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MSE 5090: Case Studies in Material Selection

Week 3 - Material Selection Process

 Weighted Property Index Methods - Problem 4
Use of Polymers in Optical Applications1
Enumerate the advantages and disadvantages of polymers as materials for use in various optical applications. Include in the presentation details of the relevant properties of some candidate optical polymeric materials.
ADVANTAGES · Low density
· Low raw material cost
· Impact resistance—one of the most attractive properties of all polymers; some optical polymers are practically unbreakable
· Excellent configuration flexibility, thereby simplifying assembly; this property is particularly useful in aspherical lens systems (Fig. Q4.1)
· High transmittance of incident light
· Safety involved in their use because when optical polymers break, the fragments tend to be large and obtuse and hence pose less danger
· Ease of application of surface texturing (which is used to disperse light or create a contrasting appearance) without the use of special decorative procedures, such as etching or controlled abrasion

DISADVANTAGES
· Low abrasion resistance: scratch-resistant coatings are, however, available for some optical polymers, such as polymethylmethacrylate (PMMA) and polycarbonate, but the coatings are expensive and necessary in only a few applications, such as ophthalmic lenses.
 

Figure Q4.1 The curved surfaces of spherical and nonspherical lenses.
 Figure Q4.2 Combination of spherical and aspherical contours in a single lens.

· Poor tolerance of large temperature functuations; this, though, might not be a serious limitation because most optical systems do not normally operate in temperatures beyond the thermal limits of most optical polymers
· Narrow choice of raw materials—this means less design freedom because of a restricted range of refractive indices and dispersions (dispersion is the variation of refractive index with wavelength and is expressed by the Abbe value);* however, creative use of polymer aspherics (Fig. Q4.2) often compensates for this drawback.

CANDIDATE OPTICAL POLYMERS

The principal ones are as follows:
· Polymethylmethacrylate (PMMA) is very hard, abrasion-resistant, mechanically stable, and very widely used.
· Polystyrene is also a widely used optical polymer, but the molding cost far outstrips the resin cost. It can be used for lens designs requiring thin cross sections, thereby reducing resin cost and offsetting the high processing cost. Its low Abbe value allows it to be used in color-corrected designs.
· Polycarbonate is a very tough polymer that gives good performance over a wide temperature range (— 140°C to 120°C). Its high impact strength makes it particularly suited for use in streetlight lenses, construction lights, and roadwork warning lights. Because the polymer cannot be easily machined, injection molding is almost invariably used.
· A copolymer consisting of 70 wt % polystyrene and 30 wt % PMMA. Its refractive index is difficult to control because it is a composite material, but many optical designs compensate for this limitation. This copolymer is particularly suitable for thin lens systems.

The relevant properties of these optical polymers are given in Table Q4. 1. Other candidate optical polymers and their properties are presented in Table Q4.2.
 

 * Abbe value or V number = (nD -1) / (nF - nc) where nD, nF, and nc. are the indices of refraction of the material at 0.5893 mm. 0.4861 mm. and 0.6563 mm, respectively. TABLE 04.1  RELEVANT PROPERTIES OF FOUR PRINCIPAL OPTICAL POLYMERS
Property
PM MA
Polystyrene
Polycarbonate
Methylmethacrylate
Polystyrene
Copolymer
Refractive index
At np = 0.589 mm
At nF 0.486 mm
At nc 0.656 mm
1.491
1.497
1.498
1.590
1.604
1.584
1.586
1.593
1.576
1.564
1.574
1.558
Critical angle (°)
42.1
39.0
39.1
39.8
Coefficient of thermal expansion (10-6 °C -1)
64.8
39.4
68.4
64.8
Maximum continuous use temperature (°C)
79
79
116
79
Density (kg m -3)
1190
1060
1200
1090
Hardness. Rockwell M(kg mm -2)
97
90
70
75
Izod impact strength 
(J m-1 notch)
0.66
0.58
23.93
  
Dielectric strength 
(MV m -1)
19.70
19.70
15.75
17.72
Dielectric constant
At 60 Hz
At 1 MHz
3.7
2.2
2.60
2.46
2.90
2.88
3.40
2.90
Power loss factor
At 60 Hz
At 1 MHz
0.05
0.03
0.0002
0.0002
0.0070
0.0075
0.006
0.013
Volume electrical
resistivity (Wm)
1016
1014
8 x 1014
1013
TABLE 04.2  CHARACTERISTICS OF SOME OPTICAL POLYMERS
Optical Polymer
Dielectric
Constant
at 10 kHz
Tensile
Strength
(MPa)
Izod Impact
Strength
(J m-1 notch)
Density
(kg m-3)
Coefficient ofThermal
Expansion
(10-6 ° C-1)
Styrene acrylonitrile
3.0
76a
0.83
1070
67
Allyl glycol polycarbonate
3.8
38a
0.50
1350
112
Polysulfone
3.0
70b
2.00
1245
56
aAt break point.
bAt yield point.
REFERENCE:
1.Lewis , G. Selection of Engineering Materials, Prentice Hall , 1990, pp 258-260
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Last update 9-12-98