WO2005000555A2

WO2005000555A2 - Method of producing articles by means of rotational moulding

Info

Publication number: WO2005000555A2
Application number: PCT/FR2004/001611
Authority: WO
Inventors: Maria-Rosa Contardi; Gérard Guillot; Christophe Lapierre; Jérôme THERBY
Original assignee: Rhodia Engineering Plastics S.R.L.
Priority date: 2003-06-27
Filing date: 2004-06-25
Publication date: 2005-01-06
Also published as: FR2856625A1; WO2005000555A3; FR2856625B1

Abstract

The invention relates to a method of producing polyamide articles by means of rotational moulding and to the articles thus obtained. The inventive method includes a step in which a polyamide-based powder is introduced into a mould for rotational moulding. The articles obtained by means of rotational moulding can be hollow seamless pieces such as, for example: basins, troughs, tanks, cisterns, cases, boxes, vats, bumpers, seats and car body elements.

Description

Manufacture of articles by rotational molding

The present invention relates to a method for manufacturing polyamide articles by rotational molding as well as articles produced by this method. The method according to the invention notably comprises a step in which a polyamide-based powder is introduced into a mold for rotational molding. The articles obtained by rotational molding can be hollow parts without welding, such as for example the articles chosen from the group comprising: reservoirs, tanks, balloons, tanks, caissons, boxes, tanks, bumpers, seats, and body parts .

The rotational molding process, also called rotational molding, has been known for a very long time and allows the production of hollow plastic articles.

This process consists in placing a plastic material in a mold and putting this mold in rotation so that all the points of the internal surface of the mold are brought into contact with the plastic material and then heating the rotating assembly so as to deposit said plastic material melted on the internal surface of the mold. Finally, a cooling step allows the part to solidify, which is then removed from the mold.

Rotational molding is appreciated because it avoids the induction in the plastic of tensions such as those which can be found in injection molded parts. Indeed, the plastic material does not undergo kneading or compaction as thorough as in an extruder or an injection device. Rotational molding allows the manufacture of large items, such as tanks and vats.

The most widely used plastic polymers for rotational molding are polyethylenes and polyvinyl chloride (PVC). Polyamides are little used for the production of articles by rotational molding. Indeed, existing manufacturing processes do not allow manufacturing polyamide articles having homogeneous characteristics, such as the regularity of the thickness of the walls. It follows that these articles have points of weakness in their structure which induces a reduction in their mechanical properties. Furthermore, the polyamide parts obtained by rotational molding having a poor surface appearance.

The present invention relates to a process for manufacturing polyamide-based articles by rotational molding in which a polyamide-based powder of fine particle size having a low moisture content is introduced into the mold. The method according to the invention makes it possible to avoid the drawbacks mentioned above. This process allows the manufacture of articles having homogeneous characteristics, such as for example a regularity of the thickness of the walls of said articles. The articles obtained have more good mechanical properties. The articles obtained according to the invention also have a good appearance of internal and external surface, without any casting, bubble or other defect. Furthermore, the temperature for the rotational molding of the powder of the invention can be reduced compared to a conventional process. The present invention relates to a method for manufacturing an article by rotational molding in which at least one polyamide-based powder is introduced into a mold having the following characteristics: i) the particle size of said powder is less than or equal to 500 μm ; and ii) said powder has a moisture content of less than or equal to 0.65% by weight. The invention relates in particular to a method of manufacturing an article by rotational molding comprising at least the following steps: a) placing in a mold at least one polyamide-based powder having the following characteristics: i) the particle size of said powder is less than or equal to 500 μm; and ii) said powder has a moisture content of less than or equal to 0.65% by weight; b) rotating the mold; c) heating the mold; d) cooling the mold and / or the article obtained; and e) unmolding the article. The polyamides suitable for the invention are preferably composed of at least one (co) polyamide chosen from the group comprising: (co) polyamide 6; 4; 11; 12; 4.6; 6.6; 6.10; 6.12; 6.18; 6.36; their copolymers and mixtures. According to a preferred embodiment of the invention, at least 90% by weight, preferably at least 99% by weight, repeating units of the macromolecular chains of (co) polyamides are chosen from the recurring units of polyamide 6 and the recurring units of polyamide 6.6.

