GB2280620A

GB2280620A - Face mask

Info

Publication number: GB2280620A
Application number: GB9316389A
Authority: GB
Inventors: Frank Robinson
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1993-08-06
Filing date: 1993-08-06
Publication date: 1995-02-08
Also published as: GB9316389D0

Abstract

A resilient, shape-retaining, cup-shaped shell suitable for use in a filtration face mask of the type intended to cover the mouth and nose of a wearer of the mask comprises at least 90% by weight of polypropylene fibres bonded to one another at intersecting points throughout the shell as it is moulded to shape. One or more shells support a filtering layer of glass or polypropylene fibres, which may incorporate active carbon or may be electrostatically charged. The layers are bonded together round the periphery of the mask.

Description

RESILIENT, SHAPE-RETAINING SHELL FOR FACE MASKS This invention relates to cup-shaped shells suitable for use in filtration face masks intended to cover the mouth and nose of a wearer and to the preparation of such shells and to face masks incorporating such shells.

Respirators or face masks have been extensively used to protect the wearers from inhalation of toxic or nuisance dusts, fumes or bacteria. Surgical masks are intended to protect both the patient from contamination by operating room staff and vice versa. Many face masks are disposable and are used for a period of time and then discarded. Such face masks generally fall into two categories, a first type which is made of planar material and flat packed, which is provided with seams, pleats and/or folds so they may be opened out to a cup-shaped configuration and a second type in which the face mask has a resilient, shape-retaining generally cup-shaped configuration. The invention is particularly concerned with face masks of the second type.

Such disposable face masks or respirators have previously been made by combining a layer of fibrous filtration media with a preformed fibrous shell. The latter establishes and maintains the shape of the face mask; the filtration media conforms to the shell and is supported and protected by it. Examples of such face masks are disclosed in GB 2077112 and US 4807619, 4850347 and 4536440.

One process for the manufacture of respirator shells involves the production of a non-woven web of polyester fibres which is then hot-moulded and sprayed with a resin emulsion. The web is then dried in a series of ovens.

The purpose of the resin is to act as a binder increasing the strength and resilience of the shell. This process has several significant disadvantages: 1. Substantial quantities of wet waste and solid design waste are generated which can not be conveniently recycled.

2. The process consumes a great deal of energy (electricity and gas).

3. The resulting respirator shells do not always meet customer expectations of respirator comfort and collapse-resistance.

Respirator shells have been formed by hot moulding a web of fibres such that sufficient fibres are bonded to one another at points of intersection throughout the web to impart resilience and shape-retention to the shell, as disclosed in U.S. 4807619.

The fibres used are usually between 1 and 200 denier and preferably, average greater than 10 denier but less than 100 denier (e.g., smaller fibres provide more fibre intersection points per unit of basis weight).

Preferably the web comprises a mixture of synthetic staple fibres, preferably crimped, and bicomponent staple fibre. The latter carries a binder material component by which the resilient shaping layer can be bonded together at fibre intersection points, e.g., by heating the layer so that the binder material on the bicomponent fibres flows into contact with adjacent fibres that are either bicomponent or other staple fibres. Fibre mixtures including staple fibre and bicomponent fibre in a weightpercent ratio ranging from 0/100 to 75/25 may be used.

Preferably, the web includes at least 50 weightpercent bicomponent fibre, and more preferably at least 75 weight-percent bicomponent fibre, since the resulting greater number of intersection bonding points increase resilience and shape-retention. Bicomponent fibre need not be used. For example, binder fibres of a heatflowable polyester can be included together with staple, preferably crimped, fibres in a shaping layer, and upon heating of the shaping layer the binder fibres melt and flow to a fibre intersection point where they surround the fibre intersection point. Upon cooling of the layer, bonds develop at the intersection points. Also, binder materials such as acrylic latex can be applied to a web of fibres being, e.g., as a supplement to binder or bicomponent fibres. Also, binder materials in the form of powdered heat-activatable adhesive resins may be cascaded onto a web of fibres, whereupon when the web is heated the fibres in the web become bonded together at intersection points by the added resin.

Suitable bicomponent fibres for the webs include, for example, side-by-side configurations, concentric sheath-core configurations. One particularly useful bicomponent fibre for producing the shaping layers of this invention comprises a generally concentric sheathcore configuration having a core of crystalline polyethylene terephthalate (PET) surrounded by a sheath of a polymer formed from isophthalate and terephthalate ester monomers. The latter polymer is heat softenable at a temperature lower than the core material. Polyester has the advantages that it contributes to resiliency and has less moisture uptake than other fibres.

