Key Takeaways
- Adhesion testing does not predict wear. A graphic can pass every tape test and disappear under a thumb in six months.
- Sunscreen and hand sanitiser are the real chemicals for handled products, and they are almost never in the specification.
- Match the abrasion mode to the service: rotary, linear and rub testing damage a surface in different ways.
- Test the finished decorated surface including any topcoat, since the topcoat provides most of the durability.
Decorated plastic surfaces rarely fail by peeling. They fail by wearing away under handling, by softening when someone cleans them with the wrong product, or by dissolving where a hand carrying sunscreen touched them repeatedly. Qualification programs built around adhesion testing measure a property that is not the one at risk.
Adhesion and Durability Are Different Properties
A tape or cross-hatch test asks whether the coating is attached to the substrate. That is a necessary property and a poor predictor of service life, because the common failure path does not involve detachment at all: the film is removed progressively from the top down by abrasion, or degraded chemically until it can be wiped away.
A printed graphic can therefore pass every adhesion test performed on it, at every stage of qualification, and still be illegible after eight months on a product that people hold. The specification was measuring the wrong thing throughout.
Choosing an Abrasion Mode
Abrasion is not a single phenomenon and the standard methods damage surfaces differently.
Rotary abrasion using loaded wheels applies a combination of sliding and rolling contact over a circular path, producing a general wear result that is reproducible and widely used. It suits flat areas and comparative work.
Linear abrasion moves an abradant back and forth along a path, which better represents wiping, sliding contact and directional wear, and can be applied to smaller and more contoured areas.
Controlled rub testing with a specified cloth, load, medium and stroke count most closely represents cleaning and hand contact, particularly when the medium is the actual cleaning product rather than a dry abradant.
Selecting the mode that resembles the real damage matters, because results do not transfer between modes. A coating that performs well against rotary abrasion may perform poorly against solvent-loaded rubbing, and the product will experience whichever one its users apply.
The Chemicals That Actually Reach the Product
Chemical resistance schedules tend to list solvents and industrial fluids while omitting the substances that cause most field failures on consumer, medical and automotive interior products.
Sunscreen is the most notorious. Its components attack many coatings and printed films aggressively, and it arrives on hands, transfers to surfaces and stays there. Products used outdoors or handled in summer see it routinely.
Hand sanitiser has become ubiquitous in medical, hospitality and consumer environments, and high-alcohol formulations soften many ink and coating systems, particularly with repeated contact.
Insect repellent containing DEET is unusually damaging to plastics and coatings alike.
Cleaning and disinfecting products in medical settings involve repeated contact with quaternary ammonium compounds, alcohols, peroxides and bleach solutions, frequently many times a day for years.
Hand creams, cosmetics and food oils transfer routinely to handled products.
Adding these to the schedule costs almost nothing and moves the test program substantially closer to reality. Test method matters too: a spot exposure for a fixed period tests one thing, while repeated wiping with a loaded cloth tests the combination of chemical attack and mechanical action that actually occurs.
Test the Surface That Ships
Where a decorated surface carries a protective topcoat, that topcoat provides most of the abrasion and chemical resistance the product has. Testing the printed layer without it measures a surface no customer will ever contact and gives results that are pessimistic to the point of being useless.
Conversely, testing must include the locations where the topcoat is thinnest — edges, corners, raised features, the boundary of the decorated area — because that is where failure initiates. Flat central regions of a test plaque systematically overstate performance.
Specifying Conditions So Results Mean Something
Abrasion and chemical test standards leave a great deal open, and different choices produce different numbers. A specification that cites a method without fixing its options will produce disputes between supplier and customer laboratories that are entirely legitimate on both sides.
The conditions that need stating are: abradant or cloth type and its replacement or reconditioning schedule; applied load; number of cycles or duration; stroke length and speed for linear methods; specimen conditioning before test; the chemical, its concentration and contact method; and the assessment criterion — whether failure is defined by visible change, by a measured optical property, or by legibility.
Abradant condition deserves particular attention. Wheels and cloths degrade with use, and a laboratory using well-worn abradant will produce systematically better results than one using fresh material. Specifying replacement intervals is what makes results comparable over time and between sites.
Building a Program That Discriminates
The purpose of durability testing during development is to choose between options, and that requires tests that separate them. A program where every candidate passes has not informed the decision.
Running to failure rather than to a fixed endpoint is the most effective change: reporting the number of cycles at which each candidate becomes unacceptable ranks them and quantifies the margin, where a pass at 500 cycles for all three tells you nothing. Where a fixed acceptance criterion is required for production, it can be set afterwards, with knowledge of where the cliff actually is.
Combining stresses is the second: aging a sample, then abrading it, then exposing it to a chemical reproduces the sequence a real product experiences far better than three independent tests on fresh specimens. Coatings that pass each test individually and fail the sequence are common, and finding that during development is considerably cheaper than finding it afterwards.
Assessing the Result Consistently
The weakest link in most durability programs is not the exposure but the judgement made afterwards. “Unacceptable” is an opinion unless the criterion is defined, and different assessors draw the line in different places on progressively worn graphics.
Instrumental assessment removes the ambiguity where it is available. Optical density or contrast measured before and after exposure quantifies loss directly. Gloss measurement captures surface degradation that precedes visible wear. Color measurement quantifies shift under ultraviolet and chemical exposure. For machine-readable codes, verification grade before and after is the natural measure and is already standardized.
Where visual assessment is unavoidable, it needs the same discipline as any other attribute measurement: reference samples defining the acceptance boundary, controlled viewing conditions and distance, more than one assessor, and a periodic check that assessors agree with each other and with themselves. Without those, a durability program can generate a great deal of data and still not distinguish reliably between a coating that will last and one that will not.
Related Reading
- Measuring Adhesion and Abrasion Durability
- Measuring Adhesion and Abrasion Durability, Part 2
- Environmental Stress Cracking
- Designing an Accelerated Aging Protocol
- Writing an Enforceable Specification
Need help with this?
The Sabreen Group provides independent engineering support for durability testing programs for printed, coated and decorated plastic surfaces. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.
Frequently Asked Questions
Why is adhesion testing insufficient for decorated surfaces?
Because it measures whether the coating is attached, not whether it will survive being touched, cleaned and rubbed for years. Abrasion removes material progressively from a well-attached film, and chemical attack softens or dissolves it. Both produce complete graphic loss on surfaces that would pass a tape test throughout.
Which abrasion test should be used?
The one that resembles the service damage. Rotary abrasion with a loaded wheel suits general wear on flat areas; linear abrasion better represents wiping and rubbing along one direction; a controlled rub test with a specified cloth, load and medium best represents cleaning and handling. Running the wrong mode gives a number that does not correlate with field performance.
What chemicals should be included?
The ones the product actually meets, which routinely extend beyond the specification. For handled consumer and medical products that means sunscreen, insect repellent, hand sanitiser, hand cream, cleaning sprays and disinfectant wipes. For automotive interiors, add cleaning products and plasticizers migrating from adjacent materials.
Should testing be done on the topcoat or the graphic?
On the finished surface as it will ship, including any protective topcoat, because the topcoat provides most of the abrasion and chemical resistance. Testing an unprotected printed layer measures something the customer will never touch and will substantially understate durability.
How are results made comparable between labs?
By specifying every condition the standard leaves open: abradant type and its replacement schedule, applied load, number of cycles, stroke length and speed, conditioning, and the assessment criterion. Different abradant lots alone can change results measurably, which is why inter-laboratory comparison without full condition matching is unreliable.
