US3727783A

US3727783A - Noneverting bottom for thermoplastic bottles

Info

Publication number: US3727783A
Application number: US00153268A
Authority: US
Inventors: K Carmichael
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1971-06-15
Filing date: 1971-06-15
Publication date: 1973-04-17
Anticipated expiration: 1990-04-17
Also published as: CA949472A; DE2229312A1; FR2141963A1; AU4320572A; GB1360107A; AU460886B2; IT956483B; BE784888A; FR2141963B1

Abstract

A generally cylindrical thermoplastic bottle for bottling liquids under pressure such as beer, soda and aerosols, having a noneverting bottom under conditions of bottling and use wherein the bottom comprises at least three lobes around the bottom perimeter of the bottom on which the bottle stands, strap sections located between each lobe and a generally circular and axially aligned, recessed section defining a re-entrant cylinder of the base. To further improve eversion resistance, the bottom of the bottle can include a reinforcing ring attached to the reentrant cylinder.

Description

United States Patent 1191 1111 3,727,783

Carmichael Apr. 17, 1973 1 1 NONEVERTING BOTTOM FOR 3,043,461 7/1962 Glassco ..220 70 THERMOPLASTIC BOTFLES 3,038,627 6/1962 Daley ..220/70 2,541,065 2/1951 Jabour 220 70 Inventor: Keith Stewart Carmichael, Wllmmg- 3,598,270 8/l97l Adomaitisetal ..215/1 (3 ton, Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: June 15, 1971 21 Appl. No; 153,268 [57] ABSTRACT A generally cylindrical thermoplastic bottle for bottling liquids under pressure such as beer, soda and "215/1 99/171 220/70 aerosols, having a noneverting bottom under conditions of bottling and use wherein the bottom comprises at least three lobes around the bottom perimeter of the bottom on which the bottle stands, strap sections located between each lobe and a generally circu- Primary Examiner-Joseph R. Leclair Assistant ExaminerStephen Marcus Attorney-Louis Del Vecchio [52] US. Cl. [51 Int. Cl. ..B65d 1/02 [58] Field of Search ..215/1 R, l C; 220/3,

220/60, 70; ISO/.5; 99/171 B [56] References Cited lar and axially aligned, recessed section defining a re- UNITED STATES PATENTS entrant cylinder of the base. To further improve eversion resistance, the bottom of the bottle can include a 3,643,829 2/1972 Lachner ..2l5/l C reinforcing ring attached to the re-entrantcylinden 3,468,443 9/1969 Marcus 150/.5

3,403,804 10/l968 Colombo ..2l5/l C 8 Clains, 12 Drawing Figures PATENTEDAPRIYIW I 3727. 783

sum 1 or 3 FIGJ l 26 *3 I INVENTOR ATTORNEY KEITH STEWART CARIICIMEL':

cab/27. 783

PATENTEI] APR 1 H975 SHEET 2 BF 3 H W w 4 m .HI/ 4 .M m 6 m. 2 I u u m H V m3 w mflmmfl/ I 4. w M F 4///////M//I(H\ \\\\\v\\\ ATTORNEY PATENTEU APR 1 7 1915 sum 3 [IF 3 FIGJZ" m T. m V m WAR] OA'RHICHAEL ATTORNEY BACKGROUND OF THE INVENTION This invention relates to the art of manufacturing thermoplastic bottles useful in bottling liquids under pressure such as sodas, beer and aerosols and is particularly concerned with providing a bottle having a bottom that will not evert during use.

It is known that thermoplastic bottles can be used to bottle beverages for consumer use. If the bottle is used to contain carbonated beverages such as soda or beer, the bottle must be designed to constrain the autogenous pressure in the bottle while remaining dimensionally stable in shape and volume.

Thermoplastics, however, by nature will deform at moderate temperatures under relatively small loads and therefore, when formed into plastic bottles and used in bottling liquids under pressure, they will deform in normal use. For example, at a temperature of about 50C. and under an autogenous pressure of about 100 psig, i.e., about the highest pressure typically found in a soda or beer bottle, plastic bottles have a tendency to deform into the shape of a sphere. One way of significantly reducing this tendency is to-make the shell of the bottle very thick. While functional, this is not economical and, furthermore, tends to make the bottle so rigid that it fractures in normal use. It has been found, however, that by making a thin-shelled bottle and molecularly orienting the polymer, the yield stress in the side walls can be improved sufficiently to resist this tendency to deform. However, it is very difficult to molecularly orient the polymer'in the bottom of the bottle. Therefore, the bottom retains this tendency to deform, i.e., evert.

