Expert Level of Dental Resins - Material Science & Technology: Detailed discussion of the formulation, production and properties of dental resins and dental resin composites.

By Ralf Janda

Resin materials are broadly used in dentistry for almost all indications, and they will gain even more importance in the future. Especially the increasing performance and efficiency of the CAD/CAM technology and 3D-printing open possibilities to use resins which were not used up to now in dentistry. Besides dentists, dental students or dental technicians, there are many other specialists such as researchers, material scientists, industrial developers or experts of adjoining professional disciplines who are technically engaged in dental resins. The "Expert Level" is the third book of the series "Dental Resins - Material Science & Technology". The "Expert Level" includes all information and data presented in the "Basic Level" and "Advanced Level" of this series, but enormously expands the knowledge base. From a total database of 8.198 references, 1.707 were selected and used for this textbook. It comprises more than 1,000 manuscript pages, 384 figures and 124 tables. The "Expert Level" describes very accurately and comprehensively all details of the material science and technology of dental polymers and composites. Furthermore, their production methods and applications are discussed in detail. Therefore, this book is a unique treatise of the complete present knowledge about dental resins and dental resin composites. This includes the discussion of the - raw/starting materials together with the explanation and presentation of their chemical structures and properties, their CAS Numbers and the names of the manufacturers. - amounts of the raw/starting materials usually used to formulate the finished products. - important material and toxicological properties of the starting materials and the finished products. - detailed description of the production processes of essential starting materials such as the syntheses of essential monomers, the silanization of inorganic fillers or the manufacturing of unfilled and filled splinter polymers. - detailed description of the formulation and the properties of the finished products. Furthermore, for many commercial endproducts rather detailed formulations as well as the exact production processes are described. All ISO standards that are relevant for dental resins are listed, too. Furthermore, many essential methods to test the mechanical, chemical and toxicological properties are also presented and explained. The "Expert Level" enables every scientist with a good chemical knowledge not only to understand how dental polymers function, but also to develop new and improved products.

• Language: English
• Publisher: tredition
• Release date: Sep 12, 2022
• ISBN: 9783347712928

Ralf Janda

Ralf Janda was born in 1953 in Berlin. He obtained his Abitur (secondary school-leaving examination in 1973 and pursued chemistry at the Free University Berlin (FUB) from 1973 to 1978, thereby obtaining the degree Diploma-Chemist (summa cum laude). While working as a scientific assistant and researcher at the FUB he wrote his doctoral thesis and graduated in 1979 as a natural science doctor, Dr. rer. nat. (summa cum laude). His professional career as a scientific assistant and lecturer at the FUB came to an end in 1980. Ralf Janda also joined the dental industry in this year as head of research and development. He worked for many internationally leading dental companies (Kulzer GmbH, Germany, Degussa AG-Dental Division, today Degudent/Dentsply GmbH, Germany, Dentsply/Detech GmbH, Germany, Dentsply INC., USA, Dentaurum GmbH & Co. KG, Germany) in different leading positions as head of: R&D, production, quality assurance, dental technology, worldwide project leader until 2003. During this time, he was a member of many dental standard commissions, and from 1987 to 2000, he was also a member of the drug commission A at the drug institute of the Federal Republic of Germany. In 2003, he joined the cosmetic industry specialized on light-curing artificial nail products and stayed there until 2017. In addition to his professional pursuits, Ralf Janda has maintained a lengthy and extensive scientific career as a researcher and lecturer at numerous universities, beginning at the FUB in 1978. From 1988 to 1990, he was a lecturer at the Faculty of Material Sciences of the Technical University Berlin, where he taught resin composite materials. From 1991 to 1999, he worked as a researcher and lecturer for non-metallic dental materials at the dental department of the Medical Faculty of the Johann Wolfgang Goethe-University, Frankfurt/M. In 1992, he obtained his Habilitation (qualification for a teaching career at universities) and the degree Privatdozent (associate professor) in dental material science at the same university. From 1999 to 2004, Ralf Janda was Privatdozent at the Center of Dental Medicine of the Medical Faculty, Charité, Humboldt-University Berlin. From 2004 to 2021 he worked as a researcher and lecturer at the dental clinic of the Medical Faculty of the Heinrich Heine University, Düsseldorf. In 2006, he was appointed as apl. Professor (adjunct professor) in dental material science. Since 2021 he put his focus on writing textbooks about dental materials.

Book preview

Preface - 1st Ed./2nd Ver. Expert Level

Editorial revisions, corrections of write errors, and an optimization of the bibliography were done in the 2nd version of this e-Book.

The Expert Level is the third book of the series Dental Resins - Material Science & Technology. From a total database of 8,198 references, 1,707 were selected and used for this textbook; it comprises more than 1,237 manuscript pages, 324 figures, and 102 tables. The Expert Level describes very accurately and comprehensively all details of material science and technology of dental resin polymers as well as their applications and thus enormously extends the knowledge base of the Basic and the Advanced Level. This includes the discussion of the:

- raw/starting materials (CAS Numbers, manufacturers) and the presentation of their chemical structures.

- material and toxicological properties of the raw/starting materials and the finished products.

- production processes of some important starting materials (e.g., syntheses of important monomers, silanization of inorganic fillers, manufacturing of unfilled and filled splinter polymers).

- detailed formulations and the production processes of the finished products.

Furthermore, for many well-known commercial end products the detailed formulations as well as the exact production processes are described. However, the trade names are not revealed.

Finally, I think that the Expert Level enables every scientist with good chemical knowledge not only to understand how dental polymers function but also to develop and formulate improved products.

Many thanks for your interest and best regards

Ralf

November 2023

Preface - Book Series

Resin materials are broadly used in dentistry for almost all indications, and they will gain even more importance in the future. Especially, the increasing performance and efficiency of CAD/CAM technology and 3D-printing open possibilities to use resins not used up to now for dental applications. Apart from dentists, dental technicians, dental students, teachers of dental universities/schools, postgraduate students and PhD candidates, there are many other specialists such as researchers, material scientists, industrial developers or experts of adjoining professional disciplines who are technically engaged in dental resins. Mainly three reasons are responsible for this interest:

a) many people dealing with dentistry feel a large desire for more profound knowledge in dental resins

b) the knowledge of many specialists is requested to develop, to investigate, to test and to evaluate dental resins

c) dental resins offer very sophisticated, highly developed properties so that they are also used in other disciplines for other purposes or are the base to develop tailor-made products for other exceptional non-dental applications.

The idea of this e-Book is to present a three-level textbook dealing with material science and technology of dental resins:

a) The Basic Level addresses students, dental technicians, teachers or all those interested in dental resins. The Basic Level gives a comprehensive insight into the chemistry, physics, and toxicology of dental resins and their technical application.

b) The Advanced Level broadens the information about the Basic Level significantly and mainly addresses teachers of dental universities/schools, postgraduate students, PhD candidates, researchers, material scientists, industrial developers or experts of adjoining professional disciplines.

c) The Expert Level gives a very profound insight into the science of dental resins and mainly addresses scientists doing research on dental resins, industrial developers or scientists of adjoining professional disciplines who are forcefully interested to become also specialists in dental resin material science. The Expert Level also describes the industrial processes that are used to manufacture dental resins. Furthermore, the exact formulations for some dental products are given; this includes know-how that has never been published before as far as the author knows.

Contrarily to print books, it is the great advantage of e-Books that improvements, corrections, additions, or enhancements can be done swiftly so that new improved editions can be produced and distributed rapidly and cheaply. Therefore, the e-Book is the ideal format to update the content immediately whenever errors or mistakes must be eliminated, or the scientific progress makes it necessary. It is the desired and planned scenario that the content of this e-Book will not become obsolete as fast as it usually happens with conventional print books, but will be refreshed in shorter periods of time.

