A PROCESS OF PRODUCING FIRE RESISTANT THERMOPLASTIC COMPOSITIONS AND COMPOSITIONS THUS OBTAINED

Abstract: A halogen-free and fire resistant thermoplastic composition is produced by preparing a blend comprising a halogen-free thermoplastic material containing both, cross-linkable and non-cross-linkable compounds, a fire resistant inorganic filler, a compatibilizing agent for said inorganic filler, and a radical generating agent; the blend is mixed and dynamically cured at a temperature above the decomposition temperature of said radical generating agent to obtain a partially cross-linked thermoplastic composition and a grafting of the compatibilizing agent on all the bulk of thermoplastic material.

Description: A PROCESS OF PRODUCING FIRE RESISTANT THERMOPLASTIC COMPOSITIONS AND COMPOSITIONS THUS OBTAINED FIELD OF THE INVENTION The present invention relates to a process of producing fire resistant thermoplastic based materials, to the polymer compositions thus obtained and to the products using said compositions . In more detail, the invention relates to halogen-free thermoplastic materials that, filled with a high level of inorganic compounds, grafted in bulk with compatibilizing substances and dinamically "cured" with free radical generator catalysts, are both fire resistant and provided with surprisingly high thermomechanical properties. BACKGROUND OF THE INVENTION Halogen-free thermoplastic materials have a broad and useful range of mechanical properties. Among them, polyolefins are a chemically homogeneous class of polymers with a wide spectrum of properties and a low level of environmental impact, so their use is growing with an impressive rate and new materials, obtained from new catalitic systems, came into the market offering new applications . These polymers, that contain mainly carbon and hydrogen, can burn and propagate the flame very easily and there has been a constant search for a way of removing this characteristic without compromising their mechanical properties and without transforming them in thermoset materials. The higher the mechanical properties the larger the application field for these materials. With hard and stiff materials structural parts can be obtained like boxes, pipes, profiles, panels; elastic and flexible materials can be used for covering walls, wires or other objects for automotive, furnishing, building or electrical industries in which fire resistance without toxic substances emission in case of fire is requested. A first, known, solution is to use inorganic compounds, i.e. metal hydroxides (e.g. Mg and Al hydroxides), oxides and/or inorganic salts, having endothermical decomposition and releasing nonflammable gas (like water or carbon dioxide) , and/or creating a protective shield when heated, as fillers in plastics or elastomers to impart flame retarding properties to said materials. The flame retarding action of these inorganic compounds is based on physical reasons. Their endothermic thermal decomposition process subtracts heat to the burning bulk, reducing the release of flammable pyrolytic gases, and further diluting it with non flammable gas released by the inorganic compounds. In addition some of them can create a protective layer that acts as a shield against the flame propagation. The main drawback of these inorganic fillers is that their effectiveness is pretty low and a high amount of filler is required to impart the polymer effective fire retarding properties (particulary with resins, like polyolefins, that can burn completely) . In fact, the required amount of filler is so high that the mechanical properties of the final material are dramatically jeopardized. By selecting the most endothermic and gas releasing compounds, such as light metal hydroxides (aluminium or magnesium hydroxides) or boron containing compounds, or by using them in combination with other flame retardants compounds - which interfere in the combustion chemistry - a slight reduction in the amount of filler was obtained. However, the required amount of filler is still so high that the mechanical properties of the final composition are pretty poor. In order to overcome these problems it was suggested to improve the adhesion between the filler and the polymer by adding compatibilizing agents that should