TRANSPARENT DRAWN ARTICLE

Abstract

The invention relates to a process for the production of a high strength transparent high density polyethylene article comprising the steps of (i) heating a high density polyethylene (HDPE) to a temperature above the melting temperature (T.sub.m) of the HDPE; (ii) molding the heated HDPE obtained in step (i) to form a hot molded HDPE article; (iii) cooling the hot molded HDPE article to a temperature below T.sub.m to form a melt-crystallized HDPE article; (iv) stretching the melt-crystallized HDPE article to a total draw ratio of at least 5 comprising at least one stretching step of the article at a temperature T.sub.1 below the melting temperature T.sub.m to a draw ratio (DR.sub.1) of at least 2 to form an oriented HDPE article, wherein the HDPE has a melt flow index (MFI) measured at 21.6 kg and 190 C. according to ASTM D1238 of at most 1.5 g/I Omin, an isotropic density measured according to ISO 1 183-1 A of at most 0.955 g/cm3 and a T.sub.m measured according to ISO 1 1357-3 of greater than 130 C. The invention also relates to high strength transparent HDPE articles and products comprising the high strength transparent HDPE article such as ballistic resistant articles, visors, car parts, train parts, plane parts, windshields, windows and radomes.

Claims

1. A process for the production of a high strength transparent high density polyethylene article comprising the steps of (i) heating a high density polyethylene (HDPE) to a temperature above the melting temperature (T.sub.m) of the HDPE; (ii) molding the heated HDPE obtained in step (i) to form a hot molded HDPE article; (iii) cooling the hot molded HDPE article to a temperature below T.sub.m to form a melt-crystallized HDPE article; (iv) stretching the melt-crystallized HDPE article to a total draw ratio of at least 5 comprising at least one stretching step of the article at a temperature T.sub.1 below the melting temperature T.sub.m to a draw ratio (DR.sub.1) of at least 2 to form an oriented HDPE article, characterized in that the HDPE has a melt flow index (MFI) measured at 21.6 kg and 190 C. according to ASTM D1238 of at most 1.5 g/10 min, an isotropic density measured according to ISO 1183-1A of at most 0.955 g/cm.sup.3 and a T.sub.m measured according to ISO 11357-3 of greater than 130 C.

2. The process according to claim 1 wherein the high strength article is a high strength fiber, tape or film.

3. The process of claim 1 wherein before or during molding the HDPE is heated to a temperature at least 10 C. above the T.sub.m of the HDPE.

4. The process of claim 1 wherein during step (ii) a shear rate of less than 10 s.sup.1, preferably less than 5 s.sup.1, more preferably less than 1 s.sup.1 and most preferably less than 0.5 s.sup.1 is applied.

5. The process of claim 1 wherein T.sub.1 is between 100 and 130 C., preferably between 110 C. and 128 C. and most preferably between 115 C. and 125 C.

6. The process of claim 1 wherein the oriented HDPE article comprises at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt % HDPE and most preferably the oriented HDPE article substantially consists of HDPE.

7. The process of claim 1 wherein the HDPE has a weight average molecular weight (Mw) of at least 200 kg/mol and/or has a molecular weight distribution (MWD) of between 2 and 10.

8. The process of claim 1 wherein the stretching of the melt-crystallized HDPE article comprises at least one further stretching step at a temperature T2>T.sub.1, preferably at a temperature T.sub.2>T.sub.m.

9. A high strength transparent HDPE article obtainable by any of the processes according to claim 1.

10. A high strength transparent HDPE article wherein the HDPE has a melt flow index (MFI) measured at 21.6 kg and 190 C. according to ASTM D1238 of at most 1.5 dg/min, an isotropic density measured according to ISO 1183-1A of at most 0.955 g/cm.sup.3 and a T.sub.m measured according to ISO 11357-3 of greater than 130 C., characterized in that said high strength transparent HDPE article has a tensile strength of at least 0.5 GPa and a transmittance of at least 70% when measured at a film thickness of 0.1 mm and at a wavelength of 550 nm.

11. A high strength transparent HDPE article according to claim 9 wherein the article comprises at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt % HDPE and most preferably the high strength transparent HDPE article substantially consists of HDPE.

12. A high strength transparent HDPE article according to claim 9 wherein the HDPE has an MFI measured at 21.6 kg and 190 C. of at most 1.0 g/10 min, more preferably at most 0.8 g/10 min and most preferably at most 0.5 g/10 min.

13. A high strength transparent HDPE article according to claim 9 wherein the HDPE has an isotropic density of at most 0.953 g/cm.sup.3, more preferably at most 0.952 g/cm.sup.3and most preferably at most 0.950 g/cm.sup.3.

14. A high strength transparent HDPE article according to claim 9 wherein the HDPE has weight averaged molecular weight (Mw) of at least 200 kg/mol, preferably at least 250 kg/mol and most preferably 300 kg/mol and/or a molecular weight distribution (MWD) of between 2 and 20, preferably between 3 and 15 and most preferably between 4 and 10.

15. A product comprising the high strength transparent HDPE article according to claim 9, preferably the product is a ballistic resistant article, a visor, a car part, a train part, a plane part, a windshield, a window or a radome.

