Plastics Materials - J. A. Brydson - 7th Edition - Chapter 15

15

Acrylic Plastics

15.1 INTRODUCTION

Poly(methy1 methacrylate) (Figure 15.1, I) is, commercially, the most important member of a range of acrylic polymers which may be considered structurally as derivatives of acrylic acid (11).

This family includes a range of polyacrylates (111), polymethacrylates (IV) and the important fibre-forming polymer, polyacrylonitrile (V).

Methyl, ethyl and allyl acrylate were first prepared in 1873 by Caspary and Tollens,’ and of these materials the last was observed to polymerise. In 1880 Kahlbaum2 reported the polymerisation of methyl acrylate and at approximately the same time Fittig”‘ found that methacrylic acid and some of its derivatives readily polymerised.

In 190 1 Otto Rohm reported on his studies of acrylic polymers for his doctoral dissertation. His interest in these materials, however, did not cease at this stage and eventually in 1927 the Rohm and Hass concern at Darmstadt, Germany commenced limited production of poly(methy1 acrylate) under the trade names

CH

- (- CH,- CH-),- I 3 - (---CH2-C--),- CH,=CH

I COOR

I COOH COOCH,

I I1 ILI

I

CH, I I

-(-CH,-CC--.),- -(-CH2--CH-),- I

CN COOR

IV V Figure 15.1

398

Introduction 399

Acryloid and Plexigum. These were soft gummy products of interest as surface coatings rather than as mouldable plastics materials. About 1930 R. Hill in England and W. Bauer in Germany independently prepared poly(methy1 methacrylate) and found it to be a rigid, transparent polymer, potentially useful as an aircraft glazing material.5

The first methacrylic esters were prepared by dehydration of hydroxyisobutyric esters, prohibitively expensive starting points for commercial synthesis. In 1932 J. W. C. Crawford6 discovered a new route to the monomer using cheap and readily available chemicals-acetone, hydrocyanic acid, methanol and sulphuric acid- and it is his process which has been used, with minor modifications, throughout the world. Sheet poly(methy1 methacrylate) became prominent during World War I1 for aircraft glazing, a use predicted by Hill in his early patents, and since then has found other applications in many fields.

Examples of commercial poly(methy1 methacrylate) sheet are Perspex (ICI), Oroglas and Plexiglas (Atoglas). Poly(methy1 methacrylate) moulding powders include Diakon (ICI), Acry-ace (Fudow Chemical Co., Japan), Lucite (Du Pont) and Vedril (Montecatini).

In addition to poly(methy1 methacrylate) plastics and polyacrylonitrile fibres, acrylic polymers find widespread use. First introduced in 1946, acrylic rubbers have become established as important special purpose rubbers with a useful combination of oil and heat resistance. Acrylic paints have become widely accepted particularly in the car industry whilst very interesting reactive adhesives, including the well-known ‘super-glues’ are also made from acrylic polymers.

During the 1970s there was considerable interest for a time in copolymers with a high acrylonitrile content for use as barrier resins, i.e. packaging materials with low permeability to gases. Problems associated with free acrylonitrile have, however, led to the virtual disappearance of these materials from the market.

Other developments in recent years have been the appearance of tough and heat-resistant materials closely related to poly(methy1 methacrylate) and to interesting cross-linked polymers. Amongst these are the so-called hydrophilic polymers used in the making of soft contact lenses.

Today a very wide range of acrylic materials is available with a broad property spectrum. The word acrylic, often used as a noun as well as an adjective in everyday use, can mean quite different things to different people. In the plastics industry it is commonly taken to mean poly(methy1 methacrylate) plastics, but the word has different meanings, to the fibre chemist and to those working in the paint and adhesives industries. Unless care is taken this may be a source of some confusion.

As with other major plastics materials, there is at present little use of the IUPAC systematic nomenclature, which is based on the nature of the repeating unit rather thafi the monomer used. The following names may, however, be noted:

Trivial name IUPAC name

Poly(acry1ic acid) Poly(acrylonitri1e) Poly(methy1 acrylate) Poly(methy1 methacrylate)

Poly-[I-(carboxy)ethylene] Poly-[I-(cyano)ethylene] Poly-[I-methoxycarbonyl)ethylene] Poly-[I-methoxycarbonyl)-I-methylethylene]

400 Acrylic Plastics

15.2 POLY(METHYL METHACRYLATE)

15.2.1 Preparation of Monomer

This successful commercial utilisation of poly(methy1 methacrylate) is due in no small measure of the process of producing the monomer from acetone developed by Crawford of IC1 which enabled the polymer to be produced at a competitive price. Some details of the process as operated by the Rohm and Hass Company of Philadelphia have been di~closed.~

Acetone is first reacted with hydrogen cyanide to give acetone cyanohydrin (Figure 15.2)

CH, I

I CN

C = O + HCN -----+ CH,-C-OH \ /

CH,

CH, Figure 15.2

The cyanohydrin is then treated with 98% sulphuric acid in a cooled hydrolysis kettle to yield methacrylamide sulphate (Figure 15.3)

CH, I

CH, I

CH,-C-OH + H,SO, - CH,=C I I CN CONH, * H,S04

Figure 15.3

The sulphate is not isolated from the reaction mixture, which passes into an esterification kettle and reacts continously with methanol (Figure 15.4).

CH, I

CH, I

I I CONH, . H,SO, COOCH,

CH,= C + CH,OH - CH, = C + NH,HS04

Figure 15.4

The esterified stream, which may contain inhibitors to prevent premature polymerisation, is then passed to a stripping column which separates the methyl methacrylate, methanol and some water from the residue made up of sulphuric acid, ammonium bisulphate and the remainder of the water. The methyl methacrylate is subsequently separated and purified by further distillation.

Because of limitations on the ready availability of HCN, particularly in Japan, processes involving the oxidation of C4 intermediates have been developed and are now replacing the older route developed by Crawford. One important process is based on the two-stage oxidation of isobutylene or t-butyl alcohol to methacrylic acid, which is then separated and esterified Figure 1 5 . 5 ~ ) .

Poly(methy1 methacrylate) 401

CH, I I

I I

CH,

CH,=C + ROH- CH,=C + H,O

COOH COOR

(a) (Japan Koka 74117425, 7795609, 79300008, 78109889, USP 3 928 462)

CH3 I

CH3 Nitrogen I -H,O

CH, - C=CH, * CH3- C-COOH CH,=C-COOH I Oxides I

OH

CH3 I CH OH 3 C H = C I COOCH,

(b) Figure 15.5

This process appears to be very similar to the process developed by the Escambia Chemical Company which has been known for over 30 years and mentioned in all the previous editions of this book (Figure 15.5b).

The monomer is a mobile liquid with a characteristic sweet odour and with the following properties:

Boiling point (760 mmHg) 100.5”C Density D4” 0.936-0.940 g/cm3

Heat of polymerisation 48.5 kJ/mole Refractive index nD2’ 1.41 3-1.416

15.2.2 Polymerisation

Methyl methacrylate will polymerise readily and the effect may be observed with non-inhibited samples of monomers during storage. In commercial practice the monomer is supplied with up to 0.10% of an inhibitor such as hydroquinone, which is removed before polymerisation, either by distillation under reduced pressure or, in some cases, by washing with an alkaline solution.


