The loading factor of washing
A high level of standardisation and
efficiency cannot be attained unless machines are loaded to the correct degree.
Overloading results in inadequate cleansing and makes efficient rinsing difficult
while under loading means loss in production and causes greater wear on the fabrics.
For cotton work, the most satisfactory loading factor is 18 litres of cage space
per kg of washing load and for woollens 25 litres per kg. Polyester/cotton blends
are usually best loaded at 22-25 litres per kg. The load in the machine absorbs
an appreciable quantity of liquor; for 400kg dry weight of cotton sheets, for example,
about 100kg of water is retained by the work at each run out, and if the overloading
is considerable, an unduly large proportion of the wash liquor is held by the load
causing an unnecessarily large quantity of the dirty liquor to be retained at the
run out and carried over to the next stage of the process.
Composition of detergent solution
and the use of the correct detergents for the type of work
When washing processes appropriate
for different types of work are designed, different alkalis and detergents are used
for different fibre types/blends in most processes, the concentrations of detergents
are different at different stages of the process. It is therefore important that
the proper types and quantities of detergents are used to ensure good results and
economy of materials.
The chief cleansing agents used in
laundry work are detergents and alkalis and in order to get good results with economy
of materials, it is necessary that quantities should be carefully controlled. The
addition of a scoopful of several handfuls of 'dry' materials to a machine is not
a very satisfactory procedure. The method is very wasteful as more materials than
necessary are often used, and additional time is required for them to dissolve in
the water in the machine. It saves time and trouble to keep stock solutions of soap,
alkali and bleach, the materials most frequently used in the laundry. By this means
it is possible to measure accurately the quantities used and when added to the machine,
the solutions are ready to function when the mixing time is completed. In order
to make up a solution of known concentration it is necessary to calculate the volume
of the tank which is to hold it, and the amount of liquid it will contain.
Alternatives to stock solutions are
now available, the major one being 'liquid feed injection'. Such systems have many
advantages over standard stock solution methods but control is still essential to
ensure their efficient and economic operation.
For the following reasons, it is
important to use the correct amount of water in the machine at any particular stage
of the washing process:
To control the amount of mechanical
rubbing to which the load is subjected.
Generally, low dips increase the amount of rubbing,
thus increasing the wear on the fabrics, but even so, they do not always give a
greater degree of dirt removal. The dip is not the same throughout all the washes
of a white work process, as greater rubbing is required towards the end to remove
resistant soiling, and therefore somewhat lower dips are used in the later washes
than in the earlier ones.
For woollens and some other fibres, where it is essential
to avoid mechanical action as far as possible, high dips are used at all stages
to minimise felting.
To control the concentration of
the solution in the machine
The quantities of detergents and
alkalis and other possible additives to be added, are calculated for known quantities
of water and load weight, and if the dip is not adjusted correctly the solution
in the machine will not be of the required concentration. This is particularly important
when bleach is used, as a solution more concentrated than necessary can have serious
weakening effect on fabrics.
To ensure efficient rinsing
The removal of wash materials and
dirt from the machine and therefore from the load, depends on the proper adjustment
of the dips at the different stages of rinsing. With a low dip level a smaller proportion
of the dirty liquor is removed from the machine than with a high one, and therefore
the quantity of dirt carried over into the next wash is greater.
Note: The difference between standing
dip and running dip should be noted. Standing dip refers to the depth of liquor
in the machine when the machine is in motion. For a given quantity of liquor, the
running dip is less than the standing, and care should be taken to avoid confusion
when designing or monitoring a process. Usually most processes quote running dips
although on the continent it is sometimes the practice to quote water volume to
unit load weight ratios.
The control of the temperature of
wash liquors is necessary because different fabrics must be washed at different
temperatures. With soiled white work a high temperature in the final wash is essential
to ensure adequate cleansing, but high temperatures might damage woollen, coloured
or heat sensitive articles and if a high temperature were used in the initial stages
of a white work process certain stains might become set and very difficult to remove.
It is therefore important that the appropriate temperature is used for any given
process or any particular stage within it and the directions given should always
be carefully followed. The control of temperature is also very important when using
hypochlorite bleach, as serious damage to the fabrics can occur if the proper temperature
is exceeded. Thermal disinfection is totally dependant on the correct temperature
control and adequate standards of disinfection can only be achieved by careful and
exact temperature control within the washing machine.
