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SAY AHHH, FOR COMPOSITE RESIN FILERS: A STUDY OF THE
COMPOSITE RESIN
by
Vishal Patel
A Thesis in Chemistry Education
Presented to the Faculty of the University of Pennsylvania in partial fulfillment of the requirement of the Degree of
Master of Chemistry Education
At
University of Pennsylvania
__________________________________________________
Constance W. Blasie Program Director
__________________________________________________
Dr. Andrew Rappe Supervisor of Thesis
__________________________________________________
University of Pennsylvania
Abstract SAY AHHH, FOR COMPOSITE RESIN FILERS: A STUDY OF THE COMPOSITE RESIN
by Vishal Patel
Chairperson of the Supervisory Committee: Professor Dr. Andrew Rappe Department of Science
Abstract – Caries are an infectious disease which can be prevented by practicing proper dental hygiene. In order to understand how caries develop it is important to understand the anatomical structure and function of enamel, dentin, and pulp. The process of demineralization of the enamel layer occurs due to the break down of sucrose which is a common sugar ingested daily. The sucrose is hydrolyzed to produce glucose and fructose. Fructose is broken down via multiple steps of glycolysis to produce lactic acid and a hydrogen ion which dissolves the enamel layer. Once the enamel is demineralized, caries begin to develop. Caries need to be treated with a proper filling which can hold up for multiple years to prevent the caries to further decompose the tooth or even possibly fracture a tooth. Amalgam has been used for many years and due to its appearance in the mouth, composite resins are being considered as an alternative. The most common dental composite resin currently used is bis-GMA; however, its high viscosity and shrinkage concerns have forced alternative composite resins to be researched. A cube structure known as silsesquioxanes have been used with organic tethers which are used to crosslink the cube structures. The crosslinking of the hard core composite resin matrix with the organic, soft tethers allow the cubes to readjust even after the initial reaction. The silsesquioxanes are one of many alternatives being reconsidered.
Patel (^) i
LIST OF FIGURES
- Anatomy of tooth ................................................................................................................ Preface.................................................................................................................................. iv
- Formation of Carries (Cavities).........................................................................................
- Amalgam Fillings .................................................................................................................
- Composite Resins and Cements .......................................................................................
- Text Box #1 (What’s free about free radicals?)
- Text Box #2 (Concreteness of Cement)..............................................................
- Post Shrinkage...................................................................................................................
- State of the Art Composite Resin..................................................................................
- Conclusion
- Bibliography.......................................................................................................................
- Appendix A: Lesson Overview......................................................................................
- Lesson Plan (Day 1)
- Experiment 1..............................................................................................................
- Experiment 2..............................................................................................................
- Experiment 3..............................................................................................................
- Lesson Plan (Day 2)
- Lesson Plan (Day 3)
- Daily Learning Log....................................................................................................
- PIM...............................................................................................................................
- Tooth Anatomy........................................................................................................ Number Page
- Steps to tooth decay ................................................................................................
- Demineralization of Enamel..................................................................................
- Sucrose .......................................................................................................................
- Hydrolysis..................................................................................................................
- Glycolysis...................................................................................................................
- Formation of Lactic Acid .......................................................................................
- Restored and repaired amalgam restoration .......................................................
- Elements used to make amalgam..........................................................................
- Symbols and Stoichiometry .................................................................................
- Silver-tin phase diagram........................................................................................
- Disposal Amalgam vial ........................................................................................
- General amalgamation reaction .........................................................................
- Schematic drawing of amalgam microstructure ..............................................
- Mechanics of Amalgam filling.............................................................................
- Amalgam photomicrograph.................................................................................
- Human Mercury Exposure ..................................................................................
- γ-methacryloxypropyltrimethoxysilane..............................................................
- Propanediyl)) (Bis-GMA)..................................................................................... 19) 2-methyl-2-Propenoic acid (1-methylethylidene) bis (4,1-phenylenoxy-2-Hydroxy-3,1-
- Stick model of bis-GMA .....................................................................................
- Ball and stick model of bis-GMA.......................................................................
- Amino-carboxylate based bonding agent (NPG-GMA) ...............................
- Carboxylate-based bonding agent.......................................................................
- Phosphate based bonding agent ........................................................................
