Description:BACKGROUND

FIELD OF THE DISCLOSURE
[0001] Various embodiments of the disclosure relate generally to polypropylene vitrimers. More specifically, various embodiments of the disclosure relate to processing polypropylene vitrimers with blends of polyolefins, in particular polyethylene.

DESCRIPTION OF THE RELATED ART

[0002] Thermoplastic polyolefins (TPO), including polyethylene (PE) and polypropylene (PP), are the most widely used plastics due to their versatility, durability, and cost-effectiveness. About 80% of the world’s consumed plastics are thermoplastics, mainly used as packaging or textile fibers, of which 50% are single-use applications. Massive consumption of TPOs has led to significant environmental concerns, primarily due to their resistance to degradation. Conventional recycling methods, such as mechanical recycling, often result in degraded material properties due to polymer chain scission and contamination, while chemical recycling can be energy-intensive and costly.
[0003] One promising approach to address these challenges involves the formation of vitrimers, which are crosslinked polymers capable of dynamic bond exchange, which allow recyclability through reversible network rearrangement. Vitrimers exhibit a unique covalent adaptive network (CAN) that can transform under appropriate conditions, enabling the material to retain its crosslinked structure while being reprocessed.
[0004] In the case of polyethylene (PE), implementing vitrimer chemistry through free radical-mediated crosslinking poses several challenges. Free radical reactions in PE often lead to uncontrolled, random crosslinking, and chain scissions, resulting in heterogeneous network structures and reduced recyclability. The high degree of crosslinking in PE vitrimer systems can also lead to processing difficulties, such as increased viscosity and reduced flow properties, hindering material reprocessing. Furthermore, achieving a balance between sufficient crosslink density for mechanical integrity and maintaining dynamic bond exchange for recyclability remains a critical technical barrier.
[0005] The branched structure of polypropylene (PP) with tertiary carbons may confer slightly higher reactivity than polyethylene for free radical reactions. However, the presence of bulky methyl side groups in PP increases steric hindrance, complicating functionalization and dynamic bond exchange processes. Balancing crosslink density for sufficient mechanical strength while maintaining recyclability remains a significant technical challenge for polypropylene vitrimers. Thus, while vitrimer technology offers a promising solution for the sustainable management of polyolefins like polyethylene and polypropylene, overcoming these technical challenges remains essential for its successful implementation in recycling systems.
[0006] Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.
SUMMARY

[0007] According to embodiments of the present disclosure, a polypropylene vitrimer is provided. The polypropylene vitrimer comprises a functionalized polypropylene comprising a maleated sidechain. The functionalized polypropylene is formed by a reaction between a polypropylene (PP), a coagent, and a maleic anhydride. The coagent comprises trimethylolpropane diacrylate, N, N’-m-phenylene dimaleimide, 1,6-hexanediol methacrylate, zinc diacrylate, copolymer of butadiene and styrene, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, or combinations thereof. The polypropylene vitrimer further comprises a dynamic crosslinker covalently bound to the maleated sidechain. The dynamic crosslinker is able to form a covalent adaptive network through a transesterification exchange reaction to form the polypropylene vitrimer.
[0008] In another embodiment, a blend comprising the polypropylene vitrimer is provided. The blend comprises a polyolefin. The polyolefin is a virgin polyolefin, a post-consumer recycled (PCR) polyolefin, a post-industrial recycled (PIR) polyolefin, or combinations thereof.
[0009] In yet another embodiment, a method of preparing a polypropylene vitrimer is provided. The method comprises performing a first extrusion of a polypropylene, a coagent, and a maleic anhydride in an extruder at a temperature in a range of 160 to 200 °C for a residence time in a range of 1 minute to 5 minutes to form a functionalized polypropylene comprising a maleated sidechain. The coagent comprises trimethylolpropane diacrylate, N, N’-m-phenylene dimaleimide, 1,6-hexanediol methacrylate, zinc diacrylate, copolymer of butadiene and styrene, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, or combinations thereof. The method further comprises performing a second extrusion of the maleated polypropylene with a dynamic crosslinker in the extruder at a temperature in a range of 160 to 200 °C for a residence time in a range of 1 minute to 5 minutes to form the polypropylene vitrimer. The dynamic crosslinker is bound to the functionalized polypropylene through the maleated sidechain and is able to form a covalent adaptive network through a transesterification exchange reaction.
[0010] In yet another embodiment, an article comprising the polypropylene vitrimer is provided.

BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a flow chart that illustrates a method of preparing a polypropylene vitrimer, in accordance with an exemplary embodiment of the disclosure; and
[0012] FIG. 2 is a reaction scheme in accordance with an exemplary embodiment of the disclosure;
[0013] FIG. 3 is a bar chart of yield strength and elongation at yield of polyoelfins and polypropylene vitrimer; and
[0014] FIG. 4 is a bar chart displaying gel content of polyoelfins and polypropylene vitrimer.
