Don't use plastic bags carelessly! Here are 4 tips to spot the trickery in the innermost layer material in just 5 seconds.

Preface 

Supermarket shopping bags, takeout bags, snack bags, and the like have long been integrated into daily life. Most plastic bags feature a multi-layer composite structure, and the material of the innermost layer directly determines their safety, food-contact suitability, and intended use scenarios. 

For professionals in the film industry, understanding the composition of the inner layer is fundamental to meeting customer needs and optimizing production (as the inner layer’s barrier properties and heat-sealing performance directly affect product quality and pass rates). For ordinary consumers, being able to distinguish between different material types can help avoid health risks associated with using non-food-grade materials to package food—for example, by completely avoiding materials containing plasticizers. PVC Packaged cooked food in an inner bag, hot oil. 

Many people mistakenly believe that “ Look at the appearance, touch the feel. ” It’s easy to misinterpret, yet it can be clearly judged. This article provides a compilation. 4 This simple, practical method, combined with a comparison of material properties, can be quickly mastered by both membrane industry professionals and general readers. 

I. First, let’s understand: Why should we pay close attention to the material of the innermost layer of plastic bags?

The outer layer of plastic bags is responsible for printing and physical protection; the middle layer enhances barrier properties; and the innermost layer, which is in direct contact with food and items, is the core layer, and there are specific requirements for material selection—here’s the underlying reason. 3 Point: 

• Safety First The inner layer that comes into contact with food must be... PE 、 PP Made from food-grade materials, free of plasticizers and heavy metals; does not leach toxins when exposed to high temperatures or oils. PVC Recycled plastics are strictly prohibited from being used as the inner layer in food contact applications. 
• Suitable for use in various scenarios The inner layer's heat resistance and oil resistance determine its application. —— The inner layer of microwave-safe plastic bags must be... PP Material: The inner layer of the hot-oil packaging bag must be oil-resistant. PE or composite materials. 
• Affect the user experience The inner layer’s flexibility and heat-sealing properties determine whether the plastic bag is easy to tear and whether its seal is secure; its barrier properties affect the shelf life of food (for example, snack bags require a high-barrier material in the inner layer to prevent moisture absorption and oxidation). 

Addition: Priority of Common Inner Layer Materials PP (The safest, microwave-safe) > PE (Most commonly used, food-grade) > PET (Composite inner layer, oil-resistant) > Other non-food-grade materials, PVC Recycled plastic must never come into contact with food. 

II. Core Method: 4 Learn practical techniques to quickly identify the innermost material—no specialized tools required! 

No specialized equipment is required—this can be done at home or in the workshop. The methods are arranged from easy to difficult, and combining multiple approaches further enhances accuracy. 

(1) Method 1 : Look at the label + Look at the structure (the most basic, accuracy) 60% ) 

Suitable for products from reputable manufacturers; pay particular attention to the food-contact labeling and the layered structure with multiple layers. 

1. Check the label. ：
        

• Marked “For Food Contact” QS/SC “Encoding” and clearly define the inner layer as PE/PP/PET Simply judge based on the labeling (reputable manufacturers do not make false claims). 
• Vague labeling, no markings, or only partial markings “ Environmental protection / Biodegradable ” The inner layer is not clearly defined; it’s highly likely not food-grade and does not come into contact with food. 
• To see PVC/ Polyvinyl chloride ” Regardless of whether the inner layer is labeled or not, direct contact with food is strictly prohibited. 

2. Look at the structure ：
        

• Single-layer bags (thin fresh-keeping bags, simple shopping bags) have no inner or outer layers and are mostly... PE (Transparent and soft) or PP (Slightly firm and transparent); 
• Multi-layer composite bags (takeout bags, snack bags) can be torn open at the corners; the innermost layer—the thinnest, softest layer that comes into direct contact with the contents—is the lining. 
• The inner layer is mostly transparent. / Semi-transparent; the outer layer may feature printing or a frosted finish, and the texture and color feel noticeably distinct. 