Mention may be made, for example, of semi-crystalline or amorphous polyamides, such as aliphatic polyamides, semi-aromatic polyamides and more generally, linear polyamides obtained by polycondensation between a saturated aliphatic or aromatic diacid, and a primary saturated aromatic or aliphatic diamine, polyamides obtained by condensation of a lactam, an amino acid or linear polyamides obtained by condensation of a mixture of these different monomers. More specifically, these copolyamides can be, for example, polyadipamide of hexamethylene, polyphthalamides obtained from terephthalic and / or isophthalic acid, copolyamides obtained from caprolactam, and one or more monomers generally used for the manufacture of polyamides, such as adipic acid, terephthalic acid, and / or hexamethylene diamine. Preferably, the (co) polyamide suitable for the invention has a melt index less than or equal to 25 g / 10 min according to ISO 1133, under a load of 2.16 kg, at a temperature of 10 ° C at above the melting point of the (co) polyamide.

According to an advantageous characteristic of the invention, the polyamide-based powder can be composed of a mixture of a (co) polyamide with one or more other polymers. It is possible to envisage a mixture of (co) polyamide with at least one polymer of the propylene polyoxide (PPO), polychloride type. vinyl (PVC), polyacrylo-butadiene-styrene (ABS), polyethylene (PE), polypropylene (PP), or (co) polyamide.

Such mixing can for example be carried out in the melt, for example by extrusion. It is also possible to envisage mixing a powder based on polyamide and a powder of another polymer.

According to the present invention, the particle size of the polyamide-based powder is approximately less than or equal to 500 μm. Preferably, at least 95% by weight of the particles of the polyamide-based powder according to the invention have a size less than or equal to 500 μm. By particles is meant the grains, in particular of spherical and / or ovoid shapes, constituting said powder. The size corresponds to the longest dimension of these particles. In the case of spherical particles, the size corresponds to the diameter of these particles. The particles of the polyamide-based powder according to the invention may have a size of between 100 and 500 μm. Preferably, these particles have an average size of between 100 and 500 μm, more particularly between 200 and 500 μm. The particles of the polyamide-based powder advantageously have an average diameter (Dm) of between 150 and 400 μm, more preferably between 200 and 350 μm. The average diameter (Dm) of the particles can be measured according to the following relationship: Dm = Σ (Pi x Di) with Pi corresponding to the percentage of particles retained with a sieve and Di corresponding to the average size of the particles in μm on the sieve. This calculation method is explained in standard ASTM D 1921-96 method A. To obtain a polyamide-based powder, any physical and / or chemical process can be used, such as for example grinding of polyamide granules . Polyamide granules are generally obtained by cutting one or more rods formed at the extruder outlet. These granules can be obtained directly after the polyamide polymerization step. The grinding of the polyamide granules can be carried out by different types of mills, such as for example a disc mill, a hammer mill, a pin mill, or an electromagnetic mill, for example piston. The grinding can be cryogenic, that is to say it is carried out at a temperature between -10 and -200 ° C, preferably between -20 and -100 ° C.

Cryogenic grinding makes it possible in particular to avoid yellowing of the articles obtained and to obtain a high flow rate of ground powder. Cryogenic grinding also makes it possible to produce a powder comprising no filaments, which is particularly suitable for rotational molding.

The grinding can be carried out under an inert atmosphere, that is to say in the absence of oxygen, for example under nitrogen.

After grinding, it is possible to measure and / or modify the particle size of the powder using rotary sieves. To determine the particle size of a polyamide-based powder, a bultage method can be used, for example, using sieves of different meshes, or a laser method.

To obtain a polyamide-based powder having a moisture content less than or equal to 0.65% by weight, several methods can be used. One can for example dry the polyamide-based powder obtained previously by grinding. This drying can be carried out under vacuum or dry air, for example at a temperature of 80 ° C. The dry air used advantageously has a dew point below -40 ° C.

According to a preferred embodiment of the invention, cryogenic grinding as defined above also makes it possible to obtain a polyamide-based powder having a low moisture content, without performing an additional drying step. In fact, the polyamide-based powder may have a moisture content of less than or equal to 0.65% by weight, at the outlet of cryogenic grinding.

