The transparency of plastics refers to their ability to allow light to pass through them. As a result, the objects behind them are distinctly seen. This desired optical property is what sets transparent plastics apart from opaque varieties. This makes them useful for many applications.
Not all plastics are transparent. Their level of crystallinity and the presence of additives affect how see-through they are. To increase transparency several additives can be added. Clarity also depends on avoiding particulate contamination and limiting absorbing compounds in the plastic.
Common transparent plastics include general-purpose polystyrene and polycarbonate. Their transparency makes them ideal for use in appliances, display cases, packaging, lenses, and windows. Advancements continue to fine-tune plastics’ refractive index and transmission. This allows the expansion of their utility in optical applications requiring high transparency. Controlling transparency allows plastics to serve functions in a wide range of technologies.
Let’s get a deeper understanding of:
Transparent Polymers — Definition and Examples
Transparency or transmission of visible light is characterized by light transmittance. It is the percentage of incident light transmitted through a standardized plastic specimen. The higher the transmittance %, the higher the transparency. A
material with good transparency will have high transmittance and low haze.
Transparency has no specific unit. Transmittance value is generally reported in the percentage of light transmitted. The transparency or translucency of plastics depends on the type and structure of the polymer/crystallinity. It also depends on the types of additives, fillers, colors, etc., used.
- Amorphous polymers – They are transparent as the light travels easily through them. For example, acrylics, polycarbonate (PC), polystyrene (PS), acrylonitrile butadiene styrene (ABS), polyetherimide (PEI), etc.
- Semi-crystalline/crystalline polymers – They are translucent or opaque. This is because the light scatters through them. For example, polyethylene (PE), polypropylene (PP), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), and polyamide (PA). They have differences in refractive indices between the crystalline regions and amorphous regions.
Although
PET and
PP are crystalline, films of these materials are transparent. This is because these films are bi-axially oriented and stretched in two directions. This orientates the polymer molecules in the plane of the film. Hence, when light passes through the film, there is no refraction because of this orientation.
Examples of transparent polymers
Factors Affecting the Transparency of Polymers
➤
Crystallinity — As the percentage crystallinity increases the polymer becomes progressively less clear. This is due to the
increase in density which in turn decreases the speed of light passing through it. However, quenching or random polymerization can improve the clarity of crystalline plastics.
➤
Pigments and additives — Colorants, fillers, and other polymer additives can reduce clarity. This is due to the reduction in light transmission.
Transparent polymers tend to be pure with few additives. However, some additives like nucleators and clarifiers can improve the clarity of plastics like PP. These additives act upon the growth and size of the crystal structure.
➤
Residual catalysts — They are left over from polymerization. They can cause a hazy appearance and reduced optical clarity. These are minimized in transparent polymers.
➤
Contaminants — Particulates, dust, or other contaminants in the polymer can scatter light. This needs to be avoided to allow
high transparency. Proper polymer processing helps to minimize contaminants.
➤
Structural regularity — Uniform polymer structures transmit light better than less ordered chains. Chain orientation also affects birefringence and polarization.
➤
Surface quality — Any surface roughness or defects can reduce transparency. This can happen from machining, molding, or handling polymers. Smooth surfaces are required for optical clarity. The appearance of scratches or surface flaws is a result of exposure image light transmittance.
➤
Thickness — Many polymers transmit light as thin films. Among thermosets, unsaturated polyester, reinforcements interfere in transmittance.
➤
Chemical change — Degradation, oxidation, or diffusion may impact transparency.
Apart from the above factors, steps such as coloration, heat treatment, and mechanical processing during production can affect optical properties.
Processing Methods of Transparent Plastics
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Extrusion: It is one of the common methods where plastic pellets are melted. They are then continuously extruded through a die into the desired plastic sheet shape. This must be carefully controlled to minimize imperfections that can cause haze/opacity. Fast cooling and smoothing rollers help transparency.
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Injection molding: Transparent molten plastic can be injected into highly polished mold cavities. They can then be cooled to produce components. Precise process control maintains optical clarity in the final molded parts.
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Calendering: Plastic films can be formed through calendering. Here the melted polymer is passed between large parallel rollers into smooth sheets. Getting uniform thickness distribution aids transparency.
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Casting: Liquid monomer and catalyst can be poured together between glass plates. These plates are spaced precisely apart and allowed to polymerize. This creates flat cast sheets without internal stresses/distortions.
Applications of Transparent Polymers
Plastics have superior impact resistance than glass. Thus, transparent plastics like
polycarbonate,
PMMA, etc., have replaced glass in many applications. Here are some of the main applications that utilize transparent polymers:
Windows and lenses
Transparent plastics like polycarbonate and acrylic are used as shatter-resistant glass replacements. They are used in
windows, lenses in glasses, camera lenses, and other
optical devices.
Packaging
Crystal clear polymer films, bottles, etc., allow high visibility of contents. They also protect the products inside. Common transparent plastics for packaging include polyethylene terephthalate (PET), polystyrene, and polyethylene.
Lighting
Transparent plastics can act as light covers, diffusers, and sheets. They help to transmit and distribute light effectively. These include materials like acrylic and polycarbonate.
Display cases
Clear plastics provide unobstructed visibility for several display objects. For example, store displays, museum exhibits, food displays, and other retail applications. In addition to this, they also help in while keeping contents protected.
Electronics
Transparent conductive films made of modified polymers conduct electricity while remaining optically transparent. They are used for touchscreens,
LEDs, flat panel displays, and solar cells.
