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Multi wall carbon nanotubes modified to resolve dispersion problems

Carbon nanotubes (CNT) have generated a lively interest in the scientific community after their discovery in 1991. CNTs are known for exhibiting unique mechanical, electrical and thermal properties, useful for a wide range of applications in materials. In the semiconductor industry, carbon nanotube-based products procure very promising additives. The electrically conductive fibrous structures of carbon nanotubes finely dispersed in materials, increase the number of electrical contacts and ensure a smoother flow of electrons. Therefore, a lower content of additive is needed for a given resistivity, in comparison with other carbon fillers or metals, when designing antistatic plastic materials for uses in clean room environment, hereby decreasing the probability of contaminating microchips and other sensitive components. The trends in electronics towards smaller components will only be possible with cleaner materials. There is a lower risk of contaminating delicate electronics during processing and handling because of different characteristics. Carbon nanotubes have antistatic properties in PEEK, PPS, PC and other polymers at a low level of addition and a precise control of electrical conductivity and therefore volume and surface resistance due to superior dispersion. Furthermore, carbon nanotubes contain a high purity with no residual sizings (like carbon fibres) or sulphur outgassing (like carbon blacks). Another important characteristic of CNTs that insures the lower risk of contaminating delicate electronics is that there is no sloughing in abrasion testing. CNTs are thin hollow tubes made of carbon atoms. A graphitic sheet is rolled up on itself to form a cylinder. If the cylinder has only one wall, it is called single-wall carbon nanotube (SWNT). When cylinders are assembled one inside another, they are called a multi-wall carbon nanotube (MWNT). The dimensions of CNTs range from a few nanometers to dozens of nanometers in diameter and from the micron scale up to the mm scale in length. This geometry imparts a very high aspect ratio, which is critical to enhance most properties of polymer composites. MWNT produced by chemical vapour deposition are the most common type of carbon nanotubes.
Carbon nanotubes are produced in a reactor in the form of a black powder. Their micro-structure consists in highly entangled agglomerates of individual cylindrical carbon nanotubes. The intense agglomeration is caused by strong Van der Waals interactions. Such a microstructure presents challenges like dispersion, stability and processability. To gain maximum benefits from CNT reinforcement in polymer composites, it is critical to exfoliate the entangled structure into an individually dispersed one. Once they are nicely dispersed, the stability of dispersion is even greater. However, multi wall carbon nano tubes have some major problems. They are:
  • Dispersion
  • Dispersion stability
  • Viscosity
  • Filtering effect
Strong surface interactions make the particles stick together. Dispersion is therefore very difficult. All parameters of dispersion process like matrix viscosity, particle loading and temperature changes should be optimized. While introducing nano particles in polymers heating can cause breakage of dispersion. The particles will then re-agglomerate inside the polymer. The composite will not exhibit the expected properties. The introduction of nanoparticles inside the polymer matrices drastically increases viscosity due to their strong intrainteractions and their high aspect ratios. If the nanoparticles are integrated inside the polymers, some processes become very difficult to perform. An ideal matrix for the production of composite materials loaded with nanoparticles should have a homogeneous dispersion quality and stability throughout the processing temperature range, a low viscosity, and avoid any filtering effect.
Demand for carbon nanotubes is increasing rapidly in electrical, mechanical and health and medical applications due to their thermal, electrical conductive and other properties. The global carbon nanotubes market is projected to exceed US$1.9 bln by 2010, at a CAGR of more than 80% as per a report by Global Industry Analysts. CNTs play a significant role in the nascent nanotechnology industry. Development of cost effective production methods and quality improvement are factors that are expected to drive carbon nanotubes market over the coming years. Rise in demand and production, and easy accessibility of carbon nanotubes would lead to extensive use of CNTs in a wide variety of applications. Nanotubes market, which was growing at a moderate pace till 2004-2005, is expected to expand at a skyrocketing pace in the coming years. United States currently dominates the world market in terms of revenues, while European market is projected to be the fastest growing market during the analysis period at a CAGR of over 100%, as stated by Global Industry Analysts, Inc. Together, United States and Asia-Pacific account for over 75% of the global CNTs market. Asia-Pacific region is poised to lead the regional market in terms of revenues and reach US$2.5 bln by the year 2012. Development and use of cost-effective production methods are expected to bolster worldwide carbon nanotubes market. Various end-use sectors such as electronics, field emission devices, batteries, research institutions, composites, and others are enabling carbon nanotubes to grow rapidly, over the last couple of years. However, high cost is one of the major issues in the carbon nanotubes industry. Expansion of manufacturing facilities by companies is expected to fuel the production of CNTs, while reducing prices.
