Oxidation of Polyethylene: A Comparison of Plasma 

and Ultraviolet Ozone Processing Techniques 

by Nicole Zander, Daphne Pappas, and Ben Stein 

ARL-TR-4701 January 2009 

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ARL-TR-4701 January 2009 

Oxidation of Polyethylene: A Comparison of Plasma 

and Ultraviolet Ozone Processing Techniques 

Nicole Zander 

Dynamic Science, Inc. 

Daphne Pappas 

Weapons and Materials Research Directorate, ARL 

Ben Stein 

Oak Ridge Institute for Science and Education 

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Oxidation of Polyethylene: A Comparison of Plasma and Ultraviolet Ozone 

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Nicole Zander,* Daphne Pappas, and Ben Stein†

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Dynamic Science, Inc., 8433 N. Black Canyon Hwy., Phoenix, AZ 85021 

†

Oak Ridge Institute for Science and Education, P.O. Box 117, Oak Ridge, TN 37831-0117 

14. ABSTRACT 

Plasma and ultraviolet (UV) ozone treatments were compared as methods to functionalize ultrahigh molecular weight 

polyethylene films in order to make them more compatible with materials or reactive for subsequent surface chemistry. 

Oxidation states and atomic compositions were determined using x-ray photoelectron spectroscopy, while surface energy and 

bond stability were probed with contact angle goniometry. Both modification techniques increased the surface energy of the 

polymer, and the bonds formed were found to be stable over a period of 30 days. Plasma treatment times were much shorter 

than UV ozone; thus, plasma was determined to be a more efficient surface modification technique. 

15. SUBJECT TERMS

polyethylene, plasma, oxidation, XPS, contact angle, UV-ozone, surface energy 

16. SECURITY CLASSIFICATION OF: 

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Nicole Zander 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Background 1

2. Materials and Methods 1

2.1 Materials ..........................................................................................................................1

2.2 Methods ...........................................................................................................................1

2.2.1 Film Preparation ....................................................................................................1

2.2.2 Plasma Modification..............................................................................................2

2.2.3 UV Ozone Modification ........................................................................................2

2.2.4 Surface Analysis ....................................................................................................2

3. Results and Discussion 3

3.1 UV Ozone Modification of UHMW PE Films ................................................................3

3.2 Plasma Modification of UHMW PE Films .....................................................................9

4. Conclusion 12

5. References 13

Distribution List 14 
iv 

List of Figures 

Figure 1. Water contact angle of untreated PE (left) and plasma-treated PE (right). .....................4

Figure 2. Water contact angle results for UV ozone-treated PE in terms of exposure time. ..........4

Figure 3. XPS overlay of oxygen peaks of UV ozone-treated PE: 15 s (red), 10 min (green), 

20 min (purple), and 45 min (blue). ...........................................................................................5

Figure 4. Relative atomic percent carboxylic acids vs. UV ozone exposure time. .........................6

Figure 5. XPS C1s spectrum of 15-s UV ozone-treated PE. ..........................................................7

Figure 6. XPS C1s spectrum of 10-min UV ozone-treated PE. ......................................................7

Figure 7. XPS C1s spectrum of 20-min UV ozone-treated PE. ......................................................8

Figure 8. XPS C1s spectrum of 45-min UV ozone-treated PE. ......................................................8

Figure 9. Water contact angle aging study of UV ozone-treated PE. .............................................9

Figure 10. Aging study of plasma-treated PE films. .....................................................................11

Figure 11. Effect of plasma treatment time on carbon and oxygen atomic percent 

concentrations. .........................................................................................................................11

List of Tables 

Table 1. Surface tension (LV), dispersive (

d

), and polar (

p

) components of the test liquids. ......2

Table 2. Initial surface energy of UV ozone-treated PE. ................................................................5

Table 3. Analysis of the C1s peak for the UV ozone-treated UHMW PE films. ...........................6

Table 4. Surface energy data for He-O2 plasma-treated PE. .........................................................10

Table 5. Analysis of the C1s peak for the plasma-treated UHMW PE films. ..............................12
1 

1. Background 

Increasingly, polymers are replacing metals and other conventional materials in high-tech and 

high-performance materials due to the advantages of low cost, corrosion resistance, and high 

strength to weight ratio. Although polymers offer many desirable attributes, the significant 

drawback of poor adhesion must be mitigated for their use in most systems (1). There are many 

routes to modify polymer surfaces, such as wet chemical etching, mechanical treatments, 

exposure to flames, ions, plasma, corona discharge, ultraviolet (UV) radiation, and UV ozone 

(1, 2). Of the aforementioned methods, those based on atmospheric or low-pressure plasmas 

offer several advantages over other techniques with the most important being the modification of 

only the surface layer of the polymer. Thus, the bulk of the material and its properties remain 

unchanged while the surface chemistry and/or adhesion properties are altered. The stability of 

the surface state after plasma modification is a major concern. For traditional plasma treatments, 

the return from the hydrophilic oxidized surface state to the hydrophobic original state is referred 

to as “hydrophobic recovery” (3). Previous research has demonstrated that different polymers 

respond dissimilarly to plasma and UV radiation. The hydrophobic recovery of the surface 

depends on the stability of the surface bonds, with polymers such as polyethylene (PE) and 

silicone forming much stronger bonds with oxidized species than polymethylmethacrylate and 

polytetrafluoroethylene (4). 

In this work, PE was oxidized with atmospheric helium/oxygen plasma and UV ozone. Contact 

angle measurements and x-ray photoelectron spectroscopy (XPS) were used to probe the surface 

energy and chemical bonding states at the surface, respectively. Aging studies were performed 

over a 30-day period to determine the stability of the chemically modified surface. 

2. Materials and Methods 

2.1 Materials 

Ultrahigh molecular weight (UHMW) PE films 75 μm thick were purchased from Goodfellow. 

Diiodomethane and formamide were purchased from Sigma-Aldrich. 

2.2 Methods 

2.2.1 Film Preparation 

UHMWPE films were cut into 1- × 2-in strips, sonicated for 10 min in ethanol, rinsed with 

ethanol, and dried with nitrogen. Samples for UV ozone treatment were taped to a standard glass 

microscope slide.
2 

2.2.2 Plasma Modification 

The atmospheric plasma system used for the surface treatments was an industrial-scale plasma 

system from Sigma Technologies (model APC 2000). Helium and oxygen were injected into the 

electrode at atmospheric pressure and allowed to diffuse, forming a filamentary glow discharge. 

Helium was used to initiate and form the plasma at atmospheric pressure before oxygen was 

introduced to the system. The operating frequency was 90 kHz, with an operating power of 

1050 W applied to a single 50- × 20-cm electrode, while the treatment time ranged from 

1.3 to 39 s. 

2.2.3 UV Ozone Modification 

Samples were placed uncovered in a UVO-Cleaner* (model no. 42) and oxidized at set intervals 

of time from 15 s to 45 min. 

2.2.4 Surface Analysis 

The setup used for contact angle measurements consisted of a 100-μL syringe held in place 

above a moveable stage. Contact angles were recorded using a goniometer equipped with a 

charge-coupled device camera and an image capture program employing LabVIEW software 

(National Instruments, USA). Small strips (5 × 1 cm) of the sample film were cut and then 

placed onto a glass microscope slide using double-sided tape to ensure a flat viewing surface. 

The glass slide was then placed onto a stage, where a 5-μL deionized water drop was dispersed 

from the syringe onto the film surface. The drop was allowed to reach equilibrium prior to 

recording the measurement and before evaporation occurred. Three to five drops were used for 

each sample, and the values were averaged to obtain a final contact angle value. This same 

process was then repeated using diiodomethane, ethanol, and formamide (see table 1) since these 

liquids cover a wide range of polarities. The surface energy was then calculated and analyzed to 

differentiate between the polar and dispersive components. 

Table 1. Surface tension (LV), dispersive (

d

), and polar (

p

) components of the test liquids. 

