CA1197033A - Preparation of curable solid polyester resin pellets and powders - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of preparing amorphous curable solid polyester resin pellets and powders which are suitable for use in injection, compression or transfer molding operations comprising: (a) preparing a mixture consisting essentially of an unsaturated polyester resin in a melted state and up to 5% by weight, based on the weight of the polyester resin, of an alkaline earth metal oxide or hydroxide, with the pro-viso that the alkaline earth metal oxide or hydroxide be added in an amount effective to provide a viscosity for the mixture characterized by a plateau region in the viscoelastic spectrum for the mixture; (b) heating the fluid mixture until the oxide or hydroxide is reacted and a desired increase in viscosity characterized by a plateau region in the visco-elastic spectrum of the mixture is attained; then (c) dis-persing into the mixture obtained in step (b) with high shear mixing an unsaturated polymerizable monomer containing a terminal ethylene group, an organic peroxide initiator and a filler, fibrous reinforcing material, pigment or mold re-lease agent, or mixtures thereof while maintaining the tem-perature of the mixture below the decomposition temperature of the peroxide initiator; and (d) extruding the mixture obtained in step (c) to form dry, free-flowing pellets or powders.

Description

This is a divisional application of copending application serial no. 353,674, filed Jund 10, 1980.
BACKGROUND OF THE INVENTION
This invention relates to a method of preparing curable solid polyester resin pellets and powders, and more particularly, to amorphous solid polyester resin pellets and powders.
Various thermosettable polyester composioitons are known which, when molded, exhibit desirable mechanical and electrical properties.
Polyester resins are widely used in molding applications in liqiud fom. Such liquid resins comprise a liquid solution of a liquid or solid polyester dissolved in a liquid cross-linking agent such as, for example, styrene. However, for many types of molding applications, it is desirable that the polyester resin be in solid form, for example, in the form of sheets, graules or powders.
Solid forms of polyester resin have been prepared from liquid polyester resin solutions of the type mentined above. For example, a normally liquid polyester resin solution can be converted into a sodi form by the addition thereto of a chemical thickening agent such as an oxide or hydroxide of magnesium or calcuim. Alternativley, the liquid polyester resin solution can be converted into a solid form by adding thereto a solid filler, such as calcuim carbonate whch absorbs the liquid resin. For example, a doughly mass or other from of soldi resin can be obtained by adding sufficinet filler to the liquid resin as disclosed in U.S. Patent No. 2,632,751.
These well known methods for converting liquid polyester resins to solid forms are not totally satisfactory for certain molding processess, more particularly injevtion molding. For example, the tough, rubbery sheets that are produced from a chemical thickened polyerst resin cannot be satifactroily gound into pellets or powders which is a desirable form of the curable resin for a molding operation such as by screw injection molding.
Another disadvantage in relying on the use of fillers to absorb sufficient amounts of a liquid resin containing an amorphious polyester in order to provid a solid resinous composition is t hat the properties of the articles made from the cured cmoposiotion can be affected adversely. For the strength of the articles made therefrom generally is reduced.
U.S. Patent No. 3,431,320 describes a proces for increasng the viscosity of unsaturated polyester resins dissolved in an ethylenically unsatllrated copolymerizable monomer by adding a mixl:ure of ca~ciuin oxide and either calc;um hydroxide or rnagnesium oxide to the polyester resin. It is reported that these compounds initially inhibit the viscosity build-up 5 which permits application of the polyester resin to fiber substrates prior to thickening. U.S. Paten$ No~ 2,568,331 describes copolymers of styrene and unsaturated polyesters and the conversion of such copolymers to solid polymers utilizing an organic peroxide in combination with an alkaline earth rnetal oxide or hydroxide.
V.S. Patent No. 2,628,209 describes the use of liquid mixtures of polymerizable vinyl compounds and polyolefinie alkyds containing free radical catalysts and magnesium oxide for impregnating or applicat;on to absorptive or porous sheet materials. After the sheet materials are treated, the magnesium oxide reacts causing the viscosity to increase. Thus when 15 the impregnated fabric is heated for curing, the viscosity remains high enough so that the resin does not run off. Alternatively, liquid polymeriz-able resins of high viscosity can be prepared by mixing low viscosity resin with pigments, dyes, catalysts, etc., and thereafter adding rnagnesium oxide to thicken the mixture.
