U. S. DEPARTMENT OF COMMERCE

HENRY A. WALLACE, Secretary NATIONAL BUREAU OF STANDARDS

LYMAN J. BRIGGS, Director

NATIONAL BUREAU OF STANDARDS MISCELLANEOUS PUBLICATION M181

JAR RINGS FOR USE IN HOME CANNING

THEIR TESTING AND A PROPOSED SPECIFICATION

By

ROLLA H. TAYLOR, HELEN G. WHEELER, and FRANCES BENEDICT

Issued, August 29, 1945

UNITED STATES

GOVERNMENT PRINTING OFFICE WASHINGTON : 1945

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price 10 cents

JAR RINGS FOR USE IN HOME CANNING: THEIR TESTING AND A PROPOSED SPECIFICATION

By Rolla H. Taylor, Helen G. Wheeler,1 and Frances Benedict 1

ABSTRACT

Canning tests were made on 20 rings from each of 50 different samples. The

rings were analyzed for rubber content, and measurements were made of their

tensile strength, ultimate elongation, stress at 100-percent elongation, swelling on

processing, indentation by a ring simulating a jar cap, and hardness. The can- ning failures in tests by the pressure-cooker method bore no relation to rubber

content or tensile properties, of the rings but were related to swelling, indentation,

and hardness, and to the change of these properties with processing. The rela- tion was not exact because some failures probably resulted from excessive variations in jars and tops. Apparatus was developed for accurately measuring profiles of sealing surfaces. The results of the investigation are expressed in terms of a proposed specification for jar rings.

CONTENTS

Page

I. Introduction 1

II. Variations in jars and tops 2

III. Laboratory processing of jar rings 5 IV. Methods of measuring physical properties of jar rings 5

1. Tensile strength and ultimate elongation 6

2. Tensile stress at an elongation of 100 percent 6

3. Swelling 6

4. Indentation t 7

5. Hardness 8 V. Chemical analysis of jar rings 8 VI. Taste imparted to canned food by jar rings 9 VII. Aging of jar rings 12 VIII. Canning tests as a basis for development of a jar-ring specification 12 IX. Correlation of properties and composition of jar rings with results of canning tests 13

1. Method of correlating results 13

2. Effectiveness of limits in excluding rings showing canning

failures 14

3. Relation of composition to properties 15 X. Proposed specification for jar rings 16

I. INTRODUCTION

An investigation of jar rings was undertaken in the spring of 1941

by the Bureau of Human Nutrition and Home Economics, United

States Department of Agriculture, in collaboration with manufacturers and the National Bureau of Standards. The original purpose

of the investigation was to determine whether satisfactory jar rings

could be made from reclaimed rubber without the admixture of new

1 Detailed by the Bureau of Human Nutrition and Home Economics, United States Department of Agriculture, to the" National Bureau of Standards for work on this project. 1
2 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

natural rubber, which at that time was becoming increasingly scarce. The results indicated that, for the experimental rings studied, there was no evident correlation between the performance of the rings and

the relative percentages of new and reclaimed rubber entering into

their composition.

In the course of the work, a study was made of the relation between

the properties of the rings as measured in the laboratory and their

performance in canning experiments. A brief study was also made of

the irregularities of the sealing surfaces of jars and tops in order to ascertain the amount that a jar ring might have to be compressed in

order to insure a satisfactory seal. As a result of these studies, a proposed performance specification

for jar rings was formulated, which is thought to be applicable to all jar rings regardless of whether they are made from mixtures of new and

reclaimed rubber, reclaimed rubber alone, or from synthetic rubber.

II. VARIATIONS IN JARS AND TOPS

The function of a jar ring is to form a seal between a jar and the

cap, or top. As the surfaces to be sealed do not fit perfectly, it is necessary to determine the approximate magnitude and distribution

of the irregularities to know how much capacity for indentation a jar

ring must have to insure a satisfactory seal. The occurrence of a

failure in canning does not necessarily mean that the jar ring was

unsatisfactory or defective. It may be that the irregularities in the

sealing surfaces exceeded those that the ring was designed to overcome. The profiles of the sealing surfaces of jars and tops were accurately

measured by means of a Last Word dial gage graduated to 0.0001 inch

and accurate to about plus or minus 0.0001 inch. The apparatus is
JAR RINGS FOR USE IN HOME CANNING 3

shown in figure 1. The procedure employed was as follows: The jar or top was placed on the turn table and was carefully centered and

leveled with reference to the axis of the table. The foot of the gage was then so set that readings could be taken at any point around a

circle in the sealing surface by rotating the table. The surface to be

measured was arbitrarily divided into 16 equal radial sectors, starting

with a molding seam or some other mark that could be subsequently

identified. The table was then rotated and readings of the gage were taken at each of the 16 points. Any irregular variations between the points were also noted. The readings thus obtained showed the departure

of the surface from a true surface. As a matter of convenience, the

lowest point of the contour was taken as the reference point.

