Week 3- Photopolymerization PDF

PHOTOPOLYMERIZATION Marcela Carrilho, DDS, PhD, FADM DENTD 1532 – Oral Health Sciences II 3rd Week Carrilho – 12/20/24 email: [email protected]  Learning Objectives By the end of this course, you are expected to: 1) identify the fundamental...

PHOTOPOLYMERIZATION Marcela Carrilho, DDS, PhD, FADM DENTD 1532 – Oral Health Sciences II 3rd Week Carrilho – 12/20/24 email: [email protected]  Learning Objectives By the end of this course, you are expected to: 1) identify the fundamentals of the polymerization reaction of dental resins; 2) differentiate between the terms “Polymerization” and “Photopolymerization”; 3) describe the main characteristics of the photopolymerization of dental resins and the factors that interfere in this reaction; and 4) describe the basic functionality of photopolymerization units (light-curing devices).  Polymerization (concept) Polymerization and OUTLINE OF THIS photopolymerization of dental resins PRESENTATION Factors changing the photopolymerization of dental resins Characteristics of light-curing units for activation of light-cured dental resins POLYMERIZATION Lecture on Adhesive Dentistry & Polymers before break… DENTD 1532 – ORAL HEALTH SCIENCES 3rd Week Adhesive Dentistry & Polymers - Carrilho email: [email protected]  Polymerization A reaction in which relatively small molecules, called monomers, join with each other to produce chainlike large molecules, called polymers. In the polymer structure, monomers are held together by chemical bonds. Several units of Monomers Polymerization reaction One Polymer chain Polymer Chain(s) Arrange POLYMER chain(s) can be arranged into different forms (see figure). Ideally, polymeric materials that are used for tooth rehabilitation should be arranged into packed cross-linked chains in order to be strong and stand masticatory mechanical stresses. Source: Craig’s Restorative Dental Materials, Chapter 7, 14th edition, 2019. POLYMERIZATION OF DENTAL RESINS  Dental Resins Polymerization Low viscous paste/fluid materials Solid structure Polymerization is necessary to transform resins into tough, hard structures. POLYMERIZATION  Dental Resins’ Polymerization Light-cured resins Self-cured resins Blue Light Chemical Camphorquinone Benzoyl peroxide (photosensitizer) (chemosensitizer/initiator) Free Radicals Free Radicals Break down double bonds, initiating curing reaction  Photopolymerization Reaction of monomers polymerization (or monomers conversion into polymer), which initiation is triggered by visible blue light. Information provided in this figure will not be asked in the exam. Phillips' Science of Dental Materials, Chapter 13, 2021.  Monomers in Dental Resins The vast majority of monomers found in restorative Dental Resins are methacrylates or dimethacrylates. C.............. C C C CH3 CH3 generic structure of a Dimethacrylate monomer  Monomers in Dental Resins UDMA HEMA Methacrylate groups, located in monomers’ ends, contain DOUBLE BONDS that are broken down into SINGLE BONDS during the polymerization reaction. This polymerization occurs by ADDITION and is initiated by free radicals. Photosensitizers* in Dental Resins 2,4,6- Camphorquinone Trimethylbenzoyldiphenylphos- (CQ) phine oxide (TPO) In 95 % of brands In 8-10 % of brands 1-phenyl-1,2-propanedione (PPD) Only in Ivoclar/Vivadent In 8-10 % of brands products * Photosensitizer = Photoinitiator Visible light absorption spectrum of different dental photoinitiators TPO from 390 to 410 nm Camohoroquinone from 425 to 495 nm Ivocerin® PPD from 390 to from 390 to 460 445 nm nm Rueggeberg et al., (2017). Braz. Oral Res. (suppl):e61 Differences in spectral absorption profiles and absolute absorption values among the dental photoinitiators when present at similar molar concentrations. Rueggeberg et al., (2017). Braz. Oral Res. (suppl):e61  Polymerization Stages 1. A initiator agent generates free radicals (reactive species that initiates the polymerization process). 2. Once a number of free radicals are generated, there will be sufficient energy to breakdown monomers’ double bonds. 3. Monomers are successively added one another. The reaction propagates by converting monomers into polymers chains. 4. The reaction terminates when free radicals are no more available (or are available in very low concentration).  Polymerization Stages INDUCTION or INITIATION R + C C R C C + C C R C C C C Free Radical Monomer PROPAGATION R C C C C + C C R C C C C C C TERMINATION R C C C C C C C C + C C C C C C C C R R C C C C C C C C C C C C C C C C R  Photopolymerization A technique that uses light (electromagnetic radiation) to induce and propagate a polymerization reaction to form a linear or cross- linked polymer structure.  CHARACTERISTICS OF DENTAL RESINS PHOTOPOLYMERIZATION Restorative Dental Resins (Polymers) Dental Adhesives Resin Composites  Shrinkage of Polymerization Volume Shrinkage (2-4%) Volume shrinkage is observed during the polymerization because monomers need to approximate one another in order to react and convert into polymer.  Degree of Conversion (DC) C C C C C C C C C C C C C C C C C C C C C C C C C C Percentage of double bonds of monomers that are converted into single bonds during the polymerization reaction. The higher the DC, the better the strength, wear resistance, and many other essential properties for resins’ good performance.  Depth of Cure Relevant only for Light-cured composites. It indicates the difference of conversion between the top and bottom of a layer of cured composite. The closer to the light source, the higher is the conversion. FACTORS CHANGING DENTAL RESINS’ POLYMERIZATION  Shrinkage of Polymerization It will always occur! 1. It is related to the molecular weight (“size”) of monomers. The polymerization shrinkage of the resin tends to be lower for monomers of greater molecular weight. 2. It depends on the degree of conversion (DC). The higher the DC, the greater is the polymerization shrinkage of the resin.  