Adhesion of carboxylate cements to hydroxyapatite. I. The effect of the structure of aliphatic carboxylates on their uptake by hydroxyapatite.
An investigation was made into the effect of the structure and functionality of aliphatic carboxylates on their sorption onto hydroxyapatite with a view to ascertaining factors affecting the adhesion of polyacrylate cements to tooth materials. In general, the amount of carboxylate sorbed was found to increase with the number of groups contained in the molecule. Thus, polyacrylate was found to be much more strongly sorbed than low molecular weight species. Sorption of the low molecular weight species appeared to be related to stereochemical factors rather than to the stability constants of their calcium chelates.
Mechanical properties of visible light-cured resins reinforced with hydroxyapatite for dental restoration.
OBJECTIVES
The purpose of this study was to measure and analyze the mechanical properties of several composite materials designed for dental restoration.


METHODS
The materials were composed of a visible light-curing monomer mixture (either Bis-GMA+TEGDMA or Bis-GMA+HEMA) as a matrix and hydroxyapatite (either microscopic or nanoscopic particles) as a reinforcing filler. The surface of the hydroxyapatite particles was modified by using a coupling agent (citric, malic, acrylic or methacrylic acid). Five specimens of 14 different composites were prepared for each mechanical test: flexural strength, Young's modulus and Vickers hardness. Mean values and standard deviations were calculated, and ANOVA and Student Newman Keuls multiple comparison tests were applied (P < 0.05).


RESULTS
The addition of 50-60 wt% of hydroxyapatite particles to the unfilled monomer mixtures led to the increase of both Young's modulus and surface hardness of the material, while the flexural strength decreased. In general, when microscopic instead of nanoscopic hydroxyapatite was used as a reinforcing filler, mechanical properties were favored. The mechanical properties were also improved by adding citric, acrylic or methacrylic acid as a coupling agent.


CONCLUSIONS
An adequate surface modification of the hydroxyapatite particles conferred enhanced mechanical properties to the final dental composite. Microscopic-hydroxyapatite particles are preferred to nanoscopic ones.
Arc flash hazard assessment in the mining industry
When it comes to electrical safety in the workplace, arc flash has become the most prominent topic over the past decade. The Occupational Safety and Health Administration (OSHA, including state chapters) is the only government body that recognizes NFPA-70E, Standard for Electrical Safety in the Workplace. However, OSHA does not have jurisdiction in mining operations. Electrical safety for the metal and nonmetal surface mining industry is covered by the Code of Federal Regulations (CFR) Title 30, Part 56, Subpart K (Electricity). The Mine Safety and Health Administration (MSHA) is responsible for enforcing electrical safety in mining operations. CFR Title 30 does not reference personal protective equipment (PPE) to protect electrical workers against arc flash hazards. This paper provides maintenance and safety personnel in surface Metal/Nonmetal mining operations with a step by step guide to implementing an electrical safety program that meets the requirements of MSHA, CFR Title 30 and NFPA-70E.