The present invention relates generally to bonding cured polyolefins
to uncured polyolefins. In particular, the present invention relates to forming
a laminate by heat fusing a sheet of cured polyolefin and a heat sealable polyolefin
to produce a substantially water impermeable seal.
Rubbery olefinic polymer sheet material has widespread use in industries
where it is desirable to provide a moisture proof membrane. For example, such
membranes are used to line water reservoirs, waste treatment tanks, sewage lagoons,
irrigation canals and industrial waste pits. Another important application is
in the installation of flat roofs for commercial and industrial buildings.
The most common material used as a roofing membrane is a cured polyolefin
polymer referred to in the industry as EPDM rubber. EPDM rubbers are formulated
from polymers of ethylene, propylene and diene monomers. EPDM is commonly compounded
with various fillers, colorants, antioxidants, extenders, or cross linking agents.
Other membrane materials are available and are formed from butyl rubber, polymers
of ethylene and propylene monomers referred to as EP rubber, and combinations
of the above. Such rubber membranes may be vulcanized by exposure to temperatures
of about 160°C for about two hours. Other membranes are also available which are
formed from PVC plastic, for example.
EPDM rubber roofing membrane is available in sheets formed into 2
to 6 m widths and a variety of lengths. The sheets are typically extruded to a
thickness of 1.50 mm and then vulcanized according to known means. The vulcanized
sheets are stored and shipped in roll form.
For many applications where rubbery membranes are used, it is necessary
to connect a number of sheets together to form a substantially continuous water
impermeable membrane over the surface to be protected. Forming a substantially
water impermeable seam between such membranes has proven difficult.
In the case of roofs, extremely rigorous demands are placed on the
membrane, particularly in the areas where the sheets are spliced together. Roof
temperatures may reach the boiling point of water when exposed to the summer sun,
or may sink to -30°C or below in the winter.
In most applications, the ability of the bond to protect the underlying
surface is critical. For instance, with roof membranes, it is critical to form
a substantially water impermeable joint which is capable of withstanding freeze
and thaw cycles, and will remain intact for the entire life of the roof.
In order to protect the membrane from wind damage, the installed
membrane is typically anchored to the substrate by mechanical means. One anchoring
method includes covering the membrane with ballast consisting of rounded washed
river rock. Another method includes providing battens anchors. Battens anchors
often are positioned at intervals along a sheet, and require mechanical fasteners
to pass through the membrane into the substrate. Piercing the membrane forms a
path for moisture to reach the substrate. Sealing the openings created by the battens
has also proven difficult and time consuming. Another anchoring method includes
installing a flock backed membrane to a substrate covered with a tacky substance
to anchor the flocking.
Apart from the difficulties in installing a roofing membrane, maintenance
is also difficult. It is often necessary to patch areas of such continuous surfaces
to stop moisture from leaking through the membrane.
Along with the problem of extreme weather conditions encountered
on a roof, an additional problem has been present when using EPDM (ethylene, propylene,
diene monomer) rubber for roofing applications. Although EPDM rubber is a very
durable roofing material, its properties are similar to wax in that its surfaces
are slick and have low surface energy. Consequently, EPDM rubber surfaces are resistant
to many adhesives. Much time and effort has gone into developing adhesives which
adhere properly to EPDM rubber and similar membrane materials.
Several methods are known for sealing two surfaces of cured polyolefin
sheets together to form a seam between two adjacent sheets of roofing membrane.
Two known seaming methods include the use of contact cement systems and the use
of pressure sensitive adhesive tape systems. For example, Fieldhouse U.S. Patent
4,480,012 discloses a pressure sensitive adhesive composition formulated for bonding
cured EPDM sheets. The adhesive includes a neutralized sulfonated EPDM elastomeric
terpolymer, an organic hydrocarbon solvent and/or an aliphatic alcohol, a para-alkylated
phenol formaldehyde tackifying resin and alkylphenol or ethoxylated alkylphenol.
The surfaces to be bonded are coated with mineral oil, are abraded, cleaned, coated
with the described adhesive, allowed to dry, and are forced together by means
The use of the adhesive described in the Fieldhouse U.S. Patent 4,480,012
patent requires numerous preparation steps and requires the use of solvents and
mineral oil to prepare and clean the surface prior to sealing. Installing such
roofing joints is tedious, time consuming and costly.