In general, the bottom of a bottle is conventionally rather flat, permitting the bottle to stand upright. In unpressurized applications, this is not a severe requirement and flat-bottomed bottles can be used successfully. ln pressurized applications, however, a flat bot-- tom is inherently a poor shape to hold rigid and the bottom tends to evert into the shape of hemisphere, increasing the volume of the bottle, distorting the bottom shape and eliminating the possibilityof the bottle being able to stand on a flat surface. Therefore, the low stress capabilities of the plastic coupled with high temperatures for extended time periods under sufficient internal pressure, will cause the plastic to creep or deform so that shape and volume change excessively even though the contents, i.e., gas and the liquid, are successfully contained within the bottle.

Therefore, it is desirable to find a way of making a plastic bottle useful in bottling liquids under pressure having a bottom that will not evert and will, at the same time, maintain a base sufficient for the bottle to stand on when used to bottle liquids under pressure.

SUMMARY OF THE lNVENTlON Accordingly, the present invention provides a thermoplastic bottle having a noneverting bottom when subjected to temperatures up to about 50C. and autogenous pressures up to about 100 psig. The bottle is a generally cylindrical, biaxially oriented, thermoplastic bottle having a shell thickness at the right cylinder section of at least about mils and at least about mils in the bottom section. The bottle is preferably prepared from a polymer having a modulus of elasticity at yield of 180,000 psi; a tensile strength of at least 5,000 psi; a Poisson's Ratio of 0.35 to 0.4; and a deformation constant equal to the slope of the log (reciprocal of the strain rate) versus strain having a value of at least about 0.65.

The bottom of the bottle consists essentially of at least three lobes around the bottom perimeter of the bottle on which the bottle stands, strap sections located between each lobe, an axially aligned re-entrant cylinder and a generally circular and axially aligned recessed disc-like section with a smooth transition of material between the lobes, straps, re-entrant cylinder and recessed sections of the base. In the following description, D the outside diameter of the bottle taken perpendicular to the major axis of the bottle, i.e., an imaginary line running from top center to the bottom center, where the bottom section of the bottle meets the generally cylindrical section of the bottle.

The strap sections each begin at the generally cylindrical section of the bottle and extend downward toward the central axis of the bottle with a radius of about 0.45 to 0.70 D, including about to of arc wherein the width of each strap is determined by the following formula:

where:

w average width of an individual strap,

N number of straps,

R outside radius of the generally cylindrical section of the bottle taken where the bottom meets the generally cylindrical section and is equal to D/2,

P= autogenous design pressure,

Syp yield stress of the thermoplastic,

C 0.75-0.95 (design stress factor),

F fraction of the total load on the lobes of the bottle to be carried by the straps,

t= shell thickness of'a strap, and

The lobes fit in between the strap sections and each lobe starts at the generally cylindrical section extending downward with a radiusof about 0.2-1.5 D measured from the plane between the generally cylindrical section and the bottom section of the bottle including about 20 to 40 of arc and is connected to a toroidal knuckle having a radius of about 0.03 to 0.1 D including about 90 to of are directed down. and inward toward the central axis of the bottle wherein the toroidal knuckle forms the standing surface of the bottle and the center of curvature of the toroidal knuckle is located a distance of about 0.3 to 0.4 D away from the central axis of the bottle. g

The inward extending portions of the lobes and strap sections, in uniform circumferential spacing, then meet around the base of a re-entrant cylinder which is axially aligned and recessed in the bottom of the bottle. The

disc and the two walls are interconnected with a toroidal knuckle having a radius of about 0.02 to 0.04 D including about 180 of arc. The wall lengths need not be the same.

In an alternate embodiment, areinforcing ring is attached to the re-entrant cylinder adding an increased amount of eversion resistance to the bottom of the bottle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view of a three-lobed plastic bottle made according to the present invention.

