GB1578472A - Polymers for extrusion applications - Google Patents

Description

(54) POLYMERS FOR EXTRUSION APPLICATIONS (71) We, CELANESE CORPORATION, a corporation organised and existing under the laws of the State of Delaware, United States of America, located at 1211 Avenue of the Americas, New York, New York, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to polymers for extrusion applications, in particular to a process for preparing a branched chain thermoplastic polymer of increased melt strength useful in extrusion applications.

In blow molding processes, molten resin must form into stable parisons for a time long enough to permit a mold to enclose the parison. If these molten resins do not possess sufficient "melt strength" or melt viscosity, the parisons will tend to elongate or draw under their own weight and either not be blow moldable or result in blow molded articles which have non-uniform wall thicknesses, low surface gloss, poorly defined sample shape, and a large number of pitmarks.

Polymers such as polyesters, polyamides, polyethers, and polyamines when melted generally form thin liquids having low melt viscosities and poor melt strengths. These low melt viscosity materials are unsuited or are only poorly suited for the manufacture of extruded shapes, tubes, deep-drawn articles, and large blow molded articles. In order to overcome this disadvantage and to convert these polymers to a form better suited for the above-mentioned manufacturing techniques it is known to add compounds to the plastics which will increase their melt viscosities. The materials which are added to increase the melt viscosity of the plastics are generally cross linking agents, as described, for example, in U.S.

Patent 3,378,532. This patent describes a technique of increasing the melt viscosity of polyamide polymers. These polymers are set forth as exemplified by polyesteramides, polyurethanes and polyamides. The melt viscosity increasing material is suitably admixed with the polymer to form a composition which is melted and then formed into a shaped article. The melt viscosity improving materials are the ester reaction products of either carbonic acid or cycloaliphatic hydrocarbon dicarboxylic acids or aryl hydrocarbon dicarboxylic acids with phenols. Further, the carbonic acid ester may be the reaction product of carbonic acid with a polyhydric phenol or alcohol.

These known cross linking agents which are added to increase the melt viscosity of the polymer are not completely satisfactory. They may, for instance, cause an excessively rapid and large increase in viscosity or form reaction products which have an adverse influence on the quality of the plastics.

Furthermore, the results obtained with the use of these known cross linking agents are not always uniform or reproducible. For example, when polyesters of carbonic acid are used to increase the melt viscosity, the degree of viscosity increase is generally dependent not only upon the amount of additive used but also upon its molecular weight and on the stage of the polycondensation reaction at which the addition takes place.

Besides having sufficient melt viscosity or "melt strength", polymers which are to be used in blow molding and related applications should also possess sufficient die swell, i.e., the molten polymer should expand as it is released from the extrusion die. This die swell is important for blow molding applications since (a) the larger the diameter of the extruded polymer, the easier it is for air to be blown into the parison, and (b) the greater the die swell the greater the expansion of the molten polymer to fit the particular mold.

Polyesters having low intrinsic viscosities are particularly difficult to blow mold. The prior art illustrates the use of numerous additives to modify various properties of polyesters. For example, U.S. Patent 3,376,272 discloses a process for the preparation of branched chain, high molecular weight thermoplastic polyesters having a multiplicity of linear non-cross linked polyester branched chains from dicarboxylic acid anhydrides, monoepoxides, and an alcohol compound by reacting these compounds at a temperature below 150"C. However, the polyesters described in this patent are formed from anhydrides and are therefore not crystalline. Non-crystalline polymers tend to take longer time to set up in a mold and thus are not suited or are only poorly suited for blow molding and related applications.

Polyisocyanates have also been used to increase the melt viscosity of polymers such as polyesters. For example, U.S. Patent 2,333,639 discloses the reaction of low intrinsic viscosity, low molecular weight polyesters with polyisocyanates (e.g., a diisocyanate) at temperatures up to 3000C to form higher intrinsic viscosity, higher molecular weight, fusible polyesters, but this process results in relatively low melting, linear, soft amorphous polyesters with poor melt viscosity and poor die swell properties.

