GB2188636A - Thermoset polyester and phenolic foams having denser outer skin - Google Patents

Abstract

A hot cured thermoset structural foam having relatively dense external layers and less dense foam core, the thermoset material being polyester or phenolic resin, may be prepared by mixing the thermosetting resin with a chemical blowing agent, which mixture decomposes with the generation of gas at a temperature between the onset of chain mobility within the thermosetting resin and the temperature of onset of cure of the resin, introducing the mixture into a mould at a temperature below the decomposition temperature of the blowing agent, heating the mixture under compression in the mould so that parts of the mixture adjacent the mould walls cure and solidify to form a skin and then enlarging the mould to a predetermined size to allow the mixture to foam and cure within the solidified skin to the size determined by the mould.

Description

SPECIFICATION Thermoset foams This invention relates to thermoset foams.

Thermoplastic materials are generally moulded with the introduction of hot thermoplastic polymeric material into a cooling mould in which mould the polymeric material cools. It is well known to produce thermoplastic foams by use of nitrogen gas, or a chemical blowing agent which decomposes under heat with the generation of gas. With the increase in viscosity of the thermoplastic polymer as it cools and the presence of the gas introduced or formed by decomposition of the blowing agent, there is obtained a foam.

Amongst the thermoplastic foams made are structural foams. These may be defined as a product (a material or moulding) having a cellular core bounded by a solid integral skin and having a high enough stiffness or strength to weight ratio for the product to be classed as load bearing. These structural foams are very successful materials and find a wide range of applications. They are comparatively strong materials but are lighter in weight and more insulating than the corresponding solid thermoplastic materials. However their applicability is restricted by the properties of the thermoplastic polymer e.g. its thermal stability.

Thermoset resins are generally speaking considerably more resistant to heat distortion than thermoplastic polymers. Moulded solid thermoset materials are obtained by the introduction of a thermosetting resin into a hot mould in which the thermosetting resin cures with the formation of the crosslinked thermoset material.

However there are of course considerable practical difficulties associated with the production of foams from thermoset materials. With thermoset foams expansion and chemical curing should be carried out more or less simultaneously. This is in contrast to thermoplastic foams where expansion occurs in the pre-existing molten polymer, without crosslinking or curing of the structure.

Thermoset foams are known. These materials have heretofore been made by cold forming methods. Such methods often have long cycle times and do not provide for good control of the foamed structure obtained.

Previously a variety of processing routes have been proposed for the production of thermoset foams utilising either two component liquid spray-up techniques or moulding techniques of varying degrees of sophistication. A process of increasing commercial interest for thermoset foams is foam reservoir moulding (Murray, T.A., Plast. Tech., 93, October (1978). In this method an open-celied flexible foam is used as the carrier for the thermosetting resin. A fibre-glass mat is used on each side of the "reservoir" and the resultant lay-up is positioned in a low pressure compression mould. As the mould closes, the resin is squeezed out of the foam, impregnating the reinforcement and curing after the mould is completely closed.A recent development (Fontana, P., Macplas, 34, January (1983)) has led to a modified hand layuplresin injection technique to enable the production of the thermoset foams. The method involves the addition of a controlled quantity of thermoset foam to a mould containing pre-placed reinforcement. On closing the mould foam impregnates the reinforcement and fills the mould cavity.

Thermoset structural foams, in particular polyurethane thermoset structural foams, have been produced by so-called reaction injection moulding. This is a cold cure system in which a mixture of reactive monomers together with the blowing agent e.g. a fluorocarbon is injected in one stream into the mould. While this method does produce structural foams it is limited in its application to certain types of thermoset polymer and does not provide for much control of the foaming in the final product.

United States Patent 3835208 discloses the use of a liquid moulding composition in a hot cure casting process in which a foamed thermoset material adopts the shape of a mould of indeterminate size. A protective skin is formed on the outside of the foam by the use of a surface active agent which collapses the foam.

However, the product is simply a soft foam with a protective outside layer, and is not a structural material.

United States Patent 4073844 uses a hot mould which enlarges under the foaming pressure of a thermoplastic resin, but there is no compression stage to the process, and thus a moulding of uniform density is produced. The stated advantage of the process is an improved surface finish on the moulding.

