FLAME RETARDANT MASTERBATCH FOR THERMOPLASTIC POLYMERS AND PROCESS FOR ITS PRODUCTION

Field of the invention

The present invention relates to a flame retardant masterbatch for thermoplastic polymers and to the process for its production.

Background art

Thermoplastic polymers have the characteristic that they can be formed (for example by moulding) when they are brought to a sufficiently high temperature, and maintain at lower temperatures the shape imparted on them during processing, for example when brought back to ambient temperature; due to this ease of processing they are applied in the production of . a vast number of types of manufactured polymer articles. The latter generally consist of a base polymer (or a mixture of polymers), to which particular additives are added in order to confer desired properties to the manufactured article, such as for example a colour, anti- adhesive, anti-static or conductive, or flame retardant properties.

The additives could be added taking each time a weight of the same suitable for the polymer batch being processed (by the English term "batch" it is meant the quantity of polymer that is processed in a single production phase).

The modality by far most commonly adopted in the art of polymers for adding an additive to a polymeric formulation is however through the use of so-called "masterbatches", composite units that consist of a polymer (so-called carrier) and the desired additive. In these units, that generally come in the form of cylinders or spheres (or similar forms) with dimensions of approximately 1 to a few millimetres, a pre-metered quantity of the additive is present and the minimum amount of a carrier polymer necessary to ensure mechanical stability of the masterbatch; the additive quantity is suitable for conferring the desired property to the whole polymer (or mixture) of a typical production batch of polymeric formulations.

Since thermoplastic polymers are combustible materials, additives commonly used in their formulation are the so-called flame retardants. The combustion process of the polymeric materials passes through the phases of: heating; decomposition (pyrolysis); ignition and combustion; and flame propagation (with thermal feedback).

In the heating (due to external heat sources) the temperature of the material increases at a speed that depends on the intensity of the heat emitted by the source and on the characteristics of the material, such as its thermal conductivity, the latent heat of fusion and vaporization and the decomposition heat. When a high enough temperature is reached, the material starts to degrade, forming gaseous and liquid compounds of lower molecular weight compared to the original polymer chains (decomposition). The speed of this phase depends on the intensity with which the polymeric material heats up. The concentration of the decomposition products, mixed with the surrounding air, increases until falling within the range of inflammability; the presence in this context of an ignition causes the combustion of the mixture to start. The produced heat is partly radiated to the material (thermal feedback), which feeds the above mentioned phases and leads to self-sustaining of the flame until the consumption of the fuel (the polymer and the vapours formed by it) or of the comburent (oxygen) causes extinction of the flame.

The action of flame retardants consists in eliminating or limiting one of the factors described above, by acting physically, chemically, or both, upon the liquid (material that melts), solid and gaseous fractions originated in the process.

The physically-acting flame retardants act by decreasing the efficiency of the thermal feedback, by diluting the combustion mixture, or :by forming a protective layer on the solid polymeric material, that is thus shielded from the oxygen-rich gaseous phase. The chemically-acting flame retardants can act through gas phase reactions, producing radicals that remove the chemically-active species involved in the maintenance and in the propagation of the flame; or through condensed phase reactions, that can consist in the formation of a protective carbonaceous layer (called "char") on the surface of the polymer, that thermally isolates the latter and slows down the contact between the pyrolysis products and the oxygen, or by forming swellings on the surface of the polymer that worsen its thermal exchange characteristics, retarding the thermal feedback process.

The flame retardants, depending on their nature, can be added to the polymer either through a genuine chemical reaction that binds them to the chain of the same, simply by physically mixing them with the material, or with intermediate modalities.

Numerous types of flame retardants are known, which act through different mechanisms, like aluminium or magnesium hydroxides, boron compounds, phosphorus compounds, or systems based on halogenated compounds. The latter, generally used together with a synergistic component like antimony trioxide, are the most widespread ones, as they offer an optimal balance of dosing quantity, cost and final performances. The combustion of these products leads however to the formation of fumes harmful to man; in particular, among the most widely used compounds, there are perbrominated diphenyl derivatives, like decabromodiphenyl ethane or decabromodiphenyl ether. In flame conditions these compounds may form dioxins and brominated furans, which often are the real cause of death in the event of fire. Halogenated compounds do in any case pose problems in the end- of-life thermodestruction phase of the product in which they are present, or in some cases also during their use; for example, some of these compounds, inserted in a manufactured article, tend to migrate to the surface with the formation of fines and therefore the manufactured articles which contain them cannot be used in the food packaging sector.