U.S. Patent 3,304,286 discloses the reaction of a polyester with a polyisocyanate such as diisocyanate. However, this process yields products having straight chain non-branched structures. These products thus lack the melt strength and die swell needed for blow molding applications.

Furthermore, U.S. Patent 3,692,744 discloses the preparation of polyester molding materials which can be injection molded by having present in the polyesterification mixture, besides the terephthalic acid and diol components, 0.05 to 3 moles percent, based upon the acid component, of a compound containing at least three ester forming groups such as a polycarboxylic acid, a polyhydric alcohol or a hydroxy carboxylic acid.

In British Specification No. 1,239,751 there is disclosed a process for the production of thick-walled shaped articles from poly (ethylene terephthalate) feed-stock by melt-shaping in which before it is shaped the poly(ethylene terephthalate) is mixed with a polyfunctional compound and optionally glass fibres, and the concentration of said polyfunctional compound and the conditions of the melt-shaping process are chosen to give in the shaped article polymer having an intrinsic viscosity of at least 0.7 decilitre gram , as measured on a solution of the polymer in o-chlorophenol at 250C. As polyfunctional compound there is used a compound each molecule of which is capable of reaction by addition or condensation with at least two molecular equivalents of groups selected from -OH and -COOH under the conditions of the shaping process.

Examples of functional groups in the polyfunctional compound are carboxylic acid groups; carboxylic acid anhydride groups; bromide groups; epoxide groups; and isocyanate groups.

We have now found that by reacting 99 to 95% by weight of a thermoplastic polymer capable of reaction with an isocyanate functionality, in the molten state, with 1 to 5 /O by weight of a branching agent which is a compound having greater than two isocyanate groups per molecule there can be produced a branched chain thermoplastic polymer having a melt strength ratio of T1,T2 of less than 2.0 at 235"C. Such polymers moreover have desirable die swell characteristics and are useful in blow molding and profile extrusion applications.

Accordingly, the present invention provides a process for preparing a branched chain thermoplastic polymer of increased melt strength useful in extrusion applications, which process comprises reacting at least one thermoplastic polymer capable of reaction with an isocyanate functionality, said polymer being in the molten state, with at least one branching agent selected from compounds containing greater than two isocyanate groups per molecule in an amount by weight of from I to 5% of branching agent and from 99 to 95% of thermoplastic polymer, to produce a branched chain thermoplastic polymer having a melt strength ratio of T,/T2 of less than 2.0 at 2350C.

The present invention also provides the melt strength improved thermoplastic polymers produced by this process.

In another aspect, there is provided a molding process which comprises forming a melt of the above-described melt strength improved thermoplastic polymer into a desired article and cooling the molten polymer.

The essence of the present invention is the discovery that when thermoplastic polymers in a molten state are chemically reacted in the stated amounts with the particularly defined isocyanate branching agents as described above, the molten, thermoplastic polymer product possesses increased melt strength. The polymers prepared according to the process of the present invention also have improved die swell characteristics, i.e. after extrusion of the molten polymer through an orifice having a particular diameter, the diameter of the extruded polymer may increase up to two or three times the diameter of the extrusion orifice.

Any thermoplastic polymer which contains functional groups capable of reacting with the isocyanate branching agent may be used in the process of the present invention. Such functional groups include carboxyl, amine, hydroxyl, epoxy, and isocyanate groups. Thermoplastic polymers include polyesters, polyamides, and allyl alcohol/styrene copolymers. Saturated thermoplastic polyesters are preferred.

The term "thermoplastic" polymer is meant to include all polymers which soften when exposed to sufficient heat and which return to their original condition when cooled to room temperature.