The Fieldhouse U.S. Patent 4,480,012 is merely one example of the
numerous pressure sensitive adhesives developed to connect roofing membranes.
Still other sealing systems utilize primers and paint-on contact adhesives. Each
of the seaming methods described above have proven to be costly, time consuming
and tedious. Many of the earlier developed pressure sensitive adhesive systems
also require the use of a variety of solvents which are believed to cause environmental
and health problems.
Methods of connecting membranes other than by using pressure sensitive
adhesives have also been developed. For example, thermoplastics have been used
as adhesives for connecting membranes. Thermoplastic adhesives do not require
the use of solvent-based primers (except to clean the surfaces to be joined, if
necessary) and therefore do not generate hazardous wastes as a result of installing
a membrane system. Thermoplastic adhesives are also less expensive to manufacture
than contact adhesives and pressure sensitive adhesives. In practice, the process
of developing thermoplastic adhesives suitable for installing in the field has
The Renstrom U.S. Patent 4,767,653 and the Levens U.S. Patent 4,732,635
for example describe the use of a strip of linear low density polyethylene mounted
onto a release liner for applying to the margin of a sheet of EPDM roofing membrane.
The polyethylene is applied by placing the strip on a cleaned surface of cured
or uncured EPDM rubber, the release strip facing away from the EPDM surface. Sufficient
heat and pressure are applied to melt the polyethylene and form a bond between
the polyethylene and the membrane.
The most preferred method of applying the polyethylene strip in the
Renstrom U.S. Patent 4,767,653 and the Levens U.S. Patent 4,732,635 patents is
in the factory prior to vulcanizing. The method includes contacting a polyethylene
surface of the release backed adhesive strip and a surface of an uncured sheet
of EPDM, and subjecting the sheet to 375 kPa pressure at 150°C for several hours
to vulcanize the membrane and at the same time form a bond between the polyethylene
and the membrane.
To connect two membranes having factory installed polyethylene strips
located along the edges of the membranes, a first membrane sheet is positioned
on the surface to be protected such that the polyethylene strip faces up. A second
membrane is positioned adjacent to the first membrane such that the polyethylene
strip faces down and is adjacent to the upward facing polyethylene strip. The membranes
are positioned such that the polyethylene strips substantially overlap. The protective
release liners are removed prior to bonding. Heat is applied either to the polyethylene
surfaces or to the upper surface of the second membrane at a temperature and for
a time sufficient to soften the adhesive strips. The strips are then pressed together,
forming a seal.
The polyethylene strips disclosed in the Levens '635 and Renstrom
'653 patents are most advantageously applied in the factory under controlled conditions.
Unfortunately, unless the membrane is completely bonded to the sheet of polypropylene,
the precise dimensions and placement of each sheet of membrane must be determined
in advance of bonding the polypropylene strips in order for the seams to be formed
where necessary on the job site.
Measurement errors, errors in the placement of particular membrane
sheets, and deviations in dimensions between blueprints and actual structures to
be protected lead the membrane installer to prefer adhesives that can be conveniently
and rapidly applied in the field. Although a membrane could be uniformly coated
on one entire surface with the adhesive film according to Renstrom U.S. Patent
4,767,653 and the Levens U.S. Patent 4,732,635, the cost would be prohibitive.
The present invention is directed to a laminate formed by heat fusing
a heat sealable polyolefin sheet to two cured polyolefin sheets to produce a water
impermeable seal between the cured polyolefin sheets. The cured polyolefin sheets
are typically roofing membranes composed of rubber, preferably formed of ethylene,
propylene and diene monomers (EPDM) or ethylene and propylene monomers (EPM).
The heat sealable polyolefin sheet is preferably selected from low density polyethylene,
linear low density polyethylene, ethylene vinyl acetate, propylene and a blend
of low density polyethylene and propylene. However, linear low density polyethylene
is most preferred.
