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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to gasket sheet materials, particularly of the type
having a corrosion-resistant surface coating on the outside surfaces.
Gasket sheet materials of the prior art generally include a flat steel
sheet as the core material and a joining sheet (ASTM F104 "F112000" or
ASTM F104 "192000"), which is a compound of asbestos or non-asbestos
fibers and synthetic rubber as a surface elastomer. Also as a surface
material a beater sheet (ASTM F104 "F125000"), impregnated, spray coated,
or roll coated with fluorine or a silicone phenol resin is used.
Gasket sheets made of a joint sheet have an advantage of better stress
relaxation properties; however, they have the disadvantage of poor
conformability because of their hard surface material. Moreover, due to
insufficient impermeability of the surface material, the mechanical
properties of the gasket sheet, such as tensile strength, tear strength
and bending strength, are affected adversely. In addition, it has been
found that the binder in the surface material disintegrates from chemical
attack causing damage to the surface material due to corrosion and/or
unwanted adhesion.
While the gasket sheets made of a beater sheet have the advantage of better
conformability, they show rather poor stress relaxation properties.
Surface treatment of gasket sheet is necessary to improve the stress
relaxation properties. Spray coating, which is applied to the beater sheet
gasket to improve its permeability, has the disadvantage that much of the
coating agent is wasted since its recovery is very difficult. Roll coating
has also a disadvantage that the coating cannot be made thick because of
agent transcription by the rolls to which the composite is subjected.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional perspective view of a gasket sheet according to
one embodiment of the invention in which an elastomer and baked film
coating have been applied to an apertured, punched hook metal profile;
FIG. 2 is a cross-sectional perspective view of a gasket sheet according to
another embodiment of the invention in which an elastomer coating and a
baked film are applied to a metal sheet core material; and
FIG. 3 is an elevated perspective view of a curtain flow coater apparatus
used to prepare a sheet gasket in accordance with the process of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention solves the abovementioned problems and is
characterized in that a coating film of a heat and corrosion resistant
sealing agent composed of a heat and corrosion resistant fluid resin, to
which is optionally added a heat and corrosion resistant ceramic or carbon
powder, is baked on the gasket sheet surfaces made of elastomers.
Baked films are formed on the sheared faces of the gasket sheet as well.
The principal ingredients of such heat and corrosion resistant fluid
resins is fluorine. Optionally, molybdenum, epoxy compounds, silicone, or
other suitable materials are included. Illustrative heat and corrosion
resistant ceramic powders include silicon carbide, zirconium, silicon
nitride, and alumina.
A curtain flow coating method is used to apply the coating of the
above-mentioned heat and corrosion resistant fluid resin or the fluid
resin containing heat and corrosion resistant ceramic or carbon powder. In
the curtain flow coating method, a sheet of gasket material passes through
a curtain-like filmy flow of a predetermined width of the coating agent.
Once applied, the coating is then baked in a furnace to form coated films
on the surfaces and sheared faces of the gaskets. This process is
described in more detail below in connection with the discussion of FIG.
3.
Since relatively insufficient coating material is applied to the departure
or exit side of the sheared faces, it is necessary to reverse the approach
direction of the gasket when coating the reverse side in the curtain flow
coating method. The curtain flow coating method is capable of reducing
consumption of the coating agent by approximately 70% in comparison with
the spraycoating method. By selecting the width of the fluid curtain flow
and movement speed of the gasket sheet passing through the fluid curtain
flow, the thickness of the coated film on the surface of the gasket sheet
can easily be adjusted in comparison with roll coating procedures.
DETAILED DESCRIPTION OF THE DRAWINGS
Various embodiments of the invention are now described referring to the
attached drawings, it being understood that the invention is not limited
to the specific embodiments that are illustrated. Referring to FIG. 1 of
the first embodiment, a core material with punched hooks 4 is provided
inside an elastomer 1 of the gasket sheet and a baked film 2 5-100 microns
in thickness is formed on the surface of the elastomer 1. In this
embodiment, a coating film 3 is also provided on all the sheared faces
(cut edges). Fluorine, the main ingredient of the heat and corrosion
resistant fluid resin, is applied to the coating film as a heat and
corrosion resistant sealing agent. Compositions of the core material, film
coating and elastomer are given below. Optionally, the heat and corrosion
resistant fluid resin also contains at least one heat and corrosion
resistant ceramic powder, such as silicon carbide, zirconium, silicon
nitride, alumina, or possibly other suitable powders, or carbon powder
(ASTM N990 or N991). Carbon powders and ceramic powders are not used in
combination. The amount of powder, be it ceramic or carbon, is in the
range of from about 3 to 10% by weight based on the weight of the resin.
