The Tribo Electrification Studies Engineering Essay

– Introduction

Electrostatic effects on solids have been known for a long clip. The ancient Greeks in the 6th or 7th century B.C. already observed that brownish-yellow attracts little objects after rubbing. In fact, the term ”electricity ” is derived from the Grecian word for gold, Zlektron ( pronounced as ”elektron ” ) . Charging in the absence of electric Fieldss occurs by contact or clash between two solid surfaces. Contact bear downing involves the direct contact and subsequent separation of two surfaces without rubbing, whereas frictional bear downing involves comparative motions of two reaching surfaces. However, these two manners of bear downing are hard to be discerned independently in physical interactions. Therefore, the term tribo-electrification is conventionally used to depict the overall procedure, even though the prefix ”tribo ” literally means friction ( Swarbrick 2007 ) . Largely, pharmaceutical pulverizations are composed of i¬?ne atoms of insulating stuff, which are in contact with each other or with the surfaces of different devices environing the pulverization. These contacts cause that negatrons may be donated or accepted during assorted procedures. Electron exchange occurs because of surface possible differences between the reaching organic structures. These possible differences are chiefly caused by different contact stuffs but may besides be ini¬‚uenced by different atoms size, surface raggedness, etc. The difference of surface potency is compensated by interchanging charges. If the separation of the organic structures occurs fast that ‘s disabled the back flow of charge and the contacting organic structures remain charged. ( Karner and Anne Urbanetz 2011 )

Basic rule of tribo-electrification

The stuffs came in contact and so separated the electrical charge is transferred from one stuff to another. This phenomenon frequently called as contact electrification or contact charging, when they rubbed against each other. The contact charging can be classii¬?ed into three classs harmonizing to the contacting stuffs. These are metal-metal contact, metal-insulator contact and insulator-insulator contact ( Matsusaka and Masuda 2003 ; Karner and Anne Urbanetz 2011 ) . As above-named pharmaceutical pulverizations chiefly are electrical dielectrics, so merely the last two classs are relevant. The theoretical accounts to depict the bear downing procedure of the insulator-insulator contact are similar to those of metal-insulator contacts, but the motion of negatrons or ions in the insulating organic structure is much more restricted. In the bear downing procedure of the metal-insulator class, three different proceedings can be distinguished ( Karner and Anne Urbanetz 2011 ) . These are negatron transportation, where the charging occurs by a i¬‚ow of negatrons ( Chowdry and Westgate 1974 ) ion transportation, where ions are exchanged by diffusion ( Diaz and Fenzel-Alexander 1993 ) and material transportation, where stuff is rubbed off one reaching organic structure and adheres on the other one ( Tanoue, Ema et Al. 1999 ) . Normally the bear downing procedure is a combination of these three proceedings. To depict the bear downing procedures of pharmaceutical pulverizations normally the negatron transportation theoretical account is used because it provides a good apprehensible description of a bear downing procedure.

Powder processing

Most of the pharmaceutical materilas are organic stuffs that have high electric resistances ( Grosvenor and Staniforth 1996 ) and charge relaxation times of proceedingss to hours ( Bailey 1984 ) . Since the rates of charge decay from atom surfaces depend on the relaxation clip ( Bailey 1984 ; Bailey 1993 ) , these solids have high inclinations to roll up charges during industrial processing. Pharmaceutical processing necessarily involves comparative motions of atoms against the solid surfaces of equipments, therefore supplying ample chances for triboelectrification. Operationss including micronization, fluidization, screening, conveying of pulverizations through pipes, bags, and hoppers, transportation into silos, and spray drying constantly generate charges ( Bailey 1993 ; Murtomaa, R & A ; auml ; s & A ; auml ; nen et Al. 2003 ; Murtomaa, Savolainen et Al. 2004 ) . Normally, the higher the energy involved in a process the greater is the attendant charging ( Bailey 1984 ) .

The charges generated from tribo-electrification frequently lead to jobs in industrial processing. The accretion of charges in a silo during pulverization burden may develop high surface electrical potencies on the turning pile, ensuing in electrostatic discharges. The pile may self-ignite if the discharge energy is greater than the minimal ignition energy of the pulverization ( Bailey 1984 ) . Discharges are particularly unsafe when flammable bluess or dust clouds are present in the locality ( Bailey 1993 ) . Attractive forces produced from atom bear downing cause atom agglomeration and adhesion to surfaces of processing equipments, which in bend alters particle size and reduces or even getas powder flow. On the other manus, electrostatic repulsive forces cut down majority pulverization denseness, accordingly the filling of containers and metering of preparation doses during industry are affected ( Bailey 1993 ) . Repulsion between atoms besides decreases the physical stableness of pulverization blends ( Staniforth and Rees 1982 )

Coherence, Adhesion and tribo-electrification ;

