GB2164329A

GB2164329A - Hydraulic cement compositions containing fibrous material

Info

Publication number: GB2164329A
Application number: GB8522710A
Authority: GB
Inventors: Stanley Richard Barker; Barry Glyn Jones; Melvyn Stanley Robson; Charles Ernest Hunt
Original assignee: BRITISH URALITE PLC
Current assignee: BRITISH URALITE PLC
Priority date: 1984-09-13
Filing date: 1985-09-13
Publication date: 1986-03-19
Also published as: GB2164329B; GB8522710D0

Abstract

An improvement in the mouldability of wet sheets formed from asbestos-free fibre reinforced cement compositions is obtained when compositions comprising hydraulic cement, alkali-resistant synthetic fibre, plasticiser, optionally a finely divided inert and particulate filler, and preferably a flocculant for the particulate components of the composition are employed. A particularly suitable composition is obtained when fibrous cellulosic material is employed in combination with inorganic synthetic fibre and organic synthetic fibre. In an example flat sheets are formed from a slurry and patterns cut from the sheets and moulded to form flue pipes, flue fittings and flue terminals.

Description

SPECIFICATION Hydraulic cement compositions containing fibrous material This invention relates to compositions suitable for use in the production of fibre reinforced cement articles by a method involving de-watering a slurry or suspension of the composition in an aqueous medium, e.g. by filtration.

Asbestos cement compositions have been widely used in the production of articles such as flat and corrugated sheets, and moulded products such as flue pipes and fittings and flue terminals. A widely employed production method involves converting a water-based slurry of the composition into a flat sheet by a method similar to that used in paper manufacture, e.g. employing a Fourdrinier or Hatschek machine, and then causing or allowing the cement to set or alternatively moulding or forming the sheet in the wet state, e.g. to produce tube or more complicated shapes or to corrugate it, and then causing or allowing the cement to set. The method includes the step of transferring the composition from the slurry on to a porous support such as a rotating drum sieve dipping into a vat containing the slurry and through which the water is filtered.Sheet is built up from the deposit formed on the support.

Because asbestos is now considered to have health risks, considerable effort has recently been expended by the industry towards finding a suitable alternative for use in fibre reinforced cement products.

While some success has been achieved in the development of a composition suitable for the production of sheet products, the subsequent moulding of the wet sheet products e.g. into tubes or other articles of more complex shape has not been successful; in particular, with the asbestos-free compositions proposed to date, it has proved difficult or impossible to obtain mouldings without cracking.

We have now developed a composition which does not require the presence of asbestos fibie and yet is not only processable on existing machinery to produce sheet products but also provides wet sheets whose tendency to crack on moulding is reduced or obviated.

In accordance with the present invention, there is provided a fibre reinforced cement composition comprising hydraulic cement, alkali-resistant synthetic fibre, plasticiser, optionally a finely divided inert and particulate filler, and preferably a flocculant for the particulate components of the composition. The invention also provides a slurry oi the composition in an aqueous medium. Such slurries can be processed on sheet-forming machinery to l-)roduce sheet which may subsequently be moulded in the wet state to produce e.g. tube or other shaped articles.Thus, further according to the invention there are provided sheets and other shaped articles obtained from the compositions of the invention by a method including the step of de-watering, e.g. by filtration, a layer of wet material deposited from a slurry of said composition in an aqueous medium, to form a sheet and optionally thereafter moulding the sheet.

The synthetic fibre may be organic or, more preferably, inorganic.

A particularly desirable combination of physical characteristics of the sheet, both in the wet and dry state and of articles moulded from the sheet is obtained if the synthetic fibre comprises a mixture of organic and inorganic fibres.

An improvement in the processability of the composition on sheet-forming machinery may be obtained if fibres having different lengths are employed. Such improvement may also be obtained by including fibrous cellulosic material in the composition.

In an embodiment which is particularly preferred for its processability on conventional machinery, including Hatschek machines the fibrous content comprises inorganic synthetic fibre, organic synthetic fibre and fibrous cellulosic material. Sheets formed from slurries of such compositions can be successfully moulded in the wet state into even quite complex or complicated shapes, e.g. as in flue terminals.

Sheet and moulded articles can also be formed from these preferred compositions to meet relevant British Standard requirements for asbestos cement products and, if desired, to meet statutory requirements as to incombustibility.

While Portland cement will normally be used, other hydraulic cements and cement mixtures may be employed in the composition. The cement is a binder for the other components of the composition and, as an important contributor to the strength of products formed therefrom, will normally form from about 65 to about 75% by weight of the composition for best results. At amounts below about 55 or 60%, the loss in strength will generally be unacceptable but above about 80% the composition becomes difficult or impossible to process and wet sheets formed therefrom are difficult or impossible to mould. Of course as the amount of cement in the composition is reduced, the time required for the product to achieve maximum strength increases.

