Patent Application
20160152820
This invention relates to reinforced epoxy nanocomposites, for example, cellulose nanocrystal (CNC)/epoxy nanocomposites, and methods for preparation thereof.
The scarcity of fossil fuel and the urgency of environment protection have driven composite research towards the development of renewable and sustainable natural fiber-based composites. Some of the key advantages of natural fiber-based composites over petroleum-based polymer and traditional composites include weight reduction, low cost, ease of recycling, and environmental friendliness. Flax, jute, hemp, and sisal fibers are just a few examples of natural fibers used in composite applications. Among different types of natural fibers, cellulose-based nanomaterials are a new class of natural nanoparticles widely studied in the field of polymer nanocomposites.
Cellulose nanocrystals (CNCs) are cellulose-based nanoparticles that can be extracted via acid hydrolysis from biological sources, such as trees and plants. These cellulose nanocrystals have high aspect ratio (3-10 nm wide and 50-500 nm long), low density, and a high degree of crystallinity. Their axial elastic modulus (EA=100-220 GPa) and tensile strength (Estimated σf=7.5 GPa) are higher than typical filler materials such as glass fiber and Kevlar. As a result, the properties of CNCs have led to research using CNCs as reinforcing materials for a variety of thermoplastic and thermosetting polymers including polyethylene, poly(lactic acid), poly(vinyl acetate), poly(vinyl alcohol), and polyurethanes.
The hydrophilic nature of CNCs has created difficulties when the CNCs are dispersed into hydrophobic polymer matrices. To disperse CNCs into a polymer matrix, three approaches have been used. One approach is chemical modification of CNCs surfaces to introduce hydrophobic side groups, which have been shown to improve CNC loading efficiency. However, this method requires extra steps and there is a loss of raw materials during the process. Another approach is utilizing water-borne polymers, in which emulsion of hydrophobic polymers or water dilution of hydrophilic polymers are chosen as the matrix materials to increase compatibility. In this method, CNC can be easily dispersed. Excess water, however, is required to emulsify or dissolve the polymer. Further, solvent-assisted dispersion uses organic solvents to reduce the viscosity of a give polymer system, which facilitates dispersion and mixing of CNCs within the polymer. However, there are environmental concerns related to emission of organic solvents. Overall, based on the choice of polymer and the final application, the proper CNC dispersion method should be selected when designing the CNC/polymer nanocomposites.
Epoxy is one of the most commonly used high performance thermosetting resins. Applications for epoxy can be found in the fields of aerospace, electronics, automobile, and construction. Most epoxy resins consist of two components: epoxy monomer and hardener/crosslinking agent. They are usually shipped separately and mixed at the point of use. There are various types of hardeners with amine, hydroxyl, or carboxyl active groups. The type of hardeners determines the crosslinking density and eventual physical properties of the cured epoxy.
Previous studies on CNC/epoxy nanocomposites have used various aromatic and aliphatic amine hardeners (Lu, et al. Compos. Part B Eng. 2013, 51, 28-34; Xu, et al. Polymer 2013, 54, 6589-6598; Ruiz, et al. Macromol. Symp. 2001, 169, 211-222; Pan, et al. Appl. Mech. Mater. 2012, 174-177, 761-766; and Tang, et al. ACS Appl. Mater. Interfaces 2010, 2, 1073-1080). Most of these studies reported that the additions of CNCs to epoxy enhanced the mechanical properties of epoxy both in the glassy and rubbery states, and also increased the glass transition temperature (Tg). However, there have not been any known studies that have used hardeners as the CNC dispersant to increase CNC dispersion within epoxy. There is a still unmet need for an alternate approach for the preparation of CNC/epoxy nanocomposites.
In one aspect, the present invention provides a method for preparing a reinforced epoxy nanocomposite.
In another aspect, the present invention provides a method for preparing a cellulose nanocrystal (CNC)/epoxy nanocomposite, the method comprising:
In some embodiments, the method of the invention further comprises a step of casting the CNC/hardener/solvent-epoxy mixture in a mold, for example, prior to the removing and curing of step (c).
