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SPECIAL APPLICATIONS

NANOPREG

FIBERPREG

PLYSTOCK

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SPECIAL APPLICATIONS

NANOPREG

	One of the most challenging issues in the fabrication of thermoplastic concentrates of micro-nano powdered solid particles, is the ideal dispersion of all individual particles in the liquid resin.

	The dispersion can be claimed to be ideal when all individual aggregates (the elements composing the agglomerates) are surrounded by the resin.

	This is not an easy task for many reasons:
	Often the flowing polymer macromolecule is bigger than the interparticles passages within the agglomerates where the liquid needs to be infiltrated The allowed time for the melt to intercalate among the particles is limited The powder is a troublesome object to handle both technically but especially for environmental (worker's health) reasons

	For all above reasons thermoplastic nanocomposites are often more academic claims than industrial realities.

	When nanoparticles show a large aspect ratio, (e.g. from 50 to 1000), typically elongated particles like tubes, fibres etc., it is important to know their RCP (Random Close Packing) value, which is a semi-quantitative rule to evaluate the maximum volume fraction for the particles to fill up a unit volume. The more aspect ratio approaches to 1 and the higher volume fraction is needed to fill up a unit volume. For the spheres RCP is about 0,74 (74% of unit volume) as discovered by Kepler in 1611, the famous Kepler conjecture, confirmed today by T.C. Hales (2005).By the other hand, for rigid needles, the RCP may drop down to very low values (<<0,001).

	When the properties to be transferred to the plastic matrix require just an interparticle material contact, it is sufficient to create a percolation network, where all or almost all particles are in mutual contiguous punctual contact. In many cases this very extended branched structure allows for two basic advantages: a) some properties depending on physical punctual contact, like electron or conduction heat transfer, may be effectively transferred to the polymeric matrix, despite the particle mass involved is very low. b) the properties (especially mechanical) of the native polymer will be preserved despite fillers, as fillers are a minimal wt percent. These advantages make possible to use nanoparticles in may applications despite their high price.

	For all nano particles, even with high aspect ratio, the agglomerates have monotonically round shaped geometry. Recalling the RCP rule for spheres we argue that many more agglomerates are needed to get a percolation network of spherical agglomerates than individual elongated particles. In other terms the worse is the dispersion and the more nanoparticles wt percent is needed. For example more than 20 times agglomerates weight is needed as compared with individually dispersed particles, to achieve similar results, for example in case of nanocomposites based on Carbon Nano Tubes (CNT).

	With some other nanoparticles the ideal dispersion aims to generate an extremely long interparticles maze, that gas molecules have to travel to pass through the polymer. This for example holds for nanoclays like montmorillonite clays dispersed in some polymer with the purpose to retard oxygen driven combustion. The flame retardant effect is achieved by slowing down the oxygen travelling through a polymeric wall.

	All above dispersions cases have in common that particles must be surrounded by the polymer in a way that the surface area interfacing solids with polymer is maximum.

	As anticipated above in many cases the conventional dispersive blending performed in screw processor, is unable to solve effectively the dispersion problem.

	Nexxus Channel has developed a novel system to disperse nanoparticles in a thermoplastic liquid splitting the dispersive process in two basic stages: 1) pre-disperse the particles in water together a proper surfactant. 2) Feed the slurry in a superheated condition, mix with the melt and remove water (vapor) under high vacuum.

	Pre-dispersion can be done easily with the most advanced technological options available: at room temperature with zero time constrains at shear rates as high as up to and more than 10⁶; s^-1, and with the aid of a liquid (water) which has a very low molecular weight and may infiltrate easily almost everywhere In a way to accurately tailor the optimum interparticle distance needed to intercalate the polymer, case-by-case.
	Mixing and water removal are made thanks to a retrofitted Nexxus Degassing Module. In this special Module slurry and melt combine and mix during a very limited time (e.g. 1-2 s). The extremely short interfacial time is needed to avoid any possible hydrolytic degradation.

	Some preliminary tests have shown this application to be very prospective. In one case electric conductive properties of a PP sample filled with 1% wt CNT exhibited better values (>2E04) than a similar PP resin for same CNT content, made by a leading company specialized with CNT filled polymer masterbatches.

	Like most water dispersions and unlike powders, NANOPREG is VOC free at the feed port

	NANOPREG is virtually applicable to all concentrates of nano-particles, micro-particles, pigments, additives in powder form, and in general in all cases where an extremely large liquid-solid interface is needed

	The application is still in the validation stage.

