PROCESS FOR PRODUCING A CROSS-LINKED POLYMER PRODUCT

Abstract: The invention relates to a process for producing a polymer product cross-linked by silane where a polymer, a grafting agent, an initiator and a cross-linking catalyst and possible additives are fed into an extruder and extruded, whereafter the grafted material obtained is cross-linked using water and the catalyst for obtaining a cross-linked polymer product. According to the process the grafting degree of the grafted material is determined using an on line method and based upon the result obtained the amounts of the components to be fed into the extruder are continuously adjusted in order to obtain the desired grafting degree.

Description: PROCESS FOR PRODUCING A CROSS-LINKED POLYMER PRODUCT The invention relates to a process for producing a polymer product cross-linked by silane. It is known in the art that the properties of polymers, such as polyethylene and other polyolefins, can be modified by cross-linking. Polyethylene, for example, can be cross-linked using a peroxide initiator, a hydrolyzing silane compound and a condensation catalyst as described in US patents 3 646 155 and 4 117 195. Said process can be carried out by injecting a polyethylene, a peroxide, for example dicumyl peroxide, a silane compound, for example vinyl trimethoxy silane or vinyl triethoxy silane and a condensation catalyst, for example dibutyltin dilaurate into an extruder and by extruding, whereby a grafted product is obtained which is then processed in condensation conditions in the presence of water or aqueous steam for providing a cross-linked product. Several drawbacks are frequently associated with prior art cross- linking processes when products of uniform quality are pursued. Such problems occur in particular when the process is continuous. For example, when cable and conductor insulations are formed using a continuous cross-linking process, where the production line is long, a product may be formed which cannot be used owing to the varying quality. The economical losses may be significant in such a case. It is very important to obtain a product of uniform quality particularly when cable and conductor insulations are produced. This obviously holds true also when producing other products, such as pipes, where a continuous process and a long production line are used. When cable and conductor insulations are concerned the cross- linking degree is of essential importance, as it directly affects the electrical and thermomechanical properties as well as the long-term endurance properties of the product. What mainly affects the cross-linking degree is the amounts of components fed into the extruder, the specific cross-linking conditions, the pressures and temperatures, and also the size of the extruder. For example, when a conductor or cable is insulated by cross-linking polyethylene as shown in the above US patent publications, the amounts of peroxide initiator, silane compound and condensation catalyst affect the cross-linking degree. The insulation properties can be affected by varying said amounts. Until now the cross-linking degree has been manually determined in a laboratory by analyzing a sample taken from the cross-linked product obtained after the condensa- tion treatment. If the cross-linking degree has been unsatisfactory, the feed of the starting components has been readjusted in order to obtain the desired product. However, the problem is that undesired changes may frequently occur in the reaction conditions of the continuous process that disadvanta- geously affect the properties of the final product. Since the manual analysis is slow, a lot of poor quality products may be formed that naturally cause economical losses. For example, when cables and conductors are insulated, and the production line is long, the amount of rejected items can be very high, if the feed of the components is adjusted on the basis of the manually obtained analyzing results of the final product. It should also be pointed out that free silane has a corrosive effect on aluminium conductors in particular. Peroxide residues, in turn, weaken the long-term stability of plastic. Minimizing the residues of both said substances thus improves the long-term endurance properties of the cable. It has now been found that the above drawbacks can be avoided, if the grafting degree of a polymer is determined at an earlier stage before cross-linking and based thereupon the amounts of substances to be fed into the process are adjusted. The grafting degree of a grafted polymer obtained as an intermediate product in the cross-linking process has to be sufficiently high in order to obtain a high enough cross-linking degree (over 60%) for the final product. By determining the grafting degree using an on line process without disturbing the production process and by continuously adjusting the amounts of starting components on the basis of the result obtained, the quality of the product to be made can be ensured, and the amount of rejected mate- rial can be reduced. The present invention relates to a process for producing a polymer product cross-linked by silane where a polymer, a silane, an initiator and a cross-linking catalyst and possible additives are fed into an extruder and extruded, whereafter the grafted material obtained is cross-linked using water and the catalyst for obtaining a cross-linked polymer product. The process is characterized in that the grafting degree of the grafted material is determined using an on line method and based upon the result obtained the amounts of the components to be fed into the extruder are continuously adjusted in order to obtain the desired grafting degree. The grafting degree is appropriately determined by measuring the contents of free and grafted silane in the grafted product. This is preferably carried out with an IR spectrophotometer using a flow through cuvette. The free silane content is determined by measuring the area of the absorption peak at 810 cm "1 in the IR spectrum of the silane, whereas the grafted silane content is similarly measured from absorption peak 1080 cm 1 . In the calibra- tion known grafted polyethylene silane mixtures are used, whose silane content as well as the silane content of a corresponding grafted material are defined by means of an NMR spectroscope, for example. Since peroxide consumed in the reaction is used as the initiator in polymer silane grafting, the amount of peroxide has to be appropriate, originally about 0.1 % by weight, in order that the silane, the amount of which is about 0.8 to 2 % by weight, is efficiently grafted so that no direct cross-linking disturbing the grafting takes place without silane. In a well-grafted material the amount of remaining peroxide is very low, hardly measurable. Also the peroxide content can be controlled by measuring the area of the absorption peak at 1155 cm "1 by means of an IR spectroscope. The calibration is carried out in the same way as in the case of silane. As the grafting degree is determined using a continuous on line measuring method, the results are rapidly obtained, and the feed of starting components can also be rapidly affected if needed. On the basis of the measuring results the feed of starting components can automatically also be adjusted using a computer as described below. The adjustment can, for example, be based on an algorithm that may be of a fuzzy logic type. Next the examples of the measurements and adjustments are shown when a silane/peroxide mixture is fed and when silane and peroxide are separately fed. Example 1 Feeding a ready-mixed silane/peroxide mixture Example 2 Feeding silane and peroxide separately Typical polymers to be cross-linked, whose grafting degree can be adjusted by the process of the invention, are polyolefins, preferably polyethylene and polypropylene. The cross-linked product is preferably a cable or conductor insulation, a plastic pipe or profile. The cross-linking catalyst is preferably a dibutyltin dilaurate. A hydrolyzing silane compound, preferably vinyl trimethoxy silane, is used as the grafting agent and the initiator is typically a peroxide compound, preferably dicumyl peroxide. Conventional additives include antioxidants and colourants. After grafting the material is cross-linked by water and the catalyst. Immediately after the polymer product has been sufficiently cooled the cross- linking degree can further be determined during the run, for example by utilizing a thermomechanical analyzer that measures the elastic properties of the material. When the measuring is carried out the thin measuring head is pressed by a constant load against the cross-linked material and the size of the elastic deflection depending on the cross-linking degree of the polymer, the temperature and the load put upon the measuring head is registered. The registered deflection values are compared with a calibration curve measured with a corresponding material at the same temperature, thus providing the cross-linking degree corresponding to the deflection. Together the information obtained from the cross-linking degree measurement and the measurement values of the grafting degree and the other measured operating parameters provide feedback to the production process concerning the quality of the final product. The feedback to the production process concerning the quality of the final product makes it substantially easier to find the correct operating parameters for the different raw materials and enables an automatic quality control of the production line. In its simplest form a thermomechanical analyzer may be composed of two wheels placed at the opposite sides of a cable or another polymer product to be run and pressed against the same, one wheel being pressed by means of a known force, and the deflection caused by the wheel in the product is measured, and of a registration apparatus for registering deflections and also the temperature of the material. The apparatus example shows only one way of measuring the elastic properties of the product on the line during the run, but the example is not intended to restrict the scope of the patent.