The most recent developments in the art have highlighted the possible use, as flame retardant additives, of compositions containing carbon nanotubes (generally abbreviated as CNTs); these compositions have a flame retardant action of the chemical type, and in particular based on the formation, during the combustion of the polymer, of a "char" layer on the surface of the same. CNTs are hollow structures of indefinite length, formed by carbon atoms that are arranged on cylindrical surfaces (one single surface in the case of single-walled nanotubes, forming nanotubes knows as "SWNT", the English acronym for "Single-Walled NanoTubes", or more concentric walls, forming nanotubes known as "MWNT", "Multi-Walled NanoTubes"). CNTs can be produced with laboratory reactors, for example by chemical-vapour deposition ("CVD") techniques, laser ablation, or others. CNTs are also marketed, albeit in low volumes, by some companies, such as Bayer, Arkema, Hyperion Catalysis International, Unidym and Nanocyl.

Patent application EP 1471 114 A1 describes thermoplastic resins, in particular polycarbonates and styrene resins, containing CNTs and a flame retardant chosen among those known.

Patent application US 2008/0293877 A1 describes flame retardant compositions containing between 0.05% and 1 % by weight of CNTs in a cross- linked silicone matrix; these compositions are produced by forming a first mixture between the CNTs and a polysiioxane containing vinyl groups; by adding to this mixture a second polysiioxane, containing hydrosilane groups; and by causing the two silane compounds to cross-link, for example by heating. The flame retardant compositions of this document have the advantage, compared to others previously known, of having a much reduced content of CNTs, but require an elaborate preparation.

Summary of the invention

Aim of the present invention is of providing a masterbatch for use as flame- retardant additive in thermoplastic polymers, and a process for the production of said masterbatch, improved compared to the prior art.

These aims are achieved with the present invention which in a first aspect concerns a flame retardant masterbatch for use in thermoplastic polymers, comprising a carrier polymer and, as functional components, a mixture between carbon nanotubes and one or more inorganic components chosen among silica, talc, calcium carbonate, zinc borates and zinc stannates; the preferred inorganic component is silica, both because of a better synergy of flame retardancy with the CNTs and for cost reasons. The inorganic component must be present in the form of powders, with maximum dimensions lower than about 30 μιτι and average dimensions (D ₅₀ ) generally lower than 10 μηι; for example, in the case of silica, powders suitable for use in the present invention are those with an average granulometry between approximately 2 and 7 μηι, and preferably around 5 μιη.

The inorganic component has a synergistic effect on the flame retardant functionality of the CNTs, maintaining and at times improving the flame retardant classifications even with a reduced quantity of CNTs in the masterbatch; the inorganic component furthermore improves the mechanical characteristics of the final manufactured article and, since it allows to reduce the quantity of CNTs required to obtain the specific product requirements, reduces the cost of the masterbatch.

Detailed description of the invention

The masterbatch contains minimum quantities of CNTs and second component of respectively 0.1 % and 5% by weight, whereas maximum quantities of these components are respectively 5% and 65% by weight, the complement to 100 being constituted by the carrier .polymer. The most commonly used carrier polymers are the linear low-, low-, and high-density polyethylenes (respectively identified in the art with the abbreviations LLDPE, LDPE and HDPE), polypropylenes (homopolymer as well as copolymers), and ethylene vinyl acetate (EVA).

The single masterbatch units of the invention generally have a weight of less than 0.1 g. With the previously described formulations, in order to impart flame retardant properties on thermoplastic polymer batches, the masterbatches of the invention must be added to the batch in such quantity as to constitute between about 10% and about 40% by weight of the total mixture.

The thermoplastic polymers that will constitute the final manufactured articles, to which the flame retardant masterbatches of the invention can be added, must be chemically compatible with the carrier polymers of the masterbatch; this condition is obviously fulfilled when the polymer of the manufactured article and the carrier polymer are the same polymer, but it is sufficient for them not to give rise to unmixing, for example LDPE can be employed as carrier polymer for a masterbatch destined to be used in polypropylene. Another general rule for choosing the carrier polymer is that its fluidity (in terms of MFI, "melt flow index") should be higher or equal to that of the thermoplastic polymer to which the masterbatch must be added.

The flame retardant masterbatches of the invention are particularly suitable for use in polyolefins, and even more so in high-density polyethylene (HDPE). For thermoplastic polymers other than HDPE, efficient flame retardant additives are commercially available, but at a high cost, to which the additives of the invention constitute an advantageous alternative. In the case of HDPE, on the other hand, there are currently no flame retardant additives available that could effectively substitute decabromodiphenylethane or decabromodiphenylether. The reason is that this polymer, compared to other thermoplastic polymers, hardly forms "char" in sufficient quantity and thus passes the tests for flame retardant properties less easily, so that in order to obtain properties useful for industrial applications flame retardant additives must be employed in high weight percentages compared to the total formulation of the final manufactured article; this, however, entails considerable modifications of the final characteristics of the manufactured article itself, such as its impact or stretch resistance.

