TRANSLUCENT POLYAMIDE BLENDS

Abstract: The invention relates to optically translucent polyamide resin compositions having excellent mechanical and thermal characteristics; a process for their preparation, their use for the production of molded parts, sheet products, and the like, and various articles produced therefrom. The composition comprises (a) a miscible blend of at least two polyamides including at least one semicrystalline polyamide; (b) glass filler; and (c) a catalyst containing phosphorus in an oxidation state of +1, +2, or +3.

Description: TITLE TRANSLUCENT POLYAMIDE BLENDS This application claims benefit of priority from Provisional Application No. 60/238,973, filed October 10, 2000. Field of the Invention The present invention relates to polyamide blends. More specifically, it relates to polyamide resins that are translucent while retaining excellent mechanical and thermal properties. Background of the Invention It is known that a characteristic property of many unfilled amorphous materials is transparence. Crystalline and semi-crystalline materials are, on the other hand, often opaque due to the fact that the crystalline domains in these materials scatter incident light. It is known that polyamides, also known broadly as nylons, are excellent in toughness, heat resistance, oil resistance, and processability. Examples of such polyamides include aliphatic polyamides such as those commonly denoted nylon 6, nylon 6,6, nylon 6,10, nylon 12, and the like. These polyamides are generally semi-crystalline. Such semi-crystalline polyamides are widely used for engineering plastics, fibers, etc., owing to the above mentioned excellent properties. As engineering plastics, they are widely used in various applications, such as electric and electronic parts and accessories for automobiles. However, a drawback of the above polyamides for many applications is that they are often opaque due to the presence in the polymers of spherulite crystals. The spherulite crystals are sufficiently large to interfere with and scatter visible light. It is a common practice to use reinforcing fillers in polyamide formulations to increase the tensile and flexural strengths of articles produced therefrom. US Patent No. 5,053,447 discloses a polyamide-based thermoplastic formulation having: a) at least 50 weight percent, based on the total weight of the formulation, unreinforced nylon selected from nylon 6,6, nylon 6, or mixtures thereof; b) about 5-50 weight percent fillers; and c) a sufficient amount of decabromodiphenyl ethane to provide a melt index value that is higher than the melt index value of the nylon alone. The fillers used are glass fibers. Glass fibers used as fillers are known to distort or interfere with the passage of light in plastics. US Patents Nos. 5, 149,897 and 4, 133,287 disclose the problem that when glass fibers are added to nylon as reinforcing or strengthening agents, they can interfere with the optical properties of the materials. It is known that certain unfilled amorphous copolyamides can be molded to produce transparent articles. US Patent No. 4,404,317 describes blends of at least one amorphous copolyamide and at least one semi-crystalline polyamide in which the amorphous copolyamide is the predominant component. The present invention builds upon this work by describing how translucent materials can be made by blending semi-crystalline materials with or without an amorphous component in the presence of a catalyst and glass filler that serve to enhance both the translucency and physical properties of the resulting materials. US Patent No. 6,022,613 describes compositions having a high degree of transparency that contain blends of select polyamide homopolymers or copolymers having balanced amino and acid terminal groups with further select polyamide homopolymers or copolymers having an excess of terminal amino groups. These requirements, however, present a major limitation regarding the range of polyamides that can be used in preparing the blends. There is a need for novel polyamide compositions that are translucent, and that possess the strength, toughness, heat resistance, chemical resistance, etc. that are known to the art as characteristic of non-amorphous polyamides. Summary of the Invention An optically translucent polyamide composition is disclosed herein, comprising: a) 59 to 96.99 weight percent of a miscible blend of at least two polyamides and wherein at least one of these polyamides is a semicrystalline polyamide; b) 3 to 40 weight percent of a glass filler; and c) 0.01 to 1 weight percent of a catalyst containing phosphorus in an oxidation state of +1 , +2, or +3. Another aspect of the invention is an improved process for the preparation of such compositions, comprising first providing the above-described miscible blend, and adding thereto the