FIG. 2 is a bottom view of the bottle shown in FIG. 1.

FIG. 3 is a cross section of the bottle shown in FIG. 1 taken along the line 3-3 of FIG. 2.

FIG.4 is a schematic showing the curves in the bottom of the bottle taken along line 3-3 of FIG. 2.

FIG. 5 is a front view of a six-lobed plastic bottle made according to the present invention.

FIG. 6 is a bottom view of the six-lobed bottle shown in FIG. 5.

FIG. 7 is a cross section of the bottle shown in FIG. 5 taken along line 77 of FIG. 6.

FIG. 8 is a schematic showing the curves in the bottom of the bottle taken along line 88 of FIG. 6.

FIG. 9 is a front view of a four-lobed plastic bottle made according to the present invention incorporating the alternate embodiments of (a) a re-entrant cylinder and (b) a reinforcing ring in the bottom of the bottle.

FIG. 10 is a bottom view of the bottle shown in FIG. 9.

FIG. 11 is a cross section of the bottle shown in FIG. 9 taken along line 11-11 of FIG. 10.

FIG. 12 is a schematic showing the curves in the bottom of the bottle taken along line 1212 of FIG. 10.

DETAILS OF THE INVENTION FIG. 1 shows a front view of a thermoplastic bottle incorporating the noneverting bottom design of the present invention. The bottle 1 is a hollow container having an opening 2 at one end used to pour liquid into or out of the bottle. The bottle is madeup of a lip portion 3 surrounding opening 2, a narrow, generally cylindrical neck section 4, a cone frustum section 5, a large, generally cylindrical section 6 and a noneverting bottom section 7 which will be described below in detail. The upper configuration of the bottle is not critical to this invention. For example, the neck portion can be short with a definite shoulder in the bottle or elongated with a smooth transition into the generally cylindrical portion of the bottle. The main portion of the bottle, namely, the generally cylindrical section, can be fluted or otherwise shaped to obtain a desirably aesthetic appearance.

The term evert" is used in its common sense, i.e., to turn outward; and a bottle having a noneverting bottom is used to mean a bottle in which the bottom will not turn outward under normal use conditions to a point where it is unsightly or would not stand. Normal use conditions are generally no greater than a temperature of about 050C. and an autogenous, i.e., internal positive, pressure up to about 100 psig. It is to be understood that under an autogenous pressure some movement, usually nominal elongation, will occur that is not eversion.

Eversion can be caused by a number of factors. Perhaps the two most important factors are (a) overstressing of the plastic material used to make the bottle, particularly where the internal pressure of the bottle causes stresses in the bottle that exceed the yield stress of the material resulting in large material deflections and deformations; and (b) geometric instability. The noneverting bottle of the present invention balances the type and amount of thermoplastic material used with a particular geometric design to the bottom of the bottle. a

The bottom geometry of the bottle will now be described in relation to FIGS. 1 to 4 of the drawings with particular reference to FIG. 4 drawn as an aid to show the curves involved in a three-lobed bottom along the line 3-3 of FIG. 2.

In describing the bottom geometry and as hereinafter used, D equals the outside diameter of the bottle taken perpendicular to the major axis of the bottle, i.e., imaginary line running from top center to bottom center, where the bottom section of the bottle meets the generally cylindrical section of the bottle. All dimensions are outside dimensions, i.e., mold dimensions. g

The set points for the bottle are three

lobes

8, 9, and 10. In between the lobes are

strap sections

11, 12, and 13. The portions of the bottom between the lobes and the strap sections curve to form smooth transitions between the lobes and the straps. The inward extending portions of the lobes and strap sections, in uniform circumferential spacing, meet around the base 14 of a reentrant cylinder 15 formed by two spaced-apart side wall sections 16 and l7joined by a toroidal knuckle 18.

Side wall 17 then connects with the center portion 19 of the bottom of the bottle which has a generally circular configuration that is axially aligned and usually recessed to a position where it is about tangent to the bottom portion of the

curve defining straps

11, 12 and 13.

