A Communication from the PLACE Div. of TAPPI

Published: April 30, 2006, By David J. Bentley, Jr., Contributing Editor

Providing practical information to the converting and packaging industries…

EXECUTIVE SUMMARY
Influence Of Electron Beam On Some Polymeric Substrates Used In Flexible Packaging
by Im Rangwalla, Energy Sciences, Inc.

The application of low voltage electron beam (EB) processors for curing coatings inks and adhesives in flexible packaging is quite promising. Testing to date indicates that the impact of low voltage and an EB dose of 20-30 kGy on various plastic substrates such as polyester, polyethylene, nylon, etc. used in flexible packaging is insignificant. Impact on bi-axially oriented OPP film from two manufacturers for flexible food packaging underwent thorough evaluation as a function of absorbed dose for mechanical, surface properties, and heat seal ability. The following resulted:

• Tensile Strength: Up to 10–15 % reduction at 30kGy for both OPP films but product was still functional for packaging applications.
• Slip Properties: No significant change for both films.
• Seal Strength: Reduction in seal strengths was a function of dose for both films; seal strengths still acceptable up to 25 kGy of dose.
• Hot Tack: EB dose severely impacts the hot tack performance. Proper choice of EB operating conditions is necessary to make the product functional. A narrowing of the heat seal temperature range occurs at higher doses. Choice of wide seal range films can compensate for this.

This work indicates that for OPP films the impact of EB dose is mostly on seal properties—most importantly hot tack. The impact on hot tack by absorbed dose on different OPP films is also different indicating that proper choice of skin resins and stabilizers may reduce the impact of EB dose.

Radiation induced in-situ polymerization reactions offer significant advantages over conventional thermal processes. The biggest advantage is use of 100% reactive and compliant chemistry that requires no thermal drying. Since the introduction of EB equipment in the early ’60s, polymer chemists have been intrigued by the ability of electrons to initiate free radical polymerization reactions without addition of any photoinitiators or photosensitizers. Immediate applications were sought in packaging using the free radical initiated chemistries since electron processing offered high speed curing—a mandatory requirement for packaging.

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EXECUTIVE SUMMARY
Polymer Processing Additives And Melt Fracture In Blown Film: Die Geometry Considerations
by Paul Neumann, Dyneon, LLC

This paper primarily discusses the effect of die geometry as it relates to the ability of PPAs to condition dies effectively. Many other factors affect PPA conditioning but are beyond the scope of this study. The effect of die geometry on the ability of PPAs to coat or condition the die during extrusion has been previously investigated. PPAs condition faster in narrow die gaps compared with wider die gaps indicating that the PPA coating mechanism is partly dependent on the shear rate at the die wall at least within a certain shear rate range. Capillary rheometry studies on the effect of the die land have also shown that PPA efficiency increases systematically with increasing length-to-gap ratios. The positive relationship between efficiency and land length was attributed to diminishing entrance effects with increasing land length of the die.

An analysis of the PPA conditioning response to changes in die geometry and throughput rate indicate that the time to clear melt fracture is affected by the shear rate at the die wall and the length-to-gap ratio of the die. Over the range studied, higher shear rates generally result in an improvement of PPA efficiency. Factors that contribute to higher shear rates such as narrower die gaps and increased throughput rates therefore also improve PPA efficiency. Additionally, larger L/G ratios reduce the time required to clear melt fracture. The effect of L/G becomes most significant as shear rates decrease.

These conclusions are consistent with a coating mechanism based on PPA migration towards the die wall. These results seem to contradict one logical conclusion inferred from a PPA coating mechanism that is based primarily on PPA wave migration along the die land where migration in the direction to the die wall is inconsequential. Implicit in this wave migration model is that dies with longer L/G ratios should require longer times for PPA to condition the die since migration time is a function of die length.

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EXECUTIVE SUMMARY
Adding Oxygen Barrier To Laminating Adhesives
by Jessica Bodine, Mica Corp.

