Development of an Advanced Engineering Polymer from the Modification of Nylon 66 by e-Beam Irradiation

When Nylon 66 was irradiated by an optimum dose of e-beam in presence of polyurethane as impact modifier in combination with triallyl isocyanurate as cross-linker, a superior performance was observed for the irradiated nylon 66. Significant improvement of properties, such as hardness, tensile strength, flexural modulus and impact strength was obtained on radiation processing of nylon 66 by e-beam. More interestingly, percent water absorption of such irradiated nylon 66 was reduced substantially. Improvement of mechanical properties and reduction of water absorption of irradiated nylon 66 were due to the cross-linking of the polymer system. Increase of cross-linking with dose of e-beam was verified by the increased gel content at higher doses. Irradiated nylon 66 showed better dimensional stability than those achieved with pristine nylon 66. The increase in dimensional stability may be attributed to reduction in crystallinity with increasing dose of e-beam as revealed by DSC studies

2.1 Materials

Injection molding grade Nylon 66, Zytel 101L from DuPont, USA was used as base polymer in this study. Molding grade Polyurethane, Estane GP 92 AE procured from Lubrizol Advanced Materials Inc, USA was used as impact modifier. LR grade Triallyl isocyanurate from Acros Organics, Belgium was used as crosslinker. Formic acid (85 per cent), LR grade from SD Fine- Chem limited, India was used as solvent in this study.

2.2 Specimen Preparation

Nylon 66 granules were dried in an air circulated oven at 80 °C for 4 h before the start of actual operation of injection molding. The dried granules were mixed with 2 phr of Triallyl-isocyanurate (TAIC) and 3 phr of polyurethane (PU) in a rotating tumbler at room temperature. The proportions of the additives mixed with the base polymer were optimized at the beginning in a separate study, which is not included in this paper. As TAIC was liquid at room temperature uniform physical blends were prepared by tumbler mixing. The mixed materials were injection molded at 270 °C on a microprocessor-based reciprocating screw-type horizontal injection molding machine of clamping force 40 MT and shot capacity 25 g of Joy D’zine, India, into various test specimens. The pristine nylon 66, and its modified form such as nylon 66/TAIC and nylon 66/PU/TAIC were molded using the same processing conditions to make the test specimens. A circular band with OD 120 mm x ID 110 mm x width 15 mm (CB1) was molded from the above three materials for the study of dimensional stability. The molded specimens were packed in polyethylene pouch and sealed immediately after molding in order to prevent moisture absorption.

2.3 E-beam Irradiation of the Molded Specimens

Nylon 66 specimens of different compositions were irradiated by e-beam at BRIT, Vashi, Mumbai using 2 MeV e-beam accelerator in air at ambient temperature. The specimens were arranged in arrays inside stainless steel trays attached to the conveyor system running at a speed of 3 cm/s and received 10 kGy dose of e-beam per pass. The penetrability of electrons in nylon 66 molded specimens was insured by optimizing the beam energy. For the test specimens, single-side radiation was enough for complete penetration. The values of radiation doses were 50 kGy, 100 kGy, 150 kGy, 200 kGy, 250 kGy and 300 kGy. As soon as the irradiation was over, the specimens were repacked in polyethylene (PE) zipper bags to minimize the moisture absorption.

2.4 Mechanical properties

Tensile strength, percent elongation and tensile modulus were determined following ASTM D 638-94 using type IV specimens on a Universal Testing Machine, Model 4302, Make Instron, UK. Flexural strength and flexural modulus were determined on the same Universal Testing Machine as per ASTM D 790-92. Rockwell Hardness was determined as per ASTM D785-93 following procedure ‘B’ using ‘R’ scale with a 12.7 mm diameter Indenter and 60 kg major load on a Rockwell hardness tester, Model RAB 250, Make Saroj Engg. Udyog, India. Impact strength was measured at room temperature on notched specimens according to ASTM D256-93 on the Ceast Universal Pendulum, Model 6545 in the Izod mode.

Ten specimens were tested for each mechanical property and the average of all values was reported. Standard deviation was calculated over the entire range of data against each of the above properties at every dose of radiation and in every case the error was found to be less than 1 per cent of the value reported.

