Rheological properties of adhesive

Polymer rheology is the science that deals with the deformation and flow of matter. Most of the plastics exhibit ‘visco-elastic’

behaviour during flow, which means that they exhibit not only viscous characteristics but also elastic behaviour in liquid state [25]. The rheological properties of polymers vary with their chemical composition and structure. One of the most important rheological properties for adhesive bonding is the viscosity of the adhesive. Viscosity can be defined as the resistance offered to the flow of liquid. It can also be defined as the resistance offered by a fluid to gradual deformation by shear or tensile stress. There are different types of viscosities depending on the type of resistance offered. The important classifications are

Dynamic Viscosity also known as shear viscosity of fluids offer resistance to shearing flow, where adjacent layer of liquid move parallel to each other. The speed of different layers varies due to

the opposition offered to the flow. Dynamic viscosity can be

Kinematic Viscosity is the ratio of dynamic viscosity to the density of the fluid and can be calculated using the equation;

𝜈 =

(

Where, ν is the kinematic viscosity and ρ is the density of the fluid.

Relative Viscosity is the ratio of viscosity of polymer solution to the viscosity of pure solvent. This can be calculated using;

𝜂

=

0

(

Where, ηr is the relative viscosity of the system, η is the viscosity of the polymeric solution and η0 is the viscosity of pure solution.

Apparent Viscosity is defined for polymers when the viscosity of the system depends on the shear rate. For dynamic or shear viscosity, the viscosity of the system is constant independent of shear rate applied to the sample, whereas, the values obtained from the systems which are influenced by the shear rate are known as ‘apparent viscosity’ or ‘apparent shear viscosity’ [26]. Using gangel test, it is possible to calculate the apparent viscosity. The system of adhesive joints is influenced by the temperature and thickness of the bond; and hence the shear rate. The equation for calculating the apparent viscosity are as follows;

𝜂 =

(

where, η is the dynamic viscosity of the adhesive (g/s.mm), m is the mass of load used during the test (g), g is acceleration due to gravity (constant: 9.81 ms^-2), h is the thickness of adhesive (mm), t is the time of failure (s), L is the length of overlap and w is the width of overlap of adhesive

Viscosity can be measured using instruments called as viscosimeters (also known as viscometers) and rheometers.

Rheometers can be called as a special type of viscometer and it is used for liquids which require more parameters to define the values of viscosity. There are two different types of rheometers.

Rheometers which control the applied shear stress on the fluid, are called as rotational shear rheometers. Rheometers which control the applied tensile stress in the fluid, are called as the extensional rheometers. Dynamic rotational shear rheometers are used for characterising and understanding high temperature rheological properties in both molten and solid state of materials, and the components used for the test can vary in design [26].

Capillary design is used for liquid which can be forced through a tube of constant cross-section and known dimensions under conditions of laminar flow. In this method, the pressure drop or flow rate is fixed and the other is measured for variations. These variations are used in computation of shear rate or shear stress.

In rotational cylinder design, the liquid is placed between the cylinders. One of the cylinders is rotated at a set speed and the shear rate of the liquid is measured. Using the resistance (force) offered by the liquid on the cylinder (torque), shear stress is calculated.

In cone and plate design, liquid is placed on a horizontal plate and a shallow cone (known angle) is placed on it. The angle between the plate and cone can be varied depending on the fluid to be

and the torque on the cone is measured, in turn the shear stress is computed.

Measurement of intrinsic viscosity is another method of measuring viscosity. In this method, the flow time of a polymer solution through a glass capillary at different solution concentrations are measured [26]. A polymer solution passing through a capillary obeys Poiseuille’s law for laminar flow through capillaries, which indicates that the pressure drop is directly proportional to the viscosity of the fluid. This method is known as the Ubbelohde method (see figure 13). The viscosity of fluid can be calculated using the equation;

𝜂 =

(

where, η is the viscosity of the fluid, ΔP is the pressure difference of the fluid in the capillary, r is the capillary radius, l is the length of capillary tube and Q is the volumetric flow rate through the capillary [26].

Figure 13 Ubbelohde method [26]

A polymeric solution of known concentration is put in the reservoir and is pumped to the upper bulb usually by creating vacuum in the chamber. Then the liquid is flows down the capillary by gravity. The time for the liquid to flow between the two marks is recorded. This method is repeated for different concentration of solution [26]. The standard for this test are mentioned in ASTM D445 and ISO 3104.