CA1122566A - Microwave plasma modification of surface properties in organic polymers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention provides a method of improving the properties of an organic or inorganic substrate so as to render the same more susceptible to coating treatment or to impart improved adhesion characteristics thereto.
The method comprises the steps of providing a solid substrate, elevating the temperature of the substrate to at least 40°C., and subjecting the substrate to a cold plasma, and if desired, coating the substrate with a coating material after the substrate has been heated.
There is also provided a method of modifying the surface properties of an aramid polymer. The method comprises providing an aramid polymer product, generating a cold plasma, and exposing the aramid polymer to the plasma whereby the surface properties of the aramid polymer are enhanced.

Description

~iZ2S66 This invention relates to the modification of surface properties of solid substrates using a "cold" plasma.
By way of background, in many industrial applications, the surface properties of various solid substrates may have to be modified for particular end uses. As a typical example, the surfaces of such solid substrates, be they of organic or inorganic nature, may have to be provided with surface coatings in order to protect such substrates against corrosion, wear, vapor sorption, etc., or on the other hand, may have to be modified in order to provide improved surface characteristics (such as adhesion) for subsequent use in other applications of the substrate.
As an example, aramid organic polymers, also known as aromatic polyamides, are well known for forming fibers which have outstanding mechanical properties. Such fibers display very high modulus and stiffness, and have a great potential for applications as high-strength textiles, in the manufacture of high-performance rubber tires, in composites, and in other applications requiring high stiffness and superior dimensional stability.
Offsetting somewhat the advantages noted above is the fact that aromatic polyamide fibers tend to have limited flame resistance and are subject to attack-by certain acid media. Fibers of these materials, for example, that are marketed under the trade mark l'Kevlar",while very strong individually, significantly tend to have limited cohesive strength between individual strands, as per ~. Konopasek and J.~.S. Hearle, J. Appl. Polym. Sci., 21, 2791 (1977). A
particular limitation, however, is the tendency of these fibers and of structures manufactured from them to adhere 2256~i weakly to other substrates, even when used with well-known cross-linking adhesives such as triazine or epoxy resins.
Thus, it would be highly desirable, for various uses, if the "Kevlar" material could be modified to possess bet~er adhesive characteristics in order to improve its versatility.
In addition to the above, there are a wide range of substrates which require, as aforementioned, a surface coating to modify the substrate for use in different fields, and typical examples would be the use of such products in the electronic, electrical and various other industrial operations such as oil fields, etc., where such protective layers are re~uired for corrosion, moisture resistance, etc.
for the substrate. The coating of substrates for this purpose has been carried on for numerous years by various known methods, e.g., by lamination, dipping, etc. But, due to the underlying method or technique of applying the coating to the substrate, be it of organic or inorganic nature, the present techniques have only resulted in limited protection for the substrate. With one aspect of the present invention, applicant has found that substrates which require a coating can be made receptive to a better coating, or alternatively, by improving the properties of the solid substrate to impart improved characteristics thereto, by the steps of providing a solid substrate, elevating the temperature of the substrate to at least 40C. and subjecting the substrate to a cold plasma and if desired, coating said substrate with a coating material after said substrate has been heated.
In another development disclosed herein, there is provided a method of modifying the surface properties of an aramid polymer by the steps of providing an aramid polymer ~122~;~6 product, generating a cold plasma and exposing the aramid polymer to the plasma whereby the surface properties, particularly those of adhesion of the aramid polymer, are enhanced to overcome the disadvantages associated with the prior art.
In greater detail, and according to one aspect of this invention, the present invention provides an improvement in the adhesion of a wide range of substrates subjected to a plasma treatment, and to the physical-chemical properties of solid substrate materials intrin-sically correlated with adhesion. Thus, the present invention speci~ically provides improved performance characteristics of protective layers against corrosion, moisture penetration, etc. for the various solid substrates.
These effects can be created by depositing onto a substrate a layer of plasma generated material or by activating the substrate and subsequently by e~posing it to the desired material. These improvements have been found to result from the fact that the solid substrate is heated to an elevated temperature, e.g., above 40C. prior to or during deposition of the coating material onto the solid substrate.
In both embodiments of the invention, use is made o a cold plasma which is a thermodynamically non-equilibrium plasma. Such a plasma may be generated by any conventional known method and apparatus, preferred, howeverl is an apparatus such as that described in Canadian Patent 972,479 with the apparatus being chosen to encGmpass or be of the type to cover the entire frequency range over the sustaining of low-pressure gas discharges which is technically feasible.

