**1. Introduction**

The French textile and clothing industry is facing many difficulties related to competition from countries with low labor costs and relocation. To cope with this, the players in the sector are adapting and diversifying their offer. Traditional industries and markets being the most affected, they are thus moving towards product niches, such as technical and high value-added textile markets, where know-how and innovation play a significant role. New products within particular advanced functionality regarding thermal comfort can help to position themselves in innovative product niches and meet consumer expectations.

The textile structure is involved in maintaining the thermal balance between the heat produced by the body and that given to the external environment. When adequately adapted, it protects the human body from hyper and hypothermia. Its resistance to heat and moisture transfer characterize it. However, its characteristics depend on external factors such as speed and relative humidity. Thermal insulation in textiles is traditionally made from voluminous composite materials where trapped air plays a key role. Since the late 1980s, a new type of insulation material has appeared on the market, phase change materials (PCMs). Integrated into clothing, they can improve their thermal behavior, regardless of their thickness, guaranteeing a greater freedom of movement for the wearer.

Phase change materials have been used for various applications in thermal energy storage since the 18th century, and many commercial products have been developed to this end. Depending on the field of application, PCMs are selected according to their temperature and phase transition enthalpy. Thus, when their phase change temperatures are relatively low (between −20 and −10°C), these materials are more likely to be used for food storage, while for temperatures

between 2 and 15°C, they are generally used for air conditioning, that is, comfort applications [1]. Higher transition temperatures allow these materials to be used for solar energy storage [2], in agriculture [3], electronic equipment protection [4], or textiles [5–7]. Thermal energy storage by solid-liquid phase change has been the subject of a census by Zalba on more than 150 existing PCMs, 45 of which are commercially available [8].

The thermal properties of phase-change materials allow them to be perceived as the material of choice for thermal insulation of the human body. The possibility of keeping the wearer as long as possible in his thermal comfort zone, and at the same time to reduce the thickness of the garment, is a conceivable objective, taking into account the thermal insulation capacity of textile support containing PCMs. Indeed, these are active during the phase change period and stop when the phase change of all PCMs is complete. This insulating effect is generally referred to as effective thermal insulation. The choice of PCMs to be used is therefore based on the feeling of comfort that the user can feel, regardless of his metabolic activity and external conditions. The effectiveness of PCMs inserted in a textile structure will depend mainly on the temperature differential between the body temperature and that of the surrounding environment. Therefore, a product ideally having a thermal window from 19°C (vasoconstriction temperature) to 37°C should contribute to localized thermal regulation.

The conventional phase change material formulations have some disadvantages related to their chemical structure [9], justifying that they cannot be used without being contained in a capsule or trapped by capillarity in a graphite matrix [10] or chemical gel [11]. Moreover, since these materials can be in a liquid state, they cannot easily be incorporated into a textile carrier without being contained in capsules. Besides, they must be as small as possible in order to facilitate their integration and thermoregulation regarding heat exchange surface, thus helping to compensate for their low thermal conductivity. Regardless of the physical state of the microencapsulated material (solid, liquid, or both), it remains trapped inside. This allows it to be integrated into a textile coating and thus to keep its functionality as long as the coating remains intact.

Microcapsules incorporated in commercial textile composites contain only one active ingredient, usually paraffin. These systems are thus limited by the thermal properties, thermal window, and enthalpy of phase change, of the microencapsulated paraffin. The objective of this work is to develop new materials with improved thermal properties before their incorporation on textile support.
