How humidity affects the cure kinetics of silicone elastomers

Humidity plays a role in the functioning and lifetime of many products, from foods to pharmaceuticals and from polymers to metals. It can significantly impact the properties of a material, such as its cosmetic surface, and mechanical and chemical properties. Temperature and relative humidity (RH) must be carefully controlled in many fields of research in order to ensure products can perform their best in a range of humidities. Here, we look at the role humidity plays in the curing of silicone elastomers, which are used across various industries from structural bonding materials to cooking utensils.

Strategies for controlling humidity vary, with many researchers using a chamber containing salts, (e.g. NaCl, RH 75%), to achieve constant RH. However, these methods are not designed to provide controlled humidity variation and as such, designing experiments with RH as a controlled variable is challenging. Dedicated humidity control systems are now being used in combination with analytical methods such as light microscopy, Raman spectroscopy, and beamline techniques such as GISAXS to characterise how materials behave in different environmental conditions. Advanced systems, such as Linkam’s RHGen Humidity Generator, are now able to control humidity across 5%–95% RH at a range of temperatures, allowing researchers to precisely control water vapour in the environment surrounding the sample for long periods of time.

Researchers at the University of Nottingham recently investigated the combined roles of temperature and humidity on the cure of a room-temperature vulcanising (RTV) silicone adhesive putty, known commercially in the UK as Sugru® and as Formerol® F10 (FormFormForm Ltd., UK) in industrial markets. Unlike many RTVs, this adhesive takes the form of a ductile solid in its pre-cured state, rather than a viscous fluid, making in situ shaping easier. It cures via condensation and hydrolysis reactions, where the formation of hydroxyl groups is catalysed by atmospheric moisture.

Various applications of mouldable, curable, silicone elastomers [Image courtesy of Sugru]

Silicone elastomer-based sealants such as Sugru are used in several industrial, commercial, and domestic applications and are favoured for their low elastic modulus, electrical insulation, hydrophobicity, and low toxicity. RTV compounds are commonly used in silicone sealants which, once dispensed from an applicator, undergo a curing process and can be manipulated in situ until the cure is complete and the sealant is fully adhered to the substrate.

Most silicone RTV sealants rely on chemical reactions with water from the atmosphere to initiate and complete the curing process, and their cure kinetics under different environmental conditions have been the subject of research interest over many years. Studies into these so-called ‘moisture-scavenging polymers’ have shown that temperature and RH have a significant impact on cure rates.

Davide De Focatiis, Associate Professor in the Department of Mechanical, Materials and Manufacturing Engineering at the University of Nottingham and the lead on this project, said: “Most silicones are fast curing (in sealing applications for example) and of low viscosity so tests such as these are difficult. Sugru is easy to handle and mould, and cures over a timeframe suitable for dynamic mechanical testing of this manner. Understanding the cure behaviour of such materials not only gives an indication of how fast the silicone will cure in different parts of the world where climates are greatly different, but can also be used to speed up or slow down cure rates where necessary (to increase moulding time or solidify quicker). In a DIY market it is perhaps not critical exactly how long it takes for a mouldable elastomer such as Sugru to cure, but in a production environment where time is money this is a much more important consideration.”

Matthew Elsmore, a PhD student in Professor De Focatiis’ group, measured the polymer’s cure progression via the shear modulus (measure of the elastic shear stiffness) as a function of time under controlled temperature and humidity conditions, to generate predictive models for cure timescales in relation to these environmental conditions. Conditions were controlled using a rheometer (MCR302, Anton Paar) fitted with a sealed CTD450 environmental chamber fed by a humidity controller (RH95, Linkam Scientific, the predeccesor to the RHGen). The results were recently published in an article in the Polymer Testing journal [1].

Experimental configuration for controlled humidity and temperature, (b) bar torsion clamping arrangement. Reproduced in accordance with CC BY licence (http://creativecommons.org/licenses/by/4.0/).

By coupling a humidity controller to a rheometer, characteristic cure timescales for a commercial and industrial silicone elastomer could be established under controlled environmental conditions. Results showed that, at constant absolute humidity, an increase in temperature from 19°C to 39°C reduced the curing timescale by approximately half (see figure below). An increase in humidity from 7.6% to 36.7% RH (at constant temperature) reduced the cure timescale from approx. 11 hours to 4 hours.

(a)  Cure progression for three temperatures as a function of cure time, illustrating the effect of temperature under conditions of approximately constant humidity [H2O] = (4.7 ± 0.7) g m− 3 (± one standard deviation). (b) Cure progression for four relative humidities as a function of time at constant temperature T = (38.9 ± 0.3) °C (±one standard deviation). Corresponding Hsich model fits shown as solid lines. Reproduced in accordance with CC BY licence (http://creativecommons.org/licenses/by/4.0/).

These results show that both humidity and temperature play key roles in the curing times of the silicone elastomers, and the group’s work to model these cure profiles enables greater accuracy in the prediction of the timeframes for curing these materials, vital for future applications where humidity and temperature may vary significantly.

Professor De Focatiis comments on the results, and future experiments: “There are still unanswered questions, such as how does cure progression in these systems affect the diffusion of water (which drives cure). In very thick specimens the curing process produces a cured shell and the interior can take considerably longer. We have some data on this already, and are aiming to construct a more detailed model of this process. This would help in predicting properties of silicone components with a more complex geometry.”

Matthew Elsmore (third from the left), Davide De Focatiis (fourth from right) and their team at the University of Nottingham

Understanding the cure behaviour of such materials not only gives an indication of how fast the silicone will cure in different parts of the world where climates differ substantially, but can also be used to speed up or slow down cure rates where necessary (to increase moulding time or solidify quicker). This is particularly important in a production environment where time savings translate into cost savings.

To find out more about this research, please contact the lead investigator, Professor Davide De Focatiis, via the article in Elsevier’s Polymer Testing [1].

For more information on Linkam’s humidity generator, and our products to control temperature and humidity around the sample for microscopy or spectroscopic characterisation, contact info@linkam.co.uk.

Reference:

1. Elsmore MT and De Focatiis DSA, Combined roles of temperature and humidity on cure of a silicone elastomer, Polymer Testing, 93 (2021) 106967. https://doi.org/10.1016/j.polymertesting.2020.106967