WO1993008231A1

WO1993008231A1 - Polymer blends

Info

Publication number: WO1993008231A1
Application number: PCT/GB1992/001912
Authority: WO
Inventors: Paul Thomas Alder; John Groves Dolden; David George Othen
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1991-10-22
Filing date: 1992-10-19
Publication date: 1993-04-29
Anticipated expiration: 1994-04-22
Also published as: GB9122327D0

Abstract

Polymer blends comprise polyethylene and a liquid crystal polymer of a random copolymer of: (a) an aliphatic dicarboxylic acid in which the carboxylic acid groups are limited by a chain containing x carbon atoms, (b) a 1,4-dihydroxy benzene, and (c) 4-hydroxybenzoic acid, wherein x is in the range 2 to 20 and the 4-hydroxybenzoic acid constitutes 10 to 90 mol % of the monomers from which the liquid crystal material is derived. The polymer blends have improved melt processability.

Description

POLYMER BLENDS

The present invention relates to blends of thermoplastic polymers having improved melt processability.

EP 30417 discloses melt processable compositions having improved processability which comprise blends of two thermoplastic polymers, at least one of which is capable of forming an anisotropic melt. The formation of an anisotropic melt is a characteristic of a liquid crystal and polymers producing such melts may be termed "liquid crystal polymers". Examples of polymers which are mentioned as one component of the blend are polyolefines. Among specific polyolefines mentioned are polypropylene and low density polyethylene. Among polymers forming anisotropic melts which may be used as the other components of polymer^" blends are polyesters described in GB1,507,207, US3,778,410, US4,067,852, US4,083,829, US4,130,545, and US4,161,470. Among specific polymers exemplified are liquid crystal polymers derived from reactions of various derivatives of aromatic hydroxyl acids and long chain alkanedioic acids or dihydroxy compounds.

Japanese published unexamined application JP-01036653A discloses blends of an ultra high molecular weight polyethylene with a polymer giving an anisotropic melt. The polymers forming anisotropic melts include copolymers of polyethylene terephthalate and p-hydroxybenzoic acid, together with copolymers of p- hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid.

These polymers forming anisotropic melts (liquid crystal polymers) give reductions in melt viscosity but there is still a need for liquid crystal materials which give greater improvements in melt properties when incorporated in blends with polyethylene.

According to the present invention a polymer blend comprising polyethylene and a liquid crystal material is characterized in that the liquid crystal material is a random copolymer of: a) an aliphatic dicarboxylic acid in which the carboxylic acid groups are linked by a chain containing x carbon atoms, b) a 1,4 dihydroxy benzene, and c) 4-hydroxybenzoic acid, wherein x is in the range 2 to 20, and the 4-hydroxybenzoic acid constitutes 10 to 90 mol % of the monomers from which the liquid crystal material is derived.

In the preparation of the liquid crystal material it is preferred to use about equimolar amounts of dicarboxylic acid and 1,4-dihydroxy benzene, except when monofunctional molecular weight limiters are used as explained below.

The preferred liquid crystal materials are those in which x is 4 to 10. Preferably the 4-hydroxybenzoic acid constitutes 10 to 50 mol% of the monomers used to make the liquid crystal material.

The liquid crystal material may optionally contain up to 20 mol% of a monofunctional carboxylic acid, e.g benzoic acid, as a molecular weight limiter. Where a monofunctional carboxylic acid is added in the preparation of the liquid crystal material then the quantity of aliphatic dicarboxylic acid may be reduced in order to provide equal numbers of carboxylic acid and hydroxy groups. Alternatively monofunctional phenols of low volatility may be used as molecular weight limiters.

Liquid crystal polymers suitable for use in the present invention are known. Liquid crystal polymers which may be used in the present invention may be described by formula:

Liquid crystal polymers which may be used in the present invention include those in which the mole fractions a, b, and c are all equal to 0.33 and x is 5. Such a liquid crystal polymer is disclosed in EP242278A. A preferred liquid crystal polymer is that wherein a = 0.3, b = 0.35, c = 0.35 and x = 8.