Illustrations and tables will increase in number with each level. The information they give is - hopefully - clear and understandable, but certainly, they will not become prettier or colored. This is a low-cost book and everything is done keeping costs to a minimum.

The author is aware that there will be errors, inaccuracies and ambitiousness, but hopefully no incorrect or even misleading information in the text despite all the care taken. The honorable readership is kindly asked for understanding, and the author will be deeply grateful for any hints and proposals to improve the content of the book or the book at all. Therefore, every type of constructive criticism will be highly appreciated.

Having said all this, I hope you will enjoy this e-Book, and you will get the information that is helpful and valuable for you and your work.

Many thanks and best regards

Ralf

Literature/Trademarks/Other

Not all the literature used to write this book is specifically cited. Common dental, chemical, or material science knowledge taken from textbooks is not specifically cited in the text. Such textbooks are.

- dentistry and dental materials [1-20]

- chemistry [21-46]

- adhesives and adhesive technology [47-50]

- material science [50-52]

Furthermore, information, figures, or tables taken from the author’s sole publications are not specifically cited; these are [53-79].

Information (terms, definitions, etc.) deriving from scientific organizations is not always specifically cited; these organizations are [80-83].

Specific information given is specifically cited.

Product names are not specifically marked as registered, even if they are so. Principally, brand names are only used when they are important in connection with the described subjects. This might be the case when only one product of a specific product category is available. Apart from that, the representatives of product categories which are presented in tables or graphics are anonymized.

Introduction

Apart from metals, alloys and ceramics, plastics and composite resins have become one of the most important material categories in all areas of daily life, such as engineering, electronics, building and construction industry, car industry and many other industries, as well as in medicine and dentistry. In 1922, Hermann Staudinger discovered these high molecular compounds and called them macromolecules [84]. This was the start of a new, until then, unknown chemistry called polymer chemistry. The development of numerous polymeric materials and combinations thereof with other organic or inorganic substances or materials gave birth to a huge number of advanced materials with exceptional properties.

In the early years, plastics were considered to be cheap and inferior materials, but today composite resins and high-performance plastics are very valuable and indispensable in all industries. The most significant aspect of the resin materials’ breakthrough is certainly the fact that for nearly every usage custom-made, often also called tailor-made, products can be developed and finally provided. Definitely, more and more new, until now, unknown, resins, or resin composites will be tailor-made for further or today even unknown applications in the future.

Resin materials (plastics, composite plastics, composite resins, resin composites) are high molecular mass products (polymers). They are manufactured by transformation of naturally occurring or by synthesis from low molecular mass substances (monomers). These low molecular mass substances (monomers) are the smallest multiple recurring units building the high molecular mass substances (polymers). The properties of each of the resulting polymers depend on how the monomers are linked, on their chemical structure, as well as on the spatial configuration of the formed macromolecules. Polymers or macromolecules do not have an exact but an average molecular mass because the single chains building the polymer/macromolecule are growing randomly and not in a well-defined manner.

Abbreviations and Chemicals

Abbreviations important in the context of this book or the dental literature are given in accordance with IUPAC [80-83]. The information given here is essential for all levels of this book series.

Monomers

4-Met = 4-methacryloyloxypropyl trimellitic acid (Fig. 308)

4-Meta = 4-methacryloyloxypropyl trimellitic anhydride (Fig. 308)

AA = acrylic acid (Fig. 35)

BADEP = N,N'-diethyl-1,3-bis(acrylamido)-propane (Fig. 262)

BDMA = butanediol dimethacrylates (Fig. 39)

Bis-EDMA(2) = bis-EMA(2) = 2,2-bis[4(3'-methacryloyloxy)ethoxyphenyl)]propane (Fig. 38)

Bis-GMA = 2,2-bis[4(3'-methacryloyloxy-2'-hydroxy)propoxyphenyl]propane (Fig. 38)

BMDU = methylene-4,4’-N,N’-bis-cyclohexylamine carbamate of 3-methacryloyl-2- hydroxypropoxy benzene (author’s knowledge) (Fig. 39)

BMP = bis-(2-methacryloyloxy)ethyl phosphate (Fig. 313)

DiPEPA = dipentaerythritol monohydroxy pentaacrylate (Fig. 36)

DDMA = 1,12-dodecandiol dimethacrylate (Fig. 38)

EDMA = ethylene glycol dimethacrylate (Fig. 39)

EHA = 2-ethylhexyl acrylate (Tab. 17)

EMA = ethyl methacrylate (Fig. 37)

Epoxy acrylate oligomer = 2,2-bis[acryloyloxy(2'-hydroxypropyloxy)phenyl]propane (Fig. 35)

FurfurylMA = Furfuryl methacrylate (Fig. 37)

GDMA = glycerol dimethacrylate (Fig. 306)

GPDM = glycerol phosphate dimethacrylate (Fig. 307)

GPTA = glyceryl propoxy triacrylate = 3-[2,3-bis(3-prop-2-enoyloxypropoxy)propoxy]propyl prop-2-enoate (Fig. 36)

HDDMA = 1,6-Hexanediol dimethacrylate (Fig. 38)

HEMA = hydroxyethyl methacrylate (Fig. 37)

HPMA = hydroxypropyl methacrylate (Fig. 37)

HPPMA = 2-hydroxy-3-phenoxypropyl methacrylate (Fig. 73b)

i-BuMA = iso-butyl methacrylate (Fig. 37)

MA = methyl acrylate (Fig. 35)

MASA = N-methacryloyl-5-aminosalicylic acid (Fig. 308)

MDP = 10-methacryloyloxydecyl dihydrogen phosphate (Fig. 307)

MDTP = MDTP = methacryloyloxydecyl dihydrogen thiophosphate (Fig. 285)

MEP = 2-methacryloyloxyethyl dihydrogen phosphate (Fig. 307)

MEPP = 2-methacryloyloxy ethylphenyl phosphate (Fig. 307)

MMA = methyl methacrylate (Fig. 37)

MPS = 3-(methacryloyloxy)propyl trimethoxysilane (Fig. 161)

NMA = N-acryloylaspartic acid (Fig. 308)

NMG = N-methacryloylglycine (Fig. 308)

NP8EO8A = nonyl phenol (EO)8 acrylate (Fig. 35)

PEG-400-DMA = polyethylene glycol 400 dimethacrylate (Fig. 39)

PENTA = dipentaerythritol pentaacrylate monophosphate (Fig. 307)

PETMP = pentaerythritol tetra(3-mercaptopropionate) (Fig. 10)

Phenyl-P = 2-methacryloyloxyethyl phenyl phosphate (Fig. 313)

PMDM = pyromellitic dianhydrate dimethacrylate (Fig. 313)

PMGDM = pyromellitic dianhydride glycerol dimethacrylate adduct (Fig. 309)

TEGDMA = triethylene glycol dimethacrylate (Fig. 38)

TMP9EOTA = ethoxylated trimethylolpropane triacrylate (Fig. 36)

TMPMP = trimethylolpropane tris(3-mercaptopropionate) (Fig. 10)

TRIM = 1,1,1-trimethylolpropane trimethacrylate (Fig. 39)

TTEGDMA = tetraethylene glycol dimethacrylates (Fig. 38)

UDA = 7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5,12diazahexadecane-1,16-dioxy-diacrylate (Fig. 35)

UDMA = 7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecane-1,16-dioxy-dimethacrylate (Fig. 38) VBATDT = 6-(4-Vinylbenzyl-n-propyl)-amino-1,3,5-triazine-2,4-dithiol (Fig. 284)

