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Cationic surfactants for fiber and fabric processing

Cationic surfactants are surface-active compounds with at least one hydrophobic alkyl chain and a hydrophilic group carrying a positive charge. Of the cationic surfactants, quaternary ammonium compounds are widely used in textile processing. Quaternary ammonium compounds are characterized by a positively charged quaternary nitrogen atom. Commercial raw materials are normally derived from natural oils which implies that homologous mixtures of surfactants with different alkyl chain lengths are used in the products. Applications of cationic surfactants include disinfectants and biocides, emulsifiers and processing additives. By volume, cationic surfactants in household products are the alkyl ester ammonium salts that are used in fabric softeners.

Surfactants in this group have long names as per abbreviations used.

ATMAC: Alkyltrimethylammonium chloride

ATMAB: Alkyltrimethylammonium bromide

DADMAC: Dialkyldimethylammonium chloride

DADMAMS: Dialkyldimethylammonium methyl sulfate

DSDMAC: Distearyldimethylammonium chloride

DTDMAC: Ditallowdimethylammonium chloride

ADMBAB: Alkyldimethylbenzylammonium bromide

ADMBAC: Alkyldimethylbenzylammonium chloride

EQ: Esterquats

DEQ: Diesterquats

DEEDMAC: Diethyl ester dimethylammonium chloride

Cationic surfactants account for only 5-6% of the total surfactant production. However, they are extremely useful for some specific uses, because of their outstanding properties. They are not good detergents nor foaming agents, and cannot be mixed in formulations which contain anionic surfactants, with the exception of non quaternary nitrogenated compounds, or when a cationic complex synergetic action is sought. Nevertheless, they exhibit two very important features. Their positive charge allows them to adsorb on negatively charged substrates, as most solid surfaces are at neutral pH. This capacity confers to them an antistatic behavior and a softening action for fabric.

Most used cationic surfactants are fatty amines, their salts and quaternary derivatives. The amine is labeled as primary, secondary or tertiary respectively when the nitrogen is linked with 1, 2 our 3 alkyl groups. If the nitrogen possesses 4 bonds with C atoms, the compound is called a quaternary ammonium. In an ammonium structure, the nitrogen atom gives two electrons to ensure the fourth bond, and thus remains with a positive charge. Alkyl-ammonium ions are produced in acid medium by the reaction of a proton with the amine. The resulting salt (in general chloride or bromide) is soluble in water.

The attainment of an amine or alkyl ammonium surfactant requires a chain of chemical reactions which are more or less selective and not necessarily complete. Consequently, only a small part of the original raw material ends up as the desired product. This is why cationic surfactants are in general more expensive than anionic such as sulfonates or sulfates. Hence, cationic surfactants are used only in applications in which they cannot be substituted by other surfactants, i.e. those which require a positive charge or a bactericide action. In this application they compete with dye and thus slow down their adsorption and help attaining a uniform coloration. Their action as corrosion inhibitor in acid environment is similar, but in this case they compete with H+ ions. Collectors for mineral floatation are often ammonium salts or quarts. Asphalts emulsions for roadway pavement and protective coatings and paints are often stabilized by fatty amine salt salts (at acid pH) or quats (at neutral pH). Benzalkonium and alkyltrimethyl ammonium chloride or bromide is used as antiseptic agents, disinfectants and sterilizing agents. They are also incorporated as additive in nonionic detergents formulation for corrosion inhibition purposes, and (in very small quantity) in anionic powdered formulas to synergize detergency.

Properties of fatty amines and their surfactant function

Applications

A: Leveling agents

Cationic leveling agents are necessary when dyeing acrylic fibers with cationic dyes to obtain level dyeing, especially pale and medium shades. Long chain quaternary ammonium compounds provide safe control of the pick speed. A distinction has to be made between retarders and migrating agents, depending of the predominance of the retarding or migratio promotion action of the cationic leveling agent. Cationic leveling agents have affinity for acrylic fibers and the affinity of retarders is greater than that of migrating agents.

K value: K value indicates the behavior of the cationic dye in combination with others. Dyes with lower K value exhaust more rapidly in combination than such with higher K values. As far as possible, dyes with similar values {0.5+ -} should be used for combination shades. This provides uniform absorption of the dye combination resulting in good levelness.

