3.4.1 General Sequence of Cleaning

Any surface preparation requires completion of one, two, or all three of the following operations [3]:

Priming, discussed in Section 3.5, may also be carried out in some cases to ensure superior durable bonds under particularly adverse environments.

3.7.1 Effect of Treatment on Plastic Surfaces

The changes that occur in the surface of plastics by applying the treatment methods are the result of the four processes of cleaning, ablation, cross-linking, and surface chemical modification (most commonly oxidation). Common treatment methods include corona, flame and plasma treatment, and chemical etching, all of which serve to increase the surface energy of plastics. The polar component of surface energy should be increased by 15–20 dynes/cm to achieve proper adhesive bonding in polymers such as polyolefin, polyphenylene sulfide, polyaramide, and others [14]. The other changes in the plastic surfaces include removal of weak materials, strengthening of the surface (by cross-linking), and increased surface roughness.

The increase in surface energy of plastic occurs through the surface oxidation of the polymer chains. For halogenated polymers, such as chlorinated and fluorinated polymers, surface modification involves significant dehalogenation, the removal of chlorine and fluorine atoms from the surface molecules. In general, surface treatments can be viewed as a transfer of energy to the surface of plastics. In almost every case, the dissipation of treatment energy on the plastic surface results in more than one change to the surface; these changes are described below.

The cleaning, or removal of contamination, including process oils, dirt, waxes, mold release agents, and exuded plasticizers, is an important result of surface treatment. Methods involving chemicals such as solvent cleaning and etching, if not properly used, can leave behind a residue that may interfere with adhesive bond formation. Clean surfaces must be protected due to rapid reacquisition of contamination from the ambient atmosphere.

Plastic surfaces often contain a low molecular polymer such as an oligomer, or loosely bonded matter, creating a weak boundary layer. There is some debate about production of low molecular weight polymers as a result of surface treatment [15]. Loosely bonded materials may originate from the plastic part itself and from additives incorporated such as pigments, processing aids, and mold release agents. They may also have been acquired externally as a result of contact with contaminated surfaces or exposure to a contaminated environment. The treatment methods remove loose material from the part surface. One mechanism for removal of material from the part’s surface material is ablation, which is the degradation and conversion of organic matter into volatile species.

The electrical discharge at the plastic surface during corona treatment reacts with the polymer molecules, generating free radicals. The presence of free radicals allows cross-linking and functionalization on the plastic surface with and without chain scission. The radicals rapidly react with ambient oxygen and produce peroxide groups that can decompose and give rise to a variety of polar groups. These groups include hydroxyl (OH), carbonyl (CO), and carboxylic acid (OCOH). The presence of these groups has been confirmed by ESCA (see Chapter 4 for a description of this technique).

Corona treatment is believed to roughen the plastics by the degradation of amorphous regions of the polymer surface [16]. The belief is that corona treatment does not impact the crystalline region of the surface, preferentially attacking the relatively weak amorphous regions. Degradation and subsequent removal of the amorphous material leads to the increased roughening of the surface of plastics such as polyethylene [17–19]. A rough surface provides a much larger adhesive contact area than a smooth surface.

Plasma treatment oxidizes the surface of the polymer in the presence of oxygen. It can thus remove organic contaminants from the surface. Early studies have concluded that the cross-linking of low molecular weight surface species is the mechanism for eliminating a weak boundary layer [20]. More recent research has attributed the effectiveness of plasma treatment to surface cleaning, ablation of surface polymer chains, surface cross-linking of polymer chains, and introduction of polar functional groups that result in increased surface energy [21].

The mechanism of flame treatment is the thermal oxidation of the polymer surface. The flame temperature may exceed 2000°C. It can clean the surface and remove the weak boundary layer by vaporizing surface contamination and low molecular weight polymers.

Chemical treatment or etching oxidizes the plastic surface similarly to corona treatment. For instance, chromic acid is used to etch the surface of polyethylene and polypropylene. An increase in etching time and temperature intensifies the surface treatment by increasing the degree and depth of oxidation.

3.7.2 Surface Cleaning

Nonmetallic material, namely plastics, should be degreased, if necessary, with an aqueous detergent solution, followed by a thorough rinse with clean water, and dried. The detergent can be substituted by a solvent. Either solvent or detergent solutions can remove mold release agents or waxes from the plastic part surface. Effective solvents include methyl ethyl ketone, acetone, and methanol, depending on the plastic type. Resistance of the plastic to the solvent should be considered during solvent selection to prevent dissolving or degrading the plastic during cleaning. The chemical surface cleaning step is unnecessary if a treatment technique, such as plasma treatment, adequately cleans the surface along with surface modification.