U.S. Patent No. 3,959,209 describes the preparation of solid, curable crystalline polyester resin compositions which does not depend upon s~hemical thickeninlr agents. Amorphous polyesters can be utili~ed in combination with the crystalline polyesters.
U.S. Patent No. 3,926,902 describes thermoset compositions which 25 are dry and free-flowing. ~he composition comprises a mixture of an unsaturated thermosetting polyester, a polymerizable cross-linking agent, a free-radical initiator and a filler material and/or reinforcing agent selected from organic and inorganic fibers. The polyesters used in the composition are those which normally melt at a temperature above about 65 C.
SUMMARY OF THE INVENTION
-It now has been discovered that curable solid polyester compo-sitions can be prepared which are dry and free-flowing pellets and powders useflù for injection, compression or transfer molding operations. In particular, the present invention relates -to a method of pre-paring amorphous curable solid polyester resin pellets and powders which are suitable for use in injection, compression or transfer molding operations comprising. (a) preparing a mixture consisting essentially of an unsaturated polyester resin in a melted state and up to 5% by weight, based on the we:ight of the polyester resin, of an alkaline earth metal oxide or hydroxide, with the proviso that the alkaline earth metal oxide or hydroxide be added in an amount effective to provide a viscosity for the mixture characterized by a plateau region in the viscoelastic spectrum for the mixture; (b) heating the fluid mi~ture until the oxide or hydroxide is reacted and a desired i~ncrease in viscosity characterized by a plateau region in the viscoelastic spectrum of khe mixture is attained; then (c) di~spersing Into the mixture obtained in step (b) with high shear-mixing an unsaturated polymerizable monomer containing a terminal ethylene group, an organic per--oxide initiator and a filler~ fibrous reir.forcing material, pigment or mold release agent, or mixtures thereof while maintai~ning the tempPrature of the mixture below the decompo-sition temperature of the peroxide initiator; and (d) ex-truding the mixture o~tained in step (c) to form dry, free flowing pellets or powders~
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_TAILED DESCRIPTI(: N OF THE IN~ENTION
This invention relates to methods of producing therl~oset polymers of increased molecular weight, and more specifically to methods of processing these higher molecular wei~ht polymers into solid polymerizable thermoset composi-tions. The higher molecular weight polymers can be distin-mab/~1J< ~

guished from their low molecular weight percursers by the appearance of a plateau region in the viscoelastic spectrum for the higher molecular weight material which is none~istant in the viscoelastic spectrum of the lower molecular weight precursers.
In one embodiment of the invention, the solid polyester resin pellets and powders are prepared by first dissolving an unsaturated polyester resin in at least one monomeric unsaturated polymerizable material containing a terminal ethylene group. In an alternative embodiment, the polyester resin is not initially contacted with the monomeric unsaturated polymerizable material. In this alternative embodiment, the polyester resin, if a liquid is mixed wIth the alkaline earth metal oxide or hydroxide, and - 3a -mab/ l,, after undergoing thicken;ng, the unsaturated material containing the term;nal ethylene group is added to the thickened polyester resin along with other desirable ingredients such as polymerizatiorl initiators. When the starting polyester resin is a low melting point solid, the alternative S embodiment comprises mixing the alkaline earth metal oxide or hydroxide with a melted polyester resin, and after heating the fluid mixture to obtain the desired increase in viscosity. The unsaturated polymeri:~able monomer containing a terminal ethylene group and other desirable components thereafter are dispersed into the thickened polyester resin.
Unsaturated polyester resins which are useful in the method of the invention are well known in the art~ The unsaturated polyesters are condensation polymers derived by the condensation of unsaturated dibasic acids or anhydrides with di-hydroxy eompoundsO The polyesters may include in the polymeric chain, varying proportions of other saturated or aromatic 15 dibasic acids and anhydrides which are not subject to cross-linking. The particular non-cross-linking components and their properties will depend upon the desired properties of the final products. Independent of the composition of the polyester resin, the polyesters should be prepared in a manner such that tne polyesters have an acid number of between abou~ 10 2Q and 100. This ac~d functionality is used to react with the alkaline earth oxides and hydroxides to produce the desired increases in molecular weight and viscosity.