Thermoplastics/Duromers

ABS = acrylonitrile butadiene styrene copolymer

APE = aromatic polyester

CA = cellulose acetate

E/P = ethylene propylene copolymer

EP = epoxy polymer, epoxide

ETFE = ethylene tetrafluoroethylene copolymer

EVA = ethylene vinyl acetate copolymer

HDPE = high density polyethylene

HMWPE = high molecular weight polyethylene

LDPE = low density polyethylene

LLDPE = linear low density polyethylene

MF = aminoplastic

PA = polyamide

PAA = polyacrylic acid

PAN = polyacrylonitrile

PBTP = polybutylene terephthalate

PC = polycarbonate

PDMS = polydimethylsiloxane

PE = polyethylene

PEEK = polyaryletheretherketone

PEMA = polyethyl methacrylate

PEI = polyetherimide

PEO = polyethylene oxide

PES = polyethersulfone

PETP = polyethylene terephthalate

PF = phenol formaldehyde resin, phenoplastic

PI = polyimide

PMMA = polymethyl methacrylate

POM = polyoxymethylene

PP = polypropylene

PPE = polyphenylene ether

PPO = polyphenylene oxide

PPS = polyphenylene sulfide

PS = polystyrene

PSU = polysulfone

PTFE = polytetrafluoroethylene

PU = polyurethane

PVAC = polyvinyl acetate

PVAL = polyvinyl alcohol

PVC = polyvinyl chloride

PVC-P = soft PVC - plasticized

PVC-U = hard PVC - unplasticized

Q = silicone rubber, silicone elastomer, polysiloxane rubber

SAN = styrene acrylonitrile copolymer

SB = styrene butadiene copolymer, high impact PS = HIPS

SP = saturated polyester

TPU = thermoplastic polyurethane

UF = urea-formaldehyde resin

UHMWPE = ultra high molecular weight polyethylene

UP = unsaturated polyester

VPE or XLPE = cross-linked polyethylene

Elastomers/Rubbers

ABR = acrylate butadiene rubber

AU = polyester urethane rubber

BR = butadiene rubber, polybutadiene

EPR = ethylene propylene rubber

E-SBR = styrene-butadiene rubber

EU = polyether urethane rubber

FKM = fluoro rubber

IIR = isoprene isobutene rubber = butyl rubber

IR = cis-1,4-polyisoprene = synthetic rubber

NBR = acrylonitrile butadiene rubber = nitrile rubber

NCR = acrylonitrile chloroprene rubber

NIR = acrylonitrile isoprene rubber

NR = natural rubber

PBR = vinylpyridine butadiene

PDMS = polydimethylsiloxane

PSO = polysiloxane

PSR = polysulfide rubber

Composite Resins/Composite Plastics

AFP = asbestos fiber-reinforced plastic

BFK = boric fiber-reinforced plastic

CFK = carbon fiber-reinforced plastic

FK = fiber-reinforced plastic

GFK = glass fiber-reinforced plastic

MFK = metal fiber-reinforced plastic

MWK = metal whiskers fiber-reinforced plastic

SFK = synthetic fiber-reinforced plastic

UD = unidirectional fiber-reinforced plastic

Initiators/Synergists/Catalysts/Stabilizers/Antioxidants

A1010 = pentaerythritol tetrakis[3-(3,5-di-tert. butyl-4-hydroxyphenyl)-propionate] (Fig. 184)

benzpinacol (Fig. 102)

BDK = benzyl dimethyl ketal (Fig. 121)

BEA = 2-n-butoxyethyl-4-(dimethylamino) benzoate (Fig. 131)

BHT = 4-hydroxy-3,5-di-tert-butyltoluene (Fig. 184)

BPO = DBPO = dibenzoyl peroxide (Fig. 100)

CQ = camphorquinone (Fig. 118)

copper naphthenate = Cu(II)-naphthenate mineral spirits (8 mass% Cu) (Fig. 111)

DHEPT = N,N-bis(hydroxyethyl)-p-toluidine (Fig. 108)

DHPBZ = 2,5-dimehylhexane-2,5-diperbenzoate (Fig. 101)

DMAEMA = DMEM = 2-(dimethylamino)ethyl methacrylate (Fig. 131)

DMAG = N,N-dimethylaminoglutethimide (Fig. 103)

DMAPAA = 4-N,N-dimethylaminophenylacetic acid (Fig. 103)

DMB = 2-(dimethylamino)ethyl benzoate (Fig. 131)

DMPT = N,N-dimethyl-p-toluidine (Fig. 103)

DMSX = N,N-dimethyl-sym-m-xylidine (Fig. 103)

TBPEH = t-butyl-per-2-ethylhexanoate (Fig. 101)

TBPIN = t-butyl-perisononanoate (Fig. 101)

EPD = EDMAB = ethyl-4-(dimethylamino) benzoate (Fig. 131)

EGDPM = 3-diethylamino-propionate methacrylate (Fig. 131)

Ester chloride = beta-phenylethyl dibutylamino acetic acid ethyl ester chloride (Fig. 111)

gamma-terpinene = p-mentha-1,4-diene (Fig. 191)

HQ = hydroquinone (Fig. 178)

HQME = hydroquinone monomethyl ether (Fig. 180)

MBF = methyl benzoylformate (Fig. 123)

NDMH = N,N'-dimethyl,-N,N'-di(methacryloxy ethyl)-1,6-hexanediamine (Fig. 131)

P1116 = 2-hydroxy-1-(4-isopropylphenyl)-2-methyl-1-propanone (Fig. 123)

P1173 = 2-hydroxy-2-methyl-1-phenyl-1-propanone (Fig. 123)

P819 = phenyl-bis-2,4,6-(trimethyl benzoyl) (Fig. 123)

PBA = 5-phenylbarbituric acid (Fig. 109)

PMP = 1,2,2,6,6-pentamethylpiperidine (Fig. 131)

succinic acid (Fig. 111)

TBB = tri-n-butyl borane (Fig. 116)

TBPIN = t-butylperisononanoate (Fig. 101)

TEA = triethanolamine (Fig. 131)

tert. arom. amine = tertiary aromatic amine

TMBA = 1,3,5-trimethylbarbituric acid (Fig. 109)

TPO = (2, 4, 6-trimethylbenzoyl)diphenylphosphine oxide (Fig. 120)

TPO-L = ethyl-2,4,6-trimethylbenzoyl phenylphosphinate (Fig. 123)

UV81 = 2-hydroxy-4-n-octyloxi-benzophenone (Fig. 187)

UV90 = 2-hydroxy-4-methoxybenzophenone (Fig. 187)

UVP = 2-(2-hydroxy-5-methylphenyl)benzotriazole (Fig. 188)

Other

M = molecular mass [g mol-1]

mass% = percent by mass, often also called wt% = percent by weight

mol = molar mass [mol] is the mass of 1 mole of a given substance divided by the amount of the substance and is expressed in g mol-1. Example: 100 g of water is about (100 g)/(18.015 g mol-1) = 5.551 mol of water

mol% = percent of mole

SEM = scanning electron microscopy

TEM = transmission electron microscopy

vol% = percent by volume

Terms and Definitions

1 Chemistry/Polymer Chemistry

Terms and definitions important in the context of this e-Book or the dental literature are explained in accordance with the IUPAC definitions [80-83] or with the literature [29-32, 85, 86].

Additive: Any type of substance that is added in small quantities to a monomer, oligomer, or polymer to improve, alter, and stabilize or to change its properties in any requested direction.

Antioxidant: A substance that inhibits or reduces the oxidation of other molecules or macromolecules, respectively. Primary and secondary antioxidants are differentiated. Primary antioxidants (mostly sterically hindered phenols or amine derivatives of higher molecular mass) are radical scavengers, but secondary are not. Secondary antioxidants (sterically hindered phenols of lower molecular mass, organic phosphites or organic sulfides) decompose hydroperoxides to form stable alcohols and, thereby, chain branching can be avoided. It is the common purpose of all antioxidants to hinder or to diminish polymer degradation due to oxidative processes and to preserve the polymer’s properties.

Catalyst: Atoms, molecules or ions which diminish the activation energy with the result that a specific chemical reaction can occur. The catalyst does not participate in the reaction but exists before and after the reaction in the same chemical condition.

