LLDPE Heat Seal Enhancement via COC Addition
The addition of TOPAS® COC 8007 to LLDPE film sealant layers increases the film’s hot-tack and its heat seal strength. When blending 8007 to a metallocene catalyzed plastomer resin a sharp increase in hot tack strength was obtained without reducing the seal initiation temperature. Additionally, using TOPAS® COC 5013 in the exterior layer of a polyethylene film enables using higher seal bar temperatures without having polyethylene stick to the seal bar. These improvements make TOPAS® COC a vital part of recyclable film solutions.
Heat Seal Strength and Hot-Tack Improvement
TOPAS® COC 8007 was added to a Ziegler-Natta LLDPE, a metallocene polyethylene plastomer, and a tubular reactor LDPE in increasing levels. The blends of LLDPE and TOPAS® COC 8007 had a steady increase in seal strength with each loading level. The blends of plastomer and TOPAS® COC 8007 resulted in seal strengths that slightly declined as the COC loading increased. The hot-tack sharply increased in both the LLDPE and the plastomer films when COC was added. The seal strength of LDPE and TOPAS blends decreased by 80% compared to the pure LDPE resin.
The addition of TOPAS® COC 8007 to the Ziegler-Natta LLDPE resulted in a slight decrease of seal strength at the 20% loading level, but then steady increases as the amount of COC in the blend increased. The increase in seal strength observed in LLDPE may be in part because Ziegler-Natta LLDPE’s do not have the high strength properties of metallocene catalyzed resins, as can be observed in the plastomer resin. The hot-tack improvement observed is likely because the high glass transition temperature and amorphous nature of TOPAS® COC enables rapid freezing of polymer chains.
As noted for the Ziegler-Natta LLDPE, the seal strength of the metallocene plastomer is already quite high, approximately 43% higher than the Ziegler-Natta LLDPE seal strength. The blending of the plastomer and TOPAS® COC 8007 resulted in seal strengths that remained steady or slightly decreased through a 60% loading of the COC resin. The hot-tack results achieved from blending 15% TOPAS® COC into the plastomer resulted in a 3.9x increase compared to the neat plastomer film. Additionally, the seal initiation temperature was constant at 102°C for the neat plastomer film and for blends up to and including 60% TOPAS® COC 8007.
Blends of LDPE and TOPAS® COC are generally not recommended, and the data in Figure 3 highlights why. The one bright spot could be that such blends could possibly be used as an easy peel formulation.
Seal Bar Heat Resistance
TOPAS® COC 5013F-04 was added to Ziegler-Natta LLDPE and sealed at increasing temperatures to observe when the film would initially stick to the seal bar and when the seal bar begins to be contaminated by picked off polyethylene. The result is that adding just 15% 5013F-04 into the LLDPE increased the initial sticking temperature from 105°C to 170°C. This data makes a compelling case for using 5013F-04 to improve heat resistance when combined with the data in Figures 1 and 2 where the seals were made with a seal bar temperature of 120°C. Figure 4, below, has the heat seal resistance data.
Conclusion
The study demonstrates that TOPAS® COC 8007F-04 can be used to increase the seal strength of Ziegler-Natta LLDPE’s and to increase the hot-tack force for both LLDPE and plastomer resins. The improvement to the seal strength in LLDPE climbed steadily as the amount of COC in the sealant layer increased. Whereas the improvement to hot-tack averaged 3.3x higher across the two grades with only a 20% addition of 8007F-04 to the sealant layer.
On the other side of the film, it was demonstrated that incorporating some TOPAS® COC 5013F-04 increased the heat resistance of the film during the sealing process. Adding just 15% 5013F-04 to the outside layer was enough to increase the heat resistance of LLDPE from 105°C to 170°C. Additional loadings did not improve the heat resistance in this test any further, but would result in increased film stiffness.
Applications
Based on this data, blends of COC resins with polyethylenes can be used to improve the processing of a variety of packaging applications. TOPAS® COC can enable recyclable packaging that doesn’t need to sacrifice heat resistance, cycle time, or strength since it is approved for recycling with polyethylene up to a 20% loading in the total structure. Specific applications where COC can help include, based off the results of this study:
- Stand-up pouches with polyethylene exterior layers
- Vertical form-fill-seal packages requiring additional hot-tack
- High speed packaging machines that require higher temperature seal bars to maintain cycle times
Methods & Materials
TOPAS® COC resins are amorphous copolymers of ethylene and norbornene that are used on their own or as a blend component with other polyolefins to adjust performance characteristics. The COC resins that were used were 8007F-04 and 5013F-04. These resins are commonly used in the packaging market for their high stiffness, exceptional chemical and moisture barrier, and temperature resistance. Basic information for these three grades is given below in Table 1:
Polyethylene resins can have different properties depending on the type of comonomer (if any), the density, the catalyst, and the molecular weight of the polymer. In this study the linear low-density polyethylene, LLDPE, was produced using a Ziegler-Natta catalyst, used 1-octene as the comonomer. The low-density polyethylene, LDPE, used in the study was produced in a tubular reactor. And the plastomer used was a gas phase, metallocene catalyzed, hexene comonomer copolymer. The melt flow index, the density, and the melt temperatures of these three resins are given below in Table 2.
For more information about TOPAS® COC resins reach out to us via the Contact Us or Find a Rep links at the top of every page on our website. Also be sure to check out the website of our parent company Polyplastics.