Molten silicon glass offers a number of advantages as a material, but manufacturers often avoid using it because of the required high temperatures. However, researchers in Germany now produce high-quality glassware at far lower temperatures using injection molding. Their technique could potentially have significant technical, economic and environmental benefits.
Silicate glass is one of the oldest known materials, produced simply by heating grains of silica until they melt. It has outstanding chemical stability, optical clarity and durability. In the 20th and 21st centuries, however, manufacturers are increasingly using polymer-based products, which often have inferior properties and become notorious environmental pollutants. One reason is that many polymers can be easily formed by injection molding – by heating the material until it softens before injection into a pre-formed mold. This usually occurs at 200-250 ° C. In contrast, silicon glass melts at about 2000˚C. Aside from the huge energy requirements, “there aren’t too many mold materials that can sustain these temperatures,” explains Frederik Kotz of the University of Freiburg in Germany.
Everyday glassware usually contains additional chemicals. “We add soda, calcium – a lot of things like that – to lower the melting temperature,” says Kotz. “But it also affects properties like thermal and chemical stability.” In precision glassmaking techniques, pure silicon glass is typically produced at ultra high temperatures before being etched with hazardous chemicals such as hydrofluoric acid and flame polished to obtain the desired shapes – although, for example, ultra pure silicon glass is usually produced for optical fibers by chemical deposition.
A potential alternative is powder injection: a technique that was first developed in the 1950s and is still used today to produce everyday goods worth billions of pounds, from metal keys to ceramic sinks. The solid particles suspended in the liquid binder are injected into the desired shape and allowed to solidify. The resulting object is then placed in an oven, causing the binder to evaporate and the particles to fuse – a process called sintering. Several research groups have previously tried to produce glassware in this way, but with limited success. “You usually get a white piece of glass – it’s more ceramic,” says Kotz.
Kotz and colleagues decided to try to use an extremely high concentration of ceramic nanoparticles. This posed a challenge: ‘You can find [ceramic nanoparticles] everywhere: in creams, in toothpaste – they act as thickeners, ‘explains Kotz. ‘Of course, it’s not useful if you want a really large amount to be well distributed.’ The researchers devised a method for retaining these solvate particles in a polymeric binder, producing a liquid that can be injected into any form. A portion of the binder was then washed and collected, before the researchers heated the material to 1300 ° C. This expelled the remaining binder and led to softening and melting of the nanoparticles. “It’s a thermodynamic effect,” Kotz explains. “They’re close enough, the surface is super big, and when you reduce the viscosity a little bit, they reduce the surface by moving together.” The researchers performed measurements that, they claim, showed that glass components of scientific quality can be produced without further processing.
Researchers have produced hundreds of kilograms of material using their technique and are working on production in several tons. Precise energy savings remain unclear, but researchers believe it exceeds 40%. Moreover, says Kotz, disposing of glass is easy. ‘The glasses are made of sand and you can grind them back into the sand.’
‘Injection molding offers the opportunity to make glass objects of a wide range of different shapes,’ says John Mauro of Penn State University. ‘However, more details are needed to understand the real potential of the new approach and to compare the quality of the resulting glass products with industry standards of high-purity molten silicate products. Also, more work is needed to estimate the energy savings of the new approach compared to conventional steam deposition and melting techniques. ‘