The heat sealable polyolefin is bonded to a porous backing on one
surface and the other surface of the heat sealable polyolefin is fused to the
cured polyolefin sheets. The porous backing functions as a reinforcing layer and
is preferably formed of heat resistant fibrous material which serves to provide
integrity to the heat sealable polyolefin sheet, especially as the heat sealable
polyolefin sheet is heat fused to the cured polyolefin sheets. The reinforcing
layer is preferably composed of fiberglass, polyester, cellulose, cotton, or nylon
fiber. The reinforcing layer has a melting point higher than the highest temperature
to which the heat sealable polyolefin sheet is heated.
In one aspect of the invention the laminate is formed by heat fusing
a heat sealable polyolefin sheet preferably of linear low density polyethylene
to two adjacent (juxtaposed) cured polyolefin sheets so that the heat sealable
polyolefin sheet covers and seals the adjacent edges of the cured polyolefin sheets.
In another aspect the laminate is formed by heat fusing a heat sealable polyolefin
sheet preferably of linear low density polyethylene to two overlapping cured polyolefin
sheets. In this case the heat sealable polyolefin sheet covers and seals the seam
formed by the overlapping cured polyolefin sheets.
The heat fusing is accomplished preferably by applying heat to the
contact area between the heat sealable polyolefin and the cured polyolefin sheet.
Heat is applied to soften or activate the heat sealable polyolefin or the cured
polyolefin sheet or both sufficiently to fuse the heat sealable polyolefin sheet
to the cured polyolefin sheet. Pressure may be applied to the contacting sheets
to facilitate their fusing to form a substantially water impermeable seal.
Figure 1 is a schematic drawing illustrating a substantially water
impermeable seal formed according to the present invention between a surface of
a sheet of heat sealable polyolefin and contact areas of two sheets of cured polyolefin
having adjacent edges in butting relationship.
Figure 2 is a schematic drawing illustrating a substantially water
impermeable seal formed between a surface of a heat sealable polyolefin sheet and
two overlapping sheets of cured polyolefin.
The present invention is directed to forming a laminate by heat fusing
a heat sealable polyolefin sheet to cured polyolefin sheets to produce a water
impermeable seal. The heat sealable polyolefin is bonded to a heat resistant
porous reinforcing layer on one surface and the other surface of the heat sealable
polyolefin is heat fused to two juxtaposed or overlapping cured polyolefin sheets
to produce a water impermeable seal.
The invention is effected by providing a cured polyolefin sheet having
a surface, at least a portion of which has a contact area and providing a heat
sealable polyolefin preferably having a softening point of at least about 80°C
and a melt index of at least 0.50 dl/g and heating a portion of the contact area
to fuse the heat sealable polyolefin to the cured polyolefin. (The melt index is
determined in accordance with ASTM D-1238.) The heat sealable polyolefin is provided
in an amount effective to at least substantially wet the contact area of the cured
polyolefin sheet. According to the present invention, either the contact area
or a portion of heat sealable polyolefin effective to wet the contact area is heated
to a temperature sufficient to activate the heat sealable polyolefin. The cured
polyolefin contact area and the activated heat sealable polyolefin are contacted
at a pressure and for a time sufficient to form a substantially water impermeable
The following definitions are provided to aid in understanding the
scope and content of the present invention.
A "joint" for purposes of this invention is a substantially water
impermeable connection between at least two sheets a cured polyolefin, or one or
more sheets of heat sealable polyolefin and one or more sheets of cured polyolefin,
including but not limited to butt joints, lap joints, patches, welds and splices.
A "seal" for purposes of this disclosure is defined as an area in
which at least two sheets of material are joined to form a substantially water
impermeable bond. Examples of seals are capped butt seals formed between two sheets
of cured polyolefin, lap joint, seals and patches applied to a damaged surface
of the cured polyolefin.
"Contact area" for purposes of this disclosure is at least a portion
of a surface of a sheet of cured polyolefin which is to be sealed. In roofing
applications, the contact area is commonly an elongated strip extending along a
length of a sheet of cured polyolefin, proximate an edge.
A "bond" for purposes of this disclosure is an interface between
a surface of a heat sealable polyolefin layer and a contact area of a cured polyolefin
which is substantially impervious to liquids.
"EPDM rubber" for purposes of this invention is a polymeric rubbery
material formed primarily of blends of ethylene, propylene and diene monomers.
