Claims:1. Process for producing thermoplastic, expandable and/or at least partially expanded polymer particles composed of a polymer matrix formed by polylactide or a polymer blend comprising at least 50% by mass of polylactide together with at least one further thermoplastic polymer, comprising the following steps:
(a) introduction of the polymer matrix into an extruder
and plasticization and homogenization of the
polymer matrix in the extruder with exclusion
of crosslinkers and/or chain extenders;
(b) addition of at least one organic blowing agent
from the group consisting of n-butane, isobutane
and propane including mixtures thereof and
dispersion of the at least one blowing agent in
the plasticized polymer matrix in the extruder;
(c) discharge of the plasticized polylactide admixed
with the at least one blowing agent from the
extruder through at least one extruder die; and
(d) pelletization of the extruded polymer strand
admixed with the at least one blowing agent
downstream of the extruder die to form the
expandable and/or at least partially expanded
polymer particles,
where the steps (c) and (d) are carried out in a cooling fluid under a pressure above ambient pressure.

2. Process according to Claim 1, characterized in that a proportion of polylactide in the polymer matrix of at least 80% by mass, in particular at least 90% by mass, preferably more than 95% by mass, is set.

3. Process according to Claim 1 or 2, characterized in that the at least one organic blowing agent is added in a proportion of from 1% by mass to 20% by mass, in particular from 2% by mass to 15% by mass, preferably from 3% by mass to 10% by mass, based on the mass of the polymer matrix.

4. Process according to any of Claims 1 to 3, characterized in that from 0 to 2% by mass, in particular from 0 to 1% by mass, based on the mass of the polymer matrix, of at least one nucleating agent, in particular based on talc, is added to the polymer matrix.

5. Process according to any of Claims 1 to 4, characterized in that a total of from 0 to 15% by mass, in particular a total of from 0 to 10% by mass, of at least one additive, in particular from the group consisting of dyes and pigments, flame retardants, stabilizers, infrared absorbers and processing aids, is added to the polymer matrix.

6. Process according to any of Claims 1 to 5, characterized in that a liquid, in particular an aqueous medium, is used as cooling fluid.

7. Process according to any of Claims 1 to 6, characterized in that the cooling fluid is kept under a pressure of
- at least 1.5 bar, in particular at least 2 bar,
preferably at least 5 bar; and/or
- not more than 30 bar, in particular not more than
25 bar, preferably at least 20 bar.

8. Process according to any of Claims 1 to 7, characterized in that the cooling fluid is kept at a temperature of
- at least 0°C, in particular at least 5°C,
preferably at least 10°C; and/or
- not more than 90°C, in particular not more than 70°C,
preferably not more than 50°C.

9. Process according to any of Claims 1 to 8, characterized in that the expandable and/or at least partially expanded polymer particles obtained in step (d) are prefoamed at a temperature in the range from 50°C to 150°C, in particular from 50°C to 100°C.

10. Process according to Claim 9, characterized in that the expandable and/or at least partially expanded polymer particles obtained in step (d) are prefoamed at essentially ambient pressure.

11. Thermoplastic, expandable and/or at least partially expanded polymer particles composed of a polymer matrix formed by polylactide or a polymer blend comprising at least 50% by mass of polylactide together with at least one further thermoplastic polymer, wherein the polymer matrix is free of crosslinkers and/or chain extenders and contains at least one organic blowing agent from the group consisting of n-butane, isobutane and propane including mixtures thereof, produced by a process according to any of Claims 1 to 10.

12. Polymer particles according to Claim 11, characterized in that they have
- a proportion of polylactide in the polymer matrix of
at least 80% by mass, in particular at least 90% by
mass, preferably more than 95% by mass; and/or
- in the compacted state according to step (d) a
bulk density of at least 150 g/l, in particular at
least 200 g/l, preferably at least 250 g/l; and/or
- after prefoaming at a temperature in the range from
50°C to 150°C for a period of 100 seconds at ambient
pressure a bulk density of not more than 125 g/l,
in particular not more than 100 g/l, preferably not
more than 75 g/l. , Description:The invention relates to a process for producing thermoplastic, expandable and/or at least partially expanded polymer particles composed of a polymer matrix which is formed by polylactide or a polymer blend comprising at least 50% by mass of polylactide together with at least one further thermoplastic polymer, and also thermoplastic, expandable and/or at least partially expanded polymer particles composed of a polymer matrix which is formed by polylactide or a polymer blend comprising at least 50% by mass of polylactide together with at least one further thermoplastic polymer which are produced in such a way.

Thermoplastic polymer particles which are expandable by means of a blowing agent or have been at least partially expanded, also referred to as polymer foam particles, are used primarily for producing polymer mouldings, with the polymer particles, whether they are already at least partially expanded or whether they are still essentially compact but expandable due to their blowing agent content, fusing to one another on the surface in an appropriate moulding tool, in particular under the action of hot steam, to form the moulding. If a proportion of the blowing agent is still present in the expandable polymer particles, the polymer particles are here expanded or foamed, as a result of which a large-area fusion of the expanded polymer particles with one another at a low density of the moulding can be achieved.

