The influence of paint film thickness and other factors on the determination of soluble lead and chromium content in coatings

Paint as a decoration, protective materials have been applied to many aspects of people's lives, such as interior walls, furniture, toys, stationery and so on. If these coatings in close contact with the human body contain lead, chromium, cadmium, mercury and other heavy metals exceed the standard, will bring great harm to human health. Soluble heavy metals were measured to simulate children mistakenly swallowing these paint films into the stomach, using a solution close to the human stomach acid 0.07 mol/L thin hydrochloric acid extraction coating dry film, the solution of heavy metal content
C
. Soluble heavy metal content measurement method is basically the same, that is, proportional mixing of the samples of various components and stirring evenly, applied to the glass plate or Teflon plate (soaked with acid), fully dried after removing the paint film, or directly from the substrate material to remove the coating as a test sample, with a certain crushing method to crush the paint film and over 0.5 mm of metal sieve. The sample powder of 0.5 g is taken, 25 mL 0.07 mol/L thin hydrochloric acid is added, pH is adjusted between 1.0 and 1.5, stirred or oscillat 1 h and then set aside 1 h, filtered with a micro-porous membrane with an aperture of 0.45 m. The filter uses flame atom absorption or ICP to determine the content of lead, chromium and cadmium, and atom absorption with hydrogenation or atomic fluorescence to test the content of mercury. There are several factors in the experiment that affect the experimental data. The literature reported that hydrochloric acidity (pH is 1.00, 1.15, 1.20, 1.30, 1.40, 1.50 when different data are produced), extraction time, the size of the crushed particles will affect the results of the determination of soluble lead content in the coating. The purpose of this paper is to compare the effects of different paint film thickness and crushed particle size on the determination of soluble lead and chromium, so the above factors mentioned in the control literature are constant values, and experiments are carried out under these conditions.
1 Experimental Part
1.1 Instruments and Reagents
Instruments: AA-6300CF Atomic Absorption Spectrograph, Izu Corporation; PCLS-10 Precision Chloride (with pH function), Shanghai Kangyi Instrument Co., Ltd.; pH electrode, Shanghai Kangyi Instrument Co., Ltd.; Magnetic mixer, IKA; Sibel shredder, Shanghai Shuai Jia Electronics Co., Ltd.; BS 224S electronic balance, Sartorius; DHG-9075A type electric blower drying box, Shanghai Heng Scientific Instruments Co., Ltd.; QSX-21A thickness meter, Tianjin Construction Instrument Factory.
reagents and solutions: hydrochloric acid, pure analysis, Beijing chemical plant; lead standard solution, chromium standard solution, National Institute of Steel Research.
1.2 Experimental Method
Mix a sample of solvent-based wood coating well and scrape it on a 15cm×20 cm glass plate (pre-soaked overnight with a solution of nitric acid, cleaned and dried). Part of the plate coated thicker, so that the thickness of the dry film is (100±10) sm, part of the plate is coated thinner, so that the thickness of the dry film is (30±5) m. After drying, the paint film of the same thickness is collected and crushed together as a batch of samples. The pre-processing process can be found in Table 1. The Pb and Cr contents of the solution were then detected by flame atom absorption. Standard solutions are ready-to-use.
: the content of soluble metals in
o'mg-samples in milligrams per kilogram (mg/kg); the test concentration of
ρ0-blank solutions in milligrams per liter (mg/L); the fixed volume of
V-hydrochloric acid solutions, in milliliters (mL);
F - dilution of test solutions;
m - the amount of sample taken in grams (g).
1.3.2 results
the standard requires corrections based on test results due to the accuracy of the test method. The correction coefficients of lead and chromium are 30%, i.e. the result is minus 30%.
2 Results and discussion of the
2.1 paint film thickness impact
different thickness of the paint film each prepared 3 batches, the same way of crushing, the degree of crushing is about the same, each batch of two parallel samples, according to the conditions of Table 1 test, the experimental results are shown in Table 2 and Figure 1.
above results can be seen, in the ambient temperature and humidity, pH, stirring speed and other conditions are the same, due to the paint film thickness and thin different data differences. In the two sets of data of membrane thickness (100±10) sm and membrane thickness (30±5) sm, the relative deviation of soluble lead was 23.5% and the relative deviation of soluble chromium was 23.6%. The limit value of soluble lead is ≤90 mg/kg, both test results are exceeded and judged to be unqualified, and the limit value of soluble chromium is ≤60 mg/kg, but one of the two measurement results is within the limit value and one exceeds the limit value, causing differences in judgment.
2.2 Impact of the size of the crushed particles
the film thickness of about 30 m of the paint film with two different crushing methods, after screening the sample, a large particle, a small particle. Tested according to the conditions of Table 1, the results are shown in Tables 3 and 2.The test results of the paint film with different crushing degrees of about 30
m thickness of
table 3 show that the determination value of soluble lead and chromium with large sample particles is low, and the measurement value of sample particles is high, which is caused by different sample crushing degree and different sample particle size distribution. Table 2 and Table 3 show that the thickness of the prepared paint film, large particles, soluble metal content of the low measurement value, paint film thin, small particles are high. This is because the lacquer film particles are easier to dissolve than heavy metals with a larger surface area, and the measurement values increase, and vice versa. According to the influence of many factors discussed in the literature on the results, it can be inferred that all the experimental conditions that are good for dissolving will increase the measurement value, such as thin paint film, small crushed particles, high indoor temperature, high acidity, etc., will lead to the increase of the measurement value. This method, as a method to evaluate the soluble metal content level in the coating, is different from the determination of the total lead content in the coating, and the resulting measurement value is not "true value", but reflects the situation of heavy metal dissolution under certain conditions. The standard is limited to the reasons for the accuracy of the method, and the results are corrected. In some cases, however, differences in decision-finding can still arise. If we want to get more uniform results, we need to unify pre-processing methods and experimental parameters to improve the consistency of results.
3 Conclusion
Above experiments show that the test data of soluble lead and soluble chromium produced by the thickness of the paint film and the different size of the crushed particles are different under the same conditions as room temperature, pH, stirring speed and crushing method. It can be inferred that any experimental conditions that are good for element dissolution will raise the measurement value. If we want to get more uniform results, we need to unify pre-processing methods and experimental parameters to improve the consistency of results.