Consumer Product Safety

NITE Alert_79

February 6, 2007

Safety alert - Prevention of burning incidents involving microwavable foot warmers

With the recent increase in use of microwavable foot warmers (*1), accidents causing burn injuries have been observed due to excessive heating. NITE implemented product safety tests by purchasing those products available in the market (*2). The results of these tests showed that in some products, the explosion of containers or contents leaks would occur when heated excessively, which may cause consequential burns/scalds.

  1. (*1)Bags made of resin film or resin cases filled with lagging materials which are designed to be heated in the microwave to retain heat for use as foot warmers, etc.
  2. (*2)Although microwave instruction manuals discourage heating non-food materials, this test is not concerned with the issue of heating non-food materials in a microwave. This is a reminder for the safe use of foot warmers.

Alert

  1. (1) Always be sure to follow the instructions for power and heating time. Excessive heating may damage or rupture bags/cases, causing high temperature heated materials to spatter and cause burn injuries.
  2. (2) Turn off a microwave immediately if the bag inflates or the contents (gel or liquid) leak. Leave enough time for the contents to cool before opening the microwave door.
  3. (3) Do not reheat while the warmer is warm, or follow reheating instructions if available.
  4. (4) Do not use a microwave in automatic mode to prevent excessive heating.
  5. (5) Stop using a foot warmer immediately if it becomes less elastic or if cracks begin to appear as the bag may deteriorate over time.
  6. (6) Be very careful when removing a foot warmer from the microwave as the bag/case may become very hot (100C) in some areas even when heating as instructed.

1. Background

NITE has received information on accidents involving microwavable foot warmers, such as consumers suffering burns from leaked contents of warmers or from cases exploding when removing them from a microwave.The Ministry of Economy, Trade and Industry (METI) alerted consumers on October 2, 2006, in response to serious burn incidents due to exploded microwavable foot warmers. METI also ordered accident reports from their manufacturers / distributors in compliance with Section 83 of the Consumer Products Safety Law. Since there are many similar products available in the market, NITE implemented product safety tests by purchasing these products.

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2. Summary of the test result

2.1 Sample products

In determining test objects, 14 samples were chosen from the market, of which 9 were gel type, 4 solid (liquefied by heat), and 1 liquid. They were purchased from retailers or by mail-order between November 2 and 13. Modifications made after purchasing are not considered in this test.

2.2 Test method

All samples were tested in the following four patterns which take actual usage patterns into consideration. In addition, a test was conducted to evaluate the probability of damage by dropping.

  1. (1) Heating as instructed
  2. (2) Heating excessively against instructions
    • Reheating while warm
    • Heating at higher output power than instructed in maximum allowed time
    • Heating continuously until damage occurs

Drop tests were also implemented after test 1 to find whether the samples would be damaged. In addition, instruction manuals were examined to see if they included safety precautions.

2.3 Test results

  1. (1) Heating as instructed
    Bags/cases of 14 samples did not indicate problems such as inflation, melting, explosion. However, in 2 samples (Sample No.10 and 14), surface temperatures reached 100C.
  2. (2) Heating excessively
    • Reheating while they are warm
      Bags were inflated in some products (No.5, 13 and 14). Repeating inflation and deflation may cause a consequent deterioration in strength on a long-term basis. Surface temperature on some samples (No. 5, 10, 13 and 14) rose to 100-140C.
    • Heating at higher output power than instructed
      While heating, the resin case of one sample (No. 11) exploded. The force burst open the microwave door, and the liquid filling, at 99 C, splashed out. There was another sample that almost exploded (No.5).
    • Heating continuously until damaged The test showed differences in effusion conditions of the contents. Resin film bags became damaged relatively quickly - in minutes, and gel filling flowed out. One of the samples (No.3) became damaged in less than 2 minutes. On the other hand, products with resin cases took as long as 15-20 minutes before damage occurred. The test showed that time to damage was generally proportional to the designated heating times given in instructions. Temperatures of the fillings that flowed out were measured at about 100 C for gel or liquid types, and over 200 C for solid types. In some products (No.9 and 11), the force of the explosion burst open the microwave door allowing filling to splash out. Also, there were some samples (No. 10, 12, 13 and 14) whose filling flowed out from the bottom of the microwave door.
    • Drop tests
      No defects were detected in drop tests with all 14 samples after reheating.
  3. (3) Comparison of descriptions in instruction manuals
    Manuals were examined for safe handling instruction content. Only one sample (No.12) covered all of the following instructions in its manual.
    • Instruction label on power output and time of heating
    • Improper heating may pose explosion of the container
    • What to do when leakage is seen / Concern about avoiding burns
    • To stop heating when the bag is visibly inflated
    • Not to open a microwave door right away when the bag is visibly inflated
    • How to reheat
    • Not to heat in automatic mode

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Appendix

1.Samples

Microwavable foot warmers vary in lagging materials, bead, gel, solid (*1), and liquid, etc. For these tests fourteen samples, which use the gel, solid or liquid lagging reported to have caused accidents, were purchased from the market (*2).

