FR2857615A1 - Manufacturing procedure for polyolefin-based expanded cellular structure uses mixture of polyolefin and expansion agent extruded in cooling chamber - Google Patents

Abstract

The procedure consists of mixing a polyolefin with an MFI of over 10 dg/min with an expansion agent made from sodium bicarbonate and citric acid and extruding it continuously in parallel layers inside a cooling chamber with compartments on either side of the layers or a chamber into which a coolant fluid is injected. The procedure consists of mixing a polyolefin with an MFI of over 10 dg/min with an expansion agent made from sodium bicarbonate and citric acid and extruding it continuously in parallel layers inside a cooling chamber with compartments on either side of the layers or a chamber into which a coolant fluid is injected. The coolant is a liquid such as water at a temperature of 50[deg]C or below fed into a cooling tank in which the chamber is immersed. The cells in the structure can be circular, elliptical or polygonal in section, depending on the extrusion speed and pressure; and have a length to width ratio of between 1 and 2:1, the length being between 4 and 30 mm and the width between 2 and 15 mm.

Description

Process for the manufacture of an expanded cellular structure based

a polyolefin

  The present invention relates to a process for the manufacture of an expanded cellular structure based on a polyolefin.

  A need met in many industries (automobile, civil constructions, naval ...) is to optimize the ratio mechanical properties / weight of the structures used. Many processes have been developed to achieve this goal and in particular, to lighten the plastic structures. Most of these methods use either the mechanical formation of macroscopic cells (by assembling solid or molten fluxes to form so-called honeycomb structures), or the physical formation of microscopic cells by gas release or expansion ( expansion or foaming with physical or chemical blowing agents). A combination of both types of processes has also been considered.

  Thus, the patent application EP 279668 discloses a process for the manufacture of foams with honeycomb structure by extrusion of rods or sheets of plastic material comprising a blowing agent in a die having apertures of appropriate size and relative location. to create, by mutual welding of the rods at the exit of the die, a continuous structure which can be in the form of a honeycomb. However, these structures are obtained by a batch process which involves the cutting of slices and their assembly in the case of large structures. Such a method therefore involves one or more manipulations that make it more cost-effective.

  A process for the manufacture of cellular structures by continuous extrusion has been proposed in application FR 2760999. According to this process: - parallel sheets of a plastic material (which may be a polyolefin) are extruded continuously inside a cooling chamber having compartments located on either side of the sheets - alternatively, in each chamber, an injection of pressurized cooling fluid and a vacuum are carried out, the two compartments situated on both sides of the same sheet being for the one subjected to the action of the fluid under pressure and for the other, to the action of the depression, and vice versa during the next alternation.

  The geometry of the die used as well as the modalities of this process are such that only very fluid resins can be used (typically having an MFI of more than 10 dg / min) and that, moreover, the temperature of the material to the The inlet of the die must be very high in order to minimize the viscosity of the resin. Such conditions are quite opposite to the conditions generally used for the expansion of polyolefins, where resins having a low MFI (typically around 2 dg / min) are foamed at a relatively low temperature and this in order to preserve a certain level of viscosity necessary for the mechanical strength of the pores.

  However, the Applicant has found that, surprisingly, this method could nevertheless be applied to a polyolefin containing an expansion agent and this with the advantage of providing a simple and fast (in a single step) a honeycomb structure based particularly light polyolefin.

  The present invention therefore relates to a process for the manufacture of an expanded cellular structure, according to which: - a polyolefin having an MFI greater than 10 dg / min is mixed with a blowing agent to form a composition capable of expansion - Parallel sheets of said composition are continuously extruded inside a cooling chamber having compartments located on either side of the sheets - alternatively, in each chamber, a cooling fluid injection is carried out under pressure and a depression, the two compartments located on both sides of the same sheet being for one subjected to the action of the pressurized fluid and for the other, to the action of the depression, and vice versa during the following alternation, end of the deformation of the sheets and their welding in pairs with the formation of cells perpendicular to the extrusion direction.

  The polyolefins that can be used in the process according to the invention are polymers of linear olefins containing from 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene and 1-octene and grafted with acidic functions, for example by radical route. These olefins preferably contain from 2 to 6 carbon atoms, more particularly from 2 to 4 carbon atoms. They may be selected from the homopolymers of the above olefins and from the copolymers of these olefins, in particular copolymers of ethylene or propylene with one or more comonomers. The comonomers are advantageously chosen from the olefins described above, from diolefins comprising from 4 to 18 carbon atoms, such as 4-vinylcyclohexene, dicyclopentadiene, methylene and ethylidene norbornene, 1,3-butadiene, isoprene and 1,3pentadiene. Preferably, the polyolefins are chosen from propylene polymers and ethylene polymers. Most preferably, the polyolefins are selected from homopolymer of ethylene, homopolymer of propylene, copolymers of ethylene, copolymers of propylene, copolymers of ethylene and propylene and mixtures thereof. hereinafter referred to as 10 resins based on propylene (PP) and / or ethylene (PE)]. Propylene-based resins (PP) are the most advantageous and in particular propylene homopolymers.

