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Файл:Chemistry, technology and properties of synthetic rubber. Tutorial
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Surfboards
Some surfboards are made with a rigid polyurethane core. A
rigid foam blank is molded, shaped to specification, then covered
with fiberglass cloth and polyester resin.
Rigid-hulled boats
Some boat hulls have a rigid polyurethane foam core
sandwiched between fiberglass skins. The foam provides strength,
buoyancy, and sound deadening.
Boat decks and outdoor marine surface areas
Some boat decks use specialized polyurethane sealants to
protect from constant moisture and harsh oceanic elements.
Flexible plastics
Tennis grips
Polyurethane has been used to make several Tennis
Overgrips. These grips are highly stretchable to ensure the grip wraps
neatly around the racquet's handle.
Watch-band wrapping
Polyurethane is used as a black wrapping for timepiece
bracelets over the main material which is generally stainless steel. It
is used for comfort, style, and durability.
Textiles
A thin film of polyurethane finish is added to a polyester
weave to create polyurethane laminate (PUL), which is used for its
waterproof and windproof properties in outerwear, diapers, shower
curtains, and so forth. PU is used in some cutting-edge swimsuits to
provide buoyancy for competitive swimmers. There are restrictions
as the buoyancy enhances swimming performance.
A still more popular use of polyurethane in textiles is in the
form of spandex, also known as elastane or lycra. Polyurethane fibers
in the form of spandex can stretch up to 600% and still return to their
original shape. Spandex is spun with other fibers, such as cotton,
nylon, or polyester, to create stretchable fibers essential for clothing
for both sports and fashion.
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Varnish
Polyurethane materials are commonly formulated as paints
and varnishes for finishing coats to protect or seal wood. This use
results in a hard, abrasion-resistant, and durable coating that is
popular for hardwood floors, but considered by some to be difficult
or unsuitable for finishing furniture or other detailed pieces. Relative
to oil or shellac varnishes, polyurethane varnish forms a harder film
which tends to de-laminate if subjected to heat or shock, fracturing
the film and leaving white patches. This tendency increases when it
is applied over softer woods like pine. This is also in part due to
polyurethane's lesser penetration into the wood. Various priming
techniques are employed to overcome this problem, including the use
of certain oil varnishes, specified "dewaxed" shellac, clear
penetrating epoxy, or "oil-modified" polyurethane designed for the
purpose. Polyurethane varnish may also lack the "hand-rubbed"
lustre of drying oils such as linseed or tung oil; in contrast, however,
it is capable of a much faster and higher "build" of film,
accomplishing in two coats what may require many applications of
oil. Polyurethane may also be applied over a straight oil finish, but
because of the relatively slow curing time of oils, the presence of
volatile byproducts of curing, and the need for extended exposure of
the oil to oxygen, care must be taken that the oils are sufficiently
cured to accept the polyurethane.
Unlike drying oils and alkyds which cure, after evaporation of
the solvent, upon reaction with oxygen from the air, polyurethane
coatings cure after evaporation of the solvent by a variety of
reactions of chemicals within the original mix, or by reaction with
moisture from the air. Certain products are "hybrids" and combine
different aspects of their parent components. "Oil-modified"
polyurethanes, whether water-borne or solvent-borne, are currently
the most widely used wood floor finishes.
Exterior use of polyurethane varnish may be problematic due
to its susceptibility to deterioration through ultra-violet (UV) light
exposure. All clear or translucent varnishes, and indeed all film-
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polymer coatings (i.e., paint, stain, epoxy, synthetic plastic, etc.) are
susceptible to this damage in varying degrees. Pigments in paints and
stains protect against UV damage, while UV-absorbers are added to
polyurethane and other varnishes to work against UV damage.
Polyurethanes are typically the most resistant to water exposure, high
humidity, temperature extremes, and fungus or mildew, which also
adversely affect varnish and paint performance.
Wheels
Polyurethane is also used in making solid tires. Industrial
applications include forklift drive and load wheels, grocery cart and,
rollercoaster wheels. Modern roller blading and skateboarding
became economical only with the introduction of tough, abrasionresistant polyurethane parts, helping to usher in the permanent
popularity of what had once been an obscure 1960s craze. The
durability of polyurethane wheels allowed the range of tricks and
stunts performed on skateboards to expand considerably.
Polyurethane is also used to make small equipment tires in the lawn
and garden industry for wheelbarrows, hand trucks, lawn mowers,
carts, etc. They provide the bounce and feel of an air-filled tire with
the benefit of no flats. They weigh about the same as air-filled tires
as well, even though they are solid polyurethane all the way through.
Other constructions have been developed for pneumatic tires, and
microcellular foam variants are widely used in tires on wheelchairs,
bicycles and other such uses. These latter foam types are also widely
encountered in car steering wheels and other interior and exterior
automotive parts, including bumpers and fenders.
Automotive Suspension Parts
Polyurethane usage has increased over the past twenty years
in the automotive industry. It is being used to replace traditional
rubber bushings which are known to fail or wear out on road surfaces
prone to large amounts of salt and chemical debris. Using
polyurethane bushings can have many benefits like maintaining the
right alignment of caster, camber and toe and thereby increasing the
overall control and handling. It also increases the lifespan, provides
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more resistance to wear out and is less pervious to oil and similar
road contaminants.
