The Case Against Oxidation as a Primary Factor for Bonding Acid Copolymers to Foil

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Application: Acid copolymers will generally adhere more strongly to foil than low density polyethylene even when processing the acid copolymer at lower temperatures. This reduces the potential for taste, odor, processing, and other problems associated with high temperature processing.

Extrusion coating is application of a curtain of molten polymer onto a substrate at high speeds. Contact with a chill roll quickly quenches the polymer. Under such conditions, obtaining good adhesion between the polymer and substrate is often difficult. One method to enhance adhesion that the extrusion coating industry practices widely is oxidation of the melt curtain by subjecting it to long exposure times of about 100 milliseconds at high processing temperatures above 280øC. Oxidation creates polar species that improve wetting of the polymer and its bonding to the substrate. This is probably the predominate mechanism for bonding low density polyethylene (LDPE) to foil. A disadvantage of this method is the oxidation of low molecular weight species that impart off-taste and odor to a food product or package headspace. The high temperatures necessary to oxidize a polymer also contribute to off-gassing, smoke, molecular weight changes—crosslinking and chain scission, thermal degradation, gel formation, and impaired heat sealing performance. The high processing temperatures can also lead to adhesion and crazing issues when coating onto metallized substrates.

Another method for improving adhesion to foil and other polar substrates in extrusion coating is the use of specialty polymers such as acid copolymers and ionomers. These polymers typically contain 3%-12% methacrylic or acrylic acid that has directly copolymerized with ethylene in the polymer chain. The acid groups provide adhesion sites for bonding to polar substrates such as aluminum foil, paper, and various primed surfaces.

While oxidation does occur, new evidence presented in this paper suggests that it is not the primary mechanism for the adhesion of acid copolymers to foil. An advantage of using an acid copolymer or ionomer is that it adheres to foil at lower processing temperatures than LDPE. This reduces the potential for odor and taste problems and other issues.

In their experiments, the authors examined the following for the effect on peel strength to foil:

  • Ozone treatment
  • Processing temperatures
  • Antioxidant
  • Orientation

Practical Implications
Acid copolymers generally have higher adhesion than low density polyethylene to aluminum foil and other polar substrates such as metallized films, primed films, and paper. They therefore often have use as tie layers for bonding LDPE to foil. The experiments by the authors suggest that oxidation is not necessary for adhering acid copolymers or ionomers to polar substrates unlike LDPE. This allows acid copolymers to run colder than LDPE and still bond to foil. Acid copolymers are typically processed at about 260øC-305øC vs. 300øC-330øC for LDPE. Some grades of acid copolymer run at even lower temperatures. By running colder, the converter reduces some problems associated with high temperature processing. In a coextrusion with LDPE, acid copolymers allow the LDPE to run colder. This reduces off-taste and odor problems associated with oxidation and thermal degradation of the polyethylene. Reduction in off-gassing, smoking, and gel problems may also result. Acid copolymers therefore have several advantages over LDPE in performance and processing. When used as sealants, acid copolymers also have lower seal initiation temperatures and better hot tack. Acid copolymers have their own special considerations. For example, they are corrosive. Most reliable equipment suppliers understand the necessary protection techniques.

The adhesion of acid copolymers and ionomers to foil is better than LDPE in the presence of aggressive products such as ketchup, orange juice, and various personal care products. Acid copolymers therefore have use in sachet and liquid packaging applications. The durability of the adhesion generally relates to the acid content of the polymer and its thickness. A higher acid level and greater thickness give better durability. To determine if processing conditions such as time in the air gap influence adhesion durability, the authors subjected the structures made during the antioxidant experiment to an accelerated aging test in acetic acid. Pouches made from the structures were filled with a 3% acetic acid solution. The pouches were stored at 40øC and periodically sacrificed for peel strength measurements. When the peel strength fell below 200g/25mm or the pouches began to swell from off-gases due to a reaction between the acid and foil, the sample was a failure. LDPE structures in this test typically fail within a day or two. Acid copolymers usually last 4-14 days depending on the acid level and thickness.

While oxidation is important, the results of four experiments suggested that it is not the primary factor controlling the peel strength of high acid copolymers to foil:

  • Ozone treatment of acid copolymers did not substantially influence peel strength.
  • Adding an antioxidant to an acid copolymer still produced good adhesion. The peel strength increased with increasing time in the air gap, but the surface oxygen did not.
  • Processing an acid copolymer well below the expected onset of oxidation effectively hindered the formation of surface oxidative species but did not significantly influence its peel strength performance.
  • Peel strength measured in the transverse direction did not show the same increase as peel strength measured in the machine direction despite both undergoing the same oxidation history. Instead, the results correlated well with the percentage of elongation of the coating. This suggests that stress and orientation may play a role.

Oxidation of LDPE and acid copolymers occurs under typical extrusion coating processing conditions. This is important particularly for LDPE and low acid copolymers. For LDPE, oxidation is the primary mechanism for adhesion to polar substrates. At high levels of acid, oxidation is less important for adhesion given the already dominant presence of acid groups on the polymer chain. The significance of oxidation as a mechanism for improved adhesion therefore decreases with increasing acid functionality built into the polymer chain. The effects of stress and orientation identified in these studies of high acid copolymers may also play a secondary role in the adhesion of LDPE and low acid copolymers.

In practice, acid copolymers can run substantially colder than LDPE. This reduces potential organoleptic and other issues associated with high temperature processing. Acid copolymers often have use in demanding packages involving aggressive food products such as ketchup and other acidic sauces. The adhesion durability of acid copolymers in a simulated aggressive environment did not depend on processing conditions such as time in the air gap. This supports previous work showing acid level and thickness have primary importance.

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