CN101672774A - Method for quickly measuring content of polyvinylidene fluoride in fluorine-carbon paint of chromatic coating steel plate by infrared spectrum method - Google Patents

Abstract

The invention relates to a method for measuring PVDF content in fluorocarbon coatings on color-coated steel sheets by infrared spectroscopy. This method selects the absorption peak of PVDF in the 894-863cm ^-1 band and the absorption peak of acrylic resin in the 1764-1696cm ^-1 band, and uses Lambert-Beer's law as the theoretical basis to calculate the peak areas respectively, and establish the ratio of PVDF to acrylate content and The linear relational model of the peak area ratio can directly obtain the PVDF content in the unknown sample by calculating the peak area of the corresponding group in the infrared spectrum of the unknown sample. Compared with the traditional method, this method is quick and simple, can improve the analysis efficiency, reduce the analysis cost, and has strong practicability.

Description

Method for rapid determination of polyvinylidene fluoride content in fluorocarbon coatings on color-coated steel sheets by infrared spectroscopy

technical field

The invention relates to a method for measuring the content of polyvinylidene fluoride (PVDF) in fluorocarbon coatings on color-coated steel plates by infrared spectroscopy.

Background technique

With the rapid development of science and technology and social economy, steel manufacturing technology and products are changing with each passing day. Color-coated steel plates with bright appearance, good corrosion resistance, easy processing, long service life, less pollution, and practical functions have entered our lives. And has been widely used in various fields such as construction industry, transportation, household appliances, furniture and office appliances. There are many kinds of coatings for color-coated steel sheets. Among them, fluorocarbon coatings have excellent outdoor durability, acid rain resistance, air pollution resistance, corrosion resistance, stain resistance and mold resistance due to the large amount of C-F bonds in the resin. It has good comprehensive performance.

At present, the fluorocarbon coatings used in industry are mainly acrylic resin/PVDF system. The fluorine content in the system is closely related to the performance of the coating, and the fluorine content directly determines the quality and performance of the coating. So far, there is no unified detection standard for fluorine content in fluorocarbon coatings in China. The traditional method for measuring PVDF content requires separation, drying, decomposition and conversion to measure fluoride ion content. The result is calculated and the process is complicated and the test period is long. It is of practical significance to establish a fast and accurate method for the determination of PVDF content in coatings.

Contents of the invention

The object of the present invention is to provide a method for measuring polyvinylidene fluoride PVDF content in fluorocarbon coatings for color-coated steel sheets by infrared spectroscopy.

The positions of the characteristic peaks of infrared spectra of different groups are different, and the CF bond vibration of polyvinylidene fluoride PVDF appears at 1404cm ^-1 , 1190cm ^-1 , 1075cm ^-1 and 882cm ^-1 in the infrared spectrum of polyvinylidene fluoride PVDF The characteristic absorption peak of the acrylic resin's infrared spectrum shows the characteristic absorption peak of the ester carbonyl group at about 1730cm ^-1 , and the characteristic absorption peak of the ether bond of the acrylic resin appears at 1250cm ^-1 -1100cm ^-1 . The absorption peak of polyvinylidene fluoride PVDF in the band of 894-863 cm ^-1 has no mutual interference with acrylic resin, and the absorption peak of acrylic resin around 1730 cm ^-1 has no mutual interference with polyvinylidene fluoride PVDF. Therefore, the peaks with wavenumber ranges of 894-863cm ^-1 and 1764-1696cm ^-1 were selected to quantify the contents of polyvinylidene fluoride PVDF and acrylic resin, respectively.

1) According to Beer's law A=abc

A is the absorbance, a is the absorbance, b is the thickness of the sample, and c is the concentration of the absorbing substance. That is, when a beam of parallel monochromatic light passes through a uniform non-scattering light-absorbing material vertically, its absorbance A is proportional to the concentration c of the light-absorbing material and the thickness b of the absorbing layer. The peak area of the infrared spectrum is directly proportional to the absorbance A, that is, the peak area S∝absorbance A. Therefore:

$\frac{{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="S"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">S</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="A"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">A</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; b</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="b\; c"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">b</figure-callout></span><figure-callout\; id="2"\; label="b\; c"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">\; </figure-callout>}{\mathrm{<span\; class="notranslate">\; </span>}}_{}{\mathrm{<span\; class="notranslate">c</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="c"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">c</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; *</span>\frac{{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="m"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">m</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; V</span>}}{{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="m"\; filenames="A2009101965280008H1.png"\; state="\{\{state\}\}">m</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; V</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; K</span>}\frac{{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; w</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

