The Notes for
Polymer and Coatings Science-
Chapter Two- part five
Poly(vinyl alcohol)
A good
on a test would be "Show the addition polymerization
of poly(vinyl alcohol.)" PVA is not prepared via addition polymerization
because of the following keto-enol tautomerism:
H H H O
| | | ||
C = C <-----> H - C - C - H
| | |
H OH H
Poly(vinyl alcohol) is prepared by the hydrolysis of poly(vinyl acetate.)
H H H H H H H H H H H H
| | | | | | | | | | | |
- C - C - C - C - C - C - ---> - C - C - C - C - C - C -
| | | | | | | | | | | |
H O H O H O H OH H OH H OH
| | |
C=O C=O C=O
| | |
CH3 CH3 CH3
PVAc is hydrolyzed by treating an alcoholic solution of PVAc with
aqueous acid or alkali.
Acid hydrolysis results in traces of the acid in the polymer which
are difficult to remove and which promote polymer instability.
Alkali hydrolysis results in the contamination of the product by
large amounts of sodium acetate which are hard to remove and have
little intrinsic value.
These difficulties are avoided by using small amounts of base as
catalyst. From the notes, the alcoholysis of PVAc in methanol in the
presense of sodium methoxide, might be an example of this.
The best method involve making solution from suspension polymers (I
take this to mean prepare a solution of poly(vinyl alcohol from a
poly(vinyl acetate) feedstock
which was polymerized by suspension polymerization.)
Poly(vinyl alcohol) has substantial head-to-tail tacticity. The
treatment of poly(vinyl alcohol) with HIO4 shows
- a reduced solution viscosity, which indicates
-
a decrease in molecular weight which in turn is attributed to
-
oxidation cleavage of the polymer chain which
-
would only occur at head-to-head linkages, so
-
there are some head-to-head linkages, but the calculations
indicate head-to-head constitutes only 1-2%.
Poly (vinyl alcohol) is soluble in highly polar solvents
such as DMSO, DMF, and water.
Poly(styrene) and Styrene Copolymers
Applications for polystyrene include commercial plastics,
styrofoam, household appliances, torp (I wonder if this is supposed
to be "tarp", as in a hugh plastic sheet to cover something outdoors
to protect it from rain), food containers, packing, and thermal
insulation.
Production is 7 to 9 billion pounds per year (date?) with a cost
of 50 cents a lb (1991.)
ethylene H H H H
+ | | -H2 | |
benzene ----> benzene - C - C - H -------> benzene- C = C - H
| |
H H
boiling pt. 139 deg C boiling pt. 145 deg C
A Friedel-Crafts reaction is carried out by treating benzene with
ethylene in the liquid phase.
The catalyst is aluminum trichloride which is used with thiol chloride
as the catalyst promoter.
Molar excess of benzene is used to reduce the formation of
poly(ethylbenzenes.) The ratio of benzene to ethylene is 10 : 6.
The reactants are fed continuously into the bottom of the reactor
while crude product is taken from near the top.
A tower, 25 to 50 ft in height, with a temperature gradient inside,
is shown below:
----------
| | The reaction is controlled by a complex
| | 100 deg C array of heating and cooling jackets and
| | coils with which the tower is fitted.
| |
| | 150 deg C As the rising fed materials traverse the
| | temperature gradient, polymerization occurs
| | and fully polymerized materials emerge from
| | 180-200 deg C from the top of the tower.
| |
----------
The notes start out talking that the polymer
comes out the top, but then they say polymer comes out the bottom, and
in the drawing in the notes, there is an arrow coming out the bottom of
the picture.
?? Does the polmer sink, or does it float?
The molten material is fed into an extruder, extruded into filaments, and
then cooled and chopped into granules.
The product contains few impurities, it has high clarity and good electrical
insulation properties.
The polymer has a broader molecular weight distribution than polymer prepared
at one temperature.
Suspension polymerization Advantages-
- The heat transfer problem associated with bulk polymerization is
simplified.
- There isn't a problem with solvent removal and recovery that is
associated with solution polymerization.
Disadvantages- There is an added drying step and this process does not
readily lend itself to a continuous operation.
The polymerization is carried out batch-wise in a stirred reactor which
is jacketed for heating and cooling. A typical formulation might be
as follows (quantities shown as parts-by-mass):
Styrene (inhibitor free) 100
water (demineralized) 70
tricalcium phosphate 0.8 suspending agent
dedecylbenzene sulphonate 0.003 suspending agent
benzyl peroxide 0.2 initiator
reaction temperature: 90 deg C.
