ISO 6691:2021

INTERNATIONAL ISO
STANDARD 6691
Third edition
2021-11
Thermoplastic polymers for plain
bearings — Classification and
designation
Polymères thermoplastiques pour paliers lisses — Classification et
désignation
Reference number
ISO 6691:2021(E)
© ISO 2021

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ISO 6691:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
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ISO 6691:2021(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3  Terms and definitions . 2
4  Classification and designation system . 2
4.1 General . 2
4.2 Data block 1 . 3
4.3 Data block 2 . 3
4.4 Data block 3 . 4
4.4.1 General . 4
4.4.2 Polyamides . 4
4.4.3 Polyethylenes. 6
4.4.4 Polyalkyleneterephthalates. 7
4.4.5 Other polymers . 8
4.5 Data block 4 . 8
5 Designation examples .10
6  Ordering information .12
Annex A (informative) Properties and applications of the most common unfilled
thermoplastic polymers .13
Annex B (informative) Fundamental application parameters.21
Bibliography .27
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ISO 6691:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 123, Plain bearings, Subcommittee SC 2,
Materials and lubricants, their properties, characteristics, test methods and testing conditions.
This third edition cancels and replaces the second edition (ISO 6691:2000), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— references in 4.5, Data Block 4 for position 3 and 4, have been made precise;
— Annexes A and B have been revised.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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INTERNATIONAL STANDARD ISO 6691:2021(E)
Thermoplastic polymers for plain bearings —
Classification and designation
1 Scope
This document specifies a classification and designation system for a selection of the most common
unfilled thermoplastic polymers for plain bearings.
The unfilled thermoplastic polymers are classified on the basis of appropriate levels of distinctive
properties, additives and information about their application for plain bearings. The designation system
does not include all properties; thermoplastic polymers having the same designation cannot therefore
be interchanged in all cases.
It also provides an outline of the properties and applications of the most common unfilled thermoplastic
polymers as well as listing some of the fundamental parameters that influence the selection of
thermoplastic polymers for use for plain bearings.
NOTE In the further course of the work, it is intended to prepare standards on “thermosetting polymers”
and “mixed polymers” for plain bearings.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 307, Plastics — Polyamides — Determination of viscosity number
ISO 527-1, Plastics — Determination of tensile properties — Part 1: General principles
ISO 527-2, Plastics — Determination of tensile properties — Part 2: Test conditions for moulding and
extrusion plastics
ISO 527-3, Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets
ISO 527-4, Plastics — Determination of tensile properties — Part 4: Test conditions for isotropic and
orthotropic fibre-reinforced plastic composites
ISO 527-5, Plastics — Determination of tensile properties — Part 5: Test conditions for unidirectional fibre-
reinforced plastic composites
ISO 1043-1, Plastics — Symbols and abbreviated terms — Part 1: Basic polymers and their special
characteristics
ISO 1133-1, Plastics — Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR)
of thermoplastics — Part 1: Standard method
ISO 1183-1, Plastics — Methods for determining the density of non-cellular plastics — Part 1: Immersion
method, liquid pycnometer method and titration method
ISO 1183-2, Plastics — Methods for determining the density of non-cellular plastics — Part 2: Density
gradient column method
ISO 1183-3, Plastics — Methods for determining the density of non-cellular plastics — Part 3: Gas
pyknometer method
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ISO 6691:2021(E)
ISO 1628-5, Plastics — Determination of the viscosity of polymers in dilute solution using capillary
viscometers — Part 5: Thermoplastic polyester (TP) homopolymers and copolymers
ISO 16396-1, Plastics — Polyamide (PA) moulding and extrusion materials — Part 1: Designation system,
marking of products and basis for specifications
ISO 16396-2, Plastics — Polyamide (PA) moulding and extrusion materials — Part 2: Preparation of test
specimens and determination of properties
ISO 17855-1, Plastics — Polyethylene (PE) moulding and extrusion materials — Part 1: Designation system
and basis for specifications
ISO 17855-2, Plastics — Polyethylene (PE) moulding and extrusion materials — Part 2: Preparation of test
specimens and determination of properties
ISO 20028-1, Plastics — Thermoplastic polyester (TP) moulding and extrusion materials — Part 1:
Designation system and basis for specification
3  Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4  Classification and designation system
4.1 General
The classification and designation are based on a block system consisting of a “description block” and
“Identity block” (see Table 1). The “Identity block” comprises an “International Standard number block”