*1:
Solid but liquefied by heat
*2:
Purchased in November 2006 at retail shops or mail order
Table1: List of test samples
No. Brand name Name of
Manufacturer / Distributor
Heating instruction
(Power / Time)
Content type Container type Country
of origin
1 Renji de yutapon Hakugen Co. 500W: 3min10sec
600W: 2min40sec
Gel Double bag Japan
2 Hokaron koarano maachi Lotte Health Products Co. 500W: 3min30sec (10C)
/ 3min (15C)
600W: 3min15sec (10C)
/ 2min45sec (15C)
Gel Double bag
3 Ohirune pokatan taiyaki Honyaradoh Co. 500W: 50sec
600W: 40sec
Gel Double bag China
4 Yu matto Terao Corporation 500W: 3min15sec
600W: 2min45sec
Gel Double bag China
5 Aroma foot warmer Natural Wave Within 3min *3 Gel Bag China
6 Hiepoka wanwan Nihon Paff Co. 500W-600W:
50sec - 1min15sec
Gel Bag
7 Kaion-kun yutanpo Okamoto Inc. 500W: 3min30sec (10C)
/ 3min (15C)
600W: 3min15sec (10C)
/ 2min45sec (15C)
Gel Double bag
8 On aisu Kenyuu Co. 500W-600W: 3min *4 Gel Bag
9 Hot & Cool Fujisho Co. 500W: within 4min
600W-1000W: within 2min
Gel Double bag *5 Taiwan
10 Yutaron Sanyo Engineering Co. Within 600W
: approx. 4min *6
Within 700(800)W:
approx. 3min30sec
Within 900(1000)W:
approx. 3min
Solid Double bag Japan
11 Yupon ENG Hashimoto 500W: 6min (within 10C)
/ 5min (10-20C)
/ 4min (20-30C)
600W: 5min (within 10C)
/ 4min (10-20C)
/ 3min (20-30C)
750W: 4min30sec (within 10C)
/ 4min (10-20C)
/ 3min (20-30C)
*7(instruction on the product)
within 6 min in 600W
Liquid Soft case
12 Renji de chinshite pokka poka DoggyMan
H.A. Co.
500/600W: 5min
700W: 4min
1000W: 2min50sec
Solid Hard case Japan
13 Ekopoka Tokyo Kikaku Hanbai 500/600W: 6min
700W: 5min
1000W: 2min50sec
Solid Hard case Japan
14 Renji heat pad KPS 600-650W: 8min,
700-750W:7min, *8
800-850W: 6min
900-1000W: 5min,
1100-1200W: 3min,
1300-1400W: 2min30sec
Solid Hard case UK
*3:
Instructions were not printed on the product, but on the package, saying "stop heating if the gel pack starts inflating, even if this occurs within 3 minutes".
*4:
Instructions were not printed on the product, but on the carry bag.
*5:
The inner bag was made of nylon and the outer bag was fabric with a rubber coating on the inside.
*6:
Instructions on the package were different to those on the product. The longer heating times (30 sec) are stated in the table.
*7:
Instructions on the cotton bag were different to those on the product and the user manual. Instructions on the product and the manual are stated in the table.
*8:
Instructions on the product were different to the manual. Instructions on the product are stated in the table

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2. Test items, test methods and results

Taking actual usage conditions into consideration, tests were implemented to confirm if any abnormal states, such as inflation or explosion, would be seen, and to measure surface temperatures at 6 points (*) on the bags, etc, and the temperatures of leaked content when samples were heated as instructed and when they were heated excessively. (*) 6 points were the center and two points one quarter of the way in from the edges on both sides. For test conditions, the temperature of the laboratory was 202C and the samples were adjusted to 20±0.5C after being kept in a thermostatic oven for four hours, in preparation for each test. The microwave oven used for the tests had adjustable power levels, 500W, 600W, 700W and 1000W, with a capacity of 30 liters, and was designed to heat from the bottom.

