WO1996026241A2

WO1996026241A2 - Polymer pellets, process for their preparation and moulded articles obtained therefrom

Info

Publication number: WO1996026241A2
Application number: PCT/US1996/001751
Authority: WO
Inventors: Graig L. Werling; Robert C. Miller; Don R. Roden
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1995-02-14
Filing date: 1996-02-08
Publication date: 1996-08-29
Anticipated expiration: 1997-08-14
Also published as: WO1996026241A3

Abstract

Polymer pellets which do not contain an interior vacuum void or contain only such a void which is reduced in size. These pellets are prepared by cutting an extruded strand which has been quenched in a liquid bath which is warm enough to avoid cooling the exterior shell of the strand and the interior core of the strand at different rates and/or which has been extruded through an elliptical die.

Description

POLYMER PELLETS WITHOUT VOIDS, A PROCESS FOR THEIR PREPARATION AND IMPROVED MOLDING PROCESSES AND MOLDED ARTICLES USING VOIDLESS POLYMER PELLETS

This invention relates to thermoplastic polymer pellets without voids and methods of forming such pellets and improved molding processes and molded articles using voidless polymer pellets.

Thermoplastic polymers, such as polycarbonate, are useful as stock to be molded into a variety of shaped articles. In a typical molding operation, polymer in the form of pellets is melted, and a polymer melt is then injected into, or is otherwise shaped by, a mold. In the manufacture of polymer pellets, polymer is recovered from the reactor in which it is prepared, and is then, typically, processed through an extruder. Once melted in an extruder, the polymer is forced through a die to form strands, which are cooled in a liquid bath, usually a water bath. The strands of polymer harden as they are cooled before they pass through a cutter to be comminuted into pellets. The pellets formed by such a procedure are often cylindrical in shape, and the length of an individual pellet can be from 1 to 10 mm, but is frequently from 2 to 5 mm. The diameter of a pellet thus formed can be from 1.5 to 5 mm, but is frequently from 2 to 4 mm. The size of a polymer pellet can also be expressed in terms of the weight of 100 of the pellets. The weight of 100 pellets cut from extrudate, as described above, can be at least 1 gram, is frequently at least 1.5 grams, and is occasionally at least 2.5 grams, and yet also is less than 5 grams, is frequently less than 4.0 grams, and is occasionally less than 3 grams. The size of a pellet can be adjusted by increasing or decreasing its length and/or diameter.

When a die through which polymer is extruded to form strands contains round holes, the resulting strand is typically found to retain such round shape. When a round strand is contacted with water or other liquid for cooling before being cut into pellets, the outside of the strand cools quickly, hardens and begins to form a rigid shell. The interior core of the strand also cools, although at a slower rate than the exterior, and the core begins to slowly contract as it cools. It is believed that tensile forces are oriented around the exterior surface of the strand in a manner such that force in one direction is off-set by an equal force in the opposite direction. Because of the presence of these equally-distributed tensile forces, the contraction of the core of the strand as it cools does not result in any significant contraction or deformation of the rigid shell. The very rapid cooling and hardening of the exterior shell of the strand and the slower cooling and contraction of the core occur independently, and there is no deformation of the strand as a single unit. To accommodate the equally-balanced tensile forces surrounding and supporting the exterior of the rigid shell of the strand, and yet relieve the stress resulting from a slower contraction of the core, a series of vacuum voids is created in the center of the polymer strand. These voids result from a contraction of the core of the strand toward the rigid shell because the shell will not contract together with the core toward the center of the strand. When a strand containing such vacuum voids is cut into pellets, the cuts may fall between the voids, with the result that a void is left intact in the center of many of the pellets. During the storage of pellets containing such a void, gases in the surrounding atmosphere, typically air and water, diffuse through the polymer shell to fill the void. When pellets with a void containing gas are melted in preparation for molding, it is difficult to remove all of the gas from the polymer melt The quality of a molded article suffers when it is molded from material containing such entrapped gas. In particular, we have found that the surface of such an article is characterized by indentions, streaks or marks caused bythe presence of bubbles of such entrapped gas at or o close to the surface at the time when resin molding and cooling to form the article occur. Thes surface defects are sometimes also referred to as splay, splay bubbles, or silver streaking.

It would consequently be desirable to have a polymer pellet which either does no contain a vacuum void in its interior or contains only such a void which is reduced in size, and to have methods for making such a polymer pellet. 5 In one aspect, this invention involves a polymer pellet having a vacuum void, the volume of which is less than 10 percent of the volume of the pellet.

