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Mechanism of a one-step method for preparing silane grafting and cross-linking polypropylene.


The silane cross-linking technique offers technological advantages over radiation and peroxide-cross-linking technique and improves many useful properties of cross-linked polymers (such as polyethylene and propylene--ethylene copolymer, etc.) [1-5]. Munteanu [6, 7] and Brown [8] reviewed the state of the technology of moisture cross-linkable silane-modified polyolefins. But, cross-linking of PP via silane grafting and moisture curing has rarely been reported. The difficulty is most likely a consequence of the nature of the PP chain scission ([beta]-scission), which is the dominant reaction in PP when subjected to free radicals at elevated temperature during processing [9, 10]. Cartasegna [11] reported that a sample of PP subjected to the same silane grafting and curing procedure as used for cross-linking PE gave a complete absence of insoluble gel. In other studies [12-17], silane-cross-linked PP has only been obtained under severe processing conditions, such as extremely high silane and initiator concentration. Furthermore, they use two-step method to prepare cross-linked PP, i.e., the silane is grafted onto PP chain first, and then cross-linking is achieved by curing the grafting products. Thus the technology is complicated and costly.

A novel one-step technique is proposed to prepare the silane grafting and cross-linking PP in this article, meanwhile high-melt-strength PP (HSMPP) with partial cross-linking is obtained. The mechanism of the one-step method is investigated in detail.


The raw material is a pelleted isotactic PP (Grade T30S; Density 0.901 g/cm; [M.sub.w] 292,000; Melt temperature 170[degrees]C; Tacticity 96.6%) supplied by the Qilu Petrochemical Company. It has a melt flow rate (MFR) of 3.1 g/10 min (ASTM D1238, 230[degrees]C, 2.16 kg). The ethenyl unsaturated silane monomer methacryloxypropyltrimethoxysilane (VMMS), the initiator benzoyl peroxide (BPO) and styrene are of analytical grade and are used as received.

Grafting and Cross-Linking Procedure

The grafting and cross-linking reaction was carried out in an SLJ-35B twin-screw extruder with a ratio of length over diameter of 32:1. PP pellets were first dry-mixed with the silane monomer, a little water, initiator, and other additives. Then the blends were fed into the extruder under the required conditions. The processing temperature for the first heating zone was kept at 80[degrees]C, while the other zones were controlled at 160-190[degrees]C. The rotating speed of screws was maintained at 80 rpm The feeding rate was controlled in the range of 8-10 kg/h. And the extruding products were pelletized.

Analysis of Grafted and Cross-Linked PP

The reaction products from extruders were extracted by using a Soxhlet method. The sample (about 0.5 g) was cut into small pieces and then packaged in a copper cloth of 200 meshes and next extracted with refluxing xylene for 12 h in a Soxhlet extractor. The insoluble part (gel) was dried in a vacuum oven at 80[degrees]C for 24 h. Then, the above process was repeated until the mass of gel became constant. The gel percentage of the samples was calculated by using formula as follows:

gel percentage = [gel weight/initial weight] x 100%. (1)

The extraction process was performed three times for each sample and the average gel percentages were calculated.

The solution achieved from the Soxhet extraction was precipitated with alcohol. The precipitate was dried at 90[degrees]C in a vacuum to a stable weight before being tested. Pure PP, precipitated PP from the extracting solution and insoluble gel PP were characterized by Fourier transformed infrared spectroscopy.

Melt Flow Rate Test

The melt flow rate was tested with a XRZ400 melt indexer (The Jilin University testing machine factory) under the condition of 2.160 kg x 230[degrees]C, based on the national standard (GB3682-83).

Melt Strength Test

The melt strength was tested with RHEOTENS 71.97 Polymer Melt Strength Tester of Geottfert GMBT. The extrusion temperature is kept at 210[degrees]C, and the extrusion speed is kept at 4 g/min with a drawing accelerating rate of 2 cm/[s.sup.2] during testing.