As an alternative, it is known that the expandable or already at least partially expanded polymer particles can be heated under the action of electromagnetic radiation, e.g. in the microwave range, radiofrequency range or the like, so that they are likewise fused to one another in the moulding tool, with, in particular, them likewise being at least partially expanded for the abovementioned reasons. If the polymers used in each case do not themselves have a sufficient absorption capacity for the respective frequency range of the electromagnetic radiation, they can be coated or wetted with a medium which absorbs electromagnetic radiation, e.g. in the microwave spectrum and/or radiofrequency spectrum, for example water.

Polymer mouldings produced in this way display, owing to the compressibility of the polymer foam of low density formed from the expanded and fused polymer particles, a high heat, sound and impact absorption capability and are therefore used predominantly for insulation materials such as insulation boards for insulating buildings or other insulating components, e.g. for roller blind boxes, window profiles, for heating engineering, for insulated containers and the like, in automobile technology, for packaging materials, as core materials of sandwich-like mouldings, e.g. sporting articles, surf boards, boat hulls, etc., for construction of models, etc. In addition, there are fields of use for loose expanded polymer particles or polymer foam particles, i.e. expanded polymer particles or polymer foam particles which have not been fused to one another to form a moulding, in, for example, filling materials for packaging purposes, for bean bags and the like, as insulation materials for blown-in insulation or as imitation snow, e.g. for decorative purposes.

The production of expandable or at least partially expanded particles composed of thermoplastic polymers occurs in practice primarily in an extrusion process, which is followed by pelletization of the polymer strand which has exited from a die or die assembly of the extruder. The thermoplastic polymer or a polymer blend of two or more such thermoplastic polymers is accordingly introduced, for example, in powder or pellet form into the extruder and plasticized and homogenized in the extruder. If a blowing agent is not already present in the polymer powder or pellets used, a blowing agent is introduced into the plasticized material, which generally occurs under superatmospheric pressure. Owing to the high pressure level in the extruder, which can be up to some 100 bar, the blowing agent is also present in the liquid and/or supercritical state at the melting point of the plasticized polymer or polymers and is, in particular, dissolved in the plasticized material. Immediately after exit from the extruder die or the die assembly in the form of, for example, a perforated plate, expansion and foaming of the polymer strand/strands occurs as a result of the abrupt pressure drop, e.g. to ambient pressure, because of rapid expansion of the blowing agent, in particular with conversion of this into the gas phase. The comminution or pelletization of the polymer strand usually occurs by means of a cutting device which is arranged downstream of the extruder and has, for example, a cutter rotating coaxially with the extruder die. While passing the polymer strand/strands through a water bath and subsequently, after sufficient cooling, pelletizing it is also known in the case of compact or unfoamed polymers, comminution of expanded or foamed polymers is carried out by means of the cutting device in, for example, a chamber filled with water in order to effect rapid cooling of the polymer strand admixed with blowing agent(s) and “freezing” of the fine-pored foam structure formed in the expansion of the blowing agent.

In order to obtain a very homogeneous, intimate fusion join between the expandable or previously at least partially expanded polymer particles to give a polymer moulding produced therefrom, it is generally desirable for the polymer particles to have a preferably spherical, but at least rounded, external contour. This results, during the production of the polymer particles themselves, in the requirement that the foaming polymer strand discharged from the extruder die be comminuted or pelletized as quickly as possible to give the polymer particles, since a spherical shape of the particles can be achieved especially while the polymer strand is still in an at least partially plastic state. On the other hand, the polymer exiting from the extruder die should however also be cooled or “quenched” as quickly as possible so that an essentially homogeneous, fine-pored foam structure can form and the bubbles formed by expanding blowing agent in the polymer matrix do not collapse.

As an alternative, it is also known that the expanding or foaming operation can be carried out separately from the extrusion operation by firstly pelletizing the polymer strand discharged from the extruder die in largely unfoamed form and subsequently effecting foaming, e.g. with initiation of a suitable blowing agent present in the polymer pellets by means of hot steam in a foaming unit.

While predominantly thermoplastic polymers derived from fossil raw materials, e.g. polystyrene (PS), polypropylene (PP), polyethylene (PE) or any other thermoplastic polymers which are obtained from fossil raw materials, are in practice employed for the production of expandable or at least partially expanded polymer particles, there is, for environmental reasons and also with a view to conservation of available petroleum resources, an increasing need for a replacement of such synthetic thermoplastic polymers by thermoplastic polymers which can be obtained from renewable, natural raw materials. Such a polymer which is in principle suitable for use in thermoplastic polymer foam particles is polylactide (PLA) or polylactic acid, which is biodegradable and readily recyclable and can be obtained, for example, by ionic polymerization of lactide, a cyclic adduct of two lactic acid molecules. In addition, polylactides can be produced directly from lactic acid, e.g. by polycondensation. Furthermore, polylactide can in principle be used in essentially pure form or else in the form of a polymer blend, known as “PLA blend”, with other thermoplastic polymers which are likewise preferably obtained from renewable raw materials for producing polymer foam particles.

One problem associated with expandable or at least partially expanded polymer particles based on polylactide is at present that, in particular, only a relatively high bulk density of the expanded or foamed polymer particles, resulting from an only relatively small pore volume of the polylactide foam particles, can be achieved in the case of a subsequent expansion or foaming operation, i.e. an expansion or foaming operation carried out after pelletization.