Figure 2 shows typical profiles that were found for two jars and two

tops of the bail type. The 16 sectors are plotted as abscissas and

the gage readings as ordinates. In this figure the maximum departure

of the sealing surface of jar 14 from a true surface is 0.0086 inch and

that of top 14 is 0.0073 inch. The maximum irregularity, then,

which a ring would have to overcome would be the sum of these, or 0.0159 inch. Obviously, if the relative positions of the jar and the

top are changed, the irregularity to be overcome may be somewhat

less than this amount, but in practical canning it would not be possible

to pick the best position of the top relative to the jar. The worst

condition that might occur must, therefore, be considered. Furthermore, for bail-type jars the relative position of the top to the jar is determined by the design, within narrow limits. The profiles of the

sealing surfaces are therefore shown approximately as they would come together.

The sealing surface of jar 2 departs much less from a true surface

than does that of jar 14. As may be seen from the figure, however,

the surface of jar 2 has sharp, narrow depressions at the molding

seams. These depressions might lead to failures in canning on account

of the difficulty in forcing rubber into such narrow spaces. Thus,

both the shape and the magnitude of the irregularities are important

from the standpoint of potential failures. Measurements made on several different types of jars and tops

showed variations in the departures of the sealing surfaces from true

surfaces ranging from approximately 0.002 inch to 0.015 inch. These

measurements are given primarily for purposes of illustration and

are not to be regarded as fully or adequately representing the jars

in common use throughout the country. The collection and measure- ment of a sufficient number of jars and tops to give a generally valid

figure for the magnitude of the irregularities and the frequency of

their occurrence would be a major investigation in itself. It was decided from the foregoing measurements and information

furnished by manufacturers of jars, that rings should be designed to take care of irregularities of the order of 0.020 inch. On this basis,

rings cut 12 to the inch would have to be capable of being indented

nearly one-fourth of their thickness under the stresses usually applied

in canning, and rings cut 16 to the inch would have to compress

about one-third of their thickness.

\ In the factory, jars and tops are usually inspected with a metal

ring having an accurately finished surface. This ring is placed against

the sealing surface, and if any rocking can be detected by a skilled
4 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

operator, the jar or top under test is rejected. It is possible, however,

for the profile to be such that significant irregularities cannot be de- tected by this means.

FRONl

/ TOP \

» NO. 2

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ui NO. 2 <

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%< T

JAR.

NO. 14- 2

<3hj /

' / FRON"

i

r \l

i X.

EQUAL SECTORS CIM SEALING SURFACE

Figure 2.

Representative profiles of sealing surfaces on jars and tops
JAR RINGS FOR USE IN HOME CANNING 5

III. LABORATORY PROCESSING OF JAR RINGS

The properties of jar rings may be affected considerably by the

temperature and the action of the steam encountered in canning. It

is important, therefore, that laboratory tests of the rings be designed

to show the effect on the rings of a standard canning procedure. In- asmuch as the pressure-cooker method seemed likely to have the

greatest effect on the rings of any of the common methods, a uniform

procedure was developed for processing the rings so as to simulate the,

effect of canning in the pressure cooker.

In this procedure the jar rings to be processed were placed on clean

absorbent paper on horizontal grids in a pressure cooker of the type

used for home canning. The number of rings processed at one time was no more than could be tested for swelling and indentation within

1 hour. The rings were placed well above the level of the water and

were distributed so that steam had access to all surfaces. The water in the pressure cooker was brought to boiling in approximately 15 minutes, and steam was allowed to escape from the vent

for 7 minutes to displace all air from the cooker. The vent was then

closed, and the temperature was brought to 121°±1° C in approximately 15 minutes and held for 120 minutes at that temperature.

The cooker was then cooled to 100° C in approximately 15 minutes.

The rings were then removed from the cooker.

The rings for the measurement of swelling were placed in water

at room temperature, care being taken that their surfaces did not

become dry, even momentarily. The rings for the indentation

measurements likewise were placed at once in water that was kept

boiling. The measurements described below were made as quickly

as possible, and in no case more than 1 hour after the removal of the

rings from the pressure cooker. No significant difference was found

between measurements of swelling immediately on removal from the

pressure cooker and after the lapse of 1 hour, during which the rings were kept in water at room temperature. The measurements of indentation were found to change considerably if the rings were kept

for any length of time in water at room temperature between removal

from the pressure cooker and testing. It was found, however, that

they could be kept in boiling water for an hour without significant

change.

