Patent Application: US-201515324201-A

Abstract:
a product for use in fire protection includes a laminate including a first layer of insulation and a second layer of hydrogel includes a first network of covalent crosslinks and a second network of ionic or physical crosslinks .

Description:
there is a pressing demand for improved fire retarding materials , which can survive in high temperature flames for extended periods of time while protecting skin from burn injuries and structures . even though there are many commercially available fire - retarding polymer fabrics , still many burn injuries and burn deaths occur worldwide . current fire - retarding polymer fabrics are expensive and decompose rapidly at high temperature flames or become very hot , e . g ., reach temperatures injurious to human skin . inexpensive fire - retarding materials were developed using hydrogels , containing about 90 % water . these products confer more protection from burn injuries compared to existing products . even though the protection from hydrogels is better than that of fabrics , the temperature can rise up to 100 ° c . after some time due to high thermal conductivity of water but remain at 100 ° c . until all the water is evaporated . laminates described herein combine hydrogels with fabrics that have low thermal conductivity . in the laminate , a hydrogel layer faces the flame and keeps the temperature at 100 ° c . for extended periods of time while the fabric with low thermal conductivity protects the skin from burn injuries . these laminates are not effective if conventional weak and brittle hydrogels are used . a tough hydrogel with superior properties was used to prepare the laminates . as these hydrogels contain mostly water , the laminates are much cheaper to make compared to existing fire - retarding fabrics . the laminates described herein have excellent heat and fire retarding properties and are useful in potential lifesaving applications such as fire - retarding blankets or apparel . severity of burns depends on the burn temperature and time of contact . if normal blood temperature of human tissue is raised from 36 . 5 ° c . to 44 ° c ., skin begins to burn . at 72 ° c . the top layer of the skin , epidermis is destroyed immediately . small changes in time of exposure and skin temperature can lead to serious burn injuries . a common method to protect skin from fire is to use fire - retarding polymer fabrics . numerous fire - fighting apparels made from polymer fabrics including fire blankets , suits , pants , jackets , gloves are used to protect people when fighting a fire . fire retarding polymers can be synthesized in two ways . one way is to incorporate flame resistant additives into polymers , which is a relatively cheaper and easy way to synthesize fire - retarding polymers . another method is to synthesize intrinsically fire resistant polymers which is a more expensive method but more efficient than additive polymers . chemically modified fabrics with additives include flame retardant cotton , wool etc . intrinsically fire - resisting fibers include aramid , modacrylic , polybenzimidazole , phenolic , asbestos , ceramic etc . asbestos has many desirable thermal properties and is cheap but the fibers are very fine and can be breathed into the lungs and promote cancer growth . glass fibers are also heat resistant but can cause skin irritation . ceramic fibers can withstand very high temperatures but have poor abrasion resistance , poor aesthetic characteristics , and high densities and are difficult to process . thus asbestos , glass fibers and ceramic fibers are not widely used in preparing fire - retarding apparel . aromatic polyamides known as aramids have been used to make fire - retarding apparels . nomex ® is the brand name of a flame resistant aramid fiber made by dupont chemical company . it has become a component in protective apparel widely used by fire fighters . nomex ® is also used in apparel worn by military pilots , combat vehicle crews , racecar drivers etc . when exposed to intense heat nomex ® fibers carbonize and thicken which creates a protective barrier between heat source and wearer &# 39 ; s skin . nomex ® is a poor heat conductor thus it takes time for heat to travel through nomex ®. aramids are resistant to temperatures around 250 ° c . for many hours but they begin to char at about 400 ° c ., e . g ., within a few seconds . at high temperature flames such as flashover fires , they provide protection only for few seconds . fire retarding fabrics made from materials including nomex ®, kevlar ®, and wool are also used as blankets to extinguish small fires . these products are helpful in temperatures up to ˜ 400 ° c . but they do not always provide protection of the level desired , particularly when exposed to substantial temperatures or flames . for example , another material suitable for use as a laminate backing is described in u . s . pat . no . 