Description:BACKGROUND

FIELD OF THE DISCLOSURE

Various embodiments of the disclosure relate generally to a method of processing polyolefins to increase a cycle life of the polyolefins. More specifically, various embodiments of the disclosure relate to a two-component composition for increasing the cycle life of the polyolefins and polyolefin-vitrimers formed thereof.

DESCRIPTION OF THE RELATED ART

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 plastics has led to significant environmental concerns, primarily due to their resistance to degradation.
Mechanical recycling is one of the most common methods of recycling thermoplastic polymers where waste plastics are collected, sorted, cleaned, shredded, and melted to form new plastic products. However, each recycling cycle may degrade mechanical properties of TPOs thus resulting in lower-quality recycled materials. Further, mechanical recycling is often limited to specific types of polyolefins and may not be suitable for complex or multi-layered plastics.
Chemical recycling has the potential to produce high-quality raw materials by breaking down the polymer into monomers or other useful chemicals. The chemical recycling process may involve solvents such as benzene for solubilization or reprecipitation thus making them less environmentally friendly. Methods like pyrolysis or catalyst-assisted pyrolysis may require significant amounts of energy thereby increasing the cost of recycling.
Energy recovery by incinerating plastic waste is another method to tackle plastic waste but is less favored when compared to the other two methods.
In a drive towards a circular plastics economy, recycling plastic waste is important to reduce dependence on fossil fuels employed in producing virgin plastic. Most recycling methods focus on open-loop recycling, where the final plastic product is different from the initial, generally of inferior quality, also called downcycling. Furthermore, an open-loop process would still require an amount of virgin polymer to meet the performance requirements of the final plastic product. In contrast, a closed-loop recycling process reduces or eliminates the use of virgin polymer since initial and final plastic product properties would be similar, resulting in final plastic products of at least the same quality, in an upcycling method.
"Post-consumer recycled (PCR)" plastics as the name indicates are after-use plastic waste generated by consumers. Recycling PCR plastic waste has its challenges. The PCR plastics vary in composition due to the different proportions of polymers and additives used by various plastic product producers. Further, PCR plastics must be sufficiently cleaned and of controlled moisture content to produce new products of acceptable quality. PCR plastics based on TPOs have low density (<1 kg/m3) and are known to degrade with usage, thus recycling TPOs is much more challenging due to a decrease in molecular weight (Mw) and molecular weight distribution (MWD) that may affect mechanical properties of the TPOs.
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

According to embodiments of the present disclosure, a polyolefin-vitrimer is provided. The polyolefin-vitrimer comprises a maleated polyolefin. The maleated polyolefin comprises a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin at a grafting yield in a range of 0.3 to 3.0. The polyolefin-vitrimer further comprises a dynamic crosslinker. The dynamic crosslinker is linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network. The dynamic crosslinker comprises 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE). A concentration of the dynamic crosslinker in the polyolefin-vitrimer is in a range of 1 to 30% by weight.
In some embodiments, the polyolefin of the polyolefin-vitrimer 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.
In some embodiments, the polyolefin of the polyolefin-vitrimer is a post-consumer recycled (PCR) polyolefin.
According to embodiments of the present disclosure, a polyolefin-vitrimer is provided. The polyolefin-vitrimer comprises a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin. The polyolefin-vitrimer further comprises a dynamic crosslinker, wherein the dynamic crosslinker is linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer. The dynamic crosslinker is 4-aminophenyl disulfide (APD).
In another embodiment of the present disclosure, a polyolefin-vitrimer is provided. The polyolefin-vitrimer comprises a maleated polyolefin comprising co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin. The polyolefin-vitrimer further comprises a dynamic crosslinker, wherein the dynamic crosslinker is linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer. The dynamic crosslinker is bis (2-hydroxyethyl) terephthalate (BHET).
In yet another embodiment of the present disclosure, a polyolefin-vitrimer is provided. The polyolefin-vitrimer comprises a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin. The polyolefin-vitrimer further comprises a dynamic crosslinker, wherein the dynamic crosslinker is linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer. The dynamic crosslinker is 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA).
In yet another embodiment of the present disclosure, a polyolefin-vitrimer is provided. The polyolefin-vitrimer comprises a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin. The polyolefin-vitrimer further comprises a dynamic crosslinker, wherein the dynamic crosslinker is linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer. The dynamic crosslinker is 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
In another embodiment of the present disclosure, a two-component composition for increasing a cycle life of a polyolefin is provided. The two-component composition comprises a first component and a second component. The first component comprises a mixture of maleic anhydride and a co-grafting agent, where the first component on extrusion with the polyolefin forms a maleated polyolefin. The maleated polyolefin comprises a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of the polyolefin at a grafting yield in a range of 0.3 to 3.0. The second component comprises a dynamic crosslinker, where the second component on extrusion with the maleated polyolefin forms a polyolefin-vitrimer having enhanced cycle life than the polyolefin. The dynamic crosslinker forms a covalent adaptive network. The dynamic crosslinker comprises 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
In yet another embodiment of the present disclosure, a method of processing a polyolefin to increase a cycle life of the polyolefin is provided. The method comprises performing a first extrusion of the polyolefin with a first component at a temperature in a range of 160 to 200 °C to form a maleated polyolefin. The first component comprises a mixture of maleic anhydride and a co-grafting agent at a ratio in a range of 1:1 to 1:1.2. The method further comprises performing a second extrusion of the maleated polyolefin with a dynamic crosslinker at a temperature in a range of 160 to 200 °C to form a polyolefin-vitrmer having enhanced cycle life than the polyolefin. The dynamic crosslinker forms a covalent adaptive network to form the polyolefin-vitrimer. The dynamic crosslinker comprises 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart that illustrates a method of processing a polyolefin, in accordance with an exemplary embodiment of the disclosure;
FIG. 2a is a reaction scheme of reaction between maleated polypropylene and 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE);
FIG. 2b is a reaction scheme of reaction between maleated polypropylene and bis (2-hydroxyethyl) terephthalate (BHET);
FIG. 2c is a reaction scheme of reaction between maleated polypropylene and 4-aminophenyl disulfide (APD);
FIG. 2d is a reaction scheme of reaction between maleated polypropylene and 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA); and
FIG. 3 is a bar diagram from recyclability tests indicating yield strengths of various samples.
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

