Stabilized synthetic pulp-cellulose blends

Abstract: A method for preparing a stabilized blend of a poly­ olefin and wood pulp suitable for use in an absorbent article, in which a polyolefin, a solvent for the polyolefin, and from 0.1-1.0% by weight, based on the weight of poly­ olefin, of an antioxidant stabilizer that is compatible with the polyolefin are combined and heated to form a homogeneous solution of the polyolefin and antioxidant in the solvent, the solution is formed into a polyolefin pulp, and the poly­ olefin pulp is mixed with from about 50 to about 95% by weight, based on the total weight of the blend, of a wood pulp, to form a blend that is not subject to ignition at processing temperatures.

Description: This invention relates to a blend of polyolefin pulp and wood pulp stabilized by an antioxidant. The blend prepared by the process of the invention is suitable for use in an absorbent article. When blends of synthetic polyolefin pulp and wood pulp are processed at elevated temperatures, they may reach a reactive condition that initiates the spontaneous emission of heat at a temperature (exotherm temperature) lower than the exotherm temperature of either of the individual components, which can lead to spontaneous ignition of the blend during processing. Such hazardous reactive conditions may, for instance, be generated in the manufacture of absorbent products by friction when defibering (i.e., fluffing) the blends in a hammermill. Therefore, a need exists for methods for producing blends of synthetic polyolefin pulp and wood pulp that exhibit higher exotherm temperatures. EP-A-261,832 discloses a fibrous molded article having 5-50 wt. % of a pulp fiber of a thermoplastic resin and 50-95 wt. % of a second hydrophilic short fiber the filaments of which are bonded together by fused thermoplastic resin pulp fiber. The pulp fiber has on its surface a composition of a propylene glycol and a phenolic antioxidant or a phosphorus acid ester type of antioxidant that is primarily intended to stabilize the glycol against thermal deterioration during fusion bonding. Therefore, there is a need for blends of synthetic polyolefin pulp and wood pulp that not only are stabilized more efficiently against spontaneous ignition but also are protected against losses of stabilizer caused by wet processing during the manufacture of the synthetic pulp or its combination with wood pulp. According to the invention, a method of preparing a fibrous article containing a blend of a synthetic pulp and a hydrophilic short fiber, is characterized in that a polyolefin, a solvent for the polylolefin, and from 0.1-1.0% by weight, based on the weight of polyolefin, of an antioxidant stabilizer that is compatible with the polyolefin are combined and heated to form a homogeneous solution of the polyolefin and antioxidant in the solvent, the solution is formed into a polyolefin pulp, and the polyolefin pulp is mixed with from about 50 to about 95% by weight, based on the total weight of the blend, of a wood pulp. Preferably 0.1 to about 0.4% by weight of the antioxidant is used, and from about 70% to about 85% by weight of the wood pulp. Preferably also the stabilizers used in this invention are thioether antioxidants, hindered phenolic antioxidants, or phosphite antioxidants. Examples of thioether antioxidants are dilauryl thiodipropionate (DLTDP) and distearyl thiodipropionate ((DSTDP). Hindered phenolic antioxidants are sterically hindered phenols that undergo fast reactions with peroxy radicals. Examples of hindered phenolic antioxidants are thiobisphenols, alkylidene-bisphenols, alkylphenols, hydroxybenzyl compounds, acylaminophenols, and hydroxyphenols proprionates. Examples of phosphite antioxidants are triesters of phosphoric acid. Synthetic pulps may be described as very fine, highly branched, discontinuous, water-dispersible fibers made from polymers. Conventionally, polyolefin synthetic pulps may be blended in all proportions with wood pulp and made into papers and boards using conventional papermaking equipment or made into low-density products by known methods. The products may be heated to melt the polyolefin, depending on the intended application. According to the invention, polymers from which polyolefin pulps are conventionally made are used, including