Читать книгу Advanced Level of Dental Resins - Material Science & Technology - Ralf Janda - Страница 13

Initiators and Catalysts

1 Introduction

The terms initiator and catalyst are very often incorrectly used in dentistry. Contrarily to catalysts initiators take part in the polyreaction whereas catalysts only start the reaction. This means that initiators change their chemical state during the reaction; their structure differs before and after the reaction. Initiators start the free radical, anionic or cationic polymerization but catalysts start the polyaddition or polycondensation.

Usually not only the initiator but initiator systems are used to start polymerization reactions. In addition to the initiator, initiator systems may contain several different initiators but also always so-called synergists. No such synergists are needed for polyaddition and polycondensation.

2 Initiators

The initiators to start polymerization reactions of dental resins are mainly classified into initiators for

- heat-curing, also called thermal polymerizing

- self-curing, also called cold-curing or autopolymerizing

- light-curing, also called photopolymerizing

resins. Heat-curing resins polymerize under the influence of heat (for instance in boiling water or in a heat cabinet), self-curing by mixing of at least two product components at room temperature and light-curing by irradiating with light of special wavelengths. All polymerization reactions are highly exothermic. Therefore, it is misleading to name the self-curing resins also cold-curing. However, cold-curing in this connection means that no external energy/heat is requested to initiate the reaction.

The above chosen categorization very clearly shows that the initiation of the polymerization reaction is not a matter of the respective (meth)acrylate but only of the chosen initiator. All three of the aforesaid initiator types are employed for the radical polymerization of dental resin products.

Initiator systems are very complex compositions not only containing the initiator or initiator blends but also other important substances are involved in the initiating process. Such important substances are the synergists often also called accelerators. Synergists increase the chemical activity of the initiator and thus the polymerization rate.

2.1 Thermal/Heat Initiators

Thermal/heat initiators start the free radical polymerization when sufficient thermal energy (warmth/heat) is applied. Dibenzoyl peroxide (DBPO also abbreviated as BPO), a typical initiator to perform thermal polymerization, is the most often used heat initiator for heat-curing dental products (Fig. 27b). DBPO significantly decomposes into radicals already at temperatures of approx. 70 °C and starts the radical polymerization.

Due to its low decomposition temperature BPO is not used in one-component resins containing unpolymerized monomers such as one-component resin filling composites, one-component denture base or crown and bridge veneer resins. Such preparations are not storage-stable at high environmental temperatures as they are in summer or in very warm regions of the world. Therefore, photoinitiators are preferred for these products because they are very temperature stable. Nevertheless, BPO is used as one initiator component in redox initiator systems for two-component self-curing resin filling materials. These products require uninterrupted cooling chains for transportation.

Fig. 27b: Dibenzoyl peroxide thermal (heat) initiator and radical formation.

Concerning all other heat-curing one-component products the problem is solved by using other peroxides (Fig. 28ba, cleavage mechanism according to [358]) or so-called C-C initiators (Fig. 45a, cleavage mechanism according to [359, 360]) that have much higher decomposition temperatures of approx. 100 °C.

For the high processing temperatures, no synergists are necessary to start the heat polymerization reactions.

Fig. 28ba: Examples of peroxy heat initiators, their initiation temperatures and mechanisms of cleavage.

Fig. 45a: Benzpinacol as an example of C-C heat initiators, its initiation temperature and mechanism of cleavage.

2.2 Redox Initiators

Redox initiator systems are used for self-curing resins that are always at least two-component products. The redox initiator system usually consists of two substances, the initiator and the synergist/accelerator, each of which is part of one of the components of the product. If these products are pasty preparations one-component is called base paste and the other one catalyst paste although it does not contain a catalyst. In case the product consists of a powder and a liquid component the initiator is part of the powder and the synergist is part of the liquid. After mixing the two product components the redox initiator ingredients are also mixed and free radicals are formed starting the polymerization.

2.2.1 Peroxide/Amine-Based Redox Initiators

Peroxide/amine redox initiator systems usually contain BPO as peroxide initiator and a tert. arom. amine as synergist/accelerator. This is due to the fact that two-component products are not cured at high but at significantly lower temperatures as for instance at approx. 40 °C, mouth temperature or even at room temperature. Therefore, the “initiation force” of BPO is also low and the reaction must be sped up by an accelerator component.

It is important to observe that products containing a tert. arom. amine as synergist might discolor yellowish or brownish even after a few months. This means that such products need to be considered as not color stable. It was found that the accelerating ability of tert. arom. amines depend on the type of the ring substituents as well as on the type of the nitrogen substituents. But the color stability is more influenced by the type of the ring than by the of the nitrogen substituents [110, 361].