Comonomer: A second monomer added to the main monomer.

Constitutional unit: A species of atoms or atomic groups in a macromolecule, polymer, or oligomer.

Composite resin/composite plastic: A resin/plastic that contains organic and/or inorganic fillers in all kinds of shapes (fibers, splinters, platelets, crystals, spheres, ligaments, etc.).

Copolymer: A polymer derived from more than one species of monomer.

Copolymerization: Polymerization of more than one species of monomer in which a copolymer is formed.

Cross-linkers: Cross-linkers are multifunctional monomers which form covalent chemical bonds between two separately growing polymeric chains to form a firm polymeric network. For polymerization reaction at least bifunctional monomers are requested, for polyaddition and polycondensation the monomers must be trifunctional at least.

Degree of crystallinity: The percentage of crystalline amount in a thermoplastic polymer.

Degree of conversion: The percentage of monomers that polymerize and form the polymer.

Degree of cross-linking: Relates to the number of groups that interconnect two materials. It is generally expressed in mole percent (mol%).

Degree of polymerization: The number of monomeric units/repeat units in a macromolecule, an oligomer, or chain. For homopolymers the number of monomeric units corresponds with the number of repeat units. For copolymers this is not always true, and occasionally the degree of polymerization is defined as the number of repeat units. Considering polyamide 66 (PA 66), for instance, the repeat unit consists of two monomeric units (-NH-(CH2)6-NH-OC-(CH2)4-CO-) with the result that a chain of two thousand monomeric units have only one thousand repeat units.

Functional group: A group of atoms in a molecule which significantly determines the reactivity or properties of the molecule (e.g., double bonds, triple bonds, aromatic compounds and hydroxyl or carboxyl groups).

Homopolymer: A polymer derived from only one specific monomer.

Inhibitor = Stabilizer: A molecule which deactivates radicals to inhibit a premature or unintended free radical polymerization. Inhibitors/stabilizers act similar to primary antioxidants.

Initiator: One or more molecules or ions forming radicals under the influence of energy and, thereby, starting the free radical polymerization. The initiator takes part in the reaction and is consumed. In case the energy involved is light, the initiator is called photoinitiator or light-initiator, in case it is heat it is called thermal or heat initiator, and in case it is chemical energy it is called redox initiator.

Ligand: Atom, molecule, ion, or radical chemically bonded to a central atom.

Macromolecule/polymer molecule: A molecule of high relative molecular mass, the structure of which derives essentially from the multiple repetitions of molecule units with relatively low molecular mass.

Macroradical: A macromolecule which is a radical.

Matrix resin: Unpolymerized monomer/oligomer blend or polymerized material that may contain different types of fillers (organic or inorganic), initiators, catalysts, stabilizers, pigments or various types of other additives.

Molecule: Two or more identical or different atoms chemically bonded to each other.

Monomer molecule, functionality: It is differentiated between mono-, bi-, tri-, tetra- or penta-functional monomer molecules. Monofunctional molecules have one reactive group, bifunctional have two, trifunctional have three and so on reactive groups to run a polyreaction. Monomers with more than one functional group are also called multifunctional or higher functional monomers; they function as cross-linkers.

Monomer molecule: A molecule which can polymerize and contributes a constitutional unit to the structure of a macromolecule. In other words: the smallest molecule which repeats oneself during a polymerization to form a polymer/macromolecule.

Monomer: A substance composed of molecules, each of which can provide one or more constitutional units to a polymer.

Monomeric unit/monomer unit: The largest constitutional unit contributed by a single monomer molecule in a polymerization process to the structure of a macromolecule or oligomer molecule.

Oligomer molecule: A substance of intermediate relative molecular mass composed of a few or more constitutional units repetitively linked to each other. The properties of an oligomer vary with the addition or removal of one or a few of the constitutional units.

Oligomer: A substance composed of oligomer molecules.

Polymer: A substance composed of macromolecules.

Polyaddition: The process of converting a monomer or a mixture of monomers into a polymer by polyaddition reaction.

Polycondensation: The process of converting a monomer or a mixture of monomers into a polymer by polycondensation reaction.

Polymerization: The process of converting a monomer or a mixture of monomers into a polymer by free radical, anionic or cationic polymerization reaction.

Polymerization rate: Can be measured and mathematically expressed. The polymerization rate describes the kinetics/growth rate of the chain propagation.

Polymerization shrinkage (often only called shrinkage): (a) Volumetric Shrinkage: The percentage of volumetric change of the unpolymerized monomer, oligomer, or substance during the polymerization. (b)Linear Shrinkage: The percentage of linear change of the unpolymerized monomer, oligomer, or substance during the polymerization. The randomly distributed monomer or oligomer molecules move towards each other when polymerized (density increases) and, therefore, need less room. In other words: The density/specific weight of the polymer is higher than of the monomer.

Polyreaction: Any type of process converting a monomer or a mixture of monomers into a polymer.

Pre-polymer molecule: A macromolecule or oligomeric molecule that provides reactive groups for further polymerization and contributes more than one monomeric unit to at least one chain of the final macromolecule.

Pre-polymer: A substance composed of macromolecules or oligomer molecules having reactive polymerizable groups.

Radical (often called: free radical): An atom or molecule that contains an unpaired electron. Mostly, radicals are very reactive substances. They are usually formed when a covalent bond breaks to leave an unpaired electron on each of the two species created by the bond breaking. The symbol is R•; the dot symbolizes the free, unpaired electron.

Relative molecular mass (typically only called molecular mass, obsolete is molecular weight): The sum of the relative atomic masses of all atoms forming a molecule.

Residual monomer: Monomeric molecules of the same or of different species that do not participate in the polymerization but remain in the polymer in their original state.

Residual monomer content: Percentage of monomer that did not participate in the polymerization but remains unpolymerized in the polymer.

Resin/plastic: No consistent and comprehensive definition was found in the literature or the internet. Therefore, it is tried to combine what was found and to formulate a definition to meet the needs of this book and of dental material science. Resin/plastic materials are polymeric materials whose main components are organic or silicon organic macromolecules. These macromolecules can be made synthetically or by transformation of natural products. To become a resin or a plastic material, the macromolecule/polymer contains additional ingredients as for instance additives (stabilizers, UV-stabilizers, plasticizers, antioxidants), pigments, dies or fillers. The differentiation between resin/plastic and polymer is not always precise. Moreover, homogeneous polymers (e.g., polyethylene, polypropylene, polyvinylchloride or polyaryletheretherketone) are called resins or plastics in case their technical application and thus their properties as a product are considered. Organic macromolecular compounds have a carbon-carbon backbone but the silicon macromolecular ones, the so called polysiloxanes (trivial names: silicones, silicon rubbers) have a silicon-oxygen-silicon backbone with organic ligands or side chains.

Synergist = Accelerator: A substance that increases the polymerization rate. More generally: a substance that increases the rate of a chemical reaction. The synergist/accelerator takes part in the reaction.

UV-stabilizer: A substance that protects the polymer against UV-light.

2 Radiometry

Radiometry is the science and technology of radiation measurement of all wavelengths within the optical spectrum. The radiometric terminology is important to understand the performance of light-curing devices. The most essential radiometric terms and their definitions for dental curing-lights are taken from [87-90].

Illuminance: is measured in lux [lx] and describes the total luminous flux incident on a surface per unit area.

Irradiance: Is expressed in Watt per square centimeter [W cm-2] and describes the radiant power [W] an object receives per unit area [cm-2].

Radiant energy: Is expressed in Joule [J] and describes the energy from the light source delivered per unit time [W s-1].

Radiant exitance/radiant emittance: Is expressed in Watt per square centimeter [W cm-2] and describes the radiant power [W] emitted from a known surface area of a light source.

Radiant exposure: Is expressed in Joule per square centimeter [J cm-2] and describes the energy an object receives per unit area [cm-2].

