BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of paper and fiber materials processed with sodium silicate material.

The invention relates to the manufacture of corrugated paperboard by means of a "cold corrugating" process in which the use of an adhesive of a predetermined sodium silicate material substantially reduces or eliminates the application of heat energy to prepare or condition the paper material for corrugating and/or to set the adhesive during manufacture and, in particular, to the use as an adhesive of a predetermined sodium silicate material having a relatively high level of chemically - bound water and a relatively low level of free water molecules therein.

The invention in part also relates to fire resistant products, and in particular to normally combustible materials having thereon a coating of sodium silicate which renders the materials fire resistant, and novel products made from such materials. The invention also relates to a novel process for rendering otherwise

combustible materials fire resistant and to products haviifg the aforesaid characteristics and to novel apparatus useful in the aforesaid process. The invention has particular utility in connection with the manufacture of fire resistant cellulosic products such as corrugated paperboard and products formed therefrom.

2. Description of the Prior Art

Sodium silicate material is known in the prior art as a fire retardant material, as opposed to a "fire- resistant" material, and as an adhesive for corrugated paperboard material at least as early as U.S. Patent No. 1,129,320 which issued on February 23, 1915 to Vail et al. The use of sodium silicate as a fire retardant and adhesive is also shown in U.S. Patent No. 2,018,800 which issued on October 29, 1935 to Morton.

U.S÷ Patent No. 3,077,222 which issued on February 12, 1963 to Shanley discloses corrugated paperboard in which sodium silicate material is used both as an adhesive in assembling the paperboard and as a stiffening material by continuously coating various surfaces of the paperboard with sodium silicate material.

In none of the prior art patents is there a teaching that sodium silicate can be used both as the adhesive in the manufacture of corrugated paperboard and as a fire- resistant agent. The application of heat energy to the paperboard to achieve setting of the sodium silicate during conventional manufacture changes the fire- resistant character to a fire-retardant.

The adhesive conventionally employed to bond the corrugated medium to the flat liner sheets in manufacturing corrugated paperboard is so-called "Stein-Hall Process" starch-based adhesive, typically

cornstarch which may contain various chemestries such as an anti-fungal agent and/or caustic soda for adjustment of the gel temperature. The starch-based adhesive also may contain borax or other tackifying agents to increase the tack of the adhesive. Due to its relatively short shelf-life and in order to prevent premature setting, a portion of the starch-based adhesive ordinarily is cooked batch-wise by heating an aqueous slurry comprising a measured quantity of starch to gradually bring the temperature of the slurry to a gellation temperature. Several processes have been developed for cooking the starch slurry substantially instaneously.

Whether a starch adhesive of the prior art or a sodium silicate adhesive of the prior art is used as the adhesive, the corrugated paperboard material must be passed through a -heating zone or "hot section" such as across hot plates having a temperature as high as 350 F to set and cure the adhesive by drawing out the free water content thereof. Accordingly there is no "cold corrugating" process in the prior art in which sodium silicate material is used as the adhesive.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method and means for rendering normally combustible paper and fiber materials fire resistant.

Another object of the invention is to provide new and improved fire resistant normally combustible products which overcome the aforesaid and other problems of the prior art.

Still another object of the invention is to provide new and improved corrugated structures which are characterized by resistance to fire or heat and low

flame spread.

An additional object of the invention is to provide new and improved corrugated container wall materials which have both good mechanical properties as well as resistance to fire or heat.

It is a further object of the invention to enable corrugated paperboard to be manufactured economically using existing equipment with only minor, low cost equipment modifications thereto and at reduced energy costs with increased yields, i.e., by the use of a "cold corrugating" process.

In accordance with the invention, new fire resistant products are provided which comprise a normally combustible carrier or substrate having applied thereto a predetermined sodium silicate material of high bound • water content and low free water content which is heat- foamable to produce a carrier or substrate capable of resisting the passage of fire therethrough in the event the carrier or substrate is exposed to flame or excessive heat. The predetermined sodium silicate material acts as an intumescent material which, upon exposure to heat or fire, forms a mechanically stable foam-like barrier which acts (a) as an oxygen denial barrier, and (b) a heat reradiator and/or thermal insulator. The foam-like barrier also limits the fuel vapor generation rate of the normally combustible carrier or substrate, thereby preventing or retarding combustion of the carrier or substrate.

In one embodiment of the invention, the normally combustible substrate comprises a laminated paper product in which the predetermined sodium silicate material also functions as the laminating adhesive. In such embodiment the predetermined sodium silicate

material typically is applied for each lamination in an amount (equivalent dry weight) of at least about three pounds per thousand square feet of the paper product. In the case of laminated paper products, the sodium silicate is applied to one or both paper elements to be laminated, the elements are then pressed into adhering contact with the predetermined sodium silicate material between them, and thereafter the predetermined sodium silicate material is permitted to set. The same results can be achieved by impregnating the product with sodium silicate.

In another embodiment of the invention, the fire resistant normally combustible product comprises a corrugated paperboard material with the predetermined sodium silicate material is being the adhesive employed for laminating the corrugated medium to-one or more liner sheets and the fire-retardant agent.

In a further embodiment of the invention, corrugated paperboard material is manufactured by use of the predetermined sodium silicate material as an adhesive and without the need of any substantial level of heat energy to set and cure the adhesive.

Still other objects and advantages of the invention will become clear from the following description taken in connection with the accompanying drawings in which like numerals denote like elements:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary section of corrugated paperboard produced in accordance with the invention.

FIG.2 is a schematic representation of apparatus for manufacturing conventional corrugated paperboard;

FIG. 3 is a schematic representation of one embodiment of the apparatus of the invention for manufacturing cold corrugated paperboard in accordance with the invention;

FIG. 4 is a fragmentary section of a fire resistant product in the form of corrugated paperboard produced in accordance with the invention;

•FIG. 5 is a schematic representation of another embodiment of the apparatus of tϋe invention for manufacturing fire resistant corrugated paperboard in accordance with the invention;

FIG. 6 is a fragmentary vertical section view of a coater/pressure impregnator of the apparatus of FIG. 5 taken along lines 6-6 in FIG. 5;

FIG. 7 is a fragmentary horizontal section showing a valve and a valve housing of the coater/pressure impregnator of FIG. 6;

FIG. 8 is a fragmentary vertical section of the valve and valve housing of FIG. 7;

FIGS. 9 to 13 are fragmentary sections of other embodiments of fire resistant corrugated paperboard material made in accordance with the invention;

FIG. 14 is a fragmentary perspective view showing an embodiment of a tubular fire resistant structure made in accordance with the invention;

FIG. 15 is a graph of heat release rate against time for various paper materials;

FIG. 16 is a graph of the inverse of ignition energy against external heat flux for various paper materials;

FIG. 17 is a graph of the cumulative actual heat release rate -against time for various paper materials;

FIG. 18 is a graph of percentage of material consumed against percent of dry silicate applied;

FIG. 19 is a graph of fuel vapor generation rate against dry silicate loading;

FIG. 20 is a graph of maximum heat release against applied silicate (dry);

FIG. 21 is a graph of combustion efficiency of paper stock against percent of dry silicate applied;

FIG. 22 is a graph of heat of combustion of stock against percent of silicate applied (dry);

-

FIG: 23 is a graph of total energy release against amount of silicate applied (dry);

FIG. 24 is a graph of total energy release against percent silicate applied (dry);

FIG. 25 is a schematic representation of fire retardant corrugated paperboard made in accordance with the invention;

FIG. 26 is a graph of the reduction of web thickness percent against time in the presence of the application of heat energy to the web thickness;

FIG. 27 is a graph of viscosity against concentration for various weight ratios of sodium silicate solutions;

FIG. 28 is ^" a graph of viscosity against density for various weight ratios of sodium silicate solutions;

FIG. 29 is a graph of viscosity against weight ratio for various solids content of sodium silicate solutions;

FIG. 30 is a graph of viscosity against temperature for various weight ratios of sodium silicate solutions; and

FIG. 31 is a graph of the weight of water lost to achieve a set against a weight of spread of sodium silicate for various weight ratios.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the invention, the terms "carrier" or substrate" are to be understood as referring to mechanical support structures. The terms "laminated paper product" and "laminate" are to be understood as referring to a structure consisting of at least one base layer-having at least, one additional layer adhesively affixed thereto. The layers may consist of flat stock, or one or more of the layers may comprise a corrugated medium as in the case of corrugated paperboard materials. The term "intumescence" refers to the property of a material to swell or foam when exposed to high temperature or fire. The term "compatible" as used in connection with the substances added to the predetermined sodium silicate material is to be understood as referring to those substances which enhance handling characteristics, adhesive characteristics and/or fire resistant characteristics of the sodium silicate material, as will be described in detail hereinafter. The terms "corrugated" and "corrugating" may be used interchangeably herein with the terms "fluted" and "fluting", respectively.

