EP2580277A1 - Bio-degradable foamable material suitable for the production of slippers - Google Patents

Abstract

A biodegradable flexible material suitable for producing at least the soles of biodegradable slippers is described, said material comprising starch, a biodegradable polyester and a plasticizer selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in non bleeding amounts of at most 15 % by weight referred to the weight of the total mixture as well as a physical expansion material such as microsheres.

Description

Bio-degradable foamable material suitable for the production of slippers

Cross References to Related Applications

This application claims the priority of Swiss patent application no. 0910/10 filed June 8, 2010 the disclosure of which is incorporated herein in its

entirety.

Technical Field

The present invention concerns biodegradable materials, in particular low weight elastic deformable materials.

Background Art

Thick-walled flexible parts, made of cellulose are already commercially available. Glycan Biotech, for example, offers plates of low density made of glycan (the term glycan refers to an oligosaccharide or

polysaccharide) . They claim 100% biodegradability (see http: / /www. glycan-biotech . com/r3. html) . Slippers have already been produced from these plates by cutting.

The use of citrate esters as low-molecular plasticizers is well known. Citrate esters are used in synthetic polymers (e.g. PVC) as well as in biopolymers, e.g. starch (see patent documents mentioned below).

Triethyl citrate and tributyl citrate are also used in thermoplastic compositions that are pharmaceutically acceptable (see EP 0 575 349) . For highly flexible starch products for e.g. slippers, it is crucial that no

plasticizers are liberated from the bulk at the surface (usually called bleeding) .

FR 2 923 488 discloses a composition

comprising as plasticizers a composition of at least one citrate and at least one lipid. Although starch is mentioned as a possible polymer, all examples were performed with poly (lactic acid). WO 2008/014573

discloses starch based compositions that also include a further polymer such as biodegradable polyester and plasticizers . The plasticizer may be selected from a broad variety comprising citrates. A much preferred plasticizer content is as high as 15 to 35 %

US 7,037,959 discloses biodegradable polymer compositions comprising hydroxyfunctional polymer, a natural polymer and a thermoplastic polymer and

optionally a plasticizer. As possible plasticizer not specified citrates are mentioned.

In WO 99/13003 biodegradable polyester compositions are disclosed that preferably comprise less than 50 % starch together with hydroxyl functional polyester and plasticizer comprising

acetyltributylcitrate . An expanded product with a high amount of gelatinized starch and water as expanding agent but without plasticizer is also addressed.

US 2003/100635 discloses a complex biodegradable polimeric composition comprising 40 to 52 % of a starch component and several other polymeric

components one of them being 8.5 to 12 % of a polyolefin homopolymer or copolymer, i.e. a not biodegradable component. As possible further ingredients plasticizers are mentioned encompassing not further specified

citrates, as well as blowing or foaming agents such as chemical blowing agents and the introduction of gases during processing. Such materials may be used for the production of flip-flops.

According to EP 0 436 689 Bl, starch

compounds can be foamed by chemical foaming. When the foaming was made on extrusion equipment for foamed tube insulating, which is an open system, a density of 0.15 g/cm³ could be realized if applying a temperature of 100- 110°C using sodium bicarbonate together with a polymeric acid, preferably an acid with carboxyl side groups like poly (acrylic acid) or ethylene acrylic acid copolymers, as foaming agent. If using injection moulding, however, a temperature of 170°C had to be used and the density obtained raised to 0.3 g/cm^.

WO 2006/027671 discloses polyurethane

products comprising microsperes and expanding agents for producing low density foamed footwear products with integral skin.

Bastioli C, "Properties and applications of Mater-Bi starch-based materials" Polymer Degradation and Stability 59 (1998) 263-272 gives an overview of Mater- Bi® products known in 1998. These products as such do not have the features needed for highly flexible and

preferably also low weight products.

Thus, there is still a need for highly flexible thick-walled products made of starch that are also injection mouldable. The advantage of injection moulding over extrusion of plates and cutting consists in being able to produce three-dimensionally shaped products by designing 3-dimensional moulds. Thus, comfortable

"foot beds" can be produced. Another advantage is that no open pores must be sealed in a further production step because - contrary to cutting - no open pores are

generated .

Disclosure of the Invention

Hence, it is a general object of the inven- tion to provide a starch material that should

(i) lack bleeding,

(ii) be highly flexible, and

(iii) preferably be light-weight,

(iv) and also preferably fulfil the criteria for "O.K. Biobased" (certification by AIB-

VINgOTTE International SA, Brussels) (v) and optionally fulfil the criteria for biodegradability, that means fulfil at least one of the critera for "OK Biodegradable" and "OK Compost"

(certification by AIB-VINcOTTE International SA,

Brussels) .

Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the

biodegradable material of the present invention is on the one hand manifested by the features that it comprises (i) a polymer composition comprising as constituents starch and a biodegradable polyester, and (ii) at least one plasticizer, wherein said polyester is an unsubstituted aromatic-aliphatic copolyester, in particular

poly (butylene-adipate-co-butylene-terephthalate) and said plasticizer is present in non beleeding amounts and selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in amounts of at most 15 % by weight referred to the weight of the total composition,

preferably at most 12.5 % by weight.

In a second aspect the present invention comprises a biodegradable flexible expansible material comprising (i) starch, (ii) a biodegradable polyester, (iii) plasticizers and (iv) a foaming agent, wherein the plasticizers are present in non bleeding amounts, wherein said plasticizers comprise at least one plasticizer selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in amounts of at most 15 % by weight referred to the weight of the total composition, and wherein the foaming agent comprises and preferably consists of expansible microspheres .

In a preferred embodiment, the starch and the polyester are used in the form of a polymer composition that optionally may comprise some additives. A preferred polymer composition comprises starch in an amount of at least 85 % referred to the polymer composition,

poly (butylene-adipate-co-butylene-terephthalate) as polyester and plasticizers, wherein about 10 % of the constituents are water soluble.