2.1 Test by heating as instructed in manuals

  1. (1) Method
    The samples were heated according to instructions (power and time). When multiple instructions were given, tests were performed for all given power/time instructions.
  2. (2) Results
    No test samples showed any abnormalities such as inflation or explosion, etc. However, the surface temperatures of parts of samples No.10 and No.14 rose to 100C as shown below.
Table 2: Test result of heating as instructed
(samples reaching high surface temperatures)
600W 700W 1000W
Sample No. Highest* Average Highest* Average Highest* Average
10 Heating duration: 4 min Heating duration: 3 min 30 sec Heating duration: 3 min
100C 81C 104C 82C 106C 80C
14 Heating duration: 8 min Heating duration: 7min Heating duration: 5 min
97C 77C 100C 75C 93C 66C
  • * Highest temperature of 6 sample points

2.2Reheat test

  1. (1) Method
    To confirm the safety of the samples when reheated while still warm, the samples were heated under the same conditions as stated above in "2.1 Test by heating as instructed in manuals "and later, when their surface temperatures cooled down to about half the temperature increase caused by heating (*9), they were heated again in accordance with "2.1 Test by heating as instructed in manuals" conditions. Also, a drop test (samples dropped from a height of 30cm onto a wooden board) simulating users dropping foot-warmers onto tables etc., when removing them from a microwave after re-heating, was conducted to see if the samples would be damaged.
    *9:
    “Temperature increase” means the difference between sample temperatures before and after heating.
  2. (2) Results
    The table below indicates the samples showing abnormal states. Inflation was seen with the bags or hard cases of samples No.5, 13 and 14, and the surface temperatures reached 100C in samples No.5, 10, 13 and 14. Temperatures for No.10 and No.14 rose as high as 140C. No defects were detected in drop tests with all samples.
Table 3: Results of Reheat test (samples showing abnormal states)
Sample No. Surface temperature of samples (C) Description of deformation/breakage
Heating power 500W 600W 700W 1000W
5 Pre-heat temperature 38 38 - - Expanded about 5cm in whole when heated at 600W
Maximum temperature 75 97
Average temperature *1 67 87
10 Pre-heat temperature - 44 44 45 No deformation/breakage
Maximum temperature 141 145 149
Average temperature *1 127 134 133
13 Pre-heat temperature - 33 34 31 Expanded when heated at 600W and 700W at filling inlet. Slightly slanted in whole after heating.
Maximum temperature 98 86 67
Average temperature *1 75 69 49
14 Pre-heat temperature - 45 46 47 Expanded gradually, but not on a grand scale. Slightly deformed resin hard case.
Maximum temperature 138 136 123
Average temperature *1 134 134 116
Note:
Heating instructions for No.5 refers to time (less than 3 min) but not to power. Test was not implemented with power of 700W or more, in compliance with the instruction alerting to stop heating when inflation is seen, as the sample started inflating while heating at 600W.
*1:
Average temperature of 6 sample points

2.3 Heating with excessive power

  1. (1) Method
    To simulate users using higher power settings by mistake, samples were heated at 700W (higher power setting of single-function microwaves) and 1000W (maximum power of domestic microwaves). The maximum heating times given in instructions were applied to the respective samples.
  2. (2) Results
    The table below indicates samples showing abnormal states such as inflation or explosion, etc. Half of sample No.5 inflated when heated at 700W, and at 1000W, this sample expanded up to 10 cm high. Sample No.11 exploded at 1000W after significant inflation. The intensity of the explosion momentarily forced open the microwave door and the liquid filling splashed out. The temperature of the exploded content was 99C. It is presumed that the high strength of the container of No.11, a soft case made of vinyl chloride resin 2.7 mm thick, allowed the inner pressure to reach high levels. Meanwhile, the surface temperature of samples No.10 and No.14 exceeded 100C.
Table 4: Test results of heating with excessive power
(Samples showing abnormal states)
Sample No. Instructed heating time Heating condition
(Power / duration)
Time started inflating Temperature of surface Temperature of contents when burst Description of leaked contents when containers broke/exploded
5 Within 3 min 700W: 3 min 2 min 30 sec 88 N/A Inflated about half of the sample
1000W:3 min 1 min 50 sec 99 N/A Kept inflating when stopped heating, and expanded up to 10 cm high
11 *10
600W: 6 min
700W: 6 min N/A 84 N/A No abnormality seen
1000W:6 min 5 min N/A 99 Inflated greatly and exploded at the top. Briefly burst the microwave door open slightly, and leaked content splashed out of the harness.
*10:
Different heating times were given on the product and in the instruction manual (see *6 of Table 1).
Table shows longest heating times indicated on the product

