High-Moisture Barrier sHDPE Gets Increasing Play in Flexible Food Packaging

A bimodal homopolymer HDPE which features up to 50% higher moisture barrier than standard resins has gained significant ground as a barrier layer in multilayer flexible food packaging, particularly in “easy-open” cereal and cracker packages. Surpass HPs167-AB, a 1.2-MI, 0.966-density resin for blown film from Nova Chemicals (U.S. office in Moon Township, Penn.) is one of the high-barrier materials made with Nova's Advanced Sclairtech dual-reactor process and single-site catalyst, referred to as sHDPE.

            HPs167-AB was addressed in a paper, “Sealable Seal Films With Enhanced Moisture Barrier Properties For Flexible Packaging Applications,” by Nova Chemical’s technical service specialist Dan Falla. In a recent exchange with Falla, he confirmed the material has been in commercial packaging applications with significant growth within the last two years. The paper points to some of the trends that are taking place to overcome the ‘catastrophic’ failure of the film and spillage of the contents in dry goods packages designed to have the seal layer peel apart while being opened.

Most of these package constructions are three-layer coextruded blow films, comprised of a peelable seal layer and an HDPE core layer. The thickness of the HDPE typically determines the moisture vapor transmission rate (MVTR) of the film. “Traditionally, the HDPE layer was conventional HDPE. The HPs167-AB resin is quickly becoming the resin of choice for a PE moisture barrier,” says Falla. He also notes that it is possible to use this sHDPE in one layer and still take advantage of the barrier properties. Also, in many cases, it is desirable to use the sHDPE where EVOH is used as the latter’s barrier properties are affected by moisture. The sHDPE has been shown to protect the EVOH from moisture.

In his paper, Falla described the use of this material in five-layer film structures produced on a vertical form fill and seal (VFFS) machine. In our discussion, he stressed how there is a limit to how much downgauging is possible without jeopardizing the film’s integrity. The film’s physical properties that can be adversely affected include tear resistance, puncture resistance, and barrier. “An important property is film stiffness. If the film is too soft, it can be difficult to run through a VFFS machine.”

 As shown in his research, Falla notes the key ways in which the use of HPs167-AB helps. First, the material has a higher density than most HDPEs, which results in higher stiffness. In addition, using the I-Beam effect (the separation of HDPE) in a five-layer film, has been shown to increase the bending stiffness of the film, allowing it to be downgauged while still retaining the same bending stiffness.

As for the peelable seal layer, the paper showed the improved performance offered by commercial ethylene propylene (EP) peel polymer and an ionomer specific to peel seal applications, compared to more traditionally used polybutene-1 (PB-1).  Key findings in this study showed:  films made with ionomer in the seal layer had the best overall physical properties; ionomer was also found to have the broadest peel seal window along with the lowest peel force; and, EP peel polymers produced films with the highest hot tack strength. “We are starting to see less PB-1. The EP and ionomer allow for a broader sealing range with more consistent peel seal properties,” says Falla. Overall, the study showed that films with either the ionomer or EP peel polymer in the seal layer and a barrier HDPE will produce an effective peelable seal film with enhanced barrier properties and, higher stiff stiffness with a barrier sHDPE will allow downgauging of the film. 

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