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clay nanocomposites for improved impact strength
Accepted Manuscript Compatibilization of polypropylene fibers in epoxy based GFRP/clay nanocomposites for improved impact strength Karanbir Singh, Tarun Nanda, Rajeev Mehta PII: DOI: Reference:
S1359-835X(17)30129-X http://dx.doi.org/10.1016/j.compositesa.2017.03.027 JCOMA 4617
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
25 August 2016 20 January 2017 25 March 2017
Please cite this article as: Singh, K., Nanda, T., Mehta, R., Compatibilization of polypropylene fibers in epoxy based GFRP/clay nanocomposites for improved impact strength, Composites: Part A (2017), doi: http://dx.doi.org/ 10.1016/j.compositesa.2017.03.027
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Compatibilization of polypropylene fibers in epoxy based GFRP/clay nanocomposites for improved impact strength
Karanbir Singh1, Tarun Nanda1*, Rajeev Mehta2
1
Mechanical Engineering Department, Thapar University, Patiala, Punjab, India, 147004
2
Chemical Engineering Department, Thapar University, Patiala, Punjab, India, 147004
*Tarun Nanda, Assistant Professor, Mechanical Engineering Department, Thapar
University,
Patiala,
Punjab,
India,
147004,
Ph.
No.:
+919463586083
e-mail id: [email protected]
ABSTRACT In the present work, glass fiber reinforced polymer (GFRP) nanocomposites containing polypropylene fibers as a reinforcement were prepared for the first time. Nanocomposites containing 1phr nanoclay and 1–3 phr polypropylene fibers were fabricated to improve the impact strength of the brittle GFRPs. In addition to impact strength, tensile and flexural properties were also investigated. Polypropylene fibers were used as a reinforcement in the as-received form and also after their compatibilization by two different methods viz. silanization, and ultra-violet assisted maleic anhydride grafting. Fourier transform infrared spectroscopy and energy dispersive spectroscopy of the treated polypropylene fibers were studied to confirm the treatment of fibers. Scanning electron microscopy of fractured impact specimens was conducted to evaluate the interfacial bonding between matrix and reinforcement in composites. Transmission electron microscopy was conducted to ascertain 1
the extent of dispersion of nanoclay in the matrix. The results showed that compatibilization of polypropylene fibers with silane agents significantly increased the impact strength of nanocomposites (maximum improvement of 44 %), though with some loss in tensile and flexural properties. However, ultra-violet assisted maleic anhydride grafting of polypropylene fibers increased the impact strength while restoring other properties of the composite system. Keywords: A. Polymer-matrix composites (PMCs), A. Fibers, B. Impact behaviour, D. Mechanical testing.
INTRODUCTION Thermosetting epoxy is extensively used in industrial applications as an adhesive and also as a matrix for fiber reinforced plastics (FRPs) owing to its high failure strength, good adhesion, ease of processing, and resistance to chemicals etc. [1–6]. Epoxy based fiber reinforced composites are used for many advanced applications. Further, the use of nanoscale fillers in fibre reinforced composites has attracted considerable interest. Such nanoscale fillers exhibit
high aspect
ratio and unique intercalation/exfoliation
characteristics, and thus enhance the performance of composites [2,4,7–9]. Fiber reinforced nanocomposites are employed in aerospace components, ship hulls, and wind-turbine structural parts due to their high specific strength and stiffness. Glass fiber reinforced epoxy based composites also find extensive engineering applications. They are used in sporting goods (vault poles, surf boards, archery bows, and arrows), piping for corrosive chemicals, automotive parts, and structural equipment etc. Unileaf glass fiber-reinforced epoxy springs are replacing steels because of substantial savings in weight [10–13]. However, for reliable 2
operation, glass fiber reinforced epoxy based nanocomposites also require high toughness, in addition to their good static mechanical properties [7,14,15]. The epoxy, while curing, gets polymerized and forms a highly cross-linked structure. This cross-linked structure improves the modulus and failure strength, but causes increased brittleness resulting in poor resistance to crack induction and propagation. This deteriorates the impact strength of these composites [13,16–19]. It is reported that impact strength can be improved by incorporating ductile fibers/elastomers as second filler, in addition to nanoclay. It is believed that addition of modifiers (polymeric fibers like polypropylene, or elastomers etc.) which are less rigid than the polymer (epoxy) matrix may behave as excellent toughners [14,16,17,20–22]. In the present work, polypropylene fibers (PP) were added as second filler (in addition to nanoclay) to glass fiber reinforced epoxy composites. Polypropylene fibers are low-cost, light-weight, and ductile and can be used as a reinforcement to improve the toughness of epoxy based GFRP/clay nanocomposites. A few researchers have investigated the role of polypropylene fibers in epoxy based composites and reported improvements in thermal and impact properties [20,23,24]. Prabhu et al. [24] reported increase in thermal stability of epoxy based composites with addition of polypropylene up to 10 wt. %. Dutra et al. [23] used PPEVASH fiber (polypropylene fiber blend with EVA copolymer) as reinforcement in epoxy and reported improvement of 270% in impact strength of epoxy/PP composite. Similarly, Dutra et al. [20] used PPEVASH fiber as additional reinforcement to carbon fiber reinforced composites and reported 68% improvement in impact strength. The authors concluded that polypropylene fiber reinforcement to the epoxy matrix yields significant improvements in impact strength only if polypropylene fibers are functionalized/modified (polypropylene fiber blend with EVA copolymer was used). There is very little literature 3
available on polypropylene fibers used as reinforcement in epoxy or epoxy/carbon fiber composites. Further, there is no literature available on the use of polypropylene fibers in epoxy/clay or epoxy based glass fiber reinforced polymer composites. Hence, there is a need to study the use of polypropylene fibers as reinforcement in such systems. The reported literature indicates that polypropylene lacks in functional groups necessary for good interfacial adhesion with other constituents of glass fiber reinforced epoxy composites systems [20,23]. To overcome this limitation, compatibilization of polypropylene fibers was done by two different methods in the present work: (i) silanization (treatment with silane agents), and (ii) ultra-violet assisted maleic anhydride grafting (UV-assisted MAH grafting). In the first method of compatibilization, polypropylene fibers were treated with two different silane coupling agents separately. Silane agents are silicon-based chemicals which act as an interface between an inorganic (e.g. glass etc.) and an organic material (e.g. an organic polymer like polypropylene etc.). Silanes contain two types of reactivityinorganic and organic which leads to increase in the interfacial bonding between two different materials [25]. The grafting of silane agents onto polymeric chains helps in crosslinking these chains and also in forming stable covalent bonds with polar fillers [25–28]. Another possibility of improving the adhesion of polypropylene fibers in composite systems is by grafting of maleic anhydride (MAH) to the fibers (by improving polarity). Researchers have compared different methods for grafting of MAH on polypropylene and found that photo-assisted grafting allows for a high level of grafting and results in improvement in mechanical and thermal properties of the resulting composites [29–31]. So, in the second method of compatibilization, polypropylene fibers were treated with maleic anhydride (MAH grafting). 4
The present research is based on the premise that proper compatibilization of relatively ductile polymer fibers added as reinforcement to glass fiber reinforced epoxy clay nanocomposites can result in significant improvements in impact strength without any appreciable loss in other mechanical properties. The presence of relatively ductile polypropylene fibers is expected to improve the impact strength of the brittle epoxy nanocomposites.
EXPERIMENTAL Materials Epoxy DGEBA based resin (Araldite LY 1564) having density of 1.1–1.2 g/cm3 (at 20 and viscosity of 1200–1400 mPa.s (at 25
)
) was used as the matrix material. Aradur 3486
was used as the hardener. The epoxy and the hardener were supplied by Huntsman Advanced Materials (India) Pvt. Ltd. Cloisite®15A clay purchased from Nanoshell, USA was used as the nano-filler. E-glass fiber mat ‘WR360’ purchased from Owens Corning Inc., India was used as the glass fiber reinforcement. Short polypropylene fibers of 3 mm length having an aspect ratio of 100 were used as the polymeric reinforcement. These PP fibers were provided by Reliance Industries Ltd, India. For compatibilization of polypropylene
fibers,
γ-methacryloxypropyltrimethoxysilane
(silane
agent),
vinylytriethoxysilane (silane agent), and maleic anhydride granules (for MAH grafting) were supplied by TCI Chemicals (India) Pvt. Ltd. Sample preparation
5
For preparation of the neat epoxy sample, predetermined amounts of epoxy and hardener in the weight ratio of 100:34 (as recommended by the supplier) were mixed by stirring with help of a mechanical stirrer (Remi, Mumbai, India) for 10 min at 500 rpm. The system was then degassed for 30 min. For preparation of epoxy/clay nanocomposites, the nanoclay was dried in an oven at 120
for 3 h. The dried clay equivalent to 1 phr (per hundred resin by
weight (where, in the present work, resin refers to epoxy-hardener system) was mixed with epoxy by homogenization at 20,000 rpm for 10 min with help of a homogenizer (T25, IKA Inc. Bangalore, India) followed by utrasonication using a probe sonicator (QSonica 700 W, Newton, USA). Utrasonication was done at 80% amplitude for 10 min, with pulse-on time of 40 s and pulse-off time of 20 s. To prevent rise in temperature during sonication, the beaker was kept in an ice bath. The epoxy/clay suspension was cooled to 40 addition of hardener.
