Date:31 December 2006
The ancient material that’s still an agent of change
Glass. What a confusing material. All our senses tell us it is a solid – it feels like a solid, has a fixed shape like a solid and certainly breaks like a solid – but the scientific community tells us it is a liquid. How in Newton’s universe can something so brittle and blatantly solid to the touch possibly be a liquid?
The answer lies in its molecular structure. Whereas solids such as ice tend to have a lattice-like molecular structure, glass is more like water in that it has an unpredictable, amorphous molecular structure – the result of a molten liquid cooling to the point of rigidity faster than the molecules can arrange themselves in an orderly fashion.
In fact, it is this rigid, amorphous molecular structure, rather than the nature of the molecules themselves, that defines a material as glass. What is defined as glass can consist of any number of substances that behave in the same way, although the glass we are most familiar with is made of silicon dioxide – basically, quartz sand – with a few additives to change certain properties. Soda ash, for example, lowers the melting point of silicon dioxide from 1 725 C to about 1 500 C, while iron oxide lends it the green tinge typical of wine bottles.
The “solidness” of glass is actually the result of its extreme viscosity at room temperature. Internal friction keeps glass molecules firmly in place, making this a liquid that simply does not flow – not even over centuries, despite the myth that this is why glass panes in ancient cathedrals are thicker at the bottom than at the top.
It is true that the panes are thinner at the top, but it would take more than 1032 years for cathedral glass to flow enough to be discernable, according to calculations by physicist Edgar Zanotto published in . As such, mediaeval glassmaking and building techniques are more likely causes for the differential in glass thickness.
Even though glass does not flow, it is observably a liquid in one important way: it is transparent. Solids, with their ordered molecular lattice, tend to be opaque because they absorb or reflect light. The amorphous molecular structure of glass, however, lets light pass straight through – like most liquids. The result is a material that we can use in our homes and cars to provide shelter from the elements without limiting visibility of the outside world.
But transparency comes at a price. Glass is fragile stuff, the result of millions of microscopic surface flaws that weaken it. All materials have these surface flaws, but in solids the molecular lattice contains them to within one or two layers of atoms. In glass, however, the liquid-like amorphous structure allows these surface flaws to form fault lines that run throughout the material – so the same property that makes glass transparent makes it brittle.
“If it weren’t for these microcracks, glass would be five times stronger than steel, thickness for thickness,” says Dr Johann van der Merwe, an independent technical glass consultant. “As it is, the cracks weaken glass to the point where it is, in practice, only about a hundredth as strong as it would be in its purest form.”
The quest for perfection
Since manufacturing a flawless pane of glass is at best a theoretical possibility, manufacturers have had to find other ways to make glass safer for architectural, motoring and aeronautical use.
One way is to toughen it. Toughening was first used to make boiler glass in 1884 and involves heating glass to temperatures in excess of 620 C and then air-cooling it on both sides. This allows the molecules in outer layers to form rigid bonds while the inner core is still in an expanded state. As this heated inner core cools, it contracts, pulling the frozen outer bonds into a state of compression. It is the compression on the outer bonds that gives toughened glass its strength.
When toughened glass breaks, it shatters into thousands of tiny shards – the pattern most people associate with “shatterproof” glass – which is both useful and problematic: useful because the glass pieces are too small to cause serious injury, and problematic because these tiny shards turn the glass opaque. As a result, toughened glass is good for applications such as shower doors, glass tables, and the side windows of cars – but not for places where visibility after breakage is an issue. It has been outlawed for use in windscreens since the early 1970s.
These days, lamination is used to make windscreens safer rather than tougher. Lamination involves gluing a thin layer of a plastic, polyvinyl butyral (PVB), between two layers of glass so that the plastic will keep the windshield intact even if the glass breaks. It is far more complicated to manufacture than toughened glass – first you have to suck out all the air out of the sandwich of glass-and-plastic, then you have to heat it to 140 C at a pressure of 12 atmospheres to make the last of the air dissolve into the plastic, says Van der Merwe. Then again, being able to see through a cracked window to swerve out of the path of an oncoming truck has probably saved more than enough lives to justify the effort.
Using a thicker plastic can add an element of security to the safety already offered by laminated glass. Take a shop front window. A 0,38 mm PVB layer between two panes of glass is enough to prevent nasty cuts should someone accidentally fall against the window. But by increasing the PVB to 1,52 mm, you would also make it difficult for an intruder to gain access to the shop, requiring 24 hammer blows to break through the 1,52 mm interlayer as opposed to three hammer blows for the 0,38 mm layer.
