BINDER

This invention relates to composite boards comprising inorganic and organic materials in which the binder system comprises latex.

Starch is well known in the art as a binder for composite boards such as ceiling tile. However, under certain environmental conditions, such as increased temperature and humidity, the boards lose dimensional stability. Ceiling tile made with starch acting as the binder sags as a result of the loss of dimensional stability when subjected to adverse environmental conditions.

It is desirable for composite boards to have adequate strength to maintain their integrity in normal handling, shipping and installation operations. A convenient measure of this desirable quality is the modulus of rupture. Some binder latexes do not impart the same degree o ^' f strength to the board that starcjn can impart.

U.S. Patent No. 4,587,278 teaches the use of soft latexes with glass transition temperatures from

30°C to 80°C as binders for sound insulating boards.

SUBSTITUTESHEET

U.S. Patent No. 4,863,979 and Reexa ination Certificate Bl 4,863,979, teach the use of latexes having relatively high glass transition temperatures as binders for latexes in composite board. The higher glass transition temperature enables a composite board to retain strength and resist sag in humid environments to a greater extent than relatively soft latexes.

It would be desirable to make a composite board with even greater strength than the prior art composite boards without sacrificing sag resistance.

The present invention relates to a composite board which, based on the dry weight of the board, comprises: from 10 to 95 percent inorganic fiber; from 1 to 25 percent latex binder which contains latex copolymer particles having particle size greater than 1800 Angstroms and a T greater than 82°C; and, optionally, up to about 35 percent organic fibers; and, optionally, up to about 60 percent inorganic filler; and, optionally, up to about 25 percent of a second binder.

The composite board has a modulus of rupture greater than 75 psi as measured by ASTM 367-78 and whereby upon exposing a 1 1/2 x 6 inch strip of the composite board to 90 percent relative humidity at 94°F for 96 hours the composite board sags less than 2 mm.

What is meant -by "dimensional stability" is a lack of movement of the composite board when exposed to

increased temperature and humidity. Dimensional stability can be measured by evaluating sag resistance of the composite board. Sag resistance is determined by exposing a 1| x 6-inch, strip of composite board of the composition as described in Example 1 to 94°F, 90 percent relative humidity for 96 hours, holding 330 g of weight and then measuring the displacement of the center of the board in millimeters (mm). Desirably, the composite board will sag 2 mm, and preferably, the composite board,will sag 1.0 mm or less, more preferably less than 0.8 mm and most preferably less than 0.5 mm.

What is meant by "strength" is that the resulting composite board of the composition has the integrity necessary to withstand handling, processing and other forces that are applied to the composite board. The necessary strength can then be determined by preparing a board by the procedure and of the composition as described in Example 1. The resulting cellulosic composite board has a' modulus of rupture

(MOR) of at least 300 psi as measured by ASTM 367-78.

More preferably, the calculated MOR of the resulting composite board will be at least about 310 psi. The most preferred MOR will be at least about320 psi.

MOR is calculated from the standard 3 point breaking loading test described in ASTM 367-78 as follows:

Modulus of Rupture (MOR) = 3 PL/bd ²

where:

P = peak force required to break the sample (lb)

L = span between the sample supports (in)

b = width of the sample (in)

d = thickness of the sample.

This modulus of rupture is corrected for density variation as shown:

MOR corrected = (MOR) D ²

where D is the density correction

D = desired density/actual density

The glass transition temperature of the copoly er of latex of the present invention is defined functionally such that the T is sufficient when the resulting composite board maintains dimensional stability when exposed to specific environmental conditions and the resulting composite board has a modulus of rupture of at least about 75 psi as measured by ASTM 367-78. The properties of the board are directly correlated to the particle size of the latex and the T of the latex used as the binder in the ceiling board. The preferred T of the latex polymer having a particle size of greater than about 1400 Angstroms preferably greater than 1800 Angstroms, is at least about 82°C. The more preferred range is between 82°C and 120°C. The most preferred range is between 85°C and 120°C.

