Thursday, 8 November 2007

Future Diaper Raw Materials?

While synthetic polymers account for only a small percentage of the oil usage, and could be obtained from coal or tar more easily than could transportation fuels, they will become increasingly costly to a point where the use of natural polymers and their derivatives will become viable again. Furthermore as consumers demand ever-more low-carbon-footprint biodegradable products based on renewable raw materials the case for such a reversion to the natural will increase.

Polylactic acid (an aliphatic polyester) has the undeniable advantage of low temperature thermoplasticity which makes it an excellent candidate for replacing polypropylene in existing nonwoven processes – whenever the price falls to parity with PP having taken the different polymer densities into account, i.e. on a cents per cubic centimetre basis. It currently sells at a premium into products where claims of “corn-based” “natural” , “sustainable” and “compostable” have value, and today this is mainly into packaging films and mouldings, wipes, coverstocks and textiles. The process used to make it does require more energy than a fossil-fuelled polyester and the current product alleviates this by carbon-offsetting. One could also argue that the naturalness associated with corn is compromised by its complete depolymerisation to dextrose followed by fermentation to lactic acid which is then polymerised to PLA. There is also the issue of the current process needing prime food growing acreage and intesive fertilisation. Future processes based on waste biomass will be significantly more sustainable in the long term.
Cotton is at the other end of the naturalness scale to PLA in that nature provides a finished fibre almost ready to be carded. Unfortunately current low-cost cotton production requires extensive petrochemotherapy and irrigation and current pricing depends on government subsidy. Furthermore, and unfortunately for nonwovens producers, it needs bleaching and special finishing if it is to be processed efficiently into absorbent products. The more attractive, sustainable and eco-friendly organic cotton with its lower yields even from irrigated agricultural land could remain far too costly and scarce for most nonwoven applications and for some time will be used primarily specialities and in high value fashion textiles. If however a subsidised expansion of organic cotton production could be part of some grander scheme, such as eliminating US cotton subsidies and reducing poverty in Africa then ethical cotton nonwovens could emerge as a more mainstream raw material. If future consumers realise that genetic modification is just a speeded up version of the natural process by which all life evolved, they may be more favourably disposed to the man-made version of GM now becoming capable of transforming our ability to live a carbon-neutral existence. Organic cotton production would be an immediate beneficiary of the new mind-set.
Other Cellulosics?
Genetic modification of trees and other biomass could likewise transform the quality and yield of cellulose from agriculture, and may even allow its efficient production from bacteria. For now, cellulose is produced in the cell walls of vegetation when sugars produced by chlorophyll-catalysed photosynthesis are polymerised by enzymes to form both lignin and cellulose. The industrial grade of cellulose used to make fibre comes from tree-farms, where specially chosen species can be grown from sapling to maturity in as little as 10 years. New trees can grow from the stumps of the cut trees, and this happens on marginal land, generally unsuitable for food crops and without the intensive use of fertilisers or pesticides. The best tree farms yield in excess of 2.5 tonnes of pure cellulose per acre per year. For comparison, cotton growing at its most intensive yields about 0.7 tonnes/acre and needs good soil. Using trees on an industrial scale can attract the wrath of environmentalists. This is of course no worse ecologically than non-intensive farming but it is important to put the usage of trees as a raw material into the correct perspective. Using very round numbers to gain an approximate impression of the impacts involved:· 100 billion tonnes of vegetation grow and decay annually on land. This represents about 12% of the planets total production of vegetation, the majority being produced in the oceans.
· 12% of this land-based vegetation is in the form of wood (trees).
· Of this annual growth of 12 billion tonnes of wood, a maximum of 4 billion tonnes is removed by man. Half of this is burnt, either as fuel or to clear land for agriculture. The other half is used by Industry. (Compare this with 6 billion tonnes of fossil reserves "mined" and burned each year.)
· Of the 2 billion tonnes of wood used by industry, half becomes timber in saw mills, and half is used raw.
· Of the 1 billion tonnes used raw, half goes into construction (pit-props, telegraph poles etc) and half is converted into pulp and chipboard.
· Of this 0.5 billion tonnes, 0.4 billion tonnes of wood become wood-pulp for the paper, board, fibre, film and chemicals industry.
· A significant proportion of this pulpmill feedstock (up to 40% in some areas) comes from forest thinnings, and saw mill waste and 6% from non-pulp sources such as straw, bagasse, hemp and cotton. This feedstock yields about 0.25 billion tonnes of pulp.
· About 0.004 billion tonnes of this pulp output are a high quality dissolving grade for forming into fibres, films, water soluble polymers and chemicals. Dissolving grade pulp is perhaps better described as industrial grade cellulose polymer, and should be considered alongside the polyester or nylon polymer chips which are the feedstocks of the synthetic fibre plants.
· Rayon manufacture consumes 0.003 billion tonnes of this cellulose, with about 2/3rds going into staple and one third into filament and tow (including acetate).
· If ever the use of trees to make fibres on this scale becomes unsustainable, we could always farm the oceans for seaweed and make the closely related alginate fibres, or even produce chitin fibres from insects or shellfish.

So, the cellulose fibres, which thrived before we learned how to make fibres from cheap petrochemicals can thrive once again as the price, both monetary and ecological, of unsustainable raw materials increases further. Since the development of efficient hydroentanglement bonding processes, they can be converted into pure, soft cellulosic nonwovens which at first sight could provide consumers with the ecofriendly biodegradable nonwovens they
Unfortunately they are not thermoplastic so conversion processes will need adapting, and they are not yet available in the form of spunbonds so the cost differentials c.f. polypropylene spunbonds will be higher. Furthermore they are inherently wettable and will need finishing with hydrophobic materials to allow them to achieve the surface dryness levels needed for diapers. These problems are soluble.. After all, about 40 years ago polypropylene was thought by some to be incapable of replacing rayon in diaper coverstock because it was impossible to card, and far too hydrophobic for coverstock use.

1 comment:

denice said...

a very good idea in making this article