Surfactant Basics 3 (Penetrants, Wetting Agents, Fabric Additives)

First, an interface is a boundary surface that exists between two substances with different properties, and interfaces exist between liquids and solids, liquids and liquids, and liquids and gases.

Surfactants enhance performance by performing functions such as washing, emulsifying, dispersing, wetting, and penetrating at this interface.

Interface = a boundary surface that exists between two substances with different properties
Liquid and solid: cup and coffee, machine and lubricant 
Liquid and liquid: water and oil 
Liquid and gas: seawater and air, soap bubbles

Examples of roles of surfactants
Cleaning ・・・ Removing dirt 
Emulsification ・・・ Dispersion ・・ Making unmixable things easier to mix
Wetting / Penetration ・・・ Makes wetting and soaking easier

-Surfactants have different structures in their molecules with different properties: lipophilic groups (oil-fitting parts) and hydrophilic groups (water-fitting parts).

-Surfactants are broadly classified into four types according to the structure of the hydrophilic group: nonionic, anionic, cationic, and amphoteric (having both anionic and cationic groups).

It consists of seven short movies for each function. 
0:00　Introduction of surfactants functions　
0:16　Part1　Washability
1:00　Part2　Permeability
2:10　Part3　Dispersion
2:55　Part4　Foaming properties
3:25　Part5　Defoaming properties
3:39　Part6　Smoothness
4:20　Part7　Antibacterial properties

Wetting agents and penetrants are used to wet or soak up water. Wetting and penetrating are similar, and wettability is important. Surfactants with almost the same composition are used for wetting and penetrating agents.

Wetting: Replacing the interface between a solid and a gas with the interface between a solid and a liquid (water) → Wettability

Penetration: Liquid (water) enters the gap between solids → soaking in

Usually, wetting can be classified into three categories.

The shape of a water droplet is used to quantitatively express wettability. The shape of a water droplet can be expressed by the contact angle θ as shown in the figure below.

 The closer the contact angle is to 0 degrees, the easier it is to wet, and the closer it is to 180 degrees, the harder it is to wet

Fig. State of wetting of solid surface

When there is no surfactant
A droplet of water tries to become spherical with the smallest surface area due to the surface tension of the water (to minimize the surface energy of the droplet).
Also, the glass surface is stabilized by lowering its energy by adsorbing gas molecules on the surface.
⇒When a water droplet comes in contact with glass, the droplet becomes spherical on the glass (i.e., not wet)

When there is a surfactant
Surfactant collects at the contact point where solid and liquid repel each other, and water droplets spread easily.
⇒The contact angle of water droplets on the glass surface becomes smaller.

If γs > γℓ⋅cosθ + γsℓ: water droplets tend to spread out
If γs < γℓ⋅cosθ + γsℓ: water droplets try to approach a spherical shape
If γs = γℓ⋅cosθ + γsℓ: the water droplet is stationary and θ at this time is the contact angle.

Further transforming this equation, the contact angle can be expressed in terms of surface tension and interfacial tension as follows.

Mechanism of action of wetting and penetrating agents

Behavior of surfactant dissolved in a drop of water
Surface tension of solid γs: Constant for different types of solids (does not change with surfactant)
Surface tension of water γℓ: Decreases with the addition of surfactant
Surface tension between water and solid γsℓ: Decreases with addition of surfactant





When a surfactant is added, the right-hand side of the above equation becomes a large number, and the contact angle θ on the left-hand side must become smaller to balance it. Thus, the surfactant's action of lowering the interfacial tension decreases the contact angle θ, which in turn increases the wetting and penetrating actions. This is the mechanism of action of wetting and penetrating agents.

In terms of wetting and penetration effects, surfactants with a hydrophilic group in the middle of the lipophilic group have the greatest penetration power. Surfactants with a linear lipophilic group and a hydrophilic group at the end have the least penetrating power.

Fig. Comparison of Penetration Power of Higher Alcohol EO Adducts

This rule of thumb is well known to be generally valid not only for nonionic surfactants but also for anionic surfactants. However, it is based on the precondition that the surfactant is in a suitable range for wetting and penetration.

Properties of treatment solutions containing wetting and penetrating agents

In the manufacturing process leading up to the production of cloth, the conditions under which the fibers are treated may be acidic or alkaline, or may contain large amounts of inorganic salts. These factors can also greatly affect the penetrating power of penetrants.

Examples of factors that reduce penetrating power
-In general, non-ionic penetrants often lose their penetrating power in alkaline conditions.
-Anionic penetrants often lose their penetrating power in acidic conditions.
-Penetrants with ester bonds in their molecules may decompose at high temperatures in strongly acidic or strongly alkaline environments, resulting in reduced penetrating power.
-Sulfate-type anionic penetrant ester salt penetrant may decompose in acidic solutions, resulting in a decrease in penetrating power during use.

