Electrostatic dissipative material and process relating thereto

The present invention relates to compositions comprising conductive particles and one or more polymers, particularly acid copolymer resins or derived ionomers, which can be extruded or heat formed into films or articles. More specifically, the method of the present invention is directed to a novel, non-uniform heating process, wherein the edge portions of a material is heated about 3-50 degrees Celsius hotter than the center or middle portions of the article or film during fabrication to thereby provide improved and substantially uniform electrostatic dissipative ("ESD") properties.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates to compositions comprising conductive 
particles and one or more polymers, particularly acid copolymers or 
ionomers which can be extruded or heat formed into films or coatings. More 
specifically, the heat processing of the present invention is directed to 
a novel, non-uniform heating method, wherein the edge portions of a 
material comprising acid copolymer and/or ionomer is heated 3-50 degrees 
Celsius hotter than the central or middle portion of the article or film 
during fabrication to thereby provide improved and substantially uniform 
surface resistivity properties. 
2. Description of The Related Art 
Plastics are often considered for use as electrical insulating materials, 
because they typically do not readily conduct electrical current and are 
generally rather inexpensive relative to other known insulating materials. 
A number of known plastics are sufficiently durable and heat resistant to 
provide at least some electrical insulating utility, but many such 
plastics are problematic due to the accumulation of electrostatic charge 
on the surface of the material. 
Such surface charge accumulation can be undesirable for various reasons. 
Such materials sometimes discharge very quickly, and this can damage 
electronic components, or cause fires or explosions, depending upon the 
environment. Sudden static discharge can also be an annoyance to those 
using the material. 
Even where sudden static discharge is not a problem, dust will typically be 
attracted to and will accumulate on materials carrying a static charge. 
Furthermore, the static charge can interfere with sensitive electronic 
components or devices and the like. 
Resistivity can be defined as involving surface resistivity and volume 
resistivity. If the volume resistivity is in an appropriate range, an 
alternative pathway is provided through which a charge can dissipate 
(generally along the surface). Indeed, surface resistivity is typically 
the primary focus for electrostatic dissipating ("ESD") polymeric 
materials. 
Surface resistivity is an electrical resistance measurement (typically 
measured in ohms per square) taken at the surface of a material at room 
temperature. Where the surface resistivity is less than or equal to about 
10.sup.5, the composition's surface has very little insulating ability and 
is generally considered to be conductive. Such compositions are generally 
poor electrostatic dissipating polymeric materials, because the rate of 
bleed off is too high. 
Where the surface resistivity is greater than 10.sup.12, the composition's 
surface is generally considered to be an insulator. In certain 
applications, such a composition is also poor electrostatic dissipating 
material, because the surface does not have the requisite amount of 
conductivity necessary to dissipate static charge. Typically where the 
surface resistivity is about 10.sup.5 to 10.sup.12, any charge contacting 
the surface will readily dissipate or "decay". Further information 
involving the evaluation of surface resistivity can be found in American 
Standard Test Method D257. 
Acid copolymer resins are a well-known class of polymers containing up to 
about 30 weight percent organic acid groups which are attached to a 
hydrocarbon or perfluorinated polymer chain. Ionomers are derived from 
these acid copolymer resins by partial neutralization of the acid groups 
with metal ions, such as zinc, sodium, or magnesium. 
Acid copolymers and ionomers generally have surface resistivities greater 
than 10.sup.12, and therefore these materials are generally not suitable 
for high performance ESD uses. 
Static charge decay rates measure the ability of an electrostatic 
dissipating material to dissipate charge. A 90% decay time as used herein 
is measured at about 15% relative humidity and at ambient temperature as 
follows: A 5 kilovolt charge is placed upon the material and the amount of 
time (in seconds) for the charge to decrease to 500 volts is measured. A 
99% decay time is measured substantially as for the 90% decay time, except 
that the amount of time measured is for the charge to dissipate to 50 
Volts. 
