CA1112786A - Transparent impact-resistant polystyrene structure - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to a method for preparing impact resistant polystyrenes having improved transparency for use in packaging. The method comprises polymerizing a solution of a diene rubber in styrene in the presence of a monoethylenically terminally unsaturated allylic bromine-containing aliphatic compound such as allyl bromide.

Description

PREPARATION OF IMPACT RESISTANT
POLYSTYRENES H~VING IMPROVED TRANSPAÆNCY

Transparent in:pact-resistant polyme.rs or resinous compo~iti.ons are highly desirable :Eor many packaging operations. Polystyrene and polymethyl-methacrylate are both resins having a hiyhly desirable s degree of transparency for many packaging applications;
however, both polystyrene and polyme-thylmethacrylate are often found to be too b~ittle to provide a relic~ble package, parti.cularly when used as containers or as vacuwm-formed oriented sheet, tubs or lids. In an e~fort to improve the impact resistance of resins such as polystyrene, rubber has been added to the polystyrene in o.ne manner or the other to provide ~ither a dispersion of so].id rubbe.r particles witl~in a polystyrene matrix or a dispersion of rubber particles containing occluded polystyrene dispersed within a polystyrene matrix.
;~ Employing such techniques a substantial increase in the ; imp~ct resistance of the polymer composition is obtained;
however, the resultant product is usually opaque or translucent and is ~lsatisfactory ~or packaging appli-cations which require a transparent material. Biaxial o.ientat.i.on of ce.rtain rubber-containing impact-resistant.
styrene polymer films results in a product having improved transparencyl; however, when such ar..~cles are obtained .

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in a heavier or thicker section the opacity is usually -too great or the impact resistance is inadequate. In an attempt to produce transparent impact-resistant styrene polymers, considerable effort has been expended in employing as a polymer matrix a copolymer of methylmethacrylate and styrene, the methylmethacrylate and styrene being employed in such a proportion that the refractive index of the reinforcing rubber and the methylmethacrylate-styrene polymer differ by an insignificant value thereby providing a resinous body which does not scatter large amounts of light and, at least to the unaided eye, in thin sections, appears to be transparent. The resultant polymers, using refractive index matching, usually are polymers which have methylmethacrylate as a major component.
Another technique of providing a rubber-reinforced styrene polymer of improved transparency is shown in U.S. 3,574,151 wherein styrene is polymerized in the presence of rubber and a small quantity of ~-bromostyrene. U.S. Patent 3,957,915 teaches the use of other bromine-containing compounds for this purpose.
This invention provides an improved process for preparing impact resistant polymers having improved transparency and composed 2Q primarily of styrene and a reinforcing rubber.
According to the present invention therefore, there is provided a process for making impact resistant polystyrenes having improved transparency by polymerizing a solution of a diene rubber in styrene or a mixture of styrene with minor amounts of vinyltoluene, t-butylstyrene or methyl methacrylate in the presence of a bromine-containing compound, 4 to 12 parts of said diene rubber being present in -lQ~ parts Qf- co~bined styrene and rubber, and polymerizing under ~ 2 ~
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free radical conditions and with a~ita-tion at least throuyh phase inversion characterized in that the polymerization is carried out in the presence of from 0.01 to 0.5 percent by weight of a mono-ethylenically terminally unsatura-ted allylic bromine-containing aliphatic compound having from 3 to 10 carbon atoms. Preferably 0.01 to 0.3~ percent by weight based on the combined weight of the rubber and styrene of a monoethylenically terminally unsaturated allylic bromine-containing aliphat:ic compound is present.
The process of this invention provides a styrene polymer structure having a dispersed phase of a reinforcing diene rubber, the diene rubber advantageously being present in a proportion of 4 to 12 parts by weight per 100 parts by weight of the combined weight of styrene and rubber. The rubber is generally in the form of a plurality of particles of cellular nature and containing occlusions of polystyrene, the particles being of both monocellular and multi-cellular configuration. Advantageously, the rubber particles have a weight average particle diameter not exceeding two microns and cell walls not greater than 0.15 micron in thickness, the dimensions and configuration of the rubber particles being determined by means of an electron microscope on a sample of the polymer which has been treated with osmium tetroxide in accordance with the procedure set forth in Polymer Engineering and Scie ce, by K. Kato, 7, 38 (1967).
Polymers prepared in accordance with this invention will have a light absorbance, measured using a wave length of 640 millimicrons, not greater than 0.10 at a thickness of 0.254 mm. The structures will preferably have a notched izod impact value when measured in accQrdance ~ith the ~merican Socie-ty for Testing Materials, ~ 3 -B

