38
Fuels & Lubes International Quarter Three 2015
VII is amorphous or crystalline in solid form, he said.
He added that higher crystallinity can make the solid
VII less tacky and easier to handle in manufacturing.
In oils, VM polymers are solubilised to dilute
concentration, and characteristics in dilute solution
are different than in solid, he explained. Whether
amorphous or crystalline in solid form, an important
concern is to avoid unwanted interactions in the fluid,
which happen at low temperatures when waxes can
start to form in the base oil or VM. For instance, the
optimised OCP has a low crystallinity in solid form,
said Fang. However, when dissolved in oil, highly
sensitive rheology testing showed negligible yield
stress, indicating no interacting networks between
polymer chains.
Fang provided additional insights on the
development and evaluation of an optimised OCP,
which involved a series of structure property studies to
demonstrate robust low-temperature properties and
advantages in the thickening efficiency, deposits and
extended shear durability. Robust low-temperature
performance is a primary focus, as demonstrated in
tests such as MRV, pour point, Scanning Brookfield,
and used-oil, low temperature tests, such as CEC L105,
with a range of PPD and base oil types. Another benefit
of optimised OCP is low deposit formation in the
high-temperature regions of the engine, due to more
thickening, leading to lower polymer treats.
EXTENDED DRAIN PERFORMANCE
Regarding extended shear stability, Fang compared an
optimised OCP and an HSI star polymer. Both were put
through extended shear cycles, and targeted to meet
the same KV100 after 90-cycle shear in ASTM D7109,
the Kurt Orbahn test. While both passed, the test was
extended to 720 cycles, eight times the 90-cycle limit, at
which point the linear optimised OCP had only sheared
an additional 5%, as opposed to significant deterioration
in the HSI structure after continuing the test beyond
the current limits. Fang pointed out that “different
chemistry really has an impact on shear stability.” The
optimised OCP demonstrated robust extended shear
stability, retaining most of its architecture instead of
breaking down. This can aid in supporting extended drain
performance, he said.
The shear stability index is a measurement of
a viscosity modifier’s ability to resist permanent
viscosity loss due to shear. As Baxter explained, “the
counterbalance to shear stability is thickening efficiency.”
Thickening efficiency is the measure of added viscosity at
100 degrees C per unit mass of polymer. Both are dictated
by molecular weight. High molecular weight molecules
are less shear stable, but more thickening-efficient,
meaning that a formulator has to add less polymer to
achieve the desired viscosity grade.
总体上讲，在“更热的”也就是更苛刻JC08测试中，使用了梳状聚合物
的 4.5 cSt ATF比使用常规PAMA的ATF多耗油0.1%，而使用了梳状
聚合物的6.0 cSt ATF则表现出0.5%的燃料经济性优势。
“我们根本不能推荐使用4.5 cSt的粘度水平，” Stoehr说，因为在
NEDC的额外城市驾驶部分和大部分的JC08中，它进入了混合润滑，
也就是说增加了燃料消耗和摩擦。
长期性能
不管用不用粘度改进剂，其目的在于无论发动机内的应力有多大，要
使发动机油的粘度保持在适当的范围内。Evonik Oil Additives亚太
区技术经理Phil Hutchinson说：“不同的粘度改进剂量具有不同的
配方粘度，即使是用于同一个SAE等级的配方中。”
SAE J300规定了KV100和HTHS150。但是，“通过配方达到两个
粘度目标，如HTHS和KV100，你得到的结果会是低VI油品和差的燃
料经济性，” Hutchinson说。
此外，对于现代发动机油而言，KV100并不是一个好的粘度指
标，HTHS150已取代了这个角色。当配方为了实现好的燃料经济性
时，配方商需要选择最能代表在特定发动机和测试循环中燃料经济
性的粘度计量和温度，他说。
不同的VM相对于燃料经济性，可以表现出非常不同的粘度值，这
取决于测量的是哪种粘度值。例如， Evonik发现，四种VM（梳状聚
合物、PMA、低乙烯OCP和星形HSI）在150摄氏度下测量时，具有类
似的粘度，但KV40的粘度则非常不同。梳状聚合物的KV40值最低，
在低温下稠化最小，在总的3-4%的燃料经济性提升中贡献了1.9%。
Hutchinson的观点对另一种有关低温粘度的观点进行了补充，
即，如果一个发动机可以用这种润滑油进行润滑，那么，可以通过使
用低粘度润滑油来提升燃料经济性。“但，到底是哪种粘度？”确定哪
种粘度计量方法可以最好的代表他们的目标是所有配方商需要考虑
的，他说。最后，看看过去SAE J300中的要求和延长测试可以提供对
粘度改进剂长期性能的更多洞察。
Torsten Stoehr
