Synthesis of chlorinated and anhydride-modified low density polyethylene by solid-phase chlorination and grafting—improving the adhesion of a film-forming polymer

Abstract

This paper is mainly focused on the modification of low density polyethylene (LDPE) by chlorinating and grafting it simultaneously in the gas–solid phase. There are two key obstacles to the success of this process. The first is the feasibility of the chlorination and grafting occurring simultaneously on the LDPE chains, and the second is how to gain control of the high chlorinated polyethylene (HCPE, Clwt% > 60%) preparation, when using LDPE as the raw material, which is a heterogeneous reaction. The feasibility of the process is discussed by FTIR and ¹H-NMR, in which LDPE is chlorinated to a high chlorine content (Clwt% = 64%) while maleic anhydride (MAH) is grafted onto the backbone chain (LDPE). This is a novel modification method patented by us, called in situ chlorinating graft copolymerization (ISCGC). The products’ molecular weight and distribution, thermal properties, and the factors affecting the modification process are analyzed by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), stereoscopic microscopy and the curve of chlorine content versus time. A graft degree (GD) of about 2% is achieved for the graft copolymer, which has a broad practical significance. The adhesion and impact strength tests of the product as a film-forming polymer in coatings indicate that it could effectively increase the adhesion when MAH is grafted onto the LDPE by ISCGC.

---

1 Introduction

The modified polyethylene (PE) prepared by chlorination is of great importance for various applications, of which the performance depends on chlorination methods, degree of chlorination and reaction conditions. Previous studies have been carried out on the chlorination of PE which has been synthesized by either suspension polymerization or solid-state chlorination conducted below the melting temperature.¹ Particularly when the degree of chlorination reached 60%wt, the high chlorinated polyethylene (HCPE) had excellent physical–chemical properties. HCPE is commonly used in corrosion-resistant paints² and many relevant patents^3–6 have been issued in recent years.

The raw material of HCPE is usually high density polyethylene (HDPE). As corrosion-resistant paint, the main drawback of HCPE made of HDPE is the high viscosity of the product, which makes it difficult to apply. This problem can be addressed by using LDPE instead of HDPE as the raw material of HCPE. However, this leads to a bad adhesion for painting. In order to make HCPE with low viscosity that possesses good adhesion, we proposed to chlorinate LDPE and simultaneously graft MAH onto it via in situ chlorinating graft copolymerization (ISCGC). This is achieved by a gas–solid phase preparation. Therefore, this paper is mainly concerned with the two key obstacles to the success of the process. The first is the feasibility of chlorination and grafting occurring simultaneously on LDPE chains, and the second is how to gain control of the HCPE preparation, when using LDPE as the raw material, which is a heterogeneous reaction.

Graft modification is the most common polymer modification method, in which maleic anhydride (MAH) is often used as a graft monomer due to its high polarity and reactivity. Of course, a lot of work can be found in this field on methods where the polymer backbones are not only confined to PE^7–9 but also include others, e.g., the grafting of MAH onto polypropylene^10–12 and chlorinated polyvinyl chloride.¹³ However, in these methods the modification is generally carried out by a free radical reaction with peroxide as the initiator. This is unlikely to achieve a high grafting degree, and requires two steps to get the chlorinated graft copolymer; the preparation of the HCPE and the grafting onto the HCPE. In contrast, ISCGC only requires one step to get the chlorinated graft copolymer.

In ISCGC, the homolysis of the chlorine molecule produces Cl free radicals under heating. Following closely on, the free radicals remove H from the LDPE chains to form macromolecular radicals. The MAH monomers are grafted onto the LDPE chains as long as they are in contact with the macromolecular radicals effectively. The LDPE macromolecular radicals can also react with chlorine molecules and be chlorinated, finally producing functionalized graft copolymers (LHCPE-cg-MAH) with low viscosity. With polyolefin as the raw material and ISCGC as the modified method, we have done a lot of previous work,^14–16 including using MAH as a monomer grafted onto HDPE with low chlorine content (about 3% chlorine content), of which the product can be used as an adhesive for plywood.¹⁷ Besides the two key points mentioned above, whether LHCPE-cg-MAH is desirable for use in paints is also discussed in this paper.

