CA2012353A1 - Polyacetal compositions stabilized with microcrystalline or fibrous cellulose - Google Patents

Abstract

TITLE
POLYACETAL COMPOSITIONS STABILIZED WITH
MICROCRYSTALLINE OR FIBROUS CELLULOSE
ABSTRACT OF THE DISCLOSURE
Incorporation of 0.05 to 5 weight percent of certain microcrystalline or fibrous cellulose polymers into polyacetal molding compositions results in improved thermal stability for such compositions.

Description

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TITLE
POLYACETAL COMPOSITIONS STABILIZED WITH
MICROCRYSTALLINE OR FIBROUS CELLULOSE
DESC]RIPTION
Technical Field This invenkion relates to certain polyacetal compositions which are characterized by improved stability and processing due to the inclusion therein of microcrystalline or fibrous cellulose, both of lo which are derived from naturally occurring cellulose and are non-malting at the temperature at which polyacetal is melt processed.
Polyacetal, also commonly referred to as polyoxymethylene, compositions are generally understood to include compositions based on homopol~mers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. Tha proportion of the comonomers can be up to 20 weight percent. Compositions based on polyacetals o~ relatively high molecular weight, i.e.
lO,ooO to lOO,oO0 are use~ul in preparing semi-finished and finished articles by any of th8 techniques commonly used with thermoplastic materials, e.g. compression molding, injection molding, extrusion, blow molding, rotational mol~ing, melt spinning, stamping and thermo~orming. Finished articles made ~rom such compositions possess desirable physical properties, including high ~tiffness, AD-5852 35 strength, low coefficient of friction, and good .. . .

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2~2~i3 solvent resistance. However, in certain applications, it would be desirable to improve the thermal stability of the polyacetal composition. By the present invention, the thermal stability of polyacetal is improved through the use o~ either microcrystalline or fibrous cellulose as a stak~ilizer, both of which are naturally occurring materials and are non-melting at the temperature at which the polyacetal is melt processed.
Back~round Art U. S. patent 4,766,168 di~closes hydroxy-containing polymers or oligomers as stabilizers for polyacetal. Naturally occurring materials are not specifically mentioned in this r~erence as being stabilizers for polyacetal. Neither microcrystalline nor fibrous cellulose, both of which are non-melting at the melt processing temperature of polyacetal, are specifically disclosed within this reference, nor are the advantages obtain~d f~om the incorporation of microcrystalline or fibrous cellulose into polyacetal disclosed therein.
U. S. patent 4,722,662 discloses a process for manufact.uring oxymethylene copolymers stabilized against thermal degradation, comprising the heating of the copolymer in an aqueous medium containing water and a disubstituted cellulose ether to hydrolyze unstable oxymathylene ends and then separating said oxymethylene copolymer from said a~ueous medium containing a disubstituted cellulose ether.
Di~ubstituted cellulosa ethexs are known thermoplastics.
U. S. patent 4,111,887 discloses polyoxymethylene molding compositions exhibiting improved physical properties comprising an admixture of a polyoxymethylene polymer, ~ fibrous reinforcement ~ ' ~123~3 which can includ~ cellulosic fiber, and a polycarbodiimide.
U.S. Patents 3,406,129 discloses melt blends of moldable cellulose polymer having free hydroxyl groups with up to 50% o~ acetal polymer and U.S.

3,406,130 discloses colloidlal dispersions of such blends with certain solvents for the cellulose polymer, which compositions, are alleged to have improved melt strength and elongation. U.S, 3,406,12g specifically teaches that the use of greater than 50 weight percent polyacetal is detrimental to the melt blend. Further, the cellulose disclosed in these references is neither microcrystalline nor ~ibrous cellulose as it is moldable, and therefore, meltable at the melt processing temperature of polyacetal.
While some o~ the reference discussed above disclose incorporating certain particular cellulosics into polyacetal compositions, none disclose the specific types of cellulo~e used .in the compositions f the present invention, nor do any disclose the unexpectedly improved stability in polyacetal compositions that results ~rom the incorporation therein o~ said types of cellulose.
SUMMARY OF T~E INVENTION
The present inYention relates to polyacetal compositions ~tabilized with 0.05 to 5 weight percent of microcrystalline or fibrous cellulose, both of which are derived from naturally occurring cellulosa and are non-melting at the temperature at which the polyacetal is melt processed. The resultant polyacetal compositions are characterized as having improved thermal stability over polyacetal alone and are useful in applications where polyacetal resin is used and where the:rmal stability in the polyacetal resin is desired.

