Optimization of breathability of ultra-soft TPU film fabric in sports shoes

1. Introduction

Thermoplastic Polyurethane film is a high-performance thermoplastic elastomer material. Due to its excellent physical properties and chemical stability, it has been widely used in the field of sports shoes. As consumers’ demand for comfort and functionality continues to increase, the breathability of sports shoes has become one of the important indicators that cannot be ignored in product design. As a lightweight, soft and durable material, TPU membrane can effectively improve the breathability of shoe materials through its unique microporous structure and molecular chain arrangement, while maintaining waterproofness and wear resistance. This article will conduct in-depth discussion on the application of ultra-soft TPU film in sports shoes, and focus on analyzing the key technologies and methods of breathability optimization.

In recent years, the global sports shoes market has continued to grow. According to Statista data, the global sports shoes market size has reached US$108 billion in 2022, and is expected to reach US$150 billion by 2027. This trend shows that technological innovation in sports shoe products has become the core driving force of market competition. As an important part of functional shoe materials, the performance optimization of TPU film not only affects the comfort of the product, but also directly affects consumers’ purchasing decisions. Therefore, studying how to improve the breathability of TPU film through material modification and process improvement is of great significance to promoting the development of the sports shoe industry.

This article will discuss from the following aspects: First, introduce the basic characteristics of ultra-soft TPU film and its current application status in sports shoes; second, analyze the principles and influencing factors of the breathability of TPU films in detail; then, discuss the At present, the main technologies and methods used in the industry to improve the breathability of TPU membranes; later, based on actual cases and famous foreign literature, the successful experience of breathability optimization of TPU membranes and their future development directions are summarized.

2. Basic characteristics and application status of super soft TPU film

(I) Basic characteristics of super soft TPU film

TPU film is a thermoplastic elastomer film made of polyurethane, with the following prominent features:

High elasticity: The tensile strength of the TPU film can reach 40-80MPa and the elongation of break can exceed 500%, making it ideal for use in sports shoes that require high elasticity and flexibility material.
Excellent wear resistance: The surface hardness of the TPU film is moderate, it can effectively resist friction and wear, and extend the service life of the shoe material.
Good weather resistance: TPU membrane has strong resistance to ultraviolet rays, oxidants and chemicals, and is suitable for outdoor sports environments.
Lightweight Design: The TPU film density is low, only 1.1-1.3g/cm³, which helps reduce the weight of the shoe material and improve the wearing experience.

The following is a comparison table of the main parameters of the super soft TPU film:

parameter name	Unit	Typical value range of super soft TPU film
Tension Strength	MPa	40-80
Elongation of Break	%	>500
Hardness (Shaw A)	–	75-95
Density	g/cm³	1.1-1.3
Abrasion resistance (Taber)	mg/1000r	<50

(II) Current application status of super soft TPU film in sports shoes

The application of TPU film in sports shoes mainly includes the following aspects:

Upper Material: TPU film is often used as a composite layer of the upper, providing waterproof and breathable function, while enhancing the softness and fit of the upper.
Midsole: By combining with EVA or PU foam, the TPU film can act as a midsole to improve the elasticity and shock absorption of the sole.
Functional Coating: TPU film can also be used as a functional coating to improve the anti-slip and stain resistance of shoe materials.

According to the report of Footwear Technology magazine, the penetration rate of TPU film in the global sports shoe market has exceeded 30%, and is showing an upward trend year by year. Especially in the field of high-end sports shoes, TPU film has a higher proportion of applications. For example, well-known brands such as Nike and Adidas have adopted TPU film technology in their flagship products.

3. Principles and influencing factors of air permeability of TPU membrane

(I) Basic principles of breathability of TPU membrane

The breathability of the TPU membrane mainly depends on the micropore structure and molecular chain arrangement inside it. Specifically, The breathable process of TPU film includes the following steps:

Gas adsorption: Water vapor or other gas molecules in the external air are first adsorbed on the outer surface of the TPU film.
Molecular Diffusion: The adsorbed gas molecules diffuse through the micropore structure inside the TPU membrane.
Gas Release: The gas molecules diffusing to the other side of the TPU membrane are finally released into the environment.

