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Cleanroom wipes have their origins in the nuclear industry when the first critical wipes were cotton twill used to control radioactive particles from leaving nuclear reactor containment. The emergence of microelectronic manufacturing required cleaner wipe options and the cotton twill was replaced by nylon monofilament. By the mid 1980s polyester monofilament became the standard critical environment wipe. The pharmaceutical and life science industry which were more concerned with viable contamination, were later to adopt critical wipes but today cleanroom wipes for all types of critical environments is big business. As manufacturing technology innovated so the demand for even cleaner wipes increased.
Substrate innovations
In the late 1980s microfibre was introduced. The definition of a microfibre is a fibre with less than 1 decitex per filament, where a decitex is a measure of linear density which is commonly used to describe the size of a filament or fibre. 1 One decitex is 9/10th of a denier. To put this into perspective it is 1/16th the diameter of a human hair. The fibres can be combined to create yarn which can be then be knitted or woven into a variety of constructions. Microfibre fabrics can be broken down into two main types, splitable microfibre and straight filament microfibre.
[Old Magic Wipe_low] Straight filament microfibres tend to made from 100% polyester. Splitable microfibre consists of very fine threads of polyester and polyamide (nylon) that are combined to form a single thread. The nylon is used to glue the fibres together until they are split later in the process. Split microfibre (Fig 1) possesses numerous wedges rather than the rounded threads found in other yarns. It is these wedges that provide the ability to collect microscopic particles off a surface. This expanded surface area and the capillary action of the fine threads dramatically increases a microfibre wipes sorbancy. A change in the percentage of the microfibre blend will yield slightly different properties.
This split structure gives the wipe the ability to pick up particles even when dry, and can easily remove residues often without solvent. The split fibres create microscopic “hooks” which collect and hold dust, dirt and particles more effectively than rounded fibres such as cotton, this can be seen in fig 2. The microfibres are also positively charged so electrostatically attract the negatively charged dirt. The virgin wipes are very soft so won’t scratch and damage surfaces, however can should be taken when re-using wipes as the very structure which makes them so good at picking up particles also means that particles get stuck in the fibres which can then scratch a sensitive surface.
Microfibre has a high sorptive capacity around 6 to 8 times its own weight in water. The fast wicking ability means a wipe can remove spills quickly and easily, so it is very suitable for mop to dry situations.
However in cleanroom environments there are some downsides to the use of microfibre, the fibres are less durable and also create much higher levels of fine particle contamination. The microfibre would need to be laundered to reduce these levels of contamination. Microfibre is high in cost so not ideally suited to being a single use material. Microfibre is most relevant to applications where mop or wipe to dry performance is paramount and the activity of continual relaundering does not create problems. A split microfibre containing nylon is not compatible with bleach based disinfectants.
Risks of relaundering microfibre
Microfibre is not an inexpensive fabric - see table 1 for a comparison of different blends of microfiber versus 100% polyester and the corresponding attributes. In many instances this high initial cost is balanced by relaundering and where relevant the resterilisation of the mop or wipe. Mops especially are laundered and reused. However, in a cleanroom environment this is not without risk. Microfibre is very delicate and can be easily damaged by high heat or harsh chemicals, this can lead to a mop or wipe degrading over time and affecting both the cleaning ability and the sorbent capacity of the mop over time.
As cleaning is carried out from the cleanest to the dirtiest area within a cleanroom complex the soil load on mops can be significantly different. It is not easy for the laundry to guarantee getting all the mops back to the same level of cleanliness every time. The structure which makes the microfibre so good at picking up particles and retaining them for removal from the surface also makes it equally difficult for them to be cleaned as small or microscopic particles remain embedded in the wedges of the microfibre structure. A question which must be asked of the laundered is what other products your mops are washed with. Mops and wipes often do not make up enough volume for a full wash load and so laundries will often combine mops and wipes (of all colours) into one washload. This can lead to cross contamination which can leave particles or other contaminants trapped in the microfibre product and returned to the critical environment.
Recent developments
The drive for a product which could provide the benefits of microfibre whilst minimising the downsides lead to the introduction of the first new category of wipe in almost 30 years. MicroGenesis combines the best features of knitted, nonwoven, and microfibre technologies to deliver unprecedented performance to critical wiping applications.
The MicroGenesis wipe is designed with a micro-terry pile to give the fast and efficient pick-up and retention of particles for which microfibre is known. However the wipe utilises 100% polyester microfibre yarn knitted into a thermally bonded polypropylene substrate limiting the amount of microfibre required and making the wipe more suitable for single use. This structure applies microfibre only to the wiping surface where it is needed while sealing the base of each stitch in a thermally bonded base. This more cost effective structure still delivers the required amount of microfibre to pick up and retain microscopic particles, dust and light oils from critical environments.
Particle attraction technology
There are other developments which can provide a wipe with exceptional pick up and retention of particle properties without the use of microfibre. All fabrics can be treated to improve different elements of their performance. One patented treatment by Anticon which permanently bonds to the fabric, traps 35 more particles than an untreated wipe and retains 95% of the particles which are trapped. Interestingly this performance is enhanced and not degraded in the presence of alcohol or solvent.
The IEST test methods for testing wipes, test how many particle and fibres a wipe releases but there is no standard test method to show how many particles a wipe picks up. In order to prove that the wipes were truly achieving this level of particulate pick up Peter K Kang and David Hildreth developed a new robust test method 2.
Basically, by creating suspensions of known particle size in deionised water and then submerging treated and untreated wipes in the particle suspension, they were able to show how many particles were both collected by each type of wipe and held onto by each type of wipe.
The test results for fine particles showed treated wipers captured over three times more small particles than traditional wipers, releasing only 14% of the particles they captured. Additionally, the test results for large particles showed that treated wipes captured over 35 times more particles than the other wipes tested. Only a very small percentage of those captured particles were released.
To paraphrase Peter Kang, cleanroom wipes need to arrive clean and leave dirty. The ideal product performance for a cleanroom wipe is that it does not add to the contamination whilst performing the removal of particles, spills, biofilms or dirt. Various innovations in both wipe substrates and wipe treatment have aided in this, creating wipes which are ideal at not only particle pick up but retention of those particles until the wipe is removed from the cleanroom. Alternative ways of blending microfibres have allowed the creation of wipes with microfibre properties at the cost of a disposable wipe.
1 Textile Terms and Definitions, 11th Edition, The Textile Institute
2 Testing Wipers for Particle Retention and Attraction, Controlled Environments Peter K Kang PhD David Hildreth