The plasma apparatus may be operated at a wide range of

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frequencies extending from low- to mid-range values. A pre-ferred plasma is a high-frequency one in the range from less than 13.5 MHz to more than 2.4 GHz. In carrying out t~e methods of the present invention, either small or large scale methods may be used employing a batch or continuous process.
In greater detail of the methods of the present invention, the apparatus may be provided with suitable heating means for heating the substrate and to this end, heaters may be placed inside the plasma generator to be in contact with the substrate and to heat it to the desired degree. Heating of the substrate is preferably carried out to a temperature between about 40 - 1000C. and more preferably, within the range of 100C. - 800C. As will be appreciated, the length of heating time will vary depending on the thickness of the substrate and the material from which the substrate is made of but in all cases, the substrate will be heated so at least the surface of the substrate achieves the desired temperature. As outlined previously, the substrate can be chosen from a wide variety of materials depending on the intended end use of the product and a wide variety of substrates may be used.
Substrates such as organic polymers, metals, glass, and cera-mics, have been treated using the method of the present invention and have been found to possess improved properties for subsequent treatment by subsequently providing a coating on the substrate. Thus, the particular choice of substrate is not critical and any suitable substrate for a given purpose may be employed in the practice of the method of the present invention utilizing the heating of the substrate prior to or during plasma treatment followed by a coating treatment.

~22S6~

In the case of the aramid polymer development of the present invention, applicant has unexpectedly found that exposure of this particular aramid polymer, exemplified by the material marketed under the trade mark "Kevlar", has been found to overcome the disadvantages of "Kevlar"
in having relatively weak adhesion properties. By exposure of the "Kevlar" material to the plasma treatment, significant improvements have been obtained so that the material is capable of being used by itself in situations requiring improved adhesiveness of the material (particularly in the form of fibers used in tire cord or other like constructions) as well as in situations involving subsequent coating of the Kevlar due to its increased adhesion properties. In the treatment of the Kevlar material, this material may also be preheated prior to or during plasma treatment.
Having thus generally described the inventionr reference will now be made to the accompanying examples, illustrating preferred embodiments only. In the examples, certain examples involve the use of strips of "Kevlar"
cloth or material using a suitable microwave plasma discharge apparatus as e.g., described in Canadian Patent 972,479. The apparatus in the examples was a 2.5 inch x 30 inch quartz tube reactor and plasma is produced by any one or a combination of several different gases flowing through the reactor at low pressure, preferably in the range of 0.1 - 10 torr. Plasma treatment times may vary widely as outlined above, although for most purposes, short ~reatments in the range of 10 - 120 seconds suffice to produce surprisingly large changes in the surface characteristics of the substrate or "Kevlar" materials.

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The plasma intensity in the treatments may also be varied broadly, although it will be appr~ciated by those skilled in the art it is to some extent determined by the electrical breakdown characteristics of the gases used. Typically, pre-ferred power applied to the plasma is in the range of less than 10W to mo~e than 3KW, corresponding to power densities in the plasma from less than 0.01W cm to several watts per cubic centimeter.
In the various examples, the low-pressure gas plasmas were divided into four categories, according to the nature of gas environment involved. Other different kinds of gases may also be used for the purpose of surface modification.
The categories used are:
(I) Inert gas plasmas - for example using argon, helium, etc.
(II) Sequential or "grafting" plasmas; in these, the surface is activated by (i) followed by exposure of the substrate to reactive monomers (vapor or liquid) as in (IV).
Alternatively, structures such as "Kevlar"
may be saturated with adhesives such as triazine resin, followed by an anchoring plasma of type (I).
(III) Reactive, but non-polymerizable (inorganic) gas plasmas (ex. N2, NH3, SO2, CO2, CO, air, etc.) singly or in combination or in sequence.
(IV) Polymerizable organic gas plasmas [e.g. allylamine (AM), propane epoxy (PE), organosiiicone monomers-e.g., hexamethyl - disiloxane (HMDS), etc.], singlyr or in combination, or in sequence.
The present teachings also apply to the combined or sequential use of any two or more of the plasmas encompassed in the above-noted categories.

