Oxidative colouring and bleaching of the hair is known to be a damaging process, but technology has recently been developed, patented and published indicating ways in which this damage might be reduced.
The hair when it emerges from the follicle in the skin of the scalp is a wonderfully engineered piece of equipment. It is very strong, flexible and in extremely good condition. The best thing to do to protect this wonderful asset would be to cover it up, never expose it to light (solar radiation) and never wash it. Fortunately for cosmetic scientists and marketing companies this is not an acceptable proposition for the modern consumer, so for many years it has been accepted practice to expose the hair to sunlight and treat it with all manner of products designed to clean, condition, style, straighten and colour as well as lighten. Generally speaking, the majority of these actions degrade the condition of the hair in one way or another.
Hair often has its own inbuilt protection mechanism from solar radiation in the form of dispersed particles of the pigment melanin, which give the hair its coloured appearance. This pigment sacrifices itself under the influence of sunlight and acts as a protectant to the many and various proteins and lipids which make up the hair structure. During this defence the conjugated melanin structure will itself be lost, along with some heat, through an oxidative mechanism, thus the hair lightens due to exposure to sunlight. Once the melanin is lost sunlight is capable of directly attacking the proteins and lipids that make up the main structural elements of the hair.
As not all hair has the same type or level of melanin some types of hair (eg blonde and white) will tend to be damaged more severely than dark hair.
Differential damage
Not all hair products damage the hair to the same extent. Some products may have little overall effect on the hair, some protect the hair and a few may even be able to repair the hair in some way. The presence of water in the hair (or moisturisation as it is sometimes described) may initially be thought to be a good thing; which it may be but only up to a certain concentration. However, water is capable of removing some of the internal forces that hold the hair together (eg hydrogen bonding and charge interactions). When this occurs the hair can swell and becomes capable of being stretched significantly. Additionally the softer, lower layers of the cuticle can swell considerably, forcing the harder outer layers to stick out away from the hair shaft, which makes the cuticle easy to remove with actions like incautious combing. This swelling of hair is found to be worse at high pH values as the hair becomes more negatively charged overall.
It is generally accepted that one of the worst ways of damaging the hair is through oxidative treatment. Oxidative treatments are used in hair products to bleach (lighten) the hair, to neutralise the hair after treatment with permanent waving lotions and during the oxidative colouring process. The main oxidising agent used in hair products is hydrogen peroxide, which has been used for this purpose since the mid-19th century.
Hydrogen peroxide used on its own (at low and neutral pH values) has a very poor and slow lightening effect on hair and to improve this performance it is necessary to increase the pH into the region of 9-11. For the greatest lightening performance it's best to use ammonia which has a specific ability to assist in the decolourising of melanin in the hair. Melanins (there are two general types) are large conjugated molecules, the structure of which has yet to be fully elucidated. Upon an oxidative attack the molecules are broken down and the conjugation (and thus colour) is destroyed. It is the ammonia/hydrogen peroxide mixture that undertakes this reaction.
Unfortunately the oxidative ammonia/hydrogen peroxide system is not that selective and is capable of attacking other parts of the hair structure in addition to melanin. Oxidative systems are capable of producing quite severe damage in hair such that the hairs strength and appearance can be severely compromised. Lengthy discussions of the effect of oxidative systems on hair can be found elsewhere in the literature.[1, 2] However, two of the most important aspects to consider here are the attack on the -S-S- bond and on thioester linkages.
One of the main structural bonds that holds separate keratin protein chains together is the cystine linkage which contains disulphide cross-links. Chemical oxidative attacks on the disulphide bonds eventually result in the destruction of this linkage resulting in two cysteic acid (-SO3-) functions. This makes the hair weaker, more porous, anionically charged and hydrophilic (water loving). This oxidative attack combined with regular washing has also been shown to remove distinctive portions of protein from the hair.
