Ultrafine dispersion of a phosphate nucleating agent in a polypropylene matrix via the microemulsion method

Abstract

To reduce the nucleator's size and improve its dispersion, microemulsions of sodium 2,2′-methylene bis-(4,6-di-tert-butylphenyl) phosphate (NA11) and NA11–sodium benzoate (SB) composite nucleators were prepared by choosing ethanol as NA11's solvent. NA11 nucleator within ethanol was dispersed on the interface of droplets, whose size is ca. tens of nanometers as characterized by dynamic light scattering and cryo-TEM. Investigations on PP's crystallization behavior as well as on the mechanical properties indicate that NA11's nucleation efficiency can be further improved using the microemulsion method. However, the blank microemulsion can adversely affect PP's crystallization and mechanical properties due to its residual oil and surfactant components in the PP matrix. A synergistic effect was found in the NA11–SB composite microemulsion, which shows a smaller size of the dispersed phase and an improvement to PP's stiffness. A possible dispersion mechanism of the nucleator microemulsions in the final PP matrix is proposed according to the microemulsions' structure and their nucleation effects.

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Introduction

Polypropylene (PP), as a semicrystalline polymer, is one of the most important thermoplastic resins and widely used in many fields owing to its low manufacturing cost and rather versatile properties.^1–3 It is widely accepted that the properties of semicrystalline polymers depend on their crystalline structure. For some semicrystalline polymers, such as PP, the addition of nucleating agents is significant for modifying the fine structure and improving the physical properties of the final products.^4–8

Commercially available PP nucleators can be broken down into two classifications, i.e. “melt sensitive” and “melt insensitive”. Melt sensitive nucleators include sorbitol-based compounds, whose melting point is below the normal processing temperature for PP-based resins. Melt insensitive nucleators include a broad range of compounds including benzoate salts and the metallic salts of organic phosphates, such as NA11. Melt insensitive nucleators act as single point nucleation sites within the matrix. Hence, it is easy to understand that nucleators with smaller sizes should initiate more nuclei than those with larger particle size at the same loading. In addition, the adequate dispersion of nucleators prior to compounding is critical for melt-insensitive nucleators.⁹

Since melt insensitive nucleators tend to aggregate in a PP matrix, many researches working on nucleators have focused on further reducing nucleators' size and improving the dispersion of nucleators in a PP matrix to improve the nucleation efficiency and mechanical properties of PP.^10–20 For example, Takahashi et al. have produced nucleating agents containing a phosphoric acid aromatic ester metal salt with micron-sized particles by a pulverization apparatus, and the resulting PP modified with these nucleating agents exhibited an improved flexural modulus.¹³ In order to improve the dispersion of nucleating agents in PP, Zhu et al. have introduced nucleating agents containing nano-CaCO₃ particles and organic phosphate into the PP copolymerization system.¹⁴ Many more nucleation sites were formed by this method, which led to a further increase of the crystallization rate of the PP copolymer. Meanwhile, the mechanical properties and heat distortion temperature have also been improved.

Microemulsions with a nanosized structure, are optically isotropic and thermodynamically stable mixtures of water, oil, and amphiphiles.²¹ They usually contain cosolvents or cosurfactants to achieve a low interfacial tension and the required packing parameters. The inner core and interface of the microemulsion can serve as a “nanoreactor” for many organic and biochemical syntheses and for controlled crystallizations.^22–28 Microemulsions are used extensively in many industrial, cosmetic, food and pharmaceutical applications.^29–35 In recent years, this technology has also been tried to disperse nucleating agents in PP. Libster et al. prepared a microemulsion containing a water soluble nucleator, HPN-68, and found that the nucleation efficiency of PP can be improved by 24% with the microemulsion method.^15,16 However, HPN-68 is not a widely used nucleator and the effect of this nucleator microemulsion on the mechanical properties of PP was not studied.

Our research interest centers on some widely used commercial nucleators, such as NA11 and SB. It is of great industrial value to further improve their nucleation efficiencies and their modification effects on PP's mechanical properties. The sizes of these commercial nucleators are usually in the range of several microns to tens of microns. The purpose of this study is to further reduce the particle sizes of these commercial melt insensitive nucleators and achieve an ultrafine/nanoscale dispersion of them in a PP matrix via the microemulsion technology. A smaller size and better dispersion will increase the nucleation efficiency of such melt insensitive nucleators, resulting in the improved crystallization and mechanical properties of PP with the nucleators with the same or even less loading.

