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Transparent thermoplastic resin with electron beam cross-linking.

INTRODUCTION

Various high performance optical modules for digital equipment have been recently developed, such as the compact camera modules of cellular phones, which show improved image quality as well as a reduction in the module size [l] and high optical efficiency of light-emitting diode (LED) lights [2]. Back-light units used for liquid crystal displays (LCDs) are required to be brighter, slimmer, and lighter and exhibit increased light-use efficiency [3, 4]. Such optical units and modules typically use plastic optical lenses, because they are low cost and lightweight. Plastic optical lenses are produced by injection molding, which enable high-volume manufacturing and cost-effective production, and injection molding technologies for optical lenses have been researched in detail [5].

In the field of electronics packaging, the packaging density continues to increase and assembly automation is becoming more easily realized. Currently, the adopted soldering method is to directly print solder on the surface of a connector which then goes through a reflow process (surface mount technology). However, increasing environmental concerns have caused the electronic packaging industry to shift from lead-containing solder to lead-free solder. The melting point of lead-free solder is higher than that of lead-containing solder; therefore, a heat resistance (reflow resistance) of 260 [degrees] C for 60 sec is required for electrical parts [6].

Recently, a novel silicone polymeric material with high thermal stability has been developed for optical device applications [7]. However, this material cannot be produced in an injection molding machine, because it must be cured with heat or UV light to withstand high temperatures. Conventional transparent thermoplastic resins used for optical modules produced using injection molding machines have poor heat-resistant characteristics and cannot withstand the reflow process. Therefore, optical modules produced using existing thermoplastic resins (cyclic olefin polymer (COP), poly(methyl methacrylate) (PMMA), or polycarbonate (PC)) require a surface mount process other than the reflow process, for example, laser soldering or a connector, so that the cost of such modules is not reduced.

Heat-resistant camera modules for cellular phones have been developed [8, 9]; an optical lens combined with a thermal curable resin body and a glass plate is formed into a hybrid lens. However, the production process of the hybrid lens is complicated compared to lenses formed with injection molding technologies.

Electron beam (EB) and any kind of radial ray cross-linking technologies for resins have been developed by many researchers. And they successfully improved the mechanical, chemical, and thermal properties of resins [10-13]. For example, Furusawa et al. reported that aqueous hydroxypropyl methylcellulose (HPMC) was irradiated with different EB power to produce gel films, and the dependence of the physical and chemical properties on the EB irradiation was investigated [14]. Recently, Kanno et al. reported that the flame resistance characteristics of Nylon66 (PA66) composites were improved mechanical strength at high temperature [15]. They observed that y-ray irradiated cross-linked composites exhibited rubber-like elasticity even at temperatures higher than the melting point of non-cross-linked PA66.

We reported that the poor heat-resistant properties of the novel transparent thermoplastic resin based on noncrystalline nylon were improved by EB irradiation cross-linking [16]. A correcting aspheric lens for a 635-nm laser diode was fabricated using an injection molding machine and was irradiated with an EB. The near-field pattern (NFP) and far-field pattern (FFP) at the focus position, and the transmittance of the lens did not change after exposure to a 260 [degrees] C reflow process for 60 s.

In this paper we experimentally investigate the dependence of the mechanical, thermal, and optical properties of the novel heat-resistant resin based on the non-crystalline nylon on EB irradiation power.

EXPERIMENTAL

Novel Heat-Resistant Thermoplastic

Table 1 shows various resins and their heat-proof temperatures and process duration times for one production cycle. Both thermally curable and UV curable resins have heat resistance temperatures higher than the 260 [degrees] C required for the reflow process. However, lenses that employ these resins must be produced by an imprint or embossing method, which results in a longer tact time than the injection molding method. Meanwhile, thermoplastic resin lenses can be produced by injection molding, and the tact time is shorter than that for other types of resin. Therefore, thermoplastic resins are more suitable for high volume production, although they cannot endure the reflow process.

TABLE 1. Heatproof temperature and production methods of existing
plastic lens materials.

Resin type             Heatproof temp.  Production           Process
                          ([degree] C)  method          duration (s)

Thermal curable resin             >260  In-printing or          >120
                                        embossing
UV curable resin                  >260  In-printing or          >120
                                        embossing
Thermoplastic resin               <180  Injection               >30
                                        molding


Okabe el al. developed a new nylon molding compound by mixing Nylon66 with a cross-linking agent and fabricated lenses by molding the mixture followed EB irradiation [17]; they successfully obtained a cross-linked and thermally stable molding made from Nylon66.