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.
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.
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.
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.
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.
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.
It is an objective of the present disclosure to provide a method of processing a polyolefin. Processing the polyolefin, in one instance 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.
FIG. 1 is a flow chart 100 that illustrates a method of processing a polyolefin through exemplary steps 102 through 104, according to embodiments of the present disclosure. At step 102, a first extrusion of the polyolefin with a first component is performed.
Example 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.
In one embodiment, the polyolefin is post-consumer recycled (PCR) polyolefin. In another embodiment, the polyolefin is virgin polyolefin. It is preferred that the polyolefin be PCR polyolefin as it enhances a cycle life thus rendering the PCR polyolefin recyclable more than once without adverse mechanical property degradation. The virgin polyolefin may be processed using the inventive method to enhance cycle life however it may critically affect certain mechanical properties such as the yield strength.
As used herein, the term “cycle life” refers to a number of times a polymer may be recycled before its melt flow index drops below 0.01 grams per 10 minutes. In one embodiment, the number of times the polyolefin may be recycled is in a range of 1 to 3 times, preferably more than 3 to 5 times, and most preferably more than 5 times.
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.
The polyolefin may be in the form of film, granules, flakes, powders, pellets, or combinations thereof. When the polyolefin is PCR polyolefin, the polyolefin may be a single layered structure, or a multilayered structure such as BOPP and BOPE. For example, the PCR polyolefins may be derived from multilayer (ML) packaging that has to be recycled. The single layered structure, or the multilayered structure may be made of same and/or different type of polyolefin material of differing compositions.
In embodiments where the polyolefin is PCR polyolefin, the PCR polyolefin is washed to remove any contaminants or residues and dried to remove moisture before processing. In one embodiment, the PCR polyolefin 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 polyolefin is washed and dried, it is cut into smaller pieces for the first extrusion.
The first component comprises a mixture of maleic anhydride and a co-grafting agent. In one embodiment, the co-grafting agent is added to maleic anhydride to form the first component. In another embodiment, maleic anhydride is added to the co-grafting agent to form the first component. The co-grafting agent may be chosen to minimize a surface energy mismatch between the maleic anhydride and the polyolefin whereby an extent of grafting of the maleic anhydride to polyolefin may be enhanced. Examples of co-grafting agents include styrene, alpha-methyl styrene, toluene, divinyl benzene, and dimethyl maleate. In some embodiments, the co-grafting agent is styrene.
A ratio of maleic anhydride to the co-grafting agent in the first component is in a range of 1:1 to 1:1.2. In one embodiment, the ratio of maleic anhydride to the co-grafting agent in the first component is 1:1.
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 polyolefin and the first component by optimizing one or more of melting of the polyolefin, homogeneous mixing between the polyolefin and the first component, and efficient reaction between the polyolefin and the first component. 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 first component and the polyolefin when compared to a single-screw extruder. The first extrusion may be performed at a temperature corresponding to the melting temperature of the polyolefin. 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 100-150 rotations per minute (rpm) and at a residence time in a range of 1 to 5 minutes.
