The influence of phage modification on the impact behaviour and dispersion characteristics of calcium carbonate / polypropylene nanocomposites

The manufacture of nanocomposites today is limited by their nano constituents, which limits the overall performance of the nanocomposite. The main limiting factors are the size, shape, and dispersion of the nano constituent. Also, when working with nanoparticles, agglomeration of particles tends to occur. This agglomeration hinders the nanocomposite from achieving its potential improvements in properties such as toughness.

In this research, calcium carbonate (CaCO₃) was added to polypropylene (PP) to improve the toughness of the PP for use in protection applications. The CaCO₃ was modified by including the bacteriophage T4 and M13 to keep the particles dispersed and further improve the toughness of PP. This technology was then transferred from PP to polycarbonate (PC). 

The interaction between CaCO₃ nanoparticles and bacteriophages were studied to help with the dispersion of the nanoparticles. Bacteriophages are hydrophilic and have a positive charged tail group that can interact with the surface of the CaCO₃ particle and offer electrostatic charge repulsion preventing agglomeration. This provides the opportunity to keep the particles dispersed after mechanical methods like attrition ball milling or planetary ball milling are used to breakdown agglomerates that may have been formed in the blank CaCO₃. 

The bacteriophages were cultured on a large scale (2L) in the lab and plaque assays were conducted to determine that the concentration was 4 × 10¹⁴  PFU/ml for T4 phage and 4 × 10¹³  PFU/ml for M13 phage. They were cultured at 37 °C as this is the optimal temperature to culture both M13 and T4.

The phage dispersed CaCO₃ in PP was shown to have a 50% improvement in impact strength compared to the blank CaCO₃ in PP. Both were shown to have an improvement in impact strength in comparison to the unmodified PP. Optical microscopy suggests that when studying the crack patterns of the polymer, both brittle and ductile failure has occurred. With the PP phage dispersed CaCO₃ composite, a standard Hertzian ring has formed with more energy being absorbed. 

SEM microscopy also showed that long and short pull outs can be seen as a form of energy transfer  during fracture in both modified and unmodified PP. It is the plastic void growth in the modified PP that changed the energy absorbing capability of the composite. The presence of the micro pull outs also in only the phage dispersed modified PP has led to further improvements in the toughness of the composite. At higher strain rates, the CaCO₃ reinforcement increases the yield stress of the PP by approximately 25 – 44%.

When transferring the technology into PC nanocomposite manufacturing, more modifications need to be made to the drying conditions for the process for the method to be successful. However, the sample retains its transparency which makes it promising for use in transparent applications.