IDS Plastics - White Paper
     
Title:
ATMOSPHERIC PLASMA -THE NEW FUNCTIONAL TREATMENT FOR EXTRUSION COATING AND LAMINATION PROCESSES  
Author:
Rory A. Wolf  
Designation:
Vice President, Business Development  
Company:
Enercon Industries Corp.  
 
 
           
INTRODUCTION

Products made from polymers of various chemistries introduce an every-growing number of conveniences to our lives. Although polymers are common in every day living, the most commonly used group of polyolefins exhibit a significant problem. As shown in Table 1, the intrinsic surface energy of these materials is rather low, and processes like lamination, printing or coating present rather difficult challenges:

Table 1: Surface Tensions of various Homopolymers

Polymer Surface Tensions [dynes/cm2]
Polyethylene (PE)
31
Polypropylene (PP)
32
Polystyrene
33
Polyisobutylene
27
Polytetraflourethylene
19

A lamination or coating bond with a substrate is lower when one or both surfaces are non-polar. Modifying surface energy to introduce polar groups to make two materials more polar increases bond strength. This is a necessary objective, since performance demands on flexible packaging constructions continue to drive the need for new innovations and efficiencies within extrusion coating and laminating processes. Down-gauging, along with changes in melt temperatures to adjust surface oxidation levels (and the subsequently induced surface polarity) and increases in production speeds, are key process variables being adjusted to meet these market demands. These strategies, in turn, require a focus on adhesion promoting process treatments such as corona or ozone to re-optimize lamination and coating adhesion. It is well known, for instance, that when either corona or ozone are used, adhesion levels can be improved as melt temperatures decrease. Pretreatment of the substrate with corona, and the extrudate with ozone, results in very good adhesion even at melt temperatures below 300ºC. Use of higher polymer coating weights can also increase adhesion in the area of contact between substrates by carrying more heat to the substrate and maximizing what is nipped into the voids of the substrate material(s). This paper is a continuation of a study of the beneficial effects of atmospheric plasma treatment in promoting adhesion, with a current focus on application within extrusion coating and laminating processes.

BACKGROUND

The most important property in extrusion coating and laminating is adhesion of the polymer to the substrate(s). Without adequate adhesion, the coating can be easily removed from the substrate. Typically, adhesion for a nonpolar polymer such as Low Density Polyethylene (LDPE) is accomplished by a combination of oxidation of the extrudate and treatment of the substrate. The level of oxidation is a function of:

  1. Melt Temperature
  2. Line Speed
  3. Air Gap
  4. Coating Weight

Some combinations of these variables will yield acceptable adhesion, but also may produce undesirable effects such as increased taste and odor or poor heat seal strength.

The most common methods used for the treatment of web substrates with ionized gases are:

  • Flame Treatment
  • Corona Treatment
  • Plasma Treatment in Vacuum
  • Plasma Treatment in Air

All of these methods are similar in that the gas at the surface of the substrates is ionized, either with the help of an electric field, or with a chemical reaction. For a differentiation of the different method one has to look at the method of ionization as well as the electron density and the electron temperature generated by the different methods.

Flame Treatment is used to promote adhesion in some extrusion processes and is generated by aiming a combustion flame, typically a propane or butane flame, onto the substrate surface. The electron density, and hence the ionization rate, in a flame is at the lower end of plasmas used for surface treatment.

Corona Treatment systems are also typical adhesion promoters in extrusion, with ionization created by applying a high frequency high voltage to an arrangement of two opposing electrodes, of which at least one is insulated with a dielectric. This creates streamer like discharges, which ionize the discharge gap. Although the technology has been long-established, it still bears some major drawbacks. The streamer-like discharge is highly non-uniform and it has been shown that once a streamer hits the polymer surface, it leaves a local discharge which in return attracts the next streamer to the exact same spot. The result is a localized treatment. The maximum achievable treatment level therefore is an average of the treated areas and untreated areas. A second disadvantage of Corona Treatment is the high voltage required to initiate the discharge. The voltage can be high enough to create a discharge on the backside of fast moving webs, resulting in treatment of the backside as well, an effect not wanted in most cases.

