These are the most well-known free sites.
There are three types of US patents: utility, design, and plant. The vast majority of patents are utility patents, which when granted are good for 20 years.
U.S. “patent documents” include both patents (PatFT database) and patent applications (AppFT database). The International Patent Cooperation Treaty requires that both be searched for prior art.
Patents issued prior to the Internet (and the computer) must be searched in the official patent office sites using the CPC subject classification system; since they are not born-digital they have not been keyword-indexed.
The USPTO recommends starting your search on their site search box, by combining a few words describing your invention with the phrase "CPC scheme," which should bring up the relevant areas of the classification system (see next tab for further information on the CPC system).
A U.S. Patent has three parts.
Patent classification systems are designed for classifying and retrieving technical subject matter such as patents. They are similar to library classification systems like Dewey and Library of Congress.
The top level of the CPC scheme is Section. There are 8 sections: A – Human Necessities, B – Operations & Transporting, C – Chemistry & Metallurgy, D – Textiles, E – Fixed Constructions, F – Mechanical Engineering, G – Physics, and H – Electricity.
US patent # 13/840,818 "Degradable polymers and method for fracking applications" has this classification: C09K8/68.
C = Chemistry & Metallurgy
C 09 K = Materials for Miscellaneous Applications, not provided for elsewhere
C 09 K 8 = Compositions for drilling of boreholes or wells
C 09 K 8/68 = ...containing organic compounds
A thorough patent search doesn't stop with U.S. patents. You need to search worldwide patent databases as well, such as PatentScope. You should also consider searching European and Asian patents using the links in the boxes below.