Electronic components
Often electronic components are protected from
environmental influence and mechanical shock by enclosing them in
polyurethane. Typically polyurethanes are selected for the excellent
abrasion resistances, good electrical properties, excellent adhesion,
impact strength, and low-temperature flexibility. The disadvantage of
polyurethanes is the limited upper service temperature (typically
121°C). In production the electronic manufacture would purchase a
two-part urethane (resin and catalyst) that would be mixed and
poured onto the circuit assembly. In most cases, the final circuit
board assembly would be unrepairable after the urethane has cured.
Because of its physical properties and low cost, polyurethane
encapsulation (potting) is a popular option in the automotive
manufacturing sector for automotive circuits and sensors.
Adhesives
Polyurethane can be used as an adhesive, especially as a
woodworking glue. Its main advantage over more traditional wood
glues is its water resistance.
The base for this system is polyether or polyester, whereas
polyurethane (PUR) is used as prepolymer. Its special features are
coagulation at room temperature and resistance to moisture.
Advantages of polyurethane glue in the bookbinding industry:
PUR is significantly better than hotmelt or cold glue. It is the most
economical glue, with a theoretical application thickness of 0.01 mm.
However, in actual use, it is not practical to apply less than 0.03 mm.
It is extremely weather-proof, and stable at temperatures from −40°C
to 100°C.
Abrasion resistance
Thermoset polyurethanes are also used as a protective coating
against abrasion. Cast polyurethane over materials such as steel will
absorb particle impact more efficiently. Polyurethanes have been
proven to last in excess of 25 years in abrasive environments where
174
non-coated steel would erode in less than 8 years. Polyurethanes are
resistance to
good
of tensile
articularly well
the main
is to
Polyurethane elastomer
ideally suited
for
applications
high
d
emands of the
material.
Lining sheet,
wheels, applied and
decorative arts items,
shafts
used in industries such as:
•
Mining and mineral processing
•
Aggregate
•
Transportation
•
Concrete
•
Paper processing
•
Power
•
Inflatable boat manufacture
Polyurethane is also used in the concrete construction
industry to create formliners. Polyurethane formliners serve as a
mold for concrete, creating a variety of textures and art.
Table 18.
Types of polyurethanes, resin mixtures and their properties
Type of
prepolymer,
polyether
Shore
number
Peculiarities Basic application
TDI, best properties
SKU-7L 76-85 A Very high
abrasion and very
performance
strength and tear.
It is p
suited for applications
where
requirement
protect against
abrasion.
SKU PFL-
100,
Adiprene
L167, L367
95А
(48D)
High abrasion, high
dynamic loads
resistance, excellent
resistance to hydrolysis.
Due to the low residual
with
compressive strain, it
gives good account at
long-term statistical
loads.
175
Vibrathane
High resistance to
mechanical influences
Sieves
slurry
pumps,
High values of stiffn
ess, life
durability, excellent impact
resistance, high lifetime,
high temperature resistance.
Wheels, shafts, bearings,
encapsulation
Processing ease
High load parts
Resistance to mechanical
influences,
frost
resistance, high rebound
Applied and decorative
arts items
cyclones
,
dampers
Ripping resistance
Different parts
Excellent processi
ng, tear,
wear resistance
Applied and decorative
arts, cleaning disks,
lining sheets, general
-
purpose parts
Excellent resistance to
mechanical influences,
ripping and abrasion
Applied and decorative
arts
,
lining, stamping
elements, cleaning
disks
B602
82А
Continuation of Table 18
, flotation,
liners
Adiprene
L325
Adiprene
L315
Vibrathane
B865,
640,876
Vibrathane
625
Vibrathane
670
Vibrathane
9086
72D
73D
65D,
75D,
75D
85А
53D
86А
TDI, moderate properties
MDI, best properties
hydrolysis,
Polyesters TDI
,
, rollers
Vibrathane
8000
55A-
55D
MDI Quasi
, sieves, flotation
176
Continuation of Table 18
Polycaprolactone TDI
Low hardness when curing
with MOCA, high
resistance to cycling
, low
permanent deformation.
H
which
provides long
term
performan
during
abrasive
Applied and decorative
arts, screen mesh, shafts
and rollers
Balance of properties,
inherent in polyethers and
polyesters, excellent frost
resistance, dynamic flexing
and wear resistance
Different parts, where
materials based on
polyesters and polyethers
do not work separately
Increased lifetime, low
viscosity, dynamic
properties and resistance to
mechanical influences
Closest analog Vulkollan
for dynamic loaded
goods, especially wheels
Polyurethane coating with
outstanding a
brasion
resistance, corrosion
protection
Flotation equipment,
classifiers, thickeners,
bins, internal pipe
coating, anti
SKU-6,
62А
Vibrathane
6060
igh elasticity,
-
ce
wear.
