Description
Improved Reinforced Thermoplastic Molding Compositions
Background of the Invention
This invention relates to glass reinforced molding compo- sitions which have improved warp resistance, hig h deflection temperature under load (DTUL.) and high impact strength in the molded article. More particularly, it pertains to composi¬ tions comprising a thermoplastic resin selected from the groupt consisting of a high molecular weight linear polyester, a ix- ture of high molecular weight linear polyesters, a mixture of a high molecular weight linear polyester and high molecular weight block polyester and a mixture of high molecular weight linear polyesters and high molecular weight block copolyester, fibrous glass reinforcement, phlogopite mica and an impact modifier.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(l, 4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction. it has been previously disclosed in cop ending appli cation ___^^
Serial No. 662, 910 filed March 1, 1976, assigned to the same assignee as herein, that glass reinforced thermoplastic compositions of a polycarbonate resin and poly ( 1 , 4-butylene terephthalate) can be molded to articles having greater resis- tance to warpage and/or improved DTTJ.L, in comparison with glass fiber reinforced poly(l, 4-butylene terephthalate) resins. It is further disclosed in co-pending application Serial No. 753, 861, filed December 23, 1976, as signed to the same assignee as herein, that zinc stearate when added to polyester polyblends elevates notched Izod impact strength, while main¬ taining unnotched impact strength, flexural strength and tensile strength and dramatically reduces sample -to -sample variability in elongation. Also, it is disclosed in co-pending application Serial No. 753, 863, filed December 23, 1976, and assigned to the same assignee as herein, that glas s fibers in combination with a mineral filler provide molded articles with improved DTUL. and/or reduced warpage. In addition, it is disclosed in co-pending application Serial No. 747, 635, filed December 6, 1976, assigned to the same assignee as herein, that compo- sitions comprising poly(butylene terephthalate), poly ( ethylene terephthalate), a polycarbonate and glass fiber have increased DTUL. and/or reduced warpage. It is disclosed in co-pending application Serial No. 778, 945, filed March 18, 1977, that blends of a poly( 1 , 4-butylene terephthalate) resin and a poly- carbonate resin reinforced with fibrous glass, when admixed with a small amount of zinc stearate poss ess even less inhere warpage in the molded article and good moldability when com¬ pared with compositions of glass fiber reinforced poly(l, 4- butylene terephthalate). i has now been discovered that compositions of high mole¬ cular weight linear polyesters, optionally in admixture with block copolyesters, fibrous glass reinforcement, phlogopite mica and an impact modifier possess very little inherent
warpage in the molded article and, in addition, are charac¬ terized with good impact strength and high deflection tempera¬ ture under load (DTUL). Description of the Invention According to this invention, there are provided reinforced thermoplastic compositions having increas ed resistance to warpage, as well as high deflection temperature under load (DTUL) and good impact strength, useful for molding, e. g. , injection molding, compres sion molding, transfer molding, the composition comprising:
(a) a thermoplastic resin s elected from the group consisting of a high molecular weight linear polyester, a mixture of high molecular weight linear polyesters, a mixture of a high mole¬ cular weight linear polyester and high molecular weight block copolyester and mixtures of high molecular weight linear polyesters and high molecular weight block copolyester where¬ in said block copolyester is derived from blocks of
(i) a terminally-reactive poly( l, 4-butylene terephtha¬ late) and (ϋ) terminally- reactive aromatic /aliphatic copoly¬ ester of a dicarboxylic acid s elected from the group consisting of terephthalic acid, isophthalic acid, naphthalene dicarboxylic acids, phenyl indane dicarboxylic acid and compounds of the formula:
in which X may be alkylene or alkylidene of from 1 to 4 car¬ bon atoms, carbonyl, sulfonyl, oxygen or a bond between the
benzene rings, and an aliphatic dicarboxylic acid having from 6 to 12 carbon atoms in the chain and one or more straight or branched chain dihydric aliphatic glycols having from 4 to 10 carbon atoms in the chain, said copolyester having at least 10% of aliphatic units being derived from a dicarboxylic acid, or
