EFFECT OF PRE-CROSS-LINKING ON MECHANICAL AND RHEOLOGICAL PROPERTIES OF SOLID SILICONE RUBBER AND ITS PROCESSABILITY IN

EXTRUSION BLOW MOLDING

Prof. Dr.-Ing. Christian Hopmann and Sarah Schäfer, M.Eng., Institute of Plastics Processing (IKV) at RWTH Aachen University, Aachen, Germany

Abstract

The blow molding process offers the possibility of reproducible, fully automatic and therefore cost-efficient mass production of complex hollow bodies. Due to the poor mechanical properties of uncured rubber, it has not yet been used for the manufacturing of elastomeric hollow parts. In this contribution, it is shown that with a defined pre-cross-linking of solid silicone rubber the blow molding of the material is possible.

With pre-cross-linking the mechanical material

properties can be adjusted precisely. This allows parison extrusion without strong drawdown. During the forming, it provides the necessary elasticity while maintaining the weldability and formability of the material. But pre-cross-linking also influences the materials rheological properties. Preliminary investigations showed pre-cross-linking has to take place in the blowing head bevor the material leaves the die. Therefore, changes in rheological material behavior are investigated and considered for the flow channel design. It is shown that the pre-cross-linking allows the blow molding of elastomeric hollow bodies with a surface stretch ratio of 3.6 to 1. However, pre-cross-linking can also lead to flow instabilities such as wall slippage and melt fracture.

Introduction

Silicone rubbers have outstanding physical and chemical material properties such as high flexibility, wide application temperature range and good chemical resistance. Furthermore, they are ozone and weather resistant, physiologically inert and have a bacteria and mold-repressive effect. They are also flame retardant, electrically insulating and resistant to vegetable and animal fat, paraffin oil and alcohol [1,2]. Silicone rubbers are mainly used in electrical (30 % of world market volume), automotive (25 %) and medical (15 %) applications. The global silicone elastomer market had a volume of 6 billion USD in 2014 and has an estimated growth of 5.8 % from 2015 to 2022 [3]. This is resulting from the rapid urbanization and industrialization in emerging countries and the demographic change and the resulting demand of medical devices [3].

Hollow bodies made of silicone rubber are for example used for packaging, transportation and as technical parts such as media lines or mechanical devices. They can be produced in injection molding or extrusion. Both production processes are time and cost consuming for hollow bodies and geometries are limited as the vulcanized product has to be removed from a metal core [4,2].

The blow molding process offers the possibility of

reproducible, full automatically and therefore cost-efficient mass production of complex hollow bodies. Thus, in the 1980s, first attempts were made to process silicone rubber in blow molding [5,6]. However, due to insufficient mechanical and extensional rheological properties of uncured silicone rubber and unadapted blow molding machinery, the process could not yet be established [5,6,7].

Objective of the research

A previous research project at the IKV (IGF-Nr. 17671N) showed that with a defined pre-cross-linking, solid silicone rubber processing in blow molding is possible [7]. For this purpose, a two stage cross-linking system was developed. The pre-cross-linking allows the preform extrusion without strong drawdown. During forming it provides the necessary elasticity, while maintaining weldability and formability of the material. Focus of this research project was the effect of pre-cross-linking on the mechanical and rheological material properties of solid silicone rubber. With this knowledge the process of blow molding can be adapted for processing solid silicone rubber.

In previous investigations, it was not possible to

ensure sufficient pre-cross-linking in the blow molding process due to insufficient material temperature control with IR-heating or the heated blow head [7]. The objective of the ongoing research project at the IKV is to develop an extrusion die for the continuous pre-cross-linking of solid silicone rubber. Usually, cross-linking of rubber inside the extrusion die is avoided in order to avoid blockage. Cross-linking should only take place after leaving the extrusion die, for example by means of infrared radiator, hot air, salt bath or microwave.

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Therefore, little is known about the design of extrusion dies for pre-cross-linking.

