Figure 1

INTRODUCTION
Surfactants provide improved wetting and penetration of penetrants into tight discontinuities. They also provide better removal of the excess penetrant from specimen surfaces.

A surfactant molecule generally has two parts: a nonpolar (designated as hydrophobic, lipophilic or oleophilic) hydrocarbon chain and a polar (hydrophilic, lipophobic or oleophobic) group. These two structures in the same molecule form a special amphiphilic molecular structure. It possesses special surface activity and is known as a surfactant. A surfactant used in a penetrant testing agent can increase wetting, penetration into a discontinuity and the solubilizing or emulsification for washing functions.

Inquiring into surfactant science, research, molecular structure and physical chemical properties provides an insight into the significance and action of penetrant testing agents' performance.

 

SURFACTANT CHEMICAL STRUCTURE/HYDROPHILIC LIPOPHILIC BALANCE
Surfactants have an amphiphilic molecular structure. That is, a structure that is polar (hydrophilic or water loving) at one end and nonpolar (lipophilic/hydrophobic or water hating) at the other. Hydrophilic groups may be cationic (organic amines — especially with three hydrocarbon chains attached to the nitrogen atom), anionic (fatty acids or sulfates with hydrocarbon chains) or nonionic (organic compounds with oxygen containing groups such as alcohols, esters and ethers). Hydrophobic or lipophilic (oleophilic) groups may be large, straight or branched chain hydrocarbons, cyclic hydrocarbons, aromatic hydrocarbons and/or combinations of them (Figure 1).

 

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Surfactants provide improved wetting and penetration of penetrants into tight discontinuities.

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Such a molecular structure poses a double property: one part can dissolve in water and the other can dissolve in oil like materials and tries to escape from water. When put into water, the hydrophilic groups dissolve in the water and the oleophilic groups close and associate together to form a hydrol or micelle structure holding some oil (micelles may be microscopic and submicroscopic spherical globules of one liquid in another; some may also be colloids). These micelles look like a tiny ball with the oil loving ends turned inside the ball and the water loving ends covering the outside. An oleophilic micelle particle can increase the retention of water in organic compounds.

These compounds can also reduce the surface tension of the liquids into which they are placed. Materials that reduce the surface tension of liquids are known as surfactants. These compounds may improve the wetting ability or the water washability of penetrants when added to them.

When a surfactant is put into an oil/water interface, it can reduce interface tension and form an emulsifying colloid or a single liquid phase, rather than two liquid phases. When surfactants' hydrophilic property is stronger, they can form water occluded oil colloids. When the oleophilic property is stronger, they can form oil occluded water colloids.

The surfactant molecular hydrophilic or oleophilic strength may be expressed as the hydrophile lipophile balance (HLB) value. When the HLB is less than 10, the oleophilic property is stronger. When the HLB is greater than 10, the hydrophilic property is stronger. An HLB of 3 to 6 can form a water/oil colloid. An HLB of 8 to 18 can form an oil/water colloid. An HLB of 13 to 15 increases the washability function and an HLB of 15 to 18 possesses a solubilizing function for a penetrant.

 

DETERMINATION OF THE HYDROPHILE LIPOPHILE BALANCE VALUE
The polyhydric alcohol fatty acid ester and fatty acid polyoxyethylene ester evaluation formula to determine the HLB value is,

(1)

where

S = the saponification number

A = the acid number of the fatty acid.

When measurement of the saponification number is difficult, another formula can be used

(2)

where

E = the weight percent of the hydrophilic content of the molecule

P = polyelement acid weight percent.

When the hydrophilic group is in the middle of the surfactant molecular configuration, the wetting function is better than when it is at the end. When the hydrophilic group is at the end of the surfactant molecular configuration, the washing activity is better than when it is in the middle.

A comparison of some other surfactant properties is useful. If the surfactant structure remains chemically similar and has the same molecular size, the wetting and penetrant function of the surfactant is better with branched than nonbranched oleophilic group configurations. As the surfactant molecular size becomes smaller, the wetting and penetrant function is better. As the oleophilic group's branched configuration becomes larger, the washing and dispersion function of the surfactant is better.

General performance of a surfactant relative to molecular structure is given in the following series: oleophilic group strength decreasing as aliphatic hydrocarbons > cyclanes > alicylic arenes > arenes; hydrophilic group strength decreasing as sodium sulfates > potassium salt of organic acids > sodium salt of organic acids > -N (tertiary amine).

 

EFFECTS OF A SURFACTANT ON PENETRANT TESTING FORMULATION

In a Penetrant Testing Formulation
In an alcohol/oil mix penetrant agent, HLB 4 (<<14) surfactants used as the emulsifier can have good penetration capabilities. In a water washed penetrant, use of HLB 14-16 surfactant in the penetrant permits water removal of the excess penetrant and water cleanup. Water based penetrants generally use an anion type surfactant that can reduce water surface tension and increase the wetting of the surface by the penetrant.

In a Cleaning Agent
In a cleaning solvent, addition of some HLB 14-16 surfactants can improve the cleaning properties. Generally, use an HLB 14-16 surfactant added into the solvent. It can emulsify the penetrant, which can then be cleaned with water.

In the Development Agent
Use an ionic type surfactant in the development solvent to improve the development properties over surfactant free formulations.

 

References
Othmer, D.F. and Raymond E. Kirk, eds., Kirk-Othmer Encyclopedia of Chemical Technology, Fourth edition, New York, Wiley, 2001.

Zhaukuoshu, Surfactant Physical Chemistry, Peking, China, University Press, 1984.

* Xiamen Chemical Machinery Plant, Xinglin Xiamen Fujian, Xiamen 361022, China.

 

Copyright © 2001 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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