A Guide to Asbestos-Free Compressed Gaskets

In the industrial sector, the choice of gasket material is crucial for ensuring the sealing performance of equipment. Asbestos gaskets once dominated the market due to their excellent load-bearing capacity, resistance to aging, compressive strength, and high-temperature endurance. However, with the growing awareness of environmental protection and the exposure of health risks, asbestos-free compressed gaskets have gradually become the mainstream choice in the market. This article will delve into the development, manufacturing processes, and engineering applications of asbestos-free compressed gaskets, helping readers gain a comprehensive understanding of this environmentally friendly and efficient sealing material.

Asbestos gaskets were once the go-to material for gaskets due to the unique properties of their fibers. These fibers not only had excellent load-bearing capabilities but also maintained stability in high-temperature and corrosive environments. However, not all asbestos gaskets possessed the same level of superior performance. The quality of the fibers in high-grade asbestos gaskets was critical, with 80% or more high-quality fibers directly affecting the gasket's performance. To reduce costs, manufacturers often mixed fibers of varying qualities, leading to performance discrepancies. Ordinary-grade asbestos gaskets had lower fiber quality and content. Although their physical properties might resemble those of high-grade products in some cases, their performance in high-temperature environments was vastly different.

Over the past 10 to 15 years, the use of asbestos gaskets in the United States has plummeted, primarily due to concerns about the carcinogenicity of asbestos. Despite this, asbestos gaskets are still used globally for some critical sealing issues because they are the only material that can ensure reliable operation. However, due to the decline in demand, many manufacturers are no longer willing to take the risk of producing high-grade asbestos gaskets.

Against the backdrop of global efforts to ban asbestos for environmental protection, asbestos-free compressed gaskets emerged. These new gaskets, primarily made of rubber and reinforced with fibers, have been widely used and accepted since the early 1980s. However, the transition to asbestos-free gaskets was not without challenges. Initially, manufacturers faced significant difficulties as the production of asbestos-free gaskets increased costs for end-users. Moreover, one type of asbestos-free gasket could not fully replace an asbestos gasket. Typically, multiple styles were needed to meet various industrial requirements. This was because synthetic or natural fibers, as substitutes, often could not match the performance of asbestos fibers.

Currently, the most commonly used alternative fibers include aramid (aromatic polyamide fibers) and carbon fibers. Compared to asbestos gaskets, asbestos-free gaskets contain very little fiber, making the design considerations for asbestos-free gaskets more complex. In asbestos gaskets, one material (such as asbestos fibers) could perform multiple functions, whereas in asbestos-free gaskets, 10 or more materials (such as aramid fibers, various fillers, etc.) might be needed to achieve the same effect. This presents a significant challenge for manufacturers in designing gasket materials with optimal performance and a wide range of applications.

The general process of manufacturing compressed gaskets has remained largely unchanged over the past few decades, with the only differences being in the manufacturing techniques and raw materials. Currently, there are two main methods for manufacturing compressed gaskets: the layer-by-layer pressing method and the pulp molding method. Since the vast majority of industrial compressed gaskets are manufactured using the layer-by-layer pressing method, this article will focus on this technique.

The first step in the layer-by-layer pressing method is to mix rubber, fibers, various fillers, and solvents to create a dough-like mixture. This "dough" is then placed into the gap between two large rollers rotating in opposite directions. One of the rollers is hot (with a temperature between 105 and 115°C), while the other is cold (with a temperature between 15 and 38°C). As the rollers turn, the "dough" thinly adheres to the hot roller and builds up layer by layer. By continuously feeding the "dough" and rotating the rollers, the desired thickness is achieved. Chemicals are added during the mixing stage to ensure that the "dough" is evenly kneaded and to prevent layering in the pressed material. The size of the final pressed sheet depends on the width and circumference of the hot roller. Most pressed sheets made using this method are 60 inches wide, as this width is easier to manufacture. However, a few companies worldwide can produce compressed sheets as large as 3 square meters.

