JPH03810A - Nylon 66 yarn having high modulus of elasticity and production thereof - Google Patents

Abstract

PURPOSE:To obtain the title nylon 66 yarn useful for cord material of belt part of radial tire, having initial modulus of elasticity close to that of polyester, specific elongation at break, high modulus of elasticity, high strength and dimensional stability. CONSTITUTION:A low-molecular nylon 66 polymer having 2.3-2.9 relative viscosity in 10mg/ml polymer concentration at 20 deg.C in dissolving the polymer in 96% concentrated sulfonic acid solution is dried in vacuum, the polymer is subjected to melt spinning at >= the melting point of the polymer, extruded from a nozzle orifice at a shear rate r shown by the formula (Q is g/second amount delivered in single hole; D diameter cm of nozzle orifice; rho is specific gravity g/cm<3> of nylon 66) of <4X10<4>sec<-1>, cooled solidified, drawn at >=30X10<-3> birefringence, subjected to high-tension orientation at one stage or several stages at a temperature in a melting point peak of DSC melt curve to give the aimed nylon 66 yarn having >=70g/d initial modulus of elasticity, <=10% breaking elongation and <=3.0 dry heat shrinkage percentage at 160 deg.C.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a high modulus nylon 66 fiber and a method for producing the same, and more specifically, the present invention relates to a high modulus nylon 66 fiber and a method for producing the same. Nylon 66 fiber and such ia
The present invention relates to a method for producing fibers.

[Prior Art] Nylon 66 fibers are industrially manufactured by melt-spinning nylon 66 having a relative viscosity of less than 4 at a temperature above its melting point, followed by hot drawing and heat treatment. Nylon 5aia fibers have been used as tire cord fibers or various resin reinforcing materials because they have superior fatigue resistance, strength, and adhesion to rubber compared to other polyamide fibers. However, although nylon 66 fibers are slightly lower than other polyamide fibers, they have the drawbacks of lower dimensional stability and lower initial elastic modulus than other materials used in industry. It is insufficient as a fiber for cords.For example, nylon 66 obtained by conventional methods
Regarding the physical properties of the fiber, the initial elastic modulus is 40~ even for high-strength type.
60 g/d, and the strength is about 6.0 to 9.0 g/d (Industrial Textile Materials Handbook 2, Japan Textile Machinery Society,
(Showa 54).

Examples of nylon 661a fibers for industrial materials include JP-A-58-60012, JP-A-58-174623, and JP-A-58.
-208413, 59-26517, 60-8
No. 13115, No. 60-28537, No. 60-104
No. 546.

Many are known in JP-A No. 81-34215, No. 61-194214, etc., and in particular, JP-A No. 58-208413,
No. 59-26517 has an initial elastic modulus of 100 g/d.
A method for producing nylon 66 fiber has been proposed. However, even with these techniques, the elongation at break and dry heat shrinkage are still insufficient, and the dimensional stability required for fibers for tire cords has not been improved.

[Problems to be Solved by the Invention] The present invention was made to solve these technical problems, and its purpose is to create a nylon 66 fiber that has high elastic modulus, high strength, and excellent dimensional stability. and to provide dimensions for manufacturing such nylon 66 fibers.

[Means for Solving the Problems] The high modulus nylon 66Ifi fiber according to the present invention is characterized in that it satisfies the following requirements (a) to (c).

(a) Initial modulus of elasticity ≧70 g/d (b) Elongation at break ≦10% (c) 1 a °C dry heat shrinkage rate ≦3.0% Moreover, the above-mentioned high modulus nylon 66 has a breaking strength of 8.0 g/d. On the other hand, the high modulus nylon 66 fibers as described above, when dissolved in a 96% concentrated sulfuric acid solution at a polymer concentration of 10 mg/ml, have a high strength. A low molecular weight nylon 66 polymer having a relative viscosity of 2.3 to 2.9 at 20°C is vacuum-dried, and then melt-spun at a temperature above the melting point of the polymer to form the following (
1) The shear rate r expressed by the formula is 4X 10'sec
It is extruded from a nozzle orifice and cooled and solidified so that the birefringence of the four threads becomes less than 30x10-3, and the birefringence of the four spun threads is taken up to be 30X10-3 or more.
It is obtained by carrying out one or several stages of high-tension stretching at a temperature within the melting point peak in the C melting curve.

