CA1184106A

CA1184106A - Process for producing cured, curved moulded articles

Info

Publication number: CA1184106A
Application number: CA000384778A
Authority: CA
Inventors: Urs Gruber; Bernd Patzeli
Original assignee: Ciba Geigy AG
Current assignee: Novartis AG
Priority date: 1980-09-01
Filing date: 1981-08-28
Publication date: 1985-03-19
Also published as: EP0047229A3; JPS5775848A; ES8206278A1; DE3166337D1; ES505068A0; EP0047229A2; EP0047229B1

Abstract

Process for producing cured, curved molded articles Abstract A process for producing cured, curved molded articles having a sandwich structure by heating-and press-molding a composite material comprising an inner layer of a solid material having hollow spaces therein and covering layers consisting of a B-stage epoxide resin/curing agent mixture reinforced with a fiber mat or fiber fleece. The process being conducted at pressure of from 0.5-1.5 N/mm2 and at temperature of from 100-200°C.

Description

~13C40/t Canada Process for producing cured, curved mo ed articles The invention relates to a process for producing cured, curved molded articles having a sandwich structure by heating and press-molding a composite material comprising an inner layer of a solid material having hollow spaces therein, and covering layers rein~orced with fiber mat or fiber fleece.

Composite materials in sandwich constructions are used in industry in many ways. Depending on the manner in which they are made up, they enable products to be produced which have high mechanical strength, good thermal or electrical insulating properties and low density, and they lead to economy in the use of materials. Products of this type can be produced for example by the covering of both sides of hard foam materials, such as polyurethane, polyvinyl chloride, or soft woods such as balsa wood, or honeycomb materials made from paper or fibres, with fibrous fabrics.
Only flat or at most slightly curved sandwich structures can however be obtained b~ this means.

There is known from the U.S. Patent Specification No.
4,0~8,477 a process for pxoducing sandwich structures which can have a greater curvature. Flexible open-cell foams are used in this process as the inner part~ the pores being for the most part filled with resin, which becomes rnostly squeezed out afterwards when -the inner part i5 compressed together with the covering layers of fibre material. Since the foam is compressed in this operation, with the smaller cavities remaining being still filled with resin, there is obtained a moulded product having a relatively high density, approximately of the order of magnitude of 700-1000 kg/m3.
A further process, by which highly and/or spherically curved sandwich structures can be produced, is known from the German Offenlegungsschrift No. 2,740,647. In this process, an inner layer, which is in the form of an absorbent felt or fleece layer containing a pasty curable synthetic resin compound and having openings likewise filled with resin, is provided with thin resistant covering layers, particularly of glass-reinforced synthetic resin, and the whole is shaped wet-in-wet and cured. Since the cured moulded product is produced in one operation, the employed resin moulding material, mixed and pro-portioned, has firstly to be applied, the pasty resinous compound then having to be carefully pressed into the felt layer. This impregnated material cannot be stored, and it likewise has a relatively high density, since the inner layer is impregnated with resin or is partially filled.
Finally, there is known the covering on one side of oam sheets formed from thermoplastic polymethacrylimide, with the bending of them being carried out in such a manner that the covering is on the convex side. Metal sheet, for example aluminium sheet~ is used as the covering layer. The curved products can then be laminated on the compressed inner side. Apart from ~he fact that the extent of curvature has to be governed by the type of covering-layer material, since this has to absorb the tensile forces, the covered matPrials cannot be bent towards both sides without harmful buckling of the covering layer occurring and the foam plastics material tearing.

