Effects of surface grafting of copper nanoparticles on the tensile and bonding properties of flax fibers

2.1 Raw materials

Flax fibers were purchased from Harbin Linen Textile Co., Ltd (Harbin, China). The average linear density of flax fiber was 0.039 g/m and 8.75 Ne in the English cotton counts. Copper chloride dihydrate salt with 99.0% purity was purchased from Tianjin Guangfu Technology Co. Ltd (Beijing, China). Polyvinyl alcohol (PVA) with a molecular weight of 24,000–88,000 was from Guohao Chenmical Company (Beijing, China). Sodium borohydride (NaBH₄) synthesizing reagent (purity≥98.0%), L-ascorbic acid (purity=98.0%), and trimethylammonium chloride (3-chloro-2 hydroxypropyl) solution were procured from Aladdin Industrial Cooperation (Shanghai, China). DGEBF (trade name is NPEF-170) having epoxide equivalent to 163.8 g/eq and density of 1.19 g/cm³ was used as the polymer matrix (Nan Ya Plastic Corporation, New Taipei City, China). MHHPA with a molecular weight of 168.19, the curing agent, was redeemed from Qingyang Chemical Co., Ltd. (Jiaxing City, China).

2.2 Synthesis of Cu nanoparticles and modification of flax fibers

Cu nanoparticles were synthesized in 0.5-g PVA solution in 100-ml distilled water. Copper chloride (250 mg) and L-ascorbic acid (6.5 g) were consequently added in 100-ml solution of PVA with intense stirring. Approximately 7.0-ml solution of sodium borohydride was injected dropwise with prolonged stirring for 2 h for the preparation of Cu nanoparticle sol. Unidirectional flax fibers were immersed in 20% trimethylammonium chloride solution maintaining a liquor ratio of 1:25 in an ultrasound sonicator (FRQ-1008HT, Front Ultrasonic Company, Hangzhou, China) for cationization process. During cationization, the temperature in the ultrasound sonicator was set at 60°C with continuous agitation. After that, 15% sodium hydroxide (4 g in 100 ml) with respect to the weight of the fiber was added in three steps at an interval of 5 min and the mixture was further stirred for 15 min.

The cationized flax fibers were removed from the ultrasound sonicator, cleansed several times with water, and dried at room temperature. The cationized fibers were then imported in the synthesized copper nano sol with concentration of 2.5 g/ml, maintaining the fiber to sol ratio of 1:25 with continuous stirring by a mechanical stirrer for 12 h so as to absorb functionalized Cu nanoparticles. Finally, functionalized Cu nanoparticles that absorbed cationized flax fibers were removed from the stirrer, rinsed with water, and dried in dark place at ambient temperature.

2.3 Preparation of flax fiber reinforced epoxy composite

Firstly, the required amount of Cu nanoparticles modified fibers were winded layer by layer adjacently (6–8 layers) on a ceramic plate. Hardener was added to epoxy resin with a ratio of 1:8, and 2 ml of initiator was added to that mixture. Furthermore, the resin mixture was poured into the modified fiber mounted plate repeatedly, so that the resin could reach every layer uniformly. Finally, the plate was kept for curing for 2 h by maintaining the temperature at 120°C for 1 h and 150°C for the next.

2.4 UV-visible absorption spectroscopy

The UV-visible absorption spectroscopy was performed by UV-Vis Spectrophotometer (UV-2450/2550, Shimadzu, Japan), with wave length of 190–900 nm and accuracy of ±0.3 nm. The Cu nanoparticles containing sol at the initial stage and after 2 weeks of synthesis were taken to conduct the UV-Vis analysis.

2.5 X-ray diffractrometry

In order to study the crystallinity nature of the fibers, the unmodified flax fibers, cationized flax fibers, and functionalized copper nanoparticles (FCuNPs) incorporated flax fibers were chopped to powdered form. X-ray diffraction was undertaken for the powdered samples with X’PERT PRO MPD (Panalytical Analytical Instrument Company, Westborough, MA, USA) having ceramic X-ray tube power of 2.2 kW, Cu as target and accuracy of 2θ±0.01°.

2.6 Fourier transform infrared spectroscopy

The Fourier transform infrared spectroscopy (FT-IR) measurement is used to record the absorption spectra of the material sample. The unmodified flax fiber and CuNPs modified flax fibers were chopped to get the powdered samples. The samples were then analyzed using a FT-IR spectrometer (Spectrum 100, Perkin Elmer Instruments, Billerica, MA, USA) with the attenuated total reflectance (ATR) technique, from the wavenumber of 4000 cm^-1 to 400 cm^-1.