In order to activate pre-cross-linking the extrusion die must be tempered in two sections. The first die section should be cooled down to avoid vulcanization and thus blocking of the flow channel. In the second section, the extrusion die should be heated to achieve pre-cross-linking. Temperature increase and continuing cross-linking are influencing the rheological material behavior. This must be considered for the flow channel design.

Experimental Materials

Two solid silicone rubbers were used for the experimental work in this contribution. Both were manufactured by Wacker Chemie AG, Burghausen (Germany). They are both heat cured silicone rubbers. In the remainder of the paper the resins will be referred to as 60SA and 70SA resin. The material properties are provided in Table 1. Table 1. Material properties of solid silicone rubbers [8,9].

60SA 70SA Density [g/cm³] (DIN 1183-1A) 1.17 1.18

Hardness [Shore A] (DIN 53505) 62 70

Tensile strength [N/mm²] (DIN 53504 S1) 10 10

Elongation at break [%] (DIN 5304 S2) 490 470

Tear strength [N/mm] (ASTM D 624 B) 34 23

Two different peroxide curing systems were used for

the investigations: Dicumyl peroxide (DCP) with a curing temperature of 180 °C and di(2,4dichlorobenzoyl) peroxide (2,4DCBP) with a curing temperature of 120 °C. The two peroxides were each blended into the two silicone rubbers at IKV on a roll mill. For each silicone rubber two compounds were thus obtained. One contains 0.74 wt% of di(2,4dichlorobenzoyl) peroxide, the other 0.57 wt% of dicumyl peroxide. The peroxide systems were provided by Pergan GmbH, Bocholt (Germany).

These two compounds provide the two stage cross-

linking system. By blending the compounds in different ratios, the pre-cross-linking degree can be varied. In the following, 6 wt% stage 1 means that 6 % of the material consists of the compound containing 2,4DCBP.

Hot pressing To analyze pre-cross-linked material samples in

tensile tests or rheological measurements, solid silicone rubber sheets were pressed and heat cured on a hot press. The dimensions of the sheets were 155 x 100 x 2 mm³. The pressing conditions were 225 bar for 5 min at a temperature of 120 °C.

Tensile testing

Tensile tests were performed on a tensile testing

machine in accordance with ISO 9026:2007. Tensile tests took place at room temperature (23 °C) with a testing speed of 100 mm/min and a maximum force of 200 N. The tension rods had a thickness of 2 mm. The clamping length was 40 mm. Additional, the tension set of pre-cross-linked samples was determined. Therefore the samples were deformed to 100 and 200 % elongation. The elongation was hold for 10 minutes and after a recovery time of 30 minutes the remaining sample length was measured. Rheological measurements

A Rubber Process Analyzer was used for the

rheological study of cross-linking behavior of the two stage cross-linking system. For curing curves, the strain amplitude was 7 % and the frequency 10.49 rad/s. The temperature was held at 120 °C for 4.5 minutes and then increased to 180 °C for another 5.5 minutes.

For rheological measurements in high pressure

capillary rheometer (HCR), samples with different levels of pre-cross-linking were taken from pressed sheets. Pre-cured samples were put into the HCR probe channel and heated to the required test temperature. The 70SA resin was tested with 0, 2, 4, 6, 8 and 10 wt% stage 1 at 110 °C and at shear rates of 5, 8, 10, 50, 80 and 100 1/s. All measurements were performed on a 10, 20 and 30 mm long capillary with a diameter of 1 mm to consider the elastic pressure loss.

Blow molding

Blow molding experiments were performed on a

laboratory scale blow molding machine, consisting of a rubber extruder with a screw diameter of 19 mm and a length of 20 D and an extrusion die consisting of deflection and pipe nozzle with an outlet diameter of 14 mm. The extruded tubes were pre-cure in a hot-air oven at 120 °C for 60 minutes. Inflation took place in a blow mold at 200 °C.

