PLASMA
The Chemistry Tool for the 21st Century

By: Stephen L. Kaplan

INTRODUCTION

One hundred years ago, Sir William Crooke stated "…. the investigation of the fourth state of matter - plasma - will be one of the most challenging and exciting fields of human endeavor". Even though it has found a major niche market (in excess of $ 1 billion) for the fabrication of semiconductor devices, plasma remains an emerging technology still in its embryonic stage in the industrial market place.

PLASMA

Gas plasmas have existed since the beginning of time, long before the advent of solids or liquids. In fact, 99 percent of the universe is a plasma. Stars are plasmas. The solar corona is a plasma. Earth's natural plasma include lightning, the Aurora Borealis, and St. Elmo's fire. Table I lists certain plasma and characterizes them by particle density and temperature.

Table I

Ranges of Particle Density and Temperature
for Various Types of Plasmas

There are many definitions of the term "plasma" depending on ones particular discipline. It has often been referred to as the fourth state of matter; the generation of plasma being analogous to the transitions that occur when energy is supplied to a material causing solids to melt and liquids to become gases. Sufficient additional energy supplied to a gas creates a plasma. In the case of cold gas plasma, typical of that to be discussed in this paper, the process is excitation of a gas at reduced pressure by radio frequency energy. A plasma contains excited atomic, molecular, ionic and radical species. While on an atomic scale, plasma generation cannot be construed as a room-temperature equilibrium process, the bulk of the material remains near room temperature. The plasma contains free electrons as well as other metastable particles which upon collision with the surfaces of polymers placed in the plasma environment break covalent chemical bonds creating free radicals on the polymer surface.² These free radicals will then undergo additional reactions depending on the gases present in the plasma or subsequent exposure to gases in the atmosphere. The result is that these gas-radical reactions form a surface potentially very different from the starting bulk polymer. Since the process is conducted in a reactor under very controlled conditions, the end result is very reproducible.

Plasma processing is not one process but a "tool box" that can be classified into three categories: 1) plasma activation, 2) plasma induced grafting and 3) plasma polymerization. Plasma activation is the simplest and most cost efficient of these processes and more than adequate to prepare plastics for painting. This paper will focus on plasma activation processes exclusively.

Plasma activation is the alteration of surface characteristics by the substitution of chemical groups or moieties for groups normally present on the polymer chain being modified. The mechanism is the reactions resulting from the bombardment of the surface by photons, ions and neutral particles, all of the active species in the plasma react with the polymer surface. The by-products, typically C0², H₂0 and low molecular weight hydrocarbons, are readily removed by the vacuum system. The use of co-reactants can serve to modify the surface chemistry obtained with a single gas chemistry or to accelerate the reaction kinetics. For example, in an oxygen plasma, the breaking of the carbon-carbon and carbon-hydrogen bonds is the rate limiting step. When tetrafluoromethane is introduced as co-reactant, the 0₂/CF₄ plasma yields excited forms of 0, OF, CO, CF₃, C0₂, and F. Since fluorine or fluorine containing species are more effective in breaking the carbon-carbon and carbon-hydrogen bonds, the reaction rate is accelerated. The use of other gases permits the incorporation of other functional groups on the polymer surface. Examples include the use of ammonia, nitrogen and nitrous oxide plasmas to incorporate nitrogen on the surface and create nitrogen based functional groups such as primary and secondary amines.

The result of plasma modification of the polymer is an increase in the surface energy of the polymer and an attendant improvement in surface wetting. Adequate wetting of the surface by the adhesive contributes to an improvement in bond strength by increasing the apparent area of contact over which the load is distributed. Published studies^3-5 suggest that this improvement in wetting directly contributes to the observed improvement in the strength of the adhesive bond. However, evidence has been presented in several studics^6-10 that indicate that the major improvement of adhesive strength is dependent on the particular functional group that has been created on the surface of the polymer. In some cases, a direct correlation is drawn relating the nature of the chemical groups on the surface, the nature of the adhesive used and the observed improvement in adhesion. In other cases, the observed improvements are related to effects of hydrogen bonding and specific surface chemical interactions that do not necessarily result in covalent bonding between the polymer surface and the adhesive.

While most, if not all, plasma equipment consists of similar components, the design of the reactor chamber, the distribution of power, the excitation frequency, and the gas dynamics can all be critical parameters influencing the efficiency and properties of plasma reactions. There has been an extensive amount of work published that shows a direct correlation between excitation frequency and plasma reactivity. Manufacturers of plasma equipment employing radio frequency excitation use either low frequencies, i.e. less than 400 KHz, or the higher frequencies at 13.56 or 27.12 MHz as specified by the FCC. For applications involving the treatment of plastics 13.56 MHz is the preferred frequency. Also important is whether the material being treated is in a primary or secondary plasma. In a secondary plasma the plasma is created either between closely spaced paired electrodes which may function as shelves or in the annulus between the vessels outer wall and a ring or cage electrode. In a secondary plasma the treatment of materials placed within the working volume depends on the diffusion of active species created in the primary plasma, i.e., within the RF field. Diffusion of these active species is the very creation of free radicals and subsequent coupling of these free radicals with active species from the plasma environment. Depending on the process gas selected, a large variety of chemical groups can be incorporated into the surface. These groups may be hydroxyl, carbonyl, carboxylic, amino or peroxyl groups. Most importantly, the insertion or substitution of these groups in the polymer chain is under the control of the operator. In this manner, the surface energies and the surface chemical reactivity of plastics can be completely altered without affecting their bulk properties.

PLASMA ACTIVATION AND REACTIONS ON THE  SURFACE

Although any gas can be ionized using RF excitation, gases such as 0₂, N₂, He, Ar, NH₃, N₂0, C0₂, CF₄, and air or some combination of these gases are generally used for surface treatment.