MDI
Vibrathane
8030,8045
80А,
95А
Adiprene
93-95А
LFM 2450
Systems for hardening
Vibraspray
80-85А
80, ЕР1690
WRM 80S,
80T,85C
80-85А
-abrasive
3.22 Thermoplastic Elastomers - General Description
They sometimes referred to as thermoplastic rubbers, this is a class
of copolymers or a physical mix of polymers which consist of
materials with both thermoplastic and elastomeric properties. While
most elastomers are thermosets, thermoplastics are in contrast
relatively easy to use in manufacturing, for example, by injection
molding. Thermoplastic elastomers show advantages typical of both
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rubbery materials and plastic materials. The principal difference
between thermoset elastomers and thermoplastic elastomers is the
type of crosslinking bond in their structures. In fact, crosslinking is a
critical structural factor which contributes to impart high elastic
properties. The crosslink in thermoset polymers is a covalent bond
created during the vulcanization process. On the other hand the
crosslink in thermoplastic elastomer polymers is a weaker dipole or
hydrogen bond or takes place in one of the phases of the material.
Types
There are six generic classes of commercial TPEs:
1. Styrene block copolymers
2. Polyolefin blends
3. Elastomeric alloys (TPE-v or TPV)
4. Thermoplastic polyurethanes
5. Thermoplastic copolyester
6. Thermoplastic polyamides.
In order to qualify as a thermoplastic elastomer, the material
must have these three essential characteristics:
1. The ability to be stretched to moderate elongations and, upon
the removal of stress, return to something close to its original shape.
2. Processable as a melt at elevated temperature.
3. Absence of significant creep
History
It was not until the 1950s, when thermoplastic polyurethane
polymers became available, that TPE became a commercial reality.
During the 1960s styrene block copolymer became available, and in
the 1970s a wide range of TPEs came on the scene. The worldwide
usage of TPEs (680000 tons/year in 1990) is growing at about 9%
per year.
Chemistry
The styrene-butadiene materials possess a two-phase
microstructure due to incompatibility between the polystyrene and
polybutadiene blocks, the former separating into spheres or rods
depending on the exact composition. Generally they offer a much
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wider range of properties than conventional cross-linked rubbers
because the composition can vary to suit customer needs.
Block copolymers are interesting because they can
"microphase separate" to form periodic nanostructures. Since most
polymers are incompatible with one another, forming a block
polymer will usually result in phase separation, and the principle has
been widely exploited since the introduction of the SBS block
polymers, especially where one of the block is highly crystalline.
Other TPE's have crystalline domains where one kind of
block co-crystallizes with other block in adjacent chains, such as in
copolyester rubbers, achieving the same effect as in the SBS block
polymers. Depending on the block length, the domains are generally
more stable than the latter owing to the higher crystal melting point.
That point determines the processing temperatures needed to shape
the material, as well as the ultimate service use temperatures of the
product. Such materials include Hytrel, a polyester-polyether
copolymer and Pebax, a nylon or polyamide-polyether copolymer.
Properties
Advantages
TPE materials have the potential to be recyclable since they
can be molded, extruded and reused like plastics, but they have
typical elastic properties of rubbers which are not recyclable owing
to their thermosetting characteristics. TPE also require little or no
compounding, with no need to add reinforcing agents, stabilizers or
cure systems. Hence, batch-to-batch variations in weighting and
metering components are absent, leading to improved consistency in
both raw materials and fabricated articles. TPEs can be easily colored
by most types of dyes. Besides that, it consumes less energy and
closer and more economical control of product quality is possible.
Disadvantages
The disadvantages of TPEs relative to conventional rubber or
thermoset are relatively high cost of raw materials, general inability
to load TPEs with low cost fillers, such as carbon black (therefore
preventing TPEs from being used in automobile tires), poor chemical
179
and heat resistance, high compression set and low thermal stability.
,
TPEs soften or melt at elevated temperature above which they lose
their rubbery behaviour. TPEs show creep behaviour on extended
use.
Table 19
TPE characteristics
TPV
Characteristics TPO TPE-S
(based
TPU TPE-E TPE-A
on PO)
Operation
temperature range
°С
Shore hardness
number
Ultimate tensile
strength, MPa
Flex modulus, MPa
Extension
elongation, %
1
-60 :
+130
A55 -
D65
-70 :
+150
A3 -
D60
4 - 28 3 - 30 2 - 28
6 -
347
200 -
700
150 -
1220
-60 :
+135
A35 -
D65
250 -
600
-50 :
+120
A60 -
D75
24 -
48
69 -
1310
250 -
700
-65 :
+165
A80 -
D82
-40 :
+170
A60 -
D72
6 - 46 29 - 57
36 570
200 -
900
16 -
370
380 -
715
1 - Characteristics are given for 23°C. The minimum and the
maximum operating temperatures are given for the group of
materials; specific brands have a more narrow range of operating
temperatures.
Processing
The two most important manufacturing methods with TPEs
are extrusion and injection molding. Compression molding is
seldom, if ever, used. Fabrication via injection molding is extremely
rapid and highly economical. Both the equipment and methods
normally used for the extrusion or injection molding of a
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