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ROSEリポジトリいばらき (茨城大学学術情報リポジトリ)
Title Morphology of Polymeric Materials Welding and their Butt-fusion Welded Joints
Author(s) Bezruk. Leonid. I. / Ustinova. Yu.L.
Citation 茨城大学教養部紀要(25): 195-204
Issue Date 1993
URL http://hdl.handle.net/10109/9692
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Morphology of Polymeric Materials Welding
and Their Butt-fusion Welded Joints.
L.1.Bezruk, Yu. L. Ustinova
ABSTRACT
Morphological data of Polypropylene(PP)and Polybutene-1(PB)welded
junctions was obtained by means of lnterference-polarizing Light Microscopy(IPLM),
Selected Area Electron Diffraction(SAED), Transmitting Electron Microscopy(TEM),
Wide Angle X-Rays Diffraction(WAXD>techniques. Several modes of plasma
treatment onto polymeric samples or specimens surface were used by adaptation a
plasma of electrodless radio-frequency discharge.
The alteration of every morphological category for a relatively wide range of
welding conditions was examined. It was shown that the most crucial zones to
mechanical tests are the oriented areas of buttwelds and the thermally affected zone
adjacent to junction.Videorecording of the thin(approx.100 micrometers in thickness)
films welding under polarized light microscope observing permitted to clarify welding
technology as a process of adhesion as well as to see morphological changes during a
course first-order phase transitions(i.e.melting, crystallization).
1.INTRODUCTION.
Welding of polymeric materlals can be considered as a technology process of the
non-detachable joint creation by means of short-time transfer of weldable surface to
the state which guarantees adhesion i.e. molecular contact of the countersubstances
and formation of a mixed zone of weldable materials having the same or different
chemical nature. Welding itself ls a relatively new field of polymeric technology. The
majority of published papers are devoted to welding equipment and technological
process description. One can find few papers only in which authors give relations
between junctions morphology and welding conditions.
It is well known that physical properties of polymeric materials are determined by
their structural parameters. Therefore an investigation of morphological changes after
welding is important for resolving the fundamental questions on the welded joints
nature. It is also of great importance for working out a scientific approach to the
196 LI. Bezruk, Yu. L. Ustinova
problems of obtaining junctions with giveh morphology as well as to make optimum
welding Parameters.
The present paper aim is to investlgate morphological changes in PP and PB
butt-welded joints which were obtained,under a wide range of welding conditions.
ll.METHODS and MATERIALS.
Original materials:(a)PP sheets,7mm in thick, melt flow index O,8;(b)PB
sheets,7mm in thick, melt flow index 3.02.
PP and PB samples were welded by hot plate cowered with teflon fabric to
make it antiadhesive. Welds were made at a number of different temperatures. These
ranged from 180℃to 340℃for PP and from 150℃to 340℃for PB. The welding
pressure(Pw)was altered from O.05 to l MPa. For each series of samples one value
was varied only.
Welded junction morphological changes were analyzed with an aid of IPLM,
SAED, WAXD and TEM techniques.
Polymeric materials are usually exposed to cutting,polishing or cleaving in a
course of sample preparation for light or electron microscopy investigation. And
surface layers of polymeric specimens always get rather strong morphological
disturbances over such mechanical treatment. Such disturbed layers should be
removed from the specimen surface in thls or that way. There exist several methods
of disturbed layer removal. We suggested to use radio-frequency plasma technique.
Here the specimen surface i『acted upon by a range of plasma particles. Electrons are
the particles with the greatest energy in a case of electrodless radio-frequency
discharge. Their mean energy does not exceed 10ev and concentration we can easily
change from lO6 to lO9㎝一3. At electron concentration higher than 107㎝一3 and under
dynamic conditions of plasma particles byproducts 鵠・・。5肇
・5晒認 generator output power. In such a case under plasma
ぎ}9晶8?P謡e昔睾?2?f。呈bき韮譜。謁:ss treatment we observe no polymer weight losses but
pユasコLa RF discharge on electron ・。n・ent・ati。n only surface optical contrast improvement, i.e.fine
morphological entities radius of curvation reduction. From the morphological point of
view this process can be called as etching(Fig.1). We believe that disturbed layer
Morphology of Polymeric Materials Welding and Their Butt-fusion Welded Joints.