[0015] Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0016] The following description illustrates some exemplary embodiments of the disclosed disclosure in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this disclosure that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present disclosure.
[0017] The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or process steps.
[0018] As used herein, the term “or combinations thereof” means that the listed components may be used individually or in any combination thereof.
[0019] All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.
[0020] These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.
[0021] As used herein, the term “cycle life” refers to a number of times a thermoplastic polymer may be recycled before its melt flow index drops below 0.01 grams per 10 minutes.
[0022] The term, “melt flow index”, as used herein, is defined as the mass of a thermoplastic polymer passing through a die of specified dimensions and properties at a specified temperature and under a known load within a time period of 10 minutes and can be measured using the International Organization for Standardization (ISO) 1133-1 or American Society for Testing and Materials (ASTM) D1238 test methods. Melt flow index (MFI) indicates flowability of the thermoplastic polymer.
[0023] The term, “functionalization” of a polymer, generally refers to the introduction of specific functional groups (for example, hydroxyl, carboxyl, amine, or anhydride) onto a polymer chain, and the resulting polymer is referred to as a “functionalized polymer”. Functionalizing the polymer modifies its chemical properties by altering reactivity, adhesion, or compatibility with other materials.
[0024] The term, “crosslinking” of a polymer, generally refers to the formation of covalent bonds between polymer chains, leading to a three-dimensional network structure. The polymer formed through this process is referred to as a “crosslinked polymer”. Crosslinking improves the mechanical and thermal properties of the polymer without significantly altering its chemical properties.
[0025] As used herein, the term “copolymer” refers to a polymer derived from more than one species of monomer, where the copolymer includes repeating units of each of the monomers.
[0026] As used herein, the term “blend” refers to a mixture of two or more polymers or copolymers that have been blended together to create a new material with different physical properties.
[0027] The term, “polyethylene”, as used herein refers to a polymer derived from ethylene monomers (CH₂=CH₂). Polyethylene may include homopolymers of ethylene as well as copolymers that contain minor amounts of one or more α-olefins or comonomers, such as butene-1, hexene-1, or octene-1. The term encompasses a wide range of polyethylenes with varying densities (e.g., low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE)), molecular weights, and molecular weight distributions. The polyethylene may be branched, linear, or crosslinked, and may exhibit amorphous, semicrystalline, or crystalline properties, depending on the synthesis method and processing conditions.
[0028] The term, “polypropylene, as used herein refers to a polymer derived from the polymerization of propylene monomers (CH₂=CHCH₃). The term includes homopolymers of propylene as well as copolymers containing minor amounts of one or more α-olefins or comonomers, such as ethylene, butene-1, or hexene-1. Polypropylene may exist in various stereoregular forms, including isotactic, syndiotactic, and atactic configurations, and can exhibit properties ranging from amorphous to highly crystalline. The polypropylene may include modifications, such as impact copolymers, random copolymers, or blends, to tailor properties such as toughness, transparency, or processability. Polypropylene may also encompass forms with differing molecular weights, molecular weight distributions, or additives, depending on the intended application or manufacturing process.
[0029] Plastic “recycling” refers to a process whereby useful products may be produced from waste plastics after reprocessing or melting the waste plastics. However, polyolefins after recycling usually possess inferior properties when compared to their virgin counterparts. The extent of degradation may depend on degradation during use, cycle life, and the severity of conditions applied during reprocessing.
[0030] Vitrimers are a class of polymers containing dynamic covalent bonds that can reorganize upon application of external stimuli allowing the material to be reshaped, repaired, or recycled while mostly retaining its original properties. Vitrimers may provide a viable strategy for polyolefin recycling.
[0031] FIG. 1 is a flow chart 100 that illustrates a method of preparing a polypropylene vitrimer through exemplary steps 102 through 104, according to embodiments of the present disclosure. At step 102, a first extrusion of the polypropylene with a coagent and a maleic anhydride is performed in an extruder at a temperature in a range of 160 to 200 °C for a residence time in a range of 1 minute to 5 minutes to form a functionalized polypropylene comprising a maleated sidechain.
[0032] Examples of polypropylene comprise virgin polypropylene, polypropylene copolymers, biaxially oriented polypropylene (BOPP), post-consumer recycled (PCR) polypropylene, post-industrial recycled (PIR) polypropylene, or combinations thereof. Post-consumer recycled (PCR) polypropylene refers to polypropylene waste generated by consumers, or in other words after-use polypropylene products. The composition of PCR polypropylene can vary significantly due to the diverse mix of polymers (for example, other polyolefins or copolymers used with polyolefins) and additives used by different manufacturers. The variation in composition makes recycling of PCR polypropylene more complex and challenging. PCR polypropylene includes multilayered polypropylene packaging. In contrast, post-industrial recycled (PIR) polypropylene is derived from polypropylene waste produced during industrial and manufacturing processes. PIR plastics are generally easier to recycle as they typically originate from a single source and are of known composition. The polypropylene used for the preparation of polypropylene vitrimers has a polypropylene content of more than 80 weight percent (wt %).
