Wool fabric plasma treatment: resulting properties evaluation and applications in field

Industry is on a continuous way to search new treatments and novel technologies to reduce costs, increase productivity, decrease waste generation and treatment. The textile industry is characterized by mature processes and its breakthrough has necessarily to come from technological transfer from research. Plasma treatment being based on a gaseous technology does not require to use many chemicals, as well as the precious resource given by freshwater. In this view, plasma treatment of textile materials appears very promising in replacing a number of current wet chemical processes, or at least, intensifying them. Plasma is commonly known as the fourth state of matter, being constituted by ionized gas in a neutral state with an equal density of positive and negative charges. It consists of a fast evolving mix of ions, electrons, free radicals, meta-stable excited species, molecular and polymeric fragments. Additionally, a high energetic content given by visible, UV and IR radiations characterizes plasmas. Decades of development have allowed researchers and scientists to confine geometrically plasma, control its energy and develop applications in material processing and waste reduction by designing devices suitable to very many industrial uses. Although plasma treatments have been used for years to process materials including semiconductors, microchips, and other electrical and electronic components, only recently the textile industry has considered the use of plasma for fabric processing with particular emphasis given to the surface of this material. Plasma treatments are classified among nanotechnologies since they interact with the fiber surface only and do not alter the properties of the fiber core. Plasmas can be classified according two major categories: thermal and non-thermal. A thermal plasma is characterized by a very high temperature and it is not suitable for applications to heat-sensitive materials. A non-thermal plasma is generated at moderate temperature and it is suitable for heat-sensitive materials such as textiles. Non-thermal plasmas are also known as low-temperature plasmas (LTP) and can be classified into many different categories depending on operating pressure, type of power supply (lowfrequency, radio-frequency and microwave) and geometrical arrangements. Plasmas modify the surface of materials by transferring energy from the excited plasma particles to the substrate. Thanks to this interaction, both chemical and physical modifications can be obtained. The mechanisms, which give origin to these modifications, include surface etching, surface activation, cross-linking, chain scission, de-crystallization, oxidation and surface chemical reactions. The reaction type depends largely on the type of gas used. For instance, inert gases such as argon and helium typically generate surface activation. Compounds that contain oxygen are commonly used as etching gases. Most likely, nitrogen is prone to cause reduction reactions. Pre-treatment and finishing of textile materials with LTP offers many advantages over conventional chemical processes, because most part of LTP surface modification treatments do not require use of water or chemicals and are characterized by an extremely low energy need. The possible applications of LTP in the textile field are commonly dyeing and finishing oriented; they include several types of functionalization such as hydrophilic enhancement to improve wetting, dyeing or adhesive bonding. Hydrophobic enhancement gives origin to water- and oil-repellent textiles. It is possible to change physical and/or electrical properties, clean surfaces, remove sizing agents, and perform surface sterilization of fiber at room temperature. Although most of the LTP treatments on polymeric materials, including textiles, developed by researcher have been carried out using low-pressure plasma, the atmospheric pressure plasma has demonstrated to be a much more interesting technology for large-scale applications. However, data and results from low-pressure plasma applications can be used to predict, compare or optimize atmospheric pressure plasma processes. In the work of this PhD research project all plasma treatments used are conducted at atmospheric pressure with a variety of gases such as nitrogen, oxygen, helium, argon a mixture of them and ambient air. Only atmospheric plasma equipment were selected because of easy applicability in a continuous mode, by considering that according to economical reasons, the evolution of textile technologies is oriented to continuous processes. In this thesis, Chapter I gives a general description of the LTP physics, main plasma generation systems and processing equipment, including also typical applications. In Chapter II a brief description of the selected textile characterization methods adopted in this work is provided. SEM, ATR-FTIR and XPS analyses, wettability tests, air and water vapor permeability measurement, tensile strength and low-stress mechanical properties test were selected to characterize and evaluate textile modifications carried out by plasma. Additionally, also the transformation mechanisms are enlightened. Chapter III comparatively describes the effect of three kinds of atmospheric plasma treatment. These treatments, finalized to wool fabric hydrophilicity enhancement, were performed in reducing, oxidizing and neutral conditions. A novel wool fabric dyeing process is described in Chapter IV. This process, requiring plasma as a pre-treatment, gives emphasis to water and energy saving, as well as to process productivity. Then, the fabric produced was characterized according to standard methods. The study presented in Chapter V deals with the comparison of a standard pad-dry-cure coating process for water- and oil-repellent finishing with respect to a plasma intensified pad-dry-cure coating process and a plasma enhanced chemical vapor deposition (PECVD) process. These two innovative processes were performed to improve coating durability, reduce chemical consumption and avoid the least use of water, as in the PECVD case.