THE EFFECTS OF HIGH DENSITY POLYETHYLENE ADDITION TO LOW DENSITY POLYETHYLENE POLYMER ON MECHANICAL, IMPACT AND PHYSICAL PROPERTIES

Polyethylene materials are the most widely used polymers which we encountered everywhere in our daily life. Some of the advantages of the polyethylenes are the diversity of production types, corrosion resistance, electrical insulation and recyclability. The main production methods are compression molding, transfer molding, rotational molding, injection molding, gas-assisted injection molding, extrusion and blown film extrusion techniques. It is possible to increase the mechanical properties of low density polyethylene materials by addition of high density polyethylene materials. Both polymer has same monomers but their chemical structure and bonding properties are different. Their physical properties can change with their chemical structure and bonding properties. In this study, 25%, 50% and 75% by weight of high density polyethylene were added to low density polyethylene. Granules were pre-mixed with a mechanical mixer before production of the samples. Plastic injection molding machine was used for specimen preparation. Density, hardness, tensile test, three-point bend test, compression test, tear test and Izod impact tests were performed. Densities and hardness values of the polymer blends decrease by the increasing amount of low density polyethylene. In general, it was observed that the mechanical properties of the polymer blends increase as the high density polyethylene content increase.

LDPE polymer. Each formula was mixed at 200 rpm for 10 minutes. The samples were dried in the oven at 60 °C for 4 hours to remove moisture. Specimens were prepared in a single screw plastic injection molding machine (diameter: 35 mm, L/D ratio: 30, clamping force: 700 kN). Mixtures were injected into mold at 170-180-190-200 °C temperatures from feeding zone to nozzle zone. Mechanical mixer, oven and plastic injection molding machine are shown in Figure 1. Mixing ratios by weight of the samples are given in Table 1. Thirty specimen groups were produced from each formulas. The specimen group includes tensile test, flexural test, tear test, conical calorimeter test, compression test, notched and unnotched Izod impact test specimens ( Figure 2).

Tests and Characterizations
All samples were stored in Nüve brand TK252 model conditioning cabinet at 23 °C and 50% relative humidity for 40 hours according to ASTM D618 standard before tests ( Figure 3). Tests were performed at 23 °C and 50% relative humidity. The density measurement was made from 6 samples from each formulas. The mass and volumes were measured and the density of the samples were calculated by dividing the mass by volume according to ASTM D792 standard. Three-point bending samples were used for density measurement (Figure 4). Hardness measurements were carried out on 6 samples in X.F Shore-D hardness tester ( Figure 5) according to ASTM D2240 standard. Impact strength properties were determined with pendulum impact test machine. Izod impact resistance test was performed at Ceast Resil Impactor device ( Figure 6) according to ASTM D256 standard. 6 samples were tested from each mixture with 7.5 J to 25 J. The dimensions of the specimens are shown in Figure 7. The size of notch is 2.54 mm depth with an angle of 45° according to ASTM D256 standard. Tear strength test was performed on 6 samples from each mixture according to ASTM D624 (Type-T) standard. The tests were performed on Shimadzu brand AGS-X 100 kN model 100 kN load cell equipped static tensile-compression tester ( Figure 8) with a tensile speed of 50±5 mm/min. The dimensions of the tear test specimens were given in Figure 9. Tear strengths of mixtures were calculated from tear tests. Tensile tests were performed in Shimadzu brand AGS-X 100 kN model 100 kN load cell equipped static tensile-compression test device at 50 ± 5 mm / min tensile speed according to ASTM D638 (Type-IV) standard. The dimensions of the tensile test specimens are given in Figure 10. Six tensile test specimen were tested from each formulas. Tensile strength, rupture strength, modulus of elasticity according to tensile condition, elongation at break and toughness values of the material were calculated from the tensile tests. Three-point bend tests were performed in Instron 8801 model 50 kN load capacity dynamic tensile and compression test device according to ASTM D790 standard with crosshead speed of 1.4564 mm/min ( Figure 11). The sizes of the three point bend test specimen is given in Figure 4 and six specimen were tested from each formula. Bending strength and flexural modulus according to bending condition values were calculated from the tests. 5% strain were taken into account to find the flexural strength values. Compressive tests were performed in Shimadzu brand AGS-X 100 kN model 100 kN load cell equipped static tensile testing machine was used with crosshead speed at 1.3 mm/min according to ASTM D695 standard. The compressive test plates were shown in Figure 8. The sizes of the compression test specimen less than 3.2 mm thickness is given in Figure 12 and six specimen were tested from each formulas. Compressive strength, yield stress in compression and compressive modulus values were calculated according to the compression state. 10% deformation values were taken into account of the specimens for compressive strength values. The compressive modulus of HDPE and LDPE were calculated from the slope of the linear region in stress-strain curve.

