Patent Publication Number: US-2018037723-A1

Title: Process for making a fabricated article from polyolefin

Description:
BACKGROUND 
     Previously, carbonaceous articles, such as carbon fibers, have been produced primarily from polyacrylonitrile (PAN), pitch, or cellulose precursors. The process for making carbonaceous articles begins by forming a fabricated article, such as a fiber or a film, from the precursor. Precursors may be formed into fabricated articles using standard techniques for forming or molding polymers. The fabricated article is subsequently stabilized to allow the fabricated article to substantially retain shape during the subsequent heat-processing steps; without being limited by theory, such stabilization typically involves a combination of oxidation and heat and generally results in dehydrogenation, ring formation, oxidation and crosslinking of the precursor which defines the fabricated article. The stabilized fabricated article is then converted into a carbonaceous article by heating the stabilized fabricated article in an inert atmosphere. While the general steps for producing a carbonaceous article are the same for the variety of precursors, the details of those steps vary widely depending on the chemical makeup of the selected precursor. 
     Polyolefins have been investigated as an alternative precursor for carbonaceous articles, but a suitable and economically viable preparation process has proven elusive. Of particular interest is identifying an economical process for preparing carbonaceous articles from polyolefin precursors. For example, maximizing mass retention during the stabilization and carbonization steps is of interest. 
     STATEMENT OF INVENTION 
     The present disclosure describes a method for preparing a carbonized article comprising: (a) providing a polyolefin resin; (b) forming a fabricated article from the polyolefin resin; (c) crosslinking the fabricated article; (d) stabilizing the fabricated article in a boron-containing oxidizing environment (BOE); and (e) carbonizing the fabricated article. The present disclosure further describes a method for preparing a boron-treated stabilized article. 
    
