The present paper investigates the effect of strain rate on different tensile properties of high density polyethylene (HDPE) and polypropylene (PP) composite. Tensile specimens of virgin HDPE-PP composites are prepared via twin screw extruder and injection moulding methods as per ASTM D638-02a (Type-I); with gage length 50 mm, width 13 mm and thickness 3 mm. Composites are fabricated with PP as reinforcing agent at a loading rate of 10%, 20%, 30%, 40% and 50% by weight. Experiments are carried out at room temperature of 23 °C and absolute humidity of 54% at a cross head speed of 30, 40, 50, 60 and 70 mm/min. Stress and strain values at yield and break points are reported. Atomic force microscopy (AFM) is used to study the distribution of polymer molecules in the mixture and surface roughness. As in last, experiments are designed by Taguchi optimization method to find out the dominating factors on tensile strength.
History reveals, the composites are mainly used for savings in secondary structures. The fibre-reinforced polymer (FRP) materials find increasing applications as load bearing structures. But in the other hand, development of polymer materials for high technology engineering applications is in demand [
A detailed review of the strain rate dependence of mechanical properties of polymer composites has been outlined by Jacob et al. [
Virgin PP of M110 Grade (homopolymer) produced by the sphericol technology and virgin HDPE of M5818 Grade (injection moulded type) produced by Mitsui Slurry CX technology are purchased from Haldia petrochemical limited, Haldia, West Bengal, India. Typical physical properties of the polymers are reported in
Polymers are collected in the form of pellets. The pellets are dried in a hot air oven at 60˚C for 8 hrs to remove moisture content followed by mixing of 10, 20, 30, 40 and 50 wt. % of PP to HDPE. Then they are mixed using a twin screw extruder (ZV20, Specific Engineering and Auto Mates, Vadodara, India) at feeder
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) | 0.90 |
speed of 51 rpm and main rotor at 54 rpm to form a homogeneous polymer blend and are collected in the form of pellets. 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 obtained pellets are moulded immediately to tensile test samples using an automatic injection moulding machine (Endura-90, Electonica plastic 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. Tensile specimens are prepared according to ASTM D638-02a type-I (gage length 50 mm, with 13 mm and thickness 3 mm) standard.
Tensile characteristics of HDPE/PP polyblends are determined using an universal testing machine (UTM-3382, Instron, UK). Tests are conducted at cross head speed of 30, 40, 50, 60 and 70 mm/min for each composite type at atmospheric temperature of 23˚C and absolute humidity of 54%. Stress and strain at yield and break points are estimated in the experiment and reported in result and discussion section. Each data corresponds to the mean value for three independent observations.
We have used AFM (Park XE 100, South Korea) to observe the topography of prepared tensile specimens. In order to see the surface behaviour of the specimens, i.e. the distribution of polymer molecules in the blend and root mean square (rms) roughness (Rq) in nanometre scale; an AFM at non contact mode and room temperature is implemented. Images were scanned by using a cantilever of tip radius 10 nm (NCHR mode), a nominal spring constant of 0.05 N/m and a scanning rate of 0.9 Hz. The scans were made on 1000 nm × 1000 nm scale (except for 60HDPE/40PP, held at 750 nm × 750 nm) and repeated five different times on five different area of the polymer surface (tensile specimen) at a resolution of 256 × 256 pixels (except for 60HDPE/40PP, held at 192 × 192 pixels) , set point of 10 nm, amplitude of 20.85 nm. The rms roughness is determined [
rms = ( Z i − Z a v ) 2 N (1)
where, Zi is the height at a particular point on an image (nm), Zav is the mean height of all pixels in the image (nm) and N is the total number of pixels in the image. The maximum range is the height difference between the lowest and highest pixels in the image.
DOE is one of the powerful statistical techniques to study the influence of the controlling factor on output. All designed experiments require a certain number of combinations of factors and levels to be tested in order to observe the results of those test combination. In our project, Taguchi optimization method is employed to find out the optimum operating parameters influencing the tensile properties using MINITAB-16 software. Tensile strength at break point is considered as response. The operating conditions implemented are given in
Full-Factorial design is conducted in accordance with 5 level L25 (56) orthogonal array. The S/N ratios for maximum tensile strength at break (in MPa) under “larger is the better characteristic” are calculated as the logarithmic transformation of the loss function as shown below.