2.5 Water Absorption

Water absorption was determined following ASTM D570-98 on molded sample pieces using a Mettler balance, Model AG 204, where the conditioned specimens were dipped in a container of distilled water maintained at a temperature of 23 ± 2°C for 24 h. The percent water absorption was calculated using the following equation

The values obtained are the average of three readings.

2.6 Gel Content

Percent gel content was determined by using about 0.5 g sample pieces collected from the molded specimens and dipped in 85 per cent formic acid16. After extraction at room temperature for three days the insoluble gels were collected by filtering through a fritted glass crucible and weighed followed by determining their percentages in the respective samples. Three samples were extracted to determine gel/sol fraction and the average was reported.

2.7 G(S)/G(X) Ratio

Radiation chemical yield of crosslinking and chain scission were calculated by using Charlesby-Pinner equation17

S + S1/2 = ( p0 / q0 ) + 1 / (q0 UD)

where S is the sol fraction, p0 and q0 are the chain scission density and crosslink density per unit dose (kGy-1) respectively, U is the number-average degree of polymerization of the polymer before irradiation, and D is the dose (kGy) of radiation. The values of p0/q0 were obtained graphically from the intercept of the experimental curve of (S + S1/2) versus 1/D. G(S), the number of polymer chain scission per 100 ev energy absorbed and G(X), the number of polymer crosslink site per 100 ev energy absorbed are related to p0 and q0 by the relation11,15 G(S) / G(X) = 2 (p0/q0)

2.8 Dimensional Stability

Dimensional stability was studied on circular band CB1 at two different temperatures, 0 °C and 50 °C. Two sets of sample for each composition were taken and their initial inner diameter (ID) and outer diameter (OD) were recorded. One set of samples were then placed in a digitally controlled low temperature cabinet at 0 °C for low temperature dimensional stability study and the other set kept in a programmable oven at 50 °C for high temperature dimensional stability study. After a fixed period of time (24 h) the samples were taken out and immediately their final ID and OD were measured by a calibrated inspection gauge to determine the dimensional change. The changes between final and initial dimensions were reported. The less the change in dimension, the more is the dimensional stability of the sample.

2.9 Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) study was carried out with the samples under nitrogen atmosphere on a Differential Scanning Calorimeter, Model Q200 of TA instruments, USA. About 8 mg of samples were taken in an aluminum DSC pan for these experiments. The samples were heated from room temperature to 300 °C at a heating rate of 10 °C/min, kept at 300 °C for five minutes and finally cooled down to room temperature at a cooling rate of 10 °C/min. Percent crystallinity was calculated using the following equation

$\text{Crystallinity}\text{\hspace{0.17em}}\left(%\right)=\frac{\Delta \text{H}\text{\hspace{0.17em}}\mathrm{exp}}{\Delta Η}×100$

where ∆H exp is heat of fusion of the sample under study, and ∆H is heat of fusion of the 100 per cent crystalline nylon 66. Heat of fusion of the 100 per cent crystalline nylon 66 was taken as 196 J/g in the calculation of percent crystallinity18.

3.1 Tensile Properties

Different sets of dumbbell specimens were prepared by injection molding of three different nylon 66 compositions followed by irradiating them under electron beam at various doses. The first set was of virgin nylon 66, while the second set was made from the combination of nylon 66 and 2 per cent TAIC. On the other hand the 3rd set comprises a blend of nylon 66, 3 per cent PU and 2 per cent TAIC respectively. The results obtained for tensile strength (TS), tensile modulus (TM), and percent elongation for different compositions are shown in fig. 1, Fig 2., and Fig. 3 respectively.

Figure 1. Tensile strength of different nylon 66 compositions vs. dose of e-beam.

Figure 2. Tensile modulus of various nylon 66 compositions vs. dose of e-beam.

Figure 3. Percent elongation of different compositions vs. dose of e-beam.

