DECEMBER 2010 3

F O C U S O N P O W D E R C O A T I N G S

comparison to charge controlagents, the charge stabilizersshow lower changing strength butgive long-term charge stability.

Environmental conditions suchas relative humidity andtemperature affect triboelectriccharging. At a given temperature,the charge decreases withincreasing relative humidity. Thiscan be due to increased leakagecaused by a decrease in theelectric resistance on the surfaceand, it is found, that chargedecreases more rapidly where thecharge particles are kept at ahigh relative humidity. Chargealso tends to decrease withincreasing temperature even whenthe relative humidity is low.

In powder handling operations,individual particles acquire chargeduring collision with walls. In gas-solids pipe flow, particlesrepeatedly collide with the innerwall, resulting in charge transfer.When a metal pipe is grounded,the charge transferred from theparticles to the wall flows to earthas an electric current. Particlesthat are being continuously fedare dispersed into airflow throughan ejector, and to increase theefficiency of the particle charging,spiral pipes can be used insteadof straight pipes. To confirm thecharge balance in a system, it isusual to assess the charge-to-mass ratio of particles at the inletand outlet of the metal pipe byconnecting them to a Faradaycage. In general, thereproducibility of the triboelectriccharging of particles is poor, butcontrol of the charge on particlesis possible using the triboelectriccharging principles. Varioustechniques are then described inthis article, for the characterizationof the triboelectric charging ofparticles.

Separation by using the largelydifferent electrostatic properties ofsolid materials is a potentialapplication for the recycling ofwaste materials. If the particles orsmall solids have widely differentelectrostatic properties then theycan be separated by triboelectricalcharging followed by feeding

these charged particles into anelectric field separator. Theparticle trajectories are deflectedin the electric field according tothe polarity and the amount ofcharge on the particles. Althoughthis method is simple and easy toapply, the charge transfer dependson the environmental conditions, sothat temperature and humidityshould be controlled within acertain range. A typical electrostaticseparator charges the particles in acyclone prior to entering theseparator zone where a DCelectric field is applied. Thetrajectory taken by these particlesdepends upon both charge andmass and the various fractions arecollected in a series of trayslocated on the bottom of theseparator.

The authors make little or noreference to the importance oftriboelectric charging in theapplication of powder coatings,but they give a comprehensiveoutline of the photocopyingapplications, which is often citedas the inspiration for applyingtribo-charged powder coatings. Inthis process, a visual image iscreated using electrostatic latentimages in the form of surfacecharge patterns on a photo-conductive surface. These visualimages consist of fine tonerparticles, which are triboelectricallycharged by mixing them withlarger carrier beads, although this“developer” mixture can also becharged by impact without theneed for carrier beads. Photo-copying is essentially a six-stageprocess. The surface of thephotoreceptor, which is aninsulator in the absence of light,is uniformly charged. Lightreflected from the image removesthe insulating effect producing alatent image – a pattern of chargeacceptance that mirrors theeventual formation of the realimage. Toner particles adhere tothe latent image transforming itinto the real image.

The developed toner on thephotoreceptor is transferred to thepaper charged on the rear side.The image is fixed by melting the

toner and the photoreceptor isfinally discharged and cleaned ofany excess toner.

In black and white copiers, thephotoreceptor can transfer theimage directly to paper. Colourcopiers form the image from fourbasic colours and the image isbuilt up initially on an intermediatesurface and then the full colourimage is transferred to paper.

The main purpose of thisarticle is to record the existingknowledge of the triboelectricphenomenon. There is still muchwork to be done before the fullpotential of this technology isachieved.

Article entitled “Triboelectric Charging ofPowders: a Review” by researchers atseveral Japanese Universities and theUniversity of Leeds, UK, published inChemical Engineering Science, 15 Nov 2010,65 (22), 5781-5807

INDUSTRYNEWS

DSM Powder Coating Resinsannounces price increase forEurope, Middle East and Africa

DSM Powder Coating Resinsannounced a price increase for itsrange of Uralac© powder coatingresins sold in Europe, MiddleEast and Africa effective for allorders shipped on or after 1 Nov2010, or as otherwise permittedby the terms and conditions ofcustomer contracts.

Prices are increased by €0.10-0.12 /kg for shipments fromEurope and by $0.13-0.15 /kg forshipments from Asia. Prices arebeing adjusted due to the strongincrease in costs for feedstocksused in the manufacture ofpolyester powder coating resins.

Press release from: DSM Powder CoatingResins, Zwolle, The Netherlands, tel: +31 (0)38 4569319, website: http://www.dsm.com(22 Oct 2010)

Tiger Coatings celebrates 80-yearanniversary

Austrian coatings manufacturerTiger Coatings in Wels celebrated