IL23792A - Cellular oxymethylene polymers and methods for making them - Google Patents

Description

Patents Form No. 3 PATENTS AND DESIGNS ORDINANCE.

SPECIFICATION.

"CELLULAR OXiMESTflYLENE POLYMERS AND METHODS FOR MAKING THEM" organised and -exi-stlng- imder the lawe of the State of J3ela are.,...Ujal.ted. Stated New ^ do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement : - This invention relates to cellular structures of oxymethylene polymers and processes of making them.

In accordance with one aspect of the invention, an oxymethylene polymer is formed into a voided, porous or foraminous structure. This may be done, for example, by contacting the polymer with a material while it is in molten or plastic state such that the polymer is caused to assume a cellular or foamed structure and solidifying the polymer while it has such cellular structure. The material contacted with the polymer which causes it to assume a cellular structure may be a gas,. Also a foaming agent of the type which generates a gas under certain conditions may be used„ In addition, the molten polymer may be mixed with a solid which may be leached out of the polymer after solidifica ion of polymer, e.g., a water soluble salt such as sodium chloride which may be dissolved out with water. Moreover, an immiscible liquid may be emulsified into the molten polymer and removed after the solidification of the polymer by washing with a volatile sol ent for the immiscible liquid and drying.

Alternatively, the polymer may be formed into a cellular structure by sintering a finely divided mass of the polymer, i.e.. heating the finely divided polymer to a temperature at which the -particles are caused to permanently stick to each othe s but not high encogt-. t cause the polymer to become completely liquid. In addition to cellular stxao rea lit which the pores or voids are of ordinary size, oxymethylene polymers having a mlcroporous structure are also contemplated under this invention. Ot:'iv.<-..r methods are described below.

■ Oxymethylene polymers are !-.'hus¾ h&ving recurring oxymethylene units and may bn prepared by the polymerization of formaldehyde or trloxane9 a cyclic trimer of formaldehyde. Suitable oxymethylene polymers include oxymethylene homopolymers and copolymers. Preferred oxymethylene copolymers are those having at least one chain containing recurring oxymethylene units interspersed with -OR-groups in the main polymer chain, where R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in the polymer chain between the two valences, with any substit^ents on said R radical being inert that is, those are free of interfering functional groups and do not induce undesirable reactions under the conditions involved. Particularly preferred are copolymers which contain from 60 to 99.6 mol per cent of recurring oxymethylene groups and from 0.4 to about 40 mol per cent of -0R-group. Most preferred are those polymers having from 85 per cent to 99.6 mol per cent of recurring oxymethylene groups and from 0.4 to 15 mol per cent of -OR- groups. In a preferred embodiment R may be, for example, an alkylene or substituted alkylene group containing at least two carbon atoms.

Among the oxymethylene copolymers which may be utilized in accordance with this aspect of the invention are those having a structure comprising recurring units having the formula ?1 (-0-CS,-(G),.-) — " wherein n is an integer from zero to 5 and wherein n is zero in from 60 to 99.6 mol per cent of the recurring units. ^ and 2 are Inert substituents, that isj substituents which are free of interfering functional groups and will not induce undesirable reactions.

A preferred class -of oxymethylene copolymers are those having a structure comprising oxymethylene a d oxye hylene recurring units wherein from 60 to 99.6 mol per cent of the recurring units are oxymethylene units.

Particularly preferred oxymethyler-e polymers are those having incorporated therein oxylalkylene units having adjacent carbon atoms which are derived from cyclic ethers having adjacent carbon atoms. These copolymers may b prepared by co olymerizing trioxane or formaldehyde with a cyclic ether having the structure where n is an integer from zero to 2.

Examples of preferred oxymethylene polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms such as the copolymers disclosed in U.S. Patent No. 3.027,352 by Cheves T. Walling, Frank Brown and Kenneth W. Bartz, which patent is assigned to the same assignee as the subject application.

Among the specific cyclic ethers which may be used are ethylene oxide; 1,3-dioxolane; 1,3,5-trioxepane; 1,3-dioxane; trimethylene oxide; pentamethylene oxide; 1,2-propylene oxide; 1,2-butylene oxide; neopentyl formal; pentaerythritol diformal; paraldehyde; tetrahydrofuran and butadiene monoxide.

