- August 01, 2003, Richard W. Halle and David M. Simpson, ExxonMobil Chemical Co.
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Application: Using metallocene catalyst technology provides an enhanced polyethylene with excellent heat sealing performance in extrusion-coated and extrusion-laminated constructions.
The unique attributes of polyethylene (PE) materials made using metallocene catalysts are well known and have been commercially available to film converters since 1991. Many well-established metallocene PE families now exist, and new polymers and families are regularly entering the market place. These families use different metallocene catalyst and reactor combinations, and each family possesses a unique set of performance characteristics. These PEs have many different generic names — metallocene PEs, plastomers, mPEs, mLLDPEs, mVLDPEs, metallocenes, etc.
Two early metallocene families were plastomers with densities below 0.910 g/cc. Several varieties of plastomers now have use in many high performance packaging film applications. The unique properties of plastomers have led to their use as heat seal or toughness layers especially in coextruded barrier films or laminations, fresh produce packaging, and batch inclusion bag applications.
Much product development effort involving metallocene PEs has focused on blown or cast film applications where linear PE materials find common use and extrusion equipment can handle LLDPE-type rheological behavior. Although the superior properties of metallocene PEs were thought equally valuable in extrusion-coated and extrusion-laminated structures, extrusion coating was not initially a focus of development. This was primarily because most extrusion coating equipment processes high pressure polyolefins such as LDPE homopolymers, EVA, EMA, ionomers, etc. With this equipment, processing of metallocene PEs that are linear PEs with relatively narrow molecular weight distributions could be a greater challenge to commercialize compared with a conventional LLDPE or VLDPE replacement in a blown film application.
The advent of metallocene plastomers has opened new windows of opportunity for designing very high performance packaging structures especially in the areas of heat sealing and toughness. Several technical presentations have presented data showing how metallocene plastomers can provide superior performance in extrusion-coated and extrusion-laminated structures. An extensive comparison of plastomers with LDPE, EVA, EMA, EnBA, and ionomers identified the attributes that provided the greatest value from each product family. When processing on extrusion coating equipment, blends of these plastomers with HP-LDPE polymers have unexpectedly good draw-down, neck-in, and draw-resonance performance. Recently an extrusion coating study demonstrated that a higher melt index mLLDPE could produce extrusion-coated and extrusion-laminated constructions with improved properties.
A new enhanced PE created with metallocene-catalyst technology is now available for extrusion coating and laminating applications. It offers high hot tack strength and low heat seal initiation temperatures with good puncture and tear resistance. The initial extrusion coating evaluation of this new enhanced PE used several stages of which this paper describes three:
- Determine the effect of LDPE blend level on processing and properties.
- Compare the extrusion coating processing and properties of this enhanced PE to a conventional LLDPE extrusion coating resin and to plastomers currently used in extrusion coating applications.
- Investigate adhesion to OPP substrates.
Extrusion Coating onto Paper
The first extrusion coating evaluation of the new enhanced PE focused on determining the impact of LDPE level on processing and comparing the performance of the enhanced PE with that of commercial extrusion coating LLDPE and selected plastomers.
Production of all structures used a pilot extrusion coating and laminating line. Monolayer extrusion coatings were generated onto 70 lb. kraft paper corona treated at 10 KW before coating. Target coating weights were 15, 25 and 50 g/m2. Resin extrudability was assessed by measuring motor load, pressure, and melt temperatures at 25, 50, and 150 rpm. Resin neck-in was determined at 25 rpm output onto untreated paper at 25, 50, 100, and 200 meters/min line speed. Maximum draw-down was measured by extruding at 25 rpm onto untreated paper and increasing line speed at 10 meters/s acceleration. Evaluation of selected film properties used the coated paper samples.
Although neck-in would normally be unacceptably high when extruding 100% enhanced PE, the first part of this work included 100% enhanced PE and extrusion coating LDPE with four blends to evaluate the processing characteristics of these materials. Previous extrusion coating studies conducted while developing specific metallocene plastomer grades for extrusion coating and laminating applications showed that some level of LDPE was necessary to achieve an acceptable balance of neck-in and draw-down. For plastomers, the minimum level of LDPE was approximately 20% LDPE for typical monolayer extrusion coating applications.
