Plasma Treatment in textile and its application
In the mid 80's a plant has been running in Russia where low pressure plasma is utilized to oxidize fiber surfaces for improved dying. This is one of the very rare examples of plasma application in textile industry. What are the reasons that prevent a broader application of this new and promising technology?
It is only rather recently that the textile industry and plasma technology took notice from each other and started looking for fields that could take advantage from plasmas. On both sides there are some specific demands. Plasma is based on low pressure and needs vacuum equipment which is not common at all in textile industry. On the other hand, textiles have a rather large specific surface area and contain usually a substantial amount of water. Both facts ask for a good vacuum system, a better one than it is needed for the treatment of films or other solid parts. The role of water in plasma treatments is an important issue that needs to be investigated in more detail for non-oxidative processes.
The surface properties alterations obtained by a plasma treatment are complex. Particle induced reactions take usually place in the upper ten nanometers of a surface. Short wavelength UV-radiation as it is emitted by low pressure plasmas initiates reactions in a thicker layer (about 100 nm). The relation between the two and the extent of both can be controlled by the process gas and other process parameters. The outermost surface, only some atom layers, sometimes less than 1 nm, determines the interaction with other media. The chemical composition of this part of a fiber is responsible for good or bad adhesion in laminates or whether the fabric is suitable for impregnation or not. Exactly this part of a fiber can be modified by a plasma. For the success of the treatment not only the process parameters contribute, but also the original surface is crucial. Trace amounts of sizings, for example, can modify the reaction condition substantially and have to be taken into consideration for almost every process.
Many different types of treatments have been done so far in batch experiments. This is a legitimate way to demonstrate that a certain treatment can create a desired effect. However, for an industrial application a continuous process is inevitable. There are basically two types of vacuum equipment to realize a continuous web treatment:
In an air-to-air system the material passes several differential pumping stages before it reaches the actual process chamber and after it had left it. This allows a real continuous treatment and is particularly suitable if one material is treated in one process for a long period of time without changes. The metalization of polymer films is done with this technology.
For the treatment of different materials, a semi-continuous or roll-to-roll technique is usually more reliable. Deviations of the materials properties as for example thickness and water content can be controlled easier than in a continuous treatment. In particular for technological investigations this is the way to go.
Application examples from our laboratory
Most of the problems that arise when textiles are treated with plasma do not apply when the surface has to be oxidized in the plasma for example to create polar and reactive functional groups. In that case the background pressure in the reactor is not so important and also the water from the webs can be utilized in the treatment. The only real danger is an overtreatment. In a prolonged plasma exposure, polymers form low molecular weight substances which can deteriorate the surface properties, can be washed off and expose a surface which is treated to a lower extent.
More sophisticated treatments have a much narrower range of treatment conditions and need much more care in process control. The hydrophobation in a fluorocarbon plasma is an example for such a type of treatment.
Example 1: Water repellent polyaramide fabric
The data in the table demonstrates that a fluorocarbon plasma treatment can reduce the soaking of fabrics in a similar way like a traditional impregnation. However, in contrast to the wet treatment, the fabric retains its flexibility after the plasma treatment.
Cotton or hemp fabric usually absorbs water immediately.
Applying a low-pressure plasma process, the fiber's surface can be altered to make it repell water. After the treatment, drops run freely over the surface while mechanical properties, the visual appearance, and the permeability for water vapor remain unchanged. The surface modification is limited to a very thin layer. A treatment as short as 2 seconds can be sufficient to achieve this effect in a batch process. Continuous treatments with a speed of more than 20 m/min are conceivable.
The stability of the modification can be seen in intermitted washing cycles of fluorocarbon treated cotton fabric. After an initial drop, the finishing remains stable for at least two hours at 95°C. The quality of the repellent effect is evaluated by putting water drops to the fabric surface. A value of 1 means that the drops run freely over the surface and do not penetrate into the material while at a value of 3 the water does not penetrate but it needs vibrations to move the drop. Obviously this evaluation depends also on the nature of the fabric.
Example 3: Wettability improvement
In an oxygen plasma the number of functional groups at the surface can be increased. The increased polarity makes the material more wettable which can be used to improve dying and sizing.
In the table we summarized examples where various polyamide fabrics were oxidized. The effect of the treatment was checked by a water rise test, i.e. a strip of the fabric was put into water end the time was measured until the water rise up 3 cm.
The test was repeated a certain time after the treatment. The results show a good stability of the treatment.
Textile materials have been widely used in medical and surgical applications, but large surface areas and the ability to retain moisture make textile fabrics more prone to bacterial growth . Cross-transmission of diseases in hospitals is a major threat . Surgical apparel should not only provide protection against microbial attack, but should also have blood barrier properties since most infections are caused by blood-borne pathogens [10, 19]. To prevent or minimize infection or transmission of diseases and to protect patients and surgical staff from infectious fluids, it is very important to treat these textile materials with antibacterial and fluid repellent finishes [5, 6]. Textile materials used in operating theatres have to meet stringent demands due to the growing threat of blood pathogens like HIV (human immunovirus) . The Occupational Safety and Health Administration has passed regulations stating that protective garments for healtheure workers must provide protection against blood and other potentially infectious materials [2, 9].