and an “individual item block”. For unambiguous coding of all thermoplastic polymers, the “individual
item block” is subdivided into four data blocks.
Table 1 — Classification and designation
Designation
Identity block
Individual item block
Description
International
block
Standard number  Data block Data block Data block Data block
block
1 2 3 4
The “individual item block” starts with a dash. The data blocks are separated by commas.
Data blocks 1 to 4 include the following information:
— data block 1: material symbol (see 4.2, Table 2);
— data block 2: intended application or method of processing (see 4.3);
— data block 3: distinctive properties (see 4.4);
— data block 4: type and content of fillers or reinforcing materials (see 4.5);
The meaning of the letters and digits is different for each data block (see 4.2 to 4.5).
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ISO 6691:2021(E)
Data block 2 comprises up to 4 positions. If at least one of positions 2 to 4 is used, but no information is
given in position 1, then the letter “X” shall be placed in position 1. The letters in positions 2 to 4 shall
be arranged in alphabetical order.
If a data block is not used, this shall be indicated by consecutive data block separators, i.e. two
commas („).
Designation examples are given in Clause 5.
4.2 Data block 1
The chemical nature of the thermoplastic polymer is designated by its symbol in accordance with
ISO 1043-1.
Table 2 — Symbols for the chemical structure of the materials
Thermoplastic polymers
Name and chemical structure
Group/Name Symbol
Polyamide PA 6 Polyamide 6; homopolymer based on ε-caprolactam
PA 6 cast Polyamide 6, cast; homopolymer based on ε-caprolactam
PA 66 Polyamide 66; homopolycondensate based on hexamethylenediamine
and adipic acid
PA 12 Polyamide 12; homopolymer based on ω-laurinlactam or ω-aminodo-
decanoic acid
PA 12 cast Polyamide 12, cast; homopolymer based on ω-laurinlactam or ω-ami-
nododecanoic acid
PA 46 Polyamide 46; a co-condensate based on 1,4-diaminobutane and
adipic acid
Polyoxymethylene POM Polyacetal (homopolymer), Polyacetal (copolymer)
Polyalkyleneterephthalate PET Polyethylene terephthalate
PBT Polybutylene terephthalate
Polyethylene PE-UHMW Polyethylene with ultra high molecular weight
PE-HD High density polyethylene
Polyfluorocarbon PTFE Polytetrafluoroethylene
Polyimide PI Polyimides from polyaddition reactions are available as thermo-
setting plastics. Polyimides from polycondensation reactions are
available as thermoplastics and thermosetting plastics, as well as
copolymers of the imide group. Some thermoplastic polyimides are
“apparent thermosetting plastics” because their thermoplastic range
lies above the decomposition temperature. Because of their inter-
mediate position, polyimides and imide copolymers are only treated
marginally in this document.
Polyaryletherketone PEEK Polyetheretherketone
Polyvinylidene fluoride PVDF Homopolymer based on vinylidene difluoride
Polyphenylene sulfide PPS Polyphenylene sulfide, linearly structured phenyl ring and sulfur
atoms (tribologically modified material)
Poly(amide-imide) PAI Poly(amide-imide) reacted by polycondensation is a hard/tough,
amorphous thermoplastic. After postcuring the PAI parts cannot be
used for re-processing (“pseudo-thermoset plastics”).
4.3 Data block 2
Position 1 gives the code for the intended application (see Table 3).
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ISO 6691:2021(E)
Table 3 — Data block 2 — Position 1
Code Intended application
E Extrusion
G General use
M Injection moulding
Q Compression moulding
R Rotational moulding
X No indication
Up to three important properties and/or additives can be indicated in positions 2 to 4 (see Table 4).
Table 4 — Data block 2 — Positions 2 to 4
Code Important properties and/or addi-
tives
A Processing stabilizer
F Special burning characteristics
H Heat ageing stabilizer
L Light and weather stabilizer
R Release agent
S Slip agent, lubricant
4.4 Data block 3
4.4.1 General
The levels of distinctive properties are coded by letters and numbers.
The properties used for the designation are different for every thermoplastic polymer.
Owing to manufacturing tolerances, single property values can lie on, or to either side of, two intervals.
lt is up to the manufacturer to state which interval will designate the thermoplastic polymer.
4.4.2  Polyamides
Polyamides are designated in data block 3 by their viscosity number, represented by two digits
(see Table 5) in accordance with ISO 16396-1 and, separated by a dash, their modulus of elasticity
represented by three digits (see Table 6).
In the last position, rapid-setting products may be indicated with the letter N.
The viscosity number shall be determined in accordance with ISO 307 using the solvents given in
Table 5. The modulus of elasticity shall be determined in the dry state in accordance with ISO 527-1,
ISO 527-2, ISO 527-3, ISO 527-4 and ISO 527-5, under the conditions specified in ISO 16396-2.
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ISO 6691:2021(E)
Table 5 — Viscosity number for polyamides
Polyamide Code Viscosity number, ml/g
Solvent
Sulfuric acid 96 % (mass fraction) m-cresol
> ≤ > ≤
09 — 90
10 90 110
12 110 130
PA 6
14 130 160
PA 6 cast 18 160 200 —
