Air Gap Dynamics: Effect on Molecular Properties and Heat Seal

Peer Reviewed Technical Papers
Following is an expanded summary of a complete paper that is available on the TAPPI web site at tappi.org. On the page, click "the PLACE" in the section designated "journals."

Application: Because the molecular structure of a polyethylene coating changes during extrusion, an extrusion coating operation should expect variations in final performance.

Earlier work by this author showed the effects of dynamic factors such as heat, oxidation, shear, and time on the mechanical, physical, and processing properties of polyethylene extrusion coatings. The study used a polyethylene having a melt index of 4.3 dg/min, molecular weight of 147,000, and density of 0.924 g/cm superscript 3. It had a broad molecular weight distribution and a high degree of side chain branching.

Changing any of the dynamic factors during extrusion coating changes the molecular structure (molecular weight) of polyethylene. In the melted state (extrudate), molecular structure determines processing characteristics. In the solid state (coating), molecular structure determines the all-important performance properties of the final product contributed by the coating.

Effects of Air Gap Dynamics
In the earlier study, the effects of the dynamics in a fixed air gap of 14 cm proved surprising when analyzing free films obtained with constant extrusion conditions. Free film coating samples used line speeds of 6, 92, 185, 305, and 460 m/min at draw ratios of 1, 14, 28, 50, and 71, respectively. The resulting free film coating thickness values were 540, 36,18, 10, and 7 microns.

Analysis of molecular weight changes occurring in the air gap from thick coatings to thin coatings showed that molecular weight of a thick extrudate decreases rapidly until a thinner extrudate reaches a draw ratio of 28. It then increases until draw ratio 50. After draw rate 50 as the extrudate becomes even thinner, the molecular weight decreases rapidly.

Repeating the experiment at a colder melt temperature gave identical results—an increase in molecular weight between draw ratio values of 28 and 50. Replacing the screw having a single flight with a Maddock mixing screw gave identical results. Again, an increase in molecular weight between draw ratios 28 and 50 occurred. (The extrudate does not know what is pushing it from the die regardless of screw design.) The Maddock mixing screw degraded the molecular structure of the polyethylene more than the other screw.

What is the explanation for the observations made with the variation in air gap dynamics? Each dynamic factor in the air gap—heat, chemical, and mechanical—determines the molecular structure of the coating.

In the case of thermal effects, relatively fluid, thick coatings retain more heat at slow coating speeds. That residual heat degrades the molecular chains to lower molecular weights and different molecular structures. For chemical effects of oxidation, slightly thinner coatings reach a critical surface to volume ratio at intermediate coating speeds. More extrudate mass therefore oxidizes. This gives oxidative cross-linking to higher molecular weights and different molecular structures. With mechanical factors of shear, the thinnest coatings have lost considerable residual heat and have become viscous at the fastest coating speeds. The molecular structures in the extrudate then mechanically shear to lower molecular weights.

Stabilizer Addition
If the increase in molecular weight between draw ratios 28 and 50 is due to oxidative cross-linking, addition of an antioxidant might prevent this cross-linking in the air gap. Modification of the highly branched, low density polyethylene with 0.2% of a strong antoxidant appeared to prevent cross-linking. It eliminated the mechanical shearing of the extrudate at the fastest line speeds. Figure 1 shows that the antioxidant also prevented the polymer from degrading below a molecular weight of 120,000. The molecular weight of the unstabilized polymer degraded to a significantly lower value of 95,000. This experiment proved the oxidative cross-linking theory to be correct.

Narrow Molecular Weight Polyethylene
An investigation of the air gap dynamics of a polyethylene with a narrow molecular weight distribution used a commercially available polyethylene with melt index of 1.7 dg/min and density of 0.926 g/cm superscript 3. The molecular weight distribution was narrow, the side chain branching was minimal, and no additives were present.

Using the same extrusion conditions as with the broad molecular weight material, this unstabilized, narrow molecular weight distribution polyethylene showed no apparent cross-linking in the air gap. The fact the broad molecular weight distribution, highly branched material did cross-link while the narrow molecular weight distribution, minimally branched material did not cross-link means that oxidative cross-linking occurs primarily with highly branched polyethylene resins.

The interesting results obtained from this led to additional experiments involving air gap variations. These variations included four polyethylene resins with varying degrees of side chain branching. Coating samples obtained for each polyethylene used variations in melt temperatures, air gaps at each melt temperature, and line speeds at each air gap. This work confirmed the results noted earlier.

Conclusions
The molecular structure of polyethylene changes significantly during extrusion under any set of conditions. Molecular weights of a coating are always lower than that of the original material. The lowest molecular weight values occur with the hottest extrusion temperatures, longest residence times in the extruder, or both. Thermal energy plays the dominant role in changing the molecular structure of polyethylene. Extrusion operations must therefore consider the dynamics in the air gap that continue to change molecular structure after extrusion. Certain properties of an extrusion coated product depend on the molecular structure of the coating indicated by molecular weight.


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