Free-radical polymerisation techniques involving peroxides or azodi- isobutyronitrile at temperatures up to about 100°C are employed commercially. The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction. It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas.

It has been observed that in the polymerisaton of methyl methacrylate there is an acceleration in the rate of conversion after about 20% of the monomer has been converted. The average molecular weight of the polymer also increases during polymerisation. It has been shown that these results are obtained even under conditions where there is a negligible rise in the temperature (4°C) of the reaction mixture.

The explanation for this effect (known variously as the gel effect, Tromsdorff effect or auto-acceleration effect) is that the chain termination reaction slows down during conversion and, as can be seen by reference to equations (2.5) and (2.6), a decrease in the termination rate constant leads to an increase in both overall rate and molecular weight. The reason for the drop in termination rate is that as the reaction mixture becomes more viscous the radical ends of the polymer chains find increased difficulty in diffusing towards each other, leading to the important mutual termination reaction. Small monomer molecules on the other hand find little difficulty in diffusion at moderate conversion so that propagation reactions are relatively little affected, until the material becomes semi-solid, when the propagation rate constant also decreases. It is of interest to note that the gel effect may be induced by the addition of already formed poly(methy1 methacrylate) or even another polymer such as cellulose tripropionate because such additions increase the viscosity of the system.

The auto-acceleration effect appears most marked with polymers that are insoluble in their monomers. In these circumstances the radical end becomes entrapped in the polymer and termination reactions become very difficult. It has been suggested that, in thermodynamic terms, methyl methacrylate is a relatively poor solvent for poly(methy1 methacrylate) because it causes radicals to coil while in solution. The termination reaction is then determined by the rate at which the radical ends come to the surface of the coil and hence become available for mutual termination.

Polymerisation in bulk

Bulk polymerisation is extensively used in the manufacture of the sheet and to a lesser extent rod and tube. In order to produce a marketable material it is important to take the following factors into account:

(1) The exotherm developed during cure. (2) The acceleration in conversion rate due to increasing viscosity. ( 3 ) The effect of oxygen. (4) The extensive shrinkage in conversion from monomer to polymer (-20%). (5) The need to produce sheet of even thickness. (6) The need to produce sheet of constant quality. (7) The need to produce sheet free from impurities and imperfections.


In order to reduce the shrinkage in the casting cell, and also to reduce problems of leakage from the cell, it is normal practice to prepare a ‘prepolymer’. In a typical process monomer freed from inhibitor is heated with agitation for about 8 minutes at 90°C with 0.5% benzoyl peroxide and then cooled to room temperature. Plasticiser, colouring agents and ultraviolet light absorbers may be incorporated at this stage if required. The resulting syrup, consisting of a solution of polymer in monomer, is then filtered and stored in a refrigerator if it is not required for immediate use. The heating involved in making the prepolymer may also be of assistance in removing oxygen dissolved in the monomer.

The preparation of a prepolymer requires careful control and can be somewhat difficult in large-scale operations. An alternative approach is to prepare a syrup by dissolving some polymer in the monomer and adding some peroxide to the mixture. As in the case of a prepolymer syrup, such a syrup will cause less shrinkage on polymerisation and fewer leakage problems.

Acrylic sheet is prepared by pouring the syrup into a casting cell. This consists of two plates of heat-resistant polished glass provided with a separating gasket round the edges. The gasket commonly consists of a hollow flexible tube made from a rubber, or from plasticised poly(viny1 alcohol). The cell is filled by opening up the gasket at a corner or edge and metering in the syrup, care being taken to completely fill the cell before closing up the gasket. The cell is held together by spring-loaded clamps or spring clips so the plates will come closer together as the reacting mixture shrinks during polymerisation. This technique will enable the sheet to be free of sink marks and voids.

It is important to use rigid glass sheet and to apply pressure to the plates in such a manner that they do not bow out as this would lead to sheet of uneven thickness.

The filled cells are then led through a heating tunnel. In a typical system the time to pass through the tunnel is about 16 hours. For the first 14 hours the cell passes through heating zones at about 40°C. Under these conditions polymerisation occurs slowly. Any acceleration of the rate due to either the rise in temperature through the exothermic reaction or due to the viscosity-chain termination effect will be small. It is particularly important that the temperature of any part of the syrup is not more than 100°C since this would cause the monomer to boil. By the end of this period the bulk of the monomer has reacted and the cell passes through the hotter zones. After 15 hours (total time) the cell is at about 97”C, at which temperature it is held for a further half-hour. The sheet is then cooled and removed from the cell. In order to reduce any internal stresses the sheet may be annealed by heating to about 140°C and, before being dispatched to the customer, the sheet is masked with some protective paper using gelatine or, preferably, with a pressure-sensitive adhesive.

When casting large blocks, the exothexm problem is more severe and it may be necessary to polymerise inside a pressure vessel and thus raise the boiling point of the monomer.

In order to compensate for shrinkage, special techniques are required in the manufacture of rod. In one process, vertical aluminium tubes are filled with syrup and slowly lowered into a water bath at 40°C. As the lowest level of syrup polymerises, it contracts and the higher levels of syrup thus sink down the tube, often under pressure from a reservoir of syrup feeding into the tubes.

Acrylic tubes may be prepared by adding a calculated amount of syrup to an aluminium tube, sealing both ends, purging the air with nitrogen and then rotating horizontally at a constant rate. The whole assembly is heated and the


syrup polymerises on the wall of the rotating tube. The natural shrinkage of the material enables the casting to be removed quite easily.

An interesting modification of the sheet casting process is the band polymerisation process due to Swedlow.8 In this process a monomer/polymer syrup is polymerised between steel bands which pass through heating zones and which are spaced according to the sheet thickness required. Whilst there may be some economic attraction of the process in some countries with high labour costs the quality of the product is generally inferior to that of cell-cast sheet. Furthermore, where lower optical qualities are tolerable extruded sheet is generally cheaper to produce. The process, as with the cast cell process, does however allow for the possibility of cross-linked polymer sheet that cannot easily be produced by extrusion processes.

Suspension polymerisation

The average molecular weight of most bulk polymerised poly(methy1 methacrylates) is too high to give a material which has adequate flow properties for injection moulding and extrusion.

By rolling on a two-roll mill the molecular weight of the polymer can be greatly reduced by mechanical scission, analogous to that involved in the mastication of natural rubber, and so mouldable materials may be obtained. However, bulk polymerisation is expensive and the additional milling and grinding processes necessary make this process uneconomic in addition to increasing the risk of contamination.

As a result the suspension polymerisation of methyl methacrylate was developed to produce commercial material such as Diakon made by ICI. Such a polymerisation can be carried out rapidly, usually in less than an hour, because there is no serious exotherm problem.

There is, however, a problem in controlling the particle size of the beads formed and further in preventing their agglomeration, problems common to all suspension-type polymerisations. The particle size of the beads is determined by the shape and size of the reactor, the type and rate of agitation and also the nature of suspending agents and protective colloids present. Suspending agents used include talc, magnesium carbonate and aluminium oxide whilst poly(viny1 alcohol) and sodium polymethacrylate are among materials used as protective colloids.

