Friction and wear of polyethylene-nylon blends

Friction and wear of polyethylene-nylon blends

341 Wear, 149 (1991) 341-352 Friction and wear of polyethylene-nylon blends* H. Ye&e, H. BenabdaI~ah and H. Richards Ecole Po&technicpe de Mont&a...

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341

Wear, 149 (1991) 341-352

Friction

and wear of polyethylene-nylon

blends*

H. Ye&e, H. BenabdaI~ah and H. Richards Ecole Po&technicpe de Mont&al, PO Box 6079, Station “A’: Mont&al, QuP. H3C 3A? (Canada) (Received

April 30, 1991).

Abstract

The objective of this work was to investigate how the mechanical properties of a high density polyethylene (HDPE) could be increased by adding nylon 6 to it while keeping its good frictional and wear resistance characteristics. Nine blends of 5%-30% nylon in HDPE were prepared with 5% of an ionomer as a linking agent. ASTM D638 traction specimens were injection moulded to control the mechanical properties of the blends, and friction and wear specimens were cut in the runners of the injection mouldings. Traction tests showed between - 13% and + 16% variations in tensile yield strength compared to pure polyethylene. Friction tests conducted on a reciprocating machine showed an increase of about 15% in the coefficient of friction over that of pure polyethyIene, but it still remained only about 60% that of pure nylon 6. The tests also showed a certain deterioration of the wear resistance for the blends compared to pure HDPE, especially for those blends having the highest content of nylon.

1. Introduction

Nylon has long been known as a good engineering thermoplastic, in particular because of its mechanical strength and sti&ess; it is, however, relatively heavy, expensive and does not have good triboiogical properties. High density polyethylene (HDPE) on the other hand is cheap, light, has good resistance to wear and a low coefficient of friction, but its mechanical yield strength S, and tensile stiffness E, are poorer than those of nylon. These comparisons are made in Table 1 for nylon 6 and HDPE [l-3]. As can be seen from Table 1, nylon has almost twice the yield strength of polyethylene, nearly 20% more rigidity and a sensibly higher melting point; on the other hand, however, nylon is about 20% heavier than polyethylene. Although exact figures for coefficient of friction p and wear coefficient K are not easy to quote, typical values are reported in Table 1. It is clear from this table that HDPE is superior to nylon in those respects. This work investigates how the best properties of nylon and HDPE could be combined to develop a new blended plastic material. The procedure chosen to do this is the blending of the two materials in various proportions with SURLYNe [4], an ionomer resin, to promote the adhesion of nylon and polyethylene in the blend [S]. The tensile yield strength, coefficient of friction and wear rate have been measured for several of these blends. *Paper presented at the International U.S.A., April 7-11, 1991.

0043-1648/91/$3.50

Conference

on Wear of Materials,

0 1991 -

Elsevier

Orlando,

Sequoia,

FL,

Lausanne

342 TABLE

1

Published

properties

of nylon 6 and high density polyethylene

E,

(HDPE) I-t

W’al Nylon 6 [l] HDPE [Z]

40.7 21-23

745 59Wao

K (in3 min Ibf-’ ft-’

210 121

1.13 0.946

0.37 0.23

h-‘)

4.5 x 10-6 3.9 X 10-7

SY, tensile yield strength; E,, tangent tensile modulus; T,, melting temperature; p, density; CL, dynamic coefficient of friction; K wear coefficient. Note: data are from ref. 1 and 2, except for p and II which were taken from ref. 3. TABLE 2 Blend compositions of high density polyethylene (HDPE), nylon 6 (PA) and ionomer (SU, Sudyn 9020 from Du Pont) as percentages of total weight Blend

HDPE (%)

PA (%)

100.0 95.oko.a 90.4 f 1 .o as.5 f 0.9 80.7 f 0.9 76.0 f 0.6 66.5 f 0.6 70.0 f 0.6 -

-

su

(%)

5.0 * 0.007

10.0*0.~7 15.0+ 0.007 zo.o*o.~ 30.0 t 0.006 30.0 + 0.005 loo.0

5.0*0.005 4.6 +- 0.007

4.5 f 0.007 4.3 t_ 0.007 4.0 + 0.004 3.5 f 0.0@5 -

This paper reports the proportions of the blends and the procedure that has been followed to produce the specimens. Then the machines and testing procedures are described before the results are presented and commented on. The main conclusions are summarized at the end.