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Specification D- 256~ of at least 0.65 foot-pounds per inch of notch (.0354 kg- meter/cm).
The rubbery reinforcing polymer may be prepared from 1,3-butadiene, isoprene, copolymers of up to 30 weight percent styrene with 1,3-butadiene or isoprene or ~ 3a ~
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mixtu~es the~eo, and advantageously has an lnherent viscosity iII the range o~ 0.9 to 2.5 as determined at 25C employing ~3 gram of rubber per deciliter of toluene. While it is preferred to employ styrene âS -the sole polymerizable monomer in the process Gf this invention one may, if desired/ utilize minor amounts, i.e , 35 weight ~ercent or less, of other copolymerizable comonoiners such as vinyll:oluene, t~butyistyrene or methyl meth-acrylateA Of such comollomers m~thyl methacrylate is advanta~eou51y employed to prc)vide products having good clarity and impact strength.

The allylic bromine-contail~ing aliphatic compound is pre~erably allyl bromide~ methallyl bromlde, 2-(bromo-methyl)-3-bromopropene, 3,3-dibromo-1-propene, 3-bromo--l~pentene, 3,4--dibromG-l-buterle, 2~(dibromome-thyl)-1-butene,

2-~bromomethyl)-1-butene r 2-(bromomethyl)-l~pentene, ~-(clib~ornomQthyi)-1-pentene, 2-(bromome~hyl)-1--hexene,

3-brol~o-1-hexener 2 (dibromomethyl)-l-hexene, or a mixture thereof.

If clesired, in the polymerization of styrene polymers o~ the present invention a diluent may be employed. Usually lt is desirahle to employ a diluent or solvent which may be presen-t in a quantity of up to 20 parts by weight per 100 parts styrene. The diluent gene.ally aids in the polymexization b~ increasing heat transfer, by reducing the viscosity of the polymerizing mixture and in easing the problem of handling viscous syrups. Suitable diluents are hydrocaxbons that are generally non-reactive under the polymerization conditions 3Q and are a solvent ~or the monomers and the polymer produced. Such diluen-ts include substituted aromatic compounds such as, for example, ethylbenzene and the xylenes.

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If desiredr a satisfactory product can be obtained usin~ thermal initiation although catalytic initiation is preferred. The preferred free radical initiators are those which decompose to produce alkoxy radical fragments or aryloxyradical fragments. The most preferred initiators are tertiary butyl perbenzoate, tertiary butyl p~ïaceta-te and l,l-bis(t-butyl-peroxy)-cyclohexane. Usually the initiators are employed at a level of 0.01 to 0.5 weight percent based on the weight of the monomer.

Useful rubbers are well known and commercially availableO Oftentimes, one or mors such rubbery polymers may be employed in the polymerization, however, the total amount of the rubber should advantageously be in the proportion of from 4 to 12 parts by weiyht of r~bery polymer per 100 parts by weight of styrene and rubber. In dissolving the rubber in styrene or a styrene mixture, it is desirable to inc]ude with the styrene a major po.tion of the solvent Qr diluent which is to be used in order to obtain the solution as rapidly as possible. Generally, a small portion of the diluent is employed to dissolve the initiator and ~he bromine--containing compound such as allyl bromide, 2-(bromo-methyl)~3-bromopropene or mixtures thereof~ The reaction mixture is then raised to appropriate polymerization initiatin~ temperature such as a temperature between 60C
and 100C at which time the solution of the initiator and bromine-containing compound is added to the reaction mixture with agitation. The temperature of the reaction mixture is then raised to a temperature within the range of 80 to 130C and maintained in this range for a period of 3 to 10 hours with agitation, a nitrogen or other inext atmosphere being maintained within the reactor.

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When the solicls content of the reaction mixture reaches from 30 to S0 percent conversion, agitation may be discontinued if desired and the temperature of the reaction mixture raised over a period of 2 to 5 hours to a temperature within the range of 170 to 190C. When polymerization, ~rom a practical standpoint, is complete, generally in a range from 70 to 95 percent con~ersion of the monomers to polymer solids, the reaction mixture is devolatilized usually at a temperature from 200 to 2~0C~
beneficially under a vacuum of ~rom 0.1 to 50 millimeters of mercury. The polymer may ~e prepared by either batch or continuous-process polymerization. The following examples serve to further illustrate this invention.