2.7 Transmission electron microscope

Transmission electron microscopy (TEM) was performed using JEOL JEM-1400 electron microscope (Tokyo, Japan) to monitor the size of the nanoparticles. A small drop of the liquid sol was spread over a carbon-coated copper grid and dried under vacuum at room temperature before measurements. To detect the nanoparticle distribution around flax fibers, a slicer was used to make slices (~70 nm) transverse to the fiber and observed by TEM.

2.8 Scanning electron microscope

The morphologies of unmodified flax fiber yarn, cationized fiber, and Cu modified flax fiber were characterized with a Quanta 200F scanning electron microscope (SEM, OR, USA). Fibers were mounted on sample holders with carbon tape and sputtered by a thin layer of gold.

2.9 Tensile property test of single flax fibers

The average diameters of fibers were measured by microscope (observation for about 50 samples). Before the tensile test, samples were glued on to a sheet of paper with length of 50 mm. Then the gauge positions of paper were cut after chucking it on the testing machine to apply tensile load. Tensile tests were carried out at ambient temperature with a crosshead speed of 0.00125 mm/s, according to the ASTM D3379-75 standard using a JQ03A single fiber tensile tester (Zhongchen Digital Technic Apparatus Co., Ltd., Shanghai, China). The test gauge length was 20 mm. At least 50 measurements were repeated for each type of samples.

2.10 Microbond pull-out test

The interfacial shear strength (IFSS) of unmodified single flax fiber/epoxy composites and Cu modified flax fiber/epoxy composites was measured by the microbond pull-out test with an equipment (FA-620, East Wing Industrial Co., Ltd., Kyoto, Japan) at a crosshead speed of 0.06 mm/min. The experimental setup is shown in Figure 1, where single flax fiber is mounted on to aluminum frame and epoxy is injected over it. The specimen was then cured at 120–150°C for 2 h. Droplets of approximately 15–25 μm in diameter were selected for testing. At least 50 measurements were conducted for each kind of fiber.

Figure 1:

Sketch of the microbond pull-out test.

2.11 Dynamic mechanical thermal analysis

The thermal properties of flax fiber composites are performed by instrument dynamic mechanical thermal analysis (DMTA Q800, TA Instruments, DE, USA). The test mode is selected as single cantilever mode at the frequency of 1 Hz, and the amplitude is set at 30.0 μm. The cross-section of the flax fiber composites samples for DMTA measurement is 35.0 mm×8.00 mm and thickness is 1.5 mm. The temperature is observed from room temperature (about 20°C) to 180°C with a heating rate of 3°C/min.

2.12 Anti-microbial analysis

Anti-microbial studies for the FCuNPs incorporated flax fiber composite and the unmodified fiber composite were investigated against the Aspergillus Niger spores, a fungus most commonly found in the environment, and were tested in construction applications. The analysis was performed for 3 weeks by keeping the specimens under incubation for 28–30°C.

3.1 Synthesis of nanoparticle sol

When copper chloride was introduced into the PVA system, the solution turned into blue, indicating the presence of CuCl₂. While adding ascorbic acid, the solution turned into pale yellow color. The effective change in the color of the sol (see in Figure 2) with the addition of the reducing agent NaBH₄, from light blue to red vine, indicates the formation of the copper nanoparticles in the solution [12].

Figure 2:

Preparation of copper sol: (A) before nanoparticle formation, (B) after adding L-ascorbic acid, (C) indication in reduction after adding NaBH₄, (D) after nanoparticle formation.

The UV-visible spectroscopic analysis (Figure 3) demonstrates that the sol at initial stage and that after 2 weeks shows a sharp edge ~510 nm. This indicates the existence of the metallic Cu nanoparticles. The peak can be observed only when the particle size is ≥5 nm, and the surface plasmon resonance is prominent in the spectrum alone with the growth of the nanoparticles [12]. As there is no observable surface plasmon resonance peak in the spectrum, this illustrates that the Cu nanoparticles is expected to be <5 nm. The color of the Cu sol was sustained for 2 weeks, which indicates that Cu nanoparticles were stable to oxidation due to the presence of anions borohydride and ascorbic acid.

Figure 3:

UV-visible spectrum of Cu nanoparticle sol at initial stage and after 2 weeks.