For
preparation of epoxy/clay/polypropylene
before
composites,
predetermined amount of treated or untreated polypropylene fibers (equivalent to 1phr, 2phr, and 3phr of PP fibers respectively i.e. 0.005, 0.012, and 0.021 fiber volume fraction of PP fibers in GFRP system) were mixed with the degassed epoxy/clay/hardener system at 500 rpm for 15 min with help of mechanical stirrer. Low concentration for PP fibers was used as steep hike in viscosity of the epoxy system was observed with addition of PP fibers. It was observed that beyond 3 phr of PP fiber concentration, the epoxy system became too viscous for satisfactory composite fabrication.
Polypropylene fiber compatibilization
6
For compatibilization, polypropylene fibers were treated with a specific compatibilizer and then mixed with resin followed by the fabrication process. Fibers were either silane treated with γ-methacryloxypropyltrimethoxysilane (MS) or vinylyriethoxysilane (VS) silane agent or were grafted with maleic anhydride (MAH). Silane treatment of polypropylene fibers was done as per the supplier recommendations. For this, ethanol solution (95%) with water was used. pH of the ethanol solution was adjusted between 4.5–5.5 pH by adding acetic acid. The silane agent was now added to the ethanol solution (in an amount equivalent to 2% volume of ethanol solution). The polypropylene fibers (in quantity as per a given composition) to be treated were dipped in this ethanol-silane solution and allowed to react for 5 min. After the reaction, the PP fibers were rinsed twice with ethanol to remove the excess silane/unreacted silane. The treated and rinsed fibers were then cured in oven at 110
for 5–10 min. These silane treated
polypropylene fibers were used as reinforcement in the nanocomposite formulations. Further, for MAH grafting, maleic anhydride (taken in amount equivalent to twice the amount of polypropylene fibers) was dissolved in acetone. The required amount of fibers was kept in a dish and the MAH/acetone solution was poured into the dish. The fibers were now completely immersed in MAH/acetone solution. The dish was now kept in a UV apparatus for four different time periods of 10 h, 20 h, 30 h, and 40 h respectively for UV assisted MAH grafting. The UV apparatus consisted of 7 UV tubes of 36 W each (see Figure 1). The distance of dish from the UV tubes was kept constant at 30 cm for all the four cases. MAH grafted samples were rinsed with acetone to remove the excess/un-grafted MAH. The treated fibers were weighed to calculate the weight gain. Table 1 shows the percentage gain in weight of PP fibers after MAH grafting for different time periods. 7
Maximum weight gain was observed for treatment time of 30 h (see Table 1). Thus, the polypropylene fibers grafted for 30 h were used as reinforcement in the epoxy composites.
Fabrication of glass fiber reinforced composites For fabrication of glass fibre reinforced epoxy based composites, the solution of epoxy/ clay/polypropylene fibers (prepared as discussed above) was used to layup the glass fiber (glass fiber volume fraction was 0.55). For tensile and flexural specimens, two layers of glass fiber mat were used and for impact specimens, twelve layers of glass fiber mat were used. Each layer of glass fiber mat was wet by the prepared solution using hand layup method on vacuum assisted resin infusion moulding (VARIM) table. After layup, the material system was covered with a separating cloth, followed by perforated sheet, and wire mesh. Breather cloth was used to make connection of the composite with vacuum ports. The vacuum bagging film was passed on sides of the VARIM mould with help of a sealant tape for providing air tight bagging. Vacuum of the order of 1 mbar was maintained in order to facilitate removal of entrapped air and for fabrication of a uniform sheet. The curing schedule followed was 4 h at 60 at 60
under vacuum, followed by post curing for 16 h
without vacuum.
Characterization and testing Fourier-transform infrared spectroscopy, FTIR (Perkin–Elmer FTIR spectrometer RXIFTIR, Massachusetts, US) and scanning electron microscopy-energy dispersive spectroscopy, SEM-EDS (JEOL JSM 6510LV, Japan) were used to verify the
8
compatibilization of polypropylene fibers. Samples for FTIR were prepared by pressing into a pellet with potassium bromide. These samples were scanned from 4000 to 400 cm-1 using FTIR. SEM analysis was also conducted to examine the fractured surfaces of impact specimens. X-ray diffraction, XRD (XPERTPro, PANalytical JDX-8030, Almelo, Netherlands) and Transmission electron microscopy, TEM (H-7500, Hitachi, Japan) was used to investigate the type of morphology (phase separated, intercalated, or exfoliated) of silicate layers (nanoclay) obtained in various nanocomposite formulations. XRD was done at low angle (2θ = 1–10°) with a speed of 0.6°min-1 and step size 0.1°. For TEM, powdered samples were prepared in a grinder and were dispersed in ethanol followed by sonication for 3 h. For each composition (composite formulation), four samples were fabricated and tested, each for tensile, flexural, and impact tests respectively. Tensile and flexural tests of composites were conducted using a Zwick/Roell Z010 TN Proline UTM (Zwick/Roell, Germany) having a load cell of 10 kN. Tensile tests were conducted at a crosshead speed of 2 mm/min. The specimens for tensile tests were prepared as per ASTM D3039. Flexural tests were conducted using a 3-point bending attachment at a crosshead speed of 1.33 mm/min and a span of 25.4 mm on the UTM. The specimens for flexural tests were prepared as per ASTM D790. The impact strength was evaluated using an ATS FAAR Izod impact tester (ATS FAAR, Italy) equipped with a load cell of 15 J. Impact specimens were prepared as per ASTM D256–02.
RESULTS AND DISCUSSION
9
This section presents the results and discussion of mechanical testing and subsequent characterization of the various nanocomposite formulations. The results of all the mechanical tests are shown in Table 2.
Mechanical properties Table 2 shows the average value (mean of four values) along with the range of values (i.e. error bar) obtained for each property for each formulation. Impact strength of the epoxy based glass fiber reinforced composite (NE) was observed as 161 kJ/m2. On addition of 1phr clay to this GFRP (E1C), a slight increase of about 4% (167 kJ/m2) was observed. Further, on addition of untreated polypropylene fibers (PP) to this nanocomposite system, the impact strength of the nanocomposite formulations viz. E1C1PP, E1C2PP, and E1C3PP decreased (148 kJ/m2, 149 kJ/m2, and 129 kJ/m2 with addition of 1phr, 2phr, and 3phr untreated PP fibers respectively). It was predicted that impact strength deteriorated because of lack of compatibility of polypropylene fibers with other constituents of composite system. For improving the compatibility, polypropylene fibers were treated with two different
silane
agents’
viz.
methacryloxypropyltrimethoxysilane
(MS)
and
vinylytriethoxysilane (VS). It was observed that addition of silane treated polypropylene fibers to the composite system increased the impact strength for a given composition (see Figure 2). A maximum improvement of 44% in impact strength (as compared to NE) was observed for the nanocomposite formulation (E1C2PPMS) containing 2 phr polypropylene fibers treated with MS silane agent. Further, it was observed that for all cases with polypropylene fiber reinforcement (treated or untreated), the impact strength of composites showed improvement till 2 phr 10
polypropylene loading and after this limit (i.e. at 3 phr PP loading), the impact strength deteriorated. This may be attributed to poor dispersion of polypropylene fibers in the composite system because of the resulting high viscosity of resin at such high concentration of polypropylene reinforcement. Since, maximum improvement in impact strength was observed for 2 phr polypropylene loading, this concentration was only selected for the other treatment i.e. maleic anhydride grafting. When maleic anhydride grafted polypropylene fibers (2 phr loading) were used as reinforcement, impact strength value showed an improvement of about 13% (as compared to NE). This improvement was attributed to the expected coupling of polypropylene fibers with epoxy with help of maleic anhydride. Thus, addition of treated polypropylene fibers (2 phr maximum) as reinforcement to the nanocomposite system enhanced the impact strength. In the present work, tensile and flexural properties were also investigated in addition to the impact strength. The tensile and flexural strengths of the epoxy based glass fiber reinforced composite (NE) were observed as 269 MPa and 197 MPa respectively. Addition of 1phr clay to this GFRP (E1C), led to improvement in both tensile and flexural strength (16.5% and 14.5% respectively as compared to NE). Also, the tensile and flexural moduli showed improvements of 6.5% and 34.5% respectively. These results were in accordance with earlier work reported of [2,3] and many others. Further, it was observed that addition of untreated or silane treated polypropylene fibers to the nanocomposite system resulted in decrease in nearly all the tensile and flexural characteristics, except for tensile strength. However, with addition of MAH grafted polypropylene fibers to the nanocomposite system, nearly all the properties (especially impact and flexural) showed a substantial
11
improvement. Flexural modulus showed maximum improvement for 2 phr MAH grafted polypropylene fiber loading (1.23 GPa; an improvement of 51% over NE sample).
