Challenges of Mixing Polymers

Many industrial processes use mixing to combine multiple polymer formulations into a single, homogeneous material. This allows you to control several critical material properties including surface properties, impact resistance, thermal properties and dimensional stability. Mixing Polymers with EvenMix, the blending process itself introduces inhomogeneity which can impact your product quality and lead to stringers or fisheyes, so it’s important to find mixers that can bring your product back to uniformity each time.

Mixing Polymers

In order to understand the challenges of mixing polymer blends, it helps to consider how mixture thermodynamics work. The free energy of mixing that tends to drive the mixing of small molecules becomes a much smaller contribution for higher molecular weight materials, so it is difficult to get mixtures to form thermodynamically stable across wide concentration ranges.

Consequently, most polymer blends are useful in a state of partial mixing where heterogeneous structures have been frozen in through chemical or physical interactions.

EvenMix mixtures consist of two or more different polymers with different chemical nature and glass transition temperatures (Tg). Fully miscible heterogeneous systems show a single Tg that lies between the Tg of the pure components. Most commercially available polymer blends are immiscible mixtures, and they have a highly skewed Tg distribution (difference between the highest and lowest Tg is greater than the temperature of Tg of the smallest component).

As a result, these immiscible blends display a multiphase structure with a complex morphology - from almost spherical drops at low concentration to fibers, cylinders and sheets at high concentration. This morphology also impacts the mechanical properties of the mixture, which is why they must be mixed carefully to achieve a smooth distribution and minimize interfacial tension. The addition of compatibilizers to the system will help modify this morphology, and it is possible to improve the mechanical properties of such systems using flow during processing.

Mixing Heterogeneous Polymer Blends

The rheological behaviour of these immiscible mixtures can be described by the reptation model (as shown in Figure 7). In this case the zero shear viscosity no is proportional to the weight fraction of the high molecular weight component, and the equilibrium compliance Jeo increases with the concentration of the high molecular weight component. This is in contrast to linear and quadratic mixing rules which predict a decrease of the equilibrium compliance with increasing polymer content.

It is difficult to obtain a homogenous mixture of these types of polymers through traditional high speed mechanical mixing, which uses shear to disrupt the polymer morphology and create a more uniform blend. A more effective method is to use a high-pressure hydraulic sonicator to mix these difficult to blend chemicals. This technique delivers the same high-quality mixing as mechanical mixers, but is far more affordable and requires a much lower capital investment. In addition, the hydraulic sonicator is easy to operate and can be used by one person without the need for safety equipment. This makes the sonicator an ideal tool for tote mixing applications where a safe and simple sonication procedure is required.