Radiant power/radiant flux: Is expressed in Watt [W] and describes the radiant energy delivered per unit time [J s-1].

Spectral irradiance: Is expressed in Watt per square centimeter and nanometer [W cm-2 nm-1] and describes the irradiance [W cm-2] at each wavelength [nm] of an electromagnetic spectrum.

Spectral radiant power: Is expressed in Watt per nanometer [W nm-1] and describes the radiant power [W] at each wavelength [nm] of an electromagnetic spectrum.

Resin Materials in Dentistry

1 Introduction

Resin materials are of high interest and importance in dentistry. They are used for numerous applications such as dentures, partial dentures, relining of dentures, artificial teeth, fillings, inlays, crowns, bridges, temporary restorations, sealants, luting purposes, adhesives, impressions etc.

Charles Goodyear ran the vulcanization of natural rubber for the first time in 1839 and, as a result, the first resin material was born [64, 91-93]. This process served to manufacture denture bases in the following period of time. Although the natural rubber used for this purpose was pink-colored, the esthetic appearance of the denture was poor because of the rubber’s high opacity. Yet, this process was used until the thirties of the 20th century. Since approx. 1870, celluloid, synthesized from nitrocellulose and camphor, has been used for denture bases aside from rubber. Celluloid denture base materials entered the market in the USA under the trademarks Hecolite and Coralite [91, 94, 95]. Around 1900, phenolic resins, developed by Baekeland and, therefore, often called bakelite, with the trademarks Aldenol and Walkerit were also used as denture base materials [53, 64, 91, 95-97].

In 1934 Pierre Castan synthesized the first epoxy polymer resin (trademark: Epoxolon) in the laboratories of DeTrey Fréres Co. (today: DeTrey/Dentsply GmbH, Germany) in Switzerland while he was searching for an improved denture base material and obtained a patent in 1940 [53, 64, 91, 98]. Although they were never used for dentures, since then, epoxy polymers have been used for products of the highest quality demands. Other plastics like benzyl cellulose (trademark: Pertax), polyamides (trademark: Protenyl), polystyrenes (trademark: Polystein), polyvinyl chloride (trademark: Hekodent, Hewodent) or polyolefins (trademark: Odenta) had similar destinies. They were only used for a short period of time as denture base materials [39, 53, 64, 91, 95-97].

The crucial ascent of polymer chemistry started in the thirties of the 20th century with Otto Röhm’s development of methyl methacrylate (also called: methacrylic acid methyl ester or MMA) from which he synthesized via polymerization polymethyl methacrylate (also called: polymethacrylic acid methyl ester or PMMA) [91, 99]. PMMA has the well-known trademark Plexiglas. In 1936, PMMA determined the great breakthrough in dental materials. The dental technician Gottfried Roth mixed milled PMMA with its monomer MMA and stirred the mixture until it became dough-like. Then he processed it the same way as it was commonly done to manufacture rubber dentures. He pressed the dough between two halves of a plaster mold representing the denture and boiled the whole assembly in a water bath until the dough was hardened [91]. This was the first time esthetically satisfying dentures were obtained, and the method, as well as the materials, were patented in 1936 [91, 100]. This process was improved and optimized in the following years and decades, and it is broadly used to manufacture dentures up to now. Later developments substituted MMA by numerous newly synthesized methacrylates, dimethacrylates or multifunctional methacrylates with sometimes very high molecular masses. This led to wholly new high-performance composite resins that can be used for nearly almost all indications in the oral cavity.

Later PMMA/MMA composites [101-103] and newly developed high molecular mass methacrylates [104-106] were the base of modern resin-based filling materials and adhesives. R. L. Bowen laid the foundation stone for modern resin composite filling materials by his crucial research work and inventions, and thus revolutionized restorative dentistry [104-111].

It was, and it is still tried to use other polymers such as polycarbonates or polyacetals (also called polyoxymethylene or POM) to manufacture dentures. These products are processed via injection molding technique, but they did not succeed on the market and are only occasionally used.

POM and polyaryletheretherketone (PEEK) processed via CAD/CAM are used today to manufacture denture bases or suprastructures which are usually made from metal. Very likely other polymers will be used for dental purposes in the future due to the upcoming innovative processing techniques like CAD/CAM grinding or milling or 3D-printing.

Since 1955 the elastomeric polymers, the polysulfides, and in 1958 [112] the polysiloxanes (also called silicones) entered the dental market and were predominantly applied in the oral cavity for performing impressions. Today, the polysiloxanes dominate this segment. Since 1966 the polyether impression materials also took a great part of this segment [113, 114]. But polysiloxanes are also used in the dental laboratory for duplicating or embedding purposes.

2 Modern Dental Resins

Today, a very broad variety of polymeric materials and composite polymers are used in dentistry. Polymethacrylates, polyacrylates, epoxies and polysiloxanes cover the largest areas of application. It is most certain that newly developed monomers and polymers will be used to develop dental materials in the future.

Still, whatever kind of resin or composite resin it is or will be, they all have or will have the general composition schematically illustrated in figure 1. Composite resins consist of very many ingredients. The respective monomer or monomer blend, also called resin matrix, forms different polymers with different chemical and physical properties. The resin matrix is the central ingredient which contains or may contain different types of fillers, pigments, dies, stabilizers or other various additives and, if necessary, initiators, or catalysts to create a finished product that fulfills the objectives.

The chemical and physical properties of polymers are determined by the:

- type of monomer

- type of monomeric link

- alignment of the monomers (primary structure)

- spatial alignment of the monomers in the polymeric chains (secondary structure)

- spatial alignment of the chains’ secondary structures against each other (tertiary structure)

There are numerous monomers available to create the resin matrix. Depending on the type of monomer different resins with different chemical and physical properties emerge. The chosen monomers determine the type of link by the functional groups reacting with each other, and thus also the type of polyreaction as well. The type of link determines the name of the polymer. The next chapter presents various polymers, also called matrix resins, their links and the respective polyreaction.

Fig. 1: Basic composition of composite resins. (1b)

Matrix Resins

1 Introduction

Matrix resins are unpolymerized monomer/oligomer blends or polymerized solid materials that may contain different types of fillers (organic or inorganic), initiators, catalysts, stabilizers, pigments or various types of other additives (Fig. 1).

Unpolymerized matrix resins are more or less viscous materials that are blended with other ingredients in special mixers or kneaders, mostly under vacuum and/or warmth. Then, the matrix resin is polymerized to solid state to obtain the final product that fulfills the requirements for the respective application.

In case the starting matrix resin is a solid polymer, no initiator or catalyst is needed. Such matrices must be thermoplastic so that they can be transformed by heat to a more or less viscous molten mass. Then, the requested ingredients are admixed. This is done, sometimes under vacuum, in special kneaders, extruders or injection devices. The final product is obtained after cooling this mass in a mold of the requested shape. Likewise, An intermediate product might be received after cooling, which is granulated by milling. To obtain the final product, the granules are plasticized again in injection devices and, then, the molten mass is injected into a mold and cooled down (injection molding).

2 Functional Groups and Monomer Links

Polymers or macromolecules are formed when numerous monomer molecules link with each other according to particular chemical principles. To transform the monomeric/oligomeric state into the solid state, the monomers/oligomers must provide special molecular groups, so called polymerizable or functional groups, which can perform the polyreaction. Depending on the type of functional group, various kinds of polyreactions can be executed and various kinds of links between the reacting molecules can be created (Figs. 2a and 2b).

Fig. 2a:Polymers, links and polyreactions. (2b-1)

Fig. 2b:Polymers, links and polyreactions. (2b-2)

The functional groups (Fig. 3) of the reacting molecules determine the types of links which are characteristic for the created macromolecule. Few different polyreactions and their characteristic links give rise to many polymers. But, also polymers which are based on the same link may significantly differ in their chemical and physical properties. This means that the type of link only classifies the polymer group as it is shown in figures 2a and 2b. A huge number of different polymers exists within each group and these polymers which can significantly differ between each other. This is because the individual molecular structures of the chosen monomers/oligomers are also a crucial factor for the properties of the synthesized polymer.

