The science of film technology hidden inside salt bags! What makes the inner film “lock in freshness and prevent leakage,” safeguarding every single grain of salt?

Preface 

Table salt is an essential seasoning in every household, yet few people pay attention to the innermost plastic film lining the salt bag. —— Although it appears fragile, this material can coexist peacefully with highly corrosive salt over the long term—remaining uneroded, leak-proof, free of harmful substance leaching, and even resistant to salt-induced caking. As a leading public account in the membrane materials field, this article uses clear, accessible language to thoroughly explain the material composition, functional roles, and core mechanisms underlying its exceptional salt-corrosion resistance, presenting the information in well-structured, bullet-point form that is both technically rigorous and easy to read. 

I. First, let’s understand: What exactly is the innermost film layer of salt packaging? 

Key conclusion: The innermost layer of the salt packaging is not ordinary plastic film, but rather Food-Grade Special Barrier Composite Film Its core function is to provide direct contact with salt, isolate corrosion, and ensure safety, while also offering moisture resistance, leak-proof performance, and easy heat-sealability—making it well-suited for industrial packaging applications. 

1. Mainstream materials: 3 Core-type thin film, compatible with various salt products. 

Depending on the type of salt and its packaging format, the material of the innermost film layer varies; all comply with the “Technical Specifications for the Production of Small-Sized Salt Packaging” ( QB/T 4510-2025 ) Requirements, the core is divided into 3 Category: 

( 1 ) Polyethylene ( PE ) Thin film: the most commonly used and cost-effective, divided into Low-density polyethylene and High-Density Polyethylene It is the preferred choice for ordinary edible salt, offering softness and easy heat sealing, making it well-suited for automated production lines. 

( 2 ) Cast polypropylene ( CPP ) Film: Modified and upgraded version, retort-grade CPP It exhibits superior salt and high-temperature resistance, making it suitable for use in iodized salt and other nutritionally fortified salts; its resistance to various media can be verified through salt-immersion testing. 

( 3 ) Composite barrier film: multilayer structure (e.g., BOPP/VMPET/PE ), the inner layer is PE/CPP A barrier layer is added to the outer packaging, making this salt suitable for long-term storage and long-distance transportation while providing both corrosion resistance and moisture- and oxygen-barrier properties. 

2. Key premise: It must be “ Food-grade ” , and complies with the specific standards 

As a food-contact film, it must meet two core requirements, which also form the basis for corrosion resistance: 

① Compliant GB 4806.7-2016 Standard: free of fluorescent agents, heavy metals, and harmful additives; long-term contact with table salt does not result in the release of harmful substances. 

② Through special modification, chemical resistance is enhanced, enabling it to withstand chloride ion attack and meet 5% The sodium chloride aqueous solution immersion test shall meet the requirements of no damage and no leaching. 

II. First, let’s clarify: the salt “ Corrosiveness ” , where exactly does it come from? 

Salt itself is not highly corrosive, but its properties can continuously erode ordinary materials; in humid environments, its corrosivity increases dramatically—this is precisely why thin films require specialized design. 

1. of salt 3 Big “ Corrosion Hazards ” (Challenges that membrane materials must address) 

( 1 ) Chloride ion erosion: table salt ( Sodium chloride ) Chloride ions released during dissolution have strong permeability, which can disrupt the molecular structure of ordinary plastics, leading to film aging, embrittlement, and leakage. 

( 2 ) Electrochemical corrosion upon moisture absorption: Salt absorbs moisture to form brine, which undergoes mild electrochemical reactions with the thin-film surface, accelerating film degradation; the extent of corrosion can be assessed using electrochemical impedance spectroscopy. 

( 3 ) Additive-assisted corrosion: Additives such as iodized salt, when dissolved in water, alter the pH of the saline solution, thereby further enhancing its corrosivity. 

2. Why won’t ordinary plastic film work? 

Common PE 、 PP Without membrane modification, the molecular structure is loose, making it easy for chloride ions to permeate. Prolonged exposure to salt—especially in humid environments—can lead to: brittle cracking and leakage of the membrane, leaching of harmful substances, and hygroscopic caking of the salt. Therefore, a specially modified membrane must be used. 

III. Core Focus: Salt-Corrosion Resistance of Thin Films 5 Core Principles (Valuable Insights for the Membrane Industry) 

The key to the long-term salt-corrosion resistance of the thin film is “ Material Modification + Structural Design + Process Optimization ” By integrating these three approaches, corrosion risks can be addressed in a targeted manner; the specific breakdown is as follows: 

1. Material modification: optimizing the molecular structure to block chloride ion penetration. 

The core step is to modify to PE 、 CPP Chloride-ion resistance: 

① Molecular crosslinking modification: induces crosslinking of polymer chains to form a dense network, reducing intermolecular voids and thereby inhibiting the penetration of chloride ions and saline solutions. 

② Add corrosion-inhibiting additives: Incorporate food-grade corrosion inhibitors and stabilizers to enhance chemical resistance and prevent film aging and discoloration. 

③ Surface smoothing treatment: reduces friction and adhesion of salt particles, prevents localized high concentrations that accelerate corrosion, and, in some cases, employs corona treatment to decrease salt particle adsorption. 

