The Plasma Advantage



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manufacturers and end-users alike
have been searching for ways to improve the surface properties of natural and man-made fibers.
Specifically, there is a need to improve adhesion, wettability, printability and dyeability; as
well as to reduce material shrinkage. Methods of modifying fiber properties to make polypropylene
(PP) dyeable, including the process of copolymerization with polymers that can be dyed, have been
evaluated. Traditional latex systems and primers with low melt points have been used to coat
fabrics to promote ink adhesion, heat-sealing and thermoforming performance. PP nonwovens have
especially been the focus of research to enhance colorfastness properties for the material because
of its excellent chemical resistance, high melting point, low cost and adaptability to many
fabrication methods. To date, the poor dyeability of PP has limited optimization of its
applications in the manufacturing of yarns and knit fabrics, upholstery fabrics and industrial
fabrics.

Fibers with polar functional groups can be dyed more easily than nonpolar fibers because
polar groups will chemically bond with dye molecules. Because the molecular chains of PP are
nonpolar and its surface is hydrophobic, the dye molecules will not bond chemically to the fibers.
PP fiber is highly crystalline as well, which also restricts its dyeability. Functional groups may
be introduced onto the fiber surface by using gas plasma treatments, improving fiber surface
properties without affecting the fiber’s bulk properties. By creating a polar layer on the fiber
surface, in reaction with functionality introduced, wettability of the fiber for dyeing is enhanced
with hydrophilicity.

A novel atmospheric plasma treatment (APT) process has been developed using atmospheric
plasma glow discharge (APGD) technology as a result of studying the reaction mechanisms between
plasma and fiber surfaces to optimize industrial system applications. The APT apparatus does not
require any vacuum systems, produces a high-density plasma and provides treatment of various
nonwoven substrates at low temperature while operating at atmospheric pressure. The wettability of
various man-made and natural fibers has been dramatically increased by this process. Electron
microscopy has shown the surface of fibers after APT is uniform and consistent, suggesting the
treatment is homogeneous. APT has a cleansing effect that removes contaminants and increases the
microroughness of the fiber
(See Figure 1).


microscrope



Figure 1: Plasma enhances the microroughness of fibers.

APT Process

Plasma is an ionized form of gas and
can be created using a controlled level of AC or DC power and an ionizing gas medium. It is an
ensemble of randomly moving, charged atomic particles with a sufficient particle density to remain,
on average, electrically neutral. Plasmas are used in very diverse applications, ranging from
manufacturing integrated circuits used in the microelectronics industry through treating polymer
films to the destruction of toxic waste. Plasma processes can be grouped into two main classes —
low-density and high-density — according to their electron temperature versus electron density. In
low-density, direct-current and radio-frequency glow discharges, the electron and heavy particle
temperatures are not equal. Low-density plasmas have hot electrons with cold ions and neutrals.
Energetic electrons collide with, dissociate and ionize low-temperature neutrals, creating highly
reactive free radicals and ions. These reactive species enable many chemical processes to occur
with low-temperature feed stock and substrates.

Well-known types of plasmas encountered in surface treatment processing techniques typically
are formed by partially ionizing a gas at a pressure well below that of the atmosphere. For the
most part, these plasmas are weakly ionized, with an ionization fraction of 10-5 to 10-1. Electron
cyclotron resonance (ECR) plasmas can have higher ionization at high power. Because these systems
are run at low pressures, vacuum chambers and pumps are needed to create and contain these plasma
processes.

The atmospheric plasma system allows creation of uniform and homogenous high-density plasma
at atmospheric pressure and at low temperatures using a broad range of inert and reactive gases.
The APT process treats and functionalizes material surfaces in the same way as the vacuum plasma
treatment process on a wide range of materials; and has unique advantages over the presently used
corona, flame and priming treatment technologies. APT production equipment testing has been
successfully performed for the treatment of various materials, including PP fiber, PP and
polyethylene (PE) nonwovens, polyester fiber, Tyvek®, nylon, wool, textile yarn, oriented PP film,
PE film, PE teraphthalate (PET) film and polytetrafluoroethylene film. The surface energies of the
treated materials increased substantially (without any backside treatment), thereby enhancing their
wettability, printability and adhesion properties.


feedrolldiagram

Figure 2: A driving roll moves textiles into plasma treatment between two electrodes one is
the grounded roll, and the other is the powered electrode with gas inlets.

If APT produces a uniform discharge
between electrodes, the system is defined as an APGD system. In the case of a nonuniform discharge,
the system is known as a corona treater. Schematically, a system to treat films or textiles at
atmospheric pressure has the structure of that shown in Figure 2 — from a feed roll, films are
driven by the grounded roll in the plasma treatment and then to a collector. Plasma is produced
between two electrodes — one is the grounded roll, and the other is the powered electrode connected
to a high-voltage frequency power supply. In the case of a corona treater, plasma is developed in
air. In the APGD system, the electrode is connected to gas inlets.

A standard corona treater and an APGD system with proprietary designed electrodes were used
for these investigations. A dielectric layer between the electrodes and an appropriate gas mixture
as the plasma medium are used to obtain a uniform discharge
(See Figure 3).


enerconimage

Figure 3: There is uniform and homogenous plasma discharge with an atmospheric plasma
surface treatment system.


