The primary purpose of dishwashing detergents (also known as light duty liquids, LDLs) is to remove soils, mainly food material residues, from kitchenware surfaces, including dishes, pots, pans, utensils and a wide range of other items. Although dishwashing detergents can be traced to ancient times, the ﬁrst modern-day liquid hand dishwashing detergent was developed in the 1940s (Lai, et al, 1997; Mizuno, 1975).
The efﬁciency and efﬁcacy of the dishwashing process is determined by several factors including the type of surfactant chassis the formulation is built on. These detergents are generally designed to facilitate soil removal and make the dishwashing job easier and more enjoyable for the consumer. Second, the amount of mechanical action is highly variable in the dishwashing process. For difﬁcult to remove soils, consumers generally apply a large amount of mechanical action by scrubbing, often with the help of an implement. High performance dishwashing detergents (Figure 1) are generally carefully formulated to work with all the key factors or variables in the dishwashing process.
Figure 1: Examples of P&G’s Hand Dishwashing Detergents
Suds generation is an important signal to the consumer that a hand dishwashing detergent is effective in removing soils and in particular, greasy oily residues. Suds longevity as it pertains to mileage, increased number of items cleaned per session, is also a signal to the consumer of the detergent’s efﬁcacy and value. Therefore, the challenge is to develop an effective dishwashing detergent that removes both particulate and greasy soils that is effective during the entire the course of a hand dishwashing cycle independent of number of items and level/type of soils on dishes, pots, pans, glasses, etc.
As cleaning demand on the surfactant system increases in hand dishwashing, the suds level drops rapidly due to the antifoam effects of particulates and grease. To mitigate this sudsing loss, weakly cationically charged polymers were investigated to stabilize and boost suds via interacting with greasy soils thereby reducing soil anti-foam effects. Previous efforts utilizing suds boosting polymers in hand dishwashing detergents focused on enhancing the bulk/ﬁlm viscosities (e.g., celluloses, guars, acrylamides, polysaccharides, etc) (Lai and Dixit, 1995). Early studies with polymers (varying backbone, charge, hydrophobicity, etc.) demonstrated the sudsing beneﬁts of polymers were achieved with hydroxypropyl cellulose, but most of these polymers had several signiﬁcant compatibility issues with liquid hand dishwashing detergents: (i) no signiﬁcant beneﬁts under high dilution product usage conditions (typically 0.004 to 0.1% in use), (ii) materials are difﬁcult to formulate into product matrix, and (iii) poor product dissolution in wash water
Therefore, charged synthetic polymers were investigated to provide a beneﬁt but remain formulatable. Importantly, having too much cationic charge within a polymer will produce solid soil ﬂocculation and polymer-surfactant precipitation. Surfactant precipitation results in cleaning loss and efﬁcacy of the detergent formulation. Critical to the innovation is to identify the cationic charge density that is high enough to have sufﬁcient interaction with the soil but sufﬁciently low enough to not precipitate out with surfactant at wash pH (Figure 2, 3). Polydimethylaminoethylmethacrylate (PolyDMAM) and derivatives with reduced charge were studied at wash pH to identify the beneﬁt range of the polymers.
Figure 2: Optimal pH: Example of pH titration curves of polymers evaluated. At high pH (>9.5 for PolyDMAM) the DMAM function is not protonated and therefore mitigating electrostatic interaction. At low pH (<7.2 for PolyDMAM) the cationic charge density is too high causing the polymer to interact too strongly with the anionic surfactant leading to sudsing negatives.
Optimal charge density was identiﬁed by using suds cylinder testing in the presence of polymer, formulated detergent and sequential soil additions (Figure 4, 5). These studies included rheology measurements, pH titrations of polymers with varying compositions of amine and non-ionic monomers, streaming potential measurements of wash solutions containing the polymers, and varying molecular structure and molecular weight of the polymer to facilitate formulation and solubility. Roughly 200 polymers were designed to optimize polymer efﬁciency taking into account performance, formulation ﬂexibility, and commercialization requirements.
Figure 4: Results from suds cylinder testing of LDL formulations with and without polymer.
The weakly charged polymer signiﬁcantly lowers the interfacial tension at the oil/water interface more that at the air/water interface which suggests why suds beneﬁts are observed despite heavy soil additions (simulating adding a food particulates to the wash solution). Increased suds mileage from the detergent formulation containing the polymer mimics was observed rheologically. Surface and interfacial tension measurements alone indicate polymer air/water surface activity is minimal up to 20 ppm polymer in solution (Figure 8). Polymer surface activity at the oil/ water interface occurs as low as 1 ppm concentration. Suds stabilization occurs via interactions (electrostatic and hydrophobic) with greasy soils, reducing soil antifoam effects.
Figure 8: Weakly charged cationic polymer significantly lowers the interfacial tension at the oil/water interface more that at the air/water interface which suggests why suds benefits are observed through soil additions.
The development of weakly cationic polymers represents the successful implementation of suds boosting polymers for hand dish washing products. This was the ﬁrst implementation of polymer technology for suds improvements in this category and has resulted in consumer noticeable performance enhancements as well as formulation opportunities for P&G’s Dawn, Fairy and Joy hand dishwashing liquids. Polymer design strategies have given for the ﬁrst time the opportunity to independently design suds and cleaning performance vectors of the company’s LDL products. Previous LDL design strategies to improve the suds proﬁle via surfactant optimization intrinsically led to compromises in the consumer beneﬁt proﬁle (e.g. better sudsing, but with rinsing issues). This new polymer class allowed P&G to offer consumers uncompromised beneﬁts on sudsing, cleaning, dissolution, rinsing and reduces total chemical load to the environment through surfactant reduction in the product and lower usage by the consumer(net a much better value to the consumer)