Patent Application
20220162138
The present application relates to processes for cleaving S—S and S—H bonds in organic compounds. In particular, the present application includes process for breaking down and optionally recovering organic compounds containing S—S and S—H bonds, including organic polymers such as sulfur-crosslinked elastomers.
The crosslinking by sulfur of unsaturated organic polymers constitutes an important technology for the formation of elastomers. The process, vulcanization of alkene-containing hydrocarbon polymers, reported by Goodyear in 1844,1 is widespread. One sector that utilizes these technologies are rubbers destined for use in transportation, sales of automobile tires that use this process, for example, were expected to reach 3 billion units in 2019.2 Vulcanization using sulfur involves radical processes that drive the crosslinking by mono- and oligosulfides of unsaturated organic polymeric chains.3 The products are very robust.
At the end of their useful lifetime, in the case of tires, some of the energy contained in the rubber is exploited as fuel in the cement industry, for instance, and some tires are turned into crumb and used as fillers,4 for example, in asphalt or cement, but a large fraction of tires constitute a form of waste that is difficult to dispose of.5 Tire fires at storage facilities are not uncommon and storage of tires can be accompanied by the leaching of their many constituents into the environment.
The sulfur-crosslinked rubbers used in automobile tires contain a wide variety of constituents, including (spent) catalysts for their formation, antioxidants, colorants, particulate reinforcing agents like carbon black and/or organosulfur-modified silica, and fibrous reinforcing agents including nylon cord and woven steel.6 However, the main component of the tire is typically a hydrocarbon-based, sulfur-cured, elastomer. There is a longstanding need to recover the organic materials from tires for sustainable reuse.
The difficulty associated with cleavage of sulfur crosslinks in organic elastomers compromises the ability to reuse or recycle rubber tires. The S—S bond strength is only 309 kJ mol−1,7 so it is somewhat surprising that practicable processes for S—S cleavage in vulcanized tires have not been reported. Reuse strategies typically involve energetically intensive, relatively inefficient pyrolytic conversion into fuel gas, low grade carbon black and other materials. Alternative chemical approaches, for example, reactive reduction with LiAIH48-9 or amines10 have been proven only moderately successful.
Hydrosilanes are mild reducing agents. Transition metal-catalyzed processes like hydrolysis/alcoholysis,11 C═C and C═O hydrosilylation;12 and reductions of radicals13 and of carbocations14 are efficient. The Lewis acid-catalyzed (typically with B(C6F5)3═BCF) reduction of carbonyls, ethers, silanols, alkoxysilanes (the Piers-Rubinsztajn reaction15-17) and benzylic, but not aliphatic, sulfides and thioacetals18 are convenient, mild and efficient processes for both organic transformations and silicone synthesis. The processes are in many cases thermodynamically driven by cleavage of weaker SiH bonds to form much stronger Si-heteroatom bonds.19 The reactions are normally easy to control, often work at room temperature, and the main experimental issues are associated with managing the co-products when they are flammable gases, including hydrogen or alkanes.
It has now been discovered that hydrosilanes may be used to cleave S—S and S—H bonds using Lewis acid-catalyzed processes, which permits the conversion of organic thiols, polysulfides and sulfur-crosslinked solid rubber tires (for example, in the form of bicycle inner tubes, solid tires or tire crumb), into silylated homogeneous solutions in good to excellent yield. In the case of automotive elastomers, the accompanying, unreactive solids, such as fillers, fiber and metal reinforcements, pigments, etc., are readily removed by centrifugation or filtration. The resulting products have been desilylated and re-oxidized to form new disulfide compounds, or in the case of polymeric compounds, new elastomers.
Accordingly, the present application includes a process for cleaving one or more S—S and/or S—H bonds in one or more organic compounds, comprising combining the one or more organic compounds with one or more hydrosilanes and a catalyst to form a reaction mixture and treating the reaction mixture under conditions to cleave one or more of the S—S and/or S—H bonds.
The present application also includes a method for de-crosslinking one or more sulfur-crosslinked elastomers comprising combining the one or more sulfur-crosslinked elastomers with one or more hydrosilanes and a catalyst to form a reaction mixture and treating the reaction mixture under conditions to de-crosslink the sulfur-crosslinked elastomers.
Other features and advantages of the present application will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the application, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the description as a whole.
The embodiments of the application will now be described in greater detail with reference to the attached drawings in which:
Unless otherwise indicated, the definitions and embodiments described in this and other sections are intended to be applicable to all embodiments and aspects of the present application herein described for which they are suitable as would be understood by a person skilled in the art.
The present application refers to a number of chemical terms and abbreviations used by those skilled in the art. Nevertheless, definitions of selected terms are provided for clarity and consistency.
In understanding the scope of the present application, the term “comprising” and its derivatives, as used herein, are intended to be inclusive or open-ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives.
As used herein, the word “consisting” and its derivatives, are intended to be close ended terms that specify the presence of stated features, elements, components, groups, integers, and/or steps, and also exclude the presence of other unstated features, elements, components, groups, integers and/or steps.
The term “consisting essentially of”, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and/or steps as well as those that do not materially affect the basic and novel characteristic(s) of these features, elements, components, groups, integers, and/or steps.
Terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies.
As used in this application, the singular forms “a”, “an” and “the” include plural references unless the content clearly dictates otherwise. For example, an embodiment including “a catalyst” should be understood to present certain aspects with one catalyst or two or more catalysts. In embodiments comprising an “additional” or “second” component, such as an additional or second catalyst, the second component as used herein is chemically different from the other components or first component. A “third” component is different from the other, first, and second components, and further enumerated or “additional” components are similarly different.
The term “and/or” as used herein means that the listed items are present, or used, individually or in combination. In effect, this term means that “at least one of” or “one or more” of the listed items is used or present.
The term “alkyl” as used herein, whether it is used alone or as part of another group, refers to straight or branched chain, saturated alkyl groups. The number of carbon atoms that are possible in the referenced alkyl group are indicated by the prefix “Cn1-n2”. For example, the term C1-10 alkyl means an alkyl group having 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms.
The term “alkoxy” as used herein, whether it is used alone or as part of another group, refers to straight or branched chain, saturated alkyl-O groups. The number of carbon atoms that are possible in the referenced alkoxy group are indicated by the prefix “Cn1-n2”. For example, the term C1-10 alkoxy means an alkoxy group having 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 carbon atoms.
The term “aryl” as used herein, whether it is used alone or as part of another group, refers to cyclic groups containing from 6 to 10 carbon atoms and at least one aromatic ring. In an embodiment of the application, the aryl group contains from 6, 9 or 10 carbon atoms, such as phenyl, indanyl or naphthyl.
The term “C1-6alkoxy-substituted” as used herein means that one hydrogen atom on the referenced group is substituted with a C1-6alkoxy group.