22 200 240
PA 66
27 240 290
32 290 340
34 340 —
11 — 110
12 110 130
14 130 150
PA 12
16 — 150 170
PA 12 cast
18 170 200
22 200 240
24 240 –
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ISO 6691:2021(E)
Table 6 — Modulus of elasticity
Code Modulus of elasticity
2
N/mm
> ≤
001 50 150
002 150 250
003 250 350
004 350 450
005 450 600
007 600 800
010 800 1 500
020 1 500 2 500
030 2 500 3 500
040 3 500 4 500
050 4 500 5 500
060 5 500 6 500
070 6 500 7 500
080 7 500 8 500
090 8 500 9 500
100 9 500 10 500
110 10 500 11 500
120 11 500 13 000
140 13 000 15 000
160 15 000 17 000
190 17 000 20 000
220 20 000 23 000
250 23 000 —
4.4.3 Polyethylenes
Polyethylenes are designated by their density represented by two digits (see Table 7) in accordance
with ISO 17855-1 and, separated by a dash, their melt flow rate (MFR) represented by one letter and
three digits (see Table 8).
The density of the base material shall be determined in accordance with ISO 1183-1, ISO 1183-2 and
ISO 1183-3 under the conditions specified in ISO 17855-2.
The melt mass-flow rate shall be determined in accordance with ISO 1133-1 at 190 °C with a load of
2,16 kg (symbol D). For thermoplastic polymers with a melt mass-flow rate <0,1 g/10 min, a test under a
load of 5 kg (symbol T) is recommended. If the melt mass-flow rate is still <0,1 g/10 min, the test should
then be carried out under a load of 21,6 kg (symbol G).
The symbols D, T and G shall precede the code for melt flow rate given in Table 8.
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ISO 6691:2021(E)
Table 7 — Density
a
Code Density
3
g/cm
> ≤
15 — 0,917
20 0,917 0,922
25 0,922 0,927
30 0,927 0,932
35 0,932 0,937
40 0,937 0,942
45 0,942 0,947
50 0,947 0,952
55 0,952 0,957
60 0,957 0,962
65 0,962 —
a
Density ranges for uncoloured and unfilled polyethylene materials.
Table 8 — Melt mass-flow rate (MFR)
Code Melt flow rate
g/10 min
> ≤
000 — 0,1
001 0,1 0,2
003 0,2 0,4
006 0,4 0,8
012 0,8 1,5
022 1,5 3
045 3 6
090 6 12
200 12 25
400 25 50
700 50 100
4.4.4 Polyalkyleneterephthalates
The distinctive property of polyalkyleneterephthalates is the viscosity number according to
ISO 20028-1, determined in accordance with ISO 1628-5, and designated by two digits (see Table 9).
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ISO 6691:2021(E)
Table 9 — Viscosity number for polyalkyleneterephthalate
Polyalky- Code Viscosity number
lene-terephtha-
ml/g
late
> ≤
06 — 60
07 60 70
08 70 80
09 80 90
PET
10 90 100
11 100 120
13 120 140
15 140 —
08 — 90
10 90 110
12 110 130
PBT
14 130 150
16 150 170
18 170 —
4.4.5 Other polymers
The coding for the distinctive properties of polyoxymethylene, polytetrafluoroethylene, poly(amide-
imide), polyimide, Polyetheretherketone, Polyvinylidene fluoride and Polyphenylene sulfide will be
included in a future edition of this document.
4.5 Data block 4
The fillers and reinforcing materials, as well as additives specific for the application in plain bearings,
are coded as follows:
— position 1: types of fillers and/or reinforcing materials, coded by a letter (see Table 10);
— position 2: physical forms of fillers and/or reinforcing materials, coded by a letter (see Table 11);
— positions 3: mass content of fillers and/or reinforcing materials, coded by two digits (see Table 12);
— positions 4: Types of lubricants, coded by two letters (see Table 13).
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ISO 6691:2021(E)
Table 10 — Types of fillers and/or reinforcing materials (position 1)
Code Type
C Carbon
G Glass
K Chalk
S Synthetic organic material
T Talcum
X No indication
Table 11 — Physical forms of fillers and/or reinforcing materials (position 2)
Code Physical forms
D Powder
F Fibre
S Spheres
X No indication
Table 12 — Mass content (position 3)
Code Mass percentage
> ≤
OX No indication
01 0,1 (inclusive) 1,5
02 1,5 3
05 3 7,5
10 7,5 12,5
15 12,5 17,5
20 17,5 22,5
25 22,5 27,5
30 27,5 32,5
35 32,5 37,5
40 37,5 42,5
45 42,5 47,5
50 47,5 55
60 55 65
70 65 75
80 75 85
90 85 —
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ISO 6691:2021(E)
Table 13 — Types of lubricants (position 4)
Code Type
GR Graphite
MO MoS (Molybdenum disulfide)
2
OL Mineral oil
PE Polyethylene
TF PTFE (Polytetrafluoroethylene)
X No indication
5 Designation examples
A summary of the designation system is given in Table 14.
Table 14 — Summary of the designation system
Description block Thermoplastic
International Standard
ISO 6691
number block
Reference
Data block Position Content
Subclause Table
1 — Material symbol 4.2 Table 2
Intended application or method
1 4.3 Table 3
of processing
2
Important properties and/or
2 to 4 4.3 Table 4
additives
Individual item block
Tables 5
3 — Distinctive properties 4.4
to 9
1 Types of reinforcing materials 4.5 Table 10
Physical forms of reinforcing
2 4.5 Table 11
materials
4
Mass content of reinforcing
3 4.5 Table 12
materials
4 Types of lubricants 4.5 Table 13
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ISO 6691:2021(E)
EXAMPLE 1 A polyamide 6 (PA 6), for injection moulding (M) with mould release agent (R), having a viscosity
2
number of 140 ml/g (14), a modulus of elasticity of 2 600 N/mm (030) and rapid setting (N) is designated as
follows:
Thermoplastic ISO 6691 -PA 6, M R 14 -030 N

