Assessing coating film strength by application

Certain requests frequently arise when discussing coating quality.

“We want a harder coating film.”
“We want a coating material that is less easily scratched.”
“We want to switch to a stronger coating material.”
“Could the current coating film be too weak?”

These are common topics of discussion in coating operations.

The product surface becomes scratched.
The film chips during conveying.
It cracks during assembly.
It wears during use.
A customer reports appearance defects.

When these problems occur, people on the shop floor tend to think that a harder coating film might solve them.

However, caution is needed here.

A hard coating film is not necessarily a good coating film.

Coating film strength cannot be judged by hardness alone.
What is meant by the required strength changes with the application.

What is a “strong coating film”?

The first question to consider is what “a strong coating film” actually means.

Although we use one expression, it has more than one meaning.

For example, strength can mean the following.

• Resistance to scratching
• Resistance to wear
• Resistance to cracking when bent
• Resistance to chipping under impact
• Resistance to peeling from the substrate
• Resistance to chemical attack
• Resistance to water and moisture
• Resistance to deterioration caused by ultraviolet light
• The ability to prevent rust
• The ability to maintain appearance over a long period

All of these are important properties required of coating films.

However, maximizing all of them at once is not easy.
Improving one property may reduce another.

This can be understood as a trade-off between properties.

The important point in coating film design is not to strengthen everything.
It is to identify which type of strength matters most for the product.

Advantages of a hard coating film

A hard coating film does, of course, have advantages.

Examples include the following.

• The surface is less easily scratched
• Good wear resistance
• Good resistance to rubbing
• Dirt may be less likely to adhere
• A hard-looking finish is easier to achieve
• Less surface deformation

Film hardness may be important for appearance parts, controls, fixtures, equipment components, and parts subject to rubbing.

Parts frequently touched by hand or subject to rubbing during transport need scratch resistance.
For such applications, selecting coating materials with an emphasis on hardness and wear resistance may be effective.

However, pursuing hardness alone can create other problems.

Risks of an excessively hard coating film

A hard coating film may resist scratching.
However, it may become less resistant to bending and impact.

Hard glass provides a useful analogy.
Its surface is hard, but it breaks under a strong impact.

The same applies to coating films.
If the film becomes excessively hard, it may fail to accommodate substrate deformation or impact, making cracking and chipping more likely.

Examples include the following defects.

• The coating film cracks when bent
• The film chips from edges during assembly
• The film cracks under impacts during conveying
• Peeling starts around bolts or at corners
• Vibration causes fine cracks
• Expansion and contraction caused by temperature changes lead to cracking

Particular care is needed when the substrate moves.

On thin metal sheet, plastic parts, rubber-like materials, parts subjected to bending, and vibrating parts, the coating film also needs to accommodate deformation.

If the substrate moves but the film cannot move with it, stress becomes concentrated in the film.
This can cause cracking, separation, or peeling.

Soft coating films also have a role

A soft coating film may sound like a weak coating film.

However, softness and flexibility can be important, depending on the application.

A flexible coating film offers advantages such as the following.

• It more readily accommodates bending
• It more readily absorbs impact
• It more readily accommodates substrate deformation
• It more readily accommodates expansion and contraction caused by temperature changes
• It is less likely to crack
• It may be less likely to peel

For example, flexibility may matter more than hardness for parts that are bent, vibrate, are made of resin or thin sheet, or experience forces during assembly.

However, an excessively soft film also has problems.

• It is easily scratched
• It wears easily
• Dirt adheres easily
• Marks readily remain on the surface
• Blocking occurs easily
• It softens easily in high-temperature environments

A softer film is therefore not necessarily better either.

What matters is the balance between hardness and flexibility.

Coating films cannot be classified simply as hard or soft

The words “hard” and “soft” are insufficient for describing coating film properties.

Films described as hard can still differ in the following ways.

• Hard, but brittle and easily cracked
• Hard, with additional toughness
• Soft, but weak and easily scratched
• Soft, but tough and resistant to cracking

The concept of toughness becomes important here.

In simple terms, toughness describes how much a material can withstand, or how much energy it can absorb, before it breaks.

Even a hard material may crack easily under impact if its toughness is low.
By contrast, appropriate flexibility and toughness make it easier to withstand deformation and impact.

When considering coating film strength, hardness, flexibility, toughness, elongation, and adhesion must be assessed separately.

A coating film deforms when force is applied

A coating film is a thin layer on the surface of metal or plastic.
Its thickness is often several tens of μm to approximately 100 μm.

Although very thin, it experiences various forces during use.

• Scratching
• Wear
• Bending
• Impact
• Vibration
• Pressure
• Thermal expansion and contraction
• Loads during assembly
• Rubbing during conveying

The film must remain adhered to the substrate and maintain its appearance and protective functions while experiencing these forces.

Under a small force, a deformed film may return to its original state.
Beyond a certain force, permanent deformation occurs.
With still greater force, the film cracks, chips, or peels.

A coating film defect is therefore not simply a surface problem.
It concerns the balance between the forces experienced by the film and its properties.

Cracking, chipping, and peeling are different defects

On the shop floor, film failures are sometimes collectively described as “weak” or “peeled off.”

In reality, cracking, chipping, and peeling are different phenomena.

