Latex
Latex is the stable dispersion (emulsion) of polymer microparticles in an aqueous medium. Latexes may be natural or synthetic.
Latex as found in nature is a milky sap-like fluid found in 10% of all flowering plants (angiosperms).[1] It is a complex emulsion consisting of proteins, alkaloids, starches, sugars, oils, tannins, resins, and gums that coagulates on exposure to air. It is usually exuded after tissue injury. In most plants, latex is white, but some have yellow, orange, or scarlet latex. Since the 17th century, latex has been used as a term for the fluid substance in plants.[2] It serves mainly as defense against herbivorous insects.[1] Many people are allergic to latex.
The word is also used to refer to natural latex rubber; particularly for non-vulcanized rubber. Such is the case in products like latex gloves, latex condoms and latex clothing.
It can also be made synthetically by polymerizing a monomer as styrene that has been emulsified with surfactants.
Contents
Sources
The cells or vessels in which latex is found make up the laticiferous system, which forms in two very different ways. In many plants, the laticiferous system is formed from rows of cells laid down in the meristem of the stem or root. The cell walls between these cells are dissolved so that continuous tubes, called latex vessels, are formed. This method of formation is found in the poppy family, in the rubber trees (Para rubber tree and Castilla elastica), and in the Cichorieae, a section of the Family Asteraceae distinguished by the presence of latex in its members. Dandelion, lettuce, hawkweed, and salsify are members of the Cichorieae. It is also present in another member of the Asteraceae, the guayule plant.
In the milkweed and spurge families, on the other hand, the laticiferous system is formed quite differently. Early in the development of the seedling latex cells differentiate, and as the plant grows these latex cells grow into a branching system extending throughout the plant. In the mature plant, the entire laticiferous system is descended from a single cell or group of cells present in the embryo.
The laticiferous system is present in all parts of the mature plant, including roots, stems, leaves, and sometimes the fruits. It is particularly noticeable in the cortical tissues. Latex may squirt out as a white glue or be difficult to see due to it being clear or not exuding very much. It can be red such as in Cannabaceae.[1]
Productive species
Latex is produced by 20,000 species from over 40 families occurring in multiple lineages in both dicotyledonous and monocotyledonous of plant. It is also found in conifers and pteridophytes. Several members of the fungal kingdom also produce latex upon injury. Notable are the milk-caps such as Lactarius deliciosus. This suggests it is the product of convergent evolution and has been selected for on many separate occasions.[1]
14% of tropical plant species create latex but only 6% of temperate ones.[3]
Defense function
Latex functions to protect the plant from herbivores. The idea was first proposed in 1887 by Joseph F. James who noted that latex“. . .it carries with it at the same time such disagreeable properties that it becomes a better protection to the plant from enemies than all the thorns, prickles, or hairs that could be provided. In this plant, so copious and so distasteful has the sap become that it serves a most important purpose in its economy.”[4]
Evidence showing this defense function include the finding that slugs will eat of leaves drained of their latex but not intact ones, that many insects sever the veins carrying latex before they feed, and that the latex of Asclepias humistrata kills by trapping 30% of newly hatched monarch butterfly caterpillars.[1]
Other evidence is that latex contains 50–1000 higher concentrations of defense substances than other plant tissues. These toxins include ones that are also toxic to the plant and consist of a diverse range of chemicals that are either poisonous or "antinutritive". Latex is actively moved to the area of injury in the case of Cryptostegia grandiflora this can be more than 70 cm.[1]
The clotting property of latex is functional in this defense since it limits wastage and its stickiness traps insects and their mouthparts.[1]
It has been noted that while there exist other explanations for the existence of latex including storage and movement of plant nutrients, waste, and maintenance of water balance that "Essentially none of these functions remain credible and none have any empirical support."[1]
Synthetic latices
Examples of synthetic latices are Styrene-butadiene Rubber, Acrylonitrile butadiene styrene, acrylic polymers, polyvinyl acetate.
Uses of latex
The latex of many species can be processed to produce many materials.
Natural rubber is the most important product obtained from latex; more than 12,000 plant species yield latex containing rubber, though in the vast majority of those species the rubber is not suitable for commercial use.[5]. This latex is used to make many other products as well, including mattresses, gloves, swim caps, condoms, catheters and balloons.
Balatá and gutta percha latex contain an inelastic polymer related to rubber.
Latex from the chicle and jelutong trees is used in chewing gum.
Dried latex from the opium poppy is opium, the source of many useful opiates and other alkaloids of high value.
Synthetic latexes are used in coatings (e.g. latex paint) and glues because they solidify by coalescence of the polymer particles as the water evaporates, and therefore can form films without releasing potentially toxic organic solvents in the environment. Manufacturers of latex polymer products must keep on top of microbiological contamination in raw materials, intermediates and final products using methods like the ATP test to prevent product spoilage.[6] Other uses include cement additives.
Latex, usually styrene based, is also used in immunoassays.
Latex clothing
Latex is used in many types of clothing. Worn on the body (or applied directly by painting) it tends to be skin-tight, producing a "second skin" effect.
Allergic reactions
Some people have a serious latex allergy, and exposure to latex products such as latex gloves can cause anaphylactic shock. Guayule latex is hypoallergenic and is being researched as a substitute to the allergy-inducing Hevea latexes. Additionally, chemical processes may be employed to reduce the amount of antigenic protein in Hevea latex, yielding alternative materials such as Vytex Natural Rubber Latex which provide significantly reduced exposure to latex allergens.
Many people with spina bifida are also allergic to natural latex rubber, as well as people who have had multiple surgeries, and people who have had prolonged exposure to natural latex.
See also
Notes
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zh:乳胶- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Agrawal AA, Konno K. (2009). Latex: A Model for Understanding Mechanisms, Ecology, and Evolution of Plant Defense Against Herbivory. Annu. Rev. Ecol. Evol. Syst. 40:311–31. doi:10.1146/annurev.ecolsys.110308.120307
- ↑ Mahlberg PG. (1993). Laticifers: an historical perspective. Bot. Rev. 59:1–23. JSTOR 4354199
- ↑ Lewinsohn TM. (1991). The geographical distribution of plant latex. Chemoecology 2:64–68
- ↑ James JF. (1887). The milkweeds. Am. Nat. 21:605–15. JSTOR 2451222
- ↑ Bowers, J.E. (1990). Natural Rubber-Producing Plants for the United States. Beltsville, MD: National Agricultural Library. pp. 1,3. OCLC 28534889.
- ↑ http://www.luminultra.com/dmdocuments/Product%20Validation%20-%20Latex_Polymer%20Emulsions%20QGOM.pdf
