WO1996005240A1

WO1996005240A1 - Structural strengthening

Info

Publication number: WO1996005240A1
Application number: PCT/AU1995/000482
Authority: WO
Inventors: Terence Alan Russell
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-08-11
Filing date: 1995-08-11
Publication date: 1996-02-22
Anticipated expiration: 1997-02-11
Also published as: AUPM739694A0

Abstract

Means for improving the strength of hollow mechanical structures, and especially for improving the impact strength of structural members in motor vehicles. Portions to be strengthened are filled with a rigid non-foamed mass with a specific gravity of preferably between 0.4 and 0.7. This can be cast in position or moulded externally and inserted when solid. The precursor mixture which hardens is pumpable and preferably comprises an epoxy or vinyl ester resin and hardener therefor, glass or plastic hollow micro-spheres, and optionally fibre reinforcement, preferably glass fibres.

Description

STRUCTURAL STRENGTHENING

This invention is directed to means for improving the strength of hollow mechanical structures, and especially for improving the impact strength of structural members in motor vehicles.

In the automotive manufacturing industry increasing standards of crash protection for occupants are constantly being demanded by consumers and by legislation. In particular, increased rigidity of the passenger compartment is being demanded. Increased stiffness of the vehicle is also highly desirable to improve handling characteristics on the road and to reduce vibrations which lead to the development of squeaks and rattles. These goals are difficult to achieve within the constraints of the lighter vehicle masses being required for improved fuel economy and exhaust emissions.

There is thus a pressing need to strengthen portions of automobiles such as chassis rails, pillars and anti-intrusion channels in doors without substantially increasing vehicular weight.

There are many examples in the prior art of proposals to increase the strength of automobile structural members by filling or partially filling them with rigid foams. Descriptions of such examples include US patents 4870113 and 4897432 assigned to Ethyl Corp, UK patent 2181998 to Dow Corning Corp, Canadian patent 2031436 to Ciba Geigy AG, Australian patent 468559 to General Motors Corporation and German patent application 3826011 by BMW. The use of foams has been considered preferable to non-foamed materials in order to reduce both weight and the cost of materials.

However substantial experimentation has now resulted in a manufacturing technique that provides, in combination to an extent that has previously been considered not technically nor economically possible, a greatly improved degree of strengthening at an acceptable weight and with simple means of application. A key feature has been the realisation that an extremely strong, light weight and impact absorbing structural void filler can be produced by the combination of a non-foamed resin with a heavy loading of added small hollow spheres. Fibre reinforcing may also be added to the mixture to further increase strength.

In conventional structural foams, gas bubbles are introduced into a liquid mass either as an injected gas or as a gas evolved during the reaction of the liquid mass, whereafter the liquid mass is caused to set hard. Such foams are lighter than the corresponding unfoamed material but are nevertheless substantially weaker. It is an aim of the present invention to produce a non- foamed material which has the light weight advantages of foamed products but has substantially greater strength.

In one aspect the present invention can be seen as a method of strengthening portion of a hollow member comprising:

(i) inserting into the hollow member a precursor mixture comprising:

the combination of both components of a 2-part resin mix, where such combination alone would have a specific gravity between 1.05 and 1.25, and

hollow micro-spheres having a shell of material different to that of said resin mix, and

(ii) permitting the mixture to set, such that said portion of the member is filled with a rigid non-foamed mass with a specific gravity between 0.4 and 0.7 and which is strongly adherent to the inside walls of the hollow member so providing a strengthening therefor.

In another aspect the present invention can be seen as a method of strengthening portion of a hollow member comprising:

(i) inserting into the hollow member a precursor mixture comprising by weight:

100 parts of the combination of both components of a 2-part resin mix, and 30 to 60 parts of hollow spheres having a shell of mateπal different to that of said resin mix and having an average diameter in the range from 10 to 100 μm, and

(ii) permitting the mixture to set, such that said portion of the hollow member is filled with a rigid non-foamed mass which is strongly adherent to the inside walls of the hollow member so providing a strengthening therefor.

In a further aspect the invention can be seen as a method of strengthening portion of a hollow structural member comprising forming with a mould an insert which is shaped to fit the inside walls of the hollow member at its portion to be strengthened, and which insert is moulded using a precursor mixture as described above, permitting the mixture to set, and fastening the insert inside the hollow member to substantially fill the portion to be strengthened.

