Antistatic laminates containing long carbon fibers

A static-dissipating laminate is made, containing at least a bottom core layer and a top decorative layer, both layers being impregnated with a resin, where at least the decorative layer has contacting, long carbon fibers uniformly distributed through it in an amount effective to provide an antistatic effect to the laminate, so that static charges accumulating on the top of the decorative layer are dissipated.

BACKGROUND OF THE INVENTION 
It is well known that if two surfaces of insulating materials are rubbed 
together and then separated, an electrostatic charge will build up between 
the two surfaces. In recent years, this problem in computer room flooring 
and desk areas has been troublesome, since the discharge of built-up 
static can result in tape or disc erasures and interference with sensitive 
equipment. Such charged surfaces in hospital surgical, and other areas 
where certain anesthetic gases can form violently explosive mixtures with 
air, has caused even greater concern that the chances of explosions caused 
by sparks or electrical discharges be minimized. In all of these cases, 
the static build-up can be caused by walking on flooring, moving 
electronic components or other equipment from place to place, and even 
utilizing the keyboard on a computer terminal. Such static build-up can 
also occur over a period of time in the wearing apparel of workers. 
The need for spark-proof flooring was recognized many years ago by Donelson 
et al., in U.S. Pat. No. 2,351,022. There, calcined magnesite, MgO, was 
mixed with from about 40 wt. % to 60 wt. % of finely divided coke 
particles, having from 1/8 inch screen size to fine dust, and liquid 
magnesium chloride, to provide a spreadable floor composition which could 
be troweled over a concrete, steel, or wood sub-floor. Such flooring was 
not very resilient, however, and caused fatigue to those who had to stand 
or walk on it all day. 
More recently, Charlton et al., in U.S. Pat. No. 3,040,210, taught a much 
more resilient, decorative, carbon containing, linoleum floor sheeting, 
laminated to a conductive base. The linoleum surface sheeting contains 
from 1 wt. % to 14 wt. % conductive carbon, homogeneously mixed with other 
conductive materials, linoleum binder, which contains oxidized drying oils 
such as linseed oil with up to 35 wt. % resin such as rosin ester gum or 
phenol-formaldehyde, and sufficient coloring pigments to provide an 
attractive appearance. The conductive backing must contain from 10 wt. % 
to 35 wt. % conductive carbon, and can be bonded to fabric for added 
strength, where the fabric itself can be made conductive by initially 
dipping it in a dispersion of conductive carbon. This provides a static 
resistant flooring having a controlled electrical resistance, which will 
wear evenly, can be applied in long sections minimizing seams, and which 
is resilient enough to help reduce fatigue for people that must stand or 
walk on the flooring for long periods of time. 
Berbeco, in U.S. Pat. No. 4,301,040, taught static-free mats containing a 
standard, non-conductive decorative laminate, such as a 0.16 cm. (1/16 
in.) thick melamine-formaldehyde laminate, or a rubber, nylon, 
polycarbonate, polyethylene or polypropylene, non-conductive sheet, as a 
top surface, adhesive bonded to, or coated with, either an electrically 
conductive solid or an open cell foam bottom backing layer. The bottom 
layer includes a polymeric material or a foam and an antistatic amount, 
generally about 2 wt. % to 40 wt. % of conductive particulate material, 
such as metal particles, aluminum salts such as aluminum silicate, 
graphite fibers, and preferably carbon black particles. Useful polymeric 
materials include butadiene-styrene resin and the like, and useful foams 
include polyurethane foams, polyester foams and the like. When a foam is 
used as the bottom layer, a flexible cushion mat results. 
Standard decorative laminates are non-conductive through their 
cross-section, and are described, for example, by McCaskey, Jr. et al., in 
U.S. Pat. No. 4,061,823. They are popular as surfacing material for 
counter and furniture tops. Because, in many cases, they must be machined, 
fillers other than coloring pigments are usually avoided. Such laminates 
generally contain 2 to 6 fibrous, Kraft paper sheets, impreganted with 
phenol-aldehyde resin, as a core for 1 high quality, fibrous, 
alpha-cellulose decorative print sheet, having a pattern or plain color, 
impregnated with melamine-aldehyde resin, and 1 top, high quality, 
fibrous, alpha-cellulose overlay protective sheet, also impregnated with 
melamine-aldehyde resin. Any pigmentation fillers would only be present in 
the decorative print sheet. 
