Thiol-functionalized cellulose nanocrystals for applications in rubber

Abstract

The present invention relates to improved cellulose nanocrystal fillers for rubber compositions, and more particularly to the grafting of disulfides and thioesters onto cellulose nanocrystals to improve performance as fillers in rubber compositions. At least one embodiment describes a method of grafting a thiol or disulfide on a CNC surface by, for example, esterification of 3-mercapto-propionic acid (MPA), 3-(acetylthio) propionic acid (APA), or dithiodipropionic acid (DTDPA). The reaction can be carried out on CNC that is highly dispersed in a suitable solvent and the reaction conditions are improved to achieve an advantageous degree of substitution DS. Embodiments further disclose how the surface thiol groups may then be protected in a second step as thio esters or asymmetric disulfides to modulate the hydrophobicity of CNC to improve compatibility with styrene-butadiene, SBR or natural rubber.

Description

Technical Field

[0001] The subject matter of the present invention relates to reinforcing fillers for rubber and, in particular, to the surface grafting of disulfides and thioesters onto cellulose nanocrystals for use in rubber compositions. Background Art

[0002] Commercial rubber products contain significant amounts of fillers and additives to regulate the viscoelastic properties of the cured products for optimal performance and lifespan. The development towards more sustainable components for rubber products provides an opportunity for chemicals and materials of biological origin, including cellulose nanocrystals (one or more "CNC"). Due to their large specific surface area, particle geometry, and reactive surface, CNCs can be used as high-performance reinforcing agents for styrene-butadiene rubber ("SBR") products. However, CNCs with polar surfaces have poor dispersibility and interfacial properties with non-polar SBR matrices by selectively and controllably generating CNC-elastomer covalent bonding interactions during rubber mixing.

[0003] Functionalized CNCs have been developed as reinforcing fillers that form covalent crosslinks with UV-cured or vulcanized rubber elastomers, where varying degrees of reinforcement have been reported in natural rubber, NR, and SBR elastomers. Despite the successful modification of nanocellulose in these cases, all these reports describe marginal improvements in mechanical properties, which are mainly attributed to the poor compatibility between the functionalized fillers and non-polar SBR. Functionalized CNCs with excellent dispersibility and compatibility in the resulting elastomer mixtures would be useful. Such functionalized CNCs can also be used as reinforcing fillers in UV-cured or vulcanized rubber elastomers. Summary of the Invention

[0004] Aspects and advantages of the present invention will be set forth in part in the following description, or may be obvious from the detailed description, or may be learned by practice of the present invention.

[0005] The present disclosure relates to cellulose nanocrystal reinforced fillers. Specifically, thiol-esterified functionalized cellulose nanocrystal reinforced fillers and methods for their preparation are provided. One embodiment of the present invention is disclosed herein, wherein a stepwise method is used in the preparation of the functionalized CNC. The first step targets high-yield esterification (e.g., grafting with methacrylic anhydride), followed by thiol-ene "click" crosslinking. The present invention also teaches that olefin hydrothiolation chemistry generates grafted functional groups introduced via monosulfide linkages, which results in the reaction of thioacids with olefins to produce thioesters. Thioacetic acid and thiobenzoic acid (both commercially available thioacids) are selected to compare the reactivity towards acyl click reactions and the compatibility of the isolated click products. Additionally, thiol-ene click chemistry can also be used to graft branched thiols onto olefin-modified CNCs. Pentaerythritol tetra(3-mercaptopropionate) (PETMP), a commercially available four-arm thiol, can be used as a crosslinking agent for branched polyvinyl monomers and other UV crosslinking polymer systems. Thiol-ene click chemistry is used to crosslink one of the four branches of PETMP with the surface-grafted alkenyl groups.

[0006] These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and the appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the detailed description, serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Reference is made to the following drawings, in which a complete and enabling disclosure of the invention, including the best mode thereof, for the ordinary skilled person in the art is set forth in the specification:

[0008] Figure 1 . Synthetic pathway and representative products of CNC thioesterification.

[0009] Figure 2 . FT-IR (ATR) spectra of DTDPA-grafted CNCs and optimization of the reaction using EDC or DCC coupling agents in DMF at 80 °C with extended reaction times.

