GB1565347A - Production of plastics articles - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO THE PRODUCTION OF PLASTICS ARTICLES (71) We, PEROXID-CHEMIE GMBH, a German company organised under the laws of Germany, of D-8023, Hllriegelskreuth, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the production of plastics articles from thermoplastic polymers in powder or granular form.

Processes of this type are known in which the thermoplastic polymer, when cold, is in the form of a powder or granules which by heating in a mould coalesce or melt on contacting the hot mould wall forming a continuous layer shaped in conformity with the shape of the mould. Such processes are known as "powder moulding". The articles which are produced by such process may be of a hollow shape such as containers, floating articles such as boats, floating bodies for fish-nets, bowls, cups or similar articles.

The walls of the articles may consist of one single layer of thermoplastic material but they may also consist of more than one plastics layer, each being in contact with the next. Part of the layers may consist of open or closed cells filled with a gas to improve insulating properties or to make the whole system more flexible so that layers of different coefficients of heat expansion can be adhered without deformation. Foam structures between the layers may also serve as specific weight reducers, for instance in boat walls, surf boards and floats. Powder moulding can also be used to produce nonself-supporting items, such as interior or exterior tube linings, roving cans, packaging forms used within supporting enclosures such as drums or tubs made of metal or wood.

Methods for forming the abovementioned articles include the so-called sintering process for thermoplastics (see for instance British Patent 1,115,639). According to the sintering process one may use a mould that rotates and/or rocks about one or more axes and is heated from the outside; and slow rotational and/or rocking movement is said to ensure even distribution of the moulding powder or granules over the inner hot mould surface, forming in this way a uniform molten layer on the inner surface.

After the formation of a layer, a powder or granule charge of the same or another composition may be introduced into the mould which on continued heating forms a second layer on the previously-formed first layer. Each time, practically the whole charge is used up for the layer formation. A third layer can be formed in the same way on the second one. The powder (in this specification the term includes granules unless specifically excepted) may contain all kinds of additives such as anti-oxidants, ultraviolet stabilizers, pigments, special copolymers such as ethylene-vinyl acetate to reduce stress cracking, anti-static compositions or compounds and flame retardants.With the addition of blowing agents, one or more foamed layers can be formed, and softening or melting of the thermoplastic composition can be prevented or retarded by the addition of cross-linking agents, as a rule containing organic peroxides as the active principle. As well as "sintering", which has a different meaning from metal sintering (the solid thermoplastic powder loses its identity by coalescing a continuous layer), other methods have been suggested such as moving the powder within a heated rotating metal tube, which may be in an inclined position, or using a fluidized bed in which the hot mould is dipped so as to be covered by a molten layer of the fluidized thermoplastic.

Although many additives are known there are still quite a few difficulties especially, but not exclusively, when fabricating larger articles. These problems may be caused for example by sagging of the fluid plastic layers, by large differences between the thermal properties of the different layers, by stress cracking and by discoloration.

The use of cross-linking peroxides in polyolefins is suggested in British Patent 1,209,530.

These peroxides are also used in sintering and other powder moulding processes. In powder moulding with the addition of crosslinking peroxides there are some disadvantages. In nearly all cases gas bubbles are enclosed in the material of the wall structure. Moreover the development of gas is a potential danger for explosion and a fire hazard. When a large container or a similar article is produced by powder moulding, the inner surface is nearly always somewhat lumpy and special measures have to be taken to improve the surface and to make it smoother.

In the making of hollow articles filled with polyurethane (such as surf riders and double-walled containers) difficulties arise because of poor adherence of the polyurethane to the walls.

Stress cracking often occurs when the inner walls of containers are contacted with liquids, especially surface-active liquids.

Discoloration is a problem that is only solved by very accurate control of temperature and despite accurate temperature control is still a problem with light colours.

Containers used in the food industry often develop an obnoxious odour. The peroxide mixtures tend to attack the mould wall which may also cause problems when releasing the article from the mould.

It has been surprisingly discovered that one or more of the aforesaid problems can be alleviated by the use of certain nonperoxygen cross-linking agents. However, the present invention is not restricted in its applicability to modifications of the known processes that are described above.

The present invention provides a process for the production of a plastics article5 which comprises the steps of (a) preparing a mixture comprising a thermoplastic polymer, especially a polyolefin, in powder or granular form and a compound of the general formula 1,

wherein each of R1 and R2, which may be the same or different, represents an alkyl group containing from I to 3 carbon atoms, and (b) heating the mixture to a temperature such that cross-linking of the thermoplastic polymer occurs.

The plastics articles which can be produced include both self-supporting articles and articles which are not selfsupporting.

