Impact and applications of different PH solutions in textile and polymer industry

pH:

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution.

Definition: pH is the negative logarithm of the hydrogen ion concentration.  pH is a measure of the concentration of hydrogens ions (H⁺ ions or protons) in a solution.

In all textile processes in which aqueous solutions are used, balancing the pH of the solution is primary. The effectiveness of oxidizing and reducing agents is pH dependent. The amount of chemicals required for a given process is directly related to the pH the solubility of substances, such as dyes and impurities, vary with pH the corrosive and scaling potential of processing solutions is also heavily influenced by pH All of these issues affect quality and costs.  Basically, pH has wide applications and impact on textile industry.

Following are some key points and out look of its impacts and applications.

Why pH changes during a Textile dyeing process?

1. Water quality
2. Reaction products
3. Additives during the process
4. Time
5. Temperature
6. Contaminants in the substrates

The control of pH in textile processing is ensured by fundamentally three different techniques, such as:

1. The maintenance of a relatively high degree of acidity or alkalinity.
2. The control of pH within fairly narrow tolerances mainly in near neutral regions.
3. The gradual shift of pH as dyeing proceeds.

Many processes of textile processing are pH dependent:

1. Scouring of cotton in highly alkaline conditions
2. Bleaching of different substrates where pH has to be maintained for proper bleaching action.
3. Solubilizing the dyestuffs
4. Exhaustion and fixation
5. Oxidation
6. Stripping
7. Finishing 

PH in textile processing:

o   Wetting and saturating process:

                                    pH plays an important role in the wetting and saturating processes. For example, caustic solutions cause interfibrillar swelling in cotton cellulose and cannot be squeezed out as easily as water, which can reduce quality in subsequent processing. The scouring of wool is a good example of a process where maintaining the pH value permits a better solubilization of certain impurities. For example, a pH of 10 is considered optimum for the removal of wool wax.

o   Bleaching:

 To effectively bleach cellulose (e.g. cotton) with a minimum amount of damage, the bleaching solution must be alkaline. This keeps the hypochlorite stable and also prevents the presence of reducing groups that cause an apparently well-bleached cloth to yellow with age. Additionally, an acidic solution will form toxic and corrosive chlorine gas. Bleaching liquor is therefore usually maintained at a pH of 9. The permanence of the white obtained is thereby increased, and the bleaching is safe

Due to environmental concerns in recent times, hydrogen peroxide bleaching has become more prevalent. Its reaction products, oxygen and water, are relatively harmless. However, hydrogen peroxide is a weak acid. Thus, its conjugate base, HO₂ -, is used to perform the actual bleaching. To ensure an adequate concentration of HO₂ -, the solution pH must be tightly controlled. Sodium hydroxide is used to maintain the pH at a very alkaline level of 12-12.5.

o   desizing:

pH must also be maintained in desizing, where hydrogen peroxide, sodium bromide, or other oxidizing agents are employed to remove the size of the yarn. A pH of >9 is required in the use of sodium bromide, for example, as it decomposes at pH 8.

o   Boiler water:

The pH of the boiler water in textile processing must also be carefully monitored and controlled. To minimize corrosion, a pH as high as 9 is desirable. However, care must be taken to avoid excess alkalinity. Alkaline substances such as sodium hydroxide, carbonate, and phosphate have a solvent action on the non-ferrous metal fittings. In pressure boilers caustic embrittlement may also occur at riveted areas and places of high stress. Additionally, it must be considered that acidic waters cannot be softened by agents such as EDTA to improve processes.

PH adjustment in textile processing:

Traditional chemicals used for pH adjustment in textile processing include sulfamic acid, formic acid, sulfuric acid, phosphoric acid and combinations thereof. These pH adjusters can exhibit one or a combination of low efficiency (i.e., require large amounts of chemical for the desired effect), high cost, difficulty in handling, hazards in handling, corrosiveness, or high acidity of the resulting effluent stream.

One of the most commonly used pH adjustment system used in carpet processing is sulfamic acid ((HO)S(O)₂ NH₂). Lower pH can be attained with sulfamic acid than with comparable amounts of formic acid. Sulfuric and phosphoric acids will lower pH more efficiently than either sulfamic or formic acids. Corrosivity tests performed on carbon steel and stainless-steel tickets show that all of these acids can be harmful to metal equipment.