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DETAILED_DESCRIPTION OF THE INVENTION
Polyacetal polymers are known to be thermally unstable in the absence of stabilizing agents. To alleviate such problem, typical commercially available poly~cetal compositions are stabilized with polyamide (such as disclosed in Alsup et al, U.S.P. 2,993~025). However, it has been found that polyamides can react with formaldehyde released during processing, resulting in the reaction products and/or decomposition produc:ts contaminating the molded article. As such, there exists a continuing need to develop effec~ive and efficient stabilizers for polyacetal compositions.
This invention relates to certain polyacetal compositions which are characterized as having improv~d stability in comparison to polyacetal alone and to polyacetal composi~ions stabilized and melt compounded with conventional stabilizers. More specifically, it relates to polyacetal compositions that are stabilized with either microcrystalline or fibrous cellulose and that are characterized as having improved thermal stability, as measured by lower evolution of ~ormaldehy~e. Both the microcrystalline and fihrous cellulosics9 which are derived from naturally occurring cellulose and which are non-melting at the temperature at which polyacetal is melt processed, used in the compo~itions of the present invention have been found to not degrade as readily as the conventional polyamide (or nylon) stabilizers.
To achieve the improvements mentioned above, i.e., lower evolution of formaldehyde, the compositions of the present invention consist essentially of ~a~ 0.05 to 5 weight percent microcrystalline cellulose or fibrous cellulose and .

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(b) 95 to 99.95 weight percen~ polyacetal pol~mer, with the weight percents being based upon the weight of component (a) and (b) only. Pref~rably, the composition~ consist essentially of 0.05 to 2 weight percent component (a) and 98 to g9.95 weight percent component (b)o Most preferably, the compositions consist essentially of ~a) 0.05 to 1 weigh~ percent component (a) and 99 to 99.95 weight percent component (b). All the above-mentionled weight perc2nts are based lo upon the weight o~ components (a~ and (b) only.
Componen (b~. Polya~etal The term ~polyacetal~ as used herein includes homopolymers of formaldehyde or of cyclic oligomers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, and copolymers of ~ormaldehyde ~r of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esteri~içation or etherification~
The polyacetals used in the compositions of the present invsntion can be branched or linear and will generally have a number average molecular weight in the range of 10,000 to 100,000, preferably 20,000 to 75,000. The molecular weight can conveniently be measured by gel permeation chromatography in m-cresol at 160C using a Du Pont PSM bimodal column kit with nominal pore size of 60 and 1000 A. Although polyacetals having higher or lower molecular weight averages can be used, depending on the physical and processing properties desired, the polyacetal molecular weight averages mentioned abovP are pre~erred to provide optimum balance of good mixing of . .

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tha various ingredienks to be melt blended into the composition with the most desired combination o~
physical properties in the molded articles made from such compositions.
As indicated above, the polyacetal can be either a homopolymer, a copolym~r or a mixture thereof. Copolymers can contain one or more comonomer~, such as those generally u~ed in preparing polyacetal compositions. Comonomers more c~mmonly used include alkylene oxides oP 2-12 carbon atoms and their cyclic addition products with ormaldehyde. ~he quantity of comonomer will not be more than 20 weight percent, preferably not more than 15 weight percent, and most preferably about 2 weight percent. The most preferred comonomer is ethylene oxide. Generally polyacetal homopolymer is preferxed over copolymer because of its greater stiffness. Pre~erred polyacetal homopolymers include those whose terminal hydroxyl groups have been end-capped by a chemical reac~ion to form ester or ether groups, preferably acetate or methoxy groups, respectively.
Component (a)._~icrocrys~alline or Fibrous Cellulose Ilhe ~tabilizer used in the polyacetal compositions herain is microcrystalline cellulose or fibrous cellulose~ Microcrystalline cellulose is preferred. Both microcrystalline cellulose and ~ibrous cellulose are dPrived from naturally occurring cellulo~e.
Microcrystalline cellulose is known in the art and is commercially available. It is described in detail in U.S. 3,023,104, incorpvrated herein by referenc~e, and it is referred to therein as ncellulose cry tallite aggregates~. Microcrystallins cellulose is also described in ~Hydrolysis and Crystallization of Cellulos~e~, Industrial and Enqineerinq Chemistr~, vol.
42, 502-507 (1950).

