The invention relates to the removal of lactide from polylactide by
ensuring effective material transfer during evaporation and relates to the recovery
of lactide from the process for the preparation of polylactide.
In recent years, interest in biodegradable polymers, i.e. biopolymers,
has greatly increased, and many companies have made efforts to launch on the market
packaging materials, hygiene products, sacks and films for agricultural purposes,
and sacks for waste. In particular, various films have gained importance. The use
of polymers of lactic acid in these applications has so far been limited by the
high price of the polymers and their susceptibility to breaking down during technical
Polyhydroxy acids can be prepared by polycondensation reactions, which
are typical in the preparation of polyesters, or by ring-opening polymerization
from cyclic lactones. Polyhydroxy acids are thermoplastic polyesters which resemble
many conventional polymers.
Polylactides, or condensation polymers based on lactic acid, are for
many reasons especially attractive, since their principal degradation product,
lactic acid, is a product common in nature, it is not toxic and is used widely
in the food and pharmaceutical industries. A high molecular weight polymer can
best be produced by ring-opening polymerization from the dilactone, also called
lactide. Lactic acid is optically active, and thus its dimer appears in four different
forms: L,L-lactide; D,D-lactide; L,D-lactide (mesolactide); and a racemic mixture
of L,L- and D,D-lactides. By polymerizing these either as pure compounds or at
different blend proportions, polymers are obtained which have different stereochemical
structures affecting their resilience and crystallinity and, consequently, also
their mechanical and thermal properties. The obtained polymers are usually hard
and optically bright.
The ring-opening polymerization of cyclic lactones of hydroxy acids,
such as lactide, glycolide, epsilon-caprolactone, etc., constitutes technology
known per se. The polymerization processes known are various, some examples
being patent US 5,378,801 relating to extrusion polymerization, patent publication
EP 0 664 309-A relating to two-step polymerization, and patent publication EP 0
499 747-A describing polymerization in a mixing reactor.
The polymer or copolymer according to the present invention can be
prepared from L-lactide, D-lactide or D,L-lactide, or blends thereof, by any polymerization
process, batch, semi-continuous or continuous. Continuous polymerization can be
carried out advantageously by polymerization in an extruder. The polymer is produced
by heating the monomer or the monomer blend to produce a homogeneous melt and by
adding a catalyst in order to polymerize the lactide, whereupon the ring opens.
The molar mass (Mw) of the polymer is approximately 20000-500000, preferably
40000-300000. Preferably the polymer is prepared from L-lactide.
The polymer is upon formation in equilibrium with its monomer, lactide.
This has sometimes also been viewed as being advantageous, since monomers and oligomers
may act as polymer plasticizers. However, it also leads to rapid hydrolysis and
causes problems of adhesion in the processing of the polymer. Furthermore, the
presence of monomer lowers the thermal stability during melt processing. If conventional
products to be produced by melt processing are aimed at, the residual monomer must
be removed from the polymer. An acceptable monomer concentration is below 2 % by
weight, preferably below 1 % by weight.
The lactide can be removed from the polymer by extraction or by evaporation
from the polymer melt. In evaporation, there is the problem of too long evaporation
times, in which case the polymer begins to break down. Evaporation methods are
listed in patent application EP 0 499 747-A. These methods include the removal
of lactide under a vacuum by evaporation from falling polylactide threads, thin
film evaporation, and the use of a vacuumized extruder-type evaporator.
In a vacuumized extruder, evaporation is ineffective owing to a poor
transfer of material when the capacity is even slightly higher.
In order to melt process the polymer, the polymer must usually first
be stabilized. A preferred stabilizing method ref. Patent FI99124 is reactive extrusion
with peroxy compounds, whereby the melt strength of the polymer is increased, and
no stabilization agent residues are left in the polymer. The stabilized polymer
material can be used for preparing products by conventional melt-processing methods,
for example films by the blowing method, or the polymer may also be used for making
flat films or sheets, or other products.
It has been found out that an efficient method for the removal of
lactide from polylactide and recovery of lactide from a lactide-containing gas
is obtained when a polymer melt traveling through a nozzle forms thin threads the
surface area of which is so large that in a normal-pressure or vacuum evaporator
the lactide evaporates from the polymer rapidly into a hot carrier-gas flow and
the polymer settles under gravity onto a collecting device and a hot lactide-containing
gas is cooled rapidly to a temperature below 100 °C, whereupon the lactide crystallizes
from gas, forming lactide crystals, which are separated from the gas.
According to the invention, the polymer melt is fed through a nozzle,
whereby it is fibrillated into thin threads, and the lactide evaporates rapidly
into a hot evaporation-gas flow from the outer surface of the formed threads. It
is essential in the invention that the threads, sufficiently thin, fall under gravity,
and under the laminar flow thereby formed the polymer melt will flow more rapidly
in the inner parts of the thread than in the surface part. Thereupon the polymer
melt flowing in the inner part of a sufficiently thin thread will form, when flowing
downwards, a new material transfer surface for lactide evaporation.
A preferred embodiment of the invention is depicted in Figure 1. According
to it, the polymer melt coming from the polymerization (1) apparatus is fed by
means of a melt pump into a nozzle part (2), which may be, for example, of the
perforated sheet type. In the nozzle the melt is compressed into numerous threads
the overall circumference of which is maximal in order to maximize the surface
area of the forming threads during the evaporation. In the normal-pressure or vacuum
drier (3) according to the figure, preferably cylindrical, lactide evaporates rapidly
into the hot carrier gas flow (4) from the outer surface of the threads formed.