6 , 358 , 608 , incorporated herein by reference . for example , the material comprises an oxidized polyacrylonitrile and one or more additional fibers , which , e . g ., are stronger but less fire retardant . exemplary additional fibers ( also called strengthening fibers ) include , but are not limited to , polybenzimidazole ( pbi ), polyphenylene - 2 , 6 - benzobisoxazole ( pbo ), modacrilic , p - aramid , m - aramid , polyvinyl halides , wool , fire resistant polyesters , fire resistant nylons , fire resistant rayons , cotton , and melamine . to make the material , in some examples , the oxidized polyacrylonitrile fibers and the strengthening fibers are each first carded into respective strands or carded together to form a blended strand . multiple strands are then intertwined together to form a yarn . alternatively , strands made from polyacrylonitrile and strengthening fibers , blended strands , or a combination thereof are felted or otherwise formed into a nonwoven mat or sheet . for example , laminate backing materials include oxidized polyacrylonitrile fibers in an amount in a range from about 85 . 5 % to about 99 . 9 % by weight of the fibers in a yarn , felt , or other fibrous blend . for example , the strengthening fibers that are blended with the oxidized polyacrylonitrile fibers are included in an amount in a range from about 0 . 1 % to about 14 . 5 % by weight of the fibers in the yarn , felt , or other fibrous blend . in some embodiments , the oxidized polyacrylonitrile fibers are obtained by heating polyacrylonitrile fibers in a cooking process between about 180 ° c . to about 300 ° c . for at least about 120 minutes . examples of suitable oxidized polyacrylonitrile fibers include lastan , manufactured by ashia chemical in japan , pyromex , manufactured by toho rayon in japan , panox , manufactured by sgl , and pyron , manufactured by zoltek . as used herein , the term “ yarn ” refers to a blend of individual strands of fibers that have been formed by , e . g ., “ carding ” one or more types of “ staple fibers ”. carding is a mechanical process that disentangles , cleans , and intermixes fibers to produce a continuous web or sliver suitable for subsequent processing . most yarns comprise two or more individual threads or strands that have been twisted , spun or otherwise joined to form a bundle of strands . this allows each strand , such as a strengthening fiber strand , to impart its unique properties along the entire length of the yarn . the individual strands within the yarn may be formed from a single type of staple fiber , or they may comprise a blend of two or more different types of staple fibers . the term “ fabric ” refers to one or more different types of yarns that have been woven , knitted , or otherwise assembled into a desired protective layer . the tem . “ felt ” refers to a more random bundle of strands typically formed by a needle punch process . the term “ fibrous blend ” refers to yarns and felts that , e . g ., include a mixture of oxidized polyacrylonitrile fibers and at least one strengthening fiber as well as fabrics knitted , woven or otherwise assembled from such yarns . the term “ fibrous blend ” also refers to individual strands formed by carding a mixture of , e . g ., oxidized polyacrylonitrile staple fibers and at least one strengthening staple fiber . the term “ fibrous blend ” encompasses any fabric that includes yarns , fabrics , felts or strands . see , e . g ., u . s . pat . no . 6 , 358 , 608 , incorporated herein by reference . in one embodiment , the laminate backing composing an oxidized polyacrylonitrile and one or more additional fibers comprises carbonx ®. a widely used fire retarding fabric is carbonx ® made from 0 - pan ( oxidized polyacrylonitrile ) fibers . it has a very high flame resistivity and does not shrink at high temperatures . carbonx ® fabric resists burning when exposed to heat or flames exceeding 1500 ° c . because the oxidized polyacrylonitrile fibers carbonize and expand , which eliminates any oxygen content within the fabric . even though carbonx ® fabric is highly flame resistant and demonstrates high thermal protection performance compared with other fire retardant fabrics , its recorded survival time under high heat / flames is still not adequate for many situations requiring protection from fire or injurious / damaging levels of heat . thus , fire - retarding materials that can provide higher survival times are still much needed . in firefighting industry , water is one of the best tools to extinguish fires . when water is sprayed , it coats the fuel and creates a barrier , which in turn , prevents oxygen from reaching the fire . fire has to put a lot of heat energy to boil water , which slows down the fire . but the disadvantage is when water is sprayed on a fire , only a percentage is effective and the rest evaporates or drips down . hydrogel slurries have been used as fire retarding materials . hydrogels have a hydrophilic polymer network swollen in water . they are advantageous compared to water as the sticky hydrogels can stay on the applied surface without dripping off . as hydrogels contain mostly water , they have very high heat capacity and high heat of evaporation . hydrogel slurries have been used in commercial products . in stunt protection , thick layers of hydrogel slurries are applied to protect from extreme heat for a short