WO2020237354A1

WO2020237354A1 - Method for improving dispersion of metal oxide in rubber by the use of cellulose nanocrystals (cncs); and rubber composites comprising cncs

Info

Publication number: WO2020237354A1
Application number: PCT/CA2020/050696
Authority: WO
Inventors: Richard Berry; Sassan Hojabr; Rachel Jessica BLANCHARD; Tizazu H. MEKONNEN; Emmanuel OGUNSONA
Original assignee: Celluforce Inc.
Priority date: 2019-05-24
Filing date: 2020-05-22
Publication date: 2020-12-03

Abstract

The disclosure relates to an aqueous dispersion comprising a rubber latex, a CNC and a curing package, said curing package comprising a metal oxide; a process for preparing a dispersion of a metal oxide in a latex composition and a process for preparing a rubber composite.

Description

METHOD FOR IMPROVING DISPERSION OF METAL OXIDE IN RUBBER BY THE USE OF CELLULOSE NANOCRYSTALS (CNCS); AND RUBBER COMPOSITES COMPRISING CNCS

FIELD OF THE DISCLOSURE

BACKGROUND OF THE DISCLOSURE

Rubber has a remarkable range of utility from its use in highly flexible items such as gloves, and condoms to its use in moderately flexible items such as tires, tracks, soles, belts and gaskets. This range is achieved through the control of the crosslinking process which is the common factor in every rubber application.

Rubber is a collective name for materials, also referred to as elastomers, that can be produced from a variety of polymer precursors. Natural rubber (NR), the latex sap of the rubber tree, is composed of polyisoprenes while synthetic rubbers are produced from, for example, styrene -butadiene and butyl-nitrile polymers. Rubber goods may contain mixtures of these classes of rubber. In all these cases, the starting material is a latex dispersion of the polymer in water which is then coagulated by the addition of acids, salts or both to form a solid. For products which need to be formed directly from the latex, coagulation is achieved on the form by applying a thin layer of coagulating agents to the surface of the form. In other cases where a thin layer of coagulating agents is not applied, coagulation of the latex unto substrates or forms can be achieved by manipulating the substrate temperature in order to accelerate the coagulation of the rubber latex unto its surface upon contact. These solids go through some level of curing in an industrial process, which causes the crosslinking of the polymers through the double bond in the polymer chain forming an inter-chain bond by the addition of sulfur. The degree of crosslinking in highly flexible goods is relatively low while that in moderately flexible goods is relatively high.

The complex crosslinking reaction, which is initiated at the double bonds in the polymer (described as cure sites) allow the formation of sulfur bridges of different lengths. Sulfur, although essential, is slow in forming crosslinks and, if used alone, requires high temperatures and long times to achieve a crosslinked structure, conditions which cause deterioration of the underlying polymer system. The process needs the addition of several types of accelerants and also activators. The accelerants, typically sulfur and nitrogen containing hydrophobic solid additives, catalyze the sulfur reactions while the activators, typically zinc compounds such as zinc oxide (ZnO) and Zinc (II) Dibutyl Dithiocarbamate (ZDBC), initiate the accelerants. These additives are not easily brought together and dispersed effectively in the system causing poor uniformity of crosslinking, less than optimal utilization of the additives and reduced product performance.

Cellulose, the most abundant of natural polymers, comprises crystalline and amorphous segments. Cellulose NanoCrystals (CNCs) can be extracted from the crystalline segment. The most common chemical process to extract CNC uses sulfuric acid hydrolysis, which results in sulfate groups on the surface of CNC. The resultant dispersion of CNC in aqueous media is acidic and can be neutralized with various bases, such as sodium hydroxide. Neutralization of CNC reduces the hydrogen bonding amongst CNC crystals. Sodium hydroxide is commonly used for the neutralization process. The most cost- efficient way to use CNC is as a dry powder.

SUMMARY OF THE DISCLOSURE

An aspect relates to an aqueous dispersion comprising a rubber latex, a CNC and a curing package, said curing package comprising a metal oxide.

A further aspect relates to a process for preparing a dispersion of a metal oxide in a latex composition comprising providing an aqueous dispersion of CNC; dispersing said aqueous dispersion of CNC and a rubber latex to provide a CNC/rubber dispersion; preparing an aqueous dispersion of a curing package, said curing package comprising said metal oxide; combining said dispersion of a curing package and said CNC/rubber dispersion to provide said dispersion of a metal oxide.