Description


International Standard


Data block 1: material symbol


Data block 2: Position 1: for injection moulding


Position 2: for mould release agent


Data block 3: Position 1: viscosity number


Position 2: modulus of elasticity


Position 3: rapid setting

Designation:  Thermoplastic ISO 6691-PA 6, MR, 14-030N
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ISO 6691:2021(E)
EXAMPLE 2 A polyamide 66 (PA 66), without indications as to use additives in data block 2, having a viscosity
2
number of 280 ml/g (27), a modulus of elasticity of 4 000 N/mm (040), rapid setting (N) and 20 % (mass fraction)
glass fibre (GF20) is designated as follows:
Thermoplastic ISO 6691 -PA 66, ,, 27 -040 N, G F 20

Description


International Standard


Data block 1: material symbol


Data block 2: no indication, therefore two commas only


Data block 3: Position 1: viscosity number


Data block 3: Position 2: modulus of elasticity


Position 3: rapid setting


Data block 4: Position 1: for glass


Data block 4: Position 2: for fibres


Data block 4: Position 3: for mass content

Designation:  Thermoplastic ISO 6691-PA 66, „ 27-040N, GF20
6  Ordering information
The purchaser and supplier shall agree on the tests to be carried out.
If mechanical and/or tribological properties are to be tested, it shall be agreed whether such tests are
carried out on
a) unmodified parts of the delivered goods,
b) test bars, manufactured from the same batch, or
c) test bars taken from a finished part to be supplied,
and whether the test shall be carried out parallel or perpendicular to the flow direction and/or
machining direction.
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ISO 6691:2021(E)
Annex A
(informative)

Properties and applications of the most common unfilled
thermoplastic polymers
A.1 Properties and applications of unfilled thermoplastic polymers
Table A.1 gives an outline of the properties and applications of unfilled thermoplastic polymers most
commonly used for plain bearings.
Table A.1 — Outline
Group of
thermoplastic
General description Chemical properties Examples of application
polymers
(symbol)
Polyamide (PA) Resistant material, extraordi- Resistant to fuels, oils, and Bearings subjected to shock
narily shock- and wear-resist- greases and to most common and vibration.
ant, good damping proper- solvents. Sensitive to mineral
Guide blocks in steel mill
ties. acid even in dilute solution,
couplings.
but not attacked by strong
High sliding resistance in
alkalis even at high concen- Bushes for brake rods in
dry running. Relatively high
trations. The use of PA 6 and wagon construction.
moisture absorption.
PA 66 in hot water requires
Bearings for agricultur-
formulations that are stabi-
al machinery. Spring eye
lized against hydrolysis. PA 11
bushes.
and PA 12 are widely resistant
to hydrolysis.
Polyoxymethylene Hard material; therefore High resistance to numerous Plain bearings having strict
(POM) higher resistance to pres- chemicals, above all to organic requirements concern-
sure than polyamide, but liquids. Only a few solvents ing dimensional stability
more sensitive to shock. Less can dissolve POM. Even at and coefficient of friction.
wear-resistant but smaller high temperatures POM-co- Good for dry running or for
coefficient of friction than polymer withstands strong deficient lubrication. Plain
polyamide. Very low moisture alkaline solutions such as bearings for fine mechan-
absorption. 50 % NaOH. Chemicals having ics, electromechanics and
an oxidizing effect and strong household appliances.
acids (pH < 4) attack POM.
Polyethylenetere- Hardness similar to that of Good weather resistance and Application for plain
phthalate (PET) POM; however, decreases high resistance to numerous bearings similar to POM.
considerably above 70 °C. Up solvents, oils, greases, and salt Mostly for plain bearings at
Polybutylenetere-
to 70 °C, wear and coefficient solutions. Sufficiently resist- temperatures below 70 °C.
phthalate (PBT)
of friction very low. Low ant to many acids and alkalis Good for dry running and
moisture absorption. in aqueous solution. Attacked for deficient lubrication.
by concentrated inorganic Plain bearings for fine
acids and alkalis. Halogen- mechanics and submerged
ated hydrocarbons such as installations, guide bushes
methylene chloride and chlo- for rods. Plain bearings for
roform lead to high swelling. oscillating movements.
Sensitive to hydrolysis at high
temperatures.
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ISO 6691:2021(E)
Table A.1 (continued)
Group of
thermoplastic
General description Chemical properties Examples of application
polymers
(symbol)
Polyethyl- PE-UHMW has high shock At room temperature, PE is Plain bearings for Instal-
ene with ultra resistance. PE-HD has low inert to water, alkaline solu- lation in waters carrying
high molecular resistance to permanent tions, salt solutions, and inor- sand.
weight (PE-UHMW) pressure. However, it is re- ganic acids (except strongly
Road and agricultural ma-
sistant to shock. About twice oxidizing acids). At room tem-
High density poly- chinery construction.
the thermal expansion of PA perature, polar liquids such as
ethylene (PE-HD)
and POM. alcohols, organic acids, esters,
Bearings for low tempera-
ketones, and the like only re-
tures.
Excellent wear resistance
sult in slight swelling. Aliphat-
against abrasive stresses. Plain bearings in chemical
ic and aromatic hydrocarbons
Good sliding and bedding installations.
and their halogen derivatives
characteristics.
are absorbed more strongly,
resulting in a decrease in
No moisture absorption. Re-
strength. After the diffusion
sistant to low temperatures.
of these media, polyethylene
can regain its original proper-
ties. Non-volatile liquids such
as greases, oils, waxes, etc.
are less active.
Polytetrafluoro- Resistant to shock, has good At temperatures below 260 °C, Plain bearings in chemical
ethylene (PTFE) bedding characteristics and is not attacked by chemi- installations, high-frequen-
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 6691
ISO/TC 123/SC 2
Thermoplastic polymers for plain
Secretariat: DIN
bearings — Classification and
Voting begins on:
2020­12­16 designation
Voting terminates on:
Polymères thermoplastiques pour paliers lisses — Classification et
2021­02­10
désignation
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 6691:2020(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
©
NATIONAL REGULATIONS. ISO 2020

