Controlling Latex Particle Size for Blush-Resistant Pressure-Sensitive Label Adhesives
September 1, 2008
Many companies that use adhesives for label application are switching from solvent-based adhesives to water-based latex pressure-sensitive adhesives. This shift has been well documented as firms put forth efforts to implement technologies with improved environmental profiles.
Water-based latex pressure-sensitive adhesives have provided the solution for many companies looking to reduce their use of solvent-based adhesives, but they sometimes have found challenges to implementing the technology in broad-based applications. One limiting factor was its use on clear labels; latex films have a tendency to turn white or blush when exposed to water. Researchers at Ashland Inc. proposed a possible explanation for the blushing phenomena, and a practical solution.
As a latex film forms, the adhesive particles initially come together at the side exposed to air (see Figure 1). Hydrophilic material becomes trapped between those particles and naturally attracts moisture. If the film composition is semi-permeable, the hydrophilic pockets will swell when exposed to water. The swollen pockets usually have a refractive index different from that of the polymer. As the pockets swell to above 40 nm, they scatter light and the film becomes cloudy.
The degree of turbidity, or blush, depends on a number of factors. For example, if the time frame of film formation is long enough, some of hydrophilic material may follow the receding water phase. This may result in pockets with higher concentrations of hydrophilic material dispersed within the film. These pockets with more hydrophilic material will continue to swell and cause a normally clear film to blush. For example, air-dried films have been observed to have more blush than oven-dried films. In addition, the blushing of an air-dried film will not be improved by post-oven treatment. In some instances water itself can impact the blush resistance. In certain areas of the Northeast, where lead pipes are still in use, the pH of the water is lower than that of water in the Midwest. If the hydrophilic material is pH sensitive, a latex film that does not blush in the Northeast may blush in the Midwest.
One way to improve blush resistance is to reduce the size of the hydrophilic-containing pockets. If the pockets are small enough, then even when they are fully swollen with water they will be less than 40 nm and not visible, which will result in a clear film. This can be achieved by lowering the average particle diameter to 80-90 nm. The relative difference in the size of the interstices between 85 and 300 nm particles is shown in Figure 2.
Lowering particle size, however, may introduce a stability problem. If a 300-nm particle is stable with 1.75% surfactant, an 85-nm particle would require 6.2% surfactant based on just the surface area. Latex made with this level of surfactant will still blush because of the increased level of hydrophilic material, which causes swelling of the pockets to diameters larger than 40 nm. The 85-nm latex needs to be made with a lower level of surfactant to take advantage of the smaller hydrophilic pockets. The use of a polymerizable surfactant improves the particle stability and reduces the amount of free hydrophilic material available. The combination of a nonionic/anionic hydrophilic tail on Hitenol BC-10 (Figure 3) produces the desired particle diameter. The propenyl group works well since it does not readily homopolymerize. In comparison, when methacrylate functional surfactants were used, the amount of water-soluble homopolymer formed was significant, increasing the blush.
The combination of small particle diameter and a polymerizable surfactant has produced latex with good blush resistance over a range of test conditions. Figure 4 shows films of the same latex that have been soaking for 24 hours. In both cases, the latex was coated on the release liner and dried. The dried latex films were then transferred to biaxially oriented polypropylene (BOPP) films. One film was dried at 90oC for two minutes and one air-dried before transferring.
As companies move to water-based latex pressure-sensitive adhesives, careful attention to potential performance issues must be considered. Often, what has been seen as a problem has led to innovative solutions that further the scientific body of knowledge. In many applications, water-based adhesives provide great technology, excellent adhesion and an improved environmental profile.
AcknowledgementsThe authors would like to thank Harvey Richards, Dennis Healy, Jonathan Burkhart and Kim Armstrong for their contributions to the development of this technology.
For more information, contact Scott Harvey, Ashland Inc., 5200 Blazer Parkway, Dublin, OH 43017; e-mail firstname.lastname@example.org.