EP1040159A1

EP1040159A1 - Method for preparing a thermoplastic composition based on polyolefins, resulting composition and application for making three-dimensional objects

Info

Publication number: EP1040159A1
Application number: EP98958960A
Authority: EP
Inventors: Frédéric JOUFFRET; Georges Fourty
Original assignee: Talc de Luzenac SA
Current assignee: Imerys Talc Luzenac France SAS
Priority date: 1997-12-16
Filing date: 1998-12-07
Publication date: 2000-10-04
Also published as: WO1999031174A1; AU1491899A; FR2772387B1; FR2772387A1

Abstract

The invention concerns a method for preparing a thermoplastic composition based on polyolefins for making three-dimensional objects, in particular thin-walled wrapping and packaging cases, which consists in incorporating into the polyolefins talc in a percentage substantially between 5 and 25 %, said talc being in the form of a powder with lamellarity index not less than 1.4 and particle size distribution such that the cut diameter D95 is not more than 18 microns and the median diameter D50 is not more than 6 microns, Said incorporation brings about a significant increase of the ratio Re/P in thermoformed three-dimensional objects, Re being the crush resistance between two surfaces of an object and P its weight.

Description

PROCESS FOR THE PREPARATION OF A THERMOPLASTIC COMPOSITION A

POLYOLEFIN BASE, COMPOSITION OBTAINED AND APPLICATION

TO THE MANUFACTURE OF THREE-DIMENSIONAL OBJECTS

The invention relates to a process for preparing a thermoplastic composition based on polyolefins for the manufacture of three-dimensional objects, in particular packaging or packaging of thin walls, in particular for food products. It particularly relates to compositions based on polypropylenes intended to be thermoformed.

The percentage of packages or packages of food products produced by thermoforming of polyolefins, in particular polypropylenes, is currently around 20% and is increasing rapidly each year, for the essential reason that these materials are easily recyclable. These packages are generally palletized and one of their first qualities is to benefit from resistance to crushing between the two load-bearing surfaces.

(Re) which is sufficient to avoid damage to the wrapping or packaging. Thus, this minimum crush resistance is a specification which conditions the quantity of material to be used and, therefore, the price of the packaging and its weight P. Parafiscal taxes depending on the weight are applied to these products in most countries to finance their recycling and we understand that the Re / P ratio constitutes the critical parameter on which the research effort of the packaging industries is focused.

It is known in the field of thermoplastics that incorporation of a mineral filler can lower the cost price of the material and increase the modulus of elasticity in bending of the latter. For example, patents EP-0 353 991 and EP-0 129 890 describe the incorporation into a polypropylene of a mineral filler such as calcium carbonate or sulphate, talc, mica ... in a high proportion by weight (50% 80% for the first patent, 19 to 69% for the second). However, this type of incorporation leads to a very significant increase in the weight of the packaging since the very large increase in density of the mineral compound added (for example average density of pure polypropylene: 0.9, average density of mineral fillers mentioned above: 2,8) is not compensated, by far, by the increase in the modulus of elasticity in bending of the material obtained; if working at constant crush resistance, the weight of the finished object is much higher, so that in most countries, the amount of recycling fees makes losing interest in incorporating such charges (the gain obtained on the quantity of polypropylene being less than the increase in costs due to taxes). Patent EP-0 476 926 also describes a polypropylene-based composition in which talcum powder is incorporated in a smaller proportion than in the preceding cases, but it does not seem that this addition leads to significant advantages as regards the Re / P ratio. For these reasons, manufacturers have given up incorporating mineral fillers into polyolefins intended for the manufacture of packaging; the only products added (in very small proportion of the order of 1%) are functional products such as pigments (titanium dioxide), antistatic agents (glycerol monostearate) or other additives. It should be emphasized that certain thermoplastic compositions are intended for producing sheets by calendering and obviously do not pose the problem of resistance to crushing of the above-mentioned three-dimensional objects. For example, patent FR-2,358,439 (corresponding to patent GB 1,575,010) describes the incorporation of a weight percentage between 10% and 60%, preferably 15% to 45%, of slate flour, dolomite , barite, calcium carbonate, kaolin, gypsum or talc, in a polypropylene intended for calendering sheets. According to this prior document, the addition of these high percentages of mineral fillers would facilitate calendering.

