CA1120223A - Method of densifying open-celled polyurethane material - Google Patents

Abstract

METHOD OF DENSIFYING OPEN-CELLED
POLYURETHANE MATERIAL

ABSTRACT
A method of densifying open-celled polyurethane foam is provided in which heated air at a temperature between about 425°F and 460°F is circulated through a mass of the foam so as to rapidly and uniformly raise the temperature of the mass to between about 365°F and 435°F, the mass is then com-pressed to a desired thickness while its temperature is main-tained between 365°F and 435°F, and the mass is then immediately permitted to cool.

Description

3L~Z~Z~23 METHOD OF DENSIFYING OPEN-CELLED
POLYURETHANE MATERIAL
. _ ~ACXGROUND OF THE II~VENTION
.
1. Field of the Invention This invention relates to methods of densifying foamed plastic material and, more particularly, to a method of densifying a mass of open-celled polyether or polyester polyurethane foam. While the method is advantageous for maker ial of any thickness, it is especially ~aluable in treating ma~erial which is several inches thick.

2, Description of the Prior Art Previous methods of densifying open-celled plastic foam articles have relied on the extended application of hea-t and substantial compressive forces to the surface of the lS article. This is often carried out by means o~ large heat~d metal molds or platens between which the foam is placed. Heat is transmit~ed to the foam by conduction from the heated platens for a substantial period of time. The low K-factors of air ~-and of polyurethane make such heating processes very time-consuming.

A commercially acceptable polyurethane foam requires a density and other physical characteristics which are quite uniform throughout. This can only be achieved by bringiny all parts of a mass of such foam into a predetermined tempera-25` ture range during processing because the matecial will return , -- 1 -- ... ~.. ;

to its pre-treatmen-t state if the treatment ternperatur~ is too low, and this necessarily causes a non--uniform product if some parts of the mass are sufficiently hot during treatment and others are not. Prior art processes inherently tend to produce non-uniform material, and are difficu]t to control because o~ the very low K-factors previously mentioned.
U.S. Patent No. 3,577,519 (Gambardella) describes a method of densifying polyurethane foam buns wherein partially cured foam is compressed while still at or near its exothermal reaction temperature. This process may resull: in non-uniform temperatures and uncontrollable densification~ at least in part, due to the relatively rapid cooling of lhe exterior of the mass and the typically large size of polyurethane reaction buns. This problem is only patially alleviatea by the use of lS heated compression means. Also, the process may not be used to densify fully cured foam. ~' Further, the heating of a mass of foam by conduction, as in the above-described prior processes, is very inefficient, as great iamounts of heat are required to initially heat the compression means, which are often massive, and then to maintain the temperature of the compression means while they are in pro-longed contact with the foam.
Another source of inefficiency in previous compression methods is the inability of those methods to adapt to continuous operating conditions. The nature of the prior processes elim-inates anything but extremely thin material for densification ~ .
on a continuous system. Prior processes were senerally operated on an intermittent basls which ~lould result in p]~ten marks on continuous strjps of foam.
Another disadvanta~e of prlor compression d~nsiEi-cation methods as applied to cured polyurethane is that very high compressive forces are required until the mass reaches the proper temperature. This requires the use o~ massive compressive equipment, the cost of which further detracts from the desirability of prior compression methods.
U.S. Patent No. 3,475,525 (Peters) describes a method of reticulating open-celled polyurethane foam in which a stream of heated gas is passed through the foam in order to heat the foam so as to melt or thermally decompose membran-ous cell walls without melting the network strands supporting the cell walls. The ~as temperature must be above about 480F
15 and is generally between about 750F and 1470F, and the thick-ness of the foam must be less than 0.75 inches to prevent degradation of the network structure. Additionally, the gas velocity through the foam must be in excess of about 3 1/~ -ft./sec.
The method of the present invention preserves the cell integrity, since gas temperature and velocity are substan-tially less than in Peters. Further, the present invention may be effectively applied to foams of thickness much greater than 0.75 inches, the upper limit of the Peters me-thod.