However, Q: single hole discharge rate (g/5ec) D = nozzle orifice diameter (cm) ρ: ratio of nylon 66 NIL (g/cm3) As described in JP-A No. 59-26517, it was believed that the higher the degree of polymerization of fibers (polymer), the better.

However, the present inventors conducted research from the three viewpoints of (1) the molecular weight of nylon 66 polymer, (2) oriented crystallization during high-speed spinning, and (2) stretchability. Crystallization during crystallization progresses significantly, and even if these undrawn yarns are drawn using normal drawing equipment, the high degree of crystallinity actually hinders the drawability and makes it difficult to achieve high physical properties. I discovered something. That is, in order to achieve a high elastic modulus using conventional polymers with a high degree of polymerization, for example, Japanese Patent Application Laid-Open No. 58-20841
It has been found that it is essential to carry out special stretching that is not accompanied by productivity, as shown in No. 3 and No. 59-28517.

Therefore, the present inventors, without being bound by conventional fixed concepts, conducted intensive research in order to achieve higher performance of nylon 66 fiber. As a result, by using a relatively low molecular weight polymer, melt-spinning this polymer to form an undrawn yarn that has been properly oriented and crystallized, and drawing it under an air surrounding the palm of your hand near its melting point, it is possible to create highly oriented and amorphous yarn. The present invention was completed based on the discovery that highly oriented crystallized threads with a high degree of elasticity can be produced, and that the obtained nylon 66 fibers have a high elastic modulus, high strength, and excellent dimensional stability. That is, in the present invention, a nylon 66 polymer having a relative viscosity of 2.3 to 2.9 when measured under certain conditions is used, and an undrawn yarn with a suppressed degree of crystallization during oriented crystallization is spun to form a fiber. By stretching the constituent polymer chains as highly as possible in the fiber direction (that is, increasing the total stretching ratio of the fiber as much as possible), (a) initial elastic modulus ≧70 g/d, (
b) Breaking elongation ≦10%, (c) 160°C dry heat shrinkage rate ≦3
High elastic modulus nylon 66iain that satisfies the requirement of 0% and more preferably has a breaking strength of 8.0 g/d or more.
was achieved.

The nylon 66 fiber used in the present invention is substantially composed of polyhexamethylene adipamide units as a main component (more than 95% mole), but does not contain other copolymer components or blend polymers. It's okay to stay. Examples of other polyamide components that can be copolymerized include ε-capramide,
Examples of such polymers include hexamethylene adibamide, hexamethylene terephthalamide, hexamethylene isophthalamide, and the like. Examples of polymers that can be blended include polymers obtained by polymerizing the above-mentioned copolymerizable components.

These nylon 66 polymers may be blended with matting agents, B ingredients, light stabilizers, heat stabilizers 9, antioxidants, antistatic agents, dyeability improvers, adhesion improvers, etc. as necessary. All can be used as long as they do not have a serious adverse effect on the characteristics of the present invention depending on their formulation.

When the nylon 8BI11 fiber of the present invention is used for industrial purposes, it is preferable to add an antioxidant for the purpose of imparting sufficient durability against heat, light, oxygen, etc. As antioxidants, copper salts such as copper acetate, cuprous chloride, cupric chloride,
Cuprous bromide, cupric bromide, cuprous iodide, copper phthalate, copper stearate.

and complex salts of various copper salts and organic compounds, such as copper complex salts of 8-oxyquinoline copper and 2-mercaptobenzimidazole, preferably cuprous iodide complex salts of cuprous iodide, copper acetate, and 2-mercaptobenzimidazole. etc., alkali or alkaline earth metal halides such as potassium iodide, potassium bromide, potassium chloride, sodium iodide, I.lium bromide, zinc chloride, calcium chloride, etc., and organic halides such as pentaiodobenzene. , hexabromobenzene, tetraiodoterephthalic acid.

