Dear Dr. Navid Taghavi,

to measure the water contact angle of PE plastic bag that, due to its structure, has a not flat surface, I would select some less altered areas, I would cut them (of a suitable size for your instrument) and I would fix them on a flat support, using the method you think is most suitable (adhesive tape, metal clips, etc.).

Relating to the second question, I suggest you to have a look at the following papers:

- Increased Wettability and Surface Free Energy of Polyurethane by Ultraviolet Ozone Treatment

Ping Kuang and Kristen Constant

In book “Wetting and Wettability” –Chapter 4 – (2015)

Available at: https://www.intechopen.com/books/wetting-and-wettability/increased-wettability-and-surface-free-energy-of-polyurethane-by-ultraviolet-ozone-treatment

-Systematic Oxidation of Polystyrene by Ultraviolet-Ozone,Characterized by Near-Edge X-ray Absorption Fine Structure andContact Angle

Robert J. Klein, Daniel A. Fischer,‡and Joseph L. Lenhart

Langmuir,24,8187-8197 (2008)

Available at: https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=854034

- Effects of UV Irradiation and Plasma Treatment on a Polystyrene Surface Studied by IR−Visible Sum Frequency Generation Spectroscopy

D. ZhangS. M. DougalM. S. Yeganeh

Langmuir, 16(10) (2000)

Available,as abstract, on RG at: https://www.researchgate.net/publication/231681888_Effects_of_UV_Irradiation_and_Plasma_Treatment_on_a_Polystyrene_Surface_Studied_by_IR-Visible_Sum_Frequency_Generation_Spectroscopy

Best regards, Pierluigi Traverso.

Dear Pierluigi Traverso

Many thanks for your time answering my questions, and many thanks for your suggestions/ comments. I will go through the papers you mentioned.

Best regards,

Water contact angel for thin film polymers general done by drope water at surface and by using microscope you can know if that material will be hydrophobic or hydrophobic and get measured for drope angle at surface of polymer specimen.

Dear Ali,

Thanks for your answer. I knew that using goniometer can show the angle. The problem is that the UV treated polystyrene instead of getting a lower contact angle had a higher angle compared to untreated plastic. Another problem is that from 8 samples which treated in the same condition when measuring the contact angle each shows a huge difference. For example, polystyrene sample 1 has a contact angle of 111 and samples two has 93 or sample three had 132.

Dear Pierluigi Traverso I read the paper you mentioned, unfortunately, they did not talk about the problem I have, they said UV make PS more hydrophilic (which is true and as I am also expected). However, the result I got was vice versa.

Dear collegue analyze the conditions of reaction treatment with UV (time, intensity). If the material became hydrophobic significs that all OH groups are interacting or their disponibility changed. These variations of values indicates that modifications happened in heterogeneous way.

Laura Maryoris Aguilar Many thanks for your time. I did not get exactly what you meant. Can you please explain in another way. All of 8 pieces of my PE and PS plastic samples were treated with UV for 3 days on each side (6 days in total). After analysis with water contact angle, the angle of PS sample 1 was 111, sample 2 was 97, sample 3 was 123, sample 4 was 90 and so on. However, the control had 85, Based on these results, plastic became more hydrophobic, which should not and instead they should be more hydrophilic due to the free-radical formation on plastic after UV treatment. So what is your explanation in this regard? Thanks.

According to the values differences maybe it is normal variation in this type of measurements. I was related with this properties when I was in oil Institute, but not working with plastic, working with rocks. Perhaps you must think about if other reaction is taking place in your material or to analyze samples of other source.

I determined a method to measure the contact angle of PE plastic fils. Since PE is not flat and has a lot of bumps and cavities, therefore it is very hard to measure the correct angle. One method which I found helpful is to rub a soaked tissue paper on a clean microscope slide. Then slowly put the PE plastic on the slide where it is soaked with water. The water on slide pulls the PE plastic and it becomes flat enough for measurement. In this way, you can determine the right contact angle. Please try and share your comments.

Regards,

For PE and PP films (including heavily oxidised/fragmented films), I have had some success with applying double-sided tape to a microscope slide and rolling the film flat so that it lies flat across the slide.

For the PS foam, could it be that you are losing degraded material from the surface, resulting in reduced oxidation of the retained surface? We found that water spray alone was enough to remove degraded particles from the surface.

Dear, Perry Higgs

Thanks for your suggestions. Unfortunately, the double-sided tape did not work in my case. Since the PE films were thin, pasting PE on the tape and then remove it from the tape could stretch and remove some part of the plastic (which could affect the pre-weighted weight). As I am going to use them later in the biodegradation test, I could not take that risk. Regarding the PS films, after UV pre-treatment, their colour changed from white to yellow (indicating oxidation of PS). However, once a drop of water contacted to the surface of PS films, the colour turns back to white again. Does it mean that oxidation will no longer exist on PS films?

Moreover, these plastics are also needed to be disinfected with ethanol (75%) and water later to be used for biodegradation test. So the statement you mentioned " even spray water can remove the oxidation result) meant that disinfection by ethanol and water can eliminate my UV -pre-treatment result on PS films?

Thanks if you can share your idea/ experience in this regard.

The colour loss is a concern since it suggests that a change is occurring. Does the colour return when the sample is dried again?

Physical erosion of the surface - particularly of foamed material will likely disproportionately affect the most degraded parts. Our experience is that degradation of PS foam is limited to the surface but is easily removed by erosion to reveal a fresh surface underneath.

You might consider FT-IR analysis before and after treatment to look for loss of carbonyl oxidation products.