The duration of any washing process
must be controlled so that on the one hand sufficient time is allowed for adequate
cleansing, and on the other, the process is not unduly prolonged, since this results
in soil-redeposition, unnecessary wear on the textile and waste of time with consequent
loss of production. In the case of coloured items, fading may be increased by a
process which is unnecessarily long. The time taken for dirty liquor to drain from
a machine should not be excessive, but as the size and construction of machines
vary it is not possible to be specific about the required time. In all cases
adequate time should be allowed for the drain period to ensure maximum removal of
dirty liquor from the machine, as otherwise an unduly high proportion of dirt remains
in the load which is then carried over to the next wash or rinse.
Sufficient time to allow complete
mixing of reagents/load/water must be allowed at the beginning of each wash. This
is known as "mixing time".
Water is referred to elsewhere as
the miracle solvent. This it indeed is, but water on its own is an inefficient
washing medium because strange as it may seem, it is not very good at making things
wet, if water is poured into a glass then emptied out again, the inside of the glass
will be quite unevenly wetted. The water has shrunk right back from some areas and
left them dry. This example demonstrates the phenomenon known as surface tension.
The main function of a detergent is to reduce this tension and to allow water to
Tension exists in the surface of
a body of water for the following reason. Water consists of innumerable molecules
which are strongly attracted operate in all directions, and the water is held in
a state of equilibrium. At the surface of the water, however, there are no forces
acting from outside. All the forces are inward and equilibrium is produced only
when molecules at the surface are drawn in towards the body of water, reducing the
surface area to a minimum. That is why the drops left at the sides of a glass after
it has been emptied tend to become spherical in shape, a sphere having the smallest
surface area for a given volume. The force which attracts molecules inwards is balanced
by a tension in the surface which prevents further shrinking. This later force is
called surface tension. In the case of washing, this tension prevents the water
from coming into intimate contact with either the surface of the material or the
dirt to be removed from it. To solve the problem and alter the condition of the
water's surface, something must be found which is sufficiently attracted to water
to dissolve in it, but is also to some extent repelled by it.
Detergents whether soapy or non-soapy,
solve the problem because they have a molecular structure consisting of a hydrophilic
('water-loving') head and a hydrophobic tails of the detergent molecules are pushed
out between the water molecules, breaking the bonds between them and so reducing
tension. In the case of a drop of water the spherical structure will collapse, since
the equilibrium has been destroyed. As it collapses the surface area increases,
pushing out more detergent molecules. So the water spreads and wets the surface.
As well as solving this basic problem
of reducing surface tension, detergents perform other very important functions in
Most soil is acidic in nature and
before any washing takes place this condition should be neutralised. If the load
which is on the acid side is not neutralised, the acidic materials react with soap
to form acid soaps, which have little or no cleaning value.
To offset this, alkalis of various
kinds are used, these are generally much cheaper than soap. Soap alone may be used,
but it's expensive.
Water soluble materials are easily
taken care of, but the proteins and albuminous material which make up the greater
part of the stains are more difficult to remove.
There are often many saponifiable
materials in soiling. Saponification is the reaction of an alkali with greasy, fatty
materials present in the load such as kitchen grease, to form soap.
Among the oily types of soil, there
are likely to be mineral oils and greases, such as lubricating oils and medicinal
oils which will not saponify. Emulsification helps get rid of this type of soil.
Emulsification is the action of an alkali or detergent which breaks up the globules
of grease and oil into very small particles. Emulsification keeps them separated
so that they do not reform into larger units.
The most difficult soil a laundry
has to remove from fabrics is solid dirt, which includes not only carbon but dust
and earth such as clay. The material is firmly held on the fabric and often penetrates
deeply into it. Water alone will not remove such soil, but a long soaking in soap
and water gradually removes and suspends it. However, this action is very slow.
It is speeded up by the mechanical action and use of alkalis. Much dirt is in the
form of tight clusters of particles, and as these clusters are removed from the
cloth in their original form, they are likely to be redeposited. To prevent this,
a binder which acts as a deflocculating agent should be used. Thus the tight clusters
are broken up into individual particles.
Suspension and prevention of redeposition
We see from the forgoing that if
soil particles are removed from the fibre, they must be prevented from re-depositing
onto the fabric. The action of holding these particles in suspension is called "suspending
power", and the action of preventing redeposition on the fabric after
these particles have been removed is called "prevention of redeposition".
Soil redeposition is referred to as "greying".
