WO1990013708A1

WO1990013708A1 - Thermally resistant container and the material for making it

Info

Publication number: WO1990013708A1
Application number: PCT/US1990/001858
Authority: WO
Inventors: Donald D. Halabisky; David G. Marsh; Dwight A. Ii Dudley
Original assignee: Weyerhaeuser Company
Priority date: 1989-05-02
Filing date: 1990-04-05
Publication date: 1990-11-15
Also published as: AU5668490A

Abstract

An insulating board (110) having at least one ply (113) containing an expanded light weight resilient particulate filler material (115) evenly dispersed throughout its structure can be manufactured by forming that ply from a slurry having 2 to 5 percent pulp fibers based on the total weight of the slurry. The filler material is dispersed throughout the slurry for that ply prior to forming the slurry into a web for that ply. It can be formed with additional plies of standard normal density fiber material. The board can be formed into cups (150), plates (170) or containers (160) for holding food or beverages.

Description

THERMALLY RESISTANT CONTAINER AND THE MATERIAL FOR MAKING IT

BACKGROUND OF THE INVENTION

Disposable containers presently used to retain heat or cold in the contained product have a number of problems. This can be seen in two examples, disposable cups for hot or cold beverages and containers used to contain fast foods. The two principal problems are that they do not perform adequately and there may be environmental problems. Many communities have or are attempting to stop the use of these containers.

Disposable cups for hot and cold beverages are presently made either of standard paperboard or of polystyrene. Usually, different materials are used for hot and for cold beverages. This increases the cup inventory and the cost for a vendor.

While paperboard makes a good disposable cup for cold beverages it does not make a good or adequate disposable cup for hot beverages. The heat transfer through a paperboard disposable cup is rapid and a cup containing a hot beverage is hot to the touch. It is difficult to hold. People attempt to overcome this problem by using a nest of. two or three cups to reduce the heat transfer to the outer surface. This increases the cost to the vendor because several cups are used for a single beverage.

A discussion of thermal conductivity in paper and paperboard may be found in Kartovaara "Conduction of Heat in Paper", Paper Making Raw Materials, Transactions of the 8th Fundamental Research Symposium, Oxford,

England, September 1985, V.l, Mechanical Engineering Publications Ltd,

London, England.

Handles have been added to paperboard cups to allow something other than the outside of the cup to be held. These cups are expensive because of the additional material required for the handle and additional steps required to add the handle to the cup. These handles are not easy to use. They are usually flimsy and inconvenient. The handle is difficult to hold. People tend to forget the handle and attempt to grasp the outside of the cup. Again, the outside of the cup is hot if it contains a hot beverage. Again, several cups are used to keep the outside of the cup tolerable to the touch. Polystyrene cups have been used for hot beverages. These cups also have a number of problems associated with them. They are not considered to be biodegradable. If they are biodegradable it is in terms of centuries, not years. They have static build-up and tend to stick together. It is difficult to obtain a single cup for use, either from a vending machine or from a stack at a coffee urn. This is the reason that most vending machines use paperboard cups. The static build-up also attracts dirt to the cup, making it unsanitary and unattractive. Polystyrene cups are thicker than paperboard cups and there are fewer polystyrene cups in a nested stack in comparison to a nested stack of paperboard cups. This is another reason that paperboard cups are usually found in vending machines. Polystyrene cups are not stiff, they are wobbly. This limits the size of a polystyrene cup. Polystyrene cups develop leaks on re-use. There can be holes in the walls of the cups.

Paperboard and foam plastic polystyrene are also used for food containers in fast food emporiums. A hamburger will exemplify the various foods that can be housed in these containers.

A paperboard container has a number of problems. Moisture emanates from the hamburger, the bun, and the condiments contained within the hamburger. This moisture is trapped within the container when the hamburger goes directly from the grill to the container and the container lid is closed. There is only a slight thermal gradient in the wall of the container because the insulating value of the paperboard wall is insignificant. For all practical purposes, the inside wall of the container is at the ambient temperature of the room which is colder than the hamburger. The moisture from the hamburger condenses on the colder inside wall of the container. The moisture enters the wall of the container and causes the container wall to become damp. The stiffness of the container wall drops dramatically, sometimes to 25 percent of its original value. The warmth of the hamburger dissipates. The grease from the hamburger also penetrates the paperboard.

This creates an unsightly container, and the grease may even transfer to surrounding surfaces.

One solution has been to coat the interior wall of the paperboard container with polyethylene. The polyethylene adds little additional insulating value to the container wall however. Moisture from the hamburger will still condense on the inner wall of the container. The moisture will not enter the wall because of the polyethylene. Instead, the condensation will flow down the inner container walls onto the bottom wall of the container and re-enter the bun, creating a soggy bun. This is not what the customer had in mind. The good feature is that grease will not enter the container wall and transfer to other surfaces.

Some of these problems have been overcome by the provision of a container made out of foamed plastic, usually a polystyrene. The container wall does insulate and there is reduced condensation of moisture. Grease does not penetrate the container wall. The material, however, is not biodegradable and presents a solid waste problem.

Another paperboard article used with hot food is the paper plate. The paper plate is poorly designed for the three functions required of it.

The paper plate should hold the food placed on it without buckling or bending. Most paper plates have a basis weight of around 138 pounds per ream. They tend to bend under the weight of the food placed upon them. This problem has been countered by using a heavier basis weight material. The better grade of paper plate now found in stores has a basis weight of 187 pounds per ream. It does not bend or collapse under the weight of the food. It is more expensive. The plate should insulate the hand or lap of the user from the heat of the food on the plate. Neither basis weight of paper plate does this. Heat transfer is the same in the paperboard plate as it is in the paperboard cup. The paperboard material has little insulation value. Heat is transferred quickly from the food on the plate to the surface on which the plate is sitting. This is often the hand or the lap of an individual. The typical solution is to use several plates. Again, this is an expensive solution.