An additional curing period at an elevated temperature may be provided after the mixture is permitted to set.

In another aspect the invention can be seen as a pumpable precursor mixture for a non-foaming structural void filler, said precursor mixture having a specific gravity between 0.4 and 0.7 and comprising a resin and hardener therefor, glass or plastic or ceramic hollow micro-spheres, and reinforcement fibres.

Preferably the precursor mixture contains 2 to 6 parts of fibre reinforcement of diameter from 5 to 25μm and substantially evenly distributed within the precursor mixture. This may be achieved by evenly distributing the fibre reinforcement in at least one liquid component of the precursor mixture, before final mixing of the precursor mixture.

Preferably the reinforcement is glass fibres, although alternatives such as Kevlar aramid fibres, carbon fibres or boron fibres may be applicable. Other organic fibres such as hemp may also be used. A mixture of fibre types can be used. Preferably the fibres are in the form of relatively short chopped fibres rather than in long filamentary form, and have a length of about from 2 to 12.5mm. A length of from 3 to 7mm is preferred. The use of fibre reinforcement substantially improves the structural integrity of the final product. The resin in the mix may be an epoxy, vinyl ester, polyurethane, polyester, acrylic or phenolic resin, or any hybrid resin system combining two or more of these. An epoxy or vinyl ester is preferred.

Preferably the micro-spheres have a glass shell and have an average diameter in the range from 10 to 10Oμm, more preferably 20 to 80μm.

Preferably the precursor mixture has a viscosity such that it is pumpable, and is inserted into the hollow member by injecting it under pressure from a conduit through a nozzle into the hollow member. Preferably a major part of the mixing of the precursor mixture occurs in a static mixer in the conduit closely upstream of the said nozzle.

Preferably the set rigid non-foamed mass has a specific gravity between 0.4 and 0.7, more preferably between 0.45 and 0.6, and even more preferably between 0.45 and 0.55.

Preferably said precursor mixture is not self levelling on standing prior to hardening.

We will first consider embodiments of the invention that involve insertion of the precursor mixture directly into the hollow structural member to be strengthened.

The precursor mixture hardens within or upon the structural member which it is intended to reinforce. The hardening process may be assisted by heating. For an application within the body of an automobile during manufacture of the automobile, such elevated temperatures may be available from the heat treatment of the auto body during its painting operations. In such situations it is preferred for the precursor mixture to be placed into its final position while the auto body is between the electrocoat baking oven and the primer coating station. The hardening process may thus be assisted by heating in the baking oven immediately after the primer application.

By way of explanation, in conventional modern auto finishing, the body is given a total dip electrocoat followed by about 30 minutes in a baking oven where the body is raised to about 1 5°C. Thereafter the body is coated with primer paint and then heated in a primer bake oven for about 20 minutes when it is raised to about 155°C. However if there is a delay in the paint line, such that the body stops in the primer bake oven, it may reach up to about 180°C. Tolerance of such an elevated temperature can be provided by use of an appropriate precursor mixture formulation.

In automotive applications, it is also highly desirable that all structural members, including those reinforced according to the present invention, are protected from corrosion as much as possible. Placing the mixture in the structural member after the member is electrocoated provides the corrosion protection. However it is most preferable for the hardened mixture to adhere strongly to the member. Placing the mixture in the structural member before it is prime coated ensures that such adhesion is not compromised by the limits of adhesion of the primer paint.

An example of putting the invention into practice will now be described by way of a preferred embodiment of the invention.

A precursor mixture is prepared by mixing two component parts which are formulated from the following ingredients readily available in Australia from the suppliers/manufacturers indicated. The composition is indicated in parts by weight.

COMPONENT PART A

100 parts EUREPOX™ 721 modified epoxy resin from Witco Australia,

- 25 to 50 (preferably 37) parts Q CELL™ 520 glass micro-balloons of nominal 45μm diameter from PQ Australia,

0 to 40 parts (preferably 20) parts EPODIL™ L adhesion promoter and diluent from Anchor Australia,

- up to 8 (preferably 5) parts TITANOX RHD2™ titanium dioxide pigment from Victor Leggo, - 1 to 15 (preferably 5) parts PPG E-GLASS™ 1156 chopped strand

6.4mm long and 10μm diam from George Fethers, and

- up to 2 parts (preferably 1 part) WACKER™ HDKN20 fumed silica from Hoechst.