    
     DETAILED DESCRIPTION 
     Unless otherwise indicated, numeric ranges, for instance “from 2 to 10,” are inclusive of the numbers defining the range (e.g., 2 and 10). 
     Unless otherwise indicated, ratios, percentages, parts, and the like are by weight. 
     Unless otherwise indicated, the crosslinkable functional group content for a polyolefin resin is characterized by the mol % crosslinkable functional groups, which is calculated as the number of mols of crosslinkable functional groups divided by the total number of mols of monomer units contained in the polyolefin. 
     Unless otherwise indicated, “monomer” refers to a molecule which can undergo polymerization, thereby contributing constitutional units to the essential structure of a macromolecule, for example, a polyolefin. 
     In one aspect, the present disclosure describes a process for producing a carbonaceous fabricated article from a polyolefin resin. Unless stated otherwise, any method or process steps described herein may be performed in any order. Polyolefins are a class of polymers produced from one or more olefin monomer. The polymers described herein may be formed from one or more types of monomers. Polyethylene is the preferred polyolefin resin, but other polyolefin resins may be substituted. For example, a polyolefin produced from ethylene, propylene, or other alpha-olefin (for instance, 1-butene, 1-hexene, 1-octene), or a combination thereof, is also suitable. The polyolefins described herein are typically provided in resin form, subdivided into pellets or granules of a convenient size for further melt or solution processing. 
     The polyolefin resins described herein are subjected to a crosslinking step. Any suitable method for crosslinking polyolefins is sufficient. In one instance, the polyolefins are crosslinked by irradiation, such as by electron beam processing. Other crosslinking methods are suitable, for example, ultraviolet irradiation and gamma irradiation. In some instances, an initiator, such as benzophenone, may be used in conjunction with the irradiation to initiate crosslinking. In one instance, the polyolefin resins have been modified to include crosslinkable functional groups which are suitable for reacting to crosslink the polyolefin resin. Where the polyolefin resin includes crosslinkable functional groups, crosslinking may be initiated by known methods, including use of a chemical crosslinking agent, by heat, by steam, or other suitable method. In one instance, copolymers are suitable to provide a polyolefin resin having crosslinkable functional groups where one or more alpha-olefins have been copolymerized with another monomer containing a group suitable for serving as a crosslinkable functional group, for example, dienes, carbon monoxide, glycidyl methacrylate, acrylic acid, vinyl acetate, maleic anhydride, or vinyl trimethoxy silane (VTMS) are among the monomers suitable for being copolymerized with the alpha-olefin. Further, the polyolefin resin having crosslinkable functional groups may also be produced from a poly(alpha-olefin) which has been modified by grafting a functional group moiety onto the base polyolefin, wherein the functional group is selected based on its ability to subsequently enable crosslinking of the given polyolefin. For example, grafting of this type may be carried out by use of free radical initiators (such as peroxides) and vinyl monomers (such as VTMS, dienes, vinyl acetate, acrylic acid, methacrylic acid, acrylic and methacrylic esters such as glycidyl methacrylate and methacryloxypropyl trimethoxysilane, allyl amine, p-aminostyrene, dimethylaminoethyl methacrylate) or via azido-functionalized molecules (such as 4-[2-(trimethoxysilyl)ethyl)]benzenesulfonyl azide). Polyolefin resins having crosslinkable functional groups may be produced from a polyolefin resin, or may be purchased commercially. Examples of commercially available polyolefin resins having crosslinkable functional groups include SI-LINK sold by The Dow Chemical Company, PRIMACOR sold by The Dow Chemical Company, EVAL resins sold by Kuraray, and LOTADER AX8840 sold by Arkema. 
     As described above, the polyolefin resin is processed to form a fabricated article. A fabricated article is an article which has been fabricated from the polyolefin resin. The fabricated article is formed using known polyolefin fabrication techniques, for example, melt or solution spinning to form fibers, film extrusion or film casting or a blown film process to form films, die extrusion or injection molding or compression molding to form more complex shapes, or solution casting. The fabrication technique is selected according to the desired geometry of the target carbonaceous article, and the desired physical properties of the same. For example, where the desired carbonaceous article is a carbon fiber, fiber spinning is a suitable fabrication technique. As another example, where the desired carbonaceous article is a carbon film, compression molding is a suitable fabrication technique. 