In another aspect, this invention involves a parcel, batch or lot of polymer pellets as to which, in a 10 gram random sample, the pellets which contain a vacuum void are less than 10 percent of the number of pellets in the sample. 0 In a further aspect, this invention involves a method of forming polycarbonate pellets by forcing melted polycarbonate through a round die hole to form a strand of polycarbonate, cooling the strand of polycarbonate by contacting it with liquid which has a temperature of at least 44^CC, and cutting said strand of polycarbonate into pellets.

In a further aspect, this invention involves a method of forming polycarbonate 5 pellets by forming a strand of polycarbonate by forcing melted polycarbonate through an elliptical die hole which has an aspect ratio greater than 1.3, and cutting said strand of polycarbonate into pellets.

In a further aspect, this invention involves a method of molding an article by

(a) providing a parcel of polymer pellets, each of which pellets has a vacuum void the volume 0 of which is lessthan 10 percent of the volume of the pellet,

(b) melting said parcel of pellets to form molten polymer, and

(c) shaping such molten polymer in the form of said article.

In a further aspect, this invention involves a method of molding an article by

(a) providing a parcel, batch or lot of polymer pellets as to which, in a 10 gram random 5 sample, the pellets which contain a vacuum void are less than 10 percent of the number of pellets in the sample,

(b) melting said parcel, batch or lot of pellets to form molten polymer, and

(c) shaping such molten polymer in the form of said article. It has been found that the pellets of this invention, when melted for molding purposes, produce articles which have few if any surface defects.

Controlling the rate of cooling of a strand in a liquid bath before it is comminuted will affect the degree and size of vacuum void formation in the center of the strand. A more gradual cooling will cause the shell of the strand to remain flexible and enable it to contract together with the core, toward the center of the strand, as the core cools. The tensile forces surrounding the shell of a round strand, which would ordinarily resist any change in the shape of such shell, will not prevent the strand from contracting as a single entity as the core cools if the shell has not been immediately hardened, as it is upon entering a low temperature bath. One of the methods of this invention, consequently, is to use a water bath to cool a molten polymer strand before pelletization which has a temperature which does not result in rapid quenching of the molten polymer as it exits the die, thereby enabling the shell and core of the strand to cool and contract together at rates which are as nearly equal as possible. When a round strand of polycarbonate, for example, is being pelletized, a temperature which is appropriate for a warm water bath, which results in such a slower rate of cooling, is at least 44°C, and is frequently at least 50°C, and yet also is less than 95°C, and is frequently less than 55°C. Cooling a polycarbonate strand in a water bath at these temperatures either eliminates, or reducesthe size of, a vacuum void in the interior of a pellet cut from such strand.

Use of a die which has holes of an elliptical or oval shape rather than round holes also produces a polymer strand which contains fewer vacuum voids than the strand typically formed through a round die hole in a low temperature bath. As is true in the case of a round die hole, the strand emerging from an elliptical die hole generally retains the elliptical shape of the hole. However, tensile forces are not oriented in an equal fashion around the surface of an elliptical strand, there being greater force on the surfaces closer to the center of the ellipse. When an elliptical strand is cooled in a liquid bath, there is consequently less resistance offered by the forces which are supporting the exterior shell to deformation of the strand as a single entity, in response to contraction of the core. There is correspondingly less tendency for the strand to form a vacuum void as the core contracts. Another of the methods of this invention therefore involves extruding polymer through an elliptical die to reduce the number and size of voids formed in pellets cut from the strand.

The elliptical die used in the methods of this invention has holes which have a ratio of longer to shorter diameter ("aspect ratio") of greater than 1 , frequently greater than 1.3, and occasionally greater than 1.5, and yet also lessthan 2.5, frequently less than 2.0, and occasionally less than 1.7. Gradual cooling of an elliptical strand in a warm water bath, as described above, is a preferred embodiment of the methods of this invention in which the likelihood of vacuum void formation is reduced to a minimum. When an elliptical strand of polycarbonate, for example, is being pelletized, a temperature which is appropriate for a warm water bath, which results in such a slower rate of cooling, is at least 28°C, is frequently at least 40°C, and is occasionally at least 50°C, and yet also is less than 95°C, is frequently less than 60°C, and is occasionally lessthan 55°C. Since the shape of an elliptical strand offers less resistance than a round shape to contraction of the shell and core together toward the center of the strand, the temperature of a bath for cooling an elliptical strand need not be as high as that used for a round strand.

