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Electron micrograph of cured epoxy shows how Nanostrength SBM “self-assembles” into a well-defined “raspberry” morphology.
A new family of acrylic block copolymers is said to be easily mixed into nanoscale dispersions with various thermoplastics, thermosets, and elastomers—and to be miscible with those matrix polymers. As a result, these nano-additives reportedly provide combinations of properties not achievable with traditional additives, including impact strength plus high rigidity and transparency. They are targeted for use as toughening agents as well as compatibilizers.
Developed by Arkema in France (U.S. office in Philadelphia), Nano strength block copolymers currently are available in two versions, each made up of three blocks. One is an SBM copolymer of polystyrene, 1,4-polybutadiene, and syndiotactic PMMA. The other is a symmetric MAM copolymer of side blocks of PMMA surrounding a center block of poly(butyl acrylate).
The SBMs are polymerized with a new patented Arkema anionic technology that yields a highly syndiotactic structure for the PMMA block. Compared with regular PMMA, sPMMA reportedly has both a higher thermal stability and higher glass-transition temperature. The polar MMA and nonpolar styrene blocks are compatible with polymers such as epoxies, styrenics, PC, PPE, PVC, and PVDF.
Because of the repulsive interactions between the three dissimilar blocks, when SBMs are blended with a polymer compatible with one block, they “self-organize” into very small domains forming a nanostructure in certain thermoplastics, epoxies, and elastomers. Their polar/nonpolar composition also suits them to act as interfacial agents for many incompatible polymers in new types of polymer blends, composites, and elastomers.
Joe Bohen, Arkema’s worldwide director of commercial development, says that unlike typical CTBN or ATBN epoxy tougheners (carboxyl-terminated or amine-terminated butadiene-acrylonitrile copolymers), Ar kema’s Nano strength provides excellent impact tough ening while maintaining both the stiffness and HDT of the epoxy matrix.
“The large do main of the CTBN or ATBN modifier in an epoxy matrix—on the order of tens of microns—makes it impossible to toughen ultra-thin applications where the thickness of the epoxy layer is also on the order of microns,” Bohen explains. “In contrast, the domain size of Nanostrength, formed via self-assembly, is on the order of tens of nanometers. The small rubber domain prevents rapid crack propagation and makes Nanostrength an excellent candidate for toughening demanding electronic applications.” Bohen adds that Arkema has been able to develop a new epoxy film technology that provides new processing opportunities, such as solventless production of B-stage composite prepregs.
There are five available Nano-strength SBM grades. Use levels are 3% to 10%. Grades E20 and E40 are designed mainly for toughening epoxy thermosets used in composites, adhesives, and coatings. Grade A123 is best suited for impact modifying thermoplastics, including engineering resins like PPE, PC, and blends. A250 is said to be an excellent compatibilizer, while A012 is targeted for modification of elastomers.
Arkema’s Nanostrength MAM group currently consists of M22, a highly dispersible grade for use in epoxy adhesives, composites, and electronic applications. Bohen says other grades in development have potential for use in epoxy advanced composites.