WO2009077328A1

WO2009077328A1 - Transparent moulded articles

Info

Publication number: WO2009077328A1
Application number: PCT/EP2008/066672
Authority: WO
Inventors: Claudio Cavalieri
Original assignee: Basell Poliolefine Italia S.R.L.
Priority date: 2007-12-18
Filing date: 2008-12-03
Publication date: 2009-06-25
Also published as: EP2222731A1

Abstract

A very transparent moulded article comprising a propylene copolymer which comprises hexene-1 as comonomer and a clarifying agent, said propylene copolymer characterized by: a) a content of hexene-1 ranging from 1.5 to 6 %wt with respect to the total weight of the copolymer; b) an MFR value according to ISO 1133 ranging from 15 to 45 g/ 10 min; c) a Xylene Soluble Fraction ranging from 1 to 4 %wt with respect to the total weight of the copolymer.

Description

Transparent moulded articles

The present invention relates to transparent moulded articles made from propylene copolymers comprising propylene andhexene-1 as comonomers and a clarifying agent. Within the term moulded article is intended any article produced by moulding techniques such as injection moulding, injection blow moulding, injection stretch blow moulding, compression moulding, thermoforming, rotational moulding and slush moulding. Moulded articles comprising copolymers of propylene and ethylene are well known in the art.

In many applications it is particularly needed an optimal transparency of the articles produced by various moulding techniques, for example by injection moulding such as food containers, drinking cups and bottles.

For such requirement, use is made of polyolefϊn copolymers, for example propylene- ethylene copolymers which are blended with nucleating and/or clarifying agents to improve the optical properties of such products. These moulding compositions have the main disadvantage that the amount of the clarifying agents necessary to improve the transparency of propylene-ethylene copolymers is relatively high and its cost has a substantive impact .Moreover, the amount of xylene soluble fraction for the propylene ethylene copolymer is relatively high so that sometimes is not possible to use such polymers in the preparation of moulded articles for food contact.

Accordingly, it is an object of the present invention to provide a moulded article having an optimal transparency, a low amount of xylene soluble matter and possibly being obtainable in a more economic way.

Furthermore, it is another object of the present invention to provide a propylene copolymer which enhances the clarifying effect of the clarifying agents and to provide transparent moulded articles by using the minimum quantity of clarifying agents even maintaining a good balance between mechanical properties such as stiffness and impact resistance.

Propylene copolymers comprising propylene and hexene-1 are already known in the art and used in various applications, for example the international application WO 2005/059210 discloses fibres for thermal bonding comprising random copolymers of propylene and hexene-1 showing good balance of mechanical properties, in particular high tenacity and good elongation at break. Use of said copolymers for moulding processes is not mentioned. The international application WO 2006/002778 discloses pipe systems made from random polymers of propylene and hexene-1 exhibiting good burst pressure resistance and rigidity.

The propylene -hexene-1 copolymers according to both the above-mentioned international patent applications do not contain any nucleating or clarifying agents.

Moreover, both said international patent applications are completely silent about the optical properties of said propylene-hexene-1 copolymers, in particular they are silent about the effect of clarifying agents on the transparency of said propylene-hexene-1 copolymers with respect to the propylene -ethylene copolymers commonly used in moulding applications requiring high transparency.

It has surprisingly been found that copolymers of propylene and hexene-1 are much more receptive to the clarifying agents than the propylene-ethylene copolymers.

Without intending to be bound to any theory or scientific explanation, the inventors deem that one of the possible explanations for the achieved technical effect might be that the structure of such propylene-hexene-1 copolymers allows a better dispersion of the finely divided clarifying agent.

