A PROCESS FOR THE CONVERSION OF MIXED PLASTIC WASTE

Abstract: The invention relates to the conversion of a mixed plastic waste by means of mechanical treatment of the starting material mixture in the presence of a mineral oil. The essence of the invention consists therein that a crushed plastic waste mixture containing about 20 % by weight of low density polyethylene with a softening point below 150 DEG C is heat-treated in an oily medium in an apparatus exerting intense kneading-moulding effect at a temperature between 150 DEG C and 250 DEG C under vigorous stirring until the formation of a quasi-homogenous, liquid-phase or meltphase bitumen-like material , then separating the bitumen-like material obtained.

Description: PROCESS FOR THE CONVERSION OF MLXED PLASTIC WASTE The invention relates to a process for the conversion of mixed plastic waste to a homogeneous product which can be utilized in the industry. It is known that from end-products manufactured in large amounts only a small proportion of plastics are recycled into the industrial utilization after use. The regeneration comprises mainly such plastic articles (e.g. bottles, cans etc.), which are easy to be recognized and thus to separate them from the waste and to re- utilize directly or to process them in form of re-granulated material after washing. The re-utilization of mixed plastic waste is for the most part unsolved the main reasons thereof are the following: four or five sorts of plastics being in general use are prevailing in the waste selectively collected by the population, however, they are available in many forms. Consequently they cannot be separated from the waste by a simple method hence the appearance of the products is very diverse according to their sorts even before utilization and this becomes more complicated after wear. The regeneration of simple packaging materials widely spread in the trade is a relatively well-solved task, however, on the one hand, it requires high costs and on the other hand, the increase of effectivity would demand such a grade of standardization, which practically cannot be solved under market conditions. Another difficulty of regeneration and re-utilization consists also in the ageing of plastic articles (products) during the period of their wear, when their physical-chemical state changes because of mechanical, thermal or other reasons and therefore, in comparison to the original ones, their colour, mechanical and chemical properties are significantly changed depending on the grade of degradation. An essential problem of the regeneration comprises that first during the wear and later during the waste processing, the plastics become contaminated by other materials (foreign materials) which are either incorporated into the waste or they remain adhered to the waste surface. The spectrum of these contaminations is extremely broad and extends from the remains of foods to oily and chemical residues, powder, metal residues etc., i.e. it is of organic or inorganic nature. The main bulk of plastics alone or together with other wastes are disposed or burnt. A relatively simple manner of utilization of mixed plastic waste consists of granulating it after cleaning but without separation, then shaping it by melting together using known plastic processing techniques. In some fields the processed products are employed for. the substitution of wood (fences, bench and the like), however, they can be produced only in a stongly coloured quality and can be sold only within a limited scope. The quality of the regenerated product significantly depends on the composition of the raw material and therefore, the utilization of mixtures with a defined composition is not possible. Considering the heterogenic state of such raw materials, the mechanical properties of the products cannot be foreseen or standardized. Thus the production of „artificial wood" has only of marginal importance and it cannot be considered that it would play a considerable role in the utilization of plastic wastes. In addition to the above difficulties, the re-granulation of plastic wastes without separation or moulding of final products directly from the melt is also problematic as it is known that the most part of plastics do not constitute a compatible homogeneous mixture even in molten state and subsequently, the mechanical properties of the manufactured products are randomly formed. In order to solve this problem EP 0 578 500 describes that high density polyethylene particles are suspended in a low density polyethylene matrix which are then moulded into plates. By adding mineral oil it is achieved that, in spite of the considerable differences of apparent densities, the composite product established by moulding and other treatment does not become decomposed. This method is suitable only for processing of a mixture of two sorts of polyethylene and both the moulding as well as cooling should be carried out by the highest care for maintaining the cohesion.. It is desired to eliminate the heterogeneous state of mixed plastic wastes containing indifferent plastic sorts mainly by specific mechanical processing and to make the product useful in this way. According e.g. to patent US 5,468,431, the mingled mixture is divided into two parts in such a way that both material groups alone are homogeneous; then the specific surface of one material group is increased and then incorporated into the other material group used as a medium. The composite prepared in this way is quasi-homogeneous, however, this method is very sensitive towards the composition of raw material and contaminations and therefore it cannot be used without a previous selection or pretreatment of the virgin material. Due to this circumstance the processing becomes uneconomical. According to patent US 5,030,662 it is tried to