Makromol. Chem., Rapid Commun. 2,749 - 755 (1981) 749

Crosslinking of Ultra-High Molecular Weight Polyethylene in the Melt by Means of 2,5-dimethyl-2,5-bis(tert-butyldioxy)-3- hexyne

Jan de Boer, Albert J. Pennings*

Laboratory of Polymer Chemistry, State University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands

(Date of receipt: October 14, 1981)

Introduction

The aim of the present study was to synthesize homogeneous networks of ultra- Ggh molecular weight polyethylene by means of peroxide curing in the melt. Ultra- high molecular weight polyethylene is a unique material in that it exhibits a combina- tion of attractive properties such as high impact strength and high abrasion resistance'). However, it may still be possible to improve these outstanding material properties by introducing crosslinks in the ultra-high molecular weight polyethylene.

It has been suggested that in order to obtain homogeneous networks, crosslinking ultra-high molecular weight polyethylene in the melt should be performed at temperatures above 150°C in order to avoid irregularities due to the presence of remnants of the initial solid state of polyethylene*). Therefore, the crosslinking was performed at 180 "C, by means of the bifunctional peroxide 2,5-dimethyl-2,5-bis(tert- butyldioxy)-3-hexyne (l), which is known to crosslink p~lyethylene~.~), and decomposes at an appreciable rate only at high temperatures".

This paper deals with the synthesis and characterization of networks from ultra- high molecular weight polyethylene, obtained by crosslinking in the melt at 180°C by means of 1. Sol-gel analysis showed that during crosslinking no mainchain scission- ing occurred. From a plot of the effective network chain density as a function of the peroxide concentration, a number average molecular weight between entanglements close to the equilibrium value for a polyethylene melt was found, suggesting that in spite of the high melt viscosity, rather homogeneous networks were obtained. IR spectroscopy showed only little chemical modification of the polyethylene upon crosslinking.

0173-2803/81/ooO2/0749/$03.00

7 50 J . de Boer, A. J. Pennings

Experimental Part

The ultra-high molecular weight polyethylene used in this study was HiFax 1900 (from Hercules), with a weight average molecular weight of 4. lo6 kg . kmol-', which was used as received. The peroxide 2,5-dimethyl-2,5-bis(tert-butyldioxy)-3-hexyne (1) (Luperox 130; from Wallace, Tiernan) was purified twice by destillation i . vac.

Mixing of the polyethylene with the peroxide was accomplished by dispersing the polymer powder in an acetone solution of the peroxide and subsequently evaporating the solvent'). After degassing the mixture, crosslinking was brought about by compression,molding at 180°C for 3 h (half-life of the peroxide at 180°C in polyethylene equals 9 min4)).

Extraction of the sol-fraction was performed in boiling p-xylene, containing 0,5 weight-% of anti-oxidant (Ionol CP 0275; from Shell). Samples were deswollen in acetone and dried i. vac. at 50 "C. The equilibrium degree of swelling was determined as described previously').

IR spectroscopy was performed using a Pye Unicam SP 3-300 infrared spectrophotometer, on films which were obtained by keeping the network at l0OoC under elevated pressure for 2 h. This temperature was chosen in order to avoid any further chemical reaction.

Results and Discussion

The amount of peroxide 1 used to crosslink the ultra-high molecular weight poly- ethylene, abbreviated as UHMWPE, was varied from 0,048 up to 1,571 weight-%. The gel content of the polymer increased from 25,5 to 100%, while the equilibrium degree of swelling decreased from 29,l down to 3,O. The results of the curing experi- ments are compiled in Tab. 1 .

