Polypropylene non-woven fabric is widely used in medical and health materials, filter materials, geotechnical building materials, clothing cloth materials, wiping materials and automotive interiors because of its excellent performance. Among them, polypropylene non-woven fabric with good filtration performance as a microporous membrane has gradually become a research hotspot in the field of filtration. Polypropylene microporous membrane is widely used in sewage treatment, gas separation, blood purification, membrane distillation, battery separator and other fields.

However, due to the hydrophobic groups in the molecular structure of polypropylene, it has high crystallinity, and has a circular fiber cross section. The fiber structure is compact and lacks micropores and gaps, so the surface wettability and hydrophilicity of the formed polypropylene spunbonded nonwoven fabric are poor. As a disposable health care material, it cannot meet the requirements for hygroscopicity and has adverse effects on health conditions. As a filter membrane, the surface of polypropylene non-woven fabric is inert and hydrophobic, which makes it easy to be polluted in the process of water filtration and biological separation, reduces the separation capacity of the membrane, increases the maintenance cost of the filtration system and reduces the service life of the membrane, thus limiting its application in industry. Therefore, hydrophilic modification of polypropylene non-woven fabric to solve its easy pollution problem can expand its application range and is of great significance in industrial applications.

Plasma surface treatment

Plasma is a gas state material with stable and excited states of atoms, electrons and ions. Under the action of an electric field, gas ionization produces highly reactive particles such as electrons, ions or free radicals in various excited states. These particles can etch and activate the polymer surface, react with the surface molecules, and form hydrophilic groups. Different discharge gases will produce different hydrophilic groups on the surface of the polymer, such as carboxyl, hydroxyl, amino and aldehyde groups.

Effect of plasma surface treatment on surface hydrophilicity of polypropylene non-woven fabric

Fig.1a is the change of surface water contact angle of polypropylene non-woven fabric before and after plasma treatment with different plasma treatment time. The results show that the water contact angle of the surface of polypropylene non-woven fabric without plasma treatment is about 133.8 °, showing hydrophobicity. After oxygen plasma etching treatment for 5 min, the water contact angle decreased to about 60.3 °. After 15 min treatment, the water droplets can directly infiltrate the polypropylene non-woven fabric, the surface presents the hydrophilic state of the Wenzel model, and the solid-liquid direct contact ; after 30 min of plasma treatment, water droplets infiltrate the non-woven fabric more quickly. Compared with the surface of polypropylene non-woven fabric treated by plasma for 15 min, the surface roughness and hydrophilicity of polypropylene non-woven fabric treated by plasma for 30 min are higher.

Fig.1 Effect of plasma treatment time on hydrophilicity of polypropylene non-woven fabric

Effect of plasma surface treatment on surface chemical composition of polypropylene non-woven fabric

In order to further verify that the hydrophilicity of the non-woven fabric after plasma treatment is related to the surface functional groups, the micro-infrared spectrometer was used to characterize the changes of the surface chemical composition of the samples before and after plasma treatment. The polypropylene non-woven fabric after untreated and plasma treated for 30 min was selected as the research object, and the results were shown in Figure 2. There are three bonding states of carbon atoms. Among them, 2850 ~ 2914cm − 1 and 2951cm − 1 correspond to the stretching vibration of CH2 and CH3, and the vibration peak at 1373 ~ 1453cm − 1 corresponds to the carbon atoms in the form of C-H. Several new absorption peaks appeared in the FT-IR spectra of the non-woven surface after etching for 30 min, including the hydroxyl peak at 3421cm − 1, the carbonyl peak at 1710cm − 1 and the ether bond at 1235-1091cm − 1. This shows that after the oxygen plasma etching treatment, the oxygen element was successfully introduced on the surface of the polypropylene nonwoven fabric.

Fig.2 Micro-FTIR spectra of polypropylene non-woven fabrics before and after plasma treatment in the wavelength range of 4 000-500 cm.

In order to further study the effect of different plasma treatment time on the surface element content of non-woven fabrics, XPS was used to characterize the element content and chemical bond state changes on the surface of the samples before and after plasma treatment. The relative atomic percentage content of each element and different groups was calculated by peak area, and the results are shown in Table 1.

Figure 3a is the XPS full spectrum of the surface of polypropylene non-woven fabric before and after plasma treatment. The results show that the obvious C1s peak and the weak O1s peak can be seen on the untreated non-woven fabric spectrum, and the oxygen element may come from the oxidation reaction in the air. After plasma etching for different time, the relative intensity of C1s peak decreased from 91.58 % to 85.96 %, while the O1s peak increased significantly, and the relative content reached 14.04 %. It shows that with the increase of oxygen plasma treatment time, the content of oxygen introduced on the surface of polypropylene non-woven fabric gradually increases, which is also the reason why the modified non-woven fabric exhibits superhydrophilicity.

Fig. 3 XPS full spectrum of polypropylene non-woven fabric surface before and after plasma treatment

The XPS spectra of C and O elements were further analyzed by peak fitting, and the chemical bond state of the surface after plasma modification was analyzed in detail. Fig.3b shows the fitting results of C element. The peak at 284.8eV is related to C-C bond, the peak at 286eV is related to C-O-C bond, and the peak at 288.5eV is related to O-C = O bond. After plasma treatment, the types of functional groups corresponding to C1s on the surface of polypropylene non-woven fabrics increased. After 30 min plasma treatment, the relative atomic content of C-C group is the lowest, but it still dominates, which proves that the modification treatment does not change the matrix structure. The peak at 531.5 ~ 532 eV is related to O2 −, and the peak at 533 eV is related to OH −. Combined with the spectra of C1s and O1s, it can be found that with the increase of oxygen plasma etching time, more oxygen-containing functional groups are introduced, the relative content of C-C groups gradually decreases, the relative content of C-O-C and O-C = O groups increases, and the relative content of OH − increases, thereby improving the surface hydrophilicity of non-woven fabrics.

Fig.3 XPS analysis results of polypropylene non-woven fabric surface before and after plasma treatment

There are only methyl groups with poor hydrophilicity in the molecular structure of polypropylene nonwovens. After oxygen plasma etching, the peak height of the bonding state of carbon atoms has changed significantly, and more functional groups and polar groups are generated on the surface. The results show that during the plasma treatment process, oxygen plasma bombards the surface of non-woven fabrics, breaks the methyl chain, produces free radicals, and then reacts with oxygen ions to generate oxygen-containing functional groups such as hydroxyl and carboxyl groups. This process is aggravated with the prolongation of plasma treatment time. Under the combined action of surface hydrophilic chemical composition and micro-nano structure, non-woven fabrics are finally infiltrated. Table 1 shows the quantitative change of the relative content of functional groups under different etching time, which further supports this view.

In summary, oxygen plasma treatment is an effective method to improve the hydrophilicity of polypropylene nonwovens. Oxygen plasma treatment introduced hydrophilic functional groups such as hydroxyl and carboxyl groups on the surface of polypropylene non-woven fabric, which promoted the modified surface to obtain extremely low water contact angle.