Crimped synthetic staple fibres, usually singlecomponent in nature, suitable for the webs include, polyethylene terephthalate (PET), the preferred material, nylon and polypropylene. Regularly crimped fibres, such as those prepared by gear crimping processes, irregularly crimped fibres such as those obtained from stuffer box crimping, or helically crimped fibres such as those obtained from the so-called "Agilon" process can be utilized to prepare webs. The number of crimps per inch can vary widely, however, generally speaking, higher crimp frequencies produce loftier resilient shaping layers. Crimp frequencies in the range of 6 to 12 crimps per inch (2 to 5 crimps per centimeter) are preferred.

Also, the percent crimp of the fibres, as measured for example, in column 6 of U.S. Pat. No. 4,118,531, is generally at least 15 percent for added crimped staple fibres. As noted above, the bicomponent fibres also can crimp during processing, which further contributes to resiliency.

The use of bicomponent fibres increases material costs compared with many monocomponent fibres. The use of added binders increases the material and processing cost and may increase flammability of the finished face masks.

It is an object of- this invention to provide alternative respirator shells and a process for their production.

According to the present invention there is provided a resilient, shape-retaining, cup-shaped shell suitable for use in a filtration face mask of the type intended to cover the mouth and nose of a wearer of the mask characterised in that the shell comprises at least 90% by weight of polypropylene fibres bonded to one another at intersecting points throughout the shell.

Also according to the invention there is provided a process for preparing a resilient, shape-retaining, cupshaped shell for a face mask which comprises shaping a layer of fibres comprising at least 90% by weight polypropylene fibres under heat and pressure to cause bonding of fibres at points of intersection throughout the layer.

This process has the following advantages: 1. No wet waste is produced and all solid design waste can be recycled.

2. Energy consumption is a fraction of that consumed in a resin reinforced process.

3. The polypropylene shell results in a respirator which has excellent comfort and collapse-resistance properties.

4. The polypropylene fibres are readily commercially available and less expensive than bicomponent fibres.

The polypropylene fibres used in the invention are typically 2.2 to 2.8 decitex (but may be 1-20 decitex) and are used in a layer of web having a basis weight in the range 150 to 250 g/m2, preferably 170 to 190g/m2. The web may be loosely bonded to facilitate handling in automatic moulding machinery. For example, the web may be formed by carding, cross-lapping and needle-punching the fibres, e.g., at 1 punch/cm2. The web comprises at least 90% by weight polypropylene fibres, preferably 95% by weight and more preferably 100% by weight polypropylene fibres. The remainder of the web may comprise any other fibre, e.g., polyester, etc. The web will generally have an average thickness of 4 to 10mm and after moulding the shell generally has an average of 0.5 to l.Omm.

Suitable polypropylene fibres are commercially available from Moplefan under the trade designations TG300 and TG380. Generally, preferred fibres are staple fibres, and particularly those with a skin or surface layer having a lower softening point than the fibre core.

The hot-moulding process involves thermally bonding the fibres of the non-woven web together by the application of heat and pressure. This is achieved by pressing the web fibres together with male and female moulds (which have the form of the shell). The moulds may be contained in a thermally-insulated enclosure (a "press oven") and heated by combustion gases (from a remote gas burner) which are recirculated through the enclosure.

The mould temperature, pressure and residence time in the mould affect the degree of bonding of the fibres.

If the conditions are too severe the fibres will melt completely forming a film. Generally the temperature is maintained within the range 135 to 150"C and typical residence times are in the range 5 to 20 secs, normally 10 to 12 secs. We have found that good results are obtained when the temperature range is no more than 10 C, preferably around 4"C. A particularly preferred mean moulding temperature is 141"C. The moulding pressure is generally in the range 28 to 85kN/m2 The resulting shells have a smooth feel, have good collapse strengths when wet and dry to both air flow and mechanical pressure and readily meet the standards for breathing resistance.

The shells of the invention are used in combination with a filtration layer to form a face mask. The fibres of the filtration are selected depending upon the kind of particulate to be filtered. Webs of melt-blown fibres, such as taught in Wente, Van A., "Superfine Thermoplastic Fibers" in Industrial Engineering Chemistry, Vol. 48, 1342 et seq (1956), especially when in a persistent electrically charged form (see Kubik et al, U.S. Pat. No.