Treatment Results

On Printing Of PP Nonwovens


Several PP nonwovens with a 0.40-mil
thickness were treated by the plasma treater at atmospheric conditions. These nonwovens were
treated on webs ranging from 27 inches to 60 inches in width. The key treatment parameters were
input power, gas type, flow and the ratio of the gas mixture.

APT was performed with both APGD and corona systems evaluating the surface tension variation
in the samples. Surface tension of the treated nonwovens was determined by ASTM D-2578 surface
tension test fluids markers. The surface energy of these nonwovens was enhanced substantially after
the plasma and corona treatments. The initial surface tension level was measured at 31 dynes per
centimeter (dynes/cm). The surface tension was increased to 52 dynes/cm using the APGD reactor with
an oxygen/helium plasma, and to 52 dynes/cm using a corona treater.

The mixture gases for APGD included helium and oxygen at flow rates of 14 liters per minute
(lpm) and 2 lpm, respectively. The treatment times for APGD and corona were both at 0.48 meters per
second. The input power was 1 kilowatt, and the operating frequency 80 kilohertz.

The nonwovens then were printed with an image of the American flag in a four-color process
using photopolymer printing plates on a Mark Andy press with Akzo Nobel Hydrokett 3000 water-based
ink. The anilox roll was a 700-line screen with a 2.1-cell volume. The nonwoven material was
printed in roll form at 60 meters per minute. The ink was dried in-line using forced air at a
temperature of 140°F. Untreated, corona-treated and plasma-treated protocols for these nonwovens
then were evaluated to determine the adhesion of the ink.

A tape test was performed for each protocol using a 1-inch by 2-inch tape peel test in which
fresh, transparent pressure-sensitive tape was applied to the printed side of the film for 60
seconds. The untreated and printed nonwoven exhibited total ink adhesion failure. The
corona-treated nonwoven retained approximately 90 percent of the image ink at its surface. The
APT-treated nonwoven displayed approximately 100-percent ink adhesion. The trial supports the role
and efficiency of atmospheric plasma in functionalizing the surface of PP nonwovens for improved
water-based ink adhesion.

A second test was performed to assess the holdout of the ink from the nonwoven fiber
structure. Since absorption of ink into printed substrates can diminish the surface perception of
the pigments used, promoting ink holdout on printed substrates can serve to increase the depth of
color within the image. A comparison was made of untreated, corona-treated and plasma-treated PP
nonwoven material relative to the reflectivity of light off of the aforementioned four-color
process image using a spectrophotometer.

The plasma-treated nonwoven demonstrated a significant increase in the reflectivity of the
flag image’s primary blue and red tones, beyond the reflectivity created by the untreated or
corona-treated nonwoven.

Additional APT trials on spunbond PET nonwoven materials utilizing gas chemistries have led
to improved dyeability using water-soluble inks and dyes. Although not a direct predictor of
pigment adhesion, dye levels of greater than 70 have been achieved for spunbond PET
nonwovens.


Surface Transformation

The gas phase and surface kinetics during APT of PP and the resulting
modification of its surface properties also have been investigated. Gas temperatures and relative
humidity were held constant during all trials. Fourier Transform Infrared Spectroscopy measurements
indicate plasma treatment of PP involves free-radical bombardment that introduces oxidized
functional groups onto the surface and may include methanol/alcohol, ketone, carboxy, ether, epoxy,
ester or hydroperoxide. This is in agreement with previous observations reporting the addition of
oxygen at the surface of PP. Grafting of nitrogen, observed in treatments with helium plasma
because of the presence of nitrogen molecules efficiently excited by helium in the discharge, is
supposed to be hindered here by the oxygen amount in the plasma.



The chemistries formed under the action of the glow discharge are responsible for the change
in the polymer surface properties. Moreover, increasing energy deposition increases the densities
of carbonyl, acid and peroxy radicals on the PP surface. For a given energy deposition, higher web
speeds also resulted in decreased concentrations of peroxy radicals, carbonyl and acid groups on
the PP surface.

enerconweb
Enercon Industries Corp.’s commercial atmospheric plasma treatment system functionalizes a
nonwoven web surface.


Conclusions

APGD can be operated at low
temperatures and at atmospheric pressure, thereby eliminating the need for any vacuum chambers or
pumps. Yet, it provides the unique advantages that plasma technology has over existing technologies
for surface treatment of nonwovens. The systems used for investigation are all commercial devices,
confirming the possibility of using APT systems in industrial places.

The surface energies of the nonwovens treated using APT have been shown to increase
substantially, significantly enhancing the wettability, printability, dyeability and adhesion
properties of these nonwovens. Furthermore, tape peel tests indicate the APT process can effect
better ink adhesion than corona treatment.

Ink holdout on PP nonwovens, printed with water-based ink and measured by wavelength
reflectivity, has been shown to be improved when compared to the same nonwoven processed using
corona treatment.


Editors Note: Rory A. Wolf is vice president, business development, at Menomonee Falls,
Wis.-based Enercon Industries Corp. Amelia Sparavigna is assistant professor, physics department,
at the Italy-based Polytechnic of Turin.

November/December 2005

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