Moulded Fibre Reinforced Plastics Articles

This invention relates to moulded articles of fibre reinforced plastics material and to a method of moulding such articles.

Heretofore, fibre reinforced plastics articles have been moulded from consolidated sheets of thermoplastics material reinforced with glass fibre mat formed from very long glass fibre strands (i.e. fibre bundles) of perhaps 200 centimeters or more in length which extend in a random serpentine manner throughout the sheet.

Thus, for example, in the manufacture of one such material, layers of glass fibre mat are interposed between layers of thermoplastics material, the composite structure being needled to produce a degree of integration of the layers, and then heated under pressure to produce the consolidated rigid sheet.

When such materials are heated to the softening temperature of the plastics matrix, prior to being fed into a forming press, slight expansion in thickness will occur. This is probably because the layers of mineral fibre, which are usually of glass, which had been pressed together during the laminating stage, spring back to partly regain their original configuration. However this expansion is relatively limited in extent and also results in the surface of the material being marred by coarse fibres and fibre bundles protruding out of the molten plastics matrix. Both of these drawbacks are particularly severe in the case of a needled construction.

The bundles of fibres which had been forced by the needling operation into loops normal to the surface of the sheet emerge and rise high above the surface to give an uneven hairy appearance. But the central layers of the web hardly expand in thickness, with the result that the stiffness of the expanded sheet hardly exceeds that of the original consolidated sheet.

Because of these problems, the hot expanded sheet is invariably formed into the required shape and cooled to below the softening temperature whilst under pressure in the mould so as to ensure that the moulded article is fully consolidated. In that condition, the moulded article will possess the maximum strength but its flexural stiffness is relatively low compared with its weight and the moulding will of course be impermeable.

According to the present invention a process for moulding articles of fibre reinforced plastics material includes heating and moulding a sheet of consolidated thermoplastics material which has dispersed therein from 20% to 70% by weight of reinforcing fibres which have a high modulus of elasticity (as herein defined) and between about 7 and about 50 millimeters long so that the stresses embodied in the glass fibres cause the matrix to expand into the configuration of the mould and become porous.

It has been found that the extent of the expansion and the relative increase in thickness of the finished moulding compared with the original sheet is much larger and more uniform than in the case of materials of known construction, resulting in an article which has a very much higher flexural stiffness and a relatively smooth coherent surface. This is especially the case when the degree of expansion is controlled by setting the dimensions of the forming tool to clearances less than the final thickness which would be achieved by free unrestricted expansion.

The relative improvement, and the method of achieving it, will become clear from the following comparative example.

Samples of consolidated glass fibre reinforced thermoplastic sheet materials were cut to a width of approximately 2 centimeters each and placed together side by side on the lower platen of a hot press. The upper platen was then closed to a height of 6 millimeters, which was considerably greater than the thckness of any of the samples. The press was then heated to a temperature of 200.degree. C., that is in excess of the softening point of the samples. After 5 minutes, the press was cooled and opened and the degree of expansion of each sample noted. The results are set out in Table 1.

It will be noted that only the two samples made by the papermaking methods expanded to fill the space between the platens and exerted sufficient pressure on the platen surfaces to produce a smooth, even surface.

The experiment was then repeated using samples made according to the process of UK Pat. Nos. 1129757 and 1329409, but using different thermoplastics as the matrix material and preheating the platens to 275.degree. C. The results are set out in Table 2 and it will be seen that the materials behaved in substantially the same way as the third and fourth examples of Table 1.

Although the flexural stiffness of these materials tend to increase during expansion, the extensional stiffness decreases so that a maximum flexural stiffness is reached at a degree of expansion that is found to be, very appropriately, 100%, that is when the expanded thickness is double that of the starting material.

It has been found that the proportion of glass fibre reinforcement used in the preparation of the material greatly affects its ability to expand. In general, samples containing more reinforcement expand more and produce a more porous surface. It is preferred to use at least 30% by weight of glass fibre in polypropylene (or an equivalent volume fraction in plastics of other densities) and at the most, 70% by weight. The best compromise for a good expansion and well bonded integral but porous surface is achieved with 50% by weight of glass fibre.