To the best of our knowledge, no similar work to this has been reported before. Both the melting point and the molecular weight of the LDPE are lower than that of HDPE, which makes it difficult to control the process of ISCGC in the gas–solid phase. Therefore, the main focus of this paper, to be exact, is whether the reaction process can ensure that the MAH is grafted onto the polymer skeleton effectively and whether the graft product can obviously improve adhesion. The product of grafting MAH onto the LDPE by ISCGC is referred as LHCPE-cg-MAH.

2 Experimental

2.1 Preparation and characterization of LHCPE-cg-MAH

2.1.1 Preparation of LHCPE-cg-MAH. LDPE was supplied by Shidai Hongye Industry & Trade Co., LTD (Wuhan, China); HDPE was supplied by Korean LG Chemical Ltd., LG 6040; MAH, acetone, dioctyl-phthalate (DOP), xylene, anhydrous ethanol, methanol and sodium hydroxide (NaOH) were AR grade reagents. Chlorine gas (Cl₂) and white carbon black were industrial products.

The preparation of the LHCPE-cg-MAH by ISCGC was as follows: The reaction was in a three-necked round-bottomed flask with a mixing plant, a thermometer, and a Cl₂ inlet. 30 g of LDPE (dried at 60 °C before use) was put in a 500 mL flask. 1.8 g of MAH was dissolved in the same quantity of acetone, and then the solution was poured into the reactor and stirred evenly. This fully mixed the LDPE and MAH, and at the same time volatilized the acetone before the reaction occurred. Before the start of the experiment, an appropriate amount of SiO₂ was added as a release agent, to prevent polyethylene powder caking. Then Cl₂ continued to pass into the reactor for 15 minutes to exclude any air. In the next step, the reaction temperature was regulated to the desired temperature (with a variation of within ±2 °C) and the Cl₂ flow rate was adjusted to meet the reaction conditions. In the initial stage of the reaction, the temperature was maintained below 80 °C. The by-product hydrogen chloride (HCl) was absorbed by water in the reaction process, and weighing the HCl by-product determined the chlorine content of the product. With an improvement of the chlorine content, the reaction temperature constantly increased and the temperature was not allowed to exceed 140 °C. After the balance reached the expected weight, the Cl₂ flow was shut off. When the temperature of the system dropped to 100 °C, the residual Cl₂ in the flask was removed by vacuum pumping. During this process, the flask was evacuated and backfilled with air many times to completely clear the residual chlorine. The experimental apparatus was as shown in Fig. 1.

Fig. 1 Schematic representation of the experimental apparatus. 1 – U flow meter; 2 – three-necked flask; 3 – thermometer; 4 – vane stirrer; 5 – buffer tube; 6 – temperature controller; 7 – water bottle; 8 – balance; 9 – NaOH solution bottle.

The chlorine content of the product may be calculated using the amount of absorbed HCl in eqn (1):

where ΔW_HCl is the mass (g) of HCl released from the reaction system and m_LDPE and m_MAH are the initial weights of the polymer and graft monomer in the reaction system, respectively. The calculation formula is based on the following reaction formulae:

In the course of the chlorination reaction, a hydrogen atom was substituted by a chlorine atom and the relative molecular mass differences of the two atoms was 34.5. The 34.5 was obtained from reaction formula (a) in the above formula. “M” in the formula represents the graft monomer.

2.1.2 Purification of products. About 1.5 g of the graft sample was dissolved in xylene (40 mL), and then the solution was heated until the sample completely dissolved at the reflux temperature. The solution was pulled slowly into another flask which contained 75 mL anhydrous ethanol under stirring conditions, which made the polymer separate completely. The deposit was filtered through a Buchner funnel, and the process was repeated twice. Finally the deposit was shredded and then put into a vacuum oven at 50–60 °C in order to dry it to a constant weight.

