Nylon 6 resin is a crystalline thermoplastic engineering material with excellent comprehensive properties, including good mechanical properties, drug resistance, gloss, and friction resistance. It is widely used in various fields of social production and life. Nylon 6 contains a large number of amide groups in its macromolecular structure, with amino and amino groups at the end of the macromolecule. It can be seen that nylon film is a highly polar material, so its water absorption rate is high, and its strength and elastic modulus decrease. However, its wet impact strength is relatively high. This is because in environments with high water content, the hydrogen bonding interactions between nylon 6 molecules weaken, the molecular chains become relatively flexible, and the impact resistance is improved. However, the impact resistance at low temperatures is relatively low. In addition, the nylon structure contains 5 non-polar methylene groups, which have a smooth surface and poor adhesion. To improve the adhesion of the adhesive to the nylon film, reduce various defects on the surface, and thus improve the quality of the composite material, nylon needs to undergo surface treatment before use.

Plasma surface treatment

Plasma exists in another form besides the three common states of matter (gas, liquid, solid), known as the fourth state of matter. It was first discovered and proposed by American scientists Langmuir and Tonks in 1929. Plasma commonly occurs in nature, such as lightning, electric sparks, auroras, etc., all of which are in the form of plasma. Plasma is usually generated by combining with neutral gas under high temperature or electromagnetic field. Ionized gaseous substances gradually become conductive, so their essence is a special electrified gas. The electromagnetic radiation generated by plasma includes ultraviolet radiation and light in the visible spectrum, and involves excited gas particles, charged ions, free electrons, free radicals, neutral reactive oxygen species and nitrogen, as well as molecular fragments. According to the existing temperature range, plasmas can be divided into high-temperature plasmas (also known as thermal plasmas) and low-temperature plasmas (also known as non thermal plasmas). The former is due to the higher ionization degree of gases, resulting in higher temperatures of electrons and particles generated by ionization, while the latter has a lighter ionization degree of gases. Although the working temperature during discharge is higher, the ion temperature is much lower than the electron temperature, resulting in an overall low-temperature state. High temperature plasma can almost completely ionize, with the same temperature between particles. Therefore, the entire high-temperature plasma system exhibits a thermodynamic equilibrium state, with a working temperature of over 5000 ℃, mainly used for studying controlled thermonuclear reactions. The temperature between particles in low-temperature plasma ionization is not the same. The electron temperature is much higher than the ion temperature, and the electron temperature can reach more than 104K. However, the temperature of molecular and atomic particles is extremely low, close to room temperature, and the entire low-temperature plasma system exhibits a thermodynamic non-equilibrium state.

In low-temperature plasma, there are more types of active particles than in general chemical reactions, and the energy of these particles is much greater than the chemical bond energy of general materials. Therefore, chemical bonds in materials are easily excited and dissociated by high-energy active particles, leading to the recombination of chemical bonds in the processed material and the generation of new functional groups that have a positive effect on the material. Due to the short processing time of low-temperature plasma and the absence of environmentally polluting substances during the treatment process, this technology is often used to modify the surface of materials.

Principle of improving adhesion by plasma treatment of nylon 6

After plasma treatment, the roughness of the nylon surface has increased to a certain extent, due to the etching effect of plasma on the material surface, which directly affects the adsorption of liquid on the material surface and improves the wetting ability of the material surface; The chemical composition of the material surface undergoes significant changes. Overall, the relative content of C element decreases, the relative content of 0 element increases, and the relative content ratio of C0 element decreases; The functional groups and their content on the surface of the material also change. The content of C-0 and O-C-0 bonds increases, and the energy in the plasma is converted to the chemical bonds on the surface of the material, resulting in an increase in the proportion of C and 0 double bond bonding.

According to relevant literature reports, the principle of plasma treatment for improving the adhesion of nylon 6 is as follows:

1. The increase in surface energy of nylon and the introduction of polar groups enhance the dipole forces between molecules

2. Provide opportunities for the formation of chemical bonds between the adhesive and the introduced polar groups at the bonding interface;

3. Increase the surface roughness of the material and improve the overall mechanical properties of the material

4. Remove the weak boundary layer on the nylon surface to avoid damage to the bond caused by weak boundary layers with poor mechanical properties.