In the early days, polymers have found very little value in detergent and cleaning formulations. Phosphate compounds such as sodium triphosphate (STP) (Figure 1) and sodium/potassium phosphate were used primarily in detergent formulations in which they comprise them up to 50 % by weight [1, 2, 3, 4, 5]. STP provides significant contributions to laundering by performing several crucial functions . They serve as builders by forming complexes with polyvalent cations such as Ca2+ and Mg2+ ions and as a result water hardness is reduced, thereby boosting the performance of the surfactants . STP also aids in removing, dispersing and suspending the dirt released and ensuring a good powder structure. However, over the years, phosphates were found to stimulate the rapid growth of algae and plants in lakes and other bodies of water, which caused the deterioration of water quality and the recreational value of the lakes [2, 4]. Due to environmental concerns and government regulations and restrictions regarding the use of phosphates, formulators pursued to reinvent the detergent compositions . Decades of efforts and millions of dollars were spent to find safe, high-performing and economical substitutes for phosphates. Some of the potential alternatives  that came out of the market include systems that are based on (1) nitrilotriacetate (NTA) [2, 6], (2) sodium citrates [5, 7] and (3) zeolites [8, 9, 10] (Figure 1).
NTA exhibits good cleaning and chelating properties but was later on suspended by National Institute of Environmental Health and Sciences (NIEHS) because it poses detrimental health threats to the development of embryo or what is known as teratogenic effect especially when combined with heavy metals such as cadmium and mercury . Sodium citrate, on the other hand, is environmentally safe and is compatible with cleaning formulations because of its solubility properties. However, in terms of cost and effectiveness, it is generally inferior compared to STP [5, 7].
Zeolite is a synthetic sodium aluminum silicate with the general formula of Nax[(AlO2)x(SiO2)x].yH2O whose primary function is similar to STP, which is to sequester Ca2+ and Mg2+ ions in wash solutions . In cases where water contains elevated amount of these ions, the zeolite system requires a co-builder, which is water soluble . Polycarboxylates have proven to be the most effective additives for this system. Their function, however, is not via complexation with the hard ions in water, but they aid in the dispersion of calcium salts such as calcium carbonate or phosphate and the soil particulates released during washing. Because these polymeric materials are available at reasonable costs and possess unique and varied multifunctionality, formulators and polymer chemists were prompted to investigate beyond polycarboxylates.
The applications of polymers in household cleaning compositions, both natural and synthetic, have grown tremendously [11, 12]. They have been an important part of detergent formulations for many years now, and 90 % of the polymers used belong to a class of polycarboxylates. The major polycarboxylates used in cleaning products are homopolymers derived from acrylic acid (PAA) and copolymer of acrylic/maleic acid (PAA/MA) (Figure 2). They are generally used as sodium salts in phosphate-free detergents to prevent soil redeposition and encrustation. Specialty polymers have also been developed recently . They provide specific and unique benefits to detergent formulations such as enhanced soil release, dye transfer inhibition, modification of rheological properties and many more.
Most synthetic polymers are toxicologically safe but not biodegradable. They are eliminated from wastewater by precipitation or adsorption to the sludge in sewage treatment facilities . Because natural polymers are renewable and biodegradable to a great extent, they have found broad appeal in detergent industry [14, 15]. Cellulose derivatives such as carboxymethyl cellulose (CMC), xanthan gums and carrageenans were among the first natural polymers to be utilized in detergent formulations. Depending on the desired applications, their physical and chemical properties were readily tailored and fine-tuned by chemical modifications.
In this chapter, we provide an overview of the applications of polymers on fabric and home care formulations. For general households, fabric and home care products include laundry and dishwashing detergents, fabric softeners and hard surface cleaning products such as floor care and bathroom and glass cleaners. Section 2 highlights the benefits provided by polymers when incorporated in the product formulations. Specific examples of synthetic and natural polymers that are widely and currently used in the formulations are discussed in detail in Section 3.
2 Benefits of polymers in fabric and home care formulations
A detergent, in general, is a cleaning agent composed predominantly of surfactant or a mixture of surfactants whose main task is to remove water-insoluble substances like dirt and grease from permeable surfaces (i. e. fabrics and clothes) and/or hard surfaces (i. e. metals, plastics and ceramics). Other than surfactant, a modern detergent formulation normally consists of builders and chelants, co-builders and other polymer additives used for specific washing effects such as soil release and anti-redeposition, dye transfer inhibition and rheology modification [16, 17]. The incorporation of polymers in fabric and home care formulations has provided numerous benefits in enhancing the action and efficiency of detergents, particularly the phosphate-free types. Consequently, the use of these materials, especially the recently developed specialty polymers, has gained increasing attention over the years because they provide specific benefits at a very low percent weight (1 % or less) . Examples of these type of polymers are polyesters based on terephthalic acid which serve as soil release agents, ethylene/propylene glycol-based polymers as anti-redeposition agents and poly(vinylpyrrolidone) as DTIs. Moreover, a detailed description of nathural and synthetic polymeric rheology modifiers is provided in Section 3. Some of the benefits provided by the addition of polymers in cleaning compositions are discussed below.
2.1 Soil release
The concept of soil release, which refers to the improved removal of soil from fabric during washing by increasing its permeability, was introduced many years ago. Two steps are involved in the general mechanism of soil release process: (1) enhanced penetration of water and detergent solution into the soil/garment interface with the aid of soil release agent and (2) solvation and transport of soil from the fabric to the wash solution by mechanical work [18, 19]. The removal of soil, however, is governed by several factors such as the kind of fabrics and soil particles, the performance of detergent solution, the washing conditions and the mechanical action of water.
Soil in particular includes stains that are water soluble, particulates, oil and grease. The water-soluble stains and the particulates can be easily removed by regular washing, while oil and grease are much more difficult to remove. They require soil release agents to facilitate their removal from the garment and prevent them from spreading on the surface of the fiber. These soil release agents, or what are commonly known as soil release polymers (SRPs), are specific additives that were developed to enable the exclusion of soil from the garments. Recently, a number of SRPs have been commercialized and used in detergent and fabric softener compositions [18, 20]. These polymers are typically low-molecular-weight polyesters that are derived from acrylic acid, terephthalic acid and polyalkylene glycols. Polyacrylates were among the first systems to be used as soil release agents.