  The polyolefins that can be used in the process according to the invention have a melt index (MFI) greater than 10 dg / min, preferably greater than or equal to 12 dg / min, or even greater than or equal to 15 dg / min. . The MFI of these polyolefins, however, is generally less than or equal to 500 dg / min, preferably 100 dg / min, particularly preferably 70 dg / min. The MFI or melt flow index of the polyolefins is measured at 230 ° C. under a weight of 2.16 kg for the propylene polymers according to ASTM D 1238 (1986) and at 190 ° C. under a weight of 5 kg for the polymers of ethylene according to ISO 1133 (1991).

  The blowing agent according to the present invention may be of any known type. It can be a so-called physical expansion agent, that is to say a gas dissolved in the plastic material under pressure and which causes its expansion during expansion at the exit of the extruder. Examples of such gases are CO2, nitrogen, water vapor, hydrofluorocarbons or HFCs (such as the mixture with 87/13% by weight of CF3CH2F / CHF2-CH3 marketed by SOLVAY under SOLKANE XG87), hydrocarbons (such as butane and pentane) or a mixture thereof. It can also be a so-called chemical expansion agent, that is to say a substance (or a mixture of substances) dissolved or dispersed in the plastic material and which, under the effect of temperature, releases the gas (s) that will be used to expand the plastic material. Examples of such substances are azodicarbonamide and mixtures of sodium bicarbonate and citric acid. These give good results.

  The amount of blowing agent used in the process according to the invention must be optimized in particular according to its nature, the properties (viscosity in particular) of the base polyolefin and the desired final density. In general, this content is greater than or equal to 0.1%, preferably 0.5% or even 1%.

  In addition to the blowing agent and the polyolefin, the composition capable of being blown according to the invention may comprise conventional additives for plastics such as fillers, pigments, stabilizers, nucleating agents, etc. According to one advantageous embodiment this composition contains, as filler, mineral fibers and in particular fibers or wood flour. However, care should be taken, especially in the case of fillers, not to excessively increase the viscosity of the polyolefin and in particular to remain at a sufficient level of MFI (greater than 10 dg / min).

  The extrusion apparatus used in the present application may be identical or similar to that described in the above-mentioned application FR 2760999. Advantageously, this apparatus is identical to that illustrated in the figures of this application and commented from page 4 to page 8.

  The extrusion conditions of the process according to the present invention are adapted in particular to the nature of the base polyolefin, to that of the blowing agent and to the desired expansion ratio. As mentioned above, the method according to the invention gives good results only if the resin is sufficiently fluid at the inlet of the die. For this purpose, the temperature of the polyolefin at the inlet of the die is preferably greater than or equal to 250 ° C., or even 260 ° C., but less than or equal to 285 ° C. or even 275 ° C.

  In order to avoid an uncontrolled expansion of the leaves parallel to the outlet of the die, care will be taken to adjust the temperature of the cooling fluid. According to an advantageous variant of the process according to the invention, the cooling fluid is a liquid, which is preferably at a temperature of less than or equal to 50 ° C. or even 40 ° C. This liquid is preferably water. In practice, the honeycomb structure is advantageously immersed in a simple tank filled with water at its exit from the die. In or after the latter, the honeycomb structure obtained is advantageously taken up by a traction train and is then preferably discharged into plates of the desired dimensions. The pulling speed and the extrusion rate will be optimized depending in particular on the size and the thickness of the cells, as well as the desired shape. - 5

  The size and shape of the cells may also be adapted by changing the melt viscosity of the polymer, the extrusion rate, and the duration of the pressure / vacuum cycles.

  The present invention also relates to an expanded cellular structure that can be obtained by the method as described above. This honeycomb structure is based on a polyolefin having an MFI greater than 10 and is expanded, ie that its constituent material has a density lower than the density of the base polyolefin. A density of less than or equal to 0.8 g / cm 3, preferably less than or equal to 0.6 g / cm 3 or even 0.5 g / cm 3 can easily be obtained, this density depending in particular on the amount of blowing agent used. This lower density is due to the presence of pores whose average diameter is generally greater than or equal to the pm, preferably to 5 pm, or even 10 mt; this diameter is, however, generally less than or equal to 500 μm, preferably 250 μm or even 100 μm.

