REFINING AND PULP CHARACTERIZATION
Refining – mechanical treatment of pulp fibers to develop their optimum papermaking properties. Refining increases the strength of fiber to fiber bonds by increasing the surface area of the fibers and making the fibers more pliable to conform around each other, which increases the bonding surface area and leads to a denser sheet. Generally a refining is a trade-off between improving fiber to fiber bonding and decreasing the strength of individual fibers. Most strength properties of paper increase with pulp refining, since they rely on fiber to fiber bonding. The tear strength, which depends highly on the strength of the individual fibers, actually decreases with refining. Refining of pulp increases their flexibility and leads to denser paper. This means bulk, opacity, and porosity values decrease with refining.
Fiber Brushing - refining at high consistency with a relatively large distance between the refiner plates increases fiber-fiber interactions that are termed fiber brushing. This tends to roughen the fiber surface, with minimal fiber cutting for improved fiber-fiber bonding.
Fiber Cutting - Operating refiners at low consistencies
with a minimal distance between the refiner plates increases fiber-bar
contact, resulting in fiber cutting. This is desired with long-fiber pulps
(redwood, cotton etc.) to increase the quality of formation on the paper
machine. In most cases, however, it is desirable to minimize fiber
cutting to maintain high paper strength.
Drainage – is the ease of removing water from pulp fibers, either by gravity or mechanical means. CSF is a measure of drainage and a useful means for determining the level of refining.
Fibrillation – production of rough surfaces on fibers by mechanical action; refiners break the outer layer of fibers, i.e., the primary cell wall, causing the fibrils from the secondary cell wall to protrude from the fiber surfaces. For example, the surface area of kraft, softwood fibers is on the order of 1m2/g at 750 CSF, but it increases to about 5m2/g at 350 CSF.
Average fiber length – statistical average length of fibers in pulp. Fiber length is measured microscopically (number average), by classification with screens (weight average) or by optical scanners - Kajaani (number average). The coarseness of fibers has an important effect on many properties of paper. Coarseness is defined as the weight per unit length of fiber expressed as milligrams per 100 m.
Pulp used to be treated in beaters , such as the Hollander beater, but
now refiners are used. The terms beating and refining are often used interchangeably,
but refining is applicable to most modern equipment.
Canadian Standard Freeness, CSF – the CSF test was developed for use
with groundwood pulps and was not intended for use with chemical pulps;
nevertheless, it is the standard test for monitoring refining in North
American mills. Refining decreases the pulp freeness, the rate at which
water will drain through the pulp. Refined pulp, therefore, has a low freeness.
A high drainage rate also means a high freeness. Obviously freeness is
of utmost importance in the operation of a paper machine. A
low freeness means that the paper machine will have to operate relatively slowly, a condition that is usually in tolerable. There are other freeness tests that are used around the world. Perhaps the most common one is the Shopper Riegler test, which is similar in concept to the CSF test. CSF results depend on the temperature and consistency.