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2~ ~23~3 Consistent with that which is taught in U.S.
3,023,104, the microcrystalline cellulose useful in the compositions of the present invention can be obtained from a commercial source or can be prepared by standard techniques o~ ,acid hydrolysi~ o~ naturally occurring cellulose. In either case, the microcrystalline cellulose should meet the puri~y req~lirements decribed belo~w. In the acid hydrolysis o~
cellulose, the acid dissolves any amorphous portion of the original cellulose chains. The undissolved portion remaining from such hydrolysis is in a particulate, non-fibrous or crystalline form, said form being the result of the disruption of the continulty of the fine structures between the crystalline and amorphous regions of the original cellulose. The undissolved portion, which is in crystalline form, is known to be non-melting at the melt processing temperature of polyacetal. The melt processing temperature of polyacetal generally does not exceed 280C~ The methods of preparing microcrystalline cellulose by the acid hydrolysis of cellulose are Xnown to those in the art and ar , for example, described in U.S. 3,023,104, column 2 and the examples.
The microcrystalline cellulose used in the compositions of the present invention has a certain level-off degree of polymerization. Level-off degree of polymerization is described in U.S. 3,023,1040 More specifically, it is described therein as being the point at which the cellulose that is subjected to acid hydrolysis reache~, a~ter a certain period of time, a su~stantially constant molecular weight. In other words, it is the point at which the number of repeating units or monomers, sometimes designated anhydroglucose units, which make up the cellulosic material, becomes relatively constant. It is then 2 ~ t~ ~ ~

apparent that the degree of polymerization of the material has leveled of~, hence the name level-off degree of pol~merization.
Consi~tent with what iB disclosed in U.S.
3,023,104~ the microcrystalline cellulo6e useful in the present compositions has a preferred average level-ofP degree of polymerization of 125 to 375 anhydroglucose units. Ideally, within this range all of the material should have the same degree of polymerization but as this is difficult, if not impossible, to achieve, it is preferred that at leask 85% of the material have an actual degree o~
polymerization not less than 50 and not more than 550.
More preferably, within the average level-o~f degree 15 f polymerization of 125 to 375, at least 90% of the material should have an actual degree of polymerization within the range of 75 to 500, and it is still more preferred that at least 95% of the material has an actual degree of polymerization in the 20 range of 75 to 450. The more preferred average level-off degree of polymerization for the microcrystalline cellulose useful herein is in the range of 200 to 300, of which material ~t least 90%
has an actual degree of polymerization in the range of 25 75 to 550. The most pre~erred average level~off degree of polymerization for the microcrystalline cellulose useful herein ranges from 175 to 225.
As taught in U.S. 3,023,104, the microcrystalline cellulose will usually have an average particle size no greater than 300 microns. For purposes of this invention, the average particle size is the point at which 50% o~ the particles are greater than average and 50% of the particles are less than average. Average particle size can be determined by 3~ standard techniques, such as microscopic inspection, .