The physical properties of the latex are correlated to the ceiling board properties on a sliding scale, the relationship can be exemplified as follows: if the T of the latex polymer is above 80°C, preferably around 95°C for a conventionally sized latex copolymer,

that is a latex having a particle size of less than about 1400 Angstroms preferably less than 1800 Angstroms, then the board resists sag and retains adequate strength when compared to starch and some other latexes. If the same sized latex is prepared to have a T less than about 80°C, then the board decreases in sag resistance. However, if the particle size of the latex is greater than about 1400 Angstroms preferably greater than about 1800 Angstroms and the T _g of the polymer is greater than about 80°C, the board strength can be increased without sacrificing sag resistance.

The particle sizes of the present invention are greater than about 1400 Angstroms. The more preferred particle sizes of the _* present latexes are between 1800 and 3200 Angstroms. The most preferred particle size is within the range of from 1800 to 2500 Angstroms.

The latex particle sizes for boards which do not contain cellulosic materials (newsprint for example) have optimum particle sizes of from 1800 Angstroms to 2200 Angstroms. The latex particle sizes for boards which do contain cellulosic materials have optimum particle sizes of from about 1400, preferably 1800 Angstroms to 2200 Angstroms.

The latex copolymer composition of this invention can be prepared by a conventional emulsion polymerization process in aqueous medium with conventional additives. Typically, the aqueous phase will contain from 0.5 to 5 weight percent (based on the monomer charge) of conventional nonionic or anionic emulsifiers, for example, potassium, N-dodecyl sulfonate, sodium isooctobenzene sulfonate, sodium

laureate and nonyl phenol ethers of polyethylene glycols.

Conventional emulsion polymerization catalysts can be employed in the foregoing latex polymerization and common examples thereof include peroxides, persulfates and azo compounds such as sodium persulfate, potassium persulfate, ammonium persulfate, hydrogen peroxide, azodiisobutyric diamide as well as catalysts such as redox catalysts which are activated in the water phase for example, by a water-soluble reducing agent. The type and amount of catalyst, as well as the particular polymerization conditions employed, will typically depend on the other monomers which are used and polymerization conditions will be generally selected to favor the polymerization of such other monomers. Typically such catalysts are employed in a catalytic amount ranging from 0.01 to 5 weight percent based upon the monomer "weight. In general, the polymerization is conducted at a temperature in the range of from -10° to 110°C, preferably from 50° to 90°C.

Similarly, conventional chain transfer agents such as, for example, n-dodecyl mercaptan, bromoform and carbon tetrachloride can also be employed in the normal fashion in polymerization to regulate the molecular weight of the polymer formed therein, and when such chain transfer agents are used, they are employed in amounts ranging from 0.01 to 10, preferably from 0.1 to 5, weight percent based upon the weight of the monomers employed in the polymerization. The amount of chain transfer agent employed depends somewhat on the particular transfer agent employed and the particular monomers being polymerized.

Similarly, conventional crosslinking agents, which can be a di- or tri- or tetra-vinyl compound, can also be employed in the normal fashion in polymerization to regulate the T and the molecular weight of the polymer formed therein. Representative examples of a crosslinking agent are a divinylbenzene, allyl methacrylate _. or a mono-, di-, tri- or tetra- ethylene glycol diacrylate or dimethacrylate. Typically, when such crosslinking agents are used, they are employed in amounts ranging from 0.01 to 4.0, preferably from 0.1 to 1.0, weight percent based upon the weight of the monomers employed in the polymerization. The amount of crosslinking agent employed depends on the monomers being polymerized.

Particularly, the latex composition with a sufficient T can be prepared with a combination of hard monomer and soft monomer. An α,β-ethylenically unsaturated carboxylic acid may also be incorporated.

The term "hard monomer" is meant to include a monomer which homopolymer has a T of at least about 80°C and by the term "soft monomer" is meant a monomer which homopolymer has a T less than about 35°C. Typical hard monomers are those conventionally known in the art, for example styrene and methyl methacrylate. Soft monomers are also conventionally known in the art and can be, for example, butadiene, ethyl acrylate or butyl acrylate.

The α,β-ethylenically unsaturated carboxylic acids include compositions of the formula:

R' RCH-C-COOH

where

R is H and R ¹ is H, C _χ -C ₄ alkyl, or -CH ₂ COOX;

R is -COOX and R' is H or -CH ₂ COOX; or,

R is CH ₃ and R' is H; and

⁵ X is H or C ₁ -C ₄ alkyl.