The mechanism of action of SRPs is via surface modification wherein they orient themselves in the surface of the garment through adsorption, making it more hydrophilic and easily penetrated by wash liquid, therefore decreasing the affinity of the dirt to the fiber . SRPs do not only improve the soil resistance of the fibers but also enhances the wetting ability with the aqueous cleaning solution especially the polyester fibers. Thus, the effects of SRPs are strongly observed after several treatments of the fabrics.
Oily soils that get impregnated in cotton fabrics are easier to remove than those infused in fabrics woven from synthetic polyester . The cellulosic fibers of cotton fabrics are composed of hydroxyl and carboxyl groups, making them more hydrophilic compared to polyester. High hydrophilicity of cotton fabrics means easy penetrability of the adsorbed soils by water and cleaning solutions, resulting in its fast removal. Methyl and hydroxyalkyl cellulose derivatives are the most common examples of cellulosic soil release agents for cotton . Polyester-containing fabrics, on the other hand, due to its hydrophobic nature absorb oil readily and tenaciously, making the removal of these stains far more challenging. Extensive research to address this issue has yielded an effective release of soil from polyester fibers with the use of polyethylene terehpthalate (PET)/polyoxyethylene terehpthalate (POET) copolymer (Figure 3) . This type of soil release agent has high affinity for synthetic fabrics because its backbone is composed of a mixture of terephthalate residues and ethyleneoxy polymeric units, which closely resembles the materials that comprise the polyester fibers of synthetic fabric.
A dispersant or dispersing agent is used primarily to diffuse and suspend the soil particulates in the washing liquor after they are released from the surfaces or fabrics. They make the dispersion process easier by adsorbing to soil particles, which results in an increase in electrostatic and steric repulsion, thus minimizing clumping, redeposition and scaling onto the garments or hard surfaces . A dispersing additive also provides uniform mixture of particles by preventing them from aggregation and settling at the bottom of the container, thereby ensuring stable formulations and storage.
The efficiency of a polymeric material as a dispersing agent depends on their affinity to the particle surface; therefore charge, size and orientation of the polymer molecule in solution are important factors. Most cleaning products rely on low-molecular-weight polymeric carboxylates as dispersant, which typically take the form of PAA or PAA/MA (Figure 2) . Their multiple charges enable them to adsorb at interfaces and act as dispersing agent for soil and inorganic salts and serve as crystal growth inhibitors.
Biodegradable polymeric dispersants were also reported, which include the polyamino acid polymers such as polyaspartate and polysaccharides such as oxidized starch . Poly(vinyl alcohol), polyalkylene glycol and copolymers of alkylene oxide and vinyl acetate are known to be effective dispersing agents for surfaces that are hydrophobic such as garment made from polyester .
In a standard laundering process, the detergent removes the soil particles from the fabric and then suspends them in wash solution and prevents these particulates from depositing back onto the surface of the cleaned fabric throughout the washing and rinsing cycle . The resettling of the soil in the textile can occur in all types of fabrics whether hydrophilic or hydrophobic . Examples of fibers made from hydrophilic materials are cotton, linen and rayon, while fabrics like polyester, nylon and acrylic are hydrophobic. Because soil is normally hydrophobic, redeposition is more likely to happen when the surface of the fabric being washed is hydrophobic. However, the degree to which this process occurs is dependent not only on the kind of the fabric being laundered but also on the detergent used, the properties of the soil particles and the washing temperature. Therefore, additives are added to cleaning compositions to enhance the release of soil and/or to inhibit redeposition of the soil.
The chemical agents added to the cleaning formulations, which help minimize and avoid the redeposition process, is known as anti-redeposition polymers (ARDs) . ARDs are usually water soluble and negatively charged. They interact and stabilize the soil in the wash water, thus preventing them from depositing back to the washed garment. There are two ways that these polymeric materials work: (1) the negatively charged ARDs adsorb on the surface of the garment causing an increase in the electrostatic repulsion between the hydrophobic soil molecules and the fabric surface and (2) entrapment of soil particles into a polymer matrix .
CMC, a polymer derived from natural cellulose, is one of the first anti-redeposition agents used in detergent systems . This polymeric material is one of the best-known ARD agent applicable to cotton fabrics. The polymeric cellulose acetate, on the other hand, is effective for a broad range of natural and synthetic fabrics. Examples of synthetic polymers that are also used as ARD agents for garments made of cotton are poly(ethylene glycol) and poly(vinyl alcohol) .
2.4 Dye transfer inhibitors
Another crucial role that polymers perform in washing formulations is as dye transfer inhibitors (DTIs). One of the many challenges in the laundering operations is the possible transfer of dyes from one fabric to another in the wash solution , a process known as color transfer. This process occurs when fabrics of different dye colors are washed together. During the wash, a garment can lose some of the dye molecules and resettle on white or differently colored fabrics . The degree to which dye colors leak out of the textile depends on the type of the fabric and dye, the freshness of the garment and the washing conditions. As a result of successive washing, dyed fabrics suffer from color fading.
DTI has become a significant ingredient of detergent formulations. They were developed for two reasons: (1) for color protection and (2) for dye transfer inhibition. As color care additives, they help the fabrics keep their original colors even after multiple washes and prolong the life of the dyed garments. They also inhibit dyes from leaching out of the garment and prevent them from staining another fabric during laundry process.
The polymeric DTIs are known to efficiently reduce the staining of textiles brought by migration of dyes by binding with the free dye molecules in wash solution, which results in the formation of polymer/dye adducts that is water soluble, therefore, allowing to wash many differently colored garments at the same time. DTI also minimizes the release of the dyes from the fabrics by adsorbing onto the surface of the fibers during washing.
The most popular polymer used as DTI in laundry detergents are poly(N-vinyl-2-pyrrolidone) (PVP) [29, 30] and poly(vinylpyridine N-oxide) (PVP-NO) [31, 32] (Figure 4). Both PVP and PVP-NO have moderately high molecular weight and are highly soluble in water. PVP is reported to be particularly effective with synthetic fibers and synthetic cotton blends; however, its efficiency decreases when mixed with anionic surfactants. Poly(vinylpyridine betaine) which contains quaternary nitrogen and carboxylate salt was proven to be effective in laundry detergents containing anionic surfactants (Figure 4) .
