This article is to introduce the influence of temperature on the coefficient of friction of materials. As well the testing condition for coefficient of friction test of film is also discussed in this article.
1. Coefficient of Friction
The coefficient of friction is a dimensionless scalar value which describes the ratio of the force of friction between two bodies. The coefficient of friction of the film surface depends on the adhesive property (surface tension and crystallinity), application of additives (slip additive, pigment, etc.), and surface polishing condition. In the following operation procedures e.g. when the film crosses the free roller, the shape of the bag is formed, the product is wrapped by cling film or when the packaging bags or other containers are stacked, the coefficient of friction of the material should be strictly controlled. Apart from the internal factors, environmental factors (such as running speed of the instrument, temperature, static accumulation and humidity) may also influence the test result of coefficient of friction.
2. Influence of Temperature on Coefficient of Friction
In view of dynamics, the state change of molecular motion of high polymer is called laxation. When the temperature rises, on one hand, the thermal movement ability of moving units could be improved, and on the other hand, because of thermal expansion, the space between molecules is increased so the free volume within the high polymer becomes larger which speed up the molecular movement and shorten the duration for laxation. Accompanied with the laxation of high polymer, the thermal property, visco-elastic property and other physical properties would be changed greatly. As one of the regular mechanical indexes, coefficient of friction could also be influenced by the increase of temperature. Generally, with the rise of ambient temperature, the coefficient of friction of the material surface would be changed. Such change varies for different materials.
3. Elevated Temperature Test
To verify the influence of temperature fluctuation on coefficient of friction, the following tests are performed with several representative materials at different temperatures.
Apparatus: Labthink i-COFTEK 3500 Friction & Peel Tester (Temperature could be controlled within the range of room temperature -99.9°C)
Test Specimen: PC film and aluminum foil complex film
Test temperature: room temperature~90.0 °C
It could be seen from Figure 1 that the rise of temperature obviously influences the coefficient of friction of material. But for different specimens, with the rise of temperature, the effect of influence is different. And for the same specimen, the variation tendency of kinetic coefficient of friction is very likely to be different from that of static coefficient of friction. For example, for PC film, the growth trend of static coefficient of friction with the rising temperature is rather steady. But the variation tendency of the kinetic coefficient of friction fluctuates greatly. However, the overall variation trend of kinetic coefficient of friction is also increasing. As for aluminum foil complex film, growth trend of coefficient of friction is not as obvious as that of PC film. And its kinetic coefficient of friction remains almost stable with the rising of temperature. It should be noted that when the test is performed above 74 degrees Centigrade, obvious stick-slip occurs in the process of test. The slippage is intermittent and not smooth.
4. Actual Application of Coefficient of Friction
In actual packaging process, friction force often acts as both dragging and resistant force. Therefore, it is necessary to control the coefficient of friction effectively within a proper range.
Generally the coefficient of friction of the inner layer of the roll material is required to be low, while the coefficient of friction of the outer layer of the roll material is required to be moderate. If the coefficient of friction of the inner layer is too low, mismatch of sealed side may occur when making bag. The coefficient of friction of outer layer should be proper. If it is too high, the resisting force in packaging process would be increased while if it is too low, the roll material may slip which may cause inaccurate operation. The content of slip additives used in inner layer, rigidity and the smoothness of the film would influence the coefficient of friction of the complex film.
In study of coefficient of friction, the influence of temperature should be paid special attention to. As is mentioned above, different temperature will cause great change in coefficient. Measured data should be obtained through actual tests. The coefficient of friction of certain material may present obvious growth or reduction with the changing of temperature. It may also maintain data stability.
As the actual temperature in practical operation of the production line is difficult to be maintained around room temperature, the coefficient of friction test of packaging materials should be tested at both room temperature and actual ambient temperature in practical operation. The ambient temperature could be changed when the test temperature is close to the room temperature. Automatic temperature control function could be applied as well. The test temperature could be controlled automatically within the range of room temperature to 99.9°C which is convenient for test operation.
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