An improved end product may be achieved by laminating together several thin layers of materials of different compositions in the moulding process. Thus for example four sheets, each of a weight of 500 grams per square meter were prepared by the process of UK Pat. Nos. 1129757 and 1329409, two containing 40% by weight of 12 millimeter long glass fibre 11 microns in diameter and two 60% by weight of the same fibre, the balance of the content of each sheet being of polypropylene. The four sheets were assembled with the two outer layers being those containing the lower glass fibre content. The four sheets were then heated in an open press and it was noted that the two centre layers expanded more than the outer layers to produce a very thick, stiff sample with a smooth, even, well bonded surface.

Table 3 is a comparison of the effects of expansion between materials made according to the invention and two commercially available products produced by other processes. It will be seen that the materials of the invention could be expanded substantially without significant surface deterioration as compared with the commercially available products.

Table 4 shows the effect of glass content on expansion characteristics and demonstrates that an acceptable flexural modulus can be maintained for appropriate application even at a substantial degree of expansion.

Tables 5 and 6 show the effect of glass content on expansion, but in four ply sheets in which the various plies have differing glass contents.

Table 7 shows the effect of expansion on stiffness, demonstrating that stiffness increased with degree of expansion.

It will be evident that the expansion process can also be used with moulded articles which may be prepared for example according to the process described with reference to the accompanying drawings in which:

FIG. 1 is a cross section through a reinforced plastics sheet used in the process of the invention.

FIG. 2 is a cross section through an infrared heating oven, and

FIG. 3 is a cross section through a mould used in the process of the invention.

Referring first to FIG. 1, this shows a sheet 1 of thermoplastics material reinforced with glass fibres 2 of between 7 and 50 millimeters in length.

Referring now to FIG. 2, this shows an oven 3 having an infrared heating element 4 and a heat resistant belt 5 which is used to move the sheet 1 into and out of the oven for heating. The belt 5 is carried around tensioning rollers 6.

After heating, the sheet 1 is transferred to a mould 7 having upper and lower parts 8 and 9. The mould parts may, for example, be part of a suitable conventional hydraulic press means having a suitable conventional mould opening stop means to permit one of the mould parts 8, 9 to be withdrawn and provide an enlarged mould cavity. The stop means effectively controls the degree of expansion of the sheet by establishing a clerance between the mould parts 8, 9 which is less than the thickness which would be achieved by free unrestricted expansion. Thus, the mould portion 8 is inserted into the cavity of the mould portion 9 at least as far as necessary for the mould portions to have a clearance less than the thickness of free unrestricted expansion of the sheet. After introduction of the sheet 1 into the mould 7 the mould part 8 is advanced by the press means and, the parts 8 and 9 are subject to pressure. After the upper mould part 8 or forming is advanced, it is withdrawn from the second or lower mold part 9 to the stop means to allow the glass fibre component of the sheet 1 to recover and expand the moulding. In this arrangement, the mould parts may have a cooling means for simply cooling the mould when a previously heated sheet is shaped. The expansion process causes air to be drawn into the moulding and render it porous.

Alternatively, the mould parts may have heating and cooling means for heating the sheet while in the mould as well as for cooling the formed article in the mould, as indicated in the Comparative Example above.

Optionally, the process described can be varied in a number of ways. Thus, for example, the upper mould part 8 may be closed again after the sheet was expanded to partly consolidate it. Or the sheet may be allowed to freely expand to its maximum extent and then subjected to moulding. It will be appreciated that the degree of compaction will affect the porosity of the sheet and can therefore be used as a means of determining the porosity of the moulded article.

The porosity of the finished moulding is of particular benefit when it is intended to coat the moulded article with an adhesive or a soft plastics foam. By applying vacuum to the other side of the article with a suitable conventional vacuum means, good adhesion can be achieved without the necessity for drilling holes through the moulding.