Plastics : Moisture contamination in the plastic industry

Moisture contamination refers to the unwanted presence of water in plastic materials, resins, or finished products. 

This water content can negatively affect processing, appearance, physical properties, and long-term performance of plastic products.

Sources of moisture contamination:

• Ambient humidity: hygroscopic resins absorbing moisture from air during storage or handling
• Improper storage: exposure to humid environments or temperature fluctuations causing condensation
• Inadequate drying: insufficient pre-processing drying of hygroscopic materials
• Process water: cooling water leaks in processing equipment
• Contaminated regrind: reprocessed material with high moisture content
• Raw material handling: transfer operations exposing pellets to ambient conditions
• Additives and fillers: some additives or fillers containing residual moisture

Hygroscopic: property of materials that readily absorb and retain moisture from the surrounding environment

Most susceptible materials:

• Hygroscopic polymers: nylon (PA), polycarbonate (PC), polyethylene terephthalate (PET), acrylics (PMMA)
• Biopolymers: PLA, PHA, and other biodegradable plastics
• Engineering plastics: ABS, polyurethanes, polysulfones
• Filled compounds: glass fiber reinforced materials, natural fiber composites
• Cellulosic materials: cellulose acetate, cellulose propionate

Nylon (PA): strong, abrasion-resistant synthetic polymer used in textiles, automotive parts, and engineering applications.
Polycarbonate (PC): tough, transparent thermoplastic known for impact resistance, used in eyewear, electronics, and automotive components.
Polyethylene Terephthalate (PET): clear, strong plastic commonly used for beverage bottles, food packaging, and polyester fibers.
Acrylics (PMMA): transparent thermoplastics with excellent optical clarity, used for displays, signs, and as a glass alternative.
PLA (Polylactic Acid): biodegradable thermoplastic derived from renewable resources like corn starch, used in packaging and 3D printing.
PHA (Polyhydroxyalkanoates): biodegradable polymers produced by bacterial fermentation, used in sustainable packaging and medical applications.
ABS (Acrylonitrile Butadiene Styrene): tough, impact-resistant thermoplastic used in automotive parts, consumer electronics, and LEGO bricks.
Polyurethanes: versatile polymers available as flexible foams, rigid foams, or elastomers, used in furniture, insulation, and coatings.
Polysulfones: high-performance thermoplastics with excellent heat and chemical resistance, used in medical devices and automotive applications.
Cellulose Acetate: plant-derived thermoplastic used in eyeglass frames, film bases, and cigarette filters.
Cellulose Propionate: cellulose-based plastic with good optical properties and durability, used in eyewear frames and tool handles.

Processing problems caused by moisture:

• Hydrolytic degradation: chemical breakdown of polymer chains during high-temperature processing
• Surface defects: splay marks, silver streaks, bubbles, or blisters on finished parts
• Voids: internal bubbles or cavities in molded parts
• Dimensional issues: warping, shrinkage variation, or poor dimensional stability
• Reduced mechanical properties: lower tensile strength, impact resistance, or fatigue performance
• Extended cycle times: need for slower injection or extrusion to prevent defects
• Inconsistent coloration: color variations or streaking
• Equipment wear: accelerated screw and barrel wear from corrosive byproducts

Long-term product issues:

• Reduced mechanical properties: loss of strength and stiffness over time
• Premature aging: accelerated deterioration of physical properties
• Delamination: separation of layers in multi-layer products
• Stress cracking: enhanced susceptibility to environmental stress cracking
• Optical issues: haze development in transparent products
• Electrical property degradation: reduction in insulation performance
• Chemical resistance reduction: increased susceptibility to chemical attack

Maximum acceptable moisture levels:

• Nylon (PA): 0.1-0.2% by weight
• Polycarbonate (PC): 0.02-0.05%
• PET: 0.005-0.02%
• ABS: 0.1%
• PLA: 0.025-0.05%
• Most polyolefins (PE, PP): less sensitive, often 0.1-0.2%

Control and prevention strategies:

Material handling:

• Sealed storage containers with desiccants
• Climate-controlled storage areas
• Just-in-time delivery practices
• Minimized exposure to ambient conditions
• First-in, first-out inventory management

Drying technologies:

• Desiccant dryers: most common for hygroscopic materials
• Hot air dryers: simple systems for non-hygroscopic materials
• Vacuum dryers: efficient systems using reduced pressure
• Compressed air dryers: systems integrating material conveying with drying
• Infrared dryers: rapid heating systems for surface moisture

Drying parameters:

• Material-specific temperature recommendations
• Appropriate residence time in dryer
• Proper air flow rates
• Dew point monitoring for desiccant dryers
• Regular maintenance of drying equipment

Testing and monitoring:

• Loss-on-drying testers: measure weight loss after heating
• Karl Fischer titration: precise chemical analysis of water content
• Near-infrared spectroscopy: non-destructive moisture measurement
• Dew point meters: monitor effectiveness of drying systems
• Process monitoring: tracking parameters that indicate moisture issues

Process adaptations:

• Vented extruders and injection units to release moisture during processing
• Modified screw designs for better degassing (process of removing trapped gases, moisture, or volatile compounds from plastic materials during processing through the application of heat, vacuum, ...)
• Vacuum systems for removal of volatiles (substances within plastic that convert to gas when heated during processing like moisture, residual solvents, ...)
• Pre-heating of materials before high-shear processing (manufacturing method that applies intense mechanical force to plastic materials as they flow through equipment, causing rapid mixing, heating, ...)

Moisture contamination represents one of the most common yet preventable quality issues in plastic processing, with proper material handling and drying being the key to consistent, high-quality production.