Suitable α,β-ethylenically unsaturated aliphatic carboxylic acids are monoethylenically unsaturated monocarboxylic, dicarboxylic and tricarboxylic- acids

10 having the ethylenic unsaturation alpha-beta to at least one of the carboxyl grσups and similar monomers having a higher number of carboxyl groups. It is understood that the carboxyl groups may be present in the acid or salt -,,- form, -COOM in which M represents hydrogen or a metal, such as for example, sodium or potassium, and are readily interconvertible by well known simple procedures.

o Specific examples of the α,β-ethylenically unsaturated aliphatic carboxylic acids are acrylic acid, methacrylic acid, fumaric acid, itaconic acid, maleic acid and aconitic acid.

r- The latex composition will typically include a hard monomer content from 50 to 99 weight percent and the soft monomer content will be in an amount from 1 to 50 weight percent. The carboxylic acid level may be from 0 to 20 weight percent. Preferably, the hard

30 monomer is present in an amount from 65 to 95 weight percent, the soft monomer is present in an amount from 5 to 35 weight percent and the carboxylic acid is present in an amount more preferably from 0.5 to 10 weight percent.

A particular copolymer family suitable for the latex of the present invention is a copolymer comprising a monovinylidene monomer and an aliphatic conjugated diene. Normally, an α,β-ethylenically unsaturated carboxylic acid termonomer is incorporated into the polymer.

The term "monovinylidene" monomer is intended to include those monomers wherein a radical of the formula:

R CH ₂ =C-

wherein R is hydrogen or a lower alkyl such as an alkyl having from 1 to 4 carbon atoms which is attached directly to an aromatic nucleus containing from 6 to 10 carbon atoms, including those wherein the aromatic nucleus is substituted with alkyl or halogen substituents. Typical of these monomers are styrene, α- methylstyrene, ortho-, meta- and para- ethylstyrene; ortho-, meta- and paraethylstyrene; o,p-dimethylstyrene; o,p-diethylstyrene; isopropylstyrene; o-methyl-p-isopropylstyrene; p-chlorostyrene; p-bromo-styrene; o,p-dichlorostyrene; o,p-dibromostyrene; vinylnaphthalene; diverse vinyl (alkylnaphthalenes) and vinyl (halonaphthalenes) and comonomeric mixtures thereof.

"Acyclic aliphatic conjugated dienes" usefully employed herein include typically those compounds which have from 4 to 9 carbon atoms, for example, 1,3- butadiene, 2-methyl-l,3-butadiene; 2,3-dimethyl- -1,3-butadiene; pentadiene; 2-neopentyl-l,3-butadiene and other hydrocarbon analogs of 2,3-butadienes, such as

2-chloro-l,3-butadiene; 2-cyano-l,3-butadiene, the substituted straight chain conjugated pentadienes, the straight chain and branched chain conjugated hexadienes, other straight and branched chain conjugated dienes having from 4 to 9 carbon atoms, and comonomeric mixtures thereof. The 1,3-butadiene hydrocarbon monomers such as those mentioned hereinbefore provide interpolymers having particularly desirable properties and are therefore preferred. The cost, ready availability and the excellent properties of interpolymers produced therefrom makes 1,3-butadiene the most preferred acyclic aliphatic conjugated dierte.

Another family of suitable copolymers is a copolymer of a monovinylidene compound as defined above, and an ester of an α,β-ethylenically unsaturated carboxylic acid, as defined below, with a T of less than about 25°C. An α,β-ethylenically unsaturated carboxylic acid termonomer can also be incorporated into the polymer.

Esters of α,β-ethylenically unsaturated carboxylic acid useful herein as soft monomers include typically soft acrylates, for example, those with homopolymers having a of less than about 25°C, such as benzyl acrylate, butyl acrylate, sec-butyl acrylate, cyclohexyl acrylate, dodecyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, ethyl acrylate, 2- ethylbutyl acrylate, 2-ethylhexyl acrylate, heptyl acrylate, hexylacrylate, isobutyl acrylate, isopropyl acrylate, methyl acrylate, propyl acrylate, etc. and soft methacrylates such as butyl methacrylate, and hexyl methacrylate. The cost, availability and known

properties of butyl acrylate and ethyl acrylate make these monomers preferred among the acrylates.