The present invention provides a hot-cured thermoset structural foam having relatively dense external layers and a relatively less dense core, the thermoset material being polyester or phenolic resin.

Such a structural foam offers the designer a number of useful properties e.g. very high stiffness to weight ratios, and the insulatory nature of the cellular material offers good thermal and acoustic properties, as well as good thermal stability and resistance to heat distortion.

Preferably, the average density is greater than 40% of the density of the same material in the solid state, typically between 50% and 80%.

Further, the invention provides a method of making a hot cured thermoset structural foam which method comprises moulding a mixture including a phenolic or polyester thermosetting resin and a chemical blowing agent, the chemical blowing agent being one which in the mixture decomposes with the generation of gas at a temperature between the onset of chain mobility within the thermosetting resin and the temperature of onset of cure of the thermosetting resin in the mixture; wherein the mixture is introduced into the mould at a temperature below the decomposition temperature of the chemical blowing agent, and the mixture is heated in the mould to a temperature sufficient to cure the resin; and wherein the mixture is subjected first to compression moulding during which the parts of the mixture adjacent the mould walls solidify to form a skin, and then the mould is enlarged to a predetermined size and the mixture is allowed to foam and cure within the solidified skin to the size determined by the mould.

Using this method, the manufacture of the foams can be well controlled particularly as regards cell size. In particular the process of the present invention can be used in the production of thermoset structural foams whereby the skin thickness and cell size can be controlled. Thus there is offered a fast method of producing structural thermosetfoams which can be subjected to good control and which can be carried out on substantially conventional injection or compression moulding apparatus.

For mass production of mouldings, it is preferred to use an injection moulding method where the mixture in warm plastic form is injected into the hot mould. Substantial further heating of the mixture will occur during rapid injection into the mould from frictional effects as the mixture passes through the nozzle and sprue into the mould cavity. This is preferred, provided of course the temperature is not too high, since thp higher the temperature of the mixture entering the mould the lower the necessary residence time to heat up in the mould and thus the faster the process.

During the compression step the solid outer skin of the structural foam is formed; the pressure is then removed and in the decompression step the material is allowed to foam. By controlling the length of time of the compression step in particular the thickness of the skin can be controlled, and thus the average density.

It is believed that above the temperature of onset of chain mobility within the thermosetting resin the polymer chains have sufficient mobility to permit expansion to occur. Below the onset of chain mobility temperature, it is not possible to obtain a satisfactory foam. The onset of chain mobility is normally associated with the glass transition temperature of the thermosetting resin, that is the onset temperature of main chain segmental mobility when the segments of the main polymer chain are able to rotate relative to each other, the glass transition temperature being the temperature at which transition from a glassy to a rubbery state occurs.

Once the resin begins to cure, i.e. crosslinking of the polymer ensues, chain mobility decreases and foaming from internally generated gas pressure (by decomposition of the chemical blowing agent) becomes increasingly difficult and stabilisation of the cellular structure occurs.

Even with particular thermosetting resins and with particular chemical blowing agents, flexibility is possible since the properties of the thermosetting resin system and the decomposition temperature of the chemical blowing agent and/or, of course, both can be adapted. The correct balance of properties here may be achieved by adapting either the thermosetting resin system or the decomposition temperature of the chemical blowing agent used or, of course, by adapting both.

Any suitable polyester or phenolic thermosetting resin which can be moulded may be used according to the present invention. The thermosetting resin may suitably comprise a prepolymer and generally a curing agent therefor such that on heating the prepolymer becomes crosslinked. There may be used phenolic e.g.

phenol-formaldehyde and polyester moulding materials.

The onset of cure of the thermosetting resin will depend in particular upon the particular curing system used. Thus by using different curing agents it is possible to affect the onset of cure of the system.

The majority of commercial polyester thermosetting moulding systems utilise a catalysed addition reaction involving the crosslinking of polyester chains by styrene monomer units activated by free radicals, liberated by thermal decomposition of the catalyst. The temperature at which catalyst decomposition, and hence radical formation, occurs can be adjusted to a large degree, thus allowing control over the temperature of cure initiation.

In addition the course of the cure reaction can be tailored by a suitable blend of curing agent and inhibitor to offer control over the duration of the cure reaction.

A suitable system for use according to the present invention includes for example dicumyl peroxide catalysed isophthallic polyester.

