Tennis ball

The present invention relates to a tennis ball comprising a spherical rubber core and a felt covering affixed thereto having the features of the preamble of claim 1.

Ordinary tennis balls generally consist of a spherical hollow core having a rubber elasticity and a felt or textile covering affixed onto the core.

As felt for the covering of the core there have been used, for instance, a woven felt called melton which is prepared by satin-weaving a woolen yarn of a blend of wool and a chemical fiber such as nylon fiber, gigging and milling the satin fabric to finish up into a felt wherein fibers are firmly intertwined to form a dense structure, and a felt prepared by needle punching a fabric.

In general, a woven felt is used for tennis balls of top-grade make from the viewpoint of good appearance. The woven felt has been generally prepared from a blended yarn of wool and nylon fiber in a ratio of from 60 : 40 to 70 : 30 by weight. It is also known to use polyester or rayon fibers instead of nylon fibers to be blended with wool (e.g. JP-C-60-29157 and JP-C-59-135079).

In general, the woven felt is prepared by satin-weaving blended yarns of wool and a chemical fiber such as nylon, polyester (GB-A-714 481) or rayon fiber with the use of cotton fiber as under thread (GB-A-666 468), subjecting the satin fabric to raising to form large quantities of raised fibers, and subjecting it to a milling processing peculiar to wool in order to make a raised fiber structure dense by intertwinement of fibers, thus finishing up the satin fabric into a thick felt of good appearance. Such an intertwinement of fibers by milling processing is peculiar to wool, and chemical fibers have no such action.

The felt covering of tennis balls reduces its thickness during playing since the felt fibers are gradually worn away and cut by repeated collision with a tennis court, thus resulting in worn state. Wool is poor in abrasion resistance. Since a felt covering made of only wool shows marked abrasion by collision with a tennis court, it has been reinforced by blending a chemical fiber, as mentioned above. Nylon 6 and nylon 66 which are superior in tensile strength and dyeability, have been generally used as the chemical fiber for this purpose.

The fineness of the chemical fibers such as nylon used for this purpose is usually 6 deniers. In case of further increasing the abrasion resistance, chemical fibers having a fineness as thick as 12 deniers are used, but the use of 12 denier fibers is not desirable in that since they are firm, namely bristle-like, it is difficult to sufficiently press down the fibers by milling processing, thus resulting in rough appearance.

When nylon is used in an amount of 50 % by weight or more in order to raise the abrasion resistance, milling effect is decreased, thus the obtained tennis balls show rough appearance, and also the ball characteristics are decreased so as to hinder playing due to marked napping during playing.

The felt covering has the function of decreasing an impact to a player at the time of striking a ball by a racket, and the function of controlling the ball speed by air resistance. These functions are also impaired by wear of the felt. That is to say, the tennis ball racketed contacts the surface of a court and rebounds after sliding a short distance, and at that time a frictional force is generated between the ball and the court, whereby the fibers of the felt surface are gradually worn and cut away to reduce the thickness of the covering. Since the speed of ball struck increases with the increase of wear, the worn ball is no longer used and is exchanged with a new ball for playing. A high wear resistance of the felt covering is desired from an economical point of view.

In recent years, all weather tennis courts easy in maintenance have been popularly used, in addition to conventionally spread clay courts and en-tout-cas courts which do not so wear the felt covering. Various materials such as asphalt-type, polyurethanes and synthetic rubbers are used as the surface materials of all weather tennis courts, and among them some surface materials remarkably wear the felt covering. A high wear resistance of the covering is desired also from such a point of view.

The object of the present invention is to provide a tennis ball with a felt covering having an improved wear resistance, a good appearance and a good napping resistance.

This object is solved by a tennis ball having the features of claim 1. Improvements are subject matter of the dependent claims.

By the present invention the wear resistance of the felt covering of tennis balls is improved with keeping good the appearance of the covering which directly affects the appearance of the ball itself and without decreasing the resistance to napping of the covering which occurs at the time of playing.