It has now been found that a composite material comprising an inner layer of a solid material having hollow spaces therein and covering layers re-inforced with fiber mats or fiber fleeces can be converted by heat and pressure to rigid articles containing any desired curvatures.
Therefore the present invention relates to a process for producing cured, curved molded articles which comprises the steps of (1) placing a storable composite material into a heated molding press, said composite material having a sandwich structure which comprises an inner layer of a solid material having hollow spaces therein and, adhered to each of the two surfaces of said inner layer, a covering layer consisting of a B-stage epoxide resin/curing agent mixture reinforced with a fiber mat or fiber fleece; (2) closing said press just until first contact is made with the projecting partof said covering layers, the area of contact corresponding to those regions which will be curved in the molded article, and maintaining said closed position until the covering layers are softened over the zones of contact; (3) fully closing said press, and shaping and curing said composite material at pressure of from 0.15-1.5 N/mm2 and at temperatures of from 100-200C; and (4) opening the press and re-moving the cured, curved molded article.
Suitable as inner layers are hard thermoplastic or duroplastic foam plastics, for example from polyacrylimide or polymethacrylimide, polyurethane, hard polyvinyl chloride or polyisocyanurate, or those based on epoxide resins.
They have a minimum thickness of about 3 mm in order that their foam structure is fully effective. As a rule, they are 4 to 8 mm thick; they can however, de-pending on the purpose of application, have a greater thickness~ for example 1 cm or more. Also suitable as inner layers are in particular honeycomb materials formed from paper or aluminium; and other fibre materials too are suitable.
Fibre mats, especially glass-fibre mats, are used as reinforcing .,, ,,i material in the two covering layers bonded to the inner layer. In these mats, the individual fibres are spaced out and slightly felted. The fibres can also ~3a-, ~ " ~

~ ~ 4 ~ ~

be preferentially orientated in one direction. It is also possible for the covering layers to contain, instead of short glass fibres which are as a rule 2-10 cm long, randomly placed continuous fibres. Suitable fibres apart from glass fibres are also synthetic fibres, such as those from polyesters, polyamide~ inorganic material or carbon.
There arP preferably used several covering layers consisting of identical or different types of fibres, for example layers of polyester fibre 1eeces alternating with glass filament mats. The fibre ma~erial is impregnated with epoxide resins and curing agents, as well as optionally accelerators, and pre-cured so that the resin still has thermoplastic properties (B stage~.
Suitable epoxide resins are for example liquid resins based on bisphenol-A or -F, or of polyglycidylated aromatic diamines, for example 4,4'-bis-(N,N~diglycidyl-aminophenyl)-methane. The curing agents used can be for example amines, such as 4,4'-diaminodiphenyl-methane, or BF3-amine complexes, such as boron trifluoride-monoethyl-amine, or dicyandiamine. It is also possible to use acid anhydrides, such as hexahydrophthalic anhydride.
There are optionally also added curing accelerators, for example those based on tertiary amines, such as benzyl-dimethylamine, tris-(dimethylaminomethyl)-phenol or N-(p-~hlorophenyl)-N',N'-(dimethyl)-urea.
The covering layers are laminated onto the porous inner-layer, and the impregnating of the fibre mats or of the fibre fleece can be performed in the same operation or beforehand. In the latter case, the pre-curing of the resin by heating can likewise be carried out before lamination~
storage-stable prepregs being then obtained. Otherwise, the formed article, coated wet-in-wet on both sides, is heated to effect pre-curing of the covering layers, for example for 5-10 minutes at 80-150C, depending on the ~ ?~

employed resin/curing agen V (accelerator) system.
The two covering layers need not be identical. They can contain fibres differing from one another, and can consist of a different number of fibre layers impregnated with epoxide resin. It is possible to apply as the uppermost layer, on one side or on both sides, additionally a highly resinous, optionally dyed, gel-like coating, which can be reinorced with fleece, or a metal layer, for example an aluminium sheet.
It is furthermore possible to produce multi~le sandwich structures ~y placing on a coverin~ layer a further rigid layer of the porous material, which in its turn is coated with a fibre~reinforced epoxide resin. This operation can be repeated depending on the desired properties of the end product.
After cooling, the composite material having a sandwich structure can be stored until it is used. It is then shaped to obtain curved moulded articles, in a moulding press with the application of heatt in the process of which the pre-cured epoxide resin is finally cured. The composite material is preferably placed in the heated moulding press.
This is firstly closed just until contact is made with the projecting parts and subsequently, as the fi~re-reinforced covering layers soften over the zones of contack, slowly fully closed. The sandwich structures can, during curing and shaping, surprisingly undergo s~vere bending, without troublesome bulges or folds occurring in the compressed zones. It is possible to produce with relatively low pressure of the order of magnitude of 0.5-1~5 N/mm , shapes having very small radii of curvature, for example

