A comparative study of the mechanical performance of PP and PP/PP-g-MAH blends reinforced with carbon fibre (CF) obtained by two different moulding techniques is presented. Three filler contents were used for fabricating the composites: 1, 3, and 5 pph (parts per hundred). The crystallisation behaviour of the composites was studied by differential scanning calorimetry. Morphological and structural features of these samples were observed by atomic field microscopy and Fourier-transform infrared spectroscopy, respectively. Mechanical properties of the injection and compression moulded composites were evaluated by means of tensile and impact resistance tests. The fracture surface of the impacted samples was observed by scanning electron microscopy. The processing method had a noticeable effect on the results obtained in these tests. Young’s modulus was enhanced up to 147% when adding 5 pph CF to a PP matrix when processed by compression moulding. Addition of PP-g-MAH and CF had a favourable effect on the tensile and impact strength properties in most samples; these composites showed improved performance as the filler content was increased.
Polypropylene possesses an attractive combination of mechanical properties which can be further improved by the modification of this polymer in order to obtain polymer blends and also by the incorporation of reinforcing fillers [
The miscibility between PP and PP-g-MAH has been studied by several authors. Depending on the molecular weight and MAH content in PP, the blending can either cocrystallise or phase separate; this in turn will influence the mechanical properties of PP/PP-g-MAH blends [
PP homopolymer XH1760 was supplied by Indelpro (Mexico) with a melt flow index of 3 g/10 min at 230°C/2.16 kg. The material was a commercially stabilized polymer without any further additives, especially without nucleating agents. Polybond 3200 with a melt flow of 115 g/10 min at 230°C/2.16 kg obtained from Chemtura was employed as PP-g-MAH compatibiliser. AGM-99 milled CF produced from PAN with a mean length of 150 microns and a diameter of 8 microns were obtained from Asbury Carbons and used as received.
A twin-screw corotating Brabender Plasti-Corder was used to prepare the PP composites and composite blends (PP/PP-g-MAH) which contained 5 pph PP-g-MAH. The screw speed was 60 rpm and they were compounded at 200 ± 10°C during 10 min. The polymer and polymer blends were reinforced with CF at three different contents: 1 pph, 3 pph, and 5 pph. These composites were moulded using compression and injection compounding. A Beutelspacher single screw extruder at a screw speed of 50 rpm and temperature profile set from 175°C in the feed section to 190°C at the die was used to prepare the extruded samples. The compounds were fabricated by a Sumitomo injection machine at a moulding temperature of 200°C. The compression moulded samples were prepared at 200°C and 5 tons for 5 min in a Carver press which was followed by cold pressing at 5 tons for 10 min.
Fourier-transform infrared (FTIR) spectroscopy was carried out on Perkin Elmer Spectrum 100 spectrometer with an ATR device. The sample films were analysed by atomic field microscopy (AFM) in tapping mode using an Innova Vecco microscope. Differential scanning calorimetry (DSC) studies were conducted on a Q2000 TA Instrument. All samples weighed approximately 6 mg and were sealed within aluminium pans. The samples were heated from −30 to 200°C and maintained at this temperature for 1 min and then they were cooled down to −30°C to be equilibrated at 1 min; both heating and cooling rates were conducted at 10°C/min. Subsequently, a similar procedure was done at 50°C/min.
Compression moulded samples (1.5 × 3 × 9 mm3) and injection moulded samples (3 × 3 × 9 mm3) were subjected to tensile test using an Instron 3365 Universal Materials testing machine at room temperature using a crosshead speed of 5 mm/min and a 5 kN full-scale load cell. All results presented are the average values of three measurements. Izod impact tests were done following ASTM D256 using CEAST Izod pendulum impact tester using a nominal hammer energy of 2 J, and measurements were done in ten samples. Morphology analysis of the impacted areas was made by scanning electron microscopy (SEM) using a JEOL JSM-7600TFE microscope at 2 kV accelerating voltage.
Extruded composites obtained from the Brabender Plasti-Corder were analysed by calorimetry in order to see the crystallisation properties of these samples. Table
Crystallisation properties of the samples.
Sample | Heating and cooling rate | |||||
---|---|---|---|---|---|---|
10 C/min | 50 C/min | |||||
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|
|
|
|
| |
Pure PP | 156.45 | 116.07 | 91.21 | 158.75 | 107.12 | 75.97 |
1 pph CF | 161.41 | 116.12 | 78.78 | 160.1 | 105.47 | 61.34 |
3 pph CF | 158.62 | 117.1 | 74.5 | 157.84 | 107.19 | 60.06 |
5 pph CF | 157.58 | 118.66 | 72.77 | 156.53 | 109.73 | 58.94 |
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PP/PP-g-MAH | 156.42 | 113.24 | 72.99 | 158.03 | 103.99 | 70.83 |
1 pph CF | 157.58 | 118.66 | 90.77 | 156.53 | 109.73 | 59.94 |
3 pph CF | 157.09 | 118.46 | 83.53 | 159.43 | 110.01 | 62.93 |
5 pph CF | 159.37 | 119.48 | 75.38 | 159.69 | 111.05 | 64.12 |
According to the fusion enthalpy values included in Table
AFM was employed to investigate the surface morphologies of the samples; Figure
AFM images of (a) PP-g-MAH, (b) PP/PP-g-MAH, (c) 1 pph CF/PP, and (d) 1 pph CF PP/PP-g-MAH.
Figure
FTIR peak assignments of iPP.
