Accelerated Degradation of Polymer

Ashley Jospeh
Department of Material Science, University of Rochester

 

Low Density Polyethylene Monomer Unit

 

Introduction

Low density Polyethylene(LDPE) is amongst some of the first polymerized materials introduced by chemical industries. The thermoplastic resin is crucial to commercial good distribution and is commonly employed in the production of single-use plastics such as plastic bags and various plastic containers. This specific grade of polymer is established to a close relationship with the manufacturing industry but also as one of the most common marine plastics contributing to ocean pollutants. Due to its extensive material properties, polymers such as LDPE have far longer life spans than most human beings which allocates for a long history of existence after disposal into natural habitats.

Depending on the grade of polymer and the conditions of the surrounding environments, scientist have found plastic degradation can still take up to hundreds of years. The plastics essentially become foreign agents now imbedded into ecosystems never fully degrading in a natural manner. As such the reality of macro and nano-plastics has become an unavoidable commonality in both our food and drinks due to easy navigation into our water and agriculture systems through connected dumping grounds.

Recognition of these facts is the driving force for the following project which concentrates on the study of LDPE plastic degradation with the support of the Scanning electron microscopy. The desire of this investigation is to explore what attributes to marine territory may have a greater impact on the eventual degradation of polymer resins such as LDPE's. The author of this text takes into consideration more specifically, degradation caused by natural events that take place in oceans and are magnified by effects of climate change. These influences constituting as ocean acidification, rising marine surface temperatures, and increase of ocean current movement driven by higher winds. As such the following focuses on distinct deformation patterns induced onto the LDPE surface morphology as a direct result of controlled conditions employed on to the plastic sample. The conditions chosen are of interest for possible correlation with degradation behaviors acquainted with plastics left in natural marine environments of our present day.

 

Sample Preparation

In order to view the plastic samples under the SEM, gold sputter coating is applied to have a conductive layer. Otherwise view of the plastics surface is bombarded with charging and several other deleterious artifacts. Four samples are prepared; three with their own individual treatments and one which undergoes all three treatments sequentially.

Control Sample

   

 Figure 1a: Control Sample at 23x

 Figure 1b: Control Sample at 114x

A plain LDPE sample with no exposed treatment is imaged in the SEM with secondary electron lens. The surface of the control appears undamaged from an eye view but a few injury marks are found along the surface. The most notable being a long stretched streak which was most likely produced from handling with tweezers. Some light scratches and some small particulates are also present across the surface. However, aside from these minor handmade injuries, the overall surface is predominately clear without any notably unique morphology.

Acid Treated Sample

     

 Figure 2a: Acid treated sample at 386x

 Figure 2b: Acid treated sample at 715x

 Figure 2c: Acid treated sample at 4.06kx

A solution of hydrochloric acid is prepared with a pH of approximately 4. The LDPE sample is then left in the solution in a sealed vial on bench top for one week. Afterward the sample is removed, rinsed with distilled water, dried, gold sputter coated (~60 thickness) and imaged in the SEM in the secondary electron lens mode with long working distance. Investigation of the surface revealed several droplets like formations spread about. A closer look at one of these droplet like formations reveals them to have crack propagations that appear to originate from the periphery.

Additionally, scattered within segments of the cracked formation are these small oval-like particles with sizes near to that of one micron. It is suspected that these particles may be macro-plastics that began to emerge as result of the treatment. Further verification of this possible event is carried out in the x-ray microanalysis portion of this study.

 

Heat Treated Sample

     

 Figure 3a: Heat treated sample at 246x

 Figure 3b: Heat treated sample at 287x

 Figure 3c: Heat treated sample at 310x

The heat treated sample is placed into a closed vial above a silicone oil bath set to heat at constant 97 degrees centigrade. The sample is left to heat for a one-week period and prepared for the SEM following the treatment. The change in morphology while not drastic does have one interesting deformation. In figure 3a, is a concave dent and surrounding it are several shrinkage lines which constrict and enclose the dent. This appearance is not seen throughout the sample surface but identifiable in some areas.

LDPE being a thermoplastic is pliable and upon heating is moldable to return to its original form or a new form. Therefore, the dents and shrinkage lines are likely correlated to this physical property of the resin. Some additional particulates are also identifiable amongst the sample surface as seen in figure 3c however the particulates are not in the same likeness as those detected in the acid sample. Therefore, they are likely attributed as salt deposited upon the surface from washing with distilled water.

 

Sonicate Treated Sample

     

 Figure 4a: Sonicate treated sample at 1.23kx

 Figure 4b: Sonicate treated sample at 2.89kx

 Figure 4c: Sonicate treated sample at 3.12kx

The sonicated sample is placed in a sealed vial filled with distilled water and then left in a sonicater to sonicate for five hours. SE2 images find a very unique transformation in the surface morphology where several open streak marks with small individual aggregated nuggets of plastic begin to layer alongside one another. The artifact is very extensive throughout the sample and heavily impacted the surface to appear nearly unrecognizable to that of the control sample.

