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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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Other fluorinated elastomers are perfluoro-elastomers
(FFKM) and tetrafluoro ethylene/propylene rubbers (FEPM). All
FKMs contain vinylidene fluoride as a monomer.
On the basis of their chemical composition FKMs can be
divided into the following types:
Type 1. FKMs are composed of vinylidene fluoride (VDF)
and hexafluoropropylene (HFP). Copolymers are the standard type of
FKMs showing a good overall performance. Their fluorine content
typically ranges around 66 weight percent.
Type 2. FKMs are composed of VDF, HFP, and
tetrafluoroethylene (TFE). Terpolymers have a higher fluorine
content compared to copolymers (typically between 68 and 69
weight percent fluorine), which results in better chemical and heat
resistance. Compression set and low temperature flexibility may be
affected negatively.
Type 3. FKMs are composed of VDF, HFP, TFE,
perfluoromethylvinylether (PMVE). The addition of PMVE provides
better low temperature flexibility compared to copolymers and
terpolymers. Typically the fluorine content of type 3 FKMs ranges
from 62 to 68 weight percent.
Type 4. FKMs are composed of propylene, TFE, and VDF.
While base resistance is increased in type 4 FKMs, their swelling
properties especially in hydrocarbons are worsened. Typically they
have a fluorine content of about 67 weight percent.
Type 5. FKMs are composed of VDF, HFP, TFE, PMVE, and
Ethylene. Type 5 FKM is known for base resistance and high
temperature hydrogen sulfide resistance.
[1]
Crosslinking mechanisms
There are three established crosslinking mechanisms used in
the curing process of FKMs.
Diamine crosslinking using a blocked diamine. In the
presence of basic media VDF is vulnerable to dehydrofluorination
which enables the addition of the diamine to the polymer chain.
Typically magnesium oxide is used to catch the resulting
141
hydrofluoric acid and rearrange into magnesium fluoride and water.
Although rarely used today, diamine curing provides superior rubberto-metal bonding properties as compared with other crosslinking
mechanisms. The diamine's capability to be hydrated makes the
diamine crosslink vulnerable in aqueous media.
Ionic crosslinking (dihydroxy crosslinking) was the next step
in curing FKMs. This is the most common crosslinking chemistry
used for FKMs. It provides superior heat resistance, improved
hydrolytic stability and better compression set than diamine curing.
In contrast to diamine curing the ionic mechanism is not an addition
mechanism but an aromatic nucleophilic substitution. Dihydroxy
aromatic compounds are used as the crosslinking agent and
quaternary phosphonium salts are typically used to accelerate the
curing process.
Peroxide crosslinking was originally developed for type 3
FKMs containing PMVE as diamine and bisphenolic crosslinking
systems can lead to cleavage in a polymer backbone containing
PMVE. While diamine and bisphenolic crosslinking are ionic
reactions, peroxide crosslinking is a free radical mechanism. Though
peroxide crosslinks are not as thermally stable as bisphenolic
crosslinks, they normally are the system of choice in aqueous and
nonaqueous electrolytes.
Properties
The performance of fluoroelastomers in aggressive chemicals
depends on the nature of the base polymer and the compounding
ingredients used for moulding the final products (e.g. O-rings, shaft
seals). This performance can vary significantly when end-users
purchase polymer containing rubber goods from different sources.
Fluoroelastomers are generally compatible with hydrocarbons, but
incompatible with ketones such as acetone and organic acids such as
acetic acid.
FFKMs have excellent resistance to high temperatures
[2]
and
chemicals. Certain grades have a maximum continuous service
temperature of 327°C. They are commonly used to make O-rings and
142
gaskets that are used in applications that involve contact with
hydrocarbons or highly corrosive fluids, or when a wide range of
temperatures are encountered.
Main trade marks & Application
•
Dipolymers of VF2/HFP, like Viton A and Tecnoflon
[1]
General purpose sealing. Automotive, Aerospace fuels & lubricants
•
Terpolymers of VF2/HFP/TFE, like Viton B and Tecnoflon:
Chemical Process plant, Power Utility Seals & Gaskets
•
Terpolymers of VF2/HFP/TFE, like Viton F and Tecnoflon:
Oxygenated Automotive fuels. Concentrated aqueous inorganic
acids, water, steam.
Peroxide curable polymers are similar to terpolymers. They
offer better polymers are typically peroxide curable with an
additional monomer of PMVE.
3.19 Polysulphide Rubber
S S
S S
Polysulphide rubber (OT, EOT) is a class of chemical compounds
containing chains of sulfur atoms. There are two main classes of
polysulfides: anions and organic polysulfides. Anions have the
general formula S
2−
. These anions are the conjugate bases of the
n
hydrogen polysulfides H2Sn. Organic polysulfides generally have the
formulae RSnR, where R = alkyl or aryl.
