Liu Xiaoxuan1,2, Xie Linghai, Zhang Weiqun, Xu Yanping, Wu Guangguo, Pang Laixing, Yang Jianwen, Zeng Zhaohua, Chen Yonglie
Department of Chemistry, Shantou University, Shantou 515063, China; Institute of Polymer Science, Zhongshan University, Guangzhou 510275, China)
Received May 19, 2003. Supported by the National Natural Science Foundation of China (No.20274023), National Ministry of Education of China (No.02114) and Guangdong Provincial Natural Science Foundation (No.021241)
AbstractThe surface modification by the photografting of r-HALS (reactive hindered amine light stabilizer) on a commercial polyolefin thin film was studied. The process proceeded by photo-initiated grafting copolymerization of 4-methacryloyloxy-1,2,2,6,6-pentamethyl-piperidine (MPMP), a reactive HALS, onto the surface of linear low density polyethylene(LLDPE) thin film, presoaked in the benzene solution containing benzophenone (BP) initiator and MPMP monomer and UV irradiation for short time under nitrogen atmosphere. All measurements of the grafted polymer surface, such as relative intensity (R.I.) of ESCA lines C1S, O1S, or N1S, and Carbonyl Value (CV) or R.I. of FTIR-ATR spectral lines of carbonyl absorbance of MPMP, and the Yellowness Index (YI) of grafted LLDPE film have been performed. The grafting yield of MPMP onto the surface of LLDPE film was calculated by the weight gain of the sample after irradiation. The results show that the increase of carbonyl absorbance (or CV) was in line with the concentration of MPMP monomer in the presoaking solution, the grafting yield and the irradiating time. The N1s signals in ESCA spectra of the grafted LLDPE surface clearly indicates that photo-grafted copolymerization has taken place and a distinct grafted layer on the surface produced. Yellowness Index (YI) of the grafted LLDPE films is much lower than that of the blank film through the 4500 hours photo-oxidation, an evidence for improvement of the photo-stability. Surface modification of LLDPE film by photo-grafted with r-HALS monomer has thus been proved efficient for the photostabilization of LLDPE.
Polyolefin resin have been applied as one of the most useful polymer materials by virtue of many merits, but photo-oxidative degradation of the polymer directly affects the service life in outdoor application, which limits its usability. Mechanism of photo-oxidative degradation caused by radical formation induced by UV irradiation has been studied and it is found that the photo-oxidizing process mainly undergoes at the surface of polymer materials [1-4]. It is well known that the surface properties of polyolefin thin film, such as the surface energy, the surface polarity and the surface hardness etc. could be modified by photochemical grafting [5-7]. Up to now, hindered amine light stabilizers (HALS) as the most effective light stabilizers have been widely applied to photostabilization of polyolefin resin and thin film . One current trend is to use higher molecular weight HALS for enhancing thermalstability and excellent extraction resistance. Thus polymeric light stabilizer has become attractive. It is of both academic and industrial interest to prepare stabilizers containing different functional groups such as reactive-HALS (r-HALS). In the previous paper, a series of r-HALS has been synthesized and characterized, and copolymerization reactivity and photo-stabilizing performance applied in UV-curable coatings have been reported [9-11]. The excellent polymerization reactivity of r-HALS ensures the monomer directly bonded onto main chain of polymer. This is an important advantage in surface modification of polyolefin thin film through grafting copolymerization. The advantage of surface grafting is that the initiating reaction are concentrated on the surface where the grafted monomers can combine with surface of substrate through chemical bonding, preventing HALS from dissipation because of migration or evaporations. In this paper, the surface modification by photo-grafting of r-HALS on a commercial polyolefin thin film has been studied. The process have been proceeded by photo-initiated grafting copolymerization of 4-methacryloyloxy-1,2,2,6,6-pentamethly-piperidine (MPMP), a reactive HALS, onto the surface of linear low density polyethylene(LLDPE) thin film, presoaked in the benzene solution containing bezonphenone (BP) initiator and MPMP monomer and UV irradiation for short time under nitrogen atmosphere. After grafting, the surface properties, especially photo-stability, are substantially improved.2. EXPERIMENTAL
2.1 Materials and instruments
A linear low density polyethylene (LLDPE) thin film with crystallinity of 38 % and without additives was used in this study. The average thickness of the film was 26.2m as measured by an A-528 micrometer with a sensitivity of 10-2 mm. The sample films were weighed on a micro photo-electronic balance TG 332-A with an accuracy of 0.01 mg. 4-methacryloyloxy-1,2,2,6,6-pentamethyl-piperidine (MPMP) was synthesized and characterized in our lab. MPMP is a light yellow liquid with b.p.= 95-97ºC/3-4 mmHg. Benzophenone (BP) was re-crystallized twice from benzene/ethanol. Other reagents used such as acetone, benzene and ethanol etc., were analytical grade, and purified by distillation before use. Infrared analysis was carried out on a Nicolet Magna-IR750 Fourier transform infrared spectrometer（FTIR）. The ESCA spectra were recorded on an ESCALab MK2 spectrometer by XPS technique with a Msource, step= 0.05.
