Nanoinjection molding (NMT) is not only an injection molding process, but also a bonding process that combines two materials at the nanoscale through injection molding. It was first developed and applied to the bonding between metals and plastics by Daihatsu Corporation in Japan. The principle is to form nanoscale pores and residual water-soluble amine compounds on the metal surface through chemical etching. The molten polymer is then bonded to the metal surface through injection molding. During the bonding process, the amine compounds undergo an exothermic reaction with the polymer, causing it to flow into the nanopores and form a local structure similar to a ship anchor with the metal substrate, which is vividly called the anchor bolt effect.

The nanoscale treatment of metal surfaces can improve the interfacial bonding strength between polymers and metals, but still faces challenges such as limited bonding area and limited polymer selection. There is still a lot of room for improvement in the bonding performance of polymers, so material innovation and process improvement are urgently needed. Plasma treatment technology, as one of the commonly used methods for surface modification, can not only achieve nanoscale treatment of metal surfaces, but also effectively improve the polarity and hydrophilicity of polymer surfaces, thereby enhancing the bonding performance between polymers and metals.

Plasma, also known as plasma, is a non condensed system generated by gas ionization, consisting of neutral atoms or molecules, excited atoms or molecules, free radicals, electrons or positive and negative ions, and radiation photons. Compared to ordinary gases, plasma is a locally conductive but macroscopically electrically neutral fluid, and the interactions between active particles in its system are much more complex than those of particles in an ideal gas. Therefore, it is often regarded as the fourth state of matter besides solid, liquid, and gas.

Principles of Plasma Surface Treatment

Low temperature plasma technology, as a new type of oxidation treatment technology, undergoes many chemical reactions during its formation process. Its high-energy electrons collide with background molecules (nitrogen, oxygen, water, etc.) to produce active particles such as secondary electrons, photons, ions, and free radicals.

Reactive oxygen species and reactive nitrogen are the main chemical active particles generated by atmospheric low-temperature plasma discharge. Among them, ROS plays a major role in the surface modification process of materials due to its oxidizing properties. ROS mainly includes free oxygen (· O), hydroxyl radicals (· OH), ozone (O3), and hydrogen peroxide (H2O2).

During the low-temperature plasma discharge process, a large amount of free oxygen is generated due to the ionization of oxygen in the air, which in turn leads to the generation of ozone; Due to the presence of water molecules in the atmosphere, H2O2 is produced through a series of reactions between high-energy oxygen atoms and water molecules, and water molecules also generate hydroxyl radicals through dissociation and ionization. The specific reaction process is as follows:

As shown in Figure 1.1, in a specific reactor, a large number of active particles (H2O2, O3, · OH, · O, etc.) are generated, which usually have high oxidation ability. Through a series of chemical reactions such as corrosion, cross-linking, and oxidation with the reactants, oxygen-containing functional groups such as - OH and - CO - and - COOH and - CHO are introduced on the surface of the reactants. At the same time, a portion of the surface molecules also undergo thermal decomposition, with large molecules degrading into small molecule fragments and detaching. Some of the degraded small molecule fragments still remain on the surface of the reactants, making the material surface uneven. This process is called etching in the field of microelectronics manufacturing and is an important processing method in micro nano process technology.

Figure 1-1 Schematic diagram of low-temperature plasma treatment technology

Nanoinjection molding is not only a molding process, but also a connecting technology. The core of the technology is the anchoring effect between polymers and metals. How to improve the bonding strength of heterogeneous materials is the main breakthrough direction of nanoinjection molding. Plasma treatment can significantly improve the surface activity of polymers, thereby enhancing the hydrophilicity of materials and the adhesion performance of heterogeneous interfaces. Therefore, plasma modification is a process control method to improve the bonding strength of polymer/metal interfaces, providing the possibility to enhance the reliability of nanoinjection molding materials.