have affinity to the polymer and the filler as well. As a matter of fact it is known that the mechanical properties of blends between polyolefins and metal hydroxides are improved when a moderate amount of a polyolefin containing succinic units, vinylsilane, or other polar groups, as side substituents, are added, because its olefinic part remains into the bulk of the polymeric part and the succinic residue bonds to the filler either chemically (salification of the hydroxide) or by means of electrostatic interaction. Alternatively, it was suggested to add to the polymer matrix compounds like peroxides, grafted silanes or sulphur that after the final product is obtained in its final shape, in a dedicated step can crosslink ("cure") the polymer matrix of the final product. By this process the mechanical properties of the polymer are improved but the polymer is transformed into a thermoset material that can not be further processed or recyled. This method is used for example for the production of cables, pipes, tyres, foamed sheets, etc. OBJECT OF THE INVENTION The aim of the present invention is to improve the mechanical properties of halogen-free fire resistant thermoplastic materials in which the fire resistance is given by an inorganic filler, by means of a chemical modification of the polymeric matrix during the blend with the filler, and to substantially maintain the thermoplastic characteristics of the starting materials in the final composition. SUMMARY OF THE INVENTION The invention relates to a reactive process to prepare halogen- free thermoplastic compositions having fire resistance characteristics, according to claim 1. According to a preferred aspect of the invention, the inorganic filler is selected from Al(OH) 3 , g(OH) 2 , CaC0 3 , boric acid, borates, CaO, silica and mixtures thereof; the radical generating agent is a peroxide and the compatibilizing agent is selected from maleic and fumaric acids, maleic and fumaric anhydrides and mixtures thereof. This invention also concerns halogen-free, fire resistant thermoplastic compositions as obtainable according to the claimed process, wherein compatibilizing agents are grafted over all the bulk of said thermoplastic material. The final composition is thermoplastic and is partially crosslinked. The invention also relates to electric cables provided with a layer made with a fire resistant thermoplastic material according to the invention. Preferably, such layer is a shield. The invention also relates to the use of a composition as above disclosed for the production of electric appliances and their parts, such as boxes, pipes, etc. The invention provides several advantageous features. As above mentioned, the invention process results in a final composition that is thermoplastic, i.e. it is further processable and recyclable. There thus is no need to give the product its final shape before curing it by heating the mixture and activating the peroxides; in fact a preferred shape for the invention composition is as pellets. The invention compositions have surprisingly good properties, namely higher values of mechanical modulus, stress strain and elongation at break, impact behaviour, abrasion resistance, compression set and softening or distortion temperature, with respect to the properties of corresponding materials made according to known techniques. These materials can be employed to obtain structural parts or to coat other objects to impart them the required characteristics of fire resistance features. They are processable by conventional processing technologies as: injection and coinjection molding, extrusion, co-extrusion, blow-molding, roto-molding, slush molding, thermoforming. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention the thermoplastic material useful for carrying out the process are selected from alpha- defines homo and co-polymers such as ethylene, propylene, ethylene/propylene, propylene copolymers containing one or more alpha olefins with 2-10 carbon atoms (e.g. ethylene, 1-butene, 1-pentene, 4-methyl-l-pentene, 1-hexene, 1-octene) ; EPM (ethylene/propylene) rubbers and EPDM (ethylene/propylene/dyene) rubbers; natural rubber; EVA (ethylene/vinylacetate) ; ethylene/1-octene copolymer; polyenes homo and co-polymers such as polybutadiene; styrol/butadiene rubbers (SBR) , hydrogenated styrol/butadiene copolymers; acrylonitrile/butadiene/styrene copolymer, their functionalizated polymers; and mixtures thereof. The starting thermoplastic material must contain both crosslinkable and non-crosslinkable compounds. The amount of thermoplastic material is usually about 5-70 wt.