The investigations leading to the present invention were started based on the commercially

available biopolymer MaterBi or Mater-Bi^® NF01U

(hereinafter referred to as "Mater-Bi") , from Novamont, Novara (I). Mater-Bi® had been developed to produce biodegradable films for packaging, e.g. shopping bags or wrappings. In order to develop a material based on Mater- Bi® suitable for slippers, the material had to be made more flexible and its density lowered. Mater-Bi® is composed of more than 85% by weight of starch, a

polyester, e.g poly (butylene-adipate-co-butylene- terephthalate) such as the polyester known as Ecoflex®, a product of BASF, and some plasticizers. The flexibility of Mater-Bi® was enhanced by addition of specific

plasticizers and the density of thereof produced articles was reduced by adding a suitable foaming agent.

Starches that may be used in the scope of the present invention are any kind of starch but preferably thermoplastic starch derived from potatoe starch and corn starch .

Examples for biodegradable polyesters are:

PHA = polyhydroxyalkanoates ,

PHB = polyhydroxybutyrate,

PHH = polyhydroxyhexanoate,

PHV = polyhydroxyvalerate,

PLA = polylactic acid,

PCL = polycaprolactone,

PBS = polybutylene succinate,

PBSA = polybutylene succinate adipate,

AAC = aliphatic-aromatic copolyesters ,

PET = polyethylene terephthalate,

PBAT = polybutylene adipate/terephthalate, PTMAT = polymethylene adipate/terephthalate, and copolymers thereof, with PBAT = polybutylene

adipate/terephthalate, e.g. poly (butylene-adipate-co- butylene-terephthalate) , a polymer that does not comprise hydrophilic groups such as hydroxy groups attached, being presently preferred.

A much preferred polymer material, i.e. the starch and polyester component, is Mater-Bi®.

While citrates are known to be advantageous plasticizers , it was found that only very specific citrates can be used and, if bleeding shall be avoided, only in limited ranges. These citrates are tributyl citrate (TBC) and acetyl tributyl citrate (ATBC) .

If Mater-Bi® is used TBC and ATBC are much preferably the only plasticizers present except for the plasticizers comprised in Mater-Bi, i.e. no plasticizers such as polyols like sorbitol, mannitol and glycerol, or lipids etc. are added to Mater-Bi®.

It was found that no plasticizer bleeding occurs even after aging if the amount of tributyl citrate does not exceed 12.5% and the amount of acetyl ,tributyl citrate does not exceed 10%.

If the material is intended for the

production of products for which reduced weight is desired or necessary such as slippers, the material comprises a foaming agent, in particular a physical foaming agent such as expandable microspheres.

In a much preferred embodiment, no plasticizer is liberated from the material at the surface and the material contains or consists of

70 to 90% Mater-Bi®

5 to 15 %, in particular 5 to 12.5% tributyl citrate or

5 to 12.5%, in particular 5 to 10% acetyl tributyl citrate or

5 to at most 12.5 % of a composition of tributyl citrate and acetyl tributyl citrate,

and optionally 0 to 18 %, in particular 0 to 15 %,

especially preferred for a light weight product about 10 to 13 % expandable microspheres

0 to 1.5 % of a dye

0 to 1.5 % of perfumes,

the sum of the above mentioned ingredients amounting to at least about 98 % by weight of the total composition, preferably to 100 %.

Using the material described above, a light weight foamed product may be produced by applying an injection moulding method comprising the steps of

metering, injection, holding, cooling, and ejection, wherein the metering is performed in a metering zone comprising at least one feeding zone, at least one heating zone and an injection zone.

In a specific embodiment, the method is performed with the injection zone having a barrel

temperature between 2°C and 35°C below the barrel

temperature causing first evidence of decomposition

(slight brown colouration) of the material, the metering zone having a back pressure between 0.5 bar and 5 bar, preferably about 1 bar, and a screw rotation speed of 20 to 50 rpm, preferably about 30 rpm, and the holding is performed at a pressure between 0 and 10 bar, preferably built by the foaming agent only.

Brief Description of the Drawings The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

Figure 1 shows a typical stress-strain curve of Mater-Bi® NF01U and a mixture of 91% Mater-Bi® NF01U + 9% acetyl tributyl citrate. Figure 2 shows the flexural modulus of Mater- Bi® plasticized with different quantities of acetyl tributyl citrate. The unity of the content means g/100 g. The 3-point flexural modulus and flexural stress at conventional deflection were determined according to ISO 178. Thickness of the samples was 4 mm.

Figure 3 shows flexural modulus vs. flexural stress at conventional deflection of Mater-Bi®

plasticized with different quantities of acetyl tributyl citrate.

Figure 4 shows a suitable, flexibility enhancing sole design in top view.

Figure 5 shows a cross section through the sole of Figure 4 along line 5-5.

Modes for Carrying out the Invention

One problem that had to be solved by the present invention was to find suitable biodegradable and preferably also biobased plasticizers that do not bleed at the amounts needed to get suitable flexibility. Thus, during the development of the present invention, many plasticizers were investigated in different amounts. Only two of them led to satisfactory results, namely tributyl citrate (TBC) and acetyl tributyl citrate (ATBC) .

1) TBC (e.g. CITRFOL® Bl from Jungbunzlauer ) was found to be a suitable plasticizer for Mater-Bi® with the following properties:

- it enhanced the flexibility of the starch compound significantly when fractions of 1% to 15% (with respect to the total mass) are added

- a preferred fraction of TBC was 5% - 12.5% (highest flexibility without bleeding)

- Products with higher amounts up to 15% usually were also acceptable. When specimens containing 10%, 12.5% and 15% had been subjected to alternating atmosphere during 5 days, only minor bleeding was

detected on the sample containing 15%. Conditions of atmosphere: temperature was altered between 20°C and 35°C (several cycles), relative humidity was altered between 30% and 80% (several cycles) .

addition of more than 15% led to a smeary surface of the material

the mixture was injection mouldable 2) ATBC (e.g. CITRFOL® B2 from Jungbunzlauer ) was found to be a suitable plasticizer for Mater-Bi® with the following properties:

it enhanced the flexibility of the starch compound significantly when fractions of 1% to 10% (with respect to the total mass) were added

a preferred fraction of ATBC was 5% to 10% (highest flexibility without bleeding)

Products with higher amounts up to 12.5% usually were also acceptable. When specimens containing 10% and 12.5% had been subjected to alternating

atmosphere during 5 days, only minor bleeding was

detected on the sample containing 12.5%. Conditions of atmosphere: temperature was altered between 20°C and 35°C (several cycles), relative humidity was altered between 30 and 80% (several cycles) .

