Ninalanyon and others make very valid points about those bottle "bricks, beyond simply doubting their durability (which I doubt too).
There are many different types of "plastic", falling into basically two classes of synthetic material, but the word has been applied to all types making any public discussion on them, difficult.
It's important to know what the words even mean. What is "Plastic"?
A Plastic substance is a solid whose shape can be distorted, and retains that new shape. Examples include wet clay at room temperature, candle-wax at higher temperatures, and steel at incandescent heat (hundreds of degrees Celsius).
[i]"Plastic" synthetic materials fall into two main classes:[/i]
Thermosetting Plastics. Mainly the synthetic resins used alone or mixed with an inert filler, forming a viscous liquid or a plastic material that can be moulded into shape before it solidifies permanently by chemical reaction into a rigid solid. They tend to be hard, with low elasticity and no ductiliity. E.g., 'Bakelite', adhesives such as 'Araldite', and the composite materials like 'Tufnol', carbon-fibre and glass-fibre.
Thermoplastics: These soften on being heated then hold their moulded form on cooling, without chemical change (unless over-heated). Many stay soft and slightly elastic but the moulded form can be damaged permanently at quite low temperature. E.g. PV, Polyethylene, Polypropylene. They do not melt into low viscosity without starting to decompose chemically.
"Plastic" natural materials include beeswax and rubber (latex). These can be melted to sufficiently low viscosity for appropriate moulding methods. Being natural they will eventually decompose but suitable binders and preservatives indefinitely but considerably extend their lives. E.g., sulphur is added to vulcanise rubber.
Clay mixed with water is plastic then fired to create rigid, brittle ceramics. Or is mixed with limestone and fired then powdered to make cement: not plastic but rigid. Steel at room temperature is rigid but elastic (varying by type and treatment); but is forged in a plastic state (heated to glowing bright orange).
Common Uses and Re-uses:
Thermosetting plastics: adhesives, electrical and heat insulators, car and boat panels, wind-turbine blades.
Can NOT be salvaged as for re-use as they are. The most that might be done with them is pulverising the scrap to create a filler for new composites using new synthetic-resin, but this might be problematical for glass-fibre and carbon-fibre.
Thermoplastics: Their many types with widely differeing natures mean overall, they are valuable in specific ways for specific uses including electrical insulation, water-pipes, building components; vehicle components; rope and textiles; bottles, toys, shopping-bags, food wrappings.
Certain types can be salvaged and pulverised into granules amenable to heat-moulding into new forms so "recycling" thwe material (not the items!); but some such "new" plastics can be chemical and physical mongrels with their own properties, and might not be especially recoverable.
Longevity
Plastic materials' lives are so heavily dependent on their individual chemistry and the conditions they meet that really, their longevity is anything from months to unknown. The synthetic-resins are stable unless exposed to strong sunlight over a long time, as sunlight's UV radiation will degrade them. Most thermoplastics behave simillarly so protectives are added where necessary, such as for uPVC building components. We cannot really know how long plastics buried or dumped in the sea, will last because they have only been around for a century!
Disposal.
The huge range of synthetic plastics and their ubiquity makes disposing of scrap plastics problematical to say the least. It is not helped by the sheer waste practised by "developed" countries demanding ever more needless or wasteful uses: toys and novelties, clothes worn once and thrown away (last week's colour, darling); "disposable" goods, "microplastic" beads in toothpaste and cleansers; tap-water sold in polyethylene bottles in regions with potable tap-water; plastic items of genuine use merely thrown in the household refuse when broken or no longer wanted. (Often including metals, as in electrical and electronic items that can and should be collected for materials-recovery.)
The natural plastics' longevity, according to processing and conditions, can be notorious as with the world's bank of umpteen millions of old tyres, made of rubber, carbon-black and "Nylon" (a thermoplastic) or steel-wire reinforcement. Some are pulverised to recover the steel and create rubber granules for artificial turf and playground surfacing.
Some of the marine plastics is from lost or discarded fishing and marine equipment. E,g, Polypropylene (a thermoplastic) is a very good rope material but decomposes fairly rapidly in sunlight to tiny flakes. Another lot are the granules added to cleansers, and some manufacturers have replaced them with natural alternatives. Most though, is pure, inexcuseable litter left or jettisoned by people too idle to take it back home for proper disposal.
Those images of people in poor countries, scraping through vast heaps of discarded plastc bottles, and laudable but desperate attempts to use them as short-term building-materials, show just how wasteful it all is.
Although using the bottles as "bricks" answers two problems at once it can only be a short-term answer and once the bottled degarde, what will happen to their fragments?
WHY are there such huge tips of the things in the first place? WHY is there so much plastic in the sea?
It is NOT the materials' fault beyond their potential but very variable longevity, and that only certain types can be salvaged..
It is the fault of people: wanting needlessly wasteful plastic products, being ignorant, lazy or downright wilful as users, but also many countries genuinely lacking proper collection systems for scrap plastics, metals, electrical and paper products.
It would be better for those countries in many ways to establish processing plants to recover and re-process those salvageable materials, rather than using mere landfills or open tips for people to scavenge from.
......
Supreme irony.....
Most of the world is slowly and painfully moving from using coal, oil-products and gas for electricity generation, initially to try to ameliorate human-wrought climate-change, but those minerals will become depleted in time anyway.
The replacements include vast wind-turbines and solar-arrays.
Both, but especially the turbines, and the associated equipment require considerable amounts of thermosetting and thermoplastic materials. The turbines, especially off-shore, also need large volumes of lubricants, hydraulic oils and high-grade paints - the last are types of synthetic-resin.
The raw chemicals for these materials and almost all synthetic plastics, are derived from Petroleum. The finite mineral, a.k.a. Crude Oil, that many of the Green types think only a "fossil-fuel".