~22566 In the following examples reference to the drawings will be made in which:
FIGURE 1 is a photograph of a control fiber;
FIGURE 2 is a similar photograph of treated fiber;
. . .
FIGURE 3 showing a graph illustrating increased density of deposited polymer; and FIGURE 4 illustrating test results graph form of the products of the present invention;
The following examples are set forth to illustrate more clearly the principles and practice of the present invention to one skilled in the art and are not intended to be restrictive but merely illustrative of the invention herein contained.
EXAMP~E 1 A strip of "Kevlar" cloth, about 6" x 1" in dimension was placed in the quartz tube reactor of a microwave frequency plasma apparatus, and the apparatus was evacuated to ~10 3 torr. Thereafter a flow of N2 gas was established, the flow rate being controlled through needle valves and ~0 flow meters, establishing a pressure of 0.5 torr in the apparatus. The "Kevlar" was exposed to a 0.5 KW plasma of N2 gas for a period of 30 sec. It was then removed from the apparatus and immediately contacted with triazine adhesive.
A doctor blade was used to establish a uniform wet thickness corresponding to a total substrate~resin weight ratio of about ~0/40. In additional experimentsl this ratio was varied from about 80/20 to a~out 50/50, with results that showed that the composi~ion which was the product marketed .

3l~225;S

under the trade mark "Triazine A" resin (Mobay Chemical Corp.) 375 g;
Methyl Ethyl Ketone 50 g; and Zinc octoate (~%), 1 ml.
- plasma effectiveness is not restricted to any particular polymer to resin ratio. The strip was now cut, and the wet halves superposed to form a 2 ply laminate, said laminate being curea by first heating in an air oven 15 min/125C., and then compression molding at 1500 p.s.i. and 175C. The peel strength was determined in an Tnstron Tester, with the two halves of the laminate being separated at 180. The ultimate peel strength was found to be 1100 -~ 100 g/cm as `
compared with a peel strength of 600 + 100 g/cm, for iden-tical laminates using untreated "Kevlar". The results of this example are further entered as article 6 in the Table of peel strength data in the following example~

In this example r many samples of "Kevlar" cloth were microwa~re plasma-treated to show the effect on laminate adhesion of various kinds of plasma pre-treatment, following the categorization given above. In all cases, 6" x 1" strips of "Kevlar'l were used, and converted to laminates for testing ~ ~-along lines already specified in example 1. The results of peel tests are summarized in the followin~ Table. Here, the four plasma categories are represented, in treatment using gas pressures of 0.2 to ~ 0.8 torr, at 100 - 700 Watt plasma intensities and treatment times in the 30 - 45 sec.-range. ;
Ultimate peel strengths (UPS) of treated and of untreated, control "Kevlar" laminates are given and, for convenience, the ratio r = (UPS) treated is also entered in the Table of ~UPS) control ~122S6~

results.
It is clear from the results, that each of the four kinds of plasma-treatment to which "Xevlar" cloth had been subjected leads to surprisingly larger increments in peel adhesion strength. The inert gas plasma using argon produces a smaller increase, but "grafting", non-polymeriz-able and polymerizable gas plasmas may be expected to pro-duce adhesion improvements ranging upward of 30 - 40%.
Maximizing the effect of any of the plasma-treatments may be obtained through judicious controls of treatment time, pressure of gas, plasma intensity, etc.

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-- 1 o LZ~6~i NOTES: 2~ Category: a= chemically inert plasma;
b= "grafting" conditions;
c= reactive gas plasma (non-polymerizable);
d= reactive gas plasma (polymerizable) 5) Nominal power (power absorbed by plasma was somewhat less).
6~ tl = duration of plasma treatment.
7) t2 = elapsed time after treatment, when sample was impregnated. Normally impregnation followed treatment within a few minutes.

