WO2018018483A1

WO2018018483A1 - Flexible lightweight fire barrier

Info

Publication number: WO2018018483A1
Application number: PCT/CN2016/092000
Authority: WO
Inventors: Xuedong Li
Original assignee: E. I. Du Pont De Nemours And Company
Priority date: 2016-07-28
Filing date: 2016-07-28
Publication date: 2018-02-01
Also published as: CN109563703A

Abstract

A lightweight fire barrier that comprises a flame-resistant coating containing expand-able graphite. The fire barrier has improved flame resistance and is flexible without undesired delamination. The fire barrier cannot be burnt through when the surface with coating is facing toward the fire, and remains flexible without cracking after 10 minutes of flame exposure.

Description

Flexible Lightweight Fire Barrier

FIELD OF THE INVENTION

This invention relates to lightweight fire barriers comprising a flame-resistant coating.

BACKGROUND OF THE INVENTION

In many cases such as emergency military and firefighter shelters， mass transportation vehicles especially aircrafts， there is a need for lightweight flame-resistant barrier. US Patent No. 6,322,022 to Fay et al. discloses burnthrough resistant systems. US Patent No. 6,670,291 to Tomkins and Vogel-Martin describes a laminate sheet material for fire barrier applications. US Patent Nos. 8,292,027 and 8,387,748 to Richardson and Knoff disclose composite laminate for thermal and acoustic barrier comprising a layer of inorganic platelets (refractory layer) . The shortcomings of the previous solutions-composite laminates are heavy， generally with an basis weight of 150-300 g/m²， and high stiffness (inflexibility) ， that fail to offer tight covering for the non-flat surfaces as well as prohibit folding on storage.

Accordingly， it is desirable to provide flexible fire barriers having excellent flame-resistance with the lightest possible material.

SUMMARY OF THE INVENTION

This invention provides a fire barrier having improved burnthrough resistance， comprising：

a flexible substrate composed of polyimide， and

a flame-resistant coating containing expandable graphite，

wherein

the flame-resistant coating is derived from a curable coating composition comprising：

(a) about 10-50 weight ％of expandable graphite， and

(b) about 50-90 weight ％of silicone elastomer，

wherein the weight ％is based on the total weight of the curable coating composition； and

the flame-resistant coating is present on at least one surface of the flexible substrate.

This invention also provides a method of preparing the fire barrier described above， comprising：

i. providing a flexible substrate and a curable coating composition；

ii. applying the curable coating composition onto at least one surface of the flexible substrate；

iii. curing the coated substrate of step ii by heating or radiation to obtain the fire barrier；

wherein

the curable coating composition， based on the total weight of the curable coating composition， comprises：

(a) about 10-50 weight ％of expandable graphite， and

(b) about 50-90 weight ％of silicone elastomer.

DETAILED DESCRIPTION OF THE INVENTION

All publications， patent applications， patents and other references mentioned herein， if not otherwise indicated， are explicitly incorporated by reference herein in their entirety for all purposes as if fully set forth.

Unless otherwise defined， all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict， the present specification， including definitions， will control.

Unless stated otherwise， all percentages， parts， ratios， etc.， are by weight.

As used herein， the term “produced from” is synonymous to “comprising” . As used herein， the terms “comprises， ” “comprising， ” “includes， ” “including， ” “has， ” “having， ” “contains” or “containing， ” or any other variation thereof， are intended to cover a non-exclusive inclusion. For example， a composition， process， method， article， or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition， process， method， article， or apparatus.

The transitional phrase “consisting of” excludes any element， step， or ingredient not specified. If in the claim， such a phrase would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase “consisting of” appears in a clause of the body of a claim， rather than immediately following the preamble， it limits only the element set forth in that clause； other elements are not excluded from the claim as a whole.

The transitional phrase “consisting essentially of” is used to define a composition， method or apparatus that includes materials， steps， features， components， or elements， in addition to those literally discussed， provided that these additional materials， steps features， components， or elements do not materially affect the basic and novel characteristic (s) of the claimed invention. The term “consisting essentially of” occupies a middle ground between “comprising” and “consisting of” .

