Polypropylene blends and processes for producing polymeric compositions
Polymer blends of polypropylene homopolymer and propylene-α-olefin interpolymer. Processes for producing polymeric compositions comprising control-cooling heated blends of polypropylene and propylene-α-olefin interpolymer. Such polymeric compositions can be employed in forming coated wires and cables.
1. A process for preparing a thermoplastic polymeric composition, said process comprising:
(a) heating a polymer blend comprising a polypropylene homopolymer and a propylene-α-olefin interpolymer to a melt temperature of (1) greater than 140° C., (2) at least the melting temperature of the polypropylene homopolymer, or (3) at least the melting temperature of the propylene-α-olefin interpolymer, whichever of (1) through (3) is greatest, for a time sufficient to form a polymer melt blend; and
(b) cooling said polymer melt blend from said melt temperature down to ambient temperature, wherein said cooling comprises:
(i) non-control-cooling said polymer melt blend from said melt temperature to an initial cooled temperature in the range of from 140 to 125° C.,
(ii) control-cooling said polymer melt blend at an average cooling rate in the range of from 0.1 to 30° C. per minute from said initial cooled temperature to an intermediate cooled temperature in the range of from 110 to 100° C., and
(iii) cooling, by either control-cooling or non-control-cooling, said polymer melt blend from said intermediate cooled temperature to ambient temperature, thereby forming said polymeric composition.
2. The process of claim 1 , wherein said polymeric composition has an alternating current (“AC”) breakdown strength that is at least 5% greater than an otherwise identical but quench-cooled polymeric composition, as measured on 100 micrometer thick films in accordance with ASTM D-149.
3. The process of claim 1 , wherein said polymeric composition has an AC breakdown strength of at least 100 kV/mm, as measured on 100 micrometer thick films in accordance with ASTM D-149.
4. The process of claim 1 , wherein said polypropylene homopolymer and said polypropylene-α-olefin interpolymer are present in said polymer blend in a weight ratio ranging from 4:1 to 1:4.
5. The process of claim 1 , wherein said initial cooled temperature is 125° C., wherein said intermediate cooled temperature is 110° C., wherein said cooling of step (iii) is non-control-cooling.
6. The process of claim 1 , wherein said control-cooling of step (ii) is performed at a rate in the range of from 1 to 10° C. per minute.
7. The process of claim 1 , further comprising, prior to said cooling of step (b), coating said polymer melt blend on a conductive core thereby forming said polymeric coating on said conductive core following said cooling of step (b).
8. The process of claim 7 , wherein said polymeric coating on said conductive core presents a direct current (“DC”) breakdown strength of at least 400 kV when measured at a thickness of 3.5 mm.
9. The process of claim 1 , wherein said polypropylene homopolymer is non-nucleated.
10. The process of claim 1 , wherein the α-olefin component of said polypropylene-α-olefin interpolymer is ethylene.