As used in the specification and claims of the subject application, the term "oxymethylene" includes substituted oxymethylene, where the substituents are inert with respect to the reactions in question, that is, the substituents are free of interfering functional groups and will not introduce undesirable reactions.

As used in the specification and claims of this application, the term "copolymer" means polymers having two or more monomeric groups, including terpolymers and higher polymers. Suitable oxymethylene terpolymers include No. 3,254,053, -those disclosed in U.S. Patent 1Aj^M*>at-i The preferred oxymethylene polymers which are treated in this invention are thermoplastic materials having a melting point of at least 150°C. and are normally millable at a temperature of 200°C. They have a number average molecular weight of at least 10,000. These polymers have a high thermal stability.

For example, if a sample of oxymethylene copolymer, which has been chemically stabilized as described below, is placed in an open vessel in a circulating air oven at a temperature of 230°C. and its weight loss is measured without removal of the sample from the oven, it will have a thermal degradation rate •of less than 1.0 wt. /min. for the first 45 minutes and, in preferred instances, less than 0.1 wt. %/min. for the same period of time. The preferred oxymethylene polymers which are treated in this invention have an inherent viscosity of at least one (measured at 60°C. in. a 0.1 weight per cent solution in p-chlorophenol containing 2 weight per cent of e^-pinene) . The preferred oxymethylene copolymers exhibit remarkable alkaline stability. For example, if the chemically stabilized copolymers are refluxed at a temperature of about 142° - 145°C. in a 50% solution of sodium hydroxide in water for a period of 45 minutes, the weight of the copolymer will be reduced by. less than one per cent.

The preferred oxymethylene copolymers are preferably stabilized by degradation of the molecular ends to a point where a stable carbon-to-carbon linkage exists at each end.

V.S. Patent No. 3» 103» 99 , Thermal degradation, as disclosed in |¾^ -ee*.eft-&effiel- by Thomas J. Dolce and Frank, M. Berardinelli, or degradation by. hydrolysis, as disclosed in Application Serial No. 102,097, filed April 11, 1961, by Frank M. Berardinelli, may be used. «re¾e- This patent ia to the same assignee as the subject application.

Other oxymethylene polymers and methods of preparation therefor are disclosed in an article Kern et al, Angewandt Chemie 73(6) 177-186 (March 21, . 1961) including polymers containing repeating carbon-to-carbon single bonds in the polymer chain by copolymerizir-g trioxane with cyclic ethers such as dioxane, lactones such as beta-propiolactone, anhydrides such as cyclic adipic anhydride and ethyleneically unsaturated compounds such as styrene, vinyl acetate, vinyl methyl ketone, acrolein, etc. Also these and other oxymethylene polymers are disclosed by Sittig in Petroleum Refiner, Volume 41, Number 11, November 1962, pages 131 through 170.

Any suitable catalyst suitable for the polymerization of trioxane or formaldehyde by themselves or with other materials may be used to provide the oxymethylene polymers which are treated in accordance with this invention. Preferred catalysts are cationic catalysts including such inorganic fluorine-containing catalysts as boron trifluoride, antimony trifluoride, antimony fluoroborate, bismuth trifluoride, bismuth oxyfluoride, nickelous fluoride, aluminum trifluoride, titanium tetrafluoride, manganous fluoride, manganic fluoride, mercuric fluoride, silver fluoride, zinc fluoride, ammonium bifluoride, phosphorous pentafluoride, hydrogen fluoride, and compounds containing these materials such as boron fluoride coordinate complexes with organic compounds, particularly those in which oxygen or sulfur is a donor atom.

Other suitable catalysts include thionyl chloride, fluorosulfonic acid, methane sulfonic acid, phosphorous trichloride, titanium tetrachloride, ferric chloride, zirconium tetrachloride, aluminum trichloride, stannic chloride and stannous chloride.

The particularly preferred catalysts are boron fluoride and boron fluoride-containing, materials such as boron fluoride monohydrate, boron fluoride dihydrate and boron fluoride trihydrate as well as boron fluoride coordinate complexes with organic compounds as mentioned previously.