Larger is the better characteristic:
S N = − 10 log 1 n ( ∑ 1 Y 2 ) (2)
where “n” is the repeated number trial conditions and “Y” is the data pertaining to tensile strength at break point.
2D and 3D topography of the prepared tensile specimens are seen, and reported in
Attributing to the above discussion, the roughness of the prepared tensile specimen confirms to be minimum for 90HDPE/10PP polymer composite. The maximum rms roughness (Rq = 3.478 nm) is resulting for 50HDPE/50PP polyblend specimen and falls with increase in HDPE load. Data pertaining to the tensile properties conducted using the UTM, are reported in
The optimisation result shows that tensile strength is maximum for 50HDPE/50PP polyblend. The tensile strength decreases with increase in HDPE
Control Factors | Level | |||||
---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | Units | |
Composition (code: C) | 50 | 60 | 70 | 80 | 90 | Weight % (HDPE) |
Speed (code: S) | 30 | 40 | 50 | 60 | 70 | mm/min |
Composite | Speed, mm/min | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
30 | 40 | 50 | 60 | 70 | 30 | 40 | 50 | 60 | 70 | |
Stress, MPa | ||||||||||
At Yield | At break | |||||||||
50HDPE/50PP | 27.96 | 25.40 | 27.42 | 26.42 | 27.49 | 25.73 | 23.22 | 26.69 | 25.57 | 26.12 |
60HDPE/40PP | 23.74 | 24.53 | 24.85 | 25.60 | 25.78 | 14.39 | 13.07 | 6.26 | 6.85 | 6.44 |
70HDPE/30PP | 25.73 | 26.11 | 26.78 | 27.47 | 27.15 | 23.57 | 24.29 | 24.15 | 25.50 | 25.17 |
80HDPE/20PP | 24.20 | 24.60 | 25.00 | 25.15 | 25.65 | 20.63 | 20.40 | 22.10 | 21.79 | 23.99 |
90HDPE/10PP | 27.95 | 28.54 | 29.13 | 29.45 | 29.48 | 2.94 | 3.03 | 5.66 | 5.40 | 7.62 |
Composite | Speed, mm/min | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
30 | 40 | 50 | 60 | 70 | 30 | 40 | 50 | 60 | 70 | |
Strain, % | ||||||||||
At Yield | At break | |||||||||
50HDPE/50PP | 7.55 | 7.62 | 7.05 | 7.62 | 7.15 | 11.46 | 12.08 | 8.82 | 9.81 | 9.78 |
60HDPE/40PP | 8.25 | 8.24 | 8.22 | 8.07 | 7.74 | 438.18 | 438.64 | 295.80 | 141.05 | 92.132 |
70HDPE/30PP | 7.87 | 7.11 | 7.50 | 7.15 | 7.04 | 12.98 | 10.90 | 12.59 | 11.13 | 11.53 |
80HDPE/20PP | 8.54 | 8.20 | 7.70 | 7.59 | 7.54 | 15.19 | 15.26 | 12.82 | 12.15 | 11.07 |
90HDPE/10PP | 7.28 | 7.16 | 6.94 | 6.91 | 7.00 | 104.35 | 84.15 | 62.59 | 68.24 | 55.07 |
content in the composite. Tensile strength is minimum at the cross head speed of 40 mm/min and maximum at 70 mm/min. The
The tensile strength behaviour at break point is designed and reported in
The analysis of variance (ANOVA) is used to analyze the influence of tensile strength parameters like composition and speed. The ANOVA establishes the
relative significances of factors in terms of their percentage contribution to the response. This analysis was carried out for a level of significance of 5% (the level of confidence 95%). “P” value, less than 0.05 for a particular parameter, indicates that it has the major effect on the responses. From
A correlation between tensile strength at break point “TSb” (non-variable factor), composition and speed (variable factors) is derived by multiple linear regressions from equation No-3. From equation No-4, it is observed that the factor “composition” has a major impact on tensile strength followed by speed.
