06-05-2013, 02:43 PM
Chemistry & Technology of Fabric Preparation & Finishing
[attachment=53468]
FABRIC PREPARATION
The term "Preparation" has two implications in textile processing. In greige
manufacturing, it is used to describe the processes which prepare yarns for weaving
and knitting. Mostly, it is used to describe slashing operations that ready warp yarns
for weaving. In dyeing and finishing, the term is used to describe those processes
that ready fabrics for the steps that follow, coloration and finishing. Fabric
preparation is the first of the wet processing steps where greige fabric is converted
into finished fabric. The steps that follow, dyeing or printing and finishing, are
greatly influenced by how the fabric is prepared. Improper preparation is often the
cause of problems encountered in the dyeing and finishing steps. Wet processing
technical conferences nearly always have sessions that stress the importance of fabric
preparation, seldom is there a conference without topics on how to prepare specific
fabrics. There are many different fabrics, many different plant set-ups and many
different machines used in wet processing. There is no universally accepted best
method for each of the wet processing steps. Nonetheless every set-up is expected to,
and more often than not, accomplish the same goals. To deal with this seemingly
infinite number of permutations, a fundamental understanding of what happens at
each step and how to control the chemical and physical parameters becomes
paramount.
PREPARATION PROCESSES
Various types of equipment can be used for preparing fabric. The ultimate goal
of any preparation process is to produce fabric that is clean and rid of all impurities
that interfere with dyeing and finishing. The preparation steps can be carried out
as either batch or continuous processes. The fabric may be transported as a rope or
as an open sheet through the equipment. The choice is often predicated on the dyehouse
itself. The distinguishing feature of batch equipment is that all of the fabric
is simultaneously submerged in the liquor. The fabric is agitated by moving it
through the liquor. In continuous processes, the fabric passes non-stop through
compartments and/or stages so that the fabric is incrementally subjected to the action
of the chemicals. The equipment used for dyeing fabrics is also suitable for preparing
fabric. In this section, the equipment used to perform fabric preparation will be
described.
YARN PREPARATION EQUIPMENT
Slashing is the process where Size is applied to warp yarns for weaving. The
purpose of size is to protect the yarn from the abrasive action of the loom. The
process is carried out on a Slasher and the application procedure is called Sizing
or Slashing. While technically this process is not considered as a step in preparing
fabric, the materials used in this operation, to a large measure, account for the bulk
of what must be removed. Because the desizing step is highly dependant on what
size was used, it is deemed instructive to discuss the slashing process as well as
discuss the nature of the sizing materials.
Figure 1 shows a schematic diagram of a slasher. In slashing, section beams
are combined to create a loom beam and at the same time apply the appropriate size
to the warp yarns. The section marked (1) is the let-off station where one or more
section beams are combined and fed through the rest of the range. The warp yarns
are let-off as a flat sheet and then pass through a size applicator (2) consisting of a
trough containing the size formulation and squeeze rolls. The yarns pass over heated
cans (3) to dry. Located at the exit end of the slasher is an arrangement of bust bars
which separate each warp end from its neighbor. The individual warp ends pass
through a reed which guides the yarn onto the take-up beam. Associated with the
slasher is a cooking station, where the sizing material (starch or polyvinyl alcohol)
is dissolved and stored. The solution is metered to the applicator as needed to
replenish what has been taken up by the warp sheet. The chemical nature of the size
will be discussed in a later section along with the conditions needed to effectively
remove the size.
FABRIC PREPARATION EQUIPMENT
Batch Machines
In batch processing, machines are used where the entire load of fabric is
immersed in the total amount of liquid needed for that process. These machines are
primarily used to dye fabric, however, in many cases they are also used to prepare
fabric prior to the dyeing cycle. The section that follows describes some of the more
popular machines.
Becks
A beck is the simplest type of wet processing batch equipment. It is a large vat
which holds the entire lot of fabric. The fabric is fed into the machine in the form of
a rope and is made into a continuous loop by sewing the two ends together. The
length of the loop is equal to a piece length, usually 100 to 200 yards. The fabric is
agitated by the action of a lifter wheel which gently moves the fabric through the
liquor by lifting and dropping the rope in and out of the liquor. Lot size is
determined by the number of loop strands that can be accommodated by the machine
and is a function of the front width of the machine. Commercial machine are
available ranging from one strand to 16 strands. A schematic of a beck is shown in
figure 2.
Jet Machines
Jet machines are similar to becks in that a continuous loop of fabric circulates
through the machine. They differ however in that a stream of liquor is forced
through a venturi tube. This provides the force to propel the fabric through the
machine. Temperatures in Jet machines that operate at atmospheric pressure cannot
exceed the boiling point of water; however, higher temperatures can be obtained in
those that are operated under pressure. An advantage of the jet machines is that
lower liquor ratios can be used. In a beck, the liquid is stationary and the fabric
moves through it. In a jet however, both the liquid and the fabric move in relation to
each other. This increases the rate of interchange between the liquid and fabric and
speeds up the process. The distinguishing features of jet machines are the venturi
tubes which create the force to circulate fabric, and circulating pumps which cycle
the bath through the venturi tubes. Figures 3 shows schematic diagrams of pressure
and atmospheric jets.