SUMMARY OF T~IE_INVENTION
It is an object of the invention to provide an improved method of densifying open-celled foam and, more ,~, ..

- ~2~3 , specifically, of densifying fully cured open-celled polyure-thane foam.
It is a further object of the invention to provide a method of densifying open-celled polyurethane foam which may be carried out so as to provide an end product of uniform density throughout.
Another object of the invention is to provide a method of densifying fully curecl open-celled polyurethane foam wherein relatively small compressive forces are re~uired to effect densification.
It is a further object of the i~nvention to provide a method of rapidly densifying open-celled polyurethane foam which may be carried out on continuous stripsas well as on separate slabs~
Yet another object of the invenltion is to provide a method of using circulating hot air to rapidly raise the foam into the proper temperature rang~ while so controlling temperature and air velocity as to avoild breaking down the membranous and sXeletal structure of the ~aterial~
Thus broadly, the invention contemplates a method of densifying a mass of fully cured open-celled poly-urethane foam which comprises the consecutive steps of ~;~
passing heated gaseous material entirely through the mass with the temperature of the gaseous material being in the 25 range of about 425F to 460F and the velocity of the gaseous material being insufficient to dest:roy the integrity of the cell walls of the mass, the gaseous material being inert }~, .

~2C~2~;3 with respect to the polyurethane and the period of exposure of the mass to the gaseous material being insufficien to allow charring of the mass yet sufficien.t to raise the entire mass substantially uniformly to a te!mpe.rature between about 365F and 435F, compressing the mass while its temperature is between about 365F and 435F to reduce :: :
its thickness and thus increase its density to a desired degree, and permitting the mass to cool.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a longitudinal central sectional view of an apparatus for practicing the method of the invention;
and, Fig. 2 is a transverse sectional view taken sub-stantially as indicated along, line 2-2 of Fig. 1.

DETAILED DESCRIPTION OF THE INVENTIOIN

Fig. 1 illustrates an apparatus, generally designat-ed 10, to permanently compress and thereby densify a mass 12 of .. .

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fully cured open-celled polyuxethane foam. The apparatus 10 comprises a base (not shown) which includes a planar horizon-tal support tnot shown), an upstanding support means (no-t shown), an endless conveyor means 14, gas he~ting and circulating means 16, and compressing means 18 which form an integral par-t of the heating and circulating means 16.
The purpose of the invention is to densi~y tll~ ma~ss 12 of chemically stable, open-celled polyurethane foam, which is preferably and illustratively a continuous strip 20, but which may be in the form of separate ~labs for batch treatment.
The mass 12 is of any thickness, with about 1/2" to about 7"
being typical, and has a density as low as about 0.~ lb/ft3.
The density of the mass 12 will generally be between 1.3 lb/f~.3 and 2.25 lb/ft3.
The conveyor 14 comprises an endless open mesh belt 22 which travels about a drive arum 24 and a pair of idler drums 26 which carry the belt 22 in the direction oi the arrow ~:
27 through a horizontal working run 28 and a return run 30.
One portion 32 o the horizontal working run 28 traverses~ and is effectively ~nclosed by a ch.amher 3~ oE the gas heating and circulating means 16, said ch.amber 34 comprising an upper portion 36 and a lower portion 38 disposed above and below the horizontal working run 28, respecti.vely.
Referring to Fig. 2, the gas heating and circulating means 16 comprises an endless condult system including both portions of said chamber 34 and a housing 40 containing a blower 42 and heating means 44. The upper portiorl 36 of the .