Methylene iodide, tributylethylammonium iodide, etc., and inorganic and organic phosphorus compounds such as sodium birophosphate, sodium phosphite, triphenyl phosphate, (9,10)-sibidro 10-(3',
5°-di-t-butyl-4'-hydroxybenzyl)-
9-Oxabar phosphaphenanthrene-10-oxide etc., and phenolic antioxidants such as tequilaquis[methylene-3-(3,5-di-t-butyl-4
-hydroxyphenyl)propionate-co-methane, (
1,3,5,)-trimethyl-(2,4,6,>-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, n-octadecyl-3-(3゜5
-di-t-butyl-4-hydroxyphenyl)-propionate, 4-hydroxy-3,5-di-t-butylbenzyl phosphate diethyl ester, etc., and amine antioxidants such as N,N'-di-β -naphthyl-p-phenylenediamine, 2-mercaptobenzimidazole, phenyl-β-naphthylamine, N, N diphenyl-p-
Examples include phenyl diamine, a condensation reaction product of diphenylamine and allyl ketone, preferably potassium iodide, 2-mercaptobenzimidazole, and the like.

The antioxidant can be added during the polymerization process of the nylon 66 polymer, or once it has been made into chips, it can be sprinkled onto the chips.

The content of antioxidant is 10 to 300% as copper for copper salt.
ppm, preferably 50-200 ppm, other antioxidants 0.01-1%, preferably 0.03-O,S%
is within the range of Antioxidants are usually 1 fffi or 2 f! of copper salts and other antioxidants. It is preferable to use one or more in combination.

The method for producing nylon 66 fibers and the properties of the fibers according to the present invention will be described in more detail below.

The nylon 66 polymer used in the present invention needs to have a relative viscosity (measuring method will be described later) of 2.3 to 2.9. This is because if the relative viscosity is less than 2.3, the spinnability deteriorates, a uniform undrawn yarn cannot be obtained, and the elastic modulus of the drawn yarn decreases.

On the other hand, when the relative viscosity exceeds 2.9, crystallization during orientation crystallization during high-speed spinning becomes noticeable, and crystallization occurs in a relatively low degree of orientation region. As a result, this high degree of crystallization suppresses the improvement of the degree of orientation, and the target degree of orientation may not be achieved, or even if it is achieved, the drawability decreases due to the high degree of crystallinity, and the drawn yarn The elastic modulus and strength decrease, and the elongation at break increases. Nylon 66 polymer having a relative viscosity of 2.3 or more and 2.9 or less is vacuum dried and then melt extruded at a temperature higher than the melting point of nylon 66. Preferably, this temperature is at least 20° C. higher than the melting point. Melt extrusion methods are not particularly limited, but include extruder type extruders, piston type extruders,
A method using a device such as a twin-screw kneading extruder is exemplified.

The conditions for extruding from the extruder through the nozzle orifice require that the shear rate r be less than 4 x 10'5ec-'. That is, r is 4X 10'5ec-1
If this is the case, melt fracture is likely to occur, making it difficult to improve the physical properties.

The shear rate r is calculated using the following formula (I). However, Q: Single hole discharge rate (g/5ec) D: Nozzle orifice diameter (cm) ρ: Specific gravity of nylon 66 (g/cm3) Extrusion The nylon 66 yarn thus obtained is cooled and solidified, and an appropriate amount of oil is applied thereto, and then taken up so that the birefringence Δn of the yarn becomes 30X10-3 or more, preferably 36X10-45X10-'. If the birefringence Δn is less than 30X10-3, the fine structure peculiar to high-speed spinning will not be noticeable, and when these yarns are drawn, the strength will improve, but the elongation and shrinkage will be large, and the elastic modulus will be low. It becomes a drawn yarn with conventionally known properties.

The undrawn nylon 66 fiber thus obtained has a temperature T1 within the peak melting point of the DSC melting curve shown in FIG. ~
High tension stretching is carried out within the T, , 2 region. In the figure, T1 is the temperature at which the melting line departs from the base line, T,2 is the melting point peak temperature,
Tm3 is the temperature at which melting of the undrawn yarn is completed.

If the stretching temperature is less than T,1, it is difficult to rearrange the fine structure formed during spinning and align the molecular chains, and the strength is 8 g/d or more and the initial elastic modulus is 70 g/d or more. As a result, it becomes impossible to maintain the dry heat shrinkage rate below 3%. Furthermore, at temperatures exceeding Twa2, the polymer used in the present invention has a relatively low molecular weight, so the yarn is likely to melt and break, making the stretching unstable.