With regard to high molecular mass dimethacrylates such as Bis-GMA, UDMA and TEGDMA it was reported that amine synergists with electron-donating para-substituents (-CH₃, -CH₂-, -CH₂OH, -CH₂COOH) very efficiently accelerated the polymerization even at 37 °C. This was not the case with electron-withdrawing para-substituents (-COOCH₂CH₃) [362]. This is important to know especially regarding the two-component paste/paste filling resin composites, two-component enamel/dentine adhesives or pit and fissure sealants which are mostly based on the aforesaid high molecular mass dimethacrylates (Fig. 18b-1).

The general mechanism of the radical formation by peroxide/tert. arom. amine redox initiators is shown in figure 46a [361, 362].

Fig. 46a: General mechanism of radical formation by peroxide/tert. arom. amine redox initiator systems (radicals are indicated by a dot).

Examples for often used tert. arom. amine synergists are N,N-dimethyl-p-toluidine (not color stable end products, Fig. 29 ba) or N,N-Bis(2-hydroxy ethyl)-p-toluidine (end products with better color stability, Fig. 30ba) [110, 361, 362].

The peroxide/amine-based initiator systems are highly reactive and the products polymerize even at room temperature with very high speed.

Fig. 29ba: Peroxide/amine redox initiator system, mechanism of radical formation - not color stable.

Fig. 30ba: Peroxide/amine redox initiator system, mechanism of radical formation - improved color stability.

2.2.2 Barbituric Acid-Based Redox Initiators

Barbituric acid-based (more correctly: barbituric acid derivative-based) redox initiator systems are mainly used for self-curing powder/liquid products such as denture base resins or veneer resins. The products based on this redox initiator system are totally color stable [91, 363-365] compared with the products based on the peroxide/amine initiator system.

The barbituric acid-based initiator system is more sophisticated than the peroxide/amine system. The powder usually contains two different barbituric acid derivatives, namely 1,3,5-trimethylbarbituric acid (TMBA) (Fig. 31ba) and 5-phenylbarbituric acid (PBA) (Fig. 47a). The liquid contains extremely low amounts of copper ions (ppm range) as catalyst to accelerate the autoxidation of both of the barbituric acid compounds. The intermediate product decomposes under the accelerating effect of chloride ions which work as synergist to form several types of radicals. The chloride ions derive from an organic compound being also part of the liquid. It is very interesting that one of the end products of the initiation reaction is oxygen which starts again the reaction by attacking a new barbituric acid molecule (Fig. 31ba) [91, 363-365].

Despite of the synergist the reaction rate of the polymerization is rather slow. Therefore, the barbituric acid-based initiator system needs a temperature impulse of approx. 45 to 55 °C to properly cure the product.

Fig. 47a: Structural formula of 5-phenylbarbituric acid.

Fig. 31ba: Reaction mechanism of the barbituric acid-based redox initiator system that is absolutely color stable.

2.2.3 Sulfinic Acid-Based Redox Initiators

To be complete, also the sulfinic acid-based redox initiator systems must be mentioned. These systems are only used in self-etch enamel/dentin adhesives (light-curing as well as self-curing) which contain highly acidic monomers and solvents (e.g. acetone, alcohol or especially water). These self-etch adhesives tend to form a thin and uncured acidic and hydrophilic top layer after curing [366-370]. It was found that subsequently applied self- or dual-curing composites showed significantly lower bond strength depending on the acidity (pH-value) of the self-etch adhesives [371] what was not the case with light-curing composites. Therefore, an incompatibility between the self-etch adhesives and self-curing composites was investigated [370-375]. It was recognized that the acidic monomers of the self-etch adhesives remaining in the uncured top layer deactivated the amine synergist of the self-curing composite so that the amine synergist lost its accelerating properties [376]. This deactivation is due to a Lewis acid-base reaction of the acidic monomer (depending on its acidity and concentration) and the amine synergist [371]. This occurs especially with self-curing composites due to their low reaction speed contrarily to the very fast polymerization of light-curing materials.

Due to the aforesaid incompatibility problem regarding highly acidic self-etch adhesives an additional bottle containing a preparation of an aryl sulfinic acid sodium salt as third initiator component was introduced in case self-curing composites were applied subsequently [377-379]. The reaction of the aryl sulfinic acid sodium salt is shown by the example of 4-Meta. 4-Meta, a commonly used adhesive monomer (Fig. 74b-2), immediately hydrolyzes in the presence of water to 4-Met a very acidic adhesive monomer promoting adhesion to the hard tooth tissues (Fig. 48a). Sodium benzene sulfonate or sodium p-toluene sulfonate reacts with 4-Met forming radicals that attack the double bonds of 4-Met or other monomers of the adhesive mixture and initiate the polymerization (Fig. 48a).