Preferably, the polyamide-based powder according to the invention has a moisture content less than or equal to 0.5% by weight, more preferably between 0.2 and 0.4% by weight, in particular less than 0.35 % in weight. The humidity can also be less than 0.2% by weight.

The polyamide-based powder can then be placed in a sealed bag so as to maintain its humidity level until it is used for rotational molding. The polyamide-based powder can also be dried just before it is used for rotational molding. To determine the moisture content of a polyamide-based powder, the Fisher method can be used according to ISO standard 15512 1999 (F), method B. Rotational molding is a process well known to those skilled in the art. The various rotational molding methods generally include the following steps: loading the mold, rotating the mold, heating the mold, cooling and demolding.

The rotation of the mold is generally carried out around two perpendicular axes. The quantity of powder to be molded introduced into the mold can be variable according to the size of the article which one wishes to obtain and the thickness of its walls.

The step of heating the mold, also called the molding step, occurs according to the invention in step c). The heating temperature can vary from 190 to 400 ° C. A molding temperature is generally used which is at least 10 ° C. higher than the melting point of the (co) polyamide which it is desired to mold. For molding, a temperature between 230 and 350 ° C, preferably between 250 and 300 ° C, can be used. For the rotational molding of polyamide 6, temperatures between 250 and 270 ° C can be used with the process according to the invention. The duration of the molding varies according to the dimensions of the article. It can be between 10 and 30 minutes, preferably around 15 minutes. The duration and the cooling time depend on the installation of the rotomoulder and on the dimensions of the article to be molded. As mentioned above, it is possible to cool the mold and / or the article contained in the mold. To cool the mold, it is possible to ventilate outside the mold air, for example at 25 ° C., and / or water spray. To cool the article inside the mold, air and / or water spray can be injected inside the mold, for example at 25 ° C.

The cooling time generally varies between 10 and 20 minutes. Demolding of the article is generally carried out when the article has a temperature between 70 and 120 ° C, preferably between 80 and 100 ° C. Rotational molding can be carried out under inert gas in the absence of oxygen. To do this, one can for example add to the mold, with the polyamide-based powder, a carbon dioxide-releasing compound, such as dry ice. Dry ice indeed generates carbon dioxide in the gaseous state during the heating stage of the molding. It is also possible to perform a nitrogen purge, by injecting nitrogen after closing the mold. The polyamide-based powder can also comprise various compounds, fillers, agents and / or additives. Many methods of mixing the (co) polyamides of the invention with these compounds, fillers, agents and / or additives can be envisaged. These can be added to the polyamide powder, or to the granules, before, during or after the grinding step. They can for example be mixed with the powder after grinding using a mixer before introduction into the mold. They can also be introduced as a mixture with the (co) polyamide in the molten state before the manufacture of the granules. Some of these compounds, fillers, agents and / or additives can be added during the polymerization of (co) polyamide. One can also consider adding these compounds, fillers, agents and / or additives to the mold with the polyamide-based powder. The granules used according to the invention and / or the polyamide-based powder can thus comprise antioxidant compounds and / or light, heat and / or UV stabilizing compounds. These additives are for example described in the work entitled "Oxidation, Inhibition in Organic Materials" edited by Jan Pospisil and Peter P. Klemchuk (1990), or in European patent application No. 0610155. By way of example, there may be mentioned as antioxidant compounds suitable for the invention, compounds comprising sterically hindered phenol functions in simple or oligomeric form (such as Irganox 1098 from Ciba-Geigy), monophenols substituted or not by alkyl groups such as 2,6-di-tert-butyl-4-methylphenol or the like; hydroquinones substituted or not by alkyl groups such as 2,6-di-tert-butyl-4-methoxyphenol; hydroxylated thiophenyl ethers such as 2,2'-thiobis (6-tert- butyl-4-methylphenol); bisphenols substituted or not substituted by alkyl groups such as 2,2'-methylene-bis (6-tert-butyl-4 methylphenol); benzene compounds such as 1, 3,5-tri- (3,5-di-tert-butyl-4-hydroxybenzyl) -2,4,6-trimethylbenzene; acylated aminophenols; hindered amines such as N, N'-di-isopropyl-p-phenylenediamine, phenothiazine, 1, 4-benzothiazine or the like.

One can also use antioxidants called "secondary" such as compounds comprising phosphite functions (such as Nrgafos 168 from Ciba- Geigy), aromatic or aliphatic phosphonites, alkaline salts of phenylphosphonic acid or hypophosphorous acid.

