Extruded polyolefin foam in thicker grades and plank and process therefor

An extruded polyolefin foam in thicknesses of at least about 12 mm and greater of improved compressive strength, extensional viscosity, and reduced aging time and a process for making the foam are disclosed. The foam is prepared using a propane blowing agent. The beneficial properties imparted to thicker grade and plank foams by a propane blowing agent can be achieved with up to as much as 75% by volume of either normal butane or isobutane or both in the blowing agent, depending on the polyolefin selected.

FIELD OF THE INVENTION 
This invention relates to extruded polyolefin foams and to a process for 
producing an expanded polyolefin foam product of low density using an 
expandable polyolefin composition. In particular, this invention relates 
to the use of blowing agents for incorporating into a plasticized 
polyolefin resin for foaming by extrusion. 
BACKGROUND OF THE INVENTION 
Thermoplastic foam products can be produced by a wide variety of processes, 
of which extrusion is but one, that are in part responsible for the wide 
variety of foam products available today. Foams range in consistency from 
rigid materials suitable for structural use to flexible substances for 
soft cushions and packaging materials. These foams range in cellular 
formation from open or interconnecting-cell foams to closed or unicell 
foams. The cell structure may range from large to fine. Electrical, 
thermal, mechanical, and chemical properties can be varied within wide 
limits depending on the thermoplastic resin composition and the method 
chosen to create the foam. Foamed thermoplastics range in density anywhere 
from about 10 kg/m.sup.3 to over 1,000 kg/m.sup.3, although the latter 
perhaps more properly are called microcellular structures. True foams are 
considered to have a density of less than about 800 kg/M.sup.3. 
Many methods have been developed for the manufacture of foamed 
thermoplastics, which generally can be classified into three groups: 1) 
methods for adding a gaseous "blowing agent" to the thermoplastic mass 
during processing, 2) methods for producing a gaseous blowing agent in the 
thermoplastic mass during processing, and 3) methods for forming a 
thermoplastic mass from granules to obtain a cellular structure. Similar 
blowing agents sometimes are used in the various methods to produce foams. 
However, the effectiveness of a particular blowing agent varies 
considerably depending on the thermoplastic resin composition, the method 
chosen, the process conditions, the additives used, and the product 
sought. 
Blowing agents work by expanding a thermoplastic resin to produce a 
cellular thermoplastic structure having far less density than the resin 
from which the foam is made. Bubbles of gas form around "nucleation sites" 
and are expanded by heat or reduced pressure or by a process of chemical 
reaction in which a gas is evolved. A nucleation site is a small particle 
or conglomerate of small particles that promotes the formation of a gas 
bubble in the resin. Additives may be incorporated into the resin to 
promote nucleation for a particular blowing agent and, consequently, a 
more uniform pore distribution. However, the foam is maintained by 
replacing the blowing agent in the cells with air. Diffusivity of the 
blowing agent out of the cells relative to air coming into the cells 
impacts the stability of the foam over time and whether the cells of the 
foam may collapse. Additives may be incorporated into the resin and 
process conditions may be adjusted to assist in controlling the 
diffusivity of the blowing agent, to promote foam stability, and to limit 
collapse of the foam to acceptable limits. 
Methods for producing a blowing agent in situ usually involve a chemical 
reaction that evolves gas. Polyethylene, silicone, epoxy, and vinyl foams 
have all been produced by adding a substance that will produce a gaseous 
blowing agent chemically. For example, dinitroso compounds and hydrazides, 
which evolve nitrogen gas on decomposition, and bicarbonates, which evolve 
carbon dioxide, have been added to thermoplastic resins to produce foams. 
Polystyrene foams often are produced by "bead molding," in which partially 
expanded granules or beads are heated in a mold in the presence of a 
blowing agent to expand and fuse the particles into a rigid unicellular 
structure. A volatile organic compound or some other gaseous blowing agent 
is impregnated into the beads. Heat is applied and the pressure is 
released to cause the beads to expand and fuse. 
There are several methods for adding a blowing agent to a thermoplastic 
resin during processing to produce a foam. Ureaformaldehyde and 
polyvinylformaldehyde foams have been produced by whipping air into a 
heated thermoplastic mass before it sets. Polyolefinic foams have been 
produced by introducing air or some other gas or volatile solvent into a 
heated thermoplastic polyolefin mass and then heating the mass or reducing 
pressure to expand the gas and form pores of a desirable size. One more 
specific method is to impregnate a thermoplastic resin with blowing agent 
under heat and pressure in a closed vessel. The pressure is released to 
expand the blowing agent to form "prefoamed," or partially expanded, 
beads. Prefoamed beads usually are further expanded in an enclosed vessel 
such as a mold to produce a molded foam product, as is discussed above. 
