High-Tech Fibrous Materials - American Chemical Society

Chapter 19

Melt—Stick Characteristics of Flame-Retardant-Treated Polyester—Cotton Blend Fabrics

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch019

Robert W. Hall Navy Clothing and Textile Research Facility, 21 Strathmore Road, Natick, MA 01760

The Navy Clothing and Textile Research Facility (NCTRF) conducted a series of heat and flame exposure tests on several blend levels of fire retardant (FR) treated polyester-cotton fabrics. The tests were conducted in an effort to evaluate the relativeburninjury potential of the various blend levels as related to melt/stick properties. A combination of four tests procedures were used to rate the fabrics. Test results showed fabrics with 65 percent polyester had high melt/stick characteristics while blends attaining up to 50 percent polyester pose little risk of melt/stick induced burn injury. Individual test failures on some fabrics containing 50 percent or less polyester indicate further testing is needed. A new test method for melt/stick testing is described. Recent develcpnent and use of durable f i r e retardant (FR) treated polyester-cotton blend fabrics i n u t i l i t y and service clothing has raised interest i n the melt/stick hazard of polyester. This study used a series of tests to evaluate and rate f i r e retardant (FR) treated polyester--cotton blend fabrics for r i s k of melt/stick induced burn injury. I t has long been f e l t that any amount of thermoplastic fiber blended with a non thermoplastic fiber would melt under heat or flame and would cling or stick t o the skin contributing t o burn injury. Ihe objective of t h i s study was t o determine the maximum amount of polyester that could be used i n a FR treated polyester-cotton blend fabric without melt/stick occurring. To provide a means of measuring the propensity of molten polymer t o stick t o the skin, current test methods were surveyed and found t o be lacking i n definition and r e l i a b i l i t y . I t was therefore decided that existing test methods would require modification or new methods developed. This study summarizes the melt/stick results for the fabrics tested, describes the test This chapter not subject to U.S. copyright Published 1991 American Chemical Society

In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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methods used and developed, and makes recommendations for selection of fabric blends to minimize melt/stick characteristics.


Background There have been several studies cxmparing the flammability of polyester--cotton blend fabrics with that of 100 percent of either fiber (1,2,3,4). The conclusions drawn from these studies indicate fabrics made from blends of polyester cotton are more flammable and pose a greater r i s k than fabrics made from 100 percent cotton or polyester. These studies also indicate that the higher the polyester content, the lower the r i s k of burn injury. Based on the methods employed and the data presented, the conclusions appear to be valid. Miles (1) showed lower burn injury potential for polyester when A-line dresses were subjected to a remote ignition source simulating a chance flame exposure. Langstaff and Trent (2) reported similar results using the Eastman f l a t plate (modified MAFT), the TRI convection calorimeter and an instrumented manikin (TIM). Burn tests on TIM showed a decrease i n heat release and depth of burn with an increase i n polyester content. Fabrics of different weights generated similar amounts of heat release since lighter weight fabrics are consumed faster than heavy fabrics. Goynes and Trask (3) found that untreated cotton fabrics burn easily and form a fragile char, while polyester shrinks and drips away from the flame. In a polyester-cotton blend, the polyester melts and flows around the cotton, supporting the char and prolonging the flame. An earlier study by Chouinard, Knodel and Arnold (4) found similar results for both nylon—cotton and polyester-cotton blends. The nylon and polyester transmit the flame slowly and often s e l f extinguish when the melting polymer f a l l s away. In these studies, both the nylon and polyester needed cotton to sustain combustion. These studies suggest that for untreated fabrics, the r i s k of burn injury i s related to the polyester content of the fabric, and that the higher the polyester content the lower the risk. In the case of polyester-cotton blend fabrics that have been finished with a durable FR treatment the polyester again melts and surrounds the cotton, but the FR treatment inhibits flame spread. The result i s a somewhat b r i t t l e char i n the direct flame contact area, with the heat causing the polyester to melt into the adjacent area forming a s t i f f , durable char. A number of tests have been used to evaluate the f i r e and flame resistance of textiles. These tests measure ease of ignition, after flame, s e l f extinguishment, heat release and several other characteristics. None of these tests measure the propensity of the melting polymer contained i n a fabric blend to melt and stick to skin. An early attempt to measure the effect of melting polymer on burn injury (4) used heat sensors placed i n a board and positioned 1/4 inch below an ignited fabric. Any molten polymer f a l l i n g from the flaming fabric dropped onto the board containing the sensors and the heat energy was measured. This study found that polyester-rich fabrics transferred less heat than the all-cotton or cotton-rich fabrics tested. The shortfalls of t h i s study were that



