Abstract

This work focus on the application of PP/PP film/foam on the reflector. We studied the properties of one mainstream commercial reflector. We try to improve the reflectance and other properties of PP/PP film/foam for other application by lamination and adding inorganic additive. Our result shows that we do improve the reflectance of the as-extruded sample and it is comparable to commercial sample somehow.

1.Introduction

Polyolefin multilayer film/foam materials are a group of polymeric material with a novel structure assembling alternating film and foam layers, which can be prepared by using lamination and co-extrusion technology.^{[1, 2]} Because of their fantastic structure, these materials combine the physical and mechanical behaviors of both solid and foamed polymers, including low density, high compressive modulus, and heat and sound insulation.3 Therefore, they are widely used in areas such as packaging, sports, automotive, electronic, and others.^[4-7] Baer et al. reported a multilayer co-extrusion technology for microlayered film/foam material (with 8, 16, and 64 layers) production in 2004.1 Such multilayer coextrusion represents an advanced polymer processing technique that the cell size in the foam layers can be significantly reduced to micro-meter scales with the increase in the layer numbers, mechanical properties also improved with the increased layer number at the same time.

However, various properties of the film/foam multilayer structure still have large room for improvement, particularly the properties of toughness, tear resistance and puncture resistance. In order to get film/foam multilayer material with improved properties, the co-extruded materials are subjected to drawing (mono-, bi- or multi-axial) while in the molten state to achieve macroscopic cellular orientation. ^{[8, 9]} The reflectance is also considered to be one of the application for film/foam. Due to the residue of foaming agent and the cell size, there is still a lot of effort to do to improve the reflectance of film/foam. This work is designed to improve the surface reflectance of PP/PP film/foam. According to figure1 ^[10], the incident light can be divided into reflective light and transmitted light. So there are two methods to increase the reflectance of film/foam.

1. With increasing of refractive index contrast at the interface between air and the surface, there would be more reflective light. Because when there is more reflectance between the surface and air, it means that there are less light going into the material. As a result, the reflectance could be improved.

2. Increasing the number of interface, we can reduce the transmitted light and collect more reflective light at every interface. The total reflective light would increase on the surface.

So we choose to use the 16 layers structure which provides enough interface. And we used lamination and inorganic fillers to improve the reflectance while keep low density around 0.5g/cm³.

2.Experimental Section

2.1 Materials

LDPE DOW 5004I (MFI=2) was used to make the skin layer for high reflectance film/foam, then 5wt% and 20wt% TiO₂ (Cristal, RCL-4) was added to increase the whiteness of LDPE layer, PP Huntsman AP6106 (MFI=6) and PP Braskem FT120WV (MFI=12) used for the fabrication of multilayer PP/PP film foam material was also supplied by the Dow Chemical Company. 1.5wt% Actafoam (AZ -130 Galata Chemicals) was used as the chemical blowing agent for foaming. The nucleating agent in the foam layer was 1 wt% Talc (Jetfine® 1H, IMERYS Talc), MCPET (Furukawa Electrical)

2.2 Production of high reflectance multilayer film/foam

In order to produce the 16 layers film/foam structures, a two-component microlayer coextrusion setup (figure2) with 3 multipliers was used. One extruder contained the film layer polymer, and the other extruder contained the foam layer polymer with blowing agent and nucleating agent. A processing temperature of 195 ^oC was used for optimum foaming based on the decomposition kinetics of Actafoam AZ 130. A 3” die was used at the end of the multipliers. And we used different pump rate at those two extruders in order to get a 30% film layer and 70% foam layer structure. A chill roll setup was used as a sheet take-off. Different numbers of multipliers were used to prepare the samples with different layer numbers.

The LDPE skin layer of different thickness is produced by compressing LDPE/ TiO₂ blend under different pressure at 200 ^oC. In order to have better dispersion, the blend is produced by twin screw extruder and then cut into pellets for further use.

Then the high reflectance film/foam was produced by lamination process. First, the as extruded film/foam was compressed at 140 ^oC to flatten the surface. Then, the final product was produced by compressing LDPE/TiO2 layer to the PP/PP film/foam at 130 ^oC.

2.3 Density measurement

A liquid displacement method (ASTM D 3575-93, W-B) was used to measure the bulk density of the film/foam samples before and after orientation.

2.4 Morphological measurement

Scanning electron microscope (SEM) was used to observe the layer structures and cell size in film/foam materials. The samples were cut in flowing direction with sharp razor blades at room temperature and the sputter coated with gold (5nm) before observation by using SEM.

2.5 Reflectance Measurement

Both total reflectance and diffuse reflectance were characterized by Cary 6000i UV-vis. The setup of the machine is showed in figure3^[11]. Cary 6000i uses the substitution method to measure the reflectance. First, a baseline is recorded with the PTFE reference disk covering the reflectance port. The sample is then mounted over the port and the reflection off the sample surface is collected by the sphere. The reflectance is therefore measured relative to the PTFE disk.