The invention is based on the discovery that by use of predetermined sodium silicate materials under

predetermined operating conditions, "cold corrugated" papefboard can be made with a substantial elimination of the heat energy used in prior art corrugation processes.

The invention is also based on the discovery that predetermined sodium silicate materials when applied to a normally flammable laminated product in sufficient quantities provides the dual functions of (1) a strong adhesive strength and (2) a fire retardant property.

The various soluble silicates are distinguished by the ratio of silicate to soda. This is given as the empirical weight percent ratio or "aspect ratio" and does not indicate any compound formation. A glass or solution containing 3.3 moles of a-0 for one mole of Si0 ₂ will have a weight percent ratio of 3.22% SiO_/l% Na-0 and is referred to as 3.22 ratio sodium silicate. In the sodium system, the mole'ratio and the weight percent or aspect 'ratio are nearly the same. The term "silicate ratio" also is used to describe the mole ratio of silicate, SiO-, to soda, Na-,0.

The term "silicate solid" or "solids content" describes the silicate content of a dehydrated specimen. This term applies to the portion of the silicate material which will not evaporate when heated above 105 C. Thus the solvent water and loosely bound water are excluded from the silicate concentration figures. The "bound water ratio", Na _^ O: H_0, depends upon the silicate ratio or aspect ratio and the dehydration temperature.

The most significant working property of an adhesive is the viscosity. This may be varied by dilution, temperature, or additives. Viscosities of various commercially available sodium silicate solutions having different aspect ratios are shown as a function of Na-0 concentration in FIG. 27. Thus the water content of a

-10- solution of a given aspect ratio increases with decreasing concentrations of Na ₂ 0. Viscosities of various sodium silicate solutions having different aspect ratios are shown as a function of density in FIG. 28. As density decreases, viscosity decreases. FIG. 29 shows the viscosity of various sodium silicate solutions of different solids content plotted against the % weight ratio of Na-0: Si0 ₂ « The graph shows that a minimum viscosity exists at or approximately at a 2:00 weight ratio. FIG. 30. shows that the viscosity of various sodium silicate solutions having different aspect ratios can be decreased by heating without evaporation.

Corrugated paperboard 40 as shown in FIG. 1 can be fabricated in accordance with the processes of the prior art or in accordance with the process of the invention. The corrugated paperboard comprises flat liner sheet or liner 41 adhered to corrugated medium material or medium 42 by adhesive 43 applied to the crests 42a of the corrugations 42b of the medium. By way of example, the liner 41 may be Kraft paper having a basis weight in lbs/1000 sq. ft of 42, 64 or 90 lbs. The medium 42 may be bogus or straw paper having a basis weight, by way of example, of 26 or 33 lbs/1000 sq. ft. Liner sheet or - liner 44 which can be similar to liner 41 is adhered to the crests of the medium opposite to the crests which are adjacent liner 41. An assembly of a single liner and a corrugated medium is referred to as single face material or single face while an assembly of two liners with a medium therebetween is referred to as double face material or single wall.

In FIG. 2, there is shown a conventional corrugating machine 45 of the type used in the prior art. The machine includes a single facer 45a and a double backer 45b which are shown separated in FIG. ^" 2. In common practice, the double backer is positioned downstream of

-li¬ the single facer on the same floor or level therewith. Liner ^" 46 is advanced from supply roll 46a, over guide roll 47, beneath roll 48, around drum 49a of preheater 49, and beneath roll 50. The preheater 49, operating at a temperature in the range of about 350°F, heats the liner 46 in order to condition the liner to receive adhesive. The heated liner 46 then advances to the single facer 45a which is a pressure single facer. Idler rolls 51 and 52 deliver the liner to pressure roll 53.

Medium 42 is advanced from supply roll 54 and over roll 55 to preconditioner 56 which comprises steam and tension drum 56a with rolls 57 and 58 to guide the medium about the drum. The drum 56a is heated in the range adjacent to about 350 F to steam and heat the medium in order to condition the medium for corrugating and for the subsequent reception of adhesive.

From the preconditioner 56 of the conventional corrugating machine, the medium advances about roll 59, roll 60, tension roll 61, and roll 62 which is adjacent top corrugating roll 63. Roll 63 has indentations or flutes extending along its surface parallel to the axis of rotation of the roll for forming the corrugations in the medium. Bottom corrugating roll 63 has indentations or flutes which are similar to those of roll 62 with the flutes of rolls 62 and 63 running in mesh. Upon passing into the nip of the corrugating rolls, the medium is set into permanent waves or flutes. As the medium passes beneath bottom corrugating roll 63, the crests 42a of the medium facing away from roll 63 contact the adhesive 64 on the surface of adhesive applicator roll 65. Metering roll 66 applies adhesive 64 to roll 65 from adhesive pan 66. As the medium passes beyond the nip of bottom corrugating roll 63 and applicator roll 65, the crests 42a of the medium come into contact with liner 46

as the medium and liner pass between the nip of roll 63 and pressure roll 53. Since the flutes of the medium are in the flutes of the bottom corrugating roll at the nip with pressure roll 53, tbe liner 46 can be pressed against the medium at a pressure of 100 psi or more, thereby insuring that a strong bond is formed.

The single face material formed by liner 46 adhered to the crests or tips 42a of corrugated medium 42 then passes over roll 67 and to the nip between moving belts 68 in the conventional corrugating machine. The single face material then advances onto a low speed conveyor (not shown) extending along the upper portion of bridge 69. The single face is accumulated with accordian-like pleats 70 on bridge 69 to provide a surplus of material to be advanced to the double backer 45b. The single face in pleats 70 advances to roll 71 and then beneath upper drum .72a of a duplex preheater 72. The heated upper drum conditions the single face for receiving adhesive on the tips of- the medium and for subsequent adhering to liner 41 in the double backer 45b. The single face then passes over roll 73 and roll 74 before passing about roll 75 adjacent to adhesive applicator roll 76. Applicator roll 76 applies adhesive 64 from adhesive pan 77 to the crests or tips of the medium 42 passing about roll 75.

Liner 41 advances from supply roll 78 of the conventional corrugating machine and across rolls 79 and 80 to drum 72b of duplex preheater 72. The liner 41 is conditioned by the heat of drum 72b which can have a temperature in a range of about 350 F. and thereby prepared for adherence to medium 42. The conditioned liner then passes about roll 81, tension roll 82, and roll 83. Liner 41 then passes between the nip formed by rolls 84 and 85 where the single face with adhesive on the tips of the medium is brought into contact with the

liner 41. The single face and medium then pass over heating section 86 having steam plates 86a for delivering heat to dry and cure the double face board comprising liners 41 and 44 with medium 42 therebetween. The steam plates are heated to a temperature in a range of temperatures adjacent to about 350°F. A continuous moving belt 87 has the lower reach thereof in contact with the surface of the upper liner of the double face. The belt is pressed against the liner by iron weight rolls (not shown) bearing upon the upper surface of the upper reach. The pressure of the belt upon the double face is customarily in the range of about 10 psi to keep the tips of the corrugated medium in contact with the liner as the adhesive sets and cures-. Such a pressure is less than that which could buckle or collapse the corrugated flutes. In more modern systems, an air plenum is used in place of the upper bolt to apply presure upon the corrugated board until the adhesive sets.

Beyond the steam plates 86a, the double face is permitted to cool before passing beyond the downstream portion of belt 87. Thereafter the double face is delivered to stations for slitting, scoring, and cutting the double face in preparation for use in making corrugated paperboard containers.

At the single facer 45a, the adhesive applicator roll clearances are typically set to give an adhesive spread of about 3 to 5 lbs. (wet) per 1000 sq. ft. and even as high as 7 to 10 lbs., (wet) per 1000 sq. ft. The adhesive spread at the double backer adhesive station is usually a little higher than at the single facer in order to take care of irregularities in the sheet. Thus the spread is about 4 to 6 lbs. (wet) per 1000 sq. ft. and as high as 7 to 10 lbs. per 1000 sq. ft. The silicate spread depends upon the related factors of

adhesive viscosity, composition, paper structure and moisture content. Generally the total spread is 8 - 10 lbs. wet per 1000 sq. ft. for double face board having "A flutes" and 15 - 30% higher for double face board having "B flutes". A heavy board may require even more adhesive.

In the prior art the tendency has been to use adhesives which will wet rapidly and yet set quickly with a minimum transfer of moisture to the paper. Accordingly silicious solutions such as a ₂ 0, 3.4 Si0 ₂ (3.4 aspect ratio) and higher aspect ratios have been used. Typically the choice for sodium silicate adhesives has been between 41°Be' Na ₂ O, 3.3 Si0 ₂ solution and 39.7°Be' Na-O, 3.5 Si0 ₂ solution. Both silicate solutions are controlled in viscosity in the prior art to permit the minimum amount of penetration into the fibers of the liner before sufficient heat can be transferred into the liner. Approximately 15 seconds is required for initial set. This enables corrugating speeds up to about 300 fpm to 400 fpm, depending upon the length of the hot plate and cooling section.