In this work, a series of microemulsions containing NA11 and NA11–SB composite nucleators were prepared for the first time, and their microstructure was characterized in detail. Then, the influence of nucleator microemulsions on PP's crystallization behavior as well as its mechanical properties was investigated. A possible dispersion mechanism of nucleator microemulsions in the final PP matrix was proposed according to the microemulsions' structure and their nucleation effects. Finally, the advantages and disadvantages of dispersing nucleators via the microemulsion technology were discussed, and some possible improvements regarding this technology were also given.

Experimental

Materials

The PP copolymer powder, ethylene content 8%, MFR 5.5 g/10 min at 230 °C, was produced by the SINOPEC Beijing Research Institute of Chemical Industry. The nucleating agent NA11 was supplied by Luoyang Zhongda Chemical Co., Ltd. The nucleating agent sodium benzoate was purchased from the Tianjin Guangfu Chemical and Industrial Institute. Antioxidant Irganox 1010 and Irganox 168, were obtained from Ciba Specialty Chemicals. Ethyl butyrate and surfactant Tween 80 (polyoxyethylene sorbitan monooleate) were purchased from the Tianjin Guangfu Chemical and Industrial Institute. Ethanol was obtained from Beijing Chemical Works.

Preparation of microemulsion

As shown in Fig. 1(a), a pseudo-ternary phase diagram of the blank system based on ethyl butyrate (oil phase), ethanol (cosurfactant), Tween 80 (surfactant) and water was initially constructed according to the method in literature.^15,16 By replacing the ethanol component with the NA11–ethanol solution at a weight ratio of 1 : 10, the nucleator-containing phase diagram was obtained (Fig. 1(b)). Based on Fig. 1(a) & (b) we could choose different spots in the 1 phase region of the phase diagram to prepare microemulsions.

Fig. 1 (a) Phase diagram of the system composed of ethyl butyrate–ethanol (1 : 1 w/w) as the oil phase, Tween 80 as the emulsifier and water at 40 °C. (b) Phase diagram of the system composed of ethyl butyrate–ethanol–NA11 (10 : 10 : 1 w/w/w) as the oil phase, Tween 80 as the surfactant and water at 40 °C.

The formulas of the blank microemulsion and NA11 microemulsion were chosen first as given in Table 1, which contain 80% of water, 5% of ethanol, 5% of ethyl butyrate and 10% of Tween 80. The microemulsions containing SB were later tried by replacing water with an SB–water solution on the basis of the formulas of the blank microemulsion and NA11 microemulsion. The specific preparation steps are as follows: according to the formulas in Table 1, ethyl butyrate and ethanol (or NA11–ethanol solution) were first mixed in a conical flask; Tween 80 was then added and stirred to form a homogenous solution; finally, water (or SB–water solution) was added dropwise to the solution with stirring to obtain the microemulsion, which is homogenous, transparent and stable for at least 30 days without demixing.

Table 1 Microemulsion formula

Sample	Tween 80 content (wt%)	Ethyl butyrate content (wt%)	Ethanol content (wt%)	Water content (wt%)
Microemulsion	10	5	5	80
NA11 microemulsion	10	5	5 (ethanol : NA11 = 10 : 1)	80
SB microemulsion	10	5	5	80 (water : SB = 10 : 1)
NA11–SB microemulsion	10	5	5 (ethanol : NA11 = 10 : 1)	80 (water : SB = 160 : 1)

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Compounding procedure

The nucleator powder or nucleator microemulsion and antioxidant were homogenized with the PP powder in a high speed mixer (GH-100, Beijing Plastic Machinery Works) for 2 min according to the composition listed in Table 2. To facilitate the melt compounding procedure, the mixture containing the microemulsion was further dried naturally at room temperature for 24 hours to remove the vaporizable components such as water and ethanol within the microemulsion. The dry-mixture was melt-compounded on a twin-screw extruder (ZSK-25, Werner and Pleiderer, Germany) at a barrel temperature of 180–200 °C and a screw speed of 300 rpm.

