The use of different emulsion types in dust control

Emulsions

Emulsions suppress the loss of surface fines as both inhalable and respirable dust particles when used in unsealed roads. The aim of this article is to discuss the three most common types of emulsions used in dust control include synthetic polymer emulsions, bitumen emulsions and emulsified lignosulfonates. I believe the multifaceted role played by synthetic polymer emulsions gives them an edge over the other emulsions which in most cases is different from one application to the other. 

Polymer Emulsions

Acrylates, polyvinyl acetates and polyvinyl chlorates synthetic polymer emulsions can be used to control dust. This group of emulsions is made by monomer polymerization in an aqueous medium. Their mechanism of action relies on evaporation kinetics, where removal of water leads to a transition from flocs into a film as a result of coalescence. The 40-60% solids by weight content are responsible for particle encapsulation and formation of a binding network adhering soil grains together. In addition to dust suppression, these emulsions can provide strength improvement whilst creating a weather-resistant surface. In some projects, application rates of 1.27l/m2 top spray seal have led to penetration values of between 25-38 mm. Various other projects utilizing 1.4 – 4.5l/m2 have shown that synthetic polymer emulsions best perform when mixed in with up to year longevity before rehabilitation and treatment cycles. 

In practice, easy application with good efficiency, increase in shear strength of material, consistent dust control performance with moderate adaptability to wet conditions are some of the advantages of synthetic polymer emulsions. Limitations include susceptibility to oxidative aging of the crust that forms on the surface over prolonged UV exposure leading to deterioration and they tend to be costly owing to a scarcity of raw materials and production processes for their synthesis. 

Bitumen Emulsions

Dust reduction through the use of bitumen emulsions of a slow setting nature is common in unpaved roads. The dispersion of the bitumen phase in the non-miscible aqueous solution of emulsifying surfactant phase at varying volume fractions is known as a bitumen emulsion. The nature of the surfactant determines the charge on the emulsion, cationic bitumen emulsions are the majority used in dust suppression. Contraction kinetics as a result of the reaction with alkaline soil surfaces happen through stages of flocculation, densification and coalescence as the bitumen phase settles out of the aqueous phase. Breaking and curing of the emulsion is a combination of phenomena based on energy fluctuations amongst forces between the dispersed particles leading to gelation over time. On the contrary siliceous soils, interact with cationic emulsions through emulsifier abstraction, deprotonation of the acidified emulsifier and droplet migration. 

Anionic emulsions interact with calcareous surfaces to break the emulsion through calcium salt formation from the surfactant whereas in siliceous surfaces the adsorption of the inorganic cation on the surface leads to counterion loss on the surfactant with both processes resulting in emulsion destabilization. Examples vary from application rates of 0.5 – 4.5l/m2 depending on road surface condition, dilution and product. The large particle size of bitumen results in lack of penetration by bitumen emulsions which poses a challenge to adequate curing as a crust forms on the surface and is prone to fragmentation under traffic and wet conditions. Dust control and a certain extent of strength can be achieved through maintenance using conventional unpaved road equipment are difficult. Dilution with water and non-volatile resins improves penetration with the latter solubilizing the asphaltenes to aid in particle size reduction leading to better penetration. On dilution with water 25% solids by weight remains as bitumen in the emulsion. Another concern is the environmental risk of bitumen emulsions is very high as it can be mobilized by rain and overland flow for at least 24 hours. Roads continuously treated with these emulsions also build up large concentrations of bitumen, which is a long-term environmental issue for disposal or rehabilitation.

Lignosulfonates

Emulsified lignosulfonates can perform as dust suppressants in unbound gravel materials. They are by-products from sulphite paper and timber making processes in which sulfonated lignin a polymer built up of phenyl propane subunits is dispersed in an aqueous solution. Their composition depends on tree species feedstock and chemicals used to extract cellulose, with neutralized lignin sulfuric acid containing sugar as their active ingredient. Calcium, sodium and ammonium lignosulfonates represents the most common types with calcium lignosulfonate used as a dust suppressant. Initially, lignosulfonate is mixed in with topical spraying as a maintenance measure and rehabilitation. In its emulsified state, lignosulfonates consist of 50% solids by weight. Application rates vary from 2.3 – 4.5l/m2 with up to 2 treatments recommended per season. 

The benefits of emulsified lignosulfonates include low cost, fast action reducing hauling delays, binding of surface particles, increase in dry strength of materials under dry conditions, non-hygroscopic in nature hence effective retention over long dry periods with low humidity, aids in plastic flexibility of clay enabling restructuring and further traffic compaction and allows for grader rework on the moist surface. The shortcomings include being biodegradable and very prone to UV degradation, the solubility of solids in water leading to loss of surface binding action as a result of leaching thus poor adaptability to wet conditions and the need for frequent reapplication, performance variations based on tree source and extraction process. 

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