Views: 0 Author: Site Editor Publish Time: 2026-06-01 Origin: Site
Polyetherimide (PEI) is one of the members of the polyimide (PI) family. Therefore, before discussing PEI, it is necessary to first understand PI itself.
As shown below, polyimides are a class of high-performance polymers whose main chains contain imide rings (-CO-NR-CO-), where R is usually an aromatic tetravalent group such as a benzene ring. Their structural units are formed through polycondensation reactions between diamine and dianhydride monomers, resulting in rigid chain-like structures containing imide groups, phenoxy groups (-O-), amino groups (-NH-), and other functional groups.
(Many PI materials are priced above RMB 1,000/kg — even more expensive than Victrex’s PEEK — placing them firmly at the top of the “plastics pyramid.”)
PI materials can be either thermosetting or thermoplastic. Common categories include PAI and PBI.
PAI, or Polyamide-imide, is a copolymer in which amide bonds (-NH-CO-) and imide rings (-CO-N-CO-) coexist alternately. Its structure can be regarded as a hybrid of polyamide (PA) and polyimide (PI). PAI has a glass transition temperature (Tg) of approximately 290°C, a 5% thermal weight loss temperature of around 510°C, and a light transmittance of 84% at 550 nm. It can also be processed into films for flexible substrate applications.
PBI, or Polybenzimidazole, contains benzimidazole rings (double nitrogen-containing heterocyclic structures) in its backbone, formed through polycondensation between tetra-amines and dicarboxylic acid monomers. PBI exhibits an RTI above 400°C and a Limiting Oxygen Index (LOI) above 40%. Composite materials based on PBI can be used in aerospace structural parts and insulation systems. It was even once considered for use in coin currency materials.
With the large-scale supply and balanced toughness-rigidity advantages of PEEK, some thermoplastic PI applications were rapidly replaced. However, this has not slowed the development of advanced PI materials in emerging applications. Market demand and performance expectations continue to rise, especially in electronics, where new applications for PI films emerge every year. This is no longer a business model defined purely by “engineering plastics” thinking.
Traditional PI products such as Kapton® film and Vespel® shapes were both invented and commercialized by DuPont. Unlike ordinary plastics businesses, they were never spun off, but instead retained alongside brands such as Nomex® and Kevlar® as high-value electronic and process-control materials. Historically, international polymer companies tended to integrate deeply from monomer synthesis all the way to end-use applications, thereby creating significant commercial barriers.
Transparent high-temperature polymers have always been a major topic, and CPI deserves special attention.
The optical transparency of Colorless Polyimide (CPI) results from the combined effects of molecular structure and condensed-state behavior. The key lies in suppressing the formation of Charge Transfer Complexes (CTCs), controlling crystallization behavior, and optimizing molecular chain arrangement.
Traditional polyimides contain rigid aromatic rings and highly polar imide groups (-CO-N-CO-) within their main chains, which easily form intramolecular charge transfer complexes (CTCs). In this structure, dianhydrides act as electron acceptors (A), while diamines serve as electron donors (D). The alternating donor and acceptor units form D-A structures, represented as D⁺δ⋯A⁻δ, where δ indicates partial charge transfer.
These interactions rely mainly on electrostatic attraction (Coulomb force) rather than conventional chemical bonds, with relatively weak bond energies (<50 kJ/mol). Electron transfer occurs from the diamine unit to the dianhydride unit, generating delocalized π-electron systems.
Strong CT absorption bands are typically formed in the 300–500 nm wavelength range, giving traditional PI its characteristic yellow color. The stronger the CTC interaction, the darker the color.
For polyimides with identical structures, higher molecular weights increase intrachain charge transfer distances, strengthening intramolecular CTC effects. Larger molecular weights also promote chain entanglement, which enhances intermolecular interactions and further strengthens interchain CTC effects.
To achieve transparency in PI materials, several molecular design strategies can be used:
Introducing non-coplanar monomers — such as alicyclic dianhydrides or diamines instead of aromatic structures — distorts the molecular backbone and prevents donor-acceptor electron cloud overlap. The trade-off is typically reduced heat resistance.
Bulky substituent groups such as trifluoromethyl or tert-butyl groups can also introduce steric hindrance, suppressing CTC formation by preventing electron cloud overlap.
For example, fluorinated ethyl structures (F₃C-) can dilute conjugation density.
