WO1999050059A1

WO1999050059A1 - Sandwich elements consisting of plastic outer layers and a core

Info

Publication number: WO1999050059A1
Application number: PCT/EP1999/002370
Authority: WO
Inventors: Wilfried GRÜDL; Manfred Gruber; Hermann Jacob; Helmut MÖSLEIN; Roland Schwall
Original assignee: Menzolit-Fibron Gmbh; KÖGEL Fahrzeugwerke AG
Priority date: 1998-03-30
Filing date: 1999-03-30
Publication date: 1999-10-07
Also published as: DE19914327A1

Abstract

The invention relates to sandwich elements consisting of plastic outer layers (3) and a core (4), a method for producing same and their use. The plastic outer layers (3) of the sandwich elements are made of a low-pressure sheet moulding compound (SMC) and partly or fully envelop the core (4).

Description

- 1 -

Sandwich elements made of plastic cover layers and a core

The invention relates to sandwich elements made of plastic cover layers and a core, processes for their production and their use.

The state of the art for trucks is box bodies made of sheet steel, of a steel-wood composite construction, and for temperature-controlled containers, in particular bodies made of sandwich elements. Such sandwich elements have a cover layer and a core on the outside and inside. The cover layers can consist of metal or plastic materials (DE-A 39 16 065).

The sandwich elements used for insulated containers (temperature-controlled containers) should preferably have a high degree of rigidity with a low weight. They must be able to cope with the load caused by forklifts, in the side wall area from the load caused by storey loading and in the roof area from the load caused by the suspension of loads. In order to be able to be used for the transport of frozen products, for example, they must ensure that the low transport temperature is maintained at high outside temperatures.

In addition to the sandwich elements with metal cover layers, plastic sandwich elements with a length of up to approximately 16,000 mm in various widths and thicknesses are known. The core of each of these elements is partially or completely enclosed by plastic cover layers. There are no metallic thermal bridges in these elements. These sandwich elements are self-supporting and achieve a high payload for the entire vehicle due to their low weight. Depending on the dimensions of the container, they are manufactured in customized shapes.

These sandwich elements are manufactured using the so-called wet-on-wet process. The molds are cleaned and adjusted to the required part size. First, the mold and the mold cover are gelcoa- - 2 -

advantage. The various glass fiber mats (depending on the type of stress different fiber structure) as well as the insulating foam are tailored to the vehicle. Then an unsaturated polyester resin with hardener is applied. Because of the small amount of hardener to be added there are always problems with the correct dosage. The resin is poured into the mobile tundish and distributed in the prepared form. Because the distribution is controlled manually, the amount of resin distributed varies greatly from element to element.

After the resin has been distributed, a surface fleece is placed on top of the surface fleece, several glass fiber and covering fabric mats, usually as a combination mat. After each layer, resin must be applied manually with the tundish. A complex extraction system must ensure that the MAK values are adhered to. Then the foam sheets, including wood or metal reinforcements, are cut to fit the vehicle.

Fabric mats are placed between the individual foam sheets to reinforce the wall. Each of these mats must also be soaked in resin by hand.

The inserted foam sheets are covered with resin from the tundish. Glass fiber and fabric mats are put on again as a top layer (combination mat). Resin is poured over the completely filled form and a cover fleece is applied. After a final pouring of resin, the mold is closed with the gel-coated lid.

The closed mold is inserted into a hot plate press and the resin is cured under pressure (approx. 1 bar) and temperature (approx. 60 ° C).

This method is very complex and also has other disadvantages. On the one hand, a high shrinkage can be observed when curing. This creates a rough surface. The liquid resin consumption is very high because - o -

To ensure process reliability, an excess of liquid resin of about 15% must be added. This production releases styrene. The MAK values must be checked and adhered to. Another crucial disadvantage is the lack of automation potential with a high value chain.

The object of the present invention was therefore to provide sandwich elements which can be produced by a simple method and which do not have the disadvantages of the sandwich elements known from the prior art.

The object on which the invention is based was achieved by sandwich elements with the features of the main claim. Embodiments are preferably characterized in the subclaims. If one speaks in connection with the invention of "core", it means, unless otherwise specified, any construction or material that keeps the two plastic cover layers at a distance.

The special feature of the sandwich elements according to the invention is that low-pressure SMC is used instead of the cover layers previously produced in situ in such cases and the reinforcements made of glass fiber reinforced plastic (GRP).