When hair first grows from the scalp it is quite hydrophobic and although it will absorb water it is quite difficult to wet. This is thought to be due to a layer of hydrophobic chains (called the hair's natural conditioner), made up of 18-methyleicanosic acid (18-MEA) which are covalently bound to the surface of the hair through a thioester linkage. However, this linkage is susceptible to oxidative attack and may be quite quickly broken during the bleaching process.
When hair is coloured using a colourant based on aromatic compounds, such as paraphenylene diamine and resorcinol, the reaction that creates the colour is initiated by an oxidising agent which is very commonly hydrogen peroxide. Thus by careful balancing of the formulation in these systems it is possible to have colouring occurring at the same time as lightening of the underlying natural (and artificial) hair colour. It is generally believed that it is the oxidative element of this process that causes the damage associated with hair colouring and not the colour creation process itself.
THE IDEAL COLOURANT
Therefore, in an ideal permanent (level 3) hair colouring product we would wish to initiate the hair colouring process (but within the hair only), oxidatively attack and destroy some of the natural hair colour whilst not damaging any of the hair's structural elements, eg proteins and lipids.
In order to understand the process and how this might be done we need to know some other facts:
l Hydrogen peroxide does seem to attack the melanin in the hair more readily than the hair's structural elements
l The hair colouring process is catalysed by the presence of hair
This latter observation is very fortuitous and important as it is desirable for the hair colouring reaction to occur preferentially inside the hair, thus giving the small dye precursor molecules time to diffuse into the hair before the reaction occurs significantly. If the colouring pigment was formed outside and on the surface of the hair the effect might be less than 'permanent'.
Recently there have been a number of publications and patents which refer to methods that claim to be able to colour and lighten the hair using commonly used dye materials as well as hydrogen peroxide but with measurably less damage to the hair structure.
P&G COLOURING TECHNOLOGY
The first recent approach to mild hair colouring systems is from Procter & Gamble and a useful summary of this work has been made available by this company.[3] This booklet has not been peer reviewed but is based on some material that has been published in papers and patents. P&G's approach is split into two parts, both of which have been developed out of the company's fabric washing technologies.
The first part is based on knowledge of the detail of the oxidative reaction that takes place when hydrogen peroxide is mixed with ammonia and brought into contact with the hair. A number of chemical species may be produced when this occurs and these are as follows:[4]
l HOO-: the perhydroxyl anion
l HOO*: the perhydroxyl radical
l HO-: the hydroxyl radical
l O2: molecular oxygen
The hair colouring process does seem to be catalysed by the presence of the hair and this is thought to be due to the presence of metal ions particularly copper or iron.[4] Copper is probably absorbed onto the hair from tap water and has a particularly high affinity for proteins, probably due to an ability to bond to deprotonated amide linkage.[5] The presence of these metals appears to promote the formation of the hydroxyl radical, which is thought to be particularly damaging to hair.
By incorporation of a metal ion chelating species into the mixture which has a high affinity for copper (to avoid it being swamped by calcium of which there is often lots on the surface of the hair), the production of damaging radicals has been shown to be reduced. The chelating agent of choice is N, N'-ethylenediamine disuccinic acid (EDDS) which has a higher affinity for copper compared to calcium. It also has the benefit of being quite biodegradable. Reduction in surface hair damage was demonstrated by the use of Fourier Transform Infrared (FTIR) based on the method described by Lewis[6] and Scanning Electron Microscopy (SEM). The use of EDDS was also shown to suppress the formation of oxygen during contact with the hair which was correlated with the suppression of free radicals. This approach is described in a number of patents see.[eg 7]
The second approach by P&G also appears to originate in its laundry bleaching technology and this has the added benefit of giving level 3 hair colouring in ten rather than the usual 20 or 30 minutes. This uses ammonium carbonate in place of ammonia (ammonium hydroxide). When added to peroxide this creates the peroxymonocarbonate ion which is claimed to be more selective towards melanin than keratin and is effective at pH values below 10, allowing a product to be formulated at lower pH values than is usual. This lower pH also does not encourage the creation of the damaging perhydroxyl anions.
