主题：模塑后的尺寸稳定性（第一部分）

Physical aging can cause slow changes in amorphous polymer dimensions and properties after molding. Stretch-blown bottles kept in a hot warehouse can suffer measurable loss of impact strength in just a few weeks. (Photo: Sidel)

Amorphous polymers are generally considered to be a molder’s insurance policy against the problems of dimensional stability.

无定型聚合物通常被认为是注塑商解决尺寸稳定性问题的保险策略。

Amorphous structures undergo a smaller and more predictable change in volume as they cool from the melt to the solid, making them easier to mold to close tolerances. If you study the shrinkage behavior of a part molded in an amorphous material, you will also find that the part achieves stable dimensions in a shorter period of time. However, over time amorphous polymers also undergo a slow and subtle structural change that can result in continued shrinkage. This process is known as physical aging and it was not well understood in polymers until the 1970s.

Any process that involves melting and re-solidifying a polymer involves a compromise between achieving the perfect structure and producing a part that can be sold at a price that the market is willing to pay. Optimal structural stability is achieved by allowing the polymer chains in a system to reach their ideal configuration in terms of spatial separation. In a semi-crystalline material, as we have already discussed, this means attaining an optimal degree of crystallinity. In amorphous polymers, where no significant level of crystallinity is obtained, the ultimate objective is something called thermodynamic equilibrium. In both cases this involves allowing the polymer chains to reach their ideal arrangement at the molecular level. This is typically achieved by maintaining the material at an elevated temperature for a prolonged period.

任何涉及到聚合物熔融和再固化的工艺，都要涉及到一种平衡：达到完美的结构和生产一种市售价格的部件之间的平衡。允许某个体系中的聚合物链达到其空间分隔状态下的理想构象，就能达到最优的结构稳定性。在一种半结晶材料中，象我们讨论的那样，这意味着获得了最大的结晶度。无定型聚合物无所谓得到显著的结晶度，其最终目标是达到所谓的热力学平衡。在这两种情形中，都涉及到允许聚合物链在分子水平达到其理想排列。在高温下延长时间就能做到这点。

Unfortunately, perfection requires too much time to be practical from an economic standpoint. So the objective of good process control is to achieve a level of structural stability that is adequate for applications in the real world. When a part is molded in an amorphous polymer, the material has not reached thermodynamic equilibrium. It spends the rest of the life of the product trying to get there. Physical aging can be thought of as the slow contraction of the polymer as the chains get closer, collapsing into the excess free volume that remained when the part was first produced. This closer approach produces a structure that is stronger and stiffer than the original matrix, however it also loses toughness in the process.

The rate at which physical aging occurs is a function of the difference between the application temperature and the glass-transition temperature (Tg) of the polymer. The closer the temperature is to the Tg the more rapidly physical aging occurs. So it is not surprising, in retrospect, that one of the first practical implications of physical aging was observed in an amorphous polymer with a relatively low Tg, PET polyester.

物理老化发生的速度是聚合物加工温度与聚合物玻璃化温度差的函数。加工温度越接近玻璃化温度Tg，物理老化就越快发生。所以，回过头来看就没有可惊讶的了，物理老化的实际含义之一可以在具有相对低的Tg的无定型聚合物—PET聚酯上得到印证。

The amorphous PET used to mold preforms and then blow beverage bottles has a Tg of about 172 F (78 C). At room temperature, the degree of physical aging needed to produce a measurable change in mechanical properties can be as long as one to two years. However, when bottles were stored in a warehouse at 120 F (49 C), this reduced the gap between the application temperature and the Tg by about 50% and accelerated the rate of physical aging by approximately an order of magnitude. Bottles stored under these conditions exhibited a measurable loss in impact strength in just a few weeks.

无定型PET用于模塑型胚，然后吹塑饮料瓶。无定型PET具有一个大约172 F (78 C)的玻璃化温度。在室温下，在物理性能上产生显著变化所需要的物理老化速度可能要长达一到两年。但是，当瓶子储存在. 120 F (49 C)的仓库中时，它就减少了曝露的温度和Tg之间的差值，可达50%，从而加速物理老化速度达1个数量级。在这个条件下储存的瓶子，只要经过几周，就会显示出冲击强度的显著损失。

Physical aging only occurs in what material scientists refer to as glasses, which are essentially any amorphous material that exists as a “solid.” All commercial polymers, even semi-crystalline ones like polyethylene and nylon, contain some amorphous regions and exhibit a corresponding glass transition. Therefore, technically it is possible for semi-crystalline materials to undergo physical aging and the associated changes in properties.

物理老化只发生在科学家们称之为玻璃态材料中间，玻璃态材料基本上可以是任何以“固态”存在的无定型材料。所有的商用聚合物，即使是PE和尼龙那样的半结晶聚合物，也含有某些无定型区域，显示出某种相应的玻璃态。所以，从技术看，半结晶材料有可能出现物理老化和相关的性能变化。

However, in most cases these changes are overshadowed by the contributions from the crystalline phase. But some semi-crystalline materials achieve a relatively low level of crystallinity that is highly dependent upon processing conditions. These tend to be the high-performance materials such as PEEK and PPS, where failure to cool the polymer at an appropriate rate will result in an almost completely amorphous structure. In these materials physical aging can occur and has been observed along with the associated property changes.

但是，在许多情况下，这些变化被结晶相的贡献掩盖了。但是某些半结晶材料只是达到了一种相对低的结晶度，因为其结晶高度依赖于加工条件。这种情况多存在于高温材料，如PEEK和PPS。在这些材料中，如不能以恰当的速度冷却聚合物，将会导致几乎完全的无定型结构。在这些材料中，物理老化以及相关性能的变化可能出现并被观察到。

Because physical aging involves a change in volume, it can be detected as a change in dimensions. This represents a relatively small dimensional change, as a percentage of the size of the part, so detection either requires making very precise measurements or the part must be very large so that this small percentage results in a readily measurable difference. Precise dimensions are not demanded in a beverage bottle, but there are industries that make measurements in microns. They have observed that parts molded in amorphous polymers with low glass-transition temperatures, such as rigid PVC, continue to exhibit a very small degree of dimensional change over a period of months. These changes are much smaller than the ones that occur due to the solid-state crystallization that we discussed previously, but they can still result in a part that drifts out of print over an extended period of time, even though the parts are only exposed to room temperature.