Thermoplastic polyesters are preferred polymers for use in the present process. Saturated thermoplastic polyesters are particularly preferred and include saturated aliphatic/aromatic polyesters and wholly aromatic polyesters. The term "saturated" polyester is meant to include all polyesters which do not contain ethylenic unsaturation in the polymer chain. The saturated, thermoplastic polyesters may be halogenated, i.e., contain halogen (e.g., bromic and/or chlorine) substitution in the polyester chain. The use of halogenated polyesters is particularly desirable when products having decreased flammability are desired.

The saturated thermoplastic polyesters useful in the present invention may be formed in a multitude of ways well known to those skilled in this art. These saturated thermoplastic polyesters may be prepared from dihydric alcohols and dicarboxylic acids or the dialkyl esters of dicarboxylic acids wherein the alkyl groups may contain from one to seven carbon atoms.

Typical dihydric alcohols include aromatic dihydric alcohols such as bisphenol A [i.e., 2,2 - bis(4 - hydroxyphenyl)propanel, phenolphthalein, 4,4' sulfbnyl diphenyl, resorcinol, hydroquinone, catechol, naphthalene diols, stilbene bisphenol, 4,4' - diphenylether diphenol, and mixtures thereof, and aliphatic dihydric alcohols such as saturated dihydric alcohols having from 2 to 4 carbon atoms, and mixtures thereof.

Halogenated dihydric alcohols may also be employed. Such halogenated dihydric alcohols include, for example, tetrabromobisphenol A, tetrachlorobisphenol A, 2,2' - [isopropylidenebis(2,6 - dichloro - p hydroxyphenylene)], and 2,2 - bis[3,5 - dibromo - 4 - (2 hydroxyethoxy)phenyl]propane.

Typical aromatic carboxylic acids include, for example, phthalic acid (including isophthalic and terephthalic), hydroxy - benzoic acid, and mixtures thereof.

Typical polyesters useful herein include the linear polyesters of an aromatic dicarboxylic acid reacted with a saturated aliphatic or cycloaliphatic diol, particularly polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, poly - 1,3 - cyclobutane terephthalate, polypentylene terephthalate, polycyclohexane - 1,4 - dimethylol terephthalate, poly - 1,5 - pentane diol terephthalate, and polyneopentylglycol terephthalate.

Typical wholly aromatic thermoplastic polyesters include the reaction product of bisphenol A, isophthalic or terephthalic acids or mixtures (50/50 or 60/40 mole %) of isophthalic and terephthalic acids. Such polyesters may additionally contain minor amounts of a saturated aliphatic dihydric alcohol having from 2 to 4 carbon atoms. Halogenated wholly aromatic thermoplastic polyesters include for example, the reaction product of tetrabromobisphenol A, and a 550 mole ratio of isophthalic and terephthalic acid (and optionally, a minor amount of ethylene glycol).

Polypropylene terephthalate, polybutylene terephthalate, and mixtures thereof as well as mixtures of polyethylene terephthalate and polybutylene terephthalate are particularly preferred polyesters.

In the process of the present invention, the thermoplastic polymer is reacted with an isocyanate branching agent containing greater than two isocyanate groups per molecule whereby high melt strength, substantially non-cross linked thermoplastic polymers are formed.

Polyisocyanates useful in the present invention contain greater than two isocyanate groups per molecule and thus include tri-, tetra-, and pentaisocyanates.

Typical polyisocyanates include polyphenylene polyisocyanate, triphenylmethane triisocyanate, benzene triisocyanate, aliphatic and cycloaliphatic polyisocyanates, and naphthalene triisocyanate. A particularly preferred polyisocyanate is polyphenylene polyisocyanate.

Mixtures of two or more branching agents may be used as long as the particular branching agents in the mixture are compatible with each other-i.e., do not reduce reactivity or branching.

Other additives, both polymeric and non-polymeric, such as flame retardents, lubricity agents, dyes, anti-oxidants, and inorganic fillers (such as glass) may be employed as long as these additives do not interfere with the reaction between the thermoplastic polymer and the branching agent. Such additives may generally be present in amounts up to 10% by weight of the total reaction mixture.

