Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
Investigating the Process of
Producing Polylactic Acid as the
Base Polymer of Biodegradable
Plastics
V.A. Fomin,1 L.P. Korovin,1 L.N. Beloded,1 Yu.A. Kurskii,2 S.I.
Shkurenko,3 E.V. Monakhova,3 and A.G. Petrov3
1“Scientific
Research Institute of Polymers” Federal State Unitary Enterprise,
Dzerzhinsk
2Institute
of Organometallic Chemistry (IMKh), Russian Academy of Sciences,
Nizhnii Novgorod
3All-Russian
Synthetic Fibre Scientific Research Institute (VNIISV) Federal State
Unitary Enterprise, Tver’
Summary
The laws governing the synthesis of polylactic acid were studied, and the
dependence of its molecular weight on the production method and the nature of
the catalyst used was established.
The production of synthetic plastics in Russia now amounts to 2–2.5 million t/
year, and almost half of them are used in the manufacture of food packaging,
disposable utensils, materials of public health and medical designation, and
also film for agriculture.
After use, all these products become waste, a negligible proportion of which
ends up at organised waste disposal sites, and the bulk of which is discarded
in residential districts, at workplaces, and in recreational areas, and which
for hundreds of years will release harmful substances into the atmosphere, a
situation that will continue to worsten with the passage of time [1–4].
This paper was originally published in Plasticheskie Massy,
number 12, 2009, in Russian
©Smithers
Rapra Technology, 2011
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
35
V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
Environmental pollution by polymer waste has become a global ecological
problem and as a result is doing irreparable damage to the health on a genetic
level.
The question arises as to how to tackle the situation that has evolved, and
of course the obvious answer is to search for and develop new classes of
high-molecular-weight compounds possessing the same physicomechanical
and service properties as the multitonnage polymers produced at present
but capable, after use, of biodegrading in the environment into harmless
components to humans [5–7].
The aim of this report is to investigate processes of producing lactic-acidbased polymers that are biodegradable under natural conditions [8, 9].
Polylactic acid comprises a polyester of the form
which is produced by a minimum of two independent methods.
1. The polycondensation of monomeric lactic acid by the scheme [10–13]
the reaction being conducted in the presence of catalysts or without them,
all depending on the magnitude of the molecular weight of the polyester
being produced.
2. The decyclisation of the lactide with the formation of a linear polymer
in the presence of catalytic systems of different nature by the scheme
[14–16]
It is to the study of questions of the influence of the nature of the initial reactants
and the influence of the type of catalyst and the temperature, pressure and
time parameters on the magnitude of the molecular weight of the polylactic
acid formed that this work is also devoted.
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Polymers from Renewable Resources, Vol. 2, No. 1, 2011
Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
EXPERIMENTAL
In the production of polylactic acid, L+-lactic acid produced by Aldrich was
used as the initial product.
The molecular weight distribution (MWD) of the polylactic acid was determined
by gel-permeation chromatography (GPC). GPC was conducted on a unit
with a set of five styrogel columns with a pore diameter of 105, 3 × 104, 104,
103, and 250 Å (Waters, USA). An R-403 differential refractometer (Waters)
was used as the detector. The eluent was tetrahydrofuran.
NMR measurements were conducted on a Bruker DPX-200 spectrometer
with a working frequency of 200 MHz for protons and 50 MHz for 13C nuclei.
The specimens investigated were dissolved in deuterochloroform produced
by Aldrich. The internal standard was TMS.
All synthetic work was conducted in a glass vessel with controlled temperature
and vacuum.
The synthesis conditions of polylactic acid by direct polycondensation of lactic
acid were investigated in an azeotropic solvent – toluene. Polycondensation
was conducted in a reactor equipped with a mechanical stirrer, a thermometer,
and a Dean–Stark trap for the removal of water. As 85% aqueous L+-lactic
acid was used as the initial product, the water present was removed at the
first stage of synthesis, and, after water removal, 1.5% p-toluenesulphonic
acid was loaded into the reaction mixture and heating was continued for 6 h
at 125–135°C until the formation of reaction water ceased.