Tab. I . Network characteristics of ultra-high molecular weight polyethylene (HiFax 1900) crosslinked in the dry state at 180°C by means of 2,5-dimethyl-2,5-bis(tert-butyldioxy)-3- hexyne (1)

Weight-% lo3. [l] lo-'. [1]-' Gel s + s ' ' ~ a) Equili- 102. V* b,

of 1 mol . dm-3 kg . mo1-l Content mol.dm-3 in % degree

of swelling

0,048 0,078 0,110 0,150 0,196 0,258 0,373 0,488 0,911 1.571

1,40 2.33 3,25 4,48 5,86 7,71 11,14 14,59 27,22 46,96

5,958 3,667 2,600 1,907 1,459 1,109 0,767 0,586 0,314 0.182

25,5 60,4 79,8 85,O 91,8 96,4 97,8 97,9 99,3

100,o

1,60 1,03 0,65 0,54 0,37 0,23 0,17 0,17 0,09 0,o

0,20 I ,05 5,61 8,28

1 I,O 12,6 13,O 13,7 16,l 19,2

a) s: sol fraction. b, v*: effective network chain density.

Crosslinking of Ultra-High Molecular Weight Polyethylene. . . 75 1

Gelation

The first subject to be considered is the gelation of the UHMWPE due to the reaction with 1. In order to determine whether any main-chain scissioning occurred during crosslinking, the results of the sol-gel analysis were interpreted according to the Charlesby-Pinner equation5), adapted for chemical crosslinking6). Upon cross- linking a polymer, with initially a random molecular weight distribution, the Charlesby-Pinner equation relates the sol-fraction to the amount of crosslinking agent. As in the present case the crosslinking reaction is thought to proceed through a cage mechanism2!’), it was thought pertinent to insert a quantity p, representing the fraction of units in primary chains capable of crosslinking8), into the Charlesby- Pinner equation. Why should this quantity p be introduced? In order to be able to answer this question, we first have to discuss the cage mechanism. According to the cage mechanism, crosslinks can only be formed if at the moment of decomposition of a peroxide molecule, two segments of polymer molecules form part of the cage surrounding the peroxide molecule. The peroxide molecule should yield two radicals, both equally capable of abstracting a hydrogen from the polymer segments. The thus obtained polyethylene radicals can combine to give a crosslink prior to radical diffusion. As not all polymer segments will be part of a cage surrounding a decomposing peroxide molecule, the cage mechanism excludes a number of poly- ethylene units from the possibility of becoming involved in a crosslink. Therefore, the quantity p may be considered as an indication for the dimensions of the cage.

The Charlesby-Pinner equation then transforms to:

in which s represents the sol-fraction, p1 and q1 are the scission and crosslinking probabilities, respectively, &fw is the weight average molecular weight of the initial polymer, [l] is the peroxide concentration in mol -g-’, and finally E stands for the number of tetrafunctional crosslinks produced per mol of peroxide 1, i. e. the cross- linking efficiency.

According to Eq. (1) a plot of s + s1l2 versus the reciprocal peroxide concentration should give a straight line. Such a plot is shown in Fig. 1. A straight line through the origin was obtained. As for any type of initial molecular weight distribution, s + sl/* will extrapolate to zero at high peroxide concentration in the absence of scissioning5), it was concluded from Fig. 1 that in the present case no chain-scissioning occurred, implying that pl/ql in Eq. (1) equals zero. By extrapolating the plot to a (s + s ” ~ ) - value of 2, corresponding to zero gel content, i.e. initial gelation, a gelpoint of 1,15 + lo-’ mol of peroxide per liter polyethylene was found. With p l / q r = 0 and s = 1, Eq. (1) reduces to the critical condition for incipient gelation for any initial molecular weight distribution*). Combining these results a value of 9,28 - for the product p - E was calculated from the gelpoint.

752 J. de Boer, A. J. Pennings

I‘

2.0

3 - -

b c - - Y Fig. I . Plot of s + s”* -

versus reciprocal peroxide concentration for ultra-high molecular weight polyethylene

- (HiFax 1900) crosslinked in the dry state at 180°C by means of 1

- 1.0 - - -

-

1.0 2.0 3.0 L O 5.0 6.0 7.0 10-2. [1]-1/(kg. mol-’)

Equilibrium degree of swelling

In the range of the peroxide concentration used, the decrease of the equilibrium degree of swelling with increasing peroxide concentration, was most pronounced at relatively low peroxide concentration, as is illustrated in Fig. 2.

