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A Guide to Providing Perfect Coating Adhesion

几乎所有美学和功能性应用的漆膜都必须与所需的基材具有附着力. 相应的, 在制定一种能为预期应用提供可接受附着力的涂料时,必须考虑多种因素. Critical considerations and how they impact adhesion include:

  1. 表面润湿
  2. Mechanical effects and internal stress
  3. Surface chemistry and bond strength
  4. 色素沉着
  5. 评价粘连

1. 表面润湿 – The relationship between 表面润湿 而附着力是优化涂层附着力的首要考虑因素. If a 涂层 in a liquid state does not spread spontaneously over the substrate surface, 然后,与基板表面形成机械和化学键的机会是有限的.

如果液体的表面张力(力/单位长度或达因/厘米)低于被涂层固体的表面自由能,液体就会自发地在材料表面上扩散. 例如, 下面的图片显示了一滴液体在被润湿的表面上不同程度的润湿特性.

Figure 1 – Images of Various Degrees of Substrate Wetting

Image of degrees of substrate wetting - A Guide to Providing Perfect Coating Adhesion

相应的, in Table 1, when the Liquid Surface Tension (LST) is lower than that of  the Solid Surface Tension (SST), then wetting of the solid will occur. The greater this difference, the greater the opportunity the liquid has to wet and spread on the surface of the solid. 与大多数有机溶剂相比,水性涂料在表面上的铺展更加困难,因为水的表面张力相对较高.

相应的, to improve wetting of 水传播的 涂料, organic cosolvents and appropriate wetting agents are normally employed. 总之, when LST < SST, wetting occurs.

表1 -液体表面张力(LST)和固体临界表面张力(SST) (dynes/cm) @ 20°C

Table of surface tensions - A Guide to Providing Perfect Coating Adhesion

2. Mechanical adhesion and internal stress – The profile of the substrate the 涂层 is to be applied to also can affect adhesion. 光滑的表面更难以粘附涂层,因为表面积更小,涂层与基材联锁的面积更小. 然而, if a 涂层 is extremely rough, it can be difficult for a liquid 涂层 to wet and 穿透 surface crevices. This is illustrated in the diagrams listed below in Figure 2.

Figure 2 Surface interactions between a 涂层 and substrate

Image of surface interactions - A Guide to Providing Perfect Coating Adhesion

图B中的微观表面轮廓将比图A提供更好的附着力,因为涂层提供了更大的机会与基板联锁. Surface C has pockets and pores that are not easily 穿透d by the 涂层, 导致气穴可以捕获水分和可溶性离子,导致水泡和腐蚀(如果基板是可氧化的metal),从而导致长期附着力差,最终导致膜失效.

总之, from a mechanical adhesion standpoint, 低表面张力和低粘度的液体涂层有助于促进更好的润湿和微观渗透(毛细管作用). Adhesion can also be adversely affected by 强调 that occur as a result of shrinkage as a 涂层 dries or cures. Environmental effects over time such as exposure to moisture, 光, 热, pollutants and thermocycling also play an eventual role to degrade adhesion.

3. Surface chemistry and bond strength  In addition to surface tension and surface profile of the substrate, 可用的基片官能团可提供与涂层组分共价和氢键结合的位点,以进一步提高与基片的粘结强度.

Table 2 – Adhesive bonding forces

Table of adhesive bonding forces - A Guide to Providing Perfect Coating Adhesion

As Table 2 illustrates, the highest bond strength to the surface is provided by 共价键, 如所提供的,例如双功能三烷氧基硅烷偶联剂在涂层和metal表面之间的反应.

大多数 metal surfaces are supplied with a thin layer of oil to slow the rate of oxidation. The oil also lowers the surface energy and thus is more difficult to wet. 因为这个原因, metal surfaces -例如 steel, 镀锌钢和铝-通常在涂漆前进行清洗以去除油,然后进行预处理以形成, 例如, a zinc phosphate or iron phosphate treated surface. The phosphate groups serve to enhance adhesion of the 涂层 through 氢键 of the metal surface to reactive sites on the polymer.

Figure 3 Example of Hydrogen bonding to a metal surface pretreated with Zn.磷酸

为mula of 氢键 - A Guide to Providing Perfect Coating Adhesion

活性组 在聚合物的脊骨上或通过添加二聚物或含有环氧树脂的多功能粘附促进剂, amino or silane functional coupling groups can further react with a suitable pretreated metal 表面形成共价键,在metal和涂层之间提供额外的粘附强度.

为 玻璃或石英 丰富的表面, 偶联剂,如氨基硅烷,也可以通过与含有环氧基团的树脂主链反应,使偶联剂的烷氧基功能硅烷部分结合到,从而增强附着力 硅 surface to form a siloxane.

塑料 因为它们的表面自由能较低,而脱模剂的存在可能会进一步降低它们的表面自由能,所以更不容易潮湿. 通过紫外线照射增加聚烯烃的表面自由能,可以提高对聚烯烃的附着力, once a photosensitizer is applied, or flame 治疗 that generates hydroxyl, carboxyl and ketone groups.

塑料表面上的这些官能团提供了更高的表面能,以改善涂层上聚合物官能团的润湿和氢键位点. 提高对热塑性塑料附着力的其他方法是在涂料中加入适当的溶剂,使塑料表面溶解,并使涂料在塑料-涂料界面上混合.

4. 色素沉着 -底漆中使用的颜料的水平和类型不仅影响涂层基材的附着力, but also how long it will adhere to the surface. 大多数底漆都是在或略低于临界颜料体积浓度(CPVC)的条件下配制的,以最大限度地提高面漆的附着力(更粗糙的底漆表面和更高的自由能)以及许多其他涂层性能(图4).

CPVC对涂层性能的影响表-提供完美涂层附着力的指南

使用更极性的颜料可以在颜料分散过程中提供容易润湿, 但可能会降低长期的附着力,因为它们更容易受到水分在涂层-基体界面的迁移和分离的影响. 板状色素和含有极低或不含水溶性成分的色素也能延长寿命.

Method A and B - A Guide to Providing Perfect Coating Adhesion

5. 评价粘连 有多种方法来确定和量化有机涂层对基材的附着力. Two of the most common means of determining adhesion include ASTM D3359 (Cross Hatch Tape Adhesion) and ASTM D4541 (Pull-Off Adhesion). ASTM D3359描述了两种测定交叉口胶带附着力的方法:方法A是简单的X, where method B is a lattice pattern. Method A is used in the field and for films > 5 mils, whereas Method B is used for lab determinations. Ratings are as illustrated below:

分类是根据用专用胶带去除的交叉舱口的面积来划分的,包括:

5B (no area removed) > 4B (less than 5%) > 3B (5 – 15%) > 2B (15 – 35%),1B (35 – 65%) and 0B (greater than 65%)

ASTM D4541 (Pull-Off Adhesion利用一种装置来测量粘在涂层表面的小车的拉力. 该装置以每平方英寸磅为单位确定剥离涂层所需的力. This not only quantifies the amount of force required to pull off the 涂层, but also the type of failure (cohesive or adhesive), how and at which layer the 涂层 fails (topcoat to primer, primer to substrate etc.).

PosiTest AT-A - A Guide to Providing Perfect Coating Adhesion
PosiTest AT-A Automatic Adhesion Tester (SOURCE: DeFelsko)

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