COMPONENT PART B

- 50 parts EUREDUR™ 350 hardener from Witco Australia,

- 12 to 30 (preferably 20) parts Q CELL™ 520 glass micro-balloons of nominal 45μm diameter from PQ Australia, up to 10 (preferably 5) parts ANCAMINE™ K54 activator from Anchor

Australia, - up to 8 (preferably 4) parts BAYFERROX™ 318 pigment from Victor Leggo, and - up to 2 parts (preferably 1 part) WACKER™ HDKN20 fumed silica from

Hoechst.

The above ingredients are mixed into the component parts indicated for storage and ease of shipment and handling. At the time of application, the two component parts are independently pumped to, and then brought together, in the weight ratio of 2 parts A to 1 part B, just upstream of a static in-line mixer which is in turn just upstream of the nozzle through which the resultant precursor mixture is injected into the structural member requiring strengthening. A priming piston displacement pump is used for material transfer.

A precursor mixture according to the above formulation which is held at a 20°C ambient temperature would gel in around 1 hour and fully cure in around 72 hours. However if the precursor mixture is injected into chassis rails, roof pillars or other structural members in an auto body under construction, and then introduced into the primer baking oven, the hardening of the precursor mixture is accelerated from that at ambient temperatures, and the filler would be hard by the time the automobile was fully assembled.

The hollow micro-spheres, also known as micro-balloons, in the above most preferred formulation comprise a 23% loading in the mixture. However from the ranges indicated one can see that variation from about 15% to about 30% are possible. The manufacturer of the micro-spheres states that although the nominal diameter of that grade is 45μm, their specification at the same time states that the product sizing is such that 95% has a diameter between 20μm and 80μm.

The hollow glass micro-spheres are added to the mixture to lower the density of the void filler and achieve weight savings. Although micro-spheres of other materials such as plastic or ceramic could be used, high quality glass is considered superior. Micro-spheres made from fly ash, although useable and glassy, are not preferred because, although they have higher strength, their higher density results in a significant undesirable increase in the specific gravity of the final mixture. Upon hardening, the density of a void filler manufactured according to the above description was found to have a specific gravity of 0.51. This was slightly lower than predicted due to incidental and non-intended entrainment of air in the mixture during the mixing process. Although such entrained air is detrimental to the performance of the void filler so produced, it will be appreciated that the present invention encompasses the inclusion of such incidentally entrained air, and such air should be distinguished from air that could be specifically included in order to foam a mixture. Foamed mixtures are not encompassed by the present invention.

In the case of conventional foamed void fillers, care needs to be taken with the placement of holes in the members being filled to ensure that, prior to hardening, the mixture does not thereby run from its intended region of use. In contrast, the precursor mixture of the present invention can be formulated to be not self levelling and thus will not significantly run from where it is placed.

Although the above formulation of mixture has been found to be particularly efficient and cost effective in reinforcing automobile chassis rails and the like, it could be varied if required for particular applications. For example 25 parts of 6mm chopped strand KEVLAR™ aramid fibres could replace the 50 parts glass fibres for improved performance albeit at an increased cost. Such a material could have anti-ballistic and other armouring applications.

Also the resultant solid void filler could be toughened by adding an elastomer such as 10-40 parts of a carboxylated nitrile rubber to the precursor mixture, using for example CTBN for an epoxy or VTBN for a vinyl ester resin. More preferably though, this would be achieved by adding a toughener such as a difunctional polyoxypropylene with a molecular weight of about 2000, for example JEFFAMINE™ D-2000 from Texaco Chemical Co. For this 10-40 parts of the Jeffamine is pre-reacted with the resin at 100°C in an inert atmosphere prior to their mixing with the other ingredients in component part A of the precursor mixture.

