PROCESS FOR REMOVING UNDESIRED SUBSTANCES IN PLASTIC MATERIALS

Abstract: A process for reducing undesired substances in the production of plastic materials, in which about 10-100 ppm of a suitably selected zeolite is added to the reactor in the polymerisation of the plastic material. For removing undesired, odour/taste-producing substances, a substantially hydrophobic aluminium silicate molecular sieve is added, having a pore diameter of at least 5.5 ANGSTROM , an Si/Al molar ratio in the crystal lattice of at least 35, and a sorption capacity for water at 25 DEG C and 4.6 torr of less than 10 wt.-%. For removing catalyst poisons, such as water and carbon monoxide, a substantially hydrophilic zeolite is added, preferably having a pore diameter not exceeding about 15 ANGSTROM . Before being added to the reactor, the zeolite can be activated by heat treatment at about 250-350 DEG C for 2-5 h.

Description: PROCESS FOR REMOVING UNDESIRED SUBSTANCES IN PLASTIC MATERIALS The present invention relates to a process for remov- ing or eliminating undesired substances in plastic mate¬ rials. The invention relates particularly to the reductio or elimination of undesired, unintentionally added sub¬ stances from polyolefin type materials, by which are henceforth meant homopolymers of ethylene or propylene or copolymers of one of these hydrocarbons with another alpha olefin. The undesired substances can especially be odour/ taste-producing substances in plastic materials for fields of application where they may contact liquids or food¬ stuffs to be consumed by human beings or animals, or where any odour or taste may be experienced as negative for hygienic or aesthetic reasons. In many contexts undesired, odour- and/or taste-pro¬ ducing substances appear owing to chemical degradation or transformation in connection with production or compound- ing of plastic materials. These substances can be of a highly varying chemical character. The occurrence thereof can constitute a health hazard or merely a source of irri¬ tation. Since the human being normally is most susceptible to odours and tastes, the substances need not appear in very high concentrations, but in most cases as low concen¬ trations as one or a few ppm are sufficient to make a taste or odour evident. To date, attempts have been made to overcome or at least reduce such odour and taste pro¬ blems in various ways, such as by degassing the plastic material. Such a method is not only lengthy and expensive; it also results in but a moderate improvement in respect of odour and taste, respectively. Some specific fields will now be mentioned where the problem with undesired substances causing odour and/or taste is of particular significance. Such a field comprises plastic pipes, more specifi¬ cally plastic water pipes, where the emission of odour- and/or taste-producing substances from the plastic mate¬ rial to the drinking water flowing through the pipe fre- quently is an annoying problem which is difficult to solve. The problem grows more serious by the fact that no details of the odour/taste-producing substance(s) are known. It is believed that they consist of a variably com posed mixture of different substances, such as oligomers, formed in the polymerisation, or volatile substances whic have been formed by oxidisation processes in conjunction with subsequent steps of processing and which even at extremely small concentrations of about 1-1000 ppm in the water give this an undesired odour and/or taste. A second field comprises film and sheeting of plas¬ tic material, more particularly film and sheeting that is caused to contact foodstuffs or pharmaceutical prepara¬ tions. Since for foodstuffs and pharmaceutical prepara¬ tions extremely high requirements as regards purity are made and no foreign substances contaminating the foodstuff or pharmaceutical preparation are allowed for reasons of health and security, it is vital that plastic film and sheeting used for such products does not emit any unde¬ sired or contaminating substances. In such plastic pro- ducts, there is also a particularly great risk that unde¬ sired substances are produced, such as when boiling the foodstuff in the wrapping, or when sterilising the drug packing by heat treatment or radiation. A closely related field is represented by containers, such as bottles, of blow-moulded or moulded plastic, for packing foodstuffs, beverages or pharamceutical prepara¬ tions. A further field applies to interior fittings of plas¬ tic, e.g. panels, for use in buildings or cars and the like. In new cars, it thus is not unusual that the odour can be very unpleasant. In all four fields mentioned above, the possible odour which one may want to eliminate can appear espe¬ cially in the compounding/processing of the plastic material and in this context constitutes a problem in the working environment. A further problem in the production of plastic mate¬ rials is the presence of moisture. The catalysts used in the polymerisation are sensitive to moisture and even small amounts of moisture