By Zachary Gatland, product development scientist, Mactac North America
Ultraviolet-cured hot-melt pressure-sensitive adhesives (UV-HMPSAs) are acrylic pressure-sensitive adhesives that can be delivered and coated as a 100% solid hot melt and cured with UV radiation. In recent years, the UV-HMPSA market has expanded, and many more commercial UV-HMPSAs have become available to tape and label manufacturers. UV-HMPSAs offer an attractive alternative to solvent-borne (SB) PSAs for several reasons including minimal volatile organic compounds (VOCs), reduced CO2 emissions, higher potential coat weights and faster processing speeds. For converters who want to coat and cure UV-HMPSAs, it is important to understand the UV cross-linking mechanism and process, which significantly differ from the process used to coat traditional SB PSAs.
Benzophenone, a Norrish type II molecule, is the basis for the most common photoinitiators used in the UV-HMPSA industry. 1 Many of the materials used to formulate UV-HMPSAs “compete” with benzophenone for UV radiation. Additives such as plasticizers, tackifiers, pigments or other functional species can have aromaticity or unsaturation that absorb the same UV radiation that is used to activate the photoinitiator (see Figure 1). Benzophenone must compete with these molecules and itself across the depth of the adhesive. Therefore, when considering the source of UV radiation for curing UV-HMPSAs, it is important to choose a source that produces radiation that will activate benzophenone at quantities high enough to be absorbed by the photoinitiator and other additives through the depth of the adhesive.
The UV-HMPSA industry typically has relied on medium-pressure mercury-vapor lamp (MVL) bulbs including Mercury (H) and Mercury+ (H+) bulbs that produce strong broadband output across the UV-A, UV-B and crucially UV-C range of radiation. In recent years, there has been interest in utilizing UV light-emitting diode (LED) systems. Though other energy-curing industries have begun to adopt and even favor LED systems, the UV-HMPSA industry has so far been unable to transition to LED curing systems due to the lack of availability of large-scale UV-C LED systems.
The study aimed to understand the effectiveness of 280 nm UV-C LEDs to cure two commercially available UV-HMPSAs (referred to as adhesive A and adhesive B). It compared rheological and cohesive shear properties of these adhesives cured by a standard H bulb MVL, a monochromatic 280 nm LED engine and a polychromatic UV LED engine.
Methods
Adhesive Casting: Adhesives A and B were solvated at a 58%w solids in a mixture of 70% ethyl acetate to 30% toluene for a minimum of 24 hours to allow for room temperature coating. Solvated adhesives were then cast using a 4 inch wide set drag bar onto the tight release side of a two-sided siliconized polycoated 72# bleach kraft paper liner. Samples were cast for dry coat weights of 1 mil and 2 mils. Samples were force dried at 80° C for 10 minutes. Once dried, a second piece of the two-sided siliconized polycoated 72# bleach kraft paper liner was laminated, easy-release side to the uncured adhesive to protect it until ready for curing. Samples were cut to a length of 6 inches and stored at standard conditions until curing.
Adhesive Curing: Cast samples of adhesives A and B were cured using three UV light sources. A standard H bulb MVL, and a custom UV LED irradiance device set to two separate configurations: a monochromatic 280 nm configuration, and a polychromatic 280 nm, 308 nm and 365 nm configuration.
H Bulb: Adhesive samples were irradiated using a Fusion systems MC6R benchtop lamp system powered by a P300 power supply source. A traditional mercury “H” bulb was used for these samples. UV-C dose was controlled using a fixed lamp power setting and varying line speed. UV dose validation was performed using an EIT2.0 UV Power Puck II.
UV LED Irradiance Device: The custom UV LED irradiance device was borrowed from Nichia Corporation, Tokushima, Japan. This prototype device was specifically developed for wavelength efficiency testing and comprises three primary components: A UV LED light engine, a water-cooled heatsink and a programmable power supply.
UV LED Light Engine: The UV LED light engine was tested in two configurations:
Monochromatic 280 nm UVC LED: This configuration consisted of 256 individual NCSU434C 280 nm diodes capable of emitting a typical output of 115 mW each at an input of 350 mA. The 280 nm LEDs possess a spectral halfwidth of 10 nm with a peak wavelength tolerance of 1.8 nm.
