ASTM D7399 PP Content Testing in PP/LDPE Blends by Infrared Spectrophotometry
The amount of polypropylene (PP) in polypropylene/LDPE mixtures using an infrared spectrophotometer can be determined using the standard test method in ASTM D7399. Thin films representing a typical portion of the material are analyzed with an IR spectrophotometer using two absorbance bands-one characteristic for PP and one for low density polyethylene (LDPE). The ratio of these two absorbances is used to assess the presence of polypropylene.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM D7399 describes a standard test method for determining the amount of polypropylene (PP) in binary mixtures of polypropylene and low-density polyethylene (LDPE) using infrared spectrophotometry (FTIR). Knowing the composition of polyolefin blends is essential for quality control, recycled content verification, and material identification in the plastics industry.
This method provides a quantitative, non-destructive analytical tool for characterizing PP/LDPE blend composition from pressed film specimens, supporting material identification in recycling streams and quality assurance in compound production.
Scope, Applications, and Benefits
Scope
ASTM D7399 applies to PP/LDPE binary blend systems and determines:
- Weight percent PP in the PP/LDPE mixture
- FTIR spectrum-based quantitative analysis using characteristic absorption bands
- Accuracy across the full 0–100% PP composition range
- Applicability to both virgin and recycled polyolefin blends
Applications
- Recycled polyolefin stream composition verification
- Polyolefin compounding blend ratio quality control
- Contamination detection in PP or LDPE production streams
- Sorting and identification of mixed polyolefin recyclates
- Raw material specification compliance for PP/LDPE blend products
Benefits
- Non-destructive analysis of a small pressed film specimen
- Rapid quantitative result without dissolution or separation
- Applicable to both production compounds and recycled material streams
- Builds on widely available FTIR instrumentation in plastics laboratories
- Simple calibration and good precision across the full composition range
Test Process
Specimen Preparation
A small amount of PP/LDPE blend material is hot-pressed into a thin film of uniform thickness for IR analysis.
1FTIR Spectrum Acquisition
The pressed film is analyzed by transmission FTIR; the full mid-IR spectrum from 4000–650 cm⁻¹ is recorded.
2Absorbance Measurement
Characteristic absorbance bands for PP (e.g., ~998 cm⁻¹) and LDPE (e.g., ~720 cm⁻¹) are integrated or measured from the spectrum.
3PP Concentration Calculation
The PP/LDPE absorbance ratio is applied to a calibration curve to determine PP wt%.
4Technical Specifications
| Parameter | Details |
|---|---|
| Standard | ASTM D7399 |
| Test Principle | Transmission FTIR of pressed film specimen |
| Applicable Materials | Binary PP/LDPE blends (0–100 wt% PP) |
| Key Absorbance Bands | PP: ~998 cm⁻¹; LDPE: ~720 cm⁻¹ |
| Measured Output | PP content (wt%) in PP/LDPE blend |
| Calibration | Gravimetrically prepared PP/LDPE reference standards |
Instrumentation Used for Testing
- FTIR spectrometer with transmission accessory
- Hot press for thin film preparation (approximately 0.1–0.3 mm thickness)
- Polished metal or KBr windows for film pressing
- Calibration standards (PP/LDPE blends at known compositions)
- FTIR data analysis software with absorbance integration capability
Results and Deliverables
- PP content (wt%) in the tested PP/LDPE blend sample
- FTIR spectrum with marked analytical bands
- Calibration curve and linearity data
- Measurement uncertainty and precision
- Analytical certificate for material identification or specification compliance
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Frequently Asked Questions
PP exhibits a distinctive absorption band near 998 cm⁻¹ (CH₃ rocking) while LDPE has characteristic bands near 720–730 cm⁻¹ (CH₂ rocking); the ratio of these band intensities is used for quantification.
ASTM D7399 is specifically validated for PP/LDPE binary mixtures; HDPE has different crystallinity and band intensities, so separate validation would be needed for PP/HDPE blends
The method is designed for blends across the full composition range; detection of very low PP levels (below ~2–5 wt%) may require more sensitive spectroscopic approaches or chemometric analysis.
Yes — for quantitative transmission FTIR, specimen thickness must be controlled and consistent; absorbance values must fall within the linear response range of Beer's Law (typically A < 1.5 absorbance units).
ASTM D7399 specifies transmission FTIR with pressed films; ATR-FTIR is a surface measurement with different penetration depth and may require separate calibration validation for quantitative PP/LDPE determination.
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