Furthermore, in certain particular applications, for example to make a polypropylene film tearable (patent EP-0 659 815), a mineral filler (talc, calcium carbonate, silica, clay, diatomaceous earth, etc.) is added to the thermoplastic material intended to form the film by calendering; this load reduces the force required to obtain the tear of the film from a notch.

The present invention proposes to prepare a thermoplastic composition based on polyolefins for the manufacture of a three-dimensional object, in particular an object with thin walls (that is to say less than 2.5 mm ), said composition making it possible to increase the Re / P ratio of the object (Re: resistance to crushing between two faces thereof; P: weight of the object) compared to the objects produced from the compositions currently known thermoplastics.

An object of the invention is thus, with constant crush resistance Re, to reduce signif icantly the weight of the object and therefore the amount of recycling taxes and its own manufacturing cost

(less amount of total material, less amount of polyolefins, a fraction of which is replaced by a cheaper material).

Another objective is to provide a thermoplastic composition capable of being implemented by thermoforming while benefiting from significant productivity gains. To this end, the process for the preparation of the polyolefin-based thermoplastic composition according to the invention is characterized in that talc is incorporated in the polyolefins in percentage by weight substantially between 5% and 25%, occurring under the form of a powder with a lamellarity index greater than or equal to 1.4 and a particle size distribution such that the cut-off diameter D95 is less than or equal to 18 microns and that the median diameter D50 is less than or equal to 6 microns. By "powder" is meant both an uncompacted product in which the particles are free from each other, as well as a densified product in which the particles or certain particles are temporarily bonded into agglomerates. By "median diameter D50, we mean a diameter such that 50% of the particles by weight have a size less than said diameter; by" cut-off diameter D95 ", we mean a diameter such that 95% of the particles by weight have a size less than said For non-spherical particles, the size is made up of the equivalent spherical diameter (Stokes diameter) All measurements of diameters D50 and D95 are carried out using a "Sedigraph" device (registered trademark) by gravity sedimentation in accordance with AFNOR Xll-683 standard.

"The lamellarity index" characterizes the shape of the particle and more particularly its flattening (large dimension / thickness). This lamellarity index is measured by the difference between, on the one hand, the value of the average dimension (Dm) of the particles of the powder obtained by a particle size measurement by laser diffraction in the wet process, (AFNOR standard NFX11-666 ) and on the other hand, the value of the median diameter (D50 ₎ obtained by a measurement by sedimentation using a "Sedigraph"

(AFNOR Xll-683 standard), this difference (Dm - D ₅₀ ) being related to the median diameter D ₅₀ . lamellarity index = ^{Dm ~ D 50} •

D 50

Reference may be made to the article "G. BAUDET and JP RONA, Ind. Min. Mines et Carr. Les Techn. June July 1990 pp 55-61" which shows that this index is correlated with the average ratio of the largest dimension from the particle to its smallest dimension. A lamellarity index greater than 1.4 characterizes an exceptional talc having very elongated particles (the lamellarity index of spherical particles is 0). It should be noted that there are many other parameters making it possible to characterize the shape of mineral particles, in particular the Anglo-Saxon parameter designated "Average Aspect Ratio". This "Average Aspect Ratio" is defined by the ratio between the average diameter of the particles measured by scanning electron microscopy and their average thickness also measured by scanning electron microscopy. The two parameters: lamellarity index and "Average Aspect Ratio" are linked by linear proportionality and the corresponding curve for talc is provided in the single figure of the appended drawing. It should be emphasized that, in patent EP-0 476 926, the "Average Aspect Ratio" of the powder incorporated is between 3 and 20, preferably between 7 and 20, and more preferably between 10 and 20. This patent therefore teaches those skilled in the art that it is advisable to choose an "Average Aspect Ratio" less than 20 to obtain a composition having satisfactory mechanical characteristics, that is to say a lamellarity index less than 1 (as shown in the correspondence curve of the drawing).

“Talc” means either the hydrated magnesium silicate mineral or the chlorite mineral

(hydrated magnesium and aluminum silicate), either a mixture of the two, associated if necessary with other minerals (dolomite, etc.), or even, a mineral substance obtained from talc and having similar properties.