197
thickness one can get after the plasma treatment is comparable with charged
particles track values. It is known that if the mean energy of electrons is 100ev, than
their tracks into polymeric substances have average values in 2,1nm. Respectively if
the electrons energy is about 10ev, the disturbed layer thickness will be comparable
with an atomic size.
Etched samples are available to light optical and different electron microscopic
modes investigations. For TEM investigations we have prepared replicas by vacuum
evaporation and depositlon onto polymeric samples surface exclusively of carbon
ultrathin films. We have chosen carbon because it is amorphous according to SAED
data and have its grain size less than lnm. Deposited carbon films than were peeled
off from Polymeric surfaces. While peeling carbon films, replica gets no plastic
deformation. An adhesion of vacuum evaporated carbon film to etched polymeric
surface is extremely high. This fact allows to make fixed the morphology of
investigated samples surface which are separated as ultrathin (approx.50nm in
thlckness)solid Iayer from an original sample as non-detachable part of peeled
replica. Hereinafter we used this polymeric layers for SAED search. We need SAED
for preparative technique correctness control. In a case of oriented polymers such
separated from original polymeric surface layers are rather suitable to answer the
main questions about chemical nature and physica1(i.e. orientation degree,
crystallinity, unit cell parameters, crystallite dimensions ets.)state of investigated
materials. For example the data on helix conformation of macromolecules is able to
calculate in accordance with the real meridional reflex positlon on the SAED pattern
[2].It takes to clean replica after SAED investigation to obtain』 =@maximum of
morphological resolution. Slnce carbon is thermostable material up to approx.1700℃
one can easily clean replica from the above mentioned polymeric layer by heating in
vacuum to the temperature about 400℃.
For IPLM investigations lO-30 μm(prepared by using glass knife)and
400-100μm(steel knife)thick cross sections were obtained by means of
“Multirange”microtome(LKB, Sweeden).
lll.MORPHOLOGY of the POLYPROPYLENE BUTT-WELDS.
The idea[3コof polymeric materials morphology implies six categories:(1)shape,
(2)dimensions(from nanometers and more),(3)new growth or new formations,
(4)inclus三〇ns,(5)structure (i.e. relative position and/or interconnection of parts as a
whole),(6)color.Change in any of them results in properties changes of materials.
Any type of polymeric material welding includes three main stages:(1)activation
of the surfaces to be weld;(2)surfhce transition to a liquid state of aggregation with a
BULLETIN, No。25,1993, CGE Ibaraki Univ.
198 L.1.Bezruk, Yu. L. Ustinova
subsequent drawing them together to van-der-Waals distances, i.e. getting an adhesion
joint;(3)solidification.
In the case of butt-welding we usually
心 Fユg.2 Scheme of the butt-welded
joints; a-fuse, b-melting, c-fin,
d-heat affected zone3
found to be positive(△n=0.002-O.0035).
crystallites was monoclinic(Fig.3a,b). Lattice
with the common known date, a-axls
According to classification these are Type I spherulites. It
besides Type I spherulites a number of Type皿
had negative birefrigance (△n=0,006-0.008).
hexagona1. In starting material these spherulites amount was about 5%
With an increase of welding pressure
simultaneously with their deformation.
there would appear in joints areas
hexagonal).
It should be noticed that the welding pressure increases the thickness of a joint’s
seam cross section decreases. One can see(Fig.4)that the seam obtained at welding
pressure more than O.3MPa consists mainly 2 zones:(a)central part-from deformed
spherulites of Type I and 皿, and lamellar areas with monoclinic and hexagonal
crystallites unit cells;(b)symmetrical melting zones, disposed on both sides of the
region marked“a”(Fig.2),i.e. zones where polymeric material softened in a course of
heating undergoes the greatest welding deformations.