For example a preferred polymer is that wherein a is p- hydroxybenzoic acid, b is hydroquinone and c is sebacic acid. If benzoic acid is present as a molecular weight limiter the preferred polymer is that wherein the mole fractions of components p- hydroxybenzoic acid, hydroquinone, sebacic acid are equal to 0.3, 0.35 and 0.35 respectively.

When describing the liquid crystal polymer as a derivative of various acids and hydroxy compounds we do not intend to restrict the synthesis to starting materials which are in the form of the free acids or hydroxy compounds. Thus in some cases starting materials may be used containing an acyloxy group which reacts in a way equivalent to a hydroxy group.

The reaction may be carried out in a liquid medium in which the reactants are dispersed, together with a catalyst such as an alkali metal acetate. High shear agitation may be desirable initially to disperse the reactants and subsequently to maintain the products dispersed as reaction proceeds. The reaction is carried out at elevated temperatures, eg, 250° - 330°C, preferably 290° - 300°C. A stream of inert gas is preferably passed through the reaction mixture to remove by-products.

Some polymerization starts to occur at about 240°C, but the reaction mixture is preferably heated to above 290°C, preferably as rapidly as is possible with the equipment used. The duration of the polymerization step, which may be defined as time at a temperature in excess of 290°C, may for example be in the range 60 to 120 minutes. Alternatively the liquid crystal polymer may be formed by melt polymerization. Melt polymerization may be carried out in two stages. The first stage is a gradual heating to the final polymerization temperature. In the second stage the reaction mixture is held at the polymerization temperature and subjected to a vacuum.

In both dispersion and melt polymerization the optimum time will depend on the removal of the products released by the condensation reaction. Thus where the hydroxy function is present as an acetoxy group it will depend on the efficiency with which acetic acid is removed from the system. For melt polymerization this will depend on the vacuum which can be achieved and the surface turnover of the melt provided by stirring. For dispersion polymerization it will depend on the inert gas flow rate and degree of agitation.

The present invention may be applied to a variety of polyethylenes. Thus it may be applied to HDPE (high density polyethylene), LDPE (low density polyethylene), and MDPE (medium density polyethylene). Polyethylenes may be characterized by their MFR (Melt Flow

Rate) (sometimes referred to as Melt Index) usually quoted in g/10 in, and by density usually quoted in kg/m .

MFR is the quantity of melt extruded in 10 minutes through a die of standard dimensions under a defined load at a defined temperature. The test is carried out in accordance with ISO 1133-1981 or BS 3412:1976.

While the invention may be applied to any pol olefin where processing improvements are desirable, it is of most value for materials of MFR less than 0.3 (2.16 kg, 190°C), especially high molecular weight grades of MFR less than 10 (21.6 kg,190°C).

Blends of the polyethylene and liquid crystal polymer may be prepared by any of the polymer melt blending techniques known in the art, e.g. batch mixers and single-screw or twin-screw extruders.

The liquid crystal polymer may be blended with the polyethylene in an amount corresponding to 0.01 - 5% by weight of the total blend. The addition of low levels of the liquid crystal materials to polyethylenes by twin screw extrusion compounding results in a reduction in melt viscosity with a reduction in temperature and specific energy during compounding. Extrusion using single screw extruders to produce pipe, film or blow mouldings leads to reduced die head pressures without substantially damaging the properties of the product. This has the advantage of increased throughput during extrusion.

In film extrusion the polymer blends eliminate sharkskin which is a non-smooth surface texture due to melt flow instability and improve film appearance.

The invention will now be described with reference to the following experiments in which examples of the invention are identified by number and comparative tests not according to the invention are identified by letter. Synthesis of liquid crystal polymers

The liquid crystal polymer used in the examples was synthesized by melt polymerization or by a dispersion polymerization process. Melt polymerization This method was used for Example 2 and comparative Tests B and C.

The polymerization was carried out in a cylindrical borosilicate glass reaction vessel with an air driven helical stirrer and fittings for distillation under inert gas or high vacuum. The flask was heated in a fluidized bed of sand in an apparatus sold as Tecam SBL- 2, and capable of controlled heating above 350°C. The apparatus was generally lagged to improve distillation rate.