In some embodiments, the CNC/hardener/solvent suspension in the method of the invention is prepared by
In another aspect, the present invention provides a cellulose nanocrystal (CNC)/epoxy nanocomposite prepared by the method of the invention. In some embodiments, the cellulose nanocrystal (CNC) in the (CNC)/epoxy nanocomposite of the invention is in an amount of from about 0.4 wt % to about 2.05 wt %. In some embodiments, the epoxy nanocomposite is cured by Jeffamine D400 (JD400), diethylenetriamine (DETA), or (±)-trans-1,2-diaminocyclohexane (DACH).
The details of one or more embodiments of the invention are set forth in the accompanying the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.
The present invention provides a method for preparing a cellulose nanocrystal (CNC)/epoxy nanocomposite, the method comprising:
In some embodiments, the method of the invention further comprises a step of casting the CNC/hardener/solvent-epoxy mixture in a mold prior to step (c).
In some embodiments, the solvent in the method of the invention is a water-miscible organic solvent known in the art. For example, the water-miscible solvent that can be used in the method of the invention includes, but is not limited to, methanol, ethanol, ethylene glycol, acetone, tetrahydrofuran, and methylethylketone. In some embodiments, the solvent is tetrahydrofuran. In other embodiments, the solvent is methylethylketone. In certain embodiments, the solvent is acetone. In some embodiments, the water-miscible organic solvent may be used singly or in combination of two or more.
The epoxy of the present invention includes, but is not limited to, a novolac based epoxy resin, a bisphenol A based epoxy resin, a bisphenol F based epoxy resin, a biphenyl based epoxy resin, a triphenylmethane based epoxy resin, and a phenol aralkyl based epoxy resin. In some embodiments, the epoxy is a bisphenol A based epoxy resin. In other embodiments, the epoxy is a bisphenol F based epoxy resin. In certain embodiments, the epoxy is bisphenol A diglycidyl ether resin (2,2-bis(4-glycidyloxyphenyl)propane). These epoxy resins may be used singly or in combination of two or more.
The hardener of the present invention can be aromatic or aliphatic amine hardeners known in the art. In some embodiments, the hardener for the invention contains one amino group. In certain embodiments, the hardener of the invention contains two or three amino groups. In some embodiments, the hardener is a polyether based amine. In other embodiments, the hardener is diethylenetriamine (DETA), Jeffamine D4000 (JD400), or (±)-trans-1,2-diaminocyclohexane (DACH).
In some embodiments, the cellulose nanocrystal (CNC) in the method of the invention is freeze-dried. In other embodiments, the cellulose nanocrystal (CNC) in the method of the invention can be used without additional drying.
In some embodiments, the removing of the solvent in the method of the invention is achieved by degassing. In some embodiments, the degassing, for example, under vacuum, can remove residual solvents and air bubbles simultaneously.
In some embodiments, the step of curing in the method of the invention is conducted at a temperature of from about 50° C. to about 200° C. In other embodiments, the curing is conducted at a temperature of from about 60° C. to about 180° C.
In some embodiments, the casting in a mold in the method of the invention, followed by curing, can prepare a plurality of sheets, films, or fibers depending on the mold used in the method of the invention.
In some embodiments, the CNC/hardener/solvent suspension in the method of the invention is prepared by
In some embodiments, the redispersing of step (e) is achieved by sonification.
In some embodiments, the formed CNC/water suspension in the method of the invention has a concentration of from about 2 wt % to about 5 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 2 wt % to about 7 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 2 wt % to about 9 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 2 wt % to about 10 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 2 wt % to about 15 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 2 wt % to about 20 wt %.
In some embodiments, the formed CNC/water suspension in the method of the invention has a concentration of from about 4 wt % to about 9 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 4 wt % to about 7 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 5 wt % to about 9 wt %. In some embodiments, the formed CNC/water suspension has a concentration of from about 5 wt % to about 7 wt %. In certain embodiments, the formed CNC/water suspension has a concentration of about 5 wt %.
In some embodiments, the solvent in the CNC/solvent organogel is calculated gravimetrically. In some embodiments, the amount of the solvent that is added to the hardener and the epoxy in the method of the invention is the same amount as the solvent in the CNC/solvent organogel. In some embodiments, no solvent is added to the hardener. In other embodiments, no solvent is added to the epoxy.