Still, a further aspect relates to a process for preparing a rubber composite comprising applying an aqueous coagulating solution on a surface of a substrate and allowing said surface to substantially dry or providing a surface of a substrate without a coagulating solution; preparing a dispersion of a metal oxide in a latex composition as defined herein; coating said dispersion of a metal oxide in a latex composition on said surface of said substrate with or without said coagulating solution; and allowing said dispersion of a metal oxide in a latex composition to cure, thereby providing said rubber composite. Still, a further aspect relates to a process for preparing a rubber composite comprising providing an aqueous coagulating solution; providing a dispersion of a metal oxide in a latex composition as defined herein, said dispersion comprising CNC, a rubber latex and a curing package comprising a metal oxide; and mixing said dispersion of a metal oxide in a latex composition and said aqueous coagulating solution to provide said coagulated rubber composition comprising dispersed metal oxide therein and curing the resulting coagulated latex to provide said rubber composite comprising dispersed metal oxide therein.

A further aspect relates to a cured latex products comprising CNC and a metal oxide, or such product prepared be a process as defined herein.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 presents the results for the assessment of: (a) Tensile strength, (b) modulus, (c) stress-strain curves and (d) force-strain curves of NR and NR-composites;

Figure 2 presents (a) the results for the assessment of vapor permeation loss of water through NR and NR- CNC composites, (b) an illustration of the proposed permeation mechanism through NR and NR-CNC composites and THF, (c) the results for vapor permeation loss of THF through NR and NR-CNC composites and (d and e) water absorption of NR and NR-CNC composites overtime;

Figure 3: presents mechanical properties of NBR and NBR-nanocomposites showing the (a) tensile strength, (b) tensile toughness, (c) Young’s modulus, (d) elongation at break;

Figure 4: presents the effect of CNC on the dispersibility of a metal oxide: percentage ZnO precipitate vs settling times at different ZnO-CNC ratios; and

Figure 5: presents the effect of CNC on the dispersibility of ZnO: ZnO precipitate vs settling times at different ZnO-CNC ratios.

DETAILED DISCLOSURE

One of the features of CNC dispersions is that they form liquid structures. These structures are interactions between individual crystals which allow networking. These networks have now been found to increase the capacity of the dispersion to suspend other particles which would normally aggregate.

CNCs also have a hydrophilic surface imparted by cellulosic hydroxyl groups and these can interact with the surface of other hydrophilic particles to allow their better initial dispersion. CNCs can therefore promote a finer dispersion and also a more stable dispersion through these two phenomena. In the case of rubber formulations, these phenomena allow the disaggregation of activator particles and the maintenance of a uniform dispersion.

The amount of CNC used can be varied according to the required properties. In lightly cured latex products (e.g. gloves, condoms, balloons), lower concentration of CNCs is preferred to maintain the flexibility/elasticity of the products. In other highly cured latex products (e.g. tires, gaskets and seals), higher concentration of CNCs is ideal to aid dispersion of the high amounts of curing package while providing reinforcement and rigidity to the product.

In one embodiment, the amount of CNC used can be varied up to 20 parts per hundred (phr), based on latex solids content, in one embodiment, the amount may be up to 10 phr, preferably up to 5 phr, including 0.5, 1,0, 1.5, 3 and 5 phr.

As used herein,“CNC” refers to CNC having sulfonic, phosphonic, carboxylic groups or mixtures thereof present on the surface of the nanocrystals.

CNC that may be used can be purchased or obtained from various approaches using controlled hydrolysis with a strong mineral acid. The use of sulfuric acid causes partial esterification of some of the primary hydroxyl groups on the exposed cellulose leaving a sulfate half-ester group which retains a single negative charge at the nanocrystal surface (J.-F. Revol et al. US patent 5,629,055).

The surface charge can be reduced by desulfation (DS) in a post-treatment after sulfuric acid hydrolysis (F. Jiang, et al. Langmuir, vol. 26, pp. 17919-17925, 2010).

Phosphoric acid can also be used to produce cellulose nanocrystals with similar suspension properties through the introduction of phosphate half-esters (S. C. Espinosa, et al. Biomacromolecules, vol. 14, p. 1223-1230, 2013).

It is also possible to extract cellulose nanocrystals by oxidation from biomass such as wood pulp. Such oxidation processes generate carboxylic acids on the surface of the nanocrystals. Persulfate salts can be used to generate a carboxylated form of charged cellulose nanocrystals (C. Woon Leung, et al. US 2012/0244357) as can hydrogen peroxide in an acidic environment (B. G. Refineries, November 2016 «The R3TM Technology - Renewable,»: http://bluegoosebiorefmeries.com/our-technology ). Both methods lead to carboxylated cellulose nanocrystals.

Sulfuric acid extracted cellulose nanocrystals can be further oxidized, for example by using the catalytic TEMPO/NaOCl/NaBr system (Y. Habibi, et al. Cellulose, vol. 13, pp. 679-687, 2006). This method leaves the sulfate-half ester group intact giving a surface bearing both sulfate-half esters and carboxylic acids.

