Assessment Of Formaldehyde Content In Leather&Fur With Multiscale Porosity And Study Of Its Release Behavior

Formaldehyde（FA）is a colorless gas with strong irritation,which can cause headache,blindness,dyspnea and even cancer.As a basic chemical raw material,FA has been used in the preparation of leather chemicals and fur processing,which is inexpensive and could not be completely replaced in a short time.The existence forms of FA in leather and fur is rather complicated,including free,capillary adsorbed,reversible and irreversible bonded molecules.The FA with the first three binding modes will do harm to human body and environment when they release and reach a certain amount during the use or storage of products.Therefore,many countries,including China,put forward strict limits on the content of free FA in leather and fur.The researches on FA in leather and fur are mainly focused on the detection method based on water extraction system,aldehyde-less synthetic tanning agent,FA capture agent and so on.However,the researches on FA release behavior in leather and fur with multi-scale porous structure have not been found in domestic or foreign.The adsorption/desorption behavior of FA in leather and fur at micro-scale and meso-scale,the macro-release behavior under the influence of the state of usage and storage media,and the relationship between the diffusion mode in pores and pore structure are not clear.This thesis has carried out research on the above problems.The specific research work includes the following aspects:The assessment of FA content in leather and fur of main producing areas in China was systematically investigated.A detection data set of FA contents in products produced in Tongxiang City for three consecutive years（2014²016）was collected and summarized.Based on replacement from a given set of observations,the bootstrap method was used to resampleand the distribution of the mean value of each bootstrap sample dataset wasestablished.A calculation and comparison of the 95%UCL（Upper Confidence Limit）for each year were performed.The Wilcoxon rank sum test was used to check the position parameters of the three-year sample data.The trend of change of FA contents in leather and fur over the past three years was confirmed.The bootstrap method overcame the defects of a non-normal distribution and small sample size.It can be universally applied for unknown data distribution morphologies and provides a new method for feature analysis of detection data.The results showed that their distributions were close to normal distribution but were highlyskewed.The one-sided 95%UCL on the mean were 95.45 mg/kg,93.02 mg/kg and 85.64 mg/kg,which decreased year by year.The Wilcoxon rank sum test indicated that the population distribution of the FA content in 2016 showed a structural improvement over those of the previous two years.The quality of products is constantly improved.But it was still higher than the standard of Class B stipulated by GB20400-2006:products with direct contact with skin≤75 mg/kg.Therefore,the environmental risk of FA in leather and fur is still serious.It is of great practical significance to carry out researches on FA release and prevention.The chromium hide powder was used as leather simulant to carry out research on the adsorption/desorption behavior with FA,which provided theoretical basis for macroscopic release behavior.Adsorption experiments were designed according to FA tanning process.The effects of initial concentration of FA,pH value of basifying,dosage of chromium hide powder,adsorption temperature and adsorption time on adsorption rate and adsorption capacity were studied.Based on the experimental data,the adsorption equilibrium and kinetics of FA on chromium hide powder were studied by mathematical method.Desorption behavior of the adsorbed sample was investigated.The effects of time,temperature and pH on the desorption behavior were studied.The results indicated that the adsorption isotherms of FA on chromium hide powder accorded with Langmuir equation and the adsorption kinetics accorded with pseudo-second-order rate equation.According to the state of adsorption,chemical adsorption was took as the principal thing,and physical adsorption was also considered.Temperature and pH had a significant effect on the adsorption/desorption behavior of FA.Temperature could promote the adsorption/desorption process.With the increase of pH,the adsorption rate of FA increased and desorption rate decreased.Based on the medium condition when the leather and fur products were dressed,a sealed bottle was proposed to simulate the FA emission environment,and the release behavior was studied by empirical model.FA release tests from FA tanned sheep leather,FA tanned cattle leather and modified glutaraldehyde tanned sheep leather were studied.The effects of temperature,water absorption rate and volume of absorbed liquid on the release behavior were investigated.The release curve equation was deduced based on the release model,and the release theory was verified by fitting the actual release data.The equilibrium release rates of different samples were investigated.The results