The size of the beads produced depends on:
- on how fast the solution is stirred
- temperature
- suspending agent
- surfactant agent
- ratio of styrene to water
When the polymerization is complete, the product is in the form of a
slurry, is washed with HCl and water to remove the suspending agent,
centrifuged, dried in warm air (about 60 deg C), extruded, and chopped.
The tacticity is predominantly head-to-tail.
I'm missing page 83 of the notes which should go here.
Expanded polystyrene is very important as a thermal insulating
material. Most commercial methods make use of expandable beads.
Styrene is polymerized as it is in solution polymerization except
that a low boiling point hydrocarbon such as n-pentane is added to
the system. This modification results in the formation of polystyrene
beads containing 5- 8% volatile hydrocarbon.
The alternative is to
treat formed PS beads under heat and pressure with the volatile
hydrocarbon. The impregnated beads are then expanded commonly by treatment
with steam. When the beads are heated in the steam, they soften and
volatilization of the low boiling point hydrocarbon and difusion of
steam into the beads causes the beads to expand to about 40 times
their original size. At this stage the beads aren't fused together.
Pressure is reduced back to atmospheric pressure, and temperature is
decreased back to room temperature, and this causes air permeation
into the beads. The beads are loaded into a mould through which
steam is passed, and they expand again by a small amount, and the
enclosure of the mold consolidates the beads into a block.
Note: the next time you use a polystyrene container, if you
look closely you might be able to see the interfaces where beads
came together to form the product in the mould.
The styrofoam product has a density as low as 1 lb per cubic foot, a low
thermal conductivity, and a low softening point.
Styrene-Acrylonitrile Copolymers (SAN)- SAN copolymers contain 20 to
30% acrylonitrile copolymerized by solution polymerization. Compared to
PS homopolymer, SAN copolymers have a higher softening point and improved
impact strength.
SAN copolymers have a higher resistance to hydrocarbons and oils
than PS homopolymer
because of the polar nature of the acrylonitrile.
This ties into the ideas of solubility parameters and compatibility. The
inclusion of the acrylonitrile monomers makes the difference in solubility
parameters between the the polymer and the solubility parameters of
hydrocarbons and oils more significant.
The higher the acrylonitrile content, the greater the heat resistance,
impact strength, and chemical resistance but the ease of molding
declines.
Higher mechanical strength usually corresponds to an increase in temperature
resistance. Molding requires that the polymer flow at temperatures
below the temperature where degradation occurs.
Styrene Butadiene Rubber was synthesized during World War II as a
substitute after the Japanese took control of the natural rubber plantations.
Acrylonitrile-Butadiene-Styrene terpolymers (ABS)- The impact
strength of SAN is higher than the impact strength of PS, but it is
still sufficiently low to make it a limiting factor in many applications.
Adding a rubbery material such as butadience would improve impact
properties. The term "ABS polymer" is misleading because it makes you
think of a terpolymer, but this is not the case. ABS polymers are
prepared by either 1) blending or 2) grafting.
Blending: SAN copolymers and ANB (acrylonitrile butadiene) copolymers
are blended together.
Example:
70 parts of a 70:30 styrene : acrylonitrile copolymer
40 parts of a 35:65 acrylonitrile : butadiene copolymer
The material must be crosslinked with peroxide type free radical reaction
in order to obtain the desired properties (the mixture is not completely
soluble in the copolymer), which are high impact strength and high
softening points.
Another way is to mix the solids on a two-roll mill. The non-crosslinked
butadiene rubber may be used as a starting material. The rubber is crosslinked
with peroxide by milling and then SAN copolymer is added.
Grafting ABS: acrylonitrile and styrene are polymerized in the presence
of a polybutadiene. Mechanically blend (parts by weight):
polybutadiene latex solids: 34
acrylonitrile 24
styrene 42
water 200
surfactant 2
mercaptants 1 (transfer agent)
potassium persulfate 0.2 (initiator)
carried out at 50 deg C
a solid product is isolated
The two methods mentioned above differ in the following:
- Blending is a blend of two copolymers done physically followed
by crosslinking.
- Grafting starts with a backbone of polybutadiene to which SAN
is grafted and then copolymerized.
The two end products don't differ much after they have been
crosslinked.
Once crosslinked, high impact strength and high softening points
are achieved.
