Method of making a structural element and the element so made

Abstract: The invention relates to a method of making a sandwich-type structural element with a mould consisting of at least two parts, which includes the steps of: a) applying a layer of gelcoat coating comprising unsaturated polyester, which contains the first free radical catalyst acting at a temperature in the range of from circa 0 to 35°C, to the inner surface of the mould, b) keeping the inner surface of the said mould so that the temperature of the gelcoat coating is in the range of from 0 and 35°C, in order to activate the first free radical catalyst and partially set the gelcoat coating, c) applying a layer of a fibrous reinforcing material comprising an unsaturated polyester and containing a second free radical catalyst, to the partially hardened layer of gelcoat coating, d) placing the solid core of the structural element on the layer of a reinforcing material, e) closing the mould around the solid core by joining together the two parts of the mould, f) keeping the mould so that the reinforcing material has the temperature required for activating the second free radical catalyst, in order to activate the second free radical catalyst and to set the reinforcing material, and g) opening the mould and removing the blank and finishing it if necessary. The invention includes the steps of I) step c) comprises applying a layer of a reinforcing material, which comprises a free radical catalyst acting at a temperature within the range of from 35 to 175°C, to a partially hardened layer of gelcoat coating, and ii) step f) comprises warming the mould so that the temperature of the reinforcing material is higher than the temperature of step b), which is in the range of from 35 and 175°C, in order to activate the said second free radical catalyst and to partially set the reinforcing material.

Description: The invention relates to a method of making a sandwich-type structural element, which includes the steps as per the Preamble of claim 1. The invention also relates to a sandwich-type structural element produced with the method, such as a door or a wall element with improved properties. Structural elements made of reinforced plastic, such as doors or wall elements, have long been in production. Often, the manufacturing method has been cold moulding or RTM in the mould at a standard temperature, which produces a rigid structural element. Such elements are generally used in wet and hygienic rooms, cold rooms, indoor swimming pools, hospitals, clean rooms of the medical and electronic industries. A cold moulded structural element, such as a structural element made in a mould by a cold moulding or RTM method, has the disadvantage of timing the setting process of the polyester. If the polyester sets too early, the surface of the product will contain blisters, and a long setting process causes a time-consuming production and weak strength properties. The production times of elements made with the methods mentioned above are also long. In addition, the high temperature resulting in an exothermal reaction, which causes weakening of colour and cracks in the product, complicates the production of larger articles. It is the object of the present invention to solve the problems mentioned above. This is achieved by the characterising part of claim 1. WO 94/07674 discloses a method as per the preamble of claim 1. The said document is quiet regarding step 1.7, i. e. with regard to placing of a solid core on a layer of a reinforcing material. According to one embodiment of the invention, in step 1.1, the unsaturated polyester of the gelcoat coating comprises a polyester, which has been selected from the reaction products of glycols, diacids or their anhydrides and unsaturated diacids and monoacids or their anhydrides, and an unsaturated monomer, selected for example from among the group of ethylenically unsaturated aromatic compounds and esters of acrylic acids. Such polyesters have been discussed in technical literature, for example, in the chapter Polyesters, unsaturated, of the Encyclopaedia of Polymer Science and Engineering, Mark, Bikales, Overberger, Menges, Second Edition (1988), Volume 12, pp. 256-290, which is enclosed here for use as reference material. The gelcoat coating typically comprises a polyester which is a reaction product of a glycol, such as ethylene glycol, diacid or its anhydride, such as phtalic acid or its anhydride, and an unsaturated diacid or its anhydride, such as maleic acid or its anhydride. A typical unsaturated monomer included in the gelcoat coating is an ethylenically unsaturated aromatic compound, such