[0010] Figure 3 . FT-IR spectra of acyl thiol-ene click-functionalized CNCs using different reaction conditions compared to the parent methacrylic acid esterified CNC (MA-CNC) precursor.

[0011] Figure 4 . Synthetic pathway for the thiol-ene click reaction of thiobenzoic acid (TBA)-CNC.

[0012] Figure 5 . Schematic of the separation and washing procedure applicable to the TBA grafting reaction of CNC.

[0013] Figure 6 . Solid state of freeze-dried TBA-CNC 13 C MAS NMR spectra.

[0014] Figure 7 . TGA isotherms of freeze-dried TBA-CNC.

[0015] Figure 8 . DSC traces of freeze-dried TBA-CNC.

[0016] Figure 9 . True secant modulus (MPa) vs. tensile strain percentage of SBR reinforced with TBA-CNC.

[0017] Figure 10 . True secant modulus TSM of rubber samples reinforced with functionalized CNC compared to carbon black CB when examined at 100%, 200%, 300%, and 400% strain.

[0018] The same or similar reference numerals are used in different figures to denote the same or similar features. Detailed Description

[0019] The present invention provides a novel cellulose nanocrystal reinforced filler for rubber compositions and methods for its preparation. For the purposes of describing the present invention, reference will now be made specifically to the embodiments and / or methods of the present invention. Each example is provided by way of explanation of the present invention and not as a limitation thereof. In fact, those skilled in the art will appreciate that various modifications and changes can be made to the present invention without departing from the scope or essence thereof. For example, features or steps described or illustrated as part of one embodiment can be used in conjunction with another embodiment or step to yield yet another embodiment or method. Accordingly, it is intended that the present invention cover such modifications and variations as fall within the scope of the appended claims and their equivalents.

[0020] Details of the results of cellulose nanocrystals CNC grafted with thiol functional groups (including thiols, thioesters, and disulfides) by different synthetic methods schematically illustrated in Figure 1 are described below.

[0021] Using Steglich esterification conditions, the disulfide 3,3'-dithiodipropionic acid (DTDPA) was grafted onto CNC using stoichiometric 4-dimethylaminopyridine, DMAP, as catalyst and dimethylformamide, DMF (80 °C) containing the coupling agent N,N'-dicyclohexylcarbodiimide (DCC). Successful grafting of DTDPA with low substitution degree SD was determined by CHNOS elemental analysis. However, by replacing the DCC coupling agent with the more reactive 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide EDC, the DS increased threefold. Fourier transform infrared FTIR analysis confirmed the signal corresponding to the C=O stretch of the DTDPA-CNC ester at 1733 cm -1 −1 ([< Figure 2 )). Dispersion tests were carried out in solvents with different polarities using the DTDPA-CNC product isolated in a paste with acetone (dispersed to 0.1 wt% in each solvent). The test showed good dispersion in ethyl acetate, toluene and limonene, even after standing for 2 h.

[0022] When characterizing DTDPA-CNC, we can quantify the extent of single grafting of DTDPA compared to double grafting, i.e., for each grafted DTDPA group, one or two carboxylic ester termini are attached. First, DTDPA-CNC was reduced at room temperature using methanol / phosphate buffer (pH 8) containing dithiothreitol (DTT) in order to expose free thiol groups by cleavage of the disulfide. Elemental analysis of the reduced product showed a halving of the mass percentage of sulfur (S% = 1.17) compared to DTDPA-CNC (S% = 2.29), thus indicating the presence of a significant proportion of singly grafted DTDPA (i.e., many cleaved disulfide groups result in loss of mercaptopropionic acid groups and corresponding loss of S%). Improvements in the total grafting degree and the double / single grafting ratio were achieved through the resulting reaction optimization. By increasing the reaction time from 5 h to approximately 20 h, the total DS was increased to DS = 0.13.

[0023] The results for CNC grafted with thioesters, thiobenzoic acid (TBA), via thiol-ene click chemistry are provided below. Two methods can be used for grafting. First, a stepwise method, where the first step aims for high-yield esterification (e.g., methacrylic anhydride grafting), followed by thiol-ene "click" crosslinking, as shown in Figure 4 . This alkene hydrothiolation chemistry yields grafted functional groups introduced via monosulfide linkages. We essentially employ acyl thiol-ene chemistry (for small molecules), which reacts thioacids with alkenes to produce thioesters.