addition of more than 12.5% led to a smeary surface of the material.

the mixture was injection mouldable

ATBC was found to be non-irritating to the skin, and reactions suggestive of contact sensitization were not observed during a human exposure study. This study was performed with 59 men and women (age range = 21-60 years), all of whom had history of diabetes, psoriasis, or active dermatoses [ (Johnson W; Intern J Toxicol 21. (Suppl. 2) : 1-17 (2001) ] . Therefore, ATBC and not TBC was used in the following further investigations. Besides the flexibility, also the weight of slippers is a relevant feature. In order to improve the weight, foaming was examined resulting in the finding that expandable microspheres (added as master batches or as pure microspheres) are favourable over chemical blowing agents. Expandable microspheres were found to be suitable to foam the compound Mater-Bi® / TCB and/or ATBC, in particular Mater-Bi® / ATBC, and to reduce the density of the material by more than 50% in an injection moulding process. The advantage of microsperes is that they do not foam due to a chemical reaction during production but have a gaseous substance incorporated within a compatible shell. The same effect may be obtained by directly using a gas, i.e. by gas injection during molding thereby avoiding the incorporation of the shell material of the microspheres. However, the

advantage of the microspheres is that they can be homogenously distributed within the polymer composition and that nospecific means for gas injection have to be provided.

A further measure to reduce weight and to enhance flexibility of slippers is via the design of the sole. A suitable and presently preferred design is shown in Figures 4 and 5.

The invention is now further described by comparing inventive compositions with comparative compositions .

Example 1: Evaluation of the biopolymer based on starch

Biograde® BM, Bioplast® 2189 and Mater-Bi® NFOIU are biodegradable materials based on starch, which can be processed by extrusion. In order to compare the flexibility, the biopolymers were melted on a laboratory roll mill with two cylinders (Collin 100T) and blended with 10% of triglycerol. Several turn-overs were

performed to obtain a sufficient homogeneity. The temperature of both cylinders was 170°C (Biograde®, Bioplast®) and 140°C (Mater-Bi®) , respectively.

Afterwards, the compounds were removed from the roll-mill and subjected to pressure moulding (conditions: 170°C (Biograde®, Bioplast®); 140°C (Mater-Bi®); 10 bar) to obtain material plates of about 140 x 140 x 3.5 mm.

Elasticity of the plates was compared using mechanical tests. The tests were performed according to ISO 527-2, but without conditioning of the samples. Dumb-bell test pieces had the following dimensions: length of narrow portion: 40 mm, width of narrow portion: 20 mm,

thickness: 0.3 mm

The mechanical properties of different biomaterials plasticized with triglycerol are shown in Table 1:

Table 1:

Mater-Bi® NF01U had the lowest E-modulus and thus the highest elasticity. Mater-Bi® also showed a high value for the strain at rupture, thus indicating that this material is not brittle in contrast to Bioplast® and Biograde®. Therefore, Mater-Bi® NF01U was selected for further investigations.

Biopolymers of the type Mater-Bi® NF are based on natural raw materials (maize starch, derivatives of vegetable oils) as well as on synthetic biodegradable polyesters (http: //www.materbi .it) . Mater-Bi® is produced by extrusion. During extrusion, the crystals of starch melt and additives can then be admixed. The producer specifies Mater-Bi® NFOIU as a polymer blend based on starch, synthetic polyester and plasticizers, with the polyester being poly (butylene-adipate-co-butylene- .

terephthalate) The infrared spectrum did not contradict this specification.

Water soluble additives were investigated by extraction of ground Mater-Bi® NFOIU with water at 100°C for 22 hours. The loss of weight was about 10%. HPLC-MS analysis showed a distinctive peak at 453 m/z, probably indicating the presence of a polyether. Additives

commonly used in biopolymers, urea, xylite or glycerol triacetate, were not detected by HPLC-MS. Nitrogen was determined according to Kjeldahl. The low content of 0.15% found indicates that proteins had not been admixed.

Mater-Bi® NFOIU is a blend of two different phases [N. Tzankova Dintcheva, F.P. La Mantia, Polymer Degradation and Stability 92(2007)630-634]. Information on Mater-Bi products can be found in literature e.g.

Gross et al., "Biodegradable Polymers for the

Environment, www . sciencemag . org SCIENCE Vol. 297 of 2 August 2002 p. 803ff (Green Chemistry) , in particular page 805, Table 1, and Gottschlich N.

"Nachhaltigkeitsbewertung eines biologisch abbaubaren Kunststoffes unter gleichzeitiger Erarbeitung von

Massnahmekatalogen", Diplomarbeit Fachhochschule Lippe und Hoxter (2004), in particular Table 5-1, page 43. The values found in literature may vary from those listed in the application due to being performed with e.g.

different sample sizes but proved to be in line with data obtained by the inventors in similar manner.

Example 2: Evaluation of plasticizers for Mater-Bi® NFOIU

All recipes described herein were produced on the laboratory scale. Materials were compounded on a laboratory roll mill with two cylinders (Collin 100T) . The

temperature of both cylinders was 140°C. In order to obtain a sufficient homogeneity several material turn- overs were performed. Afterwards, the compounds were removed from the roll-mill and subjected to a moulding press (conditions: 140°C, 10 bar) to obtain material plates of about 140 x 140 x 3.5 mm. These plates were evaluated with respect to the flexibility and a potential bleeding.

To enhance the flexibility of Mater-Bi®, addition of plasticizers was investigated and the results quoted below were obtained. Only the fractions of the additives are quoted," the remaining percentage up to 100% corresponds to the fraction of Mater-Bi®. PEG-2000 served as phase compatibilizer and was added before further additives. Dependent on their behaviour, the tested plasticizers were classified into 7 classes: 1) The following additives led to higher flexibility of the plate but also to a sticky and/ox smeary surface just after having extruded the plate or - in some cases - after having stored the plate at ambient temperature.

