Adding a suitable system is frequently necessary to blend modifications amongst polymer components in order to get the desired performance improvement. In general, proper compatibility is needed when mixing different polymer components in order to create a multi-phase blend with a high binding force between phases. The following guidelines should be taken into account to better understand compatibility with application mix systems.

(1) The comparable solubility parameter principle

The interaction between molecular chains limits the mixing process between polymers, which is basically a process of mutual diffusion between molecular chains. The solubility parameter can be used to represent the magnitude of the interaction between molecule chains. The internal energy density per unit volume is equal to the square root of the solubility parameter’s value, which has the symbol. The difference in solubility parameter can also be used to gauge how compatible two components are; the closer is, the more compatible they are.
If two polymers’ solubility properties are comparable, the difference is often less than 0.2. They cannot be combined in any ratio if the difference between the solubility properties of two polymers is greater than 0.5. PVC and NBR, for instance, have strong compatibility because their solubility parameters (A and B) are 9.4–9.7 for PVC and 9.3–9.5 for NBR, respectively. Another example is the PS/PB blend system, where their Since there is a solubility parameter difference of >0.7, there is little compatibility between the two. PVC and PS have solubility characteristics that differ by >1, making them essentially incompatible.

(2) The similar polarity principle

The compatibility between polymers improves with increasing polarity of the blend system; hence, polar and polar components as well as non-polar and non-polar components all exhibit good compatibility. As an illustration, the compatibility between PVC/EVA, PVC/NBR, and PVC/ABS is good because of their similar polarity. Understanding the fundamentals of polymer compatibility—polar/polar, non-polar/non-polar, and polar/non-polar—is crucial for designing blend modification formulations. In general, polar and non-polar materials cannot be combined, as in the cases of PVC/PC, PVC/PS, PC/PS, etc.
The rule of identical polarities does have several exceptions. For instance, while PPO/PS, two components with distinct polarity, are quite compatible, PVC/chloroprene (CR) blend systems, which have similar polarities but are incompatible, are not. good.

(3) The structural similarity principle

The compatibility will be good if the structures of the polymer blend system’s components are similar. In other words, the more similar the two polymers’ structures are, the more compatible they will be. The term “structural similarity” refers to the presence of identical or similar structural units in the molecular chains of each component. For instance, there is high compatibility between the molecular chains of PA6 and PA66 since they both include -CH2-, -CO-NH-.

(4) The similar crystallization ability principle

When a blend system is a crystalline polymer, compatibility will be high if the ease of crystallization of the various components is comparable to one another. They are only compatible with mixed crystals, such as PA/PVC, PE/PA, and PET/PBT systems. The compatibility between crystalline and amorphous, crystalline and crystalline systems is quite low. Systems with two amorphous components, such as PPO and PS, PVC and NBR, PVC and EVA, etc., work well together.

(5) The similarity concept for surface tension

The compatibility of the blend system’s components is improved by the closeness of their surface tensions. When the mixture is melted, it resembles an emulsion; the stability and dispersion of the mixture are governed by the surface tension of the two components. The wetting-contact and diffusion between the two phases will be better, and the interface bonding will be better, the lower the surface tension is. For instance, polypropylene and polyethylene are particularly compatible with butadiene rubber, natural rubber, and ethylene-propylene rubber due to their similar surface tensions. This is especially true of PP/EPDM, a common toughening blend system.