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~23~3 gravitational sedimentation, sieve analysis, and electron microscopy. The preferred method of determining particle size is gravitakional sedimentation.
It has been founcl that, for the compositions of the present inv~ntion, stability increases as the average particle 6ize of thle microcrystalline cellulose decreases. As such, although microcry6talline cellulose can have an average particle size up to about 300 microns, it is preferred that the average particle size o~ the ~icrocrystalline cellulose used herein be 100 microns or less, more preferably 50 microns or less, and even more preferably 25 microns or less, and most preferably 10 microns or less.
Fibrous cellulose is known in tha art and is commercially available. It can be prepared from naturally occurring cellulose by techniques readily available to those skilled in the art. For example, a fibrous cellulose can be made by pulverizing wood pulp and then subsequently purifying it to meet the purity requirements described below. It is also a naturally occurring substance, an example of which is cotton linter. Fibrous cellulose generally exists in the form f flat ribbons. The width and thickness of these flat ribbons o~ fibrous cellulose is not considered critical in achieving the results exemplified herein.
In yeneral, however, th~ width of these flat ribbons will be, on average, ~pproximately 25 microns and the thickness of these flat ribbons will be, on average, 6 microns. The length of the ~lat ribbons of the fibrous cellulose is also not considered critical in achieving the results exempli~ied herein. Fibrous cellulose is commercially available over a range of avera~e fiber length, including from 300 microns down through less 2 ~ s~

than 22 microns. Although average fiber length is not considered critical, it i~ expected that stability results will improve as average ~iber length decrea~es due to the increase that such a decrease in average fiber length causes in the surface area of the fibrous cellulose stabilizer.
It is important that the microcrystalline cellulose and the fibrous c:ellulose used in the compositions of the present: invention be substantially free of compounds which destabilize acetal resins.
~estabilizing impurities that are likely to occur in commercially available microcrystalline and fibrous cellulosics that are useful in the compositions of the present invention can be assayed via (1~ the pH of an aqueous suspension of 1-10 weight percent microcrystalline cellulo~e or ~ibrous cellulose, (2) the non-volatile ash content, and/or (3~ the heavy metal content.
More specifically, it i~ r~commended that for optimum results, the pH of an aqueous suspension of 1-10 weight percent o~ the microcrystalline cellulose or fibrous cellulose useful in the compositions of the present invention be in the range of 5-7 for homopolymer polyacetal an~ in the range of 5-8 for copolymer polyacetal~ It is more prePerred that the pH of the suspension be in the ran~e o~ 5-7 for both homopolymer and copol~mer polyacetal. It is reco~mended that the non-volatile ash content ~ashing is conducted at greater than or equal to 800C) of the microcrystalline cellulose or fibrous cellulose be less than 0.25%, more preferably less than 0.10%, and mo~t preferably less than 0.02%. It i8 also recommended that the hea~y metal content of the microcrystalline cellulose and the fibrous cellulose be less than 10 ppm. For maximum thermal stability 11 2~23~
results, it is recommended that the non-volatile ash content an~ the heavy metal content o~ the microcrystalline cellulo~a iand the ~i.brous cellulose be minimized.
In stabilizing ester-capped or partially ester-capped polyacetal homopolymer, the microcrystalline and fibrous cellulose should ba substantially free of basic materials which ~an destabilize the polyacetal. Basic impurities should preferably be removed to levels of not ~ore than 200 ppm and most preferably to not more than 10 ppm, measured on a dry weight microcrystalline or fibrous cellulo~e basis. In stabilizing polyacetal copolymer or homopolymer that is substantially all ether-capped, higher concentrations of basic materials in the microcrystalline and ~ibrous cellulose can be tolerated. In addition, it should be understood that if the impurity ln the microcrystalline or fibrous cellulose is only weakly basic relatively higher amounts can be tolerated. In any event, the pH range of an aqueous ~uspension of the microcrystalline or fibrous cellulose used herein should be maintained within the range of 5-8 as described above.
In using micrDcrystalline or fibrous cellulose in stabilizing both homopolymer and copolymer polyacetal, acidic impurities in the microcrystalline and fibrous cellulose should be minimized. Acidic impurities should preferably be removed to levels of not more than 250 ppm and most preferably to not more than 10 ppm~ As with the basic impurities, it should be understvod that if the impur~ty in the microcry~talline or fibrous cellulose is only we~kly acidic, relatively higher amounts can be tolerat:ed. In any event, the pH range of an aqueous suspension of the microcrystalline or fibrous 12 20~2~
cellulose used h~rein should be maintained within the range of 5-8, as described above.
Accordingly, when such acidic and/or basic impurities are present in the microcrystalline or fibrous cellulose in amounts large enough to cause destabilization of the polyacetal compositions, the microcrystalline or fibrous cellulose ehould be purified be~ore it is introduced into compo~itions of the present invention. Volatile impurities in the microcrystalline or fibrous cellulose can be removed by use of a vacuum oven. Non-volatile impurities in the microcrystalline or fibrous cellulose can be purified by washing or extracting the microcrystalline cellulose or fi~rous cellulose with an appropriate liquid, such as, for example, water.
It should be understood that the composition~ o~ the present invention can include, in addition to the polyacetal and the microcrystalline or ~ibrous cellulose, other ingredients, modifiers and additives as are generally used in polyacetal molding resins, including co-stabilizers, anti-oxidants, pigments, colorants, toughening agents, reinforcing agents, uv stabiliz~rs, hindered amine stabilizers, nucleating agents, lubricants, glass fibers, and fillers. It should also be understood that some pigments and colorants can, themselves, adversely affect the stability oP polyacetal compositions.
Preparation of the Compositi_ns The compositions of the present invention can be prepared by mixing the microcrystalline or fibrous cellulose stabilizer with the~polyacetal polymer at a temperature above the melting point of the polyacetal component of the compositions u ing any intensive mixing device conventionally used in preparing thermoplastic polyacetal compositions, such -. .