- Camphorquinone................................................................................................
- Light-curing process
- Heterolytic Cleavage...........................................................................................
- Homolytic Bond Cleavage
- Benzoyl Peroxide
- Initiation
- Electromagnetic Spectrum................................................................................
- Propagation Stage
- Chain Transfer.....................................................................................................
- Second type of chain transfer...........................................................................
- Termination
- Cement Factory...................................................................................................
- Cement Hydration
- Organic Monomers
- Ball and stick model of ethylene glycol dimethacrylate (EGDMA)..........
- Ball and stick model of triethylene glycol dimethacrylate (TEGDMA)
- Composition of dental cements.......................................................................
- Termination highlighting the unreacted double bonds
- Monomer composition......................................................................................
- Content of CQ
- Microscopic image of prosthesis
- Typical size and volume of cube.....................................................................
- Example of tether..............................................................................................
- Example of tether..............................................................................................
- Example of tether..............................................................................................
- Example of tether..............................................................................................
- Example of tether..............................................................................................
- Example of tether.............................................................................................
- Continuous organic phase................................................................................
- Non-continuous phase
- Process of how crosslinking occurs
- DDM alters nanocomposite............................................................................
- Formation of nanocomposite
- Change of setting shrinkage of composite resin..........................................
- Monomers used in Okamura’s study
Anatomy of tooth
We always manage to understand by means of research how our world, the things which are in our environment and most importantly how our bodies work to best explain to each other and ourselves what is happening chemically. Chemists and material scientists have conducted research to allow something as small as a cavity in your mouth to be filled with some sort of material, which is referred to as a composite, resin, and/or dental fillers in the dental profession. Cement is a non-metallic substance which hardens to act as a base, liner, filler, material, or adhesive to bind devices and prostheses to tooth structure or to each other. 1, 2^ A composite is a solid formed from two or more phases that have been combined to produce properties superior to or intermediate to those of the individual constituents. Fillers are best described as a combination of organic and inorganic resin particles that are designed to strengthen a composite, decrease thermal expansion minimize polymerization shrinkage and reduce the amount of swelling caused by water sorption. Not being satisfied with the quality of materials currently being used, scientist are hard at work attempting to find better materials which can be used to fill voids in teeth. This research paper briefly explains the various materials used to fill caries (cavities), chemically examines a popular monomer used in the dental profession and current research being conducted to bring about new changes in the process of treating caries. To best understand how cavities form, the anatomy of the tooth needs to be understood. Refer to the figure 1.^3
Figure 1: Tooth anatomy
The anatomical structures relevant to having a clear understanding of this paper are the dentin, pulp and enamel. The function of dentin is to act as a mechanical buffer between dead and living substances, and thus, between mechanical hard and soft material. Pulp is the vital layer of the tooth which delivers needed nutrients and blood supply to the tooth for growth and development.^4 The enamel is the primary armor of the tooth. It is one of the hardest biological substances is the enamel. Enamel has hardness greater than that of bones. The Knoop Hardness Number assigned to enamel is in the range of 340 to 431 kg/mm^2.^2 Enamel is entirely composed of calcium salts which are important in composite bonding process.
Formation of Caries (cavities)
Figure 2: Steps to tooth decay.^5
The figure above shows the tooth decay process from absolute healthy tooth to a fracture. The first tooth is a health tooth. The second tooth shows the first sign of demineralization of the enamel layer. The third tooth shows the results of enamel breakdown. The forth tooth has had an amalgam filler applied to it; however, the demineralization of the enamel has not been addressed which are visible in the fifth tooth and leading to a fracture. Had the demineralization been address the tooth could have been saved from further decay. The mechanism which causes the demineralization of the enamel layer is shown below.