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HALL

Melt-Stick Characteristics of Polyester-Cotton Blend

none of the fabrics contained a flame retardant f i n i s h , and the configuration of the test did not allow the burning, melting fabric to come i n contact with the sensors. The approach taken by the National Fire Protection Association (NFPA) for materials used i n the manufacture of firefighter's clothing i s the so-called Oven Test (5). Materials are suspended i n a convection oven for five minutes at 260° C and must not melt, separate or ignite. The test appears to have poor reproducibility and has l i t t l e definition of failure c r i t e r i a . Almost a l l fabrics containing polyester or nylon gain some stiffness i n t h i s test as a result of melting of the polymer. The amount of polyester or nylon i n the fabric influences the amount of stiffness. By definition, any resultant stiffness indicates polymer flow and test failure, but there i s no indication of melt/ stick. This test appears to have been included i n the proposed standard for station uniforms for lack of a better test. Work i n the United Kingdom by the Ministry of Defense has centered on what i s called a '"hot plate" test (6). The fabric sample i s placed on a hot plate and heated to 260° C. The amount of melted polymer v i s i b l e on the surface of the fabric indicates i t s relative melt/stick propensity. This test i s d i f f i c u l t to control since the heated fabrics tend to shrink and/or curl away from the hot plate and provide no means of measuring adhesion. Methods and Materials Oven Test Fabrics were subjected to the 260° C Oven Test for five minutes and evaluated for melt. Problems associated with t h i s test include controlling the oven temperature, the duration of the exposure and interpreting the results. For this test, the fabric ranking was based on the condition of the exposed samples. Fabrics that became s t i f f , but did not crack when folded, were rated "0", those that cracked, "1"; and those that showed melting on the surface, "2". Hot Plate Test A modification of the "hot plate" test using two layers of fabric was used to assess melt/stick i n t h i s study. I t was decided to use two layers of each fabric evaluated, since i t was f e l t that i f the polymer i n a fabric, when subjected to these test conditions, flowed i n sufficient quantity to stick to another layer of the same fabric, there was a r i s k of the fabric sticking to skin under the same conditions. For the hot plate test a 1/4 inch thick copper plate was placed on an electric hot plate and heated above 250° C. The temperature was monitored by a thermocouple set into the copper plate. Samples run with a weight on the fabrics resulted i n the fabrics being completely charred with no evidence of sticking. The test procedure was changed to heating a single layer of fabric u n t i l the fabric began to melt and char. Shrinking i n some of the fabrics required some mechanical manipulation to keep the sample i n contact with the heated copper plate. At the time of melting and charring another


