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Failure Case of Daily Used Pyrex Pot Glass CookwareMahfodzah Binti Essay

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Paper type: Essay

Failure Case of Daily Used Pyrex Pot Glass Cookware

Mahfodzah Binti Md Padzi, Maisarah Binti Mohd Bazin, Mohamad Adib Hakim Bin Mohd Affandi

[email protected], [email protected], [email protected]

Mechanical Engineering Department, Universiti Kuala Lumpur Malaysian France Institute

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Abstract:

In Consumer Reports articles, TV documentaries, complaints to the U.S. Consumer Products Safety Commission and Internet posts, the shattering of Pyrex Pot glass cookware in household kitchens has been reported [8]. This paper examines the issue from a failure case of daily used Pyrex Pot glass cookware by reviewing the reported incident scenarios that suggest thermal stress fracture. In this work, failure mode, fracture type, failure location, failure mechanism, cause of failure and suggested prevention method is discussed.

Keywords: Pyrex Pot, Borosilicate Glass, Failure Case

I. INTRODUCTION

A. Background

Pyrex pot as is a glass cookware product originally manufactured by Corning Inc. which made from low thermal expansion ceramic of borosilicate glass [1]. Many peoples refer the name Pyrex with the original borosilicate glass products. The initial Pyrex cookware was marketed as cookware “oven to icebox” or “icebox to oven,” [2] probably because the low thermal expansion coefficient of the borosilicate glass made it highly resistant to the thermal stresses that evolve during these temperature variations.

Corning holds the Pyrex name, but in 1994 the company started to license other companies to produce products under the Pyrex brand. Today, World Kitchens LLC [3] produces the Pyrex brand for consumer markets in the U.S., North America, South America and Asia under a Corning license. For the European, Middle East and African consumer markets, a separate company, Arc International [4] manufactures and markets Pyrex brand cookware. The Anchor Hocking Glass Company [5] is independently making its own line of glass cookware, and has been doing so under its own brand names for many decades.

Over the past two decades, the explosion or shattering of glass cookware has emerged as an issue, and accounts of issues have been chronicled in several news stories. The accumulated complaints collectively suggest that some glass cookware products may have a fracture problem. None of the coverage, however, dealt specifically with the scientific aspects of the reported failures. This paper looks at the technical aspects of the sudden explosion such as glass cookware consumer failure.

B. Material Properties

Smedskjaer et al. (2014) [6] in his study stated that Pyrex pot or borosilicate glass is a heat-shock resistant material that contains about 80% silica, 4% sodium oxide or potassium oxide, 2% alumina and 13% boric oxide. This type of glass is about three times as heat-shock resistant as soda-lime glass and is excellent for chemical and electrical uses. Thus, it makes possible for the borosilicate glass to be applied such products as cookware, ovenware and laboratory equipment. Table 1 and Figure 2 shows the mechanical and thermal properties and the stress-strain curve graph of borosilicate glass respectively.

Table 1 Mechanical and Thermal Properties of Borosilicate Glass [6].

Property

Value

Mechanical

Density

2.23 g / cm3

Hardness

418 Knoop

Modulus of Elasticity

60 – 64 GPa

Fracture Toughness

0.770 Mpa

Poisson’s Ratio

0.20

Thermal

Coefficient of Thermal Expansion (CTE)

4.0 x 10-6 / oC

Thermal Conductivity

1.35 W / mK

Specific Heat

0.185 cal / g oC

Maximum Working Temperature

500 oC

Figure 2 Stress–Strain Curve for Borosilicate Glass [6].

II. RESULT AD DISCUSSION

A. Failure Mode

The failure mode of Pyrex pot or borosilicate glass is thermal stress fracture. This can be proved where Read (2014) [9] on his study of Glass Cookware Failure Analysis: Pyrex where he claimed that the borosilicate glass Pyrex pot failed as a result of “thermal shock”. In addition, Consumer Reports (2011) documented and reported that the borosilicate glass cookware shattering incidents is due to the glass in consideration of thermal stresses.

B. Fracture Type

The fracture type of Pyrex pot or borosilicate glass is brittle due to its shattering condition when failed. Also, the Pyrex pot breaks suddenly instead of deforming plus the fracturing or breaking occurred with only small amount of shock (thermal shock).