~j chamber 34 is defined by said portion 32 of the horizont~l working run 28, part of an upper wall 46, an :inner wall 4~, an outer wall 50, and parts of a fron-t wall 5:2 and a rear wall 54, seen in Fig. 1.
The endless conduit system has a hood-like uppex member which includes the upper p~rtion 36 of the ch~mber ~, an upper section 56 of the housing 40, and a trough-like lower member 37 which provides the Lower port:ion 38 of the chamber 34 and constitutes the ~as inlet for the blower 42 The trough-like lower member 37 is defined by said portion 32 of the horizontal run 28, an inner wall 58, an outer wall 60 and an arcuate side and bottom.wall 62 (Fig. 1). The lower section of the housing 40 is formed by a section ~f an inner wall 66, an outer wall 68, a transverse wall 70 extending between the walls 66 and 68, and a transverse wAll (not shown) which is parallel to the wall 70.
The blower 42 is a conventional type powered by a motor 64. In operation, the blower 42 circulates ~as in the direction of the arrows shown in Fig. 2. The heating means . ~;
44 is located above, and thus downs-tream of, the blo~Yer 42 and illustratively comprises a plurality of electric heating elements 72. The upper section 56 of the housing 40 is defined by the upper margin of the heating means 44, the upper part of the inner wall 66, an outer wall.74, part of the upper wall 46, and those parts of the front and rear walls 52 and 54 which extend transversely between the walls 66 and 68.
As noted above, it is necessary to raise the temper-ature of the mass 12 to a uniformly hi~h level in order to effect uniform densification thereof. Therefore, it is nec-2~223 essary that the gas flow velocity be uniform throughout the chamber 34.
In order to ensure uniform distribution of gas flow across the chamber 34, a perforate baffle p~late 75 having a plurality of holes 75a of various sizes is disposed across the chamber 34, substantially perpendicular to the gas flow path, preferably and illustratively in the lower chamber portion 38.
In the absence of a gas distribut.ing means such as the baffle plate 75, gas flow velocities through a given cross-sectional area perpendicular to the gas flow path would tend to be distributed such that ~as flo~in,g near the walls 50, 52 and 54, i.e. relatively far from the blower 42, flows at a lower velocity than gas near the wall 48 and centrally between the walls 52 and 54. This effect i.s especially marked in large ovens. The baffle plate provides variable resistance to gas flow through the chamber 34, with relatively great resistance applied àt those points where gas velocity is highest, by means of relatively small holes, with larger holes at points of lower gas velocity. Such baffle plates are well known in the art.
It is to be noted, however, that even the smallest of the holes 75a are large enough to ensure low total resis- ~:
tance at normal flow rates which are less than 1 ft/sec.
~eferring to Fig. 1, it is to be noted that the horizontal run 28 of the conveyor 14 and the foam mass 12 carried thereon enters the chamber 3~ through an elonga-te slot 76 and that the bottom of the horizontal run 28 contacts a pair of upstanding sealing walls 78, see:n in Fig. 2 The vertical dimension of the openin~ 76 corresponds closely to .. ~ ... -..... ... . ~ : . ~

the vertical dimension of the combination of t:h~ horizon~al run 28 and the mass 12, thereby effecting a seal about khe mass 12 and the working run 28, and external uppe~ ancl lower lipped flanges 80 and 81, respectively, aid in reduci.ng the es.cape of the circulating gas. It is to be und~rs~.oc)cL, however, that the seal is intended only to contain most of t:he circulating gas and not to provide absolute containment.
The compression means 18 adjoins the gas heating and circulating means 16 at the rear wall 54 t:hereof, and encloses a section 82 of the working run 28. As seen in Fig. lt the compression means 18 comprises a ho~ising 84, including a top wall 86, a bottom wall 88, upper and lo~er rear walls 90 and 92, respectively, a frontupc;tanding wall 94 connected to the wall 88, a side wall 96 extending be-tween the rear walls 90 and 92 and the wall 94, and a second side wall (not shown~ disposed opposite the side wall 96. The housing 84 communicates with the chamber 34 through an opening 97 in the wall 54. A deflector plate 98 extends between the walls 48 and 50 of the heating chamber 34 to clirect heated ~ ~.
gas into- the.opening 97.
The housing 84 contains compressing means 99, illus- ;
tratively comprising an opposed pair of driven steel pinch rollers 100. An inclined plate 101 which receives and supports the mass.12 extends between the side~alls of the chamber 84 and terminates adjacent the surface of the lower roller 100 at a point selected such that the mass 12 is aLirected substan-tially centrally bet een the rollers 100 in older to ensure '~
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that equal compressing force is applied to each side of the mass 12, to prevent distortion thereof. It may be un~erstood that other compressin~ means, such as platens, may be used.
Platens are suitable for batchwise compression of foam slabs.
5 The section 82 of the ~or~ing run 28 travel~ over a roller 102 then under a roller 104 disposed below the roller 102, and then over a roller 106 at.the downstream end of the housing 84, whereby the working run 28 is sepa:rated from the strip 20 of polyurethane foam so as to facilitate the simul- :
taneous application of pressure to both sides of said strip.
If desired, the rollers 100 may be coated ~ith a release agent which facilitates separation of the strip 20 from.the rollers 100 after compression of the Eoam. It has - been found, however, that the use of a-release acJent is generally not necessary in the practice of the invention.. The rollers 100 must be maintained at or very near the temperature of the circulating gas and the strip 20 in order to p:revent premature cooling of the surface of the strip 20, which would result in non-uniform densification. Deflection of heated gas from the chamber 34 into the housing 84 by the deflection plate 98 and suitable heating means, such as a pair of high-intensity lamps 107 mounted on the walls 90 ana 92 and focused on the upper and lower rollers 100, respectively, maintain the tem-perature of the rollers 100 and the strip 20 at a sufficiently uniform, high level to result in uniform densification.
The strip 20 and the working run 28 lexit the housing 84, preferably into a zone at ambient temperature, through a -- . .