The coordinate complex of boron fluoride may, for example, be a complex with a phenol, an ether, an ester, or a dialkyl sulfide. Boron fluoride dibutyl etherate, the coordinate complex of boron fluoride with dibutyl ether, is a preferred coordinate complex. The boron fluoride complex with diethyl ether is also very effective. Other boron fluoride complexes which may be used are the complexes with methyl acetate, with ethyl acetate, with phenyl acetate, with dimethyl ether, with methyl phenyl ether and with dimethyl sulfide. Suitable catalysts are disclosed in U.S. Patents 2,989,505; 2,989,506; 2,989,507; 2,989,508; 2,989,509; all of which are by Donald E. Hudgin and Frank M.

Berardinelli; 2,989,510 by George J. Bruni; and 2,989,511 by Arthur W.. Schnizer. All the above patents are assigned to the same assignee as the subject application.

In addition to the above-mentioned oxymethylene copolymers, oxymethylene homopolymers of trioxane or formaldehyde may also be treated in accordance with this invention. It may be desirable to "end cap" the homopolymer molecules ' by the known methods of etherification or esterification.

It may be desirable in a preferred embodiment of this invention to incorporate one or more thermal stabilizers into the oxymetnylene polymer* The proportions of thermal stabilizer incorporated into the exymethylene polymers depends upon the specifle thermal stabilizer used, A proportion between about 0*05 and 10 t. per cent (based on the weight of polymer) has been found to be suitable for most thermal stabilizers.

One suitable thermal stabilizer system is a combination of ( ) an antioxidant ingredient such as phenolic antioxidan , and most suitabl , a substituted blsphenol, and (2) an ingredient to inhibit chain scission, generally a oompound or a polymer containing trlvalent nitrogen atoms, A suitable class of alkylene bisphenols includes compounds having from 1 to k oarbon atoms in the alkylene group and having from zero to 2 alkyl substituents on each benzene ring* each alkyl substltuent having from 1 to h carbon atoms. The preferred alkylene bisphenols are 2»2'~ methylene bis-( methyl-6- ertlary butyl phenol) and l*,!*1-butylidene ie-ie-^te tiary butyl-3-*nethyi phenol). Suitable phenolic stabilizers other than alkylene bisphenols include 2,6-ditertiary butyl-^-methylphenol, p-phenylphenol and ootylphenol.

Suitable scission inhibitors include oarboxylio polyamldes, polyurethanes, substituted polyaorylamides, t**fW9vji mm * * polyvinyl pyrollidone, hydrazines, oompounds having 1 to 6 amide groups, proteins, compounds haying tertiary amine and terminal amide groups, oompounds having amldine groups, eyoloaliphatlo amine compounds, aliphatic aoylureas and oompounds containing at least two epoxy groups. Suitable scission inhibitors as well as suitable antioxidants and proportions are disclosed in N 3,210,318 14½Λ9·ί½ ίited--by-Etadce-arrd-E¾"iCtiaT¾-53 TiTdHer""2 "Γ9"βΤ^ and French Patent No. 1,273,219» The disclosures of the above-mentioned applications and patent are incorporated herein by reference.

The thermal stabilizers may be incorporated into the oxymethylene polymer by dissolving both the polymer and the thermal stabilizer in a common solvent and thereafter evaporating the solution to dryness. Alternatively, the thermal stabilizers may be incorporated into the polymer by applying a solu t on of the thermal stabilizer to finely divided polymer, as in a slurry, and thereafter filtering the polymer and evaporating to dryn.ess. The thermal stabilizer, in finely divided dry state may be blended into finely divided polymer in any suitable blending apparatus.

One suitable method of incorporation of the chemical thermal stabilizers is by blending a dry solid thermal stabilizer into the plastic polymer, while the latter is being kneaded as on heated rolls or through an extruder. ^,.

In one preferred embodiment, an oxymethylene vjolymer may be contacted with gas, for example, by heating the polymer under superatmospheric pressure to a temperature and for a period sufficient to partially degrade the polymer and produce a gaseous product. On reduc ion- of the pressure to atmospheric pressure, the gas bubbles through the molten polymer causing the formation of a foam. The polymer is then solidified by cooling while retaining its foamed structure. This method is particularly effective when the p- lymers involved are oxymethylene copolymers which have not been stabilized by the terminal degradation process discussed above.

