Introduction: The “macro” effects of nanomaterials stem from “micro” particles
Nanomaterials (at least one dimension <100nm) exhibit "magical" extremely small addition amounts (0.1-5%) that can produce significant performance improvements
(hardness/scratch resistance/conductivity/shielding/antibacterial/UV protection)——these performance enhancements cannot be achieved by micron-scale fillers (>10-30% addition). The ultra-high efficiency of nanomaterials stems from their “nano effect”
——(1) Specific surface area effect——nanoparticles (>100m²/g) have surface atoms accounting for >50% of total atoms, with extremely high surface chemical reaction activity
(e.g., photocatalysis of nano-TiO₂——micron-TiO₂ has no activity); (2) Quantum size effect——nanoparticles (<10nm) have discretized electronic energy levels, altering optical absorption/emission wavelengths
(e.g., UVA absorption of nano-ZnO——micron-ZnO does not); (3) Nanomechanical effect——carbon nanotubes/graphene have ultra-high aspect ratio (>1000:1)
——at extremely low addition (10 times compared to micron fibers at equivalent addition.
I. Overview of the Functions of Six Nanomaterials in Coatings
| Nanomaterial | Particle size / dimensions | Addition level (%) | Core function | Key challenge |
|---|---|---|---|---|
| Nano SiO₂ | 7-40nm | 0.5-5 | Thixotropy / anti-settling / hardness improvement (H→2H→3H) / matting / scratch resistance | Dispersion (nano-agglomerates15m/s) + ultrasonication |
| Nano TiO₂ | 5-50nm | 1-5 | UV shielding (transparent) / photocatalytic self-cleaning / antibacterial | Photocatalytic activity degrades coating resin——requires inorganic coating (Al₂O₃/SiO₂) |
| Nano ZnO | 20-100nm | 0.5-3 | UV absorption (full UVA-UVB spectrum) / antibacterial / anticorrosive pigment (replacing Cr⁶⁺) | ZnO is an amphoteric oxide——dissolves at high pH (>10) and low pH (<5)——coating pH must be neutral |
| MWCNT | Diameter 10-50nm / length >1μm | 0.3-1.5 | Conductive (10³-10⁷Ω / surface resistance) / mechanical reinforcement (E+>30%) | Extremely low percolation threshold (0.05vol%)——once exceeded——resistivity drops sharply >10⁶ times——requires precise control |
| Graphene | 1-5 layers / flake size >1μm | 0.5-2 | Conductive (10³-10⁶Ω) + shielding (maze effect / permeation path +50×) / mechanical reinforcement | Very difficult to disperse (π-π stacking agglomeration) requires ultrasonication + pyrene-based dispersant + swelling pre-dispersion |
| Nano clay | Layer thickness >1nm / diameter >100nm | 2-5 | Barrier (reduces oxygen transmission rate >50%) / flame retardant (char layer enhancement) / thixotropy | Organic modification (quaternary ammonium salt intercalation)——expands interlayer spacing (d₀₀₁)——can be intercalated / exfoliated by polymer |
FAQ
Q1: The two treatments of nano-SiO₂—”hydrophilic” and “hydrophobic”—what are they respectively used for in coatings?
Hydrophilic SiO₂ (dense surface OH groups)—(1) in water-based coatings
—hydrogen bond thickening—thixotropy—good storage stability; (2) in solvent-based
—hydrophilic SiO₂ absorbs water—coating blistering—not acceptable. Hydrophobic SiO₂ (surface OH groups replaced by silane/siloxane)—(1) in solvent-based
—good compatibility—no water absorption—excellent anti-settling performance; (2) hydrophobicity imparts a “lotus leaf effect” to the coating
—water contact angle >120°.
Q2: Why is “inorganic encapsulation” needed to suppress the “photocatalysis” of nano-TiO₂ in coatings?