Sulfamic acid is a solid that can be dissolved in water up to a concentration of about 15% at room temperature. Dissolving the acid is cumbersome and represents an additional step in the textile manufacturing processes while creating another quality control step.

Since sulfamic acid is only soluble in water to an extent of about 15% by weight at room temperature, and is usually used in a 13% solids solution, it is not a very efficient pH adjustment system. Approximately 6-grams/liter of 13% sulfamic acid solution is required to lower the pH of a typical stain resist application bath to a pH of 2, depending on the concentration and type of stain blocking chemical used.

pH in dyeing:

 Effect of pH on dye adsorption is very important. Besides, controlling pH is very important in dye bath because its effect on the dyeing cycle. Generally, pH greatly influences the uptake of dye. In the case of acid dyeing, a low pH helps to form the hydrogen bonds that attach acid dyes to protein fibers, such as silk and wool, as well as nylon. In the case of most popular fiber reactive dyes, a high pH actually activates the cellulose (cotton) fiber, forming a cellulosic anion, which can then attack the dye molecule, leading to a reaction that produces a strong, permanent covalent bond.

In the case of acid dyeing, a low pH helps to form the hydrogen bonds that attach acid dyes to protein fibers, such as silk and wool, as well as nylon. Acid dyes do not work on cellulose fibers such as cotton. Optimal pH varies according to the specific type of acid dye. Some require only a very mild acid; others a more significantly low pH

Silk can be dyed at low pH (acidic) or high pH as with cellulose fibers; wool can only be dyed at low pH because it is damage by high pH Cellulose fibers, such as cotton, linen, rayon, etc., cannot be dyed at all at low pH

Different pH in Different Conditions of Wet Processing:

 

Condition	pH Should be
Neutralization after bleaching	5.5-6.5
Scouring and Bleaching Bath	10.5-11.5
Enzyme Bath	4.5-4.75
Initial Dye Bath after leveling	5.5-6.5
After Salt	6.5-7.5
After Alkali	10.5-11.2
Neutralization after dyeing	5.0-6.0
Fixing bath	4.5-5.5
Softener Bath (Cold wash)	4.0-5.0

Vat dyeing:

In the instance of vat dyeing, pH controls the solubilization of the dyes. Initially, the quantity of caustic soda present must be adequate to ensure the solubility of the leucon form. Once the dye has been exhausted, the pH is adjusted such that the dye returns to its insoluble form and is mechanically trapped in the fiber.

Fiber reactive dyes:

In the case of most popular fiber reactive dyes, a high pH actually activates the cellulose (cotton) fiber, forming a cellulosic anion, which can then attack the dye molecule, leading to a reaction that produces a strong, permanent covalent bond. Without a high pH, the dye will not fix permanently to the cellulose fiber. In dyeing cotton and other cellulose fibers with popular fiber reactive dyes such as Porcino MX or Sabra Cron F dye, sodium carbonate is used for no other reason than to increase the pH of the dye reaction, so that the fiber will react with the dye.

Chemistry of reactive dyeing:

A molecule is much too tiny to see, but we can use models to show what the dye molecule is shaped like. Each of these balls represents a different sort of atom. These Cs are carbon atoms, like we see in charcoal. The Os are oxygen, like in the air we breathe, and these Cls are chlorine, like in bleach.

Cotton, which grows on a cotton plant, is made of long strands of cellulose molecules, all twisted together. If we put two molecules, the dye and the cotton, together, nothing will happen, unless we can get some of the atoms on the surfaces to come unstuck. If the H comes off of the cellulose, and the Cl comes off of one end of the dye molecule, the molecules will be able to react with each other and stick together. How do we get the H and the Cl to get off of the cellulose and the dye? We just add another chemical, called sodium carbonate: [model of Na₂CO₃] what this does is increase the pH That's how we say that it makes it less acid. An acid has a low pH the opposite of acid is called a base. When we put baking soda in water, we get a high pH A high pH is all that is needed to get the dye and the cellulose ready to react. Sodium carbonate is stronger than baking soda, so it works better for dyeing. All we have to do to make a permanent bond between the dye and the cotton is to put the dye on the cotton and add washing soda. We can put the sodium carbonate on the fabric before or after we put on the dye. After we put the dye and the sodium carbonate on the fabric, we just have to wait a while. While we wait, the reaction is happening - chlorines are coming off the dye molecules and hydrogen is coming off of the cellulose molecules. If they do this right next to each other, the dye then attaches to the cellulose, and a permanent bond is formed. If we leave it in a warm room for a few hours, we can then wash the excess dye out. We have to rinse it in cold water and wash it with detergent in hot water to get all the extra dye off. After all the excess dye is out, the dye left on the fabric is permanent.

