Process for producing a low density foamed polyethylene

A foamed and expanded low density polyethylene sheet or tube possesses both superior buoyancy and thermal resistance characteristics. The sheet or tube is made by a process comprising mixing a low density polyethylene with a blowing agent, surface activation agent and, preferably, a separation agent, heating the mixture at different temperatures to first melt the mixture and then gasify the blowing agent to expand the polyethylene cells, cooling the melted mass to partially harden and shrink the expanded cells, further expanding the cells by introducing a second gaseous blowing agent, cutting, extruding and cooling the expanded mass, forming a sheet or tube from the cooling mass, and finally cooling the sheet or tube for at least 24 hours. A buoyant, thermal resistant, water-repellent and moisture resistant sheet or tube results, which can be used, for example, as a liner for garments, including sports, business and military garments and other related equipment.

TECHNICAL FIELD 
This invention relates to a low density, expanded and foamed polyethylene 
sheet or tube having excellent bouyancy, cold-resistant and 
moisture-resistant properties, to a process for producing such a low 
density foamed polyethylene, and to garments and equipment requiring both 
bouyant and thermal resistant properties. 
BACKGROUND OF THE INVENTION 
Garments possessing buoyancy, cold-resistant and waterproof properties are 
highly desirable in a multitude of situations. Military personnel equipped 
with uniforms and jackets having all of these properties would find them 
advantageous in a number of situations. For example, a soldier's 
maneuverability in rough terrain under adverse conditions would be greatly 
enhanced by clothing which was both cold-resistant to protect the wearer 
from the elements and simultaneously facilitated crossing a river or 
fording a stream due to its buoyant and waterproof properties. Mountain 
climbers and hikers would find such garments similarly advantageous, where 
crossing a river, dealing with a flood, or resisting the cold are all 
potential encounters. To achieve their greatest utility, such garments 
must not only possess these various properties, but they must also be 
comfortable and not bulky or cumbersome. 
Water sports enthusiasts would also find such garments of particular 
advantage. Indeed, for many activities, garments possessing buoyant, 
cold-resistant and waterproof properties would be ideal and find great 
utility. 
It is known to provide various garments with foamed materials for thermal 
insulation. Thus, U.S. Pat. No. 2,976,539 discloses a thermally-insulated 
garment having an expanded, closed-cell cellular material as a lining, 
preferably polyvinyl chloride. The garment is thick and cumbersome and 
does not possess sufficient buoyancy to support a person's weight in 
water. Similarly, U. S. Pat. No. 3,511,743 discloses a thermal insulation 
laminate for space and diving suits whose core is an open or closed-cell 
sponge or foam. A fluid impermeable rubber or plastic skin is necessary. 
The laminate gives moderate mobility, but is still fairly cumbersome and 
does not have sufficient buoyancy to serve as a life-saving device. 
Polyethylene foams have been suggested as insulation or fillers for life 
rafts and jackets. U. S. Pat. No. 3,067,147 suggests a low density 
polyethylene foam for such a purpose which is processed with 
1,2-dichlorotetrafluoroethene as a foaming or blowing agent. The 
polyethylene and blowing agent are heated under pressure and explosively 
extruded into the atmosphere to form an expanded cellular mass. The 
product is bulky and cumbersome when used in practice. Similarly, U.S. 
Pat. No. 3,819,543 discloses a molded chlorinated, cross-linked 
polyethylene foam for use in producing floats and linings for clothing. 
Various known blowing and cross-linking agents are suggested for use by 
this reference. The molded products are thick and cumbersome. 
Despite these advances, there remains a need for a thin, lightweight 
material for use as a liner for garment and outdoor equipment which 
possesses a combination of superior buoyancy, cold-resistant and 
water-resistant properties, without hampering the mobility of the wearer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the above-mentioned 
drawbacks by providing a foamed, expanded low density polyethylene 
possessing both superior buoyancy and thermal resistance characteristics. 
Another object of the present invention is to provide a process for 
producing a foamed, expanded low density polyethylene possessing both 
superior buoyancy and thermal resistance characteristics. 
Yet another object of the invention is to provide a thin and lightweight 
lining for garments and sports, business, military and other related 
equipment which is not cumbersome and bulky, yet combines both superior 
buoyancy and thermal resistance characteristics. 
These and other objects are attained in accordance with the present 
invention by a process comprising mixing low density polyethylene with a 
blowing agent, a surface activation agent and, preferably, a separation 
agent, heating the resultant mixture to form a softened mass, raising the 
temperature of the softened mass to gasify the blowing agent for foaming 
and expanding the cells of the polyethylene, reducing the temperature of 
the foamed and expanded polyethylene to partially shrink and harden the 
polyethylene cells, introducing a gaseous blowing agent into the 
polyethylene mass to cause additional foaming and expansion of the 
polyethylene cells, cooling the mass to a temperature suitable for 
cutting, cutting and heating the mass to a temperature suitable for 
extruding, extruding the mass, forming the mass into a sheet or tube, and 
cooling the sheet or tube for at least 24 hours at room temperature to 
form a low density polyethylene sheet or tube characterized by superior 
buoyancy and thermal resistance. 
The present invention further comprises a foamed and expanded low density 
polyethylene sheet or tube produced in accordance with the process of the 
present invention. 
The present invention further comprises a garment incorporating therein a 
lining of foamed and expanded low density polyethylene having a thickness 
ranging from about 0.5 mm to about 1 mm and a buoyancy such that about 300 
g of the lining will keep more than about 130 kg of weight afloat. 
In a preferred embodiment of the process according to the invention, 
azodicarbonamide is the blowing agent, zinc oxide is the surface 
activation agent, ZnC is the separation agent, freon is the gaseous 
blowing agent and a foamed and expanded low density polyethylene sheet is 
formed to be used as a liner in garments requiring buoyant, 
water-repellent and thermal resistant characteristics.

The present invention will be better understood through the following 
detailed description of the preferred embodiments. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, a low density polyethylene 
prepared by conventional process is mixed, for example in a hopper, with a 
first blowing agent, a surface activation agent and, preferably, a 
separation agent to form a dry mixture. The dry mixture is conveyed to a 
closed heat tunnel, where it is processed in a series of stages at 
different temperatures. A conventional thermocouple control box can be 
used to maintain a particular required temperature in each of the 
processing stages in the heat tunnel. 
A conventional coil or screw conveys the mixture through a first portion of 
the heat tunnel. A pressure box at the entry end of the heat tunnel 
provides pressurized air to assist in conveying the mixture through the 
heat tunnel. 
In the first stage of the heat tunnel, the mixture is heated at a 
temperature of, for example, about l70.degree.C. to form a melted and 
softened polyethylene mass. In the second stage of the heat tunnel, the 
melted mass is heated at a temperature, depending on the gasifying 
temperature of the blowing agent, sufficient to gasify the blowing agent 
and further melt the mass. The blowing agent and air begin to penetrate 
the cells of the melted low density polyethylene mass and cell expansion 
occurs. Gasifying of certain blowing agents releases nitrogen and carbon 
monoxide, which react to form carbon dioxide. The carbon dioxide then 
penetrates the cells of the low density polyethylene, causing foaming and 
expansion of the cells, i.e. blowing. 
In the third stage of the heat tunnel, the expanded polyethylene is treated 
at a temperature of, for example, about 150.degree. C. to partially shrink 
and harden the expanded cells of the mass. 
A low density polyethylene (LDPE) is well-known in the art and is 
characterized by a density ranging from about 10 to about 40 lbs/ft.sup.3 
and a density ratio of about 0.910 to about0.925 as compared to water, 
where water is considered to have a density of 1. Higher density 
polyethylenes are not suitable for use in the invention. 
Suitable blowing agents to be initially mixed with the low density 
polyethylene, preferably in powder form, include azodicarbonamide 
(commercially-available as Celogen AZ Kempore, gasifying temperature of 
about 195.degree. C.), N,N'-dinitrosopentamethylene-tetramine 
(commercially-available as Unicel NDX, gasifying temperature of about 195 
.degree. C.), and 4,4' Oxybis (commercially-available as Celogen OT, 
gasifying temperature of about 150.degree. C.). The amount of blowing 
agent added is substantially about 1% by weight of the polyethylene. 
Amounts of blowing agent substantially greater than about 1% are not 
useful for purposes of the present invention because an overexpansion of 
the polyethylene cells takes place. Similarly, amounts of blowing agent 
substantially less than 1% do not permit sufficient cell expansion for 
purposes of the present invention. 
Azodicarbonamide is the preferred initial blowing agent, especially when 
making linings for garments, sleeping bags and other articles worn or used 
intimately. It is odorless, non-toxic, causes no discoloration of the 
final product and creates a maximum amount of gas for blowing. 
Comparatively, azodicarbonamide yields about 220 cc of gas per gram of 
solid, whereas 4,4' oxybis yields only 130 cc/g and 
N,N'-dintrosopentamethylene-tetramine 210 cc/g. 
Suitable surface activation agents to be initially mixed with the low 
density polyethylene, preferably in powder form, include zinc oxide, 
cadmium oxide and calcium carbonate. The surface activation agent is added 
in an amount ranging from about 0.1 to about 0.2% by weight of the 
polyethylene. 
The surface activation agent performs several important functions. First, 
it activates the blowing process while preventing too rapid an expansion 
of the LDPE cells during blowing. Secondly, it keeps the temperature in 
the heating tunnel down during the blowing process. For example, where 
azodicarbonamide is used as the blowing agent, the surface activation 
agent assists in maintaining a temperature of around 150.degree. C. in the 
third stage of the heating tunnel. Absent this agent, the gasified blowing 
agent would raise the temperature to around 196.degree. C., causing 
excessive blowing not contemplated by the present invention. 
It is preferred, but not essential, to add a separating agent to be 
initially mixed with the low density polyethylene is ZnC. The separating 
agent, added preferably in powder form, aids in preventing the LDPE from 
sticking to the coil or walls of the heating tunnel. The amount of 
separating agent added is substantially about 0.3% by weight of the 
polyethylene. 
In a fourth stage of the heating tunnel, a gaseous blowing agent is 
introduced into the polyethylene mass to subject the mass to a second 
phase of cell expansion and strengthen the bubbled cells of the mass. This 
second blowing agent is introduced at a pressure of about 35 kg/cm.sup.2. 
The preferred gaseous blowing agent is Freon, the well-known trademark for 
dichlorofluoromethane, although liquid petroleum gas (LPG) or a mixture of 
freon and liquid petroleum gas may be used. Other suitable gaseous blowing 
agents include monochlorodifluoromethane and dichlorodifluoroethane. The 
advantage to using LPG is its inexpensiveness, although it has potentially 
toxic effects. Accordingly, in making an expanded polyethylene for linings 
of garments, sleeping bags, etc., it is preferable to use blowing agents 
having no potentially toxic effects. 
After the introduction of the gaseous blowing agent and additional 
expansion of the polyethylene mass, the mass is subjected in a fifth stage 
of the heating tunnel to a temperature of about 100.degree. C., wherein it 
begins to cool. Treatment of about 100.degree. C. prepares the mass for 
proper and efficient cutting. This completes the heating and blowing 
process. 
The first through fifth stages in the heating tunnel take from about 30 
minutes to about 1 hour to complete, each stage taking approximately the 
same amount of time. 
The expanded mass continues its travel through a second portion of the 
heating tunnel which does not have any coil or screw therein. 
In this second portion of the heating tunnel, a desired quantity of the 
expanded mass is cut, for example, by a conventional cutting blade. The 
expanded mass is still in a softened state. The desired quantity depends, 
of course, on the size of the final sheet or tube desired. 
The cut, expanded mass is next subjected to a temperature of about 
105.degree. C. in the heating tunnel and extruded through a die and 
mandrel into a free expansion zone and cooling zone at atmospheric 
pressure and room temperature. The temperature of the mass must be raised 
slightly after cutting because a temperature of about 100.degree. C. is 
too cool for proper extruding. After extrusion, the foamed polyethylene 
mass expands naturally in the atmosphere, but not explosively, and cools 
at room temperature for a short period, e.g., a few seconds. The cooling 
mass of polyethylene is then formed into a sheet or tube by conventional 
means, the thickness or diameter, respectively, being determined by the 
desired end use of the product. 
The sheet or tube can then be wound on rolls, after which it is cooled at 
room temperature (20.degree. C. to 30.degree. C., preferably 25.degree. 
C.) for at least 24 hours. The sheet or tube goes through two stages 
during this 24 hour cooling period. Initially, the cells of the expanded 
mass collapse as entrapped blowing agent and air work their way out of the 
cells. Subsequently, over the 24 hour period, air repenetrates the cells 
to expand them to an intermediate stage. 
The final product is a low density polyethylene in which innumerable small 
cells containing air form a close-packed network. The weight of the final 
product corresponds to the weight of the initial low density polyethylene. 
Thus, for example, 25 kg of low density polyethylene yield 25 kg of 
finished sheeting. 
The present invention thus provides a foamed, expanded low density sheet or 
tube of polyethylene of the formula --CH2--CH2-- having a weight ranging 
from about 12 g/yd.sup.2 to about 21 g/yd.sup.2 for sheets of thicknesses 
ranging from about 0.5 mm to about 1 mm. The sheet is characterized by 
superior buoyancy and thermal-resistant properties. For example, 300 grams 
of the finished product can float more than 130 kg of weight. The density 
ratio of the final product ranges from about 0.4 to about 0.55 as compared 
to water, where water is considered to have a density of 1. Additionally, 
the sheet or tube of low density expanded and foamed polyethylene is 
cold-resistant, water-repellent and moisture resistant. 
The expanded and foamed low density polyethylene of the present invention 
can be formed into a sheet having any thickness, but preferably a 
thickness ranging from about 0.5 mm to about 5 mm. For garments, the 
thickness of the sheet should be no less than about 0.5 mm and no more 
than about 1 mm. Sheets having thicknesses much less than 0.5 mm do not 
possess sufficient buoyancy to be practical for use in the invention. 
Conversely, sheets having thicknesses much greater than about 1 mm are not 
suitable for use in garments of the present invention, because they make 
for an uncomfortable garment. Where sheets of low density polyethylene 
according to the present invention are intended for use as liners in 
tents, sleeping bags, blankets and similar equipment, thicknesses up to 
about 5 mm may be advantageously used without making the equipment unduly 
bulky and cumbersome. 
During the mixing step, conventional additives can be added in appropriate 
amounts to impart additional characteristics to the final product, such as 
a fireproofing or anti-inflammatory agent such as tin. These conventional 
additivies increase the weight of the final product slightly, but do not 
significantly affect the buoyancy or thermal characteristics of the final 
product. Exemplary additives include about 0.1% to about 0.2% by weight of 
the polyethylene of crosslinking agents such as azobisformamide (ABFA) or 
dicumyl peroxide, which can be added in powder form to increase the 
resistance of the final product to tearing, as well as about 0.1% of an 
ultra-violet absorber to prevent discoloration. After the foamed and 
expanded polyethylene is formed into sheets, or simultaneous with the 
sheet-forming step, the polyethylene may be sandwiched between opposed 
sheets of a material such as a nylon, vinyl, plastic, gauze, sheer fabric, 
or any material covering. Since the polyethylene is still in a somewhat 
softened state at this point, the sandwiching material adheres slightly to 
the polyethylene. More importantly, sandwiching at this point facilitates 
the process of sewing a material cover to the polyethylene, as well as 
cutting and transporting of the final product. Additionally, such a 
material cover prolongs the durability of the sheet. 
No special conditions are required for storage of the low density foamed 
polyethylene produced in accordance with the invention, even during the 
final 24 hour cooling period, in view of the water-repellent and 
moisture-resistant properties of the product. 
Additionally, there is no waste of polyethylene in the process. For 
example, 10 grams of low density polyethylene starting material yields 10 
grams of expanded and foamed low density polyethylene produced in 
accordance with the invention. 
The diameter of the polyethylene tubes produced in accordance with the 
present invention varies according to the end use. The extruded tubes may 
be solid or hollow and find particular use in providing buoyant material 
around boats. 
The following example illustrates an application of the principles of the 
present invention. However, it is not intended and should not be construed 
as limiting the scope of the invention. 
EXAMPLE 
25 kg of low density polyethylene are mixed in a hopper with 250 g of 
azodicarbonamide, 40 g of zinc oxide, and 75 g of ZnC. The mix is fed into 
a heating tunnel containing a helical screw which feeds the mix through 
the tunnel. Air fed from a pressure box assists in conveying the mix. The 
mix is heated at 170.degree. C. in a first stage of the tunnel where it 
melts. The melted mass travels to a second stage of the tunnel, maintained 
at a temperature of 200.degree. C., where the azodicarbonamide gasifies 
and penetrates and expands the polyethylene cells. The mass is then 
subjected to a temperature of 150.degree. C. in a third stage to cool the 
mass, whereby the cells harden and shrink slightly. Freon is then 
introduced at 35 kg/cm.sup.2 to further expand and foam the mass. The 
temperature of the mass is next lowered to 100.degree. C. to condition the 
mass for cutting. The mass is cut with a cutting blade such that it severs 
from the next batch following it through the tunnel. The temperature of 
the mass is raised to 105.degree. C. to prepare the mass for extrusion. 
The mass is extruded into an ambient atmosphere at a room temperature of 
25.degree. C. Expansion takes place, but the mass does not explode. The 
extruded mass cools for a few seconds and then passes through rollers to 
form a sheet having a thickness of 1 mm. The continuous sheet is formed 
into 4 rolls and stored at 25.degree. C. for 24 hours. The final product 
is 4 rolls of expanded and foamed polyethylene having a thickness of 1 mm, 
with 300 m to the roll, a weight of 21 g/yd.sup.2, a thermal transmittance 
of 6.70 w/m.sup.2 C and a thermal transmission of 52.5% (ASTMD 1518) and a 
water absorption of 9.1.times.10.sup.-4 lb/ft.sup.2 (BS 3595-1969). 300g 
of the finished sheet keeps more than 130 kg of body weight afloat. 
The foamed and expanded low density polyethylene of the present invention 
finds a multitude of uses as linings for garments, sleeping bags, 
blankets, cushions and sports and leisure equipment such as tents, bags, 
backpacks, rafts and deck chairs and other related items. Since the low 
density polyethylene product of the present invention combines both 
superior flotation ability and thermal resistance, the need to dispatch 
additional safety gear on the water is eliminated. A single garment 
incorporating the lining of the present invention serves as both a thermal 
jacket against the elements and a flotation device for general flotation 
and emergencies. Morever, the present invention provides a lightweight and 
thin garment for the above purposes which is comfortable and not 
cumbersome, thus improving enjoyment as well as safety. 
The present invention provides a lining and garment which retains its 
buoyancy and flotation ability even when ripped or torn. Also, blankets 
and sleeping bags can be provided with buoyant properties to serve 
additionally as flotation devices. The cumbersome safety vest used, for 
example, by water skiers can be replaced by a thin and lightweight safety 
vest, thus improving safety and maneuverability in leisure sports. 
Conventional hunting, fishing, camping and military clothing and related 
equipment can be dramatically improved by the instant invention, 
resulting, for example, in a single garment that can be worn in the 
countryside which is thermally insulated and immediately usable in 
emergency flotation situations varying from accidentally falling into a 
river to the need to ford a stream. Application of the present invention 
to military uniforms, jackets and ponchos morever extends a soldier's 
capabilities in battle situations. The polyethylene product of the present 
invention can also be used as thermal insulation, for example, for 
military radio equipment and the like. 
It will be appreciated that while the present invention has been described 
with reference to specific and preferred embodiments thereof, this is not 
done by way of limitation, and various modifications will suggest 
themselves to those of ordinary skill in the art which fall within the 
spirit and scope of the present invention as set forth in the appended 
claims.