It can be seen that a substantial amount of heat energy is used in a conventional corrugating machine as shown in FIG. 2 to condition liner 46 at preheater 49, the single face at .the preheater drum 72a, and the other liner 41 at the preheater drum 72b. Thus, steam or other heat sources capable of achieving temperatures in the range of about 350 F. must be provided to each of the preheaters. Furthermore, such high temperatures can be deleterious to the liners as well as the single face passing over upper drum 72a of the preheater. Another substantial amount of heat is used in standard corrugating machines as shown in FIG. 2 in heating the upper corrugating roll 63 to a temperature in the range of 325 to 350°F.

The ordinary Na^O, 3.2 SiO- solution may be diluted to 37° or 40° Be' for use on the conventional single facer. Water can be used to provide the proper dilution. The heavier, more viscous grade of silicate, used on the conventional double backer results in reduced spread and decreased penetration.

When heavy liners are used such as those of 60 and 90 lbs./lOOO sq.ft. or when double wall combinations are required, the speed of a conventional corrugating machine must be appreciably reduced to provide the necessary heat transfer and to achieve proper equilibrium. When damp board or board which is more absorbent than usual is used, penetration may be extensive and require a flooding of the glue line which results in a brittle board. Such board is also more liable to staining.

It is estimated that as much as 80% of the total energy input (electrical and steam) to a conventional paperboard corrugating machine comprises the heat which must be delivered to set and cure the adhesive, whether starch or conventional silicate. In addition, such application of heat induces shrinkage of the paper. Where the mositure content varies, there is created uneven shrinkage which results in "S-warp" and "bias warp" conditions which result in waste product. It is estimated that such waste product may comprise as high as 5-10% of the total product being made. In addition, the application of heat energy to set and cure the adhesive causes shrinkage of the product of up to about 0.5%. Furthermore, the complexity of equipment to deliver heat energy to the web of paperboard being produced and the time needed for heat transfer to the web necessarily limit the maximum lineal speed of production.

In one embodiment of the invention, as shown in FIG. 3, corrifgating machine 45 has been modified and adapted to be a cold corrugating machine to make cold corrugated paperboard in accordance with the invention. One modification includes the elimination of preheater 49, preheater 72, and steam plates 86a. An additional modification is to lower the temperature of the upper corrugating roll 62 to a temperature in the range of temperatures from about 130°F to about 195°F and preferably to about 140°F. The lower corrugating roll 63 is maintained in a range of temperatures adjacent to about 100°F. Thus, the apparatus and process of the invention enables corrugated paperboard to be made by a "cold process", thereby obviating the need for conditioning the paper materials with heat and obviating the need for setting and curing the adhesive with heat.

One of the modifications of the invention to the prior art ^' process for making corrugated paperboard is the use of predetermined sodium silicate adhesives. Sodium silicates were in the past the standard adhesive for manufacturing corrugated paperboard; however, since the 1950's, sodium silicate adhesives have given way to starch-based adhesives. Commercially available sodium silicate solutions which can be used as adhesives are in a range of aspect ratios (the weight ratio of SiO ₂ /Na ₂ 0) of from about 1.60 to 3.75 with densities in the range of about 58.5°Be' to about 35.0°Be', and pH in the range of about 13.4 to about 11.1, respectively. In the past in selecting sodium silicates for adhesives in making corrugated paperboard, silicate adhesives in the range of 3.3 aspect ratio to 3.5 aspect ratio were typically used to obtain adhesives which could wet the paper rapidly and yet set quickly with a minimum of transfer of moisture to the paper. Thus, more silicious adhesive solutions having aspect ratios greater than 3.3 were typically used. In addition, sodium silicate adhesives

having higher, more concentrated liquid phases as well as he~ated adhesives were used for the same reasons.

At a SiO ₂ /Na ₂ 0 weight ratio of about 0.5, sodium silicate is believed to be substantially a monomer. At a weight ratio of about 1.0, the silicate contains multiple molecules in short chains of up to three molecules in length. These chains can readily be broken. At a weight ratio of about 1.5-1.6, the silicate solution becomes complex with moTecules formed in rings in addition to the short chains of molecules. If the weight ratio is further increased, more rings of molecules are present. At weight ratios of about 2.0- ^• 2.5, the ring structure gets further complexed with rings disposed within rings, thereby creating a spherical cluster of rings. At weight ratios of about 2.5 to 2.8, fewer monomers are present and most of the short chains are gone, leaving the spherical clusters of rings. At a weight ratio of about 2.8, there remains the spherical clusters with short chains and monomers moving off the clusters to begin to replicate the original patterns. At a weight ratiσ of 3.22 and above, few short chains are left with the result that the silicate solution is comprised substantially of spherical clusters.

It is believed that in the weight ratio of about 2.5 to about 2.8, due to the dominance of the spherical clusters of rings, enables the sodium silicate solution to set rapidly without the application of heat when the silicate solution is in a temperature range substantially corresponding to that of intermediate ambient atmospheric temperatures, such as for example in a range of about 55°F to about 80°F, and preferably in a range of about 60°F to about 70°F. Accordingly, sodium silicate solutions in the range of aspect ratios which include about 2.5 to about 2.8 comprise at least a

portion of the solutions which can be used for cold corrugating in accordance with the invention.

Predetermined sodium silicate material solutions which can be used in carrying out the "cold process" of the invention for forming fire resistant board and for carrying out the "cold corrugating" process of the invention for forming corrugated paperboard are those in the overall range of Si0 ₂ /Na ₂ 0 weight ratios of approximately 2.50 to approximately 3.25, having a range of densities of approximately 42.0° Be' to approximately 49.3 Be', and a range of viscosities of approximately 60 centipoises to approximately 960 centipoises; however, better results have been achieved in the range of approximately 2.50 to 2.80 with the best results at about 2.58.

Examples of predetermined sodium silicate adhesive materials which can be used in the invention and which are listed below are commercial products of The PQ

Corporation, P.O. Box 840, Valley Forge, PA 19482, with each identified by a product name which is a registered trademark:

SODIUM SILICATE SOLUTIONS

Product t . Ratio Density at pH Visco¬ Character- Sol.

Name SiO„/Na„0 68°F(205 2 2 °c: ) CPS sity istics Cont. %

"STIXSO RR" 3.25 42.7 11.3 830 Syrupy 39.22

"N" 3.22 41.0 11.3 180 Syrupy 37.60

"0" 3.22 42.2 11.3 400 Less Syrupy 38.65

"K" 2.88 47.0 11.5 960 Sticky 42.70

"M" 2.58 49.3 11.8 780 Syrupy 44.55

"STAR" 2.50 42.0 11.9 60 Clear 37.10

The weight ratio range of 2.88 - 3.25 is the weight ratio range of sodium silicate adhesives which the manufacturer. The PQ Corporation, suggests is the range of more siliceous silicates which "are very useful as adhesives or binders due to the higher content of polymeric silica"; however, such adhesives are not suggested by the manufacturer as being capable of setting and curing without heat. Thus, the manufacturer states that in the paperboard industry, "STIXSO RR" silicate solution, having a weight ratio of 3.25, is used as an adhesive when heat is used and that it can provide "fire-proof bonds". The manufacturer also suggests that its 'N' or "K" silicate solutions having weight ratios of 3.22, 3.22, and 2.88, respectively, are suitable for adhesives for spiral tubes; however, spiral tubes are not made by means of a continuous process involving heating as used in making corrugated paperboard. As far as the "STAR" adhesive is concerned, the manufacturer. The PQ Corporation, suggests that it is used as a deflocculant in ceramic slip castings and in slurry thinners. The manufacturer also suggests the use of the "M" solution as a cement slurry thinner. The "M" solution having a weight ratio of 2.58 is the preferred one of the solutions listed above for the cold corrugating process of the invention.

Certain of the properties of the "N" silicate solution, the " ^" M" silicate solution, and the "STAR" silicate solution are shown in FIGS. 24 and 25 and others can be interpolated in FIGS. 26 and 27.

In summary, the manufacturer. The PQ Corporation, does not disclose or suggest that any of the sodium silicate solutions listed above can be used as an adhesive in manufacturing corrugated paperboard without the application of heat for setting and curing the silicate adhesive.

