Conductive thermosetting compositions and process for using same

This invention is directed to a process and a novel composition for forming a conductive thermoset material which comprises admixing PA0 (a) particles of a polymeric material swellable at its plasticization temperature, PA0 (b) at least one liquid reactive plasticizer for (a) PA0 (c) optionally and preferably a curing agent for the reactive plasticizer, and PA0 (d) heat or electrically conductive particles, and thereafter heating the admixture for a time sufficient to flux and cure same to obtain a conductive thermoset material. Upon heating, above the plasticization temperature, the liquid reactive plasticizer plasticizes the polymer particles. This results in the swelling of the polymer particle, forcing the conductive filler to pack tightly and orderly, thereby increasing the conductivity of the plasticized conductive thermoset after curing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a conductive thermosettable dispersion 
composition which, on heating at or above the plasticization temperature, 
rapidly provides a conductive thermoset material with improved 
conductivity usable as an ink, adhesive, gasket, sealant or in EMI and RF 
shielding. 
The invention also relates to a process for forming a conductive 
crosslinked bond or seal. 
2. Description of the Prior Art 
Conductive coatings are known in the art. 
U.S. Pat. No. 3,412,043 teaches an electrically conductive resinous 
composition consisting essentially of silver flake, resinous binder and 
finally divided inert filler in specified weight ratios. Therein one 
resinous binder is an epoxy resin system which is cured by the addition of 
an amine curing agent at slightly elevated temperatures. 
U.S. Pat. No. 3,746,662 teaches electrically conductive coatings comprising 
certain epoxy resins, particles of tough polymer having carboxy, hydroxy, 
amino or isocyanate substituents which are grafted by the epoxy resin at 
the interface, finely divided metal particles and a curing agent for the 
epoxy resin. The curing is obtained by heating the composition at 
temperatures of 125.degree. C. or higher. 
U.S. Pat. No. 3,968,056 teaches a radiation curable ink comprising a 
particulated electrically conductive metal containing material in 
combination with an organic resin binder which is converted to a 
conductive coating on the surface of a substrate by exposure to either 
actinic or ionizing radiation. 
U.S. Pat. No. Re. 30,274 teaches a circuit board for activating high 
voltage flashlamps, said board including a non-conductive, thermoplastic 
substrate having a patterned electrically conductive coating on one of its 
surfaces and defining electrical circuitry for the flashlamps, said 
coating comprising an organic resin matrix curable by UV radiation and a 
particulated electrically conductive material selected from the group 
consisting of a particulated electrically conductive metal and a 
particulated electrically conductive metal containing material, including 
mixtures thereof with no more than up to about 15% by weight of said 
particulated electrically conductive material having an aspect ratio of 
diameter to thickness of a value greater than 20. 
U.S. Pat. No. 3,609,104 teaches the use of compressible, non-flowable 
particles to promote the conductivity of the conductive plastic. The 
flowable resin is one that chemically bonds to the surface of the 
non-flowable particles when it is hardened. During hardening, sufficient 
pressure is applied to distort the non-flowable particles to induce a 
conductive web from the conductive filler. For this purpose the 
non-flowable particles must be compressible. 
OBJECTS OF THE INVENTION 
One object of the instant invention is to produce a novel process and 
composition. Another object of the instant invention is to produce a 
conductive dispersion composition which is useful as an ink, shielding, 
adhesive or sealant. Yet another object of the instant invention is to 
produce a conductive dispersion composition which on curing has higher 
conductivity than conventional conductive thermosets. Still another object 
of the invention is to produce a conductive dispersion composition which 
on heating to the plasticization temperature acquires handling strength 
and cures to a conductive thermoset at or above said plasticization 
temperature. Yet another object of the invention is to produce a 
conductive, reactive, plasticized thermosetting polymer composition 
curable to a conductive, thermoset material on exposure to heat. Other 
objects will become apparent from a reading hereinafter. 
DESCRIPTION OF THE INVENTION 
This invention relates to a conductive thermosettable dispersion 
composition comprising an admixture of 
(a) particles of a polymeric material swellable at its plasticization 
temperature, 
(b) at least one liquid reactive plasticizer for (a), 
(c) optionally and preferably a thermal curing agent for (b), and 
(d) particles of a heat or electrical conductive material. 
This invention is also directed to a process for forming a conductive 
thermoset material which comprises a thermoplastic polymer, e.g., 
copolymer of styrene and maleic anhydride, commminuting and admixing said 
polymer with 
(a) a liquid reactive plasticizer therefor, e.g., an epoxy resin, 
(b) a curing agent for the reactive plasticizer, e.g., dicyandiamide, and 
(c) heat or electrically conductive particles, e.g., silver flake, 
and, thereafter, heating the admixture for a time sufficient to plasticize 
and cure same to obtain a conductive thermoset material. 