In one process described in the literature’ one part of methyl methacrylate was agitated with two parts of water and 0.2% benzoyl peroxide was employed as the catalyst. Eight to 18 g of magnesium carbonate per litre of reactants were added, the lower amount being used for larger beads, the larger for small beads. The reaction temperature was 80°C initially but this rose to 120°C because of the exothermic reaction. Polymerisation was complete in about an hour. The magnesium carbonate was removed by adding sulphuric acid to the mixture. The beads were then filtered off, carefully washed and dried.

Other additives that may be incorporated include sodium hydrogen phosphates as buffering agents to stabilise that pH of the reaction medium, lauryl mercaptan or trichlorethylene as chain transfer agents to control molecular weight, a lubricant such as stearic acid and small amounts of an emulsifier such as sodium lauryl sulphate.

The dried beads may be supplied as injection moulding material without further treatment or they may be compounded with additives and granulated.


15.2.3 Structure and Properties

Commercial poly(methy1 methacrylate) is a transparent material, and micro- scopic and X-ray analyses generally indicate that the material is amorphous. For this reason the polymer was for many years considered to be what is now known as atactic in structure. It is now, however, known that the commercial material is more syndiotactic than atactic. (On one scale of assessment it might be considered about 54% syndiotactic, 37% atactic and 9% isotactic. Reduction in the temperature of free-radical polymerisation down to -78°C increases the amount of syndiotacticity to about 78%).

Substituents on the a-carbon atom restrict chain flexibility but, being relatively small, lead to a significantly higher Tg than with polyethylene. Differences in the Tg’s of commercial polymers (approx. 104”C), syndiotactic polymers (approx. 115°C) and anionically prepared isotactic polymers (45°C) are generally ascribed to the differences in intermolecular dipole forces acting through the polar groups.

In consequence of a Tg of 104°C with its amorphous nature, commercial poly(methy1 methacrylate) is thus a hard transparent plastics material in normal conditions of use.

Because the polymer is polar it does not have electrical insulation properties comparable with polyethylene. Since the polar groups are found in a side chain these are not frozen in at the Tg and so the polymer has a rather high dielectric constant and power factor at temperatures well below the Tg (see also Chapter 6). This side chain, however, appears to become relatively immobile at about 20”C, giving a secondary transition point below which electrical insulation properties are significantly improved. The increase in ductility above 40°C has also been associated with this transition, often referred to as the @transition.

The solubility of commercial poly(methy1 methacrylate) is consistent with that expected of an amorphous thermoplastic with a solubility parameter of about 18.8 MPa’”. Solvents include ethyl acetate (6 = 18.6), ethylene dichloride (6 = 20.0), trichloroethylene (6 = 19), chloroform (6 = 19) and toluene (6 = 20), all in units of MPa’/*. Difficulties may, however, occur in dissolving cast poly(methy1 methacrylate) sheet because of its high molecular weight.

Since the polymers are unbranched (apart from the methyl and methacrylate side groups) the main difference between uncompounded commercial grades is in the molecular weight.

Cast material is stated to have a number average molecular weight of about lo6. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties.

15.2.4 General Properties of Poly(methy1 methacrylate) As indicated in the previous section poly(methy1 methacrylate) is a hard, rigid, transparent material. Commercial grades have extremely good weathering resistance compared with other thermoplastics.


Table 15.1 Some properties of methyl methacrylate polymers

Property

Molecular weight (En) Specific gravity Tensile strength

Tensile modulus

Flexural strength

Flexural modulus

Rockwell hardness Scratch hardness

(Moh's scale) Water absorption

[% in 24 h(20"C)I Izod impact strength

Vicat softening point Heat deflection

temperature(2641bf/in2) (1.82MPa)

Refractive index nD2" Volume resistivity (20°C) Dielectric constant at 10'

Hz 60% R.H.(2O0C)

Units

- -

10' Ibf/in2 MPa

lo3 Ibf/in' MPa

Io" Ibf/in2 MPa

lo3 Ibf/in* MPa - -

%

ft Ibf in-'

O C "C

a m

ASTM test

method

-

D.792 D.638

-

-

-

D.785

D.570

(B.S.) 2782

D.648

Acylic sheet*

-106 1.19

-430 (3000)

-20

-400 (2750) M.lOO

(140)

0.2

100

1.49 >loL6

3 .O

Moulding composition?

-60 000 1.18 10.5

(72.5) -350

(2400) -18

-400 (2750) MI03

-

2-3

0.3

0.40

109-112 85-95

1.49 >io17

3.1

Copolymer$

- 1.17

- -400

(2750) -18

(130)

-

0.25

80

1.49 -

* Persrx (ICI) t Diakon M (ICI) $ Astente (ICI) (withdrawn)

The properties of three types of poly(methy1 methacrylate) (sheet based on high molecular weight polymer, lower molecular weight injection moulding material and a one-time commercial copolymer) are given in Table 15.1.

As might be expected of a somewhat polar thermoplastics material, mechanical, electrical and other properties are strongly dependent on temperature, testing 'rate' and humidity. Detailed data on the influence of these variables have been made available by at least one manufacturer and the following remarks are intended only as an illustration of the effects rather than as an attempt at providing complete data.

Figure 15.6 shows the considerable temperature sensitivity of the tensile strength of acrylic sheet whilst Figure 15.7 shows how the fracturing stress decreases with the period of loading. Mouldings from acrylic polymers usually show considerable molecular orientation. It is observed that a moulding with a high degree of frozen-in orientation is stronger and tougher in the direction parallel to the orientation than in the transverse direction.

Poly(methy1 methacrylate) is recognised to be somewhat tougher than polystyrene (after consideration of both laboratory tests and common experience) but is less tough than cellulose acetate or the ABS polymers. It is superior to untreated glass in terms of impact resistance and although it cracks, any fragments formed are less sharp and jagged than those of glass and, normally consequently less harmful. However, oriented acrylic sheet such as may result


I I I

Figure 15.6. Effect of temperature on tensile strength of acrylic sheet (Perspex) at constant rate of strain (0.44% per second). (Reproduced by permission of ICI)

PERIOD OF LOADING IN 5CC

Figure 15.7. Effect of period of loading on fracturing stress at 25°C of acrylic sheet (Perspex). (Reproduced by permission of ICI)

from double curvature shaping shatters with a conchoidal fracture and fragments and broken edges can be quite sharp. Although it is harder than most other thermoplastics the scratch resistance does leave something to be desired. Shallow scratches may, however, be removed by polishing.

The optical properties of poly(methy1 methacrylate) are particularly important. Poly(methy1 methacrylate) absorbs very little light but there is about 4% reflection at each polymer-air interface for normal incident light. Thus the light transmission of normal incident light through a parallel sheet of acrylic material free from blemishes is about 92%. The influence of the wavelength of light on transmission is shown in Figure 15.8.