The plate should not transfer grease or moisture to the surface on which the plate is sitting. A paperboard plate will absorb both grease and moisture. The moisture absorbed into the plate will reduce will reduce its strength and make it more susceptible to folding or buckling and more difficult to hold. The paperboard will also absorb the grease and can transfer the grease onto the surface on which the paperboard is sitting. One method of reducing this absorption is to coat the upper face of the plate with some material which will block the moisture or grease from entering the paperboard. This adds little to the insulative properties of the paperboard plate but does add to the cost of the paperboard plate.

A representation of an apparatus for forming paperboard is shown in Figure 1. Figure 1 is a simplified diagram and discloses a mixing section 10, a forming section 20, a pressing section 40, a first drying section 50, a coating section 60, and second drying section 70 and a reel section 80. It should be understood that there are many types of equipment which can be used for each of these sections. A number of different types of apparatus for each of these sections are shown in Britt: Handbook of Pulp and Paper Technology, Second Edition, 1970, Van Nostrand Reinhold Company.

Pulp fibers are mixed with water and other chemicals in a stock or machine chest 11 of mixing section 10 a form a uniform pulp slurry. The slurry is carried from the stock chest 11 through approach pipes 12 to the headbox 21 of the forming section 20. In the forming section 20, the headbox 21 delivers the slurry to a fourdrinier or other type of wire 22. The headbox 21 forms the slurry into a web on the wire 22. Various elements 23, including suction boxes, beneath the wire 22 drain water from the web. The tension on the wire may be varied by the rolls 24.

The web passes from the forming section 20 to the press section 40. In the press section 40 additional water is forced from the web. The web passes between the press roll 41, and the back-up rolls 42 and felt 43. The web then passes from the press section 40 to the first dryer section 50 in which the web passes around a series of heated dryer cylinders 51. The web is held between upper and lower felts 52 and 53 which adsorb the water driven from the web by heat.

In the coating section 60 coating materials may be placed on the web. The web goes into the second dryer section 70 in which the web is further heated by heated dryer cylinders 71 and the water driven from the web is adsorbed by the felts 72 and 73.

The web is then wound into a roll 81 in reel section 80.

It can be seen that paper formation is actually a continuous removal of water from the web in order to form the sheet or board. The water content is reduced from 95 to 99.8 percent in the slurry to 4 to 7 percent in the resulting sheet or board at roll 81. The strength of the web increases as the water is removed from the web. The strength of the web is dependent on different mechanisms as the water is removed from the sheet. The first phase of water removal occurs between the headbox 21 and the dry line on the forming section 20. A cross-sectional view of the web

90 in this phase is shown in Figure 2. It is taken at 2-2 on Figure 1. The view is transverse to the movement of the web. The majority of the fibers 91 will be oriented substantially in the direction of travel of the web though they will not be parallel to the direction of travel. There is no strength in the sheet during this phase because the fibers 91 are separated by the water 92 and the surface tension of the water, and are not attached to one another.

At the dry line in the forming section 20, enough water has been removed from the web 90 so that air can enter the web. As a result, capillary forces are set up that draw the fibers together. These capillary forces, along with fiber to fiber friction where the fibers are in contact, give the wet web a minimum amount of strength so that it can be transferred to the press section 40.

In the press section 40 water is pressed out of sheet. Consequently, after the press section more air is introduced into the web.

Consequently, the capillaries are smaller and more numerous and the strength increases appreciably. There are, however, no strong hydrogen bonds between the fibers. Right after the press section 40, the fibers can slide by one another when the web is subjected to strain because the fibers are only held together by friction and capillary forces. Figure 3 illustrates the fiber to fiber bond after the pressing section. The two fiber sections 94 have a capillary water bond 95 holding them together. The force of the capillary bond is

F = 2rl

F = capillary force r = the radius of the miscum

1 = the length of the bond E the force on a fiber is greater than the capillary bond then either the capillary bond will move along the fibers as the fibers are moved or the bond will be broken completely if the fibers are moved too great a distance from each other.

As the sheet moves through the dryer section 50, more water is removed by heat and vaporization. The capillaries become smaller and smaller and consequently the radius of the musci becomes smaller also. Since the capillary force is inversely proportional to the misci radii, the forces, known as Campbell's forces, become appreciable and can cause the fiber cells to collapse. When this occurs, the fibers which were tube shaped originally become collapsed tubes or ribbons. As a result, the flattened portion of the fibers has more contact with other portions of flattened fibers than the original uncollapsed fiber tubes. This greatly increases the fiber area in contact with other fibers, the bond area, and increases the strength of the sheet appreciably. Since the water capillaries generally are formed where fibers cross, these areas of the fiber are more likely to collapse and result in increased bond area.

This is illustrated in Figures 4 and 5. Figure 4 shows two crossed fiber tubes 96 and the capillary 97 between them. Figure 5 shows the same two fiber tubes 96 after the capillary force has collapsed them. In both the lumen 98 has collapsed. The entire fiber tube does not collapse. Only the portion of each fiber tube at the capillary collapses into a ribbon. The fiber remains a tube on each side of the collapsed section. Toward the end of dryer section 70, that bound water adsorbed on the fiber is removed and stronger hydrogen bonds begin to form. These are much higher in strength than the capillary bonds and the bonds become more rigid. The force required to move two fibers with respect to each other at a hydrogen bond is much greater than at a capillary bond. Water is required to form a bond between fibers in the cellulose fiber web because the fibers must be attached by a capillary bond before they can be attached by a hydrogen bond. The hydrogen bond is formed during the last stages of water removal. If the hydrogen bond is broken by a force greater than the bond force, the bond cannot reform because the amount of water on the surface of the fiber is not enough to form a capillary and draw the fibers together again. Any movement of the fibers which breaks the hydrogen bonds weakens the web because these bonds cannot reform.

Not all of the bonds are formed simultaneously. The bonds are formed as the water is removed from the web. That portion of the web closest to the dryer cylinder surface will lose water first. Therefore, hydrogen bonds will form on the surface first. As the sheet advances through the dryer section, the center of the sheet will dry and bonds will form there.