Polychromatic UV LED: This configuration consisted of 120 individual NCSU434C 280 nm diodes, 104 NCSU434B 308 nm diodes (capable of emitting a typical output of 100 mW each at an input of 350 mA), and 32 NVSU233A 365 nm diodes (capable of emitting a typical output of 1540 mW each at an input of 350 mA). The 308 nm and 365 nm LEDs both possess a spectral halfwidth of 11 nm, with a peak wavelength tolerance of 3 nm.
The light engine was mounted atop a 6 x 6 x 4.5-inch testing chamber lined with 92% reflective Porex. After removing the easy-release liner, adhesive samples were placed within the testing chamber for irradiation. After irradiation, samples were removed, and the easy-release liner was re-laminated until samples were prepared for rheology or adhesive testing.
Copper Heatsink: The LED engine is cooled by a water-cooled 150 x 150-mm copper heat sink. The cooling system operates at ambient temperature and is capable of absorbing 700 W of heat at maximum output.
Power Supplies: Two identical programmable power supplies (B&K Precision Corporations model 1902B) were used to power the LED light engine. For sample irradiation, power supplies were set to 250mA output. Power was applied manually with a timer to determine the dose. A Gigahertz optics X1-UV-3726 UV radiometer was used to validate UV-C dose (unfortunately, the same radiometer was unable to be used to measure delivered dose from the H bulb and LED sources, due to spatial constraints of the experimental setup).
Rheology: Rheological analysis was performed with an Ares G2 rheometer. Constant frequency oscillatory temperature sweeps from 20° – 160° C were performed on each sample tested. The temperature was ramped at a rate of 3° C/s, with a constant angular frequency of 10 radians/s, with data taken every 10 seconds. Temperature sweeps were performed on a roughly 0.1 g sample of irradiated adhesive. For each adhesive/dose condition, two separate samples were tested. Adhesive samples were rolled into rough balls and placed between 8.0 mm parallel plate sample holders with a 1.5 mm gap. Results are reported as the ratio of the loss modulus (G”) to the storage modulus (G’), also known as the Tan(δ).
Adhesive Sample Preparation: The easy-release liner was removed from irradiated samples. 2 mil PET was then laminated to the irradiated surface of the adhesive, with pressure applied by a 10-inch, 3.7 kg siliconized roller. Than 1 x 5-inch samples were cut for shear testing.
Static Shear: Static shear measurements were performed in accordance with PSTC-107, Procedure A. 2 Each test sample was applied to a clean stainless-steel panel with a 1 x 1-inch contact area. A 2.5 kg roller was applied over the sample area twice, and 30 minutes of dwell time was allowed before testing. A 1.33 x 0.875-inch aluminum mass holder was hung from the adhesive test sample, which was looped back upon itself and secured with masking tape and staples. The panels were then hung vertically (180°) on a Cheminsturments EZ shear 30 sample shear bank, and a 1 kg mass was suspended from the test sample. Shear failure times were recorded in hours by a 30 static shear adhesion controller.
Results
UV Dosages
UV-HMPSA manufacturers and converters commonly use UV-C dose targets when determining the lamp operating intensity and exposure time required to cure an adhesive. Dose targets typically are given as total dose (mJ/cm2) or as dose per coat weight (e.g. mJ/cm2/mil) of UV-C radiation. This method of determining dose results in consistent cure levels so long as the radiation source and radiometer are not changed and well-maintained. If the radiation source or radiometer are altered, an identical measured UV-C dose could produce a different cure result because the given UV-C dose does not account for the specific UV-C spectral makeup of the radiation source. A measured 25 mJ/cm2 UV-C dose delivered from an iron-doped D bulb would have a different spectral distribution compared to a measured 25 mJ/cm2 UV-C dose delivered from a mercury bulb, and both would be different from a 25 mJ/cm2 UV-C dose delivered from an array of 280 nm LEDs. Additionally, the UV-C dose does not account for radiation emitted in the UV-B and UV-A ranges, which could potentially impact adhesive cure. The relative UV output spectra for each of the radiation sources used in this study are shown in Figure 2.
