Specially Designed Metallocene Copolymers for Extrusion Coating
- Published: March 01, 2001, By Jim Cooper and Anna Perez Moreno, The Dow Chemical Co.
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Application: New metallocene polymers have equal and sometimes better ultimate hot tack and high temperature hot tack compared with current acid copolymers and ionomers.
Metallocene copolymers produced by single site catalyst systems have made significant inroads into extrusion coating and laminating applications. A performance attribute previously lacking for these polymer systems was hot tack — especially ultimate hot tack strength and high temperature hot tack strength. Historically, acid copolymers and ionomers have provided the best options for hot tack but with significant economical disadvantages. Polyolefin plastomers (POP) that are metallocene polymers are now available with equal and sometimes better ultimate hot tack and high temperature hot tack compared with current acid copolymers and ionomers.
This work compared POP with low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ethylene acrylic acid (EAA) copolymer, ethylene vinyl acetate (EVA) copolymer, and ionomer. Evaluation of all resins used a pilot extrusion coating line. Single layer coating evaluations with the primary extruder used 50 lb kraft paper. Also, 2 mil cast film samples produced on a pilot cast film line underwent testing for physical and organoleptic properties. Other testing included hot tack and heat seal; dart, puncture, and tear; seal through contamination; adhesion; and environmental stress crack resistance.
Hot Tack
[Tests] show the hot tack strength measured for 1 mil coatings on 50 lb kraft paper. Note that POP 1 has significantly higher ultimate hot tack and broader hot tack strength compared with all other polymers tested. The ultimate hot tack of POP 1 is approximately 50% higher than the acid copolymer or ionomer materials. The EAA, ionomer, and POP 1 are best for high temperature hot tack performance. POP 1 would be an excellent choice for a broad hot tack operating window. Although POP 1 does not have ionic bonding, it still provides excellent high temperature hot tack strength.
Economics can also play a role in determining the best sealant to achieve acceptable hot tack strength. Since POP 1 gives as much as 50% improvement in hot tack strength, a reduction of sealant thickness might give the same ultimate hot tack strength. With the significantly lower cost of POP vs ionomers and acid copolymers, this could result in a major cost advantage to a converter. The significant density difference between ionomers and POP might also result in cost savings even at the same coating thickness.
Other Properties
When testing for heat seal strength, all polymers had comparable heat seal strength except LDPE that was lower. Tear strength is an example of what these new polymers can provide in packaging structures. The tear strength imparted to substrates such as paper is a significant improvement over the other polymers tested here.
Dart impact for the POP is better than any other polymers tested. Puncture resistance was similar. Tear tests for the cast films show that the POP had better results than all polymers except LLDPE. POP had the highest hot tack when sealing through flour contamination.
POP also imparts the least amount of off-flavor and off-odor due to its inherent narrow molecular weight distribution. Another key factor is the lack of any acid functionality, and POP has better adhesion and environmental stress crack resistance over the other polymers tested.
Conclusion
The development of new POP has resulted in extrusion coatings that provide the unique combination of high ultimate hot tack and a broad hot tack window. They also offer other improved performance attributes associated with POP and LLDPE coating grades such as toughness, tear strength, puncture resistance, and environmental stress crack resistance. Additional developments are underway to improve these polymers further due to new FDA food contact regulations. These allow resin designers to lower the density of the POP even further. This will result in lower hot tack and heat seal initiation temperatures and thus broaden the hot tack and heat seal window. The lower density sealants will also increase sealing speed capabilities for form, fill, and seal equipment.