C. Failure Location

Refer to Read (2014) [9] study where he stated that there were multiple origins for the failure, and these all initiated at damage sites on the bottom of the Pyrex pot. Figure 3 shows the fracture origin of the Pyrex pot.

Figure 3: A reconstructed soda lime silicate Pyrex bowl fractured by thermal shock. Arrows outline the crack paths [9].

D. Failure Mechanism

As described in Introduction to Ceramics, by Kingery et al. (1976) [7] delayed thermal stress fractures will often occur after temperature changes. This is because the maximum thermal stress is achieved only as a temperature gradient develops after the temperature change. Some of the glass cookware products are recorded to break immediately after temperature change, while other cookware fractures occur within a short time after the cookware with its contents were removed from the hot oven. The mechanism of thermal stress failures is fractures that occur at a time interval after a temperature change, such as removing the cookware from a hot oven and placing it on a wet countertop.

E. Cause of Failure

When the hot Pyrex pot was put on a cold surface, the thermal stress caused tensile stress at the bottom of the pot, causing many pre-existing damage sites to fail to start as shown in Figure 4 and Figure 5.

Figure 4: Assembled Pot Shows Several Failure Origins at Bottom Part [9].

Figure 5: Test Conducted by Consumer Reports shows bake ware glass shattering after being heated in a 450oF oven and placed on a wet countertop [9].

F. Suggested Prevention Method

The failure of the Pyrex pot is due to the thermal shock of the temperature interval. Therefore, to prevent the Pyrex pot shattering, consumer needs to avoid sudden temperature change to the Pyrex glass cookware pot by allowing the hot glassware Pyrex pot to cool by itself. Table 2 shows the Pyrex Glass Use and Care recommendations to avoid the glass cookware failure or shattering.

Table 2: Pyrex Glass Use and Care Recommendations [6].

No. Recommendation

1. Avoid sudden temperature changes to glassware. DO NOT add liquid to hot glassware; place hot glassware on a wet or cool surface, directly on countertop or metal surface, or in sink; or handle hot glassware with wet cloth. Allow hot glassware to cool on a cooling rack, potholder or dry cloth. Be sure to allow hot glassware to cool as provided above before washing, refrigerating or freezing.

2. Oven must be preheated before inserting glassware.

3. DO NOT use on or under a flame or other direct heat source, including on a stove top, under a broiler, on a grill or in a toaster oven.

4. Add a small amount of liquid sufficient to cover the bottom of the dish prior to cooking foods that may release liquid.

5. Avoid handling hot glassware (including ware with silicone gripping surfaces) without dry potholders.

6. Avoid microwave misuse. DO NOT use glassware to microwave popcorn or foods wrapped in heat-concentrating material (such as special browning wrappers), heat empty or nearly empty glassware in microwave, or overheat oil or butter in microwave (use minimum amount of cooking time).

III. CONCLUSION

In conclusion, even though many researchers and manufacturers claimed that Pyrex pot or borosilicate glass has superior mechanical strength yet the failure still occur several times. This is because due to the thermal stress where the pot experience sudden temperature change. Tensile stress will occur resulting from the thermal shock made the pot shattering due to brittle fracture which initiated at damage sites on the bottom of the Pyrex pot. Instead of having failure due to thermal stress, it can be prevented by avoid sudden temperature change to the Pyrex glass cookware pot by allowing the hot glassware Pyrex pot to cool by itself.

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REFERENCES

[1] R.C. Bradt and R.L. Martens, Shattering glass cookware, American Ceramic Society Bulletin, Vol. 91, No. 7, 2011.

[2] M.B.W. Graham and A.T. Shuldinier, Corning and the Craft of Innovation, pp. 55–58. Oxford University Press, Oxford, UK, 2001.

[3] World Kitchens, Rosemont, Ill.

[4] ARC International Cookware SAS, or ARC International Cookware Ltd., France.

[5] Anchor Hocking Glass Co., Lancaster, Ohio.

[6] Smedskjaer, M. M., Youngman, R. E., & Mauro, J. C. (2014). Principles of Pyrex® glass chemistry: Structure-property relationships. Applied Physics A: Materials Science and Processing, 116(2), 491–504.

[7] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics; pp. 816–844. Wiley, New York, 1976.

[8] “Glass Bakeware that Shatters,” Consumer Reports, 44–48, January (2011).

[9] Tom Read, Glass Cookware Failure Analysis: Pyrex, 2014.

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