POLYPROPYLENE COMPOSITIONS HAVING IMPROVED IMPACT STRENGTH PROPERTIES AT LOW TEMPERATURES AND PROCESS FOR PREPARING SAME

Abstract: Polypropylene compositions having high impact strength at low temperatures comprising as essential com­ ponents: - 100 parts by weight of polypropylene having an isotactic­ ity index > 90, - 8-25 parts by weight of an amorphous copolymeric fraction (1) of ethylene-propylene soluble in xylene at 23°C, containing 20-80% by weight of ethylene, - 2-10 parts by weight of an ethylene-propylene crystalline copolymeric fraction (2) having a crystallinity of the polyethylene type containing 50-90% by weight of ethylene; and in which the weight ratio between the total polymerized ethylene and fraction (1) is <1. The compositions are prepared by polymerization pro­ cesses comprising at least a step of homopolymerization of propylene to stereoregular polymer and a subsequent step of polymerization of ethylene-propylene mixtures in the presence of catalysts comprising a titanium compound carried on a magnesium halide in active form.

Description: The present invention relates to new polypropylene compositions having improved impact strength properties at low temperatures and to a process for preparing same. As is known, the isotactic polypropylene is endowed with an exceptional combination of excellent properties which render it suitable for a very great number of uses, including appliances at high temperatures; however, it exhibits the drawback of possessing an insufficient impact strength at relatively low temperatures. Attempts were made to obviate such drawback, without remarkably affecting adversely, however, the other properties of the polymer, either by properly modifying the synthesis process or by blending with rubbers. The modifications to the synthesis process essentially consist in introducing into the propylene stereoregular homopolymerization process one or more steps of copolymerization of ethylene-propylene mixtures. The copolymerization conditions of the ethylene-propylene mixtures are selected in such manner as to reduce as much as possible the formation of amorphous ethylene-propylene bipolymer and, on the contrary, to enhance the formation of crystalline ethylene-propylene bipolymer besides, optionally, polyethylene. This for the reason that the amorphous bipolymer is soluble in hydrocarbon solvents, and in the polymerization processes conducted in the presence of such solvents the formation of the amorphous bipolymer, besides lowering the polypropylene yield, tends to excessively increase the viscosity of the polymerization slurry with ensuing difficulties in the transfer and centrifugation steps of said slurry in order to recover the solid polymer. In the processes performed in the gas phase the presence of significant amounts of amorphous polymer tends to increase the tackiness of the solid polymeric phase, which results in the fouling of the reactor. According to the teachings of the prior art, the presence of meaningful amounts of crystalline copolymer is desirable since this results in an improvement of the impact strength properties at low temperatures without remarkably adversely affecting the optical properties (transparency, etc.) and the mechanical properties. Representative processes and compositions of the art are described in US-A-3,629,368; 3,670,053 and 3,200,173. The impact strength properties at low temperatures of the isotactic polypropylene can be improved by adding rubbers, in particular ethylene-propylene rubbers, thereto. According to US-A-3,627,852 it is necessary, however, to incorporate considerably high amounts of ethylene-propylene rubber in order to achieve a significant improvement. This involves a drastic worsening of the mechanical properties (flexural rigidity and stability to high temperatures). Studies accomplished by us allowed to ascertain that the presence of crystalline ethylene-propylene copolymer and optionally of polyethylene in the polypropylene having a high isotacticity index does not contribute to improve the impact strength properties of the polymer; on the contrary, it tends to impair the total balance of the properties. On the basis of a model studied by us it appears that the impact strength properties of a polypropylene modified with an amorphous ethylene-propylene copolymer substantially depend on the amount and quality of the copolymer. The function of the copolymer seems to be that of absorbing, at least partially, the impact energy in the area of graft and propagation of the fracture, with consequent improvement of the impact strength of the system. Contrary to any expectation, it has now surprisingly been found that it is possible to operate, in both the continuous and discontinuous processes of synthesis of modified polypropylene comprising at least a stereoregular homopolymerization step and successively a copolymerization step of ethylene:propylene mixtures, under conditions in which the resulting polymer exhibits relatively low values of the ratio between total polymerized ethylene and fraction soluble in xylene at 23°C (consisting of amorphous ethylene-propylene copolymer) without the occurrence of the drawbacks