Abstract
Silaned nano-silicon dioxide (SiO2) was used to improve the adhesion properties of carbon fibre/polyethylene (CF/PE) composites. The nano-SiO2 were treated by silane under different discharge time. And the changes on the surface properties of the treated and untreated composites were studied by impact, three-point bending tests and scanning electron microscopic analysis. The measurement showed that CF increases fracture toughness with the increase in CF content. Too much high content of CF did not further cause the increase in the toughness of CF/PE composite. The impact strength of silaned specimens is still higher than those of the unsilaned ones. The modified composite with the good matrix/fibre adhesion possessed 20% higher interlaminar shear strengths than the composite having weak interface.
Introduction
During the last decade, considerable attention was paid to inorganic–organic hybrid materials because their solid state properties could be tailored in relation to the nature and relative content of their constitutive components. Low-volume additions (1–5 wt%) of highly anisotropic nanoparticles, such as layered silicates, provide property enhancement with respect to the neat polymer that are comparable to those achieved by conventional loadings (15–40 wt%) of traditional fillers. Besides, unique value-added properties not normally possible with traditional fillers are also observed, such as enhanced strength, electrical conductivity, electrostatic discharge, remote-actuated shape recovery and ablation resistance. 1 –4
In other publications, fracture toughness was improved by adding spherical nanoparticles in which quality nanodispersion is easier to achieve. 5,6 Filler/matrix interaction and achieving quality dispersion seem to be the key factors for fracture toughness enhancement for nanoscale fillers.
One of the major concerns in designing composite structures is their susceptibility to impact loading. Fibre-reinforced polymer–matrix composites are known to be highly susceptible to internal damage caused by transverse loads, even under low-velocity impacts. 7,8 The composites can be damaged on the surface as well as beneath the surface with relatively light impacts causing barely visible impact damage, while the surface may appear to be undamaged to visual inspection. For the effective use of fibre-reinforced polymer–matrix composites for high-performance applications, understanding the causes for the formation of such damage under low-velocity impact and improving the damage-resistance characteristics of the composites are important considerations which have been the topic of extensive research for the last few years. Review articles on the impact behaviour of polymer–matrix composites covering contact laws, impact dynamics, stress analysis, damage mechanics, post-impact residual property characterization and damage-resistance improvements are available in the literature. 9,10 Many research publications are available on the impact behaviour of polymer–matrix composites covering specific aspects. 11,12
Matrix deformation and microcracking, interfacial debonding, lamina splitting, delamination, fibre breakage and fibre pull-out are the possible modes of failure in composites subjected to impact loading. Even though fibre breakage is the ultimate failure mode, the damage would initiate in the form of matrix cracking/lamina splitting and would lead to delamination. Damage-free composites are necessary for their effective use. 13,14
In this work, silicon dioxide (SiO2) is treated by silane in order to improve the surface properties of carbon fibre (CF). The purpose of this work is to study the impact properties of the polyethylene (PE) composites filled with surface-modified CF. Some insights into the impact fracture mechanisms of the PE composite are also given.
Experimental
Materials
High-density PE (T g: 85°C, T m: 130°C, p: 0.95 g cm− 3 and Mw: 125,000 g mol− 1) were all Merck (Darmstadt, Germany) products and used as purchased.
For the present investigation, the reinforcement materials were continuous polyacrylonitrile-based CFs manufactured by Shanghai Sxcarbon Technology Co. Ltd., (Shanghai, China).
The silane (KH550) was dissolved in ethanol before use. The particles of nano-SiO2 were dispersed in ethanol and subjected to ultrasonic agitation for 15 min, the silane solution was then introduced and the ultrasonic treatment continued for 1 h.
Composite preparation
PE samples were first dissolved in 40 mL of xylene in a three-necked flask. Required amount of CF was added into the above solution. Indole was also dissolved in 10 mL of xylene and added into the same flask. The reaction mixture was washed with distilled hot water several times and dried in a vacuum oven at 70°C for 24 h.
Impact and three-point bending (TPB) tests
A Charpy unnotched impact strength test was carried out on a pendulum impact tester (Model PH 125, Amsler & Co., Schaffhouse, Switzerland) with a capacity of 0.98 J. The test span was 20 mm, measured between the two specimen supports.