As examples of the use of various blowing agents for molding and prefoamed 
bead production, Kloker et al. U.S. Pat. No. 4,120,923 and Yoshimura et 
al. U.S. Pat. No. 4,464,484 disclose the use of the inert gas carbon 
dioxide as a blowing agent for molded polyolefin foam articles and for 
polymer beads, respectively. Yoshimura et al. U.S. Pat. No. 4,464,484 
disclose that a mixture of carbon dioxide and aliphatic hydrocarbons and 
halogenated aliphatic hydrocarbons, including CFCs and HCFCs, are useful 
blowing agents for polyolefin beads. Broad ranges of blends of VOCs, CFCs, 
and HCFCs with carbon dioxide are disclosed and mixtures of butane or 
dichlorodifluoromethane and carbon dioxide are exemplified. 
Another more specific method, to which the invention claimed herein 
relates, is to mix the blowing agent with molten thermoplastic resin under 
pressure and then extrude the mixture through a forming die into a zone of 
reduced pressure. Shaped extruded foams can be produced by this method 
using a forming die of particular configuration. Plank, which can be cut 
to a desirable shape, and thin foam sheets are produced in this manner. 
Many of the halogenated hydrocarbons have been used for several years as 
blowing agents in various methods for producing extruded foams from 
thermoplastic resins. The halogenated hydrocarbons include the 
chlorofluorocarbons ("CFCs") and hydrochlorofluorocarbons ("HCFCs"). CFCs 
and HCFCs are readily impregnable in thermoplastic resins and are readily 
expandable under relatively mild conditions. CFCs and HCFCs generally 
produce foams of high quality with a minimum of processing difficulty. The 
pore size is controllable, the foam has good stability with minimum 
tendency to collapse after a period of time, and the surface 
characteristics of the foam are smooth and desirable. Also, CFCs, HCFCs, 
and other halogenated hydrocarbons typically are either not flammable or 
are of low flammability, which greatly reduces the care with which they 
may be used. These compounds have the further advantage of low toxicity. 
However, governmental regulation is phasing out use of halogenated 
hydrocarbons because the halogenated hydrocarbons may be responsible for 
damage to the earth's ozone layer. 
Producers of thermoplastic foam products have been seeking alternatives to 
CFC and HCFC blowing agents for a number of years to reduce or eliminate 
altogether the amount of halogenated hydrocarbons used. A number of 
volatile organic compounds (VOCs) have been proposed, although many of 
these also are somewhat objectionable. VOCs include the light aliphatic 
hydrocarbons such as ethane, propane, n-butane, isobutane, butylene, 
isobutene, pentane, neopentane, and hexane, to name but a few. The 
diffusivity of VOCs can be many times faster than that of the halogenated 
hydrocarbons and harder to control. Foam collapse and stability problems 
have been encountered, although high quality foams have been produced 
using, for example, butane. 
VOCs typically are volatile and flammable, thus presenting handling 
problems and safety concerns. For example, Robin et al. U.S. Pat. No. 
5,314,926 describes a blowing agent comprising a mixture of one or more 
hydrocarbons or partially halogenated alkanes with a fluorinated propane, 
1,1,1,2,3,3,3-heptafluoropropane. The hydrocarbons are said to include 
propane, butane, isobutane, n-pentane, i-pentane, neopentane, n-hexane, 2 
methylpentane, 3-methylpentane, and 2,2-dimethylbuntane. The fluorinated 
propane is said to be useful even in small amounts in reducing the 
flammability of foamable plastics including polystyrene, polyvinyl 
chloride, polyethylene, and other non-polyisocyanate based foams. 
The behavior of VOCs in various thermoplastic resins and the foams prepared 
therefrom is somewhat unpredictable due to the differences in volatility 
of the various VOCs and VOC blends, the differences in the foaming 
behavior of different thermoplastic resins, the wide variation in kinds 
and amounts of processing additives that are added to the different 
thermoplastic resins, and a host of other factors too numerous to mention 
here. 
As an example of extrusion foaming, Watanabe et al. U.S. Pat. No. 4,214,054 
describe numerous volatile organic blowing agents including various CFCs, 
VOCs, and the use of decomposable gas-releasing chemical blowing agents 
for producing extruded polyolefin foams from particular resin 
compositions. 
