Moisture content is a critical quality index for silica (quartz/fused silica) filler used in plastic compounding. Excess free or bound water creates processing defects, weakens mechanical properties and degrades long-term reliability of plastic products.
1. Processing defects during extrusion and injection molding
At high processing temperatures (180–320°C for most engineering plastics), water trapped in silica vaporizes rapidly.
- Voids and bubbles: Water vapor cannot escape quickly from the melt, leaving micro-bubbles, pinholes and surface blisters on molded parts.
- Splay marks: Steam bursts out during injection, forming silvery streaks on the product surface, which ruins appearance and creates stress concentration points.
- Screw slipping and unstable feeding: High moisture makes fine silica powder agglomerate, causing inconsistent feeding and fluctuating extrusion pressure.
2. Poor interfacial bonding between silica and polymer matrix
Silica surface carries silanol groups (Si‑OH). Adsorbed water molecules compete with silane coupling agents to attach to silica surfaces.
- The silane modification layer becomes incomplete, reducing adhesion between filler and plastic resin.
- Weak filler-matrix interface lowers tensile strength, flexural strength and impact resistance. Under stress, cracks easily propagate along the particle boundary.
3. Degradation of polymer resin
Water can trigger hydrolysis of sensitive polymers, especially polyesters, polyamides, and some high-temperature engineering plastics.
- Hydrolysis breaks polymer molecular chains, reducing molecular weight and making the plastic brittle.
- For halogen-free flame retardant plastic composites, moisture may also reduce flame retardancy.
4. Rheology and dispersion issues
Excess moisture promotes silica agglomeration. Hard agglomerates do not break down during compounding.
- Agglomerates act as defect points in the finished plastic.
- Melt viscosity increases; poor dispersion causes uneven filler distribution.
5. Impact on electrical properties (critical for electronic plastics)
For silica-filled plastics used in connectors, PCB substrates and semiconductor packaging materials:
- Moisture increases dielectric loss and reduces volume resistivity.
- It may cause ion migration and insulation failure under high temperature and humidity conditions.
6. Distinction between free moisture and structural hydroxyl groups
- Free surface moisture: Unbound water, the main culprit for molding bubbles. For plastic-grade silica, free moisture is normally controlled below 0.2%, often <0.1% for electronic plastics. It can be removed by drying.
- Structural hydroxyl groups (Si‑OH): Chemically bonded inside silica particles. It cannot be removed by simple drying and will affect high-temperature long-term performance.
7. Practical handling recommendations
- Dry silica powder before compounding if moisture exceeds specification.
- Store silica in low-humidity warehouse with sealed moisture-barrier packaging.
- Preheat and dry the powder feeding system during extrusion.
- Use properly silane-treated silica to reduce water adsorption tendency.
High moisture content in silica powder causes bubbles, splay marks and poor dispersion during plastic molding. It interferes with silane coupling, weakens interfacial bonding and can hydrolyze sensitive polymers. For electronic-grade filled plastics, moisture must be strictly controlled below 0.1–0.2% to guarantee mechanical integrity and stable electrical performance.
