Polymer Blends

There are different types of homopolymers with different properties. Polymer blending is the process that creates new materials with enhanced properties of homopolymers. A polymer blend is a member of a class of materials that consists of at least two polymers that are mixed together and processed to create a new material with different properties.

Reasons for blending:

• Develop broad property range materials
• Dilute high-cost engineering resin with low-cost polymers
• Some polymers give a synergistic effect of properties when blending. (High-performance blends)
• Adjust composition to suit the customer
• Recycle industrial/municipal plastics scrap

Polymer blends combine the properties of their components, and they can also possess unique properties that cannot be achieved by the use of individual polymers only. The major advantage of the polymer blends is the possibility to control their end-use properties over a wide range, according to the requirements for specific applications. As multiphase materials, the properties of polymer blends are considerably influenced by their phase structure.

The polymer blend can be either miscible or immiscible. Miscible blends do not show any phase separation. But it can have micro-phase separation. The most common type is immiscible blends. They clearly show a phase separation. However, there is no chemical reaction between the two phases.

Examples of polymer blends

• Polyphenylene oxide (PPO) and polystyrene (PS)
• Polyethylene terephthalate (PET) and Polybutylene terephthalate (PBT)
• Polymethylmethacrylate (PMMA) and polyvinylidene fluoride (PVDF)

There are five main types of polymer blends:

1. Thermoplastic–thermoplastic blends
2. Thermoplastic–thermosetting blends
3. Thermoplastic–rubber blends
4. Thermosetting blends-rubber blends
5. Polymer–filler blends

Blending is a cost-effective method of developing polymeric materials that have versatility for commercial applications.

Methods of Blending

Polymers cannot be easily mixed together and obtain a homogeneous mixture although both polymers are polar or both are non-polar. Because polymer molecules are long-chain molecules with high molecular mass and those molecules are highly entangled. In the polymer industry, various methods are used to obtain homogeneous polymer blends.

• Mechanical mixing – this is the most common and cheapest method for blending. High shear force is applied to form a homogeneous polymer blend. Mechanical mixing results in fine dispersion with droplet diameters less than 1µm. Different types of blenders such as two roll mills and internal mixtures are used for mechanical mixing.

• Dissolution in co-solvent – both polymers are dissolved in a solvent in which both polymers are soluble. This polymer solvent can be used for film casting, freezing, or spray drying.

• Melt processing – both polymers are heated up to their melting point. At the melt, state polymers can be easily mixed together. But this method can only be used for polymers with nearly the same melting points.

• Latex blending – polymers are physically blended when they are in a latex state.

• Interpenetrating network – Here, the monomer or monomers of one polymer are used as the solvent for another polymer. The second polymer should dissolve in the monomer of other polymers. After dissolving polymerization is taken place. This creates an interpenetrating network of two polymers and generally, two polymers are un-separable after polymerization.

Morphology of Polymer Blends

The morphology of the polymer blends defines the shape and organization on a scale above the atomic level and the manner in which they are organized into more complex units.  The morphology of a polymer blend is determined by many factors which can be either material parameters or processing conditions.

Material parameters

• Blend composition
• Viscosity ratio
• Elasticity ratio
• Interfacial tension
• Thermal property differences

Processing conditions

• Temperature
• Time
• Intensity of mixing
• Nature of the flow
• Shear rate

To get co-continuous morphology for a blend, the ratio of the torque measured in an internal mixture for an individual component should be equal to the volume fraction ratio of the individual component.

• T_A - Torque value of componant A
• T_B - Torque value of componant B
• φ_A - Volume fraction of componant A
• φ_B - Volume fraction of componant B

Morphology Development During Melt Processing

The majority of polymer blends are produced by melt-mixing from powders or pellets of pure components. In the initial stage of the process, millimeter-sized solid particles (pellets) are

heated and sheared, so that the material melts and the particle size decreases into the micro- or even nano-range. The pellets in contact with the hot walls of a processing device (Ex: Extruder) are exposed to high shear stresses and disintegrate into thin sheets or ribbons. With prolonged shear stresses, holes are formed in the sheet structure (lacey structure). These lacey structures then break up into irregularly shaped particles, which in turn break up further or relax into near-spherical particles.

This mechanism leads to a rapid decrease in the dispersed particle sizes during the first few minutes of mixing.  At this stage, If the minor component softens earlier. So, it initially forms a continuous phase. Then the major component becomes liquid and the structure of the minor component disintegrates. That is a phase inversion takes place so that the major component forms a matrix surrounding the dispersed particles of the minor phase. The phase inversion time depends on the viscosity ratio of major and minor components.

• p - Viscosity ratio
• η_A - Viscosity of component A
• η_B - Viscosity of component B

For blends with p >0.1, the phase inversion occurs within the first minute of mixing. For blends with p <0.1, the time-to-phase inversion increases with the decreasing viscosity ratio

• E.g: for a blend with p = 0.003 the time taken was more than 10 min.

After the initial stage of mixing the domain size decreases only slightly such that, after some time (usually 5–10min), the phase structure no longer shows any changes. After this time, it is known as steady-state mixing.

The morphology development during mixing is a result of the competition between droplet deformation and break-up on one side, and droplet coalescence on the other side. However final morphology is a consequence of complex relationships between the viscosity and elasticity of the components, the processing conditions (type of deformation, deformation rate, time of deformation), the chemical structure of the components, and the blend composition.     

The deformation of a liquid droplet dispersed in a liquid matrix under a flow can be described using the capillary number and viscosity ratio.

Capillary number (Ca) is the ratio of the hydrodynamic stress applied to the droplet which forces the drop to deform, and the interfacial stress which tends to preserve the spherical shape of the droplet in order to minimize its surface energy.

• Ca - Capillary number
• η_m - matrix viscosity
• γ - shear rate
• d - particle diameter
• Γ_AB - interfacial tension between phases A and B

If the hydrodynamic stress is sufficiently small, Ca_Cr exceeds the capillary number, Ca of the droplets is deformed into a shape that is stable in the flow.

Ca < Ca_Cr

If the hydrodynamic stress is sufficiently high, the capillary number exceeds a certain value known as the critical capillary number (Ca_Cr). In this system, a stable shape can persist in the flow, and the droplet deforms continually until it breaks into daughter droplets. This process can continue until the droplets are so small that the interfacial stress overrules the hydrodynamic stress.

Ca > Ca_Cr

At very high values of Ca (Ca>> Ca_Cr) a quick deformation of the droplets occurs and long cylindrical threads are formed. These fibers are very stable in the flow and decay only at very high deformations.