Common curing methods for wood furniture coatings include natural curing and artificial curing (preheat curing, heat curing and radiation curing). It is generally room temperature curing type.
1. Natural curing
Natural curing is also called natural drying. It refers to the method of curing the coating under natural conditions without any artificial measures. This curing method is a method that has been used for a long time, and in most cases, it is placed on the spot in the finishing place to wait for curing. Coatings commonly used in the production of wood products (phenolic paint,
NC, PU) are room temperature curing type.
(1) Characteristics of natural curing method
1. The method is simple
The natural curing method neither requires any equipment nor complicated technology, and basically consumes no energy. Can make full use of the time and space after get off work.
2. Wide range of applications
This method is only suitable for coatings and other materials that cure quickly and do not produce serious volatile harmful gases. Such as water color, filling agent, shellac paint and so on. When there is no curing equipment and other conditions, this method can also be used for slow-curing paints such as alkyd paint and phenolic paint. But the venue must be clean. Nitro paint, polyurethane paint, acrylic paint, etc. will volatilize a large amount of harmful gas during curing, which will damage human health and easily cause fire, so it is better to place it in a special place for curing.
3. Slow curing
The natural curing coating takes a long time, the production efficiency is low, and it takes up a lot of space. Therefore, enterprises with large production volume or high production efficiency should not adopt this method.
4. Appropriate control of curing conditions
When curing naturally, it should be adapted to control the temperature, humidity and air circulation of the curing place. Generally, the temperature should be above 10 ℃, and the relative humidity should not exceed 70%.
, the air should be kept in circulation. There should be heating equipment in the winter workshop in the north.
(2) Curing method
1. Finish curing on site
Place the finished product on the spot and cure it naturally on the spot. There is an appropriate distance between the products, not less than 0.5m
;Parts can be placed on the shelf in such a way that the painted surface can fully contact with the air, and the distance between items should not be less than 25mm.
2. Natural drying room curing
Some volatile paints, such as nitro paint, alcohol-soluble paint, etc., will produce a large amount of harmful gas when the coating is cured, which is not good for the operator and is easy to cause fire. If the natural curing method is adopted, it should be cured in a special natural drying room instead of directly drying at the paint leaving work site. This kind of drying room should be able to keep warm so that the room temperature can be guaranteed in winter ( 20-25°C). At the same time, there should be a ventilation device to remove the volatile harmful gas in time to prevent fire.
2. Preheating and curing
The preheat curing method is to preheat the surface of the wood to be painted before the paint is painted. After the paint is painted, the heat accumulated by the wood is transferred to the coating, which will promote the volatilization of the solvent in the coating and the progress of certain chemical reactions. , to accelerate the curing of the coating.
The preheating method is very beneficial to the curing of the coating, because at this time the heat transfer is from the inside to the outside, that is, the direction from the wood to the coating is consistent with the volatilization of the solvent vapor. Figure 4-1 (a)
As shown, the circled arrow in the figure indicates the moving direction of solvent vapor,
Cross arrows indicate the direction of heat movement. Since the coating cures from the inside out, solvent vapors can escape from the coating without hindrance. Thereby shortening the curing time.
Methods for preheating wood are thermal contact, thermal radiation or thermal convection. The most common application is to heat wood in a thermal radiation preheating chamber. This thermal radiation preheating chamber has a simple structure, easy temperature control, and is economical and practical.
When the preheating method is used to cure the coating, the composition of the coating, the method of coating finishing and the temperature of the wood surface all have an impact on the leveling of the coating on the wood surface and the quality of the paint film. The preheating method is effective for quick-drying coatings. Slow drying paint, can only play a supporting role.
Preheat curing coating has obvious advantages, not only can shorten the curing time, but also can improve the quality of the paint film, because the viscosity of the coating drops immediately when it comes into contact with the hot wood, which helps to improve its leveling on the wood surface In addition, due to preheating, the air in the tube holes on the wood surface expands and is partially excluded, the phenomenon of foaming of the paint film is significantly reduced, and the film quality is improved.