Influence of pH in reactive dyeing at every stage of dyeing:

1        In the beginning of dyeing, the water bath should be carefully adjusted to a neutral to slightly acidic pH, as otherwise premature hydrolysis of dyestuff will take place and cause (a) uneven dyeing and (b) lighter depths than the previous batches or in other words batch to batch variation will occur.

2        If the fabric or yarn has not been neutralized properly the core alkali presence will adversely affect the dyeing, forming patchy uneven dyeing. The places were alkali residue was high having the

tendency to make deeper dyeing.

3        Lower alkali dosages and hence lower pH leads to partial reaction of reactive dyes; most of the dye may remain in water; the dyestuff that has got absorbed in to the fiber would also have less tendency to get fixed on to it, leading to poor washing and rubbing fastness.

4        Higher dosage of alkali may cause hydrolysis of dyestuff in the water itself. Thus, lower depth of shade and poor washing and rubbing fastness.

5        After dyeing is over, when the alkali still fully remains on the fiber, if we do not neutralize the alkali properly with adequate quantity of acid, that also leads to higher amount of dyestuff bleeding during subsequent soaping and hot wash operations.

6        Finally after completing the dyeing, before unloading, if we do not keep the pH neutral - alkaline pH will slowly hydrolyze the dyestuff in the fiber and acid pH will tender the cotton fiber itself.

7        Every dyestuff appears in different tone under different pH conditions. Bright Lemon yellow, if allowed dry under alkaline pH, it will turn to a dull redder yellow and similarly Turquoise blues and royal blues will appear yellowish and duller in alkaline pH and brighter and redder in acidic pH, So make sure that the pH is exactly neutral or slightly acidic during final drying process.

8        Final cationic fixation and cationic softening treatment if not done in acidic pH, that will leave higher tonal changes and improper dye fixation and improper softening effect.

pH Levels for Different Stages of Cotton Dyeing:
Initial Bath	pH 6.5~7.0.
Before Enzyme bath	pH 4.5~4.7.
Before Scouring and Bleaching	pH (With Enzyme) 5.5~5.8.
Before Scouring and Bleaching	pH (Without Enzyme) 5.5~5.8.
Scouring and Bleaching bath	pH 10.0~10.5.
After Scouring and Bleaching	pH 8.5~9.0
Before Leveling Chemicals	pH 6.5~7.0.
After Leveling Chemicals	pH 6.7~7.0.
After Adding Dyes	pH 6.2~6.35.
After Addition of Salt	pH 7.5~8.0.
After Addition of Soda	pH 10.5~11.0.
Before Hot Wash bath	pH 6.8~7.2.

pH in garments industry:

The pH values in garments can be greatly affected by:

Ø  Scouring

Ø  Bleaching

Ø  After-treatment

Ø  Final washing process

Why pH value is concerned in textile and garments industry?

Ø  Human skins are slightly acidic in nature

Ø  To inhibit bacteria growth.

Ø  Textile process involves the use of strong acids or alkalis,

Ø  With a too high or low pH

Ø  May cause irritation to skin when in contact

What material should be tested?

Natural and synthetic textile fibers.

PH in waste water treatment:

Determination of pH plays an important role in the wastewater treatment process. Extreme levels, presence of particulate matters, accumulation of toxic chemicals and increasing alkalinity levels are common problems in wastewater.

Wastewater treatment often consists of removing heavy metals and/or organic compounds from effluent streams. pH adjustment by addition of acidic/basic chemicals is an important part of any wastewater treatment system as it allows dissolved waste to be separated from water during the treatment process.

As a chemical component of the wastewater, pH has direct influence on wastewater treatability – regardless of whether treatment is physical/chemical or biological. Because it is such a critical component of the makeup of the wastewater, it is therefore critically important to treatment. Before proceeding with treatment, you have to identify the parameters, the impurities that are in the wastewater. Once you know what you are dealing with, you determine the starting and the ending pH values, along with treatment procedures; then you have to select the appropriate chemicals best suited for treatment.

