A biaxially oriented polyester film represented by polyethylene terephthalate films has been widely used as a medium for magnetic recording, utilizing the superior physical and chemical properties.  In such biaxially oriented polyester films, slipping property, resistance to scraping, scratch resistance, slitting property and winding property exert great influence on the suitability of the film production steps and processing steps, as well as the quality of the product. 
For example, magnetic tapes obtained by applying a magnetic coating on the surface of biaxially oriented polyester films have been fed at faster speeds in recent years to bring down the costs, and the feeding speed is increased in the steps of coating, calendering, slitting, housing a magnetic tape into a cassette, and duplication for manufacture of soft copies, while creating various problems. 
For example, when the feeding speed of the film is increased in the coating step, the film surfaces come into contact with a die used for the application of magnetic coating, as a result of which the surface is scraped and the scraped powder causes scratches in the magnetic layer, thus decreasing the yield of the product.  An increased speed in the calendering step brings about the need for more frequent cleaning of the white powder adhered to a calendering roll, thus again decreasing the productivity. 
Also, an increased speed at the slitting step causes insufficient winding of the film to decrease the yield of the product.  In general, an improvement in winding property involves inclusion of larger particles to decrease the coefficient of friction between films.  Thus, if such film is wound at a greater speed in the slitting step for the sake of an enhanced productivity, the film slips too much and causes disturbance in winding to the extent that, in an extreme case, the film winds around a slitter blade to lower the productivity. 
When a tape is housed in a cassette at a high speed, the scraped powder and scratch occur to possibly cause drop outs (loss of information).  Particularly in VTR use, a cheap metallic guide having an unfinished surface may be used as a guide post to be fixed in the cassette to lower the costs.  The surface of such guide post is extremely rough and a magnetic tape which is produced according to the prior art wherein slipping property and resistance to scraping of the film have been improved but a back coating is not applied [for example, a method comprising addition of inorganic particles such as silicon oxide, titanium dioxide, calcium carbonate, talc, clay and sintered kaoline (e.g., Japanese Patent Unexamined Publication No.  54-57562) and a method comprising precipitating fine particles containing calcium, lithium or phosphorus in polymerization system (e.g., Japanese Patent Examined Publication No.  52-32914)] accompanies more drop outs due to the scraped powder and scratch experienced in this step. 
To solve such problems, there has been proposed a method comprising adding alumina particles having a specific crystal form to decrease damages to the film (e.g., Japanese Patent Examined Publication No.  4-40375).  This method provides sufficient improvement in scratch resistance, but when a polyester film containing said alumina particles is run under heating, the amount of the white powder adhering to the metallic pin increases.  Therefore, for example, when the running speed of the film is increased in a calendering step in which the film is subject to heat histeresis, the amount of the white powder increases.  In addition, the high hardness of alumina causes problems when used with a high pressure homogeneous dispersion machine or media dispersion type wet grinder for the preparation of alumina slurry, since it wears SUS which constitutes these apparatuses and SUS may be mixed in the slurry as a contaminant component.  Along therewith, parts replacement frequency would become higher.  In a polyester polymerization system including alumina fine particles, moreover, the products of degraded polyester, which are adhered to or deposited in a kettle, particularly at the vicinity of gas-liquid interface, are scraped by the alumina fine particles and contained in the polyester as bulky products. 
Another method includes addition of spherical monodispersed silica obtained by removing alkali from sodium silicate or hydrolysis of alkoxysilane.  This method shows a certain degree of improvement in scratch resistance under high film running speeds.  However, the particles themselves tend to come off from the film and resistance to scraping of the film is degraded.  The particles once fell are prone to form aggromerates, and the scraped matter including the aggromerates of the fallen particles gives damages to the magnetic surface, thus increasing the number of drop outs. 
Faced with the need to increase feeding speed at the slitting step, the conventional problems such as chips from the film and build-up of the cut surfaces at slitting are desired to be solved, whereby to provide a film with less friction with a slitting blade and superior in continuous slitting.