Synthesis of low noise, fast drying packaging tape adhesive via emulsion polymerization

Adhesives are defined as materials used to bond other materials, mainly on their surfaces through adhesion and cohesion. Pressure sensitive adhesives (PSAs) are ensure instantaneous adhesion by application of a light pressure and can be organic solvent- or water-based (dispersions), or in a solvent-free form as in the case of hot melt . Water-borne adhesives comprise acrylics, natural and synthetic rubber latexes, and ethylene vinyl acetate dispersions. As known, the interest in aqueous acrylic dispersions PSA grew with the need to solve solvent emission problems. The most common applications for water based pressure sensitive adhesives are tapes and labels. The packaging tape market is a growing worldwide industrial market. Developments in dispersion polymer chemistry have largely converted this market in a safe water based polymer dispersion user. Improvements on the application machine techniques have led to significant productivity gains. Although there are a few dispersion adhesives in the marketplace, still improvements are needed in the polymer side to reduce undesired noise levels when unwinding the tapes at the packaging line as well as increase drying fastness for the coating on higher speed machines. The importance of viscosity control at high shear rates is a very well known phenomena and the particle size is the key for the control on rheology. Small particles are less affected at high shear rates which increase the importance of polymodal or broad particle size distribution. Although both solvent- and water-borne acrylic PSAs are derived from the same monomers, the adhesive and cohesive properties of emulsion PSAs are different. A likely explanation for this inferiority is given as the more heterogeneous morphology of waterborne materials. As mentioned in several resources, PSA must be soft and tacky. Thus, its glass transition temperature (Tg) should be low, ranging from -20 ◦C to -60 ◦C. Polymers with low Tg are from alkyl acrylates such as poly (butyl acrylate) and poly (2-ethylhexyl acrylate). They are inherently soft and tacky but do not possess adequate shear strength. Polar monomers are used in co-polymerization in order to improve the adhesives' mechanical properties such as cohesion which gives an improved shear strength. Their presence increases the Tg and this may be explained by increasing dipole interaction, or enhanced hydrogen bonding. Polar groups in polymers increase intermolecular forces thus reduces free volume and increase Tg. Besides Tg, these interactions mainly affect cohesive properties. -COOH groups support intermolecular dipole dipole interactions and they are able to form hydrogen bonds with hydroxy groups which helps to increase the shear strength. In this study, synthesis of acrylic copolymers by emulsion polymerization for the application of low noise, fast drying pressure sensitive adhesives has been described and especially the effect of stabilizing monomers on noise level has been studied. It has been focused on the comparison between the adhesive performance of poly (BA-co-MMA-co-AA), poly (BA-co-MMA-co-MAA), poly (BA-co-MMA-co-AAm) and poly (BA-co-MMA-co-MAAm) by using different amount of polar co-monomers. Relatively lower noise has been obtained by MAA and MAAm. To balance the performance properties of MAA including copolymers, instead of MMA, several monomers with different Tg and polarity have been used such as EA, ACN, AAm, GMA and VAM. The performances of different polar co-monomers have been detailed on high and low surface energy substrates. An additional property such as flame retardancy has also been investigated by synthesizing copolymers containing boron acrylate and multi- functional boron methacrylates which also provides hydrophobicity and relatively higher performance on non-polar surfaces. The materials used for this research were the monomers mentioned above as butyl-acrylate (BA), methyl meth acrylate (MMA), acrylic acid (AA), methacrylic acid (MAA), acrylamide (AAm), meth-acrylamide (MAAm), acrylonitrile (ACN), vinyl acetate (VAM), ethyl acrylate (EA), hydroxyl ethyl methacrylate (HEMA), glycidyl methacrylate (Gly-MA), boron acrylate, multi-functioal boron methacrylate; surfactants; ammonium persulfate, NaHCO3 and deionized water. Solid content, pH, viscosity, surface tension, free monomer, particle size and rheology analysis have been performed by dispersion, on the other hand, swelling, Tg and performance analysis have been done by polymer film. The aim of the study is not only to investigate the influence of polar monomers on PSA performance, but also to develop an acrylic polymer for low noise packaging tape adhesives for high speed coating machines.