Among them, S ₁ is the absorption peak area of polyvinylidene fluoride PVDF in the 894-863 cm ^-1 band, S ₂ is the absorption peak area of acrylic resin in the 1764-1696 cm ^-1 band; a ₁ is the polyvinylidene fluoride PVDF in the The absorption rate in the range of 894-863cm ^-1 , a ₂ is the absorption rate of acrylic resin in the range of 1764-1696cm ^-1 ; w ₁ is the mass of polyvinylidene fluoride PVDF, w ₂ is the mass of acrylic resin; m ₁ is the molar mass of polyvinylidene fluoride PVDF, m ₂ is the molar mass of acrylic resin; V is the sample volume; K is a constant.

It can be seen from the above formula that the peak area ratio is directly proportional to the concentration ratio.

2) Regression analysis method

The regression line can be expressed by the following equation

y=p+qx

In the formula, p is the intercept of the straight line, and q is the slope of the straight line.

The calculation formulas of p and q are respectively

$\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; q</span>}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; q</span>}\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}$ $\mathrm{<span\; class="notranslate">\; q</span>}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span>}{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

In the formula, x and y are the average values of x and y respectively. When p and q are determined, the linear working curve and linear regression equation are determined.

Whether the regression line is meaningful can be tested by the correlation coefficient R. The definition of the correlation coefficient is

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; q</span>}\sqrt{\frac{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; =</span>\frac{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span>}{\sqrt{\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; -</span>\stackrel{<span\; class="notranslate">\; \&OverBar;</span>}{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}$

The physical meaning of the correlation coefficient is as follows:

When aR ² =1, it means that there is a complete linear relationship between the two variables;

When bR ² =0, it means that there is no linear relationship between the two variables;

When cR ² is between 0 and 1, it means that there is a correlation between two variables.

Therefore, the closer the ^R2 value is to 1, the better the linearity.

According to above-mentioned mechanism, the present invention adopts following technical scheme:

A method for measuring polyvinylidene fluoride content in fluorocarbon coatings for color-coated steel sheets by infrared spectroscopy, characterized in that the specific steps of the method are as follows:

a. Configure a series of samples with known polyvinylidene fluoride content with polyvinylidene fluoride resin and pure acrylic resin, and measure the infrared absorption spectrum of each sample at a wave number between 400-4000cm ^-1 ;

b. Calculate the 894-863cm ^-1 band absorption peak and the 1764-1696cm ^-1 band absorption peak in the infrared absorption spectrogram obtained in step a respectively to carry out peak area integration, and the ratio of the two peak area integral values to the polypolarized bismuth in the sample Perform linear regression simulation on the content ratio of fluoroethylene resin and acrylic resin to obtain a linear working curve and a linear regression equation;

c. Measure the infrared absorption spectrum between 400-4000cm ^-1 of the sample to be tested, calculate the peak area integral of the 894-863cm ^-1 band absorption peak and the 1764-1696cm ^-1 band absorption peak, and obtain the ratio of the two peak area integral values , into the linear regression equation obtained in step b to obtain the polyvinylidene fluoride content of the sample to be tested.

The method of the present invention adopts the infrared spectrum which is relatively simple in measurement to establish the linear relational expression of the PVDF content in the fluorocarbon coating of the color-coated steel plate, can quickly complete the collection of the infrared spectrum of the unknown sample, and obtain its PVDF content, greatly improving the measurement efficiency. Improve analysis efficiency and reduce analysis cost. The testing method of the invention is rapid, simple and convenient, and the data is accurate.

Description of drawings

Fig. 1 is the infrared spectrogram of pure acrylic resin.

Fig. 2 is the infrared spectrogram of pure PVDF sample.

Figure 3 is a linear working curve, that is, the relationship between the peak area ratio of the absorption peak of PVDF resin in the 894-863cm ^-1 band and the absorption peak of acrylate resin in the 1764-1696cm ^-1 band and the content ratio of PVDF and acrylic resin.

Detailed ways

The method of the present invention adopts the relatively easy mid-infrared spectrum in measurement to set up the linear relational expression of PVDF content in the fluorocarbon coating of color-coated steel plate, below in conjunction with accompanying drawing and implementation the present invention is described in further detail:

Example 1: A series of coating samples with known PVDF content were prepared with PVDF resin and pure acrylic resin. After coating and curing, the infrared absorption spectra of various samples were measured with a Fourier transform infrared spectrometer. The corresponding data obtained by integrating the peak area of the 894-863 cm ^-1 band absorption peak and the 1764-1696 cm ^-1 band absorption peak are listed in Table 1. Taking the ratio of peak area as the abscissa, and the content ratio of PVDF and acrylic resin as the ordinate, a linear working curve and a linear regression equation are obtained through normalization.