The first extrusion produces a maleated polyolefin where the co-grafting agent-maleic-based side chain is grafted onto C-C backbone or linked to C-C backbone of the polyolefin. As used herein, the term “maleated polyolefin” refers to a polyolefin having the co-grafting agent-maleic-based side chain grafted onto C-C backbone of the polyolefin. The grafting makes the otherwise inert C-C backbone of polyolefin polar thus making it amenable for further functionalization and derivatization. A grafting yield of the maleated polyolefin may be followed to understand the extent of grafting on the C-C backbone. In some embodiments, the grafting yield is in a range of 0.3 to 3.0. In one embodiment, the grafting yield is in a range of 1.0 to 2.8. In another embodiment, the grafting yield is in a range of 2.0 to 2.4. The grafting yield may be obtained from an intensity of a characteristic infrared (IR) band of maleic moiety to an intensity of a characteristic band of the polyolefin from an IR spectrum of the maleated polyolefin. In one embodiment, 1780 cm-1 band corresponding to carbonyl symmetric stretching of maleic moiety is considered. When the maleated polyolefin is maleated polypropylene, 1167 cm-1 band corresponding to -CH3 group rocking of polypropylene is considered. The intensity of the IR band is proportional to an area under the curve of a given band in the IR spectrum. In one embodiment, the grafting yield of a maleated polypropylene may be calculated from a ratio of an area under the curve of the 1780 cm-1 band to an area under the curve of the 1167 cm-1 band from the IR spectrum of the maleated polypropylene. In the case of other polyolefins, characteristic IR band for that polyolefin may be chosen to arrive at the grafting yield. For example, in the case of polyethylene the band at 720 cm-1 may be considered to calculate the grafting yield.
A concentration of the first component extruded with the polyolefin is in a range of 1 to 20% by weight. When the polyolefin is PCR polyolefin including maleated polyolefin, the amount of the first component to be added may be estimated from the grafting yield and based on the grafting yield, a concentration of the first component to be added to the PCR polyolefin for the first extrusion may be arrived upon.
At step 104, a second extrusion is performed of the maleated polyolefin with a dynamic crosslinker to form a polyolefin-vitrimer having enhanced cycle life than the polyolefin. The dynamic crosslinker forms covalent adaptive networks based on disulphide exchange, transesterification, imine exchange, or combinations thereof to form a polyolefin-vitrimer on the second extrusion.
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 maleated polyolefin 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 maleated polyolefin 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 maleated polyolefin. 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 100-150 rpm and at a residence time in a range of 1 to 5 minutes.
The dynamic crosslinkers include 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
The dynamic crosslinkers 4-aminophenyl disulfide (APD) and bis (2-hydroxyethyl) terephthalate (BHET) have inherent exchangeable bonds which on reaction with maleated polyolefin can form dynamic covalent bonds to form the polyolefin-vitrimers. While the dynamic crosslinkers 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA), and 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE) have induced exchangeable bonds which on reaction with maleated polyolefin can form dynamic covalent bonds.
Dynamic covalent bonds are reversible covalent bonds formed under external stimuli such as heat, pH, and UV irradiation. In the present disclosure, dynamic covalent bonds are formed in the presence of heat during the second extrusion of dynamic crosslinkers with the maleated polyolefin, at step 104. Polymers containing dynamic covalent bonds are called 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 polyolefin-vitrimers. The presence of CANs renders the polyolefin-vitrimers recyclable or reprocessable. 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.
The dynamic crosslinker 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE) has the structure [I].