solve the heterogeneous state of the mixture by means of reactive additives, however, the quality of products becomes much poorer than that of the virgin material. According to the Hungarian patent HU 204,461 it is tried to solve this problem by combining two steps. During the first operation the mixed plastic waste is activated by a shear force of 500 to 5000 N within the range of softening temperature and the reactive sites formed at the molecular ends broken by the decomposition are polymerized by using a Ziegler type catalyst. The disadvantage of this process consists therein that the organoaluminum compound is pyrophoric whereby the process becomes expensive. In addition, the catalyst is decomposed and made ineffective by contaminations, mainly by moisture of the mixed wastes. A number of other known technics of processing of plastics are disclosed in the introductory part of the Hungarian patent HU 204,461. The regeneration of mixed plastic wastes for the original purpose is expensive and is substantially unsolved without a separation step of high selectivity. A different way of utilization of mixed plastic wastes is to prepare again a product of mineral oil industry, i.e. a raw material for the chemical industry or power industry from the raw material desired originally from mineral oil. According to DE 4,311,034 a liquid fuel or basic material of chemical industry is prepared from the plastic waste. The main point of this process consists therein that depolymerization products being in gas and liquid phase are prepared in oil as reaction medium and these products are processed separately. The coke-tar like residue is further utilized by partial hydrogenation. In this process a temperature between 250 °C and 450 °C and suitable pressure is necessary and a reaction time of 0.55 hour. It is evident that although by means of this process used in the mineral oil processing a product can be prepared which can be used more easily and generally than the secondary plastic product of low quality described above, however, when adapted to a waste having different composition within very wide limits in the practice, neither the composition nor the quantitative ratio of the product fractions become stable. The efficacy of this complicated process is poor since the amount of products are by an order lower than the power and investment costs of the process. Another difficulty is that the ratio of mineral oil used as reaction medium to the waste is approximately 1: 1 and therefore, a very significant quantity of oil should be maintained in the technological cycle. The process according to DE 3,037,829 has a similar target. In this process, after heating a mixture of wastes consisting of polyolefins and polystyrene over 300 % in anthracene oil, coal tar or in a liquid with similarly low volatility. Olefins and benzene homologues are prepared as products. The residue of the process is used also for a target similar to that of the original tar. A drawback of this process consists therein that a significant amount of aromatic material with high boiling point is processed and therefore, the reaction mixture should be maintained usually under pressure due to the high temperatures. The ratio of costs to the value of product is also in this case higher. The aim of the present invention is to re-utilize mixed plastic waste by the means of an economic process and to prepare a product possessing new properties, which product has wide fields of use different from those existing hitherto. The invention relates to a process for the conversion of mixed plastic waste by heat and mechanical treatment of the starting material mixture in the presence of a mineral oil and is characterized by heat treating under vigorous stirring a crushed mixed plastic waste containing at least 20 % by weight of low- density polyethylene with a softening point lower than 150 °C in the presence _of 5 to 40 % by weight (related to the weight of the plastic waste) of a mineral oil or a derivative thereof having a high flash point in an apparatus exerting an intense kneading and moulding effect at a temperature between 150 °C and 250 °C, preferably at a temperature lower than the pyrolysis temperature of the material mixture until the formation of a quasi-homogeneous liquid-phase or melt phase bitumen-like material, resp., then separating the obtained bitumen-like material. After increasing the temperature of the starting material mixture gradually up to the temperature of the intense heat treatment the mechanical and the heat treatment periods are maintained for 1 to 20 minutes. The mineral oil is mixed and added to the starting material mixture before or during the mechanical treatment; as mineral oil a substance having an open-cup flash point higher than 220 °C is added. The mechanical treatment and kneading are continued in the apparatus until the stabilization of the rate of power input thereof and subsequently, the mechanical treatment is continued for 1 to 5 minutes. The kneading apparatus is preferably a multizone mixing extruder with spiked roller of Werner-Pfleiderer type or a „multicat transfermix" transfer type equipment. However, a Banbury's mixer can also be employed, which is coupled with a continuously operating extruder. If desired, the granulated product usually having a softening point of above 100 °C is, if desired, subjected to grinding in a Prall-type or other grinding machine until a particle size lower than 10 mm is attained. The structure of the granular product obtained is not spheroidal but arborescent. On carrying out the process according to the invention, the mixed plastic waste, which is a mixture of various polymers is heat-treated in such a way that the highest temperature of heat treatment is lower than the pyrolysis temperature of the mixture. The