(iii) a terminally-reactive aliphatic polyester of a straight chain aliphatic dicarboxylic acid having from 4 to 12 carbon atoms in the chain and a straight or branched chain aliphatic glycol, said blocks being connected by inter terminal linkages consisting essentially of ester linkages;
(b) an impact modifier;
(c) fibrous glass reinforcement; and
(d) phlogopite mica. The high molecular weight linear polyesters of this invention include, in general, linear saturated condensation products of diols and dicarboxylic acids, or reactive derivatives thereof. Preferably, they will comprise condensation products of aro¬ matic dicarboxylic acids and aliphatic diols. Although the diol portion of the polyester can contain from two to ten carbon atoms, it is preferred that it contain from two to four carbon atoms in the form of linear methylene chains . It is to be understood that it is also pos sible to use polyesters such as poly (1, 4-dimethylol cyclohexane dicarboxylates, e. g. , terephtha- lates). In addition to phthalates , small amounts of other aromatic dicarboxylic acids, such as naphthalene dicarboxylic acid, or aliphatic dicarboxylic acids, such as adipic acid, can also be present in preferred compositions. The diol constituent can likewise be varied, in the preferred embodiments, by adding small amounts of cyclo aliphatic diols, such as cyclo- hexanedimethanol. In any event, the preferred polyesters are well known as film and fiber formers, and they are provided
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- 5- by methods outlined in hinfield, U. S. 2, 465, 319 and Pengilly, U. S. 3, 047, 539 and elsewhere. The preferred polyesters will comprise a poly(alkylene terephthalate), isophthalate or mixed isophthalate-terephthalate, e. g. , up to 30 mole percent isophtha- late, said alkylene groups containing from 2 to 10 carbon atoms, e. g. , poly(ethylene terephthalate) or poly(l, 4-butylene terephtha¬ late). Because of its rapid crystallization from the melt, it is preferred to use poly(l, 4-butylene terephthalate) as the linear polyester resin component of the present compositions. Also contemplated within the scope of this invention are the use of mixtures of linear polyesters, such as a mixture of polyethy¬ lene terephthalate) and poly (1, 4-butylene terephthalate). "When these mixtures are utilized the poly(ethylene terephthalate) can be used in amounts of from about 1 -40% by weight, preferably 15-30% by weight of the total composition.
The "block copolyesters" also us eful in the compositions of this invention are prepared by the reaction of terminally- reactive poly(butylene terephthalate), preferably, low molecular weight, and a terminally-reactive copolyester or polyester in the presence of a catalyst for trans esterification, such as zinc acetate, manganese acetate, titanium esters, and the like. The terminal groups are hydroxyl , carboxyl, carboalkoxy, and the like, including reactive derivatives thereof. The result of reaction between two terminally reactive groups, of course, must be an ester linkage. After initial mixing, polymerization is o carried out under standard conditions, e. g. , 220 to 280 C. , in a high vacuum, e. g. , 0. 1 to 2 m Hg, to form the block copolymer of minimum randomization in terms of distribution of chain segments. These copolyesters are described in co- pending U. S. application Serial No. 752, 325, filed on December
20, 1976, incorporated herein by reference.
The copolyester designated component (ii), hereinabove, is
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preferably prepared from terephthalic acid or isophthalic acid or a reactive derivative thereof and a glycol, which may be a straight or branched chain aliphatic glycol. Illustratively, the glycol will be 1, 4-butanediol; 1, 5-pentanediol; 1, 6 -hexane- diol; 1, 9-nonanediol; 1, 10-decanediol; neopentyl glycol; 1, 4- cyclohexanediol; 1, 4- cyclohexane dimethanol, a mixture of any of the foregoing, or the like. Illustrative of suitable aliphatic dicarboxylic acids for the mixed aromatic/aliphatic embodiment are suberic, sebacic, azelaic, adipic acids, and the like, The copolyesters may be prepared by ester interchange in accordance with standard procedures . The copolyesters designa ted (ii) are most preferably derived from an aliphatic glycol and a mixture of aromatic and aliphatic dibasic acids in which the mole ratio concentration of aromatic to aliphatic acids is from between 1 to 9 and 9 to 1, with an especially preferred range being from about 3 to 7 to about 7 to 3.
The terminally reactive aliphatic polyesters designated com¬ ponent (iii) will contain substantially stoichiometri c amounts of the aliphatic diol and the aliphatic dicarboxylic acid, although hydroxy- containing terminal groups are preferred.