Figure 3: Demineralization of Enamel
It is at the surface of the tooth or the enamel which is the hardest and most mineralized substance in the body and where the formations of caries occur. Caries are an infectious disease that destroys the tooth which is caused by bacteria and carbohydrate containing foods.^6 Bacteria forms on and around teeth in the form of a thin bio-film known as plaque which is made up of millions of bacteria which adhere to the tooth’s surface. Not all bacteria contribute to the formation of teeth caries. Streptococcus mutants, lactobacillus casei, acidophilus and actinomyces naeslunddii are the common carie causing bacteria.^7 These bacteria seek carbohydrates to survive and produce acid in the mouth. After eating sugary foods and after brushing your teeth, glycoproteins which are a combination of proteins and carbohydrate attach to teeth surface where plaque begins to build. While the plaque is forming, streptococci are hard at work forming
carries. The figures below^8 show the formation of the lactic acid. This is the acid which causes the pH on the tooth’s surface to drop. Before the intake of any foods, the pH level in the mouth is slightly more acidic than water, (6.2 – 7.0).^9 When the pH range is between 5.2-5.5, the enamel begins to be dissolved and the exposure of these foods promotes an acid attack of approximately twenty minutes after eating. The millions of bacteria that reside on the surface of the teeth ferment the sugar we intake to form lactic acid which in-turn attacks the enamel. The demineralization of the enamel is actually caused by the hydrogen ion produced by the lactic acid.
Figure 4: Sucrose^8
Figure 5: Hydrolysis of Sucrose^8
low concentration of amalgam (before 1963)^11 , caused the amalgam to weaken via corrosion because they contained gamma 2 phase (Ag 3 Sn), as shown in figure 11.^2
Figure 8: Restored and repaired amalgam restoration.^11
(The American National Standards Institute (ANSI) along with the American Dental Association (ADA) requires that amalgam alloys are mainly comprised of silver and tin.^2
Figure 9: Elements used to make amalgam.^1
The specific amounts of elements are not exactly mandated which is seen by comparing figure 9 and the following. Amalgam is comprised of mercury, and alloy power containing 40 to 70% of silver, 12 to 30% tin, 12 to 30% copper, 1% zinc and either 0.5% of palladium or indium10, 11.
Zinc is incorporated into the amalgam to improve its clinical performance.12-14^ Dental amalgam with mercury are described by the metallurgical phases (silver-tin phase diagram)5^2.
Figure 10: Symbols and Stoichiometry of Phases that are Involved in Setting of Dental Amalgams are referred to the mixture of the two metallic elements by a Greek letter.^2
The Greek letters correspond to the symbols found in the phase diagram for each alloy system^1. Amalgam fillings are extremely durable, long lasting and not likely to break. Amalgams have been known withstand multiple years of chewing stress. The silver- tin phase diagram indicates that when an alloy contains 27% tin (Sn). Tin is cooled below 480oC, Ag 3 Sn the gamma phase in the diagram is produced. Silver-tin compound is a key compound in this specific amalgam which combines with mercury to obtain the properties and characteristics sought. The silver-tin compound forms over a very narrow composition range. The silver content for silver-tin amalgam is 73%, the tin content is approximately 26% and 30% of the remaining elements used for the silver-tin amalgam are silver, copper, and zinc. If the tin concentration is less than 26%, the β phase which is a solid solution of silver and tin forms. Different varieties of amalgam are obtained by slightly altering the amount (percentage concentration) of tin or palladium. In one type of amalgam 5% indium is replaced by 5% of tin and in another type contains <1% palladium. Corrosion resistance and mechanical
properties are enhanced by the addition of palladium in amalgams .
Figure 11: Silver-tin phase diagram^2 (λ is also referred to as Ag 3 Sn. β is the solid solution of silver and tin. )
Amalgation (the process of mixing liquid mercury with one or more metals or alloys to form an amalgam) occurs when mercury contains the surface of the silver-tin alloy particles. The various other elements (copper, zinc, gold, mercury, palladium, indium and selenium)^1 are not exactly specified; however they must be in concentrations less than the concentrations of tin and silver.
Figure 12: Disposal Amalgam vial.
Amalgam is made available to dentists in a disposable vial as shown above. The mercury is kept separate from the powdered particles and the mixing pellet by a thin film. The amalgam is prepared by a method referred to as trituration (mixing of mercury with the powder particles). When the amalgam is triturated it has the consistency similar to that of a paste. When the triturated amalgam is removed from the vial it may be further worked by the dentist using a spatula. The dentist is required to mold the surface of the amalgam filling to reduce the tensile stress caused by biting forces. Amalgams are self sealing, when amalgam is applied to the tooth, corrosion occurs which fills microscopic voids between tooth and filling.
