2.1

Materials

The commercial materials used were: Neocolloid, Zhermack; Palgat Plus, 3M ESPE (both sodium alginates) and Blueprint Cremix, Dentsply DeTrey (potassium alginate). Approximate compositions are given in Table 1 . Each material was mixed according to the manufacturer’s instructions. Neocolloid was the only commercial material used for experiments on dimensional and weight changes after setting. For all other tests, all three materials were used.

Three experimental materials were developed and tested. The formulations and source of constituents for the experimental materials are given in Tables 2 and 3 . ExI is the formulation recommended by the alginate manufacturer ISP Alginates (UK) Ltd. ExII was formulated with an increased sodium pyrophosphate content to investigate the effect on setting time. It should be noted that the formulations contain magnesium oxide (MgO); not shown in most dental material textbooks . Its role will be demonstrated subsequently. A third experimental material (ExII F1) was formulated without MgO. All three materials were used for all tests, except the dimensional and weight changes, where ExII F1 was not tested.

2.2

Methods

2.2.1

Sample preparation

For commercial materials, the equivalent to one level scoop of powder and one measure of water were mixed in a rubber bowl. For all experimental materials, 10 g powder was mixed with 23 ml water for 45 s. In all cases, the water temperature was 23 °C. Wax molds were used to make rectangular samples measuring 10 mm × 60 mm that were either 1.5 mm or 3 mm thick. In all cases, one mix was used to make one sample only. To make a sample, a wax mold was placed onto a glass plate, and the rectangular cavity of the wax was filled with the mixed alginate then covered with a glass slide. Once the material had set, the glass slide was removed and the alginate sample was carefully cut out of the mold using a scalpel. Similar procedures were used to make specimens for hardness and tear testing.

2.2.2

Measuring dimensional changes

A Chesterman traveling microscope was used to measure dimensional changes in water and artificial saliva (Orthana, A.S. Pharma Ltd, UK). Corresponding studies in air are described elsewhere .

Immediately after preparation, the sample was placed in a polytetrafluoroethylene (PTFE) trough and secured at one end on a fixed pin. A movable pin was inserted into the other end of the sample. The distance (±0.0001 cm) between the fixed pin and the sample edge closest to the movable pin was measured at the following intervals during an 8 h period:

• •

  every 5 min for the first hour;

• •

  every 15 min for the second hour;

• •

  every 30 min for the remaining 6 h.

Five samples of each material (Neocolloid, ExI and ExII) were tested.

2.2.3

Determination of setting behavior

An oscillating rheometer was used to determine setting times. The rheometer was a modification of the instrument first described by Bovis et al. . This instrument subjects the specimen to an oscillating stress of constant amplitude and frequency and measures the resulting oscillating strain profile. The well known ‘wine glass’ profile is obtained as the material sets.

Before starting the experiment, the incubator was set to 37 °C and allowed to stabilize. A flask of ice water connected to the chart recorder by a copper-constantan wire was used as a 0 °C reference point. The chart speed was set at 5 mm/min and the rheometer left to oscillate for 2 min prior to testing the sample, to record the original trace length.

Each commercial alginate was mixed according to the manufacturer’s instructions (experimental materials mixed as specified previously), using tap water at 23 °C. Recording of the oscillations began simultaneously with the time of mixing, to give a true indication of setting time. The mixed alginate was then loaded onto the bottom plate of rheometer, ensuring that the 1.1 mm gap was filled. The top plate was then placed back onto the rheometer and fastened with two bolts. The experiment was terminated after sufficient time had elapsed for the material to set completely. The same procedure was adopted for the experimental materials and five samples of each material were tested. Setting time was defined as the length of time taken for a 40% reduction in chart width.

2.2.4

Shore A hardness

A Congenix Wallace Shore A hardness durometer (Wallace Instruments, UK) was used to determine hardness in accordance with the method specified by the American Society for Testing Materials (ASTM) , over a period of 15 min. A load of 10 N was applied to test specimens.

The sample preparation was essentially that given previously, except that quantities were doubled in order to give sheets of 100 mm × 80 mm × 2 mm. On setting, samples were removed from the mold and cut into three equal strips of 100 mm × 25 mm × 2 mm. For testing, three strips were placed on top of each other to give the required sample thickness of 6 mm. Six readings were taken for each sample at each time point and the mean was used as the reading. Three samples of each material were tested.

Gent has shown that the following relationship exists between Shore hardness and Young’s modulus ( E ) (also referred to as modulus of elasticity), a measure of stiffness:

2.2.5

Tear strength measurement

Tear testing was carried out at room temperature using a 5567 model testing machine by Instron (UK). Sheets of alginate materials were prepared as described above and specimens of 75 mm × 25 mm × 2 mm were cut out using a scalpel. An incision measuring 50 mm was made down the middle of specimen to make a ‘trouser test’ piece . The sample was held in place with clamps and extended at a constant rate of 100 mm/min using a 5 N load cell. Seven samples of each material were tested and the tear energy ( Г ) calculated from:

where Г is the tearing energy (J/m ² ), F the force (N), and t the thickness (m).

Eq. (2) ignores energy expended to extend and stretch the ‘legs’.

2.3

Statistical analysis

Statistical analysis of the results was carried out using the Mann–Whitney U test.

2.1

Materials

The commercial materials used were: Neocolloid, Zhermack; Palgat Plus, 3M ESPE (both sodium alginates) and Blueprint Cremix, Dentsply DeTrey (potassium alginate). Approximate compositions are given in Table 1 . Each material was mixed according to the manufacturer’s instructions. Neocolloid was the only commercial material used for experiments on dimensional and weight changes after setting. For all other tests, all three materials were used.

Table 1

Composition and mixing ratios for commercial alginate impression materials.

Neocolloid (Zhermack)	Palgat Plus (3M ESPE)	Blueprint Cremix (Dentsply)
Sodium pyrophosphate	Trisodium phosphate	Disodium orthophosphate
Zinc oxide	Magnesium oxide	Magnesium oxide
Diatomaceous earth	Diatomaceous earth	Inorganic filler
Cristobalite	Sodium alginate	Potassium alginate
Potassium fluorotitanate	Dipotassium hexafluorotitanate	Potassium hexafluorotitanate
	Calcium sulfate	Calcium sulfate dihydrate
		Polypropylene glycol
		Sodium aluminosilicate

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