(2) Method 2 Feel the texture. + Test resilience (easiest to perform, high accuracy) 70% ) 

Touch the innermost layer (after tearing it open or directly touching the inside), and combined with the toughness test, the material differences become obvious: 

1. PE Material (most commonly used inner layer) ：

• Handfeel: Soft and smooth, with no stiffness; slightly yielding when kneaded. “ Rustling ” Sound, with good ductility; 
• Toughness: It resists tearing without breaking, doesn't become brittle or crack under deformation, has moderate tearing strength, and features smooth edges. 

2. PP Material (high-end inner layer, microwave-safe) ：
        

• Hand feel: Than PE Slightly stiff and crisp, highly transparent, smooth yet lacking any softness; the sound of rubbing is crisp and clear. 
• Toughness: Extremely strong—difficult to break under tension; requires moderate force to tear open; edges may have slight serrations. 

3. PET Material (often used for composite liners, oil-resistant) ：
        

• Hand feel: Relatively hard and smooth, with extremely high transparency, poor flexibility, and difficult to knead or deform. 
• Toughness: Easily broken when pulled, cleanly torn, with sharp edges, and resistant to stretching. 

4.PVC/ Recycled plastic (non-food grade—use with caution) ：
        

• Hand feel: Relatively hard and brittle; some lower-quality recycled materials are sticky, producing a dull rubbing sound and easily sticking to hands. 
• Toughness: Very poor—breaks at the slightest pull, with rough, crumbling edges and possibly a pungent odor. 

(3) Method 3 Burn it (the most accurate—pay attention to safety and accuracy) 90% ) 

Take a small piece of the inner plastic and work in a well-ventilated area (away from flammable materials and with proper protective measures). Observe the flame, smell, and ash to make your assessment: 

1. PE Material ：
       

• Combustion: Blue flame, slight white smoke; fast and sustainable combustion. 
• Odor: Light paraffin scent, no pungent smell; 
• Ash: White and waxy, easily crumbled. 

2. PP Material ：
        

• Combustion: The top of the blue flame has a small amount of yellow flame; combustion ratio is... PE Slightly slower, sustainable; 
• Odor: Slight paraffin-like scent, no pungency; 
• Ash: A white, powdery substance that is finer and easier to crumble. 

3. PET Material ：
        

• Combustion: Difficult to ignite, yellow flame, slow burning, and produces black smoke; 
• Odor: Pungent and sour (similar to vinegar), melting and dripping; 
• Ash: A black, hard lump; after the dripping substance cools, it becomes transparent. 

4. PVC Material ：
        

• Combustion: Difficult to ignite; once ignited, it immediately goes out, producing a green flame and heavy black smoke. 
• Odor: Strong, pungent hydrogen chloride smell (similar to disinfectant), causing discomfort upon inhalation; 
• Ash: A hard, black lump with traces of burning. 

5. Recycled plastic ：

• Combustion: Unstable, with chaotic flames and black smoke, and uneven burning rate; 
• Odor: pungent, burnt smell and unusual odor (complex composition); 
• Ash: Dark gray, hard clumps that tend to cake together. 

Caution: Suitable only for small-area testing; absolutely no burning of the entire bag. Pregnant women, children, and sensitive individuals should avoid contact. Ensure adequate ventilation immediately after use. 

(4) Method 4 : Test heat resistance (for auxiliary judgment, suitable for everyday scenarios) 

Assisted by heat resistance testing, directly correlate with usage scenarios (microwave compatibility and suitability for hot oil): 

1. Testing method Take a small piece of the inner-layer plastic and place it in... 60℃ Heat from the left or right (or briefly scald), and observe whether it deforms, melts, or emits an unusual odor. Determine if it’s microwave-safe and can be placed in the microwave on medium power. 10 Second-by-second observation. 

