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issue: May 2008 APPLIANCE Magazine

Dishwashers
Putting Dishwasher-Safe to the Test


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by David S. Porter, PhD, technical associate, and Randy S. Beavers, polymer solutions executive, Specialty Plastics Business Organization, Eastman Chemical Co.

A look at how today’s plastics are standing up to the tough dishwasher environment.

Figure 1. Traditional copolyesters tend to have less residual stress than polycarbonates, but are also less resistant to heat. The Tg of most copolyesters is around 80°C, as opposed to a Tg of 150°C for polycarbonates. Tritan, a new copolyester, is said to have a Tg of 108°C.

As commonplace as dishwashers are in commercial and home kitchens, remarkably little data exist on the interaction of plastic materials and the dishwasher environment. Yet such data could provide valuable information to appliance manufacturers as they continue working to increase the efficiency and efficacy of their dishwashers. 

Consumers demand convenience, and the effects of the dishwasher environment on housewares, such as reusable plastic drinkware and food storage containers, could directly impact consumer perception of dishwasher quality. A dishwasher environment that maintains a like-new appearance of plastic household goods has the potential to foster brand loyalty, encouraging repeat brand purchases over the long term.

To address the lack of information regarding the interaction of plastics and the dishwasher, a series of tests for polycarbonate, copolyesters, and some other common kitchenware plastics were conducted to compile consistent and quantitative data on the effects of dishwasher cleaning. Although materials such as polycarbonate are generally considered dishwasher-safe, the tests reveal that the dishwasher environment may severely abuse these plastics.

Testing reproduced stresses that plastic products encounter in a dishwasher and showed how this environment can interact with a variety of other factors to affect structural integrity. This information can help determine a product’s risk of environmental stress cracking (ESC). ESC is the development of cracks and crazes due to applied stress in a specific environment and is a leading cause of performance and aesthetic failure.

Putting Materials to the Test


Figure 2. Higher Tg offers additional benefits, including reduced creep under load, reduced molding cycle time, and slower physical aging.

The harsh environment of a dishwasher is particularly aggressive toward plastics. Heat, chemicals from high-pH detergents, residual stresses from processing, applied stresses from overpacking, along with hydrolytic attack, are all elements that combine to degrade the quality of common kitchenware plastic products.

Tests were conducted in both commercial and residential dishwashers. In general, home units reach temperatures up to 70°C (158°F) and have cycles up to two hours. Commercial units, however, have much shorter cycles of 1–2 minutes and a hotter cycle (about 85°C / 185°F), or a cooler cycle (about 60°C / 140°F) which includes chemical sanitizers. In addition, it is worth noting that commercial dishwashers tend to be used several dozen times a day, which substantially increases dishwasher exposure over the lifetime of a product.

Internal temperature tests were conducted on more than a dozen commercial and residential dishwashers. Bars of polycarbonate, copolyesters, and polycarbonate-copolyester blends measuring approximately 9 in. long and 0.125 in. thick, according to ASTM standard Type II, were tested using a dishwasher with time and temperature cycles more or less average. The bars were also put under 1% stress to mimic a slightly overpacked dishwasher environment, as is common in many household and commercial kitchens, and were cleaned using a common powdered dishwashing detergent.

In this testing, the dishwashers were run over a series of cycles, and defects or changes in the material, such as warping, ESC, and loss of clarity, were recorded. It was found that dishwasher detergent in combination with the heat of the dishwasher is one of the most crucial factors in determining the long-term durability of a plastic part. In these tests, the common powdered detergent that was used tended to be more aggressive toward plastics than its liquid detergent counterpart, with a typical pH of 11 as opposed to 7 or 8 for the liquid version. Some materials began to fail soon after contact with the cleaning solution, while others maintained product integrity over multiple cleaning cycles.

This testing leads to a better understanding of the durability of plastic products and uncovers weaknesses that can be addressed in the design or molding processes, as well as directing new material development. While this study is not representative of the enormous combinations of dishwashers and detergents, the implications are significant. These results suggest that different types of plastics react quite differently in dishwasher environments and imply what design elements and material choices may be best for creating dishwasher-safe products. 

The Dishwasher-Safe Factors

Figure 3. Photo presents data measuring the stress-optical coefficient in a new copolyester material. The stress-optical coefficient enables evaluation of residual stress levels in molded parts based on comparison of birefringence within the parts, a common tool for assessing part quality. The measured values show that the stress optical coefficient for the copolyester material is significantly higher than polycarbonate, with stress-optical coefficient and residual stress being inversely related through the stress-optical rule. As a result, parts molded from the new copolyester having birefringence patterns equivalent to those seen in polycarbonate would possess lower residual stress, thereby improving the ESC performance of the copolyester.