2.4 Continuous heating test

  1. (1) Method
    The samples were heated continuously at power levels indicated in the instructions until they were confirmed broken or ruptured. The time taken for the samples to break or rupture, the temperatures of leaked contents and the internal temperatures were measured.
  2. (2) Results
    Resin film bags burst within minutes allowing the gel contents to leak out. Sample No.3 burst the fastest after 1 minute 42 seconds of heating. Gel type contents of samples No.1 through No.8 did not splash out of the microwave or flow out of the microwave through the bottom of the door. Samples No.9 and No.11 inflated and burst. The microwave door was momentarily forced open by the intensity of the burst, and a significant amount of contents splashed out of the microwave. The outer bag for No.9 is a fabric bag with a rubber coated inside, while No.11 has a soft case made of vinyl chloride resin. The bursts are presumed to have been caused by heightened inner pressures made possible by the high strength outer casings. It took as long as 15-22 minutes of heating to damage samples enclosed in hard resin cases (solid type). Although no contents splashed out of the microwave, large amounts of liquefied hot contents flowed out through the gaps between the main body and door of the microwave. In general, the time required to break a container is proportional to the instructed heating time. In addition, temperatures of contents rose to approximately 100C with gel and liquid types, and liquefied solid contents reached 200C. No.14 hit the highest temperature of 250C.
Table 5: Result of Continuous heating test
(Samples which leaked contents out of the microwave)
No Heating instruction Time required to break/explode sample
Content temperature
Description of leaked contents when containers broke/exploded
500W 600W 700W 1000W
9 500W: 4min
1000W: 2min
10min
7sec
N/A N/A 5min
33sec
Continued inflation after the inner bag burst. Explosion of outer bag opened the microwave door briefly, and significant amounts of contents splashed out up to 2.2 meters around.
101C 101C
10 600W: 4min
700W: 3min 30 sec
1000W: 3min
N/A 10min 30sec 9min 14sec 7min
4sec
Inflated then broke at the seal. Substantial amounts of contents leaked, and flowed out of microwave through the gap between main body and door.
197C 217C 202C
11 500W: 5min
600W: 4min
700W: 4min
9min 23sec 8min 8sec 6min 41sec N/A Inflated then burst. Intensity slightly opened the microwave door momentarily, and significant amounts of contents splashed out up to 1 meter around.
88C 88C 96C
12 600W: 5min
700W: 4min
1000W: 2min 50sec
N/A 20min 50sec 16min 31sec 15min The container inflated and deformed, then broke at the softened part. Hot liquefied content flowed out from the rupture, and consequently leaked out of microwave through the gap between main body and door.
221C 208C 228C
13 600W: 5min
700W: 4min
1000W: 2min 50sec
N/A 17min 12sec 14min 30sec 14min
40sec
The container inflated and deformed, then broke at the softened part. Hot liquefied content flowed out from the rupture, and consequently leaked out of microwave through the gap between main body and door.
172C 186C 194C
14 600W: 8min
700W: 7min
1000W: 6min
N/A 21min 58sec 19min 30sec 16min
50sec
The container inflated and deformed, then broke at the softened part. Hot liquefied content flowed out from the rupture, and consequently leaked out of microwave through the gap between main body and door.
218C 235C 254C

2.5 Compositional analysis of contents

  1. (1) Method
    The main components of the contents were identified by infrared spectroscopy.
  2. (2) Results
    Among samples with gel type contents, samples No.1,2,4,5 and 8 had "polyvinyl alcohol and water" as their main component, while "carboxymethylcellulose sodium and water" was the main component of samples No.3,6,7 and 9. The content of the liquid type sample (No.11) was composed of water, while the contents of No.10,12,13 and 14 (solid type) consisted of "polyethylene glycol".

2.6 Investigation of instructions

  1. (1) Objectives
    To confirm whether the instructions provided the necessary information for safe usage.
  2. (2) Results
    The table below indicates the results. Only No.12 met all the requirements. No.7 and 11 showed all instructions other than the instruction to "not to open a microwave door immediately when inflation is observed".
Table 6: Investigation if heating instructions are properly displayed
No. That misuse (heating power / time) may cause breakage or explosion of container Warnings for handling and burn hazard when the contents leak Warnings when inflation is observed Not to open a microwave door immediately when inflation is observed Reheating instructions Not to heat in automatic mode Instruction of heating time displayed on the product
1 check * check check check check
2 check check check check check check
3 check * check check check check
4 * check check check check
5 check
6 check check check check check check
7 check check check check check check
8 check check check check
9 check check
10 check check check
11 check check check check check check
12 check check N/A N/A check check check
13 check N/A N/A check check check
14 check check N/A N/A check check
  • * Handling instruction only (no warning for burn hazard)

Fore more information;
National Institute of Technology and Evaluation (NITE)
Product Safety Technology Center, Product Safety Investigation Division

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