This, of course, is provided by the
rotation of the washer. Such things as change in cage speed, the load, weight, dips
and the time of operation will increase or decrease the effectiveness of this action.
Temperature control is very important
in good laundering especially for handling coloured work, synthetics, silks and
woollens. Bleaches, setting of stains and fibre damage are also affected by temperature.
Accurate thermometers should be installed
on all machines.
Water level (dip) gauges are very
important. Every control formula is based on a given amount of water in the machine.
If a wash formula calls for a water level of 10cm, but is actually 20cm, the concentration
of supplies is totally altered. Usually the results are not what was expected. More
water than necessary during the wash means a greater weight of wash products leading
to wastage and expense. More heat energy will be required and in addition the mechanical
effect will be reduced.
Technique Of Washing
There are six principal features
of a washing process than can be varied by the process designer:
- Loading factor
- Time (duration)
- Mechanical action
- Washing materials/rations
They can be varied between one process
and another and with the exception of the first (loading factor) they can be varied
between one stage and another of a single process. The effects of varying each of
these features independently must be considered first, before considering how best
to combine them in the design of an entire process.
Loading Factor (also referred to as the degree of
This is defined as the unit weight
of work related to the unit volume of the machine cage. It is usually expressed
as weight/unit volume (e.g. kg/m³ where m³ of water = 1000 litres). If for example, a certain machine which has a cage volume of 800 litres
is used to wash a 45kg load the loading factor is 18 l/kg. If a load of 15 kg is
sometimes washed in the same machine, the loading factor is 53 l/kg.
In terms of their capacity (weight
load) washing machines cover a very wide range from a few kg to something approaching
450kg. At the same time, weights of loads washed in one given machine may be varied
within much narrower limits depending on the type of load, the weight of work available
and the preference of the individual launderer, so a machine described as of 45kg
capacity may not always be used to wash 45 kg of work. This would be the case with
a load of average soiling where the launderer may load at 55kg. Just as in the case
of heavy soiling he may load only 36kg. When washing special fabrics such as woollens
or polyester/cotton, it is common practice to load the machine to 70% of the usual
white work weight.
Once the loading factor has been
chosen, the normal weight of load for a given machine is calculated on this basis.
The machine is said to be overloaded if it is used for a heavier load, or under-loaded
if the load is lighter than normal.
Overloading has several disadvantages
unless there is some compensating feature. Firstly the amount of mechanical action
is reduced because the load has less space in which to move within the cage. Thus
the efficiency of soil removal is correspondingly decreased. Moreover, since the
heavier load carries more soiling than a load of normal weight the liquors have
to suspend a large amount of dirt. Finally, the efficiency of rinsing is reduced
because the added water cannot readily penetrate the heavy load. One way of compensating
for these disadvantages would be to lengthen the washes and rinses, but the heavier
load would then require a longer time and there would be no net benefit in terms
of increased output over the day. Nevertheless it is sometimes permissible to operate
at a higher than normal loading factor.
If the work to be washed is only
very lightly soiled, then increasing the load may not introduce more dirt than the
liquors can deal with. This is true of some hospital linen. Machines of a very large
cage diameter provide more mechanical action than smaller machines and can produce
an adequate degree of soil removal even when fairly heavily loaded.
Just as overloading decreases the
mechanical action imparted to the load, so under loading somewhat increases it,
and the first disadvantage of under loading is that it may increase the amount
of wear on some fabrics. Moreover, an under loaded machine requires virtually the
same amounts of heat, water and other washing materials as a correctly loaded machine,
so under loading increases the consumption of all these supplies in relation to
the weight of work washed. Here again, however, a lighter than normal loading factor
can sometimes be justified, in this case if the work to be washed is exceptionally
The disadvantages just described
for both overloading and under loading are of importance only if they are practiced
consistently. Occasional under loading or overloading is often unavoidable, depending
on the amount of work or the consumption of materials. Table 9 illustrates the most
commonly used loading factors established by trials in the UK.
Table 9: Most commonly used
Kg/m³ or g/l
Types of loads
Satisfactory loading factor
For heavy soiling
For very lightly soiled loads
For woollen fabrics
The term "dip" is used
to denote the depth of the liquor in the cage of a washing machine, i.e. the depth
measured from the bottom of the inner cage. References are made to a "dip"
of 15cm or a 10cm dip, and so on. Two conditions of dip are possible: standing dip
and running dip. A standing dip is that which is measured when the machine is not
running and is only of interest when the cage is at rest. Dips measured when the
cage is rotating are called running dips.
