A Comprehensive guide to Moisture-proof Storage of PVC Bags in Damp Environments
A humid environment will accelerate the physical deterioration and chemical decomposition of PVC bags, leading to problems such as plasticizer leaching, edge delamination, and microbial growth, thereby shortening the service life and contaminating the contents. The following systematically expounds the moisture-proof storage strategies for PVC bags from four dimensions: moisture-proof principles, core risks, practical operation plans, and monitoring and early warning.
——————————————————————————–
First, the damage mechanism of a humid environment to PVC bags
1. Molecular chain hydrolysis and embrittlement
Chemical erosion
Chlorine atoms (Cl) in the PVC molecular chain are prone to nucleophilic substitution reactions with water molecules when the humidity exceeds 80%, generating hydrogen chloride (HCl) gas and causing chain breakage.
Experimental data:
After continuous exposure to an environment with 90% humidity for 30 days, the tensile strength of PVC bags decreased by 32%, and the elongation at break reduced by 45%.
Microscopic electron microscopy observations revealed that pores with diameters ranging from 0.1 to 0.5μm appeared on the material surface, and the moisture permeability increased by six times.
Deterioration manifestations:
The bag body changes from transparent to foggy white, the edges show powdering, and cracks occur with slight bending.
2. The edge banding is separated from the printing layer
Failure of the adhesive layer
The polyurethane adhesive used for heat sealing the edges of PVC bags swells after absorbing water, the hydrogen bonds between molecules break, and the adhesive force decreases.
Test result:
In an environment with a humidity of 70%, the edge banding peel strength decreased from the initial 12N/15mm to 5N/15mm (72 hours).
The binder in the ink layer (such as acrylic resin) expands after absorbing water, causing the printed graphics and text to become blurred and fade.
Failure case:
The entire batch of Chinese herbal decoction pieces packaged in PVC bags by pharmaceutical enterprises was scrapped due to the delamination of the edge sealing during the rainy season, which caused the medicinal materials to become damp and moldy.
3. Risk of microbial contamination
Bacterial colony growth
A humid environment provides a nutrient substrate for molds (such as Aspergillus Niger and Penicillium), and plasticizers (such as DOP) on the surface of PVC can be decomposed by microorganisms as a carbon source.
Detection findings:
In a warehouse with a humidity of 85%, the total number of colonies on the surface of unsterilized PVC bags soared from 50CFU/cm² to 2.3×10⁴CFU/cm² within 7 days.
Organic acids secreted by mold (such as citric acid) can further corrode PVC and accelerate the aging of the material.
Health threat
If the bag contains food or medicine, mycotoxins (such as aflatoxin B1) will directly contaminate the contents, posing a risk of acute poisoning or chronic cancer.
——————————————————————————–
Second, the core strategy for moisture-proof storage
1. Active isolation: Building a physical barrier
Multilayer packaging technology
Outer protection: Wrap the PVC bag with aluminum foil composite film (such as PET/AL/PE), and take advantage of the 99% barrier property of aluminum foil to isolate water vapor.
Middle layer buffer: Fill with silica gel desiccant packs (suggested usage: Place 500g of calcium chloride desiccant per cubic meter of space) to keep the local humidity below 40%.
Inner layer sealing: Perform secondary heat sealing on the PVC bag (temperature 150-170℃, pressure 0.2-0.3MPa) to ensure that there are no air holes in the edge sealing.
Case:
Precision instrument manufacturers adopt a triple protection system of “aluminum foil bags + desiccants + vacuum extraction” for export PVC packaging bags. After being stored in sea containers (with a humidity of 90%) for 60 days, the humidity inside the bags is still less than 5%.
2. Environmental control: Inhibiting water vapor penetration
Temperature and humidity balance
Temperature control: For every 10℃ increase in storage temperature, the rate of water vapor diffusion increases by 2 to 3 times. It is recommended to maintain a constant temperature of 18 to 22℃.
Humidity threshold: Reduce the environmental humidity to below 60% through a dehumidifier (ideal value: 45-55%) to prevent the formation of condensation water.
Equipment selection:
Wareroom-type dehumidifiers (such as Songji SJ-558E) can have a daily dehumidification capacity of up to 132 liters and are suitable for a space of 200 square meters.
The portable humidity indicator card (with a color-changing range of 20-90%RH) is affixed to the shelf to monitor the local humidity in real time.
3. Material modification: Enhance moisture resistance
Hydrophobic coating treatment
Spray fluoropolymer (such as PVDF) or nano-silica (particle size 20nm) coatings on the surface of PVC bags to make the contact Angle > 150° and achieve the lotus leaf effect.
Effect verification:
The PVC bags after coating treatment were left to stand at 85% humidity for 30 days, and the weight gain rate was only 0.3% (the untreated group gained 4.7%).
The water droplets can still roll off completely after remaining on the coating surface for 10 minutes, without any trace of penetration.