Peak position, cm−1 | Vibrational modes |
---|---|
810 |
|
841 |
|
900 |
|
970 |
|
1000 |
|
1170 |
|
1380 |
|
1455 |
|
2840 |
|
2920 |
|
2950 |
|
Key:
FTIR of the extruded samples.
Figures
Young’s modulus of compression moulded samples.
Young’s modulus of injection moulded samples.
The results show the different behaviour that PP-g-MAH can provide when compounded by two different techniques, which is more evident for the injection moulded samples. In the compression moulded samples, the PP/PP-g-MAH blend showed superior modulus at 0 and 1 pph CF content, as compared to pure PP, and this was associated with a brittle behaviour of the samples, resulting in a large reduction of elongation, which is further reduced by a higher content of CF, as displayed in Table
Tensile strain at break of the compression moulded composites.
Sample | Elongation, % | Std. error |
---|---|---|
PP | 1160.45 | 53.27 |
1 pph CF | 661.09 | 87.94 |
3 pph CF | 12.83 | 1.55 |
5 pph CF | 11.86 | 2.98 |
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PP/PP-g-MAH | 114.88 | 11.03 |
1 pph CF | 143.28 | 60.27 |
3 pph CF | 11.64 | 4.84 |
5 pph CF | 12.06 | 3.79 |
Tensile strain at break of the injection moulded composites.
Sample | Elongation, % | Std. error |
---|---|---|
PP | 1016.58 | 54.30 |
1 pph CF | 1195.763333 | 27.55 |
3 pph CF | 1212.49 | 30.97 |
5 pph CF | 984.97 | 2.70 |
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PP/PP-g-MAH | 1288.83 | 64.79 |
1 pph CF | 1236.52 | 25.17 |
3 pph CF | 871.73 | 136.53 |
5 pph CF | 994.10 | 79.73 |
Addition of rigid fillers usually restricts the chain mobility of polymer molecules. Since the chains cannot move freely, the strain at break is reduced with increasing CF content, which explains the elongation behaviour found in the compression moulded samples. This behaviour is in agreement with the study conducted by Karsli and Aytac [
From the above mentioned results, it is noticeable that the performance obtained by the samples processed by injection moulding stands out when compared with compression moulded composites. The different behaviour between both processes upon the incorporation of CF can be explained in terms of the orientation of the polymer chains and the filler. Injection moulding allows the polymer chains and CF to orientate parallel to the flow direction, which is not the case for the compression moulding process. The injection moulding samples were tested in the flow direction thus favouring the elongation response; this explains the large difference between the elongation responses of both processes.
Table
Yield strength of the compression moulded composites.
Sample | Strength, MPa | Std. error |
---|---|---|
PP | 38.82 | 1.21 |
1 pph CF | 38.26 | 2.14 |
3 pph CF | 44.16 | 1.13 |
5 pph CF | 43.27 | 0.80 |
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PP/PP-g-MAH | 37.42 | 0.41 |
1 pph CF | 44.43 | 1.30 |
3 pph CF | 40.31 | 0.56 |
5 pph CF | 42.68 | 1.72 |
Yield strength of the injection moulded composites.
Sample | Strength, MPa | Std. error |
---|---|---|
PP | 61.64 | 1.42 |
1 pph CF | 60.21 | 1.67 |
3 pph CF | 60.16 | 1.02 |
5 pph CF | 57.98 | 0.72 |
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PP/PP-g-MAH | 55.59 | 1.22 |
1 pph CF | 59.46 | 1.62 |
3 pph CF | 49.34 | 4.73 |
5 pph CF | 49.73 | 2.70 |
Plastic deformation of the polymer matrix is the main energy absorbing process in impact tests [
Izod impact test values of the injection moulded samples.
Figure
SEM images of the impacted samples: (a) 1 pph CF/PP, (b) 1 pph CF/PP/PP-g-MAH, (c) 3 pph CF/PP, (d) 3 pph CF/PP/PP-g-MAH, (e) 5 pph CF/PP, and (f) 5 pph CF/PP/PP-g-MAH.
SEM micrographs of the polymer and composite fracture surfaces are included in Figure
Incorporation of PP-g-MAH has been shown to provide an advantageous effect when used as a compatibiliser in CF composites according to the DSC results which showed higher crystallisation temperatures and also higher degree of crystallinity achieved when compared to those composites based only on a PP matrix. Additionally, composites obtained by compression moulding achieved higher stiffness when the polymer blend was used and with the addition of low content of CF; on the other hand, a reduction in the modulus and a dramatic drop in the elongation of the samples were observed at higher CF content. The mechanical performance in the injection moulded composites differed more noticeably between the samples obtained from pure PP and the PP/PP-g-MAH blend. It was found that addition of CF greatly enhanced the modulus in the composites obtained from the former and the impact strength was reduced compared to the matrix. While the composites obtained from the PP/PP-g-MAH blend had a modest increase in the stiffness by the addition of CF up to the highest loading used in this work and also maintained the values of tensile strain, in these samples, the impact strength was improved with the incorporation of the filler when compared to the unreinforced sample. Impact strength was reduced in the composites obtained from the pure polymer, indicating stress region concentrations around fillers contributed mainly to this response. An absorbing energy filler behaviour was obtained in the samples prepared from the polymer blends as displayed by their enhanced impact strength.
The authors declare that there is no conflict of interests regarding the publication of this paper.
D. Pérez-Rocha wants to thank CONACYT for Scholarship no. 387175 and PROMEP for the support received. The authors are grateful to Asbury Graphite Mills for their kind donation of carbon fibres and to Indelpro for their kind donation of polypropylene (XH1760).