Method of forming a welded joint between polyolefinic members

Abstract: A polyethylene pipe 10 is joined by a welded joint to a cross-linked polyethylene coupler 14 which has been expanded to make the coupler heat recoverable. Radiant energy (eg light in the visible spectrum or infra-red light) illuminates the outer surface of the coupler 14 supplied by an array of 150W reflector lamps 20. The coupler 14 shrinks and is forced into engagement with the pipe 10 and the pipe 12 to join the pipes. A fusion welded joint is formed between the coupler 14 and each pipe 10, 12. In an alternative a pipe is drawn down and inserted in an aperture through the wall of another pipe. A lamp is inserted in the drawn down pipe and the radiant energy causes the pipe to expand into engagement with the edge of the aperture and to be joined to it by a welded joint.

Description: The invention relates to methods of forming welded joints between polyolefinic members. The invention is applicable to members made of polyethylene and other polyolefins, such as polypropylene and polybutylene for example. The invention particularly relates to forming a welded joint between members where one of the members is made of cross-linked material. It is a significant property of cross-linked material that, on reaching what would be the normal melting temperature range of, for example, polyethylene, it acquires a rubbery but still solid character. This occurs because the polyethylene crystalline structure breaks down but there remains a three dimensional polymer network due to the presence of molecular cross-links. One further consequence of this 'melting' behaviour of cross-linked polyethylene is that its optical properties change. Unpigmented cross-linked polyethylene has a milky-white appearance caused by light scattering by the crystallite structures within the material. At a temperature of about 130°C these crystallites collapse and the basic polymer structure enters the amorphous melt phase. In this condition the material is glass clear. The observation of this change in optical character prompted the idea of using radiant energy as a means of welding the cross-linked polyethylene to non-cross-linked material. Such fusion welds essentially form when surfaces of the two materials are brought into contact at a temperature exceeding their crystalline melting temperatures. To form welds between cross-linked polyethylene undergoing deformation changes, such as shrink to fit couplings, it is necessary to heat through the entire volume of the material so that the bulk of the material exceeds the crystalline melting temperature range. The need to heat through the entire wall of a cross-linked product requires considerable heat input which, by conventional electrical resistance wire, or hot iron techniques, is a slow process, because the material is a poor conductor. Furthermore, to drive heat energy through a wall by conduction requires a temperature gradient. For instance, to raise an inner wall temperature to 140°C to effect a weld may require an external surface to be maintained at temperatures in excess of 250°C, where degradation can occur. The idea of using radiant energy to simultaneously cause through-heating and interface welding overcomes these problems. The cross-linked polyethylene absorbs the electromagnetic radiation at optical and infra-red frequencies until it melts and, as it goes clear, a greater amount of the radiation passes through. If the surface to which it is to be welded is itself pigmented, there will be increased energy absorption at the interface. The major thermal advantages of this concept are: the heat-through time is shorter, the external surface temperature rise is not excessive, and total energy absorbed is lower for a given welding temperature so that cooling requirements are reduced. A major practical advantage is that equipment to obtain the welding process can be made simple and rugged. Furthermore, the energy can be supplied as radiant energy in the immediate vicinity of the weld, regardless of the location of the site at which welding takes place. US Patent No. 3953059 described a method of making a pipe connection of a first pipe high density polyethylene to the wall of a second pipe also made of high density polyethylene. A hole is formed through the wall of the second pipe and is of smaller diameter than the diameter of the first pipe. The connection end of the first pipe was flared outwardly first of all. Then, the same end was cold swaged using a swaging die, to reduce the diameter of the pipe end. The reduced diameter end of the polyethylene pipe was then pushed through the hole in the second pipe. The cold-swaged end of the first pipe recovered, owing to its mechanical memory, towards the original diameter of the first pipe and so became a firm fit within the hole in the second pipe wall. Finally, sealant or adhesive is applied around the first pipe where it joins to the second pipe. The two pipes in the method described in US Patent No.3 3953059 were not joined by a fusion welded joint. The method is described as being suitable for connecting sewer pipes but it is clearly not suited for making welded joints suitable for use in gas or water pipe systems, for example. French patent specification No. FR-A-2252191 discloses a method of forming a joint between two tubes of crosslinked polymers. One member is reduced in diameter and, when heated, recovers towards the other member and forces itself into engagement with it. European patent application No EP-A-0102919 discloses a similar method except that only an expanded member is disclosed which on heating reduces in diameter to grip the second member. In both FR-A-2252191 and EP-A-0102919 the joint formed between the two members relies on high frictional forces to keep the joint together. There is no teaching that a welded joint can be made. On the contrary, it is stated in each specification that welded joints between two members of crosslinked material are impossible. By contrast, the present invention achieves a true welded joint between a first non-crosslinked member and a second crosslinked member. According to the invention, a method of forming a joint between a first member of polyolefinic material and a second member of polyolefinic material, the second member having been reduced in diameter, the members being arranged so that a surface of one member encircles and is opposed to the outer surface of the other member, the surfaces being heated so that at least in the region of said opposed surfaces said second member recovers towards said first member and forces itself into engagement with the opposed surface of said first member is characterised in that a welded joint is made by the method between the two members, said first member being a pipe made of non-crosslinked material with a through aperture in its wall and said second member also being a pipe but made of crosslinked material and the surface of said second member remote from said opposed surfaces of the members is subjected to radiant energy so that said second member recovers towards said first member and said surface of the first member and said surface of the second member reach a temperature at which a fusion bond occurs. An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a three-dimensional view of the embodiment showing the second member as a drawn down pipe extending into an aperture in the sidewall of a pipe forming the first member; and Figures 2 and 3 are scrap views of the drawn down pipe and part of the sidewall before and after the joint has been made, respectively. In the second embodiment shown in Figure 1 and 2 the first member was a polyethylene pipe 30 of 125 mm outside diameter. The pipe 30 had an aperture 32 of 20 mm diameter cut through its sidewall and the aperture 32 was defined by the edge surface 34 of the aperture 32. The second member was a 20 mm cross-linked pipe 40. The pipe 40 was drawn down by extension in a tensile testing machine to a portion 42 having an outside diameter of 10 mm. The drawn down portion 42 was inserted in the aperture 32 and a 12V, 20W quartz-iodine projector lamp 50 was inserted in the tube 40. The inside surface of the pipe 40 was illuminated and eventually the pipe portion 42 expanded into contact with the edge 34. The lamp 50 could then be pushed into the portion 42. The portion 42 opposite the edge surface 34 was forced against that edge and a fusion welded joint was formed as shown in Figure 3. When the portion 42 expanded it went from opaque to clear and the light from the bulb 50 was transmitted more efficiently through the polyolefinic material to the interface at the opposed surfaces of the portion 42 and the edge surface 34. The pipe 40 in the finished joint was almost the same diameter throughout. The first member, the pipe 30, was slightly upset at 52 and 54 which are annular "beads" surrounding the pipe 40 and 42. The joint just described is useful in gas pipes where the pipe 30 is the main and the pipe 40 is a service pipe extending from the main to a consumer. The pipe 40 can be pushed through an existing cast iron service pipe (not shown) where it is desired to upgrade the service connection by the provision of a polyethylene new service pipe. The bulb 50 can be pushed along the service pipe from the consumer end or can be pushed into the open end of the portion 42 within the pipe 30 by a machine especially designed to effect the welded joint from within the main. When the joint has been made, the machine can be used to machine away the protruding end of the portion 42 within the main 30, as shown in Figure 3. The invention is applicable to larger pipes than those given in the above examples. For example, the main pipe may have a diameter of 1 m or more and branch pipes of 125mm, for example, can be jointed to the main pipe.