Table 1. Summary of tensile properties for various nylon 66 compositions

3.2 Flexural Properties

Figure 4 shows the increase of flexural modulus of various nylon 66 compositions with increasing dose of e-beam. Table 2gives a summary of the flexural properties of all the compositions. TAIC mixed nylon 66 showed the maximum improvement compared to the other two compositions; 21 per cent improvement in flexural modulus with respect to pristine nylon 66 was observed as compared to 4.4 per cent increase for virgin nylon 66 and 16 per cent increase for nylon 66/ PU/ TAIC at 300 kGy dose of e-beam. Apart from flexural modulus, the flexural strength of TAIC mixed nylon 66 was also superior to other compositions. This composition undergoes an improvement of flexural strength of 3 per cent even at 0 kGy and 35 per cent at 300 kGy of e-beam when compared with the value of pristine nylon 66. The observations of flexural strength and modulus of nylon 66/TAIC are in line with the results of its tensile strength and modulus and are due to the thermal crosslinking of polyamide chains before irradiation, while e-beam induced inter-chain crosslinking after irradiation. Mode of deformation in the flexure of all the compositions was yielding-type, indicating ductile failure and the trend was like that of tensile properties.

Figure 4. Flexural modulus of various nylon 66 compositions vs. dose of e-beam. >

Table 2. Summary of flexural properties for various nylon 66 compositions

3.3 Rigidity

The results obtained from Rockwell hardness indicate that surface hardness increases in every composition as the dose of e-beam increases. As shown in Fig. 5, maximum improvement in Rockwell hardness is observed for TAIC mixed nylon 66 composition and it is 9 per cent at 300 kGy e-beam irradiation. On the other hand, 6 per cent improvement of hardness for nylon 66/ PU/ TAIC composition and 4 per cent increase for nylon 66 are observed at 300 kGy e-beam compared to the pristine nylon 66. The rise in hardness being in line with the rise in tensile and flexural modulus and drop in percent elongation as mentioned earlier clearly indicates that the polymer has become more rigid on irradiation. Increased rigidity improves the machinability of nylon 66, which is one of the most desirable characteristics for a plastic material for engineering applications.

Figure 5. Rockwell hardness of various compositions vs. dose of e-beam.

3.4 Izod Impact Strength

The results of Izod impact strength for various nylon 66 compositions are shown in Fig. 6. For all the compositions, Izod impact strength was decreased with increasing dose of e-beam. However the impact strengths of TAIC mixed nylon 66 and TAIC mixed nylon 66/PU blend remained much higher compared to virgin nylon 66 in the entire range of e-beam irradiation. At lower doses (less than 100 kGy), the PU blended composition showed maximum improvement in impact strength compared to the other two compositions. The remarkable increase of impact strength of nylon 66 in the PU- modified composition at lower doses may be due the presence of rubbery PU phase, which remains immiscible to nylon 66 and absorbs the impact energy. The increase of impact strength of nylon 66 in presence of TAIC may be attributed to the absorption of impact energy by the three dimensional network structure of polyamide chains developed by isocyanurate crosslinker. On the other hand, at higher doses of e-beam may cause polyamide chain scission adversely to effect deterioration of impact strength.

Figure 6. Izod impact strength of various compositions vs. dose of e-beam.

Fracture surfaces of impact specimens for all compositions of nylon 66 were observed smooth over the entire doses of irradiation till 300 kGy, which indicates ductile failure of the samples. There were no cavities observed at the broken surfaces indicating that the specimens were free of porosity. It is worthy to note that the impact strength of both nylon 66/TAIC and nylon 66/PU/TAIC at 300 kGy are 126 J/m and 119 J/m respectively, which are even much higher than the impact strength value of 59 J/m of pristine nylon 66. Hence nylon 66/TAIC irradiated by 300 kGy e-beam or nylon 66/PU/TAIC by 100 kGy e-beam may be chosen as a suitablel material for many important industrial applications where rigidity, tensile and flexural strength and impact resistance are the critical requirements.