Density
The results of the density measurements; mean values, maximum and minimum ranges were given in Figure 13(a). The mean and standard deviation of the density values were given in Table 2. The addition of LDPE to the HDPE polymer increase the density values. The density of pure LDPE was 0.888 g/cm 3 while the density of pure HDPE was 0.951 g/cm 3 . The addition of 25% mass of HDPE to LDPE polymer increases the density by 2% to 0.905 g/cm 3 . Increasing amount of HDPE addition to LDPE polymer 50% and %75 mass increases the density by %3 and %4 to 0.913 g/cm 3 and 0.922 g/cm 3 respectively.

Hardness
Hardness value of the polymers plays key role in their usage areas. The meaning of the hardness is resistance of a material to deformation, indentation, penetration and scratching. The hardness values of polymers vary depending on their crystal phases. HDPE is more crystalline polymer (51%) than LDPE polymer (38%) [8]. The more crystalline phases bring more rigid and hard physical properties. [5]. This results are the evidence of this explanation. It has been observed that LDPE polymer hardness about 42.9 Shore-D and HDPE was 63.9 Shore-D in the hardness test. Figure 13(b) shows the average hardness values, the highest and the lowest ranges. Mean hardness and standard deviation values are given in Table 2. 25% mass of HDPE addition significantly increase the hardness values of LDPE by 34% to 57.4 Shore-D. %50 mass of HDPE addition increase the hardness values of HDPE by 35% to 57.9 Shore-D. Further addition of 75% mass of HDPE to LDPE polymer increase the hardness values by 44% to 61.6 Shore-D. The hardness value of the LDPE polymer was much lower than HDPE polymer. It was observed that significant change in the hardness values by adding HDPE to LDPE polymer.

Izod Impact Test
In spite of hammer value reached 25 J, unnotched Izod impact specimens couldn't be broken. The hammer tilted the sample and passed the other side. In the notched Izod impact experiments, no breakage was observed 25% by mass of HDPE in LDPE polymer and pure LDPE. That's because of the soft and more flexible segments of LDPE in LDPE-HDPE blend. 25% LDPE and 50% LDPE were added to the specimens were broken with a 7.5 J hammer (Figure 13(c)). Due to HDPE more crystalline than LDPE polymer and more co-monomer decrease the crystal orientation. This disrupts the impact properties of the polymer [9]. The Izod impact strength of notched pure HDPE was found 8.5 kJ/m 2 . The addition of 75% and 50% mass of HDPE to LDPE polymer decrease the Izod impact strength values compared to HDPE polymer by %11 and %20 to 7.6 kJ/m 2 and 6.8 kJ/m 2 respectively. Mean Izod impact strength values and standard deviation values are given in Table 2.

Tear Test
The highest and lowest ranges of tear strength values of the blends are given in Figure 13(d).
Mean tear strength and standard deviation values are given in Table 2. The tear strength value of pure LDPE was found 45 N/mm and pure HDPE was found 132 N/mm. The addition of 25% HDPE by mass to LDPE polymer increase tear strength values by 16% to 52 N/mm compared to LDPE. It was observed by the addition of %75 HDPE to LDPE polymer increased by 28% to 57 N/mm. The addition of 50% HDPE by mass to the LDPE polymer decreased to the lowest tear strength value by 52% to 21 N/mm. In general, it has been observed that HDPE addition to LDPE polymer increase the tear strength values. In contrast to this the addition of LDPE to HDPE polymer decreases the tear strength values remarkable except the addition of both polymer 50% weight percentage significantly decreases the tear strength values. Because of the branching network of LDPE more complicated than HDPE polymer [5][6][7][8][9]. This detoriates the tearing properties.

Tensile Test
Tensile test is important to characterize the material mechanical behaviors from the stress-strain curves under tension state. This test was revealed to find suitable application and optimum usage ratio of LDPE and HDPE polymer blends. The tensile test curves of the LDPE-HDPE samples were given in Figure 14. Mean values and standard deviation values of tensile strength, rupture strength, modulus of elasticity, elongation at break and toughness values of the composites were given in Table 3.
The average, highest and lowest range values of tensile strengths were shown in Figure 15(a). It was observed that the tensile strength of LDPE polymer was found 7.7 MPa and HDPE polymer was 30.6 MPa. The addition of 25% mass of HDPE to LDPE polymer increases the tensile strength by 121% to 17 MPa. Furthermore, it was seen that 50% and 75% addition of HDPE to LDPE polymer decreases tensile strength by 142% and 213% to 18.6 MPa and 24.1 MPa values respectively. Same manner was observed in rupture strengths, the addition of HDPE to LDPE polymer were significantly increased the rupture strengths ( Figure 15(b)). It has been determined that the rupture strength of LDPE was 7.1 MPa and HDPE was 17.7 MPa. The addition of HDPE by 25% mass to LDPE polymer increased the rupture strength by 23% to 8.7 MPa. 50% and 75% mass of HDPE addition to LDPE polymer were increased this value by 76% and 108% to 12.5 MPa and 14.8 MPa values respectively.