Table 2 Composition and mechanical properties of PP samples

Sample code	Composition^a		Flexural modulus (GPa)	Flexural strength (MPa)	Notched Izod impact strength (kJ m^−2)	Tensile strength at yield (MPa)	Elongation at break (%)
	Nucleator type	Loading
a Antioxidant (1010 : 168 = 1 : 1) content: 0.3 wt%.b PP-m refers to PP modified with the blank microemulsion. The microemulsion loading is the same as the microemulsion containing 250 ppm nucleator.
Control	—	—	1.13 ± 0.01	25.8 ± 0.1	50.7 ± 1.6	23.2 ± 0.1	590 ± 60
PP-m^b	—	—	1.05 ± 0.01	24.3 ± 0.1	47.8 ± 2.4	22.9 ± 0.1	570 ± 40
PP-1	NA11 microemulsion	250 ppm	1.21 ± 0.01	26.9 ± 0.1	54.8 ± 4.3	23.0 ± 0.1	520 ± 30
PP-2	NA11 microemulsion	500 ppm	1.23 ± 0.01	27.3 ± 0.1	59.0 ± 5.3	23.1 ± 0.1	500 ± 20
PP-3	SB microemulsion	250 ppm	1.19 ± 0.01	26.9 ± 0.1	51.0 ± 1.2	23.4 ± 0.1	570 ± 60
PP-4	SB microemulsion	500 ppm	1.25 ± 0.01	27.9 ± 0.2	50.7 ± 2.5	23.6 ± 0.1	630 ± 70
PP-5	NA11–SB microemulsion	250 ppm	1.31 ± 0.02	29.2 ± 0.1	50.6 ± 2.1	23.7 ± 0.1	560 ± 60
PP-6	NA11–SB microemulsion	500 ppm	1.32 ± 0.01	29.0 ± 0.1	56.9 ± 4.1	23.4 ± 0.1	580 ± 50
PP-7	NA11 powder	250 ppm	1.28 ± 0.01	28.6 ± 0.1	58.6 ± 4.0	23.6 ± 0.1	600 ± 50
PP-8	NA11 powder	500 ppm	1.31 ± 0.01	29.2 ± 0.1	56.2 ± 4.4	23.8 ± 0.1	630 ± 60
PP-9	SB powder	250 ppm	1.22 ± 0.01	27.6 ± 0.1	51.7 ± 1.9	23.5 ± 0.1	580 ± 50
PP-10	SB powder	500 ppm	1.24 ± 0.01	28.1 ± 0.1	53.7 ± 3.5	23.5 ± 0.1	620 ± 60
PP-11	NA11–SB powder	250 ppm	1.27 ± 0.01	28.6 ± 0.1	51.4 ± 2.2	23.7 ± 0.1	590 ± 50
PP-12	NA11–SB powder	500 ppm	1.30 ± 0.01	29.2 ± 0.2	50.2 ± 1.4	23.9 ± 0.1	620 ± 60

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Characterization

Dynamic light scattering measurements (DLS). Dynamic light scattering measurements were performed at 25 °C using a Brookhaven BI-200SM goniometer and BI-9000AT autocorrelator (Brookhaven Instrument Corp., Holtsville, NY, USA). The scattered light (MGL-III-532, Changchun new industry optoelectronics Tech, Co., Ltd.) set at 532 nm and 100 mW was measured at the angle of 90° and was collected by the autocorrelator. The hydrodynamic diameters (d) of the oil droplets were calculated by using the Stokes–Einstein equation d = k_BT/3πηD, where k_B is the Boltzmann constant, T is the absolute temperature, η is the solvent viscosity and D is the diffusion coefficient.

Cryo-transmission electron microscopy (cryo-TEM). A 3.5 μL droplet of the microemulsion was gently placed on a carbon-coated copper grid under controlled environmental conditions at 22 °C and controlled humidity (>99% relative humidity) in a Vitrobot (Mark IV, FEI company, USA). The drop was blotted by filter paper creating a thin film of the liquid over the grid, which was then immediately vitrified in liquid ethane at its freezing temperature to ensure rapid vitrification and avoid the crystallization of water. The grids were then stored in liquid nitrogen and transferred under liquid nitrogen to a cryo-TEM holder (Model 626, Gatan, Inc., USA) which was introduced into the electron microscope TECNAI 20 (FEI, Netherlands). Imaging was carried out at a temperature of about −175 °C and 200 kV acceleration voltage.