In some embodiments, the amount of the cellulose nanocrystals (CNCs) used in the method of the invention is from 0 to about 10 parts by mass based on 100 parts by mass of the hardener. In some embodiments, the amount of the cellulose nanocrystals used is from about 1 part to about 10 parts by mass based on 100 parts by mass of the hardener. In other embodiments, the amount of the cellulose nanocrystals used is from about 2 parts to about 10 parts by mass based on 100 parts by mass of the hardener. In some embodiments, the amount of the cellulose nanocrystals used is from about 4 parts to about 10 parts by mass based on 100 parts by mass of the hardener. In other embodiments, the amount of the cellulose nanocrystals used is from about 5 parts to about 10 parts by mass based on 100 parts by mass of the hardener. In certain embodiments, the amount of the cellulose nanocrystals used is from about 8 parts to about 10 parts by mass based on 100 parts by mass of the hardener.
In some embodiments, the amount of the epoxy used in the method of the invention is from about 150.0 parts to about 850 parts by mass based on 100 parts by mass of the hardener. In some embodiments, the amount of the epoxy used in the method of the invention is from about 300 parts to about 850 parts by mass based on 100 parts by mass of the hardener. In some embodiments, the amount of the epoxy used in the method of the invention is from about 600 parts to about 850 parts by mass based on 100 parts by mass of the hardener.
In some embodiments, the amount of the epoxy and the amount of the hardener used in the method of the invention are calculated based on the numbers of the amino group in the hardener and the epoxide group in the epoxy. In some embodiments, the molar ratio of the amino group in the hardener and the epoxide group in the epoxy used in the method of the invention is about 1:1.
The present invention further provides a cellulose nanocrystal (CNC)/epoxy nanocomposite prepared by the method of the invention as described herein. The cellulose nanocrystal (CNC)/epoxy nanocomposite of the invention has improved mechanical properties over an epoxy nanocomposite without reinforcing CNC. In some embodiments, in the (CNC)/epoxy nanocomposite of the invention, the cellulose nanocrystal (CNC) is in an amount of from about 0.4 wt % to about 2.05 wt %.
In some embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.4 wt % to about 1.50 wt %. In other embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.4 wt % to about 1.20 wt %. In some embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.4 wt % to about 1.0 wt %. In certain embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.4 wt % to about 0.9 wt %. In some embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.4 wt % to about 0.7 wt %. In some embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.4 wt % to about 0.6 wt %. In certain embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.5 wt % to about 1.0 wt %. In other embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.6 wt % to about 1.0 wt %. In some embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is from about 0.7 wt % to about 1.0 wt %.
In some embodiments, the (CNC)/epoxy nanocomposite of the invention is cured by Jeffamine D400 (JD400), diethylenetriamine (DETA), or (±)-trans-1,2-diaminocyclohexane (DACH). In certain embodiments, the (CNC)/epoxy nanocomposite of the invention is cured by diethylenetriamine (DETA). In other embodiments, the (CNC)/epoxy nanocomposite of the invention is cured by Jeffamine D400 (JD400). In some embodiments, the (CNC)/epoxy nanocomposite of the invention is cured by (±)-trans-1,2-diaminocyclohexane (DACH).
In some embodiments, the cellulose nanocrystal (CNC) in the nanocomposite of the invention is about 0.6 wt % and wherein the epoxy nanocomposite is cured by diethylenetriamine (DETA).
In some embodiments, the epoxy in the epoxy nanocomposite of the invention is bisphenol A diglycidyl ether resin. In some embodiments, the Young's modulus of the (CNC)/epoxy nanocomposite of the invention is increased by from about 15% to about 20% compared with an epoxy nanocomposite without reinforcing cellulose nanocrystals (CNC).
The present invention further provides materials comprising the CNC/epoxy composite of the invention. Such materials have many potential areas of uses including food or biomedical applications. The materials can be molded to sheets, films, and fibers.
In some embodiments, the present invention provides a method for preparing CNC/epoxy nanocomposites, comprising (a) dispersing a freeze-dried CNC in deionized water to reach 5 wt % suspension to achieve a CNC water suspension; (b) adding a solvent to the CNC water suspension, resulting in a top solvent layer and a bottom solvent layer; (c) creating a CNC/solvent organogel by replacing the top solvent layer with fresh solvent about every 24 hours; (d) adding a hardener to the CNC/solvent organogel to create a CNC/solvent organogel-in-hardener organogel; (e) redispersing the CNC/solvent organogel-in-hardener organogel in a hardener to create a CNC/hardener/solvent suspension; (f) mixing the CNC/hardener/solvent suspension with Diglycidyl ether of bisphenol-A (DGEBA) to create a CNC/hardener/solvent-DGEBA mixture; (g) casting the CNC/hardener/solvent-DGEBA mixture in a mold to create a plurality of sheets; and (h) curing the sheets. In some embodiments, the method comprises a step of degassing the sheets under vacuum. In some embodiments, the solvent of the method of the present invention is acetone. In other embodiments, the solvent of the method of the present invention is tetrahydrofuran.