IV. METHODS OF MEASURING PHYSICAL PROPERTIES

OF JAR RINGS

Several different types of measurements were made on jar rings for the purpose of finding characteristics that might be correlated with

the behavior of the rings in canning experiments. Tensile strength

and ultimate elongation were measured because they were used as

criteria of quality in Federal Specification ZZ-R-351 for Jar Rings,

which was in effect at the time this investigation was initiated. Measurements of load at an elongation of 100 percent were made at the suggestion of a manufacturer who had found them useful as an

index of quality. The swelling that took place during processing was

measured because it had been observed that some rings took up
6 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

moisture to such an extent that they changed significantly both in dimensions and in properties. Inasmuch as sealing by the rings depends directly upon their behavior in compression, measurements of indentation were made both before and after the rings were processed.

At the suggestion of one of the jar-ring manufacturer's, measurements

of hardness by means of the Shore Durometer were also made. The

hardness and indentation measurements are related, as they both measure the same property. The measurements of indentation,

however, were much more sensitive and simulate service conditions

much more closely than the measurements of hardness.

1. TENSILE STRENGTH AND ULTIMATE ELONGATION

Measurements of tensile strength and ultimate elongation were made on the rings as a whole by the method described in Federal

Specification ZZ-R-351 for Jar Kings. This method involved the

stretching of the rings by the procedure commonly employed for

ring specimens.

2. TENSILE STRESS AT AN ELONGATION OF 100 PERCENT

The force measured was that required to stretch a jar ring until the

mean circumference had doubled. In making this measurement

the ring was placed over two %-inch mandrels and stretched within a few seconds to the required elongation by means of a hand-operated

device. The force was registered by an accurate spring scale. As the

force decreased considerably with time, readings were made at regular

intervals over a period of several minutes. When these readings were

plotted as a function of the logarithm of the time they gave a linear

relation, which was extrapolated to give the value of the load at approximately 1 second after stretching.

Measurements were made at room temperature, about 25°or30°C,

and at 100° C. The latter temperature was obtained by stretching

the rings in a bath of boiling water. Measurements were also made on some rings that had been processed

for 2 hours at 121° C, but the number of observations was not sufficient to warrant their inclusion in the present report.

3. SWELLING

In the study of the swelling, the weight and volume of a ring were

found before and after processing, and from these the percentages

j of water absorbed by weight and by volume were calculated. In making the determinations the rings were cleaned, dried, and weighed

to the nearest milligram, first in air and then in water. They were

then processed as described above and kept under water until they 51 could be reweighed but not longer than 1 hour. Each ring was then 5 reweighed in water and in air. It was found that rings could be

weighed in air with sufficient accuracy if they were blotted and

allowed to dry superficially and were weighed within 1 minute after their removal from the water. The use of a weighing bottle was

not found necessary.
JAR RINGS FOR USE IN HOME CANNING 7 From the four weights of each ring the percentage increase in

weight and in volume were calculated as follows: Let

101= weight of ring in air before processing

w2 =weight of ring in water before process- ing

w3 =weight of ring in water after process- es

Wi= weight of ring in air after processing

^i—^100= increase in weight, percent

(w4 —

w

3 ) — {Wi— W2 ) 1An • i

v 100= increase m volume, percent. (w1 —w2 ) * A correction for the temperature of the water used for weighing

the rings before and after processing is necessary if the temperatures

differ by more than 5° C. In the present work, however, the differ- ence was never more than 2° C.

At the beginning of the investigation the swelling was measured

only in percentage increase in weight. The percentage increase in volume, however, was found from the increase in weight and the specific gravity of the rings which had been measured on different

specimens from the same lots. In this case, 100<i^^— 1^ = percentage increase in volume,

where d is the specific gravity. This relation is based on the assumption that the volume of a swollen ring is the sum of the volume of the

ring and the volume of the water separately.

The increases in thickness and diameter of rings that resulted from

processing were found from direct measurements on the rings made

by the methods described in Federal Specification ZZ-R-601a. The precision, however, was considerably less than that of the swelling measurements, and although there were indications of pronounced grain effects with certain rings, little information was secured that was not obtainable from the swelling measurements.

4. INDENTATION

The apparatus shown in figure 3 was designed to give a test simulating the conditions of pressure and loading to which jar rings are subjected in canning. It consisted essentially of two parts—a plane

metal bearing plate to represent the jar and a Duralumin indentor

plate with a flange to represent the top. The indentor used for

shoulder rings had a flange 0.2 inch high, 2.70 inches in mean diameter,

and 0.050 inch wide, with a flat bearing surface. A vacuum pump

was connected to a hole in the bearing plate, and a dial gage graduated

to read 0.0001 inch was so mounted as to indicate the changes in the

height of the cover plate above the bearing plate.

