Harold Bryant1, Michael DeGeorge1, Felicia Dixon1, Angela Ellington1, Andrew Greaves2, and Crystal Porter1 1 L’Oréal Research and Innovation, Clark, NJ, USA 2 L’Oréal Research and Innovation, France The desire for straight hair was once attributed to a “universal” vision of beauty and social status associated with straight hair; however, recent information links this style preference to improved manageability and style versatility. In order to achieve the straight look, it is necessary to transform the natural hair configuration, which has been linked to the shape of the follicle. Bernard et al. [1, 2] found that the asymmetric protein expression in curved follicles was associated with the formation of curly hair. At this time, it is not possible to straighten hair by changing the shape of the follicle. However, it is possible to straighten hair based on its chemical composition. For mature hair, the composition is generally the same and consists of roughly 90% protein with smaller quantities of water, lipids, and minerals but does not vary by degree of curl, despite differences that exist in the early stages of hair production. To understand how hair can be transformed, it is important to know the different components of hair. Hair is made up of three macro structures including the cuticle, cortex, and medulla, the latter being of little significance. The major morphologic part of the hair is the cortex, which is made up of highly organized α‐helical proteins packed in a cystine‐rich matrix. To straighten hair, an alteration of the cortex must occur. There are three types of bonds in hair: hydrogen, electrostatic salt linkages, and covalent, each consecutively requiring more energy to break. Based on the bonds that can be affected, there are two categories of straightening: temporary and permanent. To achieve temporary straightening, a lower energy process is required involving alteration of hydrogen bonds and salt linkages while permanent straightening is achieved through the modification of covalent bonds, which requires more energy. Thermal appliances can be used to disrupt and rearrange the weaker hydrogen bonds and salt linkages required for temporary straightening. Depending on the approach, the results can last from a few days to several months. However, permanent straightening is obtained through a chemical process that alters the protein structure by cleaving and reforming covalent bonds, preventing the hair from returning to its natural curly state until it grows out from the scalp. The following are usually considered when choosing the type of hair straightening process or treatment: degree of curl, degree of desired straightness, point of service, convenience, environmental conditions, and the desired frequency of straightening. All of these are important; however, the degree of curl in hair may be the most influential because it impacts other desired attributes. It is often subjectively described and ranges from various degrees of wavy to tightly curled. These descriptors are relative and can be confusing because they often overlap. Thus, the L’Oréal Curl Classification was recently developed to quantitatively describe the degree of curl in hair (Figure 34.1) [3]. This classification used hair from around the world and identified eight distinct curl types, where the degree of curl increases directly with number. People with curl types I–IV often have concerns about hair frizz and volume while higher curl types are more concerned about manageability. Thermal techniques can be used to achieve straight hair for all curl types but, typically, curl types V and above are difficult to straighten permanently without the use of hydroxide‐based systems. Figure 34.1 Hair classification types to differentiate the degree of curl in hair. Hair straightening appliances and chemicals are centuries old; however, instrument designs and formulations have evolved to improve effectiveness while limiting the negative attributes. New materials are used for the heated surface of flat irons and product formulas have been modified and developed to protect hair and scalp and aid in the ease of application. In some markets, the combination of heat and chemicals is often used. This chapter briefly reviews the most common straightening practices, including a description of the procedures and perceived advantages and disadvantages. The use of thermal appliances dates back to the Egyptian period and is still considered to be a necessity to most women in today’s world. In the Egyptian period, hot metal was used to straighten hair. A less aggressive method was popularized in the late 1800s with the invention of the blow‐dryer; the handheld version for home use became available in the 1920s. This increased the ability for women with curl types I–IV to have a variety of temporary and permanent styles (hot waving). Because curl types V–VIII were inherently more resistant to reconfiguration, there was a specific need to straighten these hair types. This was achieved with the popularization of the hot comb combined with pressing oil, attributed to Madame C.J. Walker in the early 1900s. The hot comb, still in use today, is a metal comb heated to temperatures reaching 450 °F. Whether utilizing a hot comb, professional tongs heated in a Marcel oven, or one of the electronic devices such as a blow‐dryer, curling iron, or flat iron, the process involves using heat and mechanical stress. These common thermal appliances represent an alternative way for people with naturally curly hair to achieve manageability and straight hairstyles. The combination of heat and mechanical stress, in the form of combing or brushing while blow‐drying and smoothing with the other devices, straightens hair by rearranging hydrogen bonds. Once straightened, the new configuration of the hair is only temporary and will revert back to its natural state after exposure to moisture from any source such as environmental conditions and perspiration. Temperatures of thermal appliances typically range from 150 to 232 °C (302–450 °F). While thermal processing is considered temporary in terms of styling, it can have a permanent effect on hair. For example, the proteins in hair can start to denature at high temperatures. Protein denaturation is a process by which proteins are irreversibly altered by an external stimulus, and for hair can result in decreased fiber integrity. The denaturation