Extrusion of TPE profiles using water as a physical blowing agent.

In early 1992, Advanced Elastomer Systems and Berstorff indicated their interest in cooperating to jointly develop the technology for foaming Santoprene, a thermoplastic elastomer, using water as a physical blowing agent.

One year after the initial contact between the two companies, pilot trials were carried out for a Japanese customer with the target to produce a light weight foamed profile with physical properties identical or better than the properties of conventional foamed EPDM.

Besides the raw material itself, the properties of the foam such as impact strength, bending strength, insulation value and surface quality can largely be determined by the cell structure of the foam, such as cell size, number of cells, cell uniformulation and others. To best meet the specified physical properties of the Santoprene profile, a foam with a fine, closed cell structure was targeted.

The initial trials to continuously extrude a foamed TPE profile were carried out on conventional foam extrusion equipment as typically used for foaming of thermoplastics, using single screw extruders. However, after a few trials, it was clearly shown that such standard equipment was not suitable for this new application. A new equipment concept had to be developed with shorter barrel length and different screw design.

Besides the development of a new equipment concept, the material formulation was developed also. Santoprene grades with up to 80 Shore A hardness were tested. With this grade, an excellent foam structure in view of cell uniformity, cell size and cell number was achieved resulting in the requested physical properties.

Process technology for physical foaming

Theory of physical foaming

Employing the physical foaming process, a gaseous or liquid physical blowing agent is injected at high pressure into the polymer melt and intensively dispersed. For the foaming of Santoprene, water was selected as the blowing agent. After injection of the blowing agent, the polymer melt temperature normally will be reduced. While cooling the polymer melt in the extruder, it is important to maintain a pressure above the vapor pressure of the blowing agent to avoid premature foaming in the equipment. The polymer melt containing the blowing agent can be directly formed and expanded in a suitable extrusion die.

Direct foaming in extruders

First, a homogeneous polymer melt is established in an extruder containing all the required additives. Thereafter, a gaseous or liquid blowing, agent is injected and intensively dispersed in the melt. Due to the reduction of the melt viscosity by addition of the blowing agent, the polymer temperature can be drastically reduced until slightly above the melting point, and shaped and formed in the extrusion die.

Because of the abrupt reduction in pressure after the extrusion die, the melt still containing the blowing agent is expanding immediately, resulting in further reduction of the polymer melt temperature. The calibration of the thermoplastic foam is established immediately prior to the solidification of the melt.

This continuous process is economically used for commercial manufacturing of thermoplastic foams.

New TPE-water foaming process

Based on the already briefly discussed basic rules and common equipment technology, a new, patented process has been developed for the foaming of thermoplastic elastomers (TPE-V), using water as the blowing agent.

Together with a well known raw materials supplier, extensive fundamental trials and detailed development tests have been conducted. The target of this work was to develop a process and the required equipment technology to replace conventional foamed EPDM profiles, for instance, as used in the automotive industry. So far, optimal results have been obtained with a TPE-V raw material with a Shore hardness of 68 using single screw extruders designed with a special blowing agent injection technology.

This new manufacturing system can be divided in several consecutive processing steps, depending on the final profile requirements. The main recipe for the manufacture of TPE foams includes the polymer, possibly including a color masterbatch, as well as water, which is an environmentally friendly, non-toxic and non-flammable blowing agent.

These individual components will be metered and fed into the extruder in well defined ratios and with sufficient accuracy.

Single stage extruder systems are used for the manufacture of foamed TPE profiles. The individual processing steps such as melting, mixing, melt homogenization, melt cooling and forming of the melt to profiles, are carried out in one single screw extruder only. These extruders are designed with a L/D processing length of 30:1 which is comparable with conventional plasticizing extruders.

The manufacture of such semi-finished products is based on the principle of free expansion at the extrusion tool. For the extrusion of foamed TPE profiles, the polymer melt containing the blowing agent is extruded horizontally. Final forming is determined by the extrusion die and by the three-dimensional expansion.

The extruded profile is transferred to a transporting conveyor and carefully cooled by air at first. Thereafter, additional cooling to almost ambient temperature is achieved by a spray water cooling conveyor, and finally the profile is dried by means of an airknife.

Newly developed extrusion systems for TPE foams

The main thrust of this newly developed process and equipment technology is towards profile manufacturing systems; however, it is also suitable for the manufacture of sheets and tubes. Further work to develop new products, such as heavy walled components, is still being conducted.

TPE mono profiles

The metering and feeding of the individual raw material components is accomplished by an automatic metering and mixing unit which is arranged directly above the extruder feed hopper. This volumetric feeding device is controlled by a separate controller or by the main line control system using a PLC.

The pre-mixed raw materials will be fed into the extruder feed section which is provided with a grooved liner and thereafter molten and homogenized. At approximately 18 L/D of barrel length, the blowing agent, water, is injected into the polymer melt.

The mixing of the blowing agent, as well as the necessary cooling of the polymer melt, are important processing steps, and a special designed screw geometry, as well as a sophisticated blowing agent injection technology, are essential.

The heating/cooling of the extruder barrel after the blowing agent injection point is accomplished by oil heating systems and a special designed barrel design. An extrusion head allowing a quick change of the extrusion dies is used for the final forming of the foamed TPE profile.

Thereafter, the extruded profiles are cooled in a combined air/water cooling conveyor. After cooling, the profile will be cut-to-length or wound-up depending on the customer's requirements. Figure I shows a typical profile, as used in the automotive industry.