In addition to their ease of formation by well-known proce¬ dures, both the aromatic /aliphatic copolyesters (ii) and the aliphatic polyesters (iii) are commercially available.
One source for such materials is the Ruco Division/Hooker Chemical Company, Hicksville, New York, which designates its compounds as "Rucoflex. "
The block copolyesters contemplated herein preferably comprise from 95 to 50 parts byweight of the segments of poly (1, 4-butylene terephthalate). The poly( l, 4-butylene tere- phthalate) blocks, before incorporation into the block copoly- - ■■ esters, will preferably have an intrinsic viscosity of above 0. 1 dl/g. and preferably, between 0. 1 and 0. 5 dl/g. , as
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measured in a 60:40 mixture of phenol/tetrachloroethane at o 30 c. The balance, 5 to 50 parts by weight of the copolyester will comprise blocks of components (ii) or (iii).
As will be understood by those skilled in the .art, the poly(l, 4-butylene terephthalate) block can be straight chain or branched, e. g. , by use of a branching component, e. g. , 0. 05 to 1 mole %, based on terephthalate units, of a branching component which contains at least three ester-forming groups. This can be a glycol, e. g. , pentaerythritol, trimethylolpropane, and the like, or a polybasic acid compound, e. g. , trimethyl trimeεate, and the like.
Among the impact modifiers included within the scope of the present compositions are segmented copolyester-ethers known as Hytrel (duPont), silicone-polycarbonate block copolymer, e. g. , Copel 3320 (G. E. ) and ethylene-vinyl acetate copolymers (Alathon, by duPont). In addition,, a mixture of ethylene-vinyl acetate copolymer and polyethylene homopolymer is useful. In general, the impact modifier is present in the composition of this invention in amounts within the range of from about 1 to 20%, by weight, preferably 1-15%, by weight, of the total com¬ position.
The segmented copolyester-ethers useful as impact modifiers herein are described in U. S. 3, 023, 182, U. S. 3, 651, 014, U. S. 3, 763, 109 and U. S. 3, 766, 146, which are incorporated herein by reference. Typical copolymers of ethylene and vinyl acetate include, for example, Alathon 3152 ( 15% by weight vinyl content), Alathon 3194 (25% by weight vinyl acetate), Alathon 3180 (28% by weight vinyl acetate), Vynathene EY 903, (45% by weight vinyl acetate, sold by U. S. I. Chemicals) and Vynathene EY 904 ( 52% by weight vinyl acetate). It has been obs erved that co¬ polymers of ethylene and vinyl acetate impart more impact enhancement when used in conjunction with polyethylene homo- polymer.
The fibrous glass employed as component (c) in the present compositions is well known to those skilled in the art and is widely available from a number of manufacturers. For com¬ positions ultimately to be employed for electrical use, it is preferred to use fibrous glass filaments comprised of lime- aluminum borosilicate glas s that is relatively soda free. This is known as "E" glass . However, other glasses are useful where electrical properties are not so important, e. g. , the low soda glass known as "C" glass . The filaments are made by standard processes, e. g. , by steam or air blowing, flame blowing and mechanical pulling. The preferred filaments for plastics reinforcement are made by mechanical pulling. The filament diameters range from about 0. 00030 to about 0. 0019 cm (about 0. 00012 to 0. 00075 inch), but this is not critical to the present invention.
The length of the glass filaments and whether or not they are bundled into fibers and the fibers bundled in turn to yarns, ropes or rovings, or woven into mats, and the like, are also not critical to the invention. However, in preparing the molding compositions, it is convenient to use the filamentous glass in the form of chopped strands of from about 0. 29 to about cm (about one-eighth to about 2 inches) long . In articles molded from the composition s, on the other hand, even shorter lengths will be encountered because, during compounding, considerable fragmentation •will occur. This is desirable, however, because the best properties are exhibited by thermo¬ plastic injection molded articles in which the filament lengths lie between about 0. 00025 and 0. 32 cm (about 0. 0001 and 0. 125 (one -eighth) inch). For the purposesof this invention, the fibrous glass rein¬ forcement and the phlogopite mica are employed in a combined amount of from about 1 to about 60% by weight of the total
composition, the preferred range being from about 25 to about 40% by weight.
