Effect of different nucleating agent on crystallization kinetics and morphology of polypropylene

2.1 Materials

Homogeneous PP powder (Daqing petrochemical company, China) was blended with different kinds of nucleating agent. Carboxylate nucleating agent (MD), rosin nucleating agent (WA), Sorbitol nucleating agent (NX) and Phosphate nucleating agent (NA), were purchased from Mudanjiang Beida fine chemical plant (China), Zhejiang Wanan plastic co., ltd. (China), American Milliken chemical company (America) and Asahi chemical co., ltd. (Japan), respectively.

2.2 Preparation of sample

Polypropylene powder and four different kinds of nucleating agents were respectively added into a high-speed mixer according to a fixed proportion for mixing. The mixture was melt extrude and pelletized in TSSL-25 corotating twin-screw extruder with a diameter of 25 mm and a length/diameter ratio of 36/1. The screw configuration was designed to provide better mixing and refining dispersion. The barrel temperatures from the feed zone to the die one were set as follows: 190, 210, 220, 220, 220, 220, 210, 200 and 195°C. The screw rate and feed rate were kept constant at 65 rpm and 21 rpm, respectively. The sample sheet with a specified thickness of 1 mm was prepared on a flat vulcanizing machine for mechanical property testing.

2.3 Charaterization

The infrared spectrum in the range of 600~4000 cm^-1 was recorded by a FTIR spectrometer (Mod. SPECTRUM GX 2000, Perkin/Elmer Ltd., Beacons-field, Buckshire, UK), and the resolution was 4 cm^-1. Samples for FTIR measurements were prepared by realizing films of PP pellets, which were 30 um thick by hot pressing at 210°C. According to the FTIR spectrum, the crystallinity can be calculated with the Eq. 1, wherein k is a constant, and for convenience of comparison, k is set to 1, and a crystallization spectrum band 998 cm^-1 and an amorphous spectrum band 1460 cm^-1 related to a long spiral segment are selected as internal standards for measuring the crystallinity of the PP film, and A₉₉₈ and A₁₄₆₀ is at the wave numbers 998 cm^-1 and 1460 cm^-1, respectively (20).

At room temperature, a wide-angle X-ray diffraction (WAXD) measurement was performed by the D/max-2500/PC X-ray diffractometer (Rigaku, Japan), which used CuKα radiation operated at 35 kV and 50 mA. The data were collected from 6° to 50° at 3°/min scanning rate. All samples were treated uniformly before testing. According to the XRD spectrum, the integral strength of all crystal sharp diffraction peaks and amorphous diffuse scattering peaks can be calculated by using a computer peak splitting program, and the crystallinity can be calculated according to Eq. 2, wherein Xc represents the crystallinity, Ic represents the integral strength of crystal sharp diffraction peak, and Ia represents the integral strength of amorphous diffuse scattering peak (21).

For the calorimetric investigation, about 6 mg of PP sample was weighed in Sealed aluminum foil, which is then heated from 50°C to 200°C using a Perkin - Elmer Pxrisl type differential scanning calorimeter under N₂ (50ml / min of gas flow), and kept at 200°C for 5 min (to eliminate thermal history and make it completely molten), and then DSC curves of various polypropylene samples at 10°C/min were obtained. The degree of crystallinity of PP was calculated by the Eq. 3, wherein Xc represents the crystallinity, wherein △H_m represents the melting enthalpy of PP samples, △H₀ represents the melting enthalpy of PP samples with crystallinity of 100% ( 207 J/g) (22).

For the isothermal crystallization: about 6 mg of PP sample was weighed in a crucible, and the sample was rapidly heated from 50°C to 200°C using a Perkin - Elmer Pxrisl type differential scanning calorimeter under N₂ (50ml / min of gas flow), and kept at 200°C for 5 min (to eliminate heat history and make it completely molten), and quickly cooled sample to a set temperature (110-140°C), and kept at the constant temperature until crystallization was completed, and then DSC curves of various polypropylene samples were obtained.

There are many other equations able to perform kinetics studies on the crystallization of polymer as shown in Eq. 4, in which X_t represents the relative crystallinity of the crystallization time of t, Z_t is the crystallization rate constant, which is related to nucleation rate and crystallization rate, n is Avrami index, which represents the crystallization nucleation and growth mechanism of the polymer.

The logarithm of the above formula is taken to obtain Eq. 5.

