Production of very wide and thick rubber sheets in single and laminating processes.

The shaping of a rubber sheet is only one of several steps on the way to the end product. Assembly often precedes vulcanization, so that one cannot tell that the finished articles, such as tires, v-belts, seals or shoe soles, were made from a sheet. Some exceptions include conveyor belts, roofing sheets and linings, which are put together to form a thicker or wider sheet.

Out of all the different types of rubber sheet, this article focuses on the production of innerliners and conveyor belts because the lines used to make them have undergone extensive redevelopment. There is, for example, a new type of four-roll calender for the innerliner line and a newly developed and patented roller head in use for conveyor belts.

Tire innerliners

The innerliner is the liner of a tubeless tire (figure 1). It seals the air inside the tire, thus taking over the task formerly performed by the innertube. Butyl rubbers are used for the innerliner because they are gas-impermeable. Butyl bromide rubber is preferred because it is compatible with other polymers. For small passenger car tires, the rubber sheet is plane-parallel; for larger passenger vehicle tires and truck tires, the sheet is profiled over its width. Production units include calenders, roller heads and single-roll roller die units.

[Figure 1 ILLUSTRATION OMITTED]

Conveyor belts

Conveyor belts are the central components of the conveyor line. The customer expects high reliability and a long lifespan, and the demands on the belt are many and diverse. In addition to application-oriented belt design and the right selection of materials, qualified manufacturing processes on high-quality production lines are essential to meet these demands (ref. 1).

Conveyor belts can be grouped according to their support materials into the following classifications: Steel rope belts with and without cross reinforcement, and fabric belts with subgroups of single, double and multiple layer belts of rubber, PVC and PVG. All belts consist of multiple layers or sheets. A roller head line, which will be described in detail later, can make a rubber sheet up to 18 mm thick in a single step.

Innerliner line

If the individual components are primarily assembled in the head through pure extrusion, on-line assembly becomes of increasing importance for calendered sheets and profiles. The example described by Ramm and Seidler (ref. 2) shows the options available to the project engineer in planning the line. The overall design is planned in close cooperation with the operator. The line shown in figure 1 has two roller head units (extruder-calender combination) for profiled sheet production and a single-roll roller die (extruder-single roll roller die) for profile production.

Another line design makes very thin passenger car innerliners with a single-roll roller die. In this process, either the compound must be strained or there must be an automatic screen changer between the extruder and the head. The disadvantage is that the output is decreased due to the higher compound temperature curve caused by the strainer. An alternative line design consists of a four-roll calender for sheets with parallel faces or strip production and a single-roll roller die for profiled sheet production.

Four-roll calender, type KQP 400/1200

These calenders are used to manufacture insulation with strips for track tire innerliners, passenger car tire innerliners and thin rubber sheets. Typical parameters for half-finished products include:

* Film thickness - min. 0.2 mm, max. 2.5 mm;

* tolerance - [+ or -] 0.05 mm;

* material width - min. 50 mm, max. 1,000 mm;

* tolerance - [+ or -] 0.2 mm;

* operating speed - 1.5 to 35 m/min.

This four-roll calender with one roll placed at an angle is a new design. Continuing the usual designation of the calender types with letters, this calender is assigned the letter P.

The calender consists of:

* Rolls 1, 3 and 4 vertical, roll 2 at a 30 [degrees] angle;

* roll 3 a fixed roll;

* roll barrels with a diameter of 400 mm;

* roll barrels with a length of 1,200 mm;

* all rolls are peripherally bored in order to guarantee uniform temperature control;

* rolls 1 and 4 can be inclined (the uniformity of the inclination from the left and right sides is monitored electronically, has a crossing effect);

* hydraulic pretensioning on rolls 1, 2 and 3.

Each calender roll is individually driven with a DC motor (27 kW) and water-heated and -cooled (27 kW). The calender rolls are made of chilled cast iron. The friction between the individual calender rolls can be freely adjusted. The relative deviation of the roll speeds is constant within the overall operating range.

Calender feeding for gaps I and II

For the production of insulation and strips for truck tire innerliners, the lower roll gap II is fed with QSM extruder I through the mill (figure 2). Roll gap I is fed with the cut edge of the insulation to form strips. In the production of truck tire innerliner insulation, the upper roll gap I is fed with QSM extruder II. The truck tire innerliner is shaped in the lower roll gap II. This is then fed with QSM extruder I through the mill.