Thus, EP 2 135 724 B1 describes a process for producing polylactide foam particles on the basis of a polylactic acid-based resin by homogenizing the polylactic acid-based resin, which contains both optical isomers (D form and L form) of lactic acid as monomer components, in an extruder in the presence of a blowing agent. The extrudate of the polylactic acid-based resin is then discharged through a die of the extruder and pelletized directly downstream of the extruder die by means of a rotating blade in order to produce the polylactide foam particles from the extrudate, with foaming of the polylactic acid-based resin. In order to ensure rapid cooling of the extrudate, the freshly pelletized polylactide foam particles are either sprayed with water or contact cooling can be provided by bringing the polylactide foam particles into contact with the inner cylindrical surface of a rotating drum which is arranged directly downstream of the extruder die. Possible blowing agents proposed are both chemical blowing agents such as azodicarbonamide, dinitrosopentamethylenetetramine, hydrazoyldicarbonamide and sodium bicarbonate, and also physical blowing agents such as saturated aliphatic hydrocarbons, e.g. propane, n-butane, isobutane, n-pentane, isopentane and hexane, ether compounds such as dimethyl ether, methyl chloride, chlorofluorocarbons such as 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane and monochlorodifluoromethane, carbon dioxide and nitrogen.

However, it is in this way possible to produce only polylactide foam particles which, although they are virtually type-pure and thus consist essentially entirely of renewable raw materials (polylactide), have relatively high bulk densities in the region of 210 g/l and which can be advantageous for (temporary) storage of the foam particles but have only limited suitability for later processing, whether to give a moulding or for use in a loose bed, because they have only moderate heat-, sound- and impact-absorbing insulation properties because of their low pore volume. In order to counter this, EP 2 135 724 B1 also proposes that the polylactide foam particles be firstly impregnated with an inert gas at a temperature in the range from -40°C to 25°C and a pressure of from 0.2 MPa to 1.6 MPa before being processed in a pressure-resistant moulding tool to give a moulding. The polylactide foam particles are then prefoamed under the action of heat and subsequently once again impregnated with an inert gas at a temperature in the range from -40°C to 25°C and a pressure of from 0.2 MPa to 1.6 MPa. Only bulk densities of up to at least 48 g/l can be produced in this way, especially since the multiple pressure impregnation proves to be complicated and expensive in terms of apparatus.

In addition, processes for producing thermoplastic, expandable polymer particles based on polylactide, in which the bulk density of the polymer particles produced is reduced by reacting the polylactide with synthetic polymers in the presence of crosslinkers and/or chain extenders, where the crosslinkers and chain extenders are predominantly relatively environmentally unfriendly substances and in particular substances which are hazardous to health, which additionally impair the thermoplastic properties of the polylactide and consequently also the recyclability thereof, with a type-purity of the expandable polymer particles also not being able to be achieved, are known.

EP 2 524 004 B1 describes such a process for producing expandable polymer particles based on crosslinked polylactide, which comprises the following steps:
(a) melting and mixing of the components
(i) from 50 to 98.9% by mass of polylactic acid,
(ii) from 1 to 49.9% by mass of at least one
polyester based on aliphatic and/or aromatic
dicarboxylic acids and aliphatic dihydroxy
compounds,
(iii) from 0.1 to 2% by mass of an epoxide group-
containing copolymer based on styrene, acrylic
esters or methacrylic esters and
(iv) from 0 to 10% by mass of one or more
additives;
(b) mixing of
(v) from 3 to 7% by mass of an organic blowing
agent into the polymer melt at a temperature
of at least 140°C;
(c) discharge of the polymer melt loaded with blowing
agent through a die plate; and
(d) pelletization of the melt containing blowing agent
directly downstream of the die plate under water at
a pressure in the range from 1 bar to 20 bar and,
in particular, at a temperature in the range from 5°C
to 20°C.

While predominantly n-pentane or a mixture of n-pentane and isopentane is to be used as blowing agent, other, in particular physical, blowing agents as are customarily employed in expanded polystyrene foams, for example aliphatic hydrocarbons from 2 to 7 carbon atoms, e.g. isobutane, n-butane, n-pentane and isopentane, alcohols, ketones, ethers, amides or halogenated hydrocarbons, are also proposed. Furthermore, co-blowing agents such as nitrogen, carbon dioxide, air or noble gases are used. The foam particles based on crosslinked polylactide which have been produced in such a way have relatively high bulk densities in the range from about 650 g/l to 740 g/l immediately after they have been pelletized in the water bath under a pressure in the range from 9 bar to 12 bar, and can be prefoamed to bulk densities up to at least 30 g/l by simple heat treatment under the action of flowing steam.

A similar process for producing expandable particles based on crosslinked polylactide is disclosed in EP 2 617 771 B1, with the process comprising the following steps:
(a) melting and mixing of the components
(i) from 50 to 99.9% by mass of polylactic acid,
(ii) from 0 to 49.9% by mass of one or more
further polymers,
(iii) from 0.1 to 2% by mass of a diepoxide or
polyepoxide, and
(iv) from 0.1 to 5% by mass of a nucleating agent;
(b) mixing of
(v) from 1 to 7% by mass of an organic blowing
agent and
(vi) from 0.01 to 5% by mass of a co-blowing agent
from the group consisting of nitrogen, carbon
dioxide, argon, helium or mixtures thereof into
the polymer melt at a temperature of at least
140°C;
(c) discharge of the polymer melt loaded with blowing
agent through a die plate and
(d) pelletization of the melt containing blowing agent
directly downstream of the die plate under water at
a pressure in the range from 1 to 20 bar and,
in particular, at a temperature in the range from
5°C to 20°C.