%, in any case it is such as to bring to 100 the composition. The inorganic fillers imparting fire resistance are those inorganic fillers that endotermically decompose with release of non-flammable gas and inert ashes. Examples of these fillers are aluminiun trihydroxide, magnesium hydroxide, huntite [3MgCθ3xCaC0 3 ] or hydromagnesite [Mg 5 (C0 3 ) 4 (OH) 2 x 4H 2 0], boric acid. Other suitable fillers are those that can vetrify like borate or hydrate borate, sodium bicarbonate, calcium oxide and silica. The fillers can be mixed together or diluted with inert materials like silica, or materials that decompose at higher temperatures than the polymer decomposition temperature, like calcium carbonate. Their total amount is within the range of 30- 80 wt.%, typically from 40% to 75% by weight of the final blend. The co patibilizers are molecules with chemical affinity to the filler, and able to graft, with a chemical bond, the macromolecular backbone by means of a free radical catalyzed reaction. Typically these compatibilizers are unsaturated organic acids or esters, like acrylic, methacrylic, fumaric, maleic, citraconic or itaconic acid and esters, used in a concentration ranging between 0,005 and 10% by weight of the blend. Preferred compatibilizers are maleic and fumaric acids, maleic anhydride and mixtures thereof. Another class of useful compatibilizers are the maleinized polybutadienes or vinyl- methoxysilanes that can graft the macromolecular backbone accelerating and or promoting the crosslinking reaction, used in concentration till 15% of the total blend. The preferred amount of compatibilizers is within the range of 0.01-15 wt.%. The free radicals generator agentss are preferably organic peroxides like dicumil peroxide, 2, 5-di (terbutylperoxi) -2, 5- dimethylhexane, or other molecules with a strained bond that omolitically breaks upon temperature increase, like 2,3- dimethyl-2, 3-diphenyl-butane. These free radical generator agents start radical chain reactions that both graft the compatibilizer molecule onto the macromolecules but also "cure" the polymer by crosslinking. Preferably, radical generating agents are provided in an amount of 0.01-2.0 wt.% of active material. Besides the above mentioned compounds, usual additives known in the plastics field such as antioxidants and stabylizers, plasticizers, lubricants, slipping agents and process coadiuvators are provided for in the invention composition. The type and amount of these additives are directly derivable from known formulations and are not object of this invention. According to the invention the process of preparing a thermoplastic composition that is fire resistant, free from halogens and really thermoplastic (i.e. further processable or recyclable) provides to prepare a blend comprising: a halogen- free thermoplastic material containing both crosslinkable and non-crosslinkable compounds, a fire resistant inorganic filler, a compatibilizing agent for said inorganic filler, and a radical generating agent, and to dinamically cure said blend to obtain a partially crosslinked thermoplastic composition. "Dinamically curing" the above blend or mixture means that the blend is melted and kneaded at a temperature above the decomposition temperature of said radical generating agent in order to start and carry out the reticulation of the macromolecules, i.e. the crosslinking, or the degradation of the macromolecules, according to their nature. The action of the radical generating agents occurs in the presence of the filler and of the compatibilizer to obtain the required grafting The macromolecular structure of the thermoplastic material is modified in a way that depends from its initial structure and composition: with polymers having unsaturated carbon-carbon bonds like ethylene/propylene/diene rubber or styrole/butadiene rubber, or having more than 50% (moles) of ethylenic units in the backbone like polyethylene, ethylene/1-octene (or 