Лако-красочные материалы — производство

Viscosity is a product property associated with all engineering adhesives and with most sealants. The rheology of an adhesive is the key to determining its potential field of application (gap-filling capability, strength before curing, penetrability, orientation of parts receiving adhesive, etc.) and the dispensing systems to be used. Viscosity is defined as a measure of the resistance of a fluid to flow or mathematically as the ratio of a shear stress to a velocity gradient. A measure of this fluid ‘thickness’ is traditionally expressed in centipoise (cP) values, although the SI units are milliPascal seconds (mPa-s). Fortunately, 1 cP = 1 mPa-s and so most engineers still refer to viscosity in centipoises.

Viscosity has to be considered on a logarithmic scale and the higher the number, the thicker the product. Water has a viscosity at 22 °C of 1 mPa-s and some common fluids are listed in Table 5.1 for reference purposes.

Viscosity decreases as temperature increases. This is due to the fact that an increase in temperature results in an increase in the kinetic energy of the fluid molecules that weakens the intermolecular attraction and encourages molecules to separate in the presence of shearing forces.

Rule of thumb: a 10 °C rise in temperature will half the viscosity of the liquid. Vice versa, a 10 °C drop will double the viscosity (see Section 8.1.2).

Viscous behaviour could be described approximately by Newton’s law: ‘The resistance resulting from the lack of slipping by different layers within a liquid, other factors being equal, is proportional to the rate at which these layers are separated from each other.’ In other words, the force applied is proportional to the rate of deformation.

For the majority of liquids the viscosity is independent of the velocity gradient and so they exhibit Newtonian behaviour. Many engineering adhesives, however, have non­linear behaviour and are therefore termed non-Newtonian. A thixotropic adhesive is one which will reduce in viscosity as it is sheared (or moved). When the shearing

force is removed from the adhesive, the viscosity will regain its higher value and this may be time-dependent on the history of the shearing force.

The lower viscosity adhesives (2-20 mPa-s) are often used where the adhesive needs to be post-applied as these products will tend to have good wicking and capillary characteristics.

The higher viscosity adhesives (>10,000 mPa-s) are less flowable and would be selected where the adhesive has to fill a larger gap or is required to be dispensed onto a narrow ledge and must not migrate away from the bond line (see Section 8.1).

Some adhesives — often the higher viscosity epoxies, two-part acrylics and certainly the silicone and modified silane sealants — have good ‘wet’ strength and this property can be useful for applications where an initial ‘grab strength’ is useful.