Highly electronegative fluorine atoms (electronegativity 3.98) can form low-polarizability C-F bonds, reducing chain polarity and weakening the CTC effect.
Alternatively, fluorine-free approaches can insert ether bonds (-O-) into the backbone, creating flexible oxyether linkages that interrupt conjugation continuity while forming a “rigid-flexible block” structure similar to Damascus steel.
As discussed previously in polymer transparency mechanisms, if microcrystalline regions exist within PI materials, crystal sizes can be controlled below 400 nm (smaller than visible light wavelengths) to minimize light scattering.
However, most CPI technologies focus primarily on suppressing CTC formation. The majority of CPI materials are fully amorphous and achieve transparency through complete crystallization suppression. A smaller number rely on ultra-fine microcrystal control to achieve transparency.
The distorted structures and bulky side groups in CPI create a “loose network” arrangement with low-density molecular packing.
The PEI discussed here refers specifically to bisphenol-A-based polyetherimide — the Ultem® series originally introduced by GE in 1982. It was also the last engineering plastic in GE’s product portfolio to retain a fully integrated in-house synthesis system.
Compared with other PI-family materials, PEI can be regarded as one of the easiest to synthesize. Even so, its industrial synthesis route remains highly complex. The preparation of its key precursor, BPADA (bisphenol-A dianhydride), requires sophisticated biphasic imide-anhydride exchange technology.
Traditional PI materials such as Kapton® and Vespel® contain rigid aromatic rings and continuous imide structures, requiring high-temperature imidization (>300°C) to form highly crosslinked systems characterized by “insoluble and infusible” behavior.
PEI introduces flexible ether bonds (-O-) through BPADA structures. Its secondary relaxation peak (β relaxation) occurs at approximately -60°C, far below Tg, corresponding to local cooperative motions of ether linkages and rotational movement of isopropyl side groups. This allows PEI to maintain flexibility at low temperatures.
Of course, PEI’s “flexibility” is only relative to traditional PI materials — it should not be compared with polymers such as polycarbonate.
The ether bonds interrupt continuous conjugated planar structures, suppress tight molecular packing, and lower the Tg to around 217°C, with an RTI of approximately 180°C. More importantly, PEI can be injection molded — a major advancement for the PI family.
(It is almost as if a stubborn old man suddenly realized that lowering his posture could open many more business opportunities.)
At present, PEI still belongs within the category of specialty engineering plastics — at least for now.
PEI contains ether bonds (-O-) and isopropyl groups (-CH(CH₃)₂) within its backbone, disrupting molecular regularity and suppressing crystallization. As a result, PEI is completely amorphous, exhibiting only diffuse XRD scattering peaks.
The flexible ether linkages create “kinked-stretched” chain conformations, resulting in a free volume fraction (FFV) of 0.15–0.18, significantly higher than traditional PI materials (0.08–0.12). This low packing density gives PEI membranes relatively high gas permeability (O₂ permeability ≈ 1.2 Barrer), enabling applications in gas separation.
Regarding PEI mold design, processing, mainstream applications, and PC alloy systems, abundant first-hand information can already be found on major industry websites, so it will not be repeated here.
One particularly important characteristic is its coefficient of thermal expansion (CTE ≈ 5×10⁻⁵/°C), which closely matches metals and makes PEI highly suitable for precision electronic packaging.
Additionally, compared with PPA and PPS, although PEI offers lower heat resistance, PEI and PES exhibit relatively flat mechanical modulus changes over temperature. This provides advantages in dynamic mechanical applications requiring stable mechanical properties across wide temperature ranges, such as high-temperature electrical switch levers.
Now we can gradually “distill” the commercial value of PEI.
PEI came from GE — a company renowned for its market strength — and has consistently been priced higher than PES (Polyethersulfone), despite their relatively similar performance in many applications. Much of this premium reflects GE’s strong direct engagement with end users.
Although PEI and PES differ significantly in chemical structure, they can substitute for each other in many applications. Here, we focus only on areas where PEI cannot replace PES.
The most significant advantage of PES is its superior alkali resistance.
The imide rings in PEI are susceptible to nucleophilic attack by hydroxide ions (OH⁻) under alkaline conditions, especially at elevated temperatures. This causes irreversible ring-opening hydrolysis and formation of polyamic acid salts, rapidly damaging the polymer backbone.
By contrast, the sulfone groups (-SO₂-) and ether bonds (-O-) in PES exhibit excellent resistance to alkaline hydrolysis. The structure lacks vulnerable sites for alkali attack.