Fig. 3: Functional groups, examples of unsaturated molecule structures. (3b)

3 Polyreactions

Polyreactions are chemical reactions that link appropriate monomers to polymers under appropriate conditions. There are several types of polyreactions in polymer chemistry, but for dental polymers, three types are the most important ones:

- polymerization (free radical, cationic, anionic)

- polycondensation

- polyaddition

This sounds simple, but hundreds of different monomers are known to form thousands of different polymers according to the aforesaid reaction mechanisms. A considerable variety of tailor-made resin composite materials not only for dental purposes but also for technical, medical or household applications can be produced.

This textbook considers only the basic principles of the chemical reactions which are of special interest for dental materials. Figure 4 classifies the different polyreactions.

Fig. 4: Classification of polyreactions. (4b)

3.1 Polymerization Reactions

Polymerization means the reaction of unsaturated monomeric molecules to macromolecules. Such unsaturated molecules are for instance olefins, also called alkenes, carbonyls or ring structures such as epoxides or dioxanes (Fig. 5). Polymerization reactions are started by appropriate initiators or catalysts and basically run according to the same reaction scheme:

- chain initiation

- chain propagation

- chain termination

But there are some essential facts in which the polymerization mechanisms differ and, therefore, they are subdivided in:

a) free radical polymerization

b) anionic polymerization

c) cationic polymerization

d) special forms of polymerization: ring-opening polymerization and thiol-ene polymerization

All these mechanisms will be discussed because they are important for dental resins.

The tendency of a monomer to react radically, ionically or coordinately with metalorganic complexes depends on its structure and polarity:

- terminal double bonds polymerize fast

- centrally located double bonds or double bonds placed in cyclic systems polymerize slow

- non-polar monomers polymerize preferably radically

- carbon-carbon double bonds with electron-attracting substituents polymerize preferably anionically

- carbon-carbon double bonds with electron-repellent substituents polymerize preferably cationically

As already shown in figures 2a and 2b various polymers can be synthesized via polymerization reactions and mostly the carbon-carbon link is created.

Fig. 5: Examples of unsaturated molecules. (5b)

3.1.1 Free Radical Polymerization

The free radical polymerization is a chain reaction (Fig. 6). This is undoubtedly the most important and most often performed polyreaction to synthesize dental polymers. In a first step, initiator molecules must provide energy-rich free radicals to start the free radical polymerization. The free radicals attack the monomers’ carbon-carbon double bonds, and then the monomer molecules become radicals themselves; this is called chain initiation. The newly formed monomeric radicals attack the next monomer molecules creating new but now one monomeric unit longer chain radicals (macroradicals) and so on; this is called chain propagation. In the case that two chain radicals react with each other - called recombination (Fig. 6, No. 4.a) - the chain propagation terminates; this is called chain termination. In case initiator molecules are in excess the recombination can also occur when initiator and chain radicals react with each other. The chain termination also occurs when no further monomeric molecules are available or when their concentration is too low and the chain too long so that the probability of a monomeric molecule to find a chain end-radical tends to zero. The non-converted molecules remain in the polymer and are called residual monomer.

If certain initiator molecules are exposed to certain energy forms, they create energy-rich radicals that can start the polymerization. This means that thermal or heat initiators decompose when temperature is applied and photoinitiators when light is applied. When radicals are created by chemical energy redox-initiators are involved in the process (see chapter "Initiators").

But there is also a further essential termination reaction called disproportionation. The termination by disproportionation occurs frequently and simultaneously creates new double bonds (Fig. 6, No. 4.b). Two chain radicals react in such a way that one chain radical is saturated by taking a hydrogen atom from the other chain radical, which keeps one electron and forms a carbon-carbon double bond. The double bond can start again a new chain reaction when initiator molecules are available. This is very remarkable because it makes it possible to graft a new chain onto the surface of an already existing polymer. This reaction is of high importance especially for light-curing resin composites because unpolymerized new material layers are possible to graft (bond) onto already polymerized material (see layering technique).

Atmospheric oxygen is also a good inhibitor/stabilizer because of its biradical character (Fig. 6, No. 4c). The unpaired electrons of the oxygen molecule react with chain end-radicals, forming non-reactive terminations. The inhibitory effect of oxygen becomes particularly obvious when photopolymerization (light-curing) is performed. A sticky smear layer (also called dispersion or inhibition layer) remains very often on the polymer surfaces after the reaction due to the non-reactive chain ends (for details, please see chapter Oxygen Inhibition).

Inhibitors (also called stabilizers) cause chain terminations (Fig. 6, No.4d). They are so-called radical scavengers that catch free radicals and remove them from the reaction mixture by creating new non-reactive radicals that are stabilized by mesomeric dislocation of the electrons. This will be discussed later (see chapter Stabilizers).

Polymerization controllers are further important additives which influence the reaction speed of free radical polymerizations. Although they act similar, they must not be confused with inhibitors. This will be discussed later (see chapter "Polymerization Controllers".)

Fig. 6: Basic mechanism of the free radical polymerization. (6b)

Table 1 presents some advantages and disadvantages of the free radical polymerization according to Peacock and Calhoun [115] and other literature [116].

Tab. 1: Some advantages and disadvantages of the free radical polymerization. (1a)

3.1.1.1 Oxygen Inhibition

Atmospheric oxygen is an excellent inhibitor/stabilizer because of its biradical character (Fig. 6, No. 4c). Its two unpaired electrons react with the chain end-radicals, creating new radicals. Methacrylic monomers strictly alternatingly copolymerize with oxygen during the inhibition phase (Fig. 7). The polymerization rate of oxygen with a growing chain is significantly higher compared with the rate of the monomer. The chains formed by this copolymerization are significantly shorter than the ones formed by the regular polymerization. The termination of these alternating chains predominately occurs by recombination of two macromolecules carrying oxygen radicals (Fig. 7, No. 1). The resulting copolymer is instable and partially degrades during the inhibition period [117]. The exact sequence of the oxygen inhibition was discovered by [118].

Fig. 7: Copolymerization of oxygen and monomer radicals. Termination reaction No. 1 predominates. (1a)

The inhibitory effect of oxygen becomes particularly obvious if the free radical photopolymerization (light-curing) is performed. In nitrogen saturated monomeric preparations, the reaction proceeds completely and in the case of multifunctional monomers highly cross-linked networks are obtained. But if photocuring is done the in air, inhibits the free radical polymerization is inhibited according to the mechanism shown in figure 7. This inhibition reaction is because most of the photocurable resins contain dissolved oxygen in concentrations of 10-2 to 10-3 and, furthermore, oxygen also continually diffuses into the surface [119]. According to [119] oxygen inhibition becomes especially obvious regarding the following cases:

a) if thin films are cured, oxygen can penetrate the entire layer and finally no cured material is obtained at all.

b) if highly filled or pigmented resins are photopolymerized so that light penetration is significantly reduced.

The aforesaid inhibition processes also happen with dental light-curing resins. A sticky smear layer (also called dispersion or inhibition layer) that due to low molecular mass oligomers/polymers and non-reactive chain ends remains on the polymer surface after the reaction. Inhibition layer thicknesses of 17 to 22 μm were reported [120].

The effect of the inhibition layer is impressively demonstrated by figure 8. Light-cured microfill resin composite specimens are shown after immersion in aqueous methylene blue solution in the original shade immediately after curing and after their surfaces were treated with different methods. Severe surface discolorations occur in case the inhibition layer has not been completely removed. Therefore, in practice it is significant to remove the inhibition layer after the last processing step to avoid surface discolorations caused by the different media present in the oral cavity (nutrition, bacteria etc.). The best way to remove this layer is to grind off one tenth of a millimeter and to polish carefully.