Mechanical properties. Standard flexural tests were performed on a Universal testing machine (Z010, Zwick GmbH, Ulm, Germany) according to the ISO 178 standard. The tensile test was carried out on injection-molded dumb-bell specimens in a Universal testing machine (Z010, Zwick GmbH, Ulm, Germany) at a crosshead speed of 50 mm min⁻¹ (ISO 527-2 standard). The notched Izod impact strength was measured by using a Zwick HIT 25P impact testing machine (Zwick GmbH, Ulm, Germany) according to the ISO 180 standard. For these mechanical tests, at least five specimens were tested to obtain the average value. All specimens for mechanical tests were injection molded on a HTF110X/1J injection molding machine (Ningbo Haitian Plastic Machinery Co., Ltd.). The specimens were conditioned at 25 °C for 2 days before testing.

Differential scanning calorimetry (DSC). The crystallization temperatures of the PP samples were measured on a differential scanning calorimeter (Diamond DSC 7, Perkin Elmer, USA). The samples were first heated to 210 °C, and kept at this temperature for 5 min to eliminate their thermal history and then cooled to 40 °C at a rate of 10 °C min⁻¹.

Polarized optical microscopy (POM). The morphology of the PP spherulites was observed in situ on a hot stage using a polarized microscope (DMLP, Leica, Germany). Thin layers of the PP samples with thickness of ca.100 μm were investigated. For each study a new sample was used. The molten polymer was maintained at 210 °C for 5 min and cooled down at a rate of 100 °C min⁻¹ to 150 °C for isothermal studies, or at a cooling rate of 10 °C min⁻¹ to the end of crystallization for non-isothermal studies.

Results and discussion

Characterization of nucleator-containing microemulsions

The DLS results (Fig. 2) show that the NA11 microemulsion has a larger oil droplet size (ca. 49 nm) than the blank microemulsion (ca. 37 nm). This observation should be attributed to the solubilization effect on the droplet interface. The dispersion of NA11 in the microemulsion can be described using a schematic figure (Fig. 3), which suggests that the NA11 should be dispersed on the droplet interface owing to the cosurfactant function of the ethanol component in the microemulsion.

Fig. 2 Droplet diameter of the microemulsions: (a) blank microemulsion, (b) NA11 microemulsion, (c) SB microemulsion, (d) NA11–SB microemulsion.

Fig. 3 Pictorial representation of oil droplets in the microemulsion.

As ethanol was replaced by the NA11–ethanol solution while maintaining the same ratio of ethyl butyrate–ethanol, the amount of actual cosurfactant component, i.e. the NA11–ethanol solution, increased, thus resulting in a larger droplet size. As far as the SB microemulsion is concerned, its oil droplet size is also larger than the blank microemulsion, probably due to the intercalation of nucleator into the interfacial micellar film as proposed by E. Libster.^15,16 Interestingly, for the case of the NA11–SB composite microemulsion, the droplet size reduces compared with the blank microemulsion. The specific reason for the reduction of the droplet size in this composite nucleator system is still not clear.

Besides DLS measurements, the TEM micrographs taken from cryo-TEM allow us to estimate the size of the dispersed phase in a more direct way as well. As shown in Fig. 4, the small dark dots in the large circle represent the droplets in the microemulsion, and the large circle is the hole of carbon membrane supported on copper grids used for locating cryo-TEM samples. Even though the average droplet diameter was not calculated, it can still be observed from Fig. 4 that the NA11 and SB microemulsions show relatively larger droplets than the blank microemulsion. As for the NA11–SB composite microemulsion, the size of the droplets markedly decreases compared with the single nucleator microemulsion. All these results from the cryo-TEM agree well with the DLS results. It can be verified from these characterization results that the NA11 microemulsion has dispersed phase in the scale of tens of nanometers, which indicates that NA11 nucleator within the droplet interface can exist on a nano scale via the microemulsion method.

Fig. 4 Cryo-TEM micrographs of the microemulsion: (a and b) blank microemulsion, (c and d) NA11 microemulsion, (e and f) SB microemulsion, (g and h) NA11–SB microemulsion.

Crystallization behavior of nucleator-modified PP

The characterization results from last section have proved that the size of the oil droplets in the NA11 microemulsion is in the range of tens of nanometers. It can be further deduced that the NA11 component on the droplet interface should be in nano scale as well. However, it is hard to characterize by using conventional means, such as electron microscopy, whether the final dispersion state of NA11 after drying and melt-compounding procedures can still reach the nano scale since its loading in the PP matrix is quite low, only several hundred ppm. Hence, we have to estimate if the nucleator in microemulsion can be better dispersed in the PP matrix than in its original powder form through some indirect ways such as its influence on crystallization and mechanical properties of PP.

