In the world of thermoforming, selecting the right clear thermoplastic often requires balancing formability, mechanical performance, optical clarity, and long-term dimensional stability. Two materials that sometimes come up in fabrication discussions are Cellulose Acetate Butyrate (CAB) and Polyethylene Terephthalate Glycol-modified (PETG). Each has distinct molecular structures and performance characteristics that make them suitable for certain applications — and less suitable for others.
This article provides an in-depth technical comparison of CAB and PETG, focusing on their material properties, thermoforming behavior, and practical fabrication considerations.
Material Basics
Cellulose Acetate Butyrate (CAB)
Cellulose Acetate Butyrate is a cellulosic thermoplastic engineered from cellulose derivatives. By substituting cellulose hydroxyl groups with both acetate and butyrate ester groups, CAB achieves improved elasticity, reduced moisture uptake, and enhanced surface gloss relative to pure cellulose acetate.
The resulting polymer is semi-crystalline, with moderate rigidity and excellent clarity, often used in film, sheet, optical coatings, and consumer products where visibility and surface quality are important.
Polyethylene Terephthalate Glycol-modified (PETG)
PETG is a glycol-modified version of polyethylene terephthalate (PET), a polyester widely used in packaging and thermoformed products. The addition of glycol interrupts crystallization during polymerization, resulting in an amorphous, clear thermoplastic that is easier to process and more impact resistant than unmodified PET.
PETG sheets are widely used for displays, glazing, signage, and functional parts that require transparency and toughness.
Key Mechanical and Thermal Properties
The following table summarizes typical measured properties for CAB and PETG, drawing on industry material databases:
| Property | CAB | PETG |
|---|---|---|
| Tensile Strength (Ultimate) | ~20–50 MPa | ~53 MPa |
| Flexural Modulus | 0.8–1.7 GPa | ~2.1 GPa |
| Glass Transition Temp (Tg) | ~140°C | ~80–85°C |
| Heat Deflection Temp (264 psi) | ~53–82°C | ~69°C |
| Density | ~1.2 g/cm³ | ~1.3 g/cm³ |
| Water Absorption (24h) | ~1.2–1.6 % | ~0.2 % |
From these comparisons:
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PETG exhibits higher tensile and flexural strength, indicating greater ability to carry load and resist deformation.
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CAB has a higher glass transition temperature, which may confer better thermal dimensional retention in certain environments.
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CAB absorbs more moisture, which can lead to dimensional changes with humidity, whereas PETG maintains lower moisture uptake.
These quantitative differences highlight core molecular distinctions: cellulosic polymers like CAB inherently have rigid ester groups that elevate Tg but reduce strength relative to polyethylene-based polyesters like PETG.
Thermoforming Behavior
CAB Thermoforming Attributes
CAB’s moderate stiffness, high gloss, and good flow characteristics make it attractive when optical clarity and cosmetic surface quality are primary goals. Its relatively narrow forming window and sensitivity to heat distribution means skilled temperature control is needed. The presence of butyrate groups also lends good moisture resistance and dimensional consistency when processed correctly.
In thermoforming practice, CAB can be easier to shape into tight radii and deep draws than some other rigid thermoplastics, provided temperature control is precise. Its cellulosic backbone, however, means it does exhibit higher moisture absorption and requires consideration where humidity cycles are present.
PETG Thermoforming Attributes
PETG is widely acknowledged within fabrication circles as one of the easiest clear sheets to thermoform. It softens at moderate temperatures (approximately 80°C–120°C depending on grade) and does not necessarily require pre-drying before forming, which simplifies production cycles.
PETG’s amorphous molecular structure contributes to excellent deep draw performance, low haze formation during forming, and high impact resistance, which is why it is frequently specified for display, signage, medical device housings, and protective covers.
Compared with CAB, PETG:
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Accepts higher draw ratios without cracking
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Is less susceptible to stress whitening
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Has a wider processing window for vacuum forming
These qualities make PETG the material of choice for many industrial thermoforming applications.