There may be used for example unsaturated polyester dough moulding compound, unsaturated polyester granular moulding compound, phenol-formaldehyde granular moulding compound.

Many commercially sold thermosetting systems are provided with the prepolymer and curing agent therefor already mixed. In such circumstances it of course may not be possible (or practical) to try to change the temperature of onset of cure. In these circumstances then of course it is necessary that it is the decomposition temperature of the chemical blowing agent which needs to be matched to the resin rather than the other way round.

By "chemical blowing agent" is meant a blowing agent which decomposes with heating with the generation of gas. This is in contrast to blowing for example by the introduction of fluorocarbon or nitrogen gas under pressure, so-called "physicai blowing agents".

The chemical blowing agent is selected as one which decomposes with the generation of gas between the temperature of onset of chain mobility and the onset of cure of the thermosetting resin. For best control the chemical blowing agent used should be one which decomposes with the generation of gas over a narrow temperature range. However it should be realised that gas evolution from the chemical blowing agent may occur up to, during or even beyond the onset of cure of the thermosetting resin although only gas available up to the point of maximum cure is likely to influence cell formation.

Azo esters for example are suitable chemical blowing agents.

It is well known that chemical modification to blowing agents by addition of so called "activating compounds" enables decomposition temperature to be varied. For example azodicarbonamide, which is a suitable blowing agent for use according to the present invention, can be activated with paratoluenesulphonic acid, or urea for example and p,p'-oxybisbenzenesulphonhydrazide, another suitable blowing agent, can have its decomposition temperature modified by use of both azodicarbonamide itself, or triethanolamene, ethylene glycol, urea, lead stearate.

It should be pointed out that it is the behaviour of the thermosetting resin and of the chemical blowing agent when combined in the mixture of the present invention which is important. It has been found that in practice for example onset of cure and decomposition of the chemical blowing agent of the thermosetting resin/chemical blowing agent mixture may not match those of the ingredients when separate. It is believed there may in some circumstances be some interaction between the ingredients leading to for example activation of the blowing agent, resin catalyst or both. Naturally separate determination of the temperature of decomposition of the blowing agent and of the cure onset of the thermosetting resin are a useful guide to the selection of the ingredients. However the temperatures when determined separately do not always give the same values as when used in combination.

A convenient method for use in determining for example the temperature of decomposition of the chemical blowing agent used and of determining the cure onset temperature for a thermosetting resin (either separately or in combination) is thermal analysis, using in particular differential scanning calorimetry. This technique makes it possible for a plot of energy change against temperature to be made and in this way assists in determining the temperatures at which the various changes of the different systems occur. The systems used according to the present invention do tend to be complex ones. Thus it should be borne in mind that there may be many different changes occurring and that the effects of changes occurring in the same temperature region may give rise to overlapping results.

Generally speaking the preferred amount of chemical blowing agent used lies in the range of 0.5 to 0.8 percent by weight of the total weight of the composition.

Another important aspect in the process of the present invention is the mixing of the thermosetting resin and chemical blowing agent. This mixing should yield a substantially homogeneous mixture in order to ensure that a controlled and substantially uniform cell structure is obtained in the final product. If the thermosetting resin used is in the form of a dough moulding compound, then mixing can be achieved using a conventional mixer, e.g. a Z blade mixer of suitable mixing capacity.

Similarly, if the thermosetting resin is provided in sufficiently fine particle size, uniform closed cell products can be obtained by dry mixing in a simple distributive mixer (e.g. a ribbon blender) to coat the particles of thermosetting resin with blowing agent powder. The inclusion of a small amount of paraffin oil may assist to adhere the blowing agent powder to the polymer particles.

If however the thermosetting resin is provided in large particular, e.g. granular, form, mere physical mixing may be insufficient if a uniform cell structure is to be produced. In that case then preferably the ingredients are mixed together physically and then heated to a temperature above the softening point of the thermosetting resin but below the decomposition temperature of the blowing agent and this melt subjected to mixing, e.g. in a twin screw compounding extruder. The extrudate may be cooled and converted into granular form to yield a material suitable for, for example, injection moulding.