Fig. 48a: Mechanism of the sulfinic acid initiator system (4-Met = 4-methacryloyloxypropyl trimellitic acid).

2.3 Photoinitiators

The photopolymerization, very often also called radiation or light-curing polymerization, is certainly the most important polyreaction for contemporary direct restorative resin materials. But there are also several important light-curing products for the dental laboratory. The photopolymerization can be performed as free radical as well as cationic polymerization.

The photopolymerization is initiated by photoinitiators. These substances decompose and form radicals within a fraction of a second when irradiated with light of certain wavelengths. Synergists (tertiary aliphatic or arom. amines) are always part of the photoinitiator system because only the photoinitiator is usually not able to optimally accelerate the reaction. The ratio of photoinitiator to amine influences the reactivity of the photoinitiator system. It must also be noted that the photoinitiator system may contain not only one but more photoinitiators to take advantage of the complete emission spectrum of the light-curing device. The amount of photoinitiator employed in light-curing dental resins is very low and is usually between 0.1 to 1 mass%. Also, the synergists are only used in quantities of only 0.05 to 0.5 mass% depending on the type of synergist or product.

The big advantages of light-curing products are

- very high color stability

- easy handling, one-component pastes or gels (no mixing)

- long processing times under normal environmental light

- very fast cure (within a few seconds) with appropriate light-curing devices

- very good storage stability (long shelf life)

But it must be considered that they cure fast in sunlight or when directly exposed to the surgical light.

The anionic photopolymerization is not of any interest for dental resins.

2.3.1 Conventional Radical Photoinitiators

The radical photoinitiators used from the beginning up to the present originate from technical applications and are not custom-made for dental products. Therefore, they are called conventional radical photoinitiators in this context. However, they are carefully selected to optimally meet the requirements for dental materials which are for instance fast and secure curing of the products with dental light curing devices as well as high biocompatibility.

Figure 32b presents the basic course of a photoreaction. In a first step the electrons of the photoinitiator are excited by the emitted photons of the light source. This excited state exists only for a tiny fraction of a second and decomposes to radicals which then start the free radical polymerization in the known manner.

Fig. 32b: Principle course of photoreactions.

The most often used photoinitiator for dental resins is camphorquinone (CQ, 2,3-bornanedione) that decomposes at wavelengths of approx. 460 nm. Its absorption spectrum is given in figure 36b. The photoreactions of CQ is quite good understood (Fig. 33b). CQ usually always requests an amine synergist to properly accelerate the photopolymerization [380, 381].

The combination of CQ/TPO/amine was also investigated to take advantage of the complete emission spectrum of the light-curing devices. It was found that the curing depth did not increase but the degree of conversion (DC) on top of the restoration and thus the shrinkage stress increased [382].

It was reported that lower DC and inferior mechanical properties occur in case of lower overall concentration of the photoinitiator system CQ/ethyl-4-(dimethylamino)benzoate. The overall initiator concentration of 0.5 mass% presented the lowest results, except for the 3:1 CQ/ethyl-4-(dimethylamino) benzoate ratio. No influence of the DC on the mechanical properties was observed for overall concentrations equal or greater 1.5 mass%. Therefore, it does not seem reasonable to use CQ in concentrations higher than 1.5 mass% and a ratio of 3:1 CQ/amine seems to be advantageous [383].

Despite of its high reactivity and proven quality one decisive problem of CQ is its very intensive yellow intrinsic color. This yellow color slowly disappears during the light-curing process. But too short irradiation times do not guarantee the complete conversion of CQ so that there remains a risk of further bleaching of the incorporated restoration. Therefore, the filling might become visible because its shade differs from adjacent hard tooth tissues [384-386]. It must also be noted that it is strictly advised against choosing the requested shade by comparing the uncured filling composite with the hard tooth tissues; the uncured material is always darker/yellower than the requested shade because of CQ.

Fig. 33b: Photoreaction of camphorquinone = CQ (2,3-bornanedione) with an tert. arom. amine synergist.

)TPO (2, 4, 6,-Trimethylbenzoyl diphenylphosphine oxide decomposes at wavelengths of approx. 400 nm. Its absorption spectrum is given in figure 36b.

