Transparent drawn article
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 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 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 1 below the melting temperature T m to a draw ratio (DR 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 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.
1. A process for the production of a high strength transparent high density polyethylene (HDPE) article comprising the steps of:
(i) heating HDPE having a melt flow index (MFI) measured at 21.6 kg and 190° C. according to ASTM D1238 of at most 1.5 g/10min, an isotropic density measured according to ISO 1183-1A of at most 0.955 g/cm 3 , a Tm measured according to ISO 11357-3 of greater than 130° C. and a molecular weight distribution (MWD) of between 2 and 10 to a temperature above a melting temperature (T m ) of the HDPE to form a heated HDPE;
(ii) molding the heated HDPE obtained in step (i) in the absence of solvents to form a hot molded HDPE article;
(iii) cooling the hot molded HDPE article obtained in step (ii) to a temperature below T m to form a melt-crystallized HDPE article;
(iv) stretching the melt-crystallized HDPE article obtained in step (iii) to a total draw ratio of at least 5, wherein the melt-crystallized HDPE article is stretched during at least one stretching step at a temperature T 1 below the melting temperature T m to a draw ratio (DR 1 ) of at least 2 to form the high strength transparent HDPE article.
2. The process according to claim 1 wherein the high strength transparent HDPE article is a high strength fiber, tape or film.
3. The process according to claim 1 , wherein before or during molding according to step (ii), the HDPE is heated to a temperature at least 10° C. above the Tm of the HDPE.
4. The process according to claim 1 , wherein step (ii) comprises applying a shear rate of less than 10 s −1 to the heated HDPE.
5. The process according to claim 1 , wherein T 1 is between 100 and 130° C.
6. The process according to claim 1 , wherein the the high strength transparent HDPE article comprises at least 90 wt % of the HDPE.
7. The process according to claim 1 , wherein the HDPE has a weight average molecular weight (Mw) of at least 200 kg/mol.
8. The process according to claim 1 , wherein step (iv) comprises stretching the melt-crystallized HDPE article during at least one further stretching step at a temperature T 2 , wherein T 2 >T 1 .
9. The process of claim 1 , wherein T 1 is between 110° C. and 128° C.
10. The process of claim 1 , wherein step (iv) comprises stretching the melt-crystallized HDPE article during at least one further stretching step at a temperature T 2 , wherein T 2 >T m .
11. A high strength transparent high density polyethylene (HDPE) article obtained by the process according to claim 1 .
12. A high strength transparent high density polyethylene (HDPE) article which comprises:
a high density polyethylene (HDPE) having 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 3 , a Tm measured according to ISO 11357-3 of greater than 130° C. and a molecular weight distribution (MWD) of between 2 and 10, wherein
the 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.
13. The high strength transparent HDPE article according to claim 12 , wherein the article comprises at least 90 wt % of the HDPE.
14. The high strength transparent HDPE article according to claim 12 , wherein the MFI measured at 21.6 kg and 190° C. according to ASTM D1238 of the HDPE is at most 1.0 g/10min.
15. The high strength transparent HDPE article according to claim 12 , wherein the MFI measured at 21.6 kg and 190° C. according to ASTM D1238 of the HDPE is at most 0.8 g/10min.
16. The high strength transparent HDPE article according to claim 12 , wherein the MFI measured at 21.6 kg and 190° C. according to ASTM D1238 of the HDPE is at most 0.5 g/10min.
17. The high strength transparent HDPE article according to claim 12 , wherein the isotropic density of the HDPE is at most 0.952 g/cm 3 .
18. The high strength transparent HDPE article according to claim 12 , wherein the weight average molecular weight (Mw) of the HDPE is at least 300 kg/mol.
19. A product comprising the high strength transparent HDPE article according to claim 12 , wherein 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.