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A super-toughened nylon 12 blends via anionic ring-opening polymerization of lauryllactam in a twin screw extruder, preparation, morphology, and mechanical properties.

INTRODUCTION

The rapid anionic polymerization of lactams has received considerable attention since its discovery in the 1950s (1-27). Fast reaction kinetics, clean polymerization reaction without any by-products and a crystalline end-product make anionic ring-opening polymerization of lactam a competitive choice for application of reaction injection molding, rotational molding, and reactive extrusion and subsequent continuous shaping to form extrusion profiles and melt spun fibers. Caprolactam is by far the most studied lactam, and the nylon 6 prepared by this route compares favorably in properties with that prepared by conventional hydrolytic polymerization. However, continuous polymerization of lauryllactam is seldom studied (22).

The engineering application of in situ polymerized polyamides requires substantially higher toughness than given inherently by the related system. This is probably the driving force to develop new systems. According to the dissolving property of additive in lactam, these systems can be divided into two kinds. One is homogeneous system in which additive can be dissolved in lactam. The other is heterogeneous system in which additive cannot be dissolved in lactam. Additive of the former system include polyamide, polystyrene, polyurethane, polyphenylene oxide and so on (28-30). An additive of the latter system include functionalized polypropylene, rubber and so on. Compared with PA6, much less information is available about toughened PA12. Wollny et al. used poly(ethylene-co-butylacrylate) to toughen PA 12 by in situ formation and compounding (31). In this system, poly(ethylene-co-butylacrylate) was dissolved in lauryllactam.

So far, there are few reports about heterogeneous system in which additive cannot be dissolved in lactam. Hu et al. has reported PP/PA6 blends, which were synthesized via in situ polymerization and in situ compatibilization. The effects of this method on phase dispersion and compatibility were obvious (32), (33). But they have only studied the system in which polypropylene is the matrix. What is the situation when lactam is the matrix? Moreover, the mechanical properties of such system have not been reported. Because of the large viscosity ratio of additive and monomer, the added polymers are difficult to be dispersed well in heterogeneous system when lactam is the matrix. Therefore, finding a new method to disperse polymer particles well is the key to get good properties.

In this article, Nitrile-butadiene ultrafine full-vulcanized powdered rubber (NBUFPR) with diameter of about 100 nm was used as an additive to toughen PA 12. It can be inferred from its manufacturing process that the surface part of NBUFPR has higher crosslinking degree than the interior because of a higher concentration of irradiation sensitivity near the surface part, as well as more reactions produced by the irradiation with excited molecules and ions in water. Therefore, the reversible agglomeration NBUFPR not only has good rubber properties but also can be easily dispersed when blended with plastics (34). So we hope NBUFPR can also be dispersed well in lauryllactam. However, a good adhesion between NBUFPR and matrix is also very important to display the maximum toughening effect. PA12/NBUFPR blends were synthesized via polymerization of lauryllactam in a twin screw extruder. Morphology, mechanical properties of PA 12 and PA12/NBUFPR blends were investigated. And a series of commercial PA 12 and NBUFPR blends were also prepared for comparison.

EXPERIMENT

Materials

Lauryllactam (LL) was supplied by Degussa AG. N-acetyl caprolactam (ACL) as a coinitiator was purchased from Aldrich. Sodium hydride (NaH) as initiator was purchased from Shanghai Chemical Reagents, containing 45 wt% mineral oil. NBUFPR (VP-401) was provided by SINOPEC Beijing Research Institute of Chemical Industry. The dimension of NBUFPR particles is about 100 nm. Hydrolytic-polymeric Polyamide 12 (L20G, [M.sub.n] = 21000) was purchased from EMS-CHEMIE (China) Co.

