Process for depolymerization of olefinic polymers

Abstract

Thermoplastic olefin polymers are depolymerized at moderate temperatures by a combination of thermal energy and mechanical mixing energy. The mechanical mixing energy is supplied at a specific power rate per unit mass and continues until a specific minimum amount of mixing energy per unit mass is imparted. Depolymerization can occur at temperatures well below 200°C to obtain a depolymerized product that is itself useful and / or can be efficiently converted to useful products, such as ethylene.

Description

[0001] This invention relates to a method for recovering usable hydrocarbon-value products from polyolefins.

[0002] Polyolefins are produced in the tens of millions of metric tons annually. A large proportion of this annual production is used in products such as packaging films, food packaging, bottles, packaging foams, and toys—products intended for single use or with a short lifespan. Used products represent a significant disposal problem. Polyolefins can also be used in products with a long lifespan, such as pipe and wire / cable insulation. However, even these products eventually reach the end of their life cycle and require disposal.

[0003] Despite significant efforts to recycle these materials, recycling has proven to be an inadequate solution to the disposal problem at best. Many materials end up in landfills or the environment, where they may persist for centuries or longer due to their non-biodegradability.

[0004] Polyolefins can be depolymerized through pyrolysis to produce lower molecular weight products that are usable on their own or can be readily converted into usable materials. For example, depolymerized products with melting temperatures of about 35°C to 85°C can be used as waxes. Lower molecular weight gaseous and liquid depolymerized products, especially alkanes and olefins with up to about 25 carbon atoms, form feedstocks that can be fed into crackers to produce polymerizable olefins (particularly ethylene).

[0005] Polyolefin depolymerization is typically carried out at very high temperatures (such as 400°C or higher) in the presence of a depolymerization catalyst. The energy requirements to generate the required temperatures are high, resulting in a negative carbon footprint.

[0006] Milder conditions can be used if hydrogen is supplied to the pyrolysis reaction. So-called hydrogen-assisted or hydrogenolysis methods have been reported to operate at temperatures as low as 250°C in the presence of specific combinations of depolymerization catalysts (Liu et al., Sci. Adv. 2001, 7: eabf8283). This work was carried out in a batch Parr reactor at a hydrogen pressure of 30 atm. Nevertheless, temperatures exceeding 250°C, and more typically exceeding 400°C, are still required. The required high temperatures demand a large energy input. Lower-temperature, less energy-intensive methods are needed.

[0007] It is known that shear conditions, such as those generated in single-screw or twin-screw extruders during polymer processing, can lead to shear heating. Shear heating is generated by the conversion of the mechanical energy imparted by the rotating screw into thermal energy, which heats the molten polymer and increases its temperature. Thermal degradation of polymers due to shear heating is described, for example, in Macromol. Chem. Phys. 2022, 223, 2200206 and Polym. Eng. Sci. 2022:62:815-823. However, the processing temperatures employed are well above 200°C, and no depolymerization into low molecular weight substances has been observed.

[0008] In one aspect, the present invention provides a method for depolymerizing a starting olefin polymer having a weight-average molecular weight of at least 20,000 g / mol, the method comprising heating the starting olefin polymer in the presence of a catalytically effective amount of a depolymerizing catalyst, and imparting mixing energy to the heated starting olefin polymer at a specific mixing power of 1.0 kW to 100 kW (kW / kg) per kilogram of starting olefin polymer for a duration sufficient to supply a specific mixing energy input of 1 kW-h to 1,000 kW-h (kW-h / kg) per kilogram of starting olefin polymer while maintaining a reaction temperature of 100°C to 250°C, to convert at least a portion of the starting olefin polymer into a non-crosslinked depolymerized material.

[0009] Quite unexpectedly, the starting olefin polymers depolymerize at these relatively low temperatures to form depolymerized products, which are themselves usable or can be readily converted into usable raw materials, for example, by steam cracking. By imparting mixing energy to the starting olefin polymers, the temperature required to achieve polymerization can be reduced very significantly. At a given temperature, depolymerization proceeds much faster when mixing energy is imparted to the starting olefin polymers according to the invention. On the other hand, lower specific mixing power at these lower temperatures hardly promotes depolymerization.

