Polypropylene fibers have the advantages of good mechanical properties, bending resistance, chemical resistance, and corrosion resistance, making them widely used in various fields such as cable nets, seat cushions, filling materials, asphalt modifiers, and reinforcing materials for papermaking felt. At the same time, the washability of polypropylene fibers makes them very suitable for use as protective clothing in the chemical industry. In addition, polypropylene fibers can also be used as filling materials for concrete, lime, etc., with the aim of improving the concrete's resistance to damage, tension stability, waterproofing, and thermal insulation.

The chemical formula of polypropylene fiber is (C3H6) n, and its structure is shown in Figure 1.1. From a structural perspective, polypropylene fiber is a pure hydrocarbon polymer, which has neither polar nor chemically active groups on its macromolecules, and is not easily compatible with polar polymers. Therefore, the chemical stability and hydrophobicity of polypropylene fiber are extremely strong, and suitable methods need to be selected to modify its surface.

Figure 1.1 Chemical Structure of Polypropylene Fiber

Principle of plasma modification of PP polypropylene fiber membrane

Atmospheric pressure plasma modification technology, as a non-contact surface modification method, can achieve the treatment of large-sized fiber membranes by adjusting the size of the flat plate, which has significant advantages of low cost and green environmental protection. This method can convert air into plasma at high energy levels and generate electrons with lower temperatures. While ensuring that the low melting point PP fiber membrane is not damaged, the oxygen-containing active functional groups are uniformly loaded onto the surface of the fiber membrane in a short period of time without affecting the performance of the membrane substrate.

The mechanism of plasma on the surface of polypropylene fiber membrane

Various active particles in plasma collide with the surface of materials and induce energy exchange, leading to the occurrence of many physical and chemical reactions, achieving surface modification of polypropylene fiber membranes. The interaction mechanism between plasma and the surface of polypropylene fiber membrane is shown in Figure 2, which can be divided into the following four categories according to the reaction type:

Figure 2 Schematic diagram of the action mechanism of plasma modified fiber membrane

Cleaning effect

Organic materials usually use certain fillers or additives during processing, resulting in the accumulation of surface impurities on the material surface. Plasma treatment can effectively remove impurities on the fiber surface. The cleaning effect of plasma on polypropylene fiber membrane is achieved through the synergistic effects of thermal effects, etching, and chemical reactions caused by electrons, ions, and free radicals. Due to the heating effect of electron and ion bombardment and plasma radiation on the surface of fiber membranes treated in plasma, etching can remove suspended particles on the surface and play a major role in cleaning the fiber membrane. In addition, the chemical substances generated by plasma adsorb and desorb on the surface of the fiber membrane, thereby cleaning the fiber surface through chemical reactions. As a dry method for removing impurities, there will be no waste liquid generated during the treatment process, and it will not cause environmental pollution

Surface functionalization

Low temperature plasma treatment uses plasma gas to modify the surface of fibers. After energy transfer by an electric field, gas electrons are accelerated and collide with neutral gas molecules or atoms, generating a large number of highly energetic and activated free radicals. The reaction gas is then excited to produce active particles. Plasma generates high-energy ions and photons that provide energy higher than the covalent C-C or C-H bond energy, and then bombards and destroys the C-C or C-H bonds on the surface of polypropylene fiber membranes, leading to the formation of C radicals. When different processing atmospheres are selected, different types of reactive groups can be introduced on the fiber surface.

By using gases or chemical reagent vapors containing F, Si, or Cl as reactants and mixing them with non reactive carrier gases for discharge, hydrophobic groups can be introduced onto the fiber surface. When the fiber membrane is treated with plasma generated by non reactive gases (N2, Ar, and He) or reactive gases (O2 and NH3), some reactive groups can be introduced to the fiber surface, resulting in hydrophilicity.

Etching effect

As the plasma treatment time increases, the weak boundaries on the surface of polypropylene fibers are often removed by the bombardment and sputtering of block shaped particles in the plasma. At this point, the surface morphology of the fibers changes and the surface roughness increases. Many obvious gaps can be observed on the surface of the fiber membrane based on SEM images. Therefore, in order to avoid damage to the fiber membrane substrate, lower voltage or gas consumption is usually used. When the plasma treatment time is too long, the gap depth can reach tens to hundreds of nanometers.

Grafting

By pre activating the surface of polypropylene fiber membrane with plasma, polymer materials can be grafted onto the surface of the original nanofibers to form a new surface. The specific steps include: activation of organic precursors in the gas phase, transfer of reactive film-forming substances to the substrate surface, and growth of polymers on the surface. It should be noted that the aggregation process is always accompanied by etching effects. In fact, they are two competitive processes that occur simultaneously.

In summary, plasma is composed of a mixture of charged particles (electrons and ions), excited atoms (free radicals, metastable molecules), and photons. Substances such as ions, electrons, and excited atoms will bombard the surface of polypropylene fiber membranes, producing physical and chemical reactions on the surface of polypropylene fiber membranes, playing a role in surface cleaning, surface etching, surface activation, etc.