Mechanical Properties of Hydrocolloid and Rubber Impression Materials
Hampson' has studied the elasticity of alginates under compression and has observed that materials with smaller amounts of permanent deformation were more suitable for use in the presence of undercut areas. Schoonover and Dickson2 report the breaking stress, percent strain, and percent set of alginates. Cresson3 suggested the revision of the testing procedure for compressive strengths of dental elastic impression materials that would apply the load at the rate of 18,000 Gm. per minute. Skinner et al.4 and Phillips5 point out the importance of the rate of deformation on the technic of handling alginate hydrocolloids. Warner" also shows that the breaking strength and percent deformation of alginates are functions of the loading rate where the higher the rate, the greater is the strength qualities. Anderson7 shows the recovery from deformation of alginate to be a direct function of time from the start of mixing. Donnison and Docking8 have measured the permanent deformation, stiffness, and toughness of alginates, the latter being determined as a combination of compressive strength and the strain at rupture. The ADA Specification 189 requires the compressive strength to be determined at a rate of 10,000 -+ 2 000 Gm. per minute. Based on an average strength of 7,000 Gm./cm.2 and a specimen '2 inch in diameter, the time required for rupture would be approximately half a minute. Recently, Braden'0 has determined the energy of tearing of one each of mercaptan, silicone, and alginate impression materials as a function of the rate of tearing and the temperature of testing. As the rate of tearing increased and the temperature decreased,
Suggested Revisions for Testing Dental Elastic Impression Materials
IN the development and plant control of elastic dental impression materials, of both the agar and alginate base types, it is necessary to rate materials as to their respective efficiency for practical use in order to select the best formulation or manufacturing process. The tests most commonly used for this purpose are those outlined in A. D. A. Specification #11 for hydrocolloidal impression materials' and the Medical Department Tentative Specification No. MD-1285D for algin"ate impression materials. Rating the efficiency of experimental impression materials from the data obtained in these tests does not always check the experience of operators in clinical tests. Therefore this study of measurement of the various physical properties of elastic impression compounds was instituted after preliminary investigation had indicated that the specification tests could be altered, somewhat, to permit a more accurate comparison and a better evaluation of the efficiency of the impression materials. Considering the properties such a material should possess, they would include: (1) ability to pass undercuts without rupturing, (2) ability to return to the original shape, and (3) rigidity sufficient to resist undue deformation when plaster or stone is poured into the impression material. The specifications now used for the two types of material provide tests for (1) _ aruir, o deformation caused by a given load, (2) set, or permanent deformation resulting from md (3) the compressive load required to rupture the material. These three speion tests serve as a fairly good guide in the evaluation of an impression material but do not, in every case, yield results which are comparable.
The physical properties of agar type hydrocolloid impression material.
A l t h o u g h the use of hydrocolloid mate­ rials has become widespread in restor­ ative dentistry, and it is accepted as the ultimate in a well-rounded impres­ sion technic, a more scientific analysis of the materials available and their proper manipulation is necessary before a stand­ ard technic can be evolved. A t present we are performing with these materials de­ tailed and precise procedures for which they were never intended. A complete understanding of the physical chemistry of hydrocolloid materials is essential be­ fore maximum benefits can be derived from their use and the ideal accomplished in practical application. The hydrocolloid impression materials are composed of 6 to 12 per cent agaragar as the active principle in a vehicle of water of approximately 70 to 80 per cent, depending upon the amount of ad­ ditions, such as rubber, wax, various salts and other materials, to make one hundred per cent. Agar-agar is an organic colloid derived from the cell wall of red seaweed (Gelidium cartilaginium, corneum, amansii, and so forth).1 It is essentially a hydrophilic, emulsoid polysaccharide.2 The terms “hydrophilic” (water-lov­ ing) and “ emulsoid” are nearly synony­ mous. T h e distinction of emulsoid is pro­ vided as a contrast to the suspensoid colloids which are usually inorganic, such as colloidal gold, silver, aluminum or silica, and are usually hydrophobic (water-hating). These are always com­ posed of solids so finely divided that they do not settle out of a liquid medium. In the same manner, emulsoids are suspen­ sions of minute droplets of liquids within a liquid medium. The term “ phycocolloid” is often used to distinguish gelforming materials, such as agar-agar, from the alginates which are not strictly gel-forming substances,2 although they too are derived from algae. Emulsoids of all types have many properties in common, such as viscosity higher than water, a surface tension lower than water, and the stability of their solutions depends more upon the nature of the medium in which they are dispersed than upon the electrical charge on the particles as in the case of suspensoids. An outstanding property of colloi­ dal solutions is that their particles are electrically charged. All colloidal particles in the solution carry the same charge. Some colloidal solutions are innately charged positively, and some negatively. Agar-agar carries a faint negative charge. It is the existence of the single charge which keeps the particles dispersed through the medium; they are prevented by the natural repulsion between like charges from coalescing and so forming larger particles which would finally settle out.