NITE Alert_79

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.

Back to top

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

Back to top

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

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.

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.

*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.

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		*
2
3		*
4		*
5
6
7
8
9
10
11
12			N/A	N/A
13			N/A	N/A
14			N/A	N/A

• * 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