Heat sealing performance is the key attribute of most extrusion coated structures. Adding any amount of enhanced PE to LDPE produces a significant improvement in the hot tack of a structure. Increasing the amount of enhanced PE further improves the sealing performance. Heat seal curves also showed that the seal strength improves as enhanced PE level increases in blends although the improvement is not to the same degree as the hot tack improvement. To summarize the blend results, the addition of enhanced PE to LDPE will significantly improve properties of coated paper structures. The properties continue to improve as the level of enhanced PE increases in a blend. As long as approximately 20% LDPE is in the enhanced PE extrusion coating, processing is more than acceptable.
The extrusion coating performance of the five polymers studied were compared by coating onto paper. The enhanced PE, plastomer-1, and plastomer-2 were blended with 20% LDPE to reduce neck-in. Plastomer-2 contains a very small amount of long chain branching, but the neck-in of 100% plastomer-2 was unacceptably high at the extrusion conditions used. It therefore required 20% LDPE addition. Characterization of the LLDPE indicated that it might already contain LDPE so no additional LDPE was added.
The two plastomer blends and the enhanced PE blend have very similar neck-in performance that is significantly better than the neck-in performance of the LLDPE at higher line speeds. The polymers with higher melt indices could be drawn down further as one would expect. Figure 1 shows the motor load and extruder head pressure comparison. As with the draw-down results, the motor load and pressure data correlates well with the melt indices of the polymers except the LDPE that shear thins to a greater degree than the other four polymers tested.
The property results clearly show why the use of the enhanced PE would improve the performance of extrusion coated structures. The Elmendorf tear results indicate that the enhanced PE is most similar to specialty plastomers having more than double the tear strength of the LDPE. The puncture results indicate that the plastomers have better puncture performance. The very high “coat-up” puncture energy of plastomer-2 may have been due to its lower adhesion to the substrate even though extruded under identical conditions as the other polymers.
The hot tack comparison emphasizes the benefits of using enhanced PE. Only plastomer-1 outperformed the enhanced PE. The hot tack initiation temperature of the enhanced PE was approximately 15 degrees lower than that of LLDPE and over 5 degrees lower than LDPE. The heat seal curves again confirm the excellent heat sealing performance obtained using enhanced PE. Only plastomer-1 had a lower seal initiation temperature. The polymer comparison demonstrated that enhanced PE had extrusion coating performance near that of specialty plastomers and far superior to LLDPE or LDPE.
Extrusion Coating onto OPP
A second set of extrusion coating trials determined the bonding to unprimed OPP films. Plastomers exhibit excellent adhesion to OPP, and enhanced PE should have similar performance. In addition to the polymers evaluated in the paper coating study, EVA was part of this comparison. This work used the same equipment as the study for the paper coating work. Monolayer coatings were applied to the unprimed OPP surface of a commercially available aluminum coated OPP structure. The average thickness of the substrate was 1.45 mil. The OPP surface was corona treated at 10 kW before coating at target coating weights of 15, 25, and 50 g/m2. Since adhesion normally increases as the extrusion temperature increases, a range of extrusion temperatures was evaluated. The polymers were extruded at melt temperatures between 295-315°C with the exception of the EVA at 280°C. Adhesion to OPP was determined by running peel tests on 15 mm wide strips of the 50 g coat weight samples.
Figure 2 shows the peel strength results. The enhanced PE and the three plastomer samples could not be separated from the OPP substrate. The LLDPE, LDPE, and the EVA had relatively low peel strengths. Unlike the coatings onto paper, Fig. 3 shows little differentiation in puncture performance. This would indicate that the substrate has a dominant effect on mechanical properties in this structure. The extrusion temperature had a significant impact on hot tack performance. Detailed analysis of this phenomenon will be the subject of a future paper.
A new enhanced PE made with metallocene catalyst technology produces extrusion-coated and extrusion-laminated constructions with excellent heat sealing performance. The coating performance of enhanced PE was more than acceptable when a small amount of HP-LDPE was added to adjust the draw-down/neck-in balance. Excellent adhesion to OPP makes enhanced PE the preferable choice for many extrusion-coated packaging structures.