DTIs in general are added up to 0.5 weight % to detergents that are specific for delicate and colored fabrics. Not all DTIs, however, are effective for all dyes. The overall efficiency is highly dependent on the chemical structures of both the dye and DTI.
2.5 Rheology modifier
Rheology modifier, also commonly known as viscosifier or thickener, is added to liquid compositions mainly to alter the flow behavior of the formulation, improve the aesthetics and provide specific characteristics to the product [34, 35]. The choice of the most suitable rheology modifier depends on the type of flow required based on the product thickness and pourability, the nature of the formulation as well as the performance attributes of the suspension. The rheology modifiers generally establish suitable rheological characteristics to the liquid products without imparting any undesirable properties such as unwanted phase separation and unappealing physical appearance.
Both natural and synthetic polymers have been utilized extensively as rheology modifiers for liquid detergents [35, 36]. There are two ways that these polymeric rheology modifiers can thicken a liquid composition. One is by volume exclusion (or non-associative) and the other is by association mechanism . Associative and non-associative polymers were further discussed in Section 3.1. Rheology modifiers that thicken via volume exclusion are normally water-soluble polymers that swell with water. These types of thickeners create viscosity through chain entanglement and particle flocculation. They do not associate with the surfactants or the other particulates present in the formulation. Examples of rheology modifiers under this category include cellulosic ethers such as hydroxyethyl cellulose (HEC) and alkali swellable/soluble emulsions (ASEs) such as copolymers of methacrylic acid and acrylate ester. ASE thickeners are normally acrylic-derived polymers and are pH dependent . They are insoluble at low pH, and as the pH is increased, they become soluble and the particles swell.
Associative rheology modifiers, on the other hand, are those polymers that contain hydrophobic moieties at different levels distributed randomly throughout its main backbone. These hydrophobic functionalities interact with each other via inter- or intramolecular associations, which result in an increase in hydrodynamic volume, therefore increasing the thickening ability of the polymer. The most commonly known associative thickeners are hydrophobically modified alkali soluble emulsions (HASEs) and hydrophobically modified ethoxylated urethanes (HEURs) .
Basically, HASEs are hydrophobically modified ASE. They are prepared by the introduction of hydrophobic functional groups onto the acrylate backbone of ASE. Example of HASE polymer is a terpolymer, which contains methacrylic acid, ethyl acrylate (EA) and a hydrophobic group (Figure 5) . They retain the pH-dependent behavior of ASE, but in addition to water absorption, they increase the viscosity of the solution by hydrophobic association. These hydrophobic groups can also interact with the other ingredients in solution such as the surfactants, which can contribute to the overall stability of the formulation.
HEURs are non-ionic substances that are typically of low molecular weight and consist of poly(ethylene oxide) (PEO) linked together by diisocyanates and capped at the terminal position by hydrophobic groups (Figure 6) . Unlike HASEs, HEURs do not require neutralization to activate their rheology-modifying behavior. Their thickening ability is achieved through hydrophobic interaction between the alkyl groups of the molecule and the other hydrophobic components of the formulation. HEURs have found wide applications in fabric softeners and cleaning products that are purposely formulated for kitchen and bathroom .
Some examples of polymeric rheology modifiers that are used nowadays in liquid detergents are discussed in detail in Section 3.
3 Natural and synthetic polymers
Both natural and synthetic polymers have been widely incorporated in detergent formulations. Their properties could be readily tailored depending on the needs of the household products. For synthetic polymers, the molecular weight, degree of branching and the chemical compositions in the main backbone and side chains are easily controlled and modified. Chemical modifications of natural polymers also demonstrated an improvement in the characteristics and applications relative to the unmodified materials.
3.1 Synthetic polymers
Synthetic polymers are frequently used in fabric and home care formulations for surface modifications and as rheology modifiers and DTIs. Broadly, the synthetic water-soluble polymers can be categorized into two types: (i) non-ionic polymers and (ii) charged polymers (polyelectrolytes).
(i) Non-ionic polymers: Polar, non-ionic functional groups can impart water solubility if present in sufficient numbers along to the backbone. The key technologies used in consumer products include: (1) polyacrylamide (PAM), (2) PEO, (3) poly(vinyl alcohol)-poly(vinyl acetate) copolymer, (4) poly(N-vinylpyrrolidinone), and (5) poly(hydroxyethyl acrylate). Normally, these classes of polymers are radially compatible with any kind of surfactant system (anionic, cationic or zwitterionic).
(ii) Charged polymers: Polymers possessing charges along or pendent to the molecular backbone can be classified into two categories based on the behavior in aqueous electrolyte solutions. The first category includes polyelectrolytes, which could be polyanions or polycations, and the second are polyampholytes, polymers that contain both positive and negative charges along the polymer backbone. Electrostatic effects, counterion binding, solvation and local dielectric effects are the factors that govern the phase behavior and solubility of charged polymers in surfactant system. Microstructure of polyelectrolyte can be tailored to allow conformational changes with pH, temperature or added electrolytes. The major parameters that influence the microstructure are: (1) number, type and distribution of charged monomer; (2) hydrophobic/hydrophilic balance; (3) spacing from the backbone and (4) counterion type. Polyelectrolytes are prepared by homo- or copolymerization of appropriate monomers or by modification of functional polymers.
For rheology modification, the described polymers above can be classified into two categories: (1) non-associative polymers and (2) associative polymers . Associative and non-associative polymers have been briefly discussed under rheology modifiers in Section 2.5. Here, we would only focus on the main technologies relevant for fabric and home care formulations.
3.1.1 Non-associative polymers
Non-associative polymers thicken by structuring the continuous phase and through chain entanglement. These polymers do not interact with surfactant structures, particulates or insoluble emulsion droplets. The key technology in this category are Alkali Swellable Emulsions (ASEs).
22.214.171.124 Alkali swellable emulsions
ASEs are carboxyl-containing copolymers that are prepared by the addition polymerization of ethylenically unsaturated monomers (Figure 7), and they swell or solubilize to thicken the formulations based on water .