Wetting and penetrating agents in the synthetic fiber manufacturing process

Synthetic Fiber Manufacturing Process Flow
Polymer production ⇒ Spinning ⇒ Drawing ⇒ Post-treatment process

Role of fiber oil
Fiber oil mainly refers to a spinning oil agent that facilitates yarn pulling in the prevention process. Penetrating and wetting agents are added to help the agent work effectively.

The following is an example of a fiber oil treatment process for polypropylene fiber, which is the most difficult to wet with water.
When treating polypropylene fibers with oil, the oil is emulsified or dispersed in water.

Polypropylene fibers are not easily wetted by water, which can cause problems such as "oil does not adhere easily" and "oil does not penetrate into the fiber bundle.To solve this problem, non-ionic or anionic wetting/penetrating agents are usually added to the oil to provide wetting/penetrating action.

Higher alcohol EO adducts and sulfosuccinic acid type surfactants, which have a low contact angle and excellent penetrating power, are added to make it easier for the oil to penetrate into the polypropylene fiber bundles.

Wetting and penetrating agents for refining processes

Before dyeing, natural and synthetic fibers alike are coated with oil and glue during the spinning and weaving processes, as well as with dust and machine oil. These deposits can cause a variety of problems during textile processing.（For example, they prevent dye penetration and cause uneven dyeing.)

In order to prevent these problems from occurring, the dyeing process involves a process called refining, which completely removes these adhering substances before dyeing.

Examples of penetrants used in cotton refining
Cotton is very resistant to alkali, so it is often refined using a combination of strong alkali such as sodium hydroxide or sodium carbonate and anionic or nonionic penetrants. Cotton is also resistant to heat, and is refined at high temperatures to achieve the refining effect in a short time.

Wetting and penetrating agent for gluing process

In the weaving process, glue is applied to the warp threads.

Starch and gelatin used to be used as glue, but sodium alginate, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and other synthetic glues are now being used.

The greater the amount of glue adhered to the warp, the more efficient the gluing process. Penetrants are used to increase the penetration of the glue, allowing more glue to adhere to the fibers in a shorter period of time.

Wetting and penetrating agent for bleaching process

Penetratants are used to enhance the effectiveness of bleaching agents in whitening cloth.

As bleaching methods become more mechanized, becoming a continuous process and faster in speed, it is necessary to ensure uniform penetration of the bleaching solution in a short time.

Therefore, the superiority of the penetrant becomes the deciding factor. In addition to this, low foaming is also important in continuous production, in addition to permeability.

Bleaching of cotton and rayon fabrics is often done with sodium chlorite in a weak acidic solution.
Nonionic penetrants are suitable or blends with anionic activators are used.

Wetting and penetrating agents in the dyeing process

In the case of natural fibers such as cotton and hemp, and recycled fibers such as viscose rayon, ionic groups do not exist on the surface of these fibers as adsorption sites, so direct dyes, naphthol dyes, sulfide dyes, building dyes, and reactive dyes are used as dyes to be used.

The first stage of dyeing is the penetration of dye solution into fibers, and how well it penetrates at this stage greatly affects the uniformity of dyeing. For this reason, emphasis is placed on the penetration function of the dyeing auxiliaries used.

The following are examples of penetrating agents used in dyeing.
Sodium dioctyl sulfosuccinate
Sodium alkylbenzenesulfonate
Sodium alkyl sulfates
Higher alcohol EO adduct

Wetting and penetrating agent for mercerization process

Mercerization
When cotton fibers are immersed in a 20-30% dark sodium hydroxide solution near room temperature, the cotton fibers swell and thicken due to the alkali and at the same time begin to shrink rapidly in length. At this time, if the fibers are pulled to prevent shrinkage, they become lustrous and easily dyeable cotton fibers.

Cotton golf shirts and blouses with a silk-like luster are made using this method.

Uniform and rapid penetration of the alkali solution into the cotton fabric is also important in the mercerization process, but a special penetrant is required because ordinary penetrants do not dissolve in such a thick alkali solution at all.

Alkali Penetrant
As shown in the table below, surfactants with small hydrophobic groups are suitable as alkali penetrating agents. Small hydrophobic groups are too soluble in ordinary water to show much surfactant activity, but in concentrated alkaline solutions, their solubility decreases to an appropriate level, resulting in excellent penetrating power.

Nowadays, anionic penetrants such as sulfonates and sulfates with lower alkyl groups of about 5 to 10 carbons are used in combination with solvents such as butyl cellosolve.

