Plasma Processes for Wide Fabric, Non-wovens and Film

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Plasma Processes for Wide Fabric, Non-wovens and Film[1]

Stephen Kaplan

4th State Inc.

Belmont, CA 94002

Abstract

To many people plasma is a laboratory curiosity or limited in scale. Few know that plasma is a commercial process used daily in the treatment of fabrics, non-woven webs and film. This paper reviews applications and processes used to modify materials up to 60 inches in width in a roll to roll plasma system. The applications are quite varied. Advanced reinforcing fibers in fabric form, such as Kevlar®, Spectra®, and Vectran®, are treated to enhance penetration and adhesion to resins used in the manufacture of composite structures. Kapton®, Halar®, and other engineering films are treated to enhance metallization, as well as alter surface energy.

Sometimes the process is simply to change the surface energy, while at other times far more sophisticated processes, such as plasma enhanced chemical vapor deposition processes, are employed to provide a chemical barrier or alter the tribological properties. As will be seen in this presentation, plasma is extremely versatile and applicable to high volume web applications.

Discussion

Plasma, as employed in this work, is created by supplying energy in the form of radio frequency electromagnetic radiation to ionize the process gas into a plasma. The plasma consists of electrons, ions and other energetic metastable species. A plasma at atmospheric pressure will readily attain 20,000oK and finds great utility as a torch for cutting metals and ceramics. The processes as discussed herein are conducted at reduced pressure, thus the ambient temperature during process is room temperature or slightly elevated.

As stated, plasma contains electrons and ions which, as individual particles or chemical fragments, have high energy. In a typical glow discharge, aka cold gas plasma, the energies expressed as electron volts (eV) of these particles range from 3 – 20 eV. When these energetic particles contact the material to be treated (substrate) the possibility of a chemical attack is quite high. A variety of different processes occur that may be categorized as cleaning, crosslinking, deposition, and grafting. A caveat: many of the processes are in competition, thus in characterizing the chemistry as a specific type the categorizing is by the primary effect on a specific substrate. Substrate does make a difference and what may be a the effect of any specific plasma process on one polymer may be different on another polymer. The important effects are: cleaning, cross-linking, and activation. Activation may have at least two subsets: PECVD and Grafting.

Cleaning

The first question is what are you trying to clean or remove from the surface. A close second is what is the surface. Oxygen is the workhorse for cleaning especially if you are trying to remove organics from the surface. Plastics present a variety of cleaning challenges. In addition to contamination by humans (skin oils) and aerosols in the atmosphere, the organic contaminants can include mold releases, polymer oligomers (weak boundary layers), processing aids, uv and heat stabilizers (common in plastics), et cetera.

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Plasma Processes for Wide Fabric, Non-wovens and Film

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The oxidation of these undesirables (surface contaminants) is readily accomplished, usually accompanied by their breakdown to a lower molecular weight volatile compound which boils off the surface (ablation) and is removed in the vacuum exhaust. In some instances it is desirable to supercharge the oxygen with the addition of tetrafluomethane providing the oxyfluoride radical, a very aggressive oxidizer that can accelerate the oxidation several fold to an order of magnitude. However, the plasma can not differentiate between contaminants and the polymer, thus care is necessary not to create a weak boundary layer by excessive chain scission of the polymer if the primary reason for cleaning is to promote or enhance adhesion.

First rule of plasma: discard prior knowledge and belief in contact angles and old wetting theories. With plasma there is poor correlation between contact angles and adhesion strength. Often greater bond strength is attained at mid-range surface energies than at higher surface energies. Distilled water wettability is meaningless unless your primary objective is to wet the surface with distilled water.

Often in cleaning metals oxidation is to be avoided. Argon, helium and hydrogen plasmas are routinely found to be very effective. Contamination removal occurs in a similar method, i.e. molecular breakdown of the contaminant until it boils away. However, the primary mechanism is bond scission from bombardment by the active species within the plasma.

To study the effectiveness of a cleaning process, several analytical techniques may be used. One method is Electron Sprectroscopy for Chemical Analysis (ESCA), a surface sensitive technique which provides elemental and chemical information from the outermost 40 to 80 Ǻ of a surface. Table 1 presents ESCA results on a variety of metals cleaned by plasma and other conventional processes.