[Figure 2 ILLUSTRATION OMITTED]

The stock guides of the calender gap II are positioned according to the recipe. The feed strips are automatically cut on the adjusted length of the calender gap and fed perpendicular to the roll gap.

The feed quantity is determined by the width of the feed strips. If the production speed is changed, the delivery speed of the mill is automatically adjusted. Therefore, the adjustment also applies to the corresponding conveyor belts.

Homogenizing mill 665/2100

In this mill, the drive is coupled with a DC motor (175 kW) with a spur-gear special reducer so that each roll can be coupled directly with the gearbox. The gearbox output shaft and the roll pegs are connected via cardan shafts. The relationship of the number of teeth on the coupling wheels determines the friction ratio.

Both rolls are peripherally bored (lengthwise) under the barrel surface for thorough and uniform heating and cooling. The fixed roll has zoned ribbing on its entire circumference.

Other elements of the mill are a strip blender and strip cutting device. The mill performs the following functions:

* Intake-side buffering of the extruded/preheated compound;

* plasticization and post-homogenization;

* elimination of air and moisture inclusions;

* optimal, uniform feed temperature.

The maximum throughput quantity of the mill is 1800 kg/h. Details on the QSM extruders I and II include:

* Speed - 45 rpm/min.;

* motor output - 163 kW;

* gearbox torque - 37 kNm;

* number of pins - 6 x 8 pins;

* throughput - max. 1,800 kg/h.

Output of the strips and the insulation

The strips are pulled off over roll 1 via a driven cutting roll that runs synchronously with calender roll 1. The cutting roll has four adjustable cutting knives. The edge strips are fed back into the calender gap. The strips are transported to the lamination area via side-guided belts. The lateral tendency of the strips, which results from the roll-crossing of roll 1, is compensated for by adjusting the knives.

The insulation is pulled under roll 4 via a driven cutting roll that runs synchronously with calender roll 4. The cutting roll has two adjustable cutting knives.

The insulation is carded into the lamination area with the strips via laterally guided belts. The thickness is measured on three points of the insulation with a mechanical thickness-measuring device to regulate the calender gap (roll distance and setting). An optical width measurement regulates the adjustment of the cutting knives (width tolerance of the insulation: [+ or -] 0.5 mm).

Lamination of insulation and strips

The strips are then laminated onto the insulation without centering. After the line has reached optimum efficiency, a lamination precision of [+ or -] 2 mm is achieved with the recipe.

Single-roll roller die unit

This unit is used for production of profiled butyl bromide track tire innerliners. Size range for the half-finished products includes:

* Sheet thickness - min.: 1.0 mm, max.: 6.0 mm;

* material width - min.: 550 mm, max.: 1,000 mm;

* tolerance: [+ or -] 0.5 mm.

Extruder

A QSM 150/k-16D is used with the following specifications:

* Speed: 55 rpm/min.;

* motor capacity - 180 kW;

* gearbox torque - 37 kNm;

* number of pins - 6 x 8 pins;

* output - max. 1,500 kg/h.

The compound temperature range on exiting the single-roll roller die is between 105 [degrees] C and 115 [degrees] C. The screw speed is determined depending on the recipe and is kept constant.

Single-roll roller die EWK type 150/1050

The single-roll roller die system is an ideal combination of the easy operation of the direct extrusion process and the quality-improving characteristics of the calender process for profiled rubber sheets of differing widths.

The single-roll roller die system consists of a (usually) cold-feed pin-type extruder and the single roll roller die. On the one hand, the design principle of the single roll roller die is more demanding than that of the wide extrusion head; on the other hand, it is still much less expensive than the principle of the roller head line. The single-roll roller die combines all of the advantages of both processes and thus offers additional options for production of a large number of profiled sheets and/or sheets that are not economical or not possible in the extrusion process or with the known calender techniques.

The product is shaped into its final geometry through the single-roll roller die flanged to the extruder (figure 3). The single-roll roller die consists of the head, which can be water-heated and -cooled, with built-in flow channels for pre-distribution of the material (parts 3, 4), the roll, which is also water-heated and -cooled (part 2) and a plate die (part 5).

