Dental Filling Composition of a Coefficient of Thermal Expansion Approximating That of Natural Tooth Enamel (1970)
U.S. Patent No. 3,503,128, granted on March 31, 1970, to Robert N. Boyd, Lawrence Colin, and Edward G. Kaufman (assigned to Dentsply International Inc.), describes an advanced reinforced resin composite formulated for direct dental restorations. The invention targets the long-standing mismatch between synthetic restorative materials and natural tooth structure, establishing a composite with mechanical durability, rapid intraoral curing, and thermal stability that closely matches living enamel and dentin.
This specific invention solved a persistent failure mode in restorative dentistry: marginal leakage, percolation, and recurrent secondary caries. Prior direct restorative materials (such as silicate cements and unfilled methacrylate resins) expanded and contracted at rates radically different from natural tooth enamel during normal eating and drinking. Repeated ingestion of hot and cold substances caused the fillings to break their marginal seals, allowing oral fluids and bacteria to seep around the restorations. Boyd, Colin, and Kaufman resolved this by incorporating ceramic and mineral fillers possessing negative and ultra-low coefficients of thermal expansion into low-temperature curing resin matrices.
The Innovation: Negative Thermal Expansion Mineral Counterbalance
Unfilled acrylic and methacrylate resins possess linear coefficients of thermal expansion between 80 x 10^-6 /°C and 100 x 10^-6 /°C, whereas natural human enamel sits at approximately 11.4 x 10^-6 /°C. Silicate cements offered lower expansion (7.8 x 10^-6 /°C) but suffered from low tensile strength, brittleness, and severe solubility in mouth fluids.
The inventors discovered that incorporating finely divided minerals exhibiting negative coefficients of thermal expansion directly counterbalances the high positive expansion of curing synthetic resins.
Why Negative Expansion Fillers?
- Thermal Matching: Minerals that contract slightly upon heating (such as beta-eucryptite) offset the thermal expansion of the organic polymer matrix, pulling the overall composite expansion curve down into close alignment with tooth structure over oral temperatures (0°C to 60°C).
- Marginal Integrity: Preventing differential thermal expansion eliminates the microscopic gap widening (percolation) that typically admits stain and bacteria at restorative margins.
- Structural Reinforcement: High-density inorganic particulate loading elevates compressive and tensile strength while minimizing polymerization shrinkage during hardening.
Key Chemical Components
The composition is a balanced filled-resin system where each constituent fulfills a distinct mechanical or chemical role:
| Component | Function |
| Low-Temperature Curing Resin (5% to 50%) | Fluid matrix base (unsaturated polyesters, epoxy resins, or Bisphenol A-glycidyl methacrylate adducts) that hardens rapidly intraorally. |
| Negative Expansion Filler (50% to 88%) | Minerals with negative thermal coefficients (beta-eucryptite, petalite, lepidolite, spodumene) that neutralize polymer matrix expansion. |
| Low Expansion Filler (0% to 25%) | Minerals with low positive coefficients (topaz, white beryl, zirconium silicate) or reinforcing whiskers (alumina, silicon carbide) that fine-tune physical stiffness. |
| Silane Coupling Agent (0.05% to 8%) | Organofunctional silanes (e.g., methacryloxypropyltrimethoxysilane) providing chemical adhesion between mineral filler particles and the resin matrix, while assisting bonding to the cavity walls. |
| Catalyst and Promoter System | Peroxide initiators (MEKP, benzoyl peroxide) paired with amine or cobalt accelerators (cobalt octoate, dimethylaniline) providing complete cure within 7 to 10 minutes at mouth temperatures. |
Performance: Physical Properties Comparison
The patent demonstrates that balancing synthetic polymers with negative-expansion lithia minerals elevates mechanical metrics to match or exceed natural tooth thresholds:
| Material | Tensile Strength (p.s.i.) | Compressive Strength (p.s.i.) | Coefficient of Expansion (x 10^-6 /°C) | Modulus of Elasticity (x 10^6) |
| Methyl Methacrylate | 4,000–6,000 | 10,700 | 80–100 | 0.26 |
| Silicate Cement | 700 | 23,000–27,000 | 7.8 | 2.1 |
| Natural Enamel | 1,500 | 40,000–56,000 | 11.4 | 12.0 |
| Natural Dentin | 7,800 | 50,000 | 7.8 | 2.6 |
| Reinforced Resin (Example I) | 1,600 | 8,500 | 19.1–27.9 | High stability |
| Reinforced Resin (Example V) | 5,400–7,100 | 12,000 | 39.0 | High stiffness |
The Compounding and Preparation Process
The inventors specified a controlled sequence to prepare and activate the direct filling material:
- Filler Heat-Cleaning: High-temperature baking or vacuum degassing of ceramic powders (e.g., beta-eucryptite, topaz) to strip absorbed surface moisture and impurities.
- Silane Surface Functionalization: Coating the clean mineral particulates with up to sixteen monolayers of organosilane (e.g., in an ultrasonic bath with deionized water and dilute acetic acid), followed by thermal drying at 230°F.
- Resin Blending: Dispersing the silanized mineral mixture into the liquid polyester or epoxy binder.
- Catalyst/Accelerator Activation: Incorporating peroxide catalysts and metal or tertiary amine promoters immediately prior to placement.
- Cavity Placement and Cure: Inserting the composite paste directly into the prepared tooth cavity, where it sets into a hard, insoluble mass in under 11 minutes at mouth temperature.
About the Assignee: Dentsply International Inc.
Dentsply International (headquartered in York, Pennsylvania) is one of the world’s oldest and largest dental product manufacturers, tracing its lineage back to the late 19th century.
- Clinical Innovation: During the 1960s and 1970s, Dentsply invested heavily in the transition from fragile silicate cements to modern cross-linked composite resin technologies.
- Industry Transition: The Boyd, Colin, and Kaufman patent contributed to the foundation of modern aesthetic restorative dentistry, demonstrating how silane coupling agents and high inorganic mineral loadings could overcome the thermal and physical weaknesses of early dental plastics.
Summary of Claims
The patent explicitly claims:
- A process of filling a tooth cavity using a direct-filling composition consisting of 5% to 50% by weight of a low-temperature curing liquid resin (unsaturated polyesters or epoxy resins).
- A finely divided mineral filler comprising 50% to 88% by weight of materials with a negative coefficient of thermal expansion selected from beta-eucryptite, lepidolite, petalite, and spodumene.
- The optional inclusion of 0% to 25% by weight of low-expansion mineral fillers selected from topaz, white beryl, alpha alumina whiskers/fibers, silicon carbide whiskers/fibers, and zirconium silicate.
- Pre-treatment of the mineral filler with 0.05% to 8% by weight of a silane bonding agent to couple the inorganic particles to the organic resin binder.