temperature is 235–250 °C (455–482 °F) and 155–160 °C (311–320 °F) for dry and wet hair, respectively [4–6]. The upper temperature limits for some of the appliances exceed the denaturation temperature for wet and dry hair so care must be taken to avoid repeated applications and overheating. Of all heat appliances currently available to straighten hair, the flat iron is rapidly becoming the most popular; therefore, it is the focus of technologic advancements. Important attributes of a good flat iron are the ability to provide even heat and maintain a consistent temperature. Recent improvements in temperature control promote thermal stability and coatings, including ceramic and titanium, provide durability and reduced friction. Reduced friction is critical to maintaining a smooth cuticle surface and reducing breakage during thermal processing. Other advances in materials include the incorporation of pure ceramic heating elements and minerals (tourmaline) that allow manufacturers to make claims about the positive effects of ions and far‐infrared radiation on the final state of the hair. The product offerings associated with the use of thermal appliances typically contain hydrocarbon‐based ingredients (e.g. petrolatum and mineral oil) and polymers (e.g. silicone‐derived, cationic, and nonionic) to condition, protect, and accommodate styling preferences. Because blow‐drying typically starts in the wet state when hair is vulnerable to damage, conditioning polymers (e.g. polyquaternium 10) that improve wet combing by reducing frictional forces are typically used. Prior to heat application, products that contain ingredients such as sugars and silicones can be applied. Sugars help to increase thermal integrity while silicones protect the hair by acting as a thermal barrier. Silicones also can function as lightweight films whereas hydrocarbons are often used when a heavier coating is desired for style preferences. Reducing treatments are traditionally known to curl hair (hot and cold permanent waving); however, they can also be used to straighten hair. The chemistry involves a two‐step process where the disulfide bonds in hair keratin are cleaved in the first (reducing) step followed by oxidization in the second step to form new disulfide bonds (Scheme 34.1). The difference between straightening and waving using reducing agents is the configuration of hair prior to oxidation and the form of the product during the reducing step. The reducing product for waving the hair is usually a liquid and the hair is curled using rollers before oxidizing. For straightening, the product is usually in the form of a thick cream so that the viscosity of the product can assist in holding hair fibers in a straightened configuration during manipulation. The most commonly used reducing agents in this process are ammonium thioglycolate (thiols) and sulfite. Scheme 34.1 Schematic disulfide bond reduction and oxidation. Thioglycolate straighteners come in several product strengths. Treatment procedures and strength should be based on the hair attributes according to product recommendations. For curl types V–VIII, these products usually leave the hair with residual curl so the result can be disappointing if a straight style is desired. In addition, the hair can feel dry as a result of the treatment; thus, products that contain glycerin are used to provide moisture. Thioglycolate straighteners can be used on hair that has been previously colored or permed; however, it is not recommended for bleached or relaxed hair. In addition to traditional thiol straighteners, a new technique is becoming popular that includes the incorporation of heat with thiol‐based cream products. These treatments are used to permanently straighten and/or reduce volume in curl types I–V. They are commonly used on Asian and Brazilian hair and go by several names that are listed in Table 34.1. Even though there are several names, they all use similar processes to achieve hair characteristics that consumers describe as straight, soft, and shiny. The main point of differentiation between this new treatment and the traditional straightening method is the application of heat prior to the oxidation step for the Asian and Brazilian treatments. Details about the specific procedure can be found in the appendix. This straightening technique is not recommended for natural hair that is higher than curl type V, particularly hair from people of African descent because it is inherently more fragile than other hair [7]. Table 34.1 Common names for thiol‐based straighteners that use heat. After a period of time, the processes described above will need to be repeated to the new hair growth because of its natural configuration. The treatment should only be performed on the new growth, referred to as a “touch‐up,” to minimize overlapping of treatments which may result in overprocessed hair. It is recommended that the time between treatments is maximized, with a minimum of 6 weeks, depending on the rate of hair growth. Sulfites are an additional class of reducing agents that can be used for hot waving or straightening depending on the procedure. Even though sulfite straighteners have been around for decades, they are less common than hydroxide relaxers because they tend to result in less effective straightening of highly curled hair (types VI–VIII). However, they are believed to be less irritating to the scalp. The decrease in straightening efficacy may be related back to the reactivity of the disulfide bonds, which depends on the pH of the active solution. The maximum reactivity is reached at pH 4–6, but sulfites are not stable at such low pH conditions [8, 9]. Because of this, commercial products generally range in pH from 6.5 to 7.5. At pH 7, only about 15% of the cystine residues can be reduced [10]. The first step of the reaction mechanism involves a reduction of the cystine via a nucleophilic reaction similar to thioglycolate, with the exception of the formation of Bunte salt (Scheme 34.2
CHAPTER 34
Hair Straightening
Introduction
Thermal processing
Reducing agents
Ammonium thioglycolate
Culture that introduced the technique
Popular thiol‐based treatments
Japanese/Filipino
Hair rebonding
Thermal reconditioning and restructuring
Ion retexturizing
Bio ionic system
Japanese hair restructuring
Straightening and reconditioning
Japanese straightening perm
Sulfite
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