The exact amount depends upon the nature of the gasket. This mixture is
used as a heat and corrosion resistant sealing agent for the coating film.
The heat and corrosion resistant fluid resin, or the fluid resin containing
a heat and corrosion resistant ceramic powder, or containing carbon
powder, is applied by the above-described curtain flow coating method. The
thus coated article is then baked in a furnace at the appropriate
temperature in accordance with the curing requirements of the fluid resin
composition to form the heat and corrosion resistant sealing films on the
surface and sheared faces of the gasket sheets.
Referring to FIG. 2 of the second embodiment of this invention, a core
material 4' made of a solid, imperforate metal sheet is provided inside
the elastomer of the gasket sheet. The baked film 2 is formed on the
surface of the elastomer 1 in the above-mentioned manner and the film 3
applied to all the sheared faces.
The fluid resin or the fluid resin containing ceramic powder or carbon
powder employed is the same as that used in the first embodiment of the
invention as described above.
The thickness of the coating applied depends upon several factors, but will
usually fall within the range of about 5 to 100 microns. As a practical
matter, a thickness in the range of from about 25 to about 40 microns is
recommended for most sheet gaskets.
FIG. 3 illustrates a curtain flow coating apparatus for carrying out the
process of the present invention. In the process, a gasket sheet 6 having
the elastomer layer 1 on it is passed beneath and through a flowing
curtain 5 of the coating agent. Gasket sheet 6 is carried on a conveyor
10. Curtain flow of a preset width is maintained by recirculating excess
coating agent from the coating agent reservoir tank 8 via a circulating
pump 9 to the head distributing the coating agent. The speed of traversal
of the gasket sheet 6 through the flowing curtain together with the flow
rate and thickness of the fluid curtain flow 5 are both controlled, one
necessarily related to the other, to provide a film coating 2 of the
desired thickness.
The reverse side is also coated in a second pass; however, in order to
ensure even coating of the exposed edges, the gasket sheet is inverted
end-over-end so that the end to be last coated in the first pass is the
end receiving the first coat in the second pass.
Baking times and temperatures depend on the nature of the coating film,
underlying material (such as an elastomer (if present), and the like. For
a Xylan 1010HB resin system coated on an elastomeric base, the baking time
is 240.degree. C..+-.10.degree. C. for about 10 minutes.
The procedure of the present invention: A baked film of heat and corrosion
resistant sealing agent formed on the surfaces of the gasket sheet
maintains the gasket's sealability and impermeability for a substantially
prolonged time. Accordingly, it gives good protection against corrosion of
a core sheet, elastomer, and binder, and prevents deterioration of the
mechanical properties of the gasket sheet itself, such as tensile
strength, tear strength, and bending strength. Also it avoids the direct
contact of the gasket material, which is mainly binder, with the mating
sealing surfaces to prevent sticking.
Its better wear-resistant ability also reduces the fretting phenomena of
sealing metal surfaces. The coating applied by the curtain flow coating
method enables satisfactory recovery of the coating agent and saves 70% of
agent consumption as compared with the spray coating method.
Compositions of The Coating Agent: The fluorine-containing resin used for
the baked film was Xylan 1010HB, a product of the Whitford Corporation of
West Chester, Pa. As the binder for the underlying elastomeric sheet 1 was
a synthetic rubber, the baking or curing temperature for the baked film
coating 4 was at no greater than about 250.degree. C. Other resins
believed to be suitable are various grades of TEFLON resins from duPont of
Wilmington, Delaware or POLYPRON of Daikin, Tokyo, Japan; however, the
baking temperature required for the resin must not be greater than the
maximum temperature the underlying elastomer 1 is capable of tolerating
without deterioration in properties. No particular temperature
considerations need be made for laminated metal gaskets, graphite gaskets,
or ceramic gaskets; however, with elastomer-containing gaskets, Xylan
1010HB is preferred.