At the end of the reaction, the catalyst was removed by washing, and, at a
bath temperature of 135°C, highly volatile substances in the reaction mass
were distilled off. As a result, a solid yellow product was obtained from the
initial monomeric acid in 94% yield.
In order to remove low-molecular-weight impurities from the polylactic acid,
and to obtain a colourless product, the reaction mass was subjected to
recrystallisation from organic solvents, and a product with a melting point of
117°C was formed.
The results of determining the molecular weights of the synthesised specimens
of polylactic acid by GPC in the presence of different catalysts are given in
Table 1.
A comparative analysis of the molecular weights of polylactic acid in the presence
of the given catalysts indicates that the values of the number-average (Mn) and
weight-average (Mw) molecular weights differ fairly considerably. GPC data
(Figure 1) of polylactic acid synthesised in the presence of p-toluenesulphonic
acid clearly confirm, by the shape of the curve, the high polydispersity of the
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
37
V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
Figure 1. Gel chromatogram of polylactic acid obtained in toluene in the presence of
the catalyst p-toluenesulphonic acid
Table 1. Molecular parameters of polylactic acid produced in toluene
Specimen Catalyst
number
Values of average
molecular weights
Mn
Mw
Coefficient of
polydispersity Pn
1
p-Toluenesulphonic acid
800
1700
2.1
2
Benzenesulphonic acid
870
1750
2.0
3
Sulphuric acid
850
1700
2.0
product investigated. Here, the value of the molecular weight of the polylactic
acid investigated can be characterised as 200 < Mw < 14 500.
Thus, it was shown that, in the production of polylactic acid from monomeric
lactic acid in toluene in the presence of acidic catalysts, only low-molecularweight polylactic acid is formed.
To produce polylactic acid of higher molecular weight, an investigation
was made of the process of polycondensation of lactic acid under harsher
temperature conditions and in the presence of a metal-containing catalyst.
After water had been distilled off from the initial lactic acid, 0.5% tin dichloride
was introduced into the reactor, and heating of the reaction mass was begun,
initially for 2 h at 140–150°C under a vacuum of 200–65 mmHg, after which
the temperature was raised to 170°C with a residual pressure of 10–3 mmHg,
and heating was continued for 4.5 h until the formation of condensation water
ceased.
The obtained reaction mass was a solid, dark-brown, brittle substance with
a melting point of 70–75°C. Purification by recrystallisation from organic
solvents did not give a positive result.
The molecular weight of polylactic acid produced in the presence of 0.5%
SnCl2, determined by GPC, is practically 3 times higher than the molecular
weight of the product obtained in toluene. However, as shown in Figure 2,
high fractional composition inhomogeneity with respect to molecular weights
is observed in this case.
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Polymers from Renewable Resources, Vol. 2, No. 1, 2011
Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
Figure 2. Gel chromatogram of polylactic acid obtained in the presence of 0.5% SnCl2
In order to optimise the synthesis of polylactic acid, investigations were made
of the polycondensation of lactic acid in the presence of 0.5–1.0% catalyst
SnCl2 in a wide temperature and pressure range.
Polycondensation was studied at 140–220°C under a vacuum of 200–
2.0 mmHg, with special attention paid to the time factor.
Efforts to optimise the production of polylactic acid in the presence of SnCl2
made it possible to synthesise a polymer with higher molecular weights, as
shown in Table 2. Here, the weight-average molecular weight of the product
reached values of 13 100–14 700.
Table 2. Molecular parameters of polylactic acid produced in the
presence of different SnCl2 concentrations
Specimen
number
Concentration of catalyst
SnCl2, %
Values of average
molecular weights
Mn
Mw
Coefficient of
polydispersity
Pn
1
0.5
2000
6000
3.0
2
0.7
3700
13 100
3.5
3
1.0
3300
14 700
4.4
By purifying a specimen of polylactic acid obtained in the presence of 1%
catalyst by recrystallisation from chloroform, it was possible to produce a
grey powder with a melting point of 129–131°C that was characterised by a
narrower fractional composition (Figure 3).
In spite of the use of SnCl2 as the lactic acid polycondensation catalyst, and
the carrying out of synthesis under deep vacuum at high temperature, it still
proved impossible to obtain a polyester with a high molecular weight and a
low polydispersity coefficient.
