Which substance is primarily used as a thickener in cream developer?

Cetyl alcohol is primarily used as a thickener in cream developer due to its properties that promote a creamy texture, enhance the consistency of the formulation, and improve application. It is a long-chain fatty alcohol that helps create a rich, stable cream that adheres well to the hair, allowing for more uniform application of the color. Its ability to emulsify and thicken helps ensure that the cream developer maintains its desired viscosity throughout the mixing and application process.

Other options, while they may have their own uses, do not serve the same purpose in this specific context. For example, stearyl alcohol can also act as a thickener and emollient but is less commonly used in cream developers compared to cetyl alcohol. Isopropyl alcohol and ethyl alcohol are solvents that can aid in the formulation of various haircare products, but they do not provide the thickening effect needed for a cream developer. Thus, the choice of cetyl alcohol reinforces the desired characteristics of a cream developer, ensuring a better application experience for the stylist and optimal results for the client.