Description of Invention

The present invention relates to the manufacture of non-woven webs wherein fibres are bonded together into a coherent web by a bonding agent applied to the fibres. The invention may be applied to the treatment of pre¬ formed paper. As used herein, the expression "pre-formed paper" embraces dry sheets, paper supplied in a roll and a web which incorporates fibres which have been desposited from a suspension onto a support but which web has not been dried. The invention may be applied generally to non-woven webs incorporating cellulosic fibres, webs incorporating mineral fibres and to webs incorporating synthetic fibres which are neither cellulosic nor mineral in character.

Filter elements for filtering particles of solids from fluids are commonly formed of paper. In some cases, for example where the fluid is pumped under pressure to the filter element and the filter element is not supported throughout its area, it is necessary to increase the strength of the paper by binding fibres βf the paper to one another. It is known to use for this purpose an aqueous solution of a phenolic resin. The pre-formed paper is impregnated with the solution, is then dried and is heated to cure the resin. The use of an aqueous solution of a phenolic resin suffers from the disadvantage that the solubility of phenolic resins in water is not high, although the solubility is greater in an aqueous solution of an alkali. Only resins having relatively low molecular weight can be used in this way. It is also known to use the phenolic resins in conjunction with- other water- compatible polymers, for example acrylic resins and polyvinyl acetate.

It is also known to increase the strength of a paper to be used for forming a filter element by impregnating the pre-formed paper with a solution of a phenolic resin in methanol. The solubility of phenolic resins in

methanol is better than the solubility in water and somewhat higher molecular weight resins can be used. The use of a solution of phenolic resin in methanol to increase the strength of paper is much more expensive than is the use of aqueous solution of resin. The methanol must be evaporated from the impregnated paper in a chamber to prevent escape of the methanol into the atmosphere and steps must be taken to prevent leakage of methanol vapour from the chamber. Furthermore, it is costly to recover the evaporated methanol, in order to minimise pollution of the atmosphere. In spite of these disadvantages, the use of a methanol solution of phenolic resin for increasing the strength of paper is common.

According to a first aspect of the present invention, there is provided a method of preparing a fibrous, non-woven web wherein there is applied to fibres a bonding agent comprising an emulsion wherein the continuous phase is an aqueous phase and the dispersed phase includes a ploymeric product of condensing a phenol with an aldehyde, the fibres are formed into a web either before or after application of the bonding agent, and the web is subsqeuently dried.

According to a second aspect of the present invention, the strength of pre-formed paper is increased by preparing an emulsion wherein the continuous phase is an aqueous phase and the dispersed phase includes a polymeric product of condensing a phenol with an aldehyde, impregnating the paper with the emulsion, evaporating water from the impregnated paper and curing the polymeric product.

A polymeric product having a relatively high molecular weight can be used in the emulsion. This results in a stronger or less brittle paper than can be achieved by the use of an aqueous solution of phenolic resin. Furthermore. the present invention avoids the disadvantages associated with use of an organic solvent.

The phenol used may be phenol, , Hj OH, a meta-alkyl or meta-halide substituted phenol and polyhydric phenols, for example resorcϊnol and bisphenol A. A mixture of phenols may be used. M-cresol is a suitable alkylated phenol which is readily available commercially.

The aldehyde used for condensation with the phenol will generally be formaldehyde, since this is readily available at a relatively low price.

However, other aldehydes *may be used. The amount of aldehyde used is preferably within the range I to 3 moles of aldehyde per mole of phenol.

The condensation of the phenol with the aldehyde is carried out in an aqueous solution containing a catalyst which increases the pH to a value above 7. There are also present in the aqueous solution stabilising agents for stablising an aqueous dispersion of the condensation product. Suitable stabilising agents include ethylene/maleic anhydride copolymer, hydroxyaikyiated gυar gum and carbohydrates, in particular polysaccharides, and specifically include gum arabic, gum ghatti, locust bean gum and guar gum. The total weight of gum used is typically in the region of 2.5% of the weight of phenol used.

The required quantity of gum, typically a mixture of gum arabic and guar gum in which the gum arabic predominates, is dissolved in water and the resulting solution is heated with the required quantity of the selected phenol and the required quantity of the selected aldehyde, typical ly added as an aqueous solution of formaldehyde. The reaction mixture also contains an alkaline catalyst. The particular catalyst or mixture of catalysts used and the quantity of catalyst are selected from those described in the literature as suitable catalysts for the condensation of phenols with aldehydes. An aqueous solution of sodium hydroxide is suitable as a catalyst.

The temperature of the reaction mixture is raised by heating until heat is generated by the condensation reaction at a rate sufficient to maintain the elevated temperature. The mixture is then permitted to reflux for a period within the range 15 minutes to one hour. At the end of the selected period, the reaction mixture is cooled to reduce the rate of reaction and an aqueous solution of an acid, for example phosphoric acid, is added to neutralize the alkaline catalyst. The result is an emulsion wherein the continuous phase is aqueous and the dispersed phase includes the polymeric product of the reaction of the phenol with the aldehyde. The emulsion typically comprises 35% to 60% solids by weight. This emulsion can be stored at ambient temperature for many days without settling of the polymeric product.

Paper of a grade and thickness which is commonly used for the manufacture of air filters for use with internal combustion engines is impregnated by the emulsion, either by spraying of the emulsion on to one or both faces of the paper or by drawing the paper through a bath containing the emulsion. The paper is then dried by permitting water to evaporate at ambient temperature or by .directing a current of warm air over the paper. The substantially dry, impregnated paper is then slit into appropriate sizes,

dimpled, pleated or otherwise provided with formations which are typically present in a filter element for an air f ilter used in conjunction with an internal combustion engine and the formed filter element is then provided with ^" end caps or with a housing, as appropriate. The filter elements are then ^• cured by heating them to a temperature which is typically -within the region 130° to I60°C for a period within the range ten to thirty minutes. This treatment cures the resin with which the paper is impregnated and cures any adhesTves used in the assembly of the paper element with other components of the completed filter element.

Whilst we have described the preparation of a filter element using an aqueous emulsion of a resole resin to illustrate the invention, there may be substituted for the resole resin a novalac resin prepared by reacting an aldehyde with a phenol in the presence of an acidic catalyst, the number of moles of aldehyde present being within the range 0.5 to 0.9 times the number of moles of phenol present. Stabilizing agents as hereinbefore mentioned are present in the aqueous reaction mixture so that an emulsion of the resin is formed.

The paper may be dried, prior to impregnation by the emulsion. Furthermore, the dried paper may be stored and/or transported to another site, prior to being impregnated by the emulsion. Alternatively, the emulsion may be applied to the paper after cellulose fibres have been deposited from a suspension onto a support during formation of the paper, but before the paper is dried. In this case, the emulsion is conveniently applied by spraying.

The emulsion may alternatively be incorporated in a suspension of cellulosic fibres, prior to depositing of those fibres on a support to form a paper web.

Although we have drescribed the application of the emlυsion to cellulosic fibres which are incorporated in a paper web, the emulsion may also be applied to other fibres which are required to be bonded together into a coherent web and may be applied to webs comprising other fibres. The fibres may be synthetic fibres having either an organic or a mineral character. Naturally occurring mineral fibres also may be used.

The features disclosed in the foregoing description, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method for attaining the disclosed result, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.