Table 1. Plasma cleaning of metals

ESCA Results / Carbon %
Stainless Steel 316 Control (as received) / 54
Sample 1 contaminated with WD40® / 88
After 5 minutes O2 plasma / 38
Sample 2 contaminated with WD40 / 93
After 5 minutes Argon/O2 plasma / 52
Sample 3 contaminated with WD40 / 85
Sample 3 after sonification / 47
Sample 4 contaminated with WD40 / 87
Sample 4 after sonification then 5 minutes O2 plasma / 35
Sample 5 contaminated with WD40 / 88
Sample 5 after sonification then 10 minutes Ar/O2 plasma / 27

*Data of Maria Hozbor, Adhesives Age, December 1993

The stainless steel machined surface as received exhibited a surface carbon abundance of 54 atom percent. Theoretically, a typical stainless steel contains less than 1% carbon in its bulk. In practice, however, metal surfaces are highly active, attracting 10 -30 atomic layers of adventitious material, including oxygen, water and hydrocarbons adsorbed from the ambient environment. The WD40 contaminated samples displayed carbon levels ranging between 85 and 94 atom percent. The top 40 – 80 angstroms of these coupons consists almost entirely of carbon.


The treatment employing a 5 minute argon/oxygen plasma (tailored for non-electropolished surfaces where oxide formation is a concern) reduced the initial 93 atom percent carbon level to 52 atom percent. The process employing sonification to remove bulk, as well as inorganic, contamination followed by an oxygen plasma reduced the hydrocarbon level to 35 atom percent. Increasing the plasma cleaning time further reduces the carbon abundance to 27 atom percent. Subsequent adhesive bonding tests exhibited a change from adhesive failure at the stainless steel surface on the as received stainless steel, carbon abundance of 54 atomic percent, to cohesive failure within the adhesive at carbon atomic percents less than 50.

4th State has cleaned a variety of metal foils for subsequent coating and bonding applications. In one such application, 20 mil aluminum lid stock was cleaned and then extrusion overcoated with a heat sealable PVAc coating. Prior to plasma cleaning the failure was adhesive between the PVAc and the metal foil. After plasma cleaning the failure was cohesive within the PVAc heat sealable layer. Plasma cleaning doubled the peel strength required to remove the lid and allowed the product to be exposed to higher temperatures without causing lid failure.

Activation

In our industry – non-semiconductor applications of plasma – the primary reason for cleaning is to enhance adhesion. Table 2 is a compilation of the routine level of enhancement with what most people consider difficult to bond engineering plastics. In all cases, the mode of failure changed from adhesive at the interface to either cohesive within the adhesive or tensile, i.e. cohesive through the specimen.

Table 2. Epoxy lap shear specimens[2]

Material / Control
kgf/cm2 (psi) / Plasma
kgf/cm2 (psi) / Improvement / Failure Mode
Valox 310 / 36.70 (522) / 115.59 (1644) / 3.1X / from adh to tensile
Noryl 731 / 43.38 (617) / 126.48 (1799) / 2.9X / from adh to tensile
Durel / 17.58 (250) / 151.93 (2161) / 8.6X / from adh to cohesive*
Vectra A-625 / 66.02 (939) / 87.18 (1240) / 1.3X / from adh to tensile
Delrin 503 / 11.60 (165) / 45.49 (647) / 3.9X / from adh to tensile
Ultem 1000 / 12.94 (184) / 146.59 (2085) / 11.3X / from adh to tensile
Lexan 121 / 119.87 (1705) / 157.63 (2242) / 1.3X / from adh to tensile

*cohesive within the adhesive

In Table 2 the plasma process gas has been selected to not only clean, but to also provide specific chemical functionality to react with the adhesive to form covalent chemical bonds. The covalent bonds not only enhance initial adhesive strength but, equally important, the stability or permanency of the bonds and their resistance to environmental degradation. This process is referred to as “Activation”.

Oxidative plasmas have also been used to dramatically increase the adhesive bond strength of Kapton® polyimide films for electronic applications as shown in the following table.

