Twine and method of forming same

There is disclosed a process for making fibrous material which is particularly suitable for the manufacture of baler twine. The material is preferably made from polypropylene and is produced from film which is roll-embossed and fibrillated, the roll-embossing temperature being substantially lower than the conventional roll-embossing temperature. In the case of polypropylene film this is within the range 60.degree. C to 90.degree. C, preferably about 80.degree. C. As a result, the fibrous material has a high proportion of clusters of from 2 to 5 unseparated incipient filaments. Baler twine made from the material is economical to produce and has improved physical characteristics especially good strength combined with good knot retention ability.

This invention concerns the manufacture of twine and the like. 
There has been considerable interest for several years in the production of 
various types of fibrous materials from molecularly-orientable synthetic 
thermoplastic semicrystalline organic polymer film (hereinafter called 
"film of the type described"). Basically, the film is drawn in its 
lengthwise direction, whereupon it fibrillates, or is readily 
fibrillatable, and is used, after fibrillation to a greater or lesser 
degree, to make non-woven fabrics, twine, yarn and so on. In some 
processes the film is embossed to provide a plurality of parallel 
longitudinal portions of reduced dimensions to encourage, on fibrillation, 
the formation of discrete continuous filaments. 
The general object of the present invention is to enable a fibrous strand 
to be made from film of the type described, which has, for certain end 
uses, very good characteristics. 
One particular object of the invention is to provide reasonably priced 
baler twine having an improved combination of physical characteristics. 
For many years much baler twine was made from sisal, a natural material, 
and involving the usual lengthy processing to extract from the sisal 
plant, fibres suitable for making the twine. More recently baler twine has 
been made from polypropylene film of the type described. In each case the 
film has been drawn fibrillated and twisted. In one case fibrillation 
(e.g. pin fibrillation) is not continued beyond the stage where the film 
is reduced to a largely interconnected network: the resulting twine is not 
very strong though its knot retention characteristics are reasonable. In 
another case the film is embossed whilst hot to form a plurality of 
parallel longitudinal enlarged portions connected by alternate, relatively 
fragile, parallel longitudinal portions. By subsequent drawing and 
fibrillation the enlarged portions are separated from each other to form, 
as it were, a plurality of single filaments which are then twisted 
together. The resulting twine has satisfactory tensile strength but its 
knot retention characteristics are poor. Sometimes these known baler 
twines have been unable to compete, so far as cost is concerned, with 
sisal baler twine. 
According to one broad aspect of the invention, a process for making 
fibrous material from film of the type described, which includes the steps 
of embossing the film whilst hot to form a plurality of parallel 
longitudinal enlarged portions connected by alternate, relatively fragile, 
parallel longitudinal portions, drawing the film, and fibrillating it, is 
characterised in that the temperature of the film during embossing is 
substantially lower than that which would conventionally be employed, but 
not so low as to give rise to brittleness of the film and consequent 
mechanical fracture under processing stresses. 
The film is preferably largely or wholly of polypropylene, in which case 
the temperature may be within the range 60.degree. C to 90.degree. C, 
desirably within the range 80.degree. C to 90.degree. C, and preferably 
about 80.degree. C which is at least 20.degree. C lower than the 
conventional embossing temperature. 
Such a process gives rise to surprising results. On manipulating the 
processed film, it has been found that there is fibrillation and that 
fibrous material results. However it was discovered that the fibrillation 
was of a limited extent in that the enlarged portions had not all been 
separated one from the other, but that there were a substantial number of 
"clusters" present containing various numbers of enlarged portions still 
connected together. Using a conventional embossing roller having say 20 
grooves per cm. on polypropylene film of thickness about 0.20 mm., with 
the film temperature during embossing at about 80.degree. C we found a 
tendency for clusters containing from 2 to 5 "filaments" to form, as well 
as individual filaments. Whilst the preferred temperature was 80.degree. C 
we found the tendency to be present to some useful extent using 
temperatures from 60.degree. C to 90.degree. C. When the fibrous material 
was twisted (fibrillation may be caused simply by twisting) into, for 
example, a baler twine, we found that the twine had a surprising 
combination of characteristics, namely high strength and excellent knot 
retention and efficiency. Since the cost of the product was also 
acceptably low, it is admirably suited for use as tying material, whether 
hand or machine knotted. 
According to a further aspect of the invention therefore, fibrous material, 
for making baler twine, made from film of the type described, comprises, 
whether or not single continuous filaments are present, a substantial 
proportion of clusters containing from 2 to 5 unseparated incipient 
filaments which have been formed by embossing the film. These clusters 
preferably form 90% or more of the material. 
Whilst the factors governing the manner in which the clusters are formed 
may not be fully understood, it is possible to control to some extent, at 
least, the nature of the clusters. Thus by appropriately varying the 
dimensions of every third land between the embossing roller grooves we 
can, for example, obtain a great preponderance of clusters of three. In 
this way some characteristics of the end product can be governed. 
As to other processing variables, it is preferred that the roll embossing 
pressure should be of the same order as in conventional roller embossed 
film lines; and the same applies to the stretch ratio and stretching oven 
temperature. The film thickness should preferably be not less than 60% of 
the groove depth. 
As to the twist of the twine this will depend for one thing on the final 
denier required. The invention is principally concerned with deniers of 
from about 3,000 to 50,000 and the smaller the denier, the greater the 
twist. The range is approximately 25 to 40 turns per meter. For 25,000, 
for example, a twist of about 32 turns per meter is preferred. It is most 
convenient to slit the material to the final denier prior to drawing. 
However, it has been found that for high deniers this sometimes gives rise 
to an undesirable degree of transverse molecular orientation. In this case 
the film should be slit to narrower widths and two or more used to form 
the final twine. 
These preferred criteria all apply to polypropylene film, or film 
containing at least 90% by volume of polypropylene.

