stoomcursus

Steaming

top

The Problem

The steaming of printed textiles is shrouded in mysticism. The literature contains numerous descriptions of the steaming process that are vague and sometimes downright nonsense. The underlying chemical and physical principles have, however, already been understood completely for decades. The theoretical model is based on adsorption and reaction kinetic mechanisms. It accounts quite satisfactorily for the qualitative aspects of the process. Obtaining reliable quantitative figures from the adsorption/reaction kinetics model is however extremely difficult. It is a rather complex process and the complete characterization would require a large number of variables, such as reactive group(s), molecule shape and dimensions, pH, ion strength, colour affinity with the substrate, chemicals in the printing paste, redox potentials, moisture content, temperature, type of printing process, substrate type, specific area of the textile, residual chemicals left on the textile, et cetera, and et cetera. The list of variables that play a role in the steaming process can easily be extended with dozens more. The complete model taking into account all these variables would not be very practical. The interactions between the variables would lead to a set of coupled nonlinear differential equations, which can not be solved in general.

top

Solution

A reduced qualitative model, concentrating on the dominant elements, suffices to develop a steamer and associated procedures that will guarantee optimal steaming conditions and results. The potential problems and their adverse consequences can be completely understood and adequately dealt with on the basis of an appropriately reduced description.

top

Starting Point

The objective in textile printing is the application of colour and pattern to fabrics. The end-use requirements determine the type of colorant. This can be a pigment colorant or a reactive, disperse, acid, base or vat dyestuff. The colorant can be applied to the cloth by screen printing, rotary or flat-bed, or by an inkjet printer. The auxiliary chemicals needed for the fixation can be mixed with dyestuffs and applied at the same time as the dyestuffs; this is the case in the so-called one phase fixation. But in some cases these chemicals can not be mixed with the dyestuffs or it just might be cheaper to apply these in a separate printing step. In inkjet printing the chemical are usually applied separately, prior to the actual printing.

The fixation of pigments is a very simple process. A straightforward heat treatment suffices for the cross-linking agents to transform into a tight layer encapsulating the pigment particles. The process is simple to implement and the probability that something could go wrong, is extremely low. In the sequel we will not consider the pigment colorants any further.

Startingpoint for steaming is always a piece of fabric on which the dyestuffs and chemicals are present and the goal is to achieve fixation of the dyestuffs. The steamproces of reactive dyestuffs is the most complicated and is used in our description of the proces.

top

Steaming step 1: inlet

Upon feeding the textile into the steam chamber the cold and dry cloth will heat up through the condensation of steam. The large latent heat of the condensation makes that the heating takes place in a very short time span, typically within a few tenths of a second. The substrate then has a temperature of 100 °C, while the moisture content has gone up a mere 4% since it entered the steam chamber.

top

Steaming step 2: super heating

The textiles used for reactive dyestuffs printing quite often have hydrophilic surfaces. This is a highly relevant property since it greatly influences the associated adsorption equilibrium. For a given substrate that is placed in a steam environment we have a specific equilibrium relation between the temperature and the moisture content. The function relating these two quantities is called the adsorption isobar. The general form of this relation looks as follows:

The adsorption isobar gives the equilibrium moisture content for a given temperature of the substrate. If the cloth has a temperature of 100 °C and contains less moisture than the adsorption isobar specified amount, the cloth will attract the missing water from the steam. The latent heat released by the condensation causes the cloth temperature to rise. The condensation can thus lead to temperatures that are considerably higher than the pre-set fixation temperatures; this condition with temperatures that are too high is called super-heating. Depending on substrate properties and initial moisture content, the super-heating temperature can go as high as 130 °C. The substrate temperature would then greatly exceed the optimal fixation range. The scale and impact of the condensation heat can be explained by a simple example: a 0.5% increase in moisture content can already lead to a temperature rise of 10 °C.