The development of an extrusion coating and extrusion lamination primer that has excellent oxygen barrier properties has been reported previously. Due to chemical cross linking, the primer provides excellent adhesion when applied to a variety of corona treated base webs and then extrusion coated with properly oxidized polyolefin resins. Excellent oxygen barrier transmission rates are possible. A primary factor influencing the oxygen barrier performance is continuity of the primer coating. Previous reports have detailed the steps taken to insure complete primer coverage when using the oxygen barrier primer in extrusion laminations.

Extrusion lamination can add tremendous value to flexible packaging. The technological advancements afforded by modern extrusion laminating equipment come with a price. Extrusion lamination equipment involves a considerable capital expenditure. Extrusion coaters that incorporate co-extrusion capability and coating stations have increased the cost and complication of running the equipment. The advantage of such systems is the capability to run an endless combination of extruded resins, base webs, primers, and coatings. Because of this added value, extrusion lamination can be a very economical manufacturing choice for long runs at wide widths and high speeds.

The more conventional choice for manufacture of short run, narrow width packaging has been adhesive lamination. The properties of an adhesive lamination are limited to the properties of the two webs and what has been applied to the base web at the coating station—typically the adhesive. The oxygen barrier capabilities of laminating adhesives are limited.

When adhesive laminators heard of the oxygen barrier primer used by their extrusion coating counterparts, they rushed to incorporate the primer into their structures. Several attempts at priming the base web and then over-coating the primer with various laminating adhesives were made. The attempts met with limited success. In a few cases, either the oxygen barrier properties or the adhesion were compromised.

Using a standard direct gravure coating station to apply an oxygen barrier primer to a base film before coating with solvent free or solvent based laminating adhesives allows for the incorporation of oxygen barrier properties to standard adhesive lamination stock. The oxygen barrier transmission rates of the final structures are excellent. The adhesives and the primer do not mix. The adhesion of the structures is uncompromised by the incorporation of the primer into the structure. Further experimentation to determine if previous failures were due to improper primer application or different adhesives chemistries is necessary.

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EXECUTIVE SUMMARY
How To I

Industrial hot air dryers should provide even drying or curing in the machine and cross-machine directions. The cross-machine component is certainly the more difficult to achieve for continuous webs whether they are coated, saturated, laminated, or cured. The consequences of unbalanced drying may include edge curl, ridging, sheet breaks due to over-drying, and lower throughput due to off-specification product or slower line speed required to ensure that all parts of the web are dry.

This discussion assumes that if the wet coating weight and web weight are evenly distributed across the width of the web, uneven drying would be due to an uneven cross-machine drying profile. This condition might be due to an inherent design flaw or the result of physical changes to the dryer that occurred over time. In either case, significant improvements are possible by defining the deficiency and implementing retrofittable corrections.

A common assumption is that a poor drying profile is the result of a poor air temperature profile in the dryer. Certainly the temperature profile is a major contributor to how evenly the web will dry, but other profile variables will also impact the drying or heating process including air impingement velocity, air volume, and air humidity. The resolution of a drying profile problem should therefore recognize four key requirements necessary to affect even drying, heating, or curing:

• Creation and delivery of a homogenous mixture of air at the prescribed temperature.
• Even distribution of the air (volume and velocity) across the full width of the web.
• Delivery or impingement of the drying air should be perpendicular to the web surface.
• Removal of the “spent” air away from the web should be in a perpendicular direction.

Level and stable air temperature profiles in flotation and thru-air dryers are possible by assuring that the supply air remains thermally mixed and is delivered to the product with even cross-machine velocity and volume. Burner configuration, static mixers, diffusion plates, and distributed evacuation of the “spent” air are important factors in achieving the overall result. In new dryers with web widths up to five meters, the design criteria described in this report have yielded cross-machine temperature profiles down to ±1 °C. While existing dryers may present some constraints based on their physical structure, users should expect that the implementation of these same techniques for improving the drying profile will yield comparable results.

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For information about the PLACE Division of TAPPI, access the TAPPI web page at tappi.org. To obtain the complete papers whose expanded summaries appear in this section, go to the TAPPI web site at tappi.org., then click on "the PLACE" in the section designated Journals.

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