The most preferred membranes for roofing applications also contain fillers such
as carbon black, colorants, antioxidants, extenders, cross linking agents and other
additives such as mineral oil. A typical rubber roof membrane is approximately
1/3 EPDM resin, 1/3 oil and 1/3 carbon black and other additives.
"Rubber tear bond" for purposes of this disclosure is a bond which
is stronger than the tear strength of the rubber, the rubber delaminating and
failing before the bond being tested. Rubber tear bonds typically have tear strengths
varying from 1.8 to 9.0 Kg/cm.
The term "polyethylene" for purposes of this disclosure includes
linear low density polyethylene products. The polyethylene may also include the
normally employed stabilizers, fillers, extenders, processing aids, pigments,
and the like.
The term "heat resistant" as applied to the porous reinforcing layer
to which a heat sealable polyolefin is bonded, means that the reinforcing layer
has a melting point higher than the highest temperature to which the heat sealable
polyolefin bonded thereto is heated.
The present invention, although useful for any application where
it is desirable to form a substantially water tight bond between at least one
cured polyolefin sheet and a heat sealable polyolefin, is particularly useful in
forming joints in sheets of roofing membranes.
The present invention utilizes at least one cured polyolefin sheet.
The most preferred sheet material is EPDM rubber. Although EPDM sheeting is the
most preferred membrane material for many roofing and lining applications, other
materials such as EP membranes may also be used with the present invention with
similar results. A representative rubber membrane is an EPDM rubber sheeting that
is approximately 1.5 mm thick and is available from Carlisle Syntec Corp. of Carlisle,
Pennsylvania. Although it is believed that the thickness of the membrane is not
important to the present invention, a representative membrane thickness is about
The present invention also includes providing a heat sealable polyolefin
having a melt index of at least 0.5 dl/g and having a softening point of at least
80°C. The most preferred heat sealable polyolefin has a melt index of at least
1.0 dl/g and a softening point of at least 110°C. A melt index of at least 1 dl/g
The most preferred heat sealable polyolefin film is a linear low
density polyethylene (LLDPE) such as is available from the Union Carbide Company
of Danbury, Connecticut, having a softening point (as defined in ASTM Test D-816,
Procedure 19 (January, 1988)) of about 120°C. This polymer is available under
the trade designation "G-Resin 7047 Natural". Other suitable adhesives can be selected
which are thermoplastic blends of polyethylene and polypropylene such as "Tenite"
5321E available from Eastman Chemical Company of Kingsport, Tennessee, homopolymers
of olefin monomers and polymers of two or more olefin monomers, provided that
the above softening point and melt index requirements are met.
It is also desirable that the selected heat sealable polyolefin have
an activation temperature below a temperature which degrades the cured polyolefin
sheet. The preferred EPDM rubber sheets degrade at temperatures of about 250°C
Many other polyolefins would form a suitable heat sealable polyolefin.
For example, ethylene vinyl acetate such as "Elvax" 470 available from the E.I.
du Pont deNemours Company of Wilmington, Delaware and polypropylene such as polypropylene
580A available from the Shell Chemical Company of Houston, Texas are suitable
heat sealable polyolefins.
The heat sealable polyolefin is provided in at least an amount effective
to substantially wet at least a portion of a surface of a cured polyolefin sheet
defined as a contact area.
For smoother surfaces, less heat sealable polyolefin is necessary
to form a suitable bond as compared to rougher surfaces.
Although it would be possible to provide the heat sealable polyolefin
in a particle, pellet, powder or liquid form, the most preferred form of heat
sealable polyolefin is in the form of a film. Films have the advantage of delivering
a substantially uniform amount of heat sealable polyolefin to the contact area,
and also are handled easily.
When a film is used to practice the present invention, the film may
be extruded onto a release liner, extruded directly onto a contact area, or formed
into a sheet by any known means.
Typically the heat sealable polyolefin is heated to a temperature
at or above the softening temperature in order to activate it sufficiently to
cause it to bond to the cured polyolefin.
The heat sealable polyolefin is also preferably selected such that
the temperature which activates the heat sealable polyolefin is at least 50°C
below the temperature at which the cured polyolefin membrane degrades, to protect
the membrane and to compensate for variations in process temperature.