Examples of unsaturated dibasic acids or anhydrides which are utilized in the formation of the polyester resins include maleic acid, fumaric ~S acid, itaconic acid, tetrahydrophthalic anhydride, etc. Examples of saturated aliphatic dicarboxyli~ acids include adipic and succinic acids, and examples of aromatic dicarboxylie QCidS include phthalic acid, isophthalic acid, terephthalic acid and halogenated derivati~res such as tetrachloro-phthalic acid and anhydride.
Examples of di-hydroxy and tri-hydroxy compounds include ethyl-ene glycol, propylene glycol, 1,2-butanediol, 1,3-butanediol, 194-butanediol~
diethylene glycol, dipropylene glycol, glycerol, neopentyl glycol9 and reaction products of alkylene oxides wilth, for example, 292'-bis(4-hydroxy phenylene) propane, (Bis-phenol A).
The polyesters are prepared by reacting the one or more dibasic acids or anhydrides with the di- or tri-hydroxy compounds in approximately equal proportions. Examples of such polyesters include: maleic anhydride and propylene glycol (1:1 molar ratio); isophthalic acid, maleic anhydride and propylene glycol (1:~:3 and 1:1:2 molar ratios); and adipic acid, maleic 5 anhydride and propylene glycol (1:2:3 molar ratio). The degree of polymeriæation of polyester resins is conveniently measured with respect to the acid end groups. One gram of the polyester resin is titrated with potassium hydrox;de, and the number vf milligrams of potassium hydroxide necessary to neutralize the gram of polyester is called the acid number. It 10 is preferred for purposes of this invention that the acid number of the polyester resin utilized in the method be below 100, and is generally between about 10 and 100. Moreover, the polyester resins utilized in the method of the invention are characterized as being amorphous polyesters. A wide variety of such polyester resins is available commercially from various 15 sources.
In one embodiment of the present invention, the unsaturated polyester is dissolved in at least one monomeric unsaturated polymerizable material containing a terminal ethylene group. Preferably, the terminal ethylene group is attached to an electronegative group such as the phenyl 20 group as in styrene, halogen as in vinyl chloride, the acetoxy group as in vinyl acetate or the carbethoxy group as in ethyl acrylate. Examples of such compounds include styrene, alpha-methyl styrene, chloro styrene, vinyl toluene, divinyl benzene, diallylphthalate, methyl methacrylate, etc. The particular monomeric unsaturated polymeri%able material selected for use in 25 the method of the invention should be a material which is capable of dissolving the polyester resin, preferably at room temperature. The amount of unsaturated monomer such as styrene or vinyl toluene included in the mixture generally is from about 10 to about 60% and preferably from about 30 to about 50% by weight of the mixture of monomer and polyester.
30 Alternatively, ~he liquid mixture of polyester resin alld vinyl rnonomer willcontain from about 3 to 5 parts by weight of the monomer for each 10 parts of polyester resin.
To the above described liquid mixiure, there is added an alkaline earth metal oxide or hydroxide in ~sn amount effective to provide a desired 35 increase in viscosity on standingO The increase in viscosity should be sufficient to provide a thickened mi~ture which can be converted to a free-flowing dry powder or pe~let by extrusion after addition of polymerizationiniti~tor, pigment, rnold release agents, etc. The amount of alkaline earth metal oxide or hydroxide reguired to produce the desired increase in viscosity will vary depending on the particular polyester and polymerizable 5 material containing a terminal ethylene group, but such amount can be determined easily by one skilled in the art. Examples of the alkaline earth oxides which are useful include magnesium oxide, zinc oxide and calcium oxide. The corresponding hydroxides also can be utilized ;n the method of the invention. ln the present ins~ention, magnesium oxide and zinc oxide are 10 preferred. Also in the preferred embodiment9 the liquid mixture which is prepared consisting essentially of the unsaturated polyester resin, the unsaturated monomer materi~l, and the alkaline earth metal oxide or hydroxide will contain from about 1 to about 59b by weight of the alkaline earth metal oxide or hydroxide.