 

Full Treatment Sample

     

 Figure 5a: Full treated sample at 807x

 Figure 5b: Full treated sample at 946x

 Figure 5c: Full treated sample at 1.23kx

The final sample undergoes an acid, heat, and sonicate treatment in the same fashion as the prior samples but with respect to shorter time spans. Acid treatment and heat treatment were implemented for just 24 hours and sonicate treatment was implemented for only 2 hours. The expectation of employing all three treatments is with the desire to see if unique patterns attributed to the specific treatments can be distinguished. However, this did not seem to be the case as the sample had more of a mix of the attributes of all three treatments. Figure 5a has streaks similar to want is seen from the sonicate treatment just not as pronounced; in addition to what appears to be a formation of the droplet with crack propagations like those in the acid treated sample. Instead of the rise of shrinkage formations similar to those in the heat treated sample a new form of deformation appears.

In figure 5b, vein like growths are seen protruding and branching outward across the space. This defect was not detected in any of the previous samples nor the control and is unique to its kind. Therefore, the full treatment reveals that a mixture of the conditions can mask singular deformation patterns and their overlap can also develop new patterns that are not easily comparable to a specific treatment.

 

X-ray Microanalysis

Monte Carlo Interaction Volume Simulation for LDPE

     

 Figure 6a: EDS spectra of control sample with BSD microgaph

 Figure 6b: EDS spectra of control sample with BSD microgaph

 Figure 6c: EDS spectra of acid sample with BSD microgaph

An x-ray microanalysis of the plastics is performed to reveal a more detailed survey on the chemical composition of the plastics. This is of interest to see if the presence of chemical additives such as Bis(2-ethylhexyl) phthalate could influence the observed changes in the surface morphologies of the treated samples. As expected the EDS spectra for both control and acid treated sample highest x-ray peak matches to carbon. However, the spectra did have additional detection of sodium, oxygen, calcium, chlorine, potassium, nitrogen, and sulfur. These elements can be attributed to lingering salt deposits in distilled water used to wash the samples prior to experimentation. As seen in figure 6b a matching backscatter electron image of a salt debris on the control sample to a taken EDS revealing it to be likely a calcium oxide salt. The presence of these salts are not apparent in the acid treated sample and this is likely due to the effect of the hydrochloric acid solution removing them.

Based on the EDS spectra's revealing mostly carbon and little to no oxygen there were likely no additional chemical additives incorporated in the samples. Therefore, the transformations in the plastic surface due to the treatments can be attributed fully to the character of the polymer resin with no additional influences.

 

Infrared analysis

   

 Figure 7a: IR-spectra of control LDPE; major peaks include: 2988-27544, 2693-2629, 1483-1436, 1379-1344, and 727-708 cm^-1.

 Figure 7b: IR-spectra of acid treated LDPE; major peaks include: 3040-2824, 2723-2634, 1483-1436, 11378-1355, and 738-708 cm^-1.

Infrared spectroscopy is performed on a control and acid treated sample in order to distinguish if a change of chemical functionality occurred in the polymer as a result of the acid treatment. Major peaks found replicate with relatively similar intensities between both samples. This is indicative that the original chemical structural integrity was retained despite degradation in the surface morphology taking place.

The major identifiable peaks also are in close ranges to those reported by manufacturers such as Bruker whom have performed IR on pure polyethylene samples. This also further confirms no major chemical mechanisms took place in the process of the acid treatment and likely not to the other treated samples as well.

Additional Considerations

The author of this study would like to note that the conditions used are highly controlled and also did not incorporate an even stronger influence to plastic degradation, UV-light exposure. Additionally, the influence of micro-organisms found in marine environments could not be implemented but can play as possible influences to the degradation process. This in mind there are other forms of treatment mechanisms that can be employed to further study the degradation behaviors of these thermoplastic LDPE resins not accounted for in this report.

Conclusions

Plastics are quite a versatile and tough material as seen from the results of this study. The SEM can however confirm degradation and deformation in the material that can arise but be undetected from a quick glance. The acid and sonicate treated sample observe to have the most distinct changes in surface morphology. This observation could suggest that polymers have stronger likely hood to degrade when immersed in slightly acidic conditions and stress from vigorous motion. The appearance of these artifacts as a result to changes of imposed environmental conditions suggests that natural occurrences of oceans undergoing acidification and rapid changes in current flow could have a strong impact on the plastic degradation.

Perhaps with the implementation of ESEM (environmental scanning electron microscopy) in further studies the development of these transformations leading to degraded surface morphology can be observed more readily and with greater detail. This could also potentially allow for furthering the understanding on any possible connections between the degradation of the plastic surface and the eventual dispersion of macro and nano-plastics into surrounding aqueous environments.

Acknowledgements

I would like to express my appreciation and gratitude to Brian McIntyre for not only assisting with this project but also teaching me the fundamental techniques and skills of the SEM.

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References

D Amelia, R. P., Gentile, S., Nirode, W. F., & Huang, L. (2016). Quantitative Analysis of Copolymers and Blends of Polyvinyl Acetate (PVAc) Using Fourier Transform Infrared Spectroscopy (FTIR) and Elemental Analysis (EA). Retrieved March/April, 2019, from http://pubs.sciepub.com/wjce/4/2/1/figure/1