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sample of fabric was pressed against the heated fabric and the combined sample removed from the hot plate, allowed to cool, and rated. Two additional melt/stick tests were conducted on double layer fabric samples: (1) exposure to radiant heat and (2) a hot a i r or convective heat exposure test. Since the objective of these tests was to evaluate melt/stick and not flame resistance, the hot plate, radiant and convective heat tests were run u n t i l degradation was observed rather than using a preset exposure time. A l l fabrics were exposed u n t i l melting occurred or the fabric charred. The fabrics were then allowed to cool before melt/stick evaluations were made. This was necessary because the actual time to char or melt varied considerably among fabrics. I f preset exposure times had been used, some fabrics may not have melted as they did under a more prolonged exposure. Radiant Heat Test The radiant heat exposure tests were conducted using the NCTRF radiant heat tester (Figure 1) • This tester uses two sets of quartz lamps set at a 45 degree angle to the face of the test specimen. A concentric ring sample holder was used that provides intimate contact between the fabric samples and prevents the fabric from shrinking during the test. The samples are mounted v e r t i c a l l y and were exposed to a combined heat flux of 0.9 gcal/cm spysec u n t i l charring and melting occurred. Convective Hnh AIT- Tests The convective hot a i r tests were conducted using a Black and Decker Model 9751 heat gun (Figure 2). The temperature measured two inches from the barrel of the gun was 260 to 265° C. A guide was attached to the side of the heat gun to aid i n positioning of the sample during the test. Test fabrics were mounted i n the ring holder used for the radiant heat testing, exposed u n t i l charring and melting occurred, removed from the holder and rated. Two layers of fabric were used for the hot plate, radiant and convective hot a i r tests. Melt/stick occurred when the melting polymer flowed between the fabric layers during exposure. Fabrics that did not stick were rated "0" and those experiencing minimal surface sticking were rated "1". In order to be rated "1" the sticking fabrics must separate easily with no transfer of polymer or char between fabrics, otherwise the fabric was rated "2" (melt/stick). Attempts were made to devise or use other test methods with unsuccessful results. Using a Thermal Protective Performance (TPP) tester (Figure 3), two layers of fabric were evaluated at different heat fluxes and exposure times to allow melting of the fabrics. High heat fluxes or long exposure times resulted i n completely charred fabrics with no evidence of sticking. Control of exposure allowed melting without excess char, but no sticking occurred with the samples tested. A 100 percent polyster fabric was tested with the result that the lower fabric melted away with l i t t l e or no degradation of the upper fabric. Based on these results, i t was


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Melt-Stick Characteristics of Polyester-Cotton Blend Fabrics


FABRIC

SAMPLE

Figure 1. Radiant heat.

SAMPLE

HOLDER

Figure 2. Hot air convective heat gun.


297


COOLED

FABRIC

SHUTTER

WATER

TEST

BOARD

INSULATIVE

COPPER

Figure 3. TPP tester.

CALORIMETER TO

RECORDER



19. HALL


decided that t h i s test was not a good indicator of the melt/stick hazard of these fabrics. Another test used a timed burning tablet placed on the fabric. The tablet was lighted with a match, and the heat passing throu^i the fabric measured by a water cooled heat flux transducer placed against the underside of the fabric. No sticking of polymer t o the transducer occurred and the differences i n measured heat from sample t o sample were more related t o fabric weight than fiber content. A l l fabrics used i n t h i s study were manufacturer's samples or fabrics that were available frcm previous development programs at NCTRF. No fabrics were manufactured or finished specifically for t h i s study. Table I l i s t s characteristics of the fabrics evaluated. Fabric wei^its varied from 3.8 t o approximately 11.0 ounces per square yard. No specific attempt was made t o identify the f i r e retardant finishes used or t o determine the amount of f i n i s h applied t o the fabrics. Scene of the fabrics were identified through previous work. Table I. Fabric Characteristics Fabric No. Blend m 1 25/75 2 25/75 3 30/70 4 35/65 5 40/60 6 45/55 7 45/55 8 45/55 9 50/50 10 50/50 11 50/50 12 50/50 13 50/50 14 50/50 15 50/50 16 50/50 17 50/50 18 50/50 19 65/35 20 65/35 21 65/35 22 65/35 23 65/35

Fiber

Weight Weave (oz/sq yd)

nylon/cotton 7.6 nylon/cotton 8.9 nylon/cotton 7.8 polyester/cotton 8.9 polyester/cotton 11.0 polyester/cotton 7.6 polyester/cotton 7.8 polyester/cotton 7.9 polyester/cotton 3.8 polyester/cotton 5.9 polyester/cotton 6.6 polyester/cotton 6.6 polyester/cotton 7.3 polyester/cotton 8.0 polyester/cotton 8.0 polyester/cotton 8.1 polyester/cotton 8.6 polyester/cotton 10.7 polyester/cotton 5.5 polyester/cotton 6.0 polyester/cotton 6.8 polyester/cotton 7.7 polyester/cotton 7.8

