Ball-milling Promoted Chemical Bonding between Cellulose and Plastics - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 232

9.5.2.1 Formation of an Interphase between Cellulose and
Polyethylene Phases by Ball Milling
The crystallinity of FC is retained even through melt-mixing FC and MPE 41
and, accordingly, there are almost no OH groups free from hydrogen
bonding on the surface of the FC particles that are probably capable of
forming ester bonds with MA groups of MPE. As described before, ball
milling is an effective method to activate crystalline cellulose, that is, to
change the crystalline state of cellulose into an amorphous state so that a
great number of free OH groups can be formed on the surface of the re-
sultant fine particles of amorphous cellulose. Therefore, our interest is dir-
ected to mechanochemical synthesis of composites of fibrous crystalline
cellulose (FC) with MPE by ball milling. 41
Mechanochemical compounding of FC and MPE has been performed in a
Planetary Ball Mill (Pulverisette 5, Fritsch, Germany) equipped with four jars,
each 500 cm 3 in volume and loaded with 25 balls (diameter: 1.5 cm). Milling
of FC (13.5 g) and MPE (MA graft content: ca. 0.8 wt%; 31.5 g) in each jar at
250 rpm with a cyclic mode of 50 min milling followed by a 10 min pause, for
24 h, yields a mixture with a FC content of 30 wt% (MCMix); part of the
MCMix was then further melt-mixed in an R60 mixer (Toyo Seiki; Japan) at
180 1C for 30 min to obtain a mechanochemically synthesized composite
(MCCom). For comparison, another composite (UMCom) has been prepared
with the same composition but only by melt-mixing under the same con-
ditions as described above.
The extent of MPE bonded onto cellulose in a product can be estimated by
completely removing free MPE in the product by solvent extraction. The
residue fractions of MPE are 3.1% (UMCom), 19.3% (MCMix), and 22.3%
(MCCom), based on the total weight of MPE in each material. As the weight
loss of genuine MPE is 100% by the same treatment, this demonstrates
clearly that ball milling induces extensive bonding of MPE molecules onto
cellulose even in a solid state without any solvent, in marked contrast to the
case of UMCom by usual melt-mixing.
IR spectra of the UMCom, MCMix, and MCCom are shown in Figure 9.22,
compared with those of MPE. The MPE has characteristic bands at 1792
cm 1 and at 1713 cm 1 due to saturated succinic anhydride groups 32 and
their hydrolysis derivatives, respectively, as the latter band disappears and
the former one increases in intensity after drying. Such behavior is observed
on the MCMix before and after drying (Figure 9.22d and e), suggesting free
MPE still remaining in the mixture after ball milling. Notably, the MCCom
and dried MCMix samples exhibit characteristic signals at 1733-1735 cm 1
as well as at 1717-1719 cm 1 , in marked contrast to the UCCom sample,
which shows no signals in this region. The signal at 1733-1735 cm 1 is as-
signed to ester groups between OH groups of cellulose and MA groups of
MPE. The formation of such ester bonds liberates a carboxylic acid in each
relevant MA graft, and this is consistent with the appearance of the signal at
1717-1719 cm 1 . These results indicate clearly that ball milling causes