[0033] In embodiments where the polypropylene is PCR polypropylene, the PCR polypropylene is washed to remove any contaminants or residues and dried to remove moisture before processing. In one embodiment, the PCR polypropylene is washed with an aqueous detergent solution. The washing is followed by drying in a vacuum oven at a temperature in a range of 50°C to 80°C for a time in a range of 5 to 12 hours before use to remove moisture. Once the PCR polypropylene is washed and dried, it is cut into smaller pieces for the first extrusion.
[0034] The polypropylene may be in the form of film, granules, flakes, powders, pellets, or combinations thereof. Before adding the polypropylene into the extruder it may be suitably sized into desirable dimensions of the order of a few millimeters.
[0035] The coagent comprises trimethylolpropane diacrylate, N, N’-m-phenylene dimaleimide, 1,6-hexanediol methacrylate, zinc diacrylate, copolymer of butadiene and styrene, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, or combinations thereof. In one embodiment, the coagent is triallyl cyanurate.
[0036] The first extrusion, at step 102, is performed in presence of an initiator. The initiator generates free radicals for initiating a chemical reaction between the polypropylene, the coagent and the maleic anhydride. Examples of initiators comprise benzoyl peroxide, lauryl peroxide, dicumyl peroxide (DCP), or combinations thereof. In one embodiment, the initiator is dicumyl peroxide (DCP).
[0037] A concentration of the initiator in the polypropylene vitrimer is in a range of 0.5 weight percent (wt%) to 5 wt%.
[0038] The first extrusion is performed in an extruder such as a single-screw extruder, or a twin-screw extruder. The processing parameters of the extruder may be varied to facilitate melt extrusion of the polypropylene with the maleic anhydride, the coagent, and the initiator by optimizing one or more of melting of the polypropylene, homogeneous mixing between the polypropylene, maleic anhydride, the initiator and the coagent, and efficient reaction between the polypropylene, maleic anhydride, and the coagent. Examples of such process parameters include, but are not limited to, type of extruder, geometrical design of the extruder, screw speed, residence time of material in the extruder, feed rate of the material into the extruder, temperature, and die geometry through which a product is extruded. In one embodiment, the extruder is a twin-screw extruder that facilitates enhanced mixing between the polypropylene, maleic anhydride, the initiator, and the coagent when compared to a single-screw extruder. The first extrusion may be performed at a temperature corresponding to the melting temperature of the polypropylene. In some embodiments, the melting temperature is in a range of 160 to 200°C. In some embodiments, the residence time is in a range of 1 to 10 minutes, preferably 1 to 5 minutes. In some embodiments, screw speed is in a range of 100 to 150 rotations per minute (rpm) in a twin-screw extruder. The first extrusion, in one embodiment, is performed in a twin-screw extruder at a temperature of 180°C at screw speeds of 100 to 150 rotations per minute (rpm) and at a residence time in a range of 1 to 5 minutes.
[0039] The first extrusion results in the formation of the functionalized polypropylene. As used herein, the term “functionalized polypropylene” refers to a polypropylene functionalized with maleic anhydride and contains a crosslinked coagent. The term 'crosslinked coagent,' as used herein, refers to a coagent that facilitates crosslinking of polypropylene chains through reactions initiated through free radical formation. The coagent contains reactive sites, such as carbon-carbon double bonds (unsaturation), which enable the formation of covalent bonds either within the same polypropylene chain or between different polypropylene chains. The presence of maleic anhydride in the maleated polypropylene introduces polarity to the otherwise inert carbon-carbon backbone of polypropylene, making it amenable to further functionalization. Additionally, the presence of coagent in the maleated polypropylene facilitates the formation of a three-dimensional network through crosslinking.
[0040] A concentration of the coagent in the polypropylene vitrimer is in a range of 1 weight percent (wt%) to 15 wt%.
[0041] A concentration of the maleic anhydride in the polypropylene vitrimer is in a range of 1wt% to 20 wt%.
[0042] At step 104, a second extrusion is performed with the functionalized polypropylene and a dynamic crosslinker to form the polypropylene vitrimer. The dynamic crosslinker is able to form a covalent adaptive network (CAN) through a transesterification exchange reaction.
[0043] Dynamic covalent bonds are reversible covalent bonds that can be formed or broken in response to external stimuli such as heat, pH, or UV irradiation. In the present disclosure, dynamic covalent bonds are formed during the second extrusion step (104) when the dynamic crosslinker reacts with the functionalized polypropylene under the influence of heat. Polymers containing dynamic covalent bonds are referred to as covalent adaptable networks (CANs). Vitrimers are a special class of polymers where associative CANs are formed, which means that existing covalent bonds are only broken when new ones are formed. The inventive dynamic crosslinkers form associative CANs through reversible exchange reactions of dynamic covalent bonds upon heating, to form the polypropylene vitrimers. The presence of CANs renders the polypropylene vitrimers recyclable or processable. For a thermoplastic polymer to be processable, the melt flow index (MFI) of the polymer should not be below 0.01 grams per 10 minutes.