In making a test the jar ring was placed over the guide on the bearing

plate, and the indentor was lightly placed concentrically on the ring.

The dial gage, which was supported rigidly with reference to the bear654612—45 2
8 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

ing plate, was mounted in such a position that its stem rested on the

center of the indentor plate and was adjusted to read zero. The

small chamber formed by the bearing plate, the ring, and the indentor was then evacuated through the hole in the bearing plate. This procedure caused the indentor flange to press against the jar ring with a

force approximately equal to that resulting from a pressure of 1 atmosphere acting over the area inside the indentor flange. This condition

gave a pressure of 190 ± 4 lb/m.2 on the indentor flange.

A, Indentor plate; B, indentor flange; C, center guide; D, bearing plate; E, jar ring; F, vacuum line. The pressure on the ring was equivalent to that sustained in canning.

This pressure was maintained for 1 minute, at the end of which the

indentation was read from the dial gage. Measurements were made

on the original rings and on rings that had been processed as described

above. The latter were kept in boiling water up to the time of measurement. They were then cooled quickly, blotted dry, lubricated

with castor oil, and tested.

5. HARDNESS

Measurements of hardness were made by means of the Shore Durometer according to the procedure outlined in ASTM Specification

D-676-42T. As the jar rings were not as thick as the specimens

called for by this procedure, the rings were mounted for measurement

on a rubber pad having a Shore hardness number of 60. V. CHEMICAL ANALYSIS OF JAR RINGS

One of the objects of the initial investigation was to determine how

much new rubber, if any, it was necessary to add to reclaimed rubber

in order to produce satisfactory jar rings. To this end each manufacturer collaborating in the work furnished a series of jar rings made
JAR RINGS FOR USE IN HOME CANNING 9

from compounds containing different percentages of new rubber,

usually 0, 2.5, 5, and 10 percent. The kinds and amounts of ingredients

other than new rubber were left to the discretion of the manufacturers.

It was soon found that there was no correlation between the per- centage of new rubber used and the properties or performance of the

rings. As a matter of fact, no correlation should be expected because

the larger part of the rubber in the rings was derived from reclaimed

rubber, and hence variations in the kinds and amounts of reclaim

would overshadow the relatively small differences in the percentage of new rubber. The principal reason for using new rubber was not to improve the properties of the finished rings so much as to facilitate the processing and manufacturing operations.

Some of the manufacturers stated the percentage of reclaimed

rubber used, but as they did not give its hydrocarbon content,

it was necessary to analyze the rings to reduce them to a common

denominator.2 About 40 different rings were analyzed by the method described in Federal Specification ZZ-R-601a. This method gave the total rubber

content but did not differentiate between rubber that had been

derived from new crude and that which had been derived from reclaim.

The total rubber content thus found was expressed in percent by

volume because it is the proportion of rubber by volume not by

weight that determines the physical properties of a compound.

The results are summarized in table 1 and are discussed in a subsequent section in relation to the performance of the rings.

VI. TASTE IMPARTED TO CANNED FOOD BY JAR RINGS

Oral reports have indicated that taste imparted to the canned food

by jar rings has been a prolific source of complaints regarding wartime

jar rings. Unfortunately, taste cannot be dealt with by direct,

objective measurements, such as are employed for evaluating other

characteristics and properties of the jar rings. The present investigation did not include an evaluation of taste. Taste imparted by jar rings does not come from the rubber itself but from minor constituents that are present in such small amounts

that they cannot be readily detected by the usual chemical tests. As

mentioned in a preceding section, most jar rings are made from re- claimed rubber produced from discarded tires, inner tubes, and various

other scrap-rubber articles. These articles, of course, are made for mechanical purposes without regard to taste or odor, and are likely to contain tars, asphaltic substances, organic accelerators and antioxidants, and various other materials unpleasant to the taste. These

offending materials can be avoided to a considerable extent in normal

times by a careful selection of the scrap, which is not possible under

present conditions. The reclaiming processes remove the offending

constituents to a greater or lesser extent, but at the present stage of the art do not produce a tasteless and odorless product.

The synthetic rubber GR-S is being used to some extent in the

production of jar rings. As ordinarily produced, GR-S has an appreciable taste and odor due to the residual styrene. The styrene can be

2 Reclaim, or reclaimed rubber, contains most of the fillers and other compounding ingredients that were

in the original scrap and, in addition, softeners and other materials added during reclaiming. The constit- uent of principal value is the rubber hydrocarbon, and this is commonly used as an index for comparing

different reclaims.
10 MISC. PUBLICATIONS, NATIONAL

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removed in large part at some added cost of manufacture, and the

production of a special grade of GR-S latex for use by the food process- ing industries has been authorized by the Rubber Reserve Co.