While predominantly n-pentane or a mixture of n-pentane and isopentane is once again to be used as blowing agent, other, in particular physical, blowing agents as are customarily used in expanded polystyrene foams, for example aliphatic hydrocarbons having from 2 to 7 carbon atoms, e.g. isobutane, n-butane, n-pentane and isopentane, alcohols, ketones, ethers, amides or halogenated hydrocarbons, are also proposed. Here too, predominantly inert gases such as nitrogen, carbon dioxide, air or noble gases are used as co-blowing agents. The foam particles based on crosslinked polylactide which are produced in this way have relatively high bulk densities in the range from about 650 g/l to 740 g/l immediately after they have been pelletized in the water bath under a pressure in the range from 9 bar to 12 bar, and can be prefoamed to bulk densities up to at least 30 g/l by simple heat treatment under the action of flowing steam.

WO 2017/211660 A1 is concerned with a further process for producing expanded polylactide particles having a proportion of polylactide in the range from 65% by mass to 95% by mass, a proportion of further polyesters in the range from 15% by mass to 35% by mass and a crosslinker or chain extender in the form of peroxides or epoxides. Isopentane, in particular, is envisaged as blowing agent, but a series of other blowing agents including aliphatic hydrocarbons having from 2 to 7 carbon atoms, alcohols, ketones, ethers, amides or halogenated hydrocarbons are also considered to be suitable. In all the compositions mentioned in the working examples, by means of which foam particles having a low bulk density can be produced, use is made of a chain extender marketed under the trade name "Joncryl"TM from BASF AG (Germany) (cf., for example, also Volker Frenz, Dietrich Scherzer, Marco Villalobos, Abiodun A. Awojulu, Michael Edison, Roelof van der Meer: "Multifunctional Polymers as Chain Extenders and Compatibilizers for Polycondensates and Biopolymers", ANTEC 2008, pages 1682-1686), which in turn leads to a crosslinked foam having the disadvantages indicated above.

Finally, WO 2012/020112 A1 describes a process for producing pellets of thermoplastic polyesters, which comprises the following steps:
(a) introduction of the polymer matrix, which is
predominantly polyester, into an extruder and
plasticization and homogenization of the polymer matrix
in the extruder;
(b) addition of a physical blowing agent and dispersion of
the blowing agent in the plasticized polymer matrix in
the extruder, with the physical blowing agent being, in
particular, organic blowing agents, preferably in the
form of aliphatic hydrocarbons;
(c) discharge of the plasticized polyester admixed with the
blowing agent from the extruder through a die plate;
and
(d) pelletization of the extruded polymer strand admixed
with the blowing agent downstream of the die plate to
form the expandable and/or at least partially expanded
polymer particles in a water bath at a pressure of from
1 bar to 0 bar.
Apart from many different polyesters, the working examples envisage various polyester mixtures of primarily polycarbonates of the type "MakrolonTM 2800", with, inter alia, a plasticizable polyester mixture of 79.2% by mass of polycarbonate of the type "MakrolonTM 2800" and a small proportion of 19.8% by mass of polylactide being disclosed, which is admixed with isopentane as blowing agent and processed to give foam particles having a low bulk density of 78 g/l.

It is an object of the invention to propose a simple and inexpensive process for producing thermoplastic, expandable and/or at least partially expanded polymer particles based on polylactide of the type mentioned at the outset, in which process the polymer structure of the polylactide is retained and in particular is not reacted with further reactants but expandable polymer particles can be obtained which have a relatively high bulk density but can be prefoamed by means of a simple heat treatment to give foam particles having a very low bulk density through to a bulk density below that of the prior art, while at least largely avoiding the abovementioned disadvantages. It is additionally directed to thermoplastic, expandable and/or at least partially expanded polymer particles based on polylactide which have been produced in such a way.

From a process engineering point of view, this object is achieved by a process of the type mentioned at the outset which comprises the following steps:
(a) introduction of the polymer matrix into an extruder
and plasticization and homogenization of the polymer
matrix in the extruder with exclusion of crosslinkers
and/or chain extenders;
(b) addition of at least one organic blowing agent from
the group consisting of n-butane, isobutane and propane
including mixtures thereof and dispersion of the at
least one blowing agent in the plasticized polymer
matrix in the extruder;
(c) discharge of the plasticized polylactide admixed with
the at least one blowing agent from the extruder
through at least one extruder die; and
(d) pelletization of the extruded polymer strand admixed
with the at least one blowing agent downstream of the
extruder die to form the expandable and/or at least
partially expanded polymer particles,
where the steps (c) and (d) are carried out in a cooling fluid under a pressure above ambient pressure.

To achieve this object, the invention further provides thermoplastic, expandable and/or at least partially expanded polymer particles of the type mentioned at the outset which have been produced in such a way, wherein the polymer matrix of the polymer particles is free of crosslinkers and/or chain extenders and contains at least one organic blowing agent from the group consisting of n butane, isobutane and propane including mixtures thereof.

