ADVERTISEMENT FEATURE
The new nano option for conductive plastics
Plastics are appreciated as electrical insulators but a growing number of specialist applications – including EMI and ESD – require polymers that offer permanent electrical conductivity. Carbon-based additives such as carbon black and, more recently, carbon nano- tubes (CNTs) can provide this electrical performance but typically these additives present challenges in terms of compound- ing or processing. TUBALL is a new CNT from OCSiAl that can overcome these problems.
Most commercially available CNTs are
multi-walled types (MWCNTs). OCSiAl has developed a technology for volume production of single-walled carbon nanotubes (SWCNTs), which are some- times referred to as graphene nanotubes. Its TUBALL SWCNTs feature an outside mean diameter of 1.8 ±0.4 nm and an extremely high aspect ratio (typical lengths are greater than 5 micron). Importantly for processing, the materials are available as SWCNT bundles rather than the agglomer- ates typical of MWCNTs.
Percolation threshold For any particle-like conductive additive a conductive path can be formed in a plastic matrix once a particular dosage is reached – the percolation threshold. Above this threshold, electrical resistivity drops due to formation of an intercon- necting conductive network through the matrix. A percolation threshold is also observed for tubular additives such as CNTs, but in the case of TUBALL the low diameter and high aspect ratio means the amount of material required to create this conductive network is much reduced. Key to exploiting the benefi ts of TUBALL
is achieving optimum nanotube dispersion to ensure that percolation occurs at the lowest dosage. This means overcoming the Van der Waals forces that exist between the nanotubes, allowing separation and
TUBALL nano additive from OCSiAl achieves high levels of conductivity in PP at much lower addition levels than traditional multi-wall CNTs and carbon black loaded compounds
homogenous distribution in the polymer matrix. Melt mixing is the most common and easily scalable process for dispersion of additives, pigments and fi llers in plastics and this article describes lab scale work carried out by OCSiAl to study dispersion of TUBALL SWCNTs in polypropylene. The work was focused on twin-screw extrusion using a two-step process involving production fi rst of a concentrate (master- batch) followed by dilution through a second compounding stage.
Experimental materials A high fl uidity injection moulding grade of polypropylene with a Melt Flow Index (MFI) of 25 g/10min at 230°C/2.16 kg was selected for both masterbatch preparation and dilution steps. The resin was used either as received in pellet form or ground to a powder prior to mixing with TUBALL. Preliminary investigations on micro-com- pounding equipment showed that wetting agents can facilitate SWCNT dispersion in plastics and Ionic Liquid (IL) was used in specifi c formulations with a SWCNT/IL ratio of 2:1. All ingredients were added via gravimetric feeders with the SWCNT dosage set at 2wt%.
Compounding trials were performed
on a Berstorff co-rotating twin screw extruder (25mm diameter, 48 L/D). The study compared two different screw profi les for masterbatch preparation: a distributive profi le with more mixing elements than kneading blocks and a dispersive profi le with a higher propor- tion of kneading elements. All com- pounds were then prepared using the distributive profi le.
Results and discussion Conductivity measurements were made on samples prepared under identical compression moulding conditions at 210°C and, on selected samples, disper- sion quality was assessed using optical microscopy. Table 1 shows that the masterbatch samples prepared during the experiments at 2wt% TUBALL were conductive and well above the percola- tion threshold of TUBALL in the matrix. No clear differentiation was possible between the concentrates.
In the second compounding step the masterbatches were diluted to 0.1wt% and 0.2wt% TUBALL in PP using the same twin screw equipment. To achieve the
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94
