Nanocomposites for better barriers

Materials World magazine
5 Feb 2012

A novel nanocomposite could improve the mechanical and thermal
properties and decrease the oxygen permeability of barrier packaging.

A team from the Department of Food Science and Engineering, at Ewha Womans (sic) University in Seoul, South Korea, developed the material based on polypropylene (PP) montmorillonite clay (MMT) and maleated PP (MAPP), which is said to increase the tensile and flexural strength of neat PP by up to 30% and 10%, respectively, in comparison with neat polymer and conventional composites.

Research team members Sun-Young Lee and Myong-Soo Chung explain, ‘Composites can be tailored to have the desired properties by incorporating particulate fillers into a polymer matrix to suit different applications. In particular, the smectite group of clay minerals, such as MMT, has been predominantly used due to its excellent intercalation ability.’

They claim that using a layered silicate MMT increases the contact area between the MMT and the polymer during compounding, which increases the reinforcing effect of the MMT compared to existing composites. ‘Nanocomposites produced using MMT have a smaller amount of fillers than existing composites, while maintaining their toughness.’

The process involves ball milling the MMT at 500rpm for 24 hours, before it is dried in a vacuum oven at 90ºC for a further 24 hours. Maleated PP was used as the compatibiliser.

The nanocomposites with different proportions of the PP, clay (MMT), and compatibiliser (MAPP) were prepared using one-step blending and melt compounding in a twin extruder under processing temperatures of 180-200ºC, with screw speeds of 100-160rpm. The prepared pellets were then dried at 90ºC for 24 hours in a vacuum oven to remove the absorbed water prior to manufacturing the films.

The team says, ‘Through enhanced compatibility between neat PP and clay, the polymer chains can be well-diffused into the clay layers, and the d-spacing (the distance between planes of atoms) of clay layers might be increased. Therefore, the strength properties of the composites and dispersion of clay improve the polymer. This means that the barrier properties will decrease oxygen penetration into the packaging.’

The researchers say the material is cost-effective, though the oxygen permeability of the films was lower than expected, and the intercalation and exfoliation of MMT in the PP matrix need to be improved. The technology could be used for many applications, including rice containers, though it has only been tested at a laboratory scale. At present, the team is looking to find a better dispersing agent than MAPP.

Commenting on the technology, Professor Julian Evans, of UCL Centre for Materials Research, UK, says, ‘This development needs to be seen in the context of an entire new class of composite materials that was initiated in 1985 at the Toyota Motor Corporation laboratories. Within just a few years, its nylonmontmorillonite composites were deployed in timing belt cases, making this possibly one of the shortest technical lead times for a new material class. Subsequently, the academic community began to explore other polymers such as the PP described in this example.

As this work demonstrates, apart from improving modulus and heat distortion properties, the clay reinforcement, provided it is substantially exfoliated in the polymer, reduces the diffusion of small molecules. It does this in two ways: it places a tortuous diffusion path in the way of the diffusant and it immobilises a lot of polymer on the clay surface by adsorption.

Polypropylene, of course, will not intercalate into a hydrophilic clay, so two things are needed: The clay has to be modified with a quaternary ammonium compound, and the polymer has to be modified with maleic anhydride. The impact strength of polymers nearly always falls when a filler is added. The drop in these materials is quite small, being only 11% in one case.

Of course, the modifications to both constituents raise questions about approval as a food contact layer, but the material might be deployable in a laminate as the permeation barrier.’