glass filler and catalyst mentioned above to form a blend mixture. The blend mixture is melt-blended to form a homogeneous blend. Further processing to shape the blend may include any of a variety of techniques as understood by those having skill in the art. These include without limitation injection molding, blow molding, extrusion, coextrusion, compression molding, or vacuum forming. Shaped articles of the invention may include, again without intending to limit the generality of the foregoing, bottles, sheets, films, packaging materials, pipes, rods, laminates, sacks, bags, molded goods, granules, or powders. Detailed Description of the Invention t is known that adding glass fibers to semi-crystalline polyamides improves the stiffness of the materials. However, this improvement goes hand in hand with a strong impairment in impact resistance as well as diminished optical properties. The term Optical properties' means the ability of the material in question to transmit visible light. Materials can be 'transparent', in which case they will transmit visible light without significant scattering such that items lying beyond are completely visible. Materials can also be Opaque', in which case visible light will be blocked and one cannot see through an object made from those materials. In between are materials that transmit some visible light, such that items lying beyond can be seen, but perhaps not perfectly clearly or at a distance. Such materials are referred to as 'translucent'. There are many possible applications for materials that are translucent, but not fully transparent. For example, it may be necessary that an item viewed through the material be only a short distance from the material or it may be desired that only items close to the material be visible. The degree of translucency a material can provide will often be a function of the thickness of an object made from that material. The compositions described herein are resin compositions that not only have excellent physical properties and processability, but are translucent. The compositions have three components: (A) a melt miscible/compatible blend of at least two polyamides, at least one of which is crystalline or semi-crystalline; (B) glass fibers, glass beads or other fillers that could improve heat transfer; and (C) a catalyst. Miscible blend (A) of polyamides including synthesized semi-crystalline block/random copolyamides: The first component (A) is a blend of at least two miscible thermoplastic polyamides, at least one of which is a semi-crystalline polyamide. These resins can indude semi-crystalline homopolymers, and block and random copolyamides. A thermoplastic semi-crystalline polyamide has a distinct melting point with a measurable heat of fusion, whereas an amorphous polyamide generally has neither a distinct melting point nor a measurable heat of fusion. Normally, a polyamide homopolymer, such as nylon 6,6, is a semi-crystalline polymer. Semi-crystalline polyamides are well-known in the art and widely available. They may be formed by condensation polymerization as well as addition polymerization, as discussed in The Encyclopedia of Polymer Science and Engineering, 2nd Edition, 1985, Wiley, Vol. 11, pages 318-360. The polyamides generally have molecular weights over 10,000 and can be produced by the condensation of equimolar amounts of a saturated aliphatic dicarboxylic acid containing from 4-12 carbon atoms and an aliphatic diamine containing 2-12 carbon atoms, in which the diamine can be employed, if desired, to provide an excess of amine end groups over carboxylic acid end groups in the polyamide. Alternatively, the diacid can be used to provide an excess of acid end groups. Equally well, these polyamides may be made from acid-forming and amine- forming derivatives of said acids and amines such as esters, acid chlorides, amine salts, etc. Representative aliphatic dicarboxylic acids used to make the polyamides include adipic acid, pimelic add, azelaic acid, suberic acid, sebacic acid, and dodecanedioic acid, while representative aliphatic diamines include hexamethylenediamine and octamethylenediamine. In addition, these polyamides can also be prepared from the self-condensation of a lactam. By means of example, suitable polyamides for use in the miscible blend making up component (A) include: polycaprolactam (nylon 6), polynonanolactam (nylon 9), polyundecaneolactam (nylon 1 1 ), polydodecanolactam (nylon 12), poly(tetramethylenediamine-co-adipic acid) (nylon 4,6), polyhexamethylene azelaiamide (nylon 6,9), polyhexamethylene sebacamide (nylon 6,10), polyhexamethylene isophthalamide (nylon 6, IP), polymetaxylylene adipamide (nylon MXD6), the polyamide of n-dodccanedioic acid and hexamethylenediamine (nylon 6,12), the polyamide of dodecamethylenediamine and n-dodecanedioic acid (nylon 12,12), as