3.7.3 Mechanical Treatment (Surface Roughening)

Surface roughening and sanding of plastics accomplishes the same purposes as with metals. Essentially, loose and unstable polymers are removed from the surface, thus increasing contact surface area. This step is not pertinent if there are decorative and esthetic considerations that require an even, smooth surface. For instance, if a plastic part is painted or laminated to a thin film, the application of surface roughening and sanding may be impossible or limited. When applicable, plastic surfaces are usually hand-sanded or sand-blasted to impart roughness (increased contact surface area) to the part surface.

3.7.4 Corona Treatment

Corona discharge usually takes place at atmospheric pressure in contrast to low temperature (or cold) plasma, which requires a vacuum. Corona is a stream of charged particles such as electrons and ions that are accelerated by an electric field. A corona is generated when a space gap filled with air or other gases is subjected to a sufficiently high voltage to set up a chain reaction of high-velocity particle collisions with neutral molecules, resulting in the generation of more ions. One important such species is atomic oxygen (O) generated by the UV light in the corona stream:

$2{\mathrm{O}}_2\to \mathrm{O}+{\mathrm{O}}_3$

The atomic oxygen (O) reacts with hydrogen and carbon on the film surface generating OH and CO groups which form hydrogen bonds with adhesives.

Corona discharge is applied to treat the surface of plastics, rendering them adherable (Figure 3.4). In this method, the plastic article is exposed to a corona discharge produced by high-frequency, high-voltage alternating current. The corona dosage is calculated from the following equation:

$D=\frac{P}{CB\times V}$

There are three types of treating configurations, all consisting of the same parts including an electrode, an electrical insulator (or dielectric), and a return path (or ground). The differences among the three configurations are in the location of the electrode. In a conventional system, the web passes over a roll that is covered with insulating material such as a silicone rubber. A metal electrode is suspended above the roll so that an air gap of 1.5–2.5 mm exists between the electrode and the insulated roll. High voltage operates across the air gap, ionizing it, and forming a corona discharge curtain between the electrode and the material (e.g., film) that is being treated. The conventional configuration can be used only with nonconductive material. The second configuration is called bare roll (in which the electrode is covered with a dielectric (usually ceramic) and the roll is made of anodized aluminum. In the third configuration, called double dielectric, the roll and the electrodes are covered with dielectric material.

The main parameters for the control of the treatment process include the voltage, width of the air gap between electrodes, film/web speed, and the width of the electrodes. Most machines allow treatment of one side of the web (Figure 3.5) and require two passes for two-sided treatment. There are also machines that are equipped with two sets of electrodes for one pass, two-sided treatment.

Effectiveness of corona treatment depends on the type of polymer. The duration of corona discharge treatment in air has a strong influence on the surface energy of the plastic. Surface energy increases more slowly as the crystallinity of the polymer being treated increases (from lower to higher, PE<PEP<PP). Longer exposure time is required to achieve the same level of surface energy.

3.7.4.1 Three-Dimensional Corona Treatment

Three-dimensional (3D) corona treatment, as the name indicates, is aimed at surface modification of objects with a third dimension, compared to a web (2D). Plastic objects are treated by 3D corona discharge to promote adhesion for printing, painting, coating bonding, and labeling [22–24].

A 3D corona discharge head has a dielectric enclosure with two small electrodes made of medium thickness, bare aluminum or stainless steel wire, and connected to a high-voltage power supply and fan [25]. When a high voltage exceeds the air breakdown value (30 kV/cm), an electrical arc forms. This high-current arc is blown out from the enclosure by the airstream and is usually several centimeters long and one millimeter in diameter.

When the arc strikes a solid object (Figure 3.6), it travels across the surface and returns back into the enclosure [25]. The treatment is achieved by moving the object under the discharge head or by moving the head over the object. The arc is essentially a 1D object (like a wire), and a treatment of 3D surfaces requires at least two heads. It is difficult to treat objects with complicated geometry, especially with internal cavities or channel structures, including tubing. The average power required for one head operation is about 500 W. The effective power dissipated in the arc could be as low as 50 W. Most of the ions and electrons travel between the two electrodes along the arc trajectory where the main transfer of energy occurs.

The plasma effect on the material strongly depends on the exposure time because each material requires a minimum exposure time to activate its surface. The required level of surface modification depends on the application (e.g., printing, bonding, and coating) as well as on the applied ink, adhesives, coatings, and curing process.

Occasionally, there is not a sufficient window for the necessary exposure time and thermally safe material handling for 3D corona treaters. This is especially true of tough-to-treat materials (plasmaphobic) in which the surface gets burned rather than modified. This is also the case for heat-sensitive materials, thin wall plastic objects, wires with thin insulation, fiber optics, thin coating layers, etc. This problem is partly solved by installation of several discharge heads along the process line. If the problem is not solved, other techniques are used to avoid burning.