In addition to the above materials, the liquid mixture also may contain water or an organic thickening composition in an amount of up to about 5% by weight. Examples of useful thickening agents include organic aldehydes, ketones, alcohols, amines and amides. The preferred amides are acid amides such as formamide. The use of acid amides for increasing the 20 rate of polyester reactions with alkaline earth oxides and hydroxides is described in U.S. Patent No. 3,8lg,318~
.
The above described mixtures, with or without the organic accelerator, are allowed to stand until the alkaline earth metal oxide or 25 hydroxide and the polyester c~react thereby providing a desired increase in the viscosity of the mixture. Heat may be applied to the mixture to increase the thickening rate. The desired viscosity is the ~tiscosity which will enable the mixture, after incorporation OI a polymerization initiator, to be extruded into dry, free-flowing pellets or powders. The material of 30 desired viscosity which represents an increase o mo]ecular weight can be distinguished from the low molecular weight precur.sors by the appearance of a plateau region in the viscoelastic spectrum for the higher molecular weight material. The p~ateau region is non-existent in the viscoelastic spectrum of the precursors. The degree of reaction between the alkaline 35 earth metal oxide or hydro~cide with the polyester can be monitored by ~7~33 determining the viscosily rise of the polymer solution, monitoring the increase in rela~;ation time of the polyester solution, determining the intrinsic viscosity of the resultant resin, or performing moleculas weight analysis using standard gel permeat;on chromatography. The desired degree S of reaction or viscosity increase generally is achieved in a matter of hours and is determined by the reactivity of the polye~ster resin, the type of oxide or hydroxide used, and the presence of any accelerators in lhe mixture.
Alternatively, the formation OI the mixture of polyester resin and alkaline earth metal oxide or hydroxide having the desired increased 10 YiSCosity can be accomplished in the absence of the monomeric unsaturated polymerizable material. Ll the alternative embodimentg the metal oxide or hydroxide is introduced to $he`polyester while the polyester is maintained above its melting point. The temperature of the reaction between lhe metal oxide or hydroxide and polyester is dependent upon the specific melt 15 temperature of the polyester resin employed. The time of tile reaction is considerably shorter than the solvent process described above. Typically9 desired levels of reaction can l}e achieved in a matter of minutes, arld typical reaction t;mes generally will be ibetween about 5 and 120 minutes~
- After the polyester resin mixture has been reacted to form an ~ intermediate product having increased viscosity, whether in solution or Iielta polymerization initiator and other desiràble additive components are dispersed in the mixture. lf the viscous mixture has been prepared utilizing the melt process, one of the materials added to the thickened mixture is an unsaturated polymerizable monomer containing a terminal ethylene group 25 which serves as a cross-linking agent. Any of the unsaturated polymerizable monomers containing a terminal ethylene group described above with respe~t to the solvent proeess can be utilized.
After the mixture has been reacted to provide the desired increase in viscosity, a polymerization initiator is dispersed into the 30 thickened mixture. Generally? the initiator will be a free radical initiator capable of generating free radicals that can initiate cross-linking between the monomer containing the terminal ethylene groups and the polyester resin but will not cause any significant cross-linking of the compositions at low temperatures. In other words, the polymerization initiators must be SUCIl 35 that they al~e sufficiently st~ble at the temperatures at which the pellets ~., .f~

1~9~33 and powders of the invenlion are formed so that the compositions do not prematurely cross-link and become thermoset. Preferably, the polymeriza-tion initiators are chosen from materials which contain either a peroxide group or an azo group. Example~s of peroxide compounds include t-butyl 5 perbenzoate~ t-butyl peroctoate, benzoyl peroxide, t-butyl hydroperoxide, succinic acid peroxide, cumene hydroperoxide and dibenzoyl peroxide.
Examples of typical azo compounds tvhich can be utilized include azo-bis-isobutyronitrile and t-butylaz~isobutyronitrile. Generally, amo~mts of from about 0.5 to about 2~o by weight based on the weight of the combined weight 10 of polyester and monomer are utilized in preparing the pellets and powde~s of the invention.
Other ingredients ~hich may be dispersed into the thickened polyester resin prior to extrusion to obtaîn special effects include one or more of the following: fillers, fibrous reinforcing mater;als, pigments, and lS mold release agents.