When ABS is btained by blending and is treated with a solvent such
as MEK the sample swells and only partially breaks up; this indicates
that rubber network permeates the styrene acrylonitrile copolymer
matrix.
The ABS prepared by grafting consists of mixtures of polybutadiene,
polybutadiene grafted with acrylonitrile and styrene, and SAN
copolymer.
Grafted ABS is more branched than blended ABS. Grafted ABS is
superior to blended ABS in that moulded specimens commonly have a
better surface appearance.
Applications for ABS include automobile armrests and panels, suitcases,
shoe heels, and sewer pipes.
Poly(vinyl chloride) and related polymers
Vinyl chloride is prepared from acetylene and ethylene.
Preparation of acetylene:
CaO + 3C --> CaC + CO I'm not sure if CaO + 3C simply
2 means "react calcium oxide with
graphite carbon."
-
CaC + 2H O --> H - C - C - H + Ca(OH)
2 - 2
acetylene
or
thermal
cracking -
2 CH -------> H - C - C - H + 3H
4 - 2
Ethylene can be prepared from ethane by cracking:
H H H H
| | | |
H - C - C - H --> C = C
| | | |
H H H H
and vinyl chloride can be prepared from acetylene:
H H
_ HCl | |
H - C - C - H -----> C = C
- | |
H H
The reaction is carried out in the vapor phase in a multi-tubular
reactor packed with a catalyst of mercuric chloride on activated
charcoal. The reaction is highly exothermic, thus cooling must be applied
to keep the temperature between 100 and 180 deg C. Pressures between
5 and 10 atm are used. Gases from the reactor are cooled and washed
in aqueous NaOH to remove unreacted HCl.
The product is then liquified by cooling to -40 deg C and pure vinyl
chloride is obtained by fractional distillation. Vinyl chloride
is colorless, with a boiling point of -14 deg C, a pleasant sweet
odor, and it does not need an inhibitor for storage.
Caution!- vinyl chloride is a possible carcinogen, and it is
toxic.
Preparation of vinyl chloride from ethylene:
H H Cl H H H H
| | 2 | | | |
C = C ---> H - C - C - H ---> C = C + HCl
| | | | | |
H H Cl Cl H Cl
ethylene 1,2-dichloroethane chloroethene
In the above, the gases of ethylene and chloride are allowed to
react in a solvent. A metal catalyst, ferric chloride, is used,
and the reaction temperature is kept down to avoid formation of
highly chlorinated compounds.
The 1,2-dichloroethane is dehydrochlorinated by passing it at
500 deg C and at 3 atm pressure over a catalyst of Kaolin. The
gases are quenched in a stream of unreacted 1,2-dichloroethane
(the notes use the name ethylene dichloride), and gases which do
not condense are scrubbed with water to recover HCl.
Vinyl chloride is obtained from the liquid mixture by distillation at
5 atm pressure and purified by redistillation.
Preparation of vinyl chloride from ethylene is more expensive
and the engineer must worry about the recovery of HCl.
The preparation of poly(vinyl chloride) is mainly done by suspension
polymerization, carried out batchwise in a stirred reactor, jacketed
for heating and cooling. The reaction is run at 50 to 100 deg C
and the pressure must be 100 psi. Once the reaction is completed,
the pressure is essentially gone (i.e., the monomer is all polymerized.)
This reaction requires monomer, water, surfactant, and stirring of
the initiator. The shape of the polymer particles depends on the suspending
agent used. The syndiotacticity of PVC may be increased by polymerizing
below -40 deg C, using highly active In (indium) and the result
is a more brittle product. Atactic PVC has better qualities for
commercial applications.
Properties of Poly(vinyl chloride)
- PVC is colorless and rigid
- PVC has a high P and low S point (huh?)
- PVC has a higher D and power factor than polyethylene
- The high chlorine content of PVC renders it flame retarding
- The glass transition temperature of 50-60 deg C precludes the use
of PVC for hot water pipes (the hot water would take the polymer
into the rubbery transition and over time there could be dimensional
distortion.)
- PVC is soluble in proton acceptor solvents such as THF, esters,
and MEK
- PVC is unaffected by mild acids, basic materials and water.
- exposure to temperatures above 70 deg C and exposure to UV light
have adverse effects on its properties.
- PVC is attacked by strong oxdizing agents.
PVC can be plasticized with DSP aryl phosphates (tritolyl phosphate),
esters of aliphatic aciesd (dibuytlsebocate), epoxidized oil (soybean
oil), 40-60 parts of plasticizer per 100 parts of polymer.