Figure 6 illustrates the increase in web strength, in this case web tensile strength, with the increase in solids in the web or the decrease of water in the web. The points on the curve represent different points within the paper machine and paper making process. The point A is at formation; the point B is at air intrusion into the mat; the point C is at the first strength inflection after the wet press and the point D is at the second strength inflection at hydrogen bond formation. There may be a number of variations in the paper or board making process. The paper and paperboard headbox standardly operates at consistencies under 1%. Corrugating medium has been manufactured at a headbox consistency between 1 and 1.5% using a standard headbox. Consistency is the weight of fiber in the slurry expressed as a percent of the total weight of the fiber and water. Lately there have been some paper formations using high consistency slurries. These are slurries above 2% consistency. The upper limit is usually around 5% consistency. Special headboxes are required for high consistency slurries. Typical high consistency headboxes are shown in U.S. Patents 3,565,758, 3,846,230, 3,887,428 and 4,021,296. High consistency and low consistency formation may be used on the same machine.

The web may be single ply or multi-ply. Single-ply web formation is the formation of a single layer paper web or fiber sheet using a single headbox. Multi-ply webs may be formed by the use of several headboxes, one for each ply, or a multi-opening headbox which forms several plies simultaneously.

Apparatus for the formation of single ply web was illustrated in Figure 1.

Apparatus for the formation of webs having two through five plies are shown in Figures 7 - 10. The apparatus disclosed using a separate headbox for each ply. Two ply formation is shown in Figure 7. A two ply web or sheet is formed. The first is formed by the headbox 21' in the forming section 20'. The second ply is formed by the headbox 25 near the center of screen travel of the wire 22'. The first ply is partially dewatered on the wire and the second ply is placed on top of the first ply and the two plies are dewatered together over the remainder of the wire 22'. The dewatering is by elements 23' which include vacuum boxes.

Three ply formation is shown in Figure 8. The headbox 21" places the first ply on the wire 22". The first ply is partially dewatered on the wire 22" by elements 23" which includes vacuum boxes. A second headbox 26 places the third ply on the upper wire 27. This ply is dewatered by the elements 28, including vacuum boxes, beneath the upper wire 27. A third headbox 29 places the second, central, ply onto the third ply on the upper wire 27. The two plies pass through the twin wire dewatering section 30 in which wires press against both sides of the two ply web. The two ply web is then transferred onto the first ply on the wire 22" and the three plies are dewatered over the remainder of wire 22" by elements 23".

Four ply formation is shown in Figure 9. The headbox 21'" places the first ply on the wire 22'". The first ply is partially dewatered on the wire 22'" by elements 23'" which includes vacuum boxes. A second headbox 31 places a second ply on the first ply about halfway down the wire 22'". A third headbox 26' places the fourth ply on the upper wire 27'. This ply is dewatered by the elements 28', including vacuum boxes, beneath the upper wire 27'. A fourth headbox 29' places the third ply onto the fourth ply on the upper wire 27'. The two plies pass through the twin wire dewatering section 30' in which wires press against both sides of the two ply web. The two ply web is then transferred onto the first and second plies on the wire 22" and the four plies are dewatered over the remainder of wire 22'" by elements 23'".

Five ply formation is shown in Figure 10. The headbox 21"" places the first ply on the wire 22"". The first ply is partially dewatered on the wire 22"" by elements 23"" which includes vacuum boxes. A second headbox 26" places the third ply on the upper wire 27". This ply is dewatered by the elements 28", including vacuum boxes, beneath the upper wire 27". A third headbox 29" places the second ply onto the third ply on the upper wire 27". The two plies pass through the twin wire dewatering section 30" in which wires press against both sides of the two ply web. The two ply web is then transferred onto the first ply on the wire 22"". The headbox 32 places the fifth ply on the second upper wire 33. The fifth ply is partially dewatered by the elements 34, including vacuum boxes, under the wire 33. A fourth ply is placed on the fifth ply by headbox 35. The two plies pass through the twin wire dewatering section 36 in which the two plies are dewatered. The two plies are then placed on the first three plies on the wire 22"". The five plies are dewatered over the remainder of the wire 22"" by elements 23"".

Paper mats which contain bulking agents are disclosed in several patents. U.S. Patent 3,293,114 describes a paper containing gas filled microspheres. It states that an object is to provide an improved paper which has a low thermal conductivity per unit weight.

The insulating value of a material is measured by thermal conductance. Thermal conductance is the measure of the quantity of heat passing through a material. In paperboard or the filled paperboard of the present application it is a function of the materials thermal conductivity and its thickness or caliper. The formula for determining thermal conductance is:

C = k/d

C = thermal conductance k = thermal conductivity of the material d = thickness or caliper of the material

Thermal conductance is a measure of the quantity of heat passing through the material. The higher the thermal conductance the greater the quantity of heat passing through the material; the lower the thermal conductance the smaller the quantity of heat passing through the material. Thermal conductance could also be said to be a measure of the resistance to heat passing through the material or a measure of its insulating value. The lower the thermal conductance of a material, the greater its insulating value.

U.S. Patent 3,556,934, the next patent in the series, states that there is a problem. The addition of the gas filled microspheres creates a non- uniformity in the paper or paperboard. They deposit adjacent the top side of the paper or paperboard. In other words, the microspheres float and create a two sided paper.

U.S. Patent 3,884,685 describes a paper which includes hollow glass, ceramic or metal microspheres as a low density bulking agent. It states that the bulking agent accumulates on the surface of one side of the paper creating a high coefficient of friction on that side. The paper is mechanically treated to remove or break the protruding bulking material on that side of the paper. This same problem is mentioned in Baumeister: The application of microspheres for the production of high bulk paper, Das Papier, Vol. 26, No. 10a, 716-720 (1972).