The plasticiser promotes mouldability of the wet sheet formed from the composition and in the preferred case where it comprises a finely divided water-insoluble particulate solid having lubricating properties it is normally required in an amount of at least 20% and preferably at least 25% by weight of the composition. At concentrations below 20%, wet sheets formed from the composition are difficult or impossible to mould without cracking or other adverse occurrences. However, as the amount of plasticiser is increased, the strength of the products tends to be adversely affected and in general it is undesirable for it to form more than 40%, by weight of the mixture and preferably it forms not more than 35%. While particle size is not specifically critical, it is preferred that it should not be so large as to give a granular effect in the product.Too small particles, on the other hand, tend to interfere with filtration of the wet composition during its pick up on the porous drum of a Fourdrinier, Hatschek or like sheet-forming machine. Preferably the particle size is predominantly in the range 1 to 25 microns. A preferred plasticiser is calcium carbonate e.g. as chalk but other materials having properties that would make them suitable for use as plasticisers include talc, graphite and molybdenum disulphide. Some clays having a suitably large particle size, i.e. an average particle size of at least 2 microns, also have the necessary properties for use as a plasticiser. Finely divided plastics having lubricating properties e.g. finely divided polyamide and finely divided polytetrafluoroethylene may also be mentioned.

Plasticisers which enter the liquid phase in slurries formed from the composition may also be used, either on their own or in combination with water-insoluble particulate solid plasticisers, but if used in large proportions may act as retardants and prevent the cement from setting. Preferably, if used at all they are employed in combination with water-insoluble particulate solid plasticiser, especially where little or no fibrous cellulosic material is present in the composition.

Examples of such plasticisers which enter the liquid phase in slurries formed from the composition are organic liquid or semi-liquid plasticisers and organic solids which are water-soluble.

If desired, where the material employed as plasticiser includes water-insoluble particulate solid, this solid may also act as a filler. Alternatively, other finely divided inert particulate materials may be employed as fillers, e.g. fly ash, mica, titania and finely divided siliceous materials. However, the combined amount of filler and particulate solid plasticiser preferably does not exceed about 40% by weight of the composition since otherwise the products tend to have inadequate strength. It will be understood that the comments made above in respect of the particle size of the plasticiser, where this is particulate, apply equally to any filler employed.

The function of the flocculant is to flocculate the particulate components of the composition in order to assist solids pick up by the rotating porous drum, e.g. drum sieve, and ensure adequate filtration therethrough during the processing of a slurry of the composition on a sheet-forming machine such as a Hatschek or Fourdrinier. While the compositions of the invention can be processed without a flocculant, such processing is simplified by its inclusion. Any suitable flocculating agent may be employed and the amount may be determined by simple experiment but normally will be in the range 0.25 to 2% by weight of the composition although amounts as little as 0.01% or even less may be adequate in some cases.

The fibrous content of the composition is generally at least 1% by weight and preferably at least 1.5% by weight in order to achieve adequate strength in the wet sheet and end product The upper limit will normally be about 5 to 6% but higher amounts up to e.g. about 10%, depending on fibre type, may be used if desired. A factor that controls the overall amount of fibre that may be employed is the amount of particulate material present in the composition. In general, the greater the combined amount of particulate plasticiser and inert filler present in the composition, the greater the amount of fibrous material required.

Inorganic synthetic fibre contributes to dry strength in the end product, particularly at elevated temperatures such as encountered by flue pipes in use. Preferably, such fibre forms at least about 0.25% and more preferably at least 0.3% by weight of the composition; however, as the concentration is increased beyond about 1.5% or 2% the wet sheet tends to lose integrity and to become mo-e difficult to mould and the product texture may be adversely affected, and it is generally advisable not to exceed about 4%.

The preferred inorganic fibre is glass because of its availability. Special alkali-resistant glasses have been developed for use in compositions with hydraulic cements.

Organic synthetic fibre enhances cohesion of the sheet in the wet and dry state and promotes wet strength and is generally employed in amounts up to about 2% by weight although amounts as low as 0.1% or even less, e.g. 0.05%, are also effective. Any organic synthetic fibre may be used provided it achieves the desired effect but the preferred material is polyvinyl alcohol. Others 1hat may be employed include aromatic polyamide fibres such as "Aramid" and acrylonitrile polymer fibi es.

As indicated above, a particularly valuable combination of properties in the wet sheet and end product are obtained if the composition contains both inorganic and organic synthetic fibres.