According to the present invention, it is provided a moulded article comprising a propylene copolymer comprising propylene and hexene-1 as comonomers and a clarifying agent, said propylene copolymer characterized by (percentages by weight referred to the total weight of the copolymer): a) a content of hexene-1 ranging from 1.5 to 6 %wt, preferably from 2 to 5 %wt, more preferably from 2.5 to 4 %wt; b) a MFR value according to ISO 1133 ranging from 15 to 45 g/10min, preferably from 20 to 35 g/10 min; and c) a xylene soluble fraction ranging from 1 to 5 %wt, preferably from 2 to 4 %wt. Typically the values of Polydispersity Index determined with the rhelogical method described in the characterization section approximately range from 2 to 10, preferably from 3 to 6.

The melting temperature measured on pellets is higher than 135°C, preferably is comprised between 138°C and 160⁰C, more preferably between 140 and 158°C. Although not necessary in order to obtain the advantages of the invention the propylene copolymers with hexene-1 may contain minor amounts of ethylene generally up to a value of 1.5%wt. The propylene -hexene-1 polymers according to the present invention show a haze value determined on 1 mm plaque, of less than 10 %, preferably less than 8 %, more preferably ranging from 4 to 7.5 %, the flexural modulus is higher than 800 MPa, preferably is comprised between 900 and 1200 MPa and the Izod Impact Resistance at 23°C is higher than 3 KJ/m², preferably ranges from 3 to 6 KJ/m².

The propylene -hexene-1 polymers used in the present invention can be prepared by polymerisation in one or more polymerisation steps. Such polymerisation can be carried out in the presence of Ziegler-Natta catalysts.

Preferably, the polymerization stage is carried out in presence of a highly stereospecific heterogeneous Ziegler-Natta catalyst. The Ziegler-Natta catalysts suitable for producing the propylene polymer compositions of the invention comprise a solid catalyst component comprising at least one titanium compound having at least one titanium-halogen bond and at least an electron-donor compound (internal donor), both supported on magnesium chloride. The Ziegler-Natta catalysts systems further comprise an organo-aluminum compound as essential co-catalyst and optionally an external electron-donor compound. Suitable catalysts systems are described in the European patents EP45977, EP361494, EP728769, EP 1272533 and in the international patent application WO00/63261. Preferably, the solid catalyst component comprises Mg, Ti, halogen and an electron donor selected from esters of phthalic acids disclosed in EP45977 and in particular from diisobutylphathalate, dihexylphthalate, diethylphthalate and mixtures thereof. According to a preferred method, the solid catalyst component can be prepared by reacting a titanium compound of formula Ti(OR)_n-_yX_y, where n is the valence of titanium and y is a number between 1 and n, preferably TiCU, with a magnesium chloride deriving from an adduct of formula MgCl₂^pROH, where p is a number between 0.1 and 6, preferably from 2 to 3.5, and R is a hydrocarbon radical having 1-18 carbon atoms. The adduct can be suitably prepared in spherical form by mixing alcohol and magnesium chloride in the presence of an inert hydrocarbon immiscible with the adduct, operating under stirring conditions at the melting temperature of the adduct (100-130 ⁰C). Then, the emulsion is quickly quenched, thereby causing the solidification of the adduct in form of spherical particles. Examples of spherical adducts prepared according to this procedure are described in US 4,399,054 and US 4,469,648. The so obtained adduct can be directly reacted with the Ti compound or it can be previously subjected to thermal controlled dealcoholation (80-130 ⁰C) so as to obtain an adduct in which the number of moles of alcohol is generally lower than 3, preferably between 0.1 and 2.5. The reaction with the Ti compound can be carried out by suspending the adduct (dealcoholated or as such) in cold TiCl₄ (generally 0 ⁰C); the mixture is heated up to 80-130 ⁰C and kept at this temperature for 0.5-2 hours. The treatment with TiCl₄ can be carried out one or more times. The internal donor can be added during the treatment with TiCU and the treatment with the electron donor compound can be repeated one or more times. Generally, the internal donor is used in molar ratio with respect to the MgCl₂ of from 0.01 to 1 preferably from 0.05 to 0.5. The preparation of catalyst components in spherical form is described for example in European patent application EP-A-395083 and in the International patent application WO98/44001. The solid catalyst components obtained according to the above method contain the titanium compound, expressed as Ti, generally in an amount from 0.5 to 10% by weight.