VII. AGING OF JAR RINGS

At the outset of the investigation representative rings were subjected to the accelerated aging tests that are commonly applied to rubber goods. Some of the ring were aged in air at 70° C for 7 days,

and others were subjected to oxygen at 300-lb/in. 2 pressure at 70° C

for periods of 2 days and 4 days. Measurements of tensile strength

and ultimate elongation were made before and after accelerated aging.

The results obtained indicate that the rubber probably would last for

several years without becoming soft and sticky or hard and brittle. Measurements of indentation and hardness, however, as given in

table 2, showed that the rings might undergo appreciable changes in

these characteristics as a result of natural aging within a relatively

few months. In general, tests on aged rings made before processing

showed a decreased indentation and an increased hardness ; tests made

after processing showed an increased indentation and a decreased

hardness. This effect is not necessarily associated with the failure of

rubber. It rather represents a change that is of little consequence for

many types of rubber articles, but which may be of importance in

connection with jar rings, especially if they are near the limits of

acceptable performance. A comparison of the changes in indentation

shown in table 2 with the number of canning failures shown for the same rings in table 1 indicates that the greater the change in indentation with aging the greater the number of canning failures. Table 2.

Effect of aging on swelling, indentation and hardness of jar rings

Ring number

2.

9.

23

28

40

42

Date tested

June.

Sept. Nov..

fJune.

-teept. lNov_.

fJune. ^Sept. lNov_

June.

Sept. Nov.

June.

Sept. Nov.

June.

Sept. Nov.

Swelling, %

Increase

in volume

18.0

19.0 38.0 33.4

5.6

5.8

12.7

12.6 5.1

5.2 5.0

4.6

Increase

in weight

12.2 27.6

24.1 3.8

3.3

7.5 8.0

3.5

3.5

3.4

Indentation

Unprocessed

Inch

0. 0120

.0192 0143

0105 0097

0147

0120

,0097

,0083

,0101

Processed

Inch

0. 0221

.0314

,0400 ,0482

,0141

0235

,0139 ,0140 .0107

.0128

Percentage in- crease

84 257

109 236

35

26

Shore hardness

Unprocessed ProcVIII. CANNING TESTS AS A BASIS FOR DEVELOPMENT OF

A JAR-RING SPECIFICATION

Approximately 50 different lots of shoulder rings obtained from

10 different manufacturers were used for canning tests by the Bureau
JAR RINGS FOR USE IN HOME CANNING 13

of Human Nutrition and Home Economics. These lots of rings were

duplicates of those tested by the National Bureau of Standards.

Canning tests were made on 20 rings from each lot. Ten were tested

by the hot-fill method and 10 by the pressure-cooker method. In

the hot-fill method, preheated jars of the bail type were filled with

hot water, sealed immediately, and placed in a gentian violet dye

bath to cool. Leakage was determined by the presence of dye inside

the jars. In the pressure-cooker method, the jars were filled with

a mixture of ground meat, corn, cottonseed oil, and water, partially

sealed, and processed at 121° C in a pressure cooker for 2 hours. Immediately upon removal of the jars from the pressure cooker, the

seal was completed and the jars were immersed in a cooling bath that was heavily contaminated with the spores of an especially heatresistant strain of Clostridium sporogenes . Leakage was determined

from spoilage that occurred during an incubation period or the subsequent subculturing of the contents of the jars.

The failures resulting from tests made by the hot-fill method could

not be correlated with the failures resulting from the tests made by

the pressure-cooker uiethod. In fact, the few random failures that

did occur in the hot-fill tests were probably attributable to variables

other than the jar rings, such as warped sealing surfaces. Unfortunately, the study of variations in jars and tops described in a

previous section of this paper had not been made when these canning

tests were conducted.

It seemed desirable therefore to use only the tests made by the

pressure-cooker method as a basis for the development of a specification. The results of these canning tests are summarized in table

1, along with the measurements of physical properties and chemical

composition on the same rings. The data given in this table were

used as described in a later section for establishing the limits to be

employed in a specification.

After a tentative specification had been developed a number of

additional samples of rings produced in connection with experimental

and development work were also tested. IX. CORRELATION OF PROPERTIES AND COMPOSITION

OF JAR RINGS WITH RESULTS OF CANNING TESTS

The results summarized in table 1 were examined to determine

which of the laboratory measurements could be correlated with the

failures in canning and what limits should be imposed on each of the

significant properties in order to differentiate between satisfactory

and unsatisfactory rings.