PEG-2000 (5%) / ethoxylated (20 EO) sorbitane mono stearate (10%) (trade name: Kotilen-S/1)

PEG-2000 (5%) / ethoxylated (20 EO) sorbitane mono palmitate (10-20%) (trade name: Kotilen-P/1)

PEG-2000 (5%) / sorbitane mono oleate (10%) (trade name: Kosteran-O/1 VL)

PEG-2000 (5%) / sorbitane tri oleate (10%) (trade name: Kosteran-03 VH)

Ethoxylated (75 EO) castor oil (10%) (trade name: Hedipin-R/750H)

Ethoxylated (40 EO) tallow fat amine (10%)

(trade name: Imbentin-TAM/400 )

Polycaprolactone triol, Mw = 900 (25%) . 2) Additives that did not significantly enhance or even lowered the flexibility:

Polycaprolactone diol (25%);

polycaprolactone, Mw = 14000 (25%) ; ethoxylated sorbitol (4 EO) (10%); alkyl poly glycoside (5-10%); PEG-2000 (5%) and polyvinylbutanal (10-20%); ethoxylated (200 EO) castor oil (10%) . 3) Polyeste /polyether polyurethane (25%) caused a higher flexibility but requested also

enhancement of the temperature of processing (185°C) , thus leading to a brown colour of the material. 4) The following additives led to a more flexible material without bleeding. However, the samples were not resistant against water. This means, when being in contact with water, the samples became white and maceration occurred after 30 minutes.

Glycerol (23 - 30%); diglycerol (15 -.25%); polyglycerol-3 (17 - 25%); PEG-2000 (5%) and ethoxylated (20 EO) sorbitane tristearate (10%) and diglycerol (20%) .

5) The following additive led to a more flexible material without bleeding, when the material was stored at ambient temperature. However, the surface became smeary when the sample was subjected to an

alternating atmosphere during 5 days (Temperature was altered between 20 and 35°C and humidity was altered between 20 and 80°C). In addition, the recipe was not miscible on an extruder, because the second additive was not included by Mater-Bi; it remained in the feed hopper:

PEG-2000 (5%) / ethoxylated (20 EO) sorbitane tristearate (10%) (trade name: Kotilen-S/3) . 6) Additives poorly miscible

PEG-2000 (5%), sorbitane tristearate (10%) (trade name: Kosteran-S/3G) . 7) The following additives led to a more flexible material without bleeding when stored at room conditions. The surface did not become smeary when the sample was subjected to an alternating atmosphere during 5 days (Temperature was altered between 20 and 35°C and humidity was altered between 20 and 80%.) :

Tributyl citrate (12.5%)

Acetyl tributyl citrate (10%)

With 15 % tributyl citrate or 12.5 % acetyl tributyl citrate some minor bleeding was observed that, however, should still be acceptable for several

applications.

The mechanical properties of Mater-Bi® and Mater-Bi® plasticized with 10% of acetyl tributyl citrate in comparison to Bioplast 2189 were investigated. The measurements were performed according to ISO 527-2, type 1A, and ISO 75-2 (HDT B) , respectively. In deviation of said norms, the samples were not conditioned and the thickness of the samples was 3 mm. In Table 2 the results are quoted as mean ± standard deviation.

Table 2:

Flatwise; single measurement Addition of acetyl tributyl citrate caused a significant decrease of the E-modulus as well as a strong increase of the strain at rupture, thus indicating an increase of the elasticity. The individual heat

deflection temperatures showed that the non-plasticized materials have a higher dimensional stability at

temperatures above 40°C.

Further investigations were performed with other plasticizers in addition to TBC and ATBC . These other

plasticizers did not result in good products, i.e. either the material was too smeary or the flexibility was not improved.

Example 3 :

Up-scaling to an industrial compounding machine for ATBC plasticized Mater-Bi®

Compounding was performed with a double screw extruder (Manufacturer: Anfu Plastic Machinery Co. Ltd.) using a production line consisting of the following elements: metering system (feeding) - double screw

(compounding) - water bath (cooling) - blower (drying) - granulator. The amount of 9% ATBC was added to Mater-Bi® in 3 compounding passages: 3.6% (first passage), 2.7% (second passage), and 2.7% (third passage), with

percentages being related to the produced blend (end product). ATBC was pre-mixed in an industrial mixing machine with Mater-Bi® (passage 1) or with Mater-Bi® already blended with a certain amount. of ATBC (passages 2-3) . The mixture was then charged into the feeder of the extruder. Other feeding methods would also be possible, e.g. addition of the plasticizer by side feeding. The following parameters were used:

Barrel temperatures: Feeding zone: 120°C (could be varied from 115 to 140°C)*

Barrel zone 1: 135°C (120 ... 150°C)*

Barrel zones 2-3: 140°C (130 ... 160°C)* Barrel zones 4-7: 150°C (130 ... 160°C)* Barrel zones 8-9: 140°C (115 ... 160°C)*

*The barrel temperature range in brackets indicates the range possible for the examined

composition.

Remarks: The temperature ranges applicable are rather wide, however it is preferred to keep the barrel temperatures at about the indicated values T ± 5°C to get an impeccable product. When increasing the

temperature to 160°C in the zones 4 - 7 slight brown coloration of the material was observed. According to information obtained from the producer of Mater-Bi®, Mater-Bi® would be decomposed above 200°C. Thus, the temperature of the material was significantly increased by shear forces. The more ATBC is added the lower the temperatures can be set.

Further parameters were:

Feeding speed: 300 (relative to an

internal scale of the machine)

Screw rotation speed: 135 (relative to an internal scale of the machine)

Back pressure: 1100 (1^st passage), 900 (2^nd passage), 780 (3^rd passage) (values relative to an internal scale of the machine)

Flow rate: 120 kg/h (1^st passage), 100 kg/h

(2^nd passage) , 80 kg/h (3^rd passage)

Granulation speed: 29 (relative to an internal scale of the granulator)

Characterisation of the produced compound, that is called "BioSlip" below, can be found in Table 3. Table 3:

Coloured BioSlip, e.g. BioSlip blue, was produced by addition of 1% of a coloured, e.g. blue, master batch to BioSlip in a fourth passage.

Pure Mater-Bi® was found to have the largest flexural modulus (168 N/mm²) of the samples investigated (Fig. 2) . When increasing the content of acetyl tributyl citrate from 5% to 10%, a linear decrease of the flexural modulus from 134 N/mm² to 76 N/mm² was observed. A further increase of acetyl tributyl citrate from 10% to 11% did not cause any effect regarding flexural modulus. However, the sample containing 11% of acetyl tributyl citrate was found to be slightly smeary. The maximum solubility of acetyl tributyl citrate in Mater-Bi® had obviously been exceeded.