When an isocyanate branching agent is employed in accordance with the process of the present invention, a catalyst or reaction initiator is not generally needed since the reaction proceeds at an acceptable rate in the absence of a catalyst. However, if desired a catalyst such as triphenyl phosphine as described below may be employed. Other catalysts or reaction initiators include aliphatic and aromatic amines, particularly tertiary amines, amine adducts, acids, acid anhydrides, aldehyde condensation products, and Lewis acid type catalysts, such as boron trifluoride. Particular catalysts or reaction initiators are disclosed in Belgian Patent Specification No. 849,443.

The thermoplastic polymer and branching agent may be blended in any convenient manner as long as they are in contact for a period of time sufficient for chemical reaction to take place. Thus, the improved melt strength polymers of the present invention may be prepared by coating the thermoplastic polymer with a solution of the branching agent in a solvent in which the branching agent is soluble and the polymer is insoluble. The solvent should be substantially non-reactive toward the reactants and products of the reaction. Such solvents include hydrocarbons (such as methylene chloride) and ketones. The coated polymer may be allowed to air dry and then may be heated to the temperature at which reaction between the thermoplastic polymer and branching agent takes place.

The reactants may also be prepared by blending the branching agent with solid polymer chip and then feeding this mixture to a melt screw extruder (such as a Werner-Pfieiderer ZSK twin screw extruder) which is at a temperature high enough to cause the polymer to melt and thus enable reaction between the thermoplastic polymer and branching agent to take place.

Alternatively, the thermoplastic polymer may be milled until fully molten in a plastograph (such as a C.W. Brabender Rolle type plastograph) at temperatures high enough to melt the polymer. When the polymer is in the molten state, the branching agent may be introduced directly into the polymer until a melt viscosity of generally greater than 10,000 typically greater than 20,000, and preferably greater than 60,000 poise is achieved.

In this specification, the term "melt viscosity" refers to the viscosity of the polymer in a molten or fused state. Melt viscosity is measured by dynamic viscosity evaluation in a rheometrics viscometer at 2400 C. Such a measurement may be obtained by placing a sample in a rheometer and heating to 2400C. The melt viscosity may be obtained by plotting dynamic viscosity against frequency.

The process of the present invention may be carried out at subatmospheric, atmospheric, or superatmospheric pressures, although substantially atmospheric pressures are preferred.

Also, the process of the present invention may be carried out at any temperature which is such that the thermoplastic polymer will remain in the molten state for a period of time sufficient to enable reaction between the thermoplastic polymer and the branching agent to take place. However, the temperature should not be high enough to decompose the thermoplastic polymer.

At atmospheric pressure, the reaction may be carried out at temperatures of generally from 150 to 350, typically from 180 to 300, and preferably from 220 to 280"C.

The reaction between the thermoplastic polymer and the branching agent may be conducted generally in any environment. However, because of the sensitivity of certain branching agents, and polymers to the presence of water, the reaction is preferably carried out in the substantial absence of water. Sufficient quantities of water tend to destroy the activity of certain of the branching agents, and to degrade the polymers. It is also often desirable to conduct the reaction in the substantial absence of oxygen gas. Thus, the reaction is preferably carried out in dry nitrogen, helium, and/or argon.

The molten thermoplastic polymer and the branching agent must be in contact for a sufficient period of time for chemical reaction to take place.

Reaction progress may be monitored in various ways. For example, when polyesters or polymers containing carboxylic acid end groups are reacted with the branching agent, the progress of the reaction may be monitored by observing the decrease in the carboxylic acid end groups (CEG) with time. When no further decrease in CEG takes place, reaction has ceased.

Reaction rate, of course, is a function of temperature, but in the present invention a reaction time of generally from 45 to 150, typically from 60 to 130, preferably from 90 to 120 seconds (melt screw extruder) may be employed.