decomposition occurring under effect of heat treatment and the starting temperature thereof depends on the one hand on the chemical composition and structure of the mixture and on the other hand, on the sites of faults incorporated during the manufacture of the individual plastics, which are accompanied by the molecular damage induced by mechanical, chemical or photochemical influences occurring during wear. At least 20 % by weight of the starting plastic mixture should consist of a plastic waste with low softening point, which had been subjected to product shaping and wear. Such material suffer pre-degradation and it provides the reactive radicals or ionic molecular sites, respectively, branchings and double bonds necessary for the conversion of the mixed plastic waste in the process and therefore, it induces the further degradation at a lower temperature. The additive of the starting material employed can also be considered to be a degradation initiator but simultaneously, it forms the melt or the liquid phase of the starting reaction, resp. A low density polyethylene film or any other form of low density polyethylene can be employed as such additive. Originally the mixed plastic waste is a loose aggregation of torn cut or ground plastic particles having low thermal conductivity. A mineral oil, which can be e.g. a spent oil having small volatility and an open-cup flash point of at least 200 °C is added in order to promote the further processing and homogenation, to improve kneadability and to achieve the colloidal state of the process material. The added oil is either mixed with the waste or is separately fed into the apparatus. Preferably the flash point of the oil exceeds the highest temperature of the process. An amount of oil of 5 to 6 % by weight calculated on the starting material mixture is moistening the material, improves its cohesion and heat transmission, i.e. the formation of a melt phase from particles having low softening points. In the melt phase the reaction of the total starting material mixture is started and become homogenized with the other part of material at „melting" and before attaining the solftening point. The quantity of oil additive used is determined by the grade of oil uptake capacity of the starting material mixture. Under the given conditions of the process the oil additive undergoes a thermooxidative degradation of small extent, similarly to the plastic material. Experience shows that the amount of oil additive may be 15 to 25 % by weight, preferably 5 to 10 % by weight where its softening effect already prevails. The material quality of the oil additive is not critical since in the case of oils, which can be taken into account ^ the properties of adhesion and heat conduction are nearly the same; essential is to maintain the lower limit of flash point. A crucial condition of the process of the invention is to control the temperature values of heat treatment: the lower limit is at least 115 °C and should not be higher than 250 °C in any case. At a temperature below 115 °C the reaction is not started at a suitable rate and the formation of homogenity starting with softening is extraordinarily slow even in the presence of an oil additive. At a temperature above 250 °C the pyrolysis is starting with the emission of gas and toxic decomposition products are formed. In addition to the adjustment of composition of the starting material mixture, time of heat treatment and feeding of the oil additive, the intense mechanical kneading and moulding are also indispensable from the point of view of progress of the reaction. The melt should be maintained under constant stirring during the progress of the reaction since the stirring and kneading contribute to the degradation of the viscous mass. Stirring and kneading do not induce such high shear stress which alone could induce a significant chain breaking by eliminating the bond energy of polymers; however, it contributes to a great extent to increase the rate of the chemically initiated reaction and to promote considerably the intramolecular chain transfer reactions in the polymerization proceeding concurrently with the degradation in mechanical way. By kneading the material mixture in the molten medium during the process the remaining material particles with a high softening point are transformed to microgel particles and they can uniformly be subdivided in the dispersing medium having a lower softening point. This process can be accelerated by increasing the extent of degradation but this is promoted only to an extent by which a stable colloid substance with high dispersity grade is formed from the plastic mixture. On carrying out the process of invention, at the beginning of the process, the stirring-kneading apparatus should be heated up to the temperature stated and the starting plastic mixture should be crushed to such extent that the apparatus can be stirred during the time of heating up. After being filled up with the material and after partial melting the apparatus exerts a very significant shear force which is necessary to the formation of a colloidal material system. The colloidal systems transformed from the mixed plastic waste by using the process according to the invention are similar to the bitumens or asphalts, from the point of view of rheological and many other viewpoints since asphalts are such colloids wherein asphaltene microgel dispersed particles are divided in an oily medium and the colloidal system is stabilized by so-called petroleum resins. As a result of the process, in the end product obtained the oil and the plastic material with low softening point perform the role of the oily dispersing medium; the materials with higher softening point perform the role of asphaltene; and the degradation products as well as non-thermoplastic resins and dust-like