  The shape of the cells of this structure can be approximately circular, elliptical (when the extrusion and / or traction speeds are higher), polygonal (when the applied pressure differences are more abrupt) ... These cells generally have a length L (in the direction of extrusion) larger than their width 1 (in the extrusion plane but in a direction perpendicular to that of the extrusion). In general, the shape factor (L / 1) of the cells is therefore greater than 1, or even 1.5, but generally less than 2.

  The length (L) of the cells is generally greater than or equal to 4 mm, or even 10 mm, but generally less than or equal to 30 mm, or even 20 mm. The width (1) is generally greater than or equal to 2 mm, or even mm, but generally less than or equal to 15 mm, or even 10 mm.

  The size of the honeycomb structures according to the invention is limited by the size of the processing equipment. By size, we mean in fact only the width and the height (measured perpendicularly to the extrusion plane), and not the length since this is determined by the duration of the extrusion and the cutting frequency of the extruded strip. . The height of these structures is generally greater than or equal to mm, or even 2 mm, but less than or equal to 70 mm, or even 60 mim.

  It follows from the foregoing that the present invention makes it possible to obtain one-piece honeycomb structures of infinite length or rather, the length of which is variable at infinity. - 6

  The honeycomb structures according to this aspect of the invention are advantageously used in the building (lightened ceilings, partitions, ...), the packaging (side protections, embedding objects, ...), the automobile (back range , interior door, ...).

  The present invention is illustrated in a nonlimiting manner by the following example:

Example:

  The extrusion of a PP foam honeycomb with a width of cm and a height of 17 mm was carried out under the conditions described below, using: a 35 mm DOLCI extruder long end provided with 5 separate heating zones and equipped with a die as described in application FR 2760999, with 10 slots - as polyolefin, PP MOPLEN 640P (MFI = 15 g / 10 min at 230 ° C. and 15 at 2.16 kg) ) as blowing agent, the LUVOPOR product (9539 which is based on bicarbonate and citric acid releasing, from 160 C, CO2 and water and this at a rate of 2% by weight in the composition - the following extrusion conditions: 20 Temperatures: Zone 1: 189 C Zone 2 to 5: 265 C Material temperature at the inlet of the die: 276 C Pressure at the exit of the extruder: 8 bar 25 Speed of the screw: 40 rpm Water temperature: 30 C Drawing speed of the honeycomb: 1.6 m / min Time during which the overpressure is achieved: 0.5 sec Time during which the vacuum is achieved: 0.5 sec. An expanded cellular structure was obtained with: cells of the order of 15 mm long (extrusion direction) and 8 mm wide (transverse direction) - closed pores and oriented in the direction of extrusion, included in a foam having a density of the order of 0.6 + 0.1 g / cm3.

Claims (8)

  1 - Process for the production of an expanded cellular structure based on a polyolefin, characterized in that: - a polyolefin having an MFI greater than 10 dg / min is mixed with an expansion agent to form a composition capable of the expansion - parallel sheets of said composition are continuously extruded inside a cooling chamber having compartments located on either side of the sheets - a fluid injection is alternately carried out in each chamber; pressure cooling and depression, the two compartments on both sides of the same sheet being for one subjected to the action of the fluid under pressure and for the other, to the action of the depression, and conversely during the next alternation, the end of deformation of the sheets and their welding in pairs with formation of cells perpendicular to the direction of extrusion.   2 - Process according to the preceding claim, characterized in that the polyolefin is a resin based on propylene (PP) or ethylene (PE).   3 - Process according to claim 1 or 2, characterized in that the blowing agent is a mixture of sodium bicarbonate and citric acid.   4 - Process according to any one of the preceding claims, characterized in that the expandable composition further comprises fibers or wood flour.   - Process according to any one of the preceding claims, characterized in that the cooling fluid is a liquid at a temperature of less than or equal to 50 C.   6 - Process according to any one of the preceding claims, characterized in that the cooling fluid is water and that this water feeds a cooling tank in which the cooling chamber is immersed.   7 - Expanded cellular structure obtainable by a process according to any one of the preceding claims, characterized in that it is based on an expanded polyolefin having an MFI greater than 10.   8 - alveolar structure according to the preceding claim, characterized in that its cells have a form factor (L / l) of between 1 and 2.   9 - alveolar structure according to the preceding claim, characterized in that its cells have a length of between 4 and 30 mm and a width of between 2 and 15 mm.