Chọn đúng giấy nhám trong gia công gỗ

Bước đầu tiên của gia công gỗ là chà nhám. Cho dù chà nhám bằng máy hay bằng tay thì vẫn phải chọn đúng loại giấy nhám và độ “nhám” (grit) của giấy nhám. Sử dụng sai giấy nhám có thể gây ra hư hỏng không thể khắc phục được

Chọn đúng độ “nhám” (grit)

Giấy nhám được phân loại dựa theo số hạt mài/nhám trên 1 inch vuông (6.45 cm²) của giấy nhám. Số grit càng nhỏ, giấy càng nhám/thô

Giấy nhám thường được chia thành nhám thô (40-60 grit), nhám vừa (80-120 grit), nhám mịn (150-180 grit), rất mịn (220-240 grit), cực mịn (280-320 git) và siêu mịn (360 và hơn). Chà nhám với giấy nhám mịn hơn giúp làm mất đi những vết xước do lần chà nhám trước gây ra, và làm cho bề mặt phẳng, láng mịn

Có thể sẽ có câu hỏi hỏi “Tại sao không dùng chỉ loại nhám siêu mịn cho tất cả các bước ?”. Tất nhiên là có thể, nhưng giấy nhám thô sẽ mài bề mặt cần chà nhám nhanh hơn, và sau đó dùng giấy nhám mịn sẽ giúp việc chàn nhám dễ và nhanh hơn. Hầu hết những người có kinh nghiệm gia công gỗ sẽ nói với người mới rằng, thực hiện công đoạn chà nhám càng nhanh càng tốt

Phân loại giấy nhám

Có 2 loại giấy nhám chính: loại thủ công (commercial) và loại công nghiệp (indsutrial). Có vài sự khác nhau giữa 2 loại, cụ thể là vật liệu làm hạt mài, vật liệu làm giấy mài, và keo sử dụng dán hạt mài lên giấy mài. Loại giấy nhám công nghiệp sử dụng cả 3 vật liệu trên với chất lượng cao hơn

Ngoài ra, có thể thấy giấy nhám được phân thành “open-coat” hoặc “closed-coat”. Sự khác biệt là giấy nhám closed-coat có các hạt mài phân bố gần nhau hơn, còn open-coat thì khoảng cách giữa các hạt mài lớn hơn. Giấy nhám open-coat tốt hơn cho gia công gỗ,  do nó ít bị dính/bít (clog) giấy nhám hơn khi làm việc với gỗ mềm có chứa nhiều nhựa

Phân loại vật liệu hạt mài

Có 5 loại giấy chà nhám chính (phân theo vật liệu hạt mài), nhưng không phải tất cả đều dùng được để chà nhám gỗ

Giấy nhám thủy tinh (glasspaper):  còn được biết dưới tên giấy nhám lửa (flint), thường rất nhẹ, có màu vàng nhạt. Glasspaper rất dễ bị rã/mủn ra, ít sử dụng trong gia công gỗ

Giấy nhám garnet: thường có màu đỏ nâu, dùng phổ biến trong gia công gỗ. Nó không chà nhám nhanh (hiệu quả) như các loại giấy nhám khác, nhưng cho bề mặt đẹp hơn. Giấy nhám garnet là lựa chọn tuyệt vời để chà nhám gỗ

Giấy nhám oxit nhôm (aluminum oxide): là 1 loại giấy nhám phổ biến khác trong gia công gỗ, các máy chà nhám thường dùng loại giấy này nhất. Giấy nhám oxit nhôm bền hơn giấy nhám garnet, nhưng không để lại bề mặt đẹp bằng

Giấy nhám silicon carbide: thường có màu xám đậm thậm chí là đen. Loại này thường dùng trong gia công kim loại hoặc chà nhám “ướt”, sử dụng nước như chất bôi trơn. Loại giấy này không thường dùng trong gia công gỗ

Giấy nhám sứ (ceramic paper): sử dụng loại vật liệu mài bền nhất, và có thể mài nhanh chóng bề mặt cần mài. Giấy nhám sứ thường được dùng cho máy chà nhám băng/cuộn, nhưng đôi khi cũng cho chà nhám bằng tay. Giấy nhám sứ thường để lại 1 bề mặt thô ráp, vì vậy nên thận trọng khi dùng giấy nhám sứ, nhất là với plywood và veneer, khi mà nó có thể nhanh chóng chà nhám mất lớp phía trên phá hủy xuống phía dưới

Mẹo để chà nhám tốt nhất

Trong chà nhám gỗ, thường mọi người thích chà nhám  với giấy nhám thô aluminum oxide trước, sau đó là chà nhám với giấy nhám mịn garnet để có một bề mặt láng, đẹp cho những bước staining, sơn tiếp sau