TEM observation (see in Figure 4) clarifies that the particles in the solution are few nanometers in size and are spherical in shape. By analyzing about 100 to 500 particles of the nanoparticles, the diameter was determined between 1.5 and 9 nm and the average size of the synthesized nanoparticles was nearly 3.5 nm.

Figure 4:

TEM image for synthesized Cu nanoparticles.

3.2 Flax fiber yarn treated with Cu nanoparticle sol

The prepared Cu nanoparticle sol was used to treat the flax fiber yarn. When cationized flax fibers are immersed in nanocopper sol, FCuNPs stabilized by negatively charged PVA spheres can readily be adsorbed by the positive sites of cationized fibers. A large amount of positive charges may cause the charge reduction at the fiber surface because of cationization of flax fibers, thereby increasing the adsorption potentiality of FCuNPs due to an attractive interaction between fibers and FCuNPs [10]. The CuNP-cationized flax fiber is schematically shown below:

Figure 5 shows the TEM photographs of the existence of nanoparticle grafted on the treated flax fiber. As seen, the nanoparticles form aggregation with several ten to hundreds of nanometers. The adsorption of the funtionalized Cu nanoparticles to cationized flax fiber is also depicted in the EDS measurement of the selected area, as shown in Figure 6.

Figure 5:

TEM photographs of slice perpendicular to the nanoparticle treated flax fibers, where the black aggregation is the nanoparticles formed. (A) The top right side is flax fiber and the bottom left is the embedding epoxy, (B) the bottom left side is flax fiber and the top right is the embedding epoxy.

Figure 6:

(A) SEM of Cu modified flax fibre and (B) EDS of the selected area shown in Figure 5(A).

Cellulose is considered as the main structural constituent and contributes immensely to the mechanical properties of the plant fibers. The other components like lignin and hemicellulose also play a vital role in the characteristic property of the plant fibers. Figure 7 represents the FTIR spectrum information of the untreated and treated flax fiber. The major absorbance peaks are obtained at 3405.2 cm^-1, which is due to O-H stretching, and 2903 cm^-1, which is due to C-H stretching. A peak appears to be invisible or reduced at 1723.5 cm^-1 for untreated fibers. The invisible peak at 1723.5 cm^-1 for modified fiber indicates that the fiber was almost fully covered by the FCuNPs. The development of a good fiber reinforced composite favors the use of stiff fibers for reinforcement of polymeric materials [13]. More detailed observations about the absorption peaks are given in Table 1.

Figure 7:

FTIR spectrum of unmodified and modified flax fiber.

The X-ray diffraction pattern of different flax fibers is depicted in Figure 8. X-ray diffraction peaks for both flax fibers systems are almost identical in character. Note during the fiber treatment that the small molecules of the flax fibers (e.g. lignin and hemicellulose) can be removed from the flax surfaces due to alkaline solutions [14]. The removal of the hemicellulose during the treatment leaves a less dense and less rigid interfibrillar region, allowing the fibrils to re-arrange along the fiber major axis [15]. Removing lignin makes the middle lamella joining the ultimate cells to become more plastic and homogeneous due to gradual elimination of micro voids. The rearrangement in the fibrils along the fiber axis and resulting homogeneity leads to a packing order with increased crystallinity index for the treated fiber. The curves in XRD explain the increase in the crystallinity of the flax fiber after modification with functionalized Cu nanoparticles. The major peaks observed for both materials were at 2θ diffraction angle of 14°–23°. The broad diffraction peaks near the lower diffraction angle (peaks close to 14°) come from the amorphous regions, whereas the peak close to 23° comes from the crystalline part of the fibers. For unmodified fibers, the major diffraction peaks were observed at 16.8° and 22.8°, but modified fibers show peaks at 16.9° and 23.0°. This red shift of the diffraction peak of modified fibers indicates an increase of interplanar distance. Even if both the fiber systems have shown a similar behavior in X-ray pattern, their crystallinity nature differs significantly.

Figure 8:

XRD patterns for different fibers unmodified, cationized, and Cu modified flax fiber.

According to the following equation [16]:

where CI is the crystallinity index, I₂₂ is the intensity of the diffraction peak at 22°, and I₁₆ is the intensity of the diffraction peak at 16°; the crystallinity index of the unmodified and modified fibers was determined as 55.6% and 63.2%, respectively. This X-ray diffraction results in turn confirm an effective modification of the flax fibers by Cu nanoparticles.