[0044] Examples of the dynamic crosslinker comprise 4,4’-methylenebis(N, N-diglycidylaniline) (TGDDM), Bisphenol A diglycidyl ether (BADGE), or combinations thereof. On the second extrusion at step 104, epoxide group present in the dynamic crosslinker attaches to the maleated side chain of the functionalized polypropylene through ring-opening to form CAN resulting in the polypropylene vitrimer.
[0045] In one embodiment, a concentration of the dynamic crosslinker extruded with the functionalized polypropylene is in a range of 5 to 20% by weight. The concentration of the dynamic crosslinker may be decided based on a desired property of the resultant polypropylene vitrimer. For example, by varying a concentration of the dynamic crosslinker mechanical properties of the polypropylene vitrimer such as yield strength and/or elongation at yield may be varied.
[0046] As used herein, the term “yield strength” or “yield stress’ is defined as the minimum stress at which a solid will undergo permanent deformation or plastic flow without a significant increase in the load or external force. “Elongation at yield” is the deformation of plastic material at the yield point. The yield point corresponds to a point when an increase in strain is not marked by a significant increase in stress of the material. Elongation at yield is the ability of a plastic material to resist change of shape before it deforms irreversibly. Elongation at yield is the ratio between increased length and initial length at the yield point.
[0047] In some embodiments, the second extrusion is performed in presence of a catalyst. Examples of catalysts include triazobicyclodecene, triphenylphosphine, zinc acetylacetonate, or combinations thereof. In one embodiment, the catalyst is zinc acetylacetonate.
[0048] A concentration of the catalyst in the polypropylene vitrimer is in a range of 0.5wt% to 15 wt%.
[0049] The second extrusion like the first extrusion is performed in an extruder such as a single screw extruder, or a twin-screw extruder. The processing parameters of the extruder may be varied to facilitate reactive extrusion of the functionalized polypropylene and the dynamic crosslinker, as discussed with reference to the first extrusion. In one embodiment, the extruder is a twin-screw extruder that facilitates enhanced mixing between the functionalized polypropylene and the dynamic crosslinker when compared to a single-screw extruder. The second extrusion may be performed at a temperature corresponding to the melting temperature of the functionalized polypropylene. In some embodiments, the melting temperature is in a range of 160 to 200°C. In some embodiments, the residence time is in a range of 1 to 10 minutes, preferably 1 to 5 minutes. The second extrusion, in one embodiment, is performed in a twin-screw extruder at a temperature of 180°C at screw speeds in a range of 100 to 150 rpm and at a residence time in a range of 1 to 5 minutes.
[0050] The extruded polypropylene vitrimer obtained at step 104 may be immediately quenched in a water bath and pelletized. Such pellets can be used for subsequent molding, or shaping. The polypropylene vitrimers of the present disclosure may be shaped in the form of films, sheets, foams, particles, granules, beads, rods, plates, strips, stems, tubes, etc. via any process known to those skilled in the art. Examples of such processes include extrusion, casting, compression molding and the like.
[0051] In some embodiments, the first extrusion (step 102) and the second extrusion (step 104) may be performed in presence of additives commonly used during polymer processing such as UV stabilizers, antioxidants, heat stabilizers, and the like. Examples of additives include phenolic antioxidants, phosphite, pentaerythritol tetrakis [3- [3,5-di-tert-butyl-4-hydroxyphenyl]propionate] (Irganox® 1010), octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 3,3',3',5,5',5'-hexa-tert-butyl-a,a',a'-(mesitylene-2,4,6-triyl) tri-p-cresol, 4,4'-thio-bis (3-methyl-6 tertbutylphenol, 2,2'-Thiobis(6-tert-butyl-p-cresol), thiodiethylene bis[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionate]), octylthiomethyl)-o-cresol, distearylthiodipropionate, dilaurylthiodipropionate, pentaerythritol tetrakis(β-laurylthiopropionate), or combinations thereof.
[0052] According to embodiments of the present disclosure, a polypropylene vitrimer is provided. The polypropylene vitrimer comprises a functionalized polypropylene comprising a maleated sidechain. The functionalized polypropylene is formed by a reaction between a polypropylene (PP), a coagent, and a maleic anhydride. The coagent comprises trimethylolpropane diacrylate, N, N’-m-phenylene dimaleimide, 1,6-hexanediol methacrylate, zinc diacrylate, copolymer of butadiene and styrene, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, or combinations thereof. The polypropylene vitrimer further comprises a dynamic crosslinker covalently bound to the maleated sidechain. The dynamic crosslinker is able to form a covalent adaptive network through a transesterification exchange reaction to form the polypropylene vitrimer.
