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Large diameter rolls of wipe substrate are fed into machines where they are slit to the required width. The non woven cloth is fed into a coating machine where the liquid phase is applied by a number of methods: running the non-woven through a trough of the solution, spraying sheets with the liquid through a series of nozzles or injecting into packs of folded wipes. Addition of the liquid to the wipes in a manner that gives the most even distribution of the liquid to the wipes provides the best protection to the finished product. Injection into a folded pack gives the least uniform distribution while spraying liquor from above onto the moving wipes gives the most uniform distribution. The amount of liquor on a wipe is also critical for its effectiveness and the ratio between liquor and wipes varies for different applications. For cleaning purposes either too little or too much liquor is detrimental.12

Packaging

Wet wipes may be packaged in a number of ways, depending on the eventual use of the wipes. The preferred method from a preserving point of view is individual sachets. Once packed there can be no introduction of microorganisms and no loss of moisture. Unfortunately this is the most expensive and inconvenient way to supply wet wipes and so they are generally supplied in bulk packs. Household wipes are usually packed into soft packs or tubes with pull through lids while baby wipes are most often in tubs with an opening lid for easy access. Facial cleansers are often in soft packs with a small resealable opening. Figure 2 shows a variety of typical pack types. The different packaging effects the migration of the liquid on the wet wipe but all will allow migration away from the opening. For rolled wipes in tubes, the tops of all wipes may experience drying and increased chance of microbial growth, while with tubs and soft packs the wipe nearest the opening will be most susceptible. Tubs with a large opening lid that may be left open allow greater chance of ingress of dirty fingers and microbes from the air than do the pull through tubes so may require more robust preservation. They are also most likely to be able to be refilled with new wipes so any microbial contamination from the previous load may get a head start on growing on the refills. Figure 3a shows a wet wipe protruding from a canister that appears normal. However opening the canister (Fig. 3b) reveals light fungal growth on the top of the wet wipes, but when the roll is removed from the canister (Fig. 3c) extensive fungal growth is evident. The exposed wet wipe in Figure 3a was quite dry and unable to support fungal growth, while the wipes inside the canister maintained sufficient moisture for fungi to grow. When removed from the canister the top of the wipes roll was drier than the base as the liquor had migrated towards the bottom of the canister. This migration resulted in the wipes near the top of the canister containing sufficient moisture for fungal growth but insufficient preservative to control them while the wipes towards the bottom of the container had sufficient preservative to stop the fungi from growing.

Preservative efficacy testing of wet wipes

There are a range of test methods currently in use, all with the basic concept of adding microorganisms and checking for survival. Early methods only tested the liquor and it has been shown that this was not appropriate.13 As already discussed, interaction of the liquid with the wipes may affect the efficacy of the preservative system, or heavily contaminated wipe feedstock could reduce the preservative concentration resulting in field failures in products where the liquor had passed antimicrobial preservative efficacy tests. It has also been found that preservatives may interact with the packaging reducing their effectiveness. Newer test methods rely on adding a large number of different organisms to the finished wipes and storing them under appropriate conditions for the desired length of time. Surviving organisms are then enumerated by collecting the liquor from the wipes and conducting microbial counts using various techniques. The manner of inoculation varies with one published method requiring collecting the test organisms on membranes, drying them and placing the dried membranes between two wipes in the original pack.13 Typically the inoculum is sprayed or pipetted onto the wipes. It may consist of pure cultures, with each test organism introduced to a separate test piece, or a mixture of specified test organisms added together. The manner in which the organisms are added to the test piece and treatment to ensure even distribution of the organisms has a huge effect on the test outcome and the reproducibility of the test results. The concentration and type of organisms is also critical; too low a number may give a false sense of good preservation, while an excessive concentration may require unnecessarily high levels of preservative to pass the test. The use of inappropriate test strains or strains with known preservative tolerance may also give misleading results. The test method must be designed to place the inoculum in an appropriate part of the wipes and ensure good recovery of surviving organisms. The recovery method is as critical as the inoculation to ensure that results achieved are truly representative of the number of organisms surviving on the test pieces. It is also necessary to develop pass criteria based on the reduction in the number of organisms surviving after specified time periods. The selection of an appropriate test method is critical to ensure confidence that a test result from the laboratory will truly represent what will happen in the field when the product is produced and released.

Regulatory aspects

Wet wipe preservatives, depending on their application and claims, are required to meet different regulatory requirements. Wet wipes may be used for purposes with Therapeutic claims, such as sunscreens, or other applications, like disinfectants, that may bring them under regulatory control depending on which country they are to be sold in. Preservatives used in personal care wipes will need to meet the requirements of the ASEAN Cosmetics Directive or the EU Cosmetics Regulations if they are to be exported to an EU country. Preservatives used in industrial and household applications are not limited by these restrictions. Thus the range of preservative actives available for a surface cleaning wipe can be different from a skin cleaning wipe even though the exposure risk may be similar to the person using the wipes. The influence of Eco-labels can further complicate the situation through more stringent criteria limiting preservative use. This makes it impossible to make global recommendations for preservatives in consumer applications where wet wipes predominate.

Conclusion

The preservation of wet wipes is a complex problem due to influences of the wipe and packaging on the liquid phase of the wet wipe. Generally high concentrations of a number of preservative actives are required to ensure complete long term protection against bacterial and fungal contamination. For industrial and household applications simple systems using traditional preservatives such as MCI/MI, MI/BIT and blends with formaldehyde donors are still widely used. The use of methylisothiazolinone in combination with other preservatives has been found to offer excellent protection to most personal care wipes and the use of phenoxyethanol is quite widespread. A move towards ‘natural’ products is occurring at a slower pace than other personal care sectors and the use of phenoxyethanol in combination with the organic acids may provide good protection as long as the correct pH is maintained. The addition of multifunctional ingredients, such as emollients and humectants which act as biocide boosters, improves the preservation of wet wipes ensuring good long term stability. Proven preservatives such as parabens, IPBC and methylchloroisothiazolinone/ methylisothiazolinone are still being used but with limitations.

A version of this paper was presented at the Australian Cosmetic Chemists Conference in Adelaide, 2012.


Post time: Jan-20-2021