SMC stands for "Sheet Molding Compound" and is the English name for a resin mat according to DIN 16913. SMC is generally used to refer to a flowable resin mat with isotropic properties and random fiber distribution in the mat plane. SMC is a flat semi-finished product based on unsaturated polyester resin and / Glass fibers are usually used as reinforcement fibers, but other materials are also suitable for reinforcement. A typical SMC recipe consists of approx. 30% polymer, approx. 30% filler and approx. 30% fibers, the rest settles from aggregates, such as color pigments, hardeners, dispersing agents and similar substances. SMC is manufactured from a resin master paste and reinforcement - 4 -

fibers. The resin base paste is applied to two carrier foils. These carrier foils are drawn through the SMC machine and thereby transport the resin paste onto which the reinforcing fibers are sprinkled or placed. After the reinforcing fibers have been applied, both foils are pressed together so that a kind of sandwich is created. This sandwich is pulled through an impregnation line, which uses pressure and flexing movements to ensure that the fibers are evenly wetted with the resin base paste. At the end of the machine, the semi-finished product is wound on rolls. A ripening process that can be initiated in a chemical and / or physical manner is of crucial importance. After this maturation process, the SMC can be processed further.

After the carrier films have been removed, the SMC is usually processed into molded parts in heated steel tools. The mold temperature is generally about 135 to 155 ° C.

The decomposition of the hardener is started by the mold temperature and the molding compound undergoes an exothermic crosslinking reaction. When processing in press tools, the material is usually dosed manually. Maintaining precise dosing quantities is generally very important for problem-free production. The SMC is usually prepared and weighed according to a cutting plan. Weighing out the blank serves as a control and enables a possible correction of the blank. Then the SMC is placed on the tool and the tool is closed. The pressure is built up and held until the molded part is completely hardened.

In the case of the standard SMC used up to now, in addition to the starting temperature of approx. 135 to 155 ° C, a relatively high pressure of at least 50 to 100 bar is required. This pressure level is necessary in order to distribute the standard SMC, which is flowable at high temperatures, evenly in the mold. However, these high pressures cannot be used to manufacture some components. - 5 -

The advantages of using SMC are obvious. The industrial production of the SMC enables a high degree of reproducibility in the molded parts made from SMC. When using SMC, the use of liquid resins can be omitted. In addition, the tools can be loaded mechanically and automation of the entire manufacturing process is possible.

However, the high pressures required for the standard SMC cannot be used to manufacture the generic sandwich elements. In order to be able to use the advantages of the SMC technology for the sandwich elements, the use of a special, new low-pressure SMC was necessary. Such a low-pressure SMC could be realized by developing a special SMC recipe.

The SMC according to the invention contains as a base paste a mixture of at least one unsaturated polyester resin and / or vinyl ester resin as the base component, at least one shrinkage-compensating additive, at least one hardener and additives. As the unsaturated polyester resin, the base paste preferably contains at least one resin based on maleic acid, isophthalic acid, orthophthalic acid, terephthalic acid, tetrahydrophthalic acid, adipic acid or bisphenol-A, as vinyl ester resin preferably at least one resin based on bisphenol-A or novolac or mixtures of these resins . Saturated polyesters or products based on polyurethane, polyvinyl acetate, polymethyl methacrylate, polystyrene or styrene-butadiene or mixtures of these additives, for example, are added to this base component as a shrinkage-compensating additive. Suitable components are known per se to the person skilled in the art and are commercially available for semi-finished SMC products.

Organic peroxides that start at the lowest possible temperatures are used as hardeners. Examples include t-butyl per-2-ethylhexanoate, bis- (4-t-butylcyclohexyl) peroxydicarbonate, benzoyl peroxide and methyl isobutyl ketone peroxide. Mixtures of these peroxides, which are known per se in SMC technology, can also be used. - 6 -

As further components, this resin base paste also contains the usual additives known to those skilled in the art, such as additives to ensure storage stability, additives for deaeration, thixotropic agents, thickening aids, fillers and pigments, and, depending on the application, adhesives and / or release agents. These additives include, for example, the known components which stabilize peroxide-containing UP resin batches, the known foam-inhibiting polymers, silicas such as, for example, aerosils, calcium carbonate, aluminum trihydrate, kaolin and barium sulfate.

