Light diffusion properties of polycarbonate composites

28 October 2016
Na Song, Xingshuang Hou, Peng Ding, and Liyi Shi
A two-step melt-blending process and silicate microsphere/nanotitania hybrid fillers provide optimum optical and mechanical properties.

Light diffusion materials (LDMs)—optical materials that can homogeneously diffuse a point light source into a surface light source—are used widely in several applications, including luminaires (especially LEDs), illuminated signs (especially backlit translucent signs), skylights, automotive sunroofs, and covers for automotive lights. To meet the requirements for such applications, the LDMs must exhibit both good transmittance and high haze properties. Incorporating light diffusion agents (LDAs) into a polymer matrix has previously been shown as an excellent way to obtain LDMs.1–3 With increasing LDA content, however, the transmittance of LDMs decreases and the haze increases. It is therefore necessary to find a suitable kind of LDA that can be used to produce LDMs with both the appropriate transmittance and haze characteristics.

It is thought that a synergistic effect—arising from using two different LDAs, e.g., silicate microsphere (SMS) or poly(methyl methacrylate), PMMA, with nanotitania (nTiO2)—may be a way to realize high-transmittance and high-haze LDMs. Melt-blending is the most convenient approach to produce the two-LDA substances, because it is a simple, low-cost, and environmentally friendly technique. It is thus the preferred method for industrial production of LDMs.4–6 Furthermore, it has previously been shown that the properties of light diffusion polymers are related not just to filler type and filler content, but that they are also related to the dispersion state of the fillers within the composite materials.4, 7–9

In our work,10 we have therefore used a variety of different LDAs, and melt-blending processes, to study the optical performance and mechanical properties of polycarbonate (PC) LDMs. We compared the use of SMS and PMMA particles, as well as hybrid SMS/nTiO2 and crosslinked PMMA/nTiO2 particles as the LDA fillers—at different filler contents—in our composites. We also used both one-step and (two different) two-step melt-blending processes (see Figure 1) to prepare our PC composites.


Schematic illustration of the one-step and two-step melt-blending processes used to prepare the polycarbonate (PC) composites. Two different two-step methods were used in this study: one in which a twin-screw extruder is used and one that involves melt-compounding. Silicate microsphere (SMS)/nanotitania (nTiO2) hybrid particles are the fillers in this illustrated example.

As part of our study, we assessed the transmittance and haze (i.e., key properties of LDMs) of our composite samples: see Figure 2(a) and (b). Transmittance is generally determined by the surface roughness of the LDAs, crystallization of the matrix, and difference in the refractive index of the matrix and the LDAs.11 Our results indicate that the transmittance of the samples—see Figure 2(a)—decreases with increasing filler content. In contrast, the haze value of the composites—see Figure 2(b)—increases with filler particle concentration because of the increased amount of light loss that occurs via reflection and scattering. With an SMS/nTiO2 loading of only 0.4wt%, we achieve a haze value of 77.3%. This result represents a significant (i.e., six times) increase compared with a pure PC matrix (which has a haze level of 14–16%). We also conducted an analysis of variance (ANOVA) to test the variance of the transmittance and haze results between the samples that were produced via the three different processing methods. The standard deviation (SD) values we thus obtained for the transmittance and haze results (for the PC/SMS composites) see Figure 2(c) and (d)—were 2.509–4.532, 0.992–3.542, and 0.265–2.490% for the one-step method, two-step single method, and two-step twin method, respectively.


Optical properties of the PC composite samples. (a) Transmittance and (b) haze of pure PC and composites containing between 0.5 and 2.0wt% SMS/nTiO2particles, produced via the one-step, two-step single (i.e., melt-compounding), and two-step twin (i.e., twin-screw extruder) melt-blending methods. Standard deviation of (c) transmittance and (d) haze results for the PC/SMS/nTiO2composites (with filler loadings of 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.8, and 2.0wt%) produced via the three different melt-blending processes. A comparison of the transmittance and haze characteristics of the (e) PC/SMS and PC/SMS/nTiO2 composites and the (f) PC/PMMA and PC/PMMA/nTiO2composites—where PMMA stands for poly(methyl methacrylate)—is also shown.

In addition, we have investigated how the type of hybrid filler LDA affected the optical performance of our PC composites: see Figure 2(e) and (f). For the samples we produced via the two-step twin method, we find that the transmittance of the PC/PMMA/nTiO2 samples is better than the PC/SMS/nTiO2 equivalents (i.e., when the LDA filler content is the same in both composite versions). Moreover, at a given particle concentration, the haze value of the PC/PMMA/nTiO2 composites is better than that of the PC/SMS/nTiO2 samples (mainly caused by increased Mie scattering from the dispersed LDAs).

The tensile strength and impact strength of our PC composites, containing 0–2wt% of the different LDA filler particles, are shown in Figure 3(a) and (b), respectively. We find that the tensile strength of the composites fluctuates from 58 to 63MPa, but is nearly constant with increasing filler content. Our results also indicate that the impact strength of the PC composites containing the SMS/nTiO2 hybrid particles ranges from 68 to 70kJ/m2. In contrast, the impact strength of the PC/PMMA/nTiO2 composites decreases sharply—from 70 to 18kJ/m2—with increasing filler content. We also observe—see Figure 3(c)—that the glass transition temperature of the composites decreases slightly with the addition of the various LDAs.


(a) Tensile strength and (b) impact strength curves for the PC composites containing 0–2wt% of the various light-diffusion agent fillers. (c) Heating curves of the virgin (i.e., pure) PC and some of the PC composites, revealing the glass transition temperature at about 150°C. (d) Photographs of the light source (left) and light source behind the light diffusion plate (right) used to study the light diffusion effect of the LDMs.

In the last part of our work, we used a light diffusion plate (with an LED source) to study the light diffusion effect of our LDMs. Photographs of the light source and the light source behind the light diffusion plate are shown in Figure 3(d). In our setup, the point light source is obviously diffused homogeneously into the surface light source when it passes through the light diffusion plate. In this way, the problem of dazzling can be eliminated and we can obtain a soft visual effect. We have thus demonstrated that our PC LDMs have great potential for applications in many fields (e.g., luminaires, illuminated signs, skylights, automotive sunroofs, and covers for automotive lights).

In summary, we have studied the optical and mechanical properties of polycarbonate composite light-diffusion materials that contain a variety of nanofiller particles, and that are produced via different melt-blending processes. We find that the PC/SMS/nTiO2 samples exhibit the best optical performance and that they show no deterioration of their mechanical properties. We can obtain a good balance between high transmittance (60.97%) and substantial haze (88.73%) when the SMS/nTiO2 content is 1.2wt%. Based on our work so far, we believe that there are several reasons to continue studying polycarbonate composite LDMs. In our future work we therefore plan to explore the synergistic effect of other LDAs in the LDMs, with the aim of further improving the optical properties of the composites and reducing costs. We will also modify the surface of LDAs to enhance the scattering effect and to increase the number of possible applications.


Authors

Na Song
Shanghai University

Xingshuang Hou
Shanghai University

Peng Ding
Shanghai University

Liyi Shi
Shanghai University


References

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DOI:  10.2417/spepro.006740



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