Multi walled carbon nanotubes market was estimated at US$290 mln for 2006. Revenues generated from multi walled carbon nanotubes are high due to their simple production process and low cost compared to single walled carbon nanotubes. However, in the coming years, single walled carbon nanotubes market is projected to overtake multi walled carbon nanotubes market and projected to cross US$5 bln by the year 2012, at a very high CAGR of 200%. Key players dominating the global carbon nanotubes market include Catalytic Materials, Hyperion Catalysis International, ILJIN Nanotech, Nanocyl, Nanoledge, Raymor Industries, Rosseter Holdings, Shenzhen Nanotech Port, SouthWest NanoTechnologies, Sun Nanotech, and Unidym.
Nanotubes are typically 1000 times smaller in diameter than conventional carbon fibers. SWCNTs have a typical outside diameter of 1-2 nanometers (nm) while MWCNTs have an OD of 8-12 nm. They can range in length from a typical 10 microns to as long as 100 microns and have at least a 1000:1 aspect ratio. Carbon nanotubes have 10-50 times the tensile strength of stainless steel (20 to 100 GPa vs. 2 GPa) and five times the thermal conductivity of copper, with the SWCNTs at the high end. When incorporated into a polymer matrix, they can boost electrical and thermal conductivity by orders of magnitude over the performance possible with traditional fillers such as carbon black or metal powders. Amazing physical properties are seen when a single tube is measured, but translating these properties into plastic composites has proven to be difficult. Dispersion issues need to be worked out as there is a strong need for straight, defect-free, high-purity tubes that disperse easily during processing.
Nanocyl has developed a range of products so that they solve all the problems. Nanocyl integrated its approach down the line and investigate all the byproducts that can enter the production of composite materials. These were tuned to comply with nanotube technology and some of them were converted into commercial products. The products are classified in three different families:
Sizicyl™ fiber sizing
The concept of this patented product is to “size” the fiber surface with a mixture of polymer containing carbon nanotubes. The process involves dispersing carbon nanotubes in a specifically engineered polymeric sizing agent and then applying this mixture (Sizicyl™) on the surface of fibers with conventional sizing methods. This process results in a fiber with some nanotubes already embedded on its surface that can be woven into conventional textiles. This product was designed to overcome injection-related issues (filtration effect and viscosity). The fiber can be considered as a different vehicle to integrate CNTs inside composite materials. Nanocyl has developed different Sizicyl™ formulations which are compatible with CNTs and the different fibers/sizings.
EpoCyl™ epoxy resin-based formulation and masterbatches
These products are based on CNTs in a wide range of epoxy matrices and have been commercialized for quite some time now. CNTs are integrated in the epoxy matrices in a dispersed form, enabling their use as formulated systems or masterbatches. Nanocyl has developed two ranges of EpoCyl™ products: formulated products and masterbatches.
PregCyl™ semi-finished products
The polymer system used for this range of products is EpoCyl™ NC R2 HM01, which is also available in the range of formulated products for prepreg systems.
Nanocyl™ NC 7000
Is the latest of Nanocyl’s innovative Multiwall Carbon Nanotubes. These carbon nanotubes are unique and prized world-wide because their small size and high aspect ratio (>150) lets them form a network of conductivity at a very low concentration. It is produced in multi-tons, via the Chemical Vapor Deposition (CVD) process. Nanocyl is installing a new reactor with a capacity of 400 tpa for producing its NC 7000 carbon nanotubes, ccheduled to come online in July 2010 in Belgium.
Bayer MaterialScience AG has started construction of a facility for the production of CNTs in Germany with capacity of 200 tpa. Baytubes® multi-wall carbon nanotubes are typically used in a polymer matrix or in metal systems to produce tough, strong, lightweight materials. It can be used in a broad spectrum of applications, such as rotor blades for wind turbines, transport containers and sports equipment. Bayer MaterialScience unveiled a new process for making electrically conductive MWCNTs on an industrial scale with consistent purity and considerably lower cost. Bayer sees potential for its Baytubes® in electrostatically paintable automotive parts, antistatic films for packaging electronic components, and EMI shielding of computers and cell-phone housings. Baytubes® are said to comprise up to 15 graphene layers (more than most other MWCNTs, which typically have 6-7 layers).
Most of the commercial activity involves the use of hollow multi-wall carbon nanotubes (MWCNTs), which are a single tube with 5 to 15 layers; and the closely related carbon nanofibers (CNFs). Single-wall carbon nanotubes (SWCNTs) are less advanced commercially and are which are hollow tubes with one layer. High cost creates a barrier to widespread commercialization of all carbon nano-particles, particularly for SWCNTs, as their prices are 50 to 100 times above MWCNTs or CNFs. However, all carbon nano-particles are gradually seeing a decline in price with an increase in capacities by most producers.
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