Solid-state NMR study of aging of Colorbond polymer coating

Abstract

The effect of weather and aging on the properties of polymer coatings was studied using solid-state NMR spectroscopy. Colorbond polymer coatings were aged in an accelerated weathering tester and ¹³C NMR spectra were obtained. NMR relaxation times (¹³C T₁, ¹H T₁, T_1ρ) and line width were measured to study the changes in dynamics and structure of the polymer coating due to the effect of UV light and weathering. No changes in chemical shift were observed between fresh and aged polymer coating. A difference in ¹³C T₁ and a small change in cross-polarization time between fresh and aged samples were observed. These changes in molecular mobility may be useful in understanding the physical effects of aging and correlating changes in NMR relaxation times to weathering of polymer coatings.

Introduction

Polymer coatings, including nitrocellulose, polyester, latex and acrylic (a polymer of acrylates, methacrylates and similar monomers), are widely used as paints. The top coat is required for esthetic qualities (color and gloss) and as a protective layer since it interacts with the environment and must provide resistance to the conditions encountered including irradiation, oxidation, hydrolysis and the action of aggressive fluids. Coatings are normally exposed to the environment, leading to changes in density, void space, permeability, elasticity and resistivity [1]. These changes take place with time and are known as aging of coatings.

Chemical aging of coatings involves the process of oxidative degradation. Coating structures should be designed to reduce the absorption of short wavelength solar radiation so as to avoid the generation of radicals and photochemical reactions, which speed up the aging rate of polymer coatings. Physical aging is due to the slow changes that occur in coatings with time. When amorphous polymers are cooled through their T_g (glass transition temperature), they solidify and stiffen to form glasses. In this glassy state, the rate of structural relaxation becomes very slow and the polymer conformations are fixed in a non-equilibrium state. If the temperature is held at some value below T_g, the process of relaxation towards an equilibrium configuration continues and as a result many material properties change with time. The process of physical aging is highly temperature dependent, and is fastest at temperatures closest to T_g.

Chemical and physical changes due to aging may be detected by nuclear magnetic resonance (NMR) spectroscopy [2], [3], [4]. Although solution-state NMR provides the most detailed structural information, solid-state NMR provides important information about internal molecular motions and dynamics and is applicable to insoluble samples such as cured films and paints [5], [6]. In the present work, we report changes in NMR parameters of Colorbond polymer coatings with age. Colorbond samples were chosen because of their industrial relevance to current production techniques and their improved product durability. Colorbond is a polyester paint designed to withstand the harsh outdoor Australian climate and is subject to extremes in heat and humidity. Although previous studies have reported on the aging of polymers [7], to date no work has been reported in the open literature on the aging of this class of polymers, which are directly bound to the surface.

The mobility of groups within the polymer can be measured using solid-state ¹³C NMR techniques. The polymer basically consists of a carbon backbone surrounded by protons. These protons interact with each other via a dipole–dipole interaction resulting in broad resonance lines for solid samples and, since protons have a small chemical shift range, resonance lines overlap in proton NMR spectra. ¹³C–¹³C interactions, however, are rare since ¹³C abundance is only 1.108% and, since ¹³C nuclei have a large chemical shift range, better spectral resolution is obtained. ¹³C–¹H interactions are particularly important since a ¹³C spectrum can be obtained by transferring magnetization from protons to the less abundant isotope, ¹³C. This enhances the ¹³C signals and is called cross polarization (CP). CP allows faster signal acquisition since the longitudinal relaxation times, T₁, of ¹H are shorter than ¹³C [7], [8]. Spectral resolution is enhanced by magic angle spinning (MAS), which involves placing a rotor containing the sample at an angle 54.7° to the magnetic field and spinning at several kHz. The line broadening caused by chemical shift anisotropy and dipolar interactions is reduced [9], [10].

Molecular motion can be measured using the NMR relaxation times T₁, T_1ρ and T₂. T₁ is the spin–lattice relaxation time and relates to fast molecular motion on the ω₀ time scale or Larmor frequency (MHz). Spin–lattice relaxation involves a loss of energy by the excited nucleus to the surrounding environment or lattice [7], [8], [11]. The rate at which this relaxation process occurs is measured by the spin–lattice relaxation time (1/T₁=rate). The closer the motional frequencies of the lattice are to the Larmor frequencies of the nuclei, the more efficient is the transfer of the magnetization (i.e. the shorter the T₁). T₁ measurements are usually limited to molecular motions in the 10⁷ Hz frequency range.

Both proton and carbon T₁ measurements are reported in this work. ¹H T₁ gives average information about dynamics from all the groups in the molecule since protons are ≈100% abundant [12]. The ¹³C T₁ gives information about the dynamics of individual groups on molecules since the ¹³C nuclei are rare and there is little relaxation from other ¹³C nuclei [10], [13].

T_1ρ is the spin–lattice relaxation time in the rotating frame [12]. It is similar to T₁ in the static field but due to spin–lattice interactions in the rotating frame. In CP experiments, the magnetization initially builds up to a maximum due to the carbon–proton dipolar interactions in the spin-locking field, ω₁, and decays exponentially to equilibrium with the time constant T_1ρ. The values of T_1ρ are used both to measure the molecular dynamics on the kHz time scale of ω₁ and to report on the length scale of polymer mixing [14], [15].

T₂ is the spin–spin relaxation time and is the time constant for the decay of the precessing X–Y component of the magnetization [7], [8], [11]. T₂ gives information about the distribution of resonant frequencies and about the local fields experienced by the magnetic moments of the nuclei. In liquids, the local magnetic fields fluctuate very rapidly and, therefore, quickly average to ≈0 to yield long T₂s and narrow resonance lines. The atoms in solids are in nearly fixed positions, and the internal fields are significant, which results in the rapid loss of coherence and short T₂s (microseconds). T₂ is related to the inverse of the resonance line width and thus T₂ in solids is short and the resonance lines are broad.

Changes in the NMR relaxation times T₂, T_1ρ and T₁ report on changes in molecular motion from slow to fast timescales. Colorbond polymer coatings were exposed for different periods in a weathering test situation and the effect on NMR relaxation times was studied.