The three most important Parameters which defines the pulp are. 1.)
Fiber Length, 2.) Brightness, 3.) Pulping process used.
e.g. Northern Soft Wood Bleached Kraft (NSWBK). The Northern Soft Wood tells it
is long fiber pulp. Bleached tells, it has high brightness and Kraft tells that
Kraft (Sulfate) pulping process is used to produced this pulp. Similarly
Southern Hard Woof Unbleached Kraft, will be a short fiber wood unbleached (low
brightness) pulp made by kraft process. Link to a few typical Market Pulp
data sheets
Ash content in pulp may consists of various chemicals used during
pulping/bleaching, mineral matter from wood or metallic matter from pipes
and other machinery. It is not important parameter of
pulp.
Ash is the residue left after igniting pulp at 525 0C (As per
TAPPI T211). Ash is reported in % of residue to dry pulp basis.
The standard procedure of measuring ash content is laid out in
TAPPI T211, ISO 1762
Brightness of Pulp
Brightness of paper is discussed in
Paper Properties. The paper brightness is mainly dictated by pulp
brightness. There are some modification in stock preparation which can alter
paper brightness to some extent such as filler, sizing, whitening agent,
dying etc. In short
Conductivity of Pulp
Electrical grade papers such as cable paper, condenser tissue or
insulation paper etc., require very low conductivity to electricity. The
presence of metal ion more specifically iron ion contribute to pulp
conductivity The pulp used for electrical grades are washed with
demineralized water, beater or refiner use lava or other non-metallic bars
and contacting surfaces of all equipment are made of stainless steel.
Values for the conductivity of the water extract of the
pulp are expressed in µS/m.
S =
Siemen (SI unit of electric
conductance) = 1 mho.
Dirt in Pulp
Dirt content of pulp particularly of recycled pulp is important for its
suitability to make fine paper. Dirt is any foreign material in pulp. TAPPI
defines dirt as foreign matter in a sheet which, when examined by reflected,
not transmitted light, has a marked contrasting color and has an equivalent
black area of 0.04 mm2 or more.
The standard procedure of measuring dirt content is laid out in
TAPPI T213
Drainage Time of Pulp
Here the drainage time of pulp is discussed in reference to market pulp
and/or unrefined pulp. The drainage time of pulp or freeness or slowness of
pulp is modified to have some desired properties in the paper, here that is
not discussed.
Drainage of unrefined pulp which is measured as freeness can give an
indication on : 1) Fiber Length of pulp, as long fiber pulps have more
freeness compared to short fiber pulps, 2) Damage to fiber during pulping,
bleaching or drying as short fibers or fines produced during pulping
operation, reduces pulp freeness, 3) Refining energy required to achieve
certain slowness during stock preparation.
The standard procedure of measuring pulp drainage is laid out in
TAPPI T221, T227, ISO 5267-1 and ISO 5267-2
Dry Content of Pulp
Consistency:
is the term used to describe solid content of pulp during pulp
processing. For pulp and paper maker this is the most important
process parameters. All equipments are designed to handle pulp at and up to
certain consistency. Pulp consistency is roughly divided in to three ranges:
Low Consistency: <5%
Medium Consistency: 5 - 15%
High Consistency: >15%
It is the desire of every pulp maker to keep pulp at the highest possible
consistency to minimize dilution water usage and which ends up as effluent.
Higher consistency also helps in reducing the bleaching chemical
consumption. But there are practical limitation of handling pulp at higher
consistency such as high viscosity which make pulp flow very
difficult.
The standard procedure of measuring pulp consistency (up to 25%) is laid out
in TAPPI T240.
Moisture Content of Market Pulp: is important from storage,
transportation and handling point of view. Most of the market pulp are sold,
stored, transported and used as air
dry. The useable part of pulp is dry fiber only, so the tendency is to
minimize the moisture content op pulp.
Small quantity of pulp is sold as wet lap also. Wet lap pulp is not dried
at source and transported at about 50% moisture content. It is feasible for
short distance transportation and if pulp is to be used immediately at user
end.
Extractives (Low Molecular Weight Carbohydrates) in Pulp
The low molecular carbohydrates indicates an extent of cellulose
degradation during pulping and bleaching process, which may effect pulp
strength and other properties. Pulp is treated with 1% hot NaOH solution for
one hour to estimate loss of yield due to extractives.
The standard procedure of measuring 1% Hot Alkali Solubility is laid out
in TAPPI T212
Length of fibers (arithmetic average, weighted average etc.) is one
of the most important parameters of pulp. Pulp strength is directly
proportional to fiber length and dictates its final use. A long fiber pulp
is good to blend with short fiber pulp to optimize on fiber cost, strength
and formation of paper. Softwood with pulps in general have longer fiber
compared to hard wood pulp. Pulp made from woods grown in cold climate in
general have longer fiber compared to wood grown in warmer climates.
Chemical pulps in general have higher fiber length compared to semi
chemical pulp and mechanical pulp, when made from same wood. More fibers get
damaged/shorten by mechanical action than chemical action.
There are several method to measure /report fiber length of pulp. The
'fiber length of pulp by projection' is described in TAPPI T232.
The 'fiber length of pulp by classification' is described in
TAPPI T233. "Fiber length of pulp and paper by automated optical
analyzer using polarized light' is described in TAPPI T271.
The coarseness of pulp
fiber is described in
TAPPI T234.
Kappa number is determination of relative hardness, bleachability or
degree of delignification of pulp. It is important parameter of unbleached
pulp which is to be bleached.
The method to find kappa # of pulp is described in
TAPPI T236.
Pulping Process
Though the pulping process used is directly not a pulp property but this
is one of the most important parameters used in specifying the pulp. As we
move from full mechanical to full chemical pulping process, strength
of pulp and bleachability improves. Strength improves due to less
degradation of fibers and bleachability as more lignin is removed in
chemical than mechanical pulping processes.
Yield: Pulp yield is mainly govern by the pulping process.
Mechanical pulping processes which provide high yield, retain almost all
constituents of wood. Lignin which is second highest to cellulose, does not
bond to itself or cellulose fibers as fibers do, don't contribute to any
bonding, resulting in weak pulp. Secondly lignin is brown in color and to
maintain high yield of bleached pulp, lignin is not removed during
bleaching, but only chemically modified.
Tensile Strength of Pulp
This is not the tensile strength of individual fiber, which is even higher
than or comparable with steel. The tensile strength discussed here is
maximum strength of randomly oriented pulp fiber when formed in a sheet.
This tensile strength gives an indication of the maximum possible strength
of pulp beaten under ideal condition. This again an indication of what level
of tensile strength can be achieved in real paper making environment.
One way of measuring tensile strength of pulp is "zero span breaking
strength' described in
TAPPI T231. Wet zero span tensile strength of pulp is measured using
TAPPI T273.
Viscosity of Pulp
Solution viscosity of a pulp gives an estimation of the average
degree
of polymerization of the cellulose fiber. So the viscosity indicate the
relative degradation of cellulose fiber during pulping /bleaching process.
Dissolving pulps
from wood, which contains a large proportion of alpha cellulose, give
higher viscosity values than paper pulps.
The standard procedure of measuring pulp viscosity is laid out in
TAPPI T230