The reactants containing hydroxy groups were reacted with acetic anhydride to convert the hydroxy groups to acetoxy groups before reaction.

A mixture of dried reactants were placed in the polymerization vessel and purged at least three times with dry nitrogen/high vacuum. The reactants were further dried at 100-130°C for 45-60 minutes under a vacuum of 6.67-13.33 Pa (0.05-0.1mm Hg) . The contents of the polymerization vessel were then sealed under nitrogen and the vessel was then removed from the heating bath. The bath was then heated to a temperature of about 260-280°C unless otherwise stated (i.e. about 20°C below the desired polymerization temperature. The vessel was then returned to the bath and stirring was commenced as soon as the monomers had melted. The polymerization reaction involves the liberation of acetic acid and distillation under nitrogen was carried out until approximately 80% of the theoretical acetic acid had been evolved. During distillation the temperature of the bath was gradually increased to the required polymerization temperature (280- 300°C unless otherwise stated. Distillation under a vacuum of 26.66- 66.66 Pa (0.2-0.5 mm Hg) was continued while stirring vigorously.

The polymer was removed from the reaction vessel while hot. Dispersion process

The typical procedure was as follows: The dispersing medium was a mixture of hydrogenated terphenyls, sold under the trade name Santotherm 66, used in a proportion of 1- 1.5rl w/w on acetylated monomers. The dispersing medium was deoxygenated before use by purging overnight with nitrogen. About 100 ml of the dispersing medium charge was reserved and the dispersion aid, an organo-functionalized montmorillonite clay sold under the trade name Bentone 34, (at 0.5-1.375% w/w on acetylated monomers) and catalyst (potassium acetate at 50-250 ppm on acetylated monomers) dispersed in it using a high speed disperser ("Silverson") under nitrogen. The dry monomers were charged to a flanged flask equipped with a stainless steel stirrer, nitrogen purge tube, pressure equalised dropping funnel, heater control thermocouple, and switchable reflux/distillation condenser. The apparatus was thoroughly purged with nitrogen (typically overnight), and a slow flow of gas through the apparatus maintained.

The reagents were acetylated by addition of acetic anhydride (5 mole % excess) and refluxing at 150°C for 45 min with slow stirring. The temperature was increased to 170°C and acetic acid and excess acetic anhydride distilled off. T e flow of nitrogen was then increased and the bulk of the Santotherm added, followed by the pre-dispersion. The stirrer speed was increased to about 1250 rpm to disperse the monomers and the temperature was then raised to 290-300 C. Acetic acid is collected as the polymerization proceeds. When the polymerization stage was complete the heater was switched off and the apparatus allowed to cool. Nitrogen flow and stirrer speed were maintained until the temperature dropped well below the solidification temperature of the liquid crystal polymer (LCP) (typically about 120°C for the liquid crystal polymers made.) Stirrer speed was then reduced. On cooling to below 40°C an approximately equal volume of acetone was added to the polymer/Santotherm mixture and stirred for 30 min. The polymer was filtered off and washed by re-suspending in acetone and refiltering at least twice before drying at 40°C overnight in a vacuum oven. Characterization of liquid crystal polymers

In the examples given below the materials identified as liquid crystals were shown to be such as follows.

All LC materials were shown to form LC melts by observation of a birefringent mesophase using a polarising microscope equipped with a hot stage. Melting points (crystal-liquid crystal transition) of the liquid crystal polymers used in the present invention appear to be as much dependant on thermal history as composition and MWt; they are typically 110-180°C.

Intrinsic Viscosities (IVs), as an indication of MWt, were measured as 0.5 g/100 ml solutions in p-chlorophenol at 45°C; solution flow times were measured according to ASTM D445-87 and the IV of the polymer calculated by a single-point method using the Solomon-Ciuta Equation. (Note that the IV/MWt relationship holds only for polymers of the same chemical structure). MWt data for these materials may also be obtained by Gel- Permeation Chromatography (GPC) on 0.02-0.04% w/v solutions in CHCl /CH Cl . Results are not absolute but are calculated from a scale established with polystyrene. Since the results are dependent on chain conformation in solution, they may not be strictly comparable between polymers of differing chemical structure. Example 1

A liquid crystal polymer was prepared as follows. The reactants used were p-hydroxybenzoic acid, hydroquinone, sebacic acid and benzoic acid in the molar ratio of 0.3:0.35:0.325:0.05. The reaction was carried out by dispersion polymerization as described above.

