Better stick with UV-curing

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1 Better stick with UV-curing Printable, pressure-sensitive adhesives (PSAs) are becoming increasingly important in industrial graphic applications. New printable UV PSAs have enhanced performance and are becoming a viable alternative to commercial transfer tapes. Additionally, process improvements and cost savings will factor into their use. Print-applied, UV-curable pressure-sensitive adhesives for industrial graphics The use of printable, pressure-sensitive adhesives (PSAs) has been growing in industrial-graphic applications such as durable labels, nameplates, membrane switches, and graphic overlays. Flatbed screen printing is already widely used for producing graphic overlays and circuits. Printable adhesives have the advantage that they can be used on screen-printing equipment and technology that is already in place in most industrial-graphic manufacturing operations. The advantages of printable pressure sensitive adhesives compared to transfer tapes include: - Waste reduction is possible by printing adhesive only where it is needed. Thus there is a reduction in waste when compared to the die cutting and disposal of transfer tape sections (including release liner) in areas where the adhesive is not desired. - Design advantages such as complex geometric shapes and moisture/gas flow channels can be accomplished easily by screen printing compared to the challenge of using transfer tapes. - Elimination of adhesive build up on cutting dies by printing the adhesive away from the cut line. - Reduced time and cost of screen preparation relative to the production of a cutting die for new designs. - Potential reduction of lamination steps used in membrane-switch assembly. - Improved bonding to digital inks compared to transfer tapes. UV-curing PSA as alternatives to transfer tapes A pressure sensitive adhesive (PSA) is a viscoelastic material relying on applied pressure to bond two surfaces together. Printable UV PSAs are "liquid syrups" highly viscous liquids that are screen printable and develop the desired viscoelastic properties upon UV exposure. The "liquid syrups" may be made up of oligomers, monomers, photoinitiators, and processing additives that UV cure by free radical or cationic polymerisation. The goal is to optimise the compositions in order to deliver adhesive performance properties comparable to transfer tapes that are currently being used in industrial graphics.a series of three printable UV-curable PSAs (A, B, and C) were developed to cover the adhesive needs of most industrial graphic applications. The characteristics of the cured adhesives are summarised below, and a commercial industrial transfer tape is included for comparison: - The performance properties of printable UV PSAs compares favorably with a commercial transfer tape. - The series of printable UV PSA products tested was designed to meet the needs of industrial graphics and label manufacturers. - Printable adhesive A, having similar performance to the transfer tape is recommended for membrane switch and overlay applications where high temperature resistance is required. Printable adhesive C, having high quick stick and peel resistance is designed for high performing label applications. - The performance properties of printable UV PSAs compares favorably with a commercial transfer tape. - The series of printable UV PSA products tested was designed to meet the needs of industrial graphics and label manufacturers. - Printable adhesive A, having similar performance to the transfer tape is recommended for membrane switch and overlay applications where high temperature resistance is required. Printable adhesive C, having high quick stick and peel resistance is designed for high performing label applications. Selection of the most appropriate printable UV PSA depends upon the application s assembly and final use requirements. Experimental Screen printing Three UV PSAs were screen printed at 50-microns adhesive thickness. This was followed by UV curing and testing together with the commercial transfer tape at the same film thickness. The flatbed press used was an ATMA two-post semi-automatic with a 74 mesh, 120 microns thread diameter polyester screen at 28 Newton tension force. The flood-bar pressure into the screen was minimal and the flood speed was slow. A 60 durometer, round-edge squeegee at an angle of 15 was used to print the adhesive. The squeegee speed was slow and the pressure into the screen was just light enough to transfer the adhesive to facestock over a three mm, off-contact setting. The slow speeds and low pressure minimised the amount of bubbles incorporated into the adhesive on the screen and thus resulted in fewer bubbles in the adhesive prints. UV Curing The adhesives were cured with a 160 W/cm Fusion UV Systems H bulb at 100% power on a laboratory conveyor unit. The A and B adhesives require a minimum UV dose of 450 mj/cm2 of UV-A. This correlates to a conveyor speed of 11 m/min. This conveyor speed was used to produce the samples for adhesive property measurements. An EIT UV Powermap was used to record the UV energy delivered to the adhesive (Table 1). All samples were cured in one pass under the UV lamp. The C adhesive can be cured over a range of UV exposures to tailor its performance to the application. If high tack is the primary requirement, a line speed of 15 m/min or 337 mj/cm2 UV-A can be used (see Table 2). Subjecting the adhesive to more UV energy, such as curing at 11 m/min, will reduce its quick tack but peeling will increase and some cohesive strength will be gained. While these results are for adhesive films cured with a Fusion UV Systems H bulb, a common electrode-type,