Moreover, they show a surface area (by B.E.T. method) generally between 20 and 500 m²/g and preferably between 50 and 400 m /g, and a total porosity (by B.E.T. method) higher than 0.2 cm³/g preferably between 0.2 and 0.6 cm³/g. The porosity (Hg method) due to pores with radius up to lO.OOOA generally ranges from 0.3 to 1.5 cm /g, preferably from 0.45 to 1 cm /g.

The organo -aluminum compound is preferably an alkyl-Al selected from the trialkyl aluminum compounds such as for example triethylaluminum, triisobutylaluminum, tri-n- butylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum. It is also possible to use mixtures of trialkylaluminum's with alkylaluminum halides, alkylaluminum hydrides or alkylaluminum sesquichlorides such as AlEt₂Cl and Al₂Et₃Cl₃.

The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.

Preferred external electron-donor compounds include silicon compounds, ethers, esters such as ethyl 4-ethoxybenzoate, amines, heterocyclic compounds and particularly 2,2,6,6- tetramethyl piperidine, ketones and the 1,3-diethers. Another class of preferred external donor compounds is that of silicon compounds of formula R_a ⁵Rb Si(OR⁷)_c, where a and b are integer from 0 to 2, c is an integer from 1 to 3 and the sum (a+b+c) is 4; R , R , and R , are alkyl, cycloalkyl or aryl radicals with 1-18 carbon atoms optionally containing heteroatoms. Particularly preferred are methylcyclohexyldimethoxysilane, diphenyldimethoxysilane, methyl-t-butyldimethoxysilane, dicyclopentyldimethoxysilane, 2- ethylpiperidinyl-2 -t-butyldimethoxysilane and 1,1,1 ,trifluoropropyl-2-ethylpiperidinyl- dimethoxysilane and l,l,l,trifluoropropyl-metil-dimethoxysilane. The external electron donor compound is used in such an amount to give a molar ratio between the organo- aluminum compound and said electron donor compound of from 0.1 to 500. In particular, even if many other combinations of the previously said catalyst components may allow to obtain propylene polymer compositions according to the present invention, the propylene -hexene-1 polymers are preferably prepared by using catalysts containing an ester of phthalic acid as inside donor and (cyclopentyl)2Si(OCH3)2 as outside donor. The catalysts generally used in the process of the invention are capable of producing polypropylene with a value of xylene insolubility at ambient temperature greater than 90%wt, preferably greater than 95%wt.

As already mentioned said propylene-hexene-1 polymers can be produced by a well- known polymerisation process. According to the preferred polymerisation process such polymers are produced by a polymerisation process carried out in a gas-phase polymerization reactor comprising at least two interconnected polymerisation zones. The process according to the preferred polymerisation process is illustrated in EP782587. In detail, the process is carried out in a first and in a second interconnected polymerization zones into which propylene and hexene-1 are fed in the presence of a catalyst system and from which the polymer produced is discharged. In said process the growing polymer particles flow upward through one (first) of the said polymerisation zones (riser) under fast fiuidisation conditions, leave said riser and enter another (second) polymerisation zone (downcomer) through which they flow downward in a densifϊed form under the action of gravity, leave said downcomer and are reintroduced into the riser, thus establishing a circulation of polymer between the riser and the downcomer. In the downcomer high values of density of the solid are reached, which approach the bulk density of the polymer. A positive gain in pressure can thus be obtained along the direction of flow, so that it becomes possible to reintroduce the polymer into the riser without the help of special mechanical means. In this way, a "loop" circulation is set up, which is defined by the balance of pressures between the two polymerisation zones and by the head loss introduced into the system.

