Evaluation of natural rubber latex-based PSAs containing aliphatic hydrocarbon tackifier dispersions with different softening points—Adhesive properties at different conditions P. Rajesh Raja a,n , A.G. Hagood a , M.A. Peters a , S.G. Croll b a Eastman Chemical Company, 100 N Eastman Rd, Kingsport, TN 37662, United States b Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58108, United States article info Article history: Accepted 16 November 2012 Available online 28 November 2012 Keywords: NR latex-dispersion PSAs Rheology Adhesion and cohesion Aging Hydrocarbon tackifier dispersions abstract Natural rubber latex-based water–borne pressure sensitive adhesives (PSAs) have been formulated with three aliphatic hydrocarbon water-based dispersions (varying softening points) at two different resin addition levels (25% and 50%). Time–temperature superposition analysis using WLF approxima- tions for adhesive peel has revealed that the adhesives formulated with 50% resin addition level show good adhesive behavior. It has also been determined from time–temperature superposition analysis that peel force increases systematically with softening point and peel rate. Correlation of viscoelastic behavior with adhesive properties suggests that at least 50% resin addition level is needed to bring the natural rubber-based formulations into PSA criteria as defined by Dahlquist and others. Adhesive property evaluations performed on a high surface energy substrate (stainless steel) and low surface energy substrate (LDPE) suggested that optimum tack, peel and shear properties at room temperature were obtained for a formulation containing a higher softening point dispersion (95 1C) at 50% resin addition level. Adhesive peel and tack tend to follow softening point trends as well. A 25% tackifier dispersion addition level did not provide any significant adhesion. Humid aging (50 1C and 100% relative humidity) evaluations of the water–borne adhesives seem to correlate well with the room temperature adhesive property observations. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction From as early as 1845, natural rubber-based pressure sensitive adhesives have been formulated with natural or petroleum derived resins to improve tack and processing characteristics [1–4]. Most of the early works on natural rubber-based industrial pressure sensitive adhesives were solvent-based thin film sys- tems pioneered by 3 M. Availability of natural rubber as stabilized latex emulsions increased the use of emulsions-based pressure sensitive adhesive technologies and these systems began to dominate solvent-borne technologies by the 1970s. More strin- gent health and environmental restrictions also favored the natural rubber latex-based emulsion pressure sensitive adhesive technology until World War II, when availability became an issue [5–7]. Pressure sensitive adhesives with solvent-based or water-based, natural rubber latex have been formulated with other components, such as tackifying resins and additives (plasticizers, stabilizers etc.), since the first patent in 1845. Natural rubber imparts cohesive strength characteristics to the pressure sensitive adhesive, while tackifying resins impart adhesive characteristics such as tack and peel strength [1–7]. Oldack and Bloss in 1979 reported [7] the compounding of natural latex in water-based PSAs utilizing water-based resin dispersions. They evaluated the effect of water-based tackifier dispersions of polyterpene resin, hydrogenated rosin resin and even a hydrocarbon resin dispersion in natural rubber latex-based formulation for adhesive tack and shear [8]. Although there is much reported literature on solvent-borne natural rubber pres- sure sensitive adhesives, there is limited literature regarding the natural rubber, latex-based pressure sensitive adhesive formula- tions. Recent acrylic-based emulsion availability issues are prompting more and more formulators to go back to natural rubber latex-based PSAs from a performance, availability and economics stand point. Natural rubber-based PSAs can be used in a variety of applications such as masking tape, box sealing tape, protective film tape, and tape in medical applications. Given the wide range of applications involved, the PSAs formulated should withstand different environmental and physical conditions. Therefore, understanding the adhesive properties and mechanical behavior of a PSA in a variety of operating conditions and temperatures is very critical [2]. Normally, pressure sensitive adhesive properties are charac- terized in three ways: peel, tack and shear strength (hold power). Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijadhadh.2012.11.009 n Corresponding author. Tel.: þ1 423 229 6119; fax: þ1 423 229 3217. E-mail address: rajeshp@eastman.com (P. Rajesh Raja). International Journal of Adhesion & Adhesives 41 (2013) 160–170