2~10 mm with 6 mm thick foam sheets, or 2 mm with a 12 mm high honeycomb as the inner layer. The applied pressure is lower by at least one decimal power compared with the r~

pressures required for processing sheet moulding compounds.
Although the presses usable for sheet moulding compounds can likewise be used for processing the sandwiches according to the invention, cheaper and more simple presses having a very low closing force can be used for these sandwiches.
The moulding temperatures are as a rule in the range of 100-200C.
The composite materials can be used for the rational and rapid ~roduction of automobile body components having high flexural rigidity. Even when moulded products having radii of curvature of a few millimetres are produced, neihter cracks nor folds occur in the covering layers or in the foam core.
Example 1 A layer of polyester fleece and two layers of glass filament mat having a weight per unit area o 0.45 kg/m are impregna~ed, on a flat base, with a low-viscous, solvent-free laminating system consisting oE ~9 per cent by weight of a bisphenol-A diglycidyl ether ~ith an epoxide content of 5.2 val/kg, 7.5 per cent by weight of dicyandi-amide and 3.5 per cent by weight of monuron. There is then placed thereon a 5 mm thick hard foam sheet of polymeth-acrylimide having a density of 50 kg/m ~"~ohacell" ~, reglstered trademarl~ of Rohm). Onto this are subsequently laminated, in reverse sequence, two layers of glass filament mat and one layer of polyester fleece. The whole is pre-reacted in an air-circulation oven at 130C for 5 1/2 minutes. The sandwich prepreg is only slightly sticky after cooling. The prepreg is placed between silicone-treated paper for storage, and can be stored at room temperature for several months.
After removal of the silicone-treated paper, the sandwich prepreg is pre-heated in an oven for 5 minutes at 130C, and is then placed, with the lower side facing upwards, into a metal mould heated to 170C. The upper part of the mould is advanced until contact is made with the projections of the prepreg, and is finally closed at a speed at which the proceeding softening of the foam core and of the epoxide-resin-impregnated, fibre-reinforced covering layers permits deformation without fracture. In the production of a moulded product having two 90 angles with a radius of curvature of 6 mm, the mould can be closed to the stop within 90-120 seconds.
The required closing pressure is o.6 N/mm2, and after 5 minutes the product is curedO The resulting cured sandwich, which has a thickness of 8 mm (1.5 mm are taken up by each of the covering layers and 5 mm by the inner layer), has the following properties compared with a 2 mm thick aluminium sheet.

_ . _ ~ ~ ~ ~ ~
. Sandwich accord- Aluminium sheet Propert~es ing to Example 1 ,.
weight per unit area kg/m2 4.8 5.4 density g/cm 0.6 2.698 modulus of elasticity covering layer N/mm2 8880 70000 modulus of elasticity inner layer N/mm2 90 flexural rigidity 6 5 N mm2 4.3.10 7.10 (15 mm wide specimen) _ ~

The modulus of elasticity values are determined from the bending test results with thr~e-point loading according to Standard IS0 178.

?~

A layer of polyester fleece and 2 layers of glass fila-ment rnats are impregnated with the laminating mixture given ir Example 1 on a flat base. The lami.nate obtained is pre-cured in a press at 130C with contact pressure for 2 1/2 minutes to form a prepreg. It has slightly adhesive properties, and can be stored at room temperature for several months.
After removal of the silicone paper, the resulting prepreg is placed on the upper and lower side of a 5 mm thick foam made from polymethacrylimide (I'Rohacell 51" ~ ), and the further procedure is carried out in the manner described in Example 1~ The properties of the sandwich sheets obtained are identical to those possessed by the moulded product produced by the wet-in-wet process according to Example 1.
The flexural stiffne~ss ls the product of the modulus of elasticity E and the moment of inertia I of the specimen, and is the sum of the flexural s~ifness values of the individual layers, which are calculated according to the formula ( ~ + El.bl.hl.a ~ , wherein El is the modulus of elasticity, bl is the width, hl is the height of the first layer, and al is the distance from the middle of the first layer to the neutral middle plane of the specimen.