polyethylene, polypropylene, poly(4-methyl- pentene-1), copolymers of ethylene and 1-butene, 1-hexene, and 1-octene, copolymers of ethylene and propylene, and copolymers of propylene and other olefins such as 1-butene and 1-hexene. The polyolefin pulps can be composed solely of one of these polymers or they can be composed of mixtures of two or more of the polymers. The preferred polyolefin of this invention is high density polyethylene and the preferred stabilizer is a combination of 0.125% of Irgafos 168 and 0.125% of Irganox 1010, by weight of the polyethylene. Both of these products are marketed by Ciba-Geigy; Irgafos® is a phosphite and Irganox® is a hindered phenolic antioxidant. The usual procedures used to prepare synthetic polyolefin pulps from a solution of the polymer, solution flash spinning, emulsion flash spinning, melt extrusion/fibrillation, and shear precipitation, may be used in the process according to the invention. Preferably the procedure used in this invention comprises forming a synthetic pulp from the stabilized polymer solution by allowing it to expand from a high pressure to atmospheric pressure through a suitable device for forming the desired pulp structure. This technique is called solution flash spinning and is well known in the art, for instance from "Pulp, Synthetic," Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed. (New York: 1982) Vol. 19, pp. 420-435. This synthetic polyolefin pulp containing the antioxidant is then blended conventionally with wood pulp and processed as desired. For example, the polyolefin pulp may be dispersed in a hydropulper and the wood pulp dispersed in another hydropulper. The two pulps may be blended together in the stock chest of a paper machine, formed into a sheet, and dried at a temperature below the melting point of the polyolefin. The wood pulp can be obtained from conventional sources, such as the kraft and the sulfite processes or from mechanical pulping processes. EXAMPLES High density polyethylene ("HDPE") pulp used in the following Examples were prepared by the solution flash spinning method. First, 9-12% by weight of high density polyethylene in pentane is made into a slurry by rapid agitation. To this slurry, 0.125% by weight of polyethylene of Irgafos 168 and Irganox 1010 is added. This slurry is then pressurized to 124,1-137,9 bar (1,800 - 2,000 psig) and heated to 145-155°C. This uniform solution was then passed into a specially designed spurting nozzle or spinneret, in which a controlled pressure drop was effected. Two liquid phases, one of which was polymer-rich and the other polymer-lean, resulted. This two-phase mixture exited through a small orifice at high shear into a chamber of low temperature and approximately atmospheric pressure to quickly and completely evaporate or flash the solvent. Vaporization of solvent provided the energy to form the fibrous product, and the resulting cooling caused rapid crystallization of the HDPE pulp. Under commercial conditions, the solvent would be condensed and reused. The various HDPE pulps with different amounts of antioxidant stabilizers were then tested "as is" or wet-formed into sheets with wood pulp when testing mixtures of synthetic pulp and wood pulp. All of the HDPE pulp products in the examples were surface treated with a suitable treatment for absorbent product end use, unless otherwise noted. Percentages of synthetic pulp are by weight based on the total blend composition unless otherwise noted. In the examples, the wood pulp used was a commercially available southern softwood kraft fluff pulp ("SKFP"). Percentages of stabilizer are by weight based on synthetic pulp. Trademarks or trade names used in this application for stabilizers have the following generic name: Irgafos 168® is tris (2,4-di-tert-butyphenyl)phosphite; Irganox 1010® is tetrakis[methylene (3, 5-di-tert-butyl-4-hydroxy-hydrocinnamate) ]methane; Irganox 1076® is octadecyl 3,5-di-tert-butyl-4-hydroxycinnamate; B-225 is a 1:1 blend of the Irgafos 168 and Irganox 1010; B-215 is a 2:1 blend of the Irgafos 168 and Irganox 1010: and Topanol CA is 1,1,3-tris(2-methyl-4-hydroxy-5-t-butyl phenyl)butane. Topanol is a product of the corporation ICI Americas and the other above mentioned products are from Ciba-Geigy. EXAMPLE I Samples were tested for flash ignition