For reasons of convenience, consumers want to clean virtually any kitchen utensil, plate, or storage container in the dishwasher. Based on testing performed in Eastman labs, there are several key elements defining dishwasher performance that product designers should consider.

Chemical Resistance and Hydrolytic Stability. The behavior of plastic materials in specific chemical environments is difficult to predict due to the many possible mechanisms of interaction. For example, copolyesters are well known for excellent chemical resistance, but the traditionally low heat resistance of this family of products can render it unsuitable for the dishwasher environment. Polycarbonate exhibits excellent heat resistance, but this material is known for suffering breakage in a host of chemical environments due to ESC. Stress, either applied or residual from processing, in combination with the chemical environment of a dishwasher, can create ESC in the parts made of this material.

Hydrolytic attack is another important factor in designing dishwasher-safe plastic products. In a dishwasher, plastic products need to maintain shape, appearance, integrity, and functionality when exposed to water, which can react adversely with a number of different polymers. Testing showed that damage from hydrolytic attack is a factor in combination with heat and, most significantly, applied and residual stresses.

Heat Resistance versus Residual Stress. Dishwasher-safe molded items must, of course, be able to withstand the heat of the dishwasher and maintain their shape over many cycles. Heat accelerates the rate of most chemical reactions, including hydrolysis. However, resistance to shrinkage, distortion, or other types of creep are not the only considerations. Dishwasher heat can affect a plastic product in several ways. First, it can exacerbate residual stress in the product. Secondly, heat can activate chemical reactions between plastics and foods, detergents, and, as already mentioned, water. These reactions can accelerate deteriorating effects and lead to ESC.

Applied Stresses. Although it is easy to assume that a part in the dishwasher is exposed to no applied stresses, this is far from true. Items are often tightly fitted in dishwasher compartments, creating stress on the part. These applied stresses can be added to residual stresses developed during processing, creating unexpectedly high overall levels of ESC risk.

Although consumers may be the ones who cause this stress, they still maintain a strong expectation of product performance wash after wash. A certain amount of stress in the day-to-day use of a product should be expected and countered by reinforcing the design of a product.

Again, this factor works in conjunction with other influencers. For example, testing has shown that under a 1% strain, polycarbonate can break after two to three cycles using a popular powdered detergent—not an unreasonable “real-world” scenario by any means, and one that could result in product failure much earlier than consumers expect.

Residual Stresses. Depending on the material and required manufacturing processes, residual stresses can be generated from orientation effects during processing as well as thermal contraction during cooling. Some of the stresses developed can be compressive, and some can be tensile stresses, which are more likely to cause chemical attack. Stress that occurs during the molding process will affect how the product will stand up to the rigors of everyday use, including hundreds or thousands of dishwasher cycles throughout its lifetime.

Due to brand owner and consumer demand for more high-quality, modern aesthetics in everyday items, designers are creating commercial and residential kitchenware products in a greater variety of shapes and sizes. Individual shapes, processing parameters, and design features can create new stresses that can affect durability in a dishwasher. Matching material with product functionality and shaping needs is, therefore, very important. Due to their lower temperature ceiling,
copolyesters generally enjoy much lower levels of residual stress relative to many other plastics. However, this same lower temperature ceiling prevents use in most dishwasher applications since the heat in a dishwasher can cause distortion or warping. 

For dishwasher-safe applications, polycarbonate is a popular choice. Polycarbonate has good heat resistance, with a glass transition temperature (Tg) of 150°C (302°F). Products made from this material, however, also tend to have a high degree of residual stress. This can be partially addressed through a separate annealing process to remove some of the stress, which in turn increases energy use, slows production rate, and results in additional costs. Depending upon specific conditions chosen, however, a certain level of residual stress may still remain in the product.

Residual stresses in the material have implications beyond dishwasher cracking. The toughness of molded products is adversely affected by these stresses. Minimization of stresses increases durability of that part. Although polycarbonate is considered a tough polymer, durability can be compromised through the development of residual stresses.

Conclusion

New materials are on the horizon that will make designing for the dishwasher easier, changing the standards of plastic kitchenware performance (see Figures 1–3). By studying the interior of dishwashers and creating new materials to better survive its harsh environment, the plastics industry can provide designers and brand owners with greater design freedom and ease of processing while meeting consumer expectations for high quality and durability. 

To contact the authors, please e-mail lisa.bonnema@cancom.com.

Suppliers mentioned in this article:
Eastman Chemical Co.
 

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