Examples of stretching heaters that can be used include axial radiation heaters, ceramic heaters, infrared heaters, microwaves, lasers, etc. The heating aether is not particularly limited, but air can be used.

An atmosphere of N, He, Ar, etc. is exemplified, and the heater shape may be any of a contact type, a non-contact type, a hot plate, a hot roller, a hot bottle, etc.

Next, methods for measuring the physical properties specified in the present invention will be described.

(1) Measuring method of birefringence (△n) A Nikon polarized light microscope POH model Uitsu Berek convencator was used, and the light source was a spectral light source activation device (
Toshiba 5LS-3-B type) was used (Na light source). 5-6
A sample cut at an angle of 45 degrees to the fiber axis of mm length,
Place the cut side up on a glass slide. Place the slide glass on a rotating stage, adjust the rotating stage so that the sample is at a 45 degree angle to the polarizer, insert the analyzer, set the dark field, and turn the compensator on.
Set it to 0 and count the number of stripes (n pieces). Turn the compensator clockwise and measure the scale of the compensator at the point where the sample first becomes darkest. (read up to 1/10 scale in both cases). Return the compensator to 30, remove the analyzer, measure the diameter d of the sample, and calculate the birefringence (Δn) based on the following formula (average value of 20 measurements).

Δn = r' / d r (retardation) depth nλ0+6 λO! 589.3 mμ ε: C/100 in Leitz compensator manual
1 determined from 00 and i = (a-b): Difference in convensator reading (2) Specific gravity A density gradient tube made of toluene and carbon tetrachloride is prepared, and 3
Place a sufficiently defoamed sample into a density gradient tube whose temperature is controlled to 0°C ± 0.1°C, and after leaving it for 5 hours, read the position of the sample in the density gradient tube on the scale of the density gradient tube. , Convert to specific gravity value from the density gradient tube scale to ratio calibration graph using a standard glass float. Measure with n■4, and as a general rule, read the specific gravity to 4 digits after the decimal point.

(3)! Measurement method of strength and elongation properties of am The tensile strength (strength) and elongation at break (elongation) of fibers are determined by
IS-L-1013 (1981) Noah, TENS I LON constant speed extension type universal tensile testing machine manufactured by Toyo Baldwin Co., Ltd. in a test room under standard conditions according to 5.1.
The tensile strength of single fibers was measured using tJTM-111.

However, the measurement conditions were as follows: Using a 5Jf tensile load cell, the sample was pulled at a gripping interval of 20cm, a pulling speed of 20cm/min (stretching speed of 100% of the gripping interval per minute), and a recording paper feeding speed of 50cm/min. The load when the sample is cut! (gf) is measured, and the tensile strength (gf
/d) was calculated and used as the strength (g/d).

In addition, the elongation (%) when the sample was cut was measured and defined as the elongation at break (%).

(4) Measuring method of initial tensile modulus of fiber The initial tensile resistance dust (initial tensile modulus) of fiber is determined according to JIS-
The test was conducted using the same method as the above method for measuring fiber strength according to 745°1 of JIS-L-1013 (1981), and a load-elongation curve was drawn on the recording paper, and from this figure JIS-L-1
Calculate the initial tensile resistance dust (gf/d) from the initial tensile resistance calculation formula described in 7.5.1 of 013 (1981),
It was defined as the initial tensile modulus (g/d).

(5) Relative viscosity measurement method 96.3 ± 0.1% by weight Reagent Prepare a sample solution by dissolving the sample in special grade concentrated sulfuric acid so that the polymer concentration is 10 rag/1fl, and incubate at 20°C ± 0. The relative viscosity of the solution is measured using an Ostwald viscometer at a temperature of 0.05°C and an ice fall time of 6 to 7 seconds. During the measurement, the same viscometer was used, and the falling time To (seconds) of 20 ml of sulfuric acid was the same as when preparing the sample solution, and the falling time T of 20 ml of the sample solution was measured.
From the ratio of □ (seconds), calculate the relative viscosity RV using the following formula.

RV=Tt/T.

(6) Method of Measuring Dry Heat Shrinkage The measurement was performed according to 5.7 of JIS-L-1017 (1978).