Steamers
Steamer provides dwell time, moisture and temperature. The drawing in
figure 9 shows a combination roller section - conveyer-bed steamer. The impregnated
fabric enters the roller section where the fabric is exposed to heat and moisture in
open sheet form. It is then plaited onto a conveyer-bed to provide reaction time for
the chemicals to work before the fabric is washed. Some steamers have only the
roller section. These require more floor space if the dwell time is .to be equivalent.
The combined roller, conveyor bed steamers are the result of new developments
aimed at improving cost/performance.
Open Width Washers
An open width washer can be a simple box (figure 10) containing a series of
vertically stacked rollers where the lower rollers are submerged in the wash water.
Fabric enters the box at one end and traverses the box by going over and under each
stack. Any number of these boxes can be arranged in series to provide the
appropriate amount of rinsing. More sophisticated boxes will be divided into several
compartments. Squeeze rolls are placed between them to speed up the removal of
impurities. Some are equipped with spray nozzles which also facilitate the flushing
action.
In another design, the rollers are stacked horizontally. The fabric enters
through a trough at the bottom of the box. The thread-up is such that the fabric
travels to the upper set of rollers and works its way down successive pairs of rollers
until it reaches the bottom. There it exits through another trough. Water cascades
downward through the layers so the flushing action is assisted by the physical force
of the water impacting the fabric.
YARN PREPARATION
A. Warp Size
In the production of woven fabrics, warp yarns are sized with a protective
coating to improve weaving efficiency. Movement of the warp yarn through the
heddles and mechanical actions during insertion of filling creates a great deal of
abrasive stresses on these yarns. Unprotected, the warp yarns cannot withstand the
rigors of weaving. They will break causing machine to stop and thus be responsible
for loss of productivity. Weaving efficiencies are vastly improved when the warp is
properly sized. Size or Sizing is defined as the composition of matter applied to the
warp yarn for weaving protection. Size usually consists of a water soluble, filmforming
macromolecule and a lubricant.
Requirement of a Good Size
The qualities required of a good size depends, in the main, on what type of
yarn is in the warp. For example, fine count spun yarns require more protection than
do plied yarns or course count spun yarns.
a. Spun Yarns
Spun yarns require that size contribute to yarn strength and that the
protruding fiber ends be glued down to make it less hairy. To do this, the sizing
solution must have a high viscosity to prevent penetration into the yarn. Optimum
protection is afforded when most of the size remains on the yarn surface to coat the
yarn and glue down the protruding hairs.
[attachment=53468]
FABRIC PREPARATION
The term "Preparation" has two implications in textile processing. In greige
manufacturing, it is used to describe the processes which prepare yarns for weaving
and knitting. Mostly, it is used to describe slashing operations that ready warp yarns
for weaving. In dyeing and finishing, the term is used to describe those processes
that ready fabrics for the steps that follow, coloration and finishing. Fabric
preparation is the first of the wet processing steps where greige fabric is converted
into finished fabric. The steps that follow, dyeing or printing and finishing, are
greatly influenced by how the fabric is prepared. Improper preparation is often the
cause of problems encountered in the dyeing and finishing steps. Wet processing
technical conferences nearly always have sessions that stress the importance of fabric
preparation, seldom is there a conference without topics on how to prepare specific
fabrics. There are many different fabrics, many different plant set-ups and many
different machines used in wet processing. There is no universally accepted best
method for each of the wet processing steps. Nonetheless every set-up is expected to,
and more often than not, accomplish the same goals. To deal with this seemingly
infinite number of permutations, a fundamental understanding of what happens at
each step and how to control the chemical and physical parameters becomes
paramount.
PREPARATION PROCESSES
Various types of equipment can be used for preparing fabric. The ultimate goal
of any preparation process is to produce fabric that is clean and rid of all impurities
that interfere with dyeing and finishing. The preparation steps can be carried out
as either batch or continuous processes. The fabric may be transported as a rope or
as an open sheet through the equipment. The choice is often predicated on the dyehouse
itself. The distinguishing feature of batch equipment is that all of the fabric
is simultaneously submerged in the liquor. The fabric is agitated by moving it
through the liquor. In continuous processes, the fabric passes non-stop through
compartments and/or stages so that the fabric is incrementally subjected to the action
of the chemicals. The equipment used for dyeing fabrics is also suitable for preparing
fabric. In this section, the equipment used to perform fabric preparation will be
described.
YARN PREPARATION EQUIPMENT
Slashing is the process where Size is applied to warp yarns for weaving. The
purpose of size is to protect the yarn from the abrasive action of the loom. The
process is carried out on a Slasher and the application procedure is called Sizing
or Slashing. While technically this process is not considered as a step in preparing
fabric, the materials used in this operation, to a large measure, account for the bulk
of what must be removed. Because the desizing step is highly dependant on what
size was used, it is deemed instructive to discuss the slashing process as well as
discuss the nature of the sizing materials.
