Dual elastomer gasket for protection against magnetic interference

An improved gasket shielding against magnetic waves is provided in the form of an integral two-layer component including a support layer and a cover layer, the support layer being strong and having excellent physical properties, and the cover layer being filled with ferrite or magnetite or the like.

The present invention relates to the shielding of various types of 
equipment, such as electronic equipment, to protect same against magnetic 
interference. It specially concerns gaskets for cabinet enclosures or the 
like, which cabinets are adapted to receive various types of magnetic 
sensitive equipment which shold be shielded against magnetic interference, 
the gaskets serving to shield the contents of the cabinet from damage due 
to exterior magnetic interference. 
BACKGROUND OF THE INVENTION 
Producers of commercial electronic equipment, not to mention defense 
applications, are being forced to improve the performance of shielding 
devices to counteract electromagnetic interference complicated by the ever 
higher frequencies of energy generation in and beyond the megahertz range. 
Openings in cabinet enclosures for access doors, hinges, etc. must be so 
protected by the use of suitable shielding. 
This has been an ongoing problem and various solutions have been proposed, 
these prior solutions however being unsatisfactory from one or more points 
of view. Thus, the prior devices have been insufficient from point of view 
of at least one or more of the following requirements: 
(1) Attenuation of internal and external interferences using gasketing with 
resistive values with the range of 0.1 to 0.001 ohm-cm and selectable 
attenuation form 30 to 90 dB. 
(2) Low compression set of the gasket so long term mechanical sealing is 
maintained. 
(3) Superior physical properties of the gasket to prevent failure due to 
tearing forces. 
(4) Resistance to reduced electromagnetic shielding due to oxidation. 
(5) Ease of cleanliness maintenance. 
(6) Freedom from free metal chips as occurs with the initial fitting and 
subsequent deterioration of metallic meshes and metal assemblies. 
(7) A neat appearance of the shield. 
(8) Ease of installation and replacement. 
(9) Low cost relative to alternate designs. There are presently available 
four different types of cabinet shielding means which suffer one or more 
defects. These are as follows: 
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Product Principal Inadequacy 
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(1) Carbon Filled Elastomers 
Shielding ability falls off 
in the higher frequency 
range 
(2) Metal Filled Elastomers 
Poor physical properties 
(compression set, tear, 
etc.) 
(3) Metal Mesh with Hard to cut cleanly for 
Elastomeric Core corners; chips of free 
metal 
(4) Metallic Seals Difficult to keep clean, 
(Spring-like fingers) 
oxidation 
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In addition, it is to be noted that most metals are problematic in 
shielding against magnetic waves unless the metals are in extremely thick 
layers. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention to overcome 
deficiencies in the prior art. 
It is another object of the present invention to provide an improved gasket 
shielding material for protection against magnetic waves. 
It is yet another object of the present invention to provide dual elastomer 
gasket shielding for counteracting magnetic interference. 
These and other objects are achieved according to the present invention by 
the provision of an elastomeric gasket formed in two layers, such as by 
dual extrusion, in the form of a support layer having good strength and 
other physical properties including good strength, and a relatively thin 
coating layer containing ferrite or magnetite materials or the like having 
excellent ability to counteract or attenuate magnetic waves. 
The above and other objects and the nature and advantages of the present 
invention will be more apparent from the following detailed description of 
certain specific embodiments thereof, as disclosed below.

