13 January 2008

Stiffening in the cold: more on condoms in the Canadian winter

"The elastomers—natural rubber and polychloroprene— ... are susceptible to crystallization during prolonged exposure to low temperatures. This leads to a gradual long-term stiffening. ... Stress–strain measurements have confirmed the extremely large increase (up to 100-fold) in the initial stiffness that crystallization produces." (Fuller KNG, et al. The effect of low-temperature crystallization on the mechanical behavior of rubber. Journal of polymer science: Part B: Polymer physics. 2004;42:2181-90.)

The rubber librarian has been at it again, straining hard to stretch a bit more evidence over a vast gap in the condom literature. Last month I posted on my failure to find any scientific literature on the effect of extreme cold on latex condoms. But the thrust of my investigation didn't stop there. Not trusting my own abilities to probe the strange literature of latex, I consulted with a colleague in the Sciences and Technology Library who knows well the ins and outs of the relevant databases, just to make sure that I hadn't missed anything vital. After a prolonged search he found a number of seminal papers on the influence of low-temperature crystallization on the tensile elastic modulus of natural rubber. My enthusiasm for the subject momentarily bounced back, until I realized that once again the condom was getting no respect. All that rubber research and not one mention of condoms. Undeflated however, I carried on. Here is a brief survey of the science of gelid latex, and anything useful pertaining to condoms that can be extracted from this small body of knowledge. A full bibliography with abstracts is given below as an appendage.

To get any insight into what "tensile elastic modulus" exactly means, think stretchability or "elongatability." Modulus is a mathematical term that was appropriated by the British scientist Thomas Young in the 18th century to express the physical measure of stiffness, equalling the ratio of applied load (stress) to the resultant deformation of the material, such as elasticity or shear. (A high modulus indicates a stiff material.) Having thus stretched my high school chemistry to the snapping point by reading through a torrent of exceedingly dull prose, I finally reached a partial understanding of inspissation and cold crystallization and their effect on tensile elastic modulus. To avoid undue rigidity of language, let us translate this jargon-splotched no-man's-land of technolinguistic barbed wire and chevaux-de-frises into a more flexible dube-ological vernacular (kondomswissenschaftliche Umgangssprache). The upshot of seventy years of low-temperature rubber research is that it gets hard in the cold. The non-scientific majority of humanity must be truly grateful for this remarkable advance in rubber research.

What does this all mean for the hardy condom user? Because cold tends to "crystallize" rubber, this leads to a progressive increase in density, gradual long-term stiffening, and a doubling of tensile elastic modulus ... of the condom, not its wearer. None of the literature discovered by my research actually concerns itself with the common condom, but all the science points to a Canadian winter's ability to make rubbers slightly brittle, which could possibly — and I emphasize possibly — lead to leaking or breakage. Not to elongate this explanation more than the kinetic measurements allow, it seems clear that the effect of arctic air on a condom's stress-strain characteristics, in reverse proportion to its effect on the body part for which the condom is designed, is one of stiffening and tensile swelling. Furthermore, as Natarajan cogently reminds us [6], free radicals formed during tensile testing at low temperatures are stable below the glass transition temperature of the material. (These radicals arise from main-chain fracture occurring during yielding of the material — and too-frequent reading of Bakunin in unheated garrets. Natarajan also suggests that yielding of the material which gives rise to these characteristics occurs by crazing of the material — reading Bakunin in an unheated garret during a Winnipeg winter.)

The existing research suggests that public health officials might consider ensuring that condoms for distribution by clinics and street health workers are not stored at extreme winter temperatures. Individuals should not keep their condoms in glove compartments, unheated back porches, or hidden behind the snow blower in the garage. Most package directions already recommend a normal range of acceptable temperatures for safe storage. Maybe they are right.

A condom manufacturing company with whom my local health authority has dealings responded by email to an official request for their position on condom storage. The company's reply stated that in their opinion there is no risk in storing condoms in extreme cold, as long as they are not thawed out with the application of high heat, but are allowed to come gradually to a normal temperature. A follow-up email was sent to the company asking why, this being the case, large boxes bought at wholesale containing hundreds of condoms have printed instructions not to store their contents in extreme heat or cold. To date no reply has been received. This anecdote is no proof that condom manufacturers have no answers, but it does demonstrate how the lack of research on this issue means that the concerns of public health departments cannot be resolved by resorting to corporate public relations offices.

