The reason for the whitening of rubber hot water bags upon stretching: Stress whitening (silver streak whitening)
The hot water bag is made by vulcanizing natural rubber and reinforcing fillers (such as silica or calcium carbonate). The whitening upon stretching is a physical phenomenon and does not indicate a deterioration in quality.
1. Core principle: Stress silver streak whitening
When rubber is stretched under external force:
The large molecules of the rubber are pulled, causing local micro-fractures, and a large number of tiny cavities and fine cracks (silver streaks) are generated inside;
Countless tiny air gaps refract and scatter natural light, making it appear white to the naked eye;
After releasing and contracting, the micro-cracks close, and the white gradually disappears, which is a typical stress whitening phenomenon.
2. Three reasons why hot water bags are more prone to whitening
① More fillers of white powder
Natural rubber is added with calcium carbonate or silica (to reduce cost and increase hardness), and the bonding points between the fillers and the rubber are more likely to have tiny gaps, making the whitening more obvious; pure high-elastic natural rubber rarely whitens.
② More prone to whitening at low temperatures
Low temperature → rubber becomes harder and the molecular flexibility decreases, even slight stretching causes micro-cracks, and hot water bags are particularly prone to whitening when stretched in winter.
③ Aging will aggravate whitening
After long use, the rubber slightly ages and becomes harder and brittle, with more molecular chain breaks, and a large area of whitening occurs when gently stretched, and the white mark does not disappear after recovery (if the aging is severe, it is not recommended to continue using the hot water bag, as it is prone to cracking and leaking).
3. Distinguishing normal whitening from discarded aging
✅ Normal: Whitening upon stretching, the white mark disappears automatically within a few seconds to minutes after releasing, the rubber is soft and elastic
❌ Discarded aging: The whitening mark remains permanently after stretching, the rubber becomes hard and sticky, chipping off, fine cracks, and is prone to cracking and leaking
Tips：
Try to avoid forcefully stretching the edges or mouth of the hot water bag; do not expose to direct sunlight or contact with oils (such as hand cream, engine oil will accelerate the aging, whitening, and cracking of the rubber).
