Thermal Analysis of Primers and Coatings as a Prediction Model

Thermal characteristics of primers and coatings can be a useful prediction model on how the material will perform under heat exposure from manufacturing processes and product applications. Two chemistries, poly(vinyl alcohol) and polyethylenimine, have been analyzed using the techniques of Thermogravimetric Analysis (TGA), TGA coupled to a Mass Spectrometer (TG/MS), TGA coupled to a Fourier Transform Infrared Spectrometer (TG/IR), Modulated Differential Scanning Calorimetry (MDSC), Dynamic Mechanical Thermal Analysis (DMTA). Introduction When designing and developing new packaging structures predicting performance is theoretical. Most will make the structure and then test containment and product integrity. If a simple analytical technique could be used to screen these design ideas, it would save time and money that are usually limited resources. Using an appropriate analytical experiment can predict performance of the structure in the packaging application and the manufacturing process. In this paper, two primers with different chemistries were analyzed using a variety of thermal analysis techniques. One set of experiments was designed to predict performance of the primer in the packaging application; the other set of experiments was designed to predict the feasibility of the primer performance in the manufacturing process. The first set of experiments tested a modified polyethylenimine (PEI) primer for evolved gases during the heat sealing step in the packaging fabrication. It is vital to the packaging application that only species non-reactive to the contained product be evolved during the sealing of the product into the package. The second set of experiments tested a poly(vinyl alcohol) (PVOH) based primer for thermal stability properties to determine if the primer could be sent through a tenter frame as a surface barrier coating on film. If the material decomposes or doesn’t have the proper visco-elastic properties the coating will not stretch uniformly and not have the desired properties for the targeted application. Experimental Modified PEI primer The sample was prepared by drying to a thin film on a glass slide in an oven until dry. The temperature was ~140°F. This is a typical exit web temperature for a coating station used on extrusion lines. This thin film was used for all of the experiments discussed. The first two thermal analysis techniques used were Thermogravimetric Analysis (TGA) and Modified Differential Scanning Calorimetry (MDSC). The techniques examine the thermal stability and weight loss of the material as it is heated. TGA precisely measures the weight change of a material as it is heated at a controlled rate. Explanation of modulated DSC Differential Scanning Calorimetry (DSC) is used to determine a physical property of a material, such as the glass-transition temperature (Tg), or the melt temperature (Tm). A sample and a reference material, usually an empty sample pan, are heated at a controlled rate. When a transition occurs in the sample, such as melt point, an input of energy/heat is required keep the sample and reference at the same temperature. This difference is recorded on the DSC scan as a function of temperature. Modulated DSC provides the same physical and chemical information as conventional DSC. It also provides data that is unavailable with conventional DSC. The effects of slope and curvature on the baseline are reduced, thereby, increasing the sensitivity of the analysis; and overlapping events are separated, such as molecular relaxation and glass transitions. Both modulated DSC and conventional DSC measure the difference in heat flow between a sample and an inert reference. The sample and reference cells are identical. Modulated DSC uses a different heating profile. Conventional DSC measures heat flow as a function of a constant rate of change in temperature, and modulated DSC superimposes a sinusoidal temperature modulation on this rate change in temperature. The sinusoidal change in temperature permits the simultaneous measurement of heat-capacity and kinetic effects. Typical experimental parameters for modulated DSC experiments are a heating rate from isothermal to 5°C/min, with a modulation amplitude from 0.01 to 10°C. This modulation period can vary from 10 to 100 seconds and be expressed as a frequency (10 100 MHz). Two purge gases were used in the MDSC and TGA experiments: air (oxidative environment) and nitrogen (inert environment). This will identify any decomposition products by oxidation or by internal thermal degradation. Figure 1 is a TGA curve of the modified PEI primer sample purged with an inert gas (nitrogen). Figure 2 is a TGA curve of the modified PEI primer sample purged in an oxidative environment (air). Both experiments were run from room temperature to 260°C (500°F), with a temperature ramp rate of 10°C/min and a purge gas flow at 50 ml/min. Figure 1: TGA Curve of a Modified PEI primer, purged with nitrogen. 1 3 .7 5 % (3 .3 5 6 m g ) 1 3 5 .8 7 °C 1 8 0 .3 5 °C 0 .0 0 0 .0 2 0 .0 4 0 .0 6 0 .0 8 0 .1 0 0 .1 2 D er iv . W ei gh t ( % /° C ) 8 5 9 0 9 5 1 0 0 1 0 5