The photoreaction of TPO is quite good understood (Fig. 34b). CQ and benzyl dimethyl ketal always request an amine synergist to properly accelerate the photopolymerization. Although sometimes amine synergists are also added in addition to TPO this must not necessarily be done [387]. TPO is a very reactive photoinitiator which is used to cure filling resin composites as well laboratory resin composites. As demonstrated by its absorption spectrum the shade of TPO is much less yellow than the one of CQ (Fig. 34b). Therefore, no such intensive fading occurs during the irradiation process.

The TPO-based products, even if an amine synergist is present, proved higher color stability when compared with the CQ/amine-based ones [388-390].

Fig. 34b: Photoreaction of (2, 4, 6-trimethylbenzoyl) diphenylphosphine oxide (TPO).

Benzyl dimethyl ketal (BDK) decomposes at wavelengths of approx. 350 nm. Its absorption spectrum is given in figure 36b.

The photoreactions of BDK is quite good understood (Fig. 35ba) [391]. Similar to CQ BDK always requests an amine synergist to properly accelerate the reaction [391, 392]. As shown by its absorption spectrum (Fig. 36b) BDK has no yellow shade. It is a white powder and, therefore, no bleaching occurs while it is irradiated. BDK is sometimes used in combination with CQ and amine synergists because it also accelerates the photoreaction of CQ. BDK is often used for laboratory light-curing products because the laboratory light-curing devices also emit UV-radiation.

Fig. 35ba: Photoreaction of benzyl dimethyl ketal.

Fig. 36b: Absorption spectra of different photoinitiators:1 = benzyl dimethyl ketal (BDK) 2 = (2, 4, 6-trimethylbenzoyl) diphenylphosphine oxide (TPO)3 = camphorquinone (CQ)

Figure 49a presents some more examples of photoinitiators which can be used to initiate the free radical polymerization of dental resins.

Fig. 49a: Some examples of photoiniators and their absorptions within the range of 300 to 500 nm.

2.3.2 Tailor-Made Radical Photoinitiators

Benzoyl germanium derivatives were developed as tailor-made visible-light photoinitiators for dental resins. They are characterized by [393-397]

- perfectly matching the emission spectrum of the modern LED light-curing devices

- providing very high reactivity, much higher than the CQ/amine system

- showing much higher absorption in the visible-light range than the CQ/amine system

- showing improved color stability and shelf-life than the CQ/amine system

- showing significantly less bleaching than the CQ/amine system

- the fact that no additional synergists are requested

- an excellent toxicological profile

The structures and the photoreaction mechanism of these very interesting tailor-made photoinitiators are presented in figure 50a (in accordance with [393-396]). The absorption spectra of the tailor-made benzoyl germanium derivative Ivocerin (trademark ) and CQ are schematically presented in figure 51a (following [396]).

Fig. 50a: Germanium photoiniators and their absorptions within the range of 300 to 500 nm.

Fig. 51a: Absorption spectra of Ivocerin and CQ.

2.3.3 Cationic Photoinitiators

The cationic polymerization can also be initiated by photoinitiators as it is shown by the photo-initiated formation of the Lewis acid cationic initiator BF3 in figure 37b. Start and progress of the cationic polymerization is shown in figure 38b.

Fig. 37b: Photo-initiated formation of the cationic initiator BF3.

Fig. 38b: Start and progress of the cationic polymerization.

Certainly, the above described possibility to photo-initiate the cationic polymerization is only of theoretical interest. In fact, there are very special highly reactive and well-proven cationic photoinitiators that are broadly used for technical and dental applications (Fig. 39ba).

Fig. 39ba: Examples of some commonly used cationic photoinitiators and their absorptions within the range of 300 to 500 nm.

Silorane Cationic Photoinitiator System

The first time a tailor-made cationic photoinitiator system was used in dentistry was for the silorane filling resin composites (Filtek Silorane (3M ESPE Dental Products, St. Paul, MN, USA [398, 399]). This very sophisticated photoinitiator system (Fig. 68ba) comprises the cationic photoinitiator diaryliodonium hexafluoroborate, the radical initiator camphorquinone and as synergist a tert. arom. amine. This system initiates the polymerization reaction within a light range of 400 to 500 nm similar to the radically polymerizing (meth)acrylates [398, 399]. The presented exemplary photoreaction mechanism in figure 68ba is based on several mechanisms discussed in the literature [26, 123, 126].

The combination of cationic and radical photoinitiators and conventional tert. arom. amine synergists is very interesting. It is known that radical photoinitiators (here CQ) influence the efficiency of cationic photoinitiators that are onium salts (here diaryliodonium hexafluoroborate). It was shown that a photo-redox-induced decomposition of the onium salts is achieved through an electron transfer from the free radicals. Thereby, electron-donating radicals reduce the onium salt so that the onium salt is decomposed and the proton needed to start the cationic polymerization is generated [123].

