Imagine a polymer with a low heat distortion
temperature and poor impact resistance. It is also
difficult to form into products and nearly twice
the cost of other resins. This, in a nutshell, is a
biodegradable plastic.
Despite these shortcomings, biodegradables
have come a long way. Ten years ago, Natureworks
had only just begun to commercialise polylactic
acid (PLA), its corn-derived biodegradable resin.
Now, its 140,000 t/year plant in Nebraska is
close to capacity – and the company is assessing
locations for a second manufacturing facility.
But in that time, PLA and other biodegradables
have yet to extend their reach beyond their initial
target market.
‘Most applications of biodegradable plastics are
still in packaging, or for products like agricultural
film,’ says Aniruddh Menon, a research associate
at Frost & Sullivan. ‘But a major concern for end
users is price/performance ratio – and this is one of
the factors that will decide the fate of bioplastics.
In many cases, these materials cannot match the
performance of conventional resins.’
Bioplastics suppliers recognise this. Last
year, NatureWorks held a huge congress in Las
Vegas – ‘Innovation Takes Root’ – which brought
together many players that will help PLA leap
into more demanding applications. There is a
huge effort to improve the products that already
exist – and to open up new avenues. One slightly
vexing problem will be to make the additives
themselves biodegradable – a growing demand
from end-users.
‘This will depend very much on additive
manufacturers,’ says Menon. ‘They are doing the
research to make bio-based additives. I would
expect to see joint ventures between them and
resin manufacturers.’
Tough challenge
Toughness – or impact resistance – is the amount
of physical punishment that a material will take.
A moderate improvement, achievable by adding
impact modifiers, would allow PLA to withstand
more punishing processing conditions. This would
mean faster line speeds, and less product damage
during production.
Swiss company Sukano says that its impact
modifier can help to prevent cracks and splinters
during the production of PLA film and sheet.
DuPont makes similar claims – and independent
tests confirm that its Biomax Strength impact
modifier can have extra benefits.
John Christiano, of machinery extrusion
company Davis-Standard, ran samples of
impact modified PLA through a conventional
film processing line. He says that the additive
improved impact resistance of PLA by a factor of
two or three, when added at a loading of 2%.
But it also delivered other advantages. ‘It lowered
melt temperature by around 3°C, which reduced
power consumption by up to 25%,’ he says. ‘You
would not see such a large improvement with a
conventional polymer.’
If commercialised, this kind of technique could
cut the cost of producing biodegradable film and
sheet – thanks to lower energy bills – and help it
compete with conventional polymers.
A combination of additives and process
technology is likely to overcome some of PLA’s
other potential drawbacks. At Innovation Takes
Root, Roger Avakian, chief technology officer at
US polymer compounder PolyOne, made a long
list of PLA’s shortcomings, plus a list of additives
that could solve them. The problems included high
cost and low melt viscosity, as well as poor heat
resistance. PLA can usually withstand temperatures
of around 50°C, but PolyOne has developed ways
of raising it.
‘Higher HDT [heat distortion temperature]
and improved impact strength are key enabling
properties,’ says Avakian. He says that new blends,
which Polyone expects to commercialise by the end
of this year, will deliver higher heat resistance at
low load (66psi), as well as an impact strength up
to 25 times higher than the 0.5ft-lb/inch of ‘neat’
PLA. ‘We have at least two different platforms
in development,’ he says. ‘Improvements in HDT
range from 85 to 110°C, and impact strength
from 1.5 to over 12 ft-lb/inch, depending on the
grade.’
This would lead to
applications in both
thermoforming and injection
moulding, competing with
materials such as high
impact polystyrene, ABS
and ‘some engineering
thermoplastics’. ‘We see
applications in consumer,
electronics, packaging,
and transportation,’ says
Avakian.
Some other PLA
developments revealed
at Innovation Takes
Root included making
stronger fibres and
textiles from the material;
Samsung’s use of PLA/
polycarbonate blends to
make a mobile phone
handset and Toyota’s use
of PLA reinforced with
kenaf, a natural fibre, for
various components.
