How to Solve Blocking in Water‑Based Varnishes?

Blocking (i.e., the adhesion or surface damage of coated layers when stacked after drying) is a common issue in water‑based varnish applications. The root causes typically include incomplete drying, insufficient surface hardness, or excessive intermolecular forces. Solving the problem requires targeted adjustments in four dimensions: drying process, formulation optimization, material selection, and storage conditions. The specific solutions are outlined below.

I. First, Identify the Core Causes of Blocking (Prerequisite for Targeted Solutions)

Blocking essentially results from “the coating surface not achieving sufficient dryness and hardness.” Common triggers include:

1. Insufficient drying: Water/residual solvents are not fully evaporated, leaving free small molecules inside the coating.

2. Resin performance limitations: The selected resin has too low a glass‑transition temperature (Tg) (generally ≥40°C is required); at room temperature it tends to remain in a “semi‑soft” state.

3. Missing or unbalanced additives: Insufficient anti‑blocking or slip agents, leading to high surface friction.

4. Excessive coating weight: Over‑thick coating (>10 μm greatly increases risk); drying lags behind molecular migration.

5. Environmental triggers: Storage/stacking temperature >30°C, humidity >60%, accelerating molecular‑chain mobility in the coating.

II. Scenario‑Based Solutions (Prioritized)

1. Optimize the Drying Process (The Most Direct and Effective Immediate Measure)

Insufficient drying is the primary cause of blocking. Adjustments should be made from both “drying efficiency” and “drying thoroughness” perspectives:

● Increase drying temperature/extend drying time:

○ For paper/film substrates: Raise the drying temperature from 50–60°C to 70–80°C (mind the substrate’s heat‑resistance limit, e.g., BOPP film ≤80°C). Set air velocity in the drying tunnel to 8–12 m/s to ensure >80% of surface moisture evaporates within 30 seconds (verifiable by weight‑loss test: ≤1.5% weight loss after drying is acceptable).

○ For wood/metal substrates: Adopt “staged drying” (first 60°C for 10 min to remove moisture, then 80–90°C for 20 min for curing) to avoid rapid surface sealing that traps internal moisture.

● Enhance drying‑equipment configuration: On continuous production lines, add 1–2 sets of infrared drying units (wavelength 2–5 μm) after the existing dryer to target deep‑layer moisture evaporation.

2. Adjust the Varnish Formulation (Addressing Root Issues with Resins/Additives)

● Increase resin hardness:

○ Replace or blend with higher‑Tg resins: Change from a single low‑Tg resin (e.g., acrylic with Tg 30°C) to a blend of “high‑Tg resin + medium‑Tg resin” (e.g., 70% styrene‑acrylic with Tg 50°C + 30% pure acrylic with Tg 35°C) to balance hardness (pencil hardness ≥HB) without excessive brittleness.

○ Introduce a cross‑linking system: Add 0.5–1.5% waterborne isocyanate (e.g., HDI trimer) or aziridine cross‑linker (for carboxylic‑acid‑type resins) to improve coating resistance and anti‑blocking via room‑temperature cross‑linking (ensure compatibility with the resin through small‑scale trials first).

● Strengthen anti‑blocking additives:

○ Add wax‑based anti‑blocking agents: Incorporate 2–5% micronized wax (e.g., polyethylene wax, particle size 3–5 μm) or polytetrafluoroethylene (PTFE) wax (0.5–1%). Wax particles migrate to the surface, forming a “physical barrier” that lowers surface energy (optimal surface tension: 32–36 dyn/cm).

○ Introduce silicone slip agents: Add 0.1–0.3% polydimethylsiloxane (PDMS, molecular weight 5000–10000) or silicone‑modified acrylates to create a lubricating film, reducing intermolecular adhesion during stacking (avoid over‑addition to prevent cratering).

For technical inquiries or to request samples, please contact our online engineers.

3. Control Coating Weight and Thickness

Excessive thickness significantly prolongs drying time and can trap residual solvents. Recommendations:

● Reduce wet‑film thickness: Adjust coating weight according to substrate: for paper, aim for 4–8 μm (dry film); for film/wood, ≤10 μm (dry film). This can be achieved by using a lower‑line‑count gravure roller (e.g., change from 120 to 100 lines/inch) or reducing doctor‑blade pressure.

● Adopt “multiple thin coats” technique: For high‑gloss or thick‑film effects, apply 2–3 coats (each 3–5 μm dry film), with 10–15‑minute intervals between coats (ensuring the previous layer is fully dry) to avoid single‑pass heavy coating.

4. Optimize Storage and Post‑Treatment Conditions

Even if drying is adequate, improper storage can still cause blocking:

● Control stacking environment: Storage temperature ≤25°C, relative humidity ≤55%. Stack height should not exceed 1.5 m (to avoid excessive pressure on bottom layers), and interleave stacks with breathable separator paper (e.g., kraft paper, silicone‑release paper) to reduce direct contact area.

● Post‑curing treatment: For high‑risk products (e.g., dark substrates, thick coatings), allow the dried items to rest in a ventilated area for 24 hours (natural post‑curing) to let residual small molecules fully evaporate before stacking.

5. Emergency Measures for Extreme Cases

If blocking has already occurred, minimize losses by:

● Light blocking: Separate the adhered pieces and lightly dust the surfaces with fine talc (≥3000 mesh) to absorb residual tackiness.

● Severe blocking: If the coating is undamaged, return the stack to the dryer (60°C, 30 min) for secondary drying before separation. If the surface is damaged, spot‑recoat with varnish and redry.

For technical inquiries or to request samples, please contact our online engineers.

III. Formulation‑Design Principles for Blocking Prevention (Long‑Term Solution)

To prevent blocking from the outset, the formulation should meet:

● Resin system: Tg ≥45°C (glassy state at room temperature, non‑tacky), with 5–10% high‑Tg resin (e.g., methyl methacrylate copolymer with Tg 60°C) to enhance surface hardness.

● Additive system: Routinely include 3–5% wax powder (e.g., PE wax dispersion) + 0.2% silicone slip agent to ensure a static friction coefficient ≤0.3.

● Drying standard: Coating moisture content ≤1% (tested by Karl Fischer method), pencil hardness ≥H (tested with Chinese pencils).

By following the steps above, blocking in water‑based varnishes can be fundamentally resolved. In practice, it is recommended to start with a “small‑scale diagnostic test”: measure coating moisture content (to assess drying sufficiency) → test pencil hardness (to check if resin Tg is adequate) → evaluate surface slip (to determine if additives are sufficient), then make targeted adjustments for greater efficiency.