Figure 13: The general amalgamation reaction. (λ 2 is also referred to as Sn7-8Hg. λ is also referred to as Ag 3 Sn. β is the solid solution of silver and tin. )
The physical properties of the hardened amalgam depend on the concentrations of each microstructural phase. If the percentage composition of tin is >30% or <26% it is detrimental to the amalgam. The source of amalgam’s strength is due to the Ag 3 Sn rather then the tin. The setting time or the amount of time required to fill the carie is shortened by increasing the silver content. Creep resistance is also better when Ag 3 Sn is used rather than amalgam with a higher tin content
degree of marginal deterioration, hence the higher the creep magnitude, the greater the degree of marginal deterioration.^16 As reported by Mahler and group^17 the two major factors of corrosion and creep are the determinants of the amalgam behavior which are best explained by the final concentration of mercury. The higher the concentration of mercury in an amalgam increases the possibility of the creep. Creep is not a good predictor of marginal fracture. As you can clearly see in figure 5, the amalgam fillings are easily identifiable in one’s mouth. In clinical practice, healthy tooth must be removed to allocate the needed space for the amalgam filling to hold it securely in place. Amalgam fillings can corrode over time which may lead to slight discoloration of the area of contact to the amalgam. Traditional amalgam fillings do not necessarily bond to teeth but rather sit in the enlarged cavity which explains the reason of why healthy tooth is required to be removed. Sometimes people may be allergic to mercury or be concerned about its effects. The mercury in amalgam has a very small tendency to vaporize when chewing of food especially hard foods occurs. Research supports the amount of mercury exposed from fillings is comparable to what people get from other sources in the environment.^18 To address these concerns alternative filling are also available for slightly higher cost and require a lengthier setting time.
Figure 17: Estimated human mercury exposure reported in
1991.^18
Composites Resins and Cements Composite resins are a mixture of plastic and fine glass particles and are generally applicable for small and large fillings for front teeth or the visible parts of teeth & now by popular demands rear teeth. Composite resin tends to hold up for approximately five years. Composite fillings or inlays are not noticeable since they can be matched to the tooth color selected by the dentist. Such a filling can be completed in one visit and an inlay may require two visits to complete. Composite fillings bond directly to the tooth via ionic bonding which makes the tooth structurally stronger than the amalgam which is a filling pushed into the carie. Less drilling is involved with the composite fillings because it is not necessary to create space to hold the filling securely. The bonding process holds the composite resin in the tooth. Often times indirect composite resin can are combined with glass ionomers to provide the benefits of the two materials. Composites are highly cross-linked polymeric materials reinforced by silicates or resin particles and a coupling agent (silane). The γ- methacryloxypropyltrimethoxysilane is subdivided into three parts in the figure below; M referring to the unsaturated
methacrylate group capable of copolymerizing with the unfilled resin of a composite. The R refers to the spacer ((CH 2 ) 3 ) group. The X refers to the OSi(OH) 3 or the group capable of chemically reacting with the surface. These three sub units combined together making up the M-R-X structure of the bonding agent.
Figure 18: γ-methacryloxypropyltrimethoxysilane, a typical silane which is used as a coupling agent with composite fillings.
The commonly used dental composite is bis-GMA.
Figure 19: 2-methyl-2-Propenoic acid (1- methylethylidene) bis (4,1-phenylenoxy-2- Hydroxy-3,1-Propanediyl)) (Bis-GMA, Bowen’s Resin)
Bis-GMA is an aromatic ester of dimethacrylate which is synthesized from ethylene glycol (epoxy resin) and methyl
methacrylate. The core of the two aromatic groups reduces its ability to rotate during polymerization. The two diagrams below show the steric stress caused by the two aromatic rings where the chain is forced to bring its two methacrylate groups at opposite ends of the chain together.
Figure 20: Stick model of bis-GMA.
Figure 21: Ball and stick model of bis-GMA.
Like bis-GMA, all methacrylate monomers must be diluted because of their viscosity. Diluting methacrylate monomers brings the viscosity of the composite to a consistency that is workable for the dentist. Methacrylate monomers are diluted with either ethylene glycol dimethacrylate (EGDMA) or triethylene glycol dimethacrylate (TEGDMA).
