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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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home. From automotive applications to a large variety of cooking,
baking, and food storage products; to apparel, undergarments,
sportswear, and footwear; to electronics; to home repair and
hardware, and a host of unseen applications.
Freeze-tolerant solar water heating panels exploit the
elasticity of silicone to repeatedly accommodate the expansion of
water on freezing, while its extreme temperature tolerance delivers a
lack of brittleness below freezing and excellent tolerance of high
temperatures of over 150°C. Also, its hygienic property of not having
a carbon backbone, but a chemically robust silicon backbone instead,
reduces its potential as a food source for dangerous waterborne
bacteria such as Legionella.
Non-dyed silicone rubber tape with an iron-oxide additive
(making the tape a red-orange colour) is used extensively in aviation
and aerospace wiring applications as a splice or wrapping tape due to
its non-flammable nature. The iron-oxide additive adds high thermal
conductivity but does not change the high electrical insulation
property of the silicone rubber. This type of tape self-fuses or
amalgamates without any added adhesive.
Self-healing
Recently, silicone rubber formed the matrix of the first
autonomic self-healing elastomer. The microcapsule-based material
was capable of recovering almost all of the original tear strength.
Additionally, this material had improved fatigue properties as
evaluated using a torsion-fatigue test.
Special grades
There are also many special grades and forms of silicone
rubber, including: steam resistant, metal detectable, high tear
strength, extreme high temperature, extreme low temperature,
electrically conductive, chemical/oil/acid/gas resistant, low smoke
emitting, and flame-retardant. A variety of fillers can be used in
silicone rubber, although most are non-reinforcing and lower the
tensile strength.
151
Silicone rubber is available in a range or hardness levels,
expressed as Shore A or IRHD between 10 and 100, the higher
number being the harder compound. It is also available in virtually
any colour and can be colour matched.
3.21 Polyurethane Elastomers
Polyurethane elastomer (AU, EU, PU) is a polymer composed of a
chain of organic units joined by carbamate (urethane) links.
History
Otto Bayer and his coworkers at I.G. Farben in Germany first
made polyurethanes in 1937. The new polymers had some
advantages over existing plastics that were made by polymerizing
olefins, or by polycondensation. Early work focused on the
production of fibres and flexible foams and PUs were applied on a
limited scale as aircraft coating during World War II.
Polyisocyanates became commercially available in 1952 and
production of flexible polyurethane foam began in 1954 using
toluene diisocyanate (TDI) and polyester polyols. These materials
were also used to produce rigid foams, gum rubber, and elastomers.
Linear fibers were produced from hexamethylene diisocyanate (HDI)
and 1,4-butanediol (BDO).
In 1956 polyether polyols were introduced, specifically
poly(tetramethylene ether) glycol. Polyether polyols were cheaper,
easier to handle and more water resistant than polyester polyols, and
became more popular. The availability of chlorofluoroalkane
blowing agents, inexpensive polyether polyols, and methylene
diphenyl diisocyanate (MDI) allowed polyurethane rigid foams to be
used as high performance insulation materials. In 1967, urethane
modified polyisocyanurate rigid foams were introduced, offering
even better thermal stability and flammability resistance. During the
1960s, automotive interior safety components such as instrument and
door panels were produced by back-filling thermoplastic skins with
semi-rigid foam.
152
In 70s, Bayer exhibited a new process called RIM, Reaction
Injection Molding in which the reactants were mixed then injected
into a mold. The addition of fillers, such as milled glass, mica, and
processed mineral fibres gave rise to reinforced RIM (RRIM), which
provided improvements in flexural modulus (stiffness), reduction in
coefficient of thermal expansion and thermal stability. Further
increases in stiffness were obtained by incorporating pre-placed glass
mats into the RIM mold cavity, also known broadly as resin injection
molding or structural RIM.
Starting in the early 1980s, water-blown microcellular
flexible foams were used to mold gaskets for automotive panels and
air filter seals, replacing PVC plastisol from automotive applications
have greatly increased market share. Polyurethane foams are now
used in high temperature oil filter applications.
