Abstract:
Transparent polyamide compositions with improved creep resistance comprising at least one amorphous semiaromatic polyamide and at least one semicrystalline semiaromatic polyamide. Articles of manufacture incorporating these compositions are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/601,746, filed Aug. 13, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to transparent polyamide compositions with improved creep resistance. More particularly the present invention relates to such compositions comprising melt mixed blends of at least one amorphous semiaromatic polyamide and at least one semicrystalline semiaromatic polyamide and having selected haze and light transmittance properties, and articles made therefrom.  
       BACKGROUND OF THE INVENTION  
       [0003]     Polyamide resins have excellent toughness, strength, and chemical resistance, which make them useful as engineering resins for a wide variety of applications. For many applications it would be desirable to use a transparent polyamide, but many polyamides are semicrystalline materials, and, as such, are often opaque as incident light is scattered by the crystalline domains present in the polymers. Transparent polyamides are known, but are typically amorphous, and as a result, have reduced heat resistance and articles formed from these materials tend to suffer from creeping, or deformation over time.  
         [0004]     U.S. Pat. No. 4,404,317 discloses blends of amorphous thermoplastic aliphatic polyamides copolymers with semicrystalline thermoplastic polyamides. The resulting blends could be made transparent and are disclosed to have good solvent resistance, dimensional stability, and retention of physical properties under moist or wet conditions. Japanese patent application publication 03-033157 discloses a polyamide resin composition with improved alcohol resistance prepared by compounding a copolyamide obtained from isophthalic acid, terephthalic acid, hexamethylenediamine, and an alicyclic diamine with a semiaromatic polyamide obtained from an aromatic diamine and a dicarboxylic acid.  
         [0005]     It is an object of the present invention to provide a polyamide composition that is both transparent and that has good creep resistance. Another object of the present invention is to provide polyamide compositions comprising melt-mixed blends of one or more amorphous semiaromatic polyamides and one or more semicrystalline semiaromatic polyamides, such that these compositions exhibit defined maximum haze and minimum light transmittance properties. A feature of the compositions of present invention is their ease of processing, as they are well suited to melt-blending using any of a variety of methods widely appreciated among those having skill in the field. An advantage of the compositions of the present invention is that they are readily molded using conventional melt processing techniques into a wide spectrum of articles, where optical transparency is an important feature thereof. These and other objects, features and advantages of the present invention will become better understood upon having reference to the following description of the invention.  
       SUMMARY OF THE INVENTION  
       [0006]     There is disclosed and claimed herein transparent polyamide compositions comprising a melt-mixed blend of:  
         [0000]     (i) about 50 to about 95 weight percent of at least one amorphous semiaromatic polyamide comprising repeat units derived from  
         [0007]     (a) about 20 to about 49 mole percent isophthalic acid,  
         [0008]     (b) about 1 to about 30 mole percent terephthalic acid,  
         [0009]     (c) about 25 to about 50 mole percent hexamethylenediamine,  
         [0010]     (d) optionally 0 to about 25 mole percent of at least one alicyclic diamine, and  
         [0011]     (e) optionally 0 to about 40 mole percent of one or more aminocarboxylic acids and/or lactams,  
         [0000]     wherein the amorphous semiaromatic polyamide has a heat of fusion of less than about 4 J/g; and  
         [0000]     (ii) about 5 to about 50 weight percent of at least one semicrystalline semiaromatic polyamide comprising repeat units derived from  
         [0012]     (f) terephthalic acid and, optionally, one or more additional aromatic or aliphatic diacids,  
         [0013]     (g) at least one aliphatic diamine, and  
         [0014]     (h) optionally, one or more aminocarboxylic acids and/or lactams,  
         [0000]     wherein the terephthalic acid comprises about 50 to 100 mole percent of (f) and the one or more aminocarboxylic acids or lactams comprise 0 to about 25 mole percent of the total amount of (f)+(g)+(h);  
         [0015]     and wherein the semicrystalline semiaromatic polyamide has a melting point of at least about 280° C. and a heat of fusion of at least about 5 J/g; and such that the composition has a haze of less than 12 and a light transmittance of at least about 65 percent when measured through a 2 mm thick sample of the composition, according to ASTM D1003. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     The transparent compositions of the present invention comprise at least one amorphous semiaromatic polyamide and at least one semicrystalline semiaromatic polyamide.  