in the polymerisation can poiso the catalyst and deteriorate the yield of the polymerisa¬ tion reaction. Correspondingly, also carbon monoxide acts as a catalyst poison that should be eliminated. From a general point of view, it is known to remove any undesired odour and/or taste by means of adsorbing substances, and as examples of such adsorbing substances, activated carbon, silica gel, activated aluminium oxide, diatomaceous earth and zeolites may be mentioned. An exam ple in which a number of such additives are suggested, is Japanese Patent Specification JP 1,023,970 which thus is incorporated by reference. This publication suggests the adding of e.g. aluminium hydroxide, clay, diatomaceous earth, kaolin, talc, bentonite, activated carbon or acti¬ vated carbon fibre for materials for packing foodstuffs t be heated in the packing. However, such additives have been found to be without any appreciable effect, except under special circumstances, in the case of undesired com ponents in plastic materials of the types concerned according to the present invention since, as a rule, they lose their absorbing capacity when contacting water or water vapour. Recently, US Patent Specification 4,795,482 has described a new type of zeolites intended for the elimina tion of undesired odour in certain contexts and supplied under one of the names ABSCENT or SMELLRITE. These zeo- lites are described to be crystalline, siliceous molecula sieves in which at least about 90, and preferably at leas about 95, % of the tetrahedral oxide units of the crystal lattice are SiO tetrahedra, and which have a sorption capacity for water at 25 C C and 4.6 torr of less than 10 wt.-%. These zeolites have an Si/Al molar ratio of fr 35 to infinity, preferably between 200 and 500, and a por diameter of at least 5.5 A, preferably at least 6.2 A. It is also preferred that the sorption capacity for water vapour at 25°C and a water vapour pressure of 4.6 torr is less than 6 wt.-%. These substantially hydrophobic mole¬ cular sieves are used to remove odorous organic compounds at a temperature of from -25°C to 100°C. It should here be added that the above-mentioned Si/Al molar ratio for these molecular sieves applies to the oxide units of the crystal lattice only. The Si/Al molar ratio of the zeolite, which has been determined by conventional wet analysis, can be considerably lower owin to contamination by aluminous pollutants formed in the so called dealuminification reaction which in many cases is included as a step in the production of the zeolite. The above-mentioned zeolitic molecular sieves have proved useful above all for removing undesired odour from hygienic products, such as diapers, incontinence products and the like. Reference is here also made to EP 0,348,978 As a further example of prior-art technique, US Patent 4,859,733 may be mentioned, according to which a zeolite is added to a polymer in order to remove tri- alkanolamines which are used as deactivators in the poly¬ merisation reaction. One more example of prior-art technique while using the same type of zeolite is US Patent 4,910,295. This patent relates to a specific process for reducing the odour of residual amounts of the comonomer ethylidene norbornene present in certain types .of EPDM rubber. According to the present invention, it has now sur¬ prisingly been found that by adding a zeolite in connec- tion with the polymerisation of the plastic material in the reactor, it is possible to remove or eliminate unde- sired substances, such as water, carbon monoxide or odour/taste-producing substances. It must be considered surprising that the added zeo¬ lite functions for removing the undesired substances in the invention, since one could rather expect that the zeolite would absorb other low-molecular compounds, such as the monomers present in the polymerisation reaction, o would be clogged by such compounds. However, this is not the case but, depending on the selected zeolite, this can be caused to selectively remove undesired compounds, such as water, carbon monoxide and substances having undesired odour/taste. One could also expect that the zeolite would affect the polymerisation reaction in an unfavourable manner, but it has been found that by selecting a suitable zeo¬ lite, this problem can also be avoided. According to the invention, there is thus provided a process for removing undesired substances in the produc¬ tion of plastic materials, characterised in that in the polymerisation of the plastic material in the reactor, about 10-1000 ppm of a suitable zeolite is added. Further characteristic features of the invention are evident from the specification below and the accompanying claims. According to the invention, it may be suitable that the added zeolite is dehydrated, i.e. free from water. In that case, the zeolite is heat treated or dried before being supplied to the polymerisation reactor. The condi¬ tions of the heat treatment should be such that any mois- ture or other adsorbed substances in the zeolite are removed and a dehydrated