The term “fluoro-substituted” as used herein means one or more, including all, of the hydrogen atoms on the referenced group is substituted with a fluorine atom.
The term “hydrosilane” as used herein refers to a compound containing at least one Si—H bond.
The term “linear silicone” as used herein in reference to a substituent group on a molecule means a group having the chemical formula
where
represents the point of attachment to the molecule, each Ra, Rb and Rc is independently C1-6alkyl or aryl (typically methyl or phenyl) and p represents the number of repeating OSiRaRb groups (typically 0-100). It is common for Ra, Rb and Rc to be the same.
The term “branched silicone” as used in reference to a substituent group on a molecule means a group having the chemical formula
in which one or more of the Ra or Rb groups are —(OSiRaRb)q—, resulting in a branched-type structure, and the other Ra or Rb groups, as well as Rc are independently C1-6alkyl or aryl (typically methyl or phenyl), p represents the number of repeating OSiRaRb groups (typically 1-100) and
represents the point of attachment to the molecule. It is common for Ra, Rb and Rc to be the same.
The terms “elastomer” or “rubber” as used herein, are used interchangeably refers to polymers crosslinked using at least one disulfide bond.
The term “thiol”, as used herein, refers to a compound containing an S—H bond.
The term “organic compounds” as used herein refers to carbon-based compounds comprising at least one S—S and/or S—H bonds and includes polymeric and non-polymers compounds and optionally comprised other heteroatoms, such as but not limited to O, Si, Ti and/or N
The term “organopolysulfide” as used herein, refers to a compound containing one or more sulfur-sulfur bonds, including but not limited to, a disulfide and an oligosulfide.
The present application includes a process for cleaving one or more S—S and/or S—H bonds in one or more organic compounds, comprising combining the one or more organic compounds with one or more hydrosilanes and a catalyst to form a reaction mixture and treating the reaction mixture under conditions to cleave one or more of the S—S and/or S—H bonds.
The present application also includes a process for cleaving one or more S—S and/or S—H bonds in one or more organic compounds, comprising combining the one or more organic compounds with one or more hydrosilanes to form a reaction mixture and treating the reaction mixture under conditions to cleave one or more of the S—S and/or S—H bonds.
In some embodiments, the conditions to cleave the one or more of the S—S and/or S—H bonds comprises a reaction temperature of about 20° C. to about 130° C., preferably about 20° C. to about 100° C. In some embodiments, the conditions to cleave the one or more of the S—S and/or S—H bonds comprises a reaction temperature of about 40° C. to about 80° C. In some embodiments, the conditions to cleave the one or more of the S—S and/or S—H bonds comprises a reaction temperature of below about 80° C.
In some embodiments, the one or more organic compounds are selected from one or more sulfur-containing silyl coupling agents.
In some embodiments, the one or more sulfur-containing silyl coupling agents are selected from compounds of Formula I:
In some embodiments, R1, R2 and R3 are independently selected from C1-6alkyl, C2-6alkenyl, C1-2alkylenearyl, aryl, linear silicones and branched silicones. In some embodiments, R1, R2 and R3 are independently selected from C1-4alkyl, C2-4alkenyl, CH2aryl, aryl, linear silicones and branched silicones. In some embodiments, R1, R2 and R3 are the same and are CH3, CH3CH2, (CH3)2CH, CH3CH2CH2, CH2═CHCH2 or PhCH2. In some embodiments, R1, R2 and R3 are the same and are CH3CH2.
In some embodiments, R4 is H.
In some embodiments, R4 is
In some embodiments, R5, R6 and R7 are independently selected from C1-6alkyl, C2-6alkenyl, C1-2alkylenearyl, aryl, linear silicones and branched silicones. In some embodiments, R5, R6 and R7 are independently selected from C1-4alkyl, C2-6alkenyl, CH2aryl, aryl, linear silicones and branched silicones. In some embodiments, R5, R6 and R7 are the same and are CH3, CH3CH2, (CH3)2CH, CH3CH2CH2, CH2═CHCH2 or PhCH2. In some embodiments, R1, R2 and R3 are the same and are CH3CH2.
In some embodiments, x and z are independently 1, 2, 3 or 4. In some embodiments, x and z are independently 3 or 4. In some embodiments, x and z are both 3 or 4. In some embodiments, x and z are both 3.
In some embodiments, the one or more compounds of Formula I are selected from:
wherein each x and z is independently 1, 2, 3 or 4, suitably 3 and y is selected from 2, 3, 4, 5 and 6.
In some embodiments, the one or more organic compounds are one or more organopolysulfides.
In some embodiments, the one or more organopolysulfides are selected from one or more sulfur-cured elastomers.
In some embodiments, the one or more sulfur-cured elastomers is a crosslinked polyolefin.
In some embodiments, the one or more sulfur-cured elastomers is a polyisobutylene, polyisoprene, natural rubber or polybutadiene, or a copolymer thereof, such as styrene-butadiene.
In some embodiments, the one or more hydrosilanes are selected from compounds of Formula II:
wherein
R8, R9 and R10 are independently selected from H, halo, C1-10alkyl, C1-10alkoxy, aryl, C1-2alkylenearyl, C1-6alkoxy-substituted C1-10alkyl, C1-6alkoxy-substituted aryl, linear silicones and branched silicones, provided that at least one of R8, R9 and R10 is other than H.
In some embodiments, R8, R9 and R10 are independently selected from H, Cl, C1-6alkyl, C1-6alkoxy, aryl, C1alkylenearyl, C1-4alkoxy-substituted C1-6alkyl, C1-4alkoxy-substituted aryl, linear silicones and branched silicones, provided that at least one of R8, R9 and R10 is other than H. In some embodiments, zero or one of R8, R9 and R10 are independently linear silicones or branched silicones and the remainder of R8, R9 and R10 are, independently selected from H, Cl, C1-4alkyl, C1-4alkoxy, phenyl, CH2phenyl, C1-2alkoxy-substituted C1-4alkyl and C1-2alkoxy-substituted aryl, provided that at least one of R8, R9 and R10 is other than H. In some embodiments, the compound of Formula II is selected from, PhSiH3, PhMeSiH2, Ph2SiH2, Ph2MeSiH, Ph3SiH, H2SiCl2, HSiCl3, MeSiH3, MeSiHCl2, Me2SiH2, Me3SiH, Et2SiH2, MeHSi(OMe)2, HSi(OMe)3, Et2MeSiH, Et3SiH and HSi(OEt)3, and mixtures thereof. In some embodiments, the compound of Formula II is selected from, PhSiH3, PhMeSiH2 and Ph2SiH2, and mixtures thereof.