In more particular detail, strap sections begin at the right cylinder section indicated by 20, 21 and 22 and extend downward in a circular manner toward the central axis of the bottle with a radius (R,) of about 0.45 to 0.70 D, including about to of arc-and terminating at the base 14 of the bottom of the re-entrant cylinder. The width of each strap is determined by the following formula: I

EQUATION 1 where: v

w average width of an individual strap,

N= number of straps, I

P= autogenous design pressure,

Syp yield stress of the thermoplastic,

C 0.75-0.95 (design stress factor),

= shell thickness of a strap, and

In the formula, F is the fraction of the total load on the lobes of the bottle carried by the straps and is greater than zero and less than one (O F l The most conservative value for F would be 1 in which case the straps would be designed to carry all of the pressure load on the lobes.

F can be determined by first making anexperimental lobed bottle having strap widths designed according to Equation 1 wherein the value of F is approximated, the value of yield stress is known from independent testing of the thermoplastic, a value is given to C, between 0.75 and 0.95 with the lower value being the more conservative design, a design pressure is used for P, the actual dimensions of the bottle are measured for radius R and thickness T and the number of lobes N is determined by counting the number on the bottle. Thereafter, the experimental bottle is pressurized and the amount of pressure that the bottle can withstand without undergoing permanent deformation is measured. Using this value ofpressure for P, Equation 1 is resolved for F. This value of F can then be used to design the width of the straps.

Typically, in a three-lobed bottom bottle such as that illustrated in the drawings having a capacity of ounces, an inside diameter of about 2.25 inches and three straps each 7/16 inch wide, F was about 1/3 to l/2.

The strap sections generally have a uniform width, however, they can have a varying widthin which case the average width is used in the formula. In a strap having a varying width satisfactory results have been achieved when the strap section connected to the right cylinder section is relatively wide and the width of the strap uniformly diminishes or tapers as the strap extends around toward the central axis of the bottle.

A typical calculation for the width of a strap (w) follows wherein a four-lobed bottle (N=4) is made having a radius (R,) of 1.125 inches and a shell thickness (2) of 0.03 inch. The polymer has a tensile yield stress (Syp) of 6000 psi. The bottle is designed to withstand an autogenous pressure (P) of l00 psig, the fraction of the total load (F) on the lobes of the bottle to be carried by the straps is 0.33 and the design stress factor is 0.875.

w=0.4 inch reasons or perhaps to provide increased eversion to protect the bottle from everting from some unexpected or unusual conditions. Therefore, the minimum width requirement of Equation 1 is provided to determine the lower limit on design width but may be exceeded to obtain other desirable effects.

The

lobes

8, 9, and 10 fit between the strap sections and each lobe starts at the

right cylinder section

23, 24 and 25 and extends downward with aradius (R of about 0.8-1 .5 D measured from the plane between the right cylinder section and the bottom section of the bottle and includes about 20 to 40 of are which is in turn connected to a

toroidal knuckle

26, 27 and 28 having a radius (R of about 0.03 to 0.10 D, including about to of are directed inward and upward toward the central axis of the bottle. The toroidal knuckle forms the standing surface of the bottle and the central curvature of the toroidal knuckle is located a distance (L of about 0.3 to 0.4 D away from the center line (42) of the bottle. The toroidal knuckle is met with a smooth transition of material directed into the base 14 of the re-entrant cylinder in the bottom of the bottle.

The re-entrant cylinder 15 defined by spaced apart side walls 16 and 17 wherein each side wall has a length L, and L respectively, of about 0.05 to 0.22 D. However, the side walls 16 and 17 need not be the same length. The side walls are joined by a toroidal-knuckle 18 having a radius R, of about 0.02 to 0.04 D with an arc of about The re-entrant cylinder is very significant to the structural integrity of the plastic bottles of this invention. It acts as a structural arch supporting the internal pressures of the bottle and also acts as a retaining band to retain the straps and lobes otherwise the lobes, in particular, tend to diametrically bulge, increasing the diameter of the bottom causing serious problems in packaging, shipping and storing.

The number of straps and lobes on a bottle can vary and FIGS. 5-8 show such a variation on a bottle having six lobes and six straps.

