NBSIR 74-546

A Synthetic Material for Detecting

Hazardous Edges

James E. Maness

Product Engineering Division

Product Systems Section

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

January 1975

Interim Report

Prepared for Consumer Product Safety Commission

Washington, D. C. 20207

NBSIR 74-546

A SYNTHETIC MATERIAL FOR DETECTING

HAZARDOUS EDGES

James E. Maness

Product Engineering Division

Product Systems Section

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

January 1975

Interim Report

Prepared for Consumer Product Safety Commission

Washington, D. C. 20207

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

Abstract

An investigation of poly (tetraf luorethylene) pressure-sensitive

tape (PTFE tape) was conducted to determine its suitability as an indicator of the hazard potential of exposed edges on consumer products

when used with the sharp edge testing device. In addition, criteria

were established by which suitable tape could be specified as a standard material for detection of sharp edges.

From a study of many PTFE tapes having different physical and

mechanical properties, it was concluded that in order to specify the

tape as a suitable material to be used with a sharp edge testing device,

the tape should be tested against two standard edge geometries. One

of the standard edges will cause the tape to be totally penetrated,

representing a bleeding cut or a breakthrough of the epidermis of

human skin when an unsafe edge is contacted. The other edge will not

totally penetrate the tape, representing a no-bleed cut when a safe

edge is contacted by a consumer.

Recommendations are presented to specify the proper PTFE tape

based on the tape's thickness and ability to pass a performance test

against two standard edge geometries under given force loading conditions. These edge geometries are solely for the purpose of specifying

the tape and are not intended to be interpreted as guidelines for

actual edges placed on consumer products

,

Introduction

The Consumer Product Safety Commission has been concerned with the

dangers of sharp edges on consumer products. There exists no theoretical or analytical solution to describe the cutting mechanism of materials.

The only accurate way to determine whether a sharp edge will cut human

skin is to test in vivo skin against the edge. This, of course, is

impractical in terms of personal safety. Therefore, the approach in

these studies was to test in vitro excised skin against different edge

geometries to see how they cut at different force levels. Once this was determined a further attempt was made to determine a relationship

between an edge which cut skin at a given force level and this same edge

cutting a synthetic material. Thus, a synthetic material can be substituted for tissue in testing sharp edges and their ability to cut human

skin following a given safety criterion.

This report is a continuation of the study originated by Sorrells

and Berger 1/ to establish a procedure for detecting hazardous edges.

Due to the complex nature of the problem of obtaining a suitable synthetic material which would detect dangerous exposed edges on consumer products, an empirical approach was used in this study. This approach

consists of utilizing a device which could detect hazardous edges when

used with a suitable synthetic material.

Sorrells designed a sharp edge tester (see Figure 1), as described

in the previous report, which would differentiate between unsafe edges

and safe edges under a mode previously termed "casual handling." In the

"casual handling" mode there is a normal force between the skin and the

edge, a relative velocity parallel to the edge, but no relative velocity

in the normal direction (except when cutting occurs)

.

In the previous study, forty different materials were evaluated for

their ability to discriminate edges in a manner similar to that of human

skin. As a result of this study poly (tetrafluoroethy lene) * was selected

as a test material for use with the sharp edge tester because (1) it discriminates between edges in the same way that human skin does, (2) it is

free of liquid plasticizers that may evaporate or migrate and cause changes

in the properties of the tape, (3) the polymer is highly stable, and (4) it

is relatively convenient to mount on the tester because of the adhesive

backing

.

The method of choosing sharp edge safety criteria in this study was

to select an injury level and a force developed between the edge and

human skin, that would create this type of injury. In Sorrells' study 2/

edges with many different geometries were used to cut human skin. From

these experiments two different safety criteria were chosen by the

Consumer Product Safety Commission, depending on the accessibility of

an edge. An exposed** edge would be considered safe if it did not break

*Hereafter abbreviated PTFE.

**Whether an edge is easily accessible (exposed) or not is completely

arbitrary. The manner in which the user of a product contacts an edge

will depend on the particular product, the way it is used, and for

whose use it is intended.
- 2 -

through the epidermis of the human skin when the normal force between

the edge and the skin was 89 newtons (20 pounds). No breakthrough of

the epidermis corresponds to a "no bleed" level cut. The second

criterion required that, with a force of 17.8 newtons (4 pounds), the

edge would not penetrate the human skin greater than 80 percent of the

total thickness of skin for an edge not easily accessible when a product

is in normal use. Thus, these safety criteria can be expressed as:

Safety Criterion (A) - 89 newton force, no epidermal

breakthrough

Safety Criterion (B) - 17.8 newton force, less than 80 percent

penetration of the total skin thickness

Two pairs of edges were selected from a group of test edges, against

which the synthetic material was tested for each safety criterion.

