PVC is the earliest industrialized plastic product. It has many advantages such as light weight, smooth surface, and easy milling. It is an excellent material for decorative profiles and is widely used in daily necessities, building materials, and industrial products. The surface energy of PVC is low, and there are few hydrogen bonds that can be formed on the surface. The adhesive is not easy to wet on its surface and is difficult to adhere, so the adhesion strength is low. Therefore, it is necessary to improve its bonding performance by surface treatment.

Plasma treatment technology is a surface treatment technology that emerged at the end of the 19 th century. It is a technology developed on the basis of interdisciplinary disciplines such as physics, chemistry, electronics, and vacuum technology, which can change the surface properties of materials. When the plasma bombards the surface of the material, it will be accompanied by extremely high energy, which will break some chemical bonds on the surface of the material and produce free radicals. These free radicals can bind to the ions and free radicals in the plasma, increasing the active groups on the surface of the material, thereby changing the wettability of the surface of the material and improving its interface adhesion with other materials without affecting the properties and functions of the material.

The change of water contact angle on the surface of foamed PVC plate before and after atmospheric plasma treatment

The foamed PVC plate was treated by atmospheric plasma at a processing speed of 60 mm / s, a power of 600 W, and a height of 5 mm. The surface contact angle of the foamed PVC plate before and after atmospheric plasma treatment is shown in Fig. 1. The contact angle decreased from 120o to 12.4o, a decrease of 89.6 %. Fig. 1a shows the surface water contact angle of the foamed PVC plate before atmospheric plasma treatment. Fig. 1b shows that the surface of the foamed PVC plate was hydrophobic before atmospheric plasma treatment. After atmospheric plasma treatment, the surface contact angle of the foamed PVC plate decreased significantly, from obtuse angle to acute angle. From hydrophobic surface to hydrophilic surface.

Fig.1 Surface contact angle of foamed PVC plate before and after atmospheric plasma treatment

Effect of atmospheric plasma treatment on surface morphology and chemical composition of foamed PVC sheet

The foamed PVC board is made of PVC and other additives, and its main component is polyvinyl chloride ( PVC ). The chemical structure of PVC is shown in Figure 2.

Fig.2 Chemical structure of PVC

( 1 ) Field emission scanning electron microscope analysis

After atmospheric plasma treatment and treatment at a treatment power of 600 W and a treatment speed of 60 mm / s, the electron microscopy images at different heights were significantly different. Therefore, the foamed PVC plates treated at different heights were selected for analysis. The surface microstructure of the foamed PVC plates treated by atmospheric plasma at the treatment heights of 5 mm, 10 mm and 15 mm is shown in Fig. 3 a, b, c, d. It can be seen that the effect of atmospheric plasma treatment is obvious. The surface of the untreated foamed PVC plate is relatively flat, and the surface of the treated foamed PVC plate is rough. There are many messy gullies. The increase of the treatment height makes the gully shallower and less, the wettability decreases, and the contact angle increases, which is in line with the results of the contact angle test. It shows that after atmospheric plasma treatment, the roughness of the surface of the foamed PVC plate increases, the area of the glue nail of the foamed PVC plate and the glue increases, and the bonding performance of the foamed PVC plate and the veneer is greatly improved.

Fig.3 Note : a, b, c and d are the surface scanning electron micrographs of foamed PVC sheet 

without atmospheric plasma treatment and after atmospheric plasma treatment 

at heights of 5 mm, 10 mm and 15 mm, respectively.

( 2 ) Fourier transform infrared spectroscopy analysis

Fig.4 shows the FTIR spectra of PVC before and after atmospheric plasma treatment. The treatment conditions of atmospheric plasma are as follows : treatment power 600W, treatment speed 60mm / s, and treatment height 10mm. It can be seen that there are several obvious characteristic peaks on the surface of the foamed PVC plate. The stretching vibration of C-H is near the wavelength of 2918cm − 1 and 2850cm − 1, and the stretching vibration of C-Cl is near the wavelength of 715cm − 1.There are two peaks at the wavelength of 1709 ~ 1821cm − 1, indicating the presence of C = O double bond. This group should not be present in the material composition. The foamed PVC plate used in the experiment is a plate made of PVC masterbatch as the main raw material. Some additives will be added in the production, which should be the groups in the additives added in the PVC production. It can be seen from the figure that the characteristic peaks of the infrared spectrum of the surface of the foamed PVC plate before and after atmospheric plasma treatment do not change significantly, indicating that the types of functional groups on the surface of the foamed PVC plate after atmospheric plasma treatment do not change.

Fig. 4 FTIR spectra of foamed PVC sheet surface before and after atmospheric plasma treatment

( 3 ) XPS analysis

The XPS scan of the surface of the foamed PVC plate before and after the atmospheric plasma treatment is shown in Figure 5. The treatment conditions of the atmospheric plasma are : treatment power 600W, treatment speed 60mm / s, treatment height 10mm. It can be seen that before and after atmospheric plasma treatment, there are obvious peaks at the binding energies of 285 eV, 533 eV and 199 eV on the surface of foamed PVC plates, which represent the presence of C1 s, O1 s and Cl2 p, respectively. C and Cl elements are the main elements of PVC, and O elements are derived from the additives in the production process of foamed PVC plates. It can be seen from the figure that the peak height of the O1s spectrum peak in the spectrum before and after the atmospheric plasma treatment increases, which indicates that the content of the surface elements of the foamed PVC plate changes after the atmospheric plasma treatment. Figs 7,8 and 9 are the sub-peak spectra of C1 s, O1 s and Cl2 p, respectively. It can be seen that C1 on the surface of foamed PVC plate after atmospheric plasma treatment is significantly reduced, while C2 and C3 increase, O1 decreases, O2 and O3 content increases, Cl2 p1 increases, and Cl2 p3 decreases. This may be because the functional groups on the surface of foamed PVC plate after atmospheric plasma treatment are bombarded by plasma, and chemical bonds are broken. The air on the surface of the plate is also bombarded by atmospheric plasma, generating free radicals, which are introduced into the surface of foamed PVC plate, increasing the free radicals on the surface of the plate. This shows that the atmospheric plasma treatment changes the oxygen-containing groups on the surface of the foamed PVC plate, so that the surface of the foamed PVC plate has better wettability.

Fig. 5 XPS spectra of foamed PVC plate before and after atmospheric plasma treatment : ( a ) untreated ( b ) treated

Fig. 6 Peak separation spectra of C1 s before and after atmospheric plasma treatment : ( a ) untreated ( b ) treated

Fig. 7 Peak separation spectra of O 1s before and after atmospheric plasma treatment : ( a ) untreated ( b ) treated

Fig. 8 The peak separation spectra of Cl2p before and after atmospheric plasma treatment : ( a ) untreated ( b ) treated