Insights on lane Crosslinking of PE and Tin-free Future Dr. Kerstin Weissenbach Evonik Industries Corp. Piscataway, NJ +1-732-981-5044 kerstin.weissenbach@evonik.com Aristidis Ioannidis Evonik Industries AG Rheinfelden, Germany +49-7623-91-8338 aristidis.ioannidis@evonik.com Abstract In this paper, the principle of silane crosslinking of polyethylene (PE) and currently used catalyst systems is highlighted. State-of-the-art catalyst systems for the silane crosslinking process face significant challenges. In the case of tin-based systems like DBTDL, the industry has to handle periodically stricter labeling, starting in Europe, but most likely going around the globe. Existing metal-free systems face challenges concerning corrosiveness, discoloration of the insulation and wire as well as organoleptic issues. The silane crosslinking reaction consists of two stages: The hydrolysis and the condensation step. For that reason, an effective catalyst or catalyst system must support both stages of the crosslinking reaction. This is the bench mark for potential alternatives to the state-of-the-art tin-based catalysts. Keywords: polyethylene crosslinking, silanes, moisture cure, tin-free, L-PE. 1. Introduction Crosslinked polyethylene, commonly abbreviated PE or LPE, can be produced via three different crosslinking processes: peroxide cure, silane crosslinking or e-beam crosslinking. This paper will focus on the silane crosslinking process which is mainly used for low voltage cable insulations within the wire & cable industry. L-PE for cable insulations is typically based on PE-LD or PE-LLD. 2. lane Crosslinking of Polyethylene The principle of silane crosslinking of polyethylene (PE) is based on two reaction steps (see Graph 1). The first step is the grafting of the vinyl silane onto the polyethylene (PE) chain. This is done in the presence of peroxides in an extruder. As a second step, in the presence of moisture and a suitable catalyst, the hydrolysable part of the silane is hydrolyzed and condensed afterwards, creating a crosslinked L-PE compound. The first step, the grafting process, is determined by the nature and decomposition temperature of the peroxide used. The extrusion temperature and the extrusion speed must be adjusted to the peroxide. Polyethylene (PE) Graph 1: The two process steps of moisture cure crosslinking The second step can be divided into two stages: the silane hydrolysis and condensation stage (see Graph 2). + (Me) 3 (Me) 3 Grafted PE- Compound Vinyltrimethoxysilane 1 st Stage ydrolysis 1. Grafting 2. Crosslinking Peroxid Water + Catalyst () 3 lanol generation (Me) 3 Grafted PE- 2 nd Stage Condensation Crosslinked PE (L-PE) Crosslinked PE Graph 2: The two stages of the moisture cure crosslinking process. 3. Theory of ydrolysis and Condensation of lanes In the first stage, water is essential in order to generate silanols. This reaction was studied via NMR analysis in excess of water as a model environment [1, 2]. Brand et al. found that the hydrolysis reaction can be also split into several steps (see Graph 3). 117
R R R 2 - R R R R lane lanol lane diol lane tetraol 2 - R for = R + 2 / - R 2 Graph 3 Stages in hydrolysis of functional trialkoxysilanes and tetraalkoxysilanes [1]. - R lane triol 4. Mechanism of Tin-rganic Catalyst Systems The current tin containing catalysts systems provide excellent solubility in the hydrophobic polymer matrix. This is achieved by the long hydrocarbon substituent lauryl. At the same time, the more polar ester groups close to the tin center guarantee the effective coordination of the ester group of the silane as well as the water for the hydrolysis step. In an uncoordinated form, DBDTL has a tetrahedral coordination sphere around the tin center. As many organometal compounds, it is expected that the coordination sphere changes to a octahedral formation upon coordination of the additional two ligands silane and water (see Graph 4). In order to understand the rate-determining first step of hydrolysis (from trialkoxysilane to the silanol better the dialkoxymonohydroxysilane), silanes were analyzed under different conditions. It became obvious that the rate determining step is dependent on the nature of the functional group and the nature of R, the ester used. It was concluded that hydrolysis of the studied alkoxysilanes is a (pseudo) 1st order reaction [1,2]. Known and commonly used hydrolysis catalysts for silanes in typical applications like coatings and adhesives are titanates, zirconates or mineral acids which are mainly used in primer applications. In mineral filled compounds the