An unsymmPtrical design of the sandwich structure is produced by using for the one covering layer two layers o polyester fleece and three layers of glass filam2nt mat, and for the other covering layer one layer o~ polyester fleece and one of glass filament mat. The procedure otherwise is carried out as described in Example 1. The cured moulded ~ 6 product has the foll.owing properties:
thickness : 8.3 mm, weight per unit area: 5.2 kg/m2, flexural stiffness : 3 5 1o6 N mm2 ~ n epoxide integral foam is produced, in a 6 mm wide sheet-casting mould of aluminium, by mixing together 100 parts by weight o a liquid epoxide resin based on bisphenol-A, 50 parts by weight of an amine curing agen~
based on polyaminoamide and 4 parts by weight of a blowing agent based on polysiloxane ("Silicon DC 1107" ~ of Dow Corning); the temperature is raised to 60C and is held there for 20 minutes. A foam having a weight of 110 g/litre is formed. Ater a slight roughening up of the surfaces of the foam sheet obtained, there is laminated thereon glass filament mats - ("Vetrotex M 212-40-450" ~ , Saint Gobain Pont-à-Mousson) and polyester fleece, whereby the mats and fleeces are impregnated with the epoxide resin/curing agent/accelerator system described in Example 1, to which is also added 10 per cent by weight of powdered chalk, relative to the resin/curing agent/accelerator amount.
Onto the one side of the foam sheet are applied 3 layers of glass filament mat and 2 layers of polyester 1eece, onto the other side 1 layer of glass filament mat and 1 layer of polyester fleece. The still "wetl' sandwich is pre-cured for 5 minutes at 125C in an air-circulation oven.
The sandwich when cooled is dry and storage~stable. It is placed into a mould heated to 130CI and the mould is closed within 3 minutes. The moulded specimen is removed from the mould after a further 10 minutes at 130C. The specimen has two right angles with a radius of curvature of 6 mm and llmm, respectively. The overall thickness is 8 mm, the thickness of the 1st covering layer being 2 mmg - 10 ~
of the foam sheet 5 mm, and of the 2nd covering layer 1 mm.