1. METHOD OF CORRELATING RESULTS

Table 3 gives the limits used in the two methods that were employed

in correlating the results of the physical measurements with the can- ning tests. The first method consisted in noting the limits, desiguated

as A limits, of the various physical measurements that were made on

the lots of rings showing no canning failures, and in applying these

limits to those rings with which canning failures were observed. In

the ideal case this method would suffice for the development of limits

for the purposes of a specification. The variations in the canning

tests, however, are such that reliable conclusions must be based on
14 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

averages rather than individual observations. Hence in the second

method of treating the data, narrower sets of limits, designated as B limits, were established by trial and error methods. In arriving

at the B limits consideration was given both to the significance of the individual properties and to the effectiveness of a combination

of properties in defining rings that would show the desired performance

Table 3.

Limits of physical requirements

Property

Limits re- quired to pass all rings having no

canning

failures U)

Suggested

limits

(B)

Tensile strength, lb/in. 2 Elongation, percent . Tensile strength • at 100-percent elongation, lb/in At room temperature At 100° C

Swelling on processing: Weight, percent Volume, percent Indentation (unit 0.0001 inch)

:

Unprocessed

Processed

Increase, percent. Shore hardness: Unprocessed

Processed Rubber by volume, percent.

720 min

112 min

198 min

126 min

15.3 max

20. 8 max

/168 max

\ 69 min

/271 max

\121 min

145 max

f 62 min

\ 75 max

/ 48 min

\ 69 max

19 max

43. 7 min

12 max

16 max

165 max

luO min

250 max

120 min

110 max

60 min

75 max

50 min

65 max

15 max

1 These values of tensile stress were derived from the values of load at 100-percent elongation given in table 1. The actual values of load given in table 1 were used to show the effectiveness of tensile stress in eliminating poor )ar rings. This procedure is justified by the fact that the cross-sectional area of all rings tested was approximately the same; The B limits were set relatively narrower than the A limits. They

would eliminate 58 percent of the lots of rings with which no failure

occurred, 50 percent of those with one failure, 64 percent with two

failures, 66 percent with three failures, and 100 percent of all lots showing more than three failures. It was not possible to devise

limits which would pass all the rings showing no failures and at the same time bar a high percentage of those with which failures occurred.

It was felt that it would be better to set a specification so as to re- ject some rings which might give satisfactory results than to pass

many rings with which failures are likely to occur.

2. EFFECTIVENESS OF LIMITS IN EXCLUDING RINGS SHOWING

CANNING FAILURES

The effectiveness of the two sets of limits in excluding rings with

which canning failures were found is shown in summary form in table

4. The results of canning tests are given at the top of the table in

terms of the number of failures per lot of rings. The table also shows

the percentage of rings that would be barred by the A and the B

limits for each property.

The A limits, which were arrived at by a purely mechanical process, would pass all rings with which there were no canning failures, and
JAR RINGS FOR USE IN HOME CANNING 15

would bar 30 percent of the lots in which there was one canning failure,

45 percent of the lots with two failures, 66 percent of the lots with

three failures, and all lots showing more than three failures. The application of the A limits shows clearly that certain properties

which are commonly used in specifications for rubber goods are not

effective in affording a basis for separating rings that are shown to be

unsatisfactory from the standpoint of performance. This is particularly true of tensile strength and ultimate elongation, and to a lesser extent of load at 100 percent elongation, or tensile stress which may

be calculated from the load.

3. RELATION OF COMPOSITION TO PROPERTIES

Likewise, the composition, within the range covered by the present

investigation, bore no definite relation to the results of canning tests. It had been expected that the number of canning failures would be

less the higher the percentage of rubber in the rings, but such was

not found to be the case. The rings may be divided, for example,

Table 4.

Correlation between canning tests and physical properties of jar rings Number of failures per lot

1 2 3 4 5 6 7

Results of canning tests: Number of lots .. . 19 10 11 6 3 2 1 1 Number of jars canned. 180 96 107 58 30 20 10 10 Total number of failures (jars) 10 22 18 12 10 6 7 Rings eliminated under (A) maximum

limits assigned to physical properties by: % % % % % % % %

Tensile strength Elongation. . 33 Load—100-percent elongation

Room temperature . 9 8 33

100° C 8 67 Swelling, percent- Weight 9 33 100 100 100 Volume . 9 33 100 100 100 Indentation

Unprocessed ... 20 18 14 33 100 Processed— . . 9 27 17 67 100 100 100 Increase, percent 9 9 17 100 100 Rubber by volume. . . 9 9 Shore hardness