period of time . hydrogel slurries are sprayed on to structures to protect burning from wildfire . for the fire to get in to the building , it has to evaporate all the water first thus protect the houses . hydrogels are also used in burn protection to draw the heat out of a burn . as most of the conventional hydrogels are brittle and weak , they cannot stand by themselves . thus , these hydrogels are impregnated to different fabrics to make heat - resisting blankets . but as hydrogels can rise up to 100 ° c . due to high thermal conductivity of water , they do not provide good protection for skin for a long period of time . two fire - retarding compositions were used to prepare hydrogel - fabric laminates , which can provide surprisingly better protection compared to individual materials . conventional hydrogels are weak and brittle thus cannot be used in making laminates . a tough hydrogel that contains a high level of water is a better solution for apparel . hydrogel - fabric laminates have many advantages compared to hydrogel infused fabrics . for fabrics that are infused with hydrogels , the amount of hydrogel slurry it can absorb is limited . but tough hydrogels are self - supporting , flexible and the thickness is tuned or altered according to the requirement . a hybrid hydrogel containing polyacrylamide ( paam ) and alginate , which has a record high toughness value was developed [ j .- y . sun , x . h . zhao , w . r . k . illeperuma , o . chaudhuri , k . h . oh , d . j . mooney , j . j . vlassak , z . suo , highly stretchable and tough hydrogels , nature , 489 , 2012 , 133 - 136 ]. as it can survive any damage due to high toughness and contains ˜ 90 % water , laminates using this tough hydrogel perform better as a fire retarding material compared to the available materials . the performance of hydrogels and hydrogel - fabric laminates for protecting skin was evaluated by testing the heat resistivity and fire resistivity of the commercially available fire retarding fabrics and tough paam - alginate hydrogels . a heat transfer model was developed to quantify the heat absorbed by the skin when protected with hydrogels and hydrogel - fabric laminates . a standard test known as thermal protective performance ( tpp ) test was used to measure the performance of different fire retarding materials , and the experimental data was used to calibrate the heat transfer model . a method was developed to measure the performance of fire - retarding fabrics , and the heat transfer model was used to optimize the layer thickness of the laminates for different insulating fabrics to obtain the maximum surviving time under a flashover fire . polyacrylamide ( paam )- alginate hybrid hydrogels were prepared using the following procedure : powders of alginate ( fmc biopolymer , lf 20 / 40 ) and acrylamide ( sigma , a8887 ) were dissolved in deionized water ammonium persulfate ( ap ; sigma , a9164 ), 0 . 0017 the weight of acrylamide , was added as the photo initiator for polyacrylamide . n , n - methylenebisacrylamide ( mbaa ; sigma , m7279 ), 0 . 0006 the weight of acrylamide , was added as the crosslinker for polyacrylamide . n , n , n ′, n ′- tetramethylethylenediamine ( temed ; sigma , t7024 ), 0 . 0025 the weight of acrylamide , was added as the crosslinking accelerator for polyacrylamide . calcium sulfate ( caso 4 . 2h 2 o ; sigma , 31221 ), 0 . 1328 the weight of alginate , was added as the ionic crosslinker for alginate . alternatively , a divalent cation , such as mg 2 + , sr 2 + , ba 2 + , or be 2 + , or a trivalent cation , such as al 3 + or fe 3 + , is used to crosslink the hydrogels . the solution was poured into a glass mold , 75 . 0 × 55 . 0 × 6 . 0 mm 3 , covered with a glass plate . the gel solution was cured at room temperature by exposing them for eight minutes to ultraviolet light ( oai ls 30 uv flood exposure system , 350 w power with a wavelength of 350 nm ). the samples were kept at room temperature for one day to ensure complete reaction . in order to prepare hydrogel - fabric laminates , hydrogels are threaded with nomex ® aramid strips ( mcmaster , 8796k56 ), fire retarding wool ( keane fire and safety equipment company , inc . ), and carbonx ® fabric ( cx - 6080 , concord companies , inc .). in order to test the heat resistivity , a hot plate ( dataplate digital hotplate 720 series ) was used at two temperatures ; t h = 350 ° c . and 500 ° c . samples with dimensions 55 mm * 37 . 5 mm * 3 mm were placed on the hot plate for 30 seconds and the top surfaces were observed . wool , nomex ®, carbonx ®, and hydrogels were compared for heat resistivity . a blowtorch ( home depot , bernzomatic ts3000kc self igniting torch kit ) with a high temperature flame ˜ 1000 ° c . was used to test the flame resistivity . the distance between the blowtorch tip and samples were kept at 6 cm . wool , nomex ®, carbonx ®, and hydrogel sample with length 75 mm , width 55 mm and different thicknesses were tested . tests were conducted until the flame burns through the samples . burned fabrics were observed after the test . a hotplate ( thermolyne cimarec 2 ) with a constant heat flux ( 1130 w ) was used to measure the performance of different fire retarding materials . heat flux of the hotplate was measured using a power meter ( p3 international kill a watt ez electricity usage monitor , p4460 ). 