Modifying the Mixing Polymers
Polymers are very complex materials and even when they are used as intended their properties can vary substantially. Combining chemically different types of polymers in melt mixing to form a polymer blend however increases significantly the range of possible material and property variations that can be developed. Mixing Polymers with EvenMix can be either miscible or immiscible.
Mixing of high molecular weight polymers is more difficult than for small molecules, not only because of the lower viscosity but also due to thermodynamics. The low contribution of the mixing entropy to the free energy of the system at high temperature makes it more unfavorable to mix such large molecules. This results in the majority of commercial polymer blends being immiscible and displaying the glass transition temperatures of each component of the blend (see figure 1).
Immiscible polymer combinations are typically multiphase systems that exhibit a distinct morphology. The morphology influences the mechanical properties of these materials and can be modified using compatibilizers, that improve the adhesion at the interface between the different phases of the mixture.
The morphology can be modified in many ways through the use of additives that can alter the properties, such as impact strength and thermal stability, of individual polymer components. The additives can also interact chemically with the other components of the mixture. This can result in a change in the morphology of the composite material and even the formation of new polymer structures within the composite.
One way to reduce the amount of reworking that is required to get a functional blend is to use a block copolymer of the two components of the polymer mixture. A block copolymer is a polymer made up of long segments of each polymer component joined together to form an extended chain of chains, similar to the way that a brick is formed.
Another way to improve the performance of a mixture is to add a surfactant to the solution. A surfactant is a substance that reduces the surface tension of a liquid and thus reduces the resistance to flow and shear. This can also be used to stabilize the morphology of the mixture.
The chemistry of the surfactant and its interaction with each polymer can have significant effects on the rheological behaviour of the material and its ability to disperse, particularly in the presence of water. It can also be used to control the amount of shear that is required for dispersal. It can be difficult to bring liquid products back to homogeneity, especially in EvenMix IBC totes and other open containers. Shear generating mixers, such as those found on conventional industrial mixers, often can't adequately create shear in these containers, and the product will stratify rather than being restored to homogeneity. This is a common problem encountered by distributors and on-site contractors who use portable mixers to prepare or re-mix chemicals in onsite manufacturing applications. Often, the best way to overcome this problem is to replace the traditional mixer with a EvenMix tote mixer that can achieve the desired shear.