Electrostatic elimination in plastic film production

In the production and application of thin films, static electricity has always been one of our headache problems. Friction, impact and even tension changes may cause static electricity, which may not only affect the quality of printing and bag making, but also cause safety problems. In many years of production and technical services, the author has deep feelings on the following issues, and I will exchange and discuss with you here.

●How friction generates static electricity.

The friction between the film and the metal roll is very small in production, so why do they also produce static electricity? The principle of electricity tells us that strictly speaking, friction electrification is only a form of contact electrification. One of the main reasons for static electricity is that when the two materials contact, some electron clouds overlap. When rapidly separated, one of the materials has a large ability to adsorb electrons (or other charged particles, the same below), it is possible to transfer electrons to the material, obtain electrons and show negative charge, while the other material shows positive charge. In addition, in nature, there are piezoelectricity (piezoelectricity) generated when applying force to an object and thermoelectricity generated when heating an object. Friction electrification is the comprehensive result of these effects.

●Does tribostatic electricity only occur between different materials?

A shocking fact is that static electricity can also be generated when two ice blocks of the same shape rub against each other. In January in Beijing, the author rubbed left and right according to the following figure, using the C point of a popsicle to rub all areas of B popsicle for 3 minutes. The test with eld-1 static voltmeter shows that a negative charge with high voltage appears at point C of rod a, while a positive charge with low voltage appears in the whole friction area of rod B.

The above friction is called 'asymmetric friction' in physics. Analyze the cause of static electricity. Although the work done to the two ice during friction is the same, for rod B, the temperature rise caused by friction heat is not obvious because of the large friction area, while for rod a, the temperature at point C increases relatively because the work is relatively concentrated. The rise of temperature brings about the activity and ionization of molecules - positively charged hydrogen ions and negatively charged hydroxyl ions, among which hydrogen ions are relatively active and easy to dissociate from the C point of a rod with high temperature to the vast area of B rod with low temperature, so that B rod shows positive charge, on the contrary, C point of a rod shows negative charge.

Using two polypropylene rods for the above tests, static electricity is also generated, but the electricity is opposite to that of ice.

According to the research data of Kyoto University in Japan, after two polyethylene films with the same shape are symmetrically rubbed against each other, accurate tests can find that there are many tiny positively charged points and large negatively charged areas on them. The reason is the local asymmetric friction caused by the micro unevenness of the polyethylene surface, resulting in different friction heat and temperature of each part, resulting in the uneven distribution of active charged groups.

Therefore, the mutual friction of the same materials will also lead to static electricity.

●Key electrostatic tests.

Electrostatic test includes the determination of basic electrostatic parameters, such as electrostatic voltage, electric quantity, capacitance, and the determination of material properties, such as surface resistivity and volume resistivity. For the users of plastic film production, except that some electrical packaging needs to measure the surface resistivity and volume resistivity (with a megger), general users can test the static voltage on the surface of the film.

The surface static voltage test of non-conductor such as plastic film (including the test of electrostatic polarity and potential data), because the probe cannot contact the film, is called non-contact test. According to different test principles, it can be divided into two methods: electrostatic induction and air ionization. Electrostatic induction is to test the electric field on the surface of the film by using the distorted electric field generated between the probe and the film when the test probe is close to the film. Air ionization is to use radioisotopes to ionize air, generate resistance partial voltage between the film and the input end of the instrument, and between the input end and the grounding end, and test the voltage of the film to the ground.

Table 1 Comparison of characteristics of two kinds of electrostatic voltmeters

可以看出，在车间使用感应式静电电压表时，其与薄膜的位置、薄膜面积相对固定才能降低 误差， 即所谓相对测试； 而空气电离式电位计应该在静止场合使用，并进行数据补偿。

常用静电消除器的比较：

静电消除器分无源式和有源式两类，后者又分为高压电源式、离子风式、放射源式等几种， 因为都需要另加电源，加大能耗， 高压电或放射源的安全因素，很少在塑料加工中使用。

塑料薄膜生产应用厂家， 过去一般使用无源式静电消除器中的硬毛静电刷（防静电毛刷）， 现在， 许多要求高的厂家纷纷选择一些新的静电消除器，如软毛静电刷等。

以硬毛静电刷为例，消电机理主要因为导体的静电感应，在靠近薄膜处产生与薄膜负电极性 相反的正电荷， 通过空气电离，形成电晕电流，在电晕区产生大量的正、负离子。如图3所 示，在电场的作用下，带负电的粒子向放电毛针 B 方向运动；带正电的粒子向带负电的薄 膜 C 方向运动，中和薄膜带的负电荷。由于针尖 B 附近的电场依赖于薄膜 C 本身的电位及 针尖到薄膜的距离，所以，薄膜 C 的电压越高，针尖 B 到薄膜 C 的距离越近，针尖 B 上感 应出的电荷密度越大，附近形成的电场越强，电离的带电离子数目越多，消除静电效果越好 （但太近时易产生电弧）。可以想象每一根刷毛就像伸向空中的避雷针一样， 要尖而细，才 能更好的形成感应电荷集中，早一步放电达到消除静电的目的。考察这种除静电毛刷性能主 要有两个指标： 临界电压和电晕电流。前者是能够使放电针产生电晕放电的最低电压，当然 是越低越好； 后者越大，表明单位时间内被消除的电荷越多。实际应用中，用万用表测试静 电刷的电阻即可，一般质量好的静电刷电阻小于1Ω/米，也可以用 ELD-1静电压表测试静电 刷安装前后的电位来判断。决定除静电毛刷的性能有多种因素，放电毛针针尖越细长、材料 导电性越好制成的除静电毛刷性能越好。

由于硬毛静电刷消电效果取决于薄膜静电压的感应电压，消电效果不彻底， 所以人们做了改进，把接地的 A 处加上与薄膜静电电荷性能相反的直流高压电源，如正电  荷，使消电效果大大提高，这就是直流消电器的原理。

如果把上图中 B 处硬毛换成对薄膜无损伤的软毛，则可以直接接触薄膜 C；因为软毛的“柔”， 产生静电感应后因为电荷的吸引力毛尖部分可以紧紧吸附在薄膜上，负电荷可以通过软毛静 电刷直接导入大地，消电效果也大幅度提高。据测算，TR-01接触式软毛静电刷的临界电压 比普通硬毛静电刷低80%左右， 电晕电流则大30%以上。

表2  薄膜常用消电器的特点比较

●不可忽略的问题， 消电器的安装

以硬毛静电刷为例，它的安装和性能一样重要，安装不妥，不仅会降低消电效果，有时还会 使静电加剧，甚至造成更大的危害。一般来说，安装如图5所示， 应避开图中 A 、B 、C 、D 几个位置。最好在需要消除静电部位的前面，如尽量靠近印刷和制袋前的部位或静电电位最 强的部位。安装角度垂直于薄膜运动方向，毛刷应有牢固的支撑并可靠接地。TR-01软毛静 电刷安装要在薄膜的正反面都要安装，否则前面消除后面又感应，事倍功半。

说明： 黑色三角处表示可以安装，白色三角位置不能安装，因为： A 处有摩擦的静电产生源， 前面消除后面产生，使消除工作事倍功半。B 处背面有接地体，C 处临近接地体，D 处有其 他消电装置，均可以干扰电场感应，使其达不到应有的消除效果。

试验结果表明， 使用正确的安装方法，软毛静电刷最多时可以消除95％以上的静电。