2. Material Identification ：
        

• PE Material: Heat-resistant ≤60℃ Overheating causes slight softening and minor deformation; no odor; not microwave-safe. 
• PP Material: Heat-resistant 110-130℃ Does not deform or emit odors when exposed to hot water; marked. “ Microwave-safe ” Microwaveable; 
• PET Material: Heat-resistant ≤80℃ When exposed to hot water, it softens slightly without emitting any odor; do not microwave. 
• PVC/ Recycled plastic: Heat-resistant ≤60℃ When exposed to hot water, it will soften and deform and may emit an odor. Absolutely avoid contact with hot water or hot oil. 

III. Key Comparison: Characteristics of Common Inner Materials Used in Plastic Bags + Adaptation scenario 

Combine the above methods and organize. 4 Common inner-layer material core characteristics—quick reference to avoid pitfalls: 

IV. Pitfall Guide: These Common Misconceptions + Contraindications—be sure to avoid them. 

(1) Common Misconceptions in Judgment 

• Misconception 1 “The transparent inner layer is food-grade.” —— Error! PVC It can also be transparent; the plasticizer used is non-food-grade. The key to determining its nature lies in the material itself, not in its transparency. 
• Misconception 2 “The thicker layer is safer.” —— Error! Thickness has nothing to do with safety—thicker recycled plastic layers are better than thinner ones. PE The inner layer carries higher risk. 
• Misconception 3 “Burning without any odor is...” PP 」 —— Error! PE and PP The burning smells are similar, so further judgment is needed by considering the feel and heat resistance. 
• Misconception 4 “The inner layer of the composite bag must be food-grade.” —— Error! Some low-quality takeout bags have a recycled plastic inner layer but are labeled as food-grade on the outer layer, which can easily cause confusion. 

(2) Contraindications for Use (Important! Affects Health) 

• Taboo 1 Plastic bags whose inner-layer material is not clearly identified and lack food-contact labeling should not be used to package foods that are intended for direct consumption. 
• Taboo 2 Regardless of the material, avoid storing high-oil, high-salt, and high-acid foods for extended periods, as this can accelerate material aging and cause harmful substances to migrate. 
• Taboo 3： PVC Plastic bags must not come into contact with food or hot water, and even when used to store non-food items, long-term use is not recommended. 
• Taboo 4 Recycled plastic bags (brittle, odorous, or unmarked) are to be used exclusively for trash and must not come into contact with food or utensils. 

V. Perspective from the Membrane Industry: The Logic Behind Selecting Inner-Layer Materials 

When membrane companies produce composite bags, the key consideration in selecting the inner-layer material is... “ Scene adaptation + Security Compliance ” , Core Principles 3 Point: 

• Food-contact plastic bags: The inner layer must be food-grade. PE 、 PP 、 PET , follow GB 4806.7-2016 Standards—strictly control additive residues. 
• Non-food-contact plastic bags: The inner layer can use ordinary materials. PE Or, for compliant plastic recycling, clear labeling is required. “ Food that cannot be touched ”。
•   High-end composite bag: Choose the inner layer. PET or PP Composite material that balances oil resistance and barrier properties; the outer layer is made of a material with excellent printability. 

The quality of the inner-layer material directly determines a membrane manufacturer’s reputation. Downstream customers, by clearly identifying the material type, can avoid compliance risks and ensure consumer safety. 

Conclusion 

Identify the material of the innermost layer of plastic bags—master it. “ Check the label, feel the texture, burn it briefly, and test its heat resistance. 4 This method is simple and requires no specialized skills. For consumers, it’s a helpful tip for safeguarding food safety; for professionals in the membrane industry, it’s a fundamental understanding necessary for effectively connecting with customers. 

Although the inner-layer material is small in size, it plays a crucial role in ensuring safety and compliance. Whether in daily use, film manufacturer production, or downstream procurement, giving due attention to the inner-layer material is essential to ensure that plastic bags are well-suited to their intended applications and remain safe and practical. 

In the future, as membrane material technology advances, the food-grade inner-layer materials will become safer and more environmentally friendly. Learning to identify these inner-layer materials will benefit consumers and professionals in the membrane industry for a lifetime.