[Figure 3 ILLUSTRATION OMITTED]

The gap is formed by the profiled plate die and the roll. Two hydraulic cylinders placed on the sides clamp the hinged head upper part to the fixed lower part. A wedge comb hydraulically holds and releases the plate die attached to the hydraulic hinged upper part of the head. Thus, the final plate die that determines the shape can be changed quickly and easily for production changes. In addition, the thickness of a profile can be changed by adjusting the height of the rolls using motor-driven spindles.

The turning roll causes a pulling effect in the head area and in the shaping gap. This pulling effect is influenced by the bond of the compound to the cooled roll surface. By correctly adjusting the roll temperature and the circumferential speed of the roll to match the extruder output and the take-off speed, even complex profiles with large thickness variations and long angled tips can be made without problems.

The combined effect of the feeding capacity of the extruder and the pulling effect of the roll means that the application options of this machine combination are greatly expanded over conventional production processes. The pulling effect of the roll described above means that the compound pressure decreases considerably before the extruder screw tip with respect to the extrusion and shaping with a protector head. A decrease in the compound temperature by some degrees Celsius is directly attributable to the low compound pressure. Likewise the broad spectrum of extrudable profiles such as the profiled innerliner can be directly attributed to the advantageous pulling effect of the roll.

The high product precision and low material swelling are especially noteworthy. This low material swelling simplifies the production and fitting of the profile strips. The profile corresponds very precisely to the geometry of the plate die, which sharply reduces expensive and time-consuming rework. The roll is driven by a DC motor (18 kW). A roll width of 1,050 mm means that this machine can produce a size that had been reserved for the calender or roller head line.

Take-off is performed by a driven cutting roll equipped with two cutting knives. The knives are centered symmetrically on the cutting roll. The edge strips are fed into the extruder with a conveyor belt. The knife adjustment is regulated via optical width measurement (the precision of the product width is [+ or -] 0.5 mm). The thickness of the butyl bromide is measured on three points purely as a control of the half-finished product (measurement points can be freely chosen).

Butyl bromide rubber that does not conform to the required dimensions at the beginning of production is manually rolled up on the conveyor belt and fed back in the extruder feed hopper by the operating personnel before laminating.

Take-off of butyl bromide rubber

The butyl bromide is taken off via a driven roller conveyor and conveyor belt from the single-roll roller die. The take-off speed and the cutting knife adjustment are used as a set point for the width regulation. The take-off speed is preset depending on the recipe.

Lamination of the butyl bromide with insulation and strips

The insulation is automatically transferred with the strips from the horizontal belt to the inclined laminating conveyor. The touch point on the laminating conveyor is calculated using a measuring device. The insulation is centerline controlled on the innerliner with a centering conveyor. The laminating precision is [+ or -] 1 mm (centered either by analysis of both edges or by measuring a spout of max. 0.5 mm high in the center of the half-finished products).

The lamination occurs with a laminating roll at an obtuse angle to avoid air intake (figure 4). Two factors determine how well the butyl bromide adheres to the entire width of the insulation without air intake including:

* A very smooth surface due to the single-roll roller die roll, and;

* a lamination temperature of over 95 [degrees] C, at which it is still somewhat adhesive.

[Figure 4 ILLUSTRATION OMITTED]

Cooling system 

After the inclined laminating conveyor, the half-finished products are cooled to a maximum wind-up temperature of 35 [degrees] C (inside the roll). The products are cooled on three steel belts that are indirectly cooled with water. With this system, the truck tire innerliner is intensively cooled three times on one side. For passenger car tire innerliner production, only the lowest cooling belt is used. Since the innerliner should not be directly cooled with water, two-sided air cooling can be used as an alternative. However, the conveyor in this case is almost twice as long as with single-sided steel belt cooling.

At all remaining points the material is automatically threaded up and transported tension-free. This also applies to the remainder of the inclined laminating conveyor on the steel belts (tracks and passenger cars).

Wind-up stations

All products that are produced on the line are automatically wound up in one continuous operation. An automatic crosscutting device is used to cut the half-finished products. The individual wind-up stations fulfill the following requirements:

* It is possible to laminate an already produced half-finished item on the wind-up station (lamination precision [+ or -] 1 mm);

* winding coils of different lengths are used;

* automatic spool change, i.e., automatic removal of finished goods and the empty backing spools, automatic reloading of the empty spools and the backing o the backing is threaded manually;

* the half-finished products to be transported are placed automatically.