[0010] The starting olefin polymer is a polymer of at least one monomer having a polymerizable vinyl (CH2-CHR-) group, wherein R is alkyl, substituted alkyl, phenyl, substituted phenyl, or preferably hydrogen. The olefin polymer can be a polymer of an olefin monomer containing heteroatoms, such as vinyl chloride, vinylidene chloride, vinyl alcohol, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, hydroxyethyl acrylate, tert-butyl acrylate, and methyl methacrylate. Generally, it is preferred that the starting olefin polymer is a hydrocarbon, i.e., without heteroatoms. Examples of olefin polymers include polyolefins such as polyethylene (including high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and metallocene polyethylene), polypropylene, and ethylene-propylene copolymers, as well as vinyl aromatic polymers such as polystyrene and copolymers (random, block, and / or grafted) of styrene with olefins (such as ethylene). Any one or more of the aforementioned types of waste polymers (post-consumer resins) are usable and preferred raw materials because the method of the present invention allows waste materials to be recycled and converted into usable chemicals.

[0011] The starting olefin polymer can be linear, branched, or cross-linked. In some embodiments, the starting olefin polymer is thermoplastic.

[0012] Mixtures of two or more olefin polymers can be used as starting materials. Furthermore, mixtures of one or more thermoplastic olefin polymers with one or more other polymers that are not thermoplastic polymers and / or not olefin polymers can be used as starting materials. At the end of depolymerization, polymers that did not depolymerize under the conditions of this method (such as crosslinked or other thermosetting polymers) can be separated from the depolymerization products. The ability to use mixed polymer feedstocks is a significant advantage of this invention, as it allows the processing of many types of products containing multiple types of polymer materials (such as, for example, co-extruded films and sheets, such as those commonly used in packaging applications) without separating those products into their various polymer components. Similarly, the starting olefin polymers can be blended with other materials (such as metals, wood, ceramics, inorganic fillers, etc.).

[0013] The depolymerization catalyst is any catalyst that catalyzes the depolymerization of the initiating olefin polymer. Examples of available catalysts include Fe-Cu-Mo-P supported on alumina; hydrides or other compounds of elements from Groups 3 to 12 (2016 IUPAC element table), such as scandium, yttrium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, zinc, cadmium, or mercury, preferably supported on an inorganic support (such as alumina or silica / alumina support); and zeolite catalysts, such as those described in PCT / GB / 2010 / 050932, WO Those listed in 2012 / 076890 and CA2202941A include zeolite catalysts comprising natural zeolites such as chalcogenide, mordenite, tuftite, octahedralite, and clinoptilolite; and synthetic zeolites such as zeolite A, zeolite X, zeolite Y, zeolite L, FAU-type zeolite, ZSM-5 (MFI), β (BEA), and zeolite ω; and platinum nanoparticles deposited on tungstate-zirconia, as described in Liu et al., Sci. Adv. 2021:7: eabf8283. Typically, it is suitable to provide 1 to 25 parts by weight of catalyst per 100 parts by weight of the starting olefin polymer. Preferred minimum amounts are at least 2, 3, or 4 parts by weight of catalyst per 100 parts by weight of the starting olefin polymer, and preferred maximum amounts are up to 20, 15, 12.5, or 10 parts by weight based on the same reference.

[0014] The starting olefin polymer is combined with the catalyst, heated, and subjected to mixing energy, which generates shear. All the catalyst can be added at the beginning of the method; alternatively, the catalyst can be added in one or more increments, or even continuously during the heating and mixing steps. If added incrementally, the increments can be, but do not have to be, equal increments.

[0015] The heating and mixing steps are performed in equipment that supplies both heat energy and mixing energy during operation. Mixing is also performed in the presence of a catalyst. These devices can be adapted for batch or continuous operation.

[0016] Equipment suitable for batch operation may include a container for containing reactor contents (i.e., the starting olefin polymer) and other components that may be present (such as catalysts, solvents, or diluents, if any) and the depolymerization products they form. The heating elements supply heat to the container and the mixture, such as through the container walls and / or through heated internal elements in contact with the reactor contents. The heating elements may be resistive elements that convert electrical energy into heat energy, and these elements are operated to interrupt the heat supply when a set temperature is reached. The heating elements may include equipment for supplying heated fluid to contact the container walls and / or various internal elements contained within the container. The heating elements are preferably controllable to produce a "set temperature." The set temperature is the temperature to which the contents of the equipment are heated without shear heating.

[0017] Equipment for batch operation further includes a dynamic mixing element within the container. Various rotors and rotor / stator configurations are available, as are rotating screws with various mixing elements, etc. Dual-rotor, non-meshing internal agitators are particularly suitable for generating shear. These may have roller rotors. The dynamic mixing element is preferably adapted to supply mixing energy at a rate of at least one kilowatt (kW) per kilogram of starting olefin polymer during operation.