In some embodiments, the method of the present invention comprises a step of pulling the solvent before mixing with epoxy. In some embodiments, the method of the present invention is carried out in a solvent-free environment.
The present invention provides a new approach to produce cellulose nanocrystals (CNC)/epoxy nanocomposites, where CNCs are first dispersed in the hardeners before mixing with epoxy resin. By pre-formulating the hardeners with CNCs, stable suspensions of CNC, hardeners, and acetone are achieved. The Young's modulus and tensile strength of the produced (CNC)/epoxy nanocomposites are improved, although the reinforcing effects of CNC were hardener dependent. For example, the DETA cured epoxy by the method of the invention shows increased tensile modulus, tensile strength, strain-at-failure, and work-of-fracture of ˜20%, ˜15%, ˜25%, and ˜100%, respectively at their highest at a 0.56 wt % CNC addition. The presence of acetone before curing embrittles the epoxy. However, CNC additions counteract this effect by maintaining the strain-at-failure of the epoxy resin. Dynamic mechanical analysis indicates that the addition of water and acetone could alter the degree of curing. CNCs were able to preserve the mechanical properties of epoxy despite the plasticization effect of water.
Symbols and notations as used in the present disclosure are briefly described herein.
E=Young's modulus;
σf=Tensile strength/Fracture strength;
σy=Yield strength;
εf=Strain-at-failure;
γwof=Work-of-fracture;
Tg=Glass transition temperature;
E′=Storage modulus; and
E″=Loss modulus.
In some embodiments, the term “epoxy nanocomposite” refers to the nanocomposite composition formed with epoxy resins, hardeners, and CNCs. In some embodiments, the term may refer to the aforementioned nanocomposite after curing. In other embodiments, the term may refer to the aforementioned nanocomposite in a state prior to curing; such use will be clear at the time it is discussed.
In some embodiments, the term “epoxy” refers to “epoxy resin” as known in the art.
Acetone was purchased from Marcon Fine Chemicals, Center Valley, Pa., USA. Epoxy resin (Diglycidyl Ether of Bisphenol-A (DGEBA), Equivalent Epoxy Weight (EEW)=172-176), JD400 (Mw ˜400, Amine Hydrogen Equivalent Weight (AHEW)=100), DETA (AHEW=20.6), and DACH (AHEW=28.5) were purchased from Sigma-Aldrich, St Louis, Mo., USA. All materials were used as purchased. The silicone rubber mold was created using Mold Max 40 silicone rubber from Smooth-on, Easton, Pa., USA. CNC was provided by USDA Forest Service-Forrest Products Laboratory, Madison, Wis., USA.
Tensile testing was conducted using a universal tensile testing machine (MTS insight, MTS System Corp., Eden Prairie, Minn., USA). Tensile specimens were prepared by laser cutting tensile specimens from a 12.7 cm×12.7 cm sheet following ASTM 638-10 Type IV sample dimensions and proportionally decreased by 2.27 times. The specimens were sanded to achieve a thickness close to 1 mm. Tests were completed in displacement control at rate of 5 mm/min. Five to ten replicates were tested for each type of specimen. The average and standard deviations were reported. Student t-tests were conducted on Young's modulus, ultimate tensile strength, work-of-fracture, and strain-at-failure data to determine statistical significance. The threshold level was set at 0.05.
Storage modulus, loss modulus, and tan 6 were measured using DMA Q800 (TA instruments, New Castle, Del., USA) under single cantilever mode. Specimens were laser cut into 12.78 mm×35.64 mm bars. The DETA hardened specimens were heated from room temperature to 200° C. at a rate of 3° C./min under nitrogen atmosphere. The DETA hardened specimens were heated from room temperature to 250° C. at a rate of 3° C./min under nitrogen atmosphere. The JD400 hardened specimens were heated from 20° C. to 150° C. at a rate of 3° C./min under nitrogen atmosphere. The specimens were tested at 15 μm strain and 1 Hz frequency.