Property and Influence Factors of Modified PP

To verify the realization of grafting and cross-linking PP and probe the mechanism of one-step method, much effort has been made. For example, the MFR and melt-strength of modified PP are measured, the structure of PP samples is characterized by FTIR, and the influences of catalyst and curing on MFR and gel percentage of the modified PP are investigated.

Mechanical of the Modified PP. The comparison of the properties including MFR, the melt-strength, and the mechanical properties between raw PP and modified PP, which is fabricated with 0.36 phr initiator and 1.0 phr silane, are shown in Table 1. The data of modified PP change greatly compared with raw PP. After modification, the MFR of PP decreases from 3.1 g/10 min to 0.5 g/10 min, an 84% decrease; the melt-strength increases from 161.85 to 660.22 cN, a 308% increase; the mechanical properties are also improved significantly. The changes are attributed to the grafting and cross-linking reactions, which induces the structure transforming of PP from linear to partially cross-linked. Certainly there is also a possibility of chain scission of PP occurring together with cross-linking.

FTIR Characterization of Silane Grafted and Cross-linked PP. Figure 1 shows the FTIR spectra of pure PP, precipitated PP from the extracting solution and insoluble gel PP. The spectrum of insoluble gel PP (Fig. 1B) shows clearly two new absorbance band at 1741 and 1100 [cm.sup.-1], which are characteristic absorptions of carbonyl group (1741 [cm.sup.-1]) and Si-O-Si bond (1100 [cm.sup.-1]). There is a carbonyl group in the structure of the raw silane, and the silane grafting and cross-linking produces of Si-O-Si groups. Combining the above results with the high gel percentage of the modified PP (gel percentage ~58 wt%), we can confirm that the silane cross-linking occurs in the reactive extruding process. Furthermore, the spectrum of the precipitated PP (Fig. 1C) from the extracting solution is similar to that of pure PP (Fig. 1A). This indicates that there are few grafted macromolecules in the system after the extrusion.

One probable reason is that most grafting segments, which are produced in the extrusion, cross-link directly under the catalysis of the water. The other reason is that there is a formation of silane condensation compound with two or more double-bond under the catalysis of the water, and the compound consequently reacts with PP macroradicals leading to cross-linking.



Influence of the Catalyst on MFR and Gel Percentage of Modified PP. The MFR and gel percentage of the samples with/without catalyst are shown in Table 2. Obviously, MFR of the sample with catalyst is much lower than that of the sample without catalyst. Furthermore, high gel percentage of modified PP with catalyst is obtained; on the contrary, no gel is found in modified PP without catalyst.

From the above data, it can be deduced that silane grafting occurs merely in the samples without catalyst, and the MFR decreases sharply because of the long-chain grafting. Whereas, not only grafting, but also cross-linking is achieved in the sample with catalyst, which results in the greater decrement of MFR. This is another evidence for the silane grafting and cross-linking of PP by one-step reactive extrusion.

Influence of the Curing on MFR and Gel Percentage of Modified PP. The variety of MFR and gel percentage against silane concentration between cured (in boiling water) and uncured samples is shown in Figs. 2 and 3. Obviously, when a little amount of silane is added, a great change of MFR and gel percentage of modified PP is induced, while the change becomes insignificant when the silane concentration is beyond a certain value (~0.3 wt%). There is no notable difference in the MFR and gel percentage between cured and uncured samples in the range of examined concentration. This indicates that there is almost no silane grafted PP in the final structure. It is just because there is no cross-linkable silane grafting point that the structure of modified PP has no changes during curing. So there is no difference between cured and uncured samples. The result further proves that the grafting and cross-linking reaction are achieved synchronously during the extrusion process by one-step reactive extrusion method.


Silane Grafting and Cross-Linking Mechanism

Formation of Macro-radical. First, the BPO (part life of decomposition: 1 min at 133[degrees]C) disassociates to produce peroxide radicals under the extrusion conditions. Then the peroxide radical reacts with PP chain undergoing radical transference to create PP radical. The reaction is shown below.