and pure zeolite is produced. Suitable conditions for this have turned out to be heatin at a temperature of about 250-350° for about 2-5 h, pre¬ ferably about 3-4 h. After the desorbing heat treatment, the zeolite should be stored under inert gas, such as nitrogen, before being supplied it to the polymerisation reactor. An especially preferred aspect of the present inven¬ tion concerns the reduction or elimination of odour/taste- producing substances. As mentioned above, attempts have previously been made to eliminate such undesired sub- stances by treating the finished plastic material in dif¬ ferent manners. Since, according to the present invention, the zeolite is added so as to be present during the actual polymerisation reaction and remove the substances as early as in the polymerisation, both the formation and the pre- sence of odour/taste-producing substances are counteracted in a most effective manner. As mentioned, it must be considered surprising that the added zeolite selectively removes the undesired, odour/taste-producing substances, without sorbing monomers and/or reaction products in the polymerisation reactor. Further, as far as can be judged, the efficiency of the invention for removing odour/taste- producing substances seems to be unexpectedly higher than in previous processes since, according to the invention, very small amounts of added zeolite in the order of parts per million (ppm) calculated on the weight of polymer formed are sufficient to prevent the presence of odour/ taste-producing substances. Generally, about 10-1000 ppm, preferably about 25-800 ppm, most advantageously about 50-500 ppm of zeolite is added according to the invention, calculated on the weight of polymer formed. According to the invention it has been found that not all zeolites are active to remove substances having an undesired odour or taste. Thus, it has been established that the zeolite should be substantially hydrophobic (oleophilic) to be effective in the elimination of odour and taste. A measure of the hydrophobic properties is the water sorption of the zeolite, and this should suitably be less than 10 wt.-%, preferably less than 6 wt.-%, at 25°C and 4.6 torr. The Si/Al molar ratio of the zeolite in the oxide units of its crystal lattice seems to affect the hydrophobic properties and should exceed 35 and preferably be between 200 and 500. The pore diameter of the zeolite, which according to the molecular size determines which compounds can be caught by the zeolite, should be at least 5.5 and preferably at least 6.2 A. The zeolite, which generally is in the form of a powder, should have such a small particle size that it does not detrimentally affect the appearance of the plas¬ tic material or the other properties thereof. Thus, the zeolite should have an average particle size not exceed¬ ing about 10 μm, such as about 0.1-7 μ , preferably not exceeding about 5 μm. In the above-mentioned, especially preferred aspect of the invention which concerns the reduction or elimina¬ tion of undesired, odour- and/or taste-producing sub¬ stances, the zeolite used according to the invention can advantageously be a zeolite according to the above US Patent 4,795,482. As mentioned above, a further aspect of the invention concerns the removal of catalyst poisons, such as water and carbon monoxide, from the reactor in the polymerisa- tion reaction. Like when removing undesired odour/taste- producing substances, the zeolite can be heat treated for removal of water, before being added to the reactor. How¬ ever, use is made of a hydrophilic zeolite when removing catalyst poisons, such as water and carbon monoxide, as opposed to the hydrophobic zeolite which is used for removal of odour/taste-producing substances. Furthermore, when removing catalyst poisons the zeolite should have a pore diameter which is adapted to the purpose. Suitable hydrophilic zeolites are e.g. those supplied by Grace GmbH, Germany, under the trade name "Sylosiv" and having a pore size from about 3 A to about 15 A. The method of adding the zeolite .to the polymerisa¬ tion reactor is not critical, but it can be added either together with the polymerisation catalyst or in the form of a separate flow of material. Depending on the type of polymer to be produced, the composition of the catalyst can differ, for example a so-called Ziegler catalyst or a so-called chromium oxide catalyst. It is preferred to carry out the invention in polymerisation while using a chromium oxide catalyst, for example of the type described in US Patent Specification 3,324,101. For further elucidation of the invention, a non- restrictive Example follows below. EXAMPLE For the purpose of determining the effect of the invention for removing undesired odour/taste-producing substances, different materials were tested while using a taste panel. The testing materials consisted of polyethy¬ lene produced by polymerisation of ethylene in a polymeri¬ sation reactor. In the polymerisation, use was made of a chromium catalyst of the type described in US