The adhesion fastness of offset printing ink refers to the sum of physical adsorption force and chemical bonding force formed between the dried and film-formed offset printing ink and the substrate surface. It is a core indicator measuring the bonding tightness between the two. The abrasion resistance of printed products is essentially the ability of the offset printing ink film to resist friction and avoid detachment and fading. The two are positively correlated: the stronger the adhesion fastness, the higher the abrasion resistance; conversely, problems such as ink detachment and smudging are prone to occur.

Currently, insufficient adhesion fastness of offset printing ink is mainly manifested in three situations: smudging in overprinting areas, ink detachment at the edges of substrates, and substrate exposure after scratching. Frequently used products such as food packaging and shopping bags are most severely affected, as they have higher requirements for adhesion fastness and abrasion resistance.

The compatibility between the substrate material and offset printing ink is a core prerequisite affecting adhesion fastness. Paper substrates have pores on the surface, allowing offset printing ink to penetrate and bond, resulting in good adhesion fastness. In contrast, non-absorbent substrates such as plastics and metals have smooth surfaces. If the offset printing ink formula is incompatible, ink detachment is highly likely. Additionally, dust and oil stains on the substrate surface will reduce the adhesion strength of offset printing ink.

1. Why Does the Adhesion Fastness of Offset Printing Ink Directly Determine the Abrasion Resistance of Printed Products?

The adhesion fastness of offset printing ink refers to the sum of physical adsorption force and chemical bonding force formed between the dried and film-formed offset printing ink and the substrate surface. It is a core indicator measuring the bonding tightness betwen the two. The abrasion resistance of printed products is essentially the ability of the offset printing ink film to resist friction and avoid detachment and fading. The two are positively correlated: the stronger the adhesion fastness, the higher the abrasion resistance; conversely, problems such as ink detachment and smudging are prone to occur.

Currently, insufficient adhesion fastness of offset printing ink is mainly manifested in three situations: smudging in overprinting areas, ink detachment at the edges of substrates, and substrate exposure after scratching. Frequently used products such as food packaging and shopping bags are most severely affected, as they have higher requirements for adhesion fastness and abrasion resistance.

The compatibility between the substrate material and offset printing ink is a core prerequisite affecting adhesion fastness. Paper substrates have pores on the surface, allowing offset printing ink to penetrate and bond, resulting in good adhesion fastness. In contrast, non-absorbent substrates such as plastics and metals have smooth surfaces. If the offset printing ink formula is incompatible, ink detachment is highly likely. Additionally, dust and oil stains on the substrate surface will reduce the adhesion strength of offset printing ink.

2. Testing Standards for Adhesion Fastness of Offset Printing Ink

(1) Main Domestic Testing Standards

GB/T 7706-2008 Quality Requirements and Inspection Methods for Letterpress Printed Products also applies the requirements for the adhesion fastness of offset printing ink to offset printed products. It specifies that offset printing ink must "have no obvious detachment or smudging of the ink layer". Testing shall be conducted in a standard environment of 23±2℃ and 50±5%RH. Samples shall be placed in this environment for more than 24 hours in advance. The sampling inspection ratio for mass-produced products shall be no less than 3%, and sampling shall be expanded to full inspection if unqualified.

GB/T 18724-2008 Test Method for Rub Resistance of Printed Ink Layers is the core testing standard. It specifies a friction pressure of 49N and a speed of 100 times per minute. The friction times shall be no less than 50 for ordinary books and periodicals, and no less than 100 for frequently used products such as food packaging. The qualified criterion is that there is no obvious detachment or fading of the offset printing ink film and no substrate exposure after friction.

Industry-wide adhesion grades (Grade 1 to Grade 5) for offset printing ink: Grade 1 (worst) — ink layer detachment exceeds 80%; Grade 2 — detachment ranges from 50% to 80%; Grade 3 — detachment ranges from 10% to 50%; Grade 4 — detachment is less than 10%; Grade 5 (best) — no detachment. Ordinary promotional materials require Grade 2 or above, food packaging requires Grade 3 or above, and high-end products such as children’s picture books require Grade 4 or above.

(2) Common Testing Methods and Operational Precautions

Rub Tester Method: The most commonly used and accurate method. Randomly select 3-5 samples, cut them into 100mm×100mm pieces, fix them flatly, set parameters and start the equipment. Observe the state of the offset printing ink film and friction paper. Operation precautions: ensure firm sample fixation, use standard filter paper, and keep the equipment stable to ensure accurate testing.

Tape Peel Test Method: Suitable for thin paper and plastic substrates. Apply a 25mm-wide standard 3M transparent tape flatly to the offset printing ink film, press to eliminate bubbles, leave for 10 seconds, then peel it off quickly at a 180° angle, and judge the grade by observing the state. Avoid errors such as uneven pressing, unstable peeling speed, and expired tape.