The phlogopite mica used in the practice of the present invention has a particle size of from about 10-325 mesh. Particularly preferred phlogopite micas are commercially avail¬ able from Marietta Resources, Ltd. , Montreal, and are known as Suzorite (20H, 10-20 mesh; 200 H, less than 100 mesh; 200 S, less than 100 mesh; 150 S, less than 100 mesh, 80 S, 99% less than 100 mesh. Among these, the most preferred, from a standpoint of conferring the best combination of properties, appears to be Suzorite 150 S.
Other materials which may also be included in the composi¬ tions of the present invention in conjunction with the heretofore identified impact modifiers, include polyethylene copolymers, like ethylene -propylene copolymers, ethylene -ethylene acrylate copolymers and ethylene -acrylic acid salt copolymers. Further optional materials include flame retardants like decabromodi- phenyl ether and halogenated aromatic polycarbonates. Illustra¬ tive flame - retardant additives are disclosed in U. S. 3, 833, 685, U. S. 3, 195, 926 and U. S. 3, 671, 487, which are incorporated herein by reference. Other flame retardants are disclosed in U. S. 3, 681, 281 and U. S. 3, 557, 053, U. S. 3, 830, 771 and U. K. 1, 358, 080, all of which are incorporated herein by reference. The flame retardants are generally utilized in any flame retardant quantity whereas the other optional ingredients like polyethylene, can be used in amounts of from about 0. 5 to 5% by weight of the total composition or in any conventional amounts for the purposes intended.
The compositions of this invention can be prepared by a number of procedures. In one way, the fibrous glass rein¬ forcement, phlogopite mica and impact modifier are dispersed in a matrix of the resin in the proces s . In another procedure
the glas s reinforcement, phlogopite mica and impact modifier are mixed with, for example, the poly( l, 4-butylene terephtha¬ late) resin by dry blending, then either fluxed on a mill and comminuted, or they are extruded and chopped. The glass reinforcing agent, phlogopite mica and impact modifier can also be mixed with the resin and directly molded, e. g. , by injection or transfer molding techniques.
Although it is not essential, best results are obtained if the ingredients are pre-compounded, pelletized and then molded. Pre- compounding can be carried out in conventional equipment. For example, after carefully pre-drying the polyester, e. g. , at 125 C for 4 hours, a single screw extruder is fed with a dry blend of the ingredients, the screw employed having long transition and metering sections to ensure proper melting. On the other hand, a twin screw extrusion machine, e. g. , a Werner & Pfleiderer machine, can be fed with resins and additives at the feed port and reinforcement downstream. In either case, a generally suitable machine temperature will be about 230 to 300°C. (about 450 to 570°F). The pre-compounded composition can be extruded and cut up into molding compounds such as conventional granules , pellets, etc. , by standard techniques .
The compositions can be molded in any equipment conven¬ tionally used for glas s -filled thermoplastic compositions, e. g. , Van Dorn type injection molding machine with conventional o o cylinder temperatures, e. g. , 274 C. (525 F), and conventional
, ,o o mold temperatures, e. g. , 66 C ( 150 F).
Des cription of the Preferred Embodiments
The following examples illustrate the invention. They are set forth as a further description but are not to be construed as limiting the invention thereto.
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Examples 1-5 The following formulations are mechanically blended, then extruded and molded into test pieces in a Van Dorn injection molding machine. The properties are also summarized in the following table.
5 TABLE I
Example 1* 2* 3* 4 5
Ingredients (parts by weight)
VALOX 300 63.7 58.7 43.7 51.2 46.2
Fibrous glass r einf o r cement 11 11 11 11 11
English mica C-1000 < 325 mesh 25 25 25 _ _ _ —
Suzorite mica 20H 25 25
Copel 3320b -- 5 20 12.5 17.5 c Mold release agent 0.10 o.io 0.10 0.10 0.10 d Irganox 1093 0.15 0.15 0.15 0.15 0.15
Ferro 904 0.05 0.05 0.05 0.05 0.05
Properties arpage R. T. (mm) 10 5 2 0 0
Warpage 177 'c (350 °F)/ 30 min, (mm) 16 11 7 & <ι
Warpage 149°C (300° F)/ 30 min, (mm) 13 9 2 <! <ι
Notched Izod 3.59 4.35 7.40 7.07 8.38
Impact, Kgf cm/ cm(ft. lb/in. ) (0.66) (0.80) ( (11..3366)) (1.30)(1.54) 5 Unnotched Izod 34.8 38.1 46.8 25.6 28.3
Impact, Kgf cm/ cm (ft. lb /in. ) (6.4) (7.0) (8.6) (4.7) (5.2)
DTUL at 18.6 kgf/ cm2 213 197 161 188 169
(264 psi), °C (°F) (416) (387) (321) (370) (336)
■- Control Q (a) poly ( 1, 4-butylene terephthalate) intrinsic viscosity 0.9 measured in a solution of phenol and tetrachloroethane (60:40) at 30°C. r
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(b) silicone-poly carbonate block copolymer (GE)
(c) Mold Wiz (Int. EQ-6)
(d) tetrakis 3, 5-di-t-butyl-4 hydroxyphenylpropionyloxymethyl) methane
(e) diphenyl decylphosphite
The above data clearly show the remarkable warp resistanc of the compositions of this invention which employ phlogopite mica (Suzorite), as well as the good impact strength and DTUL in comparison with similar compositions containing muscovite mica (English mica C-1000).