Variations in dip influence the amount
of mechanical action imparted to the load, bearing in mind that the action of the
machine is to lift the load repeatedly and drop it towards the bottom of the cage.
In fact the falling load strikes the liquor rather than the cage itself, so the
higher the dip the lower the degree of mechanical action. This depends to some extent
on the degree of loading employed but generally higher dips create less mechanical
movement of the load.
Dips also relate to the concentration
of washing materials in the machine. Whilst the term dip relates to the depth of
liquor in the inner cage, the gap between the inner cage and outer case varies from
machine to machine. Thus it is necessary to know the total volume of water in the
machine in order to establish correct and controllable wash material concentrations.
See diagram 13.1.
To overcome this problem, many European
countries use a volume/weight figure e.g. 4-1 15-1 usually expressed as litres per
kg (load weight), this obviously means that the inner cage dip will vary from machine
to machine but is easily controlled.
Many chemical reactions will not
take place unless heat is applied and most can be accelerated by raising the temperature.
To the extent that soil removal is achieved by chemical and physical action, there
is no exception to this general rule. The higher the temperature of the solution
up to a certain level, the more efficient the soil removal. The highest temperature
which can be attained in a typical washing machine is 100ºC, the temperature at
which the liquor boils, and it used to be thought that soil removal would be inadequate
unless this temperature was reached.
For several years it has been known
that the highest useful temperature for soil removal from most articles is 85ºC.
The efficiency improves as the temperature is raised to this level but no further
advantage is gained by exceeding this figure.
In the special circumstances of a
hospital laundry another reason for increasing the temperature during the washing
process is the need to ensure disinfection. Here again it used to be thought that
boiling is essential for this purpose, but it is now accepted that somewhat lower
temperatures are adequate, provided they are maintained for a sufficient length
of time. At 65ºC disinfection is obtained in 10 minutes. It has to be remembered
however, that the thermometer on a washing machine indicates only the temperature
of the free liquor, and if the machine is being heated the actual temperature of
the load will lag behind that displayed by the thermometer. The instrument
must therefore indicate 65ºC for at least 14 minutes, this time being extended to
18 minutes if the loading factor is high. Such times are considerably longer
than those necessary for soil removal, and would unduly prolong the washing process
so reducing the output of work from the machine. It can be preferable to use a somewhat
higher temperature. At 71ºC, 3 minutes is sufficient to ensure disinfection. The
thermometer must show this temperature for a longer time (between 7 and 11 minutes)
depending mainly on the loading factor. Some diseases can give rise to articles
which could not be adequately disinfected in the washing conditions just described.
This will of course be known to medical staff concerned and most probably this work
would be sent for autoclaving, or it may be sent with special instructions that
the washing process must include a period of at least 10 minutes at 93ºC.
Although fairly high temperatures
are necessary for both soil removal and disinfection, it does not follow that every
washing process should be operated at these temperatures from start to finish. Certain
types of staining substances can be removed readily at low temperature, but become
fixed and almost impossible to remove if subjected to high temperatures. The hottest
part of the process must therefore be preceded by a sufficient period at lower temperature
to ensure that these substances are removed early in the process.
Hospital laundry, apart from the
need for disinfection, poses particular problems of staining due to the use of disinfectants
at hospitals. Proteinaceous fluids (such as blood) and the use of chlor-hexidine
products (such as Savlon or Dettol) iodine and other medical products can cause
permanent stains if not properly handled during the washing process.
Proteinaceuos soiling and other staining
substances on articles subjected to rapid exposure to wash temperatures above 40ºC
can cause permanent staining. To avoid this, such articles should be separated and
correctly pre-rinsed below 40ºC to remove the stains.
Articles that have been soaked in
chloro-hexidine products require careful handling at laundry level. If chlorine
based bleach products (Hypochlorite bleach) are used during laundering, the chlorine
will cause the chloro-hexidine residue in the articles to set as permanent stains.
If Hypochlorite bleach is used, it should never be used at temperatures above 60ºC
as this will damage the fabric. An antichlor has to be added after the bleaching
process to remove all traces of chlorine from the load as the heat in the drying
process will accelerate damage to the fabric.
For the correct use of bleaches,
starches, brightening agents, etc. reference should be made to the laundering section
of the FCRA handbook.