Polysulfide salts
The alkali metal polysulfides arise by treatment of a solution
of sulfide, e.g. sodium sulfide, with elemental sulfur:
S2− + n S → S2−n+1
Alkali metals other than Na+ can be used. In some cases,
these anions have been obtained as organic salts, which are soluble in
organic solvents.
:
143
The energy released in the reaction of sodium and elemental
sulfur is the basis of battery technology. The sodium–sulfur battery
and the lithium–sulfur battery require high temperatures to maintain
liquid polysulfide and Na+-conductive membranes that are unreactive
toward sodium, sulfur, and sodium sulfide.
Polysulfides are common ligands in coordination chemistry.
Example of transition metal polysulfide complexes include
(C5H5)2TiS5, [Ni(S4)2]2−, and [Pt(S5)3]2−. Main group elements also
form polysulfides.
Organic polysulfides
In commerce, the term "polysulfide" usually refers to a class
of polymers with alternating chains of several sulfur atoms and
hydrocarbons. The general formula for the repeat unit is –[(CH2)m–
Sx]n–, where x indicates the number of sulfur atoms (or rank), and n
indicates the number of repeating units. Polymers containing sulfur
atoms separated by hydrocarbon sequences are usually not classified
polysulfides, e.g. polyphenylene sulfide (C6H4S)n.
Polysulfide polymers can be synthesized by condensation
polymerization reactions between organic dihalides and alkali metal
salts of polysulfide anions:
n Na2S5 + n ClCH2CH2Cl → [CH2CH2S5]n + 2n NaCl
Dihalides used in this condensation polymerization are
dichloroalkanes (such as 1,2-dichloroethane, bis-(2chloroethyl)formal (ClCH2CH2OCH2OCH2CH2Cl), and 1,3dichloropropane). In some cases, polysulfide polymers can be
formed by ring-opening polymerization reactions. The polymers are
called thiokols.
Polysulfides in vulcanized rubber
Many commercial elastomers contain polysulfides as
crosslinks. These crosslinks interconnect neighboring polymer
chains, thereby conferring rigidity. The degree of rigidity is related to
the number of crosslinks. The process of crosslinking the polymer
chains in these polymers with sulfur is called vulcanization. The
sulfur chains attach themselves to the "allylic" carbon atoms, which
144
are adjacent to C=C linkages. Vulcanization is a step in the
processing of several classes of rubbers, including polychloroprene,
styrene-butadiene, and polyisoprene, which is chemically identical to
natural rubber.
Properties
Polysulfides, as sulfides, can induce stress corrosion cracking
in carbon steel and stainless steel. Polysulfide polymers are insoluble
in water, oils, and many other organic solvents.
Applications
Because of their solvent resistance, these materials find use as
sealants to fill the joints in pavement, automotive window glass, and
aircraft structures.
3.20 Silicone Rubber
Silicone rubbers (MQ, VMQ, FVMQ, PVMQ) are widely used in
industry, and there are multiple formulations. Silicone rubbers are
often one- or two-part polymers, and may contain fillers to improve
properties or reduce cost.
History
The first silicone elastomers were developed in the search for
better insulating materials for electric motors and generators. Resinimpregnated glass fibers were the state-of-the-art materials at the
time. The glass was very heat resistant, but the phenolic resins would
not withstand the higher temperatures that were being encountered in
new smaller electric motors. Chemists at Corning Glass and General
Electric were investigating heat-resistant materials for use as
resinous binders when they synthesized the first silicone polymers,
demonstrated that they worked well and found a route to produce
polydimethylsiloxane commercially.
This new class of materials was produced in 1943. As the
unique properties of the new silicone products were studied in more
detail, their potential for broader usage was envisioned, and GE
opened its own plant to produce silicones in 1947. Wacker Chemie
145
also started production of silicones in Europe in 1947. The Japanese
C
H
C
H
+
+
company Shin-Etsu Chemical began mass production of silicone in
1953. The companies mentioned above are now still the main
competitors in the oligopoly that comprises the silicone industry.
Chemistry
When receiving silicone rubber by cation initiation there are
used strong protonic acid. Major industrial monomer is
octamethylcyclotetrasiloxane (D4).
Organocyclosiloxanes polymerization mechanism in the
presence of sulfuric acid can be represented by the following scheme.