The key advantages of ASE are: (1) they are supplied as acidic low viscous latex dispersion which makes them easy to handle on plant scale, (2) they are relatively cheap compared to other technologies, (3) they thicken the alkaline formulation almost immediately (easy dispersion) and this is important as this makes the batch time extremely small compared to most of the natural polymers such as xanthan gum, HEC, etc. and (4) they provide formulation shear-thinning behavior which is important for easy dispensing of fabric and home care products out of the bottle. As with every technology, they also have some limitations such as pH sensitivity, Ca2+, Mg2+ and Na+ ion sensitivity, and high level of ASE could give formulation a stringing behavior, which can give huge problem in dispensing the product out of the bottle and filling it during production. Some examples of typical commercially available ASE are Rheovis® AS 1130, Rheovis® AS 1125, ACUSOL™ 830 and ACUSOL™ 835.
3.1.2 Associative polymers
Associative polymers thicken by two mechanisms that can act simultaneously and synergistically, i. e. hydrodynamic volume of associative polymer and association of the extended hydrophobe groups. When these polymers are in formulation, they make a transient network made up from other hydrophobe groups of polymer and any hydrophobe present in formulation.
126.96.36.199 Hydrophobically modified polyacrylamide (HMPAM)
PAM was the first polymer used as a rheology modifier for aqueous solutions . The chemical structure of PAM is shown in Figure 8). To achieve a good thickening efficiency, the high molecular weight (Mw>1 MM g/mol) is desired in aqueous solution. The different chemical modification can be employed to improve the properties, e. g. shear resistance, salt compatibility and temperature stability. The partially hydrolyzed polyacrylamide (HPAM) is a good example where HPAM is obtained by copolymerization of sodium acrylate with acrylamide. The chemical structure of HPAM is shown in Figure 8) . The presence of electrostatic charges along the polymer backbone is responsible for prominent stretching due to monomer charge repulsion. As a consequence of high stretching of polymer, it leads to a significant increase in viscosity compared to PAM. However, the presence of charges also makes HPAM sensitive to the presence of salt in formulation.
To achieve the best thickening efficiency in formulation with surfactant molecules, HMPAM is frequently used. Figure 8(c) shows examples of HMPAM . The high thickening efficiency is a result of association with micelle structure. There are different methods of synthesizing HMPAM such as micellar, homogeneous and heterogeneous copolymerization. PAM is usually prepared via a free radical polymerization in aqueous solution. However, as is evident from the name, HMPAM cannot be synthesized using this technique as the hydrophobic monomer is not soluble in water. In order to disperse the hydrophobic monomer, it is dissolved using a co-solvent or a surfactant (micellar copolymerization) or dispersed without any additives (heterogeneous copolymerization).
The presence of hydrophobic groups suppresses the solubility of the polymer. From the above argument, it is easy to understand that an increase in fraction of hydrophobic groups above a certain percentage will lead to solubility issues, i. e. the polymer is no longer water soluble. Increasing the hydrophobicity of the hydrophobic groups will impart better thickening efficiency as a result of intermolecular association.
The hydrophobic monomer provides interesting rheological properties to the finished product. In the dilute region, intramolecular associations dominate. The hydrodynamic volume is reduced and therefore the viscosity of the subsequent product. When the polymer concentration is increased, the solution ideally moves to the semi-dilute region where intermolecular associations dominate. This leads then to network-like formations (transient network), which significantly increase the viscosity of the solution.
188.8.131.52 Hydrophobically modified alkali soluble emulsion
A typical chemical structure of HASE can contain more than three monomers, for example, in addition to acidic group (i. e. methacrylic acid) we could have a small amount of di-acid (maleic acid) and in addition to EA we could have butyl/hexyl acrylates (Figure 5) . The HASE polymers differ from ASE in that they also contain long-chain hydrophobic groups in addition to acid groups distributed throughout the polymer chain. The HASE polymers are polyelectrolytes, which have been hydrophobically modified by the introduction of few mol% of hydrophobic group. Comb-like structures where the hydrophobic groups are placed on short PEO side chains randomly distributed along the backbone of the HASE polymer chains are favored. The backbone of the HASE polymers possesses moderate high molecular weight and can reach 100,000 to 500,000 Da.
HASE polymers are synthesized using an emulsion copolymerization process. All the monomers are added to the reaction mixture along with an initiator. The hydrophobic macro-monomers act as the surfactant for the polymerization reaction. The reaction product of the emulsion polymerization is the HASE polymer. The chief microstructure parameters which plays an important role in modifying the rheology are: (1) charged to un-charged monomer ratio, (2) % of hydrophobic monomer, (3) number of EO units in hydrophobic monomer, (4) length of alkyl chain in hydrophobic monomer, (5) molecular weight and distribution of HASE polyacrylate and (6) the topology of HASE (random or copolymer).
The HASE rheology modifiers are used most frequently in fabric and home care formulations. The main reasons include: (1) the majority of fabric and home care formulations are alkaline and contain anionic surfactant and (2) they are relatively cheap and effective compared to other associative polymers. They could also be compatible with formulations, which contain cationic surfactant. To make the HASE polymer compatible with formulations containing cationic surfactant, the charge density of HASE polymer can be tuned.
The thickening efficiency of HASE in formulation depends on: (1) the intermolecular association of associative monomer, (2) the intramolecular association of associative monomer and (3) the block of EA association (Figure 9) . The thickening efficiency improves with intermolecular association. However, intramolecular association of associative monomers and block of EA decreases the thickening efficiency. Typically, it is observed that formulation with HASE polyacrylate viscosity could increase over time. The key factors responsible for increase of viscosity are the hydrolysis of EA unit which could lead to break up of EA block and the intramolecular association leading to intermolecular association. Typical commercially available HASE polyacrylates are Rheovis® AT120, Novethix™ HC200, Capigel™ 98, Novethix™ L10 and Aculyn™ 22.