Referring to the drawings, the bottle. itself 29 is generally cylindrical having a typical bottle shape from the top or lip portion down through the right cylinder section to the bottom. The bottom section is made up of six

lobes

30, 31, 32, 33, 34 and 35 equally sized and uniformly spaced around the bottom of the bottle with six

strap sections

36, 37, 38, 39, 40 and 41 equally sized and uniformly spaced in between the lobes. The width of each strap is first determined by Equation 1 above, and the lobes are sized to fit in between each strap. The lobes and straps each meet in the base 42 of the re-entrant cylinder 43 in the bottom of the bottle. One wall of the re-entrant cylinder then terminates in the central disc 50 of the base. 7

FIG. 8 illustrates the curves along line 88 of FIG. 6 involved to form the bottom of the bottle wherein R is the radius of curvature for the strap sections, R is the radius of curvature for the lobes, R is the radius of curvature for each of the

toroidal knuckles

44, 45, 46, 47, 48 and 49 on which the bottle sits and R is the radius of the toroidal knuckle connecting the two side walls of the re-entrant cylinder 43. L and L represent the length of the side walls forming the distance between the center line (Q) of the bottle and the center of the radius of curvature of the toroidal knuckles that establish the seat of the bottle. More specific geometric details are discussed above in reference to the threelobed bottle along with definitions of R R R R and L L and L in still another embodiment, the re-entrant cylinder has a reinforcing ring adding an increased amount of eversion resistance to the bottom of the bottle. FIGS. 9, 10, 11 and 12 show the incorporation of re-entrant cylinder, coupled with a reinforcing ring, in the bottom section of a four-lobed bottle wherein the straps and lobes have the same geometric configuration as described above.

Referring to the drawings, FIG. '9 shows the front view of a fourlobed bottle 51 having a typical bottle shape from the top or lip portion of the bottle down through the right cylinder section of the bottle. The bottom section, more particularly illustrated in FIGS. l0, l1 and 12, is attached to the right cylinder section and has four

lobes

52, 53, 54 and 55 and four

hemispherical strap sections

56, 57, 58 and 59 spaced between the four lobes. The lobes and strap sections have the same general geometric configuration as the lobes and straps described above, except that there are four of each instead of three.

A re-entrant cylinder 60 is formed in the same manner as that described above by two spaced-apart

side wall sections

61 and 62 each having a length of about 0.05 to 0.22 D joined by a 180 toroidal knuckle 63 having a radius of about 0.02 to 0.04 D as discussed above. A reinforcing ring 64 is formed on the toroidal knuckle 63. The reinforcing ring is formed by two concentric contacting

side wall sections

65 and 66 each having a length of about 0.05 to 0.20 D joined by a toroidal knuckle 67.

The lobes and strap sections are connected to the outer wall 61 of the re-entrant cylinder and the inner wall 62 is connected to the recessed central disc 68.

Thermoplastics useful in preparing bottles having a bottom designedaccording to the present invention are polyethylene terephthalate, acrylonitrile/styrene/met hyl acrylate copolymer, acrylonitrile/ethylene/methyl acrylate copolymer, methacrylonitrile copolymers, polycarbonate s, polys ulfones, polybis(p-aminocyclohexyl)-dodecaneamide or polyformaldehyde resin. Polyethylene,terephthalate is preferred because of its excellent strength properties, particularly a high Inherent; viscosity: natural logarithm tensile strength, excellent impact strength and relatively low creep. This high tensile strength is particularly important since. the bottles of this invention are designed with strap sections that extend from the'side walls to the reentrant cylinder and are placed in tension as internal bottle pressure is applied. The strength of the strap sections in tension plus the strength added from the axially aligned reentrant cylinder is great enoughthat the whole bottom area is not needed to support the internal pressure of the bottle, therefore,

part of the bottom area can be used to form the lobes on which the bottle stands.

Polyethylene terephthalate useful in preparing the thermoplastic articles of this invention includes (a) polymers wherein at least about 97 percent of the polymer contains the repeating ethylene terephthalate units of the formula:

with the remainder being minor amounts of ester-forming components, and (b) copolymers of ethylene terephthaiate wherein up to about 10 mole percent of the copolymer is derived from other ester-forming components which are substituted for corresponding amounts of the usual glycol and/or the carboxylicreactants. Other ester-forming components include the monomer units of diethylene glycol; propane-l ,3-diol; butane-l ,4-diol; polytetramethylene glycol; polyethylene glycol;"polypropylene glycol; 1,4-hydroxymethylcyclohexane and the like; or isophthalic, bibenzoic, naphthalene 1,4- or 2,6-carboxylic, adipic, sebacic, decanel l O-dicarboxylic acid, and the like.

