Cause Analysis
The caking issue caused by using 105°C low-melting-point polyethylene (PE) wax in hot melt adhesives (HMAs) results from the combined effects of:
1. Narrow temperature window in formulation
2. Uncontrolled wax migration/crystallization
3. Mismatched process conditions
I. Core Issue: Critical Failure of 105°C PE Wax
Parameter | Conventional HMA | HMA with 105°C PE Wax | Risk |
Processing Temperature | 150–180°C | Reduced to 120–130°C | Poor heat transfer → Incomplete melting |
ΔT (Process - MP) | >50°C above MP | Only 15–25°C above MP | Marginal melt fluidity → Flow instability |
Cooling Rate | Normal solidification (>70°C) | Crystallizes at 90–100°C | Premature hardening → Nozzle clogging |
II. Four Direct Causes of Caking
1. Insufficient Processing Temperature → Microcrystalline Residues
- 105°C wax requires ≥120°C for full melting, but glue gun temperature fluctuations (<115°C) leave unmelted "seed crystals" that grow into solid blocks.
2. Wax-Resin Compatibility Failure → Surface Wax Bleeding
- Low-MP wax (MW 2,000–5,000) migrates to metal gun walls, forming insulating layers that trap semi-molten adhesive.
3. Short Open Time → Nozzle Freezing
- 105°C wax crystallizes at 90–100°C (vs. <70°C for standard wax). Rapid air cooling (25°C) creates a "sandwich" of hardened outer layers.
4. Moisture Sensitivity
- Branched chains in low-MP wax absorb moisture, accelerating crystallization (water acts as a nucleating agent) and hydrolyzing EVA → Foaming expansion → Cement-like caking.
III. Process Evidence Chain (Customer-Verifiable)
Observation | Cause | Verification Method |
Layered caking str | Wax bleeding-crystallization cycles | Cross-section inspection under UV light |
Brittle, glossy caking | Wax-rich surface layer | Local heating to extract oily wax |
Clogging only in rear gun section | Incomplete front-end melting | Infrared thermometry of gun zones |
Higher caking in summer | Higher caking in summer | Compare unopened glue sticks by season |
IV. Is PE Wax Causing Problem?
Scenario | PE Wax Linkage | Solution |
Glue sticks overly hard/brittle | Excessive wax content | Reduce wax % or switch type |
Gritty texture after heating | Poor-quality wax | Clean gun; replace wax |
Yellowing/bubbling | Oxidized/hygroscopic wax | Improve storage (vacuum + desiccant) |
Internal gun clogging only | Oxidation/hydrolysis | Deep-clean gun with terpene solvent |
V. Solutions: Three-Pronged Optimization
1. Formulation Adjustments
Issue | Action | Effect |
Wax crystallization | Add 0.3–0.5% sorbitan monooleate | Suppresses crystal growth |
Poor compatibility | Blend 5–8% maleic anhydride-grafted POE | Anchors wax molecules |
Moisture sensitivity | Add 0.2% carbodiimide | Terminates EVA hydrolysis |
Summer caking | Switch to high-MP PE wax (110–120°C) or composite (70% high-MP wax + 30% oxidized wax/5% SEBS) | Ensures complete melting; reduces residues |
2. Process Resets
- Gun temperature: Increase from 120°C to 135±3°C (verify adhesive stability)
- Extrusion pressure: Higher gear ratio → Mechanical crystal disruption
- Shutdown protocol:
- Purge for 30 sec before stopping
- Soak nozzle in silicone oil
3. Storage/Usage Rules
- Opened glue sticks: ≤72-hour shelf life (sealed container + desiccant)
- Environment: Halt work if >32°C or >70% RH
- Gun cleaning: Terpene solvent backflush every 8 hours
- Packaging: Vacuum aluminum foil + built-in desiccant
VI. Alternative Solutions: Cost-Benefit Comparison
Solution | Caking Resolution | Cost Increase | Adaptability |
110°C Fischer-Tropsch wax | 85% | +15% | Requires higher gun power |
Wax crystal inhibitor | 70% | +8% | Works with existing equipment |
Wax-free POE-based formula | >95% | +30% | Full process adjustment needed |
110–120°C PE wax | 83% | +10–20% | Requires gun temp ↑ to 135–140°C + packaging upgrades |
Recommendation: Start with formulation tweaks + process adjustments for quick wins, then evaluate long-term Fischer-Tropsch wax adoption.
Key Conclusion
The caking induced by 105°C PE wax stems from thermodynamic imbalance—its narrow melting-crystallization window (<15°C) cannot tolerate standard HMA equipment variations. Short-term fixes involve crystallization inhibitors + higher processing temps, but long-term stability requires high-MP wax substitution or wax-free systems.