According to the invention it is essential that the size of the nozzle
perforations is sufficiently small; a suitable perforation diameter is 0.1 - 1.0
mm, preferably 0.2 - 0.3 mm.
Commonly in the removal of lactide by evaporation there is the problem
of polymer degradation. By the method according to the invention, degradation is
prevented, since owing to the effective transfer of material the evaporation time
is sufficiently short. The retention time of the polymer melt in the apparatus
according to the invention is typically less than 10 s, preferably less than 5
The evaporation gas is directed into the evaporator through a net,
perforated sheets, or the like, in order to achieve an even distribution of the
drying gas. The velocity of the gas relative to the melt threads is approx. 0.5
- 1 m/s, in order to achieve a sufficiently effective transfer of material. The
evaporation gas must be hot and preferably dry, typically nitrogen or air, but
other carrier gases can also be used. In the evaporation part the temperature is
typically 150 - 300 °C, preferably 220 - 260 °C.
It has been observed experimentally that, if dry nitrogen is used
as the evaporation gas, the temperature of the evaporation part may be above 250
°C, the molecular weight and the molecular weight distribution of the polylactide
still remaining constant.
The polymer melt threads settle under gravity on the collecting device,
such as a cylinder, of the evaporation part, and from there the polymer melt is
fed into a pelletizer. The collecting device (5) is preferably a heated cylinder
having a rotational velocity suitable for collecting the settling polymer melt
evenly on the cylinder surface, from which it is fed by means of a scraper to a
pelletizing and/or stabilizing apparatus (6).
The retention time of the polymer melt on the collecting cylinder
must also be short, preferably less than 1 min, in order to prevent degradation
reactions of the polymer.
By the lactide removal method according to the present invention,
a polymer is obtained having a lactide concentration of 0-4 % by weight, preferably
0-2 % by weight. The lactide concentration can be regulated by means of the temperature
and the retention time in the evaporation part.
According to another preferred embodiment of the invention, the nozzle
used is a film nozzle (2), in which case the polymer melt is fed as an even film
onto a horizontally rotating drum, from where it is directed as a flowing melt
film into the evaporation part (3) and from there to the lactide (10) and polymer
recovery units (6), respectively.
In the evaporation part the evaporating lactide travels along with
the drying gas to a lactide recovery unit (10), and the recovered lactide may be
recycled to polymerization.
The lactide-containing gas mixture leaving the lactide evaporator
(3) is cooled rapidly, whereupon the lactide crystallizes from gas. Typically the
gas mixture (7) is directed to an ejector or a gas nozzle (8), to which cold air
(9) is fed and sprayed into the crystallisation chamber (10). On entering the ejector
the lactide gas has a temperature of 120-300 °C, depending on the evaporation method.
In the ejector the lactide-containing gas mixed with cold air is cooled to below
100 °C, preferably to a temperature range of 20-40 °C, whereupon it crystallizes
from gas, forming small lactide crystals in the gas. Fresh air (11) is fed into
the crystallisation chamber via gas distributer unit (12). The lactide powder is
separated from the gas by cyclone (13) and filtration system (14). The lactide
powder is collected from the crystallisation chamber (10), cyclone (13) and filtration
unit (14) into the powder pot (15).
The lactide recovered by the method according to the invention is
in powder form, and it is easy to handle and can be recycled to the polymerization
process. The degree of purity of the recovered lactide is very high, and its optical
structure is correct.
The yield with the recovery method is high, higher than 90 %.
The process diagram of a preferred embodiment is shown in Figure 1.
The invention is described further in greater detail with the help
of the following examples.
The evaporation of lactide by the method according to the invention
was investigated by varying the feed rates of the drying gas and the evaporation
temperatures. The drying gas used in these experiments was dry air. The processing
parameters used and the lactide concentrations in the obtained polylactide are
shown in Table 1.
The rate of polymer fed into the evaporation apparatus was 5 kg/h;
the temperature of the feed polymer was varied.
The polylactides (PLA) used had been prepared by extrusion polymerization,
the manufacturer being Neste Oy.
PLA temperature °C
PLA lactide concentration % by weight
Drying temperature °C
Drying gas feed kg/h
Lactide concentration of product % by weight
Molecular weight of product (1000)
The lactide used in the experiments had been recovered by evaporation
after the L-lactide polymerization process.
Cold air was mixed in a gas mixer according to Figure 1 with a lactide-containing
gas having a temperature of 200 °. The ratio of the cold air feed to the hot lactide-containing
gas was 10 kg of feed air / kg of lactide-containing gas. The lactide crystallized
completely in the form of powder in the cooled gas. The crystallized lactide powder
was recovered by vibration from the chamber walls and by separating it from the
air flow by filtration, whereafter it flowed freely into the lactide-collection
The yield of recovered lactide was over 90 %.
The separated lactide was analyzed by DSC analysis, and according
to this analysis it was noted that the recovered lactide was pure L-lactide, and
it had not been racemized during the evaporation and the recovery. Figure 2 shows
a DSC diagram showing the purity of the L-lactide.
If a copolymer of D-lactide and L-lactide is prepared, the recovered
lactide is also a blend.