Although cationically polymerized resins should not have an oxygen-induced inhibition layer for the silorane filling resin composites an inhibition zone of 2 to 8 µm thickness was measured [118]. This is not very surprising because the radical initiator CQ is part of the cationic photoinitiator system (Fig. 68ba). As it was already explained earlier ring-opening polymerizations can be done either radically or cationically and, therefore, due to the presence of the radical photoinitiator CQ some silorane monomer rings are radically opened. That is why several superficial monomer radicals suffer oxygen inhibition and the well-known inhibition layer (also called smear-, dispersion- or oil-layer) is formed. This means that the polymerization of siloranes is a radical/cationic hybrid polymerization to a certain extent.

Fig. 68ba: Principal formulation and reaction of the silorane (Filtek Silorane) photoinitiator system.

Germanium Derivative Cationic Photoinitiators

For the sake of completeness, it should be mentioned that it was found that germanium derivative initiators developed for the radical polymerization are also able to accelerate the cationic photopolymerization of diaryliodonium hexafluorophosphate as it has been described above [400].

2.3.4 Radical/Cationic Hybrid Photoinitiators

Although no such products are known to be on the market yet it is thought to be important to mention that the radical/cationic hybrid photoinitiation was investigated. This very innovative method was successfully experimentally used in 3D printing technology [401]. 3D printing might become of increasing importance in dentistry, too.

3 Synergists

Synergists are very important components of initiator systems to accelerate the polymerization. Such synergists are for instance tertiary aliphatic or aromatic amines (Fig. 52a). It must be noted, that the chemical structures of the amines used for redox initiator systems very strongly differ from the structures of the amines used for photoinitiator systems. The first ones are used in quantities of approx. 1 mass% but the latter ones in much smaller quantities of only approx. 0.05 to 0.3 mass%. The amine synergists used for redox initiator systems are a major risk with regard to discoloration of the products due to their structure and rather high concentration. The amine synergists used for photoiniators systems do not cause such strong discolorations due to their different structure and much lower concentration.

The effect of amines as synergists is based on electron transfer processes concerning the self-curing as well as the photo-curing reactions [361, 381, 402].

Depending on their structures the amines differ in their effect to accelerate the polymerization reactions. Generally, it can be said that the tert. aromat. amines are more reactive than the aliphatic amines [381]. Also, the ratio of photoinitiator to amine is important to be considered. An optimal ratio for the system CQ/amine was found to be 3:1 [403]. The dependence of DC and color stability on the CQ/amine ratio has also been investigated by other literature [404]. Regarding color stability the yellow shift of the 1:1 ratio was found to be significantly lower compared to increasing levels of amine. For the 1:1 ratio it was also found that the DCs increased with increasing Bis-GMA content of Bis-GMA/TEGDMA mixtures [404].

Fig. 52a: Examples of some important synergists for dental resins and their absorptions within the range of 300 to 500 nm.

4 Catalysts

Contrarily to initiators catalysts do not take part in the chemical reaction but promote the reaction by significantly reducing the needed activation energy to start the reaction (Fig. 40b). The chemical state of the catalysts is the same before and after the reaction. Activation energy means the energy needed to start a reaction. Reactions that are thermodynamically impossible cannot be triggered by a catalyst. The catalyst is only able to increase the reaction rate or, more accurately said, it accelerates the rate the chemical equilibrium is attained.

The catalytic reaction is called catalysis and it is differentiated between heterogeneous and homogeneous catalysis. With regard to the heterogeneous catalysis the catalyst is a solid material (metal or alloy) and the reactants are liquid or gaseous. When the catalyst and the reactants are in the same phase, the process is referred to as homogeneous catalysis. This is for instance the case when the reactants and the catalyst are liquids or pastes.

The catalysis is a very complicated chemical reaction of adsorption and desorption processes of the reactants on the catalyst surface. While the reactants are adsorbed they react with each other and desorb after the end products have been formed. At the end of the reaction the catalyst is released in its original state again [405].

Polyaddition and polycondensation reactions are examples for reactions being started by catalysts. Catalysts for the polycondensation cross-linking reaction of impression materials are organic tin compounds and for the polyaddition cross-linking reaction organic platinum compounds. The catalysts for both reactions exist in same phase with the polysiloxanes and, therefore, these reactions are catalyzed homogeneously.

Fig. 40b: Reduction of the activation energy by catalyst. EA1 = activation energy without catalyst, EA2 = activation energy with catalyst.