In this study, the same technique was applied to a transparent thermoplastic resin using radiation cross-linking. The process for manufacturing the cross-linked transparent resin by EB irradiation is shown in Fig. 1. Noncrystalline nylon was mixed with a cross-linking agent, flame retardant, reinforcer, stabilizer, and a cooling agent using a twin shaft extruder, and the mixture was pelletized. This compounding resin is the TERALINK-TPNIOA produced by Sumitomo Electric Fine Polymer, Inc. The pellets were molded into given shapes, such as a lens, using an injection molding machine. The molded parts were irradiated with an EB accelerator. The nylon molecules in this novel thermoplastic were thus cross-linked, and the mechanical, thermal, and optical properties were improved.

Molding Conditions and Irradiation Process

Test pieces (50 X 5 X 1 m[m.sup.3]) were prepared using a conventional molding machine (Sodick, TR5EH). The molding conditions used for these experiments are given in Table 2.

TABLE 2. Molding conditions for die lest piece.

Item                               Molding conditions

Cylinder temperature([degrees] C)                 273
Mold temperature ([degrees]C)                     100
Holding pressure (MPa)                            150
Injection rate (mm/sec)                           150
Holding time (sec)                                  3


After molding, the test piece was irradiated in an EB accelerator with an electron accelerating voltage of 10 MeV; the EB irradiation power is given in arbitrary units (arb. units). The dependence of the mechanical, thermal, and the optical properties of the resin on the power of the EB irradiation treatment were investigated.

The elastic bending modulus and yield stress were measured using a bending tester (Imoto Factory Co. Ltd.) and dynamic viscoelasticity measurement apparatus (Rheovibron, DDV25FV, Orientec Corp.). The glass transformation temperature was measured by differential scanning calorimetry (DSC; Q200, TA Instruments), and tan $ (elastic loss modulus/elastic storage modulus) was measured with the dynamic viscoelasticity apparatus.

There have been no reports of the optical properties of thermoplastic resins cross-linked by EB irradiation. We measured the dependence of the birefringence, Abbe number, the refractive index, and the transmittance on the EB irradiation power. The birefringence was measured using a polarization microscope (CX3I-P, Olympus Corporation). The refractive index and the Abbe number were measured using a multiwavelength Abbe refractometer (DR-M4/1550, Atago Co. Ltd.). The dependence of the transmittance on the wavelength was measured using a spectrophotometer (UV3100PC Shimadzu Corp.).

RESULTS

Mechanical Properties

The mechanical properties of the resin are very important for the reflow process, because the shape of optical elements such as lenses must be maintained during the reflow process.

Bending Elastic Modulus and Yield Stress

Figure 2 shows the dependence of the bending elastic modulus and yield stress at room temperature on the EB irradiation power applied to the resin. When the EB irradiation power was increased, the bending elastic modulus increased and became saturated for an EB irradiation power over 160 (arb. units). However, the yield stress continued to increase for EB irradiation power over 160 (arb. units). The test piece became rubber-like and the yield stress was increased by approximately 34% over that without EB irradiation by EB cross-linking.

Dynamic Viscoelasticity

Figure 3 shows the elastic storage modulus as a function of the temperature for each EB irradiation power. Before EB irradiation, the resin melted at over 200 [degrees] C. We considered that the peak occurring near 200 C maybe caused by re-crystallization near melting due to residual orientations. After EB irradiation, the elastic storage modulus became flattened at over 200 [degrees] C, which indicates the rubber-like characteristics. When the EB irradiation was increased, the elastic storage modulus was increased al the same temperature. The elastic storage modulus traces at EB irradiation powers of 320 (arb. units) and 480 (arb. units) are very similar; therefore, we consider that this novel resin has transformed from the brittle to the ductile slate at elevated temperature by EB cross-linking.

Thermal Properties

Figure 4 shows the dependence of the glass transformation temperature on the EB irradiation power, as measured by DSC. The glass transformation temperature increased when the EB irradiation power was increased. Measurements of tan [delta] are shown in Fig. 5. The peak in this graph, which indicates the glass transformation temperature, was shifted to higher temperature when the EB irradiation power was increased. The traces for EB irradiation powers of 320 (arb. units) and 480 (arb. units) are similar, which corresponds with the DSC measurement results. According to both DSC and tan [delta] measurements, the glass transformation temperature was improved by 42% with the application of sufficient EB irradiation power for cross-linking. However, a glass transformation above 260 [degrees] C was not achieved, which is required for the resin to maintain its shape during the reflow process. The glass transformation temperature obtained after EB irradiation was only 165 [degrees] C. However, this resin after EB irradiation was able to transform from the brittle to the ductile state at elevated temperature by electron cross-linking, as explained with respect to the mechanical properties in the previous section. Therefore, an optical element produced using this resin could maintain its shape during the reflow process.