The specimens were impacted on the narrow 1 mm surface, with the line of impact midway between the supports, and the direction of blow normal to the plane of reinforcement.
An INSTRON 4206 (Sansi, China) electromechanical machine was used to implement the TPB test.
Single fibre pull-out tests
The key point of this pull-out test is to incorporate this resin, along with the fibre, into a stress analysis, which is presumably dominated by the wettability between the fibre and matrix.
Results and discussion
Impact strength
The results of fracture toughness measurements are shown in Figure 1. As seen in this figure, CF increases fracture toughness with the increase in CF content. Too much high content of CF did not further cause the increase in the toughness of CF/PE composite. Based on the results shown in Figure 1, it can be expected that composites made using CF show improved interlaminar fracture toughness.
The impact strength of CF/PE composite with CF content. CF: carbon fibre; PE: polyethylene.
To study the effect of silaned nano-SiO2 modification on toughness improvement of final composite through thickness cracks, the results of impact strength versus SiO2 content for notched test specimens are reported in Figure 2. It is interesting that all samples fractured through thickness and increase in modifier content led to increased impact strength. Similar type of impact test was used to evaluate impact strength of CF-modified PE composites and revealed no significant improvement in impact strength of the composite (Figure 1). This can be attributed to the fact that the PE used in that particular investigation was not ductile enough to be toughened by CF modification. This is why no obvious improvement in impact strength of the composite was observed. The contribution of fibres in absorbing the impact energy is not influenced effectively by CF modification. In addition, Figure 2 shows that impact strength of silaned specimens is still higher than those of the unsilaned ones. The modified composite with the good matrix/fibre adhesion possessed 20% higher interlaminar shear strengths compared to the composite having weak interface. Therefore, one may conclude that silaned SiO2 modification improves crack initiation energy of the composites.
The impact strength of CF/PE composite filled with SiO2 with and without treatment. CF: carbon fibre; PE: polyethylene; SiO2: silicon dioxide.
The mechanical benefit of these nanoscale materials appears to increase as stress intensity decreases. They are not capable of hindering crack propagation (fracture toughness) once a crack begins to advance, but their nanoscale dimensions enable them to better interact with the matrix before a critical crack is initiated. In addition, SiO2 maintain their high aspect ratio post-processing.
This allows for a delay in craze formation and coalescence prior to crack initiation. CFs do not present an aspect ratio that favours a strong interaction with the matrix, reducing creep improvement. On the other hand, the larger size of the CFs allows them to slow the growth of the crack by deflection, pull-out and crack bridging mechanisms. When an advancing crack is presented with a CF, it is deflected following the CF/matrix interface, eventually leading to the pull-out of the filament. The surface modification was said to help in exfoliating agglomerations and strengthening SiO2-matrix interactions.
The results of the single-fibre pull-out test are shown in Figure 3. Figure 3 depicts the effect of SiO2 on the interfacial strength of CF/PE composite. Obviously the apparent interfacial shear strength typically increases with the addition of SiO2.
The interlaminar shear strength of CF/PE composite filled with SiO2 with and without treatment. CF: carbon fibre; PE: polyethylene; SiO2: silicon dioxide.
The effect of SiO2 addition on the failure behaviour of CF/PE is illustrated by the scanning electron microscope (SEM) micrograph shown in Figure 4. Fibre pull-out is still evident on the fracture plane and no PE matrix appears to adhere to the fibre surface. The ductile failure of the PE matrix changes to a brittle one that is associated with some limited crazing. The craze remnants on the fracture surface are clearly visible. Thus, it can be expected that, during the fracture process, the resistance to crack propagation will be reduced markedly and the samples fail in a more brittle manner.
The impact fracture morphology of CF/SiO2/PE composite. CF: carbon fibre; PE: polyethylene; SiO2: silicon dioxide.
As seen, plastic deformations at the plastic zone of modified resins are accompanied by stress whitening. The size of stress whitening increases with increase in the SiO2 content; such that in 5 vol% specimen, it expands to the whole surface in front of the crack tip (Figure 3(a)). The difference in the damage zone sizes seen in Figure 3 is in agreement with fracture toughness data reported in Figure 2.
