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Synthesis of a core-shell polyacrylate elastomer containing ultraviolet stabilizer and its application in polyoxymethylene.

INTRODUCTION

Polyoxymethylene (PQM) belonging to the group of engineering thermoplastics has a wide range of applications in industry because of its high mechanical strength, excellent abrasion resistance, fatigue resistance, moldability, and chemical resistance, including in electrical and electronic applications, automotive applications, and precision machine applications. POM is a relatively unstable material though the exploitation of POM as a commercial polymer has created a great deal of interest (1-5). Owing to its poor UV aging resistance and impact toughness, the mechanical properties decline sharply after UV irradiation, especially the impact strength, which limits its applications in outdoor use. Generally, as a possible way to solve the problem of POM photostabilization, a series of ultraviolet (UV) stabilizers have been successfully used to prevent polymeric materials from aging induced by sunlight, such as benzophenones, benzotriazoles, triazines, and hindered amine light stabilizers. However, low-molecular-weight UV stabilizers have poor compatibility with POM matrix, which deteriorates the photostabilization (6-10). Furthermore, low-molecular-weight UV stabilizers can easily migrate, volatilize, and have poor solvent extraction resistance due to the low interactions between stabilizers and matrices, which results in the decrease of photostabilization for long-term use. These problems caused by the low-molecular-weight stabilizers can be overcome through the use of reactive UV stabilizers and high-molecular-weight UV stabilizers (11-15). In addition, POM is often modified with thermoplastic elastomers or structural plastics of high flexibility and deformability to enhance its impact toughness.

In this study, an elastomer containing UV stabilizer was prepared by seeded emulsion polymerization and was used as a modifier to improve the UV aging resistance and impact toughness of POM. The results showed that POM could be photostabilized and toughened by the elastomer functionalized with UV stabilizer in one step.

EXPERIMENTAL

Materials

2,4-Dihydroxybenzophenone (UV-O, purity above 99.5%), provided by Jinchun Meibang Chemical (Wuhan, China), was used as received without further purification. Analytical reagent-grade sodium bicarbonate ([NaHCO.sub.3]), sodium dodecanesulfonate (SDS), alkylphenol polyoxyethylene (OP-10), potassium persulfate (KPS), tetrahydrofuran (THF), and calcium chloride ([CaCl.sub.2]) were provided by Kelong Chemical Reagent Factory of China (Chengdu, China). Methyl methacrylate (MMA; analytical reagent) and butyl acrylate (BA; analytical reagent), provided by Kelong Chemical Reagent Factory of China, were distilled before use. UV stabilizer 2-hydroxy-4-(3-methacryloxy-2-hydroxylproroxy)benzophenone (BPMA) was synthesized according to a previous study 1161. Commercial POM copolymer, with weight-average molecular weight ([M.sub.w]) about 1.16 x [10.sup.5] g/mol and melting temperature ([T.sub.m]) 165-167[degrees]C, was procured from YunTianHua Corporation (China). Antioxidant Irganox 1010 was provided by Ciba Specialty Chemicals (Switzerland).

Preparation of Poly(MMA-BA-BPMA) and Poly(MMA-BA)

The core-shell elastomer was prepared by seeded emulsion polymerization from BA, MMA, and BPMA. The components used for the preparation of core-shell elastomer are shown in Table 1.

TABLE 1. Component for the preparation of core-shell elastomer.

                Poly (MMA-BA-BPMA)   Poly (MMA-BA)
Components     Core (g)  Shell (g)  Core (g)  Shell (g)
BA                 50.0         10        50         10
MMA                 7.0         21        10         30
BPMA                1.0          9        --         --
KPS                0.35       0.15      0.35       0.15
SDS                 1.2        0.8       1.2        0.8
OP-10               0.6        0.4       0.6        0.4
[NaHCO.sub.3]       0.2        0.1       0.2        0.1
DI water             60         30        60         30


The components for the core and the shell (as shown in Table 1, except the initiator) were added into a 250-ml four-necked round-bottomed flask and were vigorously stirred for 0.5 h at 50[degrees]C to prepare the core pre-emulsion and shell pre-emulsion. Then, the initiator was dissolved in 20 ml of deionized (DI) water to prepare the initiator aqueous solution.

