Process for making ethylene-vinyl acetate copolymers of low melt index, and wax compositions containing the copolymers

Holder: Idem (71) (74) Agent: Simonnot, Rinuy, Santarelli.

(54) Process for producing a copolymer of vinyl acetate and ethylene.

72) Invention of:

(33) (32) (31) Conventional priority:

Sale of booklets at IMPRIMERIE NATIONALE, 27, rue de la Convention - PARIS (15^e)

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The copolymers of ethylene and of vinyl acetate containing more than 15% by weight of vinyl acetate constitute advantageous molding materials giving good flexibility and good properties at low temperature "without having an elasticity analogous to that of rubber. . Since these copolymers are entirely thermoplastic and have a significant permanent elongation, they are easier to manufacture than a vulcanized product, while being more flexible and extensible than homopolymers either of ethylene or of vinyl acetate.

These copolymers are also useful as additives for waxes, in order to increase the toughness and reduce the transmission of moisture or water vapor from the wax coatings on paper, cardboard or a foil. For such applications, resins having a low melt index are very much preferred, which are generally used in proportions of between 0.05 and 1 part by weight of the copolymer per part of paraffinic or petroleum wax. Larger amounts of the copolymer provide excellent adhesive compositions in the hot melt when a high bond strength of the laminate is required.

For any given composition of these copolymers, the best tenacity and bonding strength are obtained with high molecular weights, as evidenced by a melt index at 190 ° C. which is less than 5 and preferably comprised between 0, 1 and 3 decigrams per minute. However, it is difficult and expensive to produce copolymers within this range. Higher pressures, lower temperatures, and longer durations at polymerization temperatures must be used, and therefore conversion rates are lower and localized cross-linking tends to occur, which forms gel drops ( eyes) and localized degradation. In this lower range of melt indexes, the best flexibility is obtained with the highest percentages of vinyl acetate, but this ingredient is also the most expensive and the most difficult to adjust during interpolymerization at one low melt index. On the other hand, it is relatively easy to reach a melt index of 10 or more f

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It has been found that by applying relatively low doses of ionizing radiation to cheaper and easily produced copolymers of ethylene and vinyl acetate containing 15% or more of vinyl acetate in the combined state and having an average melt index greater than 5 decigrams per minute measured at 190 ° C, it is now possible to reduce this melt index below 5 and even between 0.1 and 1, without the material is transformed into elastomer, that is to say without destroying its thermoplasticity, without reducing its solubility or without producing measurable amounts of crosslinked resin. It has also been found that at a given melt index which is less than 5 and that with a given vinyl acetate content, the irradiated copolymer prepared in this way is not only as flexible as a copolymer containing a significantly greater amount of vinyl acetate directly polymerized at this index in the molten state, but also surprisingly gives better properties to the wax mixtures.

The copolymer can be irradiated when it is in the molten state, for example, when it leaves an extruder or when it is solid in the form of a sheet, a fiber, a strip, d 'a powder or lozenges. This latter irradiation process is preferred for the sake of convenience and simplicity. Any suitable treatment apparatus or device can be used, provided that it uniformly exposes the copolymer to high energy radiation without localized overexposure which could cause crosslinking or gel formation.

The source and type of high energy ionizing radiation is of little importance. Gamma rays, Z rays, beta rays, protons, deuterons or alpha particles can be used. In the particular examples given in the present application, use is made of a cathode emission of a Van de Graff electron accelerator described by FL Foster and his collaborators in Nueleanies, October 1953, Vol. 11, No. 10, pages 14-17 (McGraw Hill Publisbing Co., Inc., New York).

The dose required for this treatment is between

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0.5 and 1.6 megarep (one megaep corresponding to 83.8 x 10 ^ ergs per gram of irradiated material), but it is in proportion to the initial melt index of the polymer. The following table A gives the typical effects on the melt index of copolymers given by way of illustration containing various quantities of vinyl acetate, using various doses included.