T S b = K 0 + K 1 A + K 2 B + K 3 C (3)
where, K i ( i = 0 , 1 , 2 , 3 , ⋯ ) is a model constant. The regression equation is given by
T S b = 36.8 − 0.287 COMPOSITION + 0.010 SPEED (4)
Finally a confirmation test was conducted to evaluate the design parameters influencing the response. The purpose of confirmation experiment is to validate the conclusions drawn during the analysis phase. For this, control parameters with optimal levels of 60HDPE/40PP for composition and 40 mm/min for speed are considered.
Our work remarks some salient features of the prepared polyblends. It is concluded that, the tensile strength decreases with increase in HDPE content. In summary, cross head speed is an important variable to decide the tensile behaviour. Tensile strengths at break point are quite identical at 40, 50 and 60 mm/min and the value is maximum at 70 mm/min. The results are quite obvious due to the earlier stated reasons. Surface roughness of the tensile specimens is
L25 (56) | Composition, Wt % HDPE | Speed, Mm/min | Tensile Strength At break, MPa | S/N Ratio, dB |
---|---|---|---|---|
1 | 50 | 30 | 25.73 | 28.21183 |
2 | 50 | 40 | 23.22 | 27.31986 |
3 | 50 | 50 | 26.69 | 28.52892 |
4 | 50 | 60 | 25.57 | 28.15631 |
5 | 50 | 70 | 26.12 | 28.33946 |
6 | 60 | 30 | 14.39 | 23.16423 |
7 | 60 | 40 | 13.07 | 22.32551 |
8 | 60 | 50 | 6.26 | 15.93981 |
9 | 60 | 60 | 6.85 | 16.72015 |
10 | 60 | 70 | 6.44 | 16.18581 |
11 | 70 | 30 | 23.57 | 27.44977 |
12 | 70 | 40 | 24.29 | 27.70927 |
13 | 70 | 50 | 24.15 | 27.65942 |
14 | 70 | 60 | 25.50 | 28.13217 |
15 | 70 | 70 | 25.17 | 28.01973 |
16 | 80 | 30 | 20.63 | 26.29251 |
17 | 80 | 40 | 20.40 | 26.19431 |
18 | 80 | 50 | 22.10 | 26.88824 |
19 | 80 | 60 | 21.79 | 26.76634 |
20 | 80 | 70 | 23.99 | 27.6006 |
21 | 90 | 30 | 2.94 | 9.378756 |
22 | 90 | 40 | 3.03 | 9.654614 |
23 | 90 | 50 | 5.66 | 15.0686 |
24 | 90 | 60 | 5.40 | 14.6527 |
25 | 90 | 70 | 7.62 | 17.64594 |
Level | COMPOSITION: C | SPEED: S |
---|---|---|
1 | 28.11 | 22.9 |
2 | 18.87 | 22.64 |
3 | 27.79 | 22.82 |
4 | 26.75 | 22.89 |
5 | 13.28 | 23.56 |
Delta (Δ) | 14.83 | 0.92 |
Rank | 1 | 2 |
significantly affected by the presence of PP; as dispersed phase in the composite. The polyblend having 50 weight % of PP and HDPE holds the highest rms roughness during moulding. Due to low material cost and traditional fabrication
Source | DF | Seq SS | Adj SS | Adj MS | F | P |
---|---|---|---|---|---|---|
COMPOSITION | 4 | 873.54 | 873.54 | 218.38 | 33.83 | 0 |
SPEED | 4 | 2.45 | 2.45 | 0.61 | 0.09 | 0.983 |
Error | 16 | 103.29 | 103.29 | 6.46 | ||
Total | 24 | 979.28 |
DF: Degree of Freedom; seq SS: The Sequential Sum of Squares; Adj SS: Adjusted Sum of Squares; Adj MS: Adjusted Mean Squares.
Optimal control parameters | ||
---|---|---|
Prediction | Experimental | |
Level | C2S2 | C2S2 |
S/N ratio, dB | 22.85 | 23.22 |
methods, the polymer composites may find suitable applications areas.
The authors declare no conflicts of interest regarding the publication of this paper.
Sutar, H., Maharana, H.S., Dutta, C., Murmu, R. and Patra, S. (2019) Strain Rate Effects on Tensile Properties of HDPE-PP Composite Prepared by Extrusion and Injection Moulding Method. Materials Sciences and Applications, 10, 205-215. https://doi.org/10.4236/msa.2019.103017