High diluted CNT magnified by SEM fully wetted by water and surfactant. This water dispersed condition is the preliminary step for NANOPREG application. NT average diameter = 9,5 nm. Average CNT length 500 nm

High diluted CNT magnified by SEM before water/surfactant pre-dispersion. CNT average diameter = 9,5 nm Average CNT length &1500 nm

NanoOnSpect BOX NEWS An excellent opportunity to validate NANOPREG is dispersion of CNT in thermoplastic matrices according to the EU project NANOONSPECT (www.nanoonspect.eu)

FIBERPREG

Thermoplastic composites reinforced with mineral fibers (e.g. GF), synthetic fibers (e.g. CF) or natural fibers (e.g. jute, kenaf) play a rapidly growing role in designing and fabricating structural parts.

		In general fibers can be embedded in the polymeric matrix in the form of chopped strands or continuous woven filaments. Either option is used depending on the application. For example continuous woven filaments cannot be used when deep thermoforming is required, while chopped strands can.

The dispersion of highly viscous liquid into multiple filament roving meets many issues. The goal is to coat all individual filaments with the polymeric liquid within a handful of seconds (e.g.10 s). Imagine a 2400 TEX roving of GF filaments D=17 microns. Since E-glass has density 2550 Kg/m³, the total number of filaments is about 4200. Obviously it is not a trivial job to disperse 4200 filaments in a few seconds. To fulfill the job in extruders usually the mechanical energy discharged on the fibers is very high thus causing fiber breakdown and shortening.

		It is a common experience that 4,5 mm or longer fiber will shorten in the composite down to fractional millimeters (e.g. 0.5 mm). When 4,5 mm long fibers are shortened down to fractional millimeters they inevitably lose most of the native mechanical properties.

To overcome the above drawbacks sometimes long pellets are used, after off line pultrusion polymer pre-impregnated, roving. Unfortunately pultruded pellets are very expensive and not always give the expected results.

		Most of the above issues vanish with a Nexxus Mixing module. Here the fibers after being introduced in a Nexxus Mixing module (this especially applies to brittle fibers like glass or carbon) the fibers first are subject to a comfortable rotational movement occurring along the tapering section of the wedge shaped chamber, then are gently but vigorously
squeezed in the last parallel portion just before entering into the parallel pumping section. The fibers dispersion takes place due the optimum combination of wedge chamber geometry, final channel depth and rotor speed. The tapered channel volume can be filled by the melt thanks to the adjustable flow resistance applied at the discharge port. If the applied resistance is small then the wedge chamber tends to remain empty and the mixing time reduces. As much as the flow resistance is made to increase then the wedge chamber gets more and more filled and the mixing time increases. The wedge geometry is the basic design parameter to generate elongational flow components. Up to three or more tapers can be designed in cascade, if it is required.

All the above design and operative parameters make possible to get very well dispersed fibers at the discharge port of the Nexxus Mixing Unit, without shortening down to the original feed length.

To further upgrade the fiber mixing process Nexxus Channel is also developing an off-line, fiber pre-treatment in order to speed up the production. This Pre-treatment is still under development.

PLYSTOCK

The state of art technology to fabricate structural, thick, thermoplastic profiles faces two limits:

The semi-molten profile tends to sag under his own body gravity when entering into the calibrator (sagging).
The line speed is low due to the typical low thermal conductivity of thermoplastic polymers (0.2 W/m°C) which restrains a fast heat removal in the cooling tank. When the ratio of sectional area to the perimeter exceeds a critical threshold (e.g. 0,005), then the conventional profile extrusion becomes profitless. An extrusion line recently investigated in North Europe, for a profile 260x250 mm sourced from city post consumer’s HDPE, compounded with glass fibers to produce railway sleepers, while meeting the technicals specs, was unable to exceed the line speed of about 13 mm per minute corresponding to about 60 kg/h, thus the project was abandoned due to too high direct costs.

Plystock® bypasses the problem by splitting the profile manufacture into two steps:

• Production of GF reinforced, thin tapes assisted by a three roll calender.

• Heat face bonding of more tapes to fabricate single, multilayer solid profile.

A line speed of up to and more than 5 to 10 m/minute corresponding to a profile throughput of 500 to 10000 Kg/h can be easily achieved, with the line speed almost exclusively depending on the profile section. Reversing what normally happens with conventional extrusion, Plystock makes the profile throughput to increase vs section.

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