Seed Emulsion. One-half of the core pre-emulsion and one-third of the initiator aqueous solution were added in a flask equipped with reflux condenser, mechanical stirrer, thermometer, constant pressure-dropping funnels, and inlet for nitrogen gas. The system was purged with nitrogen for 0.5 h with stirring moderately to remove oxygen, and then the reaction temperature was increased to 70[degrees]C. When the emulsion turned blue, the system was kept at 70[degrees]C for another 0.5 h to get the seed emulsion.

Core Emulsion. The seed emulsion was heated to 75[degrees]C in nitrogen atmosphere, and one-third of the initiator aqueous solution and the remaining one-half of the core emulsion were slowly added into the flask within 1 h. After completion of feeding, the reaction was carried out at 80[degrees]C for 0.5 h to get the core emulsion.

Core-Shell Emulsion. The shell pre-emulsion and the remaining one-third of the initiator aqueous solution were added into the core emulsion within 1 h at 80[degrees]C. The reaction was carried out at 85[degrees]C for additional 0.5 h to get the core-shell emulsion.

Core-Shell Poly(MMA-BA-BPMA) Elastomer. The core-shell emulsion was coagulated with 5% of [CaCl.sub.2] aqueous solution to get coagulum product. The poly(MMA-BA-BPMA) was washed with DI water for five times. The final product was dried at 70[degrees]C in vacuum drying oven. The core-shell poly(MMA-BA) elastomer was prepared with the same procedure, except that BPMA was not involved.

Preparation of Testing Samples

The POM/poly(BA-MMA-BPMA) blend [13 wt% of poly(MMA-BA-BPMA)] and the POM/poly(BA-MMA)/UV-0 blend 1.3 wt% poly(MMA-BA) and 1.3 wt% of UV-0) were prepared by a HT-30 corotating twin-screw extruder (Nanjing Rubber and Plastics Machinery Plant, China) at 170-180[degrees]C. The testing specimens for mechanical properties determination and UV irradiation test were prepared through injection molding by a LS-26 injection-molding machine (Nissei Plastic Industrial Company, Japan). The operating temperature was 190[degrees]C.

UV Irradiation

The specimens were placed in an exposure unit equipped with two 500-W Ga-In source lamp with a maximum intensity at 365 nm. The intensity of irradiation was 3.0 W/[m.sup.2] measured by a UV irradiance meter model UV-A tester (Photoelectric Instrument Factory of Beijing Normal University, Beijing, China). The irradiation test was carried out at 55[degrees]C [+ or -] 2[degrees]C with air circulation. Specimens for testing were taken at intervals (0, 250, 500, 750, and 1000 h).

Measurements and Characterization

Fourier transform infrared spectroscopy (FTIR) measurements of samples were carried out on a Nicolet 560 FTIR spectrometer (Nicolet Instruments Company) in the range from 4000 to 400 [cm.sup.-1] in transmission. The obtained elastomers were Soxhlet extracted with refluxing DI water for 72 h and dried. Then, the elastomers were dissolved in chloroform to prepare FTIR sample using potassium bromide tablet as supporter.

The molecular weight was determined by a HP gel permeation chromatography (GPC)-Addon GPC (HP Agilent, Taiwan) using THF as solvent.

The size and morphology of poly(MMA-BA-BPMA) particles were characterized by a HITACHI H-600 transmission electron microscopy (Hitachi Company, Japan) at an accelerating voltage of 100 kV. The sample diluted by DI water was placed on transmission electron microscope (TEM) copper grid and stained with 2% phosphotungstic acid solution.

