Electrically conductive polyethylene foam

An electrically conductive polyethylene foam which is in the form of a profile, e.g. a sheet, having a closed cell foam structure and exhibiting a surface resistivity of not more than 10.sup.5 ohms/square. The sheet is prepared by a continuous method wherein an extruded, foamable, non-crosslinked preform containing a mixture of polyethylene, ethylene copolymer, chemical blowing agent, crosslinking agent and conductive carbon black is heated to produce the foam.

FIG. 1 is a schematic view of the apparatus for producing a 
non-crosslinking formable preform or matrix sheet. Pellets of a 
concentrate of polyethylene containing conductive carbon black, pellets of 
polyethylene containing a crosslinking agent and pellets of polyethylene 
and/or ethylene/vinyl acetate copolymer containing a blowing agent are 
plasticized and mixed in a screw extruder and are thereafter extruded in 
the form of a sheet from the die head of the extruder unit 1. Metered 
amounts of the pellets containing the conductive carbon black, the blowing 
agent, the crosslinking agent and resin are, respectively, fed from 
hoppers 2, 3 and 4 into the feed hopper 5 of the extruder unit. The 
extrudate at a temperature on the order of 150.degree. C. in the form of a 
sheet 6 is passed through the nip of a pair of pressure rollers 7 wherein 
the sheet is sized or calibrated to provide a preform or matrix material 
having a uniform thickness of from about 1/8 to 1/2 of an inch and 
thereafter the size preform sheet is passed through a cooling unit 8 
wherein the sheet is cooled to room temperature by jets of a cooling 
fluid, e.g. air. The cool sheet is then rolled up on a take-up roll 9. 
The rolled up sheet of foamable, non-crosslinked, matrix material is 
removed from the take-up roll and may be stored for one or more days until 
needed for the production of the electrically conductive foam. Also it 
will be understood that the sized and calibrated sheet may be immediately 
processed into a foam, if desired. 
In order to produce the unique foam product, the preform material is heated 
to temperatures required to activate the crosslinking agent and the 
blowing agent, preferably in succession, within a multi-zone oven 10 as 
shown in FIG. 2. The interior of the oven is divided into three zones by 
flexible wall curtains which are suspended vertically within the oven 10. 
An endless screen belt 11 transports the preform sheet 6 through the oven 
at a predetermined rate of travel to allow sufficient residence time in 
each zone to effect the necessary crosslinking and/or foaming. In zone I 
the preform is heated to a temperature high enough to activate the 
crosslinking agent without activating the blowing agent. In zone II, the 
temperature is increased to activate the blowing agent and to complete the 
crosslinking reactions. Then in zone III of the oven the temperature is 
further increased to complete the formation of the foam which has a 
uniform closed cell structure and a thickness of from 1/4 to 5/8 of an 
inch or more. It has been found that in the method of the present 
invention the matrix or preform material is expanded on the order of from 
1.5 to 3 times its original thickness during the foaming operation. 
The foam is then cooled by water-filled chrome rolls in the cooling unit 12 
to provide a foam product which is very flexible and which can be easily 
stored by being rolled up on a take-up roll 13 as shown in FIG. 2. 
Advantageously, it has been found that the foam products of this invention 
can be fabricated into complex shaped articles by vacuum forming wherein a 
panel of the foam material is draped over a female or male mold which may 
be heated to a forming temperature of from 265.degree. to 320.degree. C. 
One embodiment such as a shaped article is shown in FIG. 3 wherein a 
suitable package 14 provides Faraday Cage Protection For an electronic 
device 15 positioned in a recess or pocket 16 located in the center 
portion 17 of the package. The center portion 17 is raised above the edges 
18 of the package. When the lid portion 19 is closed the electronic device 
15 is completely encased within an electrically conductive shield. The 
edges 18 may be sealed by adhesive cement or by an adhesive tape strip or 
one edge may be formed with a tab that engages the other edge. 
The electrically conductive foam product of this invention and the method 
for forming the product are further described by reference to the 
following examples wherein several foam recipes were evaluated using 
various amounts of resin, crosslinking agent, and blowing agent, all parts 
and percentages are by weight unless otherwise indicated. 
REFERENCE EXAMPLES 1-5 
Using an apparatus of the type illustrated in FIG. 1 pellets of a 
concentrate containing polyethylene and conductive carbon black, and 
pellets of polyethylene and/or ethylene/vinyl acetate copolymer containing 
conductive carbon black, azodicarbonamide (a blowing agent) or 
dicumylperoxide (a crosslinking agent) were admixed within the extruder 
unit 1 and extruded at a temperature on the order of 
140.degree.-160.degree. C. In some cases, additional pellets consisting 
entirely of resin, that is polyethylene and/or ethylene/vinyl acetate 
copolymer were also added to the extruder unit from another hopper (not 
shown). The resulting extrudate is calendered to form a sheet having a 
thickness of from 1/16" to 1/4". Samples of the sheet are heated in an 
oven which is heated step-wise to provide three distinct temperature 
levels within the oven in order to duplicate passage through a multi-zone 
oven as illustrated in FIG. 2. The first level or zone I had a temperature 
of 180.degree. C.; a second or zone II had a temperature of 200.degree. 
