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CBD-19. Caulking compounds

Originally published July 1961

Noah is reported to have used asphalt as a sealant in the construction of the Ark. The problem that lie encountered of making structures watertight and weather resistant still requires careful consideration today.

Although asphalt caulkings continue to play an important role, they are but one of a wide variety of this type of building material used extensively in present-day construction. Development of new materials, coupled with new methods of building, bas increased the importance and reliance placed on caulking compounds in preventing entry of water. It is the purpose of this paper to describe the composition and properties of a number of these materials, the significance of tests used in their evaluation and to include recommendations for application that will prolong their usefulness in service.

It is difficult to divide caulking compounds into specific types either for particular applications or on a composition basis because of the range of properties that can be obtained from the many different materials that may be used in making them. It is usual to make two rather broad classifications - one, the mastic and less elastic types of caulking, and the other the more recently developed high molecular weight elastomerics. A more meaningful distinction of the different types from the designer's point of view can be made on the basis of percentage elongation after weathering since it is this property that must be considered first in the selection of a caulking compound for a particular joint application.

The ability to remain in place and to provide a seal in a joint while subject to relatively large distortions is the primary characteristic of caulking materials. It is the amount of movement to be taken up in a joint, in relation to the joint width, which will in the first instance determine the kinds of materials that may be considered for a particular application. Where little or no joint movement is expected, conventional raw linseed oil putties may even be satisfactory. The requirement for taking up moderate movement in a joint of substantial width, which has long been a common one in building, can usually be met by the mastics made from treated drying oils providing about 10 per cent elongation after weathering. Polybutenes, butyl and neoprene rubber types can provide usable elongations up to 50 per cent. Where extreme joint movement must be accommodated in narrow joints it may be necessary to use the highly elastic caulkings such as those made from polysulphide polymers or silicone rubbers which can provide usable elongations of over 100 per cent after weathering.

When it is permissible to vary the width of the joint over a wide range, a choice may be made between the use of a caulking material of moderate or low elongation in a wide joint or of a highly elastic material in a narrow joint. More commonly, other considerations may influence the selection of joint width, and it then becomes necessary to select a material which will provide an appropriate minimum per cent elongation as fixed by the ratio of anticipated joint movement to width.

Properties Required

In addition to the elongation requirements mentioned there are a number of properties that must be inherent in a sealant to enable it to fulfil its function properly. It should be cohesive and adhere well to the material to which it is applied. Workability over a range of temperatures is also essential to ensure proper filling of the joint and a smooth and uniform surface. After application and exposure to the atmosphere it should form a non-tacky, tough, elastic skin over a flexible interior mass. The skin prevents pick-up of dirt and enables paint to be applied. It is essential that in aging a sealant retain its flexible properties with a minimum of shrinkage. Frequently when these materials are applied to porous masonry, nonstaining properties as well as alkali resistance must be present. First-grade caulkings retain these properties and a serviceable life well in excess of 10 years may be expected.

Types

Although linseed-oil putties are used almost exclusively for glazing wooden sash, they qualify as caulkings in the strict sense by serving as a sealant in preventing the entry of water. They have performed well for a long time and large quantities are still used. They are supplied in knife grade consistency only in contrast to a number of other caulking materials used for glazing which are available in a gun grade consistency. Putties are made by mixing thoroughly finely divided calcium carbonate (whiting) and raw linseed oil. They tend to harden with age and become quite brittle, but their useful life can be extended considerably by priming the sash prior to application and by following a good paint maintenance schedule.

The mastic types of caulking are composed of a vehicle, a solvent, driers and mineral stabilizers. If colour is a requirement the appropriate pigments must be included. It is the vehicle that provides the cohesion and sealing properties, usually in the form of drying oils, such as soya or linseed oil, which absorb oxygen from the atmosphere to produce a dry film. These oils have the property of increasing their viscosity upon heating under controlled conditions. It is usual to heat-treat the drying oils in making caulking, since with their heavier consistency greater control is afforded over the tendency to "bleed" into and stain porous surfaces. Non-drying oils are frequently included in the vehicle to plasticize and aid the sealant in retaining its flexibility with age. Driers are added to accelerate the rate of skin formation by oxidation of the drying oils. They are metal salts of either naphthenates or linoleates.