With these details in mind, the target UV-C dose was selected to allow comparison of the delivered irradiation across the different sources. Dose targets chosen for 1 mil samples were doubled for 2 mil samples, as dose targets for UV-HMPSAs are generally given per thickness of the coated adhesive. In some cases, actual doses varied from those predicted. Reported dosages are given as measured (see Table 1), but “like” values across lamp sources are grouped for easy comparison. For samples irradiated with the H bulb, only the lowest two dose targets were cured. These samples serve as controls, with the two dose targets chosen being reasonable doses for a UV-HMPSA that might be used in the lab or production. It is known that 280 nm radiation is absorbed by benzophenone at a lower rate than the lower wavelength UV-C radiation produced by the H bulb, so for LED-cured samples, larger UV-C doses were applied. 3
Rheology
Rheological analysis frequently is used in the PSA industry as a means of bulk adhesive characterization. By applying a rotational strain and measuring the response of the adhesive sample across a range of temperatures, the fluid loss modulus (G”) and elastic storage modulus (G’) can be determined. These properties, as well as the ratio of G” to G’ (known as Tan(δ)), can be used to predict various material behaviors over the range of temperatures analyzed. In this study, Tan(δ) is used at elevated temperatures as a measure of adhesive cure. As covalent cross-link density increases, the Tan(δ) of an adhesive will decrease at elevated temperatures. For each adhesive and coat weight, Tan(δ) at 70° C is compared across all cure conditions. Generally, for a group of samples across a given irradiance source, one observes a decrease in Tan(δ) at 70° C with increasing dose, up to the point where maximum cross-link density is achieved. For both adhesives, it is observed that at the lower level UV-C doses, the Tan(δ) is lower at 70° C for the samples cured with the traditional H bulb than for samples cured with either LED array.
The Tan(δ) values at 70° C for adhesive A (see Figure 3) are comparable across both UV LED arrays. At the lower 1 mil coat weight, even at the highest level of UV-C exposure tested, the measured Tan(δ) at 70° C remains greater than the Tan(δ) values measured for the H bulb cured samples at 40 mJ/cm2 and 55 mJ/cm2. However, the Tan(δ) for the 200 mJ/cm2 at both temperatures is only slightly higher than the 40 mJ/cm2 control. At 2 mil thickness, the Tan(δ) values of the control H bulb cured sample are greater than those of the 1 mil H bulb samples, indicating a lower overall cure. In contrast, the samples cured by the LED engines are comparable or slightly lower than those measured at the 1 mil coat weight. A comparable cure is observed for the 280 nm LED engines with five times the UV-C exposure at 1 mil, and the same comparable level of cure is seen at roughly four times the UV-C exposure at the 2 mil samples.
For adhesive B, there is more variation in Tan(δ) values between the two UV LED engines, particularly at the lower dose levels, where the multichromatic LED array records higher Tan(δ) than the monochromatic 280 nm LED on the 1 mil samples (see Figure 3). It is unclear why this relationship was observed, but the difference is less severe at the 2 mil coat weights. At both coat weights, samples cured by the UV LED engines recorded higher Tan(δ) values than samples cured by “like” doses of UV-C from the H bulb. At 1 mil, the UV LED engine-cured samples have Tan(δ) values comparable to or lower than the Tan(δ) controls at just two times the measured UV-C dose. Among the 2 mil samples, a greater discrepancy can be seen in Tan(δ) values between the samples cured by the H bulb and the UV LED. For the 2 mil samples of adhesive B, comparable Tan(δ) values are observed from samples cured by LED after a measured UV-C dose 2.8 times larger than the samples cured by the H bulb.