The dispersion of poly(MMA-BA-BPMA) particles in POM was examined by a KYKY-2800 scanning electron microscope (SEM; KYKY Technology Development, China). The testing bar was broken at liquid nitrogen temperature (-196[degrees]C) and etched by THF for 24 h at room temperature to eliminate the core-shell poly(MMA-BA-BPMA).

The UV-vis absorption spectra of poly(MMA-BA-BPMA) and poly(MMA-BA) were recorded on a T6 UV-vis spectrophotometer (PERSEE, China) in the range from 200 to 600 nm using THF as solvent.

For the differential scanning calorimetry (DSC) analysis, a TA Q200 (TA Instruments Company) with nitrogen as the purge gas was used. Powder samples scraped from the most up surface of the POM/poly(MMA-BA-BPMA) blend were heated from 50 to 200[degrees]C at a heating rate of 10[degress]C/min, and the degree of crystallinity was obtained as follows:

C = [H.sub.f]/[H.sub.f0], (1)

where [H.sub.f0] is defined as the heat of fusion of 100% crystalline POM.

Examination of Mechanical Properties of the Blends

The tensile properties were determined by a RGM-3010 tensile testing machine (Shenzhen Reger Instrument, China) with 10 kN load cell according to ISO 5272:1993. The notched Izod impact strength was measured on a UJ-40 tester (Chengde Testing Machine Factory, China) according to ISO 180:2000.

RESULTS AND DISCUSSION

FTIR Analysis of the Core-Shell Poly(MMA-BA-BPMA)

Research (17) shows that BPMA monomer has similar reactivity with MMA and can easily copolymerize with MMA because BPMA monomer is a derivative of methacrylate and has similar chemical structure with MMA. The bulky benzophenone group has no steric hindrance effect on the reactivity of BPMA, which based on the fact that benzophenone group is far away from the active center methacryloxy. Meanwhile, MMA, BA, and BPMA have similar acrylate structure with high reactivity. Therefore, the copolymer from them can be easily prepared through traditional radical polymerization such as emulsion polymerization, bulk polymerization, and solution polymerization.

The FTIR spectra of poly(MMA-BA-BPMA), poly-(MMA-BA), and monomer BPMA are shown in Fig. 1. The C--H stretching vibrations of saturated aliphatic hydrocarbons at 2958 and 2870 [cm.sup.-1], a very strong absorption peak at 1733 [cm.sup.-1] belonging to C=0 ester carbonyl stretching vibration, the C--H asymmetric and symmetric deformation vibrations of saturated aliphatic hydrocarbons at 1450 and 1380 [cm.sup.-1], two strong absorption bands at 1240 and 1160 [cm.sup.-1] assigned to C--O--C stretching vibrations from ester groups, and the --[CH.sub.2]-- rocking vibrations centered at 755 [cm.sup.-1] are also observed for poly(MMA-BA) (Fig. 1, peak a). As expected, the C--H stretching vibrations (2958 and 2870 [cm.sup.-1]), the carbonyl absorption (1733 [cm.sup.-1]), C--H asymmetric and symmetric deformation vibrations (1450 and 1380 [cm.sup.-1]), C--O--C stretching bands (1240 and 1160 cm-1), --CH2--rocking vibrations (755 [cm.sup.-1]) are similar to those observed for poly(MMA-BA-BPMA) (Fig. 1, peak b). However, a new absorption band around 3450 [cm.sup.-1] assigned to the--OH stretching vibration, a strong absorption at 1623 [cm.sup.-1] corresponding to C--O in benzophenone, absorption peaks at 1507 and 703 [cm.sup.-1] associated with benzene rings, arid a weak peak at 1340 [cm.sup.-1] originating from C--0 stretching vibration in benzophenone are also observed (Fig. 1, peak c). According to the above analysis, it could be concluded that core-shell poly(MMA-BA-BPMA) has been successfully prepared through seeded emulsion polymerization.