Figure 1 shows a schematic diagram of a slasher. In slashing, section beams
are combined to create a loom beam and at the same time apply the appropriate size
to the warp yarns. The section marked (1) is the let-off station where one or more
section beams are combined and fed through the rest of the range. The warp yarns
are let-off as a flat sheet and then pass through a size applicator (2) consisting of a
trough containing the size formulation and squeeze rolls. The yarns pass over heated
cans (3) to dry. Located at the exit end of the slasher is an arrangement of bust bars
which separate each warp end from its neighbor. The individual warp ends pass
through a reed which guides the yarn onto the take-up beam. Associated with the
slasher is a cooking station, where the sizing material (starch or polyvinyl alcohol)
is dissolved and stored. The solution is metered to the applicator as needed to
replenish what has been taken up by the warp sheet. The chemical nature of the size
will be discussed in a later section along with the conditions needed to effectively
remove the size.
FABRIC PREPARATION EQUIPMENT
Batch Machines
In batch processing, machines are used where the entire load of fabric is
immersed in the total amount of liquid needed for that process. These machines are
primarily used to dye fabric, however, in many cases they are also used to prepare
fabric prior to the dyeing cycle. The section that follows describes some of the more
popular machines.
Becks
A beck is the simplest type of wet processing batch equipment. It is a large vat
which holds the entire lot of fabric. The fabric is fed into the machine in the form of
a rope and is made into a continuous loop by sewing the two ends together. The
length of the loop is equal to a piece length, usually 100 to 200 yards. The fabric is
agitated by the action of a lifter wheel which gently moves the fabric through the
liquor by lifting and dropping the rope in and out of the liquor. Lot size is
determined by the number of loop strands that can be accommodated by the machine
and is a function of the front width of the machine. Commercial machine are
available ranging from one strand to 16 strands. A schematic of a beck is shown in
figure 2.
Jet Machines
Jet machines are similar to becks in that a continuous loop of fabric circulates
through the machine. They differ however in that a stream of liquor is forced
through a venturi tube. This provides the force to propel the fabric through the
machine. Temperatures in Jet machines that operate at atmospheric pressure cannot
exceed the boiling point of water; however, higher temperatures can be obtained in
those that are operated under pressure. An advantage of the jet machines is that
lower liquor ratios can be used. In a beck, the liquid is stationary and the fabric
moves through it. In a jet however, both the liquid and the fabric move in relation to
each other. This increases the rate of interchange between the liquid and fabric and
speeds up the process. The distinguishing features of jet machines are the venturi
tubes which create the force to circulate fabric, and circulating pumps which cycle
the bath through the venturi tubes. Figures 3 shows schematic diagrams of pressure
and atmospheric jets.
Steamers
Steamer provides dwell time, moisture and temperature. The drawing in
figure 9 shows a combination roller section - conveyer-bed steamer. The impregnated
fabric enters the roller section where the fabric is exposed to heat and moisture in
open sheet form. It is then plaited onto a conveyer-bed to provide reaction time for
the chemicals to work before the fabric is washed. Some steamers have only the
roller section. These require more floor space if the dwell time is .to be equivalent.
The combined roller, conveyor bed steamers are the result of new developments
aimed at improving cost/performance.
Open Width Washers
An open width washer can be a simple box (figure 10) containing a series of
vertically stacked rollers where the lower rollers are submerged in the wash water.
Fabric enters the box at one end and traverses the box by going over and under each
stack. Any number of these boxes can be arranged in series to provide the
appropriate amount of rinsing. More sophisticated boxes will be divided into several
compartments. Squeeze rolls are placed between them to speed up the removal of
impurities. Some are equipped with spray nozzles which also facilitate the flushing
action.
In another design, the rollers are stacked horizontally. The fabric enters
through a trough at the bottom of the box. The thread-up is such that the fabric
travels to the upper set of rollers and works its way down successive pairs of rollers
until it reaches the bottom. There it exits through another trough. Water cascades
downward through the layers so the flushing action is assisted by the physical force
of the water impacting the fabric.
YARN PREPARATION
A. Warp Size
In the production of woven fabrics, warp yarns are sized with a protective
coating to improve weaving efficiency. Movement of the warp yarn through the
heddles and mechanical actions during insertion of filling creates a great deal of
abrasive stresses on these yarns. Unprotected, the warp yarns cannot withstand the
rigors of weaving. They will break causing machine to stop and thus be responsible
for loss of productivity. Weaving efficiencies are vastly improved when the warp is
properly sized. Size or Sizing is defined as the composition of matter applied to the
warp yarn for weaving protection. Size usually consists of a water soluble, filmforming
macromolecule and a lubricant.
Requirement of a Good Size
The qualities required of a good size depends, in the main, on what type of
yarn is in the warp. For example, fine count spun yarns require more protection than
do plied yarns or course count spun yarns.
a. Spun Yarns
Spun yarns require that size contribute to yarn strength and that the
protruding fiber ends be glued down to make it less hairy. To do this, the sizing
solution must have a high viscosity to prevent penetration into the yarn. Optimum
protection is afforded when most of the size remains on the yarn surface to coat the
yarn and glue down the protruding hairs.