No\TMBER 1, 1920 



THE INDIA RUBBER WORLD 



103 



What the Rubber Chemists Are Doing 



FIREPROOFING AIRSHIP FABRICS" 



THE following notes are abstracted from a report by Guy IJarr, 

 B. A., B. Sc, of official tests on proofing airship fabrics 

 against ignition by gnni fire. 



The special fabric (B. 29) under test .was of treble cotton and 

 lubber, and had been doped on the exterior surface with alumi- 

 num dust suspended in "soluble gun-cotton." Comparison of 

 results was made with similar tests made on an ordinary yel- 

 low treble balloon fabric. 



A rubbered fabric does not continue to burn if locally sparked, 

 unless the heat supplied by the lighting agent plus tliat due to 

 the small quantity of fabric consuqjcd is sufficient to cause 

 decomposition of the subjacent rubber. If the heat reaching 

 the rubber is at any point too small to cause the distillation of 

 volatile vapors therefrom, the conflagration is not transmitted 

 to that point. The combustion of rubber requires a very much, 

 greater volume of air than that of cotton, and the supply of 

 air being limited, tlie diffusion of the inllammable rubber de- 

 composition products is the preponderating agent in the spread 

 of the fire. 



The wads used for these comparisons were made by cutting 

 out disks of cardboard of the correct diameter from a sheet 

 of such thickness that the efTect of placing one of these disks 

 on a piece of balloon fabric was the same as that caused by 

 wads removed from a few .303-inch cartridges. 



COMPARISON OF FABRICS 



When the treble yellow fabric was compared with B. 29, it 

 was found that the former was occasionally burnt completely 

 through by the smoldering wad, and nearly always damaged 

 as far as the innermost cotton layer. With B. 29 the dope was 

 not only not a source of danger in this respect, but the fabric 

 was actually somewhat protected. A wad would occasionally 

 damage the innermost layer, but usually the outer two layers 

 were alone attacked, and in no case was a hole burned right 

 through. These results were further confirmed in the pres- 

 ence of hydrogen confined under a pressure of about an inch 

 and a half of water by a piece of the fabric attached to a suitable 

 vessel. In spite of repeated attempts the fabrics were not 

 burnt through by the wad in either case, and even when, by 

 the successive application of three or four wads, a hole had 

 been burned, the gas which escaped did not catch fire. In 

 fact, wads smoldering with sufficient energy to cause their 

 complete combustion were found to be unable, at any rate in 

 the half-dozen tests made under laboratory conditions, to ignite 

 hydrogen or hydrogen-air mixtures. 



In the above cases the damage done to the fabric by a smolder- 

 ing wad was sufficient to cause a considerable local reduction 

 of strength, together with a good deal of softening of the rub- 

 ber. The position of the damage was, of course, readily visible 

 by the blackeninc: of the yellow fabric, but the aluminum-doped 

 B. 29 showed very little trace of the incident on a cursory 

 examination. It was only when the burnt spot was rubbed 

 with a pencil or other moderately hard body that the wnite 

 film of metallic powder was removed, and the scorching ren- 

 dered visible. 



FIRE RESISTANCE OF FABRICS 



The resistance to fire of these fabrics is thus somewhat small, 

 nor can any considerable protection be expected from any or- 

 dinary fireproofin.g. however efficient, applied to the different 

 plies of cotton. After consideration it was decided that the 

 most promising method of attack lay in the provision of an 



outer metallic coating. The attachment of metal foil appeared 

 to offer almost insuperable difficulties, but the following method 

 of procedure was found to afford a very gratifying measure 

 of resistance to ignition by smoldering: 



Various inventors have occupied themselves with the prob- 

 lem of spraying metallic coatings on woodwork, ironwork, 

 etc., with the idea of forming a coat to resist atmospheric in- 

 fluences. The latest and most practical apparatus for the pur- 

 pose is one due to Schoop, some details of the use of which 

 are given in a paper by Morcom (Institute of Metals). 



The principle of the method is briefly the feeding of a wire 

 of the metal to an oxy-hydrogen blow-pipe flame, where the 

 molten metal is atomized by a blast of air external to and 

 concentric with the oxy-hydrogen flame. The particles of mol- 

 ten metal are rapidly cooled by the air-blast, and reach the 

 surface to be coated at a temperature very slightly above or 

 possibly below their melting-point. In virtue of their velocity, 

 and perhaps also of their high temperature, they adhere firmly 

 to the object to be coated. The cooling and scattering is sufficient 

 to insure that no undue rise of temperature occurs on the 

 sprayed surface. 



A small sample of single rubbered fabric was sprayed with 

 aluminum by this process in order to see whether the fire- 

 proofing was of any value. It was found that a smoldering 

 wad which would burn a hole through a treble fabric scarcely 

 affected this sample beyond slightly softening a small portion 

 of the rubber. This effect was largely due to the heat con- 

 ductivity of the metallic surface. The tensile strength of the 

 sample was not affected by the spraying. It is therefore evident 

 that the metallic particles are sufficiently small and well-cooled 

 to do no damage to the cotton. This being the case, it is reason- 

 able to conclude that the rubber, and hence the permeahility 

 will also be unaffected. 



•Reports and Memoranda, No. 178, (British) Advisory Committee for 

 Aeronautics. 



THE PRESERVATION OF VULCANIZED RUBBERS 



The statement that vulcanized rubber kept in a tin box over 

 a layer of ordinary kerosene remains in a serviceable condition 

 for a much longer period than if kept in air, led Dr. H. P. 

 Stevens to test the preservative effect of kerosene and other 

 vapors. The results confirmed the claims made for kerosene 

 vapor and also showed that water vapor has a similar or even 

 more marked effect. 



From experimental results, particularly the constancy of the 

 acetone extracts after aging, it appears that the preservative 

 action of water or kerosene vapor is due to an actual chemical 

 preservation of the vulcanized rubber. These agencies inhibit or 

 retard -the chemical changes, including oxidation of the rub- 

 ber, which normally take place when vulcanized rubber is aged 

 in air, as shown by increase in weight and increase in acetone 

 extract, but ihey do not retard the physical changes sometimes 

 known as "after-vulcanization," as shown by the tensile strength 

 and reduction of distensibility (reduced final strength) of test 

 rings kept in air saturated with water vapor. We may there- 

 fore distinguish two changes which normally take place when 

 vulcanized rubber ages : firstly, a physical change comprising 

 an initial increase in tensile strength (if the specimen is not 

 apnreciably overcured), and a gradual reduction in final length; 

 Secondly, a chemical deterioration, consisting mainly in an oxi- 

 dation with a sli.ght loss of sulphur in a volatile form. The 

 extent of the chemical change is conditioned (1) by the coeffi- 

 cient of vulcanization, the higher the coefficient the more rapid 

 the oxidation, (2) by the atmosphere surrounding the specimen, 



= Tniirnal of the Society of Chemical Industry, July 31, 1920, page 251. 



