Why all the excitement about the new class of mineral-reinforced plastics known as nanocomposites? Because relatively small amounts (2-5%) of nanometer-sized clay particles can provide large improvements in mechanical and thermal properties, as well as gas barrier and flame resistance. They also reduce shrinkage and warpage. And all of these benefits are available without significantly raising the density of the compound or reducing light transmission (nano-clays are in the same size range as visible-light wavelengths).
Most of the current R&D work is focused on automotive parts and packaging. The goals are physical and thermal property enhancements and reduced permeability to gases, moisture, and hydrocarbons. Nylon-based nanocomposites are the first commercial materials to emerge, and there is now a frenzy of activity aimed at nano-reinforcing commodity thermoplastics such as polypropylene and PET.
Two research consortia on nanocomposites have been formed in recent months. The Edison Polymer Innovation Corp. (EPIC) of Akron, Ohio, got one under way in March. Headed by staff scientist Jon Collister, this three-year, $3-million program will coordinate investigations by industry and academic researchers on thermoplastics (PP, PET, PE, and acrylic), advanced composites, coatings, and elastomers.
Meanwhile, the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., has formed a consortium of government and industry scientists to explore nanocomposites' potential for reducing the flammability of thermoplastics.
Where opportunities are
The current favorite nano-clay is montmorillonite, a layered alumino-silicate whose individual platelets measure on the order of one micron diameter, giving them an aspect ratio of about 1000:1. The two major domestic suppliers of these type of nano-clays are Nanocor Inc. and Southern Clay Products.
Because it is hydrophilic, montmorillonite is not compatible with most polymers and must be chemically modified to render its surface more hydrophobic. The most popular surface treatments are organic ammonium cations, which can be exchanged for the inorganic cations existing on the silicate surface. The treated clay is then incorporated into the resin matrix either during polymerization or by melt compounding.
As an example of what can result, commercial nylon 6 nanocomposite materials provide tensile modulus, tensile strength, and HDT equivalent to 40% mineral-filled nylon, while specific gravity remains at the level of unfilled nylon 6. With lower-modulus polymers such as polyolefins, nanocomposites have the potential to upgrade physical properties to levels competitive with costlier engineering resins. Nanocomposites already look attractive for molded car parts such as body panels and under-hood components, as well as electrical/electronic parts, power-tool housings, lawnmowers, aircraft interiors, and appliance components.
On the packaging side, nanocomposites can slow transmission of gases and moisture vapor through plastics by creating a "tortuous path" for gas molecules to thread their way among the obstructing platelets. Bottles and food packaging aren't the only areas of interest here. According to EPIC, nanocomposites hold commercial benefits for reducing hydrocarbon emissions from hoses, seals, and auto fuel-system components. Nanocomposites of polypropylene might permit blow molding auto fuel tanks at lower cost than current multi-layer barrier structures.
Market debut in nylon
The first nanocomposites on the market are based on nylon 6 and produced in the reactor. Toyota Central R&D Laboratories in Japan pioneered the work on nanocomposites a decade ago. Within the last few years, Toyota has licensed its patented in-reactor nylon nanocomposite technology to companies such as Nanocor and Japan's Ube Industries. Nanocor, in turn, sublicenses the technology to firms that want to make nylon nanocomposites.
Ube was among the first to come out with a commercial nanocomposite--its "nylon-clay hybrids" (NCH) of nylon 6 and nylon 6/66 copolymer for film and structural applications. Commercial applications of NCH include nylon 6 barrier film for food packaging and a timing-belt cover for Toyota Motors. Compared with unfilled nylon 6, Ube's NCH is said to have 68% higher tensile modulus and 126% higher flexural modulus. Ube also reports that cast films of NCH-2 grade (2% clay) have half the oxygen permeability of straight nylon 6.
At about the same time, Unitika Co. of Japan introduced a nylon 6 nanocomposite for injection molding, which is marketed and distributed in North America by Toyota Tsusho. Nylon M2350 is produced with Unitika's proprietary technology, which incorporates a synthetic silicate during polymerization.
Nylon M2350 has been used by Mitsubishi Motors for an engine cover on its GDI models, where the nanocomposite is said to offer a 20% weight reduction and excellent surface finish. Lighting covers and knife handles are other applications.
