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Product safety and failure analysis

ESPEC Technology Report No.62

1

T

Technology Report

Product failure mechanisms and reliability testing

Part 2: Plastics, rubbers and electrical cables

Yasutoshi Nakagawa ESPEC CORP., Technology Development Division,

Technology Development Department

his report is an expanded version of excerpts taken by Espec from

presentations made at a product reliability seminar given by the Reliability

Subcommittee of the Kansai Electronic Industry Development Center (KEC),

and from material published in the journal KEC Jōhō (issue No. 214 of July, 2010).

Plastics (and rubbers) are widely used in various products for benefits such as their

workability, lightness and electrical insulation performance.

This report presents the findings of a survey on the mechanisms responsible for product

failures. It is based on actual cases in which plastics (and rubbers) used in products

caused serious product failures reported by the National Institute of Technology and

Evaluation (NITE). These findings are followed by a description of a reliability test

method that uses a better understanding of product failure mechanisms to reveal

potential market failures.

2.1 Breakdown by product

This section presents the breakdown of the surveyed product failures by category (for

each material responsible). These failures occurred between 2001 and 2007, and their

causes were classified by NITE as ‘C1’ (failures thought to have been caused by an old

product or a product with degraded performance from extended use).

2.1.1. Product failures caused by plastics

Figures 1 and 2 show the breakdown of the 33 plastic-linked product failures surveyed.

Oil-fired water heaters were the most failure-prone product type in this category,

accounting for about half of these cases. These oil-fired water heater failures were

mostly fires caused by fuel leaks from connections. The breakdown by years of product

uses shows that most product failures caused by plastics occurred around the 8-year

Introduction 1

Failure of products used for extended periods of time 2



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mark, with another cluster around the 15-year mark. The longest a product was used

before this type of failure occurred was 47 years.

Fig.1 Failure ratio by category (plastics)

Fig.2 Number of product failures by years of product use (plastics)

2.1.2. Product failures caused by rubbers

Figures 3 and 4 show the breakdown of the 25 rubber-linked product failures surveyed.

Fig.3 Failure ratio by category (rubbers)



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Fig.4 Number of product failures by years of product use (rubbers)

Oil-fired water heaters were again the most failure-prone product type in this category

also, followed by rubber piping, then four-wheel drive vehicles. As with product failures

caused by plastics, product failures caused by rubbers were mostly fires caused by fuel

leaks from O-rings and hoses. Most product failures caused by rubbers occurred after 20

years of product use.

2.1.3. Product failures caused by electrical cables

Figures 5 and 6 show the breakdown of the 39 electrical cable-linked product failures

surveyed.

Fig.5 Failure ratio by category (electrical cables)



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Fig. 6 Number of product failures by years of product use (electrical cables)

This category spanned many product types such as electrically heated kotatsu tables,

electric carpets and indoor wiring. Most of the failures were of indoor products.

Failure of this type most commonly occurred after 20 years of product use, followed by

15 years, 25 years, and then 37 years.

The breakdown by years of product uses therefore revealed that serious product failures

often occurred after more than 10 years of product use. Amid increasing public demand

for product safety, products are being used in the market for extended periods exceeding

the product life anticipated by the manufacturer. In the future, product development

and design work to prevent product breakdowns causing serious failures will be crucial.

2.2 Product failure cases

Table 1 provides information on several cases of serious product failures. Product

failures caused by plastics affected power tools and ski boots. They were caused by

long-term neglect of plastic parts that led to minute cracks in the material from aging.

During use, mechanical stress applied to these parts worsened the cracks, leading to

damage.

Product failures caused by rubbers affected oil-fired fan heaters and water heaters.

They were the result of aging heater connections and fuel hoses that resulted in a loss of

flexibility and the formation of minute cracks. Mechanical stress from supplying fuel

and thermal stress from operation heat worsened the cracks, leading to fires caused by

fuel leaks.



5

Product Component Years

of

use

Material

responsible

Description of

product failure

Causes

Plastics Power

tools

Pad 17 Polyurethane Pad damage

during use

Aging from

heat,

moisture

and UV

Mechanical

stress

(shocks)

Ski boots Boots 16 Polyurethane Damage from

impact during use

Office

chair

Plastic on

rear

10 Polyethylene Cracking and

damage during

use

Rubbers Oil-fired

fan

heater

O-ring 10 Silicon

rubber

Fire caused by oil

leaking from

connection

Aging from

ozone and

heat

Mechanical

stress

(pressure)

Oil-fired

water

heater

Fuel hose 25 Butadiene

rubber

Fire caused by

fuel leaking from

fuel hose

Elec.