It has been found that, by adding CNC and activators, such as metal oxides, into rubber latexes before coagulation and applying appropriate mixing energy, an improved and stable activator dispersion can be achieved in the rubber after coagulation.

In one embodiment, said metal oxide is CaO, MgO, CdO, CuO, PbO, ZnO or NiO, preferably ZnO or NiO.

The examples that follow show the effect on product properties and ZnO utilization of this improved method of incorporating ZnO into rubber as representative metal oxide activator.

As used herein,“activators” are contemplated to include those commonly used in the art such as metal oxide, fatty acid (one most commonly used fatty acid activator being stearic acid), zinc 2-ethylhexoate, and zinc dithiocarbamates . Especially, the activator is comprising at least one metal oxide, in particular at least one of CaO, MgO, CdO, CuO, PbO, ZnO and NiO, preferably ZnO and NiO, either alone or in combination.

In one embodiment, the amount of activator used can be varied, based on latex solids content as follows: metal oxide (in partcicular ZnO): 0.5 - 5 phr, stearic acid: 0.5 - 3phr, zinc 2-ethylhexoate: 0.5 - 3 phr, and dithiocarbamates: 0.25 - 5 phr.

The examples that follow show the effect on product properties and ZnO as representative activator.

Curing packages are known to those skilled in the art and may comprise various reagents that modify the kinetics and chemistry of crosslinking. Examples include accelerants, activators, retarders and inhibitors. Here, retarders and inhibitors are components of curing package which do not actively engage in the crosslinking process of the rubber latex but perform other functions such as ultraviolet light blocking and oxygen species scavenging (inhibitors) and fire and smoke suppressants (retarders) during the life cycle of the rubber latex final product.

As used herein a curing package comprises activating and curing agents, preferably comprising a sulfur- donating compound and a metal oxide, including a mixture such as sulphur, ZnO, ZDBC and KOH.

Sulfur-donating compounds may be used as part of the latex compositions. An example of such compound is octasulfur S8.

The formulations can be coagulated using a range of coagulating agents. The agent chosen is determined by the eventual use of the rubber latex formulation. Typical coagulating agents are acetic acid, formic acid and soluble calcium salts used separately or in combination. Preferably, the coagulating agents are provided in an aqueous solution referred to herein as coagulating solution.

The method can be applied to any rubber latex comprising unsaturated polymers (i.e. containing double bound) that can be cross-linked in presence of sulfur comprising polyisoprene, polybutadiene and styrene- butadiene and polycholoroprene latexes. Other rubbers such as nitrile rubbers are contemplated.

The method allows the incorporation of dispersed ZnO into a portion of coagulated rubber. This portion can then be used as a carrier that can be masticated with other rubbers in the typical process used to incorporate additives into the final rubber compound for products requiring heavy curing such as tires.

In one embodiment, the process for preparing a rubber composite is comprising applying an aqueous coagulating solution on a surface of a substrate and allowing said surface to substantially dry; preparing a dispersion of a metal oxide in a latex composition as defined herein; coating said dispersion of a metal oxide in a latex composition on said surface of said substrate; and allowing said dispersion of a metal oxide in a latex composition to cure, thereby providing said rubber composite.

EXAMPLES

Natural rubber and acrylonitrile butadiene rubber latex suspensions were obtained from Chemionics Corporation, OH, USA. Potassium hydroxide (KOH) in the pellet form, tetrahydrofuran (THF), Polyethylene glycol tert-octylphenyl ether (Triton), toluene, and zinc oxide (ZnO) in powder form were purchased from Sigma Aldrich, USA. Sulphur (99.5 % purity) was purchased from Acres Organics in powder form. Zinc (II) Dibutyl Dithiocarbamate (ZDBC) (98 wt. %), calcium carbonate, and calcium nitrate were obtained from Fisher Scientific Inc. Acetic acid, formic acid and calcium chloride (CaCl₂) were purchased from Sigma-Aldrich

Sodium hydroxide-neutralized sulfuric acid extracted CNC samples were obtained from CelluForce (under tradename CelluForce product code: NCC NCV-100).

An exemplary natural rubber (NR) and acrylonitrile butadiene rubber (NBR) latex formulations were prepared by mixing rubber latexes, individually, with activators, curing agents, and CNC as shown in Table 1. An exemplary coagulant formulation used to coat substrates with a thin layer for dipped goods production is described in Table 2. All components in Table 2 are based on 100 parts per hundred of deionized water.

Table 1. Formulation compositions for rubber latex.

* All formulations are based on NR latex solids content Table 2. Formulation compositions for coagulant solution.

* All formulations are based on Deionized water content

The rubber latex-CNC formulations are prepared by incorporating CNC in the latex formulation to provide the required concentration of CNC phr relative to the total solids content of the NR latex formulation followed by the incorporation of the pre-dispersed curing package.

