Transcript Document

ADVANCED BIO-FRIENDLY POLYMERS
Mulching foils – photochemical, hydrolytical
and biodegradation
Štefan Chmela
Plastic films for mulching of the soil are used to improve cultivation
conditions and to modify the soil microenvironment under the
covering by controlling soil humidity and temperature.
Mulching films reduce water evaporation from the soil and the
washout of nutrients into the ground water while the cultivation
area is protected against erosion.
The use of mulching film enables reduction of weeds and plant
diseases coming from the soil, thus reducing the use of pesticides.
Also plants are cleaner; the cultivation can start at lower soil
temperatures; yield is higher with mulching films.
 Foils thickness 12 – 80 µm
 Width up to 3 m
 Lifetime 2-4 months
 Colorless or pigmented (carbon black)
Current intensive and semi-intensive agricultural practices used
throughout Europe require the use of large quantities of plastics.
Recent data suggest that agriculture and horticulture is
responsible for a consumption of some 1 500 000 t/year of all
polymers in Europe.
Concerning the category of thin films, more than 130 000 t/year
mulching films are consumed per year in Europe and 2 600 000
t/year worldwide (2003–2005).
The corresponding consumption of direct cover and low tunnel
films in Europe are 72 000 and 75 000 t/year, respectively.
The extensive and expanding use of plastics in agriculture results in
increased accumulation of plastic waste in rural areas.
Part of this plastic waste may be recycled, especially the greenhouse
films, silage films and fertilizer sacks, pipes and other plastic
products.
Another part of the agricultural plastic waste is difficult to recycle for
technical and/or financial reasons. A major agricultural plastic wastes
category of low-recyclability (in general) are the thin mulching films
and in some cases, thin low tunnel and direct cover films. These films
are too thin and usually heavily contaminated by soil and foreign
materials.
The most common current disposal practices for the non-recyclable, but in many
cases also for recyclable agricultural plastic wastes, is burying in the soil (mulching
films), burning, or disposing them at the open fields or in landfills. These practices
are illegal of course and have serious negative consequences for the environment
Thus, for example polyethylene (PE) based mulching films do not break down in
soil and should never be roto-tilled or incorporated into the soil
However, the process of recovering and recycling them, following the end of the
cultivation period, is difficult as approximately 80% of the weight of the recovered
waste mulching film is foreign materials (e.g. soil, sand etc.). Also the cost of
removing from the soil and cleaning this material is prohibitively high. This is the
main reason why the farmers usually incorporate them into the soil by roto-tilling, a
practice which, apart from being illegal, also implies a serious risk for the
environment due to the accumulated PE in the soil.
An alternative option for disposing non-recyclable agricultural plastic waste is their
use as alternative fuel for energy recovery (at a high cost).
Specifically for the case of agricultural plastic wastes that cannot be easily collected and
recycled, a very attractive alternative is biodegradation. This refers to the replacement of
conventional agricultural plastics, which cannot be recovered from the field for technical
and/or financial reasons, with biodegradable mainly bio-based ones, which will biodegrade
in the soil after the end of their useful lifetime without leaving toxic or polluting remains.
The current use of biodegradable, mainly bio-based, plastics in agricultural applications in
Europe is very limited (about 2 000 t/year in 2006). However, the use of biodegradable
polymers for agricultural plastics is increasing for specific applications in the agricultural
sector.
Bio-based polymers are now moving into main-stream use for many applications
(packaging being the dominant one), and the polymers based on renewable
‘‘feedstock’’ may soon be competing with commodity plastics, as a result of the
sales growth of more than 20–30% per year.
Plastic mulching foils
PVOH
PLA
PTAB poly(butylene adipate-co-terephthalate)
PCL
PHAs
Poly(butylenes succinate) PBS
Photodegradable polyethylene with pro-oxidants
Several types of pro-oxidants (salts of iron manganese and cobalt, stearate) have
been designed for polymers used in landfill, compost and soil disposal applications.
As it is claimed from the technical guides and reports, these additives when
compounded with conventional polymers at appropriate levels control the lifetimes
of plastic films and articles.
This degradation leads to the fragmentation of the plastic waste without need for
collection and waste disposal.
It is claimed that the oxidized molecular fragments are hydrophilic, have molar
mass values reduced by a factor of 10 or more, and are biodegradable. However,
from the various reports it is stated that materials with the above additives are
fragmented into small parts, invisible to the eye, but it is not yet known if these
parts are really biodegradable i.e. accessible by microflora (fungi, bacteria and
the like) to convert and assimilate the carbon in any substrate and if yes, at which
rate.
polyethylene with pro-oxidants
Graph of fragmentation
versus biodegradation
The controversy over these materials is shown schematically in Fig.
Degradation/Fragmentation are shown to represent the fist (preliminary) stage of the
biodegradation process. Heat, moisture, sunlight and/or enzymes shorten weaken polymer
chains in this stage, resulting in fragmentation residues and cross-linking to create more
intractable persistent residues.
Biodegradation is the second stage of this process and is considered to occur only if
the fragmented residues are totally consumed by microorganisms as a food & energy source
and if this happens in an acceptable rate. Biodegradation of the fragments of photodegradable
(fragmentable) polymers based on polyethylene with pro-oxidants, remains however an open
question as it has not been proven beyond any doubt while it failed to all biodegradable
norms and standards
Europe is currently well placed in the markets for innovative bio-based products, building
on established knowledge and a leading technological and industrial position. However,
the bio-based polymers are not used widely as perceived uncertainty about product
properties and weak market transparency hinder the fast take-up of these products.
