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Corona discharge treatment is one of the surface treatments that can achieve increased wetting tensions on film surfaces. As mentioned previously, corona discharge bombards a film surface with ionized air producing oxidized surfaces of films containing ions, radicals and excited molecules via chain scission. The air between two corona treatment electrodes conducts electricity and ionizes the air. Stray electrons impact other electrons in the air making them unstable by putting them into a “higher energy orbit creating an excited molecule” [152]. The excited molecules are unstable which then decompose into radicals and ions [152]. The term corona is used to distinguish the condition of the gas or air between electrodes [152]. Placing a film to be treated between the two electrodes produces a diffuse glow rather than an arc due to interruption of the conductive path. The soft blue glow is what is referred to as corona [152].

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Multiple theories have been proposed to suggest the effects of corona discharge treatment on adhesion of polymer film surfaces: addition of polar groups through oxidation, electret formation (electric charge), and increase in surface roughness due to micro pitting, and elimination of weak boundary layers [126]. Oxidation at the film surface has been found to be the primary and most widely accepted effect of corona treatment [40]. Oxidation results in the introduction of polar groups onto the surface of a non-polar material. Some have classified this as production of a layer of low molecule weight oxidized material boundary layer (LMWOM) [146].

Others have described a second significant effect of corona discharge using more topographical methods. Corona can also increase the roughness of a film surface while simultaneously cleaning it by removing dust and debris. The surface morphology described when treating polyolefin such as polypropylene and polyethylene is pitting or

“mechanical keying” [152]. Pitting also known as micropittng can increase adhesion and wettability by producing more surface area for intimate contact between substrates.

Corona discharge treatment is applied at varying power densities required to achieve the desired wetting tension. Power density uses the units of watt/(time*surface area). (i.e. watt/(min*ft²)) Both overtreatment and under treatment can result in

insufficient wetting tension after treatment. It has been found that two series of chemical reactions can occur during corona discharge treatments. The first reaction introduces polar groups such as carbonyls, carboxyls and hydroxyl groups through chain scission. If the length of treatment was to be extended or the power density of the treater was too high for the specific material, the carbonyls can convert to ethers, which are nonpolar.

This second reaction occurs at a slower rate with increased treatment time and the

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production of nonpolar groups can reduce adhesion and wettability [126]. There are additional effects of overtreatment which can result in undesirable wetting and even lack of sealability. Overtreatment can cause what is called fracturing in the surface of the films (reorganization of the polymer chains). This can result in the polar groups produced through corona treatment migrating into the bulk of the polymer making them

unavailable at the surface. This can also occur with primers [43].

Overtreatment can also destroy the sealability of polyolefins. Corona discharge treatments can increase the molecule weight of polymers at the treatment surface via cross linking [40; 152]. According to a study conducted by Farley and Meka (1994), any amount of corona treatment has the potential to produce a change in the seal failure of LLDPE from a tear to peel. They found that the cross-linking of the polymer surface reduced chain mobility and reduced chain diffusion at the seal interface. It was also found that cross-linked polymers from corona treatment required higher temperatures to achieve the same seal strength as a non-treated film, if a seal was even achieved. Increasing the temperature or dwell time did not guarantee an achievable seal in cross-linked polymers [40].

If the proper corona discharge treatment were to be achieved on a film, there are additional factors that can cause the decay of the corona treatment over time. Many manufacturing processes include corona treatment in-line with lamination or printing processes to avoid such decay. However, this is not the case for all such manufacturing environments. Corona treatment stability can be affected by time, storage temperatures, relative humidity, migration of film additives, reorganization of polar groups, substrate type and treatment levels [40; 126; 146]. Over time, the electric charge formed on the

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surface of the film can degrade. Polar groups can rearrange changing surface morphology, and film additives such as slip additives can migrate to the surface producing a weak boundary layer [126].

Storage conditions can greatly affect the lasting effects of corona treatment. A study conducted found that 1-7% of the corona treatment was lost after 9 days in storage and 23-28% was lost after 37 days. If the storage conditions were at higher temperatures or higher humidity, the corona treatment would have been degraded further [126]. Films that have been temperature abused can result in increased crystallinity. If this were the case, the penetration depth of the corona treatment would be decreased reducing the effect of treatment [146]. High relative humidity levels can also cause the need to increase treatment duration due to interference of hydroxyl molecules in the air [126].

Although corona discharge treatment has been found to be effective in increasing the wettability and adhesion of polymer surfaces, additional surface treatments may be required. As previously stated, wettability does not necessarily produce adhesion.

Chemical compatibility is a major factor in two substrates or a substrate and liquid coating to be able to adhere to one another. Primers are a very common method of

changing the surface chemistry of a substrate for the adhesion of incompatible substrates.

Primers are very thin coatings between layers with typical laydowns of 0.04-0.4 gsm (grams per square meter) or 0.0016 to 0.016 pounds per ream [101].