2.2 Preparation of presoaking solution
MPMP and BP initiator were accurately weighed and dissolved in toluene to prepare presoaking solution with different concentration. The composition of the presoaking solution was given in Table 1. At ambient temperature, the presoaking solution was kept overnight in dark place for homogenization. The sample films of LLDPE with 3×4 cm were extracted with acetone in an extractor, dried to constant weight. The sample film was immersed in the presoaking solution for 12 hours.
Table 1. The composition of presoaking solution used for photo-grafting of LLDPE film
2.3 Photo-grafting procedures
The sample film after immersion was placed between two quartz plates to stretch smoothly, and then placed into the grafting “box”with a quartz window. The sample film was exposed in the presence of nitrogen to the UV radiation on a caterpillar UV-curing machine at a conveyor speed of 92 cm/min. The central intensity of the medium-pressure mercury lamp (2400 W) was 198 mW/cm, the main emission line of the lamp was 365 nm and the effective arc length was 20 cm. The light intensity was measured by an UV-radiometer (type UV-A, made by Beijing Normal University Photoelectric Instrument Factory), which is sensitive in the wavelength range of 320-400 nm. The sample films after irradiation were immersed in the solution with acetone for 12 hrs and extracted intensively to remove unreacted monomer and homopolymer. The sample film was weighed after drying.
The sample film grafted was irradiated with a mercury-arc lamp (GZ-500W), in air at 25ºCwhich was constantly controlled by flowing. UV irradiation of < 300 nm was filtrated by a Pyrex glass. The coloration of the sample film was characterized with Yellowness Index measured on a UV-Vis spectrometer (Model 723, Shanghai analytical instrument plant). Yellowness index was calculated by the following equation (ASTM, D1925):
where 420, 560, 680 are the transmittance of the sample films before photo-oxidation at the wavelength of 420, 560 and 680 nm respectively; 420 and 680 are the transmittance of the sample film when the exposure time of photo-oxidation equal to at the wavelength of 420 and 680 nm respectively.
3. RESULTS AND DISCUSSION
3.1 Grafting ratio of MPMP
The grafting ratiowas calculated according to Eq. (2):
Where is the original weight of the sample film before grafting; t is the weight of the film after grafting.
Figure 1. The grafting ratio and Carboxyl Value (CV) changing with monomer concentration in the presoaking solution
Using benzophenone as a photoinitiator (WPI = 2.48 %), light intensity is 1.43 mW/cm, irradiation time is 10 min.
Carbonyl Value (CV) was defined as the carbonyl absorbancefrom the FTIR spectra of the grafted film divided by the thickness of film (m):
The grafting ratio and CV of the sample film grafted increased with the monomer concentration in the presoaking solution as shown in Figure 1.
Figure 1 shows that when the irradiation time was 10 min, with the increasing of the monomer concentration in the presoaking solution, CV tends to increase in synchronism with the grafting ratio of the grafted film. In FTIR-ATR spectra, the carbonyl absorption (1750 cm-1) of the grafted film appears and increases with the irradiation time (Fig. 2), which is an evidence of the grafting of MPMP monomer onto the surface of LLDPE film although there is small absorption of carbonyl group on the surface of blank film due to oxidation occurring during the presoaking process. Grafting ratio of LLDPE with MPMP may be roughly estimated from the carbonyl values obtained by FTIR-ATR spectrum.
Figure 2. The variation of grafting ratio and CV with irradiation time
Using benzophenone as a photoinitiator (WPI = 2.48 %), the concentration of MPMP monomer in the presoaking solution is 40.13 wt%, light intensity is 1.43 mW/cm
Figure 3. Effect of irradiation time on carbonyl absorbance on the surface of LLDPE film grafted with MPMP.
Using benzophenone as a photoinitiator (WPI = 2.48 %), the concentrations of MPMP in the presoaking solution is 40.13 wt%.
Figure 4. Effect of monomer concentration in the presoaking solution on carbonyl absorbance on the surface of LLDPE film grafted with MPMP.
Using benzophenone as a photoinitiator (WPI = 2.48 %), the irradiation time is 10 min.
3.2 Characterization of the grafted film by FTIR-ATR and ESCA
The results of FTIR-ATR (Fig. 3-4) and ESCA (Fig. 5-8) spectra of the grafted film have further shown that MPMP was successfully grafted onto the surface of the polymer substrate under UV irradiation. Fig. 3 and 4 show that CV of the sample film (line b in Fig. 1 and 2) increase with the concentration of MPMP monomer in the presoaking solution and the irradiation time respectively. The data listed in Table 2 indicate that at an irradiation time of 25 min, both grafting ratio and CV reach up to maximum values of 11.63 wt% and 0.0497, respectively. Thereafter both parameters tend to decrease slowly. It is clear that the grafting coplymerization of LLDPE film with MPMP monomer has been terminated at that time. The results obtained from ESCA spectra are in accordance with that from FTIR-ATR spectra.
