Decoding Heat Cure Denture Base Polymers. By Tom Zaleske

Transcription

1 Decoding Heat Cure Denture Base Polymers By Tom Zaleske 24 Journal of Dental Technology August/September 2007

2 s dental technicians and laboratory owners trying to survive in a global marketplace, we often have to decode the international and U.S.-based designations attached to heat-cured denture base polymers. These designations refl ect a standard in quality as judged by the certifying body that granted it. But should we accept these designations on face value? How many of us actually understand who the certifying bodies are? What tests do the certifying bodies perform on denture base polymers? What do the tests actually indicate? This article will act as your decoder ring. I will define acronyms used for some certifying bodies, identify and explain some of the more prevalent certification standards and explain how heat-cured denture base polymers are tested. Then you will be able to identify what properties are part of a minimum regulated standard to which a product wearing a certain seal of certification must conform. This is important because testing confirms and establishes a standard minimum of quality that meets recognized requisite criteria for a product. Minimum requirements for heatcured denture base polymers include a degree of: 1) Color stability. 2) Dimensional stability under all conditions of service. 3) Strength, resilience, and abrasion resistance. 4) Odor and stain resistance. 5) Insolubility to oral fluids or orally ingested fluids. 6) Tasteless, odorless, non-toxic and non-irritating to oral tissues. 7) Low specific gravity. 8) A softening temperature well above the temperature of hot foods or liquids taken into mouth. 9) Repairabilty. There are many certifying bodies, but the three described below have the most involvement with heat-cured denture base polymers. The International Organization for Standardization (ISO) is an international standard setting organization composed of representatives from national standards bodies (Figure 1). It was founded in 1947 in Britain to establish manufacturing standards lacking in the manufacture of bombs during World War II. Today, it establishes worldwide industrial and commercial standards. ISO standards are the culmination of worldwide input. While ISO defines itself as a nongovernmental organization, its standards often become law through treaties or national standards. Participants include several corporations and at least one standards body from each member country. It is worth noting that an ISO certificate is not a one-time award but it must be renewed at regular intervals, usually around three years. The American Society for Testing and Materials (ASTM International) is an international voluntary standards organization that develops and produces technical standards for materials, product, systems and services (Figure 2). Figure 1 Figure 2 Figure 3 August/September 2007 Journal of Dental Technology 25

3 This organization was formed in 1848 in the United States by a group of scientists and engineers who wanted to address the frequent rail breaks plaguing the railroad industry. This led to the development of a standard for the steel used to fabricate rails. ASTM standard compliance is voluntary, but in 1995 the U.S. government passed the National Technology Transfer and Advancement Act. As a result, the government is required to use privately developed standards whenever possible and ASTM standards have been incorporated into or are referred to by many federal regulations. The American National Standards Institute (ANSI) approves standards Figure 4 that are developed by representatives of standards developing organizations, government agencies, consumer groups, companies and others (Figure 3). These standards make sure that the characteristics and performance of products are consistent, that organizations use the same definitions and terms and that products are tested the same way. This organization coordinates U.S. standards with international standards so that American products can be used worldwide. In regard to denture base polymers, ANSI has an ongoing role in the adoption of ISO 1567: 1999 requirements being reflected in the American Dental Association specification No.12, a standard established for denture base polymers. These certification bodies evaluate and certify heat-cured denture base polymers focusing on several characteristics including but not limited to: Impact Strength Flexural Strength Low residual monomer Color Stability Surface Characteristics Cadmium Free Good Working Characteristics The remainder of this article will explain how these tests are conducted. Because the standards between the differing certification bodies for the most part are similar, I will cite test methods and standards in accordance with ISO 1567:1999, which is considered to have the highest certification standard when we discuss denture base polymers. No. 1 on the list of most often mentioned properties for denture based polymers is impact strength (Figure 4). Testing can be performed using several different pendulum impact tests. The testing facility processes 8-10 sample specimens to manufacturer specifications and mills the samples to a standard identical dimension. A notch can be cut to specification in an identical location on each specimen to prevent deformation of the specimen upon impact. Each specimen is clamped into the pendulum impact test fixture with the appropriate position facing the striking face of the pendulum hammer. The hammer swings freely and strike through the specimen. The amount of energy needed to fracture the specimen is marked, recorded and the machine is reset to test the next specimen. The test is repeated until all the specimens have been struck, fractured and recorded. The average fracture force is then calculated and recorded usually in foot-pound forcer per inch. This is the amount of energy expended when a force of one 26 Journal of Dental Technology August/September 2007