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ISO/FDIS 6691:2020(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH­1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
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ISO/FDIS 6691:2020(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3  Terms and definitions . 2
4  Classification and designation system . 2
4.1 General . 2
4.2 Data block 1 . 3
4.3 Data block 2 . 3
4.4 Data block 3 . 4
4.4.1 General. 4
4.4.2 Polyamides . 4
4.4.3 Polyethylenes . 6
4.4.4 Polyalkyleneterephthalates . 7
4.4.5 Other polymers . 8
4.5 Data block 4 . 8
5 Designation examples .10
6  Ordering information .12
Annex A (informative) Properties and applications of the most common unfilled
thermoplastic polymers . .13
Annex B (informative) Fundamental application parameters.21
Bibliography .27
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ISO/FDIS 6691:2020(E)

Foreword
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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on the ISO list of patent declarations received (see www .iso .org/ patents).
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 123, Plain bearings, Subcommittee SC 2,
Materials and lubricants, their properties, characteristics, test methods and testing conditions.
This third edition cancels and replaces the second edition (ISO 6691:2000), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— references in 4.5, Data Block 4 for position 3 and 4, have been made precise;
— Annexes A and B have been revised.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 6691:2020(E)
Thermoplastic polymers for plain bearings —
Classification and designation
1 Scope
This document specifies a classification and designation system for a selection of the most common
unfilled thermoplastic polymers for plain bearings.
The unfilled thermoplastic polymers are classified on the basis of appropriate levels of distinctive
properties, additives and information about their application for plain bearings. The designation system
does not include all properties; thermoplastic polymers having the same designation cannot therefore
be interchanged in all cases.
It also provides an outline of the properties and applications of the most common unfilled thermoplastic
polymers as well as listing some of the fundamental parameters that influence the selection of
thermoplastic polymers for use for plain bearings.
NOTE In the further course of the work, it is intended to prepare standards on “thermosetting polymers”
and “mixed polymers” for plain bearings.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 307, Plastics — Polyamides — Determination of viscosity number
ISO 527­1, Plastics — Determination of tensile properties — Part 1: General principles
ISO 527­2, Plastics — Determination of tensile properties — Part 2: Test conditions for moulding and
extrusion plastics
ISO 527­3, Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets
ISO 527­4, Plastics — Determination of tensile properties — Part 4: Test conditions for isotropic and
orthotropic fibre-reinforced plastic composites
ISO 527­5, Plastics — Determination of tensile properties — Part 5: Test conditions for unidirectional fibre-
reinforced plastic composites
ISO 1043­1, Plastics — Symbols and abbreviated terms — Part 1: Basic polymers and their special
characteristics
ISO 1133­1, Plastics — Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR)
of thermoplastics — Part 1: Standard method
ISO 1183­1, Plastics — Methods for determining the density of non-cellular plastics — Part 1: Immersion
method, liquid pycnometer method and titration method
ISO 1183­2, Plastics — Methods for determining the density of non-cellular plastics — Part 2: Density
gradient column method
ISO 1183­3, Plastics — Methods for determining the density of non-cellular plastics — Part 3: Gas
pyknometer method
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ISO/FDIS 6691:2020(E)

ISO 1628­5, Plastics — Determination of the viscosity of polymers in dilute solution using capillary
viscometers — Part 5: Thermoplastic polyester (TP) homopolymers and copolymers
ISO 16396­1, Plastics — Polyamide (PA) moulding and extrusion materials — Part 1: Designation system,
marking of products and basis for specifications
ISO 16396­2, Plastics — Polyamide (PA) moulding and extrusion materials — Part 2: Preparation of test
specimens and determination of properties
ISO 17855­1, Plastics — Polyethylene (PE) moulding and extrusion materials — Part 1: Designation system
and basis for specifications
ISO 17855­2, Plastics — Polyethylene (PE) moulding and extrusion materials — Part 2: Preparation of test
specimens and determination of properties
ISO 20028­1, Plastics — Thermoplastic polyester (TP) moulding and extrusion materials — Part 1:
Designation system and basis for specification
3  Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4  Classification and designation system
4.1 General
The classification and designation are based on a block system consisting of a “description block” and
“Identity block” (see Table 1). The “Identity block” comprises an “International Standard number block”
and an “individual item block”. For unambiguous coding of all thermoplastic polymers, the “individual
item block” is subdivided into four data blocks.
Table 1 — Classification and designation
Designation
Identity block
Individual item block
Description
International
block
Standard num- Data block Data block Data block Data block
ber block
1 2 3 4
The “individual item block” starts with a dash. The data blocks are separated by commas.
Data blocks 1 to 4 include the following information:
— data block 1: material symbol (see 4.2, Table 2);
— data block 2: intended application or method of processing (see 4.3);
— data block 3: distinctive properties (see 4.4);
— data block 4: type and content of fillers or reinforcing materials (see 4.5);
The meaning of the letters and digits is different for each data block (see 4.2 to 4.5).
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ISO/FDIS 6691:2020(E)