well as copolymers thereof. Representative copolymers are the polyamide of hexamethylene adipamide and caprolactam (nylon 6,6/6), the polyamide of hexamethylene adipamide and hexamethylene-isophthalamide (nylon 6,6/6IP), the polyamide of hexamethylene adipamide and hexamethylene-terephthalamide (nylon 6,6/6T), the polyamide of hexamethyleneterephthalamide and (2-methyl)pentamethyleneterephthalamide (nylon 6T/DT), the polyamide of hexamethylene adipamide, hexamethylene azelaicamide, and caprolactam (nylon 6,6/6,9/6), the polyamide of hexamethylene terephthalamide and hexamethylene decanediamide (nylon 6T/6,10), and the polyamide of hexamethylene terephthalamide and hexamethylene dodecanediamide (nylon 6T/6,12), as well as others which are not particularly delineated here. Suitable polyamide copolymers could also be synthesized by condensation and ring opening polymerization, as will be understood by those skilled in the art. A copolymer will not necessarily be an amorphous material as many copolymers have distinctive melting points. The definition of copolymer here is a polymer synthesized by more than two kinds of monomer pair blocks (e.g., terephthalic acid, isophthalic acid, hexamethylenediamine, 1,12-diaminodedecane, caprolactam). The addition of multi-monomer copolymers into polymer blends could also effectively reduce the size of sphemlites and even significantly reduce the degree of crystallization. Suitable amorphous polyamides will be copolymers that can include, but are not limited to, copolymers made from ingredients such as isophthalic acid, terephthalic acid, hexamethylenediamine, bis(/?-aminocyclohexyl)methane, 1 ,4- bis(aminomethyl)cyclohexane, or l-amino-3-aminomethyl-3,5,5- trimethylcyclohexane, as is understood by those skilled in the art. It is critical that a blend be of at least two, but preferably more, miscible and compatible nylons, thus facilitating the reduction of the spherulite sizes in the crystalline regions of the semi-crystalline polymer components. Adding an amorphous polyamide could also facilitate a reduction in spherulite size. With optimally sized and dispersed phases and adequate interphase adhesion, the compatible polyamides provide a blend morphology conducive to useful mechanical properties. As discussed herein, by miscible blends, it is meant that the blends of two or more melt compatible polyamides, at least one of which is semi-crystalline, of the present invention behave as a single homogeneous polyamide, exhibit a single T g , and give a single-phase composition in which the stratification of the polymeric components during or after processing is generally avoided. Since immiscible blends are phase separated, they suffer from delamination at the phase boundaries because of the weak bonding between the phases. This leads to light scattering, which negatively affects the optical properties of the molded articles. Since this miscibility is crucial for translucency, the selection of nylons used for the blends will depend on their mutual compatibility. One way to judge the miscibility of two or more polyamides is by examining the appearance of the molten mixture, which should be transparent for compatible materials and cloudy for incompatible materials. For example, nylons 6 and 6,6 are fully miscible and form a transparent melt. On the other hand, nylons 6,6 and 12 are not miscible and form a cloudy melt. A cloudy melt is one in which the material contains inhomogeneous regions that scatter light to the point where objects behind the melt are not fully and clearly visible at a distance. The following are some examples of potential polymer blends. In parentheses is the appearance of the molten mixture: (1) Nylon 6 and nylon 6,6 (transparent melt) (2) Nylon 6,10 and nylon 6,12 (transparent melt) (3) Nylon 6,12 and nylon 6 (cloudy melt) (4) Nylon 6 and nylon 6,6, and nylon 12 (cloudy melt) If a molten polymer blend is not transparent, then that blend is not a good candidate for a translucent nylon material. Preferred blends making up component (A) include: a blend of (i) nylon 6,6 and (ii) nylon 6; a blend of (i) nylon 6,6, (ii) nylon 6, and (iii) an amorphous nylon; and a blend of (i) nylon 6,6 and (ii) nylon 6, and (iii) nylon 6T/DT. The blend (A) is preferably present in an amount of from 69.5 to 95.9 weight percent, and with a most preferred range of 74.6 to 95.3 weight percent. Glass fillers (B). The glass fillers when used in the form of glass fibers or glass beads are obtained from an inorganic glass composed of oxides, e.g., SiO 2 , B 2 O 3 , A- 2 O 3 , CaO, Na 2 O, and K 2 O. Preferred amounts of these and other fillers are in the range of 4 to 30 weight percent, with a most preferred range of 4.5 to 25 weight percent. Glass-based fillers were used not only to improve the physical properties of the final materials, but