As light stabilizing compounds or absorbents of UV rays, mention may be made, by way of examples, of 2- (2'-hydroxyphenyl) benzotriazoles such as 2- (2'-hydroxy-5'-methylphenyl) benzotriazole or the like; 2-hydroxybenzophenones such as 3,3'-methylene-bis (2-hydroxy-4-methoxybenzophenone); esters of substituted or unsubstituted benzoic acids such as bis (4-tert-butylbenzoyl) resorcinol; acrylates; compounds comprising sterically hindered amino functions in simple or oligomeric form (such as Tinuvin 770 from Ciba-Geigy); oxalic acid diamides, hydroxyphenyl-s-triazine; nickel compounds such as nickel complexes with 2,2'-thio-bis-4- (1, 1, 3,3-tetramethylbutyl) -phenol.

When one of these additives is present in the composition, its weight concentration is between 0.05% and 5% approximately, relative to the polyamide-based powder. The granules used according to the invention and / or the polyamide-based powder may also comprise: reinforcing and / or filling fillers preferably chosen from the group comprising fibrous fillers such as glass fibers, metallic fibers, carbon fibers, mineral fillers such as clays, kaolin, or reinforcing nanoparticles or of thermosetting material, and powder fillers such as talc. impact modifiers, such as: ethylene propylene (EP), the ethylene propylene diene terpolymer (EPDM), elastomeric copolymers such as maleic anhydride styrene (SMA) for example, ultra low density polyethylene ( ULDPE), linear low density polyethylene (LLDPE), styrene ethylene butadiene styrene (SEBS), polypropylene (PP), acrylic elastomers (such as polyacrylic elastomers), ionomer elastomers, acrylonitrile-butadiene-styrene terpolymer ( ABS) and the acrylic-styrene-acrylonitrile terpolymer (ASA). The impact modifiers can optionally comprise grafted groups such as maleic anhydride for example. It is possible in particular to use maleic anhydride grafted with ethylene propylene diene monomer. The impact modifiers according to the invention can also be combinations, mixtures, homopolymers, copolymers and / or terpolymers of the compounds mentioned above; and / or - additives, for example, flame retardants, matifiers (Ti0 ₂ ), lubricants, plasticizers, compounds useful for the catalysis of the synthesis of the polymer matrix, antistatic agents, pigments such as carbon, dyes, molding aid additives or surfactants. The present invention also relates to all uses of a polyamide-based powder having the characteristics mentioned above for the manufacture of articles by rotational molding.

The present invention also relates to articles capable of being obtained by the method described above. The articles obtained by the rotational molding method according to the invention do not include a point of weakness and have good mechanical properties. The articles according to the invention have in particular good resistance to bursting.

The articles obtained by rotational molding are generally hollow parts without welds, such as for example the articles chosen from the group comprising: tanks, tanks, balloons, tanks, caissons, boxes, tanks, bumpers, seats, and body parts . These articles may include openings and / or plastic or metallic inserts.

The walls of the articles may consist of one or more successive layers, possibly of different natures. It is thus possible to manufacture by this method articles whose walls comprise for example two or three layers. It is for example possible to produce articles having an internal and external layer of (co) polyamide, of a similar or different nature, optionally comprising antioxidant compounds and / or light, heat and / or UV stabilizing compounds, fillers, shocks, additives and adjuvants. It is also possible to produce articles having an internal layer of (co) polyamide and an external layer of polyethylene (PE) and / or polypropylene (PP).

There are several known methods for manufacturing by rotational molding articles whose walls have several layers. One can for example introduce a mixture of (co) polyamides and / or polymers having different melting points and mold at different temperatures to obtain a multilayer structure. You can also mold each additional layer before cooling the previous one. Other details and advantages of the invention are illustrated by the exemplary embodiments given below only for information.

Materials used: Granules PA 6: Melt melt index (MFI) according to ISO 1133 at 230 ° C, under a load of 2.16 kg is 18 g / 10 min. Polyamide 6 has a content of terminal acid groups of 80 meq / kg, and a content of terminal amino groups of 42 meq / kg. Viscosity index of

140 ml / g measured in formic acid according to standard ISO 307. Tinuvin 770 sold by the company Ciba-Geigy. Light stabilizing compound comprising two congested piperidines. Irganox B 1171 sold by the company Ciba-Geigy. 50% mixture of a hindered phenolic antioxidant (Irganox 1098) and a phosphite (Irgafos 168).