It appears that the expanded bulking material floats to the top of the fiber-water slurry in the early stages of formation and remains there. The bulking material is not uniformly dispersed throughout the slurry or the resulting web. It is on one side of the slurry and remains there in the web. Other problems mentioned in the patent literature in addition to the poor dispersion of the microspheres within the sheet are the picking of the material from the sheet on the dryer cylinders, the adherence of the material to the dryer cylinders which lowers the heat transfer ability of the dryer cylinders, and the poor appearance and quality of the paper formed from the web because of the bubbles or foam particles on the surface of the paper sheet. The bulking material has a tendency to peel from the paperboard in the finished board. Figure 11 illustrates a 6 ply board 100 in which the four middle plies 102 - 105 incorporate expanded microspheres 107 within the fibers 108. The two sidedness can be seen. The microspheres 107 are on one side of each of the plies. The board will split along the line between the microspheres 107 and the adjoining ply. The two outer plies 101 and 106 only have fibers 108. A number of patents describe a method of overcoming these problems through the use of unexpanded microspheres which are then expanded during the heating of the web by the dryer cylinders. This is shown by line A in Figure 6. The unexpanded microspheres will not float to one side of the furnish during the forming process and will then be expanded to their normal size in the dryer section after the sheet is formed. This process is described in U.S. Patents 3,556,934, 3,941,634, 4,133,688, and 4,619,734. Both the patents and the article about the unexpanded microspheres emphasize that the microspheres must be expanded before the bonds in the fiber matrix are formed or the microspheres will not expand to their full size. This is not easy. The art states that the dryer section must be modified because it is necessary to have water in the web in order to expand the microspheres.

The microspheres are treated in order to get them to expand at a lower temperature. However, bonds will form as heat is progressing toward the microspheres. It is difficult to completely change the drying characteristics of the dryer section. The articles appear to provide a hoped-for result in obtaining fully expanded microspheres but the reality is that the microspheres will not reach their full size if the hydrogen bonds are formed. This reduces the bulking capability of the microspheres and the insulation value of the full sheet.

The expanding microspheres rupture hydrogen bonds, decreasing the strength of the paper or paperboard. The description of Figure 2 of U.S. Patent 4,619,734 describes how the expansion of the beads moves the fibers apart. BRIEF SUMMARY OF THE INVENTION

The inventors believed that there was a need for a paperboard cup or food container that was cool enough to hold when it contains a hot beverage or hot food. They believed that there was a need for a paperboard cup or food container that would insulate and keep the contained beverage or food warm or cold. Other uses for such a container would be to contain ice cream or ice cream novelties. They also wanted to provide a paper plate in which the bottom of the plate would remain cool. They also wanted to provide paperboard cups and containers and paper plates which would have structural integrity.

They made a number of measurements and found that the insulating value of the paperboard was dependent upon the density of the paperboard. The lower the density of the paperboard the greater the insulating value of the paperboard. It was found necessary to make a low density paperboard in order to make a cup or food container that would have insulating values.

One approach that was tried was placing low density material within the paperboard. However the standard methods of placing low density material within paperboard were not effective. If unexpanded particles were expanded within the paperboard during drying then two problems could occur. The material would not expand to its full size because it was constrained by the bonded fibers, reducing the insulating value of the board. When the particle did expand then it might rupture the fiber bonds, creating a board that would not have the structural integrity required. The use of expanded particle created other problems.

Expanded particles would float to the top of the sheet creating problems within the paper machine and creating a two-sided board. The board has a tendency to delaminate.

The concept of using expanded or expandable fillers in paper and paperboard was good. The problem was incorporating these fillers in a way that would increase the bulk but not create a two-sided board. A different way of making a low density paperboard was needed.

The inventors determined that there was a different way of making each ply of the paperboard. The low density particles should be fully expanded to provide the low density paperboard ply. The low density particles would be placed throughout the paperboard ply and there would be no layering of material.

Fully expanded low density particles of material are placed in and mixed with a high consistency pulp slurry prior to the headbox. The consistency of the paperboard is from 2% to 5%. The low density particles are substantially uniformly distributed throughout the slurry. The greater number of fibers in the high consistency pulp slurry traps the particles and prevents them from floating to the top of the slurry and thereby separating from the fibers. The particles remain uniformly distributed throughout the web as it is formed on the fourdrinier wire. The fiber to fiber bonds formed in the web remain because the fibers are not forced apart in the dryer section because the fibers are formed around the pre-expanded particles and bonded together around the pre-expanded particles. A low density material having a strong fiber to fiber bond is formed. A multi-ply material may also be formed. Plies of high consistency formed paper or paperboard would alternate with layers of the low density paperboard. A three ply material in which the center ply is the lower density material would provide a strong structure at reduced weight. In the multi-ply construction the outer ply is a barrier between the dryer cylinders and the ply containing the light weight material.

It is possible to create a board having the desired properties because the expansion of the light weight particles is not dependent upon the ability to expand the particles in the dryer section.

The low density material has excellent thermal resistance properties. The maintenance of its strong fiber to fiber bond allows it to be used for containing materials. It makes an excellent container in which the contained product is a food or beverage that is to be kept hot or cold. These could be beverage cups or food containers of various types.

The container would not have the condensation of the present paperboard containers. The outside of the container would be cool and easily held.

A cup made of insulative paperboard would allow the vendor to use one type of cup for both hot and cold beverages and to reduce the different types of cups to keep in inventory. A plate made from the material would have excellent strength and thermal properties.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic view of a paper machine. Figure 2 is a cross-sectional view of a web taken along 2-2 of

Figure 1.

Figure 3 is an illustration of a fiber to fiber bond after the wet pressing section.

Figures 4 and 5 illustrate different stages in the bonding action of the cellulose fibers.

Figure 6 is a graph showing the increase in strength as the water is removed from the web.

Figures 7 - 10 are diagrammatic drawings showing the typical apparatus for the formation of paper or paperboard webs having from one to five plies.

Figure 11 discloses a prior art 6 ply construction in which the microspheres create a two sided board.

Figure 12 - 16 are cross sections of paper or fiber webs having from one to five plies respectively showing the dispersion of Ught weight filler particles. Figure 17 is a diagram of the relationship of sheet density and the percent by weight of light-weight filler particles in single-ply web containing the filler particles and three-ply web in which the center ply contains the light weight filler particles. Figure 18 is a diagram of the relationship of thermal conductivity and sheet density in single-ply and three-ply sheets.

