contaminants are often too small to be seen with the naked eye but can cause significant problems during downstream processing.
For example, in the production of fine fibers, a single large particle can cause the fiber to break, leading to downtime and scrap. By measuring the cleanliness of the melt before it is used, manufacturers can ensure that their products meet the required specifications. This is particularly important for recycled plastics, which often contain higher levels of impurities.
Screen Performance
Choosing the right size and type of filter for the test is necessary for obtaining accurate and comparable results. In the calculation of pressure filter value, the mesh size of the screen determines the size of the particles that will be trapped. Standardized test methods specify the screen configuration and the flow rate of the polymer.
The results are often expressed as the pressure increase per gram of material. The use of multiple layers of different mesh sizes can help to characterize the size distribution of the contaminants. Regular replacement of the screens is required to ensure that the test is performed under consistent conditions.
The performance of the screen is also influenced by the viscosity of the polymer and the temperature of the test.
Recycle Grade
Categorizing materials based on their cleanliness allows for their appropriate use in different applications. Pressure filter value is a key parameter used to define the quality of recycled resins. A high grade recycle will have a value that is comparable to virgin resin, allowing it to be used in high value applications.
A low grade recycle may have a very high value, limiting its use to thick walled parts where the contaminants will not cause structural or cosmetic issues. This classification helps to create a transparent market for recycled plastics and encourages the development of better cleaning and sorting technologies. By providing a clear and objective measure of quality, the test helps to build trust between the suppliers and the users of recycled materials.