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  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.