Other examples of predetermined sodium silicate materials which can be used as adhesives in accordance with the invention and which are listed below are commercial products of Diamond Shamrock Corporation, Soda Products Division, 351 Phelps Court, P. 0. Box 2300, Irving, Texas 75061:

SODIUM SILICATE SOLUTION

Product t. Ratio Density pH Solids Viscosit Name Si0 ₂ /Na ₂ 0 °Be' o @ 68 F Content% Centipoises

Grade 42 3.22 42.0 11.2 39.3 385

Grade

49FG 2.58 49.0 11.6 44.5 630

The manufacturer. Diamond Shamrock Corporation, suggest that "Grade 42" silicate is used inter alia as a fast- set adhesive for corrugated paper, solid fiber, and wall boards; however, there is no suggestion by the manufacturer that "Grade 42" silicate can be used as an adhesive in manufacturing corrugated paperboard without the conventional application of heat for setting and curing the silicate adhesive. The manufacturer. Diamond Shamrock Corporation, also suggests the use of "Grade 49 FG" silicate-as a fast-set adhesive for laminating paper and metal foil; however again there is no suggestion by

-21- the manufacturer that "Grade 49FG" silicate can be used as an" adhesive in manufacturing corrugated paperboard without the application of heat for achieving a fast-set and curing the silicate adhesive.

Tests have shown that the Type 49FG sodium silicate adhesive of the Diamond Shamrock Corporation as well as the Type M sodium silicate adhesive of The PQ Corporation, each having an aspect of 2.58, can be used in achieving cold corrugating in accordance with the process of the invention without the use of the modifications discussed below which include the addition of moisture and the use of surfactants whether in the added moisture or in the adhesive. On the other hand, tests have shown that upon using at least one of the modifications of moisture addition or the use of surfactants, sodium silicate adhesives of higher' aspect ratios for example STIXSO RR adhesive of The PQ Corporation, gave satisfactory results. Tests also show that optimum results are obtained by the use of one of the predetermined adhesives having an aspect or weight ratio in the predetermined range of aspect ratios in combination with both moisture addition and the introduction of surfactants as discussed below.

Tests have shown that the preferred temperature range for the predetermined sodium silicate adhesive is in the range of about 55 F to about 85 F and preferably in the range of about 60°F to about 70°F; however a wider range of tempeartures can be used, especially when the process of the invention is carried out under high ambient temperature conditions.

At the single facer 45a of the cold corrugating machine of the invention shown in F1.93, the adhesive applicator roll clearances are typically set to give an adhesive spread of about .75 to 13 lbs. (dry) per 1000 sq. ft.

i.e. about 1.5 to about 26 lbs. (wet) per 1000 sq. ft. The adhesive spread at the double backer adhesive station is usually a little higher than at the single facer 45a in order to take care of irregularities in the sheet. The silicate spread depends upon the related factors of adhesive viscosity, composition, paper structure and moisture content. Generally the total spread is - lbs. (wet) per 1000 sq. ft. for double face board having "A flutes" and 15 - 30% higher for double face board having "B flutes". A heavy board may require even more adhesive.

Further, in accordance with the cold corrugating process of the invention, it has been discovered that the application and setting of the sodium silicate adhesive without the application of heat to the liners and the single face by preheaters or by a "hot section" can be enhanced and improved by the application of moisture to the ^" side of the paper web which is to receive the adhesive. Thus, as shown in FIG. 3, spray heads or nozzles 88 can be provided to apply moisture to the side of liner 46 which is to receive the adhesive 64. The heads or nozzles are adapted to deliver moisture in a spray, shower, steam, or mist of water which is applied across the width of the web of the liner 46 which can extend in width up to about 96 inches. The weight of moisture to be applied is in the range of about .9% to about 15% of the basis weight of the paper, whether applied to a liner or a medium. The moisture content of the paper is the controlling variable in determining the amount of moisture to be applied. The lower the moisture content, the greater the amount of moisture is to be applied by heads 88, 89 and 90. For example, if a liner of 42 lb. per 1000 sq. ft. is used as liner 46, then the moisture to be applied can be about .4 to about 6 lbs. of moisture per 1000 sq. ft. of liner. The result of the application of moisture is to condition

the surface of the liner and facilitate the attachment and distribution of the adhesive on the corrugated medium to the surface of the liner including the fibers thereon. This in turn facilitates the setting of the adhesive without the need of the application of external heat to the paper or to the paper and adhesive after application as is required in the prior art of corrugating.

For the same reason, heads 89 are provided to apply moisture to the side of medium 42 which is to receive the adhesive 64. In addition, heads 90 are provided adjacent the side of liner 41 which is to receive adhesive on the crests of medium 100 of the single face. Tests have shown that it is desirable to use the heads to establish a moisture content in each web in the range of about 3-5%. Tests have shown that the application of moisture by heads 88, 89, and 90 can also be used to control the moisture content of the liners and medium in preventing warpage of the final product.

Since the amount of the addition of moisture to the webs of paper is a function of the rate of the area of paper passing adjacent the respective heads as well as the moisture content of the paper, the weight flow of moisture from the heads must be varied as the lineal speed of the paper webs changes. For example, tests have shown that the cold corrugating process of the invention can be carried out at web lineal speeds from 400 - 500 ft. per minute (the current maximum web lineal speeds for conventional corrugating machines) to as high as 800 ft. per minute. Since a speed increase of 500 ft. per minute to 800 ft. per minute is a 60% increase, a related % increase in the weight of moisture delivered would be provided. Accordingly, each of heads 88, 89, and 90 can be provided with flow ^" controllers 88a, 89a, and 90a, respectively, for modulating the

weight flow of moisture as a function the lineal speed of the paper webs. Thus once the heads are set by way of the controllers to deliver a desired weight flow, the controllers will thereafter vary the flow of moisture as a function of the lineal speed if the lineal speed of the webs are changed.

Double backer 45a of the corrugating apparatus of the invention can be modified by applying a surface of low friction material to what was heretofore steam plates 86a in order to reduce friction between the corrugated paperboard. On a further modification, a pair of overlying endless belts can be provided downstream of rolls 84. Rolls 84 deliver the single wall material into the nip formed by the lower reach of the upper endless belt and the upper reach of the lower endless belts.

Tests have also shown that "the addition of a surfactant or wetting agent to the moisture applied by heads 88, 89, and 90 can enhance the uniform distribution of the moisture on the paper web and the take-up of moisture thereby. This in turn enables the distributed moisture to facilitate the application and setting of the adhesive. By way of example, a surfactant or wetting agent which has been used effectively in the moisture applied by the heads 88, 89, and 90 is a disulfonated alkydiphenyl oxide which is marketed by the Dow Chemical Corporation under the trademark DOWFAX 2A1. This surfactant is characterized by high solubility in salt solutions and high tolerance for strong acids and bases. Tests have shown, by way of example, that the ratio of surfactant or wetting agent to the water to be applied as moisture to the paper webs can advantageously be in the range extending to adjacent about one part in 16,000.

Additional tests have shown that heads 88, 89, and 90 can be used to apply simultaneously both a shower or steam or mist as well as moisture with or without a surfactant in conditioning the webs to receive the sodium silicate adhesive.

It has also been determined that the viscosity of the adhesive of a preferred aspect or weight ratio can be maintained at a desired level by maintaining the temperature of the adhesive at a level within the range of about 55°F to about 80°F by the introduction of water into the adhesive pass 66 and 77. Thus flow controls 66a and 77a can deliver water to the pass to control the viscosity in accordance with the viscosity-water content relationship shown in FIG. 27. The flow controls can be activated by transducers 66b and 77b which measure the adhesive density to which viscosity is a function as shown in FIG. 28.

Still a further procedure which tests have shown can be instrumental in achieving the cold corrugating process of the invention, is to introduce a surfactant or wetting agent into the sodium silicate adhesive. The result of the addition of the the surfactant or wetting agent is to improve the application of the sodium silicate adhesive to the tips or crests of the corrugated medium by facilitating its engagement with and distribution across the receiving web. By way of example, the same surfactant DOWFAX 2A1 can be used as in the case of the moisture to be delivered by the heads. A small ratio of surfactant to adhesive such as the ratio of surfactant to water in case of the moisture delivered by the heads can be sufficient. A surfactant sold by ARCO under the trademark ULTRAWET DS has also been successfully tested as an appropriate wetting agent both in the adhesive and in the applied moisture.

One embodiment of the fire-resistant material of the invention is shown in FIG. 4 of the drawings. Referring to FIG. 4", a fire-resistant corrugated paperboard material for a container wall is shown having an external wall comprising a laminate of two liners 20 and 22. Liners 20 and 22 comprise conventional flat liner board stock, typically a basic Kraft paper stock or the like. Liners 20 and 22 are laminated to one another by a predetermined sodium silicate adhesive material 24 which typically is applied in an amount (dry weight) of about threeto about twenty-four pounds per thousand square feet of liner board, and more typically about three to about eight pounds per thousand square feet of liner board. Liner 22 in turn is adhesively affixed to a paperboard corrugated medium 26 by means of a second layer 27 of the predetermined sodium silicate adhesive material which is typically applied in an amount (dry weight) of about three to about twenty-four pounds per thousand square feet of liner board, and more typically about three to about eight pounds per thousand square feet of liner board. A third ^' liner 28 is affixed to the other side of corrugated medium 26 by means of a predetermined sodium silicate adhesive material 30 which is applied in an amount (dry weight) in the range of about four to about twenty-four pounds per thousand square feet, and most typically about three to about eight pounds per thousand square feet of liner board. In making a box or like container, the laminate of layers 20 and 22 typically is formed as the external layer of the box while liner board layer 28 is the internal layer of the box. Boxes made from the corrugated container wall materials of the invention are thermally reactive. When subjected to heat or flame, some combustion is required to generate fire resistance.

