An Introduction to Polycarbonate Lenses and Their Optical Applications

Many optical appliances commonly use polycarbonate lenses. They are more practical than traditional glass in various applications due to their low weight, high impact resistance and the cheap cost of high-volume production [1].

Some of these applications require complexity (e.g. Fresnel lens), durability (e.g. traffic light lens) or safety (e.g. safety eyewear) criteria that are difficult to meet without using plastics. Plastic lenses stand out in the field due to their ability to cheaply meet many requirements whilst maintaining sufficient optical qualities.

However, polycarbonate lenses also have limitations. The ease at which they can be scratched is the main concern for customers. Extra processes can be carried out to apply anti-scratch coatings in order to compensate for this.

Nanovea examines some important properties of polycarbonate lenses by utilizing three metrology instruments” Mechanical Tester, Profilometer and Tribometer.

This article conducts a general investigation on several important properties of a polycarbonate lens. The mechanical tester, profilometer and tribometer are able to obtain the following properties: radius of curvature, surface roughness, COF against various materials, wear rate, thickness and scratch hardness.

Example of polycarbonate lens about to be tested on Nanovea Prolometer

Example of polycarbonate lens about to be tested on Nanovea Prolometer

Example of polycarbonate lens about to be tested on Nanovea Tribometer

Example of polycarbonate lens about to be tested on Nanovea Tribometer

Example of polycarbonate lens about to be tested on Nanovea Mechanical Tester

Example of polycarbonate lens about to be tested on Nanovea Mechanical Tester

Equipment Featured

Nanovea HS200

Nanovea PS50

Radius of Curvature and Roughness

Measurement Parameters

Table 1. Test parameters for roughness and radius measurements on the lens

Test Parameter Value
Instrument Nanovea HS2000
Optical Sensor L1 Lens (200 µm Z-range)
Scan size (mm) 10 mm x 10 mm
Step size (µm) 5 µm x 5 µm
Scan time (h:m:s) 00:01:02

Figure 1 and 2 show the results from the profilometry measurements. The Figures below show 2D and 3D images of the true form of the lens.

Radius

Conducting an area scan on the lens ensures that the radius of the curvature is captured at the apex of the curve. Radius of the curvature was calculated form both the X- and Y- axis to observe the symmetry of the lens. From the front side, values of 142.1 and 135.5 mm were observed, and from the back side values of 137.9 and 139.2 mm were observed.

Prole extraction in the X-axis (left) and Y-axis (right) of front side of polycarbonate lens

Figure 5. Prole extraction in the X-axis (left) and Y-axis (right) of front side of polycarbonate lens

Prole extraction in the X-axis (left) and Y-axis (right) of back side of polycarbonate lens

Figure 6. Prole extraction in the X-axis (left) and Y-axis (right) of back side of polycarbonate lens

Roughness

The form of the sample must be removed in order to obtain roughness data. To obtain roughness height parameters, a Gaussian filter with a nesting index of 0.25 mm was applied. For the front side of the polycarbonate lens an Sa value of 26.7 nm was obtained and for the back side of the plastic lens a value of 18.6 nm was obtained. The respective Sq values were 37.77 nm and 36.02 nm. These values are very low and this makes them ideal to minimize scattering of light when the light interacts with the surface of the lens.

3D-view and height parameters of front side of polycarbonate lens after a Gaussian filter of 0.25 mm

Figure 3. 3D-view and height parameters of front side of polycarbonate lens after a Gaussian filter of 0.25 mm

3D-view and height parameters of back side of polycarbonate lens after a Gaussian filter of 0.25 mm

Figure 4. 3D-view and height parameters of back side of polycarbonate lens after a Gaussian filter of 0.25 mm

Thickness

Measurement Parameters

Table 2. Test parameters for roughness and radius measurements on the lens

Test Parameter Value
Instrument Nanovea PS50
Optical Sensor PS5 (10000 µm Z-range)
Scan size (mm) 5 mm x 5 mm
Step size (µm) 10 µm x 10 µm
Scan time (h:m:s) 00:36:32

The thickness of the polycarbonate lens was obtained. This measurement works by having a focal point at each of the surfaces. Due to the axial chromatism technique it is possible to have multiple focal points. The difference in refraction between the sample and air is corrected using the sample’s index of refraction.

False-color view of top surface (left) and bottom surface (right)

Figure 7. False-color view of top surface (left) and bottom surface (right)

False-color view (left) and height parameters (right) for thickness of polycarbonate

Figure 8. False-color view (left) and height parameters (right) for thickness of polycarbonate

Figure 7 shows the two surfaces, top and bottom. As the composition of the sample is unknown it was set to a value of common plastic: polycarbonate – 1.58. The thickness can be obtained by subtracting the two surfaces scanned. The mean thickness of the sample was scanned near the apex of the curvature and was approximately 2.611 mm.

Equipment Features

Nanovea PB100

Scratch Hardness

Measurement Parameters

Table 3. Parameters used for scratch hardness testing on polycarbonate lens

Test Parameter Value
Load type Constant
Final Load (N) 15
Scratch Length (mm) 5
Scratching speed (mm/min) 18
Indenter geometry 120° cone
Indenter material (tip) Diamond
Indenter tip radius (µm) 200

Scratch Hardness

The scratch test was conducted according to ASTM-G717. In order to minimize the error caused by the curvature of the lens, scratches were made at the apex of the lens.

A scratch hardness of 420.59 ± 8.69 MPa was obtained. The scratch hardness value is quite low due to the nature of plastics and this is to be expected. The scratch tests previously conducted on copper, steel and aluminum can be used for reference and were 0.52, 3.20 and 0.84 GPa respectively [2]. Despite different testing conditions, the scratch resistance of the polycarbonate lens seems to be the same in magnitude as a soft, scratch-prone metal like copper.

