Background and Task
Paper and board are made of cellulose, which is strongly hydrophilic due to its many hydroxyl groups. Direct contact with water therefore leads to high water absorption, fibre swelling, structural changes and loss of strength. To prevent this, so-called sizing agents (AKD or ASA) are added during paper production to render the fibres water-repellent and to increase the wet strength of the material.
To ensure that the sizing agent can fully develop its effect, it is encapsulated and stabilized in starch size prior to being added to the papermaking process. This maintains the dimensional stability, tensile strength and printability of paper and board, even under humid conditions. In addition, the writeability, printability and absorbency can be controlled—qualities that play an important role in subsequent finishing processes. Pre-sizing also improves runnability on the paper machine, enabling higher production speeds.
Challenges
The most commonly used sizing agent in paper and board production is Alkenyl Succinic Anhydride (ASA). Since ASA is inherently oily and insoluble in water, it must be introduced into the fibre suspension in the wet end of the paper machine in the form of an emulsion.
Because ASA is also a reactive molecule that rapidly hydrolyses when handled incorrectly—rendering it ineffective—the oily ASA phase is encapsulated/emulsified in a protective colloid, typically cationic starch (or another cationic, hydrophilic polyelectrolyte). Fine encapsulation in starch protects ASA from rapid hydrolysis and allows uniform dispersion of the sizing agent in the aqueous pulp suspension to ensure effective hydrophobization.
Since starch size is predominantly hydrophilic and ASA is difficult to distribute homogeneously within it, the process poses several challenges:
- Unsuitable shear forces
Excessive shear destroys the interfacial layer, causing droplets to coalesce. Insufficient shear results in overly coarse droplet distributions. - Incomplete encapsulation
Oversized ASA droplets are insufficiently coated with starch and may react prematurely; excessively small droplets increase specific surface area and therefore susceptibility to hydrolysis. - Emulsion instability
Insufficient shear prevents fine dispersion and leads to phase separation. - High energy demand
Conventional mixing techniques require long process times and high rotational speeds to combine hydrophobic and hydrophilic phases. - Quality losses
Even minimal premature ASA–water reactions prior to encapsulation impair the later sizing performance and thus the quality of the final product.
Solution
Cavitron offers a dedicated emulsification system for encapsulating ASA in starch size, featuring a rotor–stator machine engineered specifically for this application. The rotor–stator design combines perforated and slotted rotor and stator rings, while extremely tight clearances generate very high shear forces.
With the Cavitron ASA emulsifying machine, you achieve:
- Target particle/droplet size
High shear intensity ensures the optimal droplet size for effective encapsulation and emulsification of ASA in starch size. - Excellent distribution and dispersion
The geometry of the rotor–stator system guarantees uniform distribution of ASA in the starch medium, preventing the formation of “ASA nests” that cause instability or uneven sizing. - High emulsion stability
Optimized droplet size and uniform dispersion ensure long-term stability of the emulsion. - Short process times
Intensive mixing in a continuous process significantly accelerates encapsulation. - Reduced ASA consumption
Using a CAVITRON emulsifier in combination with the appropriate sizing chemistry can reduce ASA usage by up to 20%. - Improved emulsion quality
The resulting emulsion exhibits low deposition tendencies and helps avoid so-called blue spots.
Overview of Customer Benefits
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High product quality
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High process control and reproducible quality
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Controlled energy input
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Space-saving and clean
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Lower environmental impact
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Robust and efficient
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Flexible and versatile
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Tailor-made solutions
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Accessories and plant engineering
Customer Benefits in Detail
- High product quality
The rotor–stator system, designed specifically for ASA applications, ensures rapid emulsification and homogeneous distribution, resulting in a highly stable emulsion. - High process control and reproducible quality
The machine operates with precision and reliability, delivering consistent results. Continuous operation in a closed system further accelerates the process. - Controlled energy input
Rotational speed—and therefore shear intensity—can be precisely adjusted at the emulsifier. - Space-saving and clean
Compared to traditional batch production, Cavitron’s continuous system requires up to 90% less space. No sticky tanks or internal components accumulate residue, as the product is positively conveyed through the machine. This minimizes deposits and reduces cleaning demands. - Lower environmental impact and maximum occupational safety
Encapsulation occurs in a closed system. No dust escapes into the ambient air, improving worker safety and environmental protection. - Robust and efficient
Lower energy demand than comparable systems, simple maintenance and high tolerance to viscosity fluctuations. - Flexible and versatile
The high-performance Cavitron machine enables processing of various product grades and formulations. - Tailor-made solutions
Cavitron machines can be custom-configured for specific applications and customer requirements—different rotor–stator systems, shaft seals, motors and drives, heating/cooling jackets, injection ports or multistream inlets. - Accessories and plant engineering
In addition to stand-alone machines, Cavitron supplies complementary system components (hoppers, tanks, dispersers, etc.) as well as complete process plants.
For paper and board manufacturers, ASA emulsions produced with Cavitron systems exhibit high stability, excellent hydrophobization efficiency and low deposition tendencies under process conditions. Users benefit not only from improved product quality but also from significant increases in process efficiency and energy savings.
General Technical Specifications
Static seals:
Viton, Teflon, Kalrez, other elastomers.
Operating pressure:
Application-dependent, up to 100 bar; standard: up to 10 bar.
Operating temperature:
Application-dependent up to 250 °C; standard: 150 °C.
Performance and energy consumption:
Type overview available for download.
Drives and power transmission:
Three-phase motors, high-frequency motors, mechanical gear units, variable-speed drives, stepless electronically controlled and monitored systems.
V-belt and Poly-V belt drives.
Intensity levels:
Application-specific, ranging from coarse mixing to dispersion in the angstrom range.
Bearings:
Tapered roller bearings, angular contact ball bearings (double).
engineered.