According to the plot of 1n(-1n(1-X_t)) and 1nt, the linear slope n and intercept 1nZ_t can be calculated by the linear equation, and the values of n and Z_t can also be calculated, thus obtaining the information on the crystallization growth of polypropylene.

According to the non-isothermal crystallization DSC curve, the relative crystallinity X_t at any crystallization temperature can be calculated by Eq. 6. In the Eq. 6, T₀ represents the temperature at the beginning of crystallization, T_∞ represents the temperature at the completion of crystallization, T represents the crystallization temperature, and H represents the enthalpy of crystallization. By time-temperature conversion, the Avrami equation can be used to calculate the relationship between the non-isothermal crystallization time t and the relative crystallinity X_t (23, 24).

For isothermal crystallization kinetics, X_t is defined as Eq. 7 (25, 26, 27).

dH/dt is the heat flow rate and X_t is the crystallinity at time t. The relationship between the relative crystallinity t and X_t can be obtained by integrating the DSC curve with the crystallization time t. Using 1n(-1n(1-X_t)) to plot 1nt, the linear slope n and intercept 1nZ_t can be obtained from the linear equation, and then the values of Z_t and n can be obtained.

PLM polarizing microscope measurements were performed by XP-201 polarization microscope test. Firstly, the prepared 0.5 mm sheet was placed on a constant temperature slide of an electric furnace at 260°C, and the cover slide was added after melting, and the temperature was kept for 1 minute, and then the 0.5 mm sheet was quickly put into an oven at 140°C, and crystallized for 1 h, and then taken out.

According to X-ray diffraction theory, the crystal size can be calculated by Debye-Scherrer formula (Eq. 8 and 9) (28), as shown in Eq. 8, where λ = incident wavelength, D = grain diameter, β_hkl = half peak width of diffraction peak, θ = diffraction angle, wherein λ is 0.15406 nm, k = 0.9, β_hkl = β*2π/360 (converted to radians).

Crystal nucleus density, which is a physical quantity to express the size of polymer spherulites, refers to the number of crystal nuclei in the unit volume, can be calculated directly from the average spherical crystal diameter. The calculation Eq. 10 is:

3.1 Effect of nucleating agent on crystallinity of polypropylene

Four modified polypropylene samples were tested by FTIR (as shown in Figure 1), XRD (as shown in Figure 2) and DSC. DSC melt enthalpy data of four modified PP were shown in Table 1. According to Eq. 1-3, the respective crystallinity was calculated.

Figure 1

Infrared spectra of modified PP.

Figure 2

XRD spectra of modified PP.

The crystallinity calculated by various test methods is shown in Table 2. Four nucleating agents make the crystalline core of PP easier to form, and significantly increase the crystallinity of PP, in which the crystallinity of the modified PP prepared with NX nucleating agent increased greatly.

3.2 Effect of nucleating agent on non-isothermal crystallization kinetics of polypropylene

According to the non-isothermal crystallization DSC curves (as shown in Figure 3, the cooling rate was 10°C/min), the relative crystallinity X_t at any crystallization temperature can be calculated by Eq. 6. By time-temperature conversion, the Avrami equation can be used to calculate the relationship between the non-isothermal crystallization time t and the relative crystallinity X_t, as shown in Figure 4.

Figure 3

Non-isothermal crystallization DSC curve of nucleated polypropylene.

Figure 4

Non-isothermal crystallization curve relative crystallinity fraction.

According to Eq. 4 and 5, 1nt is plotted with n(-1n(1-X_t)), and the curve relationship is shown in Figure 5. It can be seen from the figure that 1n(-1n(1-X_t)) has a good linear relationship with 1nt, only at high crystallinity, the data points deviate from the linear law. The rate constants Z_t and Avrami exponent n are calculated by slope and intercept. According to the correction of cooling rate, that is, lnZ_c = lnZ_t/φ, the rate constant Z_c of the non-isothermal crystallization process can be calculated, and the calculation results are shown in Table 1. When X_t = 50%, the half crystallization time t_1/2 can also be calculated according to the calculation formula t_1/2 = (ln2/Z_t)^1/n.

Figure 5

Non-isothermal crystallization ln (-ln (1-Xt)) on a linear relationship lnt.