2. Blends Altogether, nine different blends of nylon-polyethylene were tested. The proportions of the blends are given in Table 2. Blends I and 9 are pure HDPE and pure nylon respectively. Because nylon and HDPE do not adhere strongly together, an ionomer resin is used as an additive to make a bridge between the two base polymers in an attempt to give the blend a stronger coherence. The idea for the use of an ionomer resin (Surlyn 9020 from Du Pont [4]) as a compatibiiizer for nylon 6 comes from Van Gheluwe et al. [5], who have successfully blended nylon 6 and polypropylene. In their work, Gheluwe et al. first blended the ionomer in the proportion of 5% of ionomer resin in the nylon 6, using a twin-screw extruder to achieve good mixing. In the present investigation the ionomer, nylon 6 and HDPE were combined simultaneously in the injection-moulding machine. Because of that, the proportion of ionomer was increased to 5% of the HDPE matrix in weight so as to achieve as good a dist~bution of the

343

ionomer as possible. Blend 2 is 95% HDPE and 5% ionomer resin. This blend is used to determine the possible effects of the ionomer on the mechanical and tribological properties of the HDPE. Blends 3-7 represent an HDPE matrix in which various percentages of nylon have been added. Blend 8 contains the same proportion of nylon in the HDPE matrix as blend 7 but without any ionomer. This is done to study the compatibilizing effect of the ionomer in the blend. The blends shown in Table 2 have been prepared by mechanically mixing the granulated resins in a container. The contents were then poured in the injection machine hopper and processed to produce the samples; hot mixing of the components took place in the injection machine screw. Although this cannot give a perfectly homogeneous final product, the blends appeared to be satisfactory, as indicated by the low sample standard deviations obtained for the tensile yield strength measurements. The specimens were produced with a Battenfeld (80 t) injection-moulding machine equipped with a Unilog 8000 control box. The injection parameters used are listed in Table 3; the same parameters were used for all the blends of Table 2. The nylon 6 was conditioned at 80 “C for 5 h prior to moulding. The specimens used to evaluate the mechanical strength of the blends were moulded according to the ASTM D638 standard and are shown in Fig. l(a). The specimens for testing the coefficient of friction and wear are of the pin type and were taken from the injection runners. Figures l(c) and l(D) show some of those pin-type specimens as they were prepared for the tests.

TABLE

3

Conditions

during injection

Screw speed Injection speed Mould temperature Cooling time Injection temperature Holding time Holding pressure

moulding

of specimens

(all blends)

50 rev min-’ 125 mm s-’ Non-cooled 20 s 230 “C 10 s 100 bar

Fig. 1. A, tensile test specimens; B, injection runners; C, friction specimens; in their hoider; E, wear track.

D, friction specimens

344

3. Machines

and testing

methods

Since mechanical and tribological properties were measured, testing methods for each will be presented separately.

the machines

and

3.1. Mechanical properties The mechanical property that has been measured to evaluate the mechanical resistance of the blend is the tensile yield strength. This property has been measured in accordance with the procedure recommended in ASTM D638. The machine used is a traction-testing machine (JJ Instruments, model M30K) with a constant rate of cross-head motion. In this work the samples were tested at a cross-head motion rate of 50 mm min-‘. Prior to a test the specimens were conditioned at 23 “C and 50% relative humidity by leaving them in the room for a period of at least 48 h. During the test both the force and displacement of the grips were recorded but only the maximum force at by the traction test machine is used to calculate the yield yield, F,,,, determined strength S, as

(1) where So is the original

section

calculated

as

so= wt

(2)

W is the width of the tensile specimen at the reduced section and t is its thickness. W and t were measured on each specimen with the aid of vernier callipers. The

measurements

gave

W= 5.750 f 0.050 mm

t = 3.050 f 0.050 mm These

are the values that were used to calculate

So for all the specimens.