184.108.40.206 Hydrophobically modified ethoxylated urethane
HEUR polymers are prepared by chain extension of PEO oligomer with a narrow-molecular-weight distribution (MWD) using diisocyanate and end-capping with hydrophobic group. HEUR polymers can be classified into two main classes: step-growth (S-G) HEUR and uni-HEUR. The difference between the two classes relates to the synthesis method and the corresponding MWD of the product. The S-G HEUR involves a procedure where PEO (of a given Mw) is reacted with a large excess of diisocyanate functional groups at both ends. This precursor is subsequently reacted with a hydrophobic group to end-cap both ends, yielding a telechelic polymer. This procedure leads to a HEUR polymer displaying a broad MWD. The procedure for uni-HEUR involves the direct addition of a mono-isocyanate containing a hydrophobic group to the PEO. To this, a hydrophobic group is added. The resulting polymer has a relatively narrow MWD, which is related to that of the parent PEO. The chemical structure of a hydrophobically modified PEO is given in Figure 10. Examples of commercially available HEUR are ACRYSOL™ HEUR and Rheosolve 450.
220.127.116.11 Hydrophobically modified cellulose derivative
This is the class of semi-synthetic polymers. To synthesize these polymers, the natural polymer is reacted with different kinds of hydrophobic monomer in the presence of initiator. To improve the thickening efficiency, stability and compatibility with different formulations, the natural polymers can be chemically modified, e. g. hydrophobically modified hydroxyethyl cellulose (HM-HEC) , hydrophobically modified hydroxypropyl cellulose (HM-HPC) , hydrophobically modified ethyl hydroxyethyl cellulose (HM-EHEC)  and hydrophobically modified alginate (HM-ALGINATE) . The chemical structure of above examples is shown in Figure 11.
The presence of hydrophobic groups on the polymer backbone of the polysaccharides will cause the formation of associations just like HMPAM, HEUR and HASE. The network structure is enhanced due to these groups and hydrophobic associations arise at a very low polymer concentration [42, 43, 44, 45]. The improved rheological response is a result of a formation of strong network compared to unmodified natural polymers. The type of hydrophobic group also affects the strength of the network. Increasing the hydrophobicity will increase the strength. It is also important how the hydrophobic monomers are distributed along the backbone. Inhomogeneous distribution of hydrophobic monomer could lead to a phase separation. The hydrophobic modification decreases the solubility of polymer in aqueous solution.
This class of polymer also imparts pseudoplasticity after certain critical concentration of polymers. In addition, thixotropic behavior is also observed similar to HMPAM. This class of polymer shows an opposite behavior in the presence of salt compared to HMPAM, HEUR and HASE. Indeed, the presence of salt will weaken the electrostatic repulsion. However, this leads to the enhancement of the hydrophobic associations and thus a strengthening of the polymer network. In addition to the presence of salt, the type of salt is also important in defining the thickening efficiency of polymer. The presence of salts that are able to induce a structuring of the water molecules will lead to an enhancement of the viscosity due to the increase in the number of hydrophobic association.
The interaction between hydrophobically modified polymers and a surfactant leads to different behavior depending on the concentration of the surfactant and the type of surfactant (Figure 12) . The surfactant molecule will interact with hydrophobic groups forming the mixed micelles. The formation of these mixed micelles will increase the number of hydrophobic microdomains and will enhance interpolymer associations and thus strengthen the polymer network. Increasing the surfactant concentration further will lead to electrostatic repulsions between the mixed micelles and subsequently a drop in the solution viscosity of the formulation. Increasing the surfactant aggregation number in the mixed micelles can counteract the reduction in the solution viscosity at an elevated surfactant concentration. This can be achieved by either adding screening electrolytes or oppositely charged surfactants. A polymer–surfactant formulation employing a surfactant with a lower critical micelle concentration (cmc) will reach a maximum solution viscosity at lower surfactant concentration compared to a surfactant with higher cmc.
3.2 Natural polymers
Natural polymers are renewable materials that are available in huge quantities. They are economically and environmentally attractive because of their low cost, biodegradability and low toxicity. Similar to synthetic polymers, chemical modifications can be readily achieved on natural polymers which can lead to materials with highly interesting properties. The most common and currently used natural polymers in detergent formulations include cellulose and its derivatives, gums such as xanthan and guar gums and carrageenan.
Cellulose is one of the most abundant naturally occurring polymers on earth. It contains a polymeric backbone composed of repeating units of anhydroglucose. Modifications of cellulose were easily accomplished by grafting of long-chain alkyl groups and other functionalities onto the hydroxyl groups . These modifications lead to improved physical, rheological and solution properties. The modified cellulose exhibits increase in water solubility, enhanced viscosity and surface activity. The most common examples of modified cellulose that have been used extensively in the chemical industry are cellulose ethers, which include methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), CMC, HEC and HPC . Each of these cellulosic ethers exhibits unique characteristics and properties. They serve as rheology modifier, anti-redeposition agents, thickeners and stabilizers in cleaning formulations.
MC is a non-ionic polymer that is manufactured by heating the alkali cellulose fibers and treating it with methyl chloride or methyl iodide . MC can be prepared with different degree of substitution (DS). DS refers to the average number of substituents attached to the hydroxyl groups per glucose unit. In theory, the maximum value for DS is 3.0 because there are three reactive hydroxyl units in one glucose ring. Figure 13 shows the structure of MC with DS equal to 2.0 .
The DS, however, does not only determine the properties (i. e. solubility) of the modified MC but also the distribution of the methoxy group [51, 52]. Typically, the commercially available MC materials have DS between 1.64 and 1.92. MC with DS value at this range is soluble in water and mixed organic solvent/water systems. MC with DS lower than 1.64 produces material that has low water solubility, while those with DS higher than 2.5 are soluble in organic solvents.
MC is compatible with a wide range of ionic species and metallic and inorganic salts because of its non-ionic property. It does not form a complex with these types of salts to form insoluble precipitates. It shows good stability at a pH range of 3 to 11 .
MC is marketed under the trade name Benecel™ MC, Methocel™ A and Metolose® SM. It is incorporated in cleaning solutions as thickener and is effective even at very low dosage (0.5–1 %). MC is reported to help control several important properties of formulations such as rheology, dispersion and water retention. Other benefits that MC provides aside from thickening and stabilizing effect include cold water and organic solubility and gel strength.