Example 1: Manufacture of polyamide 6 powder

The various methods for manufacturing polyamide 6 powder or granules are mentioned in Table 1 below.

The moisture content by weight is measured by the Fisher method according to standard ISO 15512 1999 (F), method B. Test 1: Polyamide 6 granules, as defined above, having a length of 2.5 mm are ground by cryogenics under internal gas at - 60 ° C, in total absence of oxygen, with a double spindle grinder. During cryogenic milling, 0.25% by weight of Tinuvin 770 and 0.25% by weight of Irganox B 1171, based on the total weight of the powder, are added to the polyamide powder using a dosing scale coupled to a screw. The polyamide 6 powder is then sieved using a sieve comprising orifices of 500 μm. The particle size distribution of the polyamide 6 powder, measured by laser with a Mastersizer 2000 from Mavern Instrument, is as follows:

The polyamide 6 powder thus comprises particles having an average diameter (Dm) according to standard ASTM D1921-96 method A of 314 μm. The polyamide 6 powder has a moisture content of 0.2% by weight. Test 2: Polyamide 6 powder is made from the polyamide 6 granules with a counter-rotating disc mill at room temperature under atmospheric air. A powder is thus obtained consisting of particles having an average size between 100 and 500 μm (average diameter (Dm) of 325 μm), by sieving. 0.25% by weight of Tinuvin 770 and 0.25% by weight of Irganox B 1171 are added, relative to the total weight of the powder. The powder is dried in a vacuum study. A humidity level of 0.20% by weight is thus obtained. Test C1: Polyamide 6 powder is made from the polyamide 6 granules with a disc mill at room temperature under atmospheric air. A powder is thus obtained consisting of particles having an average size between 100 and 500 μm (average diameter (Dm) of 325 μm), by sieving. 0.25% by weight of Tinuvin 770 and 0.25% by weight of Irganox B 1171 are added, relative to the total weight of the powder. The undried powder has a moisture content of 0.70% by weight. Test C2: Ungrinded polyamide granules having a length of 2.5 mm are used. These granules have a moisture content of 0.20% by weight. 0.25% by weight of Tinuvin 770 and 0.25% by weight of Irganox B 1171 are added, relative to the total weight of the granules. Test C3: Polyamide 6 powder is made from the polyamide 6 granules with a disc mill at room temperature under atmospheric air. A powder is thus obtained consisting of particles having an average size between 200 and 650 μm (average diameter (Dm) of 416 μm), by sieving. 0.25% by weight of Tinuvin 770 and 0.25% by weight of Irganox B 1171 are added, relative to the total weight of the powder. The powder is dried in a vacuum study. A humidity level of 0.20% by weight is thus obtained.

Example 2: Manufacturing of articles by rotational molding

The various powders of Example 1 are placed separately in molds for rotational molding. The rotational molding parameters are as follows: Reservoir mold 70 cm long and 20 cm in diameter Molding temperature: 260 ° C Molding time: 18 minutes Rotation speed: 5 revolutions / minute for the first axis and 7 revolutions / minutes for the second axis Cooling time: 13 minutes (air injection cooling outside the mold)

The articles obtained weigh 1.8 kg. The external appearance, the internal appearance and the regularity of their thickness are determined on the articles obtained. It should be noted that the parts obtained with the powder C1 are brittle when removed from the mold. The results are listed in the following table:

Table 1

The interior and exterior surface appearance is determined by cutting pieces of the article walls. The surface appearance of these pieces is observed under the microscope. The surface appearance is classified as follows:

- good: means that no bubble or a very small proportion of bubbles with a diameter between 0.2 and 0.3 mm is observed under the microscope.

- bubble and sag: means that a high proportion of bubbles with a diameter between 0.2 and 0.3 mm is observed.

- unfounded powder: means that the powder is not completely melted and that powder residues larger than 500 μm are observed.

In the column titled “regularity of thickness”, the first observation corresponds to the regularity of thickness on an article, the second observation corresponds to the regularity of thickness on 3 articles.