Many electrostatic dissipating materials generally found in the art have a 
90% decay time of greater than about 3 seconds and a 99% decay time of 
greater than about 5 seconds. However, the National Fire Protection 
Association standard (NFPA Code 56A) requires 0.5 seconds as an upper 
limit for a 90% decay time, and the U.S. Military Standard (MIL-81705C) 
requires 2.0 seconds as an upper limit for a 99% decay time. Due to high 
surface resistivities, acid copolymer and ionomer compositions generally 
cannot meet such rigorous criteria as NFPA Code 556A or MIL-81705C. 
Attempts have been made to coat an electrostatic dissipative material onto 
an insulating plastic to reduce the accumulation of static charge. 
However, surface applications have been problematic due to long term 
adhesion requirements and interference with surface properties. 
Conventional low molecular weight organic electrostatic additives typically 
work well only in the presence of high relative humidity. Such additives 
typically must bloom to the surface after blending or mixing to provide 
electrostatic dissipative performance, and such blooming may not always be 
consistent. These additives may also cause thermal stability problems 
during processing or may cause physical properties of the produced film or 
coating to deteriorate. Such additives can also wash away or abrade from 
the surface. 
High molecular weight (polymeric) electrostatic dissipating agents for 
plastic are known, but they can be expensive, can undesirably alter the 
properties of the material and can be difficult to blend or alloy into a 
polymer material. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide a high 
performance ESD material which has substantially uniform surface 
resistivity, even under conditions of low relative humidity. 
A further object of the present invention is to provide an electrostatic 
dissipating material having a 90% decay time of less than about 0.5 
seconds and a 99% decay time of less than about 2.0 seconds. 
A further object of the present invention is to provide a high performance 
ESD material which does not have the problems associated with conventional 
ESD polymeric materials requiring high loadings of ESD modifying 
additives. 
Other objects and features of the present invention will become apparent to 
one of ordinary skill in the art upon further reading of this 
specification and accompanying claims. 
SUMMARY OF THE INVENTION 
It has been surprisingly discovered that an acid copolymer or ionomer 
composition, can be loaded with conductive particles and unconventionally 
heat processed to provide a final film-type article having a substantially 
uniform electrostatic dissipative ("ESD") surface. The "heat processing 
operation" of the present invention is intended to include case film 
extrusion, extrusion coating, (including coextrusions with other 
materials) and the like. 
The acid copolymer or ionomer compositions of this invention are made 
static-dissipative by incorporating a proper amount of a rigid conductive 
additive in particle or powder form, such as conductive carbon black, 
finely divided metals, conductive powders or combinations thereof. The 
examples will show that the amount of additive to obtain the desired level 
of surface resistivity is determined by the type of extrusion process, the 
line speed of the extrusion process, and the processing temperature. 
Despite control of these variables, it has been found that extrudates, 
such as a cast film of these compositions, generally do not have uniform 
surface resistivity across its entire width. 
That is, with conventional flat cast film processing where the extrusion 
die is maintained at a uniform temperature across its width, the center 
portion may possess the desired level of surface resistivity, but the 
values approaching the outer edges of the film tend to increase and may 
exceed 10.sup.12 ohms per square. Generally, the practical consequence of 
this non-uniform surface resistivity level is that only the center portion 
of the extruded film has useful surface resistivity. Trimming of the film 
to the useful width results in loss of product and an increase in cost. 
A typical flat film extrusion dies is often equipped with a series of 
individually-controlled heaters across its width. Surprisingly, it has 
been found that by setting the temperature of the die at higher values 
near the edges with respect to the center portion, a cast film which has 
uniform surface resistivity across its entire width can be produced. 
Acid copolymers and ionomers, as with most plastics, are generally not 
capable of exhibiting turbulent flow. Hence, there is little intermixing 
as these resins flow through a heating process, but rather, the flow is 
generally quite laminar. 