Figure 14. Stress-Strain curves of LDPE-HDPE blends
The elasticity modulus of LDPE and HDPE polymer were observed 102 MPa and 920 MPa respectively. 25% addition of HDPE to LDPE polymer was increased the elasticity modulus significantly by 199% to 305 MPa. It was observed that 50% of HDPE addition to LDPE polymer was increased by 339% to 448 MPa. Furthermore, the addition of 75% HDPE to LDPE polymer increased the elasticity modulus by 413% to 523 MPa (Figure 15(c)). The elongation at break values of LDPE polymer was around 208%, while the elongation at break of HDPE polymer was 1249% (Figure 15(d)). By the addition of 25% HDPE to LDPE polymer, this value was seen to increase by 66% to 346%. The addition of 50% and 75% HDPE to LDPE polymer increased the elongation at break values by 222% and 322% to 670% and 878% respectively.
The thoughness values were observed from the tensile test curves. The areas under curves were calculated. The highest and the lowest ranges and mean values were given in Figure 15(e). Pure LDPE polymer thoughness value was calculated 1.6 J/mm 3 and HDPE polymer was 23.1 J/mm 3 . It was observed that the addition of 25% mass of HDPE to LDPE polymer increase by 106% to 3.3 J/mm 3 , 50% of HDPE to LDPE increases by 331% to 6.9 J/mm 3 and 75% of HDPE to LDPE increase by 531% to 10.1 J/mm 3 .

Three Point Bend Test
The mean and standard deviation values of flexural strength and flexural modulus of composites were given in Table 4. The addition of HDPE to LDPE polymer increased the flexural strength and flexural modulus values according to bending state. The average, highest and lowest range values of flexural strengths are shown in Figure 16(a). The flexural strength value of LDPE polymer was observed 7.8 MPa and HDPE polymer was 24.3 MPa. 25% by mass of HDPE addition to LDPE polymer was increased flexural strength value by 40% to 10.9 MPa. By mass of 50% and 75% additon of HDPE to LDPE polymer increased flexural strength by 71% and 119% to 13.3 and 17.1 MPa respectively.
The flexural modulus of LDPE polymer was calculated 345 MPa and HDPE polymer was 1109 MPa. The addition 25% by mass of HDPE to LDPE polymer increased the flexural modulus by 50% to 455 MPa, 50% mass of HDPE addition increased by 97% to 558 MPa and 75% addition increased by 241% to 875 MPa (Figure 16(b)). It was found that increasing amount of HDPE addition to LDPE polymer were increased the flexural strength and flexural modulus.

Compressive Strength Test
Compressive strength, compressive yield strength and compressive modulus of LDPE-HDPE blends mean and standard deviation values were given in Table 5. The compressive strength value of LDPE was found 10.7 MPa and HDPE was 23 MPa. The addition of 25% HDPE to LDPE polymer was increased the compressive strength values of composite by 50% to 16.1 MPa. 50% and 75% addition of HDPE polymer to LDPE polymer were increased by 60% and 63% to 17.1 MPa and 17.4 MPa respectively (Figure 17(a)).
Compressive yield strength values of LDPE and HDPE were calculated 4.2 MPa and 9 MPa respectively. The addition of 25% HDPE to LDPE polymer increased by 126% to 9.5 MPa. 50% HDPE increased 76% to 7.4 MPa and 75% HDPE increased by 157% to 10.8 MPa (Figure 17(b)). The highest and lowest ranges and mean values of compressive modulus were shown in Figure  17(c). The compressive modulus of LDPE and HDPE were found 185 MPa and 710 MPa respectively. Addition of 25% mass of HDPE to LDPE polymer increased the compressive modulus by 66% to 307 MPa. 50% HDPE addition increased by 107% to 383 MPa. Furthermore 75% addition of HDPE to LDPE polymer increased by %130 to 426 MPa.

Conclusions
In this study, it was carried out that mixing the low density and high density polyethylene in different ratios. The production method of the samples were performed by plastic injection machine. HDPE polymers were added to LDPE polymers in 25%, 50%, 75% by mass and some physical and mechanical tests were performed. As a result of the studies, the following conclusions were drawn.
• It was found out that the addition of HDPE to LDPE polymer increase the density, hardness and tear strength values. In contrast, the addition of the LDPE to HDPE polymer decrease these values.
• In Izod impact tests, although the hammer energy increased to 25 J, all unnotched specimens couldn't be broken. HDPE, L1H3 and and L1H1 specimens were broken in notched Izod impact tests. The addition of 75% and 50% HDPE to LDPE polymer decrease the Izod impact strength values compared to pure HDPE polymer. • It was found that the tensile curves obtained from the tensile test were in accordance with the pure LDPE and pure HDPE polymer tensile characteristics.
• Tensile strength, rupture strength, elongation at break, toughness and modulus of elasticity values were increased by adding HDPE to LDPE polymer from the results of the tensile tests.
• Flexural strength and flexural modulus values of the composites increased by the addition of HDPE polymer to LDPE polymer.
• It was observed that the addition of HDPE to LDPE polymer increases the compressive strength, compressive yield strength and compressive modulus. 25% and 75% of LDPE addition increase the compressive yield strength values above pure HDPE.
• In contrast to all results the addition of LDPE to HDPE polymer decreases the physical and mechanical properties of HDPE polymer.
• Thermal and combustion tests of these polymer blends will be performed in the next study.