Methods of making stretched filled microporous films

Details Methods of making stretched filled microporous films Methods of making stretched filled microporous films

1997-05-12

2001-01-30

Tentoni, Leo B. (Department: 1732)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C156S164000, C156S290000, C264S154000, C264S288800

Reexamination Certificate

active

06179939

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to stretched filled films. More particularly, the present invention relates to improved methods of making breathable stretched filled films.
BACKGROUND OF THE INVENTION
There exist a variety of fabrics today which are capable of acting as a barrier to particulate matter, water and other liquids yet which allow water vapor and air to pass therethrough. Such fabrics are commonly referred to as “breathable barriers.” Breathable barrier fabrics have been employed in outdoor fabrics, tarpaulins, garments, personal care products, infection control products, as well as numerous other articles. Moreover breathable barrier fabrics are often preferred over non-breathable barrier materials since breathable barrier fabrics allow moisture trapped beneath the fabric to escape as water vapor. Thus, apparel using breathable barriers is generally more comfortable to wear since the migration of water vapor through the fabric helps to reduce and/or eliminate discomfort resulting from excess moisture trapped against the skin.
While a variety of breathable barrier fabrics are known in the art, one particularly useful breathable barrier comprises stretched filled microporous films. Such films are typically filled with particles and then crushed or stretched to form a fine pore network which creates tortuous paths through the film. The film pore network allows gas and water vapor to pass through the film while acting as a barrier to liquids or particulate matter. The amount of filler within the film and the degree of stretching is controlled so as to create a microporous network of tortuous paths which are of a size and/or frequency to impart the desired level of breathability to the fabric. An example of stretched filled film is described in U.S. Pat. No. 4,777,073 issued to Sheth which discloses a stretched filled polyolefin film filled with about 15 to 35% by volume calcium carbonate which can be stretched to about four times its original length.
While filled microporous films are capable of providing good barrier properties and breathability, efficient commercialization and practical applications of such films requires relatively low defect rates. The barrier properties of stretched filled films may be compromised by defects such as macroscopic holes or zones of weakness in the film. This is of enormous concern where the film is intended to act as a barrier to urine, blood or other potentially hazardous materials. However, the process of stretching the filled films to the required degree, while acting to orient the film and also make the film microporous, also has the adverse effect of creating many of the defects. Thus, the process and manner in which such films are produced can have a significant impact upon the number and frequency of defects within the stretched-thinned films. It therefore follows that methods which produce less defects create superior films and have the added benefit of being more efficient in the sense of creating less unusable or defective product.
Unfortunately, the production of stretched filled films at higher production rates can significantly increase the propensity for defects within the stretched filled film as well as creating other more serious manufacturing problems which can cause process line shutdowns. However, the ability to produce stretched filled films at increased rates is often desirable although any benefits to be achieved by increased rates of manufacture are reduced and/or entirely eliminated where the increased production rate also increase the level of defects and downtime of the production line.
Thus, there exists a need for an improved method of making stretched filled films. Moreover, there exists a need for a method of making stretched filled films which have reduced levels of defects and which are more tolerant of film irregularities during stretching. Further, there exists a need for such a method of making stretched filled films and, in particular a method of making breathable barriers, which allows for increased rates of production, improved efficiencies and wider processing windows.
SUMMARY OF THE INVENTION
The aforesaid needs are fulfilled and problems experienced by those skilled in the art overcome by a method of making a breathable barrier, comprising (i) heating a precursor film comprising a thermoplastic polymer and filler; and (ii) stretching the precursor film in a series of discrete stretching steps wherein each stretching step has a strain rate of less than 100,000%/minute and further wherein the speed of the stretched filled film at the last stretching step is in excess of 500 feet per minute. The series of discrete stretching steps incrementally stretch the precursor film in substantially the same direction, desirably stretching the precursor film in the machine direction. The precursor film may be cumulatively stretched by the discrete stretching steps at least 1.5 times its original length and in a further aspect is desirably stretched from about 50% to 90% of elongation at break. In a further aspect the strain rate of each of the stretching steps can be less than 75,000%/minute and, even more desirably, less than 50,000%/minute.
In a further aspect, a support fabric, traveling at substantially the same speed as the stretched film, is superposed with the stretched film and laminated thereto such as, for example, by thermal point bonding, adhesive bonding, ultrasonic bonding and the like. The support layer may comprise any of various breathable materials such as, for example, a nonwoven web. In an additional aspect of the invention, the precursor film may comprise a polyolefin based polymer and at least 50% by weight of a particulate filler which is stretched from 2 to 7 times its original length and wherein the basis weight of the resulting stretched film is less than about 35 gsm.
In a further aspect of the invention, the methods described above can be practiced at line speeds in excess of 750 feet per minute (fpm), 1000 fpm, 1250 fpm, 1500 fpm and even 2000 fpm. Further, the average strain rate of the discrete stretching operations desirably is less than about 70,000%/minute. In a further aspect, the precursor film may be incrementally stretched over two or three stretching steps and further stretched such that the strain rate for each stretching step is less than 70,000%/minute and 50,000%/minute respectively. Films made in accord with the methods described above can have a WVTR in excess of 800 g/M
2
/day while retaining hydrohead values in excess of about 60 cm. Still further, breathable barrier materials made in accord with the present invention may have a WVTR in excess of 1500 g/M
2
/day and even 3000 g/M
2
/day. The films made in accord with the present invention may further have hydrohead values in excess of 80 cm, 150 cm and even 300 cm.


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