The specific limits on the comonome'r are governed by the glass transition temperature of the polymer. it has been found that when the glass transition temperature extends below about 50C., a copolymer having reduced mechanical properties results. Accordingly, this corresponds to the addition of no more than about 10 mole percent of a comonomer. One exception to this, for example, is the addition of bibenzoic acid where the glass transition temperature of the copolymer remains above 50C. and does not drop with the addition of more than 10 mole percent. Others would be obvious to those skilled in the art. I

In addition, the polyethylene terephthalate polymer can include various additives that do not adversely affect the polymer in use such as stabilizers, e.g., antioxidants or ultraviolet light screening agents, extrusion aids, additives designed to make the polymer more degradable or combustible, such as oxidation catalysts, as well as dyes or pigments.

The polyethylene terephthalate should have an inherent viscosity (10 percent concentration of polymer in a 37.5/62.5 weight percent solution of tetrachloroetha'ne/phenol, respectively, at 30C.) of at least 0.55' to obtain the desired end properties in the article s formed and preferably the inherent viscosity is at least about 0.7 to obtain an article having excellent toughness properties, i.e., resistance to impact loading. ,The viscosity of the polymer solution is measured relative to that of the solvent alone and the viscosity of solution fl ll az yset where C is the concentration expressed polymer per 100 milliliters of solution.

In the preferred embodiment wherein the ther-I moplastic is polyethylene terephthalate, the plastic bottle preferably has a shell thickness in the right cylinder section of at least -abo'ut'20 mils and at least about 30 mils in the bottom section with at least the following characteristics, particularly in the bottom portion of the bottle:

a. a modulus of elasticityat yield of 180,000 psi;

a tensile strength at break of at least 5,000 psi;

a Poissons Ratio of 0.35 to 0.4; and

a deformation constant equal to the slope of the log (reciprocal of the strain rate) versus strain having a value of at least about 0.65.

The modulus of elasticity at yield is the ratio of stress to strain of a specimen in tension wherein the tensile in grams of yield stress is that stress at which the specimen begins to stretch without an increase in load. The modulus of elasticity at yield is determined by ASTM D-882, Tensile Properties of Thin Plastic Sheeting.

The tensile strength at break is also determined by ASTM D886 wherein a specimen is placed under increasing tension until it breaks.

The deformation constant is a measure of creep. Creep is usually measured on polymers by placing a sample under a fixed load, i.e., stress, at a constant temperature and measuring the strain deformation as a function of time. The curves for thermoplastics have a characteristic shape in which the rate of strain decreases as a function of time. A plot of the log (reciprocal of the strain rate) versus strain results in a linear plot over a substantial part of the creep curvev The slope of the straight line segment herein referred to as the deformation constant, is mathematically expressed as:

DC= [dlog (dt/de) ]/de where DC deformation constant,

dt differential of time, and

dz differential of the strain.

This deformation constant is applicable to related thermoplastics and can be used to comparethe creep behavior by comparingthe slope values. A deformation constant equal to indicates that the sample being tested is extending at its natural strain rate or for the load indicated, the strain rate is constant. A deformation constant of infinity indicates that there is no measurable strain indicated.

For bottles prepared from polyethylene terephthalate according to the preferred embodiment of the present invention, the deformation constant is at least about 0.65, indicating a deformation of less than percent in 100 hours at 50C. with an autogenous pressure of 75 psig.

