What is the main task of binder of a coating color? What are the two main latex types used in coating colors?
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The main tasks of a binder are: to bind the pigments to the paper surface, to bind the pigment particles to each other, to fill the voids between the pigment particles, and to influence the rheology and water retention of the coating color. Two main latex types: Styrene-butadiene (S/B) latex (most common, flexible, good print gloss, but prone to yellowing) and Acrylic latex (premium, high UV and light resistance, strong and flexible, more expensive).
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What is the main task of binder of a coating color? What are the two main latex types used in coating colors?
The main tasks of a binder are: to bind the pigments to the paper surface, to bind the pigment particles to each other, to fill the voids between the pigment particles, and to influence the rheology and water retention of the coating color. Two main latex types: Styrene-butadiene (S/B) latex (most common, flexible, good print gloss, but prone to yellowing) and Acrylic latex (premium, high UV and light resistance, strong and flexible, more expensive).
Explain the working principle of an OBA! What is the targeted functionality of this additive in coating colors?
An Optical Brightening Agent (OBA) absorbs ultraviolet (UV) light (350-360 nm) and emits visible blue light (around 440 nm). This blue light compensates the yellow tint of the paper, making it appear whiter and brighter. The OBA must be carried by water-retaining agents like CMC, starch, or PVOH to remain active.
Most coating colors used for paper coating applications show viscoelastic behavior. Which important parameters can be obtained by viscoelastic measurements? What is the practical benefit of such rheological investigations?
Parameters obtained: Elastic modulus (G') — measures the solid-like behavior; Viscous modulus (G") — measures the liquid-like behavior; Phase angle — indicates the ratio between viscous and elastic behavior. Practical benefit: Helps to understand how the coating behaves during application and metering, ensuring good runnability and avoiding defects such as streaks, scratches, and web breaks.
Name two (2) types of non-impact coating technologies for paper or board and explain the operating principle of one of these technologies!
Two non-impact coating technologies: Curtain coating and Spray coating. Curtain coating principle: The coating color falls as a thin curtain of liquid from a nozzle onto the moving paper without mechanical contact. The curtain is stabilized by controlling surface tension, flow rate, and air boundary layers.
Which process parameters are used to control the final coat weight in a film coater and in a blade coater?
Film coater: Flow rate of coating color, solids content, rod diameter (profiled or smooth rod), nip pressure and load, machine speed. Blade coater: Blade angle and blade load, blade stiffness and thickness, machine speed, coating color solids and viscosity, base paper porosity and roughness.
What do we mean by "CMC" and what is this product used for in paper coating formulations?
CMC = Carboxymethyl cellulose. It is a water-soluble, non-associative thickener that improves viscosity and water retention, helps carry OBAs, and improves runnability by stabilizing the coating color.
Please describe the typical drying process for the manufacturing of coated paper or board! What are the three basic drying mechanisms in place?
Drying involves removing water by heat and mass transfer. Steps: 1. Heat-up phase (sheet temperature rises), 2. Steady-state drying (water evaporates at constant rate, coating immobilizes), 3. Falling-rate drying (remaining moisture is reduced more slowly). Three mechanisms: Radiation (infrared dryers), Convection (hot air flotation), Conduction (steam cylinders).
Define the term "water retention" and explain how this property is being measured. What practical conclusions can be drawn by measuring this parameter in coating formulations?
Water retention: ability of the coating color to hold water before immobilization. Measured by placing a known amount on filter paper and weighing water loss over time. Practical conclusions: Too low → dusting, scratches. Too high → poor drying, blistering, poor runnability.
How does the film coating technology compare in terms of machine runnability compared to blade coating? What are the main operational as well as quality issues of the film coating principle?
Runnability: Film coating has fewer web breaks, better for high-speed on-line coating. Operational issues: Film splitting (misting, streaks, orange skin), rod wear and rod spitting at high speeds. Quality issues: Lower gloss and smoothness than blade coating because pigment particles are not aligned.
Which operational and/or mineral parameters have to be taken into consideration to effectively minimize wire abrasion in paper filling?
Use soft, fine minerals with low Mohs hardness; use platy or rounded particles (e.g., talc) instead of angular ones; avoid high filler loading in wet end; choose proper wire design with fewer pores.
What kind of testing methods can be used in lab scale to simulate wire abrasion in practice?
Wire abrasion tests on laboratory sheets using controlled friction and load; test rigs to simulate suction box conditions and measure wear rate.
Give an explanation for the term "specific surface area", name available testing methods and clarify the importance of this parameter for the final paper properties!
Specific surface area (SSA): surface area per unit weight of pigment or filler. Measured by BET (gas adsorption) or air permeability methods. Importance: higher SSA leads to better light scattering and opacity but higher binder demand and viscosity.
Why are biocides practically always used in coating formulation?
To prevent microbial growth that can cause odor, viscosity changes, gas formation, and defects. They ensure coating stability and avoid contamination of the paper.
What is the purpose of adding so-called "thickeners" or rheology modifiers to paper coating formulations?
To adjust and stabilize viscosity, improve water retention, reduce fines flocculation, and enhance OBA carrying capacity.
Please explain the working mechanism of non-associative thickeners (ASE) compared to associative thickeners (HASE).
ASE: thickens by neutralization, chains extend, water is bound, viscosity increases. HASE: combines neutralization and hydrophobic interactions, micelle formation, strong thickening, sensitive to shear.
What is the most typical fluid behavior for paper coating colors and why?
Shear-thinning (pseudoplastic) — viscosity decreases at higher shear rates because particle networks in the coating color break down under shear.
Name the fluid flow type for a), b), and c), and give example of such behavior.
a) Shear-thickening (dilatant) - cornstarch in water. b) Newtonian - water. c) Shear-thinning (pseudoplastic) - coating color.
Why are fillers used in the papermaking process?
To reduce fiber usage (lower cost), improve brightness, opacity, smoothness, printability, and aging resistance.
What are the key advantages of adding fillers to graphical paper products?
Better optical properties (brightness, opacity, gloss), improved printability, cost savings, and reduced environmental footprint compared to fibers.
How can the negative impact on strength be explained and what concepts are recommended to maximize the filler loading levels?
Fillers interfere with fiber-fiber hydrogen bonding and introduce stress points. Strategies: optimize filler distribution, use retention aids, and control filler shape and size.
