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1、 The most common reason

1. Incomplete cross-linking, later development of “stress relaxation+soft segment microphase separation”

PCDL is a long soft segment polyol, and the curing of 2KPU is divided into two stages:

1) Surface dryness+initial rapid cross-linking: A dense cross-linking network is formed on the surface first, and the solvent evaporates quickly, making it hard to touch and pencil hard in a short period of time. This is apparent pseudo hardness; But there are still unreacted OH and NCO inside the system, or uneven cross-linking density, and the soft segment is limited by the hard segment.  

2) After aging at room temperature for a few days to weeks:

Residual NCO continues to react slowly and undergo network rearrangement;

The soft segment chain of PCDL gradually stretches and relaxes, and microphase separation continues to develop: the hard segment enrichment zone and the soft segment enrichment zone differentiate. The modulus of the soft segment phase itself is low. When the surface is enriched with soft segments, the surface is more prone to plastic deformation, which appears to become less scratch resistant after being left for a period of time.   ✅  Phenomenon: It is very hard after 1-3 days of curing, and after 7-14 days, the steel wool will turn white and leave marks when scraped. The pencil hardness may not change much.

2. Residual solvents/free small molecules

High solid content, high viscosity of PCDL, slow solvent evaporation:

The surface solvent goes first and quickly dries and hardens; The internal solvent is trapped in the paint film and slowly migrates outward, which is equivalent to internal plasticization and reduces the surface modulus;

Also: unreacted small molecule polyols, chain extenders, low molecular weight prepolymers, and additives migrate to the surface in the later stage; Excessive migration of lubricant can actually cause the surface to become slippery, the friction coefficient to change, and easily lead to recoverable scratches and silver scratches.

The more lubricant there is, the more resistant it is to scratching. If it migrates too much to the surface, the surface will be a weak interfacial layer that will break when scraped.

3. Water absorption plasticization

Although carbonate bonds are more resistant to hydrolysis than ordinary polyesters, they have high polarity and are easily able to adsorb water vapor from the air: the paint film is initially dense and absorbs less water; Leave it for a period of time to continuously absorb water, which acts as a reversible plasticizer. The Tg of the soft segment decreases, the surface becomes softer, the plasticity increases, and scratches are more likely to occur; The phenomenon of hot and humid environment will be amplified: after being stored at room temperature for a few weeks, it becomes less and less scratch resistant. After being put back in the oven for a period of time to dry, scratch resistance can be partially restored.  

4. R value and cross-linking density design issues

Low R value: In the initial stage, only surface crust is formed, and the overall cross-linking density is low. In the later stage, chain segment relaxation is very obvious;

Excessive R value: Excess NCO will react with H ₂ O in the air to generate urea, and the hard segments of urea will slowly aggregate, resulting in increased phase separation, uneven surface hardness, and easy scratching in some areas; The commonly used R value for the PCDL system is 1.05~1.10, and it is not recommended to set it too high.

5. The catalyst affects the uneven curing rate

Bismuth/tin catalyst is too fast: the surface quickly seals and scales, the internal solvent cannot escape, the internal reaction lags behind, and the exterior is hard and the interior is soft; Later on, the internal structure gradually matured, the network became loose, and the surface was scratch resistant and slid down. Catalyst too slow: The initial hardness is already slow, which can easily be mistaken for this problem.

6. Issues with the combination of additives

Leveling agent, organic silicon lubricant, defoamer: Some organic silicon and PU substrates have limited compatibility, initially embedded inside the paint film, and migrate to the surface over time to form a weak interfacial layer;

UV absorbers/HALS: Some low molecular weight additives migrate, which also weakens the surface layer;

Typical: Just made the sample, the steel wool test is very good; Leave it for half a month, the surface is slippery and there are marks when scraped.

7. Testing Misconceptions

Pencil hardness is a press in static hardness; Scratch resistance refers to dynamic frictional deformation. Many samples: The pencil hardness H has remained unchanged, but its dynamic scratch resistance, abrasion resistance, and stone resistance have significantly deteriorated. Essentially, this is due to a decrease in yield stress, which makes it prone to plastic deformation, rather than a decrease in hardness.

Scratches are divided into two types: plastic scratches and brittle cracking scratches, and this phenomenon is mostly the former.

2、 Quick troubleshooting and judgment methods

1. Place the sample for 7 days, dry a portion at 60 ℃ for 2 hours, and compare its scratch resistance after cooling ✅  Scratch resistance recovery after baking → Most likely residual solvent/water absorption plasticization ❌  Still not scratch resistant after baking → cross-linking network rearrangement, phase separation, and additive migration issues

2. Perform FTIR tracking: observe the changes in residual NCO peaks to determine if post crosslinking continues

3. Surface water contact angle test: After being left for a period of time, the contact angle significantly decreases, indicating the migration and water absorption of additives

3、 Improvement plan for PCDL-2KPU system

Formula end

Priority should be given to co polymerized low crystalline PCDL, which has milder phase separation and smaller chain relaxation amplitude compared to homopolymer 1,6-HD PCDL;

Polyol blending: PCDL is combined with a small amount of acrylic polyol/high Tg polyester to increase the proportion of hard segments and inhibit the enrichment of soft segments on the surface; Do not sacrifice long-term scratch resistance for the pursuit of ultimate toughness in 100% PCDL topcoat;

R value control: 1.05~1.10, not too low;

Reduce low molecular weight free components and control the amount of small molecule chain extenders added;

Choose reactive organosilicon as the lubricant to avoid free and migratory siloxanes.

workmanship

Ensure volatilization gradient, and if necessary, flash evaporation with gradient heating: first, let the solvent slowly release at low temperature to avoid premature surface coating;

Controlling film thickness makes it easier for thick films to embed solvents, which is a more prominent issue;

Control the humidity of the environment, avoid excessive humidity during spraying and curing stages, and reduce the water absorption of the paint film.

Curing system

Curing agent priority HDI trimer; The IPDI system has higher rigidity in the hard segment and is relatively less prone to later softening;

Reduce catalyst dosage or delay bismuth catalyst to avoid rapid surface crust formation.

additive

Using high molecular weight HALS/UV absorbers to reduce migration;

A small amount of gas-phase silica and nano alumina are used to construct a physical scratch resistant framework, which weakens the scratch sliding caused by chain relaxation.

4、 Difference from hydrolysis

Water absorption plasticization/chain relaxation: After drying, the performance improves, and the appearance of the paint film does not turn white or powder;

Hydrolysis failure: The paint film is irreversible and cannot be recovered by drying, accompanied by decreased adhesion, foaming after soaking in water, and powdering.