Abstract
Irradiation cross-linking of polyamide 6 (PA6) fibers in the presence of polyfunctional monomer was studied. The combination of trimethallyl isocyanurate (TMAIC) and trimethylolpropane trimethacrylate (TMPTMA) was found to enhance the irradiation cross-linking of PA6 fibers efficiently. The effect of dose rate on the irradiation cross-linking was also investigated. The highest gel content was achieved when the fibers were irradiated at a high dose rate of 10 kGy/h. In this case, the breaking strength changed slightly and the elongation at break decreased. At a low dose rate of 1 kGy/h, the breaking strength fell drastically and the elongation at break increased at first and then decreased. The onset temperature of degradation fell and the amount of non-volatile residue at 600℃ increased as the irradiation dosage and dose rate increased. A high gel fraction largely improved the melt-drip performance of PA6 fibers. Irradiation cross-linking did not change the α-phase crystal form of PA6 fibers, and crystallinity decreased as the dose increased.
Irradiation cross-linking is an elegant method to improve the properties of polymers, such as thermal performance, chemical stability, mechanical behavior, and flame resistance, without altering the manufacturing process, and even in the final processed specimens. Nowadays it is widely used in manufacturing rubbers, cables, pipes, and heat-shrinkable tubes.1,2
Polyamides play an important role in engineering plastics as well as in textiles. Among these, polyamide 6 (PA6) fibers are widely used in clothing, upholstery, transportation, and individual protection, with the merits of abradability, chromaticity, elasticity, and ductility. However, the use of PA6 fibers has limitations in the field of protective textiles and infant’s clothing because of its intensive melt-drips during burning. 3 In recent years, there have been many investigations on the modification of PA6 by high-energy irradiation cross-linking. Balabanovich studied the effects of radiation-induced cross-links on fire retardance in PA6. The optimum improvement was obtained at a red phosphorus load of 12.5 wt% at an absorbed dose of 22 kGy. 4 Dadbin investigated electron-beam cross-linking of PA6 with and without the cross-linking agent TAC, and also discussed the effects on mechanical properties and thermal stability. 5 An investigation carried out by Pramanik found that the percentage of water absorption of PA6 was reduced and mechanical properties were improved due to the cross-linking induced by electron-beam irradiation. 6 Zaharescu and Silva paid attention to the thermal degradation of PA6 in various environments and a degradation mechanism was proposed. 7 In these studies, one kind of polyfunctional monomer (PFM), triallyl cyanurate, was used to promote the irradiation cross-linking because of the very low radiation-chemical yield of cross-linking for the neat polyamides and it has been proved that using different PFMs has a distinct effect on irradiation cross-linking. Additionally, the dose rate also affects the degree of irradiation cross-linking according to the study of Spadaro. 8 In this work, trimethallyl isocyanurate (TMAIC) and trimethylolpropane trimethacrylate (TMPTMA) are combined as PFM and different dose rates are applied to sensitize the irradiation cross-linking of PA6 fibers. Furthermore, the effect of irradiation cross-linking on the melt-drip behavior of PA6 fibers was also investigated since it is a difficult to tackle the dangerous dripping performance with the addition of flame retardants.
Experimental details
Materials and methods
PA6 pellets for spinning were provided by the Foshan Lingbo Company, China, with a relative viscosity of 2.4. Formic acid (85%) from the Shanghai Guoyao Chemical Reagent Company, China, was used as received. TMAIC and TMPTMA were supplied by the Shanghai Fangruida Company.
Fiber preparation and irradiation
PA6 pellets were vacuum dried at a temperature of 105℃ for 24 h and then melt-mixed with 3 phr TMAIC and 3 phr TMPTMA by a commercial twin screw extruder. PA6/TMAIC/TMPTMA compound fibers were obtained using the conventional melting spinning method. These fibers were then gamma irradiated in air and nitrogen at room temperature at the Shanghai Atomic Nuclear Research Institution, China, with a source activity of 20 × 104 Ci. The irradiation doses ranged from 20 kGy to 200 kGy with a dose rate of 1 kGy/h and 10 kGy/h, respectively.
Characterization of samples
Gel content
The gel content of the samples was determined using a solvent extraction method.
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Portions of the dried fibers were weighed, and then immersed in 85% formic acid. Extraction was continued for three days at room temperature and a little fresh solvent was added every day. Gels were collected by filtering through a sand-core funnel and then dried to constant weight. The percent gel content was calculated using the formula:
Anti-dripping test
The anti-dripping performance of irradiated fibers was tested as follows: the same portions of fibers were weighed and carded in the same length with the same twist, and then the same flame was used to ignite the fibers and the number of drops in the burning time was recorded.
Instrumental analysis
Thermal gravimetric analysis was performed on a NETZSCH TG209F1Iris between 50℃ and 600℃ under nitrogen and at a heating rate of 10℃/min.
The X-ray diffraction (XRD) measurement of fibers was carried out using a D/Max-2550P, RIGAKU (Japan).