10 20 30 1 0 50 103.[l]/ (rnol. drn-3)

Fig. 2

10 20 30 LO 50 103 .Ill/ (rnol . d n i 3 )

Fig. 3

Fig. 2. Equilibrium degree of swelling, q, versus peroxide concentration for ultra-high molecular weight polyethylene (HiFax 1900) crosslinked in the dry state at 180°C by means of 1 Fig. 3. Effective network chain density, v*, versus peroxide concentration for ultra-high molecular weight polyethylene (HiFax 1900) crosslinked in the dry state at 180°C by means of 1

The equilibrium degree of swelling, q, was used to calculate the effective network chain density, Y*, using the recent swelling theory of Flory”. From this theory a formula for the molecular weight between crosslinks was derived’O), which can be transformed into the following equation for the effective network chain density:

Crosslinking of Ultra-High Molecular Weight Polyethylene. . . 153

where x , the Flory-Huggins interaction parameter, is given by x = 0,33 + 0,55 * 4- ' ' I ) , pi, the partial molar volume of the diluent, is 136 dm3 * kmol-', d, the network density, is 0,855 kg . dm-3, Mn, the number average molecular weight of the initial polymer, equals 1,5 * 16 kg . kmol-' and u, represents the gelfraction (2 d/mn is the correction for chain-ends). The theoretical values of Fe are given by lo):

in which @ is the crosslink functionality, p is the number of crosslinks in the network and r represents the cycle rank of the network. For a perfect @-functional network

holds - = 2/(@ - 2). The quantity K is a function of q and also of two network

parameters p and K, for which reasonable estimates are p = 2 and K = 20 9 3 lo).

Before we can combine all this information to calculate v*, we first have to consider the functionality of the crosslinks in the present system. It is known that 1 can yield two radical pairs upon decomposition'*). All four radicals produced are capable of hydrogen abstraction from the polyethylene matrix, implying that in principle each peroxide molecule can create two tetrafunctional crosslinks. However, these two crosslinks will be created within the same cage i.e. in close vicinity of each other. Upon elastic deformation, such as swelling, these two tetrafunctional cross- links will manifest themselves as one hexafunctional crosslink, when the chain in be- tween the crosslinks is too short to be elastically activei3). As a first approximation it was, therefore, thought appropriate to treat the network as one consisting of hexa- functional crosslinks, i.e. @ = 6, upon swelling. The results of the calculations of v* are presented in Tab. 1 and Fig. 3.

From Fig. 3 the following conclusions can be drawn. First of all this plot offers a second independent possibility of determining the gelpoint of the system. By extrapolating the curve to a v*-value of zero, a gelpoint of 1,3 - mol of peroxide per liter polyethylene was found, which is in excellent agreement with the value obtained from the sol-gel analysis. Secondly, the intercept on the v*-axis, obtained by extrapolating the linear part of the curve at relatively high peroxide concentration, corresponds to twice the entanglement concentration in the network2. 14). From the intercept a number average molecular weight between entanglements of 7 OOO was calculated. This value is of the same order of magnitude as the value of 4000, which is considered to be the equilibrium number average molecular weight between entangle- ments in a polyethylene melt15). Finally from the slope of this linear part of the curve a crosslinking efficiency of 0,6 mol of hexafunctional crosslinks per mol of peroxide was calculated. The calculated efficiency is rather low compared to the maximum obtainable value of 1 rnol of hexafunctional crosslinks per mol of peroxide. This phenomenon has been previously observed upon peroxide curing of polymers 16) and was found not to be caused by recombination of initiator fragments prior to hydrogen abstraction from the polymer').