Composite resin materials contain numerous components in addition to the resin matrix, inorganic filler particles, and a coupling
Figure 25: Camphorquinone (CQ)^2
The camphorquinone brings the composite paste to a slight yellow contrast making it difficult for the dentist to obtain the proper shade to match the other teeth. The chemical activation is done at room temperature. An organic amine reacts with an organic peroxide to produce free radicals that attack the carbon double bonds allowing for the polymerization process to begin.
Figure 26: The light-cure process gets activated when the camphorquinone absorbs a quantum of blue light and creates an excited-state complex with the dimethylaminoethyl methacrylate (DMAEMA) which is an electron donating amine. The figure above shows the unshared pair of electrons being donated by the amines to the ketone groups in the camphorquinone. Now that the camphorquinone is activated it extracts a hydrogen from the alpha-carbon and decomposes into the amine and CQ free radicals. The CQ free radical is readily inactivated and in the photoinitiation only the amine free radicals act to initiate the addition polymerization reaction (see figure 20).^2
What’s free about free
radicals?
Why start a reaction in your mouth? There are three steps to free radical polymerization reactions which are initiation, propagation, and termination. Free radical molecules can be typically generated by a chemical, heat, visible light, ultraviolet light, or energy transfer from another compounds which acts as a free radical. In dentistry, chemical agents, heat, and visible light are used as the initiators to start the reaction. Chemical initiators are the most common used in the profession. Initiation is the first step in the free radical polymerization reactions. The initiation step is started off by an external energy source which breaks a bond to produce the free radical(s). Free radical can be an atom or a group of atoms (a compound) with an unpaired shared electron that is used to initiate the sequences of reaction. R• can be any free radical. To better explain the initiation stage, the disassociation of hydrochloric acid to hydrogen and calcium is shown in the reaction below.
H Cl H Cl
Heterolytic Bond Cleavage
Figure 27: Heterolytic Cleavage
In the above reaction mechanism, the covalent bond in the hydrochloric acid is broken which results H+^ and Cl-. Two bard curved arrow is used to point to the chlorine in the reaction to indicate that chlorine will have two unshared electrons. When electrons are distributed unevenly, in the initiation stage, such a reaction is referred to as a heterolytic bond cleavage.
Cl Cl
Homolytic Bond Cleavage
Cl Cl
Figure 28: Homolytic Bond Cleavage
The disassociation of molecular chlorine to produce 2Cl• is a homolitic bond cleavage. Notice that the single barbed arrows are used to show the distribution of the electron pair. In a homolytic bond cleavage, once the bond is broken, one electron is distributed to each of the chlorines. Your dentist uses one of three types of initiators to start a free radical polymerization reaction.
O
O
O
O
O
O
2
benzoyl peroxide (^) benzoylradical
heat
Figure 29: Benzoyl peroxide Æ Denzoyl radical
Sufficient free radicals for polymerization may be produced at room temperatures by the reaction of a heat or chemical accelerator. Followed by this initiation stage is the quick addition of other monomer molecules to the free radical and the shifting of the free electron to the end of the growing chain.
The initiation of a methyl methacrylate molecule (Figure 20).
Figure 30: Initiation of methyl methacrylate molecule. As the unpaired electron of the free radical approaches the methyl methacrylate molecule, one of the electrons in the double bond is attracted to the free radical to form an electron pair and a covalent bond between the free radical and the monomer molecule. In the process of forming these bonds, free radical molecules are created.^2
The initiation of a dental resin, methyl methacrylate explained; as the unpaired electron of the free radical approaches the methyl methacrylate molecule (A & B), one of the electrons in the double bond is attracted to the free radical to form an electron pair and a covalent bond between the free radical and the monomer molecule (C & D). When this happens, the remaining unpaired electron makes the new molecule a free radical (D). The free radial-forming chemical used to start the polymerization is not a catalyst. This is because it enters into the chemical reaction and becomes apart of the final chemical compound. It is more accurately called an initiator because it is used to start the reaction. Many substances are able to produce free radicals and are potent initiator s for the polymerization of poly(methyl methacrylate) and other methacrylate-type resins used in dentistry. Another type of induction system is chemically activated at the ambient oral temperature. This type of system consists of at least two reactants that, upon mixing together, they undergo a chemical reaction that generates free radicals. Because of their abilities to react these reactants must be stored separate from each other. Remember that chemically induced systems consist of two or more parts. A Tertiary amine (activator) and benzoyl peroxide (initiator), which are mixed together to initiate the polymerization of a self-cured dental resins at
Figure 34: Second type of chain transfer.
