Experimental investigation of the mechanical properties of PTFE scrims under high temperature conditions

Highlights

• Directly test the mechanical properties of PTFE scrim under thermal conditions.

• Stretching results of PTFE scrim at hot state and traditional test is compared.

• The stress-strain curve of PTFE scrim changed significantly by temperature.

• The mechanical properties of PTFE scrim deteriorates severest at high temperature.

• Explore the parameters affecting PTFE scrim with grey system theory.

Abstract

Polytetrafluoroethylene (PTFE) scrim has become one of the most commonly used high-temperature flue gas filter materials due to its well-recognized performance in high temperature conditions. In practical engineering applications, some failure cases of PTFE scrim have been reported in the industry. However, few studies have been carried out to investigate the mechanical properties of PTFE substrate under such similar conditions. In this study, six common PTFE scrims were selected and subject to tensile tests at a temperature ranging from 25 °C to 260 °C. The test results show that the stress-strain curve of the PTFE scrim changes significantly with increasing temperature and PTFE scrim undergoes different stretching stages. Excessive elongation affects the service lifetime of the filter material. The elongation at break of the PTFE scrim increases slightly with increasing temperature. In the whole temperature range, the strength decreases significantly with the increase of temperature. Especially when below the glass transition temperature, the strength of the PTFE scrim decreases almost linearly with the increase of temperature. When the temperature reaches 260 °C, the strength retention of PTFE scrim is only 20% of that at 25 °C. This study also evaluates the influence of the factors affecting the strength retention of PTFE scrim by grey correlation analysis. The single yarn diameter and basic weight have the greatest impact. This finding will provide guides and insight for the PTFE scrim manufacturing industry in the aspect of optimizing the mechanical properties of product under high temperatures.

Introduction

If a large number of fine particles produced by the industry are directly discharged into the atmosphere without any effective control during the manufacturing process, serious haze weather and environmental problems will appear. With the continuous advancement of industrialization process, China has recently suffered from this problem [1], [2], [3]. Particularly, for the heavy industry, the high-temperature flue gas is the main source of air pollutants, in which dust particles are the main component. To effectively control dust particles, in the most heavily polluting industries, bag dust collector or baghouse, has been widely used to filter exhaust flow and remove dust, given its high dust removal efficiency and adaptability to even very harsh working conditions such as high temperature and high humidity [4], [5], [6], [7].

The reliability of the baghouse depends greatly on the performance of the filter bag [8], [9], [10]. The filter bag is mainly made of needle-punched filter material and the scrim. The scrim is located in the middle layer of its sandwich structure, which is a warp-weft interwoven fabric woven by staple yarn or long fibers on a loom [11]. The scrim plays an important role to maintain the three-dimensional structure and strength of the filter material [12]. The effect of with or without the scrim as a reinforcing material has been studied on the needle-punched filter material. The results show that the scrim could increase the bursting strength, thickness and filtration efficiency of filter material [13]. The scrim also can reduce the chance of size deformability of the filter material, improve its stability, and enhance the mechanical strength [14], [15].

Polytetrafluoroethylene (PTFE) scrim is one of the most common filter scrims especially for those being used for the hot gas filtration. PTFE has extraordinary chemical inertness, due to the strong bond between fluorine atoms and carbon atoms [16]. Additionally, the combination of strong bonds, a protective sheath and non-polarity make PTFE extremely chemically inert and thermally stable [17], [18], [19], [20], so that PTFE scrim can be used in the harsh conditions. Arunangshu et al. examined the degradation behavior of PTFE/P84 filter media during hot gas filtration and proved that the filter with PTFE scrim had a better tensile strength [21]. Currently, the research on PTFE scrim focuses mainly on the effect of corrosive substances in high-temperature flue gas on the performance of filter media, and the performance of PTFE composite filter media in a high temperature [22], [23], [24]. Nevertheless, few studies have been conducted on the effect of high temperature on the mechanical properties of pure PTFE filter media. Although PTFE is often regarded to be durable even at 260 °C [25], the malfunction of filter bag elongation and breakage has been reported during its actual engineering use. Fig. 1 shows the excessive elongation of original filter bags with the same length in the plant during use. With respect to the final application in hot gas filtration, any small change in elongation is of special importance for safe applications in hanging filter bags [27].

Therefore, it is necessary to study the mechanical properties of the scrim used for medium and high temperature flue gas filtration. In previous researches, the test of mechanical properties of the filter material at high temperature was carried out after being heated to a certain temperature for a period of time first and then cooling to normal ambient temperature. According to previous research, the impact of different thermal tensile test sequences on the mechanical strength of PTFE sewing varies greatly. At 230 °C, the strength retention rate of the samples that were cooled to normal temperature after heating was about 80%, while the strength retention rate of the experimental group that was directly stretched in the hot state was only 10% [26]. Therefore, the original tests cannot practically reflect the impact of the actual application conditions on the mechanical properties of the PTFE scrim.

In this study, to more practically reflect the impact of actual application conditions, a series of tests on the mechanical properties of PTFE scrim were directly performed in high temperatures ranging from 25 °C to 260 °C, without undergoing any cooling phase. In addition, to comprehensively evaluate the influence of the different factors affecting the strength retention of PTFE scrim, six different kinds of PTFE scrim commonly used for high-temperature filtration were subject to the tests. This study will contribute to gaining more fundamental understanding of the performance of the scrim in the hot temperatures, and providing insight to guide the improvement of product performance in the industry.