In some embodiments, the one or more hydrosilanes are selected from compounds of Formula III
wherein
R11, R12, R13, R14, R15, R16, R17, R18, R19 and R20 are independently selected from H, halo, C1-10alkyl, C1-10alkoxy, aryl, C1-2alkylenearyl, fluoro-substituted C1-10alkyl, C1-6alkoxy-substituted C1-10alkyl, fluoro-substituted aryl and C1-6alkoxy-substituted aryl, provided that
R12 is H, and R11, R13, R14, R15, R16, R17, R18, R19 and R20 are other than H,
R12 and R19 are H, and R11, R13, R14, R15, R16, R17, R18 and R20 are other than H, or
R14 is H and R11, R13, R15, R16, R17, R18 and R20 are other than H, or
R16 is H and R11, R13, R14, R15, R17, R18 and R20 are other than H; and
n and m are independently 0, 2, 3, 4, 5, 6, 7, 8, 9 or 10-1000. In some embodiments n and m are independently 0, 2, 3, 4, 5, 6, 7, 8, 9 or 10.
In some embodiments, R11, R12, R13, R14, R15, R16, R17, R18, R19 and R20 are independently selected from H, Cl, C1-6alkyl, C1-6alkoxy, phenyl, C1-2alkylenephenyl, fluoro-substituted C1-6alkyl, C1-4alkoxy-substituted C1-6alkyl, fluoro-substituted phenyl and C1-4alkoxy-substituted aryl, with the above-noted provisos.
In some embodiments, R11, R12, R13, R14, R15, R16, R17, R18, R19 and R20 are independently selected from H, C1-4alkyl, phenyl, CH2phenyl, fluoro-substituted C1-4alkyl, C1-4alkoxy-substituted C1-4alkyl, fluoro-substituted phenyl and C1-4alkoxy-substituted aryl, with the above-noted provisos.
In some embodiments, the compounds of Formula III are selected from one or more of
wherein R16 is C1-6alkyl or aryl, each n is independently 0, 1, 2,3, 4, 5 or 6 and m is 0, 1, 2, 3, 4, 5 or 6.
In some embodiments, the hydrosilane is
In some embodiments, the one or more hydrosilanes are cyclic hydrosilanes, such as cyclic siloxanes, including, but not limited to tetramethylcyclotetrasiloxane and pentamethylcyclopentasiloxane.
In some embodiments, the S—S and/or S—H bonds in the one or more organic compounds are reductively cleaved.
In some embodiments, one or more silyl thiol ethers are produced in a process of the application.
In some embodiments, the one or more hydrosilanes are present in an amount from about 2:1 to about 5:1 equivalents relative to the S—S and/or S—H bond content in the one or more organic compounds.
In some embodiments, the catalyst is a Lewis acid. In some embodiments, the Lewis acid catalyst comprises boron. In some embodiments, the catalyst is B(C6F5)3.
In some embodiments, the catalyst is present in an amount of about 0.01 mol % to about 5 mol %.
The present application also includes a method for de-crosslinking one or more sulfur-crosslinked elastomers comprising combining the one or more sulfur-crosslinked elastomers with one or more hydrosilanes and a catalyst to form a reaction mixture and treating the reaction mixture under conditions to decrosslink the sulfur-crosslinked elastomers.
The present application also includes a method for de-crosslinking one or more sulfur-crosslinked elastomers comprising combining the one or more sulfur-crosslinked elastomers with one or more hydrosilanes to form a reaction mixture and treating the reaction mixture under conditions to decrosslink the sulfur-crosslinked elastomers.
In some embodiments, the one or more sulfur-crosslinked elastomers are rubbers used in tires. In some embodiments, the tires are automotive tires. In some embodiments, the tires are industrial tires such as truck tires, agricultural tires or mining tires. In some embodiments the rubbers are bicycle inner tubes.
In some embodiments, the elastomers or rubbers are treated prior to combining with the one or more hydrosilanes, for example to form into a crumb and/or to extract extractible components with solvents such as acetone.
The products produced by the process of the application are silylated sulfur-containing compounds, including silylated sulfur-containing polymers. In some embodiments, these silylated sulfur-containing compounds are separated from solid or inorganic materials that may have been present in the one or more organic compounds. In some embodiments, the solid or inorganic materials include, but are not limited to unreactive solids such as fillers, fiber and metal reinforcements, and pigments. In some embodiments, the solid or inorganic materials are separated by filtration and/or centrifugation.
In some embodiments, the products produced by the process of the application are silylated sulfur-containing organic polymers and are in the form of oils.
In some embodiments, the silylated sulfur-containing compounds are desilylated, for example by treatment with fluoride, to provide the corresponding thiols, which, in some embodiments, are re-oxidized to provide other S—S containing compounds.
In some embodiments of the application, provided herein are sulfur-containing elastomers produced by desilylation and reoxidation of the silicones produced by the reductive silylation process disclosed herein. In an embodiment, after removal of inorganic materials and desilylation of the reduced organic product with fluoride, the resulting thiols may be reoxidized to generate new elastomers.
The present application also includes de-crosslinked sulfur-cured elastomers prepared using the process of the present application.
The following non-limiting examples are illustrative of the present application. As is apparent to those skilled in the art, many of the details of the examples may be changed while still practicing the methods, compositions and kits described herein.
The following non-limiting examples are illustrative of the present application:
Experimental Procedure
Materials: Bis(trimethylsiloxy)methylsilane (BisH), bis(triethoxysilylpropyl)disulfide (90%), bis(triethoxysilylpropyl)tetrasulfide and pentamethyldisiloxane (PentaH) were purchased from Gelest and used after drying over molecular sieves overnight. Dibenzyl disulfide and benzyl bromide were obtained from Sigma Aldrich and used as received. Sodium tetrasulfide was purchased from Dojindo. B(C6F5)3 (BCF) was prepared by Grignard reaction following a literature procedure.20 Naphthalene (internal standard) was purchased from Fisher. Toluene (solvent) received from Caledon (HPLC grade) was dried over activated alumina before use. Deuterated NMR solvents were obtained from Cambridge Isotope Laboratories. All glass apparatus were dried overnight at 120° C. and cooled under a dry nitrogen atmosphere for 30 min prior to use.
Methods: 1H, 13C and 29Si NMR spectra were recorded on a BrukerAvance 600 MHz nuclear magnetic resonance spectrometer using deuterated solvents chloroform-d and actone-d6. For 29Si NMR, chromium(III) acetylacetonate was used as paramagnetic relaxation agent.
GC-MS analyses were performed using an Agilent 6890N gas chromatograph (Santa Clara, Calif., USA), equipped with a DB-17ht column (30 m×0.25 mm i.d.×0.15 μm film, J&W Scientific) and a retention gap (deactivated fused silica, 5 m×0.53 mm i.d.), and coupled to an Agilent 5973 MSD single quadruple mass spectrometer. One microliter of sample was injected using an Agilent 7683 autosampler in splitless mode. The injector temperature was 250° C. and carrier gas (helium) flow was 1.1 mL/min. The transfer line was 280° C. and the MS source temperature was 230° C. The column temperature started at 50° C. and was increased to 300° C. at 8° C./min, then held at 300° C. for 15 min to give a total run time of 46.25 min. Full scan mass spectra between m/z 50 and 800 mass units were acquired after a solvent delay of 8 min.
