Comparing the mechanical properties of different types of polyethylene materials

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1.0 INTRODUCTION:

Polymer is a substance composed of molecules with large molecular mass consisting of repeating structural units or monomers, covalently chemically bonded (strong bond) with secondary weak 'Van der Walls' bonds.

Polyethylene is a polymer consisting of long chains of monomers ethylene. However, in the polymer industry; the name is shortened to PE. The ethylene which is the universally used common name of ethene C2H4 has two CH2 groups connected by a double bond CH2= CH2, thus Polyethylene are characterised by toughness, excellent chemical resistance and electrical properties, low coefficient of friction, near-zero moisture absorption and ease of processing. These wide ranges of properties, some of which are unattainable from other materials and in most cases, at an affordable cost; make them most preferable materials in engineering design and production.

Some of the benefits derive from the use of polymers in engineering design includes reduction in components through the design with plastics, reduction of many finishing operations, weight saving, noise reduction and in some situations elimination for the need for lubrication of some components (2).

PE can be created through polymerization of ethene. Some of the commonly used polymerization processes include radical, anionic addition, cationic addition and ion coordination polymerizations.

Polyethylene is classified into different categories according to density and branching as (1):

i) Low, examples are: low density Polyethylene (LDPE) and linear low density polyethylene (LLDPE). Have density range of 0.92-0.93 kg/m3

ii) Medium, example is medium density polyethylene (MDPE). Has a density of 0.94 kg/m3

iii) High, examples are: high density polyethylene (HDPE) and high density cross-linked polyethylene (HDXLPE). Have density range of 0.95-0.96 kg/m3

The main objective of this report is to examine and compare properties of low density and high density Polyethylene materials with a view of obtaining a result that offers predictability and repeatability of analysis of plastic tensile test of the specimens.

1.1 SCOPE:

The low density polyethylene (LDPE) and high density polyethylene (HDPE) are range of standard test specimens and were carefully selected for this analysis.

2.0 THEORY

One of the essential requirements for studying strength and properties of material is an understanding of the way in which materials behave under load and condition of stress.

The Young's modulus allows the behavior of a material under load to be calculated. For instance, it can be used to predict the amount a tensile material will extend under tension. It's usually obtained as the ratio of tensile stress to tensile strain. Mathematically it can be expressed as:

The SI unit of modulus of elasticity (E) is the Pascal. Given the large values typical of many common materials, s are usually quoted in megapascals or gigapascals. An alternative unit KN/mm² gives the same numeric value as gigapascals.

If a material can withstand extensive deformation without failing under high tensile stress, it is considered ductile. Ductility can be quantified by the fracture strain, which is the strain at which a test specimen breaks during a uniaxial tensile test and it's dimensionless. Reduction in cross-sectional area and elongation are common indices of ductility.

3.0 DESCRIPTION OF APPARATUS

The instrument used for the analysis is Tensometer. This is a device that is used to evaluate how much material stretches under strain (Young's modulus). The specimen is a rectangular strip of length l, width w and thickness t mounted to the machine vertically, with its lower end held rigidly, and its upper end subjected to a constant load. The measurements taken are force and extension and a tensometer gives directly the load-extension curve and not the stress-strain curve. A tensometer is connected to a computer which gives the graphical reading (output) electronically. 2.0 shows a pictorial presentation of the device.

It is possible to use a tensometer to test a material to its breaking point. The device is designed mainly to house single samples rather than a plurality of samples. The results gained from a tensometer are commonly used to plot stress-strain curves for materials that enable them to be compared without the confusion of sample dimensions (1).

4.0 PROCEDURE:

1. The specimens provided have distinctive colours of red for high density polyethylene (HDPE) and yellow for low density polyethylene (LDPE) and were identified as specimen 1 and 2 respectively.

2. The gauge lengths for the specimens were measured and the thicknesses and widths of the specimens at three (3) different locations were measured with a micrometer screw gauge and the mean value of the cross-sectional areas were calculated and results tabulated in Table 1.

3. Then, the specimen was placed between two grips of the chuck and force of 5KN was applied at the extension rate of 110mm/min. To avoid pre-stressing the specimen prior to the test, the tensometer was adjusted to zero reading before mounting the specimen onto the machine.

4. When the tension (load) button was pressed, the tensometer pulled the specimen until it's broken; the computer connected to the equipment provided directly the load extension curve as shown in s 2 and 3 for HDPE and LDPE respectively.