Rubber latex formulations used to produce film specimens were prepared by mixing either natural rubber or acrylonitrile butadiene rubber latex, activators, curing agents, and CNC as shown in Table 1. The activating and curing agents composed of sulphur, ZnO, ZDBC and KOH were pre-dispersed in water via homogenization (10 min shear mixing at 25,000 rpm) using a PowerGen™ 700 homogenizer. NR-CNC composite films were prepared by incorporating CNC in the latex formulation that provided concentrations of 0 (NR control), 0.5, 1, 1.5, 3 and 5 phr relative to the total solids content of the NR latex formulation followed by the incorporation of the pre-dispersed curing package. Formulations prepared as such were then used to prepare dipped and cast films. For the dipped film specimens, viscosity control was important and as such the formulations used were diluted as needed with de-ionized water to a constant viscosity of 60 cP using a Brookfield digital viscometer. This provided total solid compositions of 40, 35, 27, 23, and 18 wt. % for the films that contained 0, 0.5, 1.5, 3, and 5 phr CNC, respectively.

In contrast, formulations used for the cast film specimens were designed to provide uniform film thicknesses and as such the total solids in the formulations were kept constant at 40 wt.%.

The application of the process allows the nanometric dispersion of CNCs. This was best illustrated by the use of energy - dispersive X-ray spectroscopy (EDX) analysis. Since the CNCs used in this work are sulfonated, and there is additional sulfur added, EDX mapping of sulfur could not be used to determine the dispersion or aggregation of the CNCs in rubber matrices due to the presence of sulfur. EDX mapping of Zn and O showed clearly, the dispersion ZnO in the presence of CNCs. The dispersion was observed to further improve with increasing CNCs concentration.

In this case where the rubber latex is coagulated using acids, a master batch of rubber and well dispersed CNC is obtained without any curing package added. A typical example would be natural rubber (NR) with 20 phr CNCs coagulated with formic and acetic acids with and without a soluble calcium salt in this case calcium chloride (CaCl₂). Typical example of acid coagulant formulation are as follows: (i) 25% v/v acetic acid, (ii) 25% v/v formic acid (iii) a solution of 5% v/v acetic acid and 25 wt. % CaCl₂, and (iv) a solution of 5% v/v formic acid and 25% wt. % CaCl₂. The EDX mapping showed good nanometric dispersion in all cases although there are noteworthy distinctions in the dispersion quality between the acids and the acid - CaCl₂ coagulants. The formic and acetic acid-mediated coagulation particularly resulted in an excellent dispersion of the CNCs in the NR matrices despite the high loading of CNCs (20%). This implies that NR gelling and coagulation proceeds CNC aggregation that would occur with the increase in the ionic strength of the aqueous media due to the addition of the acids.

Example 1 -Dispersion of zinc oxide

An additional aspect of the disclosure is that the zinc oxide in the system is distributed in a uniquely uniform and disaggregated form. In the following experiment, the formulations of Tables 1 and 2 were used. CNCs show potential to be incorporated as a dispersant for the curing package components (activators and accelerants) while minimizing the use of either the activator or accelerant due to increased efficiency of the said components. In order to produce rubber latex master batches, CNC gel was dispersed in the latex using a blender (hand held blender, SMEG Technologies Inc., Italy) for 30 min (high setting). After the dispersion, a uniform and stable dispersion was obtained. Dispersions prepared as such were used for the co-coagulation studies.

To induce the coagulation, coagulant solutions composed of formic and acetic acids with and without a soluble calcium salt (in this case calcium chloride (CaCl₂)) were added dropwise to the dispersed mix under slow stirring (60 rpm). After coagulation was achieved (determined via turbidity measurement in the ejected water), the NR - CNC solid was separated, washed (to remove residual acid and salt), and dried. For example, 100 g of rubber latex is charged into a beaker. To this, 0 to 40 phr of solid content CNCs from CNCs gel is charged into the rubber latex slowly while undergoing mixing to disperse the CNCs. Upon dispersion of the CNCs in the latex, an appropriate coagulating agent such as 25% v/v acetic acid, 25% v/v formic acid, a solution of 5% v/v acetic acid and 25 wt. % CaCl₂ or a solution of 5% v/v formic acid and 25% wt. % CaCl₂ is chosen. The coagulant is slowly added to the stirring rubber latex and CNCs mix until the water from the mix is ejected, leaving the CNCs well-dispersed in rubber latex.

