Silk Versus Synthetic Fibers
Scientists at Oxford University and The University of Sheffield have demonstrated that natural silks are a thousand times more efficient than common plastics when it comes to forming fibers. A report of the research is published this week in the journal Advanced Materials. The finding comes from comparing silk from the Chinese silkworm to molten high density polyethylene (HDPE) - a material from which the strongest synthetic fibers are made. The researchers used polarized light shining through a disk rotating over a plate to study how the fibers are formed as the two materials are spun.
Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity. The shimmering appearance of silk is due to the triangular prism-like structure of the silk fiber, which allows silk cloth to refract incoming light at different angles, thus producing different colors.
Silk is one of the strongest natural fibers but loses up to 20% of its strength when wet. It has a good moisture regain of 11%. Its elasticity is moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.
HDPE forms filaments at over 125°C and in addition requires substantial energy input in the form of shear force applied to the material in its molten form. Silks, in contrast, in the same set-up forms filaments at ambient temperature and in addition requires only a tenth of the shear force. If the energetic costs of melting HDPE are included for comparison, silks become a thousand times more efficient.
The discovery of low-energy method for fiber formation has led the researchers to view silks as a new class of polymers they call aquamelts.
"When aquamelts are quickly stretched they lose water, which helps them lock in fibre formation like a ratchet," said Dr Chris Holland of the Oxford Silk Group, part of Oxford University’s Department of Zoology. "This doesn’t happen with your everyday plastics which need to be cooled to preserve the fibres, making them inefficient and harder to process."
"Silk produced by spiders and silk moths demonstrates combinations of strength and toughness that still outperform their synthetic counterparts. Not only are silks superior to man-made fibres, they are produced at room temperature with just water as a by-product."
Dr Chris Holland said: "Combining the best of polymer science with biology we were able to determine how much energy is required to form these two fibers. And it seems that we have discovered some fundamental differences between natural and synthetic materials. With hundreds of millions of years of R&D in fiber production it is not surprising that silkworms and spiders have found ways to conserve energy while still making superior fibers."
For further information: http://www.ox.ac.uk/media/news_stories/2011/111123.html