Figure 2 gives a description of all edges used in these experiments. One

edge in each pair was not to penetrate the material completely while the

other was to cut through the material. Thus, for the safety criteria:

Safety Criterion (A) - discriminate edge L unsafe and edge M

as safe

Safety Criterion (B) - discriminate edge A unsafe and edge B as safe

A description of the experimental testing performed in order to

obtain a method to specify properly a suitable synthetic cut indicator

for use with a sharp edge testing device is presented in this report.

In addition, this report describes a performance test for the tape and

the proper thicknesses of tape material to specify which tapes are to

be used with the sharp edge tester.

Tape Testing Experiments

Experiments were performed to find a PTFE tape that would satisfy

the dual safety criteria as mentioned in the introduction. Additional

tests were made in an attempt to relate the mechanical properties of the

tapes to their ability to meet the safety criteria. It was hoped that it

would be possible to specify the correct PTFE tape to be used on a sharp

edge tester by stating the required mechanical and physical properties.

The PTFE tapes were tested for the following properties:

1. The maximum breaking strength

2. The maximum tensile strength

3. Ultimate elongation

(
- 3 -

4. Film thickness and adhesive thickness

5. Whether the tape would discriminate safetycriteria (A) and (B)

The first three properties listed above were tested using a universal testing machine according to ASTM Standard D-882. Tape

samples were pulled apart at a rate of 50 cm/min, with an initial grip

separation of 5 cm, until they broke. The results were recorded on a strip chart and calculations were made based on the maximum load applied

to the tape.

The film thickness and adhesive thickness of each tape were measured

using a dead weight dial micrometer, having a 112 gram (4 oz) weight,

according to ASTM Standard D-374. The load was applied normal to the tape

surface which contained the adhesive coating. The adhesive coating was then removed with solvents and the film thickness measured. The adhesive

thickness was determined from the difference in the total thickness and

the film thickness.

The safety criterion (A) and (B) tests were performed with a 16.9

newton force for criterion (A) and a 4.45 newton force for criterion (B)

between the edge and the tape using the sharp edge testing device. It was established in earlier work ZJ that a 16.9 newton force applied to an edge and tape will discriminate sharp and dull edges in the same way as human skin does at a 89 newton force level between an edge and skin. In

addition, a 4.45 newton force applied to an edge and tape will discriminate sharp and dull edges similarly to human skin at 17.8 newtons . The

tape was placed on the sharp edge testing device's mandrel and allowed

to rotate against the test edges with the appropriate force applied to

the edge during one revolution. For safety criterion (A), edge L was to

penetrate the tape completely and edge M was not to penetrate the tape

completely. Safety criterion (B) required that edge A penetrate the

tape and edge B not penetrate the tape completely. The results are shown in Table 1. The manufacturing processes of most of the tapes are listed in order to see whether they correlate with the tape properties.

The breaking strength, tensile strength and elongation properties of

the tapes show little or no correlation to the tape's ability to satisfy

the discriminations (A) and (B) . The percent standard deviation of the

measurements typically varied from 5 to 15 percent.

There appears to be a strong relation between a tape's ability to

discriminate the safety criteria and its thickness. It is evident (see

results for tapes 5,6, 8 and 9) from the data that the tape should have

a film thickness of 0.075 to 0.10 mm (3 to 4 mils). These measurements

of thickness had a 2 percent standard deviation. This indicates that

most of the tapes were of uniform thickness. An examination of the

manufacturing processes of the tapes did not show a correlation to the

tape's properties.
- 4 -

An examination of the criteria data of Table 1 shows that 6 tapes were able conservatively to satisfy criterion (A) more than 80 percent

of the time. Conservative here means that in terms of consumer safety,

the tape will determine a safe edge hazardous when the edge does not pass

the safety criteria. Only two tapes were able to discriminate criterion

(B) conservatively. These two tapes were able to discriminate both

criteria (A) and (B) conservatively.

The problems encountered in trying to obtain a tape that would meet

both safety criteria were many. No relationship could be established

between the mechanical properties of the tapes and their ability to

meet the safety criteria. Nevertheless, the thickness of the tapes can

be correlated to the tape's ability to meet the safety criteria; that is,

suitable thickness should be in the range 0.075 to 0.10 mm (3 to 4 mils).