Because mixing does not take place to as great a degree in a plastograph as in a melt screw extruder, reaction times in a plastograph are generally somewhat longer.

The process of the present invention may be carried out in a batch, semicontinuous, or continuous manner, as desired.

It should be noted that in the process of the present invention, a chemical reaction is actually occurring between the thermoplastic polymer and the branching agent. This reaction is evidenced by an increase in melt strength as well as an increase in the intrinsic viscosity (I.V.). When polyesters or compounds containing carboxylic acid end groups are reacted with the branching agents, the chemical reaction is also evidenced by a concomitant decrease in CEG level.

The increase in melt strength and concomitant increase in I.V. result from chain branching of the thermoplastic polymer, which chain branching occurs when the polymer and branching agent are reacted as described hereinabove.

As indicated hereinabove, the present process provides thermoplastic polymers having increased melt strength. These increased melt strength thermoplastic polymers are useful for extrusion applications. Such applications include pipe, film, and blow molding uses such as in blow molding bottles.

Melt strength may be measured by extruding a six-inch strand of thermoplastic polymer through a constant drive index apparatus at a temperature high enough to keep the polymer molten (generally about 235"C). Melt strength (MS) may be defined as follows: T1 MS= T2 wherein the time required to extrude a polymer strand three inches (Tl) from the base of the melt index barrel is determined and without interruption the time required to extrude the same polymer to six inches is determined. The difference between the total time at six inches and the time at three inches is computed to give T2.

A melt strength value of from 1.0 to less than 2.0 is desirable when the material is to be used in extrusion applications. Ideally, a value of 1.0 is desired since this would mean that the second three-inch segment extruded at the same rate as the first segment.

For polymers with poor or low melt strengths, the second segment is extruded much more rapidly than the first segment, resulting in a Tl/T2 ratio significantly greater than 1.0.

Thus, polymers having poor or very low melt strengths have rather large values of T,/T2. By saying that certain polymers have "no melt strength" is meant that the second segment of the six-inch strand is extruded so rapidly that T2 cannot be measured.

The term "high melt strength polymers" refers to polymers having a ratio of Tl/T2 approaching the ideal value of 1.0, and the terms "poor" or "low melt strength polymers" refer to polymers having comparatively large TJT2 ratios.

Polymers having 'no melt strength" have so small aT2 value that the melt strength cannot be measured.

The melt strength of a polymer depends upon the particular polymer employed as well as the temperature. However, the improved melt strength polymers of the present invention have melt strengths of less than 2.0, typically less than 1.8, and preferably less than 1.6 at 2350C.

The improved melt strength polymers of the present invention also have improved die swell characteristics. Die swell may be described as the increase in diameter which takes place when the molten polymer is released from an extrusion die. As the polymer moves through the die, the entanglements and branches of the polymer chains are deformed or displaced from their equilibrium positions. This represents a storage of elastic energy. As the polymer is released from the die, this energy is regained by a return of the entanglements and branches to their equilibrium positions. This results in die swell.

The diameter of the improved melt strength polymers of the present invention may increase up to two or three times the diameter of the extrusion orifice. Die swell is important for blow molding applications since (a) the larger the diameter of the extruded polymer, the easier it is for air to be blown into the melt, and (b) the greater the die swell, the greater the expansion of the polymer to fit the particular mold.

The improved melt strength polymers of the present invention also have increased intrinsic viscosities. The "intrinsic viscosity" of the polymers of the present invention may be conveniently determined by the equation lim N IN.-- In- co C wherein N is the "relative viscosity" obtained by dividing the viscosity of a dilute solution of the polymer by the viscosity of the solvent employed (measured at the same temperature), and c is the polymer concentration in the solution, expressed in grams per 100 millilitres. The intrinsic viscosity of the improved polymers of the present invention in o-chlorophenol at 250C is generally from 0.85 to 1.7, typically from 0.90 to 1.65 and preferably from 0.95 to 1.6 poise.

