Identification of Polymers

Abstract

The simple identification of plastics and infrared spectroscopy are two very different methods in the analysis of plastics. I tested both of these methods in order to deduce the advantages and disadvantages of each. I found that the simple identification test is easy and cheap to complete, while the infrared spectroscopy results in much more detailed results, but takes much longer to complete and is more costly. For this reason I would recommend the use of the simple identification method as a primary test, with the infrared spectrometry as a secondary test, if the first proves to be inconclusive or if more detail is required.

Introduction

It is possible to identify polymers by a wide variety of different tests. Which test is used often depends on the level of detail required, which apparatus is available and the complexity of the material under test. A starting point is often that of simple identification of plastics due to the ease at which this may be carried out. The equipment used for this test is readily available due to the simplicity of the equipment. However, the use of additives may lead to inconclusive results and further testing may be required. For a more detailed analysis including which additives are present, infra-red spectroscopy may be used.

Simple identification uses our knowledge of the properties of the plastics tested. For this a number of different factors such as formation, appearance, density, flammability and smoke acidity must be considered. The methods require little skill to complete, but give a quick and simple result. Tables of the results from such an experiment may be compared with tables of properties in order to deduce the material through a combination of the tests previously mentioned [1]. Due to its simplicity and relative low cost, this is the test that forms the starting point. It may therefore reduce the reliance on costly, more time consuming equipment used in such tests as infrared spectroscopy.

If simple identification gives inconsistent results, infrared spectroscopy may be used. This is a non destructive method that will also highlight the use of any additives. It was first developed by Sir William Herschel in 1800 before it was fully developed during World War 2 [2]. It is subsequently readily used by chemists, physicists, biologists and engineers. It only requires a small amount of the sample and is very quick to perform [3]. The use of infrared rays relies on the principle outlined by the Beer-Lambert law. This states that the absorbance of the rays is dependant upon the chemical composition of a material. The outer electrons may be excited by the rays put into the material. This may only occur at defined energy levels. Since the test is concerned with polymers, infrared is used as it has a frequency of 0.8-25 µm, which corresponds with the frequency required for excitation of the relevant elements present [4]. As a result, the infrared light is absorbed by the electrons, which leads to the detection of lower intensity of given frequencies. Then using a footprint technique, the resulting graph of intensity against frequency may be compared to other results obtained previously and relevant conclusions drawn [3].

However the test pieces must be prepared in the appropriate manor, depending on the material involved. Materials with low carbon content in sheet form may be subjected to an attenuated internal reflectance (ATR) technique, whereby the test piece is in close contact with a crystal with high internal reflectance or diamond. If the sample is a

thermoplastic, then the melt pressing film technique may be applied [5]. This is where the plastic is melted on a high heat to approximately 20˚ above the softening temperature [6]. The plastic is then compressed between two surfaces in order to create the required thickness of 3-50µm. Finally, pyrolysis is used for rubbers. This is where the rubber molecules are broken into small fragments by the onset of heat, before being transferred onto sodium chloride plates. This may not always produce a spectrum of its parent compound, but using a fingerprint, may be used to deduce the material [2].

Both techniques have their strengths, while also containing several limitations. I will now compare and contrast these techniques to highlight these issues.

Experimental

For the simple identification of plastics, the first stage is to simply viewing the specimen, and working out how the piece may have been manufactured. The hardness and rigidity as well as the bounce and odour should be considered.

A density measurement may then be carried out. A small piece of the specimen must be cut off and dropped into a liquid such as water using the floatation process [1]. The test piece must be pushed beneath the surface and to break the surface tension.

The heating test requires the specimen to be heated gently first. A wet litmus paper should be held above the heated specimen in order to determine whether the vapour is acidic, neutral or basic. Care must be taken when smelling the odours given off during heating due to safety issues. The specimen should then be subjected to a naked flame in order to light it. This should be used to determine whether or not it is self extinguishing. The colour and odour of the flame should also be observed. Finally a Beilstein test should be done using copper wire [1].

For the infrared spectrophotometry test, the specimen must be in very specific sizes and forms. The tea spoon was tested using the attenuated total reflectance technique. The melt pressing film method was used for the coffee jar lid.

Results

Sample A seemed to be an injection-moulded thermoplastic. The fact that a smooth cut may be made to it leads to the conclusion that it is highly amorphous. The tray sank; as a result we know that this isn’t a polyolefin. The result that it reformed to a previous shape when heated gently confirms that the sample is in fact a thermoplastic. When tested with red litmus paper it indicated that the fumes created were of an acidic nature. This combined with the result of a green flame leads us to believe that an acidic form of a halogen is released which is common with polyvinyl chloride (PVC).

When Sample B was subjected to the density test, it was lighter than water so is a polyolefin. When ignited, there was a blue base to the flame with burning droplets and smelt like candles. This means that it is LDPE, HDPE or PP. Since it is relatively stiff, LDPE can be ruled out and the waxy feel would suggest that it was likely to be PP although this is not a conclusive result.

