Degradable polyamides

Abstract: Compositions comprising polyamide rendered hydrolytically labile by the incorporation of certain alkyl esters therein are useful in or as articles intended for degradation by water. Certain of the hydrolytically labile polyamides are novel compounds.

Description: This invention is concerned with biodegradable polymeric articles. The polymers and articles become brittle after long exposure to water and are especially useful as biodegradable fiber and netting, degradable plastic dishware, and degradable films for packaging. Another use of interest is in biological implants. Japanese Kokai No. 56 022324 describes the production of alternate block copolymers of low molecular weight aliphatic polyesters and low molecular weight aliphatic polyamides by ester-amide interchange which are useful for mulch films biodegradable via enzyme digestion (Rhizopus delemar lipase). In this polymer, the polyester to polyamide ratio is high in polyester, for example, 4 to 1 mole % polyester to nylon 6. Japanese Kokai No. 54 119594 describes the production of a biodegradable low molecular weight aliphatic polyester amide alternate block copolymer from polycaprolactone and nylon 6 in the presence of zinc acetate. The polyester to polyamide ratio is near 1 to 1. Again, the product polymer is subject to enzymatic (lipase) degradation. Japanese Kokai No. 54 120727 describes polyester polyamide block copolymers, high in the polyester component , which are useful for production of biodegradable films or fibers. There is no mention of marine uses of the copolymers. US-A 3,592,873 describes the preparation of polyamide esters as interpolymers to provide thermal stability in polyoxymethylenes. The polyamide esters are prepared by reacting lactams or alkyl substituted lactams with at least a four membered ring with lactones or alkyl substituted lactones, again with at least a four membered ring. J.Appl. Polymer Sci., 24(7) , 1701-11 (1979) describes the synthesis of copolyamide-esters via amide ester interchange of polycaprolactone with nylon 6, 66, 69, 11, 12, or 612. It also describes degradation of the polymers by Rhizopus delemar lipase digestion or alcoholic alkali hydrolysis. The effect of nylon/polycaprolactone ratio on the biodegradability was examined and biodegradability was found to decrease as the polyamide content increased and/or as the polyamide blocks shortened. Eur. Polym. J., 529-557 (1984) describes the preparation of copolyesteramides by anionic ring opening copolymerization of ε-caprolactam with ε-caprolactone. Alternating copolymers or random multiblock copolymers with amide to ester ratios of 90/10 to 10/90 were prepared. In addition, the reference describes cleavage by alkaline hydrolysis as well as tensile properties of films and fibers fabricated from the copolymers. Chemtech, 21 , 26-30 (January, 1991) describes the biodegradability of a variety of commercial plastics and compares their utility for medical applications. Medical uses are related to mechanical and degradative properties of many of the commercial materials. The combination of properties which polyamides (or, more specifically, nylons) exhibit make them ideal for use in fibers. Furthermore, nylon fibers have many properties which make them ideal for use in netting, including strength, light weight, and resistance to degradation, However, some of these desirable properties also result in a significant environmental problem. The lifetime for nylon netting in the ocean has been estimated as over ten years; and may be closer to thirty years. Netting which has been abandoned either intentionally or by accident continues to capture marine fish and mammals. These "ghost" nets account for enormous kills of fish, seals, whales, and dolphins. Seal deaths alone are estimated to be about 40,000 annually off the coast of Alaska alone. A similar situation is found in abandoned lobster traps where any captured lobsters are unable to free themselves from the trap's netting. Analogous situations exist in fresh water lakes, many being contaminated with fishing line either lost or abandoned. Commercial nylons are also used in biological implants. However, because of their biological inertness, uses are limited to those in which the implant is permanent or in which it can be mechanically removed. Polymers of lactic acid are well-known for their degradability under microbial attack. A copolyester of lactic and glycolic acids is used as a biodegradable suture in repairing soft tissue wounds. These polymers have also been used to fabricate degradable bone plate which is used to reinforce a broken bone during its healing period. There is no record of lactic acid/nylon copolymers being used in a similar manner. Oxalate substitution for adipate units in nylon 66, or similar units in other bi-directional polyamides for the purpose of rendering the material more degradable is not reported. Oxalate esters are highly reactive