HDPE vs. silk: Comparing the production of natural and synthetic fibres

Kayla Price
25 April 2013

Above: Caterpillar of the mulberry silkworm, or Bombyx mori (Gerd A.T. Müller)

Did you know? The silk used in ties and shirts usually comes from the cocoon of the larvae of the mulberry silkworm Bombyx mori, which are reared in captivity.

High-density polyethylene (HDPE) resin is used to create some of the strongest synthetic fibres. HDPE fibres are non-toxic, resistant to many solvents, and can be used in a variety of applications, from bottle caps to plastic bags to oil tanks. In fact, HDPE is one of the most common plastics in use today. It is also recyclable.

And yet, natural silk from the cocoons of mulberry silkworms (Bombyx mori) is stronger and can be produced more efficiently than HDPE. So while silk may normally be associated with textiles, it has a range of other potential uses. Although other challenges would make it very difficult to produce enough natural fibres to replace the volume of synthetic ones we use today, a better understanding of silk production could help make HDPE production more efficient.

Silk production likely originated in China around 2700 BCE. In 1870, Louis Pasteur (also known for his discovery of vaccines) breathed new life into the industry when he developed a method to keep disease from spreading among farm-reared silkworms. HDPE is a much newer technology. It only began commercial production in the mid-20th century.

Did you know? Shear force is the force exerted when any two objects move relative to each other while secured together by a clamp.The production of HDPE threads requires significant amounts of energy, particularly when compared to the production of natural silk. The surrounding temperature must be above 125 °C and a substantial amount of shear force is required. In contrast, recent research shows that Chinese silkworms, which can produce silk fibres at room temperature (around 21 °C), require only a tenth of the shear force needed for synthetic fibre production. When researchers considered the energetic cost associated with temperature and shear force, they determined that, when compared to HDPE, natural fibre production can be a thousand times more energy efficient. Furthermore, although only silk spun by silkworms has been scientifically tested and compared to HDPE, both spiders and silk moths produce natural fibres that are similar in terms of strength and toughness.

Did you know? While HDPE is a recyclable material, it does not biodegrade in landfills and cannot be composted.However, using natural silk to replace the same volume of HDPE currently produced may be an unrealistic goal, given the size of the infrastructure and the amount of resources that would be required. In particular, a vast number of insects would have to be fed and housed. Consider that in Western Europe alone, roughly 22 million tonnes of HDPE are used every year. By way of comparison, a grand total of about 30 thousand tonnes of raw silk is produced each year worldwide. But this level of production still requires 4.5 million tonnes of mulberry leaves to feed the silkworm larvae!

Nevertheless, studying how insects produce silk fibres allows scientists and HDPE manufacturers to learn from natural processes in an effort to make future production of plastics more energy efficient. This is one of many examples of a biological process that both inspires and educates producers of “man-made” materials.

References

General science and consumer websites

How Much Silk Does A Silkworm Produce In Its Lifetime? (Judy Time, Knoji Consumer Knowldege) History of Sericulture (Ron Cherry, insects.org) Silk Versus Synthetic Fibers (Andy Soos, Environmental News Network)

Industry publications

Environmental Product Declaration: High density polyethelyne (HDPE) (PlasticsEurope, Association of Plastics Manufacturers)

Press releases

Spider know-how could cut future energy costs (University of Oxford)

Scholarly publications

Holland C, Vollrath F, Ryan AJ, Mykhaylyk OO. 2012. Reconciling 100 Degrees of Separation: Silk and Synthetic Polymers. Advanced Materials. 24(1):105-109.

Kayla Price

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