While some progress has been made with the expansion of the use of bio-based and
biodegradable (compostable) packaging materials, the development, use and expansion of
bio-based and biodegradable materials and products in the European Agriculture is very
much limited. The two main reasons for this hysteresis are
(a) the current cost of the bio-based and biodegradable plastics compared to the
conventional ones in certain applications,
and (b) the still open discussion with regard to testing agricultural biodegradable plastics
for biodegradation in soil and under farm composting.
The second one hinders the development of a relevant certification and labelling scheme
which could be implemented, for example, in synergy with the recently developed
labelling scheme for agricultural plastic wastes . For the same reason, confusion still exists
in the marker about the performance of these materials under real soil conditions that
hinders the wider expansion of their use in agriculturalapplications.
The rationale for the acceptance of biodegradable polymers in soil
under ambient conditions may be summarised as follows:
– Complete biodegradation (not simply disintegration): 90%;
– Duration: depending on application
– No harmful effect on soil quality and environment
(heavy metals—ecotoxicity)
ASTM D20.96, Standards development protocol for degradable plastic products
General mechanism of plastic biodegradation under aerobic conditions
Ecoflex
Commercially available biodegradable mulches
•
AGROERG S
•
Plastic Suppliers Inc., EarthFirst® PLA
•
•
Rootplast International Inc., Ohio, USA
(http://www.rootblast.cc/prodinfo.asp?number=ZWBR-8)
•
Waste not, Marchant Manufacturing Co. Ltd., UK
•
BAYER AG „BAK“ (polyester amide family of biodegradable
resins for agroculture applications, Germany)
•
Baoding Fengba Modern Agricultural Facility Co., Ltd.,
China (PE mulch and greenhouse film)
•
Shanghai HiTeC Plastics, Shanghai, China
LLDPE-PLA/starch blends
•
(ERG Bieruń–Folie Sp. z o.o., Poland), based on PE
MATER Bi ® (Novamont, Italy)
naturally biodegradable and compostable (PLA)
???
Materials
PP – polypropylene, Tatren HPF, Slovnaft, Bratislava
Ecoflex® F Blend C1200
Biodegradable polyester for compostable film BASF
statistical, aliphatic-aromatic copolyester based on the monomers 1,4butanediol, adipic acid and terephthalic acid in the polymer chain. Ecoflex® F
Blend C1200 will biodegrade to the basic monomers 1,4-butanediol, adipic
acid and terephthalic acid and eventually to carbon dioxide, water and
biomass when metabolized in the soil or compost under standard conditions.
Ecoflex® F Blend C1200 has properties similar to PE-LD
Samples from STU, Faculty of food and chemical technology
PLA-M1 (Svit) PLA/PHB = 85/15 wt. + 10 % TAC
Preparation of film by pressing
Photo-degradation
Irradiation in two equipments
1. Merry-go-round apparatus, 250 W medium pressure mercury arc,
laboratory temperature
2. Suntest CPS – Xenon arc, 40 oC (minimal temperature)
Analysis – FTIR spectroscopy, GPC
FTIR spectra of original films
5.0
4.5
4.0
3.5
Absorban ce
3.0
2.5
2.0
1.5
PLA-M1
1.0
0.5
Ecoflex
0.0
PP
-0.5
3000
2000
Wavenumbers (cm-1)
1000
Comparison of PP and Ecoflex photo-oxidation – carbonyl region
1.8
1.6
1.4
Ecoflex
0.8
420 h
0.6
545 h
330
1.0
0h
Absorban ce
1.2
0.4
0h
0.2
1800
1700
Wavenumbers (cm-1)
1600
Quantification for Ecoflex– almost impossible, huge carbonyl absorption,
questionable subtraction, similar for PLA-M1 (Svit) PLA/PHB = 85/15 wt. + 10 % TAC
Comparison of PP and Ecoflex photo-oxidation – hydroxyl region
2025 h
330 h
1.0
0.8
545 h
0.6
425 h
0.4
0.2
0 h
Absorban ce
1.2
0 h
1170 h
1.4
835 h
1.6
-0.0
3800
3600
3400
Wavenumbers (cm-1)
Quantification – problem of baseline
3200
Comparison of photo-degradation - PP and ECOFLEX - 250 W mercury arc
PP integral A3080-3600 cm-1
50
ECOFLEX integral A3080-3600 cm-1
Absorption a.u.
40
30
20
10
0
0
100
200
300
400
500
Irradiation time [h]
Faster production of –OH group for ECOFLEX in comparison with PP
PLA-M1 (Svit) PLA/PHB = 85/15 wt. + 10 % TAC
No visible changes in FTIR spectra / no Norish type reactions
5
1.8x10
Mn, Da;
Mp, Da
5
molar mass (Da)
1.5x10
5
1.2x10
0 hod
75 hod 150 hod
315 hod 315 hod 580 hod
1000 hod
4
9.0x10
4
6.0x10
4
3.0x10
0.0
0S0
1S75
1S150
2SVIT
2S315
irradiated samples
Not big changes in molar mass
3S580
4S1000