Surgical gowns made from nonwovens have been treated with antimicrobial agents and fluid repellent finishes through wet processing techniques . However, there are some disadvantages to conventional fluid repellent finishes, such as the large amount of water consumed, the high cost of drying; emission of harmful chemicals presenting waste disposal problems, and the high cost of neutralizing chemical wastes [13, 14]. Some low wet pick-up techniques have also been investigated, but they are limited due to non-uniform application of finishes [15|. Moreover, traditional textile finishing operations change the bulk properties of the substrate. To prevent the change in bulk characteristics and to lower costs, surface treatments have often been investigated. Plasma treatments are surface treatments (hat are cost efficient, environmentally friendly, uniform, and applicable to many materials, and they keep the bulk properties of the substrate intact.
Plasma is an ionized gas composed of charged and neutral particles . When this plasma is applied to the substrate, the free electrons or other metastable particles, upon collision with the substrate, break the chemical bonds creating free radicals on the polymer surface [4, 20]. These free radicals can undergo additional reactions depending on the gases in the atmosphere. Thus, the substrate can be coated with different kinds of gases without changing the fabric's bulk characteristics. In textiles, plasmas have been used to provide adhesion, clean suifaces, hydrophilic surfaces, and they can change the state characteristics and sterilize medical components, etc. [7, 11, 16, 20, 21]. Atmospheric pressure plasma techniques are gaining more popularity due to the ease in incorporating them into continuous textile finishing operations 111, 23]. Plasmas containing CF^sub 4^, C^sub 3^F^sub 6^, argon, or other hydrocarbon gases can provide hydrophobicity to the fabric .
There are a very few research projects that have focused on plasma treatment of fabrics for medical protective clothing. Research in the area of acceptance as an effective barrier against blood or body fluids for surgical gowns is required. The purpose of this study is to explore the advantages of an atmospheric pressure plasma treatment us a hydrophobic finish for nonwoven surgical gowns. Therefore, our research will address the question of its acceptability as a hydrophobic finish in nonwoven surgical gowns in providing a barrier against blood and microbial activity. To answer that question, our study will have three main objectives. First, to evaluate whether the plasma treatment (C^sub 3^F^sub 6^ can provide repellency against blood and water. second, to see if the plasma in conjunction with an antimicrobial finish provides a better barrier against blood and microbial activity in nonwoven surgical gown materials. And third, to see how the different chemical classes of antimicrobial compounds respond to plasma treatment.
Fabrics were treated with three different chemical classes of antimicrobial compounds through a two-dip and two-nip pad dry cure method. Chemical A was polyhexamethylcne biguanide, provided by Avecia Inc. Chemical B was 3-(trimethoxy-silyl) propyl-dimethyloctadecyl ammonium chloride, provided by Aegis Env. Mgt. Inc. Chemical C was 5-chloro-2-(2,4-dichlorophenoxy) phenol, provided by Sanitized Inc. Chemicals A and B were applied at 2% chemical on dry weight of the fabric to ensure good antimicrobial properties as recommended. A surfactant was required for even application of chemical B onto the substrate. In this case, 0.05% Freetex® wetter (ethoxylated alcohol in water) on dry weight of the fabric was used, provided by BF Goodrich Co. The bath pH levels for chemicals A and B were 7.5 ± 0.5 and 5.0 ± 0.5, respectively, and curing temperatures for chemicals A and B were 141 and 160°C, respectively. For chemical C, the same procedure was followed with 1.0% chemical on dry weight of the fabric, pH 5.0, and a curing time of 3 minutes at 120°C. All treatments were replicated three times.
After the antimicrobial treatment, the samples were ready for plasma treatment at the Nuclear Engineering Department of North Carolina State University under the supervision of Dr. Mohamed Bourham. After doing some preliminary research on water absorbency (Table I), the test conditions were fixed. The fluorocarbon gas used to provide hydrophobicity was C^sub 3^F^sub 6^. The radio frequency required to build the electromagnetic field was 5 kHz. The electrode separation was 6 cm, and the active electrode area was 0.37 m . Treatment time was fixed at 8 minutes. The power supplied was 4.8 kW. Due to the small active electrode area and to ensure an even finish on the fabric, the sample size for plasma treatment was reduced to 20 × 20 cm. Finally, the antimicrobial and plasma treated samples were tested to see if they could provide the antimicrobial activity and fluid repellency required for surgical gowns. Various tests were conducted to determine the changes in physical properties and antimicrobial activity.
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