Adhesion Testing & Rub/Scuff Testing

Print adhesion is the bond between the ink film and the substrate, or between the lamination film and the printed surface beneath it. A print job can look perfect in the press room and fail catastrophically in the supply chain or at retail if adhesion is inadequate. Ink that peels from a flexible pouch in the filling line. Lamination that lifts from a carton during retail handling. Varnish that flakes from a cosmetic box. All of these are adhesion failures, and all of them are detectable before the product leaves the print room, using simple, low-cost adhesion tests.

Adhesion testing is particularly critical in Indian production because three conditions that impair adhesion are common: high humidity (monsoon season slows ink drying and prevents proper adhesion to lamination), variable corona treatment levels on film substrates (surface energy below 38 dynes/cm means inks and adhesives cannot bond), and pressure to speed up finishing schedules (laminating before ink is fully dry is the most common cause of delamination in India).

The tape adhesion test (based on ASTM D3359 Method B) is the simplest and most widely used print adhesion test. It requires no specialised equipment, only standard 3M 610 tape (or equivalent 25mm clear pressure-sensitive tape with defined adhesion force), a clean flat surface, and good technique. It takes less than 2 minutes per sample.

Tape adhesion test procedure

Tape adhesion grading scale (ASTM D3359)

The cross-hatch test (ASTM D3359 Method A, ISO 2409) is a more rigorous adhesion test used for inks, varnishes, and coatings on rigid or semi-rigid substrates, glass, metal, rigid plastic, and board. It is more informative than the simple tape test because it stresses the adhesion at the coating edges (where adhesion is typically weakest) by cutting through the coating in a grid pattern before applying tape.

Cross-hatch procedure

Using a cross-hatch cutter (a multi-blade tool that makes parallel cuts at defined spacing, typically 1mm or 2mm depending on coating thickness), cut a grid of 6×6 squares through the ink or coating layer to the substrate surface. Apply tape over the grid, press firmly, allow 60 seconds, and remove in a single rapid motion. Evaluate the percentage of the grid squares that show coating removal and assign a grade from 0 (no removal) to 5 (more than 65% removal). Grade 0 or 1 is acceptable for most commercial applications; Grade 2 and above requires investigation.

When to use cross-hatch vs tape test

The tape and cross-hatch tests give qualitative results, pass/fail or a grade. For applications requiring a quantitative adhesion measurement, particularly lamination bond strength, label adhesion, and heat-seal peel strength, the T-peel test (ASTM D1876) or 180° peel test (ASTM D903) measures the actual force required to peel one layer from another, expressed in N/15mm or N/25mm or g/mm.

T-peel test for lamination bond strength

Cut a 25mm wide strip of the laminated material. Separate the layers at one end for approximately 25mm to create a "T" shape, two free ends. Place in a tensile tester with one layer clamped in the upper jaw and the other in the lower jaw. Pull apart at 100–300mm/min and measure the average peel force over a 75mm peeling distance. Record in N/25mm.

Rub resistance is the ability of a printed surface to withstand mechanical abrasion without the ink or coating transferring to an adjacent surface or being visibly damaged. A print job that passes all adhesion tests may still fail in the supply chain if the rub resistance is inadequate, premium cartons rubbing against each other in transit and arriving at retail with scuffed surfaces, book covers that show white scuff marks after handling, or flexible pouches where the printed design transfers ink to the inside of the shipping case.

In Indian conditions, rub resistance failures are particularly common in three scenarios: during the monsoon season when humidity softens water-based inks and reduces dry rub resistance; in transit on rough rural roads where packages experience continuous vibration and abrasion against each other; and on fine art or premium packaging where any surface marking is unacceptable to the end client.

The Sutherland rub tester (also called the Sutherland 2000 or equivalent instruments from other manufacturers) is the standard instrument for measuring the rub resistance of printed materials. It uses a defined weight (either 2 lb or 4 lb depending on the test standard), a defined rub material (unprinted paper, printed paper, or a specific test material), and a defined number of rubs (typically 25, 50, or 100 cycles) to simulate the abrasion a printed surface will experience in its application.

Sutherland rub test procedure

Rub resistance is tested under both dry and wet conditions because ink and coating performance can differ significantly between the two. A print that passes dry rub testing may fail wet rub, particularly water-based inks and unlaminated prints are much more vulnerable to abrasion when wet.

Dry rub test

Standard Sutherland rub test at 23°C/50% RH with no moisture added. Tests the rub resistance that the print will exhibit in normal, dry ambient storage and transit conditions. The most commonly specified test for commercial print.

Pass criteria for dry rub (standard commercial applications): no visible ink removal from the printed surface; no visible ink transfer to the rub sample at the conclusion of the test. The quantified standard is typically defined by the specific brand owner's specification, common industry benchmark: no ink transfer visible to the naked eye after 25 rubs at 2 lb load on white paper. For premium packaging: no transfer after 50 rubs.

Wet rub test

The rub paper is dampened with a defined quantity of distilled water (typically 0.1 ml/25mm²) before testing. The wet rub tests resistance to abrasion when the surface is moistened, simulating: condensation on refrigerated products, moisture from humid conditions in transit, water contact during retail handling of beverage labels in ice buckets, or incidental water contact during use.

Pass criteria for wet rub are significantly more demanding than dry rub. Water-based inks on unlaminated paper typically fail wet rub, this is expected and acceptable for applications where the product will never be wet. Laminated surfaces pass wet rub because the lamination film provides a waterproof barrier over the ink. For beverage labels, wet adhesion testing is mandatory, the label must maintain adhesion and legibility when submerged or wet for extended periods.

Pass/fail criteria for adhesion and rub testing must be defined before production begins, not after a defect is found. The criteria depend on the application, the substrate, the ink system, the finishing, and the performance requirements of the end use. Using generic criteria without considering the specific application leads to either under-testing (missing failures) or over-testing (rejecting acceptable production).