Ends/Picks Char Length (in) firri 4.0 93/45 2/1 TWILL 2/1 TWILL 92/56 4.6 2/1 TWILL 81/53 3.6 3/1 TWILL 124/57 5.1 2/2 TWILL 88/39 3.7 3/1 TWILL 124/49 3.0 3/1 TWILL 128/53 4.6 2/1 TWILL 87/48 3.7 PLAIN 86/53 6.0 2x1 BASKET 104/41 5.5 2/1 TWILL 92/48 5.5 2/1 TWILL 104/48 4.6 2/1 TWILL 87/52 4.0 2/1 TWILL 98/46 4.6 2/1 TWILL 102/49 4.5 2/1 TWILL 96/48 5.0 2/1 TWILL 102/49 5.6 2/1 TWILL 81/48 3.7 2/1 TWILL 108/54 5.0 2x1 BASKET 108/72 6.4 2x1 BASKET 108/72 5.0 2/1 TWILL 87/44 3.7 2/1 TWILL 87/44 5.1

In addition t o these tests, a l l fabrics were tested for v e r t i c a l flammability, i n accordance with TM 5903 of FED STD 191, to verify their flame resistance characteristics and were shewn to be f i r e retardant, with char lengths of 3.0 t o 6.4 inches.


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Results and Discussion


A l l fabrics were rated i n each of the four tests for estimate of melt/stick bum injury potential. A t o t a l rating from a l l four tests was calculated and the fabrics ranked from highest to lowest risk. Table I I l i s t s the ratings and order of rank. For fabrics with equal t o t a l ratings the fabrics are l i s t e d by polyester content and then by weight.

Table I I . Fabric No. Blend 2 3 4 7 5 18 8 11 12 13 16 17 10 14 1 21 15 6 19 9 20 22 23

Weight

25/75 r\/c 8.9 30/70 r/c 7.8 35/65 p/c 8.9 45/55 p/c 7.8 40/60 p/c 11.0 50/50 p/c 10.9 45/55 p/c 7.9 50/50 p/c 6.6 50/50 p/c 6.6 50/50 p/c 7.3 50/50 p/c 8.1 50/50 p/c 8.6 50/50 p/c 5.9 50/50 p/c 8.0 25/75 rv/c 7.6 65/35 p/c 6.8 50/50 p/c 8.0 45/55 p/c 5.5 65/35 p/c 5.5 50/50 p/c 3.8 65/35 p/c 6.0 65/35 p/c 7.7 65/35 p/c 7.8

1/

0 no stick

Fabric Rankings 1/

Hot Radiant Heat Plate Gun 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 1 0 2 2 2 2 2

0 0 0 0 0 0 1 0 0 1 0 1 1 1 2 2 1 2 2 2 2 2 2

1 slight stick

0 0 0 0 0 0 0 0 0 0 1 1 2 0 1 2 2 2 2 2 2 2 2

Oven

Total

0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 2 2 2 2 2 2

0 0 0 0 0 0 1 1 1 1 2 2 3 3 4 4 5 6 8 8 8 8 8

2 stick

Oven Tests Five fabrics were rated "2" i n the oven test. In a l l five cases, considerable molten polymer s o l i d i f i e d on the surface of the fabrics which caused them t o become very s t i f f and fragile. Three of the five fabrics contained 65 percent polyester, one was a 50/50 polyester cotton-blend and the other was a 45 percent polyester blend. Four additional fabrics cracked and s p l i t when folded and were rated "1". Two of these fabrics contained 50 percent polyester, one 65 percent polyester and the other 25


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percent nylon. The balance of the fabrics had varying degrees of stiffness with no cracking.


Hot Plate Ttests Five fabrics rated "2" on t h i s test including four fabrics containing 65 percent polyester and one with 50 percent polyester (#9). Five additional fabrics shewed slight sticking and were rated "1". These five fabrics contained 50 percent polyester. One 65 percent polyester fabric (#21) showed no sign of melt/stick on either t h i s or the oven test. This fabric i s identical to fabric #20, except that i t was treated with a brcmine based flame retardant f i n i s h i n a latex binder, which appeared to help contain the melting polymer under these test conditions. Radiant Heat Tests A t o t a l of eight fabrics showed a tendency to stick under these test conditions. A l l five of the 65 percent polyester fabrics failed t h i s test with a rating of "2". Fabric #9 also f a i l e d t h i s test, as well as fabric #6 with 45 percent polyester, and #1 with 25 percent nylon. Six fabrics shewed slight sticking i n t h i s test, five with 50 percent polyester and fabric #8 with 45 percent polyester. Convectiw Hnt-