The description and the Examples will refer predominantly to high-density polyethylenes where the results are most surprising but similar results though generally less pronounced can be achieved with low-density polyethylene and polypropylene and copolymers of ethylene and propylene.

A preferred cross-linking agent of the general formula I is 2,3-dimethyl-2,3diphenylbutane.

Because compounds of the general formula I contain no axygen in the molecule, release of the articles from the mould and corrosion of the mould walls is generally less troublesome, and gas development is practically absent.

Moreover, the rigidifying action of the compounds of the general formula I is generally unaffected by the presence of conventional anti-oxidants, U.V. stabilizers, pigments, ethylene/vinylacetate copolymers, flame retard ants, blowing agents and antistatic compounds.

It is suggested that the advantages of the use of the compounds of the general formula I arise because in general they begin to cause cross-linking only at somewhat higher temperatures than commonly-used peroxides. However, we do not intend by this explanation to limit the scope of the invention. We here make no suggestion as to the identity of the radical which is active in cross-linking.

The present invention may be applied to the production of large to very large articles as well as in manufacturing articles of small volume, toys of any shape, foam-filled articles and powder-moulded inner or outer layers.

The compound of the general formula I may be used in a quantity of from 0.2 to 5%, preferably from O.5 to 2%, based on the total weight of the mixture.

Of course, one may add less than 0*2 /a or more than 5% of the compound of the general formula 1; but less than 0.2% produces a technically inferior result and more than 5% does not add to the advantages already obtained.

The main advantages that may be achieved according to the present invention (compared with the use of peroxides such as dicumyl peroxide or other cross-linking agents) concern the absence of bubbles on the inner surface of hollow articles, the formation of smooth surfaces, strong adherence of foams, especially polyurethane foams, and improved impact strength at lower as well as higher temperatures.

Stress-cracking properties have been observed that were distinctly superior to known cross-linked materials.

In larger open hollow structures such as boats with double walls filled with polyurethane or even, if desired, with crosslinked polyethylene foam, the foam was retained in position even after years of service. The compound of the general formula I be added to one or several charges in the moulding of a laminated article and in this way different results can be achieved. A typical construction, for instance of the walls of a food container, may consist of an inner layer of high-density polyethylene not cross-linked by means of a compound of the general formula I and an outer layer of highdensity polyethylene cross-linked by means of such a compound separated by a layer of high density foamed polyethylene which has been likewise cross-linked.

The present invention may also be advantageously applied to the extrusion of shaped articles, especially tubes. There too the extrusion speed can be increased considerably.

Especially in cases where requirements are less severe one may use a compound of the general formula I in combination with a peroxide cross-linking compound. Crosslinking activity of the compounds of the general formula I is about one third of the activity of dicumyl peroxide.

The additions of not more than 0.20/,, preferably 0.02 to 0.05 wt. %, of peroxide based on the total weight of the mixture activates cross-linking in such a way that one may take half the quantity of the compound of general formula I and maintain the same cycling time, or else lower the cross-linking temperature.

Suitable peroxides include dicumyl peroxide, tertiary butyl hydroperoxide, and di-tertiary-butel peroxide. The use of more than 0.2 wit.% of peroxide tends to cause troublesome gas development.

The invention will now be further illustrated by the following Examples.

Example I This Example concerns the moulding of a single-walled container of 3588 litre (770 Br.

gallons) capacity, diameter 1880 mm (85.4M), eight 2000 mm (76.7tut), and wall thickness 9 mm (.35n), The total weight was 100 kg (220 Ibs). Polyethylene was used having density 0.950, melt index 8. To the powdered polyethylene was added 2 wt% of 2,3dimethyl-2,3-diphenylbutane (i.e. R, and R2 were methyl groups), 2 wtO/, UV-stabilizer, 0.80% anti-oxidant and 0.75% of a yellow pigment. Use was made of a moulding unit composed of a rotatable mould, moving up and down over a total angle of 90". The rotation speed was 20 rpm, and the frequency of full up and down movements was 12 per minute. The articles could be taken out of the mould after 30 minutes (cycle time).Heating was by gas burners from the outside, and the temperature was raised to 240"C.

The container produced had a very smooth inner wall, and a practically constant wall thickness over the entire article.

Comparative Example Example 1 was repeated with the sole exceptions that the 2,3-dimethyl-2,3diphenylbutane was replaced by the same weight of dicumyl peroxide, that the temperature had to be kept below 230"C, and that the cycle time was increased to 50 minutes. Notwithstanding the lower temperature, there were many gas bubbles and also quite a number of pinholes in the container produced. The container displayed severe sagging of the plastics material on the inner side giving rise to very uneven wall strength, and also appreciable discoloration.