Again, EDX is an excellent tool to show the improvement. Scanning electron microscope (SEM) and EDX images of films produced with different loadings of CNCs were used to map the dispersion of Zn and O in the rubber films to determine the quality of dispersion and to identify the nature of any agglomerates in the SEM images. In this case, Zn rather than S was the target element because sulfur was added as part of the curing package. The presence of distinct well embedded white agglomerate islands can be observed in the SEM images of NR, 0.5 and 1.5 phr CNC-containing films. These islands are shown by the EDX mapping of the Zn within the films to be predominantly related to the zinc oxide curing agent used. From the SEM and EDX mapping images, it is clear that the zinc oxide agglomerates are decreased upon the addition of CNCs and completely disappear in films containing 3 and 5 phr of CNCs.

As will be seen in the examples below, this unexpected improvement in zinc oxide dispersion allows substantial changes in key rubber properties and does so a potential to significantly decrease the amount of zinc compounds needed in the formulation, stemming from the improved efficiency of the Zn compounds in the presence of CNCs. These changes are applicable to all cured rubber products. Example 2 - Constant viscosity CNC/NR or NBR latex formulation

The latex formulations as shown per Table 1, with constant viscosity prepared for dipped fdms were mixed at 400 rpm for 2 h using a magnetic stir bar. This was done to mature the formulation, causing it to lightly pre-cure, which helped in developing fdms during the dipping process. For the dipped fdm development, rectangular tempered glasses were first dipped in a coagulant solution (formulation shown in Table 2) for 10 s, and dried at 65°C for 20 minutes. The coagulant coated glasses were then allowed to cool for 1 min (at room temperature) and gently dipped in each of the matured latex formulations containing varying concentrations of CNCs for 40 s to develop a film. The dipped glasses were then slowly removed from the dipping bucket and allowed to cure at 100°C for 1 h in a convection oven. The cured films were then peeled from the glass plates and post-cured for an additional hour to ensure targeted curing. The films prepared by the dipping method were used for tensile and dynamic mechanical property analysis per ASTM D882-18 at cross-head speed of 500 mm/min. The results of this analysis are shown in Figures l(a)-l(d).

Similar processing step was adopted for the NBR latex formulation and fabrication of the rubber films with the exception of the of mixing, which was done for longer periods of times such as 48, 72 and 96 h. This was to apply for the pre-curing of the rubber latex since NBR requires longer mixing times to achieve pre-curing. All other steps were followed accordingly.

Figure 1(a) shows that the tensile strength of the NR films increased with increasing CNC concentration. Greater than 200% increase was observed for the composite film containing 5 phr of CNC in comparison to the NR film. It is also notable that the increase is incremental with increasing CNC concentration until 5 phr of CNC was added at which charge there is an accelerated increase. This increase in strength stems from a combination of two effects: first, the reinforcement of the rubber by the CNC through the efficient transfer of stress from matrix to filler; and second, the increase in resistance to yield and failure of the covalent bonding of the rubber chains through the crosslinking caused by the improved distribution of Zn compounds. This latter effect is more dramatic at the CNC concentration of 5 phr which has been discussed above to cause the best distribution of zinc compounds of the concentration that were evaluated

The modulus of elasticity at 50% and 100 % elongation increased with increasing CNC concentration and crosslinking (Figure 1(b)). CNCs dispersed in the NR matrix act to restrict the chain mobility and therefore reduce its ability to plastically deform and yield when an external force is applied. This leads to the stiffening of the bulk material. The increased slopes of the curves in Figure 1(c) indicate the stiffening (modulus) due to CNC and crosslinking. Likewise, the elongation is decreased with increasing crosslinking and CNC concentration (Figure 1(c)). A decrease in elongation from 613 % for NR to a minimum of 519 % with a 3 phr CNC loading is observed. Due to the restrictions of the rubber chains by the CNCs and the prevention of untangling of some of the chains because of crosslinking, plasticity is slightly reduced, resulting in fracture at lower strains. The loss in elongation for samples containing CNC was minimal while significant increases in strength were observed.

The same trend is evident in the force vs strain curves (Figure 1(d)) when testing is done with samples of progressively decreasing thickness. Even with the thicknesses of the films decreasing with increasing CNC concentration, improvement in their breaking force is observed, with again a CNC loading of 5 phr showing significantly higher results. In terms of rubber applications, these results show that while using less material and thinner films improved mechanical properties can be achieved for applications such as gloves and condoms.

Example 3 - Constant CNC/NR Film Thickness

Using Tables 1 and 2 above, a casting procedure was also employed to obtain fdms that had a constant thickness. In this method, the latex formulations always containing 40 wt.% total solids were cast on glass plates to obtain fdms with uniform thickness. The fdms were then cured at 100 °C for an hour, gently peeled off the glass plates, and post cured for an additional hour. These fdms were used to evaluate the impact of CNC incorporation on solvent barrier properties, crosslinking density, and morphology described per ASTM E96 / E96M - 16.