4 pound acts through a distance of one foot along the direction of the force. Impact resistance is a measurement of general toughness so higher numbers indicate the best performance. It is a misnomer to believe that impact strength only applies to how well the sample stands up to dropping a denture or holds up while servicing patients who are non-compliant with reline recall. Toughness or impact strength is of great benefit in the laboratory as it reduces in-house repairs because it withstands breakage in the laboratory during post process de-flasking and handling during finish. In-house laboratory repairs often are an invisible culprit of lowering laboratory profit and throwing off production schedules. High impact strength can limit those repair instances. To test flexural strength, specimen plates are processed according to manufacturer directions and machined to 64mm long, 10mm wide and 3.3mm thick (Figures 5a and 5b). Six plates are sufficient to average with. They are then stored in a water bath for 50 hours at 37º Celsius. The testing rig consists of a central loading plunger and two polished cylindrical supports. The plunger is set for a calibrated speed of decent and the machine is equipped with instrumentation for measuring the deflection of the specimen to within.025mm. Any load exerted by the deflection instrument is accounted for during calibration. If four of the six specimen plates measure not less than 65MPa, the polymer is deemed to have complied with the requirement. In results reflected by this form of flexural strength testing, a high number is always desired. Specimens are processed to test for low residual monomer according to the manufacturer directions in a round shape with a diameter of 50mm and a thickness of 0.5mm. The top and bottom should be flat. Specimens are wet ground equally on both sides to expose fresh acrylic and checked for correct size and thickness according to specification. They are then exposed to acetone and several other solvents. The specimens are tested with gas chromatography to discern levels of released free monomers. The upper levels for residual monomers are not to exceed 2.2 percent mass fraction for Type 1 heat cured polymers. If lower percentages of residual monomers are claimed by the manufacturer, the content shall not be more than.2 percent higher than that stated by the manufacturer. Worth mentioning are the higher permissible levels of residual monomer 4.5 percent mass fraction for Type 2 polymers also known as autopolymerizable polymers or self curing acrylic. Residual monomer has been identified as a possible allergen and causative factor of eczematous reactions in some patients. Patient selection based on a history of hypersensitivity is advised when using Type 2 polymers in reline and construction of dentures. When testing for residual monomer, the lowest possible percentages are desired. The color stability of denture base polymer is tested by fabricating two specimens each which are 50mm in diameter with a thickness of.5mm and possess top and bottom surfaces which are flat. One specimen is stored in the dark as a control. The test specimen is prepared by covering half with aluminum foil then being immersed in a water bath of 37º Celsius and exposed to ultraviolet light for 24 hours. Then the foil is removed and the sample compared against the base specimen that had been stored in complete darkness. When tested and inspected in accordance, specimens should not show more than a slight change in color, perceptible with difficulty. Colorstable polymer is desirable during the serviceable lifetime of a denture to assure Figures 5a and 5b relines or new opposing dentures using new acrylic will match polymer which has already been in service for a period of time. A smooth, hard and glossy exterior is the standard when it comes to surface characteristics. When tested for color stability, residual monomer, sorption and solubility the surface must retain its form without visible distortion after process. When polished, the specimen plates should present a smooth surface with a high gloss. When prepared in accordance with the manufacturer instructions, all denture base polymers must produce a specimen plate with defined edges after deflasking. Smooth, high gloss surfaces are important as they help resist the adhesion of bacterial plaque and its accumulation, which eventually turns to calculus. August/September 2007 Journal of Dental Technology 27

5 Denture base polymers are tested to ensure they are cadmium free. Cadmium was identified several years ago as having cytotoxic effects. Cytotoxicity is the quality of being toxic to cells. Cadmium was used for coloring of (poly)methylmethacrylate resin (denture acrylic) is clear in its base state. No cadmium is a good thing, but it is hardly unusual for it not to be used in denture acrylic anymore. Standards for working characteristics are defined by ISO 1567: It is judged on packing plasticity (Figure 6). The two tests to judge this property involve initial packing time and final packing to judge flow of material. A perforated brass die is used for both tests. The test for initial packing flow involves mixing a 16 grams to 20 grams Figure 6 of polymer to manufacturer specifics. Immediately prior to the recommended initial packing time, a cake shaped mass 5mm thick is placed on the upper surface of the brass die and a glass plate with a weight of 50n is put on top of the mass. After10 minutes the glass plate is removed. When the material is firm, a depth gauge-measuring tool is introduced from the reverse side of the die and each hole is probed to contact the penetrated material to determine the depth. The new mass of material is mixed to specification and, just before the final packing time recommended by the manufacturer, a 5mm cake is formed and placed on the brass die. The glass plate is placed on the mass. After 10 minutes the plate is removed and depth is measured from the reverse side.to pass at the initial packing time and final packing time standards at least two holes must be penetrated at least.5mm deep during both tests. To successfully participate and thrive, it is imperative that dental technicians and laboratory owners understand that global regulation will play a part in their business decisions. Decoding who evaluates the products we use, how minimum standards are established and what constitutes quality will have a direct bearing on our purchases and business identity in the manufacturing marketplace. To understand the difference between minimum standard and high quality, we must first identify established minimum standards, feel confident in their compliance and seek those products. Once we accomplish this, we can look for products that not only meet that minimal standard, but also have been tested by independent sources. Remember, the products we use in our manufacturing process are the first links in the chain of producing quality prosthetics. About the Author Zaleske has been a dental technician for 20 years. In addition to operating Matrix Dental Laboratory, a dental laboratory specializing in removable prosthetics, he works as a lecturer and manufacturer/laboratory consultant. NADL Note Jim McClees, CDT, is the National Association of Dental Laboratories representative on the American Dental Association s ISO dental materials committee. 28 Journal of Dental Technology August/September 2007