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A dry impregnation process based on the use of an alternating electric field to charge the powder particles serves to deposit materials in powder form on all kind of porous structures, including nonwoven materials, fabrics, papers and foams.
(Published on May-June 2006 – JEC Magazine #25)
BY JÉRÔME VILLE, TECHNICAL SALES MANAGER, FIBROLINE
This technology is compatible with insulating powders such as thermoplastics or thermosets and also conductive ones such as carbon black, allowing many different combinations of polymer matrix and reinforcing fibres, in particular natural ones. Apart from process optimization, the main challenge is to adapt powder particle size to the porosity of the substrate. Fibroline technology offers the advantage of homogeneous powder distribution, while remaining environment friendly and consuming very little energy (electric field rather than current, no water evaporation).
The Fibroline team has been carrying out feasibility and R&D studies. In the first experiments on small samples (A4 size, for instance), the powder was distributed evenly on the surface of the structure and the samples placed between two plate electrodes. An alternating field was applied and the powder started to impregnate the porous substrate. When only powder was placed between plate electrodes, electrically charged particles with the same polarity filled the inter-dielectric space exactly like a cloud, explaining the optimum distribution during impregnation. These lab trials have enabled Fibroline to define working process parameters and easily scan all relevant powders. In addition to this lab unit, a one-metre-wide pilot line has been installed in Ecully to validate the efficiency of the technology with the customer on a semicontinuous basis and allow the production of “industrial” prototype samples of a few m² in size.
A fully optimized industrial unit manufactured by Strahm. T.S. of Switzerland, is operating in line with a winding/unwinding system, a flat belt calender, and other post treatments, forming an integrated industrial line. This equipment is being used for largescale validation, and several industrial partners have already tested the process or the market before investing in the technology.
The line will also be used in the next few months to start up the production of products developed by the company, such as naturalfibre thermoplastics (NFT), glass-mat thermoplastics (GMT) and products for other specific markets.
As it is used for very different applications, the unit was designed to be very adaptable, with the capacity to switch from thick natural fibre felts to thinner porous structures.
NFT and GMT applications were the first ones investigated. The fibrous substrates were quite dense and the use of fine powders was initially necessary. The main conclusions were that the cryogenic grinding of the PP powder was an economic obstacle to the success of those applications. More recently, following a major R&D effort, the Fibroline team developed a new NFT/GMT concept starting from low-cost thermoplastic powders, recycled ones in particular. These were used in combination with different kinds of structures, leading to a cost-effective result with high flexibility. It is very common to skip from one thermoplastic reference to another (MFI or other properties) but also to adjust the surface weight of the reinforcement. A patent is being filed on this innovative industrial process.
For these applications, the company intends to exploit the process in two different ways. On the one hand, we’ve seen that very specific needs from small companies or for small volume markets are often difficult to meet, as existing large suppliers are not always interested in such markets. The easy adaptation of the Fibroline process will allow us to satisfy such markets with our own industrial line. On the other hand, Fibroline proposes, within the framework of a license agreement, to sell a complete industrial solution that includes an impregnation unit and adapted post treatment, for big volumes and low-cost price requirements.
This prepreg concept is also suitable for thermosetting resin applications. In this case, however, the Fibroline technology enters into competition with liquid technologies.
Thermosetting materials are often used in powder particles, in particular for the coating industry, thus a wide range of resins are available in powder form, depending on the properties required. The Fibroline process allows combining natural fibres with various materials, such as epoxy, PES, and phenolic resins. In this case, the main advantage of dry technology is environmental, as no VOCs are released into the atmosphere. In collaboration with car-part manufacturers, the company started working on kenaf fibres reinforced with 20 to 35% thermosetting resin because of the specifications for mechanical and insulation properties. Fibroline and powder suppliers have carried out many trials on this application to optimize the curing time and the releasing properties of the matrix. Due to high production-speed objectives, these powder resin formulations also need to have satisfying out-of-the-mould stiffness. Maybe the most important conclusion to be drawn from these trials is that Fibroline makes it possible to decrease the quantity of polymeric matrix material thanks to the optimal distribution, while preserving mechanical characteristics.
Another positive effect of the process on mechanical results from the modification of the fibre surface, and is also being investigated by Fibroline’s team. Thanks to the effect of air-particle ionisation, polar species and free radicals are created, improving the wettability of the polymer and thus adhesion to the fibre surfaces.
All this experience (mainly for automotive applications) is very useful for the team, which is currently developing other naturalfibre concepts, especially for insulating structures. Fibroline is the only technology able to distribute a tiny percentage of powder, exclusively there where the fibres crosslink, resulting in new product characteristics.
It is interesting to note that the homogeneity of the process due to similar electrical charges between particles is exactly the same with a single component as with a blend of powders. This capacity to distribute several powders simultaneously throughout the entire space of a structure provides the possibility for bi (or more)-component matrices, opening the doors to new products.
For instance, natural fibre felts could be simultaneously impregnated with a polymeric matrix and specific powders with fireretardant or bacteria-resistant properties, or inorganic fillers for added properties or for reduced cost. In particular, technical studies on epoxy/natural-fibre composites carried out in collaboration with the French Institute of Textile and Clothing (IFTH) show that the mechanical properties are diminished when the fillers are integrated during the resin formulation step (currently the process used). The mechanical properties are not affected when the fillers are mixed during the Fibroline impregnation process (see figure above).
Clearly, these characteristics depend on the inorganic filler associated with the resin.
The latest optimisations of the electrical signal – but also a better understanding of the process – have recently shown that combination of an homogeneous distribution (matrix, most of the time) can be achieved in parallel with a distribution of a functional powder in the upper layer of the product, leading to standard composites with specific surface properties (fire retardant, hot-melt properties for adhesion, etc.).
The latest composite concepts associating natural fibres with polymeric resins being developed by Fibroline concern bio-degradable polymers. PLA or other PES vegetal-based matrices probably represent a future opportunity for the automotive industry, in spite of their price although this is decreasing. For these easily grindable materials, Fibroline dry impregnation is a flexible way to obtain prepregs or plates to be moulded in a second step.
Further to this private R&D programme, an ACTRA project on “Green composites” regr ouping industrial partners (Visteon, Irisbus), technical centres (IFTH, CERMAV), and Fibroline was launched. Mechanical properties and biodegradability will be studied, keeping in mind economical requirements.
As mentioned above, Fibroline developments associating resins with natural fibre materials mainly concern semi-structural parts for the automotive sector or building products such as insulation panels and furniture. However, natural-fibre structures are also used for other kinds of applications being investigated by the company. Our R&D team has also developed innovative geotextile and agro-textile products that are scheduled to enter into production in 2006. Powder impregnation can be used for waterretention applications with a natural fibre structure, allowing water savings. The same concept with an anti-bacterial agent is being developed with promising results.
In a nutshell, this innovative process has already served to develop some interesting natural fibre applications, but Fibroline’s team is still poised to confer with partners, confident that the best is to come. The company believes that the best way to help sustainable growth in the present global competition is to use technical progress in association with the protection of intellectual property.