Polymer Recycling and Infrared Spectroscopy

Introduction

Since the start of the 21st century, the rapidly growing trend of globalization has led to a world with highly interdependent economy, which would inevitably result in global trades and distributions of natural resources. With the great expansion of worldwide population and increase in living standards, one of the major challenges we face is to fully utilize our existing resources while minimizing the amount of non-recyclable waste, and slowly shifting from a linear world economy towards circular economy.

Polymer materials used in industrial manufacturer and daily applications, especially those of engineering plastics, rubber, and polymer fibers, have brought a huge fortune for the society in the past, but also generating an enormous amount of polymer waste and biproducts.

Therefore, a huge subject of polymer research in the past decades has been focusing on the recycling and reusing of such material wastes to meet the sustainable development goals.

The reclamation of polymer/plastic could be done through reuse, mechanical recycling and chemical recycling.

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Reusing: the most convenient and common way of simply cleaning and reuse the polymer product. An example would be the reusing of plastic water bottle.

• Mechanical Recycling: waste materials are recycled into usable material without changing is chemical structure and property.
• Chemical Recycling: waste material would be reduced to its monomer form through chemical process, and re-polymerized into a new plastic product.

One of the recent advances in the field of polymer recycling is the repurposing of PC (polycarbonate) waste into Polysulfone material by placing it in alkaline conditions and transforming it to bis(phenolate).

While carbon salt is added, the bis(phenolate)participates in the polycondensation with bis(aryl fluoride). The product then could be used in reverse osmosis and water purification.

Another recent polymer recycling research reported is involved with the decomposition of PE into fuel. The process would convert few types of PE into diesel fuels and waxes. The transformation was based on alkane metathesis.

However, post-consumer plastic/polymer wastes are comprised of a large variety of different types of polymers with different shapes, sizes, physical and chemical properties. In most circumstances, products are composed of different material types. These factors need to be taken into consideration through the recycling process to ensure maximum purity of the recyclates (usually done by manual or automated sorting processes). Therefore, proper polymer identification techniques are needed to be used in the recycling process. Moreover, polymer characterization is an significant aspect when considering the industrial applications of these material by mostly using their physical and chemical properties in manufacturers and daily applications. In this experiment, the unknown polymers would be identified by differences in their physical properties, then confirm by IR spectroscopy.

Identification of Unknown Recyclable Plastics Through Physical Properties

The physical properties of the unknown plastics were first examined by tests on their relative density to water, estimated density, glass transition temperature (Tg), acetone solubility, and melting point approximation. The relative density of the plastic sample was determined by placing it in a beaker of water at room temperature. Observations were made for the plastics to record if they sink/float in a water (1g/cm3). The estimated density of the unknowns was calculated by mass/volume of the sample. The mass was recorded using an electric scale, and the volume is calculated after measuring the dimensions of the sample using a caliper.

The Glass Transition Temperature (Tg) of the sample was investigated by placing the sample in a beaker of boiling water. After the sample has been taken out, observations for the samples’ flexibility, and changes of their shape and size were recorded. The sample was then placed into a small amount of acetone, any observable changes of the sample were recorded. The melting point of the sample was estimated by using a melting point apparatus (Mel-Temps). The approximate range of temperature when the samples are observed to started melting were recorded.

Identification of unknown recyclable plastics through IR spectroscopy The unknown polymers were then analyzed through IR spectroscopy to validate its identity. The instrument used in this section was the Thermo Scientific Nicolet iS10. The IR spectra of the unknown samples are then compared to the IR spectra of each recyclable polymers.

Results

Physical Properties of Polymer Samples

The observations of tested physical properties of the two unknown samples of polymer would be shown by the following tables.

IR Spectroscopy Data:

Discussion

Physical Property Values

For this experiment, each of the recyclable plastics has unique physical and chemical properties to help us determine the identity of the polymer. In this section, results and observations from the previous section would be compared to literal values, and an early prediction of the identity would be made based on these comparisons.

Relative Density

Since sample 1 and sample 2 both floats in room temperature water, it suggests that they have a relative density lower than water, which mean it is lower than 1g/cm3.

