High Performance Copolymer Dispersions For Flexible Packaging

Transcription

1 igh Performance opolymer Dispersions For Flexible Packaging R. ooper,. Guy, N. Seung, K. Vredeveld Michelman 98 Shell Road incinnati, Ohio Phone: Fax: Abstract Traditional ethylene acrylic acid copolymer dispersions require the presence of a primer on many nonpolar substrates in order to achieve proper wet out and adhesion. This article will describe how new dispersion developments have evolved and discuss the high performance properties now delivered on a wide variety of substrates. Improvements in adhesion to nonpolar substrates, faster production speeds and lower heat seal temperatures are just a few of the developments explored in this paper. 2. Introduction The basic chemical structure of ethylene acrylic acid copolymer is displayed in Figure 1. The presence of acrylic acid reduces the crystallinity of ethylene. As the acrylic acid content in the copolymer increases, the polymer network becomes more amorphous as crystal formation is disrupted. Just as the increased branching in low density polyethylene reduces the crystallinity of polyethylene, so will the acrylic acid moiety reduce crystallinity. The structure of ethylene acrylic acid copolymer, which has pendant acrylic acid groups situated along the polyethylene chain, is similar to that of low density polyethylene with its short chain branching (Figure 2). This branching on the polymer also effectively reduces the crystallinity of the polymer. FIGURE 1 O O PE with no branching FIGURE 2 PE with branching Ethylene Acrylic Acid O O

2 opolymers of ethylene and acrylic acid have a unique set of characteristics that result from the properties of its reacted constituent monomers. The presence of ethylene provides properties such as water resistance, flexibility, crystallinity, chemical resistance and barrier, while incorporation of acrylic acid yields polarity, toughness, crosslinkability, hot tack strength and low temperature heat seal. The melt strength, tensile strength and hot tack are directly attributable to the ability of the acrylic acid groups to hydrogen bond with neighboring chains. Modification of acrylic acid content can provide the balance needed to meet specific enduse application requirements. Increasing the acrylic acid content will increase adhesion to polar substrates, reduce crystallinity, decrease softening and melting point, improve optics and increase strength. 3. The Process of Applying opolymer Dispersions in Flexible Packaging Because of its processability and low viscosity, EAA dispersions can be applied to various substrates using traditional coating equipment. Gravure, flexo, air knife and rod applicators are a few of the standard application techniques that can be used. Application rates vary, based on the intended use. For paper applications or porous substrates, the coatings should be applied at weights of gsm. When used as laminating adhesives, the coating weight should be in the range of gsm. Depending on factors such as the viscosity of the coating and the coat weight, it is important that the end user verify that the heat and air flow available are sufficient to dry the coating effectively on nonporous substrates. If the drying conditions are marginal, it is not difficult to trap moisture within the film as the outer layer skins over to form a barrier. This is best avoided by drying at temperatures exceeding the melt point of the copolymer to provide sufficient mobility for moisture to escape. Aqueous dispersions of EAA copolymers are excellent film formers and can provide continuous films at a solids content as low as 1% at room temperature; however in some cases improved performance properties can be achieved by fusing the film at elevated temperatures. When heat is employed to assist the drying process, it is just as critical to cool the coated substrate prior to winding to avoid blocking in the roll. 3.1 Wettability: The surface tension of ethylene acrylic acid copolymer dispersions will drop as the processing temperature increases (Figure 3). As a result, the coating will display improved wet out under heated conditions. Dyne / cm hemical Attraction FIGURE 3 Increasing oating Temperature Lowers Surface Tension Temperature (deg. ) The carboxyl groups present in ethylene acrylic acid copolymer dispersions provide improved adhesion to polar substrates (Figure 4). This functionality enables adhesion to foil, paper, LDPE, nylon and many other polymers. Adhesion to most polar substrates is a result of the ability of the carboxyl group to chemically interact with functional moieties at the substrate s surface. For example, adhesion to metals is believed to occur via interaction of electrons in the carboxyl group with vacant orbitals in the metal. onversely, traditional ethylene acrylic acid copolymer dispersions are not recommended for PET, PE, PS, PV and PVd films due to their nonpolar character. arboxyl Group FIGURE 4 ydrogen Bonding adds: melt strength tensile strength hot tack 1% AA 2% AA MI 3 w/ defoamer 2% AA MI 13 Low Visc. w/ipa Adhesion to polar substrates