AIT»

Tests

Nine fabrics rated "2" i n this test, including the five fabrics containing 65 percent polyester. As i n the radiant tests, fabrics #9 and #6 were also rated "2", along with two fabrics cxantaining 50 percent polyester. Fabric #1, with 25 percent nylon, and two 50/50 polyester cotton fabrics showed slight sticking. The balance of the fabrics shewed no tendency to stick i n either the radiant or heat gun exposure tests. Conclusions Of the test methods used i n this evaluation, the radiant and heat gun tests appear to be the most reliable. Test equipment can be duplicated at modest cost, the tests are simple to conduct and results are consistent. During these tests the melting polymer i s drawn toward the source of heat, which i s the face side of the fabric. When sufficient heat i s transmitted to the second layer of fabric, the polymer flows toward the front fabric. I f there i s sufficient flew, the fabrics stick together. Consequently, these tests would be more easily interpreted as a pass/fail test. Any fabric separation during handling should not be considered as "slight stick", as rated i n t h i s study. I t i s f e l t that any fabric shewing slight stick i n these tests (samples separating easily with no transfer of polymer), pose l i t t l e r i s k of melt/stick induced burn injury. A l l test results reported are the worse ratings achieved for each fabric/test condition. Replications (three) on the radiant and heat gun tests showed consistant ratings. The oven and hot


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plate tests were more difficult to control, resulting in inconsistent ratings from test to test. All of the 65/35 polyester-cotton fabrics showed poor results in the radiant and heat gun tests. Fabrics #9 and #6, containing 50 and 45 percent polyester, respectively, also showed poor results in both of these tests. Attempts to explain the poor results achieved with these two fabrics were not successful. It was originally felt that the fabrics may have contained too much finish thus preventing the melting polymer from dispersing into the cotton. As a result, some of the finish was removed and the fabrics were re-tested. This, however, provided no change in results. Two additional 50/50 polyester-cotton fabrics (#15 and #17) failed the heat gun test and fabric #11 failed the radiant test. Discounting the slight stick criteria, thirteen fabrics containing 50 percent or less polyester or nylon demonstrated little risk of melt/stick induced burn injury while five, or 28%, showed a tendency to melt/stick. Based on the test results of the recommended tests, fabrics cmtaining 65 percent or more polyester are at high risk of causing melt/stick burn injuries, while fabrics containing 50 percent or less thermoplastic fiber appear to be of low risk, although individual failures indicate more testing is needed before definite conclusions can be drawn. Thirteen fabrics showed little or no tendency to melt/stick indicating that with control of fiber blending as well as fabric construction and finishing, polyester or nylon-cotton blend fabrics could be produced that will not add to the risk of burn injury through melt/stick. This study shows that the radiant and heat gun tests can be used to estimate the relative melt/stick characteristics of FR treated thermoplastic>cotton blend fabrics although additional work is needed to determine the precision of the methods. Literature Cited 1. Miles, L. B. An investigation of the Flammability Hazard of Apparel Fabrics; Symposium on Textile Flammability; Leblanc Research Corp.: 1978. 2. Langstaff, W. E.; trent, L. C. The Effect of Polyester Fiber Content on the Burn Injury Potential of Polyester/Cotton Blend Fabrics: 1980, 7, 26. 3. Goynes, W. R.; Trask, B. J . Effects of Heat on Cotton, Polyester, and Wool Fibers in Blended Fabrics - A Scanning Electron Microscopy Study; Textile Research Journal: July 1985. 4. Chouinard, M. P.; Khodel, D. C.; Arnold, H. W. Heat Transfer from Flammable Fabrics; Textile Research Journal: March 1973. 5. Anon NFPA 1971 - Protective Clothing for Structural Firefighters, 1986 edition; National Fire Protection Association: Quincy, MA. 6. Randall, A. Correspondence, Stores and Clothing Research and Development, Ministry of Defense, U. K., Dec. 17, 1987. RECEIVED June

1, 1990


High-Tech Fibrous Materials - American Chemical Society

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