Our work aims to evaluate a complete outlook of virgin high density polyethylene (HDPE) and polypropylene (PP) polyblends. Virgin PP of 20, 30 and 50 weight% is compounded with virgin HDPE. The properties like tensile strength, flexural strength, Izod impact strength are examined. Scanning electron microscopy (SEM) and polarised light microscopy (PLM) are used to observe the surface and crystal morphology. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) tests verify the non compatibility of both polymers. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques are used to study the thermal behaviour of composites. The results manifest co-occurring spherulites for polyblends; indicating the composite to be a physical blend of continuous and dispersed phases, but on the other hand PP improves the tensile and flexural properties of HDPE.
Polymer composite is material of research in modern days. Thermoplastic polymers are of great interest due to their technical and commercial importance [
Among the thermoplastic polymers, PP possesses good mechanical strength. In addition it has high chemical resistance, low cost and easy to manufacture. PP has wide application in automobile spare parts and as well as container [
Jia-Horny Lin et al. has reinforced HDPE to PP matrix and verified the non-compatibility of both polymers, but improves the impact strength of PP [
PP (M110 Grade, homopolymer) produced by the spheripol technology and HDPE (M5818 Grade, injection moulded type) produced by Mitsui Slurry CX technology are purchased from Haldia petrochemical limited, haldia, India. Different physical properties of the polymers are reported in
Polymers in the form of pellets are collected. The pellets are dried in a hot air oven at 60˚C for 8 hrs to remove moisture content followed by mixing of 20, 30, and 50 wt% of PP to HDPE. Then they are converted into polymer blend pellets using a twin screw extruder (ZV20, Specific Engineering and Auto Mates, Vadodara, India) at feeder speed of 51 rpm and main rotor at 54 rpm. The screws are of 21 mm diameter and co-rotating type, containing three thermal barrels at 190˚C, 200˚C and 210˚C respectively. The melt and die temperatures are 224˚C and 200˚C.
The obtain pellets are dried at 60˚C for 8 hrs and moulded to test samples using an automatic injection moulding machine (Endura-90, Electonica plastic
Polymer type | Melt flow index (g/10 min) | Density (g/cc) |
---|---|---|
HDPE | 19 (2.16 kg, 190˚C) | 0.956 |
PP | 11 (2.16 kg, 230˚C) | 0.900 |
machines limited, Kolkata, India) with screw diameter of 35 mm at 177 rpm. The temperature of the nozzle is 200˚C and that of the three barrels are 190˚C, 200˚C and 210˚C respectively. Snapshot of the prepared tensile and flexural test samples are shown in
Both tensile and flexural strengths of HDPE/PP polyblends are tested using an universal testing machine (UTM3382, Instron, UK) as per ASTM D638-02a and ASTM D790 standards respectively. Tensile specimens are prepared according to ASTM D638-02a type-I; with gage length 50 mm. Tests are conducted at cross head speed of 50 mm/min. Flexural sample of size 127 mm × 12.7 mm × 3.2 mm are tested at speed of 1.365 mm/min with support span spacing of 51.2 mm (span = 16 times of thickness) at an extension up to 5%. The speed of the test and flexural strengths are calculated according to Equations (1) and (2) respectively.
Speed = Z L 2 6 d (1)
σ F max = 3 P L 2 b d 2 (2)
where, Z is Rate of straining at 0.01 mm/mm/min, L is span length (mm) and d is sample thickness (mm), σ F max is flexural strength (MPa), P is load (N), L is span length (mm) and b is sample width (mm).
Impact tests are conducted using a Izod and Charpy impactometer (IT 504 Plastic impact , Tinius Olsen ,USA ) with a V-notch cutter as per ASTM D256-A standard , possessing a pendulum energy of 13.70 J. Impact test specimens are prepared by cutting the flexural samples to a size of 63.5 mm × 12.7 mm × 3.2 mm with a V-notch of 45˚ and 0.25 mm depth.
Our investigation has used SEM (JEOL; JSM-6480 LV, Japan), Field emission SEM (Nova Nano SEM-450, USA) and PLM (Leica, DM750P, Germany). Morphology of samples is captured before and after fracture of impact test. Energy dispersive spectroscopy (EDS) analysis and carbon mapping test are conducted using FESEM at an operation voltage of 10 KV. Samples are gold coated before each test. PLM is used to observe the spherulite behaviour of the polyblends. A tiny sample is placed on a glass slide and melted at 200˚C (using the hot stage) followed by sandwiching the sample by placing a micro glass slide over it to
form a thin film. The sample is cooled at 5˚C/min (using cold stage) and spherulite morphologies are captured at 130˚C and 125˚C at magnification × 10.
In order to analyse any new phase formations after blending the polymers and to understand the chemical structure of the polyblends; the XRD (Philips, PW1720, USA) and FTIR (Perkin-Emler Spectrum 100, USA) techniques are utilised. X-ray scanning is done within a diffraction angle (2θ) range of 10 - 90˚ with Cu Kα radiation at 40 KV and 30 mA. The rate of scanning is 10˚/min and at λ = 0.154 nm. The IR Spectroscopy is observed between the waveband of 450 to 4000 cm−1.
