How Hard is 2H, Really? Why 90% of Steel Buyers Get Coating Hardness Wrong

Definition of pencil hardness of colour-coated steel sheet

The pencil hardness of colour coated sheet refers to the scratch test carried out on the surface of the colour coated sheet with a pencil of a specific hardness to assess the ability of the colour coated sheet coating to resist pencil scratching. It is a relative index to measure the hardness of the colour-coated sheet coating. Generally under national standards or industry standards, the use of a series of different hardness of the pencil, from soft to hard sequentially in the colour coated plate surface scratch to determine the hardest pencil hardness level in the surface of the coating does not leave obvious scratches or damage to the colour coated plate to express the pencil hardness of the colour coated plate. For example, the pencil hardness levels are 6H, 5H, 4H, 3H, 2H, H, F, HB, B, 2B, etc. The larger the number in front of the H indicates that the harder the pencil lead; the larger the number in front of the B indicates that the softer the pencil lead.

The pencil hardness test for colour-coated board in the national standard is mainly based on the standard GB/T 6739-2022 Determination of paint film hardness by the pencil method for colour paints and varnishes. The specific contents are as follows:

Principle

The hardness of a paint film is determined by pushing a pencil of known hardness across the film.6 The pencil hardness of a colour-coated panel coating is expressed by scratching the surface with a series of pencils of different hardness to determine the hardness level of the hardest pencil that will leave no noticeable scratches or damage on the surface of the coating.

Preparation for Testing

- Pencil Selection: Use a standard Chinese pencil with multiple grades of hardness ranging from 6H to 6B with decreasing hardness.

- Pencil Handling: Sharpen the pencil with a pencil sharpener to expose a 4 - 6mm core, hold the pencil perpendicular to the surface of a 400 # abrasive paper, and sand the pencil on the abrasive paper until the tip is sharp.

Test Procedure

- Hand Pencil: Holding the pencil at an angle of 45° to the coating, apply sufficient uniform pressure downward and forward to break the edge of the pencil or plough through the coating with a 6.5 mm stroke. The test shall start with the pencil with the hardest mark and test each marked pencil in turn. The hardness of the pencil shall be the pencil hardness of the coating to be tested. 5 strokes shall be made at each level until the pencil that does not break the coating breaks at least 4 out of 5 times.

- Instrumental pencil method: After the pencil is treated according to the manual pencil method, it is fixed on the instrumental trolley so that the tip of the pencil can freely contact the surface of the sample. Push the trolley so that it moves 6.5 mm over the sample. Test each pencil in turn as in the manual method. The hardness of the pencil is determined by the coating's resistance to the pencil until it fails to plough through the coating at least 4 times out of 5 strokes at each level.

Results

If no scratches are visible, the test should be carried out with a pencil of higher hardness until scratches of at least 3mm in length are visible; if scratches exceeding 3mm are visible, the hardness of the pencil should be lowered and the test should be repeated until the length of the scratches is no longer exceeding 3mm.6 During the inspection process, the surface of the coating can be wiped with a soft cloth, degreasing cotton wool, inert solvent, or an eraser to remove any debris from the pencil lead. Remove pencil lead debris to make it easier to assess the damage to the paint film.

Reasons for skin breakage in the pencil hardness test

Coating raw materials

- Improper selection of resin: resin is the key film-forming substance of colour-coated sheet coating, and its performance plays a key role in coating hardness. If the selected resin itself has low hardness and low glass transition temperature, such as some acrylic resin with good flexibility but insufficient hardness, it is difficult to make the coating reach high hardness.

- Pigment and filler problems: the type, dosage, and particle size distribution of pigment and filler will affect the hardness of the coating. If the hardness of pigment and filler is low, such as talcum powder, etc., the use of large quantities will reduce the overall hardness of the coating. In addition, the particle size distribution is not reasonable; the particles are too large or too small, which will make the coating structure not dense, and will also lead to a decrease in hardness.

- Additives: The role of additives is to improve the performance of the coating, but improper use will affect the hardness. For example, although plasticisers can improve the flexibility of the coating, if the dosage is too much, it will make the coating soft and reduce the hardness; leveling agent and other additives are improperly selected or excessive, which may affect the drying and curing of the coating, which in turn affects the hardness.

Production process

- Uneven coating thickness: if the coating is too thin, it is difficult to resist the scratching force of the pencil during the pencil hardness test, and scratches are easy to appear, resulting in substandard hardness; if the coating is too thick, the surface hardness may be insufficient due to inadequate internal drying and curing, and the over-thickness of the coating is prone to stress during drying, which affects the adhesion of the coating and the substrate, and indirectly leads to problems in hardness testing. Problems in hardness testing.

- Curing process is not reasonable: curing temperature is too low or curing time is too short, which will prevent the coating of the resin and other film-forming substances from being fully crosslinked, the coating structure is loose hardness. On the contrary, if the curing temperature is too high, it may cause the coating to age and become brittle, and it is also easy to break the skin and other phenomena during the test, which seems to be a high hardness, but in reality, it is an increase in the brittleness, and is not a real hardness up to standard.