The degree of fire resistance achieved in the present invention depends primarily upon the amount of sodium

silicate applied to the combustible carrier or substrate material. Acceptable fire resistance may be imparted to paper products by the application of as little as three pounds (dry weight) per thousand square feet of product, and fire resistance increases with the amount of sodium silicate applied to the product up to about sixty-six pounds (dry weight) per thousand square feet of product; however, an application of a spread as great as sixty- six pounds per thousand square feet cannot now be done on a conventional corrugating machine. Application of sodium silicate in an amount in excess of about sixty- six pounds (dry weight) per thousand square feet of product does not appear to improve fire resistance (or adhesive strength) materially; however, fire resistance can be increased by the inclusion of additional layers laminated by sodium silicate.

Nonetheless a paper producted can be impregnated with the predetermined sodium silicate material to achieve a fire retardant material. By way of example, the apparatus of U.S. Patent No. 4,411,216, of Menser can be used for such impregnation.

While not wishing to be bound to theory, it is believed that the predetermined sodium silicate material provides two separate mechanisms in sequence for resisting fire. Upon first exposure to high temperature, the sodium silicate coating undergoes detachment of bound water molecules in the form of water vapor. The bound water molecules are available since the predetermined sodium silicate material need not be subjected to elevated temperatures (such as 350°F) to achieve a set during processing of the material. Such elevated temperatures would drive off or otherwise release the bound water molecules. The release of the bound water molecules and production of water vapor is desired only in the presence of a fire condition to remove incident heat

from the surface of the corrugated container wall material, thereby generating a thermal lag. The thermal protects the otherwise combustible substrate and the contents of the box formed from the corrugated container wall material for a period of time. This water molecule depletion mechanism tends also to maintain the corrugated container wall material at a somewhat constant temperature during exposure to fire until the sodium silicate material becomes substantially depleted of bound water molecules.

Following release of the bound water molecules, the sodium silcate then undergoes intumescence, forming a mechanically stable foam consisting of the inorganic silicate. The resulting foam serves as an oxygen denial barrier and heat reflector or reradiator which provides further protection to ^" the otherwise combustible substrate and of objects contained within a box or container formed from the corrugated container wall material.- ^' The foam which is non-combustible also inhibits pyrolytic degradation of the cellulose fibers of the paper stock and, if present in sufficient quantity, thermally insulates the box and its contents so as to limit the fuel vapor generation rate of the combustible materials of the box (and contents).

It has been observed that the silicate protective foam may not prevent the cellulose fibers themselves from undergoing thermal decomposition and subsequent generation of combustible gases; however under ordinary fire conditions, the rate at .which the fibers decompose when protected by the silicate foam typically is insufficient to generate combustible gases in a sufficient quantity to form a sustaining combustible mixture at normal oxygen levels. It has been observed that the material of the invention is self-extinguishing with the removal of external fire sources. Thus,

following exposure to fire, a box or container made from corrugated paperboard material in accordance with the invention, may have the appearance of a charred substance; however, the box or container may still be intact and the contents of the box may be protected from fire. As a result, boxes or containers formed from corrugaged paperboard material made in accordance with the invention may be subjected to higher thermal flux for longer period of time than conventional boxes or containers made from conventional corrugated container wall materials using conventional paper stock and adhesives. This added margin of time safety typically will permit dousing of the fire, e.g., through automatic sprinkling systems or the like. Moreover, inasmuch as the corrugated paperboard material itself is fire resistant, it will reduce propagation of the fire as in .the case of conventional boxes or containers made from . conventional corrugated container wall materials using paper stock and adhesives.

In the process of the invention for treating paper material to make it fire resistant and in the process of the invention for achieving "cold corrugating" for making "cold corrugated" paperboard, the temperature of the paper and/or the predetermined sodium silicate material should not exceed approximately 157 F which is the crystallization temperature of the sodium silicate material. Thus the chemical treatment of paper stock in accordance with the invention must occur at lower temperatures (i.e. substantially below 157°F maximum) in order to retain a thermally activated, fire resisting, chemical phenomenon which provides fire resistance during fire exposure. A preferred temperature range for the predetermined sodium silicate material during application to the paper material is about 55°F to about 80°F and preferably in the range of about 60°F to 70°F.

In the cold corrugating process, the corrugating rollers 120 and 122 (FIG. 2) may be heated under certain circumstances such as the type of medium material to facilitate the formation of flutes without splitting or cracking the medium material. For example, the lower roller 122 may be heated to approximately 100°F and the upper roller 120 heated to approximately 140°F. The rollers 120 and 122 can even be heated to a temperature greater than approximately 157°F if the web of medium material is not heated beyond approximately 157 F. As a result of the low temperature, the treated corrugated paperboard of the invention is a product which incorporates a latent capability which is activated only upon exposure to the heat from fire or an equivalent source.

In the presence of any progressing and/or stable state fire environment, the "cold" application of sodium silicate at temperatures below 157°F (69° C) , but not below approximately 61° (16° C) to a carrier web of any substrate of fiber material generates a chemical composition such that there results a thermally reactive composite web which limits the fuel vapor generation of the web to about 30% of that of readily available organic volatiles, e.g., formalines, phenols, saccharides, lignines, and all weakly bonded hydrogen end groups in the fibers. Thus there is achieved a stable state in about 70% of the thickness of the web comprised of carbon and silicates. Utilizing "cold" silicate material achieves the goal of having a thermally reactive chemistry which prohibits further fuel vapor generation in the solid phase of the material. At an energy level (temperature) below that of large scale autogenous ignition of fuel vapors of cellulosic fibers, this thermally reactive "cold" application results in the generation of a quasi-fused to a fully-fused stratum comprised of about 70%

generated non-combustible materials of a stable state in the fire environment..

In the utilization of silicate chemistries in the prior art of adhesives for producing corrugated paperboard and laminated material, the silicate material was subjected to temperature of 340° - 350°F during setting, curing, and drying. Such temperatures reduce the silicates to the role of common non-combustible non-reacting "fillers" within the cellulosic web which have no generative thermal resistance that can inhibit the vapor generation in the solid phase since the NaSi0 ₂ is crystallized, thus eliminating the ability to intumesce and eliminating the bound water.

The corrugated paperboard product of the invention due to the chemical treatment of the invention applied during a "cold corrugating" process, possesses extraordinary fire resistance. This fire resistance is manifested by a significantly impeded ability to ignite, burn and spread flames. Moreover, due to this fire resistance, it becomes possible to retain structural integrity during exposure to fire. The chemical treatment is applied during the special "cold corrugating" process with the result that a thermally activated fire-resisting mechanism is initiated at a temperature substantially lower than that at which pyrolytic degradation occurs in.the cellulosic fibers. The application of the chemical treatment results in the surface of the fibers being coated. The thermally triggered chemical reaction acts both to impede external input to the fibers and to reduce the generation rate of combustible gases. When exposed to a fire environment, the degree of fire resistance is a function of the quantity of chemical applied relative to the fiber basis weight.

During early flammability tests of both non-treated paper" stock and paper stock treated in accordance with the invention, the low combustibility characteristics of the treated material were demonstrated, i.e., approximately a 25% reduction in heat release as compared to non-treated stock. As shown in FIG. 12, the heat release rate history of the paper treated in accordance with the invention and identified as "X DEC 69#" (69 lbs/1000 sq/ ft of liner board) can be seen to be substantially lower than non-treated papers ^~" identified as 69# liner and 26# medium materials. The rapid heat release rate of untreated (26 lbs. 1000 sq. ft. of medium) paper used as medium material had the effect of increasing the heat release rate of untreated corrugated paperboard identified as "69/26/69".

The constructions of corrugated paperboard boxes are shown in Table I below for the boxes whose test results are ^' shown in FIGS. 14-21.

_CΛBLE_I

BXPERIMENTΛL CONSTRUCTIONS QF CORRUGATED BOΛRP Paper Weight Silicate Application

Commerical Commerical Outside Inside Inside Inside To

Box Corrugated Saturating Liner- of Laminating Front Back Laminating of Total Ba

ISLumbej: Board stock Board M_____i_jm Total — Box IΔXSX Liner .. Laxer Box Silicate We

10 C 174 — —— — 174 —— —- —— -— — —— 0

10 — 64 — 34 98 / — / / — / 42

11 174 — — — 174 / — — — — — 6

15

12 174 — — — 174 / — — — — / 20

13 — 132 — 34 166 / / / / / / / / 40

20 14 — 132 — 34 166 / / / / / / / — 35

15 — — 138 34 172 / — / / / / — — 32

16 — — 138 34 172 / — / / / / — / 37

25

17 174 — — — 174 / — — — — — 11

18 174 — — — 174 / — — — — — 45

«

30 19 174 — — — 174 / — — — — — 13

35 ² 62% solids (lbs/1000 sq. ft.)

