What factors are related to the waterproof performance of EVA film?
1. Core Formula Factor: Vinyl Acetate (VA) Content
VA (vinyl acetate) is a key copolymer component of EVA resin. Its content is negatively correlated with waterproof performance and also affects the film's flexibility. It is crucial for balancing waterproofness and user experience:
Low VA content (5%-15%): EVA molecular chains are more similar to the hydrophobicity of polyethylene (PE), resulting in a dense molecular arrangement and high resistance to water penetration, resulting in strong waterproofness (typically 0.5-1.5 MPa water pressure resistance). However, the material is relatively rigid and tear-resistant, making it suitable for applications requiring high waterproofness but low flexibility (such as medical isolation gowns and outdoor tent waterproof membranes).
Medium VA content (15%-30%): The proportion of hydrophilic VA segments increases, resulting in a slight decrease in water resistance (water pressure resistance 0.3-0.8 MPa), but significantly improved flexibility (can be bent 180° without cracking), making it suitable for applications requiring both waterproofness and conformability (such as medical dressings and cold compress carrier films).
High VA content (30% and above): Hydrophilicity is further enhanced, while water resistance is significantly reduced (water pressure resistance <0.3 MPa). The material is soft, like rubber, and focuses more on "sealing and conformability" rather than "high-strength waterproofing." It is rarely used in purely waterproof applications and is mostly used as the inner layer of composite films (such as the sealant layer of food packaging).
2. Physical Structure Factors: Film Thickness
Assuming the same formula, the waterproof performance of EVA film is positively correlated with its thickness. The thicker the film, the stronger the "physical barrier" to water molecules, and the longer-lasting its resistance to water pressure and seepage:
Ultra-thin (0.01-0.05mm): Can only block small amounts of everyday water stains (such as transparent Band-Aids and electronic component dust films). Its water pressure resistance is less than 0.2MPa, and it may experience slight water seepage with long-term immersion. Its core advantage lies in its "thinness" rather than "high waterproofness."
Conventional (0.05-0.2mm): It offers sufficient basic waterproofing (water pressure resistance of 0.3-1.0MPa), meeting the needs of daily necessities (raincoat linings, bathroom waterproof bags) and medical applications (IV set packaging), and can withstand continuous rain or short-term immersion.
Thick (0.2mm and above): The waterproof barrier is significantly enhanced (water pressure resistance of 1.0-2.0 MPa). Some also contain anti-aging agents/reinforcements to improve weather resistance, making them suitable for long-term outdoor waterproofing applications (camping mats, aquaculture waterproofing membranes).
III. Processing Modification Factors: Composite Treatment and Chemical Modification
The waterproof performance of pure EVA film has limitations (e.g., weak water resistance at high VA content and poor flexibility of thick films). By combining with other materials or chemically modifying them, the waterproof limit of a single material can be exceeded while also adding functionalities:
Compound reinforcement: The most common "EVA + PE/PET/TPU" composite—PE enhances hydrophobicity, PET enhances tear resistance, and TPU improves elasticity and water pressure resistance. This type of composite film has a water pressure resistance of 2.0-5.0 MPa, far exceeding that of pure EVA, making it suitable for high-end waterproofing applications (outdoor jackets, building roof waterproofing membranes).
Coating Modification: Surface coating with hydrophobic coatings such as polytetrafluoroethylene (PTFE) and silicone creates a "superhydrophobic surface" (water molecule contact angle >120°, allowing water droplets to roll off directly). This not only improves water repellency but also repels oil and dirt, making it easy to clean. Suitable for applications such as medical face masks and kitchen aprons.
Crosslinking: Through irradiation or chemical crosslinking, EVA molecular chains are transformed into a three-dimensional network structure, minimizing the molecular gaps and making it more difficult for water molecules to penetrate. This also improves high-temperature and aging resistance, making it suitable for high-temperature and humid environments (such as automotive engine compartment waterproofing membranes and sterilized medical device packaging).
Summary
The waterproof performance of EVA film is the result of the combined effects of VA content (controlling basic waterproofness and flexibility), thickness (controlling waterproof strength), and compounding/modification (controlling high-end performance). In practical applications, the specific waterproofing requirements of the application (such as water pressure resistance, long-term immersion resistance, and oil repellency) must be considered to select a targeted combination of parameters to achieve the optimal match between "water repellency" and "usage requirements."