As depicted in Fig. 3, flexural stress and flexural modulus show pairs of values that are on a straight line having a positive slope. The linearity indicates that both parameters similarly depend on the content of acetyl tributyl citrate and that the strain - stress curves have similar shapes.

Example 4 : evaluation of foaming agents

Example 4.1: Foams produced on the laboratory scale

Materials of Example 1 were compounded on a laboratory roll mill with two cylinders (Collin 100T) . The temperature of both cylinders was set at 130°C - 140°C when the plasticizer was added. In order to obtain a sufficient homogeneity several material turn-overs were performed. After having decreased the temperature of both cylinders to 115°C - 120°C, the foaming agent was added. Afterwards, the compounds were removed from the roll-mill and subjected to a moulding press (conditions: 125°C, 10 bar) to obtain material plates of about 140 x 140 x 3.5 mm. In order to investigate the foaming process, round or rectangular coupons were cut from the plates. The coupons were heated up to the individual reaction temperature of the investigated foaming agent to initiate a quick and complete foaming process. This was done using different sources of heat: heating furnace, microwave oven, and moulding press.

Investigated chemical foaming agents :

1) Tracel NC 155 (Tramaco)

2) Tracel NCX 150 (Tramaco)

3) Tracel NCX 155 VP 62 (Tramaco)

4) Tracel NCS 175 (Tramaco) 5) Tracel PO 4155 F (Tramaco) , polymer bonded foaming agent

6) Tracel INC 7207 F (Tramaco) , polymer bonded foaming agent

7) LUVOPOR LUV/N XF (Lehmann + Voss)

With all foaming agents except Tracel INC 7207 F, the material became brown during processing on the roll mill (4 treatments, 10 min each) . This was also observed, when the foaming agents 2, 3 and 7 were

processed with Mater-Bi® and with a mixture of 85% Mater- Bi® / 15% TBC on an injection moulding machine. The brown coloration increased with increasing concentration of the foaming agent. When applying the brown coloration as criterion, the suitability of the foaming agents was evaluated as follows: INC 7207 F » NCS 175 > PO 4155 F > LUV/N XF > NCX 155 VP 62 > > NCX 150 = NC 155. The best agent to foam Mater-Bi® NFOIU was found to be TRACEL INC 7207 F. TRACEL INC 7207 is a polymer bounded salt of bicarbonate. Addition of TRACEL INC 7207 F during

injection moulding at 130-140°C led to an unsatisfactory foaming of the specimen. Increasing the temperature up to 170°C caused a cooling phase that was considered too long for large scale production on a competitive basis.

Shorter cooling phases caused swelling of the specimen after ejection.

Investigated physical foaming agents

(expandable microspheres):

8) EXPANCEL 909 DUX 80, consisting of 20% - 30% isopentane and >70% copolymer CAS 38742-70-0

9) EXPANCEL 461 DU 40, consisting of 10% -

15% isobutane and >80% copolymer CAS 25214-39-5

10) EXPANCEL 950 MB 80, comprising 10 - 14% isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70- 0, and approx. 35% ethylene vinyl acetate copolymer

(partially hydrolysed) )

11) EXPANCEL 930 MB 120, comprising 9 - 14% isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70- 0, and approx. 35% ethylene vinyl acetate copolymer

(partially hydrolysed) )

The effect of expandable microspheres was investigated by expanding plates of 140 x 140 x 3.5 mm prepared in a hot press as described above. Mater-Bi® plasticized with 10% of acetyl tributyl citrate could reproducibly be expanded by a factor of 2 if 2% or 3% of EXPANCEL 461 DU 40 were applied. Application of only 1% of EXPANCEL 461 DU 40 was not successful in most cases. When aiming at expansion factors of 2.5 to 3.5 using 2% or 3% of EXPANCEL 461 DU 40, complete expansion only on partial areas of the plate was observed. When the powders 8 and 9, which are pure expandable microspheres, were wetted with acetyl tributyl citrate previously to the addition to the plasticized polymer, no expansion of the polymer could be observed. This observation was assumed to be due to the shell of microsphere being partially dissolved by the plasticizer, thus causing loss of some gas which is responsible for expansion. According to the producer, EXPANCEL 461 DU 40 shows maximum expansion at 142°C - 150°C Since injection moulding of plasticized Mater-Bi® requires processing temperatures of 160°C - 180°C, expandable microspheres with maximum expansion at 175°C - 190°C (EXPANCEL 909 DUX 80) and at 195°C - 210°C (EXPANCEL 930 MB 120) were additionally investigated. With EXPANCEL 909 DUX 80, an expansion factor of 2.7 could be reached, when adding 1% or 2% of this agent to Mater-Bi® plasticized with 20% - 25% of glycerol

triacetate or with a mixture of 9.6% of glycerol acetate and 9.6% of acetyl tributyl citrate. Similar results have been obtained with EXPANCEL 930 MB 120. Example 4.2: Up-scaling to an industrial moulding injection machine: production of foamed slippers

Samples of slipper were produced with an injection moulding machine, type OiMA 650 190, using the following parameters:

Mould: slipper (length: 277 mm; maximum

breadth: 104 mm; minimum breadth: 69 mm; thickness: .9 mm)

Gate: diameter 7 mm

Clamping force: 190 t

Metering phase:

Barrel temperatures :

Feeding zone: 140°C (120 to 150°C)* Barrel zone 1: 160°C (140 to 175°C)*

Barrel zone 2: 175°C (165 - 185°C)*

Injection zone: 175°C (165 - 185°C)*

Screw rotation speed: 32 rpm

Back pressure : 1 bar

Metering time: 6.1 s

Metering time lag: 30 s

Metering stroke: 50 mm

* The information in brackets is the

temperature range applicable for the specific composition

Injection phase

Max. injection pressure : 180 bar Switch over point: 8 mm

Injection time: 3.88 s

Holding phase

Cushion: 7.8 mm

Holding pressure : 5 bar (built by the foaming agent)

Holding time: 5 s Cooling- phase

Mould temperature : 25°C injection-side

15°C ejection-side

Cooling time: 50 s

Global cycle time: 60.4 s

Remarks regarding the variation of the parameters:

Metering phase: The temperature of the feeding zone can be varied between 120°C and 150°C. Below 120 °C the material became more and more viscous (peak melting temperature (DSC): 116°C). In order to prevent the foaming agent from expansion in the feeding zone, the temperature should not exceed 150 °C. The temperature of the zones "barrel 2" and "injection" preferably is kept between 165°C and 185°C. Splay marks around the gate could be lowered by decreasing the temperature of the zones "barrel 2" and "injection" from 175°C to 165°C. However, this improvement went along with a delamination of a thin layer of material around the gate. This

delamination was especially distinctive when master batch of green dye was added. Below 165°C, the material became too viscous to be injected into the mould. If the

temperature of the barrel zones exceeded 185°C the material partially decomposed and became brownish. Screw rotation speed and back pressure were set as low as possible to prevent the material from shear forces and thus the expandable microspheres from collapsing.

Metering time resulted from these 2 parameters as well as from the metering stroke.

Injection phase: Again, injection speed and injection time were chosen as low and as long as

possible, respectively, to prevent the material from damage . Holding phase: When foaming with expandable microspheres, the holding pressure has to be set to zero (recommendation from AKZO NOBEL, the producer of the EXPANCEL microspheres). A higher holding pressure would hamper microspheres to expand.

Cooling phase: Cooling time was rather long because a mass of 120 g - 140 g had to be cooled. Shorter cooling times were found to 1ead to a post-expansion of the ejected sample.

Samples :

Compositions of the soles produced by

injection moulding that were investigated are listed in Table 4, wherein all amounts are given in % (w/w) related to the whole recipe (The total sum of all ingredients is 100%) .

Table 4:

Raw Foaming agent Lot Mater ATBC Composition of EXPANCEL

mateno . -Bi

Product O.

o isooctane isobutane Copolymer EVA rial

CAS

38742-70-0

BioSlip EXPANCEL 930 MB 120 7.0 1 84.6 8.4 0.7 - 1.0 0.14 - 0.42 ca . 3.5 ca. 2.4

EXPANCEL 950 MB 80 4.8 2 86.6 8.6 0.5 - 0.7 0.1 - 0.29 ca. 2.4 ca. 1.7

7.0 3 84.6 8.4 0.7 - 1.0 0.14 - 0.42 ca . 3.5 ca. 2.4

9.1 4 82.7 8.2 0.9 - 1.3 0.18 - 0.55 ca . 4.6 ca. 3.2

10.7 5 81.3 8.0 1.0 - 1.5 0.21 - 0.64 ca. 5.4 ca . 3.7

BioSlip EXPANCEL 950 MB 80 9.0 6 81.9 8.1 0.9 - 1.3 0.18 - 0.55 ca. 4.5 ca. 3.2 1% blue

BioSlip EXPANCEL 950 MB 80 9.0 7 81.9 8.1 0.9 - 1.3 0.18 - 0.55 ca. 4.5 ca. 3.2 1%

green

BioSlip LUVOPOR AZ 10/G-UT 3.8 8 87.5 8.7 3.8% LUVOPOR AZ 10/G-UT (masterbatch

consisting of a blowing agent on the basis of azodicarbonamide and a carrier (modifie alkyd resin) )

BioSlip LUVOPOR AZ 10/G-UT 7.0 9 84.6 8.4 7.0% LUVOPOR AZ 10/G-UT (masterbatch

consisting of a blowing agent on the basis of azodicarbonamide and a carrier (modifie alkyd resin) )

BioSlip -- 10 90.9 9.1 —

Legend to the Table above:

BioSlip = 91% Mater-Bi® + 9% ATBC EXPANCEL 930 MB 120 contains 9 - 14%

isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70- 0, and ca . 35% ethylene vinyl acetate copolymer

(partially hydrolysed)

EXPANCEL 950 MB 80 contains 10 - 14%

isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70-

0, and ca . 35% ethylene vinyl acetate copolymer

(partially hydrolysed)

Remarks:

BioSlip blue was produced by addition of a blue masterbatch to BioSlip during compounding according to example 2. EXPANCEL and green dye were mixed with BioSlip just before injection moulding.

The results of the tests of injection

moulding are listed in Table 4, wherein parameters are quoted when deviating from those listed at the beginning of Example 4.2. The contents of ingredient can be taken from Table 4.

Table 5:

*The sample showed strong shrinkage and brown colouration

As can be seen from the table above, the weight of sole (and hence the density of the material) decreased from 143.9 g to 119.9 g when the content of the foaming agent EXPANCEL 950 MB 80 was increased from 5% to 10% .

Further increase from 10% to 12% did no more influence the weight but an almost complete reduction of the shrinkage was observed. Addition of coloured master batch did not influence the density of the foamed

material. Foaming with EXPANCEL 930 MB 120 revealed considerably larger variation of the weight of sole from shot to shot. The samples foamed with LUVOPOR AZ 10/G-UT from Lehmann & Voss showed strong shrinkage and brown colouration. Example 5 : Testing of the foamed material Example 5.1: Stability of the recipes under alternating climate conditions

Samples of the lot numbers 4 and 6 were subjected to an alternating climate using a climate chamber Weiss SB1 300/40. Climate was alternated between 30°C ≤T <63°C and 19% ≤RH <75% according to the standard MIL-STD-810G Method 507.5-2 (RH = relative humidity).

However, the time of cycle was accelerated by a factor of 2, i.e. from 24 h to 12 h. The total time of exposure was 276 h. Samples were withdrawn after 23 h, 55 h, 78 h, 145 h, 193 h, 222 h, and 313 h and extracted by agitating in acetone at room temperature for 3 minutes. After

extraction, samples were again put into the climate chamber. The same procedure of extraction was performed before the exposure. The amount of ATBC in the extracts was determined by GC . This amount was found to be highest before exposition (200 - 250 mg / sample) and decreased within the series of extracts taken after 23 - 55 - 78 h to reach a nearly constant value for the following extracts (105 - 200 mg / sample) . These tests show that ATBC was not enriched at the surface of the samples under the used climate conditions. Example 5.2 Density of the different samples

The density of samples of the lot numbers 4, 6 and 10 was determined according to ISO 845. The density of the other samples was calculated using the weights quoted in table 4, whereas the slight shrinkage of some samples was not respected.

