NATIONAL BUREAU OF STANDARDS REPORT

4495

Progress Report

on

PRELIMINARY STUDY: TROPICAL

STORAGE OP DENTAL MATERIALS

by

Henry I. Copeland, Jr.

W. T. Sweeney

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing

of the divisions and sections engaged in technical work. In general, each section is engaged in

specialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the inside

of the back cover of this report.

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Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Organic Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure.

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Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. Components and Techniques. Digital Circuitry. Digital Systems.

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Gas Liquefaction.

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Research.

Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0708-20-3824 December 31* 1955 4495

Progress Report

PRELIMINARY STUDY: TROPICAL STORAGE

OF

DENTAL MATERIALS

by

Henry I. Copeland, Jr.*

W. T. Sweeney**

* Major, USAF (DC), Guest Worker, Dental Research

Section, National Bureau of Standards.

** Chief, Dental Research Section, National Bureau

of Standards

This work is a part of the dental research program conducted at the National Bureau of Standards in cooperation

with the Council on Dental Research of the American Dental

Association, the Army Dental Corps, the Air Force Dental

Service, the Navy Dental Corps and the Veterans Administration .

(Task 7756-41 BV No. 55-14)

. <^BS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, r APP roved for public release by the

unless permission director of the National Institute of

25 ,d,c. Suchp. Standards and Technology (NIST)

cal iy prepared if On October 9, 2015

ir in part, is prohibited

Standards, Washington

report has been specifireport for its own use.
: - ) ' - > .
PRELIMINARY STUDY: TROPICAL STORAGE

OF

DENTAL MATERIALS

Abstract

A preliminary survey of some dental materials

stored for four years in Puerto Rico by the Armed

Services was made to determine changes in properties

of dental significance. Comparison was made with

duplicates stored at the National Bureau of Standards. Alginate impression materials and investments were found to have deteriorated to an unusable

state. Methyl methacrylate monomer (liquid) containing dissolved polymer had polymerized. Some

items which had volatile and low melting constituents were shown to be improperly packed and melted.

The tests substantiated the present practice of

phase buying and preferential storage. The survey

indicates the desirability for further study of

the effect of storage on many dental items.

1 . INTRODUCTION

Deterioration of dental materials during transport and

storage is of particular interest to the Armed Services. Long

periods of storage* often under extremely adverse conditions*

can be a military necessity. Materials which are unusable or

in an inferior condition on arrival at the operational level

cause a loss of time and money as well as an interruption in

2

dental service.

In 1951* while a guest worker at the Dental Research Section of the National Bureau of Standards* Col. T. E. Fischer*

USAF (DC)* planned an investigation to determine critical

time limits for the storage of certain dental materials in

tropical climates. Although circumstances prevented carrying out the investigation as originally planned* a number of

materiel items have been returned to the National Bureau of

Standards after four years of storage* and their condition examined by visual observation and applicable specification tests.

2. CONDITIONS OF STORAGE

To avoid variations in composition* date of manufacture*

and packaging* samples for storage at three locations were selected with the same batch number.

The dental materials listed under Column A* Table 1 were

stored in a constant temperature and humidity room in the

Dental Research Laboratory of the National Bureau of Standards.

The samples were sealed in a packing case with a moistureproof liner. The room is maintained at approximately a temperature of 70 °F and a relative humidity of 60%

The items listed in Column B* Table 1 were stored on open

shelves in the penthouse of the Rodriguez Army Hospital* San

Juan* Puerto Rico. The prevailing temperature at this site was

the highest of the three locations.

The materials listed under Column C* Table 1 were stored

on open shelves at the Tropical Research Laboratory* San Juan*

- 3 - Puerto Rico, where they were exposed to year round relative

humidities of 75 to 79$.

3. RESULTS AND DISCUSSION

3.1 General Effects of Storage

The appearance of the packaging of individual samples

reflected storage conditions. Metal parts of containers

showed varying amounts of rusting. Containers stored in the

moisture-proof case at the National Bureau of Standards were

clean and relatively free of rust; the penthouse containers

were spotted with rust; and the seaside samples were very

rusty (as may be seen in Figure l) . Cardboard cartons containing materials with volatile or low melting point ingredients

were stained by these ingredients. Figure 2 shows cartons of

a protective coating material. The penthouse sample, "B",

which had been subjected to higher temperatures, is badly

discolored

Materials which were obviously affected by storage or

which had become unusable are marked by an asterisk in Table

1. They are discussed in detail below.

3.2 Investment

The investment samples showed no caking or lumping and

appeared in usable condition; however, consistency tests,

performed according to Federal Specification U-I-5^6, proved

samples from the two tropical locations to be unsatisfactory.

Figure 3 shows typical results of the consistency tests.

Samples stored under "A" conditions show normal slumping.

Samples stored under "B" and "C" conditions set so rapidly

that the test specimens supported their own weight in less
S

-

g- . 3 f

1

’ -.i

’ £’

: f-
than two minutes* a setting rate too fast for dental use.