Determination of the presence of a vacuum void in a polymer pellet is accomplished by various means. One method is to examine pellets on a light table and simply count the number of pellets in a group with vacuum voids. However, this method is not applicable to pellets which are opaque. A second method is to count the visible voids in a specified length of strand which has not been comminuted, but this method is also not applicable to opaque or pigmented strands. A third method which is less tedious than the other two, and perhaps more accurate, is to prepare a liquid medium having a known specific gravity, for example an aqueous sodium chloride solution. A sample of pellets having a known weight and density is placed in a container of the liquid medium, and the container is shaken vigorously for a period of time, perhaps one minute, to remove any air or other fluid that migh be trapped in a partially open void. The pellets which remain on the surface of the liquid medium after shaking are tapped with a stirring rod to break any surface tension or gas bubbles which may be supporting them on the surface of the liquid medium. Afterthis, the pellets that remain floating are counted, and from this count the percent by number of pellets with voids in the sample is calculated.

By this method, the size of a vacuum void in a polymer pellet can also be determined, as illustrated by the following sample calculation: A blend of a Bisphenol-A polycarbonate, acrylonitrile/butadiene/styrene copolymer ("ABS") and talc has a density of 1.20 g/cm³. An aqueous sodium chloride solution having a specific gravity of 1.10 is used as a liquid medium in which pellets of this PC/ABS/talc composition are immersed and shaken.

From this it can be determined that any pellet of such composition which will float in this NaCI solution will have a void large enough to decrease its effective density to 1.1 or less. The percent void size by volume therefore equals 100 - [(NaCI solution density/composition density)* 100] or 100 - [(1 .1 /1 .2)* 100], which equals approximately 8.3% , where * indicates a step of multiplication.

The polymer pellets of this invention are characterized in that each such pellet, if it has a vacuum void, has a percent void size by volume of less than 10 percent, preferably less than 6 percent, more preferably less than 3 percent, and most preferably lessthan 1 percent. The polymer pellets of this invention are also characterized, with respect to the extent of void occurrence, in that, in a random sample weighing 10 grams taken from a parcel, batch or lot of such pellets, the pellets having a vacuum void is less than 10 percent, preferably less than 6 percent, more preferably lessthan 3 percent, and most preferably lessthan 1 percent of the pellets in the sample by number. The random 10 gram sample is typically taken from a parcel, batch or lot of such pellets which may, for example, be a 50 pound bag, a 1 ,000 pound container such as a gaylord, or some portion of either one, although it is not required that the parcel, batch or lot be any particular weight so long as it exceeds 10 grams. " Random" is used here in the sense that each 10 gram sample contained in the parcel, batch or lot has an equal chance to be drawn therefrom as the sample to be analyzed.

The pellets of this invention are useful in a method of reducing surface defects, such as splay or splay bubbles, in a molded article. When pellets having a reduced void size or having a reduced extent of void occurrence, as described above, are used in a molding o operation, a smaller amount of gas is entrapped in the polymer melt because a smaller amount is present in the polymer pellets from which the melt is prepared. Articles having better surface quality are therefore obtained from pellets prepared according to the methods described above concerning use of a high temperature bath and/or an elliptical die during pelletization. The polymer from which the pellets of this invention are most often prepared is 5 polycarbonate, which can be made, for example, by the phase boundary process in which a bisphenate in an alkaline aqueous solution is contacted with a carbonic acid derivative such as phosgene. The bisphenate is typically formed from Bisphenol-A, although numerous other dihydroxy compounds are useful forthis purpose. Polycarbonate may also be prepared in a homogeneous solution; or by transterif ication of a dihydroxy compound with an aromatic dicarbonate at a temperature of about 250-300°C using a catalyst such as lithium hydroxide, followed by advancement to higher molecular weight in a melt or solid state condition. Processes for making polycarbonate are generally known and are more particularly described in Schnell, U.S. Patent 3,028,365; Campbell, U.S. Patent 4,384,108; Glass, U.S. Patent 4,529,791 ; and Grigo, U.S. Patent 4,677,162. Polymers other than polycarbonate are also applicable in this invention, included among which are polyacetal, polyacrylate, acrylonitrile/butadiene/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/diene block copolymer, methacrylate/butadiene/styrene copolymer, poly( inyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(vinyl ether), including derivatives and substituted varieties thereof. The pellets of this invention may also be prepared from blends of any one or more of the foregoing, with or without polycarbonate, and with or without known additives for polymers, including for example fillers such as talc or glass, stabilizers or mold release agents. The temperature of a water bath which is used to cool a strand of any of the polymers listed above which has a lower melting point or glass transition temperature ("Tg") than the Tg of polycarbonate typically need not be as high asthat which would be used for polycarbonate to obtain a comparable degree of reduction in void formation in the strand. To illustrate the practice of this invention, examples of several preferred embodiments are set forth below. It is not intended, however, that these examples (Examples 1-20) should in any manner restrictthe scope of this invention. Some of the particularly desirable features of this invention may be seen by contrasting the characteristics of Examples 1 -20 with those of various formulations (Controls A-R) which do not possess the features of, and are not therefore embodiments of, this invention.