[I]

During the second extrusion at step 104, the hydroxyl groups of dynamic crosslinker PMDE open the anhydride rings of the maleated polyolefin to generate ester bonds which undergo trans-esterification exchange reaction with dynamic imine bonds of PMDE molecule thus forming polyolefin-vitrimers having dual CANs of transesterification and imine exchange. FIG. 2a is a reaction scheme of reaction between maleated polypropylene and PMDE to form polypropylene-PMDE vitrimer.
The dynamic crosslinker bis (2-hydroxyethyl) terephthalate (BHET) has the structure [II].

[II]
On second extrusion at step 104, BHET reacts with anhydride ring of the maleated polyolefin resulting in ring-opening thus allowing for transesterification bond exchange. FIG. 2b depicts a reaction scheme for formation of polypropylene-BHET vitrimer from maleated polypropylene and the dynamic crosslinker bis (2-hydroxyethyl) terephthalate (BHET).
The dynamic crosslinker 4-aminophenyl disulfide (APD) has the structure [III].

[III]
The dynamic crosslinker 4-aminophenyl disulfide (APD) has inherent disulfide exchange bonds. On second extrusion at step 104, APD gets attached to the anhydride ring through nitrogen atoms of APD forming a single CAN to form polyolefin-APD vitrimer. FIG. 2c depicts a reaction scheme for formation of polypropylene-APD vitrimer from maleated polypropylene and the dynamic crosslinker 4-aminophenyl disulfide (APD).
The dynamic crosslinker 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA) has the structure [IV].

[IV]
FIG. 2d is a reaction scheme for formation of polypropylene-MDGA vitrimer. On second extrusion at step 104, epoxide group of MDGA attaches to the maleic moiety through ring-opening to form single CAN resulting in the polypropylene-MDGA vitrimer.
In FIGs. 2a to 2d, the maleated polypropylene is prepared by reactive extrusion of polypropylene, maleic anhydride and styrene, as discussed with respect to step 102 of FIG. 1. Here, “m” and “n” correspond to repeating units of styrene and maleic anhydride of the maleated polypropylene. The reactions depicted in FIGs. 2a to 2d correspond to the second extrusion of step 104 and may be carried out in a twin-screw extruder at a temperature of 180 °C, a screw speed of 150 rpm, and a residence time of 2 minutes (min). In FIGs. 2a, 2b, and 2d, the second extrusion is carried out in the presence of catalyst zinc acetylacetonate (Zn(OAc)2). As shown in FIGs. 2a to 2d, the dynamic crosslinkers are linked to different maleated polypropylene chains to form polypropylene-vitrimers.
In one embodiment, a concentration of the dynamic crosslinker extruded with the maleated polyolefin is in a range of 1 to 30% by weight. The concentration of the dynamic crosslinker may be decided based on the crosslinking desired and also on a desired property of the resultant polyolefin-vitrimer. For example, by varying the concentration of the dynamic crosslinker mechanical properties of the polyolefin-vitrimer such as yield strength, and/or elongation at yield may be varied.
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.
In some embodiments, the first extrusion (step 102) and the second extrusion (step 104) may be performed in presence of one or more of free-radical initiators, or catalysts. In one embodiment, the first extrusion and/or the second extrusion is performed in presence of free radical initiators such as peroxides. Examples of peroxides include, but are not limited to benzoyl peroxide, lauryl peroxide, or dicumyl peroxide (DCP). Examples of catalysts include, but are not limited to, triazobicyclodecene, triphenylphosphine, or zinc acetylacetonate. In one embodiment, the catalyst is zinc acetylacetonate (Zn(OAc)2).
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, heat stabilizers and the like. Example such stabilizers include phenolic antioxidant, phosphite, pentaerythritol tetrakis [3- [3,5-di-tert-butyl-4-hydroxyphenyl]propionate], 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 or combinations thereof.
The extruded polyolefin-vitrimer after the second extrusion at step 104 may be immediately quenched in a water bath and pelletized. Such pellets can be used for subsequent molding, or shaping. The polyolefin-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. Example of such processes include extrusion, casting, compression molding and the like.
The inventive method for polyolefin 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.
The inventive polyolefin-vitrimer comprises the maleated polyolefin and the dynamic crosslinker. The maleated polyolefin comprises the co-grafting agent-maleic-based sidechain linked to the carbon-carbon (C-C) backbone of the polyolefin. The maleated polyolefin has a grafting yield in a range of 0.3 to 3.0. The dynamic crosslinker is linked to the co-grafting agent-maleic-based sidechain and forms the covalent adaptive network based on disulphide exchange, transesterification, imine exchange, or combinations thereof to form the polyolefin-vitrimer.