These reactions represent a small sampling of those that occur in an oxygen glow discharge. As the components formed during the ionization recombine, they release energy and photons, emitting a faint blue glow and much UV radiation. The photons in the UV region have enough energy to break the carbon-carbon and carbon-hydrogen bonds in the materials on the surface that are exposed to the plasma. In the case of contaminants, the net effect appears to be degradative such that lower molecular weight materials are created. These lower molecular weight materials are subsequently removed by the vacuum. In this manner the surface that has been exposed to a plasma is cleaned. Lower molecular weigh polymer fractions that comprise the weak boundary layers on the surface are also removed in this manner. In a low pressure oxygen plasma, the following oxidation reaction scheme has been suggested.

Here the R0₂H and R0₂R’ indicate the formation of acids and esters. Not indicated in the above reaction scheme are the possible formation of alcohols, ethers, peroxides and hydroperoxides. Thus, besides the dependent on pressure; the higher the pressure, the shorter the mean free path. The mean free path is the distance active species can most likely travel before undergoing collisions that deactivate radicals or neutralize ions. Therefore, when using a secondary plasma, the concentration of active species varies either across the diameter of a barrel system or between electrode pairs as the case may be. Thus, the physical laws of nature deem that the treatment within the working volume of a secondary plasma system cannot be uniform. By contrast when working within the RF field, or primary plasma, the gas is constantly being excited. Thus, polymeric articles being treated are immersed in a constant concentration of active species. Further, since diffusion is not a mechanistic limitation, significantly higher operating pressures may be used. This allows higher process gas flow rates assuring that off gassing molecules from the polymer are sufficiently diluted providing the full benefits of the desired process gas. In addition, the primary plasma is rich in ultraviolet radiation that is often an important initiation step in polymer reactions. Since UV radiation is line of sight, uniform treatment of multiple parts can only be assured when working within the primary plasma. Otherwise any part in the shadow of another will receive different radiation and therefore the effectiveness of the treatment is expected to vary. The plasma system used in the study reported herein utilized a system designed and built by Plasma Science. All Plasma Science equipment employ primary plasma in the treatment zone.

PERMANENCY OF PLASMA TREATMENT

One of the first commercial uses of plasma outside of the semiconductor industry was an activation process to enhance adhesive bonding of molded silicone rubber gaskets in the assembly of military cylindrical connectors. Plasma activation of the silicone allowed the use of conventional epoxy adhesives to achieve adhesive bonds of silicone to metal where failure is characterized as cohesive tearing of the silicone. These cylindrical connectors have been successfully using plasma in their manufacture for more than twenty years and actual real time studies show no deterioration of the bond quality with time, even though these connectors are exposed to harsh storage and use conditions.

EXPERIMENTAL SECTION

Twelve full size fascias molded from a commercial reactor grade thermoplastic polyolefin (TPO) were painted in a commercial automotive production line. The fascias were painted with a commercial one component (IK) base and clear paints; six fascias each of white and blue metallic. Six by four inch (6" x 4") plaques were cut from six (6) different positions as follows: drivers side ear, drivers side face, drivers top deck, passenger's side ear, passenger side face and passenger side top deck. The plaques were subjected to Chrysler specification MS-PA40-1 which requires subjecting the plaques to the following tests:

RESULTS AND DISCUSSION

Two hundred eighty eight plaques (288) cut from molded bumbers were tested and the following results obtained.

All of the white painted specimens were satisfactory as tested. All metallic blue panels failed 240 hours humidity exposure due to blistering. All other tests on the metallic blue met or exceeded the specification requirements. The blisters were of the very small micro type variety. However, specification MS-PA1-1 allows no blistering, thus failure. Interestingly, the blistered samples also passed adhesion tests, an indication that the blisters were not adhesive in nature. This was further proven by Electron Spectroscopy for Chemical Analysis (ESCA) which showed TPO bound to the bottom side of microtomed blisters. This type of blister is referred to as "solvent pop" where paint solvent migrates and dissolves into the TPO bulk and is subsequently released during the elevated temperature humidity soak rupturing the substrate causing lifting and the appearance of blisters. True blisters, i.e. paint film separation from the substrate, would have no adhesive strength to the substrate. The satisfactory performance of the white paint as opposed to the humidity soak failure of the metallic blue is best be defined as a paint formulation or substrate deficiency not a failure of the plasma surface treatment.

CONCLUSIONS

Whether it is the bonding of polymers to other polymers or to other materials or the painting of polymer surfaces, adhesion is a recurring and difficult problem for all industries that use polymer materials as key components in their products. Designers often must select specially formulated and expensive polymeric materials to ensure satisfactory adhesion (albeit, even these materials often require surface preparation). In some cases, whole design concepts must be abandoned due to the prohibitive cost of the required polymer or the failure of crucial adhesive bonds. Gas plasma processes are the great equalizer making all polymers and elastomers equally paintable eliminating paint adhesion or adhesion permanency as an issue in the selection of a given polymer. Gas plasma glow discharge treatment typically allows all polymers to achieve paint adhesion strength exceeding the bulk strength of the substrate or paint. Failures do not occur at the paint film / substrate interface but within the bulk of either the paint film or the substrate. And plasma is workplace and environmentally friendly providing significant cost savings over any other means of volume surface preparation.

BIBLIOGRAPHY

1. Sheer, C. "Arc Jets In A Science and Technology", Vistas in Science (1968)

13. Kolluri, O.S., Kaplan, S. L., and Rose, P.W., "Gas Plasma and the Treatment of Advanced Fibers", SPE Advan. Poly. Composites '88 RETEC 1988