Adhesive containing zinc powder

A novel sealant composition particularly suitable for use in spot welding operations. The novel sealant composition comprises thermoplastic synthetic polymeric material and an amount of finely divided zinc up to about 15 percent by weight of the composition.

THE FIELD OF THE INVENTION 
This invention relates to a new and improved sealant composition, and to 
methods of spot welding using the sealant composition. 
DESCRIPTION OF THE PRIOR ART 
It is known, for example, in the automotive industry, to secure steel and 
other ferrous metal parts together by spot welding processes. It is also 
known to employ a sealant composition between the welded metal parts which 
is intended to prevent penetration of moisture between the parts. If the 
seal is deficient, moisture can penetrate between the welded parts which 
may lead to corrosion. The onset of corrosion has been observed 
particularly in the region of the welds. Accordingly, there is a need for 
a sealant composition which can be used in the spot weld process and which 
can provide a reduced level of corrosion within the joint. 
The use of metallic zinc in compositions for corrosion resistant 
treatments, such as paints is also known. However, in order to obtain an 
adequate level of corrosion resistance, sufficient zinc must be employed 
to insure that the corrosion resistant layer laid down from the 
composition has a substantial amount of zinc at its surface in order to 
provide a corrosion resistant effect. Usually amounts in excess of 60% 
zinc by weight of the composition are used in such corrosion resistant 
compositing. However, the use of such substantial quantities of zinc in 
permanently plastic deformable sealant compositions tends to adversely 
affect the sealing properties of the composition. 
BRIEF SUMMARY OF THE INVENTION 
The novel, improved sealant compositions of this invention are particularly 
suitable for use as spot welding sealants. The sealant compositions are 
based on thermoplastic synthetic polymeric materials and include 
comparatively small amounts--up to about 15 percent by weight of the 
composition--of finely divided zinc. The sealant compositions are 
especially useful in a spot welding process to provide joints which are 
remarkably less prone to corrosion. The compositions can be applied in the 
form of a plastic mass to metal parts and provide a deformable seal bonded 
to the metal parts after the metal parts have been secured together by 
spot welding with the sealant between them.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred sealant compositions of this invention are thermoplastic 
compositions substantially free of solvent and may be applied at elevated 
temperatures e.g., at 100.degree. to 150.degree. C., by means of a pump, 
or at room temperature as a preformed strip of foil. The preferred 
compositions are plastic or deformable to some extent under comparatively 
light pressure at room temperatures of the order of 
20.degree..+-.3.degree. C. and retain this characteristic to a high degree 
during their working life. 
An important performance characteristic of spot welding adhesive 
composition is that the composition not present safety or other hazards 
during handling or use. Temperatures reached during spot welding may be of 
the order of 1000.degree. C. or more and it is important that the sealant 
composition can be used at such temperatures. For example, the sealant 
composition must not degrade completely, nor must it flow from the joint 
to an undesirable extent. 
Another desirable performance characteristic is that the sealant 
composition have adequate tackiness and adhesion characteristics to insure 
desired permanance of bonding of the composition to the metal parts. Some 
ability to flow slightly to take up progressive changes in shape of metal 
parts may also be desirable but it is important that the sealant does not 
sag significantly or flow out of the joint during normal conditions of 
use. Additionally, it is important that the sealant composition does not 
permit transmission of moisture through the joint to a significant extent 
and accordingly a sealant composition of low moisture vapor transmission 
rate is highly desirable. 
Preferred polymeric materials for the sealant composition of this invention 
are polybutenes and polyisobutylenes, although other polymeric materials 
are contemplated, for example, ethylene vinyl acetate copolymers, 
polyethylene, atactic polypropylene, butyl rubber and mixtures thereof. 
These polymeric materials may be compounded with other polymers, e.g. 
thermoplastic block copolymers, tackifying or other resins, for example, 
rosin, rosin ester, hydrocarbon resin (e.g. a betapinene or 
coumarone-indene resin), melamine formaldehyde resins, phenolic resins, 
terpene resins, an other resins known in the adhesives art. Waxes, 
extending oils and plasticisers may also be employed in the composition to 
achieve a sealant composition having a desirable blend of properties in 
application and use as mentioned above. 
Especially preferred compositions are set forth in tabular form below. 
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Percent by Weight of Composition 
Ingredients Range Preferred Range 
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Butyl Rubber 5-50 10-20 
Polybutene 5-50 15-30 
Polyisobutylene 
0-20 4-15 
Tackifying Resin 
5-50 10-20 
Finely Divided 
10-50 20-35 
Inorganic Filler 
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The zinc used is finely divided in order to enable thorough and uniform 
dispersion throughout the sealant composition, and to facilitate heating 
of the zinc during spot welding. While particle sizes and their 
distribution in the zinc do not seem to be especially important, we prefer 
to use zinc powder having a particle size of about 10.sup.-3 mm. It will 
be appreciated that a uniform distribution of the zinc throughout the 
composition is important in order to reproducibly achieve desired 
corrosion protection. 
In carrying out a method according to the invention, metal e.g. mild steel 
parts intended to be spot welded together are prepared and brought 
together, with those parts which are to be spot welded together aligned in 
overlapping relationship. Preparation of the parts is carried out to an 
appropriate extent in accordance with the condition of the metal. Some oil 
on the metal is not detrimental, for example, freshly pressed steel sheets 
for use in manufacture of car bodies normally require no descaling or 
degreasing. However, corroded or heavily oiled steel may require descaling 
and/or degreasing prior to spot welding. Following alignment of the parts, 
a mass of sealant composition is applied to one of the metal parts e.g. as 
a foil, and the parts are pressed together with the composition between 
the parts to provide a foil of composition about 0.5 to 1 mm. thick 
between the parts. The foil is arranged to be of sufficient size to extend 
up to the exposed edge of the joint and to extend between the metal parts 
in those areas which are to be spot welded. 
