Composite resin polymerization and relevant parameters

Composite resin polymerization and relevant parameters

9

S.R. Schricker

9.1

Overview of a composite resin

Composite resins are a class of dental restorative materials that are a mixture of organic and inorganic components. The three primary organic components are the resin, the coupling agent, and the initiator, with the primary inorganic component being the filler. There are many reviews and book chapters that discuss the restorative applications of these materials.1e3 This system is versatile, and many different dental materials can be derived by modifying the resin formulation and/or the resin-to-filler ratio. Direct restorative materials such as sealants are derivations of composite resins, as are dentin adhesives and orthodontic adhesives. The focus of this chapter is to provide an overview of composite resin chemistry and properties, and to explore how varying the formulation can produce a diverse set of properties and a wide variety of dental materials.

9.2

What is a composite?

In a broad materials science and engineering context, a composite is a mixture of two or more materials that behave as a single material. In addition to many synthetic composites, many biological structural components such as seashells, bone, and teeth are considered composites. Through combining materials, composites are designed to have the advantages of the principal components while reducing the respective disadvantages. Composite resins are composed of an inorganic filler that has good wear resistance, hardness, and high elastic modulus but poor toughness or crack resistance and is difficult to process at room temperature. The resin component can be easily processed and cured at room temperature and has good toughness but poor wear resistance, hardness, and elastic modulus. The resulting composite resin has acceptable wear resistance, hardness, elastic modulus, and toughness, and can be easily cured at room temperature. The resulting properties of a composite resin are heavily influenced by the relative ratios of the components. A qualitative example is shown in Fig. 9.1. As the filler ratio increases, the hardness increases, but the viscosity of the uncured mixture decreases. This is an example of the rule of mixtures: the more filler, a harder material present in the composite, the harder the overall composite. Fig. 9.1 presents the trends, and the rule is often not linear. However, the general principle is useful for Orthodontic Applications of Biomaterials. http://dx.doi.org/10.1016/B978-0-08-100383-1.00009-6 Copyright © 2017 Elsevier Ltd. All rights reserved.

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Viscosity

Hardness

Filler ratio

Figure 9.1 Qualitative effect of the filler ratio on viscosity and hardness of a composite resin.

modifying composite properties. For example, for a restorative material, it is important to have a highly filled system because of the hardness and wear resistance necessary in this clinical application. For adhesives, it is more important for the material to flow or have low viscosity to form a good interface with the substrate; hence, these materials have low filler levels. The design of composite resins is a multifaceted problem. There are many competing criteria that have to be balanced and optimized for an environment as demanding as the oral cavity. Esthetics, water sorption, viscosity, wear resistance, degree of cure, and toughness are some of the many parameters that have to be balanced. Whether in designing the composite or selecting the material clinically, all choices will involve trade-offs in properties. It is important to match the properties of a composite with the clinical criteria. In addition to the balance between filler and resin, these individual components can be modified to change the overall properties of the composite. In the following discussion, the chemistry of the resin and filler will be examined, as well as their interaction. The role of the coupling agent and initiator will be discussed, and then the curing process. Fig. 9.2 will serve as a basis for how the composite is formed from the individual components.

Coupling agent Filler Resin

Figure 9.2 Components of a composite resin.

Composite resin polymerization and relevant parameters

9.3

155

Resin component

The resin, or polymeric component, of a composite resin is typically a mixture of dimethacrylate monomers. Fig. 9.3 is an example of the original resin system as developed by Bowen at the National Bureau of Standards (NBS), now the National Institute of Standards and Technology (NIST). There are many alternatives to this system,3e6 but the concept is still the same. The BisGMAeTEGDMA (bisphenol A glycidyl methacrylateetriethylene glycol dimethacrylate) monomer system is flowable at room temperature and upon curing will form a solid polymer. These two properties are perhaps the most important of the resin system. Fillers can be added to the monomer system to form the uncured composite resin paste. Then this paste can be cured at room temperature with visible light, allowing for a direct esthetic restoration. There are many other structural dimethacrylates and dimethacrylate-reactive diluents. Each has advantages and disadvantages, but this strategy is a method to tailor the properties of composite resins. At this stage of composite resin development, there is no universal or ideal resin formulation. Depending on the application and design parameters, different systems are formulated to achieve a beneficial outcome. Two notable and widely used structural dimethacrylates from Fig. 9.4 are ethoxylated bisphenol A dimethacrylate (BisEMA) and urethane dimethacrylate.

9.4

Dimethacrylate cross-linking

The BisGMAeTEGDMA system is composed of dimethacrylates. Upon curing, the resulting solid is highly cross-linked. A continuous, covalently bonded network is formed and is generally referred to a thermoset. Thermosets are not soluble or otherwise processable once cured. The advantages of a thermoset are high thermal and mechanical stability. This is in contrast to a resin composed of monomethacrylates, which would result in linear polymers that are soluble, thermally processable, and less mechanically stable. Monomers with three or four methacrylates are possible, but it is O

O

H2C=CC OCH2HCH2CO H 3C

HO

OCH2CHCH2O CC=CH2 OH

BisGMA = Bisphenol A glycidyl methacrylate structure of main component O

O

H2C=CC OCH2CH2OCH2CH2OCH2CH2OCH2CH2O CC=CH2 H 3C

CH 3

TEGDMA = Triethylene glycol dimethacrylate reactive diluent

Figure 9.3 The BisGMAeTEGDMA monomer system for composite resins.

CH 3

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Structural monomers O

O

BisEMA

H 2C=CCO

n

H 2CH 2CO

OCH 2CH 2 OCC=CH 2 n CH 3

H 3C O

UDMA

O

CH 3

CH 3

O

O

CH 2=CCOCH 2CH 2OCNCH 2CHCH 2CCH 2CH 2NCOCH 2CH 2OCC=CH 2 H

CH 3

CH 3

H

CH 3

Reactive diluents O H2C=CC OCH2CH2CH2CH2CH2CH2O H 3C

O CC=CH 2 CH 3

Hexanediol dimethacrylate

O

O

H2C=CC (OCH2CH2)n OCC=CH 2 H 3C

CH 3

TEGDMA derivatives

Figure 9.4 Additional structural dimethacrylates and reactive diluents. BisEMA, Ethoxylated bisphenol A dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate.

not clear if this offers a significant advantage.7,8 Details of the different methods to initiate polymerization and cross-linking will be discussed later.

9.5

Viscosity

Viscosity is the flow response of a fluid to an applied force, as discussed in Chapter 2. The lower the viscosity, the more easily a fluid can flow. For composite resins, the overall viscosity of the unpolymerized paste is a function of the dimethacrylate viscosity and the amount of filler. The higher the filler content, the greater the viscosity. Parameters such as the degree of cure and handling properties are influenced by the viscosity of the unpolymerized composite. In addition, the viscosity of the dimethacrylate component will influence how much filler can be incorporated into the composite. There is an upper limit to the unpolymerized viscosity, so the dimethacrylate component will influence the amount of filler that can be incorporated into the formulation. The reason for a mixture of dimethacrylates is the balance between mechanical properties and viscosity. BisGMA will produce a very strong, rigid polymer network. However, the monomer has very high viscosity, and the addition of fillers would result in a paste that is unworkable. TEGDMA is referred to as a reactive diluent. Its role is to reduce the viscosity of the resin while still polymerizing with the BisGMA. Table 9.1, adapted from Goncalves et al.9 demonstrates the effect of TEGDMA and BisEMA on the viscosity of the system. It is clear that the overall viscosity of the system can be easily manipulated by the formulation to suit the desired outcome. For applications where flow is important, such as sealants or a flowable composite, resins could use a different monomer composition. However, many other properties can be negatively affected by the choice of monomer

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157

Viscosity of dimethacrylate resins with varying formulations

Table 9.1

Viscosity (Pa$s) TEGDMA

TEGDMA:BisEMA (1:1)

BisEMA

33% BisGMA

0.15
0.001

0.65
0.02

3.7
0.06

50% BisGMA

0.76
0.02

3.2
0.02

14.9
0.65

66% BisGMA

5.7
0.02

14.6
0.35

29.2
0.45

BisEMA, Ethoxylated bisphenol A dimethacrylate; BisGMA, bisphenol A glycidyl methacrylate; TEGDMA, triethylene glycol dimethacrylate.

ratio, such as water sorption, degree of curing, polymerization shrinkage, and mechanical properties.

9.6

Water sorption, plasticization, and hydrolytic degradation

Water sorption is the equilibrium amount of water that can be incorporated into the composite under clinical conditions. The effect of water in a composite can influence the mechanical properties, as well as the long-term stability, of the composite. The relative hydrophilic character of the dimethacrylate resin and filler is also an important factor. The hydrophilic character of the filler is determined by the amount of coupling agent. The specifics of the coupling agent will be discussed later, but the coupling agent acts as an interface between the filler and the dimethacrylate resin. Table 9.2, adapted from Truong et al.10 demonstrates the effect of coupling agent on the water sorption of the composite resin. Note that the unsilanated resin has approximately double the water sorption while roughly four times the ethanol (EtOH)/Water sorption. An example of the effect of the dimethacrylate chemistry on water sorption is shown in Table 9.3, adapted from Venz et al.11 The chemical structures are shown in Figs. 9.3 and 9.4. The clear trend is that the more hydrophilic the resin, the higher the equilibrium water uptake. This is particularly noticeable in the comparison between Table 9.2

Equilibrium fluid uptake of composite resins

Product/material

EtOH/H2O (%)

H2O (%)

Filler (%)

Filler type

P10

0.17

0.34

69.1

Hybrid

Concept

1.72

2.10

43.1

Microfine

Experimental silanated

1.19

0.84

51.2

Coarse

Experimental unsilanated

5.80

1.90

51.2

Coarse

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Table 9.3 Equilibrium water sorption under ambient atmosphere and at six months at 100% relative humidity

Monomer

Wt% H2 O ambient

Mol H2O/Mol Monomer, ambient

Wt% H2O, 100% humidity

Mol H2O/Mol Monomer, 100% humidity

HDMA

0.2

0.024

0.52

0.08

BisEMA

0.3

0.097

0.97

0.313

TEGDMA

1.0

0.164

5.45

0.866

BisGMA/ TEGDMA

1.1

0.274

3.75

0.925

UDMA

0.8

0.209

2.67

0.697

BisGMA

1.2

0.333

3.60

1.024

BisEMA, Ethoxylated bisphenol A dimethacrylate; BisGMA, bisphenol A glycidyl methacrylate; HDMA, hexanediol dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate.

BisGMA and BisEMA. The only major chemical difference is that BisGMA has two hydroxyl (eOH) groups, which impart greater hydrophilic character. The equilibrium water sorption of BisGMA is greater than that of BisEMA. Hexanediol dimethacrylate (HDMA), with the most hydrophobic structure, has the least equilibrium water sorption. One of the major effects of water sorption is plasticization of the polymer matrix. Plasticization is caused by the presence of a small molecule in a high-molecular weight polymer matrix. This causes a lowering of the glass transition temperature and results in a softer, more ductile material. The concept of glass transition was introduced in Chapter 2; it is indicative of how much molecular motion is occurring in the polymer matrix. The higher the glass transition temperature, the less molecular motion, and in general this leads to a stiffer material with higher elastic modulus. The introduction of small molecules lowers the glass transition temperature, and if the glass transition occurs at or below room temperature, the polymer becomes more rubbery. In some cases, this is a desirable property; an example is the vinyl plastic sheet. Vinyl plastic is made from the polymer polyvinyl chloride (PVC). Typically, this is a stiff material and is the primary component of modern plumbing pipes or PVC pipes. However, with the addition of a plasticizer, this stiff polymer can become flexible and is commonly used as an inexpensive leather substitute. In the case of a composite resin, it is undesirable for the polymer to soften. Water acts a plasticizing agent in composite resins. There are many examples in the literature of the mechanical properties being reduced when the composite is exposed to water. An example is shown in Table 9.4, adapted from Truong et al.,10 demonstrating the effect of water sorption on fracture toughness. It is clear that the test specimens that have reached equilibrium water content have lower fracture toughness compared to dry samples. The results of a report on