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13 ~23~3 as rubber mllls, internal mixers ~uch as ~Banbury~ and nBrabender~ mixers, single or multiblade internal mixers with a cavity heatecl externally or by ~rickion, nKo-kneaders~, multibarrel mixers such as ~Farrel continuous Mixers~, injection molding machines, and extrud~rs, both single 6crew and twin screw, both co-rotating and counter rotating. These devices can be used alone or in combinaltion with stat~c mixers, mixin~ torpedos and/or various devices to increase internal pressure and/or thle intensity of mixing, such as valves, gates or screws designed for this purpose.
Extruders are preferred. Of course, such mixing should be conducted at a temperature below which significant degradation of the polyacetal component of the composition will occur. Generally, polyacetal compositions are melt processed between 170C to 280C, preferably between 1~5C to 240C, and most preferably 195C to 225C.
Shaped articles can be made ~rom the compositions of the present invention using any of several common methods, including compression molding, inj~ction molding, extrusi~n, blow molding, rotational ~olding, melt ~pinning and thermoforming.
Injection molding is pre~erred. Examples of shaped artiele~ include sheet, profiles, rod stock, film, filaments, fibers, strapping, tape tubing and pipe.
Such shaped articles can be post treated by orientation, stretching, coating, annealing, painting, laminating and plating. Such shaped articles and scrap therefrom can be ground and remolded~
Processing conditions used in the preparation o~ the compositions of the present invention and shaped articles made therefrom include melt temperatures of about 170-280C, prefer~bly 185-240C~ most preferably 200-230~C. When injection ,,, ~, . ..