3.2. Tribological properties As already mentioned, two tribological properties were measured: the weight loss by wear and the coefficient of friction. The tribological tests were conducted in a room maintained at 23 “C and 50% relative humidity. As for the tensile tests, the friction specimens were conditioned in the room for at least 48 h prior to the test. The friction specimens were cut first to length with a saw from the injection runners and the friction surfaces were then finished on a lathe. Once firmly secured in a holder (see Fig. l(D)), the specimens were cleaned with isopropyl alcohol and weighed. The weighing was done on a Mettler precision electronic balance (model AE240, with a display precision of 0.001 g, 0.0002 g reproducibility andf0.0003% linearity). At the end of a test, after 40 000 cycles of rubbing against the steel wear track, the specimens were postconditioned in the room, cleaned and weighed again. The difference in the weights recorded before and after the test is interpreted as the weight loss due to wear. As seen in Fig. l(D), two pin specimens are placed in a holder to assure an adequate stability of the assembly during the friction test. The total apparent contact area of two specimens in the holder is 56.5 mm’. The friction tests were conducted on the custom-made linear wear- and frictiontesting machine shown in Fig. 2 [6]. The operating principle of the machine is shown

345

Fig. 2. Reciprocating

friction

Data

and wear test apparatus.

acquisition

system

i---_----_----------__----

I;Force

L

II \

f hJear f

control

tronsduccr

/

track

Force

am

I support

v

beon

Y

Dead

weight

I I _______--____--_____~~~~~~~---

Fig. 3. Reciprocating

friction and wear test machine;

90 PSIQ

! _I

schematic

atn

4%

diagram of operating

principle.

schematically in Fig. 3. Three sections can be identified: (1) the driving section, (2) the testing section and (3) the acquisition section. The driving section consists of a single-rod pneumatic cylinder controlled by a pneumatic Iogic circuit to give a back-and-forth stroke of 150 mm. The speed of the pneumatic cyIinder is adjusted and regularized by a double-rod hydraulic cylinder and a manuaIly adjustable flow regulator. The speed of the pneumatic cylinder can be

346

adjusted between 0 and 0.2 m s-i. In the present work the tests were conducted at 0.15 m s-i. The drive mechanism is used to impart a reciprocating motion to a support beam that carries the wear track (see Fig. 1 (C)) against which the specimens are pressed. As can be seen in Fig. 2, the testing section consists of five wear tracks in parallel, all driven at the same speed by the pneumatic cylinder. It is shown Fig. 3 that the wear tracks are removable. This is to facilitate the surface finishing, the usage of various materials or the replacement of the wear tracks. For the tests presented in this paper the wear tracks were made from UNSG 10100 steel. The surface finish was produced by lapping to avoid the presence of a lead, using a 320 powder grit size. The wear tracks were refinished before each test and the surface finish was measured with a Brush Surfindicator and a Clevite Surfadriver in two mutually perpendicular directions. The centre-line average (c.1.a.) measurements indicated in fact that there was no lead on the surface, since in all cases the measurement made in the two perpendicular directions varied between 852 pm before a wear test and 6 k 2 pm after. The pressing force between the specimens and the wear track is produced by a dead-weight as shown in Fig. 3. In the present tests, the pressing force was 22.3kO.l N. On the apparent contact area of 56.5 mm’ the apparent pressure was therefore around 0.395 MPa. As shown in Fig. 3, the friction force developed between the specimens and the track is transmitted by a lever to a force transducer located at each of the five rubbing stations as can be seen in Fig. 2. The force transducers used are INTERFACE MB50 bridge-type strain gauge units. The output voltage of each transducer is amplified and fed to an ACPC-16 analogue-digital (A-D) converter card (by Strawberry Tree Computers Inc.) hosted by a microcomputer. The dual-slope-integrating A-D converters of the ACPC-16 card are programmable for 12, 14, 16 or 18 bits of precision. Of course, as the acquisition precision increases, the sampfing rate decreases. In this work the 12 bits acquisition mode was used to maximize the rate of acquisition with an acceptabie accuracy. A programme has been written in Pascal to control the ACPC-16 card and its routines. The software records and stores the friction force of all five force transducers during a complete back-and-forth rubbing cycle at predetermined programmable intervals. All five transducers are sampled in parallel. For the tests presented in this work the friction force was recorded at the first cycle, then one recording was done every 100 cycles for the next 2000 cycles; from cycle 2000 to the end of the test, cycle 40 000, recording were made every 500 cycles. The tests were conducted without interruption.