The Donelson et al. composition is applicable to dense flooring and 
requires large amounts of carbon material. The Charlton et al. material 
also requires the use of large amounts of relatively expensive carbon, and 
requires a complicated manufacturing process. The Berbeco material 
requires a non-conductive surface, through which the backing would have to 
draw static charges. Of course, standard decorative laminates are usually 
non-conductive. What is needed is a surfacing material useful for flooring 
and desk or counter tops, having outstanding antistatic properties, good 
wear properties, and an attractive appearance, and which is also 
inexpensive, easy to manufacture, and thin enough to allow ease of 
installation. 
SUMMARY OF THE INVENTION 
The above needs have been met, and the above problems solved, by providing 
a static-dissipating, high pressure decorative laminate, having print and 
preferably core sheets containing an antistatic effective amount, from 
about 1 wt. % to about 15 wt. %, of long, conductive carbon fibers, 
uniformly distributed in an intermingled, interconnecting, contacting 
relationship throughout the sheets. Paper made with thin carbon fibers, 
long enough to contact and overlap each other, will result in a floor or 
counter surface sufficiently conductive to overcome the highly electrical 
insulating nature of the thermoset resins used in such floor or counter 
laminates. 
These laminates will generally have antistatic properties through the top 
portion, i.e., at least 1/8 of their thickness, and preferably throughout 
their entire thickness, and do not rely on either a surface treatment or a 
highly conductive, extra bottom backing layer for static reduction. Since 
only from about 1 wt. % to about 15 wt. % carbon fibers are used, based on 
total unimpregnated paper and carbon fiber weight, costs are kept down, 
and the product at maximum carbon loading is medium grey, rather than 
black, with a random pattern which is attractive and is an acceptable 
decorative pattern. This eliminates the need for major amounts of colored 
pigment to tone down or modify the black surface resulting when high, 
spherical carbon particle loadings are used to provide contacting, 
static-dissipating laminate layers. In addition, these laminates wear 
well, can be applied in large area sheet form, are thin, inexpensive, and 
allow ease of manufacture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 of the drawings, a laminate 10 comprises a lay-up 
of a plurality of resin impregnated core sheets 11, and a superimposed 
resin impregnated decorative print sheet 12, which also serves as a 
protective sheet. Heat and pressure are applied to this lay-up to 
consolidate the materials into a unitary decorative structure. 
The print sheet 12 usually provides the decorative effect for the laminate. 
It is usually in the form of a decorative sheet, i.e. dyed, or pigmented 
to impart a solid color. It usually comprises a single fibrous sheet of 
high grade, absorbent, alpha-cellulose or regenerated cellulose paper 
impregnated with a thermosetting resin, such as a melamine-formaldehyde 
resin or other aminotriazine-aldehyde resin. 
The rigidity-imparting core stock layer is made of a plurality of fibrous 
sheets of Kraft paper, rag paper, cotton linter fiber paper, Dacron 
(polyethylene terephthalate) cloth, cotton cloth, glass cloth or the like, 
containing epoxy resin, or phenolic resin, such as a phenol-formaldehyde 
resin. Typically, 2 to 6 core sheets are consolidated with a single print 
sheet to form a conventional 0.16 cm. (1/16 in.) thick decorative 
laminate. 
High pressure laminating techniques are employed in preparing the laminates 
from the above described assembly of core stock layer of core sheets, and 
print overlay sheet. Temperatures ranging from about 120.degree. C. to 
about 175.degree. C. and pressures ranging from about 600 psi. to 2,000 
psi. are employed. The time required, at these temperatures, to effect a 
cure of the resinous components of the assembly will usually be from about 
3 minutes to about 25 minutes. The resulting laminate is generally allowed 
to cool to from about 50.degree. C. to 85.degree. C. before being removed 
from the press. The cooling step generally takes from about 30 minutes to 
90 minutes. Generally, the assembly will require a 15 minute to 45 minute 
warm up period before the 120.degree. C. to 175.degree. C. maximum curing 
temperatures are reached in the press. The entire cycle of warm up, cure 
and cooling will vary from 50 minutes to 160 minutes. 