Test Liquid LV 

(mN/m) 



d

(mN/m) 



p

(mN/m) 

Water 72.8 21.8 51 

Formamide 58.2 39 19 

Methylene iodide 50.8 50.8 2.3 

Young’s equation describes the surface energy as a combination of the cohesive and adhesive 

forces, which in turn dictate whether a droplet will spread on a surface. The energy of the solidliquid interface can be calculated as follows: 

   cos SL  SV  LV , (1)

 *

UVO-Cleaner is a trademark of Jelight Company Inc., Irvine, CA. 
3 

where SV and LV represent the surface energies of the solid and liquid phases, respectively; SL 

is the interfacial energy of solid-liquid, and  is the solid-liquid contact angle. The work of 

adhesion at the interface between the solid and liquid phases, Wa, can be determined from the 

following equation: 

a SV LV SL W    –  . (2) 

Equations 1 and 2 can be combined to form the following expression: 

W (1 cos ) a   LV   . (3) 

The surface energy depends upon polar (γ

p

) and dispersive forces (γ

d

) such as H-bonding/dipoledipole interactions and London forces, respectively. Good and Garifalco (5) determined that the 

attraction energy between pairs of different molecules can be described by the geometric mean of 

this energy between pairs of like molecules. The total work of adhesion (equation 4) 

corresponds to the interactions between the two different phases and forces of those interactions. 

1/ 2 1/ 2 2( ) 2( ) p

LV

p

SV

d

LV

d

SV

p

a d Wa Wa W      

 ( ) ( ) ( ) . 2 ( )

1/ 2 1/ 2 1/ 2

1/ 2

p

p SV

LV

d

d LV

p SV

LV

Wa 



 

     (4) 

A plot of 2

1

/ 2( ) p Wa LV  vs. (γ

d

LV/ γ

p

LV) 2

1

can be used to determine γ

p

SV and γ

d

SV, as all other 

variables in equation 4 can be determined experimentally. The slope of the linear plot is equal to 

(γ

d

SV) 2

1

, and the intercept is (γ

p

SV) 2

1

. 

Near surface compositional depth profiling was performed using the Kratos Axis Ultra 165 x-ray 

photoelectron spectroscopy system, equipped with a hemispherical analyzer. A 100-W 

monochromatic A1 Ka (1486.7 eV) beam irradiated a 1- × 0.5-mm sampling area with a take-off 

angle of 90°. The pressure in the XPS chamber was held between 10-9 and 10-10 torr. 

Elemental high-resolution scans for the photoelectron lines of carbon, oxygen, and nitrogen 

(C1s, O1s, and N1s) were taken at the pass energy of 20 eV. A value of 285.0 eV for the 

hydrocarbon C1s core level was used as the calibration energy for the binding energy scale. 

3. Results and Discussion 

3.1 UV Ozone Modification of UHMW PE Films 

After a few seconds or minutes of plasma and UV ozone treatment, respectively, large changes 

in surface wettability were observed, as shown in figure 1.
4 

Figure 1. Water contact angle of untreated PE (left) and plasma-treated PE (right). 

Contact angle results for UV ozone-treated PE for treatment times of 0–45 min are shown in 

figure 2. As-received (untreated) PE has a contact angle of ~106.6 o, as shown at 0-min exposure 

time. The contact angle decreases with longer UV ozone treatment times until it reaches a 

minimum at 20 min of treatment (55°). The contact angle increases slightly for longer UV 

ozone exposure times until it reaches a plateau at 45 min (65°). Table 2 displays the surface 

energy of the UV ozone-treated films calculated from the contact angle measurements 

immediately after the surface modification (see section 2). The highest surface energies were 

observed for the PE films with the longest exposure to UV ozone. The polar dispersive 

component (γ

p

) is the highest with 20 min of treatment, corresponding to the treatment time with 

the lowest water contact angle. The reason for the rise in contact angle after ~20 min exposure 

time can be explained by the XPS results shown in figures 3 and 4. 

Figure 2. Water contact angle results for UV ozone-treated PE in terms of exposure time. 

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70

Exposure Time (min)

WCA (deg)
5 

Table 2. Initial surface energy of UV ozone-treated PE.

Treatment 

(s) 

γd

(mN/m) 

γp

(mN/m) 

γtot

(mN/m) 

0.25 39.01 0.33 39.341 

10 26.94 8.62 35.56 

20 31.97 16.02 47.99 

45 34.03 15.30 49.33 

Note: 

tot = total surface energy. 