According to a preferred embodiment, a LLDPE film having a thickness
of about 0.13 mm is selected for bonding to a sheet of EPDM rubber roofing membrane.
Films as thin as 0.05 mm would be adequate to form a suitable bond if the finish
on the membrane is sufficiently smooth. The maximum film thickness is determined
by economic considerations, and by the intended function of the film. For example,
if the film were to serve also as a capping strip on a butt weld in addition to
a seal, then the selected film thickness would be much greater. Processing times
also increase with increasing film thickness.
Before the heating step, it is desirable to remove foreign substances
from the contact area of the sheet of cured polyolefin. If the contact area is
free of particulates and other matter such as water and oil, cleaning the surface
prior to applying the adhesive is not necessary. Examples of particulates which
can adversely affect the ability of the adhesive to bond include talc, mica and
dust. The surfaces to be joined may be cleaned by conventional methods such as
by applying solvent, e.g. heptane, to a cloth and wiping the surfaces.
According to the present invention, at least one of the following
group including at least the contact area of the cured polyolefin sheet and at
least an amount of the heat sealable polyolefin sufficient to wet the contact
area is heated to a temperature sufficient to raise the temperature of the contact
area of the cured polyolefin to at least about 160°C, and to activate the heat
sealable adhesive. It is to be understood that this heating may be accomplished
by heating all or a portion of the cured polyolefin sheet and not the heat sealable
polyolefin or by heating all or a portion of heat sealable polyolefin and not the
cured polyolefin sheet, or by heating all or a portion of cured polyolefin and
all or a portion of the heat sealable polyolefin.
Enough heat is provided such that when the heated contact area and
heat sealable polyolefin come into contact, the temperature of the contact area
reaches at least about 160°C, and a temperature is reached sufficient to activate
the heat sealable polyolefin.
As aforementioned, the heat curable polyolefin is preferably formed
of EPDM rubber which does not degrade below 250°C for the time during which bonding
of the roof membrane takes place. It was surprisingly discovered that by attaining
a temperature of at least 160°C on the contact surface of the EPDM sheet, and
preferably at least 177°C, and most preferably at least 205°C, when the contact
area and the activated heat sealable polyolefin are brought into contact, the
resulting bond formed between the heat sealable polyolefin and the EPDM sheet is
sufficiently strong to withstand the extreme weather conditions encountered in
many roofing applications. Although the mechanism is not precisely understood,
it is believed that by raising the temperature of the contact area to at least
160°C bonding is enhanced.
The upper limit of the selected membrane surface temperature is determined
by the membrane material selected and the heat sealable polyolefin selected. It
is necessary to select a surface temperature of the contact area which exceeds
the melt point of the polyolefin adhesive. Preferably, the selected temperature
is at least 10°C greater than the melt point of the selected heat sealable polyolefin.
The manner in which the surfaces are heated is unimportant. According
to the most preferred, method, a "Liberator Series 2000" cap welder available
from G.R. Systems, Inc. of Columbus, Ohio can be used to deliver infrared heat
to the surfaces to be joined, as well as to the adhesive. The seam welder utilizes
a "V" shaped heater capable of delivering infrared radiation and raising the temperature
of both of the surfaces to be joined at the same time to the selected temperature.
An alternative method of heating is by means of hot air or by microwave energy.
According to the present invention, the contact area and the heat
sealable polyolefin are contacted at a pressure and for an amount of time sufficient
to form a substantially water impermeable bond. According to the most preferred
method, a force of about 7 kPa or more is sufficient to achieve intimate contact.
Pressures exceeding about 138 kPa do not form a better bond than pressures within
the preferred range of about 7 to about 138 kPa. Not only is relatively little
force required to affect the bond as compared to known methods, but the amount
of time necessary to apply the pressure is relatively small. Preferably, only
2-3 seconds contact at a pressure of 7 kPa is necessary to form a suitable seal.
According to a preferred method of forming roofing seals, a pressure roller is
used for contacting the contact area and activated heat sealable polyolefin which
delivers a sufficient force of about 7 kPa for about 2 seconds, forming a substantially
water tight seal.