The aminotriazine-aldehyde resins used to impregnate the print sheet is 
well known in the art, and reference may be made to U.S. Pat. No. 
3,392,092 for exhaustive details on their production. Similarly, complete 
details on the phenolic resins used to impregnate the core sheet can be 
found in U.S. Pat. Nos. 2,205,427; 2,315,087; 2,328,592 and 2,383,430. 
Epoxy resins are also well known in the art. 
In the static-dissipating, heat and pressure consolidated laminate of this 
invention, thin carbon fibers, having lengths of from about 0.20 inch to 
about 0.75 inch, preferably from about 0.25 inch to about 0.50 inch are 
uniformly distributed throughout the fibrous print layer, and preferably 
also throughout the fibrous core layer of the laminate. The diameter of 
the carbon fibers will generally range from about 0.3 mil to about 3.0 
mils. The carbon fibers are readily commercially available. The carbon 
fibers will be present in the print layer, and when used in the core, in 
an amount of from about 1 wt. % to about 15 wt. %, preferably from about 3 
wt. % to about 8 wt. %, based on total unimpregnated, resin free sheet 
weight plus carbon fiber weight. Use of carbon fibers within the 1 wt. % 
to 15 wt. % range provides an amount of carbon fiber contact effective to 
provide an antistatic effect, so that static charges accumulating on the 
top of the decorative layer are dissipated. 
Preferably, the carbon fibers will be blended into the wood pulp, i.e., 
"felted into" the print or core sheets during paper manufacture, in an 
amount that will correspond to between the 1 wt. % and 15 wt. % values 
before described. Seldom can the carbon fibers be mixed onto the 
impregnating resins, or mixed into a resin surface coating for the paper 
with good results. When used in the resin, the carbon fibers would not 
easily remain suspended, would be badly broken during mixing, and would 
have difficulty being impregnated into the centers of the sheets, not 
achieving a uniform distribution throughout the sheets. 
Carbon fibers over about 0.75 inch long are not easily obtainable, provide 
no advantage in reducing resistivity and would add to paper felting 
difficulties. Carbon fibers less than about 0.20 inch long do not provide 
the required interconnection and contact required to lower resistivity 
substantially unless used in major amounts, increasing costs and providing 
a black surfaced material which may not be esthetically desirable in most 
commercial applications. In any case, carbon particles, i.e., spheres, are 
not desirable in the tip print overlay surface layer, since too high a 
loading is required for good anti-static contact, and at high loadings 
they would provide a black surfaced material. Carbon fiber content over 
about 15 wt. % adds significantly to expense, provides a much blacker 
surfaced material not esthetically pleasing, and doesn't improve 
anti-static properties significantly. Carbon fiber content under about 1 
wt. % will not provide enough fiber to fiber contact even if the fibers 
are relatively long to give effective anti-static properties to the 
laminate and eliminate charge accumulations at the top decorative surface. 
In the most preferred embodiment, shown in FIG. 2 of the drawings, the 
intermingled, interconnecting, contacting carbon fibers 14 will be 
uniformly distributed throughout, preferably felted into the print layer 
12, and core layer 11, to provide maximum reduction in volume resistivity. 
The distribution must be uniform and in an amount effective so that good 
electrical contact is assured, to provide electric static drain from the 
top surface 15 of the overlay layer and laminate. While not clearly shown 
in FIG. 2 for the sake of simplicity, the carbon fibers of each sheet are 
also in generally contacting relationship, providing a conductive path 
from top surface 15 to bottom surface 16 of the laminate. 