contaminations of the waste perform the role of the stabilizer. From aforesaid it can be concluded that the composition and contaminations of a given waste determine the technological parameters which are required to convert a plastic waste of optional composition for the preparation of a bitumen-like material under the given conditions. The stability of physical condition of the bitumen-like material can presumably be explained thereby that reactive molecules originating from different materials are capable to undergo a reaction with each other and thereby a degradation and polymerization or copolymerization, resp., simultaneously proceed, which promote the establishment of a transient layer at the surface of the very finely crushed particles. By using the process of the invention a bitumen-like material is obtained from a mixture of the different plastics in each case, therefore the colloidal state as a transition state always precedes the radical pyrolysis in the case of processing of plastic mixtures. Of course, the softening point, ductility and other properties of the product are depending on the substances introduced and the technological parameters employed. The particles being present in the macroscopically and microscopically homogeneous mass become homogenized. The black or dark colour of the product also indicates that the plastic mixture has been degraded concurrently with a decrease in the hydrogen content. Compared with the traditional bitumen this „synthetic" bitumen possesses also more advantageous properties, which positively promote their practical utilization. First the fact should be mentioned that the product is free of a number of ingredients causing toxic emission, such as their heating or incineration do not cause an emission in relation with sulfur compounds or originating from carcinogenic polyaromatic substances. The technological properties of the product are preferred on many fields, e.g. it has no cold-flowing property, therefore it is an ideal joint filler material. The process according to the invention can be carried out in an apparatus operating either discontinuously or continuously. It is essential that the intensity of kneading should be sufficient for the dispersing in addition to keeping the operation periods and temperatures. By carrying out the operation discontinuously it can be observed that the resistance of medium of the mixture increases very significantly at the beginning and this lasts until the dispersing of the partially molten or softened particles are prevailing. After a few minutes the resistance of medium of the mass is decreasing, then becomes stabile which can be established on the one hand from the stabilization of the rate of power input of the machine, on the other hand, from the stabilization of temperature of the mass. The properties of the obtained product become also stabile; no further essential change was observed. Thus, the process according to the invention is carried out in such a way that the mixture is kneaded until both the rate of power input of the kneading machine as well as the temperature of the mass turns stabile. The process according to the invention can be carried out by means of several known stirring-kneading (moulding) equipments used in the plastic processing; most preferably using a machine containing intense shear and breaking blades and surfaces, which is capable to solve the compression and shear of the difficultly compatible waste scraps together as well as the intense stirring with the melt or oil playing the role of a dispersion medium but which is also capable to grind the hard and non-thermoplastic resins or contaminations of extraneous materials. Another advantage of the process according to the invention is that apart from particular cases (e.g. a lot of metal contaminations, much inorganic contaminations, much more oil than the adhered oil amount, etc.), washing and pre-cleaning of the plastic waste of communal or industrial origin involving significant costs can be omitted. Carrying out the process in practice is illustrated in the following Examples. However, the scope of utilization of the process according to the invention is much broader, consequently the scope of protection is not limited by the Examples. Example 1 A coarsely pre-crushed waste (maximum grain size is 5 to 15 cm) originating from selective waste collection of the population is finely crushed in a chopper with blades in such a way that the granules become less than 10 mm in size. The waste mixture possesses the following composition (expressed as % by weight): low density polyethylene 47 % high density polyethylene 24 % polypropylene 13 % polystyrene 8 % PVC 5 % unidentifiable material 3 % The solid material was stirred with 27 % by weight of a so- called base oil which is an uncompounded mineral oil with an open cup flash point above 230 °C for the preparation of various lubricating oil and oil used for the industry. The stirring was carried out in such a way that the oil uniformly coated the surface of the waste particles. In this state the mixture could be easily treated, stored and introduced into the reactor by means of a screw feeder. The mixture was added in portions into a pre-heated twin- screw kneading-moulding reactor in such a way that the feeding should last for 5 minutes. The reactor has such shaping that in the inside thereof at a height of 10 % of radius of the cylindre from the smooth cylindre jacket tooth extrude, the cross section whereof is trapezoid from side view. A set of tooth arranged on one cylindre is fitting into the smooth cavity situated between two sets of tooth on the other cylindre, thus on rotation of the cyHndres the edge of tooth exert a crushing effect and the total of tooth exert a kneading effect while it presses the material to the