Silent Pressures: How Storage Conditions Trigger Blocking in Coated Films

In the world of film and flexible packaging production, it’s easy to blame the coating line when blocking occurs. But what if the true culprit isn’t the coating at all? What if the damage is being done long after production ends?

This article explores how invisible forces during storage conditions—namely humidity, temperature, and pallet pressure—amplify blocking, transforming seemingly perfect rolls into unusable inventory.

1. What Is Blocking and Why Storage Conditions Matter

Blocking refers to the unwanted adhesion between layers of film, especially in tightly wound rolls or stacked sheets. It's not always due to formulation or curing issues. Even well-processed films can block after storage, especially under suboptimal storage conditions.

The three most common environmental amplifiers of blocking are:

• High humidity
• Elevated or fluctuating temperature
• Excessive pallet pressure during stacking and transportation

These factors accelerate surface softening, tack, and migration of additives, all of which make blocking more likely.

2. Humidity: The Invisible Aggressor

Humidity plays a direct and indirect role in film blocking:

• Hydrophilic coatings (e.g., water-based acrylics, polyurethane dispersions) absorb ambient moisture, softening over time.
• Increased humidity reduces surface energy thresholds, making surfaces more adhesive.
• In multi-layer stacks, trapped humidity leads to micro-condensation at contact surfaces.

In one case, PET films coated with a water-based primer blocked after 5 days in a warehouse at 80% RH, despite being fully cured. The humidity had migrated into the coating, softening it just enough under pallet pressure to fuse layers together.

3. Temperature: Triggering Softening & Migration

Temperature doesn’t have to be extreme to cause blocking—just elevated enough for long enough.

• Most coatings have a softening point or glass transition temperature (Tg). Prolonged exposure to temperatures near or slightly above this threshold increases tackiness.
• Heat accelerates migration of plasticizers, surfactants, and unreacted monomers to the surface.
• Cyclical heating and cooling (day/night cycles) lead to expansion, contraction, and pressure fluctuations.

For instance, a shipment of coated PE film was stored at 30°C for 48 hours. The rolls, previously stable at 22°C, began to show blocking at the edges and core. Testing showed additive migration to the interface—driven purely by elevated storage conditions.

4. Pallet Pressure: When Weight Becomes a Weapon

Stacking coated films or storing them under heavy pallet pressure introduces two risks:

• Mechanical fusion: Even low tack surfaces can block when compressed under their own weight for extended periods.
• Edge deformation: High point loads at roll edges concentrate pressure, causing blocking rings to form.

The taller the stack, the greater the pressure at the bottom. Without separators or airflow channels, pressure-induced blocking can occur even at ambient temperatures and normal humidity.

5. Combined Effect: The Perfect Storm

Each factor alone is risky. Together, they become deadly.

Imagine this scenario:

• Films stored in summer conditions (30°C)
• Relative humidity near 70%
• Pallets stacked 6 rolls high, with no airflow
• Stored in an enclosed truck for 2 days in transit

Result? Perfectly cured films emerge with severe blocking after storage—edges fused, surfaces sticking, and entire rolls rejected by the customer.

This is not theoretical. It happens routinely when storage protocols aren’t tightly controlled.

6. Prevention Measures for Environmental Blocking

To prevent blocking caused by storage conditions, manufacturers and distributors should:

• Store in climate-controlled environments (recommended: 20–25°C, <55% RH)
• Use spacers or ventilated pallets to reduce pallet pressure
• Limit stack height to prevent excessive load on lower rolls
• Ensure rolls are aged flat for 24–72 hours before stacking
• Add anti-block layers or interleaf film between sensitive rolls
• Avoid direct sunlight, loading docks, or trucks without insulation

7. Don’t Forget the Supply Chain

Many blocking failures happen after the product leaves your plant.

Ask these questions:

• Is the warehouse temperature monitored?
• Are shipping trucks climate-stable?
• Do distribution centers stack higher than recommended?
• Is the film wrapped too tightly in shrink film (exacerbating heat buildup)?

Implementing storage condition SOPs (Standard Operating Procedures) throughout your supply chain is critical to protecting the integrity of coated films.

Summary

Even perfect coatings can fail under imperfect storage conditions. Humidity, temperature, and pallet pressure are invisible threats that can undo hours of precise coating and curing.

Don’t let your production line’s success be erased by poor storage practices. Blocking is preventable—if you manage the post-production environment with the same care you give to the coating line.

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• curing vs storage
• RH control
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READ MORE：

• Why Blocking Appears Days Later: The Delayed Danger of Improper Storage
• Invisible Traps: How Storage Conditions Undo Perfect Coatings After Shipping
• Stacked to Fail: How Pallet Pressure Turns Perfect Rolls into Blocked Waste
• Humidity at Work: How Moisture Turns Cured Films Into Sticky Failures