Research Reports of the National Instituteof Industrial Safety, NIIS-RR-2005 (2006)
UDC 537.24:667.612

静電粉体塗装用塗料の着火性に関する研究（その1）*
崔光石**, 山隈瑞樹**，鄭載喜***

Study on Ignitability of Coating Powder Used in Electrostatic Powder Coating System(Part 1)*
by Kwang-Seok CHOI**, Mizuki YAMAGUMA** and Jae-Hee JOUNG***

Abstract; Electrostatic powder coating has been successfully implemented in the painting industry as anenvironmentally friendly process. However, there have been many accidents involving dust fires and/or
explosions in electrostatic powder coating systems using corona discharge. One of the methods for avoiding fires
and/or explosions in electrostatic powder coating plants is to investigate the Minimum Ignition Energy (MIE) of
coating powders due to the capacitive discharge, such as the electrostatic discharge. This paper reports theexperimental results dealing with MIEs of the coating polymer powders (polyester, epoxy, epoxy-polyester copolymer, nylon, and polyacrylonitrile). The MIEs of five kinds of polyester powders, which were also
investigated in this study, differed with respect to pigment type, non-combustible mass fraction, and particle size.
The Hartman vertical-tube (1.2 l) apparatus (MIKE-3) was used for the ignitability (MIE) testing of dust clouds.
The important results were found as follows: (1) the ignitability of epoxy powder related to the thermal
decomposition and surface conditions was higher than that of other powders used in this study, (2) the particle
size of coating powders is more important than other factors, such as the pigment type and a non- combustible
mass fraction, with regard to their ignitability, (3) some of the sample was so sensitive that even a spark withvery low energy, such as 2 mJ, could ignite them. The values of the discharge spark energy of ignition testing set
by the BSI standards and the FM regulations related to safety of the electrostatic powder coating system are highenough to result in the ignition of some of the coating powders. Therefore, it is imperative that more appropriate
discharge spark energy values in testing be defined for safety assessment in electrostatic powder coating
systems.
Keywords; electrostatic discharge, minimum ignition energy, coating polymer powders* JJAP (Japanese Journal of Applied Physics, Vol. 44, No. 19, 2005, pp.L.599-602, 2005)で一部誌上発表** 物理工学安全研究グループ Physical Engineering Safety Research Group*** ソウル産業大学校 安全工学科Seoul National University of Technology, Department of Safety
Engineering−1−

産業安全研究所研究報告

1. INTRODUCTION
The Electrostatic Powder Coating (EPC) system
was first introduced to the industrial process in theearly 1960s. Thereafter, since the EPC system hasoffered significant environmental benefits compared
to its conventional solvent-based counterparts, it has
been widely adopted for many industrial processes.
Particularly, many countries have taken a hard line in
the regulation of Volatile Organic Compound (VOC)
emissions, which has encouraged the shift to powder
coatings or water based coatings in the industrial
finishing field.
However, accidents involving fires and/or
explosions with EPC systems using a corona
discharge of high voltage still occur even though theignition hazards arising from an electric spark are
reduced in comparisons with those at liquid paintplants. From the viewpoint of safety engineering
related to this EPC system, the Minimum Ignition
Energy (MIE) of coating powders is a very important
aspect of technical safety indices.
Various reports have been published on theignitability of dust clouds over a long period of time
1,2)
. However, the MIEs of coating powders due to a
spark have not been sufficiently investigated.
Therefore, in the present study, experiments wereconducted dealing with MIEs of several kinds ofcoating powders, which differed with respect topolymer type, pigment type, non-combustible massfraction, and particle size. In addition, a detailed
discussion is presented, which deals with the reasonsthat the ignitability of epoxy coating powder is higher
than that of other powders used in this study. A
primary objective of this paper is to publish
information regarding the ignitability of coating
polymer powders for the benefit of individuals
working to prevent and mitigate dust fires and related
explosions.

2. BRIEF SURVEY OF STANDARDSRELATED TO SAFETY OF EPCSYSTEMS AND MIE OF DUST CLOUDS
There are a large variety of standards from country
to country governing the safety of EPC systems andtesting methods associated with the MIE of powder.

