CA1075944A - Filled paper - Google Patents

Abstract

ABSTRACT

A papermaking process of the type in which prepared thick pulp stock is supplied to a head box from which it flows down to a mixing point where it is diluted with paper machine backwater, the resultant thin stock being pumped to a breast box on the paper machine, characterised in that, for manufacturing paper containing acid-reactive filler and sized by employing rosin and papermakers' alum, the filler is added to paper pulp which has already been treated with rosin size and sufficient alum for fixing the size, and further alum is subsequently incorporated in the diluted pulp before application of the pulp to the papermaking machine.

Description

--, 1075!~4 The present invention relates to the manufacture of paper containing an acid-reactive filler such as calcium carbonate and qized by the process employing rosin size and papermakers' alum. It is most desirable that this operation should be carried out simply and successfully, in particular because of the economy ~d free availability of calcium carbonate fillers based on na~ural minerals, particularly upon chaIk. These fillers ha~e other advantageq; in particular they confer permanence upon the paper.
Until recently acid-reactive fillers such as calcium carbonate have not been acceptable for papers which are sized by the rosin/alum process. This process requires acid conditions for success and the alkalinity of ~illers like calcium carbonate rapidly neutralises added acid or acid generated from the alum by hydrolysis. It is also desirable to use materials like calcium carbonate as a paper coating pigment. The recycled broke from such -~-paper coating has been a source of difficulty in the sizing of paper made from a stock containing this type of broke. Some mills have found it necessary to dissolve the whole o~ the calcium carbonæte present in the broke in acid while others have employed processes which separated the filler from the p~lp by washing. It would evidently be advantageous to avoid these complications.
It is known in the papermaking art that sizing by the rosin/alum process may be achieved under more alkaline conditions if a high free rosin emulsion type of paper is employed than if a rosin soap size is used.
It is also known that treatment of calcium carbonate to

-2- ~ -form flocs in the papermaking stock or before its incorporation into the ~tock, r&ther than co-flocs containing both filler and cellulosic fibres, reduces the rate of neutrali~ation of acid by the calcium carbonate.
In the papermaking process to which this invention particularly applies, prepared thick pulp stock is supplied to a head box from which it flows down to a mixing point where it is diluted with paper machine backwater, the resultant thin stock being pumped to the breast box on the papermaking machine, generally via cleaners.
It is an object of the invention to provide a æequence of steps in such a process, whereby undesirable interaction is minimised between sizing material and acid-reactive filler employed in the stock and present in recycled broke or backwater.
Fillers to be incorporated in the paper are generally added to the pulp in the course of its preparation and supply to the stuff box, sizing material and alum being applied æubsequently. Retention aids are added later ~
still, for insta~ce after the cleaners,- to reduce loss - ;
of additives in the course of formation of the paper web.
2~According to the invention there is provided a , j, papermaking proceæs of the a~oreæaid type, for manufacturing paper containing acid-reactive fil~er and sized by employing rosin and papermakers' alum, in which the filler 1~759~4 -- ~, is added to paper pulp which has already been treated with rosin size and sufficient alum for fi~in~ the size, but preferably before ary introduction o* broke to the pulp, and further alum is added subsequently to the diluted pulp before application of the pulp to the papermaking machine.
In this m~nner the paper stock is sized before the acid-reactive filler is added so that the effect of the latter on the siging process is minimised. ~he stage at which broke, which ma~ contain recycled calcium carbonate or other pigment from a coating process, is generally added to the pu7p is thus ef~ectively separated from the stage at which interference with si~ing by such pigment could occur hitherto, and such recycled pigment simply.joins that which, preferably in an immediately preceding stag~, i8 added as filler.
The acid-reactive fillèr may take any suitable form, the invention being of particular application to the use of chalk whiting. It is advantageous to add the fl7ler in preflocculated form, that is to say as a composition in which the filler is accompanied in aqueous suspension by a flocculant which modifies the condition and in some.
cases the effective particle size-of the filler before the compo~ition is introduced to the paper furni~h. Such a preflocculated filler compo~ition has ar-enhanced ability to flocculate onto the fibre.
~he invention is further e~plained with re~erence to the accompanying drawings, in which:- -~IGURE 1 is a schematic flow diagram of the relevant part of a conventional papermaking process; and ~07~944 FIGUR~ 2 is a schematic flow diægræm of the same part of such a process modified according t~ the invention.
In the ~igures there is indicated a papermaking machine 1 which is fed with pulp from a breast box 2 in the usual manner. The pulp has been prepared as thick stock in a preparation stage 30, for instance including a hydropulper, from water 31, paper pulp 32 and filler 41.
Ro~in size 81 and alum 61 are incorporated in the stock before the latter is supplied to a machine headbox 33 from where it flows as required through conduit 36 to fan pump 6 where the th-ck stock is diluted to thin stock by admixture of backwater 7. The pump 6 delivers the thin stock through conduit 69 to cleaners 9 from which the stock flows via line 92 to the breast box 2.
In the conventional process illustrated in igure 1, filler 41 is introduced before the headbox 33, normally before the alum 51. ~ retention aid 10 is normally added immediately before the breast bo~ 2.
~he invention is illustrated in ~igure 2. Here the sizing and rosin 82 and ~ufficient alum 52 to fix the size, are introduced at or be~ore the headbox 33. ~he pulp thick stock with the size fixed on the fibre by the alum, is joined in conduit 36 by acid-reactive filler 42, ~5 preferably preLlocculated whiting, 80 that the effect of-filler on the sizing is minimised. ~he thick stock is diluted by admi~ture of backwater 74 which in the process of the invention advantageously includes broke which may be derived from paper containing acid-reactive coating or loading filler. Such filler is accommodated by the ~07~944 process without difficulty. ~urther alum 53 is added to the thin stock in line 92 as close to the breast box 2 as possible. Retention aids may be added at this stage if desired, as in Figure 1.
In an embodiment of the invention, an aqueous suspen-sion of cellulosic papermaking fibres, i.e the pulp, which may already contain filler derived from filled or coated paper broke, is prepared to the thick stock stage at a consistency (oven dry solids content) from 0.25 per cent to 6 per cent by weight. The stock is passed to the head box m; ~ed with rosin size, preferably of the high free rosin emulsion type, added in a proportion based on the dry pulp of up t;o 5.0 per cent, more preferably from 0.15 per cent to 5.0 per cent by weight, and with papermakers' alum, preferably in aqueous solution, in a proportion based on the oven dry pulp of from 0.25 per cent to 10 per cent by weight. ~he alum serves to adjust the pH of the pulp for sizing, e.g. in the case of "Bumal"
rosin sizing to about pH 5.5, and to fix the rosin on the fibres. Calcium carbonate filler such as a preflocculated aqueous suspension of chalk whiting, is added continuously at a later point in the wet end flow system, to the stock on which the size has become sufficiently fi~ed. Such addition of filler is preferably made before the stock is continuously mi~ed with bac~water and passed to the cleaners.
~ he calcium carbonate is generally in the form of an aqueous suspension bf individual particles or preferably as a suspension of flocs or agglomerates prepared so as - --` 1075944 to impart acid resistance to them as described for instance in U.~ Patent 3pecifications Nos. 1,353,015;
1,425,114; 1~a97~280; and 49657/74.
In the preferred embodiment the second addition of alum is a continuous addition of papermakers' alum in aqueous solution as late as possible in the wet-end stock flow as is consistent with good mixing, i.e. not before the fan pump but close to the breast box, in a ~ proportion by weight based on the oven dry pulp of from 0.25 per cent to 7.5 per cent.
In this manner the flocculation effect of the alum will compensate for floc bre~kdown during pulp transport and adjust the pH to 6.5 to 7.5. Retention aids, e.g.
polyacrylamides or cationic starches, may be added just - 15 before the breast box if desired.
~ he invention also extends to paper made by the method of the invention.
~ he following Examples are given for the purpose of illustrating the invention.

