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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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Table 12.
Characteristics of Ethylene-Vinyl Acetate Copolymer
Vinyl acetate content, % by weight
Characteristics
5-7 9-14 17-22 24-30
Density at 20°С, g/cm3 0,930 0,934 0,944 0,950
Melt index 1-5 2-10 25-70 100-300
Brittle temperature, °С -100 -100 -50 -
Tensile strength, kgf/cm2 150-120 140-100 120-800 50-40
Tensile strain, % 800-700 800-600 800-700 600-500
Vicat softening point, °С 96,5 85 50 30
Shore hardness 98 90 80 76
Dielectric loss tangent at 1
MHz
Inductive capacity at 1 MHz
Electrical strength, kW/mm
1,5·10
-2
2,5·10
-2
3·10-2 5·10-2
2,4 2,6 2,8 3,0
35 34 34 33
Production
Copolymerization is carried out by a radical mechanism at
high pressure or using coordination-anionic catalysts at medium and
low pressure. For ethylene to polymers and monomers to
polyethylene grafting there are usually used radical initiators.
In industry the most common EVA types are: polyethylene,
modified by small amount you comonomers (vinyl acetate, α-olefins,
etc.); polypropylene, modified by small number of ethylene; EPDM;
block-graft copolymers of ethylene.
EVA containing 3-30% by weight of vinyl acetate is usually
synthesized using radical copolymerization at high pressure. When
including 5% vinyl acetate, a product for making films with
improved optical properties (turbidity index is 2-3% compared with
6.5% for the conventional polyethylene) and high flexibility is
received, including 25-30% results in a film with good adhesion to
131
paper and cardboard, and adhesives used in the printing, shoe and
furniture industry.
EVA containing 0.2-3% by weight of propylene or higher
olefins (1-butene, 1-hexene, 4-methylpentene) are obtained by
catalytic polymerization (mainly in presence of Ziegler-Natta
Catalyst) at the middle and low pressure in the gas or liquid phase.
These materials with increased strength to shock and crack
resistance, are used for the manufacture of pipes, containers, cans,
bottles, boxes. The most interesting is statistical EVA with 2-5% of
the olefins having uniform branched macromolecules - so-called
"linear" low density polyethylene (LDLPE).
Ethylene grafting to propylene macromolecule is carried out
for impact- and frost resistant marks of the latter. In modern methods
of polypropylene synthesis an additional reactor is usually provided,
where in the gas phase the polypropylene is copolymerized with a
mixture of ethylene and propylene; meanwhile the total ethylene
content can reach 25 or even 50% by weight. Practically, it is a
mixture of polypropylene, ethylene-propylene rubber and the block
copolymers of ethylene-propylene rubber with polypropylene.
Application
Packing. Containers (for frozen products, etc.). Covers.
Flexible toys. Flexible medical supplies. Soles and shoe parts.
Flexible fittings. Seals. Gaskets. Antivibration pads.
Trade names
Sevilen (Sevilen, Kazan)
Escorene (ExxonMobil)
Greenflex (Polimeri Europa)
Ultrathene (Equistar)
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3.14 Ethylene-Acrylic Rubber
CH2CH
CH CH R
2
n
C O
O
CH
2
m
C O
OH
3
k
Ethylene acrylic rubber (AEM, EACM) is highly saturated.
Properties
Water and ethylene glycol resistance is good, but softening
can occur after long term exposure above 100°C, good oil resistance,
with heat resistance. Owing to saturation the rubber exhibits
excellent resistance to ozone and weathering.
Table 13.
Physical Properties of Ethylene-Acrylic Rubber
Excellent Good Fair Poor
Abrasion resistance •
Compression Set •
Elongation •
Flame resistance •
Gas permeability •
Low temperature flexibility •
Tear resistance •
Tensile strength •
133
Table 14.
Chemical Resistance of Ethylene-Acrylic Rubber
Excellent Good Fair Poor
Automatic transmission fluid •
Dilute acids •
Dilute alkalis •
Ozone •
Petroleum oils and fuels •
Steam •
Water •
Weather •
Operating Temperature
-34 to 175°C
Application
Ethylene Acrylic, or AEM, rubber is generally used in
applications requiring a tough rubber that combines good oil
resistance, with heat resistance greater than nitrile or chloroprene at a
cost well below that of silicone or fluorocarbon rubbers. Good low
temperature properties are imparted by the ethylene content, while
the acrylate provides a considerable degree of oil resistance.
