Resin Fundamentals and Their Effect on Coatings Performance – Part I"A critical part of any coatings formulation is the contribution that the resin has to the ultimate performance attributes of the coating. In part I of this article, we will define some key resin parameters and how they influence resin selection in light of the desired coating properties. Some of the key definitions of resin properties we will define include thermoplastic, thermoset, glass transition temperature, tensile strength, modulus, elongation, functionality, film formation temperature, molecular weight, weight average molecular weight, number average molecular weight and molecular weight distribution.
Film Formation – Thermoset and Thermoplastic
Simply stated, a thermoplastic resin softens with heat, whereas a thermoset resin has functional groups and is thus capable of crosslinking with a reactant or self-crosslinking. Thermosetting resins react and harden in the presence of heat, moisture and/or oxygen. Film formation for a liquid paint using a thermoplastic resin takes place by water or solvent evaporation, whereas film formation for liquid coatings containing thermosetting resins occurs in two stages. In the first stage, solvent and/or water is evaporated and in the second stage, the resin self cross-links or reacts in the presence of an external cross-linker or reactant. For most applications requiring high levels of chemical resistance and toughness, thermosetting resins are normally the technology of choice. Unlike a thermosetting coating, when a thermoplastic coating is heated, it softens appreciably, as it is thermo-plastic.
Resin Characterization – Glass Transition Temperature, Molecular Weight and Molecular Weight Distribution
Two other considerations that have a pronounced influence on coating performance are glass transition temperature and molecular weight, including molecular weight distribution. The vast majority of synthetic polymers used in coatings are amorphous rather than crystalline, so they do not have a distinct melting point. Amorphous polymers have one or more temperatures in which there is a change in the rate of increase of specific volume with increasing temperature. Properly defined, the glass transition temperature is the temperature at which there is an increase in the thermal expansion coefficient (change from a glassy state where the resin is hard and brittle to a more rubbery state). The Tg can be measured using dynamics mechanical analysis (DMA) or differential scanning calorimetry (DSC).
Tg is a very important consideration in the selection of the proper resin for a given application. For example, a resin with a Tg significantly above room temperature may not properly coalesce to form a good film at room temperature. Conversely, for a thermoplastic ambient cure system, a resin with a Tg below room temperature will form a soft film that is not very mar or scratch resistant. At the Tg, the free volume and thus the mobility of the resin backbone increases. So for thermosetting resins, this also enables greater availability of resin functionalities to react or crosslink to form a film. As you can see from the above chart, a number of fundamental properties are dependent on the Tg of the resin within a given polymer type. Another important characteristic that influences the performance of a resin in a coatings formulation is themolecular weight (MW) and the molecular weight distribution. Synthetic polymers (EU) contain a mixture of polymers with different molecular weights.
There is a mixture of MW’s in a synthetic polymer. Thus the MW’s can be defined only by using a statistical distribution. The weight average molecular weight, or Mw, is defined as the summation of the products of the weight of each polymer species at a given molecular weight divided by the total molecular mass. The number average molecular weight or Mn is the molecular weight of the summation of the products of the number of molecules at each molecular weight divided by the sum of the number of molecules in a given sample. The ratio of Mw/Mn is called the molecular weight distribution or also polydispersity.
The molecular weight, as well as the molecular weight distribution, can have a profound effect on coating performance. For example, for coatings made with resin solutions, normally the higher the molecular weight, the higher the viscosity. This is a most critical consideration when formulating high solids coatings, since at the same viscosity, a higher molecular weight resin will require more solvent, resulting in a higher VOC than the same resin composition supplied at a lower molecular weight. Higher molecular weights, especially for thermoplastic polymers, normally result in higher tensile strength (toughness). For higher solids coatings, a narrow molecular weight distribution is desirable as lower viscosity results. Viscosity dependence on molecular weight is much less of an issue in water born coatings, because most of these coatings comprised of polymeric particles (EU) (emulsion, dispersion or micro-emulsion) rather than solutions."
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