2. Structural Design: Multi-layer Composite with Dual Protection (for Premium Salt Products) 

High-end salt products feature a multi-layer composite inner film for dual protection: 

① Inner layer: Modified PE/CPP , directly contacts the salt, provides corrosion resistance and leak-proof performance, and exhibits excellent heat-sealability; 

② Middle management: EVOH 、 VMPET An barrier layer is provided to prevent moisture and oxygen penetration, thereby reducing salt moisture absorption and brine formation and preventing the precipitation of aluminum ions. 

③ Outer layer: BOPP A wear-resistant film is applied to protect the inner layer, preventing damage during transportation and storage while also enhancing aesthetic appeal. 

3. Process Optimization: Strict Control of Production Details to Enhance Film Stability 

3 A key process determines corrosion resistance, in accordance with industry standards: 

① Thickness Control: 0.02-0.10mm , with uniform thickness to prevent localized damage and in compliance with relevant standards; 

② Purity Control: Strictly control the purity of raw materials to prevent impurities from serving as entry points for corrosion; raw materials must meet food-grade standards. 

③ Heat-sealing process: low-temperature heat sealing to prevent damage to the molecular structure, ensure a tight seal, and prevent leakage. 

4. Moisture-Resistant Design: Reducing Corrosion Conditions at the Source 

The corrosive effect of salt primarily stems from saline solutions; moisture control is key to corrosion resistance: 

① High barrier properties of thin films: modified PE 、 CPP Low water permeability prevents moisture ingress, thereby reducing the formation of brine. 

② Sealed packaging: The salt bags are tightly sealed, with desiccant added in some cases, to keep the salt dry over the long term and minimize contact between the film and brine. 

5. Chemical Resistance Adaptation: Withstanding Corrosion from Salt Additives 

Additional optimization of the film for salt containing additives: 

① Acid- and alkali-resistant modification: resistance to the weak acidic or alkaline environment created by additives, with a specified rate of quality change under acid and alkali conditions. ≤1%；

② Avoids chemical reactions: Does not react with additives such as potassium iodate, does not release harmful substances, and remains intact even under high temperature and pressure. 

IV. Supplement: Selection of Inner Film for Different Salt Products (Based on Practical Applications) 

Different salt products have varying compositions and storage requirements, necessitating the selection of different types of films; film industry professionals should pay particular attention to: 

1. Common refined salt: modified Low-density polyethylene Thin films offer high cost-effectiveness, easy heat sealing, and compatibility with large-scale production, making them the mainstream choice. 

2. Iodized salt / Nutritionally Fortified Salt: Cooking Grade CPP Thin film with strong chemical resistance, preventing additive reactions and iodine loss; 

3. Sun-dried salt / Coarse salt: High-Density Polyethylene The thin film boasts high hardness, excellent wear resistance, corrosion and moisture resistance, and prevents coarse salt from caking. 

4. Premium salt: multi-layer composite film—inner layer corrosion-resistant, middle layer moisture- and oxygen-barrier, outer layer wear-resistant—extending shelf life. 

V. From the Perspective of the Membrane Industry: This Thin Film 3 Key technical points 

Focusing on the technological logic of membrane materials, 3 Key points to understand its application value: 

1. The core of corrosion resistance lies in molecular-level protection: by modifying and optimizing the molecular structure to form a dense protective layer—rather than simply increasing thickness—this represents the core application of membrane material modification technology. 

2. Food-grade compliance and corrosion resistance are both indispensable: striking a balance between safety and performance requires stringent specifications for membrane raw materials and modification processes, as well as adherence to dual standards. 

3. Suitable for industrial-scale production: balances corrosion resistance with easy heat sealing and cutting, with parameters that meet the requirements of automated production lines. 

VI. Common Consumer Questions 

Answer based on everyday use. 3 The core question: 

1. Is it safe to have food come into contact with thin films? —— Completely safe: the certified film meets food-grade standards, contains no harmful additives, and does not leach under long-term contact. 

2. Are the salt bags torn and the film sticky because they have been corroded? —— It is highly likely that saltwater corrosion in humid environments will damage the film, in which case the salt is not recommended for consumption. 

3. Why is there such a large difference in the toughness of thin films? —— Depending on the modification process and thickness, low-quality salt bags may be made from unmodified conventional film, which is prone to corrosion and rupture. 

Conclusion 

The inner film of the salt bag represents a seamless integration of advanced film-material technology and the specific requirements of food packaging. Through material modification, structural design, and process optimization, it precisely addresses the risk of salt-induced corrosion, delivering core performance features such as corrosion resistance, leak prevention, and enhanced safety. 

From the perspective of the membrane industry, this thin film demonstrates the practical applicability of membrane material modification technologies. —— The value of membrane materials lies not in their high-end complexity, but in their precise fit and application. They are found in everyday salt packets, where basic membrane technologies address real-world challenges, highlighting the wide-ranging applications of membranes in daily life. 

With advancements in membrane technology, future salt packaging films will be lighter, thinner, more environmentally friendly, and more corrosion-resistant, thereby ensuring food safety while aligning with the green, low-carbon trend and continuing to provide invisible protective benefits.