3 3.3 Flow haracteristics Flow characteristics can also impact adhesion. This can include both viscosity of the coating, as well as melt flow of the polymer. As seen in the Figure 5 below, the shearing stresses of the coating at the point of application can impact the viscosity of EAA dispersions. FIGURE 5 Applicator: gravure air knife meyer rod FIGURE 6 Substrate A Aqueous Dispersion of EAA (125% nonvolatile) Drying (6 13 ) Initial coating of first substrate (film weight =.5 2 g/m2) With traditional EAA coatings, adhesion to PET, PE, PS, PV and PVd films was not recommended without the presence of a primer or modification of the substrate via flame ionization or corona discharge. Even under these conditions, the bond strengths achievable were relatively poor. New developments in these coatings have allowed adhesion to be achieved without priming the substrate, although corona treatment is still recommended. Secondly, the melt flow of the base polymer has an impact on adhesion. ertain end use applications need increased melt flow so that the polymer is mobilized and flows easily when heat is present, enabling improved adhesion. This property plays an important role in enduse applications such as medical garments and towel reinforcement. 3.4 oating EAA coatings can be applied using conventional coating equipment as illustrated in Figure 6. The process of adhesion to a substrate will rely first on the ability of the coating to form a continuous layer on the substrate which will be determined by the surface energy of the substrate and the surface tension of the coating. Meeting this requirement, however, does not guarantee that adhesion between the coating and the substrate will result. Adhesion will depend on the ability of the coating to chemically interact or bond with the substrate. Alternative approaches to achieve adhesion rely on a physical interaction such as that obtained with pressure sensitive adhesives. EAA copolymer containing 2% acrylic acid has had little utility in forming adhesive bonds with relatively nonpolar substrates such as PS, DPE and PV; however, great strides have been made recently in which the EAA copolymers have been chemically modified to achieve adhesion with these base substrates. 4. Performance Parameters of the Base opolymer The traditional water dispersible EAA copolymer contains 2% acrylic acid by weight; this translates to about one acrylic acid group for every ten ethylene groups.