as styrene. In addition to an unsaturated polyester and a free radical catalyst, the gelcoat coating may be comprised of pigments and/or thixotropic modifying agents. According to one embodiment of the invention, step 1.1 of the method comprises applying a layer of gelcoat coating directly to the surface of the mould, preferably by injection. This produces an even, non-fibrous film on the inner surface of the mould. Such a film typically has a thickness in the range of from 300 µm to 500 µm. As the gelcoat coating is usually allowed to set at room temperature or at a temperature approximating room temperature, the first free radical catalyst of the coating is usually selected from among compounds formed of peroxides and accelerators. As mere normal peroxides do not decompose into radicals at temperatures below 35 °C and thus do not catalyse a hardening reaction between the polymer and an unsaturated monomer, the free radical catalysts used in the gelcoat coating are usually said compounds of peroxides and accelerators, which normally join together to form a redox pair. A redox pair preferably used as the said first free radical catalyst is the redox pair of hydroperoxide and a metallic salt accelerator. Examples of such preferred redox pairs include the redox pairs of methyl ethyl ketone hydroperoxide (MEKP) and cobalt octoate or cobalt naphthenate, and the redox pair of cumene hydroperoxide and manganese naphthenate or manganese octoate. The most preferable free radical catalyst of the gelcoat coating is the redox pair of the said methyl ethyl ketone hydroperoxide (MEKP) and cobalt octoate or cobalt naphthenate. Auxiliary accelerators can be added to the redox pairs mentioned above, so that the first free radical catalyst of step a) of the method of the invention comprises, in addition to the said redox pair, for example a tertiary aromatic amino accelerator, preferably dimethylaniline (DMA), diethylaniline (DEA), dimethylparatoxidine (DMPT). The concentration of the first free radical catalyst in the gelcoat coating, used in step 1.1 of the method of the invention, may vary considerably, and the desired setting speed and result are obtained by optimising the concentration of the free radical catalyst in the gelcoat coating. According to one embodiment of the invention, the preferred content of the first free radical catalyst is in the range of from 0.1 to 10%, preferably between circa 0.5 and 5% and most preferably between 0.75 and 2.5% by weight of the gelcoat coating. According to a wider interpretation of the method of the invention, in step 1.1 a layer of the gelcoat coating may be applied to any inner surface or surfaces of the mould. However, it is preferable to apply a layer of gelcoat coating to both of the inner surfaces of a two-part mould. In step 1.2 of the method of the invention, the temperature of the coating on the inner surface of the mould is kept so that the temperature of the gelcoat coating is in the range of from 0 to 35°C, in order to activate the free radical catalyst and partially set the gelcoat coating. The temperature is selected so that the said chosen first free radical catalyst is activated as desired, and a bridging reaction between the polyester and unsaturated monomer causes the the gelcoat coating to set. The inner surface of the mould is preferably kept so that the temperature of the gelcoat coating is in the range of from circa 20 to 35°C, more preferably between 30 and 35°C. Reinforced plastic usually sets at the higher temperatures set out above. Preferably the gelcoat coating is partially set so that the coating is stickily dry. Such setting usually takes from circa 0.5 to 1 hour, preferably circa 1 to 3 hours. Thus, the reinforcing material connects more easily to the layer of gelcoat coating on the inner surface of the mould. In step 1.3 of the method of the invention, a layer of a reinforcing material comprising an unsaturated polyester and fibre, and comprising a second free radical catalyst, is applied to the layer of partially cured gelcoat coating. The unsaturated polyester is preferably in liquid form. According to one embodiment, the application is carried out either by hand or by injection, preferably by laminating by hand, whereupon the gelcoat coating is alternately coated with the fibre of the reinforcing material, alternately with the unsaturated polyester component of the reinforcing material, comprising the said second free radical catalyst. A practicable method of application is resin transfer moulding (RTM). The fibre of the reinforcing material of step 1.3 is usually selected from among aramid fibres (Kevlar fibres), carbon