Silane Grafting and Cross-linking. In the presence of unsaturated silane, the double bond on the silane can react with PP macroradical, which results in a stable grafted PP macroradical. The grafted macroradical disappears through radical transferring reaction to obtain a stabilized grafting point. The alkoxy on silane hydrolyzes to form silicol, followed by the condensation of the silicol to remove water; as a result, PP molecules are a cross-linked network.

In the course of one-step modified PP preparation, silane grafting and cross-linking are accomplished synchronously in the extruder. There are two potential routes for silane reacting with PP radical to achieve grafting and cross-linking.

Route One. Silane monomers graft on PP macro-radical undergoing radical transference.


Subsequently, the silane branch on the grafted PP macro-radical hydrolyzes to produce silicol and methanol in the presence of water. Mutual condensation of silicols results in cross-linking, thus the macromolecule network is formed.


Route Two. The water in the system possibly brings silane condensation to form a compound with two or more double-bond at a low temperature area in the extruder, and then the compound reacts with the PP macroradical. After which, the grafted PP can react with other PP macroradicals to form the PP macromolecule network at higher temperature area in the extruder because the compound has a multi-double-bond. This process is as follows (the example is for two silane moleculers to condense):

a. Hydrolysis and condensation of Silane:


b. Grafting reaction between the compound and PP macroradical to form a grafted PP with reactive ethenyl:


c. Cross-linking reaction between the grafted PP and another PP macroradical to form a macromolecule network:


The most possible reactive route of one-step silane grafting and cross-linking PP is considered to be the route two. In our experiments, the raw materials containing silane and water are fed into the extruder, where the feedstocks are gradually heated. When the temperature is lower than 100[degrees]C, the silane hydrolysis and condensation are easy to be carried out, whereas, it is almost impossible for the initiator to decompose at such low temperature. So no PP macroradical is produced, let alone silane grafting and cross-linking. When the temperature is higher than 100[degrees]C, the initiator begins to discompose and produce PP macroradicals, and the silane grafting should be formed. But the following hydrolysis and cross-linking of the grafted PP can't be acquired because the water has completely vaporized and escaped at this temperature. If route one is the mechanism, there must be grafted PP in the modified PP, but no evidence is revealed in the result of FTIR. The FTIR reveals that there is no grafted PP but cross-linked PP in the final chemical structure of the modified PP, i.e., grafting occurs as intermediate step. Therefore, route one is impossible under the experimental conditions. Thus route two is the most possible mechanism.


HMSPP has been fabricated by using one-step silane grafting and cross-linking technology. Compared with the raw material PP, the MFR has been decreased by 84%, the melt-strength has been increased by 308%. The partially cross-linked polypropylene in the way of one-step reactive extrusion has been verified by FTIR and other testing results and the mechanism is put forward. In this mechanism, silane hydrolyzes and condenses first with the catalytic action of water to form a compound with two or more double-bond, and then the compound reacts with the PP macroradical induced by initiator, leading to PP grafting and cross-linking.


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Shujing Yang, Guojun Song, Yunguo Zhao, Chao Yang, Xilin She

Institute of Polymer Materials, Qingdao University, Qingdao 266071, China

Correspondence to: Guojun Song; e-mail:
TABLE 1. Mechanics properties, MFR, and melt strength of raw and
modified PP.

 Melt Impact
 MFR strength Tensile yield strength
Items (g/10 min) (cN) strength (MPa) (KJ/[m.sup.2])

Modified PP 0.5 660.2 57.095 13.96
Raw PP 3.1 161.8 45.7 7.48

TABLE 2. MFR and gel percentage of samples with/without catalyst.

Items MFR (g/10 min) Gel percentage (wt%)

With catalyst 0.12 48.5
Without catalyst 0.72 0
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Author:Yang, Shujing; Song, Guojun; Zhao, Yunguo; Yang, Chao; She, Xilin
Publication:Polymer Engineering and Science
Geographic Code:9CHIN
Date:Jul 1, 2007
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