A portion of the molten polymer will flow and shear along the surface of 
the die or mold, whereas other portions of the molten ionomer will 
experience far less, if any, boundary interaction. It has been 
surprisingly discovered that the molten ionomer can be manipulated so that 
the surface will be sufficiently uniform after cooling to provide a final 
article or film which will exhibit substantially uniform ESD properties, 
making it useful for high performance ESD applications. 
This is accomplished by modifying the temperature of different zones of the 
die. A final product having substantially uniform ESD properties can be 
obtained by adjusting the edge zones of the die or mold to be about 3 to 
50 degrees Celsius hotter than the middle or center zone portions of the 
die or mold. 
It is theorized that as the molten ionomer moves into or through an edge 
zone, the shear stress or other physical interaction caused by the edge 
zone onto the molten flow causes greater separation between the particles 
or agglomerates of the rigid conductive additives. This increased distance 
between the particles causes the increase in the surface resistivity of 
the extruded film at the edges. Plotting temperature versus resistivity, 
(based upon data provided in the examples herein), and shows that as 
processing temperature is increased the resistivity decreases. Further, 
(also based upon the examples provided below) shows that as line speed 
increases, resistivity increases, and it is therefore theorized that 
correlative particle distances generally lengthen as the speed increases. 
For any particular embodiment of this invention therefore, ordinary skill 
and experimentation may be necessary in determining the optimal line speed 
and extrusion temperature, depending upon the resin and conductive 
particles chosen and also the particle loading. 
The increased heating at the outer zones reduces the melt viscosity and 
helps to maintain the average inter-particle distance in the same range as 
in the center portion of the film or coating. 
Coatings or films manufactured according to the process of the present 
invention exhibit substantially uniform ESD properties and are generally 
well suited for traditional, high performance ESD applications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Particularly useful polymer compositions comprise poly(ethylene-coacrylic 
or methacrylic acid) (hereafter "acid copolymers") or their partial metal 
salts (hereafter "ionomers"). These resins bind well to the rigid 
additives of this invention, give superior adhesion to aluminum foil which 
is frequently used as a substrate, and possess oil resistance properties 
generally superior to that of conventional polyethylene. 
The process of the present invention is particularly well suited for acid 
copolymers or ionomers, particularly in film and coating applications. The 
films of the present invention generally exhibit excellent toughness, 
adhesion and melt strength during processing. 
Preferred heat processing methods include slot-die extrusion, extrusion 
casting or extrusion coating. In any of these common film manufacturing 
processes, a thin section of polymer melt is extruded from a slot die. 
Other materials may be coextruded with the compositions of this invention 
so long as the die temperature profile is maintained. The die gap may need 
to be mechanically adjusted in order to maintain uniform thickness of the 
film or coating. 
In accordance with the present invention, the edge portions of the slot die 
are 3-50 degrees, more preferably 4-40 degrees and most preferably about 
5-30 degrees Centigrade higher in temperature than the middle portions. 
For the preferred films of the present invention, the preferred die is 
conventional coat-hanger-design die with 5 or more heater zones. For the 
most preferred embodiment (polyethylene-comethacrylic acid ionomer resin 
having 4 weight percent carbon black), a line speed of about 50-100 meter 
per minute, the two outer zones are preferably operated at a temperature 
of about 210.degree. C., the two outer intermediate zones are preferably 
operated at temperature of about 205.degree.-210.degree. C. and the middle 
zone is preferably operated at about 190.degree. C. 
Depending upon the composition chosen to be used, the optimal die 
temperature profile may need to be optimized using ordinary skill and 
experimentation. Optimization is dependent on resistivity measurements of 
the film or coating. Optimal die temperature profile is a function of the 
speed, melt viscosity and conductive particle loading. The extrudate 
exiting the slot die is typically quenched in a conventional manner, such 
as against a conventional chill roll. 