Figure 2(a) shows that the permeation of water vapour through the rubber fdms increased with CNC loading. Cellulose is much more sensitive to water than natural rubber and can interact with moisture very easily. The adsorption of water molecules on the CNCs in the fdm is the first step in the permeation process. After adsorption, the molecules leave through desorption once the CNCs surfaces become saturated. This proposed mechanism is illustrated in Figure 2(b), where it can be seen how influencing the adsorption step can cause an increase in permeated water molecules or water vapour.

Tetrahydrofiiran (THF), a volatile organic solvent, with a low solubility parameter of 18.5 MPa^{1 2} compared to that of water of 48 MPa^{1 2} and with a solubility parameter more similar to that of toluene (8.3 MPa^{1 2}) than water. The vapour permeability of THF decreased with increasing CNC content Figure 2(c). The decrease in permeability follows a similar trend to that of the mechanical properties that are related to the crosslink density that is heavily affected by the quality of the dispersion of zinc compounds. The proposed process, therefore achieves a unique and desirable combined outcome increasing the permeation of water vapour through the composite films while resisting the permeation of the vapour of solvents. Such a combination can overcome a major issue of rubber gloves by reducing the retention of sweat next to the skin while preventing the permeation of solvents, ions and pathogens through the gloves.

Although the passage of water vapour is enhanced, the new more heavily crosslinked system can still prevent the passage of liquid water or its solutions. Water absorption curves for the NR-CNC composites films are shown in Figures 2(d) and 2(e) which show the weight gain over 4 and 92 h, respectively. In Figure 2(d), the water absorption increased with increasing CNC loading over 4 hours but the curves of the films containing lower CNC loading (0.5 and 1.5 phr) increased above those with higher loading (3 and 5 phr) over 96 hours.

Example 4 - Mechanical property enhancement of NBR films due to reinforcing and crosslinking enhancement effect of CNCs

The effect of CNC on the mechanical properties of the NBR films was analyzed by studying the tensile strength, toughness, modulus and elongation at break (Figure 3). In addition, the tear strength was also studied as NBR is known to have a relatively low tear strength. From Figure 3a, it can be seen that the tensile strength of the NBR is low at ca. 4.3 MPa, similar to those found in literature ranging between 2.5 to 3 MPa. There was a slight reinforcing effect upon the addition of 0.5 phr CNC to the rubber. With further incorporation of CNC to the rubber, the tensile strength was improved by 96 and 166 % for films containing 1 and 3 phr CNC, respectively. Improvements to the tensile strength of NBR using epoxidized natural rubber as compatibilizer in nanoclay reinforcement exhibited a 60 and 100 % increase at filler loadings of 5 and 10 phr, respectively. However, higher loadings were required in this case with the need for a compatibilizer to improve interactions with the NBR. In our study, these remarkable improvements can be attributed to the load bearing capability of the CNC, the strong interface between the CNC and rubber, and increased crosslinking in the rubber system. Maturation time has minimal effect on the tensile strength.

The modulus of the NBR films revealed similar trend to that of the strength. At 0.5 phr loading of CNC, the stiffness did not exhibit significant change. In contrast, with 1 and 3 phr CNC, a remarkable increase in stiffness of 475 and 8300 %, respectively was observed. In order to highlight the remarkable results found in our investigation, we compared results from literature where a hybrid nanocomposite system of NBR was reinforced with carbon black and expanded graphite with an improvement in stiffness of 347 %. However, this was at a filler loading of 40 phr, significantly higher than that used in our system. This exceptional increase in the stiffness clearly displays the restrictive effect of the CNC on the rubber chains and suggests excellent dispersion within the rubber as well to form three dimensional networks. The effect of maturation time on the modulus of the rubber films was noticeable for samples fabricated after 3 and 4 days of maturation.

Interestingly, the tensile toughness, which is the energy required to fracture the films was improved with the addition of CNC, regardless of the restricting effect it has on the rubber changes. Again, a similar trend was observed for the tensile toughness with little or no change in films with 0.5 phr of CNC in comparison to the neat NBR. With increasing CNC loadings, the toughness has improved by a dramatic 205 and 391 % for films containing 1 and 3 phr of CNC, respectively, as shown in Figure 4b. The presence of CNC networks and increased crosslinking most likely resulted in greater energy required to disrupt these bonds, hence, leading to greater tensile toughness of the samples. The toughness is shown to mostly remain the same regardless of the maturation time.

Based on the results from the tensile strength and toughness, it was expected that the elongation at break would remain the same or be decreased, since the toughness is calculated from the area under the stress- strain curve. However, as shown in Figure 3d, the elongation at break for all maturation times improved for films containing 1 and 3 phr CNC, while that containing 0.5 phr of CNC remained mostly unchanged in comparison to the neat NBR. For all samples, the elongation decreased with increasing maturation time. The remarkable increases in toughness and elongation observed with the addition of CNC could be related to the presence of CNC networks and flexible interlayer at the interface between the CNC and rubber molecule as observed from the shifts in the tan delta peaks. This allows mobility and sliding of the rubber chains. A similar result has been observed for polyamide composites. A reduction in the tan delta was attributed to changes in the microstructure and the formation of network structure in the composites.

