Nanomaterials incorporated into adhesives and sealants is rarely publicised but could have important implications for the electronics and packaging sectors in particular.
According to the Adhesive and Sealant Council (ASC), the global adhesives and sealants market in 2011 was worth $40.5 billion. Nanoscale materials incorporated into fillers, rheological modifiers and coupling agents provide adhesives and sealants with improved properties such as increased strength and flexibility, rheological control, flame resistance, and improved durability and recyclability.
Adhesives and sealants incorporating nanomaterials are mainly found in the construction, electronics (metal oxides, silver nanowires, silver and nanotubes as electrically conductive interconnects), automotive and aerospace/aviation sectors. The main fillers used in the production of nano-enabled adhesives and sealants are fumed silica, nano precipitated calcium carbonate, carbon nanotubes and metal oxide nanomaterials. Newer nanomaterials under development include graphene and nanocellulose.
Metal oxide nanomaterials
Metal oxide nanomaterials allow for structural adhesives with a combination of thermal, electrical or thermoelectrical properties which also provide higher environmental durability due to their lower water absorption and enhanced ageing properties. Adhesives containing magnetic nanoparticles can be heated using an external magnetic filed resulting in rapid curing. This allows for a shorter bonding process and lower process temperatures.
Bayer MateralScience (www.bayercoatings.de) produce Dispercoll® S dispersions, waterborne, anionic colloid dispersion of amorphous silica with a particle size of 5 to 100 nm for use in adhesives and sealants. Applications are in the automotive, furniture, wood, textile, paper and flexible packaging sectors.
Figure 1: Dispercoll® S dispersions (Credit: Bayer MaterialScience)
Evonik (nano.evonik.com) produces Magsilica, a nanoscale iron oxide embedded in silicon dioxide for use in adhesive formulations to allow for bonding and debonding on command. The concept is to use an alternating magnetic field applied by mobile generators to heat the nanoparticles in an adhesive layer locally so that the layer can cure in the field. Subsequent heating at a higher energy rating would cause the cured adhesive to soften and debond. This technique could also be used to cure or debond adhesive materials or coatings at specific locations on a substrate. The adhesives have significantly improved tear resistance, fracture toughness and modulus and significantly improved adhesion to inorganic substrates (e.g. to glass).
Precipitated calcium carbonate (PCC)
Nanoscale calcium carbonate can greatly improve the hardness, flexibility, strength, bending resistance, abrasion resistance, the white degree and surface purity of polymer products and improve polymer processing function. Compared with the general calcium carbonate, it can better dispersion in rubber, PVC, PP and PE. PCCs act as fillers in sealants. Due to their scale (down to 60nm) they add reinforcement and increased strength. This scale allow for a large number of particles capable of interacting and building rheologcal structure, adding viscosity, yield value, control of slump and sag and ease of application. Companies producing Nanoscale PCC include Specialty Minerals (www.specialtyminerals.com).
Fumed Silica
Nano-fumed silica is a commercial nanopowder produced by high-temperature gas phase processes via flame synthesis technology. It has excellent reinforcing, thickening and thixotropic features and has found application in adhesives as well in rubbers, inks, colloid storage batteries and composite insulators. Evonik Degussa, Wacker Chemie AG, and Cabot lead the market.
Carbon Nanotubes
Nanotubes have been used to reinforce epoxy adhesives for application in packaging, sporting goods, automotive, electronics, footwear, construction repair and remodelling, textiles, consumer goods, and shipbuilding, etc. Applied Nanotech Holdings, Inc. produces CNTstix™, an ultra-strong carbon nanotube reinforced epoxy adhesive for structural applications. Tested by a leading independent laboratory, the adhesion tear strength of CNTstix™ is more than 60% higher than that of a popular adhesive manufactured by a leading industry competitor. Zyvex Performance Materials produces a Epovex Adhesive™ line of two-part epoxy adhesives they claim are stronger and less expensive than conventional aerospace adhesives. According to Zyvex laboratory tests have proven the superior T-peel and shear strength of Epovex Adhesives in composite to composite bonds, composite to metal bonds, and composite to wood bonds. In independent testing against leading industry competitors, Epovex Adhesive demonstrated 50% greater performance in T-peel strength (ASTM standard D1876) and a 15% improvement in shear strength (ASTM D1002).
Graphene
The high aspect ratio of graphene makes it an attractive filling materials for conductive adhesives, as less material is needed to form a percolative network. This conductive composite with adhesive properties can be used in forming electrical circuits and thin films on temperature sensitive substrates. XG Sciences produces graphene for application in adhesives. xGnP™ consists of exfoliated graphite nano-platelets. They are small stacks of graphene sheets made through a proprietary manufacturing process. These platelets can be produced in sizes ranging from 5 micron to 25+ microns in diameter and with various surface treatments. They are typically supplied as a dry powder.
Figure 2: Image of graphene structure
Nanosilver
Nanosilver filled conductive adhesives and pastes find application in electronics packaging. In comparison to conventional soldering-based interconnection technology, electrical conductive adhesives have a number of advantages including: finer pitch capability, lower processing temperature requirements, and are more environmentally friendly than lead-containing solders. Silver nanoparticles are being incorporated into conductive adhesives and pastes mainly to improve electrical conductivity and mechanical stability. Silver has the highest room temperature electrical and thermal conductivity among all metals.
Nanocellulose
Cellulose is a biopolymer consisting of long chains of glucose with unique structural properties, whose supply is practically inexhaustible. Nanocellulose is natural and renewable, biodegradable, biocompatible, has high strength and modulus, high surface area, high aspect ratio, chemical functionality (e.g. for modification), dimensional stability, moisture absorption properties and thermal stability (~200oC) amongst others. There are three different domains of nanocelluosic materials:
• NFC NanoFibrillar Cellulose
• NCC NanoCrystalline Cellulose
• BC Bacterial Cellulose.
In recent years, composite materials of cellulose and conductive polymers have received significant attention. Nanocellulose is being developed for strength enhancing additives for renewable and biodegradable composites, with the cellulosic nanofibrillar structures as a binder between the two organic phases are being produced for improved fracture toughness and prevention of crack formation for application in packaging, construction materials, appliances and renewable fibres. They are attractive because of their wide abundance, their renewable and environmentally benign nature, and their outstanding mechanical properties.
Applications include:
• Reinforced polymers
• High-strength spun fibres and textiles
• Advanced composite materials
• Films for barrier and other properties
• Additive for coatings, paints, lacquers and adhesives
Strength enhancement with nanocellulose increases both the binding area and binding strength for application in high strength, high bulk, high filler content paper and board with enhanced moisture and oxygen barrier properties.
Figure 3: Cross-section of a fracture surface of a cellulose nanofibril film. (Credit: Courtesy of American Chemical Society)
There are still a range of issues to overcome before nanomaterials will be widely incorporated into adhesives and sealants, especially cost. However progress is being made as industry recognizes the benefits afforded by these materials, not least of all environmental sustainability. In a recent development researchers at the University of Akron (UA) received funding to develop their research into the adhesive properties of gecko feet. The researchers at UA have created a new kind of “glueless” adhesive that sticks without the stickiness. The spin-out company, ADAP Nanotech (http://adapnanotech.com) utilizes carbon nanotubes to produce NanoTIM dry adhesives for application in microprocessors, electrical circuits, sporting goods, solar cells and display devices.