18.104.22.168 Hydroxypropyl methylcellulose
In the preparation of HPMC, the alkali cellulose fibers are treated with propylene oxide in addition to methyl chloride, to obtain the hydroxypropyl substituents in the hydroxy groups of the anhydroglucose unit (Figure 14) [49, 50]. The –OH group of the hydroxypropyl substituent can react further with methyl chloride to form a propylene glycol ether.
HPMC exhibits characteristics that are very similar to MC, but the presence of the hydroxypropyl groups affords hydrophilic domains that are lacking in MC . The HPMC materials that are available in the market consist of varying ratios of methyl and hydroxypropyl groups. They possess different attributes that influence the water and organic solvent solubility and thermal gelation temperature of aqueous solutions. Both MC and HPMC have been utilized as anti-redeposition agents in laundry detergents .
22.214.171.124 Carboxymethyl cellulose
CMC, also popular under the names cellulose gum, sodium cellulose glycolate and sodium CMC, is the most widely used cellulosic ether. Unlike MC or HPMC, CMC is an anionic polymer. It is synthesized via chemical reaction of cellulose with monochloroacetic acid and subsequent neutralization with sodium salt . An example of a chemical structure of CMC with DS of 1.0 is shown in Figure 15.
The theoretical maximum DS for CMC is also 3.0, but more often than not approximately only half of the hydroxyl units are being substituted with –CH2COOH groups [34, 49]. The highest level of substitution of CMC that is available in the market is 1.4. The structural stability of formulations induced by CMC is greatly influenced by the concentration of the polymer, while thixotropy generally increases with decreasing DS. CMC is stable over a broad pH range of 4 to 10 and is well suited for most non-ionic and anionic species as well as monovalent and divalent salts . But because of its anionic character, it is generally not compatible with cationic materials.
CMC is used in washing formulations, mainly as anti-redeposition agent, and it is mostly beneficial to the laundering of cotton fabrics . It was reported that the negatively charged carboxyl groups of CMC attract the hydrophobic dirt particles and hold on to them, therefore preventing them from resettling onto the fabrics during the washing process. In addition, CMC also provides effective thickening effects  and structural stability on the detergent slurry and liquid soap.
CMC is available in the market as purified and technical grades under the trade name CALEXIS®, CEKOL®, CELFLOW®, CELLUFIX® and FINNFIX®.
126.96.36.199 Hydroxyalkyl cellulose
Hydroxyalkyl cellulose derivatives include HEC and HPC. HEC and HPC are both non-ionic and water-soluble polymers. The general structure of HEC is given in Figure 16. The hydroxyethyl groups were introduced into the cellulose molecule by treatment with sodium hydroxide followed by ethylene oxide [49, 56]. Subsequently, the ethylene oxide that has previously substituted the hydroxyl group can react further with another ethylene oxide molecule to form an elongated side chain.
The average number of moles of hydroxyethyl and ethoxy units added per anhydroglucose ring is called moles of substitution (MS) . In the case of the structure in Figure 16, the value of MS is 2.5 (5 ethylene groups/2 glucose units) and the DS is 1.5 (3 hydroxyl groups substituted/2 glucose units). Because elongation of side chains of ethylene oxide can take place, the MS value can be higher then 3. HEC products that are commercially available have DS values ranging from 0.85 to 1.35 and MS values of 1.3 to 3.4. The water solubility of HEC depends on the values of DS and MS. At the values mentioned above, HEC is water soluble. HEC also exhibits good stability with respect to pH (2–11) and temperature and excellent compatibility with other anionic and cationic ingredients .
HEC is one of the easiest polymer derivatives to process. It dissolves easily at high temperatures and hydrates very efficiently even with just moderate mixing and agitation. HEC is used as thickening agent in detergent industry and assists in enhancing the silkiness and smoothness of the fabrics. HEC aqueous dispersions are pseudoplastic and thermally reversible. It is commercially available under the trade name Natrosol™ HEC.
The preparation of HPC is similar to HEC, wherein the alkali cellulose is treated with propylene oxide (instead of ethylene oxide for HEC) at high temperatures (Figure 17) . A side chain with more than one mole of propylene oxide can also form during the preparation of HPC.
Both HEC and HPC are soluble in water at low temperature. Similar to HEC, HPC also provides rheology control, thickening and stabilizing effects to formulations.
Gums are natural biopolymers that have found broad appeal in detergent industry. They are largely used as thickeners in cleaning compositions. Some of the examples of gums that will be covered in this section are xanthan, guar and locust bean gum.
188.8.131.52 Xanthan gum
Xanthan gum is a high-molecular-weight, anionic polysaccharide that is produced by fermentation process using the bacterium Xanthamonas campestris . It initially found broad commercial applications in food industry but later on was developed for use in detergent industry . The structural formula of a repeating unit of xanthan gum consists of β-D-(1,4)-glucose molecules in the backbone chain and a trisaccharide side chain attached at the O-3 position of every other glucose moiety (Figure 18) [49, 57]. The trisaccharide unit is composed of β-D-(1,2)-mannose attached to β-D-(1,4)-glucuronic acid and terminates with β-D-mannose. The inner mannose residue is mostly acetylated at O-6 position and the terminal mannose contains pyruvic acid linked together via keto group at the 4 and 6 positions. The carboxylic acid functionalities at D-glucuronic acid and pyruvic acid make the xanthan gum anionic.
Xanthan gum has high solubility both in hot and in cold water. Xanthan gum solutions are highly pseudoplastic and display a remarkable viscosity even at low polymer concentrations [57, 58]. Their viscosity recovers instantaneously once the shear force is removed. Xanthan gum’s shear-thinning behavior provides the desired pour viscosity and improves the stability of the formulation by preventing the suspended particles from settling at the bottom of the container throughout the shelf life of the detergent.