Unplasticized PVC is outstanding for its anti-corrosion ability.
Exposure to UV light or heat leads to degradation vi dehydrochlorination
or oxidation.
H H H H H H H H H H H H H H H H
| | | | | | | | | | | | | | | |
- C - C - C - C - C - C - C - C - ---> - C - C - C - C = C - C - C - C - + HCl
| | | | | | | | | | | | | |
H Cl H Cl H Cl H Cl H Cl H Cl H Cl
/ \ / \
| |
| |
| |
UV RADIATION ATTACK!
Dehydrochlorination can occur until all chlorine atoms are gone. The above
reaction leads to a yellowing of the plastic.
A Stabilizing agent is added to interfere with the degradation process.
Low temperature chlorination of PVC-
If PVC is chlorinated, via exposure to chlorine gas in a 50 deg C chloroform
solution, and illumination
(UV I guess, possibly just visible--I need to look this up), then chlorines
attack hydrogens on the methylene groups [methylene is -CH2- .]
H H H H H H H H Cl H Cl H Cl H Cl H
| | | | | | | | | | | | | | | |
- C - C - C - C - C - C - C - C - ---> - C - C - C - C - C - C - C - C -
| | | | | | | | | | | | | | | |
H Cl H Cl H Cl H Cl H Cl H Cl H Cl H Cl
The notes say "syntical dichloroethylene is formed." Perhaps this was
meant to read "syndiotactical dichloroethylene" but that would be a
misnomer because tacticity refers to one functional group per repeat
unit. Anyhow...
The extra chlorines increase the glass transition temperature to 100 deg
C, and there is an increase in the melt viscosity. Chlorinated PVC
is suitable for industrial and domestic (in your house) plumbing for
hot effluents (hot water.)
High temperature chlorination of PVC
- conducted in solution at 100 deg C
- substitution occurs extensively at the
Cl
|
-C-
|
H
group and chain scission also occurs
- soluble in low cost solvents such as acetone, butylacetate, and methylene
chloride
- useful for adhesives, protective coatings, spinning fibres, and chemical
filter cloth
- low softening point, low impact strength
- poor color stability
Copolymers of PVC- vinyl acetate comonomer is added to increase the
solubility and to improve the moulding characteristics by lowering the
temperature necessary for polymer flow.
Applications include records, flooring compositions, fibres, surface coatings,
solution pml. (?), and moulding application-suspension (?)
Vinylidene chloride copolymers (5-12% vinylidene chloride)
The properties are similar to the poly(vinyl chloride-co-vinyl acetate)
copolymer.
Poly(vinyl choride-co-vinylidene chloride) is used for calendering applications
and filler polymer in rigisols.
Acrylonitrile copolymer (60% VC and 40% AN)- used for nonflammable
fibres, and industrial garments. Noted for good chemical resistance.
Olefin Copolymers (3-10% ethylene or propylene)- used for blow-moulded
bottles.
Vinylidene chloride production
H H Cl H H heat H Cl
| | 2 | | -HCl | |
C = C ---> H - C - C - Cl ---> C = C
| | | | | |
H Cl Cl Cl H Cl
The liquid phase chlorination of vinyl chloride at 30 to 50 deg C under
pressure (the first arrow reaction shown above--the second arrow reaction
probably requires heat.)
or
H H Cl H H heat H Cl
| | 2 | | -HCl | |
H - C - C - H ---> H - C - C - Cl ---> C = C
| | | | | |
Cl Cl Cl Cl H Cl
The liquid phase chlorination of ethylene dichloride (the first arrow
reaction shown above--the second arrow reaction probably requires heat.)
H Cl
| |
-[- C - C -]-
| |
H Cl
The properties
(I'm not sure if the following applies to poly(vinylidene chloride) homopolymer
or to poly(vinyl chloride-co-vinylidene chloride) copolymer
include a high crystallinity
and a high crystalline melting point (220 deg C.)
Applications include filaments, car upholstery, garden chair fabrics,
toughness, chemical resistance, and packing films.
The main uses of poly(vinylidene chloride-co-acrylonitrile) copolymers
include coatings for materials such as cellophane, paper, and polyethylene.
The coatings confer moisture and gas impermeability (due to the chlorines)
and they are heat-sealable.
Prepared by the liquid phase chlorination of vinyl chloride at 30 to 50 deg C
under pressure.
Last Update- May 28, 1995- wld