Mineral spirit solvents are used to adjust the workability of a caulking by reducing the viscosity of the vehicle, and aid greatly in the ease of application. Although the addition of solvent on the job may be necessary when caulking is too stiff or when lower working temperatures prevail, an excess of solvent should be avoided. Thinning the oils too much can lead to staining, and later upon volatilization of the solvent to shrinkage and cracking. Mineral stabilizers such as asbestos fibres are added to hold caulking in position, particularly in vertical joints, immediately after application and prior to set. Fine limestone fillers add to stability by reducing shrinkage.

The above materials are oxidizing types of mastics and are used in exposed areas where painting over them may be desirable. They must be used with materials having low thermal coefficients of expansion that will not exert elongation in excess of 10 per cent on the caulking. They are ideal as general sealants in weatherproofing around windows and doors and for sealing joints between wood and masonry.

The polybutenes and asphalt caulking compounds are of the non-oxidizing type. They set through evaporation of the solvent. Both may be obtained in a variety of consistencies. When compounded with fillers and stabilizers they give excellent performance where a skin is not required and retention of plastic consistency is highly desirable. Such applications include use as an underseal with metal flashings, lapped joints and in sealing hidden joints between wood and masonry. In semi-exposed locations and where their solvent can escape, these caulkings harden with accompanying shrinkage. The asphalts may also be blended with semi-drying oils and pigmented to produce a good quality gun grade material and can be used when dark colours are acceptable. Caulkings made with the medium molecular weight polybutenes have usable elongations of 50 per cent and remain soft indefinitely.

High molecular weight elastomeric caulkings include polysulphide polymers, butyl, neoprene, hypalon and silicone rubbers. Of these the polysulphide type bas been used most extensively over the longest period of time. It is a two-component type of sealant consisting of a base compound and an accelerator. When thoroughly mixed just prior to use, chemical curing by polymerization begins; the rate of curing increases with increasing temperature and humidity. This material does not contain any solvent and sets as a soft rubber with no shrinkage. It adheres well to a wide variety of construction materials, and is used extensively in curtain wall construction for sealing critical areas where high elongations may be expected. With age it hardens slowly but has a service life in excess of 25 years. A one-part mastic type polysulphide is also available. It does not cure or harden and is used in locations where little elasticity is required.

Silicone caulking is a one-component type of sealant which cures upon application and exposure to the air. It has excellent adhesion qualities also and can be used where high elongation properties are required. This caulking is available in a number of different colours and is reported to have good resistance to weather with little shrinkage. The butyl, neoprene and hypalon caulkings are solvent types made with fillers and pigments, and are thus available in a range of colours. Butyl and neoprene rubber compounds compare favourably with polysulphide materials despite much higher shrinkage properties. Hypalon materials have many of the desirable characteristics for caulking, but unfortunately little has yet been reported on their long-term performance.

Significance of Tests

Several test methods have been developed that are helpful guides in determining the quality of caulking materials and their suitability for different needs. It is intended at this time to refer only to those that have been accepted by specification organizations such as the Canadian Government Specifications Board as more or less standard methods. Details of various test requirements and methods used are contained in relevant C.G.S.B. specifications listed at the end of this paper. Unfortunately there are no short-term tests that can be used to predict long-term performance with certainty.

Working Quality: A general requirement to assure a uniform, homogeneous mass that can, in the case of a putty, be readily worked with a knife without crumbling or stringing and hold its shape after application. Caulkings must be sufficiently fluid at the time of application to fill a void completely. As a protection, however, against materials that may be too fluid and have a tendency to sag or flow in vertical joints a slump test is used. This is made by placing the material under test in a ½- by ¾-in. channel and measuring any sag that may occur.