Line control

Truck tire innerliners

The set point of the line is the take-off speed of the single-roll roller die. The screw speed of the extruder is predetermined by the recipe and is kept constant. The speed of all belts to follow is adjusted to match the speed of the innerliner (cascading control) by dancers or ultrasound scanning. The calender speed is adjusted to the innerliner by a dancer using back bias regulation. There is adjustable friction between the innerliner and insulation.

Passenger car tire innerliners

The main factor here is the calender (take-off speed). All belts, etc., to follow are adjusted to the speed of the calender using dancers or ultrasound scanning.

Line operation

Master program

The production personnel do not have access to this program. The master program can be entered manually or generated from existing programs via teach-in.

Recipe

The recipe is loaded from the master program and constantly regenerated from the last three production data runs. The master program and the last three recipes run, from which the current recipe is averaged, are managed in the PC. The average files can be viewed. After the recipe is downloaded from the PC into the PLC, that part of the guidelines that is relevant for the current system is displayed in both CP 527 visualization systems.

The following values are preset for the three most important systems:

Four-roll calender:

* Roll distance, roll crossing;

* positions of material guides;

* positions of cutting knives;

* roll speeds and frictions;

* take-off speed;

* position of the thickness sensors;

* temperatures of the individual calender zones.

Single-roll roller die:

* Screw speed;

* cutting knife positions;

* position of the thickness sensors;

* roll speed;

* display of which extrusion dies are needed;

* temperatures of the individual zones.

Mill:

* Speed;

* knife adjustments;

* roll distance, if necessary.

As soon as the individual systems are released, the appropriate process is started or changes made. The individual systems can only be operated by the on-site visualization systems. After the partial components enter the 4-roll calender and single-roll roller die, they can be released from the individual visualization systems as well as from the lamination point from the entire line.

Modes of operation

Manual mode

All parameters of the line are freely adjustable (within safety limitations). The values (recipe) cannot be saved in the PC. The temperature set point can still be changed. The die can only be changed (opening the head) in manual operation mode.

Semi-automatic mode

All parameters except the temperature set point can be changed within the possible limits. Documentation of the line size is not possible. The values run here are only used for optimization of the recipes.

Automatic mode

The line parameters can only be changed within [+ or -] 10% of the recipe values. The line size is documented, and the trends displayed. The set values are brought in after ending automatic mode to optimize the run recipe.

Management information system

The information obtainable from the line is still determined by the production area. For example: production quantities (set/actual), product qualities (width, meters in and out of tolerance, tolerances ++, +, 0, -, - -), trend displays, etc. Labels can be printed at the wind-up stations. A label is produced for each roll.

Error messages 

Errors in the system are kept in an error message storage in the PC, and a hard copy is output from a printer.

Conveyor belt lines

This line can produce all sheets needed for conveyor belts in a single step, even with highly viscous, elastic NR compounds. The sheet is 800 to 2,300 mm wide at 1.5 to 18 mm thick. The narrower sheets are produced in double width and cut on the line.

The system consists of the following units:

* QSM 250/16D cold-feed extruder;

* WBK 2400 preform head;

* 700 x 2,600 mm two-roll calender;

* thickness measuring gage;

* roller conveyor;

* take-off device with laminating roll;

* cooling stations I-III;

* conveyor belt before laminating station;

* lamination;

* let-off stand;

* conveyor belt after laminating station;

* thickness measuring gage;

* centering belt;

* lengthwise cutting machine (center and edge);

* cross-cutter;

* pivot conveyor;

* length measuring gage;

* feed roller conveyor;

* centering and transfer conveyor;

* center winder I and II; and

* 10 dancers between the individual units.

Roller head unit

The roller head unit is used for shaping the thick conveyor belt sheets. A roller head unit refers to the combination of an extruder with a wide extrusion head and two-roll calender.

The rubber compound is already well plasticized and brought to a uniform temperature in the wide extrusion head. From here, it enters directly into the roll gap, evenly distributed over the entire working width of the head. The wide head guarantees the best possible feeding of a calender gap and good product tolerances after only one calender gap cycle.