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14 2~3~3 molding the composition6 o~ the present invention, it is preferred that the mold be as cold as possible consistent with the intracacy of the shape being produced. Generally, the mold temperature will he 10-120C, preferably 10-100~C, and most preferably about 50-90C.
XAME'LES
In the following examples, there are shown speci~ic embodiments of the present invention and certain comparisons with e~odiments o~ control experiments outside the limits of the present invention. It will be seen that the compositions of the present invention are characterized by improved stability over that of polyacetal alone. All parts and percentages are by weight, and all temperatures are in degrees Celsius unlsss otherwise ~pecified.
Measurements not originally in SI units have been so converted and rounded where appropriate.
The polyacetal polymers used in the examples below were as follows:
~ a~ PO~YACETAL ~A~ - an acetate end-capped homopolymer having a number average molecular weight o~ about 40,000 and (b) POLYACE~AL "sn - an acetate end-capped homopolymer having a number average molecular weight of about 33,000.
~ he microcrystalline cellulose ~tabilizers used in the examples below were a~ follows~
(a) ~MCll~ was a commercially available microcrystalline cellulose which was air jet milled to an average particle size of about 11 microns, (b) ~MC20" was a commercially available microcrystalline cellulose having an average particle size of about 20 microns, 2~23~3 (c) aMC50~ was a commercially available microcrystalline cellulose having an average particle si~e of about 50 microns, and (d) ~MClOOn was a commercially available microcry~talline cellulose having an average particle size o~ about 100 microns.
The average level.-off degree of polymerization for each mic:rocrystalline cellulose used in the examples ranged from 190 to ~00. Each microcrystalline cellulose used in the examples had less than 10 ppm heavy metalls and less than 0.05%
ashO The pH oE a 10~ aqueous ~uspension of each microcrystalline cellulose used in the examples ranged from about 5.5-7.
The ~ibrous cellulose us~d in the examples below were as follows:
(a) ~FCln was a commercially available fibrous cellulose having an averaye ~iber length of less than 22 microns and an ash content of about 0.20 to 0.25% ashO
(b~ ~FC2~ was a commercially available fibrous cellulose having an averag2 fiber length of less than 90 microns, an ash content o~ about 0.15%, and a pH, measured on both 5% and 10~ aqueous suspensions of cellulose, of 5.2, and (c~ ~FC3~ was a commercially available fibrous cellulose having an average fiber length of about 300 microns, an ash content of about 0.16%, and a pH, mea~ured on a 5% aqueous suspension o~
cellulose, Of 5-95-Stabilizers other than the above cellulose stabilizer~ that w2re us~d in the examples that follow ware:
(a3 ~nylon~ was a 33/23/43 terpolymer v~
nylon 66, nylon 6/10 and nylon 6, respectively and ' : ' :

3~3 (b) ~EVOH~ was ethylene/vinyl alcoholcopolymer containing 29 weight percent ethylene and 71 weight percent vinyl alcohol, and having an apparent melt viscosity at 210C o~ 9500 P, which had been purified such that it contained less than 10 ppm ash.
The antioxidants used in the examples that follow were:
(a) ~antioxidant: A~ was triethyl~neglycol bis(3-(3'-tert-butyl-4'-hyclroxy-5'-methylphenyl)pro-lo prionat~ and (b) ~antioxidant B~ was N,N'-hexamethylene his(3,5-di-tert-butyl 4-hyclroxyhydrocinnamide).
In the following examples, thermal stability of the compositions was determined using a thermally evolved formaldehyde (TEF) test procedureO A weighed sample of polyacetal composition was placed in a tube and the tube was ~itted with a cap for introduction of nitrogen to the test sample for removal of any evolved gases from the apparatus while maintaining the sample in an oxygen-free environment. The ~ample was h~ated at either 250~C or 259C, as indicated in the data tables below, in a silicone oil bath. The nitrogen and any evolved gases transported thereby were bubbled through 75 ml of a 40 g/l sodium ~ulfite in water solu*ion. ~ny evolved formaldehyde reacts with the sodium sulfite to liberate sodium hydroxide. The sodium hydroxide was continuously neutralized with standard 0.1 N HCl. The results were obtained as a chart of ml of titer versus test time. The percent evolv~d ~ormaldehyde was calculated by the formula 0.03 x 100 (V)(N) SW
where V is the volume of titer in milliliters N is the normality of the titer, and SW :is the ~ample weight in grams.

17 2 ~ 1 2 ~ ~ ?3 The Pactor ~0.03~ is the milliequivalent weight of ~ormaldehyde in g/milliequivalent.
Thermally evolvecl formaldehyde results are reported in the data tables below under columns headed with ~Wt % CH2O at x C~ he time period for which the sample was heated is a].so indicated in said data tables. The results obtained at the longer heating times are especially revea3.ing of the improved long term stability of the compositions o~ the present 10 invention, EXAMPLES 1-13. EFFECT OF MICROCRYSTALLINE OR FIBROUS
CELLULOSE ON THE THERMAL STABILITY OF
_POLYACETAh FLUFF ___ _ The components of Æxamples 1-13 and Control Examples Cl-C2 are listed in Table I, below. For each example, powdered microcrystalline cellulose or powdered fibrous cellulose stabilizer was added to polyacetal ~luff, shaken to mix, and tested fvr thermally evolved formaldehyde (CH2O) as described above. The results are reported in Table I, below.
It is ~vident from the results that both the microcrystalline cellulose and the fibrous cellulose acted to improve the stability of the polyacetal.