[0053] The formation of the polypropylene vitrimer, according to the method (100) illustrated in FIG. 1, can be better understood through a representative reaction scheme 200, as shown in FIG. 2, in accordance with the embodiments of the disclosure. The reaction scheme 200 illustrates one possible mechanism by which the reaction may proceed. It is understood that alternative pathways, intermediates, or mechanisms could also account for the observed results, and the invention is not limited to the mechanism depicted below.
[0054] In the representative reaction 200, as shown in FIG. 2, the initiator is dicumyl peroxide (DCP) and the coagent is triallyl cyanurate (TCA). Referring to FIG. 2, without wishing to be bound by any particular theory, the initiator (for example, dicumyl peroxide) decomposes thermally to form radicals that abstract hydrogen atoms from the polypropylene chain to generate polymer radicals. Typically, these polymer radicals undergo beta (β)-scission (cleavage), where the polymer chain breaks near the radical site leading to lower molecular weight polymer chains. The presence of coagent provides an alternative favorable pathway for the radical by shifting the reaction equilibrium toward crosslinking rather than cleavage. The proposed reaction proceeds through the formation of intermediate [I], where the coagent is crosslinked with the polypropylene chains. The intermediate [I] reacts with the maleic anhydride to form the intermediate product [II] corresponding to the functionalized polypropylene, obtained at step 102, of the method (100). The functionalized polypropylene of the present disclosure is functionalized with maleic anhydride and is also crosslinked to form the 3-dimensional network due to the presence of the coagent. The functionalized polypropylene (intermediate product [II]) reacts with the dynamic crosslinker, 4,4’-methylenebis(N, N-diglycidylaniline) (TGDDM) to form the polypropylene vitrimer ([III]).
[0055] The inventors have observed that the vitrimerization of polyethylene (PE) using the described method (100) did not yield a vitrimer. Polyethylene, under the outlined conditions, does not undergo vitrimerization. Without wishing to be bound by any particular theory, it is believed that in the case of polyethylene (PE), radicals formed (polyethylene radicals) during peroxide decomposition have a high tendency to form a thermoset. Without wishing to be bound by any particular theory, this may be because, at typical crosslinking temperatures, PE chains are often crystalline or semi-crystalline, which limits the mobility of polymer chains and the diffusion of coagents into the reaction zones. Hence, the methods of preparing PP vitrimer through radical initiators are not effective in preparing PE vitrimers.
[0056] However, an unexpected and significant advantage of the invention was discovered, when a blend of polypropylene (PP) vitrimer and polyethylene (PE) was utilized, the resulting material exhibited vitrimer-like properties. This approach leverages the vitrimer properties of the PP component to advantageously create a blend with enhanced recyclability and dynamic cross-linking capabilities, addressing the limitations of pure polyethylene and expanding the application potential of the invention.
[0057] In some embodiments, a blend of the polypropylene vitrimer is provided. The blend comprises a polyolefin. The polyolefin is a virgin polyolefin, a post-consumer recycled (PCR) polyolefin, a post-industrial recycled (PIR) polyolefin, or combinations thereof. The polyolefin comprises polyethylene, polypropylene, ethylene-propylene copolymer, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene (BOPE), multilayered polymer, or combinations thereof. As used herein, the term “multilayered polymer” refers to a composite material made up of two or more distinct polymer layers, each serving a specific functional purpose such as the ones used in packaging. The polymer of the “multilayered polymer” comprises polyolefin.
[0058] In some embodiments, the polyolefin comprises polyethylene, HDPE, LDPE, LLDPE, or combinations thereof, wherein the polyolefin is recyclable as part of the blend. A weight percent of polyolefin in the blend is in a range of 10 wt% to 90wt%.
[0059] It is an objective of the present disclosure to provide a method of processing a polyolefin. Processing the polyolefin, in one instance, relates to recycling the polyolefin. In one embodiment, recycling the polyolefins comprises extruding the polyolefin (for example, polyethylene) with a polypropylene vitrimer through an extruder at a temperature in a range of 160 to 200°C. In some embodiments, the processing of the polyolefin relates to forming the polypropylene vitrimer using the method (100) including polypropylene along with other polyolefins.
[0060] The polyolefin comprises polyethylene, polypropylene, ethylene-propylene copolymer, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene (BOPE), or combinations thereof. Further, the polyolefin is virgin polyolefin, PIR polyolefin, PCR polyolefin, or combinations thereof.
[0061] Processing or recycling the polyolefin may enhance a cycle life of the polyolefin. In another embodiment, processing the polyolefin may result in a polyolefin product that may have similar or superior mechanical properties to an initial polyolefin. The mechanical properties may be characterized in terms of yield strength and/or elongation at yield.
[0062] The inventive method for poyolefin processing has a recovery rate of greater than 90% when the polyolefin is post-consumer recycled polyolefin. In another embodiment, the inventive method for polyolefin processing has a recovery rate of greater than 94% when the polyolefin is post-consumer recycled polyolefin. As used herein, the term “recovery rate” corresponds to a percentage of an amount of product recovered to an amount of reactants. With such a high recovery rate it is possible to reprocess PCR polyolefins without adding virgin polyolefin to establish a closed-loop recycling process.