2.1.3 Characterization. GPC was carried out in tetrahydrofuran (THF) on a Waters 1515 gel permeation chromatography instrument. Polystyrene (PS) was used as a standard, the sample concentration was 5 mg mL⁻¹ and the flow rate was 1 mL min⁻¹ at room temperature.

DSC was undertaken using a DSC204F1 differential scanning calorimeter (German NETZSCH company). The sample was approximately 5 mg and the heating rate was 10 °C min⁻¹ in nitrogen. The range of temperatures was −50–200 °C.

The surfaces of the raw materials were observed by a stereomicroscope (Olympus SMZ1500).

The structure of the graft copolymer was analyzed with a Fourier-transform infrared spectrometer (VERTEX70, Brooke company).

Nuclear magnetic resonance spectra (NMR) were recorded in CDCl₃ using a Varian Associates Unity 500 spectrometer. Chemical shifts are reported in ppm relative to CDCl₃ (δ = 7.27).

The viscosity of samples was tested by a 4# cup viscometer at room temperature. Using the same elution volume but different efflux times allowed us to judge the viscosity.

2.2 Grafting degree (GD) measurement

Determination of GD of purified samples: About 0.5 g of purified LHCPE-cg-MAH was dissolved in hot xylene (75 mL). After being mixed, 3 droplets of water and 3 droplets of N,N–dimethyl formamide (DMF), which would accelerate the hydrolysis of the MAH, were added into the solution. The mixture was heated to keep the solution boiling for 15 min, and then the indicator, phenolphthalein–methanol solution, was added. The titration was carried out at high temperature (110–120 °C) with KOH–methanol solution (0.012299 mol L⁻¹), which was necessary so that no deposition occurred during the titration process. At the end point of the titration, the solution turned into stable light pink. The following equation (eqn (2)) was used to calculate the GD:where N is the concentration of the standard solution of KOH–methanol (mol L⁻¹), V and V₀ are the consumption volumes (mL) of the KOH–methanol solutions for specimen titration and blank titration, respectively. M is the molecular weight of MAH (98.06) and W (mg) is the weight of the polymer sample. The factor ‘2’ in eqn (2) represents the fact that one MAH particle can form two carboxyl groups after complete hydrolysis.

2.3 Preparation of the coating sample

The coating sample preparation of LHCPE-cg-MAH is as follows: the sample is dissolved in xylene to prepare a solution of 20% sample content. Appropriate amounts of the solution is coated onto tin plate (50 × 1000 × 0.50 mm), and then dried to a constant weight at 50–60 °C within a vacuum oven.

2.4 Adhesion measurement

The adhesion of the polymer was tested according to GB/T 1720-1989, and all the reported data of film properties were the average values over 5 tests. The level of the standard is divided into seven according to the integrity and testing part (I–VII) of film. If the integrity of part I is more than 80%, it will be judged to be 1 grade, the best level; if the integrity of part I is less than 80% and part II is more than 80% it will be judged to be 2 grade, others are by parity of reasoning.

The film impact strength of the samples was tested according to GB/T 1732-1993. Good test results should be free of cracks, wrinkles and spalling at 50 kg cm.

3 Results and discussion

3.1 Feasibility of preparing LHCPE-cg-MAH by ISCGC

3.1.1 Feasibility analysis. Compared with HDPE/MAH (HHCPE-cg-MAH), we found that when using LDPE/MAH as the raw material, the process of preparing LHCPE-cg-MAH by ISCGC was more difficult to control. Therefore we infer that HDPE and LDPE are totally different when they are used as raw materials of grafted MAH. The GPC results for the LDPE and HDPE are shown in Fig. 2 and Table 1.

Fig. 2 GPC curves for LDPE (1) and HDPE (2).