Several factors affect the viscosity of xanthan gum solutions including the dissolution and the measurement temperatures, the pH as well as the concentrations of the polymer and the salt in solution . Generally, the viscosity decreases with increasing temperature. However, it was observed that xanthan gum solutions that were dissolved at moderate temperatures (between 40 and 60 °C) tend to give highly viscous solutions. This observation can be attributed to the change in the conformation of the molecules when the temperature is increased. Xanthan gum molecules were found to adopt two conformations, helix and random coil, depending on the temperatures at which they were dissolved. The conformational shift observed from a low (<40 °C) to high dissolution temperature (>60 °C) corresponds to a helix–coil transition of the backbone with simultaneous release of the lateral chains followed by a progressive decrease of the rigidity of the glucose chain.
The viscosity of the xanthan gum solution is also significantly affected by the amount of the biopolymer added, that is, the higher the concentration the higher the viscosity . This observed behavior was associated with the increase in the molecular dimensions due to an increase in intermolecular interaction. Another significant attribute of xanthan gum solutions is the synergistic increase in viscosity when mixed with other gums such as locust bean and guar gum . This behavior further improves the suspending capability of xanthan gum solution.
Only very few rheology modifiers are stable over an acidic and alkaline pH ranges (pH from 1 to 13), and one of them is the xanthan gum [34, 57]. This property makes it a suitable thickener for cleaning products ranging from acid cleaners to scale removers as well as the conventional alkaline hard surface cleaners and neutral detergents . Xanthan gum also has high tolerance to both monovalent and divalent metal salts in addition to anionic and non-ionic surfactants.
Xanthan gum is widely marketed as KELDENT®, KELFLO®, KELTROL®, KELZAN®, XANTURAL® and XANVIS®.
184.108.40.206 Guar gum
Guar gum, also known as guaran, is a water-soluble polysaccharide isolated from the plant called Guar, or cluster bean Cyamopsis tetragonolobus that belongs to the family Leguminosae . Its backbone is composed of linear chain of D-mannopyranose units linked together by β-(1,4) glycoside linkages with branch points from the 6-positions linked to a single α-D-(1,6)-galactose (Figure 19) .
Similar to xanthan gum, guar gum also dissolves very quickly in cold and hot water to give highly viscous pseudoplastic solutions even at very low concentrations. However, guar gum solutions are less pseudoplastic compared to xanthan gum. Guar gum solutions display high low-shear viscosity but have strong shear-thinning property . They are thixotropic at high concentrations (~1 %) but not at lower concentrations (~0.3 %). Because guar gum is a non-ionic biopolymer, it is not easily affected by electrolytes and has high tolerance to basic and acidic environment.
Guar gum has been used effectively as natural thickening agent and stabilizer in detergent compositions. However, there are some disadvantages observed with guar gum solutions such as they tend to give formulations with a stringy rheological property and a hazy physical appearance, which is a result of the residual materials like oils and proteins present in the raw guar gums [34, 49]. These contaminants can be removed by repetitive washing of the commercially available guar gums, but this in effect results in an increase in the cost of the product.
Chemical modification of guar gum such as reaction with propylene oxide results in substitution on the hydroxyl groups (Figure 20). This functionalized guar gum is called hydropropyl guar (HP-guar) . Compared to hazy solutions obtained from guar gums, this chemically altered compound gave a much clearer aqueous solution .
HP-guar displays very similar solution characteristics to those of HPC. It is readily soluble in water and builds high viscosities at low concentrations via the mechanism of chain entanglement. It was reported to act as a foam enhancing agent in liquid detergents .
220.127.116.11 Locust bean gum
Locust bean gum, also known as Carob bean and Carubin, are extracted from the seed of the carob tree Ceratonia siliqua . Its structure is similar to guar gum (Figure 20) except that locust bean gum has fewer amount of galactose. For guar gum, there are two mannose residues for every galactose molecule, while for locust bean gum, there are four. For this reason, locust bean gum is less soluble in water and has lower viscosity than guar gum. Heating is usually necessary to obtain solutions of locust bean gum.
Locust bean gum has very similar properties with guar gum. It also has high resistance to ionic strength or broad range of pH because of its non-ionic character. It has been employed as stabilizers in detergent dispersions.
Carrageenans are family of water-soluble linear polysaccharides extracted from marine red algae (Rhodophyta). They are high-molecular-weight polysaccharide ranging from 100 to 1,000 kDa, which contains 15–40 % of sulfate ester groups and alternate units of β-D-1,3-galactose and α-D-1,4-galactose. Seven different variations of carrageenan were identified, but only three of them namely kappa-, iota- and lambda-carrageenans are of commercial interests (Figure 21) . These carrageenans were produced by different seaweeds, and the principal differences between these three are the number and position of ester sulfate groups as well as the type of substituents on the α-D-1,4-galactose moiety .
The structure of kappa-carrageenan is very similar to iota-carrageenan wherein their galactose residue contains five-membered cyclic ether group between the C3 and C6 carbon atoms, which give them rigid structures (Figure 21) [49, 64]. Both of them assume helical conformations and their structures only differ in the presence of extra sulfate ester group in the C2 carbon atom of galactose moiety of iota-carrageenan. Lambda-carrageenan, on the other hand, contains three sulfate groups per two galactose molecules (Figure 21) and does not form helical structures like kappa or iota. As a consequence, it does not form gels in aqueous solutions and are mainly used to thicken solutions.
The presence of sulfate esters in carrageenans contributes to their acidity as well as hydrophilicity. The position of the ester sulfate groups also influences their gelling abilities. High amounts of ester sulfate moieties in carrageenans are indicative of high solubility in aqueous solutions and lower gel strength. Both kappa- and iota-carrageenan require high temperatures to dissolve in aqueous solutions, but their sodium salts are readily soluble in cold water. They have the ability to form thermo-reversible gels, which can be attributed to the formation of a three-dimensional double-helix polymer network . When the carrageenans are heated to high temperatures, they exist as random coil. Upon cooling, they build up into double helices, which lead to aggregation forming a three-dimensional gel structures.
The physical properties of the carrageenans are highly dependent on the conformation of the sugar units in the polymer chain and the type of cations present in solution. For example, in the presence of cations such as potassium or calcium ions, kappa- and iota-carrageenans have the ability to form gels in aqueous solutions whereas lambda-carrageenan does not . With potassium salts in aqueous solutions, kappa-carrageenan forms brittle gels, while with calcium salts the iota-carrageenan produces soft and elastic gels. The strength of the gels produced is directly proportional to the concentration of the carrageenan as well as the amount of potassium or calcium salts in solutions. However, the use of excessive quantity of salts weakens the gel. The ionic strength of the solutions also affects the gelling and/or the melting temperatures. When the salt concentration of the aqueous carrageenan solutions in increased, the gelling temperature generally increases.