Density Calculation

Calculated Density of Sample 1: 0.91g/cm3

Calculated Density of Sample 2: 0.86g/cm3

When comparing the relative density data and calculated density with literal values, one important factor to note is that the relative density would have a smaller percent error than the calculated estimated density because it’s quite clear to observe if the sample is floating or sinking in water. Our calculated density result matches with the observations from the relative density test. Therefore, we could mostly confirm that both of the unknown samples have a density lower than 1g/cm3.

According to the presented literal values of density, it could be concluded that only Polyethylene (For both HDPE and LDPE) and Polypropylene (PP) have density below 1g/cm3. Polystyrene has a density fairly close to one, thus it is hardly possible to directly observe its relative motion in water (sinking/floating). In addition, Sample 2 has a relatively lower calculated density compared to sample 1.

Therefore, the possible identifications of both of our samples could be PE, PP and PS. The identity of sample 1 could largely be PS or PE, and the identity of sample 2 could be PE or PP.

Glass Transition Temperature Estimation

After the samples are placed in boiling water to test their glass transition temperature (Tg), no observable color/flexibility/size changes were made during the experiment. However, both of the samples are fairly flexible at room temperature before the samples are tested. It could be possible that the samples have already reached their glass transition temperature. The following figure shows the Tg values for possible polymers.

Since both of the unknown samples have a relatively high flexibility at room temperature, the possible polymers include those that have a lower Tg value than around 20°C. The possible polymers with these properties contain Polypropylene (PP) and Polyethylene (PE).

Solubility Test

From the results obtained during the experiment, there were no observable changes made to both of our samples by placing them into acetone. The following table describes the resistance of the polymers to acetone.

H = High resistanceM = Medium resistanceML = Medium to low resistanceL = Low resistance (decomposes in short time)

Because there were no change observed for the period that the samples were put in acetone, both of the samples would have a High to Medium resistance towards acetone, which means they could potentially be one of PETE, PE, PP and PLA.

Melting Point

The estimation of the melting point of Sample 1 is:

Sample 1: ~280°C

Sample 2: ~240°C

The following table describes the melting point of possible recyclable polymers.

Plastic Name Melting Temperature(°C)

One important component to mention is that during the melting point test, the data obtained could be well-over the melting point of the polymer samples due to the difficulty to closely observe the melting process. The polymer displays a similar color to its background, so it’s challenging to be able to tell the exact melting point of our samples. These data we obtained for our samples are at the point when it’s certain that the melting process of the sample has begun. Due to the small variation of ranges of the literal values of melting point, the data collected could be less convincing when the identification is being done using the melting point data.

Conclusions

In this experiment, the identification of the two unknown samples of polymers are first processed by a series of tests of its physical properties. After the initial investigation has been made, the identification of the polymer is done by IR Spectroscopy.

The Sample 1 is identified to be PE based on its relatively low density(but higher than Sample 2), low Tg value, high resistance to acetone. The identification by IR spectroscopy is practiced by finding its major peaks at 2914,2846 and 1472 cm-1 that matches the C-H stretch and C-C &C-H scissor bend. The identification of the sample is then confirmed by the high similarities it displayed as the IR spectra of PE.

The Sample 2 is identified to be PP based on its relatively low density, low Tg value, high resistance to acetone. The identification by IR spectroscopy is practiced by finding its major peaks at 2949,2916,1451 and 1375 cm-1 that matches the C-H stretch and the t-butyl structure. The identification of the sample is then confirmed by the high similarities it displayed as the IR spectra of PP.

Improvements and Considerations

In this experiment, the measurement of the two samples melting point has been very unclear and led to results that cover a large variation of polymers which makes it hardly possible to identify the unknown sample based on its melting point. One of the major reasons that led to is the difficulty to directly observe the melting point of the samples, which could be improved in future experiments. Moreover, while testing the relative density of the samples. An additional step of using some liquid other than water could further improve the accuracy of our measurements (potentially ethanol or oil) and confirm our calculations of the samples’ density in the next step.

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