The crystal structure of the composite material fundamentally guarantees the performance of the product, and the tensile process is an improvement of the existing PTFE molding process, which can fundamentally change the crystal structure and morphology of PTFE.
Therefore, when preparing modified polytetrafluoroethylene gasket sealing materials, the correct selection of tensile process parameters is very important. When selecting the drawing process parameters, it is necessary to consider the influence of drawing rate, drawing ratio, drawing temperature and other factors on the performance of the product.
Classification of drawing process:
1, one-way stretching
In practical applications, unidirectional stretching can improve the performance of polytetrafluoroethylene in the tensile direction, but the degree of improvement is still limited. However, the unidirectional drawing process is simple and can be used in the test.
In the polytetrafluoroethylene structure obtained by stretching at high temperature and high speed, nodes with uniform spatial structure can be obtained. These nodes are connected with a large number of polytetrafluoroethylene fibers to form a high-quality network, and tensile strength can be increased at high temperature and high speed.
2. Bidirectional stretching
Bidirectional stretching includes longitudinal stretching and transverse stretching, at a certain temperature and a set speed, at the same time or step by step in the vertical two directions (vertical, horizontal) of the stretching, after the appropriate heat treatment, the comprehensive performance to meet the needs of practical applications.
In the process of bidirectional stretching polytetrafluoroethylene sheet, the arrangement of molecules and chain segments is changed because the polymer has undergone a certain stretch in both vertical and transverse directions.
Therefore, the main properties of the stretched sheet have significant changes compared with the non-stretched sheet. The mechanical properties, creep resistance, resilience and flexibility are obviously increased.
3, room temperature stretching and high temperature stretching
The drawing of the sheet can be carried out in the range from room temperature to close to the melting point of PTFE (327 ° C). High temperature stretching is generally carried out between 250 and 300 ° C.
Generally speaking, when the temperature is low, the tensile ratio and the tensile rate are limited to some extent, which is determined by the mechanical properties of the polymer. When the temperature is high, the polymer chain is easy to deform, and the tensile ratio and speed can be increased correspondingly.
4. Drawing ratio and drawing rate
The drawing ratio is expressed as the ratio of the length of the specimen after drawing to the length of the specimen before drawing. With the increase of the tensile ratio, the tensile strength, elongation and flexibility of the product increase, and the apparent density decreases, so improving the tensile ratio is beneficial to the performance of the product.
But the tensile ratio is too large, the product is easy to break, forming difficult. The tensile ratio is mainly affected by resin type, drying temperature, product cross-sectional area and so on. Usually the drawing ratio should be selected between 2 and 7.
The tensile rate is generally not 1m/min, if a higher tensile strength is required, the tensile speed should be increased.
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