Today’s engineer is faced with a myriad of design considerations, coupled with an extraordinary number of available resin grades when selecting the optimum material candidate for a functional product. The problem lies in fully knowing what will be required of the product in a wide range of use/abuse environments along with understanding the true functional behavior of the polymer.
The short- and long-term effects of service conditions, including exposure to chemicals, humidity, and temperature can compromise the robustness of a plastic material. Two case studies demonstrate the effect of different service environments on the success of the product, emphasizing the need for diligent material selection.
The first case study is a component used in a beverage system. This component ensured the flow of a hot beverage into a container and also housed electronic components. The material used in the manufacture of this component was a talc-filled polypropylene resin. Several components were returned from the field after severe blistering was observed on the surface. While in use, the components were exposed to various kinds of beverages as well as steam from 175° to 205°F, and cleaned using an alkaline solution.
Examination of the components revealed numerous blisters on the surface of the plastic ranging significantly in size. In the course of the examination, a cracked blister was observed. While opening the blister to examine the inside features, a light brown liquid, consistent with coffee, was released.
Other blisters were opened throughout the component and further confirmed the presence of trapped liquid within the thickness of the part. Cross sections of the component were obtained and examined, revealing a network of cracks that extended both circumferentially within the thickness and radially at the outer surface. An analytical technique, known as infrared spectroscopy, was used to assess the plastic material at the surface of the component for potential contamination and/or degradation. The results revealed accelerated oxidation of the material on the surface of the blistered part.
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Numerous blisters were found on the surface of the plastic used in a beverage system. |
The evidence suggested that the blisters developed as a result of the expansion of trapped liquid. The liquid was likely transported into the material via capillary forces associated with the network of cracks, which had formed within the part. The increased level of oxidation at the surface of the part would lead to increased brittleness and deterioration in strength, and finally cracking. It is also possible that the liquid could penetrate the electronics, which would result in an electrical short.
The second case study is the housing of a pump used in a chlorinated water filtration system. The housing represented a field failure, which exhibited cracking after less than six months in service. The housing was comprised of a glass-filled blend of nylon 6/6, poly(phenylene ether), and polystyrene resins. The failed housing displayed two large cracks at the part outlet, along with substantial deposit buildup internally.
Examination of the fracture surface using a scanning electron microscope demonstrated what is known as a mud-cracked appearance along with debonded glass fibers. Additional examination of the deposited material revealed the presence of glass fibers and particulate debris. A comparison of the melt flow rate obtained on the failed part material to that obtained on the raw resin demonstrated a 228 percent increase. This increase was indicative of a significant reduction in the average molecular weight of the polymer.
The results of the investigation suggested that the housing failed due to direct chemical attack. The examination of the fracture surface also revealed features consistent with the degradation of the polymer matrix. Once the polymer matrix was broken down, the glass fibers were freed and then deposited on the housing surface. Degradation of the material reduced its mechanical properties and the overall structural integrity of the housing.
These are just two simple cases shortsightedness in material selection led to product failures. In both cases, the environment of the application was understood but the true functional behavior of the plastic in the environment was unfamiliar.
Diligent material selection includes going beyond the short term, room temperature, single point data provided on material datasheets by resin suppliers. It includes investigating both the short and long term effects of moisture, chemicals, and elevated temperatures experienced in the application on the polymer. Product engineers can accomplish this by reviewing engineering design data and chemical compatibility guides from resin suppliers on potential material candidates. If such information is not available at the necessary application parameters, it may be advantageous to generate material data via an internal and/or external test laboratory. As the saying goes, there is never a bad material only the wrong application.