The polyolefin-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.
The polyolefin-vitrimers of the present disclosure exhibit mechanical strength greater than a mechanical strength of PCR polyolefin from which they are derived. The term “mechanical strength” as used herein, refers to one of yield stress, an elongation at yield, or combinations thereof. In one embodiment, a yield strength of the polyolefin-vitrimer is greater than that of the PCR polyolefin. The inventive process and the polyolefin-vitrimers produced thereof result in an upcycling of PCR polyolefins. The reprocessability of the polyolefin-vitrimers overcomes the challenges of recycling TPO plastic waste, and enables transformation of TPO waste into mechanically stronger PCR polyolefins while retaining other properties of polyolefins that make them versatile. The polyolefin-vitrimers may be reprocessed multiple times without degradation of their mechanical properties when compared to TPOs not containing the inventive polyolefin-vitrimers. It is a particular advantage of the present disclosure, irrespective of the additives present in PCR polyolefin, the PCR polyolefin may be reprocessed using the disclosed method and the inventive crosslinkers to result in an upcycled PCR polyolefin.
The polyolefin-vitrimers may be recycled along with virgin polyolefin, in one embodiment. As the polyolefin-vitrimers of the present disclosure exhibit mechanical strength superior to PCR polyolefin, they may be used without blending with virgin polyolefin, which otherwise might be required to compensate for mechanical property loss. This contributes further to a closed-loop plastics economy minimizing dependence on virgin polyolefins 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.
According to embodiments of the present disclosure, a two-component composition for processing a polyolefin is provided. The polyolefins may be processed according to the method, as described with reference to FIG. 1. The two-component composition comprises the first component and the second component. The first component comprises the mixture of maleic anhydride, and the co-grafting agent, where the first component on extrusion with the polyolefin forms the maleated polyolefin. The maleated polyolefin comprises the co-grafting agent-maleic-based sidechain grafted to the carbon-carbon (C-C) backbone of the polyolefin at the grafting yield in a range of 0.3 to 3.0. The second component comprises the dynamic crosslinker, where the second component on extrusion with the maleated polyolefin forms the polyolefin-vitrimer having enhanced cycle life than the polyolefin. The dynamic crosslinker forms the covalent adaptive network based on disulfide exchange, transesterification, imine exchange, or combinations thereof. The dynamic crosslinkers include 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
The first component, or the second component, or both may further comprise a catalyst, free-radical initiators, a stabilizer or combinations thereof.
The polyolefin-vitrimers, the first component, or the second component, may further comprise additives such as carbon fibers, silica, glass fibers, UV additives, flame retardant additives, antimicrobial additives, lubricant additives, pigments, color resins, and other additives to enhance the mechanical properties and improve certain aspects of performance and processability of the resulting polymer.
In some embodiments, an article is formed using the polyolefin-vitrimers of the present disclosure. The articles 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
EXAMPLE 1
Preparation of maleic anhydride grafted polypropylene
PCR polypropylene (PP) is 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.
Reactive extrusion of the polyolefin samples, PCR-PP, multilayered PCR BOPP (ML PP) was performed with maleic anhydride (10 wt%) by extruding through DSM Xplore batch twin-screw microcompounder with a 15 cm3 capacity at screw speed of 150 rpm at 180°C in presence of dicumyl peroxide (0.5 wt%) to obtain maleic anhydride grafted PCR PP (m-PCR PP) and maleic anhydride grafted BOPP (m-PCR ML PP), respectively. Table 1 shows the composition of maleic anhydride grafted PCR-PP (m-PCR PP).
Further, m-PCR PP was re-precipitated in acetone followed by dissolving in hot xylene to to remove any unreacted maleic anhydride. IR spectrum was recorded to calculate the grafting yield. The grafting yield A1780/A1167 was found to be 0.38, where A1780 and A1167 correspond to areas under the curve of bands 1780 cm-1 and 1167 cm-1 from the IR spectrum.

