Khác
với hầu hết các loại cao su tổng hợp, vật liệu đàn hồi PU bị thủy phân ở những
mức độ nhất định. Phần tài liệu sau sẽ so sánh tính kháng thủy phân của các loại
vật liệu đàn hồi PU khác nhau và các cách hiệu quả để cải thiện tính chất này.
…
With
most commercial elastomers the property of hydrolytic stability is not even
quoted in data on their properties and they are regarded as possessing
excellent resistance to aqueous environments due to their chemical structure
being based on all carbon-carbon linked atoms. With polyurethane elastomers,
however, the presence of -COC- and -COOC- groups in the main-chain structure
results in these materials inevitably undergoing some degree of hydrolytic attack
in the course of time. It is primarily the rate of this attack which is
considered (it is assumed to occur in principle) as this varies over a very
wide range, being dependent upon the chemical structure of the polyurethane.
Hence one of the properties most frequently quoted when considering the
limitations of polyurethanes is their susceptibility to hydrolytic attack.
It
is possible to make some general comments on the relative hydrolytic stability
of different polyurethane elastomer structures. The three principal linear
polyol series used in synthesizing urethane elastomers can, for hydrolytic
stability, be ranked in the following order:
polyether >
polycaprolactone > polyester
(highest
stability → lowest
stability)
Other
useful points are:
1.
Amine cured polycaprolactones, stabilized with a carbodiimide, have similar
resistance to polyethers.
2.
Sulphur-cured urethane elastomers have the least resistance to hydrolysis
(possibly the typical amine-based sulphur-accelerator combination used
accelerates hydrolysis) (see Table 13.5).
3.
Carbodiimides (e.g. Staboxal PCD by Bayer) are effective in stabilizing all
polyester urethanes against hydrolytic attack; they are not effective when used
with polyether-based urethane elastomers.
4.
In general it is true that the longer the hydrocarbon chain of the glycol
portion of a polyglycol adipate the more resistant is the polyester to
hydrolysis.
5.
Fungus attack of polyurethanes, especially at relatively low ageing
temperatures (15-30°C) occurs as a generalization. The incorporation of a
fungicide is effective in preventing this effect.
…
Trích
đăng từ sách Polyurethane Elastomers, C. Hepburn, Elsevier Science
Publisher, 1992, trang 380 – 382
(vtp-vlab-caosuviet)