The term “comprising” is intended to include embodiments encompassed by the terms “consisting essentially of” and “consisting of” . Similarly， the term “consisting essentially of” is intended to include embodiments encompassed by the term “consisting of” .

When an amount， concentration， or other value or parameter is given as either a range， preferred range or a list of upper preferable values and lower preferable values， this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value， regardless of whether ranges are separately disclosed. For example， when a range of “1 to 5” is recited， the recited range should be construed as including ranges “1 to 4” ， “1 to 3” ， “1-2” ， “1-2 &4-5” ， “1-3 &5” ， and the like. Where a range of numerical values is recited herein， unless otherwise stated， the range is intended to include the endpoints thereof， and all integers and fractions within the range.

When the term “about” is used in describing a value or an end-point of a range， the disclosure should be understood to include the specific value or end-point referred to.

Further， unless expressly stated to the contrary， “or” refers to an inclusive “or” and not to an exclusive “or” . For example， a condition A “or” B is satisfied by any one of the following： A is true (or present) and B is false (or not present) ， A is false (or not present) and B is true (or present) ， and both A and B are true (or present) .

“Mole％” or “mol％” refers to mole percent.

In describing and/or claiming this invention， the term “homopolymer” refers to a polymer derived from polymerization of a single repeating unit； the term “copolymer” refers to polymers resulting from copolymerization of two or more repeating units.

As used herein， the term “fiber” is defined as a relatively flexible， elongate body having a high ratio of length to the width of the cross-sectional area perpendicular to that length. The fiber cross section can be any shape such as circular， flat or oblong but is typically circular. The fiber cross section can be solid or hollow， preferably， solid.

The term ″nanofibers″ as used herein refers to fibers having a number average diameter less than 1000 nm， even less than 800 nm， even between about 50 nm and 500 nm， and even between about 100 and 400 nm. In the case of non-round cross-sectional nanofibers， the term ″diameter″ as used herein refers to the greatest cross-sectional dimension.

Embodiments of the present invention as described in the Summary of the Invention include any other embodiments described herein， can be combined in any manner， and the descriptions of variables in the embodiments pertain not only to the composite laminate of the present invention， but also to the articles made therefrom.

The invention is described in detail herein under.

(a) flexible substrate

According to the present invention， the flexible substrate (a) is composed of polyimide. Polyimide is a class of thermally stable and non-flammable polymers， and commercially available polyimide films are flexible with a thickness ranging from about 5 μm to about 150 μm， thus make it an excellent material for use as the flexible substrate (a) for the present fire barrier.

From the viewpoint of practical use of the present fire barrier， the thickness of the flexible substrate (a) is preferably about 5-150 μm， or about 10-130 μm， or about 15-110 μm.

In accordance with the present disclosure， the basis weight of the flexible substrate (a) is preferably 10-50 g/m²， or 12.5-40 g/m²， or 15-30 g/m².

Polyimide typically is derived from diamines and dianhydrides or their precursors-tetracarboxylic acids. The term ″fully aromatic″ when applied to polyimide or polyamic acid means that the monomers from which the polyamic acid are produced are aromatic. Examples of the aromatic diamines include， but not limited to， 1， 4-phenylenediamine (PPD) ， 1， 3-phenylenediamine (MPD) ， 4， 4’ -oxydianiline (ODA) ， 1， 3-bis (4-aminophenoxy) benzene (RODA) ， and 3， 3’ -methylene-bisbenzenamine. Examples of the aromatic dianhydrides include， but not limited to， pyromellitic dianhydride (PMDA) ， 3， 3’ ， 4， 4’ -biphenyl-tetracarboxylic dianhydride (BPDA) ， 1， 4， 5， 8-naphthalene-tetracarboxylic dianhydride (NTDA) ， 4， 4’ -oxydiphtalic dianhydride (ODPA) ， and 3， 3’ ， 4， 4’ -benzophenonetetracarboxylic dianhydride (BTDA) .

