Due to the special physical and mechanical properties, chemical composition characteristics of wood, and the location of the building structure where wooden doors and windows are located, wood coatings for wooden doors and windows are required to meet the requirements of decorative design, but also need to meet the requirements of waterproof, breathable, anti-corrosion, deformation prevention, UV resistance and mechanical resistance. Requirements for damage, ease of maintenance and refurbishment. With reference to European and Chinese wood door and window industry experience, it is recommended to use special wood door coatings. The special wood door and window coating system consists of a permeable primer and a thick film topcoat. According to the formula and transparency, it is divided into four series: Water
-based, solvent-based, wood-colored clear lacquer (so-called water) and opaque paint (mixed water). For single-component, open the barrel for construction. In addition, the system products also include seam sealant, joint preservative, maintenance agent, polish, refurbished paint and other products. In terms of coating selection, it is determined according to the film forming characteristics of different coatings, the painting conditions of the production workshop, and the requirements of the final customer.
The new wooden windows include solid wood windows, outdoor doors (German style), German-style aluminum-clad wood doors and windows and Italian-style wood-aluminum composite doors and windows.
Such doors and windows are all made of solid wood as a profile, or partially made of solid wood as a frame, inlaid, and hollow insulating glass, so that the wooden doors and windows have special heat insulation, sound insulation and decorative effects.
Since these wood structures are more or less affected by factors such as sunlight, ultraviolet rays, temperature difference, rain water, condensed water, etc., it is necessary to protect and paint materials that conform to the characteristics of the wood itself.
China introduced European technology from the mid-1990s to produce new wooden doors and windows. Today, about 50 companies across the country have produced new types of wooden doors and windows. However, in terms of wood coating protection, there are many problems that affect the confidence of the end customer more or less. According to the experience of Germany and China, if the technical problems of wood coating are not well solved and the suitable coatings and construction techniques are standardized, the entire wood window industry will be affected.
For example, before 1990, the market share of solid wood windows in Germany was more than 35%. In the first half of the 1990s, it slipped to around 30%.
In order to investigate the reasons, Germany launched a series of studies. In 2001, the German physicist Guido Horo summarized the results of nearly 10 years of research and published a paper entitled â€œThe quality of coatings determines the quality of wooden windowsâ€, detailing the importance of coatings in wooden doors and windows. The quality of the coating affects the confidence of the end customer.
1. Technical requirements for wood door and window wood coatings
1. Characteristics of wood
Chemically, wood is mainly composed of cellulose, lignin, sugar, starch, protein, inorganic potassium, phosphate salts and other trace components. Due to its chemical composition, it is susceptible to chemical reactions (such as oxidation to the air), microorganisms, biological effects (such as mold, termites, insects, etc.), physical effects (such as water vapor, temperature changes caused by temperature changes) , UV damage, mechanical damage (such as rain, hail), etc., industrial waste gas will also accelerate the decomposition of wood fiber.
At the same time, unlike general inorganic materials, wood has a strong hygroscopic effect, which absorbs water from the air and causes swelling. In the process of water absorption and water discharge drying, the swelling/contraction is not uniform, such as the radial deformation is 3.3-4%, the metamorphism is 7-12%, and the direction of the grain is only 0.1-0.4%.
According to the German industrial standard classification, the wood of wooden doors and windows belongs to the fixed timber, during use, not
Allow the wood to deform. This requires a wood coating system that has a good ability to prevent wood deformation. In terms of mechanical properties, wood has high compressive strength and good flexural strength, which is why wood can be used as a wood door and window profile alone or in combination with other materials.
2. Factors affecting the durability of solid wood windows
2.1 Internal and external causes of durability of solid wood windows
Solid wood doors and windows, especially for the outdoor solid wood doors and windows is the most high-end door and window series, representing the trend of the integration of people and nature. Tracking research findings (Dr Richter, 1998, Dr. German, Woodwind Association, 1998), external factors affecting the durability of solid wood windows are multifaceted, UV, heat radiation, temperature changes, moisture, humidity changes and environmental concerns Mechanical factors (such as hail, wind pressure, etc.) are the main external causes. In addition, the degree of weathering and strength of solid wood windows is related to the following internal factors:
(1) Location of use: Outdoor wooden windows are most affected by the above external factors;
(2) Detail design: Improve the anti-aging ability of wooden windows by design;
(3) Wood type and quality: high-quality wood has enhanced anti-aging ability;
(4) Surface coating: The coating is the skin of the wooden window. The coating conforming to the physical and chemical principles of the building can not only visually and structurally protect the wooden window;
A study by the Wood Window Institute of ROSENHEIM, Germany, found that 68% of the aging problems associated with the use of wood windows were related to surface coatings.
