The oil absorption suitable for the construction consistency can be understood as the amount of oil (binder) in the paint when the paint reaches a consistency that is convenient for construction on the surface to be coated. The paint should be easy to apply, and should not be too thin, and should not flow on the vertical plane.
In solvent-free oil-based paints, the paintability and spreadability of the paint are determined by its thixotropy, which is the mechanical force (agitation, pressure, brush movement, etc.) Under the action of the force, it becomes diluted and has fluidity, and when the force is removed, it returns to the performance of a thick state.
Thixotropic thickening is caused by cohesion of pigment particles. In the static state, the structure formed by the cohesion of pigment particles is caused by the loss of fluidity of the paint. In order to regain the fluidity of the paint, mechanical force must be applied to destroy this structure (even a part). Therefore, the ease of painting and its flattening of the paint depend on the stability of this structure, and on the speed at which the damaged structure recovers after the deformation stress is removed. For example, a paint for brushing (paint for brushing refers to a paint suitable for brushing and French decoration) will be easy to apply if it has a small shear strength, because in this case, even Significant structural damage can occur under the small deformation stresses induced by the moving paintbrush (see page 111). Flattening is determined by the time required to restore its structure.
When its structure recovers very quickly, brush marks will be left on the coating and become obvious streaks. Coatings applied with a paint sprayer, in which case an orange peel condition or other similar pathological conditions may develop.
When the recovery of the structure is very slow, the phenomenon of paint flowing down from the vertical surface occurs. The best flattening (leveling) of the paint is compatible with a certain most suitable time for the restoration of the damaged structure in the paint.
The factors that determine the flatness of solvent-free oil-based paints are: the original colloidal properties of the components in the system that can affect the cohesion process of the pigment particles, and the quantitative ratio between the pigment and the binder. When the paint contains a solvent, the volatility of the solvent is an additional factor affecting the flattening, because the solvent evaporates rapidly from the coating, which causes the viscosity of the paint material to increase prematurely, thus causing difficulties in flattening.
The degree of dilution necessary for the paint (consistency, brushing or application) can be determined experimentally (tested by the painter during the application process).
Our standard method of determining suitable application consistency based on the "brushing" onset of bleeding is applicable only when the adhesive is a natural cooked oil. As with all other adhesives, this This kind of method is the messenger of not universal measurement. Fig. 23 shows this simple instrument sketch that can be called automatic recording brush as an example.
A brush 3 is attached to one end of the free-rotating shaft 1 on the joint (movable shaft) 2; the other end is connected to a wrong pen 5 that is in contact with the piece of paper, and the piece of paper can be moved upward at a constant speed by means of a suitable machine. Vertical movement (not shown on the figure).
The shaft moves like a pendulum, while the brush slides along the curved metal surface 6. The pressure of the brush on the surface is controlled with an adjustment screw. Apply a quantity of basecoat to the surface that has been spread with a loose brush. Thereafter, lead the shaft to its highest position, and then release it immediately; at the same time, start the automatic recording mechanism. .Under the action of the counterweight 4, the shaft starts to vibrate like a pendulum, and its amplitude gradually decreases until finally the pendulum stops swinging due to the action of the paint barrier (brake). A graph representing the gradual stop swing of the connection is drawn by the device. If it is a paint that is easy to brush, the observed number of oscillations and its amplitude will be larger than that of a paint that is difficult to brush.
Use calibrated paint samples to determine the standard curve diagram, which can also be used to determine the oil absorption suitable for the construction consistency.
In order to find out the relationship between the rheology of paint and its finishing performance, many research works have been carried out. Figure 24 is a graph showing the relationship between viscosity and deformation stress for a number of paints with different finishing characteristics.
Paints with good finishing properties (1, 3, 4, 5), their fluidity curves are in the quadrilateral of crossed lines; the fluidity curves of paints that are difficult to brush and the curves of paints with poor flattening Machines are outside the rectangle.
When testing the paint on the above-mentioned instrument, the results obtained are expressed in curves, and the curve is compared with the curve obtained when testing the standard paint. A paint whose fluidity curve intersects the Himeform area has the necessary finishing performance, which is different from a paint whose fluidity curve is outside the Himeform area.