<td colspan="5">in this range:</td>

<td colspan="2" rowspan="2">Acetate</td><td colspan="2">TABLE A</td><td rowspan="3">Final index at molten state, dg / min</td>

<td rowspan="2">Initial index in the molten state, dg / min</td><td rowspan="2">Dose, megarp</td>

<td>vinyl,</td><td> %</td>

<td> 25</td><td></td><td> 26,2</td><td> 1,12</td><td> 1,66</td>

<td> 20</td><td></td><td> 20,0</td><td> 1,50</td><td> 2,25</td>

<td> 25</td><td></td><td> 17,4</td><td> 0,765</td><td> 1,90</td>

<td> 25</td><td></td><td> 17,4</td><td> 1,00</td><td> 1,04</td>

<td> 25</td><td></td><td> 17,4</td><td> 1,28</td><td> 0,75</td>

<td> 25</td><td></td><td> 17,4</td><td> 1,38</td><td> 0,49</td>

<td> 25</td><td></td><td> 17,4</td><td> 1,60</td><td> 0,30</td>

<td> 28</td><td></td><td> 17,0</td><td> 1,00</td><td> 2,90</td>

<td> 28</td><td></td><td> 17,0</td><td> 1,25</td><td> 2,20</td>

<td> 28</td><td></td><td> 17,0</td><td> 1,60</td><td> 1,20</td>

<td> 28</td><td></td><td> 13,6</td><td> 1,00</td><td> 1,00</td>

<td> 29</td><td></td><td> 13,4</td><td> 0,78</td><td> 1,04</td>

<td> 29</td><td></td><td> 13,4</td><td> 0,85</td><td> 0,78</td>

<td> 29</td><td></td><td> 13,4</td><td> 1,15</td><td> 0,43</td>

<td> 28</td><td></td><td> 5,71</td><td> 0,75</td><td> 0,75</td>

<td></td><td>All</td><td>irradiated materials</td><td>of the whiteboard</td><td>A are whole-</td>

thermoplastic and completely soluble in hot ethyl acetate, in hot carbon tetrachloride and in hot toluene. It can be seen that the copolymers having an initial melt index of between 5 and 30 are easily transformed by the invention into copolymers having a melt index of less than 3. However, in all cases, the irradiation should not be sufficient to cause crosslinking or the formation of gel drops in the copolymer. This maximum radiation dose naturally depends on the index

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As a further example of the invention, a copolymer of ethylene and vinyl acetate containing 20% ​​by weight of vinyl acetate and having an average melt index of 20 is irradiated in the form of pellets. decigrams per minute with a dose of 1.5 megarep and a material is obtained having an average index on the molten hem of 2.25 decigrams per minute. The irradiated material obtained, which will be designated hereinafter as copolymer A, is mixed with 12.5 parts by weight of a part of a stabilizing agent under commercial ultraviolet light (a dialkylhydroxyphenylbenzotriazole sold by Geigy Industrial Chemicals Co. under the designation Tenuvin 327) and, on the other hand, a commercial antioxidant (Irganox 1093 from Geigy) for 2,475 parts of copolymer A. For comparison, another otherwise identical mixture is prepared with a copolymer of ethylene and 20% vinyl acetate having a melt index at 190 ° 0 of 2.06 decigrams per minute, prepared by direct polymerization at this melt index (control copolymer

3) »Plastic sheets are extruded from these mixtures using a die forming a tubular sheet.

<td colspan="2">make and identical extrusion conditions.</td><td colspan="2">We determine the</td>

<td>physical properties of the leaves and</td><td>they are</td><td>the</td><td>following:</td>

<td>TABLE B</td><td>Mixed</td><td>of</td><td>Mixture of</td>

<td></td><td colspan="2">copolymer A</td><td>copolymer</td>

<td>Elongation, in the direction of</td><td> 612</td><td></td><td>witness B 644</td>

<td>machine,% crosswise,%</td><td> 600</td><td></td><td> 636</td>

<td>Secant module (Rigidity) ₂in machine direction, 'kg / cm</td><td> 273</td><td></td><td> 498</td>

<td>crosswise, kg / cm²</td><td> 286</td><td></td><td> 501</td>

Light transmission,%

91,7

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The lower modulus of copolymer A is a measure indicating its greater flexibility. To obtain this lower modulus by direct polymerization at a melt index of about 2, the amount of vinyl acetate should be increased significantly.

