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粉末涂装摩擦和电晕两种方法的优劣
 
  0引言
  粉末静电涂装基本可分为两种,一种是电晕枪(CORONA),另一种为摩擦枪(TRIBO)。但在国内常用电晕粉末涂装,由于对摩擦枪接触较少,对其的报道也很少。
  摩擦枪与电晕枪相比,优势在于能够得到比较均一的涂层,无法拉第效应,也没有大量的负离子产生。但其缺点也比较明显,即一次上粉率涂装质量要求、工厂的实际条件等因素。
  1摩擦枪原理
  与电晕枪不同,摩擦喷涂时粉末涂料在摩擦喷枪管内带电,其电荷不是来自于电极尖端高压电晕放电,而是由于粉末与枪壁内的特殊高分子材料[通常是聚四氟乙烯(PTFE)和尼龙]碰撞、摩擦、接触、离合所产生的,也就是利用摩擦起电原理使粉末带电的。
  对粉末与PTFE材料的摩擦的机理,人们还不能认识得十分清楚,但可以假设其中存在材料的转移。当二种材料A和B相互摩擦,小块的材料碎落,若其中一种材料比另一种电势更低且坚硬,就像粉末带电摩擦系统材料的转移产生电荷的转移。由于材料B的电势比A低,导致负电荷向离B最近的A迁移。若摩擦中产生负电荷的碎块破落,则材料A带正电荷而材料B包括粘附着粉末涂料碎片的材料B带负电荷。
  在粉末离开摩擦枪后,无外加电场的作用,其驱动力只是气流动力,同时产生了微弱的法拉第效应,使得粉末更易进入几何形状复杂的区域,与电晕枪相比,没有大量的自由离子产生。所以无论是薄涂还是差。所以在选择涂装方式时,需综合考虑工件形状、摩擦枪带电性取决于负电荷的及时移走和正电荷的稳定化。负电荷的及时移走与喷枪的接地效果有直接关系,而正电荷的稳定化就需要选取适当的枪壁摩擦材料以及对粉末粒子进行改性。
  2影响摩擦枪上粉率的因素
  人们对摩擦枪本身曾作了大量的研究,其基本思路在于改良枪壁材料和改变摩擦管内部的几何构造。但从粉末涂料的配方和制作工艺来研究,更具有现实意义。
  2.1摩擦带电助剂
  PTFE的介电常数为2.1,也就是说,任何比其介电常数高的物质,与其摩擦后均可得到正电荷,但在粉末涂料中的聚酯树脂的介电常数仅为3.0,两者的介电常数相差太小,所以没有明显的摩擦带电性。。
  在涂料中添加季胺盐、磷酸盐和三乙胺[3]能增加粉末的带电性,但它们能与环氧基团反应,而且反应比较剧烈,从而降低了涂膜的稳定性。改性双氰胺、封闭异氰酸盐类(如Hüls公司的B1530、B1400等)、三聚氰二胺树脂、草酸苯胺类和羟烷基酰胺类等不但是成膜物质,而且对提高正电荷的稳定化也有不同寻常的作用。
  使用位阻胺类化合物作为摩擦带电剂,对粉末涂层无副作用。Zbigniew等研制的以IPDI(异佛尔酮二异氰酸酯)改性的低聚位阻胺(Eltribo)的添加量为1%~3%(质量分数)。CibA公司的TINUVIN900、TIN-UVIN110FD和TINUVIN144用途更为广泛。以TINU-VIN110FD为例,试验显示,摩擦带电剂添加到约1.5%后,粉末荷质比基本稳定,也就是说再增加摩擦带电剂对提高摩擦带电性无明显效果。位阻胺类化合物如TINUVIN144等,在与TINUVIN405配合使用时,能极大提高涂膜吸收紫外光、抗黄变的能力。
  市售的CC-150(UCB公司)和TB-1(南海药化公司)是将摩擦带电剂负载在低酸值的聚酯以及与惰性填料的混合物上。而树脂制造商在合成树脂时就已经添加了摩擦带电剂,如用氨基聚酯改性的不饱聚酯和以空间位阻胺改性的环氧树脂,不仅可以提高粉末的贮存稳定性,而且粉末原料和摩擦带电剂的混炼效果会更好。
 2.2湿度
  SteveTrigwell[10]和RichKmoch[11]等还认为粉末涂料在使用时表面的物理和化学吸附物(如水分等)会对粉末的带电性产生不良影响。因此保持粉末喷涂车间的湿度也是比较重要的。而且压缩空气的露点温度不要超过1.67℃。
  与电晕枪一样,适量的后混添加剂不仅可以吸收粉末本身的水分而且可以提高粉末的流动性。ICI公司的研究也表明在粉末涂料中添加0.1%~0.2%亲水的Al2O3和SiO2的混合物能增加粉末的摩擦带电性。
  2.3粉末的粒径分布
  SteveTrigwell等人研究表明,除了粉末本身的性质以外,对摩擦带电影响最大是粉末的粒径。同时发现较细的粉末在摩擦带电时极有可能带负电荷,从而影响粉末整体带电性能。粉末太细,还会影响其流动性。由于粉末在离开摩擦枪后,主要是靠气流的推动,所以与斯托克斯数有关,即在一定范围内,粉末的粒子越粗带电性越好。MazumderM.K.等也认为随着粒径的增大,其所带电荷的荷质比也越大,如果粒径分布比较窄,易得到均一分布的电荷。但粉末粒子表面太粗糙时,与枪壁只有简单的碰撞,所带的电荷就会较少,同时表面会吸收大量的水汽和其他污染物,影响带电性能。若再粗糙的话就会影响涂膜的平整度了。所以一般摩擦枪用粉末的平均粒径控制在35~45μM为
  当然,粉末粒子的电阻性和流动性等也是影响粉末的重要因素,但在电晕粉末涂装中人们已经研究得比较透彻,在此就不一一赘述。
  3结语
  摩擦枪涂装已占据了粉末静电涂装领域内一定的市场份额。国内主要应用在铝型材和大型管道的表面涂装。但摩擦粉末的上粉率低,直接影响了喷涂厂家的产量和经济效益。只有掌握适合摩擦涂装的粉末涂料粒子的特性,如摩擦带电剂对粉末粒子的改性,湿度以及粒径分布等才能提高粉末涂料的摩擦带电性能。
 
 
发  布  者:  admin 添加时间:  2010-1-14 点  击  数: 268
 
     
 
 
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