Wet wipes have become an increasingly popular item of everyday use in homes and workplaces. Although they were originally developed as personal care products, their success has led to the development of many products with household and industrial applications for cleaning, sanitising and polishing.
The preservation of wet wipes presents a number of challenges not encountered with other liquid products and often requires a unique solution incorporating a combination of different preservatives to provide complete protection against microbial contamination. The standard concern with matching a preservative to a particular formulation is further complicated by interactions between the liquid and substrate and the preservative and wipe substrate. A variety of different packaging types also further complicates the situation. On top off these fundamental problems, the current trend in marketing personal care products that contain no controversial preservative ingredients is leading to further difficulties for the formulator. The requirement of products to contain no parabens, no formaldehyde donors and preferably no preservatives is becoming more widespread and mainstream. Removing these long used and well understood actives from the formulators’ range of acceptable ingredients is necessitating a completely new approach to the preservation process. The use of emollients and cationic actives to enhance the effect of traditional preservatives, or even replace them, is becoming more common.
The preservation of wet wipes
All aqueous and emulsion-based products require a preservative to protect them from bacterial and fungal contamination. Alcohol based wipes with an alcohol content in excess of 15% may be self preserving and generally require no additional preservation, but should be tested to be certain. Requirements for preservatives to be used in wet wipes are set out in Table 4. The liquid phase of a wet wipe can be treated as any other personal care product when it comes to preservative selection and the usual parameters of preservative compatibility must be followed. However, it is vital to consider the wipe material as well when choosing a preservative. Some substrates that contain natural or some synthetic fibres can bind or absorb certain preservative actives reducing their efficiency. Because of this, it is important that both the solution and the formulated wet wipe are tested for preservative effectiveness. As with all personal care products preservatives are added to protect the finished product from contamination by the end user and care must be taken to ensure that raw materials used in the production of the liquid are selected in the same manner they would be for any other type of personal care product. However, extra care must be taken to consider the source of the carrier material and the production processes. Any wipe material that has been produced by a wet process is liable to contain either bacterial or fungal spores. Incorrect storage of the bulk material may also make it liable to further contamination. Excessive levels of contamination will reduce the preservative content available to protect the finished wipe in use and will also be a source of contamination of the finished packaged wipe (see Table 5).
Preservatives in use
The same familiar preservative actives used in other personal care products are used to protect wet wipes. However, there is an obvious tendency to use blends of many actives, and at high concentrations. Most manufacturers use combinations or blends of preservatives with a variety of actives at high concentrations in an effort to provide good long term preservation. Commonly used combinations of preservatives are shown in Table 6.7
While the parabens are among the safest preservatives for personal care use their image has been severely tarnished by numerous NGO campaigns and their use is in decline for personal care products. However, they are still widely used in both personal care and household products. They are good fungicides, which make them very useful for wet wipes preservation, but are less effective against bacteria requiring their use in combination with other preservatives. They also have water solubility concerns that can make them difficult to use or require they be dissolved in a solvent such as phenoxyethanol for easier addition.
DMDMH is one of a group of formaldehyde donors so it is also under public pressure to be removed. As with all formaldehyde donors there is limited activity against fungi but good efficacy against bacteria.
Bronopol is an effective broad spectrum preservative with better activity against bacteria than fungi. It is often claimed to be a formaldehyde donor but does not express its antimicrobial activity by formaldehyde release. It may cause discolouration of the finished product and does face regulatory pressure due to the possibility of nitrosamine formation in the presence of TEA.
The isothiazolinones are a diverse group of preservatives with great variation in stability, water solubility and preservative efficacy: Benzisothiazolinone: The first to be developed was benzisothiazolinone (BIT) and it has been in use in household and industrial applications for over 40 years. It has been proposed as a personal care preservative but its approval and addition to Annex V of the EU Cosmetics Regulation is still pending. BIT is effective against most bacteria but has an activity gap against Pseudomonas sp. and is poor against fungi. This can be overcome by using higher concentrations or in combination with methylisothiazolinone. Methylchloroisothiazolinone (MCI)/ Methylisothiazolinone (MI): MCI/MI has been widely used in personal care products but has a potential to cause skin sensitisation, primarily due to the MCI, so is limited by the EU Cosmetics Regulations and ASEAN Cosmetic Directive to 15 ppm for leave-on and rinse-off applications. It is widely used for shower and hair products and offers complete protection against both bacteria and fungi. Although it is permitted, this combination is not often used in leave-on products. Methylisothiazolinone: This is one of the most recent preservatives to be added to Annex V of the EU Cosmetic Directive and has also been included in the updated Annex VI of the ASEAN Cosmetics Directive. MI is very effective against bacteria but weak against fungi requiring it to be always used in conjunction with other preservatives. It has been very successfully combined with BIT and overcomes the poor fungal efficacy while also showing excellent activity against Pseudomonas sp. The combination cannot be used for personal care wipes for supply in countries with regulations based on the EU Cosmetic Regulations or the ASEAN Cosmetics Directive, but may be used in household and industrial applications. MI has also been combined with parabens, PHMB, IPBC and chlorophenesin to overcome its poor fungal efficacy. Combinations with decylene glycol have also shown excellent preservative ability. Testing has shown that methylisothiazolinone is probably the most effective preservative for wet wipes when used as part of a preservative combination.
