429 SUSTAINABLE HAIR
testing involves hair arrays being pulled through a comb (or brush) with the commensurate
measurement of the forces involved. As the hair structure becomes compromised, these
forces rise yet they can be sizably decreased by use of conventional conditioner products
that lubricate the hair surface.
Experiments can be performed in both the wet and dry states, and Figure 6 shows typical
wet-state data that illustrates the negatives associated with a bleaching treatment of the
hair, but also the ability for a conditioner treatment to mitigate the negatives and produce
a sizable benefit.
To be consumer acceptable, this lubrication must be delivered in an aesthetically pleasing
manner. This is not to be attained at the expense of an overly negative oily, greasy, or heavy
coated feel on the hair. At the same time, the technical benefit of “lubrication” is not a
consumer word however, the outcome of such useful and pleasing treatments is commonly
described by consumers as leaving the hair “softer,” “smoother,” “more manageable,”
“conditioned,” and others.
TENSILE STRENGTH
When talking to consumers about their hair care wants and needs, it likely won’t take long
until the term “strong, healthy hair” is heard. Consumers seem to equate hair’s “strength” with
its “health.” Strong hair is healthy weak hair is damaged. To a scientist, perhaps the obvious
experiment to evaluate technical strength is to stretch fibers out to break in a controlled
manner while measuring the forces involved.18 These are termed constant extension rate
experiments, and typical outcomes for hair in the wet and dry states are given in Figure 7.
Figure 6. Instrumental wet combing results showing the negative effect of damaging treatments and the
positive effect of commercial conditioner product.

430 JOURNAL OF COSMETIC SCIENCE
There are several parameters that can be extracted from such curves to provide
quantification—the most obvious being the amount of force required to snap the fiber.
However, this parameter has a strong dependence on the fiber dimensions (all things being
equal, a thicker fiber requires more force to break than a thin fiber),18 and so this factor
is usually normalized out by first measuring fiber dimensions and then reporting a break
stress (where stress =force/unit area). Figure 8 shows how the wet state break stress of hair
declines as a result of bleaching treatments of differing severity.
Similar outcomes arise from other chemical treatments (e.g. perms, permanent coloring,
relaxing, Brazilian Keratin Treatments, etc.), exposing hair to the high temperatures
encountered with curling and straightening irons,19 and exposing hair to UV radiation.20
Accordingly, there is ample evidence to support deleterious alteration of hair’s internal
structure.
Yet consumers do not assess their hair’s strength by pulling on individual fibers and
attempting to assess the forces involved. Instead, consumer perception would seem to
involve some self-assessment of the tendency for breaking. This may include noticing
broken fibers in a brush or comb during grooming or in the base of the shower after
bathing. Moreover, on a consumer’s head, it seems likely that fiber breakage ultimately
occurs as the result of a culmination of various manipulations, rather than one single
catastrophic deformation. From a mechanical testing perspective, the resistance of a
material to repeated external stimuli represents a fatiguing experiment and these have been
becoming ever more popular in the hair care world over the past decade.21,22 Commercial
equipment for performing single fiber fatigue experiments is available (Diastron Corp,
Andover, UK) and the outcomes from this testing have gradually been superseding those
from the traditional approach. For example, these newer experiments often show much
bigger differences between samples than seen by the usual tensile approach. To illustrate,
an insult that produces a 10% decrease in the dry state break stress might lead to a 10-fold
Figure 7. Typical tensile testing curves for wet and dry hair.