It has been found that when polyester fibers having specified properties are used as the synthetic fibers to be blended with wool fibers and a felt is prepared in a usual manner therefrom, the raised fiber structure can be made dense by milling processing to show a good appearance, and the abrasion resistance can be further improved as compared with conventional felts, thus it is very suitable as the covering for tennis balls and the balls can be used with decreased abrasion without causing napping of the felt during the use.

In accordance with the present invention, there is provided a tennis ball comprising a core and a felt covering affixed thereto, wherein the felt covering comprises polyethylene terephthalate polymer fibers containing at least 90 % by mole of recurring units of ethylene terephthalate, the fibers satisfying the following conditions:

• (a) hot air shrinkability/intrinsic viscosity of the polymer ${\text{ΔS}}_{\text{150°C}}\text{/IV = 5.0 to 7.5 \%}$,
• (b) birefringence ${\text{Δn}}_{\text{D}}{\text{= 170 x 10}}^{\text{-3}}{\text{to 185 x 10}}^{\text{-3}}$,
• (c) orientation factor for crystallites fc = 0.935 to 0.950, and
• (d) amorphous molecular orientation parameter F = 0.75 to 0.90.

The hot air shrinkability and the intrinsic viscosity of the polyethylene terephthalate polymer in the parameter (a) as herein shown are obtained respectively as follows:

(1) Hot air shrinkability ΔS_150°C

A sample fiber wound on a reel, namely a hank, is allowed to stand for 24 hours in a conditioning room at 20°C and 65 % RH (relative humidity). A sample having a length of ℓ₀ measured under a load corresponding to 0.1 g/d of the sample is allowed to stand under a strainless condition in an oven at 150°C for 30 minutes, the sample is taken out of the oven and the length ℓ₁ thereof is then measured under the same loading as above. The hot air shrinkability is calculated according to the following equation.$${\text{ΔS}}_{\text{150°C}}{\text{= (ℓ}}_{\text{0}}{\text{- ℓ}}_{\text{1}}{\text{)/ℓ}}_{\text{0}}\text{x 100 \%}$$

(2) Intrinsic viscosity IV

In 100 mℓ of o-chlorophenol is dissolved 8 g of a sample fiber, and the relative viscosity ηr of the resulting solution is measured at 25°C by Ostwald's viscometer (vertical capillary viscometer). The intrinsic viscosity IV is calculated according to the following equation:$$\begin{array}{c}\begin{array}{c}\text{IV = 0.0242 ηr + 0.2634}\\ {\text{ηr = (t x d)/(t}}_{\text{0}}{\text{x d}}_{\text{0}}\text{)}\end{array}\end{array}$$ wherein t and t₀ are falling times of the sample solution and o-chlorophenol, respectively, and d and d₀ are densities of the sample solution and o-chlorophenol at 25°C, respectively.

The parameters (b), (c) and (d) as herein shown are measured as follows:

(b) Birefringence Δn_D

Birefringence is obtained by Berek compensator method using a POH type polarizing microscope made by Nikon Corporation and D-line as a light source.

(c) Orientation factor for crystallites fc

With respect to an intensity distribution along equator line of Debye-Scherrer ring, a half peak width H° of the diffraction intensity of each of Debye-Scherrer rings for (010) and (100) planes is obtained. The factor fc for each of (010) and (100) planes is calculated according to the following equation and the average value thereof is defined as orientation factor for crystallites fc.$$\text{fc = (180° - H° )/180°}$$

(d) Amorphous molecular orientation parameter F

A sample is immersed in a 0.2 % by weight aqueous solution of a fluorescence reagent (trade mark "Mikephor ETN") at 55°C for 3 hours, thoroughly washed with water and air-dried. The thus treated sample is used for measurement. A relative intensity of polarized fluorescence at an excitation wavelength of 365 nm and a fluorescence wavelength of 420 nm is measured by a fluorospectrophotometer. The amorphous molecular orientation parameter F is calculated according to the following equation:$$\text{F = 1 - (B/A)}$$ wherein A is a relative intensity of polarized fluorescence in the axial direction of a fiber, and B is a relative intensity of polarized fluorescence in the vertical direction to the axis of the fiber.

