Sealant selection: a fine art
There is no such thing as a general-purpose sealant, claims Dr Mark J Bramwell of the UK-based Tremco, while highlighting the pros and cons of some of the most common sealant technologies.
01 October 2004
IT is in modern buildings, such as the monumental projects of Dubai, that the performance of the sealant joints has greater significance than in traditional buildings because of the greater design flexibility afforded by the use of new materials and construction techniques.
These developments have effectively made redundant the incorrect perception of a sealant - as simply “a gap filler”.
Joints and joint weatherproofing systems require detailed consideration to minimise the risk of failure, and the parameters for joint design and product selection need to be properly understood. The behaviour of joints in buildings has been widely studied in recent years and there is now considerable knowledge of the extent and rate of movement of a variety of material, components and their methods of assembly.
To meet the needs and criteria of contemporary building design, sealant compounds continue to be improved. Product performance targets have been set to equal or surpass industry standards to ensure long-term, weathertight joints.
Sealants & adhesives
It is important to recognise the similarities and differences between a sealant and an adhesive. Adhesives “join” and sealants “protect”. However, they clearly do so in very similar ways. The confusion often comes from the fact that they are both capable of adhering and sealing; both must be resistant to their operating environments; and their properties are highly dependent on how they are formulated and applied.
Joint design & selection
Joint design and sealant selection are, of necessity, very closely interlinked, since the movement in, and dimensions of any joint, will have a major influence on the choice of sealant. A number of factors need to be taken into account in determining the most suitable sealant:
• Extent and rate of movement: Consideration needs to be taken of not only the extent or range of movement but also the rate at which the movement occurs. Movement will occur as a result of one or more of the following effects: thermal, moisture or other.
• Joint dimensions: In addition to the requirements on joint design for movement accommodation, it is important to recognise that some sealants exhibit better resistance than others to slump and sag. Different sealant types will require particular width to depth ratios in order to give the best performance related to their dynamic properties, but in all cases the minimum sealant depth must be achieved.
• Substrate and priming: Consideration needs to taken of the substrates involved, for purposes of adhesion and compatibility. Care should be taken to ensure that joint substrates are prepared properly and that acceptable minimums are stated within the specification. Substrates should be prepared in accordance with the sealant manufacturers recommendations otherwise premature joint failure will occur.
Over 90 per cent of all joint failures result from poor or inadequate preparation. When aesthetic appearance is of importance, the compatibility between substrate and sealant should be confirmed.
In certain cases, primers may be required to ensure positive adhesion of a sealant and adjoining substrate. It is vital that the correct primer is used at the correct coverage rate.
• Conditions in service: The environmental conditions to which a sealant will be subjected will affect its performance. However, any number of local conditions can also have a bearing on the selection of the sealant. Factors to consider include: exposure to high humidity, extremes of temperature (both high and low), chemical spillage, mechanical damage, permanent submersion, high ultraviolet (UV) radiation, traffic use, bio-degradation or the requirement for colour stability.
Having identified the performance criteria that the product needs to meet, the next step is to select the best product for the job.
Sealant types
Often the first choice to be made is between a single or multi-component product. Many of the existing technologies are available as one-, two- or even three-component systems.
Multi-component sealants, generally, consist of an inert base and a reactive catalyst component. When mixed together in the correct ratio, they exhibit rapid cure. During the mixing process, a tint paste can be added to achieve the desired colour. The mixing itself can either be by hand using a high-speed paddle mixer, or via a specialist pump system as is commonly used for two-component silicone sealants for structural glazing applications.
Although this rapid cure is beneficial for many applications, it does mean that unless the product is used quickly it will cure in the mixing vessel. Specialist pump systems, need to be regularly purged or the solvent flushed in order to avoid curing of the sealant with the static mixers. In either case, this does lead to a fair amount of product wastage over a period of time.
The other aspect to consider is the application location. Whereas it is relatively simple to apply a multi-component product in a factory environment, it is generally not practical to use pump systems on-site.
Bearing this in mind, it is often preferable to use a single-component product, and the new-generation products offer faster cure and the ability to tint prior to application, thus making the decision easier.
Which technology? The most common technologies are as follows:
• Silicone: One and two-component products. They are differentiated by the by-product released during cure. Used in high-movement joints in curtain walling applications, especially suited for high UV environments, such as silicone structural glazing.
• Polyurethane: One, two and three-component products. They have a wide range of modulus and movement capability. Used in high-movement joints in stonework facades where the appearance of the finished article is of primary importance.
• Polysulphide: One and two-component products. They have been in the spotlight recently following issue with key raw material supply. Suited for applications requiring good resistance to toxic environments, such as sewage plants.
• Acrylic: Solvent-based acrylics are excellent as troubleshooters, particularly in re-seal situations and where surface preparation is poor, while water-based systems are suited for internal applications, especially where overpainting is a requirement. Smoke and fire-rated versions are readily available.
• Butyl: PIB – Hot melt primary sealant for IG (insulated glazing) units possessing extremely low MVTR (moisture vapour transmission rate). They are setting and non-setting gun grade products.
• New technology: include Kaneka MS and SPUR (both polymer-based).
The benefits of the various technologies are outlined in the table above. These benefits represent the general performance of each technology. Obviously this cannot encompass every single product available and it is worth pointing out that extremes exist in both directions and almost without exception you get what you pay for with respect to performance.
Conclusion
The importance of correct sealant specification and application can be summed up by a well-known British expression – “horses for courses”.
A structural grade silicone may be the obvious choice for structural glazing, however they are clearly not the best option for weathersealing porous substrates, such as granite and marble.
In the same way that sealant application is an acquired skill, so too is sealant selection. Understanding the various factors affecting the sealant in any given application, and recognition of the available technologies will clearly help with sealant selection. However the sealant supplier is always on hand to offer additional advice and recommendations.
Correct selection and application of a sealant, first time around, should ensure a long service life and remove the need for excessive remedial work.
* This paper was presented the Gulf Construction Conference Week held last year.