Johnson U.S. Pat. No. 3,966,373 proposes a method and apparatus for making 
relatively dense structural foam profiles having a foam core and a dense 
skin. A partially expanded extruded thermoplastic polymer composition is 
conveyed through a chilled shaping passage moving at the same rate as the 
foam to eliminate friction. The dense skin is formed by the chilled 
passage while the polymer resin is still expanding. Foaming agents are 
said to include nitrogen, carbon dioxide, lower molecular weight paraffins 
such as propane, butane, and methylchloride, lower molecular weight 
olefins such as ethylene, propylene, and butylene or mixtures of the 
above. No specific mixtures of foaming agents are disclosed. A preferred 
thermoplastic composition for extrusion to form tongue depressors or ice 
cream sticks of 320 to 1000 kilograms per cubic meter is disclosed to 
include polystyrene beads having a pentane blowing agent integrated 
therewith. 
Gilbert U.S. Pat. No. 3,488,746 discloses a process for preparing a foamed 
polyethylene layflat tube by blow extruding a foamable polyethylene resin 
composition through an annular die. Blowing agents are said to include 
lower aliphatic hydrocarbons such as ethane, propane, butane, or pentane, 
lower alkyl halides such as methylchloride, trichloromethane, or 
1,2-dichlorotetrafluorethane and inorganic gases such as carbon dioxide or 
nitrogen. Butane and isobutylene are said to be preferred. Highly active 
nucleating agents such as silica or alumina or small quantities of 
decomposable nucleating agents are disclosed in quantities of up to about 
5 percent by weight of the resin. 
Vesilyn U.S. Pat. No. 3,287,477 discloses extrusion apparatus and methods 
for preparing polymer foams and is primarily directed to polystyrene foam. 
Vesilyn discloses that the extrusion apparatus can also be operated with 
other polymers including polymers derived from ethylene monomers to 
produce foam sheets. Blowing agents are said to include examples such as 
methane, ethane, propane, butane, n-pentane, isopentane, neopentane, 
hexanes, heptanes, and a variety of others. 
Vonken et al. U.S. Pat. No. 5,484,649 describes extruded polystyrene 
expanded films and shaped articles, including building materials, that 
have been prepared using propane, butane, or mixtures of propane and 
butane as blowing agents in polystyrene melts that are treated with 
flameproofing agents and nucleating agents. The foams are said preferably 
to rest for at least about one week to adjust the amount of residual 
blowing agent to the minimum required for further expansion by heat 
treatment. The post-extrusion expansion step is said to increase the 
thickness of the extrudate by a factor of from about 1.8 to 2. 
Polystyrene typically is considered to have different foaming 
characteristics from the polyolefins. Polystyrene is an amorphous material 
whereas the polyolefins are semi-crystalline. 
Butane blowing agents normally are used for foam extrusion of low density 
polyolefin foams (over 30% expansion) in an amount of from about 15% to 
16% by weight of the resin, typically in combination with active 
nucleating agents in amounts of about 0.5% by weight of the resin. Butane 
is extremely flammable and at the levels used for blowing agent an aging 
period typically is provided to reduce residual butane blowing agents 
below explosive limits prior to shipment of the foam. 
Propane has successfully been used as a blowing agent for producing 
polyethylene thin foam sheet and foams of about 11 mm thickness or less, 
despite its volatility and flammability. However, there has been no 
disclosure of or suggestion to use propane as a blowing agent to produce 
thicker grade foams or plank and there has been no disclosure or 
suggestion of improvements in properties of such foams. 
It is desirable to continue to develop combinations of blowing agents and 
thermoplastic resins that can result in foam products having improved 
properties, including safety, foam physical characteristics, and other 
properties. 
SUMMARY OF THE INVENTION 
The invention claimed herein relates to the extrusion foaming of polyolefin 
resins into relatively low density thicker grade and plank foams having 
improved properties, particularly compressive strength, wherein the 
blowing agent is propane. "Thicker grade" foams means foams of about 12 mm 
or more. Thin foam sheet is typically thought of as 1/4-in or less (6 mm 
or so). Plank is typically thought of as 2 inches or more in thickness 
(about 50 mm). 
The benefits in compressive strength in thicker grade foams can usually be 
achieved even when the amount of butane used in the propane blowing agent 
is up to about 75% by volume, depending on the polyolefin selected. Aging 
time to reach the blowing agent lowest explosive limit can be reduced as 
compared to butane, which is important from safety and flammability 
concerns. Extensional viscosity of the melt extruded through the plank die 
can be improved. These benefits have not previously been recognized or 
appreciated in polyethylene foam sheet prepared with 100% propane blowing 
agent. 