Data block 2 comprises up to 4 positions. If at least one of positions 2 to 4 is used, but no information is
given in position 1, then the letter “X” shall be placed in position 1. The letters in positions 2 to 4 shall
be arranged in alphabetical order.
If a data block is not used, this shall be indicated by consecutive data block separators, i.e. two
commas („).
Designation examples are given in Clause 5.
4.2 Data block 1
The chemical nature of the thermoplastic polymer is designated by its symbol in accordance with
ISO 1043­1.
Table 2 — Symbols for the chemical structure of the materials
Thermoplastic polymers
Name and chemical structure
Group/Name Symbol
Polyamide PA 6 Polyamide 6; homopolymer based on ε-caprolactam
PA 6 cast Polyamide 6, cast; homopolymer based on ε­caprolactam
PA 66 Polyamide 66; homopolycondensate based on hexamethylenediamine
and adipic acid
PA 12 Polyamide 12; homopolymer based on ω‑laurinlactam or ω­aminodo­
decanoic acid
PA 12 cast Polyamide 12, cast; homopolymer based on ω­laurinlactam or ω­ami­
nododecanoic acid
PA 46 Polyamide 46; a co‑condensate based on 1,4‑diaminobutane and
adipic acid
Polyoxymethylene POM Polyacetal (homopolymer), Polyacetal (copolymer)
Polyalkyleneterephthalate PET Polyethylene terephthalate
PBT Polybutylene terephthalate
Polyethylene PE­UHMW Polyethylene with ultra high molecular weight
PE­HD High density polyethylene
Polyfluorocarbon PTFE Polytetrafluoroethylene
Polyimide PI Polyimides from polyaddition reactions are available as thermo­
setting plastics. Polyimides from polycondensation reactions are
available as thermoplastics and thermosetting plastics, as well as
copolymers of the imide group. Some thermoplastic polyimides are
“apparent thermosetting plastics” because their thermoplastic range
lies above the decomposition temperature. Because of their inter­
mediate position, polyimides and imide copolymers are only treated
marginally in this document.
Polyaryletherketone PEEK Polyetheretherketone
Polyvinylidene fluoride PVDF Homopolymer based on vinylidene difluoride
Polyphenylene sulfide PPS Polyphenylene sulfide, linearly structured phenyl ring and sulfur
atoms (tribologically modified material)
Poly(amide‑imide) PAI Poly(amide‑imide) reacted by polycondensation is a hard/tough,
amorphous thermoplastic. After postcuring the PAI parts cannot be
used for re‑processing (“pseudo‑thermoset plastics”).
4.3 Data block 2
Position 1 gives the code for the intended application (see Table 3).
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Table 3 — Data block 2 — Position 1
Code Intended application
E Extrusion
G General use
M Injection moulding
Q Compression moulding
R Rotational moulding
X No indication
Up to three important properties and/or additives can be indicated in positions 2 to 4 (see Table 4).
Table 4 — Data block 2 — Positions 2 to 4
Code Important properties and/or
additives
A Processing stabilizer
F Special burning characteristics
H Heat ageing stabilizer
L Light and weather stabilizer
R Release agent
S Slip agent, lubricant
4.4 Data block 3
4.4.1 General
The levels of distinctive properties are coded by letters and numbers.
The properties used for the designation are different for every thermoplastic polymer.
Owing to manufacturing tolerances, single property values can lie on, or to either side of, two intervals.
lt is up to the manufacturer to state which interval will designate the thermoplastic polymer.
4.4.2  Polyamides
Polyamides are designated in data block 3 by their viscosity number, represented by two digits
(see Table 5) in accordance with ISO 16396‑1 and, separated by a dash, their modulus of elasticity
represented by three digits (see Table 6).
In the last position, rapid‑setting products may be indicated with the letter N.
The viscosity number shall be determined in accordance with ISO 307 using the solvents given in
Table 5. The modulus of elasticity shall be determined in the dry state in accordance with ISO 527‑1,
ISO 527‑2, ISO 527‑3, ISO 527‑4 and ISO 527‑5, under the conditions specified in ISO 16396‑2.
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ISO/FDIS 6691:2020(E)

Table 5 — Viscosity number for polyamides
Polyamide Code Viscosity number, ml/g
Solvent
Sulfuric acid 96 % (mass fraction) m­cresol
> ≤ > ≤
09 — 90
10 90 110
12 110 130
PA 6
14 130 160
PA 6 cast 18 160 200 —
22 200 240
PA 66
27 240 290
32 290 340
34 340 —
11 — 110
12 110 130
14 130 150
PA 12
16 — 150 170
PA 12 cast
18 170 200
22 200 240
24 240 –
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ISO/FDIS 6691:2020(E)

