Sunday, April 18, 2010
Polypropylene -- The Fiber that Could
Tissue scaffolds, gym socks, and potable water…items you would never think could go together are being linked through a material you use every day. The material is one of the most industrially important polymers in the world, polypropylene (PP). The applications using natural and man-made PP fibers that link these three unrelated topics are only the beginning of what can be accomplished through the changing of the surface chemistry of the original fibers.
Research at North Carolina State University (NCSU) in the department of Chemical Engineering is making headway into changing the chemistry of the fiber surfaces through a process known as “Atomic Layer Deposition”, or ALD. ALD is the process of depositing a uniform layer of a certain chemical across the entire fiber surface, or really across any surface you want it deposited on.
Chris Hanson, an undergraduate researcher in the lab of Gregory Parsons at NCSU, has been using ALD to modify the surface chemistry of the common polymer, PP, in an effort to produce a product much more valuable than the original plastic (which is commonly used for making various consumer disposable bottles).
“Nonwoven polypropylene (which is again used for consumer bottles) is incredibly cheap to produce, but the original inert surface renders the fibers not useful for specialty applications such as advanced bio-filtration. Our goal is to show that modifying the surface chemistry of PP by adding tiny layers of aluminum oxide can alter how water adheres to the surface”, said Chris. Illustrating the change in how water adheres to the fiber surfaces shows that a change is indeed occurring on the fibers.
The research was done on small squares of PP placed inside a homemade reactor (depicted in the image). In order to obtain multiple aluminum oxide layers on the same surface, a layer of aluminum oxide must be deposited followed by a chemical to make the surface suitable to accept another aluminum oxide layer. The researchers did this cyclical procedure and obtained PP samples with aluminum oxide layers ranging from zero to 100 layers, in increments of ten (where the thickest coating of 100 layers corresponds to a thickness of ~10 nanometers). A drop of water was then placed on the layers and the angle between the water and sample surface measured to determine how well the sample repelled or attracted the water.
In this study, the researchers were able to show that depositing layers of aluminum oxide on the surface of the originally “water-repelling” polymer does change the polymer to have water-attracting properties.
So why does it matter that a surface can be changed from having water-repelling to water-attracting properties?
“Once the surface chemistry of the inert, water-repelling PP has been made to attract water, it can undergo chemical reactions much more easily and can then be used to make advanced bio-filters, clothes that destroy bacteria, water filters, etc,” said Chris. When looking at the specific application to anti-microbial coatings and what is already on the market, the new feature of this technology is that it can be used to deposit a more robust set of microbe killers than what has been used before, opening up many new opportunities.
The future of using ALD to change the surface chemistry of inert materials is promising not only to researchers, but to every person in the world who will use a product born from this technology.
Peer Reviewed Literature:
Hyde, G. K. et al "Atomic Layer Deposition and Abrupt Wetting Transitions on Nonwoven Polypropylene and Woven Cotton Fabrics." Langmuir 26.4 (2010): 2550-2558.