While not wishing to be bound by theory, it is believed that the benefits 
in compressive strength are achieved because of the greater efficiency of 
the propane blowing agent compared to other blowing agents. Foaming 
efficiency refers to the amount of blowing agent that is used to achieve a 
particular density. It is believed that more efficient blowing agents 
produce less of the blowing agent in cell walls of the foam and that the 
compressive strength of the foam is improved when less blowing agent is 
present in the cell walls of the foam. 
Thicker grade polyolefin foams and plank made in accordance with the 
invention can have an apparent extensional viscosity at a die temperature 
of about 110 degrees Centigrade of from about 1.2.times.10.sup.6 to 
7.times.10.sup.6 poise at a strain rate of from about 1.9 to 40 per 
second, respectively. As should be recognized by the skilled artisan, the 
resin typically enters the die at a much higher temperature than the die 
temperature. 
Thicker grade polyolefin foams and plank made in accordance with the 
invention can have a compressive strength about one hour after extrusion 
of from about 9 to 12 psig at 25 percent compression by unit volume. Foam 
plank made in accordance with the invention can have a compressive 
strength about two weeks after extrusion of from about 20 to 25 psig at 50 
percent compression by unit volume. 
When needle punched, foam plank made in accordance with the invention can 
reach its lowest explosive limiting value within about two weeks after 
extrusion. 
Typically, foam plank made in accordance with the invention has a cell 
count of from about 10 to 30 cells per inch and a density of from about 20 
to 120 kg/m.sup.3. 
In a more specific embodiment, foam plank made in accordance with the 
invention is prepared from a low density polyethylene resin and a blowing 
agent selected from the group consisting of propane and a blend of propane 
with up to about 65% by weight of either normal butane or isobutane. The 
polymer melt is extruded and expanded to a nominal thickness of at least 
about 12 mm or more to form a thicker grade foam or foam plank. Normally, 
the polyethylene foam also comprises a processing additive, including, for 
example, glycerol monostearate (GMS), and a nucleating agent, including, 
for example, talc. 
In another more specific embodiment, thicker grade foam and foam plank made 
in accordance with the invention is prepared from a polypropylene resin 
and a blowing agent selected from the group consisting of propane and a 
blend of propane with up to about 75% by weight of either normal butane or 
isobutane. While not wishing to be bound by theory, it is believed that 
the blowing agent efficiency of propane in polypropylene is greater than 
in polyethylene, so that less propane is required to achieve the benefits 
of the invention. 
Thus, the invention provides, among other things, thicker grade foam and 
foam plank prepared from polypropylene and low density polyethylene resins 
that are expanded with a propane blowing agent. These foams have improved 
compressive strength and reduced aging time compared to plank made using a 
100% butane blowing agent and the resins have greater melt strength for 
extrusion through the plank die. The benefits of the invention can be 
achieved in thicker grade polypropylene foams when the propane blowing 
agent includes up to about 75% by weight of isobutane or normal butane, 
and in thicker grade polyethylene foams with up to about 65% by weight of 
one or more butanes. 
The foregoing and other objects, advantages, and features of the invention, 
and the manner in which the same are accomplished, will be more readily 
apparent upon consideration of the following detailed description of the 
invention taken in conjunction with the accompanying drawing, which 
illustrates preferred and exemplary embodiments of the invention.

DETAILED DESCRIPTION 
Various processes and equipment for extrusion foaming of thermoplastic 
resins have been used for many years. Generally, solid pellets of 
thermoplastic resin are fed through a hopper to a melting zone in which 
the resin is melted, or plasticized, to form a flowable thermoplastic 
mass. The plasticized thermoplastic mass generally is then metered to a 
mixing zone where the thermoplastic mass is thoroughly mixed with a 
blowing agent under pressure for subsequent cooling and expansion of the 
resin to form a foam. Blowing agent typically is injected between the 
metering and the mixing zones. The mixture of thermoplastic resin and 
blowing agent is then forced through a die, which imparts a shape to the 
thermoplastic mass, into a zone of lower pressure, such as atmospheric 
pressure. The blowing agent expands to form the cells of the foam and the 
thermoplastic foam is cooled. 
Typical of much of the equipment used for extrusion of thermoplastic foams, 
the thermoplastic pellets are conveyed from a hopper through the melt 
zone, the mixing and cooling zones, and extruded through the die by a 
screw type apparatus. Single screw extruders are common, although double 
screw extruders sometimes are used for greater mixing and tandem extruders 
can be used to provide greater cooling of the resin prior to foaming. 
When a blowing agent is injected into the mixing zone of the screw 
extruder, the blowing agent initially forms a dispersion of insoluble 
bubbles within the plasticized thermoplastic mass. These bubbles 
eventually dissolve in the thermoplastic mass as the mixing continues and 
the pressure increases down the length of the extruder. The extruder 
should have a length to diameter ratio of at least 30:1 and a sufficient 
length of mixing zone to ensure that proper mixing occurs. 