The parts are clamped in the desired location, the location for spot welds 
is marked, and a conventional spot welding device applied to weld the 
parts together. Each weld is formed through the sealant composition. 
We have found that even small amounts of zinc are beneficial in reducing 
the incidence of corrosion on the steel parts joined by spot welding using 
a sealant composition according to the invention. Amounts as small as 
about 1% properly distributed through the sealant mass show a significant 
effect, amounts of about 5% and about 10% show more significant effects, 
and it appears unnecessary to employ amounts in excess of about 15% by 
weight of the composition in order to achieve a highly acceptable level of 
corrosion resistance. 
This is surprising in view of the previous understanding in the art that 
substantial quantities of zinc are necessary in order to achieve an 
acceptable level of corrosion resistance. The reason for this surprising 
result is not fully understood. However, it is possible that during 
application of heat at the time of spot welding, the organic content of 
the sealant composition immediately adjacent the point of welding is 
decomposed or burned away. Possibly zinc metal is vaporized at this time 
and becomes deposited on the metal immediately adjacent the weld. Perhaps 
a galvanizing of the metal occurs, or possibly an alloying occurs at the 
surface of steel parts rendering the metal parts more resistant to 
corrosion thereafter. 
In order that the invention may be more fully appreciated there now follows 
by way of Example, a detailed description of three illustrative sealant 
compositions containing different amounts of zinc power, and their use in 
spot welding of metal parts. It is to be clearly understood that the 
illustrative compositions and methods have been selected to describe 
preferred embodiments of the invention and are not limitative of the 
invention. 
EXAMPLE 
The Example compositions comprised an intimate mixture of thermoplastic 
synthetic polymeric material, fillers, extender oil and rosin together 
with varying amount of zinc powder. A masterbatch composition was made up 
comprising the following materials. 
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Ingredients Parts by Weight 
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Polybutene 25 
Polyisobutylene 5 
Butyl Rubber 15 
Finely Divided CaCo.sub.3 
30 
Extender Oil 4 
Rosin 15 
Block Copolymer (A-B-A Type) 
5 
Pigment 0.5 
Antioxidant 0.5 
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A comparative Example sealant composition (Composition 0, Table 1) 
consisted of a portion of the masterbatch composition. Illustrative 
sealant compositions (Compositions 1, 2 and 3, Table 1) comprised portions 
of the masterbatch composition, and a further 1, 5 and 10 parts by weight 
respectively of finely divided zinc powder per hundred parts by weight of 
the masterbatch composition. Accordingly, illustrative sealant 
compositions 1, 2 and 3 of Table 1 comprised 1, 4.8 and 9 percent by 
weight of finely divided zinc respectively. 
Four sets of test samples were prepared as follows. Sheets of each sealant 
composition (0, 1, 2 and 3) approximately 0.5 to 1 mm. thick were placed 
between pairs of sheets of steel 25 mm. by 25 mm. which had been degreased 
and descaled prior to application of the sealant. The metal plates were 
pressed together with the sealant between them. A center punch was used to 
mark the location of spot welds on the plates. Spot welds were formed at 
the marks by the method described above. After cooling in a flow of 
nitrogen, excess sealant was removed from the plates. 
The samples were aged at 60.degree. C. and 95% humidity. The bond strength 
of the welded joints was determined by a tensile shear test and the 
progress of corrosion within the joint was assessed by observation at time 
intervals of 1 week, 1 month, 4 months and 8 months. The results are shown 
in Table 1. 
From Table 1 it can be seen that the samples including the illustrative 
composition 3 shown considerably less evidence of corrosion than those 
samples which included comparative composition 0 particularly at the 4 and 
8 month determinations. Those samples including illustrative compositions 
2 and 3 also show an improved corrosion resistance as compared with those 
containing no zinc. 
From an inspection of the values for bond strength given in Table 1 it can 
be seen that the bond strength also is influenced by the zinc content of 
the composition. 
For samples aged for one week or one month, the bond strengths achieved are 
substantially of the same order. However, for samples aged for four or 
eight months, in line with corrosion results, the order of values for the 
bond strengths for samples including comparative composition 0 are less, 
by a factor of about two times than those for samples including 
illustrative composition 3. With samples aged four or eight months, the 
samples including illustrative compositions 1 and 2 also tend to show a 
higher bond strength than the samples including comparative composition 0. 
TABLE I 
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Aging Time Bonding Corrosion Assessment 
at 60.degree. C./95% 
Strength 
Number of 
(No. of Samples Showing Assessed Corrosion) 
Humidity 
Composition 
(Newton) 
Samples Tested 
None Slight 
Distinct 
Intense 
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1 week 0 7500 5 1 3 1 0 
1 7300 5 4 1 0 0 
2 8300 5 2 1 2 0 
3 6700 5 3 2 0 0 
1 month 0 8100 5 1 3 1 0 
1 9400 5 4 1 0 0 
2 8100 5 2 1 2 0 
3 7800 5 3 2 0 0 
4 months 
0 5900 1 0 0 0 1 
1 8600 2 0 2 0 0 
2 8200 2 2 0 0 0 
3 10200 
2 1 1 0 0 
8 months 
0 5400 2 0 0 1 1 
1 7500 5 3 1 1 0 
2 8700 3 2 0 1 0 
3 9100 3 2 1 0 0 
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