DETERMINATION OF SURFACE FREE ENERGY AND CHEMICAL MODIFICATIONS OF PLASMA TREATED ELASTOMER SURFACE

Gorazd Golob,Mladen Lovreček, Miran Mozetič, Alenka Vesel, Odon Planinšek, Marta Klanjšek Gunde, Diana Gregor Svetec

Abstract

The aim of presented work is investigation of surface free energy and chemical properties of unmodified and plasma treated NBR and EPDM based elastomer. Rubber blanket, covered with thin elastomer surface layer, is a well known ink transfer media in lithographic offset and other conventional or digital printing techniques where it is used as secondary printing forme. Characterization of the surface energy by contact angle measurements using different polar and disperse liquids combined with proper calculation methods based on Owens-Wendt, Wu or Oss&Good AcidBase theory, give us information about polar and disperse part of surface energy of the material so indirectly we can conclude about its adsorption potential and its hydrophilic/oleophilic or hydrophobic/oleophobic properties.
For oxygen plasma treatment of the samples we used a lab plasma reactor with a vacuum pump and an inductively coupled RF generator at the power of approximately 200 W. Each sample was exposed to oxygen plasma with the neutral atom density of 5x1021 m-3, the electron density of 8x1015 m-3 and the electron temperature of 35 000 K for 27 s. The samples were kept at floating potential of -15V.
For surface free energy measurements we used Krüss DSA100 apparatus to obtain contact angle and DSA software with database of water, diiodomethane and formamide test liquids properties for calculations.
We used Perkin Elmer FTIR-ATR spectrometer in mid IR area (wavenumber 500 – 4000 cm-1, 2.5 – 20 µm) to get IR spectra of untreated and plasma treated elastomer samples. Analysis of spectra was performed using KnowItAll software with spectral database to get reliable results. Typical peaks of absorption spectra give us information of elements, chemical groups or bonds on surface that are in correlation with the surface energy, polarity and other surface characteristics of the mentioned materials.
Keywords: elastomer, rubber blanket, plasma, surface energy, IR spectrum

1 Introduction

Rubber blanket is used in lithographic offset and other conventional or digital printing techniques as secondary printing forme. The surface energy of the blanket should be higher than surface energy of the ink and print areas of the printing plate and lower than the surface energy of the print substrate - usually paper. By modification of blanket surface energy we open a possibility to use a wide pallet of different printing inks, not only those based on synthetic or vegetable oil vehicle but also water-based and other types of inks.
By using different polar and non-polar liquids for contact angle measurements and proper calculation methods we should get information about polar and disperse part of surface energy of the material and indirectly we can conclude about its hydrophilic/oleophilic or hydrophobic/oleophobic properties.
During our investigation we tried to modify the surface energy of different rubber blankets to raise their surface energy and achieve nearly perfect hydrophilic surface. We also studied the stability of achieved modifications, methods for a reverse process and changes on the blanket surface at different stages during the experiments. In this paper some most important results are presented for typical NBR rubber blanket (BLUE) for conventional and EPDM rubber blanket (RED) for UV offset lithographic printing.

2 Research methods

For oxygen plasma treatment of the samples we use lab plasma reactor with a vacuum pump and inductively coupled RF generator at the power of approximately 200 W. Each sample was exposed to oxygen plasma with the neutral atom density of 5x1021 m-3, the electron density of 8x1015 m-3 and the electron temperature of 35000 K for 0, 3, 9, 27, 81, 243 and 729 s. The samples were kept at floating potential that was -15V (Cvelbar, Mozetič).
For the first set of contact angle measurements we used 3 μl drops of distilled water and a CCD camera connected to a computer to get contact angle. Contact angle measurements were performed immediately after treatment and repeated after 3 and 24 hours. At the end we used the test-pen method, too. Test-pen method results were almost the same on all samples, not in correlation with contact angle measurement results and are not presented. We obtain maximal surface free energy change after 27 s and it remains unchanged for 24 h (Golob et al).
For the second set of measurements we used the same samples, treated for 27 s by oxygen plasma under same conditions. We measured contact angles after approximately 24 hours on Krüss DSA 100 apparatus with three liquids polar and non-polar liquids with different characteristics:
•water – (total 72.8 mN/m; disp. 21.8 mN/m; polar 51.0 mN/m), 1µl drop
•diiodomethane – (total 50.8 mN/m; disp. 0 mN/m; polar 0 mN/m), 0.5 µl drop
•formamide – (total 58.0 mN/m; disp. 39.0 mN/m; polar 19.0 mN/m), 1 µl drop.
For calculation of surface free energy we used Wu (Wu), Owens Wendt (OW) and AcidBase theory (AB) methods, supported by Krüss software (Brady, Erbil). Surface free energy data of test liquids were obtained from Krüss database. For each measurement set we performed at least 6 measurements and excluded results with extreme deviation to mean value.