Fig. 8:

1) Microfill resin composite after light-curing, surface untreated.

2) No anti-inhibition varnish, badly ground and polished after 24h immersion in aqueous methylene blue solution.

3) Anti-inhibition varnish, not polished after 24h immersion in aqueous methylene blue solution.

4) No anti-inhibition varnish, not ground and not-polished after 24h immersion in aqueous methylene blue solution. (2a)

Hoyle [119] reviewed several methods to reduce or even to overcome the oxygen inhibition in the case of free radical polymerization. He differentiated between physical and chemical methods, which are discussed in the following chapters.

3.1.1.2 Physical Methods to Avoid the Inhibition Layer

Three physical or processing techniques, respectively, are available to overcome the oxygen inhibition in free radical polymerization [119]:

a) Performing the reaction in an oxygen-free atmosphere, meaning under vacuum or inert gas. For this purpose, special dental light-curing devices are available for the dental laboratory which allow conducting photopolymerization of light-curing veneer resin composites under vacuum or nitrogen as inert gas. Thus, the sticky inhibition layer is avoided.

b) Irradiation with high light irradiances in the wavelength region of the photoinitiator’s maximum absorption. Thus, high concentrations of radicals are immediately produced and react with dissolved oxygen, removing it from the reaction mixture [119].

c) A further possibility to avoid the smear layer is the application of anti-inhibition varnishes or gels (Fig. 8). These varnishes are gel-like preparations of polyvinyl alcohol solved in water or of unsilanized highly dispersed SiO2 mixed with water or glycerine. Sample formulations are shown in chapter Anti-Inhibition Varnishes/Gels. For example, when ceramic inlays are inserted with luting resin composites, the application of anti-inhibition varnishes is recommended to avoid marginal steps between tooth and ceramic. These steps are caused by the removal of excess luting composite or by abrasive processes that remove the uncured or badly cured smear layer [121] (see chapter Resin-Based Luting Composites. If samples for research work are light-cured, the samples are covered with thin foils of polyethylene or polyvinyl acetate before irradiation.

But, the inhibition/smear layer is also important to be considered when the layering/incremental technique is performed (see layering/incremental technique).

3.1.1.3 Chemical Methods to Avoid the Inhibition Layer

Many valuable suggestions were given by [122] [119] to avoid the inhibition layer by chemical methods. Some of these methods that are also of interest for dental light-curing materials are introduced in the following paragraphs.

It was found that the structures of acrylate or methacrylate monomers/oligomers significantly influence the extent of oxygen inhibition. For example, the higher the viscosity of the preparation, the lower is the oxygen inhibition. But, it is also possible to incorporate certain molecular elements into the monomeric structure that consume dissolved oxygen and thus reduces inhibition. Two of the most important intramolecular elements that scavenge oxygen molecules and thus dramatically minimize the inhibition are ethylene or propylene spacer groups. For more details about this reaction mechanisms, it is referred to the literature [119].

The monomer functionality also influences oxygen inhibition. It was found that high-functional monomers prevent rapid oxygen diffusion due to the formation of highly cross-linked polymeric networks [119]. Example formulations of such products are given in chapter Light-Curing Resins Polymerizing without Inhibition Layer. However, it must be noted that such highly cross-linked resins are very brittle and tend to break, crack or splinter easily in the case of thin films or sheets.

There are also specific comonomers that exhibit oxygen inhibition significantly when available in certain quantities in the reaction mixture. The most important comonomers of which some were also used for dental resins are:

- N-vinylpyrrolidone (Fig. 9)

- N-vinyl caprolactam (Fig. 9)

- pentaerythritol tetra(3-mercaptopropionate) (PETMP) (Fig. 10)

- trimethylolpropane tris(3-mercaptopropionate) (TMPMP) (Fig. 10)

N-vinylamides such as N-vinylpyrrolidone or N-vinyl caprolactam were reported to be excellent comonomers to avoiding oxygen inhibition [119, 123] (Fig. 9). N-vinylpyrrolidone, also already used in dental resin coatings, is soluble in almost all acrylates or methacrylates and is added to the monomeric mixture in amounts of 2 to 30 mass% [119]. This allows to increase the polymerization rate dramatically and the effect of oxygen inhibition can be reduced to such an extent that no significant inhibition layer is observed anymore. The effect of reducing the oxygen inhibition has not been completely clarified up to now. It is assumed that several mechanisms synergistically contribute to the mode of action of n-vinylpyrrolidone:

a) oxygen-scavenging due to photo-oxidation

b) formation of excited state charge-transfer complexes (or exiplexes) between oxygen and N-vinylpyrrolidone, which decays and produces radicals that can initiate chain propagation again or can consume oxygen [123].

This means that N-vinylpyrrolidone is more an additive than a real comonomer. This assumption is supported by the fact that after the reaction, high amounts of residual N-vinylpyrrolidone were found in the polymer [123]. Whatever the mechanism is, experiments indicated that polymeric films with almost no inhibition layer can be produced.

Fig. 9: N-vinylamides tested as oxygen scavengers. (5e)

Thiols can also dramatically reduce oxygen inhibition. Especially thiol-ene mixtures of pentaerythritol tetra(3-mercaptopropionate) (PETMP) or trimethylolpropane tris(3-mercaptopropionate) (TMPMP) (Fig. 10) with acrylates or methacrylates were found to greatly reduce oxygen inhibition. The polymerization rate of such mixtures is extremely fast and there are no differences in both air and nitrogen [119, 124, 125]. Multifunctional thiols like PETMP and TMPMP act quasi as multifunctional cross-linkers [126]. Adding these thiols in an amount of approx. 10 mass% to dior trifunctional acrylates or methacrylates allows producing light-curing varnishes or other products, respectively, that do not show an inhibition layer anymore. Example formulations of such mixture are shown in chapter Light-Curing Resins Polymerizing without Inhibition Layer.

Fig. 10: Thiols to excellently avoid oxygen inhibition. (6e)

The mechanism of the oxygen scavenging effect of the aforesaid thiols is quite well understood and shown in figure 11 (according to [119, 124, 125]).

Fig. 11: Oxygen scavenging mechanism of thiols during the free radical polymerization. (7e)

Thiols even initiate the free radical polymerization without adding an initiator, meaning these systems are self-initiating. Therefore, the shelf-life of ready mixed products is not very long and lasts only for some days up to approx. a month [125]. This is not very problematic for industrial products because the mixtures are processed shortly after they have been produced. But regarding end-user products, a long shelf-life of more than 3 years is an essential condition. Therefore, the application of thiol-ene chemistry was very problematic for dental resins. But, the discovery of new stabilizers or stabilizer systems (Fig. 12, according to [127]), might result in a breakthrough of this very interesting chemical polymerization type regarding dental resins, too.

Fig. 12: Stabilizers for thiol-ene preparations. (8e)

For more details about the thiol-ene chemistry and its use for dental resin composites, see chapter "Thiol-Ene Polymerization and Thiol-Michael Polymerization".

To reduce oxygen inhibition, it is also possible to add high photoinitiator concentrations (>5 mass%) to the reaction mixture. The photoinitiator immediately generates large amounts of radicals that consume oxygen, and no further diffusion into the mixture occurs [125]. Especially regarding dental photocuring resins, it must be noted that too high photoinitiator concentrations accelerate the polymerization rate dramatically so that not only the processing time under ambient light becomes too short but also too high reaction heat is generated. Furthermore, high initiator concentrations might negatively influence color stability. Therefore, such preparations are not recommendable for dental resins.

