Earlier (before 1950s), joints between different materials, such as between glass, metals, wood, concrete, etc. were filled with some traditional caulks, based on:
Oleoresins, such as linseed oil, or
Bitumen and tar in civil engineering work.
These formulations could only withstand a few percent elongation at break, and moreover they had a bad resistance to weathering.
Material
Properties
Linseed oil putties
They contain 10 to 15% of linseed oil filled with mineral fillers (calcium carbonate).
The linseed oil dries by oxidation in the air.
Oxidation continues for the whole life and the product eventually becomes quite hard, brittle, after some years, and non-flexible with very little capability of movement.
They were used mostly for the glazing of glass windows into wood or metal sash.
lmproved oleoresinous putties or caulks
They were based on blown soy or linseed oils, filled with calcium carbonate and fibrous talc, and some plasticizers were added to improve the plasticity (for instance fatty acids, DOP...).
In the best cases, the elongation at break could reach 5% which was not enough for prefabricating techniques.
Bitumen based formulations - ln civil engineering applications, the gaps between parts or works may be quite high, so high performances polymers would be too expensive to fill large volumes. Also, civil engineering people were accustomed to use bitumen and tar.
Therefore, many applications still use bitumen or tar sealants, but the formulations have been often improved, starting in the seventies, by adding rubbers, styrenic polymers such as SBS, or polyurethanes, in small amounts. The pure bitumen or tar compounds may only withstand a few percent elongation at break, and the best-modified formulation may go up to 10 -15% and the movement capabilities in service are only 20 - 25% of the elongation at break to be safe.
The fast development of prefabricated parts in the construction and development of new synthetic polymers resulted in the disappearance of these caulks from the market in the years 1950 to 1975.
Synthetic polymers allow to manufacture high performances sealants with very high elasticity and long durability and could be "tailor-made" to any specific requirement through adequate formulation. Some of the polymer classes are discussed in the table below.
Material
Properties
Polybutene
It is a low molecular weight polymer, which is liquid, tacky, non-drying and cheap.
These polymers are often blended with fillers (calcium carbonate, talc, clays) and fatty acids. A small amount of solvent may be added to control viscosity.
Sealant formulations based on polybutene are set only through drying of the solvent.
They are used in construction to make non-curing sealants for curtain walls, metal to metal joints when elasticity is not important. They are also used to manufacture preformed ribbons and tapes for glazing, bedding compounds in windows.
Polybutenes are frequently mixed with butyl rubber to act as a plasticizer.
Butyl rubber is a copolymer of isobutylene and isoprene. It contains 2 moles percent in saturation.
Butyl rubber is impermeable to gases, it has a fairly good weather and oxygen resistance. It shows some elasticity (elongation at break up to 40%, so that it may be used in joints with movements up to 15%.
Formulations include:
20% butyl rubber,
5 to 10% tackifying resins such as modified or hydrogenated rosin or hydrocarbon resin are necessary to impart good adhesion to metals and glass,
50 to 60% mineral fillers (calcium carbonate, fibrous talc, clay and others), and
20 to 25% solvents such as mineral spirits and other solvents to dissolve and mix all the components and to get the required viscosity.
Polybutene is often added as a plasticizer.
Butyl gun grade sealants may dry and set by evaporation of the solvent and absorption of the solvent into the porous and absorbing substrates (wood, concrete) but there are also curing types which cure by some slow crosslinking after a period of time.
The extruded tapes and ribbons are 100% solids so that there is no shrinking.
Butyl and polyisobutylene hot melt sealants
These are special products which are used as sealants for double (insulated) window sealing against penetration of humidity (into the space between the 2 glass panels).
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