Table 1

sample PVDF content PVDF to acrylate content ratio (y=w ₁ /w ₂ ) 1764-1696cm ^-1 peak area integration 894-863cm ^-1 peak area integration Peak area integral ratio (x=S ₁ /S ₂ ) 1 10% 0.111 1.34 0.00500 0.00373 2 30% 0.429 1.27 0.122 0.0959 3 50% 1.00 1.12 0.299 0.266 4 70% 2.33 0.719 0.515 0.716 5 80% 4.00 0.570 0.682 1.20

Take x as abscissa, y as ordinate plotting, obtain linear regression curve as shown in accompanying drawing 3, gained linear relational formula is y=3.22x+0.107, and linear regression coefficient is 0.999, and the content of PVDF=w ₁ /( w ₁ +w ₂ )=1/(1+w ₂ /w ₁ )=1/(1+y ^-1 ), substituting the characteristic peak area ratio of the sample to be tested with unknown PVDF content into this formula to obtain PVDF content.

Embodiment 2: The present invention is illustrated by examples below, but the present invention is not limited thereto. The sample is a color-coated steel plate with a known PVDF content in a steel plant. After measuring the infrared spectrum of the color-coated steel plate with the Nicolet380 mid-infrared spectrometer of Nicolet Company of the United States and the attenuated total reflection (ATR) accessory, the peak area and peak area of the characteristic peaks are calculated. Its ratio, obtains the content of PVDF in the sample to be tested by the established linear relational formula, and the result is shown in Table 2.

Table 2

sample Volume number color factory 1764-1696cm ^-1 peak area integration 894-863cm ^-1 peak area integration Peak area integral ratio PVDF to acrylic acid content ratio The measurement results Actual content 1 379xxx301 brass Berg 0.206 0.244 1.18 3.92 79.7% 80% 2 379xxx201 oyster white Berg 0.257 0.203 0.790 2.65 72.6% 73%

It can be seen from the verification results that the linear relational formula obtained by this method can quickly and simply determine the content of PVDF in PVDF coatings.

The color-coated steel plates (sample 1 and sample 2) with unknown PVDF content in a steel plant and the coatings (sample 3 and sample 4) sent to a steel plant by a paint plant were selected. After using the Nicolet380 mid-infrared spectrometer of Nicolet Company of the United States, the attenuated total reflection (ATR) accessory measures the infrared spectrum of the color-coated steel plate, calculates the peak area and its ratio of the characteristic peaks, and obtains the measured value through the established linear relationship The content of PVDF in the sample, the results are shown in Table 3.

table 3

sample Volume number color factory 1764-1696cm ^-1 peak area integration 894-863cm ^-1 peak area integration peak area integration ratio PVDF to acrylic acid content ratio The measurement results 1 173xxx501 brass Manufacturer A 0.277 0.243 0.877 2.93 74.6% 2 378xxx301 oyster white Manufacturer A 0.402 0.143 0.356 1.25 55.6% 3 / White Manufacturer B 0.812 0.386 0.476 1.64 62.1% 4 / White Manufacturer A 0.525 0.299 0.570 1.94 66.0%

Claims (1)

1, the method for content of polyvinylidene fluoride in a kind of infrared spectroscopic determination fluorine-carbon paint of chromatic coating steel plate is characterized in that the concrete steps of this method are as follows:

A. dispose the sample of a series of known content of polyvinylidene fluoride with polyvinylidene fluoride resin and pure acrylic resin, recording each sample is 400-4000cm in wave number ^-1Between infrared absorpting light spectra;

B. 894-863cm in the infrared absorpting light spectra of calculation procedure a gained respectively ^-1Wave band absorption peak and 1764-1696cm ^-1The wave band absorption peak carries out the peak area integration, and polyvinylidene fluoride resin in the ratio of this two peak areas integrated value and the sample is carried out the linear regression simulation with acryl resin content ratio, draws linear work curve and equation of linear regression;

C. measure testing sample 400-4000cm ^-1Between infrared absorpting light spectra, calculate 894-863cm ^-1Wave band absorption peak and 1764-1696cm ^-1The peak area integration of wave band absorption peak draws the ratio of two peak area integrated values, and substitution step b gained equation of linear regression draws the content of polyvinylidene fluoride of testing sample.

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