5. On the computer keyboard the F5 key was pressed at the point of breakage so as to record the results. The graphical result was saved by pressing F10 and later printed by pressing F9.

6. The width and thickness of the samples were measured at points of fracture and hence the cross-sectional areas at the break point were calculated.

5.0 RESULTS:

5.1 EXPERIMENTAL DATA

The measured dimensions of the specimen are tabulated as follows:

Table 1 Dimensions (Thickness and Width) of the specimens

Test

Specimen 1 (HDPE)

Specimen 2 (LDPE)

Thickness (mm)

Width (mm)

Thickness (mm)

Width (mm)

1

3.930

10.064

3.905

9.965

2

3.967

10.042

3.913

9.997

3

3.957

10.095

3.935

10.023

Average(mm)

3.951

10.067

3.918

9.995

Average CSA (mm2)

39.775

39.160

Based on the measured values and the readings obtained from the graphs, the following parameters were calculated and the results obtained were analyzed:

5.2 CALCULATED RESULTS

SPECIMEN 1

Gauge length =

67.00 mm

Cross sectional area CSA =

39.775 mm2

Cross-sectional area CSA @ Fracture =

2.781 x 2.985 = 8.301 mm2

Fracture load =

422.36 N

17.4 N/mm2 = 0.174GPa

SPECIMEN 2

Gauge length =

67.00 mm

Cross-sectional area CSA =

39.160 mm2

Cross sectional area CSA @ Fracture =

3.327 x 8.526= 28.366 mm2

Fracture load =

383.30 N

7.15 N/mm2 = 0.072GPa

5.4 RESULT ANALYSIS:

* The Modulus of elasticity of the high density and low density polyethylene materials were calculated through the values of the parameters obtained during the experiment. The High density Polyethylene has a modulus of elasticity (E) of 0.174 GPa, while the low density Polyethylene has 0.072 GPa.

* These values indicate that the high density PE has a high stiffness than the low density PE.

* The true fracture stress is higher in the HDPE than the LDPE, but their ductility is nearly the same, this is because their molecular structure is similar. Howver, area reduction is significant in the HDPE than in the LDPE, thus high force is needed to cause elongation and to overcome the stiffness of the HDPE.

* LDPE has a lower nominal yield of 10.72 N/mm2 when compared to 26.45 N/mm2 of HDPE.

Table 2 Comparison of mechanical properties of the samples

Specimen

True Fracture stress (N/mm)

Tensile Ductility

Nominal Yield (N/mm)

Young's Modulus (GPa)

HDPE

50.88

1.52

26.45

0.174

LDPE

13.51

1.5

10.72

0.072

6.0 DISCUSSIONS

From the results obtained, it can be observed that the maximum strength of HDPE and LDPE are 26.45N/mm2 and 10.72N/mm2 respectively, as against the standard values found in literature as 25 N/mm2 and 10 N/mm2 for HDPE and LDPE respectively. This variation can be attributed to the high extension rate applied on the specimens during the experiment which is 110mm/min, i.e. in each second; the specimen is stretched by 1.83mm, though in the literature; the recommended extension rate is 50 mm/min. Also the beam load used is higher than the manufacturer's specification of 1.2 kN.

Also, deformations at break, referred to as ductile behavior which is associated with yield and plastic flow; is higher in HDPE than in LDPE, as can be seen from the tensile ductility values.

It is interesting to note that despite the disparity with the standard; most of the values obtained from the experiment are still they within the range and depict the nature of the properties calculated. This signifies the efficiency of the equipment used in the experiment.

7.0 CONCLUSION

Within the limit of experimental error, the modulus of elasticity (E) of high density and low density polyethylene was found to be 0.174 GPa and 0.072 GPa respectively; hence they are considered to be very strong material but not tough. The comparison between the samples' mechanical properties has also shown that; the high density polyethylene is more ductile than the low density polyethylene.

REFERENCES

1. http://en.wikipedia.org/wiki/Polyetylene last modified 23 October 2007 (accessed on November 4th 2007)

2. Module 2, Laboratory Manual on comparison of the mechanical properties of Polymers, Department of Aeronautical, Civil and Mechanical Engineering. University of Salford.

3. Plastic Specimens, TO Education and Training Ltd

4. http://patentstorm.us/resources_tensometer last modified September 2007 (accessed on November 4th 2007)

5. A guide to IUPAC Nomenclature of Organic Compounds, Blackwell Scientific Publications, Oxford (1993)

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