The resulting polymer had an Inherent Viscosity (determined as described above using 4-chlorophenol) of 0.18. The polymer was shown to form a liquid crystal melt by observation of a birefringent mesophase using a polarizing microscope fitted with a hot stage. The polymer was then used to form a blend with a high molecular weight high density polyethylene. The polyethylene was a polymer commercially available under the trade name "Rigidex" HM5420XP. This was a high molecular weight HDPE blow-moulding grade. It had an MFR (21.6 kg) of 2 and a density of 0.954. The blend was formed by dry mixing particles of the liquid crystal polymer with polyethylene powder in the desired proportions. It contained 5 % by weight of the liquid crystal polymer, based on the total weight of blend.

The blend was then fed to a "Brabender" PLE 651 drive with a W 30 roller mixer head and a quick loading chute. The torque developed by the drive is related to the the viscosity of the melt under the shear conditions of the test.

The amount of blend fed was 30g. The load on the chamber was 5 kg weight and the speed was 30 rp . The chamber was heated to 200°C - 205°C and the charge of blend was added rapidly at low speed. Speed was then increased to 30 rpm and the torque recorded against time. The equilibrium torque is the value to which the torque tends once melting and mixing are complete. Equilibrium is generally assumed to have been reached when the torque is approximately constant over a ιo minute period. The equilibrium value was found to be 10.8 Nm. Comparative Test A

In a comparative test not according to the invention an equilibrium value of Brabender torque was determined as in Example 1 but without any liquid crystal polymer added to the high molecular weight high density polyethylene. The equilibrium torque was 16.5 Nm. A comparison of the equilibrium torques for Example 1 and Test A shows that the use of the liquid crystal polymer of the invention gave a considerable reduction in torque corresponding to a significant reduction in melt viscosity. Example 2

A liquid crystal polymer of the type made in Example 1 was synthesized by melt polymerization using the proportions of starting materials given in Table 1. This also gives the time for which acetic acid was distilled off and the time for which the reactants were heated under vacuum after acetic acid had distilled off.

The polymer was blended and tested as in Example 1.

The results are given in Table 1. Comparative Test B

In a comparative test not according to the invention a liquid crystal polymer consisting of units derived from polyethylene terephthalate and p-hydroxybenzoic acid was synthesized by melt polymerization of a pre-polymer having an intrinsic viscosity of 0.24. The pre-polymer was produced from 4-acetoxybenzoic acid and poly(ethylene terephthalate) by melt polymerization. The conditions used and the results obtained are shown in Table 1. Comparative Test C

A liquid crystal polymer consisting of units derived from p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid was synthesized by melt polymerization.

The conditions used and the results obtained are shown in Table 1.

TABLE 1

a : Post-polymerized from material of IV=0.24. b : ΣV=0.6 c : In pentafluorophenol (all other IVs in 4-chlorophenol) The values shown in brackets for experiment B are the equivalent quantities of monomer used to produce the pre-polymer. The control value is the value obtained when no liquid crystal polymer was added.

Examples 3 and 4 show the use of the liquid crystal polymers as process aids in polyethylene extrusion. Example 3

A liquid crystal polymer was prepared as described above from the following components: p-hydroxybenzoic acid 207.2g hydroquinone 192.7g sebacic acid 354g benzoic acid 10.7g

The polymerisation time (>290°C) was 100 min. The liquid crystal polymer was blended with Rigidex HM4516AP, a resin used in pipe compounds supplied by BP Chemicals, according to the following procedure.

The liquid crystal polymer, HM4516AP powder and Irganox 1010 anti-oxidant were pre-blended in a Pappenmeier powder blender for 5 min at 700-800 rpm. Blends containing 0, 0.25, 0.5, 1.0 and 2.0% w/w of the liquid crystal polymer were produced.