Flexibility: Flexing tests are included to determine any tendency of a caulking to lose its adhesion or rupture after exposure to alternate cycles of wetting and a range of drying temperatures. Its ability to retain elongation properties can also be measured and is of special significance in view of these same requirements in service. A test of the ability of caulking to withstand repeated flexing in service for the highly elastic types has also been developed. In this test the caulking is applied to glass and is flexed (100,000 times through % in.) over a range of temperatures. No loss of adhesion is permitted. The test simulates, in an accelerated way, the cyclic conditions in service of dimensional changes that occur in building materials imposed by changes in temperature and moisture content.

Bleeding: This test is included to measure the tendency of caulking oils to migrate when applied to porous surfaces. It is important that these oils be retained in the sealant in order to avoid premature hardening and loss of flexibility. Equally important in some instances is the effect that oil staining may have on exposed masonry.

Shrinkage: Shrinkages in caulkings are associated with loss of oils by migration or volatilization, loss of solvent and evaporation of moisture; excessive shrinkage affects the appearance of exposed joints, adhesion and hardness of the sealant.

Hardness: All caulking compounds have a tendency to harden with time. Volatilization of solvents and light oils, polymerization of drying oils and chemical reactions between the different constituents are some of the factors that may affect hardness and ultimately adhesion. Good resistance to the effect of ultraviolet radiation is necessary to avoid hardening, and is particularly essential with the drying oil types and polysulphides. Improved resistance can be had by appropriate selection of materials in compounding and maintenance after application. One standard method for determining hardness is by the use of a Shore A Durometer hardness gauge. It is usual for new materials, after application and curing, to be in the range of 20 to 35 units of the scale of this gauge. Normal rate of hardening is about one unit per year, so that after 30 years, unless acceleration bas occurred, the material is still quite pliable for use.

Application Recommendations

Despite the emphasis that may be placed on obtaining good quality sealants, the value of good material is lost if application is faulty. There are no exceptions to the requirement that all surfaces to which these products are applied must be dry and clean. Their future performance will be directly related to their initial adhesion. In the design of openings to receive caulkings it is not only essential that they be of adequate size and shape but also that they can be properly inspected for cleanliness.

With regard to the size of a joint, it is essential that movements likely to occur in it have been thoroughly considered. As it is not unusual for ranges of temperature well in excess of 100°F to occur in in any areas of Canada, many examples have been reported of butt joints closing completely during a period of high temperature. The relative positions of metal sections forming a joint under the extremes of conditions likely to occur should therefore be determined. Rabbet dimensions to receive glass and caulking require careful design. The required thickness of caulking to take care of movement caused by temperature and pressure needs to be established to minimize strain and thus lengthen the useful life of the sealant. Many suppliers do not wish their materials to be compressed in excess of 50 per cent regardless of having elongations well in excess of 100 per cent. This is understandable in view of the repetitive nature of joint movements.

With metals it is important that lacquers be completely removed from areas where caulkings are to be placed. This is essential in the case of clear coatings applied to aluminum alloys. Porous masonry or wooden surfaces are improved by priming after cleaning. If more than one type of sealant is to be used in a joint it is essential that they be compatible, for reactions may occur that can seriously stain an exposed face if they are not. Since ultra-violet radiation promotes degradation of so many caulking materials, painting of their surfaces after application may often be desirable.

Conclusion

Caulking materials are required to seal and permit movement under many conditions. They are often the only line of defence against the entry of water and frequently the successful performance of other building materials depends on them. Although the quantities of caulkings used in buildings are comparatively small, their importance is sufficient to warrant the most careful attention with respect to joint design, material selection, use and maintenance.

Specifications of the Canadian Government Specifications Board that relate to caulking compounds are: