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An In-Depth Look at Resin-Based Composites: Composition, Properties, and Applications

Resin-based Composites

Dental composite resins (better referred to as “resin-based composites” or simply “filled resins”) are dental cements made of synthetic resins.

Synthetic resins evolved as restorative materials since they were insoluble, of good tooth-like appearance, insensitive to dehydration, easy to manipulate and inexpensive.

Composite resins are most commonly composed of Bis-GMA and other dimethacrylate monomers (TEGMA, UDMA, HDDMA), a filler material such as silica and in most applications, a photoinitiator.

Dimethylglyoxime is also commonly added to achieve certain physical properties such as flow-ability. Further tailoring of physical properties is achieved by formulating unique concentrations of each constituent.

Resin-based Composites

Resin-based Composites:

Methacrylate-based monomers:

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BISGMA (Bisphenol A-Glycidyl Methacrylate)

Bis-EMA (Ethoxylated Bisphenol A Dimethacrylate)

UDMA (Urethane Dimethacrylate)

TEGDMA (Triethylene Glycol Dimethacrylate)

HEMA (2-Hydroxyethyl Methacrylate)

Formulation Details:

Resin-based Composites:

Typical composition:

Organic matrix (30-50% by weight): Monomers (e.g., BISGMA, UDMA, TEGDMA)

Inorganic fillers (50-70% by weight): Silica, quartz, zirconia, barium glass

Coupling agents (1-5%): Silane compounds to bond fillers to matrix

Initiator system: Camphorquinone + tertiary amine for light-curing

Inhibitors: Hydroquinone or butylated hydroxytoluene (BHT)

Bisphenol A-Glycidyl Methacrylate (BISGMA):

Chemical structure: Two phenol rings connected by a central carbon atom, with methacrylate groups at both ends.

Properties:

Molecular formula: C29H36O8

Molecular weight: 512.6 g/mol

High viscosity: 500,000-800,000 cP at 23°C

Glass transition temperature (Tg): ~-7°C

Refractive index: 1.55

BISGMA has high molecular weight and rigidity due to its aromatic rings, contributing to its high viscosity and mechanical strength. It’s hydrophobic and has low polymerization shrinkage (5-6%). However, its high viscosity limits filler loading.

Urethane Dimethacrylate (UDMA):

Chemical structure: Aliphatic chains connected by urethane linkages, with methacrylate groups at both ends.

Properties:

Molecular formula: C23H38N2O8 (common variant)

Molecular weight: 470.6 g/mol

Lower viscosity than BISGMA: 5,000-10,000 cP at 23°C

Glass transition temperature (Tg): ~-35°C

Refractive index: 1.48

UDMA offers lower viscosity and higher flexibility compared to BISGMA. It also provides good wear resistance and toughness.

Triethylene Glycol Dimethacrylate (TEGDMA):

Chemical structure: Linear molecule with two methacrylate groups connected by a triethylene glycol chain.

Properties:

Molecular formula: C14H22O6

Molecular weight: 286.3 g/mol

Very low viscosity: 100 cP at 23°C

Glass transition temperature (Tg): -83°C

Refractive index: 1.46

TEGDMA is used as a diluent monomer to reduce overall viscosity. It increases degree of conversion but also increases polymerization shrinkage.

BISGMA (Bisphenol A-Glycidyl Methacrylate)

Pros:

High mechanical strength due to rigid aromatic structure

Low polymerization shrinkage (5-6%)

Good wear resistance

High refractive index (1.55), allowing good color matching

Cons:

Very high viscosity (500,000-800,000 cP at 23°C), limiting filler incorporation

Hydrophobic nature can affect bonding to dentin

Potential concerns about bisphenol A release (though studies show minimal risk)

Relatively high water sorption due to hydroxyl groups

Bis-EMA (Ethoxylated Bisphenol A Dimethacrylate)

Pros:

Lower viscosity than BISGMA (5,000-9,000 cP at 23°C)

Maintains good mechanical properties

Lower water sorption than BISGMA

Allows for higher filler loading

Cons:

Slightly higher polymerization shrinkage than BISGMA

Still contains bisphenol A structure (though modified)

Less reactive than TEGDMA or UDMA

UDMA (Urethane Dimethacrylate)

Pros:

Lower viscosity than BISGMA (5,000-10,000 cP at 23°C)

High flexibility, improving toughness

Good wear resistance

Higher degree of conversion than BISGMA

Cons:

Higher polymerization shrinkage than BISGMA

Lower strength compared to BISGMA

Can increase water sorption in some formulations

TEGDMA (Triethylene Glycol Dimethacrylate)

Pros:

Very low viscosity (100 cP at 23°C), excellent as a diluent

Increases the degree of conversion in composite formulations

Improves handling properties of the composite

Cons:

High polymerization shrinkage (12-14%)

Increases water sorption due to ethylene oxide groups

Can lead to increased cytotoxicity in uncured state

Lower mechanical properties compared to BISGMA or UDMA

HEMA (2-Hydroxyethyl Methacrylate)

Pros:

Hydrophilic nature improves wetting and bonding to dentin

Low viscosity, useful as a diluent

Promotes penetration into demineralized dentin

Cons:

High water sorption, leading to hydrolytic instability

Lower mechanical properties compared to other monomers

Potential for continued release from the polymer network

Higher polymerization shrinkage than BISGMA or UDMA

Product Comparison

Viscosity: TEGDMA < HEMA < Bis-EMA ≈ UDMA < BISGMA

Mechanical strength: BISGMA > Bis-EMA > UDMA > TEGDMA > HEMA

Polymerization shrinkage: TEGDMA > HEMA > UDMA > Bis-EMA > BISGMA

Water sorption: HEMA > TEGDMA > BISGMA > UDMA > Bis-EMA

Degree of conversion: TEGDMA > UDMA > Bis-EMA > HEMA > BISGMA


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