The inventors have demonstrated that, unexpectedly, the addition in the thermoplastic material of a particular type of talc in a particular percentage range makes it possible to increase by at least 4% the Re / P ratio of the objects to three dimensions produced by thermoforming, despite the higher density of talc added. As the comparative examples provided below will illustrate, this effect is only obtained with the incorporation of a specific type of talc combining a lamellarity index> 1.4, a D95 _< 18 microns and a D50 _< 6 microns, the weight percentage of talc not to exceed 25% (beyond, the increase in the apparent density of the composition becoming too large compared to the gain in crushing resistance obtained). The value of the lamellarity index, greater than or equal to 1.4, preferably between 1.4 and 10, and very preferentially between 1.4 and 3.6, is the critical parameter for obtaining this significant improvement in Re / P report. It should be stressed that such a value greater than 1.4 is an exceptional index that only very rare talcs present. Current talcs (99% of talcs existing on the market) have a lamellarity index of between 0.4 and 1 and do not allow the above-stated result to be obtained, whatever their particle size.

This significant increase in the resistance to crushing of the object obtained from the composition in accordance with the invention is not explained at the present time and is probably due to a preferred orientation which the talc particles take in the material when the hot forming operation is performed. Other minerals do not appear to have the same effect.

The composition according to the invention in which the aforementioned talc particles are dispersed benefits from a reduced amount of recycling taxes due to its lower weight and a very reduced manufacturing cost (from 8 to 10% approximately: reduction of the total amount of material used, substitution of a fraction of the polyolefins by talc much cheaper).

In the majority of cases aimed in particular at applying the composition to the manufacture of packaging or packaging for food products, the polyolefins used are polypropylenes, homopolymers and / or copolymers, having a melt index substantially between 0, 5 and 5 g per 10 naked at 230 ° C under 2.16 kg (ISO R 1131 standard). The above-mentioned advantages are optimized by preferentially using a talc having the following characteristics: (as already mentioned) lamellarity index substantially between 1.4 and 10, and preferably between 1.4 and 3.6,

- particle size distribution such that the cut-off diameter D95 is less than 15 microns and the median diameter D50 is substantially between 0.5 and 5 microns, talc of macrocrystalline type (whose lamellar nature is most pronounced), this talc being pre-ground to present the aforementioned particle size distribution.

By "macrocrystalline talc" is meant a talc which naturally contains large elementary crystals (in particular greater than 15 microns before pre-grinding) observable on thin sections, in contrast to microcrystalline talcs naturally having small elementary crystals.

The percentage by weight of talc which seems to give the best result in most cases is appreciably between 10% and 20%.

The incorporation of talc in polyolefins can be carried out in the traditional way in extruders, in particular twin-screw, allowing a substantially homogeneous dispersion in the plastic. Of course, certain conventional additives such as pigments, anti-static agents, etc. can also be added to the composition.

Talc can be directly incorporated into polyolefins in the above-mentioned proportions. However, according to a preferred embodiment, a concentrated mixture is prepared beforehand in the form of granules comprising polyolefins and a weight percentage of talc of between 60% and 80%, and this is then diluted. concentrated mixture with polyolefins so as to obtain the aforementioned weight percentage talc / polyolefins. Thus, the transformation of the plastic material uses only granules (concentrated mixture and polyolefins) without handling of powders which are implemented at the supplier of master mixture which has the corresponding know-how. To make this concentrated mixture, it is advantageous to use the process referred to in patent FR-2 714 326, in which the talc is previously agglomerated into granules capable of disintegrating during incorporation into the polyolefins.

The invention extends to the thermoplastic composition prepared according to the process defined above as well as to the above-mentioned concentrated mixture.

The invention extends to a method of manufacturing a three-dimensional object characterized in that, in order to increase the Re / P ratio of the object, where Re is its resistance to crushing between two faces and P its weight, one chooses to carry out thermoforming, a thermoplastic composition of the type previously defined, that is to say containing talc in percentage substantially between 5% and 25%, being in the form of particles dispersed in the polyolefins, of lamellarity index greater than or equal to 1.4 and of particle size such that the cut-off diameter D95 is less than or equal to 18 microns and that the median diameter D50 is less than or equal to 6 microns. To manufacture the product in three dimensions, the composition is implemented by thermoforming either in the molten state ("Melt phase") or in the solid phase under pressure ("SPPF: Solide phase pressure forming"). We can see remarkable productivity gains (shortened cycle times) that can reach 30 to 40%. The incorporated talc increases the thermal conductivity of the polymer and therefore reduces the duration of the heating and cooling cycles.

