Factors Affecting the Release of Formaldehyde in Wood-based Panels by Perforation Extraction

Perforator method is a conventional method of free formaldehyde content of composite wood material determined, for our proposed and adopted in Europe, the principle is washed with toluene - The extracted solid extraction specimen formaldehyde, formaldehyde with water and then through a punch The liquid - liquid extraction causes the formaldehyde in the toluene to be dissolved in water and then measured in an aqueous solution. The perforation extraction method is simple in operation and short in measurement time, and is suitable for rapid determination of test pieces. The perforation extraction method actually measures all the formaldehyde that may be released in the artificial board. In this paper, the factors affecting the determination of free formaldehyde in wood-based panels by perforation extraction method are discussed for reference.

1 experiment

1.1 Materials and Instruments

          Material: High-density fiberboard, particleboard (provided by Jiangsu Yulan Wood Co., Ltd.).

          Reagents: toluene, potassium iodide, potassium dichromate, sodium thiosulfate, acetylacetone, formaldehyde ( 35%-40% ), ammonium acetate (all of which are of analytical grade). Mercuric iodide, anhydrous sodium carbonate, H2SO4 , HCI , NaOH , I2 , soluble starch.

          Instrument: Perforation extractor (Shanghai Glass Instrument Factory), 0.01g electronic balance, 0.0001g electronic balance, UV1240 ultraviolet spectrophotometer and so on.

1.2 Test principle

GB/T 17657—1999 “Test methods for physical and chemical properties of wood-based panels and veneered panels”.

1.3 calculation

          The amount of formaldehyde released is calculated as follows, to the nearest 0.1 mg .

E= ( As-Ab ) ×f×(100+H)×V /M0

         Where: E— mg of formaldehyde released per 100 g of dry test piece, mg/hg ;

As- extract absorbance;

Ab- distilled water absorbance;

F— the slope of the standard curve;

V— volume volume, 2000mL ;

H —the moisture content of the test piece, % ;

M0— the mass used to extract the test piece, g

2 results and discussion

2.1 The effect of the determination method on the results

GB/T 17657-1999 There are two methods for testing the aqueous formaldehyde solution obtained after extraction, namely iodometric method and spectrophotometry . The effects of the two methods on the experimental results are shown in Table 1 .

  Table 1 Effect of iodometric method and spectrophotometry on the results

   Note: Toluene blank is not added to the test piece during the experiment, directly added toluene, and tested according to GB/T 17657-1999 .

        It can be seen from Table 1 that the same test piece was measured by iodometric method and spectrophotometry, and the structure determined by the iodometric method was significantly higher than that of the spectrophotometric method. Moreover, the results measured by the toluene blank iodometric method were as high as 2.9 mg/hg . The iodometric titration is actually a redox reaction. The formaldehyde contained in the water and other reducing substances in the toluene participate in the reaction together. When the test piece is cooked at a boiling temperature of 110 ° C or higher, the cellulose macromolecule contains a A reducing group such as an oxy group may be cleaved or hydrolyzed to produce an aldehyde, a ketone or an alcohol compound, which is transferred to water by perforation extraction and reacted with iodine. The results of the titration test are considered to be formaldehyde in the test piece. Hemicellulose in wood is also likely to hydrolyze and produce some reducing substances. The spectrophotometric method uses a characteristic reaction of formaldehyde and acetylacetone in the presence of an excess of ammonium salt, which is highly specific and highly selective, and a small amount of aldehyde, ketone or alcohol compound has little interference with the measurement. Therefore, it is necessary to make a toluene blank when measured by the iodometric method, and the toluene blank should be deducted during the test. The author suggests that the amount of formaldehyde released should be determined by spectrophotometry in the experiment.

        2.2 The effect of the amount of toluene on the experimental results

 

GB/T 17657—1999 The amount of toluene used in the perforation extraction process is as high as 600 mL . The influence of the amount of toluene added on the experimental results is shown in Table 2 .