Example 5: Effect of CNC on the dispersibility of ZnO: percentage ZnO precipitate vs settling times at different ZnO-CNC

To investigate the dispersibility and dispersion stability of ZnO in water with and without the aid of CNC, ZnO, CNC and a combination of ZnO and CNC (ZnO:CNC ratios of 1: 1, 1:2 and 1:3) were dispersed in deionized water at a total concentration of 1 wt.%. Each sample was sonicated for 10 min to allow for proper dispersion of the particulates of each component. The vials containing the dispersed samples were left to sit for a period of 96 h while observing the changes over time. The changes in the dispersion of the suspended particles containing the CNC, ZnO, and ZnO-CNC at different ratios was observed. It was noted that the ZnO began to settle after 1 h and was clearly evident after 5 h. At the 24 h mark, the water in the vial was clear with the particulates settling at the bottom of the vials. It can be seen that there was a clear difference in the amount of settled particles at the bottom of the vials with the variation in the CNC concentration at the same time period. A trend that displays a decreasing particle settlement with increasing amount of CNC was noted. This showed that the CNC-ZnO complex formation aids the ZnO to stay suspended in the water longer due to the CNC stemming from the its excellent dispersibility in water. Without the aid of CNCs, ZnO settled rapidly due to its high density of ZnO (5.61 g/mL).

Example 6 - Effect of CNC on the dispersibility of ZnO: ZnO precipitate vs settling times at different ZnO-CNC ratios

Research on the effect of CNCs in rubber latex has so far shown that CNC improves crosslinking of the rubber chains. It is hypothesized that the complex formation between CNC and ZnO is the mechanism or chemistry involved in crosslinking improvement. Crosslinking of the rubber chains are achieved when ZnO activates sites on the chains and Sulpfur bridges the chains. However, ZnO has a high density of ca. 5.61 g/cc which makes it difficult to be dispersed and stay suspended in latex solutions. CNC on the other hand has a density of ca. 1.56 g/cc and is highly dispersible in aqueous solutions such as latex. It is therefore expected that the complex formation will aid ZnO dispersion and stay suspended for longer periods of time.

Experimental Setup

Different ratios of ZnO to CNCs (1:0, 1: 1, 1:2 and 1:3) were weighed out in separate 20 ml vials with ca. 6 mg and 6 ml of ZnO and deionized water for ratios of 1 : 0 and 1 : 1. As the CN C concentration increased, so was the amount of water used. This was done to nullify the effect of viscosity on the suspendability of ZnO in the aqueous solution. For example, at 1:2 ratio, the amount of water used was 12 ml.

Each vial was sonicated for 3 mins to completely disperse all components of the solution. As soon as sonication was done, the vials were left undisturbed and timed thereafter for 15, 60 and 120 mins. After each time interval, the vials were gently picked up not to disturb the precipitate at the bottom and the supernatant carefully decanted using a needle nose pipette. The precipitates were dried and then measured thereafter. It is worth to mention that there is residual solubilized CNC in the wet precipitate present but at minimal amount and is accounted for across all samples. Therefore, this is cancelled out across the measurements.

Results

Figure 4 shows the curves of the ZnO precipitate of the different ratios of ZnO to CNC versus settling time. It can be observed right away that after 15 mins, there is a reduction in ZnO precipitation in the presence of CNC. However, a correlation is observed for 1-2 and 1-3 ratios; the ZnO precipitate reduces with time for 1-3 and remains unchanged after 60 mins of settling for 1-2. As for 1-1, we observe the lowest settling at 15 mins and increasing settling of ZnO precipitate with time despite the presence of CNC. It can be postulated that more of the ZnO form complexes with each CNC and therefore makes it increasingly heavier which subsequently causes it to sink or settle at the bottom. Therefore, the settlement is most likely comprised of a significant amount of CNC as well. When the CNC ratio is increased, the complex formation of individual ZnO particles is spread over more CNCs while still able to stay suspended over time. It is worth knowing that difficulty in decanting the supernatant increases at and beyond the 1-2 ratio, as the precipitate is very minute with almost no clear divide between the two phases.

In order to quantify the effect of CNC on the dispersibility/suspendability of ZnO, the percentage decrease (suspension efficiency) in ZnO precipitate at different settling times of the different ratios was calculated and are reported in Figure 5 and Table 3.