Composite resin polymerization and relevant parameters

159 1

Table 9.4

Effect of water sorption on fracture toughness (MPa$m /2)

Brand

Fracture toughness, dry

Fracture toughness, wet

Occlusin

1.78

1.00

P10

1.48

1.22

Estilux, posterior

1.12

0.72

Experimental, silanized

1.58

1.00

P30

1.17

0.79

Ful-Fil

1.16

0.76

Profile

0.95

0.63

Experimental, unsilanized

1.00

0.50

Silux

0.82

0.50

Isomolar

0.79

0.45

Concept

0.77

0.35

the effect of time in water on the in vivo bond strength for a single orthodontic adhesive resin are shown in Table 9.5, adapted from Meng et al.12 The other major effect of water sorption is the hydrolytic degradation of the resin and the resinefiller interface over time. There are two major consequences to this. The first is the release of degradation by-products. Over the last several years, there

Effect of conditioning in water on adhesion of Concise orthodontic composite resin

Table 9.5

Time in water

Bond strength

Standard deviation

Day 1

0.73

0.14

Day 2

0.72

0.02

Day 3

0.72

0.01

Week 1

0.69

0.14

Week 2

0.67

0.11

Week 4

0.58

0.06

Week 8

0.62

0.09

Week 16

0.60

0.14

Week 24

0.48

0.13

Week 32

0.46

0.24

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H O H

HO

Hydrolysis of coupling agent

OH OH OR O O Si OCH2CH2CH2OCC=CH2 OR CH3

HO HO

OH

HO

OH O OH + HOCH2CH2COCC=CH2 CH3

HO HO

OH OH

OH OH

Hydrolysis of TEGDMA

H O H

O O H2C=CC OCH2CH2OCH2CH2OCH2CH2OCH2CH2O CC=CH2 CH 3 H 3C O H2C=CC OCH2CH2OCH2CH2OCH2CH2OCH2CH2OH H 3C

+

O HO CC=CH2 CH 3

Figure 9.5 Hydrolysis of coupling agent and triethylene glycol dimethacrylate.

have been many studies devoted to the potential biohazard problems caused by the release of organic compounds into the environment. While the resulting occurrence of clinical problems has not been clarified, this has been an area of concern.13e18 The other major consequence is the degradation of mechanical properties. All methacrylate polymers have ester bonds that are susceptible to hydrolytic degradation. Breaking these bonds is the equivalent of reducing cross-linking or the degree of cure. As shown in Fig. 9.5, water can be incorporated into an ester bond and break the linkage. This is a slow process that occurs on a timescale of months and years. However, the damage to the mechanical properties can build up over time and reduce clinical longevity. Fig. 9.5 also demonstrates that hydrolysis can occur between the coupling agent and the filler. This interface is very important to maintain the mechanical properties of a composite resin.

9.7

Polymerization shrinkage

Polymerization shrinkage is a very simple concept, yet it has highly important implications on the long-term stability of a composite resin. As a result of the polymerization process, the liquid or resin is converted into a solid, which results in a density change that reduces the overall volume. These phenomena can lead to internal stresses and stresses at the margins of the restoration. These stresses, over time, can lead to marginal leakage and secondary caries. Polymerization shrinkage is inherent to the curing of methacrylates.19e24 The only known methods to reduce shrinkage are to reduce the number of methacrylates or

Composite resin polymerization and relevant parameters

Table 9.6

161

Composite resin types classified by particle size

Composite type

Average particle size

Macrofill

10e100 mm

Midifill (conventional)

8e10 mm

Small particle filler

0.1e3.0 mm

Microfill

0.02e0.04 mm

Nanofilled

0.005e0.075 mm

Hybrids

Mixture of particle size

utilize a new polymerization chemistry. The silorane-based restoratives (Filtek Silorane, 3M) are an example of novel chemistry in a commercial composite resin.25e30 The other strategy is to increase the filler level, thereby reducing the overall number of methacrylates. Increasing the molecular weight of the monomer, while keeping only two methacrylate groups, will reduce the number of polymerizable methacrylates in the resin. However, either of these strategies will increase the viscosity of the unpolymerized resin. Because of the upper limit for viscosity, there is only so much reduction of methacrylate groups that is possible.

9.8

Filler

The filler or inorganic component imparts hardness and wear resistance. The properties of the composite resin can be manipulated by varying filler loading (percentage), particle size, and coupling agent. Composites are often classified by particle size (Table 9.6, adapted from Mikhail et al.31). Other than macrofills, most of these are still available, and hybrids and microfills are used widely. The coupling agent is an interfacial layer designed to bond the filler chemically to the polymer matrix. The structure and process for attaching a coupling agent to the filler is shown in Fig. 9.6. The coupling agent has a methacrylate group that can polymerize with the matrix. In addition, there are silanol groups that, when exposed to water, will bond with the alcohol groups on the filler surface. Coupling agents that provide silanation of the fillers have been shown to improve the mechanical properties of the composite resins. Condon et al.32 demonstrated that silanating the filler particles improved the wear resistance of the composites. Similar findings were reported for microfilled composites.33 Other mechanical properties are also enhanced by silanation of the fillers.34

9.9

Wear

Wear is an important property for composite resins and is a quantification of the material lost due to repeated contact with another surface.35e41 Abrasion and attrition are

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O OCH3 CH2=CCOCH2CH2Si OCH3 OCH3 CH3

H2O

O OH CH2=CCOCH2CH2Si OH OH CH3 HO

OH OH

HO

OH

HO

OH OH

RO R=

O SiCH2CH2OCC=CH2 CH3

OR OR

RO

OR

RO

OR OR

Figure 9.6 Functionalization of filler with silane coupling agent.

the two main ways in which wear is quantified in the literature. Abrasion corresponds to a situation that is analogous to sandpaper abrading a surface. Attrition adds a force vector normal to the surface and is equivalent to hitting a surface with an object and then dragging the object along the surface. The concepts are more precisely described elsewhere, but these descriptions provide a mental picture of how wear is quantified.

9.10

Filler loading

The greater the filler to resin ratio, the greater the wear resistance and elastic modulus of the resulting composite resin. In addition, the polymerization shrinkage is also lower because there are less methacrylate groups for a given volume of the composite resin. The drawback of increased filler loading is that the viscosity increases. In applications such as veneers or adhesives, filler levels are lower compared to composite resins. In these applications the ability to flow into the microstructure of the tooth to improve bonding and interfacial contact is more important than the wear resistance or elastic modulus. Some restorative applications in low-stress areas will utilize composite resins with lower filler levels compared to posterior composite resins. Flowable composites provide an example of the composite resins with lower filler loading.

9.11

Particle size

The average particle size has a significant influence on the wear and polishability of a composite resin. The smaller the particle size, the greater the wear resistance, and the

Composite resin polymerization and relevant parameters

163

Contact and flexural fatigue of dental restorative materials. Values are mean (standard deviation)

Table 9.7

Contact fatigue cycles 3 103

Flexural fatigue cycles 3 104

Brand

Type

Shofu FX

Conventional glass ionomer cement (GIC)

1.44 (0.19)

28.8 (4.6)

Shofu II

Conventional GIC

1.63 (0.720)

26.9 (1.6)

Vitremer

Resin-modified GIC

9.15 (0.91)

53.0 (10.2)

Dyract

Compomer

51.8 (5.35)

72.9 (14.7)

Silux Plus

Microfilled composite

1339 (200)

72.5 (3.4)

Z100

Hybrid composite

42.7 (18.1)

126.3 (3.1)

better the polishability.42e46 The drawback of a smaller particle size is that as the surface area increases, the viscosity increase is greater per unit weight. Flowable or microfine composites contain nanosized particles but are limited to 50% filler whereas standard composites have 65e75% filler. Composite resins with only nanosized particles have lower elastic modulus and greater shrinkage compared to conventional composites. Hybrid composites incorporate both nanometer- and micron-sized fillers. This compromise allows for composite resins that are highly filled but have some of the desirable wear and polishing properties imparted by the nanofillers. In Table 9.7, adapted from McCabe et al.,47 the resistance of various dental materials to abrasion fatigue and flexural fatigue is examined. The results measure the number of cycles of either abrasion or flexural stress before the test specimen fails. Of particular note is the difference between Silux Plus, a microfill, and Z100, a hybrid composite. Silux plus has a very high value for contact fatigue because the nanofillers provide excellent wear resistance. However, only modest resistance to flexural fatigue is observed because of lower levels of filler. Z100 displays the opposite behavior because of the higher levels of loading and overall larger average filler size.

9.12

Initiator

The goal of an initiator system is to control the curing of the methacrylate resins; it serves as a “trigger” to start the process. The polymerization of methacrylates is initiated by free radicals, which have unpaired electrons. These initiators generate a sufficient level of free radicals to sustain the polymerization process. Methacrylates having free radicaleinitiated polymerization are widely used in dentistry. Free radical polymerization is a rapid process and is not water sensitive. Moreover, compared to other systems, the methacrylates and the initiator systems are not toxic.

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OR'

R R

+

OR' Initiator

Methacrylate

Methacrylate polymer

OR' Propagation Backbiting recombination oxygen Termination

Figure 9.7 Free radical polymerization of a methacrylate (Scheme 1).

The polymerization of methacrylates can be initiated by three different methods: light, heat, and mixing. The role of the initiator system is to convert these external stimuli into free radicals.48,49 Light and mixing are the curing methods that are of the most interest to orthodontists, as they can be used in the oral cavity. Heat is commonly used in prosthetics and in other indirect restoration applications. A generic example of free radical polymerization is shown as Scheme 1 in Fig. 9.7. Note that there are several potential paths to terminate the propagation of the free radicals. The details of the termination reaction can be found elsewhere.50 The rate of the initiation and propagation must be such that they overcome any termination reactions.

9.13

Light curing

Light curing is the most common method of initiation for direct restoratives and adhesive systems.49,51 The two main advantages are that no mixing is necessary and that light provides for a command cure. As long as there is no light trigger, there is essentially an indefinite working time. The material can be shaped and manipulated until the light is applied. The two main disadvantages of light curing is a limited depth of cure and that curing will not occur under opaque substances. This is a particular issue with orthodontic brackets that are often cured from the side. The primary initiator system for light curing utilizes camphorquinone (CQ) and dimethylamino ethyl methacrylate (DMAEM). As shown in Scheme 2 (Fig. 9.8), the role of CQ is to absorb visible light and transfer a free electron to DMAEM. This free radical will then initiate the polymerization of the methacrylate monomers.

9.14

Cold curing

Cold curing is curing that is initiated by mixing two components such that the free radical is generated by a redox reaction. The main advantage of cold curing is that

Composite resin polymerization and relevant parameters

165

hv O

O

H 3C N

CQ

CH2CH2OCC=CH2 DMAEM

H3C

CH3

O

O O

O O

O

H 3C N

H3C

CH3

O + N CHCH2OCC CH2

H3C

OH

H

CH3

O

H3C N

O

CH2CH2OCC CH2

H3C

CHCH2OCC CH2

H3 C

CH3 O CH2 CCOR CH3

Methacrylate monomer

Figure 9.8 Visible light polymerization initiated by camphorquinone and dimethylamino ethyl methacrylate (Scheme 2). The incident light photon is represented by hv, where h is Planck’s constant and v is the frequency.

the composite will cure without any special instrumentation, at ambient conditions, and under opaque materials. The main disadvantages are that mixing is required, and the degree of cure is often not as great as that for light-cured composites.52 There are also limitations for the viscosity of the material as mixing is required. The primary reaction is shown in Fig. 9.9 (Scheme 3). An amine will react with a peroxide to form a free radical and initiate polymerization. Many such combinations are known as initiator systems.53,54 The composite is formulated as part A and part B, with part A containing the amine and part B the peroxide. Both parts will contain methacrylate and filler, and often these composites are dispensed in cartridges that mix the two parts.