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--` 112~S66 This example demonstrates that the surface modifications on aromatic polyamides - such as "Kevlar" -produced by lo~-pressure (glow discharge) gas plasrnas of the radio-frequency or microwave types, are affected by subse-~
quent aging of the substrate in air. Thus, a preferred con-dition for adhesive strength improvement is to allow contact between the plasma-modified surface and the fluid or surface to be bonded with the aromatic polyamide, as soon as possible after plasma treatment. Alternatively, time after treatment may ~e regarded as one of the variables in controlling the magnitude of physical property effects to be realized through plasma treatment.
Strips of "Kevlar" cloth were prepared and treated in a microwave plasma as in examples 1 and 2. Plasma using hexamethyl-disiloxane CHMDS) monomer were used and the treat-ment variables were 0.2 torr pressure, 700 Watts intensity and 30 sec-duration. One set of "Kevlar" strips was impregnated with triazine resin immediately after being removed from the plasma apparatus, and laminated as in example 1. Another set o~ strips was allowed to age in air for 2 days prior to impregnation/lamination, while a third set was aged one week ~`
prior to impregnation/lamination- The results of . .
peel tests showed that the peel strength ratio decreased from a "r" value of above 2.0 to below .75 over 7 days.
Similarly, aging time following N2 plasm~ treatment resulted in a loss of peel strength in a straight line relationship. The effect alluded to exist in all of the plasma-types referred to previously.

1~22S~i Eii EXAMP~E 4 This example shows that a wide variety of surface effects may be obtained through the exposure to plasmas of aromatic polyamide structures of the "Kevlar" type. In some instances, such as those using polymerizable monomers of ihe type already mentioned,the "Kevlar" appears to be encapsulated in a strongly-adhering layer o~ polymer the nature of which reflects the monomer used. In others, such as those using non-polymerizable gases, surface cleansing, the incorporation of polar groups and of free-radicals, etc.
is suspected and the observed beneficial effects on adhesive strength attributed to these events. Thus, both physical and chemical surface changes are attainable through the medium of plasma treatment.
The presence of new polymer on a plasma-treated "Kevlar" substrate can be shown ~y examining control surfaces and surfaces previously exposed for 90 sec to HMDS vapor plasmas at 0.5 torr., by the analytic method known as Frus-trated Multiple Internal Reflection Infra Red Spectroscopy (FMIR-IR). The resulting spectra clearly show strong absorp-tion bands at 1260, 1020 and 800 cm l in the plasma-treated sample which are absent in a control "Kevlar" sample, and which are due to vibrations of various - Si - linkages.
Surface roughening, or "cleansing" of a physical nature is inferred in a comparison of Scanning Electron Microscopy CSEM~ data, as in Figures 1 and 2. The control fiber of "Kevlar" CFigure l) is smooth whilst one exposed to an air plasma CFigure 2) is obviously roughened by the intense bombardment of the active particles in the air plasma.

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This example relates to the bond strength increments attributed to the exposure of "Kevlar" to microwave plasmas through the simpler exposure of "Kevlar" to corona discharges, such as those conventionally used in the surface pre-treatment of polymer films to be subsequently printed by comme~cial gravure inks. "Kevlar" strips were therefore exposed to -corona discharges in air, and then laminated and tested following methods herein outlined. It was found that the peel strength of "Kevlar" laminates was unaffected by corona treatment. Thus, the effects described appear to be produced by low pressure gas discharges, but insignificantly, or not at all, by atmospheric pressure discharges of the corona type.

The effect of microwave plasmas on the tensile strength of "Kevlar" fibers and cloth was examined by com-p~ring the stress/strain curves of "Kevlar" standards with similar specimens which had been exposed to microwave plasmas in argon. Typically, the ultimate tensile strength of single "Kevlar" fibers is somewhat reduced by plasma-treatment, the ratio of control ultimate tensile to plasma-treated analogue being in the vicinity of 1.5~ On the other hand, plasma treatment increases bond-strength between individual fibers.
These compare load-elongation curves of 1 inch "Xevlar"
strips with and without argon plasma-exposure of 30 sec duration. The maximum tensile is again reduced, but the load elongation curve of plasma-modified samples is significantly broadened, so that the total work needed to "break" the sample is about the same in both - ~Z25~;6 cases, areas under the load-elongation curve having a ratio of 1.0 + 0.1.