Polyurethane foam (including foam rubber) is sometimes
made using small amounts of volatile materials, so-called blowing
agents. These volatile chemicals help with density reduction,
cushioning/energy absorption and thermal insulation. By the late
1990s, the use of blowing agents such as carbon dioxide, pentane,
1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1,1,3,3pentafluoropropane (HFC-245fa) became more widespread,
especially in North America and the EU.
Rising costs of petrochemical feedstocks and an enhanced
public desire for environmentally friendly green products raised
interest in polyols derived from vegetable oils.
Chemistry
Polyurethanes are produced by reacting an isocyanate
containing two or more isocyanate groups per molecule (R(N=C=O)
hydroxyl groups per molecule (R'-(OH)
) with a polyol containing on average two or more
n≥2
), in the presence of a
n≥2
catalyst. By varying the reactants, their amounts and the reaction
conditions, one can obtain millable elastomeric gums, hard rigid
plastics, reactive liquids, and foams. While most polyurethanes are
153
thermosetting polymers that do not melt when heated, thermoplastic
polyurethanes are also available.
The properties of polyurethane are greatly influenced by the
types of isocyanates and polyols used to make it. Long, flexible
segments, contributed by the polyol, give soft, elastic polymer. High
amounts of crosslinking give tough or rigid polymers. Long chains
and low crosslinking give a very stretchy polymer; short chains with
lots of crosslinks produce a hard polymer while long chains and
intermediate crosslinking give a polymer useful for making foam.
The crosslinking present in polyurethanes means that the polymer
consists of a three-dimensional network and molecular weight is very
high. In some respects a piece of polyurethane can be regarded as
one giant molecule. One consequence of this is that typical
polyurethanes do not soften or melt when they are heated they are
thermosetting polymers. The choices available for the isocyanates
and polyols, in addition to other additives and processing conditions
allow polyurethanes to have the very wide range of properties that
make them such widely used polymers.
PU r ea cti on me cha ni sm cat aly zed b y a t ert iar y ami ne
R2O H N N
R
1
N C O
R2O
R
1
NN H
N C
R2O
O
O
R
2
N NH
N C
O
N N
O
R
2
R
1
H
gen era li zed u r eth an e r eac tio n
R1N C O R2O H
R1NHCOO R
154
2
The polymerization reaction makes a polymer containing the
urethane linkage, -RNHCOOR'- and is catalyzed by tertiary amines,
such as 1,4-diazabicyclo[2.2.2]octane (also called DABCO or
TEDA), and metallic compounds, such as dibutyltin dilaurate or
bismuth octanoate. This is often referred to as the gellation reaction
or simply gelling.
If water is present in the reaction mixture (when foams
making), the isocyanate reacts with water to form a urea linkage and
carbon dioxide gas and the resulting polymer contains both urethane
and urea linkages. This reaction is referred to as the blowing reaction
and is catalyzed by tertiary amines like bis-(2dimethylaminoethyl)ether.
A third reaction, particularly important in making insulating
rigid foams, is the isocyanate trimerization reaction, which is
catalyzed by potassium octoate, for example.
One of the most desirable attributes of polyurethanes is their
ability to be turned into foam. Making foam requires the formation of
gas at the same time as the urethane polymerization (gellation) is
occurring. The gas can be carbon dioxide, either generated by
reacting isocyanate with water or added as a gas or produced by
boiling volatile liquids. In the latter case heat generated by the
polymerization causes the liquids to vaporize. The liquids can be
HFC-245fa (1,1,1,3,3-pentafluoropropane) and HFC-134a (1,1,1,2tetrafluoroethane), and hydrocarbons such as n-pentane.
Car bon di ox id e gas f orm ed by r eac tin g w at er a nd
iso cya na te
R N C O H2O
R N C O R NH
step 1
2
R NHCOO H
step 3
R
155
NHCON R
step 2
decomposes
H
R NH
CO
2
2
When water is used to produce the gas, care must be taken to
use the right combination of catalysts to achieve the proper balance
between gellation and blowing. The reaction to generate carbon
dioxide involves water molecule reacting with an isocyanate first
forming an unstable carbamic acid, which then decomposes into
carbon dioxide and an amine. The amine reacts with more isocyanate
to give a substituted urea. Water has a very low molecular weight, so
even though the weight percent of water may be small, the molar
proportion of water may be high and considerable amounts of urea
produced. The urea is not very soluble in the reaction mixture and
tends to form separate "hard segment" phases consisting mostly of
polyurea. The concentration and organization of these polyurea
phases can have a significant impact on the properties of the
polyurethane foam.