As indicated hereinabove, when polyesters or polymers containing carboxylic acid end groups are reacted with the branching agents, the extent of reaction may be determined by measuring the change in the number of microequivalents of carboxylic acid end groups per gram of polymer. By "carboxylic acid end groups" is meant the number of carboxylic acid end groups present in the polymer, measured in microequivalents per gram of polymer. The number of carboxylic acid end groups may be measured by dissolving the polymer in a 70/30 mixture of o-cresol/chloroform solvent and potentiometrically titrating the solution with tetrabutylammonium hydroxide. When polyesters or polymers containing carboxylic acid end functional groups are reacted with the branching agent, these improved melt strength polymers may contain generally less than 65, typically less than 60, and preferably less than 55 microequivalents of carboxylic acid end groups per gram of polymer.

The present invention is further illustrated by the following examples. All parts and percentages in the examples as well as in the specification and claims are by weight unless otherwise specified.

EXAMPLE I This example illustrates the preparation of a highly branched, thermoplastic polyester useful in blow molding, using polyethylene terephthalate reacted with polyphenylene polyisocyanate.

Forty-eight grams of polyethylene terephthalate having 60 milliequivalents of carboxylic acid end groups per kilogram of polyethylene terephthalate are added to a C.W. Brabender Rolle type plastograph. The polyethylene terephthalate is heated to a temperature of 270"C such that only molten polymer is present. At this time, 2.0 grams of polyphenylene polyisocyanate (averaging more than two isocyanate groups per molecule) are added to the molten polyethylene terephthalate at 2700 C. After 5 minutes the molten polyethylene terephthalate is removed from the plastograph and cooled to room temperature.

The melt viscosity of the unmodified polyethylene terephthalate is 9,000 poise and its molecular weight is 46,000 whereas the melt viscosity of the polyethylene terephthalate as modified in accordance with the process of the present invention is 98,000 poise and its molecular weight is 47,200. The modified polyethylene terephthalate is fusible and substantially thermoplastic.

A comparison of the I.V., CEG, and MS of both the unmodified and modified polyethylene terephthalate (PET) is indicated in Table I below: TABLE I Property Unmodified PET Modified PET IV 1.0 1.2 CEG 76 40 MS 4.6 1.1 EXAMPLE II This Example illustrates the preparation of a highly branched, thermoplastic polyester useful in blow molding, using polybutylene terephthalate reacted with triphenylmethane triisocyanate.

Forty-nine grams of polybutylene terephthalate having 65 milliequivalents of hydroxyl end groups per kilogram are added to a C.W. Brabender Rolle type plastograph. The polymer is heated to 250oC and when all the polymer is molten, one gram of triphenylmethane triisocyanate is added. After seven minutes, the molten polymer is removed and cooled to room temperature.

The modified polybutylene terephthalate is fusible and substantially thermoplastic and has a melt viscosity of 95,000 poise and a molecular weight of 46,000. The melt viscosity of a polybutylene terephthalate formed in the same manner without the triphenylmethane triisocyanate branching agent has a melt viscosity of 1,000 poise and a molecular weight of 44,000.

A comparison of the I.V., CEG and MS of both the unmodified and modified PBT is indicated in Table II below: TABLE II Property Unmodified PBT Modified PBT I.V. 0.90 1.1 CEG 68 39 MS 5.0 1.2 EXAMPLE III This Example illustrates the use of a catalyst (triphenyl phosphine) to increase the rate of the reaction of Example II.

The same procedure as in Example II is employed except that 0.5 grams of triphenyl phosphine are added to the molten polybutylene terephthalate at the same time as the triphenylmethane triisocyanate. The same results are obtained in half the reaction time.

EXAMPLE IV This Example illustrates the preparation of a highly branched, thermoplastic polyester useful in blow molding, by employing polybutylene terephthalate pellets coated with polyphenylene polyisocyanate.