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Results and Discussion Two stage cross-linking system

The extrusion blow molding process places certain requirements on the material: In order to prevent sagging of the parison during extrusion the material needs sufficient melt strength. Additionally, to ensure adequate strength of the weld line the material has to be well weldable [10]. Uncured rubbers do not have sufficient green strength to be processed in extrusion blow molding [5]. In previous investigations, it was examined if a pre-cross-linking of silicone rubber is possible and how it influences the mechanical properties of the material [11].

Research aim was to find a cross-linking system that

enables a defined and processable pre-cross-linking of the material. Theoretically, it would be possible to start the cross-linking at low temperatures so that the curing proceeds very slowly and to inflate the material just when the right amount of cross-linking is achieved. But for this, the processing must be highly automated and very constant. Even little changes in timing would result in different amounts of pre-cross-linking. In the following investigations, a cross-linking system consisting of two different peroxides with cross-linking temperatures as far apart as possible is used. The peroxide 2,4DCBP has its cross-linking temperature at 120 °C and the DCP peroxide at 180 °C. This ensures a secure separation of pre-cross-linking and final-crosslinking. In Figure 1 curing curves for different degrees of pre-cross-linking are shown. The pre-cross-linking is reached after one minute and stays constant until the temperature is increased to 180 °C. Pre-cross-linking degree can be varied with the compound composition.

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Figure 1. Two stage cross-linking curves of the 60SA resin. Effect of pre-cross-linking on mechanical properties

Uncured rubber has an insufficient green strength and

breaks at very low elongations. Figure 2 shows for the 60SA resin that even small amounts of pre-cross-linking (6 wt.%) increases the elongation at break to more than

the fourfold. However, over 10 wt% stage 1 of pre-cross-linking leads to a too highly elastic material behavior. This results in a low tension set. When pre-cross-linked material is inflated during blow molding it will strongly recline. In Figure 3 the dependence of tension set on pre-cross-linking of the 60SA resin is shown. The material was stretched by 100 and 200 %.

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Figure 2. Stress-strain diagram of the 60SA resin with varying amount of pre-cross-linking.

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Figure 3. Tension set of the 60SA resin with varying amount of pre-cross-linking.

For the processing in extrusion blow molding, the weldability of the material plays an essential role. In order to examine the weldability of pre-cross-linked silicone rubber, two preliminary cross-linked silicone rubber sheets were pressed together and heated up, so that the final cross-linking occurs. The weld line lies in the middle of the tensile test sample. With these samples tensile test were carried out. The samples were made with the 60SA resin and with pre-cross-linking degrees between 6 and 50 wt% stage 1. Figure 4 shows the results of these investigations. With increasing pre-cross-linking the strength of the weld line decreases. However, for small amounts of pre-cross-linking up to 8 wt% stage 1, the failure does not occur in the weld line.

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Figure 4. Tensile strength of the weld line of samples of the 60SA resin with varying amount of pre-cross-linking [12]. Effect of pre-cross-linking on rheological properties

Previous investigations at the IKV have shown pre-

cross-linking should occur in the blow head [7]. This ensures adequate pre-cross-linking bevor the parison is extruded and sagging becomes a problem. On the other hand, pre-cross-linking changes the rheological material behavior of the silicone rubber. The following investigations aim is to determine the effect of pre-cross-linking on the material behavior in the flow channel of the blow head and to find out if adjustments are necessary. For this purpose the shear viscosities of the two resins were determined in HCR measurements. The 70SA resin is of great interest, because resins with higher Shore A hardness are even more difficult to process into hollow bodies in injection molding or extrusion. Furthermore, the viscosity of these materials is in general higher and flow instabilities are more likely to arise.

Figure 5 shows the effect of pre-cross-linking on

shear viscosity for the 70SA resin. Except for the 8 wt% stage 1 resin the viscosity decreases with higher pre-cross-linking amounts. This effect is even stronger for higher shear rates.

1,E+02

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Figure 5. Shear viscosity for the 70SA resin with varying amount of pre-cross-linking.