Referring now to the drawings, FIG. 1 is a perspective view of the 
preferred extrusion process of the present invention. The process is shown 
generally at 10. The resin compound is combined with hard conductive 
particles, such as carbon black or metal fines at a weight ratio of about 
97:3-70:30 resin to conductive particles and enters the process as shown 
at 12. As mentioned, this resin can be virtually any conventional ionomer 
or acid copolymer. The most preferred resin comprises 
poly(ethylene-comethacrylic acid partially neutralized with zinc or 
sodium. Other resins may also be appropriate for the present invention and 
ordinary skill and experimentation may be necessary in protecting any such 
alternative embodiment of this invention after reading this specification 
and accompanying claims. It is also possible to use a concentrate 
containing the hard conductive material and simultaneously blend with the 
acid copolymer or ionomer and extrude the resulting melt blend. The 
extruder must be capable of providing a uniform blend to accomplish this. 
The compounded resin is heated and forced forward by extruder 14 through 
extrusion die 16, thereby creating resin web 18 which is cooled by 
quenching rollers 20 to provide final sheet 22 and 22'. 
FIG. 2 further illustrates extrusion die 22, containing five heating zones, 
28a, 28b, 28c, 28d, and 28e. Molten resin 27 enters the extrusion die 22 
and exits the die as a melt curtain 29. The outer, non-planar extrusion 
die zones 28a and 28e are heated to a temperature about 3.degree. C. to 
about 50.degree. C. higher than die heating zone 28c. Transition heater 
zones 28b and 28d are heated to an intermediate temperature between the 
temperature of zone 28c and zones 28a and 28e. Temperature adjustment is 
made to maintain uniformity of the surface resistivity. 
The present invention is further exemplified by the following examples. 
EXAMPLES 
The following compositions were used: 
COMPOSITION 1: Poly(ethylene-comethacrylic acid) ionomer-resin partially 
neutralized with zinc containing 3.5% by weight carbon black. 
COMPOSITION 2: Poly(ethylene-comethacrylic acid) ionomer-resin partially 
neutralized with zinc containing 4% carbon black. 
CONTROL EXAMPLE 
COMPOSITION 1 was compounded on a Farrel Continuous Mixer and subsequently 
cast into a 1524 mm wide 51 .mu.m film. The cast film was produced on a 
63.5 mm single screw Sterling Extruder using a general purpose screw and a 
coat-hanger-design die with 5 heater zones. 
Extrusion conditions for the cast film are as follows, temperatures noted 
in .degree.C. 
__________________________________________________________________________ 
Extruder Zones Temp 
Die Zones Temp Die Screw 
Take Off 
Quench Rolls 
1 2 3 4 1 2 3 4 5 ADAP 
RPM M/Min 
CTR Bott 
__________________________________________________________________________ 
198 
198 
204 
200 
185 
182 
181 
185 
183 
180 30 3.0 18 15 
__________________________________________________________________________ 
Surface resistivity of the cast film was measured using a Keithly Model 617 
resistivity meter equipped with #6105 sample chamber at a constant 100 
volts. Readings measured from 10.sup.6 at the center of the film to as 
high as 10.sup.15 at the edges. The center 610 mm portion of the film was 
static dissipative within specifications but uniformity across entire 
width was not obtained. Results are given below: 
__________________________________________________________________________ 
Positions of 
102 292 457 635 787 940 1105 1245 1372 
Center of 102 mm 
Dia. Samples 
From Left, mm 
Log of Surf. 
Resist. In 
OHMS/Square 
Side 1 13.1 
14.3 
8.6 6.9 7.0 9.1 12.1 12.1 13.4 
Side 2 15.0 
15.9 
8.3 6.7 6.9 8.8 14.4 14.5 13.2 
__________________________________________________________________________ 
EXAMPLE 
COMPOSITION 2 was compounded on a Farrel Continuous Mixer was subsequently 
cast into a 1524 mm wide 51 .mu.m film using the same equipment as 
described in the control EXAMPLE. For this run there were variances in the 
temperature profile of the die as noted in the following extrusion 
conditions, temperatures noted in .degree.C. 