fibres, and fibreglass. The preferred fibre is fibreglass, preferably in the form of a chopped strand mat and more preferably in the form of glass filaments. The unsaturated polyester of the reinforcing material of step 1.3 comprises a polyester and an ethylenically unsaturated monomer, whereupon the material sets as a result of a reaction between the unsaturated linkages of the polyester and the monomer, catalysed by the said second free radical catalyst. The polyester component is usually selected from among the reaction products of glycols, diacids or their anhydrides and unsaturated diacids and monoacids or their anhydrides. Such polyesters have been discussed in technical literature, for example, in the chapter Polyesters, unsaturated, of the Encyclopaedia of Polymer Science and Engineering, Mark, Bikales, Overberger, Menges, Second Edition (1988), Volume 12, pp. 256-290, which is enclosed here for use as reference material. The gelcoat coating typically comprises a polyester which is a reaction product of a glycol, such as ethylene glycol, diacid or its anhydride, such as phtalic acid or its anhydride, and an unsaturated diacid or its anhydride, such as maleic acid or its anhydride. Examples of typical reaction products include the reaction products of glycol, such as ethylene glycol, diacid or its anhydride, such as phtalic acid or its anhydride, and an unsaturated diacid or its anhydride, such as maleic acid or its anhydride. Examples of bridging unsaturated monomer components include ethylenically unsaturated aromatic compounds, such as styrene. A particularly suitable polyester is Neste A 300 E, which is a glycoline, a product based on orthophtalic acid and maleic anhydridine. The reinforcing material of step 1.3 of the method of the invention also comprises a free radical catalyst. This second free radical catalyst can be any catalyst acting at a temperature in the range of from circa 35 and 175°C, such as an azo compound similar to azo-bis-isobutyronitrile or an organic peroxide. Preferably, however, the said second free radical catalyst is an organic peroxide included in one of the following peroxide groups: diasyl peroxides, acetyl alkyl sulphonyl peroxides, dialkyl peroxide carbonates, tert-alkyl peroxide esters, tert-alkyl peroxide alkanols, oo-tert-alcyl o-alkyl mono peroxy carbonates, di(tert-alkyl peroxy) ketale, di(tert-alkyl) peroxides, tert-alkyl mono peroxy carbonates, and ketone peroxides. Peroxides included in these groups have been discussed for example in the chapter Peroxy Compounds, Encyclopaedia of Polymer Science and Engineering, Mark, Bikales, Overberger, Menges, Second Edition (1988), Volume 11, pp. 1-21, which is enclosed here for use as reference material. A typical suitable peroxide is a tert-alkyl peroxide alkanol, such as tert-butyl peroxy 3,3,5 -tri-methylhexanol. As the invention is based on the fact that the reinforcing material is cured at a higher temperature than the gelcoat coating, the selected second free radical catalyst must be thermally degradable, that is, become activated at the said higher temperature. The temperature of the selected peroxide, at which temperature the half life is 10 hours, is preferably between 50 and 172°C, more preferably between 60 and 120°C. Naturally, this requires that the tendency of the said free radical catalyst to heterolytically decompose into initiating radicals is taken into account. Examples of organic peroxides preferably used as a second free radical catalyst include dilauroyl peroxide, tert-butyl peroctoate, dibenzoylperoxide, 2,5-dimethyl 2,5-di(2-ethyl hexanyl peroxide)-hexane, tert-butyl perbenzoate, di-tert-butyl peroxide and dicumyl peroxide. Examples of especially preferable peroxides include lauroyl peroxide, tert-butyl peroctoate, benzoyl peroxide, and Trigonox 42PR. While peroxide groups and individual peroxides suitable for use as the second free radical catalyst have been listed above, it must be taken into account that intermixtures of these peroxides, and mixtures of these peroxides and various modifying agents, such as moderator materials or accelerators, can be used in the reinforcing material of the method of the present invention. The most important point is that the curing of the gelcoat coating at a lower temperature and the curing of the reinforcing material at a higher temperature are carried out according to the main idea of the invention. The concentration of the second free radical catalyst, used in step 1.3 of the method of the invention, may vary according to the required efficiency and speed of curing. According to one embodiment of the invention, the concentration