The effect of varying frequencies of visible light on the breaking stress and strain of low-density

The effect of varying frequencies of visible light on the breaking stress and strain of low-density polyethylene

Keetan Southwell – St. Ives High School

The degradation of a polymer can be indirectly measured through the change in its breaking stress and strain before and after the degradative process. The purpose of this study is to investigate the relationship between the frequency of visible light low-density polyethylene (LDPE) is exposed to, and its breaking stress and breaking strain. LDPE test strips were exposed to three different frequencies of visible light. The stress-strain relationship of each test strip was then investigated to measure the effect that the light had on the plastic. It was found that light frequencies of 4.79*10¹⁴ Hz (red), 5.86*10¹⁴ Hz (green) and 6.43*10¹⁴ Hz (blue) caused a statistically significant reduction in the stress that the LDPE was able to withstand but had no significant impact on the strain that the plastic underwent before breaking when compared to a control group. This indicates that visible light can cause photodegradation.

1. LITERATURE REVIEW

Polymers are desired for their resistance to degradation under environmental pressures, such as temperature, acidity, and light. Polymers are also desirable for their mechanical properties, such as their strength and elasticity. Because of this durability, plastics accumulate in the environment (Ojeda 2013).

Polyethylene accounts for 37.2% of all plastic manufactured (PlasticsEurope 2018) and polyethylene, particularly LDPE is the main component in plastic bags (PlasticsEurope 2018). These two factors combined are why LDPE was chosen for this investigation over other polymers.

It is understood that ultraviolet light causes a reduction in the mechanical integrity of LDPE (Kelly & White 1996), however, an existing study relating visible light frequency to degradation in stress and strain could not be found. Visible frequencies of light were shown to reduce the mechanical integrity of LDPE when catalysed by metal oxides (Venkataramana et al. 2020). There is no significant weight loss of LDPE under shorter-term exposure (128 h) to visible light (Liu et al. 2013), in agreeance with the degradation curve provided by Schnabel (2014), showing that the degradation rate of polymers was reduced significantly after 100 h of exposure to electromagnetic radiation.

The tensile strength and strain of LDPE has not been examined following the short-term exposure to visible light (128 hours) and so this is the chosen time for this investigation.

The purpose of this investigation is to analyse how different light frequencies of blue light (ff = 6.43 ∗ 10 14 Hz), red light (ff = 4.79 ∗ 10 14 Hz) and green light (ff = 5.86 ∗ 10 14 Hz) change the breaking stress and breaking strain of the plastic under an exposure time of 128 hours. It is hypothesised that under exposure to visible light, all frequencies will cause a significant reduction in the breaking stress and strain of LDPE. The defined α for this investigation is 0.05.

There are several key structural differences between LDPE and other polyethylene (PE), variants. LDPE has shorter alkyl chained molecules with a greater number of branches compared to the longer chains of linear LDPE (LLDPE), or the even longer chains and reduced number of branches in highdensity PE (HDPE). (Khanam & AlMaadeed 2015). This gives LDPE a lower rigidity because of the more random packing of its polymer chains compared to HDPE (Khanam & AlMaadeed 2015).

Polymer degradation is a multi-step process in which a polymer undergoes a series of chemical changes. In photodegradation, the process is initiated by photons being absorbed into reactive areas of the polymer causing molecular excitement (Yousif & Haddad 2013). These more reactive groups are any functional groups that may be included in the polymer. These can include hydroperoxide groups, carbonyl groups, unsaturated carbon bonds or metal oxide imperfections in the polymer (Schnabel 2014). In theory, because polyethylene does not contain any of these active groups in its structure (CH2—CH2)n it should be stable, however, commercial LDPE such as the product used in this investigation has imperfections that occur in the manufacturing process (Rabek 1995). This allows it to be affected by visible and ultraviolet light photons (Rabek 1995). During photodegradation, photons cause the radicalisation of certain parts of the polymer chain in which carbons lose their hydrogens (Yousif & Haddad, 2013). This free carbon radical is allowed to oxidise, forming a peroxide and then a hydroperoxide. With the input of more energy in the form of photons, the radicals and the hydroperoxide splits into a hydroxyl group and a C-O* radical, allowing for chain scission to occur (Yousif & Haddad, 2013). This causes the break in the polymer chain (Reusch 2015) and in the case of polyethylene, a dicarboxylic acid is formed (Gewert et al. 2018) Generally, the length of a radical that is released during the degradation can have an alkyl chain of a length between 8 and 12, and 14 and 20 carbons with octanedioic acid, decanedioic acid and tetradecanedioic acid always forming when degraded by UV light (Gewert et al. 2018). These compounds were detected via leeching into water by Gewert et al. (2018).

Mechanical degradation occurs as the polymer chain is physically pulled apart, allowing the formation of two separate polymer radicals which can then form peroxyl radicals when they are oxidised (Yousif & Haddad 2013), similar to those formed under photodegradative processes. This then follows the same breakdown structure as photodegradation (Yousif & Haddad 2013).