During the determination of the critical energy for ignition of the paper stocks chemically treated in accordance with the invention, it was found that initial momentary ignition of the treated paper stock was

2 equivalent to that of non-treated stock, i.e. 10 KW/M ; however subsequent to ignition, the treated papers demonstrated reduced flammability. This is demonstrated by comparing the corresponding curves representing treated stock relative to 69# and 29# standard (untreated) stock in FIG. 16. It is thus indicated that a higher level of external heat is required for treated stock to generate combustion energy levels equivalent to those of untreated stock.

FIG. 17 illustrates the relationship between heat release of various small scale box specimens as a function of time. The box specimens include standard (conventional) corrugated paperboard- and corrugated paperboard specially treated in accordance with the invention. A description of the various boxes is shown in Table I and detailed results of the flammability tests are shown in Tables II and III below.

TABLE II

2 A 4" x 4" x 4" corrugated paperboard box was exposed to 40 KW/m external heat flux in normal air and natural flow conditions. The sample was ignited at the bottom with a small cotton soaked with methanol.

Sample Box Average Maximum Values or Box No. Generation Actual Heat CO Hydrocarbon

Rate of Release Generation Generation Opti Fuel Vapors Rate Rate Rate Dens fg/s) (KW) (9/s) (g/s) _ϋn

Ordinary Market Std, 0.63 6.34 0.00064 0.0 Corrugated box, 69/26/69

DEC Std. Corrugated 0.60 5.20 0.0014 0.00006 0.2 box, 69/26/69 (impregnated laminated)

DEC Interior Coated 0.56 5.33 0.00064 0.4 Corrugated box, 69/26/69 (impregnated laminated)

DEC Interior and 0.47 3.80 0.0030 0.00027 0. Exterior Coated

Corrugated box, 69/26/69

(impregnated laminated)

C 0.79 8.00 0.0010 — 0.0

10 0.37 4.00 0.0030 — 0.0

11 0.72 6.20 0.0010 — 0. 12 0.64 5.50 0.0010 — 0.

13 0.45 3.40 0.0040 — 0.

14 0.51 4.20 0.0030 — 0.

15 0.57 3.90 0.0040 — 0.

16 0.48 4.20 0.0030 — 0. 17 . 0.68 5.10 0.0020 — 0.

18 0.37 3.0 0.0050 — 0.

-36-

19 0.55 4.50 0.0030

TABLE III

A 4" x 4" x 4" corrugated paperboard box was exposed to

2 40 KW/m external heat flux in normal air and natural flow conditions. The sample was ignited at the bottom with a small cotton soaked with methanol.

Est. Total Integrated Values Over The Length of the Equipment

Sample Sil. Comb. Mat. Rat. of Box Λppli Cons. Remd. Energy Energy Hydro¬ or (% of (% of. Release co Rel. to carbon Optical 5 Box . % of t. To . Tot. (act. (at Gen. % Mat. Gen. Density No. SiO„ SiO„ Ht_J Wt.) KJ) 25Q ^C fg) Cons. fg) E£f. (m ~s ) .Ratio

Ordinary 70 0 99 01 765 760 0. 9870 7 .73 0 .556 1 .0 55 4.40 Market 10 Std.

Corrugated box,

69/26/69

15 DEC Std. 26.5 34 78 22 486 450 4. 9408 6 .23 2 .6726 , 67 140 2.92

Corrugated box, 69/26/69

(impregnated laminated) 20

DEC 26.5 43 73 27 485 430 3.8525 6 .64 1 .035 .67 213 2. 92

Interior

Coated

Corrugated 25 box, 69/26/69

(impregnated laminated)

DEC 42.5 66 60 40 305 200 4.280 5 .08 1 .60 '.42 234 1.84 30 Interior and Exterior

Coated

Corrugated box, 69/26/69 35 (impregnated laminated)

Est. Total Integrated Values Over The Length of the Experiment

Sample Sil . Comb. Mat. Rat. of '

Box APPl __ Cons. Remd. Energy Energy Hydro¬ or (% of (% of. Release CO Rel. to carbon Optical

Box % of Wt. Tot. Tot. (act. (at Gen. % Mat. Gen. Density

No. Si0 ₂ !sio ₂ Wt.) Wt.) KJ) 250 (fgg). C Coonnss.. ( (ga)l E E£fff.. ( (mm ss)) Ratic

C 0 0 96 4 745 0.541 7.76 0.014 .91 13 4.26

10 30 42 70 30 385 0.986 5.50 0.173 .91 13 3.93

5 11 3.3 6 93 7 835 1.193 8.98 0.020 1.10 16 4.80

I

12 10 20 898 11 685 1.135 7.70 0.044 .91 22 3.94 <_ VO

I

13 19 40 78 22 608 1.829 7.79 0.192 .84 29 2.66

1U

14 17 35 79 21 650 1.860 8.28 0.178 .91 21 3.92

15 16 32 79 21 625 1.741 7.91 0.116 .84 19 3.63

15 16 15 37 75 25 580 2.400 7.23 0.269 .78 28 3.37

17 6 11 89 11 578 0.886 6.49 0.054 .96 17 3.32

18 21 45 78 22 520 0.933 6.67 0.091 .67 29 2.99

20

19 7 13 87 13 656 0.943 7.54 0.067 .86 2 3.77

T e data included in the Tables have been plotted to show' ^" t e relationship between combustion properties and silicate loading in FIGS. 18 through 24. A summary of the test results from the test series and conclusions related thereto are as follows:

SUMMARY OF TEST RESULTS

As shown in FIG. 18, the percentage of paper stock consumed during burning is a direct function of the quantity of silicate material applied to it, irrespective of where it was applied. Based on this observation, the quantity of silicate applied to DEC BDB samples is estimated to be 34, 43 and 66 lbs/1000 sq ft. There was no significant difference in performance between laminated liner board and commercial liner board. The quantity'of silicate dominated.

As shown in FIGS. 19 and 20, fuel vapor generation rate, heat release rate, total heat release, heat of combustion, and combustion efficiency were all strong functions of silicate loading. Therefore fire performance was dominated almost exclusively by silicate loading.

As shown in FIG. 21, applying a minimum of 6 percent silicate material to the exterior box surface improved fire performance (decreased combustion efficiency) appreciably. The "best" performance of box No. 10 is attributed to the highest percentage silicate loading and the least amount of paper stock available for burning.

As shown in FIGS. 21 and 22, box No. 10 exhibited the poorest performance in terms of combustion efficiency and heat of combustion, both of which are related.

FIGS. 21, 22, 23 show that DEC BDB paperboard material wittvinterior and exterior coatings performed best of all tests exclusive of fuel vapor generation rate and peak heat release rate. Its performance appears to be attributed to the highest loading of silicate material. In all tests, silicate material was applied at least to the exterior liner boards.

The overall conclusion is that silicate quantity controls fire performance almost exclusively. On the basis of the foregoing, the percent loading of silicate material represents the variable quantity which results in significant reduction of flammability. The test data suggests a scheme for the application of silicate material as shown in FIG. 22.

The sodium silicate solution to be used in accordance with the invention to serve both as a fire-resistant agent and a cold adhesive for fire-resistant material or as the adhesive alone in a "cold corrugating" process is chosen to have a combination of factors which include:

(a) a predetermined weight or aspect ratio;

(b) a predetermined molecular structure of rings or clusters of rings;

(c) a predetermined viscosity which enables the adhesive to set quickly without the application of externally applied heat;

(d) a predetermined dry solids content which contains sufficient molecularly bound water to enable intumescence;

(e) a predetermined sufficiently low percentage of free water to minimize moisture removal; and

(f) a predetermined relatively high pH which facilitates interaction of the silicate adhesive with the material being laminated.

In addition to these parameters, the predetermined sodium silicate material may be provided with additives to enhance certain desired characteristics of the final product.

The aspect ratio is utilized to control both the attached water molecule characteristics of the material and the dry solids yield. Attached water molecules control the injecting rate of water vapor into the external boundary layer when exposed to fire and thereby remove some of the incident heat from the surface of the paperboard container.

Viscosity is- an ^' important factor because it affects both the ^' rate at which the chemical contacts the fibrous substrate, i.e. the rate of encapsulation of external fibers, and the drying time of the manufacturing process.

The dry solids content is an essential controlling factor since following the release of water vapor, the remaining solids at least partially in response to gasses released by the heat-decomposing cellulosic fibers react to form an intumescent heat barrier. The intumescent heat barrier provides a reradiating surface layer, and a stable state glass-like layer which retard the thermal input into the combustible fibers and impedes the rate at which the decomposing celluslosic fibers release combustible gases.

