Polypropylene (PP) is one of the most widely used plastics in our society due to its outstanding chemical, physical, thermal, and mechanical properties ^[1,2,3,4]. However, some disadvantages, such as low surface energy, non-polarity and chemical inertness, limit its further applications in the areas of poor adhesion, dyeing, antistatic performance, and poor compatibility with polar polymers, for instance. Grafting polymerization is one of the wellestablished technologies in the physical and chemical modifying approaches to overcome these drawbacks. Up to now, many strategies have been developed to prepare PP based graft copolymers or alloys. The established approaches include melt ^[5,6,7,8], solution ^{[9, 10]}, solid phase^{[11, 12,13]}, and liquid–solid phase suspension grafting ^{[14, 15]}. Melt and solution grafting have relative high reaction temperatures greater than 100 ^°C. The degradation of PP via β-chain scission reaction would take place significantly at that temperature. In addition, solid phase and liquid–solid phase suspensions also have their disadvantages, in spite of the lower reaction temperature. The reactions only take place on the surface of the PP particles, so the ratio of the graft copolymers to PP depends greatly on the surface area of PP. The GP of the products is usually about 2%–15%^{[5, 9]}, and the GE is rarely higher than 50% ^[11]. This means the products are a mixture of homopolymers and grafting polymers at a nearly equal level. Presently, the existing technologies still meet great challenges to achieve both a high grafting percentage (GP) and grafting efficiency (GE) at the same time.

In the history of PP, apart from the key milestone of the invention of Natta catalyst, reactor granule technology (RGT) invented by Paolo Galli and his coworkers at Basell is doubtlessly featured as a very significant scientific achievement ^{[16, 17,18]}. A PP granule prepared by RGT consists of agglomerated submicron particles (about 0.5 mm in diameter), which are composed of PP particles with a porous structure. Moreover, the porosity is within a range from 10 to more than 40 vol% which could be mediated by the catalyst ^[19,20]. This unique structure with high surface area can be considered as a ‘‘porous reaction bed’’ for free radical grafting polymerization. In summary, the development of a novel, highly efficient, free radical grafting polymerization process is of great value to both the academic and industrial communities.

Based on the systematic and continuous efforts on surface grafting polymerization in past decades, a very efficient and environmentally friendly strategy called water–solid phase suspension grafting polymerization (WSGP) was developed to prepare polypropylene/poly(methyl methacrylate) alloy (PP-g- PMMA) via RGT ^[21]. In this approach, grafting polymerization was elaborately regulated to occur within the micropores of PP particles. The key mechanism of the method was the surface abstracting hydrogen (chain transfer) reaction of phenyl radicals from benzoyl peroxide (BPO) decomposition. As a benefit of the special reaction system, the GP and GE could reach the highest value of 13.6% and 80.2%, respectively, in the situation of a single monomer. Compared to PMMA, PBA is not only a polar polymer, but also an elastomer with a low glass transition temperature. Therefore, significant progress should be achieved in developing an efficient grafting polymerization of BA aimed at extending the applications of PP alloy. The objective of this study is to achieve higher GP and GE and, consequently, triethylene glycol diacrylate (TEGDA) was introduced into the WSGP between PP and butyl acrylate (BA). We believed that the addition of TEGDA, as comonomer, could improve the grafting percentage of the final products by forming the cross-linked PBA microdomains, which will be helpful in improving the toughness of the prepared alloy.

Commercial PP spheres with isotacticity between 94.5% and 96.5% were provided by SINOPEC Jinan Company. The weight average molecular weight can reach about 3.7 Χ 106 to 4.1 Χ 106. Butyl acrylate is an analytical grade reagent purchased from Beijing Yili Fine Chemicals Co., Ltd. and vacuum-distilled before use. BPO, C.P. grade was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd., and purified by recrystallization in ethanol before use. Triethylene glycol diacrylate, C.P. grade, was provided by Tianjin Tianjiao Pharmacy Co., Ltd.