Since such behavior is most likely caused by partial

hydration [1]* samples of investments from "A", "B"* and

"C" were dried to constant weight at 110°C. The results*

as shown in Table 2* show the varying water content and

setting times.

Assuming that all water in the sample is in the calcium

sulfate and is removed completely by the drying process* calculations on "A n sample of investment indicate that there

w@ce approximately 30 grams of calcium sulfate hemihydrate

per 100 grams of the investment. This is a reasonable proportion for investments of this type. Comparing the percent

weight lost by "B" and "C n samples to that lost by "A" samples

it is readily apparent that partial hydration of the material

stored at "B" and "C" had occurred. Further calculations

suggest that in "B" samples approximately 9$ of the hemihydrate might be considered to be hydrated to the dihydrate form

and in "C M

* approximately 15$.

The setting rate of the hydrated investment may be slowed

and acceptable consistency obtained* by increasing the ratio

of water to investment powder when mixing. This procedure is

not recommended because it weakens the investment [2] and

changes both the setting and thermal expansion [3]* An inlay

cast into a mold made of this partially hydrated material is

not likely to fit satisfactorily.

- 5 - 3.3 Resin Materials

Dental resin materials are procured for the Armed Services

in powder/liquid form. In addition to samples of the standard

supply items* samples of several brands of "filling 11 resin

kits were also stored.

Past experience with resin liquids has indicated the

necessity for phase buying and preferential storage of these

materials to avoid premature polymerization induced by heat.

In this study* "A" samples of denture base resin liquids were

found to be still liquid while "B" and "C" samples had polymerized into clear solids* which* of course* are useless in

the dental laboratory. Figure 4 shows the effect of four

years of storage at "C" on a methyl methacrylate liquid.

It is interesting to note that all of the filling resin

liquids were still in the liquid phase. The liquid of one

brand had changed in appearance from optically clear to a

cloudy pink. The available sample was too small for analysis.

However* it is suggested that the change might be due jo some

chemical reaction involving the tertiary amine initiator. Another brand had not polymerized but the plastic seals on the

bottle caps were broken and a large portion of the liquid

had evaporated. The remaining samples were unaffected by

storage

These observations substantiate the practice of preferential storage (below 37 °C) and phase buying of denture base

resin liquids to avoid loss due to short shelf-life. It also
)

C: ‘ >

• i f

C

•«

i
- 6 - appeared that the self -cured (filling) resin liquids are more

resistant to polymerization when stored at elevated temperatures than are the heat-cured resin liquids.

Tests for polymer content were positive for the unpolymerized heat-cured resin liquids and negative for the selfcured resin liquids. Samples giving positive tests for polymer

content solidified in less than 23 hours when stored in a

constant temperature oven at 85 °C . Samples giving a negative

test resisted polymerization for more than 168 hours when

stored under these same conditions.

Tests for traces of polymer were negative for twelve of

fourteen ADA certified denture resin liquids currently on the

market. The two brands of resin liquids, which showed polymer

content, polymerized into an unusable hard mass in less than

17 hours when stored in the constant temperature oven at 85 °C.

Two brands of resin liquids, picked at random from the twelve

brands without traces of polymers, showed no evidence toward

solidification (increased viscosity) after J2 hours in the

85°C oven.

From these results it appears that resin liquids without

polymer content can be expected to have a longer shelf -life.

A more definitive study of this type might prove the desirability of a change in Federal Specification U-R -179 for Resin,

Acrylic, Dental which permits the resin liquid to be "methyl

methacrylate monomer or methyl methacrylate monomer which

has been partially polymerized."
•

.

'

f 3 i\ "• t‘.r ’J'. .

i

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- 7

- 3.4 Alginate Impression Material

All samples stored had deteriorated to an unusable condition. On mixing at standard temperature and humidity, a

gummy, grainy mass was formed in less than one and one -half

minutes

Pfeiffer [4] demonstrated that alginate impression material

deteriorates rapidly when stored at temperatures exceeding 37°C.

The fact that samples stored under standard temperature and

humidity conditions were unusable suggests some factor in

addition to temperature affects this material. Procurement

and supply agencies now avoid loss of these materials by both

phase buying and preferential storage

3.5 Materials with Volatile Solvents and

Low Melting Point Ingredients

This group of materials would be expected to be adversely

affected by high temperatures during storage. Some items were

stored only in the roof-top location, while others were stored

in two or all three locations

A protective coating material of the theobroma type was

stored at all locations, but only the samples stored at "B"

were affected to any extent . The metal tube containers had

lost approximately one-third of their contents. The remaining

portion was easily expressed from the tubes but appeared drier

and more grainy than the samples stored at ”A n and

' ! C

: '. The

material was still usable for protecting restorations. The

loss in volume was due to the effect of elevated temperatures
.

