Surface Modification of Polymers by Plasma Treatment for Appropriate Adhesion of Coatings

Abstract

In this study, recent advances in tailoring the surface properties of polymers for the optimization of the adhesion of various coatings by non-equilibrium gaseous plasma are reviewed, and important findings are stressed. Different authors have used various experimental setups and reported results that scatter significantly and are sometimes contradictory. The correlations between the processing parameters and the adhesion are drawn, and discrepancies are explained. Many authors have explained improved adhesion with the adjustment of the surface free energy or wettability of the polymer substrate and the surface tension of liquids used for the deposition of thin films. The adhesion force between the polymer substrate and the coating does not always follow the evolution of the surface wettability, which is explained by several effects, including the aging effects due to the hydrophobic recovery and the formation of an interlayer rich in loosely bonded low molecular weight fragments.

Keywords: adhesion; coatings; gaseous plasma; polymers; wettability.

Figure 1

Influence of morphology on the optimal (a) and inadequate (b) adhesion between two materials.

Figure 2

Illustration of the interaction between plasma sustained in pure helium (a–c) or a mixture of helium with ammonia water (d,e) with the surface of the PTFE.

Figure 3

Interaction of 60 MHz atmospheric pressure argon plasma with aluminum (a,b) and PEEK (c,d).

Figure 4

Interaction of air plasma with carbon fibers (a) and resulting surface finish (b).

Figure 5

Illustration of the low-pressure DC glow discharge (a); radicals and ions from the air plasma sustained by the DC glow discharge (b) interact with the polymer surface and cause rapid functionalization (c).

Figure 6

Illustration of inductively coupled plasma (a), the surface finish at low discharge power (b), and at high discharge power (c).

Figure 7

The schematic of the device useful for polymer activation and deposition of a thin metal film (a), interaction of plasma species with polymer surfaces (b), and resulting surface finish at moderate treatment times (c).

Figure 8

Illustration of capacitively coupled discharge for treatment of polymers with plasma (a) and the surface finish after a short treatment of several 10 s (b) and treatment for 120 s (c).

Figure 9

Water droplet contact angle versus the plasma treatment time [13,18,23,24,27,34].

Figure 10

The ratio between adhesion after and before the plasma treatment (adhesion increase) versus the treatment time [13,19,20,21,22,23,27,30,33,34,39].

Figure 11

The formation of agglomerates of molecular fragments on the polymer surface. (a) The polymer is exposed to plasma species, which cause functionalization (b). Moderate doses of plasma species (prolonged treatment time and/or rather large discharge power) cause the formation of well functionalized molecular fragments (c), which may agglomerate after large doses of plasma species (d).

Figure 12

Illustration of the effect of molecular fragment agglomerates on the adhesion. (a) The coating wets the substrates well, so the coating fills all gaps on the surface, but the area of good adhesion is limited to the surface free from agglomerates. (b) The agglomerates are peeled from the polymer surface after the adhesion test, so the shear strength is inadequate.