A Look at UV-curable Acrylic Hot Melt PSAs
Commonly used in labels, non-PVC graphic and specialty tape applications, UV-curable acrylic, hot-melt pressure-sensitive adhesives (UV HMPSAs) are gaining popularity due to their quick curing properties, excellent adhesion, and versatility. In particular, many converters are switching from solvent-based rubber, solvent-based acrylic, and rubber-based HMPSAs to UV HMPSAs because they offer high durability on par with most solvent-based rubber, solvent-based acrylic adhesives and overcome limitations found in rubber-based hot-melt adhesives while also keeping environmental impact to a minimum.
Understanding how tackification and coating method impact the effectiveness of UV curing helps to ensure that UV HMPSA properties are properly balanced for their intended end-use application.
Curing process challenges exist with UV HMPSAs, in particular with how the formulation and coating method (direct or transfer) can impact peel, loop tack, shear, and Williams Plasticity (WP). In this paper, we will explore how tackification and coating method impact the effectiveness of UV curing with H-Bulb. Understanding how these key variables influence performance helps to ensure that UV HMPSA properties are properly balanced for their intended end-use application.
When considering how UV HMPSAs will perform, it’s important first to understand how adhesive formulation impacts cure via a UV light. For example, many HMPSA components either absorb, or block, some of the UV light. This includes pigments and photoinitiators as well as any additives that have C = C bond unsaturation or aromaticity, such as tackifiers, plasticizers, or pigments.
Because these materials compete for UV light, addition of additives that absorb UV light limits the effective penetration of the UV light through the sample. This lower effective UV light dosage, especially at increasing concentrations of these additives, will limit the cure-through for the depth of the adhesive. Careful formulation is required to balance properties and also ensure the efficacy of the ingredients.
UV HMPSAs can be either direct or transfer coated during the manufacturing process. However, just like with the formulation, the coating method impacts end-use performance. When UV HMPSAs are direct coated, the adhesive is applied directly onto the facestock. The final adhesive performance is primarily based on the directly-irradiated surface, which has a higher level of cure compared to the rest of the adhesive layer. However, when UV HMPSAs are transfer coated, the directly-irradiated adhesive surface (high cure) is then laminated to the facestock. Therefore, adhesive performance is mostly based on the low cure surface, which is the softest portion of the entire adhesive layer (see Figure 1). Since high cure adhesives favor the cohesive properties (shear), and low cure adhesives favor the adhesive properties (tack, peel), the structural differences resulting from direct- vs. transfer-coated adhesive films drive dramatic differences in all aspects of adhesive performance.
As coat weight thicknesses increase, the variation observed in peel, tack, and static shear across direct- and transfer-coated materials can be seen in the following section. This variation increases further as adhesives are tackified and there are additional materials vying for the UV light.
The thicker the adhesive, the bigger differential will result in comparing peel strength between direct- and transfer-coated surfaces. This is only exasperated by the addition of light-absorbing materials such as tackifiers.
In general, UV HMPSAs cured with the H-Bulb demonstrate a different adhesive performance for peel, loop tack, and shear when comparing the directly irradiated (direct-coated) and indirectly irradiated (transfer-coated) surfaces. An increase in thickness and addition of additives that absorb UV light increased the variance in performance for each coating method. A difference in crosslink density at each surface due to varied light exposure leads to different viscoelastic response during testing. Cohesive strength was increased with higher UVC dose, but shear failure mode transitioned to mixed adhesive/cohesive failure, resulting in less consistent results. The results of this study highlight the considerations for coating and formulating UV HMPSAs. An understanding of the UV HMPSA formulation and interaction with light will allow for a thoughtful process to be designed for curing the adhesive to obtain the desired performance. Additionally, UV HMPSAs performance can be adjusted to manipulate the UV HMPSAs crosslink density and therefore the viscoelastic performance.
Source: Bostik / Adhesives Magazine