Table 6 — Modulus of elasticity
Code Modulus of elasticity
2
N/mm
> ≤
001 50 150
002 150 250
003 250 350
004 350 450
005 450 600
007 600 800
010 800 1 500
020 1 500 2 500
030 2 500 3 500
040 3 500 4 500
050 4 500 5 500
060 5 500 6 500
070 6 500 7 500
080 7 500 8 500
090 8 500 9 500
100 9 500 10 500
110 10 500 11 500
120 11 500 13 000
140 13 000 15 000
160 15 000 17 000
190 17 000 20 000
220 20 000 23 000
250 23 000 —
4.4.3 Polyethylenes
Polyethylenes are designated by their density represented by two digits (see Table 7) in accordance
with ISO 17855‑1 and, separated by a dash, their melt flow rate (MFR) represented by one letter and
three digits (see Table 8).
The density of the base material shall be determined in accordance with ISO 1183‑1, ISO 1183‑2 and
ISO 1183‑3 under the conditions specified in ISO 17855‑2.
The melt mass‑flow rate shall be determined in accordance with ISO 1133‑1 at 190 °C with a load of
2,16 kg (symbol D). For thermoplastic polymers with a melt mass‑flow rate <0,1 g/10 min, a test under a
load of 5 kg (symbol T) is recommended. If the melt mass‑flow rate is still <0,1 g/10 min, the test should
then be carried out under a load of 21,6 kg (symbol G).
The symbols D, T and G shall precede the code for melt flow rate given in Table 8.
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ISO/FDIS 6691:2020(E)

Table 7 — Density
a
Code Density
3
g/cm
> ≤
15 — 0,917
20 0,917 0,922
25 0,922 0,927
30 0,927 0,932
35 0,932 0,937
40 0,937 0,942
45 0,942 0,947
50 0,947 0,952
55 0,952 0,957
60 0,957 0,962
65 0,962 —
a
Density ranges for uncoloured and unfilled polyethylene materials.
Table 8 — Melt mass-flow rate (MFR)
Code Melt flow rate
g/10 min
> ≤
000 — 0,1
001 0,1 0,2
003 0,2 0,4
006 0,4 0,8
012 0,8 1,5
022 1,5 3
045 3 6
090 6 12
200 12 25
400 25 50
700 50 100
4.4.4 Polyalkyleneterephthalates
The distinctive property of polyalkyleneterephthalates is the viscosity number according to
ISO 20028‑1, determined in accordance with ISO 1628‑5, and designated by two digits (see Table 9).
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Table 9 — Viscosity number for polyalkyleneterephthalate
Polyalky- Code Viscosity number
lene-terephtha-
ml/g
late
> ≤
06 — 60
07 60 70
08 70 80
09 80 90
PET
10 90 100
11 100 120
13 120 140
15 140 —
08 — 90
10 90 110
12 110 130
PBT
14 130 150
16 150 170
18 170 —
4.4.5 Other polymers
The coding for the distinctive properties of polyoxymethylene, polytetrafluoroethylene, poly(amide‑
imide), polyimide, Polyetheretherketone, Polyvinylidene fluoride and Polyphenylene sulfide will be
included in a future edition of this document.
4.5 Data block 4
The fillers and reinforcing materials, as well as additives specific for the application in plain bearings,
are coded as follows:
— position 1: types of fillers and/or reinforcing materials, coded by a letter (see Table 10);
— position 2: physical forms of fillers and/or reinforcing materials, coded by a letter (see Table 11);
— positions 3: mass content of fillers and/or reinforcing materials, coded by two digits (see Table 12);
— positions 4: Types of lubricants, coded by two letters (see Table 13).
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Table 10 — Types of fillers and/or reinforcing materials (position 1)
Code Type
C Carbon
G Glass
K Chalk
S Synthetic organic material
T Talcum
X No indication
Table 11 — Physical forms of fillers and/or reinforcing materials (position 2)
Code Physical forms
D Powder
F Fibre
S Spheres
X No indication
Table 12 — Mass content (position 3)
Code Mass percentage
> ≤
OX No indication
01 0,1 (inclusive) 1,5
02 1,5 3
05 3 7,5
10 7,5 12,5
15 12,5 17,5
20 17,5 22,5
25 22,5 27,5
30 27,5 32,5
35 32,5 37,5
40 37,5 42,5
45 42,5 47,5
50 47,5 55
60 55 65
70 65 75
80 75 85
90 85 —
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ISO/FDIS 6691:2020(E)

Table 13 — Types of lubricants (position 4)
Code Type
GR Graphite
MO MoS (Molybdenum disulfide)
2
OL Mineral oil
PE Polyethylene
TF PTFE (Polytetrafluoroethylene)
X No indication
5 Designation examples
A summary of the designation system is given in Table 14.
Table 14 — Summary of the designation system
Description block Thermoplastic
International Standard
ISO 6691
number block
Reference
Data block Position Content
Subclause Table
1 — Material symbol 4.2 Table 2
Intended application or method
1 4.3 Table 3
of processing
2
Important properties and/or
2 to 4 4.3 Table 4
additives
Tables 5
Individual item block 3 — Distinctive properties 4.4
to 9
1 Types of reinforcing materials 4.5 Table 10
Physical forms of reinforcing
2 4.5 Table 11
materials
4
Mass content of reinforcing
3 4.5 Table 12
materials
4 Types of lubricants 4.5 Table 13

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ISO/FDIS 6691:2020(E)

