ISO 1518 Scratch Test Method for Paints and varnishes — Determination of scratch resistance

When it comes to testing the durability of paint and varnish coatings, few methods are as widely recognized as the ISO 1518 Scratch Test. This standardized method allows manufacturers and quality control specialists to measure how well a coating resists mechanical damage under controlled conditions. The ISO 1518 test method determines the scratch resistance of single coatings or multi-coat systems of paint, varnish or related products by applying a defined load to a stylus that moves across the surface.

The test works by creating a controlled scratch on the coating’s surface, revealing valuable information about its performance in real-world applications. Whether you’re developing new coating formulations or evaluating existing products, understanding this test helps you make informed decisions about coating quality and durability.

Key Takeaways

• The ISO 1518 Scratch Resistance Test uses a stylus under constant load to evaluate coating durability under defined conditions.
• Test results help you compare different coating systems and determine if they meet specific performance requirements for various applications.
• Proper preparation of test panels and calibrated equipment are essential for obtaining accurate and reproducible results when conducting the test.

Scope of ISO 1518

The ISO 1518 standard specifically focuses on determining the scratch resistance of paints and varnishes under defined conditions. This test method applies to both single coating applications and multi-coat systems.

You can use this method to evaluate various coating materials including paints, varnishes, and related products. The test helps you assess how well these coatings withstand mechanical damage from scratching.

The standard is divided into two parts. ISO 1518-1 covers the constant load method, while ISO 1518-2 addresses the variable load approach. Both provide standardized procedures for consistent testing.

When you conduct this test, you’ll be able to determine the minimum load at which visible damage occurs to the coating surface. This information is valuable for comparing different coating formulations and their durability.

The scope does not extend to other forms of mechanical resistance such as impact or abrasion testing. It is specifically designed for scratch resistance evaluation only.

This test method is applicable to laboratory samples as well as coated manufactured products. You can perform these tests on flat surfaces prepared according to the standard’s specifications.

Principles of the Scratch Test Method

The ISO 1518 scratch test evaluates a coating’s ability to resist mechanical damage by applying controlled pressure with a stylus. This method helps determine critical failure points of paint and varnish coatings.

Scratch Test Equipment

The test requires a scratch tester apparatus that consists of a hemispherical-tipped stylus and a loading mechanism. According to ISO 1518-1, the stylus is typically made of hardened steel with a tip radius of 0.5 mm. The apparatus must be capable of moving the stylus across the coating at a constant speed.

The loading mechanism applies a defined, stable force perpendicular to the coating surface. You’ll need a device to measure this force accurately, usually in newtons.

The equipment should also include a means to secure the test panel firmly to prevent movement during testing. Some advanced models include optical systems to observe and record the scratch formation in real-time.

Test Parameters Setup

Before conducting the test, you must properly configure several key parameters. The applied load is critical and can be adjusted incrementally to determine the threshold at which coating failure occurs.

The test speed must be kept constant, typically between 30 mm/s to 40 mm/s. This ensures consistent results across different test sessions.

Temperature and humidity conditions should be standardized, usually at 23°C ± 2°C and 50% ± 5% relative humidity unless otherwise specified.

Your test panels must be prepared according to specific requirements with uniform coating thickness. The surface should be clean and free of contaminants.

Rest time between coating application and testing is also important, typically minimum 7 days for air-drying paints or as specified in the product data sheet.

Preparation of Test Panels

Proper test panel preparation is crucial for accurate ISO 1518 scratch resistance testing. You must select panels that match your real-world application requirements.

For standard testing, use flat metal panels made of steel, aluminum, or tin plate. These panels should be clean and free from any defects that might affect test results.

Before coating application, you must clean the panels thoroughly to remove grease, dust, and other contaminants. Wipe the surface with a suitable solvent like acetone or isopropyl alcohol, then allow it to dry completely.