Forty-nine grams of polybutylene terephthalate having 65 milliequivalents of hydroxyl end groups per kilogram are coated with a solution of polyphenylene polyisocyanate in methylene chloride with the result that, when the methylene chloride is evaporated, there is provided polybutylene terephthalate pellets coated with about 1.5 weight percent polyphenylene polyisocyanate. These coated pellets are treated as in Example II with similar results.

The polymer melts obtained in accordance with the present invention have a uniform viscosity of a sufficiently high value to be outstandingly suited for use in fabrication techniques for production of articles, as, for example, extrusion, particularly blow molding. With the use of the branching agents in accordance with the process of the present invention, an excessively rapid increase in viscosi

The improved melt strength polymers of the present invention have improved tensile strength, percent elongation, flexural strength, flexural modulus, tensile modulus, and Rockwell hardness as indicated in Table III below (the polybutylene terephthalate is modified as in Example II).

TABLE III Property (1) Unmodified PBT Modified PBT Tensile strength 7400 7900 Percent Elongation 5.9 8.2 Flexural Strength 10,900 13,100 Flexural Modulus 3.72x 105 4.19x 105 Tensile Modulus 3.66x 10' 4.21x105 Rockwell Hardness -"m" 58 74 (1) As determined on specimens which were injection molded on a 2.5 oz. Stubbe Screw injection molding machine under the conditions listed in Table IV below: TABLE IV Nozzle Temperature ("C) 241 Barrel Temperature (OC) 235 Mold Temperature (OC) 54 RPM (Screw) 85 Cycle (Seconds) 22 EXAMPLE V The various polyesters formed in Examples I to IV are each utilized in the blow molding of a 2.2 inch diameter, 3.3 inch high barrel shape aerosol container.

Blow molding of melt viscosity increased polyesters is accomplished by charging the polymer to a 2.5 inch multi-station rotary blow molder at 241 C, and the polymer is processed under the conditions listed in Table V below: TABLE V Screw (RPM) 45 Back Pressure (psi) 1600 Blow Pressure (psi) 120 Compression Rate 3.5/1 The blow molded articles formed from the polyesters of Examples I to IV which have been modified with a branching agent are well-formed, of uniform thickness, have high gloss and no pitmarks. Thd blow molded articles formed from the various comparative polyesters of the Examples which are not modified with the branching agent, however, are not blow moldable and consequently are poorly formed, have non-uniform walls, and are generally rather rough.

A comparison of the branched modified polyester of the present invention with unmodified (non-branched) polyesters with respect to certain properties of blow molded articles are given in Table VI below: TABLE VI Blow Molded Property Modified Polyester Unmodified Polyester Wall Thickness uniform variable Surface Gloss high low Internal Roughness none poor Pinch-Off Weld good poor Pitmarks none many Shape good poorly defined WHAT WE CLAIM IS:- 1. A process for preparing a branched chain thermoplastic polymer of increased melt strength useful in extrusion applications, which process comprises

