Clear High Barrier Materials: Options & Economics
The trend towards packaging products in clear materials continues to gain momentum. Product visibility can be a powerful marketing tool. It allows for product differentiation and identification, while appealing to consumers’ desire to see what they are purchasing. Manufacturers enjoy the ability to easily inspect packaged product through the use of vision systems, metal detectors, and manual visual inspection. Because clear materials are metal-free, RFID and security/theft sensors are not inhibited, pleasing the retailer.
The material choices that we have today allow for a variety of approaches when packaging sensitive products. Clear high-barrier materials that can compete with aluminum foil on both performance and economics are a reality.
The Benchmark – Aluminum Foil
When evaluating high barrier materials, aluminum foil is the benchmark by which we measure performance. Aluminum foil, if in perfect condition (no pinholes or imperfections), is impervious to moisture and gas regardless of the thickness of the foil. It is the ultimate barrier.
However, aluminum foil at thicknesses of less than one mil (0.001 inch or 25 microns) has pinholes. These pinholes may occur randomly as the result of inclusions in the aluminum or they may be in a repeating pattern when the result of the rolling process. They are inherent to thin gauge foils and can be present in sufficient number to impact the barrier properties. As the gauge of aluminum foil decreases, the number of pinholes typically present increases. Table 1 provides a summary of the expected and typical maximum pinhole counts in various thicknesses of aluminum foil.
|Foil Gauge (in.)||Typical Count (per ft²)||Typical Maximum Count (per ft²)|
The pinholes typically range in size from 10-50 µm in diameter. While the small holes tend to be circular, the largest holes are generally oval in shape with dimensions up to 75µm by 200 µm. The effect the pinholes can have on the barrier of bare aluminum foil is significant. However, in packaging applications, aluminum foil is generally part of a multi-layer structure. The other layers in the structure minimize the impact of a pinhole in the foil layer on the overall barrier. Typical barrier properties of a PET/aluminum foil/LDPE are shown in Figure 1.
When package clarity is desired, there are a number of clear high-barrier material options many of which have barrier properties that exceed that of the thin gauge aluminum foils. The appropriate choice is dependent upon many factors:
- Type of barrier needed for the application (oxygen, water vapor, aroma, chemical, ultra-violet light, and/or microbial barriers are common needs.);
- The product itself and its compatibility with the packaging materials (For example, many options can be ruled out if the product contains water, is chemically active, or is sharp.);
- The packaging format and whether the material will be formed;
- The sterilization method (if applicable);
- Specific environmental and/or disposal requirements;
- The cost of the packaging system.
In order to understand the barrier that will be provided by the finished package, it is important to factor in not only the permeation through the face of the material but also the ingress through the seal. The sealants used in pouch applications often provide little barrier to gasses and ingress through the seal can result in a significant barrier loss. Figure 2 illustrates this phenomenon.
The rate of permeation through the sealant is impacted by both the thickness of the sealant and the width of the seal as demonstrated in Figure 3. To calculate the theoretical transmission through the sealant, see Addendum 1.
Figure 2: Ingress Through the Seal
Clear high-barrier materials are available in two forms, barrier films and barrier coatings. For more complex applications, combinations of films and coatings may be used.
Ethylene Vinyl Alcohol
Ethylene Vinyl Alcohol (EVOH) is one of the most common clear high barrier films used today. It is applied typically applied as a discrete layer in a coextrusion. However, EVOH may also be used as a sealant.
EVOH provides excellent oxygen barrier, in the range of 0.005 – 0.12 cc-mil/100 in2-day. The barrier that a particular EVOH film provides is dependent upon a number of factors:
- Mole percent – as the ethylene mole percent increases, the barrier decreases;
- Degree of crystallinity – as the degree of crystallinity increases, the barrier properties improve;
- Thickness – as with all films, as the thickness increases, the barrier increases;
- Temperature – as the temperature increases, the barrier decreases;
- Humidity – at high humidity levels, the barrier provided by EVOH drops rapidly as illustrated in Figure 4. Note: it is the humidity level at the EVOH interface rather than ambient humidity that is critical. In addition to providing excellent oxygen barrier, EVOH is also an excellent odor and aroma barrier. EVOH has a long history and is well understood.