Mechanical properties
The breaking strength and elongation at break were determined using an electronic single-fiber strength tester. The results are the average of 50 measurements. The test procedure was carried out at a gauge of 20 mm and a drawing speed of 20 mm/min under standard conditions.
Results and discussion
Sensitizing effect on irradiation cross-linking of PA6 fibers
In our previous work, we found that pure PA6 fibers were difficult to be irradiation cross-linked. There was only a 0.27% gel content when irradiated by γ-rays in air and the value was 0.5% in nitrogen.
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Figure 1 shows that the gel content increased with increasing the dose to 140 kGy at different dose rates when the sensitizers were introduced, and it exhibits a slight decrease with further increases in the absorbed doses.
Variation of gel content with absorbed dose of PA6 fibers with TMAIC and TMPTMA at different dose rates.
The dose rate affected the gel content significantly, as indicated by values from 4.38% (1 kGy/h) to 25% (10 kGy/h) at a dose of 100 kGy irradiated in air. Moreover, the gel content reached a higher level when the samples were irradiated in nitrogen; the corresponding values were observed as 11.29% (1 kGy/h) and 64.44% (10 kGy/h). Compared with the irradiated atmosphere, dose rate has a marked effect on the gel content.
Effects of irradiation cross-linking on mechanical properties
Irradiated at different dose rates, PA6 fibers with TMAIC/TMPTMA showed a different tendency in the change of mechanical properties, as seen in Figures 2 and 3. At a dose rate of 10 kGy/h, the breaking strength changed a little and there was a 4% increase at a dose of 100 kGy. The elongation at break decreased during the irradiation progress due to the formation of a three-dimensional network, which restricted the movement of the main chains. In the case of 1 kGy/h, PA6 fibers were damaged by irradiation degradation, as indicated by a 27% decrease in the breaking strength from 3.27 cN/dtex to 2.38 cN/dtex at a dose of 100 kGy, whereas the elongation at break increased first in the absence of gel and then decreased on the formation of a tighter network.
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Mechanical properties of irradiated PA6 fibers with TMAIC/TMPTMA at a dose rate of 10 kGy/h in nitrogen. Mechanical properties of irradiated PA6 fibers with TMAIC/TMPTMA at a dose rate of 1 kGy/h in nitrogen.
Thermal gravimetric analysis
As seen from Table 1, the incorporation of sensitizers caused a small decrease in the onset temperature and an increase in the non-volatile residue at 600℃ from zero to 4.69%. Irradiation with γ-rays at different dose rates weakens the thermal stability of PA6 fibers, which can be explained if the irradiation induced a large number of small fragments and their decomposition decreased the onset temperature. The lowest onset temperature for mass loss was 273.4℃ at a dose of 60 kGy and a dose rate of 1 kGy/h. At this dose, there were some residual sensitizers to volatilize and decrease the onset temperature. There was a lower non-volatile residue at 600℃ at a dose rate of 1 kGy/h, whereas the residue at 10 kGy/h increased drastically, as shown in Table 1 and Figure 4.
Thermal gravimetric curves for PA6 fibers with PFM irradiated in nitrogen and at different dose rates (see Table 1). Thermal gravimetric data for samples irradiated in nitrogen and at different dose rates
Influence of irradiation cross-linking on melt-drip performance
Variation of melt-drops of irradiated PA6 fibers with TMAIC/TMPTMA at different dose rates
Effects of irradiation cross-linking on crystalline structure
The XRD of unirradiated and irradiated PA6 fibers with sensitizers is shown in Figure 5. From the patterns we can see clearly the two strong diffraction peaks at 2θ = 20.5° and 23° among the samples, which are a distinctive feature of α-phase crystals of PA6, designated as α1 and α2, respectively. Similar peaks at the same angle were reported by Nalwa,
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Sibilia,
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and Kolhe.
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This shows that irradiation has not changed the crystal form of the PA6 fibers, but the intensity of the same peak decreased compared to that of the untreated fibers, which may be attributed to a reduction in crystallinity. During the irradiation process, the presence of sensitizers TMAIC/TMPTMA accelerated the cross-linking rate of free radicals and restrained the pyrolysis and recombination, which led to the decrease of crystallinity.
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X-ray diffraction curve of PA6 fibers with sensitizers both unirradiated and irradiated.
Conclusions
The effects of PFM and dose rate on the irradiation cross-linking of PA6 fibers were studied. The introduction of TMAIC/TMPTMA enhanced the irradiation cross-linking significantly and the highest gel content of 64.44% was achieved when the polymers were irradiated at a dose of 100 kGy, and at a high dose rate 10 kGy/h. In this case, the breaking strength changed a little, but elongation at break decreased with increasing dose. Irradiation cross-linking at a high dose rate caused a decrease in the onset temperature and a drastic increase in the non-volatile residue at 600℃, which may be attributed to the improvement of anti-dripping properties.
Footnotes
Funding
The authors would like to acknowledge the financial assistance provided by the Fundamental Research Funds for the Central Universities (12D10132).