SIST ISO 6691:2022

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 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
ii
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
iii
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.
iv
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
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).
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” bec
...

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 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
ii
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
iii
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.
iv
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
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).
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” bec
...