A BAAD™ Way to Assess Substrate Durability

UnderCurrency was established in 2019 as a technical consultancy service. We are based in Mexico and specialise in methodologies, solutions and systems for evaluating and enhancing the security, functionality and durability of secure documents and banknotes.

Our team of scientists work with central banks seeking innovative strategies to increase the lifespan of banknotes. UnderCurrency has developed an approach to predict the performance of different substrates and security features, substrate being the most influential factor in determining banknote performance in circulation.

A print trial followed by a controlled circulation trial is a traditional approach to evaluating circulation performance but is time consuming, expensive, and carries risk. This can be mitigated through laboratory testing using accelerated ageing processes designed to simulate circulation conditions.

It can still be difficult to fully simulate real circulation environments, but through the assessment presented below, it is possible to forecast the relative durability and degradation patterns of different substrates, providing valuable insights about their behaviour in circulation.

Banknote Accelerated Ageing Device (BAAD™)

Leveraging an innovative process, UnderCurrency has established the means to replicate the effects of various environmental factors on banknotes. Statistical analysis of the outcomes of these tests enables prediction of the durability and degradation patterns of different substrates and security elements. These simulations – supported by robust quantitative methods – provide valuable insights to facilitate a cost effective and informed decision-making process.

Our Durability Assessment Report ¹ examines the effect of soiling on four distinct, unprinted paper banknote substrates – I, II, and III – with IV being a reference for comparison. The key project components were accelerated ageing of the samples, comprehensive image analysis, establishing a metric to quantify the soiling, and statistical analysis of the data, including a model to estimate mean life of each substrate.

Fig. 1: Chamber of the Banknote Accelerated Aging

Device (BAAD).

 

A total of 20 samples of each of the four substrates were subjected to testing and analysis. Using a soiling mixture consisting of shredded circulation banknotes, oils, and black clay, samples were aged using the Banknote Accelerated Ageing Device (or BAAD™). This device features a cylindrical metal chamber equipped with rubber flaps, which hit and rub the samples when the shaft rotates within a range of 0-1,000 rpm (see Figure 1).

A single, two-minute ageing cycle was applied to 20 substrate samples at a time, with both sides of each sample scanned for image analysis after each cycle. Soiling level was measured in grayscale units and 10 ageing cycles were applied to each set of 20 samples. The relative change in soiling was established by subtracting a reference value measured for each sample when in pristine condition from the value obtained after each cycle.

This approach meant that specific attributes of each substrate, such as the original colour or marks, could be excluded to ensure that the measurements focused only on wear and soiling.

Analysis and data modelling 

An analysis of variance (ANOVA) test was first used to ascertain whether the variations in soiling measurements, per cycle, were statistically significant due to the inherent properties of each substrate. This was done to ensure that the outcome disparities could be used to reliably predict circulation performance differences between each substrate and the reference substrate.

Curves that best fit the soiling data for each substrate were then modelled, allowing for a comprehensive understanding of their performance. From these, the expected mean lifespan, with confidence intervals, for each substrate could be calculated, accounting for the uncertainty associated with the expected mean lifespans in circulation.

Soiling thresholds and mean lives

The average lifespan of a banknote is influenced by a central bank’s withdrawal policy, particularly the ’soiling threshold’, ie. the level of soiling at which a banknote would be deemed unfit and withdrawn from circulation. In the model, the point at which each fitted curve intersects the soiling threshold determines the mean lifespan.

Three soiling thresholds – 18, 20, and 22 – were proposed, representing the withdrawal policies for most circulation regions. This approach provides insight into expected substrate performance in different circulation environments.

Results

Figure 2 shows images of a single sample’s deterioration stages. This example illustrates how the samples appear as the number of deterioration cycles increases.

Fig. 2: Sample IV_14a (reference sample) at different stages of the deterioration process.

For substrates III and IV (the reference), the ANOVA test results showed no significant statistical difference in mean lifespan. This implies that the properties of these two substrates are similar when subjected to the BAAD deterioration process, whereas a statistically relevant difference exists between substrates I and IV and between II and IV. This suggests that their performance in circulation would also be statistically different.

Mean fitted curves were then established for each substrate (see Figure 3). These illustrate a significant initial growth rate

in soiling with a subsequent decrease as the growth rate stabilises. These curves reinforce the ANOVA test results by illustrating the similarity of substrates III and IV and the marked difference of substrates I and II compared to IV. Notably, substrates I and II have a striking similarity in performance.

Fig. 3: Mean fitted curves alongside the soiling thresholds at 18, 20, and 22.

The dashed lines represent the thresholds and the points where the fitted mean curves intersect them indicate the expected mean lives.

Figure 3 also shows proposed soiling level thresholds for withdrawal from circulation, with the expected mean life being the points where the curves intersect the threshold lines. From this, the relative mean life of substrates I, II and III compared to IV was calculated. The relative mean life is substantially better for substrates I and II, ranging from an improvement of 45 ± 9% at threshold 18 for substrate I to as much as 84 ± 4% at threshold 22 for substrate II.

Conclusions

The data acquired through image analysis, and subsequent analysis to establish mean life, was consistent with the change in visual appearance of the samples with each cycle. It is therefore possible to conclude that the image acquisition and analysis process effectively characterises the change in each sample when using the BAAD apparatus for accelerated ageing.

The data analysis, starting with the ANOVA test, established that there is a statistically significant difference in performance between substrates I and II when compared to the reference substrate (IV) but not between III and IV, which have similar outcomes.

The fitted mean curves clearly illustrate the performance of the substrates. Using proposed soiling level thresholds that align with banknote withdrawal policies in most circulation regions, relative mean life could be calculated for each substrate. The results show a significant improvement in expected circulation performance for substrates I and II but not for substrate III, as compared with the reference.

This exercise has demonstrated the BAAD apparatus is a useful accelerated ageing tool and that the analysis methods provide a robust, quantitative method for differentiating the expected circulation performance of different substrates.

The results from this approach can be used to highlight the potential impact of different withdrawal policies and for informing the decision-making process when selecting appropriate materials for specific conditions.

1 - Substrate Durability Assessment Report UC-2307, Ernesto González Candela (PhD), Melissa Valdés (MSc), Karl M. García-Ruiz (MSc), July 2023.