It is clearly seen even under light microscope that the seam morphology at
different crystallization conditions is not strictly symmetrical in respect to the joint
center(Fig.4). Interference coloring which one can get in the light microscope(IPLM)
while examine samples is very sensitive to the most delicate changes in chemical
nature, orientation and even some structural parameters of polymers, which can’t be
seen with the aid of black and white image。 We have not apply any artificial staining
of samples. It is evident that the coloring of the symmetrical welding zones is
different. The central fuse line of the seam we can identify in many cases through
interference coloring only. This is the boundary line between two weldable materials.
carry out stages(2)and(3)under some press-
ure. As a result melt always partly pressed out
and forms a fin. So butt-welds cross sections
have a vlew shown on Fig.2. And as a rule
they have two more or less symmetrical
boundaries with the starting materials. Investi-
gated samples of bulk PP contain spherulites
with an average size of 50μm. They were
According to our SAED data unit cell of its
parameters were in a good correlation
° was directed along the spherulite radius,
is worth mentioning that
spherulites rises in the joints. They
Unit cell of its crystallites was
the content of Type 皿 spherulites rises
If this pressure reached more than O.15MPa
with lamellar morphology (unit cell being
Morphology of Polymeric Materials Welding and Their Butt-fusion Welded Joints.
199
We also can find out orientation changes through the melting zone width. Melting
zone being the most textured area of a seam。 For the samples obtained under the low
welding pressure(up to O.15MPa)big axes of macromolecules in crystallites of the
melting zone are chiefly oriented under the angle to the welding line in a seam cross
section. However if pressure over than O.2MPa the big axes get to be nearly parallel
to the welding plane. Morphologically this zone consists of highly deformed spherulites
which one can find in a course of longitudinal(in the sheet plane)sections of welded
seams research, These spherulites are ellipsoid-like, having crystallites with c-axes
oriented mainly along ellipse big axes. Evidently deformation occurs in a course of
welding into the melting zone and have a similar mechanism as on uniaxial
deformation of spherulitic PP,
The most crucial areas of the welding joints are the fuse zone, forming at the
center of the seam and the melting zones(Fig.2). Molecular orientation in these zones
is one of the reasons of joint reduced mechanical strength. Frequently failure take
place on the heat affected zone neighboring the seam(i.e.areas in which a material
was subjected the action of temperature Iower than the melting one and get friable).
This area is now investigated insufficiently good.
lV. MORPHOLOGY of the POLYBUTENE WELDED SEAMS.
There are many similar features in the PP and PB welded seam morphology
(e.g. two borders with an initial material having lamellar morphology, grinding of
structural elements, texture advent). There are a range of morphological areas in
seams for PB as well as for PP. The most oriented are:the fuse zone and the
melting zone. Under welding pressure the seams PB material orients more strongly
than the PP one. Over O.2MPa PB seams have practically no n6n-oriented areas.
Morphology of PB melting zone differs from the morphological characteristics to PP
and other crystallized polymers of the spherulite morphology origin. Apparently the
main reason which causes such PB difference from the corresponding zones of other
spherulitic polymers are peculiarities of its crystalllzation from the melt. It is known
that PB crystallize from melt in two steps. A non-stable tetragonal modification
(Fig6,g)in several days is gradually converts to a stable trigonal forms(Fig.5)by
means internal annealing. One can notice that such conversion is easy to recognize by
IPLM and WAXD techniques.
Since PB crystallizes from lts melt rather slowly we used this material for
welding process modeling by means videorecording of morphological changes during a
course of PB samples welding(i.e.Heating-Cooling treatment).
Here we wish to present the result of the dynamic morphological changes[4]
BULLETIN, No.25,1993, CGE Ibaraki Univ.
200 L.1.Bezruk, Yu。 L. Ustinova
during a course of modeling welding directly under Polarized Light Microscope
(Fig.6). Mettler FP82 Microscopic Heating Table(MHT)with FP80 Central processor
of Mettler FP800 Thermal Analysis System was used to make computer control of
welding,50℃ start temperature with 5.0℃/min heating rate provided in this
modeling. PB thin(230 μm)film was cut to pieces and put against each other
between 2 microscopic support glasses. The distance between selvages to be weld
was about lmm. Such“sandwich”was placed into MHT and we carried out welding
procedure by heating our sample up to l70℃by means the start temperature and
heating rate mentioned above.