__________________________________________________________________________ 
Extruder Zones Temp 
Die Zones Temp Die Screw 
Take Off 
Quench Rolls 
1 2 3 4 1 2 3 4 5 ADAP 
RPM M/Min 
CTR Bott 
__________________________________________________________________________ 
170 
180 
180 
180 
215 
205 
190 
210 
215 
185 30 3.6 21 18 
__________________________________________________________________________ 
Surface resistivity of the cast film was measured using the same equipment 
as in the CONTROL EXAMPLE. Readings measured 10.sup.5 to 10.sup.6 across 
the entire width of the film. By adjusting the die temperatures to be 
hotter at the edges and cooler at the center uniform resistivity was 
obtained. Results are given below. 
__________________________________________________________________________ 
Positions of 
102 254 406 559 711 864 1016 1168 1321 1422 
Center of 102 mm 
Dia. Samples 
From Left, mm 
Log of Surf. 
Resist. In 
OHMS/Square 
Side 1 6.0 6.1 6.1 6.2 6.1 6.0 5.9 6.0 5.9 6.0 
Side 2 5.9 6.2 6.2 6.1 6.1 6.0 6.0 6.1 6.0 6.1 
__________________________________________________________________________ 
Further examples are provided in TABLES I, II, III, and IV in which 
Composition 1 and 2 were prepared on a #4 Farrel Continuous Mixer ("FCM"), 
using #15 mixing blades at 320 rpm, mixing chamber set at 161.degree. C., 
orifice at 90% open and a rate of 273 kg/hr. The FCM discharges to a 127 
mm single screw extruder set up with an eighty hole die (2.36 mm hole 
dia.), of which the twenty-eight perimeter holes were blocked off. The 
extruder and die were operated at 184.degree. to 195.degree. C. Polymer 
exiting from the die is underwater cut and water conveyed to a Gayla 
Spinner Dryer. 
These blends were subsequently cast into 152 mm, 51 .mu.m film using a 
Haake 19 mm single screw extruder with a 152 mm horizontal coat hanger 
design film die. 
Table 1 shows extrusion conditions and surface resistivity data measured 
with a Monroe Electronics Model 262A portable surface resistivity meter 
and the Keithly Model 617 meter equipped with #6105 sample chamber at a 
constant 100 volts. 
Based upon Table I, it is evident that carbon loading, melt temperature, 
die temperature profile (based on die width), and line speed all affect 
the final surface resistivity of the film. 
Films were made from COMPOSITION 1 and COMPOSITION 2 described above. 
Equipment used was a 89 mm Prodex single screw extruder set up with a 
general purpose screw and a 1219 mm five heater zone vertical film die. 
The film was extrusion coated to the polypropylene surface of a 
polypropylene/paper substrate. The following conditions were kept 
constant: film curtain was 7.6 mm from the center of the nip towards the 
paper roll, an air gap of 102 mm, a nip pressure of 0.0069 MPa, and 
extrusion temperature profiles as defined in Table II. Surface resistivity 
measured across the entire width of the film was performed during 
production using a Monroe model 262A portable meter. These data along with 
extruder melt, pressure, screw rpm, and film line speed are shown in Table 
III. More accurate surface resistivity measurements were made using a 
Keithly model 617 with #6105 sample chamber and constant 100 volts. 
Measurements were made at five locations across the width of the films and 
reported in Table IV using the same sample designation as in Table III. 
TABLE I 
__________________________________________________________________________ 
(SURFACE RESISTIVITY) 
FILM KEITHLY 50% RH 
MELT EXT. 
QUENCH 
TAKEOFF 
MONROE 
LOG OF SR IN OHMS/SQ 
BLEND 
TEMP .degree.C. 
RPM ROLL .degree.C. 
