Plastic Surface Modification

2	Primary Polymer Adhesion Issues with Inks, Coatings, and Adhesives

When addressing the adhesion of polymers to interfacing materials, the primary and foremost challenge is to understand the fundamental driving forces which can initiate the development of adhesion strength between polymer-to-polymer, polymer-to-metal, polymer-to-ceramic, or polymer-to-inks coatings and adhesives. These interfaces also exist in multivariate environments, such as heat and humidity, which also must be examined. Ultimately, it is the polymer and the interface chemistry that determine adhesion. However, there can be adhesion failure between the polymer and an inorganic, such as a metal, due to an oxide layer that is weakly attached.

That being said, this work will focus fundamentally on the bonding issues associated with polymers because of their unique deformation character, low modulus, and long chain structure.

Many polymeric materials inherently have a low surface energy that results in poor surface adhesion or even complete adhesion failure. This makes it difficult for inks, paints, adhesives and other coatings to properly wet-out and adhere to the surface of these substrates. Proper surface preparation of these materials will increase surface energy, improve surface adhesion properties, and add value to the product and the process. However, one must keep in mind that it is the bulk mechanical properties of the polymer that control the interfacial forces, which in turn influence adhesion. We will be subsequently reviewing various substrate orientations, from oriented and metallized films to spunbonded polyolefins and molded polymers, in order to examine their bulk structures for their ability to endure mechanically-induced deformations to allow for surface roughening and chemical covalent bonds to achieve requisite adhesions.

It is well known that polymer chain entanglement is the primary source of a polymer's strength. It is also known that over time polymeric materials can become increasingly semi-crystalline, making their surfaces even more difficult to accept surface modification techniques. The process of axially or biaxially orienting polymer films, for example, strengthens these materials as their chains become stretched. It is therefore common practice for surface modification techniques, such as corona discharge, to take place immediately following the orientation phase.

2.1	Cast and Blown Films

The cast film process involves the extrusion of various polymers which are melted through a slot or flat die to form a thin, molten sheet or film. This melted film or “extrudate” is typically laid to the surface of a water-cooled and chrome-plated roll by a blast of air from an air knife or vacuum box. Cast film extrusion orients molecules in the machine direction only, producing a large difference in machine and transverse directional properties. This means that the chain molecules become aligned in the cast direction. This will increase the tensile performance in that direction, and forms what is known as “uniaxially” oriented film, whereby the gauge of the film will be in relatively straight lanes. However, there are other cast film extrusion disturbances, notably the effect on optical properties, which can be attributed to the structure of the polymer bulk, the structure of the film surface, crystallination roughness at the surface, and surface roughness by the extrusion process. Molecular weight, molecular weight distribution, chain branching, shear strain, melt flow rate, relaxation time, elasticity, orientation, processing conditions, and cooling rate must also be considered. They contribute not only to the formation of surface roughness but also their affect on surface adhesion. For example, a narrower range in molecular weight distribution leads to a more uniform crystal size distribution and thus to lower surface roughness and better optical properties. It can also be expected that surface modification techniques applied directly after extrusion will encounter less surface crystallination, which will enable greater surface roughening and a functionalization effect on a relatively chemically inert surface to promote future interfacial adhesion.

Cast film extrusion is used in manufacturing polypropylene films and requires greater surface pretreatment power density (possibly 2‒3 times) compared to other polyolefin films. With blown film extrusion processes, polyethylene films are typically used and require pretreatment on both sides. Considerable amounts of slip additives, used to lubricate the surface of these films for processing ease, can be prevalent within the resin and migrate to the surface of the film within a few days after extrusion. Although there is potential for the additive to mask-over treatment, it is far more important to surface treat immediately after extrusion, since it will be practically impossible to do so after additive migration to improve surface properties sufficiently for ink, coating, or lamination adhesion.

It is interesting to note here that with respect to heat-sealing behavior, some research indicates that a primary effect of surface pretreatments such as corona on blown linear low density polyethylene films, for example, can be a change in the failure mode of heat seals from a normal tearing or inseparable bond to a peelable seal. More specifically, corona discharge has been determined to increase the seal initiation temperature by 5‒17 °C and decrease the plateau seal strength by 5‒20 % as the treat level, or wetting tension, increases from 31 to 56 dynes/cm. These corona treatment effects have been attributed to cross-linking during the process, which restricts polymer mobility near the surface and limits the extent of interdiffusion and entanglements across the seal interface. Results of heat-sealing studies with electron-beam-irradiated polyethylene, chemically oxidized polyethylene, and corona-treated polypropylene provide indirect evidence for the proposed surface cross-linking mechanism [1]. However, it is quite possible that this observation can also be attributed to an “over treatment” effect from discharge power densities which are higher than required. Because of the recurrent need for surface preparation optimization at extrusion, the importance of corona and other surface treatment discharge technologies requires closer process control examinations and will be discussed in depth to describe these surface mechanisms.

2.2	Metallized Films

In the metallization process, a layer of metal is deposited on plastic films using several different methods, ranging from vapor deposition to electroplating. When preparing surfaces for adhesion, one must consider the surface differences between the type of film substrates, the potential variations of film characteristics within different substrate families, variations with use of the same substrate between vacuum metallizing chambers, and any possible variations within an end-use application, even when using identical films.

It has been practical experience over many years that polyester films and oriented polypropylene have sufficient metal adhesion to be the most widely used in flexible packaging applications. As stated previously, there can be significant differences among these metallizing substrates. An initial consideration is their respective surface polarities, either inherent or pre-conditioned. In the case of the latter, a non-polar surface can be prepared to chemically and molecularly bond to the deposited aluminum layer by oxidizing the surface. This is typically accomplished by contributing oxidation, peroxides, alcohol, ester, ether, or aldehyde functional groups, which will bond well to aluminum depositions. As inferred with extruded films earlier, an overtreatment of an organic surface can actually cause metal adhesion to become quite poor. This introduces the concept that overtreatment can over-develop low molecular weight organic materials at the surface layer, causing the deposited metal to lose contact with the base polymer. This creation of what is known as a “weak boundary layer” weakens the mechanical surface bond between the metallized surface and the base film, potentially causing a failure of the metal to adhere. This metal bonding failure can also result from the surface migration of film processing additives, which are used to reduce the film's coefficient of friction for ease of processing.

As was noted earlier, a polymer surface may not necessarily need to be functionalized in order to create a strong surface bond with a metal. However, it is necessary for the surface of such polymers to be crosslinked. Schonhorn [2], for example, showed that the integrity of crosslinked surfaces will be highly dependent on the level of high-energy processing, such as the metallization process itself, or a form of surface treatment. The type of polymer used for the metallization process may be more adversely impacted at its surface by a pretreatment approach such as corona, flame, or plasma, although Schonhorn demonstrated that surface degradation effects can vary. To mitigate such an impact, more controllable surface treatment approaches, such as vacuum or atmospheric plasma, can avoid the formation of weak boundary layers by “pre-cleaning” the surface of low molecular weight organic materials and by introducing functional groups that are appropriate for the specific polymer film to be metallized (see Figure 2.1).

These processes must, however, also be controlled, most notably relative to the type of functional groups...