Picture 1.png

Figure 1: Typical FTIR-ATR spectra of RED rubber blanket sample.

Picture 11.png

Figure 2: Image of open window in spectra analyzing software KnowItAll.

We used Perkin Elmer FTIR-ATR spectrometer type Spectrum GX1 in mid IR area (wavenumber 500 – 4000 cm-1, 2.5  – 20 µm) to get IR spectra of untreated and plasma treated rubber blanket samples. For each sample we performed 64 scans. Analysis of spectra was performed using KnowItAll software with spectral database to get reliable results.
Typical IR spectra obtained using FTIR-ATR method for RED sample (untreated, oxygen plasma treated, UV laser treated and plasma and UV laser treated) are presented in Figure 1. In Figure 2 a part of spectral analyzing procedure is presented.

3 Results

Contact angle measurements and calculated surface free energy using Owens Wendt, Wu and AcidBase theory method are presented in Table 1 and 2. Calculated surface free energy according to Wu and AcidBase theory, presented in Table 2 are not reliable because of some negative values.

Table 1: Contact angles for different liquids (water – W, diiodomethane – D, formamide – F) of untreated and oxygen  plasma treated samples.

 

Contact angle (º) untreated

Contact angle (º) plasma 27 s

Sample

W

D

F

W

D

F

BLUE

100.2

49.9

76.9

52.8

39.5

38.7

RED

132.4

75.3

104.2

66.7

48.3

62.9

Table 2: Surface free energy of untreated and oxygen plasma treated samples (total – T, disperse – D, polar – P, acidbase – AB, acid – A, base – B).


Surface free energy (mN/m)

 

Owens-Wendt

Wu

AcidBase theory

Sample

T

D

P

T

D

P

T

D

AB

A

B

BLUE untreated

32.73

32.59

0.14

35.01

34.75

0.27

33.66

34.33

-0.67

0.14

0.79

BLUE plasma

50.32

33.46

16.86

52.86

35.42

17.26

39.46

39.86

-0.40

0.01

33.22

RED untreated

21.28

19.70

1.58

19.83

23.41

-3.58

21.48

19.96

1.51

0.48

1.19

RED plasma

40.15

29.34

10.81

43.25

32.12

11.13

31.52

35.22

-3.70

0.14

24.46

Results of BLUE and RED samples are presented in Figures 3 to 6 as a general overview of spectra and zoomed part where noticeable changes during plasma treatment occurred. Characteristic peaks are marked by wavenumber (frequency).

Picture 2.png

Figure 3: Absorption spectra of untreated and treated BLUE samples.

Picture 3.png

Figure 4: Zoomed part of spectra from Figure 4 for untreated and oxygen plasma treated BLUE sample.

Picture 6.png

Figure 5: Absorption spectra of untreated and treated RED samples.

Picture 7.png

Figure 6: Zoomed part of spectra from Figure 4 for untreated and oxygen plasma treated RED sample.

Significant changes between untreated and plasma treated BLUE samples were achieved at 1074.94 cm-1 peak wavenumber. This is characteristic peak for anhydrides (C-O-C), ethers at 1103 cm-1 (C-O-C), 5 ring ethers (C-O-C), silicons (Si-O-Si), sulfur (S=O) with strong peaks and some other chemical groups with weak peaks.
Significant changes between untreated and plasma treated RED samples were achieved at 1397.81, 1463.49 and 1540.19 cm-1 wavenumber. 1397.81 cm-1 is characteristic strong peak for sulfur (SO2). 1463.49 cm-1 is characteristic medium strong peak for many alkanes (CH), amides (N-H) and aromatic (ring) chemical groups and some impurities – water vapor (OH). 1540.19 cm-1 is characteristic for amides (CNH), nitro (NO2) with strong and ureas (NH) with medium peak.