Two conditions which are critical to the invention include the requirement of processing at low temperature and the reduction of free-water in the predetermined

sodium silicate compound. The low temperature (i.e. 157 F~maximum) used in the process is necessary to manufacture a corrugated product which contains latent, thermally activated, fire-resisting chemistry. The reduction of free-water in the predetermined silicate material compound reduces the need for heat during manufacturing to dry the product, thereby ridding the product of degrading effects, e.g. warping of the final product and possible release of bond water from the predetermined sodium silicate material. Both low temperature processing and low free-water chemistry act to effect substantial economies in production of the fire-resistant paperboard of the invention.

In accordance with the "cold corrugating" process of the invention, it is preferred to maintain a ratio of dry solids content with weakly bonded water molecules representing an addition of 9% to 18% to the gross molecular weight in a volume having 53% to 62% of the adhesive compound, thereby limiting the thermal requirement ordinarily associated with the manufacturing process for corrugated paperboard. The preferred practice in accordance with the invention is for the free water content to not exceed 30% to 40.%. (See Table IV).

Table IV

THE THREE MAIN GROUPS OF SODIUM SILICATES

Ratio Na 0: SiO Viscosity Dry Solids at

CPS ambient conditions

Group I

1.58 1815 47.2% 1.87 1815 48.0%

Group II

2. 40 760 47.3%

52.51% with 18% bonded water

48.51% with 9% bonded water

2.58 630 44.5% 2.84 690 43.1% 2.54 63 27.5%

Group III 3 .22 385 29 . 3%

3 .22 206 38 . 3%

3 .85 206 32 . 5%

3 . 30 303 37.9%

One method and apparatus for producing fire-retardant corrugated paperboard materials in accordance with the invention is shown in FIGS. 5 through 8. Referring particularly to FIG. 5, a web 100 of medium material from which the corrugated medium is formed is carried from a supply roll 102 to a corrugating machine having a guide roller 104, tensioning rollers 106 and 108 and a conditioning roller 110 for conditioning the web preparatory to corrugating. The web is then passed adjacent guide rollers 112, over tensioning roller 116 and 118, to adjacent guide roller 114. Thereafter the web is advanced between the corrugating rollers 120 and 122. Corrugating rollers 120 and 122 have intermeshing ribs or teeth which form flutes 100a and 100b (FIG. 4) in the web as it passes through the nip of the rollers. The first corrugating roller 120 may be heated, e.g. by means of steam, to a temperature, e.g., in the range of 125°F. to 150°F, to facilitate formation of the flutes in the web. The second fluting roller 122 may be at ambient temperature or may be cooled, e.g. by circulating water.

Located in association with the second corrugating

roller 122 is an applicator mechanism indicated generally at 124 by which the predetermined sodium silicate adhesive materials 127 of the invention is applied, for example by means of a glue roll applicator 125, to the crests or tips of the flutes of corrugated web 100. The adhesive material on the tips of the flutes subsequently binds the corrugated web to one side of liner board sheet 126 thereof.

In order to control the viscosity of the predetermined sodium silicate material, the temperature of the silicate material is controlled by means (not shown) to a temperature in the range of about 55° to about 85 F and preferably in the range of about 60 F to about 70 F which ranges are below its crystallization temperature (157 F) . Again, however, a higher range of temperatures can be used, especially under high ambient temperature conditions.

Liner 126 which is also in the form of a wound web or roll 128 is carried over a guide roller 130 and tension rollers 132 and 134 and through an adhesive application system indicated generally at 136 where the predetermined sodium silicate adhesive material 128 of the invention is applied to the underside of the liner 126. The sodium silicate application system 136 can be of a conventional construction known in the art and may comprise a gravure roller 138. It is to be understood, however, that sodium silicate application system 136 may comprise other types of applicators which are consistent with the handling of sodium silicate. For example, a trough applicator, a wave applicator, a foam applicator or the like can be used.

The sodium silicate coated liner board 126 coated with sodium silicate 128 then is passed through an ambient air circulator shroud 140 which causes the sodium

silicate coating to be dried to tack. The liner 126 is then -passed under pressure guide roller 142 which may be water' cooled. The pressure roller urges the liner into binding contact with the adhesive 127 on the crests of the corrugated medium. In this way a single face corrugated material 129 is formed which comprises corrugated web 100 and liner 126 laminated thereto which is then passed over guide rollers 144 and in between the nip of belts 146 and 147. The single face material is then advanced to a bridge conveyor 148 which serves as a buffer for differences between the operational speed of the single facer apparatus 101 just described and a subsequent double backer apparatus 201 (FIG. 5).

The resulting single face material 129 is then passed beneath a guide roller 200 and over adhesive applicator roll 212 of applicator .202 wherein ^' a film of predetermined sodium silicate material 131 is applied to the exposed crests 100b of the flutes. The coated single face material 129 comprising liner 126 and corrugated medium 100 is then withdrawn from applicator 202 and passed under a guide roller 204 between opposed guide rollers 208, 210 for application of the coated corrugated medium 100 of the single face material to a second coated liner 216. Liner 216 is advanced from a supply roll 218 and over guide rollers 220, 232, and 234 to a coater/pressure impregnator station 236 where the predetermined sodium silicate material 237 is applied to the upper side 238 of liner 216 as viewed in FIG. 5.

At this point, the liner 216 is integrally bound to the single face material 129 by passing between the nip of rollers and belts 240 and 242, thereby forming a single wall corrugated paperboard product. Subsequently, if desired, an additional liner 217 may be laminated to at least one of liners 216 and 126 ^" (FIG. 1), thereby providing a single wall corrugated product 243 with

multiple liners coated with sodium silicate. The corrugated medium 100 may also be a multiple layer stock. As shown in FIG. 7, the use of multiple layer stock with multiple sodium silicate layers increases the fire resistance as well as the strength of the finished product.

Again, the paper material can be impregnated with sodium silicate material to achieve the same effect as that of laminating. The apparatus of the Mersa patent referred to above can be used for impregnation. The degree of fire resistance achieved in the invention depends primarily upon the amount of predetermined sodium silicate material applied to the combustible carrier or substrate material; however it is not necessary for successful practice of the invention for the sodium silicate material to penetrate beneath the surface of the carrier or substrate and to impregnate the carrier- of substrate material. Thus the relative high viscosity of the predetermined sodium silicate material prevents substantial penetration or impregnation of the sodium silicate into the carrier or substrate materials.

Referring in particular to FIGS. 6 and 8, there is shown an embodiment of a coater/pressure impregnator station particularly adapted for coating the predetermined sodium silicate material onto the surface of a carrier or substrate material in accordance with the invention. The coater/pressure impregnator station 236 comprises a liquid distribution system having a liquid reservoir tank in the form of an elongated housing or enclosure 238 having a closed top 240, closed side and end walls 242 and 244, and a bottom wall 246 having an elongated slot 248. An elongated valve assembly 250 is mounted on the bottom of tank 238 in fluid communication therewith for controlling withdrawal of liquid from the tank. Elongated valve assembly 250 comprises an elongated

valve housing 252 having elongated slot 254 formed in the Housing top and bottom walls, only one of which 256 is shown. An elongated valve core 258 having a plurality of passages 272 therethrough is mounted in the elongated valve housing 252 in a pair of bushings 260 and 262 which are contained within the elongated housing. One end 264 of the valve core extends through an orifice 266 in the valve side wall 268 and is fixed to a handle member 270. Thus, the valve core 258 is mounted for rotation within the valve housing 252 and may be adjusted by rotating handle member 270.

Mounted immediately below and in fluid communication with the valve assembly 250 is a Meyer rod 280 and a Meyer rod frame assembly 282. Meyer rod frame assembly member 282 is mounted on a pair of spring assemblies 284. A pressure pad 286 mounted on a base plate 288 is disposed immediately below Meyer rod 282. Base plate 288 is fixedly mounted in a stationary position by means not shown. Completing the coater/i pregnator station are a pair of pneumatic control cylinders 290 and controls (not shown) for selectively positioning the Meyer rod frame assembly 282 vertically and the attached Meyer rod 280 relative to the pressure pad 286. In operation the amount of predetermined sodium silicate material applied to the carrier or substrate can be adjusted by controlling the flow of sodium silicate through the valve and by adjusting the pressure of Meyer rod 280 relative to pressure pad 286.

While the above described coater/pressure impregnator station 236 is particularly useful for applying (and impregnating), a substantially uniform coating of sodium silicate onto the upper side surface of a moving web (i.e. the second liner board) in accordance with the present invention, it will be understood that other means may be employed for applying the sodium silicate

material to the upper surface of the second liner board. For Example, by suitably positioning the second liner supply roll, the liner may be withdrawn from the roll, coated with sodium silicate on its lower surface, e.g. by passing through an applicator containing sodium silicate and then passing over a reversing roller for lamination with the single face material. As used herein the terms "adhere" and "adhesion" can be understood to mean "adhesion and/or lamination." Alternatively, the sodium silicate adhesive may be applied to the upper surface of the second liner by means of a conventional knife coater or the like.