Figure 19 is a diagram of the relationship of conductance and the percent by weight of a light-weight filler particles in a three ply sheet.

Figure 20 is a diagram of the relationship of sheet bulk and the percent of microspheres in a paper or paperboard, comparing expanded and originally unexpanded microspheres.

Figure 21 is a diagram of the relationship of thermal conductivity and sheet density of a paper or paperboard, comparing expanded and originally unexpanded microspheres. Figure 22 is an isometric view of a cup using the new board.

Figure 23 is a cross-section along line 22 - 22 of Figure 22.

Figure 24 is an isometric view of a food box using the new board.

Figure 25 is a cross-section along line 25 - 25 of Figure 24.

Figure 26 is an isometric view of a plate using the new board. Figure 27 is a cross-section along line 27 - 27 of Figure 26.

DETAILED DESCRIPTION

A single ply paper or fiber web or sheet containing the light weight filler particles may be formed using high consistency formation. The furnish is a slurry of pulp fibers and water. There may be other additives in addition to the light-weight filler particles. The pulp fibers form from between

2.0 to 5.0 percent of the total weight of the fiber-water slurry.

The light weight filler particles may be introduced either into the fiber-water slurry in the stock chest 11 or into the approach piping 12 to one or more of the headboxes 21, 25, 26, 29 32, or 35 depending on the number of plies and which plies are to contain the lightweight material. The approach piping is between the stock chest or machine chest and the high consistency headbox. The mixture is delivered to the high consistency headbox. The amount of light-weight filler particles in the fiber-water slurry can be from 0.1 to 20 percent by weight of the weight of the fibers in the slurry.

The light weight filler particles must have attained their maximum size before being introduced into the fiber-water slurry so that they will provide adequate bulking and insulation value. The fiber matrix forming the fiber web must remain intact and be undisrupted after formation if the material is to be used for beverage containers or other containers. It must retain its stiffness so that it can be used as a container. If the light weight filler particles expand within the fiber matrix then there is a possibility the matrix will be disrupted and the web will lose its stiffness. It will have the attributes of a tissue and not the desired attributes of a container.

The light-weight filler particles may be closed cell expanded or hollow microspheres or closed cell expanded or hollow irregular shaped particles. A particle must be capable of regaining its original size and shape after being collapsed under pressure in the press of a paper machine. Each of the closed cell filler particles will fill the volume or space it occupied in the fiber matrix prior to pressing.

The nominal diameter of the particles is the diameter if the particle were spherical. The fully expanded particles will range in size from a nominal diameter of 2 microns to a nominal diameter of 500 microns. The most desirable size is from a nominal diameter of 10 microns to a nominal diameter of 150 microns. The actual size will depend on the thickness of the web. The diameter should not be greater than the thickness of the web, and, in fact, should be less. The true density of a light weight filler is the actual density of a particle and excludes the air between particles. The true densities of the light weight fillers are from less than 10 kilograms per cubic meter to up to 500 kilograms per cubic meter. The preferred ranges are from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

A light weight filler particle typically will be an organic polymer which has been foamed by any standard commercial process. Exemplary of the organic polymers are polystyrene or polyvinylidenechloride. The blowing agents could be liquid, such as pentane or hexane, or solid, such as azodicarbonamide.

The high consistency slurry prevents the light-weight filler particles from floating to the top of the fiber-water slurry. The larger number of fibers within a given volume of slurry traps the particles within and throughout the slurry and keeps the particles distributed throughout the slurry.

The mixture of slurry and light-weight filler particles are then deposited on the fourdrinier wire. Again the greater number of fibers within a given volume of slurry keeps the low-density filler particles distributed throughout the fiber matrix formed on the fourdrinier wire. Water is removed from the slurry on the wire. The web is then pressed to remove additional water. The web is then heated and dried in the dryer section. The light¬ weight filler particles were fully expanded when they were added. There is no additional substantial expansion in the dryer section. The fiber to fiber bonds formed in the fourdrinier section are strengthened in the press and dryer section.

Multi-ply constructions may be used. The apparatus is the same as that shown in Figures 1 and 7 - 10 with the exception that a high consistency headbox is required for the high consistency material. In each of the following processes light-weight filler particles are added to the slurry for the high consistency formation using the high consistency headbox. The particles were added to the slurry before the headbox. The amount of light-weight filler particles being added to the slurry for the high consistency formation in each of the following processes is between 0.1 and 20 percent by weight of the weight of the fiber in the high consistency layer. The weight of the fiber material is between 2 and 5% of the weight of the fiber - water mixture and will be considered a consistency of 2 - 5%. The process after the forming section is the same as in Figure 1. In each of the following descriptions, the plies made of standard paperboard, not the low density paperboard, do not contain light weight particles. The apparatus for the formation of a single ply sheet or web is shown in Figure 1 except that headbox 21 is a high consistency headbox. The sheet formed by this process is shown in cross-section in Figure 12. The sheet 100 comprises fibers 101 bonded together at bonds 102. The expanded light¬ weight particles 103 are evenly dispersed throughout the fiber matrix. Again the apparatus for two ply formation is shown in Figure 7.

One layer is formed by a low consistency formation process; the other layer is formed by a high consistency formation process. The light weight filler particles are added to the slurry for the high consistency formation. The low consistency headbox 21' is at the head of the fourdrinier wire 22' and the high consistency headbox 25 is at the center of the fourdrinier wire 22'. The low consistency material is placed on the wire 22' and is partially dewatered and the high consistency material is placed on top of it and the two materials are dewatered together over the remainder of the wire. The two ply web is shown in Figure 13. The web 110 is a two-ply laminate of a standard paperboard web 111 containing only fibers 112 and a low density paperboard web 113 containing fibers 114 and light weight expanded particles 115 evenly dispersed throughout the web 113.

The apparatus for three ply formation is shown in Figure 8. The middle or second ply is formed by a high consistency formation process and the two outer plies, the first and third, are formed by a low consistency formation process. The light weight filler particles are added to slurry forming the middle layer.