FIG. 1 shows an EMI gasket 10 of typical configuration, but in accordance 
with the present invention, having a metal clip part 12 and a rubber 
gasket part 14 adhered together. FIGS. 2a-2f show various other typical 
forms which may be used. 
In accordance with the present invention, the gasket part 14 is formed of a 
non-metallic, yet preferably conductive, elastomeric inner portion 16 to 
which is permanently bonded, preferably by co-extrusion within the die, an 
outer sheathing or covering 18 of either ferrite- or magnetite-filled 
elastomer. From the embodiment of FIGS. 1, 3, 2a, 2b, and 2d-2f, it will 
be understood that such a gasket may be co-extruded and bonded during 
extrusion onto metallic and conductive non-metallic shapes for convenient 
assembly to cabinets. From FIG. 2c, it will be understood that the gasket 
material can be provided in other shapes which can be subsequently 
attached to a suitable substrate. In general, the extrusions are made in 
continuous length, shaped to any desired uniform cross-section. 
The product can be formed onto a metallic carrier, for example a flat metal 
strip (see FIGS. 2a, 2e and 2b) or a roll-formed shape (FIGS. 2d and 2f) 
by passing the metal substrate through the co-extrusion tooling in a 
continuous process. The inner portion 16 is chemically locked to the 
metallic layer thus ensuring transmission of electrical energy unimpeded 
between the co-elastomer and the metallic carrier. 
The inner layer 16 is constructed of a strong elastomeric material which 
has good physical properties, including high tear resistance, as well as 
the ability to recover shape after deformation, i.e. good elasticity. This 
material is desirably carbon filled rubber. 
The magnetic material filled exterior covering 18 provides a high degree of 
counteracting of magnetic energy than presently available non-filled 
elastomeric shielding. On the other hand, such magnetite/ferrite filled 
elastomer gasket material used alone as a shielding material suffers from 
poor physical properties, including low tearing resistance, and its 
recovery of shape after deformation is poor. The magnetite/ferrite 
particle filler does not reinforce the elastomer and acts as a diluent. 
By chemically bonding a magnetite or ferrite-filled elastomer 18 to a 
carbon-filled elastomer substrate 16, the tear resistance and compression 
recovery of the gasket 14 substantially improved. In addition, the 
shielding effectiveness is substantially improved over the use of 
carbon-filled rubber alone. The product is easily cleaned to maintain 
shielding effectiveness and presents a neat appearance. 
When the two elastomers 16 and 18 are simultaneously bonded under the 
pressure of co-extrusion, the unity of the resultant gasket is assured and 
electrical and physical results are optimum. 
It will be understood that both layers 16 and 18 serve to counteract 
magnetic interference, except that the outer layer 18 is considerably more 
effective in this regard while the inner layer 16 is less 
shielding-effective but much stronger and with better physical properties. 
They are bonded by co-extrusion in a molecular sharing way, and thus 
provide a superior electrical shield over either one used alone. 
The inner layer 16 is substantially thicker than the outer coating, 
preferably on the order of 5 to 20 times as thick when such layer 16 is 
hollow as preferred, preferably at a thickness ratio of 10:1, the outer 
coating 18 desirably being 2-8 mils thick and the inner support layer 16 
being 5-160 mils thick, preferably 20-80 mils thick. 
The inner support layer may be formed of a variety of elastomers, but 
preferably it comprises an elastomeric silicone polymer containing fumed 
silica and carbon black as reinforcers, plus a suitable cross-linking 
agent. Alternatively, the inner layer 16 may be composed of a 
non-conductive silicone elastomer, such as one comprising silicone gum, 
fumed silica and a cross-linking agent. 
As in patent application Ser. No. 07/269,350 now U.S. Pat. No. 4,968,854, 
the rubber matrix is preferably silicone rubber, although other 
elastomeric materials can be used such as, for example, natural rubber, 
nitrile rubber, or epichlorohydrin rubber. As in patent application Ser. 
No. 07/269,350 , now U.S. Pat. No. 4,968,854, gaskets of the present 
invention are preferably formed by co-extrusion. The extrusion head is 
desirably subjected to a magnetic field so that as the outer layer leaves 
the die and becomes solidified, the ferrite and/or magnetite particles 
which constitute the main functional filler are aligned into position for 
polarity. 
The quantity of ferrite and/or magnetite in the outer layer composition is 
subject to wide variation, but in general may comprise from 0.5 to 6 parts 
by weight, or even more, per part by weight of elastomer matrix. The 
quantity of ferrite and/or magnetite must not be so low as to result in 
insufficient magnetic protection, nor so high that the outer layer becomes 
brittle and/or inflexible. 
Gaskets produced according to the present invention shield against 
magnetizing field force at 200 kHz, and provide 110 dB attenuation as per 
military standard D-285. Gaskets according to the present invention 
provide the user who is concerned with magnetic protection a highly 
flexible strong, elastomeric gasket material not previously available and 
at a lower cost. Gaskets according to the present invention are not 
intended for use in frequency fields substantially higher than 200 kHz, as 
at such higher frequencies the present magnetic gaskets will not provide 
good attenuation; instead, at such higher frequencies the gaskets 
exemplified in patent application Ser. No. 07/269,350 now U.S. Pat. No. 
4,968,854, are preferred. 
The following example is offered illustratively: 
EXAMPLE 
Suitable gasket material is made using inner core material as disclosed in 
patent application Ser. No. 07/269,350, now U.S. Pat. No. 4,968,854, and 
co-extruding therewith as an outer coating a composition as set forth in 
Table 1 below. 
TABLE 1 
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100 pbw Silicone reinforced gum 
400 pbw M Fe.sub.12 O.sub.19 (M is a divalent metal 
ion such as Ba or Sr) 
2 pbw Organic peroxide (dicumyl or 2,4 
dichlorobenzoyl types) 
1-3 pbw Colorant, as needed 
0.2-1.0 pbw Dispersant aids 
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The foregoing description of the specific embodiments will so fully reveal 
the general nature of the invention that others can, by applying current 
knowledge, readily modify and/or adapt for various applications such 
specific embodiments without departing from the generic concept, and, 
therefore, such adaptations and modifications should and are intended to 
be comprehended within the meaning and range of equivalents of the 
disclosed embodiments. It is to be understood that the phraseology or 
terminology employed herein is for the purpose of description and not of 
limitation