For certain types of colorants the temperature in the steam chamber can get even higher due to exothermal chemical reactions that accompany the steaming process.

top

Steaming step 3: cooling

After the super-heating phase we are in a situation in which the cloth temperature is higher than the temperature of the surrounding steam. The lower temperature of the steam can be exploited in transporting the excess heat away from the cloth. The adsorption isobar tells us that a small decrease in substrate temperature will inextricably be bound up with an increase in moisture content (see figure ??). The extra water for this can be obtained only from the steam. The heat released in the steam-to-water process is about 2400-2600 Joules for one gram of water, which is slightly higher than the latent heat for the condensation proper. The released energy can only be removed from the steam chamber via the steam. Increasing the steam temperature from 100 to 110 °C requires about 20 Joules for one gram of steam. To get rid of the heat produced, the adsorption of 1 gram of water thus requires 125 g steam of 100 °C, or expressed in volumetric units: one needs 400 litres of steam to remove the heat produced by the condensation of 1 millilitre of water.

The enormous amounts of energy released by the condensation imply that the substrate cools very slowly and only if there is a great flow 'cold' steam around the substrate. The heat exchange between substrate and steam is a slow process. The heat-exchange coefficient is very low and increasing the flow velocity yields a limited increase only.
If the steam in the vicinity of the cloth is not replaced by cool (100 °C) steam, the cooling will not take place at all. The amount of fresh steam per kg substrate is an important parameter for practical applications.
Depending on the circumstances the dwell-time will be between a few minutes and a few dozens of minutes. The cooling process might, however, last longer than the actual dwell-time. Assuming that this is not the case, the substrate will, after proper cooling, have the same temperature as the steam and a rather high moisture content &endash; in some cases, e.g. for viscose, this moisture content can be as high as 25 %.

top

Steaming step 4: equilibrium

In the preceding steps the substrate temperature rose above 100 °C and the moisture content went up also.
When the cloth temperature reaches the desired fixation value and the moisture content of the substrate equals the condensation isobar value for this temperature, the conditions are ideal for the fixation reaction. It is crucial that the substrate reaches this equilibrium state as quickly as possible.

top

Steaming and dyestuff

The previous sections considered the physical aspects of super-heating. But the super-heating has another, also negative consequence. The moisture content is much lower if the substrate is super-heated. And the low moisture content considerably hinders the diffusion of the dye molecules to the substrate. This means that there is not enough water to allow the dyestuff molecules to move around. In the super-heated state the colorant molecules can not reach the substrate fibres and is it hence impossible for the dye to fix onto the substrate. A prolonged condition of super-heating inevitable leads to a sharp decrease in fixation efficiency.
Selecting a longer duration for the steaming process can occasionally lead to some improvements, but the fact that impact of other reactions will then also increase, renders this method useless in most cases. The colorant molecules can not only react with the substrate but also with other materials. In case the diffusion of the dyes to the substrate is impeded, the other reactions will still continue. Colorants that have reacted with other materials can not react anymore with the substrate.
In short, to obtain high colour yields and optimal fixation efficiencies one has to avoid, or at least limit, the substrate super-heating and to ensure the right moisture content for the substrate.

top

Steaming and machine design

Starting from the chemical and physical principles governing the steam process, we described the adverse effects caused by poor temperature control. A non-uniform temperature distribution in the fixation chamber, for instance a temperature difference between left and right part of the chamber, will lead to different fixation efficiencies and as a consequence to noticeable colour differences. The temperature in the steam chamber has to be homogenous and, to avoid batch differences, also reproducible.
If the super-heating of the substrate lasts too long the fixation efficiency will decrease significantly. One thus to provide sufficient cooling capacity to quickly remove the excess heat produced by the steam adsorption.
The Portafix Universal has been designed to guarantee optimal control of the temperatures in the steam chamber, even, or foremost, at the substrate surface. Quintessential in this is the enormous 'fresh' steam flow capacity. The Portafix Universal is equipped with a forced steam circulation circuit. The reused steam is reconditioned in such a way that the steam is as new when it again enters into the steam chamber.