The use of epoxy resin in the preferred mixture ensures that it obtains intimate contact and adhesion with the walls even if they are oily from earlier processing or storage. Although the embodiment described above utilises a primer bake oven to cure the reactant mass, the invention in its broader sense envisages the precursor mixture being alternatively inserted into the required positions in the body while it is elsewhere on the assembly line. In its broadest sense, the invention does not require the application of heat to cure the resin. In addition to a speeding of curing, the application of heat for curing can be advantageous where a higher glass transition temperature is sought for the cured material.

Having considered embodiments of the invention that involve insertion of the precursor mixture directly into the hollow structural member, we turn now to embodiments that involve the formation of an insert from a precursor mix and later placing this inside the hollow structural member to be strengthened.

In the preferred embodiment of this type, the precursor mix is prepared by mixing the component parts A & B of the same formulation and in the same ratio as described in detail above. Although it is possible to form satisfactory inserts from precursor mix alone, it has been found to be most convenient to pump the mix into a thin walled mould which thereafter becomes the outer surface of the insert in use. Such moulds can be conveniently made of any plastics material suited to blow moulding or vacuum forming of thin walled containers, such as a polyolefin or polyurethane.

Inserts may be fastened inside the hollow member to be strengthened by means of screwing, adhesive or by assembling the member to other structural components.

Although the invention has been described with particular reference to the manufacture of automobiles, it will be appreciated that it is similarly adapted to the manufacture of buses and other heavy road vehicles, trains, aircraft, caravans and the like. It also has application in marine superstructures and, with the incorporation of Kevlar fibres, could be useful in military applications.

Although initially developed for strengthening steel structures, the invention is also applicable to hollow members of other metals or of plastics or composite materials. It is applicable to a wide range of beam stiffening applications such as in cranes, space frame members and for aluminium window frames required to provide a significant structural strengthening role. The term hollow member is intended to include channel or trough-shaped members such as C-sections as well as tubular or box-section members. Although the invention is useful for the strengthening of structures as quantified by static tests, it has particular applicability to impact strengthening where energy absorption is an important factor.

An important feature of the void filler materials produced by the invention is the very dense packing of the micro-spheres within the matrix of the solid filler. In the preferred embodiment described, the glass micro-spheres when measured out prior to mixing are of the order of twice the volume of the final mixture into which they become incorporated. They are thus at about twice the packing density in the mixture as they are when freely resting in air. Such very dense packing has been found to be greatly aided by the use of resin components which are of extremely low viscosity and/or by relatively high rates of addition of diluent to further lower viscosity.

A significant advantage of the use of micro-spheres in the mixture is that the mixture's pumpability is increased relative to other materials of similar viscosity. It is thought that the micro-balloons act as a type of internal lubricant as their spherical shape allows the mixture to flow more freely than what would be otherwise expected, although this comment is not intended to so restrict the nature of the invention.

In the course of substantial experimentation, it has been found that in some applications the mixture as it sets can in parts reach undesirably high temperatures and even suffer some thermal degradation due to the heat liberated in the exothermic hardening reaction. Such high temperatures appear to be confined to particularly thick cross sections of filler wherein the thermal conduction path is relatively long for removal of the heat of reaction. For such applications it has been found that a heat sink means may be advantageously positioned generally central of the cross section such that it is substantially surrounded by the filler prone to high temperature elevation. Preferred forms of such a heat sink and their application are described in International patent application PCT/AU92/00468.

Claims

1. A method of strengthening portion of a hollow structural member comprising:

(a) inserting into the member a precursor mixture comprising:

(i) the combination of both components of a 2-part resin mix, where such combination alone would have a specific gravity between 1.05 and 1.25, and

(ii) hollow micro-spheres having a shell of material different to that of said resin mix, and

(b) permitting the mixture to set, such that said portion of the member is filled with a rigid non-foamed mass with a specific gravity between 0.4 and 0.7 and which is strongly adherent to the inside walls of the hollow member so providing a strengthening therefor.

2. A method of strengthening portion of a hollow structural member comprising:

(a) forming with a mould an insert which is shaped to fit the inside walls of the hollow member at its portion to be strengthened, and which insert is formed using a precursor mixture comprising:

(ii) hollow micro-spheres having a shell of material different to that of said resin mix,

(b) permitting the mixture to set to a rigid non-foamed mass with a specific gravity between 0.4 and 0.7, and (c) fastening the insert inside the hollow member to substantially fill the portion to be strengthened.