100 parts LLDPE ("G-Resin 7047 Natural 7" with a melt index of 1
dl/g); 0.2 parts ultraviolet absorber, CHIMASSORB 944 LD, commercially available
from Ciba-Geigy; 7.0 parts carbon black/polyethylene blend pigment, DENA-0038BK,
commercially available from Union Carbide Corp.; and 0.10 parts processing additive,
DYNAMAR FX-9613, commercially available from 3M Company were dry mixed for about
15 minutes until uniformly blended. The blend was then extruded at 260°C through
a 0.64 mm slot die and passed through a set of rollers resulting in an extruded
LLDPE sheet having a thickness of 1 mm. After the caliper was accurately set,
a spunbonded polyester reinforcing layer, Reemay Style 2250, commercially available
from Reemay, Inc., was laminated to one side of the hot extruded LLDPE sheet by
passing the sheet and reinforcing layer between a water-chilled/cooled TEFLON
coated rubber top roll and a steel bottom roll just as the LLDPE sheet exits from
the extruder. This allowed the reinforcing layer to be pressed into the soft extruded
LLDPE sheet while keeping the long fibers of the reinforcing layer intact and without
melting the reinforcing layer. The laminated sheet and reinforcing layer was then
allowed to cool at room temperature and was cut into 102 mm wide strips to form
a composite laminate which can be used to heat seal, splice or fuse the cured
polyolefin, e.g. EPDM rubber roofing membrane in accordance with the present invention.
When forming roofing bonds, it is highly desirable to form a seal
which meets the industry standard T-Peel test values. Other criteria used in determining
whether an acceptable bond is formed is a Cold Flex test, a Static Shear test and
most preferably the ability to form rubber tearing bonds. It was surprisingly
discovered that seals formed employing heat sealable polyolefin with a reinforcing
layer according to the present invention exceeded the industry standards in all
of the above tests, and advantageously formed rubber tearing bonds in all cases.
The T-Peel test is described below: T-Peel Test. It is necessary that a
bond formed according to manufacturers' directions in a roofing membrane receive
a minimum initial T-Peel value of at least 0.54 Kg/cm and an "aged value" of at
least about 0.2 Kg/cm. The standard T-Peel test is described in ASTM D-1876 (October,
The standard test is modified by requiring a 5 cm/min constant head
speed on the tension testing machine. In this test, two 2.54 cm wide X 15 cm long
X 1 mm thick strips of commercially available EPDM-based membrane, each provided
with a 38 micrometer layer of heat sealable polyolefin, are placed end-to-end and
are overlapped by approximately 5 cm at one end and are laminated for one minute
in a press at 160°C and at a pressure of 20 kPa. The sample is allowed to age for
7 days at room temperature (about 23°C). Conventional peel tests are then performed
in tensile testing equipment in which the jaws are separated at a rate of 5 cm/min.
In roofing applications, initial values should be at least 0.54 Kg/cm when tested
at room temperature. When subjected to any one of the conditions described below
and then re-tested, the T-peel test values should be at least the minimum values
aforementioned. In all cases, the T-Peel values of seals formed according to the
present invention exceeded the minimum values, and even more surprisingly did
not decrease in strength when subjected to any of the below enumerated "aging"
- a. Weathering Cycle. Lap seam samples are prepared by cutting two rubber sheets,
each measuring 15 cm by 30 cm to make a 30 cm long splice with a 12 cm lap. The
sheets are thoroughly cleaned with hexane and are allowed to dry. The heat sealable
polyolefin is applied to both the contact area of the first sheet and the contact
area of the second sheet according to the present invention (measuring 30.5 cm
by 12.7 cm) forming a test sheet. The test sheet is allowed to age at room temperature
for 7 days and 50% relative humidity (RH) before testing. The test sheet is then
cut along the 15 cm length into 2.54 cm wide strips.
After seven days of aging, 5 pieces are peel tested at room temperature.