3. Convection heating curing
The convective heating curing method is also called hot air drying, that is, the air is first heated, and then the coating is heated with hot air to cure the coating.
(1) Basic principles
Convection curing is a method of applying the principle of convection heat transfer to heat and cure the coating. It uses air as the heat carrier to transfer heat to the coating by convection, so that the coating is cured.
When the coating is cured by convection heating, the upper surface of the coating first contacts the hot air, and the heat conducts from the upper surface of the coating to the lower part of the coating. The curing of the coating starts from the surface and gradually expands to the inside. The place is finally dried. At this time, the direction of heat transfer is opposite to the direction of solvent volatilization. Therefore, a hard film is formed on the surface of the coating at the initial stage of curing, which hinders the volatilization of solvent vapor inside the coating, which slows down the curing speed of the coating. When the solvent vapor inside the coating is rapidly volatilized and discharged, breaking through the hard film on the surface may cause pinholes or bubbles on the surface of the paint film, which is a disadvantage of the convective heating curing method. In order to overcome the above shortcomings, the finished coating can be left at room temperature for a period of time before convection curing, so that most of the solvents can evaporate, and then further curing. According to the different volatile components in the paint, the aging time is generally
5 ~ 20min. In order to obtain a good paint film in production, a staged curing method is adopted. For example, when the lacquer coating is cured, the temperature is lower at the beginning, which is 20 to 25
℃, at this time, the solvent is violently volatilized, and then heated to 40-45 ℃, at this time, the solvent is no longer volatilized in a large amount, and finally it is lowered to 20-25
°C until the coating is cured. As for the length of each period, it needs to be determined according to the type of coating and the thickness of the coating.
(2) Features
1. Wide application and strong adaptability
Convection curing is a widely used method for wood product coating curing. It is suitable for curing the surface coating of workpieces of various sizes and different forms, not only for assembled overall wood products, but also for parts, especially for workpieces with complex shapes. When steam is used as the heat source, suitable curing temperatures are
Coating curing below 100 ℃; when natural gas and electric energy are used as heat sources, it is suitable for coating curing at various curing temperatures.
2. The coating cures faster
Convective curing can be many times faster than natural curing. For example, when the oily coating is cured, when the temperature is increased from 20 ℃ to 80 ℃, the curing time is almost 1/10
. Therefore, it is widely used in the production of furniture and wood processing enterprises at home and abroad.
3. The equipment is simple, easy to use and maintain, and the operating cost is low.
The convection curing method also has its disadvantages. When the wood coating is cured with hot air, the wood is also heated at the same time. As the temperature increases, the water in the wood evaporates, and the wood shrinks, deforms, and even cracks; the air in the wood duct expands when heated The escape may cause defects such as air bubbles and pinholes in the paint film; sometimes the color of the material will become darker due to heating. Therefore, the wood coating heating temperature should not be too high. In addition, convective heating and curing have low thermal efficiency and long heating time; the equipment is huge and occupies a large area.
(3) Hot air drying room
There are many types of hot air drying rooms. According to the operation mode, there are two types: periodic type and through type; according to the heat source used, it can be divided into hot water, steam, electricity and natural gas; according to the circulation mode of hot air in the room, it can be divided into two types: forced circulation and natural circulation. form. Periodic or pass-through forced circulation convection drying chambers are commonly used for curing coatings in the furniture industry.
1. Periodic drying room
Periodic drying chamber is also called end-type or dead-end type. It is generally a chamber type, and can be made into a single chamber type or a multi-chamber type. The periodical drying chamber is closed on three sides and only opens the door at one end. The products or parts to be dried are regularly sent in through the door, closed for drying, and then taken out from the same door after drying. This type of drying room takes a long time to load and unload, and the utilization rate is low, and it is mainly used in single-piece or small-batch production enterprises.
The periodic drying room can be divided into two types according to the way of hot air circulation.
