Fast and precise determination of total solid content of latex
The fast, reliable determination of total solid content of latex precursors is critical for further processing into medical products. In order to determine the total solid content of latex, current standards such as the ASTMD1417-16 (Standard Test Methods for Rubber Latices – Synthetic) are used.
The methods presented in chapter 4 of the ASTMD1417-16 can be very time-consuming. For example, a drying oven measurement including the cooling process requires about 40 minutes, while measurement in the infrared (IR) dryer requires about 25 minutes.
In this application note we explain how to develop a method to determine the total solid content of latex with the LMA200 microwave moisture analyser in a little over one minute. Additionally, we show that the data is comparable to recognised reference methods.
LMA200 Microwave Moisture Analyser
The Sartorius microwave drying method, LMA200, offers a fast alternative to the conventional methods based on convection heating in a drying oven. LMA200 microwave moisture/solid analysers have been specially designed for routine use. They have the resolution of an analytical balance and cover all applications in research and development, manufacturing and quality assurance. In addition, Sartorius offers individualised method development services for measuring the total solid content for specific applications.
The LMA200 moisture analyser operates according to the principle of thermogravimetry, which is a direct measurement procedure (Figure 1). In this procedure, a sample is weighed before and after the drying process. During drying, the sample is heated by microwaves. This type of heating is much faster than hot air convection heating in a drying oven. The difference in weight after drying, divided by the initial weight, yields the total solid content.
Total Solid Content = (Weight after Drying x 100 / Weight before Drying) %
Four-Step Method Development
A method for determining the total solid content of latex samples can be developed in four steps. Once the method is developed, you can begin routine measurement by closely following the same steps.
Determining the Reference Value
We determined the reference value in accordance with ASTMD1417-16. A measurement series with the drying oven method resulted in a dry content of 73.25% ± 0.17.
Step One : Preparing the LMA200
Use the parameters listed in Table 1 as guidelines for every sample type. Change only one parameter at a time and see the resulting effect before changing the next parameter. A good place to start is the microwave power output (level 70%) and the shutdown criterion (shutoff), which are the two most important parameters. The parameters listed in Table 1 have been found for a real sample of liquid latex and reflect all the results listed.
Once the LMA200 is prepared, you can begin taking measurement. Follow the instructions according to the “Operation” chapter in the instruction manual.
Step Two : Sampling the Latex
It is very important to homogenise the latex before sampling, preferably by stirring with a suitable tool (e.g., metallic spatula). Here are a few important reminders:
- Do not stir in latex that is already dried. Carefully remove these solid components beforehand as removal during a measurement series can lead to contamination.
- Do not sample from any portion of a formed skin as it can interfere with the drying process.
- Sampling with a spatula is not recommended due to sticking of sample to the spatula surface. Instead, use a disposable pipette. If the viscosity of the latex is too high for a pipette tip, then a plastic disposable syringe can be used. The graduation markings on a pipette or syringe can also be used to ensure reproducible sample weighing.
Step Three : Presentation of Sample
The best way to place a pasty sample on a microwave-based meter is between two glass fibre filters, which are microwave transparent and behave neutrally (Figure 2). The distribution of sample between the two filters increases the surface area, which results in shorter measurement time and higher precision.
With conventional methods, skin formation during the drying process prevents complete drying and leaves an Undefined amount of wet/incompletely dried blisters. Glass fibre filters have the added benefit of preventing skin formation.
You can easily press the sample between the two glass fibre filters with two smooth metal surfaces; if necessary, a household press can also be used. Avoid rolling out the sample with a roller, because it can exert excessive force and cause the sample to spill out from between the two glass fibre filters.
Step Four : Ensuring Reliability and Robustnesst
To ensure that this method provides reliable and robust results, take a series of measurements and perform statistical analysis using the formulas shown in Figure 3. To evaluate this method, we performed 11 measurements of the total solid content of latex and recorded the results in Table 2. This helps to correctly assess and evaluate the measured values in later measurements. The following evaluations have proven to be valuable: minimum and maximum values, mean value and measurement uncertainty.
With the assumption that only negligible systematic errors are included in the measurement procedure and only random normally distributed errors occur, the standard deviation can be used to calculate the measurement uncertainty.
Comparison to Other Methods
The microwave oven method described here for measuring the total solid content of latex has tangible time benefits when compared to the oven and IR drying methods in ASTMD1417-16 (Figure 4). Specifically, working with the LMA200 can save up to 40 minutes of time spent per measurement. Further, the precision of the LMA200 microwave method is similar to both oven and IR methods (Figure 5).
Conclusion
We have presented a simple method for measuring the total solid content of latex samples on the Sartorius LMA200 Moisture Analyzer, which uses a microwave drying method. We also presented guidelines on the optimal way to set up the sample to avoid artefacts like skin formation that interfere with measurement accuracy.
The microwave method yields significant time savings per measurement, allowing laboratories to be more productive. Additionally, the data quality is comparable to standard oven and IR methods.
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Further resources for your interest:
Guide to Medical Devices Quality Control and Testing