Table 1 Molecular mass and distribution index for the two kinds of PE^a

Sample	Mn	Mw	MP	Mz	Mz+1	Mv	MWD
a Conditions: All the data came from the GPC, polystyrene (PS) was used as a standard, the sample concentration was 5 mg mL^−1, and the flow rate was 1 mL min^−1 in THF at room temperature.
LDPE	1.2 × 10^4	5.2 × 10^4	3.8 × 10^4	1.3 × 10^5	2.2 × 10^5	4.5 × 10^4	4.36
HDPE	3.4 × 10^4	1.9 × 10^5	9.2 × 10^4	5.7 × 10^5	1.0 × 10^6	1.6 × 10^5	5.49

---

As Fig. 2 and Table 1 show, the number average molecular weight (M_n) of LDPE is about 1.2 × 10⁴ and the weight average molecular weight (M_w) is 5.2 × 10⁴, while the molecular weight distribution (MWD) is 4.36. Meanwhile, the M_n of HDPE is 3.4 × 10⁴, M_w is 18.7 × 10⁴, and the MWD is 5.49. Clearly, the average molecular weight of LDPE is much lower than that of HDPE. The effects of a lower average molecular weight on the preparing process have two aspects. The solubility of the polymer increases after it has been chlorinated; however, temperature control in the preparation process will be more difficult.

Fig. 3 and Table 2 show the DSC test results for the raw material. The DSC results indicate that the melting point of the LDPE is 108.2 °C, while the melting point of the HDPE is as high as 137.7 °C. The degree of crystallinity of the former is 33.3%, and that of the latter is 81.9%. All these results show the many differences between the two materials.

Fig. 3 DSC curves of LDPE (1) and HDPE (2).

Table 2 The DSC data of polymers^a

Sample no.	Tm/°C	ΔHf*/J g^−1	Crystallinity/%
a Conditions: The sample was approximately 5 mg and the heating rate was 10 °C min^−1 in nitrogen; the range of temperatures was −50–200 °C.
HDPE	137.7	223.6	81.9
LDPE	108.2	90.8	33.3

---

The lower melting point and degree of crystallinity of the LDPE suggest that LDPE which serves as a raw material has to react at lower temperatures, making it difficult to control the exothermic process. Additionally, it is also a question deserving special concern whether MAH can be effectively grafted onto the main chain under low temperatures. This is because MAH may preferentially undergo an addition reaction instead of grafting in low temperatures; moreover, a lower melting point of the raw material makes it easier for it to melt in the reaction process. In the reaction process, melting of the surface of LDPE granule will reduce the porosity of the surface, so that Cl₂ is blocked from diffusing through it. This would be a disadvantage for further grafting reactions.

The gas–solid phase reaction is a kind of heterogeneous reaction, and the structure and morphology of the solid granule surface also has a significant effect on the reaction process. For further insight into the feasibility and possible problems of solid-phase ISCGC, we observed the surfaces of the two kinds of PE granules using a stereomicroscope.

It can be seen from Fig. 4 that granules of LDPE (on the left) have different sizes, irregular shapes and a lower specific surface area. In comparison, the granules of HDPE (on the right) have a uniform size and shape, and the granularity is small meaning the specific surface area is higher. According to the above, it can be inferred that in the heterogeneous gas–solid phase ISCGC, the irregular granules of LDPE would lead to less surface contact with the chlorine, which would affect the reaction rate and, particularly, lead to a sharp decline in reaction rate in the late stage, if the initial reaction rate is fast. Furthermore, the surface of the LDPE granules is flat and smooth, while the surface of the HDPE granules is loose, porous, and abundant in inner pores, which facilitates the diffusion of chlorine molecules so they can take part in deep reactions from the surface to inside. This suggests that the reaction rate of LDPE would be much lower than that of HDPE in the late phase of chlorination.

Fig. 4 Photographs of LDPE (left) and HDPE (right). Note: the image magnification is 100 times. The scale bars are all 100 μm.