4 Distribution of the acrylic acid along the chain backbone is random. The pendant carboxyl groups associated with the acrylic acid moiety not only change the characteristics of the copolymer as previously noted, they also are the critical pathway for preparing aqueous dispersions of these materials. Typical choices for the neutralizing agent range from ammonium hydroxide to organic amines and alkali metal hydroxides. The molecular weight of the dispersible copolymers containing 2% acrylic acid is considerably lower than that of their extrusion grade counterparts. The lower molecular weight results in a higher melt index (lower viscosity) which, along with the acrylic acid content, provide improved mobility of the activated coating and enable lower heat seal temperatures to be accessed with aggressive hot tack. Recent developments have also resulted in the ability to disperse lower acrylic acid content polymers (1% by weight) that previously were limited as extrusion grade materials. The lower acrylic acid content enables improved adhesion to nonpolar substrates as the frequency of acrylic acid is now reduced to approximately one for every 22 ethylene groups. The molecular weight of the 1% copolymer is substantially higher than that of the 2% grade resulting in a much lower melt index due in part to the more extensive chain entanglements. General properties of the 1% copolymer and the 2% copolymer are listed in Figure 7. FIGURE 7 1% AA 2% AA 7, 31, Molecular Weight Mw (Mn) (13,) (8,4) Melt Index 2 24 ardness 54 5 Melt Point via DS: ( F) 95 (23) 77 (171) Blocking Temperature: ( F) 9 (194) 59 (139) 5. Dispersibility and rosslinkability: The solids content achievable with aqueous dispersions of EAA copolymers is controlled by a number of factors including the molecular weight of the copolymer, choice of neutralizing agent and the level of neutralization. The viscosity and the particle size of the dispersions formed will follow suit. In order to prepare stable dispersions of the copolymers, it is usually not necessary to neutralize all of the available acrylic acid groups present. One of the distinct advantages of EAA copolymer dispersions is that the applied film can revert back to the original state of the polymer via selection of a fugitive neutralizing agent such as ammonium hydroxide. This is achieved without the addition of any conventional surfactants which could negatively impact adhesive properties via migration through the film. Selection of an alkali metal hydroxide will fix the counterion in the polymer film after it is applied and dried. These so called ionomers yield a continuum of properties depending on the degree of neutralization. Typically, higher heat seal temperatures, improved resistance to blocking and higher tensile strengths result. Another property that can be distinctly impacted is the water resistance of the applied film. With high levels of ionomerization, the polymer film can be made to completely resolubilize in cold water which can be a useful property when used as a temporary protective coating. The acrylic acid functionality present in the copolymer is also the site for any crosslinking reactions that can take place. Although ionomerization with an alkali metal hydroxide simulates the crosslinking process, it has not yet been established whether the metal ion is able to interact with more than one acrylic acid group. The small size of the ion and the number of acrylic acid groups present in the copolymer reduces the likelihood that it can behave in this manner. onventional crosslinking agents have been used in conjunction with EAA copolymers. Examples include melamineformaldehyde resins, zinc oxide and multifunctional aziridines. Properties of the polymer can be modified via selection of a suitable crosslinking agent. 6. Market Applications EAA copolymer dispersions are used in a wide range of applications owing to the versatile combination of properties they possess. entral to the success of these applications has been the consistent quality of the base polymer and, therefore, the predictive nature of its performance. The ability to manufacture dispersions with the same level of quality and consistency has also contributed to the acceptance of EAA copolymer dispersions as the workhorse in many industries. A brief discussion of some of these applications helps illustrate the truly unique character of EAA copolymer and its utility. Traditional applications for EAA copolymers often relied on its ability to laminate or heat seal two

5 dissimilar materials and provide excellent bond strength even at low coat weights. One of the more coveted properties of EAA copolymers is the ability to form strong bonds at temperatures 23 º lower than polyethylene. The savings in energy requirements and improved line speeds are a direct consequence of the copolymer s ability to heat seal at low temperatures due to its excellent hot tack. The copolymer has also served as a coating for direct metallization of paper. Related applications have seen it used to transfer metallized aluminum from a polyester film to a paper substrate. Improved adhesion, appearance and durability result. utility of EAA dispersions well beyond the strong foundation they already have secured. 8. Summary The unique combination of properties associated with ethylene and acrylic acid in copolymer dispersions has been the solution for a wide range of commercial applications. Recent developments have expanded the potential scope of these applications even more as prior limitations of the base polymer have been addressed with the development of new products with high performance characteristics. EAA copolymer is a mainstay in the packaging of medical devices owing to its excellent adhesion to Tyvek and its ability to withstand sterilization with ethylene oxide gas. The breathable nature of the film is augmented by its excellent clarity and flexibility. The wide ranging FDA and BGVV compliance associated with the copolymer is critical in such applications. The excellent melt flow properties of EAA copolymer have helped secure applications as binders for nonwovens, such as medical garments. The EAA copolymers are also used in flexible packaging to provide barrier properties and adhesion, the graphic arts industry as an ink binder and for heattransfer printing and paper sizing for improved strength and folding endurance. 7. New Developments Although the versatile character of traditional EAA copolymers has been a critical component in the wide range of applications in which it is found, it has displayed limitations in several areas. Adhesion to nonpolar substrates has been deficient and markets are constantly looking for coatings that heat seal at even lower temperatures. New applications are constantly being explored as the family of EAA dispersions become more tailored to suit specific applications. The family of EAA dispersions continues to grow as efforts to address previous limitations have resulted in new chemically modified EAA dispersions that can now impart adhesion to polypropylene, PETG and PVd. Research and development continues in this area; however, the new generation of EAA dispersions already display improved adhesion to a wide range of substrates which could possibly even eliminate the need for primers. Greater bond strengths and faster line speeds in flexible packaging due to even lower heat seal activation temperatures promise to broaden the

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