However, thickness alone would not be enough to specify the tape. The

major problem appears to be related to the chosen safety criteria and

the selected edges to meet the safety criteria.

In reviewing the original skin cutting data several factors were observed which help explain the problems of obtaining a single tape

material to meet both criteria. Figure 3 is a plot of the percent depth

of cut* as a function of designated edge geometries, while maintaining

a 4 pound (17.8 newtons) force between the edge and the skin sample.

Figure 3 shows the human skin-cut data developed by test edges A and B at 17.8 newtons (4 pounds). Due to the similarities of edge geometries

and manner of cutting human skin there exists little difference in the

cutting potential of these edges. Results shown in figure 4 show also that

with this test there is difficulty in differentiating between cutting

potentials of edges L and M. It was concluded that it is impractical to

find a tape which could make such precise distinctions as required by these

selected edge pairs.

After discussion with the Consumer Product Safety Commission project

manager, it was decided to eliminate safety criterion (B) . It was also

decided that two new edges be selected for safety criterion (A) which

would have a larger difference in their cutting potential. Edge K (90° X 0.05 mm) was selected as a representative edge for indicating a break in the epidermis of human skin and as an unsafe edge. Two new

edges N (90° X 0.25 mm) and 0 (90° X 0.30 mm) were added as safe edges.

These additional edges were selected because there would be less

difficulty encountered in fabricating them as standard edges. A

selection of one of the two new edges as a safe edge could be made after

retesting the tapes against each of them and also edge K. The final two

edges selected could then be used to specify the mechanical properties

or the cuttability of the tape under a single force loading condition

of 16.9 newtons using the sharp edge tester. Thus, the tape could be

specified by testing it against two standard edge geometries under a single force loading, and the tape's thickness. A single safety criterion was established from these decisions against which all edges on consumer products could be tested:

Safety Criterion - 89 newton force, no epidermal breakthrough

*The percent depth of cut, D* , is the ratio of the thickness of tissue at

the bottom of a cut to that of the total thickness of skin adjacent to the

cut expressed as a percentage. This is plotted as the ordinate value in

figures 3 and 4.
- 5 -

Further testing was conducted on the tapes using the new edges

selected. This consisted of retesting the tapes for their ability to

meet the single safety criterion against edges K, N and 0 using the sharp

edge tester. The results are presented in Table 2. Edge K will cut all

the tapes tested, except tape 5 which has a thickness in excess of 0.125 mm

(5 mils), indicating edge K as an unsafe edge. Edge N did not cut through

9 of the 11 tapes tested. Edge 0 showed 10 of the 11 tapes tested as able to discriminate this edge as safe. From these results edge K and

edge N were selected as the two standard edge geometries against which

to test the tapels performance to detect hazardous edges. Edge N,

instead of edge 0, was selected because it was more similar to the geometry of edge M, which was used in the human skin cutting experiments.

Thus the safety criterion can be stated in terms of these edges as:

Safety Criterion - discriminate edge K unsafe and edge N as safe

Performance Test of Synthetic Cut Indicator

The cut indicator to be used with the sharp edge tester should be

poly (tetrafluorethylene) pressure-sensitive tape with a film thickness

of 0.075 to 0.10 mm (3 to 4 mils) and with an adhesive backing of

0.0375 to 0.075 mm (1.5 to 3 mils). The tape must also pass a performance test against two standard edge geometries: that is, the tape must

be cut (totally penetrated by the edge in at least one point on the tape

specimen) with a 90° X 0.05 mm (2 mils) edge and not be cut with a 90° X 0.25 mm (10 mils) edge when the tape is allowed to rotate against

each edge with a force of 16.9 newtons applied using the sharp edge

tester.

The standard edges should be constructed with a tolerance of + 3°

for the included angle of the edge and with a tolerance of +_ 0.0125 mm

(0.5 mil) for the radius. The edges used at NBS for making the safety

discriminations were fabricated of tool steel, oil hardened to a Rockwell

hardness of 60 - 62. The radius of the edges were finished to have an average surface roughness no greater than 8 microinches.

Summary

The PTFE sensitive tape described in the previous section can be

used as a hazardous edge detector when used with the sharp edge tester.