exhibited by the processes of the prior art. The impact strength properties at low temperature of the modified polyproplene prepared according to the process of this invention are unexpectedly better-the polymerized ethylene being equal-than those of similar products obtainable according to the processes of the prior art. Owing to the relatively low content of total polymerized ethylene necessary to attain significative improvements in the impact strength of the isotactic polypropylene, the mechanical properties of the same do not suffer a sensible worsening. The compositions of this invention comprise: 100 parts by weight of polypropylene having an isotacticity index higher than 90, preferably higher than 95; 8-25 parts by weight of a fraction (1) soluble in xylene at 23°C, consisting of an amorphous ethylene-propylene copolymer containing from 40 to 60% by weight of ethylene; 2-10 parts by weight of a fraction (2) consisting of a crystalline ethylene-propylene copolymer containing from 50 to 90% by weight of ethylene, exhibiting a crystallinity of the polyethylene type. The total content of polymerized ethylene ranges from 4 to 20% by weight. Furthermore, the compositions are characterized in that the ratio (by weight) between total polymerized ethylene and fraction soluble in xylene at 23°C (consisting of amorphous ethylene-propylene copolymer) is lower than 1 and generally ranges from 0.2 to 0.8. Such ratio increases as the content of polymerized ethylene increases. The relatively low values of the above mentioned ratio prove that the rubber-like copolymer (1) is present in prevailing amounts with respect to the crystalline copolymer. Indicatively, we may say that when fraction (1) is present for 13%, fraction (2) is lower than 5% (by -weight). The molecular weight of the various components, determined by measurements of the intrinsic viscosity at 135°C, varies as a function of the nature of the components. The intrinsic viscosity values for the various components are within the following ranges: 1-3.5 dl/g for polypropylene; 2-8 dl/g for fraction (1) and 2-15 dl/g for fraction (2). A product containing 7% by weight of polymerized ethylene and having the following compositions by weight: exhibits the following characteristics : The compositions according to this invention are prepared by polymerization processes of propylene and ethylene-propylene mixtures of known type comprising at least a stereoregular homopolymerization step of propylene and a successive polymerization step of ethylene and propylene mixtures in which use is made of high-yield and high-stereospecificity catalysts comprising a titanium compound carried on a magnesium halide in active form as are defined for example in US-A-4,226,741 and 4,149,990, in DE-A-2,933,997 and in EP-A-4597 45976 and 45975 (European patent applications 81106301.5; 81106300.7; 81106299.1). It is operated under conditions in which, in the propylene homopolymerization step or steps, the polypropylene obtained has an isotacticity index higher than 90, preferably higher than 95, and represents 65-90% by weight of the final product, and, in the ethylene propylene copolymerization step a crystalline ethylene-propylene copolymer exhibiting a polyethylenic crystallinity and an amorphous ethylene-propylene copolymer are obtained in such amounts that, in the final product, the ratio between total polymerized ethylene and fraction soluble in xylene at 23°C is lower than 1. The catalysts preferably employed are the ones described in US-A-4,149,990, in DE-A-2,904,598 and in EP-A-45977, 45976 and 45975 (European Patent Applications 81106301.5; 81106300.7 and 81106299.1). The catalysts of US-A-4,149,990 comprise the product obtained by mixing: (A) an organometallic Al compound, not containing halogen atoms directly bound to Al, partially complexed with an organic electron-donor compound; and (B) a catalyst component prepared by reacting a halogenated Ti compound soluble in hydrocarbons with an active Mg dihalide obtained by decomposition of an adduct Mg dihalide/electron-donor, the electron-donor being NH₃, an aliphatic or aromatic (thio)alcohol, a phenol, a primary or secondary amine, an amide or an aliphatic or aromatic carboxylic acid. The catalysts of DE-A-2,904,598 are prepared by mixing: (A) an Al-alkyl compound, present for at least 85% in a non-combined form with an electron-donor, in an amount corresponding to a molar ratio to the Ti compound of component (B) ranging from 1 to 30; and (B) a catalyst component consisting of a Ti compound and an electron-donor (esters of organic or inorganic oxygenated acids, anhydrides, halides and amides of said acids, ethers, ketones) both supported on a Mg dihalide in an active form, the amount of Ti compounds extractable with TiCl₄ at 80°C being lower than 50%. The catalysts of EP-A-45977 (European Pat. Applications 81106301.5) and 81106300.7 are the reaction product of: (A) an Al-alkyl compound; (B) a silicon compound containing ah least one Si-OR or Si-OCOR or Si-NR₂ bond, R being a hydrocarbyl; (C) a Mg dihalide in active form and, supported therein, a Ti halide or a halo-Ti-alcoholate and a particular type of electron-donor selected from various classes of esters. The catalysts of EP-A-45975 (European Pat. Appln. 