Optical Properties

Birefringence

Figure 6 shows a photograph of the test pieces under crossed Nicol prisms with and without EB irradiation. The flow end is the top of the test piece and the gate end is the bottom, as indicated in the photograph). The retardation of the gate area is larger than that at the flow end area. Retardation decreased for test pieces that were exposed to increased EB irradiation power.

The dependence of the birefringence on the EB irradiation power was measured at different positions (gate, center, and flow end) of the test pieces using a polarization microscope and the results are shown in Fig. 7. The birefringence gradually decreased with increase of the EB irradiation power and became saturated at all positions in the test pieces.

Figure 8 shows the relationship between the birefringence after EB irradiation and that before EB irradiation. In this graph, three plots show the birefringence at the positions (gate, center, flow end) of the test piece as shown in Fi^_,. 6. The change in birefrin gence by EB irradiation was confirmed to be dependent on the initial birefringence before EB irradiation. We considered that the molecular orientation is relaxed by EB cross-linking.

Refractive Index and Abbe Number

The refractive index and Abbe number were measured at the gate and flow end areas indicated in Fig. 6. Figure 9 shows the dependence of the refractive index and Abbe number on the EB irradiation power. The Abbe number was not dependent on the EB irradiation power. In contrast, the refractive index increased when the EB irradiation power was increased. We consider that this increase in the refractive index is due to a decrease in density explained by the Lorentz-Lorenz equation as we already reported [16]. The difference in the refractive index between the gate and the How end areas is caused by the density of each area. We consider that the increase in the refractive index was so small that the Abbe number was not changed.

Transmittance

Figure 10 shows the dependence of the transmittance on the wavelength for various EB irradiation powers. When the EB irradiation power was increased, the increase in transmittance at short wavelength was slightly shifted to longer wavelength. The transmittance was maintained at over 90% for >500 nm wavelength. The shift in wavelength is considered to be due to oxidation and an intermediate radical species caused by the EB irradiation.

DISCUSSION

A glass transformation temperature of more than 260 [degrees] C, which is required for the reflow process, was not achieved by EB cross-linking of the thermoplastic resin. However, the elastic storage modulus indicated that the resin transforms from the brittle to the ductile state at elevated temperature; therefore above the glass transformation temperature, an optical element produced using this resin could maintain its shape and could be used with the reflow process. It is very important for optical elements to reduce the optical distortion that can be caused by injection molding. This optical distortion can be slightly reduced by EB cross-linking, and the birefringence can also be reduced by relaxation of the molecular orientation due to EB cross-linking.

CONCLUSIONS

The mechanical, thermal, and optical properties of a transparent thermoplastic resin with EB cross-linking were investigated. The following results were confirmed for the EB cross-linking.

The yield stress was improved by 34% when the EB irradiation power was increased. The glass transformation temperature was increased from 116 to 1650C when the EB irradiation power was increased. Transformation from the brittle to the ductile state was achieved by EB cross-linking elevated temperature. The birefringence gradually decreased when the EB irradiation power was increased. The molecular orientation was relaxed by EB cross-linking. Transmittance (500 nm) was maintained at over 90% after EB cross-linking.

The effects of EB irradiation on the properties of a transparent thermoplastic resin were examined experimentally. A glass transformation temperature suitable for the reflow process could not be achieved by EB cross-linking; however, transformation from the brittle to the ductile state was achieved at elevated temperature. Therefore, heat-resistant thermoplastic optical elements could be successfully produced using this resin with a conventional injection molding machine and a high-temperature reflow process.

REFERENCES

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Correspondence to: Tomomi Sano; e-mail: sano-toniomi@sci.co.jp DOI I0.1002/pen.22ll8

Published online in Wiley Online Library (wileyonlinelibrary.com). [c] 2011 Society of Plastics Engineers

Tomomi Sano, (1) (2) (3) Makoto Nakabayashi,4 Hiroshi Ito3

(1.) Development Department, Japan Communication Accessories Manufacturing Company Ltd., 1-226 Higashi, Komaki, Aichi 485-0831, Japan

(2.) Optical Communications R&D Laboratories, Sumitomo Electric Industries, Ltd., 1 Taya-cho, Sakae-ku, Yokohama 244-8588, Japan

(3.) Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan Yonezawa, Yamagata 992-8510, Japan

(4.) Irradiated Products Department, Sumitomo Electric Fine Polymer, Inc., 950-1 Asashiro-nishi, Kumatori-cho, Sen-Nan-Gun, Osaka 590-0458, Japan
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Author:Sano, Tomomi; Nakabayashi, Makoto; Ito, Hiroshi
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9JAPA
Date:Mar 1, 2012
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