As a conclusion to all of the analysis above, we think that if the ISCGC of LDPE/MAH was done at low temperatures then the reaction rate could be controlled. More specifically, phase-wise chlorination can be employed to achieve this goal.

In the first stage (Clwt% ≤ 30%), the temperature was maintained at 80 °C. Then in the second stage (30% ≤ Clwt% ≤ 50%). the temperature was raised to 120 °C while the reaction occurred further. Finally in the third stage, the temperature was kept below 140 °C, and the reaction went on until it was terminated. This is totally different to the chlorination reaction of HDPE.

To lower the reaction temperature and increase the reaction rate in the early stage, adding a radical initiator is an option, and it is also possible to use UV to facilitate a low temperature reaction.

3.1.2 Feasibility validation. The reactive scheme of LDPE and MAH by ISCGC is shown in Fig. 5.

Fig. 5 The reaction processes of LHCPE-cg-MAH synthesis and the chlorination of LDPE.

To demonstrate the feasibility of the process, we carried out FTIR, ¹H-NMR and graft degree (GD) analysis on the graft product.

As Fig. 6 shows, by comparing the spectra of LHCPE-cg-MAH and LHCPE, we can conclude the following:

(1) New absorption peaks of LHCPE-cg-MAH appear at 1743 cm⁻¹ and 1779 cm⁻¹, corresponding to the stretching vibration of C=O and the coupling of the two C=O stretching vibrations, respectively.

(2) The CH bending vibration peak of LHCPE () is at 1269 cm⁻¹ and the CH bending vibration peak of LHCPE-cg-MAH () is toward 1264 cm⁻¹. It can be inferred that the inductive effect causes the dipolar distance to change. This also supports that MAH has been grafted on the product.

(3) Peaks appear at 1606–1650 cm⁻¹ in both spectra, indicating the existence of a conjugated double bond structure in LHCPE-cg-MAH and LHCPE.

(4) Both spectra have peaks at 746–798 cm⁻¹, suggesting that a vinylidene chlorine structure is present in both LHCPE-cg-MAH and LHCPE.

The above results are sufficient to prove that the MAH monomer has been successfully grafted onto the molecular chain of the LHCPE, producing the graft copolymer LHCPE-cg-MAH.

The above result was confirmed again by the ¹H-NMR spectra. Fig. 7 shows the ¹H-NMR spectra of LHCPE and LHCPE-cg-MAH. Compared with the spectrum of LHCPE, we saw a new chemical shift peak at d = 3.732 ppm in the LHCPE-cg-MAH spectrum, confirming that the MAH group had been successfully grafted onto the main chain. With the combination of FTIR and ¹H-NMR spectrum analysis, it could be proved that with LDPE as the raw material, we can obtain chlorinated polyethylene with a high chlorine content as well as graft MAH onto the main chain by ISCGC, to produce anhydride modified LHCPE.

Fig. 6 FTIR spectra of LHCPE (1) and LHCPE-cg-MAH (2).

Fig. 7 ¹H-NMR spectra of LHCPE (1) and LHCPE-cg-MAH (2). Conditions: testing in CDCl₃ at room temperature.

Furthermore, the product was subjected to calculation of the GD through an anhydrous titration method, and the result is shown in Table 3.

Table 3 The GD of LHCPE-cg-MAH^a

Samples	1	2	3
a The samples all reached the same chloride content.
GD (%)	3.44	1.67	1.65

---

The test results show that the average GD of LHCPE-cg-MAH is up to 2%. This not only indicates that the MAH has been grafted onto the backbone polymer, but also that it is high enough to make practical sense of the graft product.

Accordingly, we experimented with controlling the reaction temperature (T). The control process is as follows: Clwt% < 30%, T < 80 °C; 30% < Clwt% < 50%, T < 120 °C; Clwt% > 50%, T < 140 °C (Clwt% is the chlorine content). All of the experiments were in accordance with the standard reaction conditions. The curve of chlorine content versus time is shown in Fig. 8. Comparing the three products, the grafting system reacted relatively slowly in the latter stage.