The presence of fibrous cellulosic material in the composition enhances processability of the slurry, aids removal of the sheet from the machine and reduces brittleness in the end product in the dry state. If too much is employed, however, the wet product tends to lose cohesion. For normal purposes, the fibrous cellulosic material should form from about 0.5% or 1% to about 6% by weight of the composition, preferably from 1% to 2%. The form of the fibrous cellulosic material is also important and preferably it has been refined by conventional processing to flatten the fibres. Most preferably, it is in a form having a "Freeness", as measured by the Canadian Freeness Test, of not greater than 450 and more preferably in the range of from 100G to 400'.

While the preferred fibrous cellulosic material is wood fibre, supplied as a pulp, others that may be used include, for example, cotton, flax, hemp, processed straw and selected grasses. Usually, these will be employed in combination with wood fibre.

Where the product is to be incombustible, e.g. in accordance with British Standard Test BS 476 Part IV or the equivalent ISO 1182 test, the organic synthetic fibre and fibrous cellulosic material will together not normally form more than about 2.5%, preferably 2%, by weight of the composition.

In general, it is found that up to a fibre length of about 10 mm the strength of the products moulded from the composition tends to increase with increase in fibre length. Beyond that length, however, little or no further increase in strength is obtained. The length diameter ratio is preferably at least 3:1, more preferably at least 5:1, but since in general, the fibre diameter is preferred to be as small as possible, the ratio will usually be one or two orders of magnitude greater.

For conversion into sheet or other shaped products, the compositions of the invention are employed in the form of aqueous slurries which are then processed on conventional sheet forming machinery such as in particular a Hatschek or Fourdrinier machine. It is a particular feature of the invention that slurries can be provided which may be processed on conventional Hatschek-type machines employed for the production of asbestos cement sheets and wherein wet composition is picked up from the slurry by a cylindrical drum sieve rotating about a horizontal axis and dipping into a vat of the slurry, and through which the water is filtered, the deposit on the drum is then transferred to the underside of an endless felt belt and the layer so formed on the belt is picked up on a collector drum from which it is ultimately witndrawn as a wet sheet the thickness of which is determined by the number of revolutions of the collector drum before the sheet is withdrawn from it. In some machines, two or more drum sieves are employed in series, each depositing on to the same endless felt belt.

A preferred method of forming an aqueous slurry of the composition for use on such machines, comprises adding to a vessel equipped with an agitator, and in the order given, (a) the fibrous material, e.g.

dry inorganic synthetic fibre, and/or dry organic synthetic fibre, and preferably also fibrous cellulosic material (preferably as a pulp having about 2.5 to 3.5% solids content); (b) water in an amount about equal to that added with the pulp; (c) plasticiser and optional filler; (d) the hydraulic cement and finally, just prior to use; (e) the flocculant. The resultant slurry is agitated and then fed to the vat of a sheet-forming machine. Further dilution may be effected at this stage in accordance with the machine and process requirements. For example, with a Hatschek machine, the solids content of the slurry in the vat is preferably about 70-100 grams per litre.

Examples of products that may be formed from the compositions of the invention are pipes, channels and conduits and fittings therefor, e.g. flue pipes and fittings and flue terminals, soil pipes and fittings, rainwater goods, ventilation and fume ducts and fittings and cable conduits and troughs; containers such as flower bowls, jardinieres and cisterns; and sheet products such as wall boards, profiled cladding, roofing sheets, soffits, and roofing slates and tiles.

The invention will now be illustrated by the following Example in which all parts are by weight except where otherwise indicated.

Employing the preferred method referred to above,an aqueous slurry having a solids content of about 500 g/l was formed from the following composition: Portland Cement 67.4 parts Calcium carbonate powder (chalk) having a specific surface of 2350my/ kg which corresponds to a particle diameter of about 2.8 Fm 29.6 parts Alkali-resistant glass fibre (Cemfil available from Pilkington Brothers PLC, average length and diameter 12 mm and 20cm) 0.35 parts Polyvinyl alcohol fibre (Mewlon HM1 available from Turner Asbestos Company, average length and diameter 10 mm and 16cm) 0.15 parts Refined wood pulp with a Canadian Standard Freeness in the range 1.50 parts 250 to 300 (Refined softwood (dry weight) Kraft (Botnia)) "SnoFloc" flocculant 1.00 parts Flat sheets were then formed from the slurry on a Hatschek machine, the slurry being further diluted to about 100 g/l when fed to the breast box and still further diluted in the vat. Patterns were then cut from the wet sheets and moulded in conventional manner to form flue pipes, flue fittings and flue terminals.

The products so obtained were strong and free of cracks and met the relevant British Standard requirements for conventional asbestos cement products and also the requirements of British Standard Test BS476 Part IV for incombustability.

Similar results were also obtained when the flocculant was omitted.

