In segmented TPE materials, permeation occurs mainly in the rubbery state, in the soft segment phase of elasticity rather than through the hard segment phase. The permeation properties therefore correspond to the variation of the hard segment content. In general, the soft segment content mainly determines the water vapour permeability.
The basic polyether soft segments used for TPE films are mainly PEO and PEG or PPO/PEO copolymers, but PTMG is also used.
Complete PPO chain segments are basically not used in vapour permeable TPEs. Usually a mixture of different soft segments is used to balance the physical properties: PEO is more hydrophilic, for example, while PTMG has excellent mechanical properties and does not swell as much as the former. Increasing the CH2/O2 ratio reduces the compatibility of the soft and hard segments.
The molecular weight of the hydrophilic chain segments is within the same order of magnitude as other polyether-based TPEs, i.e. (600-4000 g/mol).
TPEs synthesised from PEO have the highest permeability, close to that of well-known hydrophilic materials such as cellulose and polyvinyl alcohol. A disadvantage of using PEO is that the film surface is too sticky.
According to the absorption-diffusion model, vapour permeability is a function of the amount of permeate and the rate of diffusive movement.
Water absorption is highly correlated with the content of PEO in the soft section and also with the molecular weight of the soft section. Increasing the molecular weight of the soft segment while improving the water absorption is believed to be related to the degree of separation between the soft and hard segments. Low molecular weight soft segments usually result in films with strong physical properties. The higher molecular weight soft segments form more pronounced PEO micro-regions, but the hard segments are weaker and able to absorb large amounts of water.
The increase in water uptake usually shows an exponential increase, which can be explained by the physical cross-linking. These crosslinks give the swelling limits of the hydrophilic micro-region. At low PEO content, the absorbed water is bound to the PEO chain segments; above a certain PEO content, free water increases.
The melting point of vapour permeable TPE films is mainly determined by the type of hard section. The high melt temperature period is seen in high temperature processes to provide thermal stability, such as laminating, waterproofing fabrics, laminating films, PA12 has 12 carbons on its repeating unit, most TPU hard segments offer a melting point of 175°C or below, while some polyester type TPEs have a melting point of up to 200°C. A range of important physical properties such as tensile strength, abrasion resistance are also largely determined by the hard segment, for example the combination of For example, films combining PA12 and TPU hard sections containing nitrogen are usually stronger.
The most common technologies currently used to manufacture breathable films are solution casting and melt extrusion.
The thermoplastic nature of TPE films allows them to be cast in solution using suitable solvents. Although TPUs are often made with MEK or other solutions, finding the right solvent for PEBA and PEE is still a topic.
Hydrophilic TPUs, for example, may be obtained from the casting of viscous solutions. After evaporation of the solvent, dense membranes can be obtained. Solution deposition into non-solvent porous membranes can also be obtained by controlled phase separation. The solution process is usually more expensive than the extrusion process.
Melt extrusion is the best film forming process.
Except for the very soft grades, most hydrophilic TPEs are suitable for extrusion.
Drying of theTPE materialis critical, as the material tends to absorb ambient moisture and higher moisture content can lead to uneven film surfaces. The film can be formed directly from the melt through the die head or blown through a round mouth mould.
The amount and speed of melt passing through the die head determines the ultimate thickness of the film and the TPE must exhibit sufficient melt strength to maintain traction. Crystallisation may occur at the end of traction to reduce non-uniformity in traction orientation and physical properties.
Blown film (Blownfilm) is by far the lowest and most efficient way.
Due to the nature of the TPE itself, special equipment is required. Slip agents or anti-tack agents are often added to the body resin to facilitate processing, especially in favour of thinner films.
Additives sometimes have a negative impact on vapour permeability, as they act at the most important moisture absorbing interface on the film surface. Blown films usually have an advantage over extruded films in that they offer better control of vapour transmission properties. Sometimes a blend of several TPE materials can be used to adjust the film properties to meet the needs of a particular application.