Fig. 8 The curve of chlorine content (Cl%) versus time for HHCPE (1), LHCPE (2) and LHCPE-cg-MAH (3).

3.2 Adhesion and impact strength of the LHCPE-cg-MAH film

HHCPE made of HDPE has a wide range of applications, but the adhesion of LHCPE made of LDPE is much lower. The adhesion and impact strengths of LHCPE, HHCPE and LHCPE-cg-MAH as film-forming polymers (the chlorine content for all three is 64% and the GD is 1.69%) were tested and the results are shown in Table 4.

Table 4 The test results^a for film impact strength and film adhesion

Samples	Adhesion test results (grade)	Impact strength test results (50 kg cm)
a All the reported data of film properties were average results over 5 tests.
LHCPE	20% or less of the various parts of the coating intact (7 grade)	No cracks, wrinkles or flaking
HHCPE	More than 80% of part I of the coating intact (1 grade)	Minor cracks, wrinkles and flaking
LHCPE-cg-MAH	More than 80% of part I of the coating intact (1 grade)	No cracks, wrinkles or flaking

---

The test results show that the grafting of the MAH to the LHCPE significantly improved the adhesion, which increased from 7 to 1 and was as high as that of HHCPE. LHCPE with LDPE as the raw material was dissolved in xylene to prepare a solution of 20% sample content. Although its viscosity was low, the adhesion was poor. However, MAH-grafted LHCPE (LHCPE-cg-MAH) had almost the same viscosity and its film was excellent. The LHCPE-cg-MAH's viscosity as tested by 4# cup was between 40 and 60 s and was much lower than the HHCPE's, which was more than 20 minutes.

Obviously, the grafting of MAH onto the polymer backbone plays an important role in the improvement of the adhesion. The grafting of MAH onto the macromolecular chain strongly improves the adhesive force between the polymer and the metal specimens. Additionally, this also demonstrates that the achieved degree of grafting generates a high adhesion, which is enough for the product to serve as a film-forming polymer.

The impact strength data in Table 3 indicate that grafted MAH doesn't increase the brittleness of the LHCPE-cg-MAH. There are two conflicting effects caused by the grafting of the MAH. On one hand, it increases the polarity of the polymer. On the other hand, due to the high steric hindrance and random distribution of the MAH group, the regularity of the macromolecule is broken and the distance between the molecules is enlarged, which improves the flexibility of the molecular chain. The experimental results show that the latter effect is superior, which is an advantage for coating film toughness. This is also supported by the DSC curves of LHCPE and LHCPE-cg-MAH shown in Fig. 8.

From Fig. 9 we find that the glass transition temperature (T_g) of the LHCPE-cg-MAH (77.4 °C) is lower than that of the LHCPE (101.2 °C). We believe that this is the result of the improved flexibility of the polymer backbone caused by the random distribution of MAH.

Fig. 9 DSC curves of LHCPE (1) and LHCPE-cg-MAH (2). Conditions: the sample was approximately 5 mg and the heating rate was 10 °C min⁻¹ in nitrogen; the range of temperatures was −50–200 °C.

4 Conclusions

According to the results of the FTIR and the ¹H-NMR spectrum, it is feasible that MAH could be grafted onto a LHCPE molecular chain via ISCGC, and that the grafting degree could reach 2. Compared to LHCPE, the adhesion of the grafting products is improved significantly and could attain 1 grade. In addition, the coating film of the grafting products has good toughness and its impact strength can meet the performance requirements for coatings.

Acknowledgements

The project was supported by the National Natural Science Foundation of China (51173090) and the Open Research Fund of the State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, natural science foundation of Shandong Province (ZR2012EMM003).