Forces that have an impact on bulk behavior of pulverizations are adhesion, coherence, new wave der waal ‘s forces and electrostatic forces. Coherence occurs between similar surfaces ( particle-particle ) and adhesion, between the atom and an instrument wall. The new wave der waal forces are the other most influencing forces, it increase when atom size lessenings and diverge with alterations in comparative humidness. Interparticulate coherence can be due to electrostatic forces, which arise from contact or frictional charging. Generally, atoms are charged statically by crunching, abrasion and hit or triboelectrification. When dry atoms are sieved, assorted and moved through a hopper, surface charge can besides be generated. Harmonizing to Staniforth ( 1982 ) excipients are usually charged negatively in contact with metal or glass surfaces, while positive charges are created with fictile surfaces ( Staniforth and Rees 1982 ) . Moisture, particulate taint and method of cleansing of treating equipment during pharmaceutical fabrication operations may act upon the electrostatic behavior of pulverizations ( Eilbeck, Rowley et Al. 2000 ) . Rowley ( 2001 ) studied pharmaceutical solid systems and found that charge acquisition was reciprocally related to particle size where contact surface taint was negligible ( Rowley 2001 ) . Electrostatic surveies on pharmaceutical pulverizations have late been made by ( Murtomaa, Ojanen et Al. 2002 ) .

4.2 – Powder commixture ;

Mix may be defined as the amalgamation of two or more dissimilar parts of stuff ensuing in the attainment of a needed degree of uniformity in the concluding merchandise ( Richardson, 1950 ) . There are three types of mixtures viz. positive, negative and impersonal mix. The first occurs spontaneously by diffusion, for illustration in mixable liquids and gases where no energy is needed, whereas with negative commixture, such as the scattering of indissoluble solid atoms with a liquid, work is required by stirring to keep the scattering. In the impersonal mix work must be done ab initio to blend the constituents as in a mixture of pulverizations ( Twitchell, 2002 ) . Powder commixture is a cardinal, of import and frequently the really first treating measure for many industrial procedures and is one of the most common unit operations utilised in many industries such as pharmaceutical, chemical, nutrient, decorative, cement, glass, and detergent. Blending public presentation appraisal, which has been a chief issue for about a decennary, is likely to go even more of import as drugs become more powerful and dosages become smaller ( Muzzio et al. , 2002 ) . Specifically, the pharmaceutical industry uses pulverization blending operations to fabricate extremely active pharmaceutical ingredients in many pharmaceutical preparations, such as pulverizations, granules, capsules and tablets. Powderize commixture can be classified either as randomized or ordered commixture ( Hersey, 1975 ; 1979 ) . In random mixtures, the constituent atoms should be free fluxing and of similar size, denseness and form. However, in pattern, the constituents of particulate solids frequently consist of different stuffs with different morphologies and sizes. When interparticulate forces of either attractive force or repulsive force are introduced in mixtures, the randomised distribution of constituent atoms will be disturbed and this type of commixture is described by ordered commixture.

4.2.1 – Forces involve in ordered commixture

Interparticulate forces have been the topic of legion probes including, Rumpf ( 1962 ) , Krupp ( 1967 ) , Capes ( 1980 ) , Stewart ( 1986 ) , and Hamby ( 1995 ) , who all focused on the types of bonds which may be between atoms. It was concluded that there are legion mechanisms by which atoms may bond together, including solid and liquid Bridgess, intermolecular and mechanical engagement. In the instance of an ordered pulverization mix, it is likely that the intermolecular forces, in peculiar new wave der Waals, will rule. However, there are three types of intermolecular forces which may be formed, van der Waals, magnetic, and electrostatic, in diminishing order of strength, all of which may take portion in the bonding procedure.

Staniforth ( 1985 ) showed that comparative humidness ( r. h. ) affected the formation of an ordered mix and increased wet content to 0.74 % w/w, of a fructose bearer conditioned at 55 % ( RH ) , resulted in stronger adhesion forces, possibly due to liquid Bridgess between the fruit sugar and adhered vitamin B6 hydrochloride.

Mechanical engagement as a signifier of adhering between bearer atoms and all right stuff, will depend upon the surface construction of the atoms in the mix. It is by and large thought that there are a figure of adhering sites on the surface of the bearer atom to which the finer constituent in the mix will adhere, but Staniforth et Al ( 1982 ) have shown that recrystallized lactose as a bearer stuff with a more porous construction, will organize stronger bonds with all right active atoms, than those formed with lactose which had a smoother surface. Possibly the fond regard of the active atoms in this instance is a consequence of mechanical engagement or lodgment of active atoms in crannies on the surface of the bearer, in contrast to an increased figure of adhering sites.

Several surveies including that by Coelho and Hamby ( 1978 ) have investigated the strength and laterality of the type of adhesion force under different conditions and with assorted pulverizations. They concluded that increased comparative humidness increased van der Waals forces, and adversely affected electrostatic forces and decreased mechanical engagement.

All the surveies conclude that molecular, chemical and physical features, such as size and surface raggedness will hold a profound consequence on the strength of adhesion. The humidness of the ambiance besides seems to be an of import consideration in finding the bond strength between atoms.

4.3- Factors impacting Tribo-electrification

The above mentioned jobs of pulverization charging may be prevented through more elaborate apprehension of tribo-electrification. A figure of more easy controlled physicochemical parametric quantities have been identified and discussed below that affect pulverization bear downing during processing.

4.3.1- Powder fundamental law




Powder fundamental law

Equipment surface

Work map

Blending velocity

Blending ratio

Powder atom size

Relative humidness

Charge dissipation