Apply the coating (paint or varnish) to the test panel according to the manufacturer’s instructions. This includes following recommended application methods, thickness, and drying conditions.

Important panel specifications:

• Size: Typically 150 mm × 100 mm
• Thickness: Minimum 0.8 mm for metal panels
• Flatness: Panels must be flat without warping

The coating should be applied evenly across the entire panel. You may need to measure the dry film thickness to ensure uniformity using a suitable measurement device.

Allow the coated panels to dry and cure completely according to the test method specifications. This typically includes:

• Drying at room temperature (23 ± 2)°C
• Relative humidity (50 ± 5)%
• Minimum curing time of 7 days (unless otherwise specified)

Before testing, condition the panels at the standard testing temperature and humidity for at least 16 hours.

Performing the Scratch Test

The scratch test requires careful execution to obtain reliable results. Proper technique and control of variables ensure consistent measurements of coating resistance to mechanical damage.

Applying the Scratch

To perform the scratch test according to ISO 1518, you must first secure your test panel firmly on a stable surface. The stylus tip should be positioned at one end of the intended scratch path.

Apply the predetermined constant load to the stylus. For the constant loading method described in ISO 1518-1, you need to select the appropriate weight based on coating type and expected performance.

Move the stylus across the coating at a steady speed of 30-40 mm/s. Maintain consistent pressure and angle throughout the test. The scratch should be approximately 10 cm long.

After completing the scratch, examine the resulting mark under good lighting conditions. Look for coating penetration down to the substrate, which indicates failure at that load level.

Control of Test Variables

Temperature and humidity significantly affect test results. You should conduct tests at 23 ± 2°C and 50 ± 5% relative humidity unless otherwise specified for specific coating types.

The test apparatus must be calibrated regularly to ensure accurate loading. Check the stylus tip condition before each test session—damaged or worn tips can invalidate results.

Substrate preparation and coating application must be standardized. Record film thickness at multiple points across the test panel.

Key variables to control:

• Speed of stylus movement
• Applied load precision (±0.1N)
• Stylus tip geometry (1 mm hemispherical)
• Panel temperature
• Coating age (fully cured)

Evaluation of Scratch Resistance

After conducting the ISO 1518 scratch test, proper evaluation of the results is crucial for determining coating performance. The assessment involves both qualitative visual inspection and quantitative measurement techniques.

Visual Inspection Criteria

The visual inspection of scratch resistance follows specific criteria outlined in the ISO 1518 standard. You should examine the tested surface under good lighting conditions, preferably using a light source at 45° to the surface.

Look for the following key indicators:

• First visible scratch mark: The lowest load at which a continuous scratch becomes visible
• Penetration to substrate: The load at which the coating is penetrated to reveal the substrate
• Coating deformation: Any signs of cracking, flaking, or delamination

You must record the results in newtons (N) to the nearest 0.1 N. For multi-coat systems, note which layer has been exposed by the scratch.

Take photographs of the scratch patterns for documentation purposes if required by testing protocols.

Measurement of Scratch Width

The scratch width measurement provides quantitative data for comparing different coating formulations. You can measure scratch width using:

• Optical microscope: With calibrated reticle or digital measuring capability
• Profilometer: For precise depth and width measurements
• Image analysis software: For automated measurement of scratch dimensions

Record measurements at several points along the scratch length, typically at 5 mm intervals. The standard testing procedure requires a minimum of five measurements to calculate an average width.

Express results in micrometers (μm) with appropriate precision. Higher quality coatings typically show narrower scratch widths at equivalent loads. Plot the scratch width against applied load to create a performance curve for comparative analysis.

Reporting Test Results

Accurate documentation of scratch resistance testing results is essential for quality control and product development. Proper reporting allows for meaningful comparisons between different coating formulations and helps determine if products meet required specifications.

