Increased Adhesion on Web Substrates
Abstract
Adhesion is a major concern in the laminating process where chemical bonding at the interface of two substrates determines the strength of the laminate. Bond strength at this interface depends on chemical group interactions on the substrate surface to promote chelation, entanglement or covalent bonding. Chelation is critical for metal/polymer bonding while entanglement is required for polymer/polymer bonding and covalent bonding is commonly seen in ink receptive substrates. Each bonding mechanisms is specific to the material being deposited and its interaction with the substrate. Many factors control adhesion between a polymer substrate (PET, BOPP and PE) and metals, polymers and inks when a vacuum deposited primer coating is applied to the surface of the substrate. By using the vacuum technique primer chemistries can be tailored to enhance adhesion and address the specific mechanism needed within the laminate structure.
Adhesion basics
There are three main factors that determine the bonding strength of one film or substrate to another, boundary layer interactions, surface chemistry functionalization and polymer morphology.1 The physical proximity of two materials being bonded is determined by the boundary layer interactions where the distance between the two interacting materials control the strength of bonding. For effective bonding through these intermolecular or van der Waals forces the distance between materials needs to be very short ~ 0.1 to 0.5 nm and the energy associated with these bond are relatively weak. One way to maximize the effects of van der Waals forces and increase the number of interfacial contact points is to employ materials with a very smoothsurface to ensure intimate contact between the two materials.
The topic of substrate surface chemistry functionalization is very broad and can involve bonding interactions like hydrogen bonding, covalent bonding, organometallic bond formation, metal chelation complexes and entanglement interactions.Surface functionalization revolves around on the preparation of the surface to receive a bonding element through treatment with flame, corona, and plasma. All of these methods are known to populate the surface of the substrate with groups that will participate in hydrogen bonding such as hydroxyl, amine, carboxyl, amide, ether, and esters (figure 1). These methods can also act to clean, remove or react with low molecular weight materials on the surface of a substrate.
The surface of a film can also be functionalized through depositing and organic layer rich in chemical functional groups of active material specifically to promote bonding to subsequent layers referred to as a primer layer. The primer layer has two functions 1) to increase the number of active functional groups on the surface of the substrate and 2) to smooth the substrate surface and thus maximize the number of interfacial contact points between the substrate and subsequent layers.
The primer surface pretreatment is designed to chemically modify the top-most layers of a web, however improvement to cohesive forces can achieved through penetration and intermingling of the primer into the bulk substrate material. The level of penetration achieved by the primer into the substrate is determined by the crystallinity of the substrate. Amorphous (non-crystalline) polymer substrates are more susceptible to intermingling due to the availability of free polymer chains to primer materials. Crystalline polymers have less free chains available to participate in intermingling and would rely more on surface functionalization to improve adhesion.
The ideal bonding situation occurs when the activated and functionalized surface of the substrate is allowed to react and form covalent bonds between the substrate and the primer layer. This is known as grafting and is a common process in the chemical industry for the production of polymer bound proton exchange resins. Figure 2 shows how functional chemical groups can be grafted onto apolymer in the production of proton exchange resins. This same grafting technique can be employed to coatings in the vacuum by the reaction of double bond found in the acrylate coating with the functional groups on the surface of the film. During plasma treatment free radical groups are formed that can participate in the polymerization process thereby effectively grafting a primer coating on to the surface. Further cohesion properties within the primer coating can be improved through crosslinking the primer layer during the curing step.
Metal Adhesion
Metals like titanium, chromium and aluminum are oxophillic (oxygen-loving) and are capable of abstracting oxygen fromchemical functional groups like ethers, esters, carboxyl, hydroxyl and ketones.2 The oxygen abstraction products from these reactions are typically olefins and reductively coupled compounds. Reductively coupled compounds, relevant to metal vapor deposition onto surfaces rich in oxygen, result in the formation of organo-metallic, metallocenes or chelated compounds that have very high bond strength. Examples include the reaction of electropositive (electron accepting) metals that possess large negative free energies like titanium, chromium, nickel, and aluminum and the gas phase oxidation to form interfacial oxides and nitrides with imides, acrylates, halogens, carbonyls, hydroxides, ketones, amines, esters, ethers, mercaptans, and other materials that possess electronegative (electron donating) functional groups.
It should be mentioned that thecrystallinity of the substrate surface can play an additional roll metal adhesion. The orientation process during the production of PET induces areas of crystallinity in the bulk material and at the surface. Metal atoms impinging on a crystalline area will not have the same level of penetration into the substrate thereby reducing the adhesion in this area. By coving the substrate surface with an amorphous (non-crystalline) polymer film (primer) allows for uniform metal penetration and increased adhesion.3
A laminate stack is made as in figure 4 and pressed together in a laminatorat temperature set point of 175oC. After lamination the stack is allowed to sit for 2 hours before it is placed in a tensile tester and pulled at 180o at a rate of 5 mm/second (1ft/min).
The HML Tool and Process
Unlike atmospheric coating processes that require the evaporation of a solvent, the HML process delivers 100% solids in unaltered formulations to the substrate surface without diluting the concentration of the formulation components. The HML technique vaporizes all components equally and subsequently delivers them to the substrate surface without a change in the component ratio. This enables the chemist a greater range of formulation design flexibility and opportunities for custom blending of components to produce a primer tailored to the specific substrate of interest.
The HML Deposition Process
A polymer film is fed from the unwind roller onto the rotating drum, as the film is unwound it passes through a plasma treatment unit to remove adsorbed water, any low molecular weight materials as well as supplying functionality to the substrate surfaceprior to deposition of an organic coating material.
The rotating drum is cooled to a temperature that corresponds to the particular organic precursor being used to insure the condensation from vapor to liquid form. Typically, the drum is rotated at a speed equal to the film between about 0.1 to 500 meters per minute.
The organic precursor(s) is then deposited on the film via a vaporizer which is supplied with liquid precursor where the monomer liquid coating material is instantly vaporized. The vaporized precursor(s) condenses on the surface of the polymer film positioned adjacent the cooled drum where it forms a thin organic film. The condensed liquid coating material is then radiation-cured using an electron beam gun (e-gun). The e-gun directs a flow of electrons onto the organic layer of the coating material curing the material to a cross-linked film. Subsequently, a second organic layer can be deposited on top of the first layer in serial stations of the HML tool, in the same manner described above.
Conclusion
Darly HML primer coatings deposited on flexible substrates can provide a real alternative to conventional solvent and water based coating processes. The method provides custom designed primers with improved adhesion and lower costs when compared with conventional wet chemistry processes. The completely enclosed vacuum chamber is environmentally friendly, inherently clean and does not employ solvents. Coatings can be applied at high speeds with a relatively low energy consumption that is cost competitive with mature liquid based coating technologies.
References
1) Suchentrunk, R., ed., (1993)Metallizing of Plastics: A Handbook of Theory and Practice. ASM International.
2) Andrews, M.P., (1990). Reactions of Metal Atoms with Monomers and Polymers. InSacher, E., ed.,Metallization of Polymers (pp. 242-264).
3) Culbertson, E., “Metal Adhesion to PET Film” AIMCAL 2006 Technical Fall Conference.
Remarks form:Jim DiBattista, Ph.D.Darly Custom Technology 276 Addison Road Windsor, CT 06095