Example 2 In the same equipment as described in Example I a similar container was made with the same volume, but with a laminated wall structure. The first (outer) layer was built up from 40 kg (88 Ibs) of a mixture of the same composition as that used in Example 1. Conditions were also indentical.

Without solidifying the first layer but after having used up the first charge, a second mixture was introduced into the mould, having the same composition as the first one except that 5 kg (11 Ibs) blowing agent had been added to 95 kg (209 Ibs) of the mixture.

Of this latter mixture 25 kg (58 Ibs) was used to build up the second (foamed) layer. For the third layer there were used 35 kg (77 Ibs) of a mixture similar to that used for the first layer but not containing any 2,3-dimethyl2,3-diphenylbutane. The temperature in all cases was above 2300 C. The inner surface of the finished article was smooth and uniform and the wall thickness did not show any unacceptable variations.

Example 3 With the equipment of Example I a container of the same volume was made which had an outer layer of UV-stabilized cross-linked polyethylene (60 kg, 132 Ibs) of the same composition as in Example 1 and an inner layer of natural high density polyethylene to which no UV-stabilizer, no anti-oxidant and no cross-linking agent was added in view of its intended use in contact with food.

Of the "natural" polyethylene, 40 kg (88 Ibs) were used.

The result was a bi-laminated container of excellent quality whereas with peroxide under the same conditions a discoloured container with a porous appearance and rough interior surface was obtained.

Example 4 An extruder used for the extrusion of pipes with an interior diameter of 2 inches showed an increase in capacity of some 30 / when the 2,3-dimethyl-2,3-diphenylbutane was added to the powdered polyethylene instead of the same quantity of tertiary butyl peroxide used previously. This was presumably partly due to the fact that the temperature could be increased by 15 to 200C without the formation of gas bubbles.

Example 5 On repeating the Examples 1 to 4 but with a compound of the general formula I in which R1 was -C2H5 and R2-C2H5 or -C3H7 very satistactory results were obtained at slightly lower temperatures. However, performance was not as good as with 2,3dimethyl-2,3-diphenylmethane, which was preferable as it showed nearly complete independence from other additives and from temperature variations.

WHAT WE CLAIM IS: 1. A process for the production of a plastics article, which comprises the steps of (a) preparing a mixture comprising a thermoplastic polymer in powder or granular form and a compound of the general formula I,

wherein each of R, and Rz, which may be the same or different, represents an alkyl group containing from 1 to 3 carbon atoms, and (b) heating the mixture to a temperature such that cross-linking of the thermoplastic polymer occurs.

2. A process as claimed in claim 1, wherein the weight of the compound of the general formula I present in the mixture prepared in step (a) is from 0.2 to 5 per cent of the total weight of the mixture.

3. A process as claimed in claim 2, wherein the weight of the compound of the general formula I present in the mixture prepared in step (a) is from 0.5 to 2 percent of the total weight of the mixture.

4. A process as claimed in any preceding claim, wherein the compound of the general formula 1 is 2,3-dimethyl-2,3diphenylbutane.

5. A process as claimed in any preceding claim, wherein the thermoplastic polymer is a polyolefin.

6. A process as claimed in claim 5, wherein the polyolefin is polyethylene, lowdensity polypropylene, or a copolymer of ethylene and propylene.

7. A process as claimed in claim 5, wherein the polyolefin is high-density polyethylene.

8. A process as claimed in any preceding claim, wherein the heating in step (b) is performed in a mould which is heated from the outside and which is rotated about one or more axes and/or rocked about one or more axes.

9. A process as claimed in any preceding claim, wherein the plastics article is a laminated article.

10. A process as claimed in any one of claims I to 8, wherein the plastics article is a tube and is produced by extrusion.

11. A process as claimed in any preceding claim, wherein in step (a) a peroxide crosslinking agent is incorporated in the mixture prepared, the weight of such peroxide crosslinking agent being not more than 0.2% of the total weight of the mixture.

12. A process as claimed in claim 11, wherein in step (a) a peroxide cross-linking agent is incorporated in the mixture prepared, the weight of such peroxide crosslinking agent being from 0.02 to 0.05% of the total weight of the mixture.

13. A process as claimed in claim 11 or claim 12, wherein the peroxide cross-linking agent is dicumyl peroxide, di-t-butyl peroxide, or t-butyl hydroneroxide.

14. A process for the production of a plastics article substantially as described in any one of Examples 1 to 4 herein.

15. A process for the production of a plastics article substantially as described in Example 5 herein.

16. A plastics article whenever produced by a process as claimed in any preceding

Claims (1)

1. claim.