Another mixture of copolymer A is prepared containing, for each proportion of 2475 parts by weight of copolymer A,

12.5 parts of both the same UV stabilizer and the same antioxidant, 2.5 parts by weight of a commercial blocking agent (a 70% mixture of stearamide and 30% palmitamide sold by General Mills, Inc. under the designation Alamide ”H-26) and 250 parts by weight of a mixture of 80% of copolymer A and 20% of very finely divided silica (Superfloss) . For comparison, another otherwise identical mixture is prepared using copolymer B. Again, plastic sheets are formed using a tubular die and identical conditions of extrusion. The physical properties are as follows:

TABLE C

Mixture of

<td></td><td>Blend of copolymer A</td><td>eopolymer witness B</td>

<td>Elongation, in the direction of</td><td> 622</td><td> 642</td>

<td>machine,% crosswise,%</td><td> 622</td><td> 624</td>

<td>Secant module (Rigidity), in machine direction, kg / çm</td><td> 352</td><td> 540</td>

<td>crosswise, kg / cm²</td><td> 312</td><td> 562</td>

<td>Light transmission,%</td><td> 90</td><td> 89,4</td>

Typical properties of the eopolymers prepared according to the present invention with various vinyl acetate contents and various melt indices are given below:

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TABLE D

Average tensile strength, kg / cm ^

Elongation,%

Modulus of elasticity kg / em2 ÿi a'acetate of

-ri nyt a, melt index of 2.25 185

26.4% vinyl acetate, melt index of 1.09 10?

29.5% vinyl acetate, melt index of 1.67 128

606

745

817

280

158

164 the greater flexibility of the copolymers treated according to the present invention confers greater flexibility on blends with polyethylene, for example. By way of illustration, a mixture of polyethylene is prepared with 862.5 parts by weight of polyethylene (density of 0.92, melt index of 0.4); 25 parts of titanium dioxide; 12.5 parts of both an UV stabilizer (Tenuvin 327), an antioxidant (Irganox 1093); 25 parts of an agent preventing solidification ('' Alamide H-26); and 26 parts of very finely divided silica (Superfloss), they are mixed in a Banbury mixer, they are put in the form of sheets on a two-cylinder mill and granulated. Mixtures are prepared, with 38% of the above ingredients and 62 respectively, of the copolymer A of the invention and of the control copolymer B, for comparison. Compression molded plates

<td colspan="3">with these mixtures respectively have the following properties:</td>

<td>TABLE E</td><td>Blend of copolymer A</td><td>Blend of the copolymer witness B</td>

<td>Molten index, 190 ° G</td><td> 1,20</td><td> 1,66</td>

<td>Elongation,%</td><td> 720</td><td> 720</td>

<td>Secant module (rigidity), kg / cm</td><td> 700</td><td> 960</td>

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Sheets prepared with these two blends of polyethylene and a copolymer are inflated using a die forming a circular sheet and identical conditions of blowing. The properties of the sheets are as follows:

¹O

TABLE F

<td></td><td>Mixture of copolymer A</td><td>Blend of the copolymer witness B</td>

<td>Elongation, in the machine direction,% crosswise,%</td><td> 596 560</td><td> 612 602</td>

<td>Secant module (rigidity), ₂in machine direction, kg / cin</td><td> 689</td><td> 960</td>

<td>crosswise, kg / cm²</td><td> 668</td><td> 910</td>

The greater flexibility of the copolymers prepared according to the present invention also surprisingly confers greater flexibility on mixtures with petroleum or paraffin waxes. For example, mixtures of 70% petroleum wax (Wax No. 1116 from Atlantic Refining Co.) and 30% of ethylene and vinyl acetate copolymers comprising 28% vinyl acetate, having an initial melt index of 15 decigrams per minute and with doses of ionizing radiation of 0, 1, 1.25 and 1.6 megarep, respectively, were molded into test sheets d '' a length of 50 mm, a width of 12.7 mm and a thickness of 1.27 mm.

The test sheets were bent 90 ° at the rate of 28 cycles per minute without failing, the results being as follows:

TABLE G

Megarep dose

1,25

1,60

Final melt index, dg / min

2,9

2,2

1,2

Average longevity, bending cycles

280 + 30 660 + 20

700

900 + 20

All of these copolymers are completely soluble in molten paraffin wax and are completely thermoplastic.