Ethylene Acrylic compounds are well suited for applications
requiring continuous exposure to hot (170°C) aliphatic hydrocarbons,
including most common automotive lubricants and hydraulic fluids.
AEM's good dampening characteristics make it well suited for
vibration mounts, pads, and isolators.
134
3.15 Polyacrylate Rubber
CH2CH CHCCH
n
O
O
CH
CH
O
CH
CH
2
Cl
3
Ethyl acrylate (95%) Chloroethyl vinyl ether (5%)
2
m
2
2
Polyacrylate rubbers (ACM) are noted for their transparency and
resistance to breakage and elasticity. Also commonly known as
acrylics.
Chemistry
Acrylate monomers used to form acrylate polymers are based
on the structure of acrylic acid, which consists of a vinyl group and a
carboxylic acid terminus. Other typical acrylate monomers are
derivatives of acrylic acid, such as methyl methacrylate, in which
one vinyl hydrogen and the carboxylic acid hydrogen are both
replaced by methyl groups, and acrylonitrile in which the carboxylic
acid group is replaced by the related nitrile group.
O
O
OH
Acrylic acid
O
Methylmethacrylate
Acrylonitrile
N
Other examples of acrylate monomers are: methacrylates,
methyl acrylate, ethyl acrylate, 2-chloroethyl vinyl ether, 2ethylhexyl acrylate, hydroxyethyl methacrylate, butyl acrylate, butyl
methacrylate, trimethylolpropane triacrylate (TMPTA).
Acrylic elastomer is a general term for a type of synthetic
rubber whose main component is acrylic acid alkylester (ethyl or
135
butyl ester). Acrylic elastomer has characteristics of heat and oil
resistance.
It is divided into old type and new type: Old types include
ACM (copolymer of acrylic acid ester and 2-chloroethyl vinyl ether)
containing chlorine and ANM (copolymer of acrylic acid ester and
acrylonitrile) without chloride. Other than the slightly better water
resistance of ANM, there are no physical differences; even
processability is poor for both types. Since prices are also high,
demand is not so high vis-à-vis the characteristics. On the other hand,
the new type of acrylic rubber does not contain any chlorine despite
its unclear chemical composition. Processability has been improved,
and most of tackiness to rolls as well as staining problems related to
molds have been solved.
Properties
Major characteristics of acrylic rubber include heat resistance
and oil resistance; it can endure a temperature of 170 ~ 180℃ under
dry heat or in oil. Since it does not have a double bond, acrylic
rubber also boasts of good weatherability and ozone resistance.
Its cold resistance is not that good, however. The saturation
point is -15℃ for the old type and -28~-30℃ for the new type. In
terms of vulcanization, the standard method for the old type is amine
vulcanization. To minimize permanent deformation, the old type
requires curing for 24 hours under a temperature of 150℃. On the
other hand, for the new type, the press curing time and follow-up
vulcanization time are significantly reduced by combining metal
soap and sulfur. It has no special characteristics. The rebound
resilience and abrasion resistance of the new type are poor, and even
its electrical characteristics are considerably poor compared with
acrylonitrile-butadiene rubber and butyl rubber.
Application
The materials are used mainly for oil seals and packagings
related to automobiles.
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3.16 Ebonite
Ebonite was a brand name for very hard rubber first obtained by
Charles Goodyear by vulcanizing rubber for prolonged periods. It is
about 30% to 40% sulfur. Its name comes from its intended use as an
artificial substitute for ebony wood. The material is known
generically as hard rubber and has formerly been called "vulcanite",
although that name now refers to the mineral vulcanite.
Properties
The material is brittle, which produces problems in its use in
battery cases for example, where the integrity of the case is vital to
prevent leakage of sulfuric acid. It has now been generally replaced
by carbon black-filled polypropylene.
Applications
It is often used in bowling balls, electric plugs, smoking pipe
mouthpieces, fountain pen bodies and nib feeds, and saxophone and
clarinet mouthpieces. It is, rarely, used for the body of high quality
clarinets. Hard rubber is also often seen as the wheel material in
casters. It is also commonly used in physics classrooms to
demonstrate static electricity.
Hard rubber was used in the cases of automobile batteries for
years, thus establishing black as their traditional colour even long
after stronger modern plastics were substituted. It is used in hair
combs made by Ace, part of Newell Rubbermaid, which survive,
essentially unchanged, from the days of the US Civil War. Ebonite is
used as an anticorrosive lining for various (mainly storage) vessels
that contain hydrochloric acid. It forms bubbles when storing
hydrofluoric acid at temperatures above room temperature, or for
prolonged durations.