3.5 Water Absorption

The most significant observation in this study is the decrease of water absorption of nylon 66 with increasing dose of e-beam for all the compositions as can be seen in Fig. 7 , which is very much desired as far as the performance of the product of these materials is concerned. Percent water absorption was decreased by 30 per cent at 300 kGy for nylon 66 /TAIC and 29 per cent for nylon 66/ PU/TAIC as compared to 14 per cent decrease for virgin nylon 66. Even at 0 kGy irradiation (i.e., before irradiation) the percent water absorption value of TAIC mixed nylon 66 was 6 per cent less and TAIC mixed nylon 66/ PU blend was 10 per cent less than that of the pristine nylon 66.

Figure 7. Percent water absorption of various compositions vs. dose of e-beam.

Sengupta and coworkers18 also found that water uptake was less for the films that received a radiation dose of 200 and 500 kGy than the un-irradiated nylon 66 film. Reduction of water absorption may be attributed to the crosslinking of polyamide chains. This is further evident from the fact that melt mixing of TAIC with nylon 66 leads to thermal crosslinking to effect a 6 per cent reduction of water absorption, even when it was not irradiated by e-beam.

3.6 Dimensional Stability

Figure 8. Change in dimension of circular band CB1 at 0 0C with e-beam dose.

Figure 9. Change in dimension of circular band CB1 at 50 0C with e-beam dose.

3.7 Gel Content

Figure 10 shows the extent of gel formation in virgin nylon 66 /TAIC sample irradiated with varying doses of e-beam. In case of virgin nylon 66, there was no gel formation till 50 kGy, followed by 48 per cent gel at 100 kGy and thereafter gradual increase to 70 per cent gel at 300 kGy. TAIC mixed nylon 66, on the other hand, underwent gel formation from the beginning of irradiation and showed 76 per cent gel at 50 kGy followed by gradual increase of gel content to 80 per cent at 300 kGy e-beam.

Figure 10. Percent gel content as a result of different doses of e-beam.

Gels are insoluble and infusible fractions constituting three dimensional crosslink network structures which are generated by crosslinking of linear polymer chains. Post polymerization crosslinking in nylon 66 is possible by high energy irradiation when crosslinking of polyamide molecules may proceed through free radical reactions in the solid phase resulting in the formation of gels. Increase of crosslinking of nylon 66 with increasing dose of e-beam has been verified by the increase of gel content in nylon 66 as the dose of e-beam increases. Formation of gel on irradiation has been enhanced further in presence of TAIC as crosslinker as was seen in case of TAIC mixed nylon 66 composition where all the mechanical properties were improved more than the improvement observed in virgin nylon 66.

As is shown in Fig. 11(a) and Fig. 11(b) , the G(s)/G(x) value of virgin nylon 66 as determined from Charlesby-Pinner plot was 1.40, whereas it was decreased to 1.32 for nylon 66/TAIC. This decrease in G(s)/G(x) value is due to more cross-linking in nylon 66 when TAIC was added as cross-linker which played very important role in accelerating the process of cross-linking because of generation of more free radicals during irradiation. The more the decrease of G(s)/G(x) value, the better is the efficiency of the cross linker to enhance the properties of nylon 66 through more cross-linking.

Figure 11. (a) Charlesby-Pinner plot of virgin nylon 66 to determine G(s)/G(x) and (b) Charlesby-Pinner plot of nylon 66/TAIC to determine G(s)/G(x).

It is interesting to note that when TAIC was blended with nylon 66 by melt mixing, the mechanical properties of nylon 66 were much affected, even though it was not irradiated by e-beam. As shown in Table 3., before the irradiation by e-beam there were significant changes in various properties of different nylon 66 compositions compared to the corresponding values of virgin nylon 66. The improvement in mechanical properties as well as decrease of water absorption at zero irradiation when cross-linker was incorporated into nylon 66 by melt mixing might be attributed to the thermal cross-linking of polyamide molecules. However, the extent of thermal cross-linking of polyamide molecules during melt mixing is not enough to form insoluble and infusible nylon 66 gel. This may be advantageous as far as the processibility of nylon 66/TAIC is concerned since thermal cross-linking will not affect the melt processing of this material for making products.