Extruders

Hot feed or cold feed - the efficiency of these lines is determined by the plasticization unit. The alternatives hot and cold feeding were discussed in detail in project planning as usual. The customer decided on cold-feeding because of the good test results. With a QSM 250/16D, all required compounds can be extruded in good quality at high throughputs. Basic data on the QSM 250/16D include:

* Speed - 22 rpm/min;

* motor power - 440 kW;

* gearbox torque - 175 kNm;

* number of pins - 12 x 8 pins;

* max. throughput - 4,500 kg/h.

Figure 5 shows the throughput for a cover sheet compound.

[Figure 5 ILLUSTRATION OMITTED]

Preform head WBK 2400

A preform head developed and patented for this product is used (ref. 3) to be able to shape the high throughputs in the required thickness range of 1.5 to 18 mm. The sheets have the following properties:

* No inclusions;

* no small parts starting to vulcanize;

* good surface over the entire thickness range;

* good thickness tolerances over the width

Why a new preform head design for NR compounds?

Symmetrical preform head (WBK) design

This preform head (WBK), the design already in use, is a wide extrusion head consisting of upper and lower halves (figure 6). The compound is distributed over the width in a so-called fishtail contour in both parts. Exchangeable plate dies are built in at the end of the contour. The plate dies make it possible for the sheet thickness or shapes being produced to be adjusted to equalize the flow differences with different compounds. The extrusion width is varied by using changeable inserts.

[Figure 6 ILLUSTRATION OMITTED]

High demands are placed on the thickness tolerance over the width - only this is determined by the preform head and calender - of rubber sheets which are produced in the roller head process. The lengthwise thickness tolerance is determined by the throughput precision of the extruder and the evenness of the take-off equipment. It should correspond to the cross tolerance.

Experience gained from the roller head process shows that the thickness tolerance that can be achieved depends on certain conditions that can have positive or negative effects:

* Thin sheets yield better tolerances than thick ones.

* A large amount of rubber in the calender gap improves the tolerance because the calender has to work harder. However, this is only possible if the compound cure rate allows the work with the temperature load. It can also lead to rough surfaces, however.

* If compound adjustments or surface demands require a small amount of rubber or even that the sheet be removed from the preform head through the calender, this can cause irregularities in the head flow that are more visible in the tolerance.

* Dead compounds, compounds with low elasticity or none at all, can be corrected relatively easily by making corrections in the plate dies of the preform head in the tolerance.

* Elastic compounds such as, for example, high-percentage NR qualities for conveyor belt covers are not as easy to adjust by adjusting the plates. Additional drag forces also occur after leaving the calender in the sheet tolerance.

To summarize, it can be stated that problems can arise due to insufficient tolerances with thickness from 8 mm and up using the conventional design to produce thicker sheets for NR conveyor belt covers.

Asymmetrical pre-form head design

The experiences described above led to the development of a new design. The goal of this design was to give the calender a stronger influence on the tolerance by connecting the preform head directly to the calender.

The plate dies were not used in this design. After leaving the flow channel contour, the compound is guided on one part of the upper roll in the gap with the lower roll (figure 7). The flow-channel contour is only in the lower part and the upper part is smooth. This design can also be used with duplex and triplex heads for the tire industry. In summary, the design offers the following advantages:

* The calender does more work yielding better tolerances with sheet thicknesses [is greater than] 8 mm;

* the plate dies are discontinued, yielding shorter lengths and lower head pressure;

* production is simpler because the flow channel geometry is one-sided.

[Figure 7 ILLUSTRATION OMITTED]

Two-roll calender KDI 70/2600

The calender calibrates the preformed sheets to finished size. The rolls are driven by a DC motor (P = 163 kW) and a gearbox with two output shafts. The lower roll is fixed. The upper roll is separated and can be jointly adjusted and hydraulically pre-loaded via two motors. Only the lower roll can be crossed, corresponding to the head principle. It was able to produce all of the 1.5 to 18 mm thick sheets made previously with very good tolerances without shrinking. This is due to the good pre-distribution of the new preform head.

Cooling

Drum or spray cooling? We have calculated the cooling of a thick sheet for comparison:

* Sheet thickness - 18 mm;

* product speed - 1.0 m/min.;

* initial water temperature - 18 [degrees] C;

* room temperature - 28 [degrees] C;

* winder temperature - 30 [degrees] C

* result - spray section 10 m, cooling drum 20 (13) m.