Processing

The reactive extrusion polymerization was carried out in an intermeshing co-rotating twin screw extruder (TSE-35, Nanjing Ruiya). The length:diameter (L/D) ratio is 60, D = 35 mm. Screw configuration and temperature of zones are shown in Fig. 1. Before the polymerization reaction, sodium hydride was reacted with melting lauryllactam. Then the melt mixture was quenched in a dry metal box placed in ice water under purging [N.sub.2], which is useful for initiator to avoid losing activity during the process. The quenched mixture containing sodium lauryllactam (NaLL) and LL was broken up and premixed with ACL and NBUFPR in a high speed mixer at speed of 1500 rpm for 5 min at room temperature. Then all the materials were fed into twin screw extruder at room temperature and protected by [N.sub.2] atmosphere. The feeding rate is 3 kg/h, and the screw speed is 100 rpm. The obtained strands were pelletized and dried at 85[degrees]C for 12 h. Specimens for mechanical test were injection-molded at 260[degrees]C. The blends synthesized by reactive extrusion were noted as BP. And 5 wt% PA12/NBUFPR blends was noted as BP5 for example. The formulation of reactants is listed in Table 1. The weight of ACL is determined in order to synthesize PA 12 with number average molecular weight of about 20,000, similar as the purchased hydrolytic-polymeric PA 12. Hydrolytic-polymeric PA 12 was also melting blended with NBUFPR for comparison. These blends were noted as BL. And 5 wt% L20G/NBUFPR blends was noted as BL5 for example.

[FIGURE 1 OMITTED]
TABLE 1. The formulation for pure PA 12 and BP blends.

      NBUFPR (g)  NaH (g)  LL(g)  ACL (g)

PAI2      0         5.24      987      8
BP1       10        5.24      977      8
BP2       20        5.24      967      8
BP3       30        5.24      957      8
BP4       40        5.24      947      8
BP5       50        5.24      937      8


Monomer Conversion

The monomer conversion in the polyamide was determined by means of thermo gravimetric analysis (TGA) using a TA SDT Q600. All TGA measurements were made under [N.sub.2]. The flow rate of [N.sub.2] was set at 20 ml/min. The temperature was increased from 50[degrees]C to 600[degrees]C. The scanning rate was 10[degrees]C/min and the initial weights of samples were 2-4 mg.

Fourier Transform Infrared Spectroscopy

FTIR in KBr pellet transmittance mode was carried out with a Nicolet AVATAR 360 FTIR spectrometer in the range from 4000 to 400 [cm.sup.-1], with a resolution of 4 [cm.sup.-1].

Mechanical Properties

The tensile properties were evaluated, according to ASTM D638 in an Instron tensile tester (CMT 4204, Shenzhen Xinsansi equipment Co.) at a crosshead speed of 50 mm/min. An extensometer strain gage with a 50 mm gap was used to obtain the modulus and yield stress values. The flexural strength and modulus (ASTM D790) were tested in the same Instron tensile tester. The dimension of the flexural specimens is 127 by 12.7 by 3.2 mm. The support-to-span ratio is 16. The maximum strain is 5.0%. Notched Izod impact tests were conducted according to ASTM D256 on an impact testing machine (model XJU-22, Chengde Experiment Equipment Company). All test specimens were 3.18 mm thick. Six specimens were tested for each mechanical property.

Scanning Electron Microscopy

The morphology was observed with a SEM (JSM-5610LV, JEOL, Co). The surfaces of samples were coated with gold under vacuum before observation. The NBUFPR particle size distribution is calculated by our laboratory software. Calculations were based on two samples, both containing at least 200 particles. The number average diameter ([D.sub.n]) and the index of diameter distribution (n) were determined as follows:

[D.sub.n] = [SIGMA]NiDi/[SIGMA]Ni (1)

n = [SIGMA][NiDi.sup.2]/[SIGMA]NiDi/[SIGMA]NiDi/[SIGMA]Ni (2)

where Ni is the number of domains having diameter Di.

RESULTS AND DISCUSSION

Monomer Conversion

Monomer conversion is an important parameter, because a little content of monomer can influence the properties of specimen remarkably sometimes. Generally, the loss weight of sample before 350[degrees]C is corresponding to lauryllactam content (22). Because the loss weight percent of pure NBUFPR before 350[degrees]C is less than 6 wt%, the influence of NBUFPR on loss weight before 350[degrees]C almost can be ignored for BP blends. The results of the monomer conversion of different samples prepared via reactive extrusion are shown in Table 2. For polyamide polymerized by anionic polymerization, such results demonstrate that all the samples have high monomer conversion, more than 96%.
TABLE 2. Monomer conversion of PA 12 and PA 12/NBUFPR blends.