     As noted above, at least a portion of the polyolefin resin is crosslinked to yield a crosslinked fabricated article. In some embodiments, crosslinking is carried out via chemical crosslinking. Thus, in some embodiments, the crosslinked fabricated article is a fabricated article which has been treated with one or more chemical agents to crosslink the crosslinkable functional groups of the polyolefin resin having crosslinkable functional groups. Such chemical agent functions to initiate the formation of intramolecular chemical bonds between the crosslinkable functional groups or reacts with the crosslinkable functional groups to form intramolecular chemical bonds, as is known in the art. Chemical crosslinking causes the crosslinkable functional groups to react to form new bonds, forming linkages between the various polymer chains which define the polyolefin resin having crosslinkable functional groups. The chemical agent which effectuates the crosslinking is selected based on the type of crosslinkable functional group(s) included in the polyolefin resin; a diverse array of reactions are known which crosslink crosslinkable functional groups via intermolecular and intramolecular chemical bonds. A suitable chemical agent is selected which is known to crosslink the crosslinkable functional groups present in the fabricated article to produce the crosslinked fabricated article. For example, without limiting the present disclosure, if the crosslinkable functional group attached to the polyolefin is a vinyl group, suitable chemical agents include free radical initiators such as peroxides or azo-bis nitriles, for example, dicumyl peroxide, dibenzoyl peroxide, t-butyl peroctoate, azobisisobutyronitrile, and the like. If the crosslinkable functional group attached to the polyolefin is an acid, such as a carboxylic acid, or an anhydride, or an ester, or a glycidoxy group, a suitable chemical agent can be a compound containing at least two nucleophilic groups, including dinucleophiles such as diamines, diols, dithiols, for example ethylenediamine, hexamethylenediamine, butane diol, or hexanedithiol. Compounds containing more than two nucleophilic groups, for example glycerol, sorbitol, or hexamethylene tetramine can also be used. Mixed di- or higher-nucleophiles, which contain at least two different nucleophilic groups, for example ethanolamine can also be suitable chemical agents. If the crosslinkable functional group attached to the polyolefin is a mono-, di- or tri-alkoxy silyl group, water, and Lewis or Bronsted acid or base catalysts can be used as suitable chemical agents. For example, without limiting the present disclosure, Lewis or Bronsted acid or base catalysts include aryl sulfonic acids, sulfuric acid, hydroxides, zirconium alkoxides or tin reagents. 
     Crosslinking the fabricated article is generally preferred to ensure that the fabricated article retains its shape at the elevated temperatures required for the subsequent processing steps. Without crosslinking, polyolefin resins typically soften, melt or otherwise deform or breakdown at elevated temperatures. Crosslinking adds thermal stability to the fabricated article. 
     The crosslinked fabricated article is heated in a boron-containing oxidizing environment (BOE) to yield a boron-treated stabilized fabricated article. The BOE includes an oxidizing agent and a boron source. In one instance the oxidizing agent is oxygen. In one instance the BOE includes air and a boron source. It is preferred that the BOE is continuously charged to the oven or other apparatus in which the stabilization process is executed to prevent depletion of the oxidizing agent and accumulation of by-products. The boron-treated stabilized fabricated article is a crosslinked fabricated article which has been heat-treated in the BOE. In one instance, the BOE contains air and a gas-phase boron-containing species. Any suitable gas-phase boron-containing species which deposits boron in the fabricated article may be used in the BOE. In one instance, boric acid is used as the BOE. In one instance a gaseous borate is used in the BOE, for example, trimethyl borate. In one instance the gaseous borate is a derivative of boric acid, for example, metaboric acid and boron oxide. In one instance, the gaseous borate is a derivative of boronic acid, for example, a substituted boronic acid (for example, alkyl substituted, such as methyl-, or ethyl-, or aryl substituted, such as phenyl-). In one instance, the gaseous borate is a derivative of borinic acid, for example, a substituted borinic acid (for example, alkyl substituted, such as methyl-, or ethyl-, or aryl substituted, such as phenyl-). In one instance, the gaseous borate is a derivative of borane, boronic ester or boroxine. In one instance, the gaseous borate is a derivative of borazine, borohydride or aminoborane. In one instance, the BOE flows over the fabricated article. In some embodiments, the temperature for stabilizing the crosslinked fabricated article is at least 120° C., preferably at least 190° C. In some embodiments, the temperature for stabilizing the crosslinked fabricated article is no more than 400° C., preferably no more than 300° C. In one instance, the crosslinked fabricated article is introduced to a heating chamber which is already at the desired temperature. In another instance, the fabricated article is introduced to a heating chamber at or near ambient temperature, which chamber is subsequently heated to the desired temperature. In some embodiments the heating rate is at least 1° C./minute. In