Veneer quality control Part 6: Bleaching method of poplar face veneer
keywors of this article : Bleached Poplar face veneer ,natural poplar veneer ,Bleached poplar production process ,bleached poplar grading rules
_. The determination of bleaching agent:
Owing to the fact that wood bleaching is achieved by oxidation and reduction reaction, both oxidative and reductive compounds have bleaching effect. Reductive bleaching agents mainly include nitrogen compounds, inorganic sulfur compounds, organic sulfur compounds and acids, which are seldom used to bleach wood. Therefore, the most commonly used method of wood bleaching is oxidative decolorization. At present, hydrogen peroxide is the most commonly used chemical agent in wood surface (veneer) bleaching.
Detailed Explanation on Bleaching Method of Poplar Surface
2. Bleaching mechanism:
_Hydrogen peroxide ions produced by dissociation of hydrogen peroxide have very strong oxidation ability, which can make some colored genes in lignin, such as pair brewing and adjacent brewing, oxidized and cracked into colorless genes, thus improving wood whiteness.
_3. Bleaching process:
_1. Alkali treatment
_Pretreatment with alkali liquor is the first stage of the bleaching process of face (veneer). It mainly plays the following roles: Firstly, purifying veneer can eliminate the stains and stains on the surface of wood; secondly, a considerable proportion of resin and soluble colored substances in wood may be leached in alkali solution, which is beneficial to reducing the consumption of bleaching agent in the main bleaching process. Thirdly, by swelling cellulose and hemicellulose, the permeability of unipolar is increased. The concentration of alkali solution can be determined by the degree of dirt on the surface of veneer and the content of wood.
_Pretreatment with alkali liquor is the first stage of Veneer Bleaching process.
_2. Washing and drying
_During the alkali treatment, the resin and organic acid dissolved in the alkali liquor and other impurities should be washed and removed repeatedly with a large amount of clean water, which can save bleaching agent and prevent the ineffective decomposition of hydrogen peroxide caused by some impurities on the veneer surface.
_3. Main drift
_Main bleaching is the most important stage in the bleaching process of poplar original color skin (veneer). In this stage, the conjugate double bonds on lignin side chains are broken, the chromogenic groups are broken, and the absorption spectrum moves to the ultraviolet region, resulting in the bleaching or lightening of veneer color, so as to achieve the purpose of veneer bleaching.
Owing to the unreacted hydrogen peroxide and other alkali liquids and additives remaining in the bleached veneer, the normal dyeing of veneer is affected, so the veneer must be thoroughly cleaned to neutral with plenty of clean water and then dried.
_Bleached poplar veneer (veneer) needs adhesive tape to prevent cracking in order to use conveniently, and after drying, the general moisture content is controlled within 16% according to the use situation.
Use of bleached poplar veneer: Bleached poplar veneer is most suitable for multi-layer veneer.