油漆 films for nearly all aesthetic and functional applications above all else must provide adhesion to the desired substrate. 相应的, one must take into account multiple considerations when formulating a 涂层 that provides acceptable adhesion for the intended application. 关键的考虑因素及其对附着力的影响包括:
1. 表面润湿 -两者的关系 表面润湿 and adhesion is the first factor to be considered in designing a 涂层 to optimize adhesion. 如果一种液体状态的涂层不能自发地覆盖在基材表面, then there is limited opportunity to form mechanical and chemical bonds with the substrate surface.
A liquid will spread spontaneously on the surface of a material if the surface tension (force/unit length or dyne/cm) of the liquid is lower than the surface free energy of the solid to be coated. 例如, the image below provides a visualization of various degrees of wetting properties for a drop of liquid applied onto the surface to be wet.
因此，在表1中，当 液体表面张力(LST的值比的值低 固体表面张力(SST)，则固体会润湿. 这种差异越大, 液体就有更大的机会湿润并在固体表面扩散. Waterborne 油漆s have a more difficult time spreading on surfaces due to the relatively high surface tension of water in comparison to most organic solvents.
相应的, 改善水性涂料的润湿性, 通常采用有机助溶剂和适当的润湿剂. 总之, when LST < SST, wetting occurs.
Table 1 – 液体表面张力(LST) and Solid Critical Surface tension (SST) (dynes/cm) @ 20° C
2. 机械粘附和内应力 -涂层要应用到的基材的轮廓也会影响附着力. Smoother surfaces are more difficult for 涂层 adhesion as the surface area is lower and provides less area for the 涂层 to interlock with the substrate. 然而, 如果涂层非常粗糙, 液体涂层很难弄湿和穿透表面裂缝. 下面的图2中列出了这一点.
The microscopic surface profile in sketch B will provide better adhesion than that in sketch A as the 涂层 provides greater opportunity to interlock with the substrate. 表面C有不容易被涂层渗透的口袋和气孔, resulting in air pockets that can trap moisture and soluble ions resulting in blisters and 腐蚀 (if substrate is an oxidizable metal) and thus poor long-term adhesion and eventual film failure.
总之, 从机械粘附的角度来看, liquid 涂料 with low surface tension and low viscosity help promote better wetting and microscopic pe净ration (capillary action). 附着力也会受到不利影响 强调 由于涂层干燥或固化而收缩的结果. 长期暴露在潮湿环境中的影响, 光, 热, 污染物和热循环也最终起到降解粘附的作用.
3. 表面化学和粘结强度 – 除了表面张力和基材的表面轮廓, available substrate functional groups may provide sites for covalent and 氢键 to the 涂层 components to further enhance the adhesive bond strength to the substrate.
如表2所示，表面粘结强度最高的是由 共价键, such as those provided 例如 the reaction of a dual functional trialkoxy silane coupling agent between the 涂层 and the metal surface.
大多数 metal 表面涂上一层薄薄的油，以减缓氧化速率. 油还降低了表面能量，因此更难以润湿. 因为这个原因, metal表面——例如钢, zinc coated steel and aluminum- are normally cleaned prior to 油漆ing to remove oils and then pretreated to form, 例如, 磷酸锌或磷酸铁处理过的表面. 磷酸基的作用是增强涂层的附着力 氢键 将metal表面的活性部位转化为聚合物上的活性位点.
活性组 on the polymer back bone or through the addition of a di or multifunctional adhesion promoter containing epoxy, 氨基或硅烷的功能偶联基团可以进一步与适当的预处理反应 metal surface to form 共价键 that provide added adhesive strength between the metal and the 涂层.
为 玻璃或石英 丰富的表面, coupling agents such as amino silanes can also serve to enhance adhesion by reacting with a resin backbone containing an epoxy group with the alkoxy functional silane portion of the coupling agent bonding to the 硅 表面形成硅氧烷.
塑料 are more difficult to wet as they have a lower surface free energy that may be further lowered by the presence of mold release agents. Adhesion to polyolefins can be improved by increasing their surface free energy through UV irradiation, 一旦使用光敏剂, 或火焰处理产生羟基, 羧基和酮基.
These functional groups on the plastic surface provide higher surface energy to improve wetting as well as 氢键 sites for polymer functional groups on the 涂层. Other ways to improve adhesion to thermoplastics is to include an appropriate solvent in the 油漆 to solubilize the plastic surface and enable intermixing of the 涂层 at the plastic-涂层 interface.
4. 色素沉着 – The level and type of pigment used in a primer not only affects 涂层 substrate adhesion, 还包括它能在表面附着多久. 大多数 primers are formulated at or s光ly below Critical Pigment Volume Concentration (CPVC) to maximize topcoat adhesion (rougher primer surface and higher free energy) as well as many other 涂层 properties (Figure 4).
The use of more polar pigments may provide ease of wetting during the pigment dispersion process, but may degrade long term adhesion as they are more susceptible to moisture migration and disbondment at the 涂层-substrate interface. Plate like pigments and pigments that have very low or no water-soluble components also enhance longevity.
5. 评价粘连 There are multiple ways to determine and quantify the adhesion of organic 涂料 to a substrate. 最常用的两种测定粘连的方法包括 ASTM D3359(交叉舱口胶带粘附)和 ASTM D4541(拉脱附着力). ASTM D3359 describes two methods to determine cross hatch tape adhesion: method A is a simple X, 方法B是一个晶格模式. Method A is used in the field and for films > 5 mils, 而方法B用于实验室测定. 评级如下所示:
Classifications are by area of the cross hatch removed by specialized adhesion tape and include:
5B (no area removed) > 4B (less than 5%) > 3B (5 – 15%) > 2B (15 – 35%),1B(35 - 65%)和0B(大于65%)
ASTM D4541(拉脱附着力) utilizes a device to measure the Pull Off Strength of a dolly glued to the surface of the 涂层. The device determines the force required to disbond the 涂层 in pounds per square inch. 这不仅量化了拉开涂层所需的力的大小, 还有失败的类型(粘性或粘性), 涂层如何失效，在哪一层失效(从面漆到底漆, 底漆到衬底等.).