[0001] The polypropylene vitrimers of the present disclosure even after 3 cycles of re-processing, or cycle life has a MFI value of 0.01 grams per 10 minutes, or above 0.01 grams per 10 minutes.
[0063] The polypropylene vitrimers of the present disclosure exhibit mechanical strength greater than the mechanical strength of PCR or PIR polypropylene. When the blend comprising the polypropylene vitrimer and polyolefin is recycled, it results in a polymer product that is superior in mechanical strength compared to the mechanical strength of individual polypropylene vitrimer or polyolefin. Thus, the inventive process and the polypropylene vitrimers produced thereof result in an upcycling of PCR or PIR polypropylene, or blends comprising polyolefins. The term “upcycling”, as used herein, refers to obtaining a polymer product that is on par or superior in mechanical properties of a polymer it is derived from. The reprocessability of the polypropylene vitrimers, or blends overcomes the challenges of recycling TPO plastic waste, and enables transformation of TPO waste into mechanically stronger PCR or PIR polypropylene, or blends while retaining other properties of polyolefins that make them versatile. The polypropylene vitrimers may be reprocessed multiple times without degradation of their mechanical properties when compared to TPOs not containing the inventive polypropylene vitrimers. It is a particular advantage of the present disclosure, irrespective of the additives present, such as in multilayer packaging (multilayered polymer); PCR or PIR polypropylene, or blends may be reprocessed using the disclosed method to result in an upcycled PCR or PIR polypropylene, or blends.
[0064] The polypropylene vitrimers, or blends of PP vitrimer with PE may be recycled along with virgin PP or virgin PE, in one embodiment. As the polypropylene vitrimer of the present disclosure exhibit mechanical strength superior to polypropylene (for example, PCR or PIR), they may be used without blending with virgin polypropylene, which otherwise might be required to compensate for mechanical property loss. This contributes further to a closed-loop plastics economy minimizing dependence on virgin polypropylene and minimizing plastic waste generation. The present disclosure thus provides a sustainable solution to address TPO plastic waste and reduce the environmental impact of plastic waste and dependence on fossil fuels.
[0065] In some embodiments, an article comprising the polypropylene vitrimers of the present disclosure is provided. The article may be formed by molding, blow molding, injection molding, filament winding, continuous molding or film-insert molding, infusion, pultrusion, RTM (resin transfer molding), RIM (reaction-injection molding), 3D printing, or any other method known to those skilled in the art.

EXAMPLES
[0066] The present disclosure will now be described in greater detail by the
following non-limiting examples. It is understood that one skill in the art will envision additional embodiments consistent with the disclosure provided herein.
EXAMPLE 1
Preparation of functionalized polypropylene
[0067] PCR polypropylene (PP) was collected from used bottles, containers, and paint buckets. It was washed with an aqueous solution of detergent followed by repeated washing in cold water and was dried in a vacuum oven at 70°C for six hours. The dried samples were chopped into small pieces to obtain PCR PP samples.
[0068] Reactive extrusion of the PCR PP was performed along with maleic anhydride (MA), dicumyl peroxide (DCP), Irganox® 1010 (antioxidant), and triallyl cyanurate (TAC) by extruding through DSM Xplore batch twin-screw microcompounder with a 15 cm3 capacity at screw speed of 150 rpm at 180°C to obtain the functionalized PP (funct-PP). The funct-PP had a composition of 84 weight percent (wt%) PCR PP, 10 wt% MA, 5 wt% TAC, 0.5 wt% DCP, and 0.5 wt % Irganox® 1010.

EXAMPLE 2
Preparation of polypropylene vitrimer
[0069] The functionalized polypropylene (funct-PP) from Example 1, a dynamic crosslinker 4,4’-methylenebis(N, N-diglycidylaniline) (TGDDM), and zinc acetylacetonate (catalyst) were extruded through DSM Xplore batch twin-screw microcompounder with a 15 cm3 capacity. The extrusion was performed at a temperature of 180 °C with a screw speed of 150 rpm for 2 minutes in the presence of zinc acetylacetonate (catalyst) to form the polypropylene vitrimer (PP vitrimer). The PP-vitrimer had a composition of 84 wt% functionalized polypropylene, 15 wt% TGDDM, and 1 wt% catalyst.
EXAMPLE 3
Preparation of a blend of polyethylene with PP-vitrimer (PE-PP-vitrimer)
[0070] The polypropylene vitrimer (PP-vitrimer) from Example 2, dynamic crosslinker 4,4’-methylenebis(N, N-diglycidylaniline) (TGDDM), zinc acetylacetonate (catalyst) and polyethylene (PE) were extruded through DSM Xplore batch twin-screw microcompounder with a 15 cm3 capacity. PCR polyethylene was obtained from used milk pouches. The milk pouches were washed with an aqueous solution of detergent followed by repeated washing in cold water and were dried in a vacuum oven at 70°C for six hours. The dried samples were chopped into small pieces to obtain polyethylene (PE) samples.