Table 3. Adhesive Bond Strength of Kapton® film with Pyralux® adhesive

Treatment / Peel Strength
Kg/cm / (ppi)
Untreated / 0.45 / (2.52)
Plasma Air Side / 1.7 / (9.6)
Plasma Drum Side / 2.02 / (11.3)

Fluoropolymers are a group of polymers in which part or all of the hydrogen has been replaced by fluorine. In general, fluoropolymers have an impressive array of engineering properties including outstanding chemical resistance and very low surface energy. The low surface energy in many cases makes it nearly impossible to achieve adequate adhesion without some type of surface preparation. Its excellent chemical resistance also provides great resistance by most chemical surface preparation techniques. Plasma is the exception, offering ease of modification providing adhesive bonds stronger than the bulk material.

Table 4. Peel Strength of Adhesive-Bonded Fluoropolymers (in kgf/cm (ppi)

PFA® / FEP®
Untreated / * / 0.018 (0.1)
Chemically Etched / 1.14 (6.4) / 1.46 (8.2)
Plasma Treated / 1.48 (8.3) / 1.86 (10.4)

*Too low to measure

Spectra™ and Kevlar™ are two high performance engineering fibers that provide an unusual combination of strength, modulus and light weight. Both fibers present matrix/fiber adhesion issues which limit their composite properties. This limitation is even greater with Spectra since it is a highly structured polyethylene fiber with absolutely no affinity to bond to typical resins employed to make composite structures. Unless the fiber bonds well to the resin matrix, stress distribution will not be uniform and the maximum benefit of these unique fibers can not be realized. Plasma treatment makes possible the use of Spectra composites for structural applications as evidence in Table 5.

Table 5. Spectra 900 Unidirectional Epoxy Composites

Treatment / Relative Shear Strength / Relative Flexural Modulus
None (control) / 1.0 / 1.0
Corona Treatment / 1.25 / 1.25
Plasma / 2.75 / 7.75

While the relative improvements in composites for Kevlar are not as dramatic they never-the-less are significant and important to an industry where weight savings is usually paramount. In addition, the standard deviation of both Spectra and Kevlar composites is greatly reduced an important design consideration permitting light weighting of the structure.

Crosslinking

As previously indicated, polymers contain a large assortment of additives critical to its performance in specific applications. Sometimes these additives are measured in parts per million or they may be a significant volume measured in tens of percents. In some applications it is desirable to minimize the migration of these additives to the surface. Crosslinking of the surface effectively reduces the rate of migration. Think of a person trying to make his/her way through a crowded reception room to the free bar; unless people make room progress is very tortuous. Crosslinking is the equivalent of cementing peoples feet to the floor, thus providing limited free volume for migration to occur.

Crosslinking is also important to stabilize a surface which has been plasma activated. Elastomers characteristically have coiled polymer chains which exhibit considerable molecular motion. If you “activate” a polymer thus making it wettable, it is not unusual to see the wettability rapidly diminish with time (often hours), thus returning to its “untreated or natural” state. Adding a functional or alien moiety to the surface creates considerable thermodynamic imbalance. The molecule desires to rotate and bury this alien group into its bulk to equalize and return to its preferred thermodynamic state. Crosslinking effectively hinders or eliminates molecular rotation. The optimum plasma chemistry in many cases has extended the effective open time between plasma treatment and intended use from hours to days, months and even years. Example of such improvement is shown in Table 6.

Table 6. Flexible PVC sheet surface energy after plasma treatment with select gases

Time
(days) / Surface Energy
(dynes/cm)
Gas 1 / Gas 2 / Gas 3
immediately after treatment / 73 / 73 / 73
0.33 (8 hours) / 66 / 62 / 73
1 / 42 / 38 / 73
2 / 38 / 38 / 73
7 / - / - / 73
14 / - / - / 73
22 / - / - / 73

Open time is the time between plasma treatment and the next step in its use. In the case of silicones the open time, depending on the plasma process, can be measured in hours, inconvenient in the manufacturing process. Proprietary processes allow weeks and possibly months of open time. However, adhesive bonds made within the effective open time do not degrade with environmental exposure and time. Bonding of military cylindrical connectors employing silicone inserts began specifying plasma treatment in the 60’s. Testing of retains and connectors in the field show little, if any, deterioration of the bond strength after 30 years of environmental exposure, and the environment and thermal cycling for these connectors can be rather extreme.