Researchers are using nanoparticles of clay to raise polymers to new capabilities. The ongoing interest in nanocomposite polymers is evidenced by two upcoming conferences on the topic scheduled later this year—one sponsored by Principia Partners in Baltimore in June and another hosted by the Canadian National Research Council’s Industrial Materials Institute in Montreal in September. The automotive area represents a lot of potential, particularly for exterior body panels and fascia, and such interior components as instrument panels. One indication of the interest level is the attention being focused on thermoplastic olefins, which is one of the fastest growing plastic groups used in exterior and interior automotive applications. Dow Chemical Co. of Midland, Michigan, and Decoma International of America in Troy, Michigan, are jointly investigating nanocomposites as part of a NIST Advanced Technology Program. The present focus of the project is understanding the fundamentals of processing, developing, and compounding nanocomposites. Dow is also looking at synthetic nanofillers, which may offer advantages in consistency over natural clay feedstocks.
In the late 1980s, Toyota Central Research Labs in Japan teamed up with Ube Industries Ltd ., a Japanese resin supplier, to produce a new composite polymer, consisting of nylon 6 interspersed with layers of montmorillonite, a layered silicate clay. The clay greatly improved the mechanical properties of the nylon with very small filler loading.
Toyota subsequently used the material for a timing belt cover, capitalizing on the material's heat resistance and dimensional stability, and proving its viability for under-the-hood automotive applications
The work was significant because the companies succeeded in creating Toyota's nylon-clay composite by working at the nanometer level. Clay particles just 10 angstroms thick were dispersed in a homogeneous mixture in the polymer matrix. Very low loadings of filler-less than 5 percent by weight-produced big gains in properties, compared with levels of 30 percent or higher with reinforcements of glass and talc. The prospect of dramatic weight savings and improvements in properties set off research activity for applying this technology to other polymer families.
"A lot of researchers believed that if this could be done with nylon, the concept of having nanocomposite plastic can occur in a whole variety of plastics," said Jim Morton, a partner of Principia Partners, an Exton, Pa., research firm that published a report on nanocomposite resins in 1999. Over the last decade, nanocomposites have been a hot research topic among resin companies, government laboratories, and universities investigating the materials as a way to boost properties of a range of polymer families, from commodity resins to engineering plastics.
The ongoing interest in nanocomposite polymers is evidenced by two upcoming conferences on the topic scheduled later this year-one sponsored by Principia Partners in Baltimore in June and another hosted by the Canadian National Research Council's Industrial Materials Institute in Montreal in September
The automotive industry remains keenly interested in nanocomposites, attracted by the possibility of significant weight reductions, improved performance and appearance, and the recyclability that these materials offer. In addition, research has opened up whole new areas of possibilities: Clay fillers were also found to have excellent gas-barrier properties, which have potential advantages for packaging. Other areas include flammability-a subject of research at the National Institute of Standards and Technology in Gaithersburg, Md.-and specialty coatings.
There are two major suppliers of montmorillonite clays for nanocomposites, Southern Clay Products in Gonzales, Texas, and Nanocor of Arlington Heights, Ill.
Monmorillonite is attractive for plastics. Consisting of a layered structure of aluminum sandwiched between two layers of silicon, these clays provide high surface areas of contact between the platelet and the polymer matrix.
The clays are also unusual because they have an electrical charge, similar to ion exchange resins that are used to purify water. The clay surface has a negative charge, which can be counterbalanced by a calcium or sodium ion. This makes it possible to do an ion exchange reaction, essentially making the clay more like an organic so that it will be more compatible with the resin.
Although much of the research activity has occurred over the last decade, patents on dispersing montmorillonite in plastics go back to the 1950s.
Douglas L. Hunter, senior scientist and team leader of the nanocomposite effort at Southern Clay Products Inc., said, "One of the key issues with nanocomposites is that this is not a well-known technology from a dispersion standpoint." There is still a great deal to be. learned to achieve desired properties, he explained.
The clays are treated chemically to allow them to disperse in the polymer. Essentially, the chemical reaction causes the clay molecules to swell; that is, the clay layers separate. The increased separation and the chemical modification make it easier for the polymer to enter between the platelet layers
In principle, there is no reason that nanocomposites cannot be made from any resins, including thermoplastics and thermosets, said Andres Garcia-Rejon, a senior scientist responsible for polymer nanocomposite research at the National Research Council Canada's Industrial Materials Institute.
In reality, some resin groups can be made into nanocomposites more easily than others. Nylons, Hunter said, are relatively easy to make into nanocomposites. On the other hand, it's more difficult to disperse the clay in polyolefins.
Clay particles are incorporated into the polymer matrix by one of two approaches: during polymerization (the reactor stage) or by melt processing, in which the clay is compounded into the plastic. Some approaches combine the two methods. Another technique is to create a solvent dispersion and then strip off the solvent during the melt blending process, Hunter said. He sees a need to study chemical treatments that aid dispersion of the clay in the polymer and the way materials are processed in the extruder, which can also affect the success of achieving dispersion.
Peter Maul, president of Nanocor, said that another focus of research is combining nanoclays with conventional fillers, such as glass. "You may achieve the properties you are looking for with far less traditional filler," he said.
