Besides the basic materials described above, the bonds of abrasive tools occasionally contain a number of additives. These are, on the one hand, necessary for the manufacturing process of the tools, and, on the other, help to create specifically defined grinding properties.
Fillers contained in the matrix of grinding wheels bonded with synthetic resin can play a reinforcing role by increasing the strength, heat-resistance, toughness and fracture resistance. The materials used as fillers for these tools are quite heterogeneous. Calcium oxide in the form of chalk powder, magnesium oxide, zinc sulphide and barium sulphate are used in order to promote the bonding of the resin while hardening and to prevent bubbles or cavities. [COLL88, N. N.7]. Graphite, pyrite, cryolite, lithopone (a mixture made of zinc sulphide and barium sulphide), potassium sulphate, potassium chloride and borax function as lubricants. Their purpose is to lower the grinding temperature and prevent oxidation on the tool surface. The sulphur dioxides that develop can also decelerate a thermally contingent decomposition of the synthetic resin and thus help increase the service life of the grinding wheel [ARMI84].
In abrasive cutting discs, lead chloride and antimony trisulphide are important fillers used for lubrication. They diminish the risk of surface hardening, because they melt during grinding, thereby taking in heat. Moreover, the fluid film which develops from this reduces friction between the grinding wheel and the tool and therefore leads to a diminished heat generation as well [N. N.8]. It must however be noted that antimony trisulphide and lead chloride are poisonous and therefore subject to label requirements. Strict MAK/TRK values are in effect for them. Thus, both additives may only be made use of in grinding wheels for stationary cutting. Accident prevention regulation VBG 7 n 6 stipulates for the continuous operation of dry grinding machines a suction or other form of removal of the accruing dust. Antimony trisulphide and lead chloride are being increasingly replaced by, for example, alkali-iron halogenide [COLL88].
In vitrified bonds, so-called burnout materials like granulated coke or paraffin are used. These sublimate or burn during the burning process. With their help, an alteration of the abrasive material structure can be achieved without changing the quantitative composition. With the extension of the bond web, there is a coagulation of smaller pores to form larger ones. This kind of additive is called an effective filling. Besides this, there is also the so-called necessary filling, which must be utilised to achieve a high pore volume and in order to ensure green strength and dimensional stability.
To increase wheel speeds, especially strong and secure bonds are employed. These high bursting speed values are achieved for vitrified grinding wheels, among other ways, by adding lithium and boron compounds [PADB93].
Sulphur and zinc oxide are used to accelerate vulcanisation for rubber-bonded abrasives.