Slot die coating is a complex process and achieving a stable thin film coating requires a deep understanding of the physics behind the deposition technique. Two types of defects may occur:
Defect due to instability of the meniscus of the coating bead as the coating process exits the stable coating window. A change in the coating parameters will result in a return to the stable coating region.
Or it can be categorized as an external factor related to fluid transport, substrate motion, or viscoelasticity of the solution. These deficiencies often require changes to the coating system or fluid to overcome the problems.
The following sections look at two types of defects and present common problems, characteristics of these defects, their sources, and methods that can be used to eliminate them.
chatter
Chatter is a defect that exists across the width of the coating. This defect appears either at the same point in the coating or periodically. feature is:
A line of varying thickness compared to the rest of the coating.
A line where a defect becomes more prominent compared to the rest of the coating.
Defects that occur at uniform intervals or frequency.
The origin of this defect is due to any of the following reasons:
Fluid delivery systems with pressure or flow rate changes
Substrate rolls/linear stages with pulsating motion or defects on the roll
Pressure change in upstream vacuum box
fluid delivery system
If the flutter defect is caused by the fluid delivery system, the cause is usually due to the pulsating flow of the solution. Piston pumps rely on the movement of discrete volumes of solution. This causes chatter defect frequencies to depend on the rate at which these discrete elements move.
For rotary pumps, this is a function of system speed. In contrast, with other positive displacement pumps based on linear motors, such as syringe pumps, it depends on the step rate of the motor. This can be mitigated by switching to a solution that delivers discrete volumes at a faster rate, either through higher RPMs or higher microstepping rates.
Pulse dampening elements can also be added to the solution feed to smooth the output of the positive displacement pump. Metal tubing can be replaced with plastic tubing, which expands and relaxes as it delivers the new fluid—effectively eliminating pulsation.
Substrate stage
Phase chatter defects depend on the type of system used. In roll-to-roll processes using drums, the shape of the drum or the motor driving the drum can be the source of defects. By checking the distance of the defect to the circumference of the roller, it can be determined whether the origin of the defect is due to the roller.
For linear stages (typically used for sheet-to-sheet deposition), chatter can be dependent on imperfections in the stage surface or a problem with the motor step rate. If the defect appears at the exact same location on the substrate, the problem may be due to a localized defect on the stage surface. For defects that occur periodically throughout the length, chatter may be due to the motor.
vacuum box
In some systems, a vacuum box is incorporated into the upstream lip of the slot die head to overcome the minimum thickness limitation. Changes in the vacuum chamber background pressure will cause changes in the stability and orientation of the coated beads. Changes in background pressure can be caused by issues such as:
chamber leak
Problems with using a Vacuum Pump
Relating the frequency of defects in the coating to changes in vacuum pressure can be difficult because these may not always be regularly spaced if they are due to leaks.
ribbed
Ribbing defects occur at regular intervals along the length of the coating and across the width of the coating. Rib features are:
Along the length of the coating, the thickness of the film is reduced compared to the rest of the film
Usually consists of multiple lines spanning the entire width of the coating
Some ribbing may appear as a single line defect. These ribs have a different origin than multiple ribs
Ribs occur when the upstream meniscus recedes towards the slot die exit. This may be due to:
High shear forces, due to fast substrate speeds moving the dynamic contact point downstream
Low pressure at slot die exit due to low viscosity or high gasket thickness
Reduced upstream pressure due to larger gap between substrate and slot die head
Localized imperfections in the head or die feed create vortices within the flow
Shear force and flow pressure
The position of the meniscus is due to the balance between the shear forces at the substrate-liquid interface and the pressures associated with flow through narrow channels. By balancing the two, the menisci are stabilized. Coating beads can be returned to a stable coating window by reducing shear or increasing flow pressure. This can be done with:
Reduce the speed of the substrate to reduce shear.
Increase the flow rate of the solution to increase the flow pressure.
Reduce the distance between the upstream lip and the substrate to increase flow pressure.
If the above parameters cannot be changed, other methods can be used to reduce the presence of ribbing. These methods are:
Reduce gasket thickness to increase pressure at tank outlet.
Increase the viscosity of the solution to increase the flow pressure.
Add a vacuum box to increase the upstream pressure gradient.
Since these methods require shutting down the coating process, they should be considered options for eliminating ribbing defects.
local defect
Sometimes the presence of ribs is due to local imperfections on the slot die head - these may be on the lip, or in the feed trough - and may cause the formation of local eddies which will cause a local drop in flow velocity. These defects can be caused by damage to the slot die head due to improper handling, material buildup in the slot die feed, or poor design of the slot die head. Damage to slot die heads can be repaired by polishing the surface to remove scratches. To reduce aggregation, the system needs to be cleaned and the solution reconstituted.