Compatibilization behaviour The main purpose of the present research was to improve the impact strength of epoxy based glass fiber reinforced nanocomposites. For improving the impact strength, second phase fillers in the form of polypropylene fibers (1–3 phr) were added to the nanocomposite system. The results showed that addition of treated polypropylene fibers (up to 2 phr) led to improvements in impact strength of the composite system. For nanocomposites reinforced with silane treated polypropylene fibers, impact strength improvement was attributed to the improved compatibility between the inorganic glass fibers and the organic polypropylene fibers. It is well reported in literature that silane agents can react with a variety of materials (viz. organic and inorganic material surfaces via covalent bonds etc.) to increase the compatibility/coupling between them. Silane agents have two types of functional groups viz. hydrolyzable group (OR group) and organo-functional group (X group). The Si-OR bonds hydrolyze quickly with water (even with moist air) to make silanol Si-OH groups which condense and result in formation of Si-O-Si bonds. This allows chemical compatibilization of dissimilar material surfaces [27,32]. Figure 3 shows the structure of γmethacryloxypropyltrimethoxysilane (MS), and vinylyriethoxysilane (VS) silane agents used in the present research. Further, for MAH grafted polypropylene fibers, there was improvement in impact strength with appreciable restoration of tensile properties of nanocomposites. The improvement in impact strength could be due to the improved bonding of MAH grafted polypropylene 12
fibers to the epoxy matrix. This is because functional groups of MAH grafted polypropylene fibers react with the hydroxyl or epoxy groups of the epoxy resin [30,33]. Figure 4 shows the schematic of possible coupling reaction of MAH grafted polypropylene fibers with epoxy in the nanocomposite system.
Surface morphology SEM analysis of untreated fibers SEM images of the as-received glass fibers and polypropylene fibers are presented in Figure 5a and Figure 5b respectively. The glass fibers showed a circular cross-section (diameter ~ 15 μm) and polypropylene fibers showed a rectangular cross-section (width ~ 35 μm). These details were noted to differentiate between the glass and polypropylene fibers during SEM analysis of the fractured impact specimens of nanocomposites containing both these type of fibers.
SEM/EDS of silane treated polypropylene fibers Figure 6a–c shows the SEM images of the un-treated polypropylene fibers and those treated with MS silane agent and VS silane agent respectively. SEM micrographs confirmed the presence of silane agents on the treated polypropylene fibers. To further confirm the SEM results with regards to silane treatment of polypropylene fibers, energy dispersive spectroscopy (EDS analysis) was conducted. The presence of silane agent on the treated polypropylene fibers was confirmed by the presence of silicon on the fiber surface. Figure 7a–c shows the results of EDS analysis. The amount of silicon in the untreated, MS treated, and VS treated fibers was observed as 0, 1.25, and 1.47% respectively. The presence of 13
silicon on surface of treated fibers confirmed successful silane coating on the fibers and thus the SEM results.
SEM/FTIR study of MAH grafted polypropylene fibers As discussed earlier, UV-assisted MAH grafting of polypropylene fibers showed maximum weight gain for treatment time of 30 h (2.18 g; see Table 1). So, for MAH grafting of polypropylene fibers, the optimum treatment time was selected as 30 h. To confirm the same, SEM analysis of the MAH grafted fibers exposed to ultra-violet rays for different treatment time periods (10 h, 20 h, 30 h, and 40 h respectively) was conducted. SEM results confirmed the observations of weight gain. SEM micrographs showed uniform grafting with minor pitting on polypropylene fibers treated for 30 h (see Figure 8c). The minor pitting observed along with uniform grafting also improves adhesion of polymeric fibers to the constituents of composite system [34]. For treatment time periods of more than the optimum period of 30 h, the SEM micrograph (Figure 8d) showed ruptured surfaces which was due to excessive exposure to UV radiations. Further, FTIR was conducted for MAH grafted polypropylene fibers to confirm the grafting. Figure 9 presents the FTIR results for both the untreated fibers and the MAH grafted polypropylene fibers. For MAH grafted samples, peaks at 841 cm-1, 1168 cm-1, 1265 cm-1, and 1636 cm-1 could be recognized as peaks due to C–H alkenes, –OH aromatic, C=C aromatic ring, and C=C alkenes respectively and the difference in shape of band between 3000–3600 cm-1 could be due to modification of self-association of –OH group due to hydroxyl group involved in interactions with maleic anhydride [35]. Absorption bands at around 1708 cm-1 and 1889 cm-1 which correspond to the asymmetric C=O 14
stretching and carboxylic acid respectively indicated grafting of maleic anhydride on polypropylene fibers [30,36–38]. Thus, MAH grafting on polypropylene fibers was confirmed by the FTIR study.
SEM analysis of impact specimens SEM micrographs of the fractured surface of impact tested specimens were investigated for some typical compositions (NE; E1C; and all nanocomposites containing 2 phr polypropylene fibers) to understand the failure behaviour of composites. Figure 10a shows the fractured surface of ‘NE’ composite formulation. The micrograph revealed a lack of interaction between glass fibers and epoxy resin. Also, the surface of epoxy appeared quite smooth indicating less resistance to failure under impact loading [2,10,39]. Figure 10b shows the fractured surface of ‘E1C’ nanocomposite. As compared to ‘NE’, the surface of epoxy in ‘E1C’ composite was more rough indicating greater resistance to impact loading before failure. Also, epoxy-nanoclay was seen on the glass fibers indicating better interaction among the E1C constituents. SEM images of fractured surfaces of epoxy/clay GFRP nanocomposite containing 2 phr of untreated polypropylene fibers (E1C2PP) are presented in Figure 10c. It was observed that there is negligible interaction of polypropylene fibers with other constituents which could be attributed to the inert nature of polypropylene [1,40]. This lack of interaction resulted in poor impact strength. Figure 10d– e shows the SEM images of fractured surface of epoxy/clay GFRP nanocomposite sample containing 2 phr silane treated polypropylene fibers (E1C2PPMS and EIC2PPVS respectively). The micrographs showed improved interaction of the treated polypropylene fibers with other constituents of the nanocomposite system. Also, some regions showing 15
fractured polypropylene fibers could be observed in the MS silane treated nanocomposite (E1C2PPMS). This indicated that silane treated polypropylene fibers used as reinforcement in the nanocomposites resisted the impact loading more effectively thereby resulting in higher impact strength of the nanocomposite system. Figure 10f shows the SEM images of the impact tested surface of epoxy/clay GFRP nanocomposite sample containing 2 phr of MAH grafted polypropylene fibers (E1C2PPMAH). The micrograph showed presence of epoxy-clay on the MAH grafted polypropylene fibers. This indicated good coupling of polypropylene fiber reinforcement with the epoxy matrix in the nanocomposite. The good mechanical properties obtained for this nanocomposite system (E1C2PPMAH) may be attributed to these reasons.
XRD analysis X-ray diffraction was conducted to examine the type of nanoclay morphology obtained in the composite systems as a result of processing method used in the present work. XRD analysis was carried out to determine if the processing method was able to achieve the desired morphology of nanoclay (i.e. exfoliated morphology) in the nanocomposites. XRD analysis was carried out for pristine nanoclay and for E1C nanocomposite. E1C composition was selected because the same processing method for clay dispersion (as was for E1C) was followed for all other nanocomposite formulations. For nanocomposite systems, absence of peak in the XRD spectrum indicates exfoliated morphology [41,42]. Figure 11 shows the XRD patterns of pristine nanoclay Cloisite 15A and E1C nanocomposite respectively. XRD pattern of pristine clay showed peak (d001) at 2.44 degree (2θ = 2.44°) and d-spacing was calculated as 36.16 Å. Further, XRD pattern for the 16
nanocomposite system (E1C) in Figure 11 showed absence of peak indicating an exfoliated morphology of silicate layers in the nanocomposite system.
TEM analysis Transmission electron microscopy (TEM) was conducted to examine the type of dispersion of nanoclay achieved in the epoxy matrix (phase separation, intercalation, or exfoliation) as a result of the processing method followed in the present research. TEM analysis was carried out only for ‘E1C’ nanocomposite to determine if the processing method could achieve effective dispersion of nanoclay. This composition was selected because the same processing method for clay dispersion (as was for E1C) was followed for all other nanocomposite formulations. Figure 12a–b presents the TEM micrographs of ‘E1C’ nanocomposite formulation. The dark/black lines represent the clay platelets and the grey background represents the epoxy matrix. Figure 12a shows that clay layers are non-parallel and randomly aligned, thus indicating exfoliated morphology. Figure 12b shows a mixed type of clay morphology where some clay layers are evenly spaced with large inter-lamellar spacing indicating uniform dispersion and exfoliated morphology of silicate layers. However, at a few places, intercalation is also observed. Thus, TEM images show largely an exfoliated clay morphology with uniform dispersion [8,39,42–47]. Thus, TEM analysis justified that the processing method used in the present research was effective in dispersing the clay in the nanocomposite system.