M/MIN 262A SIDE 1 SIDE 2 
__________________________________________________________________________ 
COMP. 1 
211 20 20 2.1 10 8 7.5 7.2 
" 198 20 20 2.1 10 12 15.3 15.3 
COMP. 2 
190 20 20 2.1 10 10 9.5 10.5 
" 200 20 20 2.1 10 7 7.1 7.0 
" 210 70 20 7.0 10 9 11.3 8.6 
" 218 95 20 9.1 10 8 7.9 6.9 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
TEMP. 
(EXTRUDER TEMP. PROFILE, .degree.C.) 
(DIE TEMP. PROFILE, .degree.C.) 
PROF. 
Z1 Z2 Z3 Z4 Z5 ADAPTER 
Z1 Z2 Z3 Z4 Z5 
__________________________________________________________________________ 
A 188 
188 
193 
199 
204 
210 227 
221 
221 
221 
227 
B 188 
188 
199 
207 
207 
216 232 
227 
227 
227 
232 
__________________________________________________________________________ 
TABLE III 
__________________________________________________________________________ 
EXTR. FILM 
TEMP 
EXT. PRESS. 
SCREW 
CHILL TAKEOFF 
RESISTIVITY 
SAMPLE ID. 
PROF. 
MELT .degree.C. 
MPa RPM ROLL .degree.C. 
M/MIN MONROE 262A 
__________________________________________________________________________ 
1. COMP. 1 
A 198 -- 12 21 6.1 10 6 
2. COMP. 1 
A 203 16.5 20 22 15.2 10 9 
3. COMP. 2 
A 204 -- 20 20 15.2 10 6 
4. COMP. 2 
A 210 28.3 40 22 30.5 10 8 
5. COMP. 2 
A 213 28.9 50 22 38.1 10 9 
6. COMP. 2 
A 216 30.3 60 23 45.7 10 10 
7. COMP. 2 
B 214 23.4 30 18 22.9 10 6 
8. COMP. 2 
B 218 27.6 50 19 38.1 10 8 
9. COMP. 2 
B 220 28.9 60 20 45.7 10 9 
10. 
COMP. 2 
B 226 31.7 80 21 60.9 10 11 
__________________________________________________________________________ 
TABLE IV 
______________________________________ 
POSITIONS OF THE 
LOG OF SURFACE RESIST. 
CENTER OF 102 mm DIA. 
IN OHMS/SQUARE SAMPLES FROM LEFT, mm 
SAMPLE DESIG. 102 279 559 838 1016 
______________________________________ 
2. COMP. 1 *SIDE 1 8.6 8.7 8.4 8.6 8.7 
COMP. 1 *SIDE 2 12.6 9.8 9.9 13.0 13.5 
4. COMP. 2 *SIDE 1 7.9 8.2 7.5 7.7 7.4 
COMP. 2 *SIDE 2 8.4 8.6 7.9 7.9 7.7 
5. COMP. 2 *SIDE 1 9.3 9.9 8.6 9.0 8.6 
COMP. 2 *SIDE 2 9.6 10.3 9.1 9.8 8.9 
6. COMP. 2 *SIDE 1 10.1 10.1 9.6 9.9 9.5 
COMP. 2 *SIDE 2 11.0 11.1 9.7 9.7 9.7 
7. COMP. 2 *SIDE 1 6.4 6.4 6.2 6.4 6.4 
COMP. 2 *SIDE 2 6.5 6.7 6.3 6.6 6.4 
8. COMP. 2 *SIDE 1 7.4 7.6 7.0 7.2 7.0 
COMP. 2 *SIDE 2 7.8 8.0 7.2 7.6 7.4 
9. COMP. 2 *SIDE 1 9.2 9.3 8.4 8.6 8.8 
COMP. 2 *SIDE 2 9.6 9.7 9.1 9.4 8.6 
10. COMP. 2 
*SIDE 1 11.3 11.8 10.5 11.3 10.5 
COMP. 2 *SIDE 2 11.8 12.0 11.8 12.5 11.2 
______________________________________ 
*SIDE 1 Denotes chill roll contact (shiny side) 
*SIDE 2 Denotes pp/paper contact (dull side)