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A much more rigorous test involves injection molding of 1.78 mm thick samples, cutting them into 12.7 mm by 41 ”3 mm sheets, clamping 15.9 mm from one end with a force of 0.0678 kgm and subjecting the projecting end to a bending of 90 °, first in one direction, then in the other, on a 38.1 mm idler roller at a rate of 30 cycles per minute. The clearance between the device holding the sample sheet and the top of the roll is 7.9 mm. Tests carried out at 25^VSC using this procedure demonstrate the marked improvement in flexibility of irradiated samples with a low melt index. The retention of flexibility by samples aged for a week before the test is particularly remarkable at a melt index of less than one decigram per minute. Particular results are given in the following table and were obtained with mixtures of 65% of a paraffinic wax (Wax N ° 1116 from Atlantic Refining Co.) and 35% of a copolymer of ethylene and acetate vinyl. The longevities indicated correspond to averages of six tests each.

TABLE H

<td>Acetate of vinyl,%</td><td>Initial melt index dg / min</td><td>Radiation dose, megarep</td><td>Final melt index dg / min</td><td colspan="2">Longevity, bending cycles after aging 4 hr 1 week</td>

<td> 28</td><td> 26,2</td><td>iN w</td><td> 26,2</td><td> 42</td><td> 21</td>

<td> 25</td><td> 20,6</td><td> 0</td><td> 20,6</td><td>S 2</td><td> 31</td>

<td> 25</td><td> 17,4</td><td> 0</td><td> 17,4</td><td>-, X</td><td> 53</td>

<td> 29</td><td> 13,4</td><td> 0</td><td> 13,4</td><td> 163</td><td> 47</td>

<td> 28 ·</td><td> 5,7</td><td> 0</td><td> 5,7</td><td> 504</td><td> 284</td>

<td> 25</td><td> 2,14</td><td> 0</td><td> 2,14</td><td> 555</td><td> 303</td>

<td> 25</td><td> 17,4</td><td> 0,765</td><td> 1,90</td><td> 546</td><td> 339</td>

<td> 29</td><td> 13,4</td><td> 0,78</td><td> 1,04</td><td> 629</td><td> 378</td>

<td> 29</td><td> 13,4</td><td> 0,85</td><td> 0,78</td><td> 684</td><td> 381</td>

<td> 25</td><td> 17,4</td><td> 1,28</td><td> 0,75</td><td> 671</td><td> 409</td>

<td> 25</td><td> 17,4</td><td> 1,38</td><td> 0,49</td><td> 734</td><td> 451</td>

<td> 29</td><td> 13,4</td><td> 1,14</td><td> 0,43</td><td> 793</td><td> -36</td>

<td> 25</td><td> 17,4</td><td> 1,60</td><td> 0,30</td><td> 1083</td><td> 596</td>

<td> 28</td><td> 5,7</td><td> 0,75</td><td> 0,77</td><td> 1024</td><td> ^74</td>

<td> 25</td><td> 2,14</td><td> 0,50</td><td> 0,74</td><td> 841</td><td> 724</td>

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It has also been noted that these mixtures of a wax and of a copolymer exhibit better hot adhesion properties, which are only obtained when the copolymer is prepared according to the present invention. These unusual results obtained with mixtures of paraffin waxes and these irradiated copolymers can be achieved with compositions containing an amount as low as 0.05 part by weight of the irradiated copolymer per part of wax and, preferably, a proportion of between 0 , 2 and 1 part of the copolymer per part by weight of the wax, which seems to give optimal results.

The physical properties indicated in the present application were determined by the following test processes:

Tensile strength: ASTM-D-412-51T Elongation,%: ASTM-D-412-51T

Melt index: ASTM-D-1238-57T, measurement made at 190 ° C Secant module: ASTM-D-638, except that the test samples were cut using the iator A of the ASTM method -D-1412. The point of deformation determined from the stress-strain curve of the automatic recorder was calculated as corresponding to 100 times the ratio of the load at a deformation of 1% to the arithmetic product of the average width and the average thickness .

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