Grafting polymerization was carried out in a 500 mL threenecked round-bottom flask. Typically, a certain amount of BPO was dissolved in acetone, and then the solution was saturated into PP spheres. The PP was dried in an exhaust hood and BPO was coated onto the surfaces of the PP particles. Then, monomer BA was mixed with the PP particles coated with BPO. About 200 mL DI water was added to the flask and placed in an oil-bath at 80 ^ºC. Then, the mixture (PP particles soaked with BPO and BA) was introduced into the flask and reacted for about 2.5 h under a gentle nitrogen flow. After the end of the polymerization, the grafted PP spheres were extracted with acetone for 24 h. The grafting percentage (GP) and grafting efficiency (GE) were calculated per the following equations:

where: W₀ is the mass of pristine PP spheres, W₁ is the mass of grafted PP spheres after extraction, W₂ is the mass of PP spheres before extraction. Data on PP grafting PBA are summarized in Tables 1 and 2.

Table 1

Table 1 The summary of the PP-g-PBA experiments.

Table 1 The summary of the PP-g-PBA experiments.

Table 2

Table 2 The summary of the PP-g-PBA experiments with the addition of co-monomer TEGDA.

Table 2 The summary of the PP-g-PBA experiments with the addition of co-monomer TEGDA.

The FT-IR spectra of PP-g-PBA were recorded with a Nicolet NEXUS 670 FT-IR spectrometer. TEM observations were performed with a HITACHI H-800 transmission electron microscope. The ultrathin sections were chemically stained in ruthenium tetraoxide (RuO₄) vapor. Surface static contact angles were measured by an OCA 20 CA system (Dataphysics, Germany), with all measurements reported as averages of at least five readings.

Fig. 1 shows the FT-IR spectra of PP-g-PBA samples after extracting with acetone. Compared to the spectrum of pristine PP, strong absorption bands appeared at 1736 cm^-1 in the grafting products. This characteristic band was attributed to the stretching of the carbonyl group of PBA. Since the homopolymer of PBA had been removed by the extraction, the conclusion that the samples were the PP-g-PBA could be drawn ^[22,23].

Fig. 1

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Fig. 1.FT-IR spectra of PP-g-PBA with different levels of GP.

Both GP and GE are important factors to verify whether graft polymerization is successful. Table 1 shows the formulae of the PP-g- PBA experiments, and Fig. 2 is the effect of monomer (A), initiator (B), and co-monomer TEGDA (C and D) concentration on GP and GE. According to Fig. 2A, GP increases from about 10.7% to 32.6% with the increasing BA feed ratio. At the same time, GE drops from about 86.1% to 78.3%. Grafting polymerization mainly occurred at outer surface and inner pore surface of porous PP particles. The sites suitable for initiating graft polymerization are limited. When the ratio of BA increased to a critical value, the homopolymerization of BA initiated by free radicals would become considerable and resulted in the decrease of GE. In our previous study, GP and GE of PP-g-PMMA are 13.6% and 61.8%, respectively ^[21]. In the situation of BA, GP and GE values were 17.6% and 84.7%. Such a difference may be attributed to more flexibility and lower steric hindrance ^[24].

Fig. 2

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Fig. 2.Effect of monomer, initiator, and TEGDA feeding on GP and GE.

To understand the effect of initiator concentration on the grafting polymerization, the addition quantities of BPO from 0.008 g to 0.512 g were carried out with a constant addition of PP (20 g) and BA (4 mL). As shown in Fig. 2B, along with the increasing of BPO, GP and GE reach their maxima simultaneously at about 17.8% and 98.3%, respectively. Then, GP remains almost unchanged, and GE drops to 92.6%. Based on the experimental design, BPO mainly locates in the micropores of PP particles; hence BPO preferentially initiated graft polymerization. When BPO increases over a certain value, primary free radicals start to initiate homopolymerization and resulted in the decrease of GE. That is to say, lower initiator concentration is beneficial to the grafting reactions.