Thermoplastic resins contemplated for use in the practice of the invention 
are the polyolefin resins, although not necessarily with equivalent 
results for different polyolefins. Polyolefin resins may be defined as 
semicrystalline polymers derived from unsaturated hydrocarbons containing 
the ethylene or diene functional groups. Semicrystalline polyolefin 
polymers are to be distinguished from polystyrene polymers and the like, 
which typically are amorphous and can have fundamentally different foaming 
characteristics. 
Polyolefin resins may include virtually all of the addition polymers, 
however, the term polyolefin typically is used for polymers of ethylene, 
the alkyl derivatives of ethylene (the alpha-olefins), and the dienes. 
Among the more commercially important polyolefins are polyethylene, 
polypropylene, polybutene, and their copolymers. 
Polyethylene is a whitish, translucent polymer of moderate strength and 
high toughness. Polyethylene is available in forms ranging in 
crystallinity from 25 to 95 percent. Polyethylene is available in low, 
medium, and high density polymer forms. For the low density material, the 
softening temperature is about 105.degree. C. to 115.degree. C. The 
softening temperature for the high density material is some 25.degree. C. 
to 40.degree. C. higher, or from about 130.degree. C. to 140.degree. C. 
Low, medium, and high density polyethylenes typically are suitable for 
extrusion foaming, including mixtures thereof, although not necessarily 
with equivalent results. 
Polypropylene is also a whitish, translucent polymer and has a softening 
point of about 168 to 171.degree. C., which is some 50 to 65.degree. C. 
higher than that for polyethylene. Polypropylene has high tensile strength 
and maintains its strength even after repeated flexing. However, 
polypropylene does not typically have the toughness of polyethylene. 
The invention is useful for producing thicker grade polyolefin foams and 
polyolefin plank. Thicker grades of polyolefin foams typically are 
considered to be about 12 mm or more thick and are often sold in 
thicknesses of about 1/2 inch, 1 inch, 11/2 inches, 2 inches or more. Foam 
plank typically is considered to have a nominal thickness of about 50 mm 
(2 inches) or more. The thicker grades are to be distinguished from 
plyolefin foam sheets and thin sheets, which are generally less than 12 
mm, and even less than 6 mm. Thicker grade foams can be used for a variety 
of purposes, including building materials, surfboards, rigid structural 
members, and insulation. Compressive strength is an important attribute of 
thicker grade foams and plank in many load bearing applications. In 
contrast, thin sheets are often used for protective purposes and 
compressive strength is not normally considered or relevant. 
The thermoplastic resin should be maintained at a temperature within a 
range above the melting point of the polymer that is sufficiently high so 
that the polymer has sufficient fluidity for mixing with blowing agent. 
This range normally will be from about 20.degree. C. to 100.degree. C. 
above the melting point of the resin. The melting zone can be maintained 
at a somewhat lower temperature due to the heat that is generated by 
friction as the plasticized resin flows through the extruder. 
After mixing, the temperature of the mixture of resin and blowing agent 
should be lowered closer to the melting point of the mixture so that the 
polymer maintains its structure upon foaming, but not so much that 
complete expansion is hindered. The blowing agent has a plasticizing 
effect on the resin reducing its viscosity, or resistance to flow, and so 
the melting point of the mixture of resin and blowing agent normally is 
below that of the resin alone. The expansion temperature, which is above 
the melting point of the mixture, is empirically determined and depends 
upon the composition of the resin, the length of the screw, whether single 
or double screws are used, on the specific resin, upon the amount of 
blowing agent, and the specific blowing agent. For a low density 
polyethylene, the expansion temperature will generally be in the range of 
from about 85.degree. C. to 120.degree. C. 
The blowing agent contemplated for use in practicing the invention claimed 
herein comprises propane. Many of the benefits of the invention normally 
can still be achieved when butane is incorporated into the propane blowing 
agent in amounts of from 10, 20, 30, or 40% or more by volume up to a 
maximum of about 75%, depending on the polyolefin selected. 
Unlike butane foams of the prior art, foam produced by the process of the 
invention with either pure propane or with mixtures of propane and either 
normal butane and isobutane typically can be shipped in a relatively short 
period. With needle punching, the residual levels of blowing agent are 
below the least explosive limit within about one week after the foam is 
produced, as explained below. 
Table 1 below shows the upper and lower flammability limits in air for 
normal butane, isobutane, propane, and ethane based on the percent by 
volume of the hydrocarbon in air. The lower explosive limit for propane is 
2.1 percent by volume in air compared to 1.8 percent for the butanes. 