[0071] The extrusion was performed at a temperature of 180 °C with a screw speed of 150 rpm for 2 minutes to obtain PP-vitrimer and PE blend (labelled as PE-PP-vitrimer) sample. A standard sample containing a blend of PE and PP were extruded (labelled as PE-PP). The compositions of the two samples are shown in Table 1.

Sample Name PE
(wt %) PP (wt%) PP-vitrimer
(wt%)
PE-PP 50 50 -
PE-PP-vitrimer 50 - 50

Table 1
Mechanical testing
[0072] The samples from Example 3 were injection molded at 3 psi pressure and 180 °C for 5 minutes to prepare samples for mechanical testing.
[0073] According to ASTM D638 (type V) stress-strain properties of the samples given in the Tables 1 were measured using a Tinius Olsen 1ST Universal Testing Machine at room temperature using Test Parameters of Load Cell: 5 kN; Preload Force: 0.1 N; Cross Head Speed: 50mm/minutes; Gauge Length: 15mm; Number of Samples: 5 per batches for consistency; and Sample Dimension: 50 mm length x 4.1 mm width x 2.1 mm thickness.
[0074] The samples were placed between clamps of the Universal Testing Machine - Tensile Testing Module such that the edges of the samples were parallel to the direction of the load. The grips were tightened to hold the samples securely within the jig. The test samples were then pulled apart at a tensile speed of 50 mm/minute until they broke. Table 2 provides the mechanical testing data, where PP corresponds to PCR polypropylene, PE corresponds to PCR polyethylene, PE-PP corresponds to blends of PCR-PP and PCR-PE, and PE-PP-vitrimer corresponds to the blend obtained from Example 3.

Sample Yield Strength (MPa) Elongation at yield (%)
PP
28 ± 0.35 16 ± 0.37

PE
14 ± 0.36 34 ± 0.24
PE-PP 19± 0.47 22± 0.51
PE-PP-vitrimer 32 ± 0.22 25 ± 0.94

Table 2
[0075] FIG. 3 is a bar chart 300 displaying the yield strength and elongation at yield of samples given in Table 2. The blend of polypropylene-vitrimer and polyethylene (PE-PP-vitrimer) sample exhibited superior mechanical properties in terms of yield strength compared to PCR PP and PCR PE. The PE-PP-vitrimer had an elongation at yield of 25% which was within acceptable values.
Recyclability test
[0076] For recyclability tests, the blend of polypropylene-vitrimer and polyethylene sample (from Example 3) was cut into small pieces, re-extruded and injection molded multiple times corresponding to multiple recycling cycles. Recyclability tests up to 3 cycles were performed, where R1, R2, and R3, correspond to recycling runs 1, 2, and 3, respectively, and the results are shown in Table 3.
[0077] There were no significant changes in yield strength and elongation at yield even after 3 reprocessing cycles. This can be attributed to dynamic covalent adaptable network (CAN) of the polypropylene vitrimers which rearrange at elevated temperatures. This confirmed that the polypropylene vitrimer of the present disclosure can be reprocessed more than 3 times without any degradation in mechanical properties. Moreover, the MFI values of the polypropylene vitrimers were 0.01 grams per 10 minutes, or more than 0.01 grams per 10 minutes at the end of 3 cycles.
Sample name Yield Strength (MPa) Elongation at yield (%)
R1 32 ± 0.22 25 ± 0.94
R3 31 ± 0.56 22 ± 0.77

Table 3
Gel content studies
[0078] Gel content (gel fraction) studies of PCR polyethylene (PE), PCR polypropylene (PP), blend of PCR-PP and PCR-PE, and blend of PE-PP-vitrimer of Example 3 were performed. The samples were weighed to obtain an initial weight (W0). The weighed samples were immersed in xylene and heated to a temperature of 115 °C for 24 hours under reflux conditions. After 24 hours, the samples were removed from xylene and dried in a vacuum oven at 100 °C overnight. The final weights (𝑊1) of the samples were recorded after drying, Gel content is given as a percentage of the final weight of the sample to the initial weight (𝐺𝑒𝑙 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛=𝑊1/𝑊0 ×100).
[0079] FIG. 4 is a bar chart 400 displaying the gel content of the above polymer samples. Gel content represents the insoluble fraction of the polymer that remains intact when exposed to solvents, indicating the portion of the material that is crosslinked. Higher gel content suggests a greater degree of crosslinking, which is essential for forming the network structure required in vitrimers. The blend of PE-PP-vitrimer exhibited maximum gel content confirming vitrimer formation.