Cationic retarders are low substantivity chemicals which can be applied prior to dyeing. These are gradually ejected by incoming molecules. These are unable to diffuse in to the fiber and they act as potential barrier to dye cation trying to enter the fibers. Many a times, they are of similar substantivity and molecular size to the basic dye molecule and any retarding effect is due to direct competition with the dye.

Some amphoteric surfactants are insensitive to pH, whereas others are cationic at low pH and anionic at high pH, with an amphoteric behavior at intermediate pH. Amphoteric surfactants are generally quite expensive, and consequently, their use is limited to very special applications such as cosmetics where their high biological compatibility and low toxicity is of primary importance.

B: Spin bath additives

The viscose process has been used for over hundred years to produce man-made fibers from cellulose. The process is very versatile; modifications at different stages in the process can be done to obtain different fiber properties. The regeneration mechanism determines the fiber tensile properties and depends on many process variables. Fiber tenacity should be proportional to the stretch applied to the filaments during the spinning stage i.e. more stretch result in more orientation, which in turn leads to higher strength. However, the higher the tenacity is, the lower the extensibility of the fiber, which means that the fiber strength and the extensibility must be weighed against each other in order to obtain desirable fiber properties. Dissolving cellulose (i.e. a pulp with low lignin and hemicelluloses content) is mainly used for production of viscose and spinning of viscose staple fibers. Fatty amine ethoxylates such as cocoamine – stearyl amine – oleyl amine – Tallow amine are used as additives to the viscose or to the spin bath to delay the regeneration and for caustic soda stability.

C: Fabric softeners

Over the three decades, extensive work has been done covering variations and combinations of new and old molecules as well as formulations of softener with different cosurfactants and functional additives. Ester quats, which are quaternary ammonium compounds having two long fatty acid chains represent a new generation of fabric softeners replacing traditional softeners. The inclusion of ester linkages significantly improves the biodegradation properties making it a unique class. This new generation fabric softening agents combines a good environmental benefit with structural features required for an effective fabric softener.

Ester quats based on triethanolamine serve as good fabric softeners. Esterfication of TEA with two equivalents of acids gives a thermodynamically controlled distribution of mono, di, and tri esters which are then quaternized with di methyl sulphate. Recent advances in this technology have resulted in significantly improved cost/performance which is capable of exceeding DETA based softeners. Cationic surfactants represent a broad family of commercial compounds, the two common types being long chain fatty amines and quaternary amine salts. Of the two, the quaternary amine surfactants cationic surfactants are very important as fabric softeners because of its hydrophilic and bacteriostatic properties.

They absorb on the surface of the fibers with their hydrophobic groups oriented away from the surface of fibers.

D: Antimicrobial agents

Quaternary ammonium compounds cationic surface active agents (cationic surfactants), including and particularly quaternary ammonium salts (QASs), are important biocides known to be effective antiseptic and disinfectant agents. As antimicrobial agents for textiles, monoammonium and “gemini'' or “dimeric'' ammonium surfactants with an alkyl, alkylaryl and perfluorinated hydrocarbon group are used. These are active against a broad spectrum of microorganisms such as Gram-positive and Gram-negative bacteria, fungi and certain types of viruses. The antimicrobial activity of QASs depends on the length of the alkyl chain and the number of cationic ammonium groups in the molecule. The antimicrobial function arises from attractive interactions between the cationic ammonium group of the QAS and the negatively charged cell membrane of the microbe; these interactions consequently result in the formation of a surfactant–microbe complex. This in turn causes the interruption of all essential functions of the cell membrane and thus the interruption of protein activity. QASs also affect bacterial DNA, causing a loss of multiplication ability. If the long hydrocarbon chain is bonded to the cationic ammonium in the structure of the QAS, two types of interactions between the agent and the microorganism can occur: a polar interaction with the cationic nitrogen of the ammonium group and a non-polar interaction with the hydrophobic chain. Penetration of the hydrophobic group into the microorganism consequently occurs, enabling the alkylammonium group to physically interrupt all key cell functions.

Occurrence in environment

Because of its positive charge, cationic surfactants sorb strongly to the negatively charged surfaces of sludge, soil and sediments. The widespread use and sorption behaviour of cationic surfactants implies that these substances are expected to be present in many environmental compartments. Particular attention is paid to the presence of distearyldimethylammoniumchloride in surface waters. On the basis of an environmental risk evaluation of DSDMAM, textile processing has switched over to readily biodegradable alternatives.

Author Details

C. N. Sivaramakrishnan

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