smooth jacket of the other cylindre. Since the tooth are not in contact with each other, the apparatus cannot break even in case of relatively big pieces but the edge of tooth is capable to shear a soft or harder material along the edge. After addition of the material a kneading time of 8 minutes is accomplished, then after the material has turned into a homogenous paste, it is discharged by opening the lower aperture. The material has a uniform black colour, good mechanical and binding properties; it is a bitumen-like product. Example 2 A material mixture originating from a mixed waste collection was processed, in which used packaging materials (plastic films, bags, plastic glasses), cable coating residues and hard plastic caps and the like were well recognizable. By subjecting the material to a two-phase (two-step) crushing (grinding and tearing) an aggregation of crushed particles of 0.1-5 mm in size were obtained. The composition thereof determined by a subsequent analysis was as follows (expressed in % by weight): low density polyethylene 55 % high density polyethylene 8 % polyamide 7 % PVC 8 % polystyrene 4 % polyurethane 6 % polypropylene 5 % unidentifiable material 6 % The material was treated similarly as described in Example 1 with the difference that 19 % by weight of a lubricating oil (so- called „spent" used oil) originating from car and free of volatile substances were added. The operation was carried out in a twin-screw continuous extruder of 7 litres of total volume in such a way that after a preheating time of 4 minutes the material gets to the kneading space of the machine being on a temperature of 190 °C, wherein it remains for 6 minutes and then it was introduced to the granulator of continuous operation through the discharge opening of the apparatus. Example 3 A plastic mixture originating from selective waste collection and having the following composition (as expressed in % by weight) was used: polyethylene (LDPE and HDPE) 70 % polypropylene 15 % polystyrene 5 % PVC 2 % others 6 % The material was crushed to particles of 2 to 5 cm size in chopper with blades , then uniformly mixed with an amount of 15 % by weight of a mineral oil having a flash point above 220 °C. An amount of 50 kg of this mixture was added into a Banbury-type mixture preheated to 140 °C. During this addition the power consumption of the machine rapidly increased and the temperature of the material increased up to a temperature of 195 °C to 205 °C. The machine attained the highest temperature within 3 minutes after the addition which lasted for 12 minutes. Thereafter the power consumption of the machine decreased about to its half value and the temperature of the material decreased to 185 °C. The kneading was continued under unchanged conditions for 1 to 2 minutes then, after stabilization, the easily flowing-out bitument-like mass was discharged from the machine and introduced into an extruder possessing water-cooled cutting head for granulation. Example 4 A plastic mixture having the following composition (expressed in % by weight) was processed according to the process of Example 3: polypropylene 12 % polyamide 0.4 % polyethylene + contaminations up to 100 % The mass obtained had a softening point starting above 100 °C and a well defined softening point at 143 °C. The material granulated was ground in an extruder at environmental temperature to particle size of between 0 to 2.5 mm size. The ground material particles obtained had an arborescent shape and were not sticking together.

Claims: C l a i m s 1. Process for the conversion of mixed plastic waste by heat and mechanical treatment of the starting mixture in the presence of mineral oil, which comprises heat-treating under vigorous stirring a crushed mixed plastic waste containing at least 20 % by weight of low density polyethylene with a softening point lower than 150 °C in the presence of an amount of 5 to 40 % by weight calculated on the weight of plastic waste of mineral oil or a derivative thereof having a high flash point in an apparatus exerting an intense kneading and moulding effect at a temperature between 150 °C and 250 °C, preferably at a temperature lower than the pyrolysis temperature of the material mixture until the formation of a quasi- homogenous liquid-phase or melt phase of a bitumen-like material resp., then separating the obtained bitumen-like material and, if desired, subjecting it to further processing. 2. A process as claimed in claim 1, which comprises keeping the period of the mechanical heat-treatment for 1 to 20 minutes. 3. A process as claimed in claims 1 and 2, which comprises heating the starting material mixture gradually to the temperature of the intense heat-treatment period. 4. A process as claimed in anyone of claims 1 to 3, which comprises mixing and adding the mineral oil to the starting material mixture before or during the mechanical treatment adding as mineral oil, preferably spent oil having an open cup flash point higher than 220 °C. 5. A process as claimed in claim 1, which comprises continuing the mechanical treatment in the kneading-moulding apparatus until stabilization of the rate of power input thereof and further for additional 1 to 5 minutes. 6. A process as claimed in claim 1, which comprises using as kneading-moulding apparatus a multizone mixing extruder with spiked roller of Werner-Pfleiderer type or an equipment with transfer system of „multicat transfermix" type or a mixer of Banbury-type. 7. A process as claimed in claim 1, which comprises granulating the mass obtained after kneading-moulding in an extruder and then further processing it.

Inventors: SZEKELY TAMAS, SZEKELY, TAMAS
Applicants: SZEKELY TAMAS, TVK ECOCENTER KFT, SZEKELY, TAMAS, TVK-ECOCENTER KFT.
Application year: 1998
Main classification: B29B17
IPC codes: B29B17, C08J11