1-hexene) copolymers, ethylene/propylene rubber, ethylene/vinylacetate, hydrogenated styrole/butadiene rubber the main result of the radical chain reactions is a macromolecular crosslinking. If the macromolecular structure has no unsaturated carbon-carbon bonds and more than 50% (moles) of substituted vinyl units (- CH2-CHR-, R different from H) like in polypropylene, the main radical reaction is a chain scission or degradation to give shorter chains. This structural change control is a crucial point to improve the material characteristics that can be customized balancing the degradation and the crosslinking by accurate polymer and additives choice. For hard and stiff materials, polymers like polypropylene, that is degradated by radical generators, without any crosslinking reaction, are frequently employed. The control of this degradation and some degree of crosslinking is possible by employing polymers rich in double bonds such as polybutadienes, polyalkenylenes, plyenes and EP(D)M rubbers that graft and join the macromolecules by free radical catalyzed reactions. For flexible thermoplastic materials, a curable rubber is frequently employed but the complete crosslinking that should transform the polymer in a termoset material is avoided by using some amount of polypropylene or other non-crosslinkable compound. The non-crosslinkable, degradable, compound substantially acts as a continuous phase in which the cured (or crosslinked) phase is dispersed. The formulation of the composition, i.e. the amounts of degradable polymers, curable polymers, peroxide and polymers rich in double bonds, is balanced according to the mechanical characteristic that are required for the final product. If elastomeric properties are required, a greater amount of crosslinkable compounds will be present in the initial thermoplastic material; if a rigid final product is required, the amount of degradable, non-crosslinkable (i.e. uncurable) compound will be greater than the amount of curable (crosslinkable) compound. The invention process provides to carry out the mixing, melting, kneading, compatibilizing and curing/degrading of the blend components substantially in one step. The inorganic filler is usually added immediately after the reaction is started on the rest of the mixture, as disclosed by the following examples. A preferred apparatus to carry out the process is an extruder, most preferably co-rotating twin screws extruders with a high dispersing and homogeneization capacity, good temperature control and high ratio L/D, where L is the barrel length and D is the barrel diameter. EXAMPLES Several starting compounds in different amounts were processed according to the invention in a MARIS ® (Turin, Italy) TM 133 co-rotating twin screws extruder with L/D=40. The filler was added at about 1/3 of the barrel length through a side feeder. Antioxidants, in any, were added at 2/3 of the barrel length. A vacuum device was used to remove moisture and volatile byproducts. The processed material was cut into pellets, cooled in water and dried in a spin dryer. The samples for assessing , the mechanical characteristics of the final material were obtained by injection molding according to ASTM D638, this being further evidence that all of the invention compositions are thermoplastic materials. The mechanical characteristics are listed in table 1 (elastomeric composition) and in table 2 (rigid compositions) . In table 1, examples 1, 5 and 6 are referring to state of the art processes. In 1 no peroxide or compatibilizing agent was used; in 5 and 6 the compatibilizing agent (comp 1 ) was previously grafted on a polymer matrix (polypropylene) and subsequently added to the invention blend, the poor results are self evident, also in ex.6 where a high amount of comp 1 was added. Example 2 shows a comparative example in which peroxide only was used. Example 3 repeats the formulation of example 2, with the addition of maleic anhydride: the value of tension at break increased from 16 to 20 MPa. This is the legenda for examples 1-8. 