Additionally, glycine is added to the colourant (at high pH it will exist as glycinate), which acts as a free radical scavenger, particularly for any carbonate radicals that may be formed. This work is described in the PR publication[3] and is also the subject of patents.[8]
HENKEL COLOURING TECHNOLOGY
The technology recently published by Henkel[9, 10] also considers the oxidative reactions occurring within the colouring process which might damage the hair. In this work a range of antioxidant materials were tested within hair colouring products to determine their effects upon hair damage. A range of test methods including Near IR spectroscopy, surface polarity, HPD scanning calorimetry and wet combing measurements were employed.
Antioxidant materials (and reducing agents) are commonly employed in hair colourant formulations to protect the oxygen sensitive hair dyes during the manufacturing and filling processes. However, it has been thought that once the colourant is mixed with the developer these materials will be the first to react with the hydrogen peroxide and will be quickly destroyed. Using an excess of antioxidants could result in the excessive generation of heat and could also affect both the colouring and hair lightening reactions. These aspects were also assessed during the study.
The material found to have the best combination of effects, ie damage reduction and lack of interference with colouring/lightening performance, was a-lipoic acid. This material significantly reduced the level of cysteic acid formation during the process
It is postulated that this material works to protect the hair structure via a sacrificial mechanism as the energy required to oxidise the -S-S- linkage within a-lipoic acid is less than that with the hair keratin. In addition there was evidence from the DSC studies that the presence of the oxidised a-lipoic acid was able to additionally stabilise the inner hair structure, perhaps by the formation of salts bridges. This is more of a stretch but not perhaps in the context of the evidence that panthenol as well as amino acids can have a measureable and beneficial effect on the inner hair structure.
The use of materials containing -S-S- linkages in hair colouring products has been suggested before and one of the drawbacks has been the odour associated with them, though this paper does not discuss this aspect of the approach.
PRODUCT COMMERCIALISATION
Commercialisation is always a difficult section as usually a new product appears between the time of writing and publication, but this is as accurate as it can be. The use of EDDS in hair colourant formulations has been employed by P&G for a while in a few shades of Nice 'N Easy (Level 3) and recently this has been extended, in the UK and US at least, to the whole range. EDDS has also been present in Natural Instincts (Level 2, not available in the UK), which raises the interesting question of the damage potential of this non-bleaching demi-permanent formulation.
Some time ago (around 20 years), hair which had been bleached or coloured badly was often found to be in a very poor condition. Since then, however, improvements in conditioning technology have been incorporated both within hair bleaching and colouring formulations as well as in after colouring products. This has done much to mitigate and limit further physical damage that has been done to the hair by these chemical processes. The technology of hair conditioning systems is a huge subject in itself.
Author
Bob Hefford, consultant
e-mail rjwhefford@btinternet.com
References
1. Chemical and physical behaviour of human hair, 4th Edition, C R Robbins, Springer, 2000, ISBN 0-387-95094X
2. The Science of hair care, 2nd edition, C Bouillon & J Wilkinson (Eds), Taylor & Francis, 2005, ISBN 0-8247-5969-9
3. Hair color research update, P&G Beauty, 2007, available from the company
4. Hair coloring systems delivering color with reduced fiber damage, J M Marsh et al, J Cosmetic Science, 2007, 58, 495 - 503
5. R J W Hefford, Stereoselectivity in metal ion dipeptide complexes, PhD Thesis, The University of Leeds, 1978
6. V Signori & D M Lewis, Int J Cosmetic Science, 1997, 19, 1 -13
7. H D Boswell, J M Marsh, J S Scott & M A Olshavsky, 2007, USP 7186275B2
8. J Marsh & J C Clarke, 2004, USP 7204861B2
9. Super mild oxidation coloring: Preventing hair damage at the molecular level, T Doerling, C Brockmann, A Wadle, D Hollenberg and T Forster, IFSCC Magazine 2007, 10, 323-329
10. T Doerling, C Brockmann, S Kainz & D Hollenberg, 2006, Patent Number WO2007140856