In a second aspect, the invention concerns a process for the production of the previously described masterbatches, that consists in mechanically mixing the components in turbo-mixers of variable speed, and extruding the mixture on co- rotating twin screw extruders, provided with degassing system, with ratio l/d > 36 and balanced screw profile between grinding zone and dispersion zone.

The inventor has verified that with particular mixing and especially extrusion parameters it is possible to create a strong bond between CNTs and additive. This bond in a situation of pyrolysis creates optimum char.

In the first phase of the process, the components of the masterbatch, i.e. the

CNTs, the inorganic component and the carrier polymer, are weighed individually and introduced into a variable speed turbo-mixer not equipped with potentiometer, and supplied with a system of mixing paddles with 4 or 5 blades and low profile (for example 3 mm), to favour the intimate mixing of the components. The proportioning of the components to be employed in the production of the flame retardant masterbatch vary as a function of the thermoplastic polymers to which they are destined and of the required flame retardant classifications. The carrier polymer is employed in the form of powders having a granulometry between 200 and 700 μπι, the CNTs in the form of aggregates with a diameter of the order of about ten nanometres and a length of the order of a few microns, and the inorganic component in the form of powders having a granulometry between 1 and 30 Mm.

Preferably, the CNTs and the polymer are premixed under mild conditions (rotation speed of the paddle not higher than 500 rpm), after which the inorganic component is added and then mixing, initially in mild conditions, is carried out with rotation speed of the paddles not higher than 500 rpm, and subsequently at a speed between 1000 and 1250 rpm, whilst rising the system temperature to values between approximately 40 and 70 °C.

Alternatively, it is possible to feed the necessary components in their turn in the form of masterbatch to the mixer, i.e. as granules comprising CNTs in a carrier polymer, granules comprising the inorganic component in a carrier polymer and, if necessary for regulating the quantities and obtaining desired concentrations of the components in the final masterbatch, granules of the carrier polymer only; the polymer used in these granules is preferably the same for all of them.

The thus obtained mixture is fed, still hot, to a co-rotating twin screw extruder, or to a kneader with rotating and oscillating axis, known in the art as "Buss kneader", produced by the company Buss AG of Pratteln (Switzerland); the extruder or the kneader must have a degassing zone. The thermal profile of the extruder or the kneader is such that, during its movement through the apparatus, the mixture passes from a temperature between approximately 120 and 160 °C at intake up to an output temperature that can be between approximately 190 and 280 °C, as a function of the melting point of the carrier polymer; the particular profile is optimized as a function of this polymer, according to principles known in the art. The extrudate obtained at outlet of the extrusion (or kneading) section is then cut into the required sizes, for example with a die of the traditional type at the head of the extruder to obtain granules of cylindrical form (2 mm x 3 mm), or by water cutting (for example with systems known in the art as "under water" or "water ring") to obtain discs or round granules (1 mm of diameter).

The thus obtained masterbatches (in any form) then undergo processes standard in the art of screening, to eliminate granules of the wrong size, and final drying. The dried product is eventually packaged in paper and/or aluminium bags.

The invention will be further illustrated by the following example.

EXAMPLE 1

2.15 g of granules of CNTs with dimensions of between 200 and 300 μπι, 42.80 g of diatomaceous earth (a natural product containing essentially silica, largely in the form of cristobalite with minor quantities of quartz, and calcium carbonate) in the form of powders with a granulometry between 1 and 30 μιη, and 55.05 g of low-density polyethylene (LDPE) powder having a "melt flow index" (mfi) of 20 and with dimensions between 200 and 700 μιτι are weighed.

The CNTs and LDPE are fed to a turbo-mixer (produced by the company Caccia of Samarate, VA, Italy) and a first mild mixing phase is set at 500 rpm for 30 seconds. The mixing is then interrupted, the turbo-mixer is opened, its walls are cleaned (letting the powder adherent to the walls fall back into the mixed mass) and silica is added. Initially, mixing is at 500 rpm for 30 seconds, to then shift to a mixing phase at 1250 rpm for 5 minutes, in the meantime heating the system until the mixture has a temperature of 70 °C. The product of the mixing appears as perfectly homogeneous, only slightly powdery and smooth.

The mixture thus obtained is fed directly to the loading hopper placed on a co-rotating twin screw extruder (model 40EV033 by the company Comae of Cerro Maggiore, Ml, Italy), in which the temperature increases along the direction of movement of the mixture from a value of 160 °C in the inlet chamber to up to 230 °C at the exit of the extruder.

The "spaghetto" is cut into cylindrical pieces at the head of the extruder using a 4-bladed cutter and subjected to three subsequent screening operations to recover only the pieces with dimensions between 2 x 3 mm, that are then packaged in bags.