Other questions come to mind. Even if the storage of condoms in extremely cold environments, caeteris paribus, has no effect on their integrity, what guarantee is there that the cold would never contribute to damage caused by the often imperfect conditions that pertain in warehouses? What if, for example, a large box full of condoms were dropped from a truck or a fork lift at a temperature well below zero, or were otherwise jostled, jounced or dented? Might the cold, having stiffened the latex, not contribute further to any resulting damage to individual condoms? Is it possible that the increased modulus and crystallization of the latex might contribute to minute tears that could cause leakage or breakage when the condom is eventually used? What is the effect of extremely low humidity and excessive cold on condom integrity? Could this combination further contribute to damage from being bumped or dropped in storage?

This will have to be our last word for now on condoms and low temperatures, until a free radical bounces upon the scene to answer all our questions, electrify the rubber world, and warm the hearts of Canadian street health workers with a path-breaking, definitive study.

Flecte quod est rigidum,
Fove quod est frigidum,
Rege quod est devium.

Bend what is stiff,
Warm what is cold,
Guide what goes off the road.

Archbishop Stephen Langton, d. 1228


1. Specifying elastomers for low temperature service. Materials and methods. 1953 Nov;38(5):114-8.
Changes occurring in rubbers as result of exposure to low temperature; how common elastomers differ in their low temperature behavior; improvements in low temperature flexibility obtained by use of certain types of plasticizers in compound; chart shows minimum temperatures at which various elastomers are useful.

2. Chenal JM, Chazeau L, Bomal Y, Gauthier C. New insights into the cold crystallization of filled natural rubber. Journal of Polymer Science, Part B: Polymer Physics. 2007 Apr;45(8):955-62.
This article is devoted to the cold crystallization of filled natural rubber with different types of filler such as carbon black, silica, and grafted silica. A large set of differential scanning calorimetry data is presented with various scanning rates, times, and temperatures of isothermal crystallization to display the factors affecting natural rubber (NR) crystallization. The crystallization kinetic measurements suggest that fillers can create a region with perturbed mobility where the kinetics of nucleation and/or growth are slowed down, the rest of the matrix being unperturbed. And, the final crystallization level indicates the existence of an excluded region for crystallization close to the filler surface. Furthermore, the presence of fillers appears less unfavorable to NR crystallization than chemical crosslinking.

3. Douglas WD. Mechanical properties of rubber in compression at low temperature. India rubber journal. 1930 Dec;80(25):9-11.
Investigation on effect of low temperatures on stress-strain characteristics of rubber in compression; tests were made upon half-inch cubes of black rubber of specific gravity 1.3 cut from single molded ring, such as might be used in springing of aircraft tail skids.

4. Fuller KNG, Gough J, Thomas AG. The effect of low-temperature crystallization on the mechanical behavior of rubber. Journal of polymer science: Part B: Polymer physics. 2004;42:2181-90.
In cold climates the correct performance of rubber components such as seismic isolators depends on them maintaining their elastic properties when exposed to prolonged periods at low temperatures. The high damping compounds developed for seismic isolation are normally especially prone to crystallization when exposed to subzero temperatures for periods of a few weeks. The effect of low-temperature crystallization on the mechanical stiffening of natural rubber is evaluated. The relationship between the shear modulus and amount of crystallization is measured using a technique in which the dimensional change and stiffness are monitored simultaneously. The relationship is found to be approximately independent of the crosslink density and the temperature of crystallization. It appears not to be realistically modeled by considering the crystals to behave as rigid filler particles but good qualitative agreement with experiment was obtained by modeling the crystals as a network of threads. Partially crystalline rubbers are found to yield under the application of a large stress like other partially crystalline polymers. Mechanisms for suppressing crystallization in rubber are discussed and the low-temperature stiffening of specially formulated rubber compounds for seismic isolation is presented. These results show that carefully formulated high damping natural rubber compounds can give adequate performance at low temperatures.