Figure 3 displays overlaid O1s spectra for PE films exposed to UV ozone for 15 s and 10, 20, 

and 45 min, in which atomic percent oxygen ranged from 4% to 15%, respectively. There is a 

large change in the atomic percent oxygen from 15 s up to 20 min; however, the difference 

between 20 and 45 min is small. This indicates that atomic percent oxygen levels out for these 

treatment times. Figure 4 shows the relative atomic oxygen percentage of carboxylic acid 

groups, which continue to increase with time. Although the atomic percent oxygen is steady 

after 20 min, the oxidation states of PE are changing with increased exposure time. Longer 

treatment times increased the amount of carboxylic acid and derivatives on the surface, which 

are less hydrophilic than hydroxyl groups, and hence the contact angle increased. 

Figure 3. XPS overlay of oxygen peaks of UV ozone-treated PE: 15 s (red), 

10 min (green), 20 min (purple), and 45 min (blue). 

A rmy Reaserch Laboratory, AP G, MD

O 1s/7

x 102

5

10

15

20

540 538 536 5 34 532 530 528 526 524 522

Binding Energy (eV)
6 

Figure 4. Relative atomic percent carboxylic acids vs. UV ozone exposure time. 

In order to obtain a quantitative understanding of the chemical bonding states on the UHMW 

polyethylene surface, we analyzed the high-resolution carbon peak. The components of the C1s 

high-resolution spectra were labeled as follows: C1 at 284.7 eV represents the –CH2 groups, C2 

at 285.5 eV assigned to –C–O or –C–OH, C3 at 286.7 eV and C4 at 288.9 eV corresponding to –

C=O and –COOR or –COOH, respectively. Peaks C5 and C6 correspond to –O–C–C=O and 

–O–C(=O)–O at 287.9 and 290.5, respectively. The relative peak areas determined by the curve 

fitting of the C1s spectra are listed in table 3. Figures 5–8 display the XPS C1s components for 

each treatment time. As illustrated next, increased UV ozone exposure increased the number of 

oxidation states of the polymer. The C1s peak for the PE exposed for 45 min has six 

components representing different chemical bonding states, while the C1s peak for the PE treated 

for 15 s contains only two bonding states (native C–C and C–O). 

Table 3. Analysis of the C1s peak for the UV ozone treated UHMW PE films. 

Treatment Time 

(min) 

C1 

(%) 

C2 

(%) 

C3 

(%) 

C4 

(%) 

C5 

(%) 

C6 

(%) 

0.25 80.1 19.9 — — — — 

10 78.8 10.9 6.0 4.3 — — 

20 69.4 15.0 8.6 6.9 — — 

45 62.2 12.9 7.7 9.3 4.6 3.2 
7 

Figure 5. XPS C1s spectrum of 15-s UV ozone-treated PE. 

Figure 6. XPS C1s spectrum of 10-min UV ozone-treated PE. 
8 

Figure 7. XPS C1s spectrum of 20-min UV ozone-treated PE. 

Figure 8. XPS C1s spectrum of 45-min UV ozone-treated PE. 
9 

Aging studies were conducted over 30 days on the UV ozone-treated films in order to determine 

the stability of the chemical bonds formed. The water contact angle data is shown in figure 9. 

The UV ozone-treated PE surface appears stable as there is very little change in the contact angle 

over time. The surface-energy aging data for UV ozone-treated PE (not shown), in general, 

agrees with the water contact angle data, suggesting stability of these oxidized films over time. 

Figure 9. Water contact angle aging study of UV ozone-treated PE. 

3.2 Plasma Modification of UHMW PE Films 

Plasma treatment of polymers under low or atmospheric pressure is a well-known method (6) for 

modifying the surface wettability. The effect is dependent upon the plasma composition; 

however, in most cases, it increases the hydrophilicity and the surface energy. 

We performed a series of wettability evaluations using the sessile drop method for four samples 

treated under He-O2, with 13% of oxygen in the plasma input gas mixture. The films were 

exposed to plasma for four different (net) exposure times: 1.3, 6.5, 19.5, and 39 s. The 

wettability measurements were taken 1 hr after the plasma treatment and the testing liquids were 

water, diiodomethane and formamide. The surface energy for each film was subsequently 

calculated. 