The interesting property of total internal reflection may be conveniently exploited in poly(methy1 methacrylate). Since the critical angle for the polymer- air boundary is 42°C a wide light beam may be transmitted through long lengths of solid polymer. Light may thus be ‘piped’ round curves and there is little loss where the radius of curvature is greater than three time the thickness of the sheet or rod. Scratched and roughened surfaces will reduce the internal reflection. This is. normally undesirable but a roughened or cut area can also be deliberately


WAVELENGTH IN 1 Figure 15.8. Light transmission of acrylic polymer (i in thick moulded Diakon. Parallel light beam

normally incident on surface). (Reproduced by permission of ICI)

incorporated to ‘let out’ the light at that point. The optical properties of poly(methy1 methacrylate) have been exploited in the development of optical fibres.

Poly(methy1 methacrylate) is a good electrical insulator for low-frequency work, but is inferior to such polymers as polyethylene and polystyrene, particularly at high frequencies. The influence of temperature and frequency on the dielectric constant is shown in Figure 15.9.

Figure 15.9. The variation of dielectric constant with temperature and frequency (Perspex) (the lines join points of equal dielectric constant). (Reproduced by permission of ICI)

Figure 15.IO. The dependence of apparent volume resistivity on time of polarisation of acrylic polymer (Perspex). (Reproduced by permission of ICI)


The apparent volume resistivity is dependent on the polarisation time (Figure 15.10). The initial polarisation current is effective for some time and if only a short time is allowed before taking measurements low values for volume resistivity will be obtained.

As may be expected of an amorphous polymer in the middle range of the solubility parameter table, poly(methy1 methacrylate) is soluble in a number of solvents with similar solubility parameters. Some examples were given in the previous section. The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions. A number of organic materials although not solvents may cause crazing and cracking, e.g. aliphatic alcohols.

15.2.5 Additives

Poly(methy1 methacrylate) may be blended with a number of additives. Of these the most important are dyes and pigments and these should be stable to both processing and service conditions. Two particular requirements are, firstly, that when used in castings they should not affect the polymerisation reaction and, secondly, that they should have good weathering resistance.

Plasticisers are sometimes added to the polymer, dibutyl phthalate being commonly employed in quantities of the order of 5%. Use in moulding powders will enhance the melt flow but somewhat reduce the mechanical properties of the finished product.

Further improvement in light stability may be achieved by addition of small quantities of ultraviolet absorbers. Typical examples include phenyl salicylate, 2,4-dihydroxybenzophenone, resorcinol monobenzoate, methyl salicylate and stilbene.

15.2.6 Processing

In commercial practice three lines of approach are employed in order to produce articles from poly(methy1 methacrylate). They are:

(1) Processing in the melt state such as by injection moulding and extrusion. ( 2 ) Manipulation of sheet, rod and tube. (3) The use of monomer-polymer doughs.

There are a number of general points to be borne in mind when processing the polymer in the molten state which may be summarised as follows:

(1) The polymer granules tend to pick up moisture (up to 0.3%). Although most commercial grades are supplied in the dry condition, subsequent exposure before use to atmospheric conditions will lead to frothy mouldings and extrudates, owing to volatilisation of the water in the heating cylinders. Particular care should be taken with reground scrap.

( 2 ) The melt viscosities at the processing temperatures employed are con- siderably higher than those of polystyrene, polyethylene and plasticised PVC. This means that the equipment used must be robust and capable of generating high extrusion and injection pressures. The injection moulding of poly(methy1 methacrylate) (PMMA) has been made much easier by the widespread use of the reciprocating screw in-line injection moulding

4 10 Acrylic Plastics

IO

I

c \ U

.- : : I e I

5 0

e

2.

w 5 lo-'. Y a z 2

10-2,,

machines. The use of a screw with a decompression zone and a vented barrel may be useful both for injection moulding and extrusion, since it is possible to remove unwanted moisture and even monomer which has been produced by depolymerisation of the polymer because of overheating.

The melt viscosity is more sensitive to temperature than that of most thermoplastics (Figure 15.11) and this means that for accurate, consistent and reproducible results, good temperature control is required on all equipment.

(3) Since the material is amorphous the moulding shrinkage is low and normally less than 0.008 cm/cm.

RIGID P.Y.C!

SOFT P.V.C.\

POLYETHYLEN€\ M.F.I. 2

150 200 250

A great number of poly(methy1 methacrylate) products are produced by manipulation of sheet, rod and tube. Such forms may easily be machined using drills, circular saws and bandsaws, providing care is taken not to overheat the polymer. It is very difficult to weld the sheet satisfactorily but cementing techniques have been highly developed. Acrylic parts may be joined using solvents such as chloroform or by use of solutions of polymer in a suitable solvent. Generally, however, the best results are obtained, particularly where there is a gap-filling requirement, by use of a monomer-polymer solution. Commercial cements of this type either contain a photocatalyst to allow hardening by ultraviolet light polymerisation or contain a promoter so that on addition of a peroxide, polymerisation of the monomer is sufficiently rapid at room temperature to harden the cement in less than one hour.

When heated above the glass transition temperature (-lOO"C), acrylic sheet from high molecular weight polymer becomes rubbery. The rubbery range extends for 60°C. Further raising of the temperature causes decomposition rather than melting. The reasonably wide rubbery range, c.f. cellulose acetate, high- impact polystyrene and polyethylene, enables the sheet to be heated in ovens rather than having to be heated while clamped to the shaping apparatus. Poly(methy1 methacrylate) is not widely suitable for normal vacuum forming operations since the modulus of the material in the rubbery state is too great to

Poly(methy1 methacrylate) 4 11

allow shaping of fine detail simply by atmospheric pressure. As a result a large number of techniques have been devised using air pressure, mechanical pressure, or both in combination, and sometimes also involving vacuum assistance.

The use of monomer-polymer doughs has been largely confined to the production of dentures. A plaster of Paris mould is first prepared from a supplied impression of the mouth. Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough. A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water. After polymerisation, which usually takes less than half an hour, the mould is cooled and opened. This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited.

A novel technique has been developed for the manufacture of tiles and sanitary ware. A dispersion of a ground sand in methyl methacrylate monomer is prepared with a solids content of about 72% by weight. The particle size is such that the dispersion has reasonable stability but is pourable. When required for use the dispersion is blended with a free-radical initiator, usually based on a peroxide, and fed into metal moulds heated to about 70°C. As the monomer polymerises there is a shrinkage of about 1 1 % by volume and this is compensated through a reduction in the volume of the mould cavity, with one mould half moving towards the other and into the other like a piston in a cylinder. The polymerised products have a remarkably good finish, are virtually stress free and have considerable flexibility in part design. Casting dispersions are available from IC1 as Asterite (reviving a name at one time used for a now-obsolete acrylic copolymer).

15.2.7 Applications

The major uses of poly(methy1 methacrylate) arise from its high light transmission and good outdoor weathering properties. It is also a useful moulding material for applications where good appearance, reasonable toughness and rigidity are requirements which are considered to justify the extra cost of the polymer as compared with the large tonnage plastics.

