Many of today's products include man-made, synthetic components, personal care absorbent articles such as diapers being but one example. Today's diapers typically include a synthetic fiber nonwoven web as the body side liner material positioned adjacent the baby's skin. Inside the diaper there is an absorbent core which may be made from natural wood pulp fiber in combination with synthetic fibers and superabsorbents. The backing materials or outercovers of diapers have traditionally been made from plastic films due to cost considerations and the liquid impermeable nature of plastic films.
While plastic films are efficient at containing liquids and other waste matters during use, the same plastic films have certain disadvantages in that they are not pleasing to the touch and they do not readily pass water vapor so that, from a wearer wellness standpoint, plastic films tend to cause skin hydration thereby making infants more prone to diaper rash. One solution has been to supplant normal nonporous plastic films with breathable plastic films as the diaper backing material. There are a number of ways of making a film breathable including aperturing and the use of fillers. When fillers are used, the film is often crushed between rollers to crack the filler or stretched so as to create small gaps between the polymer and the particles embedded in the polymer. This creates a tortuous path from one surface of the film to the other and thus provides a path for the escape of water vapor while acting as a barrier to liquids such as water and urine. Polyolefin films are often used for making breathable films. A particularly useful film for such applications is made from a linear polyolefin containing organic and/or inorganic fillers. Such filled polyolefin films provide good water vapor transmission thereby making the diapers more comfortable to the wearer. As a result, the relative humidity and temperature within the diaper or other product can be reduced by using breathable materials. Despite this, such breathable films have the disadvantage of being cold and clammy because breathable films pass moisture to the outside of the product where it condenses readily on the film surface. Consequently, another solution has been to attempt to use nonwoven materials as the backing material for diapers.
Fibrous nonwoven webs when used as the backing material for diapers alleviate the above-mentioned film problems, however, such fibrous nonwoven webs generally provide poor barriers to the passage of liquids including urine. As a result, most nonwovens, by themselves, are not suitable as backing materials. Some fibrous nonwoven webs work better than others at repelling liquids, especially when they include a layer of fine fiber nonwoven material such as a layer of meltblown. Meltblown fibrous webs are made from fibers formed by extruding molten thermoplastic material through fine die capillaries to form molten threads or filaments which are then attenuated using high velocity gas. The resultant fibers generally have very small diameters, usually 10 microns or less, and are collected on a forming surface in the form of a fibrous nonwoven batt with very small pore structures which tend to inhibit liquid flow. See for example U.S. Pat. No. 3,849,241 to Buntin et al. Even with the use of meltblown layers, however, such fibrous nonwovens do not always prove to be totally suitable as a backing material for personal care products.
In view of the foregoing deficiencies of both films and fibrous nonwovens, attempts have been made to combine the two materials thereby making it possible to rely upon the strengths of one material to overcome the weaknesses of the other. An example of combining the best attributes of a breathable film and a fibrous nonwoven is via the combination of a filled linear polyolefin film and a polypropylene or polypropylene copolymer spunbond web. In order for these two materials to work in unison, they must somehow be joined or laminated to one another. There are a number of methods for joining films and nonwovens including thermal and ultrasonic bonding, gluing, needling and sewing. For purposes of maintaining a liquid barrier, needling and sewing are generally undesirable due to the fact that the holes these processes create are relatively large and therefore permit leakage of liquids. Adhesives and gluing can be undesirable for their own reasons including undue blockage of the breathable film pores and overall stiffness of the laminate. Lamination of the film and fibrous nonwoven layers should be relatively complete. As a general matter, to achieve good lamination between a film and a nonwoven using an adhesive, either a thin, uniform layer of adhesive must be sprayed across the entire interface of the two materials or larger more localized quantities of adhesive must be used at spaced-apart intervals. Uniform applications of adhesive can and often do block the pores on one surface of the film thereby rendering the previously porous film nonporous. This is not desirable. Using larger quantities of adhesive in more localized areas reduces the amount of film surface that is being blocked by the adhesive, the drawback being that the film/nonwoven laminate tends to become stiff due to the concentrated application of adhesive. Consequently, it would be more desirable to use thermal lamination techniques.
Thermal lamination can be accomplished through the use of heat and pressure as with heated pattern rolls and with ultrasonics. Both techniques are very well suited for joining films and nonwovens when the two materials are made from the same polymer. In some cases, however, the polymers used to make the film are not the same as those used to make the fibrous nonwoven web. This can be because of both cost and physical properties. Linear low density polyethylene (LLDPE) films and polypropylene nonwoven webs are one example. These polymers are thermally incompatible with one another in that they cannot be thermally laminated to one another with a bond force of at least 5 grams. There also exists the situation where the polymers used to make the two layers are the same and therefore compatible but to bring about thermal lamination so much heat and pressure must be used that perforations end up being formed in the film layer and oftentimes the laminate is too stiff. As a result, there is a need for a process for thermally bonding such incompatible and compatible materials so that the advantages of the two materials as well as the thermal lamination process can be used. There is also a need for the resultant product. As explained in detail below, these needs have been satisfied by the present invention.