Results showed (table 4) that even after a short exposure, of 1.3 s of the discharge, the water 

contact angle of polyethylene decreased from 106.6° to 53°. By increasing the exposure time, 

the measured water contact angles continued to drop. After a 39-s treatment, the measured angle 

was 40°, showing an overall decrease of 60% compared to the control film. The hydrophilic 

character of the treated films can be interpreted as the result of the chemical modification of the 

surface, by the addition of polar groups through the plasma treatment. The attachment of new 

35

37

39

41

43

45

47

49

51

53

0 5 10 15 20 25 30 35

Days

Surface energy

15 sec

20 min

45 min

control
10 

polar functional groups was confirmed by the calculation of the total surface energy and the 

analysis of its γ

p

 and γ

d

 components. As expected, the control sample had a very low polar 

surface energy of 0.12 mN/m and a γ

tot of 39.82 mN/m, primarily due to its γd

 component. After 

a 1.3-s exposure to plasma, the γ

d

 component decreased to 32.95 mN/m, and a dramatic increase 

of the γp component was observed (18.49 mN/m). Prolonging the treatment time resulted in an 

increase of γ

p

, while γ

d

 remained almost the same. Compared to the control film and after 39 s 

treatment, we observed a significant enhancement of γ

p

 (25.2 mN/m) accompanied with a 41% 

increase of the γ

tot. 

Table 4. Surface energy data for He-O2 plasma-treated PE. 

Treatment 

(s) 

Water Contact Angle 

(o) 

γp

(mN/m) 

γd

(mN/m) 

γtot

(mN/m) 

0 106.6 0.12 39.82 39.94 

1.3 53.0 18.49 32.95 51.44 

6.5 46.8 23.14 .30.25 53.39 

19.5 42.3 24.70 31.58 56.28 

39 40.0 25.20 31.02 56.22 

Aging studies were performed to study the chemical stability of the surface after plasma 

treatment. Water contact angles were measured for 21 days, and the results are shown in figure 

10. The observed angles for all four plasma-treated samples were significantly lower than that of 

the untreated film and depended on the exposure time. Hydrophobic recovery appeared to be 

dominant in the first 3 days after the treatment, and an increase of the water contact angle was 

observed. Then, the samples seemed to reach equilibrium, and the measured values did not 

change with time. The lowest observed angle was 55.6° and corresponded to the sample treated 

for the longest time, measured 21 days after the plasma treatment. 

XPS results revealed that the modified surfaces exhibited a surface rich in oxygen-containing 

groups. It suggested that the plasma treatment induced the formation of carboxyl, hydroxyl, and 

carbonyl groups on the surface, thereby enhancing the hydrophilicity of the polymer surface. 

Figure 11 shows the atomic percent concentration of oxygen for PE films treated under a heliumoxygen dielectric barrier discharge for various treatment times, ranging from 1.3 to 19.5 s. A 

rapid increase of the oxygen concentration is observed after only 1.3 s of exposure and continues 

to increase until ~7 s of treatment and then becomes constant, indicating that the saturation level 

is reached. Carbon concentration seems to have an opposite trend as it decreases dramatically 

after 4 s (crossover point) of plasma treatment and is reduced to 75% compared to 97.5% of the 

as-received polymer. Nearly 20 atomic-percent oxygen is measured with 5 s of plasma 

treatment, whereas only 15% is observed with 45 min of UV ozone. Most importantly, the 

majority of the surface changes take place in the first 5 s of treatment. 
11 

0 5 10 15 20

40

50

60

70

80

90

100

110

 1.3 s

 6.5 s

 19.5 s

 39 s

Water contact angle (deg)

Time after plasma treatement (days)

untreated film

Figure 10. Aging study of plasma-treated PE films. 

Figure 11. Effect of plasma treatment time on carbon and oxygen atomic 

percent concentrations. 