A preferred process for preparing bottles having the bottom geometry designed according to the present invention is disclosed in US. Pat. application Ser. No. 93,571, filed Nov. 30, .1970, hereby incorporated by reference. The process produces a hollow, biaxially oriented, thermoplastic article by extruding a hollow, cylindrical, thermoplastic slug with a ramrod through an annular orifice into a slidable mold at a temperature within its molecular orientation range to a shape relatively larger than the original shape of the slug wherein the mold has an annular bead recess at one end to accept and hold one end of the extrudate while simultaneously drawing the extrudate in the direction of extrusion and expanding the extrudate by forcing a gas or liquid against the interior portions of the extrudate, expanding the extrudate to conform to the mold while sliding the mold past the extrusion orifice as continuous extrusion takes place. The lobes and strap sections of the bottle are formed by properly shaping the mold and expanding the polymer to conform to the mold. The reentrant cylinder and reinforcing ring are formed by first forming the bottle including lobes and strap sections, then reversing the direction of the mold, against the direction of extrusion forming the recessed re-entrant cylinder and/or reinforcing ring in the bottom of the bottle. Alternatively, the mold cavity is shaped to EXAMPLE 1 Plastic bottles are prepared according to the process of US. Pat. application Ser. No. 93,571, filed Nov. 30, I970, wherein a hollow thermoplastic slug is extruded through an annular orifice into a slidable mold wherein the mold has a bead recess at one end to accept the extrudate. Then the mold is made to slide by the extrusion orifice as continuous extrusion takes place, drawing the extrudate as the mold slides while the extrudate is simultaneously forced against the interior walls of the mold by introducing a fluid under pressure into the interior of the extrudate.

The slug is amorphous and is prepared from polyethylene terephthalate having an inherent viscosity of about 0.85. v

Three different molds are used each producing 10 bottles having (a) three-lobed bottoms such as that shown in FIGS. 1-4, (b) four-lobed bottoms such as that shown in FIGS. 9-12 except that the reinforcing ring is not included, and (c) six-lobed bottoms such as that shown in FIGS. 5-8. In the mold the straps are specifically formed of metal in relief to obtain the desired strap design. The lobes in the three-lobed bottle I lobed bottle have a width of about 1.08 of an inch where the strap connects to the right cylinder section tapering to a width of about 0.625 of an inch where the strap connects to the circular recessed section of the bottom. The straps in the six-lobed bottle have a width of about 0.60 of an inch where the strap connects to the right cylinder section tapering to a width of about 0.30 of an inch where the strap connects to the circular recessed section of the bottom.

Each bottle holds a capacity of about 10 ounces and has an inside diameter at the right cylinder section of about 2.25 inches with a corresponding shell thickness of about 20 mils. In the bottom section of the bottle, the average thickness of the shell is about mils.

The following dimensions also obtain:

3-lobed a (radius of seating toroidal knuckle) 0.09 0.13 0.13

4 5.1 (radius of toroidal knuckle in reentrant cylinder) 0.05 0.05 0.05

1 (side wall of re-entrant cylinder) 0.375 0.375 0.375 1 (side wall of re-entrant cylinder) 0.375 0.250 0.250 a (center line of bottle to center of toroidal knuckle on which the bottle sits) 0.625 0.75 0.75

The bottles are tested by filling each bottle with about 10 ounces of liquid (water) and pressurizing the bottle to I psig. at room temperature for about 2 minutes. In all cases, the bottle elongates in the axial direction a nominal amount but the bottom does not evert and will stand on a flat surface.

Thereafter, the internal pressure in the bottle is reduced to 80 psig. and the bottles are stored for 5 days at 50C. In all cases the bottles do not evert.

EXAMPLE 2 A four-lobed bottle is prepared in the same manner as that disclosed in Example 1 except that the bottom is provided with a re-entrant cylinder and a reinforcing ring such as that described in the specification and particularly illustrated in FIGS. 11 and 12. The contacting side walls of the reinforcing ring have a length of about 0.5 inch and an overall thickness of about 0.1 inch.

This bottle is tested by filling it with about ounces of water and pressurizing the bottle. It is found that this bottle withstands about 300 psig. pressure at room temperature for about 2 minutes. While the bottle elongates in the axial direction a nominal amount, the bottom does not evert and it will stand on a flat surface.