(7) Melting point measurement method High performance differential scanning calorimeter DSC-IOA manufactured by Rigaku Denki Co., Ltd.
was used. Cut the sample into fine powder, 5mgl1! Weigh the sample, place it in an aluminum sample pan, back it up with a sample crimper, and use it for measurement.

The measurement was carried out in an argon gas stream, with a constant temperature range: room temperature to 300°C, and a temperature increase rate of 20°C/1 lin.

Measurement range: 5 meals/see, speed = 20 cm/win, and the melting peak temperature on the chart was read, which was taken as the melting point of the sample.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the following Examples do not limit the present invention, and any design changes in accordance with the spirit of the above and below are within the technology of the present invention. It is within the scope of

[Examples] Example 1 Nylon 66 polymer having a relative viscosity of 2.6 was melt-spun using a spinneret with 15 nozzle holes at 295° C. in accordance with a conventional method, and was drawn off at a speed of 4500 m/min.

The obtained undrawn yarn was heated to a set temperature of 23 using a non-contact heater.
It was stretched at 0° C. and at a stretching ratio of 2.19 times.

The undrawn yarn supply speed was 19 m/min.

The obtained stretched fiber physical properties are as follows: tensile strength = 8.9 g
/11, elongation at break = 9.5%, initial elastic modulus 281.7
g/d, dry heat shrinkage rate: 3.0%.

Example 2 The undrawn yarn obtained in Example 1 was drawn at a draw ratio of 2.11 times using a non-contact heater at a set temperature of 245°C. The supply speed of the unstretched system was 19 m/min.

The lG fiber physical property value of the obtained drawn yarn is tensile strength:! 1.1
g/d, elongation at break: 8.8%, initial elastic modulus: 96.
9 g/d, dry heat shrinkage rate: 2.6%.

Example 3 The undrawn yarn obtained in Example 1 was drawn in two stages using a non-contact heater. The heater setting temperature is 245 in both cases.
Stretching was carried out at a total stretching ratio of 2.16 times. The undrawn yarn supply speed was 19 m/win.

The fiber physical properties of the obtained drawn yarn are tensile strength = 9.4 g
/d, elongation at break: 8.6%, initial elastic modulus: 91.8g
/d, dry heat shrinkage rate: 2.5%.

Example 4 The undrawn yarn obtained in Example 1 was drawn in three stages using a non-contact heater. The heater setting temperature is 245 in both cases.
Stretching was carried out at a total stretching ratio of 2.14 times. The undrawn yarn supply speed was 19 m/a+in.

Tan physical properties of the obtained stretched system Tensile strength: 8.4
g/d, elongation at break: 9.2%, initial elastic modulus: a9.1
g/d, dry heat shrinkage rate: 2.4%.

Example 5 The same polymer obtained in Example 1 was heated at 295°C.
Melt spinning was carried out in a conventional manner using a spinneret with 15 nozzle holes, and the fibers were drawn off at a speed of 5500 m/min.

The obtained undrawn yarn was drawn using a non-contact heater at a set temperature of 245° C. and a drawing ratio of 1.98 times. The undrawn yarn supply speed was 19 m/min.

The fiber physical properties of the obtained drawn yarn are as follows: tensile strength: 8.4 g
/d, elongation at break: 8.8%, initial elastic modulus: 91.9g
/d, dry heat shrinkage rate: 2.4%.

Example 6 The undrawn yarn obtained in Example 1 was drawn at a set temperature of 260° C. and a draw ratio of 2.19 times using a non-contact heater. The undrawn yarn supply speed was 19 m/win.

The 1a fiber physical property value of the obtained drawn yarn is tensile strength = 9.8 g
/d, elongation at break: 8.7%, initial elastic modulus near 6.8g/
d, dry heat shrinkage rate: 2.4%.

Example 7 A nylon 66 polymer having a relative viscosity of 2.8 was melt-spun at 295° C. using a spinneret with 15 nozzle holes according to a conventional method, and drawn off at a speed of 4500 m/min.

The obtained undrawn yarn was drawn using a non-contact heater at a set temperature of 245°C and a drawing ratio of 2.10 times. The undrawn yarn supply speed was 19 m/win.

The fiber physical properties of the obtained drawn yarn are tensile strength -8,98/
d, elongation at break: 8.6%, initial elastic modulus = 88.0 g/d
, dry heat shrinkage rate: 2.7%.