•

•

-

'
- 8 - which caused this low melting point material to liquefy and

evaporate

Cellulose acetate cast lacquer samples stored at "A" and

"C" had separated into a liquid phase and a gel phase.

Samples of this material from "B" had completely lost their

solvent (95$ chloroform, 5$ alcohol, by volume) through evaporation.

Wax shapes and a brand of sheet casting wax from "B"

were found to have lost a part of their more volatile ingredients, making the materials unusable for the purposes intended.

This deterioration occurred only to the samples where higher

temperatures were encountered. Another brand of sheet casting

wax and samples of a baseplate wax were unaffected by the

higher temperatures

Two of three brands of preformed plastic patterns were

also affected by the higher temperatures met in the roof-top

location, but the two brands were affected in different ways.

One brand had lost its plasticizer and had become very brittle,

whereas samples of the other brand had flowed together and were

so sticky and gummy that they could be separated neither from

themselves nor their cardboard mounts.

3.6 Cement Filling Materials

Silicate cement stored at "B" and red copper cement stored

at "A", "B", and "C" gave no evidence of deterioration.

Fischer [5] has shown that deterioration of these materials is

due to loss of water from the cement liquid. At the present

time, procurement specifications require that cement liquids
- 3 ’ _ I *)
- 9 - be packaged in sealed glass ampoules. This improvement in

packaging has been effective in preventing deterioration of

these materials during storage.

4 . SUMMARY

Deterioration of some materials is avoided only through

phase buying and preferential storage^ measures which cannot

always be utilized. Improved packaging appears to be all that

is necessary to insure long shelf -life for other materials.

The following recommendations are the result of inspecting and

testing dental materials after storage for four years

1. The condition of investments stored in humid climates

shows that the investments are inadequately protected from

moisture by the snap-in-lid metal containers in which these

materials are now supplied. Better containers should be used.

2. Low melting point materials packaged in screw-cap metal

tubes need to be more adequately protected from high

temperature by puncture-to-open seals.

3 . Specifications for the procurement of preformed plastic

patterns should contain a temperature stability requirement.

4 . An improvement in the storage stability of denture base

resin liquids is desirable.

5. There is a need for a more storage-stable elastic impression material
.

.

m
-10

BIBLIOGRAPHY

1. Farmer* G. J.* and Skinner* E. W.* Effect of relative

humidity on the setting time of plaster of paris.

Northwestern Univ . Bull. 43* 12-16 Dec. (1942).

2. Souder* W.* and Paff enbarger* G. C . * Physical properties of dental materials. Circular of National Bureau

of Standards C433* page 88. U. S. Govt. Printing Office*

Washington* D, C. (1942).

3. Volland* R. H.* and Paffenbarger*. G. C.* Cast gold inlay

technic as worked out in the cooperative research at the

National Bureau of Standards. J.A.D.A. 19.* 185 Feb.

(1932) . 4. Pfeiffer* K. R.* Harvey* J. L.* and Brauer* G. M.*

Deterioration during storage of alginate hydrocolloidal

dental impression material. U.S. Armed Forces Med. Jnl

6 S 1315-1320 Sept. (1954).

5. Fischer* T. E.* and Schoonover* I. C.* Deterioration of

silicate cements in the tropics. U.S. Armed Forces Med.

Jnl. 2, 907-913 June (1951).
.
11 ABC Table 1. Materials Stored

NBS at 70°F, 60$ R.H. Puerto Rico^ Roof-top Puerto Rico^ Seaside

Investment

Denture Resin Liquid

Acrylic Resin Powder

Clear Resin Kit P/L

Alginate Material*

Protective Coating

Baseplate Wax

Wax Shapes

3 Brands Casting Wax

Cast Lacquer*

Cavity Varnish

Copper Cement P/L

Investment*

Denture Resin Liquid*

Acrylic Resin Powder

Clear Resin Kit P/L*

3 Brands "Filling

Resin Kits P/L*

Alginate Material*

Protective Coating*

Baseplate Wax

Wax Shapes*

2 Brands Casting Wax

3 Brands.- Preformed

Plastic Patterns*

Cast Lacquer*

Cavity Varnish

Silicate Cement P/L

Copper Cement P/L

Investment*

Denture Resin Liquid*

Self-cure Resin P/L

Acrylic Resin Powder

Clear Resin Kit P/L*

Alginate Material*

Protective Coating

Baseplate Wax

Wax Shapes

3 Brands Casting Wax

Cast Lacquer*

Cavity Varnish

Copper Cement P/L

*Items marked by an asterisk were found to have been adversely

affected by storage.

12

Table 2. Water Content and Setting Time of Investment

Sample Water Content3- Setting Time 0

w (min)

A 1.8 20

B 2.2 2

C 2.5 1.5

a Average of two determinations of weight loss on

heating to constant weight at 110°C.

b On undried samples

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THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants

and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project

or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

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Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.