LC-MS analyses were undertaken using an Agilent Technologies 1200 LC coupled to an Agilent 6550 QTOF mass spectrometer. An injection volume of 2 μL was separated on a Phenomenex Luna C18(2) (150 mm×2.0 mm, 3 μm) column with 100 Å pore size (Phenomenex, Calif., USA). The mobile phases were LC-MS-grade 45/55 water/methanol with 0.5% acetic acid (A) and methanol with 0.5% acetic acid (B) at a flow rate of 300 μL/min. The column temperature was maintained at 40° C., and the autosampler storage tray was set at 10° C. The mobile phase gradient eluted isocratically with 10% B for 1.0 min followed by a gradient to 100% B over 17 min. The gradient was maintained at 100% B for 2 min and decreased to 10% B over 0.1 min. The gradient was then followed by a 5 min re-equilibration prior to the next injection. The total time for an HPLC run was 25 min. The MS parameters (for LC-MS) chosen were as follows: ESI, gas temperature at 225° C., drying gas at 13 L/min, nebulizer pressure at 20 psi, sheath gas temperature at 400° C., sheath gas flow at 12 L/min, VCap at 3500 V, Nozzle Voltage at 1000 V, fragmenter at 375 V, and October 1 RF Vpp at 750 V. The data were acquired in electrospray positive mode from m/z 50 to 1000 at a scan rate of 1.5 Hz. The mass was auto recalibrated using reference lock mass from Agilent ESI-T Tuning Mix (for Ion Trap).
Thermogravimetric analysis (TGA) analysis according to ASTM D 6370-99 (American Society for Testing and Materials) was carried out to measure the organic polymer, carbon black content and inorganic residue of the component. A small amount of test sample (2 to 5 mg) was placed into the alumina pan of the calibrated Thermogravimetric Analyzer (Mettler Toledo TGA/DSC 3+). A 100 cm3 min-1 argon purge was applied and the furnace was heated from 50° C. to 560° C. at 10° C. min-1. Then, the furnace was cooled to 300° C. and the purge gas was changed to air at 100 cm3 min-1. The temperature was allowed to equilibrate for 2 min before the furnace was heated to 800° C. at 10° C. min−1.
Stock solutions were prepared as follows:
B(C6F5)3: BCF was dissolved in dry toluene to prepare a stock solution (50 mg mL−1).
Bis(triethoxysilylpropyl)disulfide: naphthalene (0.056 g, 0.437 mmol) was added to bis(triethoxysilylpropyl)disulfide (2.21 mL, 2.27 g, 4.77 mmol) in a dried 20.0 mL glass vial (naphthalene 128.17 g mol-1, density of disulfide coupling agent: 1.025 g mL-1, density of tetrasulfide coupling agent: 1.095 g mL-1).
Bis(triethoxysilylpropyl)tetrasulfide: naphthalene (0.301 g, 2.35 mmol) was added to bis(triethoxysilylpropyl)tetrasulfide (11.77 mL, 12.89 g, 23.91 mmol) in a dried 20.0 mL glass vial).
Naphthalene in chloroform-d: solid naphthalene (4 mg, 0.031 mmol) was added to chloroform-d (2 mL, 3.0 g, 24.92 mmol) in a dried 20.0 mL glass vial.
(a) Preparation of Dibenzyl Tetrasulfide
To a pre-dried 200 mL round-bottomed flask purged with dry N2 were added sodium tetrasulfide (0.098 g, 0.562 mmol), benzyl bromide (0.209 g, 1.22 mmol) and dry THE (50 mL, 44.45 g, 0.616 mol) as solvent. The mixture was stirred for 23 d and collected by vacuum filtration. The mixture was purified using column chromatography; the low polarity impurity (S8) was removed using hexanes, after which the elution solvent was switched to dichloromethane, to give a yield of 77% (137 mg, based on the different sulfides found in the product, Note: it was not possible to detect tetrasulfide or higher polysulfide linkages in the GC-MS, which may be due to thermal degradation of polysulfide bond (when n>3), according to Stensaas et al. (2008)21
1H NMR (600 MHz, acetone-d6): δ 3.72 (s, 0.11H), 4.11 (s, 2.02H), 4.24 (s, 1.20H), 4.30 (s, 0.67H), 7.28-7.40 (m, 9.21H) ppm. 13C NMR (600 MHz, acetone-d6): δ 29.85, 43.55, 43.99, 44.54, 128.36, 129.42, 130.33, 137.54, 137.80 ppm.
GC-MS C14H14S2, MW:246: [M+-1]=65.2 (18), 77.2 (8), 91.1 (90), 121.1 (5), 181.2 (18), 246.1 (20); S8, MW:256: [M+-1]=64.1 (100), 77.1 (7), 91.1 (66), 127.9 (60), 160.0 (66), 191.9 (53), 207.1 (5), 223.9 (7), 255.8 (98), 315.1 (5). C14H14S3, MW:278: [M+-1]=65.2 (13), 77.1 (3), 91.2 (100), 123.1 (3), 181.2 (2), 213.1 (56), 278.1 (13).
(b) Dibenzyl Disulfide Reduction Using Bis(Trimethylsiloxy)Methylsilane ([SiH]/[SS]=2:1, Molar Ratio Between Hydrosilane and Disulfide)
To a pre-dried 200 mL round-bottomed flask purged with dry N2 gas, dibenzyl disulfide (0.50 g, 2.04 mmol) and bis(trimethylsiloxy)methylsilane (0.91 g, 4.09 mmol) were added together with dry DCM (3.81 g, 44.86 mmol) as solvent. Freshly prepared B(C6F5)3 stock solution was added (0.167 mL, 0.0326 mmol) after 5 min stirring. The total reaction time was 3 h, then the reaction was quenched by adding neutral alumina. The reaction was conducted under identical reaction conditions only changing the ratio of hydrosilane to disulfide [SiH]/[SS] (Table 1) to establish relative reactivity of functional groups. Conversion in the reaction was shown by peak area of the hydrogens on the carbon adjacent to the disulfide bond (—CH2SS) in 1H NMR which was plotted against different ratios of hydrosilane (SiH) to disulfide. Analogous techniques were used to follow the reduction of the tetrasulfide.
a100 mg mL−1 B(C6F5)3 stock solution
Dibenzyl tetrasulfide reduction using bis(trimethylsiloxy)methylsilane ([SiH]/[SS]=1:1): To a dried glass NMR tube (7×5 mm) purged with dry N2 gas, dibenzyl tetrasulfide (0.055 g, 0.185 mmol, a mix of oligosulfides, see above) and bis(trimethylsiloxy)methylsilane (0.048 g, 0.216 mmol) were added together with chloroform-d stock solution (0.6 mL). Freshly prepared B(C6F5)3 stock solution was added (0.023 mL, 0.0045 mmol) after 5 min sonication. Titrations were performed by adding bis(trimethylsiloxy)methylsilane (0.048 g, 0.216 mmol) and B(C6F5)3 stock solution (0.023 mL, 0.0045 mmol) in aliquots portion by portion with 3 h time interval between additions.
(c) Complete Reduction of Bis(Triethoxysilylpropyl)Disulfide Using Bis(Trimethylsiloxy)Methylsilane ([SiH]/[SiOR]=8:6, Molar Ratio Between Hydrosilane and Alkoxysilane)
To a pre-dried 200 mL round bottle flask purged with dry N2 gas, bis(triethoxysilylpropyl)disulfide stock solution (0.50 g, 1.05 mmol) and bis(trimethylsiloxy)methylsilane (1.90 g, 8.54 mmol) were added together with dry toluene (0.730 g, 7.92 mmol) as solvent. Freshly prepared B(C6F5)3 stock solution was added (0.345 mL, 0.067 mmol) after 5 minutes stirring. The total reaction time was 3 h, then the reaction was quenched by adding neutral alumina; yield was 91.9%.
1H NMR (600 MHz, chloroform-d): δ 0.03-0.12 (m, 604H), 0.29-0.30 (m, 20H), 0.37 (s, 2H), 0.66-0.69 (m, 14H), 1.17-1.18 (m, 5H), 1.55 (s, 10H), 1.69-1.72 (m, 13H), 2.36 (s, 84H), 2.50-2.54 (m, 13H), 3.78-3.80 (m, 1H), 7.48-7.49 (dd, J=7.49 Hz, 1H), 7.84-7.86 (dd, J=7.85 Hz, 1H) ppm; 13C NMR (600 MHz, chloroform-d): δ −1.86, 2.06, 5.16, 14.53, 21.81, 26.55, 30.07, 77.36, 126.16, 128.58, 138.21 ppm; 29Si NMR (600 MHz, chloroform-d, trace Cr(acac)3): δ 9.40-9.53 (m, 4Si), 7.45-7.62 (m, 16Si), −30.67 (s, 1Si), −54.49 (s, 2Si), −66.16−(−)65.90 (m, 1Si) ppm.
(d) Complete Reduction of Bis(Triethoxysilylpropyl)Tetrasulfide Using Bis(Trimethylsiloxy)Methylsilane ([SiH]/[SiOR]=12:6, Molar Ratio Between Hydrosilane and Alkoxysilane)
To a pre-dried 200 mL round bottle flask purged with dry N2 gas were added bis(triethoxysilylpropyl)tetrasulfide stock solution (0.50 g, 0.93 mmol) and bis(trimethylsiloxy)methylsilane (2.48 g, 11.1 mmol) together with dry toluene (0.90 g, 9.77 mmol) as solvent. Freshly prepared B(C6F5)3 stock solution was added (0.456 mL, 0.089 mmol) after 5 minutes stirring. The total reaction time was 3 hours, then the reaction was quenched by adding neutral alumina; yield was 59.0%.
1H NMR (600 MHz, chloroform-d): δ 0.03-0.15 (m, 964H), 0.29-0.30 (d, J=0.29 Hz, 13H), 0.37 (s, 35H), 0.67-0.69 (m, 14H), 1.70-1.73 (m, 14H), 2.36 (s, 141H), 2.51-2.54 (m, 14H), 7.48-7.49 (dd, J=7.48 Hz, 1H), 7.84-7.86 (dd, J=7.85 Hz, 1H) ppm; 13C NMR (600 MHz, chloroform-d): δ −1.86, 2.02, 5.16, 14.43, 21.81, 26.67, 28.29, 30.08, 77.36, 125.66, 128.58, 129.39, 138.21 ppm; 29Si NMR (600 MHz, chloroform-d, trace Cr(acac)3): δ 9.42-9.60 (m, 17Si), 7.46-7.64 (m, 25Si), −30.66 (s, 1Si), −32.92 (s, 4Si), −66.22−(−) 65.88 (m, 6Si), −71.05 (s, 0.12 Si) ppm.
(e) Bis(Triethoxysilylpropyl)Disulfide Using Bis(Trimethylsiloxy)Methylsilane ([SiH]/[SiOR]=3:6)
As bis(triethoxysilylpropyl)disulfide is a complex mixture of oligosulfides, calculations for the concentration of S—S groups in the monomer were adjusted according to GC-MS data showing that the major constituent was the disulfide (Table 2).
To a pre-dried 200 mL round bottle flask purged with dry N2 gas were added bis(triethoxysilylpropyl)disulfide stock solution (1.015 g, 214 mmol) and bis(trimethylsiloxy)methylsilane (1.431 g, 6.43 mmol) together with dry toluene (0.730 g, 7.92 mmol) as solvent. Freshly prepared B(C6F5)3 stock solution was added (0.259 mL, 0.051 mmol) after 5 min stirring. The total reaction time is 3 h, then the reaction was quenched by adding neutral alumina. The product was collected under reduced pressure. The reaction was conducted under identical reaction conditions only changing the ratio of hydrosilane against disulfide [SiH]/[SS] (Table 3).
a100 mg mL−1 B(C6F5)3 stock solution
bonly at this point [BCF]/[SiH] = 2.5 mol %, under which [SiH] unable to be consumed completely
(f) Bis(Triethoxysilylpropyl)Tetrasulfide Using Bis(Trimethylsiloxy)Methylsilane ([SiH]/[SiOR]=3:6)
As bis(triethoxysilylpropyl)tetrasulfide is a complex mixture of oligosulfides, calculations for the concentration of S—S groups in the monomer were adjusted according to LS-MS data showing that the major constituent was the trisulfide (Table 4).