The resulting single wall corrugated paperboard material may then be treated additionally by application of sodium silicate to one or both exterior surfaces if increased fire resistance is desired. The resulting corrugated product may then be processed.into finished products such as boxes.

Other structural designs of laminated paper products made in accordance with the present ^' invention are shown in FIGS. 9-14. Referring particularly to FIG. 9, there is shown a corrugated laminate structure consisting of a double external liner assembly 300 and double internal liner assembly 302 consisting of first and second external liners 304 and 306 and first and second internal liners 308 and 310 which are laminated together by means of sodium silicate 312 in accordance with the invention. The double external liner assembly 300 and double internal liner assembly 302 in turn are adhesively laminated to a corrugated medium 314 using sodium silicate as above described. It can be seen that the double external liner assembly and double internal liner assembly shown in FIG. 9 provide increased structural strength and increased fire resistance due to the additional loading of sodium silicate which is

available for water vapor generation and intumescence.

FIG. 10 shows yet another laminated corrugated structure made in accordance with the invention. The construction of FIG. 10 is similar to that shown in FIG. 9, except that in FIG. 10, the corrugated medium comprises two corrugated elements 316, 318, laminated to one another using sodium silicate 320. The corrugated structure shown in FIG. 7 provides additional structural strength and fire resistance.

Yet another form of corrugated structure is shown in FIG. 11. The structure of FIG. 11 is similar to the structure shown in FIG. 10 except that in the structure of FIG. 11, a third liner 322 is adhesively affixed to one side of the structure by means of sodium silicate 323. The resulting corrugated structure provides yet additional structural strength and fire resistance.

FIG. 12 shows yet another embodiment of corrugated structure made in accordance with the present invention. The corrugated structure of FIG. 12 comprises a single corrugated medium 324 which is coated on both sides with a coating 325 of sodium silicate in accordance with the invention. The single corrugated medium 324 is adhesively fixed to a single internal liner board 326 and a single external liner board 328. As before, the adhesive comprises the predetermined sodium silicate of the invention. The corrugated structure as just described has adequate structural strength and moderate fire resistance for many applications. Optionally, however, in order to increase fire resistance, an additional layer or coating 330 of sodium silicate may be applied to the exterior surface of liner board 328, and coating 330 in turn may be covered by a Kraft sheet or other decorative sheet 332 which is adhesively bonded to the structure by means of the sodium silicate layer

-51- 330 .

Yet another embodiment of the invention is shown in FIG. 13. The basic corrugated structure comprises interior and exterior liner boards 334 and 336, respectively, adhesively bound as before by sodium silicate 337 to a fluted medium 338. A graphite felt layer 340 is laminated to the outer surface of exterior liner board 336 by means of a sodium silicate layer 345, and a Kraft paper sheet or other decorative covering 342 is adhesively affixed to the graphite felt layer 340 by means of a sodium silicate layer 345. The resulting corrugated structure may then be employed to fabricate boxes or other containers, the graphite felt layer of the resulting boxes providing electrical shielding protection to the contents of the box.

While sodium silicate above described may be employed as the ^' combination adhesive and fire and/or heat barrier in accordance with the present invention, certain compatible inorganic materials may be added to the sodium silicate to further enhance handling characteristics of the sodium silicate, and/or mechanical properties and/or fire and heat resistance of the resulting product. The additives should be soluble in, immiscible with, or suspended in the sodium silicate solution. The additives should also be non-reactive with sodium silicate, or, if reactive with sodium silicate, the resulting reaction product(s) should be intumescent. A typical additive which satisfies the aforesaid criteria is fumed silica. The addition of fumed silica to the sodium silicate increases the crystallization temperature of the sodium silicate, and the fire resistance (combustion temperature) of products produced therefrom.

Other inorganic salts and oxides, such as ferric oxide.

titanium oxide, aluminum trihydrate, sodium aluminum sulfo'silicate, antimony trioxide and antimony pentoxide, mica, a carbon material such as carbon black or graphite and mixtures of one or more of the foregoing which are given as exemplary, satisfy some or all of the aforesaid criteria and are useful in accordance with the present invention. Ferric oxide can provide a degree of water resistance. Titanium oxide and aluminum trihydrate enchance the fire-retardant properties. Carbon black or graphite in the adhesive can provide a level of electrical conductivity which can minimize or control electrostatic discharges within a container or structure of board material. Application of minute quantities of diethylene glycol in solution to standard Kraft paper stock also increases the penetration rate of the sodium silicate into the paper stock and may advantageously modify the evolution rate of water vapor from the sodium silicate.

Ferric oxide also may be used in the sodium silicate material to give some degree of water resistance; however the addition of ferric oxide may reduce the bond strength of the adhesive. Compatible surfactants also may be advantageously employed in the practice of the present invention.

Other compatible materials which may be advantageously employed in combination with the sodium silicate readily may be identified by one skilled in the art following the aforesaid teachings of the present invention.

The use of sodium silicate as an adhesive for corrugated structures in accordance with the present invention offers many advantages over the prior art. The manufacturing process is safe since the sodium silicate is non-flammable. In addition the process -at least substantially reduces the energy requirements for the

heating, curing and drying steps for the adhesive as in the case of prior art systems using conventional aqueous-based starch (Stein-Hall) adhesives or conventional silicate adhesives. Furthermore, the invention may be practiced with only relatively low cost modification of existing paperboard manufacturing equipment. Typically, the cost of modifying existing manufacturing equipment will be recovered in energy savings in a short time period. Moreover, certain heating rollers, heating plates, etc. used for manufacturing corrugated structures with conventional Stein-Hall adhesives or conventional silicate adhesives, i ^" .e. in accordance with the prior art, may be omitted, thereby resulting in an overall reduction of the size of the plant and reduced energy requirements of the plant. Also, the manufacturing procedure in accordance with the invention is simplified as compared with conventional processes, through-put may be increased, and curing time may ^" be shortened as compared with conventional processes. Additionally, many of the product defects inherent in using heating plates and heating rollers and "hot sections", may be eliminated. Thus, the defect rate may be lower and the process yield higher when using the process of the invention.

Moreover, corrugated structures made in accordance with the invention are fire-resistance in a heat or flame environment of up to about 40 KW/M _2< Laminated products of the present invention may have increased structural strength and flexibility over corrugated structures made from similar paper stocks or conventional Stein-hall adhesives and conventional silicate adhesives. Moreover, fire resistance may not be adversely affected by flexure of the products of the present ^' invention. Here it is noted that although spreads of the predetermined sodium silicate material heavier than approximately twenty-four pounds per thousand square

feet of laminated board may be used to increase the level- of fire resistance, such a heavy spread can result in board material having a weight and brittleness which can make scoring, cutting and handling of the board material more difficult.

Certain changes will be obvious to one skilled in the art, any may be made in the above disclosure without departing from the scope of the invention here involved. As shown in FIG. 14, where electrical shielding is desired, a layer of electrically conducted material can be utilized. For example, a graphite felt layer 360 may be adhesively affixed by a sodium silicate layer 362 to a paper board stock 364 and the graphite felt layer 360 in turn covered with a paper fascia layer 366 adhesively bound by a sodium silicate layer 364. The resulting laminate may be formed into a cylindrically shaped- electrically shielded container ^' 368 for protecting objects such as computer tapes or the like.

Another advantage of the "cold corrugating" process and resulting corrugated board material of the invention as well as the fire-resistant material of the invention is that the use of the predetermined sodium silicate material makes the corrugated board material or the fire-resistant material capable of being recycled by hydropulping in the conventional manner. Thus, the predetermined sodium silicate material does not interfere with recycling, even when the recycling is carried out at relatively low temperatures (75°F-85 F, for example) .

While the sodium silicate has been illustrated as being applied to cellulose carriers or substrates, it should be noted that substrates formed of other man-made or natural materials such as textiles, e.g., nylon or cotton fibers or cloth may be made fire resistant by

application of a coating of the predetermined sodium silic ^' ate material. The predetermined sodium silicate material may also be applied to the surfaces of particle board material or synthetic polymer products to render them fire resistant. The sodium silicate material can be applied to a receiving surface in a continuous form such as a continuous film. For example, a blade application can produce a continuous and consistent film. In the alternative, the sodium silicate need not be applied as a continuous coating, but may be applied, for example, in an interrupted or discontinuous form such as a striped pattern or as individual dots. In such case, the stripes or dots should be sufficiently closely spaced in order that the coverage of the intumescent layer being formed upon exposure of the coated article to heat or fire is sufficent to provide the desired fire resistance. Further, if desired, a non-combustible element such as a layer of sodium silicate material may be positioned between a combustible substrate and a non-combustible material such as aluminum foil to provide heat reflection.

It is intended that all matter contained in the above description or shown in the accompanying drawings should be interpreted in an illustrative and not in a limiting sense.