High-density microcellular foams can be formed without the
addition of blowing agents by mechanically frothing or nucleating
the polyol component prior to use.
Even more rigid foam can be made with the use of specialty
trimerization catalysts which create cyclic structures within the foam
matrix, giving a harder, more thermally stable structure, designated
as polyisocyanurate foams. Such properties are desired in rigid foam
products used in the construction sector.
Careful control of viscoelastic properties - by modifying the
catalysts and polyols used - can lead to memory foam, which is much
softer at skin temperature than at room temperature.
There are then two main foam variants: one in which most of
the foam bubbles (cells) remain closed, and the gas(es) remains
trapped, the other being systems which have mostly open cells,
resulting after a critical stage in the foam-making process (if cells did
not form, or became open too soon, foam would not be created). This
is a vitally important process: if the flexible foams have closed cells,
their softness is severely compromised, they become pneumatic in
feel, rather than soft; so, generally speaking, flexible foams are
required to be open-celled.
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The opposite is the case with most rigid foams. Here,
retention of the cell gas is desired since this gas (especially the
fluorocarbons referred to above) gives the foams their key
characteristic: high thermal insulation performance.
A third foam variant, called microcellular foam, yields the
tough elastomeric materials typically experienced in the coverings of
car steering wheels and other interior automotive components.
Raw materials
The main ingredients to make polyurethane are isocyanates
and polyols. Other materials are added to help processing the
polymer or to change the properties of the polymer. The wide range
of the raw materials causes polyurethanes with different properties
and appliances, so hereinafter we will go into some details
concerning these points.
Isocyanates
Isocyanates used to make polyurethane must have two or
more isocyanate groups on each molecule. The most commonly used
isocyanates are the aromatic diisocyantes, toluene diisocyanate (TDI)
and methylene diphenyl diisocyanate, MDI.
TDI and MDI are generally less expensive and more reactive
than other isocyanates. Industrial grade TDI and MDI are mixtures of
isomers and MDI often contains polymeric materials. They are used
to make flexible foam (for example slabstock foam for mattresses or
molded foams for car seats),
[13]
rigid foam (for example insulating
foam in refrigerators), elastomers (shoe soles, for example), and so
on. The isocyanates may be modified by partially reacting them with
polyols or introducing some other materials to reduce volatility (and
hence toxicity) of the isocyanates, decrease their freezing points to
make handling easier or to improve the properties of the final
polymers.
157
OCN CH
Pure DI's
4,4'
2,4'
NCO
CH
NCO
NCO
NCO
CH
CH
NCO
2
n
NCO
2
2
2
2,2'
NCO
CH
2
OCN
Polymeric MDI's
Aliphatic and cycloaliphatic isocyanates are used in smaller
volumes, most often in coatings and other applications where color
and transparency are important since polyurethanes made with
aromatic isocyanates tend to darken on exposure to light.
[14]
The
most important aliphatic and cycloaliphatic isocyanates are 1,6hexamethylene diisocyanate (HDI), 1-isocyanato-3isocyanatomethyl-3,5,5-trimethyl-cyclohexane (isophorone
diisocyanate, IPDI), and 4,4'-diisocyanato dicyclohexylmethane,
(H12MDI or hydrogenated MDI).
Isocyanates with functionality greater than two act as
crosslinking sites.
Polyols
Polyols are most easily classified as polyether polyols, which
are made by the reaction of epoxides with an active hydrogen
containing starter compounds, or polyester polyols, which are made
by the polycondensation of multifunctional carboxylic acids and
hydroxyl compounds; they have on average two or more hydroxyl
groups per molecule. They can be further classified according to their
end use as flexible or rigid polyols, depending on the functionality of
the initiator and their molecular weight. Taking into account
functionality, flexible polyols have molecular weights from 2,000 to
10 000. Rigid polyols have molecular weights from 250 to 700.