Last fall, Bayer AG in Germany announced the development of nylon 6 nanocomposites for transparent barrier film packaging. The compounds are made in the reactor using Nanocor's nano-clay. Bayer is currently marketing nanocomposites only in Europe. It has two nylon 6-based nanocomposite grades for cast film: Durethan LPDU 601-1 and LPDU 601-2, which offer different degrees of barrier improvement. Relative to neat nylon 6, they reportedly cut oxygen transmission rate (OTR) by up to 50%,plus offer increased barrier to other gases, chemicals, and flavors/odors. The nano-films are also said to be stiffer than unfilled film.
AlliedSignal Corp. has been exploring nylon 6 nanocomposites for potential applications in automotive, consumer, and packaging markets. According to Kris Akkapeddi, team leader in plastics R&D, the company is currently evaluating this technology for some niche applications with specific customers.
Developmental products at AlliedSignal include toughened nylon nanocomposites for injection molding and high-viscosity nanocomposites for blow molding. Compared with unfilled nylon, Akkapeddi says the nanocomposites have 50-80% higher stiffness and up to 126° F higher HDT without much increase in specific gravity.
Nylon nanocomposites produced by melt compounding that reportedly approach the performance of their in-reactor counterparts are now emerging. The trick is to exfoliate (delaminate) the clay particles sufficiently so that the ultimate level of reinforcement is achieved.
Nanocor has introduced a new nano-clay grade, Nanomer I.24TC, which reportedly allows compounders to produce a nylon 6 compound with flexural modulus and HDT about 90% of the levels achieved with in-reactor blends.
RTP Co., a specialty thermoplastics compounder, announced in late April that it has produced what it believed to be the first commercially available compounded nylon 6 nanocomposite. The material is offered in injection molding grades and film grades with 3-5% loadings.
As shown in the table, RTP's injection grade shows 53% higher HDT than unfilled resin--comparable to 20-30% mineral-filled nylon--but with little change in specific gravity. Tensile strength is much better than either unfilled or mineral-filled nylon 6. Reduced flammability is also indicated by a 50% reduction in rate of heat release.
Toyota Central R&D Labs. recently succeeded in producing nylon 6 clay nanocomposites by melt processing in a twin-screw extruder. Dr. Arimitsu Usuki reports that the compounding approach improves productivity and opens the way for making nanocomposites of nylons 12, 66, and 6/66 copolymer.
Showa Denko of Japan recently started commercial production of nylon 66 nanocomposites using a kneading extruder. The materials are said to show improved flame retardancy and rigidity. Two special FR grades contain no halogen or phosphorus. Systemer FE 30600 and 30602 reportedly provide UL 94V-0 and V-2 performance, respectively, at only 1/64-in. thickness. Their flex moduli are 30-80% higher, and HDTs are 54-144° F higher, than those of standard nylon 66.
Melt-compounded nylon 6 and 66 nanocomposites are also reported to be forthcoming from BASF AG and from Solutia, respectively. (Solutia markets through Dow Plastics.)
Barriers for PET, EVOH
An agreement announced earlier this year by Eastman Chemical and Nanocor to jointly develop nanocomposites of PET and other packaging materials (e.g., polyethylene) is the first indication that commercialization of such materials may not be far off. Jim Lewis, v.p. of container plastics at Eastman, says his firm has worked with Nanocor since 1995, but several breakthroughs have occurred over the last year and a commercial product introduction is likely to come fairly soon.
Eastman is developing its first PET nanocomposites via the in-reactor approach. Its initial focus is on rigid containers. "Nanocomposites are one of the strategic technology components we are developing to enable emerging markets in food packaging that require superior barrier, including beer bottles and single-serve juice bottles," Lewis says. He hints that these applications are likely to utilize a coinjected multi-layer preform with the nanocomposite barrier layer on the inside.
Active projects on other barrier resins are also under way. Nanocor says it has made significant progress in joint development of EVOH nanocomposites for multi-layer packaging. Nanocor even offers a new nano-clay, Nanomer I.35L, designed for compounding with EVOH. Within the range of 50-80% R.H., it reduces oxygen permeability of EVOH by 66-80%. Besides packaging, EVOH nanocomposites are being investigated as a hydrocarbon barrier for automotive fuel tanks and fuel-system components.