Two-sided spray cooling is clearly more effective than dram cooling. Nevertheless, drum cooling was chosen in this case since it requires less space and is also more cost-effective. The calculation takes into consideration the fact that the thick sheets do not have optimal contact with the lower rolls. The length of the drum cooling is the sum of the contact length (13 m) and intermediate space (7 m). The cooling drum unit designed with three cooling stations requires, however, only 5 m line length. The spray section would require 12 m fully extended. Only approximately 8 m, including the upward conveyor, are needed if the spray section is built in three stories. An additional 2.5 m is also needed for a blow-off station. Other advantages of drum cooling include:

* The sheet stays dry.

* The thickness can be measured after the second cooling station. It is very close to the final dimension. Early correction of the thickness is thus possible.

* A clean film is always wound off before the first cooling station to protect the sheet from contamination.

* The entire sheet (adhesion sheet plus cover sheet) is protected from both sides.

With drum cooling, the operator is thus in a position to manufacture a better product.

Design of the cooling station and the drums

Two cooling drums are placed one on top of the other to form a so-called cooling station.

The two drums are driven by a DC motor, including gearbox with a chain. The speed of the cooling stations is adjusted using dancers. Additional rolls built on guarantee a large loop angle and thus long contact with the sheet. The cooling drums have a diameter of 1,000 mm and consist of a double-walled aluminum material. Aluminum has two advantages including:

* Much better heat conduction than steel, and;

* the spiral-shaped cooling channels always remain free, whereas cooling channels underneath steel coatings can be blocked by corrosion.

How many cooling stations are needed?

We have calculated the cooling of the 18 mm thick sheet described above. Since a one-dimensional heating transport is used here, a relatively simple calculation can provide an answer.

The maximum possible product speed is calculated from the through-put and the sheet dimensions. As a rule, only the density of the compound can be named by the processor from the physical characteristics. Values describing a critical case are selected from experience for thermal conductivity and specific heat capacity. The safety allowance must not be exceeded, however, since each extra cooling station costs space and money.

Instead of the four cooling stations planned by the operator, the calculations showed that three would be sufficient for the cooling required. Temperature measurements carded out on several products confirm the calculations. The cooling process for three cooling stations is shown in figure 8.

[Figure 8 ILLUSTRATION OMITTED]

Laminating station with let-off stand

The two let-offs are built on one stand that is laterally adjusted by a motor. Both sheets - adhesive and cover - are laminated here and edge-guided.

Cutting devices

Two narrow sheets can be cut out of one sheet with three knives. After cross-cutting, the active center winder is accelerated in order to achieve the necessary intermediate space for the winder change.

Process control

To accomplish the automatic process control function, this line also has:

* Product measuring systems for width and thickness with control and feedback to the knife position;

* gap adjustment and take-off speeds;

* measuring devices for measuring traction and sheet position with feedback to the speed of belt drives and brakes;

* production length measurement.

To ensure that the production and the recipe-controlled reproducible machine pre-sets run automatically, all line adjustments that affect the dimensional accuracy or the product quality in general are interconnected with electrically controllable control elements. This means that not only the conveyor drive and the temperature control zones are equipped with suitable reproducible working systems, but also the adjusting elements of the units that follow, such as the portability of the cut-out knife, brake pressure adjustments, pre-loading and centering.

Expanding line capacity

There are different alternatives to offer here. Maintaining the product width would mean a larger extruder and more cooling stations would mean higher production speeds.

Since a large quantity of 2,800 mm wide conveyor belts is required, such a line would have to have a preform head of approximately 3,000 mm wide. Then, for example, it could have two flow channels inside and be fed by two extruders.

Summary

The lines described above, which have already been put into practice in similar form many times, should give the reader an idea of the new options and trends in line design and automation. Although the functions for these multiple component lines may appear complicated at first, the mechanical and electrical equipment make them easy to handle. The high line capacity brings the manufacturer one step further in increasing production automation.

References

(1.) Engst, W., Herstellungsverfahren und Aspekte des Vulkanisationsprozesses von Fordergurten, Kautschuk + Gummi Kunststoffe 46 (1993) 2 pp. 129-138.

(2.) Ramm, H.-F. and Seidler, E., Zusammenfuhren von kalandrierten und extrudierten Reifenkomponenten in On-Line-Betrieb, Kautschuk + Gummi Kunststoffe 45 (1992) 3 pp. 225-231.

(3.) EP 0 330 811 B1.