Claims

1. A fibre reinforced cement composition comprising hydraulic cement, alkali-resistant synthetic fibre, plasticiser, optionally a finely divided inert and particulate filler, and preferably a flocculant for the particulate components of the composition.

2. A composition as claimed in claim 1 in which the plasticiser comprises a finely divided water-insoluble particulate solid having lubricating properties.

3. A composition as claimed in claim 1 or claim 2 in which the particle size of he plasticiser is predominantly in the range of 1 to 25 microns.

4. A composition as claimed in claim 2 or claim 3 in which the plasticiser comprises calcium carbonate.

5. A composition as claimed in any one of claims 1 to 4 in which the plasticiser is present in an amount of from 20% to 40% by weight.

6. A composition as claimed in any one of claims 1 to 4 in which the plasticiser is present in an amount of from 25% to 35% by weight

7. A composition as claimed in any one of claims 1 to 6 wherein the alkali-resistant synthetic fibre comprises inorganic fibre, organic fibre, or both inorganic and organic fibre.

8. A composition as claimed in claim 7 which contains alkali-resistant inorgani, synthetic fibre in an amount of from 0.25% to 4% by weight.

9. A composition as claimed in claim 7 which contains alkali-resistant inorganic synthetic fibre in an amount of from 0.3% to 2% by weight.

10. A composition as claimed in any one of claims 7 to 9 wherein the alkali-resistant synthetic fibre comprises glass fibre.

11. A composition as claimed in any one of claims 7 to 10 which contains organic synthetic fibre in an amount of up to 2% by weight.

12. A composition as claimed in any of claims 7 to 11 wherein the alkali-resistant synthetic fibre comprises polyvinyl alcohol fibre.

13. A composition as claimed in any one of claims 7 to 12 which contains both inorganic and organic fibre.

14. A composition as claimed in any one of claims 1 to 13 which further includes fibrous cellulosic material.

15. A composition as claimed in claim 14 in which the fibrous cellulosic material is present in an amount of from 0.5% to 6% by weight.

16. A composition as claimed in claim 14 in which the fibrous cellulosic material is present in an amount of from 1% to 2% by weight.

17. A composition as claimed in any one of claims 14 to 16 in which the fibrous cellulosic material has a 'Freeness', measured in accordance with the Canadian Freeness Test, of not greater than 450".

18. A composition as claimed in claim 17 in which the fibrous cellulosic material has a 'Freeness', measured in accordance with the Canadian Freeness Test, of from 100" to 400".

19. A composition as claimed in any one of claims 14 to 18 in which the fibrous cellulosic material comprises wood fibre.

20. A composition as claimed in any one of claims 14 to 19 in which organic synthetic fibre and cellulosic fibre together form not more than 2.5% by weight.

21. A composition as claimed in any one of claims 1 to 20 having a total fibrous content of from 1 to 10% by weight.

22. A composition as claimed in any one of claims 1 to 20 having a total fibrous content of from 1.5 to 6% by weight.

23. A composition as claimed in any one of claims 1 to 22 which includes filler and the combined amount of water-insoluble particulate solid plasticiser and filler does not exceed 40% by weight of the composition.

24. A composition as claimed in claim 23 in which the plasticiser and filler are satisfied by the same material.

25. A composition as claimed in claim 23 in which the plasticiser and filler are different.

26. A composition as claimed in claim 25 in which the particle size of the filler is predominantly in the range of from 1 to 25 microns.

27. A composition as claimed in any one of claims 1 to 26 which includes flocculant in an amount of from 0.01 to 2% by weight.

28. A composition as claimed in any one of claims 1 to 26 which includes flocculant in an amount of from 0.25 to 2% by weight.

29. A composition as claimed in any one of claims 1 to 28 in which the hydraulic cement is Portland cement.

30. A composition as claimed in any one of claims 1 to 29 in which the hydraulic cement is present in an amount of from 55 to 80% by weight.

31. A composition as, claimed in any one of claims 1 to 29 in which the hydraulic cement is present in an amount of from 60 to 75% by weight.

32. A composition as claimed in any one of claims 1 to 29 in which the hydraulic cement is present an amount of from 65 to 75% by weight.

33. A composition as claimed in claim 1, substantially as hereinbefore described.

34. A composition as claimed in claim 1, substantially as illustrated in the Example.

35. An aqueous slurry of a composition as claimed in any one of the preceeding claims.

36. A method of forming a shaped article including the steps of: (a) de-watering a layer of wet material deposited from a slurry as claimed in claim 35 to form a wet sheet; (b) optionally shaping the wet sheet, and (c) thereafter causing or allowing the hydraulic cement to set.

37. A sheet or other shaped article obtained by the method of claim 36.