The aforementioned aromatic diamines and aromatic dianhydrides may be used singly to form a polyimide homopolymer， or in the form of a mixture of two or more monomer units to form a polyimide copolymer. When the polyimide copolymer is adopted， the aromatic dianhydrides constituting the copolymer may be prepared from one or more compounds selected from the group consisting of PMDA， BPDA， NTDA， ODPA， and BTDA. Alternatively， the aromatic diamines constituting the polyimide copolymer may be prepared from one or more compounds selected from the group consisting of PPD， MPD， ODA， RODA， and 3， 3’ -methylene-bisbenzenamine.

As used herein， the polyamide homopolymers or copolymers are designated by the repeating monomer units， where the “//” is used to distinguish the aromatic dianhydride (s) and the aromatic diamine (s) ， and the “/” is used to distinguish monomers of the same functional groups. For example， the polyimide homopolymer derived from PMDA and ODA may be designated as “PMDA//ODA” ； the polyimide copolymer derived from BPDA and a mixture of PPD and MPD may be designated as ” BPDA//PPD/MPD. ” The major copolymerized unit of a polyimide copolymer may be present in the copolymer at least about 70 mol％， or about 80 mol％， or about 90 mol％or more.

In one embodiment， in the present fire barrier， the flexible substrate (a) comprises polyimide derived from at least one aromatic dianhydride selected from the group consisting of PMDA， BPDA， NTDA， ODPA， BTDA， and combinations thereof； and at least one aromatic diamine selected from the group consisting of PPD， MPD， ODA， RODA， 3， 3’ -methylenebisbenzenamine， and combinations thereof.

In another embodiment， in the present fire barrier， the flexible substrate (a) comprises fully aromatic polyimide derived from aromatic dianhydride selected from PMDA， BPDA and combinations thereof； and aromatic diamine selected from ODA， RODA， and combinations thereof.

In a further embodiment， the present flexible substrate (a) comprises polyimide derived from at least 70 mol％to 100 mol％of the monomer units that is PMDA//ODA or BPDA//RODA.

The widely practiced procedure in polyimide synthesis requires first forming polyamic acid. It involves reacting any of the aforementioned aromatic dianhydrides and any of the aforementioned aromatic diamines in a polar aprotic solvent to yield the corresponding polyamic acid (s) ， which is the polyimide precursor solution.

Examples of polar aprotic solvent include amides such as N-methyl-2-pyrrolidinone (NMP) ， N， N-dimethylacetamide (DMAc) ， N， N-diethylacetamide， N， N-dimethylformamide (DMF) ， N， N-diethylformamide， and hexamethylsulfonamide； sulfoxides such as dimethyl sulfoxide and diethyl sulfoxide； and sulfones such as dimethyl sulfone and diethyl sulfone. These solvents may be employed singly or in combination.

The amounts by mole of these aromatic dianhydrides and aromatic diamines in any of the aforementioned solvent are substantially the same， or the amount by mole of one of these components slightly exceeds that of the other component， and by allowing reaction of the resultant mixture to proceed at a temperature of preferably 100℃ or lower (more preferably 80℃ or lower) for about 0.2 to about 60 hours to obtain the corresponding polyamic acid (PAA) solution， i.e. the polyimide precursor solution.

In the present fire barrier， the flexible substrate (a) may be in the form of a polyimide film or a polyimide nanoweb.

The total monomer concentration of the PAA solution may be appropriately determined in consideration of the production methods for the polyimide film or the polyimide nanoweb. The polyimide film may be obtained by casting the PAA solution onto a surface of an appropriate support (e.g.， a metallic， ceramic， or plastic roller， a metallic belt， or a roller or belt onto which a metallic thin film tape is being supplied) by means of， for example， a die， to thereby form a film having a uniform thickness.

