How to distinguish PTFE, EPTFE, RPTFE?

In HG flange non-metallic gaskets, PTFE is mainly divided into three categories, namely: PTFE sheet PTFE, expanded polytetrafluoroethylene sheet, or with EPTFE, filled modified polytetrafluoroethylene sheet RPTFE. In the HG standard, the use temperature range of PTFE is narrow, only -50~100℃, while the use range of EPTFE and RPTFE is relatively wide, which is -200~200℃.
Teflon tape can also be used as a filling material for spiral wound gaskets for NB container flanges, HG pipe flanges, etc. However, in the NB container flange standard, the use temperature limit of the spiral wound gaskets filled with PTFE is the highest, reaching 260℃, which is higher than 200℃ of other standards, including HG American standard, European standard flange standard, SH container, and pipe selection standard, and HG/T 20583-2011 appendix A and B, see several screenshots at the end of the article.

1. PTFE sheet
According to 3.1.2 of the new national standard GB/T 5257-2018 “Polytetrafluoroethylene (PTFE) Sheet” (replacing QB/T 3625-1999), PTFE sheet can be divided into the molded sheet and turned sheet according to the process.
The molding equipment is simple and the production cycle is short. However, for large plates, the press mold is larger, and the production site requires a large space. Therefore, a large area of ​​dustproof work must be carried out. In addition, the preformed plates are extremely easy to break before entering the sintering furnace. Should be handled with care.
The molding process of large-scale molded plates: raw material inspection → crushing and screening → metering → molding → semi-finished product inspection → sintering → cooling → finished product inspection → packaging.
Polytetrafluoroethylene sheet has the advantages of good chemical stability, excellent dielectric properties, low friction coefficient, non-stickiness, etc., but it has a cold flow tendency (cold flow: the distortion of a solid under pressure, which cannot be recovered when the pressure is removed The original size of the solid distortion), should pay attention to the use temperature and other use conditions.

2. Expanded polytetrafluoroethylene
The crystals of PTFE dispersed resin are arranged in folded chains. Under the action of a certain temperature (less than 327℃) and stretching rate, the folded molecular chains are pulled apart into a fibrous structure, and the fibrous molecular chains intersect to form a fiber node. The pores between are micropores. This kind of expanded product that contains a large number of micropores in the product through the stretch forming process is EPTFE. (Ie expanded PTFE)
Due to a large number of micropores, the relative density is significantly reduced. Generally, the relative density of PTFE products is about 2.15, while the relative density of expanded PTFE products can be less than 0.1, which means that the porosity has reached about 96%. Because ePTFE has a large number of micropores, it has new functions without losing the original excellent performance, that is, separation function, softness, elasticity, and sealing.

3. Filled modified polytetrafluoroethylene RPTFE
(At present, the commonly used modification methods of PTFE can be divided into surface modification, filling modification, blending modification, chemical modification, structure modification, etc. RPTFE is also called reinforced polytetrafluoroethylene, Reinforced PTFE)
Filling modification is to improve and overcome the defects of pure PTFE by filling inorganic, metal, and organic polymer fillers in PTFE resin. On the basis of maintaining the original advantages, the composite effect is used to improve the pressure resistance, wear resistance, and cold flow.
There are four main methods for adding modified fillers: one is simply dry mixing the filler and resin, the process is simple, but the mixing uniformity is relatively poor; second, the filler is added to the lubricant to form a lubricant suspension of the filler, Then add the suspension to the dispersing resin and mix well; the third is to disperse the filler and resin in water or other solutions at the same time, after fully mixing, agglomerate and dry; the fourth is to add the filler to the dispersed PTFE emulsion, after fully mixing, the resin Coagulates with filler.
The mechanism of filling modification is that some adsorption points and some chemical points of action can be formed on the polymer chain on the inner and outer surfaces of the filler particles, and then a network structure such as chemical interlinking network, entanglement network, and filler polymer network can be formed. By changing the PTFE crystal structure And the morphology and structure play a modified role.
However, although filling modification greatly improves the physical and mechanical properties of pure PTFE, the filler also more or less affects the media resistance, electrical properties, and friction properties of the original PTFE. For example, metal-filled PTFE is not suitable for applications in electrical and certain chemical fields; inorganic filler-filled PTFE, such as glass fiber filling, is not resistant to hydrofluoric acid, carbon fiber filling is easy to oxidize; the outstanding feature of organic filler-filled PTFE is that it does not damage the mating friction surface, Easy to machine processing, good wear resistance, and heat resistance. Therefore, when selecting fillers, the required properties should be highlighted as much as possible, and at the same time, the negative effects of the fillers should be minimized as much as possible to the material application.