I
R. A. RUSSELL The Connecticut Hard Rubber Co., New Haven, Conn.
Fibrous Silicone Rubber The new material has improved compression set resistance and greater porosity than cellular elastomers
Is easily deflected
by light compressive loads
A
NEW FORM O F expanded silicone rubber called fibrous silicone rubber (3, 4, 6 ) , has been developed. I t has many properties of both foamed and sponged elastomers, but unlike either, it contains a random weblike arrangement of hollow filaments which taper to a closure a t both ends. Length and diameter of the filaments can be varied by adjusting process variables, but in a typical mat they may be 1 to 4 inches long with an average diameter of 0.01 inch. The filaments are produced by a spray process and bonded together to form a porous resilient mat in one continuous operation (7, 2).
Spray Process T h e spray process consists of four major steps : compounding silicone rubber stock, preparing the silicone dispersion, spraying and collecting the filaments, and curing the finished mat. A viscous dispersion of silicone compound is fed under pressure to a series of positive displacement gear pumps and then delivered to a spray gun. A spray gun consists of the dispersion delivery tube, which terminates in a fine nozzle, mounted concentrically within a larger tube that is connected to a source of high velocity air called primary air. The primary air issuing from the annular
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1
50
SILICONE SPONGE J - 1 FIBROUS SILICONE RUBBER
I-
z
W
space between the inner and outer tubes attenuates the stream of dispersion into fine filaments. The filaments are sprayed from a row of such guns into a chamber heated by a secondary air stream which evaporates the solvent and carries the filaments to a collecting belt of open-mesh metal fabric. Rapid evaporation of the solvent results in a hollow filament; however sufficient solvent must be retained to make the filaments tacky and bondable. Turbulence induced by the primary and secondary air streams ensures the random arrangement of the filaments on the collecting belt. The solvent-laden air is
L
w
40
W
a 30 v)
C DEFLECTION - % Figure 1. Compression-deflection characteristics of a fibrous mat at a given density may be varied by changing diameter, wall thickness, and rigidity of the hollow fllament
350
212
158
TEMPERATURE -
70
OF:
Figure 2. The excellent compression-set characteristics of the silicone matting are related to fibrous structure rather than to composition VOL. 52, NO. 5
e
M A Y 1960
405
Commercial Outlets
In the spray process, cornposition and $lament structure can be varied to give greater control over strength and dynamic properties of the resilient mat. Potential uses include:
laundry and dry cleaning press padding.
exhausted through the collecting belt to a solvent recovery system. Through proprr manipulation of the variables of the spra) process, the thickness, density, and filament structure can be controlled. As the mat builds u p on the collecting belt, it is conveyed through a curing oven, trimmed, and rolled up. The rolls of matting are subjected to additional high temperature curing in an air-circulating oven to develop the optimum properties of the silicone rubber. Physical Properties Compres sion-D efle ction. I n Figurc 1, the curves can be represented with reasonable accuracy by the equation, y = Ken" where y is the load in pounds per square inch, x is the per cent deflection, and K and n are the parameters of
the equation. The composition and filament structure for all of the samples tested were the same. The differences in density reflect the differences in the numbrr of filaments in a given unit volume of mat. For these samples, the parameter K was a linear function of the density, and the parameter n was constant. There is limited evidence a t this time which indicates that the parameter n is a function of the filament structure. This suggests that the compression-deflection characteristics of a fibrous mat a t a given density can be varied by changing the diameter, wall thickness, and rigidity of the hollow filament. Compression Set. The data shown in Figure 2 were compiled from the compression-set values obtained with a number of fibrous and sponged slabs The densities of these slabs covered a range
..
of from 15 to 22 pounds per cubic foot for the fibrous mats and a range of 27 to 30 pounds per cubic foot for the silicone sponge. The excellent compression sets obtained with the fibrous silicone rubber matting are not due to the compound from which they were made, which is relatively poor in this respect, but rather are related to the fibrous structure. As the fibrous mat is deflected by greater and greater amounts, the filaments are brought closer together without deformation until finally they collapse and pack down. Although highly simplified, this is essentially the mechanism of mat deflection which has been observed (Figure 3). It would be reasonable to expect lower, that is better, compression-set values a t deflections which did not cause the filaments to collapse, because of the presence of open passages for the escape of volatile decomposition products and the negligible deformation of the filaments. Furthermore, a fibrous mat of lower density, that is a lesser number of filamrnts in the same unit volume, should be capable
Table 1.
Difference in Compression Set a t 50 and 75% Deflection Is Greater for High Density Mats (ASTM D 395-58T) Cornprkssion Set," Mat Density. Deflection. Lb./Cu. Ft.' % % 22 15
50 75 50
19 45 14
75
21
At 300' F. for 22 hr.
The hollow filaments taper to closures on both ends
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIBROUS SILICONE RUBBER
high or low temperature sealing.
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of sustaining a greater amount of deflection before the filaments collapse and would be expected to have good compression-set values over a wider range of deflections. The data in Table I show little difference in compression set a t 50% deflection where there is little filament collapse in either sample. At 75% deflection, where the filaments of the higher density sample have been collapsed, a large difference in compression set is observed. The presence of open passages and the negligible deformation of the filaments a t deflections u p to 50% and higher for mats of lower density is considered to be the reason for the excellent compression set. Porosity and Water Absorption. The flow of air through a fibrous or cellular elastomer is related to the dimension of the passages through which the air must flow. The low resistance to air flow exhibited by the fibrous silicone rubber (Table 11) is indicative of the relatively large size of the interstices between the filaments. The magnitude of
thermal installation.