temperature and self ignition temperature according to the method ASTM D-1919, Standard Method Test for Ignition of Plastics, Procedure B. The results were as follows: This example shows that the flash and self ignition temperatures of the stabilized test sample D of the present invention are significantly higher than the wood pulp alone C or the unstabilized combination sample A. Sample D also has a higher self ignition temperature than sample B. These results demonstrate the unexpected results of the sample D. EXAMPLE II Synthetic pulps and blends of synthetic pulps with wood pulp made by a laboratory paper handsheet process were tested by being placed in an open pan in an air purge stream on a differential scanning calorimeter. These samples were heated at 10°C/min or 20°C/min. The exothermic oxidation reaction was detected by the calorimeter and the onset temperature of the exothermic reaction and the peak temperature of the reaction were used as indicators of the thermal stability of the sample, the higher those temperatures, the more stable the sample. The lower the onset and exotherm temperatures, the more rapidly the material oxidized under the test conditions. The following table shows that the stabilized samples took longer to undergo the exotherm than the unstabilized samples. Hence, this Example shows a method of determining relative thermal stability. EXAMPLE III This is a comparative example that tests the absorbency in pads bonded with unstabilized and stabilized HDPE pulp blends. Samples 1-6 are the unstabilized samples of 17-19% of spurted HDPE pulp with the remainder a wood pulp (marketed under the name Rayfloc J). Sample A has 20% of spurted HDPE, 80% of wood pulp, and 0.25% of B-225 stabilizer (marketed by Ciba-Geigy). The results show that the Sample A, made with the stabilized pulp, compared favorably to the other samples in total absorbency (GATS) and rate of absorbency in the vertical wicking test. The overall wick height is another indication of the wettability of sample A, and, hence, the suitability of the stabilized synthetic pulp for use in absorbent products.

Claims: A method for preparing a blend of a synthetic pulp and a hydrophilic short fiber wood pulp, wherein said blend is stabilized more efficiently against spontaneous ignition, characterized in that a polyolefin, a solvent for the polylolefin, and from 0.1-1.0% by weight, based on the weight of polyolefin, of an antioxidant stabilizer that is compatible with the polyolefin are combined and heated to form a homogeneous solution of the polyolefin and antioxidant in the solvent, the solution is formed into a polyolefin pulp, and the polyolefin pulp is mixed with from about 50 to about 95% by weight, based on the total weight of the blend, of a wood pulp. A method for preparing a stabilized blend as claimed in claim 1, further characterized in that the antioxidant stabilizer is a thioether antioxidant, a hindered phenolic antioxidant, or a phosphite antioxidant. A method for preparing a stabilized blend as claimed in claim 1 or 2, further characterized in that the amount of the antioxidant stabilizer is about 0.1 to about 0.4% by weight. A method for preparing a stabilized blend as claimed in claim 1, 2, or 3, further characterized in that the antioxidant stabilizer is a combination of 0.125% of Irgafos 168® and 0.125% of Irganox 1010®, by weight of the polyolefin. A method for preparing a stabilized blend as claimed in any of the preceeding claims, further characterized in that the polyolefin is polyethylene. A method for preparing a stabilized blend as claimed in claim 5, further characterized in that the polyethylene is high density polyethylene. A method for preparing a stabilized blend as claimed in any of claims 1-4, further characterized in that the polyolefin is an ethylene copolymer with 1-butene, 1-hexene, or 1-octene. A method for preparing a stabilized blend as claimed in any of the preceeding claims, further characterized in that the solution is formed into a polyolefin pulp by solution flash spinning.

Inventors: ECHT ELLIOT NMN, ENGLE EDWARD JACOB III, ECHT, ELLIOT NMN, ENGLE, EDWARD JACOB III
Applicants: HERCULES INC, HERCULES INCORPORATED
Application year: 1990
Main classification: D21H13
IPC codes: D01F6, D21H13, C08J3, D01D5, C09K15, A61L15, D21H21