2 medium-pressure mercury lamp will produce similar results. Lamp input-power settings of 120 W/cm or 160 W/cm will typically deliver sufficient irradiance (>500 mw/cm2) to cure the adhesives. Substrates Samples for testing were prepared using DuPont "Melinex ST505" heat-treated polyester as the substrate for adhesive application. The adhesion test substrate to bond the cure adhesive specimens was 403 stainless steel. The test plates were first cleaned with acetone to remove residual adhesive from prior testing. This was followed by 30 minutes of sonication, rinsing with deionised water and a final wipe with acetone. The stainless-steel plates were stored in a room at a constant temperature and humidity (24 C), 50% RH) overnight before use. Peel tests Adhesion tests of the printable UV PSAs and transfer tape were conducted according to the Pressure Sensitive Tape Council test methods [1]. All adhesive samples had a thickness of 50 microns. Peel tests were conducted at 90 with a pull rate of 305 mm/min on an Instron 4411 testing device with a 445 N (100 pound) load cell. The room temperature peel adhesion was tested 24 hours after bonding. Peel samples on stainless steel were aged in an oven at 105 C for 10 days. (Only adhesives A, B, and D were subjected to this testing.) Subsequently, the samples were allowed to equilibrate at room temperature for 24 hours before conducting peel tests. Static shear testing Static-shear testing was started after 24 hours to allow the adhesive to wet out on the surface of the stainless steel. The adhesive area was 25 mm by 25 mm with a load of one kilogram. The shear plates are at a two-degree positive angle to eliminate peel forces. Static-shear testing was carried out at room temperature (24 C) and an elevated temperature (70 C). SAFT Shear-adhesion-failure temperature (SAFT) is a test of the short-term temperature resistance of an adhesive. Samples were subjected to increasing temperature. The temperature at which the sample failed under a one kilogram load was recorded. SAFT tests use cold-rolled steel plates that are degreased and stored overnight in the constant temperature humidity room before use. The SAFT procedure involves setting up a shear sample (25 mm by 25 mm adhesive area) with a one kilogram load in an oven. Starting at room temperature, the oven temperature was increased at 2.8 C per minute and the temperature when the sample failed was recorded. Chemical resistance The chemical resistance of printable adhesive A and transfer tape D was measured. Everyday materials such as petrol, methylethylketone (MEK), vinegar (weak acid), diethanolamine (weak base), 10W30 motor oil, deionised water, and a 5% salt-water solution were used. Peel samples were constructed on stainless steel and immersed in the chemical test substance. The immersion time in petrol and MEK was one hour; with the weak acid and base, it was four hours. A three-day immersion was used for oil, deionised water and salt water. The samples were removed, wiped dry, and allowed to equilibrate in the constant temperature humidity room for 24 hours before peel testing. Test results The room-temperature 90 peel forces for the four adhesives are shown in Figure 1. Printable adhesive C was found to have the highest peel resistance (1120 N/m). The transfer tape D and printable adhesive A had very similar peel resistance (935 N/m and 866 N/m, respectively). While printable adhesive B has higher tack than printable A, it has the lowest peel resistance (753 N/m), of the tested samples. Furthermore, A, B and D showed adhesive failure with clean removal from the stainless steel. Adhesive C exhibited cohesive failure. Residue was left on both the polyester film and the stainless steel panel. Results of aging the adhesives at 105 C for 10 days are shown in Figure 2. On stainless steel, printable adhesives A and B gave the same result within experimental error (1964 N/m and 1929 N/m respectively). Both A and B exhibited cohesive failure modes during the peel test. The transfer tape D produced a peel strength of 774 N/m. A likely reason for this result is that the sample hardened during the time in the oven. The nature of the failure mode changed: the samples "zippered" during the peel test. SAFT results are shown in Figure 3. The transfer tape D had the highest SAFT of 214 C. Printable adhesive A produced a comparable result of 190 C. The probable reason is differences in crosslink density or molecular weight of the transfer tape and printable adhesive A. The other printable adhesives, B and C, had lower failure temperatures of 112 C and 66 C respectively. Figure 4 shows the results of the static-shear tests. Printable adhesives A and B matched transfer tape D with all samples reaching >10,000 minutes. Extended testing of A and D have now passed shear tests of over 21,000 minutes (350 hours). Printable adhesive C, which is known for high tack and peel, has low shear resistance (1860 minutes at room temperature and 60 minutes at 70 C). The results of the chemical resistance tests are summarised in Figure 5. Printable adhesive A gave similar peel resistance to transfer tape D for all the tests. Salt water showed the largest difference (350 N/m lower for A).In addition to the performance properties reported above, the A adhesive showed very good bonding to digital inks (HP Indigo) when compared to the transfer tape D. This advantage is important as digital printing technology is being increasingly used in industrial graphics.? Acknowledgements The author is grateful to Renae Reeves and Jason Conover for carrying out the adhesive testing. References [1] Test methods for pressure sensitive adhesive tapes; Pressure Sensitive Tape Council, 14th ed., The performance properties of printable UV PSAs compares favorably with a commercial transfer tape. - The series of printable UV PSA products tested was designed to meet the needs of industrial graphics and label manufacturers. - Printable adhesive A, having similar performance to the transfer tape is recommended for membrane switch and overlay applications where high temperature resistance is required. Printable adhesive C, having high quick stick and peel resistance is designed for high performing label applications. THE AUTHOR» Jeffrey R. Warmkessel, Ph.D., has been working in the area of radiation curable pressure sensitive and laminating

3 adhesives for ten years as a scientist for Ashland Performance Materials. The past couple of years he has spent in Product Development and Technical Service on application and commercialisation of UV-curable pressure sensitive adhesives. He has a doctorate degree in Polymer Science from the University of Akron and a B.S. degree in Polymer Science from Penn State University. * Corresponding Author. Contact: Dr. Jeffrey WarmkesselAshland Performance MaterialsTel jrwarmkessel@ashland.com This paper was presented at the RadTech Europe 07 Conference, Vienna, Austria, November Figure 4: Static shears at room temperature (24 C) and elevated temperature (70 C)

4 Figure 5: Chemical resistance results

5 Bild zu Better stick with UV-curing

6 Figure 1: 90 peel resistance 24 hours after bonding

7 Figure 2: Heat aged (105 C for 10 days) 90 peel resistance

8 Figure 3; Shear adhesion failure temperature