The effect of particle size and composition on the performance of styrene/butyl acrylate miniemulsion-based PSAs Ste ´phane Roberge, Marc A. Dube ´ * Department of Chemical Engineering, University of Ottawa, 161 Louis Pasteur St, Ottawa, Ont., Canada K1N 6N5 Received 16 July 2005; received in revised form 28 November 2005; accepted 5 December 2005 Abstract Styrene/butyl acrylate batch miniemulsion copolymerizations were performed in a 1.2 L stainless steel reactor. Conversions were monitored off-line using gravimetry and in-line using ATR-FTIR spectroscopy. The final latexes were coated on a polyethylene terephthalate carrier and dried at room temperature for 2 days. Their performance as pressure-sensitive adhesives (PSAs) was evaluated by measuring their tackiness, peel strength and shear strength. By using a constrained mixture design, the influence of particle size and copolymer composition was investigated. Particle size was found to be the most influential factor for both tack and peel strength models. Tack showed a concave upward trend whereas peel strength decreased with increasing particle size. Shear strength decreased with increasing particle size but was also significantly influenced by copolymer composition. The final forms of the models allowed 3D response surfaces to be built and an optimal adhesive performance region (highest combined tack, peel strength and shear strength) was located near the smallest particle diameter investigated with the highest styrene composition. The positive effect of smaller particles on every adhesive property relates to the tighter packing provided by smaller particles during the drying process, thus increasing the area of contact between the adhesive and the substrate. q 2005 Elsevier Ltd. All rights reserved. Keywords: Miniemulsions; Pressure-sensitive adhesives; Particle size 1. Introduction Adhesives are defined as substances capable of holding at least two surfaces together. A class of adhesives called pressure-sensitive adhesives (PSAs) is characterized by instantaneous adhesion upon application of light pressure [1]. The most common applications for PSAs are tapes, labels and protective films. The polymerization process chosen to produce PSAs can be carried out in different media such as bulk, solution or emulsion. Because of environmental concerns and government regulations to substitute solvent-based systems by water-borne products, there is a growing interest in producing PSAs by emulsion polymerization. In conventional emulsion polymerizations, the main ingredients are monomer(s), water, surfactant and initiator. When the concentration of surfactant exceeds its critical micelle concentration (CMC), the excess surfactant molecules aggregate to form small colloidal clusters referred to as micelles. In principle, polymer particles can be formed by the entry of radicals into the micelles (hetero- geneous nucleation), precipitation of growing oligomers in the aqueous phase (homogeneous nucleation), and radical entry in monomer droplets. The monomer droplets are relatively large (1–10 mm) compared to the size of monomer-swollen micelles (10–20 nm), and hence the surface area of the micelles is much greater than that of the monomer droplets [2]. Consequently, the probability for a radical to enter the monomer droplets is very low, and most particles are formed by homogeneous and heterogeneous nucleation. An alternative to conventional emulsion polymerization, miniemulsion polymerization, provides a simpler means of controlling particle size [2]. The basis for the miniemulsion polymerization process is an energetic homogenization of the reaction mixture to reduce the size of the monomer droplets and the use of both a hydrophobe and an emulsifier to protect these droplets against degradation. An efficient miniemulsion polymerization (in terms of particle formation) is very useful as it allows one to control the number and size of particles being formed in a manner different from particles formed by micellar or homogeneous nucleation. That is, miniemulsions are conducted at emulsifier concentrations well below the CMC, thus avoiding micellar nucleation altogether. The droplet size can range from 50 to 500 nm in diameter [3] and the latex Polymer 47 (2006) 799–807 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2005.12.024 * Corresponding author. Tel.: C1 613 562 5800x6108; fax: C1 613 562 5172. E-mail address: dube@genie.uottawa.ca (M.A. Dube ´).