Molecular Weight of Core-Shell Poly(MMA-BA-BPMA)

The core-shell poly(MMA-BA-BPMA), being a high-molecular-weight UV stabilizer and an impact modifier, the molecular weight should be measured. The molecular weight of core-shell poly(MMA-BA-BPM) was determined by GPC using TFIF as solvent. The number--aver-age molecular weight, weight--average molecular weight, Z-average molecular weight, and viscosity--average molecular weight are 3.4664 X [10.sup.4], 1.4936 x [10.sup.5], 3.6077 x [10.sup.5], and 1.4936 X [10.sup.5] g/mol, respectively. The molecular weight distribution (MWD) is about 4.3, as shown in Fig. 2. Therefore, the core--shell poly(MMA-BA-BPMA), as a high-molecular-weight UV stabilizer, was successfully prepared by emulsion polymerization.

Morphology of Core--Shell Poly(MMA-BA-BPMA)

TEM micrograph of poly(MMA-BA-BPMA) particles shows that a clear core--shell structure has been observed owing to the difference of electron penetrability to the core and the shell (Fig. 3). The white and light regions in the particles correspond to the core and the shell phases, respectively, which confirms the core--shell morphology of the particles. However, an interlayer (slight difference in contrast from the core and shell phases), which is the transition layer between the core and the shell phases, has also been observed. The transition layer plays a role in improving the compatibility between the core and the shell phases and in increasing the interfacial adhesion between the two phases. Furthermore, the particles are spherical in shape, with an average diameter about 60 nm. Hence, it proves that the core--shell particles have been obtained as desired through emulsion polymerization.

UV--Vis Absorption Spectroscopy Analysis

It is necessary to characterize the UV-absorbing ability Of poly(MMA-BA-BPMA). For comparison, the UV-absorbing performance of poly(MMA-BA) was also studied. The UV-vis absorption spectra of poly(MMA-BA-BPMA) and poly(MMA-BA) are presented in Fig. 4. Absorption bands of poly(MMA-BA-BPMA) at 288 and 323 nm, attributed to the [pi] [right arrow] [pi]* and n [right arrow] [pi]* transition, respectively, correspond to the results reported in previous research (18), as shown in Fig. 4 (peak a). However, it is noteworthy that no absorption bands in the range from 250 to 600 nm have been observed for the poly(MMA-BA) (Fig. 4, peak b). Therefore, the above analysis indicates that the UV-stabilizing fragment (benzophenone) chemically bonded to the copolymer can retain its UV-absorbing ability and that the obtained copolymer has stronger UV resistance at 288 and 323 nm.

Particles Dispersion

The dispersion of poly(MMA-BA-BPMA) in POM was studied by SEM. The fracture surface of POM/poly(-MMA-BA-BPMA) blend etched by THF is provided in Fig. 5. The SEM micrograph for the blend with 13 wt% of poly(MMA-BA-BPMA) clearly demonstrates two-phase morphology, as shown in Fig. 5. The holes on the surface represent the core--shell poly(MMA-BA-BPMA). The core--shell poly(MMA-BA-BPMA) elastomers are well dispersed in POM matrix, attributed to the fact that the shell phase composed of MMA-co-BPMA has good compatibility with POM.

DSC Characterization

Figure 6 illustrates the DSC curves of the POM/poly(-MMA-BA-BPMA) blend before and after irradiation. The melting point of blend decreases as the irradiation time increases, which is consistent with the previous study (19). However, the crystallinity increases remarkably with the increase in irradiation time and reaches a maximum at 500 h (59.61%). Then it slightly decreases and attains a plateau, as shown in Table 2. The decrease of the melting point is attributed to the reduction of the molecular weight of the blend exposed to UV irradiation and the formation imperfect crystalline in the amorphous regions under the testing temperature (19).

