The stretching process for TPU high-moisture-permeability waterproof films can reshape the film's microstructure and directly determine its moisture permeability characteristics. For the same TPU film, different stretching methods can lead to two distinctly different technical pathways: one produces a dense film that becomes "more dense the more it is stretched," while the other produces a microporous film that forms water vapor transport channels.
I. Stretching Process: The "Shaper" of the Microstructure of TPU Film
The moisture transmission process in TPU films essentially involves water vapor molecules undergoing a "adsorption–diffusion–desorption" transport process within the film matrix. Whether this process proceeds smoothly depends on the film's microstructure, particularly the arrangement of its molecular chains and whether the film contains channels through which water vapor can pass. The stretching process is the key step in applying directional forces to this microstructure.
The effect of stretching is to induce the molecular chains within the film to reorient along the direction of the applied force. The orientation direction, the degree of stretching, and whether the process is combined with pore formation directly alter the degree of molecular-chain ordering and the magnitude of free volume, while also determining whether interconnected micropores can form within the film. It is precisely these microscopic differences that enable a single raw material to be processed along two distinct pathways-dense and microporous-thereby determining the range of moisture permeability of the resulting film.
II. Dense TPU film: unidirectional orientation stretching- "the more it is stretched, the denser it becomes"
(1) Process Logic
Compact TPU films are generally produced using unidirectional orientation stretching. Unidirectional orientation involves applying tensile force to the film along a single direction, causing the molecular chains to align regularly in the direction of the applied force. After this type of stretching, the molecular chains become more ordered, the packing between molecules becomes tighter, and the free volume within the film consequently decreases.
(2) Moisture permeability manifestations
Free volume serves as a critical pathway for the diffusion of water vapor molecules. Unidirectional oriented stretching aligns the molecular chains in a more ordered arrangement and reduces the free volume; consequently, the diffusion pathways for water vapor molecules become constricted, significantly impairing the film's moisture permeability. In simple terms, along the path of producing a dense film, the higher the degree of stretching, the denser and more compact the film tends to become, and the lower its moisture transmission rate.
(3) Applicable Scenarios
Compact membranes are designed for a dense structure and offer advantages in applications that require higher mechanical strength and superior water-pressure resistance; they are therefore suitable for waterproof diaphragm applications that demand strong physical protection and high pressure resistance standards. If the application places greater emphasis on barrier and mechanical performance while having relatively low requirements for moisture transmission, a compact membrane solution is more appropriate.
III. Microporous TPU film: biaxially stretched additive pores that "open moisture-permeable channels"
(1) Process Logic
Unlike the unidirectional orientation of dense films, microporous TPU films employ a technical approach combining biaxial stretching with pore-forming treatment. The film is stretched simultaneously in both the longitudinal and transverse directions, in conjunction with pore-forming additives; upon completion of stretching, a network of interconnected micropores forms within the film matrix.
(2) Moisture permeability manifestations
Once the micropores have been formed, water vapor molecules no longer rely solely on diffusion through the free volume between molecular chains; instead, they can pass directly and rapidly through the membrane via these continuous microporous channels. As a result, water vapor transmission can increase substantially, far exceeding that of dense membranes of equivalent thickness. Thanks to their excellent water vapor transmission efficiency, microporous membranes have become a widely adopted solution in high-water-vapor-permeable yet water-resistant fabrics.
(3) Applicable Scenarios
When the primary performance requirements of a product are high moisture permeability and high breathability-such as in outdoor sportswear or high-performance waterproof and breathable fabrics-the microporous membrane approach is the superior choice. It simultaneously delivers both waterproof and moisture-permeable properties, making it an ideal material for creating functional fabrics that are "breathable and non-suffocating."
IV. Comparison between Dense Membranes and Microporous Membranes
Comparison dimension Compact TPU film Microporous TPU film
Stretching method Unidirectional tensile drawing Biaxially stretched and perforated
micromechanism Regular molecular chains with tightly packed packing Continuous micropores form internally
Moisture permeability characteristics Free volume decreases, and moisture permeability declines. Water vapor passes through microporous channels, causing the moisture permeability to increase exponentially.
5. How to Select the Appropriate Option: Derive the Process from the Application Scenario
The core value of understanding the stretching process lies in the ability to "work backward" from actual application requirements to determine which route should be chosen, rather than rigidly applying a fixed film material specification. This can be assessed from the following dimensions:
• We aim for high moisture and air permeability, giving priority to microporous membranes with dual-directional stretch and additive pores, thereby delivering a wearable experience characterized by high moisture and air permeability without causing perspiration retention.
• For applications requiring higher strength and water pressure resistance, or those that place greater emphasis on physical protection, water pressure resistance, and barrier properties, the dense-film approach is more suitable.
• For composite requirements that fall between these two extremes, it is recommended to communicate directly with the manufacturer, allowing engineers to comprehensively match the tensile processing with the hole-forming solution based on criteria such as moisture permeability, tensile strength, and water pressure resistance.
VI. Selecting the appropriate tensile process is essential for achieving the matching moisture permeability.
The stretching process can be regarded as the "watershed" for the moisture transmission performance of TPU films, because it directly shapes the film's microstructure: uniaxial orientation makes the film denser and reduces its moisture transmission capacity; whereas biaxial stretching combined with pore formation creates continuous micropores, substantially increasing moisture transmission. Neither approach is inherently superior; the criterion for evaluation depends on whether it is suited to your specific application scenario.
If you are looking for suitable TPU moisture-permeable and waterproof films for outdoor fabrics, medical breathable membranes, or other functional applications, please feel free to contact us.
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