C.; and a third level or zone III had a temperature of 220.degree. C. 
TABLE 1 
______________________________________ 
Ref. Ref. Ref. Ref. Ref. 
Example No. 
Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 
______________________________________ 
Polyethylene 
39.3 19.2 20.0 18.1 18.1 
MI 2.0 
MI 20.0 60.7 -- -- -- 14.7 
Ethylene/ -- 61.5 73.9 73.4 58.8 
Vinyl Acetate 
(VA) 9% VA, 
MI 8.0 
18% VA, -- -- -- -- -- 
MI 8.0 
18% VA, -- 19.3 6.1 8.5 8.4 
MI 2.5 
22% VA, -- -- -- -- -- 
MI 23 
Conductive 
6.0 12.6 15.0 15.0 15.0 
Black (Ket- 
jenblack (EC) 
Azodicarbo- 
5.6 4.0 7.0 8.0 8.0 
namide 
Dicumyl 1.2 1.25 1.25 1.02 1.02 
peroxide 
Total Parts 
112.8 117.85 123.25 124.02 
124.02 
by Weight 
Surface 10.sup.15 
&lt;10.sup.4 
&lt;10.sup.4 
&lt;10.sup.4 
&lt;10.sup.4 
conductivity 
ohms/sq. 
Foam density 
9.0 12.0 8.0 6.0 6.0 
P.C.F. 
______________________________________ 
It will be observed from the tabulation of data provided in Table 1 that in 
Reference Example 1 the foam had a density of 9 P.C.F., but it was not 
conductive. Also, the foam was brittle and had a blistered surface. In 
Reference Example 2, the loading of conductive black was increased and the 
foam produced from this run was conductive and not brittle, but it had a 
density of 12 pounds per cubic foot which is greater than the desired 
density of 6 pounds per cubic foot. Also this foam produced a very high 
head pressure of 2800 psi on a laboratory extruder having an extruder 
screw with a 2 inch diameter and at a rate of 60 pounds per hour. 
In Reference Example 3, using a higher level of blowing agent, it was 
possible to reduce the density of the foam. However, it was found that 
this formulation also provided a high head pressure on the laboratory 
extruder and it was not possible to produce a foam with a density of 6 
pounds per cubic foot. In the fourth run, (Reference Example 4) a 
formulation was employed which has a reduced level of crosslinking agent 
and a slightly higher level of blowing agent and it was found that this 
formulation produced a blister-free foam that is conductive and had a 
density of 6 pounds per cubic foot. Head pressure during this extrusion on 
the laboratory extruder was still too high, that is on the order of 2800 
psi. 
In the Reference Example 5, a formulation was employed using a blend of the 
ethylene/vinyl acetate copolymer and conductive black concentrate and a 
low density polyethylene-conductive black concentrate. It was found that 
in this run the head pressure in the laboratory extruder dropped to 2100 
psi. However, the head pressure of 2100 psi in the laboratory extruder 
showed that this formulation would not be suitable for a production 
extruder wherein the extrudate is formed at a rate of 500 pounds per hour. 
EXAMPLES 1-6 
In these examples, additional runs were conducted using the laboratory 
extruder having a two inch extruder screw as used in the reference 
examples and runs were conducted using a production extruder having a six 
inch screw extruder. The formulations used and the results obtained are 
tabulated in Table 2 below. 
In Example 1 in order to reduce the head pressure when using a concentrate 
containing the ethylene/vinyl acetate copolymer and conductive carbon 
black, a crosslinking concentrate was prepared using a 20 melt index low 
density polyethylene resin. Advantageously it was found that this 
formulation when extruded lowered the head pressure to 2100 psi. However, 
since it is necessary during production extrusion levels to have head 
pressures no more than 2000 psi, this formulation was considered to be 
marginal and not acceptable for production extrusion rates. 
In Example 2 a new blowing agent concentrate was used employing a 20 melt 
index low-density polyethylene and an 8 melt index ethylene vinyl acetate 
copolymer. This formulation was extruded and found to have a lower head 
pressure of 1700 psi in the laboratory extruder. It appeared to provide a 
formulation which would be suitable for use in a production large scale 
extruder. 
However, as shown in Table 2 it was found that using the same formulation 
in Example 3 using a the six inch extruder at a rate of 500 lbs/hr. caused 
very high amperage loadings and during the run the head pressure increased 
steadily. After approximately one and three-quarter hours, it was 
necessary to terminate the run because the amperage load on the extruder 
drive was at the maximum and the head pressure was on the order of 2800 
psi. The foam produced during the initial portion of the trial was found 
to be satisfactory although the cell size was larger than the laboratory 
extruder trials. At the end of the run just prior to termination, the foam 
was blistered. 