other embodiments the heating rate is no more than 15° C./minute. In yet another instance, the chamber is heated step wise, for instance, the chamber is heated to a first temperature for a time, such as, 120° C. for one hour, then is raised to a second temperature for a time, such as 180° C. for one hour, and third is raised to a holding temperature, such as 250° C. for 10 hours. The stabilization process involves holding the crosslinked fabricated article at the given temperature for periods up to 100 hours depending on the dimensions of the fabricated article. The stabilization process yields a boron-treated stabilized fabricated article which is a precursor for a carbonaceous article. Without being limited by theory, the stabilization process oxidizes the crosslinked fabricated article and causes changes to the hydrocarbon structure that increases the crosslink density while decreasing the hydrogen/carbon ratio of the crosslinked fabricated article. Without being limited by theory, the stabilization process in the presence of boron modifies the oxidation chemistry and increases the crosslink density. 
     Unexpectedly, it has been found that including boron in the oxidizing environment improves mass retention of the subsequently produced stabilized carbonaceous article. Unexpectedly, it has been found that including a boron-containing species in the fabricated article during the stabilization step improves mass retention of the subsequently produced stabilized carbonaceous article. It has also been found that treating the crosslinked fabricated article with a boron-containing species improves form-retention of the subsequently produced carbonaceous article. 
     In another aspect, the present disclosure describes a boron-treated stabilized fabricated article which is formed from a polyolefin precursor (resin). In one instance, the boron-treated stabilized fabricated article is formed according to the process described herein. 
     In yet another aspect, a carbonaceous article and a process for making the same are provided. Carbonaceous articles are articles which are rich in carbon; carbon fibers, carbon sheets and carbon films are examples of carbonaceous articles. Carbonaceous articles have many applications, for example, carbon fibers are commonly used to reinforce composite materials, such as in carbon fiber reinforced epoxy composites, while carbon discs or pads are used for high performance braking systems. 
     The carbonaceous articles described herein are prepared by carbonizing the stabilized fabricated article by heat-treating the boron-treated stabilized fabricated articles in an inert environment. The inert environment is an environment surrounding the boron-treated stabilized fabricated article that shows little reactivity with carbon at elevated temperatures, preferably a high vacuum or an oxygen-depleted atmosphere, more preferably a nitrogen atmosphere or an argon atmosphere. It is understood that trace amounts of oxygen may be present in the inert atmosphere. In one instance, the temperature of the inert environment is at or above 600° C. Preferably, the temperature of the inert environment is at or above 800° C. In one instance, the temperature of the inert environment is no more than 3000° C. In one instance, the temperature is from 1400-2400° C. Temperatures at or near the upper end of that range will produce a graphite article, while temperatures at or near the lower end of the range will produce a carbon article. 
     In order to prevent bubbling or damage to the fabricated article during carbonization, it is preferred to heat the inert environment in a gradual or stepwise fashion. In one embodiment, the boron-treated stabilized fabricated article is introduced to a heating chamber containing an inert environment at or near ambient temperature, which chamber is subsequently heated over a period of time to achieve the desired final temperature. The heating schedule can also include one or more hold steps for a prescribed period at the final temperature or an intermediate temperature or a programmed cooling rate before the article is removed from the chamber. 
     In yet another embodiment, the chamber containing the inert environment is subdivided into multiple zones, each maintained at a desired temperature by an appropriate control device, and the boron-treated stabilized fabricated article is heated in a stepwise fashion by passage from one zone to the next via an appropriate transport mechanism, such as a motorized belt. In the instance where a boron-treated stabilized fabricated article is a fiber, this transport mechanism can be the application of a traction force to the fiber at the exit of the carbonization process while the tension in the stabilized fiber is controlled at the inlet. Some embodiments of the invention will now be described in detail in the following Examples. 
     In the Examples, overall mass yield is calculated as the product of oxidation mass yield and carbonization mass yield (calculated as provided below). PHR refers to parts per hundred resin (mass basis). MI refers to melt index which is a measure of melt flow rate. Wt % refers to parts per 100 total parts, mass basis. PE refers to polyethylene. BA refers to boric acid. Definitions of measured yields: 
     
       
         
           
             
               Oxidation 
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               mass 
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               yield 
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                 : 
               
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                 Y 
                 O 
               
             
             = 
             
               
                 m 
                 OX 
               
               
                 m 
                 PE 
               
             
           
         
       
       
         
           
             
               Carbonazation 
                
               
                   
               
                
               mass 
                
               
                   
               
                
               yield 
                
               
                 : 
               
                
               
                   
               
                
               
                 Y 
                 C 
               
             
             = 
             
               
                 m 
                 CF 
               
               
                 m 
                 OX 
               
             
           
         
       
       
         
           
             
               Overall 
                
               
                   
               
                
               mass 
                
               
                   
               
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               Overall 
                
               
                   
               
                
               mass 
                
               
                   
               
                
               
                 yield 
                  
                 
                     
                 
                 ( 
                 
                   carbonaceous 
                    
                   
                       
                   
                    
                   mass 
                    
                   
                       
                   
                    
                   per 
                    
                   
                       
                   
                    
                   initial 
                    
                   
                       
                   
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                   mass 
                    
                   
                       
                   
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                   of 
                    
                   
                       
                   
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                   PE 
                 
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     Where m PE  is the initial mass of polyethylene; m OX  is the mass remaining after oxidation; m CF  is the mass remaining after carbonization; M % PE  is the mass % of polyethylene in the origin formed article. 
     Soxhlet extraction is a method for determining the gel content and swell ration of crosslinked ethylene plastics, also referred to herein as hot xylenes extraction. As used herein, Soxhlet extraction is conducted according to ASTM Standard D2765-11 “Standard Test Methods for Determination of Gel Content and Swell Ratio of Crosslinked Ethylene Plastics.” In the method employed, a crosslinked fabricated article between 0.050-0.500 g is weighed and placed into a cellulose-based thimble which is then placed into a Soxhlet extraction apparatus with sufficient quantity of xylenes. Soxhlet extraction is then performed with refluxing xylenes for at least 12 hours. Following extraction, the thimbles are removed and the crosslinked fabricated article is dried in a vacuum oven at 80° C. for at least 12 hours and then weighed, thereby providing a Soxhlet-treated article. The gel content (%) is then calculated from the weight ratio (Soxhlet-treated article)/(crosslinked fabricated article). 
     Comparative Example 1 
     An ethylene/octene copolymer (density=0.941 g/cm 3 ; MI=34 g/10 min, 190° C./2.16 kg) is melt blended with Esacure ONE, a commercially available photoinitiator sold by Lamberti, at 2 wt % (2.04 PHR) at 180° C. in a Haake mixer under nitrogen. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked (30 s exposure time) using a 600 W/in H-type mercury UV lamp fitted with a parabolic (non-focused) reflector. Gel fraction is determined to be 35.5% by hot xylenes extraction. Ten (10) smaller circular films are sectioned from the prepared films and weighed. The films are oxidized in a convection oven at 270° C. for 10 hours under air (21% oxygen content). The ten (10) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table I. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min. Mass retention during carbonization (carbonization mass yield) is reported in Table I. Calculated overall mass yield is reported in Table I. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                 Film 
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Section 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 28.75 
                 56.68 
                 16.30 
               
               
                   
                 B 
                 28.77 
                 56.94 
                 16.38 
               
               
                   
                 C 
                 35.56 
                 57.04 
                 20.28 
               
               
                   
                 D 
                 31.86 
                 56.72 
                 18.07 
               
               
                   
                 E 
                 30.05 
                 54.81 
                 16.47 
               
               
                   
                 F 
                 38.01 
                 55.88 
                 21.24 
               
               
                   
                 G 
                 32.61 
                 55.82 
                 18.20 
               
               
                   
                 H 
                 30.60 
                 56.05 
                 17.15 
               
               
                   
                 I 
                 28.74 
                 55.78 
                 16.03 
               
               
                   
                 J 
                 31.27 
                 55.80 
                 17.45 
               
               
                   
                   
               
            
           
         
       
     
     Example 1 
     An ethylene/octene copolymer (density=0.941 g/cm 3 ; MI=34 g/10 min, 190° C./2.16 kg) is melt blended with Esacure ONE, a commercially available photoinitiator sold by Lamberti, at 2 wt % (2.04 PHR) at 180° C. in a Haake mixer under nitrogen. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked (30 s exposure time) using a 600 W/in H-type mercury UV lamp fitted with a parabolic (non-focused) reflector. Gel fraction is determined to be 35.5% by hot xylenes extraction. Nine (9) smaller circular films are sectioned from the prepared films and weighed. The films are placed in a convection oven. Sixteen (16) compression molded polyethylene films containing 20.4 phr boric acid by melt blending and UV crosslinked in the manner previously described are also placed in the oven. The films containing boric acid are of the same film thickness and size as the films prepared according to this example. The films are oxidized in the convection oven at 270° C. for 10 hours under air (21% oxygen content); a gaseous boron-containing species is generated in situ by heating the polyethylene films containing boric acid in the oven. The nine (9) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table II. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min Mass retention during carbonization (carbonization mass yield) is reported in Table II. Calculated overall mass yield is reported in Table II. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 46.95 
                 52.31 
                 24.56 
               
               
                   
                 B 
                 45.72 
                 52.73 
                 24.11 
               
               
                   
                 C 
                 46.75 
                 52.69 
                 24.63 
               
               
                   
                 D 
                 46.10 
                 47.54 
                 21.91 
               
               
                   
                 E 
                 46.61 
                 53.46 
                 24.92 
               
               
                   
                 F 
                 42.34 
                 49.94 
                 21.14 
               
               
                   
                 G 
                 51.94 
                 50.27 
                 26.11 
               
               
                   
                 H 
                 48.48 
                 44.20 
                 21.43 
               
               
                   
                 I 
                 45.61 
                 50.61 
                 23.08 
               
               
                   
                   
               
            
           
         
       
     
     It is observed that mean oxidation mass yield of A-I (Example 1) increases by 47.8% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-J (Comparative Example 1). It is further observed that mean overall mass yield of A-I (Example 1) increases by 32.5% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-J (Comparative Example 1). 
     Comparative Example 2 
     An ethylene/octene copolymer (density=0.941 g/cm 3 ; MI=34 g/10 min, 190° C./2.16 kg) is melt blended with Esacure ONE, a commercially available photoinitiator sold by Lamberti, at 2 wt % (2.04 PHR) at 180° C. in a Haake mixer under nitrogen. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked (30 s exposure time) using a 600 W/in H-type mercury UV lamp fitted with a parabolic (non-focused) reflector. Gel fraction is determined to be 35.5% by hot xylenes extraction. Nine (9) smaller circular films are sectioned from the prepared films and weighed. The films are oxidized in a convection oven at 270° C. for 5 hours under air (21% oxygen content). The nine (9) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table III. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min Mass retention during carbonization (carbonization mass yield) is reported in Table III. Calculated overall mass yield is reported in Table III. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 32.50 
                 50.22 
                 16.32 
               
               
                   
                 B 
                 31.11 
                 48.04 
                 14.94 
               
               
                   
                 C 
                 33.39 
                 45.25 
                 15.11 
               
               
                   
                 D 
                 34.80 
                 44.44 
                 15.46 
               
               
                   
                 E 
                 47.29 
                 25.86 
                 12.23 
               
               
                   
                 F 
                 32.39 
                 44.20 
                 14.32 
               
               
                   
                 G 
                 33.97 
                 41.32 
                 14.03 
               
               
                   
                 H 
                 34.49 
                 39.94 
                 13.77 
               
               
                   
                 I 
                 30.64 
                 50.86 
                 15.58 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     An ethylene/octene copolymer (density=0.941 g/cm 3 ; MI=34 g/10 min, 190° C./2.16 kg) is melt blended with Esacure ONE, a commercially available photoinitiator sold by Lamberti, at 2 wt % (2.04 PHR) at 180° C. in a Haake mixer under nitrogen. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked (30 s exposure time) using a 600 W/in H-type mercury UV lamp fitted with a parabolic (non-focused) reflector. Gel fraction is determined to be 35.5% by hot xylenes extraction. Eight (8) smaller circular films are sectioned from the prepared films and weighed. The films are placed in a convection oven. A vessel containing boric acid is placed in the oven. The films are oxidized in the convection oven at 270° C. for 5 hours under air (21% oxygen content); a gaseous boron-containing species is generated in situ by heating boric acid in the oven. The eight (8) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table IV. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min Mass retention during carbonization (carbonization mass yield) is reported in Table IV. Calculated overall mass yield is reported in Table IV. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 57.17 
                 54.76 
                 31.31 
               