Referring first to FIG. 1 the machinery line consists of, in succession, an 
extruder 10 fitted with a film-forming die 12, a water quench bath 14 an 
REF (roller embossed film) unit 16, slitting means 18, first godet rolls 
20, a hot air oven 22, second godet rolls 24, and a winder 26. The path of 
the film is indicated by the arrowed line. 
Twisting is preferably carried out as a separate operation, for example on 
a ring-twister. 
There now follow two examples of how the machinery may be used to produce 
material from which baler twine is then made. 
EXAMPLE I 
The barrel of extruder 10 was 85 mm diameter, with a 25:1 length to 
diameter ratio. The slot of die 12 was 600 mm long, the lip separation 
being 0.635 mm. An Aspin REF unit 16 was used, the embossing roll being 50 
cm wide with approximately 19 grooves per cm, the included groove angle 
being 55.degree.. Such units may be obtained from the firm Aspin Shaw 
Limited, of Manchester, England. 
Tbe film material was Shell CARLONA polypropylene grade KY61. The three 
regions of the extruder barrel were at 178.degree. C, 190.degree. C and 
210.degree. C respectively, whilst the die adaptor was maintained at 
230.degree., as was the die. An extruder screw speed of 43 r.p.m. was 
selected. The extruded film thickness was 0.21 mm. As to the REF unit the 
embossing roll was at 83.degree. C; the nip pressure was 170 kg per cm of 
nip width; and the embossing speed 10 m per minute. The stretch ratio 
between the godet rolls 20, 24 was 10:1 and the stretching oven 
temperature 200.degree. C. 
The stretched embossed film, slit by slitter 18 into four tows each of 
approximately 20,000 denier, was wound up. On later twisting on a ring 
twister at approximately 40 turns per meter, which caused, or completed, 
the fibrillation of the film, a baler twine resulted having a tenacity of 
5.2 gms per denier, and a baler type thumb knot efficiency of 58.4% (i.e. 
the tensile stress to break at the knot was 58.4% of the normal breaking 
stress of the twine). Knots formed in the twine showed no undesirable 
tendency whatever to unform. 
EXAMPLE II 
In this example the only processing changes compared with Example I were 
that the extruded film thickness was 0.25 mm, the embossing roll 
temperature 80.degree. C, and the stretching oven temperature 190.degree. 
C. The tows, consequently, were approximately 25,000 denier each. The 
resulting baler twine had a tenacity of 4.96 gms per denier and a knot 
efficiency of 67.0%. This, as in Example I, compared favourably with even 
substantially higher denier twines produced by conventional fibrillated 
film methods. 
It is to be noted in passing that winding up and twisting appeared, in both 
Examples to proceed in a rather unexpectedly trouble-free manner. 
It will be appreciated that the embossing roll temperatures of 83.degree. C 
and 80.degree. C are 20.degree. C or more lower than normal embossing roll 
temperatures. 
Reference will now be made to FIGS. 2 and 3 which show, by highly magnified 
partial cross-section views, a typical conventional baler twine made from 
roller embossed film and a typical baler twine made according to the 
invention. In the former it is clear that virtually all the constituent 
filaments A are "single" whereas, in the latter there is a high proportion 
of clusters B of 2 to 5 unseparated incipient filaments.