TECHNICAL INFORMATION ON BUILDING MATERIALS

FOR USE IN THE DESIGN OF LOW-COST HOUSING

TIBM - 2k

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C,

August 1, 1936.

PLASTIC CAULKING AND POINTING MATERIALS

Tests are now "being conducted at the National Bureau of Standards

to determine the "behavior of representative plastic caulking compounds.

It is hoped that this investigation will afford information as to their

performance and provide a "basis for a specification which will assure

more reliable compositions. As no definite conclusions have as yet "been

reached, this paper will deal with the more apparent characteristics of

the materials.

Nature and Uses ; Plastic caulking compounds are designed mainly

for the weatherproof sealing of suaces "between door or window frames

and masonry, and vertical and horizontal joints of copings and cornices.

They should possess three outstanding qualities: adhesiveness,

plasticity and permanence ; and lesser ones such as durability of color

in harmony with the masonry, a non-staining oil content, and ease of

application.

Their compositions are variable, most of the materials consisting

of non-drying or treated drying oils reduced to a plastic condition

with pulverized silica, whiting, "^srium sulphate, titanium oxide, etc.,

with additions of asbestos fiber an! some coloring matter. Most

materials contain small amounts of drying oil and a volatile solvent,

which cause a film or "skin" to be formed on the surface when exposed

to the air. This is intended to serve as a protection against further

evaporation or oxidation of the oils and is also desirable when the

joint is to be covered with paint.

Plastic caulking materials are of two consistencies, the usual

one being plastic enough to be forced through a l/4 inch or 3/8 inch

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tube and referred to as the "gun grade", and the other of stiffer con- sistency like that of glazing putty, and known as the "knife grade"

,

to he applied with a putty knife or other hand tool.

Service Requirements ; The purpose of these materials is to maintain airtight and watertight joints. The materials must remain appreciably plastic and have good bonding properties in order to function

properly since the widths of joints are subject to change by settlement

of the building, expansion and contraction, and other causes. Most

masonry materials are porous; hence, capillary attraction tends to draw

the oil from the plastic mass and if appreciable amounts of oil are so removed, the composition loses its plasticity and shrinks sufficiently

to form cracks.

Plastic caulking materials must be resistant to such weathering

agents as heat, cold, moisture, and sunlight. Such materials are less

plastic in cold weather at a time when they are subject to more tensile

strains caused by contraction of the masonry. Fillers of a calcareous

nature will, if not protected by the "skin", be attacked by the action

of the acid present in rainwater. Sunlight appears to have some injurious effect on caulking at the exposed surface, and sometimes produces appreciable changes of color.

Application Methods : When caulking around door or window frames,

it is advisable to fill the joint to within an inch of the surface

with oakum or rock wool and the remaining portion with caulking material,

but before doing so, the joints should be clean and free from dust and

dirt. Joints in porous stone masonry should be treated with a waterproofing material of the stearate type which is quite satisfactory for

this purpose and does not discolor the masonry, if some of the treatment

spreads over the exposed face. This treatment tends to prevent the

excessive absorption of oils which is probably the main cause of the

disintegration of the material. The masonry joint to which this material

is applied should be dry, as considerable moisture adversely affects

adhesion

.

The caulking should be forced into the joint to a depth of at

least l/2 inch and preferably 1 inch. Application with a caulking gun

frequently does not obtain such depths of filling, particularly for

narrow joints, unless the nozzle is small enough to completely enter

the void. Application with a putty knife is slow and expensive, but

satisfactory results are more often obtained by this method.

When applied around door or window frames, it is sometimes desirable to paint the caulked joint. This gives a better appearance

and serves to protect the caulking material from the weather. A wood

bead or staff moulding placed around the frames and over the joints

after caulking is considered good practice.

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Stains from Plastic Caulking : Stone masonry is usually more apt

to "be stained "by oils, from caulking materials than brick or concrete;

the denser stones, such as granite and marble, being more susceptible

than the porous varieties. The removal of oil from the plastic usually causes excessive shrinkage, cracking, and ultimate failures resulting

in an objectionable appearance. Oil stains on the more porous tyoes of

masonry usually disappear within a few weeks, but in the denser materials, such as marble and granite, they persist indefinitely.

Slumping of Caulking Materials ; Most of the caulking materials

used are of the "gun grade", which is Quite plastic. In cold weather,

this grade may become so stiff that it would be difficult to force it

through the nozzle of a gun. Manufacturers, in an effort to supply a grade that will work well in cold weather, sometimes make it so plastic

that it will flow or slump somewhat in vertical joints when used in hot

weather. Some producers supuly two consistencies, one for winter use and one for summer use. However, if shelf stocks are carried over from

one season to another, the winter grade may be sold for summer use or vice versa, which may lead to difficulties. Identification of the

proper consistency for a particular season should be made before purchase.

Specifications and Tests : Caulking materials are usually specified

by brand name or purchased without quality requirements. Specifying this

product by composition is not feasible because of difficulties encountered

in identifying its ingredients and further it is fairly well proven that

widely differing comoositions give equally satisfactory results.

A rigorous specification for this material that requires an ex- cellent product without reference to composition is one used by the

Supervising Engineer of the Treasury Department:

"Pointing compound shall be light in color,

elastic and waterproof. It shall not stain

limestone, marble or terra cotta nor corrode

copuer. It shall not be affected by long

exposure to extremes of outside temperatures.

It shall gradually form a thin, tough "skin"

on exposed surfaces, but underneath the

surface, it shall remain plastic indefinitely.

It shall be mixed to the proper consistency at

the factory and shall be used as delivered,"

A test procedure has been devisr d and is now being used at the

National Bureau of Standards on all samples of caulking compounds

submitted by Government departments. This concerns physical tests

entirely and, although it does not determine durability, it does

reject materials of poor early performance. The procedure has been

outlined in detail in a paper published in the Proceedings of the

American Society for Testing Materials, vol . 35, part II, p. 581 (1935).

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