To a pre-dried 200 mL round bottle flask purged with dry N2 gas were added bis(triethoxysilylpropyl) tetrasulfide stock solution (0.508 g, 0.943 mmol) and bis(trimethylsiloxy)methylsilane (0.626 g, 2.81 mmol) together with dry toluene (0.339 g, 3.67 mmol) as solvent. Freshly prepared B(C6F5)3 stock solution was added (0.114 mL, 0.022 mmol) after 5 min stirring. The total reaction time is 3 h, then the reaction was quenched by adding neutral alumina. The product was collected under reduced pressure. The reaction was conducted under identical reaction conditions only changing the ratio of hydrosilane against disulfide [SiH]/[SS] (Table 5).
a100 mg mL−1 B(C6F5)3 stock solution
Results
Model compounds demonstrated the efficiency of the reduction reaction. The B(C6F5)3-catalyzed (BCF) reductions of dibenzyl disulfide 1 (n=1) with a small hydrosiloxane HSiMe(OSiMe3)2 (BisH) 2 provided a sense of the relative reactivity of the various sulfur bonds (
Analogous outcomes were observed when the reductions were repeated with analogous commercial coupling agents that are used to modify silica in ‘green tires’ including, without limitation, alpha-functional coupling agents and gamma-functional coupling agents, such as 8, 9 and 10 (
1H NMR data following the addition of aliquots of 2 to 10 demonstrated that Sβ-Sβ bonds distal to carbon were somewhat more reactive than C—Sα-Sβ bonds, and that S—S bonds were preferentially cleaved over the Si-OEt groups (thus, relative reaction rates are S—H>S—S>Si—OR), but complex mixtures resulted until excess reducing agent was added, at which point the identical product 14 was formed from all three starting materials 8-10 (
The use of substoichiometric quantities of silicone reducing agents permits the synthesis of monomers useful for the synthesis of sulfur-containing silicones 11-14 and analogous compounds with different alkoxy groups and alternate linear and branched silicone groups are similarly available, for e.g. 15-18 (
Experimental Procedure
Silane materials: Tetramethyldisiloxane (TetraH) was purchased from Gelest and used after drying over molecular sieves overnight.
Rubber samples: Bicycle inner tube (Chaoyang 700×38/45C bicycle inner tube, China), Truck tread 1: a piece of truck tread, not part of a complete tire, was found at a local garbage dump (origin unknown), Truck tread 2: (Sailun 225/70R19.5), EPDM (pond liner, purchased at a local garden center, producer unknown), Crumb-1 (Canadian Eco Rubber Ltd., Emterra, Canada), Crumb-2 (AI's-RC, Amazon, Canada) were used as received. From both truck treads, samples were cut only from the external, road contacting tread part. The tread and side wall samples—cross sections—were cut from different parts of a used car tire (snow tire, Cooper 185/65R4).
Methods: The polymer constituents of rubber samples were estimated from carbon high-resolution magic angle spinning (13C HR-MAS) NMR spectroscopy. The oils produced by the silylating reductions were characterized primarily by NMR and TGA as described in Example 1. Molecular weight of recovered organic oil and polydispersity index (PDI) were estimated from gel permeation chromatography (GPC) using a Waters 2695 Separations Module equipped with a Waters 2996 photodiode array detector, a Waters 2414 refractive-index detector, and two Jordi Labs Jordi Gel DVB columns. Polystyrene standards were used for calibration, and THF was used as the eluent at a flow rate of 1.0 mL min−1. The Young's moduli of rubber samples were determined using a MACH-1 micromechanical system (Biomomentum Instruments) with a 0.500 mm hemispherical indenter radius, and Poisson ratio of 0.3. All measurements were conducted at 22° C. and in triplicate, with error bars representing the standard deviation of the replicate measurements.
To prepare powdered rubber, raw rubber samples of different shapes and sizes as well as rubber crumb samples of broad dispersity, with average particles sizes of 2.14±0.06 mm for Crumb-1 and 1.30±0.09 mm for Crumb-2, were obtained. A cryogenic grinding progress was used to obtain rubber powder samples with homogeneous particle sizes for comparable experiment. Liquid nitrogen was employed to cool the rubber samples below their glass transition temperature before they were pulverized with a coffee grinder (KitchenAid) to give powders with an average particle size of ˜330 μm. Bulk samples were cut directly from different parts of a car tire using a reciprocating saw.
As raw rubber samples may contain organic additives, including oil or other additives that could influence the reductive cleavage process, in a control experiment, the extractable components were extracted prior to reduction. The most commonly used extraction solvent23 acetone, was used to remove resins, free sulfur, acetone soluble softeners and antioxidants, processing rubber additives, mineral oils, waxes, organic accelerators and their reactive products and fatty acids. The Soxhlet extraction procedure was as follows: 5.0 g raw truck tread 1 rubber powder was placed inside Whatman cellulose extraction thimble (33 mm×118 mm). The sample containing the thimble was extracted with 200 ml refluxing acetone 56° C. for 72 h in a standard Soxhlet apparatus. After this purification, the sample was dried in 100° C. oven overnight. The weight of collected sample was 4.48 g.
The general experimental procedure (with powders or coupons) is as follows: The cryogenically ground rubber powder was allowed to swell in dry toluene (12 mL) for 30 min. Pentamethyldisiloxane 7 was added to the reaction mixture. Then the stock catalyst solution was added to initiate the reaction. The suspension was heated in a 60° C. oil bath for 48 h. The residual undissolved rubber powder was washed with toluene and separated by centrifugation (Eppendorf, Centrifuge 5424, at 12000 rpm for 20 min). The extraction process was repeated two times to completely remove soluble compounds. The supernatants were mixed, and the solvent was removed by rotary evaporation. Any residual (organic) volatiles were removed under a stream of N2 over 24 h. The residual rubber powders were dried at 100° C. overnight and then examined by TGA. The reaction conditions for the reduction of rubber powder are as follows: 300 mg powdered sample; pentamethyldisiloxane 7 1.5 mL (1.14 g, 7.7 mmol), BCF/Rubber=10 wt %, 12 mL toluene, 60° C., 48 h. The recovered organic liquid was characterized by NMR.
The organic yields were calculated from TGA profiles:
Where m0 is the mass of starting rubber and mi is the mass of residual solid. W0 is the organic weight % in starting rubber. Wi is the organic weight % in residual solid. Both W0 and W1 is from TGA.
The experimental procedure for the reduction of bulk samples is as follows: The raw tread bulk (cross-section, 2.083 g, ˜1.200 cm×1.519 cm×1.125 cm, containing metal and fiber) was allowed to swell in dry toluene (80 mL) for 6 h. Pentamethyldisiloxane (7.6 g, 51.35 mmol, 10 mL) was added to the reaction mixture. Several ceramic beads were added to increase shear force while stirring with a magnetic stirrer. BCF catalyst was added portion by portion each 24 h (6+2+2+2 wt % B(C6F5)3). The reaction mixture was heated in a 60° C. oil bath for 6 d. The residual undissolved rubber bulk was washed with toluene and dried in a 100° C. oven. The suspension was centrifuged, then washed, and re-centrifuged (repeated twice). The supernatants were mixed, and the solvent was removed by rotary evaporation. The volatile organics were removed by blowing with a stream of N2 for 48 h. The residual (now smaller) rubber bulk (0.460 g, broken in two pieces: 0.675 cm×1.331 cm×0.445 cm, 0.754 cm×0.937 cm×0.340 cm) and powder (0.527 g) were separately examined using TGA. The recovered organic liquid was characterized by NMR.