The conductive, reactive dispersion when plasticized can be used as a 
gasket, sealant or adhesive. 
Originally, plastisols were produced by dispersion of a fine particle size 
PVC resin in a plasticizer. The PVC resin particles are uniformly 
distributed throughout the continuous plasticizer phase. A plastisol 
undergoes several morphological changes upon heating at a temperature 
above its fusion temperature. First, the PVC particles swell into a 
pregellation state at which the particles increase their sizes by the 
absorption of plasticizer molecules and remain flowable in the dispersion 
state. In the progress of plasticization, the dispersion becomes gel and 
the liquid phase virtually disappears. The gellation phenomonium provides 
no physical strength to the gelled dispersion until it turns to the fusion 
stage. A fused plastisol is a clear continuous plasticized PVC having 
final physical properties: [A. C. Werner, Tappi, 50(1), 79A (1967)]. 
Theoretically, the phenomonium of a plastisol is controlled by 
thermodynamics and transport kinetics. To form a stable dispersion, the 
plasticizer molecules should not diffuse into the polymer particles to 
solvate the segments of polymer backbone. That is, the size of the 
plasticizer molecules should be larger than the free volume of the polymer 
particles at storage temperature. As soon as the free volume of the 
polymer particles increases to a certain size, which occurs on heating, 
the plasticizer begins to diffuse into and solvate the polymer. 
Plastization is a process in which the plasticizer migrates into the three 
dimensional lattice of tne polymer particles resulting in a solvation of 
the polymer segment by the plasticizer molecules. This reduces the number 
of points of interaction between segments, but the gel structure from 
entanglement of polymer molecules retains a degree of resilience and 
resistance to deformation. 
The condition for thermodynamic equilibrium between the p-mixed phase 
(polymer plus swelling plasticizer) and pure swelling liquid is given by 
EQU .DELTA.G.sub.1 =O 
where .DELTA.G.sub.1 is the Gibbs free energy of dilution, defined as the 
change in the Gibbs free energy of the system resulting from the transfer 
of unit quantity of swelling liquid from the pure liquid phase to a very 
large quantity of the swollen polymer. For flexible linear polymers, 
.DELTA.G.sub.1 is given by the well known Flory-Huggins equation (M. L. 
Huggins, J. Chem. Phys., 9, 440 (1941); P. J. Flory, ibid. 9, 660 (1941): 
##EQU1## 
where v.sub.2 is the volume fraction of the polymer, .gamma. is the ratio 
of molar volumes of polymer and solvent and x is an interaction parameter 
that generally varies from -1.0 to slightly over 0.5. If 
x&lt;1/2,.DELTA.G.sub.1 is negative for all values of v.sub.2 and the polymer 
is soluble or swellable to equilibrium; if x&gt;1/2, there is a particular 
value of v.sub.2 for which .DELTA.G.sub.1 =O and at this value the polymer 
is swollen to equilibrium. 
Using kinetic and thermodynamic controls based on the principles outlined 
above, the plastisol concept can be extended to other polymeric materials. 
Any liquid organic material can be used as plasticizer for a plastisol 
preparation when the plasticizer is compatible with the polymer and 
provides a stable dispersion.. 
This invention utilizes this particular concept to prepare conductive 
thermosets having improved conductivity over the conventional ones 
containing only a dispersion of conductive filler in a thermosetting 
resin. The conductive thermosetting material in this invention contains a 
conductive filler and a non-crosslinked polymer powder dispersed in a 
thermosetting plasticizer. Upon heating at curing temperature, polymer 
powders are swollen by the reactive plasticizer to their maximum volume 
and, hence, reduce the volume of the liquid phase containing conductive 
filler. This causes the conductive filler to pack tightly and to form a 
conductive path web after the conductive thermosetting composition is 
plasticized at an elevated temperature. 
Several advantages can obviously be realized from this novel approach. 
First, at the same loading of conductive filler, the conductivity of the 
thermoset prepared from this invention is higher than the conventional 
conductive thermoset. Second, to reach the same conductivity, the 
conductive thermoset in this invention can contain less conductive filler 
than conventional ones resulting in improved rheology and physical 
properties and cost saving, especially since the conductive filler is an 
expensive metal such as gold or silver. 
FIG. 1 shows the sequence of the morphological change of the conductive 
thermosets described herein. In the figure, 1 represents the swellable 
polymer particles, 2 represents the conductive particles, and 3 represents 
the liquid reactive plasticizer. FIG. 1A is a stable dispersion under 
storage conditions containing a reactive plasticizer or a mixture of 
reactive plasticizers 3, a polymer powder 1 and a conductive filler 2. 