Table 6. Density of the different samples

lot number

1 2 3 4 5 6 7 8 9 10

0.72 0.72 0.69 0.60 0.60 0.60 0.60 -- -- 1.20 Example 6: Sole design

In the scope of the present invention it has been found that the flexibility of a sole 1, in

particular if produced with the material described above, can be significantly enhanced if the sole 1 is provided with grooves 2 arranged in different directions, e.g. longitudinal grooves 2b of different length are arranged in the center of the sole 1, and grooves 2a perpendicular thereto are arranged in the lateral regions whereas central parts 3 comprise fewer grooves in order to provide better stability. A presently preferred

orientation, number and dimension of such grooves is indicated in the Figures 4 and 5. The benefit of a structured sole is not only the higher flexibility but also the further reduced weight and reduced risk for slipping .

Summarizing the embodiments of the present invention, the invention concerns

- a biodegradable flexible material comprising (i) a polymer composition comprising as constituents starch and a biodegradable polyester, and (ii) at least one plasticizer, wherein said polyester is an unsubstituted aromatic-aliphatic copolyester, in particular poly (butylene-adipate-co-butylene- terephthalate ) , and wherein said plasticizer is present in non bleeding amounts and selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in amounts of at most 15 % by weight referred to the weight of the total composition, preferably at most 12.5 % by weight,

- the above described material wherein the polymer composition comprises starch in an amount of at least 85 % referred to the polymer composition, a

poly (butylene-adipate-co-butylene-terephthalate) and about 10 % water soluble constituents, - one of the materials described above, wherein the polymer composition is Mater-Bi® NF01U.

- one of the materials described above, wherein the plasticizer comprises _.up to 15 %,. preferably up to 12.5 % tributyl citrate, or wherein the plasticizer comprises up to 12.5%, preferably up to 10 % acetyl tributyl citrate.

- one of the materials described above, comprising a foaming agent.

- one of the materials described above, wherein the foaming agent is expansible microspheres,

- the material described above wherein the expansible microspheres are composed of

9 - 14% isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70-0, and ca. 35% ethylene vinyl acetate copolymer (partially hydrolysed) , e.g. EXPANCEL 930 MB 120 or

EXPANCEL 950 MB 80,

- one of the materials described above containing or consisting of

70 to 90% Mater-Bi® NF01U

5 to 15 %, in particular 5 to 12.5% tributyl citrate or

5 to 12.5%, in particular 5 to 10% acetyl tributyl citrate or

5 to at most 12.5 % of a composition of tributyl citrate and acetyl tributyl citrate,

and optionally

0 to 18 %, in particular 0 to 15 %,

especially preferred for a light weight product about 10 to 13 % expandable microspheres

0 to 1.5 % of a dye

0 to 1.5 % of perfumes,

the sum of the above mentioned ingredients amounting to at least about 98 % by weight of the total composition, preferably to 100 %, - a method for producing a foamed product comprising one of the materials described above, by applying an injection moulding method comprising the steps of metering, injection, holding, cooling, and ejection, wherein the metering is performed in a metering zone comprising at least one feeding zone, at least one heating zone and an injection zone,

- the above described method, wherein

a) the injection zone has a barrel temperature between 2°C and 35°C below the barrel

temperature causing first evidence of decomposition

(slight brown colouration) of the material,

b) the metering zone has a back pressure between 0.5 bar and 5 bar, preferably about 1 bar, a screw rotation speed of 20 to 50 rpm, preferably about 30 rpm, and

c) the holding is performed at a pressure between 0 and 10 bar, preferably built by the foaming agent only,

- a biodegradable flexible expansible

material comprising (i) starch, (ii) a biodegradable polyester, preferably a non substituted aliphatic- aromatic copolyester, (iii) plasticizers and (iv) a foaming agent, wherein said plasticizers comprise the plasticizers in non bleeding amounts, and wherein said plasticizers comprise at least one plasticizer selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in amounts of at most 15 % by weight referred to the weight of the total composition, and wherein the foaming agent is expansible microspheres.

- the biodegradable flexible expansible material, wherein the starch and the polyester are constituents of a polymer composition comprising starch in amounts of at least 85% by weight of the polymer composition and wherein about 10 % of the constituents are water soluble, - one of the biodegradable flexible

expansible materials described above, wherein the

polyester is a poly (butylene-adipate-co-butylene- terephthalate) ,

- one of the biodegradable flexible

expansible materials described above, wherein at least the starch and polyester components, preferably all components except the citrate plasticizers, are Mater-Bi® NF01U,

- one of the biodegradable flexible

expansible materials described above, wherein the

plasticizer comprises up to 15 %, preferably up to 12.5 % tributyl citrate, or wherein the plasticizer comprises up to 12.5%, preferably up to 10 % acetyl tributyl citrate,

- one of the biodegradable flexible

expansible materials described above wherein the

expansible microspheres are composed of

9 - 14% isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70-0, and ca . 35% ethylene vinyl acetate copolymer (partially hydrolysed) , e.g. EXPANCEL 930 MB 120 or

EXPANCEL 950 MB 80,

- one of the biodegradable flexible

expansible materials described above containing or consisting of

70 to 90% Mater-Bi®

5 to 15 %, in particular 5 to 12.5% tributyl citrate or

5 to 12.5%, in particular 5 to 10% acetyl tributyl citrate or

5 to at most 12.5 % of a composition of tributyl citrate and acetyl tributyl citrate,

and optionally

0 to 18 %, in particular 0 to 15 %,

especially preferred for a light weight product about 10 to 13 % expandable microspheres

0 to 1.5 % of a dye

0 to 1.5 % of perfumes, the sum of the above mentioned ingredients amounting to at least about 98 % by weight of the total composition, preferably to 100 %,

- a method for producing a foamed product comprising the material of any one of claims 1 to 17, by applying an injection moulding method comprising the steps of metering, injection, holding, cooling, and ejection, wherein the metering is performed in a metering zone comprising at least one feeding zone, at least one heating zone and an injection zone, and

- the method described above, wherein

a) the injection zone has a barrel temperature between 2°C and 35°C below the barrel

temperature causing first evidence of decomposition

(slight brown colouration) of the material,

b) the metering zone has a back pressure between 0.5 bar and 5 bar, preferably about 1 bar, a screw rotation speed of 20 to 50 rprn, preferably about 30 rpm, and

c) the holding is performed at a pressure between 0 and 10 bar, preferably built by the foaming agent only.