For many years the market growth for poly(methy1 methacrylate) was much lower than for other major thermoplastics. For example, UK production in 1950 was about the same as that for polystyrene, in 1965 (when the first edition of this book was being completed) it was about 40% and by the end of the 1970s it was down to about 10%. There was, however, an upsurge in the late 1980s and early 1990s and world production capacity was estimated at 1.7 X 106t.p.a. in 1996. This is about 17% of the capacity for polystyrene. During the late 1990s there was a considerable capacity build-up in Asia and already by 1996 this area claimed about 38% of global capacity followed by America with 34% and Europe 28%. While the overall market is roughly divided between mouldings and sheet products extruded sheet is making inroads into the cast sheet market and in 1997 in the USA it was estimated that less than 25% of PMMA products were produced from cast (mainly sheet) materials. In Western Europe the market has been assessed at auto applications 30%, illumination engineering 20-25%, building industry 15%, optical industry 10-15%, household goods 8-lo%, and other 15%.

The material is eminently suitable for display signs, illuminated and non- illuminated, and for both internal and external use. The properties of importance here are weatherability, the variety of techniques possible which enable a wide range of signs to be produced and, in some cases, transparency.


In lighting fittings poly(methy1 methacrylate) finds an important outlet. Street lamp housings originally shaped from sheet are now injection moulded. Ceiling lighting for railway stations, school rooms, factories and offices frequently incorporate poly(methy1 methacrylate) housings. In many of these applications opalescent material is used which is effective in diffusing the light source. Poly(methy1 methacrylate) is the standard material for automobile rear lamp housings.

The methacrylic polymer remains a useful glazing material. In aircraft applications it is used extensively on aircraft which fly at speeds less than Mach 1.0. They form the familar ‘bubble’ body of many helicopters. On land, acrylic sheet is useful for coach roof lights, motor cycle windscreens and in do-it yourself ‘cabins’ for tractors and earth-moving equipment. Injection mouldings are frequently used for plaques on the centre of steering wheels and on some fascia panelling.

Transparent guards for foodstuffs, machines and even baby incubators may be fabricated simply from acrylic sheet. It should, however, be pointed out that due to rather rapid surface deterioration and the lack of ‘sparkle’ the material is not ideally suited as a cover for displayed goods.

Acrylic sheet is also employed for many other diverse applications, including baths and wash-basins, which have considerable design versatility, are available in a wide range of colours, and are cheaper and much lighter than similar products from other materials.

Extruded sheet is cheaper than cast sheet but because there is some residual molecular orientation, is somewhat less satisfactory optically and more difficult to machine. On the other hand, no doubt a function of its lower molecular weight, it may be thermoformed more easily.

The energy crisis that began in the 1970s has led to much interest in solar heating. Because of its excellent weathering properties, transparency and light weight compared with glass the material is being used for the dome-shaped covers of solar collectors. In this application it is important to use a heat-resistant film between the acrylic dome and the absorbing material, both to reduce heat loss and to protect the acrylic material if there is an accumulation of heat due to failure of the liquid circulation in the absorber.

In contrast to the above use PMMA sheet has been used as the ‘bed’ in indoor ultraviolet lamp operated solaria. Here the ultraviolet radiation is so intense as to require the use of special formulations with adequate ultraviolet resistance.

PMMA has not been able to compete in the field of compact discs, the market having gone to the polycarbonates (see Chapter 20). It is, however, suitable for optical data storage using large video discs. Large-scale acceptance in the field of optical fibres has been held back by problems of obtaining material of an acceptable level of purity.

As described in the previous section, casting dispersions based on monomer and fine sand are now finding use in high-grade sanitary ware and tiling.

Decorative plaques are produced by injection moulding poly(methy1 methacrylate) and then coating the back of the transparent moulding with a thin coat of metal by the vacuum deposition technique or with a paint by spraying. By suitable masking, more than one metal and more than one colour paint may be used to enhance the appearance. These plaques are frequently used in the centre of car steering wheels, refrigerators and other equipment where an eye-catching motif is considered desirable.

Methyl Methacrylate Polymers with Enhanced Impact Resistance 41 3

If the surface of an acrylic sheet, rod or tube is roughened or carved, less light is internally reflected and the material is often rather brighter at these non- polished surfaces. The use of this effect enables highly attractive carvings to be produced. Similarly, lettering cut into sheet, particularly fluorescent sheet, becomes ‘lit-up’ and this effect is useful in display signs.

The use of acrylic materials for dentures has already been mentioned.

Tensile strength (5 mm/min strain rate)

Tension modulus Impact strength Notched impact strength

Elongation at break (5 mm/mm strain rate)

Light transmission Vicat softening point

15.3 METHYL METHACRYLATE POLYMERS WITH ENHANCED IMPACT RESISTANCE AND SOFTENING POINT

MPa %

MPa N mm mm-z N mm mm-*

% “C

As with other rigid amorphous thermoplastic polymers such as PVC and polystyrene (see the next chapter) poly(methy1 methacrylate) is somewhat brittle and, as with PVC and polystrene, efforts have been made to improve the toughness by molecular modification. Two main approaches have been used, both of which have achieved a measure of success. They are copolymerisation of methyl methacrylate with a second monomer and the blending of poly(methy1 methacrylate) with a rubber. The latter approach may also involve some graft copolymerisation.

An early approach was to use butadiene as the comonomer but the resultant copolymers have largely been used only in latex form in paper and board finishes and are no longer believed to be important.

Copolymers of methyl methacrylate and butyl acrylate gave polymers that were somewhat tougher and slightly softer than the homopolymers. Materials believed to be of this type were marketed in sheet form by IC1 as Asterite for a short while in the 1960s (the name having been recently revived for another product as described in Section 15.2.6).

Rather more recently Rohm and Haas GmbH have introduced Plexidur plus which is a copolymer of acrylonitrile and methyl methacrylate. It is best considered as a glazing material for use in schools, sports halls and vehicles. The material also has good clarity, rigidity and surface hardness. Some typical properties compared with PMMA are given in Table 15.2.

Following the success in blending rubbery materials into polystyrene, styrene- acrylonitrile and PVC materials to produce tough thermoplastics the concept has been used to produce high-impact PMMA-type moulding compounds. These are two-phase materials in which the glassy phase consists of poly(methy1 methacrylate) and the rubbery phase an acrylate polymer, usually poly(buty1 acrylate). Commercial materials of the type include Diakon MX (ICI), Oroglas

Table 15.2 Some properties of a methyl methacrylate-acrylonitrile copolymer compared with a general purpose poly(methy1 methacvlate) compound at 23°C and 50% R.H (German DIN tests)

Property Units I I

MMA-ACN copolymer

85 60

4500 40

3 90 80

PMMA

80 5.5

3300 12 2

92 115

... "

N

mm

m

mr

nm

rn

m

mm

m

mm

mm

U

-u

U

c e 8 8

Nitrile Resins 415

DR (Rohm and Haas) and Plex 8535-F (Rohm GmbH). Some typical properties of these materials compared with straight PMMA and with the competitive ABS and ASA polymers (discussed in Chapter 16) are given in Table 15.3.

In comparison with the styrene-based and better known ABS and ASA materials the high-impact methacrylates have generally lower values for mechanical properties such as tensile strength, impact strength and modulus. However, long-term weathering tests show the marked superiority of the methacrylates over ABS and even ASA materials to degradation. In a typical test the impact strength of unnotched high-impact PMMA rods was about sixfold that of both ABS and ASA materials.