[0018] Particularly suitable equipment for batch operations includes those manufactured by Thermo Fisher Scientific under the brand name Haake. ® Mixing and heating equipment for sale, such as those sold as Haake Rheomix 3000 and Haake Polylab QcRheomix 600.

[0019] Suitable equipment for continuous operation includes tubular and / or loop reactors having heating elements as described with respect to batch equipment, dynamic mixing elements (such as one or more in-line stirrers), and means for moving the reactor contents through the equipment (such as pumps or internal forwarding elements). The dynamic mixing elements are preferably adapted to supply mixing energy at a rate of at least one kilowatt (kW) per kilogram of starting olefin polymer during operation.

[0020] The apparatus for either batch or continuous operation preferably further includes means for measuring the temperature of the mixture of contents of the container, such as an internal temperature probe.

[0021] The starting olefin polymer is introduced into the device and subjected to thermal energy due to the operation of the heating element and to mixing energy due to the operation of the mixing element. This increases the temperature of the reactor contents. During the mixing energy imparting step, the temperature of the reactor contents (i.e., the reaction temperature) should be maintained between 100°C and 250°C. Preferred reaction temperatures are at least 120°C, at least 130°C, or at least 140°C. An advantage of the invention is that only a moderately elevated temperature is required to depolymerize the starting olefin polymer; therefore, preferred reaction temperatures during the mixing energy imparting step are at most 225°C, at most 200°C, or at most 190°C. These temperatures refer to the actual temperature of the device contents, which is higher than the set temperature produced by the operation of the heating element itself due to shear heating. The temperature tends to decrease as the reaction proceeds because the viscosity of the reactor contents typically decreases as depolymerization occurs, resulting in the formation of lower molecular weight depolymerized products, which in some cases can act as solvents or plasticizers for the retained higher molecular weight substances.

[0022] Depolymerization is achieved by subjecting the starting olefin polymer to mixing energy while maintaining it within the aforementioned temperature range. The mixing energy is imparted to the starting olefin polymer at a specific mixing power (mixing power per unit weight of starting olefin polymer) of at least 1 kW / kg. In specific embodiments, the specific mixing power is at least 2 kW / kg or at least 3 kW / kg, and, for example, up to 100 kW / kg, up to 50 kW / kg, up to 30 kW / kg, up to 20 kW / kg, or up to 10 kW / kg. Depending on the specific equipment, the power can be measured or estimated in various ways. The electrical power consumed in operating the dynamic mixing element can be measured directly. The power of the rotating mixing element can be determined by torque measurement, rotational speed, and geometric factors, as described, for example, in Example 1 below.

[0023] At least a portion of the mixing energy is converted into heat via shear heating, thereby increasing the temperature of the contents of the apparatus. The amounts of thermal energy and mixing energy applied to the reactor contents are selected to maintain the reaction temperature as indicated above, provided that the mixing power is as previously described.

[0024] Typically, it is preferred to provide sufficient thermal energy to reach the "set temperature" (i.e., the temperature to which the reactor contents will be heated in the absence of shear heating). When the reactor contents exceed this set temperature, it is preferable to control the heating element to interrupt the supply of thermal energy. The set temperature is typically 5°C to 100°C below the reaction temperature, and preferably 5°C to 40°C below the reaction temperature. Generally, the set temperature should be high enough to melt and soften the starting olefin polymer. In cases where the starting olefin polymer is thermoplastic and semi-crystalline, the set temperature can, for example, be 0°C to 40°C or 0°C to 25°C above the crystallization melting temperature of the starting olefin polymer. In specific embodiments, the set temperature is at least 80°C, at least 100°C, or at least 120°C, and preferably up to 225°C, up to 200°C, up to 175°C, up to 150°C, or up to 140°C.

[0025] The temperature rise due to shear heating can be, for example, at least 5°C to at most 100°C, but more typically 15°C to 75°C or 20°C to 65°C. The temperature rise due to shear heating may or may not be constant throughout the reaction. As depolymerization proceeds, the viscosity of the reactor contents typically decreases. As viscosity decreases, less mixing energy is converted into shear heating, and therefore, at a constant specific mixing power, the temperature rise due to shear heating tends to decrease over time. Unless additional thermal energy is provided to compensate (or the specific mixing power is increased), the temperature of the reactor contents typically decreases during the depolymerization reaction. For example, during the depolymerization reaction, the temperature rise due to shear heating can decrease, for example, from 100°C to 15°C or from 65°C to 20°C. If necessary, the specific mixing power can be increased over time to maintain a constant temperature rise due to shear mixing. As the temperature rise due to shear heating decreases, thermal energy is provided as needed to maintain the reactor contents within the aforementioned temperature range.