2. Continuous drying room
The continuous drying room is also called the pass-through drying room. Both ends of this drying room have doors, and the workpiece to be painted is driven by the conveyor, enters from one end and moves to the other end, and the coating is cured during the movement. The movement method can be divided into two types: continuous and intermittent, and the latter moves a certain distance at regular intervals. The pass-through drying chamber usually forms several sections with different temperature, wind speed and air exchange rate, and the coating can be cured according to the alternating curing procedure. Commonly used workpiece transportation devices include mobile multi-storey trolleys, belt, plate, suspension and roller conveyors, etc.
The coating curing speed of the convective heating curing method is many times faster than that of the natural curing method, and it is a coating curing method commonly used in the production of wood products at home and abroad.
4. Radiation curing
Radiation curing is a method of curing the coating by irradiating it with various rays (infrared rays, ultraviolet rays, etc.). Depending on the radiation used, the coating cures in different ways. Divided into infrared radiation, ultraviolet radiation curing and so on.
（一）红外线辐射固化
1. 工作原理
利用红外线辐射被涂饰的工件及涂层后，辐射的能量被吸收并被转化为热能，从而固化涂层。
红外线是一种不可见射线，介于可见光和微波之间，波长在 0.72 ～ 1000 μ m
，人们按波长把红外区又划分为二或三部分，分别称为“近”、“远”，或者“近”、“中”、“远”红外线。
红外线有两个重要特性，一个是能穿过空间，称透射能力，另一个是射到物体上，容易使物体变热，称热效应。红外线加热，主要由红外辐射和红外吸收两方面组成，而各种物体对红外辐射的吸收能力是不一样的，就是同一物体对不同波长的红外线的吸收能力也不相同。对涂层固化过程来说，应选择能尽可能地透过涂层而很少透过木材的某种波长范围的红外线，使其刚好在涂层与木材的交界处转化为热能，从而使涂层由内向外地固化。
2. 特点
（ 1 ）固化速度快，生产效率高 与热空气对流干燥相比，固化时间可缩短 3 ～ 5 倍。特别适用于大面积表层的加热干燥。
（ 2 ）升温迅速，热效率高
辐射干燥不需中间媒介，可直接由热源传递到涂层，故升温迅速。它没有中间介质引起的热消耗，减少了部分热气带走的热量。用远红外线加热时，约 50%
左右的辐射能被涂层吸收， 45% 被基材吸收，只有 5%
被反射掉，所以其效率要高于近红外线固化，同时，远红外线几乎适用于所有涂料的固化（紫外线、电子束固化涂料除外）。因此热效率高。
（ 3 ）固化质量好
红外线能穿过涂层到达木材表面，使之变热，这时涂层下面受热首先开始固化，而表面的涂层仍然处于液体状态，下层的溶剂蒸汽可以自由地向上散发，可避免在漆膜表面形成气泡、针眼等缺陷。另外，红外线干燥不需要大量循环空气流动，飞扬的灰尘少，漆膜表面清洁。
（ 4 ）设备紧凑，占地面积小 红外线干燥设备结构上比热空气对流干燥设备简单、紧凑，故长度短，占地面积小。同时使用灵活、操作简单。
红外线干燥的缺点是由于红外线是直接传播，需要是直射被固化的涂层表面上才有效果，对于形状比较复杂的曲面上的涂层，则很难均匀的固化。另外，电能消耗很大。
3. 红外线干燥设备