Subsequently， the casted film is heated through a thermal cycle with temperatures ranging from about 100℃ to about 500℃， or about 150℃ to about 450℃， until at least 90％of the amic acid functionality has been converted to imide functionality (i.e. imidization) ， where the thermal cycle may be appropriately determined by a skilled artisan. The thermal imidization may be carried out by means of any heating apparatus such as a hot air furnace or an infrared heating furnace. Preferably， the flexible substrate (a) is a polyimide film has a degree ofimidization (DOI) of at least 90％， or 95％， or 99％， or 100％.

The ″degree of imidization″ (DOI) is defined herein as the ratio of the infrared IR absorbance of the imide C-N stretch (typically located at or near 1375 cm-1) to the absorbance of the aromatic C-NH stretch (typically at or near 1500 cm-1) . For any given monomer pair (repeating unit) ， the process for determining DOI involves the steps of running and IR spectrum and establishing the exact location of these absorbances.

Polyimide nanowebs may be prepared by the process described in detail by T. J. Dennes et al， in U.S. Patent Application Publication No. 2013/0005940.

The term ″nanoweb″ as applied to the present invention refers to a nonwoven web constructed by greater than 50％of the fibers in the web being nanofibers. The nanoweb of the invention can also have greater than 70％， or 90％， or it can even contain 100％of nanofibers.

The term ″nonwoven″ means here a web including a multitude of randomly oriented fibers. By ″randomly oriented″ is meant that the fibers have no long range repeating structure discernible to the naked eye. The fibers can be bonded to each other， or can be unbonded and entangled to impart strength and integrity to the web.

The PAA solution is formed into a nanoweb by electroblowing， as described in detail in U.S. Patent Application Publication No. 2013/0005940 A1. The electroblown nanoweb may be calendered first. The dried and calendered nanoweb may then be thermally imidized at a temperature ranging from about 200℃ to about 500℃ for a period of 5 seconds to 10 minutes.

Alternatively， the electroblown nanoweb may be thermally imidized at a temperature ranging from about 200℃ to about 500℃ for a period of 5 seconds to 10 minutes， and calendered subsequently.

″Calendering″ is the process of passing a web through a nip between two rolls. The rolls may be in contact with each other， or there may be a fixed or variable gap between the roll surfaces. Advantageously， in the calendering process， the nip may be formed between a soft roll and a hard roll. The ″soft roll″ is a roll that deforms under the pressure applied to keep two rolls in a calender together. The ″hard roll″ is a roll with a surface in which no deformation that has a significant effect on the process or product occurs under the pressure of the process. An ″unpatterned″ roll is one which has a smooth surface within the capability of the process used to manufacture them. There are no points or patterns to deliberately produce a pattern on the web as it passed through the nip， unlike a point bonding roll. The calendaring process may also use two hard rolls.

Imidization of the polyamic acid nanoweb so formed may conveniently be performed by first subjecting the nanoweb to solvent extraction at a temperature of about 100℃ in a vacuum oven with a nitrogen purge； following extraction， the oven is then heated to a temperature of about 200 to about 500℃ such that the nanoweb is heated thereby for at least 5 seconds， or about 10 minutes or less， preferably 5 minutes or less， more preferably 2 minutes or less， and even more preferably 1 minute or even 30 seconds or less， to sufficiently imidize the nanoweb.

In one embodiment， the polyimide nanoweb consists essentially of polyimide nanofibers formed from the monomer units： pyromellitic dianhydride (PMDA) and oxy-dianiline (ODA) .

Suitable polyimide films and polyimide nanowebs may be obtained from commercial sources， for example， under the trade nameNH from E. I. Du Pont de Nemours and Company (Wilmington， DE， USA) .

a flame-resistant coating

In accordance to the present disclosure， the flame-resistant coating (b) is derived from a curable coating composition comprising a silicon elastomer and expandable graphite.