The above-mentioned particular polyethylene terephthalate fiber used in the present invention has a low birefringence Δn_D and a small amorphous molecular orientation parameter F. This means that the orientation of amorphous molecules present between crystalline regions is low and the amorphous molecules is in a very relaxed orientation state and, therefore, the differentiation between the crystalline layer and the amorphous layer is definite. It is considered that the presence of the amorphous molecules in the relaxed orientation state absorbs and lightens an impact frictional force that tennis balls receive at the time of contacting a court, thus raising the abrasion resistance.

The polyethylene terephthalate polymer which constitutes the fiber is a polymer comprising at least 90 % by mole of ethylene terephthalate recurring units. The polymer may contain less than 10 % by mole of units of dicarboxylic acids other than terephthalic acid and/or less than 10 % by mole of units of diol compounds other than ethylene glycol.

The polyethylene terephthalate fiber is used as a constituent fiber of the felt covering. The fiber is usually made up into woven felts with wool fiber, and optionally with other synthetic fibers in a known manner. Needle felts prepared by using polyethylene terephthalate fiber may of course be used as the covering for the tennis balls.

In case of preparing woven felts, it is necessary to use wool. The amount of the polyethylene terephthalate fiber is from 5 to 50 % by weight, preferably 15 to 40 % by weight. The amount of wool fiber is from 50 to 95 % by weight, preferably 60 to 85 % by weight. Nylon fiber may be used with these fibers in order to reduce the cost, and the woven felt may contain at most 45 % by weight, preferably at most 25 % by weight, of nylon fiber.

The polyethylene terephthalate fiber used in the present invention is required to satisfy the value of hot air shrinkability/intrinsic viscosity of polymer (ΔS_150°C/IV) within the range of 5.0 to 7.5 %. When the value is more than 7.5 %, durability, particularly abrasion resistance, of the obtained felt decreases, and when the value is lower than 5.0 %, a satisfactory intertwinement of fibers is not achieved by milling processing, so no good appearance is obtained.

The polyethylene terephthalate fiber is required to have a birefringence of 170 x 10⁻³ to 185 x 10⁻³, since when the birefringence of the fiber is outside this range, the durability, particularly abrasion resistance, of the felt decreases.

The polyethylene terephthalate fiber is also required to have an orientation factor for crystallites fc of 0.935 to 0.950. When the factor fc of the fiber is more than 0.950, the feeling of hand touching of the obtained felt is hard, and the feeling of striking balls becomes bad. Also, when the factor fc is less than 0.935, the fiber strength decreases, thus resulting in decrease of durability.

Further, the polyethylene terephthalate fiber is required to have an amorphpous molecular orientation parameter F of 0.75 to 0.90. When the parameter F is more than 0.90, the durability decreases since the degree of orientation in the amorphous region becomes too large. When the parameter F is less than 0.75, creeps generate partly in yarns in the raising processing, giving a poor appearance.

The polyethylene terephthalate fibers which satisfy the above conditions (a) to (d) are, for example, commercially available under the trade mark "Toray Tetoron" #1500-360-705M (product of Toray Industries, Inc.).

Felts prepared by using the above-mentioned particular polyethylene terephthalate fibers can be used as the covering for covering cores of both pressurizied tennis balls and pressureless tennis balls. Known cores prepared from various core rubber compositions are adoptable in the present invention.

The tennis balls of the present invention are prepared in a usual manner by using the above-mentioned particular felt covering.

The present invention is more specifically described and explained by means of the following Examples, in which all % are by weight unless otherwise noted.

Examples 1 to 4 and Comparative Examples 1 to 5

Woven felts containing the polyethylene terephthalate fiber (hereinafter referred to as "PET") shown in Table 1 in the proportion shown in Table 2 were prepared and punched out into cocoon shape to provide covering materials. Two sheets of the cocoon shaped felts were stuck onto the surface of a hollow spherical core having 0.9 kg/cm² higher internal pressure than atmospheric pressure in a usual manner to cover the surface of the core, thus providing a pressurized tennis ball.

In Table 1, PET-1 is a polyethylene terephthalate fiber according to the present invention which satisfies the above-mentioned conditions (a) to (d). PET-2 is a conventional polyethylene terephthalate fiber which does not satisfy the conditions (a) to (d).