Suspension Efficiency (%) = (Initial dry weight of ZnO used - dry weight of ZnO precipitate)/ Initial dry weight of ZnO used (equation 1)

As observed from Figure 5 and Table 3, there is definitely a change in the ZnO suspendability over time. The 1-2 ratio looks to be the most efficient at suspending ZnO. It also looks as though beyond 30 mins of sonication, there is a steep decline in the ZnO precipitate observed. This could imply that the complexation between ZnO and CNC is increases significantly, subsequently leading to more ZnO being suspended for longer periods of time.

Table 3: Suspension efficiency of ZnO at different ZnO-CNC ratios over time

Conclusion

Results from this study demonstrate that CNC has a significant effect on the suspendability and therefore dispersibility of ZnO since it it well known that CNC disperses well in water. Also, it is worth noting that the study was done on samples allowed to sit undisturbed which significantly reduces the contact between the ZnO and CNC and therefore reducing ZnO-CNC complex formation. Hence, since this result demonstrates clearly that CNC aids dispersion of ZnO, then in turbulent conditions such as high rate of mixing, the efficiency is expended to significantly improve.

Claims

1. An aqueous dispersion comprising a rubber latex, a cellulose nanocrystal (CNC) and a curing package, said curing package comprising a metal oxide.

2. The aqueous dispersion of claim 1, wherein the curing packages is comprising one or more of accelerants, activators, retarders and inhibitors.

3. The aqueous dispersion of claim 1, wherein the curing packages is comprising a sulfur-donating compound and a metal oxide.

4. The aqueous dispersion of any one of claims 1 to 3, wherein the metal oxide is CaO, MgO, CdO, CuO, PbO, ZnO or NiO.

5. The aqueous dispersion of any one of claims 1 to 4, wherein the said metal oxide is ZnO or NiO.

6. The aqueous dispersion of any one of claims 1 to 5, wherein the rubber latex is comprising polyisoprene, polybutadiene, styrene-butadiene, polycholoroprene or nitrile rubbers and combinations thereof.

7. A process for preparing a dispersion of a metal oxide in a latex composition comprising providing an aqueous dispersion of cellulose nanocrystal (CNC); dispersing said aqueous dispersion of CNC and a rubber latex to provide a CNC/rubber dispersion; preparing an aqueous dispersion of a curing package, said curing package comprising said metal oxide; combining said dispersion of a curing package and said CNC/rubber dispersion to provide said dispersion of a metal oxide.

8. The process of claim 7, wherein the amount of CNC is up to about 20 parts per hundred (phr), based on latex solids content.

9. The process of claim 7, wherein the amount of CNC is up to about 5 parts per hundred (phr), based on latex solids content.

10. The process of any one of claims 7 to 9, wherein said CNC is comprising sulfonic, phosphonic, carboxylic groups or mixtures thereof present on the surface of the nanocrystals.

11. The process of any one of claims 7 to 10, wherein said CNC is CNC obtained from sulfuric acid hydrolysis.

12. The process of any one of claims 7 to 11, wherein said metal oxide is CaO, MgO, CdO, CuO, PbO, ZnO or NiO.

13. The process of any one of claims 7 to 11, wherein said metal oxide is ZnO or NiO.

14. The process of any one of claims 7 to 11 wherein the amount of metal oxide (in particular ZnO) is about 0.5 to about 5 parts per hundred (phr), based on latex solids content.

15. The process of any one of claims 7 to 14, wherein the rubber latex is comprising polyisoprene, poly butadiene, styrene-butadiene, polycholoroprene or nitrile rubbers and combinations thereof.

16. A process for preparing a rubber composite comprising applying an aqueous coagulating solution on a surface of a substrate and allowing said surface to substantially dry or providing a surface of a substrate without a coagulating solution; preparing a dispersion of a metal oxide in a latex composition as defined in claim 7 to 15; coating said dispersion of a metal oxide in a latex composition on said surface of said substrate with or without said coagulating solution; and allowing said dispersion of a metal oxide in a latex composition to cure, thereby providing said rubber composite.

17. The process of claim 16, wherein the coagulating solution is comprising acetic acid, formic acid or soluble calcium salts, separately or in combination.

18. The process of claim 16, wherein the coagulating solution is a solution of one or more soluble calcium salts.

19. A cured latex product comprising cellulose nanocrystal (CNC) and a metal oxide, or cured latex product prepared by the process as defined in any one of claims 16 to 18.

20. A process for preparing a rubber composite comprising providing an aqueous coagulating solution; providing a dispersion of a metal oxide in a latex composition as defined herein, said dispersion comprising CNC, a rubber latex and a curing package comprising a metal oxide; and mixing said dispersion of a metal oxide in a latex composition and said aqueous coagulating solution to provide said coagulated rubber composition comprising dispersed metal oxide therein and curing the resulting coagulated latex to provide said rubber composite comprising dispersed metal oxide therein.