Temperature and humidity have a significant impact on test results. Strictly control the environment at 23±2℃ and 50±5%RH. Place samples and equipment in this environment for 24 hours in advance to avoid deviations caused by abnormal temperature and humidity.

3. Improvement Paths for Abrasion Resistance of Printed Products

(1) Optimization of Offset Printing Ink Formula: Enhancing Adhesion at the Source

Selection of Binders Matching Substrates: Use resin-based binders such as rosin-modified resin and acrylic resin for paper substrates; use solvent-based binders such as polyurethane resin for plastic and metal substrates. Adjust the viscosity of offset printing ink to 15-25s (25℃, No. 4 cup) to avoid insufficient drying or poor adhesion.

Rational Addition of Additives: Add 1%-3% adhesion promoters, 5%-10% curing agents for solvent-based offset printing ink, and a small amount of cobalt drier. Control the addition ratio to prevent offset printing ink discoloration and ensure stable performance.

Optimization of Pigment Dispersion: Select pigments with good dispersibility, add 5%-8% polycarboxylate dispersant, and grind with a sand mill for 30-60 minutes to ensure uniform pigment dispersion and avoid uneven ink film and poor adhesion.(2) Adjustment of Printing Process: Reducing Adhesion Loss During Production

Control Water-Ink Balance: Minimize the dosage of fountain solution and maintain its pH value at 4.5-5.5 to prevent emulsification of offset printing ink. Monitor the ink state in real time and adjust or add anti-emulsifiers in a timely manner to ensure printing stability.

Optimize Printing Pressure and Speed: Printing pressure for paper substrates: 0.3-0.5MPa; for plastic and metal substrates: 0.5-0.8MPa. Ordinary offset printing speed: 8,000-12,000 sheets per hour; for UV offset printing ink: 12,000-15,000 sheets per hour, balancing drying effect and production efficiency.

Improve Drying Process: Hot air drying at 60-80℃ for 30-60 minutes for traditional solvent-based offset printing ink; curing power of 80-120W/cm for UV offset printing ink; infrared drying at 50-70℃ for 20-40 minutes for water-based offset printing ink to ensure complete drying of the ink film.(3) Substrate Pretreatment: Improving Compatibility with Offset Printing Ink

Surface Cleaning: Use an electrostatic dust remover to remove dust from paper substrates; wipe plastic and metal substrates with alcohol or acetone to remove oil stains. Print immediately after cleaning to prevent secondary contamination and ensure effective bonding between offset printing ink and the substrate.

Surface Modification: Subject plastic and metal substrates to corona treatment (intensity: 38-42dyn/cm) and print within 24 hours. Metal substrates can undergo additional phosphating treatment to enhance the adhesion fastness of offset printing ink.(4) Post-Processing: Auxiliary Improvement of Abrasion Resistance

Laminating/Varnishing: Laminating with PET or BOPP film (laminating temperature: 100-120℃, pressure: 0.2-0.3MPa) increases abrasion resistance by 3-5 times. Applying and drying UV or water-based varnish enhances the hardness of the offset printing ink film and prevents ink detachment and smudging.

Curing and Conditioning: Store printed products in an environment of 23±2℃ and 50±5%RH for 24-48 hours. Avoid high temperature, humidity and overstacking. Conduct cutting and packaging after conditioning to ensure stable adhesion of the offset printing ink film.

4. Industry Cases 

Case 1: A medium-sized offset printing packaging enterprise producing food packaging once suffered from insufficient abrasion resistance (only 30 friction times), with a rework rate of 18% and a monthly loss exceeding 50,000 yuan. By optimizing the offset printing ink formula, adjusting the printing process and adopting electrostatic dust removal pretreatment, the abrasion resistance was increased to more than 120 friction times, the adhesion fastness of offset printing ink reached Grade 4, the rework rate dropped to below 2%, and the monthly loss was reduced by 45,000 yuan.

Case 2: A small offset printing enterprise producing shopping bags once had problems of offset printing ink detachment and smudging, with an adhesion fastness of only Grade 2. By subjecting substrates to corona treatment, optimizing the UV offset printing ink formula, adjusting the curing power and adding laminating, the adhesion fastness was increased to Grade 3 or above, the abrasion resistance reached 80 friction times, and the quality pain point was solved.

5. Conclusion 

In summary, insufficient abrasion resistance of printed products stems mainly from the insufficient adhesion fastness of offset printing ink, which is affected by factors such as offset printing ink formula, printing process and substrate pretreatment. Standards including GB/T 7706-2008 and GB/T 18724-2008 provide a basis for testing, and enterprises can control the quality of offset printing ink through relevant methods. Optimizing the formula, adjusting the process, conducting proper pretreatment and post-conditioning can effectively enhance the adhesion fastness of offset printing ink, solve abrasion resistance problems, and promote the high-quality development of the offset printing industry.