Nomenclature
A0 – Surface area (cm2)
Ie – Incident energy (MJ/cm2)
P – Lamp power (W)
PS – Polystyrene
Ps – Incident power (W/cm2)
R – Distance form UV source to the sample (cm)
t - time (s)
UV – B – Ultraviolet B radiation
Wi – initial mass of the sample (g)
Wo – mass of the metal cage (g)
Ws – dry insoluble mass (g)
ΔHm – Heat of fusion (J/g)
ΔHm’ – Heat of melting for 100% crystalline PS
Introduction
Polystyrene (PS) is one of the most consumed rigid thermoplastic polymers worldwide owing to its high refractive index, good chemical resistance, low moisture absorption, density, electrical conductivity, ease of coloring, processability, and low cost of production [1]. The versatility of PS makes it useful in various industrial applications, such as the components of machines, vehicles, electronics and appliances, as well as in refrigerators, electric motors, seats, sanitary parts, heels and soles of shoes, stationery, and disposables [1-5].
In several of these applications, failure due to fatigue is caused by the application of cyclic stresses, under values below the fracture limit. Fatigue failure is observed by the formation of microscopic cracks that are nucleated at stress concentrators or beneath the material surface until catastrophic failure occurs. Additionally, when employed outdoors, the action of environmental agents on the surface of polymers can promote structural changes of a physical, chemical, and mechanical nature [6-9].
Thus, the objective of this work was to evaluate the influence of ultraviolet (UV) radiation on the mechanical behavior of PS subjected to fatigue. Macromolecular changes induced by the degradative process at different exposure times were analyzed.
Materials and Methods
The PS used in this work is an all-purpose commercial product. This as-received PS was processed by extrusion in the form of 6 mm thick plates. Samples with dimensions of 30 × 30 cm were produced by mechanical machining of the as-received plate and were previously approved by visual inspection to ensure accordance with the technical standards.
Exposure to ultraviolet radiation (UV-B), corresponding to the UV-B wave spectrum region (290–315 nm), was performed at room temperature in a chamber of an accelerated nonmetallic aging system (Comexim C-UV) adapted to ASTM G154-12 [10]. Each sample face was exposed to UV radiation for 336 and 575 h without simulating rain and/or mist. Comexim fluorescent lamps with 40 W intensity were used as a source of UV radiation in the range of 280–320 nm, with a peak seen at 313 nm.
The incident energy in the samples was calculated using Equations 1 and 2, by employing fundamental physical relationships.