- Coating process problems: In the coating process, if the coating speed is too fast or uneven, it will make the coating thickness uneven, affecting the uniformity of hardness. In addition, poor precision and stability of the coating equipment can also lead to unstable coating quality and difficulty in meeting the standard hardness requirements.

Aspects of the production environment

- Temperature and humidity: Too low a temperature or too high a humidity in the production environment will slow down the drying speed of the coating, and the curing reaction will be incomplete, affecting the hardness of the coating. For example, in a low temperature and high humidity environment, the water in water-based coatings evaporates slowly, the curing time of the coating is prolonged, and even phenomena such as whitening and softening may occur, resulting in substandard hardness.

- Air quality: Dust, oil, and other impurities in the production environment are mixed into the coating, which will destroy the uniformity and denseness of the coating and reduce the hardness of the coating. At the same time, harmful gases in the air, such as sulphur dioxide and nitrogen oxides, may react chemically with the components in the coating, affecting the curing and performance of the coating.

Surface treatment of the substrate

- Insufficient surface cleanliness of substrate: The presence of impurities such as oil, dust, and rust on the surface of the substrate will hinder the close bonding of the coating with the substrate and reduce the adhesion and overall hardness of the coating. During the pencil hardness test, the coating is easily detached from the substrate surface, or scratches appear.

- The surface roughness of the substrate is not suitable: the substrate surface is too smooth, the mechanical occlusion between the coating and the substrate is weak, the adhesion is poor, and the coating is easy to peel off during the hardness test; the surface is too rough, which will lead to uneven thickness of the coating and make it difficult for the coating to be cured completely in the peaks and valleys of the roughness and affect the hardness of the coating.

Methods to eliminate the phenomenon of broken skin in the pencil hardness test

A. Optimise coating formulations to improve coating hardness and adhesion

(1) Select a high hardness resin system

Epoxy resin, polyurethane, highly cross-linked polyester, and other resins with high hardness can enhance scratch resistance.

Appropriately increase the wear-resistant filler (such as silicon dioxide, silicon carbide) to improve surface hardness.

(2) Adjust the proportion of curing agent

Ensure that the amount of curing agent (e.g., isocyanate, amino resin) is sufficient to avoid incomplete curing, resulting in a soft coating.

DSC (Differential Scanning Calorimetry) test can be carried out to confirm whether the curing reaction is sufficient.

(3) Control the viscosity of the coating

Too low viscosity will lead to sagging, uneven film thickness, and affect the hardness; too high viscosity may affect the leveling.

Adjust the solvent ratio to ensure stable viscosity when painting (e.g, using Ford Cup 4#, 20-30 seconds).

B. Ensure that the surface treatment of the substrate is qualified

(1) Thoroughly clean the substrate

Degreasing: Use alkaline or solvent cleaning to ensure that there is no oil (can be examined by the contact angle test or the water film continuity method).

Phosphate/passivation: Enhance the adhesion of the coating to prevent the substrate from rust and corrosion affects the bonding.

(2) Optimise substrate roughness

sandblasted (Sa 2.5 grade) or chemically etched, so that the surface roughness (Ra) is controlled at 0.5-2.0 μm, to improve mechanical occlusion.

Avoid too smooth substrate (Ra <0.3μm)or too roughness(Ra>3μm), otherwise it will affect the coating uniformity.

C. Optimise the coating process

(1) Ensure uniform coating

Adopt a high-precision roller coating machine or electrostatic spraying to ensure uniform film thickness (e.g., 20-30μm).

Avoid fluctuation of coating speed. It is recommended to use a closed-loop control system to adjust the coating amount.

(2) Control curing conditions

Baking temperature: Ensure that the curing temperature required for the coating is reached (e.g., PMT 220-240°C).

Baking time: to ensure full curing (such as 20-30 minutes), to avoid the 'surface dr,y inside not dry'.

Temperature uniformity: Regularly check the temperature of each area of the oven to avoid local temperature shortages. 

D. Improve the production environment

(1) Control temperature and humidity

The temperature is recommended to be 20-30 ℃, humidity is ≤ 70% RH (especially for water-based coatings need to be strictly controlled by humidity).

When the temperature is low in winter, preheat the substrate or increase the drying time.

(2) Reduce environmental pollutants

The paint shop should be kept clean and dust-free (an air filtration system can be added).

Avoid oil mist, sulphur dioxide (SO₂), ammonia (NH₃), and other corrosive gases can affect the coating curing.

E. Enhance quality testing

(1) Standardization of the pencil hardness test

Perform according to GB/T 6739 or ASTM D3363 to ensure:

Pencil angle 45°, load 750g or 1kg.

Use the incremental hardness method (from 6B→9H) to test step by step to avoid misjudgment.

(2) Other Related Tests

Scribing Adhesion Test (ISO 2409) to confirm coating bonding.

Solvent-resistant wipe test (e.g., alcohol or acetone) to verify the degree of curing.

Contact Now to Get More