Processing Technology of Film-coated and Plastic-coated Bamboo Plywood

Processing Technology of Film-coated and Plastic-coated Bamboo Plywood

Film-coated or plastic-coated bamboo plywood is a kind of composite bamboo plywood with film paper as its surface layer. Film-coated bamboo plywood is a kind of bamboo wood-based panel which is first pressed, sanded and then coated. Plastic-coated bamboo plywood is a kind of composite board, which is made up of plastic film paper and one or two different components, and is hot-pressed at one time, so it is also called one-time plastic-coated method. The laminated bamboo plywood has the advantages of small thickness tolerance and small color difference. It is a kind of high-grade concrete formwork, but it has the shortcomings of many processing procedures and high product cost. The plastic-covered bamboo plywood obtained by one-time Plastic-Covering method is also a kind of high-strength concrete formwork, which has the advantages of simple technology and low cost, but also has the shortcomings of large thickness tolerance and color difference of the product. From this, we can know the common characteristics and uses of the two kinds of products, but because of the different production methods, they have their own advantages and disadvantages. Radial bamboo strip curtain composite board is also mainly used as concrete formwork. It not only has the utilization ratio of bamboo as high as 90%, but also has low cost and good performance. In the same production line, two different grades of products can be produced by Film-covering method and one-time Plastic-Covering method, respectively.

(1) Film-coated bamboo plywood

The laminated bamboo plywood is based on sanded bamboo plywood with fixed thickness and core layer. A laminated veneer, a phenolic resin film paper and a melamine resin film paper are arranged on the top and bottom of the laminated bamboo plywood to form the slab structure of the laminated bamboo plywood.

Because the high temperature and pressure are needed for hot pressing, the defects on the substrate board will be reflected on the surface to varying degrees. Especially for the cracks of bamboo sheets on the base material, such as cracks in joints, etc., when hot-pressing film covering, because of the small pressure on these cracks, strip “white flowers” appear because of the insufficient plasticization degree, which not only affects the appearance quality, but also these strip “white flowers” film is easy to fragile and rupture. Therefore, the quality of the base plate will directly affect the quality of the cladding board, so the cracks and defects of the base plate more than 3 mm must be repaired with the same material before the cladding can be carried out.

Bamboo plywood is made of softened and unfolded bamboo sheets which are vertically collocated with each other. The surface of the plywood is rough and uneven, and the thickness of the plywood has some errors. In order to ensure the thickness accuracy and smoothness of the laminated bamboo plywood, it is necessary to sanding the plywood to ensure that all the substrates are within the error range of + 0.2mm.

A transverse veneer is arranged between the plywood base material and the film paper of the slab. This is due to the poor repressibility of the bamboo plywood (because it has been hot pressed once when it is made), the hardness of the plywood is higher, the impregnated paper has only two layers, and the compressibility is also small. Adding a layer of soft veneer can reduce the surface quality defects caused by the uneven pressure caused by the thickness errors due to various reasons such as pressing plate, metal cushion plate and material of the base material of the hot press, such as color difference, corrugation, exposed bottom, poor local gluing, etc., so as to make the lamination possible. The surface quality of the film has been greatly improved. On the other hand, the bamboo plywood base material is mostly three-layer structure. The static bending strength and modulus of elasticity are quite different between the longitudinal and transverse directions. Adding one layer of transverse wood veneer to the two surfaces of the base material can greatly increase the transverse strength and modulus of elasticity, thus reducing the strength difference between the longitudinal and transverse directions. Therefore, the role of the veneer layer is very great.

(2) Plastic-coated bamboo curtain plywood

Plastic-coated bamboo curtain plywood is made of bamboo curtain as the main material as the core layer, bamboo mat or wood veneer as the inner layer, and surface paper impregnated with phenolic resin as the bottom layer.

The purpose of this structure is to provide the mechanical strength of the product by the core layer about 85% of the thickness of the board. The inner bamboo mat or veneer is used to cover the grooves of the bamboo curtain and improve the surface smoothness of the board. The surface film paper is used to increase the wear resistance and water resistance of the board on the one hand, and to improve the gloss and finish of the board on the other hand. Through the composite effect, the use requirements of high strength concrete formwork can be achieved.

After bamboo curtain and mat are dried to 8% – 12% moisture content, the sizing method is to impregnate low concentration water-soluble phenolic resin.

In the process of dipping, the moisture content of low-concentration adhesives enters the interior of bamboo curtain and mat in large quantities, which makes the moisture content of bamboo curtain and mat increase greatly. Therefore, drying must be carried out. In order to avoid the pre-curing phenomena in the drying process, the drying temperature must be controlled within 95 C. The bamboo curtain and mat can be dried continuously by jet mesh belt dryer or periodically by drying kiln.

Base paper is usually bottom paper, and its quantity is 80-100 g/cm2. Titanium white decorative paper with reddish brown color is better if its quantity is 100-120 g/cm2. Titanium dioxide can cover the defects of inner bamboo mat (or veneer) because of its strong covering power on base paper. The color of reddish brown is similar to that of phenolic resin adhesive, which can deepen the color of board surface and help to eliminate the color difference of board surface.

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