Degree of Conversion and Depth of Cure 1. They depend on the type and concentration of monomers, sensitizers and accelerators in the resin. 2. For light-cured resins, they will be also dependent on the transmission of the light through the material and 3. Accordingly, the transmission of the light through the resin is controlled by the wavelength of the light unit, amount of absorbed energy and volume of inorganic fillers in the material. Final properties of Composites affected by Degree of Conversion Shrinkage of Polymerization Biocompatibility Mechanical strength DEGREE OF CONVERSION Chemical stability Color stability Water sorption/Solubility CHARACTERISTICS OF LIGHT-CURING UNITS FOR ACTIVATION OF LIGHT- CURING DENTAL RESINS  Light-curing units The most common light sources used in dentistry to photoactivate dental resins are quartz-tungsten-halogen (QTH) and blue light-emitting diode* (LED). * Most common in the U.S.A. QTH units: The bulb in these units consists of a tungsten LED units: A light-emitting diode (LED) is a semiconductor filament enclosed in a transparent quartz casing, filled with light source that emits light when current flows through it. a halogen-based gas. Electrons are forced to traverse from one side of the semiconductor to a substrate having an electron deficiency. As electricity flows through the filament, because of the wire resistance, heat is generated sufficient to cause Electrons in the semiconductor recombine with electron tungsten atoms to vaporize from the wire surface. holes, releasing energy in the form of photons. During this process, light is emitted requiring low power, no filament Typically, these units require tremendous amount of filtering and no optical filter. to remove heat and excess of visible light that are not required for photocuring. These units provide much greater photon-generating efficiency. Case: Quartz Medium: Filament: halogen- Tungsten based gas Electromagnetic spectrum with correlated depictions of frequency and wavelength Rueggeberg et al., (2017). Braz. Oral Res. (suppl):e61 Terms used to describe light-curing sources * * 1 Joule (J) = 1000 mW × s Craig’s Restorative Dental Materials, Chapter 9, 14th edition Spectral emission of QTH vs. LED QTH LED QTH and LED light-curing units The spectral emission of these light sources comprises wavelengths varying from 390 to 490 nm for most QTH units and from 430 to 470 nm for LED units.  Spectral emission profile of QTH and LED curing units regarding the absorption wavelength of photoinitiators blue Spectral Irradiance (mW/cm2/mm) QTH LED Camphorquinone Camphorquinone Rueggeberg et al., (2017). Light curing in dentistry and clinical implications: a literature review. Braz. Oral Res. (suppl):e61 QTH and LED light-curing units The peak wavelength (430 to 470 nm) of these light sources is the best spectral output to activate the photosensitizer camphorquinone.  Peak wavelength of QTH and LED curing units regarding the absorption wavelength of photoinitiators blue QTH LED Spectral Irradiance (mW/cm2/mm) peak wavelength peak wavelength Camphorquinone Camphorquinone Rueggeberg et al., (2017). Light curing in dentistry and clinical implications: a literature review. Braz. Oral Res. (suppl):e61  Quartz-tungsten-halogen (QTH) Important parameters: QTH units’ irradiance (intensity) ranges from 400 to 800 mW/cm2, but higher- intensity units are available. A 2 mm-thick layer of resin can achieve adequate polymerization by receiving 16 J/cm2 of radiant energy density (400 mW/cm2 × 40 seconds = 16000 mW s/cm2), which is considered the ideal energy for good conversion (degree of polymerization). Craig’s Restorative Dental Materials, Chapter 9, 14th edition  Quartz-tungsten-halogen (QTH) Biosafety concerns: The cumulative exposure to high intensity blue light may cause ocular damage, thus one should avoid looking directly at the tip or the reflected light from the teeth. Blue-light filters (orange blue-blockers) can reduce the transmission of light and are encouraged to be worn by operators and patients. Some lamps produce considerable heat at the curing tip, which may produce pulpal irritation. Avoid long periods (>40 s) of light exposure. Craig’s Restorative Dental Materials, Chapter 9, 14th edition  Blue light-emitting diode (LED) Important parameters: It is extremely effective for curing resins containing camphorquinone as major photosensitizer. LEDs emit light irradiance ranging from 1000 to 3000 mW/cm2, which allows more efficient and deeper curing depths. On the other hand, the heat generated from these high-intensity devices can represent a risk of harm for pulp tissue. To achieve good polymerization, it is recommendable to photoactivate increments of resin not thicker than 2.0 mm.  Blue light-emitting diode (LED) Biosafety concerns: The employment of blue-light blockers are also required when using LED photo- curing units. LED curing units have a long lifespan, and do not produce as much heat as QTH devices. Heat generations becomes a concern for pulp tissue, when LED unit emit irradiances higher than 1200 mW/cm2 for an exposure time exceeding 25 s.  Factors causing decreasing in the intensity of curing lights Critical for: QTH & LED UNITS ONLY QTH UNITS ONLY QTH UNITS QTH & LED UNITS QTH & LED UNITS QTH & LED UNITS QTH & LED UNITS MORE CRITICAL FOR QTH QTH & LED UNITS Craig’s Restorative Dental Materials, Chapter 9, 14th edition  References 1. Craig’s Restorative Dental Materials, Chapter 4, 14th edition, 2019. 2. Craig’s Restorative Dental Materials, Chapter 9, 14th edition, 2019. 3. Phillips' science of dental materials, Chapter 1, 13th edition, 2021. 4. Phillips' science of dental materials, Chapter 13, 12th edition, 2021. 5. Polymerization video: https://www.youtube.com/watch?v=HpPHN7fcLHI 6. Rueggeberg et al. Light curing in dentistry and clinical implications: a literature review. Braz. Oral Res. (suppl):e61, 2017 [email protected] Happy Holidays!!!