In this connection, it is striking to note that tricovalent phosphites and phosphines commonly used as antioxidants (see chapter "Antioxidants") also accelerate photopolymerization dramatically so that oxygen inhibition can be significantly reduced. Especially triethyl phosphite was found to speed up the reaction rate significantly even when used in tiny amounts of approx. 1 mass% [125]. The mechanism of this reaction is shown in figure 13 (according to [125]). It is really remarkable and surprising that this substance class stabilizes and accelerates thiol-ene reaction mixtures (see also thiol-ene stabilizer).

Fig. 13: Reaction of oxygen with phospines and phosphites. (9e)

3.1.2 Cationic Polymerization

Cationic polymerizations are started by Lewis acids (BF3, AlCl3), protonic acids (HCl, H2SO4, CH3COOH) as well as by carbenium (RR’R’’C+X-, R = organic rests) and onium (e.g., NH4+X-, PH4+X-, H3O+, H2Cl+) salts. The activation energy of ionic polymerizations is very low so that they run very vigorously even at low temperatures (below 0 °C). Lewis acid initiators only start the reaction in the presence of traces of water so that addition compounds are formed which transfer protons to the alkene. The chain initiation is started by the carbenium ion - formed by the initiator - that undergoes addition to the alkene and builds a new carbenium ion (Fig. 14). The chain propagation proceeds by adding further alkenes. The chain is terminated by bases (e.g., OH-).

Since the cationic polymerization can also be initiated by photoinitiators it is used for dental filling resin composites, too. Combinations of radically and cationically induced polymerizations are also possible and even realized in filling resin composites.

Product formulations with cationic initiators also undergo very slow, thermally induced polymerization and, therefore, must be stabilized with special inhibitors to avoid premature polymerization and to guarantee good shelf life [128].

Due to their reaction mechanism, ionic polymerizations are theoretically not inhibited by oxygen. Therefore, it is assumed that their surfaces always polymerize completely and dry even under air access, what is not easily achievable with the free radical polymerization. But it has to be noted that this does not fully apply to the dental silorane filling resin composites (see silorane inhibition layer).

Fig. 14: Cationic polymerization mechanism. (7b)

Figure 15 exemplarily shows a cationically polymerizing difunctional vinyl ether monomer that was used for dental applications [129].

Fig. 15: Difunctional vinyl ether monomer that was tested for dental applications. (19e)

Table 2 presents some advantages and disadvantages of the cationic polymerization according to Peacock and Calhoun [115] and other literature [116].

Literature gives more profound knowledge about the cationic polymerization [28, 130-135].

Tab. 2: Some advantages and disadvantages of the cationic polymerization. (2a)

3.1.3 Anionic Polymerization

Anionic polymerizations are started by Lewis bases (e.g., OH-, NH3) or alkali metals (e.g., Na, K). The electron pair donor creates a carbanion which reacts with the double bond of the monomer so that a new carbanion is formed and so on (Fig. 16). The inhibitors responsible for the termination of the anionic polymerization are proton donors, as for instance water, acids or aliphatic halogen compounds. Since anionic polymerizations are difficult to carry out, they are only done industrially.

Fig. 16: Basic mechanism of anionic polymerizations. (8b)

The anionic polymerization is insensitive to temperature and can be performed at high as well as at low temperatures. A special case of the anionic polymerization is the synthesis of so-called living polymers which have dianionic character and are stable at sufficiently low temperatures over a longer period of time. These polymeric dianions can be reactivated by adding appropriate monomers and new polymers can be formed.

Important products for medical and dental purposes which are synthesized or cured, respectively, by anionic polymerization are cyanoacrylate adhesives (Fig. 17). The anionic polymerization is initiated by catalytic amounts of water on the substrate’s surface but is inhibited on acidic surfaces. Cyanoacrylates must be stabilized to avoid premature polymerization during storage, and it is necessary to prevent the entry of traces of moisture. The cyanoacrylates mainly differ in their organic rests R. Important cyanoacrylates are skin and surgical adhesives with R = n-butyl-group or 2-octyl-group and dental root canal sealers with R = i-propyl-group. More details about anionic polymerizable technical and medical cyanoacrylates are given by the literature [136].

Fig. 17: Anionic polymerization of cyanoacrylates. (8b-1)

Table 3 presents some advantages and disadvantages of the anionic polymerization following Peacock and Calhoun [115] and other literature [116] and for a comprehensive treatise, it is referred to [36].

Tab. 3: Some advantages and disadvantages of the anionic polymerization. (3a)

3.1.4 Ring-Opening Polymerization

Ring-opening polymerizations are of great and increasing interest not only for scientific reasons or many technical applications [137] but also for dental resins. The great advantage of ring-opening polymerizations is their very low shrinkage because space is gained by the opening of the ring structures during the reaction. It has even been succeeded to synthesize monomeric ring structures that allow the resulting polymer not to shrink at all but even to expand [131, 138, 139]. Ring-opening polymerization can run radically [137, 138], cationically [131, 137, 139] or anionically [137] (Figs. 18a and 18b). As already mentioned above and as it will be discussed in more detail later, the radical and cationic ring-opening polymerizations have been realized in resin-based filling composites, too (see chapter Silorane Filling Composites and paragraph "Bulk-Fill Composites with Ring-Opening Molecular Segments") [140-146].

Fig. 18a:Basic reaction mechanism of the cationic ring-opening polymerization. (9b-1)

Fig. 18b:Basic reaction mechanism of the free radical ring-opening polymerization. (9b-2)

Apart from the above-mentioned ring-opening systems, there were many further attempts to develop special ring-opening monomers/oligomers especially for dental purposes. A very comprehensive review is given by [129]. It must be noted that all systems presented in the following were only used for research work up to now, as far as the author knows.

3.1.4.1 Free Radical-Ring-Opening Polymerization

Many authors investigated the free radical ring-opening polymerization of cyclic monomers such as cyclic disulfides, cyclic ketene acetals, spiro orthoesters or 2-vinylcyclopropanes that shrink significantly less compared to linear vinyl monomers such as acrylates or methacrylates [129, 147-151].

The 1,1-disubstitued 2-vinylcyclopropanes proved the best stability in humid, acidic and basic surroundings as well in the presence of fillers such as silicon dioxide [129, 150]. Their biggest problem was the significantly lower reactivity compared with acrylates or methacrylates. Subsequent investigations found that bicyclic derivatives and combinations with acrylic units improved the polymerization rate considerably [129, 147, 149]. The highest reactivity was found for 2-(bicyclo)[3.1.0]hex-1-yl)acrylate. Moszner et al. [129] proposed a ring-opening mechanism that is presented in figure 19. But, the glass transition range was only Tg ≈ 52 °C what is much too low for dental composites and the shrinkage was still higher compared with Bis-GMA [129].

Fig. 19: Ring-opening polymerization of 2-(bicyclo)[3.1.0]hex-1-yl)acrylate (heat-polymerized: initiator 2,2'-azo-bis-isobutyronitrile). (13e)

Better results were obtained by polymerizing derivatives of 2-(bicyclo)[3.1.0]hex-1-yl)acrylate (Fig. 20, according to [152]) with only approx. 3 vol% shrinkage. But, the polymerization rate was lower in comparison with the unsubstituted substance. However, mixtures of bicyclic cyclopropyl acrylates - used as diluents - with conventional dimethacrylates (Fig. 38) showed very promising results after photocuring also about the mechanical properties [152].

Fig. 20: Structure of the derivative of 2-(bicyclo)[3.1.0]hex-1-yl)acrylate; shrinkage 3.0 vol%, Tg = 107 °C. (14e)

A further approach investigated liquid crystal monomers carrying free radical ring-opening polymerizable groups [129, 153-155]. Two liquid crystal monomers were synthesized by [153, 154]. The liquid crystal monomer No. 1 of figure 21 showed a polymerization shrinkage of only 0.2 vol%, but it was solid at room temperature. Liquid crystal monomer No. 2 of figure 21 had a degree of polymerization of only 15 to 16 (Fig. 21 according to [129]). Therefore, both of these monomers were no strong candidates which could be