1) comp is a compatibilizing agent "POLYBOND 3150" ® by Uniroyal Chemical 2) Dicumyl peroxide by Oxido 3) Polypropylene homopolymer Daplen DS10 by PCD 4) Ethylene/1-octene copolymer Engage 8150m by DuPont-Dow Elastomers 5) Ethylene/propylene copolymer CO059 by Enichem Elastomeri 6) ethylene/propylene copolymer Stamylan 56M10 by DSM 7) ethylene/propylene random copolymer Daplen CHC 3007 by PCD In table 2 examples 3-10 refer to rigid compositions and show the surprising results of invention composition 11 with respect to comparative examples 9 and 10. The added legenda references for table 2 are: 8) ASTM 1238 (230°C, 2.16) 9) ASTM D 256 10) polypropilene homopolymer: Valtec MH113Y by Himont 11) EPDM: Dutral Ter 4038 by Enichem Elastomeri TABLE 1 Eampl. pp elastomer Mg(OH) 2 CaCO 3 MAh comp * p-fc«e * «er.o* .«x-r. *■ t toennes a *att h brrΩea*alk-» elong a * -*tt brr-eaa •-»!k*■ % % % % % % % MPa % 1 7.48 3 29.79 4 60.5 2,23 0 0 0 6.5 1075 2 7.48 3 29.79 4 60.5 2.18 0 0 0.05 16 225 3 7.48 3 29.79 4 60.5 2.00 0.18 0 0.05 20 210 4 7.48 3 29.79 4 60.5 1.96 0.18 0 0,09 23 198 5 6.00 3 29.99 5 60.5 2.00 0 1 ,51 0 8,3 730 6 6.00 3 15.8 5 60.5 2.00 0 15,7 0 14,1 130 7 7.48 6 29.79 4 60.5 2.00 0.18 0 0.05 17.5 227 8 7.48 7 29.79 4 60.5 2.00 0.18 0 0.05 19.4 218 TABLE 2 ampl. PP elastomer Mg(OH) 2 Ca iCCOC > 33 MMAAhh ccoommpp 1 ppeerrooxx 22 M MFFII β8 F Flleexxuurraall mmoodduulluuss I IZZOODD"" Notched 23 β C % % % % % % % % MPa MPA J/m 9 35 10 0 62 3 3 0 0 0 10 3800 28 10 34.47 10 0 62 3 3 0.5 0 0.03 12 3850 35 11 25 10 9.45 11 42 2 233 0.5 0 0.03 6 2500 45

Claims: CLAIMS 1. A process for the preparation of a fire resistant thermoplastic composition, wherein a blend comprising a halogen- free thermoplastic material containing both crosslinkable and non-crosslinkable compounds, a fire resistant inorganic filler, a compatibilizing agent for said inorganic filler, and a radical generating agent, is mixed and dinamically cured at a temperature above the decomposition temperature of said radical generating agent to obtain a partially crosslinked thermoplastic composition. 2. A process according to claim 1, wherein said blend comprises 30-80 wt.% of said inorganic filler, 0.01-2.0 wt.% of active radical generating agent and 0.01-15 wt.% of compatibilizing agent. 3. A process according to claim 1 or 2, wherein said fire- resistant inorganic filler is selected from Al(OH) 3 , Mg(OH) 2 , CaC0 3 , boric acid, borates, CaO, silica, and mixtures thereof; said radical generating agent is a peroxide and said compatibilizing agent is selected from maleic and fumaric acids, maleic anhydride and mixtures thereof. 4. A process according to any previous claim, wherein said non- crosslinkable compound comprises a propylene polymer and said crosslinkable compound is a polyene and/or a rubber selected from EPM and EPDM rubbers, natural rubber, EVA, polyalkenylenes, ethylene/1-octene copolymer, styrol/butadiene rubbers (SBR) , hydrogenated styrol/butadiene copolymers, acrylonitrile- butadiene-styrene copolymer, their functionalizated polymers, and mixtures thereof. 5. A process according to claim 4, wherein said rubber is oil extended and said thermoplastic material comprises a polymer rich in double bonds such as polybutadiene. 6. A process according to any previous claim, wherein a first blend is prepared, melted and kneaded to decompose said radical generating agent, said fire-resistant inorganic filler is added to said blend and heating and kneading of the complete blend is continued to obtain said partially crosslinked thermoplastic composition. 7. A thermoplastic, fire resistant composition as obtainable according to the process of any claim 1 to 6, comprising a halogen-free thermoplastic material containing both a crosslinked and non-crosslinked portion, a fire resistant inorganic filler, a compatibilizing agent for said inorganic filler, said compatibilizing agent being grafted over all the bulk of said thermoplastic material. 8. A composition according to claim 7, in the form of pellets. 9. An electric cable, characterized in comprising a layer made of a composition according to claim 7. 10. The use of a composition according to claim 7 for the manufacture of fire resistant portions of electric appliances.

Inventors: MASCIA FRANCESCO, PUPPI CRISTIANO, MASCIA, FRANCESCO, PUPPI, CRISTIANO
Applicants: COMMER SPA, MASCIA FRANCESCO, PUPPI CRISTIANO, COMMER S.P.A., MASCIA, FRANCESCO, PUPPI, CRISTIANO
Application year: 1997
Main classification: C08L23
IPC codes: C08K3, C08K5, C08L23, C08L51, H01B3