Unprocessed 9 8 34 67 100 Processed .... .. 18 48 67 100 100 100 Decrease ...... 9 9 34 33 100 Total eliminated 1 29 45 66 100 100 100 100 Rings eliminated under (B) suggested

limits assigned to physical properties by: Swelling, percent- Weight 11 9 9 33 100 100 100 Volume 11 9 28 17 67 100 100 100 IndentationUnprocessed 22 40 26 66 67 100 100 Processed ... _ ... . 6 9 27 17 67 100 100 100

Increase, percent. . 6 20 9 34 100 100 Shore hardness

Unprocessed 9 8 17 67 100 Processed 21 20 36 48 67 100 100 100 Decrease . 16 20 28 50 33 100 100 Total eliminated i

58 50 64 66 100 100 100 100

1 The total percentage of rings eliminated is the total number of rings eliminated by 1 or more tests multiplied by 100 and divided by the number used in canning. Sometimes different tests eliminate the same

rings, sometimes different rings.
16 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

into two groups, 1 group of 19 lots containing more than 50 percent

of rubber by volume, and the other group of 22 lots containing less than

50 percent. The first group shows a total of 41 canning failures

and the second, likewise, 41 canning failures. It should not be con- cluded from this that the rubber content is immaterial, but rather

that within the range of compositions here dealt with there were no differences that could be attributed to the rubber content. The

lack of correlation between the number of failures per lot and the

percentage of rubber by volume is clearly shown by figure 4.

fe"

7 HJ 6

uj _ a 5 u 4

_j< 3 2 2 2i i

40 42 44 46 48 50 52 54 56 58 60 62 64

RUBBE.R - PEIRCENT" BY VOLUME

Figure 4.

Relation of canning failures to percentage of rubber by volume. X. PROPOSED SPECIFICATION FOR JAR RINGS

The results presented in the foregoing sections of this paper were

used as the basis for drawing up a proposed specification for jar rings.

This specification deals only with the performance of the rings and

hence should be applicable to any rings irrespective of the composition. PROPOSED SPECIFICATION

FOR

RUBBER RINGS: JAR (FOR HOME CANNING)

A. APPLICABLE FEDERAL SPECIFICATIONS

A-l. The following Federal specification of the issue in effect on date of invi- tation for bids shall form a part of this specification.

ZZ-R-601. Rubber Goods: General Specifications (Methods of physical

tests and chemical analysis). B. TYPES AND GRADES

B-l. Rings shall be of a single grade in each of the two types described below. Type I. Top seal. Type II. Shoulder seal. C. MATERIAL AND WORKMANSHIP

C-l. Jar rings shall be free from defects in material and workmanship. (See

paragraph 1-1.)
JAR RINGS FOR USE IN HOME CANNING

D. GENERAL REQUIREMENTS

D-l. Jar rings shall be suitable for use in the canning of vegetables, fruits, meats, and other food products by the hot-fill, the water-bath, the oven, and the

pressure-cooker methods. Jar rings shall not impart off flavors to the canned

food. E. DETAIL REQUIREMENTS

E-l. Dimensions.—Jar rings shall have dimensions within the following limits. TYPE I TYPE II Max. Min. Max. Min.

Thickness, inch 0. 086 0. 080 0. 086 0. 080 Width of flange, inch .28 .25 .28 .25

Internal diameter, inches 2. 00 2. 20 E-2. Application.—Jar rings shall have sufficient strength and elasticity to permit application to jars without tearing or other damage to the ring. When

applied the rings shall fit snugly without buckling. E-3. Mechanical properties.—Jar rings shall meet the following requirements

when tested in accordance with the methods described in section F: Property Max. Min.

indentation:*

Indentation of original rings, inch 0. 0165 0. 010

Indentation of rings after processing, inch .0250 .012

Increase of indentation on processing, percent 110 swelling:

Increase of volume on processing, percent 16

Increase of weight on processing, percent 12 hardness:*

Hardness of original rings, Shore scale 75 60 Hardness after processing, Shore scale____ 65 50 Decrease of hardness on processing 15

*It is not intended that both indentation and hardness shall be made on the same sample of rings

since they both measure the same property. Hardness measurements may be made in lieu of indentation at the descretion of the purchaser. Indentation measurements shall be made in all cases of dis- agreement. F. METHODS OF SAMPLING, INSPECTION, AND TEST.

F-l. Sampling.—Two dozen rings shall be taken at random from each 1,000

gross or fraction thereof. F-2. Indentation.—For purposes of this specification the indentation is that which is produced by a metal circular indentor flange placed concentrically on a

jar ring while the jar ring is lying on a flat plate equipped with a center guide.

The pressure on the indentor surface shall be 190 ±4 pounds per square inch. This pressure is approximately equal to that which would result from a pressure

of 1 atmosphere acting on the area inclosed within the metal indentor flange

specified in F-2a and F-2b.