hydrolytically. Thus, incorporation of oxalate units into nylon provides sites for attack and chain cleavage, whether biologically or by simple hydrolysis. We have now found amide polymer compositions which are used as articles degradable in fresh or ocean water. Such articles can be used for biological implants or as degradable nylon film. The invention also provides a process for the efficient production of compositions suitable for the use as articles degradable by water. The invention involves incorporating into a nylon, or reacting with a nylon precursor, an appropriate ester, cyclic ester, or polyester such that the ester functionality is incorporated into the polymer backbone. Incorporation can be accomplished via three major routes; reaction of preformed polymers, for example, nylon 6 with polycaprolactone; reaction of a prepolymer with a monomer, for example, nylon 6 with dimethylglycolide; or copolymerization of two monomers, for example, caprolactam with caprolactone. We have discovered that by controlling the extent of incorporation, the desirable properties of the nylon are largely maintained while at the same time the nylon becomes subject to hydrolysis by water, presumably (although we are not to be bound by such theoretical considerations) at the incorporated ester functionality. These ester functionalities, which are hydrolytically unstable, act as weak links. The strength of the nondegradable nylon is preserved. However, as the ester functionalities are hydrolyzed, the chain falls apart. The hydrolysis may be uncatalyzed, catalyzed by acids or bases, or biologically catalyzed. Furthermore, since the hydrolysis results in creation of an acidic polymer end group, it can also be autocatalyzed. As the polymer chain length becomes shorter due to the hydrolytic degradation, the nylon becomes embrittled and, therefore, weaker. Thus, netting produced from ester modified nylon fibers becomes so weak over time that the struggles of a captured animal are sufficient to break the fiber, freeing the animal; biological implants degrade at a rate commensurate with their replacement by normal tissue; and films degrade at a predetermined rate. Although some of the explanation in this disclosure refers to nylon as the polymer, the invention is applicable generally to polyamides. Hydrolytically labile polyamides useful in this invention comprise: (a) from about 90 to about 99 weight percent of polyamide, such as nylon, including nylons 6, 6/6, 6/12, 6/9, 6/10, 11, 12, 4/12, 12/12, and the like; and (b) from 1 to 10 weight percent (based on (a) plus (b)) of ester comprising oxalate of formula I (bidirectional esters) and/or dimethylglycolide of formula II (unidirectional esters): wherein R 1 and R 2 are the same or different radicals selected from hydrogen and C 1 - C 4 alkyl such as, for example, dibutyl oxalate, dimethyl oxalate, lactic acid cyclic dimer, and the like. The oxalate ester can also be in the form of a polymer (that is where the R 1 and R 2 radicals form connecting links between oxalate units). Examples of these polymeric oxalates include those wherein R 1 and R 2 are derived from ethylene glycol, propylene glycol, 1,4-butanediol, and the like. The oxalate, oxalate polymer, or glycolide dimer is randomly incorporated into the polyamide chain as single, diad, triad, or oligomeric units. This invention is directed to the use as biodegradable articles, or to impart biodegradable properties to articles, of the polyamides or of polyamides which have been made hydrolytically labile by randomly incorporating into the polymer chains of the polyamide, singularly or in combination, of from 1 to 10 weight percent of alkyl ester selected from compounds of the formula III, IV, V, or VI; wherein R 1 and R 2 are the same or different radicals selected from H, or C 1 - C 4 alkyl; R 3 is selected from hydrogen or C 1 - C 20 alkyl; n is 1 - 10; m is 2 - 6; and x varies with the polymer molecular weight range; such as, for example, 3-hydroxybutyric esters, polycaprolactone, caprolactone, adipic esters, and the like. Furthermore, the invention includes the use of copolymers of amide and ester functionality such as, for example, caprolactam with caprolactone, caprolactam with dimethylglycolide, adipic ester and hexamethylenediamine with dimethyl oxalate. The polyamides useful in this invention include any of the materials typically referred to as nylons, both bi-directional (produced from dibasic acids and diamines) and unidirectional (produced from amino-substituted acids or lactams). Lactam monomers can also be used. Similarly, the ester can be bi-directional such as, for example, dimethyl adipate, dibutyl oxalate, oxalic acid polymers such as those with ethylene glycol, propylene glycol, or 1,4-butanediol, or the ester can be unidirectional, such as caprolactone or a polyester, such as polycaprolactone, polylactic acid, polyglycolic acid, or poly-3-hydroxybutyrate. Acids, esters, or polymers of α-, β-, or