neck constriction
Neck defects occur at the edges of the coating length. Neck defects are characterized by:
在基材的移动长度上逐渐减小薄膜的涂层宽度
薄膜边缘的增厚，随着涂层宽度的减小，边缘处的厚度增加变得更加突出
颈缩缺陷的起源是由于：
从唇部和基材之间的受限溶液流向活塞流的过渡导致流动动力学的变化
流速和基板速度之间的不匹配会导致流体加速和剪切力
剪切力导致涂层珠粒向中心收缩，导致边缘处的流速更高
流动动力学的变化
从约束通道流向活塞流的变化随着唇缘和基材之间的间隙高度的增加而发生。当溶液不能再与缝模唇形成弯月面时（当缝涂过渡到幕涂时），就会发生这种情况。间隙越大，发生的缩颈程度越高 - 因此降低间隙高度可以减少缩颈缺陷的出现。
溶液加速
溶液流速和涂层基材速度之间的差异会导致溶液沿基材的行进方向加速。这导致剪切力的形成，导致帘幕或槽珠的边缘向涂层中心后退，从而导致颈缩。如果这是原因，降低涂层网的速度可以减少缩颈的出现，此外，较小的垫片厚度将增加溶液离开间隙的速度，从而在基材和溶液之间实现更好的速度匹配。
解决方案收缩
这可能会受到解决方案属性的影响。例如，具有高粘度和高表面张力的溶液通常会发生较高程度的缩颈。这些溶液类型在溶剂分子之间具有很强的相互作用。对于具有高表面张力的溶液，添加表面活性剂有助于减少这些相互作用。
边缘缺陷
边缘缺陷可能出现在涂层的所有边缘。然而，当在卷对卷系统中持续运行时，涂层的前边缘和后边缘不被视为问题。边缘缺陷的特点是：
涂膜边缘的厚度变化，通常表现为边缘区域的增厚
涂层边缘位置的变化。对于前缘和后缘，它们通常是弯曲的而不是直的；对于侧边，这些可以沿着涂层的长度进出
边缘缺陷的起源是由于：
涂层开启或关闭之间的过渡是导致前缘和后缘边缘缺陷的主要原因
溶液的表面张力和粘弹性能导致湿膜在沉积和干燥阶段移动
基材上不同区域的表面能差异会改变溶液润湿表面的方式
后缘和前缘过渡
后缘和前缘缺陷的主要原因是涂层珠粒可以稳定和不稳定的速率。通过更改处理步骤以在末端有目的地使珠子不稳定，可以很好地定义该边缘。这可以通过停止溶液流向头部来完成 - 或者甚至通过将溶液拉回来从珠子中收回溶液。也可以增加基材的速度以快速剪切涂层珠粒。对于更出色的系统，槽模头可以从表面缩回，以快速增加槽模和基板之间的距离。结合所有这些方法可以产生非常明确的涂层边缘 - 特别是在处理间歇涂层时。
溶液特性
在较低粘度下，溶液的表面张力主导着这种边缘增厚。通过降低表面张力和改善溶液在基材上的润湿性，边缘增厚可以保持在几毫米的范围内。对于高粘度溶液，表面张力对边缘缺陷的影响很小，改变溶液的表面张力不太可能提高边缘质量。对于溶液发生压缩和膨胀的粘弹性材料，当压力下降时，槽模出口末端会发生膨胀。这会导致涂层宽度和厚度的变化。
不幸的是，消除所有边缘缺陷非常困难，并且需要复杂的工程。一些方法的例子是：使用空气喷射去除珠子，使用刮刀剪切厚区域，或在缝模出口边缘引入溶剂以稀释珠子边缘的溶液。通常，涂层边缘缺陷的可接受尺寸约为几毫米，许多基材通过具有可去除的牺牲涂层区域来考虑这一点。
表面处理
湿膜可以固定在特定位置，以改善涂层边缘的清晰度。这可以通过在需要润湿薄膜的特定位置处理基材表面来完成。一种方法是使用紫外线臭氧处理来降低基材在特定位置的表面能。这将导致处理区域优先润湿，从而形成明确的涂层位置。
通常，边缘缺陷只是处理大规模生产时的主要问题。在小规模原型制作中，通常使用牺牲区域（以允许存在边缘缺陷）。然而，在卷对卷或片对片工艺（吞吐量高）中，较厚的区域将具有较慢的干燥时间，如果这些区域仍然潮湿，可能会导致设备污染。在卷对卷系统中，较厚的区域可能会导致卷筒纸无法正确缠绕 - 当在生产线下游使用接触印刷方法时，涂层区域的中心可能不会像外边缘那样接触.