The stock paste according to the invention contains 10 to 34% by weight, preferably 17 to 34% by weight of unsaturated polyester resin and / or vinyl ester resin, 1 to 15% by weight, preferably 1.5 to 13.5% by weight, of shrinkage-compensating additive, and at least one hardener in amounts which exclude undercuring, preferably 0.1 to 2% by weight hardener. Up to 0.2 wt.%, Preferably 0.03 to 0.15 wt.% Inhibitor, up to 0.2 wt.%, Preferably 0.03 to 0.15 wt.% Deaerator can be used as additives , up to 2% by weight, preferably 0.15 to 1% by weight thixotropic agent, up to 0.2% by weight, preferably 0.015 to 0.1% by weight thickening aid, 1 to 30% by weight , preferably 4 to 24% by weight of fillers and up to 4% by weight, preferably 0.15 to 2% by weight of pigment substances, and depending on the application 1 to 15% by weight, preferably 1.5 to 13, 5 wt .-% adhesive and / or 1 to 2 wt .-%, preferably 1, 2 to 1, 8 wt .-% release agent are added.

These components are dispersed intensively in appropriate dissolvers. The necessary thickening can be initiated both chemically and physically. In chemical thickening, the resin paste is mixed with a thickener. Alkali and / or alkaline earth oxides or hydroxides and / or isocyanates can be used as thickeners. Magnesium oxide, magnesium hydroxide and / or calcium hydroxide may be mentioned by way of example. 0.1 to 2% by weight, preferably 0.15 to 1.7% by weight, of thickening agent can be added to the stock paste. The resin paste according to the invention is applied to two carrier foils. These are then pulled through the SMC machine as usual - 7 -

sprinkled or covered with the reinforcing fibers. Depending on the required strength, 10 to 70% by weight, preferably 20 to 60% by weight, of reinforcing fibers are embedded. The reinforcing fibers can be used as a mat, fabric, fleece, continuous fibers or as clippings or as combinations of these shapes. Possible reinforcing fibers are: glass fibers, carbon fibers, kevlar, aramid, polymer fibers or fibers made from renewable raw materials such as flax, hemp, banana or coconut. Combinations of these materials can also be used. The rest of the process is the same as for standard SMC. After the ripening process (chemical and / or physical thickening), the low-pressure SMC according to the invention thus produced can be processed further.

With the help of the SMC recipe according to the invention, it was thus possible for the first time to provide a low-pressure SMC which can be processed at low temperatures and even at a pressure of 1 to 2 bar.

This low-pressure SMC is used to produce the sandwich elements according to the invention. For this purpose, the shape required for the elements is cleaned as usual and adjusted to the required dimensions. The mold and the mold cover are provided with a release agent in order to be able to remove the finished element from the mold without problems. A gel coat (fine-layer resin) with a self-separating effect is preferably used. It consists, for example, of unsaturated, thixotropic polyester resin based on dicarboxylic acid neopentyl glycol in a highly reactive setting and is characterized by high water resistance and mechanical resistance. Alternatively, the gel coat can also be integrated in the low-pressure SMC according to the invention.

Instead of the gel coat, a single-layer or multilayer film or lacquer film can also be inserted into the mold or into the mold cover. The insertion of the film can expediently be facilitated by suction using a vacuum. If a multilayer film is used, it expediently consists of a carrier film with an applied layer of colored lacquer and clear lacquer. Even more - 8th -

Other layers of paint can be applied. The sandwich element can be individually designed using such a single-layer or multilayer or lacquer film. In this way, painting, so to speak, can be integrated into the manufacturing process of the sandwich elements according to the invention.

After a surface fleece, for example made of glass fibers or polyester fibers and impregnated with UP resin, is inserted into the mold as described above to compensate for the structure, the first layer of the low-pressure SMC webs is inserted. The foam sheets tailored to the vehicle, including any necessary reinforcements made of wood, metal or reinforcements made of low-pressure SMC sheets, are inserted into the mold. Subsequently, further low-pressure SMC webs and the cover fleece are put on and the mold is coated with the release agent, gel coat, single or multi-layer or. Lacquered lid closed. The foam sheets, which serve as spacers and insulation, should have a low density, high pressure resistance, low thermal conductivity and should also be environmentally friendly. Polyurethane plates or plates made of thermoplastic material are advantageously used as foam plates.

In principle, a multi-layer low-pressure SMC structure of the sandwich elements is of course also possible, for example by providing not only cover layers and reinforcements made of low-pressure SMC, but also intermediate layers or other structures made of low-pressure SMC.

The closed mold is inserted into the hot plate press and at a temperature which depends on the hardener used and is sufficient for the hardener to start, preferably at about 60 ° C., a pressure which is sufficient for the internal connection of the foam plates to the low pressure SMC is sufficient, preferably pressed at a pressure of 1 to 2 bar.