of the said second free radical catalyst is in the range of from 0.1 to 10%, preferably from circa 0.5 to 5%, and more preferably from 1.0 to 2.5% by weight of the reinforcing material. The reinforcing material can be applied to any inner surface of a mould consisting of at least two parts, including surfaces not coated with gelcoat coating. However, in step 1.3), it is preferable to apply a layer of the said reinforcing material to a layer of gelcoat coating on both of the inner surfaces of a two-part mould. After a layer of the reinforcing material has been applie to a layer of the partially cured gelcoat coating, the mold is closed (step 1.4). The solid core comprises a solid substance, inert in relation to curing, which can be a powder, a crushed aggregate, a granulate, or consist of smaller or larger objects of varying forms or monolithic objects. According to a preferable embodiment, the solid core of the structural element is a laminar structure. Such laminae are generally used in hatches and doors, as well as panels, both outdoors and indoors. The solid core of the structural element, such as a laminar structure, can consist of any solid material, such as stone, ceramics, wood, mineral or fibreglass or plastic materials. The material used is preferably a fire-resistant material. It may also be a laminate or a mixture of these materials. It is preferable that the solid core material of the structural element is a cellular plastic selected from among the following group: epoxy resin, phenolic plastic, polyvinyl chloride (PVC) plastic, polyurethane plastic, urea formaldehyde resin, expanded elastomer, polyethylene plastic, polypropylene plastic, silicone plastic. Especially suitable are rigid cellular plastics, such as epoxy resin, phenolic plastic, polyvinyl chloride (PVC) plastic, polyurethane plastic, urea formaldehyde resin, preferably polvurethane plastic, polyvinyl chloride plastic, polyethylene plastic, and most preferably polyurethane plastic. Suitable cellular plastics have been described in, for example, the chapter "Cellular Materials" of the Encyclopaedia of Polymer Science and Engineering, Second Edition (1985), Volume 3, pp. 1-59, which is enclosed here for use as reference material. Generally, the solid core of the structural element, described in step d) of the method of the invention, is placed on the reinforcing material on one of the two inner surfaces of a two-part mould. After this, step e) of the method comprises closing the mould around the solid core. Thus, the two parts of the mould are preferably joined together so that the amount of air in the mould remains at an acceptable level. After the mould has been closed, step 1.5 comprises warming the mould so that the temperature of the reinforcing material is in the range of from 35 to 175°C. A more suitable temperature is in the range of from circa 35 to 100°C, preferably in the range of from 40 to 80°C. Even though the temperature is selected from the described range, the desired thermal degradation temperature of the second free radical catalyst has a deciding effect on the selection. At this temperature, the second free radical catalyst becomes activated and the reinforcing material sets. As the reinforcing material is cured at a higher temperature than the gelcoat coating, the partially cured gelcoat coating also sets finally at this stage of the method. The warming in step 1.5 of the method is preferably carried out with a simultaneous use of pressure. Such pressure is preferably in the range of from circa 1 to 10 N/cm 2 , more preferably from circa 1 to 5 N/cm 2 and most preferably from circa 1 to 3 N/cm 2 . According to one embodiment, the mould is warmed in step 1.5 by employing the pressure in short periods, for example, in periods of 10 to 15 seconds. In step 1.6 of the method of the invention, the mould is opened and the blank consisting of the core material surrounded by the reinforcing material and a layer of gelcoat coating is removed and trimmed ,if necessary. The finishing stage may include typical late stages of polyester laminating and compression moulding, such as: secondary setting trimming finishing and fittings. Fittings are used when doors, windows, hatches, etc., are made. After step 1.6 the solid core of the structural element is placed on the layer of a reinforcing element (step 1.7). As mentioned above, the invention also relates to a sandwich-type structural element made with the method set out above. Such elements include doors, hatches, panel parts, etc. The new improved method introduced here improves the strength properties of these elements, and the elements will