The term conditioning is used by consumers rather than scientists, and consequently there is danger in trying to technically define the term. It seems logical, however, to presume that a perceived improvement in hair condition is somehow involved. Accordingly, the ability to mask a rough and degrading surface structure can be perceived as a significant improvement in the condition of hair. As such, surface lubrication represents the primary function of conditioning products, with improved manageability through easier detangling and grooming becoming an additional benefit. Traditional rinse-off conditioner products accomplish this task by depositing a lamellar liquid crystalline structure of water, quaternary amine surfactants and fatty alcohol co-surfactants onto the hair surface. The lubricating performance may be further supplemented by including silicones, which also help mediate a potentially waxy dry-state feel. The deposition of silicones from a shampoo base is the mechanism supporting 2-in-1 shampoo plus conditioner products; however, certain quaternary amine functionalized polymers may also provide lubrication and/or facilitate silicone deposition.
It is helpful to have technical measurements of product performance that provide guidance to the formulation chemist, while also potentially being useful in product marketing. Of course, these instrument-based evaluations contain no information about the aesthetics of formulations, which are considered to be at least equal in consumer importance. Consequently, lubrication at the expense of a particularly greasy, oily or slimy feel would clearly lead to an unacceptable formula.
In addition, the presence of such surface deposits could have a negative effect on other attributes such as weighing down fine, limp hair and reducing the perception of volume and body. Therefore, companies manufacturer products with a spectrum of performance—where lighter-depositing formulas are intended for those with fine, thin hair; while heavier-deposition is desirable for longer, thicker, damaged hair. In short, there are dangers in assigning any “superiority” rank to formulas based on instrumental testing alone. Nevertheless, there is still a strong desire for such measures, particularly at a time when quantitative claims (i.e., 5 times stronger, 4 times smoother, etc.) are popular.
Standard testing protocols are absent from the hair care industry; however, there are a number of methods and approaches that have become widely adopted (although each laboratory likely performs them in a slightly different manner). The most widely utilized approach for evaluating lubrication of hair involves instrumental combing experiments, where the attraction lies with the consumer relevance of the assessment. The methodology involves the use of a mechanical testing device manufactured by companies such as Instron, Dia-Stron and Stable Micro Systems that measure the friction generated between a comb/brush and hair during grooming. Therefore, lubrication is evaluated through a decrease in grooming forces for product-treated hair relative to an unconditioned control. As already mentioned, these experiments can be setup in a variety of ways depending on available resources. However, all approaches should yield quality results after paying due diligence to method validation. Figure 1 shows an experimental test setup where a hair tress is suspended from a load cell that is connected to the cross-head of a tensile testing device. Thus, a hair tress can be pulled through the comb, with an evaluation of the “combing force.”