Polymer Wear and Its Control - ACS Publications - American Chemical

24

Effect

of

the

Degree

of

Crystallinity

on

Friction

and Wear of Poly(ethylene terephthalate)

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 4, 2016 | http://pubs.acs.org Publication Date: September 12, 1985 | doi: 10.1021/bk-1985-0287.ch024

Yoshinori Yamada and Kyuichiro Tanaka Department of Precision Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan

Measurements of f r i c t i o n and wear were made on poly (ethylene-terephthalate) (PET) s l i d i n g against a smooth s t e e l disk. The f r i c t i o n was little dependent upon c r y s t a l l i n i t y , while the wear rate increased with increase i n c r y s t a l l i n i t y , e s p e c i a l l y remarkable i n the range above about 40%. I t was found that the r e c i p r o c a l of wear rate was closely related to the Vickers hardness i n PET of d i f f e r e n t c r y s t a l l i n i t y . The wear of PET with a low c r y s t a l l i n i t y as small as 8% was mainly due to transfer. On the other hand, wear of high c r y s t a l l i n i t y PET is d i f f e r e n t from that of the low c r y s t a l l i n i t y PET and seemed to be mainly due to a surface fatigue during s l i d i n g . The differnce between the mechanisms i n PET of low and high c r y s t a l l i n i t y was e s s e n t i a l l y due to the differnce i n morphological structures. I t i s w e l l known, t h a t t h e m o r p h o l o g i c a l and m o l e c u l a r s t r u c t u r e s o f polymers p l a y an i m p o r t a n t r o l e i n t h e i r wear b e h a v i o r . I t seems t h a t t h e degree o f c r y s t a l l i n i t y i s a l s o a s t r u c t u r a l f a c t o r o f s e m i c r y s t a l l i n e polymers i m p o r t a n t t o t h e i r wear. L o n t z e t a l . Q.) r e p o r t e d t h a t t h e wear o f p o l y ( t e t r a f l u o r o e t h y l e n e ) , ( P T F E ) d e c r e a s e d w i t h t h e i n c r e a s e i n c r y s t a l l i n i t y . Tanaka e t a l . (2) s t u d i e d t h e wear o f h e a t - t r e a t e d PTFE specimens and c o n c l u d e d t h a t the wear r a t e was a f f e c t e d by t h e w i d t h o f t h e band i n t h e f i n e s t r u c t u r e r a t h e r than c r y s t a l l i n i t y . R e c e n t l y , Hu e t a l . ( 3 ) have s t u d i e d t h e e f f e c t o f c r y s t a l l i n i t y on wear o f PTFE u s i n g v a r i o u s h e a t - t r e a t e d specimens. They have shown t h a t t h e wear d e c r e a s e s w i t h t h e i n c r e a s e i n c r y s t a l l i n i t y , when m o l e c u l a r w e i g h t i s c o n s t a n t . E i s s e t a l . (4) reported t h a t p o l y ( c h l o r o t r i f l u o r o e t h y l e n e ) o f a c r y s t a l l i n i t y o f 65% e x h i b t e d h i g h e r wear than t h a t o f 4 5 % . The r e s u l t s o b t a i n e d by t h e a u t h o r s mentioned above i n d i c a t e t h a t t h e e f f e c t o f c r y s t a l l i n i t y on the wear o f polymers i s somewhat c o m p l i c a t e d and f u r t h e r i n v e s t i g a t i o n i s needed t o c l a r i f y t h e e f f e c t o f c r y s t a l l i n i t y on polymer wear. I t i s known t h a t p o l y ( e t h y l e n e - t e r e p h t h a l a t e ) , (PET) has a p e c u l i a r p r o p e r t y t h a t i t s c r y s t a l l i n i t y v a r i e s i n a v e r y wide range

0097-6156/85/0287-0363$06.00/0 © 1985 A m e r i c a n C h e m i c a l Society

Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER WEAR AND ITS CONTROL

364

after heat treatment. In the present work, therefore, the effect of c r y s t a l l i n i t y on f r i c t i o n and wear of PET s l i d i n g against a smooth s t e e l surface at a r e l a t i v e l y low speed was studied within a cryst a l l i n i t y range of 8% to 75% . In addition, the f r i c t i o n experiment using a s t e e l sphere s l i d i n g on a f l a t PET surface was also carried out at a low speed under various load i n order to study the fundament a l f r i c t i o n a l properties of the PET specimens with various c r y s t a l linity.


Experimental Procedures for Studying the Fundamental F r i c t i o n a l Properties of PET To study the fundamental f r i c t i o n a l properties of PET specimens of various c r y s t a l l i n i t y , a s t e e l sphere of 2.38 mm i n radius was s l i d on the PET specimens mounted on the glass plate at a speed of 0.25 mm/s under loads i n a range from 0.5 to 8.0 N. After a constant load was applied for 30 seconds, s l i d i n g was started and the f r i c t i o n a l force was measured using s t r a i n gauges attached to the spring p l a t e s where the s t e e l sphere was mounted at one end, while the other end was connected to the s t e e l arm supported by a b a l l bearing. A s t e e l b a l l for b a l l bearing was used as the s t e e l sphere i n the experiments. The PET specimen surfaces were abraded with 1500-grade abrasive cloth i n water and made as clean as possible using ethylalcohol. f