                        PA12  BP1   BP2   BP3   BP4   BP5

Monomer conversion (%)  97.3  96.4  97.1  96.2  96.7  97.5


FTIR Characterization

The FTIR spectra of PA12 and NBUFPR are shown in Fig. 2. The weak peak at 2237 [cm.sup.-1] (Fig. 2a) is attributed to C[equivalent to]N stretching vibration. Some researchers have reported that alkaline hydrolysis of polyacrylonitrile (PAN) could lead to the formation of poly[(sodium acrylate)-co-acrylamide] (35), (36). Besides, a reddish brown color appears at the very beginning of hydrolysis and disappears at almost complete conversion of the C[equivalent to]N groups. The coloration should definitely be caused by PAN cyclization, which might be assumed as one of some possible intermediate stages of hydrolysis. In this experiment, NBUFPR turned from light yellow to brown immediately after being mixed with other reactants in melt state. It is because the nitrile groups can be easily attacked by anion in strong alkaline condition during anionic ring-opening polymerization of lauryllactam. The reaction mechanism of nitrile groups on the surface of NBUFPR with anion is as shown in Fig. 3. Structure B which is formed via reaction of nitrile group and anion of sodium lauryllactam can immediately transform to structure C, showing brown color. Note that the conjugated sequences of C=N bonds are short. Thus the lauryllactam group covalent bonding to carbon atom of C=N group can be an active dot to induce the propagation of polyamide 12 chain on the surface of NBUFPR particles. Similar structure has been reported by Luisier et al. (37).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

To confirm this graft reaction, NBUFPR-g-PA12 was synthesized as follows. All reactants used for preparation of 5 wt% PA12/NBUFPR blend except for N-acetyl caprolactam were blended in flask at 180[degrees]C, and reacted for 10 min. The product was extracted by ethanol to remove residual monomer for 48 h first, then was extracted by N, N-dimethylformamide to remove homopolymerized PA12 for 96 h. The final product was characterized by FTIR (Fig. 2b). Some typical peaks of PA12 at 3297 [cm.sup.-1], 1641 [cm.sup.-1], and 1560 [cm.sup.-1] appear on this spectrum. Therefore, the assumption that PA12 chains can graft from NBUFPR particles is proved. Because the surface part of NBUFPR is crosslinked, the amount of C[equivalent to]N groups participating graft reaction is little in total amount of C[tbomd]N groups. Similar result had been reported by our lab. Hou and Yang (38) let styrene-co-acrylonitrile (SAN) hydrolysis in [epsilon]-caprolactam at 180[degrees]C for 300 min. The FTIR spectrum showed that caprolactam could covalently bond to SAN. However, he did not let SAN react in caprolactam with the existence of sodium caprolactam. In this work, NBUFPR can react fast with sodium lauryllactam, which is a much stronger alkali than caprolactam to form active dots. And in the presence of a catalysis (sodium lauryllactam), NBUFPR-graft-PA12 formed.

Mechanical Properties

The mechanical properties of PA12, L20G, BP, and BL blends are shown in Fig. 4. Both of the tensile strength and yield strength of BP blends decrease with increasing NBUFPR content. But this tendency is not obvious in BL blends. The elongation at break changes a little for both series blends. The flexural strength decreases dramatically with increasing NBUFPR content for BP blends, especially when the NBUFPR content is above 4 wt%. On the contrary, the flexural properties of BL blends decrease more gently. What is the most interesting? The notched Izod impact strength of BP blends rises rapidly when NBUFPR content is more than 3 wt%, and achieves 989J [m.sup.-1] when 5 wt% NBUFPR was added. In contrast, the notched Izod impact strength of BL blends changes little.

[FIGURE 4 OMITTED]

Morphology

SEM pictures in Fig. 5 show how the fracture mode of the specimen alters with increasing NBUFPR content. For the pure PA12, it shows no ductility after crack tip blunting (Fig. 5a). However, for the PA12/NBUFPR blends, the failure mode was totally different, which is well documented in Fig. 5b-d. At lower NBUFPR content, namely less than 4 wt%, the patchy appearance is because of competing processes between crazing and particle-caused secondary cracking before fast fracture. Secondary cracking is generated by NBUFPR particles themselves. At 4 wt% and 5 wt% NBUFPR content (Fig. 5c and d, respectively) the failure is fully ductile albeit with superimposed secondary cracking. Figure 5d shows lotus leaf-like strips on the fracture surface. However, for BL5 (Fig. 5e), the fracture surface shows a patchwork type pattern just ahead of the razor notch and which is the result of the fast breaking up of a less developed craze region.