CONCLUSIONS
17
It was expected that despite the lower strength and modulus of polypropylene fibers, the impact strength of composites reinforced with them would increase (because of the good ductility of polypropylene fibers) while tensile/flexural properties may decrease. However, addition of the as-received (untreated) polypropylene fibers to the nanocomposite system deteriorated all the mechanical properties viz. tensile/flexural and also the impact strength. This decrease in mechanical properties of nanocomposites was due to the inert nature of polypropylene fibers resulting in their poor compatibility with other constituents. To overcome this problem, two different compatibilization procedures for polypropylene fibers were investigated. Both the compatibilization procedures viz. silane treatments and MAH grafting of polypropylene fibers proved successful in improving the impact strength of nanocomposites. The reinforcement of treated polypropylene fibers with 2 phr loading in the nanocomposite system provided the best results. Thus, for the first time, GFRP nanocomposites with clay as the nano-filler and compatibilized polypropylene fibers as the micro-sized filler were fabricated successfully providing improved impact strength without much drop in the tensile/flexural properties.
Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors are thankful to CIPET, Amritsar, India for providing facilities for the impact testing of nanocomposites. The authors also acknowledge the contributions of SAIF Labs, Punjab University, Chandigarh, India for extended support for TEM and FTIR facilities.
18
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Figure 1: Schematic of UV treatment set-up for MAH grafting. Figure 2: Impact strength of various composite formulations. Figure 3: Structure of (a) γ-Methacryloxy silane, MS and (b) Vinyltyriethoxy silane, VS. Figure 4: Schematic of coupling reaction between MAH grafted polypropylene and epoxy. Figure 5: SEM images of the as-received (a) glass fibers, and (b) polypropylene fibers. Figure 6: SEM micrographs showing surface of polypropylene fibers (a) un-treated, (b) VS silane treated, and (c) MS silane treated.
22
Figure 7: Results of EDS of polypropylene fibers (a) untreated, (b) treated with VS silane agent, and (c) treated with MS silane agent. Figure 8: SEM micrographs showing polypropylene fibers subjected to UV-assisted MAH-grafting for exposure time period of (a) 10h, (b) 20h, (c) 30h, and (d) 40 h. Figure 9:
FTIR results of untreated and MAH grafted polypropylene fibers. PP = untreated
polypropylene fibers, PPMAH = MAH grafted polypropylene fibers. Figure 10: SEM micrographs showing fracture surfaces of the GFRP composite (a) GFRP without clay/PP (NE), (b) nanocomposite without PP (E1C), (c) nanocomposite with 2phr PP (E1C2PP), (d) nanocomposite with 2phr MS treated PP (E1C2PPMS), (e) nanocomposite with 2phr VS treated PP (E1C2PPVS), and (f) nanocomposite with 2phr MAH grafted PP (E1C2MAHPP). PP = polypropylene. Figure 11: XRD patterns for the nanoclay Cloisite 15A and the E1C nanocomposite. Figure 12: TEM micrographs of the epoxy-clay nanocomposite showing clay morphology.
Figure 1: Schematic of UV treatment set-up for MAH grafting.
23
275
Im pac t S trength (kJ /m
2
)
250
225
200
175
150
125
100
NE
E1
C E1
C1
PP E
1 1C
PP
S M E
1 1C
PP
VS
E1
C2
PP
E
2 1C
PP
S M E
2 1C
PP
VS
E1
P C2
S S PP AH PV PM C3 PM 3P 3P E1 C C E1 E1
C om pos ite F orm u la tion
Figure 2. Impact strength of various composite formulations.
Figure 3: Structure of (a) γ-Methacryloxy silane, MS and (b) Vinyltyriethoxy silane, VS [48,49].
24
Figure 4: Schematic of coupling reaction between MAH grafted polypropylene and epoxy [30].
Figure 5. SEM images of the as-received (a) glass fibers, and (b) polypropylene fibers.
25
Figure 6. SEM micrographs showing surface of polypropylene fibers (a) un-treated, (b) VS silane treated, and (c) MS silane treated.
26
Figure 7. Results of EDS of polypropylene fibers (a) untreated, (b) treated with VS silane agent, and (c) treated with MS silane agent.
Figure 8. SEM micrographs showing polypropylene fibers subjected to UV-assisted MAH-grafting for exposure time period of (a) 10h, (b) 20h, (c) 30h, and (d) 40 h.
27
50 45 40
PP
35
%T
30 25 20
PPMAH
15 10
1887 841
5
1168
3058
1707 1636
0 4000
3500
3000
2500
2000
1265
1500
1000
500
-1
cm
Figure 9. FTIR results of untreated and MAH grafted polypropylene fibers. PP = untreated polypropylene fibers, PPMAH = MAH grafted polypropylene fibers.
28
Figure 10. SEM micrographs showing fracture surfaces of the GFRP composites (a) GFRP without clay/PP (NE), (b) nanocomposite without PP (E1C), (c) nanocomposite with 2phr PP (E1C2PP), (d) nanocomposite with 2phr MS treated PP (E1C2PPMS), (e) nanocomposite with 2phr VS treated PP (E1C2PPVS), and (f) nanocomposite with 2phr MAH grafted PP (E1C2MAHPP). PP = polypropylene.
29
100000
Pristine Clay, Cloisite 15A Nanocomposite, E1C
80000
Intensity
60000
36.16 A
O
40000
20000
0
-20000 0
1
2
3
4
5
6
7
8
9
10
11
12
Angle (2
Figure 11. XRD patterns for the pristine nanoclay and the nanocomposite system.
Figure 12.TEM micrographs of the epoxy-clay nanocomposite showing clay morphology.
30
Table 1: Weight gain by PP fibers during MAH grafting. Table 2: Mechanical properties of various composite formulations.
Table 1: Weight gain by PP fibers during MAH grafting. Grafting time (h)
Initial weight before grafting (g)
Final weight after grafting (g)
Weight gain by PP fibers (g)
Percent weight gain by PP fibers (%)
10
2.00
3.31
1.31
65.5
20
2.00
3.88
1.88
94.0
30
2.00
4.18
2.18
109.0
40
2.00
3.21
1.21
60.5
Table 2. Mechanical properties of various composite formulations.
Sample Description (Sample Designation)
Avg* s Avg* s Avg* s Epoxy/1 phr clay/1 phr MS treated Avg* polypropoylene fiber GFRP (E1C1PPMS) s Epoxy based GFRP with no clay and no polypropylene fiber (NE) Epoxy/1 phr clay GFRP with no polypropylene fiber (E1C) Epoxy/1 phr clay/1 phr polypropylene fiber GFRP (E1C1PP)
Avg* s Avg* s Avg* s Epoxy/1 phr clay/2 phr VS treated Avg* polypropoylene GFRP (E1C2PPVS) s Epoxy/1 phr clay/2 phr MAH grafted Avg* polypropoylene GFRP(E1C2PPMAH) s Epoxy/1 phr clay/3 phr polypropoylene Avg* GFRP (E1C3PP) s Epoxy/1 phr clay/1 phr VS treated polypropoylene fiber GFRP (E1C1PPVS) Epoxy/1 phr clay/2 phr polypropoylene fiber GFRP (E1C2PP) Epoxy/1 phr clay/2 phr MS treated polypropoylene fiber GFRP (E1C2PPMS)
Impact Strength (kJ/m2)
Tensile Strength (MPa)
Tensile Modulus (MPa)
Flexural Strength (MPa)
Flexural Modulus (MPa)
161±2 4 167±15 30 148±8 16 178±8
269±3 7 313±6 12 215±4 8 292±3
7315±40 81 7780±133 265 5410±20 93 6110±22
197±5 11 226±4 8 156±8 17 82±2
8125±159 318 10928±379 759 6318±132 263 5618±58
16 177±12 23 149±13 25 232±21 42 185±8 16 181±1 2 129±5 9
6 278±8 16 247±8 17 242±2 5 258±2 4 266±8 17 183±4 8
43 5970±58 115 5835±76 152 5225±86 148 5225±38 76 6515±131 262 4385±136 273
3 104±2 3 193±3 6 104±4 8 85±3 6 223±4 9 167±7 15
116 9995±792 1583 10440±626 1252 7753±390 779 5880±520 1039 12275±525 1050 6635±308 617
31
150±6 215±10 Epoxy/1 phr clay/3 phr MS treated Avg* polypropoylene GFRP (E1C3PPMS) s 11 19 142±7 205±6 Epoxy/1 phr clay/3 phr VS treated Avg* polypropoylene (E1C3PPVS) s 15 13 Avg* = Mean value ± Range of values; s = Standard deviation of values
32
4708±54 108 4415±64 129
63±7 15 43±0 1
5135±712 1423 3125±253 505
S1359-835X(17)30129-X http://dx.doi.org/10.1016/j.compositesa.2017.03.027 JCOMA 4617
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
25 August 2016 20 January 2017 25 March 2017
Please cite this article as: Singh, K., Nanda, T., Mehta, R., Compatibilization of polypropylene fibers in epoxy based GFRP/clay nanocomposites for improved impact strength, Composites: Part A (2017), doi: http://dx.doi.org/ 10.1016/j.compositesa.2017.03.027
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Compatibilization of polypropylene fibers in epoxy based GFRP/clay nanocomposites for improved impact strength
Karanbir Singh1, Tarun Nanda1*, Rajeev Mehta2
1
Mechanical Engineering Department, Thapar University, Patiala, Punjab, India, 147004
2
Chemical Engineering Department, Thapar University, Patiala, Punjab, India, 147004
*Tarun Nanda, Assistant Professor, Mechanical Engineering Department, Thapar
University,
Patiala,
Punjab,
India,
147004,
Ph.
