WHAT SEALANT DO I USE? There are many different sealants available on the market and all are marketed for a variety of different applications. This can be extremely confusing when choosing the right product but the basic differences are generally related to quality of the product, toolability, drying times, flexibility and movement and adhesion properties. So in a nutshell to help you understand sealant technology we have tried to simplify what the manufacturers are talking about:
The main product types are Silicone, Water-Based (commonly referred to as acrylics in the Trade), Hybrid Polymers and Polyurethane (PU); some are more compatible with certain materials than others. Here’s an overview with their benefits and limitations to help you make a more informed choice:
SILICONES Silicones can generally be split into two groups; ACETOXY CURE (Acetic Acid) and NEUTRAL CURE. All one part silicones cure by moisture present in the air surrounding them. All have different characteristics, some harder or softer than others. Some adhere to certain materials better than others. No silicones are suitable for use with bitumen, asphalt or any tar product and are not over paintable.
ACETOXY CURE - see pages 8 and 9. The most common and generally cheaper option. These smell of vinegar which can be quite pungent. These can be used, without issue, in many applications in conjunction with many materials. However due to Acetic Acid being evolved during the curing process, Acetoxy silicones do have their limitations. These include:
1. When used with untreated / unprimed metals such as lead, copper, brass, steel, etc. The Acetic Acid vapours can oxidise the surface, creating a powdery residue between the bonds and subsequently lead to adhesion failure.
2. When used with any material containing cement such as concrete, plaster, mortar and render. Cement is alkaline, as Acetoxy silicones are acidic when curing there is a risk of reaction causing adhesion failure.
3. With some plastics, especially with silicones extended with solvents. Testing is always recommended before use. Plastics can be compounded with other unknown materials.
4. Tiles made of natural stone which will react to the acid in the silicone.
NEUTRAL CURE - see page 11. Neutral cure silicones generally have better adhesion than acetoxy to most common materials, including plastics. Adhesion promoters are easily added to neutral cure silicones. Being pH neutral whilst curing, these are compatible with metals and substrates containing cement. Fewer odours can be expected when using them.
NEUTRAL CURE OXIME - see page 10. Oximes, which have only become prevalent in recent years, give off a chemical called MEKO when curing. There is some evidence that MEKO is carcinogenic (cancer causing). Of late, low MEKO and zero MEKO formulations are available with emissions that fall within the permitted exposure limits. These are often extended with solvent and fillers, they possess very good adhesion to the likes of PVCu giving a cost effective product for perimeter sealing in the glazing industry.
NEUTRAL CURE ALKOXY Alkoxy silicones offer excellent adhesion without evolving MEKO during the curing process. Can be used in a wide range of applications from glazing and engineering to general construction. Note: 100% neutral cure alkoxy products are the only silicones that can be used for ‘bedding in’ double glazed units. This is because they are neutral cure and don’t contain any solvents that will degrade the insulating glass seals.
WATER BASED SEALANTS - see pages 13 to 15. These are blends of fillers and other materials bound together by a water based polymer emulsion and cure by evaporation. Depending on the polymer used, some are more water resistant than others and can be harder or softer. Drying times can vary substantially depending upon conditions and the water content of the sealant.
Unlike silicones which will cure faster in warm humid conditions, water based sealants will dry more slowly. Warm, dry, well ventilated conditions will provide faster dying times. Water based sealants are over paintable. All have an element of ‘flexibility’, however ‘flexibility’ should not be confused with the stretch, elongation or elasticity a silicone sealant will offer. Silicones will generally FAR out perform water based products in all departments when used as sealants.
POLYURETHANE SEALANTS - see page 16. One part moisture cure Polyurethane offers excellent adhesion to most materials, it offers good resistance to chemicals and abrasion when used as a sealant. PU sealants are generally harder than silicones and unlike silicones they are overpaintable. PU’s are not normally found being used in domestic applications; they are more widely used in engineering, automotive, construction and marine applications. Being tough and flexible with excellent adhesion to construction materials, PU is ideal for expansion joints.
HYBRID SEALANTS - see pages 18 and 19. Hybrid Polymer, sometimes known as MS, sealant technology has only recently arrived on the scene but is growing in popularity. They are among the most versatile sealants available. They are neutral and cure by moisture to form a tough resilient, permanently flexible seal; resistant to UV, abrasion, salt water, mild solvent and acid, are overpaintable and can be applied in damp conditions. Due to the wide range of materials that Hybrid Sealants will adhere to they are used in a wide range of applications including; construction, engineering, glazing, marine, automotive and are becoming more available as an ‘All In One’ product for general domestic use.
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APPENDICES: Silicones & Sealants
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