Examples 6 - 9
The following formulations are mechanically blended, then extruded and molded into test pieces in a Van Dorn injection molding machine. The properties of the compositions are also summarized in Table 2.
Table 2
Example Ingredients H 1 H 2.
(parts by weight)
VALOX 300 48.7 51.7 28.7 31.7 fibrous glass reinforcement 11.0 11.0 11.0 11.0
Suzorite mica 20H 25.0 25.0 25.0 25.0 Hytrel 4056 15.0 10.0 15.0 10.0
Microthene FN-510
(polyethylene) _-. 2.0 -- 2.0
2 poly(ethylene terephthalate) -- -- 20.0 20.0
Irganox 1093 (antioxidant) 0.15 0.15 0.15 0.15
Ferro 904. (antioxidant) 0.05 0.05 0.05 0.05 Mold Wiz (mold release agent) 0 0..1100 0.10 0.10 0.10
Table 2 cont'd
Properties
Warpage R. T. (mm) 0 0 0 0 Warpage 177°C (350°F)/
30 min. (mm) 0 <1 <1 0
Warpage 149° C (300°F)/
30 min. (mm) 0 0 1 0
Notched Izod Impact 7.13 6.91 6.96 6.85
Kgf cm/ cm (ft/lb/in) (1.31) (1.27) (1.28) (1.26) Unnotched Izod Impact 23.4 22.8 27.2 26.7
Kgf cm/ cm (ft/lb/in) (4.3) (4.2) (5.0) (4.9)
DTUL at 18.6 Kgf/cm2 159 168 151 159
(264 psi), °C (°F) (319) (335) (303) (319)
1) segmented copolyester-ether (duPont)
2) intrinsic viscosity of 0.62 measured in a 60/40 solution of phenol and tetrachloro ethane.
The resistance to warpage of the above compositions is excellent.
Examples 10 -12 The following formulations are mechanically blended, then extruded and molded into test pieces in Van Dorn injection molding machine. The properties are summarized in the table below:
QMPI V-
Table 3
Example 10 11 12
Ingredients (parts by weight)
VALOX 300 41.8 31.8 36.8 fibrous glass reinforcement 11.0 11.0 11.0
Suzorite mica 200 S 25.0 25.0 25.0 poly(ethylene terephthalate)* 20.0 20.0 20.0
Alathon 3194 - 10.0 -
Alathon 31522 - - 7.0
Microthene FN-510 2.0 2.0 -
Irganox 1093 0.15 0.15 0.15
Ferro 904 0.05 0.05 0.05
Properties
Warpage R. T. 1.5 Warpage 177°C (350°F)/ 30 min. (mm) 2.5
Warpage 149°C (300°F)/ 30 min. (mm) 1.5 2.5 4.8
Notched Izod Impact, 4.35 5.22 5.00 Kgf cm/ cm (ft/i /in) (0.80) (0.96) (0.92)
Unnotched Izod Impact, 35.9 40.8 38.6 Kgf cm/ cm (ft/lb/in) (6.6) (7.5) (7.1)
DTUL at 18.6 Kgf/cm 187 178 194 (264 psi), °C (°F) (368) (352) (382)
(1) ethylene-vinyl acetate copolymer (25% vinyl acetate, duPont) (2) ethylene-vinyl acetate copolymer (15% vinyl acetate, duPbnt) (3) polyethylene, average particle size less than 20 microns, sold by U.S.I. Chemicals, New York, New York, ^(intrinsic viscosity same as Table 2)
Example s 13-16 The following formulations are mechanically blended then extruded and molded into test pieces in a Van Dorn injection
molding machine. The properties are summarized in Table 4.