NIIS-RR-2005（2006）

The standards, regulations, and research have been
screened and are presented in this Section.
Initially, safety regulations and standards related tospray finishing operations were prescribed by thefollowing: (1) the National Fire Protection
Association (NFPA) Nos. 33 and 68, (2) Factory
Mutual (FM) Class Nos. 7261 and 7264, (3)Occupational Safety and Health Standards (OSHA)
CFR 1910.107, (4) American National Standards
Institute (ANSI) Z9.3, (5) British Standards
Institution (BSI), EN 50050-2001, (6) DMT 03
ATEX E092, (7) National Swedish Board ofOccupational Safety and Health, Safety Regulations
No. 12:2, (8) Hauptverband der gewerblichen
berufsgenossenschaften in Germany, ZH 1/444, (9)Korea Occupational Safety and Health Agency(KOSHA) Code E-3-2002, (10) Occupational Healthand Safety Regulation 2001, New South Wales inAustralia, and (11) Uniform Fire Code (UFC), Part V,
Article 45. In general, these standards deal with safety
from a variety of viewpoints. However, several
aspects of these standards have not been evaluatedquantitatively and systematically.
The second part is devoted to the standards relating
to the testing methods for assessing the MIE of dust
clouds as follows: (1) the International
Electrotechnical Commission (IEC, 1994), 1241-2-3,(2) the National Fire Protection Association (NFPA,
1993), 77, (3) the Draft European Standard (DES,1998), (4) CENTC 305/WG1/SG:1.2, (5) the British
Standards Institution (BSI, 1991), BS5958, (6) the
American Society for Testing and Materials (ASTM,
2001), 2019-99, and (7) The Research Institute ofIndustrial Safety, Japan, RIIS-TR-87-1. Thesestandards emphasize the use of the MIE of dust
clouds as very important safety indices in practice andprovide basic information about dust clouds ignition
processes.
In the third part, important researches related toprevention and mitigation of dust explosions are
specifically addressed as follows: (1) Eckhoff (1994,
1996 and 2000) reviewed a large number of studies
on dust explosions. His papers reviewed in detail
three aspects of dust explosions: basic research,
applied research and prevention and mitigation ofdust explosions in industry. Furthermore, his work
covered the testing of the ignitability and explosibility−2−静電粉体塗装用塗料の着火性に関する研究（その1）*of dusts 3-5). Gibson (1993) presented a valuablesummary of methods for preventing the ignition of
powders and dusts in drying operations 6). Electrostatic
hazards in connection with industrial use of big
flexible bags were discussed by Rogers (1994) and
Dahn et al. (1994) 7,8). Pratt (1994) presented threecase histories in which an electrostatic spark
discharge was generated during pneumatic transportof powders 9). Matsuda (1995) gave an account of
ignition hazards of combustible dusts and theirprevention 10). Siwek et al. (1995) summarized theignition behavior of dust clouds. Test apparatus andprocedures were introduced, including the significant
parameters on the safety indices 11). Jaeger et al. (1998,
2001) described the determination, prevention andmitigation of potential hazards due to the handling of
powders during transportation, charging, discharging
and storage. They also outlined a "risk analysis" toensure and maintain safety in the chemical industry
12,13). Kodama (1999) gave an overview for MIE andthe incendiary potential of gases and powders
exposed to ESD 14) . These kinds of researchemphasized the use of the minimum ignition energy(MIE) of dust clouds as very important safety indexes
in practice and also provided basic information about
dust cloud ignition processes.

3. EXPERIMENTAL
3.1 Apparatus and method

The Hartman vertical-tube (1.2 l) apparatus (MIKE3), which is illustrated in Fig. 1, is one of the piecesof equipment used for the general ignitability (MIE)
testing of dust clouds mentioned in Section 2 above
15). The control range of the dust concentrations of the
MIE apparatus used in this study is from 0.125 kg/m3
to 3 kg/m3. For MIE measurements, a single electric
spark from a capacitance is carried via a transformer
to pass between the two electrodes with a diameter of
2 mm. The energy of the spark is varied by changing
the values of the capacitor and the applied voltage inthe discharge circuit. The dust is dispersed by meansof air stored in the reservoir (0.7 bar), which issuddenly released into the chamber by means of asolenoid valve. Observations are made until thelowest possible energy at which the flame propagates
through dust clouds is found under a constantcondition of testing (inductance, 1mH; ignition delaytime, 120 ms; and electrode spacing, 6mm). This test
conditions were described in the international
standard of IEC 15). In the case of the capacitiveinductive (LC) sparking circuit used in this study,
since virtually all the stored energy on the capacitor
appears in the spark, the net energy (in J) was givenby 1/2 CV2, where C is the capacitance in the circuit(in F) and V is the potential at the moment ofdischarge (in V).
In this experiment, "ignition" was acknowledged tohave taken place when the dust cloud caught fire byelectric sparking within 10 successive attempts. All
the test conditions were 26 ± 2 ℃ and 35 ± 5 %RH.
3.2 Coating polymer powder