A possible way to solve these problems is to investigate the process of
polylactic acid production using lactide as the initial feedstock. The lactide
is produced from lactic or oligolactic acid by the following scheme:
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
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V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
The use of a lactic acid oligomer as the initial feedstock [17] is preferable to
the use of monomeric acid [18], as in the former case the reaction system
for lactide production contains a much smaller amount of volatile products/
impurities (water, lactic acid, pyroracemic acid) which contaminate the final
product. In connection with this, only the process of lactide production via a
lactic acid prepolymer was investigated.
The process of producing the prepolymer itself from monomeric lactic acid was
studied in an azeotropic solvent to remove water in the presence of catalysts of
different nature: p-toluenesulphonic acid, o-phosphoric acid, and also zinc oxide.
As a result of a package of investigations, the process of polycondensation of
lactic acid in the presence of p-toluenesulphonic acid but without a solvent was
chosen as the optimum variant for lactic acid oligomer production. Synthesis
was conducted in the presence of 1.5–2.0% catalyst at a temperature of
140–160°C for 2 h and 3 h at 160°C under a vacuum of 70–50 mmHg. As
a result, a product was obtained with an acid number of 48–52 mg KOH/g
product, which corresponded to an average molecular weight of 1100–1200.
After oligomerisation, a sample was taken from the reactor, in which the
content of p-toluenesulphonic acid was determined. The latter was neutralised
with a calculated amount of 30% solution of sodium hydroxide. After this,
residual water was removed from the reaction vessel at 110–130°C under a
vacuum of 150–100 mmHg, and a light lactic acid oligomer with an average
acid number of 50 mg KOH/g product was obtained. It was this oligomer that
was subsequently used for lactide production.
The lactification of lactic acid oligomer with a degree of polycondensation
of 12–16 was investigated in the presence of tin octoate and zinc powder
catalysts [19–21].
It must be noted that, in the presence of 0.5% tin octoate at a reaction mass
temperature of 170–190°C, no appreciable lactide formation was observed,
and only when the catalyst concentration was brought up to 1.3% and the
temperature to 210°C under a vacuum of 200–100 mmHg did the lactide yield
amount to 20% of the theoretical value. Further increase in the tin octoate
concentration to 2% did not lead to any increase in the lactide yield.
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Polymers from Renewable Resources, Vol. 2, No. 1, 2011
Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
Zinc powder was investigated as a second catalyst. The initial catalyst
concentration amounted to 0.5% of the acid oligomer.
Lactide synthesis was conducted at a temperature of 180–220°C under a
vacuum of 200–100 mmHg, and here the distillate was collected in an icecooled reception vessel, which solidified into a waxy mass.
It must be pointed out that, with increase in the amount of catalyst to 1.5%,
lactification proceeded more intensely. The raw lactide yield amounted to 62–74%.
The raw lactide obtained was a bright-yellow waxy product with the acrid
odour of burnt pitch. The lactide was purified by recrystallisation from ethyl
acetate. The raw material was dissolved in a minimum amount of ethyl acetate
at 45–60°C and then held at 7–10°C. The final product precipitated in the form
of white crystals. After triple recrystallisation, an odourless white lactide with a
melting point of 94.5°C was obtained, which was similar to published data [22].
To determine the purity and individuality of the obtained product, it was
subjected to analysis by NMR spectroscopy. In the 1H NMR spectrum in CDCl3
there are two groups of lines: a doublet at 1.65 ppm (CH3) and a quartet at
5.10 ppm (CH) (Figure 4).
Figure 4. 1H NMR spectrum of the lactide obtained
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
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V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
In the 13C NMR spectrum there are three lines: 15.7 ppm (CH3), 72.5 ppm
(OCH) and 167.7 ppm [C(O)O] (Figure 5). No impurities were found in the
lactide. It was this lactide that was subsequently used as the initial feedstock
when investigating the process of polylactic acid production.
Figure 5. 13C NMR spectrum of the lactide obtained
Lactide polymerisation was conducted in the presence of catalyst tin octoate
[16, 23].