2~1~3S63 TABLE I. EFFECT OF MICROCRYSTALLINE OR FIBROUS
CELLULOSE IN POLYACETAL FLUFF
Wt. % Wt. ~ Wt % C~12O ~ 250C
E~. No PAc tabilizer 15 min 30 min 60 min 5C1 100 A - 0O26 1.08 2.16 1 99.8A 0.2 MC20 0.29 0.69 1.12 2 99.5A 0.5 MC20 0.15 0.42 0.77 3 99.0A 1.0 MC20 0.11 0.36 0,69 4 98.0A 2.0 MC20 0.09 0.38 0.85 95.0~ 5.0 ~C20 0.09 0.6~ 1.17 10C2 100 B - 0.72 1.02 2.08 6 99.75B 0.25MC20 0.38 0.28 0.68 7 99.5B 0.5 MC20 0.24 0.32 0.52 8 99.0B 1.0 MC20 0.24 0.26 0.54 9 98.0B 2.0 MC20 0.20 0.43 0.54 99.75B 0.25 FCl 0.38 0~64 0.92 11 99.5 B 0.50 FC1 0.34 0.63 0.96 1512 99.0 B 1.0 FC1 0.35 2.22 1.34 13 98.0 B 2.0 FCl 0.30 1.24 1.70 PAc = polyacetal EXAMPLES 14-19 E~FECT OF MICROCRYSTALLINE CE~LULOSB ON
THE THERMAL STABILIT~ OF
_ _ PO~YACETAL tmelt processed~
The components of Examples 14 19 and Control Examples C3-C6 are listed in Table_IIA and Table IIB, below. For each example, the components were mixed together and melt compounded on a 28 mm Werner and Pfleiderer twin screw extruder with barrel temperature settings of 150C to 180C, die temperature setting of 200C and screw speed of 150 rpm. The temperature of the melt as it exited ~he die for the examples ranyed from 221C to 224C. The melt compounded sample was tested by the T~F test, described above. The results, as reported below in Table IIA and Table IIB, showed that the microcrystalline cellulose stabilizer 3S imparted better thermal stability, on average, to :. :: , :' ~2353 lg polyacetal than did the conventional stabilizers at longer test times.
TABLE IIA. EFFECT OF 2~ICROCRYSTALLIiNE CELLULOSE IN
__MELT PROCESSED_POLyAc ETAL _ _ Wt % CH20 @ Z50 C
Eg. Wt % Wt % Wt % 15 30 60 120 180 PAc Stab. _ AQ mln min ~ln min min 14 99.4A 0.5MC20 0.1A 0.15 0.60 1.21 2.19 3.08 99.4A 0.5MC20 O.lB 0.18 0.55 1.09 1.95 C3 98.9A l.OEVOH O.lB 0.20 0.78 1.96 4.60 6.84 C4 98.9A l.Onylon 0.1A 0.11 0.60 3.40 14.00 21.08 PAc - Polyacetal Stab. = Stabilizer TABIE IIB. EFFECT OF MICROCRYSTALLINE 5:ELLULOSE
_ IN MEI,T PROCESSED POLYACETAL
Wt % CH20 ~ 259 C
20 EgO Wt % Wt % Wt % 15 30 No. PAc St~bilizex AO mln min 16 98.9A 1.0 MCll O.lB 0.15 0.63 17 99.65A 0~25 MCll O.lB 0.21 0.74 18 99 . 4A O . 5 MCll O . lB O . 27 0 . 87 ~S
19 99.4A 0.5 MC20 O.lB 0.20 0.66 C5 99.7A 0.2 EVOH 0.lB 0.18 0.99 C6 9809A 1.0 EVOH 0.1B 0.07 0052 PAc = polyac:etal 2~23~
~o EX~MPLE 20 ~ C8 EFFECT OF FIBROUS CELI.ULOSE ON THE
THERMAL STABILITY OF POLYACETAL (melt _ _ processed~ _ The components of Example 20 and Control Examples C7-C~ are listed i.n TABLE I~I, below. For each example, the components were mixed together, melt compounded, and extrud~d under the same conditions as for examples 14-19. Each sample was subj~cted to the TEF test, described above.
The results are reported in TABLE III, below. Control ~xample C8 shows the type of results obtained when an impure fibrous cellulose (pH less than 5.5) was added to the polyacetal.
TABLE III. EFFECT OF FIBROUS CELLULOSE IN
MELT PROCESSED POLYACETAL _ Wt % CH2O ~ 259~C
~g. Wt % Wt % Wt % 15 - 30 No. PAc Stabi izer AO min mln 94.90B 5O0 FC3 0.095B 0.03 0.27 ~0 C7 99.1 B 0.8 EVOH 0.1 B 0.06 0.47 C8 94.90B 5.0 FC2 0.095B 0.68 3.49 _ _ _ . _ PAc = polyacetal EXAMPLES 21-26. POLYACETAL WITH MICROCRYSTALLINE
z5 CELLULOSE OF VARYING PARTICLE SXZE
The components of Examples 21-26 and Control Example C9 are listed in Tables_IVA and IVB, below.
The components were mixed together and melt compounded as described for Examples 14 19. Each sample was subjected to the TEF test, described above.
The results for Examples 21-23 are reported in Table IVA, below. The results ~howed that as the average particle size of the microcrystalline cellulose decreased, the thermal stability of the polyacetal composition increased.