Comparative Example 1 The same polymer used in Example 1 was heated at 295°C.
Melt spinning was carried out using a spinneret with 15 nozzle holes according to a conventional method, and the fiber was drawn off at a speed of 3500 m/m1n.

The obtained undrawn yarn was drawn using a non-contact heater at a set temperature of 245° C. and a drawing ratio of 1.98 times. The undrawn yarn supply speed was 19 m/min.

The fiber physical properties of the obtained drawn yarn are tensile strength: 9.9 g/
d, elongation at break: 9.4%, initial elastic modulus: 80.2 g/d
, dry heat shrinkage rate=3.4%.

Comparative Example 2 The undrawn yarn obtained in Example 1 was drawn using a non-contact heater at a set temperature of 185° C. and a drawing ratio of 1.68 times.

The undrawn yarn supply speed was 19 m/11 inch.

The fiber properties of the obtained drawn yarn are tensile strength: 7, Og/
d, elongation at break: 12.8%, initial elastic modulus: 58.4g
/d, dry heat shrinkage rate: 5.2%.

Comparative Example 3 An attempt was made to stretch the undrawn yarn obtained in Example 1 at a set temperature of 265° C. using a non-contact heater, but the yarn was fused and could not be stretched.

Comparative Example 4 Nylon 66 chips with a relative viscosity of 3.3 were melt-spun at 295° C. using a spinneret with 15 nozzle holes according to a conventional method, and the spinneret was taken off at a speed of 4500 m/win.

The obtained undrawn yarn was drawn using a non-contact heater at a set temperature of 245°C and a drawing ratio of 2.00 times.

The undrawn yarn supply speed was 19 m/m1n.

The fiber physical properties of the obtained drawn yarn are tensile strength = 7.7 g/
d, elongation at break: 9.4%, initial elastic modulus: 7o, sg/d
, dry heat shrinkage rate: 2.8%.

Comparative Example 5 Nylon 66 polymer having a relative viscosity of 2.2 was melt-spun at 295° C. using a spinneret with 15 nozzle holes according to a conventional method, and taken off at a speed of 4500 m/+in, but leakage properties were observed. This made stable winding impossible.

Detailed data of Examples 1 to 7 and Comparative Examples 1 to 5 are collectively shown in Table 1.

[Effects of the Invention] The present invention is configured as described above, and the nylon 66 fiber according to the present invention has excellent strength, elongation, 8 dimensional stability, and elastic modulus, and can be used as a tire cord material and Ideal as a material for belts. In particular, it has an initial elastic modulus close to that of polyester, and it is thought that it can be used as a coating material for the belt portion of radial tires, which was previously considered unsuitable.

However, the use of the fiber of the present invention is not limited to the above, and it goes without saying that it can be used in general fields as well.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the DSC melting curve of nylon 66 fiber.

Claims (3)

[Claims] (1) A high-modulus nylon 66 fiber that satisfies the following requirements (a) to (c). (a) Initial elastic modulus≧70g/d (b) Breaking elongation≦10% (c) 160°C dry heat shrinkage rate≦3.0% (2) Claim (1) whose breaking strength is 8.0 g/d or more
The high modulus nylon 66 fiber described in . (3) Polymer concentration is 10 mg/m in 96% concentrated sulfuric acid solution
When dissolved so that the relative viscosity at 20°C is 2.
After vacuum drying a low molecular weight nylon 66 polymer having a molecular weight of 3 to 2.9, it is melt-spun at a temperature higher than the melting point of the polymer, and the shear rate r expressed by the following formula (I) is 4 x 1.
It is extruded from a nozzle orifice and cooled and solidified so that the birefringence of the spun strips is less than 0^4sec^-^1.
Claim (1): By taking off the film so as to obtain a temperature of ×10^-^3 or more, and performing one or several stages of high-tension stretching at a temperature within the peak melting point in the DSC melting curve, either by winding once or without winding. Or high elastic modulus nylon 66 characterized by obtaining the high elastic modulus nylon 66 fiber described in (2)
Fiber manufacturing method. r=(32Q/πD^3)×(1/ρ)...(I)
However, Q: Single hole discharge rate (g/sec) D: Nozzle orifice diameter (cm) ρ: Specific gravity of nylon 66 (g/cm^3)