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Bubbling and cracking after putty powder application are common stubborn issues that trouble many painters and homeowners. These problems not only affect wall aesthetics but may also indicate deep quality hazards. This article deeply analyzes the root causes of bubbling and cracking and focuses on the scientific optimization of the core additive — Hydroxypropylmethylcelulóza (HPMC).
Ⅰ.Why Does Your Putty Powder Bubble and Crack?
1. Main Causes of Putty Powder Bubbling
- Insufficient water retention, excessively fast drying: This is the most critical reason. When the water retention of éter celulózy (such as HPMC) is insufficient, the water in the putty paste is quickly absorbed by the substrate (e.g., cement wall) or evaporates. The air on the substrate surface is trapped during scraping, and rapid water loss prevents the paste from flowing and leveling to release the air, forming bubbles.
- Excessive viscosity, poor leveling: High-viscosity HPMC provides excellent water retention but makes the paste too thick, making it difficult for bubbles to escape.
- Improper construction: Overly dry substrate with strong water absorption or excessively thick single-layer application will aggravate bubble formation.
- Poor solubility of cellulose ether: HPMC that does not fully dissolve forms “fish eyes” in water, and these colloidal particles themselves become bubble nucleation points.
2. Main Causes of Putty Powder Cracking
- Excessive drying shrinkage stress: After water evaporates, the putty layer shrinks. If the internal stress generated exceeds the bonding strength and tensile strength of the putty itself, cracking occurs.
- Insufficient bonding strength: Poor bonding performance of cellulose ether fails to effectively bind inorganic fillers (heavy calcium, ash calcium) together or achieve firm adhesion with the substrate.
- Inconsistent internal and external drying: The surface dries quickly while the interior remains wet, causing the hardened surface to shrink while the interior is still soft, resulting in differential shrinkage and cracking.
- Lack of flexible components in the formula: Under temperature and humidity changes, an overly rigid putty layer cannot withstand stress variations and cracks.
Conclusion: Whether it is bubbling or cracking, hydroxypropyl methyl cellulose (HPMC) plays a pivotal role. It directly determines the water retention, viscosity, lubricity, and bonding strength of the putty paste.
Ⅱ.Scientific Optimization of Cellulose Ether (HPMC) Formula
Optimizing the HPMC formula requires a three-dimensional approach: model selection, dosage control, and compound usage.
1. Selection of HPMC Model
HPMC Viscosity (mPa·s) | Zadržování vody | Application Feel | Leveling Performance | Crack Resistance | Doporučené použití |
Low viscosity (40,000–60,000) | Dobrý | Light & smooth | Excellent, bubbles easily escape | Fair | Recommended: finish coat putty pursuing ultimate smoothness |
Medium viscosity (80,000–100,000) | Vynikající | Smooth with texture | Dobrý | Vynikající | Recommended: universal base coat putty, balancing water retention and crack resistance |
High viscosity (>150,000) | Extremely good | Thick & heavy | Poor, bubbles hard to escape | Good (but prone to internal cracking due to uneven drying) | Use with caution: only for extremely water-absorbent substrates |
2. Precise Control of HPMC Dosage
HPMC Dosage (kg per ton of putty) | Water Retention Effect | Construction Performance | Cost & Side Effects |
Too low (3.0–3.5 kg) | Insufficient, prone to bubbling & cracking | Fast drying, rough feel | High quality failure rate |
Moderate (3.8–4.5 kg) | Sufficient & stable | Smooth, long open time | Best cost-performance balance |
Too high (>5.0 kg) | Excessive | Overly thick, poor leveling, edge curling | Higher cost, slow drying & strength development |
3. Golden Compounding Solutions for HPMC
A single HPMC has performance limits. Compounding with other additives can achieve “1+1>2” effects.
Compound with Hydroxyethylcelulóza (HEC)：
Function: HEC’s molecular structure better locks surface water and forms an excellent “surface water-retaining film.” Compounding with HPMC significantly reduces bubbles and increases compactness.
Recommended ratio: HPMC (100,000 mPa·s) : HEC = 3.5 : 0.5 (kg/ton)
Compound with Škrobový éter:
Function: Only 0.5–1.0 kg/ton starch ether dramatically improves sag resistance and application feel, making thicker scraping easier without sacrificing leveling.
Recommended ratio: HPMC (4 kg/ton) + Starch ether (0.5 kg/ton)
Ⅲ. Summary & Final Recommendations
To completely solve putty powder bubbling and cracking, systematic optimization must start from the cellulose formula:
- Abandon the myth of high viscosity: Prefer 80,000–100,000 mPa·s HPMC, or adopt medium-low viscosity compounding strategy for base/finish coats.
- Precisely control dosage: Stabilize HPMC at around 4.0 kg/ton and fine-tune as needed.
- Master compounding technology: Use HEC, starch ether, wood fiber, etc., to target improvements in water retention, crack resistance, and application feel.
- Never ignore other factors: High-quality fillers, appropriate polymer powder, and standardized construction (substrate wetting, thin multi-layer application) are equally critical to success.
Master these points, and putty bubbling and cracking will no longer be problems!
