Effects of UV degradation on surface hydrophobicity, crack, and thickness of MWCNT-based nanocomposite coatings

Abstract

Surface degradation is a common problem in polymeric coatings when they are exposed to sunlight, moisture, and oxygen. In order to reduce their surface degradation, thus keeping the coatings’ original properties, multi-wall carbon nanotubes (MWCNTs) were added, and the coatings were exposed to UV light and salt fog for various lengths of time. At 0 days of UV exposure, contact angle values of 0%, 0.25%, 0.5%, 1%, and 2% MWCNT-based nanocomposite coatings of 75 μm (∼3 mil) thickness were between 85° and 89°. However, after 16 days of UV exposure, contact angle values of the same samples were reduced to 11°, 13°, 34°, 50°, and 54°, respectively. Longer UV exposures resulted in several microcracks on the surface of the coated samples in the absence of nanoscale inclusions, while very minimal cracks or degradation appeared on the MWCNT-loaded samples. Test results also showed that UV exposure along with salt fogging reduced the coating thickness up to 24% at 0% CNTs; in contrast, this reduction was only 7% with a 2% MWCNT coating. These results clearly indicate that MWCNTs added to polymeric coatings reduce UV degradation, lessen surface cracks, protect the film thickness, and hence increase the lifetime of the polymeric coatings.

Introduction

Polymeric coatings are commonly used for the purpose of protecting surfaces against environmental attacks, including UV light, moisture, and oxygen [1]. These organic films, including polyurethane, polyamide, polyester, resin, and epoxy, play a crucial role as a barrier layer to avoid the transportation of corrosive species, such as chloride and hydroxyl ions, oxygen, water, pollutants, pigments, and other substances [2]. These unwanted elements/species have a high affinity to reacting with material surfaces when they interact with the interfaces [3].

Polymeric coatings experience physical, chemical, and physicochemical deterioration as the result of environmental interactions [4]. The degradation of polymeric materials can develop in the form of swelling, cross-linking, dissolution, water absorption, oxidation, and color changes [5]. Additionally, at higher temperatures, some gas species may form from the coatings, thus changing the molecular weight (MW), density, gloss, and glass transition temperature (T_g), and hence increasing the porosity and brittleness of the polymeric coatings [3], [4], [5], [6]. The combined effects of environmental interactions can also take place on the polymeric surfaces, resulting in the alteration of corrosive, electrical, thermal, and optical properties, as well as other chemical and physicochemical properties of the materials [6]. Fig. 1 shows the major atmospheric influences on an organic coating and corrosion formations on a metal substrate [7], [8].

Ultraviolet (UV) light is electromagnetic radiation with wavelength ranging from 10 nm to 400 nm and energies from 3 eV to 124 eV which is shorter than that of visible light, but longer than X-rays. The reason behind the naming UV is the frequency of its spectrum which consists of electromagnetic waves with frequencies higher than those that human eye identifies as the color violet [4]. According to ISO solar irradiance standard (ISO 21348 process for determining solar irradiances compliance), the electromagnetic spectrum of UV can be subdivided into the following main groups [4], [5]:

• Ultraviolet A (UVA): 99% of the total ultra-violet light that can reach the surface of the earth is Ultraviolet A which is the radiations with wavelengths between 320 nm and 400 nm. UVA radiations are responsible for some photosensitivity reactions and it can increase the harmful effects of ultraviolet B radiations.

• Ultraviolet B (UVB): Ultraviolet B is the radiations with wavelengths between 290 nm and 320 nm. 1% of the total ultra-violet radiations that can reach the surface of the earth are UVB. It causes a number of damaging photochemical reactions.

• Ultraviolet C (UVC): Radiations with wavelengths between 200 and 290 nm are ultra-violet C. UVC is filtered out by the ozone layer and mostly cannot reach the surface of the earth.

The present study deals with solar ultraviolet radiations (mostly medium and long wavelengths) and their effects on organic coating materials. Note that severe UV effects can exist with short UV light, which is fortunately absorbed by the ozone layer in the atmosphere before it reaches the surface of the Earth and cause fatal defects on the organic coatings [31], [32], [33].

Sunlight has a high intensity of UV light, which cause the formation of free radicals on polymeric surfaces [9]. These radicals are simply groups of atoms/molecules with an excess of electrons that have an affinity for paring with other electrons in the polymer structure [10]. Therefore, this process breaks the covalent bonds of polymer molecules into small molecules and initiates the cross-linking reactions for extra polymerization, oxidation, or both [4]. The amount of energy absorbed by a molecule must exceed the bond energy in order to cause the degradation. The excitation energy per mole can be obtained using the following equation [5], [9]:

$$E=Nh\nu =\frac{Nhc}{\lambda }=\frac{119627}{\lambda }(\text{kJ/mol})$$

where E is the energy of radiation of a given wavelength, N is Avogadro's number (6.022 × 10²³ mol⁻¹), h is Plank's constant (6.63 × 10⁻³⁴ J s), v is the frequency of radiation, c is the velocity of light (2.998 × 10⁸ m/s), and λ is the wavelength of radiation (nm). The most important degradative mechanisms are associated with the absorption of ultra-violet (UV) light with energies between 300 kJ/mol and 450 kJ/mol for the great majority of synthetic macromolecules [10]. Table 1 shows the electromagnetic spectrums and their excitation energies [7]. As soon as UV energy exceeds the polymer's bond strength, degradation takes place through free radical formation [5]. Table 2 provides the bond strength in polymeric molecules [7]. Other environmental conditions, such as humidity, temperature, oxygen, acidity/basicity, and pollutants will drastically accelerate the level of UV degradation [11], [12], [13].

As a result of long-term UV exposure, synthetic and naturally occurring polymeric coatings cannot withstand environmental attacks [14], [15], [16], [17]. Especially at higher levels of cross linking and oxidation, polymeric materials can degrade, and eventually crack propagation can occur on the protective coatings [5]. Consequently, the surfaces of metals and alloys under the coatings can be degraded or dissolved as oxide or other compounds in an aqueous media, which will reduce the lifetime of coatings and substrates under the coatings [18], [19].