PET-1

PET-2

Fineness (denier)

6

6

Strength (g/d)

8.06

9.28

Intrinsic viscosity IV

0.93

0.92

ΔS_150°C/IV

6.12

11.1

Birefringence Δn_D (1 x 10⁻³)

177

192

Orientation factor for crystallites fc

0.944

0.935

Amorphous molecular orientation parameter F

0.880

0.958

Each of the obtained tennis balls was used in tennis ball playing on an asphalt court for 1 hour by two players, and the state of abrasion of the felt covering was estimated visually according to the following criteria.

The results are also shown in Table 2.

Abrasion resistance of felt covering

Ⓞ:

Very excellent

○:

Excellent

□:

Good

X:

Bad

Example

Comprative Example

1

2

3

4

1

2

3

4

5

Fiber constitution (%)

Wool

65

65

65

65

65

65

65

65

65

Nylon 6

27

20

15

0

35

27

20

15

0

PET-1

8

15

20

35

0

0

0

0

0

PET-2

0

0

0

0

0

8

15

20

35

Abrasion resistance

□

○

Ⓞ

Ⓞ

X

X

X

X

X

As apparent from Tables 1 and 2, tennis balls of Comparative Examples 2 to 5 wherein PET-2 (conventional fiber) having a large amorphous molecular orientation parameter F, namely a low degree of the relaxation of molecular orientation in amorphous region, was used as a constituent fiber of the felt covering, were poor in abrasion resistance of the covering, in spite of having a larger fiber strength than PET-1 according to the present invention, as well as the tennis ball of Comparative Example 1 having a felt covering consisting of wool and nylon 6 fiber.

In contrast, the tennis balls of Examples 1 to 4 according to the present invention all were superior in the abrasion resistance of the felt covering, though the superiority varied depending on the amount of PET-1 used.

Also, the tennis balls of Examples 1 to 4 had a good appearance of the covering, and the napping resulting from the use was a little.

With respect to the tennis balls of Examples 1 to 4, forward deformation, return deformation and rebound were measured according to the following method together with the weight of balls. The results are shown in Table 3.

Forward deformation (mm)

A tennis ball was subsequently compressed about 2.54 cm in three directions at right angles to each other. This procedure was repeated 3 times. That is to say, the ball was compressed 9 times total. In 2 hours after the above preliminary compression, the deformation was measured by a Stevens compression tester in the following manner.

The ball was compressed with an initial load of 3.5 pounds (1.575 kg) and the deformation was measured, and the ball was then compressed with a load of 18 pounds (8.165 kg) and the deformation was measured. The forward deformation is expressed by the difference (mm) between the deformation by a load of 3.5 pounds and the deformation by a load of 18 pounds.

Return deformation (mm)

After measuring the deformation in the above forward deformation test, the ball was further compressed up to a deformation of 2.54 cm. Then the compression was reduced to a load of 18 pounds (8.165 kg), and the deformation was measured.

Rebound (cm)

A tennis ball was dropped from a height of 100 inches (254 cm) onto a concrete base, and the bound of the ball (height from the concrete base to the bottom of the ball) was measured. The measurement was repeated 3 times and the average was obtained.

1

Ex. 2

Ex. 3

Ex. 4

Regulation of International Tennis Federation

Weight (g)

57.2

57.4

57.1

57.2

56.7 to 58.5

Forward deformation (mm)

6.2

6.3

6.7

6.1

5.6 to 7.4

Return deformation (mm)

9.2

9.1

9.2

9.2

8.9 to 10.8

Rebound (cm)

142

141

143

142

135 to 147

As apparent from the results shown in Table 3, the tennis balls of Examples 1 to 4 according to the present invention all come up to the regulation of International Tennis Federation, and there is no problem in physical properties of balls.

From the results shown in Tables 2 and 3, it would be understood that the abrasion resistance of covering of tennis balls can be improved, with keeping the appearance of the covering good and with restraining occurrence of fuzzing during playing, by using a specific polyethylene terephthalate fiber as a constituent fiber of a felt covering for cores of tennis balls.

In addition to the ingredients used in the Examples, other ingredients can be used in the Examples as set forth in the specification to obtain substantially the same results.