Plastics : Storage and handling issues in the plastic industry

Below is a comprehensive introduction to the problems that occur when plastic materials (resins, compounds, additives) deteriorate or become contaminated due to improper warehouse conditions, packaging methods, or material handling practices before processing.

Types of degradation during storage:

Moisture absorption:

Many plastics, particularly engineering resins like nylon, polycarbonate, PET, and PBT attract and hold water molecules. 

When exposed to humid environments these materials absorb moisture from the air, and absorbed moisture causes hydrolysis (chemical breakdown) of polymer chains

During processing, this moisture can turn to steam, causing:

• Surface defects (splay marks)
• Voids and bubbles
• Reduced mechanical properties
• Increased viscosity variations

Oxidative degradation:

• When plastic resins are exposed to oxygen, especially at elevated temperatures
• Free radical reactions break polymer chains
• Molecular weight decreases
• Melt flow rate increases (material flows more easily but has reduced strength)
• Discoloration occurs (yellowing or browning)
• Mechanical properties deteriorate
• Processing window narrows

UV degradation:

• Exposure to sunlight or fluorescent lighting can cause:
• Photo-oxidation of polymers
• Chain scission (breaking of polymer chains)
• Crosslinking (unwanted bonding between chains)
• Color shifts
• Surface chalking
• Brittleness

Thermal degradation:

• Even below melting temperatures, extended exposure to heat can cause:
• Accelerated oxidation
• Volatilization of plasticizers or additives
• Premature aging
• Increased crystallinity in semi-crystalline polymers
• Changes in flow characteristics

Biological degradation:

• In warm, moist environments:
• Mold and mildew can grow on material surfaces
• Biodegradable plastics may begin decomposition prematurely
• Contamination with microorganisms affects product quality

Critical storage parameters:

Temperature control:

• Most resins should be stored between 10-25°C / 50-77°F
• Temperature fluctuations accelerate degradation
• Hot spots near heating elements or direct sunlight cause localized degradation
• Cold temperatures can cause moisture condensation when containers are opened

Humidity management:

• Ideal relative humidity: 40-60%
• Higher humidity accelerates moisture absorption
• Desiccant packs or climate-controlled warehousing may be necessary
• Material "breathing" (temperature cycling) causes repeated moisture absorption

Light exposure:

• UV-sensitive materials require opaque packaging
• Warehouse lighting considerations (UV filters)
• Window exposure management
• Protection during transportation

Airborne contaminants:

• Dust particles
• Chemical vapors from other materials
• Cross-contamination between different resins
• Forklift exhaust contamination

Material-specific concerns:

PVC compounds:

• Plasticizer migration and evaporation
• Heat stabilizer depletion
• Color shift due to stabilizer consumption
• Plate-out of additives

Polyolefins (PE, PP):

• Antioxidant depletion during storage
• Auto-oxidation catalysis from metal contamination
• Slip agent migration to surface
• UV stabilizer consumption

Engineering resins:

• Higher moisture sensitivity
• More rapid property changes
• May require specialized packaging

Masterbatches and compounds:

• Pigment settling or separation
• Additive migration
• Compatibility issues after extended storage

Packaging methods and solutions:

Moisture barrier packaging:

• Aluminum foil liners
• Multi-layer barrier films
• Heat-sealed packaging
• Vacuum packaging for critical materials

Container types:

• Gaylord boxes (risk of crushing and contamination)
• Lined fiber drums (moderate protection)
• Plastic drums (better barrier properties)
• FIBC (Flexible Intermediate Bulk Containers) with various liner options
• Octabins (octagonal containers with liner options)

Desiccants and oxygen absorbers:

• Silica gel packets
• Molecular sieves
• Clay-based desiccants
• Oxygen scavenging sachets
• Active packaging technologies

Handling challenges:

Material transfer:

• Static electricity generation causing dust attraction
• Contamination from transfer equipment
• Material segregation (fines separation from pellets)
• Bridging in hoppers and silos
• Rat-holing in storage containers

Bridging occurs when granular plastic materials form an arch or bridge across the hopper opening, creating a void space below while material remains stuck above. This stops material flow despite the container having plenty of material inside.

Rat-holing is when plastic material flows only through a narrow vertical channel (resembling a rat hole) directly above the outlet, while the surrounding material remains stationary against the container walls

Equipment contamination:

• Cross-contamination between material types
• Color contamination
• Residual moisture in pneumatic systems
• Metallic contamination from wear

Material tracking:

• First-in, first-out (FIFO) implementation challenges
• Lot tracking and traceability
• Material age monitoring
• Quality degradation tracking

Industry best practices:

Storage environment:

• Climate-controlled warehousing
• Air filtration systems
• Regular cleaning regimes
• Segregation of incompatible materials
• Limited access to storage areas

Material rotation:

• Strict FIFO protocols
• Maximum storage time guidelines by material type
• Regular quality testing of aged inventory
• Computerized inventory management systems

Pre-processing preparation:

• Proper drying protocols before processing
• Material testing before use
• Blending strategies for aged materials
• Additive replenishment for aged stocks

Economic impact:

Direct costs:

• Material write-offs due to degradation
• Processing inefficiencies from degraded materials
• Higher scrap rates
• Increased energy consumption for processing degraded materials
• Additional testing requirements

Material write-offs are financial losses recorded when inventory (raw materials, compounds, or finished products) must be removed from accounting books because it has become unusable due to degradation, contamination, obsolescence, or damage, and can no longer be sold or processed at full value.

Indirect costs:

• Downtime for troubleshooting
• Customer returns due to quality issues
• Reputational damage
• Warranty claims
• Excess inventory to compensate for quality variations