1. General Safety Critical Values
The acid value of the resin in the entire leafing aluminum paste system must be ≤7; If the acid value is >7, the carboxyl groups in the resin will damage the stearic acid leafing coating on the aluminum sheet surface, resulting in a significant decrease in leafing value, graying, weak metallic appearance, and poorer leafing sheet quality.
2. Optimal Usage Range
• Best leafing Effect: Acid value 1-5 (preferred), intact leafing, bright mirror silver finish, stable leafing value above 65%
• Barely Usable Range: 5-7, leafing ability gradually weakens, only suitable for ordinary coarse-finish leafing silver
• Strictly Prohibited: >7, completely loses leafing ability, becoming almost non-leafing silver effect
3. Comparison of Acid Values between leafing and Non-leafing
• leafing Aluminum Paste: Resin acid value ≤7, preferably 1-5
• Non-leafing Aluminum Paste: Resin acid value ≤15 is sufficient, not limited by fatty acid coating
4. Supplementary Key Points
1. leafing Aluminum paste relies on the stearic acid film on the aluminum sheet surface for buoyancy. Acidic groups will saponify and dissolve the fatty acid film, directly ruining the leafing performance;
2. In addition to acid value, leafing Aluminum paste should be used with low-polarity, weak-polarity resins, and strong-polarity ketones and alcohols should be used sparingly;
- For alkyd resins and rosin-modified resins, acid value should be strictly controlled. For acrylic silver paint, low-acid-value hydroxyl acrylics should be selected as much as possible.
Comparison Table of Acid Values for leafing Aluminum Silver Paste and Various Resins
I. Core Usage Principles
leafing aluminum silver paste relies on the stearic acid coating layer on the surface of aluminum powder to float. If the resin carboxyl groups (acid value) are too high, they will saponify the stearic acid, destroying the leafing film and resulting in issues such as failure to leafing, haziness, dullness, sinking of silver particles, lack of mirror finish, and color differences before and after spraying. Stronger polarity makes it easier to damage the leafing film.
II. Suitable Acid Values and Usage Instructions for Various Resins
1. Alkyd Resins (Most Commonly Used in leafing Silver Paint)
1) Long-Oil Alkyd Resins
• Suitable Acid Value: 2～5 mgKOH/g
• Optimal Range: 3～4
•Characteristics: Low polarity, good silvering effect, stable floating, suitable for furniture topcoats, industrial leafing silver paints, and hardware silver powder paints
• Contraindications: Acid value > 6 significantly reduces leafing performance; short-oil alkyd resins have high polarity and should be used sparingly.
2) Medium-Oil Alkyd Resins
• Suitable Acid Value: ≤4 mgKOH/g
• Optimal Range: 2～3
• Notes: Higher polarity than long-oil resins, only suitable for low-acid-value products for leafing silver; higher acid values only suitable for non-leafing silver applications.
3) Rosin-Modified Alkyd Resins
• Suitable Acid Value: ≤3 mgKOH/g
• Key Points: Rosin contains carboxyl groups, which easily damage the leafing film. The lower the acid value, the better. An acid value >4 is strictly prohibited for leafing silver. It is mostly used in low-grade anti-rust leafing silver paint.
2. Acrylic Resin (Self-drying/Baked leafing Silver Paint)
1) Self-drying Hydroxyl Acrylic Resin
• leafing silver acid value limit: 1～5 mgKOH/g
• Preferred: 2～4
• Note: Most commercially available self-drying acrylic resins have an acid value of 8～15, and can only be used for non-leafing silver; special low-acid acrylic resin is required for mirror-finish leafing silver.
2) Baked Hydroxyl Acrylic Resin
• leafing silver acid value limit: ≤5 mgKOH/g
• Preferred: 2～4
• High acid value baked acrylic resin (6~12) is only suitable for non-leafing aluminum silver paste.
3) Thermoplastic Acrylic (Single-component Self-drying)
• leafing silver acid value limit: ≤4 mgKOH/g
• 1. Preferred 1-3; Thermoplastic acrylics have high polarity, and relaxing the upper limit of acid value easily leads to loss of floatability.
3. Amino Resins (for Baking Paint)
1) Methyl Etherified Amino (Fully Methyl Ether)
• The overall acid value of the mixed system should not exceed 5.
• Amino resins themselves have low acidity; the main focus is on controlling the acid value of the main acrylic resin.
2) Butyl Etherified Amino Resins
• Slightly higher acidity; when used with leafing silver, the acid value of the main resin should be kept as low as possible to 1-3.
4. Epoxy Resins
• High-acid epoxy resins are prohibited for leafing silver.
• Can be used: Low molecular weight bisphenol A type epoxy, acid value ≤2 mgKOH/g.
• Most epoxy resins are highly polar and have high acidity due to trace amounts of hydroxyl groups; they are rarely used in leafing silver systems and are primarily used only for non-leafing silver systems.
5. Polyester Resins (Roll Materials, Hardware Baking Paint)
1) Saturated Polyester
• leafing silver acid value ≤4 mgKOH/g, preferably 2-3.
2) Unsaturated Polyester
• It is not recommended to use with leafing aluminum silver paste, as the system is highly acidic and polar, making leafing difficult and unstable.
6. Chlorinated rubber resin
• leafing silver acid value ≤ 5 mgKOH/g, preferably 2-4; commonly used in shipbuilding and anti-corrosion leafing silver paints.
7. Nitrocellulose resin/Nitrocellulose cotton
• Nitrocellulose cotton itself has almost no acid value; when used with alkyd/acrylic resins, the acid value should be controlled to ≤ 5; nitrocellulose leafing silver topcoats are very common.
III. Overall Acid Value Requirements of the System (Key Point)
Not just looking at the main resin, but the overall acid value after mixing the entire formula:
1. High-end mirror-finish leafing silver: Overall acid value 1-4
2. Ordinary glitter leafing silver: Overall acid value 4-6
3. Red line critical: The overall acid value of the entire system must not exceed 7.
IV. Key Points for Matching Solvents (Affecting leafing, Supplementary Information)
1. Prioritize: Xylene, Trimethylbenzene, High-boiling-point aromatics, Aliphatic hydrocarbons (low polarity)
2. Minimize: Ethyl acetate, Butyl acetate, Isophorone, Cyclohexanone (Strongly polar esters and ketones damage the leafing film)
3. Minimize the use of alcohol solvents (ethanol, isopropanol, butanol)
V. Simple Troubleshooting
1. Silver surface appears grayish and lacks brightness → Resin acid value is too high
2. aluminum paste settles after a period of time → Acid value exceeds standard, stearic acid coating is damaged
3. Initially bright after spraying, but becomes darker with continued spraying → System acidity gradually damages the leafing film