Sample C was injection moulded and showed whitening when flexed which would suggest that we are looking at a thermoplastic. Again, it sinks so is not a polyolefin and because it merely softens when heated we know that it is a semi crystalline thermoplastic. The Beilstein test did not highlight any halogens so PVC may be ruled out. When ignited the flame bubbles and burning drops come off suggesting that this is in fact polystyrene. Another trait of PS is that it is very brittle, which was conducive to Sample C.

Sample D was also an injection moulded Thermoplastic. When cut, it gives a nice soft edge, which again concludes it is highly amorphous. It is lighter than water so is obviously a polyolefin, and when burned, there is a blue based flame with burning droplets and smelt like candles. This leads us to believe it is a Poly-styrene, LDPE or HDPE.  Looking at how it cuts leads us to believe it is LDPE.

All of the test pieces were then subjected to the infrared spectrometry test. The resulting transmitions and resolutions are shown by the graphs. By comparing with previous experiments and using a fingerprint technique, it was possible to deduce that the suggestions made from the Simple ID tests were correct. [6].

Discussion

The simple identification test was shown to be a very effective way in which to identify materials. However, the use of additives may cause inconsistent results. For this reason the material should be tested using infrared. This gives a fingerprint of the material. However, both tests have advantages and disadvantages which must be considered before taking the tests.

The simple identification test is, as its name suggests, simple. If the analysis of the additives contained within a material is required then a more complex chemical or physical method must be used. As well as this there are limitations in identifying the plastic if the sample is not a homopolymer [1]. For example, the copolymer of PP would result in contrasting results, showing some characteristics from each of its polymers. Additional care must be taken when viewing the sample, as it may be layered with more than one polymer.

However, it does have a lot of benefits over other tests. The test may be carried out with great speed due to its simplicity, with the material being identified within minutes of beginning the testing some cases. The above results for the Sample A and Sample B were of this nature. Other tests take much longer such as the infrared spectrometry, which may take eight minutes or more to complete. There is also very little preparation time with the simple identification method. For each test, the sample may be used in any form, including that of a finished product.  A part may simply be cut off for each test, except the Beilstein test, where the sample may simply be singed with the hot copper wire.

The infrared test is an alternative that may be used if the simple identification tests do not conclude the correct results. In fact, the infrared spectroscopy has a much larger scope and may bring more results about the material than the simple identification. As well as the identification, the footprint method may be used to identify all major components, additives, stereoregularity, crystallinity and copolymer composition [5]. Therefore the test has a high

yield of results relevant to the effort input to undertake these tests. It gives much more definite results than the simple identification test, which may lead to it being the preferred test to identify a plastic. In comparison with tests

other than the simple identification, the preparation of test pieces is relatively simple and the sample is not required to be in a solution [3].

There are drawbacks to this technique however. The test piece must be of prepared in specific ways depending upon how thick, reflective, scattering it is and whether or not the specimen is in sheet form. It is also dependant on how crosslinked the sample is. For example a vulcanised rubber which is by definition highly crosslinked, will need to be sat in a solution in order to break it down before being heated until it takes a sheet form. This leads to another limitation; the material type must be known in order to assess how to produce the sample. The test piece must be 3-50µm thick for the melt pressing film technique [5]. Since the spectroscopy simply highlights the composition and not the behaviour of the material, HDPE and LDPE may not be differentiated. They both have the same composition; yet behave differently [5]. The final consideration is that of the background air. As the test is not carried out in a vacuum, the constituents within the air are highlighted in the test. It is up to the scientist or engineer to recognise these elements using the footprint technique and disregard them [2].

Conclusion

1. The simple identification technique is highly beneficial as a preliminary test.
2. The simple identification test may be preferred where time and cost are limiting factors
3. The infrared spectrometry is more detailed than the simple identification technique.
4. The FTIR is a lengthier process.

Reference:

[1]        Dietrich Braun ‘Simple Methods for Identification of Plastics’, 1982, Macmillan Publishing Co., New York

[2]        A. Lee Smith ‘Applied Infrared Spectroscopy: Fundamentals, Techniques, and Analytical Problem Solving’, 1979, John Wiley & Sons, Inc., New York

[3]        Edward A. Collins, Jan Bares and Jr. Fred W. Billmeyer ‘Experiments in Polymer Science’ 1973, John Wiley & Sons, Inc, New York

[4]        J. Urbanski et al. ‘Handbook of Analysis of Synethetic Polymers and Plastics’, 1977, Ellis Horwood Ltd., Chichester

 [5]       James Mark, Kia Ngai, William Graessley, Leo Mandelkern, Edward Samulski, Jack Koenig and George Wignall ‘Physical Properties of Polymers – Third Edition’, 2004, Cambridge University Press, Cambridge

[6]        Dieter O. Hummel ‘Infrared Spectra of Polymers in the Medium and Long Wavelength Regions’, 1966, John Wiley & Sons Ltd., New York