Architectural Polyurethane

Architectural Polyurethane

Main types of construction sealants

Sealants are pasty substances used to seal and waterproof various joints in construction. They cure to be elastic masses so as to withstand movements due to temperatures or moisture variations, wind action, settlement etc.

Sealants protect from penetration of water and water vapor. They also protect from intrusion of noise, fire, dust etc.

Our main interest here is the waterproofing aspects of sealants.

Sealants belong to the same family as adhesives do- with the latter being stronger and less deformable.

Sealants in order to play their role successfully need to have:

* strong adhesion to the substrates- even with the help of a primer and

* adequate elasticity so as to comfortably expand and contract following substrate movements.

It’s absolutely vital that these properties will be kept for long spans of time.

Most times it is extremely difficult to replace failed systems of sealants.

In a few words sealants are used:

a) to bridge gaps (joints)

b) to create some kind of barrier.

SOME TYPICAL JOINTS

- Expansion joints in concrete constructions

- Control joints in floor slabs

- Joints between window frames and masonry

- Joints between panels

- Joints around penetrations

- Joints between corrugated sheets

- Joints around bathtubs

- Joints in glazing assemblies

- Joints in dry construction

- Joints in swimming pools – water tanks (under hydrostatic pressure)

- Floor joints in chemical plants etc. (resistance to chemicals needed)

Main types of sealants used in construction:

* Acrylic sealants

* Bituminous sealants

* Polysulfides

* Polyurethanes

* Silicones

* MS Polymers

MAIN TYPES OF SEALANTS FOR JOINTS WATERPROOFING IN CONSTRUCTIONS

ACRYLIC SEALANTS

Acrylic sealants are massively used in Europe. Although there are solvent based products in the market, most of these products are based on dispersions.

Traditional acrylic sealants can accommodate joint movements in the region of 10-15%. These products are characterised as plastomeric.

Some companies have formulated acrylic sealants with a Movement Accomodation Factor (MAF) of 25%. These new formulations are more like elastomers than plastomers. Things change rapidly!

Generally, acrylics exhibit a very strong adhesion on most construction materials. On porous surfaces a primer prepared from the sealant + water can be used.

Although they possess a fair UV resistance, my personal preference is to use them for internal applications. Furthermore their water resistance is rather poor and they shouldn’t be applied on damp surfaces, floors and generally surfaces which come in contact with water.

Take into account that these formulae shrink by about 15%, maybe more!!

On the other hand they are very easily applied, are user friendly, can be easily overpainted and are available in a big variety of colours.

And they are very – very cheap!!!

Main uses:

* in dry construction to seal and reduce the STC of partitions

* to seal cracks in plasters

* jointing around door and window frames etc.

Addition of silanes in acrylic formulae can considerably expand their application range by improving adherence on non porous surfaces.

BITUMINOUS SEALANTS

Bituminous sealants are mainly used:

- in roofing installations as an accessory for SBS and APP membranes waterproofing

- in civil engineering works: water retaining structures, concrete decks at airports, roadways, pavements etc.

Raw materials – asphalts – come from the fractional distillation of crude oil. Asphalt fractions are modified with the addition of small quantities of elastomers as neoprene, styrene butadiene copolymers etc. Other ingredients could be: fibers – other than asbestos – and fillers as calcium carbonate.

Bituminous sealants are very suitable for permanent contact with water but not suitable for trafficable floors.

Combinations of these products with epoxies give fuel resistant sealants used in tarmacs / airport decks.

Another interesting version is bitumen – urethane or bitumen – polysulfide sealants which possess excellent characteristics at an affordable cost.

Bituminous sealants can be cold or hot applied. Modified products can serve temperature ranges from -30oC to + 90oC.

POLYSULFIDE SEALANTS

USA company Thiokol (greek etymology: glue based on sulfur) invented polysulfides back to 1960s. They come in two-parts or one-part syntheses and are mostly used in civil engineering and glazing. They are considered as the oldest elastomeric sealants. Two components formulae are usually catalysed with manganese dioxide.

Polysulfides bond very well to most of the usual substrates in constructions. Though, they usually need a primer to ensure the right grip and most definitely they need a primer when used under water or in dilatation joints.

Take care: polysulfides are unsuitable for many plastic substrates including PVC, plexiglass, ABS and polycarbonate.

Polysulfides exhibit a natural resistance to UVs, water, solar radiation and other weathering factors.

Elasticity: they have a M.A.F. of 20-25%.

Curing rate: two part polysulfides cure depending thickness, temperature and relative humidity. Their curing rate is considered slow.

