تحليل FTIR للبوليمرات والبلاستيك

The Short Answer

FTIR analysis for polymers works because every organic functional group absorbs infrared light at a wavenumber that depends on its bond strengths and atomic masses, so a spectrum is a readable inventory of the chemistry present. You do not identify a polymer by matching a whole spectrum against a library and accepting a score; you identify it by finding the bands that only that chemistry can produce. Polypropylene shows a methyl bend near 1376 cm⁻¹ that polyethylene cannot show, because polyethylene has no methyl branches in its repeat unit. ABS shows a nitrile stretch near 2237 cm⁻¹ that polystyrene cannot show. PET shows an ester carbonyl near 1715 cm⁻¹ that a polyolefin cannot show 12.

The second half of the answer is practical: the spectrum you get depends less on the spectrometer than on how the sample was presented to it. أتر, transmission film, hot-pressed film and diffuse reflectance each sample a different depth and each suppresses different bands, which is why the same pellet can give two defensible but different-looking spectra 3. If you are building a quality-control decision flow around identification rather than interpreting spectra, our FTIR polymer identification QC workflow guide covers that side; this article stays with interpretation and sample preparation.

What FTIR Actually Sees in a Polymer

Functional groups, not molecules

An infrared spectrum records which frequencies of infrared light were absorbed as the beam passed through or reflected from the sample. Absorption happens when the frequency matches a vibrational mode of a bond and that vibration changes the molecule’s dipole moment. What this means in practice is that FTIR reports on bonds and groups, not on molecular identity directly. Two different polymers containing the same ester group will both show an ester carbonyl, at slightly different positions, and the difference in position is the information.

This is also the reason the technique has blind spots. A homopolymer with a highly symmetric, non-polar backbone absorbs weakly — polyethylene is the classic example, where the spectrum is dominated by C–H stretching and bending with little else to distinguish it. And a pigment such as carbon black absorbs across the whole range, which is why heavily filled black compounds are difficult by transmission and need either ATR with good contact or a different technique.

The fingerprint region and why libraries work

Below about 1500 سم⁻¹, absorption patterns become highly specific to the whole molecular skeleton rather than to individual groups. That region is called the fingerprint region, and it is what makes library matching viable: two polymers that share a functional group still differ in the fine structure of their fingerprint bands. ASTM E1252 sets out the general practice for obtaining infrared spectra for qualitative analysis and remains the reference point for good measurement technique [1]. ASTM E168 covers the quantitative side where peak height or area is turned into a concentration 4.

The honest caveat is that a library match score is a starting hypothesis, not a result. Published validation work on ATR FT-IR for polymer identification notes that identification is assigned on the basis of a set of characteristic absorption bands and that materials yielding too few identifying bands have to be reported as unknown [3]. That is the correct discipline: confirm the diagnostic bands by eye, then accept the library assignment.

Why FTIR analysis for polymers is a chemistry answer, not a compliance answer

This distinction causes most of the friction between a laboratory and a purchasing department. FTIR analysis for polymers answers “what chemistry is present”, and it answers it extremely well. What it does not answer is “does this batch conform”, because conformity is defined by physical properties — melt flow, density, tensile strength, thermal transitions — that have no infrared signature. A regrind stream that is chemically identical to virgin material can fail a melt-flow specification completely, and FTIR will not see the difference.

The practical consequence is that FTIR belongs at the front of a testing sequence, not at the end. Use it to sort incoming material into polymer families, to catch a wrong resin before it enters a machine, and to screen for gross contamination or oxidation. Then move the sample to a rheological or thermal method for anything the specification actually calls a number on. Laboratories that try to make FTIR carry the whole specification end up either over-testing or, worse, releasing material on a spectrum that never measured the property in dispute.

The Diagnostic Bands You Actually Use

Positions below are the working values a laboratory uses for assignment. Expect a shift of roughly ±10 cm⁻¹ depending on sampling method, crystallinity and instrument calibration, and never read a single band in isolation.

Telling the polyolefins apart

PE and PP are the pair that most often gets mis-assigned, because both spectra are dominated by C–H stretching and bending and both look “simple”. The discriminator is the methyl symmetric bend near 1376 سم⁻¹: polypropylene has a methyl group in every repeat unit, polyethylene in its homopolymer form does not. Polyethylene instead shows the 730/720 cm⁻¹ doublet from CH₂ rocking in crystalline regions. Read both, and read the 973–841 cm⁻¹ set for isotactic polypropylene helices if the grade matters.