Nano-TiO₂ (anatase) generates ·OH radicals under UV — (1) Positive — photocatalytically decomposes organic pollutants/bacteria/mold on the coating surface
— self-cleaning — antibacterial; (2) Negative: OH radicals also attack the organic resin of the coating — “decompose” the resin — coating accelerates chalking
. Inorganic encapsulation (nano-TiO₂ particle surface coated with a dense >2-5nm Al₂O₃/SiO₂ shell) — physically isolates the photocatalytic activity of TiO₂ from the resin
— the resin does not sense the photocatalysis of TiO₂ — the coating does not chalk — but retains the UV shielding of TiO₂
(refractive index of TiO₂ >2.5 — UV photons are scattered by TiO₂ particles — do not enter the coating resin) — uncoated nano-TiO₂ will “eat its own coating” under UV.
Q3: Dispersion of graphene in coatings—why is it the “most difficult” nanomaterial?
The π-π stacking interaction between graphene sheets is extremely strong
(van der Waals force / per layer approx. >100 mJ/m²)—graphene exists in solvent/resin >99% as agglomerates (>1 μm diameter / stacked >100 layers)
—not as monolayers (20 kHz / 500 W / >30 min)—breaks graphene agglomerates “apart by vibration”
into monolayers / few layers—but within hours after ultrasonication—few-layer graphene “re-agglomerates” again
—(2) Dispersant—contains pyrene-based (Pyrene) anchoring groups
—the pyrene group and graphene’s π electron cloud undergo strong π-π stacking adsorption
—the dispersant’s long chains (polymer segments) provide steric hindrance
—preventing graphene from re-agglomerating—pyrene-based dispersants are the “key” to dispersing graphene in coatings.
Q4: The “intercalation/exfoliation” mechanism of nanoclay?
Natural montmorillonite (MMT)——interlayer contains Na⁺/Ca²⁺——hydrophilic——incompatible with organic resins. Organic modification with quaternary ammonium salt (long-chain alkyl ammonium/QAS)
——(1) The positive charge (N⁺) of the quaternary ammonium salt undergoes ion exchange with Na⁺ in the MMT interlayer
——The quaternary ammonium salt “inserts” into the interlayer——the interlayer spacing d₀₀₁ expands from >1nm to >2-3nm “intercalation”; (2) During coating processing——polymer segments or monomers “squeeze into” the interlayer
——further expansion——eventually the MMT sheets are “completely exfoliated” into monolayers (100nm diameter) “exfoliation”
——The exfoliated MMT sheets form overlapping “nano-barriers” in the coating
——The permeation path of O₂ and H₂O is extended by >50 times——the barrier performance of the coating is significantly improved.
Q5: Nano ZnO as an anti-corrosion pigment—mechanism for replacing Cr⁶⁺?
Anti-corrosion mechanism of ZnO—(1)Slow dissolution of ZnO in water
(ZnO+H₂O→Zn²⁺+2OH⁻—Zn²⁺ forms a Zn(OH)₂/ZnO precipitation layer at the coating/steel interface
—dense—blocks subsequent corrosion); (2)Inhibition of cathodic reaction by Zn²⁺
(Zn²⁺+2OH⁻→Zn(OH)₂—consumes OH⁻ generated at the cathode—lowers interfacial pH—slows corrosion). The anti-corrosion efficiency of ZnO is weaker than Cr⁶⁺ (>50%)—but non-toxic/environmentally friendly
—is one of the feasible alternatives to Cr⁶⁺.
Related Reading
Summary
Six nanomaterials—SiO₂ (thixotropy/hardness), TiO₂ (UV shielding/self-cleaning), ZnO (UV absorption/anticorrosion), MWCNT (conductivity), graphene (conductivity + shielding), and nanoclay (barrier)—each excel in their respective roles in coatings. Nanodispersion (ultrasonication/ball milling/pyrene-based anchoring/intercalation-exfoliation) is the “key technical barrier” to the quality of nanocoatings. Kexin New Materials provides customers with full-set nanomodified coating formulations and dispersion technical support.