         [0017]     As used herein and as will be understood by those skilled in the art, the terms terephthalic acid, isophthalic acid, and dicarboxylic acid refer also to the corresponding carboxylic acid derivatives of these materials, which can include carboxylic acid esters, diesters, and acid chlorides.  
         [0000]     Amorphous Semiaromatic Polyamide  
         [0018]     The at least one amorphous semiaromatic polyamide comprises repeat units derived from about 20 to about 49 mole percent isophthalic acid, about 1 to about 30 mole percent terephthalic acid, and about 25 to about 50 mole percent hexamethylenediamine. The polyamide may optionally comprise 0 to about 25 mole percent (and if used, preferably about 1 to 25 mole percent) of one or more alicylic diamines, where the mole percentages are based on the total amount of amorphous polyamide. Up to about 40 mole percent of the repeat units of the amorphous polyamide may optionally be derived from one or more aminocarboxylic acids (or acid derivatives), and/or lactams, and/or aliphatic dicarboxylic acids containing 4-20 carbon atoms, and/or aliphatic diamines containing 4 to 20 carbon atoms.  
         [0019]     The alicyclic diamine preferably contains at least one cyclohexane moiety. Examples of suitable alicyclic diamines include 1-amino-3-aminomethyl-3,5,5,-trimethylcyclohexane; 1,4-bis(aminomethyl)cyclohexane; and bis(p-aminocyclohexyl)methane. Any of the stereoisomers of the alicyclic diamines may be used.  
         [0020]     The amorphous polyamide has a heat of fusion of less than about 4 J/g, measured as described further below in the detailed description.  
         [0021]     The amorphous polyamide is present in about 50 to about 95 weight percent, or more preferably in about 70 to about 92 weight percent, based on the total amount of semicrystalline and amorphous polyamide present.  
         [0000]     Semicrystalline Semiaromatic Polyamide  
         [0022]     The at least one semicrystalline semiaromatic polyamide comprises repeat units derived from terephthalic acid monomers and one or more aliphatic diamines monomers with 6 to 20 carbon atoms. The semicrystalline polyamide may optionally contain repeat units derived from one or more additional aliphatic or aromatic dicarboxylic acid monomers. Moreover the semicrystalline polyamide may optionally contain repeat units derived from one or more aminocarboxylic acids (or acid derivatives) and/or lactams.  
         [0023]     Suitable examples of additional dicarboxylic acid monomers will preferably include those having 6 to 20 carbon atoms and include, but are not limited to, isophthalic acid, dodecanedioic acid, sebacic acid, and adipic acid. The terephthalic acid monomers comprise about 50 to 100 mole percent of the dicarboxylic acid monomers used to make the semicrystalline polyamide.  
         [0024]     The aliphatic diamine monomers may be linear or branched. Preferred aliphatic diamines include hexamethylenediamine, 2-methyl-1,5-pentanediamine, 1,10-diaminodecane and 1,12-diaminododecane. Suitable examples of aminocarboxylic acids (or acid derivatives) and/or lactams from which repeat units of semicrystalline semiaromatic polyamide can be derived include, but are not limited to caprolactam, 11-aminoundecanoic acid, and laurolactam,  
         [0025]     If used, the one or more aminocarboxylic acids and lactams will preferably be present in from about 1 to about 25 mole percent of the total monomers used to make the semicrystalline polyamide.  
         [0026]     Examples of preferred semicrystalline polyamides include hexamethylene adipamide/hexamethylene terephthalamide copolyamide (polyamide 6,T/6,6), hexamethylene terephthalamide/2-methylpentamethylene terephthalamide copolyamide (polyamide 6,T/D,T), poly(dodecamethylene terephthalamide) (polyamide 12,T), poly(decamethylene terephthalamide) (polyamide 10,T), decamethylene terephthalamide/decamethylene dodecanoamide copolyamide (10,T/10,12), and poly(nonamethylene terephthalamide) (polyamide 9,T).  
         [0027]     The semicrystalline polyamide will have a melting point that is at least about 280° C. and is preferably less than about 340° C. The semicrystalline polyamide will have a heat of fusion that is at least about 5 J/g. Herein heats of fusion are determined by ASTM D3418-82, at a heating rate of 20° C./min. The peak of the melting endotherm is taken as the melting point. The heat of fusion is taken as the area under the melting endotherm. All of these are measured on the second heat, meaning that the sample is heated at 20° C./min until the melting point and/or glass transition point, whichever is higher, is exceeded, and then the sample is cooled at 20° C./min to 30° C. The heating cycle begins again and measurements are then taken on a second heat, also done at 20° C./min.  