This breathable mechanism is similar to “selective penetration”, in which the TPU membrane allows small molecules such as water vapor to pass through, and prevents the penetration of larger molecules (such as liquid water). Therefore, the TPU film can achieve efficient breathable effect while maintaining waterproof performance.

(II) Key factors affecting the breathability of TPU membrane

The breathability of TPU films is affected by a variety of factors, mainly including the following points:

Micropore structure: The size, shape and distribution of micropores directly determine the diffusion path and velocity of gas molecules. Studies have shown that TPU films with micropore diameters in the range of 0.1-1 μm have excellent breathability.
Molecular Chain Arrangement: The arrangement of the molecular chains of the TPU membrane will affect its breathability. TPU films with higher hard segment content usually have higher breathability because the micropores formed in hard segment areas are more regular.
Thickness: The thickness of the TPU film is negatively correlated with its breathability. Generally speaking, the thinner the thickness, the better the breathability.
Temperature and Humidity: Changes in ambient temperature and humidity will also affect the breathability of the TPU film. Under high temperature and high humidity conditions, the breathability of the TPU film usually decreases.

The following table lists the degree of influence of different parameters on the breathability of TPU membranes:

Influencing Factors	The degree of impact (high/medium/low)	Remarks
Micropore structure	High	The bigger the micropore, the better the breathability
Molecular chain arrangement	in	The higher the content of the hard section, the better the breathability
Thickness	High	The thinner the thickness, the better the breathability
Temperature and humidity	in	Breathability decreases in high temperature and high humidity environments

IV. Techniques and methods for optimizing breathability of TPU membrane

In order to further improve the breathability of TPU films, a variety of technologies and methods have been developed in the industry. The following will introduce in detail from three aspects: material modification, processing technology and composite structure.

(I) Material Modification

Blending Modification: By blending the TPU with other polymers (such as silicone rubber or polyether), the microporous structure and breathable properties of the TPU film can be improved. For example, BASF, Germany, has developed a TPU/silicon rubber blended film, which has a breathability of more than 30% higher than that of ordinary TPU films.
Nanofiller addition: Introducing nanoscale fillers (such as silica or carbon nanotubes) into the TPU matrix can form more microporous channels, thereby enhancing breathability. According to research from the Massachusetts Institute of Technology (MIT), adding an appropriate amount of nanofiller can increase the breathability of the TPU film by more than 50%.

(II) Processing technology

Bidirectional tensile technology: Through the bidirectional tensile process, a uniform microporous structure can be formed inside the TPU film, significantly improving breathability. Toray, Japan’s TPU film produced by this technology, has more than twice the breathability of similar products.
Foaming: Using chemical or physical foaming technology, a large number of micropores are generated inside the TPU film, thereby improving breathability. South Korea’s LG Chemical Company has developed a new foamed TPU film, which has a breathability of 60% higher than that of traditional TPU films.

(III) Composite Structural Design

Multi-layer composite structure: By combining the TPU film with other functional materials such as PTFE film or nonwoven fabric, a balance of breathability and other properties can be achieved. For example, the Gore-Tex composite film launched by W.L. Gore, a company in the United States, combines the flexibility of TPU film and the high breathability of PTFE film, and is widely used in high-end sports shoe products.
Gradar Structure Design: By designing the gradient structure of the TPU film, the surface has different micropore sizes and distributions, thereby optimizing the breathable performance. A gradient TPU film developed by Montelli, Italy, has a breathability of 40% higher than that of ordinary TPU films.

5. Actual case analysis

(I) Nike Flyknit series

Nike Flyknit series sneakers use TPU film as upper material, and achieve excellent breathability by optimizing the microporous structure and composite design. According to Nike’s official data, the breathability of this series of shoes is more than 70% higher than that of traditional shoes, and has been highly praised by consumers.

(II) Adidas Boost Series

Adidas Boost series sneakers apply TPU film to the midsole, which enhances the breathability and comfort of the sole by combining it with EVA foam. According to evaluation by a third-party testing agency, the Boost series of shoes is 50% more breathable than ordinary sports shoes.