No.:
+919463586083
e-mail id: [email protected]
ABSTRACT In the present work, glass fiber reinforced polymer (GFRP) nanocomposites containing polypropylene fibers as a reinforcement were prepared for the first time. Nanocomposites containing 1phr nanoclay and 1–3 phr polypropylene fibers were fabricated to improve the impact strength of the brittle GFRPs. In addition to impact strength, tensile and flexural properties were also investigated. Polypropylene fibers were used as a reinforcement in the as-received form and also after their compatibilization by two different methods viz. silanization, and ultra-violet assisted maleic anhydride grafting. Fourier transform infrared spectroscopy and energy dispersive spectroscopy of the treated polypropylene fibers were studied to confirm the treatment of fibers. Scanning electron microscopy of fractured impact specimens was conducted to evaluate the interfacial bonding between matrix and reinforcement in composites. Transmission electron microscopy was conducted to ascertain 1
the extent of dispersion of nanoclay in the matrix. The results showed that compatibilization of polypropylene fibers with silane agents significantly increased the impact strength of nanocomposites (maximum improvement of 44 %), though with some loss in tensile and flexural properties. However, ultra-violet assisted maleic anhydride grafting of polypropylene fibers increased the impact strength while restoring other properties of the composite system. Keywords: A. Polymer-matrix composites (PMCs), A. Fibers, B. Impact behaviour, D. Mechanical testing.
INTRODUCTION Thermosetting epoxy is extensively used in industrial applications as an adhesive and also as a matrix for fiber reinforced plastics (FRPs) owing to its high failure strength, good adhesion, ease of processing, and resistance to chemicals etc. [1–6]. Epoxy based fiber reinforced composites are used for many advanced applications. Further, the use of nanoscale fillers in fibre reinforced composites has attracted considerable interest. Such nanoscale fillers exhibit
high aspect
ratio and unique intercalation/exfoliation
characteristics, and thus enhance the performance of composites [2,4,7–9]. Fiber reinforced nanocomposites are employed in aerospace components, ship hulls, and wind-turbine structural parts due to their high specific strength and stiffness. Glass fiber reinforced epoxy based composites also find extensive engineering applications. They are used in sporting goods (vault poles, surf boards, archery bows, and arrows), piping for corrosive chemicals, automotive parts, and structural equipment etc. Unileaf glass fiber-reinforced epoxy springs are replacing steels because of substantial savings in weight [10–13]. However, for reliable 2
operation, glass fiber reinforced epoxy based nanocomposites also require high toughness, in addition to their good static mechanical properties [7,14,15]. The epoxy, while curing, gets polymerized and forms a highly cross-linked structure. This cross-linked structure improves the modulus and failure strength, but causes increased brittleness resulting in poor resistance to crack induction and propagation. This deteriorates the impact strength of these composites [13,16–19]. It is reported that impact strength can be improved by incorporating ductile fibers/elastomers as second filler, in addition to nanoclay. It is believed that addition of modifiers (polymeric fibers like polypropylene, or elastomers etc.) which are less rigid than the polymer (epoxy) matrix may behave as excellent toughners [14,16,17,20–22]. In the present work, polypropylene fibers (PP) were added as second filler (in addition to nanoclay) to glass fiber reinforced epoxy composites. Polypropylene fibers are low-cost, light-weight, and ductile and can be used as a reinforcement to improve the toughness of epoxy based GFRP/clay nanocomposites. A few researchers have investigated the role of polypropylene fibers in epoxy based composites and reported improvements in thermal and impact properties [20,23,24]. Prabhu et al. [24] reported increase in thermal stability of epoxy based composites with addition of polypropylene up to 10 wt. %. Dutra et al. [23] used PPEVASH fiber (polypropylene fiber blend with EVA copolymer) as reinforcement in epoxy and reported improvement of 270% in impact strength of epoxy/PP composite. Similarly, Dutra et al. [20] used PPEVASH fiber as additional reinforcement to carbon fiber reinforced composites and reported 68% improvement in impact strength. The authors concluded that polypropylene fiber reinforcement to the epoxy matrix yields significant improvements in impact strength only if polypropylene fibers are functionalized/modified (polypropylene fiber blend with EVA copolymer was used). There is very little literature 3
available on polypropylene fibers used as reinforcement in epoxy or epoxy/carbon fiber composites. Further, there is no literature available on the use of polypropylene fibers in epoxy/clay or epoxy based glass fiber reinforced polymer composites. Hence, there is a need to study the use of polypropylene fibers as reinforcement in such systems. The reported literature indicates that polypropylene lacks in functional groups necessary for good interfacial adhesion with other constituents of glass fiber reinforced epoxy composites systems [20,23]. To overcome this limitation, compatibilization of polypropylene fibers was done by two different methods in the present work: (i) silanization (treatment with silane agents), and (ii) ultra-violet assisted maleic anhydride grafting (UV-assisted MAH grafting). In the first method of compatibilization, polypropylene fibers were treated with two different silane coupling agents separately. Silane agents are silicon-based chemicals which act as an interface between an inorganic (e.g. glass etc.) and an organic material (e.g. an organic polymer like polypropylene etc.). Silanes contain two types of reactivityinorganic and organic which leads to increase in the interfacial bonding between two different materials [25]. The grafting of silane agents onto polymeric chains helps in crosslinking these chains and also in forming stable covalent bonds with polar fillers [25–28]. Another possibility of improving the adhesion of polypropylene fibers in composite systems is by grafting of maleic anhydride (MAH) to the fibers (by improving polarity). Researchers have compared different methods for grafting of MAH on polypropylene and found that photo-assisted grafting allows for a high level of grafting and results in improvement in mechanical and thermal properties of the resulting composites [29–31]. So, in the second method of compatibilization, polypropylene fibers were treated with maleic anhydride (MAH grafting). 4
The present research is based on the premise that proper compatibilization of relatively ductile polymer fibers added as reinforcement to glass fiber reinforced epoxy clay nanocomposites can result in significant improvements in impact strength without any appreciable loss in other mechanical properties. The presence of relatively ductile polypropylene fibers is expected to improve the impact strength of the brittle epoxy nanocomposites.
EXPERIMENTAL Materials Epoxy DGEBA based resin (Araldite LY 1564) having density of 1.1–1.2 g/cm3 (at 20 and viscosity of 1200–1400 mPa.s (at 25
)
) was used as the matrix material. Aradur 3486
was used as the hardener. The epoxy and the hardener were supplied by Huntsman Advanced Materials (India) Pvt. Ltd. Cloisite®15A clay purchased from Nanoshell, USA was used as the nano-filler. E-glass fiber mat ‘WR360’ purchased from Owens Corning Inc., India was used as the glass fiber reinforcement. Short polypropylene fibers of 3 mm length having an aspect ratio of 100 were used as the polymeric reinforcement. These PP fibers were provided by Reliance Industries Ltd, India. For compatibilization of polypropylene
fibers,
γ-methacryloxypropyltrimethoxysilane
(silane
agent),
vinylytriethoxysilane (silane agent), and maleic anhydride granules (for MAH grafting) were supplied by TCI Chemicals (India) Pvt. Ltd. Sample preparation
5
For preparation of the neat epoxy sample, predetermined amounts of epoxy and hardener in the weight ratio of 100:34 (as recommended by the supplier) were mixed by stirring with help of a mechanical stirrer (Remi, Mumbai, India) for 10 min at 500 rpm. The system was then degassed for 30 min. For preparation of epoxy/clay nanocomposites, the nanoclay was dried in an oven at 120
for 3 h. The dried clay equivalent to 1 phr (per hundred resin by
weight (where, in the present work, resin refers to epoxy-hardener system) was mixed with epoxy by homogenization at 20,000 rpm for 10 min with help of a homogenizer (T25, IKA Inc. Bangalore, India) followed by utrasonication using a probe sonicator (QSonica 700 W, Newton, USA). Utrasonication was done at 80% amplitude for 10 min, with pulse-on time of 40 s and pulse-off time of 20 s. To prevent rise in temperature during sonication, the beaker was kept in an ice bath. The epoxy/clay suspension was cooled to 40 addition of hardener.