A 4 mm thick hard polyurethane foam ("Hartmoltopren" ~ , Bayer) having a weight of 40 g per litre is coated on both sides and pre-cured in the manner described in Example 1. The resulting sandwich prepreg is moulded in a press at 130C to obtain an object having two right-angles with a radius o curvature of 6 mm and 11 mm, respectively;
overall thickness: 7 mm, thickness of foam: 4 mm. There is no cracking in the foam and no bulging or folding on the compressed sides. If by way of comparison the glass fibre mats are replaced ~y glass filament fabric (Type 917459 Intergla~, there is obtained a shaped object, of which the foam inner layer also shows no cracking, but on the compression side there is a slight warping of the fabric.
~.~
A 4 mm ~hick poLyisocyanurate foam ("Isovit-PIR" ~ , Isovit AG) having a weight of 38 g per litre is coate~
and shaped as described in Example 5. The results both in the case of the san~wich coated with glass fibre mats and in the case of that coated with glass filament fabric are ~he same as those described in Example 5.
Example 7 There is used as the inner layer polyamide paper consistin~ of m-phenylenediamide and isophthalic acid ("Nomexl' ~ , Du Pont~ in honeycomb form t'lAeroweb" ~ -honeycombs, Ciba-Geigy Ltd., Duxford) for a sandwich;
honeycomb diameter: 6 mm, hPight of honeycomb: 6 mm, paper thickness: 7.62.10 2 mm, weight- 40 kg/m3 r The one covering laminate contains 3 layers of glass fiLament mat and 2 layers of polyester fleece, and the other laminate 1 layer of each of these. They are impregnated with a mixture of a) 100 parts by weight of a mixture of 50 parts by weight of 4,4'-bis-(N,N-diglycidylaminophenyl)-methane with an epoxide content of 8.0 val/kg and 50 parts by weight of a liquid epoxide resin based on bisphenol-A with an epoxide content of 5.3 val/kg, b) 29 parts by weight of 4,4'-diaminodiphenylmethane, and c) 0.23 part by weight of boron trifluoride-monoethylamine.
The cured mixture has a glass transition temperature of at least 180C.
The two covering laminates are left for 4 hours at room temperature. The sandwich is then assembled and exposed for 1/2 hour to a temperature of 80C. The resin is converted to the B condition~ and is scarcely no longer sticky at room temperature. The sandwich in this state is storage-stable for several weeks.
The sandwich is preheated for 6 minutes at 120C, and is then placed into a mould at 150C9 which within 30 seconds is closed to the stop. The moulded specimen is removed from the mould after 10 minutes, and i~ after-cured up ~o 180C. A moulded product exhibi~ing no cracking or other defects is obtained. No bulging is visible on the compressed inner side (right-angle having a radius of curvature of 2 mm).
Example 8: Production of a right-angle ~ray There is prepare~ analogously to Example 7 with the same materials a prepreg sandwich con~aining ~he following layers:
1 fleece ~ 1 mat + 1 fleece ~ 6 mm thick "Nomex~ honey-comb + 1 fleece ~ 1 mat + 1 fleece. The briefly preheated prepreg sandwichis placed over a mould at 100C for a right-angle tray. The countermould, likewise at 100C, is approached until contact is made with the sandwich, and is subsequently closed to the stop within 30 seconds.
The prepreg sandwich is pressed into the tray mould, in the process of which no folds or kinks are formed. The temperature of the press is raised to 150C, and after 1 hour the right-a~gle (17 x 17 cm) tray is removed. The walls are cut off at a height of 3 cm. The right-angled edges and corners with a 6 mm inside radius are formed fully satisfactorily. It can be observed in transmitted light that even at the corners the hexagonal honeycomb structures on the outside are neither overstretched nor torn, whilst being on the inner surface uniformly compressed.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing cured, curved molded articles which com-prises the steps of (1) placing a storable composite material into a heated molding press, said composite material having a sandwich structure which comprises an inner layer of a solid material having hollow spaces therein and, adhered to each of the two surfaces of said inner layer, a covering layer consisting of a B-stage epoxide/curing agent mixture reinforced with a fiber mat or fiber fleece;
(2) closing said press just until first contact is made with the projecting part of said covering layers, the area of contact corresponding to those regions which will be curved in the molded article, and maintaining said closed position until the covering layers are softened over the zones of contact;
(3) fully closing said press, shaping and curing said composite material at pressures of from 0.5-1.5 N/mm2 and at temperatures of from 100-200°C; and (4) opening the press and removing the cured, curved molded article.

2. The process according to Claim 1, wherein said composite material has a solid foam plastics material as the inner layer containing hollow spaces.

3. The process according to Claim 2, wherein the foam plastics material consists of polyacrylimide or polymethacrylimide.

4. The process according to Claim 2, wherein the foam plastics material is based on epoxide resins.

5. The process according to Claim 2, wherein the foam plastics material consists of polyurethane.

6. The process according to claim 1, wherein said composite material contains a honeycomb material of paper or aluminium as the inner layer having hollow spaces.

7. The process according to claim 1, wherein the covering layer of said composite material contain glass-reinforced B-stage epoxide resin/curing agent mixtures.

8. The process according to claim 1, wherein the covering layers of said composite material contain a B-stage epoxide resin/curing agent mixture reinforced with polyester fibers.