[0080] It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described, but can be practiced with modification and alteration within the scope of the appended claims. , Claims:CLAIMS
We Claim:
1. A polypropylene vitrimer comprising:
a functionalized polypropylene comprising a maleated sidechain, wherein the functionalized polypropylene is formed by a reaction between a polypropylene (PP), a coagent, and a maleic anhydride, and wherein the coagent comprises trimethylolpropane diacrylate, N, N’-m-phenylene dimaleimide, 1,6-hexanediol methacrylate, zinc diacrylate, copolymer of butadiene and styrene, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, or combinations thereof; and
a dynamic crosslinker covalently bound to the maleated sidechain, wherein the dynamic crosslinker is able to form a covalent adaptive network through a transesterification exchange reaction to form the polypropylene vitrimer.
2. The polypropylene vitrimer as claimed in claim 1, wherein the dynamic crosslinker comprises 4,4’-methylenebis(N, N-diglycidylaniline) (TGDDM), Bisphenol A diglycidyl ether (BADGE), or combinations thereof.
3. The polypropylene vitrimer as claimed in claim 1, wherein the polypropylene comprises polypropylene, ethylene-propylene copolymer, biaxially oriented polypropylene (BOPP), or combinations thereof.
4. The polypropylene vitrimer as claimed in claim 1, wherein the polypropylene comprises virgin polypropylene, post-consumer recycled (PCR) PP, post-industrial recycled (PIR) PP, or combinations thereof.
5. The polypropylene vitrimer as claimed in claim 1, wherein the reaction between the polypropylene (PP), the coagent, and the maleic anhydride is initiated in presence of an initiator, and wherein the initiator comprises benzoyl peroxide, lauryl peroxide, dicumyl peroxide (DCP), or combinations thereof.
6. The polypropylene vitrimer as claimed in claim 1, wherein a concentration of the coagent in the polypropylene vitrimer is in a range of 1 wt% to 15 wt%.
7. The polypropylene vitrimer as claimed in claim 1, wherein a concentration of the dynamic crosslinker in the polypropylene vitrimer is in a range of 5 wt% to 20 wt%.
8. The polypropylene vitrimer as claimed in claim 1, wherein a yield strength of the polypropylene vitrimer is greater than a yield strength of the polypropylene it is formed from.
9. A blend comprising the polypropylene vitrimer as claimed in claim 1, wherein the blend comprises a polyolefin, and wherein the polyolefin is a virgin polyolefin, a post-consumer recycled (PCR) polyolefin, a post-industrial recycled (PIR) polyolefin, or combinations thereof.
10. The blend as claimed in claim 9, wherein the polyolefin comprises polyethylene, polypropylene, ethylene-propylene copolymer, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene (BOPE), multilayered polymer, or combinations thereof.
11. The blend as claimed in claim 10, wherein the polyolefin comprises polyethylene, HDPE, LDPE, LLDPE, or combinations thereof, wherein the polyolefin is recyclable as part of the blend, and wherein a weight percent of polyolefin in the blend is in a range of 10 wt% to 90 wt%.
12. A method (100) of preparing a polypropylene vitrimer comprising:
performing a first extrusion of a polypropylene, a coagent, and a maleic anhydride (102) in an extruder at a temperature in a range of 160 to 200 °C for a residence time in a range of 1 minute to 5 minutes to form a functionalized polypropylene comprising a maleated sidechain, wherein the coagent comprises trimethylolpropane diacrylate, N, N’-m-phenylene dimaleimide, 1,6-hexanediol methacrylate, zinc diacrylate, copolymer of butadiene and styrene, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, or combinations thereof; and
performing a second extrusion of the functionalized polypropylene with a dynamic crosslinker (104) in the extruder at a temperature in a range of 160 to 200 °C for a residence time in a range of 1 minute to 5 minutes to form the polypropylene vitrimer, wherein the dynamic crosslinker is bound to the functionalized polypropylene through the maleated sidechain and able to form a covalent adaptive network through a transesterification exchange reaction.
13. The method as claimed in claim 12, wherein the first extrusion (102) is performed in presence of an initiator, and wherein the initiator comprises benzoyl peroxide, lauryl peroxide, dicumyl peroxide (DCP), or combinations thereof.
14. The method as claimed in claim 12, wherein the second extrusion (104) is performed in presence of a catalyst, and wherein the catalyst comprises triazobicyclodecene, triphenylphosphine, zinc acetylacetonate, or combinations thereof.
15. The method as claimed in claim 12, wherein the polypropylene vitrimer has a cycle life of more than 3 cycles with a melt flow index of 0.1 gram per 10 minutes (g/10min), or more than 0.1 (g/10min).
16. The method as claimed in claim 12, wherein the first extrusion (102), or the second extrusion (104), or both is performed along with a polyolefin, and wherein the polyolefin comprises polyethylene, polypropylene, ethylene-propylene copolymer, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), biaxially oriented polypropylene (BOPP), biaxially oriented polyethylene (BOPE), multilayered polymer, or combinations thereof.
17. An article comprising the polypropylene vitrimer as claimed in any of the claims 1-11.