IPBC is a true fungicide with negligible efficacy against bacteria. It is used in combination with other preservatives that lack good fungal activity or where the finished product is particularly prone to fungal attack. The EU Cosmetics Regulation was altered to restrict the use of IPBC in preparations for use on children under three years-of-age (except in bath products/shampoos and gels) and in products used over large areas of the body and these changes have been adopted in the updated Annex VI of the ASEAN Cosmetics Directive. This was due to concerns of iodine intake that the SCCP concluded should not exceed 20% of the recommended daily intake of 150 ìg.8,9 Like many parts of the world the populations of ASEAN countries are deficient in iodine and add it to food stuffs such as salt to increase the daily intake.10 This loss of an efficient fungicide is particularly worrying as the conditions within the packaged wet wipes tend to favour the development of moulds and yeasts, particularly where ‘skin compatible’ pHs of 5-6 are used. The parabens are very effective against fungi but with their decreased use of specific fungicides may be required to help control the fungi and the removal of IPBC limits available solutions. Chlorophenesin is a viable alternative but can be difficult to use due to low water solubility. IPBC can be still be used in personal care wet wipes in Australia where regulations do not prohibit its use, and for non personal care wet wipe products in all countries.
The use of phenoxyethanol is quite common, possibly due to its useful properties and lack of controversy. It has good activity against Pseudomonas sp. while acting as a solvent for other actives. It lacks efficacy against fungi and needs to be used at high concentrations if used alone. Phenoxyethanol is widely used in combination with the parabens, formaldehyde donors, bronopol, IPBC and MCI/MI, usually with at least two other actives.
There are problems with the use of some preservative actives. Apart from the usual issues of dissolving in water and migration to the oil phase in emulsions there is also the possibility of absorption onto the carrier or container. Polyaminopropylbiguanide (PHMB) is gaining popularity, particularly for antibacterial wipes, but due to its strongly cationic nature there is the risk of being absorbed onto anionic viscose or cellulose fibres, so it must not be used with viscose or viscose blend wipes. Table 8 shows results of work conducted where 10 gm of fabric was soaked in 100 mL of PHMB solution (3000 ppm) and the liquid analysed for PHMB content after four and eight days’ contact. The results clearly show a significant loss of PHMB when it is exposed to viscose.11 Antibacterial wipes will often contain PHMB or other cationic substances such as quaternary ammonium salts as the antimicrobial active ingredient that will also double as the preservative. Care must be taken when using these to ensure they are not bound to the wipe and thus unavailable to act against microorganisms. These products usually make simple claims such as ‘antibacterial, kills 99.9% of germs’. These products may be regulated in some countries, depending on specific claims made on labels.
Organic acids, alcohols and glycols
There has been an increase in the use of organic acids and alcohols, with glycols and emollients being used as biocide boosters. The use of organic acids requires an acidic pH for the liquid phase to ensure the acids remain active. Unfortunately this pH increases the susceptibility of the wet wipes to fungal contamination, as they prefer an acidic environment. Also, some wipes affect the pH of the liquid raising it by half to one pH unit. This is particularly important when using airlaid wipes where interaction between the coating polymer and liquor may raise the pH. If organic acids are being used this may be sufficient to raise the pH above the pKA value of the acid and reduce the efficacy of the preservatives. Therefore the pH needs to be controlled in the finished product, not just the liquid, and should be checked in liquor expelled from the wipe. Although the organic acids may be used on their own in combination, they are very often used in combination with phenoxyethanol. Glycols, in particular the 1,2 alkane glycols, are becoming more widely used in personal care products and their use in wet wipes is also increasing. The glycols lower water activity and some offer specific antimicrobial properties by interfering with the cell membrane of microorganisms. The two most common used are caprylyl glycol and decylene glycol. They are not classified as preservatives and are listed as humectants or emollients, imparting sensory characteristics to the skin. They offer relatively good antimicrobial properties against both bacteria and fungi but only have moderate activity against microorganisms when used on their own. They may exhibit compatibility issues in O/W emulsions, have low water solubility and can be expensive. The EU Cosmetics Directive Annex VI contained a preamble as follows: 1. Preservatives are substances which may be added to cosmetic products for the primary purpose of inhibiting the development of microorganisms in such products. 3. Other substances used in the formulation of cosmetic products may also have antimicrobial properties and thus help in the preservation of the products, as, for instance, many essential oils and some alcohols. These substances are not included in the ANNEX. The recast EU Cosmetics Regulation updated the preamble to read: Preservatives mean substances which are exclusively or mainly intended to inhibit the development of microorganisms in the cosmetic product. This was changed to control the practice of using non-listed preservatives under the guise of including them for another purpose. Unfortunately the revised Annex VI of the ASEAN Cosmetics Directive has not included this change. As the glycols are not approved as preservatives, care needs to be exercised in using them in personal care wet wipes that will be exported to countries that follow the EU Regulation. While there is a move away from traditional preservatives such as the parabens, formaldehyde donors, IPBC and MCI/MI in personal care products, they are still used often in wet wipes. However, there is a push to move away from these preservatives particularly for natural products, although this market appears to be lagging behind other sectors of the personal care market. The traditional preservatives are still widely used in household wet wipes. There is no universal preservative system for use in wet wipes. Wet wipes are difficult to preserve and care must be taken to ensure a robust system is chosen.
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.