The possible effect of heat on some
dyes may have to be taken into account when designing processes for coloured work.
The dyes on many coloured articles are fast at the temperature necessary for soil
removal and disinfection, but others suffer fading and may do so even at lower temperatures.
It is increasingly unlikely that the less fast coloured will be used in hospital
departments, because of the accepted need to use disinfecting temperatures in washing
but personal work from staff and patients must always be expected to include some
coloured items on which dyes are not fast at 65ºC. Unfortunately there is no way
of judging from the appearance of a colour whether the dye is fast or not, even
if this were possible, and if the articles were separated according to their degrees
of fastness, this would result in a number of very small loads which would be uneconomical
and impractical to wash separately. The only practical alternative is to wash them
altogether as a low temperature miscellaneous coloured load, in which some dyes
may be fast and others not. A typical temperature used in a low temperature wash
Another feature which may prevent
the use of high temperature is the nature of the fibres. The heat sensitive character
of some fibres may not allow this. If the temperature is too hot some man made fibres
soften causing distortion and loss of shape of garments.
High temperatures must also be avoided
on garments where special fabric finishes have to be retained. One example of this
is the flame retardant finish imported to nightwear or protective work wear which
may be affected by high temperatures.
Temperatures in rinsing need not,
in general, be deliberately controlled. It is normal practice to use only cold water,
but it does not follow that the rinses can properly be described as cold. The load
and the liquor retains carry-over heat from the final wash, and this warms the water
added for rinsing. This procedure is repeated from each rinse to the next, so the
temperature falls in a step pattern: firstly if it is desired to bleach in a rinse
the temperature must be controlled precisely for the reason already stated. The
second exception is with washer-extractors. It is sometimes good practice to use
a hot water final rinse in order to improve the hydroing efficiency.
The total duration of a washing process
is made up of the times occupied by individual washes and rinses, the time taken
to run in water for each stage and the times required to drain the machine at the
end of each stage. Time is needed in a wash for soil removal to be achieved. However
effective the cleansing properties may be, the dirt is not removed instantaneously.
Rather it is removed gradually over a period of several minutes, but the rate at
which it is removed is not constant. The rate is highest at the commencement of
the wash and gradually becomes less at the time passes until dirt is being removed
only very slowly.
However good the suspending power
of a liquor may be it is never perfect. Redeposition of the removed dirt is taking
place continuously, and when the rate of soil removal falls to a level not much
higher than the rate of redeposition there is no point in continuing the wash. Dirt
still remaining in the work is better left to be removed in a further wash. In practice
it is found that about 6 minutes is the longest useful time for washing most rotary
machines, but in many processes it is useful to extend the final wash as it is at
this stage that the most resistant dirt has to be removed.
These times are appropriate for machines
running at conventional cage speeds. As mentioned earlier some modern machines are
designed to run at higher speeds and in these cases the wash times may be reduced.
In all cases the times are sufficient only if the washing materials are already
in solution when added to the machines. If dry materials are added, extra time must
be allowed for them to dissolve in the wash liquor before they can be fully effective.
Rinsing is achieved, as already explained
by diluting the 'carry over' from the final wash in order to reduce the concentration
of washing materials and removed dirt. In many machines the method of "batch
rinsing" is operated, whereby this procedure is repeated to give 3 separate
rinses, or sometimes 4. In each rinse the added water is allowed to mix with the
carry over, the machine is drained, more water is added and so on. Complete mixing
of the carry over and the added water to produce a uniform concentration throughout
all the liquor in the machine, takes about 3 minutes. It should be noted that the
time for rinsing to be effective (about 3 minutes) is the same whether or not there
is a low speed extract after the final wash and between rinses. The alternative
to "batch rinsing" is "continuous rinsing" which has already
been explained. Continuous rinsing can achieve the same efficiency as batch rinsing
but will require different water quantities which will vary with time.
Another component of the total process
time is the time required to mix water and materials into the machine for each stage
of the process. This time is known as "mixing time" and must be included
into the running time of the wash.
Finally the draining times contribute
to the total process time. This is a recurring feature of the process, as the machine
has to be drained at the end of every wash and with batch rinsing - every rinse.
If the time is excessive the prolonging effect is considerable and clearly it is
important that the draining times should be as short as possible. Most modern machines
drain in 1 minute.