         [0028]     The semicrystalline polyamide is present in about 5 to about 50 weight percent, or more preferably in about 8 to about 30 weight percent, based on the total amount of semicrystalline and amorphous polyamide present.  
         [0000]     Additives  
         [0029]     The compositions of the present invention may contain any of a variety of additives either alone or in combination with one another. These additives may be selected from ultraviolet (UV) light stabilizers, optical brighteners, lubricants, antioxidants and/or heat stabilizers, colorants, and nanoparticle fillers, and according to the properties of interest. Each of these additives are discussed in further detail immediately below.  
         [0030]     The composition of the present invention may optionally comprise about 0.05 to about 2.5 weight percent, based on the total weight of the composition, of ultraviolet (UV) light stabilizers. Suitable UV stabilizers include hindered amine light stabilizers, benzotriazoles, resorcinols, benzophenones, and salicylates. A preferred UV light stabilizer is N-(2-ethoxyphenyl)-N′-(2-ethylphenyl)-ethanediamide.  
         [0031]     The composition of the present invention may optionally comprise about 0.001 to about 3 weight percent, based on the total weight of the composition of at least one optical brightener. Examples of optical brighteners include stilbenes such as bis(benzoxazolylstilbenes), diaminostilbenes, and bis(triazoyl)stilbenes; bis(benzoxazoyls); pyrazoles; and distyrylbiphenyls. A preferred optical brightener is 4,4′-bis(2-benzoxazolyl)stilbene, which is available as Eastobrite® OB-1 from Eastman Chemical.  
         [0032]     The composition may optionally comprise about 0.001 to about 1 weight percent based on the total weight of the composition, of lubricants and processing aids. A preferred lubricant is aluminum distearate.  
         [0033]     The compositions may optionally comprise other additives such as about 0.01 to about 1 weight percent antioxidants and/or heat stabilizers; about 0.1 to about 1 weight percent colorants; about 0.5 to about 3 weight percent nanoparticles; and the like, provided that their presence does not appreciably or deleteriously increase the haze or decrease the light transmittance of the composition. Examples of antioxidants and heat stabilizers include hindered phenols and hydroquinones. Examples of nanoparticles include nanoclays.  
         [0034]     The compositions of the present invention are made by melt-blending the components using any known methods. The component materials may be mixed to homogeneity using a melt-mixer such as a single or twin-screw extruder, blender, kneader, Banbury mixer, etc. to give a resin composition. Or, part of the materials may be mixed in a melt-mixer, and the rest of the materials may then be added and further melt-mixed until homogeneous.  
         [0035]     The compositions of the present invention are transparent. By transparent, it is meant that they have a haze of 12 or less and a light transmittance of at least about 65 percent, or preferably at least about 75 percent, as measured following ASTM D1003 through a 2 mm thick sample of the composition.  
         [0036]     The compositions may be formed into the articles using any suitable melt-processing technique, such as injection molding, extrusion, blow molding, injection blow molding, thermoforming and the like.  
         [0037]     The compositions of the present invention are suitable for the manufacture of any of a wide variety of articles where optical transparency and good mechanical properties are desirable. Without intending to limit the generality of the foregoing, articles formed from the compositions of the present invention can include sight windows for tanks or reservoirs for fuel and/or oil. Other suitable applications are those that require the use of parts that are both optically transparent and have good mechanical properties and creep resistance.  
       EXAMPLES  
       [0038]     Haze and light transmittance are measured on 2 mm thick samples by a integrating sphere hazemeter following ASTM D 1003-61. A sphere of diameter 200-250 mm has a matte-white internal surface. The photocell detects the uniform intensity of scattered light inside the sphere. A circular light trap captures all the incident beam, if no specimen is present, with an annular gap of 1.3 degree. From these measurements the total transmittance T t  is calculated as T t =I2/I1. And the diffuse transmittance T d  as:
 
 T   d =( I 4 −I 3( I 2/ I 1))/ I 1 
 
 Where: 
 
         [0039]     I1: incident light  
         [0040]     I2: total light transmitted by specimen  
         [0041]     I3: light scattered by instrument  
         [0042]     I4: light scattered by instrument and specimen  
         [0000]     Haze=100×(Td/Tt)  
         [0043]     Elongation at break, tensile properties, and notched Izod impact strengths were measured using standard ISO testing procedures at 23° C. on samples dry-as-molded.  