not have blisters or other construction defects. Thus, the present invention solves the problem of timing the setting process of polyester in the making of a sandwich-type door or other structural element, as well as improves the strength properties and structure of the reinforced plastic laminate in comparison with a door made with a cold moulding or RTM process. The invention significantly shortens the manufacturing time of a door or other structural element. In the following, the invention will be described in detail with reference to an example, comprising a sandwich-type door structure, where a core is surrounded by a reinforced plastic laminate surface, which has a thickness of circa 2.5 mm. As core, for example, freon free polyurethanes or other hard expanded plastics can be used. The layer of reinforced plastic surrounds the sides of the door seamlessly, whereupon the door plate becomes tight and will not let water or dampness pass through. As a reinforced plastic structure, fibreglass-reinforced polyester can, for example, be used. A door structure corresponding to the invention can be made, for example, as follows. To the inner surfaces of door moulds, having a temperature of from 30 to 35°C, there is injected a layer of an unsaturated polyester gelcoat coating, which is set with MEKP peroxide with an accelerator content of 1.5%. After the gelcoat coating has become stickily dry, both sides of the mould are loaded as follows: both door halves are loaded with (a required number of) layers of chopped strand mats and unsaturated polyester, for example, Neste A 300 E, with added thermally degradable peroxide Trigonox 42PR (1.5-2%). Neste A 300 E is an unsaturated polyester resin, which is based on a reaction product of glycol, orthophtalic acid and maleic anhydride. Trigonox 42PR is a tert-butyl peroxy 3,3,5-trimethyl-ethanol. The core material of the door, into which the required holding blocks for mounting the fittings have been assembled, is placed in one side of the mould on the layers of fibreglass-reinforced plastic. The mould is closed and transferred into a flat press, whose compression force is increased by periods of 10 to 15 seconds up to a pressure corresponding to 1.9N/cm 2 . The temperature of the planes of the press is 60°C. After the mould has been placed on the press, the heat is gradually conducted from the press planes to the mixture of polyester and glass through the metallic mould. When the mixture reaches the temperature of 40°C, the thermally degradable peroxide initiates the setting process of the polyester. After the reinforced plastic laminate has set, the mould is removed from the press and opened, and the door is removed from the opened mould. The blank is trimmed and the door is finished and equipped with hinges, locks and other fittings.

Claims: A method of a making a sandwich-type structural element which comprises a solid core coated by a layer of fibrous reinforcing material and by an outer layer of gelcoat with a mould consisting of at least two parts, which includes the steps of: a) applying a layer of gelcoat comprising an unsaturated polyester, which contains a first free radical catalyst to the inner surface of the mould, b) keeping the inner surface of said mould at a first temperature of the gelcoat layer in order to activate the first free radical catalyst and partially set the gelcoat layer, c) applying a layer of a fibrous reinforcing material comprising an unsaturated polyester and a second free radical catalyst, acting at a temperature of 35 to 175°C, to the partially hardened layer of gelcoat, d) placing a solid core of the structural element on the layer of fibrous reinforcing material, e) closing the mould around the solid core by joining together the two parts of the mould, f) keeping the mould so that the layer of fibrous reinforcing material has a second temperature of 35 to 175°C in order to activate the second free radical catalyst and to set the fibrous reinforcing material, and g) opening the mould and removing the blank and finishing it if necessary, characterised in that i) said first free radical catalyst acts at a temperature in the range of from circa 0 to 35°C, and that ii) said first temperature is lower than said second temperature and in the range of from 0 to 35°C. A method according to claim 1, characterised in that step a) comprises applying a layer of gelcoat coating directly to the inner surface of the mould, preferably by injection. A method according to claim 1or 2, characterised in that the first free radical catalyst of step a) has been selected from among the compounds formed of peroxides and accelerators. A