In Maul's view, although nanocomposites can theoretically be made of any resin family, economics as well as benefits need to be considered in assessing research potential. He noted that some areas may represent a volume that is too small to pursue.
Packaging and automotive are probably the application areas furthest along, according to Morton of Principia. Another promising area is construction, he added, which could benefit from the improved stiffness and mechanical performance of the composite materials.
EYE ON THE FUTURE: Nanotechnology
Kris Akkapeddi, the team leader for new nylon technology at Honeywell, said nylon-based nanocomposites can improve oxygen barrier properties by a factor of three compared to standard unfilled nylon in humid conditions
Several nanocomposite nylon resins have been introduced by various companies, including Eastman Chemical, Honeywell, and RTP Corp., for packaging with high gas-barrier properties.
The National Institute of Standards and Technology recently concluded a consortium studying the flammability aspects of nanocomposites. According to fire researcher Jeffrey Gilman, who was involved in the project, dispersed montmorillonite appears to be a char-forming catalyst. Information in a recently published paper posited that natl0- dispersed clay particles can convert non-char-forming polymers, such as polystyrene, to form char, offering improvements in fire retardancy and physical properties.
Nano and Automotive Applications
The automotive area represents a lot of potential, particularly for exterior body panels and fascia, and such interior components as instrument panels. One indication of the interest level is the attention being focused on thermoplastic olefins, which is one of the fastest growing plastic groups used in exterior and interior automotive applications. Yet because TPOs are non-polar-that is, lack a charge-it is more difficult to disperse the clay.
One resin supplier that has been working on TPO nanocomposites is Basell Polyolefins of Wilmington, Del. Basell and General Motors formed a joint development programs in 1997 to develop TPO nanocomposites for exterior body panels. At the time, Basell's Automotive Business Group and GM Research and Development displayed prototypes of exterior door and rear quarter panels.
Since then, Basell has developed a cOl1IDlercial nanocomposite TPO grade, HiFax DX277-for GM's exclusive use-for exterior cladding applications, such as body side moldings and rocker panels. Grades have been developed for both paintable and molded-in-color versions. Paint adhesion, paint durability, and ductility of the painted part are similar to unfilled and mineral-filled grades, according to Basell's program manager, Jim Keeler.
Keeler sees weight savings as a major advantage of TPO nanocomposites, because of loadings of less than 5 percent. In addition to less weight, the low filler content results in excellent low-temperature ductility, he added. The new material is targeted to replace mineral-filled TPO, he said. Keeler said nanocomposite TPOs also offer a wider processing window, which can mean reduced injection molding cycle times, lower molding pressures, and fewer appearance defects.
Bob Ottaviani, laboratory group manager of advanced polymers in the materials and processing laboratory at GM Research and Development, expects the automaker to specify a commercial nanocomposite exterior part within a year on either a new or current car platform.
"Looking at individual parts on the exterior, we can get from 7 to 21 percent reduction in weight," he said. He added that the company is seeking to replace more expensive engineering plastics now used in exterior panels.
Mixed Composite Systems
Research is also focused on mixed composite systems, in which nanoclays and conventional fillers such as glass are combined in the same polymer matrix. Nanocomposite polymers also offer some processing advantages, according to Ottaviani. He said that in some parts, molders have been able to reduce fill times-the time it takes an injection mold to fill with plastic-by a factor of three.
Ford has been investigating nanocomposites for the last three or four years, according to Jeff Helms, manager for materials at Ford Research. Helms sees typical applications as body panels, bumper fascia, tonneau covers, and truckbed liners, as well as interior trim. Nanocomposite resins are currently not used on any Ford vehicles.
Work has focused on polypropylene as well as some polyethylene resins. Olefins have rather low melting points and a fairly large coefficient of thermal expansion, which means they rely on inorganic filler content for stiffness, he noted. Very stiff materials require higher loading, which decreases ductility. Nanocomposite olefins offer the intriguing prospect of making an engineering plastic out of something that is basically a fairly inexpensive commodity resin.
Ford's focus has been primarily on compounded plastics; that is, taking the existing plastic and melt-blending the clay particles. "We are getting higher stiffness and some changes in thermal expansion, but not as high as we need," Helms said. "We lose in ductility, so low-temperature impact is sacrificed." He said the clays show variable results with different grades of resins, even within the same family.
Dow Chemical Co. of Midland, Mich ., and Decoma International of America in Troy, Mich. , are jointly investigating nanocomposites as part of a NIST Advanced Technology Program. Decoma is a supplier of automotive exterior fascia and body panels, and is affiliated with Magna International of America, which manufactures interior automotive systems such as instrument panels.
According to Jane Palmieri, development manager of new materials at Dow Automotive in Auburn Hills, Mich., the present focus of the project is understanding the fundamentals of processing, developing, and compounding nanocomposites. Dow is also looking at synthetic nanofillers, which may offer advantages in consistency over natural clay feedstocks, she said.
She sees the major benefit of nanocomposite polymers as high modulus. The main drivers for the industry are weight savings and possible cost savings, she said.
Palmieri noted that the performance of nanocomposites varies with the resin system. 'Just because you see a certain improvement and certain attributes when you use it with one resin doesn't mean you will see the san1e impact on performance in another resin. They are all unique."