In Examples 1-3 and Controls A-B, pellets of a 3 MFR Bisphenol-A polycarbonate are prepared. ("MFR" is melt flow rate as determined according to ASTM Designation D 1238- 89, Condition 300/1.2.) The polycarbonate is melted in a 40 mm Berstorf 40A co rotating twin- screw extruder and is fed at a rate of 30 Ib/hr/die hole through a round die of appropriate size to make pellets having a size in the range of 1.75 to 2.0 g/ 100. The strands are cooled in a water bath having either a 5.5 or 8.5-foot length at various temperatures. The percentage of pellets with voids in each example or control is determined by manual counting on a light table. The bath conditions and percentage of pellets having voids in 50 grams are shown belo in Table I for Examples 1-3 and Controls A-B.

Table I Examples 1 -3 and Controls A-B

Bath Length of Percent of Temperature Bath in feet Sample with

Void

Example 1 53°C 5.5 1

Example 2 53°C 8.5 1.5

Example 3 44°C 8.5 2

Control A 40°C 5.5 24

Control B 35°C 5.5 33.5

In Examples 1-3, the use of a warmer bath temperature results in a lower percentage of pellets which have voids.

In Examples 4-7 and Controls C-D, pellets of a 3 MFR Bisphenol-A polycarbonate are prepared. The polycarbonate is melted in a 40 mm Berstorf 40A corotating twin-screw extruder, is fed at a rate of 50 Ib/hr/die hole through a three-hole die, and is cooled in a water bath to form 9-foot strands. The strands are cooled at three different water bath temperatures, and strands are prepared with both a round and oval die at each different wate bath temperature. The oval die hole has an aspect ratio of 1.6. The number of voids in each se of three strands is counted. The bath temperature, shape of die and number of voids in three 9-foot strands are shown below in Table II for Examples 4-7 and Controls C-D.

Table II Examples 4-7 and Controls C-D

Bath Temperature Die Shape Number of Voids

Example 4 28°C oval 13

Example 5 40°C oval 25

Example 6 50°C oval 10

Control C 28°C round 629

Control D 40°C round 232

Example 7 50°C round 88

In Examples 4-7 and Controls C-D, the use of a round die with a higher temperature water bath, as in Example 7, reduced the production of voids, and the use of an oval die reduced the production of voids at all temperatures. In Examples 8-13 and Controls E-J, polycarbonate strands are produced in the same manner described above. The feed rate through each die hole is 50 Ib/hr, and both round and elliptical die holes are used. The elliptical die has an aspect ratio of 1.6. A count is made of the number of voids visible in three 9-foot strands of the polycarbonate produced in each example or control. The strands are cooled in a water bath 8.5 feet long at different temperatures. However, in Example 1 1 and Control F, the strand is lifted out of the water after passing through 3 feet of the bath, and is reimmersed in the bath after exposure to air over a distance of 14 inches. The aspect ratio of each strand is measured in two places. The resulting values are averaged to determine an aspect ratio for each strand. The bath temperature, shape of die, number of voids counted in three 9-foot strands, and aspect ratio are shown below in Table 111 for Examples 8-13 and Controls E-J. Table III Examples 8-13 and Controls E-J

Bath Shape of Number of Aspect Temperature Die Voids Ratio

Example 8 50°C oval 12 2.02

Example 9 41°C oval 13 1.87

Example 10 39°C oval 25 1.85

Example 11 29°C oval 15 1.80

Example 12 28°C oval 30 1.88

Example 13 51°C round 90 1.55

Control E 41°C round 225 1.66

Control F 40°C round 110 1.54

Control G 37°C round 605 1.58

Control H 31°C round 600 1.62

Control J 27°C round 620 1.51

In Example 8-13 and Controls E-J, the use of a round die with a higher temperature water bath, as in Example 13, reduced the production of voids, and the use of an oval die reduced the production of voids at all temperatures.