Generally, the condition of fast fluidization in the riser is established by feeding a gas mixture comprising the relevant monomers to said riser. It is preferable that the feeding of the gas mixture is effected below the point of reintroduction of the polymer into said riser by the use, where appropriate, of gas distributor means. The velocity of transport gas into the riser is higher than the transport velocity under the operating conditions, preferably from 2 to 15 m/s.

Generally, the polymer and the gaseous mixture leaving the riser are conveyed to a solid/gas separation zone. The solid/gas separation can be effected by using conventional separation means. From the separation zone, the polymer enters the downcomer. The gaseous mixture leaving the separation zone is compressed, cooled and transferred, if appropriate with the addition of make-up monomers and/or molecular weight regulators, to the riser. The transfer can be carried out by means of a recycle line for the gaseous mixture.

The control of the polymer circulation between the two polymerisation zones can be carried out by metering the amount of polymer leaving the downcomer using means suitable for controlling the flow of solids, such as mechanical valves.

The operating parameters, such as the temperature, are those that are usual in olefin polymerisation process, for example between 50 to 120 ⁰C.

The operating pressures can range between 0.5 and 10 MPa, preferably between 1.5 to 6

MPa.

Optionally, one or more inert gases, such as nitrogen or an aliphatic hydrocarbon, are maintained in the polymerization zones, in such quantities that the sum of the partial pressures of the inert gases is preferably between 5 and 80% of the total pressure of the gases.

The polymerization catalysts can be fed up to the riser at any point of the said riser.

However, they can also be fed at any point of the downcomer. The catalyst can be in any physical state, therefore catalysts in either solid or liquid state can be used.

The propylene-hexene-1 polymers of the present invention are then blended with at least one clarifying agent.

With the term "clarifying agent" it is intended any additive which gives rise to polymer crystallites so small that they scatter significantly less visible light therefore increasing the transparency of polyolefϊn parts.

Usually, said clarifying agents also belong to the class of nucleating agents.

Suitable clarifying agents include the acetals of sorbitols and xylite Is as well as phosphate ester salts. Many such clarifying agents are disclosed in U.S. Pat. No. 5,310,950. Specific examples of acetals of sorbitols include dibenzylidenesorbitol or its Ci-Cs-alkyl- substituted derivatives such as methyldibenzylidenesorbitol, ethyldibenzylidenesorbitol or dimethyldibenzylidenesorbitol. Examples of suitable commercially available sorbitol- acetal clarifying agents are those designated as Millad 3940 and Millad 3988, both available from Milliken Chemical. Specific examples of phosphate ester salts include

2,2'-methylenebis(4,6,-di-tert-butylphenyl)phosphate sodium or lithium salt. Examples of commercially available phosphate ester salts for use as clarifying agents include ADK stabilizer NA-71 and ADK Stabilizer NA-21, both available from Amfine Chemical

Corp. Particularly preferred clarifying agents are 3,4-dimethyldibenzylidenesorbitol; aluminum-hydroxy-bis[2,2 '-methylene -bis(4,6-di-t-butylphenyl)phosphate]; sodium 2,2'- methylene-bis(4,6-ditertbutylphenyl)phosphate and other clarifying agents different from sorbitols and phosphate ester salts such as N,N',N"-tris-isopentyl-l,3,5-benzene- tricarboxoamide, bicyclo[2.2.1]heptane-2,3-dicarboxylic acid disodium or calcium salt

(1R,2R,3R,4S) or the commercial nucleating agent NJ Star PCl . Combinations of any of the above may also be employed.

The at least one clarifying agent may be added to the propylene-hexene- 1 polymers by known methods, such as by melting the propylene-hexene- 1 polymer under shear condition in a conventional extruder and blending in it the clarifying agent.

Preferably, the propylene-hexene- 1 copolymers comprise up to 2500 ppm, more preferably from 500 to 2000 ppm, of at least one clarifying agent.

