Representing a $38 billion global market, the flexible packaging industry is growing rapidly. With the demand for flexible packaging growing at an average rate of 3.5 percent per year, flexible materials need to meet and exceed the high expectations of consumers and the stressors of the supply chain. Increased competition between suppliers, along with government regulations, have resulted in innovations in films that enhance product and package performance, as well as address worldwide concerns with packaging waste.
One such innovation is polymer nanocomposite technology, which holds the key to future advances in flexible packaging. In a December 2003 article, packaging industry expert Dr. Aaron Brody stated that "nanocomposites appear capable of approaching the elusive goal of converting plastic into a superbarrier—the equivalent of glass or metal—without upsetting regulators."
Nanocomposites, defined as polymers bonded with nanoparticles to produce materials with enhanced properties, have been in existence for years, but are recently gaining momentum in mainstream commercial packaging use. (References to nano materials simply mean that the material or activity can be measured in nanometers. In the metric system of measurement, a "nano" equals a billionth and therefore, a nanometer is one-billionth of a meter.)
The U.S. is leading in nanotechnology research, with more than 400 research centers and companies involved and more than $3.4 billion in funding. Europe has more than 175 companies and organizations involved in nanoscience research, with $1.7 billion in funding, and Japan has more than 100 companies working with nanotechnologies. Globally, the market for nanocomposites is expected to grow to $250 million by 2008, with annual growth rates projected to be 18 percent to 25 percent per year.
Polymer nanocomposites are constructed by dispersing a filler material into nanoparticles that form flat platelets. These platelets are then distributed into a polymer matrix, creating multiple, parallel layers that force gases to flow through the polymer in a "tortuous path," forming complex barriers to gases and water vapor. The more tortuosity present in a polymer structure, the higher the barrier properties. The permeability coefficient of polymer films is determined using two factors: diffusion and solubility coefficients, i.e., P = DxS.
Effectively, when there is more diffusion of nanoparticles throughout a polymer, the permeability is significantly reduced. According to the Natick Soldier Center of the U.S. Army, "The degree of dispersion of the nanoparticles within the polymer relates to the improvement in mechanical and barrier properties in the resulting nanocomposite films over those of pure polymer films."
Nanoparticles allow for much lower loading levels than traditional fillers to achieve optimum performance. Usually, additional levels of nanofillers are less than 5 percent, which significantly impacts the weight reduction of nanocomposite films. This dispersion process results in a high aspect ratio and surface area, creating higher-performance plastics than with conventional fillers.
Different types of fillers are utilized; the most common is a nanoclay material called montmorillonite—a layered, smectite clay. Clays in a natural state are hydrophilic, while polymers are hydrophobic. To make the two compatible, the clays' polarity must be modified to be more "organic." One way to modify clay is by exchanging organic ammonium cations [positively charged ions] for inorganic cations from the clays' surface.
Additional nanofillers include carbon nanotubes, graphite platelets and carbon nanofibers. Other fillers are being investigated, such as synthetic clays, natural fibers (hemp or flax) and POSS (polyhedral oligomeric silsesquioxane). Carbon nanotubes—a more expansive material than nanoclay fillers, which are more readily available—offer superb electrical and thermal conductivity properties. Two major suppliers of nanoclays are Southern Clay Products (www.scprod.com) and Nanocor (www.nanocor.com).
There are three common methods used to enhance polymers with nanofillers to produce nanocomposites: melt compounding, in-situ polymerization and the solvent method. Melt compounding, or processing, of the nanofillers into a polymer is done simultaneously when the polymer is being processed through an extruder, injection molder or other processing machine. The polymer pellets and filler (clay) are pressed together using shear forces to help with exfoliation (the process of separating the particles into the right shape and layer structure) and dispersion. With in-situ polymerization, the filler is added directly to the liquid monomer during the polymerization state. Using the solution method, fillers are added to a polymer solution using solvents such as toluene, chloroform and acetonitrile to integrate the polymer and filler molecules.
Since the use of solvents is not environmentally friendly, melt processing and in-situ polymerization are the most widely used methods of nanocomposite production.
The advantages of nanocomposite films are numerous, and the possibilities for application in the packaging industry are virtually endless. Because of the nanocomposite process's dispersion patterns, the platelets result in largely improved performance in the following properties:
Gas, oxygen, water, etc. barriers
High mechanical strength
Good optical clarity (since particles are nano-size).
A majority of consumer products that use nanocomposite packaging are in the beverage industry. Many different types of commercial plastics, flexible and rigid, are utilized for nanocomposite structures, including polypropylene, nylon, polyethylene terephthalate and polyethylene.
Nylon nanocomposites, used as barrier layers for multilayer PET containers, prove to perform better—as much as two to three times better—than the traditional ethylene vinyl alcohol barrier layers, since nylon has a 50-deg-F-higher melt temperature.
Studies show that nylon 6 nanocomposites can achieve an oxygen transmission rate (OTR) almost four-times lower than unfilled nylon 6. In the case of Honeywell's (www.aegisnylon.com-) Aegis(TM) OX barrier nylon resins, which were designed for multilayer bottle applications, the nanoclay layers act as a trap to retain the active oxygen scavengers in the polymer while reducing OTR one-hundredfold. Aegis OX's oxygen-scavenging component provides beer bottles with a shelf life of six to 12 months—comparable to glass bottles. Aegis has been used in a three-layer, 1.6-L structure for a brewery in Asia.
