Acrylic Pressure Sensitive Adhesives Exhibiting Enhanced Adhesion to Low Surface Energy Substrates

Prevailing trend towards the use of lighter and lower cost engineered plastics in automotive, construction, aerospace, electronics and other industrial end uses have created a need for pressure sensitive materials that can bond well to these new, inherently low surface energy plastics. Reported herein are novel all acrylic compositions with inherently lower surface energy that display significantly improved adhesion to low surface energy substrates such as polyethylene and polypropylene. Additionally, evidence will be presented herein, where these new compositions demonstrate compatibility with tackifiers historically known to be incompatible with more traditional acrylic pressure sensitive polymers. Introduction Pressure sensitive polymer compositions have been utilized for well over 50 years. Many types of polymers can be made pressure sensitive via various formulation methods. (Meth)acrylic copolymers are one of the most widely used polymer classes for the production of pressure sensitive adhesives because they are relatively low cost, thermally and oxidatively stable, optically clear, and require little to no formulation to be a useful pressure sensitive material. There exists a wide variety of (meth)acrylic monomers of which pressure sensitive copolymers can be made. The large selection of available monomers enables a vast range of viscoelastic performance characteristics. Various chemically functional monomers provide a diverse selection of cross-linking options that can be tailored to specific applications. (Meth)acrylic copolymers can be polymerized and used industrially in a waterborne, solvent cast, melt, or monomer polymer syrups. Any of these formats of delivery can be selected depending on final adhesive performance required, the manufacturing assets available and cost requirements. Many high performance applications require the improved coat quality, coating thickness and material properties achievable via solvent cast, or melt and syrup techniques. Historically (meth)acrylic pressure sensitive adhesives (PSAs) have delivered adequate adhesion to a broad base of materials used in the industry. Ever evolving trends in the market place have seen, and continue to see the replacement of metal, glass and wood assembly or construction materials with lower cost, lower weight plastic alternatives. This continuing trend toward lighter weight, lower cost materials has challenged the traditional acrylic pressure sensitive adhesives ability to adhere to these new substrates because in most cases these new materials are much lower surface energy (LSE) than traditional materials. Traditional acrylic pressure sensitive adhesives adhere very well to relatively polar substrates such as steel, aluminum, tin, glass, and wood. These types of materials tend to have higher free surface energy or surface tension than that of the pressure sensitive adhesive as seen in Figure 1. 1 Figure 1. Surface energy of various materials and common PSA. Given the fact that an adherend must have higher surface energy than the corresponding adherent, the common acrylic PSA has no difficulty wetting the materials in Figure 1 to form a bond surface. These types of materials have been or are continuing to be replaced in new material construction in various applications including, automotive assembly, building and construction, electronics and medical devises. Many of these market areas are moving toward lighter weight and often lower cost plastic materials, but still require PSAs to bond various components. These lighter weight plastic components can be challenging to adhere to because they are generally much lower in surface energy as seen in Figure 2. 1 Figure 2. Useful plastic materials surface energy compared to common acrylic PSA Tin 526 Aluminum 840 Zinc 852 Silica Glass ~1000 Copper 1362 Stainless Steel 650-1200 Common Acrylic PSA 35-45 Material Surface Energy (dynes/cm) Polyterafluroethylene 19 Polydimethyl Siloxane 23 Natural Rubber 24 Polyethylene 30 Polypropylene 30 Acrylonitrile Butadiene Styrene 35 Polymethylmethacrylate 41 Polytheylene Terephthalate 42 Polycarbonate 46 Common Acrylic PSA 35-45 Material Surface Energy (dynes/cm) As mentioned previously there exist a wide variety of (meth)acrylic monomers to select from when designing an acrylic pressure sensitive adhesive. These materials can be fairly polar or non-polar depending on the length and chemical nature of the ester side chain. A monomers polarity can be expressed as solubility parameter 2 (cal/cm 3 ) 1/2 and selecting monomer with lower solubility parameters will result in a final adhesive that can wet lower surface energy materials. Some common acrylic base monomers, functional monomers and polar and non-polar glass transitions temperature (Tg) modifying monomer and their corresponding glass transition temperatures and solubility parameters can be found in Figure 3. 3 Figure 3. Common acrylic monomer and their Tg’s and solubility parameters. Designing an acrylic PSA with monomers that have lower solubility parameters will inherently enable the PSA to wet a wider variety of materials that can include various plastic substrates. 2-Ethylhexyl Acrylate -60 9.22 Butyl Acrylate -55 9.77 Isooctyl Acrylate -60 9.22 Methyl Acrylate 10 10.56 Ethyl Acrylate -25 10.2 Vinyl Acetate 30 10.56 Methyl Methacrylate 105 9.93 Acylic Acid 110 14.04 Methacrylic Acid 230 12.54 Hydroxy Ethyl Acrylate -15 13.5 t-Butyl Acrylate 50 9.36 t-Butyl Methacrylate 105 9.07 Isobornyl Acrylate 96 9.71 Isobornyl Methacrylate 155 9.5 Chemically Functional Non-polar Tg Modifing Use Monomer Tg Solubility Parameter (cal/cm)