[0024] Alkene-grafted CNC (e.g., methacrylic anhydride, MA-CNC) can be used as a precursor for grafting thiols and thioesters via thiol-ene and acyl thiol-ene click chemistry, as inFigure 4 as shown. Thioacetic acid can be used as a thioacid and reacts with MA-CNC at elevated temperature or room temperature. Additionally, a UV-initiated reaction is possible by employing the thiolactonization procedure of mercaptoalkanoic acids. The reaction mixture is typically quenched with methanol and worked up by centrifugation / washing with a solvent (e.g., methanol or acetone or a combination thereof). The reaction carried out with triethylamine base in a heated solvent, which is thought to facilitate the “Michael addition” mechanism, is most successful in converting the olefin into the thioester click product, achieving a DS of 0.15 in toluene, representing approximately 30% conversion. FT-IR analysis of the lyophilized products from each of the toluene, DMF, or UV-reactions (shown in Figure 3 ) shows a small, broadened peak for the shoulder methacrylate C═O stretch measured at 1726 cm -1 −1, which is assigned to the thioester C═O expected at 1696 cm -1 −1.

[0025] To improve the compatibility of the product, thiobenzoic acid TBA is used in the click reaction to generate a more hydrophobic thioester group and thus improve the dispersibility of the functionalized CNC in SBR. The Michael addition type reaction can be carried out at 110 °C to produce a product with a moderate degree of grafting of TBA groups, close to a DS of 0.2. Among the solvents screened, DMF gives a product with the highest DS = 0.18, which corresponds to almost 50% conversion of the grafted alkenyl groups into the acyl click product (thioester product). The click products are generally dispersible in the more polar solvents, ethyl acetate and toluene, but precipitate relatively rapidly in limonene and heptane. It should be noted that the UV light-initiated reaction gives a lower DS, approximately 0.04 - 0.08, whether in DMF solvent or in pure liquid TBA at room temperature, which may be due to the inactivation of the photoinitiator during the reaction duration.

[0026] Overall optimization of the reaction conditions shows that the TBA reaction, typically carried out in excess TBA at 80 °C - 110 °C for 5 h, is insensitive to the base catalyst used, the reaction time is not long, and generally 0.1 < DS < 0.15 is obtained. Based on these results, the base catalyst can be removed, thus simplifying the procedure. The variability of the S% of the TBA-CNC batches determined by CHNS elemental analysis is correlated with the slightly reddish color of the product. For some reactions, using the same conditions (DMF with triethylamine base, 110 °C), the color of the reaction mixture can be more variable, from amber to dark brown / reddish brown. This is thought to be the result of TBA decomposition, and whether this may lead to impurities in the TBA-CNC product is unknown.

[0027] As with the successful acyl click reactions with thioacetic acid and thiobenzoic acid, similar reaction conditions can be employed for the click grafting of methacrylated MA-CNC with commercially available branched thiols, pentaerythritol tetra(3-mercaptopropionate) or PETMP, such as Figure 1 shown. Toluene and DMF solvents can be screened with catalytic triethylamine and excess PETMP at 110 °C, and a UV light-initiated reaction can also be carried out in pure liquid PETMP at room temperature. Both the Michael addition type reaction and the UV light-initiated reaction heated in pure PETMP yield similar DS values of 0.06 - 0.07, indicating comparable reactivity for both pathways. Each grafted PETMP molecule theoretically has 3 exposed thiol groups, and the "DS of exposed thiols" is estimated to be closer to 0.18 - 0.2. The grafting of PETMP with 4 carboxylic acid groups per molecule can be confirmed by FTIR through the strong peak at 1732 cm -1 . It is noteworthy that the reaction product in toluene is rapidly dried in air to produce a free-flowing powder that can be used in rubber formulations, as it is not desirable to add reinforcing fillers as a paste in flammable solvents to the rubber mixture. PETMP-CNC is a highly reactive system that can produce highly rigid structures at low strain levels when compounded with rubber.