It should be realised that wash time
is money, so wash cycle times should be kept to the effective minimum, thus allowing
high production from the washing machines. Another reason for avoiding excessive
times in the wash cycle is to prevent fabric damage caused by excessive mechanical
This is provided by vigorous movement
of the load as the cage rotates, so that the fabrics rub against one another and
fall to strike against the cylindrical wall of the cage. If the cage had no internal
fitment it would merely slide past the load and produce little or no mechanical
action, so it is fitted internally with "lifters", these are devices which
lift part of the load at a time and allow it to drop. Rubbing and the impact as
the work strikes the liquor at the bottom of the cage produces the necessary mechanical
action. Several precautions can be taken to limit the mechanical action. Very high
dips can be employed in the washes and rinses to reduce the drop. The loading factor
is restricted to 25 l/kg to reduce the amount of rubbing action. The speed of rotation
of the cage can be reduced, or, the speed remains unaltered but the motion is interrupted
from time to time. The machine may be run for say 15 sections and then be stopped
for the same or somewhat longer period and so on alternately. Some machines have
a mechanism which affects this interruption automatically and which is sometimes
called an "interrupter gear". To increase mechanical action would be reversal
of the procedure described above.
Washing materials - ratios
Basic essential materials for adequate
cleansing during a washing process are:
These substances can be varied
to suit different types of soils and work to be processed but within the requirement
of a specific wash process, the quantities of each must be well defined.
Detergents (as has been seen) can
wet efficiently and are important in the removal of soiling matter. Alkalis whilst
assisting the detergent are valuable in their own right having an effect on the
removal of ingrained and greasy soil removal, especially by saponification.
The detergent/alkali ratio can be
seen as important when designing a wash process since the requirements of each wash
can be fulfilled e.g. high detergent/low alkali for early washes, low detergent/high
alkali for later hotter washes.
Detergent and alkali are often
purchased as dry substances and the necessary quantities are usually specified by
weight in the washing process. It is not convenient however to weigh each separate
amount of detergent and alkali for each wash. This difficulty is overcome by preparing
a stock solution for each. A stock
solution being a solution of known concentration, such as 50 grams/ litre, so that
any measured volume contains a known weight of substance in solution. Thus any required
weight of detergent of alkali can be obtained merely by measuring the corresponding
volume of solution. Measurement is supplied and the materials are already
in solution when added to the machine.
A concentration of 50 grams/litre is commonly used i.e. 1 litre contains 50g of
material so that if the washing process calls for, say 300g of detergent the quantity
is simply translated into 6 litres of stock solution. The concentration chosen can
be whatever is most convenient in the light of all prevailing circumstances.
With the use of stock solutions it
is necessary to know the capacity of the storage tank and decide the concentration
of the stock solution, it is then a simple matter to calculate the weight of washing
material to be dissolved to prepare a full tank.
Stock solutions may be tested to
check that they have been prepared properly or whether the concentration has changed
since it was first prepared because of re-heating. Heating causes the evaporation
of water from the solution, which thus becomes more concentrated. If heating
is by open-ended steam pipe the steam entering the tank condenses to form water
which dilutes the solution.
Alternatives to stock solutions are
now available, the most popular being 'liquid feed' injection. Concentrated, pre-formulated
detergents, alkalis and bleaches are added to washing machines using peristaltic
pumps, creating a convenient space saving situation.
Once the materials are in the washing
machine further control is necessary to ensure that correct conditions are produced
in the process.The quantity of detergent can be readily judged by the appearance
of the lathers. In a typical wash a rich lather should form and should persist throughout
the wash. A thin or non-existent lather may indicate lack of sufficient suspension
power, with the risk of disposition of the removed dirt. This depends on the type
of detergent used. The main objection to a very heavy lather is that it represents
a waste of detergent and therefore money but it may also be detrimental in that
it reduces the mechanical action imported to the load. A lighter lather should be
observed on the first rinse. This indicates suspending power which is essential
at the beginning of rinsing.
Unfortunately there is no simple
visual indication of whether an appropriate amount of alkali is present in the wash.
It is therefore necessary to test a sample of the wash liquor by titration. Alkali
will be present in water used in the wash process and this alkalinity can be determined
by testing the process water separately by the same method used for the wash liquor,
the figure that remains is the "net alkalinity" of the liquor i.e. the
alkalinity due to washing materials. This in turn is due partly top soap and partly
to alkali. Soap contributes relatively little to the net alkalinity which is largely
due to the alkali so a test to determine the net alkalinity of a wash liquor indicates
mainly whether the amount of alkali in the liquor is appropriate.