In general, a coating powder consists of a polymer,
such as, polyester, epoxy, nylon, or acrylic, and asmall amount of pigment. Thus, the coating powders
used in the experiment were polyester, epoxy, epoxypolyester co-polymer, nylon, and polyacrylonitrile.
Five kinds of polyester powders, which were also
used in this study, differed with respect to pigment
type, non-combustible mass fraction, and particle
size. Before being subjected to the testing, the coating
powders were dried in a desiccator at 25 ℃ for 24hours.
Fig.1 Schematic drawing of the MIE measurement
apparatus (MIKE-3) for coating powder.

−3−

産業安全研究所研究報告

4. RESULTS AND DISCUSSION
Figure 2 shows the ignition energy as a function of
the dust concentration of coating powders. Each data
point in the figure indicates the lowest energy at
which ignition occurs within 10 successive attempts
to ignite the dust/air mixture.
μ
μ
μ
μ
μ

Fig.2 Ignition energy as a function of the dust
concentration in coating powders (*:epoxypolyester co-polymer).
Table 1 Median particle size, D 50 and MIEamong the five kinds of coating powder
used in this study.

The mean particle size distribution, D 50 measured
by the laser diffraction method (LDA Win 1.21, Wettype) and the MIE values estimated by the use of theprobability of ignition are listed in Table 1. It should
be noted that the MIE is usually quoted as a range;
the lower value represents the highest energy at which

NIIS-RR-2005（2006）

no ignition is found in at least 10 experiments. The
higher value, on the other hand, is the lowest energy
at which the dust air mixture is just ignited: noignition (W1) < MIE <ignition (W2) 11). The MIE value
can be estimated by the use of the probability ofignition as follows16): MIE = 10^(log W2-I [W2] ・log
W2 - log W1/(NI+I) [W2] +1)), where, I is number oftests with ignition at the energy W2, (NI+I) is total
number of tests at the energy W2.
The first noteworthy point in the experimental
results in Fig. 2 and Table 1 is that coating powder
was so sensitive that even a spark with energy as low
as several tens of mJ could ignite them. Anotherinteresting point is that, nevertheless, D 50 of theepoxy powder, 93 μm, was almost three times aslarge as those of the other powders. Its MIE, 24 mJ,
was similar to those of the other powders. This value
was much smaller than expected. It is well known thatepoxy powder has high ignitability. This is evident
from previous papers by Eckhoff et al. and Pidoll etal. 1,2) . It remains to be demonstrated why theignitability of epoxy powder is higher than that ofother powders. This is discussed in detail later. First,
as to the ignition phenomena of dust clouds, theincreased temperature of particles is initiated by spark
discharge, and, subsequently, the combustion reactionof particles starts and forms a kernel for flame
propagation, leading to ignition. From this point ofview, the thermal decomposition of coating powder
has an effect on the ignitability of dust clouds.
Thermal analysis of the coating powders underatmospheric pressure was conducted with adifferential scanning calorimeter (DSC 2920). The
heat-increasing rate and the mass of the coating
powder in the test were 10 ℃/min and 1.5 mg,
respectively. A sample pan (aluminum, open type)and a GPIB interface for communication with thecontroller were used for this testing.
The heat flow associated with transition for threekinds of coating powders as a function of thetemperature is shown in Fig. 3. As indicated in the
figure, it is clear that epoxy coating powder has alower exothermic onset temperature, Ta [℃], and ahigher heat of the decomposition, Q [J/g], than theother powders. The value, 8362 J/g of Q, for epoxy
coating powder was almost twice as large as the 4099
J/g of polyester powder. These characteristics result−4−

静電粉体塗装用塗料の着火性に関する研究（その1）*in the acceleration of the combustion reaction of
epoxy coating powder particles. In addition, in case of
the epoxy-polyester co-polymer, the values of Ta andQ lie between the values of epoxy and polyester, as
expected. Strictly speaking, the MIE for epoxypolyester co-polymer is lower than that of polyester inalmost the same size as that given in Table 1. This
experimental result is also related to the values of Ta
and Q.