In addition to providing excellent oxygen barrier, EVOH is also an excellent odor and aroma barrier and is an ideal choice for holding organic solvents. EVOH can be thermoformed for three-dimensional applications.
If sterilization is required, EVOH can be irradiated or sterilized via ethylene oxide.
Cyclic Olefin Copolymers
Cyclic Olefin Copolymers (COC) or Cyclic Olefin Polymers (COP) are a good moisture barrier providing approximately 0.2 g-mil/100in2-day. When using COCs/COPs to achieve barrier, they can be coextruded as a discrete layer or, for improved economics, blended to a level of 60 – 70 percent with polyolefins. COCs/COPs will enhance stiffness making them ideal for use in stand-up pouch applications. They have excellent clarity and, if the appropriate grade is selected, can be used in retort/autoclave applications.
COCs/COPs can be used as a sealant. They have low extractables, and therefore can be used for chemically sensitive products. Three-dimensional packaging formats are possible with COCs/COPs since they can be thermoformed.
COCs/COPs are not appropriate for all applications. While they are a good choice for polar organics, COCs/COPs are attacked by nonpolar solvents such as toluene and naptha and have limited resistance to ethyl acetate. COCs/COPs are a poor choice for holding oils or greasy products. COCs/COPs also tend to be brittle although blending can reduce that issue.
Most COC grades can also undergo sterilization by gamma and e-beam radiation, retort/autoclave, and ethylene oxide.
Amorphous polyester (APET) is a good moisture barrier at approximately 0.4 cc-mil/100in2-day. APET has excellent chemical resistance except to alkalis and can be used to protect other materials from reactive gasses. It will seal to itself and other polar materials. By modifying APET, peelable sealants can be created. For applications requiring outstanding organoleptics or a chemically clean sealant, APET is an excellent choice. APET can also be thermoformed.
A variety of sterilization methods can be used with structures containing APET. In addition to radiation sterilization, APET can be used when autoclave/retort sterilization is required and is uniquely appropriate for very demanding dry heat sterilization.
Polyacrylonitrile (Barex®) is a good oxygen barrier, in the neighborhood of 0.7 cc-mil/100in2-day. It is chemically inert making it very popular for pharmaceutical applications. Polyacrylonitrile can be used as a sealant and is thermoformable. However, the sole commercial manufacturer of polyacrylonitrile has discontinued its production.
Depending upon the chemistry of the product being packaged, APET, COCs, and EVOH are suitable replacements.
Polychlorotriflouroethylene (PCTFE) provides excellent moisture barrier. It is available as a homopolymer with water vapor transmission rates of approximately 0.038 g-mil/100in2-day and 0.016 g-mil/100in2-day respectively. PCTFE is inert and has excellent clarity. Although PCTFE is commonly used in blister (thermoformed) packages where moisture barrier is required, it can also be used for pouch applications.
Nanocomposites are polymer structures that contain fillers, typically silicate nanoclays, with at least one dimension in the nanometer range. The fillers separate into tiny platelets that disperse into a matrix of layers. Because the matrix of layers creates a tortuous path for gasses trying to permeate through the film, the barrier properties of the modified polymer are improved. However, the challenge is to ensure that that the filler dispersion is consistent. In addition to better barrier properties, nanocomposites modified films also have improved dimensional stability and stiffness and, because crystallinity is increased, enhanced clarity.
Nanocomposite masterbatches are commercially available for nylon and polyolefins. The oxygen barrier of nylon nanocomposite films can as much as 50 percent higher than a nonmodified nylon. Polyethylene and polypropylene nanocomposite structures have shown improvement in gas barrier of 25 to 50 percent and in water vapor of 10 to 15 percent in laboratory settings. Achieving consistent barrier properties on a commercial scale remains challenging.