A net alkalinity of at least 1,2
g/l is necessary to ensure adequate soil removal for washing process for white work
or fast coloureds. Even with careful control, however, variations in the actual
alkalinity must be expected and it is preferable to aim at a somewhat high concentration,
usually 1,4 g/l to allow a margin of safety to meet the occasions when the alkalinity
is unexpectedly low. If the concentration is consistently low the efficiency of
soil removal is likely to be inadequate. If the alkalinity is consistently high
not only is alkali and money being wasted but the difficulty of ensuring adequate
rinsing is increased.
For classifications which might be
adversely affected by excessive alkali much lower net alkalinites are necessary,
in the order of 0,6 g/l or even less. The test to determine the alkalinity of a
wash liquor can be applied also to rinse liquor. In practice there is little point
in testing the liquor during the early stages of rinsing but a test at the
completion of rinsing can indicate how efficiently the load has been rinsed. Even
better from this point of view is a test of the effluent when the machine is on
extract, because this liquor is forced out from within the fabric, where it cannot
be reached while the load is in the washing machine and may have a higher alkalinity
than the free liquor which surrounds the load.
If too much alkali is left in the
fabric at the conclusion of the washing process the fabric is likely to become yellowed
when subjected to the high temperature of finishing on a calendar or press. This
fault is called "galling" and is especially likely to occur if the total
alkalinity of the liquor left in the load exceeds 0,28 g/l. The aim of rinsing is
to reduce the net (or added alkalinity) as much as possible.
It has already been stated that a
washing process must remove dirt from the load, it must hold dirt in suspension
and it must discharge the removed dirt into the drain. It has also been shown that
requirements may be met by suitable variation of certain features of the process,
notably the degree of loading, the dip, temperature, time, the amount of mechanical
action and the detergent/alkali ratio. A careful consideration of the effects of
these variations, will show, however, that they give rise to some apparently conflicting
requirements. For example a low dip is necessary for maximum soil removal, but in
order to discharge the dirt from the machine a higher dip is required. Again a low
temperature must be maintained to prevent the setting of certain stains, but
good soil removal calls for the temperature to be raised. Finally for soil removal
a low detergent/alkali ratio is necessary to ensure suspension of the removed soiling.
Clearly it is impossible to incorporate all these features at the same time, at
a low dip and high dip, at a low temperature and a high temperature and with the
detergent/alkali ratio both low and high. For this reason it cannot be expected
that a process which includes only one wash can ever by satisfactory in a conventional
rotary machine unless perhaps the load is initially so very small. A process must
therefore include at least 2 washes. Once this is accepted it is obvious that each
wash can be of a different character from the other, and can be designed to perform
a different function.
It has already been stated that the
greater proportion of dirt is relatively easily removed therefore conditions at
the first washing stage need to ensure the best loose soil removing properties.
The detergent/alkali ratio must be high to provide a good wetting and suspending
power and the dip must be high to ensure that a large proportion of the removed
dirt is discharged to the drain. Also the temperature must be low to ensure that
no stains are set.
By the time the final wash is reached,
only the most resistant dirt should remain to be removed. Very good oily soil removing
properties are required and are achieved by operating at lower dip levels, at high
temperature and with a low detergent, high alkali ratio. These are not conditions
which always ensure good suspending power or the discharge of a large proportion
of dirt to the drain but this is of little consequence as there is likely to be
only a small amount of this very resistant dirt.
For most of the work to be laundered
2 washes on these lines should be sufficient but there is often a small amount of
heavily soiled work where it would be unwise to attempt to remove all the dirt in
only 2 washes. The process for such work should therefore include another wash between
the first and final washes described above. Clearly this additional wash would have
to deal with dirt of a type which is not so easily removed as that encountered in
the first wash. The conditions therefore would be such as will remove this type
There may even be occasional loads
which are so very heavily soiled that 4 stages would be advisable particularly where
polyester/cellulose fabrics are concerned. This should seldom arise, but a similar
pattern would be followed with a gradual change of conditions throughout the successive
washes, the dips decreasing, the temperatures rising and the detergent/alkali ratio
decreasing. These principles are referred to as graded principles and most satisfactory
washing processes for use in conventional rotary machines are designed on them.
This information courtesy of Division
of Building Technology, CSIR.