Carbonyl-containing polymers

Once a carbonyl is present, the exact position is the identification. Carbonate in polycarbonate sits near 1770 سم⁻¹, ester in PET near 1715 سم⁻¹, ester in PMMA near 1730 سم⁻¹, and amide in polyamide near 1640 سم⁻¹. The amide is lower because the nitrogen donates electron density into the carbonyl and weakens the bond. That ordering — carbonate, ester, amide, descending — is stable and worth memorising.

Sample Preparation by Physical Form

The physical form of the sample, not the polymer, usually decides the sampling method.

أتر: when it is the right choice

ATR has become the default for solid polymers because it needs almost no preparation, and because the sampling depth is shallow and well defined — typically a fraction of a micron to a few microns, depending on crystal, wavenumber and contact. That shallowness is a feature for a moulded part, since it reads the surface as supplied, and a limitation when the surface is oxidised, coated or contaminated differently from the bulk. Where surface and bulk must be separated, microtome a slice and run transmission.

Crystal choice matters more than most datasheets admit: our ATR crystal selection guide covers ZnSe, diamond and germanium in detail.

الانتقال, films and hot pressing

Transmission remains the route when you need quantitative work on a known path length or when the additive levels are low. Hot pressing a film from pellets gives a homogeneous path, removes scatter from pellet surfaces, and is often the fastest way to turn a granule into a quantitative sample. The trade-off is thermal history: hot pressing can drive off volatile additives or oxidise the surface, so a film is not always representative of the part as moulded.

What Else FTIR Reads in a Compound

Fillers, plasticisers and copolymers

FTIR sees the organic phase well and the inorganic phase poorly. Calcium carbonate, talc and glass fibre all produce broad, strong bands that can dominate a spectrum and mask the polymer bands underneath, so a filled compound often has to be read on its weaker polymer features. Plasticisers are easier: a flexible PVC shows a strong ester carbonyl near 1730 cm⁻¹ from the plasticiser on top of the PVC pattern, and the size of that band is a rough guide to plasticiser level.

Copolymers are read by looking for two sets of characteristic bands and judging their relative intensities. EVA, for example, shows the polyethylene pattern plus an acetate carbonyl near 1740 سم⁻¹; the vinyl acetate content tracks with the relative intensity of that carbonyl. This is a semi-quantitative judgement unless you have built a calibration, which is where ASTM E168 practice applies [4].

Degradation and the Carbonyl Index

Polymer degradation is one of the most valuable routine applications of FTIR in a plastics laboratory, because oxidation introduces carbonyl groups into a polymer that did not have them. Polyolefins are the clearest case: a fresh polyethylene has no carbonyl band, and a thermally or UV-aged one develops a broad absorption between about 1700 و 1750 سم⁻¹. Tracking the ratio of that carbonyl band against an internal reference band that does not change — the 1463 cm⁻¹ methylene bend is commonly used for polyethylene — gives a carbonyl index that can be trended over a weathering or oven-ageing programme.

Two rules keep the number honest. Always ratio against an unchanged reference band rather than quoting the carbonyl peak height alone, and always record the sampling method, because ATR and transmission weight the near-surface region differently and oxidation is a surface-dominated phenomenon.

ASTM D3895 provides the oxidative-induction time measurement for polyolefins by differential scanning calorimetry 5; ايزو 11357-1 sets out the general principles of DSC for plastics 6. Neither replaces the carbonyl index, but together they separate “how much oxidation has happened” من “how much stabiliser is left”.

Where FTIR Stops and Another Method Starts

The pattern is that FTIR answers chemistry while thermal and rheological methods answer physics. A specification that asks FTIR to confirm a grade is usually asking the wrong instrument.

Instrument Configuration

The instrument itself is rarely the limiting factor in FTIR analysis for polymers. Resolution of 4 cm⁻¹ and a mid-infrared range covering roughly 4000 ل 400 cm⁻¹ is adequate for essentially all polymer identification and degradation work; what actually determines whether a laboratory gets usable data is the sampling accessory, the detector, and the purge or desiccant regime that keeps water vapour out of the beam path. Water vapour produces a dense set of narrow rotational lines between about 1300 و 1900 cm⁻¹ and above 3500 سم⁻¹, and those lines sit directly on top of the amide and carbonyl regions that polymer work depends on. A spectrometer with a tired desiccant cartridge will therefore produce spectra that look noisy in exactly the places that matter most.

For a wider view of where FTIR sits alongside absorption spectroscopy, see FTIR versus UV-Vis و what an FTIR spectrometer is used for.

