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Expanding rubber through the years.

The goal of this article is to review the significant methods, techniques and blowing agents used to expand rubber during the 20th century. Numerous patents, papers and articles have been reviewed to provide the details. Also, many interesting discussions were held with veteran sponge compounders.

Sponge rubber is a general term used for both open cell and closed cell materials, usually made with chemical blowing agents.

Closed cells are the network of non-interconnecting cells formed by the entrapment of inert gases as discrete bubbles in a matrix of rubber or plastic. In open cell sponge the walls between the cells are broken and air passes freely between the cells.

Steam cured open cell sponge balls and bath mats were among the first commercial expanded products.

Several patents issued in the late 1930s covered the use of high pressure chambers to produce closed cell sheets. In one, nitrogen was injected at about 5,000 psi and in another chemical blowing agents were used. Among the most important chemical blowing agents (CBA) are:

* ADCA, azodicarbonamide;

* OBSH, p,p' oxybis (benzene sulfonyl hydrazide);

* DNPT, dinitroso pentamethylene tetramine.

Early days of the sponge industry

The use of blowing agents to expand rubber dates back to the very beginning of the rubber industry. In 1846 patents (refs. 1 and 2) were issued to Hancock and others covering the use of ammonium carbonate and volatile liquids in making a natural rubber sponge.

In 1856 the first commercial sponge was produced in England on a very limited basis.

The first large scale production of sponge rubber in the United States was in about 1902. A.C. Squires (ref. 1), B.F. Goodrich Co., used ammonium carbonate to produce a sponge with large interconnecting cells. The introduction of efficient rubber accelerators and antioxidants in the 1920s was a major factor in the rapid growth of sponge production for household and industrial applications (ref. 1).

The Sponge Rubber Products Co. had its beginning in 1923 with the production of steam cured open cell natural rubber play balls. Somewhat later they started to make bath mats. They produced bath mats for about 50 years. The compounds, process and equipment were improved, but the steam vulcanizer was still the heart of this operation.

Press molded closed cell sponge

The closed cell process was developed about 90 years ago in Germany. The natural rubber closed cell products were called "Rubatex." The closed cells were produced by gasing with nitrogen at about 35 kPa. In the 1940s a refined version of this method was often referred to as the "gun barrel process" because huge guns could be modified to produce a suitable high pressure chamber. In the years that followed some improvements were made in this product in Great Britain and in other parts of Europe.

In the early 1930s Rubatex Products was established and it later became Virginia Rubatex Corporation. Their commercial process of producing gas expanded rubber was covered in the Denton patent, 1,905,269 (ref. 3). During the next few years numerous patents were issued to this fledgling company. They extended and refined the gasing process (ref. 4) and developed a new chemical blowing agent process where the compound was confined in a mold during the precure, blowing and final steps of the process (refs. 5 and 6).

A patent for a parallel process was issued to the United States Rubber Co. (ref. 7). This patent also described the use of a hydraulic steam heated press to confine and heat the mold. The company produced chemically expanded rubber from 1937 through about 1950. Expanded SBR closed cell sheet and hardboard were also produced.

These efforts lead to the development of the high pressure press precure method of making large closed cell sheets or buns as they are commonly called in the industry. The rapid expansion of the press molded closed cell industry after World War II was led by Virginia Rubatex and United States Rubber Company (Uniroyal Chemical Company). Many of the other producers of these pads had their roots in these pioneer companies.

Process for press molded closed cell sponge sheet

In this process, an excess weight of compound is placed in the mold cavity. As the press is closed, the compound completely fills the mold expelling the air and sealing the cavity. The closing and restraining pressure is generally in the range of 7 to 14 kPa based on the cavity area. The typical compound is relatively low in plasticity, flows readily in the mold, coalesces and eliminates trapped air blisters. As the stock temperature rises, the cure starts to develop and the decomposition of the blowing agent begins.

Nitrogen is released and the cells start to form. As the decomposition progresses, an exotherm develops and pressure builds up. These factors accelerate the curing rate. The press is opened before the cure has been completed. Owing to the high internal pressure in the partially cured compound, the pad expands rapidly when the press is opened and a material very small closed cells is obtained. The extent of expansion is controlled through compound design and regulation of the press temperature and time. This step is known as the precure.

Most producers prefer about 50% expansion during the precure step. A typical precure with a 2.5 cm thick mold is 30 minutes at 140 [degrees] C. The pads are expanded to the final size and the cure is completed in an oven set about 15[Degrees]C higher than the precure temperature. This step generally takes about 90 minutes. The press specifications are shown in table 1.
Table 1- press specifications
Multi cavity - 4 to 8 platens
Capacity - 14 kN
Platens - 1 00 x 1 00 x I 0 cm (40 x 40 x 4 inch)
Molds - steel blank - 1 00 x 1 00 x 2.5 cm (40 x 40 x 1
Cavity - beveled edge - 50 x 75 x 2.5 cm (20 x 30 x 1
Opening between platens - 1 0 cm (4 inch)
Time from start to full open all platens about 20 seconds
Blank size about 75 x 1 1 2 x 38 cm (30 x 45 x 1.5 inch)
Blank weight about 14 kg (32 pounds)

Chemical blowing agents


The first organic chemical blowing agent was Unicell (refs. 1 and 8), diazo amino benzene (DAB) which was introduced by Du Pont in 1940. DAB produces a dark colored sponge and it may stain other materials it contacts. Nevertheless, DAB was extremely useful despite the discoloration and staining problems. It was used to produce expanded hard rubber products for military use during World War 11. These products were flotation boards, flotation discs for life vests and boarding nets plus structural pieces for aircraft.


Azobis (isobutyronitrile (refs. 1 and 8) was used on a large scale by Germany during World War II to produce both flexible and rigid PVC foams. After the war it was also used in the United States. In the early 1950s its use was abandoned because its residue, tetramethyl succinonitrile was toxic.


The big breakthrough in chemical blowing agents for closed cell sponge came in 1946 with the introduction of Vulcacel BN, dinitroso pentamethylene tetramine (refs. 1 and 8), DNPT by ICI. DNPT is a very efficient and effective CBA. The exotherm of decomposition contributes substantially to the development of the cure. The main disadvantage with DNPT is the pronounced amine or fish type odor in the sponge. The use of urea and borates somewhat reduces the level of this odor, however it still persists.

While DNPT was the key to the early growth of the closed cell industry, two more CBAs extended the scope and versatility of sponge produced in the large high pressure presses. These new chemical blowing agents were OBSH and ADCA.


OBSH, p-p'-oxybis (benzene sulfonyl hydrazide) (Celogen OT) was discovered by Dr. Loren Schoene (ref. 9), Naugatuck Chemical (Uniroyal Chemical Company) in 1951. U.S. Patent 2,552,065 for this symmetrical sulfonyl hydrazide was awarded at that time.

OBSH is by far the most useful and popular of the sulfonyl hydrazide type blowing agents. OBSH was the prime CBA used in the early days of the production of nitrile-vinyl insulation tubing.


ADCA, azodicarbonamide was discovered in 1892. The first use of ADCA as a CBA was by Germany in World War 11 also to expand PVC. This use was reported by an Allied technical investigation team after the war in the B.I.O.S. reports (refs. 8 and 10).

The first impression of this new CBA in the rubber industry was that the decomposition temperature of about 210 [degrees] C (410 [degrees] F) was too high for use in sponge. Naugatuck Chemical discovered that glycols and glycerol effectively lowered the decomposition temperature. The patent application in 1953 was approved in 1957. This work keyed the development of the use of ADCA in the rubber and plastics industries.

The first patent for the manufacture of azodicarbonamide in the U.S. (ref. 12) was to Uniroyal Chemical (2,692,281). The product, Celogen AZ, was first produced in 1954.

These blowing agents OBSH and ADCA are odorless, nontoxic chemical blowing agents that proved to be very useful in the development of nitrile-vinyl foams (sponge) for insulation, flotation and athletic applications.

Shoe soling

Cellular shoe soling made an appearance in the late 1940s. The cellular soles were made via a process somewhat akin to the press molded closed cell pads. However, the precures were generally at about 160 [degrees] C. The pads popped out of the press when the closing pressure was released. The shoe soling is then stabilized at about 120 [degrees] C for about six hours. These pads were initially expanded with DNPT. The documentation of this industry is somewhat sparse. We believe that two friendly competitors, Biltrite and Monarch started to produce slab forms of microporous cellular shoe soling in the late 1940s. The basic polymers being GRS (SBR) with polystyrene resin reinforcement. The specific gravity was about 0.5.

Other companies followed soon thereafter, most notably, United States Shoe and Quabaug Rubber Co. These companies were among the world leaders in cellular shoe soling.

In about 1980 the Monarch Rubber Company took a leading position in the development of EVA low density soling. This material is available in bright clean colors. Cellular EVA is now the major type of expanded shoe soling having replaced the drab tans, off-reds and blacks of the SBR types. Also, with time the DNPT was replaced by activated forms of ADCA like Celogen 754 and Cellmic Cap W.

Automotive sponge

In the 1920s the sides of most of the cars were open. The few closed cars or sedans did not have either the door and trunk seals nor the cushions and gaskets that are so important as comfort factors in modern cars. There were no means of effectively keeping out the wind, rain and snow or to reduce the noise level. Also, there were no cushions, no dust seals or means of reducing vibrations and no padding on the doors, dash boards or arm rests (ref. 13).

One of the first records of the use of sponge was in a 1927 automotive blue print (ref. 13). By 1934 the sponge strips were molded to shape (ref. 13). A natural skin was formed by contact with the mold. These rectangular strips with rounded edges were cemented against the flange of the door. In 1935 another improvement was the use of an applied dense rubber skin covering the exposed surface of the sponge strip (ref. 13). The skin was applied to the individual sponge strips prior to molding. The skin reduced water absorption and improved the abrasion and ozone resistance. These strips covered only the upper part of the doors.

In 1938 the sheet loading technique for the skin and sponge strips was developed (ref. 13). The strips were designed to utilize this new technique in multiple cavity molds. These molds enabled improvements in both productivity and quality. By the early 1950s strips were being molded, 1.5m (60") to 2.3m (90") long.

The NcNeil, clam shell or book, press was another innovation. The open work area enabled the operator to load very complex sections and parts.


Windlace was the designation of an open cell sponge rod about 12mm in diameter covered with a fabric. This part provided a windseal in combination with the weatherstrip.

Two significant processes were invented to produce the open cell cord.

United States Rubber developed a unit consisting of an extruder, a heated pipe with a slot at the top and a pulling device. The inside diameter of the pipe was about 1.6 cm and the length was about 15 meters. The extruder was used to feed a rubber strip or rod into a fabric trough that was used to convey the rod through the tube. The fabric was about 8 cm wide. The expansion and cure was usually completed at about the 3/4 point of the tube line.

Since the blowing pressure forced the fabric against the walls of the tube, an effective device was required to pull the fabric and expanding strip through the unit. Initially, a dust was used to provide the release of the rod from the fabric.

Eventually suitable fabrics and dies were developed to withstand the heat of the blowing and curing process. This led to the blown in windlace.

Continuous cord machine

At about the same time the Sponge Rubber Products Co. developed a continuous die type curing machine (ref. 15) for the cord strips. The machine consisted of two sets of dies, top and bottom, fastened to heavy continuous chains.

The individual dies were about 9.3 cm (3.7") wide by 2.5 cm (1") thick and 0.7 m (27") long.

The unit had full sets of similar top and bottom dies. Each die contained 36 cavities of one-half of the cross section. A landing about 6 mm wide separated each of the cavities. The top and bottom dies were offset about one-half the width of the die. These dies travelled a rectangular path about 1/2 meter high by five meters wide with semicircular ends.

The windlace and cord machines provided low cost strips that helped reduce the drafts in the cars from about 1940 to about 1966.

Continuous weatherstrip machine

These were the days of great imagination and innovation. By 1940 Sponge Rubber Products had also developed a unique continuous cantilever machine (ref. 16) to produce skin and sponge weatherstrips. In this unit the bottom dies were about 1.5 m long and 0.7 m wide. They contained multiple cavities for the skin covered portion of the strip. The curing section of the machine was about 4.5 m long. The machine had six bottom dies. Part of a die was always at the entrance and exit ends of the machine and the sixth die was in transit from the exit end to the front or feed end of the machine via a roller bed. An elevator lifted the die in place at the proper time.

The top dies contained the cavities for the exposed sponge or cementing surface of the strips. The top die arrangement of the cord and weatherstrip machines were similar.

Latexfoam weatherstrip

The General Motors Co. was also busy in the innovation field. A 1951 United States patent 2,579,072 (ref. 17) developed a process to make a latex foam strip. The foam strip was covered by either a Neoprene latex or a Hypalon solution to seal the surface. The coating provided the desired weather protection. These latex strips were used in the top of the line GM cars for about 40 years. The foam strips are now being phased out.

In the late 1950s the weatherstrip suppliers started work on automotive weatherstrip profile extrusions. At first, Neoprene and SBR blends were used. With the development of EPDM in the 1960s there was a steady switch to Neoprene and EPDM blends and then to EPDM. EPDM is now the polymer of choice for automotive extrusions.

By 1966 the change from skin and open cell sponge weatherstrips to either extruded closed cell strips or latex foam strips was in full swing.

By the early 1970s the skin and sponge strips were a minor factor. The development of the uni-body construction for cars was the key factor in this change. With uni-body came the metal clip assembly to fasten the strip to the welded flange of the body frame. The next step was the dual durometer strip. This strip consists of a soft cellular bulb adhered to a U-shaped metal clip fastener which was coated on the outside with a dense rubber. A cross head extruder die was used to combine the soft bulb with the dense rubber coated continuous metal clip strip. The combined part known as dual durometer was then cured and expanded in an oven.

This technique was developed more or less jointly by General Tire, Sheller Globe, Schlegel, Standard Products and the Inland Division of GM.

As with the other sponge areas the extrusion lines have undergone extensive revisions.

The extruders have gone from a L/D of about 8 to 1 to the current L/D of 20 to 1. The units are now fully programmed to provide customized control of six temperature zones.

The first curing ovens were crude inefficient units with a lack of control of air velocity and temperature across and along the length of the line.

The producer may now select from high velocity hot air, high velocity hot air plus microwave or liquid curing medium, LCM lines. The transit time from extruder to windup may be less than five minutes.

The change to the extruded closed cell strips led to the development of the transfer molding technique for making sponge comers, transition pieces and end pieces. Transfer molding was developed by B.F. Goodrich and the Hood Rubber Co. in about 1964. A weighed slug is placed in a warm pot (60 [degrees] C). A piston forces the rubber through an orifice into a mold cavity at about 180 [degrees] C. The cavity contains the ends of the extruded strip or strips being joined by the sponge. By this method very complex sections can be molded. Also metal inserts can be molded into the part.

Continuous open cell sponge

A brief return to open cell sponge will show the development of the continuous process. By the late 1930s several companies had developed continuous sheet machines to produce open cell sponge. The sheet machine consists of a series of U-frames supporting top and bottom steam heated platens. Continuous belts or aprons run between the sets of platens. The gap between the platens may be set or adjusted to provide the desired opening as the sponge expands during the passage through the machine.

The first units were about 1 m (40") wide and 5 m (15') long. The cure time varied with the thickness, about 30 minutes for the maximum thickness at that time of about 12 mm (1/2").

By 1960 the sheet machines were about 30 m (100') long and up to 2 m (80") wide. Line speeds were up to 0.25 m/sec (50 feet/minute). A calendar is used to form the continuous sheet for the unit. The first sheet machines made industrial sponge sheet and standing mats.

Rug cushion

By the end of World War II flat sheets and the early version of the bubble, waffle or ripple type rug cushions were being marketed. The waffle type marketed by United States Rubber was formed in patterned aluminum molds. The bubble or ripple types are formed by dropping a sheet of sponge into a chain or wire. The pattern formed is a function of the wire design and the spiral height. By this technique a cushion may be produced having a substantial height or thickness relative to the unit weight. Rug cushion reached its sales zenith in about 1970. Several producers still make limited quantities of flat and ripple rug cushion types. The major competition has been from densified and rebounded urethane types.

Nitrile-vinyl foams

The cellular nitrile-vinyl industry has generally used the term foam to describe their products since inception in about 1947.

The concept of blending nitrile with vinyl dates back to 1940 when Goodrich filed a patent application (ref. 18). The patent 2,330,353 was issued in 1943. The nitrile rubber provided oil resistance and flexibility while the PVC provided chemical and ozone resistance to the blend which could be vulcanized (ref. 19). Incidentally, blends of this type were used to make aircraft fuel cells during World War II.

Flotation and athletic applications

The development work on press molded closed cell pads made with nitrile-vinyl blends began at U.S. Rubber's pilot plant in Mishawaka, Indiana in early 1947. A team was established to develop a low density nitrile-vinyl foam for buoyancy and shock absorbing applications. A patent covering this work was approved in October 1951 (ref. 20).

The claims covered a rubbery co-polymer of the Buna N type with either PVC or a PVC-vinyl acetate blend. The blowing agents covered were AIBN, DAB and DNPT. The process consisted of the following: a precure mold is filled completely and heated under pressure to 149 [degrees] C (300 [degrees] F) for 20 minutes. The mold is cooled to 27 [degrees] C (80 [degrees] F) and the pad is demolded. Then the blank is expanded at 121 [degrees] C (250 [degrees] F) in an oven. The finished product absorbs less than 10% moisture when immersed for 72 hours. The reported density was less than 112 kg/[m.sup.3] (7 pounds/[foot.sup.3]). The patent further states the cellular material was ideal for life saving jackets and buoyant cushions.

Production of Ensolite was started in 1950 at the pilot plant. Foam inserts were made for Navy life vests. Sales for the first year were $5,311. By 1954 the sales had grown to over $500,000. Production was started in 1958.

The Cornell University Aeronautics Laboratory and the Colorado State Medical Society were very helpful in the evaluation of shock absorbing properties. Soon boxing ring mats were being produced for New York State.

Early in the program the chemical blowing agent of choice was changed to azodicarbonamide. In 1967 Monsanto chose Ensolite for Astroturf. In 1970 the first continuous rolls of Ensolite Astro Turf backing were shipped to Monsanto.


The insulation portion of the nitrile-vinyl industry had its beginning with the change in air conditioning units from straight line to dual temperature lines. Basically, the same unit is used to provide heat in the winter and to cool in the summer time. Conventional insulation was being used with the straight line units, i.e., fibre glass and 85% magnesia. These types were very difficult to apply to tubing with its many twists and turns.

In about 1953 at an Armstrong Cork plant in Texas, sponge rubber was used to wrap the tubes to provide insulation. After a session at high temperature the sponge depolymerized and flowed off the pipe.

An investigation by their R&D provided the idea on how to use the concept to provide a new product. The first approximation was via an SBR - polystyrene sponge. The initial installation looked adequate but problems developed with ozone resistance. The next step was to a nitrile-vinyl blend. Armstrong Cork (ref. 21) received U.S. patent 2,849,028 in 1958 covering this development work.

The first production tubes were made at Armstrong Cork in late 1955. In 1956 over 305,000 meters (one million feet) of insulation tubing were produced at this facility. The first pieces of insulation tubing were expanded in corrugated troughs. Not long thereafter the continuous ovens were developed.

Also, soon thereafter, B.F. Goodrich, Rubatex, United States Rubber and Presstite were also producing insulation tubing.

The insulation tubing industry has grown by leaps and bounds. The ovens, extruders and compounds have been improved immeasurably. There are still many producers of insulation tubing including Armstrong World Industries, Halstead Industries and the Rubatex Corporation.


We have reviewed many innovative developments in a dynamic industry. Replete with almost a constant state of change, early large volume segments have passed into obscurity with the tides of progress. However, new concepts, methods and formulations breathe fresh air into this ever expanding market . The future seems to be in blends of rubber and plastics using techniques native to either group. [TABULAR DATA 2 OMITTED]


(1.) Henry Lasman formerly National Polychemical: Blowing Agents Encyclopedia of Polymer Science and Technology, Volume 2, 532-560,1965. (2.) Sidney A. Brazier, Some Problems in Sponge Rubber Manufacture Transactions Institution of the Rubber Industry 6,526 (1931). (3.) Geoffrey Price Denton, U.S. patent 1,905,269; April 25, 1933. (4.) Hans Pfleumer, U.S. patent 2,258,804, October 14, 1941 (Rubatex) and U.S. patent 2,297,022, September 29, 1942. (5.) Lester Cooper and Dudley Roberts, U.S. patent 2,283,316, May 19,1942 (Rubatex). (6.) Dudley Roberts, Roger Charles Bascom and Lester Cooper, U. S. patent 2,299,593, October 20, 1942 (Rubatex). (7.) George R. Cuthbertson, U.S. patent 2,291,213, July 28, 1942 (United States Rubber Company). (8.) Byron A. Hunter, Chemical blowing agents chemistry a decomposition mechanisms, ASP 4455 (Uniroyal Chemical Company). (9.) D. Loren Schoene, U.S. patent 2,552,065, May 8, 1951 (Uniroyal Chemical Company). (10.) B.I.O.S. Final Report 1150 (PB 79428), pp. 22-3. (11.) Wesley B. Curtis and Byron A. Hunter, U.S. patent 2,806,073, September 10, 1957 (Uniroyal Chemical Company). (12.) T.H. Newby and J.M. Allen, U.S. patent 2,692,281, October 19,1954 (Uniroyal Chemical Company). (13.) Calvin S. Yoran, Brown Rubber Company: Various papers to the Detroit Rubber and Plastics Group, February 17,1966. (14.) Calvin S. Yoran, Brown Rubber Company, "Engineering with sponge rubber," Rubber Age, May 1948, pp. 192-203. (15a.) F.M. Daley and LH. DeWyk, Sponge Rubber Products Company, U. S. patent 2,200,262, May 14, 1940. (15b.) L.H. DeWyk, Sponge Rubber Products Company, U.S. patent 2,218,527, October22, 1940. (16.) L.H. DeWyk, Sponge Rubber Products Company, U.S. patent 2,292,366, August 11, 1942. (17.) E.P. Harris, U.S. patent 2,579,072, December 18, 1951, (General Motors Corporation). (18.) Donald E. Henderson, U.S. patent 2,330,353, September 28,1943, B.F. Goodrich Company. (19.) Robert A. Emmett, Acrylonitrile-butadiene Copolymers Industrial and Engineering Chemistry, Volume 36, Number 8, August 1944, pp. 730-4. (20.) Lawrence E. Daly, Robert W. Pooley, U.S. patent 2,570,182, October 9, 1951, United States Rubber. (21.) Lawrence Clark, et al, U.S. patent 2,849,028, August 26, 1958.
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Author:Rowland, Donald G.
Publication:Rubber World
Date:Aug 1, 1993
Previous Article:Performance of a novel activator for azodicarbonamide for sponge and cellular rubber.
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