The cured specimens were observed using a Carl Zeiss inverted microscope equipped with two crossed polarizers. The specimens were sanded to remove surface defects. Images were taken when polarizers were at full extinction.
CNC/hardener/acetone suspensions were created for all three types of hardeners at various concentrations of CNCs. In detail, freeze-dried CNCs were first dispersed in deionized water to reach 5 wt % suspension. Following the previous solvent exchange sol-gel process developed for dispersing CNCs in polymer by Capadona et al (Nat. Nanotechnol. 2007, 2, 765-769), 15 mL of acetone was added to 2 mL of CNC water suspension. To create the CNC/acetone organogel, the top acetone layer was replaced with fresh acetone every 24 hours. After 48 hours, the hardener was added to the CNC/acetone organogel and allowed to immerse for one hour. The CNC/acetone organogel was then redispersed in hardener using a sonifier (S-250D, Branson Ultrasonics Corp., Danbury, Conn., USA) at 25% amplitude and one-second on/off cycles until a transparent suspension was achieved.
The CNC/hardener/acetone suspension was mixed with DGEBA using a vortexer (VWR, West Chester, Pa., USA) under 1:1 amine to epoxide ratio. The mixture was casted in a silicone rubber mold to create 12.7 cm×12.7 cm sheets. The specimens were degassed under vacuum to remove the residual acetone and air bubbles. The JD400 and DETA specimens were cured at 60° C. for 12 hours, followed by 80° C. for 2 hours, and then 125° C. for 3 hours. The DACH specimen were cured at 60° C. for 12 hours, followed by 80° C. for 1 hours, and then 177° C. for 2 hours.
The residual acetone in CNC/acetone organogel was calculated gravimetrically. The same amount of acetone was added to hardeners and DGEBA during mixing to create the corresponding EQA specimens. Acetone was subsequently removed during the degassing step. The EQA specimens were cured following the same procedure as their corresponding CNC specimens. All types of specimens created were listed in Table 1.
Good dispersion of CNCs within epoxy is necessary to maximize performance of the resulting CNC/epoxy nanocomposite. CNC/epoxy nanocomposites are generally prepared through co-mixing epoxy, hardeners, and CNCs in situ. In the method of the present invention, a approach was taken by dispersing CNCs in hardeners first before mixing with epoxy resin. Bisphenol A (BPA) based epoxy is generally hydrophobic, which makes CNC dispersion difficult, while the hardeners are typically more hydrophilic. The amine group on the hardeners can form cationically charged moieties that can interact with the negatively charged CNC surface, which may increase CNC dispersion. Once predispersed, the CNCs would then be easier to disperse in the BPA epoxy phase. The hardeners are acting similar to dispersants to minimize aggregation. Additionally, the CNCs may be kinetically trapped by the higher viscosity or form charged complexes leading to higher dispersion. Due to these potential benefits, the predispersion of CNC into the hardeners was performed.
To disperse CNCs within the hardeners, an acetone/water sol-gel solvent exchange method was used (Tang, et al. ACS Appl. Mater. Interfaces 2010, 2, 1073-1080; Capadona, et al. Nat. Nanotechnol. 2007, 2, 765-769). An ultrasonifcation step was required to redisperse the CNC/acetone organogel in hardeners to allow the formation of stable CNC/acetone/hardener suspensions.
Similar behaviors were observed in all three types of hardeners. At low CNC concentrations, the birefringent patterns observed in all three hardeners were polychromatic. At similar concentrations, suspensions of CNC in water and other organic solvent suspensions displayed monochromatic patterns. The shear thinning effects of the CNC/acetone/hardener indicated that there was a reversible interaction between CNC, hardener, and acetone. Similar effects were also observed by others in CNC/dimethylsulfoxide (DMSO) suspensions under shear. It is believed that the hydrogen bonding between the amine groups on the hardeners and the hydroxyl groups on the CNC had created a weak and reversible physical interaction.
To evaluate the dispersion state of CNCs within the cured epoxy, specimens were evaluated using optical microscope under polarized light.
