- September 01, 2003, Günter Schubert, Hydro Aluminum Deutschland GmbH
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Application: The composition and chemistry of an aluminum foil surface are important to achieve good wettability, bond formation, and durable adhesion on this substrate.
Aluminum foil production involves cold rolling strip in subsequent rolling steps to reduce the thickness by almost half in each step. The strip for cold rolling can originate from continuously cast ingots that have been hot rolled to a thin strip or from continuously cast strip in a thickness of approximately 2-4 mm. The solidification of the metal melt during ingot casting occurs slowly compared with quenching during strip casting. In principle, precipitations become finer and more homogeneous in strip casting. Some special foils or foil alloys exhibiting extraordinarily high tensile strength or elongation are manufactured using continuously cast strip. In general, the performance properties of the two types of foil are very high making them equivalent or interchangeable for most applications.
For a final foil thickness below 60 µm, two layers of foil are usually wound together before the last rolling step. Rolling oil is sprayed between the two layers as a release agent, and the “twin foil” is rolled to the desired final thickness. After the final rolling step, the two foil webs are separated, slit, and wound to the desired length and width. In the form of a coil, most hard foil undergoes annealing to recrystallize the grain structure and remove the rolling oil.
Besides these tasks, the main purpose of annealing is to generate a surface that has good and even wettability with high chemical resistance and provides reliable adhesion properties. Maintaining good unwinding behavior especially for thin foil is essential to avoid uncontrolled oxide growth that can lead to the foil sticking together during unwinding.
After rolling, a continuous amorphous oxide layer forms immediately due to the reaction between oxygen and humidity from the environment and the freshly generated metal surface. During annealing at temperatures of approximately 300°C, this oxide grows thicker due to elevated diffusion of oxygen through the oxide and reactivity with the metal in the oven atmosphere. The oxide growth occurs at the interface between oxide and metal. The oxide on top changes during annealing accompanied by a loss of water leading to a more compact oxide of generally higher resistance.
Since mechanical strength is often an important consideration, alloys with a specific grain structure and distribution and number and size of precipitations are necessary. Some of these precipitations from alloying that are necessary to achieve the desired mechanical strength disrupt the continuous oxide layer if they are in the surface. Pure aluminum foil has the best oxide layer integrity without defects. This provides the best resistance but insufficient mechanical strength.
Wetting, Spreading, and Adhesion
To show the different behavior of a foil web with different oxide thickness and wettability between the middle and the edges, laboratory work used a foil that was very thick (15 nm) at the edges with a thinner (5 nm) oxide in the middle — both perfectly wettable. Ethylene acrylic acid (EAA) films with 6% acrylic acid were melt coated onto this foil in a laboratory oven at 200°C by continuously laying a molten film on the foil as in slow motion extrusion coating. Complete elimination of air entrapment was not possible, but the number and size of bubbles formed between the metal and the coating showed significant differences. At the edges, bubbles were less numerous but some were very large. In the middle, more and smaller bubbles were visible. This indicates that the attraction between melt and oxide is higher in the middle — spreading occurs faster and less selectively. At the edges, the reactivity is lower and more selective. Although wetting of the melt on the foil occurs at a slow speed compared with extrusion coating, adhesion levels show the same tendency as in actual extrusion coating. This type of foil with an extraordinary thick oxide layer leads to lower adhesion to an EAA resin at the edges than in the middle. In the laboratory coating, the EAA adhered inseparably to the foil in the middle of the web but was easily peeled at the edges. In practice, normal coils do not exhibit such thick oxide layers at the edges meaning that effects on adhesion are less extreme.
Extremely high wettability is not really necessary for good extrusion coating and can do more harm than good as Fig. 1 shows. The reason for this is the presence of water at the edges of higher wettability foil as indicated by infra-red analysis. The predominantly non-polar character of a polymer melt leads to partial repelling from the highly polar surface and therefore contributes less to adhesion. The functional acid groups might not react to form soap bonds but partially only form hydrogen bonds. The true reason in this case is unknown.
Stand-up pouches usually use a two-component polyester-polyurethane laminating adhesive. During retorting of filled pouches, water vapor can pass through the plastic layers. Around undesirably coarse precipitations in the surface, the aluminum can react continually with water-generating hydrogen and forming bubbles. This oxide growth finally leads to adhesion failure.
Retorting does not always adversely affect adhesion. Sometimes it has a positive impact. Silanol groups of silane-functionalized adhesives can react progressively with a hydroxylated oxide surface. In Fig. 2, the hydroxylation of the oxide is substantial because only the hydroxylated oxide reacts entirely with the silanol groups.
When discussing extrusion coating adhesion on polyethylene, the degree of oxidation of the adhering film is primarily responsible for adhesion. The time in the air gap, the draw ratio, the temperature, and the character of the plastic all have an influence. Since functional groups do not completely cover the adhering interface of the plastic, other interactions also play a role — hydrogen bonds or Van der Waals interactions, weaker, but acting on a larger portion of the interface. Internal stresses in the coatings from melt quenching affect adhesion adversely.
Polypropylene extrusion coatings on aluminum that carry acid groups commonly disguised as maleic anhydride adhere well when internal stresses are decreased and additional bonds are formed. Reheating such coatings after an extrusion coating step eliminates these stresses and allows the coating to form additional bonds and makes them sufficiently resistant to withstand retorting even at 130°C for 30 minutes.
Adhesion between metal and coating also occurs as a result of soap formation. The best adhesion is reached when each bonding aluminum atom carries a single soap group. Even at room temperature, acetic acid can easily pass through a coating infiltrating chemical bonds by advanced attack and leading to adhesion failure. Note that particular components of package contents such as fatty acids can also enter into a coating. During pasteurization or steam retorting, fatty acids can react progressively with the upper oxide layer. The degree of saponification increases and adversely influences adhesion finally leading to adhesion failure. This occurs by opening and replacing chemical bonds in the oxide by saponification in the second and third stage as Fig. 3 shows. The coating is then no longer chemically grafted onto the metal. Infrared spectra of metal coating interfaces exhibit the presence of aluminum carboxylates in the case of adhesion failure.
This is not a hopeless case because such specific migration problems can be overcome by choosing the most appropriate coating resin. Improvements to the fat barrier of the coating may be sufficient to solve the problem associated with the fatty components.
Efficient wetting and well-chosen adhesives or tie-resins may not be sufficient to achieve and ensure effective and robust adhesion. Since the surface of aluminum acts as a barrier in a laminate, absorbants or migrating matter such as moisture or fat can have an influence on adhesion. They can accumulate in front of the barrier or can react at the interface to the aluminum-like fatty acids at elevated temperature. Adhesion failure occurs as a result of the replacement or the infiltration of interfacial bonds. Demanding foil applications require particular attention to the proper functional barrier in the plastic coating when initially designing the laminate.