When people talk about improving barrier properties in multi-layer food-packaging films, many of them think only of special barrier resins and overlook the role of the polyethylene that makes up the bulk of most films. The PE in a coex film can also be optimized to contribute to barrier.
Moisture barrier is important in films for food packaging, such as liners for cereal and cracker boxes and cake mixes. Good moisture barrier in a PE also improves the performance of moisture-sensitive high-barrier resins like EVOH. And since the degree of moisture barrier of a PE resin correlates with its level of oxygen barrier, better moisture barrier means better oxygen barrier, too.
PE's moisture barrier is directly affected by basic resin properties, by the resin manufacturing technology and catalyst used, by extrusion process conditions, and by film gauge and layer structure. Packaging engineers should know how these factors interact in order to get the best total barrier performance from their multi-layer films.
Basic properties of polyethylene that affect barrier are density, melt index, and molecular-weight distribution (MWD). PE with higher density provides better moisture barrier or lower water-vapor transmission rate (WVTR). Foods requiring low moisture or oxygen exposure would use a higher-density PE to get optimum barrier. Conversely, lower-density PE would be chosen for low-barrier applications.
Barrier properties of PE also generally improve with higher melt index (MI) and/or narrower MWD because both affect the film's crystalline structure. High-MI and narrow-MWD resins make blown and cast films with relatively balanced crystalline orientation, thereby increasing barrier performance. These resins are also less sensitive to changes in processing conditions, while barrier properties of low-MI or broad-MWD resins are significantly affected by process conditions.
Resins with long-chain branching, such as LDPE, LDPE blends, and certain HDPEs, show improved barrier as gauge increases. But barrier properties can vary dramatically depending on process conditions.
Factors in PE resin manufacturing that affect barrier include the type of polymerization reactor and catalyst, efficiency of catalyst removal, pelletizing, and antioxidant level. While this information may not be something processors typically consider, they should be aware of these factors when designing barrier films.
HDPE resins are generally made in either a solution or slurry process. The solution process makes molten polymer in a single fluid phase. The slurry process makes polymer solids in a carrier fluid, either gas or liquid. The solution process has inherent advantages over slurry technology in making high-barrier packaging resins because solution technology has short reactor times, requires less shear to pelletize, and offers better ways to incorporate antioxidants.
Long reaction times can generate high-molecular-weight species, which can reduce barrier performance because of gels or unbalanced crystalline orientation. Certain catalysts also produce resins with long-chain branching and/or HMW tails, which increase crystalline orientation, reduce barrier, and raise a resin's sensitivity to process conditions.
More long-chain branching provides advantages in low-barrier packaging applications because it affects crystalline orientation and reduces barrier. Long-chain branching can be produced in a high-pressure LDPE reactor or in a slurry process with chromium catalyst or in certain solution processes with metallocene catalysts. Blending LDPE into another PE is also a common way to introduce more chain branching.
Under certain process conditions, some catalysts also generate in-situ comonomer, which lowers resin density and reduces barrier properties. Removing rather than deactivating or neutralizing catalyst residues can lower the resin's crystallization rate and the tendency to form gels, thus increasing barrier performance. But removing catalyst residues adds cost. It's generally done only with vanadium catalyst in order to improve color stability. Equistar Chemicals removes vanadium from PE made with its DuPont solution process in Victoria, Texas. Nova Chemicals Corp. removes vanadium from PE made in its DuPont Sclairtech plant in Sarnia, Ont.
How resin producers compound and pelletize PE also affects barrier. Quality of antioxidant dispersion affects resin stability because oxidation reduces barrier. Pelletizing imparts heat and shear, which can increase molecular weight, broaden MWD, and cause oxidation, long-chain branching, and gels--all of which also lower barrier performance.
Film extrusion conditions, including blow-up ratio (BUR), frost-line height, and gauge all affect WVTR of PE films made from resin with broad-MWD or long-chain branching. As the frost-line goes up, the WVTR decreases (moisture barrier improves). This effect on a broad-MWD HDPE is enhanced as blow-up ratio increases. Processing conditions have little effect, however, on the barrier properties of narrow-MWD resins.
Adjusting processing equipment can also improve permeation rates. The effect on barrier of using different die gaps to blow HDPE films from a broad-MWD resin. At a given BUR, barrier is better for film made with a narrow die gap. So, to maximize barrier, extrude PE blown film with a high BUR, high frost-line, and narrow die gap.
Changes in film thickness can also significantly affect barrier properties, depending on the type of PE. Narrow-MWD resins have relatively constant barrier properties per unit of thickness--i.e., permeation rate is inversely related to gauge. In contrast, WVTR for resins with broad MWD or long-chain branching can be disproportionately higher at thinner gauges (less than 50 microns). Higher permeation rates are attributed to less balanced crystalline orientation in thinner films.
Moisture barrier for HDPE with a density of 0.96 g/cc is about the same for thick films, regardless of whether the resin has broad or narrow MWD. But the narrow-MWD resin has nearly 20% higher barrier in thin gauges. Likewise, LDPE and LLDPE resins with a density of 0.92 g/cc have nearly identical barrier properties in thick gauges, but the LLDPE (with less chain branching) has over 25% higher barrier in thin gauges.
Barrier performance of multi-layer PE films is affected by layer configuration. Compare the relative barrier performance of three different HDPE films, each containing three layers of equal gauge and composed of two resins with MI's of 1 and 2. The 2-MI resin has somewhat narrower MWD.
Figure shows three film structures. Structure 1 has three layers of 1-MI resin with regrind blended into the middle layer. Structure 2 sandwiches a 2-MI layer between skins of 1 MI. Structure 3 is 2 MI/1 MI/1 MI. Structure 3 has the best barrier, and its barrier advantage increases as film gauge is reduced.
Layer position affects barrier because each layer experiences different conditions during extrusion. A film's outermost skin layer is subject to more intensive shear and cooling forces than the interior layers. So a resin's barrier performance in a skin layer will more closely resemble its performance in thin monolayer film, while the barrier of resins in interior layers will be similar to its performance in thick film. Thus, Structure 3 excelled in barrier because it was the only one to have a skin layer of higher-MI resin with narrower MWD, both of which contribute inherently to better barrier.
James Krohn is marketing manager for medium-MW HDPE film resins at Equistar Chemicals, LP, Houston. William Todd is senior advisor in process development at Equistar's PE plant in Victoria, Texas. John Culter retired from General Mills after over 20 years in packaging and is now president of Advanced Materials Engineering, Inc., Naples Fla., a consulting firm for packaging and converting. He is also an adjunct professor at the School of Packaging, Michigan State University in Ann Arbor.