3 mm thick wool , nomex ® carbon x ®, hydrogel and laminates with 3 mm hydrogel - 3 mm wool and 3 mm hydrogel - 3 mm nomex ®, and 3 mm hydrogel - 3 mm carbonx ® were tested . all the samples tested had an area of 18 cm * 18 cm similar to the area of the hotplate . a sample was placed on top of the hotplate which was immediately covered by 18 cm * 18 cm * 2 cm insulating board which has a copper calorimeter attached to the surface facing the fire - retarding material . copper calorimeter is a disc with 4 cm diameter and 1 . 5 mm thickness and two thermocouples ( mcmaster , fluke thermocouple thermometer , 40255k32 ) were attached to the rear side of the calorimeter facing the insulating board . temperature rise of the copper disc was recorded as a function of time . heat resistivity was tested when fire - retarding materials are placed on a hotplate for 30 seconds at temperatures t h = 350 ° c . and 500 ° c . as shown in fig1 ( a ) . three commercially available fire retarding fabrics : fire - retarding wool , nomex ®, and carbonx ® are compared with hydrogel . at 350 ° c ., wool started to carbonize and became very brittle compared to soft fabric at initial state as in fig1 ( b ) . at 500 ° c . it melted on top of the hot plate and cannot be recovered . nomex ® fabric resisted heat at 350 ° c . and still remained similar to the initial state according to fig1 ( c ) . but at 500 ° c ., it started carbonizing and shrinking . nomex ® fabric made from aramid fibers begins charring ˜ 400 ° c . [ a . r . horrocks , s . anand , handbook of technical textiles , crc press , 2000 ]. even though it did not melt at high temperature , it shrank rapidly and became very stiff and brittle . carbonx ® resisted heat at both 350 ° c . and 500 ° c . as shown in fig1 ( d ) and only a slight change in color is observed at the rear side of the fabric . fig1 ( e ) shows that except for few parts , the hydrogel remained similar to the initial state still soft and flexible compared to nomex ® and wool . these data indicate that both carbonx ® and hydrogel survive very high temperatures ˜ 500 ° c . without damaging the samples . in order to observe the temperature of the top surface of samples during hot plate test at 500 ° c ., infrared thermal imaging ( flir thermal camera ) was performed as shown in fig2 . wool , nomex ® and carbonx ® heated up very rapidly , but hydrogel remained at a low temperature for a long period of time . even though carbonx ® fabric at high temperature looked similar to the initial state ( fig1 ( d ) ), the temperature of the carbonx ® fabric rose to very high level . when compared with existing fire retarding fabrics , hydrogels provide better protection at high temperatures . excellent heat resistivity is attributed to high heat capacity and high heat of evaporation of water inside the hydrogel . fire resistivity of wool , carbonx ®, and nomex ® fabrics were compared with hydrogels under a high temperature flame . wool and nomex ® fabrics burned within few seconds when exposed to flames of 1000 ° c . temperature as shown in fig3 ( a ) and 3 ( b ) . but carbonx ® and tough hydrogels withstood the high temperature flame for a long period of time before burning through as shown in fig3 ( c ) and 3 ( d ) . photos are shown for samples with 6 mm thickness . front and rear views of the hydrogel when burning were observed as shown in fig4 a - b . un - burned rear views show that hydrogel survive extreme high temperature flame for a long period of time without burning through . images were taken just after the flammability test as shown in fig3 ( a )-( d ) ). wool and nomex ® became very stiff and brittle within few seconds but carbonx ® remained similar to its initial state when compared after 1 min . tough hydrogels when observed even after 1 min , still had lot of un - burned hydrogel in it . even though the part subjected to high temperature became carbonized , it was only a thin layer and the rest of the hydrogel still remained same , i . e ., in a hydrated state and flexible as shown in fig3 d and 4 b . when exposed to a flame , water in the hydrogel facing the flame starts to evaporate and a thin dry polymer layer is created . the thickness of the dry polymer layer increases with time until all of the water in the hydrogel is evaporated . the thickness of the dry polymer layer depends on the flame temperature and time of exposure to the flame . the thickness effect is plotted in fig3 e . wool and nomex ® were found to have a small improvement in the flame resistivity with increased thickness , but carbonx ® and hydrogel enhanced the flame resistivity dramatically . even though carbonx ® has a better fire resistivity than hydrogel , it heats up rapidly compared to hydrogel and thus does provide as much protection to a wearer when compared to hydrogel . as the tough hydrogel withstood such a high temperature , these data indicate that it can withstand a flashover fire , which can go up to 600 ° c . in contrast , most of the currently available fabrics decompose or heat up rapidly under such conditions . the laminate constructs described herein improve and extend the use and performance of such existing fabrics . three different fire - retarding mechanisms to protect skin are discussed as follows . fire - retarding polymer fabrics usually have high decomposition temperature and low thermal conductivity . many fire - retarding fabrics retard fire due to formation of protective coating or char to insulate the fabric from the heat source [ h . zhang , fire - safe polymers and polymer composites , ph . d . dissertation , university of massachusetts , 2003 ]. as shown in fig5 ( a ) , when a heat flux is exposed to a fire - retarding polymer fabric it retards fire due to bather formation or other mechanisms [ h . zhang , fire - safe polymers and polymer composites , ph . d . dissertation , university of massachusetts , 2003 ] and low thermal conductivity of the fabric does not carry the flame heat towards your skin thus protects the skin . but when the temperatures go much higher than its decomposition temperature , as the fabric does not carry the heat away due to low thermal conductivity , the surface facing the flame can be very hot allowing it to decompose rapidly . from heat resistivity test and fire resistivity tests it was observed that both wool and nomex ® fabric decompose rapidly when exposed to high heat and high temperature flame ( fig1 b - c and 3 a - b ). thus at temperatures above polymer decomposition , these fabrics cannot provide enough protection for skin . even though carbonx ® fabric resisted heat and flame ( fig1 ( d ) and 3 ( c ) ) due to expansion of fibers and reducing oxygen content in fabric , it cannot provide much protection to skin as the temperature becomes too high ( fig2 ( c ) ). another method for fire protection is to use hydrogels . hydrogels contain mostly water , and water has high specific heat capacity ( 4187 j / kgk ) and high heat of evaporation ( 2 . 26 * 10 6 j / kg ). when the heat flux is exposed , part of heat from flame is carried away due to evaporation of water as shown fig5 ( b ) . when water is evaporated it generates two layers ; dry polymer layer where all the water is evaporated and the hydrogel layer which still has water . the two regions are separated by a moving boundary corresponding to a temperature of 100 ° c . this behavior is observed using a thick hydrogel as shown in fig6 . due to the high thermal conductivity of water (˜ 0 . 58 w / mk ), the other side facing skin can reach 100 ° c . after some time and remain at that temperature for a longer period of time until all of the water is evaporated . laminating hydrogels with low thermal conductive fabrics make better fire - retarding materials compared to hydrogels and fabrics as individual materials . both materials participate in providing better performance . as shown in fig5 ( c ) hydrogel layer carries the heat away and keeps the temperature of the hydrogel - fabric interface at 100 ° c . until all the water is evaporated . even though how high the flame temperature is , the maximum temperature observed by the fabric facing hydrogel is 100 ° c . for a long period of time . by selecting a fabric with low thermal conductivity that has the ability to survive a temperature of 100 ° c ., skin can be protected for a long period of time , e . g ., the side of the fabric facing skin remains at a safe temperature level for skin . the burn protection from hydrogel and hydrogel - fabric laminates is tested using the following models . the procedure used to model the heat transfer through hydrogels and hydrogel - fabric laminates is described below . the evaporation process in the protective layer is approximated as a one dimensional heat transfer problem with a moving phase boundary , which is solved with an enthalpy method [ j . crank , free and moving boundary problems ( 2nd ed ) oxford university press , new york ( 1975 )]. similar enthalpy models are used for materials made with cement mortar and polymer gels with a moving boundary [ z . f . jin , y . asako , y . yamaguchi y and m . harada , fire resistance test for fire protection materials with high water content , int j heat mass transfer , 2000 , 43 , 4395 - 4404 ; y . asako , t . otaka , y . yamaguchi , fire resistance characteristics of materials with polymer gels which absorb aqueous solution of calcium chloride , numer . heat transfer , part a , 2004 , 45 , 49 - 66 ]. any flow of water or steam inside of the material was neglected . fig5 ( c ) shows the hydrogel - fabric laminate with thickness t in its reference state . it consists of one layer of hydrogel of thickness t gel and one layer of fabric of thickness t f . the hydrogel consists of a polymer network with density ρ p and heat capacitance c p and water with density ρ w and heat capacitance c w . the heat of evaporation of water at its boiling temperature t b is h f . the concentration of polymer by weight is w p . the insulating fabric has a density ρ f and a heat capacitance c f . when heat is added at the bottom of the hydrogel , it heats up until water starts to evaporate . the zone in which phase transition takes place is modeled as an infinitely thin boundary at boiling temperature , which travels through the hydrogel as shown in fig5 ( c ) . the boundary at x = x s ( t ) divides the hydrogel in two regions . the material in region i ( x & gt ; x s ( t )) the temperature consists of the original hydrogel at temperatures below the boiling temperatures . in region ii ( x & lt ; x s ( t )) the water is evaporated and the remaining polymer network is at a temperature above the boiling temperature . while the boundary moves through the hydrogel , the whole sample becomes thinner due to drying of the hydrogel . the layer of insulating fabric constrains the deformation of the hydrogel such that it dries under plane strain condition . is valid , where q is the heat flux per area , t is the temperature , and k i has to be replaced by the heat conductivities k i and k ii in the corresponding regions . energy balance in both regions requires where h is the enthalpy of the material and v the speed at which the material moves due to drying of the hydrogel . the enthalpy of the hydrogel can be described as t & gt ; t b : h ={ tilde over ( h )} f ρ p c p ρ p ( t − t b ) ( 6 ) to simplify the mathematical analysis , all quantities are expressed in a material coordinate . x describes the location of a material point with respect to the original swollen hydrogel . in this coordinate system , the phase boundary is at a location the stretch λ is the ratio of the thicknesses of the dry gel and the swollen gel . under plane strain , condition λ can be determined to be where h = h / λ is the enthalpy of the material in the material coordinate system . by introducing the transformation { circumflex over ( t )}= t − t b in region i and { circumflex over ( t )}=( t − t b ) k ii /( k i λ ) both equations combine in ( 10 ) to { circumflex over ( t )}& gt ; 0 : h ={ tilde over ( h )} f ρ p + λ 2 c p ρ p k i / k ii ( t − t b ) ( 12 ) equations ( 11 ) and ( 13 ) are integrated over time with an explicit euler algorithm and use a central difference scheme to approximate the special derivatives [ j . crank , free and moving boundary problems ( 2nd ed ) oxford university press , new york ( 1975 )]. after each integration step equation ( 12 ) is used to update the temperature at each node [ j . crank , free and moving boundary problems ( 2nd ed ) oxford university press , new york ( 1975 )]. the heat transfer model is calibrated with a standard test , and it is used to predict the performance of hydrogel and hydrogel - fabric laminates . tpp test is a standard test to measure the performance of fire retarding materials according to the nfpa ( national fire protection association ) 1971 test standards [ http :// www . nfpa . org /]. this test has been widely used to measure the thermal protection for fire - retarding fabrics [ w . p . behnke , thermal protective performance test for clothing , fire technol , 1977 , 1 , 6 - 12 ; w . p . behnke , predicting flash fire protection of clothing from laboratory tests using second - degree burn to rate performance , fire and materials , 1984 , 8 : 2 , 57 - 63 ]. tpp test measures the fabric &# 39 ; s ability to block the heat , which can cause second - degree burns when exposed to 2 cal / cm 2 s heat flux . this heat flux is chosen to replicate a flashover fire . the standard test involves exposing a combination of radiant and convective heat flux to the fabric and a copper calorimeter placed above the specimen records the heat transferred through the specimen . the heat / time curve obtained in this test is compared with human tissue tolerance to heat to get a tpp rating [ w . p . behnke , thermal protective performance test for clothing , fire technol , 1977 , 1 , 6 - 12 ; w . p . behnke , predicting flash fire protection of clothing from laboratory tests using second - degree burn to rate performance , fire and materials , 1984 , 8 : 2 , 57 - 63 ]. the tpp test was modified as follows . the purpose of performing this test is to compare different fire - retarding materials and to calibrate our heat transfer model . a reduced heat flux ( 0 . 8 cal / cm 2 s ) compared to the standard test is used due to the limitation of the maximum heat flux of the available hotplate . the test set up is shown in fig7 ( a ) where a hotplate was used to apply the heat flux . total heat absorbed by the calorimeter is obtained by the temperature change in copper calorimeter multiplied by mass and specific heat of the copper calorimeter . the hydrogel - fabric laminates are prepared according to fig7 ( b ) by threading hydrogel and fabric together . as the hydrogel described herein is very tough , it can survive threading similar to a fabric , an important and advantageous attribute compared to conventional weak and brittle hydrogels . total heat absorbed by the calorimeter is plotted for different materials as shown in fig7 ( c ) . heat absorbed by 3 mm thick nomex ®, carbonx ®, and wool rise up faster . even at this heat flux , they do not provide enough protection as the temperature of the hot plate ( 500 ° c .) is above the fabric decomposition temperature . a 3 mm hydrogel has better protection compared to the fabrics but as the thermal conductivity of water is high , it also can heat up faster . hydrogel - fabric laminates are prepared using 3 mm thick hydrogels and 3 mm thick fabrics . the laminates show better performance compared to their parent materials . the heat transfer model was used to obtain heat / time curves for hydrogel and laminates . the following properties for fabrics are used in the model . for nomex ®, specific heat is used as 1748 j / kgk , thermal conductivity 0 . 