Still a third family of copolymers useful in the latex of the present invention comprises a copolymer of two or more esters of ethylenically unsaturated carboxylic acids, one of which has a homopolymer with a T less than about 25°C and the other of which has a homopolymer with a T of greater than about 80°C, as defined above, such as methyl methacrylate and ethyl acrylate. The copolymer can also include a termonomer of an α,β-ethylenically unsaturated carboxylic acid.

Esters of α,β-ethylenically unsaturated carboxylic acids having a homopolymer which T is greater than about 80°C typically include hard methacrylates such as tert-butyl methacrylate, isopropyl methacrylate and methyl methacrylate. The cost, availability and known properties of methyl methacrylate make it preferred among the methacrylates.

Additional monomers, such as those described above, can be added to any of the families of copolymers described above. Additional monomers which can also be added to any of the families of copolymers are termonomers having homopolymers with an intermediate T of greater than about 25°C but less than 80°C such as: 4-biphenyl acrylate, tert-butyl acrylate, vinyl acetate, sec-butyl methacrylate, cyclohexyl methacrylate, ethyl methacrylate, isobutyl methacrylate, propyl methacrylate, isobutyl methacrylate, n-propyl methacrylate and tert-butyl amino ethyl methacrylate.

The particle size of the latex copolymer can be controlled by the use of an appropriate amount of seed particles according to the following formula:

Parts of seed (based on monomer)=

seed diameter , _{x 100}

Final Latex Diameter,

10 For a further discussion the preparation of latexes of a uniform particle size see Emulsion Polymerization Theory and Practice, by D.C. Blackley, John Wiley & Sons, New York 1975.

15 The composite board of the instant invention can be prepared by any conventional process. Examples are water-felted formed on a Fourdrinier, water-borne (slurried in, water) or cast molded.

20 The inorganic and cellulosic materials of the composite are conventionally known fillers and fibers. For example, mineral wool (inorganic fiber), perlite or clay (inorganic fillers) and cellulose (cellulosic _-. material). Therefore, the boards can be prepared with formulations comprising either cellulosic or non- cellulosic materials, or a mixture thereof.

In a typical formulation, typical amounts of the binder are from 1 to 25 percent by weight; the

30 cellulosic material, from 5 to 50 percent by weight; the inorganic fiber, from 10 to 70 percent by weight; and the inorganic filler, from 5 to 90 percent by weight, based upon the total weight of the composite board.

The binder system is not limited exclusively to synthetic latex, but may include other binder components, such as artificial latexes, thermosetting resins and starch. For some applications, mixtures of various synthetic latexes, artificial latexes and starch are desirable.

Ceiling tile is usually prepared by dispersing cellulosic material and mixing the dispersion with binder material, inorganic fiber and filler material. Flocculant is added and the resultant furnish is poured into a mold, diluted and drained. Alternatively, the flocculant may be mixed with the other components prior to the addition of the binder. Generally, the order of addition does not matter. The resultant wet mat is then pressed and dried.

The invention is further illustrated, but is not limited by the following examples wherein all parts and percentages are by weight unless otherwise specified.

Example 1

A styrene/ethyl acrylate/acrylic acid latex having the same monomeric ratios but various particle sizes, which showed the increase in advantageous properties achieved with the larger particle size latexes, and a T _g of about 95°C was prepared by conventional polymerization methods.