In Examples 14-17 and Controls K-N, pellets of a 3 MFR Bisphenol-A polycarbonate, or a blend of Bisphenol-A polycarbonate, ABS and talc ("PC/Blend") in a 60/32/8 weight ratio, are prepared. The polycarbonate individually, or the blend components together, are melted in a 40 mm Berstorf 40A corotating twin-screw extruder, and are fed through a round or oval die hole to form a strand. The strands are cooled in either warm or cold bath conditions prior to pelletization. The number of pellets containing voids in a 10 g sample of each example or control is calculated by shaking the sample in an aqueous NaCI solution of known specific gravity. The content, bath temperature, die shape, specific gravity, and number of pellets containing a void are shown below in Table IV for Examples 14-17 and Controls K-N. Table IV Examples 14-17 and Controls K-N

Number of

Bath Die Specific Pellets

Content Temperature Shape Gravity Containing a

Void

Control K PC/Blend 23°C round 1.148 55

Control L Polycarbonate 23°C oval 1.148 15

Example 14 Polycarbonate 53°C oval 1.148 0

Example 15 PC/Blend 53°C oval 1.148 2

Control M PC/Blend 23°C round 1.180 200

Control N Polycarbonate 23°C oval 1.180 40

Example 16 Polycarbonate 53°C oval 1.180 8

Example 17 PC/Blend 53°C oval 1.180 8

In Examples 14-17 and Controls K-N, formation of voids is reduced at higher bath temperatures.

In Examples 18-20 and Controls P-R, pellets of a 3 MFR Bisphenol-A polycarbonate, or a blend of Bisphenol-A polycarbonate, ABS and talc ("PC/Blend") in a 60/32/8 weight ratio, are prepared. The polycarbonate individually, or the blend components together, are melted in a 40 mm Berstorf 40A corotating twin-screw extruder, and are fed through a round or oval die of appropriate size to make pellets having a size of either 2.0 g/100 in a range of 3.5-4.0 g/ 100. The strands are cooled in either warm or cold bath conditions prior to pelletization. The percentage of pellets containing a void in a 10 g sample of each example or control is calculated by shaking the sample in an aqueous NaCI solution having a specific gravity of 1.148. The pellets are dried in a Conair hopper drier at 210°F for a minimum of 4 hours, and are then fed to a 300 Demag injection molding machine to produce a 6.5^" x 3.5" 2.5" open box, or tray. Each test piece is evaluated for the evidence of splay and the presence of splay bubbles. Each separate indication of splay is noted as a "splay event" , and the number of splay bubbles is counted. The content, bath temperature, die shape, pellet size, average percentage of pellets containing a void, total number of splay events, number of test pieces with a splay event, total number of splay bubbles, and number of test pieces with a splay bubble, are shown below in Table V for Examples 18-20 and Controls P-R. Table V Examples 18-20 and Controls P-R

Percent Total Pieces Pieces

Bath Die Total

Content Pellet Size with Splay with with Temp. Shape Bubbles Void Events Splay Bubble

Example 18 PC/Blend 53°C oval 2.0g/100 <2 11 8 2 2

Example 19 PC 53°C oval 2.0g/100 < 2 11 8 2 2

Example 20 PC/Blend 53°C oval 3.5- <2 4 4 0 0 4.0g/100

Control P PC/Blend 23°C round 2.0g/100 > 30 35 18 7 6

Control Q PC 23°C round 2.0g/100 >30 38 15 10 8

Control R PC/Blend 23°C round 3.5- >30 35 18 7 7 4.0g/100

In Examples 18-20 and Controls P-R, molding with pellets having a reduced level of void content produces test pieces having fewer imperfections from splay and splay bubbles.

Claims

1. A method of forming polycarbonate pellets comprising forcing melted polycarbonate through a round die hole to form a strand of polycarbonate, cooling the strand of polycarbonate by contacting it with liquid which has a temperature of at least 44°C, and cutting said strand of polycarbonate into pellets.

