PRESERVATIVES
43 Benzoic Acid ■ Phenoxyethanol ■ Ethylhexyl glycerin ■ DMDMH ■
HLB (Griffin) 6 7 8 9 10
be seen by the many ‘free-from’ claims appearing on various packaging, calling out the exclusion of specific ingredients such as parabens or phenoxyethanol, and even claiming that no preservatives are used at all. Given that this has been the situation in
our industry for a few years already, there were already attempts to address and solve some of these problems. Improved manufacturing conditions, ‘free-from’ preservative systems based on combinations of organic acids and glycols, and other alternatives gave manufacturers some relief, but unfortunately, did not address the problems in full. In order to provide a more holistic solution
to the problem, it is critical to dive deeper and understand if there are additional reasons that should be addressed. We took into account that some aspects are
constant and not going to change as we work on a preservative system, such as packaging type, consumer behaviour and product claims.
Non-woven effect on bacteria growth We explored the assumption that it is the non-woven itself that is the culprit in the preservation issue. To prove this, we went to a comparative challenge test, based on EP and USP protocol. In Figure 1, we have a challenge test performed on a standard wipe formula, just the aqueous solution without any non-woven fabric. The formula was preserved with a blend of organic acids. The challenge test results show that the preservative system provides full protection for the formula against gram-negative and gram-positive bacteria, yeast and mould. Simultaneously, the same formula was
tested again, this time with the presence of the non-woven fabric. The challenge test results as can be seen in Figure 2, show a different story: the same preservative system which was proven to provide full control on the formula alone, is unable to provide the same protection for the formula once the non- woven is part of the test, resulting in a failed challenge test. Therefore we can conclude it is indeed the presence of the non-woven that is raising the microbial risk. Once this is established, we turn to explore the additional factors impacting the ability of a preservative system to protect wet wipes.
www.personalcaremagazine.com 11 Figure 3: Calculation of HLB on non-woven fibres & preservatives
Structure & composition of wet wipes As already established, the fabric is the source of complexity, therefore it is important to understand the composition of the fabric itself. A big segment of personal care wipes fabrics consists of synthetic fibres made from polypropylene or polyester (PET), which are known for having good strength, in combination with viscose fibres, which have good absorbency and are known for their softness. Other alternatives exist such as wood pulp and cotton, and more technologies continue to enter the market slowly. Considering the hydrophilic–lipophilic
balance (HLB) is another key step in understanding the preservation challenge of wipes. HLB is a way of measuring a substance solubility within water or oil. It is an important tool for formulators when designing a product. The process of determining the HLB requirement of the application and components can be complex. Calculating the HLB of non-woven fibres
reveals we can set the fibre composition on a scale of HLB values, as can be seen in Figure 3. At the same time, the HLB values of commonly used preservatives are also calculated. From this scale, we can see that most of the preservatives are close to the value of the PET fibres However, the microbial contamination risk is
in fact found in the viscose fibres and of course in the water phase of the formula itself.
By looking at this bar we understand that most preservative systems used today are set outside the desired boundaries of the nonwoven fabric composition. Some preservatives with more suitable HLB value exist of course, for example DMDM hydantoin, but surely this is not a desired system from a regulatory, safety as well as consumer appeal. Therefore, we can now understand another
reason for wipes being so challenging to preserve - as the profile of the preservatives used today on wipes is not ideal for this application, regardless of level of use, pH modifications, formulation components and more. We stated above that the microbial risk is
coming mostly from the viscose fibres, and in fact not from the polyester component of the non-woven fabric. We will further expand on this statement. We argue that it is not only microbial growth that is a concern when working with a high ratio of viscose in the non-woven. So what is more alarming than the microbial growth? Well – a biofilm.
Biofilm formation Biofilm is defined as a bacterial population composed by cells steadily adhering between them onto a solid surface, creating micro colonies. It is a polysaccharide coating, shielding the bacteria and promoting colonies adhesion on viscose fibres. Biofilm formation is a concern to many
Figure 4: Comparing biofilm formation of P. aeruginosa on PET fibres
manufacturers, not only personal care product manufacturers. This risk is associated with the production environment. Many cases of biofilm formation in production equipment pipes have been reported over the years – and not only pipes. The biofilm risks are clear – from corrosion and risk of the equipment integrity to microbial contamination of the products. This risk is common, globally described and normally controlled by plant hygiene processes. When it comes to wipes, the biofilm risk does not originate from the production equipment, it comes from within – from the product itself. An in-depth study shows that non-woven fabrics which have viscose fibres, are at risk of biofilm formation. And the higher viscose ratio in the fibre, the greater the risk. The study was performed by introducing
February 2022 PERSONAL CARE 15 16
PET Viscose
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