Mixing Polymers with EvenMix
Polymers are often mixed together to obtain new material properties not available from a single component. Typical reasons include cost effectiveness and improved processing performance. Some examples are combinations of crystalline and amorphous materials to provide transparency, stiffness, and dimensional stability; or a thermoplastic with an elastomer for high resilience and toughness. Mixing Polymers with EvenMix can also be formulated to meet specific mechanical or chemical performance criteria.
The rheology of complex polymer mixtures can be studied using a variety of rheological testing equipment including parallel plate, steady-state, oscillatory, capillary, and elongational tests. The rheological behavior observed is then compared to theoretical, numerical or empirical model predictions. The results from these studies are used to guide the design of mixing and processing equipment for polymer blends as well as to optimize existing systems.
Typical commercially available industrial polymers are composed of many individual molecules. These molecules have distinct morphologies which are regulated by thermodynamics and flow-imposed morphology. Depending on the specific combination of polymers and morphologies, the resulting mixture can be miscible or immiscible. Immiscible mixtures are typically characterized by small spheres of the minor component dispersed in a matrix of the major component. When these mixtures are under load they exhibit low stress relaxation times and lower tensile strength compared to samples of the pure components.
To increase the processability of immiscible polymer blends, compatibilizers can be added to the formulation. These chemicals are typically surfactants or other agents with hydrophobic and hydrophilic regions that can interact with the interfaces of the two phases and decrease interfacial tension. Compatibilizers can also have an effect on the morphology of immiscible blends. Adding a small amount of compatibilizer to the blend can cause the spheres of the minor component to grow larger, which reduces the number of contact points between them and increases the mechanical properties of the blend.
There are a variety of methods for preparing and sorting immiscible polymer blends for recovery and reuse. One common method involves bringing the plastic mixture into contact with supercritical carbon dioxide at a certain temperature and pressure, followed by gravity separation. Other methods use dielectric heating, fluidized bed sorting tanks, or specific gravity separation.
When blending complex polymer systems, the key to success is understanding that a complete and uniform distribution of all of the individual polymer particles is required to achieve good rheological properties. Achieving this goal requires careful selection of mixing equipment and a thorough knowledge of the physical characteristics of the polymer blend, which includes characterization using rheological test equipment such as parallel plate, rotational, steady-state, oscillatory, and capillary.
In addition to the rheological characterization, the characterization of physical properties such as density, viscosity, tensile strength and impact resistance is required for accurate and successful design of mixing equipment. These measurements are used to help select mixers, dewatering equipment, and processing equipment that can be optimized for the desired rheological response of the blend. This is particularly important when blending high molecular weight industrial polymers such as those used in the manufacture of tires, automobiles, packaging, and construction materials.