In carrying out this method, the polymer may be initially heated to a temperature, for example, in the range -of 230 to 300°Go and a pressure of at least 75 psi for a period of, for example, 30 to 45 minutes to cause substantial generation of gas, and then cooled. The heating is preferably conducted in an inert atmosphere, e.g. of nitrogen or argon. The partially degraded polymer is then transferred to a mold or form which is itself under atmospheric pressure, and the resulting liberated gas causes the polymer to assume a foamed structure. Alternatively, the molten polymer may be confined in a mold during the initial heating step and the pressure regulated to a level which will cause the generation of gas to form a shaped foamed structure directly without transferring molten polymer under pressure to a mold at a lower pressure. The mold may be heated by such methods as the use of electrical heating elements or heat transfer mediums, such as hot oil, superheated steam, etc.

Another method of contacting the molten polymer with a gas in forming it into a foamed structure is to physically mix a gas supplied from an external source into the molten polymer. The gas used is preferably substantially inert to the polymer at the temperature of the molten polymer and may be, for example, , itrogen, argon, carbon dioxide, helium, neon, krypton, xenon, the Freons, methylene chloride, carbon tetrafluoride and other fluoro-carbons.

Various types of commercial machinery may be used to foam the polymer with the inert gas. For example, sigma blade and paddle type mixers can be used to disperse the inert gases which are injected through valved jets into the body of the molten polymer under high pressure. The inert gas may be preheated to preserve the polymer in molten condition while it is being formed into a foamed structure. Moreover, the injection of inert gas may be carried out under elevated pressure so that the gas initially dissolves "somewhat in the molten polymer and is liberated when the pressure is released.

Still another method of contacting the oxymethylene polymer with a gas so as to form it into a cellular structure is to first mix the polymer with a foaming or blowing agent and then heat the mixture to a temperature sufficient to liberate the foaming gas. Some foaming agents which may be sed are azodicarbonamide, azobisisob tyronitrile-, benzenesulfonylhydrazide-diazoaminobenzene, . N,N*-dimethyl-N,. N* -dinitrosoterephthalamide, dimitrosopent-amethylenetetramine, 4,4'-diphenyldisulfonylazide, and 4,4'-oxybis (benzene-sulfonylhydrazide). The molten polymer-foaming agent mixture may be heated, for example, to a temperature of at least 155°C. to form the foamed structure. This method of foaming may be carried out directly in the mold to form specific foamed shapes directly. The foaming agent may be used, for example, in an amount of 0.5 to 20 wt. % based on the weight of the polymer.

The oxymethylene polymer may also be formed into a cellular structure by mixing the polymer with a granular solid material, e.g., a salt, which may subsequently be removed after the polymer solidifies, e.g., by leaching out with water. Some granular solids which may be used are salts such as NaCl, KC1, Na2S02, or other suitable soluble materials such as starches, etc. . In one type of procedure for carrying out this method, the soluble material and molten polymer are mixed, cooled, and put into a processable form, e.g., molding pellets The mixture is then processed or molded into its final form. After molding, the item is soaked in the solvent and the soluble material is leached out.

Still another method of forming an oxymethylene polymer into a cellular structure is to sinter the polymer in polymer form, i.e. heat the particles of polymer to a temperature which causes the particles to stick to one another but is insufficient to completely melt the particles into a molten or plastic mass. An important advantage of this type of process is that the specific structure of the cellular polymer, e.g., the size of the voids, can be closely controlled by controlling the size of the original particles and the temperature and pressure of sintering. Moreover, the polymer particles can be sintered directly in a compression mold to form desired shapes of the cellular oxymethylene polymer. The size of the particles used may be, for example, in the range of 0.010 to 0.125 inch diameter, the temperature of sintering may be, for example, in the range of 150 to 250°C. and the pressure of sintering may be, for example, in the range of 15 to 250 psi.