The blends were compounded using a Brabender DSK 42/7 contra- rotating twin-screw bench top extruder at 25 rpm and a 3mm round, conical entry die with the following process temperatures:

Heating Zone : 1 2 3 Die Temperature °C : 170 185 200 205

The resulting strand was collected and pelletised using a Betol pelletiser.

Blend processability was then evaluated by extrusion through a Brabender 19/25D single-screw bench top extruder at a range of screw speeds typically 10, 20-25, 40-50 and 65-75 rpm. The screw compression ratio was 3:1 and a 1.5mm round, conical entry die was used. The processing temperatures were as for the compounding operations.

Both the die pressure and output were monitored and readings taken at regular intervals, generally three sets of readings at each screw speed.- Pressure readings were taken at the mid-point of the range observed over a 1 minute period.

The results are given in Figure 1 as plots of the die melt pressure against extruder throughput. It can be seen that for a given die pressure the throughput increases markedly with the content of the liquid crystal polymer. For example at a die pressure of 1400 psi the throughput increases by 11, 17, 38 and 78% at 0.25, 0.5, 1 and 2% w/w respectively of the liquid crystal polymer. Example 4 Five liquid crystal polymers were prepared having the components listed in Table 2. TABLE 2

The liquid crystal polymers (A-E) were compounded with Rigidex HM5420XP using a 30mm intermeshing co-rotating twin screw extruder (Werner S Pfleiderer ZSK30), a Tron T-20 volumetric feeder and a Cumberland pelletiser.

The ZSK30 extruder had 8 barrel sections with 5 separate temperature control zones and melt pressure measurement at the die. A die plate with a single 5 mm hole was fitted.

The liquid crystal polymer was blended with the HM5420XP powder in a tumble blender prior to extrusion.

A total of six blends containing 2% by weight of the liquid crystal polymer were produced under the conditions given in Table 3.

For comparison blends 1 and 2 did not incorporate any liquid crystal material.

S.M.E. = Specific Mechanical Energy Input The screw speed was 100 rpm and the barrel set temperature control zones were 170, 200, 200, 210 and 235°C.

A drop in extruder torque of approximately 30% relative to the control was noted for all blends implying a significant reduction in extruder power consumption as shown by both the reduced specific mechanical energy input and the reduced melt temperature.

The resultant pelletised blends were used to blow bottles on a Kautex KEB4 blow moulder fitted with a 50mm single screw. The extruder barrel had a grooved feed zone and six separate temperature control zones along the barrel.

The machine was fitted with a low compression head and a diverging 11mm die with a 9mm mandrel. A 4oz. stress resistance bottle mould was used.

The blow moulding conditions are given in Table 4.

TABLE 4

The temperature profile used in all blends was 170, 190, 210, 210, 220 and 230°C.

All the liquid crystal polymer blends gave substantial reductions in die pressure. Example 5

This example shows the reduction in surface effects eg sharkskin observed with the liquid crystal polymer blends. A liquid crystal polymer^"of MW=6500 having the components hydroxybenzoic acid, hydroquinone, sebacic acid and benzoic acid in the respective proportions 0.3, 0.35, 0.35 and 0.0175 was prepared according to the dispersion polymerization method described above. Innovex LL7206AA a film grade linear low density polyethylene supplied by BP Chemicals, the liquid crystal polymer and a standard antioxidant/stabiliser additive were powder blended and compounded on a Werner fi Pfleiderer ZSK30 twin-screw compounding extruder. The additive comprised 0.03% w/w Irganox 1010, 0.1% w/w Irgaphos PEP Q and 0.1% w/w calcium stearate. The following blends were prepared:

A polyethylene + additive

B polyethylene + 1% LCP + additive

C polyethylene + 0.5% LCP + additive

D polyethylene + 0.2% LCP + additive.

The processing conditions were are follows:

The heating zone temperatures used were as follows:

The liquid crystal polymers reduced the die pressure, machine torque and melt temperature.