To the lactide was added 0.1% catalyst, and the mixture was heated to 110°C
during constant stirring for 30 min. At the end of this time it was assumed that
the solution was homogenised and that the catalyst was evenly distributed
throughout the lactide.
The mixture obtained was charged into four ampoules and heated. The
temperature and time parameters of the polycondensation regime are given
in Table 3.
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Polymers from Renewable Resources, Vol. 2, No. 1, 2011
Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
Table 3. Polycondensation conditions of lactide in the presence of
tin octoate
Specimen
number
Heating time of specimen, h, at different polymerisation temperatures
125°C
130°C
150°C
160°C
170°C
1
1
1
2
1
1
13
—
—
13
4.5
—
3
1
4
1
1
13
4.5
5
1
13
4.5
9
At the end of heating, all ampoules were opened and the contents were readily
taken out in the form of white solid rods. The polylactic acid was odourless.
To determine the influence of the temperature and time parameters on the
molecular weight of the polylactic acid formed, all four specimens were
studied to establish the extent to which their properties depended on the
production conditions.
Firstly, specimens 1 to 4 were subjected to purification to remove lowmolecular-weight impurities by dissolving them in chloroform with a ratio of
polylactic acid to chloroform of 1:10 respectively. After the polylactic acid
had dissolved entirely, methanol was added to the solution formed in a ratio
of acid solution in chloroform to methanol of 1:1.
When methanol was added, the mixture became cloudy and comprised a
milky white solution. The latter was held for 1 h at room temperature and
subjected to separation of the polymer on a Schott filter.
Filtration was conducted under a vacuum of 300–200 mmHg, which made
it possible to separate the polymer from the solvent fairly rapidly and fully.
The precipitated polymer was dried in an oven at 100°C to constant weight,
which produced a white, granular, odourless powder.
The results of reprecipitation of the polylactic acid polymer and the properties
of the reprecipitated product are given in Table 4.
Table 4. Yield and properties of reprecipitated polylactic acid
Specimen number
Yield of reprecipitated
product, %
Melting point of
reprecipitated product, °C
1
72.1
172
2
79.1
179
3
68.2
175
4
86.5
177
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
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V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
It is noteworthy that the yield of reprecipitated polymer increases with increase
in the heating time and with increase in the polymerisation temperature, with
the exception of specimen 3, for which the yield and the melting point depart
from the general dependence, although it should be noted that the melting
points of the reprecipitated products are roughly equal.
Purified specimens 1 to 4 were subjected to analysis and assessment of their
characteristics using gel-permeation chromatography and NMR spectroscopy.
All four specimens of polylactic acid were presented for investigation by
GPC, but only specimens 1 to 3 were subjected to analysis; the polymer of
specimen 4 was insoluble in tetrahydrofuran.
The molecular parameters of specimens 1 to 3 were calculated according
to polystyrene calibration. This method does not enable the true values of
the molecular weights (MWs) to be obtained, but it does make it possible
to conduct a correct comparison of the molecular weight distribution of
specimens of the same nature.
Chromatograms of polylactic acid specimens 1 to 3 are presented in Figure 6.
As follows from Figure 6, polylactic acid specimen 1 contains a polymer with
a MWD of 2100–250 100, with a maximum of 18 600. However, a ‘shoulder’
with M = 28 800 can be seen on the chromatogram. Specimens 2 and 3
contain fractions with the same weights of 2100–250 100, but, instead of the
shoulder (M = 18 600), a second peak appears, i.e. a bimodal distribution of
specimens 2 and 3 is observed.
The molecular parameters presented in Table 5 indicate that the weightaverage molecular weights of specimens 2 and 3 are identical: 27 500 and
27 900 (experimental error 10 rel.%).
Figure 6. Gel chromatograms of polylactic acid specimens 1 to 3 44
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
Table 5. Values of the molecular weight characteristics of specimens
of polylactic acids 1 to 3
Specimen number
Average molecular weights
Polydispersity Pn
Mn
Mw
1
16 200
22 400
1.4
2
18 300
27 500
1.5
3
16 100
27 900
1.7
For specimen 1, the weight-average MW is 22 400, which is lower the Mw
values of specimens 2 and 3; this is probably due to its lower polymerisation
temperature compared with specimens 2 and 3.