The propylene-hexene- 1 polymers of the invention may also be blended with any other additives commonly employed in the art, such as antioxidants, light stabilizers, heat stabilizers and any other nucleating agents selected among talc, aromatic carboxylic salts, salts of monocarboxylic or polycarboxylic acids, e.g. sodium benzoate or aluminum tert- butylbenzoate.

As mentioned above, the propylene-hexene- 1 polymers disclosed in the present invention provide very transparent moulded articles such as, for example, bottles, food and drugs packages, containers for syrups and juice drinks, cups, thin walled containers, automobile parts, medical syringes an houseware containers, cooking utensils and plates. All such articles can be produced according to the known technique in the art.

The following not limiting examples are given to better illustrate the present invention.

Examples

The following characterization methods were used in testing the propylene copolymers produced. Determination of the comonomer content: by infrared spectroscopy (IR spectroscopy).

Polydispersity Index (PI): Determined at a temperature of 200⁰C by using a parallel plates rheometer model RMS-800 marketed by RHEOMETRICS (USA), operating at an oscillation frequency which increases from 0.1 rad/sec to 100 rad/sec. From the crossover modulus one can derive the P.I. by way of the equation:

P.I.= 10⁵/Gc in which Gc is the crossover modulus which is defined as the value (expressed in Pa) at which G'=G" wherein G' is the storage modulus and G" is the loss modulus.

Solubility in xylene: 2.5 g of polymer are dissolved in 250 ml of xylene at 135° C under agitation. After 20 minutes the solution is allowed to cool to 25° C, still under agitation, and then allowed to settle for 30 minutes. The precipitate is filtered with filter paper, the solution evaporated in nitrogen flow, and the residue dried under vacuum at 80° C until constant weight is reached. Thus one calculates the percent by weight of polymer soluble and insoluble at room temperature (25° C) - Melt Flow Rate (MFR"L^M): Determined according to ISO 1133 (230⁰C, 2.16 Kg).

Flexural modulus: Determined according to the ISO 178 method.

Izod impact resistance: Determined according to the ISO 180/1 A method.

Melting temperature: Determined by differential scanning calorimetry (DSC) according to the ISO 11357/3 method.

Haze (on 1 mm plaque):

According to the method ASTM D 1003 used, 5x5 cm specimens are cut molded plaques of 1 mm thick and the haze value is measured using a Gardner photometric unit connected to a Hazemeter type UX-10 or an equivalent instrument having G.E. 1209 light source with filter "C". Reference samples of known haze are used for calibrating the instrument. The plaques to be tested are produced according to the following method. 75x75x1 mm plaques are molded with a GBF Plastiniector G235/90 Injection Molding Machine, 90 tons under the following processing conditions:

Screw rotation speed: 120 rpm

Back pressure: lO bar

Melt temperature: 260⁰C

Injection time: 5 sec

Switch to hold pressure: 50 bar First stage hold pressure: 30 bar Second stage pressure: 20 bar

Hold pressure profile: First stage 5 sec

Second stage 10 sec Cooling time: 20 sec

Mold water temperature: 40⁰C

Examples 1 and 2

Propylene -hexene-1 polymers are prepared by polymerising propylene and hexene-1 in the presence of a catalyst under continuous conditions in a polymerisation apparatus comprising two interconnected cylindrical reactors, riser and downcomer. Fast fluidisation conditions are established in the riser by recycling gas from the gas-solid separator.

The catalyst employed comprises a catalyst component prepared by analogy with example 5 of EP-A-728769 but using microspheroidal MgCl₂-LTC₂H₅OH instead of

MgCl₂^-IC₂HsOH. Such catalyst component is used with dicyclopentyl dimethoxy silane

(DCPMS) as external donor and with triethylaluminium (TEA).