PH in polymer industry:

pH-sensitive polymers can be defined as polyelectrolytes that include in their structure weak acidic or basic groups that either accept or release protons in response to a change in the environmental pH, the acidic or basic groups of these polyelectrolytes can be ionized just like acidic or basic

groups of monoacids or mono bases; however, complete ionization of these systems is more difficult due to electrostatic effects exerted by other adjacent ionized groups.

The pH range where the reversible phase transition happens can generally be modulated in two ways:

Ø  selecting the ionizable moiety with a PK matching the desired pH range

Ø  selecting between polyacid or poly base and incorporating hydrophobic moieties into the polymer backbone (selectively control their nature, amount and distribution)

If the polyelectrolyte chains are hydrophobic when unionized in a poor solvent, they collapse into globules and precipitate from solution. The interplay between hydrophobic surface energy and electrostatic repulsion between charges dictates the behavior of the polyelectrolyte. Since the degree of ionization of a weak polyelectrolyte is controlled by pH and the ionic composition of the aqueous medium, pH-sensitive polymers dramatically change conformation in response to minute changes in the pH of the aqueous environment. Medium pH controls poly base conformation changing from an expanded state to a folded state depending on the ionization degree of the pH-responsive polymer

 pH-responsive polymers contain

Ø  weakly acidic (e.g., carboxylic acid)

Ø  basic (e.g., ammonia) groups

 these either release protons or accept free protons, respectively, in response to environmental pH

 

Effect of decreasing PH on concrete properties

Depends on the way you want to lower the pH of the concrete? If you use carbon dioxide you are adding material to your concrete. The matrix becomes a higher density and with it a slightly better compressive strength. As mentioned above the big disadvantage is the loss of rebar protection. If this is no problem for your investigation...

If you are going to dilute the alkaline you are taking material from your matrix. The matrix density decreases. I know concrete wall surfaces from drinking water facilities which could be removed up to 2 cm deep by using my finger.

pH is the important parameter in studying the properties of concrete. Low and high pH both creates problem in concrete in terms of corrosion and spalling. For this ASTM has recommended 0.6% alkalinity of cement or mineral admixtures (if alkaline) for use in concrete. 

High pH of the cement is due to presence of portlandite (CaOH₂) and after adding in concrete mix the pH of the concrete decreases after setting due to utilization of portlandite in the formation of hydration products like CSH, ettringite and others. This formation of CSH and other hydration products dense the matrix and reduces the permeability of the chloride or reduces the carbonation resulted in reduced corrosion.

Several studies have been reported on the use of mineral admixtures (like SCMs etc.) that reduces the high alkalinity of cement-concrete, fills the voids and hardens the matrix leads to reduction in porosity and/or permeability. The addition of SCMs decreased the pH or alkalinity up to certain limit that is necessary for hydration reaction and additional supply of Al, Si from the admixtures enhance the formation of additional hydration products.

The decrease in pH of the concrete mix before casting and molding affects the early hydration and strength but improves the later age concrete properties

In concrete media, the pH lowering from 12.5 cause the decrease of pore liquids acidity and thus decrease of instability of hydration product. This phenomena in first hour, lead to accelerate of ettringite large crystals and low strength even in early age and durability problems in long term. Decrease of pH also cause to decrease the amount of calcium carbonate and decrease the reaction rate of produce of CSH as hardening element in concrete. It is clear if the pH decrease, be larger, the destructive effect is more and lead to formation of very porous concrete with low strength

Future trends:

  The next generation of biomaterials looks toward the development and clinical use of smart materials which will allow better control over processes occurring post-implantation. The host site may itself control the material through local changes in pH, ionic strength or other specific molecular inter-actions. The use of supramolecular assemblies of responsive polymers (e.g., shell or core cross-linking structures) can be utilized to achieve long-term structural stability. The detection motifs exhibiting more sensitive and selective responses should be further developed and incorporated into responsive polymer matrices, aimed at sensing and discriminating subtle changes in the gradients and concentrations of pH, temperature, glucose, bioactive small molecules and other biorelevant macromolecular species.