TABLE 2. DSC data of POM/poly(MMA-BA-BPMA)

                          0 h   250 h   500 h   750 h  1000 h

[T.sub.m]([degrees]C)  163.46  161.95  160.03  160.44  159.52
[DELTA](J/g)            119.8   175.1   196.7   182.0   183.1
Crystallinity (%)       36.30   53.06   59.61   55.15   55.48


The increase in the crystallinity of the POM/poly(-MMA-BA-BPMA) blend after UV irradiation is affected by two factors. On the one hand, the crystallization of noncrystalline regions in POM matrix and the fragments produced from scission of the molecular chains of POM under UV irradiation continues under the testing temperature, being much higher than the glass transition temperature of POM, which makes the polymer chains have ability to rearrange (testing temperature = 55[degrees]C; glass transition temperature of POM =-30[degrees]C). On the other hand, the crystallinity deceases owing to degradation of crystalline regions in POM under UV irradiation. The crystallinity increases within the range from 0 to 500 h as the irradiation time increases, attributed to the fact that the crystallization process of the amorphous regions is faster than the cleavage of chains from the crystalline regions. However, as the amorphous regions decrease, the crystallization rate of the noncrystalline regions equals the degradation rate of the crystalline regions. Therefore, the crystallinity of the blend reaches a plateau.

Mechanical Properties

Pure POM exposed to sunlight undergoes degradation, resulting in rapid deterioration of mechanical properties, especially the impact strength and the elongation at break, because of its poor UV resistance. Therefore, it is necessary that POM should be toughened and photostabilized simultaneously. Poly(BA-MMA-BPMA) is a high-molecular-weight UV stabilizer with a soft rubber core and hard shell structure, which can also be used as impact modifiers to improve the impact toughness of pure POM. Our previous work about toughening of POM shows that POM blended with 13 wt% of modifiers has a balance between the tensile strength and the impact strength. The mechanical properties of POM/poly(MMA-BA-BPMA) and POM/ poly(MMA-BA)/UV-0 blends before and after UV irradiation are shown in Figs. 7-9.

The tensile strength for both of the modified POM blends is lower than pure POM, owing to the addition of impact modifiers. It is noted that the tensile strength for pure POM decreases with the increase of the irradiation time; however, the tensile strength for both of the modified POM blends is smooth, as shown in Fig. 7. It can be attributed to the combined effects of photodegradation and increase of crystallinity. On the one hand, degradation deteriorates the tensile strength. On the other hand, the increase of crystallinity improves the tensile strength. For lack of protection from UV irradiation, degradation is the major cause for the deterioration of tensile strength for pure POM. However, the two effects are equal for both of the modified POM. However, both photodegradation and increase of crystallinity result in the deterioration of elongation at break and notched Izod of impact strength. The elongation at break for pure POM decreases more quickly as the irradiation time increases than that for modified POM (Fig. 8), owing to the photostabilization by UV stabilizers. In addition, the elongation at break for POM/poly(BA-MMA-BPMA) system is higher than POM/poly(BA-MMA)/UV-0, especially after 750-h UV irradiation. This implies that poly(BA-MMA-BPMA) could provide long-term protection to POM. A similar finding has been observed for the notched Izod of impact strength, as shown in Fig. 9. In addition, the elongation at break and the impact strength for pure POM have been increased by 102%, 43.4%, and 84.1% and by 31.5% for poly(MMA-BA-BPMA) and poly(MMA-BA)/UV-0 systems.

CONCLUSIONS

A core--shell polyacrylate elastomer containing UV-stabilizing fragments was successfully prepared by emulsion polymerization, and the elastomer containing UV stabilizer exhibited UV-absorbing ability when compared with that without UV stabilizer. POM modified by the elastomer showed outstanding UV resistance and better impact strength in comparison with pure POM. Besides, the elastomer containing UV stabilizer confers a long-term and more effective protection for POM from UV aging than the low-molecular-weight UV stabilizer.

Correspondence to: X. Ren; e-mail: xiancren@sina.com

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50873069.

DOI 10.1002/pen.23194

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Bin You, Cubo Wu, Shiling Zhang, Fan Yang, Xiancheng Ren

College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China
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Author:You, Bin; Wu, Guibo; Zhang, Shiling; Yang, Fan; Ren, Xiancheng
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Nov 1, 2012
Words:3350
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