Process for fabricating polypropylene sheet

Abstract

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 any preceding claim 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 any preceding claim 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 any preceding claim 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 any preceding claim 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 any preceding claim wherein the compaction pressure does not exceed 10 MPa. 
     
     
         13 . A process as claimed in any preceding claim 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 any preceding claim wherein the fibres are melt formed fibres. 
     
     
         16 . A monolithic article manufactured by a process as claimed in any preceding claim, 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  or  17  wherein the failure strength of the matrix phase is at least 20 MPa. 
     
     
         19 . A monolithic article as claimed in  claim 16 ,  17  or  18  wherein the failure strain of the matrix phase is at least 5%. 
     
     
         20 . A monolithic article as claimed in any of  claims 16  to  19  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 any of  claims 16  to  20  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 any of  claims 16  to  21  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.