EXAMPLE 1 A polyamide 6 (PA 6), for injection moulding (M) with mould release agent (R), having a viscosity
2
number of 140 ml/g (14), a modulus of elasticity of 2 600 N/mm (030) and rapid setting (N) is designated as
follows:
Thermoplastic ISO 6691 ­PA 6, M R 14 ­030 N

Description


International Standard


Data block 1: material symbol


Data block 2: Position 1: for injection moulding


Position 2: for mould release agent


Data block 3: Position 1: viscosity number


Position 2: modulus of elasticity


Position 3: rapid setting

Designation:  Thermoplastic ISO 6691-PA 6, MR, 14-030N
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EXAMPLE 2 A polyamide 66 (PA 66), without indications as to use additives in data block 2, having a viscosity
2
number of 280 ml/g (27), a modulus of elasticity of 4 000 N/mm (040), rapid setting (N) and 20 % (mass fraction)
glass fibre (GF20) is designated as follows:
Thermoplastic ISO 6691 ­PA 66, ,, 27 ­040 N, G F 20

Description


International Standard


Data block 1: material symbol


Data block 2: no indication, therefore two commas only


Data block 3: Position 1: viscosity number


Data block 3: Position 2: modulus of elasticity


Position 3: rapid setting


Data block 4: Position 1: for glass


Data block 4: Position 2: for fibres


Data block 4: Position 3: for mass content

Designation:  Thermoplastic ISO 6691-PA 66, „ 27-040N, GF20GR
6  Ordering information
The purchaser and supplier shall agree on the tests to be carried out.
If mechanical and/or tribological properties are to be tested, it shall be agreed whether such tests are
carried out on
a) unmodified parts of the delivered goods,
b) test bars, manufactured from the same batch, or
c) test bars taken from a finished part to be supplied,
and whether the test shall be carried out parallel or perpendicular to the flow direction and/or
machining direction.
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Annex A
(informative)

Properties and applications of the most common unfilled
thermoplastic polymers
A.1 Properties and applications of unfilled thermoplastic polymers
Table A.1 gives an outline of the properties and applications of unfilled thermoplastic polymers most
commonly used for plain bearings.
Table A.1 — Outline
Group of
thermoplastic
General description Chemical properties Examples of application
polymers
(symbol)
Polyamide (PA) Resistant material, extraordi­ Resistant to fuels, oils, and Bearings subjected to shock
narily shock‑ and wear‑resist­ greases and to most common and vibration.
ant, good damping properties. solvents. Sensitive to mineral
Guide blocks in steel mill
acid even in dilute solution,
High sliding resistance in couplings.
but not attacked by strong
dry running. Relatively high
alkalis even at high concen­ Bushes for brake rods in
moisture absorption.
trations. The use of PA 6 and wagon construction.
PA 66 in hot water requires
Bearings for agricultur­
formulations that are stabi­
al machinery. Spring eye
lized against hydrolysis. PA 11
bushes.
and PA 12 are widely resistant
to hydrolysis.
Polyoxymethylene Hard material; therefore High resistance to numerous Plain bearings having strict
(POM) higher resistance to pres­ chemicals, above all to organic requirements concern­
sure than polyamide, but liquids. Only a few solvents ing dimensional stability
more sensitive to shock. Less can dissolve POM. Even at and coefficient of friction.
wear­resistant but smaller high temperatures POM­co­ Good for dry running or for
coefficient of friction than polymer withstands strong deficient lubrication. Plain
polyamide. Very low moisture alkaline solutions such as bearings for fine mechan­
absorption. 50 % NaOH. Chemicals having ics, electromechanics and
an oxidizing effect and strong household appliances.
acids (pH < 4) attack POM.
Polyethylenetere­ Hardness similar to that of Good weather resistance and Application for plain
phthalate (PET) POM; however, decreases high resistance to numerous bearings similar to POM.
considerably above 70 °C. Up solvents, oils, greases, and salt Mostly for plain bearings at
Polybutylenetere­
to 70 °C, wear and coefficient solutions. Sufficiently resist­ temperatures below 70 °C.
phthalate (PBT)
of friction very low. Low ant to many acids and alkalis Good for dry running and
moisture absorption. in aqueous solution. Attacked for deficient lubrication.
by concentrated inorganic Plain bearings for fine
acids and alkalis. Halogen­ mechanics and submerged
ated hydrocarbons such as installations, guide bushes
methylene chloride and chlo­ for rods. Plain bearings for
roform lead to high swelling. oscillating movements.
Sensitive to hydrolysis at high
temperatures.
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ISO/FDIS 6691:2020(E)

Table A.1 (continued)
Group of
thermoplastic
General description Chemical properties Examples of application
polymers
(symbol)
Polyethyl­ PE­UHMW has high shock At room temperature, PE is Plain bearings for Installa­
ene with ultra resistance. PE­HD has low inert to water, alkaline solu­ tion in waters carrying sand.
high molecular resistance to permanent tions, salt solutions, and inor­
Road and agricultural ma­
weight (PE­UHMW) pressure. However, it is re­ ganic acids (except strongly
chinery construction.
sistant to shock. About twice oxidizing acids). At room tem­
High density poly­
the thermal expansion of PA perature, polar liquids such as
Bearings for low tempera­
ethylene (PE‑HD)
and POM. alcohols, organic acids, esters,
tures.
ketones, and the like only re­
Excellent we
...