Sưu tầm

Increasing Adhesion of UV Coatings on Wood

Dr. Jan Weikard and Dr. Wolfgang Fischer / Bayer AG, Leverkusen, Germany Coatings on wooden substrates for furniture, flooring and interior are a main application for radiation curing lacquers. Increased adhesion is an important target for development of UV coatings on wood. Superior adhesion leads to better whitening behavior (coin test) and abrasion resistance. Guidelines to improve adhesion on wood are: Swelling of wooden fibres with waterborne UV primers to increase the substrate's surface area and its interaction with the coating. Better wetting of oily surfaces e.g., teak, mahogany or rosewood by using coatings with low surface tension. Chemical crosslinking of coating and substrate. "From a theoretical point of view, adhesion is the result of the action of surface forces, of wetting and of the coating's curing process at the substrate coating interface."1 The porous nature of wood is a main difference in comparison to other substrates such as metal and plastics. The liquid coating material is partly absorbed into the wood by capillary forces. This presents an additional challenge for UV curing coatings as the absorbed material will only be cured if there is sufficient UV energy. Uncured material - especially low-molecular-weight reactive diluents - can cause loss of adhesion and sweating of the cured coating. The surface of wood is seldom homogeneous. Varying surface tension (e.g., caused by oily components from the wood) and phenolic components with inhibiting effects on UV cure have a negative influence on the adhesion of coatings. Additional problems may arise from the very fast UV curing reaction during which a high amount of internal stress is built up in the coating.

Waterborne UV Primers Figure 4 / Water-Reducible Urethane Acrylates The right choice of waterborne UV curing primers solves many of the above-mentioned adhesion problems. Water, together with the dispersed or dissolved polymers, is rapidly absorbed by the wood pores and capillaries. Fibers are swollen and lifted up, which increases the substrates surface area. Waterborne UV curing coatings are available as water-reducible/water-containing resins, emulsions or dispersions2 of different chemical nature. Figures 1-2 compare the relative properties of emulsions and dispersions. The best overall properties are achieved with polyurethane systems. Polyurethane dispersions are generally produced by the polyaddition of diisocyanates with bivalent compounds that react with isocyanates. Emulsifying groups such as polyether or ammonium carboxylate (internal emulsifiers) are also incorporated into the polyurethane structure. The key factor in UV curing PUR dispersions is how the acrylic double bonds enter the polymer. Previously, the only dispersions known were those in which hydroxyfunctional acrylate monomers such as 2-hydroxyethyl acrylate were used as the UV curing component. As these acrylate monomers are monofunctional components, which stop the formation of the polyurethane polymer, it is very difficult to produce high molecular weights with an adequate density of double bonds. Recent developments no longer use monomeric acrylates as the double bond components but rather oligomeric or polymeric compounds such as polyester and epoxy acrylates, which have their own hydroxyfunctionality. Figure 3 shows the structure of high-molecular-weight PUR dispersions. The combination of different components allows the specific design of UV curing polyurethane dispersions. By varying the saturated and unsaturated polymer building blocks, by choosing different diisocyanates, by using monomeric diols or diamines - also mono- or polyfunctional building blocks - and by varying the process technology, the properties of the polymers can be customized. Examples of such properties are physical drying, UV reactivity, hardness and elasticity, glass-transition temperature (Tg), and other solids-related properties of the paint film. Also urethane acrylates that can be formulated with about 10% water for roller coating application are well-suited for application on wood (see Figure 4). Here, the amount of water that has to be forced out of the coating during drying is minimized. Figure 5 summarizes the advantages of PUR dispersions, PUR emulsions and water-reducible urethane acrylates.

Optimizing UV Resins If the use of waterborne material is not possible for a given coating process, resins with a reduced shrinkage during polymerization and improved wetting are needed. Figure 6 compares a standard polyester acrylate with an unsaturated polyester in DPGDA. With the latter, good adhesion - especially on oily wood types like mahogany, teak or rosewood - is achieved. The lower double-bond density, however, means a reduced UV reactivity. Wood contains many phenolic compounds, which have similar chemical structures compared to the substances that are commonly used to stabilize acrylate resins. In a model experiment, commercial-grade hexanediol diacrylate (HDDA) was used to extract shredded teak wood (3 pbw. HDDA on 1 pbw. teak). Analysis of the extract by means of GC-MS revealed substantial amounts of the expected phenolic type species. Varying amounts of teak HDDA extract were further used to formulate coatings based on the above-mentioned unsaturated polyester in DPGDA. The formulations were tested for UV reactivity in relation to the amount of teak HDDA extract added. Reduced reactivity in comparison to a formulation with standard HDDA was found only with high amounts of teak HDDA extract. This simple experiment shows that there is an inhibiting effect of the wood extractables. But under conditions closer to an actual coating process, it can be estimated that this inhibiting effect is low.