3. A method of strengthening portion of a hollow structural member comprising:

(a) inserting into the hollow member a precursor mixture comprising, by weight proportions:

(i) 100 parts of the combination of both components of a 2- part resin mix, and

(ii) 30 to 60 parts of hollow spheres having a shell of material different to that of said resin mix and having an average diameter in the range from 10 to 100 μm, and

(b) permitting the mixture to set, such that said portion of the hollow member is filled with a rigid non-foamed mass which is strongly adherent to the inside walls of the hollow member so providing a strengthening therefor.

4. A method of strengthening portion of a hollow structural member comprising:

(a) forming with a mould an insert which is shaped to fit the inside walls of the hollow member at its portion to be strengthened, and which insert is formed using a precursor mixture comprising, by weight proportions:

(i) 100 parts of the combination of both components of a 2- part resin mix, and

(b) permitting the mixture to set to a rigid non-foamed mass, (c) fastening the insert inside the hollow member to substantially fill the portion to be strengthened.

5. A method according to claim 3 or 4 wherein the set rigid non-foamed mass has a specific gravity between 0.4 and 0.7.

6. A method according to any one of the preceding claims wherein the resin is an epoxy or vinyl ester resin.

7. A method according to claim 2 or 4 wherein the mould is thin walled and the insert in use continues to carry the mould as an outer skin.

8. A method according to any one of the preceding claims wherein, after the set rigid non-foamed mass comprising the strengthening mixture is in the hollow member, an additional curing period is provided at elevated temperature.

9. A method according to claim 8 wherein the hollow member is part of a motor vehicle body and the elevated temperature is provided by heat treatment of the vehicle body during its painting operations.

10. A method according to claim 9 wherein the strengthening mixture is placed in the hollow member between the vehicle body's heating in an electrocoat baking oven and its painting at a primer coating station.

11. A method according to any one of the preceding claims wherein the micro-spheres have a glass shell and have an average diameter in the range from 10 to 100μm.

12. A method according to any one of the preceding claims wherein the micro-spheres have a glass shell and have an average diameter in the range from 20 to 80μm.

13. A method according to any one of the preceding claims wherein the precursor mixture contains, using the same weight proportions, 2 to 6 parts of reinforcement fibres of diameter from 5 to 25μm and length from 2 to 12.5 mm evenly distributed within the mixture.

14. A method according to claim 13 wherein the reinforcement fibres are glass fibre.

15. A method according claim 13 or 14 wherein the reinforcement fibres are evenly distributed in at least one liquid component of the precursor mixture before final mixing of the precursor mixture.

16. A method according to any one of the preceding claims wherein the precursor mixture has a viscosity such that it is pumpable, and is injected under pressure from a conduit through a nozzle into the hollow member or the insert mould.

17. A method according to claim 16 wherein a major part of the mixing of the precursor mixture occurs in a static mixer in the conduit closely upstream of the said nozzle.

18. A method according to any one of the preceding claims wherein the set rigid non-foamed mass has a specific gravity between 0.45 and 0.6.

19. A method according to claim 18 wherein the set rigid non-foamed mass has a specific gravity between 0.45 and 0.55.

20. A pumpable precursor mixture for a non-foaming structural void filler, said precursor mixture having a specific gravity between 0.4 and 0.7 and comprising a resin and hardener therefor, glass or plastic or ceramic hollow micro-spheres, and reinforcement fibres.

21. A mixture according to claim 20 wherein the resin is an epoxy or vinyl ester resin.

22. A mixture according to claim 20 or 21 wherein the reinforcement fibres are glass fibres with a length of 2mm to 12.5mm.

23. A mixture according to claim 22 wherein the reinforcement fibres have a length of 3mm to 7mm.

24. A mixture according to any one of claims 20 to 23 wherein the hollow micro-spheres have a glass shell with an average diameter in the range from 20 to 80μm.

25. A mixture according to any one of claims 20 to 24 which is not self levelling on standing prior to hardening.

26. A mixture according to any one of claims 20 to 25 having a specific gravity between 0.45 and 0.55.