The crystallization behaviors of the PP samples were first studied using DSC. As shown in Fig. 5(a), where the nucleator loading is maintained at 250 ppm, the crystallization temperature (T_c) of virgin PP is 120 °C, and increases with introducing nucleator in powder form. For instance, the T_c of PP modified with NA11 powder, SB powder and NA11–SB composite powder can increase to 126.2, 127.1 and 127.5 °C, respectively. It can also be seen from in Fig. 5(a) that the blank microemulsion has an adverse effect on the crystallization of PP. After introducing the blank microemulsion into PP, T_c decreases from 120 to 117.9 °C. As the oil and surfactant components within the microemulsion can hardly be removed during melt compounding process, they are inevitably left in the PP matrix like plasticizers, thus hindering the crystallization of PP.

Fig. 5 DSC cooling curves of PP samples with nucleator loading: (a) 250 ppm, (b) 500 ppm.

However, the NA11-containing microemulsion shows a better nucleating effect compared to the NA11 powder. Although residual components of the microemulsion have negative effect on PP's crystallization, the T_c of PP modified with the NA11 microemulsion increases to 128.5 °C, which is higher than that with the same amount of NA11 powder. In contrast, the T_c of PP modified with the SB microemulsion is lower than that of PP modified with 250 ppm SB powder. The highest T_c of 129.7 °C is achieved for the PP modified with the NA11–SB composite microemulsion, suggesting a better final dispersion state of nucleators in the PP matrix. Despite the adverse effect of the blank microemulsion, NA11 and NA11–SB nucleators' nucleation efficiency can still be improved by the microemulsion method, due to their much better dispersion in the PP matrix.

When the loading of nucleator is increased to 500 ppm (Fig. 5(b)), the results are similar with the 250 ppm case, but the differences between nucleator powder and nucleator microemulsion become marginal. In particular, the T_c of the sample modified with the NA11–SB composite microemulsion is even lower than that with the NA11–SB composite powder, which is contrary to the 250 ppm case. This should be ascribed to the negative influence of residual components within the emulsion on the crystallization of PP, especially when a larger amount of microemulsion is incorporated into the modified PP sample. To offset such a negative influence, the loading of nucleator-containing microemulsion should be controlled as low as possible.

Besides DSC measurements, POM was employed to study the morphology of PP spherulites, which can help us deduce the dispersion state of nucleators in the PP matrix in another way. The non-isothermal crystallization results corresponding to a nucleator loading of 250 ppm are shown in Fig. 6. As observed in Fig. 6(a) and (b), the PP sample modified with the blank microemulsion displays larger spherulites in comparison with the virgin PP, which implies that the blank microemulsion has an adverse effect on PP nucleation and crystallization, which agrees with the DSC results above. The samples modified with SB powder and SB microemulsion show the same trend, i.e. the sample modified with SB microemulsion has larger spherulites compared with the SB-powder-modified sample (Fig. 6(e) and (f)), suggesting a lower nucleation efficiency in the SB microemulsion system. The differences between samples modified with NA11 powder and with NA11 microemulsion are not pronounced. Both samples show much higher nuclei density compared with SB modified samples, indicating a much higher nucleating efficiency of NA11 than SB. While for the NA11–SB composite nucleator system, as shown in Fig. 6(g) and (h), the NA11–SB composite nucleator contained in the microemulsion seems to be better dispersed with the help of the microemulsion than the original nucleator powder, as more nuclei can be found in the sample modified with the NA11–SB composite microemulsion.

Fig. 6 POM micrographs of PP samples modified with 250 ppm nucleating agents at 110 °C: (a) control, (b) blank microemulsion, (c) NA11 powder, (d) NA11 microemulsion, (e) SB powder, (f) SB microemulsion, (g) NA11–SB powder, (h) NA11–SB microemulsion.

In order to observe the morphology of PP spherulites more clearly, an isothermal crystallization experiment at 150 °C was also carried out. As compared with the non-isothermal crystallization experiments, much larger spherulites can be observed for the virgin PP (Fig. 7(a)). The effect of the blank microemulsion on crystallization as well as the variation trends of the nuclei density with the incorporation of nucleator microemulsion are all in agreement with the results of non-isothermal crystallization, except that more distinguished differences between nucleator powder and its corresponding microemulsion can be observed.