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. The improved melt strength polymers of the present invention have improved tensile strength, percent elongation, flexural strength, flexural modulus, tensile modulus, and Rockwell hardness as indicated in Table III below (the polybutylene terephthalate is modified as in Example II). TABLE III Property (1) Unmodified PBT Modified PBT Tensile strength 7400 7900 Percent Elongation 5.9 8.2 Flexural Strength 10,900 13,100 Flexural Modulus 3.72x 105 4.19x 105 Tensile Modulus 3.66x 10' 4.21x105 Rockwell Hardness -"m" 58 74 (1) As determined on specimens which were injection molded on a 2.5 oz. Stubbe Screw injection molding machine under the conditions listed in Table IV below: TABLE IV Nozzle Temperature ("C) 241 Barrel Temperature (OC) 235 Mold Temperature (OC) 54 RPM (Screw) 85 Cycle (Seconds) 22 EXAMPLE V The various polyesters formed in Examples I to IV are each utilized in the blow molding of a 2.2 inch diameter, 3.3 inch high barrel shape aerosol container. Blow molding of melt viscosity increased polyesters is accomplished by charging the polymer to a 2.5 inch multi-station rotary blow molder at 241 C, and the polymer is processed under the conditions listed in Table V below: TABLE V Screw (RPM) 45 Back Pressure (psi) 1600 Blow Pressure (psi) 120 Compression Rate 3.5/1 The blow molded articles formed from the polyesters of Examples I to IV which have been modified with a branching agent are well-formed, of uniform thickness, have high gloss and no pitmarks. Thd blow molded articles formed from the various comparative polyesters of the Examples which are not modified with the branching agent, however, are not blow moldable and consequently are poorly formed, have non-uniform walls, and are generally rather rough. A comparison of the branched modified polyester of the present invention with unmodified (non-branched) polyesters with respect to certain properties of blow molded articles are given in Table VI below: TABLE VI Blow Molded Property Modified Polyester Unmodified Polyester Wall Thickness uniform variable Surface Gloss high low Internal Roughness none poor Pinch-Off Weld good poor Pitmarks none many Shape good poorly defined WHAT WE CLAIM IS:-

1. A process for preparing a branched chain thermoplastic polymer of increased melt strength useful in extrusion applications, which process comprises

reacting at least one thermoplastic polymer capable of reaction with an isocyanate functionality, said polymer being in the molten state, with at least one branching agent selected from compounds containing greater than two isocyanate groups per molecule in an amount by weight of from 1 to 5% of branching agent and from 99 to 95% of thermoplastic polymer, to produce a branched chain thermoplastic polymer having a melt strength ratio of Tilt2 of less than 2.0 at 2350C.

2. A process according to claim 1 wherein the branching agent is polyphenylene polyisocyanate.

3. A process according to claim I wherein the branching agent is an aliphatic or cyloaliphatic polyisocyanate.

4. A process according to claim 1 wherein the branching agent is triphenylmethane triisocyanate, benzene triisocyanate or naphthalene triisocyanate.

5. A process according to any one of the preceding claims wherein the polymer is a saturated thermoplastic polyester.

6. A process according to claim 5 wherein the polymer is polybutylene terephthalate.

7. A process according to any one of the preceding claims wherein said branched chain thermoplastic polymer produced has a melt strength ratio of Tl/T2 of less than 1.8 at 2350C.

8. A process according to any one of the preceding claims wherein the reaction is carried out at a temperature of from 150 to 3500C and at substantially atmospheric pressure.

9. A process according to claim 8 wherein said reaction is carried out at a temperature of from 220 to 2800 C.

10. A process according to any one of the preceding claims wherein said branched chain polymer produced has a melt strength ratio of Tl/T2 of less than 1.6 at 2350C.

11. A process according to claim 1 substantially as hereinbefore described in any one of Examples I to IV.

12. A branched chain thermoplastic polymer suitable for extrusion applications having a melt strength ratio of Tl/T2 of less than 2.0 at 2350C when produced by a process according to any one of claims 1 to 10.

13. A branched chain thermoplastic polymer according to claim 12 having a melt strength ratio of T1/T2 of less than 1.8 at 2350C.

14. A branched chain thermoplastic polymer according to claim 13 having a melt strength ratio of Tint2 of less than 1.6 at 2350C.

15. A branched chain thermoplastic polymer according to any one of claims 12 to 14 produced from a saturated thermoplastic polyester having carboxylic acid end functional groups and capable of reaction with an isocyanate functionality.

16. A branched chain thermoplastic polymer according to claim 12 substantially as hereinbefore described in any one of Examples I to IV.

17. A molding process which comprises forming a melt of a polymer according to any one of claims 12 to 16 into a desired article and cooling the molten polymer.