Table 3. Change in properties of different nylon 66 compositions before irradiation as compared to the respective values of virgin nylon 66

3.8 Crystallinity

DSC thermograms of TAIC mixed nylon 66 as presented in Fig. 12 reveal that crystalline melting temperature (Tm and crystallization temperature (Tc) of nylon 66 in presence of TAIC have decreased considerably with the increase of dose of e-beam irradiation. Tm of nylon 66 was reduced from 263 °C to 250 °C while its Tc was decreased from 234 °C to 219 °C, when irradiated by 0 to 300 kGy e-beam in presence of TAIC. The decrease of Tm and Tc of nylon 66 with increasing dose of e-beam may be due to the chain scission effect of e-beam at higher doses causing reduction in size of molecular chains of nylon 66, which melt faster and crystallize slowly at comparatively lower temperature.

Figure 12. DSC Thermograms for TAIC mixed Nylon 66 at 0, 100, 200 and 300 kGy.

Figure 13. Percent crystallinity of samples as a result of application of different doses of e-beam.

Electron beam radiation is proved to be a powerful means to improve the mechanical properties of nylon 66, thus can overcome the inherent limitations of such polymer. This radiation processing technique is highly beneficial since all the changes are brought about in solid-state, as opposed to inconvenient alternative chemical and thermal reactions processes.

Physical and mechanical properties improved significantly when nylon 66 irradiated by electron beam in presence of TAIC as crosslinker. Such crosslinked Nylon 66 may be useful for preparing rigid and dimensionally stable articles with superior mechanical properties and improved serviceability.

Blending of PU with nylon 66 in presence of TAIC at lower doses of e-beam improved the impact resistance of pristine nylon 66 manifold. PU modified e-beam crosslinked nylon 66 may find applications in high impact industrial products Decrease of water absorption of e-beam irradiated of nylon 66 in presence of crosslinker is another important achievement as it helps reduce the inherent hygroscopic nature of nylon 66 to effect improved dimensional stability and enhanced shelf life of the product. Development of cross-linked structure in irradiated nylon 66 is evident from the formation of infusible and insoluble gel where gel content increased with increasing dose of e-beam irradiation. Presence of crosslinker has decreased G(s)/G(x) value in irradiated nylon 66 indicating occurrence of more cross-linking than chain scission in irradiated sample than that occurred in virgin nylon 66.

The authors would like to acknowledge the financial assistance provided by Board of Research in Nuclear Sciences (BRNS), Mumbai for carrying out this research work. The authors also offer their special thanks to BRIT, Mumbai for making available the E-beam accelerator facility for the irradiation work carried out in this study.

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 Mr Nilay Kanti Pramanik completed Master of Science in Polymer Science from Tezpur University, Assam. He is pursuing Ph.D from Jamia Hamdard on radiation processing of polyamides using gamma rays and e-beam. He has more than six years research experience in polymer processing and fabrication. Polymer characterization using state of the art instrumentation facility is also a subject of his interest. He was the Young Scientist Award Winner in Engineering Sciences, 2010 organized by Indian Science Congress Association. He has six publications in international journal to his credit
 Dr R.S. Haldar received his MTech (Polymer Science & Technology) and PhD from IIT Delhi. He is presently working as Senior Assistant Director at Shriram Institute for Industrial Research, Delhi. He has an experience of twenty eight years in the field of polymer processing technology. His areas of interest include: New product development in polymers, engineering polymers, polymer blends and alloys, radiation processing of polymers, shape memory polymers, shear thickening polymers, biocompatible polymers, bioresorbable polymers and morphology of polymers. He has published 30 research papers, filed two patents, edited two books.
 Dr U.K. Niyogi obtained MSc (Tech) (Technology of Plastics) from U.D.C.T, Mumbai and was awarded PhD (Tech.) from University of Calcutta. He is presently working as Joint Director and Head, Material Science Division of Shriram Institute for Industrial Research, Delhi and engaged in various R&D projects on polymeric materials, specialty chemicals and waste utilization. He has more than thirty years of experience in the field of R&D, synthesis and processing of polymers and resins. He is life member of various professional institutes such as Institution of Materials, Minerals and Mining, U.K., Indian Plastics Institute, Indian Institute of Chemical Engineers, Institution of Engineers, India, etc. He has 47 research papers to his credit.