Effect of Corona and Flame Treatments
Various levels of corona and flame treatments were applied to aluminum foil surfaces and then extrusion coated with LDPE. The changes in extrusion coating performance (adhesion, hot tack, and heat seal) as a function of treatment type and level were monitored. Overall, the adhesive strength between the LDPE and aluminum foil interface improved with treatments, flame and corona. Hot tack and heat seal performance were not significantly impacted as a result of surface treating the aluminum foil surfaces.
The role of acid-base interactions in LDPE/aluminum foil adhesion has been previously explored. Higher adhesive strengths can be obtained between these two materials by maximizing the basic sites on one surface and acid sites on the other.
A relatively simple technique has been developed using contact angle of acid and basic solutions on the foil surface to predict the acid-base character of the substrate. For the work presented here, flame and corona surface treatment increased the basic characteristics of the foils. This increase in surface basicity correlated to increased adhesion.
This method of measuring the relative surface acidity and basicity of foil surfaces can also be used to predict package performance for LDPE/aluminum foil structures.
Aluminum Foil, Low Density Polyethylene (LDPE), Adhesion, Hot Tack, Heat Seal, Acid-Base Interactions, Contact Angle, Extrusion Coating.
Low density polyethylene (LDPE) coated aluminum foil structures are used extensively in the food packaging industry. This structure exhibits good sealing properties and provides suitable barrier for most products; however, the adhesive strength between the foil and LDPE can sometimes be too low for many applications. Various techniques exist to overcome this shortcoming, for example, changing the relatively non-polar polyethylene surface into a polar one. These techniques include high extrusion coating temperatures and ozonation of the polymer web, which increase the surface oxidation of the polymer.(1,2)
Surface treating aluminum foils is another technique that can be used to increase the adhesive strength to polyethylene. Corona or flame treating the foils results in improved wettability which is believed to be related to burning off the retained rolling oils which oxide layer that contributes to the polar nature of aluminum foil.
The polar nature of polyethylene and aluminum foil surfaces and the importance of chemical interactions on adhesive strengths between LDPE and aluminum foil surfaces has been explored using a variety of techniques, including X-ray Photoelectron Spectroscopy and Flow-Microcalorimetry. Some of these techniques have clarified the role acid-base interactions play in strength of bonds formed between polymer-polymer and polymer-aluminum interfaces. Previous work has described a relatively simple technique using the contact angle of acidic and basic solution on the surfaces of the substrate being studied.(4) Changes in contact angles give an indication of surface acidity or basicity and also correlate to adhesive strengths between interfaces.
Surface treating aluminum foils, using varying levels of flame and corona discharge, and its subsequent effect on the acidic and basic nature of the foil surface were examined in the work reported here. The changes in relative surface acidity and basicity were then correlated to extrusion coating packaging performance; adhesion, hot tack, and heat seal.
The LDPE used in this study was LDPE 724 produced by The Dow Chemical Company, having a 0.916 g/cc density and 8 g/10 min. melt index. Samples were prepared using a laboratory extrusion coater equipped with a 3.5Ó, 301 L/D extruder set at 85 rpm (250 lbs/hr output) with a matte chill roll. The average melt temperature was 320°C (608°F) and a 6 inch air gap was maintained for the experiments. The aluminum foils were 0.009 mm thick with ÒAÓ wettability. Corona treated film and foil samples were covered with MYLAR and kept dry in a desiccator until they were measured (1-14 days).
In addition to aluminum foil adhesion, heat seal and hot tack performance was measured for the coronal and flame treated samples. Although both types treatment impacted the foil adhesion (as seen in Fig. 1) heat seal strength was no significantly affected. AS shown in Figures 4 & 5, the seal strength remained essentially the same for different levels (low, med., high) of treatment, as well as, for different types of treatment (flame vs. corona).
The hot tack performance illustrated in Figure 6 did not change as a result of corona treating the foils. However, the hot tack results for the flame treated foil samples showed slightly higher values for the low and medium treatment levels. These results are shown in Figure 7.
Aluminum adhesion improves with surface treatments.
The highest level of flame treatment produced the best overall adhesion to LDPE and the highest level of flame also generated more basic surface sites on the aluminum foil. The more basic the foil, the better the adhesion to LDPE>
Increases in surface acid site on the foil did not correlate to increase adhesion to LDPE.
Surface treating the aluminum foil did not significantly affect heat seal or hot tack performance.
Contact angle measurement using acidic and basic solutions are a viable method for predicting the adhesive performance of LDPE/aluminum foil structures.
The information in this paper is presented in good faith, but no warranty is given, nor is freedom from any patent to be inferred.
1. D. Briggs, D.M. Brewls M. B. Konieczko, European Poly. J., 14, p. 1 (1978)
2. D. Brigg, D.M. Brewis, M.B Konieczko, J. of Mat. Sci., 12 p. 429 (1977).
3. H. Gyner, TAPPI Polymers, Lamination & Coatings Conference Proceedings, p. 89, (1989).
4. M.H. Hansen, M.F> Finlayson, J.D. Goins, M. Castille, TAPPI Journal, 76, 2, p. 171 (1993).