Claims: A method of forming a joint between a first member(30) of polyolefinic material and a second member(40) of polyolefinic material, the second member(40) having been reduced in diameter, the members being arranged so that a surface(34) of one member encircles and is opposed to the outer surface(42) of the other member, the surfaces being heated so that at least in the region of said opposed surfaces said second member(40) recovers towards said first member(30) and forces itself into engagement with the opposed surface(34) of said first member(30) characterised in that a welded joint is made by the method between the two members(30,40), said first member(30) being a pipe made of non-crosslinked material with a through aperture(32) in its wall and said second member(40) also being a pipe but made of crosslinked material and the surface of said second member(40) remote from said opposed surfaces(34,42) is subjected to radiant energy so that said second member(40) recovers towards said first member(30) and said surface(34) of the first member(30) and said surface(42) of the second member(40) reach a temperature at which a fusion bond occurs. A method according to claim 1, in which said radiant energy is light or infra-red radiation. A method according to claim 1 or claim 2, in which said radiant energy is light supplied by a bulb inserted inside said second member.

Inventors: SMART ANDREW, STAFFORD TREVOR GEORGE, SMART, ANDREW, STAFFORD, TREVOR GEORGE
Applicants: BRITISH GAS PLC, BG PUBLIC LIMITED COMPANY
Application year: 1992
Main classification: B29C65
IPC codes: B29C35, F16L47, B29K23, B29C65, B29L31