（ 1 ）干燥室结构 红外线干燥室是由一定数量的红外线辐射器组成通过式干燥室。被涂饰的零部件或制品用传送装置运送，在干燥室中通过时使涂层固化。
（ 2 ）辐射加热器
辐射加热器又称辐射元件，是指能发射远红外线的元件。它是远红外干燥室中的主要设备。远红外辐射元件由远红外涂层、热源和基体（包括附件）组成，按辐射元件的形状有灯式、管式和板式等型式。
板式辐射器正面为碳化硅板，把电阻线夹在碳化硅板的沟槽中，背面设有保温盒，其内部填充保温绝热材料，外表涂一层远红外涂料。板式远红外辐射器适用于大面积的工件。装卸维修简便，不用反射板，如图
4 — 5 所示。
管式辐射器，如图 4 — 6 所示，内部有一电热丝，外面是一根无缝钢管，
空隙中填满结晶态的氧化镁粉，压实后具有良好的导热性和绝缘性，管壁外涂一层远红外辐射涂料，电热丝通电加热后，套管的表面就会辐射出一定波长范围的远红外线。管式辐射器适合于加热小型工件或形状不复杂的平板工件。
灯式辐射器外形似红外线灯泡，如图 4 — 7
所示，配有反光罩，将发出的大部分远红外线经反射罩汇聚成平行线后辐射出去，适合于形状复杂的工件的涂层固化。
（ 3 ）热源 热源的作用是给辐射元件提供足够的热量，使其辐射出远红外线。通常采用电、煤、蒸汽等作为热源，但应用最多的是电阻丝加热，即电热远红外线。
4. 远红外线辐射固化工艺
影响红外线固化的因素主要有辐射器表面温度、辐射波长、辐射距离和辐射均匀性等。
（二）紫外线辐射固化
1. 固化原理
紫外线是一种波长在 10 ～ 400nm 之间的电磁波。用于光敏涂料固化的主要是波长为 300 ～ 400nm
的紫外线，在光敏涂料中加有少量的光敏剂，在紫外线的照射下，光敏剂吸收特定波长的紫外线，分解产生活性基团，引发成膜物质的聚合反应发生，形成网状结构而使涂层固化。
现在应用的光敏涂料有两大类，即不饱和聚酯树脂和丙烯酸酸改性树脂（丙烯酸酸环氧树脂，丙烯酸酸聚氨酯），在它们的分子中有乙烯基不饱和键的结构。在涂料中加入光敏剂，在紫外线照射下，分解而产生活性基团，引发聚合作用，在涂料高分子之间进行交联反应，形成网状结构而使涂层固化。
2. 特点
（ 1 ）涂层固化快效率高
紫外线照射时间十分短，涂层在几十秒甚至几秒钟内迅速固化。由于固化快，干燥装置长度短，被涂饰部件一经照射即可收集堆垛。节约车间的生产面积，生产周期短，是机械化连续涂饰流水线上很快的固化装置。因而生产效率极高。
（ 2 ）漆膜质量好节省涂料 光敏漆是无溶剂型漆，涂料转化率接近 100%
，固化后的漆膜收缩极少，没有因漆膜收缩而产生的各种缺陷，漆膜平整光滑。另外，光固化时，当涂层已固化而基材未被加热，基材含水率可保持稳定，避免或减小了因含水率变化而引起的变形、翘曲等。
（ 3 ）装置简单，投资少，维修费用低
紫外线辐射固化的缺点是只能固化平表面的零部件，某些形状复杂的构件不能保证固化质量。紫外线对人的眼睛和皮肤有危害，操作时不能直接用肉眼向照射区内窥望。
3. 固化装置
生产中用光敏漆组成一条光固化流水线，这种装置涂料涂饰设备与紫外线辐射装置两个主要部分组成，用运输机连接起来，安装在密闭的隔离室内。紫外线辐射固化装置主要由照射装置、冷却系统、传送装置、空气净化及排风系统等。
（ 1 ）照射装置 照射装置由光源、冷却系统、照射器、漏磁变压器等部分组成。其中最主要的是紫外光源。
常用的紫外光源主要是低压汞灯和高压汞灯，此外还有属于近紫外光源的氙气灯等。低压汞灯的外壳是软质玻璃，内壁涂有黑光粉，管内充有高纯度水银液和稀有气体。其紫外线放射率较高，达
18%
，发热量低，但输出小；高压汞灯的外壳是石英玻璃管，内有高纯度水银液和惰性气体，启动后，管壁温度达数百度，水银呈完全蒸发状态，达几个大气压，放射效率低（
7% ～ 8% ），但输出量大，有效的紫外线强度远比低压汞灯大。
（ 2 ）排气通风及空气净化系统
排风通气的目的是排除预固化区的热量，排除部分溶剂所挥发出的有害气体以及高压汞灯所产生的臭氧。
空气净化的目的是为了保证漆膜的质量，整个淋涂装置及照射装置都安装在密闭的隔离室内，进入隔离室的空气需要净化。在风机吸风口前设置布袋式粗过滤器，风口后设置泡沫过滤器以达到净化要求。另外为了避免外界灰尘进入，须使隔离室内维持正压，这就要求进风量大于排风量。
4. 紫外线辐射固化工艺条件
影响紫外线辐射固化的因素有紫外线波长、紫外线长度、涂层厚度及涂层温度。
（三）电子线固化
电子线固化就是用电子线照射涂层，使涂膜高分子发生交联固化。
电子线固化装置由电子加速器、高压发生器、控制台、运输机和通风系统组成。主体为加速器，由电子枪和加速管组成，从电子枪发射出来的电子靠加速管内高压的作用而获得很高的能量。加速的电子流经过交变磁场，该磁场将电子流导向涂层，并从运输带的一边移到另一边。固化装置的主要部分设在混凝土或铅制的防护墙内，以防电子遇到金属时产生的
X 光射到外面。
所有的不饱和化合物、不饱和聚合物等都可用电子线进行固化。
电子线固化的优点是：常温固化、漆膜质量好、时间极短（几钞钟即可），能固化不透明涂料。能量利用率高，但设备价格高。
五、固化规程