In the face of global recycling issues, nanocomposite polymers can help to reduce packaging waste and enable recycling efforts.
Iperm(R), produced by Mitsubishi Gas Chemical Co. (www.mgc-a.com), has similar results when added to a multilayer PET structure. Imperm's oxygen barrier is two times that of Mitsubishi's standard Nylon-MXD6 resin, and its carbon dioxide barrier is four times that of N-MXD6. It also requires no adhesion tie layers to PET and is very recyclable. When used in a 16-oz beer bottle, Imperm guarantees an almost seven-month shelf life.
Tensile strength, tensile modulus and heat-distortion-temperature (HDT) characteristics are also improved with the use of nanotechnology. One example is Cloisite(R), a nylon nanocomposite produced by Southern Clay Products, with a clay loading of 15 percent. Nanoclays in nylon increase tensile strength in this example by 23 percent, the tensile modulus by 69 percent and the flexural modulus by 56 percent. In addition, HDT is raised by 68 percent. The amount of change in mechanical properties is directly related to the quantity of nanofiller used in the particular nanocomposite. For example, by adding 2-percent nanoclay to a nylon 6 nanocomposite, the tensile strength is increased by 98 percent. This pattern also applies to the HDT and flexural-modulus characteristics. Other nylon nanocomposite polymers have increased mechanical properties similar to Cloisite.
As the global flexible packaging market increases, we will see more and more specialized products utilizing films. Nanocomposites would ease the transition between current packaging with metal layers and glass containers to flexible pouches or rigid plastic structures. Many current structures require multiple layers, which render the packaging nonrecyclable, but in the face of global recycling issues, nanocomposite polymers can help to reduce packaging waste and enable recycling efforts. Waste reduction is a very pressing issue in the world, and the U.S. military's application is a good example of how nanocomposite polymers can positively impact the environment.
Since 2002, Natick has been conducting extensive research into the use of no-foil polymer nanocomposite structures for military food rations, Meals Ready-to-Eat (MREs). The goal of the research is to reduce the amount of solid waste associated with the current packaging, as well as reduce costs through material savings. Each year, 14,177 tons of MRE packaging waste is generated because of the foil layer, which is susceptible to pinholing and does not allow the pouch to be recycled. One army ration creates 1.04 lb of waste, while a navy ration creates 3.8 lb of solid waste.
Current MRE packaging, which consists of three- to four-layer retortable pouches with a foil layer, do not meet the rigorous standards of the military. MRE packaging needs to be air-droppable and provide a minimum three-year shelf life at 80 deg F and a six-month shelf life at 100 deg F. By using nanocomposite polymers, which offer high barrier properties, Natick can extend the shelf life and provide greater product protection for its military rations.
Nanocomposite ration pouches are not in production at the moment, but according to Natick, they are in the advanced stages of development for future use. Natick researchers are working with a variety of different materials, including low-density PE, nylon, EVOH and others to find the right blend of polymers and nanofillers when delivered through various extrusion processes, such as cast film, multilayer film, blown film, single-screw and twin-screw.
It is possible that technologies developed by Natick would be accessible to commercial food packagers to increase processed-food shelf life, as military standards are more rigid.
According to U.S. Army research, costs of the future nanocomposite structures are estimated to be 10-percent to 30-percent less than the current pouches. Expected savings come from less material cost, improved manufacturability with more automation, and less waste-handling costs. Overall cost savings are estimated at between $1 million and $3 million.
Despite the prosperous future of nanocomposites, there are a few issues that warrant concern about the mass commercialization of these polymers. According to University of South Carolina researchers, there are four main issues surrounding the production and use of nanocomposites: exfoliation, orientation, compatibility and reaggregation.
When clay fillers got through the process of exfoliation, they need to be very thin—1 nanometer—and very wide—500 nanometers—to be able to achieve optical gas permeability without affecting optical quality. Particle orientation also has an effect on the success of a nanocomposite. Nanoparticles need to be dispersed throughout polymer so they are parallel to the material's surface. This position ensures a maximum tortuous path for the gasses when migrating through the polymer. For converters, proper particle orientation is an ongoing problem.
Compatibility between the nanofillers and the polymer substrate may cause issues, as well, depending on how they interact with each other. Certain nanofillers need to be prepared so they can perform well with the substrate. Another concern is that during the processing state, there is a possibility of reaggregation, where the particles clump together. If this happens, the creation of the nanocomposite is unsuccessful.
By 2009, it is estimated that the flexible and rigid packaging industry will use 5 million lb of nanocomposite materials in the food and beverage industry. By 2011, consumption is estimated to be 100 million lb. Beer is expected to be the biggest consumer by 2006, with 3 million lb of nanocomposites. Carbonated soft drink bottles are projected to surpass that, to use 50 million lb of nanocomposites by 2011.
Polymer nanocomposites are the future for the global packaging industry. Once production and materials costs decrease, companies will be using this technology to increase their product's stability and survivability through the supply chain to deliver higher quality to their customers while saving money. The advantages that nanocomposites offer far outweigh the costs and concerns, and with time, the technology will be further refined and processes more developed. Research continues into other types of nanofillers, allowing new nanocomposite structures with different, improved properties that will further advance nanocomposite uses in many diverse packaging applications.