3D corona treaters also generate ozone. To reduce high ozone concentration, special filters are required to comply with clean room environment regulations for the medical and semiconductor industries. A high-voltage signal applied to the electrodes usually has an audible frequency (60 Hz and upwards). Electrical breakdown takes place during each half-period of the cycle and produces a small shock wave with a distinctive sound. This may generate significant amounts of noise in multiple head discharge systems, requiring noise abatement and hearing protection.

3.7.4.2 Corona Treatment Under Chemical Atmosphere

One of the most important commercial polyolefin films is biaxially oriented polypropylene (BOPP) film. Because of its use in a variety of applications such as food packaging, multipurpose packaging, and adhesive-tape packing [2–4], BOPP films have low hydrophilicity and do not allow printing ink or adhesives well. Its surface is thus modified to improve wettability and adhesion. The most common method for surface modification of BOPP and other polyolefin films is corona treatment which offers the benefits of continuous and inline operation and cost effectiveness.

Corona treatment introduces [26] polar functional groups such as hydroxyl, carbonyl, and carboxylic groups onto the film surfaces. The presence of these functional groups on the film surface raises the film surface-free energy. Thermodynamic driving forces and the small sizes of these groups, however, result in the overturning or “inversion” of the polar group which migrates below the film surface [27,28]. This phenomenon is called hydrophobic recovery. In just 30 days after treatment, surface-free energy of a film like polypropylene can lose the effect of modification significantly, thus rendering it practically useless for most applications [29].

The mobility of polymer chains at the surface of the film is the reason polar groups invert inward. To eliminate the time dependence of corona treatment, the mobility of polymer chains at the surface may be constrained by grafting bulky functional groups [30]. The key issue is, however, that postcorona treatment surface grafting involves additional steps, which significantly lengthen the process and increase the cost of the film. Technology has been developed to introduce organic grafting compounds into the atmosphere of the corona process, which is usually air.

Introduction of vapor of acrylic acid (AAc) monomer into the corona region has proved successful. Surface wettability of BOPP films were improved (surface energy increased) by comparison with the standard air corona treatment (Table 3.4). The hydrophilicity lasted for more than 90 days of aging (Figure 3.7).

Table 3.4

Surface-Free Energy of Aged BOPP Films at Ambient Temperature [26]

Corona Energy (kJ/m2)	Surface-Free Energy (mJ/m2)
	Before Treatment	After Treatment		After Aging (90 days)
		Air-Corona	AAc-Corona	Air-Corona	AAc-Corona
15.3	34.8	48.0	54.1	42.4	51.0
38.2	34.8	49.6	58.4	44.3	53.5
76.4	34.8	55.7	64.0	47.0	56.8

3.7.5 Flame Treatment

Flame treatment is a commercial process to render polyolefins and polyethylene terephthalate adherable. The polymer article (e.g., film) is passed over an oxidizing flame formed by an oxygen-rich (relative to stoichiometry) mixture of hydrocarbon gas. Variables affecting the extent of oxidation include the flame characteristics (e.g., excess oxygen) as well as the speed of the article movement. Gas flame contains excited fragments and species such as atomic oxygen (O), NO, OH, and others that can abstract hydrogen from the surface of the polymer that is replaced by oxygenated functional groups (mostly CO and OH). Figure 3.8 depicts the flow diagram of flame treatment system.

Polyethylene and polypropylene have a surface tension of 30 dynes/cm that must be raised to a minimum of 38 dynes/cm (preferably to 42 dynes/cm) to render their surfaces adherable. Flame treatment is the most widely used method of pretreatment. It is flexible and reliable if carefully controlled. It enables the treatment of uneven and curved surfaces. It uses a mixture of air at 138–345 kPa and a fuel gas at a (low) pressure of 1.7 kPa. The gas can be butane, propane, natural gas (methane), and coal gas.

For the flame to be effective, it must be oxidizing (represented by a blue flame). Correct flame control is very important. A basic flamer will complete simple work; specially designed flame control systems are recommended for routine operations and long production runs. These systems are fitted with gas and air control valves to compensate for pressure fluctuations, ensuring that the mixture is always at its optimum. Safety devices, such as flame failure shutdown, are fitted as standard. Automatic ignition is also standard. Flame nozzle design is important for these normally single or double row ribbon burners because they give a more stable flame shape and characteristic. “Flame throwers” are inefficient and unreliable. Flame control and position of the item in the flame are critical. Setting up the flamer is also very important (Figure 3.9). Over-flaming will damage the surface of the product, while under-flaming can cause failure with the ink adhesion [34].