This example demonstrates the unique capability of high frequency plasmas to produce strongly adhering films of high and controlled density through the appropriate choice of the substrate temperature. Plasma polymers of HMDS were deposited onto various substrates which were in good thermal contact with an electric heater, and substrate temperature was varied up to 500 C, all other deposition variables being kept constant. The density of the deposit was deter-mined by independent gravimetric and thicknes~ measurements, the latter by multiple beam intererometry. Figure 3 shows that the density of HMDS, plasma polymerized at room temper-ature, exceeds the density of conventionally polymerized polymethylsiloxane (0.:98 g cm 3~ by about 70% and, secondly, that the density of the plasma polymer further increases to about 1.9 g cm 3 with increasing substrate temperature. Peel tests using "Scotch tape" were performed on deposits produced at different substrate temperatures. Films deposited at room temperature peeled quite readily from the substrate, but in the case of films deposited at tempera~ures above about 150C
it was impossible to remove the films by this t~st.
The inference of plasma polymer films having the characteristics of high density and strong adhesion is high resistance to the penetration of liquids and vapors and, therefore, highly beneficial properties in corrosion protection, vapor transmission, etc. The following example elaborates on this.

1~22~66 This example illustrates the moisture barrier properties of inherently hydrophilic polymer foils which have been modified by the presence of a plasma polymer deposit 50um thick polyimide foils at various temperatures were ex-posed to HMDS plasma under constant plasma conditions. The moisture permeation characteristics of these composite structures were subsequently determined by weight change data using the procedure of ASTM E 96. Figure 4 illustrates typical results ~ollowing 24 hours' exposure to water vapor, and pertains to 0.5 um thick plasma polymer deposits. Clearly, moisture uptake is greatly reduced by plasma polymer deposits produced at elevated substrate temperatures.
The substrate temperature range involved in the present example is specific to the organosilicone monomers used here. The principle of controlling properties of plasma polymers through substrate temperature control is held to be common to many "monomers", however.

With respect to heating the substrate, in addition to the fact that heaters can be placed inside the plasma generator, it will also be understood that the substrate may be preheated before entering the reactor, in the case of a continuous treatment process, or heaters may be placed inside the plasma generator to be in contact with the substrate and to heat it to the desired degree, or the substrate may be heated by radiant energy from infra-red sources external to the plasma reactor.

2~;6 It will be understood that various modifications can be made to the above-described embodiments without departing from the spirit and scope of the invention.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of modifying the surface properties of an aramid polymer comprising providing an aramid polymer product, generating a cold plasma and exposing said aramid polymer to said plasma whereby the surface properties of said aramid polymer are enhanced.

2. A method as defined in Claim 1, wherein said aramid polymer is initially heated prior to or during exposure of said aramid polymer to said cold plasma.

3. A method as defined in Claim 1 wherein a coating is deposited onto said aramid polymer product by plasma generation.

4. A method as defined in Claim 1, wherein said aramid polymer is exposed to an inert gas plasma.

5. A method as defined in Claim 1, wherein said aramid polymer is the product marketed under the trademark "KEVLAR".

6. A method as defined in Claim 1, wherein said plasma used has a power density in the range of from less than 0.01 W cm-3 to more than 10 W cm-3.

7. A method as defined in Claim 1, wherein the material being heated is heated to a temperature of between 100°C and 500°C

8. A method as defined in Claim 1, wherein the material to be treated is exposed to a high frequency plasma in the range from less than 13.5 MHz to more than 2.4 GHz.

9. A modified aramid polymer having improved surface adhesion properties obtained by exposing said aramid polymer to a cold plasma.

10. The product of Claim 9, wherein said polymer is the product marketed under the trade mark "KEVLAR".