showed that FA emission could be divided into three stages:the linear freerelease,barrier release and chemical release stagein sealing system.The linear release time inversely correlated with mass transfer coefficient of leather.The temperature had significant promotion impact to release during linear release time.The higher the temperature was,the faster the FA release.Different types of leather showed different release sensitivity to temperature because of porosity.The water absorption of the sample increased linearly with time,and the increase of temperature could promote the water absorption of the sample.The water absorption of the sample had a positive linear correlation with the FA emission.The increase of water absorption could promote the FA release.The synergistic effect of temperature and humidity was more obvious.In the sealed system,the release rate of FA-tanned samples was only about 20%even under the condition of high temperature（60°C）and water,while that of modified glutaraldehyde-tanned samples was 43.60%at 40°C.The residual FA in FA tanned samples will continue to release after environmental factors change,and the harm was long-term.Based on the medium condition when the leather and fur products weren’t in wear,the environment chamber was proposed to simulate the FA emission environment,and the release behavior was studied by mass transfer model.The FA tanned leather samples were placed in an environmental chamber to simulate the release.The effects of environmental temperature,relative humidity,ventilation rate and loading rate on the release behavior were studied.Based on mass transfer theory,a mathematical model describing FA emission process in leather was established and solved.The effects of environmental temperature and relative humidity on key parameters（diffusion coefficient and initial release concentration）of FA emission were investigated.The results show that the increase of temperature and relative humidity could promote the FA release,and the effect of temperature was more obvious.The maximum release concentration increased by 111.2%when the temperature increased from 23°C to 35°C,and the maximum release concentration increased by 19.6%when the relative humidity increased from 40%to 60%.The increase of loading rate and the increase of release source which led to an increase in FA emission.Displacement ventilation can effectively reduce FA concentration and improve indoor air quality.When the relative humidity was constant,the initial release concentration C₀ increased with the increase of temperature,and the diffusion coefficient D_m increased with the increase of temperature.The temperature increased from 23°C to 35°C,the D_m increased by 15.2%,while the C₀ increased by 103.3%.When the temperature was constant,C₀ increased with the increase of relative humidity,D_m decreased with the increase of relative humidity,relative humidity increased from 40%to 60%,C₀ increased by 55.7%,and D_m decreased by 15.0%.Based on the mass transfer theory,a mathematical model describing the FA emission process in leather was established and its solution process was given by MATLAB.It provided a new idea and approach for the study of harmful substances in leather industry,and had certain practical significance.The relationship between the pore structure and the FA release behavior was clarified at multi-scale level.The multi-scale pore structure of leather was characterized by means of modern chemical and instrumental analysis.The pore structure characteristics of leather were obtained.Based on the mass transfer theory of gas in porous solids,the basic diffusion mode of FA in the pores of leather was determined.Based on the hypothesis,three models of pore interconnection were established and validated by emission test.The connection mode between pores was determined,and the relationship between pore structure and FA macroscopic release behavior was clarified.The MIP（Mercury Intrusion Porosimetry）test indicated that leather was a porous medium with self-similar micro pore structure and the fractal dimension of leather is 2.19.Based on MIP and BET（Brunaur-Emmett-Teller）test,the pore size of leather samples ranged from 1.5 nm to 182μm,with an average pore size of 2.26μm.The volume fraction of micro-pore（≤3.53 nm）was 0.09%,that of meso-porous（<3.53μm）was 4.64%,and that of macro-pore（≥3.53μm）was 95.27%.Based on Knudsen number,Ficke diffusion occurred in macro-pores,transitional diffusion occurred in meso-pores and Knudsen diffusion occurred in micro-pores.The diffusion of FA in the pores of leather could only consider the molecular diffusion and transition diffusion,and the meso-porous pores were the control channels of FA release,which affected the whole release process.Independent LIFE（Liquid-Inner tube diffusion-Film-Emission）tests showed that the pore structure of the leather conformed to Xiong model,which was to say,the macro-pore and meso-pore in the leather were considered to be connected by series.The model was useful for predicting VOCs diffusion coefficient and could be applied to the design and development of low VOCs emission leather and fur products.