Need to stop a bullet? No problem. Simply add a few extra layers of glass and PVB to your basic lamination sandwich. The number of layers and the thickness of the panes depend entirely on the calibre of the bullet the glass is meant to stop. F or example, it takes a 70 mm window pane, consisting of six layers of glass of varying thickness interspersed with five pieces of plastic, to stop three AK-47 rounds over 100 mm. If you need to stop more rounds than that, our guess is no pane of glass is going to keep you alive.
Fortunately, most of us lead peaceful lives and need glass only to protect us from the elements – rain, wind and noise. But what about the Sun? Even though you can’t really burn behind glass (it blocks UVB rays), it still lets in the heat and UVA rays, known for causing age spots and contributing towards cancer. Tinting the glass to block the heat and rays helps, but that often means viewing the world as if from inside a wine bottle. A more elegant solution is to coat glass with a thin metallic sheen to reflect the heat of the Sun.
In fluorescent lights, electricity makes the gas particles sealed in the tube bump against the metal electrodes on either side, knocking off the molecules that form metallic deposits on the inside of the glass. This accident of physics has been turned into a tool to coat glass panes with metal; only here the electrode is much bigger and made out of stainless steel, titanium or silver. The electron is housed in a chamber filled with nitrogen or argon gas through which an electric current is passed while the glass passes under it to pick up the coating.
“It’s amazing,” says Van der Merwe. “The whole chamber lights up.” Because the metallic coating easily rubs off, the glass is then laminated – metal coating facing in – using a clear or tinted PVB layer and a second, uncoated piece of glass.
If the Sun doesn’t scare you, perhaps carrying a bucket of water up a tall ladder to clean the windows does – in which case you will be pleased to hear that international glass group, Pilkington, has designed a self-cleaning glass. Launched in 2001, self-cleaning glass has a 50-nanometre layer of titanium oxide that acts as a photocatalyst, meaning that it uses UV light to break down organic dirt particles.
It’s also hydrophilic,
in that it attracts water, so that when the rain comes to wash off the broken-down dirt particles, the water forms a thin film on the surface rather than watermark-forming droplets. Although manufactured overseas, self-cleaning glass is available in South Africa.
Small beats all
It is ironic that plastic, the very thing that is used to make panes of glass safer through lamination, needs glass in the form of glass fibres to make it stronger in turn. Glass fibres are microscopic filaments of varying length that measure no more than 10 microns in diameter and are, in the words of Dr Kashif Marcus of the Centre for Materials Science at the University of Cape Town, “the workhorse reinforcing agent to make materials stronger.”
To understand just how glass fibres reinforce a material, think of a piece of A4 paper in landscape position. That’s the raw material – in the case of glass fibre pools, this would be polyester. Now, if you were to paste a number of 3 cm strips of sticky tape (the glass fibres) along the length of the paper, positioning the strips randomly but keeping them parallel to the horizontal, you would find it difficult to tear the paper in half from top to bottom – more so than if you were to tear it from left to right, where you stand a chance of tearing between the strips of paper.
Similarly, glass fibres significantly improve the strength of a material along a certain orientation, depending on the positioning of the fibres. Jumbling up the glass fibres to form a random mesh rather than a regiment of parallel filaments will strengthen the material from all orientations, although the magnitude of strength will be diminished.
Glass fibres also increases the ability of plastic to withstand high temperatures. “Polymers are usually used in low-temperature applications,” says Marcus, comparing nylon, which is stable to about 150 C, with glass, which is stable to about 600 C. Put the two together and you get a material that is lighter than metal and just as durable, but that does not distort under high temperatures.
“Our aim is to increase the range of mechanical thermal properties of polymers so that they can be used in the manufacture of aeronautical parts. We have already designed some composite materials that are being used in the manufacture of car parts,” he says.
Big or small, glass remains a simple substance. There aren’t many unexplored ways to change it on a molecular structure: glass is glass, give or take a pinch of selenium for colour or boron to improve thermal endurance. It will never bounce like a rubber ball or burn like petrol. But that doesn’t mean we’ve seen the last of new developments in this material.
“The way ahead is to cross-purpose existing glass technology for new applications,” says Van der Merwe, citing the way motor manufacturers have stolen fighter-jet technology to heat windows for demisting and project dashboard readings on the windscreen. “The projections are done by making the PVB interlayer a little thicker at the bottom than at the top, so that the glass is angled slightly to improve refraction where the readings are projected,” he explains. “It’s been used in aircraft for ages.”
Old technologies are constantly being incorporated into glazing products for new applications. Electronic sensors are being fired into windows and windscreens, ready to sense changes in light and weather to switch on the headlights or windscreen wipers. Superfine metal coatings are changing the surface properties of glass, and materials scientists are pushing the boundaries of what the PVB layer can do.
Glass is glass and it will never bounce, but that doesn’t mean it isn’t burning with potential.