Medical devices
Transparent biocompatible polymers allow visualization for components like intravenous (IV) tubing, fluid reservoirs, and syringe barrels. Common materials include PVC, styrene copolymers, PETG, and cyclic olefin polymers.
Check out different grades of transparent polymers used in various applications:
Common Standard Tests to Measure Transparency
ASTM D1003
It determines the
haze and luminous transmittance of transparent plastics. It evaluates the specific light-transmitting and wide-angle-light-scattering properties of planar sections of materials. For example, transparent plastics. Two procedures are provided for the measurement of luminous transmittance and haze.
- Procedure A uses a haze meter
- Procedure B uses a spectrophotometer
Material having a haze value greater than 30% is considered diffusing and should be tested in accordance with Practice E2387. Haze and luminous-transmittance data are especially useful for quality control and specification purposes.
ASTM D1746
It determines the transparency of plastic sheeting. This method measures the regular transmittance of plastic sheets and films. Regular transmittance is the percentage of incident light that is transmitted through the sample without being deviated from its original path.
Light Transmittance Values of Several Plastics in Percentage
Click to find the polymer you are looking for:
A-C |
E-M |
PA-PC |
PE-PL |
PM-PP |
PS-X
| Polymer Name |
Min Value (%) |
Max Value (%) |
| Amorphous TPI, Moderate Heat, Transparent |
58.0 |
58.0 |
| Amorphous TPI, Moderate Heat, Transparent (Food Contact Approved) |
58.0 |
58.0 |
| Amorphous TPI, Moderate Heat, Transparent (Powder form) |
58.0 |
58.0 |
ASA - Acrylonitrile Styrene Acrylate
|
1.050 |
1.070 |
ASA/PC Blend - Acrylonitrile Styrene Acrylate/Polycarbonate Blend
|
1.150 |
1.150 |
CA - Cellulose Acetate
|
90.00 |
90.00 |
CAB - Cellulose Acetate Butyrate
|
90.00 |
90.00 |
| Celllulose Diacetate-Gloss Film |
92.70 |
92.70 |
| Celllulose Diacetate-Integuard Films |
90.26 |
90.26 |
| Celllulose Diacetate-Matt Film |
16.80 |
16.80 |
| Cellulose Diacetate-High Slip Film |
92.70 |
92.70 |
| Cellulose Diacetate-Semitone Films |
54.00 |
54.00 |
| CP - Cellulose Proprionate |
90.00 |
90.00 |
COC - Cyclic Olefin Copolymer
|
91.00 |
91.00 |
ETFE - Ethylene Tetrafluoroethylene
|
95.00 |
95.00 |
EVA - Ethylene Vinyl Acetate
|
80.00 |
80.00 |
FEP - Fluorinated Ethylene Propylene
|
96.00 |
96.00 |
HDPE - High Density Polyethylene
|
80.00 |
80.00 |
LDPE - Low Density Polyethylene
|
80.00 |
80.00 |
| MABS - Transparent Acrylonitrile Butadiene Styrene |
88.00 |
88.00 |
PA 11 - (Polyamide 11) 30% Glass fiber reinforced
|
80.00 |
80.00 |
| PA 11, Conductive |
80.00 |
80.00 |
| PA 11, Flexible |
80.00 |
80.00 |
| PA 11, Rigid |
80.00 |
80.00 |
| PA 12, Flexible |
80.00 |
80.00 |
| PA 12, Rigid |
80.00 |
80.00 |
| Polyamide semi-aromatic |
80.00 |
80.00 |
| PC (Polycarbonate) 20-40% Glass Fiber Flame Retardant |
80.00 |
80.00 |
PC - Polycarbonate, high heat
|
88.00 |
89.00 |
PET - Polyethylene Terephthalate
|
70.00 |
90.00 |
PETG - Polyethylene Terephthalate Glycol
|
88.00 |
91.0 |
PFA - Perfluoroalkoxy
|
93.00 |
93.00 |
PMMA - Polymethylmethacrylate/Acrylic
|
80.00 |
93.00 |
| PMMA (Acrylic) High Heat |
80.00 |
93.00 |
PMMA (Acrylic) Impact Modified
|
80.00 |
92.00 |
PP (Polypropylene) Copolymer
|
85.00 |
90.00 |
PP (Polypropylene) Homopolymer
|
85.00 |
90.00 |
| PP Homopolymer, Long Glass Fiber, 30% Filler by Weight |
1.100 |
1.100 |
| PP Homopolymer, Long Glass Fiber, 40% Filler by Weight |
1.200 |
1.200 |
| PP Homopolymer, Long Glass Fiber, 50% Filler by Weight |
1.300 |
1.300 |
PP, Impact Modified
|
0.880 |
0.910 |
| PPA - Polyphthalamide 30% Mineral |
0.00 |
0.00 |
| PPA, 33% Glass Fiber-reinforced |
0.00 |
0.00 |
| PPA, 33% Glass Fiber-reinforced High Flow |
0.00 |
0.00 |
| PPA, 45% Glass Fiber-reinforced |
0.00 |
0.00 |
| PS (Polystyrene) Crystal |
88.00
|
88.00
|
| PS, High Heat |
80.00
|
90.00
|
PVC, Plasticized
|
75.00
|
85.00
|
| PVC, Plasticized Filled
|
1.150
|
1.350
|
PVC Rigid
|
80.00 |
80.00
|
SAN - Styrene Acrylonitrile
|
86.50
|
91.00
|
| SAN, 20% Glass Fiber-reinforced
|
1.200
|
1.400
|
SMA - Styrene Maleic Anhydride
|
1.050
|
1.080
|
SMMA - Styrene Methyl Methacrylate
|
80.00
|
91.40
|
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