Over the years many attempts have been made to produce commercial acrylic polymers with a higher softening point than PMMA. The usual approach was to copolymerise MMA with a second monomer such as maleic anhydride or an N-substituted maleimide which gave homopolymers with a higher Tg than PMMA. In this way copolymers with Vicat softening points as high as 135°C could be obtained.

In the early 1990s attention appeared to be focusing on the imidisation of acrylic polymers with primary amines.

CH; CH, CH, CH,

._ I ,CH* \c/ I CH2. ... .. C

+ H H "/

I CH,

I CH, +

2CH,OH

As might be expected from a consideration of the factors discussed in Section 4.2, the imidisation process will stiffen the polymer chain and hence enhance Tg and thus softening points. Hence Vicat softening points (by Procedure B) may be as high as 175°C. The modulus of elasticity is also about 50% greater than that of PMMa at 4300MPa, whilst with carbon fibre reinforcement this rises to 25 000 MPa. The polymer is clear (90% transparent) and colourless.

Such materials, known as poly(methy1 methacrylimides) or PMMI, are marketed by Rohm and Haas in the USA as Kamex, and there is a small production by Rohm in Europe, where the product is marketed as Pleximid.

Hard-coated poly(methy1 methacrylimide) sun-roofs have already been specified for American sports cars, whilst the polymer might be expected to make some inroads into the polycarbonate market, with one specific target being auto headlamp diffusers.

15.4 NITRILE RESINS

Poly(acrylonitri1e) has found little use as a plastics material because it softens only slightly below its decomposition temperature of about 300°C. In addition it does not dissolve in its monomer so it cannot be shaped by bulk casting. It will,

4 16 Acrylic Plastics

however, dissolve in solvents such as dimethylformamide and tetramethylene- sulphone. In consequence poly(acrylonitri1e) and closely related copolymers have found wide use as fibres (e.g. Orlon, Acrilan).

Copolymers of acrylonitrile and vinylidene chloride have been used for many years to produce films of low gas permeability, often as a coating on another material. Styrene-acrylonitrile with styrene as the predominant free monomer (SAN polymers) has also been available for a long time. In the 1970s materials were produced which aimed to provide a compromise between the very low gas permeability of poly(viny1idene chloride) and poly(acrylonitri1e) with the processability of polystyrene or SAN polymers (discussed more fully in Chapter 16). These became known as nitrile resins.

Table 15.4 illustrates that though the nitrile resins had a gas permeability much higher than has poly(acrylonitri1e) the figures for oxygen and carbon dioxide are much lower than for other thermoplastics used for packaging.

Poly( acrylonitrile) Nitrile resins Poly(viny1idene chloride) Poly(viny1 chloride) High-density polyethylene

Table 15.4 Permeability ( P ) of nitrile resins compared with other polymers

0.14 0.23 2.3-3.6 4.5-9

3.6 14-23 23-32 40-1 80 900 2000

Polymer

I O2 I

In the mid-1970s many major plastics materials producers marketed or were actively developing materials of this type. They included American Cyanamid, Borg-Warner, Dow, Du Pont, ICI, Marbon, Monsanto, Solvay, Union Carbide and Vistron (Sohio).

The common feature of these materials was that all contained a high proportion of acrylonitrile or methacrylonitrile. The Vistron product, Barex 210, for example was said to be produced by radical graft copolymerisation of 73-77 parts acrylonitrile and 23-27 parts by weight of methyl acrylate in the presence of a 8-10 parts of a butadiene-acrylonitrile rubber (Nitrile rubber). The Du Pont product NR-16 was prepared by graft polymerisation of styrene and acrylonitrile in the presence of styrene-butadiene copolymer. The Monsanto polymer Lopac was a copolymer of 28-34 parts styrene and 66-72 parts of a second monomer variously reported as acrylonitrile and methacrylonitrile. This polymer contained no rubbery component.

The main interest in these materials lay in their potential as beverage containers although other suggested uses included such, presumably, diverse materials as barbecue sauces, pesticides and embalming fluids. However, in 1977 the US Food and Drugs Administration proposed a ban on these materials for beverage applications and suggested stringent levels of allowable acrylonitrile residual monomer migration. This led to companies withdrawing from manufacture of these resins. Shortly afterwards this particular market was penetrated by polyester resins of the poly(ethy1ene terephthalate) type (see Chapters 21 and

Acrylate Rubbers 417

25). In 1984 the use of nitrile resins was re-approved by the Food and Drugs Administration with specific limits on the level of residual unreacted monomer.

This has resulted in some resurgence of interest in these materials. At the time of writing the only manufacturer is BP Chemicals, who acquired the rights to manufacture Barex in 1987 and doubled the manufacturing capacity to 20 000 t.p.a. in 1990. Whilst this copolymer graft has barrier properties inferior to those of poly(viny1idene chloride) and ethylene-vinyl alcohol (EVOH), it is markedly better than for polypropylene and poly(ethy1ene terephthalate) (PET). The ability to save some 30-40% on materials helps to offset the price of the material, which at the time of writing is about twice that of PET. The material is also attractive because of its processing versatility, with film and blow moulding operations dominating. There is some interest in the use of nitrile resins as an internal barrier layer in a co-extruded product, so that the barrier layer is not in direct contact with the foodstuff.

About 95% of material produced is used for packaging, with food packaging accounting for about 70% in the USA and 40% in Europe. This difference in usage has been ascribed to longer shelf life requirements in the USA and hence more demanding specifications. Processed meat dominates the food packaging field. The excellent chemical resistance of the material has led to uses in such diverse fields as containers for petrol (gasoline) additives, nail polishes, lemon juice, air fresheners, nicotine patches and toothpaste packs.

Another area of potential interest is in refrigerator liners. The move away from the ozone-layer-damaging chlorofluorocarbons (CFCs) to HCFCs in the USA and pentanejcyclopentane blends in Europe has not been without problems. These newer materials have an adverse effect on ABS whereas the nitrile resin appears satisfactory, if more expensive.

15.5 ACRYLATE RUBBERS

The acrylic or acrylate rubbers were first introduced in 1948 by B. E Goodrich in consequence of earlier work carried out by the Eastern Regional Laboratory of the US Department of Agriculture. The original materials were a copolymer of ethyl acrylate with about 5% of 2-chloroethyl vinyl ether acting as a cure site monomer (eventually marketed as (Hycar 4021) and a copolymer of butyl acrylate and acrylonitrile (Hycar 21 21x38). These materials found some limited use in oil seals and other automotive uses where nitrile rubbers had insufficient heat resistance or tended to be cross-linked by sulphur-bearing additives in the oils. In heat resistance they are in fact superior to most rubbers, exceptions being the fluororubbers, the silicones and the fluorsilicones. Amongst the heat- resisting, oil-resisting rubbers they are, however, inferior in low-temperature properties (Le. they stiffen at higher temperatures) to the silicones, the fluorsilicones and the epichlorhydrin rubbers.

Subsequently, several other companies have entered the acrylic rubber market (e.g. Thiokol, American Cyanamid, Goodyear, Polymer Corporation and US Rubber) and this has led to many technical developments. These may be categorised into the three main areas:

(1) Attempts to improve low-temperature properties without loss of oil resistance.