For
preparation of epoxy/clay/polypropylene
before
composites,
predetermined amount of treated or untreated polypropylene fibers (equivalent to 1phr, 2phr, and 3phr of PP fibers respectively i.e. 0.005, 0.012, and 0.021 fiber volume fraction of PP fibers in GFRP system) were mixed with the degassed epoxy/clay/hardener system at 500 rpm for 15 min with help of mechanical stirrer. Low concentration for PP fibers was used as steep hike in viscosity of the epoxy system was observed with addition of PP fibers. It was observed that beyond 3 phr of PP fiber concentration, the epoxy system became too viscous for satisfactory composite fabrication.
Polypropylene fiber compatibilization
6
For compatibilization, polypropylene fibers were treated with a specific compatibilizer and then mixed with resin followed by the fabrication process. Fibers were either silane treated with γ-methacryloxypropyltrimethoxysilane (MS) or vinylyriethoxysilane (VS) silane agent or were grafted with maleic anhydride (MAH). Silane treatment of polypropylene fibers was done as per the supplier recommendations. For this, ethanol solution (95%) with water was used. pH of the ethanol solution was adjusted between 4.5–5.5 pH by adding acetic acid. The silane agent was now added to the ethanol solution (in an amount equivalent to 2% volume of ethanol solution). The polypropylene fibers (in quantity as per a given composition) to be treated were dipped in this ethanol-silane solution and allowed to react for 5 min. After the reaction, the PP fibers were rinsed twice with ethanol to remove the excess silane/unreacted silane. The treated and rinsed fibers were then cured in oven at 110
for 5–10 min. These silane treated
polypropylene fibers were used as reinforcement in the nanocomposite formulations. Further, for MAH grafting, maleic anhydride (taken in amount equivalent to twice the amount of polypropylene fibers) was dissolved in acetone. The required amount of fibers was kept in a dish and the MAH/acetone solution was poured into the dish. The fibers were now completely immersed in MAH/acetone solution. The dish was now kept in a UV apparatus for four different time periods of 10 h, 20 h, 30 h, and 40 h respectively for UV assisted MAH grafting. The UV apparatus consisted of 7 UV tubes of 36 W each (see Figure 1). The distance of dish from the UV tubes was kept constant at 30 cm for all the four cases. MAH grafted samples were rinsed with acetone to remove the excess/un-grafted MAH. The treated fibers were weighed to calculate the weight gain. Table 1 shows the percentage gain in weight of PP fibers after MAH grafting for different time periods. 7
Maximum weight gain was observed for treatment time of 30 h (see Table 1). Thus, the polypropylene fibers grafted for 30 h were used as reinforcement in the epoxy composites.
Fabrication of glass fiber reinforced composites For fabrication of glass fibre reinforced epoxy based composites, the solution of epoxy/ clay/polypropylene fibers (prepared as discussed above) was used to layup the glass fiber (glass fiber volume fraction was 0.55). For tensile and flexural specimens, two layers of glass fiber mat were used and for impact specimens, twelve layers of glass fiber mat were used. Each layer of glass fiber mat was wet by the prepared solution using hand layup method on vacuum assisted resin infusion moulding (VARIM) table. After layup, the material system was covered with a separating cloth, followed by perforated sheet, and wire mesh. Breather cloth was used to make connection of the composite with vacuum ports. The vacuum bagging film was passed on sides of the VARIM mould with help of a sealant tape for providing air tight bagging. Vacuum of the order of 1 mbar was maintained in order to facilitate removal of entrapped air and for fabrication of a uniform sheet. The curing schedule followed was 4 h at 60 at 60
under vacuum, followed by post curing for 16 h
without vacuum.
Characterization and testing Fourier-transform infrared spectroscopy, FTIR (Perkin–Elmer FTIR spectrometer RXIFTIR, Massachusetts, US) and scanning electron microscopy-energy dispersive spectroscopy, SEM-EDS (JEOL JSM 6510LV, Japan) were used to verify the
8
compatibilization of polypropylene fibers. Samples for FTIR were prepared by pressing into a pellet with potassium bromide. These samples were scanned from 4000 to 400 cm-1 using FTIR. SEM analysis was also conducted to examine the fractured surfaces of impact specimens. X-ray diffraction, XRD (XPERTPro, PANalytical JDX-8030, Almelo, Netherlands) and Transmission electron microscopy, TEM (H-7500, Hitachi, Japan) was used to investigate the type of morphology (phase separated, intercalated, or exfoliated) of silicate layers (nanoclay) obtained in various nanocomposite formulations. XRD was done at low angle (2θ = 1–10°) with a speed of 0.6°min-1 and step size 0.1°. For TEM, powdered samples were prepared in a grinder and were dispersed in ethanol followed by sonication for 3 h. For each composition (composite formulation), four samples were fabricated and tested, each for tensile, flexural, and impact tests respectively. Tensile and flexural tests of composites were conducted using a Zwick/Roell Z010 TN Proline UTM (Zwick/Roell, Germany) having a load cell of 10 kN. Tensile tests were conducted at a crosshead speed of 2 mm/min. The specimens for tensile tests were prepared as per ASTM D3039. Flexural tests were conducted using a 3-point bending attachment at a crosshead speed of 1.33 mm/min and a span of 25.4 mm on the UTM. The specimens for flexural tests were prepared as per ASTM D790. The impact strength was evaluated using an ATS FAAR Izod impact tester (ATS FAAR, Italy) equipped with a load cell of 15 J. Impact specimens were prepared as per ASTM D256–02.
RESULTS AND DISCUSSION
9
This section presents the results and discussion of mechanical testing and subsequent characterization of the various nanocomposite formulations. The results of all the mechanical tests are shown in Table 2.
Mechanical properties Table 2 shows the average value (mean of four values) along with the range of values (i.e. error bar) obtained for each property for each formulation. Impact strength of the epoxy based glass fiber reinforced composite (NE) was observed as 161 kJ/m2. On addition of 1phr clay to this GFRP (E1C), a slight increase of about 4% (167 kJ/m2) was observed. Further, on addition of untreated polypropylene fibers (PP) to this nanocomposite system, the impact strength of the nanocomposite formulations viz. E1C1PP, E1C2PP, and E1C3PP decreased (148 kJ/m2, 149 kJ/m2, and 129 kJ/m2 with addition of 1phr, 2phr, and 3phr untreated PP fibers respectively). It was predicted that impact strength deteriorated because of lack of compatibility of polypropylene fibers with other constituents of composite system. For improving the compatibility, polypropylene fibers were treated with two different
silane
agents’
viz.
methacryloxypropyltrimethoxysilane
(MS)
and
vinylytriethoxysilane (VS). It was observed that addition of silane treated polypropylene fibers to the composite system increased the impact strength for a given composition (see Figure 2). A maximum improvement of 44% in impact strength (as compared to NE) was observed for the nanocomposite formulation (E1C2PPMS) containing 2 phr polypropylene fibers treated with MS silane agent. Further, it was observed that for all cases with polypropylene fiber reinforcement (treated or untreated), the impact strength of composites showed improvement till 2 phr 10
polypropylene loading and after this limit (i.e. at 3 phr PP loading), the impact strength deteriorated. This may be attributed to poor dispersion of polypropylene fibers in the composite system because of the resulting high viscosity of resin at such high concentration of polypropylene reinforcement. Since, maximum improvement in impact strength was observed for 2 phr polypropylene loading, this concentration was only selected for the other treatment i.e. maleic anhydride grafting. When maleic anhydride grafted polypropylene fibers (2 phr loading) were used as reinforcement, impact strength value showed an improvement of about 13% (as compared to NE). This improvement was attributed to the expected coupling of polypropylene fibers with epoxy with help of maleic anhydride. Thus, addition of treated polypropylene fibers (2 phr maximum) as reinforcement to the nanocomposite system enhanced the impact strength. In the present work, tensile and flexural properties were also investigated in addition to the impact strength. The tensile and flexural strengths of the epoxy based glass fiber reinforced composite (NE) were observed as 269 MPa and 197 MPa respectively. Addition of 1phr clay to this GFRP (E1C), led to improvement in both tensile and flexural strength (16.5% and 14.5% respectively as compared to NE). Also, the tensile and flexural moduli showed improvements of 6.5% and 34.5% respectively. These results were in accordance with earlier work reported of [2,3] and many others. Further, it was observed that addition of untreated or silane treated polypropylene fibers to the nanocomposite system resulted in decrease in nearly all the tensile and flexural characteristics, except for tensile strength. However, with addition of MAH grafted polypropylene fibers to the nanocomposite system, nearly all the properties (especially impact and flexural) showed a substantial
11
improvement. Flexural modulus showed maximum improvement for 2 phr MAH grafted polypropylene fiber loading (1.23 GPa; an improvement of 51% over NE sample).