~X~MP~E 1 A ten-hour trial was run on a high-speed printing quality paper machine (using soft water) with the acid rosin sizing system and whiting additlon according to the invention in order to assess whether the strength achieved with 16 per cent clay loading could be maintained with 21 per cent whiting loading using this process. ~he whiting had been preflocculated by means of a starch/alginate combination. The sizing chemical additions were as follows:

* 107S944 "Bumæl" rosin ~.5~ ~r cent 1st alum 1.~ per cent 2nd alum 1.45 per cent The average whiting loading ~as 21.8 per cent whilst maintaining strength, and the ~ritish Standard 2~44/1955) Cobb water penetration test avera~ed 20, thus indicating that sizing had been satisfactorily achieved with a higher filler loading.
In this test the "Bumal" size was added at the machine chest, the first alum between the machine chest and the headbox. The preflocc~lated whiting was added before the fan pump and the second ~mount of alum was added down-stream of the cleaners.

~r.PL~ 2 In a second trial (using hard water) to achie~e 7 per cent higher paper loading content without detriment to machine running or paper properties, the "Bumal" was added between the fine chest and the fine pump, and the ~irst addition of alum was made between the fine pump and the midget refiners. The preflocculated whiting was added after the head bsx and the second addition of alum was made to the backwater at the mixiA~g pump inlet. A
polyacrylamide retention aid (Hydracol 852) was added after the selectifiers during most of the trial.
~he æizin~ chemical addition~ ~ere as follows:-"Bumal" æize 0.5 per cent 1st alum 1.5 per cent 2nd alum 1.5 pe~ cent * a Trade Mark A

` ~ 1075944 The average whiting loading was 22 per cent and the results of the various strength tests (bul~st, tensile, etc.) were as good as the normal specifications of paper containing only 15 per cent loading of the filler. ~he Cpbb water penetration test averaged 18.

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Claims (4)

What is claimed is:-

1. A papermaking process of the type in which prepared thick pulp stock is supplied to a head box from which it flows down to a mixing point where it is diluted with paper machine backwater, the resultant thin stock being pumped to a breast box on the paper machine, characterised in that, for manufacturing paper containing acid-reactive filler and sized by employing rosin and papermakers' alum, the filler is added to paper pulp which has already been treated with rosin size and sufficient alum for fixing the size, and further alum is subsequently incorporated in the diluted pulp before application of the pulp to the papermaking machine.

2. A process according to Claim 1, wherein the filler is added to the pulp before paper broke is added to the pulp.

3. A process according to Claim 2, wherein the filler is added in preflocculated form.

4. A process according to Claim 3, wherein the filler comprises calcium carbonate.