         [0044]     Creep resistance was determined in a dynamic mechanical analyzer using an accelerated creep testing method based on a time-temperature superposition principle. The method is described generally in  Journal of Thermal Analysis and Calorimetry,  69 (2002) 37-52. Time is simulated by heating the sample. For each sample, a 10×42×4.2 mm flexural bar was clamped in a dynamic mechanical analyzer and subjected to a constant stress load of 2000 psi. Experiments were started at ca. 26° C. and temperatures were increased at 5° C. increments to ca. 110° C. At each temperature the sample was allowed about 30 minutes to equilibrate to temperature, experienced load for 15 minutes, and allowed to recover for 60 minutes.  
       Examples 1-4  
       [0045]     The ingredients shown in Table 1 were combined and melt-blended by extrusion. A 58 mm bilobal twin-screw extruder was used in the case of Examples 1 and 2 and a 57 mm trilobal extruder was used in the case of Examples 3 and 4. The melt temperatures were in the range of about 340 to about 345° C.  
         [0046]     Amorphous polyamide A is a polyamide derived from about 50 mole percent hexamethylendiamine, about 15 mole percent terephthalic acid, and about 35 mole percent isophthalic acid. Semicrystalline polyamide A is polyamide 6,T/D,T with a melting point of about 300° C. The UV light stabilizer is N-(2-ethoxyphenyl)-N′-(2-ethylphenyl)-ethanediamide, sold as Sanduvor VSU by Clariant. The optical brightener is Eastobrite® OB-1, available from Eastman Chemical.  
                                                         TABLE 1                                       Example   Example   Example               1   2   3   Example 4                        Amorphous polyamide A   81.3   68.83   81.29   81.3       Semicrystalline   18   30   18   18       polyamide A       Aluminum stearate   0.1   0.17   0.1   0.1       UV light stabilizer   0.6   1   0.6   0.6       Optical brightener   —   —   0.01   —       Elongation at break (%)   11.2   4.4   25   16.5       Tensile strength at break   88.6   85.4   91   92.5       (MPa)       Tensile modulus (GPa)   4.76   3.53   3.74   2.89       Notched Izod impact   9.1   9.8   8.5   —       strength (kJ/m 2 )       Light transmittance (%)   —   —   —   89       Haze   —   —   —   4                  
 
 Ingredient quantities are given in weight percent relative to the total weight of the composition 
 
       Examples 5-9 and Comparative Example 1  
       [0047]     The ingredients shown in Table 2 were combined and melt-blended by extrusion using a 40 mm twin-screw extruder. The melt temperatures were in the range of about 340 to about 345° C. Amorphous polyamide B is a polyamide derived from about 47 mole percent hexamethylendiamine, about 3 mole percent of an alicyclic diamine, about 15 mole percent terephthalic acid, and about 35 mole percent isophthalic acid. Semicrystalline polyamide A is polyamide 6,T/D,T with a melting point of about 300° C. Semicrystalline polyamide B is polyamide 6,T/6,6 with a melting point of about 312° C.  
         [0048]     The creep resistance of each of the compositions was measured as described above and percent total strain (the sum of elastic strain and creep strain) after simulated total duration of up to 10 5  hours is shown in Table 2. The initial strain reported in Table 2 is the elastic strain. A lower percent strain corresponds to greater creep resistance.  
         [0049]     A comparison of Comparative Example 1, an amorphous polyamide, with Examples 5-9, blends of an amorphous polyamide with semicrystalline semiaromatic polyamides, indicates that the presence of the semicrystalline semiaromatic polyamides significantly improves the creep resistance of the compositions.  
                                                                                       TABLE 2                                       Comp.                    Ex. 1   Ex. 5   Ex. 6   Ex. 7   Ex. 8   Ex. 9                        Amorphous   100   95   90   87   90   90       polyamide B       Semicrystalline   —   5   10   13   —   5       polyamide A       Semicrystalline   —   —   —   —   10   5       polyamide B       Strain (%):       Initial   0.55   0.59   0.58   0.54   0.57   0.57       1 h   0.60   0.63   0.62   0.57   0.60   0.60       10 h   0.64   0.66   0.65   0.59   0.62   0.62       10 2  h   0.73   0.72   0.71   0.62   0.66   0.66       10 3  h   0.97   0.86   0.82   0.70   0.75   0.72       10 4  h   1.82   1.22   1.12   0.96   0.95   0.84       10 5  h   —   —   2.11   1.95   1.51   1.14                  
 
 Ingredient quantities are given in weight percent relative to the total weight of the composition