Fig. 7 POM micrographs of PP samples isothermally crystallized at 150 °C: (a) control, (b) blank microemulsion, (c) NA11 powder, (d) NA11 microemulsion, (e) SB powder, (f) SB microemulsion, (g) NA11–SB powder, (h) NA11–SB microemulsion.

Mechanical properties

It has been proved in the previous sections that the NA11 nucleator dispersed at the nano scale in its microemulsion can increase the T_c and reduce spherulite size of PP in a more effective way than in its powder form, indicating a better dispersion of nucleator in final PP matrix via the microemulsion method. From a practical point of view, it is more important to know whether the good dispersion of the melt insensitive nucleator can improve the mechanical properties of PP, especially the stiffness.

The mechanical properties of PP samples modified with nucleator powder and microemulsion are listed in Table 2. As far as stiffness is concerned, nucleator modified samples all show higher flexural modulus and strength than the PP control sample. For samples modified with various nucleator powders, the effectiveness of improving the stiffness of PP samples is in the order of NA11 > NA11–SB > SB with a nucleator loading of 250 ppm, suggesting that NA11 is more effective than SB, and that the NA11–SB composite nucleator's efficiency is in between. Furthermore, by increasing the loading of nucleator powder from 250 ppm to 500 ppm, the stiffness of PP samples can be further improved.

As for samples with the microemulsion, the flexural modulus of virgin PP decreases from 1.13 to 1.05 GPa after introducing the blank microemulsion with the same loading of the 250 ppm NA11 microemulsion, owing to the negative influence of residual oil and surfactant components within the microemulsion on the stiffness of PP. Compared to the blank microemulsion, the microemulsions loaded with nucleators result in a higher stiffness. For example, the flexural moduli of PP modified with 250 ppm NA11 or SB microemulsions are 1.21 and 1.19 GPa, respectively, which are much higher than that of PP modified with the blank microemulsion. These values, however, are still lower than those of PP samples modified with the corresponding nucleator powder due to the negative effect of the residual microemulsion components in the final PP samples. It should be noted that the flexural modulus of PP modified with the NA11–SB composite microemulsion reaches 1.31 GPa, which is much higher than those of the PP samples modified with NA11 or SB microemulsions alone, as well as with the corresponding NA11–SB composite powder. The negative effect of blank microemulsions on the stiffness is counteracted in the composite nucleator system. This may be attributed to a better dispersion of NA11 within the composite microemulsion, since the previous DLS and cryo-TEM results all show that the composite nucleator microemulsion has the smallest dispersed droplets, which may result in a finest dispersion of NA11 in the final PP matrix. As the loading of nucleator microemulsion increases to 500 ppm, the same trend of flexural modulus can be observed.

The tendency of flexural strength is similar to that of the flexural modulus. The PP modified with the NA11–SB composite microemulsion achieves the highest flexural strength at a microemulsion loading of 250 ppm. But a slightly lower flexural strength value is found in comparison with those modified with the NA11–SB composite powder or NA11 powder at a loading of 500 ppm.

As far as the impact properties are concerned (Table 2), the PP samples modified with nucleators are basically the same or slightly higher than the control sample, except that the blank microemulsion causes the reduction of impact strength. It is generally accepted that large spherulites can adversely affect impact properties and the spherulitic borders represent weak zones.³⁶ When the blank microemulsion was introduced in PP, it hindered the crystallization process and induced larger spherulites than the control sample, thus reducing the impact strength of PP. While for the PP samples modified with nucleators, the nucleation density increases and the size of PP spherulites decreases, resulting in the improvement of impact strength. It is worth noting that the PP samples modified with NA11 all show a higher impact strength than those with SB, which should be ascribed to a better nucleating ability of NA11 compared with SB.

With regard to the tensile properties presented in Table 2, the changes are not pronounced. The blank microemulsion causes a slight reduction in tensile strength. Most nucleator-modified samples show a very slight enhancement in the tensile strength compared with the control sample. This indicates that even though the nucleator can make the spherulite morphology finer, this change will not have a substantial influence on the tensile properties. Some published research results also de-emphasize the importance of spherulite size/morphology on the extensional properties of semicrystalline polymers.³⁶

Discussion of the dispersion mechanism of nucleator via the microemulsion method

According to the preparation and characterization of nucleator-containing microemulsion, as well as its effect on PP's crystallization and its mechanical properties, we propose a schematic diagram (Fig. 8) to summarize the dispersion mechanism of nucleators in the PP matrix via the microemulsion method.