When we characterize the diffuse reflectance, the specular exclusion port is designed to absorb the specular reflective light. As a result, the machine can get the diffuse reflectance. So when we replace the specular exclusion port with another PTFE disk, the machine would get the total reflectance.

3.Results and Discussion

3.1 The properties of commercial sample

The commercial sample, what we call microcellular PET (MCPET), is foamed by supercritical CO2 fluid. The density of MCPET is 0.38 g/cm³. From Figure 4 ^[12], we can see that The commercial MCPET consists of a condensed shell (10μm), a foam core with cell size around 5μm and a foam layer in between having cell size around 10-20μm. The microcellular structure provides hundreds of interface for the MCPET.

3.2 The effect of lamination on density change of PP/PP film/foam

We choose 2 different concentrations of TiO₂ 5wt%, 20wt% and two different thickness 100μm, 200μm. First, we investigate the effect of lamination on the density change of film/foam sample. From table1, we can get that at the same thickness, the higher the concentration of TiO₂, the higher the density. And at the same concentration of TiO₂, the density increases with thickness of skin layer. And the PP/PP film/foam with 20wt% TiO₂ has the highest density which is 0.53 g/cm³ . But, the overall density doesn’t increase too much comparing to the original density 0.48 g/cm³.

3.3 Morphology of the laminated PP/PP film/foam

Figure 5 shows clear structure of film/foam and the cell size didn’t change too much during the lamination. All the cell sizes are around 50-300μm and the medium cell size is at 180μm. It means that we can get the laminated PP/PP film/foam without significant change in the structure during compressing.

3.4 Reflectance Property of PP/PP film/foam

Figure 6 shows the total reflectance of the PP/PP film/foam sample and the commercial sample. The commercial sample has highest reflectance which is 98.6% (λ=660nm). And PP/PP film/foam with 20wt% TiO2 skin is better than the one with 5wt% skin. And for 100μm skin, the total reflectance increases from 86.9% to 89.0% (λ=660nm) and for 200μm skin, the total reflectance increases from 87.7% to 91.0% (λ=660nm). The reflectance of 20wt% TiO2 becomes more stable while the 5wt% one deteriorates from 400nm to 500nm. Because the 20wt% TiO₂/LDPE is less translucent than 5wt% one, the yellow substrate is easier to be exposed to light as a result, the 5wt% laminated film/foam is weaker in this 400-500nm range.

Figure 7 shows the diffuse reflectance of the commercial sample and PP/PP film/foam. We can get similar result that the commercial has the highest diffuse reflectance which is 97.0% (λ=660nm) and the sample with 20wt% TiO2 skin is better than the one with 5wt% skin. For 100μm skin, the diffuse reflectance increases from 84.0% to 86.7% (λ=660nm) and for 200μm skin, the total reflectance increases from 84.5% to 88.1% (λ=660nm). So adding higher concentration of TiO2 increases the reflectance of film/foam sample.

3.5 Mechanical properties of PP/PP film/foam and commercial sample

From Figure 8 and table 2, it shows that the MCPET has similar Young’s modulus (308 MPa) with our PP/PP film/foam (311MPa). But the MCPET can be stretched further than PP/PP film/foam and the stress at break is much higher than PP/PP film/foam.

4.Conclusion

Adding 5wt% TiO2 could increase the refractive index of LDPE film layer from 1.49 from 1.54 while 20wt% could make it increase from 1.49 to1.7. So it explains that why 20wt% TiO2 works better than 5wt%. But there is still one question that PET has refractive index at 1.58 which should be lower than the LDPE skin with 20wt% TiO2. It’s because that the PP/PP film/foam has much larger cell size which ranges from 50nm to 300nm than the microcellular. In the vertical direction, the PP/PP film/foam would only have tens of air/polymer interfaces while the microcellular structure provides hundreds of interfaces.

It explains why the current maximum reflectance for PP/PP film/foam is only at 91% which is still lower than the MCPET. So the next step to improve the reflectance of film/foam should be focused on the number of interface. But when compared to the as extruded film/foam, we can see that skin improves the original reflectance which is only 69% to 91%. It makes the PP/PP applicable on the reflector area.

Appendix: Figures

Figure 1. Schematic of reflected light and refracted light on multilayer interface

Figure 2. Schematic of multilayer co-extrusion of film/foam

Figure 3. The setup for reflectance measurement

Figure 4. The SEM morphology of microcellular PET

Figure 5. SEM images of PP/PP film/foam a) without skin, b) compressed with 100um LDPE layer containing 5wt% TiO₂, c) compressed with 200um LDPE layer containing 5wt% TiO₂, d) compressed with 100um LDPE layer containing 20wt% TiO₂, and e) compressed with 200um LDPE layer containing 20wt% TiO₂.

Table 1. Density change of PP/PP film/foam after lamination procedure

Figure 6. Total reflection of PP/PP film/foams with and without lamination skin layer.

Figure 7. Diffuse reflection of PP/PP film/foams with and without lamination skin layer.

Figure 8. Stress-strain curve of PP/PP film/foam and commercial sample

Table 2. Mechanical Property of PP/PP film/foam and Commercial Sample

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