Compatibilization behaviour The main purpose of the present research was to improve the impact strength of epoxy based glass fiber reinforced nanocomposites. For improving the impact strength, second phase fillers in the form of polypropylene fibers (1–3 phr) were added to the nanocomposite system. The results showed that addition of treated polypropylene fibers (up to 2 phr) led to improvements in impact strength of the composite system. For nanocomposites reinforced with silane treated polypropylene fibers, impact strength improvement was attributed to the improved compatibility between the inorganic glass fibers and the organic polypropylene fibers. It is well reported in literature that silane agents can react with a variety of materials (viz. organic and inorganic material surfaces via covalent bonds etc.) to increase the compatibility/coupling between them. Silane agents have two types of functional groups viz. hydrolyzable group (OR group) and organo-functional group (X group). The Si-OR bonds hydrolyze quickly with water (even with moist air) to make silanol Si-OH groups which condense and result in formation of Si-O-Si bonds. This allows chemical compatibilization of dissimilar material surfaces [27,32]. Figure 3 shows the structure of γmethacryloxypropyltrimethoxysilane (MS), and vinylyriethoxysilane (VS) silane agents used in the present research. Further, for MAH grafted polypropylene fibers, there was improvement in impact strength with appreciable restoration of tensile properties of nanocomposites. The improvement in impact strength could be due to the improved bonding of MAH grafted polypropylene 12
fibers to the epoxy matrix. This is because functional groups of MAH grafted polypropylene fibers react with the hydroxyl or epoxy groups of the epoxy resin [30,33]. Figure 4 shows the schematic of possible coupling reaction of MAH grafted polypropylene fibers with epoxy in the nanocomposite system.
Surface morphology SEM analysis of untreated fibers SEM images of the as-received glass fibers and polypropylene fibers are presented in Figure 5a and Figure 5b respectively. The glass fibers showed a circular cross-section (diameter ~ 15 μm) and polypropylene fibers showed a rectangular cross-section (width ~ 35 μm). These details were noted to differentiate between the glass and polypropylene fibers during SEM analysis of the fractured impact specimens of nanocomposites containing both these type of fibers.
SEM/EDS of silane treated polypropylene fibers Figure 6a–c shows the SEM images of the un-treated polypropylene fibers and those treated with MS silane agent and VS silane agent respectively. SEM micrographs confirmed the presence of silane agents on the treated polypropylene fibers. To further confirm the SEM results with regards to silane treatment of polypropylene fibers, energy dispersive spectroscopy (EDS analysis) was conducted. The presence of silane agent on the treated polypropylene fibers was confirmed by the presence of silicon on the fiber surface. Figure 7a–c shows the results of EDS analysis. The amount of silicon in the untreated, MS treated, and VS treated fibers was observed as 0, 1.25, and 1.47% respectively. The presence of 13
silicon on surface of treated fibers confirmed successful silane coating on the fibers and thus the SEM results.
SEM/FTIR study of MAH grafted polypropylene fibers As discussed earlier, UV-assisted MAH grafting of polypropylene fibers showed maximum weight gain for treatment time of 30 h (2.18 g; see Table 1). So, for MAH grafting of polypropylene fibers, the optimum treatment time was selected as 30 h. To confirm the same, SEM analysis of the MAH grafted fibers exposed to ultra-violet rays for different treatment time periods (10 h, 20 h, 30 h, and 40 h respectively) was conducted. SEM results confirmed the observations of weight gain. SEM micrographs showed uniform grafting with minor pitting on polypropylene fibers treated for 30 h (see Figure 8c). The minor pitting observed along with uniform grafting also improves adhesion of polymeric fibers to the constituents of composite system [34]. For treatment time periods of more than the optimum period of 30 h, the SEM micrograph (Figure 8d) showed ruptured surfaces which was due to excessive exposure to UV radiations. Further, FTIR was conducted for MAH grafted polypropylene fibers to confirm the grafting. Figure 9 presents the FTIR results for both the untreated fibers and the MAH grafted polypropylene fibers. For MAH grafted samples, peaks at 841 cm-1, 1168 cm-1, 1265 cm-1, and 1636 cm-1 could be recognized as peaks due to C–H alkenes, –OH aromatic, C=C aromatic ring, and C=C alkenes respectively and the difference in shape of band between 3000–3600 cm-1 could be due to modification of self-association of –OH group due to hydroxyl group involved in interactions with maleic anhydride [35]. Absorption bands at around 1708 cm-1 and 1889 cm-1 which correspond to the asymmetric C=O 14
stretching and carboxylic acid respectively indicated grafting of maleic anhydride on polypropylene fibers [30,36–38]. Thus, MAH grafting on polypropylene fibers was confirmed by the FTIR study.
SEM analysis of impact specimens SEM micrographs of the fractured surface of impact tested specimens were investigated for some typical compositions (NE; E1C; and all nanocomposites containing 2 phr polypropylene fibers) to understand the failure behaviour of composites. Figure 10a shows the fractured surface of ‘NE’ composite formulation. The micrograph revealed a lack of interaction between glass fibers and epoxy resin. Also, the surface of epoxy appeared quite smooth indicating less resistance to failure under impact loading [2,10,39]. Figure 10b shows the fractured surface of ‘E1C’ nanocomposite. As compared to ‘NE’, the surface of epoxy in ‘E1C’ composite was more rough indicating greater resistance to impact loading before failure. Also, epoxy-nanoclay was seen on the glass fibers indicating better interaction among the E1C constituents. SEM images of fractured surfaces of epoxy/clay GFRP nanocomposite containing 2 phr of untreated polypropylene fibers (E1C2PP) are presented in Figure 10c. It was observed that there is negligible interaction of polypropylene fibers with other constituents which could be attributed to the inert nature of polypropylene [1,40]. This lack of interaction resulted in poor impact strength. Figure 10d– e shows the SEM images of fractured surface of epoxy/clay GFRP nanocomposite sample containing 2 phr silane treated polypropylene fibers (E1C2PPMS and EIC2PPVS respectively). The micrographs showed improved interaction of the treated polypropylene fibers with other constituents of the nanocomposite system. Also, some regions showing 15
fractured polypropylene fibers could be observed in the MS silane treated nanocomposite (E1C2PPMS). This indicated that silane treated polypropylene fibers used as reinforcement in the nanocomposites resisted the impact loading more effectively thereby resulting in higher impact strength of the nanocomposite system. Figure 10f shows the SEM images of the impact tested surface of epoxy/clay GFRP nanocomposite sample containing 2 phr of MAH grafted polypropylene fibers (E1C2PPMAH). The micrograph showed presence of epoxy-clay on the MAH grafted polypropylene fibers. This indicated good coupling of polypropylene fiber reinforcement with the epoxy matrix in the nanocomposite. The good mechanical properties obtained for this nanocomposite system (E1C2PPMAH) may be attributed to these reasons.
XRD analysis X-ray diffraction was conducted to examine the type of nanoclay morphology obtained in the composite systems as a result of processing method used in the present work. XRD analysis was carried out to determine if the processing method was able to achieve the desired morphology of nanoclay (i.e. exfoliated morphology) in the nanocomposites. XRD analysis was carried out for pristine nanoclay and for E1C nanocomposite. E1C composition was selected because the same processing method for clay dispersion (as was for E1C) was followed for all other nanocomposite formulations. For nanocomposite systems, absence of peak in the XRD spectrum indicates exfoliated morphology [41,42]. Figure 11 shows the XRD patterns of pristine nanoclay Cloisite 15A and E1C nanocomposite respectively. XRD pattern of pristine clay showed peak (d001) at 2.44 degree (2θ = 2.44°) and d-spacing was calculated as 36.16 Å. Further, XRD pattern for the 16
nanocomposite system (E1C) in Figure 11 showed absence of peak indicating an exfoliated morphology of silicate layers in the nanocomposite system.
TEM analysis Transmission electron microscopy (TEM) was conducted to examine the type of dispersion of nanoclay achieved in the epoxy matrix (phase separation, intercalation, or exfoliation) as a result of the processing method followed in the present research. TEM analysis was carried out only for ‘E1C’ nanocomposite to determine if the processing method could achieve effective dispersion of nanoclay. This composition was selected because the same processing method for clay dispersion (as was for E1C) was followed for all other nanocomposite formulations. Figure 12a–b presents the TEM micrographs of ‘E1C’ nanocomposite formulation. The dark/black lines represent the clay platelets and the grey background represents the epoxy matrix. Figure 12a shows that clay layers are non-parallel and randomly aligned, thus indicating exfoliated morphology. Figure 12b shows a mixed type of clay morphology where some clay layers are evenly spaced with large inter-lamellar spacing indicating uniform dispersion and exfoliated morphology of silicate layers. However, at a few places, intercalation is also observed. Thus, TEM images show largely an exfoliated clay morphology with uniform dispersion [8,39,42–47]. Thus, TEM analysis justified that the processing method used in the present research was effective in dispersing the clay in the nanocomposite system.
