Process for fabricating polypropylene sheet
A process for production of a monolithic article from a web of fibres of oriented polypropylene homopolymer or copolymer having a weight average molecular weight (M w ) of at least 250,000 includes the steps of subjecting the web to elevated temperature and pressure sufficient to melt a proportion of the polymer and compact it, and thereby yielding an oriented phase and a matrix phase, and effecting a heat treatment selected from (i) subjecting the compacted web to a retarded rate of cooling down to a lower temperature at or below the temperature a which the recrystallisation of the matrix is complete; and (ii) annealing the compacted web at an annealing temperature within 15° C. of the temperature at which the matrix phase is completely melted. The resultant articles have good stiffness and strength, and acceptable ductility, yet corresponding articles made with polypropylene of lower M w are brittle.
1 . A process for production of a monolithic article from a web of fibres of oriented polypropylene homopolymer or copolymer having a weight average molecular weight (M w ) of at least 250,000, the process comprising the steps of subjecting the web to elevated temperature and pressure sufficient to melt a proportion of the polymer and compact it, thereby yielding an oriented phase and a matrix phase, and effecting a heat treatment selected from
(i) subjecting the compacted web to a retarded rate of cooling down to a lower temperature at or below the temperature at which the recrystallisation of the matrix is complete; and
(ii) annealing the compacted web at an elevated annealing temperature.
2 . A process as claimed in claim 1 wherein for the heat treatment step (i) the mean cooling rate from the compaction temperature down to said lower temperature is not greater than 5° C./minute.
3 . A process as claimed in claim 2 wherein for the heat treatment step (ii) the mean cooling rate from the compaction temperature down to said lower temperature is not greater than 3° C./minute.
4 . A process as claimed in claim 1 wherein for the heat treatment step (ii) the elevated annealing temperature is within 15° C. of the temperature at which the matrix phase is completely melted.
5 . A process as claimed in claim 1 wherein for the heat treatment step (ii) the annealing temperature is below the temperature at which the matrix phase is completely melted, and up to 10° C. below that temperature.
6 . A process as claimed in claim 5 wherein for the heat treatment step (ii) the annealing temperature is below the temperature at which the matrix phase is completely melted, and is up to 5° C. below that temperature.
7 . A process as claimed in claim 1 wherein for the heat treatment step (ii) the compacted web is within the defined temperature range for at least 3 minutes.
8 . A process as claimed in claim 7 wherein for the heat treatment step (ii) the compacted web is within the defined temperature range for at least 5 minutes.
9 . A process as claimed in claim 1 wherein the heat treatment is carried out with the web restrained against dimensional change.
10 . A process as claimed in claim 9 wherein the heat treatment is effected with the compacted web under tension.
11 . A process as claimed in claim 9 wherein the heat treatment is effected with the compacted web retained in the compaction apparatus.
12 . A process as claimed in claim 1 wherein the compaction pressure does not exceed 10 MPa.
13 . A process as claimed in claim 1 wherein the weight average molecular weight (M w ) of the fibres is in the range 250,000 to 400,000.
14 . A process as claimed in claim 13 wherein the weight average molecular weight (M w ) of the fibres is in the range 300,000 to 400,000.
15 . A process as claimed in claim 1 wherein the fibres are melt formed fibres.
16 . A monolithic article manufactured by a process as claimed in claim 1 , having a matrix of polymer produced by selective melting of oriented fibres during the process, and oriented fibres remaining from that selective melting.
17 . A monolithic article as claimed in claim 16 wherein the Young's modulus of the matrix phase is at least 0.9 GPa.
18 . A monolithic article as claimed in claim 16 wherein the failure strength of the matrix phase is at least 20 MPa.
19 . A monolithic article as claimed in claim 16 wherein the failure strain of the matrix phase is at least 5%.
20 . A monolithic article as claimed in claim 16 wherein the Young's modulus in the longitudinal direction of the oriented fibre phase is at least 4 GPa.
21 . A monolithic article as claimed in claim 16 wherein the failure strength in the longitudinal direction of the oriented fibre phase is at least 250 MPa.
22 . A monolithic article as claimed in claim 16 wherein the failure strain in the longitudinal direction of the oriented fibre phase is at least 5%.
23 . A process for fabricating a monolithic article of a polypropylene polymer, or a monolithic article thus formed, substantially as hereinbefore described with particular reference to the accompanying examples.