81106299.1) are the reaction product of: (A) an Al-alkyl compound; (B) a silicon compound containing at least one Si-OR or Si-OCOR or Si-NR₂ bond, R being a hydrocarbyl; (C) a Mg dihalide in active form and, supported therein, a Ti halide or a halo-Ti-alcoholate or said Ti compound and a silicon compound as defined in (B). The polymerization processes are Conducted either continuously or discontinuously according to conventional techniques, operating in the liquid phase either in the presence or in the absence of an inert hydrocarbon diluent, or in the gas phase, or according to liquid-gas mixed techniques. The polymerization process in the liquid phase in the presence of inert hydrocarbon solvents (suspension process) includes, if conducted continuously, the homopolymerization in two steps, either with or without degassing for recovering the unreacted propylene, and the ethylene-propylene copolymerization in a third step. The polymeric suspension is centrifuged in order to separate the solvent and to recover the atactic polymer dissolved herein. The product is then conveyed to the drying and granulation steps. The process in the liquid monomer, if conducted continuously, includes the homopolymerization of propylene in one step or in more steps, followed by a copolymerization step of ethylene-propylene in liquid propylene. It follows a flash of propylene and other gases with recovery of the monomer and the step of drying and granulation of the product. The mixed process, if conducted continuously, includes a homopolymerization step of propylene in the liquid monomer in one or more steps and the copolymerization of ethylene in a system consisting of a fluid bed reactor with external recycle of the gases. A finishing and granulation step follows. The process in the gas phase, if conducted continuously, comprises a hompolymerization step of propylene in the gas phase in one or more steps and the copolymerization of ethylene in another final step, in the gas phase as well. The reactors may be either of the fluidized bed type or of the agitated bed type: in any case they are characterized by the absence of dispersants such as solvents and liquid monomers. It follows a degassing step of the polymer from the unconverted monomers and the granulation step. The polymerisation process is accomplished in the presence of stereospecific coordination catalysts comprising a titanium compound carried on a magnesium halide in active form having a surface area larger than 3 m²/g and/or having a X-ray spectrum in which the line of maximum intensity appearing in the spectrum of nonactivated magnesium halide is broadened or substituted by a halo. Examples 1 and 2 Two continuous suspension polymerization tests were carried out under the conditions specified in Table 1. The obtained polymers, after finishing and granulation, exhibited the characteristics indicated in said table. A comparison between the products exemplified in Examples 1 and 2 shows the effect exerted on the characteristics by the amount and quality of the ethylene/propylene bipolymer produced. The product of Example 2, although containing a lower amount of total ethylene, shows a balance of characteristics better than type 1. The catalyst employed in Example 1 was prepared by operating according to Example 1 of US-A-4,226,741. The Al/donor ratio was maintained at such values as to obtain an isotacticity index equal to 94 in the 1st homopolymerization step. The catalyst of Example 2 was prepared by operating according to Example 9 of EP-A-45977 (European patent application 81106301.5). Comparative Examples 1-2 The tests were conducted in a 1.3-liter autocalve according to a polymerization process in suspension in a solvent (hexane), operating discontinuously and in two steps. The first step (homopolymerization) comprised the introduction of propylene and catalyst in a hexane suspension. The second step (copolymerization) comprised the introduction, into the same autocalve, after degassing of the unreacted propylene, of an ethylene-proylene mixture in the desired ratios. After degassing of the unreacted monomers, the slurry was centrifuged and the polymer granulated. The conditions employed in the homopolymerization step and in the copolymerization step as well as the rigidity and impact strength characteristics of the products are recorded on Table 3. The catalyst employed in all examples was prepared by operating according to Example 1 of US-A-4,226,741. The determinations according to ME methods indicated in the tables or cited in the description were carried out according to the following modalities: ethylene determination (ME 15600) by infrared spectroscopy; the polymer fraction soluble in xylene at 23°C (ME 15558) by solubilization of the product in xylene at 23°C and filtration; the brittle-ductile transition temperature (ME 17116) by using a BALL-DROP type instrument and by taking the temperature at which 50% of the examined test pieces break in a brittle manner; the impact energy (ME 17142) by means of a BALL-DROP instrument with autographic evaluation of the impact energy.