Combined with the lower level of usage that is possible for propane 
blowing agent, 100 percent propane presents a concentration in fresh foam 
that rapidly declines below explosive limits, whereas butane often remains 
above the lowest explosive limit for a period of time. 
TABLE 1 
______________________________________ 
Flammability Limits Vol. % In Air 
Lower Higher 
______________________________________ 
Butane (n or iso) 
1.8 8.4 
Propane 2.1 9.5 
Ethane 3.0 12.5 
______________________________________ 
Propane has a higher foaming efficiency than the butanes. For a given 
weight, propane generates more gas volume than butane. Less propane 
typically is required than is required of the butanes to achieve a 
comparable foaming efficiency. Only about 12 percent by weight of the 
resin of propane is used as a blowing agent when pure propane is used, 
compared to about 16 percent for pure butane. Therefore, the plasticizing 
effect of propane as a blowing agent is much less than in the case of 
butane. The temperature of the resin in the extruder and die tends to 
increase and nucleation is more difficult to control with the typical 
active nucleators. Propane blowing agent used in the same amounts and with 
the same nucleators as butane produces small bubbles and a high density 
foam that is subject to collapse. Prefoaming in the extruder and 
corrugation of the foam can occur. 
In the practice of the invention, process conditions and nucleation are 
carefully controlled to produce low density foams from a propane or 
propane and butane blend blowing agent that are dimensionally stable. A 
minimum die opening sufficient for preparing plank can be used to maintain 
sufficient pressure to prevent prefoaming. However, the minimum die 
opening increases the shear and heat and sensitivity of the system to 
nucleation. 
Very low levels of relatively inactive metal oxide nucleators, from 0 to 
0.8 percent by weight of the resin, such as zinc oxide and zirconium 
oxide, have been used in combination with propane blowing agent in 
accordance with the invention for foam extrusion of polyethylene foam. 
Another nucleator comprising sodium bicarbonate and citric acid is useful 
for producing fine cells in foams. The sodium bicarbonate and citric acid 
nucleator blend is available from BI Chemicals in Winchester, Va. under 
the trade name Hydrocerol. 
While the foam can be made with a single screw extruder, it is helpful in 
the process of the invention to use tandem extruders where the first 
extruder is used for mixing and the second extruder is used to maximize 
cooling of the resin prior to foaming. 
While no butane need be present when propane is used as a blowing agent in 
accordance with the invention, the benefits of the invention are still 
realized in foams produced with high percentages of butane in admixture 
with propane. The butane can be present in a ratio of from about 0:1 to 
3:1 by weight for producing polypropylene foam, which is from 0 to about 
75 percent by weight of butane. The butane can be present in a ratio of 
from about 0:1 to 1.9:1 by weight for producing polyethylene foam, which 
is from 0 to about 65 percent by weight of butane. 
The blowing agent is mixed into the plasticized polyethylene polymer resin 
in proportions to achieve the desired degree of expansion in the resulting 
foamed cellular product. Stable foam densities from 40 kg/m.sup.3 down to 
as low as 20 kg/m.sup.3 may be made by practice of the invention. Stable 
foams of higher density, up to about 100 to 120 kg/m.sup.3, can also be 
produced, if desired. 
The blowing agent generally is mixed with the resin in a ratio of about one 
part blowing agent to ten parts resin. The maximum useful proportion of 
blowing agent in the plasticized resin is density related and is related 
to the pressure that is maintained on the resin in the extrusion die 
passage, as is believed to be well known to the skilled artisan. 
The benefits of using the blowing agent in accordance with the invention 
claimed herein may be enhanced in preparing polyethylene foams by using a 
combination of a nucleation agent and an aging modifier to control cell 
size and development and to control the replacement of blowing agent with 
air in the cells of the foam, respectively. In particular, it has been 
found that a combination of low levels of relatively low activity metal 
oxide nucleation agents, such as zinc oxide, zirconium oxide, talc, and 
others, in combination with an aging modifier, including, for example, 
glycerol monostearate (GMS), is useful to further reduce the density of 
polyethylene foams and results in a thickness increase. 
The aging modifier is mixed with the polyethylene resin prior to melting in 
an amount sufficient to produce a desirable rate of exchange of air with 
blowing agent in the cells of the foam. More specifically, GMS is mixed 
with the polyethylene resin prior to melting in an amount from about 0.5 
to 5 kg per 100 kg of polyethylene resin. Still more specifically, GMS is 
added in an amount of 1 kg per 100 kg of polyethylene resin. 