~2~)%~:3 shallow elongate opening 108 b~tween the walls 90 and 92.
~hile the vertlcal dimension of the opening 108 is not so small as to form an absolute seal about the strip 20 and the working run 28, it is sufficiently small to prevent the loss of any substantial amount of heated gas. l'he slot 76 and the s}lallow opening 108 allow moderate escape of heated gas from the system, and the entry of make-up gas.
In operation, the mass 12 is placed on the conveyor 14 and is carried into the chamber 34 where it is contacted by the circulating heated gas. The gas is commonly and illus-tratively air, but may be another gas or mixt.ure of gases in certain situations. The circulating gas must be chemically inert with respect to the polyurethane, and n.ot otherwise have a tendency to impart undesirable characteristics to the foam.
The circulating air is heated to a temperature between about 425F and 460F by the heating means 44, and is directed into the chamber 34 where it is drawn downwardly through the mass 12 and the working run 28 by the suction produced by the blower 42, which creates a lower pressure in the lower por-tion 38 than in the upper portion 36 of the chamber 34.
The open-celled s-tructure of the foam allows the heated air to intimately contact the entire interior cell struc-ture of the foam, thereby rapidly heating the mas s 12 to a . temperature which is substantially uniform t:hroughout the mass, but which is generally lower than the temperature of the cir~
culating air, e.g. between about 365F and 435F. At gas ~.
velocities less than about 1 ft/sec, uniform heating oE the mass to the desired temperature level is typically accomplished within 10-15 sec.. It is important that the mass 12 does not - :
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- . . . ,. - : , : .
, ~2~23 reach a temperature above approxima-~ely 475~F nor remain at a temperature above about 365~E for periods lonyer than about one minute, as undesirable charring of polyure-thane m~y beyin under such conditions.
After the temperature of the mass 12 is raised to a temperature between 365F and 435F, the mass 12 exits th~
chamber 34 and enters the housing 84 via the opening 97, ~here the mass 12 is maintained at the desired tempe!rature, and is fed between the steel pinch rollers lO0 while separated from the working r~n 28.
The internal resistance of the mass 12 to compressive force is greatly reduced when the temperature of the mass is between about 365F and 435F. In that temperature range, the state of the mass approximates that of its exothermal reaction s-tage, during which it has low resistance to com-pressive force and little resilience. As a result, a per-manently densified strip 20a may be produced by the application of a relatively small compressive orce for a comparatively short time relative to those necessary with previous compression methods. The pressure may be less than 10% of that required by previous methods; and after a brief period of compression, the compressive force may be removed at once without fear of ` subsequent expansion.
Cooling of the mass to substantially below 365F
immediately after compression insures permanence o~ the densi-fication. Cooling occurs very rapidly because of the low specific heat and relatively low density of the mass even after . . . ~.
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densification. This allows hi~her production ra-tes than were possible with older meth~ds of densification. Also, the need for heavy molds or platens is obviated, therehy allowing for decreased capital cost. The rel~tive vcrtical spacin~ of the rollers 100 is adjustable, thereby allowing for different de- ~
sired final thicknesses and, thereore, different final densities of the densified strip 20a. The final aensity of the strip 20a may be up to about 30 lb/ft3.
As an alternative to pinch rollers, two movable platens, or one movable platen and a stationary surface, may be employed as the compressive means 99. If SO employed, a ~irst platen is disposed above the mass 12 with its horizontal surface substantially parallel to the upper surface of the mass, and a second platen (or s-ta-tionary surface) is disposed below the mass 12, with its surface substantially parallel to the surface of the first platen. The upper platen, or both platens, are moved ~ertically toward one another so as to apply compressive forces to the upper and low~er surfaces of the mass.
It may be noted that certain aspects of the operation are different if platens are used as the compressing means, rather than pinch rollers. Operations with platens are nec-essarily performed in a discontinuous manner, as it is neces-sary-~o~ stop the movement of the conveyor when the platens `
are in their closed position around the mass 12. The use of platens is especially suitable for use with individual slabs of foam as opposed to continuous strips, although platens may : . ~., ~ `:

~12~;2%3 be used to successively densify portions of a strip, thereby ef~ecting densification of the entire strip.
The surfaces of the compressing means 99 may be smooth or embossed, depending on the desir~d sur~ac~ app2~lr-ance of the finished product.
After compression, the densified strip 20a and the working run 28 exit the housing 84 through the openin~ 108;
and after the strip 20a has cooled, it may be further cut, shaped, etc.
It may be noted that due to the extremely rapid nature of the cooling process, fur-ther operations may be under-taken almost immediately after the strip 20 exits -the housing 84.
The following specific examples will serve to more ~ully illustrate the practice of the present invention.
A series of foam densification tesls were conduc-ted on an apparatus such as shown in Figs. 1 and 2. The rollers 100 were of steel and were each 8" in diameter, wi~h spacinys of 0.14" and 0.125" in successive series of tests. The rollers were heated by two 400 watt quart~ infra-red lamps focused on the roller surfaces, as shown in Fig. 1.
The belt 22 was 93% open steel mesh and was chain driven by a 1/4 Hp. capacity D.C. gear head motor. The rollers 100 were driven by the same mo-tor, and were intercoImected .
with the belt 93 through a gear arrangement allowing independent varia-tion of -the speeds oE the rollers 100 ana the belt 93, with a maximum belt speed greater than 125"/min.

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;23 Air temperature ~ras controlled b~ control of voltac~e to the finned strip heaters 72 by a Variac.
The blower 42 had a capacity of 160 f-t3/min. The cross-sectional area of tlle chamber 34 was a~o~lt 2 ~t ~.iV.it~
a maximum air flow velocity of about l.3 ft/sec. The length of the heatiLng chamber 34 between the fronf~ and rear walls 52 and 54 was about 20". Actual air veloc:ity was believed to be abou. 0.5-0.7 ft/sec, due to resistance from the foam ' ' mass 12.
It was found that the conveyor and roll,er system was easily stalled by feeding a l4" wide, 3/~" thic~ strip - of unheated polyurethane foam having a nominal density of 2 lb/ft3 and an ILD (indentation load deflection) character-istic of 36 to the rollers at about 96"/minute. However, lS there was negligible resistance when an identical strip was heated to temperatures between about 365F and 435F.
The results o- the te~ts are summarized in Tab1e I-'~