The starting mixture according to the present invention may contain other ingredients as required or desired according to the method of manufacture used and the properties required for the final foam. For example the mixture may contain a release agent or lubricant, to ease release of the moulded material from the mould. Solid thermoset materials often suffer from shrinkage and thus in conventional thermoset moulding systems there is usually used a low shrink or low profile additive. There may be used similar materials according to the present invention but generally speaking the materials according to the present invention suffer less shrink than do solid thermoset materials. It is believed that residual internal pressure reduces shrinkage with the foamed material according to the present invention. Thus the addition of low shrink or low profile additive may be avoided.

The mixtures used according to the present invention may also contain conventional fillers. Generally speaking due to the inherent resistance to flamability of the base polymer the final foams have good flame retardant properties but, if their proposed use demands, flame retardant fillers such as aluminium trihydrate may also be included. The use of glass fibres assists to strengthen and toughen the final foam. Also there may be used, as fillers, hollow spheres. The use of such hollow sphere fillers provides a hot cured thermoset material which is both chemically blown and syntactic.

In addition to the choice and mixing of the starting materials, the conditions of moulding are most important according to the present invention. The structure and concomitant properties of thermoset structural foams obtained according to the present invention are a function of the processing conditions. Optimisation and control of foam structures requires strict attention to, and reproducibility of processes. Precise moulding conditions will vary according to the material being processed.

As mentioned above the preferred moulding process is injection moulding. Most preferably the moulding procedure is essentially an adaptation of conventional injection moulding techniques for thermosets using a single screw preplasticising injection unit, maintained at a temperature to soften the polymer without curing it and a hot mould into which the resin formulation is injected, foamed and cured. In order to produce a product having a solid integral skin and uniform foamed close cellular core an injection-compression-decompression moulding sequence is employed, to densify the outer surface of the material and then allow subsequent foaming of the core. It has been found that vertical flash tooling is generally required to achieve this objective.

To accommodate the expansion reaction in an injection mould with a controlled degree of flash it is necessary to use vertical flash tooling.

The moulding of thermoset structural foams according to the present invention is described with reference to Figure 1 of the accompanying drawings.

The apparatus illustrated in Figure 1 comprises a screw extruder 1 and vertical flash mould 10. The screw extruder 1 comprises a screw 2 within a barrel 3 provided with two heat jackets 4, 5 and a nozzle 6. There is also provided an inlet 7 for introduction of starting mixture.

The vertical flash mould 10 comprises a fixed front plate 11 having sprue inlet 12 and a movable back plate 13, the plates 11 and 13 defining between them the mould cavity 14.

The apparatus of Figure 1 is used as follows: In the first (feed/plasticisation) stage of the procedure, illustrated in Figure 1(a), the starting mixture is - introduced via inlet 7 to the screw extruder 1 and fed forward in the barrel 3 by means of screw 2. The material is heated in the screw extruder by means of the heat jackets 4 (at temperature T1) and 5 (at temperature T2) about barrel 3 and in addition by friction of the screw 2. Thus as it approaches the nozzle in the screw extruder the mixture is heated, to a temperature (T3) below the decomposition temperature of the blowing agent and below the curing temperature of the mixture but sufficient to ensure that the mixture is softened and will readily flow under pressure.

The next (injection) stage, illustrated in Figure 1(b), comprises the injection of the required amount of mixture from extruder 1 into the cavity 14 of mould 10. The density of the final foam moulding is dictated, inter alia, by the shot weight of material delivered for injection. The plasticised material is injected through nozzle 6 into the heated partly open mould 10, the temperature of mould plate 11 being TF and that of mould plate 13 being To; the distance between the two plates being X1. Injection parameters are optimised as the maximum injection speeds and pressures possible without shear heating in the sprue inlet 12 causing blowing agent decomposition to occur.

When all the material has been injected, the full clamp pressure of the mould is applied by closing of the mould and the screw 2 is withdrawn. This is the (compression/screwback) stage illustrated in Figure 1(c). The time of compression hold at full clamp pressure determines the skin thickness and ultimately the overall foam density.

Finally there is the decompression stage illustrated in Figure 1(d). The pressure is relaxed by moving plate 13 to a predetermined position (separation between mould plates 11 and 13 X2) so that the material foams against the existing solid skin to give a moulding thickness determined by the mould open position.