As used herein， a “silicone elastomer” is a silicone composition when subsequently cured or vulcanized， provides a silicone elastomer or rubber. Silicone refers to organopolysiloxanes containing siloxane units independently selected from (R₃SiO_0.5) ， (R₂SiO) ， (RSiO_1.5) ， or (SiO₂) siloxy units， where R may be any organic group. These siloxy units can be combined in various manners to form cyclic， linear， or branched structures. The silicone elastomer useful in the curable coating composition， may be selected from any silicone elastomer known in the art， such as those considered as high consistency silicone rubbers. The silicone elastomer may also be derived from those considered to be “liquid silicone rubbers” (LSRs) .

LSRs typically consist of a methylhydrogensiloxane as a cross-linking agent and a silicone polymer containing vinyl groups that vulcanize under the catalysis of a noble metal catalyst (e.g.， Pt (0) complexes) by means of a hydrosilylation reaction are usually employed. The LSR may include inhibitors for hydrosilylation reactions， especially those catalyzed by platinum based catalysts， for example， acetylenic alcohols such as 3-methyl-1-butyn-3-ol， 3， 5-dimethyl-1-hexyn-3-ol， 1-ethynyl-1-cyclohexanol， and phenylbutynol； ene-yne compounds such as 3-methyl-3-penten-1-yne， and 3， 5-dimethyl-3-hexen-1-yne； tetramethyl-tetrahexenylcyclotetrasiloxane； and benzotriazole. The hydrosilylation reaction inhibitor is preferably present in the LSR formulation in a proportion sufficient to ensure that the resulting coating cures quickly enough as well as allow well mixing with the expandable graphite.

Commercially avaible LSRs typically are sold in two-part formulation that may have a specific viscosity after mixing the two parts at a specified ratio. The silicone polymer containing vinyl groups (e.g.， alkenyl organopolysiloxane) and the noble metal catalyst are usually contained in the first part of the formulation， and the organohydrogensiloxane and/or the hydrosilylation reaction inhibitor are contained in the second part. The commercial source of the LSRs typically may suggest a weight ratio between the two parts， that may range from 10∶1 to 1∶1. Preferably， the silicone elastomer is liquid silicone rubbers that are suitable for fabric coating.

Exemplary SLRs in 2 parts formulation are commercially avaible from Dow Corning Co.， under the trade name of

In accordance to the present disclosure， the expandable graphite employed in the curable coating composition to form the flame-resistant coating (b) ， has an average particle diameter (d_s0) of about 20-300 μm， or about 50-150 μm. The average particle diameter is measured by， for example， a laser diffraction/scattering particle size distribution measuring device. Also in accordance to the present disclosure， the expandable graphite used herein is substantially free of sulfur. That is， the expandable graphite used herein has a sulfur content level of less than 200 ppm.

Suitable expandable graphite may be obtained commercially from Boading Action Carbon Co. Ltd. (Hebei， China) ， for example.

The curable coating composition may be prepared by mixing a desired amount of expandable graphite with the two parts of the LSR formulation simultaneously. Alternatively， the expandable graphite may be mixed with the first part of the formulation， and the second part of the formulation is subsequently added to the mixture.

The amount of the expandable graphite in the curable coating composition may be at a level of about 20-70 weight ％， or about 20-60 weight ％， or about 25-50 weight ％， based on the total weight of the curable coating composition. The curable coating composition comprising the expandable graphite in this amount can still be easily applied to the flexible substrate (a) while keeping an excellent flexibility of silicone elastomer.

Regardless of the preparation process being mixed simultaneously or sequentially， the curable coating compositions should have a viscosity that is appropriate to the particular application， machinery used for applying to the flexible substrate (a) ， but generally it is in the range of 2-120 Pa·s at 25℃. Preferred curable coating compositions have a viscosity in the range 5-40 Pa·s at 25℃， and are able to cure within one-half minute at a temperature of 150-200℃ to provide an elastomeric coat on at least one surface of the flexible substrate (a) . The curable coating compositions according to the invention should be capable of remaining at workable viscosities for at least 3-6 hours when stored at temperatures less than about 40℃.