Cables

Wiring

device

Covering 30 Polyethylene,

PVC

Fire caused by

short-circuiting of

extension cable’s

core wires

Aging from

heat, oxygen

and UV

Mechanical

stress

(twisting

and pulling)

Table 1. Serious product failures

Product failures caused by electrical cables were caused by a drop in insulation

performance, itself due to aging from long-term neglect of the cable covering. Core wires

of opposite polarities short-circuited, causing the product to catch fire or emit smoke.

Serious product failures in this category were also caused by partial disconnections, but

this discussion only applies to product failures caused by deterioration of cord

insulation. The next section discusses the mechanisms for product failures caused by

plastics and rubbers.

3.1 Aging

Figure 7 illustrates plastics and rubbers aging process compared to metals. There are

several known causes for the deterioration of plastic and rubbers.

Product failure mechanisms 3



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The mechanisms for deterioration by the most typical causes (heat, moisture and ozone)

are described below. 1, 2

Fig.7 Material decline of plastics and rubbers relative to usage time2

3.1.1. Thermal deterioration

There are two types of thermal deterioration during product use

1) Changes in the mechanical strength of formed/ molded products or in the

dimensions of products that generate or absorb heat, and

2) The decline of physical properties due to increasing deterioration from

thermal oxidation.

Thermal deterioration is caused by materials absorbing heat that results in more

energetic molecular motion. As more heat is absorbed, overall molecular vibration

becomes more pronounced, and oxidation favors cross-linking reactions, hardening (loss

of flexibility), and loss of strength.

Deterioration from thermal oxidation causes the product to deteriorate progressively

through the action of heat and oxygen, starting from its surface. Since the product’s

operating temperature is lower than the temperature it was formed or molded at,

deterioration from oxidation progresses slowly. Surfaces in contact with oxygen

gradually break down. Molecular breakdown starts at the product surfaces. It decreases

molecular weights and generates cross-linking, causing the surface layer to become

brittle, and minute cracks to form. Stress applied to a material in this state enlarges

these cracks and concentrates on them, significantly reducing the material’s ability to

withstand shocks, pulling or stretching.

Natural materials

(leather, cotton, silk, etc.)

Plastic products

Rubber products

Inorganic metals

Material

retention rate

Usage time



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3.1.2. Moisture deterioration (hydrolysis)

Moisture causes hydrolysis in polymers with ester bonds in their molecular structure.

Hydrolysis becomes more pronounced as the temperature rises, and even minute

amounts of moisture can cause hydrolysis when the temperature is near the material’s

melting temperature. Hydrolysis is further boosted by the presence of alkaline

substances.

Figure 8 Hydrolysis of urethane rubber

Fig.8 illustrates how hydrolysis takes place in urethane (polyurethane) rubber.

Hydrolysis occurs when moisture is absorbed into the surface and other exposed areas

of aging material, causing the ester bonds on its polyester molecules to react with the

water and break down into alcohol and acid. The molecular weight of the hydrolyzed

material decreases, resulting in cracks forming in the material’s surface. Polyurethane

foam is known to be particularly prone to hydrolysis due to the synergetic effect

between the capillary action of its minute foamed holes and the effect of deterioration

over long-term use.

3.1.3. Ozone deterioration

Diene-based rubbers with a structure containing double bonds in the polymer ’s main

chain deteriorate from atmospheric ozone. Ozone deterioration is caused by the action of

ozone on the double bonds of the molecular chains in vulcanized rubber. It breaks apart

the molecular chains, causing minute cracks (ozone cracks) to form on the surface of the

material. These ozone cracks grow into larger cracks in the direction perpendicular to

the direction where stress (strain) is applied. The higher the temperature and

magnitude of the stress (strain) applied the more ozone deterioration progresses.

Urethane bond

Urethane rubber polyester Polyester



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3.2 Product failure mechanisms

This section discusses the mechanisms by which aging causes serious product failures.

