Design of Polymer-Additive-Solvent Coatings for Enhancing Drying

Abstract

Polymeric coatings are composed of polymeric materials and can be applied to a variety of surfaces by different methods. Polymeric coatings should provide good adhesion to the substrate as well as environmental protection. Residual solvent needs to be minimized since its presence impacts the properties of coatings such as glass transition temperature and causes coating defects. Residual solvent is one of the important factors in polymeric coatings also because the solvents used can be harmful to the environment and consumers. Waterborne coatings are biodegradable, non-inflammable, non-toxic, and show low environmental impact but have issues of high residual solvent along with long preparation time. There is a need to alleviate their drying. Additives enhance the drying properties of the films. A lot of literature is available on the use of plasticizers and surfactants for various purposes. However, there is a dearth of literature on the role of modifiers in enhancing drying of thin polymeric films. In the present work, the coatings were prepared by solution casting method. In the first part of the work, waterborne coatings of PVA-PEG6000-water, PVA-PEG400-water, PVA-FS-water, and PVA-TPP-water systems were considered. For PVA-PEG6000-water coatings, the maximum drop in the residual solvent was 87% at 0.50 wt% PEG6000 loading in the coating of initially 5% PVA and 1000 μm thickness. The effect of doubling the initial wet thickness from 500 μm to 1000 μm was first examined. The minimum residual solvent significantly reduced from 1.30% to 0.59%, and occurred at the same PEG6000 loading of 0.50 wt%. SEM results also revealed that at the optimum PEG6000 loading a smooth and dense coating was obtained. TGA/DTG and DSC studies exhibit thermal stability. Films with good mechanical properties were obtained. When the initial polymer concentration was doubled from 5% to 10%, the optimum PEG6000 loading lowered from 0.50 wt% to 0 wt%. Simultaneously, a marked increase in the minimum residual solvent from 1.30% to 3.27% occurred, caused also by the hydrophilicity of PVA and enhanced solution viscosity. On comparison with FS modifier, PEG6000 yields significantly better maximum reduction in residual solvent at substantially lower optimum loading by employing higher wet film thickness. PEG400 of lower molecular weight exhibited significantly more reduction in residual solvent with a value of 0.20% at 2.50 wt% PEG400 loading, 1025 μm initial film thickness. In the study of PVA-FS-water system, the residual solvent dropped by 85% with the optimum modifier loading of 2.50 wt% in the coating of initially 5% PVA and 500 μm thickness. SEM images conformed to these results as the dried coatings up till 2.50 wt% loading yielded smooth, dense, and defect-free morphology, whereas higher loading values showed otherwise. On doubling the initial solution thickness from 500 μm to 1000 μm, the same optimum loading of 2.50 wt% was exhibited. However, the minimum residual solvent increased significantly from 0.51% to 1.16%, and drying rate slowed down significantly. On doubling the initial polymer concentration from 5% to 10%, the optimum loading reduced from 2.50 wt% to 1.00 wt%. The minimum residual solvent respectively increased significantly from 0.51 to 1.20%, which can also be attributed to the water affinity of PVA and three-fold increase in solution viscosity. TGA/DTG and DSC studies exhibit thermal stability. Films with good mechanical properties were obtained. In the study of PVA-TPP-water system, the residual solvent dropped by 94% with the optimum loading of 2.50 wt% in the coating of initially 5% PVA and 600 μm thickness. On doubling the initial solution thickness from 600 μm to 1200 μm, the same optimum loading of 2.50 wt% was exhibited. Again, a significant drop of 87% in residual solvent was attained by the 5% PVA and 1200 μm thickness coatings. On doubling the initial polymer concentration from 5% to 10%, the optimum loading reduced from 2.50 wt% to 0.50 wt%. The minimum residual solvent respectively increased significantly from 0.20% to 4.03%, which can also be attributed to the increase in solution viscosity. SEM results also revealed that at the optimum TPP loadings smooth and dense coatings were obtained. TGA/DTG and DSC studies showed thermal stability of the films. Films with good mechanical strength were obtained. FS showed considerable reduction in residual solvent with a minimum value of 0.51%. Modifier PEG400 showed significantly more reduction in residual solvent of 0.21% at higher thickness and is of lower cost, more environmentally friendly, and biodegradable. TPP showed the lowest residual solvent of only 0.20%; however it is more expensive than PEG. All the waterborne coatings exhibited good mechanical properties. In a nutshell, although FS and TPP showed better results overall, yet low amounts of PEG6000 (0.50 wt%) can be used to significantly lower down the residual solvent and cost by employing higher wet film thickness for the solution casting of waterborne PVA coatings. This was attained due to reduced skinning at lower drying rate. For the same reason PEG400 also yielded lower residual solvent but significantly higher amount of modifier (2.50 wt%) was required. The drying conditions throughout the first part were 25 ± 2 °C, 39 ± 2% RH, and no air flow. In the second part of this work, polystyrene–p-xylene coatings were studied at different initial parameters and operating conditions. Modifier TPP was used for polystyrene–p-xylene coatings, since it exhibited good results in the first part. In the design study, first, two initial wet film thicknesses of 800 μm and 2400 μm were implemented in the films. The 800 μm films showed a lower value of the minimum residual solvent due to a lower mass transfer resistance. For the 800 μm films, when the initial polymer content was changed from 7% to 14%, the corresponding optimum TPP content shifted from 2.50 wt% to 5.00 wt%. The 7% PS films yielded a lower value of the minimum residual solvent attributed also to lower solution viscosity. For the 7% PS and 800 μm films, the effects of molar mass and temperature were studied. Three wide ranging PS molar masses of 35,000, 192,000, and 280,000 gmol-1, and the drying temperatures of 9 ± 2 °C and 25 ± 2 °C were implemented. Remarkably low values of the minimum residual solvent of 0.0110%, 0.0154%, 0.100%, were attained at 25 ± 2 °C, 39 ± 2% RH, and no air flow for the three molar masses, respectively. In line with the fundamentals of polymer solutions, the corresponding optimum content of TPP was 2.50% irrespective of these operating parameters. Lower residual solvent was achieved at lower PS molar mass attributed to higher segmental mobility and lower solution viscosity. SEM showed that the films prepared at low PS molar mass, PS content, and thickness, and at high temperature and optimum TPP content were dense and smooth with minimum defects. The corresponding films with no TPP were phase-separated and non-uniform. The TGA and DSC results showed that the higher molar mass films were thermally more stable. A detailed comparison with literature is presented. The films hold potential importance since TPP can act as plasticizer, drying enhancer, and fire retardant in the inherently inflammable PS films. Coatings with good mechanical and thermal properties were obtained in the present work, which can be of use in practical applications.