Patent Spe- cification 3,324,101. When producing a reference sample, the polymerisation was performed without adding a zeolite, whereas in the tests according to the invention, varying amounts of zeolite were added at the same time as the catalyst. The zeolite used was a hydrophobic zeolite of the type described in US Patent Specification 4,795,482 and marketed under the trade name SMELLRITE. Before the zeolite was added to the polymerisation reactor, it was heat treated at about 300°C for 4 h and then protected by nitrogen gas, before being mixed with the catalyst and added to the reactor. After the polymerisation, the poly¬ mer material was pelletised in connection with the extru¬ sion, and from each test, about 32 g of pellets was taken out and stirred for 4 h at a temperature of 30 ± 1°C by a magnetic agitating means in a flask fitted with a ground- in plug and containing 1000 ml of pure distilled water. Five sub-samples were then prepared from each sample by dilution according to Table 1 below. TABLE 1 Sub-sample Sample water Dilution water Taste/odour leve No. ml ml degree 0 1 70 1.5 100 2 150 4 175 8 The level of taste and odour is here defined as fol¬ lows: (a + b)/a wherein a is the amount of sample water in the sub-sample in which odour or taste is barely detect- able, while b is the amount of dilution water in the same sub-sample. The lower the value of the taste and odour level the better the result. A value of 1.5 and below is acceptable. Seven trained taste testers are allowed to estimate each sample and determine their personal taste level. The average value of the seven people is considered to produce an objective estimation thereof. The testing is blind, i.e. the taste panel cannot distinguish the samples. A total of 3 samples were examined according to this procedure. The results are shown in Table 2. TABLE 2 Sample 1 (Reference) 199 — 0.55 25 944.0 7 5 3 1 1 1.5-2 2 790 53:47 0.65 24 944.8 4 1 0 0 0 1-1.5 3 292 76:24 0.51 24 943.2 3 2 2 0 0 <1 * Parts per million by weight of the quantity of produced polymer. Determined by determining the ash content of the polymer. In Table 2, MI., indicates the melt index of the pro¬ duced polymer, determined according to ISO-R-292 at a con¬ stant load of 5 kg and a temperature of 190°C. MFR stands for "melt flow ratio", i.e. a ratio of different melt indexes. MFR 2 , ,*- concerns the ratio of I 21 to MI,-. As is evident from Table 2, the added zeolite is effective for removing odour/taste-producing substances. It is also evident that the properties of the produced polymer are not notably affected by the added zeolite, while the catalyst activity can decrease slightly as the amount of added zeolite increases. This appears from the fact that the values of "amount of catalyst + zeolite" increase, thus indicating that the formed amount of poly¬ mer per unit of weight of catalyst decreases, i.e. the activity of the catalyst decreases. An optimal amount of zeolite seems to be about 1/4 of the amount of catalyst + zeolite.

Claims: CLAIMS 1. Process for reducing undesired substances in the production of plastic materials, c h a r a c t e r i s ¬ e d in that in the polymerisation of the plastic material in the reactor, about 10-1000 ppm of zeolite is added thereto. 2. Process as claimed in claim 1, c h a r a c - t e r i s e d in that about 25-800 ppm of zeolite is added to the reactor. 3. Process as claimed in claim 2, c h a r a c ¬ t e r i s e d in that about 50-500 ppm of zeolite is added to the reactor. 4. Process as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the amount of zeolite is about 25-75 wt.-% of the total amount of zeolite, and catalyst. 5. Process as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the zeolite has an average particle size not exceeding 5 μm. 6. Process as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the zeolite is a substantially hydrophobic aluminium silicate molecular sieve having a pore diameter of at least about 5.5 A, an Si/Al molar ratio in the crystal lattice of at least 35, and a sorption capacity for water at 25°C and 4.6 torr of less than 10 wt.-%. 7. Process as claimed in claim 6, c h a r a c - t e r i s e d in that the molecular sieve has an Si/Al molar ratio in the crystal lattice of between 200 and 500, and that it has a sorption capacity for water at 25°C and 4.6 torr of less than 6 wt.-%. 8. Process as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the zeolite is a sub¬ stantially hydrophilic aluminium silicate molecular sieve having a pore diameter not exceeding about 15 A. 9. Process as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the zeolite i dehydrated by drying at about 250-350°C for about 2-5 h, before being added to the reactor. 10. Process as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the zeolite is stored under an inert gas before being added to the reac¬ tor.

Inventors: GUSTAFSSON BILL, JOHANSSON SOLVEIG, GUSTAFSSON, BILL, JOHANSSON, SOLVEIG
Applicants: NESTE OY
Application year: 1992
Main classification: C08F2
IPC codes: C08F2