4 Discussion

Rubber consists of ten or more components like one or more polymers, fillers, additives for crosslinking of basic monomer, additives for hardness and elasticity control, pigments and dyes (Salamone). Surface structure was typical for ground finished surface layer, it remained almost unchanged after plasma treatment and obviously there was no significant etching of polymer on surface layer. Stability of the samples hydrophilicity gave us opportunity for contact angle measurements using different liquids after 24 h at another location. After oxygen plasma treatment surface free energy has arisen in all samples.
During the investigation we did not achieve very hydrophilic stage with contact angle near 0º on our rubber blanket samples. On many other metal and organic materials we achieved super hydrophilic surface in very short time during oxygen plasma treatment.
Results of contact angle measurements and surface energy calculations are not reliable and we have to repeat this part of investigation. Reasons could be in inappropriate handling with equipment, test liquids with characteristics that are not the same as in database, unhomogenity and roughness of rubber blanket surface, differences in air temperature and humidity or some other reason. It is well known from references that different methods for surface free energy give us different results and that some methods are not appropriate for all solid materials. At this stage of investigation we got enough information about influence of oxygen plasma on different types of rubber to continue with measurements of surface free energy using other types of rubber blankets and raw materials like basic polymers and fillers for rubber production.
It is interesting that so called polar BLUE rubber shows us almost no polar part of surface free energy before treatment and non-polar RED rubber has higher amount of polar part compare to BLUE NBR type of rubber blanket sample.
After treatment with oxygen plasma we have achieved higher level of total surface free energy and polarity with BLUE compared to RED rubber. The ratio of surface cleaning effect compared to chemical modification during oxidation process is still unknown. The untreated samples were wiped with ethanol five minutes before measurements and plasma treatment but obviously this treatment was not efficient and therefore we try to determine chemical structure of untreated and plasma treated samples by IR spectra analysis using FTIR-ATR method.
Comparing spectra analysis of BLUE and RED we see that significant changes of spectra peaks are at different wavenumber. At BLUE sample after oxygen plasma treatment amount of chemical groups with bonded oxygen and sulfur were higher (higher peak). At RED sample spectra peaks after plasma treatment were lower, changes occurred on NO2, SO2 and other groups without bonded oxygen. There were no additional peaks formed by new chemical groups. In general spectral curve of BLUE sample was positioned slightly higher compared to untreated sample due to higher roughness. At RED sample both spectral curves remain almost at the same level with exception of some peaks.

5 Conclusions

To get more reliable results the analysis of pure basic materials before and after plasma treatment is necessary. We should continue our investigation using typical EPDM, NBR and other types of crude rubber and different fillers to get surface free energy of untreated and plasma treated basic rubber components. After that new set of rubber samples should be prepared for further investigations.
XPS analysis should give us chemical fingerprint of the sample surface. Surface free energy measurements and FTIR-ATR spectra analysis method are not sufficient to determine hydrophilic/hydrophobic or oleophilic/oleophobic character of the surface before and after treatment.
Impact of impurities at the rubber surface layer is still unknown and with repeated series of plasma treatments and defunctionalisation we should get pure surface to perform measurements of surface free energy and chemical analysis.
It can be concluded that oxygen plasma treatment gives us a rubber blanket with new characteristics that open new opportunities for improvements of their characteristics and new functionality of blankets in different printing processes.

Acknowledgements

We would like to thank colleagues from Savatech, Kranj for technical support.

References

Brady, Robert: Comprehensive Desk Reference of Polymer Characterization and Analysis, Oxford University Press, 2003, ISBN 0 8412 3665 8.
Cvelbar Uroš, Miran Mozetič: Method for improving of electrical connection properties of a surface of a product made from a polymer matrix composite, international patent WO 2006/029642.
Erbil, H. Yildrim: Surface Chemistry of Solid and Liquid Interfaces, Blackwell Publishing, 2006, ISBN 1 4051 1968 3.
Gojo, Miroslav; Lovreček, Mladen: Characterisation of Surfaces on the Offset Printing Plate, Proceedings of Lectures and Posters, 1st International Symposium of Novelties in Graphics, Ljubljana : Faculty of Natural Sciences and Engineering, 1998. 253-260.
Golob, Gorazd; Mozetič, Miran; Eleršič, Kristina; Junkar, Ita; Đorđević, Dejana; Lovreček, Mladen. Rubber blanket surface energy modification using oxygen plasma treatment. 36th International Research Conference iarigai, 13-16 September 2009, Stockholm, Sweden. Advances in printing and media technology : [book of extended abstracts]. Stockholm: INNVENTIA AB: International Association of Research Organizations for the Information, Media and Graphic Arts Industries, 2009.
Lovreček, Mladen; Gojo, Miroslav; Dragčević, Krešimir: Interfacial Characteristics of the Rubber Blanket - Dampening Solution System, Bristow, J., Anthony (ur.). Leatherhead, Surrey, UK : PiraInternational, 1999. Str. 370.
Mozetič Miran, Alenka Vesel, Cvelbar Uroš: Method and device for local functionalization of polymer materials, international patent WO 2006/130122.
Mozetič, Miran: Controlled oxidation of organic compounds in oxygen plasma. Vacuum. [Print ed.], 2003, vol. 71, p. 237-240.
Salamone, J.: Polymeric Materials Encyclopedia, CRC Press, Taylor & Francis LLC, 1996, ISBN 9780849324703.

 

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