The existence of birefringent domains could also be stress related. Epoxy, even though isotropic in nature, could exhibit birefringent behavior when subjected to stress (Bettany, et al. Br. J. Appl. Phys. 1963, 14, 692-695). Previous studies indicated that shear stress could lead to orientated CNC domains, which may change the birefringent behavior of casted CNC films (Reising, et al. J. Sci. Technol. For. Prod. Process. 2012, 2, 32-41; and Diaz, et al. Biomacromolecules 2013, 14, 2900-2908). There were no noticable change in birefringent effects when specimens were rotated under the microscope. For DETA specimens, the concentration of birefringent domains did not increase as CNC loading increased. While there may be no correlation between the birfriengent domains and CNC aggregation, the posibility of microscale CNC aggregation could not be excluded due to resolution limitations of the optical microscopy used in this disclosure.
In the method of the present invention, CNC/acetone organogels were redispersed in hardeners to create a stable suspension. Aceteone was initially left in the suspension to maintain low viscosity and prevent potential CNC aggregation, and then removed by vacuum after the CNC/acetone/hardener suspensions were mixed with epoxy. Vacuum was also applied on neat epoxy specimens to remove large bubbles generated during mixing. To evaluate the impact of acetone on the nanocomposite system, equivalent acetone (EQA) specimens were created. An equivalent amount of acetone to the CNC/epoxy nanocomposites was added to the neat epoxy during mixing and removed afterward with vacuum. Mechanical and thermal properties of CNC/epoxy nanocomposites and their corresponding EQA specimens were analyzed via tensile testing and dynamic mechanical anaylsis. It is noted that although acetone is used herein as one embodiment of the present disclsoure, other solvents may be used, including tetrahydrofuran (THF). The process may also be conducted in a solvent-free environment.
JD400 is a difunctional short chain hardener used to increase flexibility and decrease brittleness of cured epoxy. The hydrophilic oxypropylene repeating units in the backbone and low vicosity can potentially increase CNC dispersion. The tensile properties of the specimens cured with JD400 are shown in
All specimens cured with JD400 had shown necking behaviors during tensile testing. As shown in
Similar trends were also observed with the strain-at-failure and work-of-fracture data. As indicated in
DETA is a trifunctional small molecule hardener, and is one of the most commonly used epoxy hardeners. DETA has higher amine content than JD400 and DACH, and as such it can potentially form more hydrogen bonds with CNCs to increase dispersion as well as form higher cross-linking denisty networks than JD400 and DACH.
Tensile properties of CNC/epoxy nanocomposites cured with DETA and the corresponding EQA specimens are shown in
The strain-at-failure and work-of-fracture data for DETA cured specimens showed similar trends. There were no significant change between CNC specimens and neat specimens except at 0.56 wt % CNC. Unlike JD400 cured specimens, the DETA cured specimens were not embrittled by the addition of CNCs and acetone. This indicates that the more cross-linked epoxy was less likely to be affected by the acetone caused defects. It can be concluded that CNC improved modulus and strength of epoxy while not scarificing ductility. Further, CNC additions to DETA before being incorporation into the epoxy resin simultancously increased Young's modulus, tensile strength, strain-at-failure, and work-of-fracture, which is difficult to achieve in nanocomposites.
DACH is a difunctional cycloaliphatic hardener. They are less reactive, therefore are usually cured at higher temperatures and because of this DACH cured specimen can provide additional insights to CNC reinforcing characteristic after a high temperature curing stage. Further, DACH has two active amine groups that are in close proximity to each other, which provide structural variety to the cured epoxy as compared to the other two aliphatic amines. DACH cured epoxies have similar mechanical properties as DETA cured epoxies.
The CNC reinforcing effects depended on the molecular structure of the hardeners and the crosslinking network formed between the epoxy and hardeners. DETA, which has five active amine hydrogens, formed a high density crosslinking network with epoxy. DACH cured epoxies, which were cured at elevated temperature, formed epoxy network with high degree of crosslinking. JD400, due to long crosslinker length, formed flexible epoxy networks with more freedom of movement between polymer chains. As a result, the tensile modulus and strength of the specimen cured with JD400 was lower in comparison with those of the DETA and DACH cured specimens.
Despite the differences in epoxy network structure, Young's modulus were increased for both JD400 cured and DETA cured specimens. Similar improvement of Young's modulus was observed by Xu et al (Polymer 2013, 54, 6589-6598). Xu et al also showed a lowering of strain-at-failure and tensile strength when CNC loading were below 2 wt %. However, in this study, only the JD400 cured specimen exhibited such behavior. This further indicated that the CNC reinforcing effects were hardener dependent. DETA cured specimens had the best combination of mechanical properties improvement among the three hardeners evaluated. The mechanical properties improvement, however, did not increase with CNC loading for CNC content greater than 1% wt. This could result from two possible mechanisms: the formation of CNC aggregation and/or the presence of residual acetone and water that led to increasing numbers of defects and lowering of cross-linking density. A combination of both might also be applied.