Multiple reductions of bulk rubber: The residual solid after the first reduction (300 mg) was ground cryogenically (under liq. N2) and subjected to a secondary reduction using fresh catalyst (30 mg) and pentamethyldisiloxane (1.14 g, 1.5 ml). The reduction was carried out for 48 h in a 60° C. oil bath, followed with an additional wash sequence. A similar protocol was followed using a piece of sidewall (1.595×1.427×0.6096 cm, 1.475 g; which led to 1.427×1.409×0.549 cm). Experimental conditions for the first reduction: BCF/Rubber=12 wt % (added portion by portion: 6+2+2+2), 6 days, 60° C. The residual bulk materials from the first reduction were ground into a powder prior to the second reduction. Experimental conditions for the second reduction: BCF/Rubber=10 wt % (added all at once), 48 h, 60° C. Only metal was removed from the elastomer matrix in first reduction step; polymeric fiber remained bound to the residual bulk solid.
Reduction with tetramethyldisiloxane: Cryogenically ground inner tube powder (300 mg) was placed in dry toluene (12 mL), followed by tetramethyldisiloxane (1.5 mL, 8.5 mmol). The stock catalyst solution (600 μL, stock catalyst concentration: 50 mg/mL in toluene, catalyst concentration in reaction: 10 wt %/inner tube) was added immediately afterwards to initiate the reaction. The suspension was heated in a preheated 100° C. oil bath for 30 min. The reaction flask was put into a room temperature water bath to quench the reaction and followed by a separation process using the same protocol as described above. The organic yield was 87%.
Results
Inner tubes (bicycle) and side walls (automobile) frequently contain high quantities of polyisobutylene (PIB), while tread rubber often contains higher fractions of PIB, polybutadiene (PDB), polyisoprene (PIP) and natural rubber (NR). The constitution of the elastomeric component of the rubber materials was determined by NMR and, particularly, by their thermogravimetric degradation profile between 50-560° C. (Table 6) butyl rubber (inner tubes) decomposes from 387-430° C., Tmax at 412° C.24 polyisoprene from 300-450° C., Tmax at 365° C.; polybutadiene from 350-485° C., Tmax at 465° C. and styrene-butadiene rubber from 325-465° C., Tmax at 447° C.;25 inorganic carbon (carbon black) thermally decomposes from 560-800° C. in oxygen.26 TGA and differential thermal analysis (DTA) data show the constituents of the rubber samples tested (
Elastomers including EPDM (ethylene propylene diene monomer terpolymer, ‘pond liner’), PIB (bicycle inner tube), truck tread, automobile side wall, automobile tread, and commercially available ‘rubber crumb’ (scrap rubber from automobile tires formed by shredding tires from multiple sources to remove metal wires and polyester cord and grinding the resulting product to various crumb sizes) were exposed to reductive silylation conditions. In the samples tested, the organic rubber content was approximately 60 wt % (Table 7).
aEPDM terpolymer of ethylene, propylene and diene, PIP/NR polyisoprene/natural rubber, PIB polyisobutylene, PBD polybutadiene. Constituents were determined by a combination of TGA, which showed separate decomposition temperatures for PIB and PIP, and 1H NMR.
With PentaH as reducing agent, the yields for reduction—as judged by the fraction of organic oils that could be separated by filtration from inorganic constituents, including carbon black and other excipients (e.g. metal wires can be magnetically separated from the bulk rubber after reduction)—ranged from about 36% up to 93% (Table 8). Two separate sources of commercial crumb were compared for their reactivity under the reducing silylation conditions. The reduction process was readily visible by eye, as black dispersions were converted to yellow oils. Much more BCF catalyst (10 wt % compared to the rubber starting material) was required to achieve reasonable yields of reduction with rubbers than with the model compounds (<1 mol %), which is not surprising given the complexity of the mixed rubber starting materials and the fact that they have been exposed to degradation and various environments during use. The efficiency of the reduction of rubber samples was dependent on catalyst concentration. With 1 wt % BCF catalyst concentration, (60 μL of catalyst containing 3 mg of B(C6F5)3 for 300 mg of inner tube powder in 12 mL of toluene with 113 μL of PentaH, 60° C., 48 h), 41% (organic yield calculated by TGA data) of the organic compound in rubber could be reduced to soluble polymers, while at higher catalyst concentrations (B(C6F5)3/Rubber=10 wt %), the organic yield reaches 93%. The TGA data before and after reduction for the different rubbers is found in
aExperimental conditions: 300 mg powdered sample; PentaH 1.5 mL (1.14 g, 7.7 mmol), BCF/Rubber = 10 wt %, 12 mL toluene.
bBased on TGA.
cIncludes carbon black and thermally stable inorganic moieties.
dFraction of available elastomer converted into organic soluble oils.
eResidual solid contains carbon black, inorganics and residual polymeric rubber.
f49.0 mg of residual powder and 188 mg of residual coupon still containing elastomer.
g2000 mg of starting elastomer was used, PentaH 10.0 mL, B(C6F5)3/Rubber = 10 wt %, 80 mL toluene, 60° C., 48 h.
hThe rubber powder was allowed to swell in reaction mixture for 3 h before adding B(C6F5)3 catalyst.
iCatalyst solution was added immediately after addition of hydrosilane.
jProcess for degradation of Crumb-1 using multiple reduction cycles; 169.0 mg of starting material were used in the 2nd reduction and 65.0 mg in the 3rd (FIG. 8).
Since elevated, but still mild, temperatures were more effective than room temperature, in this work, 60° C. was selected as a convenient reaction temperature to avoid the possibility of BCF degradation beyond 80° C. in the presence of moisture.27 These studies with rubber reduction, however, showed this not to be problematic. A 93% yield of recovered organic polymer (PIB from inner tube) was achieved at 100° C. after 18 hours, but an 87% yield had already been achieved in the first 30 minutes (Table 9). This result suggests that reduction processes at 100° C. or higher, for example up to 130° C., could be adapted to a continuous process.
aExperimental conditions: 300 mg powdered sample; PentaH 1.5 mL (1.14 g, 7.7 mmol), BCF/Rubber = 10 wt %, 12 mL toluene.
bBased on TGA.
cIncludes carbon black and thermally stable inorganic moieties.
dFraction of available elastomer converted into organic soluble oils.
eResidual solid contains carbon black, inorganics and residual polymeric rubber.
f49.0 mg of residual powder and 188 mg of residual coupon still containing elastomer.
g2000 mg of starting elastomer was used, PentaH 10.0 mL, B(C6F5)3/Rubber = 10 wt %, 80 mL toluene, 60° C., 48 h.
hThe rubber powder was allowed to swell in reaction mixture for 3 h before adding B(C6F5)3 catalyst.
iCatalyst solution was added immediately after addition of hydrosilane.
jProcess for degradation of Crumb-1 using multiple reduction cycles; 169.0 mg of starting material were used in the 2nd reduction and 65.0 mg in the 3rd (FIG. 8).