1) Initiation:
CH
SiCH
3
O
Si
CH
3
CH
3
O Si
3
O
SiO
CH
3
CH
CH
3
CH
CH
CH
CH
CH
3
3
+
Si
HSO
CH
3
4
3
3
3
+
H
+
HSO
3
4
O Si
CH
OH Si
3
CH
3
O Si O
3
2) Chain growth:
CH
3
3
O Si
SiCH
O Si
CH
CH
3
OH Si
3
CH
CH
CH
3
O Si O
CH
3
CH
3
3
+
Si
HSO
CH
3
4
3
3
n
+
CH
3
O
CH
CH
3
O
SiOSi
CH
3
CH
3
3
O Si
CH
CH
3
OH Si
3
4
CH
CH
n
3
O Si
3
CH
CH
CH
3
O Si O
CH
3
CH
3
3
+
HSO
Si
CH
3
3
3) Chain transfer reaction. Main reaction of chain transfer
during cationic initiation due to the formation of sulfate bridges:
CH
CH
CH
CH
CH
O Si
CH
CH
3
OH Si
3
CH
CH
4
n
O Si
3
O Si
3
CH
CH
CH
CH
3
OH Si
3
CH
3
O Si O
CH
3
CH
CH
4
n
3
3
3
O Si
3
CH
Si
CH
CH
CH
3
+
HSO
3
CH
3
O Si O
CH
3
3
3
CH
4
CH
CH
4
3
O Si
O Si O
CH
CH
3
3
n
CH
CH
3
3
O Si
O Si O
CH
CH
3
3
n
O Si
O Si
CH
CH
CH
OH Si
3
3
OH Si
3
H2SO
4
4
CH
3
3
+
Si
HSO
+
4
CH
3
3
O
3
3
Si
O S
CH
O
3
3
O
CH
3
3
Si
O S
CH
O
3
3
+
HSO
H
H
O
H
O
4
4
O Si
CH
CH
3
OH Si
3
CH
CH
O
CH
CH
3
O Si O
CH
3
CH
3
Si
CH
3
3
O Si
CH
3
4
n
3
O S
3
CH
Si
O
CH
O
CH
CH
3
CH
3
CH
CH
CH
3
3
O HSi
CH
3
3
+
H
+
OSi
4
n
HSO
4
3
3
OSiO
OSi
CH
3
3
146
While interacting the polymer and water formed sulfo ether
C
H
groups and terminal sulfate groups are converted into hydroxyl
groups:
O
CH
CH
CH
O Si
1.
CH
3
OH Si
3
CH
4
CH
CH
3
O Si O
CH
3
CH
3
3
Si
O S
CH
O
3
3
3
O Si
CH
3
n
CH
3
Si
O
CH
CH
3
CH
CH
3
OSiO
OSi
CH
3
CH
3
3
CH
3
3
+
HOH
OSi
O HSi
CH
3
3
4
n
CH
CH
CH
CH
3
O Si O
CH
3
CH
3
OSi
CH
3
CH
3
3
Si
O H
CH
3
3
CH
3
3
OSi
O HSi
CH
3
3
4
3
3
O Si
OH Si
O Si
CH
CH
CH
3
3
n
4
O
CH
Si
O S
2.
OH
CH
O
CH
CH
3
3
OSiO
CH
CH
3
3
O
CH
Si
O S
+
OH
CH
O
+
HOH
n
CH
CH
3
SiO
CH
3
CH
CH
3
3
O
3
OH
3
CH
SiO
3
OSi
O HSi
CH
3
3
4
n
CH
CH
3
OSi
O
CH
3
CH
3
3
CH
3
3
H2SO
+
OSi
O HSi
CH
3
3
4
n
OSi
CH
3
CH
CH
3
Si
CH
CH
3
The catalysts of organo cyclosiloxanes polymerization are
strong bases.
The mechanism of octamethylcyclotetrasiloxane anionic
polymerization can be represented as the following scheme:
1) Initiation:
CH
SiCH
3
O
Si
CH
3
CH
3
O Si
3
O
SiO
CH
3
CH
CH
3
CH
CH
3
+
+
OH
K
3
OH Si
CH
CH
3
3
Si
O
O
CH
3
3
Si
CH
3
O Si
3
CH
CH
3
+
O
K
3
2) Chain growth:
CH
OH Si
CH
CH
CH
CH
CH
CH
3
3
3
O
3
Si
CH
Si
O
CH
3
O Si
3
CH
3
+
+
O
K
3
SiCH
3
O
n
CH
3
CH
3
O Si
3
O
SiOSi
CH
3
CH
CH
3
3
3
CH
3
+
O
CH
K
3
n
+1
4
O Si
H
3) Chain transfer reaction. The main reason for chain
termination is the interaction of potassium-siloxanolate groups and
water:
4
CH
3
O Si
H
CH
3
+
O K
n
+1
4
HOH
++
CH
O Si
CH
3
OHH
4
3
+
K
OH
+
+1
n
147
Therefore, in the industry to produce low molecular weight
(liquid) silicone rubbers highly concentrated aqueous alkaline
solutions are used, and to obtain high molecular silicone rubbers the
reaction product of dry alkali D4 (potassium oligosiloksanolate) is
used as a catalyst.
