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Core principle: The hydroxyl value represents the hydroxyl (- OH) content in the resin and is the active site for reacting with isocyanate NCO; The hydroxyl value determines the crosslinking density and directly affects the hardness, durability, flexibility, curing ratio, and viscosity of the paint film. Definition: Hydroxyl value (mgKOH/g): the milligrams of KOH required to neutralize the acetic acid generated by the acetylation of hydroxyl groups in 1g of resin. The higher the hydroxyl value, the more hydroxyl groups per unit mass of resin.

The effect of hydroxyl value on the performance trend of paint film

High hydroxyl value hydroxypropyl resin (typical: 120-200 mgKOH/g)

✅ Advantage

1. High hydroxyl groups and cross-linking density: high hardness and Tg; MEK has better resistance to wiping, alcohol, acid and alkali, and salt spray; Anti sticking and anti pollution after drying.

2. Under the same solid content, there are sufficient reaction sites with the curing agent, and the chemical resistance limit is high after film formation, making it suitable for topcoats, metal topcoats, and high weather resistant industrial topcoats.   ⚠️ shortcoming

3. High cross-linking density, increased brittleness, decreased impact/flexibility, easy cracking, thick coating, and significant risk in low-temperature environments.

4. For resins with the same molecular weight, hydroxyl groups are polar groups, and the higher the hydroxyl value, the higher the polarity of the resin: viscosity increases; The formula does not have a high solid content, and VOCs are prone to increase (solvent based); The viscosity of water-based lotion increased.

5. Consuming more isocyanate curing agents leads to an increase in curing agent usage and cost.

6. There are many hydroxyl groups, and residual unreacted hydroxyl groups will increase the hydrophilicity of the paint film; If the curing is not complete, the water resistance will actually deteriorate.

Hydroxypropyl resin with medium hydroxyl value (typical: 60-110 mgKOH/g, most commonly used universal grade)

Balanced range, the main selection of 2KPU topcoat paint. Balancing: hardness and flexibility, acceptable chemical resistance, moderate dosage of curing agent, wide construction window.

Around 80: The most classic universal hydroxypropyl, the preferred industrial topcoat, balances comprehensive performance.

Low hydroxyl value hydroxypropyl resin (typical: 20-55 mgKOH/g)

✅ Advantage

1. Low hydroxyl groups, low cross-linking density, excellent flexibility, high elongation, good impact resistance, and not easy to crack; Suitable for primer, elastic coating, plastic substrate, thin coating flexible system.

2. Low polarity, lower viscosity of the resin body, which can achieve higher solid content and reduce VOC; The amount of curing agent added is small, and the formula cost is low.   ⚠️ shortcoming

3. Insufficient cross-linking density: Low hardness, poor resistance to solvents, alcohols, and chemicals, easy to scratch and stain.

4. Low cross-linking network, easily permeable to water and solvents, with decreased resistance to water and salt spray.

3. Key influencing points of formula operation

① Curing agent addition amount

Equivalent resin feeding: The higher the hydroxyl value, the more HDI/IPDI curing agent is required.

For example, the hydroxyl value is 40 vs 120, with the latter having a curing agent dosage that is nearly three times higher. Fixed NCO/OH ratio, changes in hydroxyl value must be recalculated for curing agents, and the ratio cannot be fixed based on experience.

② Applicable period

Same type of resin, the higher the hydroxyl value and concentration, the faster the reaction rate, and the shorter the applicable period; Especially for water-based 2KPU, the system contains water, and high hydroxyl value system NCO is more easily consumed, resulting in a significant decrease in shelf life.

③ Table work/practical speed

High hydroxyl: Multiple reaction sites, faster development of hardness and hardness; Low hydroxyl: slow curing reaction, slow hardness climb, and longer curing period in the later stage.

④ Water absorption rate and weather resistance of paint film

The higher the hydroxyl value, the better the water resistance

Complete curing: high hydroxyl → dense cross-linked network, better water resistance;

Incomplete curing: A large amount of unreacted hydroxyl groups remain in the paint film, polar hydroxyl groups absorb water, and are resistant to sudden drops, foaming, and loss of gloss.

⑤ Compatibility with pigments and additives

Hydroxyl groups are polar groups: high hydroxyl resins have better coloring effects on pigments and improved dispersion stability; But if the polarity is too high, it is easy to have compatibility problems with some acidic additives and wetting agents.

4. Selection recommendation window

High durability and high hardness industrial topcoat: hydroxyl value of 100-160 mgKOH/g, combined with HDI trimer curing agent

Universal 2KPU topcoat (balanced hardness and toughness): hydroxyl value of 60~90 mgKOH/g

Primer, plastic substrate, requiring impact resistance, high solid and low VOC: hydroxyl value 25~55 mgKOH/g

5. Attention to matching: hydroxyl value ≠ molecular weight

⚠️ Two independent parameters, do not confuse:

1. Same molecular weight: hydroxyl value ↑=number of hydroxyl groups ↑

2. Same hydroxyl value: The higher the molecular weight, the sparser the distribution of hydroxyl groups on the molecular chain, and the more resilient the cross-linked network; Low molecular weight, dense cross-linking points, and more brittle paint film.

The formula must be considered simultaneously: hydroxyl value+number average molecular weight Mn+Tg