Acrylic Polymers

7/27/2019 Acrylic Polymers

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Monomers

The principal commercial processes for the

production of acrylate esters are based on

ethylene cyanohydrin, the carbonylation ofacetylene, or the polymerization and

depolymerization of B-propiolactone to give

acrylic acid or an acrylate ester.


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Methyl methacrylate is made by heating acetone cyanohydrin(from the addition of hydrocyanic acid to acetone) with sulfuricacid to form methacrylamide sulfate.

OH CH3

l H2SO4 l CH3OH

CH3-C-CN--------CH2=CCNH2.H2SO4-------->

l 125oC ll H2O

CH3 O

CH3

l

CH2=CCOCH3

ll

O


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Polymerization

Poly methyl methacrylate for molding or

extrusion is made by bulk or suspension

polymerization. The production of castsheets, rods, and tubes is carried out by bulk

polymerization starting in most cases with a

syrup of partially polymerized methyl

methacrylate with a convenient viscosity forhandling.


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Properties

Polymethyl methacrylate is a linearthermoplastic, about 70-75% syndiotactic.

Because of its lack of completestereoregularity and its bulky sidegroups, it isamorphous. Both isotactic and syndiotacticpolymethyl methacrylate have been preparedbut have not been offered commercially.Polymethyl methacrylate is resistant to manyaqueous inorganic reagents, including dilutealkalis and acids.


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It is quite resistant to alkali saponification, incontrast to the polyacrylates. It undergoespyrolysis almost completely to monomer by achain reaction.

Perthaps the outstanding property ofpolymethyl methacrylate is its optical clarityand lack of color. Coupled with its unsuallygood outdoor weathering behavior, its opticalproperties make it highly useful in allapplications where light transmission isimportant.


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The mechanical and thermal properties of the

polymer are also good. Tensile strength ranges as

high as 10,000 psi. Impact strength is about equal to

that of the impact-resistant styrene copolymaers.

Heat deflection temperatures are above 90 oC for theheat resistant grades of polymethyl methacrylate

molding powder. Electrical properties are good but

not outstanding. Fabricability is quite good; only

slightly higher temperatures are needed fro moldingpolymethyl methacrylate than for polystyrene.

Polymethyl methacrylate is less susceptible to

crazing than is polystyrene.


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A limitation to the optical uses of the material

is its poor abrasion resistance compared to

glass. Despite considerable effort, attempts

to improve the scratch resistance or surfacehardness of polymethyl methacrylate have so

far been accompanied by deterioration in

other properties, such as impact strength.


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Application

Injection-molded acrylic articles includeautomotive lenses, reflective devices.

Instrument and appliance covers, opticalequipment, and home furnishings.

Acrylic sheeting is used for signs, glazing (inparticular, aircraft windows), furniture,

partitions, and lighting-fixture diffusers.About 0.5billion of acrylic plastics was sold in

1982.


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Ciopolymers of methyl methacrylate, ethyl

acrylate, and monomers containing reactive

functional groups are widely used as

thermosetting resins in baked enamelapplications. The functionality can be derived

from amines (acrylamide, methyl

methacrylates or methacrylates).


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POLYVINYL CHLORIDE

Monomer

Vinyl chloride is a gas boiling at -14oC. It is

produced by the dechlorination o9f ethylene

dichloride, which is made by reactingethylene with chlorine. Almost all the vinyl

chloride produced is used in polymerization,

with greater care being taken throughout

because of the carcinogenic nature of themonomer.


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Polymerization

Suspension polymerization is used for the

production of well over 80% of polyvinyl

chloride. Small amounts are made bysolution emulsion, and bulk polymerization,

despite difficulties in the latter process

resulting from the insolubility of the polymer in

its monomer.


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Structure

Polyvinyl chloride is a partially syndiotactic

material, with sufficient irregularity of structure

that crystallinity is quite low. Its structuralcharacterization is complicated by the

possibility of chain branching and the

tendency of the polymer to associate in

solution.


