The effect of a substrate on fabric combustibility

A large number of publications has been devoted to the study of fabric combustibility, and this is related to the important practical significance of this problem. Particular attention in this case is devoted to the nature of the fabric [1, 2] and the choice of the antipyrenes (flame-retardant combustion inhibitors) [1]. The effect of fabric surface density (i.e., the mass of a unit of fabric area) [3], the moisture content of the fabric [4], the presence of an external source of radiated heat [5], the velocity of the gas flow streamlining the specimen [6] was also examined. The velocity of flame propagation in various directions relative to the warp of the fabric was investigated in [7]. It is paradoxical that in the available li terature there are no experimental results with respect to the influence of a substrate on the combustibility of fabrics, although this question is of obvious practical significance for all manner of clothing and upholstery fabrics. At the same time, there exists an entire series of publications [8-10] in which the effect of a substrate on the combustion of plane or cylindrical layers of certain polymers has been examined. A theoretical analysis of the influence of a number of parameters on the velocity and structure of a flame propagating along the surface of a solid-fuel specimen in a gaseous oxidizer was undertaken, for example, in [11, 12], although in these studies no substrate was present for the specimen. When investigating the role o fa substrate, we have to separate out two cases: 1) material in contact with the substrate virtually without any clearance; 2) there is a gap between the material and the substrate through which air and the products of combustion might pass, and under certain conditions, through which a flame might propagate. It would be interesting to examine the question as to the influence exerted by a substrate on the critical conditions of combustion and on the velocity of flame propagation over the surface of a material. F o r the purposes of our experiment we used a cotton-polyester fabric with a surface density of 290 g/m 2, classified under article 3303 in GOST 12239--76. Aspec imen 80 • 700 mm in size was glued at the edges to a textolite frame (10 mm thick) with a rectangular cutout 50 x 500 mm in size. The textolite insert could be fitted tightly into this cutout and assume a particular fLxed position. This allowed us to establish a A = 0-10 mm wide gap between the insert and the fabric. Such an assembly could be set up at any angle ~o to the horizontal. The tests were carried out at ~o = --90 ~ (the combustion proceeding vertically downward), at --60, ---45, --30 ~ and 0 (horizontally); 30, 45, 60, and 90* (the combustion proceeding vertically upward). The fabric was ignited with a Nichrome spiral (the diameter of the spiral was 10 ram, and its width was 50 mm). All of the tests were conducted in atmospheric air at a temperature of 18-20~ and a relative humidity of 60-80%. The flame velocity W = l / r was calculated on the basis of the time r (measured with a stopwatch) required for the flame to pass over a segment of fabric of length 1. The movement of the flame was established on the basis of the extent to which these fabrics became blackened. The critical clearance between the substrate and the fabric A* = (A + + A--)11, where A + represents the minimum clearance at which the entire test segment was subjected to stable burning, while A-represents the maximum gap for which the combustion was extinguished, without reaching the end of the fabric. The critical flame velocity was defined as W* = W/A +. It should be noted that the accuracy in the measurement of W for combustion proceeding vertically downward was considerably higher than in the case of upward combustion. In the latter case, the flame was speeded up during the course of the combustion, while the blackening front changed shape and was shielded by tongues of flame. The results o f these tests are shown in Table 1. We can see that in the test without a substrate (A ~ 0o) the value of W for +90 >__ ~o > --30 ~ drops rapidly as !o diminishes, while in the interval ~ = --90 to --30 ~ it remains virtually constant. When A = 10 mm the fabric burns throughout the entire range of ~ values and the flame velocity drops off by no more than 10-20% from its value in the absence of a substrate, while for the case in which A < 9 mm the specimen no longer burns for all values of ~: the narrower the gap, the smaller the range of ~o in which combustion is stable. For each fixed ~o the value of W quickly drops in proport ion to the reduction in A and at some A* combustion ceases. In proportion to the rise in ~o the quantity A* diminishes (weakly when ~ = --90 ~ to 0 and markedly when ~o = 0 to +60~ cp, degi ~90 ~60 --45 --30 0 4-30 +45 4-60 A*,mra 9,5 8.5 8,5 8,5 7,2 3:8 3,0 1,5