Wear-Testing Apparatus and Experimental procedure. To study the e f f e c t of c r y s t a l l i n i t y on wear of PET specimens with d i f f e r e n t degrees of c r y s t a l l i n i t y , the pin-on-disk type wear-testing apparatus used i n our previous work (5) was again employed. With t h i s apparatus, measurements of f r i c t i o n a l force and wear depth can be made continuously during the wear process. When the wear-testing apparatus was used, experiments were carried out at a s l i d i n g speed 0.1 m/s under a load 10 N. The PET specimens of 3 mm i n diameter were bonded on the f l a t ends of polypropylene pins of the same diameter i n order to mount the specimens to the pin holder and were rubbed against the disks made of a stainless s t e e l (SUS 304). F i n a l l y , the f r i c t i o n a l surface of the disk was abraded with a 1500-grade waterproof abrasive cloth i n water and i t had about 0.02 m c l . a . roughness. The diameter of the f r i c t i o n a l tracks on the disks was 5 cm. After the specimen was mounted to the holder, the specimen was i n i t i a l l y rubbed against a 6/0 grade Emery paper placed on the disk. This pre-rubbing was useful for allowing a uniform contact between the specimen surface and the disk. After the pre-rubbing, the specimen and disk surfaces were made as clean as possible by rubbing with a soft cloth wetted with ethylalcohol. A l l experiments i n this work were carried out i n the room a i r of 20 + 2 C and of 55 ± 10 % r e l a t i v e humidity. PET Specimen, The PET plate of 2-mm thick and of very low c r y s a l l i n i t y was supplied by the T e i j i n Ltd.. By annealing the plate, the PET specimens of two d i f f e r e n t c r y s t a l l i n i t y ( 39 % and 55 % ) were obtained. On the other hand, the PET f i l m of very high c r y s t a l l i n i t y ( 75 % ) was also used as the specimen wich was obtained by annealing a f i l m of 0.5-mm thick and of 60 % c r y s t a l l i n i t y . The o r i g i n a l f i l m was produced by Toray Industries, Inc. The c r y s t a l l i n i t y of


24. YAMADA AND TANAKA

Crystallinity Effect on Friction and Wear

365

specimens was determined by measuring their density and using the values of 1.328 g/cm and 1.450 g/cm as the values of the density of amorphous and c r y s t a l l i n e phases, respectively. The c r y s t a l l i n i t y , density and annealing condition for the specimens are l i s t e d Table I. 3

TAble I. Specimen

3

Condition of Annealing and C r y s t a l l i n i t y Conditionof Annealing Density (g/m ) none 1*337


3

1-0

Degree of C r y s t a l l i n i t y ( % ) 8

1-L

O r i g i n a l , 1-0, Annealed; 120 C, 20 min

1.373

39

1-H

O r i g i n a l , 1-0, Annealed; 210 C, 60 mir

1.392

55

2-H

O r i g i n a l , Density 1.399, 1.417 D.C. 60 % Annealed; 200 C, 90 min

75

Morphology of the Specimens. In order to examine the morphology of specimens, the sections perpendicular to the surfaces used as the f r i c t i o n a l surface were cut from the plate and f i l m specimens and were observed by a p o l a r i z i n g microscope with crossed-Polaroid. Figures 1(a) and (b) show the p o l a r i z i n g micrographs of the sections of various specimens at a c e r t a i n position under a p o l a r i z i n g microscope and those at the position turned 45 degrees from the micrograph (a), respectively. The structure of the specimen 1-0 i s almost amorphous and very d i f f e r e n t from that of the other specimens, because any morphological structure cannot be observed i n the micrographs of the specimen 1-0. The bright bands seen i n the micrograph of the specimen 1-0 must be due to the l o c a l drawing during the sectionning process. The specimen 1-H has a spherulite structure and the spherulites are very small. With the specimen 1-L; however, the micrograph i n Figure 1 (a) shows a maze-like pattern and that i n Figure 1 (b) shows a mesh-like pattern. Thus, the specimen 1-L may have a structure somewhat d i f f e r e n t from the spherulite structure of the other semicrystalline polymers such as polyethylene and nylon. On the other hand, micrograph (b) of the specimen 2-H shows a f i b e r - l i k e pattern and t h i s may be due to a strong drawing during a f a b r i c a t i o n process of the f i l m . Experimental Results and Discussion S l i d i n g of the Steel Sphere on F l a t PET Surfaces. Figure 2 shows t y p i c a l f r i c t i o n traces i n the s l i d i n g of a s t e e l sphere at a speed o.25 mm/s under a load 8 N. I t i s seen that the s t a t i c f r i c t i o n i s considerably higher than the k i n e t i c f r i c t i o n and there i s no s t i c k s l i p phenomenon. Comparing the f r i c t i o n of PET with that of other polymers obtained i n the s l i d i n g experiment (6) similar to that i n the present work, i t was found that PET exhibited r e l a t i v e l y lower


366


POLYMER WEAR AND ITS CONTROL

Figure 1 P o l a r i z i n g m i c r o g r a p h s o f t h e s e c t i o n s of v a r i o u s PET specimens. The s e c t i o n s i n t h e m i c r o g r a p h (b) a r e t u r n e d 45 degrees from the p o s i t i o n i n t h e m i c r o g r a p h ( a ) 0

0


Crystallinity Effect on Friction and Wear

24. YAMADA AND TANAKA

367

f r i c t i o n . The c o e f f i c i e n t of s t a t i c f r i c t i o n , ys obtained from the f r i c t i o n traces measured under various loads. As shown i n Figure 3, μ decreases with the increase i n load and i s represented by the r e l a t i o n y

Polymer Wear and Its Control - ACS Publications - American Chemical

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