The PTFE tape must meet the proper thickness specifications and pass

the performance test against two standard edge geometries using the

sharp edge tester. An edge may be considered hazardous if the tape is

completely penetrated at a contact force of 16.9 newtons (3.8 pounds),

the equivalent of a breakthrough of human skin when the contact force is

89 newtons (20 pounds) between skin and the edge. This procedure then

may establish a safety criterion by which edges on consumer products may

be tested.
- 6 -

The two selected edge geometries to test the tapes are intended

solely for the purpose of indicating whether or not the PTFE tape is

suitable for use as a cut indicator when used on the sharp edge testing

device. In no way should these edge geometries be interpreted as guidelines for edges placed on consumer products. The edges presented

in Figure 2 show many different edge geometries that will cut human

skin at the 16.9 newton (20 pound) force level. Edges on consumer

products will have to be tested individually against the PTFE tape to

insure that they are not hazardous to the consumer.
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Table 2. Discrimination Test Results

Edge K Edge N Edge 0

Sample # No. of Tests 90° X 2 mils 90° X 10 mils 90° X 12 mils

The percent of successful discriminations

are shown for each edge

1 20 100% 45% 58%

2 10 100% 100% 100%

3 8 100% 100% 100%

4 10 100% 100% 100%

5 8 0 100% 100%

7 10 100% 100% 100%

8 10 100% 100% 100%

10 10 100% 100% 100%

11 10 100% 100% 100%

12 10 100% 100% 100%

14 10 100% 90% 100%


EDGE ANGLE (a) RADIUS TYPE

degrtet, radions

A 60». T/Z .000 in GROUND

B »/2 .000 GROUND

C SHEARED

D SHEARED

E I05*, .000 GROUND

F .000 GROUND

6 1 5* n09 f mm \ b tXUU IM U

H 30* 002 GROUND w lA VV n L/ I 60* .002 GROUND

J 15 • .004 (.lOmm) GROUND

K 90* .002 GROUND

t 90* .004 GROUND

M 90* .008 (.20fnm} GROUND

N 90* .010 (.25mm) GROUND

0 90» .012 (.30mm) GROUND

I |<— .059 In (1.5mm) —|

| — .I25in (3.2mm)

TYPICAL SHEARED EDGE TYPICAL GROUND EDGE

FIGURE 2. DESCRIPTION OF TEST EDGES
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References

1. Sorrells, J.R. and Berger, R.D., "An Inspection Procedure for

Detecting Hazardous Edges," National Bureau of Standards Report

NBSIR 74-428 (April 1974).

2. Sorrells, J.R. and Berger, R.D., "Some Cutting Experiments on Human Skin and Synthetic Materials," National Bureau of Standards

Report NBSIR 73-262 (October 1973).

3. Op cit

USCOMM-NBS DC
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUHI.K A HON OR Ri: PORT NO.

NBSIR 74-546

2. (lOv't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A Synthetic Material for Detecting

Hazardous Edges

5. Publication Date January 1975

6. Performing Organization Code

446.04

7. AUTHOR(S)

James E. Maness

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4460502

11. Contract/Grant No.

12. Sponsoring Organization Name and ( omplete Address (Street, City, State, ZIP)

Consumer Product Safety Commission

Washington, D.C. 20207

13. Type of Report & Period Covered

Interim

14. Sponsoring Agency Code

IS. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of moat significant information. If document includes a si^ifleant bibliography or literature survey, mention it here.)

An investigation of poly (tetraf luorethylene) pressure-sensitive tape (PTFE

tape) was conducted to determine its suitability as an indicator of the hazard

potential of exposed edges on consumer products when used with the sharp edge

testing device. In addition, criteria were established by which suitable tape

could be specified as a standard material for detection of sharp edges.

From a study of many PTFE tapes having different physical and mechanical

properties, it was concluded that in order to specify the tape as a suitable

material to be used with a sharp edge testing device, the tape should be tested

against two standard edge geometries. One of the standard edges will cause the

tape to be totally penetrated, representing a bleeding cut or a breakthrough of

the epidermis of human skin when an unsafe edge is contacted. The other edge

will not totally penetrate the tape, representing a no-bleed cut when a safe edge

is contacted by a consumer.

Recommendations are presented to specify the proper PTFE tape based on the

tape's thickness and ability to pass a performance test against two standard

edge geometries under given force loading conditions. These edge geometries are solely for the purpose of specifying the tape and are not intended to be inter17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated try semicolons

)

Consumer; discrimination; edges; geometry; hazardous; injury; safety

criterion; sharp edge tester; skin; synthetic cut indicator; tape

18. AVAILABILITY O Unlimited

|Xj^ For Official Distribution. Do Not Release to NTIS

1 1 Order From Sup. of Doc, U.S. Cjovernment Printing Office Washineton. D.C. 20402. SD Cat. No. C13

Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

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(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

15

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