[0026] The starting olefin polymer is maintained at the aforementioned temperature and specific mixing power for a period of time sufficient to convert at least a portion of the mass of the starting olefin polymer into a non-crosslinked depolymerized material. The non-crosslinked depolymerized material may have a number average molecular weight, for example, not greater than 25% or not greater than 10% of the number average molecular weight of the starting olefin polymer. The non-crosslinked depolymerized material may include oligomers having a number average molecular weight, for example, from 250 g / mol to 2,000 g / mol or from 400 g / mol to 1,000 g / mol. The non-crosslinked depolymerized material may include straight-chain and / or branched alkanes and olefins having 2 to 150, 5 to 150, or 7 to 75 carbon atoms. Preferably, heating is continued under shear until at least 5% or at least 10% of the starting olefin polymer is converted into a non-crosslinked depolymerized material having a molecular weight not exceeding 25% of the number average molecular weight of the starting olefin polymer. Heating under shear can continue until 5% or at least 10% of the starting olefin polymer is converted into a non-crosslinked depolymerized substance with a molecular weight of 2,000 g / mol or less. For the purposes of this invention, substances having a melting temperature below their degradation temperature and / or being soluble in at least one solvent are considered non-crosslinked. In some embodiments, heating under shear continues to convert at least a portion of the starting olefin polymer into linear and / or branched C-chains suitable for use as oils or waxes. 15 -C 100 Aliphatic hydrocarbons. In a particularly preferred embodiment, heating is continued under shear to convert at least a portion of the starting olefin polymer into straight-chain and / or branched C2-C hydrocarbons suitable for use as feedstock for steam cracking. 20 Aliphatic hydrocarbons are used to produce olefins (such as ethylene). For example, heating under shear can be performed for 10 minutes to 72 hours, 30 minutes to 72 hours, 1 hour to 72 hours, preferably at least 2 hours, and more preferably at least 4 hours, and preferably up to 36 hours, up to 24 hours, up to 18 hours, up to 12 hours, or up to 6 hours. The molecular weights of the starting materials and decomposition products are determined by gel permeation chromatography (GPC) relative to suitable standards. When the starting olefin polymer is a polyolefin, a polyethylene standard is preferred; when the starting polymer is a vinyl aromatic polymer, a polystyrene standard is preferred.

[0027] During the heating step under shear, a certain amount of char or other cross-linked material may be formed. This can be removed from the non-cross-linked depolymerized material using any convenient solid / liquid or solid / gas separation method, such as by filtration or centrifugation. If some or all of the non-cross-linked depolymerized material is a room-temperature solid, it can be heated to produce a melt that can be separated from the char and other cross-linked materials. Similarly, the residue of undepolymerized polymer in this method can be similarly separated from the non-cross-linked depolymerized product.

[0028] The specific mixing energy input (SMEI) (i.e., the total amount of mixing energy imparted to the reactor contents during the heating at the aforementioned temperature and the steps of imparting mixing energy to the heated starting olefin polymer) can be, for example, at least one kW-hr (kW-h / kg) (3600 joules / g) per kilogram of starting olefin polymer. SMEI can be at least 2 kW-h / kg (7,200 J / g), at least 5 kW-h / kg (21,000 J / g), at least 10 kW-h / kg (36,000 J / g), at least 20 kW / kg (72 kJ / g), or at least 40 kW / kg (144 kJ / g), and in some embodiments up to 1000 kW-h / kg (3,600 kJ / g), up to 500 kW-h / kg (1,800 kJ / g), up to 250 kW-h / kg (900 kJ / g), or up to 100 kW-h / kg (360 kJ / g).

[0029] SMEI is the product of mixing power and time, and therefore increases with increasing reaction time. The heating and energizing steps can continue for a period of time, for example, at least 10 minutes, at least 30 minutes, at least 60 minutes, at least 2 hours, or at least 4 hours, provided that the mixing power is sufficient to achieve the necessary SMEI as described above within such a period. The heating and energizing steps can continue for any arbitrarily longer time, such as up to 72 hours, up to 48 hours, up to 24 hours, up to 18 hours, up to 12 hours, or up to 6 hours.

