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The Influence of Aluminum Gauge on the Adhesion of Extruded Polyester to Foil Using Adheisve Primers

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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: Adhesion primers are effective when the thickness of polyester resin extrudate to aluminum substrate thickness exceeds a ratio of 0.7.

Unique properties of poly(ethylene terephthlate or PET) compared with polyethylene (PE) make it useful in extrusion coating applications. For example, the higher stiffness and use temperature of PET provide an alternative to foil in packaging structures. The high gloss, good chemical resistance, and good organoleptic properties of PET make it a possible enamel replacement in metal packages.

Since extruded PET does not adhere well to metals, we started to develop an adhesive primer to bond the materials. When we develop a new adhesive primer, typically, we characterize it in the laboratory for bonding latitude by generating a time and temperature heat seal profile. Bond strength usually improves with increase in time or temperature. Eventually, the ultimate bond strength occurs. It does not increase with additional heat or may even decrease due to material degradation. Such laboratory data for a PE primer correlate well with production. A similar heat seal profile for a developmental primer designed to adhere PET to paper and aluminum substrates looks similar to that for the PE primer, except the bond strengths of the PET primer are higher at equivalent heat seal temperatures or dwell times.

Although the adhesion of PET to primed substrate was excellent in the laboratory, considerable bond strength variation occurred on an aluminum substrate during trials on extrusion coating equipment. The bonds were excellent sometimes; no bonding occurred at other times. Excellent agreement between the laboratory and production equipment occurred with paper substrates. We hypothesized that a critical factor for adhesion to aluminum is the gauge of aluminum, since it can influence cooling of PET melt. This paper estimates the temperature equilibration rate and equilibrium temperature in a PET to aluminum structure. It also compares the resulting interfacial temperature to the phase transition temperatures of PET and draws a conclusion about the effect on adhesion.

Rate of Temperature Equilibration
Previous work shows that temperature in a PE extrudate and paper substrate varied across the thickness of the PE/paper structure at 0 and 1 milleseconds after contact between layers in the extrusion coating process. When using primers at the polymer/substrate interface, the temperature and time at this interface should be as high as possible for maximum bonding because heat can have a dramatic effect on bond strength.

In a PET/aluminum structure, the flow of heat from the melt to the substrate is much faster than with a paper substrate. To estimate the rate of temperature change with time relates to thermal diffusivity. Estimating these values for aluminum and paper shows the thermal diffusivity for aluminum is about 100 times higher than paper. This means that a melt/aluminum interface cools in microseconds rather than in milliseconds for a paper substrate. Like the cooling from a chill roll on the other side of the melt, this is essentially instantaneous.

We did no modeling to predict how melt temperature changes over time through the thickness of a structure for this paper. We expect that the temperature drop at a PET/aluminum interface is higher for thick aluminum than for thin aluminum because the mass is higher. As the aluminim becomes infinitely thick, the profile should mimic that of the chill roll side.

Equilibrium Temperature at PET/Aluminum Interface
The preceeding text described the rate temperature change in a PET/aluminum structure. We wanted to estimate the temperature at the interface after heating equilibration.

First, we assumed the chill roll absorbed half the heat from the PET. The aluminum absorbed the remaining half. Assuming an adiabatic system, the aluminum would absorb any heat lost by the PET until the temperatures of the PET and aluminum were equal.

We made a calculation for several PET and aluminum thickness values and plotted the resultant equilibrium temperature at the interface. As expected, our calculations show the temperature at equilibrium increases as PET melt thickness increases or the gauge of the aluminum foil decreases.

PET Transitions
We examined the thermal transitions of PET to see if any transitions from room temperature to the melt temperature existed. The fastest crystallization occurs from 150°C to 190°C. The half time of crystalliation in this temperature rate is about one minute. This is many order of magnitude slower than temperature equilibration time in a PET/aluminum structure. Therefore, we believe that little if any crystallization occurs at a PET/aluminum interface.

The glass transition temperature (Tg) of PET is about 80°C to 90°C. This is important for bonding. Above this value, the material is rubbery and can flow. Below the value, little motion of the molecules occurs. The polymer is glassy and will not flow.

To our plotted chart, we added the Tg of the PET to the curves we previously plotted. We noted the equilibrium temperature of the PET at the interface may actually be below the Tg of the PET. When this occurs, little molecular motion exists in the polymer chains. Polymer to polymer mixing with a primer is difficult; therefore, adhesion will be poor. This phenomenon probably accounts for variation in adhesive bonding of PET to aluminum.

We also added the Tg for PE. It will always be below the equilibrium temperature because the equilibrium temperature can never be lower than the temperature of the aluminum substrate before it contacts the melt.

For good adhesion, the PET thickness to aluminum thickness must be greater than a ratio of about 0.7. Having a PET temperature above the Tg is necessary but not sufficient for adhesion. A polymer primer that bonds to the aluminum and PET also is necessary on the aluminum surface as a tie layer.

Transfer of heat from melt to aluminum is more critical in PET than PE because of the Tg of PET lies between the aluminum and melt temperatures. In PE, the Tg is well below the aluminum temperature. Therefore, no transitions interfere with adhesion. Primes can adhere well if the PET thickness to aluminum exceeds a ratio of 0.7.

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