Nanocor has another new nano-clay, Nanomer I.28MC, for compounding with MXD6 aromatic nylon from Mitsubishi Chemical. Applications could include packaging films for moisture- and oxygen-sensitive foods and electronics.
Add muscle to PP; TPO
Automotived-driven development of PP and TPO nanocomposites is one of the hottest areas of current interest. Because PP is a strongly nonpolar resin, it has been especially challenging to come up with an appropriate surface treatment for the clay. Nonetheless, late last year, three development partners--Montell North America, General Motors R&D in Warren, Mich., and Southern Clay--jointly announced the world's first TPO-based nanocomposite.
According to Jim Keeler, advanced program manager at Montell's Automotive Business Group, the partners are testing several TPO nanocomposites for exterior and interior applications. The initial focus is on exterior door and rear quarter panels. Although the nanocomposites are produced by melt compounding, Montell is using its Catalloy in-reactor TPOs in order to simplify the overall compounding process.
According to Keeler, previous PP/nano-clay compounds achieved only 10-15% exfoliation of the layered clay particles, whereas the current work has achieved much greater exfoliation and thus enhanced property development. TPO nanocomposites with specific gravity as low as 0.91 have been produced with better dimensional stability than unfilled TPOs and comparable to talc-filled grades (CLTE of 5 x 10-5 mm/mm/°C).
These developmental TPO nancomposites also have flexural moduli of 150,000 to 250,000 psi. In a 5-mph impact test, the TPO remains ductile down to -30 C. The partners also report that the nanocomposite has improved surface appearance due to the low filler loading.
Work on nanocomposites is also well under way at Ford Motor Co., according to Jeff Helms, director of research and materials science. "We have been focusing on anything polypropylene-based, including TPOs, impact-modified PP, and PP alloys," he says. "We have explored several different methods via compounding with which we can make PP nanocomposites."
Ford's initial target is interior trim--instrument panels, A and B pillars, door panels, and consoles. The chief challenge, Helms says, is to maximize coupling between the silicate and the polymer matrix. He indicates that this could prove simpler with surface-treated synthetic clays. "With natural clays, you may need a surface treatment plus a compatibilizer." He says his group recently has achieved very good results in terms of stiffness/impact balance. The next step is to team up with material suppliers.
Meanwhile, Toyota Central R&D Labs recently produced PP nanocomposites in a twin-screw extruder using 5% clay and maleic anhydride-modified PP oligomer as a compatibilizer. According to Toyota, the "polypropylene clay hybrid" (PPCH) provides higher stiffness than the same loading of talc or glass.
Dow Plastics and automotive processor Magna International, Southfield, Mich., are in their second year of a joint effort to develop nanocomposites of PP and TPO. The five-year project, which is partially funded by the U.S. Commerce Dept.'s Advanced Technology Program, is aimed at developing methods for dispersing the nano-clay in such resins and demonstrating cost-effectiveness of nanocomposites for exterior front and rear car parts.
For auto under-hood and electronics uses, Showa Denko just commercialized the first acetal nanocomposite. It reportedly shows low warpage and few sinks. Compared with unfilled acetal copolymer, it has 40% higher flexural modulus and 45° F higher heat resistance.
Nano fire extinguisher
Flame-retardant properties of nanocomposites are of interest on many fronts, as indicated by ongoing work at NIST and its new consortium. Over the last three years, NIST fire researchers demonstrated that the rate of heat release for nylon 6 nanocomposites with 5% clay is 63% lower than the RHR of pure nylon 6. There was also no increase in carbon monoxide or soot generated during combustion. Though the mechanism of nanocomposites' FR action is not known for sure, a durable carbon-silicate char was observed to form very rapidly in the burn tests.
NIST's work last year showed that the reduced flammability of nanocomposites extends to other thermoplastics, including PP and PS. In one study, PP samples were grafted with 0.4% maleic anhydride to make them more compatible with the clay. The chart shows an RHR reduction of more than 75% with the PP-g-MA nanocomposite containing 4% silicate.
Fire researcher Jeffrey Gilman thinks that nano-clay in combination with reduced amounts of conventional FR fillers such as ATH and MgOH at lower use levels are likely to meet FR standards such as 94V-0 and V-2. Gilman notes that Showa-Denko has reported V-0 performance of nylon 6 nanocomposites containing melamine.