CONCLUSIONS
17
It was expected that despite the lower strength and modulus of polypropylene fibers, the impact strength of composites reinforced with them would increase (because of the good ductility of polypropylene fibers) while tensile/flexural properties may decrease. However, addition of the as-received (untreated) polypropylene fibers to the nanocomposite system deteriorated all the mechanical properties viz. tensile/flexural and also the impact strength. This decrease in mechanical properties of nanocomposites was due to the inert nature of polypropylene fibers resulting in their poor compatibility with other constituents. To overcome this problem, two different compatibilization procedures for polypropylene fibers were investigated. Both the compatibilization procedures viz. silane treatments and MAH grafting of polypropylene fibers proved successful in improving the impact strength of nanocomposites. The reinforcement of treated polypropylene fibers with 2 phr loading in the nanocomposite system provided the best results. Thus, for the first time, GFRP nanocomposites with clay as the nano-filler and compatibilized polypropylene fibers as the micro-sized filler were fabricated successfully providing improved impact strength without much drop in the tensile/flexural properties.
Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors are thankful to CIPET, Amritsar, India for providing facilities for the impact testing of nanocomposites. The authors also acknowledge the contributions of SAIF Labs, Punjab University, Chandigarh, India for extended support for TEM and FTIR facilities.
18
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Figure 1: Schematic of UV treatment set-up for MAH grafting. Figure 2: Impact strength of various composite formulations. Figure 3: Structure of (a) γ-Methacryloxy silane, MS and (b) Vinyltyriethoxy silane, VS. Figure 4: Schematic of coupling reaction between MAH grafted polypropylene and epoxy. Figure 5: SEM images of the as-received (a) glass fibers, and (b) polypropylene fibers. Figure 6: SEM micrographs showing surface of polypropylene fibers (a) un-treated, (b) VS silane treated, and (c) MS silane treated.
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Figure 7: Results of EDS of polypropylene fibers (a) untreated, (b) treated with VS silane agent, and (c) treated with MS silane agent. Figure 8: SEM micrographs showing polypropylene fibers subjected to UV-assisted MAH-grafting for exposure time period of (a) 10h, (b) 20h, (c) 30h, and (d) 40 h. Figure 9:
FTIR results of untreated and MAH grafted polypropylene fibers. PP = untreated
polypropylene fibers, PPMAH = MAH grafted polypropylene fibers. Figure 10: SEM micrographs showing fracture surfaces of the GFRP composite (a) GFRP without clay/PP (NE), (b) nanocomposite without PP (E1C), (c) nanocomposite with 2phr PP (E1C2PP), (d) nanocomposite with 2phr MS treated PP (E1C2PPMS), (e) nanocomposite with 2phr VS treated PP (E1C2PPVS), and (f) nanocomposite with 2phr MAH grafted PP (E1C2MAHPP). PP = polypropylene. Figure 11: XRD patterns for the nanoclay Cloisite 15A and the E1C nanocomposite. Figure 12: TEM micrographs of the epoxy-clay nanocomposite showing clay morphology.
Figure 1: Schematic of UV treatment set-up for MAH grafting.
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275
Im pac t S trength (kJ /m
2
)
250
225
200
175
150
125
100
NE
E1
C E1
C1
PP E
1 1C
PP
S M E
1 1C
PP
VS
E1
C2
PP
E
2 1C
PP
S M E
2 1C
PP
VS
E1
P C2
S S PP AH PV PM C3 PM 3P 3P E1 C C E1 E1
C om pos ite F orm u la tion
Figure 2. Impact strength of various composite formulations.
Figure 3: Structure of (a) γ-Methacryloxy silane, MS and (b) Vinyltyriethoxy silane, VS [48,49].
24
Figure 4: Schematic of coupling reaction between MAH grafted polypropylene and epoxy [30].
Figure 5. SEM images of the as-received (a) glass fibers, and (b) polypropylene fibers.
25
Figure 6. SEM micrographs showing surface of polypropylene fibers (a) un-treated, (b) VS silane treated, and (c) MS silane treated.
26
Figure 7. Results of EDS of polypropylene fibers (a) untreated, (b) treated with VS silane agent, and (c) treated with MS silane agent.
Figure 8. SEM micrographs showing polypropylene fibers subjected to UV-assisted MAH-grafting for exposure time period of (a) 10h, (b) 20h, (c) 30h, and (d) 40 h.
27
50 45 40
PP
35
%T
30 25 20
PPMAH
15 10
1887 841
5
1168
3058
1707 1636
0 4000
3500
3000
2500
2000
1265
1500
1000
500
-1
cm
Figure 9. FTIR results of untreated and MAH grafted polypropylene fibers. PP = untreated polypropylene fibers, PPMAH = MAH grafted polypropylene fibers.
28
Figure 10. SEM micrographs showing fracture surfaces of the GFRP composites (a) GFRP without clay/PP (NE), (b) nanocomposite without PP (E1C), (c) nanocomposite with 2phr PP (E1C2PP), (d) nanocomposite with 2phr MS treated PP (E1C2PPMS), (e) nanocomposite with 2phr VS treated PP (E1C2PPVS), and (f) nanocomposite with 2phr MAH grafted PP (E1C2MAHPP). PP = polypropylene.
29
100000
Pristine Clay, Cloisite 15A Nanocomposite, E1C
80000
Intensity
60000
36.16 A
O
40000
20000
0
-20000 0
1
2
3
4
5
6
7
8
9
10
11
12
Angle (2
Figure 11. XRD patterns for the pristine nanoclay and the nanocomposite system.
Figure 12.TEM micrographs of the epoxy-clay nanocomposite showing clay morphology.
30
Table 1: Weight gain by PP fibers during MAH grafting. Table 2: Mechanical properties of various composite formulations.
Table 1: Weight gain by PP fibers during MAH grafting. Grafting time (h)
Initial weight before grafting (g)
Final weight after grafting (g)
Weight gain by PP fibers (g)
Percent weight gain by PP fibers (%)
10
2.00
3.31
1.31
65.5
20
2.00
3.88
1.88
94.0
30
2.00
4.18
2.18
109.0
40
2.00
3.21
1.21
60.5
Table 2. Mechanical properties of various composite formulations.
Sample Description (Sample Designation)
Avg* s Avg* s Avg* s Epoxy/1 phr clay/1 phr MS treated Avg* polypropoylene fiber GFRP (E1C1PPMS) s Epoxy based GFRP with no clay and no polypropylene fiber (NE) Epoxy/1 phr clay GFRP with no polypropylene fiber (E1C) Epoxy/1 phr clay/1 phr polypropylene fiber GFRP (E1C1PP)
Avg* s Avg* s Avg* s Epoxy/1 phr clay/2 phr VS treated Avg* polypropoylene GFRP (E1C2PPVS) s Epoxy/1 phr clay/2 phr MAH grafted Avg* polypropoylene GFRP(E1C2PPMAH) s Epoxy/1 phr clay/3 phr polypropoylene Avg* GFRP (E1C3PP) s Epoxy/1 phr clay/1 phr VS treated polypropoylene fiber GFRP (E1C1PPVS) Epoxy/1 phr clay/2 phr polypropoylene fiber GFRP (E1C2PP) Epoxy/1 phr clay/2 phr MS treated polypropoylene fiber GFRP (E1C2PPMS)
Impact Strength (kJ/m2)
Tensile Strength (MPa)
Tensile Modulus (MPa)
Flexural Strength (MPa)
Flexural Modulus (MPa)
161±2 4 167±15 30 148±8 16 178±8
269±3 7 313±6 12 215±4 8 292±3
7315±40 81 7780±133 265 5410±20 93 6110±22
197±5 11 226±4 8 156±8 17 82±2
8125±159 318 10928±379 759 6318±132 263 5618±58
16 177±12 23 149±13 25 232±21 42 185±8 16 181±1 2 129±5 9
6 278±8 16 247±8 17 242±2 5 258±2 4 266±8 17 183±4 8
43 5970±58 115 5835±76 152 5225±86 148 5225±38 76 6515±131 262 4385±136 273
3 104±2 3 193±3 6 104±4 8 85±3 6 223±4 9 167±7 15
116 9995±792 1583 10440±626 1252 7753±390 779 5880±520 1039 12275±525 1050 6635±308 617
31
150±6 215±10 Epoxy/1 phr clay/3 phr MS treated Avg* polypropoylene GFRP (E1C3PPMS) s 11 19 142±7 205±6 Epoxy/1 phr clay/3 phr VS treated Avg* polypropoylene (E1C3PPVS) s 15 13 Avg* = Mean value ± Range of values; s = Standard deviation of values
32
4708±54 108 4415±64 129
63±7 15 43±0 1
5135±712 1423 3125±253 505