The manufacturing process of asbestos-free sheets is slightly different from that of asbestos fiber sheets. The formula for asbestos-free sheets may contain up to 30 components, while that for asbestos sheets may have only 10 components. Therefore, the manufacturing process for asbestos-free sheets is more complex. The interaction of the many additives requires the use of complex equipment with hydraulic, pneumatic, and precise monitoring, rather than the old-fashioned gear and lever-driven machinery.

When designing flanged piping systems, engineers must consider some key properties of compressed gasket materials, such as temperature resistance, chemical resistance, and the gasket coefficient. For compressible asbestos-free materials, these three properties must be directly obtained from the manufacturer, as characteristics like material strength are determined by the specific formulas used by each manufacturer. Manufacturers are usually willing to analyze based on specific information and offer suggestions. However, sometimes manufacturers do not have all the information about a project, resulting in the production of a "one-size-fits-all" composite gasket. While this gasket can be barely used in various situations, it is "at a loss" when it comes to selecting gasket materials for special purposes.

The gasket coefficient is one of the most familiar design criteria for engineers, but it is also the least understood. Years ago, ASTM established a procedure for determining the gasket coefficient, through which the ASTM-recommended M and Y values could be obtained. When this procedure was created, almost all gaskets were made of compressed asbestos material. Since high-quality compressed asbestos gaskets contained at least 80% asbestos fibers, different gasket products could be categorized into several gasket coefficients, which was acceptable for flange design. However, after the development of asbestos-free gasket materials, the procedure for determining the gasket coefficient became inaccurate due to the significant differences in coefficient values for the same type of gasket produced by different manufacturers. Most manufacturers have modified their procedures to make their M and Y values acceptable for engineering design. Nevertheless, differences in M and Y values still exist, and from a design perspective, there is still much work to be done.

Currently, the issue with M and Y values is that they are not easily reproducible in different laboratories. About 20 years ago, ASME began developing a more convenient gasket coefficient calculation procedure. Since most engineers are trained to calculate the gasket coefficient based on M and Y, it will take some time for the new gasket coefficient to be accepted by engineers. The promotion of the new ASME gasket coefficient calculation procedure will lead all manufacturers to adopt the same procedure, which means that even if this coefficient is not used for design, it can enable more precise comparisons between competitive products, provided there are no other issues.

In engineering design, tensile strength is an important performance indicator. However, for compressible asbestos-free materials, tensile strength is not a significant indicator of sealing performance. In contrast, when comparing compressible asbestos gasket materials, tensile strength is an important factor that cannot be ignored, as the quality of compressed asbestos gaskets is proportional to the quality and strength of the asbestos fibers. In the asbestos-free field, it may be more appropriate to regard tensile strength as a quality control indicator. Manufacturers can use this indicator to quickly determine whether the manufacturing process is carried out as designed. For compressible asbestos-free products, the actual difference between 1500 psi and 2000 psi is not important unless the operating conditions are close to the upper limit of the product. If tensile strength is truly important, the wise choice would be to consider switching to another product that can provide a higher safety factor.

With the increasing awareness of environmental protection and health risks, the market demand for asbestos-free compressed gaskets will continue to grow. Manufacturers are constantly improving the formulas and manufacturing processes of asbestos-free gaskets to enhance their performance and reduce costs. At the same time, engineers are also working to better understand and apply the gasket coefficient to ensure the accuracy and reliability of their designs. In the future, with the advancement of technology and changes in the market, asbestos-free gaskets are expected to play an important role in more industrial applications and become the preferred choice for sealing materials.

In summary, the emergence of asbestos-free compressed gaskets represents a significant advancement in the field of industrial sealing materials. They not only meet the requirements of environmental protection and health but also provide reliable sealing performance. By gaining a deep understanding of their manufacturing processes and key factors in engineering design, we can better utilize this material to ensure the safe operation of equipment and environmental protection.