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New developments in biodegradable fibres - II
Fibres from the first mentioned EcoPLA - polymer were said
to be: reminiscent of PET or PS in some forms and of PP and PE in others. Fully
biodegradable under composting conditions. Convertible into nonwovens by dry-air-wet-spunmelt-laying
systems. Capable of giving improved resilience, moisture transport, breathability
and wet strength. The price was, in 1998, said to be $ 3-6 per kg, but capable
of reduction to $ 1.1 per kg at full-scale production.
Clearly PLA can be manufactured with a range of properties,
if only because the lactic acid, being chiral and with two asymmetry centres
exists in four different forms. It also appears to have the properties required
to meet our biodegradable diaper requirements, being easily converted into film,
fibre, spunbond and meltblown products on existing extrusion equipment. However,
the new 140,000 tonne polymer plant is said to be costing $ 300 million so whether
or not the polymer can be produced down to a price that would guarantee success
remains to be seen. Several other producers are active. At Index 99, NKK (Japan)
showed a PLA spunbond nonwoven at 15 gsm with apparently excellent formation
and properties. Kuraray (Japan) showed PLA fibres and provided some data on
their properties and biodegradation rates. PLA polymer is made in Japan by Mitsui
Toatsu (under the LACEA brandname.
Biodegradable fibre properties
On the information currently available, PLA looks like
an excellent fibre with the right technical credentials to replace polypropylene
in nonwovens. As noted by Carothers, the melting point still appears too low
for it to challenge the supremacy of aromatic polyester in mainstream textiles,
but this may be to the nonwoven industry’s advantage. If it were that good,
PLA marketing effort would be concentrated first on higher value textile applications,
and it’s biodegradability would not feature so prominently.
Other biodegradable synthetics
Monsanto’s BIOPOL (originally developed by ICI-Zeneca,
who planned a 10,000 tpa plant in 1987) was based on a random copolymer of 3-hydroxybutyrate
and 3-hydroxyvalerate made by bacterial fermentation. It was sold for around
$ 15 per kg for speciality products and was expected to appear in fibre form
at a much lower price. At Index 99, Monsanto was said to have stopped the project
in January 1999. However, their press release in October 99 notes that direct
production of PHBV and other poly (hydroxyalkanoates) in plants, the latter
said to be viable alternatives to expensive fermentation processes.
Polycaprolactone has been used in blend with other
plastics to make biodegradable films since the late 1970s. Unitika and Nippon
Unicar developed a hydroentangled nonwoven based on 80 per cent cellulose and
20 per cent PCL fibres. Freudenberg (Germany) developed biodegradable spunbonds
based on 50 per cent PCL and 50 per cent “conventional fibre forming polymer”
for scrub-suits, incontinence products and bandage holders.
Bayer’s BAK, a polyester amide polymer is based
on hexamethylene diamene, butane diol and adipic acid. Butane diol is also the
basis for Bionolle, Showa Polymer’s biodegradable synthetic.
Spun-laid cellulosics
All evidence to date suggests that biodegradation will
only become a major marketing issue if it can be delivered in a diaper without
sacrificing any of the attributes of the current leading brands. Biodegradable
nonwovens usable as coverstocks already exist in many forms, one of the best
being hydroentangled rayon, but none are available at a price to compete with
thermal bonded spun-laid or dry-laid polypropylene. Furthermore to perform as
well as PP in wet-back testing the HE rayon would need an additional finishing
process.
If we consider the cost of cellulosic nonwoven at the
converter, the greatest savings can in theory be achieved by moving from dry-laying
staple fibres to a suitably sized spun-laying operation in a fibre plant, ideally
on the site of a pulp mill. Much work has already been done in this field, most
of it before environmental issues such as biodegradation and renewable resources
had reached the public consciousness.
Viscose/Cupro routes
Several cellulosic fibre producers have already attempted
to improve the performance/cost ratio by making nonwovens themselves. Courtaulds
(now Acordis), Rhone-Poulenc and Enka Glanstoff AG (now Acordis) developed spun-laid
rayon nonwovens on a pilot scale in the late 60s and earlyseventies. Courtaulds
relaunched an improved fabric based on a new process in 1978 and withdrew in
1982. Kanebo and Daiwabo researched similar techniques. Asahi worked with viscose
and cuprammonium pilot lines before commercialising the cupro route as Bemliese
nonwovens. Mitsubishi Rayon developed a process (later sold to Futamura) based
on hydroxymethyl cellulose xanthate in which the webs were point-bonded in a
thermal calender before regeneration. Kosabura Miura spun viscose vertically
downwards from oscillating spinnerettes onto a conveyor, and oversprayed the
liquid filaments with acid. The Tachikawa Research Institute developed a polynosic
viscose spun-laid process. The surviving processes, Asahi’s Bemliese and
Futamura’s TCF, both make coverstock weights but on a relatively small
scale and at premium prices. In fact there’s nothing here to persuade today’s
diaper manufacturers that these cellulose based spun-laid processes could be
a viable contender for a biodegradable coverstock.
Of the numerous other routes to cellulosic fibres researched
in the last three decades, within the context of this paper, four are worth
mentioning.
Carbamate route
Turunen and Struszczyk reacted cellulose with urea
to form a stable derivatised “pulp” which could be stored indefinitely
and was easily dissolved in sodium hydroxide. The resulting solution was also
spinnable into dilute acid or alkali to yield fibres of cellulose carbamate
or regenerated cellulose (or mixtures of the two). Most effort went into making
dope for bonding sausage skins or into regenerated cellulose fibres to compete
with viscose. From our current point of view, the cellulose carbamate fibres
themselves appear more interesting: fibres with a nitrogen content of 2 per
cent resisted biodegradation without being toxic to the organisms. They also
had high water absorbency and were soluble in 8 per cent caustic soda. They
were self-bonding when wet-laid. Fibres became progressively more biodegradable
and insoluble in alkali as the nitrogen content was reduced (and the cellulose
regenerated).