4. Results

As for the machines parts, namely mechanical

and testing methods, the results and tribological results.

will be presented

in two

4.1. Results of mechanical tests Six to eight traction tests were performed for each of the blends of Table 2. The results of these tests are presented in Table 4 and Fig. 4; the values for the tensile yield strength S, shown in Table 4 represent the arithmetic average of all the measurements made for each blend with their sample standard deviation. The measured

347 TABLE 4 Experimental results: yield stress S,, dynamic friction coefficient p and weight loss due to wear, AW, for all nine blends as a function of weight percentage of nylon 6 (PA) Blend

PA (%)

0 0 5

10 15 20 30 30 loo

S, (MPa)

+q

%

jI,

t+

%

AW (8)

i6(Aw)

%

23.6 21.9 23.4 23.3 24.6 24.0 25.3 24.5 38.5

1.1 0.5

5

0.181 0.192 0.216 0.210 0.212 0.229 0.222 0.213 0.603

0.012 0.026 0.012 0.009 0.006 0.010 0.010 0.022 0.025

7 14 6 4 3 4 5 10 4

0.0030 0.0012 0.0033 0.0051 0.0064 0.0144 0.0152 0.0228 0.0140

0.0013 0.0007 0.0023 0.0023 0.0013 0.0053 0.0020 0.0037 0.0014

42 58 68 45 20 37 13 16 10

0.5 0.8 0.4 0.6 0.8 0.5 2.2

2 2 3 2 3 3 2 6

9

Fig. 4. Yield strength

S, as a function

of weight percentage

of nylon 6 for all nine blends.

348 yield strengths for blends 1 and 9 reproduce within a good accuracy the figures published by the manufacturer and reported in Table 1. The dispersion in the results

shown in Table 4 is quite small, less than 6%. From this, one can conclude that the parts produced were of good and uniform quality and that the testing procedure followed was accurate. As can be seen from Fig. 4 and Table 4, the addition of 5% ionomer to the pure HDPE, blend 2, reduces its mechanical strength by approximately 7%. However, the addition of only 5% nylon, blend 3, recovers the strength of the pure HDPE resin. The addition of higher percentages of nylon to the HDPE matrix does not improve the mechanical properties significantly; the small increase in yield strength from blend 3 to blend 7 lies within the experimental variation of the results. It seems that there is not as good adhesion between the polyethylene and nylon resins as might be suggested in ref. 5, since it was expected that the addition of 30% nylon to HDPE would have increased the mechanical strength more significantly. The presence of the ionomer in the blends also seems to be of small benefit, as indicated by the small difference in the yield strength of blends 7 and 8. An explanation may lie in insufficient or inadequate mixing of the components 4.2. Results of tribological tests For the tribofogicai properties, three tests were done for each of the blends in Table 2. As mentioned before, the tests were conducted at the same speed and load for all the bIends. Two variables were measured, namely the friction force and the wear. Figure 5 reproduces a typical recording of the friction force on specimens of blend 1 at 20 000 cycles. This recording shows how the friction force varied as the specimens travelled along the wear track. The reversal in force sign corresponds to the reversal of direction of the wear track. This figure identifies the portion of the

0.0

0.5

Fig. 5. Variation

1.0 t,me, 5

1.5

of friction force measured

2.0

during one cycle.

