Ball-milling Promoted Chemical Bonding between Cellulose and Plastics - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 225

MA onto PP in melt state 33 is usually operated for 10-20 min but at high
temperature, ranging from its melting temperature to 200 1C, accompanied
inevitably by the degradation of the polymer. In contrast, mechanochemical
grafting of MA by ball-milling can be performed at lower temperatures,
though longer times are needed. 25,26
Mechanochemical grafting of MA onto i-PP 25,26 has been performed by use
of a Planetary Ball Mill (Pulverisette 5, Fritsch, Germany) equipped with four
jars, each of which is 500 cm 3 in volume and loaded with 25 balls of 15 mmf
diameter. Usually, 50 g of PP is loaded in each jar with given amounts of MA
as well as benzoyl peroxide (BPO) as an initiator, and is milled at a rotational
speed of 300 rpm for 8 h, with a cyclic mode of milling (50 min) followed by a
pause (10 min). The temperature inside the jars is 75-80 1C during the
milling steps, which is much lower than that in the melt-mixing. Figure 9.15
shows that the grafting of MA onto i-PP proceeds abruptly during the re-
action time of 1-2 h to almost finish after 4 h; the melt flow index (MFI)
increases significantly from the beginning to 1 h, and then quickly decreases
until 2 h, after which it almost maintains a plateau (Figure 9.16). These
results indicate that b-scission occurs at the beginning, generating frag-
ments with smaller molecular weights, until a certain amount of macro-
radicals with tertiary carbon radicals is attained, beyond which grafting of
MA on the macroradicals occurs competitively with combination of fragment
radicals (secondary carbon radicals) resulting from b-scission of the mac-
roradicals. It is, therefore, expected that the grafting of MA onto i-PP de-
pends strongly on the initially added amounts of MA as well as BPO. The
amounts of MA and BPO hereafter are given in the unit of wt% that is based
on the weight of i-PP used.
The graft yield is plotted against the initial amount of MA under a con-
stant amount of BPO of 1.0 wt% in Figure 9.17. The graft yield is ca. 90%,
which is very high, in the range of MA r 1.0 wt%, beyond which it decreases

1.4 100

1.2

80

1.0

60

0.8

0.6

40

0.4

20

0.2

0.0 0

02468 0 2 4 6

Reaction time (h)

Figure 9.15 Changes of graft degree ( J ) and yield of MA onto i-PP ( & ) with reaction
time under a weight ratio of i-PP/MA/BPO ¼ 100/2/1.
Reprinted from ref. 26 with permission of John Wiley & Sons.