method according to claim 3, characterised in that the first free radical catalyst of step a) comprises a redox pair of an organic hydroperoxide and a metallic salt accelerator, preferably a redox pair of methyl ethyl ketone hydroperoxide (MEKP) and cobalt octoate or cobalt naphthenate, or the redox pair of cumene hydroperoxide and manganese naphthenate or manganese octoate, most preferably a redox pair of methyl ethyl ketone hydroperoxide (MEKP) and cobalt octoate or cobalt naphthenate. A method according to any preceding claim, characterised in that the concentration of the first free radical of the gelcoat layer is in the range of from 0.1 to 10%, preferably in the range of from circa 0.5 and 5% and most preferably in the range of from 0.75 and 2.5% by weight of the gelcoat layer. A method according to any preceding claim, characterised in that step a) comprises applying a layer of said gelcoat layer to both sides of the interior of a two-part mould. A method according to any preceding claim, characterised in that step b) comprises keeping the inner surface of the said mould so that the temperature of the gelcoat layer is in the range of from 20 to 35°C, preferably from circa 30 to 35°C. A method according to any preceding claim, characterised in that step b) comprises keeping the inner surface of the said mould, so that the temperature of the gelcoat coating is in the range of from circa 0 to 35°C, until the surface of the layer of gelcoat coating has become stickily dry. A method according to any preceding claim, characterised in that the fibre used in the fibrous reinforcing material of step c) is fibreglass, preferably in the form of a chopped strand mat A method according to any preceding claim, characterised in that in step c), the second free radical catalyst has been selected from among the following: lauroyl peroxide, tertiary butyl peroctoate, benzoyl peroxide, preferably Trigonox 42PR. A method according to any preceding claim, characterised in that in step c), the concentration of the second free radical catalyst is in the range of from 0.1 to 10%, preferably from circa 0.5 to 5%, most preferably from 1.0 to 2.5% by weight of the polyester component. A method according to any preceding claim, characterised in that step c) comprises applying a layer of said reinforcing material to the layer of gelcoat coating on both sides of the interior of a two-part mould. A method according to any preceding claim, characterised in that step d) comprises placing the inner core of the structural element, which is a laminar structure, on the layer of reinforcing material. A method according to any preceding claim, characterised in that step d) comprises placing on the layer of reinforcing material the solid core material, which is a rigid cellular plastic selected from among the following group: epoxy resin, phenolic plastic, polyvinyl chloride (PVC) plastic, polyurethane plastic, urea formaldehyde resin, preferably polyurethane plastic, polystyrene plastic, polyvinyl chloride plastic, polyethylene plastic, and most preferably polyurethane plastic. A method according to any preceding claim, characterised in that step d) comprises placing a solid core material on the reinforcing material on one of the two inner surfaces of a two-part mould. A method according to any preceding claim, characterised in that step e) comprises closing the mould around the solid core material by connecting tightly the two parts of the mould so that the amount of the air in the mould remains at an acceptable level. A method according to any preceding claim, characterised in that step f) comprises warming the mould so that the temperature of the reinforcing material is in the range of from circa 35 to 100°C, preferably from 40 to 80°C, to activate the second free radical catalyst and to cure the reinforcing material. A method according to any preceding claim, characterised in that step f) comprises warming the mould with the simultaneous use of pressure, which is preferably in the range of from 1 to 5 N/cm 2 , more preferably from 1 to 3 N/cm 2 . A method according to any preceding claim, characterised in that step f) comprises warming the mould by employing the pressure in short periods, for example, in periods of from 10 to 15 seconds. A sandwich-type structural element, such as a door or a panel, preferably a door, which is produced with the method according to any preceding claim, which structural element has improved strength properties and does not have blisters.

Inventors: AHO ESKO, AHO, ESKO
Applicants: MUOVILAMI OY
Application year: 1996
Main classification: B29C70
IPC codes: B29C70, B29C37