The timing of each of the stages is determined by the tool face temperatures. The usable temperature range is effectively limited to temperatures greater than the blowing agent decomposition temperature but below temperatures at which appreciable mould staining occurs. Normally the tool temperatures will lie between 160to 170 C.

The structural thermosetfoams obtained according to the present invention have improved properties both compared with structural thermoplastic foams and compared with their solid thermoset counterparts. They have good thermal stability. Moreover they have a high stiffness to weight ratio in flexural loading and a significant improvement in acoustic and mechanical damping properties compared with the solid thermoset materials. Also, in the fibre reinforced thermoset compositions according to the present invention, it has been observed that there is an increase in energy absorption to failure over the corresponding solid materials.

Density reductions of up to 50% relative to solid material have been readily achieved.

The invention is further illustrated with reference to the following Examples. In the Examples thermal analysis, differential scanning calorimetry, measurements were carried out using a Perkin Elmer DSC-2 differential scanning calorimeter interfaced with a TADS computerised data analysis system. The sample, contained within a sealed aluminium planchet, was maintained at the same programmed temperature, during a 200C min-1 scan rate, as a thermally inert reference material. The differential power required to maintain a temperature of the sample holder the same as the reference holder was recorded as a function of time, to allow the reaction exotherpisto be quantified.

Example 1 There was prepared an unsaturated polyester dough mixing compound formulation as follows: Percent by weight Isophthailic polyester resin (Scott Bader - D4029) 17 Low shrink additive (Union Carbide - LP40A) 7 Curing catalyst (Dicumyl peroxide) 0.4 Calcium carbonate filler (Croxton & Garry- Hydrocarb) 38 Flame retardant filler (Aluminium trihydrate) 20 Chemical blowing agent-activated azodicarbonamide (FBC - Genitron LE) 0.7 Release agent (Zinc stearate) 1.75 Glass fibres (Fibreglass - 6mm chopped strand) 15 The thermal analysis of the chemical blowing agent, Genitron LE, is shown in Figure 2 of the accompanying drawings.

It can thus be seen from Figure 2 that the onset decomposition of the chemical blowing agent (Do) occurred at approximately 4000K with the maximum decomposition (Dm) occurring at approximately 420 K.

The ingredients were mixed in a Z blade mixer to substantial homogeneity.

A thermal analysis of a sample of that mixture is shown in Figure 3 where there can be seen the onset of chain mobility temperature (ore) (at 375 K), the onset of decomposition of the chemical blowing agent (Do) maximum decomposition of blowing agent (dry) and the temperature of maximum cure (Mc) (at 450 K) of the thermosetting resin. However, presumably because of peak overlap, it is not possible to identify the onset of cure of the thermosetting resin here. The onset of cure occurs at approximately 420 K.

The mixture was subjected to injection moulding using the process described with reference to Figure 1.

The moulding conditions were as follows: Extruder T1 60 C T2 650C T3 approximately 90 to 950C Screw rotate speed 50 rpm Screw back pressurp approximately 1 MPa Injection speed ' Maximum available on machine.

Mould To 165 C TF 170"C X1 = X2 variable Injection time 10 to 30 seconds Compression time variable depending on desired skin thickness.

Figure 4 is a photograph taken with a scanning electron microscope of a section of a foam obtained.

Example 2 There was a granular polyester formulation as follows: Percent by weight Uncatalysed nodular polyester compound (Scott Bader-Crystic Impel) 97.45 Curing catalyst (Dicumyl peroxide) 0.3 Chemical Blowing Agent -activated azodicarbonamide (FBC-Genitron EPE) 0.75 Release agent (Zinc stearate) 1 50 The ingredients were first tumble blended in a ribbon blender and then extrusion compounded in a co-rotating twin screw compounding extruder with optimised screw geometry using the facility described by Hornsby, P.R., Plastics Compounding Sept/Oct., pp 6571, (1983).

Figure 5 of the accompanying drawings shows the thermal analysis of the chemical blowing agent Genitron EPE used. Thus onset of decomposition (D0) of this blowing agent occurred at approximately 432 K and maximum decomposition at approximately 434=4380K. It is to be noted that in this system a double peak was produced.

Figure 6 of the accompanying drawings shows the thermal analysis of the formulation obtained on mixing.