When jar rings to be tested are of such sizes as to preclude the use of the above

indentor, other indentor rings shall be provided and the load shall be adjusted so that the pressure on the indentor surface is 190 ± 4 pounds per square inch. F-2a. Apparatus.—The apparatus shown in figure 3 for measuring the indentation of jar rings shall consist of a flat metal bearing plate, D, with a center guide,

C, an indentor plate, A, with an integral circular indentor flange, B, and a dial gage graduated to 0.0001 inch mounted on the bearing plate in such a way that

it will bear in the center of the indentor plate when the indentor plate is placed on the jar ring concentric with the guide on the bearing plate. The bearing ulate and the indentor plate shall conform to the following requirements

F-2a (1). For type I jar rings.—The bearing plate, except for the center guide,

shall not deviate from a flat surface by more than 0.0005 inch. The center guide on the bearing plate shall be 2.187 ±0.005 inches in outside diameter,

and shall be approximately 0.075 inch high.

The indentor plate including the indentor flange shall weigh approximately

5.3 oz (150 g). The indentor flange shall be 2.50 ±0.005 inches in mean diameter, 0.046 ±0.001 inch wide, shall have a flat bearing surface, and shall pro- trude not less than 0.2 inch from the indentor plate.
18 MISC. PUBLICATIONS, NATIONAL BUREAU OF STANDARDS

F-2 a (2) . For type II jar rings.—The apparatus used for measuring the indentation of type II jar rings shall conform to the requirements given in F-2a, except

that the outside diameter of the guide on the bearing plate shall be 2.325 ±0.005

inches; the mean diameter of the indertor flange shall be 2.700 ±0.005 inches,

and the width of the flange shall be 0.050 ±0.001 inch. F-2b. Procedure.—The indentor ring shall be wiped with castor oil before each measurement.

Indentation shall be measured by means of a gage accurate to within ±0.0001

inch, and the indentation reading shall be taken 1 minute after the load has been

applied. In making the indentation test, three jar rings shall be cleaned and dried and

the indentation measured in accordance with the above instructions. Three additional rings from the same lot shall then be processed in accordance with para- graph F-4 and the measurements repeated. F-3. Swelling.—Three jar rings shall be cleaned, dried, and weighed in air and

in water with an accuracy of ±0.001 g. These rings shall be processed as di- rected in paragraph F-4, and shall again be weighed, first in water, and then in

air. The weighing in air shall be made within 30 seconds after the ring has been

removed from water and blotted or wiped dry. The drying may also be effected by immersing the rings in alcohol for 5 seconds and quickly shaking off the excess, and allowing the remainder to evaporate within a few seconds. Calculations of the percentage increase in weight and in volume may be made

as follows: Let wx =weight of ring in air before processing w2 =weight of ring in water before processing

Wz= weight of ring in air after processing ^4 =weight of ring in water after processing. Wz w

Wl

y. 100 — increase in weight on processing, percent w _ w

-X 100 = increase m volume on processing, percent.

The temperatures of the water used for weighing the rings before and after processing shall not differ by more than 5° C. F-4. Processing.—Jar rings to be processed shall be placed on clean absorbent paper on horizontal racks or grids in a pressure cooker of the type used for home

canning. The rings shall be placed above the level of the water and shall be

distributed so that steam has access to all surfaces. The water in the pressure cooker shall be brought to boiling in approximately

15 minutes and steam shall be allowed to escape from the vent for 7 minutes to

displace all air from the cooker. The vent shall be closed and the temperature

shall be brought to 121° ± 1° C in approximately 15 minutes and held for 2 hours

at that temperature. The cooker shall then be cooled to 100° C in approximately

15 minutes. The rings shall then be removed from the cooker. Those for swelling measure- ments shall be placed in water at room temperature and those for the measure- ment of indentation, in water which is kept boiling. The rings shall be kept in water at the respective temperatures until measurements are to be made. All measurements shall be completed within 1 hour after the removal of the rings

from the pressure cooker. Rings for indentation test shall be cooled in water and tested within one

minute.

F-5. Hardness shall be measured as directed in ASTM Specification D-676,

Indentation of rubber by means of the Durometer.

I. NOTES.

1-1. This specification is intended to give requirements essential to the per- formance of jar rings, irrespective of the material from which they are made.

Consequently, rings furnished under the specification may be made from any

material suitable for the intended purpose.

1-2. It has been found that the results of swelling tests are not significantly altered by keeping the rings in water at room temperature for periods up to

1 hour after processing. Similarly, the results of indentation tests are not changed significantly if the rings are kept in boiling water up to 1 hour. Washington, April 10, 1945.

o