It is imperative to find the correct flame treatment suitable to the component in question. Key factors include the gas type, gas to air ratio, burner type, gas flow rate, flame distance, and flame passage speed (treatment time). Excessive treatment of the surface results in degradation of the polypropylene and, therefore, poor adhesion. Insufficient treatment, however, fails to modify the surface adequately, leading to poor adhesion. It is often a rather delicate procedure to find the proper conditions and great care must be taken in the experimentation (Figure 3.10).

During flame treatment, all hydrocarbon gases combine with oxygen to produce heat, as shown here for methane:

${\mathrm{CH}}_4+2{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2$

The above equation indicates complete combustion of the fuel gas methane. The volume of air required for complete theoretical combustion of gas (such as methane) is called the stoichiometric volume (100%). A flame containing an excess (>100% stoichiometry) of air is referred to as an oxidizing flame; a flame that does not have sufficient air (<100% stoichiometry) to complete combustion is a reducing flame. Figure 3.11 [34] shows the effect of using more or less air than the stoichiometric volume on the tape peel strength of polyethylene or polypropylene. A maximum value develops at about 115% stoichiometry or 15% excess air.

In Figure 3.12 [26], the influence of the flame passage speed over the plastic on the adhesion (shown as percentage of maximum adhesive bond strength) between the adhesive and glass matte-reinforced thermoplastic (GMT) is presented. The machine supplier often provides assistance with the optimization of the system.

The speed of the conveyer greatly effects the flaming. As a rule, the higher the speed of passage of the item through the flame, the less likelihood there is of damage to the surface. High gloss surfaces are susceptible to blooming, which reduces the gloss. Higher speeds help reduce blooming. Conveyers are constructed from metal mesh, which must be of sufficient length to allow the mesh to cool. Hot meshes will mark plastic components.

Flame treatment is not effective in the adhesion treatment of perfluoroplastics. The data in Table 3.5 reveal a large increase in the bond strength of polyvinyl fluoride (PVF) and ethylene chlorotrifluoroethylene (ECTFE) after flame treatment. The fluorine-to-carbon (F/V) ratio of PVF remained unchanged but the O/C ratio increased significantly. In the case of PTFE, the F/C ratio actually increased, which could explain the drop in the bond strength as a result of flame treatment. The flame most likely removed contamination that had previously masked some of the F atoms on the surface.

3.8.1 Dyne Liquids

This method is common for estimating the treatment level of plastic surfaces in manufacturing/production environments. It involves using solutions made from a mixture of two chemicals that produce liquids (dyne) with surface tension in the range of 30–70 dynes/cm. The test consists of placing droplets of the various dyne liquids on the treated surface and observing the spreading of the drops in 2 s. Successive liquids with different surface tensions allow narrowing of the surface tension range of the plastic surface. This method is subjective, but it provides a rapid means of assessment of the treatment level, particularly useful in a production environment. There are also pens that operate similarly to dyne liquids. A more quantitative approach is the measurement of contact angle, which decreases with an increase in treatment level. A perfect wetting liquid forms a contact angle of zero on the solid surface.

3.8.2 Water-Break Test

This test depends on the observation that a clean surface (one that is chemically active, or polar) will hold a continuous film of water, rather than a series of isolated droplets. This is known as a water-break-free condition. A break in the water film indicates a soiled or contaminated area. Distilled water should be used in the test, and a drainage time of about 30 s should be allowed. Any trace of residual cleaning solution should be removed to avoid a false conclusion. If a water-break-free condition is not observed on the treated surface, it should not be used for bonding. The surface should be recleaned until the test is passed. If failures continue to occur, the treating process itself should be analyzed to determine the cause of the problem [3].

3.8.3 Contact-Angle Test

Wettability may also be determined by measuring the contact angle between the polymer surface and the drop of a reference liquid such as distilled water. A small contact angle indicates that the liquid is wetting the polymer effectively, while large contact angles show that the wetting is poor. Every surface has a critical surface tension, γ_c, of wetting. Liquids with surface-free energies below γ_c will have zero contact angles and will wet the surface completely. In contrast, liquids with surface-free energies above γ_c will have finite contact angles. The critical surface tension is in units of dynes/cm at 20°C. Contact angles for untreated materials vary from 37° to 48° for relatively polar materials such as nylon to respective highs of 100° and 97° for the nonpolar, unbondable silicone and polyethylene resins. After exposure to activated argon plasma, contact angles are reduced to 40° for polymethyl methacrylate (PMMA) and to 19° or less for nylon, polystyrene, polyethylene, and room temperature vulcanization (RTV) silicone [7]. Zissman and others [36,37] have written comprehensively on surface tension phenomena relating to adhesion.

Table 3.6 gives the treatment values recommended for a number of plastics as a function of the adhesive coating system and the manufacturing processes. Note that, as discussed earlier in this section, waterborne adhesives universally require higher surface energy levels than solvent-based systems.