15 w / mk and density 446 kg / m 3 . for wool , specific heat is used as about 1200 j / kgk , thermal conductivity 0 . 04 w / mk and density 162 kg / m 3 . for carbonx ®, specific heat is used as 1200 j / kgk , thermal conductivity 0 . 04 w / mk and density is used as 75 kg / m 3 . agreement between the experimental data and the model was observed . at low heat level data , the most relevant heat level as human tissue cannot absorb more than 4 - 5 cal / cm 2 s heat during this time period without second - degree burns . hydrogel - fabric laminates have better performance compared to parent materials ( fig7 ( c ) ). the thickness of hydrogel and fabric layers can be altered and therefore optimized to tailor performance . the heat transfer model is used to optimize the thickness for hydrogel - wool and hydrogel - nomex ® laminates . even though the tpp test is a standard test used to measure the performance of fire retarding fabrics , it has drawbacks . it is reported that firefighter &# 39 ; s suits , which reported 17 . 5 seconds surviving time from tpp test , can only survive around 10 seconds in a real scenario . tpp is a test designed to measure the performance during a short duration and does not produce detailed information to evaluate thermal performance of protective clothing over a range of conditions . thus , the performance measured by tpp test has been questioned in the literature [ w . e . mell , j . r . lawson , a heat transfer model for fire fighters &# 39 ; protective clothing , fire technology , 2000 , 36 , 39 - 68 ; j . f . krasny , j . a . rockett , d . huang , protecting fire fighters exposed in room fires : comparison of results of bench scale test for thermal protection and conditions during room flashover , fire technology , 1988 , 24 , 5 - 19 ]. in the tpp test , the insulating layer blocks the heat flux output from the fabric , which is different from the real scenario . as the heat flux is blocked , the effectiveness of the laminates does not show during this test . thus , the test method was modified to measure the surviving time of fire - retarding fabrics . instead of blocking the heat flux , skin temperature ( 37 ° c .) is imposed at the top surface of fabric that faces the skin and the heat output from the fabric is measured . the initial temperature of the setup is assumed to be 37 ° c . a heat flux of q in = 2 cal / cm 2 s is used as the flame heat input to mimic the flashover fire and the heat output from the fabric that can go to the skin is calculated . the setup is shown in fig8 ( a ) . t gel / t is the thickness ratio of hydrogel to the total sample thickness . thermal conductivity of the fabric is denoted as k f . heat output of the fabric , q out or in other words heat that will be absorbed by the skin is calculated for both hydrogel - nomex ® and hydrogel - wool laminates as shown in fig8 ( b ) and ( c ) corresponding to different t gel / t values . the total thickness used in this analysis is t = 9 mm . literature data for heat exposures on human skin were used to determine the level of heat energy that would create a second - degree burn [ a . m . stoll and m . a . chianta , method and rating system for evaluation of thermal protection , aerospace med ., 1969 , 40 , 1232 - 1238 ]. the heat flux was varied and the time that creates a second - degree burn was measured . a second - degree burn is where a blister forms and the outer layer of human skin ; the epidermis is destroyed . the heat flux vs time curve was integrated to get the total heat absorbed by the skin . this curve is known as the “ stoll curve ” and shown in fig8 ( b ) and 8 ( c ) in black color . this curve sets an important criterion when designing fire - retarding materials to protect skin . as long as the heat passing through the fire retarding fabric is below the heat set by the stoll curve , the material protects the skin . heat output from hydrogel - fabric laminates is compared with the stoll curve for heat absorbed by skin to avoid second - degree burns . the intersection points give the surviving time and they are plotted in fig8 ( d ) . hydrogel - fabric laminates provided better surviving time compared to fabric and hydrogel itself . even with pure hydrogel it is possible to maintain 37 ° c . without second - degree burns for a longer time compared to fabrics . but with laminates better surviving time were observed . hydrogel - wool laminates have better protection compared to hydrogel - nomex ® laminates due to the low thermal conductivity of wool compared to nomex ®. according to the model , the lower the thermal conductivity of the fabric , the better the