5. Ho CC, Khew MC. Low glass transition temperature (Tg) rubber latex film formation studied by atomic force microscopy. Langmuir. 2000;16(6):2436-49.
Latex with very low glass transition temperature (Tg) polymers forms a continuous film on drying. The physical and mechanical properties of the film are dependent on the extent the latex particles are able to coalesce and fuse into each other. Any hindrance to the film formation process would result in a poorly formed film and a drop in performance. The film formation process of natural rubbert (Tg approx. -65C) latexes and synthetic latexes with low Tg are monitored as a function of time using atomic force microscopy (AFM). The influence of the leaching method of the film, the presence of additives (some added after preparation) and nonrubber materials [specific for natural rubber (NR) latex only], and gel content on film morphology and flattening of the particles in the film is studied. The influence of the leaching procedure on the effectiveness of nonrubber removal from NR latex films and their effect on film formation is highlighted. The effects of nonrubbers and high gel content of NR latex in slowing down the NR film formation is discussed and contrasted with the synthetic polyisoprene and chloroprene latexes. The change of the surface mean roughness, Ra, with time provides a convenient means of comparing the rate of flattening of the polydisperse particles in these films.

6. Natarajan R, Reed PE. Molecular fracture in natural rubber during tensile testing at low temperatures. Journal of Polymer Science, Macromolecular Reviews. 1972 Apr;10(4):585-98. Sulfur-cured natural rubber and other elastomers subjected to tensile tests at low temperatures and low strain rates are found to swell and left double quote foam right double quote after testing when brought to room temperature. Free radicals formed during tensile testing are studied by (ESR) techniques. It is found that the free radicals observed at the low temperatures are stable below the glass transition temperature of the material, and it is suggested that these radicals arise from main-chain fracture occurring during yielding of the material. It is also suggested that yielding of the material which gives rise to these characteristics occurs by crazing of the material.

7. Spanos P. Cure system effect on low temperature dynamic shear modulus of natural rubber. Rubber world. 2003 Nov;229(2):22-7.
The effects of cure system on low temperature properties of natural rubber are discussed. High crosslink density cure systems were used for minimizing the crystallization induced shear modulus increase at low temperatures. Modulus measurements were made using dual lap shear samples on a servohydraulic dynamic test machine. The results show that the most pronounced changes in modulus occurred with the lowest sulfur formulation. All of the modified cure systems showed a much smaller change in modulus with decreasing temperature.

8. Stevenson A. Crystallization stiffening of rubber vulcanizates at low environmental temperatures. Kautschuk und Gummi Kunststoffe. 1984 Feb;37(2):105-9.
In environments with low ambient temperatures, several types of rubber vulcanizate can stiffen due to the formation of a crystallite structure in the rubber. The elastic modulus can increase by up to two orders of magnitude. The paper discusses the relevance of the stiffness changes to the performance of various rubber engineering components in cold environments - e. g. bridge bearings, helicopter rotor bearings and offshore mooring bearings. The stiffening of several natural rubber and polychloroprene vulcanizates, specified for engineering applications, has been studied at temperatures from minus 40 degree C to plus 5 degree C, using direct measurements of elastic modulus. The paper also reports on the correlation between changes in elastic modulus and results from the existing standard (ISO) tests - low temperature compression set and low temperature hardness.

9. Stevenson A. Influence of low-temperature crystallization on the tensile elastic modulus of natural rubber. Journal of Polymer Science, Polymer Physics Edition. 1983 Apr;21(4):553-72.
Data are presented which show that when natural rubber crystallizes at low temperatures, there is an increase in elastic modulus of up to two orders of magnitude. This phenomenon has beens studied at various temperatures in the range 0 to minus 55 degree C for samples held at tensile strains of up to 500%. There is an induction period associated with the nucleation of crystallites, before any increase in modulus is observed. The induction period increases with decreasing strain and passes through a minimum with increasing temperature at minus 25 degree C. The growth rate subsequent to nucleation is successfully described in terms of Avrami-type rate relationships. The Avrami rate coefficient is independent of temperature and follows a simple exponential function of strain. The equilibrium extent of the modulus incease has also been studied by means of experiments of up to three months' duration. The equilibrium modulus increases with decreasing temperature - as predicted by Flory's thermodynamic theory.

10. Yu HQ, Liu, XH. [Study on shear properties of low-temperature modified nature rubber]. Guti Huojian Jishu/Journal of Solid Rocket Technology. 2006 Jun;29(3):222-4. Chinese.
Compared with the relationship between shear strength and strain of nature rubber, the shear properties of the low-temperature modified nature rubber at -30-50C. was investigated. The analysis results show that the shear modulus of the low-temperature modified nature rubber kept for 6 hours at -30C or 50C is close to shear modulus at room-temperature when the shear stress is up to 343 N. Furthermore, the adhesive quality of the modified nature rubber with metals and other composite materials is very good, which can meet the adhesive strength demand of component in the temperature range -30-50C.