349 recording that has been retained to evaluate the friction force for that number of cycles. The beginning and end of the recording are eliminated because of transient phenomena. The negative recording is also eliminated because of the level of vibrations in the testing machine. The same phenomena are observed at all cycles with all the blends. During the tests it appeared also that the friction force, evaluated at each cycle as discussed above, varied considerably from the start of the test until approximately 1000 cycles. This is shown in Fig. 6, which plots the recordings made for blend 1 during one whole test. The friction force increased from cycle 1 to 100, decreased from 100 to 1000 cycles and remained almost stable from there on for the rest of the test. The same behaviour has been observed in all the tests. Since the friction force did stabilize over 1000 cycles, it has been decided to use only the recordings made after 1000 cycles to calcutate the average friction force F,,,, of the test by F imean

ZQ =

number

recorded

of recordings

The coefficient

of friction

of the test is then calculated

as

F fmean P=

-

N

where

N=22.3

N.

The values of the coefficient of friction p calculated by eqn. (4) and the sample standard deviation &A. are presented in Table 4 and plotted in Fig. 7. Despite the uncertainties involved in the evaluation of the friction force, the standard deviation of the calculated values is relativeIy small, exceeding 10% in only two cases and being below 7% in all others. 8.5

0.65 T-

-r-

El

8

nd #1

/

7.5

0.6

9

0.55 ; 0.5 p:

z 7 . s b 6.5

2 0.45 & 0.4

6 6 c v ; 5.5 IL

ii0.35 ; : 0.3 !_I.

5

0.25

4.5 4

I

10

llrm

100

Fig. 6. Variation Fig. 7. Coefficient

1000 10000 Number of cycIes

100000

0

10 20

30

40 50 60 70 ;:of nylon 6

80

90

100

of friction force with number of cycles for blend 1. of friction as a function of weight percentage

of nylon 6 for all nine blends.

350

The value of the coefficient of friction measured for the pure HDPE, blend 1, is in fairly good agreement with that published in ref. 3 and reported in Table 1. However, this is not the case with blend 9, pure nylon; the value measured in this work is almost double that reported in Table 1. Many factors may explain this discrepancy and one important one is the surface finish. So many variables are implicated in measuring the coefficient of friction of materials that we will not attempt to compare the present results with those of others, other than in a relative manner. In that sense Table 4 shows that the coefficient of friction of polyethylene is lower than that of nylon as reported in Table 1. In the literature this is also the general consensus. Therefore we conclude that the tendency of the results presented in this work is correct and that they can be used to compare the various blends. From Table 4 and Fig. 7 it appears that the coefficient of friction of all blends is lower than that of nylon alone. The addition of the ionomer to the HDPE increases the coefficient of friction only by a marginal amount; the difference lies within the experimental variation. The addition of only 5% nylon to the polyethylene matrix, however, clearly causes an increase of the coefficient of friction that seems to level out with further increases in nylon content. Removing the ionomer, blends 7 and 8, again causes only a marginal change in the coefficient of friction, but this time in the opposite direction such that it brings it to the level of the pure polyethylene within experimental error. These results seem to indicate that the ionomer in a small percentage has as much influence on the coefficient of friction of the blend as nylon in much larger percentages. The effect of nylon is to increase the friction while that of the ionomer is not well identified. The other tribological measurement done was the weight loss by wear. The weight loss AW and the sample standard deviation s(AW) are also shown in Table 4 and plotted in Fig. 8. In this case the experimental variations are much more important than for strength and friction. These variations can be explained by the wear debris produced on the side of the specimens when they rubbed on the track. Depending on how many of these wear debris particles remained attached to the specimens, the second weighing value could vary considerably, causing the large variations observed. As the nylon content was increased, a skin became visible on the wear specimens, aggravating the problem of the wear debris (see Fig. l(C)). The result is clearly seen in Fig. 8, where the sample standard deviation increases considerably for blends 6 and 7. The weight loss and standard deviation are at their maximum for blend 8. However, the tendency shown by the results is again in the right direction, since the wear rate of pure nylon is about five times larger than for pure HDPE. As seen in Table 4, the addition of ionomer to pure polyethylene, blend 2, reduces the wear losses. The addition of nylon in this case increases the wear of the blend systematically. Finally, the addition of 20% or 30% nylon to the polyethylene brings the loss of weight by wear to the same level as for pure nylon. The addition of 30% nylon without ionomer clearly gives a blend that has a poorer resistance to wear than pure nylon. In the case of wear resistance, both the ionomer and nylon cause an increase in the wear rate of the blends.