In Example 4 a new conductive black concentrate was prepared using an 
ethylene/vinyl acetate copolymer containing 22% by weight of vinyl acetate 
and blending it equally with a 20 melt index low density polyethylene. The 
head pressure achieved in this run was on the order of 1500 psi and the 
foam had extremely good appearance and excellent electrical conductivity. 
In Example 5 and in Example 6, additional formulations for producing a foam 
in the production scale extruder were prepared and were evaluated. It will 
be seen from Table 2 that the unique results of the present invention are 
achieved when resins having a higher melt index, that is on the order of 
from 15 to 25, are used in place of resins having a lower melt index. 
TABLE 2 
______________________________________ 
Example No. 
Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 
______________________________________ 
Polyethylene 
4.8 -- -- -- -- -- 
MI 2.0 
MI 20.0 12.6 16.9 16.9 54.8 61.3 61.3 
Ethylene/ 
73.8 74.2 74.2 -- 
Vinyl Ace- 
tate (VA) 
9% VA, 
MI 8.0 
18% VA, -- 8.9 8.9 8.6 3.4 3.4 
MI 8.0 
18% VA, 8.7 -- -- -- 
MI 2.5 
22% VA, -- -- -- 36.6 35.3 35.3 
MI 23 
Conductive 
15.0 15.0 15.0 15.0 14.0 14.0 
Black (Ket- 
jenblack 
(EC) 
Azodicarbon- 
8.0 8.0 8.0 8.0 5.0 4.2 
amide 
Dicumyl 0.95 0.90 0.90 1.0 1.0 1.0 
peroxide 
Total. Parts 
123.85 123.9 123.9 124.0 120.0 119.0 
by Weight 
Surface &lt;10.sup.4 
&lt;10.sup.4 
&lt;10.sup.4 
&lt;10.sup.4 
&lt;10.sup.4 
&lt;10.sup.4 
conductivity 
ohms/sq. 
Foam density 
6.0 6.0 6.0 6.0 9.0 12.0 
P.C.F. 
______________________________________ 
EXAMPLE 7 
In this example a panel of the foam product obtained in Example 4 having 
dimensions of 9".times.12" was formed into a electrically conductive 
package as shown in FIG. 3 by using a female type vacuum molding device at 
a temperature of 300.degree. F. 
Advantageously it was found that this foam product exhibited excellent 
uniformity in the surface resistivity with the variation of no more than 
15% over the surfaces of the article. In all cases the surface 
resisitivity was less than 1.times.10.sup.4 for ohms/square. 
EXAMPLE 8 
In this example, an additional run was conducted using the laboratory 
extruder previously used in Examples 1 to produce a foamable preform from 
a formulation containing 100% of polyethylene (P.E.): the formulation of 
the admixture formed into the preform is as follows: 
______________________________________ 
100% Polyethylene Foam 
______________________________________ 
P.E. Resin 100.00 
M.I. 20 
Dicumyl Peroxide 
1.0 
Azodicarbonamide 
7.0 
Ketjenblack 15.0 
Total Parts by Weight 
123.0 
Foam Density 9.0 P.C.F. 
______________________________________ 
From an evaluation of the foam obtained from the preform, it was found that 
the product was an extremely brittle matrix which would be very difficult 
to work with in a production process. This matrix had a density of 9 
P.C.F. At this density, the foam product may be used in a table mat 
application or the like. 
EXAMPLE 9 
In this example, an additional run was conducted using the laboratory 
extruder previously used in Example 1 to produce a foamable preform from a 
formulation containing a resin made up of 100% of an ethylene/vinylacetate 
copolymer. The formulation in this run is as follows: 
______________________________________ 
100% EVA Foam 
______________________________________ 
EVA (9% VA) 100.00 
M.I 7.0 
Dicumyl Peroxide 1.0 
Azodicarbonamide 7.0 
Ketjenblack 15.0 
Total Parts by Weight 
123.0 
Foam Density 6.0 P.C.F. 
______________________________________ 
An evaluation of the foam product obtained from the resulting preform 
showed that the foam had a density of 6 P.C.F. However, it was determined 
during the processing of this formulation that a formulation containing a 
blend of polyethylene and ethylene/vinylacetate copolymer molds much 
better than a formulation employing the copolymer alone. 
It will be understood that in accordance with the present invention is it 
much preferred to use an admixture of low density polyethylene and 
ethylene copolymer to produce an electrically conductive ethylene polymer 
foam since this admixture has significantly greater workability during the 
extruding and cooling operations and provides a more flexible and useful 
product, especially in terms of moldability of the foamable preform. Also, 
it should be recognized that the density of the foam products may be more 
than 12 P.C.F. depending upon the intended application and a density up to 
30 P.C.F. may be obtained.