Lyocell route
Employing the cyclic amine n-methyl morpholine n-oxide
to dissolve cellulose prior to spinning the dope into water, the lyocell process
has been fully developed and scaled up for textile applications by Enka and
Courtaulds (now together in Acordis) over the last 20 years.
It offers an environmentally acceptable way of converting
natural cellulose into a fully biodegradable premium quality rayon fibre with
tensile proerties approaching those of polyester. Furthermore the technology
offers the potential for converting pulp to fibre on a scale and at a cost that
would give polyester and cotton some serious competition. The high capital requirement
of the first plant necessitated launching the fibre (in 1990) at a premium price
into the fashion-apparel market. This proved highly successful and led to rapid
expansion up to the current capacity of around 100,000 tonnes. However this
represents only about one half of one percent of the current cellulosic fibres
market, and lyocell fibre is still only available commercially from two sources,
Acordis and Lenzing. Only Lenzing appear to have had even the theoretical capability
of integrating lyocell with pulp production, and have chosen not to.
Lyocell makes excellent nonwovens, especially in those
processes that allow it’s superior aesthetics to shine through, like needle-punching
and hydroentanglement. Its high strength is of little intrinsic value in disposables,
but it enables the nonwoven producer to reduce basis weight while meeting strength
targets. It’s freedom from shrinkage and high wet stability allows higher
area yields in HE processes, and its high modulus prevents it from collapsing
in the wet to the same extent as viscose rayon. Fibrillation, the development
of surface microfibres on wet abrasion or in high-pressure entanglement, adds
an additional dimension for the nonwoven developer. Unfortunately, while it
has established itself in several profitable niches, its premium positioning
has so far prevented it’s use in mainstream disposables.
The nonwoven industry enjoys the economies of polypropylene
because PP is a by-product of the energy industry. Viscose rayon requires dissolving
pulp, a premium product of the timber industry. Lyocell is currently similar,
but its simple production process has the so far unexplored potential to use
cheaper pulps and hence the potential to achieve the economies of scale that
may ultimately interest the major diaper producers.
Direct dissolution of cellulose in soda
It has been known for many years that wood pulp partly
dissolves in very cold dilute caustic soda, and there have been several attempts
to improve such solutions to the point where textile fibres could be spun from
the dope. Kamide et al working in Asahi’s Fundamental (Fibre) Research
Laboratory described fibres spun from soda solutions of cellulose. Here the
key step was steam-explosion of the woodpulp to improve the accessability of
the cellulose chains prior to contact with the sodium hydroxide.
Struszczyk et al used cellulase enzymes to modify the
structure of cellulose to allow it to dissolve in soda. While these soda routes
make fibres with properties inferior to viscose rayon, they are more than adequately
strong for spun-laying or melt blowing. Once again his ‘old’ technology
coupled with “Super Site” thinking could become the basis of a low-cost
biodegradable nonwoven process.
Dissolution in phosphoric acid
Cellulose dissolves readily in 85 per cent phosphoric
acid without degradation and can be regenerated by spinning into water to make
strong high modulus fibres. Reconcentration of the dilute phosphoric acid for
reuse in dissolution has however been too expensive to allow an economic fibre
process. It is, therefore, interesting to see in a recent series of patents
from Akzo-Nobel, one postulating the centrifugal spinning of fibrous mats, another
the manufacture of superabsorbent cellulose gel-fibres in fibrid form. Direct
synthesis of cellulose fibrils worthy of mention if only because related technology
to make polyesters is mentioned above, is the fact that bacteria, in this case
Acetobacter Aceti produces fine fibrils of cellulose when cultured under the
correct conditions.
Weyerhauser commercialised Cellulon fibrils based on
this technology, later selling out to Monsanto. With filament diameters of less
than 0.1 micron the fibres were hard to process even on wet-lay systems.
Conclusion
Biodegradable thermoplastic fibres made from PLA have
the potential to bring the production and marketing of biodegradable disposables
one step nearer reality. Fibres of this sort appear spinnable on conventional
melt spinning equipment into coverstocks that will work in conventional disposable
diaper manufacturing plants. The ability to vary the properties of the PLA by
careful selection of the blend of isomers and the polymerisation route appears
to make it possible to vary the fibre properties from amorphous to crystalline
thereby creating a range of melting points, biodegradation rates, fibre strengths,
and even bicomponency. Clearly the fibres can be used in a wide variety of applications
and it will be interesting to see how the producers prioritise these applications.
When sums of the order of $ 300 million are spent on a polymer at the start
of it’s learning curve the economic pressure to develop the higher value
applications first is enormous. In the case of PLA, however, the melting points
appear to be similar to polypropylene rather than polyester, and its ability
to replace polyester in conventional textiles could be similarly restricted.
In nonwovens, compared with the cellulosics, it has the key advantages of simple
conversion into fibre and spunlaid nonwovens coupled with the resilience and
bulk necessary for good surface dryness in coverstock. It appears to have the
potential to beat current cellulosic nonwovens on price but we need to await
actual prices of the product due to emerge from the 1,40,000 tonne plant in
2002 before we can be sure. As for future cellulosic nonwovens, this year’s
announcements of PLA expansion reduces the already minimal likelihood of any
investment in their really large scale manufacture either by the current technologies
or by the visible future technologies linked to “Super Site” thinking.
If trees ever produce thermoplastic cellulose the conclusion might well change,
but by then polyesters will be synthesised directly by plants, their extraction
in pulp mills obviating the need for the relatively expensive fermentation and
polymerisation processes now being developed.
(Source: Calvin Woodings Consulting)
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