Surface Modification to Control Wettability of Paper Using Argon-Carbon Plasma Treatment

Paper is an abundant, eco-friendly, and inexpensive material widely applied in fields such as printing, packaging, paper-based devices, and sensors. Paper has hydrophilic properties and can absorb large amounts of water because the bonds between cellulose molecules in the paper structure are primarily composed of hydrogen bonds involving hydroxyl groups. These hydroxyl groups of the cellulose structure cause the polarity of the paper surface so that the paper is hydrophilic. It is complicated to make paper have water-resistant properties, but a process of controlling water-resistance properties is essential for applications in various fields.
This study introduces a straightforward method to transform ordinary paper into water-resistant paper using argon-carbon (Ar-C) plasma generated by a direct current pulsed sputtering device equipped with parallelly arranged magnets. Argon gas was used as a sputtering gas, and the carbon element was supplied from the graphite target. Plasma was generated using a negative pulse power source with a square pulse and a frequency of 60 kHz. The plasma treatments were performed at powers of 80 W (plasma current: 300 mA), 135 W (500 mA), and 200 W (700 mA) with an exposure time of 20 seconds. All processes were conducted at room temperature.
Damage from energetic plasma ions bombarding the paper surface after plasma treatment was minimal. This result could be explained by the fact that the paper was exposed to plasma for a short time, and the pulsed power reduced damage. Analysis in the C 1s region revealed an inverse relationship between C-C and C-OH bonds, while C=O and O-C=O bonds showed little variation. The variation ratio of the C-C bond increased from approximately 5% to 11% with the increase in plasma current. These results meant that the Ar-C plasma removed the hydroxyl groups on the paper surface, and the C elements were substituted at the site of the removed hydroxyl groups. The replacement of C elements led to hydrophobic properties on the paper surface. Therefore, the degree of substitution of the hydroxyl group can be controlled using Ar-C plasma, which leads to the control of the paper's wettability degree.
The contact angle decreased gradually with time due to the water absorption of the paper. The reduction ratio of the contact angle was linear with time, decreasing from 0.945 to 0.182 (°/s) with plasma current increasing from 300 to 700 mA. The water droplet on the ordinary paper surface was absorbed immediately and spread widely. As the plasma current increased, the water droplet did not spread and was absorbed under the droplet. Notably, the contact angle of the paper plasma-treated at 700 mA remained unchanged for 20 seconds.
In conclusion, the water-resistant properties of the paper surface were successfully achieved and controlled through Ar-C plasma treatment. The wettability of the paper was controlled by adjusting the substitution ratio of hydroxyl groups with carbon components through Ar-C plasma treatment according to the plasma current. This substitution neutralized the polarity originating from hydroxyl groups on the paper surface, thereby imparting hydrophobic properties. Since Ar-C plasma-treated paper does not contain polymers or harmful substances, it can be easily recycled after use. The Ar-C plasma process is a simple, easy, and inexpensive method that can be applied to mass production. It is also an eco-friendly method that does not use harmful gases. In addition, this technique is effective for treating weak sheets, including paper and polymer sheets.