Common Mistakes in Polymer FTIR

الأسئلة المتداولة

Can FTIR tell polyethylene from polypropylene?

نعم, and reliably. Polypropylene shows a methyl symmetric bending band near 1376 cm⁻¹ because its repeat unit carries a methyl group; polyethylene in homopolymer form does not. Polyethylene instead shows the 720 و 730 cm⁻¹ doublet from methylene rocking in crystalline regions. Reading both features together separates the two without any ambiguity.

Why does my spectrum of a black plastic look so poor?

Carbon black absorbs infrared light across the whole range, so transmission through a filled compound is usually impossible. Use ATR with firm, even contact so the beam only samples a shallow surface layer, and expect the polymer bands to sit on a sloping baseline. Very heavily filled compounds may need a different technique altogether.

Do I need transmission, or is ATR enough?

ATR is enough for identification of solid parts, pellets and films in the overwhelming majority of routine work, and it needs almost no preparation. Transmission becomes necessary when you need a defined path length for quantitative work, when additive levels are low, or when you need to separate a surface layer from the bulk of the material.

What does a band near 1730 cm⁻¹ mean in a polyolefin?

In a polyolefin it almost always means oxidation or an additive. A fresh polyethylene has no carbonyl absorption at all, so a band appearing between roughly 1700 و 1750 cm⁻¹ after heat or UV exposure indicates carbonyl groups formed during degradation. Ratio it against an unchanged reference band to get a carbonyl index you can trend.

How accurate are the band positions in published tables?

Treat published positions as working values with a tolerance of roughly ±10 cm⁻¹. The exact position moves with the sampling method, the crystallinity of the polymer, and the calibration state of the instrument. The relative ordering of bands is far more reliable than any single absolute position.

Can FTIR confirm that a batch is the grade we ordered?

Usually not on its own. FTIR confirms chemistry, so it can distinguish polymer families and detect copolymers and some additives, but it cannot measure melt flow, thermal transitions or mechanical performance. Grade confirmation normally needs ISO 1133 melt mass-flow rate or ISO 11357 differential scanning calorimetry alongside it.

Which standard should I cite for FTIR measurement practice?

ASTM E1252 covers the general techniques for obtaining infrared spectra for qualitative analysis, and is the usual citation for good practice. ASTM E168 covers the quantitative side, where peak height or area is converted into a concentration or an index such as the carbonyl index.

خاتمة: How to Decide

FTIR analysis for polymers rewards a laboratory that decides the question before it decides the method, because the sampling route follows the physical form and the acceptance criterion follows the question.

1. Decide the question first. Identification, additive screening and degradation monitoring are three different jobs with three different sampling routes.
2. Match the sampling method to the physical form of the sample, not to the polymer — a pellet, a film and a foam need different preparation even when they are the same resin.
3. Confirm the diagnostic bands by eye before accepting any library assignment, and read the carbonyl position together with the fingerprint region.
4. Ratio oxidation bands against an unchanged reference band so the carbonyl index is comparable between runs and between laboratories.
5. Add a thermal or rheological method wherever the specification is about grade rather than chemistry.

BRT supplies FTIR spectrometers and sampling accessories configured for plastics and polymer work, from incoming inspection to degradation studies. Tell us which physical forms you need to measure and which polymers you expect, and we will map that to a spectrometer, an ATR crystal and a sample preparation kit. Start from the FTIR spectrometer range, or read what an FTIR spectrometer is used for if you are new to the technique.

عن المؤلف. This article was prepared by the BRT spectroscopy application team at Shenyang Baoruitong Automation Equipment, which manufactures FTIR and UV-Vis instrumentation for polymer, pharmaceutical and environmental laboratories. Band positions are working values for assignment; standard numbers and editions were checked against ASTM and ISO catalogue entries on the dates given below.

مصادر

1. ASTM International — E1252-98 (reapproved 2021) Standard Practice for General Techniques for Obtaining Infrared Spectra for Qualitative Analysis
2. NIST — Validation of ATR FT-IR to identify polymers of plastic debris
3. AZoM — Failure Analysis of Plastics Using IR Spectroscopy
4. ASTM International — E168-16 Standard Practices for General Techniques of Infrared Quantitative Analysis
5. ASTM International — D3895-19 Standard Test Method for Oxidative-Induction Time of Polyolefins by Differential Scanning Calorimetry
6. UNI Ente Italiano di Normazione — ISO 11357-1:2023
7. UNI Ente Italiano di Normazione — ISO 1133-1:2022