Five pieces are subjected to a weathering cycle including four repetitions of
four phases for a total of 28 days. Phase one includes placing the strip in a circulating
air oven set at 80°C for 24 hrs. Phase two includes immersing the same strip in
water at 80°C for 72 hrs. Phase three includes placing the same strip in a freezer
at -18°C for 8 hrs. Phase four includes immersing the same strip in water at 80°C
for 64 hrs. The four phases are then repeated in the same order three more times
to complete the weathering cycle test. Temperature tolerances in the oven and
freezer are ± 2°C. Time tolerances for the weathering cycles are ± 0.5 hrs. Five
samples which are aged but not subjected to the weathering cycle are tested according
to ASTM D-1876 (October, 1972) but modified by using a 5 cm/min constant head
speed and the results are averaged to determine an "initial value." The other five
samples are subjected to the Weathering Cycle and tested according to the same
T-Peel test procedure. The weathered sample test values are averaged to provide
an "aged value." The samples are allowed to equilibrate for 16-32 hrs at room
temperature prior to testing.
Other samples may be made and tested according to the above procedure
and tested to evaluate bonds. The T-Peel criteria listed above also applies to
the tests listed below.
- b. High Humidity. T-peel samples are exposed to 38°C and 100% RH for one week,
are removed, dried and tested at room temperature.
- c. Heat Aging. Samples are placed in a 70°C oven for one week, removed and
tested at room temperature.
- d. Heat Resistance. T-peel samples are heated as in sub-paragraph "e" but are
tested at 70°C.
- e. Weather Resistance. Samples are exposed to the artificial weathering conditions
provided by a "Weatherometer" machine in accordance to ASTM Test D-750 (June,
1985). Tests are performed after 250 and 500 hrs.
- f. Freeze-Thaw Resistance. T-peel samples are immersed in room temperature
water for one week and are then placed in a -18°C freezer for one week. They are
removed and tested at room temperature.
In tests b-f, the formed seal was water impermeable and the formed
bond was a rubber tear bond, i.e., its strength was greater than tear strength
It was surprisingly discovered that each seal formed according to
the present invention had a T-peel strength of at least 1.2 Kg/cm. Even more surprising,
it was discovered that the samples subjected to tests a-f did not have lower aged
values than initial values.
Other tests used to evaluate the quality of the seal include a Cold
Flex test and a Static Shear test.
Static Shear Test. Strips of rubber sheeting 2.54 cm wide
x 15.2 cm long are overlapped 2.54 cm at an end and bonded together. The strip
is then hung vertically in a 70°C oven with a 300 g weight attached to the free
end. Failure should not occur in less than 24 hrs.
Cold Flex Test. Spliced EPDM composite is conditioned at -30°C
for 24 hrs and then wrapped around a 6.4 mm mandrel. No cracking should occur when
flexed. Each of the polyolefin adhesives tested yielded suitable Static Shear
and Cold Flex results provided that the surfaces to be joined were at least at
a temperature of 150°C prior to contacting with the softened film. Each of the
water impermeable seals formed according to the present invention when tested
yielded acceptable results.
The Figures show two ways to practice the present invention.
The present invention may advantageously be used to form a substantially
water impermeable seal over a butt joint. Figure 1 shows two sheets of cured polyolefin
30 and 32, each having an edge which is butted together forming a butt joint 34.
Although Figure 1 shows a butt joint in which there is no space between the edges,
the present invention may also be used when the edges are spaced apart. To form
a substantially water impermeable seal, a portion of each of the upper surfaces
36 and 38 defining contact areas 40 and 42 proximate the butt joint 34 are contacted
with sheet 44 of capping laminate 46. Sheet 44 is formed of a heat sealable polyolefin
according to the invention. The preferred sheet 44 has a thickness of about 1.5
mm and is thick enough to protect the butt joint as well as providing enough heat
sealable polyolefin to form a substantially water impermeable seal. Heat sealable
polyolefin sheet 44 is preferably formed of LLDPE.
Sheet 44 is preferably compounded with UV light absorbers as described
in the Example when laminate 46 or sheet 44 is exposed to outdoor conditions such
as in a roofing application. Such additive need not be included when sheet 44
or laminate 46 is intended to be used for lining a water reservoir where no UV
light is present.
