Aim for the Stars: Exploring Polymeric Surfactants

The crop protection market is facing unprecedented challenges arising from environmental matters such as reduction of active ingredient (a.i.) residue levels, increasing weed resistance and spray drift.  As a result, agrochemical formulations are becoming more complex, often requiring enhanced bioefficacy and inclusion of multiple a.i.s. In the preparation of an emulsion or dispersion which can be deemed as the most common formulation types in the agrochemical sector, the formulator’s choice of an emulsifying dispersing or system is crucial to be able to produce a stable product.

 

There are many different mechanisms in which stabilisation can be achieved by the surfactant system and one of the most common is by steric stabilisation.  An a.i. is said to be sterically stabilised when the surface of the solid’s particles are completely covered by the dispersant, making particle to particle contact impossible.  As particles approach each other, the adsorbed dispersant chains intermingle, which is thermodynamically unfavourable and presents a barrier to further attraction.  

 

The force-distance curves below indicate only one minimum whose depth depends on several factors, importantly one of which is the adsorbed layer thickness. When this exceeds a certain value, the dispersion or emulsion approaches thermodynamic stability. Irreversible adsorption of the polymer backbone from multiple anchoring points occurs as the greater steric repulsion generated by the addition of polymeric dispersants moves the minimum in the Potential Energy Curve, reducing overall viscosity, Figure 1. Polymeric surfactants also offer greater steric repulsion due to their considerably higher molecular weights compared to traditional monomeric surfactants. 

 

 

Structural features of polymeric surfactants can allow multiple functionality binding sites to give durable adsorption layers upon many physically and chemically different particles.  However, surface properties of the particles are critical to the effectiveness of the polymer.   Optimal steric stabilisation is achieved when the polymer chains are fully solvated by the medium of the formulation. Steric stabilisation has a number of advantages, including the possibility of achieving unique emulsification and dispersion effects (in both aqueous and non-aqueous systems), the formation of thermodynamically stable colloidal systems and most importantly, design and use of tailor made molecules.

 

Polymeric surfactants fall into several categories but two most common architectures are block copolymers and random copolymers.  At Croda we have a number of polymeric surfactants each designed with a specific architecture. An example is Croda’s Atlox™ 4912 which is a block copolymer and can be described as an A-B-A make-up with a molecular weight around 5,000 Daltons (Da), based on 12 poly-hydroxysteric acid and polyethyleglycol (PEG). It has a low hydrophilic-lipophilic balance (HLB) ~5-6 and medium polarity, making it an effective water in oil (W/O) emulsifier.  It can tolerate a high level of inorganic salts or water miscible organic materials in the aqueous phase. The oil phase used can range from paraffinic mineral oils, hydrocarbon aromatic solvents, diesel oil, kerosene, vegetable oils and fatty acid esters.  The optimum usage level is 5% w/w (on water), which typically gives a droplet size of 3-5 µm average diameter and these emulsions are stable to heat and high shear.  An example of a random copolymer is Croda’s Atlox 4914 which is an alkyd-PEG resin with a low HLB ~5-7 and good solubility in a wide range of hydrocarbons which forms water-in-oil (W/O) emulsions. When combined with Atlas™ G-5000, a hydrophilic AB block copolymer, oil-in-water emulsions can be formed. 

 

One example of a polymeric dispersant with a higher HLB value ~11-12 is Croda’s Atlox 4913, Figure 2.  Atlox 4913 is a hydrophilic methyl methacrylate graft copolymer and a useful dispersant for organic or inorganic active ingredients in water as it adheres at a particulate interface.  Dispersions formed using Atlox 4913 are sterically stabilised by the hydrated PEG extending from the acrylate polymer backbone and are relatively insensitive to the presence of salts or solubilised organic materials in the aqueous phase.  It is produced by low temperature polymerisation with low free monomers (typically < 100 ppm) and low solvent levels (typically < 250 ppm).  Hydrophilic PEG chains extend into the water phase and the hydrophobic polymer backbone sits on the interface of the dispersed particle covering the surface and the extended polymer chains prevent the particles from coming into contact with other particles. 
 

 

In the past, is has been common to see formulation ingredients from other markets being used in agrochemical formulations.  In today’s market, agrochemical formulations are becoming more challenging requiring enhanced bioefficacy, the inclusion of multiple a.i.’s often with different physical chemical properties which might be antagonistic and then subjecting these to complex tank mixtures.

 

Due to work on overcoming resistance or differentiating products in the market, there is increasing interest in the synthesis of tailor-made polymeric surfactants to provide advanced or specific roles. Although not as well-defined as small-molecule surfactants, polymeric surfactants offer greater opportunities in terms of flexibility, diversity and functionality.  This is especially true in the light of recent advances in controlled living radical polymerisation chemistry, as exemplified by atom transfer radical polymerisation (ATRP) and, to a lesser extent, reversible addition fragmentation transfer (RAFT) polymerisation.   These types of polymer chemistry have enabled synthetic polymer chemists to make new, well-defined amphiphilic block copolymers, many of which exhibit interesting surfactant behaviour.

 

Star polymers are another class of polymeric surfactants gaining interest because of their characteristic rheological and dilute solution properties.  Increasing the number of arms decreases the degree of dynamic entanglement for star shaped polymers compared to a linear polymer of the same molecular weight.  These inherent design principles have been the focus for the development of Croda’s new star shaped polymer Atlox 4916.

 

This new molecule is a high molecular weight polymeric emulsifier and dispersant with low HLB developed to provide excellent stability when used in a variety of crop formulations.   With its unique ‘star structure’, the design of this molecule is targeted at providing multiple chain anchoring points into both the aqueous and the oil phases.

 

Atlox 4916 has superior anchorage at the oil/water/particle interface providing unrivalled stability even in the most challenging formulation conditions. For example, enhancing emulsion stability through increased steric hindrance of irreversible flocculation.  The star consists of a sorbitol base reacted with ethylene oxide (EO). This product is then further reacted with a polymerised fatty acid resulting in between 2-6 polymerised fatty acid chains randomly distributed across the star shape.

 

This unique star shaped structure is what gives Atlox 4916 its superior stability.  Figure 3 shows the oil/water interface as occupied by three surfactant molecules of differing molecular weight and shape and as shown. There is a larger portion of the Atlox 4916 molecule anchored in both the water and the oil phases when compared to the smaller sorbitan ester emulsifier molecule and even the block copolymer Atlox 4912. This creates a larger steric barrier effectively preventing droplet coalescence and so the stability of the emulsion is increased.
 

 

With multiple anchoring points into the oil phase derived from the addition of polymerised fatty acid chains, the star polymer can perform as an exceptional emulsion stabiliser. There is also a hydrophilic aspect to the molecule that arises from the unreacted arms of the ethoxylated sorbitol base structure.

 

The ratio between the two gives the molecular composition a calculated HLB ~6.  Atlox 4916 in combination with a high HLB emulsifier, such as Atlas G-5002L for example, can help with both chemical and physical instability in O/W formulations, forming emulsions with narrow particle size distribution, excellent room and high temperature stability and showing no undesirable viscosity increase over time. 

 

In conclusion, polymeric surfactants offer the formulator a toolbox of materials in which greater stability of formulations can be achieved.  In today’s continually challenging environment, where multiple and often antagonistic a.i.s are required to reside together, addressing this complexity with tailor-made polymeric surfactants is key.