Fig. 8 Schematic diagrams of the nucleator dispersion mechanism via the microemulsion method: (a) blank microemulsion, (b) NA11 microemulsion, (c) SB microemulsion, (d) NA11–SB microemulsion.

The blank microemulsion is discussed first as a control. As shown in Fig. 8(a), the evaporable water and ethanol components within the microemulsion were removed after the dry-mixing and drying procedure, with the oil and surfactant components left on the surface of PP powder. Through the melt compounding and injection molding procedures, most of the oil and surfactant components were still left in the PP matrix. These components act as plasticizer in the PP matrix and hinder the crystallization of PP, as proved by the DSC and POM results. These residual components are also harmful to the mechanical properties of the final PP samples.

As far as the NA11 microemulsion is concerned, the NA11 nucleator dissolved in ethanol is dispersed on the interface of the nano-size droplets, as evidenced by the DLS and cryo-TEM results. With the evaporation of ethanol and water, NA11 will crystallize and precipitate from the ethanol phase. The crystallites of precipitated NA11 may still be at the nano scale, which, however, is quite difficult to be proved using conventional characterization techniques. The DSC results suggest that the NA11 microemulsion has a higher nucleation efficiency compared to its powder. This may indirectly prove that NA11 in the microemulsion can achieve a finer dispersion in the PP matrix than its original micron scale powder.

As for the SB microemulsion (Fig. 8(c)), the SB nucleator is dissolved in the continuous water phase. After the dry-mixing and drying procedure, SB will crystallize in larger size on the surface of the PP powder, at least at the micron scale. On the other hand, as water evaporates, SB will be covered by the surfactant component. This will reduce the contact area of SB with the PP matrix and lower the nucleation efficiency.

Fig. 8(d) depicts the NA11–SB composite microemulsion. It appears to be a simple combination of the NA11 microemulsion with the SB microemulsion, but this combination has led to some special synergistic effect on promoting PP's crystallization and improving its mechanical properties. The previous DLS and cryo-TEM results have verified that the droplet size of the NA11–SB composite microemulsion is the smallest among all the prepared microemulsions. Accordingly, finest crystallites of NA11 should be formed on the PP powder surface with the evaporation of ethanol. At the same time, SB also crystallizes from the water phase with the evaporation of water and combines with NA11 to form a structure with SB as a core covered by fine NA11 crystallites. These two factors may finally result in a synergistic effect between the NA11 and SB nucleating agents.

As discussed above, the microemulsion method can help NA11-SB composite nucleators disperse better in the PP matrix than incorporating the nucleators in powder form directly. The microemulsion method will inevitably incorporate the oil and surfactant components into the PP matrix, inducing an adverse effect on PP's crystallization and mechanical properties. To alleviate such negative influence, a lower loading of the microemulsion is preferred. However, this adverse effect can be minimized or eliminated if we could choose some special oil or surfactant components that are also the necessary functional components in the final PP products. For example, we may choose some liquid antioxidants or liquid UV stabilizers as the oil phase to prepare a multi-functional microemulsion containing both nucleators and other functional liquid components. We have carried out some research in this area and achieved some positive results, which will be further organized and published.

Conclusions

This study focuses on finding a way to ultrafinely disperse some of the most widely used commercial melt insensitive nucleators in PP. A finer dispersion of a phosphate nucleator, NA11, was successfully achieved by forming a microemulsion based on ethyl butyrate (oil phase), ethanol (cosurfactant), Tween 80 (surfactant) and water. The NA11 nucleator within ethanol can be dispersed on the interface of droplets in the range of tens of nanometers, while the composite NA11–SB composite nucleator microemulsion possesses an even smaller dispersed phase.

The NA11 microemulsion can disperse better in the PP matrix and improve the nucleation efficiency compared to its powder. The mechanical properties of PP, especially the stiffness, can be further improved when using the NA11 microemulsion at a loading of 250 ppm. However, the oil and surfactant components within the microemulsion can be incorporated into the final PP matrix and adversely affect the stiffness as well as the crystallization ability of PP. Therefore a higher loading of the nucleator microemulsion is not preferred. The NA11–SB composite microemulsion shows the best performance in promoting PP's crystallization and improving its mechanical properties. The smallest droplet size of the microemulsion as well as a composite structure of the nucleator crystallites may cause this synergistic effect between the two nucleators.

Notes and references

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