Another suitable method of providing a foamed polymer involves mixing an oxymethylene copolymer of the type described previously with an esterified oxymethylene homopolymer. In addition, a material which decomposes to a base is incorporated in the mixture. Under suitable conditions, the base generating material decomposes and the base attacks the esterified oxymethylene homopolymer degrading it to form formaldehyde gas which acts as the gaseous foaming agent.

It should be noted that bases do not attack oxymethylene copolymers of the type described previously.

A preferred embodiment of the cellular polymeric structure has a density between about 0.3 and about 1.35 grams/cc. The cavities preferably comprise between about 5 and about 80 per cent of the volume of said oxymethylene polymer. The preferred cavities have an average diameter between about 0.0001 and about 0.1 inch.

When a separate material is mixed with the oxymethylene polymer this material maybe regarded as a cavity-producing material which upon appropriate treatment, such as heating, generates a gaseous material which, in. urn, produces a cavity within the polymeric material. In other embodiments the cavity-producing material may be one which is subsequently removed from the polymeric structure and by its removal produces cavities. In another embodiment terminal oxymethylene groups generate gas upon degradation and thus the terminal groups may be regarded as a cavity-producing material.

In the following examples, unless otherwise specified, the oxymethylene copolymer is produced from trioxane and about 2 weight per cent ethylene oxide.

This polymer has been stabilized by removal of the terminal oxymethylene groups as previously described and has also been chemically stabilized by the addition of 0.1 per cent of cyanoguanidine and about 0.5 per cent of 2,2' -methylene bis-(4-methyl-6-tertiary butyl phenol).

Example I Particles of an oxymethylene copolymer are mixed with about 2 weight per cent of Ν,Ν' -dinitrosopentamethylenetetramine and are placed in a mold which is then closed and heated to a temperature of about 180°C. The mixture is sloshed about in the mold and at this temperature the polymer becomes molten and the foaming agent generates a gas. The mold is then cooled to room temperature and, upon ." opening the mold, a foamed structure is found which in some instances had a hollow interior.

Example II A previously foamed oxymethylene copolymer, using the same foaming agent as in Example I, is placed in a mold then .heated to about 180°C. as above. A foamed polymeric structure is obtained.

Example III An oxymethylene copolymer; and about 2 weight per cent of azodicarbon- amide are loaded into the mold and tamped to completely fill the mold. The mold is then closed and placed in a pressure clamp whereupon it is heated to a temperature of about 180°C. which melts the polymer and activates the foaming agent. The mold is then cooled to room temperature and opened. A formed foamed structure is obtained.

Example IV An oxymethylene copolymer is made from trioxane and about 2 weight per cent ethylene oxide. This raw copolymer is not stabilized as discussed previously by removing the comparatively unstable oxymethylene end groups.

This material is placed in a mold with a mold pressure of about 500 psi and a temperature- of about 230°C. Under these conditions, the polymer partially degrades by removal of its comparative unstable oxymethylene end portion until a carbon-to-carbon bond is reached in the polymer chain. The product of this terminal degradation process is gaseous formaldehyde which acts as the gas to form the cavities within the polymer. After the mold is cooled the polymer is removed and is found to have a porous structure of the "closed-cell" type.

Example V Any oxymethylene copolymer was heated in an autoclave equipped with , paddle mixers to a temperature of about 200°C. After 5 minutes at this temperature an inert gas, in this case nitrogen, is injected into the molten mass at a pressure of 100 psi by means of jets. The mixture, of the molten polymer and the gas is beat into a foam by the paddle mixers. The foam is then.removed from the mixture and is placed into the cooling trays to form slab stock or into molds to form appropriate structures. The foams produced by this method are ;.r-normally of the "closed-cell" type.

Example VI An oxymethylene copolymer was dissolved in butyrolactone at a temperature of 145°C. and is mixed with Ν,Ν' -dinitrosopentamethylenetetramine, a foaming agent. The foaming agent-polymer mixture was then cooled to room temperature to precipitate the mixture which gives a raw molding material having a foaming agent widely and uniformly dispersed therein. This material . was then molded at a temperature of approximately 190°C. and resulted in a foamed structure having "closed-cell" structure.