It has surprisingly been found that the combination of the use of an organic blowing agent from the group consisting of n-butane, isobutane and propane including mixtures thereof together with pelletization of the freshly extruded polymer strand admixed with such a blowing agent downstream of the extruder die or an extruder die assembly, e.g. of the type of a perforated plate, in a cooling fluid under a pressure above ambient pressure not only makes it possible to produce expandable and/or at least partially expanded polymer particles based on polylactide which have a high bulk density approximately comparable to the abovementioned prior art as described in EP 2 524 004 B1 and EP 2 617 771 B1 but, in addition, the expandable polymer particles based on polylactide which have been produced in such a way can also be prefoamed by means of a simple heat treatment (for further details, see below), which does not require any impregnation in a pressurized space corresponding to the abovementioned EP 2 135 724 B1 to give expanded or foamed polymer particles having a very low bulk density which can even be below the values disclosed in EP 2 524 004 B1 and EP 2 617 771 B1 (for further details, see also below). The expandable and/or at least partially expanded polymer particles based on polylactide which are obtained during step (d) of the process of the invention can consequently be prefoamed merely under the action of heat to a very low bulk density, with the residual content of the blowing agent(s) according to the invention then still present being able to ensure further slight foaming in any fusion of the polymer foam particles to give a moulding, so that good filling of the moulding tool and areal and permanent fusion of the foam particles with one another to form the moulding of low density are achieved.

It is here found to be an important aspect of the invention that the process of the invention makes this possible with exclusion of any crosslinkers and/or chain extenders including those based on epoxides, so that not only is the use of such environmentally damaging substances and substances which are hazardous to health made dispensable but, additionally, the polymer particles produced in such a way have, in particular, a polymer matrix which is formed by more or less pure, uncrosslinked, polylactide or by a polylactide blend, preferably with at least one other thermoplastic polymer obtained from renewable raw materials, for example starch including derivatives thereof, cellulose including derivatives thereof, e.g. cellulose acetates and/or propionates, polyhydroxybutyrates (PHB) and the like. If desired, it is also possible, as an alternative or in addition, to mix one or more blending partners in the form of synthetic thermoplastic polymers, for example polyolefins, e.g. polyethylene (PE), polypropylene (PP) and the like, polystyrene (PS), polyesters, e.g. polyalkylene terephthalates, preferably polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polybutylene adipate-terephthalate (PBAT) etc., or the like into the polylactide. The proportion of polylactide can consequently be, in particular, up to about 100% by mass, and in a polymer blend is preferably at least about 60% by mass, most preferably at least about 70% by mass, for example at least about 80% by mass, particularly preferably at least about 90% by mass, or in particular more than about 95% by mass. If production of expandable and/or at least partially expanded polymer particles from a thermoplastic polymer blend composed of polylactide with at least one further thermoplastic blend partner is envisaged, the polylactide and the at least one blend partner are consequently introduced, either separately from one another or in the form of, for example, a previously blended masterbatch, as per step (a) into the extruder and plasticized and homogenized together with exclusion of crosslinkers and/or chain extenders, with, according to step (b), at least one organic blowing agent from the group consisting of n butane, isobutane and propane including mixtures thereof being added and dispersed in the plasticized polymer matrix of the polylactide with the at least one blend partner in the extruder. The polymer matrix composed of the plasticized polylactide admixed with the at least one blowing agent and the at least one blend partner thereof is then discharged as per step (c) through at least one extruder die from the extruder, after which the extruded polymer strand of polylactide together with the at least one blend partner, which has been admixed with the at least one blowing agent, is pelletized as per step (d) downstream of the extruder die to form the foam particles based on polylactide, where the above steps (c) and (d) are carried out in a cooling fluid which is under a pressure above ambient pressure.

The at least one organic blowing agent from the group consisting of n-butane, isobutane and propane can advantageously be added in a proportion of from about 1% by mass to about 20% by mass, in particular from about 2% by mass to about 15% by mass, preferably from about 3% by mass to about 10% by mass, based on the mass of the polymer matrix (pure polylactide or, in the case of a polymer blend, polylactide including the blend partner(s)).

In one variant of the process of the invention, from about 0 to about 2% by mass, in particular from about 0 to about 1% by mass, based on the mass of the polymer matrix, of at least one nucleating agent can be added to the polymer matrix based on polylactide. Such, in particular finely particulate, nucleating agents are known per se from the prior art and ensure, in particular, good bubble formation during expansion or foaming and also a high crystallinity of the polylactide and also high heat distortion resistance of the latter. Examples of suitable nucleating agents include, in particular but not exclusively, those of natural origin, e.g. talc (magnesium silicate hydrate) or other sheet silicates.

In addition, a total of from about 0 to about 15% by mass, in particular a total of from about 0 to about 10% by mass, of at least one additive can be added to the polymer matrix based on polylactide. The additives are additives from among those known from the prior art, which can be used as a function of the purpose of the expendable and/or at least partially expanded polymer particles based on polylactide which are produced, so as to adapt the properties of the particles to the desired purpose. Examples of such additives encompass, in particular but not exclusively, dyes and pigments, flame retardants, stabilizers, infrared absorbers, processing aids and the like.

Although the cooling fluid used in steps (c) and (d) of the process of the invention can in principle also be a gas or gas mixture, in an advantageous embodiment the cooling fluid used is a liquid, in particular an aqueous medium, for example water, which owing to a comparatively high heat capacity is able to ensure a high cooling rate of the expandable polymer particles produced in step (d).