A heat resistant porous backing 45 which functions as a reinforcing
layer is bonded to the heat sealable polyolefin 44, e.g., low density polyethylene
to form a sealing tape or laminate 46 by pressing it into contact with the polyethylene
sheet 44 while hot, that is, just as the sheet 44 exits the extruder. Reinforcing
layer 45 is preferably formed of a sheet of heat resistant fibrous material such
as woven or non-woven fiberglass, spunbound polyester fiber, cellulose fiber,
cotton and nylon fiber. A suitable nylon fiber for the reinforcing layer is available
under the trade designation "CEREX" 4803-23 from James River Corp. A suitable spunbound
polyester is available under the trade designation "Reemay" Style 2250 from Reemay,
Inc. As heat is applied by any of the methods above described, the heat sealable
polyolefin sheet 44 becomes molten and fuses to the two sheets of cured polyolefin
rubber membranes 30 and 32 forming a tight seal along butt joint 34.
Reinforcing layer 45 desirably has an openness or porosity such that
it will allow molten heat sealable polyolefin 44, e.g., molten low density polyethylene
to seep into and through the layer 45 thickness. This provides a strong bond between
reinforcing layer 45 and the heat sealable polyolefin 44. Reinforcing layer 45
holds the heat sealable polyolefin 44, e.g., low density polyethylene, in place
as it becomes molten and thus provides integrity to laminate 46 during subsequent
heating. Reinforcing layer 45 also reduces the chance of the polyolefin sheet
44 splitting if too much heat is accidentally concentrated on the surface of sheet
44. Another advantage of the porous reinforcing layer 45 is that if one heat sealable
laminate 46 is crossed over another like laminate, the molten low density polyethylene
from the top laminate 46 will be able to flow through the reinforcing layer of
the underlying like laminate and thus fuse with the low density polyethylene of
the underlying laminate. This type of crossing is desirable in roofing application
in order to conveniently splice intersecting seams of the cured polyolefin rubber
membranes 30 and 32.
The fiber density of reinforcing layer 45 should be such to as to
create an openness or porosity of at least 20% of the reinforcing layer's exposed
planar surface area. That is, upon viewing the planar top or bottom surface of
reinforcing layer 45, the open area between fibers should amount to at least 20%
of the planar surface area. The percent openness or porosity of reinforcing layer
45 by volume should amount to at least 1% of the total reinforcing layer volume.
Reinforcing layer 45 preferably has a thickness between about 0.025
to 0.5 mm and more preferably between 0.075 to 0.125 mm. Reinforcing layer 45
is preferably of relatively light weight having a basis weight of at least about
6.8 g/m2 and preferably between about 6.8 and 850 g/m2 for
any given thickness between 0.025 to 0.5 mm. Reinforcing layer 45 advantageously
has a melting point at least about 4°C higher and preferably at least about 38°C
higher than the temperature required to fusing the low density polyethylene 44
to the cured polyolefin rubber roofing membranes 30 and 32. The temperature at
which the low density polyethylene sheet 44 is fused to the cured polyolefin membrane
30 and 32 is typically between about 160°C and 232°C. Reinforcing layer 45 desirably
has a tensile strength of 0.35 Kg/cm width in the machine direction (i.e. longitudinal
direction). The reinforcing layer having the above mentioned properties reinforces
the linear low density polyethylene 44 and gives it structural integrity and support
when the polyethylene 44 is heated to a molten state.
Although preferred, it is not necessary that the cured polyolefin
sheets be in a juxtaposed, that is, in a side by side arrangement as in Figure
1. The cured polyolefin sheets 30 and 32 can be overlapping as shown in Fig. 2.
In that embodiment the heat sealable polyolefin 44 is placed over the seam 34a
formed by the overlapping cured polyolefin sheets 30 and 32 so that a portion
of heat sealable polyolefin sheet 44 contacts a portion of the surface of cured
polyolefin sheets 30 and 32 on either side of seam 34a. The heat sealable polyolefin
sheet 44 is bonded to reinforcing layer 45 to form capping laminate 46. The composition
and thicknesses of the heat sealable polyolefin sheet 44 and reinforcing layer
45 as shown in Fig. 2 are the same as described with reference to the embodiment
shown in Fig. 1. If the heat sealable polyolefin sheet 44 of Fig. 2 is formed
of low density polyethylene, sheet 44 is preferably fused to the cured polyolefin
membranes 30 and 32 at a contact temperature typically between about 160°C and