Claims: Polypropylene compositions having a high impact strength at low temperatures comprising the following essential components: 100 parts by weight of polypropylene having an isotacticity index higher than 90; 8-25 parts by weight of a copolymeric fraction (1) soluble in xylene at 23°C consisting of an amorphous ethylene-propylene copolymer containing from 40 to 60% by weight of ethylene; 2-10 parts by weight of a fraction (2) consisting of a crystalline ethylene-propylene copolymer exhibiting a polyethylenic crystallinity, containing from 50 to 90 % by weight of ethylene; and in which the total content of polymerized ethylene, referred to the weight of polypropylene and of fractions (1) and (2), ranges from 4 to 20%, the weight ratio of total polymerized ethylene to fraction (1) in said compositions being lower than 1 and preferably ranging from 0.2 to 0.8 and obtainable by a process for polymerizing propylene and ethylene-propylene mixtures carried out either continuously or discontinuously in consecutive steps comprising at least a step of stereoregular homopolymerization of propylene and a single step of copolymerization of mixtures of ethylene and propylene, said process being characterized in that the polymerization is accomplished in the presence of stereospecific coordination catalysts comprising a titanium compound carried on a magnesium halide in active form having a surface area larger than 3 m²/g and/or having a X-ray spectrum in which the line of maximum intensity appearing in the spectrum of nonactivated magnesium halide is broadened or substituted by a halo, by operating under conditions in which in the homopolymerization step a polypropylene having an isotacticity index higher than 90 is obtained in an amount ranging from 65 to 90% by weight of the final product, in the copolymerization step a crystalline ethylene-propylene copolymer exhibiting a polyethylenic crystallinity and an amorphous ethylene-propylene copolymer extractable with xylene at 23°C are obtained in such amounts that the weight ratio of the total polymerized ethylene to said amorphous ethylene-propylene copolymer is lower than 1, and the total content of polymerized ethylene in the final product ranges from 4 to 20% by weight. A process for preparing the compositions as claimed in claim 1, by means of a process for polymerizing propylene and ethylene-propylene mixtures carried out either continuously or discontinuously in consecutive steps comprising at least a step of stereoregular homopolymerization of propylene and a single step of copolymerization of mixtures of ethylene and propylene, said process being characterized in that the polymerization is accomplished in the presence of stereospecific coordination catalysts comprising a titanium compound carried on a magnesium halide in active form having a surface area larger than 3 m²/g and/or having a X-ray spectrum in which the line of maximum intensity appearing in the spectrum of nonactivated magnesium halide is broadened or substituted by a halo, by operating under conditions in which in the homopolymerization step a polypropylene having an isotacticity index higher than 90 is obtained in an amount ranging from 65 to 90% by weight of the final product, in the copolymerization step a crystalline ethylene-propylene copolymer exhibiting a polyethylenic crystallinity and an amorphous ethylene-propylene copolymer extractable with xylene at 23°C are obtained in such amounts that the weight ratio of the total polymerized ethylene to said amorphous ethylene-propylene copolymer is lower than 1, and the total content polymerized ethylene in the final product ranges from 4 to 20% by weight.

Inventors: GALLI, PAOLO, SPATARO, MARIO
Applicants: MONTEDISON S.P.A.
Application year: 1982
Main classification: C08F297
IPC codes: C08L23, C08F297