Depending on the desired bulk density of the expandable polymer particles based on polylactide which are produced in step (d), the cooling fluid can be kept under a pressure of
- at least about 1.5 bar, in particular at least about
2 bar, preferably at least about 5 bar; and/or
- not more than about 30 bar, in particular not more
than about 25 bar, preferably at least about 20 bar.
Here, a comparatively high pressure of the cooling fluid of at least about 5 bar is able, for example, to counter expansion or foaming of the polymer particles during pelletization thereof in step (d), so that the at least one organic blowing agent based on n-butane, isobutane and/or propane which is present in the polymer particles is prevented from outgassing due to the fact that the cooling fluid cools the polymer particles relatively quickly to below their glass transition temperature, so that the particles solidify, and expandable polymer particles loaded with blowing agent, which have a relatively high bulk density and can, for example, be prefoamed by means of a simple heat treatment to give expanded foam particles having very low bulk densities and consequently a very large pore volume, are produced to a large extent. If, on the other hand, the pressure of the cooling fluid is set to a comparatively low value of up to about 5 bar, at least partial expansion or foaming of the polymer particles can be ensured as early as during pelletization as per step (d), so that lower bulk densities of the resulting, partially foamed but still further-expandable polymer particles compared to a higher pressure of the cooling fluid are obtained. These lower bulk densities can, for example, be useful when the at least partially foamed polymer particles are not to be temporarily stored for a prolonged period of time or transported further but instead are to be processed further more or less immediately.

The cooling fluid can for the purposes mentioned be maintained at, for example, a temperature of
- at least about 0°C, in particular at least about
5°C, preferably at least about 10°C; and/or
- not more than about 90°C, in particular not more than
about 70°C, preferably not more than about 50°C.

As indicated above, the process of the invention makes it possible, in particular, for the expandable polymer particles based on polylactide, which are obtained in step (d) and are, depending on the pressure of the cooling fluid set, still essentially compact or are already partially expanded or foamed but can still have a relatively high bulk density, to be prefoamed at a temperature in the range from about 50°C to about 150°C, in particular from about 50°C to about 100°C, for example from about 50°C to about 80°C, in order to obtain expanded polymer particles or polymer foam particles having an extraordinarily low bulk density of up to less than 20 g/l and consequently having a very high pore volume in relatively short heat treatment times of less than 2 minutes without additional impregnation of the polymer particles with a pressurized gas (cf. also the working examples further below), as has hitherto not been possible in the prior art even when the polylactide has been crosslinked with other polymers with addition of a crosslinker based on epoxides. Here, the desired bulk density of the polymer foam particles can be set in a simple way to the desired value within relatively wide intervals of, for example, from greater than 200 g/l to less than 20 g/l by varying the duration of the heat treatment and/or the proportion of the organic blowing agent according to the invention (cf. likewise the working examples further below). The heat treatment for prefoaming the polymer particles based on polylactide can be effected in virtually any way, e.g. by means of appropriately heated steam, air, water or other heat transfer fluids, by exposure of the polymer particles to electromagnetic radiation, for example in the infrared range by means of a heat source, by means of microwave radiation, radiofrequency radiation or the like.

Of course, it is in principle possible to set a pressure which is increased or reduced relative to ambient pressure during the above-described prefoaming of the expandable polymer particles based on polylactide which are obtained in step (d), whether they are, depending on the pressure of the cooling fluid which is set, still essentially compact or are already partially expanded or foamed, at elevated temperature, but the polymer particles obtained in step (d) can, for the abovementioned reasons, nevertheless be prefoamed in a simple and inexpensive manner, in particular at essentially ambient pressure, in order to achieve the above-described very low bulk densities or the very high pore volumes.

While the expandable and/or at least partially expanded polymer particles based on polylactide which are produced by means of the process of the invention can, as described above, have bulk densities which are variable within wide limits, they can, in an advantageous embodiment, have
- a proportion of polylactide in the polymer matrix of
at least 80% by mass, in particular at least
90% by mass, preferably more than 95% by mass;
- in the compact state as per step (d) (i.e. before
any prefoaming) a bulk density of at least about 150 g/l,
in particular at least about 200 g/l, preferably at
least about 250 g/l, so that they can be maintained in a
relatively compact state during storage and/or
transportation; and/or, in particular,
- a bulk density after prefoaming at a temperature in the
range from 50°C to 150°C over a period of 100 s at
ambient pressure of not more than about 125 g/l, in
particular not more than about 100 g/l, preferably not
more than 75 g/l, for example not more than about
50 g/l.

Further features and advantages of the invention may be derived from the following working examples.
Working Examples 1 to 4:
Production of thermoplastic polymer foam particles composed of polylactides of different types using isobutane as blowing agent:

For carrying out Working Examples 1 to 4, polymer foam particles were produced from various formulations of polylactides "PLA 1", "PLA 2" and "PLA 3" having different crystallinities using isobutane as blowing agent and talc as nucleating agent, with the formulations used being shown in Table 1 below:

Table 1: Composition of the formulations for Working
Examples 1 to 4.