Upon heating at the curing temperature, the polymer particles 1 are 
swollen through plasticization or solvation by tne reactive plasticizer 3 
as shown in FIG. 1-B. The volume fraction of polymer particle 1 is 
increased and, therefore, the packing densitiy of conductive filler 2 is 
increased as well. When the polymer particles 1 containing reactive 
plasticizer 3 swell to their maximum volume, a conductive web of 
conductive particles 2 is formed. The conductive web of conductive 
particles 2 and the size of swollen polymer particles 1 and 3 become 
permanent after the polymerization or the crosslinking reaction of the 
reactive plasticizer as shown in FIG. 1C. 
Hence, this invention relates to the use of swellability of polymer powder 
at an elevated temperature above the plasticization temperature to cause 
the conductive filler to pack tightly and to arrange orderly and, hence, 
to increase the conductivity of the conductive thermoset. The reactive 
plasticizer such as a liquid epoxy does not need to have low viscosity. 
The essential requirements for the reactive plasticizer are (1) not to 
swell the polymer powder at room temperature, (2) to maintain the 
viscosity of the dispersion, (3) to be able to plasticize the polymer 
powder at an elevated temperature at or above the plasticization point and 
(4) to be polymerizable or curable. Therefore, any polymerizable or 
thermosettable resin can be used as the reactive plasticizer when it meets 
these requirements. 
The polymer powder has to be swellable by the reactive plasticizer upon 
heating at or above the plasticization temperature. Additionally, the 
particles of polymeric material can have reactive functional groups such 
as --COOH, --OH, --NH.sub.2 or --NCO present but such groups are not 
necessary, and higher conductivity for a given amount of conductive 
fillers is dependent on the solvation of the polymer particles by the 
reactive plasticizer and the plasticizers' subsequent polymerization or 
curing. 
In this invention, a series of conductive thermosets prepared by 
conventional method and containing only silver particles and epoxy resin 
(Epon-828) was used as reference to illustrate the conductivity 
improvement of the thermosets when the concept is used. The polymer 
powders utilized for demonstration include an isobutene-maleic anhydride 
copolymer and a styrene-maleic anhydride copolymer. Of course, any polymer 
powder swellable in a reactive plasticizer can be applied to this 
invention. The polymer powder in the instant invention can be, but is not 
limited to, polyethylene, polypropylene, polyphenylene oxide, 
polymethacrylate, polyacrylate, polyvinyl chloride, polystyrene, 
polyacrylonitrile, polyimide, polyvinyl acetate, polyvinyl acetal, 
polyphenylene sulfide, poly(aromatic sulfone), polyvinyl alcohol and many 
others, and copolymers of these homopolymers. 
In the present invention a reactive plasticizer is a liquid material which 
can solvate polymer powder at a temperature equal to or above the 
plasticization point and is polymerizable or crosslinkable under 
polymerization or curing conditions. Therefore, the reactive plasticizer 
or a mixture of reactive plasticizers in the dispersion will become a 
plastic, either thermoplastic or thermoset, interpenetrated in the swollen 
powdered polymer after the plasticization and polymerization or curing. 
Reactive plasticizers applicable to this invention include various types 
of monomers and thermosetting resins. Monomers include, but are not 
limited to, styrene, methacrylates, acrylates, epoxides, diisocyanates, 
diols, dianhydrides, diamines and dicarboxylic acids which are all 
suitable as reactive plasticizers. The thermosetting reactive plasticizers 
include, but are not limited to, epoxy resin, polyfunctional isocyanate, 
melamine resin, phenolics, polyols, polyamines and the like. 
The curing agent employed in the instant invention is dependent upon the 
type of liquid reactive plasticizer. In certain instances the curing agent 
is not necessary but can be optionally employed. Examples of this type of 
liquid reactive plasticizer are acrylic or methacrylic terminated 
monomers, oligomers or prepolymers which materials are self-polymerizing 
on heating. However, to increase the reaction rate of the polymerization, 
free radical generators such as organic peroxides are usually employed. 
Other liquid reactive plasticizers which are polymerized or crosslinked by 
free radical generators include, but are not limited to liquid butadiene 
copolymers and reactive unsaturated olefins. In the instances where free 
radical generators are used, they are usually present in an amount ranging 
from 0.001 to 10% by weight of the liquid reactive plasticizer. In other 
instances such as in the polymerization or crosslinking of epoxy resin 
with cationic BF.sub.3 amine complex or anionic amine initiators, the 
amount of the initiator ranges from 0.001 to 10% by weight of the liquid 
reactive plasticizer. In the instance where the liquid reactive 
plasticizer is an epoxy resin and the initiator is dicyandiamide or an 
amine adduct, amounts of initiator present range up to the stoichiometric 
amount necessary to react with the epoxy groups present in the 
plasticizer. In the instance where a mixture of reactive plasticizers is 
employed which require different curing agents, a combination of curing 
agents including those operable for each of the reactive plasticizers 
should be used. Thus, for example, when an acrylate terminated reactive 
plasticizer is admixed with an epoxy plasticizer, both an organic peroxide 
and either a cationic or anionic initiator or dicyandiamide should be 
combined to insure that both reactive plasticizers are cured. 