References

1. S. Stoeva, D. Tsocheva and L. Terlemezyan, Thermal behavior and characterization of solid-state chlorinated polyethylenes, J. Therm. Anal. Calorim., 2006, 85(2), 439–447.
2. J.-R. Zhao, Y. Feng and X.-F. Chen, Graft-modified HCPE with methyl methacrylate by the mechanochemistry reaction. II. Physical-mechanical properties and processability, J. Appl. Polym. Sci., 2004, 91(1), 282–287.
3. K. W. Cho, Economic tube for paints with polyethylene material, KR Pat. 10-2012-0079555 A, 2012.
4. P. U. Yangnan, Polysiloxane modified high chlorinated polyethylene coating, CN Pat. 102153918 A, 2011.
5. Jiangxi Copper Co Ltd, Novel HCPE high chlorinated polyethylene anti-corrosive paint, CN Pat. 101654576 A, 2010.
6. P. U. Yangnan, Fluorine-contained high-chlorinated polyethylene coating, CN Pat. 101851449 A, 2010.
7. N. Petchwattana, S. Covavisaruch and S. Chanakul, Mechanical properties, thermal degradation and natural weathering of high density polyethylene/rice hull composites compatibilized with maleic anhydride grafted polyethylene, J. Polym. Res., 2012, 19(7), 9921.
8. S. Charoenpongpool, M. Nithitanakul and B. P. Grady, Melt-neutralization of maleic anhydride grafted on high-density polyethylene compatibilizer for polyamide-6/high-density polyethylene blend: effect of neutralization level on compatibility of the blend, Polym. Bull., 2013, 70(1), 293–309.
9. A. I. Vicente, S. G. Pereira, T. G. Nunes and M. Rosário Ribeiro, ¹H-NMR study of maleic anhydride modified ethylene-diene copolymers, J. Polym. Res., 2011, 18(4), 527–532.
10. S. H. P. Bettini, L. C. de Mello, P. A. R. Muñoz and A. Ruvolo-Filho, Grafting of maleic anhydride onto polypropylene, in the presence and absence of styrene, for compatibilization of poly(ethylene terephthalate)/(ethylene–propylene) blends, J. Appl. Polym. Sci., 2013, 127(2), 1001–1009.
11. Y. Ye, J. Qian and Y. Xu, Ultrasonic induced grafting of maleic anhydride onto polypropylene in melt state, J. Polym. Res., 2011, 18(6), 2023–2031.
12. G. S. Ezat, A. L. Kelly, S. C. Mitchell, M. Youseffi and P. D. Coates, Effect of maleic anhydride grafted polypropylene compatibilizer on the morphology and properties of polypropylene/multiwalled carbon nanotube composite, Polym. Compos., 2012, 33(8), 1376–1386.
13. A. G. Filimoshkin, G. A. Terentieva, E. M. Berezina, T. V. Pavlova and L. P. Gossen, A novel behavior of copoly(vinylchloride–maleic anhydride), J. Polym. Sci., Part A: Polym. Chem., 1993, 31(7), 1911–1914.
14. L. Zhang, Y. Sun, Y. Ma, J. Zhao, Y. Feng and J. Yin, In situ chlorinating-graft copolymerization on isotactic polypropylene in gas–solid phase, Polym. Bull., 2009, 63(3), 341–354.
15. C. Xu, S. Wang, L. Shao, J.-R. Zhao and Y. Feng, Structure and properties of chlorinated polyvinyl chloride graft copolymer with higher property, Polym. Adv. Technol., 2012, 23(3), 470–477.
16. Z. Du, C. Xu, Z. Zhao, J.-R. Zhao and Y. Feng, Blend toughening of poly(vinyl chloride) by chlorinated polyethylene/hydroxy ethyl acrylate graft copolymer, J. Appl. Polym. Sci., 2011, 121(1), 86–96.
17. L. Tang, Z.-G. Zhang, J. Qi, J.-R. Zhao and Y. Feng, A new formaldehyde-free adhesive for plywood made by in-situ chlorinating grafting of MAH onto HDPE, Eur. J. Wood Wood Prod., 2012, 70(1–3), 377–379.

---