Increasing the scale from 300 to 2000 mg did not significantly change the efficiency of the process (
Loss of organic polymer led to elastomer shrinkage after a first reduction, leading to a more highly crosslinked residual structure, as shown by an increase in Young's modulus (Table 10), but maintained their shape. Attempts were made to undertake second and third reduction steps using fresh catalyst and hydrosilane in the hopes of improving overall conversion of elastomer to linear oils (
aExperimental conditions for the first reduction: BCF/ Rubber = 12 wt % (added portion by portion: 6 + 2 + 2 + 2), 6 days, 60° C.
bThe residual bulk materials from the first reduction were ground into a powder prior to the second reduction.
cExperimental conditions for the second reduction: BCF/Rubber = 10 wt % (added all at once), 48 h, 60° C.
dNote: Only metal was removed from the elastomer matrix in step 1; polymeric fiber; remained bound to the residual bulk solid.
Oligosulfides were converted into silyl thiol ethers during reduction. Therefore, the residual solids and recovered non-volatile liquids often exhibited a weight gain when compared to the starting rubber mass. The constituent rubber components, as shown by 1H NMR, are consistent with the TGA data on the starting elastomers (
Several factors were manipulated to improve the efficiency of the reduction process. A variety of hydrosilicones are commercially available that vary in the density of SiH groups. Model studies on the reduction of the organic sulfides or automotive rubbers were undertaken with 2 or 7, respectively, because the use of small molecules facilitated characterization of the reaction products. However, either compound is too expensive for practical use. Attempts to facilitate reduction of rubbers with the inexpensive, high SiH density polymer Me3Si(OSiMeH)nSiMe3 19 were unsuccessful because the silicone product of the reduction is a network polymer, which led to the formation of intractable tars. By contrast, the use of inexpensive, high SiH density HMe2SiOSiMe2H 20 (TetraH) led to efficacious, rapid reduction of rubbers (Table 9,
Experimental Procedure
Tetrabutylammonium fluoride trihydrate (TBAF; Bu4NF) and iodine (I2) were obtained from Sigma Aldrich and used as received. The silylated organic oil 21 in
The desilylated organic oil 2 (
Results
Initial studies for reusing/re-crosslinking the recovered oil focused initially on the regeneration of thiols from silyl thio ethers. Me3Si—S—SiMe3 is very labile, undergoing rapid degradation simply in the presence of water (vapor) to form H2S and Me3SiOH. By contrast, the hydrolysis of silicone-based thio ethers was much less facile. Alcoholysis of 2 yielded 12% product only once acetic acid was added to isopropanol solutions (the less sterically hindered thio ether PhCH2SSiMe2OSiMe3 underwent rapid, quantitative cleavage under the same conditions). The silylated polymeric oils derived from elastomers 21 were yet less reactive. It was necessary to use more aggressive nucleophiles for silicon, such as TBAF to regenerate the silyl free thiols 22 (
Once cleaved, the free thiols on the organic polymers remained susceptible to oxidative coupling. Addition of iodine in isopropyl alcohol to a solution of the oil recovered from truck tread (origin unknown) led to a new elastomer 3 (
Experimental Procedure
A silicone mold was prepared with a two-part liquid component kit (Sylgard 184). Two components were mixed at the recommended ratio of 10 parts (10.0 g) base to 1 part curing agent (1.0 g). The mixing process was performed using a planetary centrifugal mixer (FlackTek Inc.) with a duration of 5 min at a speed of 3000 rpm. In order to fabricate bubble free elastomer, the mixed uncured PDMS was thoroughly degassed in a vacuum desiccator at low pressure for 30 min. The right front tire of a toy car (outer diameter˜2.5 cm) was removed from the toy and placed in the degassed, uncured mixture. The mixture was cured in an 80° C. oven overnight. The tire was removed from the cured mold.
Used rubber powder from the truck tread (Sailun) (2.0 g) was reduced by PentaH in a 100° C. oil bath for 18 h to give a polymeric oil 21 (81.5% yield). The oil was accompanied by residual undissolved rubber powder and excipients that was washed with toluene and separated by centrifugation (Eppendorf, Centrifuge 5424, at 12 000 rpm for 20 min). Solvents in the supernatants, after centrifugation, were removed by rotary evaporation, any residual volatiles and silicone by-products were removed using a stream of N2 over 24 h. The recovered solids were dried in an 80° C. oven for 18 h, and ground into powder using a mortar and pestle. The silylated organic oil 21 from the former step (comprised of PIP/NR derivatives, 0.707 g) was dissolved in hexanes (10 mL), benzoyl peroxide (BPO, 0.01 g, 1 wt %, 0.0413 mmol) and, optionally, ground residual inorganic solids (from the preparation of 21, 0.3010 g), were added sequentially and mixed to give a homogeneous dispersion. After the solvent was removed by rotary evaporation, the mixture was placed in the silicone mold and degassed under vacuum in a desiccator for 30 min. The curing process was performed at 100° C. for 18 h. The formulations for rubber with different residual solid are listed in Table 11.
Results
It was found that if silylated polymers were derived from PIP/NR or PBD and possessed residual alkenes, re-crosslinking did not require removal of the silyl groups; simply adding a radical initiator such as benzoyl peroxide (BPO) and heating led to new elastomers 24 (
Experimental Procedure
Rubber contaminated steel that is usually considered as waste from tire recycling was received from eTracks (Oakville, Canada). The rubber contaminated steel (1.0 g
Results
It was found that after treating rubber contaminated steel with 0.5 wt % BCF/steel and excess tetramethyldisiloxane, most of the metal was clean. The small amount of residual rubber adhering to the metal surface (
It should be noted that the reaction solution could be used multiple times. Addition of a second portion of contaminated steel (1.0 g) to the solution of the former treatment lead to clean steel (
The degradation of rubber crumb was performed in a Haake Toque Rheomix mixer with a rotor speed of 60 rpm at 140° C. Prior to reaction, rubber crumb (190 g) was premixed with hydrosilane (e.g., MHD4MH, HMe2Si(OSiMe2)4OSiMe2H, 20 ml) or non-functional silicone oil (MD25M, Me3Si(OSiMe2)25OSiMe3) for about 5 min. The premixed crumb was allowed to mix in the Haake chamber for 40 min. The obtained admixture was an oily black solid crumb. After reaction, 1 g of the crumb was soaked in 10 ml toluene for 24 h and the residual solid was centrifuged down. The extraction process was repeated 5 times. The supernatants were combined and the solvents were removed by rotary evaporation. Small amounts of residual volatiles were removed under a stream of N2 flow until the mass stabilized. The residual solid was dried in vacuum oven for 24 h. The extraction results are summarized in Table 12.
Results
As shown in
aCrumb without being reacted in Haake
While the present application has been described with reference to examples, it is to be understood that the scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. Where a term in the present application is found to be defined differently in a document incorporated herein by reference, the definition provided herein is to serve as the definition for the term.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2020/050455 | 4/6/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62829677 | Apr 2019 | US |