Polyols with molecular weights from 700 to 2 000 are used to add
stiffness or flexibility to base systems, as well as increase solubility
158
of low molecular weight glycols in high molecular weight polyols.
Herewith, it is the length of the polyol chain and the functionality
that contribute much to the properties of the final polymer.
Polyether polyols come in a wide variety of grades based on
their end use, but are all constructed in a similar manner. Polyols for
flexible applications use low functionality initiators such as
dipropylene glycol (f=2), glycerine (f=3) or a sorbitol/water solution
(f=2.75).
[15]
Polyols for rigid applications use high functionality
initiators such sucrose (f=8), sorbitol (f=6), toluenediamine (f=4),
and Mannich bases (f=4). Propylene oxide is then added to the
initiators until the desired molecular weight is achieved. Polyols
extended with propylene oxide are terminated with secondary
hydroxyl groups. In order to change the compatibility, rheological
properties, and reactivity of a polyol, ethylene oxide is used as a coreactant to create random or mixed block heteropolymers. Polyols
capped with ethylene oxide contain a high percentage of primary
hydroxyl groups, which are more reactive than secondary hydroxyl
groups. Because of their high viscosity (33 Pa·s at 25°C),
carbohydrate initiated polyols often use glycerine ordiethylene glycol
as a co-initiate in order to lower the viscosity to ease handling and
processing (5.5 Pa·s at 25°C). Graft polyols (also called filled
polyols or polymer polyols) contain finely dispersed styreneacrylonitrile, acrylonitrile, or polyurea (PHD) polymer solids
chemically grafted to a high molecular weight polyether backbone.
They are used to increase the load-bearing properties of low-density
high-resiliency (HR) foam, as well as add toughness to microcellular
foams and cast elastomers. PHD polyols are also used to modify the
combustion properties of HR flexible foam. Solids content ranges
from 14% to 50%, with 22% and 43% being typical. Initiators such
as ethylenediamine and triethanolamine are used to make low
molecular weight rigid foam polyols that have built-in catalytic
activity due to the presence of nitrogen atoms in the backbone. They
are used to increase system reactivity and physical properties build,
and to reduce the friability of rigid foam molded parts. A special
159
class of polyether polyols, poly(tetramethylene ether) glycols are
made by polymerizing tetrahydrofuran. They are used in high
performance coating, wetting and elastomer applications.
Polyester polyols fall into two distinct categories according to
composition and application. Conventional polyester polyols are
based on virgin raw materials and are manufactured by the direct
polyesterification of high-purity diacids and glycols, such as adipic
acid and 1,4-butanediol. They are distinguished by the choice of
monomers, molecular weight, and degree of branching. While costly
and difficult to handle because of their high viscosity, they offer
physical properties not obtainable with polyether polyols, including
superior solvent, abrasion, and cut resistance. Other polyester polyols
are based on reclaimed raw materials. They are manufactured by
transesterification (glycolysis) of recycled
poly(ethyleneterephthalate) (PET) or dimethylterephthalate (DMT)
distillation bottoms with glycols such as diethylene glycol. These
low molecular weight aromatic polyester polyols are used in the
manufacture of rigid foam, and bring low cost and excellent
flammability characteristics to polyisocyanurate (PIR) boardstock
and polyurethane spray foam insulation.
Specialty polyols include polycarbonate polyols,
polycaprolactone polyols, polybutadiene polyols, and polysulfide
polyols. The materials are used in elastomer, sealant, and adhesive
applications that require superior weatherability, and resistance to
chemical and environmental attack. Natural oil polyols derived from
castor oil and other vegetable oils are used to make elastomers,
flexible bunstock, and flexible molded foam. Copolymerizing
chlorotrifluoroethylene or tetrafluoroethylene with vinyl ethers
containing hydroxyalkyl vinyl ether produces fluorinated (FEVE)
polyols. Two component fluorinated polyurethane prepared by
reacting FEVE fluorinated polyols with polyisocyanate have been
applied for make ambient cure paint/coating. Since fluorinated
polyurethanes contain high percentage of fluorine-carbon bond
which is the strongest bond among all chemical bonds. Fluorinated
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