                                                    Table 2 Effect of the amount of toluene on the results

 

        From Table 2 , as the amount of toluene increases, the test result of formaldehyde emission increases, and it does not change much when it reaches 600 mL and more than 600 mL . The quality of the test piece specified in GB/T 17657-1999 is 105-110g , and the amount of toluene in the extract should not be too high or too low. Toluene and distilled water liquid - liquid extraction time, although the polarity of the water is higher than toluene, most of the formaldehyde can be extracted in distilled water, but if the amount of toluene is too high, there is still a small portion of residual formaldehyde in a toluene solution, so that the test result When the toluene and the test piece are subjected to liquid - solid extraction, if the formaldehyde concentration of the test piece is high, the amount of toluene is too low, and since the solubility of formaldehyde in toluene is constant, toluene cannot sufficiently extract the formaldehyde.

2.3 Influence of test piece quality on experimental results

        The effect of test piece quality on the experiment is shown in Table 3 . The quality of the test piece specified in GB/T 17657-1999 is 105-110g .

 

                                               Table 3 Effect of test piece quality on results

        It can be seen from Table 3 that the quality of the test piece has little effect on the final test result, but the quality of the test piece cannot be too small, otherwise it will affect the representativeness of the test piece.

        2.4 Influence of reflux time on experimental results

The reflux time specified in GB/T 17657-1999 is 2 h and the reflux rate is 30 mL/min . The effect of reflux time on the experimental results in the case of a constant reflux rate is shown in Table 4 .

                                            Table 4 Effect of reflux time on experimental results

It can be seen from Table 4 that in the case where the reflux rate is 30 mL/min , when the reflux time exceeds 1 h , the formaldehyde in the test piece has been extracted sufficiently, and the test results have little effect.

       2.5 Influence of moisture content of test piece on experimental results

       The formaldehyde released from the urea-formaldehyde rubber board mainly comes from the wood itself, the free formaldehyde in the urea-formaldehyde glue, the formaldehyde released by the uncured adhesive and the formaldehyde formed by the hydrolysis of the cured adhesive. As the moisture content of the sheet increases, the adhesive may be hydrolyzed, which increases the amount of formaldehyde released, and may also increase the rate of release of free formaldehyde from the sheet. Therefore, the moisture content of the panel has a great influence on the amount of formaldehyde released. In Jann et al., the water cut correction coefficient was proposed . The measured value at a water content of 6.5% was a certain value. The measured values ​​at other moisture contents were multiplied by a moisture correction coefficient. The correction factor is already in the European particleboard and medium. Density fiberboard formaldehyde detection standards have been applied.

        China GB/T 4897-2003 "Particle Board" and GB/T 11718-1999 "Medium Density Fiberboard" although the water content correction coefficient is also used, but GB 18550-2001 "interior decoration materials" The formaldehyde release limit in wood-based panels and their products does not take into account the effect of this correction factor. In order to artificially control the moisture content of the wood-based panel and not affect its formaldehyde emission, the author is not enough in this aspect. However, according to reports, the effect of water content on the perforation value is very significant, the water content is from 3% to 16% , the amount of formaldehyde released is very fast, and the perforation value has a good linear relationship with the water content. In the range of 3% to 13% moisture content, it is necessary to use a water content correction factor.

       3 conclusions

 

1 ) Spectrophotometry is highly specific, with strong selectivity and little interference, no need to do toluene blank experiment; iodometric titration is a redox reaction, the reducing substance in toluene and the test piece cellulose macromolecule The reducing group such as methoxy group has a great influence on the test results, and it is indispensable to do the toluene blank test. It is recommended to measure the amount of formaldehyde released by spectrophotometry as much as possible.

2 ) The quality of the test piece has little effect on the experimental results. The amount of toluene in the extractant is 600 mL . The reflux time is about 2 h at a reflux rate of 30 mL/min .

 

3 ) The moisture content of the test piece has a great influence on the experimental results. It is recommended to correct the perforation extraction value in the mandatory standard of GB 18580-2001 .

 

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