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Stability

Polyvinyl chloride is relatively unstable to

heat and light. Thermal initiation involves

loss of chlorine atom adjacent to somestructural abnormailty which reduces the

stability of C-Cl bond. The chlorine radical so

formed abstracts a hydrogen to form HCl; the

resulting chain radical then reacts to formchain unsaturation with regeneration of a

chlorine radical.


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The reaction can also be initiated by ultra

violet light which is absorbed at

unsaturated structures with liberation of an

adjacent chlorine atom. In the presence ofoxygen, both chain reactions are accelerated,

and ketonic structures are formed in the

chain.


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Stabilizers are almost invariably added to

improve the heat and light stability of the

polymer. Metallic salts of lead, barium, tin,

or cadmium are used. Oxides, hydroxides, orfatty-acids are most effective. Epoxy

plasticizers aid materially in stabilizing the

resin. Free radical acceptance appears to be

a prominent mechanism or stabilization.


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Vinyl Resins

a) Rigid compounds the term rigid vinyls

usually refer to unplasticized polyvinyl

chloride or compositions with only a fewpercent of a plasticizer such as epoxy resin.

Often other polymers are mixed physically with

the polyvinyl chloride to improve impact

resistance.


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Copolymers

The advantages in polymer properties resultingfrom the copolymerization of small amounts of vinylacetate with vinyl chloride were discovered around

1928.The lower softening point and higher solubility of

the copolymers make fabrication very much easier.Stability is improved over that of homopolymer, colorand clarity are also better. Polymerization methods

are similar to those for vinyl chloride homopolymerexcept that emulsion polymerization has had lesssuccess.


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Polymers containing around 10% vinylidene

chloride have better tensile properties than

pure polyvinyl chloride. Copolymers

containing 10-20% diethyl fumarate or diethylmaleate have improved workablity and

toughness and retain the high softening point

of polyvinyl chloride. Acrylic esters have also

been used to impart improvements insolubility and workability.


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Plasticization

Many properties of polyvinyl chloride andvinyl chloride vinyl acetate copolymers are

improved by plasticization. The large majorityof commercial production of vinyl resins is inthe form of plasticized compositions. The firstimportant plasticizer for the vinyls wastricresyl phosphate, which has since beenreplaced by other esters because of itstendency to cause low-temperaturebrittleness in plasticized compounds.


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The plasticizer content varies widely with the

end use of the material, but typically may be

around 30% by weight.


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Plastisols and Organisols

These liquid compositions are produced by spraydrying lattices obtained from the emulsionpolymerization of vinyl chloride. The latex particles

are dispersed into plasticizers to make plastisols, orinto mixtures of plasticizers and volatile organicliquids to make organisols. Other ingredients suchas stabilizers, fillers, colorants, surfactants, andpossibly blowing agents or gelling agents are also

present. The polymer particles do not dissolve in theliquids, but remain dispersed until the mixture isheated. Fusion plus loss of solvent then yield thefinal product.


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Application

About 55% of polyvinyl chloride is used as

rigid resins, the remainder plasticized. The

single largest use, in the rigid field, is pipe,accounting for 40% of production. It is used

for water supply distribution, agricultural

irrigation, chemical processing, drain, waste,

and vent pipe, sewer systems, and conduitsfor electrical and telephone cables.


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The building construction market accounts forabout another 30% of polyvinyl chlorideproduction, including siding, window frames,gutters, and interior molding and trim (12%);flooring (5%); wire and cable insulation (7%);and wall coverings, upholstery, showercurtains, gaskets, and so on (5%).

Automotive uses, meat and food packaging,

bottles, footwear, outerwear, phonographrecords, sporting goods, and toys are othersignificant matrkets.


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Properties and Uses

Polyvinyl chloride are insensitive to

moisture and have high wrinkle resistance

and good resistance to chemicals, insects,fungi, and the like. They have poorer

dimensional stability at high temperatures,

and somewhat less strength, elasticity, and

abrasion resistance.

Documents

Acrylic Polymers