Microsphere

EXPANCEL expandable microspheres are used in the production of fine paper in order to improve bulk. With 1% retained microspheres in the product, bulk and thickness will increase by 18 22%. The increased thickness implies improved stiffness at constant grammage. Thus microspheres reduce disturbances in running paper in a copying machine and a printing press. Mills are sometimes compelled to exceed the specified basis weight in order to reach the specifications for caliper and stiffness. To avoid such an over weight, which may lead to customer complaints, small amounts of EXPANCEL can be added.

In fine paper with a high filler load EXPANCEL is used to compensate for the loss in thickness and stiffness.

Replacing fibre with EXPANCEL is profitable when fine paper is purchased based on area or volume, because the volume of expanded microspheres is cheaper than the volume of mechanical pulp.

Other properties that can be improved by using EXPANCEL are:

- Thermal insulation

- Soft touch

- Anti-slip

PROPERTIES OF EXPANCEL

MICROSPHERES

EXPANCEL consists of white, spherically formed particles with a mean diameter of about 10 12 um. The shell, a copolymer of vinylidene chloride and acrylonitrile, encapsulates the blowing agent, liquid isobutene, under pressure. When the microspheres are heated to temperatures above 70^oC the thermoplastic shell becomes soft and elastic, which allows the spheres to expand.

The diameter of the microsphers increases 4 to 5 times at full expansion, which corresponds to a volume increase of more than 60 times. The initial true density of approx 1250 kg/m³drops to less than 20 kg/m³.

THE APPLICATION ON PAPER MACHINE

EXPANCEL 820, the microsphere grade often used in paper applications, is delivered as a 40% by weight, ready-to-use slurry. The slurry may

contain a sodium salt, which reduces the expansion starting temperature by about 10^oC and also prevents setting of microspheres.

To further avoid sedimentation, small percentage of starch is added as a thickener.

EXPANCEL 820 is available as EXPANCEL 820 SL 40, a grade well suited for fine paper and for anti-slip coating, and as EXPANCEL 820 SL 80, which is the ideal choice for board. An un salted version, EXPANCEL 820 SLU 40, is also available.

The standard package is a 1000 litres steel plated, one-way, polyethylene container, net weight 1000 kg, EXPANCEL content 400 kg. For trial purpose, 120 litre drums are also available.

Preparation

Unexpanded microspheres have a true density of 1250 kg/m³and will therefore settle to some degree in the water-based salt-treated slurry. Therefore, the slurry has to be re-dispersed with a mixer before dosage. If a sediment occurs at the bottom of the container, it is very important that stirring takes place in the liquid phase just above the sediment. The mixer should be lowered as the sediment is gradually dispersed.

A suitable mixer may have the following characteristics:

(an adjustable mounting and double propeller are recommended).

Power: 0.75 1.5 kW

Speed: 1000 1500 rpm

Propeller diameter: 150 200 mm

Shaft: 1200 mm

The microsphere slurry is pumped to the thick stock at the level box or at a corresponding point before the mixing pump. Dilution of slurry before dosage is not recommended as this may reduce the effect of the salt treatment. For dosage EXPANCEL recommends peristaltic pumps equipped with wear resistant tubing. In some special cases microspheres may be added by spraying on the surface of one or several centre layers. Such an arrangement requires good spraying equipment to provide an even distribution of the spheres.

Productivity

It is possible to use EXPANCEL to improve dewatering through a higher hip load in the press section. The compressed sheet will regain its bulk when the microspheres expand in the dryer section. The improved dewatering could be used either to increase machine speed or to reduce energy consumption. This effect can also be seen when the basis weight is reduced and the caliper is maintained through substitution of fibres with microspheres.

Retention of microspheres

Microspheres have a negative surface charge. It is therefore often necessary to use effective retention aids in order to reach a sufficient retention of microspheres in fine paper. The requirements on the retention system increase with increasing machine speed or decreasing grammage.

Good rention is required partly in order to fully utilize the relatively costly product and partly in order to improve the bonding of microspheres to the sheet. At low retention microspheres will be concentrated to the top side of a fine paper produced on a Fourdrinier machine. This may cause severe dusting in the dryer section and also increase the linting tendency in a printing press.