I claim:

1. In a generally cylindrical thermoplastic bottle, biaxially oriented at least in the generally cylindrical section, the improvement wherein the bottom configuration consists essentially of at least three lobes around the bottom perimeter of the bottle on which the bottle stands; strap sections located between each lobe; an axially aligned re-entrant cylinder; and a generally circular and axially aligned recessed section with a smooth transition of material between the lobes, straps, re-entrant cylinder, and recessed sections of the base wherein a. the strap sections begin at the generally cylindrical section of the bottle and extend downward toward the central axis of the bottle with a radius of about 0.45 to 0.7 D, including about 75 to 90 of arc and terminating in the base of the re-entrant cylinder and the width of each strap is determined by the w average width of an individual strap,

N= number of straps,

R outside radius of the generally cylindrical section of the bottle taken where the bottom meets the generally cylindrical section and is equal to D/2, P= autogenous design pressure,

Syp= yield stress of the thermoplastic,

C =0.75-0.95 (design stress factor),

F= fraction of the total load on the lobes of the bottle to be carried by the straps,

t= shell thickness of a strap, and

b. the lobes fit in between the strap sections and each lobe starts at the generally cylindrical section of the bottle extending downward with a radius of about 0.2-1.5 D measured from the plane between the right cylinder section and the bottom section of the bottle, including about 20 to 40 of arc and is connected to a toroidal knuckle having a radius of about 0.03 to 0.1 D including about to of are directed down and inward toward the central axis of the bottle terminating in the base of the re-entrant cylinder wherein the toroidal knuckle forms the standing surface of the bottle and the center of curvature of the toroidal knuckle is located a distance of about 0.3 to 0.4 D away from the central axis of the bottle; and

c. the axially aligned re-entrant cylinder in the bot tom of the bottle is defined by two concentric walls each having a length of about 0.05 to 0.22 D wherein one wall is connected to the inward extending portions of lobes and strap sections, in uniform circumferential spacing around the base of this wall, the other wall is connected to the recessed central disc and the two walls are interconnected with a toroidal knuckle having a radius of about 0.02 to 0.04 1) including about of are where D equals the outside diameter of the bottle taken perpendicular to the major axis of the bottle where the bottom section of the bottle meets the generally cylindrical section of the bottle.

2. The bottle of claim 1 having three lobes and three strap sections.

3. The bottle of claim 1 having four lobes and four strap sections.

4. The bottle of claim 1 having five lobes and strap sections.

5. The bottle of claim 1 having six lobes and six strap sections.

6. The bottle of claim 1 having a reinforcing ring of thermoplastic on the interior surface of the toroidal knuckle connecting the walls of the re-entrant cylinder wherein the reinforcing ring is formed of two concentric contacting side wall sections each having a length of about 0.05 to 0.20 D.

7. The bottle of claim 1 wherein the thermoplastic is polyethylene terephthalate having an inherent viscosity of at least about 0.55.

8. The bottle of claim 7 having a shell thickness of at least 20 mils in the generally cylindrical section and 30 mils in the bottom section, prepared from a polymer having a modulus of elasticity at yield of at least about l80,000 psi, a tensile strength of at least 5,000 psi, a Poisson's ratio of 0.35 to 0.4 and a deformation constant equal to the slope of the log (reciprocal of the strain rate) versus strain having a value of at least about 0.65.

five

Claims

1. In a generally cylindrical thermoplastic bottle, biaxially oriented at least in the generally cylindrical section, the improvement wherein the bottom configuration consists essentially of at least three lobes around the bottom perimeter of the bottle on which the bottle stands; strap sections located between each lobe; an axially aligned re-entrant cylinder; anD a generally circular and axially aligned recessed section with a smooth transition of material between the lobes, straps, re-entrant cylinder, and recessed sections of the base wherein a. the strap sections begin at the generally cylindrical section of the bottle and extend downward toward the central axis of the bottle with a radius of about 0.45 to 0.7 D, including about 75* to 90* of arc and terminating in the base of the reentrant cylinder and the width of each strap is determined by the following formula:

2. The bottle of claim 1 having three lobes and three strap sections.

3. The bottle of claim 1 having four lobes and four strap sections.

4. The bottle of claim 1 having five lobes and five strap sections.

5. The bottle of claim 1 having six lobes and six strap sections.

8. The bottle of claim 7 having a shell thickness of at least 20 mils in the generally cylindrical section and 30 mils in the bottom section, prepared from a polymer having a modulus of elasticity at yield of at least about 180,000 psi, a tensile strength of at least 5,000 psi, a Poisson''s ratio of 0.35 to 0.4 and a deformation constant equal to the slope of the log (reciprocal of the strain rate) versus strain having a value of at least about 0.65.