silane acts as adhesion promoter at the interface of the mineral and polymer. The inorganic surface provides the necessary p (acting as catalyst) to catalyze the hydrolysis reaction. Water is usually present in stoichometric excess. In the case of crosslinking of PE, there are major differences in the system: There is no mineral present, no excess of water and the hydrophobic polymer matrix is not supporting the presence of water either. Yet still, hydrolysis must occur first on the way to a silane crosslinked PE compound but at the same time also the condensation reaction is challenged by the limited mobility of the grafted (vinyl-) silane. Consequently, the presence of a catalyst is more than necessary. This catalyst must be soluble in the polymer matrix yet still highly active to make the best use of the limited moisture present. Ideally the catalyst must act as a hydrolysis and condensation catalyst at the same time. R Graph 4: Proposed transition state for DBDTL as a hydrolysis catalyst As both reaction partners (water and alkoxysilane) are expected to be coordinated at the vertical axis. An exchange of the ligands can explain the catalytic effect during the hydrolysis reaction. The same formation can also explain the catalytic effect during the condensation step (see Graph 5). R =, Y(R)2 R = Cn2n+1 Y(R) Y(R) Currently tin-organic catalysts like dibutyltindilaurate (DBTDL) and dioctyltindilaurate (DTDL) are the state of the art catalysts for moisture cured L-PE. Another known system for ethylenesilane-copolymers is based on organo-functionalized sulphonic acids. Current catalyst systems for the silane crosslinking process face significant challenges. In the case of tin-based systems, the industry has to handle periodically stricter labeling, starting in Europe but most likely going around the globe. The existing metal-free systems face challenges concerning corrosiveness, discoloration of the insulation and wire as well as organoleptic issues. R =, Y(R)2 Y(R) Graph 5: Proposed transition state for DBDTL as a condensation catalyst 118
5. Designing a Model System for a New Class of Catalyst Systems for lane Crosslinking of PE For a better understanding of the catalytic ability of different classes of substances, a screening study on a model system was performed. exadecyltrimethoxysilane as a representative of a very hydrophobic and slowly reacting silane was chosen to mimic the situation in a polyethylene matrix. This model system allows obtaining relatively fast screening tests results by observation of skin formation on the surface of the silane. This skin formation resembles the hydrolysis and condensation step in the crosslinking reaction. Ambient moisture promotes the hydrolysis reaction. The skin consists of condensated silane oligomers or siloxane polymers which are no longer miscible with the monomeric silane. In this model system different groups of catalyst were studied by analyzing the required time of skin formation on the surface of the silane. The longer the time, the less efficient the catalyst is. n the other hand, if the reaction is too fast, so the time too short, the catalyst is also not qualified as the crosslinking reaction will most likely already occur in the extruder and not under controlled conditions after extrusion during storage in a sauna or under water. 6. The Results in the Model System Graph 6 shows the different groups of catalysts. Despite the fact that DBTDL is under close observation with respect to its toxicity profile, other metal containing systems were tested. All of them are currently not labeled as toxic. Bismuth-based blend cat 1 91% exadecyltrimethoxysilane + 9% catalyst mixture in open cup measurement of the film forming time Distannoxane-based cat 2 Distannoxane-based cat 1 DBTDL rganic tin-based cat 2 rganic tin-based cat 1 rganic cat 1 Basic cat 2 Basic cat 1 Acid cat Bismuth-based cat 2 State-of-the-art Possible catalyst alternatives Too slow The reaction could not be controlled. Even low concentrations lead to immediate skin formation indicating that this group of catalysts is not be feasible for use in an extruder process. 7. The Conclusions of the Model Trials During the study it became obvious that different classes of substances catalyze differently and not all were suited to be used as catalysts of silane PE crosslinking. The correct choice of reactivity and velocity is critical to the overall success of a new catalyst system. Based on the medium speed catalyst alternatives, further tests were performed to yield potential candidates leading to encouraging results for future commercially viable solutions for tin-free silane-based crosslinking systems for polyethylene. 