漆膜质量的好坏与固化工艺规程是否合理有很大关系，制定涂层固化规程的主要According to是涂料中成膜物质的种类，涂层厚度及其固化方法等。
油性涂料干燥的快慢与温度的高低有关，温度高，漆膜干燥快，但一般情况下，温度宜在 80 ℃以下。用皮胶、骨胶胶合的拼板制作的制品，漆膜干燥温度不宜超过
60 ℃，否则会降低胶接强度。油性漆的涂层厚度对固化时间影响很大，随着涂层厚度的增大，固化时间也大为延长，所以油性漆的涂层不宜过厚。
硝基涂层干燥时最高温度不能超过 50 ℃，干燥初期，如温度稍高，涂层就易起泡，所以，规定涂层进入干燥固化装置的最初 5 ～ 10s
钟之内，温度应逐渐地升高，或者使工件进入干燥固化装置之前，预先在室温下陈放一段时间，固化后不磨光的涂层，在 40 ～ 50 ℃的温度下，固化 15
～ 20min 就可以，需漆膜修整的涂层，则需在室温下固化 48h 。如用对流方式固化涂层，在 50 ℃下固化 3 ～ 4h 。辐射方式固化涂层，在
50 ℃温度下，需固化 2h 。
聚氨酯漆是一种反应性很强的涂料，对环境很敏感，漆膜性能与施工的关系密切，双组分漆的配比对漆膜的性能影响很大，故须严格按照规定比例配漆，并充分搅拌均匀；放置
15 ~ 20min, use after the bubbles disappear. The prepared paint should be used up within 4 hours to avoid gelling and scrapping due to too long time.
During the paint mixing and painting process, avoid contact with water, acid, alkali or alcohol and other substances. The moisture content of the wood should not be too high, and the underlying water-based materials (filler, colorant, etc.) are completely dry before applying polyurethane paint.
If viscosity h is required, use industrial anhydrous xylene or industrial anhydrous cyclohexanone at a ratio of 1:1
After mixing, thin the polyurethane paint. The diluted paint can only be used as a primer, not as a top coat, otherwise it will affect the gloss of the paint.
The coating thickness is generally 15 μm per layer, and the coating should be dried for about 1 hour. If the coating is too dry, the cross-linking between layers will be poor when the next layer is applied, which will affect the adhesion between layers.
Different varieties of polyester paint have different curing procedures. For wax-type unsaturated polyester paint, the curing temperature is 15-30 °C. If it is higher than 40 °C, the quality of the formed paint film is not good.
℃, paraffin will crystallize in the coating, making the paint film blurred. Coating thickness should be above 200 μm, if less than 100 μm
, the wax film is not easy to form, which affects the sealing and oxygen barrier of the coating. The curing temperature of the non-wax type is related to the type of initiator. For example, the lowest initiation temperature of cyclohexanone peroxide is 20-60 °C, and that of benzoyl peroxide is 60 °C.
~120°C.