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~lZ~ZZ3 All foam density values are nomirlal and, where a stretch or thickness value is no-t ~iven, no measurement was made.
The time required to bring ~he temperature of the circulating air and the rollers to a desired level was about thirty minutes, as opposed to the required start-up time of four hours or more experienced with prior plateN compression systems.
It was noted that when the roller temperature was reduced, the surface of the product foam was fuzzy and not fully densified, thus showing that, if desired, foam of non-uniform density may be produced.
Exposure time of the foam to heated air varied be-tween about 10 seconds at a conveyor speed of 125 in/min to about 15 seconds at a conveyor speed of 84 in/min. It is not believed that exposure times longer than 15 seconds give improved results, except when the initial foam thickness is greater than about 7 in.
Of course, it will be apparent that required foam/
gas contact time is a function of foam thickness and gas tem-perature, with longer con-tact times being required for very thick (i.e. greater than 7 in.) foams and in cases where the gas is heated o~ly to relatively low temperatures. Ho~ever, ` -~it is believed that contact times substantially greater than 15 seconds will rarely, if ever, be required at adequate heat.
The 1/4 Hp. motor was more than sufficient to power the system when the foam was heated before and during feeding . . .. ...

~LZ0;~23 ~o the rollers, thus indica-ting that required pressure is negligible compared to prior systems. It is believed that the required horsepo~er increases with increased roller size, which in turn increases ~7ith the initial thickness of the mass 12. It is also ~elieved that horsepower requirements increase with the desired degree of densification of a given thickness of foam~
It is to be noted that the foam may be elon~ated if desired by increasing the speed of the rollers relative`
to that of the conveyor belt.
It is to be understood tha-t the use of the subject invention is limited to those foams which have an open-celled structure which allows penetration of the foam by the flow o heated gaseous material. It should also be! noted that either open-celled polyester or polye-ther urethane may be employed with equally good results. It is believed that the process of the invention may be advantageously applied to ~;
foams of synthetic resins other than polyurethane.
It will be readily apparen-t from the foregoing detailed - 20 description and test results that the present invention provides a foam densifica-tion process whereby capital and operating ex-penditures are significantly decreased due to the absence of the requirement of masslve pressure and heat application equip-ment.
It is believed that the time required to effect foam densification accordin~ to the present invention is only about 10% of that required by prior methods, thus a:Llowing signifi-cantly increased production ra-tes.' The relatively short required start-up and turn-around periods also-contribute significantly to increased production rates.

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Z~3 The ~oregoing deta:iled description is given for clearness of understanding only, and no unnecessary limit~tions should be understood therefrom as modifications will be obvious to those skilled in the art.

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Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of densifying a mass of fully cured open-celled polyurethane foam, said method comprising the consecutive steps of:
passing heated gaseous material entirely through the mass, the temperature of said gaseous material being in the range of about 425°F to 460°F and the velocity of said gaseous material being insufficient to destroy the integrity of the cell walls of the mass, said gaseous material being inert with respect to the polyurethane, and the period of exposure of the mass to the gaseous material being insuffi-cient to allow charring of the mass, yet sufficient to raise the entire mass substantially uniformly to a temperature between about 365°F and 435°F;
compressing the mass while its temperature is between about 365°F and 435°F to reduce its thickness and thus increase its density to a desired degree; and permitting the mass to cool.

2. The method of claim 1 wherein the gaseous material is air.

3. The method of claim 1 wherein the gaseous material is circulated downwardly through the mass.

4. The method of claim 1 wherein the mass is main-tained in the temperature range of 365°F to 435°F for a time not exceeding about one minute.

5. The method of claim 1 wherein the mass is moved into a zone at ambient temperature immediately after it is compressed.

6. The method of claim 1 wherein the mass is moved continuously endwise and is compressed between pinch rollers.

7. The method of claim 6 wherein the mass is main-tained in the temperature range of 365°F to 435°F for a time not exceeding about one minute.

8. The method of claim 7 wherein the mass is permitted to cool by moving it endwise out of the pinch rollers into a zone at ambient temperature.

9. The method of claim 1 wherein the mass is moved in a discontinuous fashion and is compressed between two flat parallel surfaces.

10. The method of claim 9 wherein at least one of said flat parallel surfaces comprises a movable platen.