Fillers are added to the resin mixture as extenders and to impart such properties as reduction in shrinkage and tendency to craclc during curing. Fillers al~so tend to improve stiffness and heat resistance in molded articles. Examples of fillers that can be utilized in the method of the 20 invention include alumina trihydrate, calcium carbonate, clays, calcium silicate, silica, talcs, mica, harytes, dolomite; solid or hollow glass spheres of various densities.
The particular filler chosen may be dependent upon the cost of such filler, the effect of the filler on mix viscosity and flow properties 25 during extrusion, or the effect that the filler has on proper~ies such as shrinkage, surface smoothness, chemical resistance, flammability and/or the electrical characteristics of the cured molded article. The amounts of filler included in the polyester resin formulations may vary up to about 70% by weight, and the amount preferred for any particular formulation can be 30 determined readily by one skilled in the art. Useful fillers generally will have an average particle size of from about 1 to about 50 microns.
Fibrous reinforcing materials can be added to the polyester resin compositions of the invention for the purpose of imparting strength and other desirable physical properties to the cured products formed therefrom.
35 Examples of fibrous reinforcements that can be utilized inc]ude glass fibers, g~

asbestos, synthetic organic fibers such as acrylonitrile7 nylon, polyamide, polyvinyl alcohol and polyester fibers, and natural organic fibers such as cotton and sisal. The preferred fibrous reinforcements generally will be g]ass fibers which are available in a variety of forms including, for example, 5 mats of chopped or continuous strands of glass, glass fabrics, chopped glass and chopped glass strands.
The amo~mt of fibrous reinforcing material included in the compositions of the invention can be varied over a wide range including amounts OI up to about 75% although much smaller amounts will be required 10 if any of the above described fillers are included in the formulation. The amount of fibrous reinfol cing material to be utilized in any particular formulation can be determined readily by one skilled in the art.
Mold release agents also can be included in the formulations used in the method oi the ;nvention, and these are typically zinc~ calcium7 15 magnesium and lithium salts of fatty acids. Specific examples of mold release agents include zinc stearate, calcium stearate, magnesium stearate, lithium stearate9 calcium oleate, zinc palmitate, etc. Amounts of up to about 5% of the mold release aO~ent) and preferably from about 1 to about 5%
of the mold release agent can be included in the mixture based upon the 20 weight of the polyester and monomer.
Pigments also can be included in Ule formulations OI the invention. Typical examples OI pigments include carbon blacks~ ir on oxides9 titanium dioxide and phthalocyanines. The pigment can be dispersed into the mixtures prior to extrusion as dry pigment po~ders or pre-dispersed forms in 25 non-reactive carriers.
Thermoplastic polymeric materials which reduce the shrinkage of the polyester resin formulations during molding also ~an be included in the iormulations used in the method o the invention. These thermoplastic materials produce articles having surfaces of improved smoothness. Ex-30 amples of useful thermoplastic polymers include homopolymers of ethylene9styrene, vinyl tol~lene, alkyl methacrylates and alkyl acrylates. Addi~ional examples of thermoplastic polymers are copolymers of: vinyl chloride and vinyl acetate; styrene and acrylonitrile; methyl methacrylate and alkyl esters OI acrylic acid; methyl methacrylate and styrene; snd methyl 35 methacrylate and acrylamide. Up to about 20% of a thermoplastic polymer can be included in the mixture.
.v ~'7~3 As mentionecl above~ the polymerization initiator, fillers, fibrous reinforcing materials, pigments, anti-shrink polymer~ or mold release agents, or mixtures thereof are dispersed into the mixture after the desired increase in viscosity has been attained. It has been found in accordance with the process of this invention that the desired free-flowing pellets are not obtained if the$e components are added to the polyester resin prior to undergoing a viscosity increase. Since the com-ponents are added after the desired viscosity increase, it is necessary to employ high shear mixers to adequately disperse the components in the thickened polyester resinO The useful types o~ high shear mixers include Banbury~ mixers, two roll mills, high shear compound screw extrudcrs and similar mixers. The composition is mixed until an adequate dispersion of all of the filler and other materials is achieved. Generally, thîs type of mixing results in a significant increase in the temperature of the material being mixed, and this temperature must be maintained below the critical temperatures which would resul~ in decomposition of the poly-merization initia~ors and which would cause premature polymerization of the molding composition.
