Cyclic-Olefinic Copolymers as Non-Migrating, Polymeric Slip Additives in LDPE Cast Films

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


Email the author(s) at: dave.constant@ticona.com


Application: Cyclic-olefinic copolymers (COC) have use in flexible packaging as a blend component or as a discreet layer in multi-layer polyolefin films. They typically enhance stiffness and heat resistance in food packaging. As a core layer in laminated or coextruded multilayer films, they provide high moisture barrier and exceptional clarity and stiffness.

Non-migrating slip additives have been of general interest to the flexible packaging community for many years. They are especially of interest in form-fill-seal operations where high coefficient of friction (COF) from “too little remaining” conventional slip additives can actually stop operations. In the worst case scenario, high COF can cause costly down time. Too low COF can cause other issues with “too high” levels of the additives causing sealing, printing issues, or both. Additive migration can also influence food product quality. Difficulties encountered in controlling “bloom rate” of various slip additives including erucamides, oleamides, and variants have driven the research for these additives. New offerings are continually undergoing investigation, and a few are commercially available. All seem to come with compromises that are generally in film clarity or cost.

The following is one synopsis of the state of the technology development for non-migratory slip:

  • Inorganic compounds

    Silica and talc essentially work by roughening the surface of a film. Used alone, they provide anti-blocking characteristics but do little to directly reduce COF.

    Glass spheres can reduce COF significantly when used in polyolefin films. They are most effective when used in thin skin layers below 1 micron. This maximizes the concentration of particles at the film surface. They can decrease COF to 0.35 – 0.45. Addition of amide slips will further reduce COF. These find use primarily in oriented polypropylene (OPP) applications.

  • Organic Compounds

    Thermoset silicone powders find use in the same way as glass spheres. They can reduce COF to the 0.25 – 0.30 range. Addition of amide slips will further reduce COF. As with glass spheres, they are most effective in thin skin layers below 1 micron. They are premium non-migratory slip materials.

    Thermoplastic silicones that are melted and dispersed throughout a polymer matrix during extrusion also provide non-migratory COF reduction. They typically find use in co-extrusion. Only the skin layers incorporate the silicone additive. Different opinions exist on the total effectiveness of these additives for COF reduction and how these additives impact seal strength and printability.



Cyclic-olefinic polymer (COC) is another candidate for non-migratory slip applications. As organic, amorphous, polymeric materials based on ethylene, COC materials can blend well with polyolefins. They generally increase modulus too and allow for downgauging. Low level dosing of COC into polyethylenes with the use of “lower” temperature processing conditions can make COC materials viable alternatives for COF reduction.

COC materials may or may not be “miscible” in LDPE depending on the Tg of the COC. As a general rule for miscible polymeric blends, the Tg or µ relaxation of the blend varies monotonically (shifts) between the Tg values of the blend components. This is generally evident in LDPE blends with “lower” Tg COCs. If both materials exhibit independent and separate Tg values and exist as separate microphases, they are immiscible. This is more prevalent in LDPE blends with “higher” Tg COCs.

COC materials do blend well in a practical sense with many polyethylenes. With proper melt temperature selection, they process easily with these polyethylenes and do not delaminate in final film structures. Domains of COC materials are generally well distributed as a minor phase within a LDPE matrix.

The COC materials are relatively “clear” fillers in a sense with domain size affecting both surface haze and COF of a film. Larger domain size equates to increased surface roughness, decreased film clarity, and decreased COF. Melt process temperature influences domain size with lower temperature providing a larger domain size.

Experimental
The experimental work for this study examined two different COC materials in LDPE blends using a 25-mm extruder and evaluating the films produced for COF. The cast film samples of polyolefin/COC blends also underwent examination by various microscopy techniques to assess miscibility and surface “smoothness” of the two component blends. The samples were embedded in epoxy and microtomed to produce thin cross sections approximately 4 mm thick. The thin section samples underwent examination by an optical microscope using phase contrast mode. The cross sectional surfaces were then etched with chloroform for two minutes and examined using scanning electron microscopy (SEM). Contact mode atomic force microscopy (AFM) was also performed on the film surfaces as-received and on etched film samples using a commercially available instrument. Measurement of surface roughness used commercially available software.

A commercially available dynamic mechanic analyzer (DMA) determined the dynamic moduli. Dynamic mechanical measurements used film fixtures in a tension mode (E’, E’’) as a function of temperature at constant frequency of oscillation. The mechanical relaxations of the materials were investigated by running temperature sweeps at constant frequency of oscillation and constant amplitude. The temperature was scanned at a rate of 3°C/min using a frequency of 10 Hz. The amplitude of oscillation was set to 30 mm; the amplitude was chosen as a compromise between the upper force limit of the instrument and a good signal-to-noise ratio. The film specimens were cut into strips of width 12 mm and length approximately 0.37 mm.

Results
Addition of five to ten percent COC as a pellet blend to LDPE can reduce film to film dynamic COF by a factor of 2 with absolute values decreasing from approximately 0.6 to approximately 0.3. To accomplish this, cast film process melt temperature must be “low.” This temperature is a maximum of approximately 180°C for an 80°C Tg COC and approximately 240°C for a 136°C Tg COC. Glass transition temperature of the COC also has a significant effect on COF of the blends. Higher Tg promotes film surface roughness at a “higher” process melt temperature than does lower Tg COC. DMA data suggests at least “partial” miscibility between 12 MI LDPE and the 70°C Tg COC where an intermediate transition appears at 77°C. Phase constrast optical microscopy shows that domain size of COC in the blend increases with decreased process temperature. AFM images confirm increased surface roughness. Increased Haze also coincides with decreased COF.

Viscosity of the matrix polyethylene plays a smaller role at least within the range of the LDPE materials tested. It does influence the imposed and resultant melt temperature to process the blend into an acceptable quality cast film. Higher temperatures typically observed for lower MI LDPE materials decrease domain sizes that in turn negatively influence COF data.

Density of the LDPE matrix does not affect COF within the range of densities investigated. Initial internal data generated independently from this work indicates that COC materials lose effectiveness as a slip additive in very soft, very low density linear polyethylenes. Increased COC level within the range investigated generally decreases COF in LDPE cast film but not to commercially acceptable levels. It also increases haze in the LDPE film at these levels.

Conclusions
Cyclic olefinic copolymers can reduce COF. to commercially acceptable levels in cast LDPE films. Pellet blend levels in the range of five to ten percent COC by weight can achieve COF values in the 0.2–0.4 range. Resulting films are medium slip films useful for form-fill-seal operations. Process melt temperature for the blends and Tg of the COC used are both significant factors affecting COF. A process melt temperature of approximately 180°C maximum is necessary for blends containing a 80°C Tg COC, and a melt temperature of approximately 240°C maximum is necessary for blends containing a 136°C Tg COC.

Use of these lower temperatures does adversely affect haze percentage but should not affect printability. Use of the lower Tg COC should not significantly affect sealing properties. COC materials are viable alternatives as non-migratory slip additives when processed at relatively low temperatures.



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