A Study on the Photopolymerization Kinetics of Selected Dental Resins Using Fourier Infrared Spectroscopy (FTIR)

At a Glance

Metadata	Details
Publication Date	2022-08-25
Journal	Materials
Authors	Mirosław Kwaśny, Jakub Polkowski, Aneta Bombalska
Institutions	Military University of Technology in Warsaw
Citations	10
Analysis	Full AI Review Included

Executive Summary

This study investigated the photopolymerization kinetics and final Degree of Conversion (DC) of eleven commercial dental resin composites using Fourier Infrared Spectroscopy (FTIR) under varying clinical conditions.

Core Achievement: Established a highly precise, reference-band-independent method for calculating DC based solely on the kinetics of the monomer C=C absorption band (1638 cm^-1), eliminating systematic errors associated with traditional methods.
Polymerization Kinetics: The conversion process follows three distinct stages: a rapid initial phase (70-75% of maximum DC achieved within 5 s post-irradiation), a moderate phase (15-20% increase over 15-20 min), and a very slow termination phase (5-10% increase over 5 days).
Optimal Parameters: For 8 out of 11 tested resins, the optimal clinical parameters were determined to be 400 or 1000 mW/cm² irradiance, 10 s exposure time, and a maximum layer thickness of 2 mm.
Irradiance Impact: Increasing irradiance above 1000 mW/cm² (e.g., to 1600 mW/cm²) did not significantly improve the final DC, primarily affecting only the speed of the initial conversion phase.
DC Range: Final DC values typically ranged from 45-60% for most materials, suggesting that conversion is often limited by the mobility of the polymer chains or insufficient photo-initiator concentration, rather than light dose alone.
Methodology: DC kinetics were successfully measured using both transmission (for long-term aging tests up to 7 days) and Attenuated Total Reflection (ATR) modes.

Technical Specifications

The following hard data points define the experimental setup and key results derived from the study of dental resin photopolymerization.

Parameter	Value	Unit	Context
Irradiance (Power Density)	400, 1000, 1600	mW/cm²	Woodpecker LED source settings tested
Exposure Time	10, 20, 40	s	Light curing duration tested
Resin Layer Thickness (ATR)	0.76 to 3.70	mm	Range tested in reflection mode
Resin Layer Thickness (Transmission)	~0.2	mm	Required thickness for KBr crystal setup
FTIR Wavenumber Range	1200-1800	cm^-1	Spectral range for analysis
FTIR Resolution	4	cm^-1	Spectral measurement detail (64 scans)
Monomer Absorption Peak	1638	cm^-1	C=C stretching vibration (used for DC calculation)
Traditional Reference Band	1608	cm^-1	C=C stretching vibration (found unreliable in this study)
Max DC (Typical Range)	45-60	%	Final conversion for majority of tested materials
Max DC (Estelite)	>80	%	Highest conversion observed
DC Increase (Slow Phase)	5-10	%	Increase observed between 20 min and 5 days post-irradiation
Confidence Interval (L)	± 3.1	%	For DC value of 44.2% (p = 99%, n = 5)

Key Methodologies

The polymerization kinetics were investigated using two primary FTIR measurement modes: transmission (for thin layers and long-term aging) and Attenuated Total Reflection (ATR) (for thicker layers and real-time kinetics).

Sample Preparation (ATR Mode):
- Resin material was placed in a 9 mm diameter hole on a plate situated over a diamond crystal.
- The sample was pressed with a 0.5 mm microscope slide to ensure intimate contact with the crystal surface, simulating the resin-tooth interface.
- Absorption signals relate only to the top layer (several micrometers) due to limited IR penetration.
Sample Preparation (Transmission Mode):
- Resin layer thickness was strictly limited to approximately 0.2 mm due to strong absorption.
- The resin was sandwiched between two KBr crystals (25 mm diameter, 3 mm thickness) using a 0.2 mm spacer.
Irradiation Setup:
- A Woodpecker LED light source (8 mm diameter light guide) was used for exposure.
- Irradiation parameters were varied: Power Density (400, 1000, 1600 mW/cm²) and Exposure Time (10, 20, 40 s).
FTIR Measurement:
- Spectra were collected using a Thermo Scientific IS50 ATR Module FTIR spectrophotometer.
- Measurements were taken in absorbance mode over the range 1200-1800 cm^-1, with 4 cm^-1 resolution and 64 scans.
- Kinetics were tracked from 0 s immediately post-irradiation up to 7 days (in transmission mode).
Degree of Conversion (DC) Calculation:
- DC was calculated based on the loss of the monomer C=C stretching vibration peak at 1638 cm^-1.
- The calculation used the formula: DC(t) = (1 - A_t / A_m) * 100%, where A_t is the absorbance at time t and A_m is the initial monomer absorbance.
- Crucially, this method avoided the use of a reference band (e.g., 1608 cm^-1), which was shown to be unstable during the initial polymerization stages, thereby increasing measurement accuracy.

Commercial Applications

This research provides critical data and methodology for the development, testing, and clinical application of light-cured dental composite resins.

Dental Product Development: Provides manufacturers (e.g., Micerium, 3M Oral Care, Ivoclar Vivadent) with precise kinetic data necessary to optimize photo-initiator concentration and monomer composition (e.g., Bis-GMA derivatives) to achieve higher final DC values (above the typical 45-60% range).
Clinical Protocol Standardization: Results directly inform best practices for dental practitioners, confirming that layer thickness should not exceed 2 mm and that 10 s exposure at moderate power (400-1000 mW/cm²) is sufficient for most materials.
Curing Light Technology: Data on the relationship between irradiance and DC speed guides the design of next-generation LED curing lights, particularly those employing gradual irradiance increases to manage polymerization stress.
Material Longevity and Safety: Maximizing DC reduces the release of unreacted cytotoxic monomers, improving the biocompatibility, mechanical properties, and long-term durability (resistance to water sorption and solubility) of dental restorations.
Quality Assurance Testing: The non-reference band FTIR methodology offers a robust, high-precision standard for quality control testing of new composite materials entering the market.

View Original Abstract

The aim of the presented study was a comparative analysis of the polymerization kinetics of dental resin-based composites currently used in dentistry in different environmental conditions (irradiance, activation time, layer thickness). The photopolymerization kinetics of eleven dental resins were investigated using a Woodpecker LED source. The DC was measured by FTIR in transmission mode and attenuated total reflection (ATR) from 5 s to 7 days. In the transmission mode, the spectra from parallel optical layers (about 0.2 mm thick) of samples placed between the KBr crystals were recorded. In the reflection mode, an ATR attachment with a diamond window was used. The DC calculation method was applied based on the application of a monomer absorption band at 1638 cm−1 (stretching vibration double bond C=C of the vinyl group) without using a reference band. The data were analyzed by performing an ANOVA test comparison between sample groups at the significance level α = 0.05. For all tested materials, the polymerization kinetics consist of three stages. The fastest stage occurs during the irradiation, and the achieved DC value is 70-75% of the maximum value 5 s after the irradiation. Another 15-20% DC increase at a moderate speed takes about 15-20 min. There is also a very slow further increase in DC of 5-10% within 5 days after irradiation. For 8 out of the 11 tested fillings, the optimal photopolymerization conditions are as follows: a power density of 400 or 1000 mW/cm2; an exposure time of 10 s; and a thickness of the irradiated resin layer of up to 2 mm. The influence of various conditions and factors on the reaction kinetics is dominant only in the early, rapid phase of the conversion. After longer times, the DC values gradually level out under different light conditions. The DC of the dental resins are dependent on the irradiance, light source, filler type, time after irradiance, and monomer thickness.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15175850

References

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