2.2 Durability of wood, aluminum and window wood
The wood of aluminum-clad and wood-aluminum composite windows is not directly exposed to rain and sunlight, and the durability is relatively long. However, such windows are still deformed and moldy due to condensation, while sunlight and ultraviolet light cause discoloration, and still require different protective materials for interior furniture.
3. Technical requirements for special coatings for wood window wood
Since 1995, the German IFT Institute, the Federal German Coatings and Preservation Technology Committee: "BFS Federal Board for Paints and the Protection of Values" and the German Window and Facade Manufacturing Industry Association VFF (VFF=Verband der Fenster-und Fassaden â€”hersteller, eV) has issued specifications to specify the technical requirements for wood window coatings.
Since 1999, the European outdoor wood protective coating industry standard EN927 (Lacke und Anstichstoffe: Beschichtungsstoffe und Beschichtungssysteme fuer Holz im Aussenbereich, Teil2: Leistungsanforderungen. March 1999) has been put into trial, and the technical requirements to be achieved as a special coating for wood doors and windows have been proposed. Passed tests and recommended test items and test standard methods.
According to the above specifications and more than 30 years of experience in Europe. Wood coatings for wooden windows need to meet the following basic requirements: anti-corrosion, insect proof, waterproof and breathable, toughness, UV resistance, aging resistance and ecological safety.
3.1 Anti-corrosion and pest control
Wood contains certain sugar, starch, protein, etc., and is easily damaged by microorganisms and bacteria. Therefore, the wood needs to be treated with antisepsis and mildew resistance. Generally, the preservative can be added to the primer to achieve the purpose. However, its effectiveness and durability need to be tested by specialized agencies, and there are certificates, such as the German authoritative RAL certificate.
In general, wood that uses wooden windows is less likely to be exposed to insects and termites, so it is generally not necessary to treat wood against insects and termites. However, in southern China, such as Guangdong and Fujian, the air is humid, and the average relative air humidity is about 70%. According to the German industrial standard DIN68800, it is necessary to carry out insect and termite treatment.
3.2 Waterproof and breathable
The deformation of the wood installed in the wall causes the cracking of the joint material itself and the joint portion of the profile on the one hand, and the peeling of the sealant on the other hand, which affects the compactness of the window. In addition, when the wood moisture content exceeds 20%, the wood is prone to mildew. Therefore, the paint must have a good waterproof function.
According to the European standard DIN EN 927-5, the 72-hour water absorption of the coated wood is less than 175 grams.
Water includes external rainwater, external and internal condensate (condensation). Liquid or gaseous water can cause changes in the volume of the wood. Water-repellent coatings are also needed in invisible areas to prevent condensation and moisture from entering the laminate.
While ensuring the waterproofness of the coating, it is required to have a certain gas permeability, otherwise, the paint film will foam and fall off. The breathability and sealing properties of wood door and window coatings are required to reach a balance. The ideal is one-way breathing: the water outside the wood does not enter the wood, and the water inside the wood can evaporate.
There is currently no uniform standard for breathability. It is generally considered (VFF, 1999; URBAN, 2000;), according to the German industrial standard DIN52615 sd = 0.5 - 1.2m, that is, at 23 Â° C, in the relative humidity of 95% -50% (wet cup method), The 24-hour water vapor transmission rate is 17-40 grams, which is more suitable. This not only ensures that the coating has good sealing properties, but also has a certain gas permeability.
3.3 Toughness and hardness
Wood is a tough organic material that requires a certain degree of elasticity and toughness to be applied to the paint film on its surface so that the paint will be durable and not easily peeled off and separated. Today, polyester modified alkyd resins and core-shell PU-acrylate paints have certain strength and toughness. In contrast, wood door and window paint does not require the same hardness as wood floor paint. High-hardness paint will crack and fall off.
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