There can be seen the onset of chain mobility temperature (ore) of the resin (at 380"K), the onset of decomposition of blowing agent (Do) and the maximum cure (at 460 C) (Mc). A corresponds to blowing agent decomposition and B to resin cure. The onset of blowing agent decomposition is lower in the resin composition than when analysed separately. This is believed to be due to activation by the resin formulation.

Again there is overlap between the onset of resin cure and decomposition of gas from the blowing agent. The onset of resin cure occurs at approximately 430"K.

The mixture was subjected to injection moulding using the process described with reference to Figure 1, the mould conditions were as follows: Extruder T1 65 C T2 70 Cì T3 approximately 1050C Screw rotate speed 50 rpm Screw back pressure approximately 1 MPa Injection speed Maximum available on machine.

Mould To 1650C Tf 1700C X, =X2 (variable depending on desired cell size/foam density) Injection time 10 to 30 seconds Compression time variable depending on desired skin thickness.

(typically 2 to 10 seconds) Figure 7 is a photograph taken with a scanning electron microscope of a section of a foam obtained.

Example 3 There was prepared a formulation using a proprietary grade of phenol formaldehyde thermosetting resin powder (X34/45-DF006 Bakelite Ltd) and 0.7% by weight of Genitron EPE blowing agent as used in Example 2.

These ingredients were tumble blended together with 0.5% by weight liquid paraffin to aid homogeneous distribution of the blowing agent, in a ribbon blender.

The mixture was subjected to injection moulding using the process described with reference to Figure 1, the moulding conditions were as follows: Extruder T1 90 C T2 950C T3 approximately 120 C Screw rotate speed 50 rpm Screw back pressure approximately 1 MPa Injection speed Maximum available on machine.

Mould To 165 C Tf 1700C X, =X2 (variable depending on desired cell size/foam density) Injection time 10 to 30 seconds Compression time variable depending on desired skin thickness.

(typically2to 10 seconds) Figure 8are photographs taken at different magnifications with a scanning electron microscope of a section of the foam obtained.

Figure 9 illustrates the different average density obtained by varying the compression time (in seconds) in the mould for a phenolic resin. This assumes exactly the same moulding composition and conditions, in particular the mould enlargement for the decompression stage. The compression time determines the thickness of the solid outer skin of the moulding. The graph shows that in this instance compression of 15 seconds or more will produce a solid product which is effectively completely cured before forming is allowed.

Claims (12)

1. A hot-cured thermoset structural foam having relatively dense external layers and a relatively less dense foam core, the thermoset material being polyester or phenolic resin.

2. A structural foam as claimed in Claim 1, wherein the average density is greater than 40% of the density of the same material in the solid state.

3. A structural foam as claimed in Claim 2, wherein the average density lies in the range 50% to 80% of the solid state density.

4. A structural foam as claimed in Claim 1,2 or 3 wherein the material includes a proportion of glass fibres.

5. A method of making a hot cured thermoset structural foam which method comprises moulding a mixture including a phenolic or polyester thermosetting resin and a chemical blowing agent, the chemical blowing agent being one which in the mixture decomposes with the generation of gas at a temperature between the onset of chain mobility within the thermosetting resin and the temperature of onset of cure of the thermosetting resin in the mixture; wherein the mixture is introduced into the mould at a temperature below the decomposition temperature of the chemical blowing agent, and the mixture is heated in the mould to a temperature sufficient to cure the resin; and wherein the mixture is subjected first to compression moulding during which the parts of the mixture adjacent the mould walls cure and solidify to form a skin, and then the mould is enlarged to a predetermined size and the mixture is allowed to foam and cure within the solidified skin to the size determined by the mould.

6. A method as claimed in Claim 5, wherein the moulding is carried out using injection moulding apparatus.

7. A method as claimed in Claim 5, wherein the moulding is carried out using compression moulding apparatus.

8. A method as claimed in any of Claims 5 to 7, wherein the thickness of the solidified skin and thus the average density of the structural foam is controlled by adjustment of the time for which the compression moulding is continued.

9. A method as claimed in any of Claims 5 to 8, wherein the mould temperature lies between 1 600C and 170 C.

10. A hot cured thermoset structural foam produced by a method as claimed in any of Claims 5 to 9.

11. A method of making a hot-cured, thermoset structural foam substantially as herein described.

12. A hot-cured, thermoset structural foam substantially as herein described.