When a propagating chain has interacted with a passive segment as formed in figure 23, another type of chain transfer has occurred. During this type of interaction the passive segment becomes active and the active segment becomes passive. 2
Figure 35: Termination.
The final stage in the free radical polymerization is termination. Termination is reached when all of the free radicals have interacted and formed covalent bonds. 2 Free radical polymerization is how composites reach their high durability. The polymerized resin is highly cross-linked because of the dysfunctional carbon double bonds. The polymerization process of the light-cured composites varies according to the distance from the composite to the light and the time of exposure to the visible blue light. Monomethacrylate and dimetharylate monmers polymerize by the initiation of the free radicals.^2 The free radicals which are required can be produced either by chemical
activation or by external energy activation such as heat or light. Considering the product produced at the termination stage, a hypothesis can be made that the carbonyl groups alter to create cross linkage, which leads to high resonance along the terminal ends of the highly conjugated structure. The aromatic rings at either terminal ends provide the composite resin tensile strength and make the resin bite worthy. Composite resins have become stronger and more resistant to wear. Polymer research has not developed a composite with the similar characteristics to that of amalgam. The use of composite fillers increases the chair time of a patient by approximately 10 to 20 minutes or longer. When large carries have formed, composite fillings may not last as long as amalgam fillings. A major concern which has dentists resisting the use of composite fillings is their ability to shrink after curing. Often times, fillers can be added to reduce the shrinkage of the composite; however, shrinkage cannot be prevented.
You are probably familiar with cement and how it is used as a material in construction work. Just look around you when you are outside and bring your attention to the structures made of concrete. How is cement made? What are its chemical properties? What is happening chemically to make it hard and strong? These are some questions you probably though about but never had answered. Cement is the binding agent in portland cement concrete (PCC). PCC is an inorganic material, or a mixture of inorganic materials, that sets and develops strength by chemical reaction with water by forming
hydrates. This hydraulic powder type mixture solidifies when combined with water. There are several ways in which portland cement is manufactured. Regardless of which method is used, they all require similar chemical components and raw materials. Concrete is approximately 70% to 80% aggregate (filler material such as various grade rocks and/or sand) depending on which brand and type of concrete cement. The chemical components limestone (CaCO 3 ), clay, shale (2SiO 2 y Al 2 O 3 ), iron oxide (Fe 2 O 3 ), silica sand (SiO 2 ).20, 21^ The afore mentioned materials are placed in a kiln and heated approximately 1400 to 1700oC. 3CaO ● SiO 2 , 2CaO ● SiO 2 , 3CaO ● Al 2 O 3 , 4CaO ● Al 2 O 3 ● Fe 2 O 3 , are formed when the raw materials is heated to such extreme temperature which allows for them to react chemically. The product at the end of this heating process is the cement which is available for purchase at your local hardware store.
Figure 36: Cement Factory
The described cement manufacturing process can be reviewed in depth at http://www.cement.org/basics/images/flasht our.html.^22
Figure 37: Cement hydration^21
Figure 27 provides a visual representation of cement hydration. The process begins with dissolution of grain particles followed by a solution of ionic concentration. Then compounds begin to form and upon reacting the point of saturation, solids begin to precipitate out as the products of the hydration process.^21 A chemical reaction occurs when water and cement are mixed together, in chemistry we refer to this type of reaction as a hydration. Cement hydration is an exothermic reaction. The chemical compounds that harden the quickest are tricalcium aluminate and tetracalcium aluminoferrite. Gypsum is added to prevent the quick curing of the cement caused by tricalcium aluminate. Heat is generated by the hydration of tricalcium aluminate. The color of cement is due to tetracalcium aluminoferrite. Tetrachlcium aluminoferrite is used to vary the composition of cement. The hydration of tricalcium silicate provides the cement strength while it cures (hardens). The dicalcium silicate is responsible for the