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vi bration absorption and.
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Table II. Air Flows Rather Freely through Fibrous Silicone Rubber Air Flow Rate, Ft. / M i a
84
119
147
170
Pressure drop, mm. of water Fibrous silicone 3.2 rubber" Polyurethane foamrubberb 6 . 6
5.1
7.1
8.9
10.7
14.2
18.8
E '/pin. thick mat having a density of 20 lb./cu. ft. S/m-in. thick flexible foam having a density of 2.3 Ib./cu. ft.
sterilizable cushioning VOL. 52,
NO. 5
MAY 1960
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0 0 0 Q Q Q 0-0 Q Q 0 0 0
Table V. Dynamic Properties of Fibrous Silicone Rubber Sample
25 %
0%
z
50 %
75
ASTM D 1056-6ST
Figure 3. As the per cent of defl ction increases, the filam nts are brought closer together without deformation until they finally collapse and pack down
Table 111.
Water Drains Rapidly from Silicone Rubber Mats (ASTM D 1056-56T) Wt.
Material Fibrous silicone rubber
7cIn-
crease After Immersion Dens., After Lb./ 5 min. Cu. Ft. Initial drain 14 17 ~. 19 2
Polyurethane foam
35
17
18
10 ~.
~~
22 1900
12 1700
Table IV. By Varying the Base Compound, Mats Having Different Strengths Con Be Prepared ElonDensity, Tensile gaLb./ Strength,a tion, Compound Cu. Ft. P.S.I. yo Fibrous silicone rubber 20 10 100 27 22 20 27
65 50 5 70
125 200 100 200
Silicone foam Silicone sponge a From break strength of 1-inch wide strips.
this difference is effectively demonstrated in Table 111-rapid drainage of water from the fibrous mats as compared to the slower rate of drainage from the polyurethane foam filrther illustrates the difference between the fibrous and cellular structurr. Microscopic examination of fibrous mats after water submersion for several weeks has shown that little if any water enters the hollow filaments, except of course for the small percentage of filaments severed at the cut edges of the matting. Thermal Conductivity. Thermal conductivity of fibrous silicone rubber has been determined for a mat having a density of 20 pounds per cubic foot and a thickness of 0.25 inch. T h e value of K ohtained was 0.56 B.t.u. per square foot per hour per O F. per inch. The good insulating properties of the fibrous mat despite its high porosity and open structure which are conducive to convective heat transfer is believed to result from the presence of the sealed hollow filaments. Tensile Strength a n d Elongation. T h e fibrous rubber spray process can be used with any silicone rubber compound which forms a satisfactory dispersion, Variation of the base compound and the
Thick-
Resonant Fre-
Density, Lb./Cu. Ft.
ness,
quency,
In.
C.P.S.
Transmissibility
20 20 31
1/4
22.0 24.0 29.0
8.0 6.0 10.0
5/16
s/s
amount of curing can result in fibrous mats which have different strength properties although the density and filament structure are the same (Table IV). None of the silicone compounds evaluated as yet are of the class termed high strength silicone rubbers. and it is reasonable to anticipate that future developments will lead to fibrous mats with strengths far greater than any attainable with cellular silicone elastomers. Dynamic Properties. Transmissibiliry measures the capacity of a material to absorb energy and act as a vibrarion insulator (5). For the three samples listed in Table V, the input double amplitude was increased in subsequent testing. However, the fibrous mats developed additional modes of vibration a t the higher input amplitudes which distorted the sinusoidal wave form and rendered the test results inaccurate. I t has been found that the hysteresis and rebound resiliency of the fibrous mats can be varied within rather wide limits by changing the filament structure and composition of the mat. This suggests that dynamic properties and transmissibility of the fibrous mats can also be changed by such variations. Acknowledgment
fr,
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TO SOLVENT RECOVERY
The author wishes to thank Alfred Kidwell for his many helpful recommendations, Irving Allen for his lucid drawings, and T h e Connecticut Hard Rubber Co. for permission to publish this report. Literature Cited
ROLL WIND-UP
AIR
BELT
FEED TANK
GEAR PUMPS
PRIMARY AIR BLOWER
TRIM MING
DISKS
In the spray process, a hollow filament i s formed b y rapidly evaporating the solvent with a secondary air stream
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Francis, C. S., Jr. (to American Viscose Corp.), U. S. Patent 2,483,404 (Oct. 4,1949). (2) Zbid., 2,483,406 (Oct. 4, 1959). ( 3 ) Kilbourne, F. L., Jr., Kidwell, .4.S., Moroney, T. S., Materials €8 Methods 45, No. 5, 140 (1957). (4) Ibid., No. 6 , 114 (1957). (5). Macduff, J. N., Curreri, J. R., “Vibration Control.” McGraw-Hill, Xew York, 1958. ( 6 ) McGregor, R. R., “Silicones and their Uses,” Ibid., 1954. RECEIVED for review June 5, 1959 ACCEPTED February 12, 1960 Division of Rubber Chemistry, 75th Meeting, ACS, Los Angeles, Calif., May 1959.