Z_c is a non-isothermal crystallization kinetic rate constant, and the larger the value, the faster the crystallization rate of polypropylene is. It can be seen from Table 3, the n value of each modified polypropylene is not different, which indicates that each nucleating agent makes the polypropylene exhibit heterogeneous crystallization, and changes the crystallization nucleation and growth mode of the polypropylene. Compared with other nucleating agents, NX nucleating agents significantly increase the crystallization rate of polypropylene.

3.3 Effect of nucleating agent on isothermal crystallization kinetics of polypropylene

According to isothermal crystallization curve and Eq. 7, the relationships between crystallization time t at different crystallization temperatures and relative crystallinity X_t can be obtained by integrating DSC curves of different nucleating agents modified polypropylene (Figure 6)

Figure 6

Isothermal crystallization relative crystallinity fraction curve.

The crystallization time of modified polypropylene increases with the increase of crystallization temperature, which indicates that the crystallization rate of modified PP decreases gradually. Meanwhile, compared with pure PP, nucleating agent can greatly reduce the semi-crystallization time and increase the crystallization rate of polypropylene, thus shortening the forming and processing period of the product. According to Eq. 5, ln(-ln(1-X_t)) was plotted as shown in Figure 7.

Figure 7

Isothermal crystallization ln (-ln (1-Xt)) on a linear relationship lnt.

There is a good linear relationship between ln(-ln(1-X_t)) and ln_t. The rate constants Z_t and Avrami exponent n can be obtained from the intercept and slope of the curve, so that X_t = 50%, and the half crystallization time t_1/2 can be calculated. The specific numerical values are shown in Table 4.

Table 4 that Avrami index has little change at different temperatures, indicating that nucleating agent has little effect on the crystallization mode of polypropylene. The crystallization rate constant Z _t decreases with the increase of crystallization temperature, which indicates that the crystallization temperature increases, and the growth rate and nucleation rate both decrease. In addition, the addition of nucleating agent shortens the semi-crystallization time, in which the semi-crystallization time of NX-modified polypropylene is 4.12 min, and the kinetic constant Z_t value is 0.106, which indicates that NX-nucleating agent has good nucleating effect.

3.4 Effect of nucleating agent on crystallization rate of polypropylene

At present, the crystallization rate is characterized by t_1/2 (semi-crystallization time), △T_c (difference between initial

crystallization temperature T_c^on and peak temperature T_c^p), and S (initial slope of crystallization peak). These three kinetic rate parameters are obtained by DSC test. t_1/2 is calculated by avramni equation. According to Figure 3, t_1/2, △T_c, S and other parameters are shown in Table 5.

△T_c value represents the crystallization rate. The smaller the value is, the faster the crystallization rate goes. From Table 5, the crystallization parameters of the modified

polypropylene such as T_c^on, T_c^P, S values are significantly improved, where the S value represents the initial nucleation rate, and the larger the absolute value is, the faster the crystallization rate goes. By comparison, NX nucleating agent has the best effect on increasing crystallization rate, the △T_c value is the smallest, the S value is the largest, and the semi-crystallization time is the shortest.

3.5 Effect of nucleating agent on crystallization temperature of polypropylene

The melting and crystallization behavior of polypropylene were measured by DSC method. The melting temperature T_m and crystallization temperature T_c of modified PP are shown in Table 6.

Nucleating agents significantly increases the crystallization temperature of polypropylene (Table 6). In addition, because the nucleating agent reduces the nucleation free energy, the polypropylene is crystallized at a high temperature, the crystallization is more perfect, and the crystallization melting point is higher, wherein the nucleating agent NX has better effect on improving the crystallization and melting temperature of the polypropylene.

3.6 Effect of nucleating agent on spherulite size of polypropylene

In order to verify the change of nucleating agent on the spherulite size of polypropylene, the crystal size was characterized by XRD.

The average grain diameter size of each PP and nucleated PP was calculated according to the above Eq. 8 and 9, as shown in Table 7.

Nucleating agent can change the spherulite size of polypropylene, increase the crystallization rate and decrease the spherulite size.

According to Eq. 10, crystal nucleus density of polypropylene were calculated, as shown in Table 8.

From Table 8 (nucleated PP spherulite size and nucleation density), we can see the crystal nucleus density of the modified polypropylene increases and

the spherulite size decreases. The nucleating agent NX modified polypropylene has the smallest spherulite size and the largest crystal nucleus density, which indicates that the nucleating effect is the best.