[0030] The heating step under shear can be carried out in air, oxygen, an inert atmosphere (such as nitrogen, helium, or argon), or a reactive atmosphere (such as hydrogen). Without being bound by any theory, a reactive atmosphere of hydrogen can be advantageous because depolymerization can proceed thermodynamically more favorably in the presence of hydrogen, and the product can be hydrogen-saturated, making it suitable as a feedstock for downstream applications.

[0031] Plasticizers and / or solvents may be present for the starting olefin polymers. In cases where the polymer is semi-crystalline, the presence of plasticizers and / or solvents can reduce the set temperature required to soften the starting olefin polymer to below its crystallization melting temperature.

[0032] The product mixture obtained by this method is preferably separated into various components. The gaseous products of the depolymerization reaction are readily separated from the solid and liquid components of the product mixture by venting or similar methods. Various types of solid-liquid separation devices known in the art are used to readily separate the solid and liquid components of the product mixture. Hydrocarbons having up to about 20 to 25 carbon atoms can exist as vapors, which can be condensed to separate them from more volatile materials such as unreacted hydrogen and methane. The carbon can be burned to recover energy or disposed of.

[0033] If necessary, spent catalysts or catalyst residues can be recovered, regenerated (by calcination or other thermal regeneration), and, if required, recycled or reused in the method of the present invention by introducing them into the heating zone of a non-remixing tubular reactor.

[0034] The following examples are provided to illustrate the invention and are not intended to limit the scope of the invention. Unless otherwise specified, all parts and percentages are by weight.

[0035] Example 1

[0036] Low-density polyethylene (LDPE) with a number-average molecular weight of approximately 15,000 and a weight-average molecular weight of approximately 77,000 g / mol was depolymerized in a Haake PolyLab high-shear stirrer. 35 g (0.035 kg) of LDPE was combined with 5 g of calcined ZSM-5 zeolite catalyst and the mixture was loaded into the stirrer. The mixture was heated in the stirrer under nitrogen atmosphere, and the equipment was adjusted to a set temperature of 120 °C. The rotor speed was set to 250 rpm. The start time (time = 0) was the time when the rotor speed reached 250 rpm; at this point, the actual temperature of the reactor contents was 136.2 °C. The actual temperature of the stirrer contents, rotor speed, and applied torque (in N-mm) were then continuously measured until operation was interrupted after 24 hours. The peak temperature of 182.1 °C was reached after approximately 38 minutes. Table 1 indicates the average torque for each time interval during operation. The average torque for each interval is the arithmetic mean of the start and end torques for that interval. The average mixing power (in kW) for each interval is calculated from the average torque using the following relationship: Average mixing power = Torque / 1000 * 2π * rpm / 60. The energy for each interval is the average power multiplied by the number of hours in that interval. The specific mixing power and SMEI for each interval are the average mixing power and mixed energy divided by the weight of the LDPE (in kg), respectively. The cumulative SMEI indicated for each time interval is the total SMEI from the start time to the end of the indicated time interval. These parameters are described in Table 1.

[0037]

[0038] At the end of 24 hours, the contents of the high-shear stirrer were cooled to room temperature, and samples were obtained for analysis relative to polyethylene standards by gel permeation chromatography (GPC). 21% of the product mixture was soluble in the GPC solvent. Insoluble materials included residual catalyst, crosslinking materials, and char. The soluble depolymerization products had a number-average molecular weight of 480 g / mol and a weight-average molecular weight of 1180 g / mol. The number-average molecular weight of the soluble depolymerization products was reduced to 1 / 32 compared to the starting LDPE.

[0039] Similar results were obtained when the experiment was repeated in air instead of nitrogen.

[0040] Compare and run A

[0041] Comparative run A was performed in the same manner as in the example, except that the rotor speed was only 25 rpm. The start time (time = 0) was the time when the rotor speed reached 25 rpm; the temperature at the start time was 116.3°C. The set temperature was reached after approximately 6 minutes. Thereafter, the actual temperature of the agitator contents, rotor speed, and applied torque (in N-mm) were continuously measured until the run was interrupted after 24 hours. The peak temperature of 142.7°C was reached after approximately 28 minutes. The average torque, average mixing power, mixing energy, SMEI, and cumulative SMEI for each time interval are reported in Table 2.