( 2 ) Provision of more active cross-link sites. (3) Development of new cross-linking systems.

Whilst increasing the length of alkyl side chain can, to some extent, depress Tg and improve low-temperature properties this is at the expense of oil resistance. On the other hand lengthening of the side chain by incorporation of an -0- or an -S- linkage could often depress Tg and reduce swelling in hydrocarbon oils. This led to the commercial development of copolymers of either ethyl or butyl acrylate with an alkoxy acrylate comprising some 20-50% of the total composition. Typical of such alkoxy compounds are methoxyethyl acrylate (I) and ethoxyethyl acrylate (11):

CH, =CH CH, =CH I I COO. CH, . CH, . O . CH, COO. CH, . CH, . O . C,H,

(1) (E)

Because of processing problems 2-chloroethyl vinyl ether has now been replaced with other cure site monomers. These include vinyl and allyl chloracetates and allyl glycidyl ether.

Curing systems have also radically changed. With early grades aliphatic amines and then ammonium salts were used, whilst in the late 1960s the so-called soap-sulphur systems became paramount. More recently, four-part curing systems have become more popular which contain curative, accelerator, activator and retarder. Such a typical system would be sodium stearate 3-5 (curative); 3-(3,4-dichloropheny1)-1, 1 -dimethyl urea 2-6 (accelerator); high activity magnesium oxide 0-1 (activator); and stearic acid 0-3(retarder). This system shows good scorch safety, fast cure and low compression set without causing many of the difficulties exhibited by the earlier systems. Post-curing is still advisable for optimum compression set resistance.

The changes in acrylic rubber compounds have increased the scope of these materials as heat-and oil-resisting materials able to meet many of the increasingly stringent demands being imposed on rubbers for use in automotive applications.

15.6 THERMOSETTING ACRYLIC POLYMERS

Acrylic and methacrylic acids and their esters are highly versatile materials in that the acid and ester side groups can partake in a variety of reactions to produce a very large number of polymerisable monomers. One particularly interesting approach is that in which two methacrylic groupings are linked together so that there are two, somewhat distant, double bonds in the molecule. In these cases it is possible to polymerise through each of these double bonds separately and this will lead eventually to a cross-linked network structure.

In recent years these materials, which as a class have been known for a very long time, have found use in two areas:

(1) As anaerobic adhesives (see Section 15.7). ( 2 ) As laminating resins in competition with polyester laminating resins.

Acrylic Adhesives 41 9

Laminating resins have been offered by Akzo (Diacryl lOl), Dow (Derakane Vinyl Esters) and Showa (Spilac). Typical of these is Diacryl 101, which is manufactured by esterification of the addition product of ethylene oxide to bis- phenol A with methacrylic acid. They exhibit lower curing shrinkage than the polyester laminating resins during cure. The structure of Diacryl 101 is

CH, I I

CH, = C

C O . O . C H , . C H , . O

CH,-C - CH, I Q

15.7 ACRYLIC ADHESIVES

Methyl methacrylate has been used for many years as a reactive adhesive for joining together poly(methy1 methacrylate). To reduce curing shrinkage it is usually thickened with its polymer although alternative materials could be used which might be cheaper but generally cause a loss in clarity. The bond sets by polymerisation which may be brought about by ultraviolet light or by the use of peroxides. Room temperature setting with peroxides is achieved by the use of amines as promoters.

The alkyl 2-cyanoacrylates have become well-known adhesives, often popularly known as super-glue.

In dry air and in the presence of polymerisation inhibitors methyl and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they may be polymerised by free-radical methods, anionic polymerisation is of greater significance. A very weak base, such as water, can bring about rapid polymerisation and in practice a trace of moisture on a substrate is enough to allow polymerisation to occur within a few seconds of closing the joint and excluding the air. (As with many acrylic monomers air can inhibit or severely retard polymerisation).

Cyanoacrylate adhesives are particularly valuable because of their speed of action, which allows the joining of intricate parts without the need for complex jigs and fixtures. Within very broad limits the more monomer that is used to make a joint the less will be the strength. These adhesives have in fact no gap- filling ability, nor can they be used on porous substrates. Whilst they have good heat and solvent resistance their weathering behaviour is limited and joints should not be in frequent contact with water.


The reluctance of acrylic monomers to polymerise in the presence of air has been made a virtue with the anaerobic acrylic adhesives. These are usually dimethacrylates such as tetramethylene glycol dimethacrylate. The monomers are supplied with a curing system comprising a peroxide and an amine as part of a one-part pack. When the adhesive is placed between mild steel surfaces air is excluded, which prevents air inhibition, and the iron present acts as a polymerisation promoter. The effectiveness as a promoter varies from one metal to another and it may be necessary to use a primer such as cobalt naphthenate. The anaerobic adhesives have been widely used for sealing nuts and bolts and for a variety of engineering purposes. Small tube containers are also available for domestic use.

To overcome brittleness these materials are sometimes blended with rubbery materials and with polyurethanes. These polymers may contain unsaturated groups, particularly at the chain ends, so that graft structures may be produced rather than simple mixtures.

15.8 HYDROPHILIC POLYMERS

The successful development of eye contact lenses led in turn to a demand for soft contact lenses. Such a demand was eventually met by the preparation of copolymers using a combination of an acrylic ester monomer such as methyl methacrylate, a cross-linkable monomer such as a dimethacrylate, and a monomer whose homopolymer is soluble or highly swollen in water such as N-vinyl pyrrolidone. Such copolymers swell in water (hence the term hydrophilic), the degree of swelling being controlled by the specific type and amount of the monomers used. In use the lens is swollen to equilibrium in water, a typical soft lens having a water content of about 75%.

Such lenses may be made by machining from rod. More recently processes have been developed where the monomers are cast polymerised in tiny plastics moulds whose cavity corresponds to the dimensions of the lens and using procedures very reminiscent of those described for the manufacture of acrylic sheet (see Section 15.2.2).

15.9 POLY(METHACRYLIM1DE)

Poly(methacry1imide) has the structure

H

and should not be confused with poly(methy1 methacrylamide) discussed in Section 15.3.

Materials containing the above structure in the polymer chain may be made from copolymers of methacrylic acid and methacry lonitrile. Ammonia-producing additives (such as urea and ammonium hydrogen carbonate) are added to the

Methacrylate and Chloroacrylate Polymers and Copolymers 42 1

copolymers at a temperature above Tg(- 140°C) but below the decomposition temperature (-240°C)

CH, CH,

Complete imidation will not occur but that which does will be accompanied by the formation of a cellular structure to produce a rigid cellular polymer.

The foams, marketed by Rohm as Rohacell, are stable at room temperature to hydrocarbons, ketones, chlorinated solvents and 10% sulphuric acid. They may be used under load at temperature up to 160°C. Uses quoted for these materials include bus engine covers, aircraft landing gear doors, radar domes, domes, ski cores and tennis racket cores. Their potential is in applications demanding a level of heat deformation resistance, solvent resistance and stiffness not exhibited by more well-known cellular polymers such as expanded polystyrene and the polyurethane foams.