Description

METHODS

[0030] Melting temperature (T.sub.m ), and heat of fusion (H.sub.F) were established by differential scanning calorimetry according to ISO-11357-3 by evaluation of the second heating curve at a heating rate of 10 C./min in the interval from room temperature to 200 C.
The crystallinity (X.sub.c) was calculated from the equation: X.sub.c=H.sub.F/H.sub.F.sup.0, where H.sub.F.sup.0 is the heat of fusion of perfect crystalline HDPE which is assumed to be equal to 280 J/cm.sup.3.

SEC-MALS

[0031] The molecular mass distributions (Mn, Mw, Mz, Mw/Mn) were measured using a PL-210 Size Exclusion Chromatograph coupled to a multi-band infrared detector (IR5 PolymerChar) and a multi-angle light scattering (MALS) detector (laser wavelength 690 nm) from Wyatt (type DAWN EOS). Two PL-Mixed A columns were used. 1,2,4-trichlorobenzene was used as the solvent, the flow rate was 0.5 ml/min, and the measuring temperature was 160 C. Data acquisition and calculations were carried out via Wyatt (Astra) software. The HDPE should be completely dissolved under such conditions that polymer degradation is prevented by methods known to a person skilled in the art.

Transparency/Haze/Transmittance

[0032] Transmittance spectra were measured in the range of 400-700 nm on a Shimadzu (Japan) UV-3102 PC spectrophotometer with a 1-nm interval at 50% humidity and 23 C. The distance between samples and the detector is around 150 mm. A blank measurement was performed, without a sample and the transmitted light to the detector at each wavelength was set to 100%. The recorded light transmission at each wavelength was normalized to the blank measurement and the transmittance value was obtained.

Tensile Properties

[0033] The Young's modulus and tensile strength of the drawn samples were measured at room temperature on a Zwick Z100 tensile tester at a crosshead speed of 100 mm/min. The Young's moduli were calculated from the tangents of the engineering stress-strain curves below a strain of 0.1%. In all cases, at least three strips were measured and the mean values of Young's modulus together with tensile strength were calculated and reported. For calculation of the tensile strength, the tensile forces measured are divided by the cross-sectional area, as determined by measuring thickness and width of the molded drawn tapes; values in GPa are calculated with the density of the molded article of 0.96 g/cm.sup.3.
Isotropic density was determined according to ISO 1183-1 method A on an isotropic sample obtained by annealing the sample for 1 hour at 160 C.
MFI have been measured according to ASTM D1238, at a weight of 21.6 kg and a temperature of 190 C. Values are commonly expressed in the historical g/10 min which is identical to the SI unit dg/min.

Intrinsic Viscosity (IV)

[0034] IV is determined according to ASTM-D1601/2004 at 135 C. in decalin, the dissolution time being 4 hours, with DBPC as anti-oxidant in an amount of 2 g/l solution, by extrapolating the viscosity as measured at different concentrations to zero concentration.

EXPERIMENTAL

[0035] HDPE samples with different physical properties (Table 1) have been selected to produce HDPE sheets. Polyethylenes 1 to 4 were homogenized in a co-rotating twin screw extruder at 160 C. for 10 minutes. Extrudates were cooled down in air to room temperature, followed by granulation. Polyethylene 5 was homogenized by casting from a 4 wt % xylene solution.

Substantially isotropic melt-crystallized HDPE sheets with a given thickness of approximately 1 mm were then produced by compression moulding at 160 C. for 10 minutes, followed by quenching to 20 C. Dumbbell-like samples with gauge dimensions 122 mm.sup.2 and a thickness of 1 mm were then cut from the compression-moulded sheets. These dumbbell-like samples were subsequently drawn at different temperatures to various draw ratios in air using a Zwick Z100 tensile tester at a crosshead speed of 100 mm/min to identify the drawing temperature and draw rate resulting in highest tensile properties for each polyethylene. Such optimal drawing conditions and properties are reported in Table 2. The thickness of the drawn samples was calculated by weighing, assuming a density of the oriented PE equal to 0.96 g/cm.sup.3. Young's modulus, strength and transmittance of the tapes after uniaxial drawing were measured and are reported in Table 2.

TABLE-US-00001 TABLE 1 M.sub.w MWD T.sub.m X.sub.c Density MFI Sample (kg/mol) () ( C.) (%) (g/ml) (g/10 min) PE1 80 3.2 132.4 75.1 0.964 8.0 (190 C. @2.16 kg) PE2 140 4.7 133.6 70.6 0.960 0.9 (190 C. @2.16 kg) PE3 134 3.6 134.0 70.7 0.958 21 (190 C. @21.6 kg) PE4 408 5.1 132.6 61.5 0.947 0.6 (190 C. @21.6 kg) PE5 1,070 5.9 131.8 54.2 0.940 <0.1 (190 C. @21.6 kg)

TABLE-US-00002 TABLE 2 Mod- Transm. Draw ulus Strength Thickn. @550 nm Sample temp. DR (GPa) (GPa) (m) (%) Comp. PE1 60 20 19.6 0.54 100 58 Exp. 1 Comp. PE2 80 20 22.1 0.76 100 57 Exp. 2 Comp. PE3 80 20 18.2 0.64 100 15 Exp. 3 Example 1 PE4 100 15 16.9 0.96 100 90 Example 2 PE5 120 13 17.2 1.1 140 67