Plastic Density and Infrared (FTIR) Testing: Methods, Standards, and Applications

Plastic density and infrared (FTIR) testing is a combined analytical approach that determines the physical density of polymeric materials while simultaneously confirming their chemical identity through infrared spectroscopy. Accredited third-party laboratories apply standardized methods such as ASTM D792, ISO 1183, and FTIR spectroscopy to support incoming material inspection, failure analysis, recycling, and regulatory compliance across the plastics industry. This article explains the technical principles, sample requirements, test methods, and typical applications of density measurement and infrared identification for plastic materials.

Technical Overview

Density, expressed as mass per unit volume (g/cm³ or kg/m³), is one of the most fundamental physical properties of plastic materials. Because each polymer family occupies a characteristic density range, density measurement provides a rapid first-line screen for material identification, lot-to-lot consistency, and detection of fillers or contamination. For example, polypropylene typically falls between 0.89 and 0.92 g/cm³, polyethylene between 0.91 and 0.97 g/cm³ depending on grade, polycarbonate near 1.20 g/cm³, and rigid PVC between 1.35 and 1.50 g/cm³.

Infrared spectroscopy, most commonly performed with a Fourier-transform infrared (FTIR) spectrometer, complements density data by revealing the chemical structure of the polymer. When infrared radiation in the mid-infrared region (approximately 4000 to 400 cm⁻¹) interacts with the sample, molecular bonds absorb energy at characteristic vibrational frequencies. The resulting spectrum acts as a molecular fingerprint, allowing analysts to identify the base polymer, detect copolymers and blends, and observe additives, oxidation products, or contaminants.

Used together, density testing and infrared analysis provide fast, mutually reinforcing verification: density constrains the list of candidate materials, and FTIR confirms the chemical identity. Commonly applied standards include ASTM D792 and ISO 1183 (Part 1 and Part 2) for density, ASTM D1505 for the density gradient column method, ASTM E1252 for infrared qualitative analysis of polymers, and GB/T 1033 and GB/T 6040 as the corresponding Chinese national standards. Samples are typically conditioned at 23 °C and 50% relative humidity in accordance with ISO 291 or ASTM D618 before measurement.

Testing Samples

Virgin Resins, Pellets, and Powders

Pelletized or powdered raw materials are submitted for incoming inspection and grade verification. A small portion of the lot is hot-pressed or molded into a solid plaque for accurate density determination, while individual pellets can be analyzed directly by ATR-FTIR to screen for mixed or mislabeled material.

Molded Parts and Finished Components

Injection-molded, blow-molded, extruded, or 3D-printed parts can be tested by cutting small sections from the component. This is common in automotive, electronics, consumer goods, and medical device applications where material substitution or counterfeit resin is suspected.

Films, Sheets, and Fibers

Thin polyolefin films, multilayer packaging films, and synthetic fibers are well suited to both techniques. Density measurements help classify film grades such as LDPE versus LLDPE, while FTIR can distinguish individual layers or coatings in multilayer structures.

Recycled and Regrind Materials

Recycled flakes, regrind, and reprocessed pellets are tested to verify polymer purity and detect cross-polymer contamination, which is critical for maintaining quality in closed-loop recycling streams and for meeting recycled-content specifications.

Unknown Particles and Contaminants

Foreign particles isolated during production—such as black specks, discolored inclusions, or unknown deposits—are analyzed microscopically and by micro-FTIR to identify their origin, whether degraded polymer, packaging residue, rubber, or inorganic filler.

Testing Items

Typical test items covered under plastic density and infrared testing services include:

• Density of solid plastics in g/cm³ or kg/m³, measured per ASTM D792, ASTM D1505, ISO 1183-1/1183-2, or GB/T 1033
• Specific gravity (relative density), the dimensionless ratio of material density to water density
• Apparent or bulk density of pellets and powders per ASTM D1895 for material handling and hopper design
• Polymer identification by FTIR spectroscopy, confirming materials such as PE, PP, PS, ABS, SAN, PC, PA6, PA66, PET, PBT, POM, PMMA, PVC, PTFE, PPS, PEEK, and TPU
• Screening for polymer blends and copolymers, such as PC/ABS, PPO/PS, and PP/EPDM
• Detection of characteristic additives, plasticizers, ester lubricants, and oxidation by-products
• Identification of contaminants, coatings, adhesives, and surface residues
• Spectral comparison against reference samples, datasheets, or spectral libraries for lot conformity assessment

Testing Methods

Density Determination

The most widely used technique is the hydrostatic immersion method (ASTM D792 Method A, ISO 1183-1 Method A), based on Archimedes' principle. The sample is weighed in air and again while fully immersed in a liquid of known density, usually distilled water with a wetting agent. Density is calculated from the ratio of the mass in air to the apparent mass loss, with corrections applied for the suspension wire and liquid temperature. For samples lighter than water, a nonwetting liquid such as ethanol or the addition of a sinker allows complete immersion.