One of the main concerns encountered with gels that are produced from kappa-carrageenan is the spontaneous separation of water through the surface of gel, a phenomenon known as syneresis . The degree of syneresis increases as the amount of potassium salt in solution is increased. In order to improve this condition, kappa-carrageenan is frequently combined with locust bean gum  or guar gum . This combination has shown not just a decrease in the level of syneresis but also a substantial increase in the gel strength, an enhanced texture of the gel, from brittle to elastic, and improvement in water-binding capability. Iota-carrageenan, on the other hand, does not exhibit syneresis.
Iota carrageenan aqueous gels are known to display thixotropic rheological properties at low concentrations, that is, the gels can reorganize to its original form once it is destroyed . This thixotropic property is particularly useful to suspend insoluble particles. Aqueous gels formed with kappa-carrageenan, however, do not display this thixotropic property.
At neutral and basic pH conditions, carrageenan aqueous solutions exhibit good stability. At high temperatures and acidic conditions, the glycosidic linkages of the carrageenans are prone to undergo hydrolysis, causing it to lose its viscosity and gelling capabilities. But once the gel is formed, hydrolysis can no longer take place; therefore it remains stable even at low pH.
Because of the instability of the free sulfonic acid, carrageenans are commonly marketed as the sodium, potassium and calcium salts or a combination of these. They are normally used as thickening, gelling and stabilizing agents . The solution viscosity is strongly influenced by the concentration, the type and molecular weight of carrageenan used, the temperature and the presence of other solutes. Viscosity is directly proportional to the concentration and molecular weight of the carrageenan but inversely proportional to the temperature.
Carrageenans are marketed by CP Kelco under the names: GENULACTA®, GENUGEL®, GENUTINE®, GENUVISCO®, GENU® PLUS and GENU® Texturizer.
Alginates are anionic polysaccharides that are extracted from marine brown algae (Phaephyceae) . Its structural formula, shown in Figure 22, consists of β-d-(1,4)-mannuronic acid and α-l-(1,4)-guluronic acid which can be linked glycosidically as poly(mannuronic acid) and poly(guluronic acid) or mixture of both [49, 69]. The sequence of the mannuronate and glucuronate residues can vary in the naturally occurring alginate depending on the algal source. Alginates owe their highly anionic character to the C6 carboxylate groups of both mannuronic and guluronic acid moieties.
The viscosity of aqueous solutions of alginate is affected by change in pH because of the existence of carboxylic acid groups . The isoelectric point of carboxylic acid is close to pH equal to 4, and this is where the effect of pH on viscosity is most evident. Similar to carrageenans, alginates are not stable at vey low pH because of the tendency of the glycosidic bonds to undergo hydrolysis.
The presence of cations (i. e. calcium ions) in solution also influences the viscosity and gelation of the alginates . Polyvalent cations are known to cause the formation of “junction zones” in alginate solutions. The formation of these junction zones is a result of the interaction between the carboxylic acid substituent of the guluronic acid residues and the calcium ions. These interactions enhance the viscosity of the alginate solutions especially at higher concentration of the cations and eventually cause gel formation. However, a very high amount of the polyvalent cations can cause the alginic acid to precipitate out of the solution . Alginates, therefore, can be used either as thickeners or gellants depending on the matrix and kind of alginates used. It is marketed under the trade name ZENVIVO™.
Synthetic and naturally derived polymers have been of great use to fabric and home care products whether in solid or liquid formulations and for manual or machine washings. In addition to conventional cleaning effects, significant improvements and specific benefits have been successfully delivered to detergent formulations with the use of these polymers. The incorporation of these polymeric materials in the cleaning solutions has created one or several effects and offered many novel performance advantages. The synthetic polymer poly(vinyl pyrrolidone), for instance, was developed to effectively inhibit dye transfer in synthetic fabrics. The natural polymer CMC has been used as an efficient anti-redeposition agent after stain removal in household detergents both in washing machines and in hand washing.
The recent developments on synthetic polymers have considerably enhanced the performance attributes of the household products. Polycarboxylates were among the first and most extensively investigated class of synthetic polymers. To get the desired properties, synthetically derived polymers were easily tailored and controlled through modifications of the molecular weight, the degree of branching and chemical functionalities of the polymer main and side chains. The easy manipulation of the characteristics of these materials is one advantage that synthetic polymers have over most natural materials. However, synthetic polymers, particularly the specialty ones, can be costly.
The market price for the natural polymers, on the other hand, is low compared to most synthetic polymers and does not normally affect the final price of the finished products. Hence formulations that are based on these polymers are cost-effective. Various quantities of naturally derived polymer have been used to obtain the preferred properties. Their qualities and versatilities were also further improved by chemical modifications. Most often than not, chemical alterations of these class of polymers lead to materials that are of interesting and remarkable properties.
Manufacturers believed that the future of household formulations is not only linked to enhanced cleaning action but also to sustainability and biodegradability. In general, the goal would be to bring out formulations that would be of sound effects in terms of properties, economic and environmental considerations. Natural polymers have high rate of biodegradability; thus, they can be used in the formulations at the commercial level on a large scale. Synthetic polymers are only biodegradable to some level, but they can be removed from wastewater with the aid of treatment facilities. These amenities can quickly degrade water-soluble polymers by providing high degree of bioactivity. In some cases, this is a limitation that caused synthetic polymers to become comparatively unappealing commercially.
In the following years, we expect that major producers will continue to provide innovations on the finished formulations of fabric and home care products based on evolving needs of the people and trend toward sustainability. The design and development of novel polymeric materials will continue to expand the capabilities and effectiveness of cleaning formulations. Entirely new products with much improved properties will continue to emerge in the market place to better serve the users. These new innovations will involve new chemistry, formulating techniques and functionalities from commercial polymers, whether synthetic, semi-synthetic or natural.
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