top	The Problem
	The steaming of printed textiles is shrouded in mysticism. The literature contains numerous descriptions of the steaming process that are vague and sometimes downright nonsense. The underlying chemical and physical principles have, however, already been understood completely for decades. The theoretical model is based on adsorption and reaction kinetic mechanisms. It accounts quite satisfactorily for the qualitative aspects of the process. Obtaining reliable quantitative figures from the adsorption/reaction kinetics model is however extremely difficult. It is a rather complex process and the complete characterization would require a large number of variables, such as reactive group(s), molecule shape and dimensions, pH, ion strength, colour affinity with the substrate, chemicals in the printing paste, redox potentials, moisture content, temperature, type of printing process, substrate type, specific area of the textile, residual chemicals left on the textile, et cetera, and et cetera. The list of variables that play a role in the steaming process can easily be extended with dozens more. The complete model taking into account all these variables would not be very practical. The interactions between the variables would lead to a set of coupled nonlinear differential equations, which can not be solved in general.
top	Solution
	A reduced qualitative model, concentrating on the dominant elements, suffices to develop a steamer and associated procedures that will guarantee optimal steaming conditions and results. The potential problems and their adverse consequences can be completely understood and adequately dealt with on the basis of an appropriately reduced description.
top	Starting Point
	The objective in textile printing is the application of colour and pattern to fabrics. The end-use requirements determine the type of colorant. This can be a pigment colorant or a reactive, disperse, acid, base or vat dyestuff. The colorant can be applied to the cloth by screen printing, rotary or flat-bed, or by an inkjet printer. The auxiliary chemicals needed for the fixation can be mixed with dyestuffs and applied at the same time as the dyestuffs; this is the case in the so-called one phase fixation. But in some cases these chemicals can not be mixed with the dyestuffs or it just might be cheaper to apply these in a separate printing step. In inkjet printing the chemical are usually applied separately, prior to the actual printing. The fixation of pigments is a very simple process. A straightforward heat treatment suffices for the cross-linking agents to transform into a tight layer encapsulating the pigment particles. The process is simple to implement and the probability that something could go wrong, is extremely low. In the sequel we will not consider the pigment colorants any further. Startingpoint for steaming is always a piece of fabric on which the dyestuffs and chemicals are present and the goal is to achieve fixation of the dyestuffs. The steamproces of reactive dyestuffs is the most complicated and is used in our description of the proces.
top	Steaming step 1: inlet
	Upon feeding the textile into the steam chamber the cold and dry cloth will heat up through the condensation of steam. The large latent heat of the condensation makes that the heating takes place in a very short time span, typically within a few tenths of a second. The substrate then has a temperature of 100 °C, while the moisture content has gone up a mere 4% since it entered the steam chamber.
top	Steaming step 2: super heating
	The textiles used for reactive dyestuffs printing quite often have hydrophilic surfaces. This is a highly relevant property since it greatly influences the associated adsorption equilibrium. For a given substrate that is placed in a steam environment we have a specific equilibrium relation between the temperature and the moisture content. The function relating these two quantities is called the adsorption isobar. The general form of this relation looks as follows:

	The adsorption isobar gives the equilibrium moisture content for a given temperature of the substrate. If the cloth has a temperature of 100 °C and contains less moisture than the adsorption isobar specified amount, the cloth will attract the missing water from the steam. The latent heat released by the condensation causes the cloth temperature to rise. The condensation can thus lead to temperatures that are considerably higher than the pre-set fixation temperatures; this condition with temperatures that are too high is called super-heating. Depending on substrate properties and initial moisture content, the super-heating temperature can go as high as 130 °C. The substrate temperature would then greatly exceed the optimal fixation range. The scale and impact of the condensation heat can be explained by a simple example: a 0.5% increase in moisture content can already lead to a temperature rise of 10 °C. For certain types of colorants the temperature in the steam chamber can get even higher due to exothermal chemical reactions that accompany the steaming process.
top	Steaming step 3: cooling
	After the super-heating phase we are in a situation in which the cloth temperature is higher than the temperature of the surrounding steam. The lower temperature of the steam can be exploited in transporting the excess heat away from the cloth. The adsorption isobar tells us that a small decrease in substrate temperature will inextricably be bound up with an increase in moisture content (see figure ??). The extra water for this can be obtained only from the steam. The heat released in the steam-to-water process is about 2400-2600 Joules for one gram of water, which is slightly higher than the latent heat for the condensation proper. The released energy can only be removed from the steam chamber via the steam. Increasing the steam temperature from 100 to 110 °C requires about 20 Joules for one gram of steam. To get rid of the heat produced, the adsorption of 1 gram of water thus requires 125 g steam of 100 °C, or expressed in volumetric units: one needs 400 litres of steam to remove the heat produced by the condensation of 1 millilitre of water. The enormous amounts of energy released by the condensation imply that the substrate cools very slowly and only if there is a great flow 'cold' steam around the substrate. The heat exchange between substrate and steam is a slow process. The heat-exchange coefficient is very low and increasing the flow velocity yields a limited increase only. If the steam in the vicinity of the cloth is not replaced by cool (100 °C) steam, the cooling will not take place at all. The amount of fresh steam per kg substrate is an important parameter for practical applications. Depending on the circumstances the dwell-time will be between a few minutes and a few dozens of minutes. The cooling process might, however, last longer than the actual dwell-time. Assuming that this is not the case, the substrate will, after proper cooling, have the same temperature as the steam and a rather high moisture content &endash; in some cases, e.g. for viscose, this moisture content can be as high as 25 %.
top	Steaming step 4: equilibrium
	In the preceding steps the substrate temperature rose above 100 °C and the moisture content went up also. When the cloth temperature reaches the desired fixation value and the moisture content of the substrate equals the condensation isobar value for this temperature, the conditions are ideal for the fixation reaction. It is crucial that the substrate reaches this equilibrium state as quickly as possible.
top	Steaming and dyestuff
	The previous sections considered the physical aspects of super-heating. But the super-heating has another, also negative consequence. The moisture content is much lower if the substrate is super-heated. And the low moisture content considerably hinders the diffusion of the dye molecules to the substrate. This means that there is not enough water to allow the dyestuff molecules to move around. In the super-heated state the colorant molecules can not reach the substrate fibres and is it hence impossible for the dye to fix onto the substrate. A prolonged condition of super-heating inevitable leads to a sharp decrease in fixation efficiency. Selecting a longer duration for the steaming process can occasionally lead to some improvements, but the fact that impact of other reactions will then also increase, renders this method useless in most cases. The colorant molecules can not only react with the substrate but also with other materials. In case the diffusion of the dyes to the substrate is impeded, the other reactions will still continue. Colorants that have reacted with other materials can not react anymore with the substrate. In short, to obtain high colour yields and optimal fixation efficiencies one has to avoid, or at least limit, the substrate super-heating and to ensure the right moisture content for the substrate.
top	Steaming and machine design
	Starting from the chemical and physical principles governing the steam process, we described the adverse effects caused by poor temperature control. A non-uniform temperature distribution in the fixation chamber, for instance a temperature difference between left and right part of the chamber, will lead to different fixation efficiencies and as a consequence to noticeable colour differences. The temperature in the steam chamber has to be homogenous and, to avoid batch differences, also reproducible. If the super-heating of the substrate lasts too long the fixation efficiency will decrease significantly. One thus to provide sufficient cooling capacity to quickly remove the excess heat produced by the steam adsorption. The Portafix Universal has been designed to guarantee optimal control of the temperatures in the steam chamber, even, or foremost, at the substrate surface. Quintessential in this is the enormous 'fresh' steam flow capacity. The Portafix Universal is equipped with a forced steam circulation circuit. The reused steam is reconditioned in such a way that the steam is as new when it again enters into the steam chamber.