In the instance where the polymer powder has functional groups for curing 
the reactive plasticizer, the additional curing agent may not be needed in 
the conductive thermosetting composition. 
The electrically conductive material herein can be in the form of 
particles, spheres, beads, powder, fibers, flakes or mixtures thereof. By 
"electrically conductive material", as used herein, is meant the 
electrically conductive material, per se, not including any substrate on 
which it may be coated. Aside from the noble metals and noble metal coated 
substrates which can be used as the electrically conductive material 
herein, the use of other metals such as copper, aluminum, iron, nickel and 
zinc are also contemplated. Also employable are silver coated glass 
spheres sometimes referred to as "beads" which have an average diameter of 
about 6 to 125 microns. These materials are made from glass spheres 
commonly employed as reflective filler materials and are commercially 
available. Additionally, glass fibers coated with silver, copper or nickel 
as shown in French Patent No. 1,531,272 can also be employed. Electrically 
conductive material used herein also includes carbon black and graphite. 
In the instant process the amount of the electrically conductive material 
needed for conductance is in the range 1 to 80 weight percent of the 
conductive composition employed, preferably 5-70 weight percent on the 
same basis with the balance being the thermoset material consisting of the 
particles of the polymeric material, reactive plasticizer and curing agent 
for the plasticizer. 
The electrically conductive material employed herein can be used in various 
sizes depending on its form. For best results, the major dimension of the 
electrically conductive material should be no greater than about 400 
microns. Prefably, the electrically conductive material has a major 
dimension in the range 10 to 60 microns. 
In the thermoset material the amount of the particles of the polymeric 
material can range from 0.0001 to 70%, preferably 0.1 to 30% by weight, 
with the balance making up to 100% by weight being the liquid reactive 
plasticizer. 
In carrying out the instant invention, the conductive thermosettable 
dispersion composition is heated to the plasticization temperature of the 
plasticizing components. This temperature will vary in the range 
40.degree. to 250.degree. C. depending on which polymeric material and 
which reactive plasticizer is used. The crosslinking or polymerization 
reaction of the liquid reactive plasticizer is carried out at a 
temperature in the range 40.degree. to 250.degree. C. dependent upon the 
liquid reactive plasticizer and curing agent. 
The heating step can be carried out by various means. In simple systems 
wherein the conductive thermoset material is to be used as an adhesive, 
the adhesive can be applied by manual means to an adherend, contacted by 
another adherend and the assembled system heated in a forced air oven 
until a conductive thermoset bond results. Additionally, electromagnetic 
heating including induction heating and dielectric heating can also be 
utilized for faster cures.

The following examples are set forth to explain, but expressly not limit, 
the instant invention. Unless otherwise noted, all parts and percentages 
are by weight. Conductivity measurements were made using a two-probe 
Simpson meter on a cured sample, 50 mm. in length, 3.2 mm. in width and 
with a thickness measured with a micrometer. 
EXAMPLE 1 
A known amount of silver (&lt;74.mu.) was dispersed in 10 g of liquid 
diglycidyl ether of bisphenol-A, commercially available from Shell under 
the tradename "Epon-828", containing 0.6 g of dicyandiamide. The sample 
was cured in a cell with a diameter of 0.5" and a thickness of 0.06" at 
170.degree. C. for 30 minutes. The cured plate was placed between two 
electrodes to measure its conductivity. The results are shown in FIG. 2. 
EXAMPLE 2 
A known amount of silver (&lt;74.mu.) was dispersed in a plastisol containing 
2 g of isobutene-maleic anhydride copolymer, 8 g of Araldite CY-179 
(3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate; Ciba-Geigy). 
The curing and conductivity measurement procedures are the same as those 
described in Example 1. The results are also shown in FIG. 2. 
EXAMPLE 3 
A known amount of silver (&lt;74.mu.) was dispersed in a plastisol containing 
2.5 g of a commercially available styrene maleic anhydride copolymer and 
7.5 g of Epon-828. The curing and conductivity measurement procedures are 
the same as those described in Example 1. The results and the calculated 
silver savings are also shown in FIG. 2. 
The conclusions from FIG. 2 are (1) the conductive thermoset (Example 1) 
without the addition of polymer powder has the lowest conductivity of the 
three examples at the same level of silver content; (2) the enhancement of 
conductivity depends on the type and amount of the polymer powder in the 
composition; and (3) to maintain the same level of conductivity, the 
conductive thermoset containing polymer powder requires a lower silver 
content than the conventional conductive thermoset.