EXAMPLE 2
PCR PP
(wt %) Maleic anhydride
(wt %) Styrene
(wt %) DCP
(wt %) Irganox
(wt %)
m-PCR PP 89 10
- 0.5 0.5
m’-PCR PP 78.38 10
10.62 0.5 0.5
Preparation of styrene-maleic anhydride polypropylene
Reactive extrusion of the polyolefin samples (PCR-PP), multilayered BOPP (ML PP) was performed with maleic anhydride (10 wt%) and styrene (10.62%) by extruding through DSM Xplore batch twin-screw microcompounder with a 15 cm3 capacity at screw speed of 150 rpm at 180°C in presence of dicumyl peroxide (0.5 wt%) to obtain styrenyl-maleic anhydride grafted PCR-PP (m’-PCR PP) and styrenyl-maleic anhydride grafted ML PP (m’-PCR ML PP), respectively. Table 1
shows the composition of styrenyl-maleic anhydride grafted (maleated polyolefin) PCR-PP (m’-PCR PP).

Table 1
m’-PCR PP was re-precipitated in acetone followed by dissolving in hot xylene to to remove any unreacted maleic anhydride and styrene. IR spectrum was recorded to calculate the grafting yield. The grafting yield A1780/A1167 was found to be 2.25. The presence of styrene enhanced the grafting of maleic anhydride as confirmed from the grafting yield values. This results in more number of maleic anhydride moieties being linked to C-C backbone of the polyolefin which may inadvertently increase a loading achievable with dynamic crosslinkers to form pololefin-vitrimers.
EXAMPLE 3
Preparation of 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE)
Terephthaldehyde (0.149 moles, 20g) was dissolved in xylene (150 mL) at 80 °C in a 250 mL round-bottom flask. The hot solution was cooled down to room temperature and 2-aminoethanol (0.298 moles, 18.21g) was added dropwise to the reaction mixture for over 20 minutes while stirring at room temperature. The resulting mixture was set under reflux condition at 60 °C for 18 hours. The mixture was cooled to room temperature and filtered to obtain a crystalline light yellow colored precipitate. The precipitate was washed twice with 100 mL of hot xylene and 100 mL of methanol using a separating funnel. The final product was kept for drying in a vacuum oven at 80 °C overnight and cooled to room temperature to obtain a yield of 43.37 %.
EXAMPLE 4
Preparation of Polyolefin-vitrimers
Maleic anhydride grafted PCR-PP (m-PCR PP) of Example 1 and styrenyl-maleic anhydride grafted PCR-PP (m’-PCR PP) of Example 2 were extruded through DSM Xplore batch twin-screw microcompounder with a 15 cm3 capacity along with crosslinker PMDE (from Example 3). The twin-screw microcompounder has a co-rotating conical screw profile and a recirculation channel that permits control over the residence time. The extrusion was performed at a temperature of 180 °C with a screw speed of 150 rpm for 2 minutes in presence of dicumyl peroxide (DCP), an antioxidant (Irganox 1010), and zinc acetate (catalyst) with varying concentrations of crosslinkers and PP samples. Table 2 shows compositions of PP-PMDE samples with varying concentrations of PMDE, where 5PMDE, 10PMDE refer to percentages of 5 wt% and 10wt% of PMDE, respectively.
Sample Name m- PCR PP (wt%) m’-PCR PP (wt%) PMDE (wt%) Zn(OAc)2 (wt%)
m-PCR PP-5PMDE 94 - 5 1
m-PCR PP-10PMDE 89 - 10 1
m-PCR PP-20PMDE 79 - 20 1
m’-PCR PP-5PMDE - 94 5 1
m’-PCR PP-10PMDE - 89 10 1
m’-PCR PP-15PMDE - 84 15 1
m’-PCR PP-20PMDE - 79 20 1
Table 2

The addition of 15 wt% of PMDE gave the best properties in terms of mechanical strength. Hence, other crosslinkers 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), and 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA)) were added at 15 wt% loading with m’PCR-PP to form the polyolefin-vitrimers and are listed inTable 3.

Sample Name m’-PCR PP (wt%) Crosslinker (wt%) Zn(OAc)2 (wt%)
m’-PCR PP-15PMDE 84 15 1
m’-PCR ML PP-15PMDE 84 15 1
m’-PCR PP-15APD 85 15 0
m’-PCR PP-15MDGA 84 15 1
m’-PCR PP-15BHET 84 15 1
Table 3
Infrared (IR) study
IR spectrum of styrenyl-maleic PCR polyolefin with PMDE (m’-PCR PP-15PMDE) (polyolefin-PMDE vitrimer), confirmed ring-opening of maleic anhydride by the crosslinker PMDE with the appearance of a new peak at ~1697 cm-1 corresponding to the C=O stretching of the carbonyl peak of maleic acid. With the increase in concentration of PMDE, the intensity of the 1697 cm-1 peak increased indicating more and more acid group formation. The disappearance of peaks in the range of 1706-1850 cm-1 corresponding to the anhydride groups further confirmed ring-opening. Another significant peak was the imine peak in the range of 1635 cm-1 signifying the presence of dynamic imine bonds in polyolefin-PMDE vitrimer.
Mechanical testing
According to ASTM D638 (type V) stress-strain properties of the samples given in the Tables 1-3 were measured using 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.
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. Further, the broken samples were cut into small pieces and again extruded followed by injection molding for several cycles for recyclability tests.

Sample name Yield stress (MPa) Elongation at yield (%)

PCR PP
28 ± 0.35
16 ± 0.37

m- PCR PP
24 ± 0.36 13 ± 0.24

m’-PCR PP
29 ± 0.61 18 ± 0.22
Table 4
Table 4 provides the yield stress of PCR polypropylene (PCR PP), maleic anhydride-grafted PCR PP (m-PCR PP), and styrenyl-maleic grafted PCR PP (m’-PCR PP) expressed in Mega Pascals (MPa). As shown in Table 4, m’-PCR PP had a yield stress and elongation at yield higher than PCR PP, and maleated PCR PP (m-PCR PP).