The latex was then incorporated as the binder into composite boards in the following manner:

Printed newsprint was dispersed in water at 2.0 percent solids. A Cowles blade was used with an air

stirrer at high rpm to redisperse the cellulose to a Canadian Standard Freeness of 250 to 300 mis. Water (4,400 g); perlite (72.1 g); mineral wool (30.1 g); and the dispersed newsprint (34.9 g) are mixed for three minutes with moderate agitation using a Cowles blade. Latex (12.8 g of polymer) was added and the slurry was mixed for 30 seconds. Flocculant (cationic polyacrylamide) was added until the latex was- completely flocculated (determined by the point when the water became clear). Flocculation was carried out with less than moderate agitation. The flocculated furnish was poured into a Noble and Wood Sheet mold apparatus and was diluted to approximately 1.0 percent solids. The furnish was dispersed and drained on a retaining wire. The wet mat was pressed to a thickness of 630 mils and dried at 375°°F to 400°F in a forced air oven. The resulting board was approximately 7.5 x 7.5 inches, had a thickness of about 0.7 inches and a density of about 12 lbs/ft ³ .

TABLE I Ceiling Board Evaluation

The table above illustrates the high MOR which starch imparts as a binder to ceiling tile but the starch imparts little or no sag resistance. The ceiling tile made with the latex binder having a particle size less than about 1800 Angstroms shows good sag resistance but a relatively low MOR compared to the larger particle sized latexes.

The latex binders of the instant invention have both excellent strength (high MOR) and retain sag resistance, with a MOR of at least 300 psi and a sag of 1 mm or less.

Example 2

A styrene/ethyl acrylate/acrylic acid latex having the same monomeric ratios but particle sizes of 900 and 2100 Angstroms and a T _g of about 95°C was prepared by conventional polymerization methods. The 900 Angstrom latex was a comparative Example to demonstrate the advantages of the larger particle size latex.

The latex was then incorporated as the binder into ceiling tiles in the following manner:

Water, (4,000 g); perlite, (18.0 g); mineral wool, (100.5 g); (180.00 g) of a 5 percent solution of starch in water by weight and clay (9.0 g) are mixed using a Cowles blade. 30 Grams of latex (about 13.8 g of polymer) was added and the slurry was mixed for 30 seconds. Flocculant (cationic polyacrylamide) was added until the latex was completely flocculated (determined by the point when the water became clear). Flocculation was carried out with less than moderate agitation. The flocculated furnish was poured into a Williams Sheet

mold apparatus and is diluted to approximately 1.0 percent solids. The furnish was dispersed and drained on a retaining wire. The wet mat was pressed to a thickness of 630 mils and dried at 375° to 400°F in a forced air oven. The resulting board was approximately 7.5 x 7.5 inches, had a thickness of about 0.6 inches and a density of approximately 15 lbs/ft ³ .

TABLE II Ceiling Board Evaluation

Example 3

Into a 1-liter glass reactor immersed in a temperature controlled water bath were added 342 g of deionized water, 3.4 g of a 1 percent active aqueous pentasodium diethylenetriamine pentaacetate solution and 2.8 g of a 30.4 percent solids seed latex containing polystyrene polymer particles. The reactor was purged with nitrogen and heated to 90°C. Then over a 3 hour periodwas added a monomer stream containing 272 g of styrene, 20.4 g of acrylic acid and 47.6 g of ethyl acrylate.

Beginning at the start of the monomer addition was also added over a 3 and 1/2 hour period, an aqueous stream containing 71 g of deionized water, 1.7 g of sodium persulfate, 0.34 g sodium hydroxide and 3.8 g of a 45 percent active surfactant solution. Following the

addition of the monomer and aqueous streams, the reactor was maintained at 90°C for 30 minutes, then cooled. The particle size of the latex was 2130 angstroms.

Other latex examples of different particle sizes were prepared by using the same polymerization recipe but changing the amounts of seed latex according to the following formula.

Parts of seed (based on monomer)=

Formulation 1 is for a composite board made with the latex of Example 3, and the data shows the increase in MOR with increasing particle size of the latex copolymer particle.

Formulation 1:

96 hrs/95%/R .H . /95°F

BINDER PARTICLE SIZE MOR (psi) SAG (mm)

3-A 900A 102 1.77

3-B 1850A 101 1.84

3-C 2130A 125 1.01

Formulation 2 is for a composite board with a high content of cellulosic fibers. The daca shows an increase in MOR with increasing particle size of the latex copolymer particles.

Formulation 2:

Newsprint (Wall Street Journal)

Perlite

Mineral Wool

Latex

134 hrs/95°F/98%/R.H.