               
                   
                 B 
                 62.03 
                 53.00 
                 32.87 
               
               
                   
                 C 
                 61.05 
                 50.08 
                 30.57 
               
               
                   
                 D 
                 64.66 
                 53.11 
                 34.34 
               
               
                   
                 E 
                 68.03 
                 51.37 
                 34.95 
               
               
                   
                 F 
                 60.00 
                 56.20 
                 33.72 
               
               
                   
                 G 
                 61.62 
                 56.20 
                 34.63 
               
               
                   
                 H 
                 58.59 
                 50.28 
                 29.46 
               
               
                   
                   
               
            
           
         
       
     
     It is observed that mean oxidation mass yield of A-H (Example 2) increases by 78.6% when crosslinked polyethylene films are oxidized in the presence of a gas-phase boron species when compared to A-I (Comparative Example 2). It is further observed that mean overall mass yield of A-H (Example 2) increases by 124% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-I (Comparative Example 2). 
     Comparative Example 3 
     A vinyl trimethoxysilane-grafted ethylene/octene copolymer (MI=7 g/10 min, 190° C./2.16 kg; 1.6 wt % grafted silane content) is used as a precursor resin. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked by treating the films with a commercial aryl sulfonic acid catalyst in isopropanol solution (Nacure B-201, King Industries) for 12 hours, followed by moisture curing at 60-80° C. for 72 hours. Gel fraction is determined to be 81.8% by hot xylenes extraction. Nine (9) smaller circular films are sectioned from the prepared film and weighed. The films are oxidized in a convection oven at 270° C. for 5 hours under air (21% oxygen content). The nine (9) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table V. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min. Mass retention during carbonization (carbonization mass yield) is reported in Table V. Calculated overall mass yield is reported in Table V. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE V 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 43.50 
                 51.01 
                 22.19 
               
               
                   
                 B 
                 42.19 
                 43.59 
                 18.39 
               
               
                   
                 C 
                 41.58 
                 50.52 
                 21.00 
               
               
                   
                 D 
                 43.81 
                 45.03 
                 19.73 
               
               
                   
                 E 
                 45.31 
                 42.46 
                 19.24 
               
               
                   
                 F 
                 40.26 
                 52.07 
                 20.96 
               
               
                   
                 G 
                 43.87 
                 43.11 
                 18.91 
               
               
                   
                 H 
                 49.09 
                 42.74 
                 20.98 
               
               
                   
                 I 
                 41.85 
                 41.13 
                 17.21 
               
               
                   
                   
               
            
           
         
       
     
     Example 3 
     A vinyl trimethoxysilane-grafted ethylene/octene copolymer (MI=7 g/10 min, 190° C./2.16 kg; 1.6 wt % grafted silane content) is used as a precursor resin. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked by treating the films with a commercial aryl sulfonic acid catalyst in isopropanol solution (Nacure B-201, King Industries) for 12 hours, followed by moisture curing at 60-80° C. for 72 hours. Gel fraction is determined to be 81.8% by hot xylenes extraction. Eight (8) smaller circular films are sectioned from the prepared film and weighed. The films are placed in a convection oven. A vessel containing boric acid is placed in the oven. The films are oxidized in the convection oven at 270° C. for 5 hours under air (21% oxygen content); a gaseous boron-containing species is generated in situ by heating boric acid in the oven. The eight (8) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table VI. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min. Mass retention during carbonization (carbonization mass yield) is reported in Table VI. Calculated overall mass yield is reported in Table VI. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE VI 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 61.94 
                 55.77 
                 34.55 
               
               
                   
                 B 
                 62.23 
                 52.71 
                 32.80 
               
               
                   
                 C 
                 60.88 
                 53.29 
                 32.44 
               
               
                   
                 D 
                 61.03 
                 50.54 
                 30.85 
               
               
                   
                 E 
                 61.96 
                 51.19 
                 31.72 
               
               
                   