higher, hydroxy-substituted carboxylic acids can also be used. The ester can also be in the form of a glycolide, such as dimethylglycolide. Ester levels, within the range of one to ten weight percent, are selected depending upon the degradation time desired. The ester-amide interchange or derivatization is readily accomplished in a melt. Melt temperatures can be in the range of about 180°C to about 300°C. Preferred melt temperatures vary with the ester to be incorporated. For example, from 240°C to 280°C is preferred for polycaprolactone while from 200°C to 220°C is preferred for dimethylglycolide. In the case of extrusion, ester incorporation is dependent on the extent of mixing and the residence time in the extruder. Twin-screw extruders are generally preferred over single-screw extruders because of the greater extent of mixing. The reaction can be catalyzed by acidic catalysts, such as tin octoate or zirconium acetylacetonate; by base catalysts, such as lithium or sodium t-butoxide, sodium or lithium hydride; or, when lactam monomers are used, their sodium or potassium salts. Acidic catalysts are preferred. Most preferred is zirconium acetyl acetonate which we have found to be unexpectedly efficient in reducing the time required for incorporation of the ester into the polyamide. Catalyst levels of 0.0% to 0.5% can be used. However, levels of 0.05% to 0.2% are preferred. Most preferred is a level of about 0.1%. The final polymer compositions can include typical additives known in the art, for example; impact modifiers such as ABS (acrylonitrile/butadiene /styrene), MBS (methyl methacrylate/butadiene/styrene), all acrylic types; fillers such as hydrated alumina, glass or other fiber reinforcements, talc or other minerals, metallic particles; colorants (e.g. pigments and dyes), toners, and color agents or concentrates such as those described in the Color Index (Society of Dyers and Colourists, U.S.A.), including Pigment Black 7, Pigment White 6, Pigment White 21, Pigment Green 7, Pigment Blue 15, Solvent Orange 60, Solvent Red 179, Solvent Green 28, Solvent Blue 45, Solvent Blue 101, Solvent Violet 14, Disperse Yellow 54, toner Irisol N™ (1-p-toluidino-4-hydroxyanthraquinone); lubricants such as high molecular weight alcohols (such as those with 12 - 24 carbons), esters (especially long-chain alkyl esters of high molecular weight acids including butyl or stearyl stearate), monoesters of glycols (such as ethylene glycol monostearate); antioxidants such as organophosphites (such as tris(aryl)- or tris(alkylaryl)- or tris(alkyl)phosphites), organophosphonites (such as trisaryl-, trisalkaryl-, or aryldialkaryl-phosphonites); thioesters (such as dilauryl thiodipropionate, ditridecyl thiodipropionate, and distearyl thiodipropionate); and ultraviolet stabilizers such as hydroxybenzophenones, salicylate esters, benzotriazoles, hindered amines [such as bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, 2,2,6,6-tetramethyl-4-piperdinyl benzoate, 1,2,3,4-tetrakis-(2,2,6,6-tetramethyl-4-piperdinyl) butane-tetracarboxylate, 1,2-bis-(2-oxo-3,3,5,5-tetramethyl-1-piperdinyl)ethane, 1-(3,5-di- tert -butyl-4-hydroxyphenyl)-2,2-bis-(2,2,6,6-tetramethyl-4-piperidinyloxycarbonyl)hexane, poly(1-oxyethylene-(2,2,6,6-tetramethyl-1,4-piperidinyl)-oxysuccinyl, N,N'-bis-(2,2,6,6-tetramethyl-4-piperidinyl)-1,6-hexanediamine, and bis-(4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyl)ethane; blowing agents; flatting agents; antistatic agents; conductive materials; odor control agents. In addition, for items discarded on land, a hydrolytic mechanism of degradation may not be entirely effective simply because of a lack of sufficient humidity or water to promote degradation at an appropriate rate. To enhance degradation under these conditions, a photosensitizer may be added to the degradable nylon. Photochemically active organic materials such as, for example, benzophenone, Rose Bengal, or inorganic salts, such as salts of iron, nickel or cobalt, may be added. The polyester-amide copolymer can be fabricated into monofilament fibers, netting, films, biological implants, disposable dinnerware and utensils, and similar articles using standard techniques practiced by those familiar with the specific area. This invention is not to be construed as limited to any particular method of preparing the fibers, netting, film, or implants. In the following Examples, which are given for the purposes of illustration only, "parts" are "parts by weight". Examples 1 to 6 - Unidirectional polymer. Pellets of commercially available nylon 6(Capron®8207; Allied Chemical Co.) were combined with dimethylglycolide (DMG) and extruded by means of a Leistritz twin-screw extruder. The mixtures were extruded in a single pass. The amounts of each component used are found in Table 1. Nylon 6 DMG Catalyst Ex.No. g. % g. % g. % 1 1500 100 - - - - 2 1485 99 15 1 - - 3 1455 97 45 3 - - 4 1410 94 90 6 - - 5 1453.5 96.9 45 3 1.5 0.1 Catalyst = Zirconium acetylacetonate Extruded strands were pelletized and the pellets were then injection molded into notched Izod bars. The Izod bars were exposed to 60° C. sea water for periods of 7, 14, 21, 35, or 60 days and then tested for brittleness (Izod). Test results are in Table 2. Notched Izod Behavior Units = Joules/m C (ft.lbs./ in.) of notch Day Ex. No. H/C 7 14 21 35 60 % % % % % 1 H 100 1280 (24.00) 100 1308 (24.52) 100 1243 (23.29) 80 1320 (24.74) 100 1315 (24.65) C - - - - - - 20 3.49 - - 2 H 100 1204 (22.97) 100 1284 (24.07) 100 1261 (23.64) 20 1248 (23.38) 20 1265 (23.70) C - - - - - - 80 419 (7.85) 80 141 (265) 3 H 100 1209 (22.65) 80 12443 (23.29) 40 1159 (21.72) 60 1179 (22.21) 40 1233 (23.11) C - - 20 228 (4.28) 60 192 (3.59) 40 168 (3.14) 60 99 (1.85) 4 H 60 895 (16.77) 40 1121 (21.01) - - - - - - C 40 203 (3.80) 60 180 (3.37) 100 125 (2.35) 100 163 (3.06) 100 68 (1.27) 5 H 100 1115 (20.90) 40 941 (17.63) - - - - - - C - - 60 271 (5.07) 100 189 (3.54) 100 149 (2.80) 100 196 (3.68) H = hinged break C = clean break Change in notched Izod results over the course of 60 days immersion in 60°C. sea water. However, copolymers formed by incorporation of dimethylglycolide into nylon 6 show switch from hinged to clean breaks in notched Izod tests. These results are consistent with a material which gradually becomes embrittled. Example 7 - Bidirectional polymer Dimethyl adipate (1 part), 1,6-hexanediamine (1.1 parts), and dimethyl oxalate (0.1 parts) were combined in a round bottom flask and then heated to 100°C. The mixture was stirred while methanol formed during reaction was allowed to distill off slowly. When methanol distillation ceased, the temperature of the mixture was increased to 125°C. Upon cooling a solid polymer was obtained. Example 8 - Catalyst Effect on Ester Incorporation Nylon 6 (9 parts), polycaprolactone (1 part), and a catalyst (0.01 parts), all parts by weight, were combined and heated rapidly to 270°C. The temperature was then held constant at 270°C. Samples were periodically withdrawn and analyzed by differential scanning calorimetry (DSC). Over time, the peak in the DSC plot due to polycaprolactone diminished and finally disappeared, reflecting full incorporation of the polycaprolactone into the nylon 6 backbone. Time to full incorporation, that is, disappearance of the polycaprolactone peak, for three different catalysts is in Table 3. The significant advantage of the zirconium acetylacetonate catalyst is apparent. Catalyst Time to Full Incorporation Tin(II) octoate > 4 hours Zinc acetate > 3 hours Zirconium acetylacetonate 1.5 - 2 hours

Claims: The use as an article degradable by water, or to impart properties to an article which render it degradable by water, of a composition comprising (a) polyamide and (b) from 1 to 10 weight percent, based on polyamide, of one or more alkyl esters effectively randomly incorporated into said polyamide such that said polyamide is hydrolytically labile; wherein the alkyl ester is selected from compounds of the formula; wherein R 1 and R 2 are the same or different radicals selected from hydrogen and C 1 -C 4 alkyl; R 3 is selected from hydrogen or C 1 - C 20 alkyl; n is 1 - 10; m is 2 - 6; and x varies with the polymer molecular weight range. The use of Claim 1 wherein either the polyamide comprises unidirectional nylon, preferably nylon 6, and the alkyl ester comprises one or more compounds of formula II, III, IV, or V, preferably polycaprolactone and/or poly-3-hydroxybutyrate, or the hydrolytically labile polyamide comprises a copolymer of caprolactam and one or more alkyl esters of formula III or V. The use of Claim 1 or 2 wherein the composition further comprises photochemically active material and/or impact modifier polymer. The use of Claim 1 wherein (A) the polyamide is symmetrical and the ester comprises an oxalate, preferably dibutyl oxalate or (B) the polyamide is nylon 6/6 and the ester is selected from an oxalate or a polymer of an oxalate and an alkanediol, preferably dibutyl oxalate; or (C) the polyamide is unidirectional, preferably comprising nylon 6, and the ester comprises a glycolide, preferably dimethylglycolide. The use of Claim 1 of a hydrolytically labile polyamide comprising random copolymer of dimethyl oxalate, dimethyl adipate, and hexamethylenediamine, or random copolymer of: a. from 90 to 99 weight percent of a lactam, and b. from 1 to 10 weight percent of a glycolide of formula II. Fishing line, fibre netting, film, or biological implant embodying the use as claimed in any of Claims 1 to 5. A method of preparing a composition suitable for use according to any of Claims 1 to 5 comprising melt blending the components in the presence of zirconium acetylacetonate catalyst.

Inventors: BORTNICK NEWMAN MAYER, HOLY NORMAN LEE, BORTNICK, NEWMAN MAYER, HOLY, NORMAN LEE
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1993
Main classification: C08K5
IPC codes: C08K5, A61L27, C08G69, C08L77, C08L101, A61L17