The regularity of the thickness on an article is determined by measuring the thickness of 10 pieces of walls recovered by cutting from an article.

- good: means that the variation in the thickness of the article is between 0 and 20%, compared to the average thickness of this article. - medium: means that the variation in the thickness of the article is between 20 and 50%, compared to the average thickness of this article.

- bad: means that the variation in the thickness of the article is greater than 50%, compared to the average thickness of this article.

The regularity of the thickness on three articles is determined by measuring the thickness of 30 pieces of walls recovered by cutting from the three articles (10 pieces per piece).

- good: means that the variation in thickness is between 0 and 20%, compared to the average thickness of the three articles.

- average: means that the variation in thickness is between 20 and 50%, compared to the average thickness of the three articles.

- bad: means that the thickness variation is greater than 50%, compared to the average thickness of the three articles.

Claims

1. Method for manufacturing an article by rotational molding in which at least one polyamide-based powder is introduced into a mold having the following characteristics: i) the particle size of said powder is less than or equal to 500 μm; and ii) said powder has a moisture content of less than or equal to 0.65% by weight.

2. Method according to claim 1, comprising at least the following steps: a) placing in a mold at least one polyamide-based powder having the following characteristics: i) the particle size of said powder is less than or equal to 500 μm ; and ii) said powder has a moisture content of less than or equal to 0.65% by weight; b) rotating the mold; c) heating the mold; d) cooling the mold and or the article obtained; and e) unmolding the article.

3. Method according to claim 1 or 2, characterized in that the polyamide is composed of at least one (co) polyamide chosen from the group comprising: (co) polyamide 6; 4; 11; 12; 4.6; 6.6; 6.10; 6.12; 6.18; 6.36; their copolymers and mixtures.

4. Method according to any one of claims 1 to 3, characterized in that at least 90% by weight of the repeating units of the macromolecular chains of (co) polyamides are chosen from the recurring units of polyamide 6 and the recurring units of polyamide 6.6.

5. Method according to any one of claims 1 to 3, characterized in that the (co) polyamide has a melt index less than 25 g / 10 min according to ISO 1133, under a load of 2.16 kg , at a temperature of 10 ° C above the melting point of the (co) polyamide.

6. Method according to any one of claims 1 to 5, characterized in that the particles of the polyamide-based powder have an average size between 100 and 500 microns.

7. Method according to any one of claims 1 to 6, characterized in that the particles of the polyamide-based powder have an average diameter (Dm) between 150 and 400 microns, preferably between 200 and 300 microns.

8. Method according to any one of claims 1 to 7, characterized in that the moisture content of the polyamide-based powder is less than or equal to 0.5% by weight.

9. Method according to any one of claims 1 to 8, characterized in that the moisture content of the polyamide-based powder is between 0.2 and

0.4% by weight.

10. Method according to any one of claims 1 to 9, characterized in that the polyamide-based powder is obtained by cryogenic grinding at a temperature between - 10 and - 200 ° C.

11. Method according to claim 10, characterized in that the cryogenic grinding is carried out under an inert atmosphere.

12. Method according to any one of claims 2 to 11, characterized in that the temperature in step c) is between 230 and 350 ° C.

13. Method according to any one of claims 2 to 11, characterized in that the temperature in step c) is between 250 and 300 ° C.

14. Method according to any one of claims 1 to 13, characterized in that the rotational molding is carried out under inert gas.

15. Method according to any one of claims 1 to 14, characterized in that the polyamide-based powder comprises at least one compound chosen from the group comprising an antioxidant and a stabilizing compound with regard to light, heat and / or UV.

16. Method according to any one of claims 1 to 15, characterized in that the polyamide-based powder comprises at least reinforcing and / or filling fillers, agents modifying the impact resistance, and / or additives.

17. Article capable of being obtained by the manufacturing process by rotational molding according to any one of claims 1 to 16.

18. Article according to claim 17, characterized in that it is chosen from the group comprising: tanks, bins, tanks, tanks, caissons, boxes, tanks, bumpers, seats and body parts.

19. Polyamide-based powder for manufacturing articles by rotational molding having the following characteristics: i) the particle size of said powder is less than or equal to 500 μm; and ii) said powder has a moisture content of less than or equal to 0.65% by weight.