It is possible to notice several stages of welding process. At first we saw a
thermal expansion of PB films by means of spherulites dimensions increase and
correspondingly decrease of the distance between selvages to be weld. Close to 88℃
this distance became twice as less(Fig.6a>. The original color of spherulites slightly
changed by annealing. It means unit cells parameters changes of polymeric crystals
have already started. In 23 minutes we has reached l20℃temperature(Fig.6b). At
this temperature conditions PB color change becomes highly noticeable. It means we
are about the lst order phase transition temperature of PB(i.emelting temperature).
However completely state of melt began at l30℃(Fig.6c).
Since we did not apply a welding pressure to joint together the counterparts of
PB it took further heating till l70℃. About 165℃we have seen an adhesion contact
appearance by means of dividing line onto Fig.6c disappearance. So butt-weld of PB
has occurred.
After that cooling process was carried out to see morphological changes in a
course of crystallization.
Among a number of crystallizable polymeric materials perhaps PB only is the
best to enjoy us by its slow morphological changes during a course of crystallization
from melt(Fig.6d-h). Since monochromatic view of PB melt is changing to
polychromatic one we say again about lst order phase transition temperature,
i.e.crystallization. Of course for this way of phase transition determination a melting
temperature is equal to crystallization one. At first several primary spherulites appear,
than th6ir dimensions increase little by llttle(Fig.6d,e). Approximately about 105
℃(Fig.6f)we have the best crystallization temperature conditions, but for so called
tetragonal modification of PB crystallites(Fig.6f,g). As it was mentioned above such
unstable form of PB crystallites gradually(approx.7 days at room temperature)
transforms into stable trigonal crystals modification. Spherulites with such forms of
PB crystallites are shown at Fig.5. Fig.6h demonstrates such gradualness:
morphologically it seems almost like as Fig。6g, but has differences in color only. The
picture of Fig.6h has taken in 5 days after welding.
Morphology of Polymeric Materlals Welding and Their Butt-fusion Welded Joints.
201
So as a result of butt-fusion welding morphological changes occur in the welded
seams.1t terms in appearance of the range of areas which differs on shape,
dimensions, structure and color of morphological units. The most crucial areas of
welded鼻eams are oriented zones. It takes to disturb orientation in these zones by
some other technological processes, e.g。 by annealing. Another way is to make
orientation much stronger in some fixed directions. One can get it with reinforceme-
nt.
REFERENCES
1.Hudis,M, Plasma Treatment of Solid Materials./in Techniques and Application of
Plasma Chemistry, N.-Y., A Wiley-lnterscience Publication,1974, P.138.
2.Bezruk L., Graboshnlkova V., Grinyuk V. Morphology of Polymeric Materials as
Revealed by Methods of Diffraction and Transmission Electron Microscopy//
Proceed. of the XVth Czech. Conf. on Electron Microscopy with International
Participation, VoLB, Prague, Czech.Acad. of Sci.,1977, P.442.
3.Bezruk L., Yesaulenko G, Uskov I. Morphology of Polybutene Welded Seams//
Proceed. XVIIth Intern. Conf.“Morphology of Polymers”, Vol.9E, Prague,
Czech.Acad, of Sci.,1985, p.77.
4.Esaulenko G., Bezruk L, Kondratenko V. Study of strength characteristics of
welded butt-joints in polyethylene and development of testing methods. VoL29, N
9/10,London, Welding in the World,1991, PP.301-306.
BULLETIN, No,25,1993, CGE Ibaraki Univ,
202 L.Is Bezruk, Yu. L. Ustinova
a
b
Fig.3 TEM patterns of the PP Type I spherulites:a-diffraction patterns from its center, b-pe-
riphery;c-morphology.
Fig.40ptical micrograph of the PP welded seam with P=0.3MPa, hot plate temperature 300℃.
Seam borders with the initial material are oriented. White color spherulites belong to the Type
III one,S.
Morphology of Polymeric Materials Welding and Their Butt-fusion Welded Joints,
203
a
b
Fig.5 Two variants of the PB spherulites with the trigonal modification of its crystals observ-
ing:a-in IPLM mode;b-in PLM(i.e. without interference plate)mode.
BULLETIN, No.25,1993, CGE Ibaraki Univ.
204L.1.Bezruk, Yu. L. Ustinova
a
b
Fig.6 Fragments of PB welding by means dynamic morphological changes.