In all likelihood, polylactic acid specimen 4, which was insoluble in THF, had
an even higher Mw, as it was heated at 170°C for 9 h.
The coefficients of polydispersity Pn of the investigated specimens of polylactic
acid increase from 1.4 to 1.7, which is due to increase in the area of the
second peak (with higher molecular weights) of the bimodal distribution of
polymer specimens 1 and 3.
It must be noted that lactide-based polylactic acid not only has a higher
molecular weight but also has a narrower molecular weight distribution than
products based on monomeric lactic acid.
In the investigation of polylactic acid specimens 1 to 4 by means of NMR
spectroscopy (Figures 7 and 8) it was established that, in 1H NMR spectra,
intense doublets are observed at 1.58 ppm, and a quartet at 5.17 ppm, relating
to protons of the CH3 and CH groups of the main chain. Furthermore, a weak
doublet and a quartet at 1.49 and 4.36 ppm are observed, relating to the end
fragment of the chain of polylactic acid H[OCH(CH3)CO]nOH.
Only at one end of the polylactic chain is there a methine carbon atom HO–
CH– bound to a hydroxyl group, and all remaining methine fragments of the
polymer chain are bound with an ester group –C(O)O–CH.
The ratio of integral intensities of protons of the methine fragments of the
main chain and the end methine fragment makes it possible to determine the
average length of the polylactic acid chain, and consequently the molecular
weight of the polymer (Table 6).
As shown in Table 6, the values of the number-average molecular weights
of specimens of the polylactic acid obtained, calculated by means of 1H
NMR, correlate to a certain degree with the values of the molecular weights
obtained by GPC.
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
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V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
Figure 7. 1H NMR spectrum of polylactic specimen 1
Figure 8. 13C NMR spectrum of polylactic acid specimen 1
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Polymers from Renewable Resources, Vol. 2, No. 1, 2011
Investigating the Process of Producing Polylactic Acid as the Base Polymer of Biodegradable Plastics
Table 6. Properties of polylactic acid according to the results of NMR
spectroscopy
Specimen number
Number of monomer units in
chain
Number-average molecular
weight Mn
1
110
7940
2
126
9090
3
110
7940
4
153
11 030
Both methods confirm the regularity of increase in molecular weight during
lactide polycondensation (with the exception of specimen 3) with increase
in the time and increase in the temperature of the polymerisation process.
13C NMR spectra of polylactic acid of specimens 1 to 4 consist of three lines at d
16.7, 69.1, and 169.6 ppm, relating to CH3, CH, and C(O)O carbons respectively.
The absence of splitting indicates that, in the course of polylactic acid formation,
the racemisation of the initial optically active lactide does not occur.
In the case of racemisation, fragments of the polymer chain (diads, triads,
etc.) of different stereoregularity would appear, which would lead to the
appearance of additional signals.
The synthesised biodegradable polylactic acid with Mw = 27 000 has
successfully undergone tests as the base thermoplastic in the production of
new specimens of adhesive melts.
CONCLUSIONS
1. Variants of the synthesis of polylactic acid from monomeric lactic acid in
the presence of acidic and metal-containing catalysts were investigated.
It was shown that, by this method, it is possible to produce a polylactic
acid with a maximum weight-average molecular weight of 14 700 with a
high polydispersity.
2. A method for synthesising oligolactic acid with a degree of polymerisation
of 12–16 was devised, and, on its basis, the conditions of lactide
production in the presence of a catalyst – zinc powder – with a 62–74%
yield have been optimised.
3. Through the polymerisation of lactide in the presence of tin octoate,
polylactic acid with a weight-average molecular weight of 27 000 has
been obtained. Methods of gel-permeation chromatography and 1H
NMR have been used to investigate the dependence of the molecular
weights of the polymers formed on the temperature and time parameters
of lactide polymerisation.
Polymers from Renewable Resources, Vol. 2, No. 1, 2011
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V.A. Fomin, L.P. Korovin, L.N. Beloded, Yu.A. Kurskii, S.I. Shkurenko, E.V. Monakhova, and A.G. Petrov
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