P r

754 J. de Boer, A. J. Pennings

Sol-gel analysis rendered a value of 9,28. for the product p * E. So, knowing E, p can be calculated. However, the efficiency E from the Charlesby-Pinner plot referred to tetrafunctional crosslinks, i.e., twice the value obtained from the curve of v * versus the peroxide concentration. With E = 1,2, which is in good agreement with previously published results‘), p = 7,7 implying that only small amount of the polyethylene units will be involved in a crosslink.

Infrared spectroscopy

In order to detect any chemical modification of the polyethylene chains upon crosslinking, IR spectra were taken of the initial polyethylene and the most dense crosslinked network. Apart from the occurrence of a small absorption at 1710 cm-I, probably due to carbonyl groups, and the appearance of a weak absorption at 962 cm- I, due to vinylene unsaturation, the chemical modification upon crosslinking was undetectable small.

To calculate the amount of unsaturation, the absorption at 962 cm-I was determined, using a standard technique to construct the baselinel79 18). With a molar absorptivity of 168 for the vinylene unsaturation at 962 cm-I 1 9 ) , a concentration of one vinylene group for every 11 000th carbon atom was calculated.

In summary, it was shown that rather homogeneous networks from UHMWPE can be synthesized in the melt by means of 1, using the technique described above. Little chemical modification of the polyethylene was detected upon crosslinking. As only a few crosslinks were needed to cause gelation, the loss of crystallinity at low tempera- tures may be small. It is our intention to report crystallization studies, performed on these networks, in the near future.

The authors would like to thank J. J . lager for performing part of the experimental work and W. R. Beukema for his assistance in the infrared spectroscopy work.

This study was supported by the Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).

K.-C. Chen, E. J . Ellis, A. Crugnola, “Technical Papers”, SOC. Plast. Eng., Inc., Brook- field Center, 1981, vol. 27, p. 270

2, A. Posthuma de Boer, A. J. Pennings, J. Polym. Sci., Polym. Phys. Ed. 14, 187 (1976) 3, M. Narkis, J. Miltz, J. Appl. Polym. Sci. 13, 713 (1969) 4, G. A. Harpell, D. H. Walrod, Rubber Chem. Technol. 46, 1007 (1973) 5, A. Charlesby, S. H. Pinner, Proc. Roy. SOC. London, Ser. A: 249, 367 (1959) 6, J. Barton, J. Polym. Sci., Part A-I, 6, 1315 (1968) 7, G. W. van Dine, R. G. Shaw, Polym. Prepr., Am. Chem. SOC. Div. Polym. Chem. 12, (2),

8, W. H. Stockmayer, J. Chem. Phys. 12, 125 (1944) 9, P. J. Flory, Macromolecules 12, 119 (1979)

713 (1971)

Crosslinking of Ultra-High Molecular Weight Polyethylene. . . 755

lo) M. A. Llorente, J. E. Mark, Macromolecules 13, 681 (1980)

‘’1 F. Tang, E. S. Huyser, J. Org. Chem. 42, 2160 (1977) 13) J. E. Mark, Makromol. Chem., Suppl. 2, 87 (1979) 14) J. D. Ferry, “Viscoelastic Properties of Polymers”, Wiley, New York 1970, 2nd edition,

Is) A. N. Gent, J. Polym. Sci., Polym. Symp. 48, 1 (1974) 16) J. D. van Drumpt, H. H. J. Oosterwijk, J. Polym. Sci., Polym. Chem. Ed. 14, 1495 (1976) 17) W. J. Potts, Jr., “Chemical Infrared Spectroscopy”, Wiley, New York 1963, vol. 1, p. 165 ”) H. W. Siesler, K. Holland-Moritz, “Infrared Spectroscopy and Raman Spectroscopy of

19) R. J. Kock, P. A. H. M. Hol, J. Polym. Sci., Polym. Lett. Ed. 2, 339 (1964)

A. N. Gent, V. V. Vickroy, Jr., J. Polym. Sci., Part A-2, 5, 47 (1967)

p. 265

Polymers”, Dekker, New York 1980, p. 148