Friction graph obtained from the scratch test

Figure 9. Friction graph obtained from the scratch test

Scratch hardness measurement conducted under an optical microscope. The blue dotted lines are positioned at the edge of the scratch to obtain scratch width.

Figure 10. Scratch hardness measurement conducted under an optical microscope. The blue dotted lines are positioned at the edge of the scratch to obtain scratch width.

Table 4. Results from scratch hardness test

Measurement 1 (MPa) Measurement 2 (MPa) Measurement 3 (MPa)
Scratch 1 432.19 418.89 412.52
Scratch 2 431.51 416.25 413.4
Scratch 3 431.71 421.55 409.8

Scratch Imaging with Optical Profilometry

In order to closely inspect the outcome of the scratch test, the polycarbonate lens was profiled with the profilometry instrument. Figure 13 shows the large amount of material found surrounding the area where the scratch took place. The volume of material lost (Hole) and the volume around the scratch (Peak) are about equal. This study shows that the soft plastic appears to have been displaced easily during the scratch. This suggests that the material has low scratch resistance.

The mean depth of the scratch into the surface was 7.864 ± 0.2652 mm. This was obtained by extracting a series of profiles across the scratched area and then taking an average of the maximum valley depth (Pv) of each profile (Figure 14).

False-color view of a scratch made on the lens

Figure 11. False-color view of a scratch made on the lens

3-D view of scratch made on the lens

Figure 12. 3-D view of scratch made on the lens

Volume of a hole/peak analysis on the scratch created

Figure 13. Volume of a hole/peak analysis on the scratch created

Extracted series of proles (left) and their primary prole parameters (right). Red line indicates the mean prole.

Figure 14. Extracted series of proles (left) and their primary prole parameters (right). Red line indicates the mean prole.

Equipment Featured

Nanovea T50

Coefficient of Friction

Measurement Parameters

Table 5. Parameters used for coecient of friction testing on polycarbonate lens

Test Parameter
Load (N) 0.5
Test Duration (min) 5
Speed (rpm) 10
Radius (mm) 0.0-5.0
Total Distance (m) 0.78
Pin Geometry Ball
Pin Material Rubber, PTFE, ZrO2, Al3O2, SS440C
Pin Diameter (mm) 6

Table 6. Pin-On-Disk material combinations

Combination Disk Material Pin Material
1 Polycarbonate Rubber
2 Polycarbonate PTFE
3 Polycarbonate ZrO2
4 Polycarbonate Al3O2
5 Polycarbonate SS440C

Coefficient of Friction

To ensure that the pins would pass over an unworn region throughout all tests, a Pin-On-Disk Spiral Test was performed. The first five resolutions were cropped from the graphs to remove data when the radius was near zero (minimal tangential movement). When analyzing the COF data, the curvature of the lens must be kept in consideration.

The test results rank the following material from the highest COF to the lowest COF: Rubber, Al203, ZrO2, PTFE, SS440C. In order to minimize the effects of wear on the sample, the tests were conducted with a small normal force.

COF graphs of 1) Rubber, 2) PTFE, 3) ZrO2, 4) Al2O3, 5) SS440C

Figure 15. COF graphs of 1) Rubber, 2) PTFE, 3) ZrO2, 4) Al2O3, 5) SS440C

Table 7. Results of COF testing on Plastic Lens

Pin Material Max COF Min COF Average COF
Rubber 0.947 0.277 0.734
PTFE 0.210 0.027 0.089
ZrO2 0.193 0.043 0.106
Al3O2 0.215 0.051 0.120
SS440C 0.175 0.022 0.072

Linear Wear

Measurement Parameters

Table 8. Parameters used for linear wear testing on polycarbonate lens

Test Parameter Value
Load (N) 20
Test Duration (min) 20
Speed (rpm) 100
Amplitude (mm) 10
Total Distance (m) 40
Ball Material ZrO2
Ball Diameter (mm) 6

To minimize the effects of curvature the linear test was conducted near the apex of the lens. Two stages of wear can be observed from the COF graph. The two surfaces are adapting to the surface of the sample at 0-200 revolutions. Significant wear begins to occur after 200 revolutions. Three-body abrasion wear is created by loose particles which are created from the wear test and are now rampant along the surface of the worn area.

In order to accurately calculate the wear rate, the volume loss was determined by profiling the wear track, analytically removing the curvature from the lens and then conducting a volume of a hole study (Figure 18). There was a total volume loss of 577,479,379 mm was lost. The average wear of the zirconium oxide into the plastic lens was 61.69 ± 6.830 mm.

Friction graph from linear wear testing

Figure 16. Friction graph from linear wear testing

Extracted series of proles (left) and their primary prole parameters (right). Red line indicated the mean prole.

Figure 19. Extracted series of proles (left) and their primary prole parameters (right). Red line indicated the mean prole.

Table 9. Linear wear testing results

Max COF Min COF Average COF Volume Loss (µm3) Wear Rate x 10-5 (mm3/Nm)
0.459 0.037 0.336 577479379 72.185

Conclusion

Nanovea’s metrology instruments were used to investigate important properties of polycarbonate lens. For material selection and quality control processes it is important that it is possible to accurately measure and quantify properties of materials.

Nanovea can target a wide variety of specific applications with these instruments, as is shown by the different types of testing. The humidity, temperature, corrosion and lubrication modules can be used to easily apply environmental conditions.

Progressive Tribology Mapping of Flooring

References

[1] Kogler, Kent. "Selection of plastics for optical applications.” Advanced materials and processes technology (1999).

[2] Li, Duanjie. "SCRATCH HARDNESS MEASUREMENT USING MECHANICAL TESTER." (2014).

Nanovea.

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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