                 F 
                 60.25 
                 50.58 
                 30.47 
               
               
                   
                 G 
                 62.52 
                 49.97 
                 31.24 
               
               
                   
                 H 
                 61.47 
                 52.90 
                 32.52 
               
               
                   
                   
               
            
           
         
       
     
     It is observed that the mean oxidation mass yield of A-H (Example 3) increases by 41.5% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-I (Comparative Example 3). It is further observed that the mean overall mass yield of A-H (Example 3) increases by 61.6% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-I (Comparative Example 3). 
     Comparative Example 4 
     An ethylene/octene copolymer (density=0.941 g/cm 3 ; MI=34 g/10 min, 190° C./2.16 kg) is melt blended with Esacure ONE, a commercially available photoinitiator sold by Lamberti, at 2 wt % (2.04 PHR) at 180° C. in a Haake mixer under nitrogen. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) by micrometer. All films are crosslinked (30 s exposure time) using a 600 W/in H-type mercury UV lamp fitted with a parabolic (non-focused) reflector. Gel fraction is determined to be 35.5% by hot xylenes extraction. Three (3) smaller circular films are sectioned from the prepared film and weighed. The films are oxidized in a convection oven at 250° C. for 10 hours under air (21% oxygen content). The three (3) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table VII. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min Mass retention during carbonization (carbonization mass yield) is reported in Table VII. Calculated overall mass yield is reported in Table VII. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE VII 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 36.24 
                 43.96 
                 15.93 
               
               
                   
                 B 
                 34.19 
                 46.64 
                 15.94 
               
               
                   
                 C 
                 32.42 
                 47.19 
                 15.30 
               
               
                   
                   
               
            
           
         
       
     
     Example 4 
     An ethylene/octene copolymer (density=0.941 g/cm 3 ; MI=34 g/10 min, 190° C./2.16 kg) is melt blended with Esacure ONE, a commercially available photoinitiator sold by Lamberti, at 2 wt % (2.04 PHR) at 180° C. in a Haake mixer under nitrogen. Films are compression molded using a Carver press at 180° C. into thin films measuring 3 millimeters (76.2 microns) thick by micrometer. All films are crosslinked (30 s exposure time) using a 600 W/in H-type mercury UV lamp fitted with a parabolic (non-focused) reflector. Gel fraction is determined to be 35.5% by hot xylenes extraction. Two (2) smaller circular films are sectioned from the prepared film and weighed. The films are placed in a convection oven. Fourteen (14) compression molded polyethylene films containing 8.16-20.4 phr boric acid by melt blending and UV crosslinked in the manner previously described are also placed in the oven. The films containing boric acid are of the same film thickness and size. The films are oxidized in the convection oven at 250° C. for 10 hours under air (21% oxygen content); a gaseous boron-containing species is generated in situ by heating the polyethylene films containing boric acid in the oven. The two (2) films are weighed after air oxidation. Mass retention during air oxidation (oxidation mass yield) is reported in Table VIII. Oxidized films are then carbonized under nitrogen from 25° C. to 800° C. using a ramp rate of 10° C./min. Mass retention during carbonization (carbonization mass yield) is reported in Table VIII. Calculated overall mass yield is reported in Table VIII. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE VIII 
               
               
                   
                   
               
               
                   
                   
                 Oxidation 
                 Carbonization 
                 Overall 
               
               
                   
                   
                 Mass Yield 
                 Mass Yield 
                 Mass Yield 
               
               
                   
                 Example 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 54.68 
                 49.83 
                 27.2 
               
               
                   
                 B 
                 55.23 
                 48.84 
                 27.0 
               
               
                   
                   
               
            
           
         
       
     
     It is observed that the mean oxidation mass yield of A-B (Example 4) increases by 60.3% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-C(Comparative Example 4). It is further observed that the mean overall mass yield of A-B (Example 4) increases by 72.4% when crosslinked polyethylene films are oxidized in presence of a gas-phase boron species when compared to A-C(Comparative Example 4).