4,215,682), are especially useful. Preferably these melt-blown fibres are microfibres having an average diameter less than about 10 micrometers (herein referred to as BMF for "blown microfiber"). The use of polypropylene BMF is preferred since any design waste may be recycled along with the design waste from the polypropylene shells. Electrically charged fibrillatedfilm fibres as taught in van Turnhout, U.S. Pat. No. Re.

31,285, are also especially useful. Rosin-wool fibrous webs and webs of glass fibres are also useful, as are solution-blown, or electro-statically sprayed fibres, especially in microfilm form.

Another material suitable for use in the filtration layer is available from Hepworth (Kendal., UK) under the trade name Technostat and comprises two dissimilar fibres which are carded together and needle-punched into a supporting scrim. The fibres are electrostatically charged by the carding process.

For certain applications, it is advantageous to incorporate absorbent particles, e.g., activated carbon, in the filter layer.

The filtration layer may be formed from one or more layers of such fibres and generally has a basis weight in the range 25 to 350 g/m2. The filtration layer is generally positioned on the upstream side of the shell, i.e., outside the shell, so that it is fully supported by the shell and will not move or collapse during inhalation. The filtration layer may be sandwiched between 2 shells or preferably is positioned between a shell and a coverweb, e.g., spun bonded polypropylene with a basis weight of about 5 to 25g/m2. The web and optionally the filter layer may be secured to the shell by welding, e.g., heat or ultrasonic welding around the perimeter of the shell, in the normal manner. The mask is provided with straps in any conventional manner, e.g., welding or stapling elastic material or buckles to the mask. An exhalation valve may be fitted to the mask.

Claims

CLAIMS:

1. A resilient, shape-retaining, cup-shaped shell suitable for use in a filtration face mask of the type intended to cover the mouth and nose of a wearer of the mask characterised in that the shell comprises at least 90% by weight of polypropylene fibres bonded to one another at intersecting points throughout the shell.

2. A shell as claimed in Claim 1 which comprises at least 95% by weight of polypropylene fibres.

3. A shell as claimed in Claim 2 consisting of polypropylene fibres.

4. A shell as claimed in any preceding Claim in which the polypropylene fibres are 2.2 to 2.8 decitex.

5. A shell as claimed in any preceding Claim in which the shell is formed from a web having a basis weight of from 150 to 250g/m2

6. A shell as claimed in Claim 5 in which the shell is formed from a web having a basis weight of from 170 to 190g/m2.

7. A shell as claimed in Claim 1 substantially as herein described.

8. A process for preparing a resilient, shaperetaining, cup-shaped shell for a face mask which comprises shaping a layer of fibres comprising at least 90% by weight polypropylene fibres under heat and pressure to cause bonding of fibres at points of intersection throughout the layer.

9. A method as claimed in Claim 8 in which the moulding is conducted at a temperature of 135 to 150 C.

10. A method as claimed in Claim 8 or Claim 9 in which the residence time in the mould is from 5 to 20 seconds.

11. A method as claimed in any one of Claims 8 to 10 in which the layer comprises at least 95% by weight of polypropylene fibres.

12. A method as claimed in Claim 11 in which the layer comprises 100% by weight of polypropylene fibres.

13. A method as claimed in any one of Claims 8 to 12 in which the polypropylene fibres are 2.2 to 2.8 decitex.

14. A method as claimed in any one of Claims 8 to 13 in which the layer has a basis weight of from 150 to 250g/m2

15. A method as claimed in Claim 14 in which the layer has a basis weight of from 170 to 190g/m2

16. A method as claimed in any one of Claims 8 to 14 in which the fibres of the layer are carded, crosslapped and needle-punched.

17. A method as claimed in Claim 2 substantially as herein described.

18. A face mask comprising a shell as claimed in any one of Claims 1 to 7.

19. A face mask as claimed in Claim 18 additionally comprising a layer of filtering material supported by the shell.

20 A face mask as claimed in Claim 19 in which the filtering material comprises electrically charged fibres.

21. A face mask as claimed in Claim 20 in which the filtering material has a basis weight of from 25 to 350g/m2.

22. A face mask as claimed in any of Claims 19 to 21 in which the filtering material is on the outside of the shell and is protected by a coverweb.

23. A face mask as claimed in any of Claims 19 to 21 and comprising two shells as claimed in any one of Claims 1 to 7 in which the filtering material is sandwiched therebetween.

24. A face mask as claimed in Claim 18 substantially as herein described.