Example VII An oxymethylene copolymer is mixed with a oeocerite paraffin wax having a melting point of about 80°C. The mixing takes place at a temperature about 180°C. for about 15 minutes in a nitrogen atmosphere.. After mixing, the pellets are molded at a temperature of about 190°C. The finished articles are then placed in a benzene bath and the paraffin is dissolved therein leaving an "open-cell" molded article. Other low melting<-point materials, such as fats and other waxes (e.g. beeswax, candelilla, carnauba, moritan, etc.) may ί t ■■ be used.

Example VIII An oxymethylene copolymer is mixed with NaCl .at a temperature of 200°C. in an autoclave equipped with paddle mixers until a foam results.

This foam is placed in pans where it cools to a solid. The NaCl also is in solid form dispersed throughout the polymer. The mixed solid is then made into pellets and is subsequently molded. After molding, the NaCl is leacKed out by water which is a solvent therefor. This results in a foamed "open-cell" structure.

Example IX 0.5 grams of guanidine carbonate were dissolved at about : 40 milliliters of water and the solution was filtered through fine filtering paper. A small amount of acetone was added to the solution to act as a wetting agent. The solution was mixed with 45 grams of an oxymethylene copolymer as described previously. The slurry was allowed to'*dry in a hood temperature of about 50°C. 5 grams of a commercial esterified oxymethylene homopolymer. were added to this mixture which was then plastographed at 30 revolutions per minute in a nitrogen atmosphere at a temperature of 188°C. for 5 minutes. After cooling this material was ground to particles which would pass through a AO-mesh screen. This material was then heated at a temperature of 220°C. which liberated guanidine, which is a base and which attacked the homopolymer, causing evolutions of formaldehyde gas. A foam resulted in 3 minutes time.

The above examples all produce foamed cellular structures having densities between about 0.3 and about 1.35 grams/cc, with cavities having an average diameter between about 0.0001 and about 0.1 inch. The cavities comprise between about 5 and about 80 per cent of the volume of the oxymethylene polymer.

It has been found that foamed articles made by the above methods are useful in making thermoinsula ion materials, acoustical panels, mechanical items such as light-weight seats, arm rests, crash pads, packages, etc., electrical dielectric applications such as potting materials and wire spacers, self-lubricating light-weight, bearings, molded toys, floatation panels, etc.

It is to be understood that the foregoing detailed description is merely given by way of illustration and that many variations, may be made therein without departing from the spirit of our invention.

Claims (9)

jHaVIKO WOW particularly desc ibed and ascertained! the X¾¾¾¼ nature of our Said invention and in what manner the same is to be performed, XBL decla e that what we claim is J

1. A solid shaped article comprising an oxymethylene polymer having at least 60 mol per cent of oxymethylene groups and having a plurality of cavities therein thereby providing a cellular polymeric structure having a density between about 0.3 and about 1.35 grams/cc, said cavities comprising between about 5 and about 80 per cent of the volume of said oxymethylene polymer, said cavities having an average diameter between about 0.0001 and about 0.1 inch.

2. The shaped article of claim 1 wherein said oxymethylene polymer is a copolymer having at least one chain containing between about 85 and about 99·6 mol per cent of oxymethylene groups and between about 0.4 and about 15 mol per cent of -0-R- groups wherein R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in said chain between the two valences, any substituents in said R radical being inert.

3. Process of making a cellular polymeric structure as in Claim 1 or Claim 2 which comprises dispersing a cavity- producing material throughout at least a portion of the oxymethylene polymer and producing cavities in the structure until the desired density is obtained.

4. Process as in Claim 3 wherein the cavity-producing material is an inert gas.

5. Process as in Claim 3 wherein the cavity-producing material is a foaming agent.

6. Process as in Claim 3 wherein the cavity-producing material is a comparatively unstable oxymethylene polymer the terminal groups of which degrade to formaldehyde.

7. Process of making a cellular polymeric structure as in Claim 1 or Claim 2 which comprises sintering a mass of the oxymethylene polymer until the desired density is obtained.

8. Shaped articles of oxymethylene polymers substantially as hereindescribed.

9. Process of making a cellular polymeric structure substantially as hereindescribed. Dated this twe ty third day of June 1 65 AE* OLPOBD Ageht for Applicants)