The flat and almost featureless image seen in Figure 4(a) indicates the typical brittle fracture of neat PE. On the other hand, the rough fracture surface of 3 vol% specimen seen in Figure 4(b) shows the significant amount of plastic deformation occurred in this material prior to fracture. It is found that cavitation is a prerequisite for massive shear deformation of the matrix. The latter is known as the main source of energy absorption in fracture of rubber-toughened PE. However, comparison between Figure 4(c) reveals less cavitation and void grows on the damaged surface of the impact test specimen. Decrease in cavitation and void growth corresponds to decrease in plastic deformation and energy absorption. These regions are vulnerable to stress due to stress concentration and consequent poor strength compared to the regions with no porosity.
Hackling type morphology is observed for unmodified matrix remaining between the fibres (Figure 4(a)), and in the case of modified resin (Figure 4(b)), treated SiO2 particles are observed between fibres with no hackling in resin surface (Figure 4(c)). Figure 3 shows some typical fibres on the fracture surface of composite samples. This figure illustrates that the fibre in unmodified specimen (Figure 4(a)) contains less remaining resin on its surface in comparison with treated SiO2 samples (Figure 4(b)).
It is interesting to note that the surface of the fibres in CF/PE composite (Figure 4(a) and (b)) is partially covered with a sheath of PE matrix. The presence of SiO2 seems to contribute to the interfacial bonding between CF and PE matrix. The effectiveness of SiO2 particles in improving the toughness of thermoplastic matrices is strongly controlled by their ability to act as stress concentrating sites to induce multiple crazing and shear yielding, thereby forming a large damage zone.
Therefore, it confirms that treated SiO2 has more adhesion to fibre surface than unmodified one. The surface appearance of CF shown by the SEM micrograph in Figure 3 provides a good indication that matrix deformation still persists.
The positive contribution of SiO2 modifiers and CF reinforcement in improving the fracture toughness of PE is due to the favoured pull-out of the matrix. The surface of the CF is well coated with the matrix, indicating for sheathed pull-out.
TPB properties
Figure 5 shows the TPB value obtained from the tests of CF/PE and CF/SiO2/PE composites, and it can be seen that all TPB values of CF/SiO2/PE composites are significantly higher than that of CF/PE composite, the former is 1.2–1.4 times the latter; in addition, the composite made up of treated SiO2 shows a higher value of interlaminar shear strengths. These phenomena can be attributed to the change in interfacial adhesion induced by the variety of the treatment of SiO2 in the composite interface.
The three-point bending of CF/SiO2/PE composites. CF: carbon fibre; PE: polyethylene; SiO2: silicon dioxide.
The SEM micrographs of the cross-section and longitudinal section surfaces of various composites after TPB tests are shown in Figure 6. For CF/PE composite (Figure 5(a)), few resin is left on the surfaces of fibres, and some cavities appear between the fibres and matrix resin, reflecting that CF is easily pulled out of the matrix owing to its poor interfacial adhesion with PE matrix. However, with regard to CF/SiO2/PE composites, a larger amount of fibres are buried and covered by PE matrix as shown in Figure 6(b); besides, fibres tend to break rather than being pulled out of the PE resins; and the pulled out fibres are surrounded by a large amount of the matrix resins, suggesting that the interfacial adhesion of CF/SiO2/PE composite is much stronger than that of CF/PE. This is in good agreement with the results of TPB.
The bending fracture morphology of CF/SiO2/PE composite. CF: carbon fibre; PE: polyethylene; SiO2: silicon dioxide.
Conclusions
When the SiO2 volume fraction increased in the CF/PE matrix from 1 to 5 vol%, the impact improved. Silaned SiO2 modification improves crack initiation energy of the composites. Acid treatment efficiently improves the interfacial adhesion of the clay-filled composites.
Treatment with silaned SiO2 would increase the interlaminar shear bonding strength of CF/PE composites, thus the TPB of CF/SiO2/PE composite increased significantly.
Footnotes
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.