9.15

Dual cure

Dual cure composites are a combination of light-curing and cold-curing mechanisms. These systems have a part A and a part B redox system, along with a visible light

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O C

O

O O

O

C

C

O

CH 3 CH 2CH 2OH H 3C

H 2C

N CH 2CH 2OH

O

C COCH 3

CH 3 H 2C O

C

R

COCH 3

Figure 9.9 Cold cure initiation of methacrylate polymerization by redox reaction between peroxide and amine (Scheme 3).

initiator. Initial mixing is required, but light is then used to cure the composite while the redox reaction will cure where the light does not penetrate. These systems are particularly useful in bulk-fill composites and in areas where light might not contact all of the composite.55 These advantages are beneficial for deep restorations where layering is not performed, with only the inconvenience of a mixing step. As lightinitiated curing will yield a higher degree of conversion, greater levels of curing occur on surfaces that encounter the highest stresses and wear.

9.16

Degree of cure

One of the most important parameters of an initiating system is the resulting degree of cure. This is a measure of how many methacrylate groups are polymerized, divided by the number of starting methacrylate groups. Light-cured composites have a degree of cure ranging from 55% to 75%.56 In general, the higher the degree of cure, the greater the mechanical properties. This is true of elastic modulus, tensile strength, and other physical parameters such as water sorption and wear.34 Condon et al.32 found that increasing the degree of cure from 56% to 66% increased the resistance of the composite to wear. Table 9.8, adapted from Condon et al.,32 presents the results of this investigation. A higher degree of cure also reduces the amount of unpolymerized monomer that can be leached out of the composite. Anseth et al.56 estimated that at 75% cure, 6.25% of the monomer is unreacted and not incorporated into the cross-linked network. As with the hydrolytic breakdown products, there are concerns about the effect of unpolymerized monomer escaping from the composites.

Composite resin polymerization and relevant parameters

167

Effect of degree of cure on wear properties. Average wear (standard deviation) in microns

Table 9.8

Cure time

Baseline

6 months

1 year

2 years

9s

10(5)

81(40)

97(44)

144(62)

12 s

8(4)

62(34)

74(38)

112(51)

25 s

5(3)

35(24)

43(28)

69(40)

40 s

5(4)

25(19)

32(24)

50(31)

9.17

Parameters that affect cure

Many factors will affect the degree of cure. Two of the most important are viscosity and the presence of oxygen. Oxygen is known to terminate free radical polymerization and is observed in the oxygen inhibition layer common to sealants and composite resins. At the surfaces, where oxygen concentration is high, the inhibition is sufficient that polymerization is effectively prevented. The viscosity of the unpolymerized resin will also affect the degree of cure. For a free radical to propagate, it must come into contact with an unpolymerized methacrylate group. Once the resin matrix has reached a certain viscosity, termed vitrification, no further polymerization is possible. As the curing process proceeds, the viscosity increases until vitrification is reached. There is insufficient molecular mobility for the free radicals to bond with the monomer. For any given resin system, the vitrification point is a fixed value; the greater the initial viscosity, the less conversion is needed to reach the vitrification point. Two other significant factors that will affect the degree of cure are the light source and the length of exposure. There are many studies which demonstrated that these two parameters are important for composite resins and orthodontic adhesives. Table 9.9, adapted from Santini et al.,57 compares the degree of cure for three orthodontic adhesives using three different light sources.

Degree of cure (%) as function of light source. (Parentheses are standard deviations, and letters indicate values that are not significantly different.)

Table 9.9

Adhesive

Value for dual peak

Blue phase dual peak

Blue phase single peak

APCþ

61.0 (6.5)a

59.8 (4.2)b

63.3 (5.9)c

Opal bond

60.9 (5.3)a

59.6 (5.2)b

61.5 (5.3)c

Light bond

46.3 (5.1)

45.9 (3.2)

45.8 (2.7)

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