The polymer particles exiting the reactor are subjected to a steam treatment to remove the reactive monomers and volatile substances and then dried. The polymer particles are extruded with a usual packing of stabilisers (500 ppm of calcium stearate, 1000 ppm of Irgafos 168, 500 ppm of Irganox 1010, 500 ppm of Atmer 129 and 500 ppm of OB 22 AT) and 1800 ppm of a clarifying agent (Millad 3988) in a Maris extruder. After 7 days the polymer particles are characterised. Comparative Example 1 Example 1 is repeated with the difference that ethylene is used in place of hexene-1.

Table 1 shows the operative conditions of the polymerization process of the propylene - hexene-1 polymers of the examples 1 and 2.

Table 2 shows the properties measured on the propylene -hexene-1 polymers of the examples 1 and 2 and the propylene -ethylene copolymer of the comparative example 1. Table 1

Example

TEA/solid catalyst component, g/g 5 5

TEA/DCPMS, g/g 4 4

Temperature, ⁰C 85 85

Pressure, bar 24 24

H2/C3-, mol/mol 0.0373 0.0375

C6- feeding, Kg/h 27 32

Table 2

1 2 Comparative Ex

Example 1

Hexene-1 content, wt% 2.9 3.3 0

Ethylene content, wt% 0 0 3

Nucleant content, ppm 1800 1800 1800

XS, wt% 3 3.1 5.5-

MFR, g/10 min 27 18 26

PI — 3.7 —

Melting Temperature, ⁰C 146 146 149.5

Crystallization Temp., ⁰C 113.8 114 114

Haze 1 mm plaque, % 7.5 7 10

Flexural modulus, MPa 1140 1110 1105

Izod Impact Resistance, at 23°C,

KJ/m² 3.8 4.3 4.4

The propylene -hexene-1 polymers of the present invention show good stiffness and impact resistance comparable with those of the propylene-ethylene copolymer of the comparative example 1. The haze value is an index of transparency: lower said value is, better the transparency is. From the above data, it is evident that, maintaining the same quantity of the clarifying agent, the propylene -hexene-1 polymers of the present invention are substantially more transparent than the commonly used propylene-ethylene copolymers. From another point of view, Fig.l showing the haze values relating to the propylene - hexene-1 polymers according to Ex. 1 and to the propylene-ethylene copolymers according to the comparative Ex. 2 at different amounts of clarifying agent, makes clear that using the propylene -hexene- 1 polymers of the invention for producing moulded articles allows to obtain a very transparent injection moulded article having a haze value about 10 % by using lower amount of clarifying agent.

By using about 1200 ppm of clarifying agent it can be obtained a propylene -hexene- 1 polymer having a transparency comparable to that of the propylene-ethylene copolymer blended with 1800 ppm of clarifying agent.

Claims

1. A molded article comprising a propylene copolymer which comprises hexene-1 as comonomer and a clarifying agent, said propylene copolymer characterized by: a) a content of hexene-1 ranging from 1.5 to 6 %wt with respect to the total weight of the said propylene copolymer; b) a MFR value according to ISO 1133 (230⁰C, 2.16 Kg) ranging from 15 to 45 g/10min; c) a xylene soluble fraction ranging from 1 to 4 %wt with respect to the total weight of the said propylene copolymer.

2. The moulded article according to claim 1 wherein the propylene copolymer has a haze value measured on lmm plaque less than 10%.

3. The moulded article according to claim 1 wherein the propylene copolymer comprises up to 2500 ppm of the said clarifying agent.

4. The moulded article according to claim 1 wherein the clarifying agent is selected from acetals of sorbitols and xylitols or phosphate ester salts.

5. The moulded article according to claim 1 wherein the clarifying agent is 3,4- Dimethyldibenzylidenesorbitol.

6. The moulded article according to claim 1 wherein the propylene copolymer has a flexural modulus higher than 800 MPa.

7. The moulded article according to claim 1 wherein the propylene copolymer has a Izod Impact Resistance at 23⁰C higher than 3 KJ/m .