Contamination
Ebonite contamination was a big problem when it was used
for electronics. The ebonite was rolled between metal foil sheets,
which were peeled off, leaving traces of metal behind. For electronic
use the surface was ground to remove metal particles.
137
3.17 Propylene Oxide-Allyl Glycidyl Ether Copolymer
OCH2CH
CH
OCH2CH
n m
3
CH2OCH2CH CH
2
Propylene oxide rubber (PO, GPO) is a copolymer of propylene
oxide with allyl glycidyl ether: The content of allyl glycidyl ether
units is approx. 2 mol. %. Mp 70°C, Tg -74°C; ρ=1.02 g/cm3.
Properties
It is soluble in toluene, THF, poorly soluble in hexane. PO
has very flexible molecular chains due to a low barrier of rotation
around the ether bonds, and is impermeable to gas.
PO plasticizes and is easily milled, Mooney viscosity is 50-
60. It is notable for low capacity in relation to carbon black. PO is
vulcanized with sulfur, organic peroxides (without precure).
Vulcanizates are characterized by high tensile strength (see table),
flexing life, low heat buildup, high cut growth resistance. POs are
suitable for using from -60 to 150°C, thermal and ozone resistant,
slightly swell in the mineral oils, especially high in naphthenes
content, are stable in diluted alkalis, fail by mineral acids.
Table 15.
Chatacteristics of Propylene oxide rubber
Characteristics Unfilled
vulcanizate
σ
, MPa 16.1 22.6
tens
Filled
vulcanizate*
Elongation, % 690 630
Elongation set, % - 16
Tensile stress at 300%, MPa 2.1 7.9
Resilience / Rebound 77.0 49.0
Shore A hardness 50.0 73.0
Brittle temperature, °C - -64.0
138
Characteristics Unfilled
Heat ageing coefficient (48 h,
150°C):
at σ
tens
at elongation
Freeze-proof factor at -45°C
at rubber nerve
vulcanizate
-
-
-
Filled
vulcanizate*
0.68
0.5
0.72
* The filler is carbon black; vulcanization during 45 min at 153°C.
Production
PO is obtained copolymerizing monomers in solution
(Catalysts are alkyls A1 or Zn; cocatalyst - H2O). For smooth
catalyst hydrolysis, the system is fed by ether, while to increase the
activity of the catalyst and to regulate molecular weight of rubber the
acetyl acetone is used. The polymerization influenced by the
products of incomplete trialkylaluminium hydrolysis alkilaluminoxane, probably follows the coordination and ionic
mechanism.
Application
POs are used substantially in cars parts production (hoses,
gaskets and other general mechanical rubber goods). It is promising
to use POs at tires production, ozone-resistant coatings, antivibrators
and shock absorbers making.
Trade names
Propylene oxide rubber is known as Danagen and Parel.
3.18 Fluorocarbon Rubber
A fluoroelastomer (FPM, FKM) is a special purpose fluorocarbonbased synthetic rubber. It has wide chemical resistance and superior
performance, especially in high temperature application in different
media. All FKMs contain vinylidene fluoride as a monomer.
Fluoroelastomers are more expensive than neoprene or nitrile rubber
139
elastomers partly because they provide additional heat and chemical
resistance.
History
Originally developed by DuPont (Viton), FKMs today are
also produced by Daikin Chemical (Dai-El), 3M's Dyneon (Dyneon
Fluoroelastomers), Solvay Specialty Polymers (Tecnoflon) and
HaloPolymer (Elaftor).
Chemistry
FKMs can be divided into different classes on the basis of
either their chemical composition, their fluorine content or their
crosslinking mechanism.
Fluoroelastomers are categorized under the ASTM D1418 &
ISO 1629 designation of FKM (FPM by ISO). This class of
elastomers is a family comprising copolymers of
hexafluoropropylene (HFP) and vinylidene fluoride (VDF or VF2),
terpolymers of tetrafluoroethylene (TFE), vinylidene fluoride (VDF)
and hexafluoropropylene (HFP) as well as perfluoromethylvinylether
(PMVE) containing specialties. The fluorine content of the most
common grades varies between 66 and 70%.
CH2CF
2
Viylidene Fluoride
CH2CF
Cl
CF2CF
CF
CF2CF
CF2CF
CF
Chlorotrofluoroethylene
Haxafluoropropylene
3
Tetrafluoroethylene
2
Perfluoro (methyl vinyl ether)
2
2
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