Selecting the right sealant for the job
S M Musabbar, technical manager of Saudi-based Construction Material Chemical Industries (CMCI), presents a detailed insight into the world of sealants and gives tips on selecting the best sealant for various kinds of jobs.
01 September 2009
SEALANTS are required to seal two substrates together, to fill the gaps/holes and to form a protection barrier. Sealing problems usually arise where leakages of air or liquid would affect the functional capability of the structure. As an example, sealing problems lead to seepage of water, causing corrosion of reinforcement in concrete members and deterioration of the structure.
In order to avoid problems, a suitable sealant has to be methodically selected at the time of designing a joint, based on the movement capability of the sealant, dimensions of the joint with the annual weather patterns prevailing at the location of the structure. The selection of the best sealant depends on the technical and performance requirement of a sealant, the most suitable type of sealant, and knowledge of established sealant specifications for some common applications in the Middle East.
Link for graph (Impact of Temperature):
Technical/performance
Sealant manufacturers position various products for different applications. These include facade joint sealing; window sealing; concrete precast members; car-parks, walkways, industrial and commercial floor joint sealing; sewage water plants; water purification plants and potable water reservoirs; chemical-resistant seals; gasoline station/tank seals; and bridge, airport, and road construction.
A sealant formulated for glazing may not provide high chemical resistance and hence while specifying a sealant, a designer should look into important aspects like the movement accommodation factor (MAF), elasticity, elastic recovery and elastic modulus, and specify properties according to the application’s environment such as temperature exposure, durability in continuous immersion, ageing resistance and chemical resistance, as most sealants are designed to meet these basic criteria.
Chemical resistance: This depends on the concentration of the chemical, its reacting temperature, exposure time and frequency and mixture of different chemicals. Chemical-resistant properties of a sealant should be considered when it is used at the framework of groundwater protection, in chemical plants, filling stations, and loading points for liquid chemicals, airfields, in motor vehicle repair and maintenance areas, washing plants, farms and breweries.
Temperature exposure: Sealant performance is affected by temperature and depends on the temperature at installation and during service. The magnitude of joint movements and the rate of temperature change affecting those movements are equally important. Sealants generally perform better at higher temperatures, and when movement at a joint occurs at a slow and uniform rate.
Sealants not designed for very high temperatures cause the plasticisers in the product to migrate, resulting in staining and crazing. Also, the resin and flexibilisers should be incorporated with suitable additives to provide high ultraviolet (UV) resistance. On the other hand, a sealant for low-temperature areas like cold rooms, should possess the required Tg (glass transition temperature) to avoid becoming brittle.
Special properties: Sealants with special physical, chemical and microbiological properties are used in purification and waste water plants where resistance against high water-pressure, acid or alkali content material and microbiological effects is required. Sealants for potable water exposure should comply to standards such as BS 6920 and with FDA (Food and Drug Administration) for contact with food products.
Grade: Vertical joints require non-sag grade (gun grade) sealants, and horizontal and very low slope joints require self-levelling (pourable) grade types.
Colour: Nowadays, sealants are available in various colours and manufacturers such as CMCI are customising colours to client requirements. Colour stability depends on the type of resin used.
Hardness: Sealants of high elastic modulus and shore A hardness are generally used for floor joints. Sealants meeting standards such as ASTM C920, Use T and FS TT-S-00227e, are suitable for trafficable floors and those subject to frequent cyclic loads. A sealant should also be elastic enough to hold an elongation of 100 per cent for 24 hours without the loss of adhesion or cohesive failure, after immersion in chemicals, according to DIN 52452, Part 2.
Link for graphg (Typical Properties of Sealants):
Types of sealants
Sealants are basically available in multi-component kits or in single-component sausages or cartridges, depending on the chemical nature of the resin. In multi-component sealants, the base component is generally chemically cross-linked with a curing agent. The transformation from a pasty state into the functional final state of forming a rubber takes place via a chemical reaction within the material once applied. Depending on the chemical reacting system, the procedure is known as cross-linking or vulcanisation. The general chemical categories include:
• Acrylic: These are used for sealing non-movement or low movement joints. They offer strong adhesion properties and are paintable. On the downside, they are subject to shrinkage and afford low resistance to water;
• Silicone: Available as single or two-component sealants, they offer good temperature and UV resistance and high MAF and flexibility. However, they are generally non-paintable, slow curing at low temperature and humidity sensitive;
• Polysulfide: Available as single or two-component sealants, these sealants provide excellent resistance to fuel, ozone and weathering and good adhesion. Their disadvantages include their poor recovery rate and slow curing at low temperature;
• Polyurethanes: These single or two-component sealants provide a high elongation and elastic recovery rate, good adhesion, abrasion resistance and chemical resistance. However, they are non-environment-friendly;
• STP (silane-terminated polyurethane) polymer: These sealants offer the advantages of very high aging resistance, high adhesion and abrasion as well as high elongation and recovery rate.
A point to note is that in single-component polyurethane, the chemical reaction compellingly needs humidity. Hence, the speed of transition to the flexible condition depends on air humidity and temperature. Therefore, fluctuations will always develop during the process according to the place or time of the application. In single-component MS polymer sealants, the tackiness causes an increased bonding of dust, which can dirty the joint surface. The indication “over paintable” should be confirmed in the product data sheet by stating the test and coating system as stipulated in DIN 52 452 Part 4. Failing this, such confirmation must be obtained before the job starts.
For single-component STP sealants such as Cemtec 30A, whose backbone is polyurethane, the curing mechanism of silane-terminated polyurethane ensures that features such as good adhesion to various substrates (primer-less) and no formation of bubbles even in high humidity (no formation of carbon dioxide). Also, it can be painted over, is free of isocyanate, has good weatherability, high elasticity, and is durable.
Bibliography:
• ALIPS – Aliphatic Polysulphides by Heinz Lucke;
• Bayer Material Science;
• Momentive Performance Products;
• The Sealants Manual by AkzoNobel;
• BASA – Industrial Guide To The Professional Application Of Construction Sealant On Site;
• ACI 504R-90 Guide to Sealing Joints in Concrete Structure.