The three ply web is shown in Figure 14. The web 120 is a laminate of central low density ply 121 and two outer standard paperboard plies 122. The outer paperboard webs 122 are composed of fibers 123 and the central low density paperboard web 121 is composed of fibers 124 and light weight expanded particles 125 evenly dispersed throughout the fiber matrix of the center ply.

The apparatus for four ply formation is shown in Figure 9. The two middle plies are formed by the high consistency formation process and the two outer plies are formed by the low consistency formation process. The light weight filler particles are added to the slurry for the high consistency formations. The four-ply web is shown in cross-section in Figure 15. The web 130 has two outer plies 131 of standard paperboard and two inner plies 132 of low density paperboard. The two outer phes 131 contain fibers 133 and the two inner plies 132 contain fibers 134 and light-weight particles 135 evenly dispersed throughout the fiber matrix.

The apparatus for five ply formation is shown in Figure 10. The middle and two outer phes are formed by the low consistency formation process and the two intermediate phes are formed by the high consistency formation process. The light weight filler particles are added to the slurry for the high consistency formations. A cross-section of a five ply web is shown in Figure 16. The web 140 has two outer standard paperboard plies 141 and a central standard paperboard ply 142. Each of these plies contains only fibers 143. A low density paperboard ply 144 is between each of the outer paperboard plies 141 and the central paperboard ply 142. Each of the plies 144 contains fibers 145 and light weight expanded particles 146 evenly dispersed throughout the fiber matrix.

The use of plies that do not contain light-weight filler particles has an added advantage in the dryer section. The non-filled ply would be placed against the dryer cylinders and assure that no filler particles would contact the dryer cylinders and remain on the dryer cylinders.

The paperboard product that is formed has a basis weight that can be from 100 pounds per ream (3000 square feet) to 300 pounds per ream. The actual basis weight of the material will depend upon the product to be formed by the material.

It has been found that the paperboard material containing the light-weight filler particles has good insulating properties. The addition of light weight filler particles to a fiber web will reduce the density of the fiber web. The density of the fiber web itself will depend upon the type of fiber in the web, the amount the fiber has been refined, and the pressing and drying conditions. The amount the density will be reduced by the light weight filler particles will depend in part on the type of filler material.

The relationship between web densities and the amount of light weight filler particles added to the fiber web was measured. Figure 17 shows this relationship. The amount of light-weight filler particles added is stated as a percent by weight of the total weight of the web. Typical filler materials were used for these measurements. As can be seen, the density of the filled fiber web that were measured in these experiments decreased rapidly until about 7 percent by weight, based on the total weight of the web, of light weight filler particles has been added. The density of the filled fiber web then levels off. In a single ply sheet the density at 7 percent by weight of light-weight filler particles is between one-half and one-third of the unfilled sheet.

The relationship between board densities and thermal conductivity was also measured. Figure 18 shows this relationship. It shows that there is a straight line relationship between board density and thermal conductivity and that the thermal conductivity decreases with a decrease in density.

The relationship between conductance and amount of light weight filler was also determined. This relationship is shown in Figure 19. It can be seen that the addition of light-weight filler particles greatly reduced the conductance of the three ply material. The addition of 4 percent by weight, based on the total weight of the web, of light-weight filler particles decreased the conductance of the three-ply sheet from 170 to 60.

There are two methods of lowering the thermal conductance of a material. The thermal conductivity of the material can be lowered or the thickness of the material can be increased. The addition of light weight filler particles to a fiber web can reduce both the density and the thermal conductivity of the fiber web. The addition of the light-weight filler particles will reduce the density of the fiber web and this in turn will reduce the thermal conductivity of the fiber web. The reduction of thermal conductivity is shown in Figures 17 and 18.

The addition of light-weight filler particles can also increase the thickness or caliper of the board. Either reducing the thermal conductivity of the fiber web or increasing the thickness of the fiber web will increase the insulation value of the fiber web by reducing the thermal conductance of the web. The reduction of conductance is shown in Figure 19 for a given basis weight.

Figures 20 and 21 illustrate the difference in properties when of pre-expanded or unexpanded microspheres are used in the web. Figure 20 compares sheet bulk and the weight of microspheres in the web as a percent of the total weight of the web. Figure 21 compares the thermal conductivity and sheet density.

The trials were conducted on a Noble & Wood pilot machine using both pre-expanded and unexpanded microspheres. The pilot machine has a single-nip press, a dryer section and a recirculating white water system. Press conditions and dryer drum temperatures were maintained for all trial conditions. The sheet basis weight was 156 grams/square meter. The pre-expanded microspheres were Expancel grade 551 DE and the unexpanded microspheres were Expancel grade 820 WU, the papermaking grade. The unexpanded microspheres expand in the dryer section of the paper machine.

The results of the bulk tests are shown in Figure 20. The curves indicate that pre-expanded microspheres develop higher sheet bulk, lower density, at a given microsphere level, and are a more efficient bulking material on a weight basis. The results of the thermal conductivity tests are shown in Figure

21. These tests were run at 75 degrees C. The test indicate that the pre- expanded beads show a greater potential for conductivity reduction.

This material is an insulating paperboard having good stiffness and rigidity. It can be used for a number of new products and as a replacement for products presently on the market. It has a number of attributes that the present products do not have.

Cups made of this insulating paperboard have the attributes of cups made of paperboard but without the liabilities. They also have the insulating attributes of cups made of polystyrene without the liabilities. They are for the most part biodegradable. They have a thinner caliper than expanded polystyrene cups and more cups can fit in the same space. They have less static build-up than expanded polystyrene cups. They can be more easily separated and do not attract dirt. They are usable in vending machines. They have better stiffness than polystyrene cups and can be made in larger sizes. They can be used for multi-cup beverage containers. They do not transfer as much heat to the outer wall of the cup as the paperboard cup so that they can be held in the hand when containing hot beverages. They do not need handles. The cups could have an inner polyethylene lining.

