Unappreciated materials (6): Straw

Latest posts by Prof. Mike Ashby (see all)

straw-thumbnailSome materials catch headlines, are held in awe, but not all.  Some get little respect, despite having changed the world.  They have become commonplace, anonymous, ignored and (particularly if they are cheap) cast aside when no longer wanted.  If they had feelings, they would be hurt. This brief series of blogs is to draw attention to their plight.

Rumpelstiltskin could spin straw into gold.  Materials Scientists can’t do that yet.  But they can do other things with straw.  The value-added may be lower but the business-case has a firmer scientific base.  But first: what is straw?

A selection of images showing the use of StrawStraw is what’s left over when cereals are harvested.  The US harvest leaves behind it 128 million tonnes of straw; globally the quantity is about 2000 million tonnes.  That is about the same as the weight of steel we use in a year.  The volume of straw we produced in a year exceeds that of any other material, albeit, in the case of straw, without really wanting to produce it.  Straw cannot be used to feed livestock – straw is not hay – so there is an overabundance.

The perception of straw as insubstantial and worthless has evolved from fairy tales and become embedded in language.  The last straw is the final, additional small burden that makes the sum of your burdens unbearable.  To clutch as straws is cling to hopes that are insubstantial and have little hope of success.  To set up a straw man is to advance a proposal that is easily knocked down.  Straws in the wind are wisps that hint that something might happen.  To make bricks without straw is to embark on a project without the proper means to achieve it.

With a reputation like that straw does not attract the research interest and funding that accrue to “modern” materials.  Yet straw has many uses – straw hats, straw mats (tatamis), straw doilies, straw bedding in stables and pig-sties – but these are not going to need two billions tonnes of straw.  Buildings use straw on a larger scale.  Mud bricks reinforced with straw have been used as a building material since the first settlements of ancient Egypt.   Adobe, with an equally long history, is a composite of clay, sand and straw.  Cob, found in Europe and North America, is almost the same mixture.  Both are still in use; modern versions are stabilized with additions of emulsified asphalt or Portland cement.

Straw compressed into bales can be used for load-bearing structures. It is appealing as a building material for several reasons.  In areas of grain production it is inexpensive. It is an excellent thermal insulator.  Its embodied energy is low and it absorbs almost its own weight of CO2. In construction, bales of straw are stacked to form thick insulating walls, arches and vaults up to two stories high. Increasing awareness of environmental issues makes straw bale appear an attractive choice of building material.

It is not, however, one that lends itself to mass construction. The properties of straw vary with climate.  The R-value of straw bale walls depends on humidity and cannot be guaranteed. It is prone to attack by insects and fungi.  Erecting and finishing straw-bale housing can be slow.  And the bales have to be available locally:  if they are transported more than 40 miles the cost and energy required to do so exceed those of the straw itself.  It is a material of niche construction.

Surely 21st century materials science can transform this enormous resource of a natural material into something, even if it is not gold.  There have been many attempts to do so: hot pressing to make binder-free straw board for packaging; injection-molding of shredded straw in a natural binder to make profiles like flower pots; extrusion of straw-reinforced plastic profiles for construction; and more.  Few have been economically successful and, because processing is energy-intensive, even fewer offer overall environmental benefits.  The aims are admirable but the implementation is strewn with impediments.

This makes straw seem pretty useless stuff, indeed 20th century farmers simply burnt it to get rid of it.  But earlier generations have found it useful – the evolution of housing in vast areas of the world would have been very different without it.  Perhaps some 21st century Rumpelstiltskin can find a new way to make it valuable.

One thought on “Unappreciated materials (6): Straw

  1. Prof. Rupendra M. Anklekar

    The article sheds a new perspective of how even an ordinary material like straw which is often ignored by everyone in society can be engineered properly using Materials Science principles to make it a useful material for host of traditional and newer applications.


Leave a Reply

Your email address will not be published. Required fields are marked *