Blending modification
Adding 5-8% chlorinated polyethylene (CPE) or acrylate impact modifiers can enhance the moisture and heat aging resistance of PVC, and increase the notched impact strength by 300%.
——————————————————————————–
Third, response plans for special scenarios
1. Moisture-proof for long-term storage
Management of three-dimensional shelves
The goods should be stored off the ground (pallet height ≥15cm) to prevent the capillary rise of moisture on the ground.
The spacing between the shelves should be maintained at 0.8 to 1 meter, and industrial fans should be used to form convection air ducts to accelerate the discharge of moisture.
Periodic detection
Randomly inspect the edge sealing strength of PVC bags every month (using a tensile testing machine, with a standard value ≥8N/15mm);
Microbial indicators should be submitted for inspection every quarter (total colony count ≤100CFU/g, and no mold or yeast should be detected).
2. Protection during transportation
Moisture-proofing of containers
Start the container drying rods (such as TopDry L1000) 24 hours before loading the container to reduce the humidity inside the container from 85% to 35%.
Cover the top of the goods with rainproof canvas and fix them with binding straps to prevent the packaging from being damaged due to jolts during transportation.
Cold chain transportation
Use phase change material (PCM) cold storage agents instead of traditional ice packs to avoid temperature difference condensation.
Stick hygroscopic pads (such as montmorillonite composite materials) on the inner walls of the insulation box to absorb condensate water.
3. Emergency dehumidification treatment
Local moisture remediation:
For slightly damp PVC bags, treat them in a 60℃ hot air circulation oven for 2 hours (excessive temperature will cause the plasticizer to volatilize).
After processing, immediately put it into a sealed bag containing silica gel desiccant and let it stand for 24 hours before use.
Disposal of large-scale mold:
Wipe the surface of the bag with 75% ethanol solution to remove mold spores.
Fumigate with an ozone generator (concentration 3mg/m³) for 2 hours to oxidize and decompose the residual organic matter.
——————————————————————————–
Fourth, monitoring and optimization of moisture-proof effect
Intelligent monitoring system
Wireless sensor network
Deploy LoRa temperature and humidity sensors (such as SHT31-DIS-B) to achieve real-time cloud upload of warehouse temperature and humidity data;
Set threshold alarms (such as triggering SMS notifications when humidity exceeds 65%) and start the dehumidification equipment in a timely manner.
Visual dashboard
Generate humidity heat maps through Power BI to locate high-risk areas (such as corners of walls, near doors and Windows);
Predict the risk of mold by correlating historical data and issue a 15-day early warning.
2. Packaging iteration verification
Accelerated aging test
Place the PVC bags in a constant temperature and humidity chamber at 85℃/85%RH, and test the mechanical properties once every 24 hours.
The shelf life at room temperature (25℃) was calculated through the Arrhenius equation, and the formula and process were optimized.
User feedback loop
Collect the moisture complaint cases of end customers (such as food factories and pharmaceutical enterprises) and establish the FMEA (Failure Mode Analysis) library;
Targeted improvements should be made for high-frequency failure points (such as edge banding cracking), such as switching to reactive polyurethane hot melt adhesive.
——————————————————————————–
Fifth, key conclusions and practical suggestions
Principle of graded protection:
High-value items (such as electronic components and medicines) : Three-level protection of “aluminum foil bag + desiccant + vacuum” is adopted;
For general materials (such as daily necessities and industrial parts) : Use the combination of “PE inner lining bag + silica gel desiccant + sealed box”.
Cost-benefit balance
The initial investment in dehumidification equipment is relatively high (for example, about 50,000 yuan is needed for a 500-square-meter warehouse), but it can reduce the packaging loss rate by 15-20%.
The unit price of coating modification is 0.2-0.5 yuan per square meter higher than that of ordinary PVC, but the shelf life is extended by 2-3 times.
Full-chain control
From raw material procurement (requiring suppliers to provide moisture-proof test reports) to terminal use (training customers on correct storage methods), a closed-loop moisture-proof system is constructed;
Promote the “moisture-proof label” technology (such as the Timestrip humidity indicator card), allowing users to directly determine whether the packaging has expired.
Through systematic moisture-proof design, precise environmental control and dynamic monitoring optimization, the shelf life of PVC bags in a humid environment can be extended from several weeks to more than two years, while avoiding quality accidents and economic losses caused by moisture.
We Committ To Serve All Kinds Of PVC/ EVA Bags, Cosmetic Bags, Canvas Bags And Paper Bags With Custom Service!
Shanghai Chenhuan Industrial is a manufacturer which is specialized in the research, development and production of all kinds of bags and packaging. Our products include PVC bags, EVA bags, TPU bags, PE bags, paper bags, cosmetic bags, canvas bags and tote bags. We provide OEM service to the material, the size, the style and the printing.Official website Address:https://www.chenhuanind.com/