also to improve heat transfer from within the material during crystal formation period. Since crystallization is a thermodynamic process, a rapid cooling will tend to both reduce the rate of crystallization and the size of the resulting crystalline domains. Anything that enhances the rate of heat transfer from within the material would also be expected to reduce the degree of crystallization. Alkali-free glass and alkali-containing glass are useful in the instant invention (for example, E glass.C glass and A glass) with E glass being particularly preferred since it is most commonly used to reinforce engineering resins. Preferred glass fiber is in the form of glass rovings, glass chopped strands, and glass yarn made of continuous glass filaments 3-20 micron meters in diameter, commercially available as PPG 3531 , PPG3660 and PPG 3540 from Pittsburgh Plate Glass Company. The refractive index of E-glass fiber is 1.554 as measured by white light and index matching fluids (Composites, Part A (1998), Volume Date 1999, 30A(2), 139-145). To keep the blends translucent, the glass refractive index has to be fairly closely matched to that of the polymer matrix. The refractive index of nylon 6 and nylon 6,6 is 1.53 (ref. V-8, Polymer Handbook Second Ed., Brandrup, Wiley Interscience Publication). Catalyst (C). The third component (C) is a phosphorous catalyst, which promotes transamidation between the different semi-crystalline nylons. Useful catalytic oxidation states of phosphorus compounds are +1 , +2, and +3. (see Phosphorus: an Outline of its Chemistiy, Biochemistiy, and Technology, Fifth Ed., D. E. C Corbridge, Elsevier, 1995 p. 25,). For example, phosphites and hypophosphites of Group I, Group II, zinc, manganese, and aluminum salts can be used. Phosphite and hypophosphite esters are also included. Preferred catalysts are sodium hypophosphite, potassium hypophosphite, and manganese hypophosphite, The amount of the catalyst to be added will vary depending on the blend, the amount of glass fiber, and other factors known to those skilled in art. However, it is effective in a surprisingly small amount, preferably ranging from 0.1 to 0.5 weight percent and most preferably from 0.2 to 0.4 weight percent. Other components such as pigments, dyes, anti-oxidizing agents, or weathering agents may be incorporated into the polyamide resin composition in the present invention in so far as they do not affect the optical properties, moldability, and physical properties thereof. Typically such conventional additives are added to the composition in a mixing step and are included in an extrudate of the composition. Preparation. The method of mixing the components of the polyamide formulation of the present invention is not particularly limited, and any known method can be employed. Blending or mixing of the constituents that comprise the composition may be by any effective means that will effect their uniform dispersion. All of the constituents may be mixed simultaneously or separately by a mixer, blender, kneader, roll mixer, extruder, or the like in order to assure a uniform blend of the constituents. In the alternative, the constituents making up the polyamide blend component may be blended or mixed first by a mixer, blender, kneader, roll mixer, extruder, or the like in order to assure a uniform blend of the polyamide blend and the resultant polyamide mixture is melt-kneaded together with the glass fibers, catalyst, and any additives in an extruder to make a uniform blend. The uniform composition is then extruded into strands, and subsequently chopped into pellets. The pellets may be subsequently provided to the feed hopper of a molding apparatus used for forming articles. The novel blend is useful for both molded and film applications. The shaped articles formed from the compositions of the present invention, are generally formed by a known molding method for thermoplastic resins such injection molding, extrusion molding, blow molding, transfer molding, or vacuum molding. Examples Materials. The materials used in the examples described below were as follows: Nylon 6,6: Zytel®101 supplied by DuPont. Nylon 6: Ultramid® B3 supplied by BASF. Amorphous nylon: Zytel® 330 supplied by DuPont PPG3540: Glass fibers supplied by the Pittsburgh Plate Glass Company. SHP (sodium hypophosphite): Supplied by OxyChem as EN grade. Al distearate (aluminum distearate): Supplied by Shepherd Chemicals. Irganox® 1098: Supplied by Ciba. Eastobrite® OB- 1 (4,4'-bis(2-benzoxazolyl)stilbene): Supplied by Eastman Chemical Products, Inc. S-EED (Nylostab® S-EED®): Supplied by Clariant. Material Preparation. A 40 mm Werner & Pfleiderer twin-screw extruder was used to prepare thoroughly mixed blends of polymers, glass fibers, catalysts, and additives. The temperatures used were typically 270-300 °C and the resulting