5. Conclusions The objective of this work was to investigate how the mechanical properties of a high density polyethylene can be increased by adding nylon 6 to it while keeping its good frictional and wear resistance characteristics.

0.03

0.025

0.02 cp . z .!0.015

tf,

” L !? s

c7

0.01

_+5

0 Fig. 8.

10 20 30 40 50 60 70 % of nylon 6

Weight

L

I

80 90 100

loss due to wear as a functjon

of weight percentage

of nylon 6 for all nine

blends. The results show that the mechanical strength of the polyethylene was not improved very much by the addition of nylon to it; this may be attributed to the blending method. On the other hand the coefficient of friction increased slightly, though it remained well below the coefficient of friction of pure nylon. The percentage of nylon added to the blend did not appear to have a major influence on either the yield strength or the coefficient of friction; almost the same effect can be obtained with 5% or 30% nylon added. It appears, however, that the wear resistance of the blends deteriorated significantly compared to polyethylene and that it reached or even surpassed the wear rate of pure nylon. In this case the percentage of nylon added is very significant. The use of an ionomer did not seem to favour adhesion between polyethylene and nylon. The addition of ionomer has a tendency to decrease the strength of the blend while it increases the coefficient of friction. However, as far as the wear resistance is concerned, the addition of ionomer proved to beneficial. Hot mi&ing of the component resins in the injection machine screw gives a good homogeneous blend for low nylon percentages (less than 15%). The blends containing higher percentages of nylon showed evidence of large nylon inclusions in the HDPE matrix. This could be improved by blending the nylon and ionomer in a twin-screw extruder prior to injection moulding. However, the set of data generated shows only a small dispersion, which makes it reliable.

This work has been made possible by an Engineering Council of Canada (ENSRC) grant.

and Natural Science Research

352 References ZYTEL” nylon resins for moulding and extrusions, Cafalug E-42267, 1971 (Du Pont Canada Inc., Plastics Division, PO Box 2200 Streetville, 7070 Mississauga Rd., Mississauga, Ont. L5M 2H3, Canada). Technical Catalog SC-2.54, June 1978 (Du Pont Canada Inc., Plastics Division, PO Box 2200 Streetville, 7070 Mississauga Rd., Mississauga, Ont. L5M 2H3, Canada). N. P. Suh, Tribophysics, Prentice-Hall, Englewood Cliffs, NJ, 1986. SURLYN’ ionomer resins, Cukzlog H-04376, 1988 (Du Pont Canada Inc., Plastics Division, PO Box 2200 Streetville, 7070 Mississauga Rd., Mississauga, Ont. L5M 2H3, Canada). P. Van Gheluwe, B. D. Favis and J.-P. Chatifoux, Morphologi~aI and mechanica properties of extruded pol~ropylene/nylon-6 blends, J. Murer. Sci., 23 (23) (1988) 3210-3220. S. M. H. Benabdallah, Reciprocating sliding friction and wear test apparatus, P&n. Test, 9 (1990) 195-211.