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PCDL is a high-end 2KPU soft segment polyol with a main chain of carbonate bonds, which is different from ordinary adipic acid polyester, PCL polycaprolactone, and PPG polyether. Its core positioning is a high-performance topcoat/elastic coating system with long life, high hydrolysis resistance, wear and weather resistance, and light and color retention. It is generally used with aliphatic curing agents for solvent based 2KPU and can also be used for water-based 2KPU polyol components such as Asahi Kasei Co., Ltd.

1、 PCDL endows 2KPU paint film with core advantages

1. Water resistance is much better than ordinary adipic acid polyester. Ordinary polyester PU is prone to ester bond hydrolysis, powdering, and adhesion collapse due to long-term wet heat, salt spray, and water immersion; The hydrolysis stability of PCDL carbonate bonds is stronger, and the long-term performance retention rate of wet heat aging and salt spray is high (it still slowly hydrolyzes in alkaline environments, which is a weakness).

2. Wear resistant, scratch resistant, stone impact resistant, good toughness. Carbonate polarity is high, strong cohesion, and the paint film balances hardness and fracture elongation, making it suitable for coatings that require abrasion resistance and stone impact resistance; At the same time, it has high tear strength and the elastic coating is not easily cracked by repeated bending.

3. Resistant to chemicals, oil, solvents, engine oil, hydraulic oil, alcohols, weak acids, suitable for engineering machinery, chassis, and automotive parts topcoats; Compared to polyether, it has a significant advantage in medium resistance.

4. Weather resistance, light and color retention, low yellowing, slow gloss decay under UV aging, and small color difference; Long term exposure to outdoor sunlight is not easy to lose luster and powder, making it the preferred soft segment for high-end outdoor 2KPU topcoats; Aromatic curing agents (TDI/MDI) will still turn yellow and are not recommended for outdoor use.

5. Good thermal stability, less mechanical attenuation at high temperatures, not easily softened or sticky under high temperature conditions, suitable for engine peripherals and high-temperature automotive parts coatings.

Comparison Chart

PPG polyether: best hydrolysis resistance, but poor oil resistance, weather resistance, wear resistance, and easy loss of gloss

Ordinary adipic acid polyester: high cost performance, oil and wear resistance, but poor hydrolysis and easy failure due to moisture and heat

PCL polycaprolactone: well balanced, but prone to crystallization, hardening at low temperatures, viscosity drift during storage, and weaker hydrolysis resistance than PCDL

PCDL: Comprehensive durable ceiling, with balanced resistance to hydrolysis, weather, wear, and oil, and the highest cost

2、 Common molecular weight selection

PCDL1000 (Mn1000, hydroxyl value ≈ 112mgKOH/g): short soft segment, hard coating, high hardness, good wear resistance, suitable for high gloss topcoat, metal topcoat, and engineering machinery topcoat; Low elongation rate.

PCDL2000 (Mn2000, hydroxyl value ≈ 56mgKOH/g): the most commonly used mainstream grade, with a balance of toughness, hardness, and elasticity. It is preferred for automotive exterior, flexible coatings, elastic topcoats, and stone impact resistant coatings; Co polymerized PCDL (5/6 carbon mixed diol) has low crystallization, lower viscosity, better flowability, and better formulation.

PCDL3000 and above: The soft segment is longer, the paint film is extremely soft, and has high elongation. It is mostly used for elastic coatings, leather finishing, and soft matte paint, with low hardness.

3、 Formula application method

Route 1: Directly as a hydroxy polyol

Component A: PCDL+small molecule chain extender (BDO, 1,6-HD)+solvent (ethyl acetate, PMA, xylene)+leveling, defoaming, UV absorber, antioxidant; Component B: HDI trimer (such as TPA-100)/IPDI trimer; R-value (NCO/OH) conventional: 1.05~1.15 (commonly used for topcoats, 1.1 to improve crosslink density and chemical resistance) ✅ Suitable for: high-performance industrial topcoat, automotive parts, motorcycle exterior, agricultural machinery, engineering machinery topcoat.

Practical application: Very few 100% single use PCDL, mostly blended with acrylic polyols and ordinary polyester polyols: PCDL accounts for 15% to 40% of the total polyol content, used to improve durability and control costs; The pure PCDL system has high cost and high viscosity.

Route 2: First synthesize PCDL based polyurethane prepolymer/hydroxy PU resin, then mix with 2K curing agent

PCDL+IPDI/H ₁₂ MDI synthesis of hydroxyl containing polyurethane resin, followed by component A; Advantages: Better compatibility, flat paint film, better adhesion, suitable for high-end soft touch paints, automotive interior soft touch coatings, and elastic protective coatings.

4、 Typical application scenarios

1. Automotive field: Automotive exterior 2KPU topcoat, bumper, rearview mirror, chassis stone resistant coating, interior soft matte paint

2. Construction machinery/agricultural machinery: outdoor heavy-duty topcoat, high salt spray, humid and hot environment, scratch resistance, hydraulic oil resistance

3. Motorcycles and high-end bicycles: The frame and covering parts are coated with aliphatic 2KPU topcoat, which is light and sun resistant

4. Anti corrosion and protective elastic coating: flexible protective topcoat for pipelines and metals, balancing toughness and medium resistance

5. Leather and synthetic leather coating: 2KPU coating agent with surface friction resistance and solvent resistance

6. Water based 2KPU: PCDL is used to synthesize water-based hydroxyl PU dispersion, which improves the water resistance and weather resistance of water-based two-component paint film

5、 Formula difficulties and risks

1. High cost: The price of PCDL is significantly higher than that of ordinary polyester PCL， Generally only producing high-end product lines, not suitable for general cargo topcoats.

2. High viscosity: The viscosity of PCDL with the same molecular weight is higher than that of ordinary polyester, and it is more difficult to make high solid and low viscosity with a high solid 2K formula. Therefore, co polymerized low crystalline PCDL is preferred.

3. Still hydrolyzed in alkaline medium: It is not always hydrolyzed. Long term strong alkaline immersion will still degrade the carbonate bond, and it cannot be used in strong alkaline conditions.

4. Crystallization risk: Poly-1,6-hexanediol (PCDL) will crystallize during low-temperature storage, and can only dissolve when heated; Priority should be given to selecting copolymer grades for coatings.

5. Compatibility: The compatibility with some acrylic polyols and low polarity solvents is average, and a compatibility test should be conducted in the preliminary experiment.

6. Moisture sensitivity: The 2K system strictly controls moisture, and the moisture content of PCDL entering the factory must be less than 0.05% to avoid reacting with the curing agent, foaming, and tank swelling.