GMW 14698 Scratch Resistance of Organic Coatings and Self-Adhesive Films (2)

GMW 14698 Scratch Resistance of Organic Coatings and Self-Adhesive Films

The “GMW 14698 Scratch Resistance of Organic Coatings and Self-Adhesive Films” standard, developed by General Motors Worldwide (GMW), describes a test method used to evaluate how well coated or film-coated surfaces resist scratching when subjected to controlled mechanical contact. This standard is particularly important for automotive interior and exterior components where maintaining surface appearance and durability is crucial.

The primary purpose of the GMW 14698 standard is to determine the scratch resistance of an automotive surface under defined test conditions. During vehicle manufacturing and normal use, surfaces can be scratched by:

• quotes
• apparatus
• Keys and other hard objects
• Proximal components
• Cleaning equipment
• Assembly operations
• Contact with other vehicle parts
• General passenger use

A scratch doesn't just create a physical groove; it can also cause whitening, discoloration, changes in gloss, or other visible surface damage. Therefore, scratch testing provides an objective way to assess whether a coating or film has sufficient resistance for its intended application.

The GMW 14698 standard primarily applies to the following:

• Organic coatings
• Single-layer coating systems
• Multilayer coating systems
• Coatings applied to metallic surfaces
• Coatings applied to polymeric surfaces
• Self-adhesive films / foils

This test is particularly useful for automotive interior upholstery and appearance-related surfaces. Test laboratories list the GMW 14698 standard under automotive surface and scratch resistance testing. This standard does not simply test whether a material can withstand scratching; it provides controlled methods for applying a defined mechanical force and then evaluating the resulting damage.

The GMW 14698 standard offers three main test methods:

• Method A Hardness test bar: This method uses a spring-loaded hardness/scratch test bar. The test bar is placed vertically to the sample surface and moved across the surface. The force is controlled via a spring mechanism. The bar is pulled a specific distance across the test surface. This standard defines a test stroke of approximately 50 mm or more. This method is particularly useful for determining the resistance of coatings and films to controlled linear scratching.
• Method B Five-finger scratch test: This method uses a five-finger scratch tester. Each finger carries a steel scratch tip and can apply a defined force to the sample. This standard defines the following standard forces: 5 N, 8 N, 13 N, 15 N, and 20 N. Other forces may be used as required by the relevant material specification or the test engineer.

This method is particularly useful when several force levels need to be evaluated on the same sample. The five-finger arrangement allows for the systematic application of different scratch loads, making it possible to determine the force at which visible damage begins.

• Method C Linear hardness test: This method uses a linear hardness/scratch testing device. The applied force can be adjusted between approximately 0,5 and 20 N. By moving a defined test tip across the surface under a controlled load, the device allows for the quantitative assessment of the surface's resistance to scratching.

The equipment used in the tests depends on the chosen method. Typical equipment includes:

• Hardness test bar: Used for Method A. The bar contains a spring mechanism that determines the applied force.
• Five-finger scratch tester: Used for Method A. The device includes five movable arms/fingers equipped with replaceable scratch tips. Commercial five-finger scratch testers used in automotive applications typically provide different standardized loads and use approximately 1 mm hemispherical scratch tips.
• Linear hardness testing machine: Used for Method C. The load is adjustable and the test probe is moved across the sample.
• Scratched tips: This standard specifies polished steel or hard metal tips. A key tip configuration is a hemispherical tip with a diameter of approximately 1,0 plus/minus 0,1 mm.

In tests, the finished product component should be tested whenever possible. If the finished part cannot be easily placed in the test equipment due to its size, shape, or geometry, representative samples of the component may be taken. Coatings or films should be applied according to the relevant material specification. At least three samples should be tested. At least one sample is required for routine quality control tests.

Environmental conditioning is important in testing because polymeric materials and coatings can exhibit different behaviors depending on temperature and humidity conditions. The GMW 14698 standard refers to GMW 3221 standard Code D for conditioning, unless another condition is specified by the relevant test procedure. This helps ensure that different laboratories and suppliers perform the test under comparable conditions.

After the scratching process, the sample is visually inspected. One of the important effects evaluated for molded colored plastics is the whitening of the scratched area.

The GMW 14698 standard rating scale includes ratings from 1 to 5. Higher ratings represent better scratch appearance/resistance. For molded colored plastics, the referenced requirement is typically a rating of 4 or higher, where this requirement is specified in the relevant material or component specification.

A simplified interpretation is as follows:

• 5: Visible pitting but no actual whitening.
• 4-5: Mild whitening under certain viewing conditions
• 4: Mild whitening
• 3-4: Slight or minimal whitening depending on the viewing angle.
• 3: Noticeable whitening
• 2-3: Noticeable to severe whitening
• 2 or fewer: Significant apparent damage

The definitive acceptance criterion should always come from the relevant General Motors material, component, or drawing requirement, rather than being inferred from a general testing procedure.

The basic principle is this: higher required scratch strength generally implies better scratch resistance. However, strength alone should not be interpreted without considering the specified tool geometry, test method, substrate, coating system, and evaluation criteria.

The GMW 14698 standard is important because automotive surfaces are subjected to repeated mechanical contact throughout their lifespan. For example, a vehicle interior might include:

• instrument panels
• Door trims
• Center consoles
• Key frames
• Decorative panels
• Polymer flooring
• Coated plastic components
• Self-adhesive decorative films

These surfaces must maintain an acceptable appearance despite use and contact. Therefore, scratch resistance is a crucial part of the durability of the automotive look.

Poor scratch resistance can lead to the following:

• Visible surface damage
• Whitening
• Loss of brightness
• Coating damage
• Exposure of the underlying substrate
• Decrease in perceived quality
• Customer complaints

In conclusion, the GMW 14698 standard can be used during material development, supplier qualification, component verification, manufacturing quality control, and failure investigation.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the GMW 14698 standard.