melt temperatures were typically 280-330 °C. The extruder and screw were set up to accommodate main feeding and side feeding. Polymers, catalysts, and additives were fed into the extruder through the main feed throat and glass fibers or beads were fed the extruder through the main feed throat and glass fibers or beads were fed through a side feeder. The melting zone has to be severe enough to obtain the intimate mixing that is required to achieve a thorough compatibilization of multiple polyamides at the molecular level. A less severe melting zone could lead to inadequate mixing, which could result, upon cooling, in the formation of undesirably large crystals that would decrease the translucency of the resulting material. By examining a given well-mixed molten polyamide blend before glass or other filler is introduced to the extruder, it is possible to assess the suitability of the blend for a translucent mixture by visually inspecting it. A transparent melt is indicative of a compatible blend and a cloudy melt of an incompatible blend. The molten material containing all ingredients was then extruded into strands, and chopped into pellets. General Test Procedures. The materials were molded into test bars. The following tests were performed on samples dry-as-molded (DAM): Elongation at break (E@B) and tensile strength (TS) measurements were determined as described in ASTM D-638 or ISO 527. Flexural modulus (FM) measurements were determined as described in ASTM D-790 or ISO 178. Notched Izod (NI) and unnotched (UNI) impact testing was done as described in ASTM D-256, ASTM D-4812, or ISO 180. Heat deflection temperatures (HDT) were determined as described in ASTM D-648. Differential scanning calorimetry (DSC) scans were taken in a TA Instruments device. The heating and cooling ramps were 10 °C/min. Yellowness index (YI) measurements were determined as described in ASTM E313. Characterization of Translucency. Materials from each group to be compared were molded into 1.6 mm or 4 mm thick bars. The molded bars were, in turn, placed under the same lighting conditions on top of a sheet of paper marked with a thick line or printed words. The markings were easily legible through the bars. Each bar was then lifted from the paper until the markings were no longer legible through the bar. The distance in millimeters between the top of the bar and the paper at the point at which the markings are no longer legible was used to characterize the translucency of the material. These numbers are given in the tables below under the heading of 'part translucency', where the thickness of the bars used is also indicated. Bars with a higher degree of translucency will have longer distances indicated than less translucent bars. The Effect of Immiscibility Upon Physical Properties. The purpose of the examples shown in Table 1 is to illustrate how physical properties can deteriorate when two incompatible semi-crystalline polymers are melt blended. Nylon 6,6 by itself has a 55% elongation at break. Nylon 6,12 alone has an 80% elongation at break. However, when 20% nylon 6,12 is melt blended with 80% nylon 6,6, the elongation at break of the mixture is only 19%. The melt has a milky appearance, which indicates that nylon 6,6 and nylon 6,12 are not fully miscible and that this system would not be suitable for inclusion in transparent or translucent blends. Table 1 Blends of Semi-Crystalline Polymers. The blends used in this group of examples were made by melt-blending two commonly used semi-crystalline polymers, nylon 6,6 and nylon 6. Together they formed a homogeneous melt that was totally clear, indicating that the two polymers were compatible. PPG3540 glass fibers, a catalyst and other ingredients were added to the mixture as indicated in Table 2. After the materials were prepared, each blend was molded into bars for physical testing and 1.6-mm-thick bars for translucency testing. The results are shown in Table 2. The surprising results are (1) that the addition of glass fibers did not prevent the materials from being translucent, and (2) that the addition of glass fibers enhanced the translucency of the resulting materials more in the nylon 6/nylon 6,6 blends than in either the pure nylon 6 or nylon 6,6 systems. That the glass fibers did not negatively impact the translucency was a result of a combination of two factors. First, the refractive index difference between the polyamide and PPG3540 glass fiber is quite small. Second, the presence of the glass probably accelerated the cooling time in the molded part, which would tend to lower the rate of crystallization. Table 2 DSC Analysis Results. DSC analysis was used to characterize the effect of using a catalyst, which, in these examples, was SHP. Melting and freezing points (abbreviated MP and FP, respectively), and the corresponding heats