Sample name Yield stress (MPa) Elongation at yield (%)

m-PCR PP-5PMDE
27 ± 0.45 15 ± 0.35

m-PCR PP-10PMDE
30 ± 0.42 17± 0.33

m-PCR PP-20PMDE
29 ± 0.37 8± 0.22

m’-PCR PP-5PMDE
32 ± 0.53 18 ± 0.25

m’-PCR PP-10PMDE
32 ± 0.42 16 ± 1.06
m’-PCR PP-15PMDE
34 ± 0.51 15 ± 0.46
m’-PCR PP-20PMDE
35 ± 0.66 12 ± 0.27
Table 5
Table 5 presents the yield stress of polyolefin-vitrimers prepared using maleic anhydride-grafted PCR polypropylene (m-PCR PP) and styrenyl-maleic PCR polypropylene (m’-PCR PP) using the crosslinker PMDE with varying concentrations of PMDE. Such studies provide a guidance on deciding the desired loading of the dynamic crosslinker in polyolefin-vitrimers for optimizing mechanical properties.

Sample name Yield stress (MPa) Elongation at yield (%)

m’-PCR PP-15PMDE
34 ± 0.26 15 ± 0.46

m’-PCR ML PP-15PMDE 31 ± 0.26
13.7 ± 0.39

m’-PCR PP-15APD
34 ± 0.68 15 ± 0.33

m’-PCR PP-15 MDGA
34 ± 0.49 20 ± 0.69

m’-PCR PP-15BHET
32 ± 0.49 15 ± 0.21

Table 6

Table 6 presents the yield stress of polyolefin-vitrimers prepared using maleic anhydride grafted PCR polypropylene (m-PCR PP) and styrenyl-maleic PCR polypropylene (m’-PCR PP) using the crosslinker PMDE (m’-PCR PP-15PMDE), APD (m’-PCR PP-15APD), MDGA (m’-PCR PP-15MDGA), and BHET (m’-PCR PP-15BHET), at crosslinker concentrations of 15%. The polyolefin-vitrimer samples exhibited superior mechanical properties in terms of yield stress compared to PCR PP (Table 4), maleic anhydride PCR PP (Table 4) and styrenyl-maleic PCR PP (Table 4).
Recyclability test
Recyclability tests up to 3 cycles for m’-PCR PP-PMDE vitrimer were performed.

Sample name Yield stress (MPa) Elongation at yield (%)
m’-PCR PP-15PMDE 34 ± 0.51 15 ± 0.46
m’-PCR PP-15PMDE (R1) 33 ± 0.63 14 ± 0.5
m’-PCR PP-15PMDE (R2) 33 ± 0.44 14 ± 1.3
m’-PCR PP-15PMDE (R3) 33 ± 0.56 14 ± 0.77
Table 7
Table 7 provides the recyclability test results of m’-PCR PP-PMDE at R1, R2, and R3, where R1, R2, and R3 represent recycling cycle numbers 1, 2 and 3, respectively. There were no significant changes in yield strength and elongation at yield even after 3 reprocessing cycles carried out through an extruder. This can be attributed to dynamic covalent adaptable network (CAN) of the polyolefin-vitrimers which rearrange at elevated temperatures. This confirmed that the polyolefin-vitrimer of the present disclosure may be reprocessed more than 3 times without any degradation in mechanical properties. Moreover, the MFI values of the polyolefin-vitrimers were 0.01 grams per 10 minutes, or more than 0.01 grams per 10 minutes at the end of 3 cycles.
The styrenyl-maleic grafted PCR polypropylene PMDE (15wt%) (m’-PCR PP PMDE) had a recovery rate of 96.3% at the end of the third cycle life. In the case of maleic anhydride grafted PCR polypropylene (m-PCR PP PMDE ), the recovery rate was 94.5%. PCR-PP had a recovery rate of 84.1% thus confirming the superior recyclability of the inventive polyolefin-vitrimers.
FIG. 3 is a bar diagram 300 of yield strength at cycle numbers 0 to 3. In FIG. 3, 302 corresponds to sample PCR PP, 304 corresponds to m-PCR PP PMDE and 306 corresponds to m’-PCR PP PMDE. As shown in FIG. 3, m’-PCR PP PMDE had maximum yield strength and the yield strength was retained even after 3 recycling cycles.
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.