Polymerization using bases has several advantages over the
polymerization by means of acidic catalysts. It allows obtaining
high-molecular silicone rubbers (with a molecular weight of up to
106 without maturing) and liquid rubbers in a short time with the
catalyst concentration 10-2-10-4% (by weight). It is able to regulate
the molecular weight of the synthesized rubber. In contrast to the use
of an acid catalyst using alkaline catalysis does not require
neutralization of the terminal active sites by water washing. During
alkaline catalysis neutralization of active sites is carried out by their
stabilization with Aerosil or phosphoric acid.
Liquid silicone rubber is a high purity platinum-cure silicone.
It is typically supplied in two parts with one of the parts containing
the platinum catalyst. These are then automatically mixed with any
colours and ingredients which may be required. The mixing produces
a very homogeneous material that leads to products that are not only
very consistent throughout the part, but also from part to part.
Properties
Silicone rubber offers good resistance to extreme
temperatures, being able to operate normally from −55°C to +300°C.
At the extreme temperatures, the tensile strength, elongation, tear
strength and compression set can be far superior to conventional
rubbers although still low relative to other materials. Organic rubber
has a carbon to carbon backbone which can leave them susceptible to
ozone, UV, heat and other ageing factors that silicone rubber can
withstand well. This makes it one of the elastomers of choice in
many extreme environments.
Compared to organic rubbers, however, silicone rubber has a
very low tensile strength. For this reason, care is needed in designing
products to withstand even low imposed loads. The material is also
148
very sensitive to fatigue
from cyclic loading. Silicone rubber is a
highly inert material and stable and does not react with most
chemicals. Due to its inertness, it is used in many medical
silicone rubber chain
differ from other polymers in that their
er polymers that
contain carbon backbones. Polysiloxane is very flexible due to large
bond angles and bond lengths when compared to those found in more
basic polymers such as polyethylene. For example, a C
bond angle of 112°, whereas
O has a bond length of 1.63 Å and a
of silicone rubber
The siloxane backbone differs greatly from the basic
ielding a much more flexible polymer.
Because the bond lengths are longer, they can move farther and
change conformation easily, making for a flexible material.
Polysiloxanes also tend to be chemically inert, due to the strength of
congener
silicon analogues of carbonaceous compounds generally exhibit
different properties, due to the differences in electronic structure and
between the two elements; the silicon
applications and inmedical implants.
Structure
Polysiloxanes
backbones consist of Si-O-Si units unlike many oth
unit has a bond length of 1.54 Å and a
the siloxane backbone unit Sibond angle of 130°.
Si
O
Si
n
repeat unit
O
Si
polyethylene backbone, y
the silicon-oxygen bond. Despite silicon being a
-C backbone
of carbon,
electronegativity
149
-oxygen bond
in polysiloxanes is significantly more stable than the carbon-oxygen
bond in polyoxymethylene (a structurally similar polymer) due to its
higher bond energy.
Mechanical properties
Hardness, shore A 10–90
Tensile strength 11 N/mm²
Elongation at break 100–1100%
Maximum temperature +300 °C
Minimum temperature −120 °C
Applications
Due to its properties and its ease of manufacturing and
shaping, silicone rubber can be found in a wide variety of products,
including: automotive applications; cooking, baking, and food
storage products; apparel such as undergarments, sportswear, and
footwear; electronics; medical devices and implants; and in home
repair and hardware with products such as silicone sealants.
During manufacture, heat may be required to vulcanize the
silicone into its rubber-like form. This is normally carried out in a
two stage process at the point of manufacture into the desired shape,
and then in a prolonged post-cure process. It can also be injection
molded.
Once mixed and coloured, silicone rubber can be extruded
into tubes, strips, solid cord or custom profiles according the size
restrictions of the manufacturer. Cord can be joined to make O-rings
and extruded profiles can be joined to make seals. Silicone rubber
can be moulded into custom shapes and designs. Manufacturers work
to set industry tolerances when extruding, cutting or joining silicone
rubber profiles. In the UK this is BS3734, for extrusions the tightest
level is E1 and the widest is E3.
Becoming more and more common at the consumer level,
silicone rubber products can be found in every room of a typical
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