15.10 MISCELLANEOUS METHACRYLATE AND CHLOROACRYLATE POLYMERS AND COPOLYMERS

A large number of methacrylate polymers have been prepared in addition to poly(methy1 methacrylate). In many respects the properties of these materials are analogous to those of the polyolefins described in Chapter 8.

As with other linear polymers the mechanical and thermal properties are dependent on the intermolecular attraction, the spatial symmetry and the chain stiffness. If the poly-(n-alkyl methacry1ate)s are compared it is seen that as the side chain length increases the molecules becomes spaced apart and the intermolecular attraction is reduced. Thus as the chain length increases, the softening point decreases, and the polymers become rubbery at progressively lower temperatures (Figure 15. 12)." However, where the number of carbon atoms in the side chain is 12 or more, the softening point, brittle point and other properties closely related to the glass transition temperature rise with increase in chain length. As with the polyolefins this effect is due to side-chain crystallisation. It is to be noted that in the case of the polyolefins the side-chain crystallisation has a much greater effect on melting point than on the glass temperature. In studies on the methacrylates the property measured was the brittle point, a property generally more associated with the glass temperature.

A number of higher n-alkyl methacrylate polymers have found commercial usage. The poly-(n-butyl-), poly-(n-octyl-) and poly-(n-nonyl methacry1ate)s have found use as leathering finishes whilst poly(laury1 methacrylate) has become useful as a pour-point depressant and improver of viscosity temperature characteristics of lubricating oils.

As is the case in the polyolefins, the polymethacrylates with branched side chains have higher softening points and are harder than their unbranched isomers. The effect of branching of Vicat Softening point is shown in Table 15.5."


Figure 15.12. Brittle points of n-alkyl acryl acrylate and methacrylate ester polymers. (After Rehberg and Fisher," copyright 1948 by The American Chemical Society and reprinted by permission of the

copyright owner)

This effect is not simply due to the better packing possible with the branched isomers. The lumpy branched structures impede rotation about the carbon- carbon bond on the main chain, thus giving a stiffer molecule with consequently higher transition temperature.

Methyl methacrylate has been widely copolymerised with a variety of other monomers and several of the copolymers have been commercially available. Copolymerisation with styrene gives a material with improved melt flow characteristics whilst methyl methacrylate-a-methylstyrene copolymers have improved heat resistance. As described earlier, enhanced impact strength is obtained by copolymerising the methacrylate with either butadine or acrylonitrile or alternatively by the use of a poly(methy1 methacrylate)-rubber blend. Such improvements in toughness are gained with a commensurate loss in clarity, water-whiteness and weathering resistance. Copolymerising with a second acrylic monomer such as butyl acrylate gives products which are softer, and sheet made from the copolymer may be formed without difficulty. A material of this

Table 15.5 Vicat softening of methacrylate polymers from monomers of type CH2 = C(CH,)COOR(T)

* Too rubbery for testing

Other Acrylic Polymers 423

type was available in the early 1960s (Asterite, ICI) but later withdrawn. Typical properties of such a copolymer are given in Table 15.1.

Latices of butadiene-methyl methacrylate copolymer have been used in paper and board finishes.

Terpolymers based on methyl methacrylate, butadiene and styrene (MBS) have been increasingly used in recent years both as tough transparent plastics materials in themselves and as additives for PVC (see also Chapters 12 and 16).

Mention may also be made here of a number of polyfunctional compounds such as allyl methacrylate and glycol dimethacrylates which have been used to produce a cross-linked sheet of enhanced heat resistance compared with conventional poly(methy1 methacrylate). Some manufacturers supply the sheet in an incompletely cross-linked state which allows a limited amount of forming after which the sheet may be further heated to complete the cure.

Sheet from poly(methy1 a-chloroacrylate) has also been available. This material has a higher softening point than poly(methy1 methacrylate). It is, however, expensive, difficult to obtain in a water-white form and the monomer is most unpleasant to handle. It is because of these disadvantageous features that the polymer is believed to be no longer commercially available.

15.11 OTHER ACRYLIC POLYMERS

A number of acrylic polymers other than those already described have been produced but these are not generally of interest as plastics materials

Poly(acry1ic acid) is insoluble in its monomer but soluble in water. It does not become thermoplastic when heated. The sodium and ammonium salts have been used as emulsion-thickening agents, in particular for rubber latex. The polymer of methacrylic acid (Figure 15.13 (VI)) is similar in properties.

CH, I CH,=C

I CH,=CH

I COOH CN

VI VI1 Figure 15.13

A large number of organic acrylic ester polymer have been prepared in the laboratory. Poly(methy1 acrylate) is tough, leathery and flexible. With increase in chain length there is a drop in the brittle point but this reaches a minimum with poly-(n-octyl acrylate) (see Figure 15.12.). The increase in brittle point with the higher acrylates, which is similar to that observed with the poly-a-olefins and the poly(alky1 methacrylate)s, is due to side-chain crystallisation.

Poly(methy1 acrylate) is water-sensitive and, unlike the corresponding methacrylate, is attacked by alkalis. This polymer and some of the lower acrylate polymers are used in leather finishing and as a textile size.

A number of thermosetting acrylic resins for use as surface coatings have appeared during recent years. These are generally complex copolymers and terpolymers such as a styrene-ethyl acrylate-alkoxy methyl acrylamide


polymer. Coating resins have also been produced by blending methyl methacrylate with a non-drying alkyd.

The ease with which acrylic monomers may polymerise with each other and with other monomers has led to a host of compositions, frequently of undisclosed nature, being offered for use as moulding materials, casting resins, coating resins, finishing agents and in other applications.

References 1. CASPARY, w., and TOLLENS, B., Ann. 167, 241 (1873) 2. KAHLBAUM, G. w. A,, Ber, 13, 2348 (1880) 3. F I ~ I C , R., Ber, 12, 1739 (1879) 4. FITTIG, R., and ENGELHORN, E., Ann. 200, 65 (1880) 5. U.S. Patent 1,980,483; British Patenr 395,687 (ICI) 6. U S . Patenr 2,042,458; British Patent 405,699 (ICI) 7. SALKIND, M. RIDDLE, E. H. and KEEFER, R. w., fnd. Eng. Chem., 51, 1232, 1328 (1959) 8. SWEDLOW INC., Neth. Appl., 6, 613,600 (1967): 6, 613,601 (1967) 9. HORN, M. B. Acrylic Resins, Reinhold, New York (1960)

10. REHBERG, c. E., and FISHER, c. H., fnd. Eng. Chem., 40, 1431 (1948) 11. CRAWFORD, J. w. e. SOC. Chem. fnd., 68, 201 (1949)

Bibliography HORN, M. B. Acrylic Resins, Reinhold, New York (1960) RIDDLE, E. H. Monomeric Acrylic Esters, Reinhold, New York (1954) SCHILDKNECHT, c. E. Vinyl and Related Polymers, John Wiley, New York (1952)

Reviews BUCK, M. Kunstoffe, 77, 1012-16 (1987) BUCK, M. Kunstoffe, 80, 1132-36 (1990) GEISSLER, c., ALBRECHT, K., and WUNDERLICH, w., Kunstoffe, 86, 1484-1488 (1996)

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Plastics Materials - J. A. Brydson - 7th Edition - Chapter 15