Cracking occurs when the film cannot withstand substrate deformation or internal stress and develops fissures.
Chipping occurs when impact or loads at corners break away part of the film.
Peeling occurs when adhesion is lost at the interface between the film and the substrate.

Their causes differ.

For cracking, check film hardness, flexibility, thickness, drying conditions, and substrate deformation.
For chipping, check impact resistance, film thickness at corners, conveying conditions, and assembly loads.
For peeling, check the substrate surface, surface preparation, wettability, and drying and curing conditions.

An incorrect classification of the defect can lead to the wrong corrective action.

“Make it harder” is not always the right answer

Suppose a customer says, “The coating film gets scratched, so we want to make it harder.”

Immediately proposing a high-hardness coating material is risky.

Certain points must be checked first.

What causes the scratches?
Is it wear?
Rubbing during conveying?
Contact during assembly?
Handling by operators?
Contact with packaging materials?
Insufficient film thickness?
Insufficient drying?

If the cause is the conveying method or packaging, increasing hardness may not solve the underlying problem.
It may also lead to more cracking or chipping instead.

Similarly, if the request is “We want a stronger coating material because the film cracks,” simply making it harder may not be the solution.
Greater flexibility or toughness may be needed instead.

What matters is translating the request for greater strength into specific phenomena.

Performance priorities change with the application

The priority of each coating film property depends on the application.

For appearance parts, color, gloss, and scratch resistance may be important.
For parts requiring rust protection, adhesion, film thickness, rust resistance, and water resistance matter.
For parts subjected to bending, flexibility and the ability to accommodate deformation matter.
For parts exposed to chemicals, chemical resistance matters.
For parts used outdoors, weather resistance, water resistance, and resistance to chalking matter.
For parts exposed to impact during conveying or assembly, impact resistance and control of the coating at corners matter.

The meaning of a “good coating film” therefore changes with its application.

Coating material selection must consequently begin with the following questions.

What must this coating film protect the product from?Which property is most important for this film?

Film thickness also affects coating film strength

Film thickness is another important factor when assessing coating film strength.

An excessively thin film may provide insufficient protection, hiding power, or rust resistance.

An excessively thick film, however, also causes problems.

• Poor drying
• Insufficient curing
• Cracking
• Sagging
• Blistering
• Increased internal stress
• Poor adhesion
• Higher costs

Particular care is needed when applying a hard film at a large thickness.
Thicker films are more likely to retain internal stress, which may cause cracking or peeling.

Film thickness also tends to be uneven at corners and edges, where chipping or rust can begin.

A thicker coating film is not always better.
It is important to maintain the appropriate thickness for the application.

Drying and curing conditions also determine coating film strength

Film hardness and strength are affected by drying and curing conditions, as well as the coating material itself.

For example, an incorrect mixing ratio in a 2-component coating material makes film performance unstable.
Insufficient actual workpiece temperature in a bake-curing coating causes undercuring.
If an excessively thick film does not dry sufficiently inside, the surface may be hard while the interior remains weak.

Conversely, excessive baking or unsuitable drying conditions may make the film brittle.

Coating film strength therefore changes with process conditions, as well as the type of coating material.

When a defect occurs, check the drying and curing conditions as well as the coating material’s name.

Points to check on site

When discussing coating film strength, it is important to check the following points.

• Which specific property does the customer mean by a “strong coating film”?
• Is the problem scratching, cracking, chipping, peeling, or wear?
• When does the defect occur?
• Where on the product does it occur?
• Does the product experience bending, vibration, or impact?
• Is force applied to the coated surface during assembly?
• Does rubbing or impact occur during conveying?
• Is the film thickness appropriate?
• Are drying and curing conditions appropriate?
• Can the film accommodate substrate deformation?
• Are flexibility and impact resistance assessed as well as hardness?

Checking these points makes it possible to consider film performance suited to the application, rather than simply looking for a hard coating material.

How CoatureVietnam can help

Instead of immediately turning a request for “a strong coating film” into a coating material name, CoatureVietnam examines the application and the defect involved.

Examples of available support include the following.

Paint Voyager
We identify the required hardness, flexibility, impact resistance, wear resistance, and chemical resistance for the application and consider suitable coating material candidates.

Paint Optimizer
While retaining the existing coating material, we review film thickness, thinning ratio, drying conditions, and application methods to help stabilize film performance.

Local Optimer
When considering locally available alternative coating materials, we check the balance of properties, including hardness, flexibility, adhesion, and durability, as well as color.

Work standards and process control
We organize the causes of coating film defects from the 4M perspective of materials, methods, equipment, and people, and support the development of SOPs and inspection criteria to prevent recurrence.

Summary

A hard coating film is not necessarily a good coating film.

A hard film may resist scratching and wear.
However, it may be less resistant to bending and impact, making cracking and chipping more likely.

A soft film may accommodate bending and impact more readily.
However, it may be less resistant to scratching and wear.

Coating film strength must be considered as a balance between hardness, flexibility, toughness, adhesion, impact resistance, wear resistance, film thickness, and drying conditions.

The essential point is to design coating film performance for the intended application.

Rather than merely “make it harder,”
clarify what it needs to resist.

That is the first step toward reducing coating problems and achieving consistent coating film quality.

Coating the Future with Science and Care
CoatureVietnam considers film performance suited to the application with you, rather than focusing only on coating material names.