, C , C , Claims:We Claim:
1. A polyolefin-vitrimer comprising:
a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin at a grafting yield in a range of 0.3 to 3.0; and
a dynamic crosslinker linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer, wherein the dynamic crosslinker comprises 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE), and wherein a concentration of the dynamic crosslinker in the polyolefin-vitrimer is in a range of 1 to 30% by weight.
2. The polyolefin-vitrimer as claimed in claim 1, wherein polyolefin of the polyolefin-vitrimer is a post-consumer recycled (PCR) polyolefin.
3. The polyolefin-vitrimer as claimed in claim 2, wherein a mechanical strength of the polyolefin-vitrimer is greater than a mechanical strength of the post-consumer recycled (PCR) polyolefin.
4. The polyolefin-vitrimer as claimed in claim 1, wherein polyolefin of the polyolefin-vitrimer 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.
5. A polyolefin-vitrimer comprising:
a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin; and
a dynamic crosslinker linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer, and wherein the dynamic crosslinker is 4-aminophenyl disulfide (APD).
6. A polyolefin-vitrimer comprising:
a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin; and
a dynamic crosslinker linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer, and wherein the dynamic crosslinker is bis (2-hydroxyethyl) terephthalate (BHET).
7. A polyolefin-vitrimer comprising:
a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin; and
a dynamic crosslinker linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer, and wherein the dynamic crosslinker is 4,4’-methylenebis(N, N-diglycidylaniline) (MDGA).
8. A polyolefin-vitrimer comprising:
a maleated polyolefin comprising a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of a polyolefin; and
a dynamic crosslinker linked to the co-grafting agent-maleic-based sidechain and forms covalent adaptive network to form the polyolefin-vitrimer, and wherein the dynamic crosslinker is 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).

9. A two-component composition for increasing cycle life of a polyolefin comprising:
a first component comprising a mixture of maleic anhydride and a co-grafting agent, wherein the first component on extrusion with the polyolefin forms a maleated polyolefin, and wherein the maleated pololefin comprises a co-grafting agent-maleic-based sidechain grafted to a carbon-carbon (C-C) backbone of the polyolefin at a grafting yield in a range of 0.3 to 3.0; and
a second component comprising a dynamic crosslinker, wherein the second component on extrusion with the maleated polyolefin forms a polyolefin-vitrimer having enhanced cycle life than the polyolefin, wherein the dynamic crosslinker forms covalent adaptive network, and wherein the dynamic crosslinker comprises 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
10. The two-component composition as claimed in claim 9, wherein a ratio of maleic anhydride to the co-grafting agent in the first component is in a range of 1:1 to 1:1.2 by weight, and wherein a concentration of the first component extruded with the polyolefin is in a range of 1 to 20% by weight.
11. A method of processing a polyolefin to increase a cycle life comprising:
performing a first extrusion of the polyolefin with a first component (102) at a temperature in a range of 160 to 200 °C to form a maleated polyolefin, wherein the first component comprises a mixture of maleic anhydride and a co-grafting agent at a ratio in a range of 1:1 to 1:1.2; and
performing a second extrusion of the maleated polyolefin with a dynamic crosslinker (104) at a temperature in a range of 160 to 200 °C to form a polyolefin-vitrimer having enhanced cycle life than the polyolefin, wherein the dynamic crosslinker forms covalent adaptive network based on disulphide exchange, transesterification, imine exchange or combinations thereof to form the polyolefin-vitrimer, and wherein the dynamic crosslinker comprises 4-aminophenyl disulfide (APD), bis (2-hydroxyethyl) terephthalate (BHET), 4,4’-methylenebis(N,N-diglycidylaniline) (MDGA), or 2-2’-[p-phenylenebis(methylidynenitrilo)]bisethanol (PMDE).
12. The method as claimed in claim 11, wherein the method of processing the polyolefin comprises processing a post-consumer recycled (PCR) polyolefin.
13. The method as claimed in claim 11, wherein the method has a recovery rate of greater than 94% when the polyolefin is a post-consumer recycled (PCR) polyolefin.
14. The method as claimed in claim 11, wherein a concentration of the first component extruded with the polyolefin is in a range of 1 to 20% by weight.
15. The method as claimed in claim 11, wherein a concentration of the dynamic crosslinker extruded with the maleated polyolefin is in a range of 1 to 30% by weight.
16. The method as claimed in claim 11, 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), or combinations thereof.
17. The method as claimed in claim 11, wherein the first extrusion (102), or the second extrusion (104), or both is performed in presence of a stabilizer, a catalyst, a free radical initiator, or combinations thereof.
18. The method as claimed in claim 11, wherein the polyolefin-vitrimer has a cycle life of more than 3 times with a melt flow index of 0.1 gram per 10 minutes (g/10min), or more than 0.1 (g/10min).
19. The method as claimed in claim 11, wherein the first extrusion (102) and the second extrusion (104) are performed in a twin screw extruder at a screw speed of 100 to 150 rotations per minute (rpm) and at a residence time in a range of 1 to 5 minutes.
20. An article formed using the polyolefin-vitrimer as claimed in any of the claims 1-8, and 11.