The curable coating composition may be applied onto the flexible substrate (a) by any coating techniques including gravure， knife blade coating， and transfer coating techniques. Although， some curable coating compositions may be cured at ambient temperature， curing of the curable coating composition preferably is carried out by heating or radiation. Heating is the preferred method， and preferably at a temperature ranging from about 100℃ to about 200℃ ， or from about 130℃ to about 170℃.

One major advantage of present curable coating composition is that good adhesion can be achieved after relatively short curing times that range from 30 seconds to an hour， generally 1-30 minutes， and typically 1-15 minutes at about 150℃. An increase in curing temperature allows a shortening of the curing time.

The thickness of the flame-resistant coating (b) derived from the curable coating composition and present on at least one surface of the flexible substrate (a) is preferably 10-180 μm， or 15-160 μm， or 20-135 μm， from the viewpoint of providing sufficient burnthrough resistance without impairment of the lightweight and flexible properties of the present fire barrier.

The thickness ratio of the flexible substrate (a) to the fire barrier (i.e.， the sum of the thickness of the flexible sub strate (a) and that of the flame-resistant coating (b) ) is preferably 0.75 or less， or 0.50 or less， or 0.25 or less， from the viewpoint of preventing impairment of the flexibility of the film. No particular limitation is imposed on the minimum of the thickness ratio， so long as the effects of the present invention are not impaired. The thickness ratio of the flexible substrate (a) to the thickness of the fire barrier is preferably 0.01 or more， or 0.05 or more， or 0.1 or more.

Although the contemplated use of the fire barrier of the present invention includes flexible， extended time flame barriers in applications such as electrical cable trays， fuel line， steam and process pipeline protection， structural steel fireproofing， and fire-rated wall assemblies， it is to be understood that other end uses are intended. Typical applications include， for example， as a liner to be encapsulated between and within the doors， walls， or roofs for building structures such as emergency military and firefighter shelters， or transportation vehicles such as airplanes， trains， automobiles， or boats， can provide additional heat and flame protection by slowing down heat transmission. Other uses for the fire barrier of the present invention include， for example， a protection cover for equipment shrouds， support members， electrical circuit panels， medical gas boxes and elevator call boxes； or a wrap for fuel lines， steam and process pipelines， and electrical cables.

Without further elaboration， it is believed that one skilled in the art using the preceding description can utilize the present invention to its fullest extent. The following examples are， therefore， to be construed as merely illustrative， and not limiting of the disclosure in any way whatsoever.

EXAMPLES

The abbreviation “E” stands for “Example” and “CE” stands for “Comparative Example” is followed by a number indicating in which example the thermal protective garment is prepared. The examples and comparative examples were all prepared and tested in a similar manner.

Materials

Polyimide film (a1) ： available from E. I. du Pont de Nemours Co.， USA under the trade name of

50HN. The polyimide film had a basis weight of about 18.8 g/m² and a thickness of 12.7 μm (0.5 mil) .

Polyimide nanoweb (a2) ： was prepared according the procedure described in U.S. Patent Application Publication No. 2013/0005940. The polyimide nanoweb was imidized first， then calendered to obtain a nanoweb having a basis weight of about 17.0 g/m²， a thickness of 25.6 μm， DOI is 0.630， tensile strength of 40.7MPa， and tensile elongation at break of 18.0％. Tensile strength and tensile elongation at break of the nanoweb (a2) were measured by cutting the nanoweb (a2) into strips of 25 mm in width and 100 mm in length. The testing was performed on an Instron 5566 universal testing machine according to a modified ASTM D882-12 method. The gage length of testing was 50 mm and the crosshead speed was 100 mm/min.

Polyimide nanoweb (a3) ： was prepared according the procedure described in U.S. Patent Application Publication No. 2013/0005940. The polyimide nanoweb was calendered first， then imidized to obtain a nanoweb having a basis weight of about 23.8 g/m²， a thickness of 72.5 μm， DOI is 0.584， tensile strength of 19.0MPa， and tensile elongation at break of 9.0％. Tensile strength and tensile elongation at break of the nanoweb (a3) were measured according the same method described above for the nanoweb (a2) .