of fusion and crystallization were determined for two successive cycles of heating and cooling. Heats of fusion and crystallization are reflective of the degree of crystallinity possessed by a material. If, in the case of a blend, the melting point, freezing point, and associated heats have changed between the first and second heating and cooling cycles, that is a good indication that chemical reactions between the various components have occurred. This is illustrated by the examples given in Table 3. In the case of a single polymer such as nylon 6,6, there is no significant change in the melting point, freezing point, and associated heats between the first and second heating and cooling cycles. However, in the multi-polyamide systems, a significant reduction of the melting points, freezing points, and associated heats is observed between the two heating and cooling cycles. This is a good indication that a transamidation reaction between the nylon 6 and nylon 6,6 components has taken place and reduced the degree of crystallinity. The result is a system in which enough crystallinity is preserved to maintain good physical properties, but in which the crystalline domains have been reduced sufficiently to allow for significantly improved optical properties. Table 3 Semi-Crystalline Polyamides Blended With an Amoφhous Polyamide. In these examples, two semi-crystalline polyamides, nylon 6 and nylon 6,6 were blended with glass fibers, a catalyst, additives, and an amoφhous polyamide. The amoφhous polyamide, which has a refractive index of 1.588, was synthesized by condensation polymerization. The diamines used are bis(p- aminooyclohexyl)methane, and hexamethylenediamine. The diacids used are isophthalic acid and terephthalic acid. The amoφhous polyamide is fully miscible with nylon 6 and nylon 6,6 at all concentrations and the ternary mixture forms a transparent melt. The blends were molded into 4-mm-thick bars and the translucency test described above was applied. The translucency and physical testing results for six different compositions are given in Table 4. The presence of the amoφhous resin has improved the optical properties of the resulting materials, but the presence of the semi-crystalline components and glass fibers has endowed the materials with good physical properties Table 4 nt = not tested

Claims: WHAT IS CLAIMED IS: 1. An optically translucent polyamide composition comprising: a) 59 to 96.99 weight percent of a miscible blend of at least two polyamides and wherein at least one of said polyamides is a semicrystalline polyamide; b) 3 to 40 weight percent of a glass filler; and c) 0.01 to 1 weight percent of a catalyst containing phosphorus in an oxidation state of +1 , +2, or +3. 2. The composition of Claim 1 comprising 69.5 to 95.9 weight percent of said blend (a), 4 to 30 weight percent of said glass filler (b), and 0.1 to 0.5 percent of said catalyst (c). 3. The composition of Claim 2 comprising 74.6 to 95.3 weight percent of said blend (a), 4.5 to 25 weight percent of said glass filler (b), and 0.2 to 0.4 weight percent of said catalyst (c). 4. The composition of Claim 1 wherein said at least one semi-crystalline polyamide in said blend (a) is selected from synthesized semi-crystalline block/random copolyamides. 5. The composition of Claim 1 wherein said glass filler (b) is E glass. 6. The composition of Claim 1 wherein said glass filler (b) is fibrous and selected from the group consisting of glass rovings, glass chopped strands, and glass yarn. 7. The composition of Claim 1 wherein said catalyst (c) is selected from the group consisting of sodium hypophosphite, potassium hypophosphite, and manganese hypophosphite. 8. A process for the preparation of an optically translucent polyamide composition comprising: a) providing a 80 to 97 weight percent of a miscible blend of at least two polyamides and wherein at least one of said polyamides is a semicrystalline polyamide; b) adding thereto from 3 to 40 weight percent glass filler and from 0.01 to 1 weight percent catalyst containing phosphorus in an oxidation state of +1, +2, or +3, to form a blend mixture; and c) melt-blending said blend mixture forming a homogeneous composition. 9. The process of claim 8, further comprising the shaping said homogeneous blend by any of injection molding, blow molding, extrusion, coextrusion, compression molding or vacuum forming. 10. A shaped article according to the process of claim 8. 11. The shaped article of Claim 10 selected from the group consisting of bottles, sheets, films, packaging materials, pipes, rods, laminates, sacks, bags, molded goods, granules, or powders.

Inventors: CHENG PAUL P, CHENG, PAUL, P.
Applicants: DU PONT, E.I. DUPONT DE NEMOURS AND COMPANY
Application year: 2001
Main classification: C08L77
IPC codes: B29B7, B29K77, B29K105, C08K3, C08K5, C08L77