Table 4 Example
Ingredients
(parts by weight) 11 ii 11 ii
VALOX 300 16.8 19.8 13.8 13.8
Fibrous glass reinforcement 11.0 11.0 11.0 11.0
Suzorite mica 200 S 25.0 25.0 25.0 --
Suzorite mica 150 S _- __ -- 25.0 poly( ethylene terephthalate)* 20.0 20.0 20.0 20.0
ALATHON 3194 7.0 7.0 7.0 7.0
Polyethylene 2.0 2.0 2.0 2.0
RL-1624 (G.E.)1 13.0 10.0 16.0 15.0
Antimony oxide 5.0 5.0 5.0 6.0
Irganox 1093 0.15 0.15 0.15 0.15
' Ferro 904 0.05 0.05 0.05 0.05
Properties
Warpage, R. T. 0 0 0 0
Warpage, 177°C (350°F)/
30 min. (mm) 11 8.5 8 < . o o
Warpage, 149 C (300 F)/
30 min. (mm) 4 7 4 <'
Notched Izod Impact 4.84 4.84 4.90 5.44 Kgf cm/ cm (ft /lb /in) (0.89) (0.89) (0.9) (1.0)
Unnotched Izod Impact, 34.8 35.9 33.7 32.1 Kgf cm/ cm (ft/lb/in) (6.4) (6.6) (6.2) (5.9) , 2
DTUL at 18.6 Kgf/ cm 162 162 162 162
(264 psi), °C (°F) (324) (324) (324) (323)
UL Standard 9 94 V-O 94 V-O 94 V-O 94 V-O
(1) Flame -retardant, aromatic (copoly-)carbonate 50:50 mole ratio of bisphenol A and tetrabromobisphenol A. ^(intrinsic viscosity same as Table 2. )
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Example 17
The following formulation was mechanically blended, extrud and molded into test pieces in a Van Dorn injection molding machine. The composition and properties are summarized below:
5 Ingredient 17 (parts by weight)
Valox 300 34.8 fibrous glass reinforcement 11.0
Suzorite mica 150S 25.0
1n poly( ethylene terephthalate) 20.0
Alathon 3194 7.0 polyethylene 2.0
Ferro 904 (antioxidant) 0.05
Irganox 1093 (antioxidant) 0.15 iς Properties
Warpage R. T. (mm) 0
Warpage, 177°C (350°F)/30 min. (mm) __l
Warpage, 149°C (300°F)/30 min. (mm) l
Notched Izod impact 5.44 Kgf cm/cm (ft/lb/in) (1.0)
Example 18 The following formulation was mechanically blended, extruded and molded into test pieces in a Van Dorn injection molding machine. The composition and properties are summarized below:
Ingredient 18
(parts by weight ) block copolyester of poly( l, 4-butylene terephthalate) and poly(l, 6-hexylene-(0. 7) azelate-(0. 3) isophthalate molecular weight of block 300 34. 8 fibrous glass reinforcement 11. 0
Suzorite mica 150 S 25. 0 poly (ethylene terephthalate) 20. 0
Alathon 3194 7. 0
, _ polyethylene 2. 0 15
Ferro 904 (antioxidant) 0. 05 Irganox 1093 ( antioxidant) 0. 15
Properties -0 Warpage R. T. (mm) 0
Warpage, 177 C (350 F)/30 min. (mm) 7 Warpage 149°C (300°F)/30 min. (mm) <1 Notched Izod Impact Kgf cm/cm (ft. lbsJin. ) 6. 85 ( 1. 26)
DTUL at 18. 6 Kgf/ cm (264 psi), °C (°F) 158
25 (316)
Obviously, other modifications and variations of the present invention are possible in light of the above teachings. For example, small amounts of materials such as dyes, pigments, stabilizers, plasticizers and the like can be added to the present compositions. It is to be understood, therefore, that changes may be made in the particular embodiments of fee invention described which are within the full intended scope of the invention as defined by the appended claims.
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