Static Shear
The static shear adhesion test is often used to determine the effectiveness of a given tape construction to support a hanging weight, but for curable adhesives, it is also useful as a tool to measure adhesive cure. For a given PSA formulation, as covalent cross-link density increases and the cohesive structure increases, the recorded time to failure generally increases. The measured failure time of the samples across all radiation sources was compared. For sample A, results are observed that correlate well with those from the rheological analysis (see Figure 4). As the UV-C dose increases across all radiation sources, so does failure time. Comparable failure times are achieved by LED-cured samples after UV-C doses roughly five times greater at 1 mil and four times greater at 2 mils (these results correlate well with rheological observations). A similar pattern is observed with the samples of adhesive B (see Figure 4). At 1 mil, samples cured by LED reached comparable or greater failure times after UV-C doses less than two times greater than samples cured by the H bulbs. At 2 mils, comparable performance is seen only for samples cured by the multichromatic UV LED system, and only after roughly four times the UV-C exposure as samples cured by the H bulb. Last, for both adhesives at the higher coat weight, higher failure times are observed for samples cured with the multichromatic UV LED compared to the 280 nm only LED engine. Interestingly, no similar relationship was observed from the rheological data.
Analysis
The study found that rheological and cohesive shear properties of UV-HMPSAs are affected significantly by the source and magnitude of UV-C radiation. These measures of adhesive cure, Tan(δ) and shear failure time, clearly show that adhesives A and B require significantly higher doses of UV-C radiation produced by 280 nm LEDs to achieve an equivalent cure to samples cured by a wide spectrum mercury vapor lamp (roughly 4 to 5 times for adhesive A and 1.5 to 4 times for adhesive B, depending on coat weight). These observations are not surprising as benzophenone, the basis for the photoinitiator used by both adhesives, absorbs 254 nm radiation about five times as frequently as 280 nm light, and mercury vapor lamps produce strongly in the 250 nm to 260 nm range (about one-fourth of an H bulb’s total UV-C output). It is also clear that when compared to the H bulb, the UV-C LED systems were more effective in curing adhesive B than adhesive A. Some effects in this study appear to be based on coat weight, but there is no clear pattern. Because 280 nm radiation is absorbed with less frequency, it makes sense that it could penetrate more deeply into an adhesive film. The possibility that 280 nm UV-C radiation could facilitate the ability to cure heavier coat weights warrants further investigation. Similarly, the introduction of 308 nm and 365 nm UV LED radiation in addition to the 280 nm radiation may also affect UV-HMPSA cure, but more study is required to better understand where these wavelengths may improve cure or adhesive performance.
Conclusion
For many commercially available UV-HMPSAs, if a converter wanted to use 280 nm UV-C LEDs as a radiation source, they would need to provide a significantly higher UV-C dosage to achieve comparable cure and adhesive performance. The results of this study suggest that a sufficiently powerful UV-C LED source (or a system that allows for long enough exposure time) would be a viable option for converting UV-HMPSAs. Alternatively, to facilitate UV LED-curable adhesives, UV-HMPSA manufacturers could formulate adhesives using photoinitiators that are activated by wavelengths of radiation emitted by common LEDs, or PSA converters will need to wait until commercially viable ~250 nm LEDs are available.
Thank you to the Nichia Corporation for supplying the UV LED light engine used in this study.
Works Cited
- Comer, M. Christopher, McGuire, Kelsey, Kilian, Lars. Impact of Adhesive Composition and Differential Cure on the Performance of Acrylic Ultraviolet Light Cured HotMelt Adhesive. May 21, 2023.
- Test Methods for Pressure Sensitive Adhesive Tapes, 17th Northbrook, IL: Pressure Sensitive Tape Council, 2000.
- Polymer Innovation Blog UV Curing: Part 2; A Tour of the UV Spectrum. [Online] May 15, 2025. https://polymerinnovationblog.com/uv-curing-part-2-tour-uv-spectrum/
Zachary Gatland is a product development scientist at Mactac specializing in technical tapes with an emphasis on UV-cured hot-melt pressure-sensitive adhesives. He has been in the UV-HMPSA industry for three years and was recently named to the RadTech Board of Directors. Before entering the industry, Zachary completed his master’s in physics at Case Western Reserve University, where he utilized single-molecule fluorescence microscopy to study corrosion and other material processes. Gatland can be reached at email: Zcgatland@mactac.com, www.mactac.com.