条纹
条纹是湿膜涂布技术中的常见特征，并且可以由多种来源产生。条纹缺陷的特点是：
源自一个点并继续向上游延伸的单个线缺陷 - 看起来类似于彗星。缺陷的下游面上出现材料堆积，而上游出现一条厚度减小的线
沿涂层长度延伸的具有减小厚度的细长线
条纹缺陷的来源是：
基材缺陷例如阻碍溶液流动的灰尘或污垢颗粒，形成源自污垢颗粒所在位置的条纹缺陷。
狭缝模头或模唇处的障碍物会导致厚度减小的延伸线，甚至是未涂层区域。这些障碍物要么是槽模进料内的聚集物，要么是大到足以被夹在唇部和基板之间的灰尘颗粒。
狭缝模头的损坏会导致在薄膜内形成不可逆条纹缺陷，这是由于该特定位置的涂层珠粒不稳定。
基材缺陷
基材表面存在灰尘和污垢是湿膜涂布技术不可避免的问题。彻底清洁基材将去除污垢和灰尘。然而，空气中的灰尘颗粒会随着时间的推移重新污染基材。通过在洁净室环境中工作可以减少发生这种情况的机会，其中灰尘颗粒的存在被最小化。在线工艺通常在基材到达缝模涂布机之前集成高压空气喷射器或橡胶擦拭器，以吹走或推走表面的灰尘颗粒。
阻塞
在存在大灰尘颗粒且槽模唇口和基材之间的间隙与这些颗粒的大小大致相同的区域中，可能会阻塞上游或下游唇口。这导致涂层珠粒的不稳定和条纹的形成。可以使用上面列出的步骤来减少和去除基材上的这些灰尘颗粒，但是这些步骤永远不会 100% 成功。其他方法（例如快速振荡的缝模头）可用于去除这些被捕获的颗粒。
形成条纹缺陷的另一个障碍源是狭缝模头涂布机的进料槽内的障碍物 - 通道宽度较低。在这里，溶液中形成的任何可能的聚集体都可能被困住并扰乱下游溶液的流动。如果聚合成为您正在使用的解决方案的一个重大问题，您可能需要重新配制墨水以减少聚合的存在。减少这些阻塞机会的简单方法是增加通过头部的流速（以减少停留时间），或增加槽模通道宽度。更出色的方法可以包括在槽模头上添加一个加热元件，或者一个内部刮板，它可以快速穿过进料通道的整个宽度，以清除被困的聚集体。
阻塞的来源之一是在溶液到达槽模头之前溶液中存在夹带的空气。这种夹带的空气会导致气泡的形成，这些气泡可能会被困在进料槽中以及唇缘和基材之间。为帮助防止这种情况，溶液应在进入系统之前彻底过滤和脱气。此外，应检查用于传输溶液的连接器和适配器是否存在任何潜在泄漏。
槽模头损坏
虽然槽模头由坚固的材料（如不锈钢）制成，但在关键区域（如槽模唇部）可能会损坏头
部。槽模头应小心处理和存放，以减少发生任何损坏的机会 - 尤其是唇口和进料通道。如果确实发生任何损坏，可以使用低粒度研磨膜去除小划痕。对于头部的大凹痕，可以对该区域进行铣削以平滑受损区域 - 或者可能需要完全更换头部。
气泡
气泡可以出现在涂层中任何点的薄膜中。气泡缺陷的特点是：
圆形或椭圆形的薄膜缺陷。这些缺陷的大小可以变化。
出现气泡缺陷的薄膜的厚度会明显变薄，或者可能不含任何材料。
沿涂层宽度的缺陷位置是随机的。
气泡的来源是：
原始溶液中的空气夹带进入泵送系统。
流体输送系统内的泄漏导致空气滞留。
由于上游或下游弯液面不稳定而截留空气。
空气夹带
空气夹带是在溶液进入溶液计量系统之前将空气截留在溶液中。在溶液制备过程中，这可能由于多种原因而发生，例如将溶液加载到泵送容器中，或者通过不正确的设置和用溶液清洗管线。对于高粘度溶液，气泡的存在通常被夸大，因为它们的寿命更长。此外，低表面张力会导致更多气泡的形成——尤其是在使用表面活性剂产生这些低表面张力的情况下。
溶液可以在放入流体输送系统之前脱气。如果存在气泡缺陷问题，则在将溶液装入流体泵储液罐时应格外小心。溶液计量系统应保持在低于槽模组件的高度，并且应避免管道弯曲，以减少起初冲洗管线时气泡的滞留。
管线泄漏
当管线内可能发生泄漏时，它们会在溶液进入槽模头之前将气泡引入溶液中。这些泄漏有可能发生在使用连接器和适配器的地方。检查所有这些是否密封 - 使用 PTFE 胶带（用于密封螺纹）有助于保持紧密密封。
弯月面卡压
When the coating process exits the stable coating region, the position of the meniscus will shift towards the upstream and downstream menisci. Since the position of these menisci is quite dynamic, the menisci can oscillate (in localized areas) between stable and unstable. This causes air to become trapped within the wet film, creating bubble defects. By moving the coating process deeper into the stable coating window, these random fluctuations in meniscus position—and thus the presence of bubble defects—can be significantly reduced. In general, the ratio of gap height to thickness should be reduced in order to return the process to a stable coating window. This can be achieved by moving the slot die coating lip closer to the substrate, reducing the web speed,
epilogue
Slot die coating is a powerful processing technique for depositing highly uniform thin films through scalable deposition processes. Uniform films can be deposited over very large areas by adjusting the geometric parameters of the slot die head, solution properties, and processing parameters. Since the process is a pre-metered deposition technique, the thickness of the sample is determined simply by varying the flow rate of the solution or the speed of the web - while keeping the process within a stable coating window.
