The shelf life of a dispersion refers to the length of time that a product remains stable. Over time, destabilisation mechanisms occur in dispersions, changing the quality of the products. A recognised method for analysing these processes is static multiple light scattering. The performance of such measurements is specified in standards such as ISO/TR 13097 and ASTM D7061.

What is the shelf life?

The ISO/TR 13097 standard defines the term shelf life as the recommended period during which a product can be stored and during which the defined quality of a particular characteristic of the product remains acceptable under the expected (or specified) conditions of distribution, storage, display and use. It is based on how the stability indicators change. A faster change means a shorter shelf life. Predicting the shelf life of a dispersion is of great importance in many practical applications, as the stability of the dispersion determines the quality and safety of a variety of products.

Dispersions are heterogeneous systems consisting of at least two immiscible phases. The phases are distinguished as the disperse and continuous phases. The dispersed phase consists of smaller particles or droplets that are distributed in the continuous phase. The continuous phase is usually present in larger quantities and surrounds the dispersed components.

Over time, the position and ratio of the molecules and particles of the two phases change relative to each other. These changes are referred to as 'destabilisation processes'. Various destabilisation processes can occur in dispersions. These processes lead to changes in the dispersion and thus influence its stability. Important destabilisation processes include sedimentation, creaming, coalescence, agglomeration and aggregation. The shelf life of these systems is defined as the period during which such processes can be avoided.

Static multiple light scattering method for predicting the shelf life

Various measurement methods can be used to assess durability and long-term stability. One such method is the static multiple light scattering method (SMLS). Such tests can be carried out using a dispersion stability analysis system, such as the MultiScan MS 20 from DataPhysics Instruments. These systems can detect even the smallest changes in dispersion.

To carry out the static multiple light scattering method (SMLS), the liquid sample is filled into a round vessel and placed in a measuring tower. The measuring tower contains two light sources and a light detector. One light source is located opposite the detector and shines through the sample; the detector then records the transmitted light. The second light source is located next to the detector, which detects the backscattered light. The detector and light sources scan the sample from bottom to top at regular intervals. This enables changes in the transmitted and backscattered light to be determined in terms of position and time.

Shelf life prediction according to ISO/TR 13097

Some standards regulate measurements that serve to predict shelf life. The standard ISO/TR 13097 Guidelines for the characterisation of dispersion stability deals with the characterisation of the stability of liquid dispersions (suspensions, emulsions, foams and their mixtures) for the development of new products, the optimisation of existing products, quality control during processing and during the use of the product.

According to ISO/TR 13097, the stability of a dispersion is defined as the change in one or more physical properties over a certain period of time. ISO/TR 13097 describes two methods for predicting shelf life: comparative analysis and predictive analysis.

In comparative analysis, the stability index is compared with a reference sample. This approach is reliable, does not require complex calculations and can be used for all types of dispersions. It is the quickest and easiest method to predict how long a dispersion will remain stable.

In predictive analysis, data is modelled and extrapolated to check whether the stability metric remains within the criteria for the desired time period. However, the models are currently not well suited to predicting the stability of complex systems.

ISO/TR 13097 explicitly recommends instrumental methods as they are the most objective and reproducible. Furthermore, due to their high sensitivity and reproducibility, they can detect destabilisation at an earlier stage more reliably than conventional visual observations. It is also recommended to analyse the sample in its original state without dilution or interference.

Shelf life prediction according to ASTM D7061

ASTM D7061 defines the Standard Test Method for Measuring the Phase Separation of Asphaltene-Containing Heavy Oils Induced by n-Heptane as a Separation Number Using an Optical Scanning Device. Understanding the stability of heavy oils is an important issue in the petroleum industry. The destabilisation of asphaltenes in heavy fuel oil leads to a number of problems, such as the fouling of heat exchangers or significant damage during oil processing. In addition, flocculation and sedimentation of asphaltenes can be caused by incompatible heavy oil mixtures. Therefore, it is of great importance to predict the stability of asphaltenes in heavy fuel oils.

With dispersion stability analysis systems such as the MultiScan MS 20 from DataPhysics Instruments, optical stability and ageing analysis is possible, for example in accordance with the ASTM D7061 standard.