Such an object (packaging, packaging, container, etc.) is characterized by a Re / P ratio increased by at least 4% compared to an object of the same external shape produced from the same polyolefins in the absence of talc.

The single figure of the appended drawing provides, in the case of talc, the correspondence curve between the lamellarity index used in the description and in the examples and the Anglo-Saxon parameter "Average Aspect Ratio". Examples 1 to 5 which follow illustrate the process of the invention, while the comparative tests C1, C2 and C3 describe manufacture of different characteristics in order to illustrate, by comparison, the effectiveness of the invention.

Example 1:

A polypropylene homopolymer manufactured by the company MONTELL under the name "JE 6100", in the form of granules, having a melt index close to 2 is used.

In order to prepare a concentrated mixture, talc (ground macrocrystalline product, chlorite weight rate: 3.5%, BET specific surface area: 7 m / g) of characteristics indicated in Table I is mixed with polypropylene in the molten state. a twin-screw extruder, type "Clextral BC21", so as to obtain a weight percentage of talc of 75%. The concentrated mixture is in the form of granules with an average size of 2 mm.

The concentrated mixture is then diluted with the same homopolymer polypropylene so as to obtain a talc content by weight of 15% in the final mixture, directly in an "Reifenhâuser RM 801" type extruder capable of calendering the mixture in a sheet of constant thickness equal to 950 μm and of width 315 mm; said sheet is thermoformed by the "SPPF" process on an "Illig type RDM53K" machine fitted with a preheating system (heating to temperatures close to the melting temperature of the mixture: 155 ° C., and forming under a pressure of 6 , 5 bars in a three-dimensional mold).

The finished object is a generally rectangular rectangular container, flared towards its open face opposite the bottom, (dimensions: height 6 cm, length 13 cm, width 9 cm), it is ejected from the mold thanks to an air system compressed. The characterization of the object is made by a measurement of resistance to crushing between its bottom and its open face on a dynamometer of the type "Adamel Lhomargy DY24" provided with force sensors. The force measurement in Newton is made at a speed of 100 mm / min after deformation in compression of 7 mm. The essential parameters and the results obtained are summarized in Table I below:

Example 2:

The composition referred to in this example is produced in an identical manner to that of Example 1 with the same percentage of talc, but the latter of a deposit different, (ground macrocrystalline product, chlorite weight content: 2.5%; BET specific surface: 5 m ² / g) has the characteristics indicated in Table II. The finished object is prepared in the same way as that of Example 1. Table II below summarizes the results obtained and essential parameters implemented:

Comparative test Cl:

The composition of this comparative test C1 contains only polypropylene without addition of filler. The finished object is prepared in the same way as those of Examples 1 and 2 but by extruding a thicker sheet, of thickness equal to 1100 μm, so as to obtain an equal crush resistance for the finished object.

Comparative tests C2, C3: Compositions manufactured in a similar manner to those of Examples 1 and 2 contain talcs described in Table III, having respectively a cut-off diameter D95 equal to 20 μm for the first and a lamellarity index less than 1 , 3 for the second. The finished articles are prepared in the same way as those of Examples 1 and 2, starting from a sheet of thickness equal to 950 μm.

Table III below summarizes the data in Tables I and II and summarizes the parameters and results of the comparative examples C1, C2, C3.

This table shows that only the type of talc targeted by the invention having a high lamellarity index associated with a fine particle size as described makes it possible to produce a polypropylene tray whose Re / P ratio is increased. The increase in this ratio is approximately 10% compared to the comparative test C1 (no charge) and to the comparative test C2 (relatively lamellar talc but of different particle size), and is approximately 17% by report on the comparative test C3 (charge constituted by a talc with standard lamellarity). This comparative test C3 leads to a Re / P ratio which is lower than that of the test Cl carried out in the complete absence of talc; this test highlights the essential character of the lamellarity index which must be greater than 1.4 to allow the obtaining of a significantly increased Re / P ratio. This effect is completely unexpected.