Alternative methods include the liquid pycnometer technique (ISO 1183-1 Method B) for small granules, and the density gradient column (ASTM D1505, ISO 1183-2), in which calibrated glass floats within a liquid column of continuously varying density provide very high precision—commonly used for polyethylene and polypropylene resin characterization, where density correlates directly with crystallinity and stiffness.

Infrared Spectroscopy (FTIR)

Attenuated total reflectance FTIR (ATR-FTIR) is the preferred routine technique because it requires minimal sample preparation: the specimen is pressed against a diamond or ZnSe crystal and a spectrum is collected, typically at 4 cm⁻¹ resolution with 16 to 32 scans across 4000 to 400 cm⁻¹. Transmission FTIR is used for thin films and for powders dispersed in a potassium bromide pellet. The acquired spectrum is matched against reference libraries and in-house standards.

Interpretation relies on characteristic absorption bands, such as C–H stretching between 2850 and 3000 cm⁻¹ for polyolefins, carbonyl stretching near 1720 cm⁻¹ for polyesters and acrylics, the carbonate band near 1775 cm⁻¹ for polycarbonate, and aromatic ring vibrations near 1600 and 700–800 cm⁻¹ for styrenics. Combining the infrared result with the measured density enables confident confirmation—for instance, a density of approximately 1.20 g/cm³ together with a strong carbonate carbonyl band confirms polycarbonate.

Reporting and Data Interpretation

Test reports typically state the measured density, test method and liquid used, temperature, the FTIR spectrum with library or reference match quality, the identified polymer, and any observed deviations such as oxidation, contamination, or the presence of mineral filler indicated by an unexplained density increase.

Applications

Plastic density and infrared testing supports a broad range of industrial and regulatory objectives:

• Incoming material inspection and supplier quality verification at resin receiving or molding facilities
• Detection of counterfeit, substituted, or off-specification resins in supply chains
• Failure analysis of cracked, discolored, or degraded plastic parts, including oxidation and hydrolysis screening by FTIR
• Quality control in recycling operations, including polymer sorting and purity verification of recycled flakes and pellets
• Polyethylene and polypropylene grade classification, since density directly reflects crystallinity, stiffness, and melt behavior
• Research and development support for new formulations, blends, and additive packages
• Compliance evidence for customer specifications, data sheet verification, and certification programs
• Identification of unknown materials in customs inspection, intellectual property disputes, and reverse engineering studies

FAQ

Can density alone identify the type of plastic?

No. Many polymers share overlapping density ranges—for example, polystyrene and ABS both sit near 1.05 g/cm³. Density narrows down the candidate list quickly and cheaply, but infrared spectroscopy is needed to confirm the chemical identity and detect blends or additives.

Is FTIR a quantitative method?

FTIR is primarily a qualitative identification technique. With proper calibration standards, it can be used semi-quantitatively for estimating copolymer content or degree of oxidation. Inorganic fillers such as glass fiber are infrared-inactive and are better quantified by ash testing or thermogravimetric analysis, although their presence is often signaled by an elevated density.

What sample size and condition are required?

For immersion density testing, a solid piece of roughly 1 to 10 grams with a smooth, void-free surface is recommended. For ATR-FTIR, a flat area of only a few millimeters is sufficient. Samples should be clean, dry, and conditioned at 23 °C before testing to ensure accurate and repeatable results.

Can black or heavily filled plastics be tested?

Yes, with certain limitations. ATR-FTIR works well on black parts and typically returns a clean spectrum of the base polymer. However, high carbon black loadings can mask infrared absorption, in which case the polymer may need to be recovered by extraction, hot pressing, or microtome sectioning before analysis.

How long does the testing take?

Individual density and ATR-FTIR measurements are completed within minutes once samples are conditioned. Full identification work, including library matching, report review, and, where necessary, supplementary analysis, is typically completed by accredited third-party laboratories within three to seven working days.

What is the difference between density and specific gravity?

Density is an absolute property expressed in mass per unit volume, while specific gravity is the dimensionless ratio of the material's density to that of water at a specified temperature. Because water has a density close to 1.0 g/cm³ near room temperature, the two values are numerically almost identical for most plastics.