Overall, the emission for the method for producing the sandwich elements according to the invention by using the low-pressure SMC - 9 -

reduced by hydrocarbons. With the use of the low pressure SMC it is possible for the first time to obtain a significantly improved surface structure. In addition, the amount of waste liquid resin is significantly reduced. The new process offers a high level of automation compared to the state of the art.

Figure 1 shows the schematic structure of a sandwich element according to the invention. The numbers mean: (1) gel coat; (2) surface fleece, (3) hardened low pressure SMC, (4) foam sheet.

The sandwich elements according to the invention can be used in a variety of ways. For example, worktops, residential units, for example residential containers, cold stores, box bodies, cooling units, refrigerated bodies, cold rooms, sanitary rooms, for example wet cells, isolation cells or clean rooms, can be produced from the sandwich elements according to the invention. Other areas of application are wagon, bus and shipbuilding. They are preferably used in commercial vehicle construction and in the manufacture of containers.

Claims

- 10 patent claims

1. Sandwich elements made of plastic cover layers and a core, characterized in that the plastic cover layers are essentially made of low-pressure SMC.

2. Sandwich elements according to claim 1, characterized in that the low-pressure SMC contains a mixture of at least one unsaturated polyester resin and / or vinyl ester resin, at least one shrinkage-compensating additive, additives and reinforcing fibers.

3. Sandwich elements according to claim 1 or 2, characterized in that the low-pressure SMC as unsaturated polyester resin at least one resin based on maleic acid, isophthalic acid, orthophthalic acid, terephthalic acid, tetrahydrophthalic acid, adipic acid or bisphenol-A, as vinyl ester resin, preferably at least one resin based on bisphenol-A or novolack or mixtures of these resins and at least one vibration-compensating additive selected from the group consisting of saturated

Contains polyester, or products based on polyurethane, made of polyvinyl acetate, polymethyl methacrylate, polystyrene, styrene butadiene or mixtures of these additives, additives and reinforcing fibers.

4. Sandwich elements according to one or more of claims 1 to 3, characterized in that the core contains foam sheets, preferably polyurethane sheets or sheets made of thermoplastic material.

5. Sandwich elements according to one or more of claims 1 to 4, characterized in that reinforcing elements are integrated in the core. - 1 1 -

6. Sandwich elements according to one or more of claims 1 to 5, characterized in that the core contains reinforcing elements made of wood, metal or low-pressure SMC.

7. Sandwich elements according to one or more of claims 1 to 6, characterized in that the low-pressure SMC consists of a fiber structure

Contains mats, fabrics, fleece, continuous fibers or clippings or combinations of these shapes.

8. Sandwich elements according to one or more of claims 1 to 7, characterized in that the low-pressure SMC contains a fiber structure made of glass, carbon, Kevlar, aramid, polymer fibers or fibers from renewable raw materials or mixtures of these fibers.

9. Sandwich elements according to one or more of claims 1 to 8, characterized in that the low-pressure SMC is provided on the side facing away from the core with a gel coat, a single or multi-layer or a lacquer film.

10. sandwich elements according to one or more of claims 1 to 9, characterized in that the multilayer film consists of a carrier film with an applied layer of colored lacquer and / or clear lacquer.

1 1. A process for producing a sandwich element according to one or more of claims 1 to 10, characterized in that the core is partially or all-round surrounded by low-pressure SMC and is pressed into a sandwich element from a mold and mold cover under suitable pressure and temperature .

12. A method for producing a sandwich element according to claim 11, characterized in that the mold and the mold cover are gel-coated. - 12 -

13. A method for producing a sandwich element according to claim 11 or 12, characterized in that the mold and the mold cover are provided with a release agent.

14. A method for producing a sandwich element according to one or more of claims 11 to 13, characterized in that a single-layer or multilayer or lacquer film is inserted at least into the mold.

15. A method for producing a sandwich element according to one or more of claims 11 to 14, characterized in that the sandwich element is pressed in a mold from the mold and mold cover at about 60 ° C and a pressure of 1 to 2 bar.

16. Use of the sandwich elements according to one of claims 1 to 10 for the production of worktops, residential units, for example residential containers, cold stores, box bodies, cooling units, refrigerated bodies, cold rooms, sanitary rooms, for example wet cells, isolation cells or clean rooms.

17. Use of the sandwich elements according to one of claims 1 to 10 in bus, wagon and shipbuilding.