Expandable Graphite： available from Boading Action Carbon Co. Ltd. Hebei， China. The expandable graphite had an average particle size of 75 μm.

Liquid silicone rubber (bl) ： available from Dow Corning Co.， consisting of

3600A (49.5 weight％) ，

3600B (49.5 weight％) ， and a curing retardant-Dow

LC-9608 (1 weight％) .

Liquid silicone rubber (b2) ： available from Dow Corning Co.， consisting of 90.9％of

9151-200P Part A and 9.1％of

9151-200P Part B

Preparing the Curable Coating Compositions CC1-CC3

CC0： A mixture of

3600A (49.5 parts by weight) ，

3600B (49.5 parts by weight) ， and Dow

LC-9608 (1 parts by weight) were mixed uniformly to obtain a comparative coating composition as CC0 that contained no expandable graphite.

CC1： A mixture of

3600A (49.5 parts by weight) ，

3600B (49.5 parts by weight) ， Dow

LC-9608 (1 parts by weight) and expandable graphite (60 parts by weight) were mixed uniformly to obtain a curable coating composition as CC1 that contained 37.5 weight ％of expandable graphite. The freshly prepared curable coating composition should be applied to a specified flexible substrate within 48 hours.

CC2： A mixture of

3600A (49.5 parts by weight) ，

3600B (49.5 parts by weight) ， Dow

LC-9608 (1 parts by weight) and expandable graphite (15 parts by weight) were mixed uniformly to obtain a curable coating composition as CC2 that contained 13 weight ％of expandable graphite.

CC3： A mixture of

9151-200P Part A (91 parts by weight) ，

9151-200P Part B (9 parts by weight) ， and expandable graphite (26.7 parts by weight) were mixed uniformly to obtain a curable coating composition as CC3 that contained 21.1 weight ％of expandable graphite.

Preparing the Coated Specimens E1-E10 and CE1-CE7

The freshly prepared curable coating composition according to what was specified in Tables 1-3 was spread uniformly on a PTFE impregnated glass fabric using a knife to cover an square area of 18 x18 cm². A piece of polyimide film or nanoweb of the same size， pre-weighed， was then placed on top of the curable coating composition. The assembly having the curable coating composition sandwiched between the PTFE fabric and the polyimide flexible substrate was then flipped over， and by removal of the PTFE fabric now on top of the assembly， the curable coating composition was transferred to the polyimide flexible substrate. The coated polyimide flexible substrate was cured for 4 minutes in an oven of at 160℃ to provide a specimen， which was an embodiment of the present fire barrier.

Test Method

Basis weight (g/m²) ： weights of the uncoated and coated specimens were measured on a METTLER TOLEDO precision balance model PR8002. The basis weight was calculated by dividing the weight (g) over the area of the uncoated or coated specimens (m²) .

Thickness (μm) ： thicknesses of the polyimide film or nanoweb before and after coating were measured according to ASTM D1777 with a presser foot of 28.7 mm in diameter and pressure of 4.14kPa.

Burnthrough Test： the testing was done by adjusting the propane flow of a Bunsen burner to provide an inner flame with a height of 70 ± 2 mm and an outer flame with a height of 100 ± 2 mm， as measured from the burner head. The temperature of the outer flame rose from 100℃ to 600℃ in 25 seconds， to 700℃ in 35 seconds， and to 800℃ in 47 seconds. The specimen was clipped horizontally with the coated side facing toward the burner on a supporting frame having an exposed area of 10 cm x 10 cm. The burner was placed under the sample and positioned at 45 degree angles to the vertical providing a flame that the tip of the inner flame just touched the coating of the sample to ensure fully contact with the outer flame. The “burn through time” was the duration between the testing started till the flame penetrated the sample， usually resulting a specimen with a hole or crack. If the sample remained intact for more than 10 minutes， then the testing was stopped and “no burn through” was recorded for this sample.