3.2.1. Carbon monoxide leak caused by oil-fired fan heater hose deterioration3

One of the surveyed product failures was caused by a hose in the air supply system of an

oil-fired fan heater- the ‘secondary air hose’ used to supply the air needed for complete

combustion. Cracks in this hose led to carbon monoxide poisoning. The secondary air

hose was an S-shaped component that retained stress from the twisting needed to

attach it. The stress concentrated on the surfaces of the hose curves. Since the hose was

made of acryl nitrile butadiene rubber (NBR), it was oxidized by atmospheric ozone,

particularly around the surface layer of its stress retaining curves. And with aging

promoted by the heat of combustion, cracks formed on the hose. The result was a

decrease in the amount of air supplied to the combustion chamber, causing a gradual

increase in the carbon monoxide concentration from incomplete combustion. The heat

generated by the fan heater during this time also promoted crack growth on both the

outside and inside hose surfaces, until the cracks became holes. When the holes reached

a certain size, the air supply pressure in the secondary air hose dropped, creating a

backflow from the combustion chamber of exhaust gases that were not completely

combusted. These exhaust gases started leaking form the holes, generating a large

amount of carbon monoxide.

3.2.2. Fire from electrical wiring short-circuit4

The mechanism by which aging degrades electrical cable insulation is as follows. The

action of ambient heat and atmospheric oxygen (autoxidation reaction) decreases

molecular weights as molecules are gradually broken down, causing the material’s

surface layer to become brittle and form minute cracks. This deterioration significantly

lowers material properties such as shock resistance and pull strength. Crack growth

progresses from heat exposure and stress loads, reducing insulation performance.

Degraded electrical wiring insulation can cause short-circuits due to either of two

failure modes:

1) Physical contact between conductors of opposite polarities, or

2) Arc discharging between opposite poles at areas of destroyed insulation

material.

I will now discuss the relationship between short-circuits and the covering material

(vinyl or rubber insulation) on live electrical wires. Table 2 lists the conditions used for

short-circuit tests performed on several types of wiring and insulation. Figure 9 shows

the physical state of the wiring before and after the tests. The tests found that the type

of electrical wire and magnitude of the radiant heat determined not only the time until



9

short-circuiting occurred, but also the cause of the short-circuiting. The following

conclusions can be drawn:

Short-circuiting of vinyl-insulated electrical wiring was caused mainly by contact

between the core wires when the covering material melted. When an external force

such as twisting or clamping was present, short-circuiting occurred at radiant heat

of 10kW/m3.

At a radiant heat of 40kW/m3, arc short-circuiting was observed between nearby

points on the core wires of vinyl-insulated electrical wiring.

At a radiant heat of 20kW/m3 or more, rubber-insulated electrical wiring exhibited

arc short-circuiting due to insulation destruction, but no contact short-circuiting.

The time until short-circuiting occurred was generally longer than for

vinyl-insulated electrical wire.

Electrical wiring insulated with thick rubber (#5) did not short-circuit even at a

radiant heat of 40kW/m3.

No. Conductor

diameter (mm) x

number

Wiring insulation

material, thickness

(mm)

Jacket material,

thickness (mm)

Outer

dimensions

(mm)

#1 0.18 x 50 PVC, 0.9 None 6.3 x 3

#2 0.18 x 30 PVC, 0.7 PVC, 1.0 7.4

#3 2.1 x 1 Rubber, 1.0 PVC, 1.2 9.8 x 6.4

#4 0.18 x 50 Rubber, 1.0 None 7.0 x 3.5

#5 0.18 x 50 Rubber, 1.0 Rubber, 1.5 10.5

#6 0.18 x 50 Rubber, 1.0

Paper, cloth

Cloth 7.0 x 5.0

Table 2. Tested wiring (and covering) types4

(a)#1：40 kW/m3, (b)#1：10 kW/m3,

(c)#2：40 kW/m3, (d)#3：40 kW/m3

Fig.9 Tested wiring types before (left) and after (right) testing4



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4.1 Common reliability testing

Table 3 lists commonly used reliability test items and signs of material deterioration.

Service life evaluations generally use the reliability test items shown in table 3 (such as

temperature, humidity, pulling and bending) to enable service life estimates by

evaluating material characteristics, analyzing failures, and verifying accelerated

market conditions.

