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Home / News / Industry News / Film Flat and Smooth After Application — Why Does It Crack After Long-Term Use?
Industry News
Wood Coatings · Furniture · Industrial & Decorative Coatings
In wood coatings, furniture lacquers, industrial coatings, and decorative coatings, a situation arises consistently: the film is flat and smooth after application, adhesion and hardness both meet requirements, but after a period of use the surface gradually shows fine cracks, crazing, or even localised fractures. In most cases, this is not caused by application quality. It results from the combined action of environmental change, material ageing, and film stress during long-term use, causing the coating to gradually lose stability.
Seven Reasons Why Cracking Appears Over Time
1A Flat Film Does Not Mean Long-Term Durability
At the time coating is completed: surface is flat and smooth, appearance is uniform and complete, initial performance is normal. These characteristics only indicate that the film was in good condition at the application stage — they cannot fully reflect long-term durability performance during use. A flat film does not mean cracking will not occur later.
2Environmental Change Continuously Affects Film State
During long-term use: temperature repeatedly changes, humidity continuously fluctuates, and light continuously acts. The film continuously experiences expansion and contraction, and internal stress gradually accumulates.
3Film Ageing Reduces Flexibility
As service time increases: the resin structure gradually changes, film flexibility decreases, and resistance to deformation weakens. When the film can no longer adapt to external change, cracks are more likely to form.
4Long-Term Internal Stress Accumulation Can Cause Cracking
After film curing: residual stress may remain inside, the film is continuously affected by external forces and environment, and stress is continuously released and accumulated. When stress exceeds the film's bearing capacity, cracking may occur.
5Substrate Movement Also Affects Film Stability
Particularly for wood products: wood absorbs and loses moisture with the environment, minor dimensional changes occur, and the film needs to adapt simultaneously to the deformation. If the film's deformation capacity does not match the substrate, cracks can easily develop after long-term use.
6The Longer the Service Time, the More Clearly Problems Appear
In the early stages of use: the film remains stable, and surface change is not obvious. But as time extends: material gradually ages, stress continuously accumulates, and micro-cracks continuously expand — ultimately forming visually apparent cracking.
7Why No Problems Are Found at Factory Inspection
Because when the product has just been completed: film structure is complete, the ageing process has not yet begun, and long-term environmental effects have not yet been reflected. After long-term use: material properties gradually change, environmental factors continuously act, and film stability continuously declines — cracking problems only gradually appear.
Summary
A flat film that cracks after long-term use is fundamentally the combined result of material ageing, environmental change, internal stress accumulation, and substrate deformation. When analysing long-term film cracking problems, in addition to focusing on application quality, it is also necessary to comprehensively evaluate the resin system, flexibility, substrate compatibility, and long-term weathering performance.
Frequently Asked Questions
Is there a way to predict whether a film will crack in long-term use before the product is shipped?
Standard measurements at delivery — hardness, adhesion, surface appearance — reflect the product in its zero-service-history state. Accelerated ageing tests combining thermal cycling, UV exposure, and humidity cycling, along with flexibility measurements at multiple intervals during ageing, give a more useful picture of how the film will perform over its intended service life. No single initial test reliably predicts long-term crack resistance.
Does higher hardness always improve long-term cracking resistance?
Not necessarily — hardness and crack resistance are distinct properties. A harder, more densely crosslinked film may actually be more brittle and more prone to crack initiation under cyclic environmental stress. For products exposed to temperature and humidity variation over their service life, the flexibility and elastic recovery of the film matter more than hardness alone.