Data Representation

When reporting ISO 1518 scratch test results, you should include both numerical and visual documentation. Record the critical load at which coating failure occurs in Newtons (N) for constant load tests according to ISO 1518-1. For variable load tests, document the minimum load that causes coating penetration.

Include photographs of the scratch patterns when possible. These provide visual evidence of the failure mode and help with future reference.

Create a standardized table format that includes:

• Sample identification
• Test date
• Applied load(s)
• Number of test repeats
• Mean critical load value
• Standard deviation

Each test report should clearly indicate which part of ISO 1518 was used (Part 1 or Part 2) as the test methodologies differ significantly.

Interpretation Guidelines

You should establish clear pass/fail criteria before conducting tests. Define the minimum acceptable scratch resistance value based on product specifications or industry standards.

When interpreting results, consider the following factors:

• Failure mode: Distinguish between complete penetration to substrate versus surface deformation
• Pattern consistency: Evaluate if failures occur uniformly or at random points
• Comparative analysis: Compare results against reference materials or previous batches

Note that higher critical load values indicate better scratch resistance. However, interpret results in context of the coating’s intended application environment.

Be careful not to overinterpret minor variations between samples. Statistical significance should be established before concluding meaningful differences exist. Consider environmental factors like temperature and humidity which may influence test outcomes.

Quality Control and Reproducibility

Ensuring reliable test results with the ISO 1518 scratch test requires strict quality control measures. This qualification is essential for industries where coating durability directly impacts product performance.

To maintain reproducibility, you must regularly calibrate your testing equipment. The scratching tool, particularly the hemispherically tipped needle mentioned in ISO 1518, requires inspection for wear or damage before each test session.

Environmental conditions play a crucial role in test consistency. You should conduct tests at standard temperature (23 ± 2°C) and relative humidity (50 ± 5%) unless otherwise specified.

Key Quality Control Factors:

• Equipment calibration
• Needle condition verification
• Consistent sample preparation
• Standardized environmental conditions
• Trained operators

Sample preparation techniques must follow the guidelines outlined in ISO 1518-1:2019. This includes proper curing time and uniform application thickness.

Test operators should receive thorough training on both equipment operation and result interpretation. Your interpretation criteria must be consistent across all testing personnel.

For multi-lab testing scenarios, round-robin testing helps verify consistency. You should establish acceptable variance ranges for your specific testing application.

Documentation is essential for quality assurance. Your records should include all test parameters, environmental conditions, and detailed observations of coating behavior during testing.

Maintenance and Calibration of Equipment

Proper maintenance and regular calibration of scratch test equipment ensure accurate and repeatable results when testing paint and varnish surfaces according to ISO 1518 standards. These practices are essential for maintaining the integrity of test data over time.

Routine Maintenance

Clean the scratch tool tip after each test to prevent cross-contamination between samples. Inspect the hemispherical tip regularly under magnification for signs of wear, deformation, or damage that could affect test results.

Replace the needle tip when any flattening or irregularities are observed, as this will compromise the validity of your scratch resistance measurements. Most manufacturers recommend tip replacement after approximately 100 tests.

Store the scratch tester in a dust-free environment and cover when not in use. Regularly check moving parts and loading mechanisms for smooth operation, applying appropriate lubricant as specified by the equipment manufacturer.

Wipe down guide rails and load-bearing surfaces monthly with a lint-free cloth. Examine the base plate for flatness and clean test surfaces thoroughly before mounting specimens to ensure proper contact.

Calibration Procedures

Calibrate your scratch tester at least every six months or after 500 tests, whichever comes first. Use certified reference materials with known scratch resistance values to verify equipment performance.

The load application system must be calibrated using certified weights traceable to national standards. Check that the applied force matches the displayed or set values within ±2% across the full range of test loads.

Calibration Check Points:
- Zero point verification
- Linear response across load range
- Speed control accuracy (5 mm/s ±0.5 mm/s)
- Horizontal movement stability

Maintain a calibration log documenting dates, procedures, results, and any adjustments made. When calibrating the scratch tip geometry, use a calibrated microscope to verify the 1 mm diameter hemispherical shape as specified in ISO 1518-1.