Fig. 4 Surface conditions of the polyester andepoxy coating powders observed with
SEM.

Fig. 3 Heat flow associated with the transition forcoating powder as a function oftemperature.

Second, the surface conditions and the shape of thecoating powder particles could be responsible for its
ignitability. Coating powders (polyester and epoxy)observed with scanning electron microscopy (SEM)
are shown in Fig. 4. The epoxy-polyester co-polymer
had almost the same surface condition as polyester
powder; thus, the SEM image was omitted in Fig. 4.
As shown in Fig. 4 (a), the polyester particles were
almost spherical. However, epoxy particles (see Fig.
4 (b)) were irregular in shape. It was also found thatthe surface of epoxy coating powder had innumerable
variety in the size of holes from several μm toseveral ten μm, which does not occur in the surfaceof other powders.

Both of these characteristics, irregular shape andsurfaces with holes, suggest that the combustion rateincreases because the total contact surface area
between the particle and air has increased, resulting ina very small amount of energy required for theignition of epoxy powder.
There are two possible explanations with regard to
the holes. First, with the rapid progress of powder
technologies in recent years, these holes may improve
the performance of the charge of coating powder andthe high-quality decorative finish. Secondly, the holes
may occur unexpectedly due to problems during the
manufacture of the coating powder. Accordingly, theMIE for the same types of coating powder may differ
according to the technology used by the
manufacturer.
The ignition energy with dust concentrations ofpolyester coating powder (specimen taken from the E
company) in various colors is shown in Fig. 5. Table
2 gives the fundamental characteristics, such as
pigment type, particle size, and non-combustible massfraction of coating powder used in Fig. 5, including
MIE values.
Black polyester had the smallest MIE value, 2.4 mJ,
the smallest one being 7.5 μm. Other coating
powders, except for the black one, gave
approximately the same MIE values in a similar
particle size irrespective of the pigment type as well−5−

産業安全研究所研究報告

as a non-combustible mass fraction. This result
suggests that the particle size of coating powders ismore important than other factors with regard to theirignitability. However, it was also found that theminimum ignition dust concentrations (MIC)
decreased as the content of non-combustible material
decreased. This result agreed with the data obtainedby Eckhoff 1). A more detailed discussion will bepresented concerning several tests in another report.
Concerning the relationship between the particle
size of the coating powders and their ignitability, we
assumed that the epoxy coating powder mentioned inFig. 2 included some particles that were similar insize to those in the polyester coating powder shown in
Fig. 5.
The dependence of the ignition behavior (MIE) on
D 50 can, thus, be described by the following
equation: MIE2=MIE1×(M2/M1)2.5 ---(1), where Index
1: measured, Index 2: estimated, MIE1: 24 mJ, M1: 93
μm (D 50), and M2: 7.5μm (D 50). As estimated,
the MIE2 of epoxy coating powder with 7.5 μm was
far lower than 1 mJ.
The previous data obtained by Pidoll et al. indicatethat the MIE for epoxy coating powder with a mean
particle size of 30 μm was 1.7 mJ 2). Besides, the MIE
for epoxy powder (D50; 58 μm, color; green) takenfrom another company (F) was 7 mJ depending
strongly on the particles size.
These values generally agreed with the data
calculated in Equation (1) above as 1.4 mJ and 8 mJ,
respectively. Moreover, Eckhoff showed that the true
MIE for sensitive dusts, such as epoxy coating
powder, was considerably lower, by at least one orderof magnitude, to below 3 mJ 1). In this sense, it could
be argued that extreme low energy below 1 mJ could
ignite fine epoxy coating powder with several μm.

NIIS-RR-2005（2006）

Fig. 5 Ignition energy with polyester dust
concentrations in various colors.
Table 2 Fundamental characteristics of polyester
powders with various colors.

*:Non-combustible mass fraction. **: value estimated by
the use of the probability of ignition within 10 successive
attempts.