[0028] It is possible to notice the variability in the gravimetric yield determination based on mass gain, which is caused by material loss during washing / purification. Therefore, as Figure 5 shown, a control scheme for the separation and washing procedures for the thiol grafting reaction with CNC was established. Using Figure 5 Separation Process 1, adding excess acetone to quench the reaction may lead to complete or partial precipitation of the material. When complete precipitation occurs, the precipitate is purified with acetone as usual to obtain the product. However, when precipitation is incomplete after quenching with acetone, the precipitate is washed with acetone while adding a co-solvent (in this case water) in a 1:1 volume ratio to the supernatant to induce precipitation. However, Separation Process 2, also shown in Figure 5 , is carried out by quenching the reaction with ethyl acetate. In this case, complete precipitation can be observed, and purification is restarted with acetone.

[0029] Solid-state 13 C MAS NMR of freeze-dried TBA-CNC (shown in Figure 6The carbonyl carbon signal at 175 ppm was confirmed. The two carbonyl carbons (CNC ester and thioester) can overlap within this region, or the thioester signal expected at approximately 190 ppm can be indistinguishable from the baseline. The lack of a clear thioester peak can also indicate cleavage of the thioester during synthesis. Another small peak can be assigned to residual DMF, with a carbonyl at approximately 167 ppm. The signal at 128 ppm can be assigned to the aryl of the thioester, while the signal at 137 ppm can be assigned to the methacrylate olefin carbon from any ungrafted MA groups. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to analyze the thermal properties of the lyophilized TBA-CNC, and the results are shown in Figure 7 and Figure 8 . In the TGA test, the heating rate was 20 °C / min. For DSC, the temperature was first increased at a rate of 10 °C / min and then cooled at the same rate. Figure 7 The TGA curves of Figure 8 showed onset and peak decomposition temperatures at 300 °C and 323 °C, respectively. At room temperature, the material contained approximately 2.2% moisture.

[0030] Therefore, the thiol-functionalized CNC is compatible with SBR and can be directly blended with rubber components. The thiol-functionalized CNC is used as a renewable and sustainable reinforcing agent in rubber, which can replace non-renewable materials such as carbon black. Table 1 provides two example formulations for rubber blending. The rubber properties can be adjusted by regulating the dosage of specific components (e.g., ZnO and / or crosslinking agents) in the blend. The mixing of rubber with thiol-functionalized CNC can be carried out using conventional rubber processing equipment (e.g., internal mixer), and the material can be added as a dry powder or as a paste with a solvent-solid content of about 7%-40%. Conventional rubber processing equipment and procedures can be used, such as a Haake mixer equipped with a pair of Banbury rollers. In the first step, the elastomer can be mixed with olefin-functionalized CNC, ZnO, stearic acid, SAD, and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine 6PPD at specific temperatures and speeds (e.g., 90°C - 120°C and 70 rpm - 100 rpm, respectively). Then, S and N,N-dicyclohexyl-2-benzothiazolesulfenamide CBS can be mixed at a lower temperature (e.g., 50°C and 30 rpm). The compounded rubber can be vulcanized in a steel mold using a hydraulic press at a high temperature (e.g., 150°C). If the functionalized CNC is used as a paste, the mixing and solvent evaporation can be carried out in one of two methods. Method 1: Mix the paste with the elastomer in a Haake mixer and evaporate it, for example, at a temperature above the boiling point of the solvent. Method 2: Dissolve the elastomer in a solvent miscible with the paste and mix them thoroughly.

[0031] Table 1. Example of a rubber compounding formulation. (*Equivalent of 27 phr CNC + 8.1 phr TBA grafted, with its equal volume percentage to N234 carbon black .)

[0032]