A typical cup 150 is shown in Figures 22 and 23. The cup side wall 151 and base 152 are made of the low density insulating board described above. It would have at least a two ply construction with a standard normal density fiber ply 153 as the outer wall and a low density ply 154 as the inner wall. Light weight resilient paniculate filler material 155 would be evenly dispersed throughout the low density ply. The inner surface of the cup would be coated with layer 156 of polyethylene or other liquid impervious material. Very large cups might include additional layers of low density and normal density plies on the outside surface. It would have a construction similar to that shown in Figure 16 except that a coating layer 156 would be substituted for standard fiber layer 141. A three ply insulating board such as shown in Figure 14 having a coating 156 on its inner side could also be used.

A container, such as a hamburger box, made from the insulating paperboard and having an inner polyethylene lining would have all the attributes of the foamed plastic container and is to a great extent biodegradable. The walls would insulate and condensation would be minimized. There would not be the grease problem of standard paperboard containers. The food would remain warm. It would maintain the food cold if used for ice cream or ice cream confections.

A typical food container 160 is shown in Figures 24 and 25. Again, the side walls 161, cover 162 and base 163 would be of the insulating paperboard. It would be at least three ply as shown in Figure 25. The center ply 164 would be low density paperboard having the light weight resilient particulate filler material 165 evenly dispersed throughout its structure. The two outer phes 166 would be of standard normal density fiber material. The inner face of the container would be coated with a water resistant material 167 such as polyethylene. The insulating board could also be a five ply material such as shown in Figure 16 with the coating material 167 on the inner face of the container. The low density insulating paperboard could also be used for paper plates. It could be coated with polyethylene on the upper surface. It would have insulating properties that present paper plates do not have and would be more comfortable to use with hot foods. It would provide a greater stiffness than present day paper plates. This means that a plate made of the insulating paperboard would have greater stiffness than a paper board plate of the same weight. A paper plate made from an insulating paperboard having the same 138 pound per ream basis weight as the present paper plate would have a greater stiffness than the standard paper plate. It would not bend as easily under the weight of food. It would not create a problem for the user. It would not burn the lap or hand of the user. It appears that a paper plate made from the insulating paperboard which has a basis weight of 138 pound per ream, the same basis weight as the light weight paper plate, would be as stiff as a paper plate made from the paperboard having a basis weight of 187 pounds per ream. In addition it would have more insulative properties than either of the present paper plates.

A paper plate 170 is shown in Figures 25 and 26. The plate 170 is made of the low density insulating paperboard. It would be three ply and the center ply 171 would be low density material having the light weight resilient particulate filler material 172 evenly dispersed throughout its structure. The two outer plies 173 would be standard normal density fiber material. The upper face of the plate would be coated with a water resilient material 174 such as polyethylene. The main purposes of the insulating paperboard are to insulate and to provide a structure that has integrity. In a cup containing a hot beverage the purpose of the insulating paperboard is to maintain the outer wall of the cup at a temperature that is comfortable to the hand and to keep the beverage warm for a longer period of time than would be possible with paperboard alone. In a cup containing a cold beverage the purpose of the insulating paperboard is to maintain the beverage at a cool temperature and to minimize the amount of condensation on the outside of the cup. In a food container the purpose of the insulating paperboard is to keep the food warm and to maintain the inner wall at a high enough temperature so that condensation within the container is minimized. In a plate the purpose of the insulating paperboard is to keep the bottom of the plate at a comfortable temperature and to keep the plate stiff enough for its intended use.

Claims

1. A cup comprising a bottom wall and a side wall extending upwardly from said bottom wall, said side wall wall being formed of two-ply paperboard, the outer ply of said two-ply paperboard consisting essentially of fibers, the inner ply of said two-ply paperboard comprising an undisrupted fiber matrix containing light-weight compressible resilient particulate filler materials evenly dispersed therein, a coating of water impervious material on the inner surface of said cup.

2. The cup of claim 1 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

3. The cup of claim 2 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

4. The cup of claim 1 in which the light weight filler materials are closed cell structures.

5. The cup of claim 4 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

6. The cup of claim 5 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

7. The cup of claim 1 in which the light weight filler materials are present in an amount in the range of 0.1 to 20% of the total weight of the ply.

8. The cup of claim 7 in which the density of the light weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

9. The cup of claim 8 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

10. The cup of claim 1 in which the light weight filler materials have nominal diameters in the range of 2 microns to 500 microns.

11. The cup of claim 10 in which the density of the light weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

12. The cup of claim 11 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

13. The cup of claim 1 in which the coating material is polyethylene.

14. A cup comprising a bottom wall and a side wall extending upwardly from said bottom wall, said side walls being formed of three-ply paperboard, the two outer plies of said three-ply paperboard consisting essentially of fibers, the inner ply of said three-ply paperboard comprising an undisrupted fiber matrix containing light-weight compressible resilient particulate filler materials evenly dispersed therein, a coating of water impervious material on the inner surface of said container.

15. The cup of claim 14 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

16. The cup of claim 15 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

17. The cup of claim 14 in which the light weight filler materials are closed cell structures.

18. The cup of claim 17 in which the density of the Ught weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

19. The cup of claim 18 in which the density of the Ught weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

20. The cup of claim 14 in which the Ught weight filler materials are present in an amount in the range of 0.1 to 20% of the total weight of the ply.

21. The cup of claim 20 in which the density of the Ught weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

22. The cup of claim 21 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

23. The cup of claim 14 in which the Ught weight filler materials have nominal diameters in the range of 2 microns to 500 microns.

24. The cup of claim 23 in which the density of the Ught weight fiUer materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

25. The cup of claim 24 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

26. The cup of claim 14 in which the coating material is polyethylene.

27. A container comprising a bottom waU, side waUs and a cover, said walls and cover being formed of three-ply paperboard, the two outer phes of said three-ply paperboard consisting essentially of fibers, the inner ply of said three-ply paperboard comprising an undisrupted fiber matrix containing Ught-weight compressible resiUent particulate fiUer materials evenly dispersed therein, a coating of water impervious material on the inner surface of said container.