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The results of Examples 11-13 are reported in Table IVB, below. For these examples, TEF values were obtained at test times longer than 30 minutes.
The results showed again that the smaller the average particle size of the microcrystalline cellulose, the better the thermal ~tability of the polyacetal.

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Claims (17)

1. A thermoplastic polyacetal composition consisting essentially of (a) 0.05 to 5 weight percent of a stabilizer selected from the group consisting of microcrystalline cellulose and fibrous cellulose, and (b) 95 to 99.95 weight percent of polyacetal polymer, provided that the above-stated percentages are based on the total amount of components (a) and (b) only and wherein the average particle size of the microcrystalline cellulose is 100 microns or less.

2. The composition of Claim 1 wherein the component (a) stabilizer is substantially free of acidic materials.

3. The composition of Claim 1 wherein the component (b) polyacetal is homopolymer and the component (a) stabilizer is substantially free of acidic materials and basic materials

4. The composition of Claims 1, 2 or 3 wherein component (a) comprises 0.05 to 2 weight percent of the composition.

5. The composition of Claims 1,2 or 3 wherein component (a) comprises 0.05 to 1 weight percent of the composition.

6. The composition of Claim 1 wherein the component (a) stabilizer is microcrystalline cellulose.

7. The composition of Claim 6 wherein the average particle size of the microcrystalline cellulose is 50 microns or less.

8. The composition of Claim 6 wherein the average particle size of the microcrystalline cellulose is 25 microns or less.

9. The composition of Claim 6 wherein the average particle size of the microcrystalline cellulose is about 10 microns or less.

10. The composition of Claim 6 wherein the microcrystalline cellulose has a level off degree of polymerization of 125 to 375 anhydroglucose units.

11. The composition of Claim 6 wherein the microcrystalline cellulose has a level off degree of polymerization of 175 to 225 anhydroglucose units.

12. The composition of Claim 1 wherein the component (a) stabilizer is fibrous cellulose.

13. The composition of Claims 1 or 2 wherein the component (b) polyacetal polymer is a copolymer.

14. The composition of Claims 1, 2, or 3 wherein the component (b) polyacetal polymer has a number average molecular weight of 10,000-100,000.

15. The composition of Claims 1, 2, or 3 further comprising at least one of co-stabilizers, antioxidants, pigments, colorants, reinforcing agents, UV stabilizers, hindered amine stabilizers, nucleating agents, glass fibers, lubricants, toughening agents, and fillers.

16. Shaped articles made from the composition of Claims 1, 2, or 3.

17. A method of preparing the composition of Claims 1 comprising mixing the stabilizer with the polyacetal polymer at a temperature above the melting point of the polyacetal polymer and below the temperature at which degradation of the components will occur.