Hardness: Depending on use, various formulations can lead to a Shore hardness from 25 up to more than 50.

Polysulfides mains uses are:

- in gas stations and airport tarmacs due to their excellent resistance to fuels

- insulation of glass windows (they are completely vapor proof and possess an enhanced UV resistance)

- sealing of expansion joints in civil engineer projects.

Polysulfide – epoxy combinations are available to be used in projects where high flexibility, excellent adhesion and ultimate chemical resistance are strongly desired.

Polysulfides are in retreat these years due to ecological considerations .

Both silicones and polyurethanes have made serious inroads in the polysulfide market.

POLYURETHANE SEALANTS

Polyurethane sealants may be of one component or two components.

One component polyurethane sealants are widely used worldwide due to their application simplicity and their excellent general characteristics:

- ultimate elongations up to 600-650%

- a high range of elastic moduli to suit all kinds of applications

- elastic recoveries to exceed 95%

- strong adhesion on most substrates in construction industry (primers are recommended according individual uses)

- high resistance to aging

- low shrinkage

- generally overpaintable

- M.A.F.s up to 25% (newest products even better)

- abrasion resistance (good choice for floor joints).

Inclusion of harmful isocyanates is a serious concern.

They cannot be considered user and environment friendly.

Also some other issues of concern are their relatively poor resistance to humidity, UV radiation and most chemicals. (Although some specialized formulae overcome to a certain extent these problems).

Polyurethane sealants come in a big variety of formulae:

- one or two components

- suitable for flooring

- façade compatible

- sealants and adhesives in one product

- pasty or pourable

- lower to higher elasticities

- different Shore hardnesses

- products that can be sanded etc.

One component products polymerize with the aid of environmental humidity, form a superficial skin and thereafter polymerise towards the core at a speed of approximately 2mm/day.

Polyurethane sealants are additionally used in automotive industry and have a good share at the relevant marine market.

Polyurethane sealants will be here for the years to come but they will have a very strong competition: MS polymers sealants.

SILICONE SEALANTS

Silicone sealants are the most universally known sealants. There are many types of silicones and their application range is really vast.

Generally these products exhibit:

- high resistance to water: they are extensively used for construction joints waterproofing

- elevated resistance to temperature variations, very low and high temperatures

- enhanced flexibility which they keep for many years

- the highest Movement Accomodation Factors in the market: could reach 50%.

Silicones are very popular because their cost was largely reduced during recent years and there are many types of them, very well suited to various individual applications.

Namely there exist:

* one or two component products

(latter products used for architectural glazing)

* silicones with different moduli suitable for a wide range of applications

* acetic silicones which during polymerization exhibit the characteristic odour of acetic acid (contain acetoxy groups)

* neutral silicones, oximic or alkoxy with the latter gaining ground due to health considerations pertaining to oxime cured sealants

* pressure sensitive silicone sealants which are permanently tacky

* thermoset silicones activated by heat

* silicones resistant to high temperatures used in gaskets (resistance up to 270oC-300oC)

* mold resistant silicones used in bathrooms + wet rooms

* special silicones for facades that won’t stain natural stonesand marbles

* silicone specially conceived for aquariums (very strong adhesion to glass and safe for fish)

* a huge variety of nuances to suit individual aesthetic preferences.

*Overpaintable silicones which combine excellent waterproofing properties with the desired finishing.

(Generally silicones cannot be overpainted and this is one serious drawback.)

MS POLYMER SEALANTS

The most recent technology in elastomeric sealants. Firstly developed in Japan and now rapidly expanding in USA and Europe. Based on MS Polymers (silyl terminated polyethers) an innovative polymer from Kaneka corporation.

I see MS Polymer sealants as safer, more environmental friendly, more ecological alternatives to polyurethane sealants.

They possess all the excellent mechanical characteristics of polyurethanes plus:

- they don’t contain any isocyanates or other harmful substances

- they have an unparalleled aging resistance

- their adhesion is superb on most surfaces. There is no other sealant with such a brood adhesion spectrum

- are easily paintable (mainly with water based paints)

- have the lowest shrinkage rate

- won’t stain marbles and other natural stones

- are easily extruded even at low temperatures.

It sounds like a miracle and probably is!

MS Polymer based sealants’ use is expanding and their main victim will be polyurethane based sealants.

Chris Strogilis

About the Author

Civil engineer with postgraduate studies in MBA and Marketing.
christos@macon.gr
http://maconwaterproofing.blogspot.com/
http://totalfitness-christos.blogspot.com