Examples 1 1~
A plurality of polymerization runs were carried out employing the following procedure. A two-liter jacketed reactor having an agitator was employed in all polymeriæations. The agitator was a rotatable hollow sha~t having two helical vanes mounted external to the hollow shaft. The vanes had a clearance of about 1 mil.
from the inner wall of the reaction vessel. When the agitator was rotated, the helical vanes forced the reaction mixture downwardly along the shaft and adjacent walls of the reaction vessel (More conventional agitators with horizontal cxossbars can also be used satisfactorily).
Styrene plus 95% o~ -the diluent employed and rubber were added to the reactor. The reactor was ~; nitrogen-purged and heated to a temperature of about 90C with stirring. The polymerization initiator and bromine-containing compound dissolved in the remaining 5~ of diluent was then added. The agitator was set at a speed of about 20 rpm and the temperature of the reaction mixture raised to about 105C generally cycling .. ~ . : . .. .
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from 100 to llQ for a period of 4 to 5 hours. During -` this period, nitrogen was maintained over the reaction mixture. Periodically, samples of the reaction mixture `. were removed from a sampling connection on the bottom of the reaction vessel and when the reaction mixture contained between about 38 ancl 42 percent solids, the mixture was transferred to tubes and polymerization . completed in a heating block programmed to raise the temperatuxe of the reaction mixture from about 100~
to 175C over a period of 5 hours. At the end of that time, the solids content of the reaction mixture was from about 78 to 8~ percent. The mixture was then forced from the tubes with nitrogen pressure and devola-tilized in a vacuum oven maintained at a temperature of about 210C and under a pressure of about one ,' millimeter of mercury for a period of about one and , one-half hours. A-t the end of one and one-half hours of devolatilization, the product was removed from the . oven and maintained in an inert atmosphere .or a few minutes to minimize surface ye,llowing. The cooled slab was then granulated and portions molded into test ~ plaques for light absorbance testing. Light absorbance was measured using a Beckman Model B spectrophoLometer ` wherein the film sample was placed approximately 9 centimeters from the center of the photocell light detector. The wavelength employed was 640 millimicrons and the reading obtained was corrected to 10 mils .~. .. _ . .(0.254 mm) thickness.. Notched Izod impact tasts were : conducted employing procedure ASTkl D-256A.
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The rubbers employed were:
Diene 55: a polybutadiene rubber of 2.30 deciliter/~ram inherent viscosity at a concentration of 0.3 gram per deciliter; and * Trade Mark ~1 ~B.

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;. 8 -Stereon S-700*: approximately 80 parts by weight butadiene and 20 parts by weight .styrene in which 18~ is randomly present and 2% is as block.

- Ta}ctene* a polybutadiene rubber whose mole cular structu.re is approximately 98% cis~1,4 confiyura-tion, the remaining is vinyl-1,2O
' The results are set forth in the following Table~

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TABLE (Conlinuefl) 1. Numbers indicate wei~ht percent o:E styrene and rubber 2r Tsps = terti.ary-butyl perbenzoate 3~ AB = allyl bromide 4e TBPA = tertiary-butyl peracetate 5. Bz202 - benzoyl peroxi.de 6. TBC = 4-t-butyl~].-1-bis(t-butylperoxy)cyclohexane 7. BMBP -- 2-bromom~thyl-3-bromopropene , - ~

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Resins of the foregoing examples within the scope of th~ presen-t invention had wei~ht average particle diameter of less than two microns and cell walls not greater than 0.15 micron in thickness. The majority of the particles showed occlusions of polystyrene.
Some particles were monocellular while others were multi-cellular.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Process for making impact resistant polystyrenes having improved transparency by polymerizing a solution of a diene rubber in styrene or a mixture of styrene with minor amounts of vinyltoluene, t-butylstyrene or methyl methacrylate in the presence of a bromine-containing compound, 4 to 12 parts of said diene rubber being present in 100 parts of combined styrene and rubber, and polymerizing under free radical conditions and with agitation at least -through phase inversion characterized in that the polymerization is carried out in the presence of from 0.01 to 0.5 percent by weight of a monoethyl-enically terminally unsaturated allylic bromine-containing aliphatic compound having from 3 to 10 carbon atoms.

2. Process of claim 1 characterized in that the allylic bromine compound is allyl bromide, methallyl bromide, 2-(bromomethyl)-3-bromopropene, 3,3-dibromo-1-propene, 3-bromo-1-pentene, 3,4-dibromo-l-butene, 2-(dibromomethyl)-1-butene, 2-(bromomethyl)-1-butene, 2-(bromomethyl)-1-pentene, 2-(dibromomethyl)-1-pentene, 2-(bromo-methyl)-l-hexene, 3-bromo-1-hexene, 2-(dibromomethyl)-1-hexene, or a mixture thereof.

3. Process of claim 2 characterized in that the allylic bromine compound is allyl bromide.

4. Process of claim 2 characterized in that the allylic bromine compound is 2-bromomethyl-3-bromopropene.