2. The method of Claim 1 wherein the liquid has a temperature of at least 50°C and less than 95°C.

3. The method of Claim 1 wherein the polycarbonate is blended with one or more of the following: polyacetal, polyacrylate, acrylonitrile/butadiene/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/- diene block copolymer, methacrylate/butadiene/styrene copolymer, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(vinyl ether).

4. The method of Claim 1 wherein 100 of said pellets weigh at least 2.5 grams.

5. A method of forming polycarbonate pellets comprising forming a strand of polycarbonate by forcing melted polycarbonate through an elliptical die hole which has an aspect ratio greater than 1.3, and cutting said strand of polycarbonate into pellets.

6. The method of Claim 5 wherein the elliptical die hole has an aspect ratio greater than 1.5.

7. The method of Claim 5 further comprising a step of cooling the strand of polycarbonate, after it has been forced though the die hole, by contacting it with liquid which has a temperature of at least 28°C.

8. The method of Claim 7 wherein the liquid has a temperature of at least 40°C and less than 95°C.

9. The method of Claim 5 wherein the polycarbonate is blended with one or more of the following: polyacetal, polyacrylate, acrylonitrile/butadiene/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/- diene block copolymer, methacrylate/butadiene/styrene copolymer, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(vinyl ether).

10. The method of Claim 5 wherein 100 of said pellets weigh at least 2.5 grams.

1 1. A polymer pellet having a vacuum void, the volume of which is less than 10 percent of the volume of the pellet.

12. The pellet of Claim 11 which is a polycarbonate pellet.

13. The pellet of Claim 1 1 which is a pellet of polycarbonate blended with one or more of the following: polyacetal, polyacrylate, acrylonitrile/butadieπe/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/diene block copolymer, methacrylate/butadiene/styrene copolymer, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(vinyl ether).

14. The pellet of Claim 1 1 wherein 100 of said pellets weigh at least 2.5 grams.

15. A parcel, batch or lot of polymer pellets as to which, in a 10 gram random sample, the pellets which contain a vacuum void are less than 10 percent of the number of pellets in the sample.

16. The parcel, batch or lot of pellets of Claim 15 which are polycarbonate pellets.

17. The parcel, batch or lot of pellets of Claim 15 which are pellets of polycarbonate blended with one or more of the following: polyacetal, polyacrylate, acrylonitrile/butadieπe/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/diene block copolymer, methacrylate/butadiene/styrene copolymer, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(vinyl ether).

18. The parcel, batch or lot of pellets of Claim 15 wherein 100 of said pellets weigh at least 2.5 grams.

19. The parcel, batch or lot of pellets of Claim 15 asto which the vacuum void, in those pellets which have a vacuum void, is less than 10 percent of the volume of the pellet.

20. A method of molding an article comprising (a) providing a parcel of polymer pellets, each of which pellets has a vacuum void the volume of which is less than 10 percent of the volume of the pellet,

(b) melting said parcel of pellets to form molten polymer, and

(c) shaping molten polymer in the form of said article.

21. The method of Claim 20 wherein the polymer pellets are polycarbonate pellets.

22. The method of Claim 20 wherein the pellets are pellets of polycarbonate blended with one or more of the following : polyacetal, polyacrylate, acrylonitrile/butadiene/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/diene block copolymer, methacrylate/butadiene/styrene copolymer, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(vinyl ether).

23. The method of Claim 20 wherein 100 of said pellets weigh at least 2.5 grams.

24. A method of molding an article comprising (a) providing a parcel, batch or lot of polymer pellets as to which, in a 10 gram random sample, the pellets which contain a vacuum void are less than 10 percent of the number of pellets in the sample, (b) melting said parcel, batch or lot of pellets to form molten polymer, and (c) shaping molten polymer in the form of said article.

25. The method of Claim 24 wherein the polymer pellets are polycarbonate pellets.

26. The method of Claim 24 wherein the pellets are pellets of polycarbonate blended with one or more of the following: polyacetal, polyacrylate, acrylonitrile/- butadiene/styrene copolymer, polyamide, poly(alkylene oxide), polyester, polymethacrylate, olefin homo- and copolymers, poly(phenylene ether), polystyrene, styrene/acrylonitrile copolymer, polyurethane, styrene/diene block copolymer, methacrylate/butadiene/styrene copolymer, poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), and poly(viπyl ether).

27. The method of Claim 24 wherein 100 of said pellets weigh at least 2.5 grams.

28. An improved molded article prepared by a method according to Claims 20 or 24.