Polyethylene (PE) and polypropylene (PP) are both common plastics with diverse applications, from packaging to automotive parts. Generally, PE tends to be less expensive than PP. This price difference can be attributed to PE's simpler polymerization process and the fact that it is produced in larger quantities worldwide, which scales down its cost. However, prices can fluctuate based on market demand, oil prices (as both are petroleum-derived products), and the specific type or grade of the polymer. Polyethylene is preferred for its flexibility and durability, making it ideal for products like plastic bags and containers. Polypropylene, while slightly more costly, offers higher melting points, making it suitable for applications requiring resistance to heat, such as microwaveable containers. Ultimately, the choice between the two materials depends on the intended application's specific requirements and the budget constraints of the project.

The concept of "particle size" in the context of epoxy is primarily relevant when referring to epoxy composites or fillers, rather than the epoxy resin itself. Epoxy resin, in its basic form, is a liquid polymer that doesn’t have particles in the conventional sense, but when it is used in applications that require fillers or reinforcements (like fiberglass, carbon fiber, or microballoons), particle size becomes an important factor. These fillers can range widely in size from nanometers (nm, for nanocomposites) to several millimeters (mm) for structural components. The specific particle size chosen for a project depends on the desired properties of the final product, such as strength, flexibility, and surface finish. Smaller particles can enhance strength and surface finish but may require more careful handling and dispersion. In contrast, larger particles are easier to work with but might result in a rougher surface finish and different mechanical properties.

Dipole-dipole forces are stronger than London dispersion forces due to the presence of permanent molecular dipoles. In molecules with dipole-dipole interactions, positive and negative ends of polar molecules attract each other, leading to a stronger, more stable attraction. London dispersion forces, in contrast, are temporary polarizations caused by the momentary distribution of electrons across a molecule. These fluctuations result in temporary dipoles that induce dipoles in adjacent molecules, leading to a weaker, transient attraction. The strength of dipole-dipole interactions, originating from the permanent separation of charges, is therefore inherently greater than the strength of London dispersion forces, which rely on fleeting changes in electron distribution.