Optical and Surface Performance
Both CAB and PETG offer high transparency and light transmission suitable for clear parts. However:
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PETG frequently outperforms in impact resistance and crack resistance, making it preferred for safety glazing and protective covers.
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CAB’s cellulosic structure provides a distinct surface luster and high gloss that some designers favor for cosmetic or decorative parts.
While optical clarity is comparable between the two, PETG’s moisture resistance and toughness often deliver better long-term dimensional stability in varied environments.
Chemical and Environmental Considerations
PETG provides good chemical resistance to acids, bases, and many solvents — yet can be prone to surface scratching and may yellow without UV stabilization.
CAB, while clear and weatherable, is incompatible with strong solvents and some alkaline environments and exhibits higher moisture uptake, which can impact part tolerances over time.
From an environmental perspective, PETG is recyclable under category #1 recycling streams and widely accepted in automated recycling systems.
Applications and Use Cases
Where CAB Excels
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Cosmetic sheet goods with high surface gloss
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Optical films and decorative glazing
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Situations requiring higher heat resistance without mechanical load
Where PETG Excels
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Vacuum formed covers and enclosures
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Machine guards and protective barriers
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Retail displays and signage
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Medical device housings and FDA-compliant parts
PETG’s ease of forming, toughness, and processing flexibility often make it the default choice in thermoforming operations for production parts.
Conclusion
In a head-to-head comparison of CAB and PETG for thermoforming:
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PETG typically offers superior overall thermoformability, mechanical strength, and impact resistance, making it more versatile for industrial parts.
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CAB provides high surface gloss, moderate thermal stability, and unique cellulosic properties, but demands tighter process control and has limitations in moisture sensitivity.
For high-volume fabrication of clear parts with deep draws and robust performance, PETG remains the workhorse material. CAB may be selected for niche cosmetic applications where its higher Tg and surface aesthetics are prioritized, but it generally does not match PETG in toughness or environmental stability.
Citations
Material Property & General Info Sources
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MakeItFrom — Cellulose Acetate Butyrate (CAB)
https://www.makeitfrom.com/material-properties/Cellulose-Acetate-Butyrate-CAB -
MakeItFrom — Compare CAB vs PETG
https://www.makeitfrom.com/compare/Cellulose-Acetate-Butyrate-CAB/Glycol-Modified-Polyethylene-Terephthalate-PETG-PET-G -
Acme Plastics — What Is PETG?
https://www.acmeplastics.com/what-is-petg?srsltid=AfmBOooXfC8RTMuDqQ9mO8oscmOkOafcIajJlz4DP6Hi77qOa_PTreFv -
Piedmont Plastics — PETG Uses
https://www.piedmontplastics.com/blog/petg-uses?srsltid=AfmBOopPo_KVpVyZZaq5ltN9lC0OjIxlE4beUqSEDlIm4NuIrLGrX4w- -
RapidMade — PETG Thermoforming Guide
https://rapidmade.com/petg-plastic-strengths-weaknesses-and-how-to-get-crystal-clear-thermoformed-parts/ -
Desu Plastic — PET vs PETG vs COC for Thermoforming
https://desuplastic.com/pet-vs-petg-vs-gag-for-thermoforming-comparison-guide/
Technical Property Data
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MakeItFrom — CAB Material Properties
https://www.makeitfrom.com/material-properties/Cellulose-Acetate-Butyrate-CAB -
MakeItFrom — PETG Material Properties (via Compare)
https://www.makeitfrom.com/compare/Cellulose-Acetate-Butyrate-CAB/Glycol-Modified-Polyethylene-Terephthalate-PETG-PET-G -
Laird Plastics — PETG Properties & Recycling
https://lairdplastics.com/resources/petg-plastic-properties-uses-amp-advantages-2025-update/?srsltid=AfmBOoosOWgbSRkjmPHc-5mxmHtMP34eF5jgmKtfyiodwdn_IIGE6AVP
Aerospace & Flame Context (General)
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14 CFR Part 25, FAA Appendix F
https://www.law.cornell.edu/cfr/text/14/appendix-F_to_part_25