Working
Example PLA 1
type "6302D"
(amorphous) PLA 2
type "4060D"
(amorphous) PLA 3
type "8052D"
(partially crystalline) Isobutane as blowing agent Talc as nucleating agent
1 100% by mass 0 0 5% by mass 0.5% by mass
2 0 100% by mass 0 5% by mass 0.5% by mass
3 50% by mass 0 50% by mass 5% by mass 0.5% by mass
4 0 0 100% by mass 5% by mass 0.5% by mass

The above formulations of the various types of polylactide with 0.5% by mass of talc as per Working Examples 1 to 4 were introduced into a twin-screw extruder model "Leistritz iMaxx 27" having an L/D ratio of 48 and extruded at a throughput in the range from 15 kg/h to 25 kg/h at a temperature in the range from 140°C to 180°C. The blowing agent in the form of isobutane was introduced into the extruder in each case via a "LEWA ecofoam" gas metering device. The extrudate which had been admixed in this way with isobutane was discharged from the twin-screw extruder either through a die assembly in the form of a perforated plate having 12 die openings each having a diameter of 1 mm (Working Examples 1, 3 and 4) or through a single die having a diameter of 2.2 mm (Working Example 2), which were in each case maintained at a temperature in the range from 190°C to 270°C, and transferred directly into a "GALA LPU" cooling-pelletization device. Water at a temperature of 40°C and a pressure of 9 bar (Working Examples 1, 3 and 4) or 5 bar (Working Example 2) was used as cooling fluid. Finally, the bulk density of the expandable polylactide particles obtained in this way was determined.

The bulk densities of the expandable polylactide particles produced, including the process parameters set in each case, are summarized in Table 2 below.

Table 2: Process parameters of Working Examples 1 to 4
including bulk densities of the expandable
polylactide particles produced in this way.

Working Example Throughput Extrusion
temperature Die
temperature Die
diameter Pressure and temperature of cooling fluid Bulk density of PLA particles
1 25 kg/h 180-150°C 270°C 1.0 mm x 12 9 bar / 40°C 486 g/l
2 15 kg/h 180-140°C 190°C 2.2 mm x 1 5 bar / 40°C 190 g/l
3 25 kg/h 180-160°C 250°C 1.0 mm x 12 9 bar / 40°C 470 g/l
4 25 kg/h 180-160°C 250°C 1.0 mm x 12 9 bar / 40°C 430 g/l

The expandable polylactide particles produced in the above way were finally, in order to effect further processing to give expanded polylactide foam particles, subjected to a prefoaming operation by exposing them to steam having a temperature in the range from 52°C to 73°C at ambient pressure for in each case two different foaming times in a prefoamer model "Erlenbach ED2HP". The bulk densities of the polylactide foam particles which had been prefoamed in this way were subsequently determined.
Specimens of mouldings were furthermore produced from the prefoamed polylactide foam particles by fusing them together by means of hot steam in an automatic moulding machine model "Erlenbach EHVCE 870/670" to give the moulding, after which the density of the mouldings obtained in this way was determined.

Table 3 below summarizes firstly the prefoaming parameters of the prefoamed polylactide foam particles including the bulk densities thereof obtained in each case, and secondly the densities of the polylactide mouldings obtained from the prefoamed polylactide foam particles.

Table 3: Prefoaming parameters of Working Examples 1 to 4
including bulk densities of the polylactide foam
particles prefoamed in this way and also densities
of the mouldings produced therefrom.

Working Example Prefoaming time Prefoaming temperature Bulk density of PLA particles Density of moulding
1 100 s 58°C 25 g/l 27 g/l
60 s 54°C 66 g/l 69 g/l
2 100 s 52°C 70 g/l 94 g/l
30 s 52°C 120 g/l ---
3 100 s 56°C 25 g/l 29 g/l
65 s 58°C 55 g/l 70 g/l
4 100 s 73°C 16 g/l 38 g/l
65 s 58°C 65 g/l 77 g/l
Comparative Examples 1 and 2:

Production of thermoplastic polymer foam particles composed of polylactides of different types using isopentane instead of isobutane as blowing agent:

For carrying out Comparative Examples 1 and 2, polymer foam particles composed of the polylactide "PLA 1" were produced in a manner corresponding to Working Example 1 above using various proportions of isopentane instead of isobutane as blowing agent and talc as nucleating agent, with the formulations used being shown in Table 4 below:

Table 4: Composition of the formulations of Comparative
Examples 1 and 2.

Comparative Example PLA 1
type "6302D"
(amorphous) PLA 2
type "4060D"
(amorphous) PLA 3
type "8052D"
(partially crystalline) Isopentane as blowing agent Talc as nucleating agent
1 100% by mass 0 0 5% by mass 0.5% by mass
2 100% by mass 0 0 7% by mass 0.5% by mass

The above formulations of the polylactide "PLA 1" with talc as per Comparative Examples 1 and 2 were introduced, in a manner corresponding to Working Example 1 above, into the twin-screw extruder model "Leistritz iMaxx 27" having an L/D ratio of 48 and extruded at a throughput of 25 kg/h and a temperature in the range from 160°C to 180°C. The blowing agent in the form of isopentane was in each case introduced into the extruder via the "LEWA ecofoam" gas metering device. The extrudate which had been admixed with isopentane in this way was discharged from the twin-screw extruder through the die assembly in the form of a perforated plate having 12 die openings each having a diameter of 1 mm (once again corresponding to Working Example 1 above), which were maintained at a temperature in the range from 250°C to 270°C, and transferred directly to the cooling pelletization device model "GALA LPU". Water at a temperature of 40°C and a pressure of 9 bar (corresponding to Working Example 1 above) was once again used as cooling fluid. Finally, the bulk density of the expandable polylactide particles obtained in this way was determined.