In some instances, where a thin laminate is used, and where surface 
resistivity reduction is primarily desired, only the print layer 12 need 
contain the uniform distribution of the carbon fibers. In all instances, 
the laminate will be electrically conductive into its interior. As shown 
in FIG. 2, no backing layer is used or desired next to the core layer 11, 
at bottom surface 16, to provide or enhance conductivity. 
In all cases, the standard ASTM-D257-54T surface resistivity will be at or 
below about 1.times.10.sup.6 megohms, and when the carbon fibers are 
included in the laminate core, the standard ASTM-D257-54T volume 
resistivity will be at or below about 1.times.10.sup.5 megohms/cm. These 
laminates, can be used alone as a surfacing material, and can be easily 
applied in large sheet form to wood, concrete, or plaster, to provide 
superior, inexpensive, attractive, antistatic surfaces for computer room 
or hospital floors, walls, counters, and the like. 
EXAMPLE 
Long sections of 66 lb. (per 3,000 sq. ft. ) basis weight, alpha cellulose 
paper stock, containing 1.2 wt. %, 5 wt. %, and 10 wt. % respectively of 
intermingled, contacting carbon fibers, about 1.5 mils in diameter and 
from 0.25 inch to 0.44 inch long, were impregnated with 
melamine-formaldehyde resin. Another long section of these carbon fiber 
containing paper stocks were impregnated with phenol-formaldehyde resin. 
Control sections of stock containing 100 wt. % paper fibers, i.e., no 
carbon fibers, inpregnated with melamine-formaldehyde and 
phenol-formaldehyde resin were also made. The sections were all cut into 5 
ft..times.12 ft. sheets. 
Twelve stack-ups, each containing one melamine impregnated sheet with 
carbon fibers, and six phenolic impregnated sheets, as a core, with carbon 
fibers, were assembled, Samples A, B and C, appropriately placed between 
press plates and heated platens in a flat bed press, and molded, using a 
60 minute heating plus cooling cycle, with peak platen temperature of 
about 132.degree. C., and a pressure of about 1200 psi. Additionally, in a 
similar construction and fashion, twelve stack-ups, using only a top 
melamine impregnated sheet with carbon fibers, Sample D, and control 
sheets with no carbon fibers, Control Sample E, were molded, where, 
however, the Sample D core sheets were 156 lb. basis weight Kraft paper. 
After cooling and press release, the resulting laminates were tested for 
surface and volume resistivity by the standard ASTM-D257-54T method. The 
results are shown in Table 1 below, where lower megohm values mean better 
laminate antistatic properties: 
TABLE 1 
______________________________________ 
Carbon Surface 
Sample Fibers Resistivity 
Volume Resistivity 
______________________________________ 
A. 1 melamine top sheet 6 phenolic core sheets 
1.2 wt. % 1.2 wt. % 
4.8 .times. 10.sup.5 megohms 
##STR1## 
B. 1 melamine top sheet 6 phenolic core sheets 
5 wt. % 5 wt. % 
1.3 .times. 10.sup.5 megohms 
##STR2## 
C. 1 melamine top sheet 6 phenolic core sheets 
10 wt. % 10 wt. % 
6 .times. 10.sup.4 megohms 
##STR3## 
D. 1 melamine top sheet 6 phenolic core sheets 
10 wt. % 1 .times. 10.sup.5 megohms 
##STR4## 
E.* 1 melamine top sheet 6 phenolic core sheets 
0 wt. % 0 wt. % 
1 .times. 10.sup.8 megohms 
##STR5## 
______________________________________ 
*Comparative control sample 
Core and top sheets are alpha cellulose in Samples A to C & E, with Sampl 
D having alpha cellulose top and Kraft core sheets 
As can be seen, even the use of the carbon fibers only in the top sheet of 
a seven sheet laminate dropped surface resistivity by a factor of 10.sup.3 
megohms from the control sample. Use of carbon fibers throughout the 
laminate, in the best sample, dropped surface resistivity by a factor of 
over 10.sup.4 megohms, and more importantly, dropped volume resistivity by 
a factor of over 10.sup.6 megohms from the control sample.