The polyblends thermal behaviour is analysed using a DSC (Perkin-Elmer DSC 7, MA, USA) and TGA (Perkin-Elmer TGA, MA, USA) analysers. The DSC tests are performed under nitrogen flow rate of 50 ml/min. Polymer samples of around 10 mg are scanned at a heating rate of 10˚C/min from ambient temperature to 200˚C. The samples undergo three thermal cycles. Heating, cooling and reheating under the same condition to follow an identical thermal history for all polymer blends.
The degree of crystallinity (XC) of the polyblends is evaluated by Equation (3)
X C ( % ) = Δ H f ø Δ H f 0 × 100 (3)
where, Δ H f = Melting enthalpy of HDPE or PP in the blend, Δ H f 0 = Enthalpy corresponding to melting of 100% crystalline HDPE or PP and Ø = weight fraction of HDPE or PP in the blend. In TGA test, polymer samples with masses of approximately 10 mg are heated from atmospheric temperature to 600˚C, at heating rate of 10˚C/min and nitrogen flow rate of 50 ml/min, to observe their degradation behaviour. Data corresponding to Δ H f 0 are referred from Roger L. Blaine [
Tensile strength results are shown in
The experimental outcomes for flexural tests are reported in
The impact strength of polymers are expressed in three different ways and reported in
The chemical and crystal structure of HDPE/PP polyblends are analysed by XRD and FTIR.
From the DSC study the melting temperature (Tm) of PP and HDPE are 168.6 and 134.6˚C respectively.
Group | Wave number (cm−1) | Vibration type | Assigned to |
---|---|---|---|
-C-H | 2985 - 2810 | Stretching | PP |
-CH2 | 2950 - 2850 | Stretching | HDPE |
-CH2 | 1475 - 1440 | Bending | PP |
-CH3 | 1380 - 1370 | Bending | PP |
-CH2 | 1470 - 1460 | Bending | HDPE |
-CH2 | 730 - 700 | Rocking | HDPE |
Polymer Type | Δ H f , J/g | Tm, ˚C | Tc, ˚C | X c , % | Δ H c , J/g |
---|---|---|---|---|---|
HDPE | 213.9 | 134.6 | 115.0 | 73.0 | 253.2 |
PP | 55.7 | 168.6 | 122.1 | 26.9 | 74.94 |
50HDPE/50PP | 119.7a/30.5b | 134.4a/166.1b | 115.2 | 81.70a/29.46b | 212.4 |
70HDPE/30PP | 138.1a/29.6b | 134.4a/163.7b | 115.5 | 67.33a/47.66b | 201.5 |
80HDPE/20PP | 131.0a/25.1b | 134.8a/162.6b | 120.8 | 55.88a/60.62b | 148.6 |
The superscript abcorresponds to cite HDPE and PP respectively.
composite to be a physical mixture of both the polymers. The existence of PP in HDPE does not alter the melt peak temperature significantly.
The weight loss of a polymer with respect to time or temperature is usually predicted by using TGA technique. The thermal degradation is an irreversible process. Our work focused to predict the degradation temperature (TD). It is defined in our project as; the temperature at which the weight loss of the polymers just starts to fall immediately.
The surface morphology of polymers before fracture is reported in
Sample | TD, ˚C | Weight % at TD | Residual weight % | Inflection Point | |
---|---|---|---|---|---|
˚C | %/˚C | ||||
HDPE | 368.56 | 97.45 | 0.7495 | 452.5 | 2.264 |
PP | 356.87 | 97.86 | 1.206 | 438.0 | 3.008 |
50HDPE/50PP | 392.34 | 96.20 | 0.5303 | 447.68 | 3.002 |
70HDPE/30PP | 388.54 | 96.45 | 0.6602 | 447.65 | 2.94 |
80HDPE/20PP | 320.174 | 96.16 | 1.972 | 433.16 | 1.378 |
similar morphology of all the polymer blends is observed. Prepared sample’s surfaces are smooth and difficult to differentiate. The continuous and dispersed phases for un-fractured polyblend samples are difficult to identify. To evaluate the changes in the properties; carbon elemental mapping and EDS tests are conducted and disclosed in
Crystal structures of the polyblends during solidification from molten stage are reported in
Our project promisingly combines PP with HDPE. The dispersion of PP in HDPE improves tensile and flexural strengths. The results show that a 50 wt% PP increases the tensile strength of the composite by 29%, and is maximum among the polymer blends. The magnitude of the flexural strength for all the polyblends are close to 23 MPa and improved by 44%. The XRD, FTIR and DSC tests prove the polyblend to be a combination of two dispersed matrices. No changes in chemical structure are observed, confirming the composite to be a physical blending. PLM tests authenticate; reinforcement of PP particles to HDPE retards the crystal growth and spherulites lap over. TGA tests disclose the degradation characteristics; showing a maximum degradation temperature and weight loss for polymer blends is for composite with 50 wt% PP. Because of the
conventional methods are adopted for preparing the HDPE/PP blends with low manufacturing cost, the composite blends may find suitable application areas.
Sutar, H., Sahoo, P.C., Sahu, P.S., Sahoo, S., Murmu, R., Swain, S. and Mishra, S.C. (2018) Mechanical, Thermal and Crystallization Properties of Polypropylene (PP) Reinforced Composites with High Density Polyethylene (HDPE) as Matrix. Materials Sciences and Applications, 9, 502-515. https://doi.org/10.4236/msa.2018.95035