Examples 3, 4 and 5: The compositions referred to in Examples 3, 4 and 5 are produced in an identical manner to that of Example 1 with the same percentage of talc (15%), the talc having been the subject of a different grinding according to the process described in the international application PCT / FR 98/00569 in order to give it the following lamellarity indices: example 3: 2.9; Example 4: 3.6; example 5: 10.

The finished object is prepared in the same way as that of Example 1.

Table IV below summarizes the results obtained and essential parameters implemented.

In the three examples, there is a significant improvement in the Re / P ratio (compared to the comparative tests). This improvement increases with the lamellarity index, but in practice it will be advantageous to be limited to a lamellarity index less than 10 because, beyond, talcs are very expensive. In practice, a range of 1.4 to 3.6 leads to maximum savings on the manufactured object (taking into account the increasing price of talc beyond this value).

Claims

1 / - Process for the preparation of a thermoplastic composition based on polyolefins for the manufacture of three-dimensional objects, in particular packaging or packaging, characterized in that talc is incorporated in the polyolefins in weight percent substantially between 5 % and 25%, in the form of a powder with a lamellarity index greater than or equal to 1.4 and with a particle size distribution such that the cut-off diameter D95 is less than or equal to 18 microns and that the median diameter D50 either less than or equal to 6 microns.

2 / - Process according to claim 1, characterized in that a talcum powder having a lamellarity index substantially between 1.4 and 10, preferably between 1.4 and 10, is incorporated into the polyolefins.

3.6.

3 / - Method according to one of claims 1 or 2, characterized in that there is incorporated into the polyolefins a talcum powder having a particle size distribution such that the cut-off diameter D95 is less than 15 microns and the median diameter D50 is substantially between 0.5 and 5 microns.

4 / - Method according to one of claims 1, 2 or 3, characterized in that there is incorporated into the polyolefins a weight percentage of talc substantially between 10% and 20%.

5 / - Method according to one of claims 1, 2, 3 or 4, characterized in that one incorporates into the polyolefins a talc of macrocrystalline type, ground to present the aforementioned particle size distribution.

6 / - Method according to one of claims 1, 2, 3, 4 or 5, characterized in that one prepares beforehand a concentrated mixture in the form of a granule comprising the polyolefins and a weight percentage of talc of between 60% and 80%, and this concentrated mixture is diluted with polyolefins so as to obtain the aforementioned weight percentage talc / polyolefins.

7 / - Method according to one of claims 1 to 6, wherein the talc is incorporated into the polyolefins in an extruder under suitable conditions to obtain a substantially homogeneous dispersion. 8 / - Method according to one of claims

1 to 7, in which polypropylenes, homopolymers and / or copolymers are used as polyolefins, having a melt index substantially between 0.5 and 5 g per 10 i at 230 ° C under 2.16 kg (ISO standard R1113) . 9 / - Thermoplastic composition comprising polyolefins, characterized in that it comprises talc in percentage substantially between 5% and 25%, being in the form of particles dispersed in the polyolefins, with a lamellarity index greater than or equal to 1 , 4 and of particle size such that the cut-off diameter D95 is less than or equal to 18 microns and that the median diameter D50 is less than or equal to 6 microns.

10 / - Thermoplastic composition according to claim 9, based on polypropylenes, comprising a weight percentage of talc substantially between 10% and 20%.

11 / - Concentrated mixture of polyolefins and talc for the preparation of a thermoplastic composition according to one of claims 9 or 10, characterized in that it comprises talc in percentage substantially between 60% and 80% , being in the form of particles dispersed in polyolefins, of lamellarity index greater than or equal to 1.4 and of particle size such that the cut-off diameter D95 is less than or equal to 18 microns and that the median diameter D50 is less than or equal to 6 microns.

12 / - Method for manufacturing a three-dimensional object, characterized in that, in order to increase the Re / P ratio of the object, where Re is its resistance to crushing between two faces and P its weight , a sheet formed from a thermoplastic composition according to one of claims 9 or 10 is thermoformed.

13 / - Three-dimensional object, in particular conditioning or packaging, produced by implementing the method according to claim 12, characterized by a Re / P ratio increased by at least 4% compared to an object of the same external shape made from the same polyolefins in the absence of talc.