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

Claims

1. A biodegradable flexible material

characterized in that it comprises (i) a polymer

composition comprising as constituents starch and a biodegradable polyester, and (ii) at least one

plasticizer, wherein said polyester is an unsubstituted aromatic-aliphatic copolyester, in particular

poly (butylene-adipate-co-butylene-terephthalate ) , and wherein said plasticizer is present in non bleeding amounts and selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in amounts of at most 15 % by weight referred to the weight of the total composition, preferably at most 12.5 % by weight.

2. The material of claim 1 wherein the polymer composition comprises starch in an amount of at least 85 % referred to the polymer composition, a

poly (butylene-adipate-co-butylene-terephthalate) and about 10 % water soluble constituents.

3. The material of claim 1 or 2, wherein the polymer composition is Mater-Bi® NF01U.

4. The material of any one of claims 1 to 3, wherein the plasticizer comprises up to 15 %, preferably up to 12.5 % tributyl citrate.

5. The material of any one of claims 1 to 3, wherein the plasticizer comprises up to 12.5%, preferably up to 10 % acetyl tributyl citrate.

6. The material of anyone of the preceding claims comprising a foaming agent.

7. The material of claim 6 wherein the foaming agent is expansible microspheres.

8. The material of claim 7 wherein the expansible microspheres are composed of

9 - 14% isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70-0, and ca . 35% ethylene vinyl acetate copolymer (partially hydrolysed) , e.g. EXPANCEL 930 MB 120 or

EXPANCEL 950 MB 80.

9. The material of anyone of the preceding claims containing or consisting of

70 to 90% Mater-Bi® NF01U

5 to 15 %, in particular 5 to 12.5% tributyl. citrate or

5 to 12.5%, in particular 5 to 10% acetyl tributyl citrate or

5 to at most 12.5 % of a composition of tributyl citrate and acetyl tributyl citrate,

and optionally

0 to 18 %, in particular 0 to 15 %,

especially preferred for a light weight product about 10 to 13 % expandable microspheres

0 to 1.5 % of a dye

0 to 1.5 % of perfumes,

the sum of the above mentioned ingredients amounting to at least about 98 % by weight of the total composition, preferably to 100 %.

10. A method for producing a foamed product comprising the material of any one of claims 1 to 9, by applying an injection moulding method comprising the steps of metering, injection, holding, cooling, and ejection, wherein the metering is performed in a metering zone comprising at least one feeding zone, at least one heating zone and an injection zone.

11. The method of claim 10, wherein ■ a) the injection zone has a barrel temperature between 2°C and 35°C below the barrel

temperature causing first evidence of decomposition

(slight brown colouration) of the material,

b) the metering zone has a back pressure between 0.5 bar and 5 bar, preferably about 1 bar, a screw rotation speed of 20 to 50 rpm, preferably about 30 rpm, and

c) the holding is performed at a pressure between 0 and 10 bar, preferably built by the foaming agent only.

12. A biodegradable flexible expansible material comprising (i) starch, (ii) a biodegradable polyester, preferably a non substituted aliphatic- aromatic copolyester, (iii) plasticizers and (iv) a foaming agent, wherein said plasticizers comprise the plasticizers in non bleeding amounts, and wherein said plasticizers comprise at least one plasticizer selected from tributyl citrate, acetyl tributyl citrate and mixtures thereof in amounts of at most 15 % by weight referred to the weight of the total composition, and wherein the foaming agent is expansible microspheres.

13. The material of claim 12, wherein the starch and the polyester are constituents of a polymer composition comprising starch in amounts of at least 85% by weight of the polymer composition and wherein about 10 % of the constituents are water soluble.

14 The material of claim 12 or 13, wherein the polyester is a poly (butylene-adipate-co-butylene- terephthalate ) .

15. The material of any one of claims 12 to

14, wherein at least the starch and polyester components, preferably all components except the citrate

plasticizers, are Mater-Bi® NF01U.

16. The material of anyone of claims 12 to 15, wherein the plasticizer comprises up to 15 %,

preferably up to 12.5 % tributyl citrate.

17. The material of anyone of claims 12 to

15, wherein the plasticizer comprises up to 12.5%, preferably up to 10 % acetyl tributyl citrate.

18. The material of anyone of claims 12 to 17 wherein the expansible microspheres are composed of

9 - 14% isooctane, 2 - 6% isobutane, >50% copolymer CAS 38742-70-0, and ca. 35% ethylene vinyl acetate copolymer (partially hydrolysed) , e.g. EXPANCEL 930 MB 120 or

EXPANCEL 950 MB 80.

19. The material of , anyone of claims 12 to 18 containing or consisting of 70 to 9.0% Mater-Bi®

5 to 15 %, in particular 5 to 12.5% tributyl citrate or

5 to 12.5%, in particular 5 to 10% acetyl tributyl citrate or

5 to at most 12.5 % of a composition of tributyl citrate and acetyl tributyl citrate,

and optionally

0 to 18 %, in particular 0 to 15 %,

especially preferred for a light weight product about 10 to 13 % expandable microspheres

0 to 1.5 % of a dye

0 to 1.5 % of perfumes,

the sum of the above mentioned ingredients amounting to at least about 98 % by weight of the total composition, preferably to 100 %.

20. A method for producing a foamed product comprising the material of any one of claims 12 to 19, by applying an injection moulding method comprising the steps of metering, injection, holding, cooling, and ejection, wherein the metering is performed in a metering zone comprising at least one feeding zone, at least one heating zone and an injection zone.

21. The method of claim 20, wherein a) the injection zone has a barrel temperature between 2°C and 35 °C below the barrel

temperature causing first evidence of decomposition

(slight brown colouration) of the material,

b) the metering zone has a back pressure between 0.5 bar and 5 bar, preferably about 1 bar, a screw rotation speed of 20 to 50 rpm, preferably about 30 rpm, and

c) the holding is performed at a pressure between 0 and 10 bar, preferably built by the foaming agent only.