[0033] The final product can be dried using air drying, vacuum drying, freeze drying, spray drying, or any other suitable technique that allows direct removal of the solvent from the system. Subsequently, this dry material can be mixed with rubber components and vulcanized. It is also possible to use a solvent paste containing functionalized CNC and directly blend it with rubber components while drying the solvent. The rubber reinforced with thiol-functionalized CNC exhibits a strain behavior of about 350%-450% and a true secant modulus approaching 20 MPa at maximum strain. This material is also characterized by a high true secant modulus at low strain values. For example, for vacuum-dried TBA-CNC, the true secant modulus is about 9 MPa at 100%, about 11 MPa at 200%, and about 15 MPa at 300%. Figure 9 and Figure 10Describes the basis for the excellent reinforcement of thiol-functionalized CNCs in rubber when compared to carbon black, a non-renewable material commonly used for rubber reinforcement. Figure 9 Shows the true secant modulus curves for five vacuum-dried samples (replicates). The figure clearly illustrates the excellent hardening and reinforcement at strains < 150%, which is attributed to the good connectivity of the thiol-functionalized CNC nanoparticles and the development of an interpenetrating network. When the material is stretched to 300%, the true secant modulus (TSM) remains high, achieving > 14 MPa, which is more than twice the TSM of rubber reinforced with CB, as Figure 10 shown in. This indicates excellent dispersion and interfacial properties between the thiol-functionalized CNC and SBR, as well as the effectiveness of the former in generating an effective interpenetrating network for reinforcement and in achieving good cross-linking via thiolation between the elastomer and the thiol-functionalized CNC. Figure 9 and Figure 10 also indicates that if the thiol-grafted CNC is applied as a paste and then dried, or vacuum-dried from a solvent-elastomer mixture form, the results and thus the reinforcement potential are improved.

[0034] Selected combinations of aspects of the disclosed technology correspond to various different embodiments of the invention. It should be noted that each of the exemplary embodiments presented and discussed herein should not be construed as limiting the subject matter. Features or steps described or depicted as part of one embodiment may be used in combination with aspects of another embodiment to yield yet additional embodiments. Additionally, certain features may be interchanged with similar devices or features not explicitly mentioned that perform the same or similar functions.

[0035] The terms "a," "an," and the singular form of a word shall be considered to include the plural form of the same word, such that these terms mean providing one or more of something. The terms "at least one" and "one or more" are used interchangeably. A range described as "between a and b" includes the values of "a" and "b."

[0036] The citation of any document does not admit that the document is prior art with respect to any invention disclosed or claimed herein, nor does it admit that the document teaches, suggests, or discloses any such invention, alone or in combination with any other one or more reference documents. Additionally, in the event of any conflict between the meaning or definition of a term in this document and the meaning or definition of the same term in a document incorporated by reference, the meaning or definition given to the term in this document shall prevail.

Claims

1. A functionalized cellulose nanocrystal filler for a rubber composition, comprising: Cellulose nanocrystals having a sulfur-containing ester functional group.

2. The functionalized cellulose nanocrystal filler according to claim 1, wherein the sulfur-containing ester functional group is a thiol.

3. The functionalized cellulose nanocrystal filler according to claim 1, wherein the thiol is grafted onto the cellulose nanocrystals by esterification with an acid selected from the group consisting of 3-mercaptopropionic acid, 3-(acetylthio)propionic acid, and dithiodipropionic acid.

4. The functionalized cellulose nanocrystal filler according to claim 1, functionalized with thioesters and thiobenzoic acid by thiol-ene click chemistry.

5. The functionalized cellulose nanocrystal filler according to claim 4, wherein the cellulose nanocrystal filler is first esterified with high-yield esterification and then subjected to thiol-ene "click" crosslinking.

6. The functionalized cellulose nanocrystal filler according to claim 5, wherein the esterification is carried out by grafting with methacrylic anhydride.

7. The functionalized cellulose nanocrystal filler according to any one of the preceding claims, wherein the filler is dry.

8. The functionalized cellulose nanocrystal filler according to claim 7, wherein the filler is dried by any technique belonging to the technical group consisting of air drying, vacuum drying, freeze drying, spray drying, and combinations thereof.

9. A rubber composition, comprising: A diene elastomer; and The functionalized cellulose nanocrystal filler according to any one of the preceding claims.

10. A method for preparing a functionalized cellulose nanocrystal filler for a rubber composition, comprising: Esterifying cellulose nanocrystals with a molecule containing at least one thiol group and at least one sulfur atom; Protecting the cellulose nanocrystal filler thiol ester by reacting with thiol-ene "click" crosslinking.

11. The method according to claim 10, wherein the at least one sulfur atom is at least one thiol group.

12. The functionalized cellulose nanocrystal filler according to claim 11, wherein the molecule is grafted onto the cellulose nanocrystals by esterification with an acid selected from the group consisting of 3-mercaptopropionic acid, 3-(acetylthio)propionic acid, and dithiodipropionic acid.

13. The functionalized cellulose nanocrystal filler according to claims 10 to 13, functionalized with thioesters and thiobenzoic acid by thiol-ene click chemistry.

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