Application Note | Print Durability
Nanomechanical Testing of Thermal Transfer Print
Ranking Label and Barcode Print Durability by Nano Scratch and Hardness
Research & Experimental Testing
Camille Murray
Visual Design & Editorial
Andrew Shore
Introduction
Digital thermal transfer printing, unlike direct thermal printing, uses the process of melting a coated ribbon onto the material on which the print is applied. There are three main types of thermal ribbons, wax, wax/resin or pure resin, and they are almost always black. These different types have varying abilities to survive harsh environments and are selected to print on different material surfaces.
Importance of Nano Scratch Testing for Quality Control
Digital thermal transfer printing is used primarily for its print durability in applications such as bar codes and printed labels. These applications demand high resistance in harsh environments for long periods of time. Therefore, the formulas of the different types of digital thermal transfer ribbons should have a known, tested resistance to scratch and adhesion failure. Using the nano scratch testing method, the failure of digital thermal transfer printing can be tested and compared to identify the most durable formulation.
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Measurement Objective
We simulate the process of scratching in a controlled and monitored manner to observe the sample’s behavior. In this application, the NANOVEA Mechanical Tester, in nano scratch testing mode, is used to measure the load required to cause failure of 3 µm wax/resin prints on coated paper. A 2 µm, 90° cone diamond-tipped stylus is used at a progressive load ranging from 0.10 mN to 20 mN to scratch the printed surface, and the points of failure are reviewed. In addition, nanoindentation mode is used to obtain the hardness and elastic modulus of the sample prints.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Parameters
| Parameter | Samples 1 and 3 | Sample 2 |
|---|---|---|
| Load type | Progressive | Progressive |
| Initial load | 0.10 mN | 0.10 mN |
| Final load | 20 mN | 15 mN |
| Loading rate | 40 mN/min | 30 mN/min |
| Scratch length | 2 mm | 2 mm |
| Scratching speed, dx/dt | 4 mm/min | 4 mm/min |
| Indenter geometry | 90° cone | 90° cone |
| Indenter material (tip) | Diamond | Diamond |
| Indenter tip radius | 2 µm | 2 µm |
Table 1: Nano scratch test parameters.
Results
The main numerical results are summarized in Table 2 and the individual scratches in Table 3. Initial delamination is the point at which the coating shows the first signs of failure, seen as it sporadically delaminates to the substrate. Because the ink coating on Sample 1 was unevenly distributed, its scratch did not demonstrate a clear delamination.
| Sample | Initial delamination (mN) | Complete delamination (mN) |
|---|---|---|
| 1 | 2.28 ± 0.85 | 11.09 ± 0.66 |
| 2 | 3.84 ± 0.42 | 11.44 ± 0.86 |
| 3 | 4.81 ± 0.12 | 15.44 ± 1.93 |
Table 2: Summary of main numerical results (average ± standard deviation of three scratches).
| Sample | Initial delamination, scratches 1 to 3 (mN) | Complete delamination, scratches 1 to 3 (mN) |
|---|---|---|
| 1 | 1.59, 2.02, 3.22 | 10.32, 11.52, 11.42 |
| 2 | 4.16, 4.00, 3.36 | 11.80, 10.46, 12.07 |
| 3 | 4.74, 4.95, 4.74 | 14.05, 14.62, 17.65 |
Table 3: Critical loads of the individual scratches.
Additional Nanoindentation Results
| Test parameter | All tests |
|---|---|
| Maximum force (mN) | 1 |
| Loading rate (mN/min) | 2 |
| Unloading rate (mN/min) | 2 |
| Creep (s) | 20 |
| Computation method | ASTM E2546 and Oliver & Pharr |
| Indenter type | Berkovich diamond |
Table 4: Nanoindentation test parameters.
| Sample | Hardness (Vickers) | Hardness (GPa) | Young’s modulus (GPa) | Max depth (nm) |
|---|---|---|---|---|
| 1 | 4.40 ± 0.16 | 0.0465 ± 0.0017 | 1.35 ± 0.25 | 1037 ± 35 |
| 2 | 4.77 ± 0.62 | 0.0505 ± 0.0066 | 2.11 ± 0.37 | 970 ± 63 |
| 3 | 8.00 ± 0.74 | 0.0847 ± 0.0078 | 3.06 ± 0.61 | 756 ± 27 |
Table 5: Nanoindentation results.
Conclusion
As seen in our analysis, the scratch adhesion failure of the thermal prints has been identified. The technique identified Sample 3 as the most resistant and Sample 2 as slightly better than Sample 1. This data was then correlated with nanoindentation hardness measurements, which also identified Sample 3 as the hardest.
Other tests that could have been done include linear wear testing under various conditions to identify the effect of continuous rubbing on the print, performed with either the Mechanical Tester or the Tribometer. Additionally, the 3D Non-Contact Profilometer could have been used to image the surface of the print and/or the volume lost to wear or scratching.
Frequently Asked Questions About Testing Thermal Transfer Prints
Why test thermal transfer prints?
Thermal transfer printing is used for durable barcodes and labels that must survive harsh environments for long periods, so ribbon formulations should have a known scratch and adhesion resistance.
How were the prints scratched?
Progressive nano scratches from 0.10 mN to 20 mN (15 mN for Sample 2) over 2 mm with a 90° cone diamond stylus of 2 µm tip radius, three scratches per sample, on 3 µm wax/resin prints on coated paper.
What failures were recorded?
Initial delamination, when the coating sporadically delaminates to the substrate, and complete delamination.
Which print performed best?
Sample 3: initial delamination at 4.81 mN and complete delamination at 15.44 mN, against 3.84 and 11.44 mN for Sample 2 and 2.28 and 11.09 mN for Sample 1.
Did hardness agree with scratch resistance?
Yes. Nanoindentation to 1 mN with a Berkovich tip found Sample 3 hardest at 0.0847 GPa, with a modulus of 3.06 GPa, against 0.0505 and 0.0465 GPa for Samples 2 and 1.
What other tests could be added?
Linear wear testing to study continuous rubbing on the print, and 3D non-contact profilometry to image the print surface or the volume lost to wear or scratching.