28. The container of claim 27 in which the density of the Ught weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

29. The container of claim 28 in which the density of the Ught weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

30. The container of claim 27 in which the Ught weight filler materials are closed ceU structures.

31 The container of claim 30 in which the density of the Ught weight fiUer material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

32. The container of claim 31 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

33. The container of claim 27 in which the Ught weight fiUer materials are present in an amount in the range of 0.1 to 20% of the total weight of the ply.

34. The container of claim 33 in which the density of the Ught weight fiUer materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

35. The container of claim 34 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

36. The container of claim 27 in which the light weight filler materials have nominal diameters in the range of 2 microns to 500 microns.

37. The contamer of claim 36 in which the density of the Ught weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

38. The container of claim 37 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

39. The cup of claim 27 in which the coating material is polyethylene.

40. A plate comprising a three-ply paperboard, the two outer phes of said three-ply paperboard consisting essentially of fibers, the inner ply of said three-ply paperboard comprising an undisrupted fiber matrix containing Ught-weight compressible resflient particulate filler materials evenly dispersed therein, a coating of water impervious material on the upper surface of said plate.

41. The plate of claim 40 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

42. The plate of claim 41 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

43. The plate of claim 40 in which the light weight filler materials are closed cell structures.

44. The plate of claim 43 in which the density of the Ught weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

45. The plate of claim 44 in which the density of the Ught weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

46. The plate of claim 40 in which the Ught weight fiUer materials are present in an amount in the range of 0.1 to 20% of the total weight of the ply.

47. The plate of claim 46 in which the density of the Ught weight fiUer materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

48. The plate of claim 47 in which the density of the Ught weight fiUer materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

49. The plate of claim 40 in which the Ught weight fiUer materials have nominal diameters in the range of 2 microns to 500 microns.

50. The plate of claim 49 in which the density of the Ught weight fiUer materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

51. The plate of claim 50 in which the density of the Ught weight fiUer materials is from 15 kilograms per cubic meter to 120 Mlograms per cubic meter.

52. The cup of claim 40 in which the coating material is polyethylene.

53. A paperboard comprising a two-ply structure,one ply of said two-ply structure consisting essentiaUy of fibers, the other ply of said two-ply structure comprising an undisrupted fiber matrix containing Ught-weight compressible resilient particulate filler materials evenly dispersed therein, said other ply having a lower density than said one ply, a coating of water impervious material on the outer surface of said other ply.

54. The paperboard of claim 53 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

55. The paperboard of claim 54 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

56. The paperboard of claim 53 in which the light weight filler materials are closed ceU structures.

57. The paperboard of claim 56 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

58. The paperboard of claim 57 in which the density of the light weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

59. The paperboard of claim 53 in which the Ught weight filler materials are present in an amount in the range of 0.1 to 20% of the total weight of the ply.

60. The paperboard of claim 59 in which the density of the light weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

61. The paperboard of claim 60 in which the density of the light weight fiUer materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

62. The paperboard of claim 53 in which the Ught weight filler materials have nominal diameters in the range of 2 microns to 500 microns.

63. The paperboard of claim 62 in which the density of the Ught weight fiUer materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

64. The paperboard of claim 63 in which the density of the Ught weight ffller materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

65. The cup of claim 53 in which the coating material is polyethylene.

66. A paperboard comprising a three-ply structure, the two outer phes of said three-ply structure consisting essentiaUy of fibers, the inner ply of said three-ply structure comprising an undisrupted fiber matrix containing Ught-weight compressible resiUent particulate fiUer materials evenly dispersed therein, a coating of water impervious material on at least one outer surface of said paperboard.

67. The paperboard of claim 66 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

68. The paperboard of claim 67 in which the density of the Ught weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

69. The paperboard of claim 66 in which the Ught weight filler materials are closed ceU structures.

70. The paperboard of claim 69 in which the density of the light weight filler material is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

71. The paperboard of claim 70 in which the density of the Ught weight filler material is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

72. The paperboard of claim 66 in which the Ught weight filler materials are present in an amount in the range of 0.1 to 20% of the total weight of the ply.

73. The paperboard of claim 72 in which the density of the light weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

74. The paperboard of claim 73 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

75. The paperboard of claim 66 in which the light weight filler materials have nominal diameters in the range of 2 microns to 500 microns.

76. The paperboard of claim 75 in which the density of the light weight filler materials is from 5 kilograms per cubic meter to 500 kilograms per cubic meter.

77. The paperboard of claim 76 in which the density of the light weight filler materials is from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

78. The cup of claim 66 in which the coating material is polyethylene.

79. The method of forming a low density paperboard comprising forming a slurry comprising fiber and water, said fiber being present in the amount of 2 to 5 percent of the weight of the total slurry, adding Ught weight resilient particulate filler material to said slurry, thoroughly mixing said filler material with said fibers in said slurry, forming said slurry into a web, drying said slurry to form a fiber matrix having said light weight resilient particulate material evenly dispersed throughout the matrix, said material reducing the density of the web.

80. The method of claim 79 in which said Ught weight resiUent particulate material is added in an amount in the range of 0.1 to 20 percent of the weight of the fibers in the slurry.

81. The method of claim 80 in which the Ught weight resiUent particulate material has a density in the range of 5 kilograms per cubic meter to 500 kilograms per cubic meter.

82. The method of claim 81 in which the Ught weight particulate material has a nominal diameter in the range of from 2 microns to 500 microns.

83. The method of claim 82 in which the Ught weight particulate material has a nominal diameter in the range of from 10 microns to 150 microns.

84. The method of claim 81 in which the Ught weight resiUent particulate material has a density in the range of from 15 kilograms per cubic meter to 120 kilograms per cubic meter.

85. The method of claim 84 in which the Ught weight particulate material has a nominal diameter in the range of from 2 microns to 500 microns.

86. The method of claim 85 in which the Ught weight particulate material has a nominal diameter in the range of from 10 microns to 150 microns.