performance is . thus , if a fabric which has thermal conductivity lower than wool is used , an even better surviving time is achieved . the dashed line indicates the complete drying of the hydrogel when all the water is evaporated . as soon as hydrogel dries out , it can go beyond 100 ° c . and when the temperature reaches the decomposition temperature of the fabric , it can protect the skin for only a few seconds . the curves to the left of the intersection of dash line might collapse with the dashed line ( fig8 d ) in a flashover fire scenario , as our model does not include decomposition of fabric . in the laminates , the fabric portion of the construct does not have to be a fire - retarding material , because its purpose is to survive 100 ° c . temperature and provide thermal insulation to the skin . applications for hydrogel - fabric laminates include fire - retarding products such as blankets or apparel . for example , these hydrogels are integrated with suits that are used by fire fighters or other critical applications that require more protection . these products are inexpensive compared to most of the highly engineered fire - retarding polymer fabrics and can be readily available in many places . the hydrogel - fabric laminates can be stored at homes or any other places in case of a fire emergency . for example , a possible scenario is a burning room ( e . g ., a hotel room , cruise ship , boat , and the like ) and an occupant can grab a hydrogel - fabric blanket to wrap around and escape from the building . the tough hydrogels contain about 90 % water . thus , it cannot be stored in open air for a long period of time as the water tends to evaporate . thus these materials need to be kept sealed to avoid losing its function . in addition , the blankets may be packaged in a single - use form ( e . g ., disposable after use ). alternately , the products are stored in a dehydrated state and are rehydrated prior to use , e . g ., for use of a boat or other situation with easy access to water . macroporous dehydrated hydrogels can be stored in many places , e . g ., places that have access to water , and they can be rehydrated within a few minutes prior to use . widely used fire - retarding polymer fabrics protect skin from burn injuries mainly due to high decomposition temperature and low thermal conductivity . above the decomposition temperatures they do not provide good protection . hydrogels are also used in fire retarding applications but cannot be used for a long period of time as they reach 100 ° c . but remain there for a long period of time until all the water is evaporated . by combining the two mechanisms prepared are hydrogel - fabric laminates that are much better than hydrogels and fabrics . the laminates are made by securing the layers together or using other methods such as fusing or sewing the layers or gluing layers together . hydrogel layer protects from high heat flux and stay at 100 ° c . for a long time until all the water is evaporated while fabric with low thermal conductivity keeps the skin at a safe temperature . 1 . http :// www . ameriburn . org / resources_factsheet . php . 2 . zhang , h . fire - 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4404 . 22 . y . asako , t . otaka , y . yamaguchi , fire resistance characteristics of materials with polymer gels which absorb aqueous solution of calcium chloride , numer . heat transfer , part a , 2004 , 45 , 49 - 66 . 23 . http :// www . nfpa . org /. 24 . w . p . behnke , thermal protective performance test for clothing , fire technol , 1977 , 1 , 6 - 12 . 25 . w . p . behnke , predicting flash fire protection of clothing from laboratory tests using second - degree burn to rate performance , fire and materials , 1984 , 8 : 2 , 57 - 63 . 27 . s . lee , c . park , d . kulkarni , s . tamanna , t . knox , heat and mass transfer in a permeable fabric system under hot air jet impingement , proceedings of the international heat transfer conference , ihtc14 , 2010 , 1 - 10 . 28 . http :// www2 . dupont . com / energy_solutions / en_us / assets / downloads / 418_419 . pdf 29 . a . handermann , oxidized polyacrylonitile fiber properties , products and applications , zoltek corporation ( http :// www . zoltek . com / white - paper - oxidized - polyacrylonitrile - fiber - properties - products - and - applications /). 29 . w . e . mell , j . r . lawson , a heat transfer model for fire fighters &# 39 ; protective clothing , fire technology , 2000 , 36 , 39 - 68 . 30 . j . f . krasny , j . a . rockett , d . huang , protecting fire fighters exposed in room fires : comparison of results of bench scale test for thermal protection and conditions during room flashover , fire technology , 1988 , 24 , 5 - 19 . 31 . huang t . j ., william j . h ., chapmen m . r ., u . s . pat . no . 6 , 287 , 686b1 , 2001 . while the invention has been described in conjunction with the detailed description thereof , the foregoing description is intended to illustrate and not limit the scope of the invention , which is defined by the scope of the appended claims . other aspects , advantages , and modifications are within the scope of the following claims .