But unlike inorganic materials such as metals, plastics and rubbers are prone to

extreme drops in original physical properties (deterioration) due to the physical and

chemical action of aging, a characteristic that requires careful consideration (figure 7).

For example, it would be risky to guarantee the service life of a plastic or rubber for 10

years with a bend fatigue rating of about 36,500 bends (10 bends per day x 365 days x

10 years) just because it passed a 38,000-count bend test. Estimating a product service

life using only simple accelerated life testing is problematic since the aging of the

material itself also needs to be considered (such as the aging of the rubber in the

secondary air hose on the carbon monoxide poisoning case previously described).

Table 3. Reliability tests and signs of deterioration (examples)

4.2 Reliability testing designed to reveal potential product failures

Figure 10 illustrates an approach to evaluate a product time to failure. The time to

failure of a product used for an extended period should be evaluated and determined by

combining the ‘fatigue life’ elements of the conventional accelerated life testing

approach (changes in strength from repeated use) with ‘aging life’ elements that

Test Sign of deterioration

Weather meter test Discoloration, minute cracks

Ozone exposure test Ozone cracks

High temperature/humidity test Softening, cracks

High temperature test Hardening, brittleness, reduction in strength

Low temperature test Hardening, brittleness, reduction in strength

Temperature cycle test Crack growth

Acid (alkaline) resistance test Degeneration, reduction in strength

Pull/compression test Permanent deformation, destruction

Bend fatigue test Cracks, fatigue destruction

Tracking test Tracking, burning

Reliability testing designed to reveal potential product failures 4



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consider material aging (changes in strength from aging).

This approach could enable service life

predictions with a high correlation to

market failures by moving the focus

on ductile destruction (mainly elastic

deformation), and more toward brittle

destruction with aging-induced plastic

deformation.

Figure 10 Relationship between polymer aging and fatigue life

Figure 11 shows the graph of tensile elongation versus immersion time revealed by the

service life testing performed on a hose5.

Fig. 11 Hose service life test results (example)5

The graph indicates that

1) The tensile elongation of the hose decreases as immersion time increases,

and

2) The rate of the decrease varies according to ambient temperature.

To accurately predict the service life of plastics or rubbers using this graph, it is

important to understand the material deterioration caused by the product’s operating

environment (such as the temperature and humidity).

Strength Fatigue life elements

Aging life elements

Combined life elements

No. of repetitions

Aging

Set service life cutoff line

Tensile

elongation

(%)

Immersion time (days)



12

Evaluations in product’s actual operating environment

(factors causing material deterioration)

Temperature

Humidity

Ozone

UV

Other

Reliability testing designed to reveal potential market

failures

Evaluations in product’s actual operating environment (factors causing mechanical

deterioration)

Pulling

Compression

Bending

Tracking

Other

Stress

Fig. 12 Reliability testing designed to reveal potential market failures (example)

The survey findings showed that over long-term use, the accumulation of thermal,

chemical and mechanical actions on plastics (and rubbers) often caused cracks,

hardening, contraction and other reductions in functionality (primary product failures)

that led to serious (secondary) product failures.

Testing that combines the element tests of accelerated life testing (for material

deterioration) with practical testing (for mechanical deterioration) should be effective

for predicting product service life by anticipating market failures.

Conclusion 5

Element tests Practical tests

Time



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In the future, researchers will need to gain a precise understanding of the signs of

deterioration mechanisms for element tests. This understanding should be used to

devise reliability tests and verify the correlation of the results to market failures.

Bibliography

1. Tsutomu Nakamura and Yoshito Otake: Journal of the Society of Heating,

Air-Conditioning and Sanitary Engineers of Japan [in Japanese], p. 55. Volume 79, No.

11 (2005)

2. Yoshito Otake: Journal of the Society of Heating, Air-Conditioning and Sanitary

Engineers of Japan [in Japanese], p. 69. Volume 80, No. 1 (2006)

3. “Encyclopedia of Product Failures”: Nikkei BP, pp. 406 to 410 (2009)

4. Yasuaki Hagimoto, Norimichi Watanabe and Katsuhiro Okamoto: “Short-Circuiting

of Electrical Wiring From Radiant Heat”, Bulletin of Japan Association for Fire Science

and Engineering, Volume 54, No. 2 (2004)

5. Union of Japanese Scientists and Engineers: “Failure Physics and Service Life”

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