A fatal accident with an EPC system using epoxy
powder paint occurred in Japan 17). For the safety ofspray-finishing operations and in accordance with
BSI (BS EN) standards, a test gas mixture with adischarge spark energy of 2 mJ should be used for
ignition tests on electrostatic spray apparatuses for
flammable coating powders with an electrical highvoltage generator 18). In FM regulations, the value ofthe energy of the discharge spark mentioned above isdefined as 5 mJ 19). The values of the energy of thedischarge spark with both standards and regulations
were higher than the MIE values of some coating
powders. Therefore, it is important that the most−6−

静電粉体塗装用塗料の着火性に関する研究（その1）*

appropriate value of a discharge spark be determined
for assessing the safety of an EPC system.

5. CONCLUSIONSTo improve the understanding of the ignitability ofcoating powders due to a spark, such as anelectrostatic spark, their MIEs were investigatedexperimentally. The results are summarized asfollows:(1) The ignitability of epoxy powder related to thethermal decomposition and surface conditions
was higher than that of other powders used in thisstudy.
(2) The particle size of coating powders is more
important than other factors, such as the pigment
type and a non-combustible mass fraction, withregard to their ignitability.
(3) As powder technologies have been rapidly
progressing, a variety of new fine powders has
been produced and used in various industrial
processes. Therefore, it is imperative that themost appropriate discharge spark energy value intesting for the safety assessment in an EPC
system be renewed as soon as possible.

ACKNOWLEDGEMENTS
We thank Mr. Shin Dae-Sung of Department of
Safety Engineering, Seoul National University ofTechnology, Korea, for his assistance in theexperiments. We gratefully acknowledge helpful
discussions with Dr. T. Ando, Dr. M. Yashima, andDr. M. Kumasaki of Chemical Safety Research Groupin NIIS, on several points in this paper. We also wish
to thank the coating powder companies for providing
powder specimen to us. They want to remain
anonymous.

REFERENCE(1) Rolf K. Eckhoff, Geir H. Pedersen, Tommy A
rvidsson: J. of Hazardous Materials, 19, p.1(1988)(2) U. Von Pidoll, H.Kramer: J. of Electrostatics, 30,
p. 103 (1993)(3) R. K. Eckhoff: Proc. 6th Int.Coll. Dust Explosions,
Shenyang, P.R.China, 29 Aug.-2 Sept., eds Deng

Xufan and Wolanski, P., pp.5-34 (1994)(4) R. K. Eckhoff: J. Loss prev. Process Ind., Vol.9,
No.1, pp.3-20 (1996)(5) R. K. Eckhoff: 3th Int. Sym. on Hazards,
Prevention, and Mitigation of Industrial
Explosions Proc., pp.7-20 (2000).
(6) N. Gibson: Paper given at the European Summer
School on; Dust Explosion Hazards; Their
Assessment and control, Cambridge, UK,
organized by IBC technical Services Ltd. In
association with BMHB and IELG (1993).
(7) R. L. Rogers: Preprints for seminar on Explosion
Safety and Related Risk Control, Gent. Belgium,organized by Technological Inst.〓KVIV, in
cooperation with EuropEx (Kontich, Belgium),pp.169-179 (1994).
(8) C. J. Dahn, B.N. Reyes and A. Kashani: Proc. 28.
AIChE Ann. Loss Prev. Sym., Session No. 12 on
Electrostatic Hazards, Atlanta, USA, April 17-21.
American Institution of Chemical Engineers, 345
E. 47. Street, New York (1994).
(9) T. H. Pratt: Proc. 28. AIChE Ann. Loss Prev.
Sym., Session No. 12 on Electrostatic Hazards,
Atlanta, USA, April 17-21. American Institutionof Chemical Engineers, 345 E. 47. Street, NewYork (1994).
(10) T. Matsuda: J. Insti. Electrostatics Japan,
Vol.19, No.1, pp.35-40 (1995)(11) R. Siwek: Process safety progress, Vol.14, No.2,
pp.107-118 (1995)(12) N. Jaeger: Process safety progress, Vol.17,
No.1, pp.74-81 (1998)(13) N. Jaeger: J. of Loss prev. Process Ind., Vol.14,
No.1, pp.139-151 (2001)(14) T. Kodama: J. Insti. Electrostatics Japan,
Vol.23, No.5, pp.227-233 (1999) (In Japanese)(15) IEC, International standard (1241-2-3) (1994)(16) A. D. Kuhner: Instruction Manual of MIKE3(1999)(17) Countermeasure for Accident due toElectrostatic Discharge (In Japanese), Japan
Paint Manufacturers Association (JPMA) p.87
(2002)(18) British Standard Institution (BSI), BS EN 50050(2001)(19) Factory Mutual (FM) Class No. 7260 (1996)−7−（平成17年10月23日受理）