Figure 5 shows ESCA spectra of LLDPE film, both grafted and blank, at different concentration of MPMP in the presoaking solution. The characteristic peaks of C1s, O1s and N1s changed with increasing of concentration of MPMP. For detailed structural analysis the peaks of C1s, O1s and N1s in Fig. 5 are amplified and shown separately in Figs. 6-8.
In Fig. 6, a shoulder peak of C1s (around 288 eV) appears at higher concentrations of MPMP, this peak is ascribed to carbon atoms from -C-O- and carbonyl groups, which demonstrates that the monomer has been successfully grafted onto the polymer surface by photo-grafting copolymerization.
Relative intensity (R.I.) and the area of C1s peaks decrease with the increment of MPMP concentration owing to the grafted MPMP onto the polymer substrate, meanwhile the ratios of N1s/C1s and O1s/ C1s increased clearly. It can be seen from Figs. 7 and 8 that with the increasing of MPMP concentration in the presoaking solution, both O1s peak (at 530 eV) and N1s peak (at 398 eV) appear and increase remarkably. The above results further demonstrate that MPMP can be successfully grafted onto the surface of LLDPE film by UV-induced copolymerization using BP as an initiator.
Table 2. Variation of CV and grafting ratio with irradiation time by photo-grafting of MPMP on LLDPE film
|Grafting ratio (%)||CV (A1725 cm-1m)|
Figure 5. ESCA spectra of the surface of LLDPE film photo-grafted with MPMP
Using benzophenone as a photoinitiator (WPI = 2.48 %), the irradiation time is 10 min. Concentrations of MPMP in the presoaking solution. changed from 0 to 40.13 wt%
Figure 6. ESCA spectra of C1s on the surface of LLDPE film photo-grafted with MPMP
Using benzophenone as a photoinitiator (WPI = 2.48 %), the irradiating time is 10 min. Concentrations of MPMP in the presoaking solution. changed from 0 to 40.13 wt%
Figure 7. ESCA spectra of O1s on the surface of LLDPE film photo-grafted with MPMP
Using benzophenone as a photoinitiator (WPI = 2.48 %), the irradiating time is 10 min. Concentrations of r-HALS monomer are changed from 0 to 40.13 wt% in the presoaking solution. Irradiating time is 10 min.
Figure 8. ESCA spectra of N1s on the surface of LLDPE film photo-grafted with MPMP
Using benzophenone as a photoinitiator (WPI = 2.48 %), the irradiating time is 10 min. Concentrations of r-HALS monomer in the presoaking solution. changed from 0 to 40.13 wt%
3.3 The photo-stabilizing performance of LLDPE-g-MPMP film
The photo-stabilizing performance of LLDPE-g-MPMP film during the photo-oxidation aging by UV exposure up to 4500 hours is shown in Fig. 9. Yellowness index (YI) of blank film showed a rapid increase after UV exposure time of 3000 hours and the film changed to very brittle. Although photo-oxidation degradation mechanism of photo-grafting surface is similar to that of other in terms of chemical route, but there are different details because HALS completely distributes throughout the polymer surface and chemically bonded with each other. Therefore, the multi-layers of HALS with functional bonds such as piperidine ring on photo-grafting surface layer are regularly arranged and form excellent protecting layer against photo-aging. This effect is especially manifested as the >N-CH in the piperidine ring could be quickly transferred to >N-O· during the photo-grafting procedure orat the early stage of photo-oxidation. The fact that the >N-CH in the piperidine ring can be quickly transferred to >N-O· has been proved in the study on the reaction kinetics of photo-induced polymerization of r-HALS in detail
Figure 9. Photo-stabilization of LLDPE film photo-grafted with MPMP
The concentration of MPMP in the presoaking solution is 10.07 wt% (line-1), 19.39 wt%(line -2) and 30.02 wt% (line 3) respectively, and line 0 is the blank.
Figure 9 shows that Yellowness index (YI) of the sample film changed with the exposure time. After the 1800 hours, YI of the blank began to increase quickly, in contrast to the grafted film which was still under lower coloration level. Better photo-stabilizing performance is obtained when higher MPMP concentration in the presoaking solution is used.
Surface modification of linear low density polyethylene (LLDPE) thin film by the method of photo-grafting copolymerization of MPMP (r-HALS) has been studied. Effects of monomer concentration in the presoaking solution and the irradiation time on the grafting ratio of LLDPE film were investigated by FTIR-ATR and ESCA spectra. All measurements of the grafted polymer surface, such as relative intensity (R.I.) of ESCA lines of C1S, O1S or N1S, and Carbonyl Value (CV) or R.I. of FTIR-ATR spectral lines of carbonyl absorbance of MPMP, and the Yellowness Index (YI) of grafted LLDPE film were performed. The results show that the carbonyl absorbance (or CV) increases in line with the concentration of MPMP monomer in the presoaking solution, the grafting ratio and the irradiation time. The N1s signals in ESCA spectra of the grafted LLDPE surface clearly indicated that photochemical-grafted copolymerization has taken place and a distinct grafted layer produced on the surface of the LLDPE film. After grafting, the surface properties, especially photostability, is substantially improved. Surface modification of LLDPE film by photo-grafting of r-HALS monomer is an effective way to improve the photostability of LLDPE film.