TPE co-extruded profiles

Figure 2 shows an extrusion system for co-extruded profiles.

The co-extrusion system consists of two extruders with melt discharge pumps and a special co-extrusion profile die.

The extruder for the foamed components is of similar design as previously described for the single extrusion line. The dense component requires a second extruder, which is a standard three (3) stage melt extruder.

At the discharge of both extruders, melt discharge pumps are provided to convey the melts into the co-extrusion die.

The newly developed foaming process also required the design of a new co-extrusion die.

The downstream equipment, such as cooling conveyors, take-off, etc., are similar as described in the single extrusion system; however, some of the profiles will require an additional calibration.

TPE co-extruded profiles with reinforcement with steel or dense

plastic materials

Co-extruded profiles may require an insert for reinforcement. Inserts made of steel, aluminum or dense plastics are typically used. The incorporation of such inserts is identical as used for conventional EPDM profiles.

Further development of this technology is still required even as the first production system of this kind is currently being commissioned. This manufacturing system is identical to the co-extrusion system as previously described; however, it also includes additional equipment for the feeding and insertion of a steel band to the co-extrusion head. Furthermore, the extrusion head is equipped with additional guiding tools and dies and is also designed for application of vacuum.

The profiles produced with this extrusion system will be used in the automotive industry for door and trunk seals.

Application of TPE foam profiles

The initial success of foamed TPE profiles lies in the automotive industry. Mitsubishi is the first automotive manufacturer to commercially use such profiles in one of their model's door sealing of motor hoods. The experience was so positive that Mitsubishi also decided to use such TPE seals in other areas of their cars; trunk and door seals, also in clearly visible areas.

Besides the automotive industry, several other markets show a demand for physically foamed profiles: consumer goods; household products; industrial; gasketing; construction; seals and joint fillers; furnitures; gaskets and seals.

Since the beginning of the development work for the manufacture of foamed TPE profiles, several single and co-extrusion lines have been sold into the above mentioned markets. Additional applications in the construction and furniture industry, as well as applications for sound and heat insulation, are being tested.

Additionally, further development work is being conducted to use this foaming technology for profiles with larger cross sections, as well as for foamed sheeting.

Advantages of TPE-water foaming process

The water foamed TPE profiles offer essential advantages when compared with conventional rubber profiles, and therefore can be used in numerous traditional applications. The advantages are:

* Quality of raw material. TPEs are manufactured in a continuous process compared to the batch process as used for conventional elastomer compounds.

* Process without curing. The described processing parameters are only related to the foaming process and not to the curing process. Only water is being used as a blowing agent.

* Process ecology. No environmentally harmful by-products are generated and only minor air pollution will occur.

* Profile design. Computer programs are available to design and optimize extrusion dies providing flexibility in production.

* Recycling. Production waste can be blended with virgin material and reprocessed without major expenditures. The same material can be re-processed several times. See tables 1 and 2.

* Coloring. Santoprene can be colored easily.

* Weight reduction. The finished profile is lower in weight compared with a comparable EPDM profile.

Table 1 - recyclability A

Number of recycle              0      1      2      3      4      5

Shear viscosity (Pa s)      20.8   19.7   19.2   17.8   16.1   14.7
Surface roughness ([Mu]m)   8.74   9.47   9.63   8.13   7.21   6.20
Compression set (%)           14     16     13     54     49     59
Tensile strength (MPA)      0.60   0.43   0.59   0.67   1.29   0.89
Elongation (%)               163    132    171    219    219    154
100% modulus (MPA)          0.52   0.43   0.53   1.06   1.06   0.83
Specific gravity            0.15   0.18   0.20   0.30   0.42   0.37
Diameter(*) (cm)           0.663  0.589  0.531  0.462  0.356  0.399

(*) Foamed profile diameter

Table 2 - recyclability B

Recycled TPO (%)       0      10      20      30      40      50
Virgin TPO (%)       100      90      80      70      50      50
Diameter (*)(cm)   0.678   0.660   0.696   0.648   0.650   0.650
Specific gravity    0.17    0.15    0.15    0.15    0.16    0.15

(*) Foamed profile diameter

Result

The development of the technology to foam TPE profiles using water as the blowing agent must be considered a significant advancement, not only in view of the technology itself, but also in view of the development of new markets and products.

It has been proven that TPE profiles manufactured using this technology with the advantages of Santoprene such as recycling, generally superior physical properties and improved versatility, can compete with profiles made of conventional elastomers sudh as EPDM.

Some of the physical properties of TPE foam with different densities are summarized in tables 3 and 4.

Properties                            Norm        Development
                                                     sample

Density                             ASTM D 792         0.35
Load deflection                     ASTM D 1056         25
(N/20 cm)
Compression set (%)                 ASTM D 1056
22 hrs./25% defl./70 [degrees] C                        31
22 hrs./40% defl./100 [degrees] C                       25
22 hrs./50% defl./100 [degrees] C                       33
Tensile strength (MPA)              ASTM D 412         1.18
Elongation (%)                      ASTM D 412         156

The best results have been achieved with Santoprene with a Shore A hardness of 68. Foaming of other TPE grades is possible; however reduced output rates and physical properties may have to be expected.

Currently, extrusion of profiles with a maximum cross section of 1" x 1" is possible. Further development work will be necessary to extrude profiles with larger cross sections.

Commercially, standard foam extruders with 100 and 200 lb./hr. of TPE foam are being offered.

This process is commercially viable. Several single and co-extrusion lines are already in commercial operation.