With Plasma Treatment in vacuum, the low pressure levels in vacuum coating chambers allow the generation of uniform plasma, usable for highly effective treatment of polymer surfaces. The technology is widely used for web coating application and for the treatment of 3D objects like automotive bumpers. The uniformity of the plasma allows for high treatment levels.

The high functionality of a uniform plasma discharge in vacuum has driven many efforts to establish a uniform glow discharge at atmospheric pressure, making this technology applicable to extrusion processes at atmospheric pressure. Recently, Enercon Industries and Sigma Technologies International have developed a line source which can produce an Atmospheric Plasma Treatment (APT) stable glow discharge. The differences to a Corona Treatment system are as follows:

  1. Injection of Plasma Gas directly into the discharge gap.

    In order to achieve a stable glow discharge free of streamers it is required to use noble gases, whose high metastable phases allow the steady glow discharge. In order to reduce the consumption of these gases to a minimum the gas is injected directly into the discharge gap. Doing it this way also allows the injection of other treatment gases, which get highly ionized in the discharge and allow users to address the specific chemistry of the treated polymer.
  2. Creation of a stable, homogeneous and uniform glow discharge.

    Unlike Corona, where the discharge occurs in streamers, the APT creates a uniform glow in the discharge gap. This allows for uniform treatment on the substrate surface, thus a higher treatment level of the surface is achievable.
  3. Lower Voltage.

    The voltage required to initiate the gas discharge is greatly reduced compared to Corona Treatment Systems. This prevents backside treatment, one of the major drawbacks of corona.

EXPERIMENTAL DESIGN

The objective of the experimental design was to determine differences in the peel adhesion strength of LDPE film which was untreated, treated with corona discharge, and treated with the APT process.

Materials

  • 1 mil. gauge – 25 micron LDPE, MD elongation – 330%, TD elongation – 550%, MD tensile – 23MPa, TD tensile – 19MPa.

Equipment

  • Enercon Covered Roll corona treating system w/ metal electrodes
  • Enercon/Sigma APT lab station
  • Thwing-Albert Model 225-1 Friction Peel Tester with sled and peel clamp

The LDPE film was pre-treated under the following conditions:

Table 2: Variables in Experimental Design

Substrate Treatment
Corona Treatment
APT Treatment
No Treatment
Line Speed
50 fpm
50 fpm
n/a
Air Gap
.040
.040
n/a
Watt Density(w/ft²/min)
9
9
n/a
Pre-Trial Dyne
30
30
30
APT Chemistry
n/a
He/C2H2
n/a
Initial Post-Trial Dyne
46
60
30

The peel adhesion test was measured in accordance with ASTM test method D3330A and utilized a 180 degree peel fixture with 3M 800 acetate film acrylic adhesive tape.

RESULTS

The peel adhesion tests took place from within thirty minutes to seventeen days of the pretreatment protocol to determine peel adhesion strength and longevity. Longevity of treatment and average peel adhesion statistics are summarized in Figure1 and Figure 2, respectively.

Figure 1. Longevity of Treatment of LDPE in Dynes/cm over 17 day period

Figure 2. Average Peel Adhesion of LDPE in lbs./in over 17 day period

CONCLUSION

The data indicates that the surface functionalization capability of APT treatment can significantly improve the surface tension and adhesion of LDPE film compared to standard corona treatment. Moreover, the specific chemistries utilized with APT treatment in this trial protocol provided a significant increase in treatment longevity, with LDPE retaining over 44% more surface adhesion strength over comparable periods of time compared to corona discharge.

The application of the APT process on extrusion coating and laminating lines to increase the bond between substrates for liquid, flexible and paperboard packaging structures appears to offer promising process advantages. Post-treatment on these lines using APT systems would also appear to offer unique and longer-term surface functionalizations for downstream converting operations compared to corona treatment.

REFERENCES

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