Record ambient conditions during calibration, as temperature and humidity can influence test performance. If your equipment fails calibration checks, contact the manufacturer for service before conducting further tests.

Safety Considerations

When performing the ISO 1518 scratch test for paints and varnishes, you should always prioritize safety. The test involves sharp instruments that could cause injury if mishandled.

Always wear appropriate personal protective equipment (PPE) during testing. This includes safety glasses to protect your eyes and gloves to protect your hands from sharp tools and chemicals.

The test may involve working with paints and varnishes that contain volatile organic compounds (VOCs). Ensure your testing area is well-ventilated to prevent inhalation of potentially harmful fumes.

Keep your workspace clean and organized. This reduces the risk of accidents and ensures accurate test results. Store all testing equipment properly when not in use.

Essential Safety Equipment:

• Safety glasses
• Protective gloves
• Lab coat
• Proper ventilation system

Be aware of the physical hazards associated with the testing apparatus. The scratch testing needles have sharp tips that can cause puncture wounds if mishandled.

When disposing of test samples and materials, follow local regulations for chemical waste disposal. Some coating materials may be classified as hazardous waste.

If you’re operating automated scratch testing equipment, ensure you’re familiar with the manufacturer’s safety instructions before use. Never bypass safety features on testing equipment.

Document any safety incidents during testing and review procedures regularly to prevent future occurrences. Your safety should always be the primary concern when conducting any laboratory testing.

Frequently Asked Questions

The ISO 1518 scratch test method includes specific parameters for measuring coating durability and resistance to mechanical damage. These common questions address testing equipment, applicable coatings, and result interpretation.

What types of coatings can be evaluated using the ISO 1518 scratch test method?

The ISO 1518 scratch test method can evaluate a wide range of coating materials. This includes single coatings or multi-coat systems of paint, varnish or related products.

You can test both decorative and protective coatings on various substrates. The method works well for automotive finishes, industrial coatings, and architectural paints.

What apparatus is required to conduct the scratch resistance test as per ISO 1518 standards?

The primary instrument needed is a scratch tester with a hemispherically tipped needle. This device must allow controlled application of force during the scratching process.

You’ll also need proper sample preparation tools and a means to measure the applied load. Some versions require a constant-load apparatus while others use a progressively increasing load mechanism.

How does the ISO 1518 method measure the scratch resistance of paints and varnishes?

The ISO 1518 method measures scratch resistance by drawing a hemispherically tipped needle across a coated surface under defined conditions. The test evaluates how well the coating withstands this mechanical stress.

You can observe at what load the coating begins to show damage. The higher the load required to produce visible damage, the better the scratch resistance of the coating.

What are the differences between ISO 1518-1 and ISO 1518-2 standards regarding scratch resistance testing?

ISO 1518-1 uses a constant load method where a specific weight is applied throughout the test. This provides a straightforward pass/fail assessment at predetermined loads.

ISO 1518-2 employs a variable load technique where force increases progressively during the test. This allows you to determine the exact load at which coating failure occurs.

How are scratch resistance test results interpreted under ISO 1518 guidelines?

Results interpretation depends on whether you’re using the constant load (ISO 1518-1) or variable load (ISO 1518-2) method. For constant load tests, you report whether the coating showed damage at each tested load.

For variable load tests, you record the minimum load that caused coating penetration. Higher numerical values indicate better scratch resistance properties.

Can ISO 1518 scratch test method be applied to both water-based and solvent-based coatings?

Yes, the ISO 1518 scratch test method works equally well for water-based and solvent-based coating systems. The test evaluates physical resistance properties regardless of the coating chemistry.

You can compare different coating technologies using this standardized approach. This makes it valuable for product development and quality control across various coating formulations.