(Like rival “factory beauties,” PEI and PES belong to different industrial groups, each publishing extensive data highlighting the weaknesses of the other side — “ours is better.” Similar examples exist everywhere. I have seen reports claiming PPE is more hydrolysis-resistant than PPS, and other reports claiming the opposite, both with convincing data. Usually, a specific grade from one side is compared with a carefully selected competitor grade from the other. Such comparisons often say more about commercial strategy than the intrinsic nature of the material itself.)
The competition between PEI and PES may eventually conclude because PPSU (Polyphenylsulfone), with superior overall performance, has become significantly cheaper — now around USD 10/kg, approaching the current synthesis cost of PEI itself.
There are two application areas where PEI maintains overwhelming dominance after 40 years of market selection:
Fiber Optic Connectors (FOC)
Aerospace composite materials
FOC stands for Fiber Optic Connector — one of the few high-temperature engineering plastic applications featuring the iconic blue color associated with PEI.
Both GE and later SABIC training materials emphasized that PEI was selected primarily because its CTE (~5×10⁻⁵/°C) closely matches aluminum. However, PES exhibits similar CTE values, and CTE itself can be modified through formulation changes, so this alone cannot fully explain PEI’s dominance.
The real differences likely emerge in long-term application behavior.
According to the white paper Push The Polymer Envelope:
In one 5G base station test, PES optical connectors exhibited optical power losses of 2.1 dB after operating at 75°C for 2000 hours, while PEI connectors showed only 0.3 dB loss.
PEI’s amorphous structure enables nanometer-level optical surface finishes (Ra < 50 Å), whereas weakly crystalline PES can form >10 μm spherulites during molding, leading to surface microcracks and poorer polishing performance.
PES shows a refractive index thermal drift coefficient approximately 50% higher than PEI, increasing optical axis deviation under high temperatures.
Under UV exposure, PES exhibits significantly stronger yellowing than PEI.
In humid environments, sulfone groups in PES interact strongly with water molecules, leading to swelling and stress cracking.
These differences explain why PEI continues to dominate modern communication precision components. In many FOC applications, only glass can realistically compete with PEI.
PEI also performs exceptionally well in aerospace composite systems.
Cone calorimeter testing shows PEI exhibits very low heat release rates. According to datasheet values:
PEI: V-0 @ 0.4 mm
PES: V-0 @ 1.0 mm
The LOI values are also dramatically different:
PEI: LOI ≈ 50
PES: LOI ≈ 36
PEI’s superior flame resistance comes from the aromatization of imide structures during combustion, forming dense protective char layers. In contrast, PES decomposes to release SO₂ gases, weakening char integrity and generating more toxic emissions.
In confined environments such as aerospace, rail transit, and nuclear facilities, where composite materials are widely used, PEI offers major advantages as a matrix resin.
(Realistically speaking, materials may eventually burn. The critical questions are: how fast they burn, how much smoke they generate, what toxic gases are released, and how much heat is emitted.)
Over the past 15 years, PEI has also found impressive applications in eyeglass frames, slow juicers, and laboratory animal cages.
However, its brittleness limits some opportunities. It cannot become the “golden baby bottle” material, nor can it easily be pulverized into non-stick cookware coatings.
Like many specialty polymers, PEI lacks the broad “platform-style” growth potential of more versatile materials. As other amorphous high-temperature plastics continue lowering in price, some PEI applications are gradually being eroded.
Although SABIC’s core PEI synthesis patents have already expired, few Western chemical companies have invested in PEI synthesis capacity. Meanwhile, domestic Chinese companies have actively entered the field, often pursuing “stamp-collection-style” completion of missing material categories.
As discussed above, PEI clearly possesses unique strengths and deeply rooted application advantages. The remaining question is simply how large the market can ultimately become.
From injection molding and extrusion to films, powders, fibers, composites, and alloys, the broader the processing ecosystem surrounding a polymer, the more likely it is to survive long-term.
Topics such as mold design, injection molding, stress relief, and chemical resistance are already well covered in widely available technical materials, so they will not be repeated here.
One final note:
Market reports issued by securities firms are generally not recommended — getting even 50% accuracy is already impressive. Likewise, many “feasibility study” reports are filled with unrealistic assumptions and optimistic fantasies. Although there are certainly excellent exceptions, the average reliability may only be around 30%.
Reading too many of them can become toxic.