Research Reports of the National Instituteof Industrial Safety, NIIS-RR-2005 (2006)

抄録

静電粉体塗装用塗料の着火性に関する研究（その1）崔 光石，山隈瑞樹，鄭 載喜
近年、静電粉体塗装は一般の吹付塗装に比べ，生産効率が高く，環境にやさしいという大きなメリッ
トから普及率が極めて高い。しかし，静電粉体塗装
は高電圧印加により塗料を帯電させ，接地した被塗物に向かって移動させる工程であり，放電による粉塵爆発・火災の発生が危惧されていることから，粉体塗料の最小着火エネルギー(MIE)を測定した。測定には，国内外で標準的に用いられているIEC規格に準拠した吹上げ方式着火試験装置（ハルトマン式、
MIKE-3）を使用し，着火試験用粉体塗料としては，ポリマーを主成分とする粉体塗料（ポリエステル，
エポキシ，ポリエステル/エポキシ, アクリル, ナイロン）及び5種類(色別)のポリエステル粉体塗料の計10種類を用いた。その結果，粉体塗料は数mJの小さい
放電エネルキーでも着火する危険性が明かとなった。特に，粉体塗料の粒径を考慮すると、エポキシ
粉体塗料の方が他の粉体塗料に比べて，着火しやす
いという結果が得られた。また，粉体塗料に含まれている顔料などの成分によっては，MIEはほとんど変化しないことが明らかになった。
（図5，表2，参考文献19）

噴霧・噴出帯電の静電気危険性評価法の検討
大澤 敦
配管やノズルなどから液体が噴出すると液体および液滴に静電気が帯電して着火源となることがある
ので，各種工程の現場において簡便に噴霧・噴出帯
電の静電気危険性を評価する方法を構築することは工程の安全化と静電気による着火の防止対策の指針を与え，安全工学の立場からも重要である。本研究では噴霧・噴出の静電気危険性を評価するための測定技術とこの測定データを元に評価する手法を構築することを目標としている。測定技術として接地円
筒ケージと電界計による空間電荷密度の測定，吸引ファラデーチューブによる空間電荷密度の測定またはフローティングプローブによる電位測定の３つの方法を検討した。また，評価手法としては，各測定データを元にポアソンの方程式とE = −∇V を用いて噴霧空間の電界強度分布を求め，静電気放電の可能性を評価する方法を検討した。圧搾空気ドライフ
ォグ２流体ノズルを用いた噴霧帯電のモデル実験により，これらの３つの測定方法を検討して，接地円
筒ケージ・電界計と吸引ファラデーチューブの測定が妥当な結果を導き，現場で簡便に測定できることを考慮すると接地円筒ケージ・電界計による方法が適していることを示した。また，静電気放電の可能性を評価するための簡便なモデルも提案している。
（図7，参考文献4）

破砕を伴う落石現象の物理モデル化に関する研究
伊藤和也，豊澤康男，日下部治
落石は道路，鉄道，住宅等へ影響を及ぼす斜面災
害の中でも発生頻度が比較的高い災害現象の一つである。また，落石に起因する労働災害について調査したところ，過去10年間で40件程度報告されていた。
中には落石が突破・跳躍して落石対策工により保護されている箇所にいた労働者が被災する事例も報告されており，落石の運動形態や衝突現象など，落石対策の計画・設計に必要な事柄についても未だ十分には解明されていないのが現状である。そこで本研
究は，多くのパラメーターが自明となり，応力条件を等価にすることが出来る遠心模型実験手法を用い，破砕を伴いながら衝突と跳躍を繰り返すような落石現象について物理モデル化を行い，その運動形
態・衝突現象の解明を試みた。本報では，新たに開発した遠心場落石発生装置の概要と，それを用いて行った落石実験の落石軌跡および落石の破砕状況を確認した。その結果，球形タイプの落石形態は回転運動が主であるが，破砕を伴うと回転運動から跳躍運動に変化し，破片は大きな跳躍をすることが確認された。
（図4，表5，写真12，参考文献28）− 75 −