[0042]

[0043] The reaction mixture was cooled at the end of the 24-hour reaction and evaluated by GPC as in Example 1. 63% of the product mixture was soluble in the GPC solvent. The soluble depolymerized product had a number-average molecular weight of 6790 g / mol and a weight-average molecular weight of 39,670 g / mol. Lower specific mixing power and cumulative SEI resulted in significantly less depolymerization than seen in Example 1. The weight-average molecular weight of the soluble depolymerized product was reduced to only about 1 / 2.2.

[0044] Compare run B

[0045] Example 1 was repeated, this time with the amount of LDPE increased to 40 grams and the catalyst omitted. The average torque, average mixing power, mixing energy, SMEI, and cumulative SMEI for each time interval are reported in Table 3.

[0046]

[0047] At the end of the 24-hour reaction, the reaction mixture was cooled and evaluated by GPC as in Example 1. 40% of the product mixture was soluble in the GPC solvent. The soluble depolymerized product had a number-average molecular weight greater than 5000 g / mol. This data demonstrates the effect of the catalyst; compared to Example 1, almost no depolymerization was observed in the absence of a catalyst.

Claims

1. A method for depolymerizing a starting olefin polymer having a weight-average molecular weight of at least 20,000 g / mol, the method comprising heating the starting olefin polymer in the presence of a catalytically effective amount of a depolymerizing catalyst, and imparting mixing energy to the heated starting olefin polymer at a specific mixing power of 1.0 kW to 100 kW (kW / kg) per kilogram of starting olefin polymer for a duration sufficient to supply a specific mixing energy input of 1 kWh to 1,000 kWh (kW-h / kg) per kilogram of starting olefin polymer while maintaining a reaction temperature of 100°C to 250°C, to convert at least a portion of the starting olefin polymer into a non-crosslinked depolymerized material.

2. The method of claim 1, wherein the steps of heating the starting olefin polymer in the presence of a catalytically effective amount of depolymerization catalyst and imparting mixing energy to the heated starting olefin polymer are performed in air.

3. The method of claim 1, wherein the steps of heating the starting olefin polymer in the presence of a catalytically effective amount of depolymerization catalyst and imparting mixing energy to the heated starting olefin polymer are performed under an inert atmosphere selected from nitrogen, helium, argon, or any mixture of two or more thereof.

4. The method of claim 1, wherein the steps of heating the starting olefin polymer in the presence of a catalytically effective amount of depolymerization catalyst and imparting mixing energy to the heated starting olefin polymer are performed in an atmosphere containing hydrogen.

5. The method according to any one of claims 1 to 4, wherein mixing energy is imparted to the heated starting olefin polymer at a rate of at least 2 kW / kg for a period of time to supply a specific mixing energy input of 10 kW-h / kg to 250 kW-h / kg.

6. The method according to any one of claims 1 to 4, wherein mixing energy is imparted to the heated starting olefin polymer at a rate of at least 3 kW / kg for a period of time sufficient to supply a specific mixing energy input of 20 kW-h / kg to 250 kW-h / kg.

7. The method according to any one of claims 1 to 4, wherein mixing energy is imparted to the heated starting olefin polymer at a rate of 3 kW / kg to 20 kW / kg for a certain period of time to supply a specific mixing energy input of 20 kW-h / kg to 100 kW-h / kg.

8. The method according to any preceding claim, wherein the non-crosslinked depolymerizing material has a number-average molecular weight (M0) of 250 g / mol to 2,000 g / mol. p (e.g., as measured by GPC relative to polyethylene standards).

9. The method according to any of the preceding claims, wherein the starting olefin polymer is thermoplastic.

10. The method according to any of the preceding claims, wherein the starting olefin polymer is a polyolefin.

11. The method according to any of the preceding claims, wherein the starting olefin polymer is polyethylene.

12. The method according to any of the preceding claims, wherein the steps of heating the starting olefin polymer and imparting mixing energy to the heated starting olefin polymer are performed in a dynamic stirrer.

13. The method according to any of the preceding claims, wherein the steps of heating the starting olefin polymer and imparting mixing energy to the heated starting olefin polymer are performed in the presence of 3 to 20 parts by weight of depolymerization catalyst per 100 parts by weight of the starting olefin polymer.

14. The method of claim 13, wherein the depolymerization catalyst comprises one or more of zeolite and platinum nanoparticles deposited on a tungsten-zirconia catalyst.

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