Flexibility Test： the specimen remained intact after the burnthrough test was subjected to this test. The specimen was folded at the burnt area along one direction then was folded further along a perpendicular direction to form a 4-fold square which is a quarter of the original size. The specimen passed this test would be marked with “○” if it could be folded and unfolded without a portion of the specimen exhibiting cracking or separation of the coating layer， or a portion of the specimen to fall away from the specimen construction； otherwise it would be marked as “×”.

The results of the above mentions tests were reported in Tables 1-3.

Table 1

Table 2

Table 3

The test results of Tables 1-3 clearly show the superior flame retardancy of the fire barriers of the present invention (El-E9) .

Comparison of the burnthrough testing results， the specimens comprising silicone elastomer without expandable graphite (i.e. CE2， CE3， CE5 and CE6) were quickly burned through within 2 minutes. In contrast， the specimens of Examples 1-9， i.e. embodiments of the fire barriers of the present invention， show superior flame retardancy by remaining intact after 10 minutes of flame exposure.

Other advantages observed during the burnthrough test of the fire barriers of the present invention is that the specimens of the comparative examples 1-7 generated a lot of smoke and objectionable combustion gases during the testing and became brittle and friable afterwards； whereas the fire barriers of Examples 1-9 didn’t generate smoke or gases during the testing and remained intact after the burnthrough test.

Another advantage of the flame barrier of the present invention， as compared to the specimens of Comparative Examples 1-3， is that the fire barriers of Examples 1-9 are much more flexible， being able to be folded and unfolded without showing cracks.

While the invention has been illustrated and described in typical embodiments， it is not intended to be limited to the details shown， since various modifications and substitutions are possible without departing from the spirit of the present invention. As such， modifications and equivalents of the invention herein disclosed may occur to persons skilled in the art using no more than routine experimentation， and all such modifications and equivalents are believed to be within the spirit and scope of the invention as defined by the following claims.

Claims

A fire barrier having improved burn-through resistance, comprising:

(a) a flexible substrate composed of polyimide, and

(b) a flame-resistant coating containing expandable graphite,

wherein

the flame-resistant coating is derived from a curable coating composition comprising:

about 10-50 weight ％ of expandable graphite, and

about 50-90 weight ％ of silicone elastomer,

wherein the weight ％ is based on the total weight of the curable coating composition； and

the flame-resistant coating is present on at least one surface of the flexible substrate.
The fire barrier of claim 1, wherein the fire barrier has a basis weight of about 20-200 g/m², or about 25-175 g/m², or about 30-150 g/m².
The fire barrier of claim 1, wherein the amount of the flame-resistant coating is about 10-180 g/m², or about 15-160 g/m², or about 20-135 g/m².
The fire barrier of claim 1, wherein the fire barrier has a total thickness of about 45-550 μm, or about 60-430 μm, or about 75-310 μm_.
The fire barrier of claim 1, wherein the flexible substrate is a polyimide nanoweb or a polyimide film.
The fire barrier of claim 1, wherein the flexible substrate has a thickness of about 5-150 μm, or about 10-130 μm, or about 15-110 μm.
The fire barrier of claim 1, wherein the thickness ratio of the flexible substrate to the fire barrier is about 0.01-0.75.
The fire barrier of claim 1, wherein the silicone elastomer contained in the flame-resistant coating is a liquid silicone rubber.
A method for preparing the fire barrier of claim 1, comprising:

i. providing a flexible substrate composed of polyimide and a curable coating composition；

ii. applying the curable coating composition onto at least one surface of the flexible substrate；

iii. curing the coated substrate of step ii by heating or radiation to obtain the fire barrier；

wherein

the curable coating composition, based on the total weight of the curable coating composition, comprises:

about 10-50 weight ％ of expandable graphite, and

about 50-90 weight ％ of silicone elastomer.
Use of the fire barrier of claim 1, wherein the fire barrier is a liner of doors, walls or roofs for building structures or transportation vehicles； a protection cover for equipment shrouds, support members, electrical circuit panels, medical gas boxes and elevator call boxes； or a wrap for fuel lines, steam and process pipelines, and electrical cables.