8. Checking Performance in a Model Crosslinking System The next step is to prove the positive results from the model system in a more realistic situation. PE-D was grafted with vinyltrimethoxysilane in a twin screw extruder 1. The grafted PE was mixed together with 5% of a catalyst containing masterbatch in a laboratory kneader 2. The polymer-mix was finally pressed to plates at approx. 200 C. The plates were Crosslinked in a water bath at 80 C for 4 and 22 h. The gel content 3 of the plates (non-crosslinked and cross-linked) was measured. The results of the screening of the various substances which were defined as potential catalyst alternatives are shown in Graph 7. According to their nature and functionalities the various catalysts led to significant differences in the crosslinking performance. In some cases, the crosslinking speed was even slower compared to the sample without any catalyst. It is not clear at the moment which characteristics of the catalyst are primarily controlling the crosslinking performance in the polymer matrix. Reasons for the reduced reactivity in the PE matrix can be, e.g. insufficient distribution of the catalyst in the polymer matrix because of the catalyst functionality or steric hindrances. Several prospective solutions (see Graph 7) have been identified within the screened catalyst field. These catalysts are used as the base for the ongoing developments of tin-free catalyst systems for the silane crosslinking process. Inorganic tin-based cat 2 Too fast Inorganic tin-based cat 1 0 2 4 6 8 10 12 14 Time until film forming on the surface of an open cup [min] Graph 6: Results of the screening test with alternative catalyst systems - film forming speed of silane/catalyst mixtures in an open cup. Bismuth and some different tin based catalyst systems appeared to be not efficient enough. They are even slower in reactivity than the state-of-the-art DBTDL. Inorganic tin-based catalyst systems show almost immediate reaction in the model system. 1 Co-rotating, l/d = 33, d = 25 mm, 3 kg/h, 100 rpm, Temperature profile: -/150/160/200/200/210/210/210 C 2 30 rpm, Temperature profile: 3min@140 C, in 2 min upto 210 C, 5min@210 C, 50g 3 Boiling p-xylene, Soxhlett extractor, 8h 119
Graph 7: Results of the PE crosslinking screening study 9. Conclusions The development path to a new, tin-free catalyst solution for silane crosslinking of polyethylene has been presented starting from a screening study on model systems of a large variety of substances. In summary, this paper proved that there is a possible alternative to the currently used catalyst systems for moisture cure crosslinking processes for the cable industry, avoiding toxic substances like organotin catalysts leading into a new era of silane crosslinking. 10. Acknowledgments Special thanks to the continuing hard work of our R&D group in Rheinfelden for providing all the experimental. 11. References [1] Marcus Brand et al., NMR-Spectroscopic Investigations on the ydrolysis of Functional Trialkoxysilanes, Zeitung für Naturforschung 54b, 155-164 (1999) [2] Fatmir Beari et al., rganofunctional Alkoxysilanes in Dilute Aqueous Solution: New Accounts on the Dynamic Structural Mutability,Journal of rganometallic Chemistry 625 (2001) 208 120
Dr. Kerstin Weissenbach Evonik Degussa Corp. 2 Turner Place Piscataway, NJ 08855, USA Dr. Kerstin Weissenbach studied chemistry at the University of Konstanz (Germany). er PhD thesis focused on organometallic chemistry. She started in the year 2000 in the R&D and Applied Technology department of the BL Functional lanes. From 2003-2006, she worked on an expatriate assignment in the US, afterward she headed the Plastics and Mineral Fillers & Pigments R&D and Technical Service group in Rheinfelden (2006-2010). nce March 2010, she is the Technical Manager of the Functional lanes Applied Technology group in NAFTA and is based in NJ. Aristidis Ioannidis Evonik Industries AG Untere Kanalstrasse 3 79618 Rheinfelden Germany Mr. Aristidis Ioannidis studied chemical engineering at the University of Stuttgart (Germany) focusing on plastics technology (Institut für Kunststofftechnologie IKT). Major topics were processing and compounding of plastics and especially the chemical modification of polymers. nce 1996, he has worked in the R&D and Applied Technology department of the BL Functional lanes, which belongs to the BU Inorganic Materials within Evonik Industries AG, in the application fields of Plastics and Mineral Fillers & Pigments... 121