As a crosslinking agent, triethylene glycol diacrylate has two polymerizable double bonds, which allows it to connect the grafting chains and homopolymer. This characteristic may be beneficial in producing new, longer grafting chains, and improving GP and GE of the final products. Fig. 2C shows the effect of a constant TEGDA concentration (3 wt%) of BA on GP (Table 2, RUN 1–7). As expected, TEGDA increases GP remarkably compared with the situation without a crosslinking agent. When there is no TEGDA, GP increases from about 10.7% to 32.6% with the increasing BA feed ratio. At the similar addition of BA and BPO, GP is improved from 16.2% to 36.3% under the influence of both BA and TEGDA. As is known, acetone is a good solvent of PBA, and, therefore, the long and rigorous extraction can remove the PBA homopolymer completely. TEGDA provides an opportunity of the connection between grafting chains and homopolymer by chemical bonds. Fig. 2D shows the effect of TEGDA in a constant amount of BA (30 mL) and BPO (0.3 g) on GP and GE (Table 2, RUN 8–13). The concentration of TEGDA is the only variable, so the amount of crosslinking sites in the reaction system is the influencing factor of GP and GE. According to the figure, both GP and GE improve with the ratio of TEGDA and it could be concluded that the crosslinking agent is favorable to the grafting polymerization.

The microphase structure of PP-g-PBA samples after kneading in a Haake torque rheometer was observed by TEM. As presented in Fig. 3 with the greater magnification of 10⁴, it clearly shows that PP is the continuous phase (the dark region stained by RuO₄) and the PBA is the dispersed phase with the size of the microdomains less than 500 nm. With the increase of GP, the volume fraction of the PBA phases increases. Interestingly, when TEGDA is added in the reaction system, a dispersed phase with a clear boundary is not observed in the visual field. Instead, the grafting phase exhibits a single-phase behavior. It could be concluded (or assumed) that some PBA grafting chains may be connected to the PP main chains under the effect of TEGDA. This preliminary result shows the crosslinking agent, TEGDA, is potentially an effective compatibilizer between PP and grafting PBA. Furthermore, owing to its hydrophobicity, TEGDA has a tendency to permeate into the depths of the micropores which prevents phase separation.

Fig. 3

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Fig. 3.TEM images of PP-g-PBA after kneading.

Water contact angle (WCA) measurement is a simple method to evaluate the polarity of the modified alloy. WCAs of the PP before and after grafting were measured, and the results shown in Fig. 4. The unmodified PP shows a WCA approximately 102.6^º, indicative of relatively hydrophobic surfaces. Grafted with PBA, all thewater contact angles of the alloy exhibit a significant decrease. When the GP reaches a relatively high value (GP = 28.1%), WCA can decrease to 94.6^º, which indicates that the grafting polymerization allows reduction of their hydrophobic tendency. Since it is a polar polymer, the addition of the polar poly(butyl acrylate) molecular chains provides enhanced hydrophilicity of the final products.

Fig. 4

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Fig. 4.Effect of GP on water contact angle.

With the porous PP spheres prepared by RGT as the precursor, the PP-g-PBA was synthesized by WSGP. The GP could be as high as 32.6% with the BA alone as monomer, and with the addition of TEGDA as crosslinking agent, the GP increases up to 36.6%. The PBA domains could be finely dispersed in the PP matrix and controlled less than 500 nm in diameter. Moreover, co-monomer TEGDA could hinder the phase separation and further improve the compatibility between PP and PBA. With the GP increased, the PP-g-PBA exhibited an ascending hydrophilicity proved by measuring the water contact angle.

The financial supports of The Cultivation Fund of the Key Scientific and Technical Innovation Project (No. 705008), the Ministry of Education of China and The Young Scholars Fund of Beijing University of Chemical Technology are greatly appreciated.