R(C5-C10)-SO3Na
R(C5-C10)-OSO3Na
C4H9OCH2CH2OH (butyl cellosolve)

Example of washing process of clothes
1) Process of penetration of detergent solution into the crevices of fibers
2a) The process of separating stains from the surface of fibers
2b) The process of dispersing and protecting the stain
3) The process of removing stain from the cleaning system

Of these three processes, process (1) is to wet the fibers and allow the detergent solution to penetrate into the gaps between the fibers and make contact with the stains in order to remove the stains, which is almost unnecessary when cleaning smooth solid surfaces such as tableware and metals.

Next, the penetrating cleaning solution pulls the stain away from the fibers in the process of 2a).
In process (2b), the detached dirt particles are dispersed into smaller particles, and the emulsifying, dispersing, and anti-re-agglomeration action of the detergent is responsible for protecting the dirt particles from re-agglomeration once they are dispersed.

γow：Interfacial tension between cleaning solution and oil stain,
γws：Interfacial tension between cleaning solution and fiber,
γos：Interfacial tension between fiber and oil stain,
θ：Contact angle

Fig. Concept of washing mechanism

For more information on cleaning, please also see the following pages.
Surfactants Basics 1 (Detergent)

Although it is possible to dissolve and use surfactants such as penetrating agents and detergents when using an oxidant, the problem of oxidative degradation of surfactants during storage arises when an oxidant and surfactant are used together. Therefore, it is necessary to select a surfactant that does not have easily oxidizable bonds.

When using hydrogen peroxide
Nonionic surfactants such as higher alcohol EO adducts can be stably blended.

When using sodium hypochlorite
Alkalinity is required to keep sodium hypochlorite stable.
⇒ Must be stable in oxidative stability and alkalinity.
Since higher alcohol EO adducts are oxidatively degraded, sulfonic acid type anionic surfactants are usually used.

Surfactants used as Spreading Agents for Agriculture
Among the above, surfactants are mainly used as wetting agents in emulsions and hydrates, as shown in the table below. Surfactants derived from silicones have superior surface tension lowering ability compared to hydrocarbon-based surfactants.

Usually, the use of 0.01~0.1% of wetting agent as a spreading agent for agriculture, above the critical micelle concentration (c.m.c.), is sufficient in cases such as tridecyl alcohol EO adduct, but in many cases, plants are targeted, and sufficient attention should be paid to chemical damage to plants.

Dry spot is the term for turf dieback that occurs on golf courses during hot weather. Anaerobic bacteria present in the roots of the turf produce a waxy substance through metabolism, which creates an impermeable layer near the soil surface, making it difficult for rainwater or sprinkled water to penetrate into the soil, which is then heated by the sun, resulting in a waterlogged condition.

The surfactant used in this process acts as an infiltrating agent that facilitates water penetration and emulsifies a waxy substance to prevent the formation of an impermeable layer. Therefore, as with agricultural spreading agents, the required functions of dry spot inhibitors are excellent surface lowering ability and low chemical damage.

As with spreading agents, the amount of surfactant used as a dry spot inhibitor should be sufficient as long as the concentration is above the critical micelle concentration, and the amount and frequency of application should also be carefully considered to prevent chemical damage.

Aqueous emulsion paints consist mainly of resin emulsions and inorganic pigments, to which are added dispersants and other additives.

Inorganic pigments are made up of agglomerates (secondary particles) consisting of several to several dozen fine particles (primary particles) ranging from 0.01 to several μm in diameter.

The dispersion solution in which the pigments have been dispersed shows vivid colors because the pigments are dispersed as fine particles. The paint is then made by adding a resin emulsion and a small amount of additives.

If no dispersant is added at all...?
(1) Dispersion efficiency by machine is reduced (dispersion takes a long time or requires strong physical dispersion force). If the dispersant alone is difficult to wet, a penetrant may be added separately.

(2) The color of the paint becomes dull due to reagglomeration of dispersed fine particles. Also, since agglomerated particles tend to settle, problems such as separation of the paint are more likely to occur.

Aqueous dispersants
Higher alcohol EO adduct
Sorbitan fatty acid esters
Sodium dioctyl sulfosuccinate
Formalin condensate of sodium naphthalene sulfonate
Sodium polystyrene sulfonate
Sodium polyacrylate
Carboxymethyl cellulose

Nonaqueous dispersants
Polyacrylic acid partial alkyl esters
Polyalkylene polyamine

In general, linear alcohol surfactants have excellent emulsifying and dispersing power, but poor penetrating power. 

Our proprietary ethylene oxide addition technology enables us to narrow the molar distribution of addition and synthesize surfactants with the targeted balance of hydrophilic and lipophilic properties, thereby increasing the penetration power of linear alcohol type surfactants.

For the Narrowacty ID series product introduction page (link to corporate website) 
 NAROACTY ID-40
 NAROACTY ID-60
 NAROACTY ID-70

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