Green additives
Researchers at the University
of Kassel in Germany have assessed the
strengthening effects of different natural fibres
on PLA. They say they have managed to improve
physical properties using wheat and rye husks.
‘Natural bio-fibre composites are emerging as
a viable alternative to synthetic fibre-reinforced
composites, especially in the automotive and
building industries,’ says Abdullah Al Mamun, a
researcher on the project.
These fillers are of particular interest because
they are waste products: the husks account for
around 20% of the crop’s weight, but are inedible.
The researchers also looked at the effect of soft
wood fibre, as a comparison.
Each type of fibre was mixed (at 40% weight)
with PLA, then injection moulded and subjected to
physical testing. The two additives showed quite
different effects. Compared with the wood fibre
composite, wheat husks had a 20% increased
effect on tensile strength, while rye husks were
40% lower. In the Charpy strength test – which
measures the energy absorbed by a material during
fracture – wheat and rye husks were 35% and 20%
better than soft wood fibres, respectively.
‘This type of lignocellulosic waste is ecofriendly,
available and cheap,’ says Al Mamun. ‘Wheat
and rye husk reinforced composites could be
used together with wood fibre composites, or as
a substitute.’
Film awards
PLA’s main application – as a film material – is far
from forgotten. A widespread technique used to
make barrier films is being increasingly applied to
PLA. Biaxial orientation involves stretching a film
in two directions to improve strength and barrier
properties. It is routinely used to make films out of
polypropylene (‘BOPP’) and PET (‘BOPET’). Now it is being applied to PLA to create ‘BOPLA’, which
currently accounts for less than 1% of ‘bi-oriented’
production.
There are even some natural advantages: PP
is stretched around five times in the machine to
create BOPP – this is just three-and-a-half times
for PLA. In the transverse direction, the elongation
is a factor of 10 for PP, but just five for PLA.
‘Biodegradable film is quite a young and
booming market,’ says Christian Aigner, head of
marketing at film machinery company Brueckner.
‘Growth rates in this area are expected to be well
above 10 per cent per year.’ He points to a number
of existing applications that could be replaced
with BOPLA, such as twist-wrap cellophane, for
confectionery; multi-layer food packaging film;
and shrink sleeve films.
Direct material substitutions are often
impractical, but Aigner says that existing production
lines – used to make BOPP, for example – can be
adapted to produce BOPLA. They would need to
take account of differences between PLA and other
plastics, such as PLA’s sensitivity to humidity. Other
changes are needed for resin handling, extruder
layout and the electrical system.
‘Each line needs an individual check and
recommendation for a retrofit,’ says Aigner.
Once film has been stretched, it is commonly
metallised – and this has begun to happen with
PLA. Celplast Metallized Products (CMP), a US film
specialist, has developed a metallised version of
PLA, called Enviromet. A clear roll of PLA film is
placed in a high vacuum metallising chamber, air
is pumped out, then high purity aluminium wire
is evaporated at the bottom of the chamber and
film is run over this vapour at high speeds. The
aluminium condenses on the film surface, creating
a high barrier layer.
Such a technique will be used on a
groundbreaking snack food package. Frito-Lay’s
SunChips, crisps which are sold widely in the US, are
packed in three-layer bags. The outer layer recently
shifted to PLA – and the manufacturer, a subsidiary
of PepsiCo, expects to make all three layers out of PLA
by next year. Dante Ferrari, chief operating officer of
CMP, says: ‘This is a large volume application that is
likely to switch to metallised PLA by next year.’
Further in the future – perhaps within five years
– Aniruddh Menon says bioplastics could develop
into a specialist resin for medical products. ‘One
key trend in medical devices is towards home care
treatment,’ he says. ‘These devices are commonly
designed to be disposable – but they are made
from conventional plastics.’
If the performance of biodegradables could
be raised to compete with engineering resins
such as polyamide or polycarbonate – and they
can gain the necessary medical approvals – he
thinks they could have a significant impact on the
market. ‘The medical devices market is growing
fast, and could be a key area for bioplastics,’
he says.
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