Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published September 1998

TECHNICAL PAPER SERIES

For many years, glass, paper, and metal were the dominant primary materials for packaging food, beverages, and medical products. However, over the past several years, high-barrier plastics have increasingly replaced these conventional packaging materials. It has become important to understand the variables that influence barrier properties in plastics, and being able to predict barrier performance prior to data testing can result in substantial cost savings.

Today's polyolefin packaging can have a range of functional requirements, among them high flexibility, optical clarity, low water loss, sealability, extended shelf life, slow permeability to oxygen and carbon dioxide, lack of leachables, dimensional stability during sterilization, flex resistance, biocompatibility and environmental compatibility, and low cost. These requirements can often conflict with one another. Fortunately, today's cutting-edge coextrusion and lamination technology provides various solutions to satisfy gas-permeation requirements while simultaneously meeting other functional needs. The use of mathematical modeling to predict the diffusion of various gases such as oxygen, water vapor, and carbon dioxide in and out of containers during shelf life also plays an integral part in designing optimized packaging materials.

In general, the permeability of plastics depends on crystallinity, molecular orientation, chain stiffness, free volume, cohesive energy density, temperature, and moisture sensitivity.^1–3 Higher crystallinity, molecular orientation, chain stiffness, and cohesive energy density lead to lower permeability. In this study, we attempt to understand the correlations between material density and modulus, internal haze, and water-vapor transmission rate (WVTR).

MATERIALS AND EXPERIMENTAL METHODOLOGY

WVTR characteristics of a wide variety of commercially available polyethylene and polypropylene films were studied, as shown in Tables I and II. The polyethylenes that were examined ranged from ultra-low-density (ULDPE) to high-density (HDPE) formulations, while the polypropylenes varied in density from 0.875 to 0.90 g/cm³. Material density was measured using a density gradient column (Techne Model DC4), and the WVTR was measured using a Mocon Permatran 3/31 at two temperatures of 30° and 40°C, with RH of 0% on one side of the film and 35% on the other side. The modulus of the films was measured with a tensile tester (Instron Model 4201) at 20 in./min, and the haze was determined by a haze meter (Haze Guard Plus). A differential scanning calorimeter (Thermal Analyzer 2100 series) at a scanning rate of 20°C/min was used to analyze film thermal properties.

RESULTS AND DISCUSSION

The density and WVTR properties of several polyethylenes and polypropylenes are shown in Tables I and II. Figure 1 shows that WVTR is linearly correlated to the density or crystallinity on a semilogarithmic scale; it is also a function of temperature, as shown in Figure 2. The higher the density (or crystallinity), the lower the WVTR. It is clear that WVTR can be reduced by almost an order of magnitude by increasing density from 0.90 to 0.95 g/cm³. With this information, one can readily determine the thickness of a polyethylene barrier film needed to meet a required WVTR.

Figure 1. Water-vapor transmission rate versus density for polyethylenes at 30° and 40°C.

Figure 2. Water-vapor transmission rate as a function of reciprocal temperature for polyethylenes.

Figure 3. Differential scanning calorimetry trace for polyethylene of 0.9015-g/cm³ density.

Figure 2 shows the WVTR as a function of reciprocal temperature plotted in an Arrhenius fashion for all grades of the polyethylene films studied. These plots show that the permeability of LDPEs is more temperature dependent than those of HDPEs, especially at higher temperatures. This increase in slope as polyethylene density decreases can be attributed to the increase in the ratio of amorphous to crystalline content, as shown in Figure 3. A fraction of crystals melt as the testing temperature increases from 30° to 40°C. The effect of crystallinity on permeability can be expressed by the equation

where P_c and P_a are the permeability of the crystalline and amorphous fractions, respectively; is the fraction of amorphous content; and b is approximately 2.25, for water.¹ The plots in Figure 2 also allow us to interpolate the WVTR properties at temperatures between the two extremes. This is important, since it becomes possible to predict the shelf life of the packaging material at one temperature if it is known at another temperature within the range.

Figure 4 shows the plot of modulus as a function of polyethylene density on a semilogarithmic scale. Once again, a linear relationship is evident. As shown in this plot, the modulus is an exponential function of density. Thus, while polyethylenes develop better water vapor barrier properties as density increases, they also become stiffer—an example of the conflicting requirements of packaging materials. Ideally, of course, one would prefer a combination of high water-vapor barrier with low modulus (stiffness) for ease of handling.

Figure 4. Modulus as a function of density for polyethylenes.

Figure 5. Internal haze versus density for polyethylenes.

Figure 5 shows the internal haze data for polyethylenes as a function of density. It is clear that haze increases exponentially with density. Thus, while HDPEs have better water-vapor barrier properties, as shown in Figure 1, they also have lower optical clarity, as shown in Figure 5. (In this case, the ideal would be a combination of high water-vapor barrier with low haze.) The density-dependence of these properties is the same for polypropylenes as they are for polyethylenes. Figure 6 shows the effect of temperature and density on the WVTR of polypropylenes.

Figure 6. Water-vapor transmission rate as a function of reciprocal temperature for polypropylenes.

One way to accommodate these conflicting requirements for packaging materials is to employ multilayered structures to achieve an optimal balance of properties: one layer for barrier properties, one for sealability, one for gloss, one for flexibility, and so on. Fortunately, advances in coextrusion technology have provided the impetus for the development of new structures that can be designed to satisfy complex use requirements.

Table III compares theoretically predicted values for haze and WVTR with corresponding experimental values for two multilayer polyolefin films. A fair degree of agreement was achieved. Film B is exactly twice the thickness of film A, with each layer increased proportionally. As expected, both theoretical and experimental values turned out to be proportional to thickness. The discrepancy between the theoretical and experimental values for internal haze for film A can be explained by the differences in processing conditions. It is known, for example, that a quenched film is usually clearer than a slow-cooled one.

CONCLUSION

The density or crystallinity of polyolefin films is a key parameter that ultimately determines properties such as stiffness, refractive index, internal haze, and permeability of various gases. In this study, we successfully predicted the haze and WVTR of two composite polyolefin films, obtaining values that agree within 10 to 15% with experimentally determined values. The correlations of internal haze, WVTR, and modulus with density presented here can offer a priori predictions of properties for complex, multilayered polyolefin film structures. The approach will also enable processors to optimize the structures before coextruding final films for a given application. This will allow for a reduction of the number of iterative trials required to arrive at an optimized film structure, potentially resulting in substantial cost savings.

REFERENCES

1. Salame M, Plastic Film Technology, vol 1, Finlayson KM (ed), Lancaster, PA, Technomic, p 132, 1989.

2. Peters JE (ed), Packaging Encyclopedia, vol 30(4), Newton, MA, Cahners, 1985.

3. Briston JH, Plastic Films, New York, Wiley, 1984.

All of the authors are current employees of Baxter Healthcare Corp. (Round Lake, IL). Ketan Shah, PhD, is an engineering specialist concentrating on polymer processing and polymer structure-property relationships. Michael T.K. Ling is an engineering specialist involved in medical product design and troubleshooting and in developing applications for polymeric materials. Lecon Woo, PhD, a Baxter distinguished scientist, specializes in biomedical polymer development and polymer rheology and processing. Gregg Nebgen is an engineering specialist working in polymer extrusion, while Scott Edwards is an engineering specialist involved in product development management. Lillian Zakarija is a product development engineer whose specialization is container development.

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