[FIGURE 5 OMITTED]

Generally, good impact property is usually corresponding to good dispersion and adhesion. Figure 6 shows the dispersion of NBUFPR in BP5 and BL5 after cryogenic fracture. Surface of the particles pulled out in Fig. 6b is smooth. This indicates that NBUFPR particles have poor adhesion with matrix in BL5. Contrarily, ambiguous particles and good adhesion between the two phases can be seen in BP5 (Fig. 6a). It is attributed to the in situ formed NBUFPR-g-PA12 copolymer improved the compatibility of two phases. Figure 6 also show the dispersed particle size. The number average diameter ([D.sub.n]) of NBUFPR particles were calculated and listed in Table 3. For the BL5, the dimension of NBUFPR particles is in the range of 0.5 [micro]m ~ 1 [micro]m. While in case of BP5, it is decreased sharply to the scope of 200 nm ~ 300 nm. Moreover, the distribution index (n) is 1.12 for BP5, smaller than 1.20 for BL5. According to Wu's theory, the thickness of matrix ligament is shown to be the single parameter determining whether a polyamide/rubber blend will be tough or brittle (39-41). When the average ligament thickness is smaller than a critical value, a blend will be tough; otherwise, it will be brittle. With the same NBUFPR content, it can be predicted that the tearing of the matrix is strongly favored with decreasing modifier size and thus decreasing interparticle distance. Besides, the grafted PA12 on NBUFPR particles also decreased the interparticle distance. Thus, NBUFPR particles in PA12/NBUFPR blends could absorb the impact energy effectively and achieved brittle-tough transition when only 4 wt% NBUFPR was added.

[FIGURE 6 OMITTED]
TABLE 3. Number average diameter of NBUFPR and index of diameter
distribution.

     Dn/nm    n

BP5  212    1.12
BL5  570    1.20


Why better dispersion of NBUFPR can be achieved via reactive extrusion? The blending evolution of NBUFPR along reactive extrusion process is shown in Fig. 7. The lauryllactam particles could be broken up easily in a high speed mixer, and it premixed with NBUFPR powder better than PA12 granules mixed with NBUFPR powder. After feeding into the twin screw extruder, the melt monomer with very low viscosity could penetrate into the gap among NBUFPR particles, further dispersing NBUFPR powder. Because the matrix viscosity rose as the polymerization continued, the viscosity ratio of NBUFPR and matrix was closing to 1. Therefore, the NBUFPR could be dispersed better under shear force. On the contrary, the viscosity ratio of NBUFPR and PA12 (L20G) deviated from 1, because the PA12 molecular chains could be broken under shear force by simply melt blending. Thus, NBUFPR was dispersed better in reactive extrusion than simple melt blending.

[FIGURE 7 OMITTED]

CONCLUSIONS

Blends of nylon 12 and NBUFPR were successfully synthesized via anionic polymerization in twin screw extruder. The tensile and flexural strengths of PA12/ NBUFPR blends decrease with an increasing NBUFPR content. However, the notched Izod impact strength of BP blends improves significantly at NBUFPR content >3 wt%. But brittle-tough transition does not appear in BL blends within the same range of NBUFPR content. NBUFPR particles have better adhesion with matrix and much smaller size in BP blends than those in BL blends. This is primarily attributed to two factors. One is the in situ formed NBUFPR-graft-PA12 on surface of NBUFPR particles leads to good adhesion of two phases. The other is good dispersion property of NBUFPR during the reactive extrusion process, leading to smaller interparticle distance. Thus the impact resistance of BP blends was improved remarkably.

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Libo Du, (1), (2) Guisheng Yang (1), (3)

(1) CAS Key Laboratory of Engineering Plastics, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People's Republic of China

(2) Graduate School of Chinese Academy of Sciences, Beijing 100039, People's Republic of China

(3) Shanghai Genius Advanced Materials Co., Ltd, Shanghai 201109, People's Republic of China

Correspondence to: G.S. Yang; e-mail: ygs@geniusen.com

Published online in Wiley InterScience (www.interscience.wiley.com).

[C]2010 Society of Plastics Engineers

DOI 10.1002/pen.2l640
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Author:Du, Libo; Yang, Guisheng
Publication:Polymer Engineering and Science
Date:Jun 1, 2010
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