Production of fiber webs by the airlaid process

Details Production of fiber webs by the airlaid process Production of fiber webs by the airlaid process

2000-04-17

2002-10-01

Theisen, Mary Lynn (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming articles by uniting randomly associated particles

Stratified or layered articles

C264S113000, C264S121000, C264S122000

Reexamination Certificate

active

06458299

ABSTRACT:

This invention relates to a process for producing fiber webs by the airlaid process of laying down fibers and a pulverulent binder in an air stream, to the webs obtainable thereby and to their use.
WO-A 96/39553 gives an account of the background art of manufacturing airlaid nonwoven materials used, for example, in hygiene products, household articles or filter media. Natural fibers, for example, cellulose fibers (fluff pulp), are blown by air onto a wire, the air is aspirated and the sheetlike fibrous structure thus obtained is consolidated using an aqueous binder or thermoplastic fibers under the influence of heat, pressure and/or water jets. WO-A 96/39553 itself concerns the manufacture of non-wovens by the airlaid process by bonding the fiber using an aqueous polymer dispersion and controlling the degree of penetration of the latex binder into the laid fibrous structure through the spray pressure and/or the vacuum applied.
The disadvantage of airlaids bound using aqueous binders only is the inadequate throughbonding of the web at high basic weights, so that web layers on the inside may delaminate. The reason for this is that, in the case of thick webs, the polymer fraction of the binder dispersion does not fully penetrate and it is just water which gets through to the interior of the fibrous structure. Heavy airlaids having basic weights of>60 g/m
2
are accordingly finished using an additional costly production step, for example, lamination with hotmelt adhesives.
The binding of airlaids using thermoplastic binding fibers, predominantly based on a polyolefin, presents difficulties due to dusting or linting in production and finishing since these materials are insufficiently bonded with regard to very short natural fibers. Similarly, the adhesion of these binding fibers to the polar fluff pulp fibers is inadequate because of their apolar character, necessitating increased binder quantities. At the same time, the hydrophobic binding fiber significantly reduces the absorption capacity with regard to aqueous fluids, which conflicts with any use as an absorption medium in hygiene articles, one of the main applications for bulky airlaids.
WO-A 90/11171 describes the production of fibrous structures by an airlaid process in which natural fibers, preferably wood fibers, are sprayed with binder latex and dried, so that they are completely impregnated with a thermoplastic binder layer. The fiber is consolidated in a second step by means of heat and pressure. The disadvantage with complete impregnation is the change in the physical properties of the completely enrobed natural fiber surface. For instance, the fiber's absorption capacity for aqueous fluids may deteriorate as a result, so that such a process is not suitable for producing absorbent airlaids. In addition, this reference advises against processing conditions which will cause the dispersed binder particles to dry, since the view is taken that dry binders have little if any adhesion to the fiber.
The use of pulverulent binders such as a phenolic resin powder or a polypropylene powder is known with regard to the production of card webs (randomizer cards). In WO-A 90/14457, for example, glass fibers are carded into a random web which is besprinkled with powder. The powder-containing web is subsequently folded in such a way that a plurality of layers are in superposition and is consolidated by the action of heat and pressure. The carding of fibers, unlike the airlaid process, is used for producing very thick fibrous structures having a basic weight of 2000 to 4000 g/m
2
, generally from fibers>20 mm in length. The disadvantage is the complicated laydown technology, which at the low basic weights typical of airlaid products, produces a non-uniform web whose non-uniformities only disappear with increasing basic weight.
The use of crosslinkable polymer powders in processes wherein the powder is sprinkled into previously laid, optionally pre-consolidated, fiber materials is described in EP-B 687317 and EP-A 894888. The disadvantage is that the airlaying of a fiber/powder mixture is associated with considerable powder losses and relatively thick airlaids are not obtainable by simply laying a fiber/powder mixture, but the products obtainable in such a way would have to be expensively laminated.
It is therefore an object of the present invention to provide a process for producing fiber webs by an airlaid process whereby even thick and bulky airlaid webs having a basic weight of>60 g/m
2
and optimal throughbonding are obtainable without the need for complicated laminating steps and without restricting the absorbency of the fiber.
This object is achieved by a process for producing fiber webs by the airlaid process of laying down the fibers and the pulverulent binder in an air stream, which comprises
a) a first step of laying a fiber web or fibers up to a basic weight of 10 to 50 g/m
2
,
b) a subsequent step of laying down fibers and a thermoplastic polymer powder based on polymers of one or more monomers selected from the group of the vinyl esters and (meth)acrylic esters separately or as a mixture in the air stream in an amount of 10 to 300 g/m
2
and, if appropriate, repeating this step until the desired basic weight is obtained, and
c) consolidating the fiber material at temperatures of 80° C. to 260° C. and, if appropriate, at a pressure of up to 100 bar.
Useful fiber materials include all natural and synthetic fiber materials. There is no a priori restriction with regard to the choice of fiber materials; all fiber raw materials which are used in the non-wovens industry are contemplated for use, for example polyester, polyamide, polypropylene, polyethylene, glass, ceramic, viscose, carbon, cellulose, cotton, wool and wood fibers. Preference is given to polyester, polyamide, glass, cellulose, cotton, wool and wood fibers. Particular preference is given to natural fibers such as cellulose, cotton, wool and wood fibers, especially cellulose fibers such as pulp fibers.
Suitable thermoplastic polymer powders are polymers of one or more monomers selected from the group of the vinyl esters of branched or unbranched carboxylic acids having 1 to 12 carbon atoms and the esters of acrylic acid and methacrylic acid with branched or unbranched alcohols having 1 to 12 carbon atoms. Preference is given to short-chain vinyl esters having 1 to 4 carbon atoms in the carboxylic acid moiety such as vinyl acetate, vinyl propionate, vinyl butyrate and 1-methylvinyl acetate. Preference is also given to short-chain methacrylic esters or acrylic esters having 1 to 4 carbon atoms in the ester moiety such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate and n-butyl methacrylate.
If desired, the polymer on which the powder is based is a copolymer which additionally contains units derived from 0.05 to 10.0% by weight, based on the total weight of the monomers, of polar comonomers selected from the group consisting of ethylenically unsaturated mono- and dicarboxylic acids and their amides, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, acrylamide, methacrylamide; ethylenically unsaturated sulfonic acids and their salts, preferably vinylsulfonic acid, 2-acrylamidopropanesulfonate and N-vinylpyrrolidone.
Preference is given to hydrophilic polymers whose hydrophilicity is retained by partial hydrolysis of the vinyl ester or methacrylic ester units and, if appropriate, by a polar comonomer fraction in the polymer. Hydrophilic polymers for the purposes of the present invention are therefore polymers which contain not less than 50% by weight of the abovementioned short-chain vinyl ester or (meth)acrylic ester units or at least 5% by weight of the polar comonomers mentioned, each percentage being based on the total weight of the monomers. In the case of core-shell polymers, the weight percentages mentioned apply to the shell polymer.
For applications in the building construction sector in particular, it may be useful, if appropriate, to

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