The bulk densities of the expandable polylactide particles produced, including the process parameters set in each case, are summarized in Table 5 below.

Table 5: Process parameters of Comparative Examples 1 and 2
including bulk densities of the expandable
polylactide particles produced in this way.

Compar-ative Example Throughput Extrusions tempera-ture Die tempera-ture Die diameter Pressure and temperature of cooling fluid Bulk density of PLA particles
1 25 kg/h 180-160°C 270°C 1.0 mm x 12 9 bar / 40°C 750 g/l
2 25 kg/h 180-160°C 250°C 1.0 mm x 12 9 bar / 40°C 726 g/l

Finally, the expandable polylactide particles produced in the above way were, to effect further processing to give expanded polylactide foam particles, once again subjected to a prefoaming operation by exposing them, in a manner corresponding to Working Example 1 above, to steam having a temperature in the range from 58°C to 65°C for a foaming time of in each case 100 seconds at ambient pressure in the "Erlenbach ED2HP" prefoamer. The bulk densities of the polylactide foam particles which had been prefoamed in this way were subsequently determined.

Table 6 below summarizes the prefoaming parameters of the prefoamed polylactide foam particles including the bulk densities thereof obtained in each case.

Table 6: Prefoaming parameters of Comparative Examples 1
and 2 including bulk densities of the polylactide
foam particles prefoamed in this way.

Comparative Example Prefoaming time Prefoaming temperature Bulk density of
PLA particles
1 100 s 58°C 240 g/l
2 100 s 65°C 200 g/l

It can be seen from the working examples that the combination of the use according to the invention of an organic blowing agent from the group consisting of n butane, isobutane and propane (here: n-butane) with pelletization in a pressurized cooling fluid (here: water) makes it possible to produce polylactide foam particles having a bulk density which is lower than in the prior art or having a larger pore volume than in the prior art without the polymer structure of the polylactide being changed and, in particular, without it being crosslinked by means of any crosslinkers or chain extenders.

As can be seen from Table 2 above, it is possible in step (d) of the process of the invention firstly to produce expandable but still essentially compact polylactide particles which have a relatively high bulk density, as can be desirable for the purposes of very compact storage or for transport, if the pressure of the cooling fluid is selected so as to be greater than about 5 bar (cf. Working Examples 1, 3 and 4 as per Table 2), while in the case of a comparatively lower superatmospheric pressure of the cooling fluid of about 5 bar or less it is possible to produce partially expanded polylactide particles which have a lower bulk density because of partial foaming in the cooling fluid (cf. Working Example 2 as per Table 2). The desired bulk density can in this way be matched in wide ranges to the desired use.

In particular, however, prefoaming, which is here carried out under atmospheric pressure and is consequently very simple and inexpensive in terms of apparatus, of the expandable polylactide particles obtained according to step (d) of the process of the invention over short prefoaming times of about 100 s gives extremely low bulk densities of the expanded or prefoamed polylactide particles which are below those which in the prior art are possible only with the assistance of crosslinkers or chain extenders, with bulk densities of significantly below 20 g/l being able to be produced without problems (cf. Working Examples 1, 3 and 4 in Table 3 above). Here, it is possible to produce relatively low albeit comparatively somewhat higher bulk densities even when the pressure of the cooling fluid in step (d) of the process of the invention is set to a pressure of from about 1.5 bar to about 5 bar which is only slightly higher than ambient pressure, at which the polylactide particles obtained in the pelletization operation partially foam (cf. Working Example 2 in Table 3 above). In addition, the desired bulk density of the prefoamed polylactide foam particles can be varied in a very simple manner via the prefoaming time at a moderate prefoaming temperature and also via the proportion of the organic blowing agent according to the invention (cf. once again Table 3 above), with the bulk densities which are significantly lower compared to the prior art being achieved even in the case of prefoaming times of about 100 seconds.

The density of a polymer moulding produced from the polylactide foam particles produced in such a way can consequently be set in a corresponding way, wherein the residual content of the organic blowing agent according to the invention which is always still present in the polylactide foam particles even after their prefoaming ensures building up an (additional) pressing together of the foam particles during fusion of the foam particles to give the moulding, bringing about further expansion of the foam particles during their shaping to give the moulding and consequently ensuring intimate fusion over a large area of the foam particles with one another, resulting in a very low density of the corresponding polymer moulding.

As can be seen from Comparative Examples 1 and 2, comparably low bulk densities of foam particles composed of pure, i.e. uncrosslinked, polylactide cannot be achieved when, under otherwise corresponding process conditions, a blowing agent which has hitherto been considered to have an equivalent effect in the prior art, here: isopentane, is used. The bulk density of such polylactide particles cannot be brought by means of prefoaming to a value comparable to that of the polylactide foam particles of the invention even when the proportion of blowing agent is increased (here: from 5% by mass to 7% by mass) (cf. Comparative Examples 1 and 2 in Table 6 above).

Last but not least, it may be mentioned that a specimen of a moulding made from the polylactide foam particles produced according to the invention (here: as described in Working Example 3 above) was subjected to the flame protection test in accordance with DIN 4102 B2 (EN 13501), with it being found that the moulding passes the flame protection test without flame retardants having been additionally added to the polylactide.