It is also worth noticing that the hydrophilic main chain of JD400 was not a factor on the CNC reinforcing effects, since the improvement of Young's modulus were not significantly different between JD400 and DETA cured specimens (16% increase for JD400 and 19% increase for DETA at close to 2 wt % CNC). This indicates the main reinforcing mechansim was physical interactions instead of chemical bonding. The molecular structure of JD400 and DACH indicates higher tendancy to form a more flexible crosslinking network. These flexible networks caused both epoxies to be less flaw tolerant. The maintenance of strain-at-failure of DACH cured specimens indicated that CNC enhanced the flaw tolerance of the epoxy network by preventing defect propagation. For the JD400 cured sample, flaw tolerance did not improve, which was likely due to the ductile nature of JD400 cured epoxy.
The presence of CNCs within the epoxy, while preventing defect propagation, also limited polymer chain movement within the loose cross-linking network and therefore did not maintain high strain-at-failure. However, the reinforcing mechanism could not be clearly identified due to the possible existence of defects created by the residual acetone and water, which could alter the fracture mechanism dramatically. In addition, previous studies indicate that the presence of solvent and water during the epoxy curing could also affect the degree of cure and the curing kinetics of the epoxy resin, which can also influent the mechanical properties. Nevertheless, the mechanical properties of the CNC/epoxy nanocomposites developed in the method of the present invention were not only unaffected but also improved in some hardener systems. This indicates that the CNC reversed the plasticizing effect through its superior mechanical reinforcing efforts.
A similar trend was observed for the DETA cured specimens (
In
In addition, the temperature dependent curing process could have been affected as solvents absorbed heat during evaporation. The residual solvent may also inhibit the epoxy curing process as dipole-dipole interaction between the solvent and the amine groups of the hardeners could prevent the amine groups from reacting with epoxy. In addition, Liu et al (J. Mater. Sci. 2012, 47, 6891-6895) suggested that nanoparticles could also selectively absorb resin or hardener at its surface, limiting the reaction between epoxy resin and hardener. In this study, since CNC was exposed with hardener first, the weak hydrogen bonding interactions between CNCs and the hardeners could possibly inhibit the epoxy/hardener reaction. These weak interactions of the hardeners, acetone, and CNCs could have caused incomplete curing in some regions of the epoxy and led to formation of inhomogeneous crosslinking networks. In addition to decreasing Tg, there was a decrease in the degree of cure caused by the presence of water.
Side-by-side properties comparisons between CNC/epoxy nanocomposites cured with JD400, DETA, and DACH are given in
For small molecule trifunctional hardeners such as DETA cured epoxy, CNC additions improved Young's modulus, tensile strength, strain-at-failure, and work-of-fracture properties, despite the presence of residual solvent and water, which may have depressed its Tg. For high temperature cured cyclic structured difunctional hardeners such as DACH, there was minimal influence on Young's modulus, tensile strength, work-of-fracture, and strain-at-failure with CNC additions. The Tg of DACH cured specimens was also lowered with the presence of residual water and acetone during the curing reaction.
From these results, it can be concluded that the method of dispersing CNCs in the hardener first before mixing with epoxy was a viable approach to produce epoxy with improved properties. The degree of improvement depended on the choice of hardeners. Small molecule trifunctional hardeners such as DETA had the highest increase of mechanical properties. The residual acetone and water from the solvent-exchange step affected the curing process of the epoxy and led to plasticization of the cured epoxy. Further, limiting water and solvent during the epoxy curing process is the key to improving the current method.
Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.
This application claims the benefit from U.S. Provisional Patent Application Ser. No. 62/083,028, filed on Nov. 21, 2014, which is incorporated herein by reference in its entirety.
This invention was made with government support under DGE1144843 awarded by the National Science Foundation, under FA9550-11-1-0162 awarded by the United States Air Force Office of Scientific Research (USAF/AFOSR), and under 11-JV-11111129-118 awarded by the United States Department of Agriculture. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62083028 | Nov 2014 | US |
