By Penny Carmichael, on 25 February 2013
– Article by Abigail Mountain
Silicones have had a lot of media attention in the last year owing to the PIP implant scandal. We’ll return to this issue later but first let’s talk about the other uses of silicones and why they’re cropping up everywhere. As I’m a chemist, my mum thought it was apt to supply me with loads of silicone baking ware in the past few years, hoping to rumble my inner-geek. She was successful. Indeed, many kitchen utensils are now being made of the stuff. But why? And where else can we find silicones? Mike Southon from Basildon Chemicals gave this week’s CPS lecture, which gave a welcome and interesting insight into these versatile compounds.
Of course, the multitude of uses of silicones is related to their structures and properties. The most basic silicone has trimethylsilyl end-groups and a repeating middle unit consisting of a (…Si-O-Si-O…) backbone with methyl constituents attached to the Si atoms. The way in which these are units are put together governs their chemical and physical properties, and thus their uses. Silicone oils – long chains of middle units – are so-called due to their hydrophobic and lubricating characteristics. They are very commonly found in conditioners and cosmetics due to these properties. The reason they have these properties are as follows: there is lots of space along the zigzagging backbone, allowing the hydrophobic methyl group to orientate themselves on one side, leaving the hydrophilic Si-O chain atoms on the other. When the chains coil up the methyl groups remain on the outside, resulting in a hydrophobic oily molecule. The Si-O bonds have a very low energy barrier to rotation, meaning the molecule is easily deformed and allows it to be a fantastic lubricant.
Cyclic silicones have low boiling points and are therefore used in many deodorants and suncreams. They’re still oily, so still feel nice to slather on, but quickly evaporate without feeling cold to leave just the active ingredients. This is the only difference between the cyclic and oils, the rest of the following properties are shared. Their high water repellency lets them be used in waterproof jacket coatings, rising damp preventatives, medicine bottle coatings, and many many more. Their antifoaming ability was very nicely demonstrated by dropping a tiny amount of one such silicone into a large beaker of bright yellow foam which almost instantaneously dropped down to just a meniscus. This is down to the spreading properties of the silicone once it has entered into the bubble film. Such antifoams are used in the beer industry, antibiotic delivery systems and in vegetable processing.
But those are just the basic silicones. Functional silicones can have anything you like added to the backbone. Platinum is the catalyst used to functionalise the building blocks. Polyethers make them hydrophilic, leading to personal care applications; amino groups make them stick to surfaces; alkyl groups let them be overpainted. Taking the functional silicone and your regular basic silicone, you can equilibrate them, giving you a silicone with properties tunable to how much of each you add. Another great demo showed two colourless functional silicones reacting before our eyes to give a thick yellowy (Pt catalyst changing oxidation state) rubber. Pretty cool. Silicone rubbers, of baking ware fame, are cross-linked functional silicones. Non-stick and a thermal stability up to around 200°C make them perfect for such uses. A plethora of other functional compounds exist with a wide range of uses, too many to go in to here.
It’s fair to say that silicones are a very interesting and widely used groups of materials that I think will only become utilised in more and more applications in the future. They are highly biocompatible, which brings me back to the PIP implants. This story has probably given silicones a bad name. And fair enough, it’s put a lot of women through a horrible experience. It must be clear, however, that it was the way in which the silicones used in the PIP implants were made that made them so dangerous, not the silicone gel itself. They consisted of industrial grade silicone which still contains many impurities, including the transition metal catalyst and organic side-products. It is these which pose a serious risk to the women’s health if the implant casing were to rupture. So no hating on silicones please. I think they’re pretty cool.