Measurements of Coating Colors
The shear-viscosity and viscoelastic properties of paper coatings and their time-dependent behavior are important attributes that need to be measured for better understanding and optimization of the process.
Viscosity is measured with rheometers or viscometers. The methods used include rotational deformation, squeezing deformation, extrusion (capillary) flows, and free surface stretching. For rotational instruments, there are two modes of opera¬tion: controlled strain and controlled stress. Rotational measurements can be fur¬ther subdivided into different measuring methods (flow, oscillatory, stress relaxa¬tion, and creep) and different measurement devices (spindle, cone-and-plate, par¬allel plate, concentric cylinder). As for a capillary rheometer, there are two modes of operation: controlled flow (strain) and controlled pressure (stress).
Viscoelasticity refers to rheological behavior that is a combination of viscous (liq-uid-like) and elastic (solid-like) behavior. Ideal elastic behavior is called Hookean, where the stress is directly proportional to the strain. A Hookean solid deforms as long as stress is applied. Once stress is removed, it fully recovers its original shape. This behavior can be modeled by a spring that stores energy under deformation and then releases it. Ideal viscous behavior is called “Newtonian”, where the stress is directly proportional to the rate of strain.
A Newtonian fluid flows as long as a stress is applied and retains its final shape once the stress is removed. This behavior is modeled by a dashpot that consists of a piston moving in a viscous liquid.
Paper coatings can be generally considered as viscoelastic fluids. The viscoelas¬ticity of a material is most often measured by applying oscillatory shear instead of steady shear. There are two common types of oscillatory tests. A strain sweep test is carried out at a fixed frequency while the amplitude of the oscillation is varied. As the amplitude is increased, more energy (or shear) is applied to the sample.
This energy may begin to break down the internal structure of the material. This is analogous to stretching a spring too far. The strain at which the coating structure begins to break down is called the critical strain, which gives some information on the nature of the internal structure of the material. For example, flocculated disper¬sions have critical strains of about 1 %, polymer solutions have critical strains of about 10 %, and polymer melts have critical strains of about 100 %.
Most paper coatings have critical strains of about 0.5–5 %, indicating that their elastic behavior is a result of a weak network structure between pigment particles and/or thickener. The second common type of oscillatory test is a frequency sweep where the ampli¬tude of oscillation is fixed while the frequency of oscillation is varied. The ampli¬tude for the frequency sweep is chosen to be within the linear viscoelastic region, as determined by the amplitude sweep measurement.
An alternative way to measure the viscoelastic properties of a material is to apply a step change in stress or strain and then measure the response of the material over time. This is the basis for stress relaxation and creep testing of materials. In a stress-relaxation test a constant strain (or shear) is applied for a period of time and then removed.
Upon removal of the strain, the stress begins to relax. Typically, one measures the relaxation time needed for the stress to relax to half of its equilib¬rium value at long times. Closely related to stress relaxation is a stress-growth test. Here a sudden increase in strain is applied to the sample, and the growth in stress is monitored over time. Both stress-relaxation and stress-growth measurements are used to characterize paper coatings.
A correlation between the stress-relaxation time and the healing of coating defects can be observed in practice: Coatings with a shorter relaxation time (more fluid-like) show narrower residual widths of an induced blade streak. Stress-relaxation properties of a coating also correlate with improved leveling of orange peel patterns for high-speed metered film coatings.
During coating consolidation the liquid coating color on the base paper transforms into an immobilized (stiff ) coating layer. The runnability of a coating color in coating is determined by the properties of the liquid color, i. e., wet coating struc¬ture, whereas the final quality of the coated paper depends on the dry coating structure. The transfer from wet to dry coating structure is a result of the aqueous phase penetrating from the coating color into the base paper and, later, due to evaporation on drying. A prerequisite for liquid transport in porous material is the presence of a driving force. Some common examples for water transport in paper are capillary pressure, external pressure, vapor pressure, concentration gradient, and temperature gradient. Several driving forces may be present at the same time, which makes the transport mechanism complex.
The various water retention measurement techniques can be divided into static (direct and indirect) as well as dynamic methods. The first provides a quantifica¬tion of the amount of aqueous phase leaving the coating color and penetrating into the base paper. The measurement principle is based on filtration of coating color under the influence of external pressure. The indirect methods measure other parameters reflecting the penetration of aqueous phase into the base paper, such as electric conductivity, ultrasonic transmittance surface gloss, and coating vis¬cosity. The dynamic techniques are usually based on measurement of solids in¬crease in the coating pan or various scrape-off experiments during laboratory and pilot coater trials.
The right solids content of pigments is important as pigments are the biggest portion in the coating color recipe. The solids content of the coating color being too high or too low, directly affects the coat weight, the runnability of the coating unit and the necessary drying energy. The solids content is measured by drying a sam¬ple in an oven and calculating the amount of dry components as a percentage.
Because the traditional oven method takes several hours to measure solid content, quicker methods have been developed for “just-in-time quality control”. In these quick methods, infrared and microwave drying are used.
In practice it is easy and quick to ascertain pH from aqueous suspensions and chemicals. However pH is sensitive to temperature variations; therefore pH meas¬urements should be performed under constant conditions and the calibration rules of each individual pH gauge have to be followed carefully. The normal coat¬ing color pH is between 8 and 8.5, but higher levels are needed when using some
7.9 Measurements of Coated Surface
synthetic thickeners or pH-dependent latexes. The ISO 787–9 method should be followed for pH value measurements.
This is defined as the percentage of coarse particles in the measured material after screening through a given Mesh-number screen (e. g. for pigment slurries and finished coating colors a Mesh number of 300 is used, and for latex Mesh numbers 150 and 80 are used). If blade stripes or other similar coating problems occur, it is worth measuring the screening residue and determining whether any larger parti¬cles are mixed in the coating color.
A high bacteria level of coating color chemicals may influence the machine runn¬ability. Furthermore, for coated paper that comes into contact with food the bacte¬ria level has to be low. The bacteria level of a material can be counted by the so-called Easicult Combi test. This method of control is easier than doing a plate cultivation (see also section 3.7.4).