The extruder blends were pelletised using an Accrapak 750/6 pelletiser. The pelletised blends were extruded into film using a Betol BK32 32mm single screw film extruder with a die gap of 0.6mm and a screw speed of 80 rpm.

The heating zone temperatures during extrusion were 200/150/190/190/190/200 and 200°C (die).

The processing conditions and effect on sharkskin are shown below. Sharkskin is a surface defect due to melt flow instability on extrusion at high shear rate which reduces gloss and clarity of the film.

It can be clearly seen that after running for a short time period the liquid crystal polymer effectively eliminates sharkskin and markedly reduces die pressure. Example 6

This example shows the reduction in die pressures observed during pipe extrusion of the liquid crystal polymer blends.

A liquid crystal polymer having the components as shown in Example 5 and a MW = 6200 was blended with Rigidex HM4516AP, a high density polyethylene pipe grade material, together with the standard stabilizer/colour additive package for PC4049 grey ducting grade. The additive consisted of:

0.25% UVS

0.15% Irganox 1010 antioxidant

0.85% FNP 1605 colour masterbatch.

Liquid crystal polymer levels of 0, 0.2, 0.5 and 2% were used for the tests. The blends were produced on the 50Kg scale using a Werner £ Pfleiderer ZSK53 co-rotating twin screw extruder, pelletised and used to make pipe on a Davy 6.5 cm pipe extruder.

Figure 2 represents a plot of die pressure against output rate and shows a substantial reduction in die pressure for a given output. However this required an increase in screw speed to maintain output as may be expected for a true reduction in bulk viscosity. Example 7

This example shows the rheological properties of the liquid crystal polymer blends described in Example 6.

These properties were measured by capillary rheometry using a Gottfert Rheograph 2000 twin barrel extrusion capillary rheometer at 220°C using 30mm x 1mm and 5mm x 1mm dies. Figures 3 and 4 represent plots of shear stress and viscosity respectively against shear rate and show a systematic drop in viscosity with increasing liquid crystal polymer content across the shear rate range examined.

Claims

Claims:

1. A polymer blend comprising polyethylene and a liquid crystal material characterised in that the liquid crystal material is a random copolymer of:

(a) an aliphatic dicarboxylic acid in which the carboxylic acid groups are linked by a chain containing x carbon atoms,

(b) a 1,4-dihydroxybenzoic acid, and

(c) 4-hydroxybenzoic acid, wherein x is in the range 2 to 20, and the 4-hydroxybenzoic acid constitutes 10 to 90 mol % of the monomers from which the liquid crystal material is derived.

2^'. A polymer blend according to claim 1 wherein the liquid crystal material has the formula:

3. A polymer blend according to either of the preceeding claims wherein x is in the range 4 to 10.

4. A polymer blend according to any of the preceeding claims wherein the 4-hydroxybenzoic acid component constitutes 10 to 50 mol % of the liquid crystal material.

5. A polymer blend according to any of the preceeding claims wherein the mole fraction component (a) is equal to 0.3, and that of (b) and ^• (c) is 0.35, and wherein x = 8.

6. A polymer blend according to any of the preceeding claims wherein the liquid crystal material additionally contains a molecular weight limiter.

7. A polymer blend acccording to claim 6 wherein the molecular weight limiter is present in up to 20 mol% of the liquid crystal material.

8. A polymer blend according to either of the two preceeding claims wherein the molecular weight limiter is a monofunctional carboxylic acid.

9. A polymer blend according to claim 8 wherein the monofunctional carboxylic acid is benzoic acid.

10. A polymer blend according to any of the preceeding claims wherein the liquid crystal material is present in an amount in the range 0.01 to 5 % by weight of the total polymer blend.

11. A liquid crystal material comprising a random copolymer of:

(a) p-hydroxybenzoic acid

(b) hydroquinone, and

(c) sebacic acid.

12. A liquid crystal material according to claim 11 wherein the mole fraction component of (a) is equal to 0.3 and that of (b) and

(c) is equal to 0.35.

13. A liquid crystal material according to claim 11 which additionally includes a molecular weight limiter.