3.3 Tensile properties of the single flax fiber

The tensile properties of the FCuNPs grafted flax fiber were tested with a single flax fiber, which was carefully extracted from flax fiber yarn. The untreated flax fiber was set as control specimen. The tensile strength of the untreated flax fiber is increased by 75% with the incorporation of nanoparticles, as shown in Figure 9A. From Figure 9B, an increase in modulus by 50% was noticed. XRD results have already confirmed the increase in crystallinity, which leads to the improvement of the tensile modulus. This increase in tensile strength and modulus is attributed to the reduced defects or flaw of the treated fibers and the strengthening effects of the nanoparticles on the fiber surfaces. FCuNPs can uniformly cover the flax fiber and reduce the defects and flaws, leading to enhanced tensile strength [14]. FCuNPs on the fiber surface may work as a strengthening sheath and contribute to the load sharing during tension of the fibers, bringing in strengthening effects.

Figure 9:

Tensile strength (A) and tensile modulus (B) of unmodified and FCuNPs modified flax fibers.

Figure 10 presents the stress-strain curve of control and grafted single flax fiber. As indicated, tensile strength, modulus, and strain at break were all enhanced by the grafting. In addition, linearity did not vary with the treatment.

Figure 10:

Strain-stress curve of control and grafted single fibers.

3.4 Bonding of the flax fiber to epoxy

An enhanced bonding occurred between the FCuNPs treated fiber and the epoxy resin matrix compared to the bonding between untreated fiber/epoxy, as shown in Figure 11. This is due to the incorporation of nanoparticles. The epoxy resin in the interphase zone was considered to be reinforced. The mechanical properties were enhanced, as commonly nanoparticles reinforce resin system. Consequently, the efficiency of the stress transfer from resin matrix (relative soft) to the flax fiber (more stiff) through the nanoparticle reinforced resin layer (interphase) was enhanced, leading to effective adhesion between the fiber and resin matrix.

Figure 11:

Interfacial shear strength for control flax fiber and FCuNPs grafted fibers to epoxy droplet tested by microbond pull-out test.

3.5 DMTA of flax-epoxy composite

The incorporation of FCuNPs impregnated flax fire with the DGEBF epoxy resin resulted in the decrease in tan δ intensity and also shifted the peak to low temperature from DMTA analysis (Figure 12). It is well established that the higher the peak tan δ value, the greater the mobility of polymeric chains [17]. Tan δ value for FCuNPs modified flax fiber reinforced composite sample decreased comparatively than that of unmodified fiber based one. This is due to the long chain of chemical bondage that happens during the cationization of fiber before the nanoparticles are grafted on the surface of the fiber. It is also observed from Figure 12 that with respect to the unmodified fiber reinforced epoxy, the intensity of the peaks reduced for nanocomposites. This is because of the increased interfacial interaction between polymer molecules and the nanoparticles, which restricts the molecular mobility.

Figure 12:

Tan δ curves of FCuNPs modified flax FRP composites.

As illustrated in Figure 13, storage modulus value increases with the incorporation of nanoparticles. The increase in the storage modulus is due to the restricted chain mobility imparted upon the reinforcement.

Figure 13:

Storage modulus of control and CuNPs modified flax FRP composts.

3.6 Anti-microbial results

Anti-microbial tests were implemented for the FCuNPs modified flax fiber composite and untreated flax fiber composite against Aspergillus niger (AS3.315) spores. The experiments were carried out by surface spread plate method, i.e. culturing fungi in an agar medium at 120°C on sterilization for 20 min. The medium was allowed to grow for a week. The spore formation occurs on the medium at times. The spores were extracted from the medium and spread on to the plates, where the composite samples were placed. Finally, the specimens were kept under incubation at 20–30°C for 3 weeks. Regular examination of the specimen was done by taking pictures, as shown in Figure 14, every week. From the analysis, we could realize that spore formation is taking place around the untreated flax fiber composite, and also, there is a salient growth of the fungus on to the surface of the unmodified flax fiber composite. This confirms that Cu nanoparticles incorporated in the flax fiber inhibited the growth of fungus on the composite. Hence, a composite that imparts anti-microbial activity with increased mechanical property can be achieved.

Figure 14:

Photograph of FCuNPs modified flax FRP composites (A) and control flax FRP (B) subjected to microbial environments after 3 weeks.