0 5 10 15 20

0

5

10

15

20

25

30

35

60

70

80

90

100

Oxygen (at.conc.%)

Treatment Time (s)

Carbon (at.conc. %) 
12 

As mentioned earlier, after the plasma treatment, the carbon signal from the surface decreases, 

giving rise to an increased oxygen signal. In order to obtain a quantitative and qualititative 

understanding of the chemical groups grafted on the UHMW PE surface, we analyzed the 

high-resolution carbon peak. The components of the C1s high-resolution spectra were labeled as 

follows: C1 at 285 eV represents the –CH2 groups, C2 at 286.6 eV assigned to –C–O or 

–C–OH, C3 at 288 eV and C4 at 289.2 eV corresponding to –C=O and –COOR or –COOH, 

respectively. The relative peak areas determined by the curve fitting of the C1s spectra are listed 

in table 5. The C1 intensity decreased from 97.9% to about 70% after the plasma treatment, 

indicating the oxidation of the surface through its interaction with the plasma active species. The 

C2 peak corresponding to –C–O or –C–OH exhibited a fivefold increase after a short exposure of 

7.8 s. The C3 peak reached saturation when the substrate material was treated for times longer 

than 7.8 s; further treatment did not increase the intensity of the peak. Finally, the –COOH bond 

was enhanced and was 9.4%, 11.2%, and 13.2% for the treatment times of 7.8, 23.4, and 70.2 s, 

respectively. The C5 and C6 peaks observed in the 45-min UV ozone-treated PE film were not 

observed in the plasma treated films. We speculate that this was due to the much shorter plasma 

treatment times and less surface oxidation and possibly degredation. 

Table 5. Analysis of the C1s peak for the plasma-treated UHMW PE films. 

4. Conclusion 

In this work, two different methods of surface oxidation of polyethylene were compared in order 

to improve the adhesion properties of the material. UV ozone treatment proved to be an 

effective method to increase the surface energy of PE, and modified films were stable for at least 

30 days. A linear trend of relative atomic percent carboxylic acids vs. treatment time was 

observed. Thus, the number of surface carboxylic acids could be effectively controlled with 

treatment time, which can be very useful for controlling subsequent surface chemical reactions. 

The main drawback with UV ozone was the long treatment times required (20–45 min). Less 

robust polymer systems could become brittle or otherwise experience a reduction in mechanical 

properties with such exposure times. Plasma oxidation, on the other hand, was an extremely 

effective way to modify the PE surfaces in terms of surface energy modification as well as short 

treatment time. Higher atomic percent oxygen was achieved in a few seconds with plasma 

treatment as compared to several minutes for the UV ozone method. In addition, plasma-treated 

films were stable for at least 21 days. 

Treatment Time 

(s) 

C1 

(%) 

C2 

(%) 

C3 

(%) 

C4 

(%) 

Untreated 97.9 2.1 0.0 0.0 

7.8 72.3 10.4 7.9 9.4 

23.4 72.3 10.4 6.1 11.2 

70.2 70.0 9.9 6.9 13.2 
13 

5. References 

1. Wertheimer, M.; Fozza, A.; Holländer, A. Industrial Processing of Polymers by Low-Pre 

ssure Plasmas: The Role of VUV Radiation. Nucl. Instrum. Methods Phys. Res., Sect. B

1999, 151, 65–75. 

2. Truica-Marasescu, F.; Guimond, S.; Wertheimer, M. VUV-Induced Nitriding of Polymer 

Surfaces: Comparison with Plasma Treatments in Nitrogen. Nucl. Instrum. Methods Phys. 

Res., Sect. B 2003, 208, 294–299. 

3. Pascual, M.; Balart, R.; Sánchez, L.; Fenollar, O.; Calvo, O. Study of the Aging Process of 

Corona Discharge Plasma Effects on Low Density Polyethylene Film Surface. J. Mater. Sci. 

2008, 43, 4901–4909. 

4. Weikart, M.; Yasuda, H. Modification, Degradation, and Stability of Polymeric Surfaces 

Treated with Reactive Plasmas. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3028–3042. 

5. Good, R.; Garifalco, L. A Theory for Estimation Surface and Interfacial Energies. III. 

Estimation of Surface Energies of Solids from Contact Angle Data. J. Phys. Chem. 1960, 

64, 561–566. 

6. Shenton, M. J.; Stevens, G. C. Surface Modification of Polymer Surfaces: Atmospheric 

Plasma Versus Vacuum Plasma Treatments. J. Phys. D: Appl. Phys. 2001, 34, 2761. 
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