Materials will be experimented on using tensile testing methods. Tensile testing involves the observation of the stress acting on a material (pressure on the material’s cross section) and the strain (change in length as a ratio between final length and original length) which can be plotted against each other to form a graph known as a stress-strain curve which is unique to each material (University of Arizona 2013).

2. SCIENTIFIC RESEARCH QUESTION

What is the relationship between the frequency of visible light that LDPE is exposed to and the breaking stress and breaking strain that the material is able to withstand?

3. SCIENTIFIC HYPOTHESIS

Under exposure to visible light, all frequencies will cause a significant reduction in the breaking stress and strain of LDPE.

4. METHODOLOGY

4.1 Preparation of Test Strips

Polyethylene test strips were cut out of a black plastic garbage bag with a width of 16 mm and an arbitrary length. The test strips were cut into dumbbell shapes to minimise areas of higher stress as per the findings from Feng & Jasiuk (2010) by using a razor blade and cookie-cutter for consistency. Each test strip out of 100 samples was assigned a random number and then using this number was placed into a random sample group. Test strips were then placed into completely dark boxes with LED light strips over the top. There were 25 strips per sample group: a dark control group, red light (ff = 4.79 ∗ 10 14 Hz), green light (ff = 5.86 ∗ 10 14 Hz), and blue light (ff = 6.43 ∗ 10 14 ). The test strips were irradiated for 128 hours in a dark room with a temperature between 14.5°C and 17.5°C.

4.2 Other Materials

A Gardenline 6.5-ton hydraulic log splitter with a 2200 W motor was used to break the plastic to maintain a constant strain rate. Each strip was placed inside a custom clamp designed for this experiment.

Image 1: Testing rig loaded with a test strip.4.3 Testing of plastic

Each test strip was placed individually inside of the testing rig (Image 1) in a random order based on the system used to number each test strip. The machine was then activated, and the test strips were pulled apart at a constant rate of 25.57 mms -1 (strain rate 1.17 s -1 ). When the strip broke, the machine was switched off and the clamps were reset with a new test piece. During the experiment, individual videos were taken of each test strip for later analysis.

4.4 Analysis of results

Measurements of the stress and strain acting on the plastic were taken every 1 of 3 a second. At each time, the force read from the digital scale and the change in its length was recorded. To measure the strain, the built-in positional indicator on Adobe Premiere Pro (see appendix) was placed exactly on the inside edge of a dot

drawn on each piece of plastic with one dot at either end to obtain the initial length and the final length of the piece of plastic.

5. RESULTS

Figure 1: Stress-strain curves for LDPE irradiated with 3 different frequencies and one control group. A: No light exposure, B: Green, C: Red, D: Blue. Obtained through the analysis of recorded videos by taking a point of stress and pairing it with a point of strain. The shape of each curve is due to the relationship between the stress and strain as a material deforms. As the graph increases linearly at the beginning, stress and strain are directly proportional, however, the material later begins to stretch and deform proportionally more than the stress on it increases. This is due to the amount that the material stretches. Finally, the small inflection at the end occurs as the material reaches its breaking point and snaps, with the upwards inflection being caused by a sudden increase in the stress on the material as it is unable to stretch any further. Stress was measured to an accuracy of 4 significant figures and strain to an accuracy of 5 significant figures.

Figures 2 and 3 shown below demonstrate the statistical significance of measurements taken. For the strain data in figure 1, it has been concluded that there is no relationship between the exposure to any frequency of light and the strain on the material (p > α). For the stress on the test strips, there was a proven link between the stress that the material was able to withstand before breaking and the frequency of the light that it was exposed to (p < α).

Figure 2: Average breaking strain for three different light frequencies. P-values for difference from the control mean: Red: 0.21; Green: 0.094; Blue: 0.18.

Figure 3: Average breaking stress for three different light frequencies. P-values for the difference from the control mean: Red: 2.7*10 -6 ; Green: 0.050; Blue: 9.6*10 -9 .

6. DISCUSSION

6.1 Significance of results

LDPE was irradiated with three different frequencies of light: 4.79 ∗ 10 14 Hz (red), 5.86 ∗ 10 14 Hz (green) and 6.43 ∗ 10 14 Hz (blue). After 128h of exposure, the LDPE was tested for its breaking stress and strain to determine if the frequency of light that LDPE is exposed to affects its mechanical properties. Outliers were excluded at a boundary of 2 standard deviations from the mean. After outlier exclusion, a one-tailed t-test was conducted comparing the coloured sample mean to the control group mean in both the stress and strain relationships (see Figure 2 and Figure 3 descriptions for confidence intervals).

For all breaking stresses, there was a significant reduction in the tensile strength of the material leading to the on average lower breaking stresses of 12.6 MPa, 13.3 MPa and 12.2 MPa for red, green, and blue, respectively compared to the initial 14.0 MPa from the control group (p < 0.05). For the breaking strains, there was no significant reduction by any colour of light (p > 0.05).