Chemical Crosslinking Chemical crosslinking between coating and wooden substrate can be achieved with "dual-cure" formulations. Dual-cure formulations are taken here to mean those that crosslink not only by radical photopolymerization but also by an additional mechanism, namely the reaction of isocyanates with highly polar groups. The polar groups may be a constituent of one of the components in the formulation or may form through the effect of atmospheric moisture on the isocyanates. Alternatively, they may be bound in the substrate (see Figure 7). The use of urethane acrylates bearing free isocyanate groups results in significantly improved adhesion of the coating on wood. We conducted a model experiment to find out how exactly the chemical crosslinking takes place considering three probable mechanisms. 1. Moisture from the wood leads to the formation of urea groups at the coating substrate interface, 2. Aliphatic hydroxy groups of the cellulose fibers react with the isocyanates to yield aliphatic urethane groups or 3. Phenolic hydroxy groups, e.g., from lignin form aromatic urethanes. We applied an aliphatic hexamethylene diisocyante isocyanurate polymer on wood and, as a control, on glass. Both coated substrates were stored under dry nitrogen atmosphere to exclude any influence from atmospheric moisture. Infrared spectra were recorded using a probe head that was moved along the z-axis through the coating layer. After one week, the control on glass showed no change in the IR spectra. The coating on wood, however, after a couple of hours showed a new peak as a shoulder at 1687 cm-1. This occurred in those spectra that were recorded close (< 2 Km) to the coating wood interface. The peak was evaluated as typical for an aromatic urethane by comparison with reference spectra from the literature. Aliphatic urethanes and ureas could not be detected in this experiment. Therefore, the most probable mechanism for the crosslinking of isocyanate functional urethane acrylates with wood is formation of additional urethane bonds with lignin type components of wood. For dual-cure applications, the range of isocyanate functional urethane acrylates for outdoor use (Figure 8) is supplemented by a new range of low-viscosity products for indoor use (see Figure 9). An application example for a parquet flooring coating process is given in Figure 10. The use of a dual-cure adhesion promoter leads to significant improvement in adhesion. The drawback of the primer formulation is the reduced pot life. There are, however, two advantages of using isocyanate modified urethane acrylates instead of just adding polyisocyanates to an acrylate resin formulation. Since the unmodified polyisocyanate is unreactive under UV irradiation, it will lower the UV reactivity of the formulation. Second, some of the polyisocyanate will not yet have reacted when the topcoat is applied. The polyisocyanate, therefore, can be extracted and induces a pot life on the topcoat roller coater that is applying a one-component system. Both problems can be avoided by using isocyanate functional urethane acrylates that link the isocyanate functions to the acrylate network.

Conclusion Figure 7 / Dual-Cure Adhesion Promotion Improving adhesion of UV curing coatings on wood can be achieved by different approaches. Using waterborne UV primers results in significantly enhanced adhesion. If water as solvent is not wanted, similar improvements are possible with dual-cure adhesion promoters. Here, isocyanate functional urethane acrylates are the resins of choice. If the pot life of dual-cure systems is not acceptable, 100% resins with optimized adhesion properties but lower UV reactivity are available.

References 1 Cited after H.Pecina, O. Paprzycki, "Lack auf Holz", Vincentz Verlag, Hannover, 1995.  2 In physics, the term emulsion usually describes insoluble liquid oligomers and polymers dispersed in water. The (umbrella) term dispersion allows no conclusion as to the state of aggregation of the disperse phase. In the coatings industry, both terms are often used to describe the state of a film after evaporation of the water. Emulsions yield tacky and dispersions tack-free films.

Lubrizol Introduces New Wood Coatings Mobile App

Hi all,

Chọn nguyên liệu phù hợp với yêu cầu của công thức, ứng dụng luôn là bài toán khó với một formulator. Vừa qua, Lubrizol ra 1 ứng dụng chạy trên điện thoại, giúp chọn nhanh những sản phẩm của họ, cho ứng dụng/yêu cầu cụ thể trong lĩnh vực sơn gỗ. Ứng dụng này khá hay (đã checked), xin giới thiệu đến mọi người

Link để tải ứng dụng từ Google Play: https://play.google.com/store/apps/details?id=com.lubrizol.woodcoatings

Còn dưới đây là thông báo chi tiết hơn về ứng dụng này để tham khảo

Lubrizol Introduces New Wood Coatings Mobile App

The Lubrizol Corporation has introduced a new mobile application - the Wood Coatings Product Guide - designed to help formulators of wood coatings identify advanced Lubrizol chemistries that help achieve a wide-range of performance needs.

"We are very excited to launch this new mobile app," explained Dan Latas, global market segment manager, Lubrizol Performance Coatings group. "Formulators have often inquired about an electronic tool capable of simplifying the process of finding the right materials to solve their unique coatings challenges, and now one is available. The Wood Coatings Product Guide app allows formulators to search for the best chemistries for their coating products based on their application, the type of coating and their most important performance attributes."

The app allows users to narrow product recommendations by global location, preferred coating system (solvent-borne, water-borne or EB-UV), coating type (primer, sealer, stain, topcoat), technology type (dispersants, surface modifiers or resins/binders), desired performance benefits and more. A list of available coating technologies that meet those search criteria is displayed. Users can further access detailed information such as product descriptions, literature, technical data sheets, as well as request product samples.

"We expect the app to get a lot of attention and use," continued Latas. "This is just a beginning, but we think it's a significant step forward. We look forward to users sharing feedback that can make the tool even better in the future."

The Wood Coatings Product Guide mobile app is now available on both the Apple App Store and on Google Play Store for Android devices.



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