Claims: CLAIMS I. A process for producing a polymer product cross-linked by silane where a polymer, a grafting agent, an initiator and a cross-linking catalyst and possible additives are fed into an extruder and extruded, whereafter the grafted material obtained is cross-linked using water and the catalyst for obtaining a cross-linked polymer product, characterized by determining the degree of the grafted material using an on line method, and based upon the result obtained, continuously adjusting the amounts of the components to be fed into the extruder in order to obtain the desired grafting degree. 2. A process as claimed in claim 1, characterized by determining the grafting degree using IR spectrometry. 3. A process as claimed in claim 1 or 2, characterized by also determining the cross-linking degree of the cross-linked polymer product. 4. A process as claimed in any one of claims 1 to 3, charac- terized by determining the cross-linking degree using a thermomechanical analyzer. 5. A process as claimed in any one of claims 1 to 4, characterized by using a polymer, which is a polyethylene. 6. A process as claimed in any one of claims 1 to 4, charac- t e r i z e d by using a grafting agent, which is a silane compound. 7. A process as claimed in claim 6, characterized by using a silane compound, which is a vinyl trimethoxy silane. 8. A process as claimed in any one of claims 1 to 7, characterized by using an initiator, which is a peroxide. 9. A process as claimed in claim 8, characterized by using dicumyl peroxide as an initiator. 10. A process as claimed in any one of claims 1 to 8, character i z e d by using dibutyltin dilaurate as a cross-linking catalyst. I I . A process as claimed in any one of claims 1 to 10, where the grafted product is a cable or conductor insulation. 12. A process as claimed in any one of claims 1 to 11 , w h e r e the grafted product is a pipe.

Inventors: HARLIN ALI, HIRVENSALO MATTI, HARLIN, ALI, HIRVENSALO, MATTI
Applicants: HARLIN ALI, HIRVENSALO MATTI, NEXTROM HOLDING SA, HARLIN, ALI, HIRVENSALO, MATTI, NEXTROM HOLDING S.A.
Application year: 2000
Main classification: C08F255
IPC codes: B29C47, B29K23, B29K105, B29L9, B29L23, C08F255, C08J3, C08J5, H01B13