The joint sheet material onto which the coating films of the present
invention are applied are typically asbestos fiber reinforced or, more
recently, non-asbestos fiber reinforced. Typical formulations are for an
asbestos joint sheet in approximate weight percent:
asbestos: 75%
binder (EPDM rubber): 15%
other additives (reinforcing agents, antioxidants, antiozants, etc.) 10%
while an illustrative non-asbestos joint sheet contains:
reinforcing fibers (KEVLAR polyamide): 75%
binder (EPDM rubber): 15%
miscellaneous additives (as above): 10%
While the above description has featured elastomer-containing, metal
reinforced gaskets, other types of flat sheet gaskets are considered
suitable for the coating process. These include laminated metal gaskets,
graphite gaskets and ceramic gaskets.
Test Results: Products produced according to the present invention have
been tested using the following test pieces:
The test pieces have a structure that consists of a core material of a flat
steel sheet and asbestos joint sheets bonded on both sides of the sheet.
They have the coated films of polytetrafluoroethylene (PTFE) resin,
processed by the curtain flow coating method and baked at 220.degree. C.
in a furnace for 10 minutes. The baked film has a thickness of 50 microns,
are annular in shape, and measure 55 mm outside diameter (O.D.) and 32 mm
inside diameter (I.D.) by 1.35 mm thick.
TEST 1
The above-described test piece was attached to a testing jig to which
pressure was applied. The testing jig included a first metal block
provided with an opening 32 mm in diameter to simulate a cylinder block of
the internal combustion engine, and a second metal block and having a flat
surface covering the first metal block.
The jig was so constructed that a pressurized fluid may be filled in the
opening of the first metal block through a duct. The test piece was
interposed between these two blocks and the tightening torque was set at
40 kgf/cm.sup.2. Engine oil (ASTM #3) was used as a pressurizing fluid.
The temperature of the oil was set at 140.degree. C. and the pressure was
raised at a rate of 0.5 kgf/cm.sup.2 every 20 minutes up to the maximum
pressure of 10 kgf/cm.sup.2 was reached.
The test piece was kept under the maximum pressure for 6 hours and 40
minutes. No leakage of the fluid was observed. When the test piece was
taken out and visually checked, there was no infiltration of the fluid
into the material and the material could be easily removed from the
testing jig without sticking.
For comparison, a similar test was conducted using a test piece of
conventional construction which did not have the baked film of the
invention. The test piece became stuck to the surface of the metal block
of the testing jig and it was difficult to release the test piece from the
jig. There was a visual observation of oil infiltration on the surface of
the material of the test piece in this case.
TEST 2
A test similar to the above Test 1 was conducted using a mixture of
automotive anti-freeze and water as the pressurizing fluid. The
temperature of the anti-freeze and water mixture was set at 95.degree. C.
The pressure was applied up to the maximum of 3 kgf/cm.sup.2 by increasing
the same at a rate of 0.5 kgf/cm.sup.2 every 20 minutes. The maximum
pressure was maintained for 8 hours. There was no leakage of the fluid
observed. When the test piece was taken out and visually checked, there
was no infiltration of the fluid inside the material and it could be
easily removed without sticking from the testing jig.
For comparison, a similar test was conducted using a test piece with the
conventional structure which was not provided with the baked film of the
invention. This sample became stuck to the surface of the metal block of
the testing jib. When the test piece was removed from the testing jig, it
suffered considerable damage.
TEST 3
Three gasket sheets according to the first embodiment of the invention were
used as test pieces. Three conventional gasket sheets without a baked
film, though of a similar configuration, were also prepared. These six
gasket sheets were immersed in engine oil at 140.degree. C. for 3.5 hours
and the samples were weighed to determine the change in weight before and
after the test. The result was that the weight increase was 1.31-1.44%
with the test pieces according to the invention on the surface of which
was performed a baked film, which the weight incrase was 1.77-2.15% with
the test pieces with the conventional structure.
Each gasket sheet was cut and the change in the condition of the cut face
was visually observed. Infiltration of black engine oil was observed in
the conventional gasket sheets while no such infiltration was observed in
the gasket sheets according to the present invention.
TEST 4
A test was carried out by mounting the gasket sheet of the first embodiment
to a turbocharged diesel engine, developing 195 HP, in a dynamometer cell.
The engine was subjected to 50-hour continuous operation by
accelerating/decelerating the speed every 2.5 minutes from idling (500
rpm) to full load (3,000 rpm). The temperature of the gasket sheet during
the operation was substantially at 180.degree. C. The cylinder head of the
engine was then removed and the test piece was inspected. The test piece
could be completely removed from the engine. No damage or infiltration of
oil was visually observed.
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