When looking at the HCR-strings at shear rates of 100 1/s (Figure 6), it can be seen that with increasing amount of pre-cross-linking compound the strings surface

is getting rough. This observation indicates wall slipping of the pre-cross-linked silicone rubber. Wall slipping occurs when wall shear stress reaches a certain degree. At this point the material now longer sticks to the wall but begins to slip. While slipping the wall shear stress decreases until the stress is below the critical degree and the material sticks to the wall again [13]. This process repeats periodically and leads to the observed surface roughness. This can explain the unexpected results of the HCR measurements. Wall slipping decreases pressure loss in the flow channel and the viscosities are calculated from too low pressure values.

Another effect that can lead to the unexpected results

in HCR measurement is the temperature increase because of shear heating. Pre-cross-linking increases the viscosity this leads to higher shear rates which heats the material more strongly. This effect is neglected in HCR measurements as it is not easy to detect [14].

0 wt% stage 1 2 wt% stage 1 4 wt% stage 1

6 wt% stage 1 8 wt% stage 1 10 wt% stage 1

Figure 6. HCR-strings for the 70SA resin with varying amount of pre-cross-linking. Processability in extrusion blow molding

Mechanical and rheological investigations on pre-

cross-linking have shown strong effects even for small amounts of pre-curing. For the following investigation on the processability in blow molding the 60SA resin was used with an amout of 6 wt% stage 1. This amount of pre-cross-linking is very promising because on the one hand it increases the elongation at break strongly and on the other hand weldability remains and tension set is not too much decreased.

For first blowing experiments the extruded parison

was pre-cured in a hot air oven and afterwards inflated inside a heated blow mold. The wall thickness distribution of the blow molded samples was analyzed and burst pressure tests were carried out.

In Figure 6 the wall thickness distribution of the blow

molded bottles is shown. The wall thickness was

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measured for seven lines with 4 measuring points on each line. The blow molding experiments show that a certain pre-cross-linking enables the inflation of silicone rubber with compressed air into hollow bodies. The blow molded parts had a surface stretch ratio of 3.6 to 1. Because of the low modulus of pre-cured silicone rubber very low air pressure of 50 mbar is needed for inflation. Wall thickness of the bottles varies between 0.5 and 1.5 mm. The lowest wall thickness is at the top of the bottle. It increases towards the bottles bottom. The wall thickness distribution and how it is affected will be subject of further investigations.

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Figure 6. Wall thickness distribution of blow molded bottles made of the 60SA resin with 6 wt% stage 1 of pre-cross-linking.

In burst pressure tests, the bottles were inflated with compressed air until failure. Because of the uneven wall thickness distribution and very thin areas the failure occurred at low pressures of 200 mbar. However, the weld line did not cause the failure.

Conclusions and perspective

Blow molding of solid silicone rubber can improve the processing of complex hollow bodies made of silicone rubber and open new fields of applications. For the processing in extrusion blow molding, the silicone rubber must have a certain degree of stability in order to be blown into its final shape. This stability is not provided by uncured rubber but with a slight degree of pre-curing the material could get enough stability to be formed in blow molding. The investigations in this paper show the potential of the extrusion blow molding of pre-cross-linked solid silicone rubber.

In order to adjust the processing of silicone rubber, a

tempered blow head will be developed, which allows the pre-cross-linking. For the development of this blowing head the dependence of the rheological material behavior on the pre-cross-linking has to be considered. Next steps using the tempered blow head will be the examination of the pre-cross-linking in the blow head and the quality of the parison. When a good pre-cross-linking and quality is ensured further investigations will deal with the inflation

of the parison in the hot mold and the resulting part quality.

Acknowledgements

The research project 18911 N of the Forschungsvereinigung Kunststoffverarbeitung has been sponsored as part of the "industrielle Gemeinschaftsforschung und -entwicklung (IGF)" by the German Bundesministerium für Wirtschaft und Energie (BMWi) due to an enactment of the German Bundestag through the AiF. We would like to extend our thanks to all organizations mentioned.

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