The blowing agent efficiency of propane is relatively high and various 
polyolefin resins can have different foaming characteristics. For example, 
polypropylene has a softening temperature that is about 50 to 65.degree. 
C. higher than that for polyethylene. Less blowing agent is required to 
prepare polypropylene foams than is required to prepare polyethylene 
foams. Aging modifiers, including GMS are not typically needed to prepare 
polypropylene foams in accordance with the invention, although nucleation 
agents are used. GMS and similar compounds can be used to reduce friction 
induced static in polypropylene resins and foams. 
Nucleation agent is mixed with polyolefin resin in an amount sufficient to 
promote nucleation and to develop a pore structure of the desired size. 
More specifically, nucleation agent is mixed with the resin in an amount 
of from about 0.05 to 0.5 kg per 100 kg of polyolefin resin. Generally, 
low activity metal oxide nucleators have proved useful for propane blown 
foams and the propane and butane blends described herein. As the 
percentage of CO.sub.2 increases, it is helpful to use a low activity 
nucleator. Similar nucleation agents in similar amounts are beneficial in 
producing both polypropylene and polyethylene foams in accordance with the 
invention. 
EXAMPLES 
Example 1 
Low density polyethylene resin having a melt index of 2 (Novacor 219) was 
prepared by adding 1 pph of talc and 1.2 pph of glycerol monostearate 
(GMS) to the plasticized resin and mixing thoroughly. Selected blowing 
agents were thereafter injected through a single port in the primary 
extruder of a tandem screw extruder. In one case, 7.5 parts per hour 
isobutane was injected as a blowing agent. In a second case, 6.4 parts per 
hour propane was injected as a blowing agent. In each case, the resin was 
then cooled in the secondary extruder of the tandem system and dispersed 
in a slab die for even expansion to form foam plank that was 54 
millimeters thick (2.1 inches) and 64.5 centimeters wide. 
These foams, whether produced with isobutane or propane blowing agent had 
an initial density of about 38.5 kilograms per cubic meter. The hot foam 
plank was allowed to reach room temperature naturally. Typically, a few 
hours is required for the plank core to reach room temperature. At one 
hour, 25 percent compression was applied to each foam sample to check the 
center strength. Plank made with isobutane had a compressive strength of 
7.5 to 8.5 psig at 25 percent compression. The propane plank sample had a 
compressive strength of 9.5 to 11.0 psig at 25 percent compression. After 
two weeks storage, the isobutane plank had a compressive strength of 19 to 
21 psig at 50 percent compression. The propane plank had a compressive 
strength of 21 to 23 psig at 50 percent compression, which shows an 
improvement in compressive strength. 
Example 2 
The propane and isobutane foams of Example 1 were needle punched in 
accordance with the procedures set forth in U.S. Pat. Nos. 5,424,016 and 
5,585,058. Needle holes were applied 16 millimeters apart to accelerate 
removal of the residual blowing agent. Testing with a hand held 
hydrocarbon sniffer show that the propane plank took 2 to 3 weeks to reach 
8 to 10 percent of the lowest explosive limit. The lowest explosive limit 
for propane is 2.1 percent by volume (Table 1). By comparison, the 
isobutane plank took 12 to 16 weeks to reach a similar level. 
Example 3 
The same low density polyethylene resin prepared as in Example 1 was mixed 
with a blowing agent comprising 35 percent by volume of propane and 65 
percent by volume of isobutane at a concentrations shown in Table 2. The 
resin was extruded on a 150 millimeter twin screw extruder. The results 
are shown in Table 2 below. 
TABLE 2 
______________________________________ 
25% 
Resin Rate B/A Rate Density 
Cell Count 
Comp. 
Kg/Hr B/A comp. Kg/Hr Kg/m.sub.3 
#/inch psi @ 1 hr 
______________________________________ 
295 isobutane 20.9 37.6 23 8.4 
295 35/65 i-Bu/Pro 
19.1 -- 21 -- 
295 35/65 i-Bu/Pro 
19.1 36.6 22 9.0 
______________________________________ 
As shown in Table 2, a foam prepared in accordance with the invention has 
improved compressive strength at 25% compression 1 hour after extrusion 
compared to a foam prepared with 100% isobutane blowing agent of 
comparable density. 
Example 4 
A polypropylene resin was prepared in the absence of talc or glycerol 
monostearate or other aging modifiers and nucleating agents. The resin was 
mixed with either isobutane, propane, or a blend of isobutane and propane 
blowing agents and extruded to form a foam on tandem extruders. The 
primary extruder was 8.9 centimeters in diameter. The secondary extruder 
was 11.4 centimeters. The results are as shown in Table 3 below. 
TABLE 3 
______________________________________ 
Resin B/A Cell 30% 
Rate Rate Thick- 
Count Comp./ 
Kg/ N/A B/A Kg/ Density 
ness #/ Time 
Hr pph Comp. Hr Kg/m.sub.3 
cm inch psi/min 
______________________________________ 
118.2 
1 isobutane 
4.54 64 2.69 25 39.5*/100 
118.2 
2 isobutane 
4.54 58.9 2.13 16 30.0/100 
118.2 
0.7 35/65 4.09 -- 1.85 20 37.5/70 
I-Bu/Pro 
118.2 
-- propane 4.0 63.4 1.83 20 -- 
118.2 
0.2 propane 4.0 48.2 1.60 13 40.0/40 
______________________________________ 
*39% Compression 
As clearly shown in Table 3, 100 percent isobutane blowing agent takes much 
longer to reach the same or comparable levels of compressive strength for 
foams of comparable density in comparison foams prepared in accordance 
with the invention. 
Example 5 
Extensional viscosity was evaluated as follows in accordance with the 
Cogswell formula as shown in F. N. Cogswell, Polymer Melt Rheology, p.55, 
Woodhead Publishing, London, (1981) and as applied in a similar setup by 
S. T. Lee and N. S. Ramesh, Adv. Polym. Tech. 15, 4 (1996). Cogswell 
developed the extensional viscosity equation for the orifice die. The 
Cogswell formula is a useful tool for evaluating resin foaming 
characteristics and has been used to generate yield force data for 
calculating extensional viscosity. 
When the extensional viscosity of a resin melt in an extruder is measured, 
the resin melt undergoes acceleration in the direction of flow when the 
cross-sectional flow area is decreased, such as when the polymer melt 
passes through a constriction such as a slab die. The polymer tends to 
react and the resistance toward the stretching that is induced by 
acceleration of the polymer in the flow direction is evaluated as 
"extensional viscosity." For semi-crystalline, low density polyethylene, a 
higher extensional viscosity correlates with a higher degree of branching 
then enhances melt strength for better forming. 
In contrast, in the Cogswell method, as applied to a transition state 
polyolefin, a rod of plasticized polyolfin resin is extruded through a 
capillary die and then grabbed by a roller outside the capillary die to 
accelerate the rod until it breaks. Foam is starting to appear at this 
stage. The extruded rod is in a transition state between the plasticized 
polyolefin in the extruder and an expanded cellular product. The force 
required to drive the roller, the roller speed, and the die exit velocity 
are taken into account to calculate the extensional viscosity, which is 
considered to be an indication of the melt strength of the resin in the 
machine direction. It is believed that this property is similar in the 
transition state, it which some foaming has occurred, to the same property 
in the fully expanded cellular state. It is also believed that the 
properties should be somewhat similar in the machine direction, the cross 
direction, and the thickness direction. 
The trials were carried out on a co-rotating Haake twin-screw extruder 
having a capillary die. Low density polyethylene resin of Example 1 was 
used. The plasticized resin was extruded in the absence of blowing agent, 
with a 100% propane blowing agent, and with a 100% isobutane blowing 
agent. The results are shown in FIG. 1. 
As shown in FIG. 1, the extensional viscosity of plasticized polyolefin 
resin having a propane blowing agent mixed therewith is close to the 
extensional viscosity for the plasticized resin in the absence of blowing 
agent, indicating that the melt strength of the resin is not significantly 
reduced by the presence of propane blowing agent. FIG. 1 shows that resin 
incorporating a propane blowing agent has a much better melt strength than 
resin incorporating an isobutane blowing agent. 
Resin with isobutane was shown to have an apparent extensional viscosity of 
at least about 4.5.times.10.sup.5 to 4.6.times.10.sup.6 poise at a strain 
rate of from at least about 1.9 to 40 per second. Resin with no blowing 
agent was shown to have an apparent extensional viscosity of at least 
about 1.8.times.10.sup.6 to 6.1.times.10.sup.6 poise at a strain rate of 
from at least about 2.1 to 28 per second. Resin with propane, which is a 
blowing agent for use according to the invention, is shown to have an 
extensional viscosity of from at least about 1.2.times.10.sup.6 to 
7.times.10.sup.6 poise at a strain rate of from at least about 1.9 to 40 
per second. 
The invention claimed herein has been described hereinabove with respect to 
particular preferred embodiments. These embodiments should be considered 
illustrative of and not in limitation of the invention claimed herein. The 
full scope of the invention should be judged in accordance with the 
appended claims and equivalents thereto.