Nanocomposite technology is very much an emerging science. It shows a great deal of promise and as more options become available for film applications it will have a significant impact on barrier material options.
Polyvinylidine chloride (PVdC) is a widely used barrier material with a long history. While PVdC is available as an oriented film and a discrete layer in coextruded films, it is more commonly used as a barrier coating. PVdC has good oxygen and moisture barrier properties and provides an excellent aroma and flavor barrier. Common transmission rates for PVdC coated substrates are shown in Table 2.
|Material||O²TR (cc/100 in² -day)||WVTR (g/100 in² -day)|
|PVdC-coated Nylon 6||0.5||0.65|
The barrier level of PVdC coatings are dependent upon the coating thickness. Much lower transmission rates can be achieved by applying more coating as is common in the pharmaceutical industry.
Because PVdC contains chlorine, hydrochloric acid can be generated under certain conditions. As a result, specialized equipment must be used to apply the coating and the proper equipment must be used if the packaging material is to be incinerated.
Polyvinyl alcohol (PVOH) is also available as a film or a coating. As a coating, PVOH provides excellent oxygen barrier as shown in Table 3.
|Material||O₂TR (cc/100 in² - day)|
The barrier of a PVOH coating is dependent upon the coating thickness. PVOH is moisture sensitive and will dissolve when exposed to water or high humidity.
Silicon Oxide and Aluminum Oxide
Silicon oxide (SiOx) and aluminum oxide (Al2O3), coatings are generally applied as a vacuum deposition onto films such as polyester and nylon. SiOx and Al2O3 coatings provide excellent oxygen and water vapor barrier properties in a variety of ranges as shown in Table 4.
|Material||O₂TR (cc/100 in² -day)||WVTR (g/100 in² -day)|
A concern is often expressed regarding the ability of silicon oxide coatings to maintain their barrier when flexed. The reality is that most grades perform favorably when compared to aluminum foil and metallized composites. Figure 6 illustrates the relative impact of 20 gelbo flex cycles on aluminum foil as compared to ClearFoil® X.
SiOx and Al2O3 coating are available in grades appropriate for retort/autoclave applications.
The conventional wisdom is that composite packaging materials that use SiOx or Al2O3 coatings as the barrier will be more expensive that aluminum foil composites. This conclusion is often reached after comparing the cost of the base aluminum foil (unlaminated) to the base SiOx or Al2O3 coated product. However, it ignores that fact that aluminum foil must be protected on both sides whereas the SiOx or Al2O3 coated product may only need a sealant. The cost of the additional layer and processing necessary for aluminum foil often results in a composite structure that is more expensive than a comparable SiOx or Al2O3 composite. Table 5 provides a relative cost comparison of various structures.
|Material||O₂TR (cc/100in² - day)||WVTR (g/100in² - day)||Relative Cost Comparison*|
|48 ga PET / PE / 50 ga Foil / 2 mil LDPE||0.001||0.001||100%|
|48 ga PET / 2 mil LDPE||4.5||0.30||49%|
|48 ga metalige PET / 2 mil LDPE||0.1||0.05||51%|
|48 ga PVOH coated PET / 2 mil LDPE||0.2||0.30||52%|
|48 ga saran coated PET / 2 mil LDPE||0.5||0.30||56%|
|48 ga ClearFoil® V / 2 mil LDPE||0.04||0.07||68%|
|48 ga ClearFoil® X / 2 mil LDPE||0.004||0.003||80%|
There is no one ideal barrier material for all pouch applications. Achieving the correct balance of barrier, performance, and economics is very much application dependent. Fortunately, we have a wide variety of clear high barrier materials from which to choose. Figure 5 shows barrier properties of common materials as compared to aluminum foil.
With the advances in SiOx and Al2O3 coated products, clear barrier options exist that can provide barrier properties comparable to or even better than aluminum foil and often at a lower price. Today, even the most demanding products can be packaged in clear materials.