Consumables can represent a significant cost in a cleanroom operation, but today's increasingly stringent requirements for cleanliness mean that while they may be disposable, they are certainly not dispensable.
Although cleanroom wipers may typically represent a relatively low percentage of the total cost, their impact on the process and on the product can be very high. A typical cleaning process with a wiper can last from a few seconds to several minutes. In Western Europe, where labour costs are relatively high, the workplace costs of a semiconductor fab are estimated to be around €1 a minute. When choosing a cleanroom wiper, the user needs to balance out the total cost against the cleanliness profile of the operation and the performance of the wipe, including elements such as the sorbency rate and capacity, as well as the ability to pick up and remove contaminants. Two different cost factors must be borne in mind in selecting a wipe: the material or purchase price; and the cost of using the wipe, that is to say, a paid person cleaning a sensitive surface in a certain amount of time with a specific result. The choice of wiper fibre substrates will similarly depend on several considerations: • The cleanroom class • The actual cleaning application • Specific mechanical needs Knitted fabrics made from continuous filament polyester yarn have proved to be one of the cleanest products available but for many applications the sorbency of this inherently hydrophobic material is not satisfactory.
Increasing sorbency There are various possible ways to increase the sorbency capacity and sorbency rate of polyester. One option is to use surface active chemicals (surfacants) but this typically results in unacceptably high values for extractables and potential ion contamination of the surface or part that needs to be cleaned. Another route is to blend the polyester – for example, in a non-woven material – with other more absorbent materials like cellulose, but this typically results in high values for fibre and particle shedding. However, Milliken & Company, the world's largest privately owned textile and chemical company and a producer of functional technical fabrics for industry, has taken a different route to improve significantly the sorbency of polyester. Using the company's polyester processing know-how, a process was patented that allows specific functional groups to be chemically attached to free valencies of the polyester polymer chain. This process gives properties to the polyester yarn that make it especially interesting for the critical environment industry: • Maximum sorbency for a polyester fabric with a given fabric weight • High absorption speed (or wickability) • Lowest values on particles and fibres as the final product is still a modified continuous filament polyester • Lowest values on extractables and ions as no additives, lubricants or other surface tension reducing ingredients are used This extra polyester modification process obviously increases the cost of the raw material, but against this additional cost, the following performance parameters must be weighed to assess the total cost impact: • Given a specific cleaning need, how many wipers are used by the typical work force to perform this cleaning action? • How fast can a typical cleaning action be performed with the various products? • How safe is the wiper with respect to making sure it does not "import" any contaminants into the process like fibres, particles, ions, non-volatile residues (NVRs)? In an application where sorbency is a key requirement, a wiper with maximum sorbency will ensure that fewer wipers are required for the same cleaning action. This offers the company two benefits: it not only reduces the overall cleaning cost, it also cuts the total number of wipers that are introduced into the plant – a factor that is often overlooked. Earlier this year Milliken introduced Matrix, an ultra-microdenier wiper with a denier per filament of approximately 0.2. This gives the Matrix wiper outstanding particulate holding properties, the company says, together with excellent wickability and low fibre counts.
Unique construction In the area of highly absorbent non-woven materials Milliken also recently introduced the Captura family of wipers that uses a unique 100% Rayon construction. The products are said to combine the best absorption of a "natural" material in a continuous filament and to offer a balance of very low particles and fibres with very high sorbency capacity that has not previously been achieved. The wipers are made by Milliken using Asahi's Bemliese spunbonded rayon technology. "The fact that spunbonding is a continuous filament process lends itself to cleanliness and low fibres," explains Chris Roman, Milliken's Anticon products business manager. "There are no additives or lubricants and the absorbency of rayon is extremely high compared with other raw materials. Our Bemliese technology partner, Asahi Kasei, has worked very closely with our company to ensure that the proper attributes have been designed in." The properties of the Captura range make it suitable for less critical environments of ISO Class 5 (Class 100 Fed. Std. 209D) and higher, such as those used in life science and other applications. The products are available already sterilised or can be autoclaved in the customer's own facility. "Milliken recognises that there are numerous unmet needs in the areas of non-volatile residues, sorbency and fibres, and we are striving to find solutions," Roman conclude