After thoroughly mixing the components in a high shear mixer, the mixture is extruded into a pellet or powder form using either a screw extruder o~ a ram extruder equipped with a dry face pelletizing die, Typical of such extruders are: an NRM* 4.5 inch Screw Extruder and a 10 inch Barwell* Sin~le Ram Extruder. Alternatively, the mixer and extruder can comprise a single unit consisting of an extruder having a mixing zone and an extrusion pelletizing zone. Examples oE co~mercially available continuous mixing extruders are those manufactured by FCM* (Farrell Con-tinuous Mixer); Werner Pfleriderer*; and Buss-Condux*.
The following example illustrates the process of the invention.
Unless otherwise indicated all parts and percentages are by weightO
Example 1 A polyester alkyd (1900 grams) available commercially from Pioneer Plastics under the trade mark "Pioester 1100" comprising maleic anhydride~ isophthalic acid and propylene glycol is melted by heating to a temperature of about 175C. To this melted polyester resin is added 40 grams of magnesium oxide and 66 grams of water. Although much of the water is converted to steam, the water does initiate the polyester-*trade mark mab/~'l~

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magnesium oxide interaction. ~fter the magne~ium oxide is added, about 15 minutes of mixing is required to produce entanglement coupling of the polyester resin as demorlstrated by producing a plateau zone in the elastic and the loss shear modulii when plotted against frequency utilizing a ~heometrics* Dynamic Spectrometer.
The mixture is cooled to room temperature, and 140 grams of the reaction mi~ture is mixed on a 2-roll mill with the following ingredients:
styrene 60 grams t-butyl perbenzoate3 grams zinc stearate 10 grams carbon black 15 grams ground alumina trihydrate 400 grams 1/~ inch glass fibers150 grams The ingredients are mixed on the mill until there is sufficient dispersion of all of the ingredients to form a homogeneous mass while keeping the temperature of ~he mass below 90C. The homogeneous mass then is con-verted to 1/4 inch diameter pellets.
The curable solid polyester resin pellets and powders pre-pared in accordance with the invention and the specific examples can be molded and cured to useful articles such as appliance housings and electrical equipment components utilizing injection or transfer or com-pression molding techniques. ~olding and curing pressures and tempera-tures which are used with other types of curable polyester resin composi-tions can be used to mc,ld and cure the solid polyester resins of the present inyention, Typical molding pressures of about 200 to 2000 psi and typical temperatures~ of about 120 to 18~C can be utilized. Specific e~a~ples of molded parts which can be prepared from the curable pellets and powders of the inventiQn include electrical circuit hreakers~ elec trical switch gear, and appliance handles.

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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. A method of preparing amorphous curable solid polyester resin pellets and powders which are suitable for use in injection, compression or transfer molding operations comprising:
(a) preparing a mixture consisting essentially of an unsaturated polyester resin in a melted state and up to 5% by weight, based on the weight of the polyester resin, of an alkaline earth metal oxide or hydroxide, with the proviso that said alkaline earth metal oxide or hydroxide be added in an amount effective to provide a viscosity for said mixture characterized by a plateau region in the viscoelastic spectrum for said mixture;
(b) heating the fluid mixture until the oxide or hydroxide is reacted and a desired increase in viscosity characterized by a plateau region in the viscoelastic spectrum of said mixture is attained; then (c) dispersing into the mixture obtained in step (b) with high shear mixing an unsaturated polymerizable monomer containing a terminal ethylene group, an organic per-oxide initiator, and one or more fillers, fibrous reinforcing materials, pigments or mold release agents, while maintaining the temperature of the mixture below the decom-position temperature of the peroxide initiator; and (d) extruding the mixture obtained in step (c) to form dry, free-flowing pellets or powders.

2. The method of claim 1, wherein the mixture of step (a) also contains an organic thickening accelerator.

3. The method of claim 2, wherein the thickening accelerator is water.

4. The method of claim 2, wherein the thickening accelerator is formamide.