6.2 Comparing to the literature

Based on a study of the mechanical properties of LDPE conducted by Jordan et al. (2016), the true stress, acting on LDPE upon breaking was 19 MPa. However, this data uses true stress which considers the change in the thickness of the material, whereas the data collected her measures nominal stress, which does not consider the change in the width of the material (since stress = Force / Cross Sectional Area) and so achieves the control value of 14.0 MPa. It also uses a slightly different strain rate of 0.2 s -1 , compared to 1.17s -1 , although this variation should not make a large impact. Since the strain rate used in this investigation was higher than that used by Jordan et al. 2016, the results should have shown a higher breaking stress. This can be accounted for by several factors including the aforementioned slight difference in measuring the true stress versus nominal stress. There is a potential for a difference in crystallinity to impact the results (Jordan et al. 2016) and a difference in molecular weight to impact the results (Balani et al. 2015).

Crystallinity is the degree of ordered packing in the polymer molecules (Balani et al. 2015). Tighter molecule packing creates a stronger material since intermolecular forces are stronger.

Molecular weight also has the potential to slightly impact the results, but only if the molecular weight of the polymer used in this investigation was exceptionally low or the one used by Jordan et al. (2016) was exceptionally high (Balani et al. 2015). Since both these factors can impact the strength of the polymer, this limits the ability to compare the data gathered in this experiment to other studies.

6.3 Experimental Design

During the irradiation process, the temperature was kept relatively constant at between 14.5°C and 17.5°C with an average temperature of 16°C. This temperature was kept cooler to minimise the impact of thermal degradation, although degradation generally occurs at much higher temperatures, beginning significantly at roughly 700 K depending on the rate of heating (Das & Tiwari 2017). For the same reasons, testing was conducted at 20°C to limit the effect of temperature on the results, since according to Kelly and White (1996), the temperature at which the test is conducted does matter.

Stress was measured to 4 significant figures using a digital force meter and strain was measured to 5 significant figures using Adobe Premiere Pro’s position indicator (see appendix for more detail) so there is a high degree of precision in the measurements.

Each light colour had a small variation in its intensity (see appendix for intensity graphs). This means that exposure to each colour can only be compared independently to the control group and not between colours.

Certain tests were deemed invalid for several reasons. Primarily a test was deemed invalid if the digital scale tipped too far to the side, making the display unreadable. Tests could also be deemed invalid if the digital scale locked itself on a specific force reading, however, this only happened once.

6.4 Sources of error

The greatest potential for random error occurs from how each test strip was cut. During preliminary investigation, it was found that slightly compromising the plastic with a cut that was perpendicular to the direction of the applied force significantly decreased the tensile strength of the material. Because of this, when the plastic was cut out of the original sheet it was ensured that the razor knife did not create any cuts in the sides of the plastic, although, it should be noted that there is still the potential for this factor to affect the results. This was compensated for by the large sample size of 25 tests for each colour, including any outliers and tests deemed invalid. The sample size remained large enough to compensate for these random errors.

There is a slight capacity for systematic error in this investigation. The clamp that held the test strips applied a slight weight force to the test strips and so may have increased the observed stress on the material. It is also not discernible if this had any bearing on the results since as discussed under section 5.2, the values cannot reasonably be compared to other studies.

6.5 Future testing

For future testing directions, the molecular weight and the degree of crystallinity should be determined so the results can be compared to other studies. Given that Jordan et al. (2016) uses true stress, the true stress of the material should be calculated alongside the nominal stress of the material.

Since red light was shown to have a significant impact on the breaking stress of LDPE, future experimentation should extend this to lower frequencies including infrared light. Conversely, to investigate if photodegradation will, at any point, impact the breaking strain of LDPE. In future investigations, a lighter clamp should also be used as the weight of the clamp could increase the reading on the scale which would increase the observed stress on the material.

Future testing should also investigate the effect of visible light intensity of a specific frequency on the breaking stress and strain of LDPE as each light colour had a different intensity.

7. CONCLUSION

The relationship between the frequency of light that LDPE is exposed to compared to the breaking stress and strain was investigated. It was found that all three frequencies of light (red: 4.79 ∗ 10 14 Hz; green: 5.86 ∗ 10 14 Hz; blue: 6.43 ∗ 10 14 Hz) made a significant impact on the breaking stress of LDPE, however, there was no relationship between the breaking strain and the exposure to visible light for short time frames of 128 hours. Thus, the hypothesised result for this experiment that as the frequency of light increases, there will be a reduction in breaking stress and strain, is only partially correct.

The result serves as a measurement device to quantitatively track the photodegradation of LDPE under visible light. This result is not reflective of a similar test conducted by Liu et al. (2013), which used the lack of mass reduction in the polymer to show that there was no degradation under short term visible light exposure. Similarly, Venkataramana et al. (2020) showed that photodegradation by visible light would occur when catalysed. Despite this divergence from the existing literature, tests were conducted validly, therefore, it is reasonable to conclude that breaking stress is affected by exposure to visible light frequencies while breaking strain is not.

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